Application of hydroxyapatite-modified carbonized rice husk for the adsorption of Cr(VI) from aqueous solution

Abstract

Agricultural and forestry waste biomasses have been widely used as adsorbents for environmental remediation with the advantages such as sustainability, economic efficiency, and excellent adsorption performance. A biochar-supported nano-Hydroxyapatite-modified carbonized rice husk (HAP@CRH) with high-efficiency and low-cost has been developed in our previous study. In this study, the effects of application condition parameters on the adsorption performance of HAP@CRH for Cr(VI) ions are further investigated. The results illustrate that 99.28 % Cr(VI) ions are removed at 50 °C, pH 2.0 with the adsorbent dosage of 5.0 g/L and the initial Cr(VI) ions concentration of 40 mg/L after adsorption equilibrium. The kinetic data is highly consistent with the pseudo-second-order model, indicating that the adsorption is mainly controlled by chemisorption. It also can be well-described by the the intra-particle diffusion model and Boyd’s film-diffusion kinetic model, indicating the external mass transport is a major process where particle diffusion is the rate-limiting step. Moreover, adsorption data is consistent with the Langmuir isotherm with an adsorption saturation capacity of 14.28 mg/g. It can be speculated that the adsorption mechanism includes physical adsorption, chemical adsorption, and ion exchange. Overall, the value of adsorption enthalpy change (ΔH) is 49.57 kJ/mol through thermodynamic calculations, which is over 40 kJ/mol, suggesting that the adsorption process is chemisorption, whcih is inherently spontaneous and exothermic, and the elevation of the temperature can result in the promotion of adsorption performance. It is anticipated that the composite of HAP@CRH with high adsorption capacity has the potential to be a newly developed adsorbent for Cr(VI) ions removal with a bright application prospect in environmental protection.