Chemical Fractionation of Trace Elements in a Metal-Rich Amphibolite Soil Amended with Municipal Solid Waste Composts

Abstract

The high concentrations of metals naturally present in soils developed on mafic and ultramafic rocks such as amphibolite could be mobilized in response to modification of soil physicochemical properties after compost amendment. However, so far, research with this type of soils is insufficient. With the objective of understanding the potential risk associated to trace elements in this case, we have studied the chemical distribution of trace metals in an amphibolite agricultural soil amended with several composts and incubated during 3 months in the laboratory. An agricultural acid soil developed on amphibolites was amended with 5% (dry weight) of five commercial composts (produced mainly from municipal solid wastes) and incubated in the laboratory during 90 days. Trace element potential mobility and bioavailability in the soils were studied by means of chemical analyses: extraction of immediately available trace elements with 0.01 N CaCl2 and the normalized BCR sequential extraction procedure. High concentrations of 0.01 N CaCl2 soluble metals occurred in the non-amended soil (especially Zn and Ni), that decreased after compost addition for Pb, Zn, Ni and Cd, with no effect on Cu and Cr. In general, composts had little effect on the BCR fractionation of the trace elements, because of their high native concentrations in the studied soil. Even so, the addition of the composts with the highest metal contents increased the oxidizable (organic matter-bound) and reducible (iron oxides-bound) fractions of soil Cu and the soluble and reducible fractions of soil Zn and Ni. None of the composts modified Pb fractionation, which was mostly found in the reducible fraction, nor that of Cr, present in more than 80% in the residual non-extractable fraction. This study showed that, in soils developed over metal-rich parent materials such as amphibolites, chemical fractionation of metals after compost amendment is more influenced by the nature of the metals previously present in soil than by the composition of the composts.