Fungal biosorption is a method for removing dissolved metal ions from water by using fungal biomass as a binding material. It is important to define this process accurately: metals cannot be biodegraded or destroyed in the way that many organic pollutants potentially can. Biological treatment may change a metal’s chemical form, oxidation state, solubility, mobility or bioavailability, but the element itself remains present. Fungal biomass can bind, concentrate, immobilise, precipitate or transport metals out of the dissolved water phase; it does not make them disappear.
What is fungal biosorption?
In its most specific use, biosorption means the passive removal and concentration of dissolved metal ions by biological material. The material may be living or dead fungal biomass, cell-wall fragments, extracellular polymers or processed fungal residues. Because passive biosorption does not depend on cellular metabolism, non-living biomass can retain useful binding activity after drying, heat treatment or inactivation.
Living fungi may also remove metals through bioaccumulation. In this case, ions cross the cell membrane through transport systems and may subsequently bind to intracellular proteins or peptides, associate with sulfur-containing compounds, or become sequestered in compartments such as vacuoles. A living fungus can therefore combine rapid surface binding with slower, metabolism-dependent intracellular uptake. Intracellular accumulation should not be treated as synonymous with passive biosorption: it depends on cell viability and can be limited by toxicity, nutrient availability, oxygen and other environmental conditions.
Why fungal biomass can bind metal ions
Fungal cell walls are chemically diverse structures. Their inner framework commonly includes chitin and beta-glucans, while outer layers may contain mannans, glycoproteins, melanin and other polymers. Composition varies among fungal groups, species and growth conditions, but these materials provide functional groups capable of interacting with dissolved metals.
Relevant groups may include carboxyl, hydroxyl, amino, amide and phosphate-containing groups, together with carbonyl, sulfur-containing and, where present, sulfhydryl or thiol groups. The fungal surface can carry a negative charge when acidic groups are deprotonated. This may attract positively charged metal species, although the availability and charge of those sites change with pH.
Mechanisms involved in metal removal
Adsorption and electrostatic attraction
Metal ions may accumulate at or near the fungal surface through electrostatic attraction and interactions with accessible surface sites. This process can be relatively rapid, especially with dried, dead or chemically treated biomass. The term adsorption is often used broadly in experimental studies and does not necessarily demonstrate that removal is caused only by weak physical forces.
Ion exchange
Metal cations can exchange with protons or naturally associated ions such as sodium, potassium, calcium and magnesium on the biomass. This helps explain why pH and background ionic composition have such strong effects. Hydrogen ions may occupy or neutralise binding sites, while other cations can compete with or displace the target metal.
Complexation and chelation
Metal ions may form coordination complexes with oxygen-, nitrogen- or sulfur-containing ligands on the fungal surface. Carboxyl, hydroxyl and phosphate groups can provide oxygen donor atoms, while amino and thiol-containing groups may offer nitrogen or sulfur donors. Chelation is a more specific form of coordination in which multiple donor sites hold the same metal ion. Fungal polymers and compounds released by living fungi may provide such sites, but chelation should not be claimed unless the evidence supports that interpretation.
Precipitation and biomineralisation
Not all decreases in dissolved metal concentration result from surface binding. Under suitable conditions, metals may form poorly soluble hydroxides, carbonates, phosphates or sulfides at or near the fungal surface. Living fungi can promote precipitation by changing pH, releasing metabolites or supplying ligands, although precipitation may also occur without biological activity. Analytical measurements therefore need to distinguish adsorption from precipitation, co-precipitation and deposition where possible.
Intracellular accumulation by living fungi
Living fungi may take up metal ions through membrane transport systems, sometimes because toxic metals resemble essential nutrients. Once inside the cell, metals can bind to proteins and peptides or become compartmentalised in vacuoles. This can contribute to tolerance as well as removal from water. However, high concentrations may damage or kill the organism, making living systems less predictable than passive biomass under toxic conditions.
Metals studied in fungal biosorption
Research has examined fungal interactions with lead, cadmium, mercury, copper, chromium, nickel, zinc, cobalt, manganese and uranium, among other elements. Results are not interchangeable. A fungus that performs well with one metal may perform poorly with another, and different strains or biomass preparations of the same species can behave differently. Metal oxidation state, solution chemistry and the availability of competing binding sites all affect the outcome.
Factors that affect biosorption performance
- pH: pH changes fungal surface charge, functional-group protonation, metal speciation and the likelihood of precipitation. Very acidic conditions can cause protons to compete strongly for binding sites, while higher pH may increase binding but also promote metal hydroxide precipitation.
- Initial metal concentration: Increasing concentration can raise the amount captured per gram until sites approach saturation. Percentage removal may fall because the available biomass cannot bind the larger metal load.
- Competing ions and water chemistry: Calcium, magnesium, iron, sodium and other metals can compete for sites. Chloride, sulfate, carbonate, phosphate and dissolved organic matter can alter metal speciation, solubility and bioavailability.
- Temperature: Temperature affects diffusion, viscosity, membrane transport and binding energetics. Reported effects vary among fungi and metals; there is no universal temperature rule.
- Biomass preparation: Fresh mycelium, dried biomass, heat-killed material, acid- or alkali-treated biomass, pellets and immobilised particles may have different surface areas and accessible functional groups. Pretreatment can improve capacity but adds cost, chemical requirements and possible residues.
- Living versus dead biomass: Dead biomass is generally easier to store and is not vulnerable to metal toxicity or nutrient shortage. Living biomass can grow and accumulate metals internally but requires suitable operating conditions.
- Contact time: Uptake often begins quickly at readily accessible sites and then slows as sites fill or diffusion into less accessible regions occurs.
- Surface area and particle size: Smaller particles can provide more accessible area and shorter diffusion paths, but fine powders may be difficult to separate and can cause turbidity or clogging.
Measuring adsorption capacity
A common measure is equilibrium uptake, expressed as milligrams of metal per gram of dry biomass:
qe = (C0 − Ce)V / m
Here, C0 and Ce are the initial and final dissolved-metal concentrations, V is solution volume and m is the dry mass of biosorbent. Studies may also report a fitted Langmuir maximum capacity, or qmax. This is a model parameter measured under defined conditions, not a universal property of fungi.
Capacity values should be reported with the fungal species or strain, metal and oxidation state, pH, temperature, concentration, biomass dose, particle size, pretreatment, contact time and whether the solution contains one metal or several. For example, cadmium uptake reported for different Trichoderma species has varied substantially under the same general study conditions, illustrating species-specific performance. Results from simplified laboratory solutions, particularly those using unusually high concentrations, cannot simply be assumed for industrial wastewater.
Batch tests and continuous-flow treatment
In a batch experiment, a known mass of biomass is mixed with a known volume of metal solution. Batch studies are useful for screening fungi and testing pH, dosage, concentration, temperature and contact time. They can also support kinetic and isotherm analysis. However, they may overlook hydraulic constraints, solids separation, pressure drop, long-term exhaustion and changes in water chemistry.
Continuous systems commonly use pellets, granules or immobilised biomass in a fixed-bed column. Performance depends on flow rate, bed height, particle size, empty-bed contact time and competing contaminants. A breakthrough curve tracks effluent concentration relative to influent concentration as the bed becomes loaded. A column may remove metal effectively at first while the leading part of the bed is already saturated, so repeated-cycle, breakthrough and regeneration tests are needed for realistic evaluation.
Advantages and limitations
Fungal biomass offers a chemically varied surface, and filamentous fungi can produce substantial biomass through cultivation. Waste biomass from fermentation or food-processing operations may provide a useful feedstock. Dead biomass can operate without nutrient supply, while pellets and granules may be easier to recover than loose fungal powder. Concentrating metals into a smaller stream may also support recovery when the metal value and process economics justify it.
Limitations include variable selectivity, competition in mixed-metal wastewater, fouling by organic matter and difficulty separating fine particles. Living systems can be inhibited by toxic concentrations or unsuitable conditions. Drying, sterilisation, immobilisation and chemical pretreatment add costs. Regeneration may damage the biomass, reduce later capacity and produce a concentrated metal-bearing eluate.
What happens after capture?
Metal-loaded fungal biomass is a concentrated contaminated material, not harmless organic waste. Possible next steps include controlled desorption, metal recovery, stabilisation or regulated disposal. Acidic, alkaline, salt or chelating solutions may release captured metals, but the resulting eluate requires containment and further treatment. Thermal processing may also produce metal-bearing ash or emissions that must be controlled.
Spent biomass should not be composted, land-applied, used as animal feed or discarded as ordinary waste without assessing its metal content, leachability and applicable regulations. The central principle is simple: fungal biosorption changes the location and chemical environment of a metal; it does not destroy the metal element.