## 3 Technologies for Treating Landfill Leachate
### Advanced Oxidation Progresses
#### 3.2.4 Photocatalytic Oxidation  
Photocatalytic oxidation has emerged as a cutting-edge remediation technology for landfill leachate treatment, leveraging semiconductor-mediated photoredx mechanisms. The process initiates through photon absorption (UV spectrum, \(\lambda\) < 380 nm) by wide-bandgap semiconductors (e.g., TiO\({}_{2}\) [3.2 eV], ZnO [3.37 eV], and CdS [2.4 eV]), inducing electron excitation from the valence band (VB) to the conduction band (CB) with subsequent generation of electron-hole (e\({}^{-}\)-h\({}^{+}\)) pairs possessing redox potentials of +2.4-3.0 V vs. NHE [127]. Photocatalysis can degrade the macromolecular pollutants from leachate into small molecules or directly mineralize the pollutants into CO\({}_{2}\) and H\({}_{2}\)O [29; 128; 129]. Recent studies on semiconductor material modification have significantly advanced the application efficacy of photocatalytic technology in landfill leachate treatment. [PERSON] et al. [130] synthesized p-type TiO\({}_{2}\) nanoparticles through silicon doping (2.5 wt.%), achieving 84.7% COD removal efficiency under visible-light irradiation (\(\lambda\) > 420 nm) with optimized parameters (calcination temperature 500 \({}^{\circ}\)C, pH 6). Their subsequent research [28] comparatively evaluated n-type, p-type, and n-p heterojunction TiO\({}_{2}\) photocatalysts for treating high-strength leachate (initial COD 2050 mg/L), demonstrating COD removal rates of 46.0%, 60.0%, and 64.0%, respectively, after 46 h irradiation, which substantiated the enhanced photocatalytic activity through interfacial charge separation effects in heterojunction systems. [PERSON] et al. [131] further developed perovskite-type CaTiO\({}_{3}\) nano-photocatalysts, achieving simultaneous removal of multiple pollutants under UV-A irradiation (33 W, 8 h) at optimal conditions (catalyst dosage 3.33 g/L, pH 6): 72.3% COD removal, 43.6% chroma reduction, 41.8% turbidity improvement, 59.0% total dissolved solids elimination, 35.7% NH\({}_{3}\)-N degradation, and heavy metal removal efficiencies of 52.3% for chromium (Cr), 63.5% for nickel (Ni), and 87.5% for zinc (Zn). Table 4 lists the efficiency of AOPs for landfill leachate treatment. These technologies can completely degrade pollutants into CO\({}_{2}\) and H\({}_{2}\)O, but the degradation efficiency depends on several factors, such as the oxidation potential, pollutant concentration, and reaction kinetics. AOPs degrade pollutants by generating highly oxidative free radicals (e.g., -OH and SO\({}_{4}\)\({}^{-}\)); the oxidation potentials of hydroxyl radicals and sulfate radicals are 2.8 V and 2.6 V, respectively, theoretically enabling the mineralization of most organic compounds [132]. However, in practical applications, the selection of reactive species must align with the specific properties of the target pollutants. Pollutant concentration also significantly impacts degradation efficiency: excessive concentrations can lead to rapid oxidant consumption, insufficient radical generation, and accumulation of intermediate products [130]. Additionally, factors like pH, temperature, and catalyst