Recently, the application of FeVI in water decontamination has sparked much attention, while owing to the instability of FeVI, there is limited information to clarify its behavior in removing organic contaminants (OCs) under acidic conditions. This work discovered that altering the reaction pH from 7.0 to 3.0 caused two patterns of performance variation in the FeVI system during the elimination of nine representative OCs. Specifically, the best removal of OCs containing electron-donating moieties was observed at pH 6.0, while that of other OCs with electron-withdrawing moieties was presented at pH 3.0. Mechanism research indicated that during FeVI oxidation, high-valent Fe species were active oxidants at pH above 6.0 and •OH as a secondary active species would be derived from the Fenton process at pH below 6.0. Due to the pH-dependent activity of Fenton chemistry, •OH formation exhibited a clear pH dependence. The relative contributions of high-valent Fe species and •OH in oxidizing OCs highly relied on the substrate-specific reactivity. Despite the presence of different active species at pH 3.0–7.0, the FeVI system still effectively immunized the most common water matrices and favored the detoxification of OCs. These results greatly enrich the fundamental knowledge of FeVI oxidation behavior while pointing to considerable potential for designing more effective and rapid FeVI oxidation processes.

The continuous electron supply for oxidant decomposition-induced reactive oxygen species (ROS) generation is the main contributor for the long-standing micropollutant oxidation in the iron-based advanced oxidation processes (AOPs). Herein, as a new class of co-catalysts, metal borides with dual active sites and preeminent conductive performance can effectively overcome the inherent drawback of Fenton-like reactions by steadily donating electrons to inactive Fe­(III). Among the metal borides, tungsten boride (WB) exhibits a significant co-catalytic performance run ahead of common heterogeneous co-catalysts and exceptionally high stability. Based on qualitative and semi-quantitative tests, the hydroxyl radical, sulfate radical, and iron­(IV)-oxo complex are all produced in the WB/Fe­(III)/PDS system and Fe­(IV)-induced methyl phenyl sulfoxide decomposition is up to 72%. Moreover, the production efficiency of ROS and relative proportions of radical and nonradical pathways change with various experimental conditions (dosages of PDS, WB, and solution pH) and water matrices. The rate-determining step of Fe­(II) regeneration is greatly accelerated resulting from the synergetic effect between exposed metallic reactive sites and nonmetallic boron with reductive properties of WB. In addition, the self-dissolution of surface tungsten oxide and boron oxide leads to a renovated surface for sustainable Fe­(III) reduction in long-term operations. Our discovery provides an efficient and sustainable strategy in the field of enhanced AOPs for water remediation.

Carbon nanotubes (CNTs) and their derivatives have been widely exploited to activate various oxidants for environmental remediation. However, the intrinsic mechanism of CNTs-driven periodate (PI) activation remains ambiguous, which significantly impedes their scientific progress toward practical application. Here, we found that CNTs can strongly boost PI activation for the oxidation of various phenols. Reactive oxygen species analysis, in situ Raman characterization, galvanic oxidation process experiments, and electrochemical tests revealed that CNTs could activate PI to form high-potential metastable intermediates (CNTs−PI*) rather than produce free radicals and 1 O 2 , thereby facilitating direct electron transfer from the pollutants to PI. Additionally, we analyzed quantitative structure− activity relationships between rate constants of phenols oxidation and double descriptors (e.g., Hammett constants and logarithm of the octanol−water partition coefficient). The adsorption of phenols on CNT surfaces and their electronic properties are critical factors affecting the oxidation process. Besides, in the CNTs/PI system, phenol adsorbed the CNT surfaces was oxidized by the CNTs−PI* complexes, and products were mainly generated via the coupling reaction of phenoxyl radical. Most of the products adsorbed and accumulated on the CNT surfaces realized phenol removal from the bulk solution. Such a unique non-mineralization removal process achieved an extremely high apparent electron utilization efficiency of 378%. The activity evaluation and theoretical calculations of CNT derivatives confirmed that the carbonyl/ketonic functional groups and double-vacancy defects of the CNTs were the primary active sites, where high-oxidation-potential CNTs−PI* were formed. Further, the PI species could achieve a stoichiometric decomposition into iodate, a safe sink of iodine species, without the generation of typical iodinated byproducts. Our discovery provides new mechanistic insight into CNTs-driven PI activation for the green future of environmental remediation.

With the development of industry and agriculture, the increasingly serious problems of environmental pollution and water quality deterioration that affect human health need to be solved. Materials containing both Fe and Mn, the two most abundant metal elements, have been widely used for wastewater decontamination since they are nontoxic, low-cost, easy to prepare, and highly efficient. This review systematically summarizes the synthesis methods of Fe–Mn-based unloaded materials (such as spinel-type, perovskite-type, layered double hydroxides, metal–organic framework, and its derivatives) and loaded materials (carbon materials, oxides, and other loaded materials). Furthermore, the progress and problems of the application of Fe–Mn-based materials in the fields of Fenton, photo-Fenton, electro-Fenton, persulfate, and sulfite catalysis processes are analyzed. Different activation mechanisms of Fe–Mn-based materials, such as radical pathways and nonradical pathways (like direct electron transfer pathways, singlet oxygen, and high valence metals, etc.) are described. Finally, the application potential of Fe–Mn-based materials in environmental remediation is clarified and the future research directions are pointed out. It is expected that this review can provide new inspiration for subsequent research on the application of Fe–Mn based materials for wastewater treatment.

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.