Bioremediation of hexavalent and trivalent chromium using Citrobacter freundii: a mechanistic study

Abstract

The mechanisms involved in the bioremediation of hexavalent chromium (Cr(VI)) and trivalent chromium (Cr(III)) by a Gram negative bacterium, Citrobacter freundii were independently investigated. The biosorption isotherms of Cr(VI) and Cr(III) for C. freundii exhibited a typical Langmuirian behaviour. The Gibbs free energy (ΔG) was determined to be -25.5 and −27.2 kJ mol−1, respectively for Cr(VI) and Cr(III), suggestive of chemisorption between the functional groups of bacterial surface and Cr. The relatively lesser amount of desorption of Cr(VI) or Cr(III) from the bacterial cells attested to the irreversible nature of biosorption. FTIR studies revealed the involvement of carboxyl, amino and hydroxyl groups in the biosorption of Cr(VI) and Cr(III). X-ray photoelectron spectroscopic studies provided evidence in support of the bioreduction from Cr(VI) to Cr(III). Electrokinetic studies revealed that the bacterial cells become less electronegative after interaction with Cr species. The mechanisms of bioremediation have been delineated to involve both biosorption and bioreduction processes for Cr(VI), while for Cr(III), biosorption is the governing process of remediation.