Abstract
Scanning transmission X-ray microscopy (STXM) was utilized for analysing the bioremediation of Cr(VI) by Citrobacter freundii, a species of gram-negative bacteria. The biosorption and bioreduction processes were analysed by the chemical mapping of cells biosorbed at different concentrations of Cr(VI). STXM spectromicroscopy images were recorded at O K-edge and Cr L-edge. A thorough analysis of the X-ray absorption features corresponding to different oxidation states of Cr in the biosorbed cell indicated the coexistence of Cr(III) and Cr(VI) at higher concentrations. This signifies the presence of partially reduced Cr(VI) in addition to biosorbed Cr(VI). In addition, the Cr(III) signal is intense compared with Cr(VI) at different regions of the cell indicating excess of reduced Cr. Speciation of adsorbed Cr was analysed for the spectral features of biosorbed cell and comparison with Cr standards. Analysis of absorption onset, L3/L2 ratio and absorption fine structure concludes that adsorbed Cr is predominantly present as Cr(III) hydroxide or oxyhydroxide. The evolution of absorption features in the duration of biosorption process was also studied. These time lapse studies depict the gradual decrement in Cr(VI) signal as biosorption proceeds. A strong evidence of interaction of Cr with the cell material was also observed. The obtained results provide insights into the biosorption process and chemical speciation of Cr on the cells.
Highlights
Heavy metal contamination arising from anthropogenic activities is becoming a major concern
We demonstrate the utilization of Scanning transmission X-ray microscopy (STXM) in analysing the bioremediation of Cr(VI) by Citrobacter freundii bacteria
Dried samples were re-dispersed in DI water and sufficiently diluted to obtain isolated cells suitable for STXM analyses. 1 μL of each of the diluted samples were drop-casted on silicon nitride window (50 nm thick, 0.5 mm × 0.5 mm; frame 200 μm, 5 mm × 5 mm) and air dried
Summary
Heavy metal contamination arising from anthropogenic activities is becoming a major concern. The obtained results pave way for further in-situ STXM studies required for a molecular level understanding of the process of biosorption and subsequent bio-reduction.
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