Valorization of Spent Metal-Finishing Solutions into Ferrite-Based Sorbents for Sustainable Fe2+ Removal from Water

The valorization of metal-containing industrial waste into functional sorbents offers opportunities for resource recovery and wastewater treatment. This study investigates the conversion of spent etching and electroplating solutions into ferrite-based materials for Fe2+ removal from water. Zn2+, Ni2+, Cu2+, and Co-containing ferrites were synthesized at Fe2+/Me2+ ratios of 1.5:1 and 3:1, with selected samples thermally treated at 400 °C. Phase composition was characterized by X-ray diffraction. Fe2+ removal was evaluated in laboratory-scale batch experiments at 10 °C for 24 h, using a sorbent dosage of 1.2 g/L and initial Fe2+ concentrations of 2–120 mg/L. Experimental results were fitted using nonlinear Langmuir and Freundlich models. Thermal treatment increased crystalline ordering but generally reduced apparent removal capacities. For ZnFe2O4, NiFe2O4, and CoFe2O4, fitted Langmuir capacities decreased from 67.7 to 47.2, 90.2 to 51.8, and 70.8 to 57.5 mg/g, respectively. Bentonite and volcanic tuff showed the highest model-fitting quality (R2 > 0.93), whereas Cu-containing materials exhibited poor agreement with both models. Regeneration with 0.1 M HCl achieved 83% Fe2+ desorption in the first cycle, decreasing to approximately 38% after five cycles. Since Fe2+ oxidation and precipitation could not be excluded, the results represent a laboratory-scale proof of concept. Further investigations involving real wastewater, metal leaching, and long-term sorbent stability are required to establish practical applicability.