Upcycling of Electric Arc Furnace Slag for Phosphate Removal: A Green Solution for Water Treatment

The excessive discharge of phosphorus into aquatic systems is a major environmental concern, contributing to eutrophication and biodiversity loss. This study investigates the reuse of electric arc furnace slag (EAF-slag), an abundant steelmaking byproduct, as a low-cost, upcycled adsorbent material for phosphorus removal and recovery, in line with circular economy and sustainable waste management principles. The slag, rich in calcium and reactive oxides, was extensively characterized using Raman spectroscopy, scanning electron microscopy, and X-ray fluorescence, and tested for phosphate adsorption across a wide range of concentrations (5-400 mg/L) and solid-liquid ratios (from 1:10 to 1:50 expressed as g of slag in mL of P-solution). Excellent removal efficiency was achieved under all conditions, with complete phosphate uptake and a maximum sorption capacity of 10.97 mg/g at S/L = 1:30. Post-adsorption analyses (Raman mapping, X-ray photoelectron spectroscopy, and cross-sectional energy-dispersive X-ray spectroscopy) confirmed the formation of hydroxyapatite on the slag surface. Importantly, the performance of EAF-slag was also evaluated in more complex aqueous systems containing phosphate, nitrate, and sulfate ions (50 and 100 mg/L each). The phosphate adsorption capacity remained unaffected, and the slag simultaneously removed nitrate and sulfate, confirming its multifunctional sorption behavior. Additional recycling experiments demonstrated that the spent slag can be reused after resting, maintaining satisfactory phosphorus removal efficiency (≈60%) even after 5 or 10 days. Moreover, the hydroxyapatite-enriched slag showed potential for use as a fertilizer, enabling phosphorus recovery and reuse. These findings demonstrate the potential of EAF-slag as an effective, low-cost, and sustainable material for phosphorus recovery and valorization of secondary raw materials, laying the groundwork for its future application in wastewater treatment and environmental remediation.