Biocatalytic and enzymatic systems for heavy metal removal from aquatic environments: mechanisms, applications, and...

Heavy metal contamination in aquatic environments poses a severe threat to ecosystems and human health due to the non-biodegradability, bioaccumulation, and toxicity of metals such as lead, cadmium, mercury, chromium, and arsenic. Conventional treatment methods often suffer from limitations, including high operational costs, incomplete removal, and secondary pollution. In this context, biocatalytic and enzymatic systems have emerged as promising green alternatives for heavy metal remediation. This review comprehensively examines the current state of enzymatic and biocatalytic approaches for removing heavy metals from water systems. Specific focus is placed on naturally occurring and genetically engineered enzymes, including metallothioneins, phytochelatins, oxidoreductases, and peroxidases, as well as microbial biocatalysts and enzyme-immobilized composites. The underlying mechanisms, such as enzymatic reduction, chelation, biosorption, and bioaccumulation, are discussed in detail. Key factors affecting efficiency, including pH, temperature, enzyme stability, and metal ion speciation, are critically analyzed. Additionally, recent advancements in nano-biocatalysts and immobilized enzyme systems are highlighted for their potential in enhancing selectivity and recyclability. This review not only elucidates the strengths and limitations of biocatalytic systems but also outlines the future directions toward scalable, cost-effective, and sustainable water treatment technologies.