ADAPTATION OF INDIGENOUS IRON-OXIDIZING BACTERIA FOR BIOLEACHING OF HEAVY-METALS IN CONTAMINATED SOILS

Indigenous iron-oxidizing bacteria in soil were acclimated in three di fferent contaminated soils to remove toxic heavy metals. The adaptatio n was carried out at a pH of 4.0 and a room temperature of 24 +/- 2-de grees-C in a gyratory shaking incubator. The iron-oxidizing microflora was adapted in three to four successive transfers. Ferrous iron was s upplemented by addition of FeSO4.7H2O. The oxidation of ferrous iron w as associated with an increase in ORP up to 500 to 600 mV. The final p H in the soil solution after bacterial adaptation was lower than 2.5 f or all the soils tested. Amendment of the soil with ammonium sulphate and dipotassium hydrogen phosphate increased the rate of iron oxidatio n by iron-oxidizing microflora. The acclimation of the indigenous micr oflora reduced the metal leaching period from 18-30 days to 2-8 days. The metal leaching capacity of the indigenous iron-oxidizing bacteria seems to be influenced by the speciation of the metals in the differen t soils. This process of bioleaching appeared to be equally efficient for the removal of Zn (57 +/- 32%) and Mn (57 +/- 28%) whereas Cu (49 +/- 13%) was solubilized to a lesser extent. On the whole, metals were more solubilized in Montreal soil (80 +/-24%) than in Stratford (54 /- 11%) and Weedon (29 +/- 8%) soils.