Reduction of bioavailability and leachability of heavy metals during agitated pile composting of Salvinia natans weed of Loktak lake

Purpose Composting of harvested Salvinia natans weedof Loktak lake of Manipur, India can protect the preciouslake from the unwanted growth of the weeds but there is aneed to assess the total amount and bioavailable forms ofheavy metals and minerals of the final compost beforeapplication as a soil conditioner.Methods Studies were conducted on the transformationsof the physico-chemical parameters and the total amount,bioavailable and leachable forms of heavy metals andminerals during agitated pile composting of S. natans withrice husk and cattle manure in five different combinations[trial 1 (5 S. natans: 4 cattle manure: 1 rice husk), trial 2 (6S. natans: 3 cattle manure: 1 rice husk), trial 3 (7 S. natans:2 cattle manure: 1 rice husk), trial 4 (8 S. natans: 1 cattlemanure: 1 rice husk) and trial 5 (10 S. natans: 0 cattlemanure: 0 rice husk)].Results The highest temperature (52.2 C) was monitoredin trial 2 having 30 % cattle manure during the process.Highest reduction of moisture content and volatile solid intrial 2 corroborated the temperature profile. Total concentrationsof heavy metals (Zn, Cu, Mn, Fe, Ni, Pb, Cd andCr), total concentration of nutrients (Na, K, Ca, and Mg)and the water-soluble forms of the nutrients were enhancedsignificantly. The water-soluble, plant-available andleachable metals decreased favorably with maximumreduction observed mostly in trial 2. The study also revealedthat the total concentrations of Zn (211–254 mg/kg), Ni (310–345 mg/kg) and Pb (805–891 mg/kg) of thefinal composts were higher than that of Cr (140–207 mg/kg) but the water-soluble forms of Zn (3.6–4.7 % of totalZn), Ni (NIL) and Pb (0.7–1.0 % of total Pb) were lowerthan that of Cr (13.3–19.3 % of total Cr) indicating that Crin the composts had higher toxicity potential. The leachableheavy metals were within the threshold limits prescribedfor agricultural application.Conclusions Composting of S. natans biomass with appropriateproportion of cattle manure reduced thebioavailable and leachable forms of the heavy metals in thebiomass.