Zinc speciation and isotopic exchangeability in soils polluted with heavy metals

Correct characterization of heavy metal availability is a prerequisite for the management of polluted soils. Our objective was to describe zinc (Zn) availability in polluted soils by measuring the isotopic exchangeability of Zn in soil/solution ( E value) and in soil/plant systems ( L value), by assessing the transfer of Zn and ⁶⁵Zn in the fractions of a six-step selective sequential extraction (SSE) in incubated soils and by identifying Zn forms in soils by means of extended X-ray absorption fine structure (EXAFS) spectroscopy. We distinguished three pools of exchangeable Zn: the pool of Zn exchangeable within 1 minute, which is observed in all soils, Zn exchangeable in the medium term, and the slowly and not exchangeable Zn. The amount of Zn present in the first two pools was similar to the L value measured with Thlaspi caerulescens . The first three steps of the SSE solubilized the first pool and a fraction of the second pool. Most of the second pool and a fraction of the third pool were extracted in the fourth step of the SSE, while the rest of the third pool was extracted in the final steps of the SSE. The EXAFS study conducted on two soils showed that more than half of the Zn was present in species weakly bound to organic compounds and/or outer sphere inorganic and organic complexes. Other species included strongly sorbed Zn species and Zn species in crystalline minerals. The EXAFS study of selected SSE residues showed that the specificity and the efficiency of the extractions depended on the properties of the soil studied.