Spinel CuB₂O₄ (B = Fe, Cr, and Al) Oxides for Selective Adsorption of Congo Red and Photocatalytic Removal of Antibiotics

A polyacrylamide gel method has been used to synthesize spinel type nano-CuB2O4 with cubic phase (B = Al) and tetragonal phase (B = Cr and Fe). The phase structure, functional group information, and charge state characterization confirm that the CuAl2O4 oxides contain no impurities, while CuCr2O4 and CuFe2O4 contain CuO impurities. The CuAl2O4 oxides exhibit a small particle size of 25 nm, high optical band gap (Eg) value, high adsorption capacity for the adsorption of Congo red dye, and high photocatalytic activity for the photocatalytic degradation of tetracycline hydrochloride (TCH), which are much better than those of the CuCr2O4 and CuFe2O4 oxides. The high adsorption capacity for the CuAl2O4 oxides can be ascribed to the larger zero charge point (pHPZC) values, unique crystal structure, pore filling, electrostatic interaction, and hydrogen bond force. Photocatalytic degradation of antibiotics by CuB2O4 (B = Al, Cr, and Fe) shows high selectivity and B ion and crystal structure dependencies. The CuAl2O4 oxides exhibit high photocatalytic activity for the photocatalytic degradation of TCH and oxycline hydrochloride (OCH) and poor photocatalytic activity for the photocatalytic degradation of metformin hydrochloride (MFH) and naproxen sodium (NPS). Cyclic stability tests and XRD results further confirm that CuAl2O4 oxide is recyclable and stable. The high photocatalytic activity of CuAl2O4 oxides was explained in terms of the synergetic effects of the optical absorption coefficient, specific surface area, crystal structure, adsorbed oxygen concentration, chemical structure of the antibiotic, and conduction band and valence band potentials. The trapping experiments confirmed that TCH photodegradation in this reaction system occurs via photogenerated holes (hVB+) and superoxide radicals (•O2–).