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The effect of process parameters on kinetics and mechanisms of Co2+ removal by bone char

Bone char powder, composed mainly of poorly crystalline hydroxyapatite (Ca10(PO4)6(OH)2), carbon and CaCO3, has potential applicability in the removal of Co2+ ions from contaminated effluents. In the present study, the influence of process parameters: particle size, agitation speed, initial pH and initial sorbate concentration, onto kinetics and mechanism of Co2+sorption was studied and discussed. In order to describe and compare time evolution of the process under different conditions, the experimental data were analyzed using pseudo–first, pseudo–second and Vermeulen's kinetic models. Generally, experimental results were best fitted with the pseudo–second-order model, which accurately predicted the equilibrium sorbed amounts. The pseudo–second-order rate constant was the most influenced by variations in initial metal concentration and pH, in the investigated ranges. The conclusions about sorption mechanism were derived based on Co2+ amounts sorbed during time, as well as considering solution pH changes, changes of Ca2+ amounts released into liquid phase and Ca2+/Co2+ molar ratios. It was concluded that rapid sorption stage was governed by surface complexation reactions, whereas the contribution of the ion-exchange mechanism increased with time and became more significant in the second, slower phase. Experimentally determined maximum sorption capacity towards Co2+, under optimal conditions, was found to be 0.38 mmol/g. The results show that bone char represents cost-effective alternative to synthetic hydroxyapatite sorbent.