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Biosorption
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Biosorption is a physiochemical process that occurs naturally in certain biomass which allows it to passively concentrate and bind contaminants onto its cellular structure.[1] Biosorption can be defined as the ability of biological materials to accumulate heavy metals from wastewater through metabolically mediated or physico-chemical pathways of uptake.[2] Though using biomass in environmental cleanup has been in practice for a while, scientists and engineers are hoping this phenomenon will provide an economical alternative for removing toxic heavy metals from industrial wastewater and aid in environmental remediation.

Environmental uses

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Pollution interacts naturally with biological systems. It is currently uncontrolled, seeping into any biological entity within the range of exposure. The most problematic contaminants include heavy metals, pesticides and other organic compounds which can be toxic to wildlife and humans in small concentration. There are existing methods for remediation, but they are expensive or ineffective.[3] However, an extensive body of research has found that a wide variety of commonly discarded waste including eggshells, bones, peat,[4] fungi, seaweed, crab shells,[5] yeast, baggase [6] and carrot peels [7] can efficiently remove toxic heavy metal ions from contaminated water. Ions from metals like mercury can react in the environment to form harmful compounds like methylmercury, a compound known to be toxic in humans. In addition, adsorbing biomass, or biosorbents, can also remove other harmful metals like: arsenic, lead, cadmium, cobalt, chromium and uranium.[8][9]

The idea of using biomass as a tool in environmental cleanup has been around since the early 1900s when Arden and Lockett discovered certain types of living bacteria cultures were capable of recovering nitrogen and phosphorus from raw sewage when it was mixed in an aeration tank.[10][11] This discovery became known as the activated sludge process which is structured around the concept of bioaccumulation and is still widely used in wastewater treatment plants today. It wasn't until the late 1970s when scientists noticed the sequestering characteristic in dead biomass which resulted in a shift in research from bioaccumulation to biosorption.[8]

Differences from bioaccumulation

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Though bioaccumulation and biosorption are used synonymously, they are very different in how they sequester contaminants:

Biosorption is a metabolically passive process, meaning it does not require energy, and the amount of contaminants a sorbent can remove is dependent on kinetic equilibrium and the composition of the sorbents cellular surface.[9] Contaminants are adsorbed onto the cellular structure.

Bioaccumulation is an active metabolic process driven by energy from a living organism and requires respiration.[9][12]

Both bioaccumulation and biosorption occur naturally in all living organisms [13] however, in a controlled experiment conducted on living and dead strains of bacillus sphaericus it was found that the biosorption of chromium ions was 13–20% higher in dead cells than living cells.[9]

In terms of environmental remediation, biosorption is preferable to bioaccumulation because it occurs at a faster rate and can produce higher concentrations.[9] Since metals are bound onto the cellular surface, biosorption is a reversible process whereas bioaccumulation is only partially reversible.[9]

Factors affecting performance

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Since biosorption is determined by equilibrium, it is largely influenced by pH, the concentration of biomass and the interaction between different metallic ions.[3]

For example, in a study on the removal of pentachlorophenol (PCP) using different strains of fungal biomass, as the pH changed from low pH to high pH (acidic to basic) the amount of removal decreased by the majority of the strains, however one strain was unaffected by the change.[14] In another study on the removal of copper, zinc and nickel ions using a composite sorbent as the pH increased from low to high the sorbent favored the removal of copper ions over the zinc and nickel ions.[15] Because of the variability in sorbent this might be a drawback to biosorption, however, more research will be necessary.

Common uses

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Even though the term biosorption may be relatively new, it has been put to use in many applications for a long time. One very widely known use of biosorption is seen in activated carbon filters. They can filter air and water by allowing contaminants to bind to their incredibly porous and high surface area structure. The structure of the activated carbon is generated as the result of charcoal being treated with oxygen.[16] Another type of carbon, sequestered carbon, can be used as a filtration media. It is made by carbon sequestration, which uses the opposite technique as for creating activated carbon. It is made by heating biomass in the absence of oxygen. The two filters allow for biosorption of different types of contaminants due to their chemical compositions—one with infused oxygen and the other without.

Figure 1. A Sorption Column Using Biosorbents alt text
Figure 1. A Sorption Column Using Biosorbents.

In industry

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Many industrial effluents contain toxic metals that must be removed. Removal can be accomplished with biosorption techniques. It is an alternative to using man-made ion-exchange resins, which cost ten times more than biosorbents.[17] The cost is so much less, because the biosorbents used are often waste from farms or they are very easy to regenerate, as is the case with seaweed and other unharvested biomass.

Industrious biosorption is often done by using sorption columns as seen in Figure 1. Effluent containing heavy metal ions is fed into a column from the top. The biosorbents adsorb the contaminants and let the ion-free effluent to exit the column at the bottom. The process can be reversed to collect a highly concentrated solution of metal contaminants. The biosorbents can then be re-used or discarded and replaced.

References

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Biosorption

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Biosorption is a passive, metabolism-independent process in which biological materials, such as microorganisms, algae, fungi, or derived biomass, adsorb and accumulate pollutants—primarily heavy metals, dyes, and organic compounds—from aqueous solutions through surface binding mechanisms like ion exchange, complexation, and electrostatic interactions.[1] This technique leverages the functional groups on cell walls, including carboxyl, hydroxyl, amino, and phosphate groups, to achieve high selectivity and efficiency in pollutant removal, often outperforming traditional physicochemical methods in cost and environmental compatibility.[2] Originating from early 20th-century wastewater treatments like activated sludge processes, biosorption gained prominence in the late 1970s as a sustainable alternative for addressing heavy metal contamination in industrial effluents.[2] Key mechanisms of biosorption include physical adsorption via van der Waals forces, chemical sorption through covalent bonding or chelation, and ion exchange where metal ions replace lighter ions on the biosorbent surface, all influenced by factors such as pH (typically optimal at 3.0–6.5), temperature, and initial pollutant concentration.[3] Dead biomass is commonly used for its simplicity and avoidance of toxicity issues, while living biomass can incorporate active processes like bioaccumulation for enhanced removal, though this requires metabolic maintenance.[1] Applications span municipal and industrial wastewater treatment, metal recovery from mining operations, and remediation of radionuclides, with notable examples including the removal of chromium(VI) at capacities up to 330.84 mg/g using brown algae like Sargassum horneri.[2] The advantages of biosorption lie in its eco-friendliness, low operational costs, and utilization of abundant waste materials—such as agricultural residues, sewage sludge, or microbial cultures—as biosorbents, promoting circular economy principles.[1] Advances from 2019–2020 focused on biomass modification through chemical treatments or immobilization in matrices like calcium alginate to improve reusability and capacity, achieving capacities as high as 698.48 mg/g for dyes like methylene blue with seaweed-derived sorbents.[1] Subsequent developments as of 2025 include surfactant-modified biosorbents for enhanced efficiency and microbial approaches targeting pharmaceuticals and heavy metals from battery waste.[4][5] Despite these benefits, challenges remain in scaling up for commercial use and optimizing kinetics, modeled via isotherms like Langmuir or Freundlich and pseudo-second-order equations.[3] Overall, biosorption represents a versatile, green technology integral to modern environmental engineering for mitigating water pollution.[2]

Definition and Fundamentals

Definition and Principles

Biosorption is defined as the ability of biological materials, including dead or inactive biomass, to accumulate heavy metals or other pollutants from aqueous solutions through passive physico-chemical processes, independent of the organism's metabolic activity.[2] This process relies on the inherent affinity of biological surfaces for sorbates, enabling the sequestration of contaminants without requiring cellular energy input or viability of the biosorbent.[6] Unlike bioaccumulation, which involves active transport and metabolism in living cells, biosorption is a rapid, reversible adsorption mechanism suitable for non-living materials derived from microorganisms, plants, or algae.[7] The fundamental principles of biosorption center on interactions such as surface binding, ion exchange, and complexation between pollutant ions and functional groups on the biomass surface. For instance, heavy metal ions like Cd²⁺ and Pb²⁺ bind to electron-donating groups such as carboxyl (-COOH) and hydroxyl (-OH) on the biosorbent, forming stable complexes through electrostatic attraction or covalent linkages.[8] These mechanisms exploit the heterogeneous chemical composition of biological materials, including polysaccharides, proteins, and lipids, which provide a diverse array of binding sites.[9] Ion exchange often involves the displacement of lighter ions (e.g., H⁺ or Ca²⁺) from the biomass by heavier metal cations, enhancing selectivity for toxic pollutants.[5] In practice, the biosorption process begins with the contact of the biosorbent with the contaminated solution, typically under agitation to facilitate mass transfer, followed by the achievement of equilibrium where the rate of adsorption equals desorption.[10] This equilibrium immobilizes the pollutants on the biomass surface without external energy, allowing for straightforward separation of the loaded biosorbent from the treated effluent.[11] Originating from pioneering studies in the 1980s on microbial metal uptake, particularly by researchers like Bohumil Volesky, biosorption has evolved into a recognized eco-friendly alternative to conventional chemical treatments for wastewater remediation.[12]

Historical Context

The concept of biosorption emerged in the late 1970s and early 1980s from studies on microbial tolerance to heavy metals, where researchers observed that certain microorganisms could passively accumulate metals through surface binding rather than metabolic processes.[12] Pioneering work by Bohumil Volesky and colleagues demonstrated this phenomenon using waste microbial biomass for uranium and thorium removal, marking the initial formulation of biosorption as a distinct process. A key example was the 1981 study by Tsezos and Volesky, which explored biosorption using fungal biomass, highlighting its potential for treating nuclear waste effluents. During the 1990s, biosorption research expanded toward practical applications, with efforts to scale up processes for industrial wastewater treatment, including pilot-scale systems for metal recovery from mining effluents.[13] Seminal reviews, such as Volesky and Holan's 1995 publication in Biotechnology Progress, synthesized mechanisms and biosorbent performance, influencing subsequent developments and establishing biosorption as a viable alternative to chemical methods.[14] In the 2000s, focus shifted to genetic engineering of biosorbents, with modifications to bacteria and fungi enhancing metal-binding capacities through overexpressed metallothioneins and surface proteins.[15] Post-2010 advancements integrated nanotechnology with biosorption, such as hybrid nanocomposites combining algal biomass with nanoparticles to improve selectivity and efficiency for heavy metal removal.[16] Recent 2020s studies emphasize sustainable biosorbents derived from waste biomass, like agricultural residues and food industry byproducts, promoting circular economy approaches. Recent studies (2024–2025) have explored surfactant-enhanced biosorption and agroindustrial waste-derived sorbents for improved efficiency in dye and heavy metal removal.[1][17] This evolution reflects a transition from living cells, which faced toxicity limitations, to dead or inactivated biomass for robust, non-metabolic uptake.[18][10]

Mechanisms and Processes

Core Mechanisms

Biosorption primarily involves several physico-chemical mechanisms that enable the passive binding of pollutants, such as heavy metals, to the surfaces of biological materials without requiring cellular energy. These mechanisms include physical adsorption, where solutes adhere to the biomass surface through weak van der Waals forces or hydrophobic interactions; chemical adsorption, involving stronger chemical bonds; ion exchange, in which metal cations displace lighter ions like protons (H⁺) or calcium (Ca²⁺) from binding sites on the biosorbent; surface complexation, where metal ions form coordination complexes or chelates with ligands such as amino (-NH₂) or sulfhydryl (-SH) groups; and microprecipitation, wherein metal ions precipitate as insoluble compounds directly on the biomass surface. These processes occur predominantly on the cell walls of biosorbents, leveraging their inherent polymeric structures.[14][19] The interactions driving biosorption are mediated by specific functional groups on the biosorbent, including carboxyl (-COOH), hydroxyl (-OH), and amino (-NH₂) groups, which facilitate binding through electrostatic attraction—particularly when oppositely charged—or covalent bonding for more stable associations. For instance, at acidic pH, protonated groups like -COOH can exchange with positively charged metal ions, while at higher pH, deprotonated sites enable electrostatic uptake of anions or chelation of cations. A notable example is the biosorption of hexavalent chromium (Cr(VI)), where biomass such as the brown alga Sargassum species not only adsorbs Cr(VI) but also reduces it to the less toxic trivalent form (Cr(III)) via electron transfer from surface functional groups, enhancing overall removal efficiency. This reduction is confirmed through techniques like X-ray photoelectron spectroscopy (XPS), revealing shifts in chromium oxidation states post-biosorption.[14][19] Unlike metabolically active bioaccumulation, which relies on energy-dependent transport across cell membranes, biosorption is metabolically independent, occurring with dead or inactive biomass through passive diffusion and rapid attainment of equilibrium. This passivity allows for quick uptake kinetics, often within minutes to hours, and reversibility under changing conditions, distinguishing it from slower, enzyme-mediated active transport processes. The synergy of multiple mechanisms—such as combined ion exchange and surface complexation—amplifies biosorption capacity, with certain fungal biomasses like Rhizopus arrhizus achieving uptakes of 100-300 mg/g for heavy metals, representing up to 25-30% of the biomass dry weight. This multi-faceted approach underscores biosorption's versatility for pollutant sequestration.[14][19]

Biosorption Kinetics and Isotherms

Biosorption kinetics describe the rate at which sorbate ions are removed from solution by biosorbents, providing insights into the controlling mechanisms and time required for equilibrium. These models are essential for designing efficient biosorption systems, as the process is often rate-limited by diffusion steps, including external film diffusion, intraparticle diffusion, and adsorption onto active sites. Common kinetic models include pseudo-first-order, pseudo-second-order, and intraparticle diffusion, which are applied to experimental data to determine rate constants and predict uptake over time. In heavy metal biosorption studies, such as those involving lead (Pb²⁺) and cadmium (Cd²⁺), these models reveal that chemisorption frequently dominates, leading to better fits with pseudo-second-order kinetics.[20] The pseudo-first-order model, originally derived from solid-liquid phase reaction kinetics, assumes that the rate of sorbate occupation of biosorption sites is proportional to the number of unoccupied sites. It is expressed in nonlinear form as:
qt=qe(1−e−k1t) q_t = q_e (1 - e^{-k_1 t})
where $ q_t $ is the amount sorbed at time $ t $ (mg/g), $ q_e $ is the equilibrium sorption capacity (mg/g), and $ k_1 $ is the rate constant (min⁻¹). To derive this, start with the differential equation $ \frac{dq_t}{dt} = k_1 (q_e - q_t) $, which integrates to the logarithmic linear form $ \log(q_e - q_t) = \log q_e - \frac{k_1 t}{2.303} $ for parameter estimation. This model applies best to physisorption-dominated processes or initial rapid uptake phases but often underestimates equilibrium capacities in chemisorption scenarios, as seen in biosorption of Cu²⁺ by algal biomass where it yielded lower correlation coefficients compared to other models.[20][21] In contrast, the pseudo-second-order model assumes chemisorption control, where the rate is proportional to the square of available sites, suitable for valence force interactions between sorbate and biosorbent. The nonlinear equation is:
qt=k2qe2t1+k2qet q_t = \frac{k_2 q_e^2 t}{1 + k_2 q_e t}
Derivation begins with $ \frac{dq_t}{dt} = k_2 (q_e - q_t)^2 $, rearranging to $ \frac{dq_t}{(q_e - q_t)^2} = k_2 dt $, and integrating from 0 to $ t $ and 0 to $ q_t $, yielding the linear form $ \frac{t}{q_t} = \frac{1}{k_2 q_e^2} + \frac{t}{q_e} $ for plotting $ t/q_t $ versus $ t $. This model is widely adopted in biosorption due to its high fit (R² > 0.99) for heavy metals like Pb²⁺ on fungal biomass, indicating chemisorption dominance, and initial sorption rates $ h = k_2 q_e^2 $ that highlight rapid equilibrium attainment.[20][22] The intraparticle diffusion model addresses mass transfer limitations within the biosorbent, assuming sorption rate is proportional to the square root of time. It is given by:
qt=kpt1/2+C q_t = k_p t^{1/2} + C
where $ k_p $ is the intraparticle diffusion rate constant (mg/g·min^{1/2}) and $ C $ relates to boundary layer thickness. Derived from Fick's second law for radial diffusion in spherical particles, multi-linear plots of $ q_t $ versus $ t^{1/2} $ identify stages: external diffusion (steep slope), intraparticle diffusion (gradual), and equilibrium (plateau). This model is applied when diffusion controls the rate, as in Pb²⁺ biosorption by bacterial biomass, where $ k_p $ values decrease with increasing initial concentration, confirming diffusion as a rate-limiting step in heterogeneous biosorbents.[20] Biosorption isotherms model equilibrium distribution of sorbate between solution and biosorbent, enabling prediction of maximum uptake and surface behavior. These are crucial for scaling processes, as they indicate saturation points; for instance, in heavy metal studies, isotherms fit data to estimate capacities, often revealing favorable adsorption (separation factor <1). Key models include Langmuir, Freundlich, and Temkin, selected based on surface homogeneity and interaction assumptions.[20] The Langmuir isotherm assumes monolayer adsorption on homogeneous sites with no lateral interactions, deriving from ideal localized adsorption. The nonlinear form is:
qe=qmKLCe1+KLCe q_e = \frac{q_m K_L C_e}{1 + K_L C_e}
where $ q_m $ is the maximum capacity (mg/g), $ K_L $ is the Langmuir constant (L/mg), and $ C_e $ is equilibrium concentration (mg/L). Linearization as $ \frac{C_e}{q_e} = \frac{1}{q_m K_L} + \frac{C_e}{q_m} $ allows parameter calculation via plotting. It applies to uniform biosorbent surfaces, yielding q_m up to approximately 126 mg/g for Pb²⁺ on acid-modified rice husk, and the dimensionless separation factor $ R_L = \frac{1}{1 + K_L C_0} $ (0 < R_L < 1) confirms favorable biosorption.[20][23] The Freundlich isotherm, an empirical model for heterogeneous surfaces, assumes multilayer adsorption with exponentially decreasing energy. It is:
qe=KFCe1/n q_e = K_F C_e^{1/n}
where $ K_F $ (mg/g·(L/mg)^{1/n}) relates to capacity and $ 1/n $ (0 < 1/n < 1) indicates intensity. The linear form $ \log q_e = \log K_F + \frac{1}{n} \log C_e $ is used for fitting. Suitable for non-ideal biosorption like Cd²⁺ on algal biomass, it explains varying affinities on diverse functional groups, though it lacks a finite saturation limit.[20][24] The Temkin isotherm considers uniform adsorption energy distribution and indirect sorbate interactions, assuming heat decreases linearly with coverage. The equation is:
qe=RTbTln⁡(KTCe) q_e = \frac{RT}{b_T} \ln(K_T C_e)
where $ b_T $ (J/mol) relates to heat, $ K_T $ (L/g) is the equilibrium binding constant, $ R $ is the gas constant (8.314 J/mol·K), and $ T $ is temperature (K). Linearized as $ q_e = B \ln K_T + B \ln C_e $ with $ B = RT/b_T $, it applies to systems with interaction effects, such as Zn²⁺ biosorption on phosphoric acid-modified rice husk, where it estimates sorption heat around 25 J/mol, indicating physisorption.[20][24] In practice, model selection relies on statistical fits (e.g., R², chi-square), with pseudo-second-order and Langmuir often prevailing in heavy metal biosorption due to chemisorption and monolayer tendencies, aiding in process optimization without delving into underlying mechanisms.[20][21]

Biosorbents and Materials

Natural Biological Biosorbents

Natural biological biosorbents encompass unmodified biomass derived from microorganisms and agricultural byproducts, offering cost-effective options for heavy metal sequestration due to their abundant functional groups and natural availability.[25] These materials are categorized primarily by their biological origins, including bacteria, fungi, algae, and yeast, each exhibiting distinct cell wall compositions that facilitate ion binding through mechanisms such as ion exchange and complexation.[5] Agricultural wastes, such as rice husk and peanut shells, further extend this category as lignocellulosic biosorbents with inherent porosity and low acquisition costs.[26] Bacterial biosorbents, like Bacillus subtilis, leverage their peptidoglycan-rich cell walls to provide high surface areas and numerous binding sites for metals such as lead and cadmium.[27] This gram-positive bacterium demonstrates biosorption capacities of approximately 60-100 mg/g for Pb(II) under optimal conditions, attributed to its robust exopolysaccharide layers that enhance metal affinity.[28] Fungal biosorbents, exemplified by Aspergillus niger, utilize chitin and glycoproteins in their cell walls to target radionuclides like uranium(VI).[29] Algal biosorbents, such as Sargassum species, rely on alginate and carboxyl groups in their cell walls for high-affinity binding, with reported capacities up to 100-200 mg/g for various metals including Pb(II) and Cu(II).[30] Yeast biosorbents, including Saccharomyces cerevisiae, employ mannoproteins and chitin in their walls to sorb metals like chromium(VI), removing up to 99.66% from effluents at neutral pH.[31] The efficacy of these biosorbents often favors dead biomass over living cells, as the former enables passive, metabolism-independent sorption without the need for nutrient supply or toxicity concerns, while allowing regeneration and reuse through desorption.[32] Living biomass, conversely, can incorporate active bioaccumulation but requires controlled environments to sustain viability.[1] For instance, dead algal biomass exhibits enhanced stability in continuous systems compared to living counterparts.[33] Biosorption capacities vary, with algae demonstrating up to approximately 23 mg/g for Cd(II) in brown seaweed like Sargassum fusiforme.[34] Agricultural wastes serve as economical natural biosorbents; rice husk, rich in silica and lignin, achieves 90% removal of Pb(II) and Cr(III) in lab-scale tests from synthetic wastewater.[35] Similarly, peanut shells, with their high cellulose content, enable over 90% Cd(II) removal when pretreated minimally, as shown in 2021 studies on ionic liquid-enhanced variants, though unmodified forms retain substantial efficacy.[36] These wastes highlight the scalability of biosorption using agro-residues abundant in developing regions.[37] Preparation of natural biological biosorbents typically involves straightforward processes like oven-drying at 60°C to inactivate cells and grinding to uniform particles, preserving native functional groups without chemical alterations.[25] Immobilization in alginate beads or biofilms further enhances mechanical durability for practical applications, as seen with entrapped Sargassum achieving 100% Pb(II) removal at 200 mg/L.[33] Such methods ensure biosorbents remain unmodified while improving handling and reusability.[38]

Engineered and Modified Biosorbents

Engineered biosorbents are created by altering natural biological materials through targeted modifications to enhance their adsorption capacity, selectivity, stability, and reusability for heavy metal removal from aqueous solutions. These modifications address limitations of unmodified biosorbents, such as low mechanical strength and difficulty in separation, by introducing functional groups, improving surface area, or enabling easy recovery.[39] Developments in the 2010s and 2020s have focused on sustainable, low-cost approaches, drawing from agricultural and microbial wastes to produce high-performance materials suitable for industrial-scale wastewater treatment.[40] Chemical modifications involve treating biosorbents with acids, bases, or reagents to alter surface functional groups, thereby increasing binding sites and selectivity for specific ions. For instance, acid treatment protonates carboxyl and amino groups, enhancing attraction to anionic metals like Cr(VI), while alkaline treatment deprotonates sites for cationic metals. Surface grafting, such as attaching polyethyleneimine, introduces additional amine groups for improved chelation. A representative example is sulfuric acid-treated acorn waste, which achieved a Pb(II) adsorption capacity of 96.8 mg/g at optimal conditions, following pseudo-second-order kinetics, outperforming untreated biomass by increasing surface negativity.[41] Another approach, esterification via poly(amic acid) modification of biomass, boosts selectivity for Pb(II) and Cd(II) by esterifying carboxyl groups to form more stable complexes with target ions.[42] Acid-modified waste fungal biomass, such as that from Aspergillus niger, has demonstrated high Cr(VI) removal efficiency at low pH and short contact times.[43] Physical modifications primarily employ immobilization techniques to encapsulate biosorbents in supportive matrices, improving durability and facilitating separation without compromising adsorption efficiency. Common methods include entrapment in calcium alginate beads, where microbial cells or biomass are mixed with sodium alginate and crosslinked with CaCl₂, yielding mechanically robust beads with high reusability. For example, Bacillus subtilis immobilized in calcium alginate beads exhibited a Cd(II) capacity of 251.91 mg/g and maintained performance over 5 cycles, with only minor capacity loss due to stable bead integrity.[38] Similarly, Trichoderma viride entrapped in alginate achieved 40.1% Cr(VI) removal and 75% Ni(II) recovery across 5 cycles, enhancing reusability compared to free biomass.[38] These techniques often result in 9-10% higher efficiency than free cells for metals like Cu(II) and Zn(II), as immobilization prevents biomass aggregation and leaching.[38] Biological modifications leverage genetic engineering to overexpress metal-binding proteins or transporters in microorganisms, creating highly selective biosorbents with enhanced intracellular sequestration. Bacteria like Escherichia coli are engineered to express heterologous genes, such as metallothioneins (MTs) or specific permeases, increasing uptake without toxicity. A key example is E. coli modified with the NiCoT permease from Helicobacter pylori, achieving 4.8 mg/g dry weight Ni(II) accumulation from 50 μM solutions, with improved selectivity over native strains by facilitating targeted ion transport.[40] Another advancement involves overexpressing Pisum sativum MT in Serratia marcescens combined with MerT/P transporters, reducing Hg(II) from 2 g/L to 6.3 ng/L through efficient import and binding.[40] For Cd(II), Lactobacillus plantarum MntA transporter in E. coli lowered concentrations from 1 mg/L to 0.2 mg/L, demonstrating up to 80% higher accumulation via overexpressed surface-binding proteins.[40] These 2010s innovations, including extremophile chassis like Deinococcus radiodurans for uranium precipitation (90% efficiency), enable operation in harsh conditions.[40] Notable advancements include magnetic biosorbents, developed in the 2010s, where iron oxide nanoparticles (e.g., Fe₃O₄) are impregnated into biomass for rapid magnetic separation post-adsorption. Fe₃O₄-impregnated biochar composites removed up to 95% Cd(II) and Cr(VI) from wastewater, with surface area increases of 20-50% over unmodified versions, facilitating easy recovery without filtration.[39] Regeneration of modified biosorbents is typically achieved via acid desorption, such as 0.2 M HCl, restoring 90-94% capacity for metals like Cu(II) and Cd(II) over 9-10 cycles before significant decline.[44] Hybrid biosorbents combining biomass with activated carbon further elevate performance; for instance, date seed waste-Ganoderma lucidum hybrids attained 365.9 mg/g Pb(II) capacity at pH 4.5, leveraging synergistic porosity and functional groups for >98% removal in combined systems.[45] Nanoparticle-coated algal composites, like Fe₂O₃-algal biomass, have shown enhanced Cu(II) biosorption, with capacities doubling relative to unmodified algae due to increased active sites and magnetic recoverability.[46]

Influencing Factors

Physicochemical Factors

The pH of the solution profoundly influences biosorption efficiency by governing the speciation of metal ions and the ionization state of functional groups on the biosorbent surface, such as carboxyl, hydroxyl, and amino groups. In the optimal pH range of 4-6 for most heavy metals (e.g., Cu²⁺, Pb²⁺, Cd²⁺, and Zn²⁺), deprotonation of these sites imparts a negative charge, promoting electrostatic attraction, ion exchange, and complexation with cationic pollutants.[47] At lower pH levels, excessive H⁺ ions protonate the binding sites and compete directly with metal cations, significantly diminishing uptake. Temperature exerts an inverse effect on biosorption capacity in many systems, with optimal performance typically observed between 20°C and 30°C, where molecular interactions between pollutants and biosorbents are maximized without thermal disruption. Beyond this range, elevated temperatures weaken sorbate-sorbent bonds—often due to the exothermic nature of the process—and can reduce efficiency by desorbing bound metals or altering biosorbent structure.[48] This temperature dependence aligns with the Arrhenius equation framework, which relates the process rate constant to activation energy (E_a), illustrating how moderate temperatures enhance diffusion and collision frequency while excessive heat favors desorption over adsorption.[33] The initial pollutant concentration drives the mass transfer gradient in biosorption, with higher levels accelerating initial uptake rates by increasing the availability of metal ions for binding. However, as concentrations rise, binding sites on the biosorbent saturate more rapidly, limiting overall capacity and shifting the process toward equilibrium sooner; for instance, lead(II) biosorption by immobilized biomass shows positive uptake correlation up to 200 mg/L before plateauing due to site exhaustion.[33] Ionic strength further modulates these interactions through competition from co-ions like Na⁺, which screen electrostatic forces or vie for sites, typically reducing heavy metal binding by 20-30%; studies on lignocellulosic sorbents report up to 25% sorption decline in solutions with conductivities of 300-1500 μS/cm.[49]

Operational and Environmental Factors

The biosorbent dosage significantly influences the efficiency of the biosorption process, with higher dosages generally increasing the percentage of pollutant removal due to the greater availability of binding sites and surface area.[5] For instance, increasing the dosage from 0.1 to 2.0 g per 50 mL can elevate metal ion adsorption from 72% to 89.2%.[50] Optimal dosages typically range from 2 to 5 g/L, balancing removal efficiency and practicality in laboratory and pilot-scale applications.[44] However, excessively high dosages may lead to particle aggregation, which reduces the effective surface area available for sorption and can decrease overall uptake capacity.[51] Contact time is another critical operational parameter, as biosorption is a time-dependent process where initial rapid uptake slows until equilibrium is reached. Equilibrium is commonly achieved within 30 to 120 minutes, depending on the biosorbent type and pollutant concentration, after which further exposure yields minimal additional removal.[52] Agitation during contact enhances mass transfer by reducing boundary layer resistance around the biosorbent particles, thereby accelerating the rate of pollutant binding and improving overall efficiency.[53] Moderate agitation speeds promote optimal sorbate-sorbent interactions without causing excessive shear that might disrupt biosorbent integrity.[54] Environmental factors, particularly in real-world applications, can substantially impact biosorption performance by introducing interferences. The presence of competing ions, such as sodium or calcium, can reduce sorption efficiency by 10-40% through site competition, as observed in simulated wastewaters containing multiple metal ions.[55] Similarly, organic matter in effluents forms complexes with target pollutants or blocks binding sites, further diminishing removal rates in complex matrices like industrial wastewater.[56] These matrix effects highlight the need for pre-treatment or process adjustments to mitigate interference in practical settings. Biosorption systems operate in either batch or continuous modes, each with distinct operational considerations affecting performance. Batch systems allow for controlled, discrete treatment cycles ideal for laboratory optimization, while continuous fixed-bed columns enable scalable, uninterrupted processing but are sensitive to flow dynamics. In column setups, lower flow rates of 1-5 mL/min extend breakthrough time—the point at which effluent concentration reaches a predefined threshold—by providing sufficient residence time for effective sorption, whereas higher rates accelerate saturation and reduce overall capacity.[57][58]

Applications and Uses

Environmental Remediation

Biosorption has emerged as a vital technique for restoring polluted water bodies, including rivers and lakes impacted by heavy metal discharges from mining operations. Algal biosorbents, such as species from the genera Chlorella and Scenedesmus, leverage their cell wall polysaccharides and proteins to passively adsorb metals like copper, lead, cadmium, and zinc through ion exchange and complexation mechanisms. These biosorbents have demonstrated high removal efficiencies for heavy metals in various effluents.[59] In soil remediation, immobilized biosorbents enable in-situ treatment of contaminants such as arsenic, minimizing excavation and promoting ecological recovery. Microbial biomass, such as bacteria or fungi encapsulated in alginate beads or attached to carriers like biochar, is incorporated into contaminated soils to facilitate biosorption and subsequent leaching of arsenic into treatable forms, thereby lowering its mobility and bioavailability. Studies indicate potential effectiveness of such immobilized systems in reducing arsenic levels.[60][61] Recent initiatives in the 2020s, particularly in India, have applied microbial biomass for biosorption of dyes and heavy metals from industrial discharges entering natural water systems, supporting broader environmental restoration efforts. For instance, bacterial species have been used in studies to treat textile and mining effluents, achieving substantial removal of dyes and metals.[62][63] A key advancement in biosorption applications involves its integration with constructed wetlands, where microbial and algal biosorbents in the rhizosphere complement phytoremediation by plants like Phragmites australis for comprehensive heavy metal cleanup. In these hybrid systems, biosorption contributes significantly to total metal removal through substrate-associated biofilms, enhancing overall efficiency in treating stormwater and agricultural runoff while fostering habitat restoration.[64][65] As of 2025, recent studies have explored hybrid microbial-algal systems in wetlands for improved scalability in heavy metal remediation.[66]

Industrial and Wastewater Treatment

Biosorption plays a crucial role in treating industrial wastewater streams contaminated with heavy metals, dyes, and other pollutants, offering an eco-friendly alternative to conventional methods in controlled manufacturing environments. In the textile industry, effluents laden with synthetic dyes and metals like copper and chromium are effectively remediated using biosorbents such as immobilized algae and treated agricultural wastes; for instance, alkali-modified sunflower seed hulls have achieved significant dye removal from textile wastewater through surface complexation and ion exchange mechanisms.[67] Similarly, in mining operations, biosorption addresses acid mine drainage by sequestering metals like copper from leached tailing solutions, with fungal and bacterial biomass demonstrating high uptake capacities in continuous flow systems.[68] Electroplating wastewater, rich in chromium and zinc, benefits from macroalgal biosorbents like Ecklonia sp., which remove these ions effectively in batch and column setups via passive binding to cell wall functional groups.[69] Bacterial biosorbents, such as immobilized Bacillus laterosporus, have demonstrated nickel(II) removal from aqueous solutions.[70] Beyond remediation, biosorption facilitates the recovery of valuable metals in industrial processes, enhancing resource efficiency. Baker's yeast (Saccharomyces cerevisiae) exemplifies this by selectively biosorbing gold(III) from acidic cyanide leaching solutions derived from mining and e-waste processing, reducing Au(III) to metallic gold with near-complete recovery in neutral to low-pH conditions.[71] This approach integrates well into zero-liquid discharge (ZLD) systems, where biosorption concentrates metals for downstream recovery, minimizing liquid waste and enabling water reuse in closed-loop operations, as demonstrated in pilot-scale setups treating metal-laden streams.[72] Pilot-scale implementations underscore biosorption's scalability for wastewater treatment. In Europe, post-2015 projects like the BIOMETAL demonstration plant have utilized biosorption-based systems to treat electroplating and mining effluents, achieving effective metal removal in continuous operations. Modified algal biomass, such as in the ALGBIO initiative, has been deployed in pilot facilities for municipal and industrial wastewater, processing volumes up to 1000 m³/day while simultaneously capturing CO₂ and nutrients.[73] These systems highlight biosorption's potential for large-scale adoption, with capacities supporting industrial demands without excessive energy input. Economically, biosorption offers 30-50% cost savings over ion exchange resins due to lower material prices ($10-15/kg for biosorbents versus $30-50/kg for resins) and reduced operational expenses.[74] Furthermore, biosorbents can be regenerated using dilute acids or bases, permitting reuse in 5-10 cycles with minimal capacity loss, further enhancing sustainability.[44]

Comparisons and Evaluations

Differences from Bioaccumulation

Biosorption and bioaccumulation represent two distinct mechanisms for the removal or uptake of heavy metals and other pollutants by biological materials, differing fundamentally in their biological requirements and operational characteristics. Biosorption is a passive, metabolism-independent process that involves the binding of contaminants to the surface of non-living biomass, such as dead algal or bacterial cells, achieving equilibrium typically within minutes to hours.[75] In contrast, bioaccumulation is an active, metabolism-dependent process occurring in living organisms, where pollutants are transported across cell membranes and accumulated intracellularly over extended periods, often spanning days to weeks.[75] This distinction arises because biosorption relies on physicochemical interactions like ion exchange and adsorption on cell walls, without requiring cellular energy, whereas bioaccumulation integrates with the organism's metabolic pathways, including transport proteins and enzymatic processes.[76] A key process difference lies in the localization and reversibility of pollutant uptake: biosorption primarily occurs extracellularly on the biomass surface, allowing for potential regeneration of the sorbent and avoiding intracellular toxicity since the cells are inactivated. Bioaccumulation, however, results in intracellular storage, which can lead to toxic effects on the living organism and is generally less reversible due to integration into metabolic cycles.[76] For instance, in environmental remediation contexts, biosorption using dead algal biomass enables safe external binding of lead ions (Pb²⁺) without risking cellular damage, as demonstrated in studies with species like Sargassum sp. achieving high uptake capacities through surface mechanisms.[76] Conversely, bioaccumulation in living fish involves Pb²⁺ uptake through gills, ingestion of contaminated prey, and internal sequestration in organs like the liver and kidneys, potentially causing physiological stress and oxidative damage.[77] These differences extend to ecological implications, particularly regarding biomagnification risks. Biosorption avoids biomagnification—the progressive increase in pollutant concentrations up the food chain—because it employs non-viable biomass that does not enter trophic transfers, thereby protecting higher organisms like fish and humans from amplified exposure.[75] In bioaccumulation scenarios, such as Pb²⁺ in aquatic ecosystems, contaminants transfer from algae or invertebrates to predatory fish, leading to elevated levels in top consumers and biomagnification factors greater than 1, as observed in riverine food webs.[77] Furthermore, 1990s pioneering studies highlighted biosorption's superior kinetics, enabling rapid equilibrium in hours compared to the slower metabolic integration over days.[75] This kinetic advantage underscores biosorption's preference for efficient, large-scale wastewater treatment without the ecological drawbacks of bioaccumulation.

Advantages, Limitations, and Comparisons to Physicochemical Methods

Biosorption offers several key advantages over traditional wastewater treatment methods, primarily due to its reliance on inexpensive, abundant biological materials such as agricultural waste or microbial biomass. The operational cost of biosorbents is notably low, typically ranging from 0.5 to 2 USD per kg, making it accessible for large-scale applications without requiring expensive synthetic materials.[78] Additionally, the process is eco-friendly, producing no chemical sludge or secondary waste, and operates under mild conditions without the need for additional nutrients or energy-intensive steps.[10][1] Biosorption demonstrates high selectivity for target pollutants like heavy metals, driven by functional groups on the biosorbent surface that enable specific ion binding across a wide pH range (3–9).[10] Regenerability further enhances its appeal, with biosorbents often retaining significant efficiency after multiple desorption cycles using mild acids like HCl.[79] It is particularly versatile for treating low-concentration effluents (<100 mg/L), where conventional methods become inefficient or costly.[10] Despite these strengths, biosorption has notable limitations that can hinder its widespread adoption. The process is generally slower than chemical precipitation for high pollutant loads (>500 mg/L), as biosorption relies on passive surface interactions that require equilibrium times of 30–120 minutes, whereas precipitation achieves rapid removal through bulk reactions.[80][81] Biosorbents are prone to degradation over repeated cycles due to mechanical instability or chemical breakdown, reducing long-term performance without immobilization.[10] Optimization of parameters like pH, temperature, and contact time is essential for maximal efficiency, adding complexity to process design.[1] Adsorption capacities for biosorbents typically range from 100–300 mg/g for heavy metals, which is lower than synthetic resins (up to 500 mg/g), limiting their use in high-volume industrial settings.[82][83] When compared to physicochemical methods, biosorption provides a more sustainable alternative, particularly in resource-limited contexts. Versus ion exchange, biosorption is cheaper (up to 50% lower operational costs) but offers lower selectivity in multi-ion solutions due to competition effects, with ion-exchange resins achieving 55–95% removal versus 20–70% for biosorbents like crab carapace.[10][55] Compared to adsorption on activated carbon, biosorption yields similar efficiencies (70–100% for heavy metals) but is more sustainable, utilizing renewable waste materials and avoiding the high energy demands of carbon activation.[82] Relative to membrane filtration, biosorption avoids clogging and fouling issues, operates at lower pressures, and reduces energy costs by 20–40%, making it preferable for decentralized treatment in developing regions.[82] Recent meta-analyses from the 2020s indicate biosorption is more cost-effective than these methods for low-concentration wastewater in developing countries, emphasizing its role in addressing global remediation gaps.[84][85]

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