Isolation of cadmium-resistant microbial strains and their immobilisation of cadmium in soil

Six cadmium (Cd)-resistant microbial strains were isolated and their ability to immobilise Cd²⁺ in soil investigated. Cd-1, Cd-2, Cd-5, and Cd-6 were identified as Stenotrophomonas sp., Cd-3 as Achromobacter sp., and Cd-7 as Staphylococcus sp. The six strains showed a wide adaptation range for salinity and a strong tolerance to Cd²⁺. The effects of the initial Cd²⁺ concentration (1-100 mg/L), duration (18-72 h), temperature (10-40 °C), and pH (5.0-9.0) on the efficiency of Cd²⁺ removal were analysed. The results revealed that the Cd²⁺ removal rate was higher at an initial Cd²⁺ concentration of 5-100 mg/L than at 1 mg/L. The maximum Cd²⁺ removal effect was at a culture duration of 36 h, temperature of 10-35 °C, and pH of 5.0-7.0. X-ray diffraction (XRD) analysis revealed that the Cd²⁺ was immobilised by Stenotrophomonas sp. Cd-2 and Staphylococcus sp. Cd-7 through bio-precipitation. X-ray photoelectron spectroscopy (XPS) revealed that the Cd²⁺ was adsorbed by Stenotrophomonas sp. Cd-2, Achromobacter sp. Cd-3, and Staphylococcus sp. Cd-7. Fourier transform infrared spectroscopy (FTIR) analysis revealed that the isolates reacted with the Cd²⁺ mainly through the O-H, protein N-H, C-N, lipid C-H, fatty acid COO, polysaccharide C-O, P-O, and other functional groups, as well as with lipid molecules on the cell wall surfaces. Scanning electron microscopy (SEM) analysis revealed that there was little difference in the cells after Cd²⁺ treatment. The results of the soil remediation experiments indicated that the toxicity of Cd in soil could be effectively reduced using certain strains of microbe.