An Effective CuFeZnO Photocatalyst for Formaldehyde Degradation and Chromium Adsorption Using Multilayer Model & Statistical Interpretation

Indoor formaldehyde (HCHO) vapor and aqueous hexavalent chromium (Cr(VI)) are pervasive pollutants typically addressed with distinct, purpose-specific materials. In this study, a ternary Cu–Fe–Zn oxide (CuFeZnO) was synthesized via co-precipitation and evaluated as a dual-function material for both photocatalytic HCHO degradation and Cr(VI) adsorption. The catalyst exhibited a narrowed optical band gap of 2.3 eV, compared to 3.2 eV for TiO₂, consistent with enhanced visible-light absorption. Under low-intensity visible irradiation (single 18 W daylight lamp, 8.8 mW cm⁻²), 20 mg of CuFeZnO achieved 92.1% HCHO removal within 2 h, outperforming commercial TiO₂ (P25, 78.2%) and laboratory-synthesized TiO₂ (68.3%) under identical conditions, and retaining 90.8% conversion after seven reuse cycles. For Cr(VI) removal, CuFeZnO achieved 98.98% removal from a 50 mg L⁻¹ solution at pH 8 with a 0.2 g dose. Adsorption kinetics followed a pseudo-second-order model, and equilibrium data were best described by the Freundlich isotherm (R² = 0.992). A double-layer statistical physics model further captured the temperature-dependent equilibrium behavior (298–318 K) with high accuracy (R² = 0.997–0.998), yielding a near-unimolecular site occupancy (n ≈ 0.999 at 298 K) and a maximum adsorption capacity of 111.7 mg g⁻¹. The first-layer adsorption energy increased from 13.4 to 27.6 kJ mol⁻¹ with rising temperature, indicating a physisorption-dominated mechanism. Notably, CuFeZnO achieved comparable or superior HCHO conversion under substantially lower light intensity (18 W) than reported photocatalysts requiring 27–300 W sources. These results demonstrate CuFeZnO as a low-energy, reusable, and multifunctional material for simultaneous air and water remediation.