Sorption of phosphates on Al-pillared smectites and mica at acidic to neutral pH

To investigate sorption behaviors and mechanisms of phosphates, phosphate sorption (hereafter, P sorption) experiments were carried out for synthetic montmorillonite, beidellite, saponite, and fluor-tetrasilicic expandable mica pillared with cationic, Keggin ion-like, Al clusters (Alclst). The P sorption experiments were done in the final pH range of 3 to 7 at room temperature and at an initial concentration of 3.2 mmol/l P. The P sorption maxima for the Alclst samples were 0.66–0.87 mmol/g, which are higher by about 2 orders of magnitude than those for smectites without Alclst pillars. The sorption ability of the Alclst samples is nearly as high as that of Al-bearing minerals and materials such as boehmite and amorphous Al hydroxide, which are regarded as effective absorbents to remove P from water. Examinations by X-ray diffraction analysis, infrared spectroscopy, scanning electron microscopy, and analytical transmission electron microscopy indicated that the P sorption is mostly related to the OH functional groups of the Alclst pillars, but little to the P species in solution, the reactive sites on the edge surface of smectites and mica, or possible precipitates. The P sorption maxima increased with increasing an amount of Alclst pillars in the Alclst samples, suggesting that the P sorption maximum is determined by the number of the OH functional groups of the Alclst pillars and is independent of the structures and chemical compositions of the host phases.