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

Abstract

Accelerating the rate-limiting Fe 3+ /Fe 2+ 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 Fe 3+ /Fe 2+ cycle and promote H 2 O 2 activation. The reduction of Fe 3+ 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.

Unrecognized Role of Organic Acid in Natural Attenuation of Pollutants by Mackinawite (FeS): The Significance of Carbon-Center Free Radicals

“…Due to the intricate nature of mineral microscopic interfaces and the structural diversity of organic acids, it is challenging to attribute ROS production to either of the aforementioned two processes. In addition, organic acids are readily decomposed by ROS, such as nonselective • OH, therefore impacting the effectiveness of contaminant transformation. − The trade-off between the accelerated production and inevitable elimination of ROS should be paid more attention. − However, there is also a large knowledge gap about the generation of free radicals and their role in pollutant removal in FeS-organic acid systems under dark condition.…”

Main Controlling Factors Affecting the Viscosity of Polymer Solution due to the Influence of Polymerized Cations in High-Salt Oilfield Wastewater

“…These structural changes reduce the hydration shell of the polymer, lowering the hydrodynamic radius of the solution, which further decreases its viscosity. In the compressed double layer environment, the free radical reactions catalyzed by Fe 2+ ions more readily promote the breakage of polymer chains, as the charge shielding between molecules is weakened, making them more prone to contact and reaction [31][32][33]. Overall, the role of Fe 2+ ions in polyacrylamide solutions is multifaceted.…”

Electro-Fenton-Based Membrane System for Organic Micropollutant Removal: New Trend and Prospect

“…Hetero-electro-Fenton has been explored as an effective alternative to expand the pH application range . Fenton catalysts are stable in solid catalysts and activate H 2 O 2 to form surface-bound radicals, (eq ) particularly Fe-, − Mn-, , Cu-, − and other metal-based electrocatalysts (e.g., Au, TiO 2 , Pd, CeO 2 ). − However, there are still some key obstacles to be overcome. The conventional hetero-electro-Fenton system has limited reactant transfer efficiency for the complex three-phase reactions (O 2, H + , Fenton catalysts, H 2 O 2 , • OH, and pollutants), which significantly reduces ROS utilization efficiency .…”