## 3 Technologies for Treating Landfill Leachate
### Advanced Oxidation Progresses
#### 3.2.2 Ozone Oxidation  
Ozonation (O\({}_{3}\)), as a robust advanced oxidation process (AOP), has gained prominence in leachate remediation for its exceptional capability in mineralizing refractory organic contaminants (ROCs) through radical chain reactions (-OH-mediated oxidation). The decomposition of O\({}_{3}\) generates -O\({}_{2}\)\({}^{-}\) and -OH\({}^{-}\) radicals in water (as shown in Equations (1)-(6)), as the highly reactive oxidative radicals non-selectively attack refractory organic compounds in landfill leachate (e.g., humic acids and polycyclic aromatic hydrocarbons, denoted as CHOSs) through electron transfer, hydrogen abstraction, and addition reactions. These radical-mediated processes progressively cleave large organic molecules into low-molecular-weight, biodegradable intermediates such as carboxylic acids and ketones, while simultaneously oxidizing them through bond-breaking mechanisms. Ultimately, the degraded organic constituents are mineralized into environmentally benign end-products, predominantly carbon dioxide (CO\({}_{2}\)) and water (H\({}_{2}\)O). Figure 2 shows macromolecular substances can produce highly resoluble small molecular substances, and even degrade them into CO\({}_{2}\) and H\({}_{2}\)O [116].  
The total reaction is as follows:  
\[O_{3}+H_{2}O\to 2\cdot OH+O_{2} \tag{1}\]  
The chain reaction is as follows:  
\[O_{3}+HO^{-}\rightarrow\cdot O_{2}^{-}+HO_{2} \tag{2}\]  
\[O_{3}+\cdot OH\rightarrow O_{2}+HO_{2}. \tag{3}\]  
\[O_{3}+HO_{2}\cdot\to 2O_{2}+HO\cdot \tag{4}\]  
\[2 HO_{2}\cdot\to O_{2}+H_{2}O_{2} \tag{5}\]  
The degradation reaction is as follows:  
\[\cdot OH+RH\rightarrow\cdot R+H_{2}O_{2}\to Further\ oxidation\to CO_{2}+H_{2}O \tag{6}\]Recent advancements in landfill leachate treatment have demonstrated the progressive development of ozone-based oxidation technologies. [PERSON] [118] implemented an ozonation process for pretreatment of high-concentration leachate, achieving 80.38% COD removal efficiency through direct oxidative degradation. However, recognizing the inherent limitations of single-stage ozonation in handling complex leachate matrices, [PERSON] [119] engineered a hybrid O\({}_{3}\)/supercritical water oxidation (O\({}_{3}\)/ScWO) system, establishing an innovative synergistic oxidation platform. Under optimized operational parameters (reaction temperature 450 \({}^{\circ}\)C, pressure 25 MPa, O\({}_{3}\) dosage 15 mg/L), this integrated process attained remarkable removal efficiencies of 95% BOD\({}_{5}\), 92% COD, and 79% TOC, representing 1.6-2.3-fold enhancement compared to standalone ozonation processes.  
[PERSON] [120] engineered a manganese-cerium composite oxide supported on a gamma-alumina (MnCeOx/\(\gamma\)-Al\({}_{2}\)O\({}_{3}\)) catalyst to enhance ozone activation in landfill leachate treatment. Comparative analysis revealed significant performance enhancement in catalytic ozonation systems: COD removal efficiency increased by 48% (from 55.8% to 82.4%) and UV\({}_{254}\) absorbance reduction improved by 15% (from 83.6% to 96.3%) compared to conventional ozonation. This performance enhancement stems from the catalyst's ability to generate reactive oxygen species (ROS), including hydroxyl radicals (\(\bullet\)OH) and superoxide radicals (O\({}_{2}\bullet\)-), which exhibit superior oxidative potential for refractory organic compound mineralization. Recent mechanistic studies [121] have further validated that heterogeneous catalytic ozonation provides threefold advantages: (1) enhanced organic mineralization rates (>85% TOC reduction), (2) optimized ozone utilization efficiency (up to 92% mass transfer efficiency), and (3) facile catalyst separation via \(\gamma\)-Al\({}_{2}\)O\({}_{3}\)'s macroscopic structure, making it technically viable for continuous-flow operations.