Removal of HF via CaCl2-Modified EAF Slag: A Waste-Derived Sorbent Approach

This study evaluates CaCl2-modified electric arc furnace (EAF) slag for fluoride removal from synthetic hydrofluoric acid (HF) wastewater. Adsorption performance was assessed under different particle sizes (850 μm–1.7 mm, 250–850 μm, and <250 μm), temperatures (25–45 °C), and initial pH values (2–11), using oxidized (EOS) and reduced (ERS) slags in raw and modified (C1, C2) forms. Characterization included isotherm modeling (Langmuir and Freundlich), X-ray diffraction (XRD), and inductively coupled plasma mass spectrometry (ICP-MS). The CaCl2-modified slags (particularly EOS-C2 and ERS-C2) demonstrated stable performance under all conditions. ERS-C2 achieved the maximum adsorption capacity of 16.13 mg/g at 600 mg F−/L. EOS-C2 maintained capacities above 8.0 mg/g across pH 2–11, whereas unmodified slag showed a decline in performance above pH 5, with residual concentrations exceeding 250 mg F−/L and capacities dropping to 1.14–2.14 mg/g. XRD analysis indicated increased amorphization and enhancement of dicalcium silicate and brownmillerite phases after modification. Isotherm fitting showed better agreement with the Freundlich model, suggesting multilayer adsorption. Leaching tests confirmed that Cr, Cu, and As concentrations were within safe limits, while Pb and Cd were not detected. These results demonstrate the strong potential of CaCl2-modified EAF slag as an efficient, pH-stable, and environmentally safe adsorbent for treating HF-containing industrial wastewater.