Reduction of the short-term availability of copper, lead and zinc in a contaminated soil amended with municipal solid waste compost

The effect of two municipal solid waste composts on the availability of Cu, Pb and Zn in a soil contaminated in the laboratory was evaluated. An agricultural acid soil developed on granite was amended with the composts at two rates (3% and 6% dry weight), contaminated with 1000 mg kg−1 of Cu, Pb and Zn, and incubated in the laboratory for three months. Determinations of soil pH, CaCl2-extractable and EDTA-extractable Cu, Pb, and Zn were run monthly during the incubation. At the end, a leaching test (TCLP) and selective extractions were performed for these elements. The analysis of the CaCl2-extractable elements demonstrated a strong capacity of both composts to decrease the solubility of the metals added to the soil, specially for Cu and Pb. The percentage of reduction of the soluble forms with respect to the initial addition was higher at the highest rate of compost, and reached 99% for Cu and Pb, and 80% for Zn in the compost-amended soil, whereas the soil without amendment was able to reduce Cu availability by a 94%, but not Zn or Pb availability. The TCLP test showed that compost also reduced the leachability of the three elements. Nevertheless, EDTA extracted a major amount (around 90%) of the elements added in all the treatments. Given that EDTA has a strong ability to extract elements bound to organic matter, it can be hypothesized that the main mechanism of the observed insolubilization was the formation of low-solubility organo-metallic complexes with both soil and compost organic matter. The selective extractions confirmed that compost reduced the exchangeable fraction of the elements, and that the organically bound fraction (pyrophosphate-extractable) was the main one for the three elements.