Enhanced phosphate removal by zeolite loaded with Mg-Al-La ternary (hydr)oxides from aqueous solutions: Performance and mechanism

A novel La-incorporated nanostructured ternary (hydr)oxides adsorbent (MALZ) was prepared and employed for enhanced phosphate removal in this study. The results of scanning electron microscopy (SEM) and X-ray diffraction (XRD) indicated that MALZ presented an amorphous surface with Mg, Al and La homogeneously dispersed on the outer region of zeolite. Effective phosphate removal was obtained over a wide pH range of 4-10 and the equilibrium pH after phosphate adsorption was in near neutrality. A relatively fast adsorption of phosphate was observed in the kinetics experiment, and the maximum adsorption capacity of MALZ (12.5 wt% La) was determined at 80.8 mg P/g at pH 6.6 in the equilibrium adsorption isotherm study. The coexisting anions (e.g., Cl−, SO42− and NO3−) had insignificant effects on phosphate adsorption. The adsorption-desorption experiment suggested that MALZ could be regenerated by 0.5 M NaOH solution and maintained 75.8% of its initial phosphate adsorption capacity at the fourth adsorption-desorption cycle. The combined results of zeta potential analysis, SEM, XRD and X-ray photoelectron spectroscopy demonstrated that electrostatic attraction, ligand exchange of surface hydroxyl groups by phosphate and the complexation of forming inner-sphere complex (LaPO4·xH2O) were the main adsorption mechanisms. Furthermore, La played a key role in the complexation of phosphate, while the introduction of Mg and Al further facilitated the dispersion of La and enhanced the surface charge of the adsorbent.