Atomic Level Understanding of Orthophosphate Adsorption by Magnesium Aluminum-Layered Double Hydroxides-A Multitechnique Study

The phosphate (P) adsorption properties of three magnesium aluminum layered double hydroxides (LDH) intercalated with nitrate and/or carbonate (MgAl-NO3/CO3 LDH) with targeted Mg/Al ratios of 2, 3, and 4 were investigated to understand how phosphate interacts with the LDH using powder X-ray diffraction (PXRD), solid-state NMR spectroscopy, vibrational spectroscopy, scanning electron microscopy, and inductively coupled plasma-optical emission spectrometry. The solid products after phosphate exposure contained four different phosphate species which were quantified by P-31 MAS NMR: phosphate intercalated in the interlayer (A), phosphate adsorbed to the surface of the LDH particles (B and C), and phosphate adsorbed to amorphous aluminum hydroxide (D). Their relative concentrations depend on the Mg/Al ratio, the phosphate loading, and the intercalated anion in the parent LDH. Surprisingly, phosphate intercalated by anion exchange, which has been assumed to be the dominant mechanism of P adsorption, is less than 30%. Instead phosphate adsorbed to the surface of the LDH particles is the predominant site (52-88%). Moreover, a non-negligible fraction (8-39%) of phosphate is adsorbed to an amorphous aluminum hydroxide (AOH) impurity (site D) especially at high phosphate concentrations and exposure time. AOH is a known impurity in LDH prepared by co-precipitation at constant pH, the most commonly used method in phosphate removal studies. This indicates that MgAl-LDH have higher affinity for phosphate than AOH. Furthermore, no significant dissolution of the LDH followed by precipitation of phosphate minerals was observed by PXRD and Al-27 and( 31)P MAS NMR.