Phosphate recovery using porous monolith MgAl-LDH: insights from NMR spectroscopy, physisorption and microscopy

Monolithic layered double hydroxides (LDH) may alleviate the challenging separation process after ion-exchange of conventionally polycrystalline (powdery) LDH, which have been extensively tested as potential phosphate sorbents. However, the structure, ion-exchange properties, and stability of monolithic LDH have been sparingly studied and a detailed understanding of these properties is lacking. The phosphate (P) removal properties, structure, including porous network and stability of two MgAl-LDH and an Al(OH)₃ monolithic composite with a high surface area (130-211 m² g^-1) were investigated in detail. The LDH monoliths are a mixture of amorphous Al(OH)₃ and nano-crystalline MgAl-LDH (≈50%) and their phosphate removal capacity (≈50 mg P per g) is comparable to polycrystalline LDH, but only around half of the phosphate is removed by the LDH phase based on solid state ²⁷Al, ³¹P and ³⁵Cl NMR spectroscopy. Single pulse and 2D EXSY ¹²⁹Xe NMR spectroscopy identified micro-, meso- and macro-porous environments. Micropore formation is related to the Al(OH)₃ phase, which forms first during the synthesis, whereas the meso- and microporous environments are assigned to the LDH-phase subsequently formed. The main porous environments are well connected at a low Mg content, but to a lesser extent at higher Mg content. After P sorption, the monolith experienced a decreased crystallinity and dissolution of some Al(OH)₃ (4-10%) or LDH (10%). Moreover, the pores are generally stable but disconnected likely by the phosphate species formed, based on ¹²⁹Xe EXSY NMR. Thus, monolithic LDH suffers from the same challenge as polycrystalline LDH in relation to stability in a phosphate solution.