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 (PO₄) recovery and recycling from waste streams. However, PO₄ recycling in agriculture might be limited by incomplete desorption of PO₄ from the minerals. This study was set up to identify the factors explaining irreversible PO₄ sorption ("fixation") on LDHs by comparing the isotopic exchangeability (³³PO₄) with the PO₄ 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 PO₄ prior to ³³PO₄ isotopic exchange (0-20 days, 1 mM NaHCO₃) or PO₄ desorption (0-20 days, NaHCO₃ concentrations increasing from 0 to 20 mM). The isotopic PO₄ exchangeability was 85-95% of total PO₄ after 72 h in the phase pure LDHs with intercalated PO₄ 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 PO₄ fractions were only 55-63% for the phase pure LDHs, indicating that not all of the isotopically exchangeable PO₄ can be desorbed. Samples at different stages of desorption (different initial NaHCO₃ concentrations) were subjected to isotopic exchange after desorption. In the LDHs with PO₄ intercalation, PO₄ was increasingly less isotopically exchangeable as the initial NaHCO₃ concentrations increased, while this trend was not observed for samples without intercalated PO₄. This suggests that PO₄ becomes increasingly less accessible for isotopic exchange as the fraction binding sites occupied with HCO₃^- increases. The interlayer outward diffusion of PO₄ might be increasingly rate limited upon H₂PO₄^-/HCO₃^- exchange, which explains the PO₄ fixation in LDHs.