Adsorption of tripolyphosphate from aqueous solution by Mg–Al–CO 3-layered double hydroxides

Abstract

The adsorption of tripolyphosphate anions (P 3O 10 5−) from aqueous solution on Mg–Al–CO 3-layered double hydroxides (Mg–Al–CO 3-LDHs) and its calcined product (Mg–Al–CO 3-CLDH) were investigated. Experiments were carried out as a function of contact time, temperature, initial concentration of solution, adsorbent dosage, initial pH of solution and different phosphate species. The results show that the Mg–Al–CO 3-LDH and Mg–Al–CO 3-CLDH can be used as effective adsorbents for the removal of P 3O 10 5− from aqueous solution. The adsorption of P 3O 10 5− on Mg–Al–CO 3-LDH and Mg–Al–CO 3-CLDH are by ion exchange and ‘memory effect’, respectively. The adsorption processes are followed the pseudo-second-order kinetic model. The rate constants are increased with temperature. The activation energy values on Mg–Al–CO 3-LDH and Mg–Al–CO 3-CLDH are 11.70 kJ mol −1 and 98.40 kJ mol −1. The adsorption data are fitted well with the Langmuir adsorption isotherm equations. The maximum adsorption capacities on Mg–Al–CO 3-LDH and Mg–Al–CO 3-CLDH from 298 K to 313 K are 34.52–38.00 mg P 3O 10 5− g −1, 118.67–132.92 mg P 3O 10 5− g −1, respectively. The adsorption capacities are increased with temperature shows that the adsorption processes are endothermic. The removal rate is changed insignificantly for a wide initial pH of 4.0–10.0. The affinity sequences of different phosphate species on Mg–Al–CO 3-LDH and Mg–Al–CO 3-CLDH are both HPO 4 2− > H 2PO 4 − > P 3O 10 5−.