Fe oxides

This study evaluates the possibility to immobilise toxic metals (Cd, Cu, Ni, Pb and Zn) in contaminated soils by applying a chemical treatment consisting in repeated cycles of soil saturation with 0.1 M FeSO4, air drying and pH neutralisation with Ca(OH)2. The treatment was tested on a contaminated area soil analyzing exchangeable (Mg(NO3)2 extractable), phyto-available (DTPA/TEA extractable) and orally bio-accessible (PBET extractable) metals, after applying 1, 4 and 8 cycles. After the 8th cycle, the treatment decreased the exchangeable fractions of Cd, Cu, Pb and Zn by 70%, 87.7%, 73.5% and 49.8% respectively, whereas those of Ni and Mn showed a marked increase. DTPA-extractable fractions followed the same trend with a decrease of 89%, 83% and 63% for Cd, Cu, Pb and Zn, respectively. Oral bio-accessibility of toxic elements showed a marked decrease for all metals ranging between 61% to 80% for Cu. The decrease is noticeably related to a leaching first induced by ammonium, was especially evident for Cu and Zn (65% and 61%, respectively) and highly significant for Cd (31% and 47%, respectively), whereas Pb mobility was unaffected. The subsequent chemical process caused immobilisation of all metals except Ni, which showed surface saturation and metal adsorption onto freshly precipitated Fe (hydr)oxides, followed by occlusion of metals at each subsequent cycle. There is great potential for using repeated additions of Fe(II) to enhance the effectiveness of remediated surface and subsurface soils.