Heavy Metals: An Overview of their Sources and Biosorption Processes

363 Nigerian Research Journal of Engineering and Environmental Sciences 10(2) 2025 pp. 363-375 p ISSN: 2635-3342; e ISSN: 2635-3350 Review Article Heavy Metals: An Overview of Their Sources and Biosorption Processes 1Korode, I.A., 2Fadeiye, J.K. and *3Emegha, J.O. 1Department of Petroleum Engineering and Geosciences, Petroleum Training Institute P.M.B 20, Delta State, Nigeria. 2Department of Industrial Safety and Environmental Technology. Petroleum Training Institute, P.M.B 20, Delta State, Nigeria. 3Department of Physics, Hensard University, Toru Orua, P.O. Box 1036, Bayelsa State, Nigeria. *jjjem[email protected]; [email protected] http://doi.org/10.5281/zenodo.18061158 ARTICLE INFORMATION ABSTRACT Article history: Received 10 Sep. 2025 Revised 11 Oct. 2025 Accepted 18 Oct. 2025 Available online 30 Dec. 2025 Recently, the use of biosorption has shown promising ways to reduce environmental pollution from heavy metals. Even though several physical and chemical techniques are employed for cleaning up heavy metal-contaminated sites, green biosorption methods are gaining significant recognition due to their economical and sustainable decontamination capabilities. Much research has explored how biosorption could improve wastewater treatment. Different biosorbents from various sources can be changed to gain superior qualities, which helps them specifically absorb heavy metals in the environment. Therefore, this review focuses on using biological biosorbents to remove heavy metal ions from wastewater. It looks at key factors like temperature, pH, contact time, and biosorbent features influencing biosorbents efficiency, and suggests possible directions for future research. © 2025 RJEES. All rights reserved. Keywords: Heavy metals Biosorption Environment Wastewater Ecosystem 1. INTRODUCTION In recent decades, the unchecked discharge of wastewater from a variety of sources has emerged as a pressing global environmental challenge (Luong Nguyen, 2021; Oliomogbe and Emegha, 2024). Effluent discharged from industrial activities often harbors a complex mixture of heavy metals (HMs) and metalloids, including arsenic (As), chromium (Cr), cadmium (Cd), mercury (Hg), and lead (Pb) (Wang et al., 2022). The presence of these contaminants poses a significant threat to environmental health and public wellbeing. However, the inherent toxicity and bioaccumulation potential of these elements necessitate continued vigilance, as their persistence in the environment jeopardizes the health of ecosystems and human populations (Ukhurebor et al., 2023a; Ukhurebor et al., 2023b). Even at minimal concentrations, these metals can be detrimental to human health when they enter the food chain. For instance, plants and aquatic organisms can absorb these contaminants, which then move up the food chain, ultimately 364 I.A. Korode et al. / Nigerian Research Journal of Engineering and Environmental Sciences 10(2) 2025 pp. 363-375 affecting humans (Hussain et al., 2023). The accumulation of these metals in human tissues can lead to a range of adverse health effects, including toxicity, neurotoxicity, carcinogenicity, and mutagenicity (Xue et al., 2022; Oliomogbe et al., 2024). These effects can manifest in various ways, such as developmental disorders, neurological damage, cancers, and genetic mutations, underscoring the critical need for stringent monitoring and control measures (Oliomogbe et al., 2024). The sources of heavy metal exposure are diverse, encompassing both natural and anthropogenic origins (Younas et al., 2023; Fadeiye et al., 2025). Natural sources include volcanic activity, the weathering of rocks, and geothermal processes. Volcanic eruptions, for instance, release significant quantities of metals like mercury and arsenic into the atmosphere, which can then be deposited into water bodies. Similarly, the natural weathering of mineral-rich rocks releases metals such as cadmium and lead into the soil and water. On the other hand, human activities significantly exacerbate this issue. Industrial processes such as oil refining and coal-fired power generation release large quantities of heavy metals into the environment (Ukhurebor et al., 2021). High-voltage power lines and the manufacturing of textiles and plastics also contribute to the release of these contaminants (Egboduku et al., 2023). The production of microelectronics involves the use of metals like lead and cadmium, which can enter the environment through improper disposal or during the manufacturing process (Ukhurebor et al., 2020) Wood preservation and paper milling are other significant sources of heavy metal pollution (Jomová et al., 2024). Chemicals used in these processes often contain metals that can leach into water sources. Mining and smelting activities release metals like mercury and arsenic into the environment, while the tanning of leather involves chemicals that contain chromium (Ukhurebor et al., 2021). Additionally, the use of pesticides in agriculture introduces various metals into the soil and water, which can then be taken up by crops and enter the food chain (Alsafran et al., 2023; Emegha et al., 2024a). The remediation of heavy metals in water has necessitated the development of diverse treatment techniques. Traditional or conventional methods encompass chemical precipitation, membrane separation, and activated carbon adsorption, while newer approaches explore ion exchange, reverse osmosis, and the utilization of clay minerals, biosorbents, and biochar (Mudila et al., 2019). Recent researches have looked into the potential of organic materials like fruit and vegetable peels, agricultural waste, and marine algae as cost-effective adsorbents (Liu et al., 2019; Oliomogbe et al., 2023). Inorganic alternatives such as iron oxides, modified zeolites, and nanomaterials are also being explored (El Nemr et al., 2023). Studies have shown that these varied sorbents, including their derivatives, exhibit high removal efficiencies for diverse heavy metals under various experimental conditions (Sheikhi and Rezaei, 2021). The economic burden associated with conventional heavy metal removal methods has spurred the exploration of cost-effective alternatives, particularly those utilizing waste materials (Mudila et al., 2019). Unlike their organic counterparts, heavy metals persist in the environment, relentlessly accumulating within living organisms. This bioaccumulation acts as a ticking time bomb, potentially triggering a cascade of diseases and disorders that can ultimately jeopardize human well-being (El Nemr et al., 2023; Oliomogbe et al., 2023). However, it is crucial to understand that not all heavy metals possess identical properties and effects. Certain elements, like iron, zinc, copper, and cobalt, play a vital role as micronutrients in our bodies, but only in minute quantities. Exceeding these essential levels transforms them from allies into antagonists, unleashing their toxic potential. Conversely, some heavy metals, such as mercury, cadmium, lead, and arsenic, are inherently villainous, exerting their harmful effects even at trace concentrations (Jyothi, 2020). Recently, the specter of heavy metal contamination looms large across the globe, affecting millions and emanating from various sources, as depicted in Figure 1. These metallic menaces operate as systemic toxins, capable of inflicting damage on multiple organs, even when present in minuscule amounts (Tchounwou et al., 2012). The severity of these health consequences hinges on a complex interplay of factors. An individual's genetic makeup acts as the battleground, influencing susceptibility. The dose and route of exposure determine the intensity of the assault (Tchounwou et al., 2012). The underlying mechanism of heavy metal toxicity lies in their ability to wage chemical warfare at the cellular level. They bind to essential components like proteins, enzymes, and membranes, disrupting 365 I.A. Korode et al. / Nigerian Research Journal of Engineering and Environmental Sciences 10(2) 2025 pp. 363-375 their normal functions. This cellular chaos cripples fundamental processes like growth, proliferation, differentiation, repair mechanisms, and programmed cell death (apoptosis) (Balali-Mood et al., 2021). Mercury, cadmium, arsenic, and lead stand as the most nefarious heavy metals, their severe and chronic effects targeting a multitude of biological processes and vital organs throughout the body (Fadeiye et al., 2025). Over the years, various categories and subcategories of conventional wastewater treatment strategies have been employed with diverse results (Egboduku et al., 2023; Emegha et al., 2024a). Despite the effectiveness of these methods, several limitations persist (Mojiri, 2023). Furthermore, readily available information regarding the performance, mechanisms, and processes of each conventional method for removing different types of pollutants remains undesired (Mojiri, 2023). However, the evolution of biosorbents—naturally occurring or modified/synthetic materials of biological origin like agricultural by-products, microorganisms, or natural polymers—that can adsorb or absorb and remove pollutants, particularly heavy metals and organic compounds, from aqueous solutions, has effectively mitigated some of these reported limitations (Oliomogbe et al., 2023; Molebatsi et al., 2025). Consequently, biosorbents contribute to overcoming the shortcomings of conventional wastewater treatment strategies through biological means. Figure 1: Heavy metals pollution sources (Karnwal, 2024) Reportedly, biosorbents demonstrate exceptional performance due to their intrinsic physicochemical characteristics, the presence of metal-ion binding functional groups, and their synergistic interactions with eco-friendly components (Oliomogbe et al., 2023). These attributes differentiate biosorbents from conventional synthetic adsorbents, which frequently depend on non-biodegradable substances and energy-intensive manufacturing processes (Oliomogbe et al., 2023). As a form of bioremediation, biosorption provides a more environmentally sound and economical substitute for standard wastewater treatment approaches, utilizing the natural capacity of diverse biomasses to capture heavy metals (Staszak, and Regel-Rosocka, 2024). The distinctive qualities of biosorbents, including their varied molecular weight, charges, affordability, and the length of their functional groups, among other features, enable their versatile application (Razzak et al., 2022). They have garnered significant research interest owing to their active sites, specific surface area, pore volume, pore size distribution, ease of separation, and reusability, all of which are conducive to the remediation of heavy metalloid and organic contaminants. Despite their significant role in the biosorption of toxic heavy metals, the existing literature offers limited information regarding the limitations of conventional methods and the advantages of biosorptive heavy metal removal over these techniques. This knowledge is vital for 366 I.A. Korode et al. / Nigerian Research Journal of Engineering and Environmental Sciences 10(2) 2025 pp. 363-375 comprehending and developing effective biosorption processes for heavy metal removal from aqueous solutions, enabling optimized parameters for efficient utilization. Therefore, this review article begins with a broad introduction to heavy metals, addressing their origins, toxicity, impacts, and biosorption properties. It also emphasizes the application of biosorbents for the biosorption of heavy metal ions and other contaminants in aqueous solutions under varying process conditions. Finally, the review identifies the gap between current knowledge and future research directions. 2. CONVENTIONAL TECHNIQUES FOR HEAVY METAL REMOVAL The remediation of wastewater contaminated with heavy metals necessitates the implementation of effective treatment strategies. Fortunately, a well-established repertoire of conventional methods exists for the removal of these metallic pollutants from industrial effluents (Ivanova et al., 2016). This section will explore the most commonly employed techniques: chemical precipitation, ultrafiltration, ion exchange, and reverse osmosis, as depicted in Figure 2. Figure 2: Methods for heavy metals removal (Bayou et al., 2023) 2.1. Ion Exchange Ion exchange technology utilizes specialized resins containing functional groups with specific charges. These charged groups function as selective binding sites, attracting and exchanging unwanted heavy metal ions for harmless ions already present within the resin structure (Zhao et al., 2016). This process resembles an ion exchange program, where the targeted heavy metal ions are swapped out for innocuous alternatives. The selectivity of the resin can be tailored to target specific heavy metal species, rendering this method highly effective for the removal of specific contaminants (Egboduku et al., 2023). For example, cation exchange resins containing negatively charged functional groups, such as sulfonate (SO3⁻) groups, can selectively bind positively charged heavy metal cations like copper (Cu²⁺) and nickel (Ni²⁺) from wastewater. The metal cations exchange with the harmless cations (e.g., H⁺ or Na⁺) already present on the resin, achieving targeted heavy metal removal. Ion exchange offers several advantages, including high removal efficiencies for specific metal ions, the ability to regenerate the resin for reuse, and the potential for selective targeting of specific heavy metals. However, limitations also exist (Table 1) (Bolisetty et al., 2019). The effectiveness of this method can be influenced by the presence of competing ions in the wastewater, and the regeneration process can generate concentrated waste streams that require proper management (Oliomogbe et al., 2023). 367 I.A. Korode et al. / Nigerian Research Journal of Engineering and Environmental Sciences 10(2) 2025 pp. 363-375 2.2. Chemical Precipitation Chemical precipitation is a frequently utilized method that exploits the principle of transforming soluble heavy metal ions into insoluble precipitates (Bolisetty et al., 2019). This transformation effectively sequesters the metals, rendering them inert and facilitating their subsequent separation from the treated wastewater (Kato and Kansha, 2024). The successful application of this technique hinges on the selection of appropriate precipitating agents that exhibit a high affinity for the target heavy metals (Emegha et al., 2024a). These precipitating agents react with the metal ions in a carefully controlled stoichiometric manner, forming insoluble compounds that can be readily removed from the solution through sedimentation or filtration processes (Gunatilake, 2015). For instance, the removal of lead (Pb) ions from wastewater can be achieved through precipitation with sulfide (S²⁻) ions. The addition of a sodium sulfide (Na2S) solution to the wastewater would lead to the formation of highly insoluble lead sulfide (PbS) precipitates according to the following reaction: Pb²⁺ (aq) + S²⁻ (aq) → PbS(s) (Yu et al., 2020). The selection of precipitating agents can be tailored to target specific heavy metals. For example, hydroxides (OH⁻) are commonly employed for the precipitation of various metal cations, while phosphates can be effective for capturing certain heavy metal cations like lanthanum (La³⁺) and cerium (Ce³⁺). However, chemical precipitation has limitations (Table 1). It can generate a significant amount of sludge as a byproduct, requiring proper disposal considerations (Bolisetty et al., 2019). Additionally, the effectiveness of this method can be influenced by factors like pH, the presence of complexing agents that compete for metal binding, and the solubility of the formed precipitates. 2.3. Reverse Osmosis Reverse osmosis (RO) leverages a pressure-driven mechanism to achieve the separation of HMs from wastewater. This technique utilizes a semi-permeable membrane that allows water molecules to pass through under the application of pressure, while retaining dissolved contaminants, including heavy metal ions, on the pressurized side of the membrane (Bolisetty et al., 2019). In essence, RO acts as a sophisticated filter that employs pressure to force clean water through a barrier, leaving behind the unwanted heavy metals in the concentrated brine solution. While RO offers a highly effective means for removing a broad spectrum of contaminants, including heavy metals, it necessitates a significant energy input for its operation. The effectiveness of RO for heavy metal removal is primarily dependent on the membrane characteristics and the operating conditions. The membrane's rejection rate for specific heavy metal ions is a critical factor, with tighter membranes exhibiting higher rejection efficiencies. Additionally, factors like pressure, feedwater temperature, and the presence of other dissolved solutes can influence the performance of RO systems. While RO offers a versatile solution for removing a wide range of contaminants, it has limitations (Table 1). The high energy consumption associated with the pressure-driven process is a major concern. Additionally, the treatment process generates a concentrated brine stream containing the rejected HMs, requiring proper disposal or further treatment. 2.4. Electrodialysis Electrodialysis (ED) offers a unique approach to heavy metal removal from wastewater. This technique utilizes electrical driving forces to separate charged ions, including heavy metal cations. The ED system comprises an alternating arrangement of anion-selective and cation-selective membranes within a chamber containing the contaminated wastewater (Zhao et al., 2016). When an electric current is applied, the charged membranes act as selective barriers, allowing the passage of specific ions while repelling others (Ivanova et al., 2016). Cations, including heavy metal ions, migrate towards the cathode (negative electrode) through the cation-selective membranes, while anions migrate towards the anode (positive electrode) through the anion-selective membranes. This creates a demineralized stream depleted in heavy metals in the central compartment, while the concentrated solutions containing the removed heavy metals accumulate in the compartments adjacent to the electrode chambers. Electrodialysis offers several advantages, including the ability to remove a broad spectrum of ionic contaminants, its modular design allowing for scalability, and the potential for lower energy consumption compared to reverse osmosis in certain applications (Ivanova et al., 2016). However, limitations also exist (Table 1). The effectiveness of ED can be influenced by factors like the conductivity of the wastewater, the presence of competing ions, and the specific type of heavy metal being targeted. Additionally, the concentrated streams generated during the process require proper management. 368 I.A. Korode et al. / Nigerian Research Journal of Engineering and Environmental Sciences 10(2) 2025 pp. 363-375 Table 1: The primary disadvantages associated with conventional heavy metal remediation techniques Treatment methods Disadvantages References Ion exchange The approach is susceptible to changes in pH, carries a risk of fouling, and is not effective for high concentrations of metals. Senila et al., 2022; Chemical precipitation Sludge generation, extra operational cost for sludge disposal Kurniawan et al., 2006 Reverse Osmosis High operational costs, a lower pH than regular tap water—which could affect taste and potentially interact with plumbing over time—and the removal of essential minerals. Aziz et al., 2024; Onyenanu and Onyenanu, 2025 Electrodialysis High operational costs stemming from membrane fouling and substantial energy consumption. Onyenanu and Onyenanu, 2025 Although these conventional or traditional methods have been widely used, they come with certain limitations. The most common challenges encountered include the generation of secondary pollutants, high costs, time-consuming procedures, and overall inefficiency (Oliomogbe et al., 2023; Emegha et al., 2023). To overcome these problems, scientists are diligently working to develop more environmentally friendly and sustainable processes for wastewater decontamination. Recently, biosorption has captured significant scientific interest due to its ecological benefits and cost-effectiveness. 3. BIOSORPTION: A GREEN APPROACH TO HEAVY METAL REMOVAL Biosorption has emerged as a promising technique for the removal of heavy metals from wastewater. This method leverages the unique properties of biomass, a broad term encompassing both intact living cells and various derivatives of biological origin, including waste materials and charcoal (Yu et al., 2020; Emegha et al., 2023). Notably, biosorption can utilize both living and dead biomass as sorbents, offering a versatile approach to heavy metal remediation. Dead biomass offers a passive and metabolism-independent approach to heavy metal removal. These contaminants bind to the dead biomass through various mechanisms, including ionic interactions, chemical complexation, and physical adsorption (biosorption) (Carvalho Costa et al., 2021). The absence of metabolic activity simplifies the process compared to living biomass, making it less susceptible to complex biological interactions. The utilization of dead biomass for biosorption presents several advantages. Firstly, it eliminates concerns associated with potential toxicity towards the biomass itself. Additionally, dead biomass requires minimal maintenance and can be stored for extended periods without compromising its effectiveness. Furthermore, regeneration procedures are generally more feasible for spent dead biomass sorbents compared to living biomass (Carvalho Costa et al., 2021). The inherent properties of dead biomass also offer practical benefits. Dead biomass can be easily processed through methods like cutting and grinding to achieve a desirable particle size, facilitating efficient contact with the contaminant-laden wastewater. Moreover, dead biomass can function effectively across a wider range of environmental variables, offering greater operational flexibility in wastewater treatment processes. While dead biomass offers a simple and manageable approach to biosorption, it may possess a lower overall capacity for heavy metal removal compared to living biomass. Living biomass possesses the additional advantage of metabolic activity, which can contribute to the removal of pollutants through mechanisms like active transport and bioaccumulation within the cells (Zhang et al., 2020). Additionally, living biomass may exhibit enzymatic activity that can transform or degrade certain pollutants, potentially offering additional benefits beyond simple biosorption. An additional advantage of living biomass lies in its potential for biotransformation and biodegradation. Certain living biomass systems possess enzymatic activity that can transform or degrade the captured pollutants, offering a potential pathway for complete pollutant removal beyond simple biosorption (Wang et al., 2020; Emegha et al., 2024b). While offering enhanced removal capabilities, living biomass comes with its own set of drawbacks. Utilizing living biomass necessitates the implementation of culture systems, a consistent supply of nutrients for the organisms, and a method for cell harvesting after treatment. These requirements can significantly increase the overall cost of the process compared to using dead biomass (Carvalho Costa et al., 2021). It's important to note that dead biomass can also incur additional costs if it 369 I.A. Korode et al. / Nigerian Research Journal of Engineering and Environmental Sciences 10(2) 2025 pp. 363-375 undergoes chemical modification, carbonization, or grinding to improve its effectiveness. However, living biomass generally offers a simpler and more cost-effective method for separation from the treated wastewater. A crucial consideration to maximize the advantages of living biomass is the selection of organisms exhibiting high resistance to the toxic effects of the target pollutants. A critical aspect of optimizing a biosorption/bioaccumulation process that leverages living biomass lies in the strategic selection of the organisms employed. Microbial strains exhibiting superior tolerance to the target pollutant can demonstrably enhance the removal capacity. A recent study by Xu et al. (2020) exemplifies this principle. Their research demonstrated that a strain of Pseudomonas sp., specifically chosen for its multimetal resistance, proved significantly more effective for cadmium removal compared to its non-resistant counterpart when utilized as living biomass. This burgeoning interest in harnessing microorganisms as the foundation for developing biosorbents stems from their exceptional sorption capabilities and remarkable resilience to the deleterious effects of pollutants. A plethora of studies have explored the potential of fungi, bacteria, yeast, and microalgae against a diverse range of contaminants, yielding highly encouraging results (Xu et al., 2020). 3.1. Factors Affecting Biosorption Various elements influence the effectiveness of biosorption, including the nature of the biomass, temperature, ionic strength, pH, and the presence of free-floating cells. The application of chemical treatments, such as alkali, often enhances biosorption capacity, especially in certain microbial systems where chitin deacetylation leads to the creation of more effective chitosan-glucan complexes with a stronger affinity. Numerous characteristics and parameters that affect the biosorption, of HMs have been identified and briefly discussed (Figure 3) (Ukhurebor et al., 2023b; Oliomogbe et al., 2024; Ukhurebor et al., 2024). Figure 3: Factors affecting Biosorption processes (Ahmad and Zaidi, 2020). Solution pH exerts a profound influence on metal biosorption due to its impact on both the functional group charge on the biomass and the speciation of the metal ions. As the pH changes, the charge state of these functional groups also undergoes alterations, ultimately affecting the strength of the interaction with the metal ions. For the majority of metals existing primarily as cations in aqueous solutions, a more negatively charged biosorbent translates to a higher metal biosorption capacity. Therefore, a pH range of 7.0-8.0 is generally considered most favorable for metal biosorption (Blagojev et al., 2019). At lower pH values, competition for binding sites arises between hydrogen ions and metal ions. Conversely, higher pH values promote the precipitation of metal ions as hydroxides, diminishing the pool of free metal available for biosorption. This behavior is an exception for certain metals with dominant anionic forms, such as chromium, arsenic, or molybdenum (Blagojev et al., 2019). Under acidic conditions (pH 2.0-4.0), these anionic species are attracted to the positively charged biomass, leading to enhanced biosorption. Understanding these pH-driven mechanisms empowers researchers to tailor biosorption processes for 370 I.A. Korode et al. / Nigerian Research Journal of Engineering and Environmental Sciences 10(2) 2025 pp. 363-375 specific heavy metal profiles in wastewater streams. This targeted approach facilitates the development of more efficient and effective heavy metal remediation strategies. However, Salehi et al. (2020) developed a unique biosorbent by cross-linking chitosan magnetic beads and altering them with methionine-glutaraldehyde Schiff's base. The pH of biosorbents is commonly used for heavy metal removal. The resulting biosobent was then utilized to remove heavy metals. At optimal working conditions, including pH, the biosorbent had removal efficiencies of 172.4, 175.4, and 163.9 mg/g for Cu(II), Pb(II), and Cd(II), respectively. Oliomogbe and Emegha (2024) studied the green microalga Scenedesmus quadricauda biosorbent for the removal of copper and zinc ions from synthetic wastewater. The results demonstrated remarkable removal efficiency, with Scenedesmus quadricauda removing 81% copper and 73% zinc. Further tuning of pH and biosorbent dosage improved performance, with copper removal reaching 91% at pH below 6 and zinc removal at 82.85% at 0.5 g/L. The data indicate that Scenedesmus quadricauda is particularly well-suited for synthetic wastewater types with pH values close to neutral (about pH 5-6) and moderate Cu and Zn concentrations, where competition from other ions is low. Also, Oliomogbe et al. (2024) looked at the potential of Chlorella sorokiana, a green alga, biosorbent for removing Cu (II) and Zn (II) from synthetic wastewater. Batch studies were carried out at 28°C, pH (3–7), and a contact time of 60 minutes. Chlorella sorokiana shown promising biosorption capacity, removing 85% of Cu(II) and 75% of Zn(II), respectively. Further adjustment resulted in improved removal efficiencies at pH 5 (88.5% Cu(II) and 82% Zn(II)). At pH 4 and 6, removal effectiveness decreases, whereas pH 3 has the lowest removal efficiency. Daneshvar and Hosseini (2018) investigated the biosorption of Cr (VI) with a magnetic biosorbent. The resulting nano-biosorbent demonstrated good effectiveness in the removal of Cr (VI), with a removal efficiency of 92% with pH set at 5. Gouran-Orimi et al. (2018) examined the creation of a biosorbent utilizing functionalized polydopamine-coated zeolite. This resulting nano-biosorbent proved highly effective in nitrate removal, demonstrating exceptional efficiency with removal rates ranging from 98.5% to 99.8% at a pH of 3. Table 2 illustrates the removal of several heavy metals by a biosorbent at an optimized pH. Table 2: Heavy metal removal at various pH values Biosorbents Heavy metal removed pH References Silica-Coated core-shell nanoparticle Cu(II) 6 Plohl et al. 2019 Nanofibers Cr(VI) 7 Leung et al. 2016 Posidonia Oceanica Cd(II), Pb(II) and Cu(II) 7 Boubakri et al. 2017 Modified biopolymer chitosan Cr(VI) 1.5 Khalil et al. 2021 Beyond pH, temperature significantly influences biosorption, primarily impacting reaction rates. Higher temperatures generally accelerate the process by increasing both surface activity of the biosorbent and the kinetic energy of metal ions (sorbates) (Blagojev et al., 2019). The influence of temperature on the maximum biosorption capacity (MBC) of metals remains a topic of ongoing investigation. The dominant theory posits that biosorption is an endothermic process, absorbing heat from the surrounding environment (Libatique et al., 2020). Consequently, increasing temperature typically leads to an enhanced MBC. Higher temperatures may induce structural modifications that improve the biosorbent's affinity for metal ions. Also, elevated temperatures can disrupt existing bonds within the biosorbent, creating new sites for metal binding. However, exceptions exist. In exothermic biosorption processes, which release heat, a rise in temperature can lead to a decrease in MBC. This decline might be attributed to thermal damage of the biosorbent's surface, hindering its ability to effectively bind metals (Libatique et al., 2020). Understanding these competing effects of temperature is crucial for optimizing biosorption processes for specific metalbiosorbent combinations. The influence of temperature on biosorption capacity exhibits a dependence on the type of biomass employed. Living biomass displays a more pronounced response to temperature compared to dead biomass. In living cells, metabolic activity increases with temperature, reaching an optimum value. This heightened metabolic activity facilitates the uptake of metals into the cell interior, leading to a greater increase in biosorption capacity compared to dead biomass at elevated temperatures (Ahmad et al., 2018). However, exceeding this optimal temperature can damage living material, causing a sharper decline in biosorption capacity than observed with dead biomass. This showcases the importance of considering the type of biomass (living or dead) when optimizing biosorption processes for specific metals and operating temperatures. At 30°C, Olukanni et al. (2014) used Pseudomonas Aeruginosa to investigate heavy metal 371 I.A. Korode et al. / Nigerian Research Journal of Engineering and Environmental Sciences 10(2) 2025 pp. 363-375 biosorption, specifically cadmium, zinc, and silicate. The biosorbent achieved its maximum zinc biosorption capacity. The sorption remained constant as the temperature was raised. However, the optimal temperature for biosorption of heavy metals ranged from 30 ˚C to 50 ˚C for the three metals studied. Thus, the biosorption methodology was a superior option to the conventional methods in terms of material availability, costeffectiveness, and the absence of precipitates and slurry. Also, Ritixa and Monika (2015) assessed the biosorption ability of Bacillus licheniformis for copper and iron ions. Bacteria were cultured at various pH levels (3, 4, 5, 6, 7 and 8) and temperatures (30⁰C, 35⁰C, 40⁰C, 45⁰C, 50⁰C and 55⁰C). Bacteria were found to be effective at decontaminating Fe and Cu ions at pH 8, with 92% and 93%, respectively. Fe ions were removed 92% at 30⁰C, whereas Cu ions were removed 94% at 45⁰C. Metal ion removal was most effective at 30⁰C for iron and 45⁰C for copper, with 92% and 94% efficacy, respectively. The findings indicated that the living biomass of Bacillus licheniformis exhibited enhanced metal removal efficiency, which was influenced by temperature fluctuations. Another key factor influencing metal biosorption is ionic strength. Generally, a rise in ionic strength leads to a decrease in the amount of metal bound. This is because other cations present in the solution compete with the target metal ions for binding sites on the functional groups of the biosorbent (Barquilha et al., 2019). The competition among various cations in complex effluents highlights the necessity of selecting highly effective biosorbents. Biomass-derived materials are particularly suitable for this purpose due to their abundant and varied functional groups, such as carboxyl, hydroxyl, sulfate, phosphoryl, and amino groups. These groups enable strong interactions with metal ions, making these materials highly efficient for metal removal (Oliomogbe et al., 2024;). For example, algae-based biosorbents demonstrate high adsorption capacities, typically between 1 and 10 g/L (Oliomogbe et al., 2024;). Recent innovations have focused on enhancing the biosorption capabilities of microorganism-derived biomass through genetic modification. This approach has shown significant promise in improving metal removal efficiency. For instance, genetically engineering bacteria to express the EC20 protein, a synthetic phytochelatin, has resulted in improved biosorption of metals such as lead, zinc, copper, cadmium, manganese, nickel, and platinum. Similarly, incorporating a cadmium-binding protein from Lentinula edodes into Escherichia coli has significantly increased its capacity for cadmium biosorption (Dong et al., 2019). Moreover, genetically modifying Saccharomyces cerevisiae with metallothionein genes from Populus trichocarpa has produced strains with higher cadmium uptake compared to non-modified strains (De Oliveira et al., 2020). Similarly, Aranda-García et al (2020) investigated the variation in Ni2+ biosorption capacity as a function of biosorption time across NaCl ionic strengths ranging from 0.2 to 2000 mM. The kinetic profile of Ni2+ biosorption at an ionic strength of 0.2 mM shows no significant differences were observed in the Ni2+ biosorption capacities. This indicates that Ni2+ biosorption is not affected by a NaCl ionic strength of 0.2 mM. Conversely, the Ni2+ biosorption capacity gradually decreased as the ionic strength increased from 0.2 to 200 mM. However, no significant differences were observed at very high ionic strengths of 200 and 2000 mM. A finding that suggest that NaCl ionic strengths equal to or greater than 2 mM negatively affect the biosorption of Ni2+. Additionally, it has been shown that the ionic strength of NaCl has a detrimental effect on the biosorption of Ni2+ by grape stem waste (Villaescusa et al., 2004), filamentous fungi such as Rhizopus sp., Mucor sp., and Penicillium sp. (Mogollón et al., 1998), and barley straw (Thevannan et al., 2010). Furthermore, the NaCl ionic strength decreased the sorption capacity of other heavy metals, such as Cu2+ by HNO3-pretreated newspaper scraps, HNO3-pretreated maize spatha (Djemmoe et al. 2016), and by an exopolysaccharide of Wangia profunda (Zhou et al. 2009), as well as Cd2+ by an exopolysaccharide of Wangia profunda (Zhou et al. 2009), Pb2+ by Sargassum filipendula (Verma et al. 2016), and Cr6+ by lignin (Albadarin et al. 2011). Thus, biosorption offers the additional benefit of enabling the recovery of metals from contaminated solutions, thereby enhancing its appeal as a remediation method (Oliomogbe et al., 2024). This technique also holds potential for industrial-scale applications, where the biosorbent must be suitable for commercial modification and use. Fortunately, biomass-derived materials often possess these desirable properties, making them highly adaptable to various industrial applications (Zoroufchi Benis et al., 2020). 4. CONCLUSION Heavy metals pose a significant challenge for environmental remediation. Biosorption presents a potential solution for cleaning up polluted environments. Research on biosorption continues to demonstrate highly promising results for removing contaminants and heavy metals. The mechanism offers a wide array of possibilities and combinations, showcasing considerable adaptability in its application. Despite its potential,