Speciation of heavy metals during co-composting of livestock manure

Heavy metals determination in manures provides beneficial information to predict the availability and the potential contamination of environment. Therefore, a study was carried out to investigate heavy metals changes in livestock manure during a co-composting process. The metals were stepwise fractionated into exchangeable, adsorbed, organically bound, carbonate precipitated and residual forms by extracting with 0.5M KNO₃, de-ionized water, 0.5M NaOH, 0.05M Na₂EDTA and 4M HNO₃, respectively. During the extraction process, the amount of metals remained highly dependent on an extraction method and stage of manure composting. Extractions released heavy metals contents in the final composted manure as Zn > Cu > Pb > Cr > Ni > Cd. Irrespective of the days of composting, extracting agents differed in the order HNO₃ > Na₂EDTA > NaOH > KNO₃ > H₂O for all heavy metals. Higher concentrations of heavy metals were observed during the early stages of composting. Conversely, there was a progressive increase in the concentrations of K, Ca and Mg during composting of manure. Total C in the manure reduced with composting days.