Critical review on biological treatment of microplastics and heavy metals for biohydrogen production: Integrated mechanisms, ecotoxicological insights, and sustainable remediation solutions

Microplastics and heavy metals are common and persistent pollutants that affect large areas and have adverse effects on both the health of humans and the health of aquatic ecosystems. The presence of heavy metals in conjunction with microplastics may increase the potential risk associated with both. Microplastics are able to act as carriers of heavy metals, changing the form of the heavy metal's bioavailability and ecological impact. Long-term success and cost-effectiveness are significant limitations for traditional physical or chemical techniques of remediating these pollutants. Environmental friendly alternatives to these methods are urgently needed. The aim of this study is to provide a semi-systematic overview of the literature on the biological strategies used to remediate microplastics and heavy metals for the years 2015-2025 as obtained from the Scopus, Web of Science and PubMed Databases. The outcome of this review will identify key biological remediation mechanisms for microbial remediation, algal remediation, fungal remediation and plant remediation. An overview of the crucial processes through which biological remediation takes place, i.e., biosorption, bioaccumulation, biomineralisation, enzymatic degradation, bioflocculation and Phytoremediation is presented. In particular, it discusses the ways in which microorganisms are being employed to deal with both microplastics and heavy metals and the processes by which they are likely to reduce the toxicity and immobilise the contaminants. The studies reviewed provide a foundation for developing environmentally sound, highly efficient and effective means of biological removal of waste microplastics and heavy metals under controlled laboratory conditions.