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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 Sr(2+) concentrations were varied to examine the effect of these process parameters on the amount of Sr(2+) sorbed. The kinetics of Sr(2+) 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 Sr(2+) 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 > 10 sorbed amounts of Sr(2+) increased due to attractive electrostatic forces between negatively charged sorbent surface and positively charged metal ions. On the basis of the amount of Ca(2+) released and final pH decrease in respect to the point of zero charge of bone char (pH(PZC)), two possible mechanisms of Sr(2+) sorption were identified: ion-exchange and the formation of complex compounds with HAP and carbon active surface sites. The amounts of Sr(2+) leached from bone char increased with the increase of Ca(2+) content and the decrease of solution pH. In comparison with synthetic HAP, bone char represents a cost-effective alternative for Sr(2+) sequestering.

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