Integrated Microalgal Bioprocesses for Heavy Metal Removal: Linking Bioremediation Efficiency with Circular Bioeconomy and Bioproduct Valorization

The growing global concern over heavy metal pollution has increased interest in microalgal bioremediation as an eco-friendly and sustainable approach to treating wastewater. This review critically examines recent advances in microalgal heavy metal remediation by integrating metal uptake mechanisms, factors affecting removal efficiency, and the valorization potential of the resulting biomass. The review shows that heavy metal removal by microalgae is governed by the interaction of biosorption, bioaccumulation, and detoxification processes, whereas the overall performance is strongly influenced by pH, initial metal concentration, biomass dosage, contact time, and wastewater composition. A central finding is that the value of microalgal remediation should not be assessed solely based on removal efficiency, as heavy metal accumulation directly affects biomass quality, downstream processing, and product safety. In this context, thermochemical conversion pathways, particularly pyrolysis and hydrothermal liquefaction, have emerged as the most feasible valorization routes for metal-laden biomass because they enable energy recovery while retaining most heavy metals in solid residues, thereby reducing the risk of secondary contamination. In contrast, high-value applications and some biochemical conversion pathways are more constrained because of contaminant carryover and metal-related inhibition issues. Despite promising technological advances, including immobilized systems, hybrid separation methods, and engineered strains, large-scale implementation remains limited by metal toxicity, reduced biomass productivity, and process instability under actual wastewater conditions. Therefore, this review highlights that safe biomass management, realistic valorization strategies, and integrated process design are essential for advancing microalgal bioremediation within the circular bioeconomy framework.