Bimolecular versus Trimolecular Reaction Pathways for H2O2 with Hypochlorous Species and Implications for Wastewater Reclamation.

The benchmark advanced oxidation technology (AOT) that uses UV/H2O2 integrated with hypochlorous species exhibits great potential in removing micropollutants and enhancing wastewater treatability for reclamation purposes. Although efforts have been made to study the reactions of H2O2 with hypochlorous species, there exist great discrepancies in the order of reaction kinetics, the rate constants, and the molecule-level mechanisms. This results in an excessive use of hypochlorous reagents and system underperformance during treatment processes. Herein, the titled reaction was investigated systematically through complementary experimental and theoretical approaches. Stopped-flow spectroscopic measurements revealed a combination of bi- and trimolecular reaction kinetics. The bimolecular pathway dominates at low H2O2 concentrations, while the trimolecular pathway dominates at high H2O2 concentrations. Both reactions were simulated using direct dynamics trajectories, and the pathways identified in the trajectories were further validated by high-level quantum chemistry calculations. The theoretical results not only supported the spectroscopic data but also elucidated the molecule-level mechanisms and helped to address the origin of the discrepancies. In addition, the impact of the environmental matrix was evaluated by using two waters with discrete characteristics, namely municipal wastewater and ammonium-rich wastewater. Municipal wastewater had a negligible matrix effect on the reaction kinetics of H2O2 and the hypochlorous species, making it a highly suitable candidate for this integration technique. The obtained in-depth reaction mechanistic insights will enable the development of a viable and economical technology for safe water reuse.

Chlorination of Emerging Contaminants for Application in Potable Wastewater Reuse: Disinfection Byproduct Formation, Estrogen Activity, and Cytotoxicity.

With increasing water scarcity, many utilities are considering the potable reuse of wastewater as a source of drinking water. However, not all chemicals are removed in conventional wastewater treatment, and disinfection byproducts (DBPs) can form from these contaminants when disinfectants are applied during or after reuse treatment, especially if applied upstream of advanced treatment processes to control biofouling. We investigated the chlorination of seven priority emerging contaminants (17ß-estradiol, estrone, 17α-ethinylestradiol, bisphenol A (BPA), diclofenac, p-nonylphenol, and triclosan) in ultrapure water, and we also investigated the impact of chlorination on real samples from different treatment stages of an advanced reuse plant to evaluate the role of chlorination on the associated cytotoxicity and estrogenicity. Many DBPs were tentatively identified via liquid chromatography (LC)- and gas chromatography (GC)-high resolution mass spectrometry, including 28 not previously reported. These encompassed chlorinated, brominated, and oxidized analogs of the parent compounds as well as smaller halogenated molecules. Chlorinated BPA was the least cytotoxic of the DBPs formed but was highly estrogenic, whereas chlorinated hormones were highly cytotoxic. Estrogenicity decreased by ∼4-6 orders of magnitude for 17ß-estradiol and estrone following chlorination but increased 2 orders of magnitude for diclofenac. Estrogenicity of chlorinated BPA and p-nonylphenol were ∼50% of the natural/synthetic hormones. Potential seasonal differences in estrogen activity of unreacted vs reacted advanced wastewater treatment field samples were observed.

Overlooked Transformation of Nitrated Polycyclic Aromatic Hydrocarbons in Natural Waters: Role of Self-Photosensitization.

Photochemical transformation is an important process that involves trace organic contaminants (TrOCs) in sunlit surface waters. However, the environmental implications of their self-photosensitization pathway have been largely overlooked. Here, we selected 1-nitronaphthalene (1NN), a representative nitrated polycyclic aromatic hydrocarbon, to study the self-photosensitization process. We investigated the excited-state properties and relaxation kinetics of 1NN after sunlight absorption. The intrinsic decay rate constants of triplet (31NN*) and singlet (11NN*) excited states were estimated to be 1.5 × 106 and 2.5 × 108 s-1, respectively. Our results provided quantitative evidence for the environmental relevance of 31NN* in waters. Possible reactions of 31NN* with various water components were evaluated. With the reduction and oxidation potentials of -0.37 and 1.95 V, 31NN* can be either oxidized or reduced by dissolved organic matter isolates and surrogates. We also showed that hydroxyl (•OH) and sulfate (SO4•-) radicals can be generated via the 31NN*-induced oxidation of inorganic ions (OH- and SO42-, respectively). We further investigated the reaction kinetics of 31NN* and OH- forming •OH, an important photoinduced reactive intermediate, through complementary experimental and theoretical approaches. The rate constants for the reactions of 31NN* with OH- and 1NN with •OH were determined to be 4.22 × 107 and 3.95 ± 0.01 × 109 M-1 s-1, respectively. These findings yield new insights into self-photosensitization as a pathway for TrOC attenuation and provide more mechanistic details into their environmental fate.

Merits and Limitations of Radical vs. Nonradical Pathways in Persulfate-Based Advanced Oxidation Processes.

Urbanization and industrialization have exerted significant adverse effects on water quality, resulting in a growing need for reliable and eco-friendly treatment technologies. Persulfate (PS)-based advanced oxidation processes (AOPs) are emerging as viable technologies to treat challenging industrial wastewaters or remediate groundwater impacted by hazardous wastes. While the generated reactive species can degrade a variety of priority organic contaminants through radical and nonradical pathways, there is a lack of systematic and in-depth comparison of these pathways for practical implementation in different treatment scenarios. Our comparative analysis of reaction rate constants for radical vs. nonradical species indicates that radical-based AOPs may achieve high removal efficiency of organic contaminants with relatively short contact time. Nonradical AOPs feature advantages with minimal water matrix interference for complex wastewater treatments. Nonradical species (e.g., singlet oxygen, high-valent metals, and surface activated PS) preferentially react with contaminants bearing electron-donating groups, allowing enhancement of degradation efficiency of known target contaminants. For byproduct formation, analytical limitations and computational chemistry applications are also considered. Finally, we propose a holistically estimated electrical energy per order of reaction (EE/O) parameter and show significantly higher energy requirements for the nonradical pathways. Overall, these critical comparisons help prioritize basic research on PS-based AOPs and inform the merits and limitations of system-specific applications.

Enhancement of bio-S0 recovery and revealing the inhibitory effect on microorganisms under high sulfide loading.

Biodesulfurization is a mature technology, but obtaining biosulfur (S0) that can be easily settled naturally is still a challenge. Increasing the sulfide load is one of the known methods to obtain better settling of S0. However, the inhibitory effect of high levels of sulfide on microbes has also not been well studied. We constructed a high loading sulfide (1.55-10.86 kg S/m3/d) biological removal system. 100% sulfide removal and 0.56-2.53 kg S/m3/d S0 (7.0 ± 0.09-16.4 ± 0.25 µm) recovery were achieved at loads of 1.55-7.75 kg S/m3/d. Under the same load, S0 in the reflux sedimentation tank, which produced larger S0 particles (24.2 ± 0.73-53.8 ± 0.70 µm), increased the natural settling capacity and 45% recovery. For high level sulfide inhibitory effect, we used metagenomics and metatranscriptomics analyses. The increased sulfide load significantly inhibited the expression of flavin cytochrome c sulfide dehydrogenase subunit B (fccB) (Decreased from 615 ± 75 to 30 ± 5 TPM). At this time sulfide quinone reductase (SQR) (324 ± 185-1197 ± 51 TPM) was mainly responsible for sulfide oxidation and S0 production. When the sulfide load reached 2800 mg S/L, the SQR (730 ± 100 TPM) was also suppressed. This resulted in the accumulation of sulfide, causing suppression of carbon sequestration genes (Decreased from 3437 ± 842 to 665 ± 175 TPM). Other inhibitory effects included inhibition of microbial respiration, production of reactive oxygen species, and DNA damage. More sulfide-oxidizing bacteria (SOB) and newly identified potential SOB (99.1%) showed some activity (77.6%) upon sulfide accumulation. The main microorganisms in the sulfide accumulation environment were Thiomicrospiracea and Burkholderiaceae, whose sulfide oxidation capacity and respiration were not significantly inhibited. This study provides a new approach to enhance the natural sedimentation of S0 and describes new microbial mechanisms for the inhibitory effects of sulfide.

Characterization of a multi-stage focusing nozzle for collection of spot samples for aerosol chemical analysis.

Concentrated collection of aerosol particles on a substrate is essential for their chemical analysis using various microscopy and laser spectroscopic techniques. An impaction-based aerosol concentration system was developed for focused collection of particles using a multi-stage nozzle that consists of a succession of multiple smooth converging stages. Converging sections of the nozzle were designed to focus and concentrate a particle diameter range of 900-2500 nm into a relatively narrower particle beam to obtain particulate deposits with spot diameters of 0.5-1.56 mm. A slightly diverging section before the last contractions was included to allow for better focusing of particles at the lower end of the collectable diameter range. The characterization of this multi-stage nozzle and the impaction-based aerosol concentration system was accomplished both numerically and experimentally. The numerical and experimental trends in collection efficiency and spot diameters agreed well qualitatively; however, the quantitative agreement between numerical and experimental results for wall losses was poor, particularly for larger particle diameters. The resulting concentrated particulate deposit, a spot sample, was analysed using Raman spectroscopy to probe the effect of spot size on analytical sensitivity of measurement. The method's sensitivity was compared against other conventional techniques, such as filtration and aerosol focused impaction, implementing condensational growth. Impaction encompassing the multi-stage focusing nozzle is the only method that can ensure high sensitivity at Reynolds numbers greater than 2000, that can be supported by small pumps which renders such method suitable for portable instrumentation.

Defect Engineering Modulated Iron Single Atoms with Assist of Layered Clay for Enhanced Advanced Oxidation Processes.

Single-atom catalysts (SACs) feature maximum atomic utilization efficiency; however, the loading amount, dispersibility, synthesis cost, and regulation of the electronic structure are factors that need to be considered in water treatment. In this study, kaolinite, a natural layered clay mineral, is applied as the support for g-C3 N4 and single Fe atoms (FeSA-NGK). The FeSA-NGK composite exhibits an impressive degradation performance toward the target pollutant (>98% degradation rate in 10 min), and catalytic stability across consecutive runs (90% reactivity maintained after three runs in a fluidized-bed catalytic unit) under peroxymonosulfate (PMS)/visible light (Vis) synergetic system. The introduction of kaolinite promotes the loading amount of single Fe atoms (2.57 wt.%), which is a 14.2% increase compared to using a bare catalyst without kaolinite, and improved the concentration of N vacancies, thereby optimizing the regulation of the electronic structure of the single Fe atoms. It is discovered that the single Fe atoms successfully occupied five coordinated N atoms and combined with a neighboring N vacancy. Consequently, this regulated the local electronic structure of single Fe atoms, which drives the electrons of N atoms to accumulate on the Fe centers. This study opens an avenue for the design of clay-based SACs for water purification.

Overlooked Formation of H2O2 during the Hydroxyl Radical-Scavenging Process When Using Alcohols as Scavengers.

Hydroxyl radical (•OH) is an active species widely reported in studies across many scientific fields, and hence, its reliable analysis is vitally important. Currently, alcohols are commonly used as scavengers for •OH determination. However, the impacts of alcohols on the reliability of •OH detection remain unknown. In this study, we found that adding different types and different amounts of alcohols in water samples treated with ultraviolet irradiation undesirably produced substantial amounts of hydrogen peroxide (H2O2), which is a known •OH precursor. This means that the conventional •OH determination method using alcohols is likely unreliable or even misleading. Through careful investigation, we revealed an overlooked reaction pathway during H2O2 and •OH transformations. Varying oxygen concentrations, pHs, alcohol dosages, and types altered H2O2 formation, which can affect •OH determination accuracy. Among alcohols, n-butanol is the best scavenger because it quenches •OH rapidly but re-forms little H2O2.

New Insight into a Fenton-like Reaction Mechanism over Sulfidated ß-FeOOH: Key Role of Sulfidation in Efficient Iron(III) Reduction and Sulfate Radical Generation.

Sulfidation can greatly improve the efficiency of utilization of reducing equivalents for contaminant removal; however, whether this method benefits Fenton-like reactions or not and the possible mechanism are not well understood. In this study, we revealed that surface sulfidation can greatly promote the heterogeneous Fenton activity of ß-FeOOH (Fe3S4@ß-FeOOH) by 40 times, in which not only the •OH formation was enhanced but also SO4•- as a new oxidation species was generated. Moreover, their contribution to metronidazole (MTZ) degradation was 52.5 and 37.1%, respectively. In comparison, almost no HO2•/O2•- was detected in the Fe3S4@ß-FeOOH/H2O2 system. These results were different from some previously reported Fenton counterparts. Based on the characterization and probe experiments, sulfur species, including S2-, S0, and Sn2-, as an electron donor and electron shuttle were responsible for efficient conversion of Fe(III) into Fe(II) other than via the Haber-Weiss mechanism, leading to excellent •OH generation via a Fenton-like mechanism. Most importantly, HSO5- can be generated from SO32- oxidized by •OH, and its scission into SO4•- was not dependent on the extra electric potential or Fe-O2-S(IV) intermediate. These findings provided new insight for utilizing sulfidation to improve the activity of iron-based Fenton catalysts.

Mechanistic Understanding of Superoxide Radical-Mediated Degradation of Perfluorocarboxylic Acids.

Perfluorocarboxylic acids (PFCAs) exhibit strong persistence in sunlit surface waters and in radical-based treatment processes, where superoxide radical (O2•-) is an important and abundant reactive oxygen species. Given that the role of O2•- during the transformation of PFCAs remains largely unknown, we investigated the kinetics and mechanisms of O2•--mediated PFCAs attenuation through complementary experimental and theoretical approaches. The aqueous-phase rate constants between O2•- and C3-C8 PFCAs were measured using a newly designed in situ spectroscopic system. Mechanistically, bimolecular nucleophilic substitution (SN2) is most likely to be thermodynamically feasible, as indicated by density functional theory calculations at the CBS-QB3 level of theory. This pathway was then investigated by ab initio molecular dynamics simulation with free-energy samplings. As O2•- approaches PFCA, the C-F bond at the alpha carbon is spontaneously stretched, leading to the bond cleavage. The solvation mechanism for O2•--mediated PFCA degradation was also elucidated. Our results indicated that although the less polar solvent enhanced the nucleophilicity of O2•-, it also decreased the desolvation process of PFCAs, resulting in reduced kinetics. With these quantitative and mechanistic results, we achieved a defined picture of the O2•--initiated abatement of PFCAs in natural and engineered waters.

Visible Light-Induced Catalyst-Free Activation of Peroxydisulfate: Pollutant-Dependent Production of Reactive Species.

Activation of peroxydisulfate (PDS, S2O82-) via various catalysts to degrade pollutants in water has been extensively investigated. However, catalyst-free activation of PDS by visible light has been largely ignored. This paper reports effective visible light activation of PDS without any additional catalyst, leading to the degradation of a wide range of organic compounds of high environmental and human health concerns. Importantly, the formation of reactive species is distinctively different in the PDS visible light system with and without pollutants [e.g., atrazine (ATZ)]. In addition to SO4•- generated via S2O82- dissociation under visible light irradiation, O2•- and 1O2 are also produced in both systems. However, in the absence of ATZ, H2O2 and O2•- are key intermediates and precursors for 1O2, whereas in the presence of ATZ, a different pathway was followed to produce O2•- and 1O2. Both radical and nonradical processes contribute to the degradation of ATZ in the PDS visible light system. The active role of 1O2 in the degradation of ATZ besides SO4•- is manifested by the enhanced degradation of contaminants and electron paramagnetic resonance spectroscopy measurements in D2O.

Mechanistic Study of the Effects of Agricultural Amendments on Photochemical Processes in Paddy Water during Rice Growth.

The photochemical properties of paddy water might be affected by the commonly used amendments in rice fields owing to the associated changes in water chemistry; however, this important aspect has rarely been explored. We examined the effects of agricultural amendments on the photochemistry of paddy water during rice growth. The amendments significantly influenced the photogenerated reactive intermediates (RIs) in paddy water, such as triplet dissolved organic matter (3DOM*), singlet oxygen, and hydroxyl radicals. Compared with control experiments without amendments, the application of straw and lime increased the RI concentrations by up to 16.8 and 11.1 times, respectively, while biochar addition had limited effects on RI generation from paddy water in in situ experiments under sunlight. Fluorescence emission-excitation matrix spectroscopy, Fourier transform ion cyclotron resonance mass spectrometry, and structural equation modeling revealed that upon the addition of straw and lime amendments, humified DOM substances contained lignins, proteins, and fulvic acids, which could produce more RIs under irradiation. Moreover, the amendments significantly accelerated the degradation rate of 2,4-dichlorophenol but led to the 3DOM*-mediated formation of more toxic and stable dimeric products. This study provides new insights into the effects of amendments on the photochemistry of paddy water and the pathways of abiotic degradation of organic contaminants in paddy fields.

Reactive High-Valent Iron Intermediates in Enhancing Treatment of Water by Ferrate.

Efforts are being made to tune the reactivity of the tetraoxy anion of iron in the +6 oxidation state (FeVIO42-), commonly called ferrate, to further enhance its applications in various environmental fields. This review critically examines the strategies to generate highly reactive high-valent iron intermediates, FeVO43- (FeV) and FeIVO44- or FeIVO32- (FeIV) species, from FeVIO42-, for the treatment of polluted water with greater efficiency. Approaches to produce FeV and FeIV species from FeVIO42- include additions of acid (e.g., HCl), metal ions (e.g., Fe(III)), and reductants (R). Details on applying various inorganic reductants (R) to generate FeV and FeIV from FeVIO42- via initial single electron-transfer (SET) and oxygen-atom transfer (OAT) to oxidize recalcitrant pollutants are presented. The common constituents of urine (e.g., carbonate, ammonia, and creatinine) and different solids (e.g., silica and hydrochar) were found to accelerate the oxidation of pharmaceuticals by FeVIO42-, with potential mechanisms provided. The challenges of providing direct evidence of the formation of FeV/FeIV species are discussed. Kinetic modeling and density functional theory (DFT) calculations provide opportunities to distinguish between the two intermediates (i.e., FeIV and FeV) in order to enhance oxidation reactions utilizing FeVIO42-. Further mechanistic elucidation of activated ferrate systems is vital to achieve high efficiency in oxidizing emerging pollutants in various aqueous streams.

Oxidative dehydrogenation of ethane: catalytic and mechanistic aspects and future trends.

Ethene is a commodity chemical of great importance for manufacturing diverse consumer products, whose synthesis via crude oil steam cracking is one of the most energy-intensive processes in the petrochemical industry. Oxidative dehydrogenation (ODH) of ethane is an attractive, low energy, alternative route to ethene which could reduce the carbon footprint for its production, however, the commercial implementation of ODH requires catalysts with improved selectivity. This review critically assesses recent developments in catalytic technologies for ethane ODH, and discusses how insight into proposed mechanisms from computational studies, and CO2 assisted ethane dehydrogenation (CO2-DHE), provide opportunities for economically viable processes to meet growing demands for ethene while reducing carbon emissions. Future trends and emerging technologies for ethane ODH are also discussed.

Effects of Experimental Conditions on the Signaling Fidelity of Impedance-Based Nucleic Acid Sensors.

Electrochemical impedance spectroscopy (EIS), an extremely sensitive analytical technique, is a widely used signal transduction method for the electrochemical detection of target analytes in a broad range of applications. The use of nucleic acids (aptamers) for sequence-specific or molecular detection in electrochemical biosensor development has been extensive, and the field continues to grow. Although nucleic acid-based sensors using EIS offer exceptional sensitivity, signal fidelity is often linked to the physical and chemical properties of the electrode-solution interface. Little emphasis has been placed on the stability of nucleic acid self-assembled monolayers (SAMs) over repeated voltammetric and impedimetric analyses. We have studied the stability and performance of electrochemical biosensors with mixed SAMs of varying length thiolated nucleic acids and short mercapto alcohols on gold surfaces under repeated electrochemical interrogation. This systematic study demonstrates that signal fidelity is linked to the stability of the SAM layer and nucleic acid structure and the packing density of the nucleic acid on the surface. A decrease in packing density and structural changes of nucleic acids significantly influence the signal change observed with EIS after routine voltammetric analysis. The goal of this article is to improve our understanding of the effect of multiple factors on EIS signal response and to optimize the experimental conditions for development of sensitive and reproducible sensors. Our data demonstrate a need for rigorous control experiments to ensure that the measured change in impedance is unequivocally a result of a specific interaction between the target analyte and nucleic recognition element.

Hydroxyl Radical-Involving p-Nitrophenol Oxidation during Its Reduction by Nanoscale Sulfidated Zerovalent Iron under Anaerobic Conditions.

Sulfidated zerovalent iron (S-ZVI) has been extensively used for reducing pollutants. In this study, the oxidation process in the reductive removal of p-nitrophenol (PNP) by S-ZVI was confirmed under anaerobic conditions. We revealed that a PNP oxidation process involving •OH resulted from the H2O2 activation by surface-bound Fe(II) in S-ZVI, in which H2O2 was generated via a surface-mediated reaction between water and FeS2. Only the PNP reduction process occurred for ZVI. Herein, efficient PNP degradation by S-ZVI resulted from two functions: reduction into p-aminophenol due to enhanced electron transfer and PNP oxidation into p-benzoquinone by •OH radicals from the interaction of surface-bound Fe(II) and in situ generated H2O2, the contributions of the oxidation and reduction processes to PNP degradation over S-ZVI were 10 and 90%, respectively. Sulfur in S-ZVI suppressed the pH increase in the reaction media and produced more surface-bound Fe(II) than ZVI for •OH generation via the heterogeneous Fenton reaction process. Since different degradation pathways could lead to different effects on the water environment, such as toxicity, our findings suggest that the oxidizing process induced by S-ZVI during groundwater decontamination should be considered.

Photogeneration of Reactive Species from Biochar-Derived Dissolved Black Carbon for the Degradation of Amine and Phenolic Pollutants.

Due to agricultural waste combustion and large-scale biochar application, biochar-derived dissolved black carbon (DBC) is largely released into surface waters. The photogeneration of reactive species (RS) from DBC plays an important role in organic pollutant degradation. However, the mechanistic interactions between RS and pollutants are poorly understood. Here, we investigated the formation of DBC triplet states (3DBC*), singlet oxygen (1O2), and hydroxyl radical (•OH) in straw biochar-derived DBC solutions and photodegradation of typical pharmaceuticals and personal care products (PPCPs). Laser flash photolysis and electron spin resonance spectrometry showed that DBC exhibited higher RS quantum yields than some well-studied dissolved organic matter. The RS caused rapid degradation of atenolol, diphenhydramine, and propylparaben, selected as target PPCPs in this study. The 3DBC* contributed primarily to the oxidation of selected PPCPs via one-electron-transfer interaction, with average reaction rate constants of 1.15 × 109, 1.41 × 109, and 0.51 × 109 M-1 s-1, respectively. •OH also participated in the degradation and accounted for approximately 2.7, 2.5, and 18.0% of the total removal of atenolol, diphenhydramine, and propylparaben, respectively. Moreover, the photodegradation products were identified using high-resolution mass spectrometry, which further confirmed the electron transfer and •OH oxidation mechanisms. These findings suggest that DBC from the combustion process of agricultural biomass can efficiently induce the photodegradation of organic pollutants under sunlight in aquatic environments.

Formation of Nitrite and Hydrogen Peroxide in Water during the Vacuum Ultraviolet Irradiation Process: Impacts of pH, Dissolved Oxygen, and Nitrate Concentration.

Photolysis via vacuum ultraviolet (VUV) irradiation is a robust technology capable of inactivating pathogens and degrading micropollutants, and therefore, its application has recently attracted great interest. However, VUV irradiation of water may yield nitrite (NO2-, a regulated carcinogenic contaminant) and hydrogen peroxide (H2O2, a compound linked to aging, inflammation, and cancer), thus motivating us to better understand its risks. By applying a novel H2O2 detection method insensitive to coexisting compounds, this study clearly observed concurrent and substantial formations of NO2- and H2O2 during VUV irradiation of various synthetic and real waters. Increasing pH and/or decreasing oxygen promoted the conversion of nitrate (NO3-) into NO2- but suppressed the H2O2 formation, suggesting that there was a transition of radicals from oxidizing species like hydroxyl radicals to reducing species like hydrogen atoms and hydrated electrons. Under low light dose conditions, both NO2- and H2O2 were formed concurrently; however, under high radiation dosage conditions, the patterns conducive to NO2- formation were opposite to those conducive to H2O2 formation. Real water irradiation proved the formation of NO2- and H2O2 at levels near to or greater than current drinking water regulatory limits. Hence, the study reminds of a holistic view of benefits and disbenefits of a VUV process.

Reactivity and reaction mechanisms of sulfate radicals with lindane: An experimental and theoretical study.

Advanced oxidation technologies (AOTs) have been intensely used to eliminate various organic pollutants in engineering waters. In this context, we investigated the kinetics and mechanisms of the sulfate radical (SO4-)-mediated degradation of lindane in UV/peroxydisulfate system, and compared results with previous studies on SO4--based AOTs for destruction of lindane. The second order rate constant (k) value between SO4- and lindane was determined to be (8.95 ± 0.29) × 106 M-1 s-1via competition kinetics using p-cyanobenzoic acid as reference compound, which is close to the theoretically calculated value of 4.41 × 107 M-1 s-1, that was performed at SMD/M05-2X/6-311++G**//M05-2X/6-31+G** level of theory using density functional theory (DFT) approach. H-atom abstraction pathway was calculated to be thermodynamically favorable and kinetically dominant. In the combined experimental and theoretical study, we aim for a better understanding on the degradation kinetics and mechanisms of lindane, serving as a starting point for more attention to SO4--mediated degradation kinetics of cycloaliphatic compounds in future.

Graphene-modified graphite paper cathode for the efficient bioelectrochemical removal of chromium.

Metal-free electrocatalysts have been widely used as cathodes for the reduction of hexavalent chromium [Cr(VI)] in microbial fuel cells (MFCs). The electrocatalytic activity of such system needs to be increased due to the low anodic potential provided by bacteria. In this study, graphite paper (GP) was treated by liquid nitrogen to form three-dimensional graphite foam (3DGF), improving the Cr(VI) reduction by 17% and the total Cr removal by 81% at 30 h in MFCs. X-ray absorption spectroscopy confirmed the Cr(VI) reduction product as Cr(OH)3. Through the spectroscopy characterizations, electrochemical measurements, and density functional theory calculations, the porous structures, edges, and O-doped defects on the 3DGF surface resulted in a higher electroconducting rate and a lower mass transfer rate, which provide more active sites for the Cr(VI) reduction. Additionally, the scrolled graphene-like carbon nanosheets and porous structures on the 3DGF surface might limit the OH- diffusion and result in a high local pH, which accelerated the Cr(OH)3 formation. The results of this study are expected to provide a simple method to manipulate the carbon materials and insights into mechanisms of Cr(VI) reduction in MFCs by the 3DGF with in situ exfoliated edges and O-functionalized graphene.