A framework for microalgal biosorption of heavy metals: mechanisms, modeling, and critical insights

Heavy metal contamination of aquatic systems poses serious risks to human health and the environment, requiring efficient and sustainable treatment strategies. Owing to the abundance of functional groups, high surface reactivity, and structural plasticity of microalgal biomass, microalgal biosorption has emerged as a promising approach for heavy metal remediation. This review provides a comprehensive and critical evaluation of microalgal biosorption, focusing on its mechanistic basis, comparative performance, environmental and operational controls, biomass diversity, and modeling approaches. Major physicochemical processes governing heavy metal uptake, including ion exchange, surface complexation, electrostatic interactions, and precipitation, are systematically discussed, with an emphasis on the roles of surface functional groups and biomass characteristics. The effects of key operational parameters, including pH, temperature, ionic strength, initial metal concentration, contact time, competing ions, and biomass state, are critically evaluated, highlighting their interactive effects on biosorption performance. Comparative analysis further demonstrates that biosorption efficiency is highly system-dependent and varies with the target metal, microalgal biomass, wastewater composition, and treatment configuration. Commonly applied kinetic, isotherm, and thermodynamic models are critically examined, revealing their limitations in representing heterogeneous biomass surfaces, multimetal competition, mass-transfer, and complex real wastewater matrices. Advanced spectroscopic and theoretical methods, including XPS, XAS/EXAFS, and density functional theory, are further considered to clarify metal-specific coordination environments and the energetic basis of molecular interactions. A unified conceptual framework linking interfacial mechanisms, environmental and operational drivers, biomass diversity, and model-based interpretations is proposed. The review identifies the need to move beyond isolated parameter optimization and capacity-centered evaluation toward mechanism-informed and predictive biosorption frameworks supported by standardized experimentation, realistic wastewater validation, and systematic assessment of regeneration, scalability, techno-economic feasibility, and environmental sustainability.