High value-added utilization of secondary aluminum ash & carbide slag: preparation of a high-performance adsorbent for rapid removal of fluoride from wastewater

TA-CS@SAA (tartaric acid modified calcium carbide slag doped with aluminum ash) with a large specific surface area was prepared by calcination and modification with tartaric acid and used as an adsorbent for removing fluoride from wastewater. TA-CS@SAA showed that the optimal pH for removing fluoride is 5 at an adsorbent dose of 5.0 g L⁻¹ with an adsorption time of 30 min at 318 K. These conditions yielded the highest removal efficiency and adsorption capacity of 86.52% and 173.04 mg g⁻¹, respectively. Kinetic data were better described by a pseudo-second-order kinetic model. The theoretical maximum adsorption capacity obtained from an optimum fitting Langmuir isotherm was 172.41 mg g⁻¹. Thermodynamic data (ΔG^θ = −14.44 kJ mol⁻¹, ΔH^θ = −31.16 kJ mol⁻¹, and ΔS^θ = −0.05 J K⁻¹ mol⁻¹ at 318 K) revealed that the removal of fluoride from wastewater is a spontaneous, exothermic, and entropy-decreasing process. SEM-EDS demonstrated that the interaction of fluorine with Al, Ca and O occurs on the surface of the adsorbent in the adsorption process. FT-IR and XRD analysis indicated that electrostatic attraction on an adsorbent surface with a positive charge, surface coordination and ion exchange between fluoride and hydroxide ions were the dominant mechanisms. The effects of co-ions Br⁻, Cl⁻, Ac⁻, HCO₃⁻, and SO₄²⁻ had a negligible interference on adsorption of fluoride. Therefore, it can serve as a potential adsorbent in practical industrial applications.