Redox-Active Polymers as Robust Electron-Shuttle Co-Catalysts for Fast Fe3+/Fe2+ Circulation and Green Fenton Oxidation

• Published in: Environmental science & technology Vol. 57; no. 8; pp. 3334 - 3344
• Main Authors: Zhou, Hongyu, Peng, Jiali, Duan, Xiaoguang, Yin, Haoxiang, Huang, Bingkun, Zhou, Chenying, Zhong, Shuang, Zhang, Heng, Zhou, Peng, Xiong, Zhaokun, Ao, Zhimin, Wang, Shaobin, Yao, Gang, Lai, Bo
• Format: Journal Article
• Language: English
• Published: Easton American Chemical Society 28.02.2023
• Subjects: Catalysts; Decontamination; Density functional theory; Dissolution; Electron transfer; Heavy metals; Hydrogen peroxide; Hydroquinone; Hydroxylamine; Ionization; Ionization potentials; Iron; Metal sulfides; Molecular orbitals; Organic wastes; Oxidation; Pollutant removal; Polyanilines; Polymers; Quenching; Reagents; Reducing agents; Solid phases; Wastewater; Wastewater treatment; Water purification
• ISSN: 0013-936X; 1520-5851
• DOI: 10.1021/acs.est.2c07447

Accelerating the rate-limiting Fe3+/Fe2+ circulation in Fenton reactions through the addition of reducing agents (or co-catalysts) stands out as one of the most promising technologies for rapid water decontamination. However, conventional reducing agents such as hydroxylamine and metal sulfides are greatly restricted by three intractable challenges: (1) self-quenching effects, (2) heavy metal dissolution, and (3) irreversible capacity decline. To this end, we, for the first time, introduced redox-active polymers as electron shuttles to expedite the Fe3+/Fe2+ cycle and promote H2O2 activation. The reduction of Fe3+ mainly took place at active N–H or O–H bonds through a proton-coupled electron transfer process. As electron carriers, H atoms at the solid phase could effectively inhibit radical quenching, avoid metal dissolution, and maintain long-term reducing capacity via facile regeneration. Experimental and density functional theory (DFT) calculation results indicated that the activity of different polymers shows a volcano curve trend as a function of the energy barrier, highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO–LUMO) gap, and vertical ionization potential. Thanks to the appropriate redox ability, polyaniline outperforms other redox-active polymers (e.g., poypyrrole, hydroquinone resin, poly(2,6-diaminopyridine), and hexaazatrinaphthalene framework) with a highest iron reduction capacity up to 5.5 mmol/g, which corresponds to the state transformation from leucoemeraldine to emeraldine. Moreover, the proposed system exhibited high pollutant removal efficiency in a flow-through reactor for 8000 bed volumes without an obvious decline in performance. Overall, this work established a green and sustainable oxidation system, which offers great potential for practical organic wastewater remediation.