Heterogeneous electro-Fenton degradation of methyl orange: Multivariate optimization and voltammetric monitoring

The degradation of methyl orange (MO) was investigated using the heterogeneous electro-Fenton (het-EF) process, with synthetic goethite (S-Goe) as an iron-based catalyst in an undivided cell equipped with a Dimensionally Stable Anode (DSA) and an air-diffusion cathode. A full factorial design (FFD) combined with response surface methodology (RSM) was used to evaluate the effects and interactions of the variables pH and applied current on MO degradation. The best-performing conditions within the investigated experimental domain were obtained at pH 3.0 and 100 mA, decreasing the MO concentration from 20 mg L⁻ 1 to 1.04 ± 0.11 mg L⁻ 1 by spectrophotometry and to 0.860 ± 0.10 mg L⁻ 1 by differential pulse voltammetry (DPV), respectively, corresponding to approximated removal efficiency of 95%. The catalyst exhibited good stability and activity over four consecutive cycles, demonstrating its effectiveness in the het-EF process under the investigated conditions. Monitoring of Fe 2 ⁺, Fe 3 ⁺, and total dissolved Fe indicated limited iron leaching during treatment, supporting the contribution of heterogeneous Fenton reactions alongside homogeneous reactions involving dissolved iron species. DPV showed suitable analytical performance for monitoring MO, with limits of detection and quantification of 9.98 × 10 –7 mol L⁻ 1 and 4.73 × 10 –7 mol L⁻ 1 , respectively. The novelty lies in integrating S-Goe into the het-EF system, combining effective MO degradation with measurable and limited iron leaching, good catalyst stability (confirmed by XDR and FTIR), multivariate optimization, and electrochemical monitoring.