The photocatalysis technology is widely applied for effective degradation of environmental contaminants. This study aims to examine the effectiveness of heat-treated iron oxide nanoparticles photocatalysts in the photocatalytic degradation of oxalic acid. To provide a comprehensive characterization of the photocatalysts, a range of techniques were utilized. These techniques included transmission electron microscopy (TEM) for the analysis of particle morphology, diffuse reflectance spectroscopy for the calculation of the energy bandgap, Brunauer–Emmett–Teller (BET) analysis for the determination of surface area, and X-ray diffraction (XRD) for the identification of crystal structure. The catalyst that was produced demonstrated an increased specific surface area, as well as a band gap of 2.2 eV. These characteristics provide a basis for understanding its photocatalytic performance. This work additionally examines the phenomenon of oxalic acid adsorption on iron oxide nanoparticles that have undergone heat treatment at different temperatures, employing Operando ATR-FTIR spectroscopy. Following this, the same approach is used to investigate the photocatalytic breakdown of the adsorbed oxalic acid. Subsequently, a study was undertaken to examine the surface electronic configuration of iron oxide nanoparticles that had undergone heat treatment. This analysis employed Fe–L edge X-ray absorption spectroscopy in the total electron yield (TEY) mode. It yielded valuable insights regarding the distribution of Fe3+/Fe2+ sites on the surface of the nanoparticles. The presence of Fe2+/Fe3+ ions are determined using operando ATR—FTIR spectroscopy, by analyzing the adsorption of oxalic acid onto the surface of iron oxide nanoparticles. The sample subjected to low-heat treatment showed an improved effectiveness in absorbing the oxalic acid, which is related to the presence of Fe2+ sites. The photocatalytic process was further investigated using quantitative analysis methods, namely through kinetic studies. The observed kinetics followed a pseudo first- order model, with a rate constant of 0.010 M−1S−1. The low heat-treated nanoparticles shows greater photocatalytic rate constant and these results highlight the efficient and quick degradation capabilities of these nanoparticles.Graphical Abstract: