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

• Zr-Mt and mixed Zr/Al-Mt were prepared for phosphate removal for the first time.
• Zr/Al-Mt exhibited significantly enhanced phosphate removal capacity.
• The electrostatic attraction played a main role in phosphate adsorption of Zr/Al-Mt.
• The risk of Zr4 + leakage was almost negligible in a wide pH range of 3.0–9.0.
• The adsorbent could be regenerated and repeatedly used for phosphate removal.

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 17.2 mg 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 SO42 −, slightly impacted the phosphate adsorption; whereas the introduction of CO32 − 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.

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