Reaction of Amino Acids with Ferrate(VI): Impact of the Carboxylic Group on the Primary Amine Oxidation Kinetics and Mechanism

Ultra-efficient degradation of isoquinoline from shale gas wastewater with the diethylamine-ferrate(VI) system: The key role of Fe(IV)/Fe(V) active species

Although isoquinoline (IQL) in shale gas wastewater contributes minimally to chemical oxygen demand, its potential high toxicity makes it an environmental risk factor that cannot be overlooked. This study introduces a synergistic diethylamine (Di)/ferrate (Fe(VI)) system for efficient degradation of IQL. Compared with Fe(VI) alone, the Di/Fe(VI) system demonstrated superior performance, achieving degradation efficiency of 80.5 %. The degradation rate constant of the Di/Fe(VI) system was almost 3-fold larger than that measured with Fe(VI) alone in the degradation of IQL. Mechanistic studies, including radical quenching, electron paramagnetic resonance, pre-mixed experiments, Raman spectroscopy, and probe compounds tests suggested that high-valent iron intermediates (Fe(IV/V)) were responsible for IQL degradation in the Di/Fe(VI) system. The presence of Di promoted the generation of Fe(IV)/Fe(V) by donating electrons. Based on the intermediates identified with GC-MS measurements and density functional theory calculations, three reaction pathways for IQL degradation were proposed. ECOSAR prediction and Escherichia coli toxicity tests showed that the toxicity of IQL was significantly reduced after treatment with Di/Fe(VI) system. Optimal IQL degradation occurred at higher Fe(VI)/Di concentrations and lower pH, with minimal interference from common ions or matrix components. The system also effectively degraded other organics (e.g., 2,4-di-tert-butylphenol, 6-methylquinoline, diclofenac, carbamazepine), demonstrating broad applicability for refractory pollutant treatment.

Degradation of 1,2,3-trichloropropane by ferrate(VI) oxidant: Mechanisms, influencing factors and oxidative iron species

The ferrate(VI) ion is a green, versatile oxidant employed eliminate organic contaminants. Research on the degradation of saturated chlorinated hydrocarbons such as 1,2,3-trichloropropane (1,2,3-TCP), by Fe(VI) is limited. In this study, we investigated the degradation of 1,2,3-TCP by Fe(VI). Four oxidants (Fe(VI), sodium persulfate, hydrogen peroxide and potassium permanganate) were also used, among which only Fe(VI) could degrade 96 % and 91 % of 1,2,3-TCP (0.33 mM) in pure- and polluted-water samples, respectively, within 4 days. During degradation, the intermediate product 2,3-dichloropropylene (2,3-DCP) was formed. The main mechanisms of 1,2,3-TCP and 2,3-DCP degradation by Fe(VI) were alkaline hydrolysis and oxidation, respectively. A higher 1,2,3-TCP degradation efficiency was achieved in an alkaline environment than in acidic or neutral environment. The reaction rate for 2,3-DCP was high at pH levels between 6.0 and 7.0, and the final degradation efficiency increased at pH 8.0-9.0. Electron paramagnetic resonance and radical quenching experiments confirmed that the dominant intermediate ferrate species (Fe(IV) and Fe(V)) were responsible for the 2,3-DCP oxidation. NH4+, Cu2+, CO32-, HCO3- and humic acid (HA) tended to inhibit the degradation of 2,3-DCP by Fe(VI). Meanwhile, the degradation of 1,2,3-TCP was slightly affected by HA, although it was considerably affected by Ca2+ and Mg2+. This study shows the great application potential of Fe(VI) in pump and treat systems for the ex-situ elimination of high-concentration chlorinated hydrocarbons.

Revisiting the Electron Transfer Mechanisms in Ru(III)-Mediated Advanced Oxidation Processes with Peroxyacids and Ferrate(VI)

The potential of Ru(III)-mediated advanced oxidation processes has attracted attention due to the recyclable catalysis, high efficiency at circumneutral pHs, and robust resistance against background anions (e.g., phosphate). However, the reactive species in Ru(III)-peracetic acid (PAA) and Ru(III)-ferrate(VI) (FeO42-) systems have not been rigorously examined and were tentatively attributed to organic radicals (CH3C(O)O•/CH3C(O)OO•) and Fe(IV)/Ru(V), representing single electron transfer (SET) and double electron transfer (DET) mechanisms, respectively. Herein, the reaction mechanisms of both systems were investigated by chemical probes, stoichiometry, and electrochemical analysis, revealing different reaction pathways. The negligible contribution of hydroxyl (HO•) and organic (CH3C(O)O•/CH3C(O)OO•) radicals in the Ru(III)-PAA system clearly indicated a DET reaction via oxygen atom transfer (OAT) that produces Ru(V) as the only reactive species. Further, the Ru(III)-performic acid (PFA) system exhibited a similar OAT oxidation mechanism and efficiency. In contrast, the 1:2 stoichiometry and negligible Fe(IV) formation suggested the SET reaction between Ru(III) and ferrate(VI), generating Ru(IV), Ru(V), and Fe(V) as reactive species for micropollutant abatement. Despite the slower oxidation rate constant (kinetically modeled), Ru(V) could contribute comparably as Fe(V) to oxidation due to its higher steady-state concentration. These reaction mechanisms are distinctly different from the previous studies and provide new mechanistic insights into Ru chemistry and Ru(III)-based AOPs.

How Should We Activate Ferrate(VI)? Fe(IV) and Fe(V) Tell Different Stories about Fluoroquinolone Transformation and Toxicity Changes

Many studies have investigated activation of ferrate (Fe(VI)) to produce reactive high-valent iron intermediates to enhance the oxidation of micropollutants. However, the differences in the risk of pollutant transformation caused by Fe(IV) and Fe(V) have not been taken seriously. In this study, Fe(VI)-alone, Fe3+/Fe(VI), and NaHCO3/Fe(VI) processes were used to oxidize fluoroquinolone antibiotics to explore the different effects of Fe(IV) and Fe(V) on product accumulation and toxicity changes. The contribution of Fe(IV) to levofloxacin degradation was 99.9% in the Fe3+/Fe(VI) process, and that of Fe(V) was 89.4% in the NaHCO3/Fe(VI) process. The cytotoxicity equivalents of levofloxacin decreased by 1.9 mg phenol/L in the Fe(IV)-dominant process while they significantly (p < 0.05) increased by 4.7 mg phenol/L in the Fe(V)-dominant process. The acute toxicity toward luminescent bacteria and the results for other fluoroquinolone antibiotics also showed that Fe(IV) reduced the toxicity and Fe(V) increased the toxicity. Density functional theory calculations showed that Fe(V) induced quinolone ring opening, which would increase the toxicity. Fe(IV) tended to oxidize the piperazine group, which reduced the toxicity. These results show the different-pollutant transformation caused by Fe(IV) and Fe(V). In future, the different risk outcomes during Fe(VI) activation should be taken seriously.

Enhanced removal of phenolic compounds by ferrate(VI): Unveiling the Bi(III)-Bi(V) valence cycle with in situ formed bismuth hydroxide as catalyst

The use of 2-hydroxybenzophenone (2-HBP) in personal care products is of great concern due to its potential negative effects on the ecosystem and public health. This paper presents the degradation of 2-HBP by bismuth(III) (Bi3+)-ferrate(VI) (FeVIO42-, Fe(VI)) (Bi3+-Fe(VI) system). Experimental studies at different pH and dosages of Bi3+ and Fe(VI) showed that the Bi3+-Fe(VI) system increased the degradation rate and removal efficiency of 2-HBP compared to Fe(VI) alone. The in situ formed flake-like white flocculent precipitate of Bi(OH)3 showed catalytic performance through the Bi(III)-Bi(V)-Bi(III) valence cycle which was demonstrated through spectroscopic measurements. The hydrogen transfer-mediated reactions between Fe(VI) and Bi(OH)3 as well as subsequent formation of Bi(V) were supported by performing density functional theoretical (DFT) calculations. Seventeen identified transformation products of 2-HBP by Fe(VI) with and without Bi3+ revealed hydroxylation, bond breaking, carboxylation, and polymerization reaction pathways. Significantly, Bi3+ facilitated the polymerization reaction and the dioxygen transfer-mediated hydroxylation reaction pathways. The ions (anions and cations) and humic acids (HA) present in the Bi3+-Fe(VI) system had minimal influence on the removal efficiency of 2-HBP. Reusability tests and use of real water samples as well as toxicity assessments of transformation products unveiled the practical application aspect of the Bi3+-Fe(VI) system. Finally, the results showed that the system exhibits good removal efficiency for all 12 phenolic compounds, indicating theuniversality. The Bi3+-Fe(VI) system may be an easy-to-implement cost-effective method for the catalytic degradation of benzophenones by Fe(VI).

Elucidation of the Photochemical Fate of Methionine in the Presence of Surrogate and Standard Dissolved Organic Matter under Sunlight Irradiation

The abiotic fate of dissolved free amino acids considerably contributes to the cycling of dissolved sulfur and nitrogen in natural aquatic environments. However, the roles of the functional groups of chromophoric dissolved organic matter (CDOM) and the fate of free amino acids under sunlight irradiation in fresh waters are not fully understood. This study aims to elucidate the fate of photolabile methionine in the presence of three CDOM surrogate compounds, i.e., 1,4-naphthoquinone, 2-naphthaldehyde, and umbelliferone, and two standard CDOM by coupling experimental measurement, quantum chemical computations, and kinetic modeling. Results indicate that excited triplet-state CDOM and hydroxyl radicals are able to cleave the C-S bond in methionine, resulting in the formation of smaller amino acids and volatile sulfur-containing compounds. Singlet oxygen forms methionine sulfoxide and methionine sulfone. The distribution of phototransformation products offers an improved understanding of the fate of nitrogen- and sulfur-containing compounds and their uptake by microorganisms in natural aquatic environments.

Insights into manganese(VII) enhanced oxidation of benzophenone-8 by ferrate(VI): Mechanism and transformation products

Benzophenones (BPs) are commonly used as UV filters in cosmetics and plastics products and are potentially toxic to the environment. This paper presents kinetics and products of BPs oxidation by ferrate(VI) (FeO₄^2-, Fe(VI)) promoted by permanganate (Mn(VII)) . Degradation of 10.0 µM 2,2'-dihydroxy-4-methoxybenzophenone (BP-8)were determined under different experimental conditions ([Mn(VII)] = 0.5-1.5 µM, [Fe(VI)] = 50-150 µM, and pH = 7.0-10.0). The addition of Mn(VII) traces to Fe(VI)-BP-8 solution enhanced kinetics and efficiency of the removal. Similar enhanced removals were also seen for other BPs (BP-1, BP-3, and BP-4) under optimized conditions. The second-order rate constants (k, M^-1s^-1) of the degradation of BPs showed positive relationship with the energy of the highest occupied orbital (E_HOMO). The possible interaction between Mn(VII) and BP-8 and the enhanced generation of Fe(V)/Fe(IV) and •OH was proposed to facilitate the oxidation of the target benzophenone, supported by in-situ electrochemical measurements, theoretical calculations and reactive species quenching experiments. Thirteen oxidation products of BP-8 suggested hydroxylation, bond breaking, polymerization and carboxylation steps in the oxidation. Toxicity assessments by ECOSAR program showed that the oxidized intermediate products posed a tapering ecological risk during the degradation process. Overall, the addition of Mn(VII) could improve the oxidation efficiency of Fe(VI).

Oxidation of Pharmaceuticals by Ferrate(VI)-Amino Acid Systems: Enhancement by Proline

The occurrence of micropollutants in water threatens public health and ecology. Removal of micropollutants such as pharmaceuticals by a green oxidant, ferrate(VI) (FeVIO42-, Fe(VI)) can be accomplished. However, electron-deficient pharmaceuticals, such as carbamazepine (CBZ) showed a low removal rate by Fe(VI). This work investigates the activation of Fe(VI) by adding nine amino acids (AA) of different functionalities to accelerate the removal of CBZ in water under mild alkaline conditions. Among the studied amino acids, proline, a cyclic AA, had the highest removal of CBZ. The accelerated effect of proline was ascribed by demonstrating the involvement of highly reactive intermediate Fe(V) species, generated by one-electron transfer by the reaction of Fe(VI) with proline (i.e., Fe(VI) + proline → Fe(V) + proline•). The degradation kinetics of CBZ by a Fe(VI)-proline system was interpreted by kinetic modeling of the reactions involved that estimated the rate of the reaction of Fe(V) with CBZ as (1.03 ± 0.21) × 106 M-1 s-1, which was several orders of magnitude greater than that of Fe(VI) of 2.25 M-1 s-1. Overall, natural compounds such as amino acids may be applied to increase the removal efficiency of recalcitrant micropollutants by Fe(VI).