Sodium dodecylsulfate-layered double hydroxide and its use in the adsorption of 17β-estradiol in wastewater

Abstract

Modified Mg₃Al layered double hydroxide (LDH) intercalated with dodecylsulfate anion composites, which were designated as SDS-LDH composites, were synthesized by coprecipitation. The samples were characterized using SEM, EDX, FT-IR, zeta potential analysis, and XRD. The results showed that the SDS-LDH composites contain a thicker and larger porous interconnected network than inorganic LDH due to the enlarged inter-layer distance. The outstanding adsorption performance of SDS-LDH composites toward 17β-estradiol (E2) was investigated under different conditions, including solution pH, adsorbent dosage, ion strength, reaction time, and temperature. When the solution pH was 7 and the adsorbent dosage was 2 g L^-1, the removal rate of E2 reached the maximum at 94%, whereas inorganic LDH displayed a poor E2 removal rate of 10%. The presence of various ions (Na⁺, SO₄ ^2-, CI^-, and H₂PO₄ ^-) in aqueous solution exerted no significant adverse effects on the adsorption process. The adsorption equilibrium was reached within 20 min, and the adsorption fitted well with the pseudo-second-order model and the Freundlich isotherm. The thermodynamic test revealed that the adsorption process was spontaneous and endothermic. Phosphorus was selected as the index for evaluating the adsorption capacity of SDS-LDH composites for inorganic ions. The removal rates of total phosphorus and PO₄ ^3- were 43.71% and 55.93% for SDS-LDH composites at 2 g L^-1. The removal rate of PO₄ ^3- reached up to 85% when the contact time was 120 min and the dosage was 3 g L^-1 for SDS-LDH composites, which were approximately close to those of inorganic LDH of 30 min and 2 g L^-1, respectively. This finding indicates that the removal capacity of SDS-LDH composites for PO₄ ^3- decreased after the dodecylsulfate anions intercalated into the interlayer. The composites retained their high efficiency and stability after desorption and regeneration with alkali treatment. This study demonstrated that SDS-LDH composites are a promising adsorbent for the recovery and abatement of trace-level E2 in secondary effluents of wastewater treatment plants.

Scheme 1. Synthetic routes and schematic illustrations of SDS-LDH composites.

Fig. 1. SEM of inorganic LDH composite (left) and SDS-LDH composites (right).

Fig. 2. XRD spectra of SDS-LDH and inorganic LDH composites.

Scheme 2. Structures of inorganic LDH composites (left) and SDS-LDH (right).

Fig. 3. FT-IR spectra of SDS-LDH and inorganic LDH composites.

Fig. 4. Effect of solution pH for the adsorption of 17β-estradiol onto SDS-LDH composites and zeta potential of SDS-LDH composites (dosages = 2 g L⁻¹, solutions = 0.319 mg L⁻¹, V = 200 mL, time = 45 min, temperature = 298 K).

Fig. 5. Effect of adsorbent dosage for the adsorption of 17β-estradiol on removal rate and equilibrium adsorption amount q_e (inset) (solutions = 0.320 mg L⁻¹, V = 200 mL, pH = 7, time = 45 min, temperature = 298 K).

Fig. 6. Effect of ion strength for the adsorption of 17β-estradiol onto SDS-LDH composites (dosages = 2 g L⁻¹, solutions = 0.379 mg L⁻¹, V = 200 mL, pH = 7, reaction time = 45 min, T = 298 K).

Fig. 7. Effect of contact time for the adsorption of 17β-estradiol on removal rates and the adsorption amount q_t (inset) (dosages = 2 g L⁻¹, solutions = 0.302 mg L⁻¹, V = 500 mL, pH = 7, T = 298 K).

Fig. 8. Effect of temperature on the adsorption process (dosages = 2 g L⁻¹, solutions = 0.260 mg L⁻¹; 0.275 mg L⁻¹; 0.301 mg L⁻¹, V = 200 mL, pH = 7, reaction time = 45 min).

Fig. 9. Effect of contact time for the adsorption of PO₄³⁻ on removal rate (dosages = 2 g L⁻¹ for SDS-LDH and inorganic-LDH; 3 g L⁻¹ for SDS-LDH, C_TP = 2.952 mg L⁻¹, C_PO4³⁻ = 2.379 mg L⁻¹, V = 200 mL, pH = 7, reaction time = 180 minutes, T = 298 K).

Fig. 10. Removal rates of TP, PO₄³⁻ by SDS-LDH composites (C_SDS-LDH = 2 g L⁻¹, C_TP = 2.952 mg L⁻¹, C_PO4³⁻ = 2.379 mg L⁻¹, V = 200 mL, pH = 7, reaction time = 180 min, T = 298 K).

Fig. 11. Nonlinear fitting of the pseudo-first-order and pseudo-second-order models for the adsorption of E2 onto SDS-LDH composites.

Fig. 12. Adsorption isotherm for the adsorption of E2 onto SDS-LDH composites.

Fig. 13. Linear fitting of the Langmuir and Freundlich (inset) models for the adsorption of E2 onto SDS-LDH composites.

Scheme 3. Illustration for the hydrophobic of SDS-LDH composites.

Scheme 4. Mechanistic illustration for the adsorption of E2 onto SDS-LDH composites.

Fig. 14. Effect of recycling time for the adsorption of E2 on the equilibrium adsorption amount q_e and the removal rate (inset).