An investigation of the phosphate removal mechanism by MgFe layered double hydroxides

The phosphate adsorption properties including kinetics and uptake pathway of Mg _x Fe layered double hydroxides (LDH) with targeted x values of 2 and 3 and intercalated with nitrate (NO ₃ ⁻) and chloride (Cl ⁻) anions were investigated at a phosphate concentration of 16.31 mgP·L ⁻¹ and pH (pH = 5, 7, and 9) similar to wastewater conditions. The synthesized Mg _xFe-LDH show fast adsorption (90–98%) of the final phosphate removal within 3 h and remove 68–73% of the total phosphate content after 24 h. The pristine Mg _x Fe-LDH and solid products after phosphate exposure were characterized by PXRD, FT-IR, zeta-potential, SEM, and solid-state ³¹ P MAS NMR spectroscopy to investigate the phosphate removal pathway. Interestingly, the four Mg _x Fe-LDH phase had similar Mg:Fe ratios irrespective of the Mg:Fe ratio of the reactants based on PXRD. The Mg _xFe-LDH plate-like morphology was preserved after phosphate exposure and no sign of phosphate intercalation were evident by PXRD, FT-IR, and SEM albeit a small amount of the Mg _xFe LDH was dissolved based on ICP-OES, PXRD and the final pH. Partial dissolution of the LDH was clearly evident by SEM for Mg ₂ Fe-LDH exposed to 200% theoretical P capacity. The FT-IR spectra and zeta-potential values indicated that the phosphate ions are adsorbed by surface complexation. The ³¹P MAS NMR spectra of Mg _xFe-LDH exposed to phosphate has an intense spinning sideband manifold and only minor paramagnetic shifts (<30 ppm) are observed. Thus, phosphate is in the proximity of iron, but not directly bound to the paramagnetic iron via Fe-O-P bond (inner-sphere complexation or mineralization). Thus, the dominating phosphate uptake pathway by Mg _x Fe-LDH is surface complexation similar to MgAl-LDH and possible the formation of ill-defined magnesium phosphate due to partial dissolution of the MgFe-LDH, especially at high P loading.