# Microbial Biopolymers: From Production to Environmental Applications--A Review
## 6 Applications of Microbial Biopolymers  
### Microbial Biopolymers as Adsorbents  
Adsorption is a surface phenomenon that has gained popularity in water treatment in recent years. It is a well-known equilibrium separation process recognized by its simplicity, effectiveness, and efficiency [261]. This process is favored because it does not require the use of extra chemicals, excess water, or energy, thereby allowing for a low operating cost [262]. Synthetic polymers are generally derived from coal and petroleum, which are unable to fit in recycling systems. Contrarily, microbial biopolymers outperform petroleum-based polymers in terms of affordability, environmental sustainability, and usability [263]. Microbial biopolymers are composed of a variety of functional groups, including methoxyl carboxyl, phenolic hydroxyl, amines, and hydroxide. Due to chelating metal ions establishing surface complexes, these functional groups of microbial biopolymer-based adsorbents enhance the adsorption efficacy of pollutants from different samples [264]. Generally, adsorption occurs due to attractive forces between the adsorbate, which is the compound being adsorbed, and the surface of the adsorbent, the compound to which the adsorbate gets attached. Up to date, a variety of adsorbents have been used, and the common types can be classified depending on the material nature. Thus, adsorbents can be from organic, mineral, or natural origins [261]. Among the natural group, microbial biopolymers and biopolymer composites, (Table 5) play a significant part due to their porous structure, highly specific surface area, durability, and cost-effectiveness. Additionally, their abundant functional groups present on the surface can speed up decontamination and the removal of soluble particles, greasy substances, dyes, and other pollutants in water. Likewise, these functional groups have an affinity to combine with heavy metals like mercury, copper, cadmium, lead, nickel, chromium, and zinc. Biopolymers also offer a promising solution to address emerging contaminants, particularly surfactants, in wastewater. [PERSON] and [PERSON] [265] discussed the use of biopolymers such as chitosan, alginate, tannin composites, and more for the removal of surfactants. Recent progress in the development of chitosan-based adsorbents, shedding light on their remarkable potential for various pollutant removal, such as heavy metal ions, and cationic and anionic dyes, has been emphasized by [PERSON] et al. [266]. As highlighted by [PERSON] et al. [267], the abundant amino and hydroxyl groups on the surface of chitosan can be used to chelate heavy metal ions, specifically Cu\({}^{2+}\), Hg\({}^{2+}\), Pb\({}^{2+}\) and Zn\({}^{2+}\), with a maximum adsorption capacity of 79.94 mg/g, 109.55 mg/g, 58.71 mg/g and 47.15 mg/g, respectively. Additionally, alginate, with its hydroxyl and carboxyl groups distributed across its surface, can capture metallic ions through an ion-exchange mechanism [268]. However, despite such unique advantages, limited functionality, including poor mechanical, chemical, and physical properties, has been observed. Therefore, biopolymers are often subject to modification using various synthetic or natural monomers, and they are combined with diverse materials for applications in water treatment [259]. As a low-cost adsorbent, cellulose can be chemically modified by esterification, etherification, oxidation, halogenation, and chelation. In the context of chelation, amine, carboxyl, amide, and imidazole binding ligands were seen to have better efficiency, owing to their large surface area, cost efficiency, and sustainability [171]. Another promising way to further enhance biopolymer adsorbents' ability to remove heavy metals and dyes is the utilization of low-cost sorbent hydrogels. A sodium alginate/polyethylene amine compound hydrogel has shown an adsorption capacity of 322.6 mg/g and 344.8 mg/g for the absorption of Cu\({}^{2+}\) and Pb\({}^{2+}\) ions, respectively, in wastewater [269].