The Use of Biosorbents in Water Treatment

Abstract

Biosorbents are materials of biological origin (microbial, biomass-derived waste, or industrial by-products) used to adsorb or absorb pollutants. They have been used to remove various contaminants, including heavy metals, dyes, and pharmaceuticals. Their effectiveness is due to the different functional groups that interact with pollutants, including hydroxyl, amino, carboxyl, and phosphate groups. This review explores the various kinds of biosorbents (classification), mechanisms, and factors influencing biosorption, such as biomass content, time, temperature, pH, and concentration of pollutants, synthesis methods of biosorbents, and the current state of research on biosorbents. The review highlights the advantages of biosorbents, along with the challenges encountered, such as difficulty in regeneration and variability in performance. Finally, the review identifies research gaps and future directions, including exploration of modified/synthetic biosorbents for the removal of multi-component pollutants.

1. Introduction

Water is essential to our lives; it plays a pivotal role in maintaining public health, promoting social well-being, and is a critical driver of economic growth [1,2,3]. In Botswana, mining activities, industries, and agricultural processes add problems to the already scarce freshwater resources due to the country’s semi-arid climate. Studies show that wastewater from Gaborone treatment plants is not well treated before discharge, leading to contamination of surface and groundwater sources [4].

Within Africa, industrialization, poor sanitation, and climate change have also caused water scarcity and pollution. For example, a review by the United Nations stated that many African countries struggle with access to clean water, and that the wastewater in some countries in Africa is mostly discharged untreated due to a lack of sustainable water treatment technologies. This lack of clean water systems is not only an African challenge but a global problem, and creates risks to human health and the environment [5,6]. Addressing the water challenges requires innovative, low-cost, and sustainable solutions, and biosorbents present that opportunity.

Access to clean and safe drinking water as outlined in the United Nations Sustainable Development Goals (SDG 6), which focuses on ensuring access to clean water and sanitation for all [7], is crucial for preventing waterborne diseases, reducing child mortality, and ensuring overall community health, and aligns with SDG 3, which aims to promote good health and well-being for people everywhere [8,9]. The importance of water cannot be overemphasised; water flushes waste out of the body through perspiration and urination. It transports minerals and oxygen throughout the body and regulates body temperature [10]. Access to clean water is essential for maintaining overall health as it prevents medical conditions by lubricating the joints and regulating body temperature. It also improves strength and endurance, especially during physical activities [11].

Industries and businesses require a reliable water supply for their operations. Access to safe water supports agricultural activities, enabling farmers to irrigate their crops and ensure food security. Furthermore, it fosters entrepreneurship by providing a foundation for small-scale businesses, encouraging economic growth at the local level [12].

Therefore, to maintain water safety, the World Health Organization (WHO), the Environmental Protection Agency (EPA), and the Bureau of Standards from different countries are statutory bodies that safeguard human life and the environment by removing pollutants from water resources.

1.1. Types of Pollutants

Pollutants compromise the water quality, making it necessary to purify water to make it safe. They come from various sources like industries, domestic waste, and agriculture (pesticides, fertilizers, and animal waste), effluent discharges, mining activities, pharmaceuticals, tanks, and pipeline leakages [13]. Polluted water may contain organic, radioactive, inorganic substances, emerging, and microbial pollutants [14,15].

Examples of organic pollutants are dyes (for example, methyl red, methyl orange, methylene blue, etc.), phenolic compounds, petroleum products, surfactants, pesticides, and pharmaceuticals [16,17]. Inorganics include cations of heavy metals e.g., lead (Pb), mercury (Hg), copper (Cu), cobalt (Co), manganese (Mn), iron (Fe), and chromium (Cr), and anions (chlorides (Cl⁻), fluorides (F⁻), nitrates (NO₃⁻), phosphates (PO₄⁻), nitrites (NO⁻), and sulfates (SO₄²⁻) [15,18,19,20]. The physicochemical parameters, including biochemical oxygen demand (BOD), chemical oxygen demand (COD), pH, total suspended solids (TSS), dissolved oxygen (DO), electrical conductivity (EC), and total dissolved solids (TDS) [21]. These are critical indicators of water quality.

In many parts of Africa, domestic waste is primarily composed of faecal waste, which contaminates underground water sources that supply drinking water through wells and boreholes, especially in underdeveloped communities. These communities rely on the use of pit latrines, where there are no maintained sewage lines, and urban road runoff [22]. Countries like Uganda, Nigeria, Malawi, Kenya, Ethiopia, Ghana, South Africa, and Zimbabwe face challenges as pit latrines are located close to boreholes, and this leads to elevated concentrations of microbial contaminants, and anions amongst other pollutants, posing a risk of cholera [23,24].

A study conducted in South Africa advised that the use of pit latrines in areas where groundwater is used for drinking must be discontinued because the levels of nitrates were found to be above the acceptable limit of 10 ppm, and Escherichia coli (E. coli) counts above 10,000 colony-forming units (E. coli/100 mL) [25]. In Botswana, groundwater pollution was linked to pit latrine usage, especially in villages surrounding the capital city, Gaborone. High levels of nitrates were reported in Ramotswa, which resulted in the closure of the Ramotswa wellfield. Other areas like Molepolole, Serowe, Palapye, Pallaroad, and in the northeast were also affected in a similar way [26].

Polluted water sources can contribute to diseases such as cholera, dysentery, and diarrhea, particularly affecting vulnerable populations, including children, the elderly, and those with compromised immune systems.

Table 1. Effects of pollutants on human health.

Pollutant Class	Pollutant	Effects
Organics	Volatile organic compounds, for example, toluene, benzene, vinyl chloride, etc.	Exposure can be through ingestion or inhalation and causes chronic illnesses like cancer and reproductive disorders, and affects the central nervous system. Inhalation brings lung problems, and a mild problem associated with ingestion is ulcers [27,28].
	Persistent organic pollutants (POPs): Polychlorinated Biphenyls (PCBs), DDT (Dichloro diphenyl trichloroethane),	These can cause stroke, hypertension leading to heart failure, metabolic disease, reduced fetal growth, endocrine disruption (when hormones are interfered with), and cancer [8].
	Pesticides, e.g., benzamide, found in Fluopyram	They cause skin irritation and allergic reactions [29].
	Pharmaceuticals like amoxicillin, ciprofloxacin, ibuprofen, and diclofenac	Prolonged or even low-level exposure may lead to various organ dysfunctions. May also lead to hormonal imbalances to decreased fertility, as some of them are contraceptives, developmental abnormalities, and even congenital conditions in future generations [30].
	Dyes: methylene blue, turmeric, congo red, crystal violet	Dye pollution in water brings allergies, cancer, dermatitis, and skin irritation [31].
Inorganics	Nickel (Ni)	It has harmful effects on human health. These include gastrointestinal complications, headache, anemia, dizziness, allergies, lung, and kidney failure [32].
	Total dissolved solids (TDS) Fluoride, F	Prolonged exposure to high amounts of dissolved salts leads to kidney stones. Fluoride consumption causes dental problems like teeth discoloration and weakening [33].
	Arsenic, As	In addition to causing cancer, elevated levels of arsenic are associated with discoloration of the skin, stomach pain, nausea, diarrhea, and vomiting.
	Mercury, Hg	Causes neurological disorders and may lead to abortion in pregnant women. Methylmercury causes cardiovascular diseases and dementia in elderly people [34].
	Copper, Cu Chromium, Cr	Nausea, diarrhea, liver, and kidney damage [19,35].
	Cadmium, Cd Lead, Pb	Excessive amounts cause damage to the nervous system and destroy the blood circulation. Irritation of the respiratory system [36,37,38].
Biological pollutants	Viruses, protozoa, and bacteria	These produce harmful toxins and are cholera-causing organisms. Exposure causes liver damage and skin problems [33]. Mild problems are diarrhea, nausea, vomiting, headaches, and dehydration.

illustrates how various pollutants impact human health and the environment.

To address water pollution challenges, a wide range of water treatment methods has been developed, such as adsorption, flocculation, photocatalysis, sedimentation, neutralization, and membrane technologies [39,40,41,42]. Membrane technologies involve the use of semi-permeable barriers to remove pollutants from water based on size, charge, and other physicochemical properties. They include microfiltration, ultrafiltration, nanofiltration, and reverse osmosis [43]. Adsorption is when pollutants adhere to solid surfaces, while flocculation uses coagulants (e.g., alum) to aggregate suspended particles. Studies show that flocculation, while effective, produces a significant volume of sludge, and its disposal can be a major challenge and a high-cost process. Photocatalysis, where light-activated catalysts (such as titanium dioxide, TiO₂) degrade organic contaminants, is highly dependent on pH, catalyst dosage, and light intensity, making the technique costly [44]. These methods are simple, fast, and target various pollutants, but often come with high operational costs, limited removal efficiencies of dissolved pollutants, and environmental impacts [31]. Activated carbon is a porous carbon material with a vast surface area and high heavy metal removal efficiencies of 75–96% depending on dosage, pH, and target ions, and can be regenerated, but at significant cost [45,46]. Ion-exchange resins offer high ion-specific selectivity, whereas biosorption may be less selective; however, through functionalization, their performance can be improved. Unlike many conventional sorbents, biosorption combines multiple mechanisms such as electrostatic, complexation, and ion exchange, while offering an economical, eco-friendly alternative because biosorbents are biodegradable and often require minimal processing and disposal [47].

In recent years, biosorption has emerged as an eco-friendly and cost-effective alternative, yielding high pollutant removal efficiencies [27,43]. Biosorption is a process that uses materials of a biological nature, known as biosorbents, to remove contaminants from water. It is an attractive option for water treatment because of its cost-effectiveness and the biodegradability of the materials, especially in areas with limited access to advanced filtration technologies [42]. Biosorbents, including microbial, plant-based, or biomass-derived, and other materials, are extensively used in the removal of heavy metals from industrial effluents, ensuring compliance with environmental regulations [44]. Further, biosorbents are useful in decentralized water purification systems, especially in remote or disaster-stricken areas, providing cost-effective and sustainable water management practices through biosorption [48].

1.2. The Purpose of the Review

The purpose of the review is to explore the current state of research on biosorbents, with emphasis on their classification, synthesis, effectiveness, mechanisms, and applications. To the best of the authors’ knowledge, this is the first comprehensive study to critically evaluate the use of biosorbents in drinking and wastewater treatment across a wide range of pollutants. Recent studies have demonstrated their remarkable ability to adsorb heavy metals, organic compounds, and other contaminants [27,28,30,48,49,50,51,52]. Most reviews limit their scope to specific pollutants or biosorbent types, whereas this review consolidates results from diverse sources and applications to provide future research opportunities in biosorption technologies. The review also compares biosorption with common sorption-based separation methods (e.g., activated carbon, ion-exchange resins).

2. Biosorbents

Biosorbents are naturally occurring or modified/synthetic materials of biological origin, such as agricultural by-products, microorganisms, or natural polymers, that can adsorb/absorb and remove pollutants, especially heavy metals, and organic compounds, from aqueous solutions [47,50]. Examples of biosorbents are fruit and vegetable waste, shells, microbiological, and other biomass-derived materials. They are utilized in industries to treat effluents containing pollutants, preventing their discharge into water bodies and ensuring compliance with environmental regulations [53].

What makes a material a biosorbent is that it must be of biological origin, and its unique structure and chemical composition, which allow it to interact with pollutants effectively. Biosorbents contain functional groups (such as hydroxyl, carboxyl, sulfate, sulfide, amino, and phosphate groups) on their surfaces [38,54]. The functional groups interact with pollutants through various mechanisms, including ion exchange, complexation, adsorption, and micro-precipitation. The mechanism of biosorption usually starts with a quick attachment of pollutants onto the functional groups on the surface of the biosorbent [33]. The attachment is based on the affinity of the pollutant to the functional group available on the surface. Then the pollutant ions slowly move deeper into the biosorbent’s internal structure, ensuring they remain bound over time.

Biosorbents’ unique properties, such as varying molecular weight, charges, cost-effectiveness (usually low cost), and length of functional groups, among others, allow the material to be used in any desirable pattern [51]. They have attracted increasing research attention due to their active sites, specific surface area, pore volume, pore size distribution, easy separation, and reusability, which are suitable for the remediation of heavy metalloid and organic pollutants [55].

2.1. Classification of Biosorbents

Biosorbents used in water treatment are diverse and can be broadly classified into natural (biomass-derived and microbial) and modified/synthetic biosorbents [56,57]. The diverse sources of biosorbents, ranging from biomass-derived residues to microorganisms, offer a wide array of materials to choose from, each with specific adsorption properties tailored to several types of pollutants [58]. This versatility enhances their overall effectiveness in water treatment applications. Natural biosorbents have demonstrated impressive pollutant removal, but their limited selectivity is due to hydroxyl (OH), amine (NH₂), carboxyl (COOH), and phenolic groups. Modified/synthetic biosorbents are chemically or physically treated natural biosorbents that enhance adsorption properties. Examples include acid-treated sawdust and chemically modified peanut shells. Agricultural wastes, including seeds, peels, roots, and bagasse, offer abundant and low-cost sources for biosorption. Microorganisms, including bacteria, fungi (such as molds, mushrooms, and yeasts), and algae (microalgae and macroalgae), as well as natural substrates like tree bark and cellulose, and biopolymers like chitin and chitosan, have demonstrated significant potential for adsorbing a variety of pollutants [59].

It is essential to note that natural biosorbents are seldom utilized in their unmodified form due to their weak interaction with pollutants. They are usually physically or chemically modified to enhance their adsorption capacities. In this regard, the classifications in the subsequent sections are primarily based on the origin of the biosorbent, rather than on the strictest definition of natural versus modified biosorbent, as cited in the reported studies.

2.1.1. Natural Biosorbents

The natural biosorbents are derived from natural sources, either without or with little chemical modification. They encompass materials derived from nature, such as plants, and algae, which possess inherent adsorption properties due to their unique surface structures [52]. Activated carbon, as a natural biosorbent, is widely used for its high surface area and porosity, making it effective for adsorbing a broad range of contaminants, including organic compounds and heavy metals [60]. Natural biopolymers, like those used in biosorbent applications, are effective for pollutant binding due to their functional groups, particularly ether (−C−O−C−) and hydroxyl (−OH) groups [18]. Some examples of natural biosorbents are discussed below.

Biomass-Derived Biosorbents

These are plant materials and can be used as biosorbents due to their porous structures and high surface area. They include plant-based materials like rice husks, coconut shells, vegetable and fruit peels, seeds, cellulose, plant bagasse, and sawdust. These materials are readily available, are of low cost, and have high surface areas, making them effective for adsorbing contaminants [18,61].

A study carried out by Gautam et al. (2023) [62] showed that acid-treated rice husks (as agricultural waste) were modified for the removal of cadmium (Cd²⁺) ions in water. Acid treatment increased their surface area and adsorption capacity by introducing carboxyl functional groups. Rice husk biochar modified with potassium permanganate (KMnO₄) introduced oxygen functionality and removed up to 93% of Cd²⁺ at a concentration of 50 ppm, while biochar modified with nitric acid (HNO₃) furnished 86% removal [62]. The modified biochar’s surface, enriched with carboxyl and hydroxyl groups, increased the biochar’s capacity for electrostatic interactions and complexation by binding with Cd²⁺ ions in aqueous solutions. Carboxyl groups, with their negatively charged COO⁻ parts, form stronger inner-sphere complexes with Cd²⁺ through coordinate covalent bonding, while hydroxyl groups contribute mainly via hydrogen bonding and weaker electrostatic attraction. This is why they form stable complexes and is the reason for the observed differences in removal performance.

Alkali-treated coconut shells are another example of biomass-derived biosorbents, where chemical treatment improved the removal of organic contaminants from industrial wastewater. The coconut shells were washed, dried, and treated with alkali to produce activated carbon. This process improved the porosity and increased the number of hydroxyl functional groups for enhanced adsorption performance of methylene blue in wastewater. The research demonstrated that under an optimal pH of 7, a temperature of 30 °C, and a dosage of 1 g/L, the biosorbent achieved up to 95% removal efficiency [63]. Water hyacinth, a widely available plant (Pontederia crassipes-formerly Eichhornia crassipes), demonstrated its biosorption potential by removing 66% of Cd²⁺, achieving a maximum adsorption capacity of 21.6 mg/g. The plant biomass was used in its dried and ground form without chemical modification. However, this process led to a chemical oxygen demand (COD) increase to 292 mg/L, highlighting a secondary pollution issue. A microalgae–endophyte symbiotic system (MESS) was combined with water hyacinth tissues with a Cd²⁺ tolerant microalgal strain and its associated endophytic bacteria, and significantly improved performance to 99.2% Cd²⁺ removal while reducing COD to 77 mg/L within three days. The synergistic approach showed the potential of integrating plant-based biosorbents with biological enhancements for efficient and sustainable wastewater treatment [64].

Microbial Biosorbents

These include bacteria, fungi, and algae, which are living organisms that can accumulate heavy metals and other pollutants through metabolic processes or surface binding [65]. Bacterial biosorbents, including species like Pseudomonas putida, Escherichia coli, and Bacillus subtilis, exhibit metal-binding capabilities through biosorption and bioaccumulation, providing an eco-friendly approach to heavy metal removal [49]. Algae, such as Spirogyra and Chlorella, are valuable for their ability to accumulate metals and nutrients, making them suitable for nutrient removal in wastewater treatment [66].

An example of a study that used microbial biosorbent is where Fucus vesiculosus algae were used for the removal of copper (Cu²⁺) and zinc (Zn²⁺) ions. To prepare the biosorbent, the algae were collected, rinsed, air-dried, crushed, rinsed again, and air-dried. To boost its effectiveness, a portion of the dried algae was treated by shaking it with a calcium nitrate solution for a day. This promoted ion exchange where the Ca²⁺ ions were replaced by the metal ions. It led to high removal rates for metal ions, with 67% for Cu²⁺ and 55% for Zn²⁺ [51,66].

2.1.2. Modified/Synthetic Biosorbents

These are natural materials that are engineered through physical or chemical methods to enhance adsorption [67]. The physical and chemical modifications involve heat treatment, functional group interconversions, and the use of enzymes to improve the adsorption capacity of the biosorbent. Many materials can be incorporated into natural biosorbents to further improve their efficiencies by integrating them with non-biosorbent materials such as clay minerals, zeolites, metal–organic frameworks (MOFs), graphite, and silica, which serve as structural supports or functional additives [49]. While these materials are not biosorbents themselves, their incorporation into biosorbent matrices can significantly improve surface area, porosity, stability, selectivity, and overall adsorption efficiency. For example, clay minerals such as montmorillonite, smectite, kaolinite, illite, and chlorite are often combined with natural biosorbents to form composite materials with enhanced properties. Among these, kaolinite, montmorillonite, and illite are most widely used due to their favorable structural characteristics [68]. These properties make them effective in adsorption and catalytic applications. A study investigated the use of bentonite–chitosan composite beads for the removal of Pb²⁺ ions from aqueous solutions. The composite beads were prepared by mixing bentonite clay with chitosan. Through ion exchange, Pb²⁺ ions replaced cations in the bentonite structure, and chelation through the NH₂ groups of chitosan. This combination facilitated efficient Pb²⁺ removal under optimal conditions of adsorbent dose of 0.4 g, contact time of 14 h, and pH 5. Under these conditions, the composite achieved a maximum adsorption capacity of 344.83 mg/g. The process followed the Freundlich isotherm model and pseudo-second-order kinetics, indicating a multilayer adsorption process [69].

Zeolites, porous aluminosilicate minerals with high ion-exchange capacity, are often incorporated into biosorbent materials to create modified/synthetic biosorbents with enhanced adsorption performance. Their negatively charged framework arises from the substitution of tetrahedral silicate cations with aluminum cations within their structural lattice [70]. Zeolites have a large surface area suitable for creating binding sites for the removal of heavy metals, organics, and microorganisms. Additionally, engineered zeolites are crafted for a selective ion-exchange mechanism, making them highly efficient in adsorbing heavy metals and organic pollutants in water treatment applications. A study investigated the use of zeolite-modified sugarcane bagasse for the removal of Cd²⁺ ions from aqueous solutions. The biosorbent was prepared by impregnating sugarcane bagasse with natural zeolite, which enhanced the surface area and introduced additional ion exchange sites. The removal mechanism involved was ion exchange between Cd²⁺ ions and the native cations in zeolite, as well as complexation with the hydroxyl and carboxylic functional groups [71].

Silica is an inorganic solid made from silicon dioxide (SiO₂), frequently utilized as an effective modified biomass-derived adsorbent in water treatment. It possesses a high surface area, chemical stability, enhanced porosity, and abundant surface functional groups, offering numerous sites for grafting [72]. Functionalized silica gels are modified to effectively capture metals and organics. For instance, chitosan coated with silica was developed for the removal of Cr⁶⁺ ions from aqueous solutions, achieving a maximum monolayer adsorption capacity of 294.1 mg/g. This was achieved at pH 3, room temperature, and moderate initial Cr⁶⁺ concentrations (50–150 mg/L), with equilibrium reached after approximately 180 min. The removal mechanism involved electrostatic attraction between protonated amine groups of chitosan and the negatively charged Cr⁶⁺ ions, as well as complexation through amine and hydroxyl functionalities [73]. This example highlights how silica modification can synergistically combine the advantages of inorganic supports and bio-based functional groups for effective water treatment.

Among currently available synthetic adsorbents, carbon-based materials and metal–organic frameworks (MOFs) stand out as highly effective and widely researched options. Carbon materials are described as a class of materials composed primarily of carbon atoms arranged in various forms and structures, each with distinctive properties. Examples of carbon materials are carbon nanotubes (CNTs), activated carbon, and carbon nanofibers, and these are characterized by their high surface area, porosity, and structural stability [74].

Carbon-based adsorbents are particularly valued for their affordability, high thermal stability, and hydrophobic properties, which make them resistant to moisture. Additionally, they exhibit diverse structural features such as varying pore sizes, pore structures, and surface areas that enhance their adaptability and effectiveness in adsorption applications. This versatility in texture and structure is key to their suitability across different adsorption processes [75].

A study integrated metal-organic frameworks (HKUST-1 and ZIF-8) with 2,2,6,6-tetramethylpiperidin-1-oxyl-4-yl (TEMPO)-oxidized corncobs (OCBs), for the removal of methyl orange and microbial contaminants (Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus)). The adsorption capacity improved significantly, from 8% removal using unmodified corncobs to 55% with HKUST-1/OCB’s and 84% with ZIF-8/OCBs for methyl orange. The biosorbents demonstrated excellent antibacterial activity, achieving 90.2% microbial inactivation with HKUST-1/OCBs and 44.8% with ZIF-8/OCB’s [76]. The mechanism of adsorption was electrostatic interaction, as the TEMPO oxidation of corncobs introduced carboxyl groups, hence increasing the surface’s negative charge, which enhances attraction to the positively charged sites.

Magnetic nanoparticles can also be incorporated into natural biosorbents to make modified/synthetic biosorbents, enabling magnetic separation post-adsorption. Chitosan derived from shrimp shells embedded with magnetic nanoparticles was synthesized to remove vanadium (V⁵⁺) and palladium (Pd²⁺) ions from water. Electrostatic interactions from the NH₂ group of the chitosan and the embedded magnetic nanoparticles enhanced the adsorption. The research focused on optimizing key factors such as pH levels (3–5), contact time (60 min), and agitation speed (200 rpm) to maximize the adsorption process. Removal efficiencies of 99.9% for V⁵⁺ and 92.3% for Pd²⁺ showed that a biosorbent, when combined with magnetic nanoparticles, can play a significant role in environmental remediation [77].

Graphene oxides provide extensive surface areas and customizable functional groups for enhanced pollutant capture. For example, keratin/graphene oxide modified/synthetic biosorbent showed excellent results, achieving up to 99% removal of metal ions of As, Se, Cr, Ni, Co, Pb, Cd, and Zn from synthetic wastewater. The study highlighted electrostatic interaction, chelation, and complexation as primary mechanisms for the removal of these metal ions [78].

The advantages of modified/synthetic biosorbents are that they offer higher adsorption capacity with improved selectivity for specific pollutants. They have greater resistance to harsh environmental conditions and high versatility in applications. The challenge with modified/synthetic biosorbents is the increased cost due to modification processes, the potential environmental impact from chemical modifications, and limited biodegradability [79].

Other classifications reported in literature are based on the origin and chemical composition; biosorbents can be classified into diverse groups, including lignocellulosic materials, chitosan-based biosorbents, and microbial biomass, each with distinct properties suited for specific pollutants and water treatment scenarios.

2.2. Characteristics of Biosorbents

It is important to understand the physical and chemical characteristics of biosorbents to inform theoretical and practical applications. Biosorbents possess some characteristics that determine their effectiveness in water treatment. These characteristics include porosity, functional groups, charges, surface area or size, biodegradability, and chemical composition [52].

2.2.1. Physical Characteristics

Physical characteristics of biosorbents refer to the structural and textural properties that influence their adsorption capacity. Examples of these properties are surface area, porosity, particle size, and morphology [80]. Porosity refers to the presence of small spaces or pores within a material. For example, a porous structure is a crucial characteristic, as it provides a large surface area for adsorption, allowing contaminants to be trapped effectively within the spaces/pores. A study by Bai et al. (2021) [81] demonstrated that biochar with a high porosity (surface area of 213 m²/g) achieved an 89% removal efficiency of Pb²⁺ from water (pH 5 and 25 °C). This high porosity allowed a greater surface area for Pb²⁺ ion adsorption [81].

In a study examining the removal of Cr⁶⁺ from solutions using modified peanut shells, sawdust, and Cassia fistula leaves, it was observed that the larger surface area of Cassia fistula leaves made it a more effective biosorbent compared to the others. The biosorbent prepared from Cassia fistula leaves (surface area of 3.42 m²/g) showed the highest removal efficiency of 87.9%, with 76.9% for sawdust (surface area of 2.01 m²/g) and peanut shells (surface area of 1.23 m²/g) having a slightly lower efficiency of 70.4% [82].

Morphology describes the physical appearance and the arrangement of the material. A study showed that rough, porous morphologies in peanut shell biochar led to a Cd²⁺ removal efficiency of over 85%, compared to smoother biochar, which removed less than 50%. Rougher surfaces tend to have more active sites for adsorption, and materials with high porosity typically have a larger surface area for adsorbing pollutants [50].

2.2.2. Chemical Characteristics

Chemical characteristics involve the chemical interaction through functional groups, composition, and the reactive nature of the biosorbent [83]. Functional groups on the surface of biosorbents, such as hydroxyl, carboxyl, and amino groups, play a vital role in binding contaminants through chemical interactions. They provide numerous active sites for the adsorption of pollutants through chelation, electrostatic attraction, ion exchange, and complexation (discussed in the next section) [84]. Functional groups can become negatively charged in aqueous solutions, allowing them to attract and bind positively charged metal ions; they can form chelates with the ions or exchange protons with metal ions.

Dey et al. (2021) [85] used orange peels as a biosorbent for the removal of ammonia and nitrates. The study stated that the surface charge highly influenced the biosorption capacity because of the combination of both acidic and basic media on the surface of the orange peel. In an acidic medium, pH 5.5, the acidic groups remained protonated, supporting the electrostatic attraction of negatively charged pollutants. In a basic medium, carboxyls may lose protons and become negatively charged, becoming more favorable for attracting positively charged ions [85]. Biosorbent stability and resistance to physical and chemical changes during the adsorption process are essential to ensure long-term effectiveness.

Chitosan was used for the removal of dyes and Cr⁶⁺ ions, with removal efficiencies that exceeded 90% at a pH range of 3–5 and a temperature of around 30 °C. The presence of the amino groups on the chitosan created active sites for electrostatic interaction and complexation [86]. In general, functional groups enhance biosorption of heavy metals and dyes, achieving over 80% removal. The carbon-rich structure improves π-π interactions with aromatic compounds in dyes, and high oxygen content binds pollutants via complexation and hydrogen bonding. Chemical modifications improve stability under acidic or alkaline conditions, maintaining functional group integrity for effective binding. These chemical characteristics increase the density of binding sites for pollutants, enhancing the capacity for ion exchange and complexation [87,88,89].

2.3. Synthesis of Biosorbents

Biosorbents, as discussed, are obtained from biomass, microorganisms, and natural polymers. This section discusses some of the common techniques used in the synthesis of biosorbents.

2.3.1. Physical

The physical synthesis method involves physical processes, such as size reduction, activation, or structural modification, without significant chemical alteration. Size reduction is usually achieved by grinding the material, heat treatment to break the material, and pyrolysis. Grinding usually involves washing the material, drying, then finally crushing it into smaller particles to create a large surface area. In thermal treatment, the material is subjected to higher temperatures either by direct heating or in water at a controlled pressure, and the latter method is called hydrothermal treatment. Pyrolysis is the application of a high temperature to the biomass in the absence of oxygen [90].

Activated sludge was washed and ground to effectively remove heavy metals from aqueous solutions. The study found significant removal efficiencies with Cd²⁺ increasing from 39.12% to 61.11%, Cu²⁺ from 31.24% to 50.28%, Ni²⁺ from 13.13% to 27.54%, and Zn²⁺ from 10.26% to 36.28% as the adsorbent mass increased from 0.5 g to 3.0 g [91]. In a 2007 study by Yasemin and Tez, the preparation of hazelnut and almond shells as adsorbents was entirely physical, involving no chemical agents to activate or treat the adsorbent surfaces before use. The shells were mechanically ground to increase their surface area, making more active sites available for the adsorption of heavy metals. This grinding step served as the primary modification method, aimed at enhancing the adsorption capacity without altering the shell material chemically. The shells showed the highest affinity for Pb²⁺, followed by Cd²⁺, and then Ni²⁺ [92].

In a one-step ball milling process, wood feedstock was used to create two types of biosorbents aimed at removing Congo red and crystal violet dyes. The wood underwent acidic and alkaline ball milling, producing two distinct biosorbents. After milling and drying for 12 h at 80 °C, the acidic biosorbent achieved a high removal rate of 87.9% for Congo red, while the alkaline biosorbent removed 76.9% of crystal violet [93].

2.3.2. Chemical

This method involves the use of reagents to enhance materials’ capacity to bind pollutants because of the affinity strength on the surface of the material. In a 2016 study, Masoumi, Hemmati, and Ghaemy synthesized a low-cost nanoparticle sorbent from modified rice husk and a copolymer poly (methylmethacrylate-co-maleic anhydride) (poly(MMA-co-MA)) to remove Pb²⁺ and crystal violet dye from water. The process used a chemical modification technique. The rice husk was chemically treated by incorporating active functional groups that improve its binding affinity. The synthesized nanoparticles included crosslinked poly(MMA-co-MA), enhancing both the stability and specificity for Pb²⁺ and crystal violet adsorption. The study reported that the modified nanoparticles followed a Langmuir adsorption isotherm, suggesting a high affinity for Pb²⁺, with a maximum adsorption capacity of 93.45 mg/g [94].

A chemical method involving electrochemical reduction was used to enhance the efficiency of gold ion (Au) adsorption onto activated carbon through electroreduction. This process involved electrochemical techniques to explore the reduction kinetics of AuCl₄⁻ ions on activated carbon derived from plant materials. The results indicated that the reduction of gold ions onto the carbon surface was diffusion-controlled. The rapid electron transfer at the carbon surface, as observed in the study, highlighted the high efficiency of this method for electro-sorbing gold [95].

2.4. Equilibrium Isotherms: Langmuir and Freundlich Models

Isotherms describe how contaminants interact with biosorbents at equilibrium, providing insights into surface properties, adsorption capacity, and binding mechanisms [96]. The Langmuir isotherm assumes monolayer adsorption on a homogeneous surface with finite, identical sites and no interaction between adsorbed molecules expressed as:

$${\mathrm{q}}_{\mathrm{e}}={\displaystyle \frac{{\mathrm{q}}_{\mathrm{m}\mathrm{a}\mathrm{x}}\mathrm{b}{\mathrm{C}}_{\mathrm{e}}}{1+\mathrm{b}{\mathrm{C}}_{\mathrm{e}}}}$$

(1)

where q_e is the equilibrium adsorption capacity (mg/g), q_max is the maximum adsorption capacity, b is the Langmuir constant (L/mg) related to binding affinity, and C_e is the equilibrium concentration (mg/L) [97]. A study examined the efficiency of three Aspergillus fungal species, Aspergillus clavatus, A. oryzae, and A. fumigatus, in capturing heavy metals (zinc, cadmium, lead, and nickel) from aqueous solutions through biosorption. Researchers fitted the data to the Langmuir isotherm model and computed constants, including the maximum adsorption capacity (q_max) and the separation factor (RL). The Langmuir model indicated that biosorption occurred as a monolayer on a uniform, homogeneous set of adsorption sites on the fungal biomass rather than multilayer physical adsorption [98]

Another study on acid-treated rice husks demonstrated that Cd²⁺ adsorption followed the Langmuir model, with a maximum adsorption capacity (q_max of 93.45 mg/g) [99]. This high q_max value and good Langmuir fit (R² > 0.98) suggested a dominant monolayer chemisorption process driven by ion exchange and surface complexation mechanisms. Similar trends were observed for other modified/synthetic wastes, confirming the model’s suitability for describing metal ion uptake under controlled conditions [100].

The Freundlich Isotherm is an empirical model that describes adsorption on heterogeneous surfaces with sites of varying affinities, allowing for multilayer adsorption [101]. It is expressed as:

$${\mathrm{q}}_{\mathrm{e}}={\mathrm{K}}_{\mathrm{f}}{\mathrm{C}}_{\mathrm{e}}^{1/\mathrm{n}}$$

(2)

where K_f indicates adsorption capacity and 1/n represents adsorption intensity. A study tested Phragmites australis (common reed) biomass as a low-cost biosorbent for removing several heavy metals from aqueous solutions. The biomass was cleaned, dried, and ground into powder, and the point of zero charge (pHpzc), scanning electron microscopy (SEM), and Fourier Transform infrared (FTIR) spectroscopy were used to examine the surface morphology and functional groups. Langmuir and the Freundlich isotherm models were fitted, which the authors interpreted as indicating a mix of monolayer (Langmuir) and heterogeneous/multilayer interactions (Freundlich). Langmuir q_max for Mn²⁺ ≈ 6.3 mg/g, Cd²⁺ ≈ 5.1 mg/g [102]. Figure 1 shows that the experimental data match the Langmuir adsorption isotherm model well, and the plot of q_e versus Ce for the Freundlich model shows a strong correlation, with R² values exceeding 0.90.

2.5. Biosorption Kinetics

Kinetic models describe the rate at which adsorption occurs and help elucidate the controlling mechanisms, such as surface reaction, pore diffusion, or a combination of both [103]. The pseudo-first-order model assumes that the rate of occupation of adsorption sites is proportional to the number of unoccupied sites [103] and is expressed as:

$$\log ({\mathrm{q}}_{\mathrm{e}}-{\mathrm{q}}_{\mathrm{t}})=\mathrm{l}\mathrm{o}\mathrm{g}{\mathrm{q}}_{\mathrm{e}}-({\mathrm{k}}_1/2.303)\mathrm{t}$$

(3)

where q_e and q_t are the adsorption capacities (mg/g) at equilibrium and time t, respectively, and k₁ is the rate constant (1/min). Although widely used, it often does not fit biosorption data across the entire adsorption period [104]. The pseudo-second-order model assumes that the adsorption rate is proportional to the square of the number of unoccupied sites and often suggests chemisorption involving valence forces through electron sharing or exchange [104]. It is expressed as:

$${\displaystyle \frac{\mathrm{t}}{{\mathrm{q}}_{\mathrm{t}}}}={\displaystyle \frac{1}{{\mathrm{k}}_2{\mathrm{t}}_{\mathrm{e}}^2}}+\mathrm{t}/{\mathrm{q}}_{\mathrm{e}}$$

(4)

3. Biosorption

Biosorption is a physicochemical process that uses materials of a biological nature to remove contaminants by binding the ions onto the functional groups present on the surface of the biosorbents [80,105,106]. The advantages of biosorption are its efficiency, ease of operation, and applicability across a range of pollutants. Therefore, biosorption offers an environmentally friendly process and cost-effective technique used for the removal of pollutants [35]. However, the high cost of some adsorbents and the need for regeneration limit their use. The mechanisms of biosorption are physical adsorption (physisorption) and chemical adsorption (chemisorption). Physical adsorption is where molecules are held by van der Waals forces, hydrogen bonding, electrostatic forces, and hydrophobic interactions. Chemical adsorption involves the formation of chemical bonds between the adsorbate and the adsorbent (Figure 2). A substrate surface is an adsorbent material, mostly called an adsorbent, while an adsorbate is a liquid or gas that gets adsorbed on the surface [107]. The mechanism depends on several factors, such as the nature of the adsorbent and adsorbate, temperature, surface area of the adsorbent, and concentration of the adsorbate [33].

3.1. Mechanisms of Biosorption

The mechanisms of biosorption are adsorption (physical or chemical), electrostatic interactions, ion exchange, chelation, complexation/coordination, covalent binding, van der Waals forces, aggregation/precipitation (microprecipitation), and redox reactions, among others [109,110,111]. These mechanisms are often discussed separately from the broader categories of physical and chemical adsorption. This distinction is made to provide a more detailed understanding of the specific interactions involved, as each mechanism operates under different conditions and with varying strengths of interaction [109,110,111].

One of the key factors contributing to the effectiveness of biosorbents is their large surface area and abundant functional groups. Biosorbents possess surface functional groups, including hydroxyl, carboxyl, and amine, aldehyde, carbonyl, phenolic, and/or ether groups, which offer selective binding mechanisms with pollutants [106]. Other interactions (discussed in subsequent sections) include ion exchange, electrostatic attraction, and covalent bonding, depending on the nature of the contaminant and the biosorbent material [112,113]. The combination of these mechanisms allows biosorbents to efficiently remove a wide range of contaminants from water, making them highly effective in water treatment applications.

3.1.1. Physical Adsorption

Physical adsorption or physisorption happens because of weak van der Waals attraction forces [114]. Van der Waals forces are weak attractive forces that result from induced dipoles or temporary charge imbalances in the adsorbate and adsorbent molecules [115]. This force can be overcome by kinetic energy caused by an increase in temperature or pressure. This means that when the adsorbate molecules are adsorbed on the surface of the adsorbent, the freedom of movement of the molecules is limited.

The key characteristics of physisorption are that there is no formation of chemical bonds, it requires lower temperatures, and it involves the formation of multiple layers of adsorbed molecules [106]. The adsorbate attaches to the adsorbent layer without changing its chemical structure. Since the molecules are held by weak van der Waals forces, they require a small amount of energy to break the forces. Therefore, temperature must be increased and pressure lowered to reverse the process.

Walnut shell powder, naturally rich in secondary metabolites, was used as a biosorbent for Cr⁶⁺ removal, leveraging its polyhydroxy, carboxyl, and amine functional groups. Three derivatives were explored: native walnut powder, alkali-treated with NaOH, and citric acid-modified. The removal efficiencies of Cr⁶⁺ were 64.82 mg/g for native walnut, 69.56 mg/g for alkali-modified walnut, and 75.26 mg/g for citric acid-modified walnut. At low pH, the biosorbents became protonated, acquiring a positive charge on their surface. This positive charge facilitated the adsorption of the anionic form of Cr⁶⁺, which was drawn to the surface through electrostatic attraction (Figure 3) [116]. The electrostatic attraction between the negatively charged Cr⁶⁺ and the positively charged surface of the biosorbent is a physical process, not a chemical one, making it a form of physisorption.

A study by Lakshimi and colleagues on the removal of phenol using activated carbon from wood waste (biomass-derived biosorbent) showed that physisorption took place. This is because the phenol molecules were attached to the surface of the activated carbon without the formation of any chemical bond, but instead by weak van der Waals forces. It was also noted that the process was easily reversed by heating the activated carbon, leading to desorption of the phenol without changing its structure. The results of this study showed the removal efficiency of phenol by activated carbon from wood waste at 99% [117].

In another study, a metal–organic framework (synthetic biosorbent) was used to remove methylene blue from wastewater. The adsorption was attributed to physisorption because of the high surface area of the MOF, allowing the dye to be adsorbed without any chemical bonds, yielding a removal efficiency of 97%. Since the structure of the dye remained unchanged, this showed that there was no chemical reaction between the MOF and the dye, showing a physisorption [118].

Biochar (biomass-derived biosorbent) was used for the removal of Pb²⁺ and Zn²⁺ from contaminated water. The biochar had a high surface area, forming multiple layers to allow for these metal ions to adsorb onto its surface, yielding a removal efficiency of 95% for Pb²⁺ [119].

3.1.2. Chemical Adsorption

Chemisorption is a strong chemical interaction between adsorbates and adsorbent surface functional groups [106]. Niu et al. (2021) [120] used an orange peel modified with ethylenediamine as an adsorbent for the removal of Cr⁶⁺ ions in wastewater. The ethylenediamine brought an amino functional group that favored the chemisorption process through the formation of strong covalent bonds with Cr⁶⁺ ions, as explained below:

I.

Step 1: Cr⁶⁺ ions, either as chromate (CrO₄²⁻) or dichromate (Cr₂O₇²⁻), are adsorbed onto the biosorbent surface: Biosorbent-NH₂ + CrO₄²⁻ → Biosorbent-Cr(VI);

II.

Step 2: Cr⁶⁺ undergoes reduction by receiving electrons from the biosorbent. The general reduction half-reaction is: Cr⁶⁺ + 3e⁻ → Cr³⁺

III.

Step 3: Protonation of the reduced Cr³⁺ Species: CrO₄²⁻ + 8H⁺ + 3e⁻ → Cr³⁺ + 4H₂O;

IV.

Step 4: Cr³⁺ ions are formed and remain adsorbed on the biosorbent: Cr³⁺ + 3NH₂⁻ → Cr(NH₂)_{3 (S)};

V.

Step 5: Cr³⁺ ions are then stabilized on the biosorbent surface through complexation- Biosorbent Cr³⁺ (complex)

Heavy metals are easily chemisorbed because they undergo redox reactions, and in this study, the Cr was reduced from Cr⁶⁺ to a less toxic form, Cr³⁺, which was chemisorbed on the surface of the modified peel [120]. Maximum adsorption capacity was obtained at a pH of 6, which was ideal for the chemisorption of the Cr. Other parameters like temperature and time were investigated to determine the optimum conditions for chemisorption and were found to be 45 °C and 120 min, respectively.

Graphene oxide (synthetic biosorbent) is derived from the oxidation of naturally occurring graphite to enhance its adsorption capacity. Graphene oxide was used for the removal of Pb²⁺ through a chemisorption mechanism. The Pb²⁺ ions formed strong bonds with the hydroxyl and carboxyl groups on the graphene oxide. This formation of bonds makes adsorption irreversible, even with a change in temperature or pH [121]. Hg²⁺ was removed from a thiol-functionalized MOF (synthetic biosorbent) by forming covalent bonds with the sulfur atoms in the thiol group. The bond formation was specific to Hg ions, and it was not easily broken, indicating a chemisorption process [122]. About 85–95% of Cu²⁺ ions were removed from contaminated water using Chlorella vulgaris (microbial biosorbent) at a pH of 5.5. The process was chemisorption since Cu²⁺ ions formed covalent bonds with the functional groups in the algal biomass. These functional groups were amino, hydroxyl, and carboxyl groups, resulting in the formation of a complex [123].

Ion Exchange

During ion exchange, the target ions move from the bulk solution onto the biosorbent, undergoing exchange reactions with the functional groups on the biosorbent’s surface [120]. Metal ions in solution may replace other cations originally bound to the functional groups through ion-exchange processes. This exchange occurs due to differences in the affinity of metal ions for the functional groups and the concentration between the solution and the biosorbent surface. Several mechanisms may occur depending on the nature of the biosorbent and the metal ions involved, where metal ions bind to functional groups such as hydroxyl (-OH), carboxyl (-COOH), and amine (-NH₂), serving as active sites for metal ion adsorption [124].

An example of an ion-exchange reaction involving modified chitosan for the removal of Cu²⁺ and Cd²⁺ is shown in Figure 4 [125]. The amino and hydroxyl groups from the chitosan are protonated in the acidic medium (pH 4–5) with exchangeable H⁺ ions. When the water containing the metal ions is passed through the chitosan, the metal ions displace the H⁺ through ion exchange. The Cu²⁺ and Cd²⁺ ions adsorb onto the surface of the chitosan, and the H⁺ ions are released into the water.

The specific mechanism of ion exchange can vary depending on factors such as the solution’s pH, the presence of competing ions, and the surface chemistry of the biosorbent. For example, in the case of rice husks, metal ions such as Pb²⁺, Cd²⁺, or Cu²⁺ can bind to hydroxyl groups present on the surface of silica-based structures in the rice husk. The metal ions are exchanged with other cations in the solution, leading to their removal from the wastewater. Ho et al. (2006) [104] investigated the mechanism of heavy metal ion adsorption onto various biosorbents, including biomass-derived and biopolymers [104]. It was found that ion exchange and complexation were the predominant mechanisms involved in metal ion removal. The strong electrostatic interactions were from the negatively charged functional groups, hydroxyl, carboxyl, and amino groups, with the metal ions.

The optimal pH for a certain study showing an ion exchange mechanism was found to be around 4, where the electrostatic interactions between the negatively charged sludge and positively charged metal ions were most favorable. The negative charges were from the carboxyl and hydroxyl functional groups on the surface of the sludge. Contact time also played a crucial role; longer exposure periods enhanced adsorption efficiency. Additionally, higher initial metal ion concentrations increased the driving force for adsorption, further improving efficiency [91].

Another study used orange peels as a biosorbent for the removal of ammonia and nitrates. Characterization of the biosorbent was performed using FTIR, SEM-EDX, X-ray diffraction (XRD), and Brunauer–Emmett–Teller (BET) surface area analysis. FTIR analysis revealed the presence of functional groups such as hydroxyl, carbonyl, and amine groups, which were responsible for the adsorption of NH₄⁺ and NO₃⁻ ions. SEM analysis showed that the biosorbent had a rough and porous surface, providing a large surface area for adsorption, as shown in Figure 5 below.

The study stated that the surface charge (negative) highly influenced the biosorption capacity because of the combination of both acidic and basic media on the surface of the orange peel. In an acidic medium, pH 5.5, the acidic groups supported the electrostatic attraction of negatively charged pollutants, as shown in Figure 6 below. In a basic medium, carboxyls may lose protons and become negatively charged, becoming more favorable for attracting positively charged ions [85].

Algae usually have a carboxyl (COO⁻) group that can exchange ions with positively charged metals in solution. For the removal of Pb²⁺ in solution using algae as a biosorbent material, the reaction Equation (5) is as follows:

$$\mathrm{R}-\mathrm{C}\mathrm{O}{\mathrm{O}}^{-}+{\mathrm{P}\mathrm{b}}^{2+}\to {\left(\mathrm{R}-\mathrm{C}\mathrm{O}\mathrm{O}\right)}_2\mathrm{P}\mathrm{b}+2{\mathrm{H}}^{+}\mathrm{W}\mathrm{h}\mathrm{e}\mathrm{r}\mathrm{e}\mathrm{R}:\left({\mathrm{C}}_6{\mathrm{H}}_8{\mathrm{O}}_6\right)\mathrm{n}$$

(5)

In this reaction, Pb²⁺ replaces H⁺ on the COO⁻, effectively adsorbing the Pb onto the biosorbent [126]. At a lower pH of between 1 and 3, the availability of free -COO⁻ groups decreases because they tend to protonate, becoming -COOH. This reduces the Pb²⁺ ability to bond, lowering adsorption. At a higher pH of more than 8, deprotonation is favoured, enhancing Pb²⁺ binding. Higher temperatures often increase ion-exchange rates by providing the energy needed for the ions to move faster and interact with the binding sites.

Competitive biosorption experiments were conducted at pH 4, with algal biomass of 1 g per batch system, across initial metal concentrations of 10–200 mg/L. FTIR analysis revealed that exposure to the metals produced the most significant alterations in the functional groups of the algal biomass for Pb²⁺, indicating its highest affinity among the tested Ca²⁺, Mg²⁺, and Na⁺, as shown in Figure 7. The findings showed that metal removal was through ion exchange, with FTIR confirming the involvement of specific binding sites in the algae’s matrix [127].

Complexation

Complexation is a mechanism between metal ions and a biosorbent involving the formation of coordination complexes, where metal ions bind to specific functional groups present on the biosorbent’s surface through electron pair donation [128]. These functional groups typically contain oxygen and nitrogen, such as hydroxyl (-OH), carboxyl (-COOH), and amine (-NH₂) groups. The stability and effectiveness of metal ion removal depend on the affinity of the functional groups for the metal ions and the overall surface chemistry of the biosorbent [129]. Examples of complexation reactions are shown in Equations (6)–(8):

$${\mathrm{A}\mathrm{g}}^{+}+{2\mathrm{N}\mathrm{H}}_3\to {\left[\mathrm{A}\mathrm{g}\right(\mathrm{N}\mathrm{H}}_3{)}_2{]}^{+}$$

(6)

$${\mathrm{Z}\mathrm{n}}^{2+}+{4\mathrm{O}\mathrm{H}}^{-}\to {\left[\mathrm{Z}\mathrm{n}\right(\mathrm{O}\mathrm{H})}_4{]}^{2-}$$

(7)

$${\mathrm{H}\mathrm{g}}^{2+}+{4\mathrm{C}\mathrm{l}}^{-}\to {[\mathrm{H}\mathrm{g}\mathrm{C}\mathrm{l}}_4{]}^{2-}$$

(8)

In the study by Michalak et al. (2019) [130], biochar was produced from the freshwater macroalga Cladophora glomerata through pyrolysis at a temperature of 450 °C. FTIR analysis revealed biochar contained functional groups such as silicate (Si-O-), aliphatic C-H, and hydroxyl (-OH) groups, which aided the adsorption of metal ions as shown in Figure 8. SEM images showed a porous surface structure, which increased the surface area, and ICP-OES analysis confirmed the presence of metal ions in the biochar, indicating successful adsorption. The study observed that the biosorption capacity of the biochar increased with higher pyrolysis temperatures, demonstrating removal efficiencies of 89.9% for Cr³⁺, 97.1% for Cu²⁺, and 93.7% for Zn²⁺ [130].

A study on metal and the released polysaccharide (RPS) interactions conducted at pH 6, a biosorbent dosage of 100 mg/L, and 25 °C investigated the role of functional groups in binding through FTIR spectroscopy. The spectra in Figure 9 show the characteristic bands of polysaccharides, including O-H, C-H, amide, carbonyl, and sulfate vibrations. Upon metal exposure, shifts in the O-H band, the appearance of a new peak at ~1420 cm⁻¹, and the disappearance of the C=O band at 1730 cm⁻¹ indicated that hydroxyl, carboxyl, and carbonyl groups were actively involved in metal coordination.

These spectral changes confirm complexation as a key mechanism, with metal ions binding to oxygen-rich sites within the RPS matrix. ICP-OES measurements quantified substantial metal removal, supporting strong metal–ligand interactions, while SEM-EDX mapping confirmed the presence and distribution of bound metals on the RPS surface, further validating the complexation mechanism [131].

In the study by Ajaykumar et al. (2008) [132], the chemistry of chemical biosorption involved the interaction between metal ions (Cd²⁺, Cu²⁺, Ni²⁺, and Zn²⁺) and functional groups (carboxyl, hydroxyl, phosphate, and amino) on the surface of activated sludge [132]. Metal ions (like Pb²⁺ or Cu²⁺) interact with electron-donating functional groups (e.g., -OH, -COOH) on the sludge.

An example of a biosorption mechanism was that with chitosan, which has an -NH₂ group, that formed stable complexes with Cu²⁺ [126]. The Cu²⁺ coordinates with the NH_2, creating a metal-biosorbent complex. Among other factors, this study showed that moderate temperatures improve the kinetics of adsorption, but extreme temperatures could affect the stability of chitosan, leading to weaker adsorption sites.

Exopolymeric substances (EPSs) from Bacillus cereus were incorporated into a sodium alginate solution, followed by gelation in a calcium chloride bath, resulting in spherical beads. Functional groups such as -COO, -OH, and -NH₂ were present on the EPS, facilitating metal ion binding. Complexation occurred through these functional groups, enabling the biosorbents to effectively remove heavy metals, as shown in Figure 10. The removal efficiencies were 97.54% for Cu, 98.51% for Pb, 90.83% for Zn, 82.88% for Cd, and 97.35% for Cr [133].

About 98% of Cr⁶⁺ was removed at an optimum pH of around 2 using a dead fungal biomass called Aspergillus niger through surface complexation. The fungi have carboxyl, hydroxyl, and amine groups, which chemically interact with the Cr⁶⁺ ions. The Cr⁶⁺ was reduced to Cr³⁺ (a less toxic form), which binds strongly to the biomass and is an irreversible process [134]. Hickory wood-based biosorbents were created via acidic and alkaline ball milling, enhancing oxygen-containing functional groups and pore channels. These features promoted electrostatic interaction, ion exchange, and surface complexation with Congo red and crystal violet, achieving 87.9% and 76.9% dye removal, respectively [93].

3.2. Factors Influencing Biosorption

Biosorption involves the interaction between various metallic ions and the biosorbent. Therefore, the equilibrium is significantly influenced by pH, temperature, concentration of the biomass, and particle size [135].

3.2.1. pH

This is a measure of the alkalinity or acidity of a solution [136]. The pH conditions determine the type of interaction between the metal ion and the binding site of the biosorbent. In more alkaline solutions, there is deprotonation of functional groups such as hydroxyl and amine. These functional groups are used in complexation mechanisms with monovalent and divalent metals. Acidic environments enable the carbonyl and hydroxyl groups to exchange metal ions with hydrogen ions [105]. Anionic cations, for example, chlorine, bromine, and sulfur, may form weaker complexes and require alternative sites such as oxygen donor atom metals, leading to precipitation. A study using eggshells as a biosorbent investigated the process kinetics within a pH range of 3–5. It was found that the system reached equilibrium within 1 h. An increase in pH led to higher sorption capacity and faster sorption rates, with the optimum pH being 5 [137]

Lower pH levels resulted in reduced metal uptake due to the positive charge on the sludge surface, which limited the binding sites. As pH increased, the negative charges on the sludge surface enhanced attraction to positively charged metal ions, facilitating greater adsorption. The optimal pH for maximum removal efficiency was found to be around 4, where the balance between metal solubility and adsorption was favourable for effective biosorption [91]. In a 2000 study by Dorris et al. [138] on Cu²⁺ removal from water using sawdust, the optimal pH for copper adsorption was found to be around 5 to 6. Lower pH levels hindered Cu²⁺ uptake, likely due to the high concentration of H⁺ competing for binding sites. As pH increased, the sawdust’s capacity to bind Cu²⁺ improved, up to the point where Cu²⁺ began to precipitate at higher pH values. Under optimized conditions, such as a near-neutral pH level and sufficient contact time, the sawdust was able to remove a significant portion of Cu²⁺, with efficiencies reaching up to around 90% [138].

Biosorption capacity for both Pb²⁺ and Cd²⁺ by Crassostrea gasar (Bivalve) biomass material increased significantly as the pH of the solution was adjusted to more alkaline conditions. Specifically, the optimal pH for maximum adsorption was determined to be around pH 10. At this pH, the surface of the biosorbent is more negatively charged, which enhances the electrostatic attraction between the positively charged metal ions and the active sites on the biomass. In contrast, at lower pH values, the biosorbent’s surface tends to become protonated, leading to a reduction in the adsorption capacity, as the metal ions may remain in their more soluble, unadsorbed forms [139].

In aqueous solutions, Cu²⁺, Pb²⁺, and Cd²⁺ undergo pH-dependent speciation that critically influences how they can be removed. Cu²⁺ tends to precipitate as Cu(OH)₂ at around pH 6.5 to 7, as its solubility decreases sharply under these conditions [140]. Pb²⁺ remains mostly as free ions at lower pH, but above pH ~7, Pb(OH)₂ and carbonate complexes emerge, reducing its solubility and favoring precipitation or surface complexation [141]. Accordingly, at high pH (e.g., pH 10), removal is dominated by chemical precipitation rather than biosorption. Thus, biosorption is most effective at moderate pH (typically pH 5–7), where ions remain soluble yet are electrostatically attracted to the biosorbent and chemically complexed or exchanged at surface sites.

The effect of pH on biosorption is closely related to the point of zero charge (pHpzc), which is the pH at which the biosorbent’s net surface charge is zero. Below the pHpzc, the surface carries a net positive charge, while above it, the charge is net negative. Functional groups such as carboxyl (-COOH), hydroxyl (-OH), amine (-NH₂), and phosphate influence this charge behaviour. For instance, P. australis biomass has a pHpzc of about 6.2, meaning that at pH 7–8 the surface becomes negatively charged, enhancing cation removal through electrostatic attraction and complexation [102].

Different biosorbent materials have pHpzc values that shift depending on composition. Algal cell walls rich in polysaccharides and uronic acids often have pHpzc ~4–5, while fungal cell walls with chitin/amines may exhibit pHpzc ~6–7 [142,143]. Natural orange peel biosorbent exhibits a pHpzc around 3.85, which indicates a negatively charged surface above this pH and thus promotes cation binding via electrostatic and coordination interactions. FTIR revealed that orange peel surfaces are rich in -OH, -COOH, C=O, and ester groups. These oxygen-bearing parts tend to deprotonate above the pHpzc, increasing surface negativity and enhancing the capture of positively charged species [144].

3.2.2. Temperature

This is a form of energy that can be used to break the arrangement of particles and influence the rate of the reactions. Elevated temperatures may negatively impact the biosorption capacity of the biosorbent material [114]. For example, a higher temperature in an exothermic reaction would result in low metal removal, while a lower temperature would support metal removal. The converse is also true for an endothermic reaction [13]. A study using activated sludge showed that at an increased temperature, the kinetic energy of the metal ions also increased, leading to enhanced diffusion rates and improved interactions with the adsorbent. Higher temperatures generally promote adsorption, but the specific optimal temperature was not explicitly stated. The findings, though, highlight the importance of thermal conditions in the effectiveness of heavy metal removal [91].

A study showed that the adsorption capacity of Cd, Cu, Ni, and Zn decreased as temperature increased (Figure 11) [91].

Another study investigated the adsorption performance of hazelnut and almond shells as adsorbents for removing heavy metals like Ni²⁺, Cd²⁺, and Pb²⁺ from water. The adsorption process relied on ion exchange, where metal ions in the solution were bound to functional groups on the shell surfaces, especially influenced by pH, initial metal concentration, and temperature. It was found that higher temperatures of 60 °C generally improved metal ion adsorption, suggesting an endothermic process where metal uptake by the shells increased with rising temperature [92]. To quantify this temperature dependence, the Arrhenius equation can be applied:

$$\mathrm{k}=\mathrm{A}·{\mathrm{e}}^{-\mathrm{E}\mathrm{a}/\mathrm{R}\mathrm{T}}$$

(9)

where k is the rate constant, A is the pre-exponential factor, Ea is the activation energy, R is the universal gas constant, and T is the temperature in Kelvin. An increase in temperature leads to an increase in the rate constant k, implying that the adsorption process becomes more efficient at higher temperatures. Given that the study observed positive values for ΔH, the van ’t Hoff Equation (10) predicts that the equilibrium constant K increases with temperature, further supporting the endothermic nature of the adsorption process.

$$\mathrm{l}\mathrm{n}\mathrm{K}=-{\displaystyle \frac{∆\mathrm{H}}{\mathrm{R}}}\left({\displaystyle \frac{1}{\mathrm{T}}}\right)+∆\mathrm{S}/\mathrm{R}$$

(10)

where ΔH is the enthalpy change, ΔS is the entropy change, R is the gas constant, and T is the temperature in Kelvin. A study on activated sludge biosorption (treated with NaOH) examined how the adsorption capacity for Cd²⁺ changed with temperature (25, 30, 38, and 45 °C) under constant pH 5 and 2 h contact time. The results showed a clear negative trend: as the temperature rose from 25 to 45 °C, the maximum adsorption capacity (q_max) dropped by approximately 18.3%, indicating reduced affinity at higher temperatures. Therefore, the biosorption of Cd²⁺ onto activated sludge is exothermic (ΔH° < 0), aligning with the van ’t Hoff Equation (10), which relates the equilibrium constant K (proportional to adsorption affinity) to temperature [145].

3.2.3. Concentration

Biosorption of metal ions leads to toxicity to the biosorbent, which affects the interaction activity and may disrupt the arrangement of particles. This can lower the effectiveness of biosorption as compared to conventional treatment methods. High metal concentrations result in saturation of ion exchange sorption sites and spread onto physically sorbed metal ions. The spreading increases the contribution of physical adsorption [105]. The success and efficiency of biosorption do not just depend on the biosorbent’s properties but also on the initial concentration of the metal ions in the solution. As metal concentration increased, though, removal efficiency dropped as Cd²⁺ decreased from 56.06% to 40.48%, Cu²⁺ from 51.78% to 18.03%, Ni²⁺ from 20.88% to 11.35%, and Zn²⁺ from 40.61% to 15.97%. This effect was mainly due to adsorption sites becoming fully occupied during the process, leaving some sites unused [91]. The experimental data were fitted to both the Langmuir and the Freundlich isotherm models, with the Langmuir model providing a better fit, indicating monolayer adsorption onto a surface with a finite number of identical sites. Also, at higher concentrations, adsorbent particles tend to clump together, reducing the available surface area and lengthening the path for ions to reach active sites, which lowers overall efficiency.

A study examined the biosorption of Cu²⁺ onto dried activated sludge with varying sludge ages (5, 20, and 40 days). The researchers observed that the adsorption capacity increased with the initial Cu²⁺ concentration, with maximum monolayer adsorption capacities of 40.32, 37.04, and 24.27 mg/g for dried activated sludge aged 5, 20, and 40 days, respectively. The data were well-fitted to both the Langmuir and the Freundlich isotherm models, indicating that the biosorption process was influenced by the initial metal ion concentration [146]. Equilibrium isotherm models, like Langmuir and Freundlich, help describe how adsorption sites are occupied, and kinetic models further explain how adsorption speed depends on concentration, with some following first-order and others pseudo-second-order behavior. Though these models rely on simplifying assumptions such as equilibrium and homogeneous surfaces, they remain essential for understanding and predicting biosorption trends, especially the increase in uptake with concentration until surface saturation occurs [147].

3.2.4. Particle Size

This represents the dimensions, shape, and surface area of a particle. Small particles may increase the adsorption rate of a reaction because of their high surface area compared to larger particles with a smaller surface area. Therefore, a biosorbent with a high surface area would increase biosorption, but depending on other parameters like pH, temperature, and time [13]. However, even though smaller particle sizes increase the surface area for adsorption, excessively fine particles may lead to aggregation, reducing the available surface area.

In a study, as the amount of sludge increased, the removal efficiency improved because there were more binding sites available for the metal ions. However, at a certain point, this efficiency levelled off or even started to decrease. This happened because, with too much biomass, the cells blocked each other’s active sites, which reduced the space available for ion binding. Specifically, as sludge mass went from 0.5 g to 3 g, Cd²⁺ removal efficiency increased from 37.61% to 61.11%, Cu²⁺ from 23.36% to 50.28%, Ni²⁺ from 12.04% to 27.54%, and Zn²⁺ from 9.2% to 36.28% [91].

Smaller particle sizes result in higher adsorption capacities, but the increase in efficiency becomes less pronounced at very fine sizes due to limitations in handling and filtration. According to the study, the peanut shell biosorbent with a particle size of around 0.5 mm had the highest adsorption efficiency, and a corresponding Cr⁶⁺ removal efficiency of 87.9%. In contrast, sawdust and Cassia fistula leaves had lower surface areas and, therefore, lower removal efficiencies of 76.9% and 70.4%, respectively [82].

In the study titled “Effect of Biosorbent Particle Size on Biosorption of Pb²⁺ from Lengkeng Seeds and Shell (Euphoria logan Lour),” researchers explored how varying particle sizes of lengkeng seeds and shells influenced biosorption of Pb²⁺ from aqueous solutions. The biosorbents were prepared in different particle sizes: 106 μm, 150 μm, 250 μm, and 300 μm. The smallest particle size of 106 μm had the highest adsorption capacities, with 25.626 mg/g for seeds and 30.095 mg/g for shells. This indicates that reducing the particle size enhances the biosorption efficiency, likely due to the increased surface area and availability of active sites for Pb²⁺ interaction [148].

4. Current Status of Research on Biosorbents

Different biosorbents are currently being actively researched and developed for water treatment applications, focusing on the removal of heavy metals, ions, and dyes, as summarized in

Table 2. A summary of the types of adsorbents used for the removal of different pollutants and the removal efficiencies.

Type of Adsorbent	Synthesis Method and Active Functional Group	Pollutant Removed	The Amount Removed and the Mechanism of Biosorption
Natural-biomass-derived biosorbent: Modified walnut shells biosorbent comprising 41.2% cellulose	Chemical treatments were employed to enhance the adsorption efficiency. The adsorption capacity was due to the activation of hydroxyl and carboxyl functional groups. The experiment was carried out at pH 6 within 240 min of contact time, using a biosorbent dosage of 0.2 g.	Methylene blue	As the size of the walnut shell was reduced, the methylene sorption rate increased from 17.2% to 39.2% [55]. The mechanisms of biosorption were ion exchange and complexation.
Natural-biomass-derived biosorbent: Acorn shell of Quercus crassipes Humb. and Bonpl. (QCS)	Acorn shell biosorbent was cleaned, dried, and ground without chemical modification. The lignocellulosic components with hydroxyl, carboxyl, and phenolic groups contributed to metal ion binding. Biosorption was conducted at an optimal pH of 8.0, with a biosorbent dosage of 1 g/L, and contact times monitored for up to 120 h.	Chromium Nickel	Removed the hexavalent chromium. An increase in NaCl ionic strength increased the biosorption of Ni2+. The main adsorption mechanism is ion exchange, where divalent nickel ions exchange with cations on the acorn shell’s active sites [32].
Natural-biomass-derived biosorbent: Coconut husk and tree sawdust	The material was modified with acids and bases to enhance its adsorption capability. Hydroxyl and carboxyl groups were active groups in binding the ions.	Cd(II), Cr(III) and Hg(II) ions Acid violet and acid brilliant blue Rhodamine B and methylene blue	The material removed about 84% to 96% of the metal ions. There was an 85% removal of rhodamine B at a lower pH of 3, while methylene blue was removed at about 97–100% at a pH of 6.9. The mechanisms of adsorption were chemical, that is, specifically ion exchange and surface complexation, since the positively charged ions were attaching to the negatively charged functional groups [31].
Natural-biomass-derived biosorbent: Coconut coir	Both physical (particle size reduction) and chemical (use of acids) processing were employed. Hydroxyl and carboxyl groups were the binding surfaces for the adsorption of Zn and Pb ions.	Lead Zinc	91% of Zn was removed at pH 4.5 and 97% at pH 2.5 for Pb. The mechanism of adsorption was ion exchange and complexation [31].
Natural-biomass-derived biosorbent: Orange peels	Physical synthesis: The peels were washed, dried, and ground into powder. The amine group binds and traps the ammonia molecule, while a combination of carbonyl and hydroxyl groups has high affinities for pollutants. At pH 5.5, with an orange peel dosage of 4 g and a contact time of 60 min, the biosorbent showed optimal removal performance for both ammonia and nitrate.	Nitrate (NO3) Ammonia (NH)	100% removal of nitrates and 70–100% of ammonia on the increased amount of orange peel. The mechanism of adsorption is ion exchange [85]. The work also included characterization of the material via XRD, FTIR, BET, and SEM-EDX to elucidate its structure and adsorption functionality.
Natural microbial biosorbent: Biochar from freshwater macroalga Cladophora glomerata	Chemical synthesis: biochar was tested at pH 5, using a dosage of 1 g/L, over a contact time of 3 h under agitation (150 rpm, ~20 °C), pyrolysed at a maximum temperature of 450 °C. The active groups are hydroxyl, silicate, aliphatic, and aromatic.	Chromium (III) Copper (II) Zinc (II)	The removal efficiency for Cr(III) was 89.9%, Cu(II) was 97.1%, and for Zn(II) ions was 93.7%. The adsorption mechanism is ion exchange [130].
Natural-biomass-derived biosorbent: eggshell and Java plum seed	Physical preparation: drying and grinding of eggshells and seeds. The eggshells have calcium carbonate, but the surface is predominantly hydroxyl groups.	Arsenic (III)	As(III) elimination was 78–87% at a pH of 7 using ion exchange as an adsorption mechanism [150]. The pH values for the eggshell were 4.1 and 5.3 for Java plum seed.
Natural-biomass-derived biosorbent: Tea waste and pomegranate peel	Physical: drying and grinding. The active group is carboxyl and hydroxyl. Maximum arsenic biosorption occurred at 2 h, following kinetics best described by the Langmuir isotherm and pseudo-second-order model.	Arsenic	Arsenic removal by tea waste was 74%, and pomegranate peel yielded 65%, which shows tea waste to be more effective than pomegranate peel. The adsorption mechanism is physisorption [150].
Natural microbial biosorbent: Marine green algae	Physical: drying and grinding. The active groups are carboxyl and hydroxyl. The alkaline-treated algae biomass was most effective at pH 6 with a contact time of 240 min and a biosorbent dose of 1 g/L.	Cadmium (II)	The biosorption capacity for Cd(II) ions was 53.68%. The mechanism of adsorption is ion exchange [36].
Modified/synthetic biosorbent: Magnetic biochar	Physical through high-temperature pyrolysis.	4-nitrotolouene nitrotoluene	80% removal of nitrotoluene [149].
Modified/synthetic Biosorbent: Amine-crosslinked starch (ACS)	The chemical synthesis method was used by crosslinking starch with amine-containing agents to introduce amine groups.	Dyes: Brilliant Blue (BB) and Amaranth (ART) Pharmaceutical: Diclofenac Sodium (DS)	Amount Removed (Adsorption Capacities): BB: 1287.7 mg/g ART: 724.6 mg/g DS: 595.2 mg/g The mechanism of adsorption is physical and ion exchange, where the protonated amine group interacts with negatively charged pollutants (e.g., anionic dyes and pharmaceuticals) [30].
Modified/synthetic and biomass-derived biosorbent: Magnetic biochar prepared from chicken bones	Physical through low-temperature pyrolysis.	Methylene blue Rhodamine B dye Tetracycline	70% removal of methylene blue and 75% removal of other organic contaminants [149]
Natural-microbial biosorbent: Macro algae (Fucus vesiculosus) Natural: Crab shells (Cancer pagurus) Natural-biomass-derived biosorbent: Wood chippings Natural: Iron-rich soil Modified/synthetic material: Biochar (commercial) Modified/synthetic material: Activated Carbon (commercial)	Physical: breaking the particles into smaller ones. Chemical by the use of modifiers and acids to expose chitin in the case of crab shells. Sulfonic groups on the surface of the algae. Amino and hydroxyl groups from the chitin. A phenolic group from the lignin of wood chippings.	Copper Zinc	Copper activated carbon (100%) and biochar (96–99%) removal. Iron-rich soils removed 96%. Crab shells removed 92%. The algae removed 59–67% while wood chipping had a low removal efficiency of 31%. Zinc The highest removal was achieved from activated carbon and biochar, with 100% removal. Crab shells removed 96% Algae removed 39–87% Iron soils had 66% removal, while wood chippings had the lowest removal of 57%. Ion exchange, chelation, precipitation, electrostatic attraction, complexation, and mechanism [35]
Modified/synthetic biosorbent: Magnetic activated carbon	Physical synthesis: pyrolysis	Lead (II) Cadmium (II) Chromium (VI) Copper (II)	The removal efficiencies were: Pb 99.4%, Cr 66.7%, Cu 99.1% [149].

. The types of adsorbents are mostly plant material (shells, coir, seeds, corn, husk, waste, flowers, and peels), charcoal, polymers, metal nanoparticles, activated carbon, iron oxides, magnetites, or a combination of adsorbents [149]. The contaminants targeted include Cr, Mo, Se, Co, Pb, Cu, Ni, and Zn, while dyes include methyl orange, congo red, methylene blue, crystal violet, and rhodamine B. Inorganic pollutants in water include phosphates (PO₄), fluoride (F), cyanide (CN), and nitrate (NO₃). Some adsorbents are used for the removal of other parameters found mostly in wastewater, for example, color, COD, TOC, BOD, total solids, and total hardness [31]. The field has made significant progress, with many studies focusing on modification techniques, hybrid materials, and large-scale applications.

5. Challenges and Future Directions

While biosorbents offer a sustainable solution for removing pollutants from wastewater, they face challenges such as limited selectivity, potential toxicity, and variable performance in different environmental conditions, as shown in

Table 3. Summary of parameters that need investigation, the challenges faced, and proposed future directions.

Parameter that Requires Further Investigation	Challenge	Proposed Future Direction
Enhancement of biosorption capacity through modification of the biosorbent	Low biosorption capacities: natural biosorbents show low removal efficiencies; therefore, there is a need for further enhancement to improve their capacity to adsorb pollutants effectively.	Modified/synthetic biosorbents show improved removal efficiencies; therefore, methods should be developed and optimized to modify biosorbents, such as chemical treatments, physical alterations, or the incorporation of functional groups, to significantly increase their biosorption capacity.
Assessment of biosorbents under multi-component pollutants	Limited information on biosorption of multi-component pollutants: most studies on natural biosorbents focus on single pollutants, which do not represent real-world scenarios.	Modified/synthetic biosorbents have shown efficiency in multi-component pollutants. These adsorbents have different functional groups, such as -OH, -COOH, -NH2, and -SO3H enhance interactions with pollutants via ion exchange, hydrogen bonding, or complexation.
Investigation with real industrial effluents	Limited reports on studies of real wastewater samples: laboratory studies often use synthetic wastewater, which does not accurately reflect industrial conditions.	Perform investigations using industrial effluents to validate the performance of biosorbents in practical applications and ensure their effectiveness in real-world conditions.
Regeneration studies	Limited information on regeneration studies of biosorbents: the studies that show their materials being regenerated do not report the removal efficiencies after regeneration.	More studies can be dedicated to the efficiency and cost-effectiveness of regeneration techniques to improve the recyclability of biosorbents.

. Future research should focus on enhancing biosorbent efficiency through modifications, optimizing regeneration methods, and scaling up applications for industrial use to fully harness their potential for environmental remediation.

6. Conclusions

In conclusion, safe water is not just a necessity; it is a cornerstone of healthy communities and sustainable development. Water pollution has become an environmental concern, affecting not only ecosystems but also human health and global development. The increase in industrialization and urbanization causes the release of pollutants into water bodies, requiring innovative and sustainable solutions to address this escalating problem.

Biosorbents represent a promising and sustainable solution for water pollution control, offering cost-effective and efficient removal of contaminants, particularly metal ions, from aqueous solutions. The unique properties of biosorbents, because of their natural origin and renewability, make them an attractive alternative to conventional adsorbents. However, further research is needed to enhance their biosorption capacity and to understand their complex sorption mechanisms. Biosorbents offer an immense opportunity and potential to ease waste generation and, in general, create sustainable practices in the environment. One of the challenges is improving the performance of these materials so that maximum pollutants can be removed under various conditions. Factors such as surface area, porosity, and the presence of functional groups significantly influence the adsorption capacity of biosorbents. To overcome these limitations, modifications (both physical and chemical) have been carried out. These modifications may be physical (thermal activation or mechanical), to enhance the surface area and porosity of the material, thus allowing more available surfaces (active sites) for the adsorption of pollutants. With an aqueous environment, thermal activation is a great way to alter the micropore volumes of the material, so the biosorbent can accommodate small pollutants. Mechanical treatment, such as grinding or sonication, may also be used to improve particle size distributions or surface morphology.

Chemical modifications primarily focus on either introducing additional functional groups or enhancing existing functional groups that interact with specific pollutants and contaminants. For example, acid-base treatment is one technique that has been shown to modify the surface chemistry of biosorbents to improve their affinity for certain pollutants, such as heavy metals or dyes. The availability of acid-treated biosorbents provides mechanical stability. Chemical modifications to biosorbents may have improved durability and multifunctional properties (such as adding some antimicrobial properties). For example, using existing biosorbents with the addition of nanomaterials may enhance the biosorbent’s adsorption efficiency, while also adding antimicrobial properties to the adsorbent, which is valuable in treating waters contaminated with microorganisms. While there are a lot of positives to learn about biosorbent modifications, further research is needed to overcome the issues of scalability and costs.

Most studies utilize lab-based biosorbent adsorption concepts, but the transition to scale should not be underestimated. Utilizing methods that are inexpensive and energy-friendly will be another important avenue to enable biosorbents to be used more widely in water treatment systems. Following the above challenges opens significant opportunities for biosorbents as they improve their material property, whereas employing new applications will encourage biosorbents to become a new frontrunner in sustainable technology in water treatment systems. With pollution reaching new limits, funding biosorbent research and outreach is necessary not only to do the right thing for the environment but also to foster green technologies for everyone’s benefit. With scientific inquiry and collaboration, the use of biosorbents can be a game-changer for the future to achieve clean, safe water for all.