<p>A mechanistic understanding of electro-Fenton (EF) degradation processes, in terms of reaction pathways and their energetic feasibility, is essential for improving their efficiency in water treatment applications. In this work, FeSO₄ waste from the steel industry was used as the iron catalyst in an EF process, which served as an experimental platform to investigate degradation pathways of an organic pollutant. The effects of applied voltage, FeSO₄ waste dosage, and supporting electrolyte concentration on pollutant degradation were evaluated to determine the operating conditions that provided the best results. Under these optimized conditions, the process achieved 93.2% pollutant removal, and 78.6% COD elimination, with an energy consumption of 4.77 kWh kg⁻¹ COD, following first-order kinetics, with a rate constant of 0.048&#xa0;min⁻¹. Eight intermediates were identified by HPLC–MS, and, together with Density Functional Theory calculations, four possible reaction pathways were proposed. The reaction energies associated with each elementary stage were determined, enabling a comparative evaluation of the proposed routes. Based on this analysis, the following mechanistic hierarchy was established: Pathway 1 ≈ Pathway 4 &gt; Pathway 3 &gt; Pathway 2. These results provide a thermodynamic perspective on EF degradation pathways and demonstrate that industrial waste can be effectively used as a source of iron without altering the fundamental reaction mechanisms of the EF process.</p>