Transfer of lead, zinc and cadmium from mine tailings to wheat (Triticum aestivum) in carbonated Mediterranean (Northern Tunisia) soils

Abstract

Carbonate-rich mine tailings are exposed to severe erosion and dispersion in Mediterranean climates. In Jalta mining area, the dispersion of particles that contain (Pb, Zn and Cd) results in the contamination of the surrounding agricultural soils. These soils have high concentrations of Pb (17,229mg/kg), Zn (4813mg/kg) and Cd (42mg/kg). Mobilization of heavy metal rich particles from mining waste occurs from aeolian and hydric erosion. The vertical distributions of heavy metals show superficial accumulation resulting from the spreading of mining wastes by runoff and atmospheric fallout. Heavy metal-bearing particles in the surface horizons include sulfides, carbonates, iron oxyhydroxides and sulfates. Selective extractions used to estimate the risks of toxic element mobilization show that a very low proportion of heavy metals are water soluble and exchangeable (mean water soluble fractions are 481, 446 and 6.3μg/kg for Pb, Zn and Cd, respectively). Similarly, the mean exchangeable fractions are 198, 0.17, and 4.4μg/kg for Pb, Zn and Cd, respectively. The transfer of heavy metals from agricultural soils to cultivated plants is a principal hazard. Heavy metal concentrations in wheat grown on contaminated soils are high (Pb contents of 1824 and 54mg/kg and Zn contents of 1546 and 636mg/kg in the roots and aerial parts, respectively). These toxic metal concentrations exceed the thresholds plant toxicity which are 20 and 400mg/kg for Pb and Zn, respectively (national research council (US) 2005).