Use of Additives in Composting Promotes Passivation and Reduction in Bioavailability of Heavy Metals (HMs) in Compost

Abstract Composting influences heavy metal (HM) speciation by transforming the exchangeable and reducible forms of HMs to oxidisable and residual forms, promoting the redistribution of HMs to a more stable form thereby reducing their mobility, leachability, and bioavailability. This enhances HM passivation hence reducing environmental contamination and possible HM-related toxicity. In this review, we explored the impact of additives/ameliorants on HM passivation during composting and the reduction in HM bioavailability in compost. Using additives in composting improved the passivation of HMs and reduced the bioavailability through several mechanisms. Additives may not reduce total HM concentration but reduce the bioavailable concentration, which is of importance in risk assessment. However, vermicomposting is exceptional, as it reduces total HM concentration. Improving humification and microbial activity is at the heart of HM passivation in composting. Overall, conditions and substances that promote humification, thermophilic phase, and improve microbial community structure enhance HM passivation in compost. Combining suitable composting additives depending on HMs of interest and vermicomposting the final compost is most likely to yield the best result in reducing HM bioavailability cum total concentration to achieve a safe final product.