Phosphate Recovery using Porous Monolith MgAl-LDH: Insights from NMR Spectroscopy, Physisorption and Microscopy

Abstract

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) 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 mgP/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.