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). Biosorption properties of yeast were studied using various forms, including free vs. immobilized cells, dead vs. live cell, engineered vs. non‐engineered cell, etc. (Park et al., 2003). The yeast cells in free form are not suitable for biosorption as there is a problem in solid‐fluid phase separation. In flocculating cells, this problem tends less effective (Veglio and Beolchini, 1997). Yeast cells must be pretreated to increase the surface‐to‐volume ratio of metal binding sites (Mapolelo, 2004).
Table 2.3 Biosorption of heavy metals by different fungi.
| Fungal biomass (biosorbent) | Metal ions (biosorbate) | Functional groups | References |
|---|---|---|---|
| Pleurotus ostreatus | Chromium | Carboxyl, amine groups | (Arbanah et al., 2013) |
| Hydrilla verticillata | Cadmium | Carboxyl, hydroxyl, amine groups | (Acosta Rodríguez et al., 2013) |
| Aspergillus terreus | Copper | Carboxyl groups | (Gulati et al., 2002) |
| Trametes versicolor | Nickel | Carboxyl, hydroxyl, amine groups | (Subbaiah and Yun, 2013) |
| Penicillium chrysogenum | Arsenic | Carboxyl, amino groups | (Mamisahebei et al., 2007) |
| Phanerochaete chrysosporium | Lead | Hydroxyl, amino groups | (Haluk Ceribasi and Yetis, 2004) |
| Pencillium simpliccium | Zinc | Carboxyl, amino groups | (Fan et al., 2008) |
| Aspergillus fumigatus | Mercury | Amino, hydroxyl groups | (Mamisahebei et al., 2007) |
Different parameters of metal ions such as valency, diameter, cell age, culture conditions, concentration of metals, temperature, and pH influence the process of biosorption (Wang and Chen, 2006). Biosorption involves amino, carboxyl, and hydroxyl functional groups. The isothermal model of Langmuir was better adapted to a biosorption ion exchange mechanism (Luo et al., 2010). Yeasts’ huge size, on the other hand, makes them potential candidates for metal bioremediation. Table 2.4 lists the many yeast strains utilized in biosorption.
Biosorbents Derived from Plant and Animal Waste
Solid wastes derived from flora and fauna are plentiful, low‐cost, renewable resources. They're made in vast quantities every year, and disposing of them is usually a problem. An important area of research is to find meaningful uses for these materials. They can be used to minimize waste and create cost‐effective products (Kulkarni, 2014).
Plants disposed of as agricultural waste and food industry waste can be used as biosorbents. This is a method of repurposing and recycling discarded materials, so using plant materials has no significant cost (Ali Redha, 2020). Plant‐derived wastes are predominantly made up of cellulose, with structural components such as lignin, proteins, hemicellulose, carbohydrates, lipids, and starch (Rajapaksha et al., 2015). Plant biosorbents can absorb water because of the presence of carboxylic and phenolic functional groups in the cellulosic matrix and components linked with cellulose, such as hemicellulose and lignin (Abdi and Kazemi, 2015). Based on cation exchange between binding sites and metal ions, the metal ions bind with functional groups, resulting in biosorption and, therefore, the removal of the metal ions from media (Abdi and Kazemi, 2015). Table 2.5 lists the various plants employed as biosorbents.
