Removal of cadmium ions by magnesium phosphate: Kinetics, isotherm, and mechanism studies

Abstract

In this study, magnesium phosphate with needle structure has been prepared by simple co-precipitation method. XRD, FTIR and SEM-EDX analyses indicated the successful fabrication of Mg3(PO4)2·5H2O phase. Subsequently, the adsorption performance of the material was tested toward Cd(II) removal from aqueous media. Batch experiments indicated that the solution pH highly influences the removal efficiency of Cd(II) with a maximum uptake observed at pH=3. Besides, the examination of the adsorption kinetics highlights that the data could be fitted by both pseudo 2nd order and Elovich models. This suggests that adsorption/desorption process in the limiting step in the kinetics of Cd(II) uptake. The Langmuir maximum adsorption capacity was 943 mg/g and the calculated energy from Dubinin–Radushkevich model was E=8.75 kJ/mol. This result indicates that the uptake of Cd(II) onto Mg3(PO4)2·5H2O is essentially an ion exchange process. Thermodynamic study suggests spontaneous and exothermic adsorption process. Besides, the XRD diffractogram of the adsorbent after saturation with Cd(II) revealed the presence of characteristic peaks related to Cd5H2(PO4)4·4H2O phase.

Keywords

• Metal phosphates
• Heavy metals
• Adsorption
• Ion exchange mechanism