Effective phosphate adsorption by Zr/Al-pillared montmorillonite: insight into equilibrium, kinetics and thermodynamics

Zirconium-pillared montmorillonite (Zr-Mt) and zirconium/aluminum-pillared montmorillonite (Zr/Al-Mt) were prepared by intercalating Zr4+ and Zr4+/Al3+ polyhydroxy-cations into the interlayer spaces of natural montmorillonite (Mt). Zr-Mt and Zr/Al-Mt exhibited greater specific surface areas and pore volumes, as compared with Mt. In particular, the enhancement in the phosphate removal performance of Zr/Al-Mt was more significant in terms of phosphate adsorption capacity and rate, in relation to Zr-Mt. Herein, several important parameters, such as contact time, temperature, initial solution pH and competing anion, were investigated in detail to evaluate the phosphate adsorption performances of Zr/Al-Mt. The pseudo-second-order kinetic model fitted our acquired phosphate adsorption data best, in comparison with the use of the pseudo-first-order or the pseudo-third-order kinetic model. The Langmuir model appeared to fit the adsorption process better than that of the Freundlich model, with a maximum phosphate adsorption capacity of 17.2mg P/g at 25°C. The thermodynamic parameters (δG°, δH° and δS°) were also determined, which revealed that the phosphate adsorption process was spontaneous and endothermic in nature. A high adsorption capacity was observed at pH=3.0, which decreased by increasing pH. The presence of competitive ions, e.g. Cl-, NO3 -, and SO4 2-, slightly impacted the phosphate adsorption; whereas the introduction of CO3 2- caused the greatest adverse effect. The study on the Zr/Al-Mt leaching strongly suggested that the risk of Zr4+ leakage during adsorption process be negligible within a wide pH range of 3.0-9.0. After three adsorption-desorption cycles, there was no significant loss in the adsorption performance of Zr/Al-Mt.