Adaptation of indigenous iron‐oxidizing bacteria for bioleaching of heavy metals in contaminated soils

Abstract

Indigenous iron‐oxidizing bacteria in soil were acclimated in three different contaminated soils to remove toxic heavy metals. The adaptation was carried out at a pH of 4.0 and a room temperature of 24 ± 2°C in a gyratory shaking incubator. The iron‐oxidizing microflora was adapted in three to four successive transfers. Ferrous iron was supplemented by addition of FeSO₄.7H₂O. The oxidation of ferrous iron was associated with an increase in ORP up to 500 to 600 mV. The final pH in the soil solution after bacterial adaptation was lower than 2.5 for all the soils tested. Amendment of the soil with ammonium sulphate and dipotassium hydrogen phosphate increased the rate of iron oxidation by iron‐oxidizing microflora. The acclimation of the indigenous microflora 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 different 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.

Keywords:

• Contaminated soils
• iron‐oxidizing bacteria
• T. ferrooxidans
• adaptation
• heavy metals