Assessment of heavy metal removal technologies for biowaste by physico-chemical fractionation

In the Netherlands, the heavy metal content of biowaste-compost frequently exceeds the legal standards for heavy metals. In order to assess heavy metal removal technologies, a physico-chemical fractionation scheme was developed to gain insight into the distribution of heavy metals (Cd, Cu, Pb and Zn) over the physical entities of biowaste and to determine the binding strength of heavy metals to these physical entities. The scheme was based on a particle fractionation on size and density followed by sequential chemical extraction of the fractions. The organic fraction >1 mm contributed to 40% of the mass of biowaste and this fraction met the legal standards for compost after composting. About 28% of biowaste was composed of sand, which can be reused in road and building construction as the heavy metal content is very low. The heavy metals were concentrated in the organic particles from 0.05-1 mm and the organo-mineral fraction >Zn>Pb>Cu. The order of extractability for Pb and that for Zn were sometimes reversed most probably because the extraction is not only determined by the adsorption strength but also by the accessibility of adsorbed metals. The results of sequential chemical extraction showed that the amount of heavy metals available for extraction was sufficient. Citric acid turned out to be the best option for the removal of heavy metals from solid organic waste streams.