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

Abstract

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.

Keywords:

• Bone char
• Co²⁺
• sorption kinetics
• kinetic models

Acknowledgments

This work was supported by the Ministry of Science and Technological Development of the Republic of Serbia, under Project No. 43009.