Enhanced ferrate(VI) oxidation of organic pollutants through direct electron transfer.

Fe(VI) is a versatile agent for water purification, and various strategies have been developed to improve its pollutant removal efficiency. Herein, it was found that in addition to intermediate iron species [Fe(IV)/Fe(V)], direct electron transfer (DET) played a significant role in the abatement of organic pollutants in Fe(VI)/carbon quantum dots (CQDs) system. Around 86, 83, 73, 64, 52, 45 and 17% of BPA, DCF, SMX, 4-CP, phenol, p-HBA, and IBP (6 µM) could be oxidized by 30 µM of Fe(VI), whereas with the addition of CQDs (4 mg/L), the oxidation ratio of these pollutants increased to 98, 99, 80, 88, 87, 66 and 57%, respectively. The negative impact induced by solution pH and background constituents on Fe(VI) abatement of pollutants could be alleviated by CQDs, and CQDs acted as catalysts for mediating DET from organic pollutants to Fe(VI). Theoretical calculation revealed that iron species [Fe(VI)/Fe(V)/Fe(IV)] was responsible for the oxidation of 36% of phenol, while DET contributed to the oxidation of 64% of phenol in the Fe(VI)/CQDs system. Compared with iron species oxidation, the CQDs mediated DET from pollutants to Fe(VI) was more efficient for utilizing the oxidation capacity of Fe(VI). The DET mechanism presented in the study provides a prospective strategy for improving the pollution control potential of Fe(VI).

Highly Efficient Utilization of Ferrate(VI) Oxidation Capacity Initiated by Mn(II) for Contaminant Oxidation: Role of Manganese Species.

Manganese ion [Mn(II)] is a background constituent existing in natural waters. Herein, it was found that only 59% of bisphenol A (BPA), 47% of bisphenol F (BPF), 65% of acetaminophen (AAP), and 49% of 4-tert-butylphenol (4-tBP) were oxidized by 20 µM of Fe(VI), while 97% of BPA, 95% of BPF, 96% of AAP, and 94% of 4-tBP could be oxidized by the Fe(VI)/Mn(II) system [20 µM Fe(VI)/20 µM Mn(II)] at pH 7.0. Further investigations showed that bisphenol S (BPS) was highly reactive with reactive iron species (RFeS) but was sluggish with reactive manganese species (RMnS). By using BPS and methyl phenyl sulfoxide (PMSO) as the probe compounds, it was found that reactive iron species contributed primarily for BPA oxidation at low Mn(II)/Fe(VI) molar ratios (below 0.1), while reactive manganese species [Mn(VII)/Mn(III)] contributed increasingly for BPA oxidation with the elevation of the Mn(II)/Fe(VI) molar ratio (from 0.1 to 3.0). In the interaction of Mn(II) and Fe(VI), the transfer of oxidation capacity from Fe(VI) to Mn(III), including the formation of Mn(VII) and the inhibition of Fe(VI) self-decay, improved the amount of electron equivalents per Fe(VI) for BPA oxidation. UV-vis spectra and dominant transformation product analysis further revealed the evolution of iron and manganese species at different Mn(II)/Fe(VI) molar ratios.

Interpreting the degradation mechanism of triclosan in microbial fuel cell by combining analysis microbiome community and degradation pathway.

Microbes play a dominant role for the transformation of organic contaminants in the environment, while a significant gap exists in understanding the degradation mechanism and the function of different species. Herein, the possible bio-degradation of triclosan in microbial fuel cell was explored, with the investigation of degradation kinetics, microbial community, and possible degradation products. 5 mg/L of triclosan could be degraded within 3 days, and an intermediate degradation product (2,4-dichlorophen) could be further degraded in system. 32 kinds of dominant bacteria (relative intensity >0.5%) were identified in the biofilm, and 10 possible degradation products were identified. By analyzing the possible involved bioreactions (including decarboxylation, dehalogenation, dioxygenation, hydrolysis, hydroxylation, and ring-cleavage) of the dominant bacteria and possible degradation pathway of triclosan based on the identified products, biodegradation mechanism and function of the bacteria involved in the degradation of triclosan was clarified simultaneously. This study provides useful information for further interpreting the degradation mechanism of organic pollutants in mixed flora by combining analysis microbiome community and degradation pathway.

Role of iron(II) sulfide in autotrophic denitrification under tetracycline stress: Substrate and detoxification effect.

Autotrophic denitrification using inorganic compounds as electron donors has gained increasing attention in the field of wastewater treatment due to its numerous advantages, such as no need for exogenous organic carbon, low energy input, and low sludge production. Tetracycline (TC), a refractory contaminant, is often found coexisting with nutrients (NO3- and PO43-) in wastewater, which can negatively affect the biological nutrient removal process because of its biological toxicity. However, the performance of autotrophic denitrification under TC stress has rarely been reported. In this study, the effects of TC on autotrophic denitrification with thiosulfate (Na2S2O3) and iron (II) sulfide (FeS) as the electron donors were investigated. With Na2S2O3 as the electron donor, TC slowed down the nitrate removal rate, which decreased from 1.32 to 0.18 d-1, when TC concentration increased from 0 mg/L to 50 mg/L. When TC concentration was higher than 2 mg/L, nitrite reduction was seriously inhibited, leading to nitrite accumulation. With FeS as the electron donor, nitrate removal was much more efficient under TC-stressed conditions, and no distinct nitrite accumulation was observed when the initial TC concentration was as high as 10 mg/L, indicating the effective detoxification of FeS. The detoxification effects in the FeS autotrophic denitrification system mainly resulted from the rapid adsorption of TC by FeS and effective degradation of TC, as proven by a relatively higher living biomass area. This study offers new insights into the response of sulfur-based autotrophic denitrifiers to TC stress and demonstrates that the FeS-based autotrophic denitrification process is a promising technology for the treatment of wastewater containing emerging contaminants and nutrients.

Ferrate self-decomposition in water is also a self-activation process: Role of Fe(V) species and enhancement with Fe(III) in methyl phenyl sulfoxide oxidation by excess ferrate.

To reveal the role of ferrate self-decomposition and the fates of intermediate iron species [Fe(V)/Fe(IV) species] during ferrate oxidation, the reaction between ferrate and methyl phenyl sulfoxide (PMSO) at pH 7.0 was investigated as a model system in this study. Interestingly, the apparent second-order rate constants (kapp) between ferrate and PMSO was found to increase with ferrate dosage in the condition of excess ferrate in borate buffer. This ferrate dosage effect was diminished greatly in the condition of excess PMSO where ferrate self-decomposition was lessened largely, or counterbalanced by adding a strong complexing ligand (e.g. pyrophosphate) to sequester Fe(V) oxidation, demonstrating that the Fe(V) species derived from ferrate self-decomposition plays an important role in PMSO oxidation. A mechanistic kinetics model involving the ferrate self-decomposition and PMSO oxidation by Fe(VI), Fe(V) and Fe(IV) species was then developed and validated. The modeling results show that up to 99% of the PMSO oxidation was contributed by the ferrate self-decomposition resultant Fe(V) species in borate buffer, revealing that ferrate self-decomposition is also a self-activation process. The direct Fe(VI) oxidation of PMSO was impervious to presence of phosphate or Fe(III), while the Fe(V) oxidation pathway was strongly inhibited by phosphate complexation or enhanced with Fe(III). Similar ferrate dosage effect and its counterbalance by pyrophosphate as well as the Fe(III) enhancement were also observed in ferrate oxidation of micropollutants like carbamazepine, diclofenac and sulfamethoxazole, implying the general role of Fe(V) and promising Fe(III) enhancement during ferrate oxidation of micropollutants.

Highly efficient removal of p-arsanilic acid with Fe(II)/peroxydisulfate under near-neutral conditions.

As a common animal feed additive, p-arsanilic acid (p-AsA) is thought to be excreted with little uptake and unchanged chemical structure, threatening the environment by potentially releasing more toxic inorganic arsenic. We herein investigated the removal of arsenic by in situ formed ferric (oxyhydr)oxides with the promotion of p-AsA degradation in Fe(II)/peroxydisulfate (PDS) system. Results showed that under acid conditions, p-AsA degraded very quickly and over 99% of p-AsA (5 µM) was degraded within 10 min at the optimal dosage of Fe(II) (100 µM) and PDS (150 µM) at pH 3, while less than 66.4% of arsenic was removed at pH 3-5. Higher pH (3-7) would inhibit the degradation of p-AsA but promote the arsenic removal. At pH 6-7, over 98.5% of total arsenic was removed, while the degradation efficiency of p-AsA was lower than 52.4%. HPLC-ICP-MS results indicated that the arsenic group was cleaved from p-AsA in the form of As(III) and then rapidly oxidized to As(V). FTIR and XPS analysis indicated that both As(V) products and residual p-AsA were bonded to ferric (oxyhydr)oxides via hydroxyl groups. Common cations (e.g., Na+, Ca2+, Mg2+) and anions such as Cl-, SO42-, CO32- had no significant influence on arsenic removal, while SiO32-, PO43- and HA inhibited the removal of total arsenic, mainly by affecting the zeta potential of iron particles. In summary, the Fe(II)/PDS process, as an efficient method for partial oxidation and simultaneous adsorption of p-AsA under near-neutral conditions, is expected to control the potential environmental risks of p-AsA.

Enhanced Permanganate Oxidation of Sulfamethoxazole and Removal of Dissolved Organics with Biochar: Formation of Highly Oxidative Manganese Intermediate Species and in Situ Activation of Biochar.

Sulfamethoxazole (SMX) is a broad-spectrum antibiotic and was largely used in breeding industry. The reaction rate of SMX with KMnO4 is slow, and the adsorption efficiency of biochar for SMX was inferior (less than 11% in 30 min). By adding biochar powder into SMX solution with the addition of permanganate, the oxidation ratio of SMX surged to 97% in 30 min, and over 58% of the total organic carbon (TOC) was simultaneously removed. KMnO4 interacted with biochar and resulted in the formation of highly oxidative intermediate manganese species, which transformed SMX into hydrolysis products, oxygen-transfer products, and self-coupling products. Brunauer-Emmett-Teller (BET) analysis showed that surface area, total pore volume, and micropore volume of biochar increased by 32.1%, 36.4%, and 80.6%, respectively, after reaction process. This in situ activation of biochar with KMnO4 enhanced its adsorption capacity and led to great improvement of TOC removal. Besides KMnO4 oxidation, biochar also enhanced TOC removal in Mn(III) oxidation (KMnO4+ bisulfite) and ozonization of SMX. Considering that KMnO4 could react with biochar and result in the formation of intermediate manganese species, while biochar can be simultaneously activated and exhibit high capacity for organic adsorption, the combination of biochar with the chemical/advanced oxidation could be a promising process for the removal of environmental pollutants.

Comparative study on ferrate oxidation of BPS and BPAF: Kinetics, reaction mechanism, and the improvement on their biodegradability.

Bisphenol S (BPS) and bisphenol AF (BPAF) were increasingly consumed and these compounds are resistant to environmental degradation. Herein, ferrate oxidation of BPS and BPAF was investigated, and biodegradability of the oxidation products was examined. The second-order reaction rate constants of ferrate with BPS and BPAF were 1.3 × 103 M-1s-1 and 3 × 102 M-1s-1, respectively, at pH 7.0, 25 °C. In the oxidation process, some BPS molecules dimerized, while other BPS molecules were oxidized through oxygen-transfer process, leading to the formation of hydroxylation products and benzene-ring cleavage products. The dominant reaction of BPAF with ferrate was oxygen-transfer process, and BPAF was degraded into lower molecular weight products. The variation of assimilable organic carbon (AOC) suggested that the biodegradability of BPAF and BPS was largely improved after ferrate oxidation. Compared with the BPS oxidation products, the BPAF oxidation products were easier to be bio-consumed. Pure culture test showed that BPAF inhibited the growth of Escherichia coli, while ferrate oxidation completely eliminated this toxic effect. Co-existing humic acid (HA, 1 mg C/L to 5 mg C/L) decreased the removal of BPS and BPAF with ferrate. Compared with BPAF, more oxidation intermediates formed in the ferrate oxidation of BPS may be reduced by HA to the parent molecular. Thus, the inhibition effect of HA on the ferrate oxidation of BPS was more obvious than that on BPAF.

Impact of Phosphate on Ferrate Oxidation of Organic Compounds: An Underestimated Oxidant.

Ferrate (K2FeO4) is a powerful oxidant and up to 3 mol of electrons could be captured by 1 mol of ferrate in the theoretical conversion of Fe(VI)-Fe(V)-Fe(IV)-Fe(III). However, it is reported that the utilization efficiency of the ferrate oxidation capacity is quite low because of the rapid autodecomposition of intermediate iron species, which negatively influences the potential of ferrate on organic pollutants control. We accidentally found that for the ferrate oxidation of carbamazepine (CBZ), bisphenol S (BPS), diclofenac (DCF), and ciprofloxacin (CIP), the determined reaction rate constants were 1.7-2.4 times lower in phosphate buffer than those in borate buffer at pH 8.0. For the reaction of ferrate with 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) at pH 7.0, the determined reaction stoichiometries were 1:1.04 in 100 mM phosphate buffer, 1:1.18 in 10 mM phosphate buffer, and 1:1.93 in 10 mM borate buffer, respectively. The oxidation ability of ferrate seems depressed in phosphate buffer. A kinetic model involving the oxidation of ABTS by Fe(VI), Fe(V) and Fe(IV) species was developed and fitted the ABTS•+ formation kinetics well under different buffer conditions. The results showed that phosphate exhibited little influence on the oxidation ability of Fe(VI) and Fe(IV) species, but decreased the specific rate constants of ABTS with Fe(V) species by 1-2 orders of magnitude, resulting in the outcompeting of Fe(V) autodecomposition pathway. The complexation between phosphate anions and Fe(V) species may account for the inhibition effect of phosphate buffer. Considering that many studies regarding ferrate oxidation were carried out in phosphate buffer, the actual oxidation ability of ferrate may be underestimated.

Highly effective oxidation of roxarsone by ferrate and simultaneous arsenic removal with in situ formed ferric nanoparticles.

Roxarsone (ROX) is used in breeding industry to prevent infection by parasites, stimulate livestock growth and improve pigmentation of livestock meat. After being released into environment, ROX could be bio-degraded with the formation of carcinogenic inorganic arsenic (As) species. Here, ferrate oxidation of ROX was reported, in which we studied total-As removal, determined reaction kinetics, identified oxidation products, and proposed a reaction mechanism. It was found that the apparent second-order rate constant (kapp) of ferrate with ROX was 305 M-1s-1 at pH 7.0, 25 °C, and over 95% of total As was removed within 10 min when ferrate/ROX molar ratio was 20:1. Species-specific rate constants analysis showed that HFeO4- was the dominant species reacting with ROX. Ferrate initially attacked AsC bond of ROX and resulted in the formation of arsenate and 2-nitrohydroquinone. The arsenate was simultaneously removed by ferric nanoparticles formed in the reduction of ferrate, while 2-nitrohydroquinone was further oxidized into nitro-1,4-benzoquinone. These results suggest that ferrate treatment can be an effective method for the control of ROX in water treatment.

Removal of Organoarsenic with Ferrate and Ferrate Resultant Nanoparticles: Oxidation and Adsorption.

Many investigations focused on the capacity of ferrate for the oxidation of organic pollutant or adsorption of hazardous species, while little attention has been paid on the effect of ferrate resultant nanoparticles for the removal of organics. Removing organics could improve microbiological stability of treated water and control the formation of disinfection byproducts in following treatment procedures. Herein, we studied ferrate oxidation of p-arsanilic acid ( p-ASA), an extensively used organoarsenic feed additive. p-ASA was oxidized into As(V), p-aminophenol ( p-AP), and nitarsone in the reaction process. The released As(V) could be eliminated by in situ formed ferric (oxyhydr) oxides through surface adsorption, while p-AP can be further oxidized into 4,4'-(diazene-1,2-diyl) diphenol, p-nitrophenol, and NO3-. Nitarsone is resistant to ferrate oxidation, but mostly adsorbed (>85%) by ferrate resultant ferric (oxyhydr) oxides. Ferrate oxidation (ferrate/ p-ASA = 20:1) eliminated 18% of total organic carbon (TOC), while ferrate resultant particles removed 40% of TOC in the system. TOC removal efficiency is 1.6 to 38 times higher in ferrate treatment group than those in O3, HClO, and permanganate treatment groups. Besides ferrate oxidation, adsorption of organic pollutants with ferrate resultant nanoparticles could also be an effective method for water treatment and environmental remediation.

Interpreting the effects of natural organic matter on antimicrobial activity of Ag2S nanoparticles with soft particle theory.

Natural organic matter (NOM) ubiquitously exists in natural waters and would adsorb onto the particle surface. Previous studies showed that NOM would alleviate the toxicity of nanomaterials, while the mechanism is seldom quantitatively interpreted. Herein, the effects of humic substances [Suwannee River fulvic acid (SRFA) and Suwannee River humic acid (SRHA)] and biomacromolecules [alginate and bovine serum albumin (BSA)] on the aggregation and antimicrobial effects of silver sulfide nanoparticles (Ag2S-NPs) were investigated. The aggregation kinetics of Ag2S-NPs in electrolyte solutions were in agreement with the results based on Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. The dynamic light scattering (DLS) results showed that the SRFA, SRHA, alginate and BSA molecules coated on the Ag2S-NPs surfaces. The NOM coating layer prevented salt-induced coagulation of Ag2S-NPs, and the effects of BSA and SRHA on Ag2S-NPs stabilizing were more obvious than that of SRFA and alginate. Flow cytometry analysis results suggested that BSA and SRHA were more effective on alleviating the Ag2S-NPs induced cell (Escherichia coli) membrane damage than SRFA and alginate. After interpreting the electrophoretic mobility (EPM) data of the NOM coated Ag2S-NPs by Ohshima's soft particle theory, it was found that the thickness of the NOM coating layers followed the orders of BSA > SRHA > alginate > SRFA. The E.coli cell membrane damage level was negatively correlated with the thickness and softness of the coating layer. NOM coating may physically alleviate the contact between NPs and E. coli cells and thus attenuate the extent of cell membrane damage caused by the NP-cell interaction. This work provides a new perspective for quantitatively interpreting the influence of NOM on the environmental behaviors and risks of nanomaterials.

Rapid oxidation of iodide and hypoiodous acid with ferrate and no formation of iodoform and monoiodoacetic acid in the ferrate/I-/HA system.

Toxic and odorous iodinated disinfection byproducts (I-DBPs) could form in the chemical oxidation of iodine-containing water. A critical step for controlling the hazardous I-DBPs is to convert the iodine species into stable and harmless iodate (IO3-) while inhibiting the accumulation of highly reactive hypoiodous acid (HOI). Herein, the oxidation of I- and HOI with ferrate was investigated, and the formation profile of HOI was determined based on 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) coloring method through a stopped-flow spectrophotometer. The second-order rate constants (kapp) of ferrate with HOI decreased from 1.6 × 105 M-1s-1 to 8.3 × 102 M-1s-1 as the solution pH varied from 5.3 to 10.3, which were 7.5, 7.2 and 13.8 times higher than that of ferrate with I- at pH 6.0, 7.0 and 8.0, respectively. Compared with other oxidants such as ozone, hypochlorous acid, chloramine and potassium permanganate, ferrate would swiftly oxidize HOI formed in the I- oxidation process. For the ferrate oxidation of I-containing water, HOI was swiftly oxidized to IO3- from pH 5.0 to 9.0. Phosphate buffer promoted the oxidation of I- while inhibited the oxidation of HOI with ferrate. When 5 mgC/L of humic acids (HA) existed in the solution, no formation of iodoform and monoiodoacetic acid (MIAA) was observed in the oxidation of iodide (20 µM) with ferrate (from 10 µM to 80 µM). These results suggested that ferrate oxidation could be an effective method for the control of I-DBPs in iodine-containing water treatment.

Chlorination of bisphenol S: Kinetics, products, and effect of humic acid.

Bisphenol S (BPS), as a main alternative of bisphenol A for the production of industrial and consumer products, is now frequently detected in aquatic environments. In this work, it was found that free chlorine could effectively degrade BPS over a wide pH range from 5 to 10 with apparent second-order rate constants of 7.6-435.3 M-1s-1. A total of eleven products including chlorinated BPS (i.e., mono/di/tri/tetrachloro-BPS), 4-hydroxybenzenesulfonic acid (BSA), chlorinated BSA (mono/dichloro-BSA), 4-chlorophenol (4CP), and two polymeric products were detected by high performance liquid chromatography and electrospray ionization-tandem quadrupole time-of-flight mass spectrometry. Two parallel transformation pathways were tentatively proposed: (i) BPS was attacked by stepwise chlorine electrophilic substitution with the formation of chlorinated BPS. (ii) BPS was oxidized by chlorine via electron transfer leading to the formation of BSA, 4CP and polymeric products. Humic acid (HA) significantly suppressed the degradation rates of BPS even taking chlorine consumption into account, while negligibly affected the products species. The inhibitory effect of HA was reasonably explained by a two-channel kinetic model. It was proposed that HA negligibly influenced pathway i while appreciably inhibited the degradation of BPS through pathway ii, where HA reversed BPS phenoxyl radical (formed via pathway ii) back to parent BPS.

Highly efficient removal of trace thallium from contaminated source waters with ferrate: Role of in situ formed ferric nanoparticle.

Thallium (Tl) is highly toxic to mammals and relevant pollution cases are increasing world-widely. Convenient and efficient method for the removal of trace Tl from contaminated source water is imperative. Here, the removal of trace Tl by K2FeO4 [Fe(VI)] was investigated for the first time, with the exploration of reaction mechanisms. Six different types of water treatment agents (powdered activated carbon, Al2(SO4)3, FeCl3, δ-MnO2, MnO2 nano-particles, and K2FeO4) were used for the removal of Tl in spiked river water, and K2FeO4 showed excellent removal performance. Over 92% of Tl (1 µg/L) was removed within 5 min by applying 2.5 mg/L of K2FeO4 (pH 7.0, 20 °C). XPS analysis revealed that in the reaction of Tl(I) with K2FeO4, Tl(I) was oxidized to Tl(III), and removed by the K2FeO4 reduced ferric particles. The removal of Tl by in situ formed and ex situ formed ferric particle was examined respectively, and the results revealed that the removal of trace Tl could be attributed to the combination of adsorption and coprecipitation processes. The hydrodynamic size of the reduced particle from K2FeO4 ranged from 10 nm to 100 nm, and its surface was negatively charged under neutral pH condition. These factors were conducive for the efficient removal of Tl by K2FeO4. The effects of solution pH, coexisting ions (Na+, Ca2+, and HCO3-), humic acid, solution temperature, and reductive environment on the removal and desorption of Tl were investigated, and the elimination of Tl in polluted river water and reservoir water was performed. These results suggest that K2FeO4 could be an efficient and convenient agent on trace Tl removal.