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 (33 PO4 ) 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 33 PO4 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 H2 PO4 - /HCO3 - exchange, which explains the PO4 fixation in LDHs.