Assessment of heavy metal availability (Pb, Cu, Cr, Cd, Zn) and speciation in contaminated soils and sediment of discharge by sequential extraction

The accumulation of the heavy metals in the soil received a particular interest because of their toxicity and retention time in the soil which is slower than in other compartments of the biosphere. Knowledge of the total concentration of metals in soils and sediments is frequently insufficient to ascertain environmental risk. Simple and sequential extractions are useful tools for estimating the mobility of metals. In this study we were interested in highly toxic metals such as cadmium, lead, chromium, copper, and zinc in the soil of a controlled dump and the witness soil as well as the sediment of this dump in order to assess the mobility of these metals and their toxicity. The physicochemical parameters pH, organic matter, total calcium, cation exchange capacity, and total nitrogen were determined on the samples. Results show that average contents of heavy metals exceed the threshold recommended by the AFNOR NF U 44-041 standard. The results of the sequential extraction of heavy metals in the composite samples of soil and sediment according to the Community Bureau of Reference method show that cadmium is mainly associated with the exchangeable fraction (for sediment 77.7 % and for soil 40 %). Cadmium is therefore mainly associated with the mobile fraction and the risk of its transfer is high. Zinc is mainly bound to the metallic oxyhydroxides and carbonates, while Cu and Pb are mostly bound to organic matter and metallic oxyhydroxides in the proportions of 96.12 % (Cu) and 84.38 % (Pb), respectively.