From Waste to Resource: Upcycling Spent Coffee Grounds into Biochar Functionalized with CaFe2O4 for Phosphate Removal

Abstract Eutrophication caused by excess phosphorus in water bodies represents a significant environmental challenge, requiring efficient, selective, and sustainable technologies for its mitigation. In this study, a magnetic biochar (BiMag) was developed through the valorization of residual coffee grounds and functionalized with calcium ferrite (CaFe2O4) for phosphate removal from aqueous media. The synthesis of the composite was optimized using response surface methodology (Box–Behnken), evaluating the effects of precipitation time, CaCl2 volume, temperature, and calcination time. The optimal condition obtained consisted of 102 min of precipitation at 20 °C, 18 mL of CaCl2 (3 mol/L), calcination at 400 °C for 120 min, with 11 mL of FeCl3 (3 mol/L). Material characterization confirmed the incorporation of CaFe2O4, a highly porous rough morphology, and superparamagnetic behavior. Adsorption studies indicated high efficiency and selectivity for phosphate, with minimal influence of pH and the presence of competing anions. Adsorption kinetics were well described by the Elovich and pseudo-second-order (PSO) models, while equilibrium data were adequately represented by the Sips isotherm, with a maximum phosphate adsorption capacity of 97 mg/g. The adsorption process was spontaneous and endothermic, involving combined mechanisms of surface complexation, ligand exchange, and precipitation of calcium and iron phosphates. These results demonstrate that BiMag is an efficient, selective, and regenerable adsorbent with high potential for application in wastewater treatment and sustainable phosphorus recovery, in alignment with the principles of the circular economy.