Algae as Cost-Effective and Efficient Biosorbents for Heavy Metal Removal from Wastewater: Recent Progress, Limiting Factors, and Mechanistic Insights

Heavy metal pollution in water bodies is a serious environmental and public health concern, as these contaminants are toxic, persistent and bioaccumulative in ecosystems and human tissues. Conventional remediation technologies are expensive, require constant monitoring and do not fully remove them. Recent studies have shown the potential, sustainability and cost-effectiveness of biosorption using algal biomass. This review gives a detailed assessment of the potential of algae and cyanobacteria as cheap biosorbents for the removal of heavy metals from wastewater. The sorption efficiency of algae and cyanobacteria is critically evaluated in terms of important operating parameters such as pH, temperature, initial metal concentrations, biomass loading and contact time. The diversity of metal-binding functional groups such as carboxylate, amine, imidazole, phosphate, sulfhydryl, sulfate and hydroxyl groups present on the surface of algal cells is discussed in detail, highlighting the complex algal biochemistry. Recent developments in functionalized algal materials are also discussed, with emphasis on their potential to improve adsorption capacity, selectivity, regeneration, and practical applicability. However, this review also identifies some limitations such as energy requirements for the drying of biomass, limitations of batch systems for microalgae applications, and challenges for large-scale implementation. Future research directions are suggested to highlight the urgent need for functionalized algal materials, optimization of large-scale applications, and integration of biosorption with other treatment technologies in the framework of a circular economy.