Table 2.2 Biosorption of heavy metals by different algae.
| Algal biomass(biosorbent) | Metal ions(biosorbate) | Functional groups | Reference |
|---|---|---|---|
| Palmaria palmate | Chromium | Amino, carboxyl, sulfur groups | (Murphy et al., 2008) |
| Ulva lactuca | Cadmium | Amino, hydroxyl, carboxyl groups | (Lupea et al., 2012) |
| Spirulina platensis | Copper | Amino, carboxylic groups | (Çelekli et al., 2010) |
| Oedogonium hatei | Nickel | Amino, amide, hydroxyl and carboxyl groups | (Gupta et al., 2010) |
| Maugeotia genuflexa | Arsenic | Carboxyl, hydroxyl, amide groups | (Sarı et al., 2011) |
| Cladophorasp | Lead | Carboxyl, amino, amide, hydroxyl groups | (Lee and Chang, 2011) |
| Laminaria japonica | Zinc | Carboxyl, hydroxyl groups | (Davis et al., 2003) |
| Spirogyra hyalina | Cobalt | Amino, hydroxyl groups | (Viraraghavan and Srinivasan, 2011) |
| Sargassum sp. | Mercury | Amino, sulfur groups | (Subhashini et al., 2011) |
The cell wall components differ between algal species. Brown algae have three primary components in their cell walls: cellulose that provides structural support, alginic acid (mixture of polymers with their respective salts), and sulfated polysaccharide with large carboxylic groups implicated in metal biosorption. Red algae have garnered attention for biosorption due to the presence of sulfated polysaccharides formed from galactans (high concentrations of hydroxyl and carboxyl groups). Green algae have polysaccharide‐bound cellulose with a large amount of protein that includes amino, sulfate, hydroxyl, and carboxyl functional groups (Flouty and Estephane, 2012).
The algal cell wall consists of polysaccharides that include chemical groups (sulfate, amino, hydroxyl, phosphate, imidazole, mannan) known to function as binding sites for metal (Romera et al., 2007). Electrostatic interaction attracts cations due to the negative charge of the cell wall. In addition to the charge, which plays an essential role in biosorption, hydrophobicity, species and structure, ionic strength of the metal, and chemical makeup of the ionic solution of metals are also important factors to consider when choosing algae for biosorption. (Adewuyi, 2020).
The mechanism of metal absorption by algae is similar to bacteria: metal ions are bound to the outer membrane, followed by internalization. Algal biosorption involves one of two mechanisms:
Ion exchange process: ions such as calcium, magnesium, sodium, and potassium are replaced by metal ions on the algal surface.
Functional groups and metal ions combine to form complexes.
Ionic strength and covalent interactions are hypothesized in the case of the metal uptake process. Carboxyl and sulfate groups form ionic bonds, but amino and carboxyl groups require covalent interaction between the metallic ligand and the chemical group. Phytochelatins are formed within the algal body in response to metal ions (Abbas et al., 2014).
Multiple researchers worked on exploiting algal biomass to remove heavy metals from polluted water sources. Algal biosorption capacity is 15–84% more than other microorganisms, according to research (Mustapha and Halimoon, 2015). As a result, algal biomass is seen as a cost‐effective and ecologically beneficial wastewater treatment option.
Fungi as Biosorbents
Fungi are eukaryotic species that include yeasts, mushrooms, molds, and so on. They are used as biosorbents due to their distinguishing features: i.e. easy to grow, greater biomass yield, and ease of alteration either genetically or chemically (Mulligan et al., 2001). Both dead and living forms of fungi can be used as biosorbent material (Wang and Chen, 2006). The cell wall of fungal organisms possesses outstanding binding characteristics because of the presence of chitin, mannans, and glucans in addition to lipids, polyphosphates, and proteins (Javaid et al., 2011). The fungal cell wall is rich in polysaccharides (90%) and glycoproteins that contain different metal‐binding groups, such as amines, phosphates, carboxylate, and hydroxyls (Remacle, 1990). Active and passive metal absorption by fungi have been reported:
Active or intracellular absorption or bioaccumulation depends on the metabolism of the cell.
Passive absorption, or biosorption, involves metallic ions binding to the exterior of the cell membrane and is unrelated to cell metabolism.
Uptake of metals in active mode occurs only with living cells. In this circumstance, metal ions may bind with cell surface functional groups via ion exchange complex formation or simple physical binding. The metal absorption process, which is independent of energy, may be influenced by temperature, metabolic inhibitors, etc. (Shamim, 2018). Physical and chemical treatments such as thermal treatment, dimethyl sulfoxide, detergents, orthophosphoric acid, glutaraldehyde, formaldehyde, and alkali can alter the biosorption potential of fungal populations (Das et al., 2008). At the industrial level, fungi can be easily generated to adsorb metal ions from huge amounts of polluted supplies. Moreover, biomass can be created using inexpensive growth media or byproducts from a variety of fermentation processes. Furthermore, fungi are somewhat sensitive to nutritional variations as well as other process factors, including temperature, pH, and aeration. They are easily separated by simple techniques such as filtration due to their filamentous existence. As a result, they are also regarded as cost‐effective and environmentally beneficial biosorbents (Leitão, 2009). Table 2.3 lists some fungal species that have been utilized as biosorbents.
Yeasts as Biosorbents
Yeasts are single‐celled organisms in which the bulk of the biomass either biosorbs a wide range of metals or is selective for a single metal ion. Yeasts also possess a high potential to accumulate and, as a result, can be utilized as biosorbents to absorb heavy metals. Saccharomyces cerevisiae is a model organism for biosorption research. They remain non‐pathogenic and easy to cultivate, and they produce a large amount of biomass using a basic growing media (Gaensly et al., 2014).