Solid-state speciation of interlayer anions in layered double hydroxides.

Layered double hydroxides (LDH) have been proposed for phosphate (PO) recovery and recycling from waste streams due to their high anion exchange capacity, good stability and high affinity towards PO. The high affinity towards PO strongly relates to the electrostatic interaction with PO, and thus the charge of PO. However, the anion speciation of intercalated PO, i.e. either HPO, HPO or PO is often overlooked. This study was set up to measure solid phase PO speciation through ion exchange stoichiometry, X-ray diffraction (XRD), attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIR) and modelling. Six phase pure Mg/Al LDH materials were synthesized using co-precipitation of metal nitrate (NO) solutions with varying M/M ratio at pH 12 and pH 10. The LDHs synthetized at pH 12 contained larger equivalent fractions of intercalated OH, smaller fractions of NO and smaller interlayer distance than those prepared at pH 10, likely because of the higher OH concentration in the more alkaline synthesis solutions. Two high charge LDHs prepared at pH 12 or 10 were selected, exchanged with PO (0-20 mM initial PO, 24 h) at one starting pH (7.20); desorption was subsequently performed with carbonate (3 mM, initial pH 8.4) during 480 h. The resulting solution concentrations of NO, PO and CO and the pH allowed the identification of the anion exchange stoichiometry. The LDH synthesized at pH 12, which had a large fraction of exchangeable OH, adsorbed PO as HPO/PO, in exchange for both NO and OH anions. The material synthesized at pH 10 containing a lower fraction of exchangeable OH, therefore, adsorbed mainly HPO in exchange for NO anions. The carbonate exchange was consistent with adsorption of divalent CO. The pH dependent speciation modelling showed that the exchanged PO ions have higher charge compared to those in the contacting solution. This study suggests that the highest P content of LDH is obtained in high charge materials holding divalent PO anions, i.e. materials synthesized at lower pH and/or exchanged in solutions with low alkalinity.