Biosolids-based Co-composts reduce the bioavailability of heavy metals

Biosolid samples from the Bolivar Wastewater Treatment Plant (South Australia) were mixed with lime (1, 3 and 5%; w/w basis), red mud (Comalco Alumina Refinery, Queensland, Australia), fly ash (Alinta Energy, South Australia, Australia) and bentonite (IPOH Pacific Ltd., Queensland, Australia) (5, 10 and 20%; w/w basis). They were incubated under aerobic conditions at room temperature for seven months so that the effects of various additives on transformation of heavy metals in biosolids could be examined. The specific objectives were to study, (i) the redistribution of metals in the biosolids; and (ii) the mobilization of metals and their subsequent release to pore water. Both the NH₄NO₃ extractable concentration of Cd (Cd _NN), Cu (Cu_NN) and Pb (Pb_NN) from biosolids and the concentration of these metals in pore water were investigated. Co-composting biosolids using alkaline materials and clay mineral have been shown to immobilize metals. The pH of both biosolids and pore water increased while dissolved organic carbon (DOC) decreased on co-composting biosolids with inorganic amendments. The addition of red mud and lime resulted in the highest pH increase, and red mud proved to be most efficient in the sorption of DOC, Cd, Cu and Pb in comparison to lime, coal fly ash, and bentonite. The NH ₄NO₃ extractable Cd (Cd_NN), Pb (Pb _NN) and Cu (Cu_NN) from biosolids and these metals' total concentration in pore water were lower in all amendments than biosolids alone. The effect of amendments on immobilizing biosolids-derived Cd, Pb and Cu varied according to both the nature and level of amendments being added. While red mud and lime were most effective in the immobilization of Cd and Pb in the biosolids and their subsequent release to pore water, bentonite was most effective in immobilizing Cu in the biosolids.