Fabrication of Cu-doped iron oxyhydroxide for phosphate removal: A comprehensive spectroscopic, equilibrium, and thermodynamic approach

Phosphate remediation from natural and wastewater systems is of great importance, and the use of adsorption and ion exchange processes are efficient means for its decontamination. In this study, pure and Cu-doped iron oxyhydroxide was synthesized by the co-precipitation method and was successfully applied for the decontamination of phosphate from aqueous solutions. The solid sample was characterized using a surface area analyzer, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), pH_iep, and Fourier transform infrared (FT-IR) spectroscopy. The results reveal that the surface area of iron oxyhydroxide increases from 35.1 m²/g to 126 m²/g with Cu ion doping, which may be due to the difference in ionic radii of Fe⁺³ (0.645 Å) vs. Cu⁺² (0.73 Å), thus creating crystal defects. The isoelectric point (pH_iep) for the Cu-doped sample was found to be 3.05 in 0.1M KNO₃ and decreased to 2.5 when treated with phosphate anions in solution. Elemental analyses confirmed by XPS indicated that 0.1% by weight Cu was incorporated into the doped iron oxyhydroxide, changing the overall surface parameters. The adsorption process was evaluated with different doses of Cu-doped iron oxyhydroxide, contact time, phosphate anion concentrations, and pH of solution over a range of temperatures (303–323K). The adsorption of phosphate increased with increasing phosphate anion concentrations and decrease with increase in temperature. Langmuir models fittedthe adsorption data well with a good correlation coefficient (R² = 0.99). The sorption maxima for pure iron oxyhydroxide were found to be 0.04 mmol/g at pH 3 and 303K. With Cu ion doping, sorption maxima increased elevenfold to 0.442 mmol g⁻¹ at 303K. Thermodynamic specifications indicated that the sorption process was spontaneous and endothermic in nature. Thermodynamic parameters were evaluated from the Langmuir constant X_m (mmol.g⁻¹) and B (L.mmol⁻¹). FT-IR analyses revealed that phosphate species were adsorbed via electrostatic interactions on the positive sites of the Cu-doped iron oxyhydroxide along with ion exchange mechanism on the surface of the solid.