Characterization of the water soluble fraction (WSF) of different composts and their interaction with Pb(II) and Cu(II) ions

In order to elucidate the role of the water-soluble fraction (WSF) of compost in Me-contaminated acidic soils, we studied the interaction of WSF from two municipal solid waste composts (C1-WSF and VV-WSF) with Pb(II) and Cu(II) ions at pH 4.5. The addition to the WSFs of solutions containing Pb(II) or Cu(II), in the 0.4-6.4 mM concentration range, gave rise to the formation of soluble Me(II)-organic complexes (as highlighted by the FT-IR spectroscopy), as well as promoted a decrease in the solubility of both ions, whose extent was greater for Pb(II) rather than Cu(II) at all the concentration tested. This latter effect was due to the association of the metal ions with the water-soluble organic carbon (WSOC) and anions leading to the formation of precitates [1.10 and 0.62 mmol•L-1 and 2.06 and 0.42 mmol•L-1 of Pb(II) and Cu(II) precipitated from C1-WSF and VV-WSF respectively, when 6.4 mmol•L-1 Me(II) was added]. In particular, a loss of the WSOC of ~30% and <10% from C1-WSF and VV-WSF respectively, was detected in the systems containing 6.4 mM Me(II). The WSOC fractions involved in the Me(II) precipitation, as highlighted by the FT-IR spectra of the precipitates, were compounds rich in aromatic structures and carboxylate groups. A strong contribution in the formation of Pb(II) precipitates was also given by phosphate, chloride and sulfate anions present in the WSFs, since their initial concentrations in both WSFs decreased of about 80, 25 and 90%, respectively. A decrease of about the 30% of phosphate, as well as any precipitation of chloride or sulfate in both WSFs was found in the systems containing Cu(II) 6.4 mM. The results obtained showed that the precipitation of copper from compost-WSF was almost exclusively ascribable to the formation of Cu(II)-organic complexes precipitates, while the Pb(II) precipitation could be assigned to the formation of Pb(II)-organic and -inorganic precipitates. Taken together, the results herein obtained show an important role of the water soluble fraction of mature compost in affecting the solubility of Cu(II) and Pb(II) in acidic polluted soils. This aspect has a crucial importance for the understanding of the compost effectiveness as Me(II)-immobilizing agents in acidic polluted soils.