Stability of magnetic LDH composites used for phosphate recovery

Abstract

Layered double hydroxides (LDH) and their magnetic composites have been intensively investigated as recyclable high-capacity phosphate (P) sorbents but with little attention to their stability as function of pH and phosphate concentration. The stability of a Fe3O4@SiO2-Mg3Fe LDH P sorbent as function of pH (5-11) and orthophosphate (Pi) concentration (1-300 mg P/L) was investigated. The composite has high adsorption capacity (approx. 80 mg P/g) at pH 5 but with fast dissolution of the LDH component resulting in formation of ferrihydrite evidenced by Mössbauer spectroscopy. At pH 7 more than 60 % of the LDH dissolves within 60 min, while at alkaline pH, the LDH is more stable but with less than 40 % adsorption capacity as compared to pH 5. The high Pi sorption at acid to neutral pH is attributed to Pi bonding to the residual ferrihydrite. Under alkaline conditions Pi is sorbed to LDH at low Pi concentration while magnesium phosphates form at higher Pi concentration evidenced by solid-state 31 P MAS NMR, powder X-ray diffraction and chemical analyses. Sorption as function of pH and Pi concentration has been fitted by a Rational 2D function allowing for estimation of Pi sorption and precipitation. In conclusion, the instability of the LDH component limits its application in wastewater treatment from acid to alkaline pH. Future use of magnetic LDH composites requires substantial stabilisation of the LDH component.

Super‐stable mineralization effect of layered double hydroxides for heavy metals: Application in soil remediation and perspective

“…Furthermore, LDHs also possesses high buffer capacity in broad range of pH attributed to extensive OH groups on the surface of layer. [84,85] Inspired by the advantages of LDHs, in our recent work, we adopted CaAl-LDH as the amendment for the remediation of Cd-contaminant soil in lab scale and in practice, [86] which displayed excellent performance as follows: The maximum capture capacity for Cd 2+ reached to 592 mg/g in solution with fast kinetics, surpassing most of the reported adsorbents (Figure 3A); As a soil amendment, the immobilization efficiency for Cd 2+ reached up to 96.9% in soil remediation within 28 days (Figure 3B), much higher than that of quicklime (53.1%) and hydroxyapatite (23.8%). Furthermore, the practice remediation performance of CaAl-LDH as the amendment was investigated in the Cd-contaminated agriculture soil with a scale of 10 ha.…”

“…Furthermore, LDHs also possesses high buffer capacity in broad range of pH attributed to extensive OH groups on the surface of layer. [ 84,85 ]…”

First-Principles Study on Interlayer Spacing and Structure Stability of NiAl-Layered Double Hydroxides

“…The same anion can be intercalated into the interlayer of different basal layers. Therefore, due to the interchangeability of the basal layer cation and guest anion in LDHs, LDHs exhibit significant potential for applications in the fields of catalysis, , adsorption, , ion exchange, , biomedicine, light and magnetism, and other functional materials fields and have received widespread attention from academia and industry …”

Coupling the Batch Adsorption Enrichment using Ternary LDHs with Struvite Crystallization in a Fluidized Bed Reactor for High-efficiency Phosphorus Recovery from Wastewater

“…A recent study shows that ultrathin MgAl-LDHs nanoparticles adsorb up to ~ 55% more P than conventional MgAl-LDHs 19 , but this will raise new challenges in recycling these P-adsorbed LDH nanoparticles. To prevent the loss of P-adsorbed adsorbent during the recycling process, such LDHs are usually either bound to porous carriers [20][21] or assembled with magnetic particles [22][23][24] . In general, P adsorption on LDHs is often subjected to competition from various co-occurring anions, e.g., nitrate, sulfate, chloride, and bicarbonate ions 15,25 .…”