Boosting Degradation of Persistent Organic Compounds in Landfill Leachate with ZnO-Fly Ash Composites and Peroxymonosulfate in a Hybrid Ozonation System

This study investigates the efficacy of a hybrid ozonation system utilizing ZnO-fly ash (FA@ZnO) composites and peroxymonosulfate (PMS) for the degradation of persistent organic compounds (POCs) in landfill leachate. The FA@ZnO catalyst, synthesized via a modified sol-gel method with ZnO ratios of 10%, 20%, and 30%, was characterized using SEM, EDX and XRD, confirming successful ZnO integration and structural stability. SEM revealed a transition from fibrous FA to granular FA@ZnO with mesoporous structure (BET surface area 15.29 m2/g, pore volume 0.0463 cm3/g), while EDX confirmed Zn (5.77 wt%) and Fe (2.23 wt%) content, enhancing PMS activation via Fe-mediated redox cycles. Optimal conditions were identified as pH 9.0, a PMS dosage of 300 mg/L, and a catalyst dosage of 1.25 g/L, achieving a maximum color removal of 90.92% and a total organic carbon (TOC) removal of 73.01% after 80 minutes, with an initial TOC of 421.0 mg/L. The system’s performance, driven by synergistic generation of hydroxyl (•OH) and sulfate (SO4−•) radicals, outperformed standalone ozonation (34.01% color removal, 15.03% TOC removal). Kinetic analysis revealed a pseudo-first-order rate constant that peaked at 0.0138 min−1 at a catalyst dosage of 1.00 g/L. Further modeling showed excellent fit with pseudo-second-order (PSO) kinetics (qe = 370.37 mg/L, k2 = 1.6382 × 10−4 L/mg·min, R2 = 0.9941) and moderate Langmuir-Hinshelwood (L-H) fit (k = 0.6883 mg/L·min, K = 3.48 × 10−3 L/mg, R2 = 0.6609), indicating chemisorption-dominated degradation; mass transfer analysis confirmed reaction-limited kinetics with kLa = 0.114 min−1 (gas-liquid) and ksas = 14.13 min−1 (liquid-solid). Scavenger studies with CO32- and Cl− highlighted the critical role of •OH radicals, with efficiency dropping to 60% and 70% (color) and 40% and 50% (TOC), respectively. The O3/FA@ZnO/PMS system achieved 87.26% color and 68.93% TOC removal in the first cycle for stabilized leachate, declining to 67.95% and 43.69% after five cycles, due to pore blockage (BET surface area reduced to 9.61 m2/g), carbon accumulation (28.85% to 32.14% wt), and Zn/Fe leaching (Zn: 5.77% to 4.92% wt, Fe: 2.23% to 2.01% wt), as evidenced by post-cycle SEM/EDX/XRD. For real leachates, the system achieved 72.30% color and 65.18% TOC removal for the young leachate (TOC: 1386.0 mg/L, color: 3.55 ABS), and 64.61% color and 57.04% TOC removal for the aged leachate, indicating higher biodegradability in younger matrices.