Evidence of trace metal association with soil organic matter using particle size fractionation after physical dispersion treatment

Abstract

Fractionation methods including physical rather than chemical dispersion were applied to soil samples in order to study interactions between soil constituents and trace metals. In particular, the role of soil organic matter was highlighted by particle size fractionation combined with density fractionation. In order to take into account the trace metal background content, experiments were carried out on both control and sludge-treated soil samples coming from a field trial on a crusting loamy soil 12 years after the end of sewage sludge application. Comparison of trace metal distributions between these two samples permitted a speciation approach. The collected samples were separated into eight particle size fractions, and the most abundant fraction in mass among coarser fractions, the 50–100 µm fraction, was also separated by flotation on water into two density fractions. Total chemical analyses of all these fractions were performed for major elements and Cd, Cr, Cu, Ni, Pb and Zn. In both field trial plots, retention of trace metals was found not only in the finest fractions (<2 µm), but also in the coarser fractions (>50 µm). The organic carbon contents in the particle size fractions followed the same trends, but different organic C distributions between the two experimental treatments were observed. The light isolated samples after density separation had high organic C contents, and in sludge-treated plots their trace metal contents were higher than in any other bulk particle size fractions. In control plots, however, they were also abnormally high. These facts are discussed, and a simple test is proposed to define soil pollution status.