Equilibrium, thermodynamics and mechanisms of Ni2+ biosorption by aerobic granules

Aerobic granules with a mean size of 1 mm were used as biosorbent to remove soluble nickel ion from aqueous solution. Investigation of the mechanisms of Ni2+ biosorption by aerobic granules showed that Ni2+ biosorption was associated with a significant release of Ca2+ ion, e.g., 1 meq of Ni2+ adsorbed would result in a release of 0.68 meq of Ca2+. Compared to fresh aerobic granules, analysis by powder X-ray diffraction (XRD) further revealed that no new crystal was formed in the aerobic granules after Ni2+ biosorption. These seem to imply that a chemical precipitation mechanism would not be largely involved in the Ni2+ biosorption by aerobic granules. The contribution of extracellular polymeric substances (EPS) to the Ni2+ biosorption by aerobic granules was also examined. It was found that the EPS-associated Ni2+ biosorption by aerobic granules only accounted for about 14.2% of the total Ni2+ removed. The biosorption isotherm equation previously derived from a thermodynamic principle was applied to this study and it can provide a satisfactory description for Ni2+ biosorption by aerobic granules at various temperatures. It was found that the equilibrium constants of the Ni2+ biosorption increased with increase of temperature from 25 to 55 °C, i.e., the biosorption of Ni2+ by aerobic granules is favored at high temperature. The thermodynamic parameters (ΔG°, ΔH° and ΔS°) were also determined, and it was found that the Ni2+ biosorption by aerobic granules was an endothermic process. Meanwhile, the energy dispersive X-ray (EDX) analysis indicated that Ni2+ ion could penetrate into the core of the aerobic granule, and the distribution of the adsorbed Ni2+ in the aerobic granule seemed uniform.