Kinetics and mechanisms of mercury biosorption by an exopolysaccharide producing marine isolate Bacillus licheniformis

Eight exopolysaccharide (EPS) producing metal-removing marine bacteria were screened for mercury (Hg) sorption. Bacillus licheniformis with the highest MIC values and Hg sorption ability was selected for further study. Biosorption of Hg from aqueous solution by Bacillus licheniformis was studied with respect to the metal concentration, adsorbent concentration, pH, different contact times, and in the presence of other metal ions. Under optimum conditions, more than 70% mercury was removed by 25 mg dried biomass of Bacillus licheniformis at pH 7.0 after 1 h of contact time. Freundlich adsorption isotherm was acceptable at studied Hg concentrations as compared to Langmuir isotherm model. Pseudo-second-order kinetic model was found to be more suitable for data presentation in contrast to pseudo-first-order kinetic model. Involvement of external mass transfer was prominent as compared to intraparticle diffusion model. Desorption of Hg was more effective with acids from all the studied eluents, showing 49.36 and 33.8% eluting capacity for 0.1 N HCL and 0.1 N HNO_3, respectively. Scanning electron microscopy exhibited altered cell surface morphology of the cells under the influence of mercury. The spectral images of energy dispersive spectroscopy showed the presence of metal ions on the surface of cells.