Enhanced oxidative degradation of decabromodiphenyl ether in soil by coupling Fenton-persulfate processes: Insights into degradation products and reaction mechanisms

Comparative study of Fe(II)/sulfite, Fe(II)/PDS and Fe(II)/PMS for p-arsanilic acid treatment: Efficient organic arsenic degradation and contrasting total arsenic removal

As a widely used feed additives, p-arsanilic acid (p-AsA) frequently detected in the environment poses serious threats to aquatic ecology and water security due to its potential in releasing more toxic inorganic arsenic. In this work, the efficiency of Fe(II)/sulfite, Fe(II)/PDS and Fe(II)/PMS systems in p-AsA degradation and simultaneous arsenic removal was comparatively investigated for the first time. Efficient p-AsA abatement was achieved in theses Fe-based systems, while notable discrepancy in total arsenic removal was observed under identical acidic condition. By using chemical probing method, quenching experiments, isotopically labeled water experiments, p-AsA degradation was ascribed to the combined contribution of high-valent Fe(IV) and SO4•-in these Fe(II)-based system. In particular, the relative contribution of Fe(IV) and SO4•- in the Fe(II)/sulfite system was highly dependent on the molar ratio of [Fe(II)] and [sulfite]. Negligible arsenic removal was observed in the Fe(II)/sulfite and Fe(II)/PDS systems, while ∼80% arsenic was removed in the Fe(II)/PMS system under identical acidic condition. This interesting phenomenon was due to that ferric precipitation only occurred in the Fe(II)/PMS system. As(V) was further removed via adsorption onto the iron precipitate or the formation of ferric arsenate-sulfate compounds, which was confirmed by particle diameter measurements, fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. Through tuning solution pH, complete removal of total arsenic could achieve in all three systems. Among these three Fe-based technologies, the hybrid oxidation-coagulation Fe(II)/PMS system demonstrated potential superiority for arsenic immobilization by not requiring pH adjustment for coagulation and facilitating the in-situ generation of ferric arsenate-sulfate compounds with comparably low solubility levels like scorodite. These findings would deepen the understanding of these three Fe-based Fenton-like technologies for decontamination in water treatment.

A critical review on BDE-209: Source, distribution, influencing factors, toxicity, and degradation

As the most widely used polybrominated diphenyl ether, BDE-209 is commonly used in polymer-based commercial and household products. Due to its unique physicochemical properties, BDE-209 is ubiquitous in a variety of environmental compartments and can be exposed to organisms in various ways and cause toxic effects. The present review outlines the current state of knowledge on the occurrence of BDE-209 in the environment, influencing factors, toxicity, and degradation. BDE-209 has been detected in various environmental matrices including air, soil, water, and sediment. Additionally, environmental factors such as organic matter, total suspended particulate, hydrodynamic, wind, and temperature affecting BDE-209 are specifically discussed. Toxicity studies suggest BDE-209 may cause systemic toxic effects on living organisms, reproductive toxicity, embryo-fetal toxicity, genetic toxicity, endocrine toxicity, neurotoxicity, immunotoxicity, and developmental toxicity, or even be carcinogenic. BDE-209 has toxic effects on organisms mainly through epigenetic regulation and induction of oxidative stress. Evidence regarding the degradation of BDE-209, including biodegradation, photodegradation, Fenton degradation, zero-valent iron degradation, chemical oxidative degradation, and microwave radiation degradation is summarized. This review may contribute to assessing the environmental risks of BDE-209 to help develop rational management plans.

Degradation efficiency and mechanism of 2,2',4,4'-tetrabromodiphenyl ether (BDE-47) by thermally activated persulfate system

Polybrominated diphenyl ethers (PBDEs) as a typical brominated flame retardant (BFR) have attracted worldwide attention due to the high environmental risk and resistance to conventional remediation processes. In this study, thermally activated persulfate (TAP) process was applied to degrade 2,2',4,4'-tetrabromodiphenyl ether (BDE-47), which is the most toxic and representative PBDEs in e-waste dismantling sites. Impact factors such as PDS dosage, heating temperature, and initial pH were evaluated. Results showed that BDE-47 can be 100% degraded within 180 min under the condition of PDS:BDE-47 = 1000:1, 60 °C, and pH = 7. Quenching experiments combined with EPR analysis further proved the important role of SO₄^·- in oxidating BDE-47. According to high-resolution mass spectrometry (HRMS) analysis, only one oxidation product of low toxicity was detected during the oxidation process. Theoretical calculations further revealed that the oxidation process mainly involved radical attack at C-Br bond, cleavage of C-Br bond, and fission of ether bond, and HSO₄· may also play an important role in BDE-47 degradation in TAP system. In addition, TAP system exhibited universality as all selected PBDE congeners can be degraded, and the degradation rate of PBDEs was greatly affected by the number of substituted Br atoms in a negative trend. Overall, these findings indicate that TAP can be applied as an effective method for removal of PBDEs, and we provide a new insight for the practical application of TAP technology in BDE-47 degradation from experimental and theoretical aspects.

Removing emerging e-waste pollutant DTFPB by synchronized oxidation-adsorption Fenton technology

Liquid crystal monomers (LCMs), an emerging group of organic pollutants related to electronic waste, have been frequently detected from various environmental matrices, including landfill leachate. The persistence of LCMs requires robust technology for remediation. The objectives of this study were to evaluate the feasibility, performance and mechanism of the remediation of a typical LCM 4-[difluoro(3,4,5-trifluorophenoxy)methyl]- 3,5-difluoro-4'-propylbiphenyl (DTFPB) via synchronized oxidation-adsorption (SOA) Fenton technology and verify its application in DTFPB-contaminated leachate. The SOA Fenton system could effectively degrade 93.5% of DTFPB and 5.6% of its total organic carbon (TOC_DTFPB) by hydroxyl radical oxidation (molar ratio of Fe²⁺ to H₂O₂ of 1/4 and pH 2.5-3.0) following a pseudo-first-order model under 0.378 h^-1. Additionally, synchronized adsorption of DTFPB and its degradation intermediates by in situ resultant ferric particles via hydrophobic interaction, complexation, and coprecipitation contributed to almost 100% of DTFPB and 33.4% of TOC_DTFPB removal. Three possible degradation pathways involving eight products were proposed, and hydrophobic interactions might drive the adsorption process. It was first confirmed that the SOA Fenton system exhibited good performance in eliminating DTFPB and byproducts from landfill leachate. This study provides new insights into the potential of the Fenton process for the treatment of emerging LCMs contamination in wastewater.

A Comparative Study on the Oxidation Mechanisms of Substituted Phenolic Pollutants by Ferrate(VI) through Experiments and Density Functional Theory Calculations

In this work, the oxidation of five phenolic contaminants by ferrate(VI) was comparatively investigated to explore the possible reaction mechanisms by combined experimental results and theoretical calculations. The second-order rate constants were positively correlated with the energy of the highest occupied molecular orbital. Considering electronic effects of different substituents, the easy oxidation of phenols by ferrate(VI) could be ranked as the electron-donating group (-R) > weak electron-withdrawing group (-X) > strong electron-withdrawing group (-(C═O)-). The contributions of reactive species (Fe(VI), Fe(V)/(IV), and •OH) were determined, and Fe(VI) was found to dominate the reaction process. Four main reaction mechanisms including single-oxygen transfer (SOT), double-oxygen transfer (DOT), •OH attack, and electron-transfer-mediated coupling reaction were proposed for the ferrate(VI) oxidation process. According to density functional theory calculation results, the presence of -(C═O)- was more conducive for the occurrence of DOT and •OH attack reactions than -R and -X, while the tendency of SOT for different substituents was -R > -(C═O)- > -X and that of e^--transfer reaction was -R > -X > -(C═O)-. Moreover, the DOT pathway was found in the oxidation of all four substituted phenols, indicating that it may be a common reaction mechanism during the ferrate(VI) oxidation of phenolic compounds.

Experimental and theoretical study on the degradation of Benzophenone-1 by Ferrate(VI): New insights into the oxidation mechanism

The oxidation of Benzophenone-1 (BP-1) by ferrate (Fe(VI)) was systemically investigated in this study. Neutral pH and high oxidant dose were favorable for the reaction, and the second order rate constant was 1.03 × 10³ M^-1·s^-1 at pH = 7.0 and [Fe(VI)]₀:[BP-1]₀ = 10:1. The removal efficiency of BP-1 was enhanced by cations (K⁺, Ca²⁺, Mg²⁺, Cu²⁺, and Fe³⁺), while inhibited by high concentrations of anions (Cl^- and HCO₃^-) and low concentrations of humic acid. Moreover, intermediates were identified by LC-MS, and five dominating reaction pathways were predicted, involving single hydroxylation, dioxygen transfer, bond breaking, polymerization and carboxylation. Theoretical calculations showed the dioxygen transfer could occur by Fe(VI) attacking the CC double-bond in benzene ring of BP-1 to form a five-membered ring intermediate, which was hydrolyzed twice followed by H-abstraction to generate the dihydroxy-added product directly from the parent compound. Dissolved CO₂ or HCO₃^- might be fixed to produce carboxylated products, and Cl^- led to the formation of two chlorinated products. In addition, the toxicity assessments showed the reaction reduced the environmental risk of BP-1. This work illustrates Fe(VI) could remove BP-1 in water environments efficiently, and the newly proposed dioxygen transfer mechanism herein may contribute to the development of Fe(VI) chemistry.

Ferrate(VI) oxidation of bisphenol E-Kinetics, removal performance, and dihydroxylation mechanism

Bisphenol E (bis (4-hydroxyphenyl) ethane, BPE), as a typical endocrine disrupting chemical, is commonly detected in source water and drinking water, which poses potential risks to human health and ecological environment. This paper investigated the removal of BPE by ferrate(VI) (Fe^VIO₄^2-, Fe(VI)) in water. Under the optimal condition of [Fe(VI)]₀:[BPE]₀ = 10:1 and pH = 8.0, a removal efficiency of 99% was achived in 180 s. Sixteen intermediates of BPE were detected, and four possible reaction pathways were proposed, which mainly involved the reaction modes of double-oxygen and single-oxygen transfer, bond breaking, carboxylation and polymerization. The double-oxygen transfer mechanism, different from traditional mechanisms, was newly proposed to illustrate the direct generation of di-hydroxylated products from parent BPE, which was demonstrated by theoretical calculations for its rationality. Significantly, NO₂^-, HCO₃^-, Cu²⁺, and humic acid, constituents of water promoted the removal of BPE. Additionally, samples from river, tap water, synthetic wastewater, and secondary effluent were tested to explore the feasibility of Fe(VI) oxidation for treating BPE in water. It was found that 99% of BPE was degraded within 300 s in these waters except for synthetic wastewater. The toxicity of BPE and its intermediates was evaluated by ECOSAR program, and the results showed that Fe(VI) oxidation decreased the toxicity of reaction solutions. These findings demonstrated that the Fe(VI) oxidation process was an efficient and green method for the treatment of BPE, and the new insights into the double-oxygen transfer mechanism aid to understand the reaction mechanisms of organic pollutants oxidized by Fe(VI).

Ferrate (VI)-mediated transformation of diethyl phthalate (DEP) in soil: Kinetics, degradation mechanisms and theoretical calculation

Diethyl phthalate (DEP), as a kind of universally used plasticizer, has aroused considerable public concern owing to its wide detection, environmental stability, and potential health risks. In this work, the highly efficient removal of DEP by ferrate (VI) (Fe(VI)) was systematically explored in soil environment. The effects of the oxidant dosages, soil types, as well as the presence of coexisting cations and anions in tested soil on DEP removal were evaluated. When the dosage of Fe(VI) was 20 mM, complete removal of DEP (50 μg/g) was achieved in the tested soil after 2 min of reaction. Furthermore, the removal rate of DEP was closely related to the soil types, and the degradation rates were decreased obviously in red soil (RS), black soil (BS) and paddy soil (PS), probably due to the acidic condition and high content of organic matters. Moreover, the presence of Ca²⁺, Mg²⁺ and Al³⁺ in soil can inhibit the removal of DEP by Fe(VI), while SO₄^2- has an slightly promotion effect. Six oxidation intermediates were detected in the reaction process of DEP, product analysis revealed that the transformation of DEP was mainly through two pathways, including hydrolysis and hydroxylation reactions, which were probably mediated by oxygen atom transfer process of Fe(VI). Based on the frontier electron density theory calculation, two ester groups of DEP were prone to be attacked by Fe(VI), and the hydroxyl addition tended to occur at the para-position of one of the ester groups on the benzene ring. This study provides a novel approach for phthalate esters removal from soil using Fe(VI) oxidation and shows new insights into the oxidation mechanisms.