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

Layered double hydroxides (LDH) have been proposed for phosphate (PO4 ) recovery and recycling from waste streams due to their high anion exchange capacity, good stability and high affinity towards PO4 . The high affinity towards PO4 strongly relates to the electrostatic interaction with PO4 , and thus the charge of PO4 . However, the anion speciation of intercalated PO4 , i.e. either H2 PO4 - , HPO4 2- or PO4 3- is often overlooked. This study was set up to measure solid phase PO4 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 (NO3 - ) solutions with varying M2+ /M3+ ratio at pH 12 and pH 10. The LDHs synthetized at pH 12 contained larger equivalent fractions of intercalated OH- , smaller fractions of NO3 - 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 PO4 (0-20 mM initial PO4 , 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 NO3 , PO4 and CO3 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 PO4 as HPO4 2- /PO4 3- , in exchange for both NO3 - and OH- anions. The material synthesized at pH 10 containing a lower fraction of exchangeable OH- , therefore, adsorbed mainly HPO4 2- in exchange for NO3 - anions. The carbonate exchange was consistent with adsorption of divalent CO3 2- . The pH dependent speciation modelling showed that the exchanged PO4 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 PO4 anions, i.e. materials synthesized at lower pH and/or exchanged in solutions with low alkalinity.