An Overview of Electrochemical Advanced Oxidation Processes for Pesticide Removal

Abstract

This article provides an overview of the use of electrochemical advanced oxidation processes (EAOPs) applied to the treatment of water contaminated by pesticides. Given the global increase in the use of pesticides and the ineffectiveness of conventional treatment methods, EAOPs emerge as promising alternatives. They stand out for their efficiency in the degradation of organic compounds, minimal reliance on additional chemical reagents, and minimal generation of waste. The main methods addressed include anodic oxidation, photoelectro-oxidation, electro-Fenton and photoelectro-Fenton, which use hydroxyl radicals, a potent non-selective oxidant, to mineralize pollutants. A total of 165 studies were reviewed, with emphasis on the contributions of countries such as China, Spain, Brazil, and India. Factors such as electrode type, presence of catalysts, pH, and current density influence the effectiveness of treatments. Combined processes, especially those integrating UV light and renewable sources, have proven to be more efficient. Despite challenges related to electrode cost and durability, recent advances highlight the sustainability and scalability of EAOPs for the treatment of agricultural and industrial effluents contaminated with pesticides.

1. Introduction

Water pollution is a significant global concern, which directly affects its availability. The level of pollution of water resources has been progressive, and pesticides, in turn, assume a prominent role as contaminants. With their unbridled use, the presence of pesticides in water for human consumption has become a concern in both national and international contexts, fostering concern about the adverse effects on the environment and human health [1,2,3,4].

According to the report by the Food and Agriculture Organization (FAO), entitled: “Statistical YearBook: World Food and Agriculture 2023” [5], there was a 62% increase in the use of pesticides between 2000 and 2021, with the Americas responsible for 50% of this use. In 2021, approximately 3.5 million tons of pesticides were used globally, with Brazil leading consumption (0.72 million tons), followed by the United States (0.46 million tons) and Indonesia (0.28 million tons). While essential to agricultural productivity, intensive pesticide use contributes to environmental contamination and the development of resistant pest species [6,7].

The main forms of contamination by pesticides come from the sanitation of storage containers, application equipment, pesticide factories, agro-industrial effluents, and drainage from intensive agriculture [8]. Even at trace levels, these compounds pose ecotoxicological risks due to their bioaccumulation in aquatic environments, which can generate chronic and toxic effects for living organisms [9].

Given the inefficiency of conventional treatment methods for pesticide removal, Advanced Oxidative Processes (AOPs) emerge as promising alternatives, standing out for their capacity to degrade and mineralize these organic compounds [10,11,12]. These methods are based on the in situ generation of hydroxyl radicals (^·OH), capable of non-specific oxidation of a wide range of pollutants (E° = 2.8 V) [13].

Among AOPs, electrochemical advanced oxidation processes (EAOPs) stand out for their advantages, such as reduced need for chemical addition, rapid degradation of organic compounds, and minimal formation of harmful by-products [14]. However, their large-scale application still faces challenges, such as high operating costs and electrode degradation [15]. Research has been exploring strategies to overcome these limitations, including developing more efficient electrodes and integrating these processes with renewable energy sources [16].

In view of this scenario, the present study aims to present the main foundations of EAOPs and to carry out a systematic review of the literature from the last 10 years on the application of electrochemical advanced oxidation processes in pesticide degradation. The review seeks to map the main techniques used, the most studied types of pesticides, and recent advances in the improvement of these processes.

2. Electrochemical Advanced Oxidation Processes (EAOPs)

EAOPs are being applied for the remediation of environmental pollution, especially focusing on the remediation of wastewater contaminated with pesticides and pharmaceuticals [17,18,19,20]. EAOPs consist of heterogeneous processes, where ^·OH is generated on the surface of the electrode, and/or homogeneous processes, where ^·OH is electrogenerated in situ.

These processes receive great attention from the scientific community in the treatment of wastewater due to their high performance in degrading organic substances, using relatively simple equipment, being an effective and ecological technology due to the absence or addition of low levels of non-toxic chemicals, and operating under ambient conditions of temperature and pressure, with ease of scaling up to industrial scale [17,18,19,20,21,22]. The main EAOPs are called anodic oxidation (AO), photo-electro-oxidation (PEO), electro-Fenton (EF), and photoelectro-Fenton (PEF).

2.1. Anodic Oxidation (AO)

Anodic oxidation, also called electro-oxidation, allows the oxidation of pollutants in two main ways: by direct or indirect electrolysis. Indirect electrolysis can, in turn, be reversible or irreversible, and the redox reagent involved is electrogenerated through an anodic or cathodic process [21]. Direct electrochemical oxidation occurs when there is direct transfer of electrons on the electrode surface, without the aid of other substances. The indirect form, on the other hand, is mediated by some electroactive species that act as intermediaries for the transport of electrons between the electrode and the organic compounds for their oxidation, which is possible at low potentials, i.e., before the onset potential for O₂ evolution [17,18,19,20,21].

The efficiency of hydroxyl radical production and, consequently, the oxidation of organic compounds depends on the materials used as electrodes. Since the formation of ^·OH is a heterogeneous process, in the case of using metal oxide electrodes, or oxide anodes (MO_x), the conversion/combustion process begins with the discharge of water produced by adsorbed hydroxyl radicals (MO_x(^·OH)) (reaction (1)) [23].

MO_x + H₂O → MO_x(^·OH) + H⁺ + e

(1)

The interaction between the adsorbed hydroxyl radicals and the oxygen present can lead to the transition of these radicals from the adsorbed form to the oxide anode structure, promoting the formation of higher oxides (MO_x+1), as illustrated in reaction (2).

MO_x(^·OH) → MO_x+1 + H⁺ + e

(2)

Reactions (1) and (2) demonstrate the existence of two distinct states of active oxygen on the surface of oxide electrodes: physical adsorption of oxygen in the form of hydroxyl radical and chemical adsorption of oxygen as higher oxides [23,24]. When there are no oxidizable organic compounds, the adsorbed oxygen results in the formation of O₂, which regenerates the electrode surface, as explained in reactions (3) and (4).

MO_x(^·OH) → ½ O₂ + H⁺ + e + MO_x

(3)

MO_x+1 → ½ O₂ + MO_x

(4)

When oxidizable organic compounds are present, the dynamics of adsorbed oxygen change. Physically adsorbed oxygen MO_x(^·OH) tends to preferentially promote mineralization to CO₂, according to reaction (5). On the other hand, chemically adsorbed oxygen in the form of higher oxides (MO_x+1) favors selective oxidation of organic compounds, leading to the formation of oxidized organic species (RO), as shown in reaction (6) [23,24].

R + MO_x(^·OH) → CO₂ + ZH⁺ + Ze + MO_x

(5)

R + MO_x+1 → RO + MO_x

(6)

For the desired type of occurrence, it is essential to prioritize the form of oxygen adsorption on the electrode surface. If the concentration of adsorbed hydroxyl radicals is higher than that of higher oxides, complete oxidation of organic compounds tends to be the dominant pathway. However, if the concentration of oxides is higher, selective oxidation will probably predominate [20,23,24].

The mechanism is different in the case of a non-active anode, such as boron-doped diamond (BDD) [25]. Water oxidation leads to the formation of BDD(^·OH). This compound is responsible for the oxidation of organic pollutants, initially generating by-products and then promoting their complete mineralization into CO₂, which in turn regenerates the initial material (reactions (7)–(9)) [24,25].

BDD + H₂O → BDD(^·OH) + H⁺ + e

(7)

BDD(^·OH) + RH → BDD + by-products

(8)

By-products + BDD(^·OH) → BDD + CO₂ + H₂O + inorganic ions

(9)

During indirect oxidation, the anodic oxidation or cathodic reduction reaction (of water or species present in the medium) results in the formation of intermediate products or oxidizing agents such as ozone, hydrogen peroxide, and other active species on the electrode surface under the action of external current. The oxidizing agents generated interact with organic pollutants, promoting their oxidation and the subsequent formation of several intermediate compounds. While the initial oxidation reaction takes place on the electrode surface, the subsequent degradation of the pollutants occurs predominantly in the solution phase beyond the electrode surface [24,25].

2.2. Photo-Electro-Oxidation (PEO)

Photo-electro-oxidation has emerged as a promising technology for the removal of emerging contaminants in industrial and drinking water wastewater, especially when present in low concentrations, such as pesticides, pharmaceuticals, and dyes [26]. This process is based on the combination of the energy of photons incident on the surface of the electrodes with the electron flow generated by the potential difference in an electrochemical cell. For optimal efficiency, the electrode must have a photoactive surface, usually deposited on a conductive support, such as TiO₂. During PEO, UV radiation activates a semiconductor, promoting charge separation: valence band (VB) electrons absorb photons and migrate to the conduction band (CB), leaving positive holes in the VB (TiO₂ bandgap ~3.2 eV). This phenomenon generates reactive oxygen species, known as photon-ROS, while the polarization of the electrodes induces the discharge of water molecules on the anode surface, forming electron-ROS, such as ^·OH [27]. The mechanisms involved in this process are like those of photohydrolysis and electrochemical oxidation (EO), enhancing the degradation of pollutants [26,27,28,29,30].

2.3. Combined Process: Electro-Fenton (EF) and Photo-Electro-Fenton (PEF)

Based on these principles, several variants of electrochemical processes have been explored to increase the efficiency of pollutant degradation. Among them, the combination with the Fenton reagent results in the processes called electro-Fenton and photo-electro-Fenton. These processes combine electrochemical action with other advanced oxidation mechanisms, enhancing the removal of contaminants and reducing the formation of undesirable by-products [10,11,12,31].

Fenton reagent consists of the reaction between hydrogen peroxide (H₂O₂) and ferrous ions (Fe²⁺) in an acidic medium to produce ^·OH. In this process, iron acts as a catalyst and H₂O₂ as an oxidizing agent, promoting the formation of ^·OH and the continuous regeneration of Fe²⁺ through the cathodic reduction of Fe³⁺, ensuring the propagation of the Fenton cycle (reaction (10)). The ideal pH for this reaction is around 3, as higher values favor the precipitation of Fe³⁺ in the form of hydroxides, reducing the efficiency of the system [32,33].

Fe²⁺ + H₂O₂ → Fe³⁺ + ^·OH + OH⁻

(10)

In the electro-Fenton (EF) process, ^·OH production is intensified through the electrochemical generation of H₂O₂. In addition to the oxidation promoted by the homogeneous ^·OH formed in the solution, reactions with heterogeneous ^·OH on the electrode surface and with secondary oxidizing species, such as active chlorine, may also occur, depending on the experimental conditions [32]. The most used cathode materials for in situ H₂O₂ production are based on carbon (carbon nanotubes, graphite, lattice glassy carbon (RVC), carbon felt, or carbon sponge).

In the photo-Fenton process, UV light provides enough energy to activate the Fenton reaction, promoting the activation of iron and hydrogen peroxide. This process is more efficient, faster, and effective compared to conventional Fenton due to its high capacity to degrade organic pollutants, even with lower concentrations of ferrous ions. Furthermore, its application in a wider range of environmental conditions makes it a promising alternative for effluent treatment. The photo-Fenton system is a process that occurs in the presence of UV radiation; first the Fenton process reaction occurs, and then Fe³⁺ is photoreduced to Fe²⁺ according to reaction (11). Furthermore, direct photolysis of H₂O₂ also produces ^·OH (reaction (12)) [32,33].

Fe³⁺ + H₂O₂ + hν → Fe²⁺ + ^·OH + H⁺

(11)

H₂O₂ + hν → 2 ^·OH

(12)

In photo-electro-Fenton (PEF), which uses UV light (e.g., 254 nm), and in solar photo-electro-Fenton (SPEF), ultraviolet radiation applied to the solution plays an additional role in the degradation of pollutants. This effect occurs due to the photolysis of Fe(III) complexes with organic compounds, such as carboxylic acids (reaction (13)), and the photoreduction of the [Fe(OH)]²⁺ species, resulting in the regeneration of Fe²⁺ and the additional production of ^·OH (reaction (14)) [34,35].

Fe(OOCR)²⁺ + UV → Fe²⁺ + CO₂ + R^·

(13)

[Fe(OH)]²⁺ + UV → Fe²⁺ + ^·OH

(14)

The combination of processes resulted in better results in relation to the degradation of organic compounds compared to the processes applied separately; that is, the synergistic effect is efficient [10,12,34,35]. As a major advantage, EAOPs use as inputs the energy required to perform electrolysis and that used by the UV radiation source, with the only reactants being electrons and photons.

3. Overview of Literature

A literature review was conducted, focusing on the application of EAOPs in treating effluents containing pesticides. A search was performed in the Scopus, SciELO, and Web of Science databases, using the following keywords: electrochemical, wastewater, effluent treatment, pesticide, herbicide, anodic oxidation, electro-oxidation, photo-electro-oxidation, electro-Fenton, and photo-electro-Fenton (The Boolean search strategy applied was as follows: (((“electrochemical”) AND (“pesticide” OR “herbicide”) AND (“wastewater” OR “effluent treatment”)) AND ((“anodic oxidation” OR “electro-oxidation”) OR (“photo-electro-oxidation” OR “photo-electro-catalysis”) OR (“electro-fenton” OR “electrofenton”) OR (“photoelectrofenton” OR “photo-electro-fenton” OR “photoelectro-fenton”))).

As a result, 165 documents were identified, covering both primary articles and review articles, with no temporal delimitation. The distribution of studies over the years can be seen in Figure 1, evidencing the evolution of scientific interest in the topic.

There has been an increase in publications focused on research on the application of EAOP in pesticide treatment in the last 10 years, with around 128 publications in this period (2014–2024). There was an increase of around 41% in the last three years (2021–2024) compared to the previous three years (2017–2020). Figure 2 shows the representation of the main countries that published the most on this topic in the last 10 years. Spain and China led the publications, followed by Brazil and India.

Among the 128 published documents included in the analysis, 77% were primary articles, while 16% were reviewed articles. The main technical approaches described in the selected studies involve anodic oxidation, photo-electrocatalysis, electro-Fenton, and photo-electro-Fenton, demonstrating the diversity and complementarity of electrochemical processes applied in the treatment of compounds originating from pesticides. The articles selected for the construction of this overview, after full-text review and evaluation of methodological rigor, are presented in

Table 1. Selected literature references that applied electrochemical advanced oxidation processes (EAOPs) to degrade pesticides and the best results obtained.

Pesticide	EAOP	Experimental	Best Results	Ref.
Bentazon C10H12N2O3S (herbicide)	Anodic oxidation with electrogenerated H2O2, electro-Fenton, solar photo-electro-Fenton.	BDD, RuO2 i = 16.6 mA cm−2 Electrolytes: Na2SO4, NaCl pH = 3.0 360 min.	The photo-electro-Fenton treatment with BDD, in sulfate media, allowed a high reduction of total organic carbon (TOC) of 86.8%, although the solar photo-electro-Fenton treatment obtained the best efficiency, of 96.0%.	[36]
Clopyralid C6H3Cl2NO2 (herbicide)	Electro-Fenton.	Two types of anodes (mixed mineral oxides and BDD) and two cathodes (RVC and Al foams modified with carbon black, CB, and polytetrafluoroethylene, PTFE) i = 20 mA cm−3 Electrolyte: Na2SO4 pH = 3.0 120 min	The best electrode pair was BDD and CB/PTFE-Al, resulting in the complete removal of the herbicide quickly with lower energy consumption compared to the other electrodes studied.	[37]
Pyrimethanil C12H13N3 (fungicide) and Methomyl C5H10N2O2S (insecticide)	Solar photo-electro-Fenton. (UV photons collected at a solar CPC photoreactor)	Nb-BDD (anode) and GDE (gas diffusion electrode, cathode) i = 100 mA cm−2 Electrolyte: Na2SO4 pH = 3.0 30 min	The proposed combined action allowed the removal of more than 50% of both pesticides in 5 min, showing the efficiency in combining EAOPs.	[38]
2,4-D C8H6Cl2O3 (herbicide)	Photo-electrochemical	Photoreactor with BDD i = 10, 30, 300 and 500 mA cm−2 Electrolyte: Na2SO4 pH = 4.5 60 min	The study demonstrated that the combined photo-electrochemical process is more efficient for the removal of 2,4-D compared to electrochemical or photochemical processes alone. The application of higher current densities in the combined system enabled faster degradation, with complete removal of the herbicide in 40 min at 500 mA cm−2.	[39]
Bromacil C9H13BrN2O2 (herbicide)	Electro-Fenton in mixed tank cell (MTC) and flow-through cell (FTC)	BDD (anode) and carbon felt (cathode) i = 0.1 to 1.2 A Electrolyte: Na2SO4 pH = 2.5–3.0 480 min	Bromacil was degraded using MTC and FTC; however, higher mineralization rates are obtained in MTC, which was explained because hydrogen peroxide is not effective in the complete mineralization of bromacil but only in its conversion to its oxidation intermediates.	[40]
Tebuthiuron C9H16N4OS (herbicide)	Anodic oxidation, eletro-Fenton, and photo-electro-Fenton in a flow-by reactor	BDD (anode) and GDE (cathode) i = 10 to 125 mA cm−2 Electrolyte: K2SO4 pH = 3.0 120 min	The combination of oxidative processes (photoelectron-Fenton) was found to be the most effective technique for Tebuthiuron removal; this technique presented fast kinetic degradation, a high mineralization rate (~95%), and a great degree of versatility once it could be applied under a wide pH range.	[41]
Thiamethoxam, imidacloprid, acetamiprid, and thiacloprid. (insecticides)	Anodic oxidation	BDD Electrolytes: Na2SO4, NaCl, NaNO3 and HK2PO4 i = 5.86 to 34.14 mA cm−2 buffered pH solutions 60 min	Anodic oxidation is effective in removing insecticides from surface waters, with efficiencies ranging from 71% to 90.8%, depending on the compound. However, current density was the most influential factor in degradation kinetics, while the choice of electrolyte impacted the speed of the process.	[42]
Triclopyr C7H4Cl3NO3 (herbicide)	Electro-Fenton and Photo-electro-Fenton (with UVA light or sunlight)	Anodes: BDD or DSA. i = 16.7 mA cm−2 Electrolyte: Na2SO4 + NaCl pH = 7.0 360 min	The photo-electro-Fenton/solar process with BDD yielded better results than photo-electro-Fenton/UVA for triclopyr removal. A total of 78% TOC removal was achieved.	[43]
Tricyclazole C9H7N3S (fungicide)	Electro-Fenton	Stainless steel and carbon fiber as electrodes. i = 2.22 mA cm−2 Electrolyte: Na2SO4 pH = 3.0 180 min	The reactor used proved to be a promising technology for the pretreatment of pesticide effluents, considerably reducing the organic load and the fungicide tricyclazole. However, the residual toxicity points to the need for further investigation into the byproducts formed before the application of subsequent biological treatments.	[44]
Chlordimeform C10H13ClN2 (insecticide)	Photo-electro-Fenton	TiO2 nanoparticles embedded into an iron–chitosan matrix. i = 70 mA Electrolyte: Na2SO4 pH = 3.0 360 min	The TiO2/Fe3O4-CS magnetic catalyst was shown to be efficient, stable, and reusable in the photo-electro-Fenton process for the treatment of effluents containing recalcitrant pesticides such as chlordimeform. The simple synthesis approach and the possibility of magnetic recovery of the catalyst make this strategy promising for AOPs at an environmental scale.	[45]
Thiamethoxam, imidacloprid, acetamiprid, and thiacloprid (insecticides)	Anodic oxidation	BDD (anode), and carbon-felt (cathode) i = 5.86 to 34.14 mA cm−2 Electrolytes: Na2SO4, NaCl and NaNO3 pH = 3.0 60 and 120 min	The electrochemical process applied proved to be highly effective in the total removal of emerging contaminants in WWTP effluents, with significant levels of TOC mineralization, representing a promising alternative for the reuse of wastewater. This technology stands out for its versatility, efficiency, and applicability in different aqueous matrices, with the potential to compose sustainable strategies for advanced wastewater treatment.	[46]
1H-1,2,4-triazole C2H3N3 (fungicide)	Anodic oxidation	meso-flower PbO2 layer electrode (MF-PbO2). i = 5 to 25 mA cm−2 Electrolyte: Na2SO4 pH = 3.0 90 min	The pesticide residues of 1H-1,2,4-Triazole were completely removed by MF-PbO2 from an actual pesticide tailwater. The complete removal of triazole, associated with significant improvements in water quality parameters, highlights the promising potential of this technology for full-scale applications.	[14]
Thiamethoxam C8H10ClN5O3S (insecticide)	Electro-Fenton and photo-electro-Fenton	Ti/Ru0.3Ti0.7O2 (DSA) (anode) i = 50 mA cm−2 Electrolyte: Na2SO4 pH = 3.0 120 min	The best degradation efficiency of thiamethoxam, from a commercial product, was obtained with the application of the photo-electro-Fenton process. The results showed 79% TMX degradation and 83% COD removal, with a low estimated cost of USD 1.01 dm−3.	[31]
Methomyl C5H10N2O2S (insecticide)	Anodic oxidation (indirect electrolysis mediated by gaseous oxidants (ClO2))	Mixed metal oxide (MMO) electrodes i = 79.5 mA cm−2 Electrolyte: Na2SO4 pH = 3.5 300 min	The electrochemical processes applied are capable of removing methomyl, but the indirect approach, mediated by ClO2, showed greater energy efficiency, being able to completely remove 0.1 mM of methomyl in 500 mL of solution with a consumption of only 50 Wh, a value considerably lower than that required in direct oxidation, which achieves less than half of this removal with the same energy consumption. This is mainly due to the greater efficiency in the mass transfer of gaseous oxidants in the diluted solutions.	[47]

.

4. Conclusions and Prospects

This overview of recent years shows progress and interest in the development and application of electrochemical advanced oxidation processes for the degradation of pesticides in aqueous matrices. The studies analyzed demonstrated that the efficiency of the processes depends on factors such as the type of electrode (e.g., BDD, Ti/RuO₂, TiO₂, and PbO₂), the presence of catalytic agents, and the combination with UV radiation or peroxides, as well as the optimization of operational parameters (pH, current density, treatment time, etc.). Also noteworthy are innovative approaches such as the use of electrodes with specific morphological structures and processes mediated by gaseous oxidants, which combine high degradation efficiency with reduced energy consumption.

Due to the increasing environmental contamination caused by the indiscriminate use of pesticides and the inefficiency of conventional water treatment methods, such as flotation, coagulation, filtration, and decantation, EAOPs have emerged as sustainable, effective, and versatile technological alternatives. Furthermore, the phytotoxicological and ecotoxicological assessment of the treated effluents reinforces the environmental viability of EAOPs, indicating a reduction in residual toxicity and an increase in the biodegradability of the remaining compounds. In this context, these processes not only promote the mineralization of recalcitrant pesticides but also contribute to mitigating risks to human health and aquatic biodiversity. These findings support the role of EAOPs in circular economy and sustainable agriculture frameworks.

Thus, it is concluded that EAOPs represent a promising front in the treatment of effluents contaminated with pesticides, with potential for large-scale application and other persistent organic pollutants. The continuous improvement of materials, integration with renewable energy sources (solar-driven EAOPs [35]), and evaluation in real systems are strategic paths to consolidate their industrial applicability.