OH Yield Research Articles

Acceleration of goethite-catalyzed Fenton-like oxidation of ofloxacin by biochar

While carbon materials have been well studied to stimulate the homogeneous Fenton-like processes, little was known about their impacts on iron mineral-catalyzed heterogeneous Fenton-like reactions. Here, it was found that biochar prepared at 300 °C or 600 °C (BC300 or BC600) greatly stimulated the degradation of ofloxacin (OFX) in a goethite (Gt)-mediated Fenton-like system. In 4 h, while only 38.4 % and 48.4 % OFX were removed in Gt/H2O2 and BC600/H2O2 systems, the removal efficiency reached over 94.0 % in Gt/BC600/H2O2 system. And the pseudo-first-order rate constant of Gt/H2O2, BC600/H2O2 and Gt/BC600/H2O2 systems were 0.12, 0.16 and 0.72 h−1, respectively, indicating the occurrence of synergistically catalytic degradation. •OH was identified as the major oxidant. Both the •OH yield and the H2O2 utilization efficiency of Gt/BC600/H2O2 system were higher than those of Gt/H2O2 and BC600/H2O2 systems. BC600 showed better stimulation effects than BC300. The persistent free radicals (PFRs) of BC could activate H2O2 and partly contribute to •OH production in the Gt/BC/H2O2 system. While BC could not directly reduce Fe(III) in Gt, it improved the cycling of Fe(III)/Fe(II) through complexing Fe(III) with its carboxyl group. Potential pathways were proposed for OFX degradation in the Gt/BC/H2O2 system.

Superfast degradation of refractory organic contaminants by ozone activated with thiosulfate: Efficiency and mechanisms

Thiosulfate (S2O32-) is frequently used as an ozone (O3) quenching agent when investigating the ozonation of organic contaminants and the kinetics thereof. In this study, however, O3 is activated by S2O32-, resulting in a superfast degradation of O3-refractory contaminants. Therefore, the focus of this study is the exploration into the enhancing role of S2O32- in the degradation of refractory organic contaminants by O3, which has been overlooked thus far. Results obtained from scavenging experiments and electron paramagnetic resonance (EPR) spectra verify that •OH generated from the reaction of S2O32- with O3 is mainly responsible for the superfast degradation of O3-refractory contaminants. The •OH yield from the O3/S2O32- process is determined to be 0.216. A plausible mechanism for the generation of •OH from the O3/S2O32- process is proposed with the implementation of density functional theory (DFT). Initially, ozone reacts with a sulfur of S2O32- to form OOOSSO32-. The adduct then rearranges to OO(O)SSO32- or HOO(O)SSO32- in the presence of H+, which cleaves to give a sulfoxide radical cation and O2•-/HO2•. O2•-/HO2• is rapidly transformed into •OH by O3 through a series of steps. Degradation efficiency of O3-refractory contaminants of this process highly depends on the molar ratio of S2O32- and O3 ([S2O32-]:[O3]). The optimal [S2O32-]:[O3] is pH dependent in synthetic water (e.g. 0.3 at pH 7). The presence of bicarbonate inhibits the degradation of refractory contaminants by the O3/S2O32- process. Humic acid exhibits a slight enhancing effect at low concentrations (0.1–0.2 mg-C/L), and an inhibiting effect at higher concentrations (≥0.4 mg-C/L). In addition, the efficacy of the O3/S2O32- process in real water matrices is also confirmed.

Magnetite CaF2 near-infrared photocatalysts fabricated with Ca-enriched ferrites derived from electroplating wastewater

The hydroxide precipitation with CaO or Ca(OH)2 is frequently applied in the ferrite formation process of electroplating wastewater, but the excess Ca2+ ions present often weaken the quality of the resultant mixed-ferrite (M−Fe3O4) precipitates. In this work, for resource utilization of the M−Fe3O4 and the Ca2+ sources introduced from Ca(OH)2 in the hydroxide precipitate process, the upconversion assisted magnetite near-infrared (NIR) photocatalysts of M−Fe3O4/Tm3+/Yb3+-doped CaF2/TiO2 (FCT) are synthesized. The M−Fe3O4 phases possess the advantages of magnetic separation, broadband spectrum absorption, and the heterostructure with TiO2, and absorb the strong upconversion luminescence emitted from Tm3+/Yb3+-CaF2 for photocatalysis. The TiO2 phase contributes to the large improvement of upconversion luminescence and photocatalytic activity. Because of the stronger UV–Vis-NIR absorption, upconversion luminescence, and electron-hole pair separation for the FCT sample with Ti: Ca molar ratio of 3: 1 (FCT3), higher yields of OH and O2− radicals are generated, which enable FCT3 to possess much enhanced degradation rate (87.3%) of ciprofloxacin compared with those of other FCT samples under UV–Vis-NIR light irradiation. The fabrication of the upconversion magnetite NIR photocatalysts from electroplating wastewater is a promising strategy for its resource utilization and the environmental remediation.

Generation of hydroxyl radical during chlorination of hydroxyphenols and natural organic matter extracts

The generation of hydroxyl radicals (•OH) during the chlorination of air saturated solutions of different hydroxyphenols (hydroquinone, resorcinol, catechol, gallic and tannic acids) at pH 7 has been determined by the formation of phenol (in presence of benzene in excess) or 2-hydroxyterephthalic acid (in presence of terephthalic acid). Formation of •OH was only detected during the chlorination of o- or p-hydroxyphenols, compounds that react with chlorine by electron transfer forming the corresponding semiquinones/quinones. In aerated solutions, oxygen is reduced by the semiquinone to the superoxide radical, O2•−, which reacts with HOCl to •OH. Compared to the studied o-hydroxyphenols, the lower reactivity of hydroquinone towards chlorine favours the reaction between chlorine and O2•−, and its •OH formation potential is ∼50 times higher. The extent of •OH generated increased with the concentration of the hydroxyphenol and chlorine, but the •OH yield (moles formed per mole of hydroxyphenol eliminated), decreased due to the formation of the quinone, that acts as O2•− scavenger. The yield was almost not affected by the pH (6 ≤ pH ≤ 7.5), whereas a strong impact of dissolved O2 was observed. The •OH production was null in absence of O2 and 2.5–3 times higher at oxygen saturated conditions compared to air-saturated. Contrary to chlorination, during bromination of hydroquinone •OH was not formed, which can be attributable to a much faster consumption of the oxidant, with no chance for O2•− to react with bromine.Formation of •OH during the chlorination of different NOM extracts (SRHA, SRFA, PLFA and Nordic Lake NOM) and water from Lake Greifensee (Switzerland) was also studied using terephthalic acid as •OH scavenger. For SRHA, SRFA and Nordic Lake NOM (all of allochthonous origin and presenting high electron-donating capacity, EDC), •OH yields expressed as moles formed per mole of DOC0 (%), were between 1.1 and 2.0, similar to that of hydroquinone (∼1.5). For PLFA and Lake Greifensee water (autochthonous, lower EDC) much lower •OH yields were observed (0.1–0.3). Both chlorination rate and EDC, the later favouring the formation/stabilization of O2•−, seem to be key factors involved in •OH generation during the chlorination of NOM. A mechanism for these findings is proposed based on kinetic simulations of hydroquinone chlorination at pH 7.

Oxidative Degradation of Organic Contaminants by FeS in the Presence of O2.

Reductive transformation of organic contaminants by FeS in anoxic environments has been documented previously, whereas the transformation in oxic environments remains poorly understood. Here we show that phenol can be efficiently oxidized in oxic FeS suspension at circumneutral pH value. We found that hydroxyl radicals (•OH) were the predominant reactive oxidant and that a higher O2 content accelerated phenol degradation. Phenol oxidation depended on •OH production and utilization efficiency, i.e., phenol degraded per •OH produced. Low FeS contents (≤1 g/L) produced less •OH but higher utilization efficiency, while high contents produced more •OH but lower utilization efficiency. Consequently, the most favorable conditions for phenol oxidation occurred during the long-term interaction between dissolved O2 and low levels of FeS (i.e., ≤1 g/L). Mössbauer spectroscopy suggests that FeS oxidation to lepidocrocite initially produced an intermediate Fe(II) phase that could be explained by the apparent preferential oxidation of structural S(-II) relative to Fe(II), rendering a higher initial •OH yield upon unit of Fe(II) oxidation. Trichloroethylene can be also oxidized under similar conditions. Our results demonstrate that oxidative degradation of organic contaminants during the oxygenation of FeS can be a significant but currently underestimated pathway in both natural and engineered systems.

TiO2 electrocatalysis via three-electron oxygen reduction for highly efficient generation of hydroxyl radicals

The use of heterogeneous catalysts to improve the Electro-Fenton (EF) process has attracted significant attention. However, complex redox reactions and multiple mass transfer steps during the EF process result in limited current efficiency with only a low rate of generation of hydroxyl radicals (OH). Herein, we establish a three-electron oxygen reduction reaction (ORR) process coupling adsorbed H2O2 (H2O2ad) generation with an in situ EF-like reaction on TiO2 during cathodic electrolysis. Anatase TiO2 was composited with graphite, enabling the electrochemical reduction of Ti4+ to Ti3+. H2O2ad was formed during the ORR, electrocatalyzed by TiO2, then the H2O2ad was reduced in situ to OH without desorption. The generation and decomposition of Ti-OH during the electrochemical reduction were directly observed by in situ Raman spectroscopy. This result was further confirmed by density functional theory (DFT) calculations. Based on this three-electron ORR mechanism, the OH yield reached 2.69 μg cm−2 min−1 and the current efficiency approached 92.8% at a current density of −0.80 mA cm−2 within 2 h during ORR on the TiO2/graphite (TiO2/C) cathode.

Contaminant Degradation by •OH during Sediment Oxygenation: Dependence on Fe(II) Species.

It has been documented that contaminants could be degraded by hydroxyl radicals (•OH) produced upon oxygenation of Fe(II)-bearing sediments. However, the dependence of contaminant degradation on sediment characteristics, particularly Fe(II) species, remains elusive. Here we assessed the impact of the abundance of Fe(II) species in sediments on contaminant degradation by •OH during oxygenation. Three natural sediments with different Fe(II) contents and species were oxygenated. During 10 h oxygenation of 200 g/L sediment suspension, 2 mg/L phenol was negligibly degraded for sandbeach sediment (Fe(II): 9.11 mg/g), but was degraded by 41% and 52% for lakeshore (Fe(II): 9.81 mg/g) and farmland (Fe(II): 19.05 mg/g) sediments, respectively. •OH produced from Fe(II) oxygenation was the key reactive oxidant. Sequential extractions, X-ray diffraction, Mössbauer, and X-ray absorption spectroscopy suggest that surface-adsorbed Fe(II) and mineral structural Fe(II) contributed predominantly to •OH production and phenol degradation. Control experiments with specific Fe(II) species and coordination structure analysis collectively suggest the likely rule that Fe(II) oxidation rate and its competition for •OH increase with the increase in electron-donating ability of the atoms (i.e., O) complexed to Fe(II), while the •OH yield decreases accordingly. The Fe(II) species with a moderate oxidation rate and •OH yield is most favorable for contaminant degradation.

Determination of Hydroxyl Radical Production from Sulfide Oxidation Relevant to Sulfidic Porewaters

Hydroxyl radical (·OH) production from the reaction between aqueous total sulfide ([H2S]T = [H2S] + [HS–] + [S2–]) and dissolved oxygen is potentially an environmentally important reaction when anoxic, sulfidic water is exposed to oxygen. Using hydroxyterephthalate (hTPA) formation from the reaction of terephthalic acid (TPA) with ·OH as a probe for ·OH production, hydrogen peroxide was verified as an essential intermediate, and the production of free ·OH, versus lower energy hydroxylating agents, was established. The optimal conditions for the quantification of ·OH production kinetics and yield were determined by varying TPA and total sulfide concentrations. An initial total sulfide concentration of 10 μM and TPA concentration of 2 mM was used to find a yield of 15 mmol of ·OH per mole [H2S]T. Additionally, a pseudo-first-order model elucidated a maximum rate of production of 1.04 (±0.05) × 10–4 moles of ·OH per hour per mole of [H2S]T. Experiments with sulfidic wetland porewaters containing up to 294 mg/L of dissolved organic carbon (DOC) revealed that [H2S]T, and not reduced DOC, was the dominant source of ·OH. A simple model considering a water containing [H2S]T, DOC, and methane exposed to a constant concentration of oxygen (∼50 μM) gave steady state values of [·OH] ranging from 5.7 × 10–19 to 3.2 × 10–18 M. The results indicate that [H2S]T should be considered a source of dark formation of ·OH in addition to ferrous iron and reduced DOC.

Deep eutectic solvent-mediated synthesis of ceria nanoparticles with the enhanced yield for photocatalytic degradation of flumequine under UV-C

This study investigated the use of a deep eutectic solvent (DES) for promoting the yield and stability of ceria nanoparticles used for the degradation of flumequine (FLU) under UV-C irradiation. The characterization by Fourier transform infrared spectroscopy, X-ray diffraction spectroscopy, Raman spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, high resolution transmission electron microscopy, thermogravimetric analysis, and BET surface area analysis revealed the synthesis of highly stable, highly crystalline, and mesoporous ceria nanoparticles using DES which led to the high removal, i.e., 50 % and 94 % of FLU using DES-Ceria and UV-C/DES-Ceria, respectively. Removal of FLU by the UV-C mediated ceria nanoparticles was due to OH and thus factors that influenced the reactivity and yield of OH retarded the removal efficiency of FLU. The pH of aqueous solution affected the removal of FLU by the photocatalysts and removal of FLU was inhibited at highly alkaline and acidic pH. The degradation pathways of FLU were established from the pattern of its degradation and nature of the degradation products. Acute and chronic toxicities of FLU as well as its products were measured. The photocatalyst synthesized in DES was found to be environmentally benign and showed significant potential in the remediation of FLU.

Efficiently dedicated oxidation of long-chain crude oil in the soil by inactive SOM-Fe

To find out the mechanism of efficiently dedicated oxidation of long-chain crude oil in the Fenton system with inactive SOM-Fe, two crude oil-contaminated soils with different concentrations (S1: 14579 mg/kg; S2: 15491 mg/kg) were used as the treatment targets in this study. The results showed that the removal of long-chain crude oil (C21–C30) with inactive SOM-Fe (3120–6930 mg/kg) was much higher than that with active SOM-Fe (2493–4424 mg/kg). Moreover, about 62%–71% OH was allocated for direct oxidation of long-chain crude oil with inactive SOM-Fe compared to that with active SOM-Fe. Meanwhile, the highest removal of long-chain crude oil by unit OH yield was 1345 mg/kg for inactive SOM-Fe, which was 3.6 times that for active SOM-Fe. It can be seen that less OH were consumed by organic matter and more OH was allocated to dedicatedly oxidize long-chain crude oil in inactive SOM-Fe Fenton system. Further analysis found that, for inactive SOM-Fe the low content of active organic matter (protein Ⅰ and protein II) and the active organic functional group (CO), while high content of inactive organic matter (fulvic acid, microorganism and humic acid) and inactive organic functional groups (CH and OH) was observed. Therefore, the allocation of OH was changed after SOM was inactivate, which result in less consumption of OH by SOM and efficiently dedicated oxidation of long-chain crude oil in the soil by inactive SOM-Fe.

Perovskite-derived trimetallic Co-Ni-Cu catalyst for higher alcohol synthesis from syngas

Trimetallic Co-Ni-Cu catalyst from perovskite precursor was prepared by co-precipitation method and then examined for its ability to form higher alcohols from syngas. A superior C2+OH yield of 142.1 mg·gcat.−1·h−1 was achieved at 315 °C, H2/CO = 2, P = 3.0 MPa and GHSV of 10,995 ml·gcat−1·h−1, which was almost two times higher than that obtained with monometallic Co and bimetallic CoNi perovskite catalysts. The addition of Cu alters the electronic interactions with the Co and Ni species, thereby promoting the reduction of perovskite catalysts and forming Co-Ni-Cu alloy after reduction. It was found that the substitution of Cu facilitated the formation of Co2C species during the reaction. Both Co-Ni-Cu alloy and Co2C species were observed in the spent catalyst, suggesting that the formation of Co-Ni-Cu alloy and Co-Co2C interface play key roles in the improvement of higher alcohol formation.

Laser Photolysis Kinetic Study of OH Radical Reactions with Methyl tert-Butyl Ether and Trimethyl Orthoformate under Conditions Relevant to Low Temperature Combustion: Measurements of Rate Coefficients and OH Recycling.

Methyl tertiary butyl ether (MTBE) and trimethyl orthoformate (TMOF) are potential biofuel ethers and could replace conventional fossil fuels, or act as additives to aid combustion. Laser flash photolysis with laser-induced fluorescence detection of the OH radical has been used to measure the rate coefficients of the OH reaction with these ethers, from 298 K to approximately 740 K. The temperature dependence of the rate coefficients is parametrized as kOH+MTBE(298-680 K) = 9.8 × 10-13× ( T/298)2.7 × exp(2500/R T) cm3 molecule-1 s-1 and kOH+TMOF(298-744 K) = 8.0 × 10-13 × [( T/298)2.6 + ( T/298)-8.1] × exp[2650/R T] cm3 molecule-1 s-1. The room temperature (298 K) bimolecular rate coefficients were measured as kOH+MTBE = (2.81 ± 0.32) × 10-12 cm3 molecule-1 s-1 and kOH+TMOF = (4.65 ± 0.50) × 10-12 cm3 molecule-1 s-1 where the errors represent statistical uncertainties at the 2σ level in combination with an estimated 10% systematic error. Regeneration of OH radicals was observed for both reactions at higher temperatures in the presence of O2 via biexponential OH decays, which were observed above 489 K and 568 K, for TMOF and MTBE respectively. The OH yield from MTBE/O2, between 620 and 700 K, was invariant with the concentration of oxygen (1015-1018 molecules cm-3) at (36 ± 5)%. Mechanisms for OH regeneration from MTBE are briefly discussed and compared with those in the literature and from dimethyl and diethyl ether. The lower OH yield from MTBE, compared to these other ethers, is most likely due to competition with an HO2 formation channel.

The oxidation of dehydroascorbic acid and 2,3-diketogulonate by distinct reactive oxygen species.

l-Ascorbate, dehydro-l-ascorbic acid (DHA), and 2,3-diketo-l-gulonate (DKG) can all quench reactive oxygen species (ROS) in plants and animals. The vitamin C oxidation products thereby formed are investigated here. DHA and DKG were incubated aerobically at pH 4.7 with peroxide (H2O2), ‘superoxide’ (a ∼50 : 50 mixture of and ), hydroxyl radicals (•OH, formed in Fenton mixtures), and illuminated riboflavin (generating singlet oxygen, 1O2). Products were monitored electrophoretically. DHA quenched H2O2 far more effectively than superoxide, but the main products in both cases were 4-O-oxalyl-l-threonate (4-OxT) and smaller amounts of 3-OxT and OxA + threonate. H2O2, but not superoxide, also yielded cyclic-OxT. Dilute Fenton mixture almost completely oxidised a 50-fold excess of DHA, indicating that it generated oxidant(s) greatly exceeding the theoretical •OH yield; it yielded oxalate, threonate, and OxT. 1O2 had no effect on DHA. DKG was oxidatively decarboxylated by H2O2, Fenton mixture, and 1O2, forming a newly characterised product, 2-oxo-l-threo-pentonate (OTP; ‘2-keto-l-xylonate’). Superoxide yielded negligible OTP. Prolonged H2O2 treatment oxidatively decarboxylated OTP to threonate. Oxidation of DKG by H2O2, Fenton mixture, or 1O2 also gave traces of 4-OxT but no detectable 3-OxT or cyclic-OxT. In conclusion, DHA and DKG yield different oxidation products when attacked by different ROS. DHA is more readily oxidised by H2O2 and superoxide; DKG more readily by 1O2. The diverse products are potential signals, enabling organisms to respond appropriately to diverse stresses. Also, the reaction-product ‘fingerprints’ are analytically useful, indicating which ROS are acting in vivo.

Synergistic degradation of trans-ferulic acid in aqueous solution by dielectric barrier discharge plasma combined with ozone.

Trans-ferulic acid (FA), extensively used in pharmaceutical and olive oil industries, causes huge risks to ecological environment due to its biotoxicity and phytotoxicity, leading to the difficulty of biochemical processes in treating FA wastewater. In this study, synergistic degradation of FA via dielectric barrier discharge (DBD) plasma and O3 (plasma-ozone) was studied. The results showed that FA degradation efficiency reached 96.9% after a 40-min treatment by plasma-ozone process, and the energy efficiency of FA degradation was increased by 62.5 and 24.5% compared to single DBD plasma and ozonation treatment. Moreover, FA degradation rate constant in plasma-ozone process was 41% higher compared with the sum of single DBD plasma and ozonation, indicating a significant synergistic effect. Radical diagnosis experiments reveal that a profound increase of ·OH yield through peroxone (H2O2/O3) and UV/O3 pathways is the important mechanism of synergistic degradation of FA in plasma-ozone process, while eaq- played little role in FA degradation. A degradation pathway of FA by plasma-ozone was also proposed according to the detected intermediates from EEM and LC-MS. This work revealed that plasma-ozone process is an alternative process for FA treatment, and the findings are helpful for understanding FA degradation characteristics and synergistic mechanisms in plasma-ozone process.

The oxidation regime and SOA composition in limonene ozonolysis: roles of different double bonds, radicals, and water

Abstract. Volatile organic compounds play an important role in air quality and climate change, largely because they contribute to the formation of oxidizing compounds and secondary organic aerosol (SOA). In this study, a series of products, including peroxides and carbonyl compounds in both gaseous and particulate phases, were simultaneously detected to investigate the oxidation regime and SOA composition in limonene ozonolysis. The roles of different double bonds (DBs), radicals, and water were also examined. In our first investigation, we focused on representative oxidizing compounds produced in limonene ozonolysis, including stabilized Criegee intermediates (SCIs), OH radicals, and peroxides. The dependence of H2O2 and hydroxymethyl hydroperoxide (HMHP) formation on RH demonstrates that the reaction with water is an important reaction pathway for limonene SCIs, and the lower limit SCI yields of endocyclic and exocyclic DBs were estimated to be ∼0.24 and ∼0.43, respectively. The OH yield was determined by adding sufficient amounts of an OH scavenger, and the OH yields of endocyclic and exocyclic DBs were ∼0.65 and ∼0.24, respectively. These results indicate that in limonene ozonolysis the endocyclic DB is inclined to generate OH radicals through the hydroperoxide channel, while the exocyclic DB has a higher fraction of forming SCIs. Additionally, other gas-phase and particle-phase peroxides were also studied in this work. The formation of performic acid (PFA) and peracetic acid (PAA) was promoted significantly by increasing RH and the degree of oxidation, and the discrepancy between the experimental and model results suggested some missing formation pathways. Considerable generation of H2O2 from SOA in the aqueous phase was observed, especially at a high [O3] ∕ [limonene] ratio, which was mainly attributed to the hydration and decomposition of unstable peroxides in SOA such as peroxycarboxylic acids and peroxyhemiacetals. Different DBs and OH scavengers had a large impact on the particulate peroxides, and their stability indicated that the types of peroxides in SOA changed under different conditions. As for the contribution of peroxides to SOA, the results demonstrated that the mass fraction of particulate peroxides in limonene SOA was less than 0.2 at a low [O3] ∕ [limonene] ratio, while the mass fraction was 0.4–0.6 at a high [O3] ∕ [limonene] ratio. The partitioning behavior of peroxides showed that multi-generation oxidation helped produce more low-volatility peroxides, which partially explained the higher SOA yield. The partitioning behavior of carbonyls was also examined and the experimental partitioning coefficients (Kp) were found to be typically several orders of magnitude higher than the theoretical values. This study provided new insights into the oxidation regime and SOA composition in limonene ozonolysis, and limonene showed its specificity in many aspects when both endocyclic and exocyclic DBs were ozonated. We suggest that the atmospheric implications of terpenes containing more than one DB and the SOA composition, especially particulate peroxides, need further study.

Reactive quenching of electronically excited NO<sub>2</sub><sup>∗</sup> and NO<sub>3</sub><sup>∗</sup> by H<sub>2</sub>O as potential sources of atmospheric HO<sub><i>x</i></sub> radicals

Abstract. Pulsed laser excitation of NO2 (532–647 nm) or NO3 (623–662 nm) in the presence of H2O was used to initiate the gas-phase reaction NO2∗+H2O → products (Reaction R5) and NO3∗+H2O → products (Reaction R12). No evidence for OH production in Reactions (R5) or (R12) was observed and upper limits for OH production of k5b/k5<1×10-5 and k12b/k12<0.03 were assigned. The upper limit for k5b∕k5 renders this reaction insignificant as a source of OH in the atmosphere and extends the studies (Crowley and Carl, 1997; Carr et al., 2009; Amedro et al., 2011) which demonstrate that the previously reported large OH yield by Li et al. (2008) was erroneous. The upper limit obtained for k12b∕k12 indicates that non-reactive energy transfer is the dominant mechanism for Reaction (R12), though generation of small but significant amounts of atmospheric HOx and HONO cannot be ruled out. In the course of this work, rate coefficients for overall removal of NO3∗ by N2 (Reaction R10) and by H2O (Reaction R12) were determined: k10=(2.1±0.1)×10-11 cm3 molecule−1 s−1 and k12=(1.6±0.3)×10-10 cm3 molecule−1 s−1. Our value of k12 is more than a factor of 4 smaller than the single previously reported value.

Ozone and chlorine reactions with dissolved organic matter - Assessment of oxidant-reactive moieties by optical measurements and the electron donating capacities

Oxidation processes are impacted by the type, concentration and reactivity of the dissolved organic matter (DOM). In this study, the reactions between various types of DOM (Suwannee River fulvic acid (SRFA), Nordic Reservoir NOM (NNOM) and Pony Lake fulvic acid (PLFA)) and two oxidants (ozone and chlorine) were studied in the pH range 2–9 by using a combination of optical measurements and electron donating capacities. The relationships between residual electron donating capacity (EDC) and residual absorbance showed a strong pH dependence for the ozone-DOM reactions with phenolic functional groups being the main reacting moieties. Relative EDC and absorbance abatements (UV254 or UV280) were similar at pH 2. At pH 7 or 9, the relative abatement of EDC was more pronounced than for absorbance, which could be explained by the formation of UV-absorbing products such as benzoquinone from the transformation of phenolic moieties. An increase in fluorescence abatement with increasing pH was also observed during ozonation. The increase in fluorescence quantum yields could not be attributed to formation of benzoquinone, but related to a faster abatement of phenolic moieties relative to fluorophores with low ozone reactivity.The overall •OH yields as a result of DOM-induced ozone consumption increased significantly with increasing pH, which could be related to the higher reactivity of phenolic moieties at higher pH. The •OH yields for SRFA and PLFA were proportional to the phenolic contents, whereas for NNOM, the •OH yield was about 30% higher.During chlorination of DOM at pH 7 an efficient relative EDC abatement was observed whereas the relative absorbance abatement was much less pronounced. This is due to the formation of chlorophenolic moieties, which exert a significant absorbance, and partly lose their electron donating capacity.Pre-ozonation of SRFA leads to a decrease of chloroform and haloacetic acid formation, however, only after a threshold of > ∼50% abatement of the EDC and under conditions which are not precursor limited. The decrease in chloroform and haloacetic acid formation after the threshold EDC abatement was proportional to the relative residual EDC.

Solar light driven degradation of norfloxacin using as-synthesized Bi3+ and Fe2+ co-doped ZnO with the addition of HSO5−: Toxicities and degradation pathways investigation

In this study, solar light responsive Bi3+ and Fe2+ doped ZnO were synthesized and used for photocatalytic degradation of norfloxacin (NOR), an emerging water pollutant. Analysis with Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), and UV–vis diffuse reflectance spectroscopy (DRS) confirmed coupling of dopants (i.e., Bi3+ and Fe2+) or co-dopants (Bi3+/Fe2+) with ZnO and suggested small narrow size as well as highly crystalline and porous nature of the doped ZnO. Metal ions doping lowered band gap energy and inhibited recombination of electron-hole pair in the as-synthesized ZnO. Consequently, BiFe-ZnO showed enhanced photocatalytic activity and reusability, giving 80% removal of NOR as compared to 36% by un-doped ZnO at a reaction time of 120 min. The analysis with photoluminescence (PL) technique verified high yield of OH from solar light activation of BiFe-ZnO compared to Bi-ZnO, Fe-ZnO and ZnO. Performance of the as-prepared photocatalysts was promoted with the added HSO5− and increased with increasing [HSO5−]0. Degradation of NOR by BiFe-ZnO with the added HSO5− was due to OH and SO4− and NOR showed high reactivity with OH and SO4−. The removal efficiency of NOR was inhibited using OH and SO4− scavengers. Photocatalytic activity of the as-synthesized photocatalysts with the added HSO5− was highly dependent on [NOR]0, [pH]0, co-existing ions, and natural organic matter. Degradation pathways were suggested from the pattern of NOR degradation and identified transformation products. The mineralization of NOR as well as formation of non-toxic end product suggests co-doped ZnO with added HSO5− to be a potential technology for treating antibiotics contaminated water.

Photocatalytic, Fenton and photo-Fenton degradation of RhB over Z-scheme g-C3N4/LaFeO3 heterojunction photocatalysts

Z-scheme g-C3N4/LaFeO3 heterojunction photocatalysts were prepared by calcining the uniformly mixed g-C3N4 nanosheets and LaFeO3 nanoparticles at 300 °C for 1 h. The as-prepared g-C3N4/LaFeO3 composites were systematically characterized by XRD, SEM, TEM, XPS, UV–vis DRS, PL spectroscopy, EIS and photocurrent response. It is demonstrated that LaFeO3 nanoparticles (average size: ca. 35 nm) are uniformly assembled onto the surface of g-C3N4 nanosheets through chemical bonding, leading to the formation of g-C3N4/LaFeO3 heterojunctions. The photocatalytic, Fenton and photo-Fenton performances of the samples were investigated by degrading RhB using simulated sunlight as the light source. Compared to bare LaFeO3 and g-C3N4, the g-C3N4/LaFeO3 composites exhibit significantly enhanced photocatalytic and photo-Fenton degradation of the dye, and the maximum activity is observed for the 15%g-C3N4/LaFeO3 composite. Moreover, the photo-Fenton performance of the composites is much higher than the photocatalytic or Fenton performances. The much improved photo-fenton activity is ascribed to the efficient separation of photogenerated electron-hole pairs due to the Z-scheme electron transfer and the synergistic effect between LaFeO3 and g-C3N4. •OH radicals were examined by PL spectroscopy using TPA as a probe molecule, clearly revealing the •OH generation, and moreover, the •OH yield and the dye degradation rate show a similar variation between different cases of the catalysis process. This suggested that •OH is the dominant reactive species causing the dye degradation. The underlying photo-Fenton mechanism of the composites is discussed.

Preparation of RuO2-TiO2/Nano-graphite composite anode for electrochemical degradation of ceftriaxone sodium

Graphite-like material is widely used for preparing various electrodes for wastewater treatment. To enhance the electrochemical degradation efficiency of Nano-graphite (Nano-G) anode, RuO2-TiO2/Nano-G composite anode was prepared through the sol-gel method and hot-press technology. RuO2-TiO2/Nano-G composite was characterized by X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy and N2 adsorption-desorption. Results showed that RuO2, TiO2 and Nano-G were composited successfully, and RuO2 and TiO2 nanoparticles were distributed uniformly on the surface of Nano-G sheet. Specific surface area of RuO2-TiO2/Nano-G composite was higher than that of TiO2/Nano-G composite and Nano-G. Electrochemical performances of RuO2-TiO2/Nano-G anode were investigated by cyclic voltammetry, electrochemical impedance spectroscopy. RuO2-TiO2/Nano-G anode was applied to electrochemical degradation of ceftriaxone. The generation of hydroxyl radical (OH) was measured. Results demonstrated that RuO2-TiO2/Nano-G anode displayed enhanced electrochemical degradation efficiency towards ceftriaxone and yield of OH, which is derived from the synergetic effect between RuO2, TiO2 and Nano-G, which enhance the specific surface area, improve the electrochemical oxidation activity and lower the charge transfer resistance. Besides, the possible degradation intermediates and pathways of ceftriaxone sodium were identified. This study may provide a viable and promising prospect for RuO2-TiO2/Nano-G anode towards effective electrochemical degradation of antibiotics from wastewater.