## 1 Introduction
Both direct and indirect oxidation can occur during EO. In direct oxidation, a hydroxyl radical (\(\bullet\)OH) is produced, which causes oxidation of organic matter on the anode surface. Organic matter undergoes indirect oxidation when byproducts of the solution (\(\bullet\)OH, O\({}_{3}\), and HClO) are produced [13]. [PERSON] and [PERSON] [26] asserted that boron-doped diamond (BDD) anodes remove organic matter at a high rate. The presence of colloidal and hydrophobic humic substances (HA and FA), which can affect process efficiency and increase energy costs, is a challenge in the treatment of stabilized leachates through the EO process. The development of a separation alternative, such as pretreatment, would increase the effectiveness of the treatment process by eliminating interference from these colloidal species [27; 28; 29]. The treatment of leachates is difficult and the organic matter involved is not sufficiently characterized. [PERSON] et al. [30] examined more than 210 scientific papers on leachates and their treatments published between 2009 and 2021 and found that pH (95%) was the most frequently evaluated parameter, followed by COD (85%), BOD\({}_{5}\) (78%), chloride (Cl\({}^{-}\)) (61%), and ammonia nitrogen (NH\({}_{3}\)-N) (55%). Monitoring the development of organic matter throughout the process is required to improve the leachate treatment systems. By characterizing organic matter, we can better understand its evolution in terms of composition, origin, and biodegradability, which supports the choice of treatment options. Because stabilized leachates could contain humic substances that the EC process can eliminate [31], this study suggests that the EC process is the initial stage of treatment, followed by EO. From the suggested treatment system (EC-EO), the contribution of this research was to create, in addition to the distinctive measurements of COD and dissolved organic carbon (DOC), a complete characterization of the present organic matter. It was challenging to track and explain the evolution of organic matter in various fractions because of the scarcity of information on the characterization of leachates and effluents. This study used analytical and spectroscopic methods to measure and track various fractions of organic matter in a stabilized leachate treatment system in order to close this data gap. The methods of analysis included determination of COD fractions (soluble COD-CODs, biodegradable COD-CODb, and particulate COD-CODp) and fractions of humic substances (HA, FA, and hydrophilic fraction -HvL). In addition, spectroscopic techniques, including ultraviolet-visible (UV-VIS) and Fourier transform infrared spectroscopy (FTIR), were used. These findings will be useful in determining how the process transforms recalcitrant organic matter and for understanding and improving stabilized leachate treatment systems. The characterization of organic matter in wastewater has recently been the subject of numerous investigations using spectroscopic methods [32; 33], including leachate analysis [21; 34]. Various studies have monitored humic substances in leachates [35; 36; 37], including COD fractionation [23; 38; 39; 40]. However, there are knowledge gaps in the use of these techniques as tools to understand processes and make decisions to determine the best treatment strategy. As far as we know, this study is the first to follow the evolution of organic matter in a system with specific characteristics, as suggested by this project.