Enhanced soil toxic metal fixation in iron (hydr)oxides by redox cycles

The possibility to foster toxic metals (Cd, Cu, Ni, Pb and Zn) fixation on Fe (hydr)oxides, either native or added as ameliorants, was investigated at the laboratory scale in two heavily polluted arable and grassland soils by inducing repeated cycles of the reduction and oxidation. Soil samples were treated with Fe-rich waste (FeW) alone or together with fresh organic waste (OW) to accelerate reduction step. The FeW (242 g Fe kg−1) was an air dried sediment produced by treatment of olive mill waste waters and theOW(410 g C kg−1) was obtained by source separate collection of solid urban waste. The efficiency of metals immobilization was evaluated in terms of biological availability. Phyto-availability was assessed either by 1 M NH4 acetate extraction (exchangeable fraction) and by 0.005 M DTPA/0.1 M triethanolamine (pH 7.3) extraction, whereas hazard to humans from direct soil ingestion (oral bio-accessibility) was tested by the physiologically based extraction test (PBET). Aerobic incubation of soils treated with FeW+OW did not cause any significant change to the exchangeable fraction of toxic metals in both the arable and the grassland soils, whereas one to three redox cycles, applied after FeW+OWaddition, reduced Cd and Cu in both soils and Pb in the arable soil below detection limits. Exchangeable Zn was reduced in the FeW+OW treated soil by 50 and 90% in the arable and grassland soils, respectively. DTPA-extractable Cd, Cu and Zn were unaffected by treatments when soils were maintained oxic, but on the contrary, decreased significantly after one to three redox cycles by 88%, 93%, 36% and 95%, 98%, 65% in the arable and grassland soil, respectively. Lead showed a different behaviour with a general tendency to increase its availability. This result could be explained with the progressive dissolution of Mn oxides during the redox cycles. DTPA-extractable Ni showed in the arable soil a progressive increase, but remained constant in the grassland soil. A further reduction in DTPA-extractable metals, particularly evident for Ni, Pb and Zn, was observed after 150 d ageing of treated soils. Soil treatments were also effective in reducing potential hazard from direct soil ingestion in grassland soil (95%, 100%, 27% and 30% for Cd, Cu, Pb and Zn respectively), whereas the effect was much lower in the arable soil. The mechanisms of metal fixation are compatible with chemisorption and co-precipitation on Fe (hydr)oxides. These results suggest that redox cycles can be applied to FeW treated soils to reduce biological availability of many toxic metals in neutral soils, but their efficiency varies according to the element and soil composition. The addition of OW generally fosters metal fixation by intensification of the reductive process.