Degradability of chlorophenols using ferrate(VI) in contaminated groundwater

Improvement of Fe(VI) oxidation by NaClO on degrading phenolic substances and reducing DBPs formation potential

Ferrate(VI) is a green oxidant and can effectively oxidize micropollutants. However, the instability of Fe(VI), i.e., self-decomposition, in the aqueous solution limited its application. Herein, it was found that the degradation of phenolic substances had been substantially improved through the combination of Fe(VI) with NaClO. At the condition of pH 8.0, 50 μM of Fe(VI) degraded 18.66 % of BPA (bisphenol A) at 0.5 min or 21.67 % of phenol at 2 min. By contrast, Fe(VI)/NaClO (50/10 μM) oxidized 38.21 % of BPA at 0.5 min or 38.08 % of phenol at 2 min with a synergistic effect. At the end of the reaction, the concentration of Fe(VI) in Fe(VI)/NaClO (50/10 μM) was 28.97 μM for BPA degradation, higher than the 25.62 μM of Fe(VI) group. By active species analysis, intermediate iron species [i.e., Fe(V) and Fe(IV)] played a vital role in the synergistic effect in Fe(VI)/NaClO system, which would react with the applied NaClO to regenerate Fe(VI). In natural water, the Fe(VI)/NaClO could also degrade phenolic substances of natural organic matter (NOM). Although the NaClO reagent was applied, disinfection by-products (DBPs) formation potential decreased by 22.75 % of the raw sample after Fe(VI)/NaClO treatment. Significantly, THMs, mainly caused by phenolic substances of NOM, even declined by 29.18 % of raw sample. Based on that, this study explored a novel ferrate(VI) oxidation system using the cheap NaClO reagent, which would present a new insight on ferrate(VI) application.

Carbonate and bicarbonate ions impacts on the reactivity of ferrate(VI) for 3,4-dichlorophenol removal

Carbonate and bicarbonate ions are common constituents found in wastewater and natural water matrices, and their impacts on the reactivity of ferrate(VI) (Fe(VI)) with 3,4-dichlorophenol (3,4-DCP) were investigated by determining second-order rate constants of 3,4-DCP removal by Fe(VI) in the presence of CO₃^2- and/or HCO₃^-. The second-order rate constants decreased from 41.75 to 7.04 M^-1 s^-1 with an increase of [CO₃^2-] from 0 to 2.0 mM, indicating that CO₃^2- exhibits an inhibitory effect on 3,4-DCP removal kinetics, and experiments on pH effect, radical quenching, and Fe(VI) stability were conducted to explore possible reasons for its effect. Under identical pH conditions, the rate constant in NaOH medium was always higher than in Na₂CO₃ medium, suggesting that the inhibitory effect partially comes from an increase in alkalinity. Furthermore, the scavenging of hydroxyl radical by carbonate ion also contributed to the inhibitory effect of CO₃^2-. On the other hand, the enhancement effect of CO₃^2- depending on the increase in Fe(VI) stability was found, but did not exceed its inhibitory effect. In addition, 3,4-DCP removal kinetics was not affected by HCO₃^-, while synergistically inhibited by CO₃^2-/HCO₃^-. Moreover, 3,4-DCP removal efficiency was substantially suppressed in the presence of CO₃^2-, while the slight enhancement effect of HCO₃^- and the synergistic inhibitory effect of CO₃^2-/HCO₃^- were observed. The experimental results clearly demonstrated that carbonate and bicarbonate ions play an important role in the process of 3,4-DCP removal by Fe(VI) and should not be considered only as scavengers.

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.

Response of 2,4,6-trichlorophenol-reducing biocathode to burial depth in constructed wetland sediments

Biocathode systems could be used for in-situ bioremediation of chlorophenols (CPs) in constructed wetland (CW) sediments. However, little is known regarding whether or how cathode burial depths affect the dechlorination of CPs in sediments. Here, 2,4,6-trichlorophenol (2,4,6-TCP)-dechlorinating biocathode systems were constructed under a cathode potential of - 0.7 V (vs. a saturated calomel electrode, SCE) at three different cathode burial depths (5, 10, and 15 cm). The 2,4,6-TCP removal efficiency and average transformation rate with the biocathode increased by 21.46-36.86% and 14.63-34.88% compared to those in the non-electrode groups. Deeper cathode burial depths enhanced the 2,4,6-TCP dechlorination performance. Furthermore, the oxidation-reduction potential (ORP) of the sediment decreased with sediment depth and the applied potential created a more favorable redox environment for the enrichment of functional bacteria. Deeper cathode burial depths also promoted the selective enrichment of electro-active and dechlorinating bacteria (e.g., Bacillus and Dehalobacter, respectively). The biocathode thus served as the carrier, electron source, and regulator of functional bacteria to accelerate the transformation of 2,4,6-TCP (2,4,6-TCP → 2,4-dichlorophenol → 4-chlorophenol → phenol) in sediments. These results offer insights into the effects of cathode burial depth on 2,4,6-TCP dechlorination in sediments from a redox environment and microbial community structure standpoint.

Kinetics and mechanisms of phenolic compounds by Ferrate(VI) assisted with density functional theory

In this paper, Phenol, 4-Chlorophenol (4-CP), 2,4-Dichlorophenol (2,4-DCP) and 2,4,6-Trichlorophenol (2,4,6-TCP) were selected as model pollutants to explore the oxidant mechanism by ferrate (Fe(VI)). The reactions between ferrate (1000 μM) and four phenolic compounds (100 μM) were conformed to the second-order reaction kinetics at pH 9.2, and the order of k_obs followed as: k_4-CP (129 M^-1 s^-1) > k_2,4-DCP (96 M^-1 s^-1) > k_2,4,6-TCP (44 M^-1 s^-1) > k_Phenol (12 M^-1 s^-1). Meanwhile, the degradation rates of all four compounds by Fe(VI) increased with increased pH (3.1-9.2). A total of 14 degradation products were identified by Liquid chromatography-Time-of-Flight-Mass Spectrometry (LC-TOF-MS), and two pathways including hydroxylation of benzene ring and substitution of chlorine atom were proposed. Hydroxyl radicals, played a vital role during the degradation of phenolic compounds. Moreover, density functional theory calculations were used to explore the degradation mechanisms. The results showed that the hydroxyl radical was more favorable to substitute chlorine atom than hydrogen atom, and the substitution on ortho-position was more favorable than para-position for all four compounds. The findings of this study could greatly improve our understanding on the degradation mechanism of chlorophenol-like compounds by Fe(VI) for environmental remediation.

Ferrate(VI) oxidation of pentachlorophenol in water and soil

Although the use of ferrate (VI), an emerging green oxidant, has been widely investigated to remove organic pollutants in water, its ability to remediate contaminated soils has been scarcely evaluated. Here, we explore the use of ferrate (VI) to degrade a polychlorinated persistent compound, the pentachlorophenol (PCP), in aqueous solution and in an aged contaminated soil under batch, water-saturated and water-unsaturated flow conditions. The first results showed the prominent efficiency of ferrate (VI) over conventional oxidants (e.g. H₂O₂ and persulfate) in both matrices and at different oxidant doses. In aqueous solution, more than 80% of PCP was degraded by ferrate (VI) while complete removal was observed in soil under batch conditions. In column experiments, PCP removal by ferrate (VI) remained efficient but dependent on the flow rate and water saturation. Maximum PCP removal (95%) in columns was observed under water saturated conditions when ferrate (VI) (0.2 g g^-1 of soil) was injected at a low flow rate (i.e. 0.025 mL min^-1). This study has strong implications in the development of new sustainable processes based on ferrate (VI) for the remediation of different environmental compartments.

A study of the reaction of ferrate with pentachlorophenol - kinetics and degradation products

Pentachlorophenol (PCP) is a persistent pollutant which has been widely used as a pesticide and a wood preservative. As PCP is toxic and is present in significant quantities in the environment, there is considerable interest in elimination of PCP from waters. One of the promising methods is the application of ferrate. Ferrate is an oxidant and coagulant. It can be applied as a multi-purpose chemical for water and wastewater treatment as it degrades a wide range of environmental pollutants. Moreover, ferrate is considered a green oxidant and disinfectant. This study focuses on the kinetics of PCP degradation by ferrate under different pH conditions. The formation of degradation products is also considered. The second-order rate constants of the PCP reaction with ferrate increased from 23 M^-1 s^-1 to 4,948 M^-1 s^-1 with a decrease in pH from 9 to 6. At neutral pH the degradation was fast, indicating that ferrate could be used for rapid removal of PCP. The total degradation of PCP was confirmed by comparing the initial PCP molarity with the molarity of chloride ions released. We conclude no harmful products are formed during ferrate treatment as all PCP chlorine was released as chloride. Specifically, no polychlorinated dibenzo-p-dioxins and dibenzofurans were detected.