An Effective CuFeZnO Photocatalyst for Formaldehyde Degradation and Chromium Adsorption Using Multilayer Model and 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.