Atomic Level Understanding of Orthophosphate Adsorptionby Magnesium Aluminum-Layered Double HydroxidesA MultitechniqueStudy

The phosphate (P) adsorption properties of three magnesium aluminum layered double hydroxides 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 interact with the LDH using powder X-ray diffraction, solid state NMR, vibrational spectroscopy, scanning electron microscopy, and ICP-OES. The solid products after phosphate exposure contained four different phosphate species which were quantified by 31P 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 29 %. Instead phosphate adsorbed to the surface of the LDH particles is the...