Proposal of a simple screening method for a rapid preliminary evaluation of “heavy metals” mobility in soils of contaminated sites

Purpose Risks associated to “heavy metals” (HM) soil contamination depend not only on their total content but, mostly, on their mobility. Many extraction procedures have been developed to evaluate HM mobility in contaminated soils, but they are generally time consuming (especially the sequential extraction procedures (SEPs)) and consequently applicable on a limited number of samples. For this reason, a simple screening method, applicable even “in field”, has been proposed in order to obtain a rapid evaluation of HM mobility in polluted soils, mainly focused on the fraction associated to Fe and Mn oxide/hydroxides. Materials and methods A buffer solution of trisodium citrate and hydroxylamine hydrochloride was used as extractant for a single-step leaching test. The choice of this buffered solution was strictly related to the possibility of directly determining, via titration with dithizone (DZ), the content of Zn, Cu, Pb and Cd, which are among the most representative contaminants in highly mineralised soils. Moreover, the extraction solution is similar, aside from for the pH value, which is the one used in the BCR SEP second step. The analysis of bivalents ions through DZ titration was exploited in order to further simplify and quicken the whole procedure. The proposed method generically measures, in few minutes, the concentration of total extractable “heavy metals” expressed as mol L⁻¹ without distinguishing between elements. Results and discussion The proposed screening method has been developed and applied on soil samples collected from rural, urban and mining areas, representing different situation of soil contamination. Results were compared with data obtained from the BCR procedure. Conclusions The screening method demonstrated to be a reliable tool for a rapid evaluation of metals mobility. Therefore, it could be very useful, even “in field”, both to guide the sampling activity on site and to monitor the efficacy of the subsequent remediation action.