Solid-state speciation of interlayer anions in layered double hydroxides

Abstract

Layered double hydroxides (LDH) have been proposed for phosphate (PO4) recovery and recycling from waste streams due to their high anion exchange capacity, good stability and high affinity towards PO4. The high affinity towards PO4 strongly relates to the electrostatic interaction with PO4, and thus the charge of PO4. However, the anion speciation of intercalated PO4, i.e. either H2PO4-, HPO4 2or PO4 3is often overlooked. This study was set up to measure solid phase PO4 speciation through ion exchange stoichiometry, x-ray diffraction (XRD), attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIR) and modelling. Six phase pure Mg/Al LDH materials were synthesized using co-precipitation of metal nitrate (NO3-) solutions with varying M 2+ /M 3+ ratio at pH 12 and pH 10. The LDHs synthetized at pH 12 contained larger equivalent fractions of intercalated OH-, smaller fractions of NO3and smaller interlayer distance than those prepared at pH 10, likely because of the higher OHconcentration in the more alkaline synthesis solutions. Two high charge LDHs prepared at pH 12 or 10 were selected, exchanged with PO4 (0-20 mM initial PO4, 24h) at one starting pH (7.20); desorption was subsequently performed with carbonate (3 mM, initial pH 8.4) during 480 h. The resulting solution concentrations of NO3, PO4 and CO3 and the pH allowed the identification of the anion exchange stoichiometry. The LDH synthesized at pH 12, which had a large fraction of exchangeable OH-, adsorbed PO4 as HPO4 2-/PO4 3-, in exchange for both NO3and OHanions. The material synthesized at pH 10 containing a lower fraction of exchangeable OH-, therefore, adsorbed mainly HPO4 2in exchange for NO3anions. The carbonate exchange was consistent with adsorption of divalent CO3 2-. The pH dependent speciation modelling showed that the exchanged PO4 ions have higher charge compared to those in the contacting solution. This study suggests that the highest P content of LDH is obtained in high charge materials holding divalent PO4 anions, i.e. materials synthesized at lower pH and/or exchanged in solutions with low alkalinity.

Optimization of phosphate recovery from urine by layered double hydroxides

“…The amount of NO3in the materials was measured (Table 1). The LDH synthesized at pH 12 have a lower NO3content compared to those prepared at pH 10, which is likely related to the larger intercalation of OHat the higher pH (Everaert et al, 2019).…”

“…Especially LDHs with a low M 2+ /M 3+ content have a high selectivity for OHand CO3 2anions, as a result of the high charge density of these anions (Ashekuzzaman and Jiang, 2014;Das et al, 2006). Alternatively, higher P uptake at lower pH might be interpreted as more efficient use of the AEC by sorbing monovalent H2PO4 -, instead of HPO4 2or PO4 3at higher pH (Everaert et al, 2019).…”

“…The effect of a lower P uptake at increasing solution pH confirms the results of Cheng et al (2009); Das et al (2006) and Yang et al ( 2014). The effect can be attributed to i) a more pronounced competition between PO4 anions and OHions (Das et al, 2006), ii) a higher concentration of CO3 2-, causing more competition for sorption of P (Ashekuzzaman and Jiang, 2014;Das et al, 2006) and iii) a change in P speciation of the P solution, and as a result also the speciation of P sorbed on the LDH (Everaert et al, 2019). At a pH below 7.2, P will be present mostly in the monovalent species (H2PO4 -), whereas at the higher pH values, the divalent HPO4 2will be prevalent (Cheng , 2009;Das et al, 2006).…”

Layered Double Hydroxides as Slow-Release Fertilizer Compounds for the Micronutrient Molybdenum

“…From the elemental composition of the as-synthesized LDHs ( i.e. , the measured molar Zn/Al ratio), the theoretical AEC of the materials was determined, which were 3.51 meq c g –1 for Zn 2 Al, 2.70 meq c g –1 for Zn 3 Al, and 2.13 meq c g –1 for Zn 4 Al (Table ). The basal spacing decreased slightly from 7.87 Å for the Zn 4 Al LDH to 7.73 Å for the Zn 2 Al LDH, as derived from the position of the (003) XRD reflection (Figure a and Table ).…”

“…Three ZnAl LDHs were prepared by varying the molar Zn/Al ratio in the metal precursor solution of either 2, 3, or 4 using a co-precipitation method at room temperature. 30 These precursor solutions were obtained by dissolving ZnCl 2 and AlCl 3 •6H 2 O in MilliQ water at the required molar ratios while maintaining a total metal concentration of 1 M. These metal solutions were added to a 200 mL MilliQ solution (adjusted to pH 9 with NaOH) at a rate of 10 mL h −1 using a Perfusor pump (B. Braun). During precipitation in a N 2 atmosphere, the pH was maintained at 9.0 ± 0.1 with the controlled addition of a 2 M NaOH solution, performed by a Metrohm Titrino 702.…”