Recovery of soils contaminated with heavy metals: preliminary results

This paper assesses the heavy metal absorption capacity of a natural zeolite to evaluate whether it can be used to reduce metal mobility and bioavailability, and also to recover polluted soil. In the experiments zeolite was artificially polluted at controlled pH and temperature values with solutions containing increasing concentrations of three heavy metals (Pb, Zn and Cd). The results showed that the zeolite absorption preference for the heavy metals was as follows: Pb > Cd > Zn. The isotherm for Pb was L-TYPE, which is typically found when metal affinity is high in the initial steps of the absorption. Zn and Cd were absorbed less by the zeolites. Their isotherms were C and L-TYPE, respectively. After contamination all samples were treated with solutions of gradually increasing extracting capacity (H2O, Ca(NO3)2 0.1N, EDTA 0.05N) to study the role of different heavy metal-zeolite interaction mechanisms. The results showed that the non-extracted Pb fraction was constant in the different metal concentrations in solution. By contrast, the non-extracted Zn and Cd fractions increased as the concentrations of the two heavy metals increased. Thus we conclude that the zeolite examined may be profitably used for recovering soils polluted with Pb, Zn and Cd, but in different amounts depending on the particular chemical species.