Fate and bioavailability of heavy metals during vermicomposting of various organic wastes-A review

Vermicomposting process employing earthworms is increasingly being used for conversion of both municipal and industrial wastes to vermicast with reduced metals content. The aim of the paper is to review recent developments and approaches for determination of bioavailable metal fractions through speciation and consequently the fate of both essential and xenobiotic heavy metals during vermicomposting of various organic wastes. The review also provides a comprehensive summary on the occurrence of heavy metals in wastes vis-à-vis metals standards worldwide and the process efficacy in limiting the mobility of heavy metals. Past and present researches demonstrated a high-degree of agreement that vermicomposting results in effective reduction of both total and bioavailable metal contents owing to the formation of stable metal complexes and/or silicate fractions. Further, it has been concurred that earthworms and associated microbial communities during vermicomposting tend to decrease the mobility of metals by enhancing enzymatic activities in both gut- and cast-associated processes. In addition, the decrease in total metal content has been attributed to accumulation and immobilization of mobile metal fractions in earthworm tissues. In summary, it can be inferred that vermicomposting potentially converts various municipal and industrial wastes to nutrients-rich organic manure for agronomic applications with reduced metals bioavailability.