Geochemical interactions in the trace element–soil–clay system of treated contaminated soils by Fe-rich clays

Clays have been widely applied in contaminated soils in order to reduce the mobility of potentially toxic elements (PTEs), such as Pb, Zn and Cu. In the present study, three Fe-rich clays from Greece were selected as amendments of three contaminated soils with distinct physicochemical and mineralogical characteristics. The amendments consisted of palygorskite-rich (PCM), Fe-smectite-rich (SCM) and natural palygorskite/Fe-smectite-rich (MCM) clays. The changes induced in the environment of the soil–PTE–clay system were assessed by examining the water-labile fraction of Pb, Zn and Cu, as well as the bioaccessibility of Pb, in the contaminated soils. The initial water-leachable concentrations of PTEs in soil were within the range 1826–6160 μg/kg Pb, 152–645 μg/kg Cu and 370–4052 μg/kg Zn. All three Fe-rich clays exhibited high retention efficiency toward PTEs, following the order Pb (55–70%) > Zn (45–55%) > Cu (0–45%). The high reactive surface area of the clay particles acted as a substrate for the deposition of Fe–Al oxides with a concomitant removal of PTEs that were transported through the colloidal fraction. Furthermore, the decrease in relative bioaccessibility of Pb (5–10% compared to the control) suggests dissolution of primary clays followed by entrapment of the element in secondary Fe-rich precipitates. In conclusion, the use of Fe-rich clays as soil amendments may have a positive effect in reducing the environmentally significant PTE fraction in soils, especially when different clay phases coexist.