Microalgae-based bioremediation of heavy metals: Mechanisms, optimization, and environmental applications

Heavy metal (HM) contamination in aquatic environments remains a persistent global challenge driven by industrialization, mining activities, and agricultural runoff. Microalgae-based remediation has emerged as a sustainable and cost-effective technology, utilizing both passive surface biosorption and active intracellular bioaccumulation mechanisms. This review comprehensively synthesizes recent advances in microalgae-mediated HM removal by integrating mechanistic insights at molecular, cellular, and process aspects. The performance of dried, living, and chemically modified biomass is comparatively evaluated across major HMs, including Cd, Pb, Cr, Cu, Hg, Ni, Zn, and As, under varying operational conditions. Microalgae are categorized into four major phyla to comparatively evaluate their HM adsorption capacities. Quantitatively analysis indicates that biosorption onto cell surfaces is the predominant mechanism while bioaccumulation, i.e., the uptake of HM into cells, is in general secondary but in some cases substantial. Transporter-regulated uptake and intracellular detoxification mechanisms are also discussed. Kinetic, isotherm, and thermodynamic modeling approaches are examined comprehensively, despite recurring inconsistencies in model selection, parameter interpretation, and reporting practices in the literature. Adsorption enhancement strategies, including physical and physiological conditioning, chemical modification, immobilization, biofilm cultivation, and multi-species co-cultivation, are assessed for their roles in improving HM removal efficiency, regeneration stability, and system robustness. By bridging mechanistic understanding with engineering implementation, this review provides a comprehensive framework to advance microalgae-based HM remediation.