Influences of natural zeolite on speciation of heavy metals during rotary drum composting of green waste

Abstract The presence of heavy metals in water hyacinth compost is the main limiting factor for its land application. Water hyacinth can accumulate heavy metals in its tissues. Application of natural zeolites (clinoptilolite) during water hyacinth composting may be helpful to immobilise heavy metals. The present study was carried out on the speciation of heavy metals (Zn, Cu, Mn, Fe, Pb, Ni, Cd and Cr) during rotary drum composting of water hyacinth mixed with cattle manure, sawdust and natural zeolite. The Tessier sequential extraction method was used for heavy metal speciation determination. The water hyacinth, cattle manure and sawdust at a ratio of 6:3:1 ratio was mixed with 5, 10 and 15% natural zeolite by weight. Influences of physicochemical parameters such as temperature, pH and organic matter degradation on speciation of heavy metals were studied during the process. A rotary drum composter has been shown to be highly efficient for organic matter degradation. The most bioavailable fractions (exchangeable and carbonate fractions) were reduced significantly due to the combined effects of zeolite addition and metal complexing with stabilised organic matter as a result of the high degradation of organic matter in a rotary drum. Ni, Pb and Cd were not found in the reducible and oxidisable fractions. Overall, the addition of an optimum quantity of natural zeolite significantly reduced the bioavailability fractions of heavy metals during rotary drum composting of the water hyacinth.