The batch study of Sr2+ sorption by bone char

Abstract

Considering the excellent sorption properties of synthetic calcium hydroxyapatite (HAP) towards many divalent cations, the potential application of bone char, the natural source of HAP, for sequestering Sr(2+)ions from aqueous solutions has been studied in batch conditions. Contact time, initial solution pH and initial Sr2+ concentrations were varied to examine the effect of these process parameters on the amount of Sr2+ sorbed. The kinetics of Sr2+ sorption was found to be a 2-step process, with contact time of 24 h required for attaining equilibrium. The sorption isotherm was well fitted with Langmuir and DKR theoretical models. Sorption of Sr2+ on bone char was found to be a favorable, thermodynamically feasible and spontaneous process, with the maximum sorption capacity of 0.271 mmol/g and sorption energy of 11.09 kJ/mol. The sorption was pH-independent in the initial pH range 4-10, as a result of excellent buffering properties of bone char (constant final pH), while for pH GT 10 sorbed amounts of Sr2+ increased due to attractive electrostatic forces between negatively charged sorbent surface and positively charged metal ions. On the basis of the amount of Ca2+ released and final pH decrease in respect to the point of zero charge of bone char (pHPZC), two possible mechanisms of Sr2+ sorption were identified: ion-exchange and the formation of complex compounds with HAP and carbon active surface sites. The amounts of Sr2+ leached from bone char increased with the increase of Ca2+ content and the decrease of solution pH. In comparison with synthetic HAP, bone char represents a cost-effective alternative for Sr2+ sequestering.