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 H2PO4-, HPO42- or PO43- 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 HPO42-/PO43-, in exchange for both NO3- and OH- anions. The material synthesized at pH 10 containing a lower fraction of exchangeable OH-, therefore, adsorbed mainly HPO42- in exchange for NO3- anions. The carbonate exchange was consistent with adsorption of divalent CO32-. 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.status: publishe