Enhanced adsorption removal of phosphate from water by mixed lanthanum/aluminum pillared montmorillonite

In order to explore the feasibility of improving phosphate adsorption ability of aluminum pillared clays (Al-PILCs) by doping with lanthanum, mixed lanthanum/aluminum pillared clays (LaAl-PILCs) were prepared and investigated for their ability to remove phosphate from water. The structures of LaAl-PILC and Al-PILC were confirmed by XRD characterization. The applicability of adsorption models for the present isotherms approximately followed the order: Freundlich > Langmuir > Temkin. As the isotherms indicated, LaAl-PILC has a phosphate adsorption capacity of about 1.3 times the value of Al-PILC. The thermodynamic analysis indicated that the adsorption process was spontaneous and exothermic in nature. Pseudo-first-order kinetic equation can provide satisfactory fitting of the kinetic data. The pseudo-fist-order kinetic constant (k1) of LaAl-PILC, 1.0086 h−1, is much greater than that of Al-PILC, 0.4369 h−1. The phosphate adsorption on the PILCs tends to increase with a decrease of pH, maximum adsorption occurred at around pH <5.0 and <4.0 for LaAl-PILC and Al-PILC, respectively. The concomitant anions have profounder adverse effect on phosphate adsorption of LaAl-PILC than on that of Al-PILC and the effects follow the order: HCO3− > Cl− > SO42− ≈ NO3−. The concentration of lanthanum released from LaAl-PILC decreased gradually with the increase in pH and cannot be detected at pH higher than 4.5. LaAl-PILC was successfully tested as an adsorbent for the removal of phosphate from a contaminated water samples containing phosphate produced in process of yellow phosphorus and regenerated for reuse by removing the adsorbed phosphate with NaOH. The present study confirmed that LaAl-PILC would be one of candidates for phosphate adsorbent.