Adsorption mechanism of Chromium(III) from water solution on bone char: effect of operating conditions

The adsorption capacity of bone char (BC) towards Cr(III) from an aqueous solution was studied in this work. The characterization of the BC showed that the BC is mainly composed of hydroxyapatite, Ca₁₀(PO₄)₆(OH)₂, and its point of zero charge was 7.7. The infrared spectroscopic analysis of the BC loaded with Cr(III) revealed that the Cr(III) adsorbed on BC interacted with the phosphate of the hydroxyapatite. The adsorption capacity increased 2.6 times by raising the solution pH from 3 to 5. This tendency was due to the interactions between the surface charge and the cationic species of Cr(III) in aqueous solution. The adsorption capacity was enhanced 2.5 and reduced 1.3 times when the temperature was increased from 15 to 25 °C and from 25 to 35 °C, respectively. The effect of temperature on the adsorption capacity of BC showed an anomalous behavior since the adsorption capacity exhibited a maximum at T = 25 °C. It was demonstrated that the main adsorption mechanism of the Cr(III) species was coordination to the phosphate ions and the Ca(II) in the BC was substituted or replaced by the Cr(III) species from the solution. This latter result was further corroborated by XPS analysis.