Mechanistic studies on the bioremediation of Cr(VI) using Sphingopyxis macrogoltabida SUK2c, a Cr(VI) tolerant bacterial isolate

•55% biosorption of chromium ions was achieved under optimum conditions.•After bioremediation by S. macrogoltabida only Cr(III) present in bulk solution.•XPS studies confirmed the presence of Cr(III) on the bacterial cell surface.•Extracellular bioreduction of Cr(VI) validated by chromate reductase assay tests.•Biosorption and bioreduction processes govern Cr(VI) bioremediation mechanism. Bioremediation studies of toxic hexavalent chromium (Cr(VI)) were investigated using an indigenous bacterial strain namely, Sphingopyxis macrogoltabida SUK2c, isolated from water samples collected from the Sukinda Valley in Odisha, India. A maximum Cr(VI) biosorption of about 55% could be achieved using the isolate for an initial Cr(VI) concentration of 4 mg L−1. The Cr(VI) biosorption isotherm was found to follow a typical Langmuirian behaviour. The Gibbs free energy value of Cr(VI) biosorption obtained was -25.6 kJ/mol, indicative of the involvement of chemical binding forces. The Cr(VI) biosorption process followed pseudo second order kinetics. FTIR spectral studies revealed that carboxyl, hydroxyl, amino and phosphate groups present on bacterial surface were involved in the complexation process. XPS studies confirmed the involvement of Cr(III) in addition to Cr(VI) ions with the bacterial cell surface. Zeta potential studies showed that the bacterial cells became less negative after interaction with Cr(VI), which further corroborated the binding of positively charged Cr(III) on the cell surface. The marginal shift in iso-electric point for Cr(VI) interacted bacteria further testified to the involvement of chemical binding forces in the bioremediation process. The results of the chromate reductase and Bradford protein assay tests performed on the extracellular component of the isolate also confirmed the involvement of extracellular protein in the reduction of Cr(VI) to Cr(III).