Metals distribution in the organic and inorganic fractions of soil: a case study on soils from Sicily

The content of Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb and Zn, has been determined in the organic and inorganic fraction of eight soil samples from Sicily. A modified type-Tessier sequential extraction procedure has been used to recognize metals in five different fractions categorised as follows: (a) exchangeable metals, (b) metals bound to carbonates, (c) metals bound to iron and manganese oxides, (d) metals bound to organic matter, and (e) metals in residual fraction. The mineralogical composition of soils samples was characterized by X-ray diffraction. Humic substances were extracted by means of an alkaline extraction procedure. It has been found that both the mineralogical composition and the different content of humic fraction of organic matter in the soils investigated are responsible for the observed trace metal distribution. The obtained data show that a significant amount of trace metals is bound to the organic fraction where different binding sites are present simultaneously. In particular, Pb and Cu seem to be the elements more tightly linked to the organic fraction of soils, whilst cadmium is mostly concentrated into the iron and manganese oxides fraction. The presence of clay minerals in soil is significant for metal distribution, playing a fundamental role in binding metal ions. Besides, it has been observed that the calcophile metal mobility is strongly dependent on the presence of sulfide phases. Applying the Risk Assessment Code to the analyzed soils, it appears that they contain, generally, less than 30% of metals in the exchangeable and carbonate fractions. Calcic brown soil from Piano Zucchi exhibits a medium up to very high risk for Pb, Cd, Co, Zn and Mn. It also been recognized the elevated presence of Cd in the exchangeable and carbonate fractions of andic brown soil on volcanites from Pachino.