## 3 Technologies for Treating Landfill Leachate
### Advanced Oxidation Progresses
#### 3.2.3. Fenton Oxidation  
Fenton oxidation employs hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)) as an oxidizing agent to produce hydroxyl free radicals (\(\bullet\)OH) in the presence of ferrous ions, which then attack organic compounds as active component of the Fenton reagent, resulting in their partial or complete oxidation and eventually producing CO\({}_{2}\) and H\({}_{2}\)O [57,122] (as shown in Equations (7)-(15)).  
\[Fe^{2+}+H_{2}O_{2}\to Fe^{3+}+\cdot OH+OH^{-} \tag{7}\]  
Figure 2: Proposed mechanism of organics’ transformation through the synergy between chlorine, a coagulant, and ozone [117].  
\[RH+\cdot OH\to H_{2}O+R\cdot \tag{8}\]  
\[Fe^{3+}+H_{2}O_{2}\to Fe^{2+}+HO_{2}\cdot+H^{+} \tag{9}\]  
\[Fe^{3+}+HO_{2}\cdot\to Fe^{2+}+O_{2}+H^{+} \tag{10}\]  
\[\cdot OH+H_{2}O_{2}\to H_{2}O+HO_{2}\cdot \tag{11}\]  
\[Fe^{2+}+HO_{2}\cdot\to Fe^{3+}+HO_{2}^{-} \tag{12}\]  
\[\cdot OH+Fe^{2+}\to Fe^{3+}+OH^{-} \tag{13}\]  
\[COD/TOC+H_{2}O_{2}\left(Fe^{2+},Fe^{3+}\right)\to Intermediate\ product \tag{14}\]  
\[H_{2}O_{2}\left(Fe^{2+},Fe^{3+}\right)+Intermediate\ product\to CO_{2}+H_{2}O+ Inorganic\ salt \tag{15}\]  
The Fenton oxidation process, operating through the in situ generation of hydroxyl radicals (\(\bullet\)OH) via Fe\({}^{2+}\)/H\({}_{2}\)O\({}_{2}\) catalytic reactions under acidic conditions (pH 2.5-3.5), has emerged as a benchmark chemical oxidation technology for leachate pretreatment [58]. Fenton oxidation degrades landfill leachate by generating highly reactive hydroxyl radicals (\(\bullet\)OH) through the reaction of Fe\({}^{2+}\) with H\({}_{2}\)O\({}_{2}\) under acidic conditions (pH 2-4). These radicals non-selectively attack C-H bonds, aromatic rings, and functional groups (such as carboxyl and amino groups) in organic molecules, gradually mineralizing refractory organic compounds (e.g., humic acids and polycyclic aromatic hydrocarbons) and metal complexes into CO\({}_{2}\), H\({}_{2}\)O, and inorganic salts.  
Recent advancements in Fenton-based technologies for landfill leachate treatment have demonstrated significant progress in process optimization and catalyst development. [PERSON] et al. [123] pioneered the aeration-enhanced electrochemical Fenton process for domestic leachate treatment, achieving exceptional removal efficiencies of 99% COD and complete color elimination (100%) in stabilized leachate under optimized aeration parameters. Parallel developments by [PERSON] et al. [124] in bio-electro-Fenton systems demonstrated 70% COD removal under baseline conditions (pH = 2, applied potential 0.6 V), which significantly improved to 91.3% through cathode modification using platinum-group metal coatings, highlighting electrode engineering's critical role in process enhancement.  
Despite these advancements, conventional homogeneous Fenton processes face three fundamental limitations, as critically analyzed by [PERSON] et al. [125]: (1) iron sludge production (0.8-1.2 kg/m\({}^{3}\) treatment capacity), (2) stringent pH requirements (2.5-3.5), and (3) non-recoverable catalysts. Addressing these challenges, [PERSON] et al. [126] developed a novel heterogeneous Fenton system employing iron oxide-impregnated granular activated carbon (Fe-GAC) catalysts for mature leachate treatment. Through orthogonal experimental design, optimal conditions (H\({}_{2}\)O\({}_{2}\)/COD = 1.5, Fe\({}^{2+}\)/H\({}_{2}\)O\({}_{2}\) = 0.3, reaction time = 90 min) achieved 93.7% COD removal, 65.5% NH\({}_{3}\)-N reduction, and 84% decolorization in batch mode. Continuous operation in a fluidized-bed reactor demonstrated process stability with >90% COD removal sustained over 12 h of operation. The electro-Fenton variant exhibited superior performance, attaining 97.4% COD removal, complete color elimination (99%), TN reduction of 94.2%, and full nitrification of NH\({}_{3}\)-N (100%) through synergistic radical oxidation (\(\cdot\)OH yield = 2.3 mM) and electrochemical reduction mechanisms.