Immobilisation of soil toxic metals by repeated additions of Fe(II) sulphate solution

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 arable 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%, 80%, 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% for Ni to 80% for Cu. The decrease in mobility, estimated by a leaching test performed on soil columns, was especially evident for Cu and Zn (65% and 61%, respectively) and highly significant for Cd and Ni (31% and 47%, respectively), whereas Pb mobility was unaffected. The metals fixation obtained by this treatment seems compatible with a mechanism involving co-precipitation and metal adsorption onto freshly precipitated Fe (hydr)oxides, followed by occlusion of adsorbed metals at each subsequent cycle. Ageing of precipitates further increased immobilisation of all metals except Zn. After 48 h since the end of the treatment, soil microbial biomass C and enzyme activities slightly decreased compared to the control, but recovered after 192 h of incubation. Soil treatment effectively reduced toxicity of metals to microrganisms as shown by increased microbial growth and enzyme activities in the treated soil after addition of glucose.