Bioavailability and Leachability of Heavy Metals during Composting – A Review

Composting is the best-known processes for biological stabilization of sewage sludge, municipal solid waste and other biodegradable wastes. Prepared compost can be used as a soil conditioner in agricultural applications. However, the presence of non-biodegradable and high level of toxic heavy metals in the compost frequently hinders agricultural land application. Uptake of heavy metals by plants and subsequent accumulation along the food chain is a potential threat to animal and human health. The availability of metals in soil depends on the nature of the chemical association between a metal with the organic residual and soil matrix, the pH value of the soil, the concentration of the element in the compost and the soil, and the ability of the plant to regulate the uptake of a particular element. The water soluble fraction is positively the most biologically active and highest potential for contamination of food chain, surface water and ground water. The diethylene triamine penta acetic acid (DTPA)-extractable fraction of metals might represent a supplemental approach to check the bioavailability of heavy metals in the soil and sludge amended soil for plant uptake. The phase association and solubility of metals changes with composting time thereby altering metal availability. The toxicity characteristic leaching procedure (TCLP) is designed to determine the mobility of both organic and inorganic analytes present in liquid, solid, and multiphasic wastes. If an analysis of any one of the liquid fractions of the TCLP extract indicates that regulated heavy metals are present at such high concentrations that, even after accounting for dilution from the other fractions of the extract, the concentration would be above the regulatory level for those metals, then the waste is hazardous. Therefore this review examines the water solubility, DTPA extractability and leachability of heavy metals in the compost, which has to be applied to the agricultural field.