Effects of bacteria on metal bioavailability, speciation, and mobility in different metal mine soils: A column study

Purpose Successful phytoremediation depends mainly on the bioavailability of heavy metals in the soil. Recently, soil microbes possess several mechanisms that are able to change metal bioavailability in the soil, which provides a new strategy for investigating biogeochemical cycling of metals in contaminated soils. Three metal mines soils with elevated concentrations of Cd, Pb, and Zn from China were applied in this column study to (1) evaluate the effects of metal tolerant bacterial inoculation (Burkholderia cepacia, accession number: AB051408) on metal release, (2) monitor the migration of metals in the rhizospheric horizon (0-20 cm), and (3) investigate metal speciation and sequential fractions in soil. Materials and methods All soils were air dried, crushed to pass through a 2-mm sieve and then autoclaved at 121°C for 2 h. The control soil was then mixed with different metal polluted soils, respectively, at four percentages (w/w), prior to being packed into polyvinyl chloride (PVC) columns. Four PVC columns were added with B. cepacia peat-based inocula, and the final concentration of the bacteria is 3.0×108 cell/g soil. The other four columns received the same amount of the autoclaved peat-based inocula (without bacteria) in order to keep the soil texture and other properties similar. The columns were applied with 80 mL deionized water daily for 12 weeks. At the end of the experiment, the soil samples from two horizons (0-10 and 10-20 cm) were collected for physico-chemical and metal speciation analyses. Results and discussion Results indicated that bacterial inoculation significantly enhanced the extractability of water-soluble Cd, Cu, and Zn (single-chemical extraction) in all metal mine soil treatments. However, inoculation of bacteria had a minor effect on As and Pb extractability. Increasing water-soluble and Olsen-P in soil solution induced by the presence of bacteria may contribute to the decrease of water-soluble Pb through sedimentation of Pb phosphate. Bacterial inoculation also increased the mobility of heavy metals (sequential chemical extraction). HOAcsoluble Cd and Zn increased as the pH value decreased since the metal adsorption on colloids weakened due to a decrease in pH-dependent negative charge. In addition, a significant increase in water-soluble N, P, NaHCO₃-extractable P, and soil acid phosphatase activity were observed. Conclusions Based on the above results, it can be concluded that metal tolerant bacteria are efficient in increasing the bioavailability of Cd and Zn when expressed relative to the control treatment, which might be of great significance for the successful phytoextraction of metal polluted sites.