A Review on Bacterial Exopolysaccharides for Heavy Metal Remediation: Mechanisms, Challenges, and Sustainable...

Heavy metals are generally defined as metals having densities greater than 5 g cm^-3 and atomic weights between 63.5 and 200.6 g mol^-1. Trace levels of certain heavy metals (e.g., Pb²⁺, Hg²⁺, As³⁺, and Cd²⁺) exhibit significant toxicity and have been widely recognized as environmental pollutants. They are hazardous materials that may harm humans, the environment, and plants. Some bacteria that have extracellular polymeric substances (EPSs) have the capacity to resist harmful contaminants. Bacteria such as Pseudomonas sp. (85%-98% removal rate for Pb²⁺; for Cd²⁺, 70%-95%; for Cu²⁺, 65%-95%; and for Ni²⁺, 55%-85%), Azotobacter sp., Arthrobacter, Agrobacterium, Bacillus, and Azotobacter all have high potential to promote the growth of many plants in metal-contaminated environments. These bacteria contribute to phytoremediation by immobilizing or detoxifying heavy metals, thereby enhancing plant growth in contaminated environments. EPS is produced by the bacteria when they face stress in their surroundings. The two layers of EPS interact with harmful environmental contaminants through the process of mixing two liquids that do not naturally mix, known as emulsification, the process of making something that does not usually dissolve in a liquid dissolve, called solubilization, binding, precipitation, complexation, and ion exchange. Various EPS functional groups including hydroxyl, amide, carboxyl, and phosphoryl remove hazardous materials from contaminated environments. This review explains the importance of bacterial exopolysaccharides in mitigating heavy metal pollution through biosorption. Mechanisms such as ion exchange, complexation, and emulsification are discussed. The role of EPS-producing bacteria like Bacillus and Pseudomonas in bioremediation is highlighted. There remains a significant research gap in understanding the molecular mechanisms and environmental factors influencing EPS composition, stability, and metal-binding specificity under real-world conditions. This review is especially needed now because of the growing urgency of eco-friendly, sustainable solutions for metal-contaminated environments, combined with recent advances in microbial biotechnology that have unveiled novel insights into EPS composition, metal-binding mechanisms, and potential for large-scale application-yet these findings remain fragmented across disciplines and require synthesis to guide future research and practical implementation.