Comparative study on synchronous adsorption of arsenate and fluoride in aqueous solution onto MgAlFe-LDHs with different intercalating anions

Abstract

In this study, a series of MgAlFe-LDHs (Cl^-, NO₃ ^-, intercalation, and calcined products of a CO₃ ^2- interlayer) was synthesized and used for adsorption of arsenate and fluoride in individual contaminants and coexisting pollutant systems. Effects of various factors such as initial pH of solution, dosage of materials, coexisting ions, contact time, and initial pollutant concentrations were evaluated. Experimental results showed that different intercalating anions had a significant effect on adsorption performance of arsenate and fluoride in water. The adsorption of arsenate and fluoride on MgAlFe-CLDH, MgAlFe-Cl-LDH or MgAlFe-NO₃-LDH can be described by different adsorption isotherm equations. During the simultaneous removal process, arsenate and fluoride competed for adsorption sites of the adsorbent materials, and the fluoride ions had advantages in the competitive adsorption on MgAlFe-Cl-LDH and MgAlFe-NO₃-LDH. MgAlFe-NO₃-LDH was used to adsorb arsenate and fluoride in coexisting pollution systems (the concentration of each pollutant was 2 mg L^-1, the adsorbent dosage was 1.5 g L^-1). The remaining arsenic concentration was reduced to less than 10 μg L^-1 and the remaining fluoride ion concentration to below 20 μg L^-1 which meets the World Health Organization's, EPA's and China's drinking water standards for arsenic and fluoride limits. A possible mechanism is discussed with support from further XRD, SEM, and XPS analysis of the materials after their adsorption.

Fig. 1. (a) XRD patterns of MgAlFe-CLDH, MgAlFe–Cl-LDH, and MgAlFe–NO₃-LDH. (b) FTIR spectra of MgAlFe-CLDH,MgAlFe–Cl-LDH, and MgAl-e–NO₃–LDH.

Fig. 2. (a) N₂ adsorption/desorption isotherms of MgAlFe-CLDH. (b) N₂ adsorption/desorption isotherms of MgAlFe–Cl-LDH. (c) N₂ adsorption/desorption isotherms of MgAlFe–NO₃-LDH. (d) Pore size distribution of three materials (BJH method).

Fig. 3. SEM image of the materials (a) and (b) MgAlFe-CLDH, (c) and (d) MgAlFe–Cl-LDH, (e) and (f) MgAlFe–NO₃-LDH.

Fig. 4. (a) Isothermal study of adsorption of arsenic with MgAlFe-CLDH. (b) Isothermal study of adsorption of arsenic with MgAlFe–Cl-LDH. (c) Isothermal study of adsorption of arsenic with MgAlFe–NO₃-LDH. (d) Kinetic fitting curve of arsenic adsorption on the three materials.

Fig. 5. Effect of coexisting anions on arsenic removal by materials in water. (a) MgAlFe-CLDH. (b) MgAlFe–Cl-LDH. (c) MgAlFe–NO₃-LDH.

Fig. 6. (a) Isothermal study of adsorption of fluoride with MgAlFe-CLDH. (b) Isothermal study of adsorption of fluoride with MgAlFe–Cl-LDH. (c) Isothermal study of adsorption of fluoride with MgAlFe–NO₃-LDH. (d) Kinetic fitting curve of fluoride adsorption on the three materials.

Fig. 7. Effect of coexisting anions on fluoride removal by materials in water. (a) MgAlFe-CLDH; (b) MgAlFe–Cl-LDH; (c) MgAlFe–NO₃-LDH.

Fig. 8. (a) Isothermal study of adsorption of arsenateon MgAlFe-CLDH in coexistence with fluoride. (b) Isothermal study of adsorption of fluoride with MgAlFe-CLDH in coexistence with arenate.

Fig. 9. (a) Isothermal study of adsorption of arsenate on MgAlFe–Cl-LDH in coexistence with fluoride. (b) Isothermal study of adsorption of fluoride with MgAlFe–Cl-LDH in coexistence with arsenate.

Fig. 10. (a) Isothermal study of adsorption of arsenateon MgAlFe–NO₃-LDH in coexistence with fluoride. (b) Isothermal study of adsorption of fluoride with MgAlFe–NO₃-LDH in coexistence with arsenate.

Fig. 11. Effect of MgAlFe–NO₃-LDH dosage on the adsorption of As(v) and F⁻ binary system.

Fig. 12. XRD patterns of materials after adsorption arsenate and fluoride. (a) MgAlFe-CLDH. (b) MgAlFe–Cl-LDH. (c) MgAlFe–NO₃-LDH.

Fig. 13. (a) SEM image of MgAlFe-CLDH after adsorption of arsenate. (b) SEM image of Mg–Al–Fe-CLDH after adsorption of fluoride.

Fig. 14. (a) XPS spectra of MgAlFe-CLDH after adsorption of As(v) and F⁻ peak of F 1s. (b) XPS spectra of MgAlFe-CLDH after adsorption of As(v) and F⁻ peak of Mg 2p and As 3d.

Fig. 15. (a) XPS spectra of MgAlFe–Cl-LDH after adsorption of As(v) and F⁻ peak of F 1s. (b) XPS spectra of MgAlFe–Cl-LDH after adsorption of As(v) and F⁻ peak of Mg 2p and As 3d.

Fig. 16. (a) XPS spectra of MgAlFe–NO₃-LDH after adsorption of As(v) and F⁻ peak of F 1s. (b) XPS spectra of MgAlFe–NO₃-LDH after adsorption of As(v) and F⁻ peak of Mg 2p and As 3d.