Humification kinetics and heavy-metal stabilization performance in a two-stage integrated composting technology

The management of water hyacinth through conventional composting is constrained by prolonged processing times despite its potential for effective humification and heavy metal stabilization. This study addresses this limitation by exploring an integrated Rotary Drum Composting (RDC) and Vermicomposting (VC) approach to accelerate humification and enhance metal immobilization, offering a more efficient pathway for aquatic biomass valorisation. Fourier Transform Infrared Spectroscopy (FTIR) and Gas Chromatography-Mass Spectrometry (GC-MS) analysis demonstrate that RDC + VC technology drives a synergistic and accelerated conversion of key organic precursors into humic compounds. The simultaneous addition of Eisenia produced the highest rate of organic matter decomposition (44.33%) and the highest nitrogen content (124%), likely due to increased microbial activity and earthworm-driven mineralization. Additionally, Eisenia was greatly helped in its conversion from fulvic acid (FA) to humic acid (HA). HA content rose by 210% and HA/FA ratio by 76.57% in comparison to the RDC. The rates of HA and DP formation were 4 g/Kg and 0.18 g/Kg/d, respectively, whereas the kinetic constants of HA and DP synthesis were 0.08 d^-1 and 0.06 d^-1. Notably, the bioavailable fractions of Cu decreased by 38.71% after 27 days of composting period. The 27th day compost has a 63.01% lower Zn leaching potential. This study offers fresh perspectives on improving the humic material content of the final compost and accelerating humification during composting.