Gypsum-based idol immersion waste as a novel additive for improved heavy metal immobilization during in-vessel composting of organic waste.

Idol immersion waste (IIW) generated during Indian festivals is an emerging source of aquatic pollution due to heavy metals released from gypsum-based idols and synthetic pigments. This study investigated laboratory-scale in-vessel composting as a circular strategy for detoxification and resource recovery of IIW. Six bioreactors containing food waste, garden waste, cow manure, biochar, and varying gypsum proportions were operated for 30 days. ICP-OES analysis identified elevated Cr, Cu, Hg, Ni, Pb, and Zn in idol-derived residues. Sequential extraction revealed progressive transformation of metals from labile fractions to stable oxidizable and residual forms, indicating effective immobilization. Near-complete stabilization of Pb, Ni, and Hg was achieved their bioavailable fractions and water-soluble concentrations declining below analytical detection limits (immobilization factor 82.8-97.4%) respectively, in optimized treatments. FE-SEM-EDX revealed surface precipitates and metal-calcium co-location consistent with secondary mineral formation and possible co-precipitation. Germination index values (125-344%) and compliance with fertilizer standards demonstrated agronomic safety. The findings establish composting as a scalable pathway for converting hazardous festival waste into value-added organic amendments.