Mobility of Potentially Toxic Elements (Pb, Zn, Cd, As, Sb) in Agricultural Carbonated Soils Contaminated by Mine Tailings (Northern Tunisia): A New Kinetic Leaching Approach with Organic Acids

Abstract

The present study was carried out to show the potential of root exudates to mobilize potentially toxic elements (PTE) present in rhizospheric carbonated soils. Five different contaminated rhizospheric soils were collected from five former mining districts of northern Tunisia (Jebel Hallouf (H3), Sidi-Bouaouane (B1), Jebel Ghozlane (G7), Hammam Zriba (Z2) and Jalta (J2)). The abundant minerals in these soils are quartz, calcite and clays. These soils contain significant PTE amounts compared to the local geochemical background (LGB). The important concentrations of Pb, Zn, Cd, As and Sb are, respectively, in the order of 17,350 mg·kg−1 in B1, 37,000 mg·kg−1 in G7, 205 mg·kg−1 in G7, 683 mg·kg−1 in B1 and 145 mg·kg−1 in B1. Kinetic leaching tests were conducted with a mixture of low molecular weight organic acids (LMWAOs) for increasing times up to 16 h (initial pH = 2.8) to study the mobility of PTE in the rhizospheric soils. The results showed an increase in the pH of the solution (2.8) to values up to neutrality together with the increase in Ca and Mg concentrations in the leachate, resulting from the dissolution of carbonates (calcite and dolomite). Additionally, leaching tests showed important extractions of Cd and Zn (25% for Cd and 11% for Zn). Pb was also mobilized but to a lesser extent (5%). The extractability of metalloids (As and Sb) was, in contrast, relatively low, except for Jebel Hallouf and Sidi Bouaouane soils, with an extraction percentage of no more than 1% for Sb and 0.1% for As, respectively. The mobility of Zn, Pb and Cd was thought to be controlled by both the solubility of their host minerals (e.g., sphalerite, hemimorphite, cerussite and jordanite) and the high pH. In contrast, As and Sb mobility was dependent on secondary carrier phases such as iron oxyhydroxides.