Removal of Inorganic Pollutants and Recovery of Nutrients from Wastewater Using Electrocoagulation: A Review

Abstract

Water pollution is a major concern due to its detrimental effects on the environment and public health. The particular danger of inorganic pollutants arises from their persistent toxicity and inability to biodegrade. Recently, electrocoagulation (EC) has been demonstrated as an alternative sustainable approach to purifying wastewater due to the increasingly strict pollution prevention rules. In particular, EC has been used to remove inorganic pollutants, such as Cr, Zn, Pb, or As. EC has emerged as a sustainable tool for resource recovery of some inorganic pollutants such as N and P that, when recovered, have value as plant nutrients and are critical in a circular economy. These recovered materials can be obtained from diverse agricultural drainage water and recycled as fertilizers. In this work, a state-of-the-art technique is reviewed describing the advances in contaminant removal and nutrient recovery using EC through an in-depth discussion of the factors influencing the contaminant removal process, including operating pH, time, power, and concentration. Furthermore, limitations of the EC technology are reviewed, including the high-power consumption, fast deterioration of the sacrificial electrodes, and the types of contaminants that could not be efficiently removed. Finally, new emerging constructs in EC process optimization parameters are presented.

1. Introduction

Supplying clean water is crucial, and therefore, it is a concerning issue for humans [1]. For this, several coagulation techniques have been developed to treat water from contaminants. Two coagulation techniques take the most interest, which are chemical coagulation and electrocoagulation processes (ECs). However, EC seems to be more effective and greener. In this regard, Hussein et al. investigated the removal of reactive blue dye from wastewater using both chemical coagulation and electrocoagulation methods. Their results involved comparing different coagulants, such as ferric sulfate (Fe₂(SO₄)₃), poly aluminum chloride (PACl), magnesium chloride (MgCl₂), and aluminum chloride (AlCl₃). They demonstrated that the chemical coagulation by Fe₂(SO₄)₃ achieved a maximum dye removal of approximately 96% under optimal conditions, while EC with aluminum electrodes showed higher efficiency and reached a 98% removal efficiency [2]. This comparison shows the advancement of EC as a technique over the chemical coagulation process in addition to dealing with a lower quantity of chemicals than conventional processes, which means more environmentally friendly. However, inorganic pollutants present significant hazards to humans and all living tissue. In contrast to organic pollutants, heavy metals cannot be broken down through biological processes, as they possess a tendency to accumulate in living bodies, while several heavy metal ions are acknowledged to possess hazardous or carcinogenic effects [3]. Inorganic pollutants, such as Cr, Zn, Pb, P, N, Hg, and other heavy metals, have extremely negative effects on the ecological system and the overall health of the population if their concentration exceeds the WHO-specified limits [1,4]. As a result, it is imperative to devise efficient and dependable processes for treating diverse wastewater sources [1]. Fish and other dwellers of aquatic ecosystems are easily able to absorb heavy metals that have been dissolved in water [5]. Metal contamination may cause a range of reactions, including structural and functional alterations of the biota with dramatic ecological repercussions. For example, in reaction to chemical stressors found in mine effluents, factors such as algae growth rates, activity of photosynthesis, chlorophyll-a levels, and nutrient absorption capacity can be affected [6]. Algal groups need photosynthesis to participate in biofilm activities, while toxicity from metals has been demonstrated to encourage its suppression.

In particular, inorganic contaminants (IOCs) can cause vital health issues for living systems. Nitrate and cyanide ions, Cd, Cr, Hg, Se, Ba, Sb, Be, Ni, and Tl result from industrial waste [7,8]. Zinc (Zn) is an example of the significant elements that are necessary for various physiological and biochemical processes and functions in living tissues. However, excessive intake of zinc can lead to hard health complications, including abdominal discomfort, skin rashes, and anemia [3]. In addition to gastrointestinal upset, pulmonary fibrosis, and skin rashes, Ni exposure beyond the critical threshold can result in significant lung and kidney issues [9]. Additionally, Ni is a proven human carcinogen [9]. Otherwise, As is mostly found in water and wastewater in its various forms, such as (As(III)) and (As(V)) [10]. As(V) is often present in aerobic environments, whereas As(III) is common in anaerobic ones [10]. The pH and redox of the solution have a significant impact on the ionic form of arsenic species [10]. On the other hand, in animal metabolism, copper (Cu) plays a vital role. However, excessive intake of copper can give rise to hazardous toxicological concerns, including increased mortality or symptoms like vomiting, cramping, and convulsions [11]. Cd presents serious dangers to human health. Chronic exposure to cadmium causes kidney disease, and extreme exposure can be fatal. Cd leads to renal damage, hypertension, and atrophy [12]. Cr is essential for the kidney and liver at low concentrations [13]. However, its higher concentration causes lung cancer and skin allergy [12,14]. In the aquatic environment, Cr mostly exists in the forms of Cr³⁺ and Cr⁶⁺. Generally, Cr³⁺ is less hazardous than Cr⁶⁺. Cr⁶⁺ causes changes in human physiology, accumulates in the food chain, and leads to significant health problems, including lung cancer and minor skin irritation [15]. In addition, Ag, Co, As, Ni, and Pb represent particularly high-toxicity contaminants [16,17]. They are long-lasting contaminants in the environment. These are derived from the leather, fertilizer, glass, battery, and metal plating industries [18,19,20,21]. For example, Pb is considered one of the most dangerous elements in the aquatic environment [12,22]. It is related to several severe health problems associated with the brain, nervous system, and kidneys [23,24]. Furthermore, the use of vanadium (V) increased from 2011 to 2022 by around 45%, and it is expected to increase by 27% in 2024 [25]. Due to its harmful effects on living tissues, it was determined that the maximum limit of vanadium in drinking water is 50 μg/L [25].

For these reasons, primary standards were established for public water systems. These standards set both the maximum permissible levels and target goals for inorganic contaminants in drinking water, as outlined in

Table 1. The Maximum Contaminant Level Goal (MCLG) and the Maximum Contaminant Level (MCL) of inorganic contaminants for drinking water according to NPDWR [26].

Contaminant	MCLG (mg/L)	MCL or TT (mg/L)
Antimony	0.006	0.006
Arsenic	0.0	0.010 as of 01/23/06
Asbestos (fiber > 10 μm)	7 million fibers per liter (MFL)	7 MFL
Barium	2.0	2.0
Beryllium	0.004	0.004
Cadmium	0.005	0.005
Chromium (total)	0.1	0.1
Copper	1.3	Action Level = 1.3
Cyanide (as free cyanide)	0.2	0.2
Fluoride	4.0	4.0
Lead	0.0	Action Level = 0.015
Mercury (inorganic)	0.002	0.002
Nitrate (measured as Nitrogen)	10	10
Nitrite (measured as Nitrogen)	1.0	1.0
Selenium	0.05	0.05
Thallium	0.0005	0.002

MCLG: Maximum Contaminant Level Goal—This represents the concentration of a contaminant in drinking water that is considered safe, with no anticipated health risks. MCLGs include a safety margin and serve as non-enforceable public health targets. MCL: Maximum Contaminant Level—This refers to the maximum allowable concentration of a contaminant in drinking water. MCLs are established as close to MCLGs as possible, taking into account the best available treatment methods and cost considerations. MCLs are legally enforceable standards. TT: treatment technique—A mandated procedure designed to lower the concentration of a contaminant in drinking water.

.

Numerous methods were used to help in solving the problem of the inorganic contaminants from diverse water sources for human consumption [1,27]. Physical, chemical, and biological technologies (like evaporation, precipitation, ion exchange, membrane separation, and adsorption), as well as thermal-based technologies, are divided into numerous categories [1]. Physical methods such as sedimentation, filtration, and adsorption are commonly used to remove solid particles and suspended solids from wastewater [28]. These methods rely on the physical properties of pollutants and their ability to be separated or adsorbed onto different media. Chemical methods, including coagulation, flocculation, and precipitation, involve the addition of chemicals to induce the formation of flocs or precipitates (these flocs can be separated with various techniques easily). Biological processes for treatment utilize microorganisms to reach the degradation of the organic pollutants present in wastewater [29,30]. These processes are effective in removing organic matter and nutrients, but they may require longer retention times and larger treatment facilities. Despite all of the advancements in traditional wastewater treatment techniques, there is growing attention to electrochemical methods due to their importance as efficient pollutant removal and energy savings [31,32].

Electrochemical techniques involve the use of electric current and electrodes to induce chemical reactions that lead to the removal or transformation of pollutants. Among the electrochemical methods, electrocoagulation (EC) has gained significant attention and recognition [33]. EC is an electrochemical method used to add coagulants and eliminate suspended solids, colloidal substances, metals, and dissolved solids in general from water and wastewater. The process involves the sacrifice of electrode plates, which dissolve into the solution and cause the precipitation of contaminants as stable precipitates. One of the advantages of EC is its ability to treat a wide spectrum of contaminants, and this spectrum includes heavy metals, organic compounds, and even microorganisms. The process can effectively remove pollutants that are extremely hard to treat via conventional methods. Moreover, EC offers benefits such as shorter retention times, lower energy consumption, and the potential to remove multiple contaminants simultaneously [34].

EC not only offers a significant solution for removing heavy metals from wastewater but also provides an environmentally friendly approach. Not only does EC focus on the removal of pollutants but also allows valuable resources to be recovered. Nitrogen and phosphorus, which are common contaminants in wastewater, can be efficiently recovered during the EC. These nutrients can be transformed into forms suitable for reuse, such as ammonium and phosphate ions, through various electrochemical reactions. The recovered nitrogen and phosphorus can then be utilized as fertilizers or in other applications, contributing to resource conservation and circular economy practices. Thus, the integration of EC technology enables simultaneous pollutant removal and resource recovery, promoting sustainability in wastewater treatment processes. In particular, the nutrient recovery process utilizes EC for the transformation of contaminants into usable forms, allowing for their potential reuse or further utilization [35]. Nitrogen and phosphorus, in particular, are commonly found in wastewater and can be recovered through various electrochemical reactions during the EC [36,37]. One of the primary sources of nitrogen in wastewater is in the form of ammonia (NH₃) or ammonium ions (NH₄⁺) [38,39]. These nitrogen compounds can be transformed into more stable forms, such as nitrate (NO₃⁻) or nitrogen gas (N₂), depending on the conditions of the operations and electrode materials used in the EC system [40,41]. The recovery of nitrogen can be achieved through processes like electrooxidation or electrochemical reduction [42]. During electrooxidation, ammonium ions are oxidized at the anode, and therefore, the formation of nitrate ions occurs [43,44]. On the other hand, electrochemical reduction at the cathode can convert ammonium ions into nitrogen gas, which can be released into the atmosphere [45]. In addition, phosphorus, which is often present in wastewater as phosphate ions (PO₄³⁻), is another valuable resource that can be recovered during the EC [46]. Phosphorus recovery is typically accomplished through electrochemical reactions such as electrocoagulation–flotation (ECF) or electrochemical precipitation [47,48]. In ECF, phosphorus is coagulated and subsequently floated to the surface as precipitates, which easily separate from the treated wastewater [49]. The recovered phosphorus-rich precipitates can then undergo further processing to produce phosphorus-based fertilizers or other products. The recovery of nitrogen and phosphorus in EC not only contributes to the conservation of valuable resources but also helps to mitigate their environmental impact. Moreover, the recovered nitrogen and phosphorus can be utilized in various applications promoting resource conservation and circular economy practices. In agriculture, for example, the recovered nutrients can be used as fertilizer substitutes providing essential elements for plant growth while reducing the reliance on traditional chemical fertilizers [50]. This not only reduces the environmental impact of fertilizer production but also helps to close the nutrient loop by recycling valuable resources back into the agricultural system. Furthermore, the recovered nitrogen and phosphorus can find applications beyond agriculture. They can be used in the production of animal feed, biogas generation, or even in the synthesis of chemicals and pharmaceuticals. By incorporating recovered nutrients into these processes, the EC enables the sustainable utilization of resources, reducing the dependency on finite raw materials and promoting a more circular and resource-efficient economy.

1.1. The Operational Principle of EC

Numerous chemical and physical processes are used in the EC [51]. The following are the several steps of the EC, as stated by Singh et al. [51]. These steps are shown in Figure 1. The pollutant migrates to an electrode with an opposite charge (electrophoresis) and aggregates as a result of neutralizing the charge, (ii) the interaction of either the cation or hydroxyl ion (OH) with the contaminant leads to the creation of a precipitate, (iii) the reaction between the metallic cation and hydroxyl ions results in the formation of a metal hydroxide with strong adsorption capabilities, which adheres to the contaminant (bridge coagulation), (iv) these hydroxides develop larger lattice-like formations that move through the medium (sweep coagulation), (v) pollutants undergo oxidation, transforming into less harmful compounds, and (vi) pollutants are eliminated through flotation or precipitation, along with their attachment to bubbles.

The EC unit is equipped with an electrolytic cell that includes an externally linked cathode and anode that are submerged in an electrolyte solution, as illustrated in Figure 1. When a current is applied, anodes in electrolytic cells undergo oxidation, resulting in the release of electrons [52]. Although the electrodes are made up of identical materials, the phenomenon of dissolution exclusively takes place at the anode [53]. As indicated in Equations (1) and (2), the metallic anode electrode separates into a metal ion. The metal ions produced on the cathode interact with the hydroxyl group on the electrode to create the metal hydroxide, the pH of the solution impacts whether the metallic hydroxide is monomeric or polymeric, either insoluble or soluble. Because of their enormous surface area, metallic hydroxides M(OH)_n are excellent pollutant adsorbents with a strong tendency to form bonds with them to generate flocs. Various additional responses occur in the EC cell, including the formation of H₂ and O₂ gas at both the anode and cathode, correspondingly [52].

Anode:

2H₂O→4H⁺_(aq) + O_2(g) + 4e⁻

(1)

M_(s) →Mⁿ⁺ _(aq) +ne⁻

(2)

Cathode:

nH₂O + ne⁻ → (n/2) H_2(g) + nOH⁻_(aq)

(3)

As indicated in Equation (3), the separated H⁺ ion reacts with another H⁺ ion present in water to form H₂ gas. The H₂ and O₂ gases are critical for the removal of flocs that fail to settle. The H₂ gas produced lifts the flocs to the solution’s surface, a process known as electro-floatation, and the resulting flocs may be removed by skimming. The oxygen generated at the anode creates hydrogen peroxide, an intermediate that aids in the process of oxidation of both non-toxic and hazardous molecules. The formed heavy flocs drop to the bottom of the system, leading to sludge formation, which can be cleaned in several ways [52].

1.2. The Merits of EC

• There is no possibility of secondary contamination occurring via side reactions of high chemical concentrations, as no chemicals were added [54].
• Pollutants can be removed more readily by floating on the surface of the solution due to the gas bubbles created by EC [54].
• Due to the simplicity of the equipment, full process automation is achievable [55].
• Clear, colorless, and odorless water is produced through EC treatment of wastewater [55].
• Due to the ability of the electric current to speed up, impact, and promote coagulation, EC may eliminate even the smallest colloidal particles [54].

The EC approach has shown great potential in removing contaminants from a variety of water sources, including water from municipalities, lake water, river water, and seawater, but it has not yet been widely used. Therefore, more investigation is required to determine the effects of the novel reactor designs examined in this work. Moreover, additional research is needed to look at the processing conditions, electrode dissolution phenomena and development of scalable EC reactors, and electrode design and configuration. To ascertain the operating circumstances that allow for the industrial scale-up of these techniques, additional evaluations are also required. The current review specifically focuses on the removal of inorganic contaminants, as well as recovery of the nutrients, and encompasses a wide range of operational parameters, such as pH, time, contaminant concentration, power, electrode spacing, and choice of anode materials. These parameters were thoroughly examined concerning their impact on the EC and the removal of inorganic elements. The review provides valuable insights into the mechanisms and effectiveness of EC for addressing inorganic contaminant removal, as well as nutrient recovery.

2. Inorganic Contaminants

2.1. Hydrated Silica and Related Minerals

Since silica salts adhere to the surfaces of pipes and various unit operations, and hydrated silica in large quantities will cause operational issues. Contact with hydrated silica particles mostly impacts the lungs, which raises the chance of developing lung cancer, chronic bronchitis, chronic obstructive pulmonary disease, silicosis, and other respiratory diseases [56]. This is why it is crucial to remove hydrated silica from groundwater [34]. Silica, also defined as silicon dioxide (SiO₂), is typically regarded as one of these scalants (minerals such as calcium sulfate) that is most responsible for the inorganic clogging of membrane surfaces [57]. Because of the limited solubility and the restricted ability to dissolve, silica scaling is a major difficulty in membrane separation procedures [58]. The presence of silica in the feed solution poses a significant challenge to obtaining safe drinking water in the industries of desalination. Removing silica from the membrane surface, especially in its non-crystalline forms, is extremely challenging. It becomes even more difficult when silica forms a crystalline scale layer on the membrane surface [59]. The solubility of solubilized silica in brackish water is approximately 120 mg/L, but the concentration of silica in saltwater is between 1 and 12 mg/L [60]. While the concentration of silica in saltwater is relatively low, the presence of polymers with less solubility can lead to forming additional deposits on the surface of the membrane. Research indicates that polymeric silica, one of the three types of silicates (including monomeric and filterable silicates), is primarily responsible for membrane scaling, contributing to a significant amount of deposition [61]. Among the most effective methods for removing silica is to apply alkaline conditions to minimize silica precipitation. This treatment can significantly increase the pH of the water, which might cause silica to be adsorbed onto magnesium hydroxide flocs or the development of magnesium silicates, which complicates the removal procedure [62]. López et al. investigated the effect of the EC on removing hydrated silica from real groundwater and found that, with an initial concentration of 42 mg/L and current density of 12 mA/cm², pH maintained at 8.02, and electrode material made of Al, the optimal elimination was around 83% [34]. The authors discussed that the Al electrode dissolves to form Al³⁺ and loses three electrons, which is considered the oxidation reaction. On the cathode, water is reduced to form hydroxyl ions (OH^-) and hydrogen gas (H₂). This leads aluminum cations to form aluminum hydroxide and oxide (Al(OH)₃ and Al₂O₃). It was also reported that, if the process optimized to remove silica and arsenic, this would prevent removing fluoride due to the inhibition of hydroxyl substitution by fluoride. This indicates a competition between silica, arsenic, and fluoride to exploit the active sites on flocs [34]. Castañeda et al. studied the elimination of arsenic and hydrated silica using EC from real groundwater [56]. A concentration of 154 mg/L was used as the initial, pH 7.5, a conductivity of 550 μS/cm, and electrode material made of Al [56]. The current density of 7 mA/cm² was found to be optimal [56]. These parameters led to 96.4% removal efficiency [56]. Another work used the EC to remove some minerals from groundwater, and the trials included hydrated silica [63]. With a pH value of 7.5, initial concentration of 160 mg/L, and water conductivity of 450 μS/cm, the optimum current density was 5 mA/cm with Fe and Al electrodes, and the elimination efficiency reached nearly 80% [63]. Chow et al. used Fe anode to remove silica using EC. The results demonstrated that the efficiency of removing was over 95%, while, in the case of Al anode, the removal efficiency of silica reached 99% at pH from 7.7 to 8.9 [64]. The authors reported that using Fe⁰ led to the precipitation of iron sulfide (FeS) and facilitating the adsorption of silica on the FeS surface, while using Al⁰ led to the adsorption of silica on the surface of aluminum hydroxide but without removing sulfides [64].

2.2. Silver (Ag)

Ag has been used in different fields, for example, medicine and chemical production. Due to its various applications in smart devices, pictorials, and medicine, the discharge of wastewater contaminated with Ag causes severe damage to the environment and the public population as a result of its high carcinogenicity and neurotoxicity [65]. As a result of the high consumption of metals, Ag faces a problem of high demand and low supply, highly used in nearly all fields of elements [65]. Because of this, the extraction of Ag from sewage is crucial for both the environment and the world’s needs [65], and EC can be used for this purpose. Xie et al. removed Ag through an EC. They used Al as the main electrode material, while the gap was maintained at 1 cm, and the initial concentration of Ag in the process was 20 mg/L. In addition, they set the pH value to be 5.4, while the conductivity of the solution was 20 mS/cm, and the current density was 5 mA/cm² [66]. They managed to remove 97.4% with an operation time of 20 min [66]. Yaychi et al. used Ag nanoparticles in their experiment, and after 30 min, reached an efficiency of 99.4% using Al-Fe alloy, 90% purity as the anode and the cathode material with a gap between the electrodes equal to 2 cm with the existence of polyvinylpyrrolidone (PVP) as a stabilizer [67]. When the electric field was applied during the EC process, the Al³⁺ ions and Al(OH)₃ coagulants formed and neutralized the surface charge of AgNPs by removing the stabilizers. Also, sodium citrate was used and showed slower degradation compared to PVP. This means AgNPs stabilized by sodium citrate exhibit greater resistance to the electric field, while PVP-stabilized AgNPs are faster in degradation [67]. Matias et al. used the EC to remove 99.9% of the Ag nanoparticles in 10 min [68]. They used Al as the electrodes (anode and cathode), and they set the gap between the electrodes to be 2 cm [68].

2.3. Arsenic (As)

Arsenic is considered a toxic and life-threatening substance [69]. Prolonged intake could result in several illnesses, such as psychiatric disorders, skin conditions, and cancer [70]. Arsenic is listed as a toxic substance by some countries [71]. According to experts, the most severe episode of environmental contamination on record affected around two million people in Bangladesh owing to prolonged intake of groundwater tainted with arsenic [70]. According to the International Agency for Research on Cancer, arsenic is a first class carcinogenic material (IARC) [72]. About 200 million individuals are believed to be subjected to excessive quantities of arsenic in water sources on a global scale [72]. It is primarily linked with tungsten ore, and all the processes that remove arsenic can be used to purify tungsten ore from arsenic [71]. Valentín-Reyes et al. eliminated arsenic from wastewater with an initial concentration of 32.45 µg/L and with a pH value maintained at 8.36. The conductivity of the aqueous medium was 533 µS/cm, the current density value was 8 mA/cm², and the overall efficiency was 96.6% using four 1018 steel plates as the electrodes [73]. M. Kobya et al. treated potable water from As with EC. After 2.5 min of the operation time, about 94.1% of As was removed using Fe electrodes. On the other hand, Al electrodes removed about 93.5% of As after 4 min [74]. They also reported that, at the anode, the metallic ions of Fe²⁺, Fe³⁺, or Al³⁺ are released, while, at the cathode, hydrogen gas and OH^- ions are formed. They create together the metal hydroxides such as Fe(OH)₃ and Al(OH)₃ that serve as coagulants. This leads to two primary mechanisms, which are adsorption and coprecipitation. The As(V) ions can be adsorbed on the surface of metal hydroxide via ligand exchange. This is done by replacing hydroxyl groups and forming insoluble surface complexes (coprecipitation) [74]. This process is highly pH-dependent, with adsorption more favorable below the isoelectric point of the hydroxides. Through this combined action of coagulation, adsorption, and co-precipitation, arsenic is effectively removed from water. Moreover, Nguyen et al. used the EC and reached an efficiency of 92% in eliminating arsenic with an initial concentration of 0.1 mg/L and used stainless steel plates as the electrodes in addition to maintaining the gap between the electrodes as 1 cm, and the operation took 5 min to be completed with a voltage of 7.5 Volts [75].

2.4. Cadmium (Cd)

Despite the extremely small levels, cadmium complexes are exceedingly hazardous [76]. Cadmium is highly soluble in water and considered a highly carcinogenic metal that reacts with the cells of the body and might cause cancers [76]. Cadmium is mainly used in the production of nickel–cadmium batteries that are used for several applications, including backup energy in consumer products, electric cars, airplanes, and digital flashing devices, which is one of its most significant uses [76]. Stylianou et al. used Al plates as the main electrode and kept the gap at 0.5 cm. They achieved an efficiency of 96% with a current density of 20 mA/cm² after 150 min of processing [77]. Thakur et al. used Fe electrodes (anode and cathode) with a gap between the electrodes equal to 1.0 cm and an initial concentration of 17 mg/L. They reached an efficiency of 99.9%, adjusting the pH to 5, while the process took 40 min with a current density of 140 A/m² [12]. They reported that, in the case of Cd, the process is highly influenced by pH. When the pH was lower than 5, the removal efficiency was lower due to the existence of protons, which reduced the H₂. This limited the formation of hydroxide ions. At higher pH, the removal reached 98% by neutralizing the charge by cationic iron species of Fe(OH)₂. These species led to a reduction in the solubility of Cd and promoted sweep coagulation, where amorphous Fe(OH)₃ traps Cd particles. When the pH values increased more, the removal decreased slightly due to the existence of fewer hydroxides for effective coagulation [12].

2.5. Manganese (Mn)

Manganese is a valuable and distinctive metal with several commercial and metallurgy uses. This is due to its adaptability and essential qualities that greatly contribute to its rising demand for a broad range of applications [78]. Manganese is harmful to people if it is abundant in the atmosphere, water, or foodstuffs at high levels. It can also induce convulsions when its levels are too low in the bloodstream [79]. Long-term neurological abnormalities could develop due to the inhaling of particulate manganese, which frequently travels straight to the brain without first being processed by the liver [79]. A neurological condition involving cognitive, mental, and motor impairments can also be brought on by an excessive buildup of manganese in the brain, which might potentially hasten a particular kind of Parkinsonism [79]. Stylianou et al. used the EC to eliminate several metals, including manganese. The removal time was 150 min, and the efficiency reached 99.9%, while the current density was 20 mA/cm², the initial concentration was 100 mg/L, and the electrode material was chosen to be Al [77]. They showed that the main mechanism is converting metals into their insoluble forms, which allows them to precipitate. In addition, they reported that lime treatment can raise pH to help metals reach their insoluble state [77]. Das et al. managed to use an EC in removing metals, including manganese. They removed 98% of the manganese present in the sample. The electrodes were combinations of Al and Fe with a 1 cm gap [80]. The initial concentration was 0.96 mg/L, and the pH value was 7, the whole operation took one hour to be completed [80].

2.6. Calcium (Ca)

High-salt content water has been used in industrial applications, such as a cooling agent [81]. Due to the shortage of freshwater sources, high-salt water is a good candidate source for industrial applications [81]. Calcium ions are a hazardous problem in this water source due to the limescale that increases the manufacturing price and decreases the performance [81]. Additionally, once limescale bits enter the abdomen, they interact with the hydrochloric acid to start releasing calcium ions and carbon dioxide. The former could cause illnesses like kidney and renal stones, and the latter may increase the risk of stomach punctures in individuals who already have stomach ulcers [81]. Due to the high salt concentration of the water, fewer bacteria can stand the sewage water [81]. The process took 6 days, and the concentration of the calcium declined from 1200 mg/L to 21–159 mg/L. This process reached an efficiency of 87–98% [81]. Zhang et al. treated shale gas drilling wastewater with an EC [82]. They used electrodes made from Fe, and the gap between them was 2 cm, and kept the pH value at 4.4 [82]. The process took 20 min to be finished, and the overall efficiency was 56.5% [82]. It was also mentioned that the metal cations and their hydroxides can be formed at the anode and destabilize contaminants. These contaminants are then removed through processes like adsorption, interparticle bridging, precipitation, and entrapment within floc structures [82].

2.7. Phosphorus (P)

Phosphorus (P) is a crucial ingredient in the environmental life cycle, and it is necessary for the manufacturing of pesticides and medicines, as well as for the food supply, which accounts for 85% of the world’s total phosphorus consumption [83]. It is a significant component in the creation of adenosine triphosphate (ATP), which is an important substance for the energy production process of photosynthesis [84]. Phosphorylation is a key step in the metabolism of inorganic compounds (e.g., HPO₄²⁻ and H₂PO₄⁻) [84]. The most prevalent and reactive form of P in most sludges is orthophosphate (PO₄³⁻) [84]. P is essential for both bacteria and microalgae [85]. The high P concentrations in surface waters have a detrimental influence on water suitability for biological usage, mostly by promoting the growth of algae, which spurred various legal institutions to promote a more sustainable development of actions to be made against eutrophication [86]. The influence of eutrophication on lakes and reservoirs has sparked interest across the world [2]. Although traditional biological treatment techniques are known for their cost-effectiveness, they suffer from prolonged hydraulic retention times and substantial volume requirements, which can decrease their overall efficiency. Moreover, the significantly increased land and facility demands associated with these methods make them less desirable compared to physicochemical treatments. In contrast, physicochemical approaches offer advantages such as shorter retention times, enhanced efficiency, expedited installation, and simplified operation and maintenance [3].

Sewage sludge contains a significant quantity of phosphorus, which, after recovery, may provide 12–15% of the overall phosphorus needed. In this regard, Sanni et al. removed P and COD from wastewater using a combination of EC and electrooxidation [87]. The results showed that about 97% and 95% of P and COD were removed from wastewater, respectively [87]. Azerrad et al. coupled EC with advanced oxidation processes [88]. The results showed success in the recovery of about 99% of ammonia from sewage [88]. With two titanium plate electrodes as the cathode and two Al and Fe plate electrodes as anodes, pH = 4, the current density is 20 A/m², voltage 120 Volts, and DC between 0 and 20 A. Omwene et al. reached a 99.99% percentage of P recovery using EC [89]. In this regard, metal ions (Al³⁺ or Fe³⁺) are generated at the anode in addition to hydrogen gas released at the cathode, which aids in the flotation of flocculated particles. Further, PO₄³⁻ ions bind strongly to the generated ions at the anode to undergo ligand exchange with surface hydroxyl groups on aluminum and iron oxides, which leads to inner or outer surface complexation. This leads to creating precipitates like Al_1.4PO₄(OH)_1.2 and Fe_2.5PO₄(OH)_4.5, along with hydroxides such as Fe(OH)₃ and Al(OH)₃. These substances effectively remove phosphate through adsorption, precipitation, and co-precipitation. Sanni et al. made a combination system between EC and electro-oxidation to obtain a percentage of 97% P recovery using four anode Fe electrodes and four cathode graphite electrodes with a 1 cm spacing distance each. The current maximum intensity of 70 A and a maximum voltage of 40 Volts were employed with pH conditions of 7.9 ± 0.4 and current density = 13.6 mA/cm² for 90 min, as shown in

Table 2. Recovery of phosphorus (P) from wastewater and agricultural drainage water using EC. It must be noticed that (---) means not available.

NO.	Recovered Substance	Anode	Cathode	Gap (cm)	Concentration (mg/L)	Efficiency η (%)	pH	Time (min)	Power (A/m2)	Ref.
1	P	Al-Fe	Ti	---	---	100	4	80	20	[89]
2	P	Fe	Graphite	1	22.9	97	7.9	90	91 and 136	[87]
3	P	Al	Al	2.5	2	99	5–8.83	60	---	[90]
4	P	Fe-Al	SS	1	10	98	4	2–5	100	[94]
5	P	Al	Activated carbon	0.5 to 2	---	99	7–7.2	360	6–8	[95]
6	P	Al-Fe	Fe-Al	0.7	---	93	5	60	100	[96]
7	P	Al	Al-Fe	---	---	89	6.88–7.05	15 to 210	---	[97]
8	P	Steel rod	steel pipe	---	---	99	6–7	60	---	[92]
9	P	Two carbon brushes and 1 Fe plate	carbon felt		---	87	5.15	---	---	[91]
10	P	Fe	Graphite	1	22.9	97	3–7.2	45	91	[87]
11	PO₄3⁻	Fe	SS-Al	1	1.3	98 ± 2		2	100	[94]
15	Phosphate	Fe, Al	SS	----	5.5,	99,	----	----	94	[88]
	Carbonate				75,	88–98
	DOM				300	40–50
16	HS, arsenic, and phosphates	Al	Al	----	161, 22	40	7.6	----	40–100	[98]

[87]. Franco et al. used two EC reactors that were composed of a one square Plexiglas tank that could store 1 L of liquid and Al electrodes, 50 mm × 82 mm for each, as the anode and cathode in each reactor. The electrodes were separated from each other by 2.5 cm. To provide a preset current and voltage to the electrodes, a 30 Volts/5A single-output DC power supply was employed. For the solutions with pH of 5–8.83, a 99% decrease in P concentration after 60 min of reaction duration was attained [90]. Wu et al. used a system with a mixed anode for each cell, two carbon brushes and one Fe plate were employed, and as an air cathode, carbon felt was applied. The system obtained 87% of the total phosphorus. The duration of this procedure was extended from 5 to 63 days (5 min on and 5 min off) at a constant voltage of 3 Volts and pH = 5 [91]. Liu et al. used the EC technique to reach a P recovery efficiency of 99.4%. As an anode in the EC system, a steel rod was attached in the center of the column reactor, and as a cathode, an encircling steel pipe was put against the reactor’s inner wall. The current was kept constant at 5 A. The pH level was kept at around 6–7 [92]. There was an experiment was completed by Zhang et al. to treat 100 L of liquid manure per day to remove P and fine particles [93]. They showed that the process recovered between 76.3% and 88.9% of P₂O₅ from shallow-pit swine manure, while 65.9% to 78.4% was recovered from deep-pit manure [93]. This treatment led to a concentrated P content in the resulting sludge, which ranged from 5.9% to 10.2% of P₂O₅ in dry matter. There were low nitrogen ratios (N:P₂O₅) observed in the sludge, between 0.52 and 0.78, which was an indication of the high concentration of phosphorus, which means that it is suitable for application as a phosphorus-enriched fertilizer. After treatment, the volume of manure was reduced by 95% for shallow-pit manure and by 90% to 92% for deep-pit manure. The pilot EC process recovered 79.3% of P₂O₅ from the dry matter, which indicates its effectiveness in phosphorus recovery from agricultural waste [93].

2.8. Nitrogen (N)

In the natural N cycle, the primary nitrogenous substances are N₂ gas, organic N, ammonium ions, and nitrate. There is a lot of N₂ in the atmosphere, but because it is not reactive, living organisms cannot directly absorb it. To absorb N₂ in its reactive form by nitrogenase and lighting, biological N fixation is frequently used for crops and plants. The manufacturing of traditional fertilizers is unsustainable due to their non-renewable and fossil-based origins, raising questions about their long-term supply [99]. Therefore, extracting ammonium by other techniques became critical. One of these methods is the EC, which can coagulate the ammonium around metal hydroxide flocs. That means ammonium can be recovered using the EC mechanism, which is another advantage of EC. EC can be used for contaminant recovery of compounds. Different systems with varying operability, prices, and possible applications of the recovered material have been offered for N-recovery from flows in wastewater treatment that involve N, including dewatering liquor, sewage sludge, effluent, sludge ash, and digester supernatant [100]. A. Mohammadi et al. used the EC to eliminate N from wastewater [101]. As in Table 3, the trials showed maximum efficiency and the best conditions of an 81.6% recovery rate [101]. The trials were done after 100 min using a combination of Fe and Al, which resulted in efficiency flocculation in the efficiency, 53.4% recovery rate for Al/Fe, and 42% for Al/Al [101]. Also, the reaction time was tested and found that, when the reaction time increased, the recovery rate increased as well [101]. When the pH value was fixed at 7.2, these trials showed that, when the current was 0.3, 0.5, and 0.9, the efficiency was 70.5%, 72.1%, and 75.9%, respectively [101]. Furthermore, the agitation speed was also investigated and found that, when the agitation speed was 100, 200, 300, and 400 rpm, the efficiency was 79.12%, 80%, 81.59%, and 79.56%, respectively [101]. The reported removal mechanism of N from anaerobic digestion (AD) effluent via EC shows multiple complex processes. Primarily, ammonium nitrogen dominates due to the extended retention time in AD reactors, though other nitrogen compounds like organic nitrogen, nitrite, and nitrate are also present. Chloride ions, which exist in wastewater, are electrolyzed to form chlorine gas, which is hydrolyzed into hypochlorous acid and hypochlorite and then oxidizing ammonium into nitrogen gas. At the same time, nitrates near the cathode are reduced to nitrogen gas and ammonia, with the ammonia that is subsequently oxidized. Furthermore, iron ions from the anode form hydroxo-polymeric complexes that adsorb nitrite and nitrate and, consequently, lead to their removal through flocculation and precipitation. Some light flocs bind to hydrogen bubbles and rise to the surface as foam for removal. Moreover, at pH levels above 9.6, ferric hydroxide species electrostatically attract and bind positively charged nitrogen species, facilitating their coagulation and precipitation [101]. J. Lu et al. reached a 100% recovery rate of ammonia from wastewater [102]. This result was obtained under the optimum conditions [102]. In the trials, they investigated the different parameters, and they concluded that the best conditions for the experiment were 120 min, a current density of 350 A/m², a pH value of 8.4, and 2 g/L NaCl supporting electrolyte solution [102]. They also investigated other conditions, such as a current density of 50 A/m², pH value of 3, 2 g/L Na₂SO₄ supporting electrolyte solution, and experiment time of 60 min, and the efficiency was 50.9% [102]. They proved that, as the reaction time increased, the rate of recovery also increased [102]. Additional previous results are summarized in Table 3.

M. Malakootian. et al. recovered N from wastewater using the EC [103]. The recovery efficiency of N reached 87.9% [103]. On the other hand, M. M. Emamjomeh et al. used monopolar EC to optimize the recovery efficiency of nitrates [74]. The results showed that nearly 87.9% of nitrates were recovered at a pH of 10 after 68 min [74]. Further, P. I. Omwene et al. recovered phosphorus (P) from domestic wastewater using the EC and hybrid anodes [89]. The result showed a recovery efficiency of 99.99% of P at a pH of 4 with a current density of 20 A/m² after 80 min of operation [89]. Moreover, K. S. Hashim et al. used EC to recover phosphate from a river [104]. Using Al electrodes, the recovery efficiency of phosphate reached 99% at a pH of 6 with a current density of 60 A/cm² at 60 min [104]. Furthermore, V. Kuokkanen et al. used Al-Fe electrodes to recover phosphate from wastewater using the EC technique [96]. The optimum parameters were a pH of 5, current density = 100 A/cm², and operation times of 30 and 60 min [96]. The results showed that the recovery efficiency of phosphate was approximately 79% and 93%, respectively [96].

Table 3. EC conditions in the recovery of N ions accompanied with their concentrations, pH values, and time of processing, as well as the obtained efficiency.

NO.	Removed Element	Anode	Cathode	Gap (cm)	Conc. (mg/L)	η (%)	pH	Time (min)	Power (A/m2)	Ref
2	NO3	Al	Al	----	100	88–94	7.25	120	----	[105]
3	NO3	Al	Al	----	100	94	7	----	----	[106]
5	NO3	Al	Al	----	2500	87	7,	100	29.4–1030	[107]
7	fluoride	Al	Al	1.8	100	88,	5.94	180	90.9	[108]
	NO3					42
8	N	Fe	Fe	2.5	----	80	6.72–7.25	----	----	[109]

Table 3. EC conditions in the recovery of N ions accompanied with their concentrations, pH values, and time of processing, as well as the obtained efficiency.

NO.	Removed Element	Anode	Cathode	Gap (cm)	Conc. (mg/L)	η (%)	pH	Time (min)	Power (A/m2)	Ref
2	NO3	Al	Al	----	100	88–94	7.25	120	----	[105]
3	NO3	Al	Al	----	100	94	7	----	----	[106]
5	NO3	Al	Al	----	2500	87	7,	100	29.4–1030	[107]
7	fluoride	Al	Al	1.8	100	88,	5.94	180	90.9	[108]
	NO3					42
8	N	Fe	Fe	2.5	----	80	6.72–7.25	----	----	[109]

2.9. Zinc (Zn)

Heavy metal contaminants in wastewater, particularly waste from electroplating, steel, and heavy industries, represent the biggest impact on the environment and human health [110]. Metal contaminants that cannot be remediated in biological systems are most likely to accumulate in microorganisms [111]. As an example, the high concentrations of Zn cations in drinking water can cause major health concerns, such as nausea, irritability, and anemia [112]. To enhance water quality and maintain compliance with environmental standards, industrial-scale heavy metal removal technologies are required [113]. Many processes are used to remove Zn from wastewater, including electro-flotation. However, there is difficulty with energy usage. For instance, the photocatalysis process depends on sunlight, while coagulation-flocculation might cause the formation of secondary contaminants, chemical precipitation, which requires excessive chemicals, and ion exchange, where the resin becomes contaminated. Das et al. mentioned that about 40% of Zn is removed from wastewater using Al-SS as electrodes in an electrode area of 35 cm², and the time was 120 min [80]. Aji et al. used the EC technique to obtain 96% of zinc removal with a current density of 25 mA/cm² [114]. Further, Pociecha et al. used the EC to remove Pb, Zn, and Cd from the washing water of the soil [115]. With a pH of 7.52, the removal efficiency of Pb, Zn, and Cd reached 53%, 26%, and 52%, respectively [115]. Dura et al. accomplished a zinc removal efficiency of 99% when Fe or SS420 was used as the anode, while the efficiency was 92% when SS310 was used as the anode at the current density of 11.7 mA cm⁻² [116]. Xiujuan Chen et al. conducted several experiments to investigate the optimum conditions for the removal of Zn from wastewater [117]. In the first experiment, they set a 20-min timer with a current density of 83 A/m², while, with a pH value of 5.3, they found that 50 mg/L of Zn was eliminated from the wastewater [117]. However, when they set the timer to 50 min, the removal rate increased to 250 mg/L of the eliminated zinc [117]. Al was used in the trials as both anode and cathode with an effective surface area of 24 cm² [117]. M. Kobya et al. conducted trials to investigate the different parameters affecting the elimination of Zn and other contaminants from wastewater [118]. They found that, with a current density of 60 A/m², pH value of 5, experiment time of 25 min, and Al electrodes, the elimination rate reached 96.7%, but when they used Fe electrodes with 60 A/m², pH value of 3, and experiment time of 15 min, they reached a 97.8% elimination rate [118]. When the current density was fixed at 60 A/m², the time varied concerning the electrode material, as, with Fe, it took 15 min, while, with Al, it took 25 min of operation [118]. The results showed that the removal efficiency of Zn was more than 93%, while the removal efficiency of Cu exceeded 92% within 15 min [10]. Mansoorian et al. compared alternating and direct currents in addition to other parameters, and they found some interesting conclusions [119]. By using an alternating current, with a current density of 60 A/m², the Fe electrodes reached a 95.2% elimination rate, but when using SS electrodes, with a current density of 80 A/m², the elimination rate reached 93.3% of Zn [119]. However, the direct current showed a different rate when using Fe electrodes and a current density of 60 A/m², as the elimination rate reached 95.5% removal efficiency of Zn, and the SS electrodes reached 92.5%, while the current density was 80 A/m² [119]. However, the most sludge was precipitated by using alternating currents [119]. Marinos Stylianou et al. completed trials to investigate different heavy metals and the effects of different parameters on their elimination rate [77].

Kuokkanen et al. managed to achieve a zinc removal percentage of 93% using an EC system consisting of Al/Fe electrodes as anode and cathode, respectively, with an electrode spacing of 7 mm in pH = 5 and current density of 100 A/m² at a voltage between 13.0 and 13.5 Volts and current intensity of 13.6 A [96]. The best conditions for these trials were a current density of 20 A/m² and an agitation speed of 600 rpm [77]. They reached an efficiency of 99.9% with zinc, and this was at 90 min, using Al electrodes, with an effective area of reaction 20 cm², finally, they set the gap between the electrodes to be 0.5 cm, and in their trials, they proved that Al electrodes with their conditions were better than Fe electrodes in the overall elimination rate [77]. Sohail Ayub et al. conducted different trials on the removal of different heavy metals, including Zn [120]. The trials showed that, with a 60-min experiment, they reached a 99.2% removal rate of Zn with the best conditions, despite a 90% removal rate at the first 20 min [120]. In their trials, they concluded the best conditions to be a pH value of ~4, the current was 2 A, and the experiment time was 60 min [120]. They used plates of Al as their electrode materials, with a 1.5 cm gap between the electrodes [120]. At the end of the trials, it was clear that the EC was an effective method of eliminating heavy metals from wastewater [120].

2.10. Boron (B)

The boron element is present everywhere in the natural world. As with any natural resource, it is frequently discovered in granites and pegmatites under the action of natural crumbling [121]. Water in volcanic regions exhibits elevated concentrations of boron. Typically, freshwater sources contain boron levels as low as 0.1 mg L⁻¹, while seawater exhibits an average boron content of 4.5 mg L⁻¹. However, in certain regions like the Mediterranean Sea, boron concentrations can surpass 9 mg L⁻¹ [121]. Additionally, boron may be released through trash incineration, biomass burning, glass manufacture, the soap and detergent industry, and mining operations [121]. Several water sources have been found to contain boron, which is harmful [121]. A higher bioaccumulation index results from its accumulation and leaking into groundwater and surface water, endangering the health of people, animals, and even agriculture [121]. However, boron is a crucial micronutrient for the growth of animals, people, and plants. For some deboronation treatment procedures, it is still challenging to meet these criteria [121]. According to reports, boron improves bone development and parathyroid hormone release, which are important for brain function. Additionally, osteoarthritis, infertility, and tooth decay might also be avoided [121]. The global need for boron in industrial sites has grown recently, coupled with the prevalence of boron found in aquatic resources, and as a result, water quality regulations have become increasingly strict. Many other treatment methods have been employed up to this point, including membrane technologies, ion exchange, coagulation/flocculation, adsorption, and EC [122,123]. Ribeiro et al. utilized EC with Al anode to remove boron from wastewater [124]. About 70% of boron has been removed successfully [124]. Yilmaz et al. used Al as an electrode material [125]. The optimum pH can be found by value 8, and the maximum boron removal efficiency can reach 99% [125]. Also, as the pH increases more than this value, the boron removal decreases. The current density that is applied can be averaged from 1.2 to 6 mA/cm2. The quantity of energy that is consumed varies from 6.33 to 141.33 kW h/m³ [125]. Ezechi et al. used Al as electrodes. The optimum pH value obtained was 7 at an initial concentration for boron of 15 mg/L to achieve 98% boron removal efficiency [126]. The applied time is from 15 min to 90 min. The optimum current density can be found at the value of 2400 Ah/m³ [126]. From another point of view, Yao utilized Fe as an anode. As the potential increases from R1 (−0.5 to −0.2 Volts) to R3 (0.5–0.8 Volts), the efficiency can increase from 7% to 16% [127]. At pH 7.5, the initial tests were carried out. The current density can record 60 mA/cm². The distance between the two electrodes was noted at 1 cm at room temperature [127]. The total boron removal efficiency can be above 92% [127].

2.11. Chromium (Cr)

Anthropogenic emissions and natural processes contribute to a higher prevalence of heavy metals in water systems in emerging countries [128]. The main natural processes that increase the amount of Cr discharged into rivers, lakes, seas, and estuaries are soil erosion and rock weathering by water from rains [128]. These activities result in contaminants in soils and groundwater through mineral dissolution, precipitation, and chemical species adsorption and desorption [128]. The trivalent state of Cr, or Cr (III), predominates in the natural world [128]. In nature, chromate is a metallic transition element that is a steely-grey, shiny, brittle metal. Because of their instability in the environment, Cr (VI) and Cr (III), which exist in nine valence states, affect the environment. When compared to Cr (VI), Cr (III) is less toxic and mobile in terms of toxicity [128,129,130,131]. As a result of its widespread usage in the context of industrial activity such as electroplating, metallurgy, wood preservation, leather tanning, and refractory, Cr is one of the very toxic trace elements that are disseminated to surface water and groundwater [128,129,131]. Because it may easily pass through biological membranes, Cr (VI) has been claimed to be naturally carcinogenic and to cause a variety of health problems for humans [128,131]. Inhaling Cr (VI) ions from a polluted environment can cause liver damage, internal bleeding, respiratory issues, dermatitis, skin ulcers, and chromosomal abnormalities in humans [128,129]. Cr(VI) alters the subcellular regulation of the RAD51 gene, which ultimately results in the degradation of lymphocyte DNA, interfering with the biological process of homologous recombination and playing a significant role in DNA disruption [129]. Additionally, it has been demonstrated that Cr (VI) has negative effects on the environment, including decreased plant germination and development, increased earthworm mortality and reproduction rates, organ damage in crayfish, negative effects on the survival, growth, and post-exposure copepods, negative effects on kidney, gill, and liver cells, and perhaps diatom death [128]. Ion-exchange resin, chemical precipitation, reverse osmosis, membrane filtering, adsorption, and coagulation are some of the several methods established to remove Cr ions from aqueous solutions. The majority of these methods have financial limitations due to insufficient removal, high operating costs, and the production of unpleasant metal sludge [128,131]. Yu et al. succeeded in removing Cr(VI) from wastewater using EC with an efficiency equal to ~95% [132]. Das et al. used EC with Fe and Al electrodes to remove heavy metals from wastewater [80]. It could be observed that about 99.5% of Cr is removed successfully [80]. Martín-Domínguez et al. used ferrous sulfate (Fe_SO₄) to remove Cr by EC. Ferrous sulfate has been used to reduce Cr (VI) to Cr (III) by the oxidation of Fe(II) to Fe(III). Fe(III) interacts with water and forms ferric hydroxides that can act as absorption again for Cr due to having a large surface area. An optimum ratio was required between Fe and Cr ≥ 3 to attain high-efficiency Cr removal from wastewater. The chromium removal efficiency can reach more than 99%. The redox interaction between Cr (VI) and Fe(II) can occur at a pH of more than 6.5. After 5 h, high efficiency at a voltage of 3.7 Volts was obtained [133]. From another perspective, Khan et al. utilized Fe and Al electrodes. At the anode, the process of oxidation occurs to Fe and converts Fe(II) to Fe(III). On the other hand, at the cathode, the process of reduction occurs to Cr, where Cr (VI) is reduced to Cr (III). The optimum parameters of Cr removal efficiency reach 100% at PH value 3, with 1.48 A an application current. The initial Cr (III) concentration was 49.96 ppm, and the time was found to be 21.47 min for 100% Cr (VI) removal. The energy consumption was recorded to be 12.97 W-hour per gram removal of Cr (VI) [134]. Wang utilized NiO/NF electrodes for the removal of Cr (VI) from drinking water. It was discovered that O₂-annealed NiO/NF has more Ni3+ and performs better at adsorbing Cr (VI)/OH anions. Additionally, the NiO/NF showed outstanding removal ability for additional metal ions that were present concurrently. The removal mechanism was examined, and it was ultimately shown that there are three possible routes: reduction of Cr (VI) to Cr (III) at the cathode, coprecipitation with Ni(OH)₂ floc to make Cr(OH)₃, and direct complexation interactions with recently formed amorphous Ni hydroxides. At the applied potential of 0.97 Volts, it exhibited a high removal efficiency of 99.5% in 20 min against a hydrogen-generating rate of 1.1 mmol g⁻¹ h⁻¹ [135]. Zhang et al. also used Fe as an anode. The outcomes demonstrated that RED-EC can eliminate 99% of Cr (VI) in 2 h and an applied power density of 0.482 W/m². In addition, 5.71 Volts was the highest voltage that RED-EC could produce. The maximum elimination of total Cr was marginally greater at pH 5 than it was at pH 3, since the precipitation of Fe³⁺/Cr³⁺ hydroxides was more effective at pH levels above 3 [136]. Khezmi et al. eliminated Cr from wastewater using the EC technique [15]. They used Fe electrodes to remove 100% of Cr ions from wastewater [15]. Moreover, Akbal et al. removed 100% of Cu, Cr, and Ni from wastewater using the EC with a pH of 3 [137].

2.12. Iron (Fe)

Iron is considered the most familiar water contaminant found in many water bodies [138]. Fe is mainly found in groundwater all over the globe [138]. Due to the greater levels of Fe within the lithosphere, as well as the release of Fe-containing sewage into waterways, Fe is among the heavy metals that are frequently found in both surface and groundwater (between 0.5 and 50 mg/L) [139]. The WHO encourages reducing the concentration of Fe to less than 0.3 mg/L in consumable water [138]. Also, the European Commission Directive suggests reducing the concentration to less than 0.2 mg/L for the average consumer [138]. Even though Fe is a necessary component for humans, high levels can lead to a variety of illnesses, such as cognitive dysfunction [139]. A variety of techniques were studied for the removal of Fe from water sources. These include methods such as lime softening and oxidation, followed by precipitation and filtration, subsurface iron removal, ion exchange, and the use of activated carbon, along with other filtration materials. Additionally, approaches like bioremediation, supercritical fluid extraction, the application of aerated granular filters, and various adsorption techniques have also been explored [138]. Many parameters influence one method over the other, such as getting rid of the product waste, cost-to-efficiency ratio, time, and reconstruction of new contaminants [138]. The previous methods can suffer from these parameters [138]. In this work, the main focus was the EC method, as it is cost-effective and fairly fast with high efficiency. Abdulhadi et al. [139] used EC to eliminate nearly most of the Fe present in their sample. In their experiment, they implied Al as the electrodes, the other parameters such as pH, time, concentration, and current density were 7.0, 50 min, 10 mg/L, and 30 A/m², respectively [139]. Their article proved that, at different current densities, there are several efficiencies, with the lowest efficiency at a current density of 15 A/m² [139]. Vasudevan et al. reached a 98.4% elimination using only a current density of 6 A/m², the electrodes were a combination of magnesium anode, and the cathode was a sheet of galvanized Fe [138]. The best efficiency was at a pH value of 6, and the experiments were operated for 60 min [138]. In addition, Abdulhadi et al. utilized a continuous flow EC to remove Fe from wastewater [139]. They succeeded in removing 99.9% of Fe using Fe electrodes [139]. The operation parameters were pH 7, current density 73 A/cm², and time of 50 min [139]. Stylianou et al. investigated the influence of different parameters on EC [77]. They managed to remove nearly 100% of Fe within 5 min, they used Al electrodes for this process [77]. These results were obtained with the gap between the electrodes at 0.5 cm [77]. Ghosh et al., with the use of Al electrodes, were able to reach an elimination efficiency of 99.2% in just 35 min, with an initial concentration of 25 mg/L [140]. The initial pH was set to 7.5 and at the end of the trials, the pH varied with different current densities that were used in the trials [140]. The best current density used was 0.04 A/m, at this value, the time used for concentrations of 5–10 mg/L was just 5 min [140]. As with the other elements, Fe removal involves the dissolution of a sacrificial metal anode, such as an iron or aluminum electrode, which leads to generating coagulant species. These coagulants aggregate and precipitate suspended particles, while dissolved contaminants are adsorbed. At the same time, bubbles of hydrogens and oxygens are produced during water electrolysis. These bubbles assist in floating pollutant particles to the surface. Also like the previous elements, the process depends on the pH and redox potential and the iron existing as Fe(II) or Fe(III). This mainly controls the hydroxide formation or precipitated compounds in general. The flocs, formed by this process, can float via hydrogen bubbles or settle as sediment [140]. K. S. Hashim et al. managed to reduce the Fe concentration from 20 mg/L to 0.3 mg/L in 20 min using 15 A/m² [141]. The initial pH in the experiment was 6, and the gap between the electrodes was set to be 0.5 cm, they used Al as the electrodes with a combination of 6 plates in a new design of the EC [141]. The utilization of Fe as anode and cathode showed a substantial impact on removing, which is about 99.1% of As geothermal fluid at a pH value of 8.6 and a current of 2.9 A at 15 min [10]. Abdulhadi et al. removed Fe from wastewater using a continuous-flow EC. The results showed that the Al electrodes efficiently removed about 99.9% of the Fe from wastewater [139]. A comparison between the use of EC for contaminants removal from wastewater upon their cost is summarized in Table 4.

2.13. Fluoride (F)

Fluoride ions in water can have both positive and negative effects on health and the environment. At an optimal concentration of 1 mg/L, fluoride helps prevent dental cavities, offering protection against tooth decay [142]. However, long-term consumption of water with excessive fluoride levels can result in dental and skeletal fluorosis and, consequently, cause damage to teeth and bones [142] Khatibikamal et al. used aluminum electrodes for fluoride removal from industrial wastewater. These samples are from the steel industry. As the main idea of EC, aluminum dissolves at the anode, forming aluminum hydroxide, which adsorbs and removes pollutants (fluoride) through charge neutralization and enmeshment in precipitates. For an initial fluoride concentration of 4.0–6.0 mg/L, the authors reported that the process achieved 93% fluoride removal within only 5 min [143].

2.14. Mercury (Hg)

Nanseu-Njiki et al. evaluated the effectiveness of the EC process for removing mercury(II) from synthetic water solutions and river water. This process is based on iron and aluminum electrodes [144]. They showed a 3 cm distance between electrodes, a current density of 2.5 to 3.125 A/dm², and charge loading of 9.33 F/m³ for iron and 15.55 F/m³ for aluminum. In addition, the mercury removal efficiency exceeded 99.9%. They reported and demonstrated in their study that iron was more effective than aluminum, with iron achieving the highest mercury removal within 15 min compared to 25 min for aluminum [144]. The removal process was not significantly affected by organic matter in river water, with efficiencies remaining high. The results indicated that electrocoagulation using iron electrodes was faster and more efficient than aluminum for mercury(II) ion removal [144].

2.15. Lead (Pb)

A study by Mansoorian et al. assessed the efficiency of EC for lead removal from wastewater in the battery manufacturing industry using direct and alternating currents [119]. The results showed that, with alternating current at a current density of 6 mA/cm², the removal of lead reached 96.7% using iron electrodes. In contrast, stainless steel electrodes at a current density of 8 mA/cm² achieved a lead removal efficiency of 93.8%. On the other hand, by using direct current, iron electrodes provided an even higher lead removal efficiency of 97.2% at the same current density, while stainless steel electrodes yielded 93.2%. The optimum energy consumption for lead removal was 0.69 kWh/m³ with iron electrodes under alternating current, compared to 1.97 kWh/m² with direct current. Maximum sludge production was noted at 0.084 kg/m³ for alternating current and 0.091 kg/m³ for direct current [119]. These results demonstrate that EC is an effective method for lead removal from wastewater, especially with alternating current due to its higher efficiency and lower energy consumption.

2.16. Selenium (Se)

In a study by Hansena et al., the EC technique was evaluated as a potential solution for treating selenium in wastewater from a petroleum refinery [145]. They used a batch airlift reactor with air stirring and iron sacrificial electrodes to obtain ferrous ions. This process reached a selenium removal efficiency of 90% after 6 h, which reduced the concentration from 0.30 mg L⁻¹ to 0.03 mg L⁻¹. They identified the current density as a critical factor influencing removal efficiency. The increase in the current density from 76.7 A m⁻² to 153.4 A m⁻² nearly doubled the removal efficiency over a 240-min treatment period. Although the treatment significantly lowered the selenium levels, the residual concentration of 0.03 mg L⁻¹ still exceeded the maximum allowable limit of 0.01 mg L⁻¹ set by local regulations [145].

Table 4. Inorganic ions (cations) removed from wastewater using the EC method with their concentrations, pH values, and time of processing besides the obtained efficiency.

NO.	Removed Element	Anode	Cathode	Gap (cm)	Conc. (mg/L)	η (%)	pH	Time (min)	Power (A/m2)	Ref
1	Ag	Graphite	Al	1	20	97	5.4	20	50	[66]
4	As(V)	ZVI-PVDF	----	----	5.12	99	7	120	----	[146]
7	Ni (II)	Fe sheets	Ti	----	100	99.	7	25	7	[147]
8	Cr(VI),	Al	Al	3	50	~95,	5	60	250	[132]
	Ni(II)					~96,
	Cu(II)					~100
9	Boron	Fe-based	304 SS	1	----	6–13	7.5		600	[127]
10	Zn,	Al	Al	0.7	0.096,	95 for Heavy metals and 97 for microplastics	6	20	120	[148]
	Ni,				0.022,
	Cu,				0.045,
	Cr,				0.036,
	Pb,				0.003,
	microplastics				48.1
11	As	SS	SS	----	0.01	92	----	----	----	[75]
12	As	Fe2+	Fe2+	----	0.1, 0.3, 0.4, and 1	----	7–8	60	20.83	[149]
13	Ni(II)	platinum-coated titanium mesh	nickel plate	----	325	97	4.5	120	50	[150]
14	Fe,	Al	Al	----	100	99,	----	5, 90, 150, 45, 150, +435, +435	200	[77]
	Zn,					99,
	Mn,					99,
	Cu,					99,
	Ni,					98,
	Cd,					96,
	Cr					88
15	Cu-EDTA	RuO2–IrO2/Ti	Al	----	50	99	7	60	102.9	[151]
17	Pb,	Fe	Fe	1	18, 451, 17	99,	5	40	140	[12]
	Cr,					94,
	Cd					99
18	Fe	Al	Al	----	10–30	99	----	10–50	15–45	[139]
19	Fluoride	Fe	Fe	----	----	----	6		75.44	[152]
20	Ca(II) (COD)	Al, Mg, and Fe	Ti	5	----	64, 75	7.5	60	115	[153]
21	Zn,	steel anode with polypyrrole modification	SS-Al	2	----	99,	4	70	300	[154]
	Ni					80
22	Fluoride	Al	Al	----	1.37–48	90	4–9	----	----	[155]
23	As	Fe	Fe	----	15.2–41.5	----	----	----	3	[156]
24	Cu	Al	Al	1	----	95	----	10	40	[157]
25	Cr (VI)	Fe	----	----	1, 5, and 10	----	----	----	13	[158]
27	Fluoride	Al	----	----	61	----	4.8–5	----	50	[159]
28	NO3,	Al3+ or Fe2+	H2	----	----	62,	7	240	100	[160]
	C−,					30,
	SO42−,					42,
	K(I),					29,
	Mg(II),					83,
	Ca(II),					31,
	Na(I)					27,
	F-					69
29	Fluoride	Al	----	----	7.35	86	----	10	100	[161]
30	Sb(V) and Co(II)	Fe	SS	1	60	95	----	20	50	[162]
31	Na, Cr, Cu, Pb, and Ni	Al	H2	0.5–20	10.2	85–98	3.6–8.7	20	20–80	[163]
32	Boron	Al	H2	----	----	70	7.35	----	187.5	[124]
33	Fe(II)	Al	SS	1	330	----	6.34	45	20	[164]
34	Ca(II),	Mg	SS	1	----	52,	8	60	142.9	[165]
	Mg(II)					94
35	Sb(V)	Fe	Fe	----	> 4.0	99		30	----	[166]
36	fluoride and HS	AL	Al	----	4.08, 90	96	7.38	----	40–70	[167]
38	Fe,	Al, Fe	Fe, Al	1	----	99,	7.09	60	68.50	[80]
	Cr,					99,
	Pb,					99,
	Mn,					98,
	Cu					73
39	Br-, Cl-, TDS, and SO42−	Al	Al	----	----	84	8	10	20	[168]
40	Ca, Mg, silica and dissolved organic matter	Ti	SS	----	17–25	60–65	11.5	----	220	[169]

Table 4. Inorganic ions (cations) removed from wastewater using the EC method with their concentrations, pH values, and time of processing besides the obtained efficiency.

NO.	Removed Element	Anode	Cathode	Gap (cm)	Conc. (mg/L)	η (%)	pH	Time (min)	Power (A/m2)	Ref
1	Ag	Graphite	Al	1	20	97	5.4	20	50	[66]
4	As(V)	ZVI-PVDF	----	----	5.12	99	7	120	----	[146]
7	Ni (II)	Fe sheets	Ti	----	100	99.	7	25	7	[147]
8	Cr(VI),	Al	Al	3	50	~95,	5	60	250	[132]
	Ni(II)					~96,
	Cu(II)					~100
9	Boron	Fe-based	304 SS	1	----	6–13	7.5		600	[127]
10	Zn,	Al	Al	0.7	0.096,	95 for Heavy metals and 97 for microplastics	6	20	120	[148]
	Ni,				0.022,
	Cu,				0.045,
	Cr,				0.036,
	Pb,				0.003,
	microplastics				48.1
11	As	SS	SS	----	0.01	92	----	----	----	[75]
12	As	Fe2+	Fe2+	----	0.1, 0.3, 0.4, and 1	----	7–8	60	20.83	[149]
13	Ni(II)	platinum-coated titanium mesh	nickel plate	----	325	97	4.5	120	50	[150]
14	Fe,	Al	Al	----	100	99,	----	5, 90, 150, 45, 150, +435, +435	200	[77]
	Zn,					99,
	Mn,					99,
	Cu,					99,
	Ni,					98,
	Cd,					96,
	Cr					88
15	Cu-EDTA	RuO2–IrO2/Ti	Al	----	50	99	7	60	102.9	[151]
17	Pb,	Fe	Fe	1	18, 451, 17	99,	5	40	140	[12]
	Cr,					94,
	Cd					99
18	Fe	Al	Al	----	10–30	99	----	10–50	15–45	[139]
19	Fluoride	Fe	Fe	----	----	----	6		75.44	[152]
20	Ca(II) (COD)	Al, Mg, and Fe	Ti	5	----	64, 75	7.5	60	115	[153]
21	Zn,	steel anode with polypyrrole modification	SS-Al	2	----	99,	4	70	300	[154]
	Ni					80
22	Fluoride	Al	Al	----	1.37–48	90	4–9	----	----	[155]
23	As	Fe	Fe	----	15.2–41.5	----	----	----	3	[156]
24	Cu	Al	Al	1	----	95	----	10	40	[157]
25	Cr (VI)	Fe	----	----	1, 5, and 10	----	----	----	13	[158]
27	Fluoride	Al	----	----	61	----	4.8–5	----	50	[159]
28	NO3,	Al3+ or Fe2+	H2	----	----	62,	7	240	100	[160]
	C−,					30,
	SO42−,					42,
	K(I),					29,
	Mg(II),					83,
	Ca(II),					31,
	Na(I)					27,
	F-					69
29	Fluoride	Al	----	----	7.35	86	----	10	100	[161]
30	Sb(V) and Co(II)	Fe	SS	1	60	95	----	20	50	[162]
31	Na, Cr, Cu, Pb, and Ni	Al	H2	0.5–20	10.2	85–98	3.6–8.7	20	20–80	[163]
32	Boron	Al	H2	----	----	70	7.35	----	187.5	[124]
33	Fe(II)	Al	SS	1	330	----	6.34	45	20	[164]
34	Ca(II),	Mg	SS	1	----	52,	8	60	142.9	[165]
	Mg(II)					94
35	Sb(V)	Fe	Fe	----	> 4.0	99		30	----	[166]
36	fluoride and HS	AL	Al	----	4.08, 90	96	7.38	----	40–70	[167]
38	Fe,	Al, Fe	Fe, Al	1	----	99,	7.09	60	68.50	[80]
	Cr,					99,
	Pb,					99,
	Mn,					98,
	Cu					73
39	Br-, Cl-, TDS, and SO42−	Al	Al	----	----	84	8	10	20	[168]
40	Ca, Mg, silica and dissolved organic matter	Ti	SS	----	17–25	60–65	11.5	----	220	[169]

3. Operating Parameters Affecting the EC Process

For the improvement of the efficiency of the EC, several parameters may be taken into consideration, such as pH, power (current density), and time [12]. The two most prominent factors impacting the operating expenses of the system are power usage and electrode material [170]. Therefore, adjusting the right values for these parameters might improve the overall efficiency of the removal system [12]. It also may cut the cost of the cell, and the EC procedure may be suggested to be used in many aspects of treating wastewater [171]. At the end of the experiment, the overall cost defines the most practical scenario [171]. Power consumption may be the main cost of the cell.

3.1. pH Value

Most of the preceding articles have found a significant connection between EC efficiency and effluent pH [172]. Adjustment of the pH value improves the efficiency of the process [172]. Márquez et al. managed to remove brilliant green (BG) tannery dye with an efficiency of 100% at pH 6 with a current density of 6 mA/cm² [173]. The pH was set using a diluted solution of sulfuric acid or sodium hydroxide [173]. Lach et al. studied the pH value influence on the EC [174]. They found that pH values between 5.0 and 9.0 resulted in mononuclear species or complex polynuclear Al being generated and were efficient for coagulating the waste [174], while, in the elevated pH values, the generated hydroxides were not efficient for coagulating the toxic materials [174]. The current used in this study was 4.0 A, and the initial concentration was 0.10 g/L [174].

Lu et al. eliminated heavy metals from acid wastewater and studied the efficiency of the electrodes with a pH adjustment [171]. They adjusted the pH with NaOH and HNO₃ [171]. Cu(II) and Cr(III) could produce hydroxide precipitates more easily in an alkaline pH, which was also beneficial for their chemical precipitation, but in the range of 4–8 with Al, the electrode led to a more efficient precipitation of the material [171]. With an initial pH of 2.0, the efficiency was low, while, at a pH of 3.0, the efficiency expanded, and with pH = 4.0–6.0, the efficiency was better with a removal rate for Cu(II) and Cr(VI) around 86.8–97.9% and 84.2–94.5%, respectively [171]. Yu et al. investigated the influence of different pH values (5, 7, and 9) on their experiments. The results showed that, at pH 9, the polymeric species that appeared was impractical in removing waste, and the removal rate was reduced when investigating pH of 7 and 5. They also found that the reduction was better at pH 5 with Al electrodes [132].

3.2. Applied Power

Power consumption is a major factor determining the overall cost and efficiency ratio of the EC cell [171]. The electrical current is a crucial component affecting the removal efficiency, because it affects the rate of the transferred electrons and the coagulant formation in the EC procedure [172]. Lu et al. compared the efficiency when the current density varied from 5.79 A/m² to 11.57 A/m². The results exhibited that the removal efficiency of Cu(II) and Cr(VI) changed significantly from 59.0% to 99.9% and 79.1% to 98.0%, respectively [171]. This work concluded that there is an optimum value of the used power. In other words, there is a certain value of power that could be preferred for each contaminant to be removed. When the power is transferred from this value, there is no remarkable development that can be obtained in the removal efficiency, as observed in Figure 2, and this is adapted from [171].

Yu et al. investigated the relationship between the power and the removal rate with different current densities. They found that, at 25 mA/cm², the removal rate of Cr(VI), Ni(II), and Cu(II) was elevated from 6.5% to 91.5%, 11.1% to 92.1%, and 14.1% to 97.1%, respectively. The process took place over one hour, as in Figure 3 [132]. Oladipo et al. investigated a variety of voltages from 4.5 to 12.0 Volts. They found that, in the voltages from 4.5 through 9 Volts, there were significant variations in the removal rate, and in the elevated voltages of more than 9 Volts to 12 Volts, there were no significant removal rate expansions [172]. The current density also elevated as the applied voltage increased. However, at elevated voltages, there were unnecessary processes that caused an increase in power consumption and loss of efficiency [172].

The number of electrons released by the anode is an important parameter. Further parameters that affect removal efficiency are current density, inter-electrode distance, pH of the solution, and electrode configuration [175,176,177]. The current density denotes the number of electrons per unit area, which is a crucial parameter in determining the removal efficiency [178]. An increase in current density means more electrons in the cathode for oxidizing contaminants and hydrolysis. At the same time, more cations are formed in the water for metal hydroxide formation. However, there is an upper limit for the current density, and above this certain value, the efficiency decreases. The surplus of current might lead to a secondary reaction and overdose coagulation. As a result, the electrode life might be shortened. In addition, forming a thin layer on the electrodes increases the depletion region and Ohmic drop [177]. Therefore, less EC performance can be obtained if the current density exceeds the limit.

The inter-electrode distance is the distance between the electrodes. This distance controls the resulting electrostatic field. The decrease in distance between the cathode and anode increases the electrostatic field [179]. There is a critical value of the distance, since the smaller values cause the field to decrease due to the violent collisions, leading to a deterioration of metal hydroxides and flocs [180].

The value of the pH has a significant role in determining the efficiency of the EC. Slightly acidic and neutral conditions show higher efficiency than basic in the case of some materials, while others exhibit higher EC degradation efficiency at alkaline conditions [181,182].

Electrode configuration describes the connection between the anode and cathode [183]. There are different types of connections that can be in series or parallel and monopolar or bipolar. There are two series configurations and only one parallel configuration. If two anodes are connected on the positive side of the power source in a parallel connection, the cathodes are on the negative side in parallel. Thus, the configuration is called monopolar-parallel (MP-P) [184]. When they are in a series, it is called monopolar-series (MP-S) [185]. Further, if the inner electrodes are not connected while the outer ones are connected to the power source, a polarization is induced on the surfaces of the inner electrodes and called bipolar-series (BP-S) [186].

3.3. Time

The value of time in the EC may be an influential factor in power consumption [132]. Expanding the period under the same conditions, the efficiency of removing the wastes increases significantly [132]. Stylianou et al. proved in their article that different elements would take different periods to coagulate [77]. At the current density of 20 mA/cm², the 90%+ removal rate of Fe, Cu, Mn, Cd, Zn, Ni, and Cr would take 5, 45, 90, 150, 150, 90, and 150 min, respectively, as displayed in Figure 4, adapted from [77]. Each element has its own time to be coagulated concerning the current density and the initial pH value [77]. Thakur et al. tested an EC cell to remove heavy metals from synthetic wastewater and found that, when the working time was 20 min, the elimination rate of lead was 95%. However, the rate was 100%, with an additional 20 min. Moreover, the chromium was eliminated at 87.7% in 20 min and hit an extreme value of 97% within 40 min [12]. On the other hand, cadmium hit a 98% elimination rate within 20 min and total elimination within 45 min [12]. This proved that, expanding the period of the process, more of the metals coagulate, and the cell has a higher elimination efficiency [12].

3.4. Electrode Spacing

The electrode spacing has an impact on how well the ions produced by the reaction on the electrode diffuse [172]. Once the distance is ideal for the reaction, it requires less time to reach equilibrium, which accelerates the removal of wastes [172]. Oladipo et al. managed to achieve great results by providing the best spacing for their reaction. They tried 1, 2, and 4 cm spacing between the electrodes and found that, when the spacing value was 1 cm, the reaction took time to reach equilibrium, and that slowed down the reaction. Furthermore, when they investigated 2 and 4 cm, they found that the duration required to achieve a state of equilibrium was reduced, however, with 4 cm, the removal rate was cut down compared to 2 cm [172]. Moreover, they found out that, by reducing the spacing between Fe electrodes with an initial spacing of 5 cm and final spacing of 2 cm, the efficiency increased by around 8% [172]. That study and the previous ones proved that shrinking the distance between electrodes decreased the movement resistance since the particles had a shorter path, using less electrical energy, which increased the effectiveness of the process [172]. Lu et al. tried varieties of distances (0.5, 1, and 1.5) and found that, when increasing the distances between the electrodes from 0.5 to 1.0 cm, the efficiency of eliminating Cu(II) increased from 96.5% and reached 99.9%, and further expansions led to decreasing the efficiency of the cell, but the enlargement led to higher power consumption rates [171].

3.5. Concentration of Contaminants

According to several research works, the effectiveness of removal declines as the influent contaminant concentration rises, while the current density is kept unchanged [172]. Lach et al. found that, as the starting concentration increased, the heavy metal removal ratios fell and vice versa [132]. The elimination percentages of Cr(VI), Ni(II), and Cu(II) fell from 84.18% to 53.7%, 99.97% to 68%, and 99.5% to 70.8%, respectively, for one hour process. As the starting concentration increased from 10 to 100 mg/L, the number of heavy metal ions eliminated rose as the starting content of the solutions rose, despite the elimination rate declining [132]. Oladipo et al. compared different concentrations of tetracycline and found that, when raising the concentration from 5 mg/L to 10 mg/L and the time kept unchanged, the elimination rate rose from 78.8% to 97.9% [172]. They also tried to increase the tetracycline concentration to 15 mg/L and found that the elimination rate fell off to reach 96.9% with the same conditions, but when the time increased to reach one hour, the elimination rate reached 99.3% [172].

3.6. Anode Material

The anode is extremely important, as it is the element in that system that oxidizes the contaminants and corrodes them into the solution as metal cations. Metal oxides play a key role in immobilizing heavy metals in the water and changing their form from reactive to inert forms [187]. The mixed metal oxide anode encourages the oxidation of ammonium and organic contaminants directly or indirectly using locally produced electrogenerated reactive oxygen/chlorine species [187].

In the Fe anode, Fe(II) and reactive oxygen/chlorine combine to generate additional potent oxidants, which facilitate the conversion of contamination [187]. It has been demonstrated that the formation of flocs during the Fe-based coagulation process depends on the hydrolysis of Fe(III). Due to the sluggish oxidation process from Fe(II) to Fe(III), Fe hydroxide/oxide compounds adhere in the pores of typical electrocoagulation membrane (ECM) reactors, allowing for the oxidation of a portion of Fe(II) and the hydrolysis of Fe(III). The oxidation process (from Fe(II) to Fe(III)), which restricts the oxidation and hydrolysis processes to the bulk solution, will be improved based on the aforementioned mechanism, which will significantly lessen the inorganic fouling of the membrane [188]. Zeng et al. fabricated a coupled EC with electrooxidation to collect N and P. The anodes were mixed metal oxide (MMO)/Fe and played a significant role in the absorption of P from the solution. Additionally, the dual MMO/Fe anode system had beneficial effects on decreasing the mobility of Cu in the spiked sediment [187]. The results showed that the concentration of copper was reduced by 16–23% from the solution, while NH₄⁺ and organic carbon both fell by 32–63% and 56–71%, respectively [1]. Zhao et al. studied the secondary treatment of wastewater using a ferrate-enhanced EC/ultrafiltration system to determine how oxidation and coagulation could work together. The results showed that the usage of Fe as an anode played a key role in the removal of protein and dissolved organic carbon [188]. The efficiency of the removal was about 70.8% [188]. Al anode showed better behavior in removing the microplastics from the wastewater than the iron anode [189].

According to reports, the EC method’s removal effectiveness when it was used to treat As(V) using the aforementioned electrodes ranged from 75% to 99% [75]. High As concentrations were also generated using stainless steel electrodes. Even though Fe electrodes are the most often used electrodes in the EC. They might cause an issue with water turning yellow, because they produce tiny rust particles. This obstacle has been addressed using stainless steel anodes [75].

Total cost reduction poses numerous concerns about the usability and viability of EC technology [154]. Fe and/or Al anodes might theoretically dissolve or corrode, and these dissolved metal ions could operate as active coagulants during the operation of an EC to help heavy metals to become precipitated. Anode longevity may be somewhat shortened if anodic plates are consumed excessively and needlessly [154]. Finding a solution to apply anode protection is therefore vital to decreasing wasteful anode consumption and lengthening anodic periods [154]. Conducting polymers (CPs) have garnered a great deal of interest in a variety of fields, including corrosion protection coatings, electrode materials, and capacitor material. This is because of their distinctive chemical and biochemical properties, high stability, and ease of synthesis [154]. Under various conditions, conductive polymers (CPs) deposited onto the surface may successfully limit the corrosion of 430 SS by acting as physical barriers to stop and isolate the attack of hostile ions [154]. Both polypyrrole (PPy) and polyaniline (PANi) were widely used to reduce metal corrosion [154]. In corrosive environments, the doped PPy can be reduced by the electrons produced during the oxidation of the metal substrate, and the reduced PPy could subsequently transfer the electrons to oxygen during self-oxidation to return to the doped state [154]. Thus, to increase corrosion resistance, the repeated oxidation and reduction reaction of PPy considerably avoids direct contact with oxygen [154]. Additionally, PPy is more ecologically friendly than PANi due to its biocompatibility and environmental stability as opposed to PANi’s high poisonousness [154]. However, a method of using a stainless anode modified with PPy to prevent corrosion during the EC is still a secret [154]. Sorayyaei et al. removed methyl orange from the solution using adsorption and EC. The stainless steel anode played a vital role in removing about 93.1% of methyl orange from the solution [190]. Further, Nguyen et al. removed arsenate from wastewater. The result showed about 92% of the arsenate was removed from the wastewater [75]. Liu et al. increased the steel anode’s lifetime using polypyrrole modification during the EC method of treating wastewater from electroplating. Zinc and nickel were successfully removed from the electroplating sewage with removal efficiencies of 99.9% and 80%, respectively. In addition, even after ten further EC treatment batches, the corrosion inhibition efficiency reached 30% [154].

4. Limitations and Opportunities of the Low-Cost EC Process

4.1. Drawbacks of the EC Process

There are several drawbacks of electric flocculation, such as:

• The electrode produces gas, while the flocculant is a result of wear, necessitating regular plate replacement [148].
• The using of electricity and electrode passivation may raise energy consumption and cost [191].
• Excessive electrolyte concentrations might result in the creation of hazardous compounds, so pretreatment is necessary in some cases before the EC [148].
• Formation of sludge.

4.2. New Developments and Opportunities

Due to the widespread use of the EC for the treatment of wastewater, recent studies have attempted to enhance its performance by reducing cost, sludge production, and the energy consumption of the process using new configurations [191]. To overcome the power consumption problem, drilled electrodes have been used. The most common design is rectangular reactors with planar rectangular electrodes. Determining a new EC reactor that would reduce power consumption by changing external wastewater mixers for drilled electrodes was the initial step in the present research [139]. Abdulhadi et al. developed a new EC reactor to remove Fe from wastewater with low power [139]. They overcame the power problem using drilled electrodes. This unique reactor comprises four rectangular pierced Al electrodes with an effective area of 280 cm² that are placed within a rectangular Perspex container. To guarantee that the anode holes are 5 mm offset from the cathode holes, each electrode contains 35 holes, each measuring 5 mm in diameter (the very next electrode). By forcing the solution to flow along a winding direction, the electrode distribution of holes increases the water stirring efficiency even without external stirrers. This concept could be a more affordable option than traditional EC reactors, which rely on mechanical or magnetic stirrers and require additional electricity [139]. To increase the water mixing efficiency without using external stirrers, the electrode distribution of holes pushes the solution to flow in a complicated course [139].

On the other hand, to overcome the electrode passivation problem, new shapes of electrodes, such as rotating or cylindrical electrodes, with different types of electrodes, such as Mn or Cu, have been made. Also, applying AC provides better control of electrode passivation. Employing a high-pressure EC reactor, solar energy instead of an electrical source of power, simultaneous electro-flotation (EF), or a separate flotation chamber for improved sludge flotation and separation, could accelerate the conversion of ferrous to ferric ions [191]. The electrode passivation increases the electric power consumption and creates secondary contaminants, with a small quantity of coagulant formation through the sacrificial anodes, which are some of the disadvantages of the EC treatment technology, which restricts its uses in real applications. With this idea in mind, the source of Fe²⁺ that serves as either a reducing agent (oxidizing Fe²⁺ into Fe³⁺) or a coagulant was chosen to be the salt FeSO₄·7H₂O [192].

Further, cathode passivation, anode degradation, and the short lifespan of electrodes could be drawbacks of using direct current (DC). Researchers have suggested using alternating current (AC) to increase the lifespan of electrodes, because AC interacts with electrodes in a different way than direct current. It has been proposed that replacing conventional direct current (DC) with sinusoidal alternating current (AC) or pulsed direct current (PDC) waveforms in EC might assist in decreasing passivation and ameliorating its related negative consequences [193]. The use of AC (Al electrodes) in the EC treatment of simulated wastewater led to the uniform dissolving of anodic metal and the avoidance of passivation.

To overcome the problem of the cost due to anode passivation, one suggested employing recyclable metallic wastes as electrode materials as a sustainable option [194]. The use of Al foil, beverage cans, scrap mild steel, and scrap Fe as sacrificial electrodes for the EC method of treating grey water (GW) was suggested [194]. However, their subsequent usage as electrodes brought up some significant issues. During electrolysis, Al foil experiences rapid and significant perforation and surface area loss [194]. It was discovered that the scrap Fe and the mild steel increased the ions of Fe in the treated GW, which led to metal contamination [194]. Metal scrap wastes may be used to make sustainable replacement electrodes [194]. The running costs are viewed as key elements of the EC regarding energy consumption and the most effective method of manufacturing the metallic wastes into an appropriate form to be reused in the EC approach [194]. Bani-Melhem et al. evaluated the use of scrap metallic waste electrode materials in wastewater EC. According to the results, it was possible to significantly lower the COD, turbidity, color, and electric conductivity by 88%, 99%, 97%, and 89%, respectively, when metallic wastes were employed as sacrificial electrodes [1]. A brief comparison of the cost of EC and some other techniques for wastewater treatment is in Table 5.

Additionally, because the necessary electricity to operate could be generated by a solar panel, employing this technology became feasible in rural regions [195]. As a result, EC technology has been deployed to eliminate nitrate in water and wastewater, either alone or in combination with other techniques [195]. Hashim et al. utilized a novel flow column reactor and energy-efficient Ec. An experimental, statistical, and financial strategy is used to reduce nitrate from drinking water. To remove nitrate from synthesized drinking water samples, the modern EC reactor, FCER, has been employed while taking into consideration the impact of important operational factors, including the starting pH, CD, and GBE. The collected findings demonstrated that the EC technique of nitrate removal from drinking water prefers an alkaline environment. Furthermore, it has been shown that the freed coagulants from the Al anode are equivalent to the integral of the applied current and the electrolyzing period, which, in turn, affects the denitrification process [195]. Nazlabadi et al. used a new multi-response optimization method, electrocoagulation–flotation (ECF), for the simultaneous removal of nitrate and nitrite. Fifty-seven experiments created using the response surface method (RSM) were completed for this reason [196]. Selected factors, such as the beginning number of electrodes, initial nitrate concentration, current intensity, response duration, pH, and their interactions, were assessed for their effects [196]. Eight optimum settings with residual nitrite and nitrate concentrations of less than 10 mg/L and 50 mg/L, respectively, and operating costs of 100.05 USD/(kg NO₃ removed), were achieved [196].

Table 5. Cost comparison between the EC technique and some other techniques for the treatment of wastewater.

NO.	Removed Substance	Removal Technique (1)	Removal Technique (2)	Energy Consumption/Cost (1)	Energy Consumption/Cost (2)	Ref
1	PE	EC	---	0.91 USD·L−1	---	[148]
2	MPs	EC	---	0.03 and 3.85 USD/m3	---	[197]
3	COD	EC	---	31.9 kWh/m3 and 20.4 kWh/kg	---	[198]
4	Acid Red 18	EC	---	3 USD/Kg	---	[199]
5	Cobalt	EC	---	0.204 kWh	---	[200]
6	Cr	EC	---	137.2 KWh m−3	---	[201]
7	Cr, P, COD, and turbidity contents	EC	---	2.21 kWh/m3	---	[202]
8	azo dyes	EC	---	1.5925 KWh m−3	---	[203]
9	oxytetracycline hydrochloride	EC	---	0.0014 kWh/L and 0.19 kWh/L	---	[204]
10	Pb, Cd, and Cu	EC	---	12.71 kWh/m3	---	[205]
11	Blue dye	EC	photo-assisted chemical oxidation	0.0481 USD and 0.6418 USD	1.0267 USD and 1.04337 USD	[206]
12	Dye	EC	Ozonation	1.58 kWh/m3	8.41 kWh/m3	[207]
13	Dye	EC	Fenton	1.58 kWh/m3	4.71 kWh/m3	[207]
14	Dye	EC	Photo-Fenton	1.58 kWh/m3	11.2 kWh/m3	[207]
15	Boron	EC	chemical coagulation	0.8 $/kg	1.8 USD/kg	[208]
16	COD	EC	Photo-electrocoagulation	65.06 kWh/kg	119.34 kWh/kg	[209]
17	COD	EC	Peroxi-electrocoagulation	65.06 kWh/kg	32.14 kWh/kg	[209]
18	COD	EC	Peroxi-photoelectrocoagulation	65.06 kWh/kg	77.55 kWh/kg	[209]
19	COD	EC	Chemical coagulation	1.336 USD/kg	0.591 USD/kg	[210]
20	COD	EC	Chemical coagulation	1.336 USD/kg	0.318 USD/kg	[210]
21	COD	EC	Ultrafiltration	1.336 USD/kg	0.044 USD/kg	[210]
22	Dye	EC	Chemical coagulation	0.34–0.52 US$/kg	0.32 USD/kg	[211]
23	COD	EC	Electrochemical Fenton	2.73 kWh/kg	3.38 kWh/kg	[212]
24	COD	EC	Electro-Fenton	2.73 kWh/kg	63.64 kWh/kg	[212]
25	COD	EC	Peroxi-coagulation	2.73 kWh/kg	23.19 kWh/kg	[212]
26	TOC, COD, TP, and color	EC	electro-Fenton	1.27 EUR/m3	1.42 EUR/m3	[213]

Table 5. Cost comparison between the EC technique and some other techniques for the treatment of wastewater.

NO.	Removed Substance	Removal Technique (1)	Removal Technique (2)	Energy Consumption/Cost (1)	Energy Consumption/Cost (2)	Ref
1	PE	EC	---	0.91 USD·L−1	---	[148]
2	MPs	EC	---	0.03 and 3.85 USD/m3	---	[197]
3	COD	EC	---	31.9 kWh/m3 and 20.4 kWh/kg	---	[198]
4	Acid Red 18	EC	---	3 USD/Kg	---	[199]
5	Cobalt	EC	---	0.204 kWh	---	[200]
6	Cr	EC	---	137.2 KWh m−3	---	[201]
7	Cr, P, COD, and turbidity contents	EC	---	2.21 kWh/m3	---	[202]
8	azo dyes	EC	---	1.5925 KWh m−3	---	[203]
9	oxytetracycline hydrochloride	EC	---	0.0014 kWh/L and 0.19 kWh/L	---	[204]
10	Pb, Cd, and Cu	EC	---	12.71 kWh/m3	---	[205]
11	Blue dye	EC	photo-assisted chemical oxidation	0.0481 USD and 0.6418 USD	1.0267 USD and 1.04337 USD	[206]
12	Dye	EC	Ozonation	1.58 kWh/m3	8.41 kWh/m3	[207]
13	Dye	EC	Fenton	1.58 kWh/m3	4.71 kWh/m3	[207]
14	Dye	EC	Photo-Fenton	1.58 kWh/m3	11.2 kWh/m3	[207]
15	Boron	EC	chemical coagulation	0.8 $/kg	1.8 USD/kg	[208]
16	COD	EC	Photo-electrocoagulation	65.06 kWh/kg	119.34 kWh/kg	[209]
17	COD	EC	Peroxi-electrocoagulation	65.06 kWh/kg	32.14 kWh/kg	[209]
18	COD	EC	Peroxi-photoelectrocoagulation	65.06 kWh/kg	77.55 kWh/kg	[209]
19	COD	EC	Chemical coagulation	1.336 USD/kg	0.591 USD/kg	[210]
20	COD	EC	Chemical coagulation	1.336 USD/kg	0.318 USD/kg	[210]
21	COD	EC	Ultrafiltration	1.336 USD/kg	0.044 USD/kg	[210]
22	Dye	EC	Chemical coagulation	0.34–0.52 US$/kg	0.32 USD/kg	[211]
23	COD	EC	Electrochemical Fenton	2.73 kWh/kg	3.38 kWh/kg	[212]
24	COD	EC	Electro-Fenton	2.73 kWh/kg	63.64 kWh/kg	[212]
25	COD	EC	Peroxi-coagulation	2.73 kWh/kg	23.19 kWh/kg	[212]
26	TOC, COD, TP, and color	EC	electro-Fenton	1.27 EUR/m3	1.42 EUR/m3	[213]

5. Summary and Path Forward

EC is characterized by simplicity and effectiveness. The process depends on different parameters that are discussed in detail. The main idea is precipitating the contaminations on flocs of the metal hydroxide, which are created by OH ions from the cathode with the metal cations of the anode. This process is simple and works for different water contaminants. In a way, it can be used for industrial wastewater, and in another way, it can treat drinking water. Further, this process might be combined with other techniques such as advanced oxidation for obtaining the highest results. From an economical aspect, EC is lower in costs than the other technique compared to its efficiency. The cost depends on the dissolved electrode if there is an additional electrolyte (additional chemicals) and the energy used for electricity. Therefore, using the optimum configuration of electrodes and parameters can decrease the used energy and, consequently, the cost. In this regard, it was reported by Kobya et al. that removing 95% of Ni and 93% of Cd cost them 2.45 USD/m³ and 1.85 USD/m³, respectively [206]. This indicates how costly and effective the process is. More investigations for optimal conditions must be done in the future to obtain an overview of most cases and materials for saving more energy consumption. Furthermore, the operating cost was estimated by Ebba et al. due to its importance and its implementation in large-scale industrial applications [214]. This total cost is a result of several elements, which include the material expenses for electrodes, which is approximately 0.7107 USD for a pair of aluminum electrodes. This means around 23.15 USD/kg and about 10.557 USD/kg for iron electrodes [214]. In addition, the electrical energy costs are approximately 2.184/m³ USD for the aluminum combination and 2.016 USD/m³ for the iron combination, based on electricity of 0.056 USD/kWh. There are also fixed costs related to labor, maintenance, and sludge disposal assumed to be around 1 USD/m³. Consequently, the overall operating costs are 4.15 USD/m³ for the Al–Al combination and 4.01 USD/m³ for the Fe–Fe system [214]. Moreover, some limitations were found, such as studying the different families and generations of antibiotics and the non-effectiveness of this process for different antibiotics, such as ampicillin. Furthermore, various types of drugs have lacked investigation with the EC technique. Therefore, more investigations for different fields are needed to find the optimum conditions in the effectiveness and economical aspects.

Both the number of new toxins emitted by companies and the contaminants themselves have significantly increased as a result of the growing population. The review showed that EC has been widely used to treat various kinds of wastewater that contain contaminants that cannot be efficiently eliminated by traditional treatment techniques. On the other hand, there are numerous factors affecting the process: the electrolysis time, the type of power source, the pH of the solution, the conductivity of the solution, the gap between the electrodes, the current density, and the original temperature of the wastewater. There is also a relationship between these factors, according to recent experimental investigations. In-depth knowledge of these mechanisms and parameters is important, because EC is used to successfully remove an array of inorganic pollutants from an extensive amount of sewage resulting from industrial, domestic, and medical purposes. Any variation in these parameters will impact the effectiveness of contaminant removal. The effectiveness of EC for the elimination of new, emerging pollutants and advancements in EC technology, such as enhancements in coagulation processes that now call for minimal electrical power and development, as well as the accessibility of less expensive raw materials, have also contributed to a renewed focus on EC in the past few years. The removal of inorganic materials and their hazardous effect have been discussed in this work. In addition, the optimum parameters to remove these materials and removal efficiencies have been mentioned. Not only the removed materials but also the recovered materials have been discussed due to their importance to the soil.

Moreover, integrating EC powered by direct photovoltaic solar systems will be crucial for enhancing wastewater treatment, which has already been applied in the past few years. This is significant, because harnessing solar energy to power the EC process can facilitate the operation and reduce operational costs in addition to the carbon footprints while ensuring a sustainable energy source. This allows for utilizing renewable energy, which makes the process more environmentally friendly. Furthermore, pairing EC with other techniques such as advanced oxidation processes (AOPs) and membrane filtration can be beneficial in improving the treatment of complex wastewater matrices. AOPs might handle more pollutants that EC may not fully eliminate, while membrane filtration can separate and recover valuable resources from the resulting flocs. This combination approach between the various techniques not only maximizes the pollutant removal efficiency but also promotes sustainability in wastewater management. Future research should focus on optimizing the integration of EC with different techniques and systems to establish cost-effective and efficient treatment solutions.