CoAl-LDH decorated with cerium oxide as an efficient adsorbent for restoring low-concentration phosphate in wastewater

Abstract

The requirement for the removal of phosphorus (P) from wastewater has become progressively stringent, therefore, it is essential to remove low-concentration phosphate from secondary effluents through a tertiary treatment. One of the biggest challenges in removing phosphate from wastewater is the development of low-cost, green, and pollution-free adsorbents. In this study, novel, eco-friendly and low-cost CeO₂ nanosphere modifying CoAl-LDH nanosheets (CoAl-LDH/CeO₂) were successfully fabricated using a classical hydrothermal strategy. The microstructure and morphology of CoAl LDH/CeO₂ were characterized using SEM, TEM, FTIR, XRD, TG, XPS, and BET techniques. The performance of the P adsorption from water for CoAl-LDH/CeO₂ was investigated. The influences of adsorption parameters, such as adsorbent dosage, pH, phosphate concentration, adsorption time, and experimental temperature, were investigated through batch adsorption experiments. The batch adsorption experiments showed that the P removal by CoAl-LDH/CeO₂ could reach 93.4% at room temperature within 60 minutes. CoAl-LDH/CeO₂ showed ultrafast and high-efficiency adsorption for low concentration P contaminated wastewater. Pseudo-second order model exhibited better fitting with the kinetics of the phosphate adsorption, while the Freundlich model well-described the isotherm results (R² > 0.999). Although Cl^-, NO₃^-and SO₄^2- coexisted in the solution, CoAl-LDH/CeO₂ still possessed favourable selectivity for phosphates. More importantly, the adsorption capacities of CoAl-LDH/CeO₂ retained over 85% after five cycles. Therefore, the low cost and sustainable utilization of CoAl-LDH/CeO₂ for the phosphate removal from secondary effluent with phosphate at a low concentration highlights its potential application to alleviate eutrophication.

Fig. 1. Schematic process for the facile preparation of CoAl-LDH/CeO₂.

Fig. 2. SEM micrograph for CoAl-LDH (a), CeO₂ (b) and CoAl-LDH/CeO₂ (c), TEM micrograph for CoAl-LDH (d), CeO₂ (e) and CoAl-LDH/CeO₂ (f), element composition and SEM-EDX analysis spectrums of CoAl-LDH/CeO₂ (g–i).

Fig. 3. (a) FT-IR spectra of CoAl-LDH, CeO₂ and CoAl-LDH/CeO₂. (b) TG curves of CoAl-LDH, CeO₂ and CoAl-LDH/CeO₂. (c) XRD patterns of CoAl-LDH, CeO₂ and CoAl-LDH/CeO₂. (d) Zeta potential of the CoAl-LDH, CeO₂, CoAl-LDH/CeO₂ before and after phosphate adsorption curves.

Fig. 4. (a) XPS full spectrum of CoAl-LDH and CoAl-LDH/CeO₂. High resolution spectra of (b) Co 2p, (c) Al 2p, and (d) Ce 3d of CoAl-LDH and CoAl-LDH/CeO₂, respectively.

Fig. 5. (a and b) Nitrogen adsorption/desorption isotherms of CoAl-LDH, CeO₂ and CoAl-LDH/CeO₂.

Fig. 6. Adsorption isothermal model of phosphate by CoAl-LDH/CeO₂.

Fig. 7. Adsorption kinetics model of phosphate by CoAl-LDH/CeO₂.

Fig. 8. (a) Effect of initial concentration on removal rate of CoAl-LDH/CeO₂, (b) effect of CoAl-LDH/CeO₂ dosage on adsorption properties, (c–e) effect of time, temperature and initial solution pH on adsorption properties, respectively, (f) comparison of the adsorption effect of CoAl-LDH, CeO₂ and CoAl-LDH/CeO₂ on phosphate.

Fig. 9. Effect of coexisting anions (SO₄²⁻, Cl⁻, NO₃⁻ and CO₃²⁻, HCO₃⁻) on the adsorption of phosphate by CoAl-LDH/CeO₂.

Fig. 10. Reuse performance ratio of CoAl-LDH/CeO₂ adsorbed phosphate.

Scheme 1. A schematic diagram of the adsorption mechanism by CoAl-LDH/CeO₂.