Assessment and mobility of heavy metals in carbonated soils contaminated by old mine tailings in North Tunisia

• Total concentrations of Pb, Zn and Cd in soils exceed the local geochemical background.
• Heavy metals dispersion from the tailings are due to surface runoff and strong winds.
• Low mobility of metals follow the decreasing order: DTPA ≫ LMWOAs > CaCl2 ≈ H2O.
• Heavy metals adsorption and retention are weakly influenced by pH, CEC and carbonates and governed by mineralogy of soils.

The activity of the former Pb–Zn mine of Jebel Ghozlane, located in the extreme north of Tunisia, generated during the last century large quantities of tailings (extraction, flotation, gravimetry), which are deposited without concern for environmental issues. The tailings contain both primary (galena, sphalerite, pyrite) and secondary (cerussite, smithsonite, iron oxi-hydroxides) heavy metals bearing minerals. The gangue minerals are composed of calcite, dolomite, barite, celestite and quartz. The concentrations of Pb, Zn and Cd in the tailings vary from 1.2% to 5.7%, 9.3% to 13.6%, and 185.6 to 410.6 mg kg−1, respectively. The soils surrounding the tailings piles, developed on a calcareous bedrock, are characterized by alkaline pH (H2O) ranging from 6.8 to 7.9. In addition, cation-exchange capacity (CEC) was low with a mean value of 4.25 cmol kg−1 and organic matter (OM) content varied between 2.23% and 6.13%. These soils contain high amounts of Pb, Zn and Cd, that can reach 3052 mg kg−1, 22,086 mg kg−1 and 99.3 mg kg−1, respectively. The contamination levels by heavy metals (HM) in the soils were evaluated using the modified degree of contamination (mCd). This evaluation shows that 60% of the analyzed soils are between the high degree and the ultra-high degree of contamination classes. The extraction tests using deionised water, salt (CaCl2), low-molecular-weight organic acids (LMWOAs) and chelating agent (DTPA) solutions were performed to evaluate the mobility of Pb, Zn and Cd in soil samples. Results show that, the extractability of heavy metals is slightly influenced by pH, CEC and total carbonate (TC) content of the soil. The amounts extracted with water and CaCl2 are negligible. The highest percentages of total heavy metals are extracted with DTPA (0.7–19.5% for Pb, 0.3–6.2% for Zn and 0.4–11.9% for Cd) and at a lesser degree by LMWOAs (0.1–0.8% for Pb, 0.1–2.5% for Zn and 0.1–1.9% for Cd). These results showed that the mobility of metals is generally very low and influenced by the high carbonate content of soils.