The isotopic exchangeability of phosphate in Mg-Al layered double hydroxides.

Layered double hydroxides (LDH) of Mg/Al are anion exchangers that are candidate materials for phosphate (PO) recovery and recycling from waste streams. However, PO recycling in agriculture might be limited by incomplete desorption of PO from the minerals. This study was set up to identify the factors explaining irreversible PO sorption ("fixation") on LDHs by comparing the isotopic exchangeability (PO) with the PO desorption from LDH materials and from boehmite as a P fixing reference mineral. Six different Mg-Al LDH materials were synthesized, by varying the synthesis pH and exposing obtained materials to hydrothermal (HT) treatment. Phase pure LDH materials were obtained from syntheses at pH 10 and 12, while at pH 8 Al-rich phase impurities such as a boehmite or gibbsite were formed. Crystallite size increased significantly during HT treatment. The LDHs were first loaded with PO prior to PO isotopic exchange (0-20 days, 1 mM NaHCO) or PO desorption (0-20 days, NaHCO concentrations increasing from 0 to 20 mM). The isotopic PO exchangeability was 85-95% of total PO after 72 h in the phase pure LDHs with intercalated PO whereas this value was 40-54% in the presence of Al-rich phase impurities in non-HT treated materials and in boehmite. In contrast, the maximally desorbed PO fractions were only 55-63% for the phase pure LDHs, indicating that not all of the isotopically exchangeable PO can be desorbed. Samples at different stages of desorption (different initial NaHCO concentrations) were subjected to isotopic exchange after desorption. In the LDHs with PO intercalation, PO was increasingly less isotopically exchangeable as the initial NaHCO concentrations increased, while this trend was not observed for samples without intercalated PO. This suggests that PO becomes increasingly less accessible for isotopic exchange as the fraction binding sites occupied with HCO increases. The interlayer outward diffusion of PO might be increasingly rate limited upon HPO/HCO exchange, which explains the PO fixation in LDHs.