Assessment of porous sorbents for phosphate recovery using NMR spectroscopy

Phosphate is a limited resource and research into how to recover and reuse phosphate from wastewater is extensively done. A promising sorbent material, which is cheap to synthesize, reusable and environmentally friendly, is the magnesium aluminum layered double hydroxide (MgAl-LDH). The MgAl-Cl-LDH have in this project been synthesized using the coprecipitation or the epoxide mediated sol-gel method accompanied by phase separation. The LDH synthesized by the coprecipitation did undergo extensively characterization and phosphate sorption experiments (removed 42−54 mgP/g). It was not possible to dope Zr(IV) into the MgAl-LDH. Consequently, the Zr(IV) existed as Zr(OH)4/ZrO2 in the LDH, which significantly decreased the P removal capacity to 16−20 mgP/g. The MgAl-LDH (without Zr(IV), but 23−29% amorphous aluminum hydroxide, AOH) removed >62% of the phosphate, the rest were removed by impurities (i.e., AOH, MgP, Al-P) according to 31P MAS NMR.


Following this, advanced 1D and 2D NMR experiments were used to identify the phosphate removal pathways for two phase pure MgxAl-Cl-LDH (x = 2 and 3), i.e., not containing any AOH. Thus, a general procedure for the synthesis of phase pure MgAl-Cl-LDH have been obtained. It was determined that the charge density (x) have a significant influence on the 31P removal pathways: the Mg2Al-LDH mainly (75%) removed phosphate through anion-exchange by intercalation of HPO4 2- and H2PO4 - (δiso( 31P) > -2 ppm), whereas the Mg3Al-LDH removed phosphate through anion-exchange (≈33%) and grafting (≈66%) of H2PO4 - , (δiso( 31P) > -5 ppm). Importantly, 25 mol% of the Mg2Al-LDH dissolved upon P exposure, this is not ideal with a view of application of the LDH on a large-scale facility (i.e., wastewater treatment plant). Only 8 mol% dissolved for the Mg3Al-LDH. Further experiments revealed that the Mg3Al-LDH is more stable over a broad pH range (5−12), whereas the Mg2Al-LDH is more reactive and at pH 12 form a shigaite-type LDH (d(003)- spacing = 10.2 Å, δiso( 31P) = 7(1) ppm), with a double interlayer of phosphate anions.


The LDH synthesized by the coprecipitation method result in a powdered material. Powdered material is difficult to extract at large scale facilities. Thus, investigation of LDH granulates with pores synthesized by the sol-gel method accompanied by phase separation were done. As the material is granulates, not powder, pores are necessary to facilitate the transport of anions into the inner part of the monolith. Extensive research revealed three pore sizes: micro-, meso- and macropores. The meso- and macropores are related to the LDH-formation and the micropores to the Al(OH)3 formation. Results indicated that the LDH grow on the surface of the Al(OH)3. 129Xe EXSY showed that meso- and macropores are exchanging, but micropores are isolated. The MgAl-Cl-LDH monolith removed ≈ 50 mgP/g and the LDH structure and pores remained after phosphate sorption, albeit with a decrease in crystallinity and homogeneity and some dissolution of the LDH.


To summarize, the work added to the knowledge of the application of MgAl-LDH as phosphate sorbents. The work illustrated an instability of the LDH which needs to be addressed. The presented work also detailly described the pathway behind phosphate removal using 31P NMR for LDH synthesized by the coprecipitation and the sol-gel method accompanied by phase separation.