The effects of operational parameters (pH, contact time, initial Cd(II) concentration, and temperature), kinetics, equilibrium, and thermodynamics of Cd(II) biosorption by compound bioflocculant (CBF), produced by mixed culture of Rhizobium radiobacter F2 and Bacillus sphaeicus F6, were investigated. Desorption-reuse study and Fourier transform infrared (FTIR) spectra were also examined towards a better standing of the biosorption mechanism. The maximum biosorption capacity was obtained at pH of 7.0 and contact time of 60 min. The biosorption data obeyed pseudo-first-order model better than pseudo-second-order equation and Elovich equation. The equilibrium data were analyzed using the Langmuir, Freundlich and Redlich-Peterson isotherm models, and Langmuir isotherm gave the best fit. The thermodynamic parameters indicated that the biosorption process was endothermic and spontaneous. The adsorbed Cd(II) on CBF could be effectively desorbed by HCl and NaOH, and about 65% of the initial biosorption capacity was regained after 2 cycles of biosorption-desorption-elution using HCl as the desorbing agent. The FTIR characterization indicated that -OH, -NH, -C=O, and –COOH and C-N groups may be involved in the interaction. It was concluded that CBF in this study can be used as an effective and environmental friendly biosorbent for the removal of Cd(II) ions from aqueous solution.
Keywords: Biosorption, cadmium, compound bioflocculant, desorption, EPSs, extracellular polymeric substance, equilibrium, FTIR, heavy metal, pollution, isotherm, kinetics, mechanism, reuse, thermodynamics, water treatment.