Comparative study of chromium biosorption by Mesorhizobium amorphae strain CCNWGS0123 in single and binary mixtures

The appearance of chromium in the aqueous effluent is a major concern for the modern industry. In this work, Mesorhizobium amorphae strain CCNWGS0123 was investigated as a biosorbent to remove chromium from aqueous solutions. The optimum pH for Cr(III) and Cr(VI) biosorption were 4 and 2, respectively. This isolate showed an experimental maximum Cr(III) adsorption capacity of 53.52 mg L(-1), while the result was 47.67 mg L(-1) for Cr(VI), with an initial 100 mg L(-1) Cr ions and 1.0 g L(-1) biomass. In terms of time equilibrium, Cr(III) ion was more readily adsorbed than Cr(VI) by this isolate. The biosorption data of both ions fit the Langmuir isotherm better than that of Freundlich model. Meanwhile, this organism exhibited a good capability to release Cr ions, with desorption efficiency of 70 % for Cr(III) and 76 % for Cr(VI). Fourier transform infrared spectroscopy analysis showed that -OH, -COO, -NH, amide I, and C=O were involved in Cr(III) and Cr(VI) binding. The biosorbent was further characterized by scanning electron microscopy and energy-dispersive X-ray spectrometry, which indicated an accumulation of chromium on the cellular level. In the binary mixtures, the removal ratio of total Cr and Cr(III) increased from pH 2 to 4. The highest removal ratio of the total Cr was observed in the 25/25 mg L(-1) mixture at pH 4. In addition, the removal efficiency of Cr(VI) was closely influenced by Cr(III) in the mixture, decreasing to 23.57 mg g(-1) in the 100/100 mg L(-1) mixture system, due to the competition of Cr(III). The potential usage of the chromium-resistant rhizobium for the remediation of chromium-contaminated effluents has been demonstrated based on the above results.