Freezing-enhanced degradation of azo dyes in the chloride-peroxymonosulfate system.

In this study, we investigated the freezing-induced acceleration of dye bleaching by chloride-activated peroxymonosulfate (PMS). It has been observed that the oxidation of chloride by PMS generates a free chlorine species, such as hypochlorous acid (HOCl), under mild acidic and circumneutral pH condition. This process is the major reason for the enhanced oxidation capacity for electron-rich organic compounds (e.g., phenol) in the chloride-PMS system. However, we demonstrated that the chloride-PMS system clearly reduced the total organic carbon concentration (TOC), whereas the HOCl system did not lead to decrease in TOC. Overall, the chemical reaction is negligible in an aqueous condition if the concentrations of reagents are low, and freezing the solution accelerates the degradation of dye pollutants remarkably. Most notably, the pseudo-first order kinetic rate constant for acid orange 7 (AO7) degradation is approximately 0.252 h-1 with 0.5 mM PMS, 1 mM NaCl, initial pH 3, and a freezing temperature of -20 °C. AO7 degradation is not observed when the solution is not frozen. According to a confocal Raman-microscope analysis and an experiment that used an extremely high dose of reactants, the freeze concentration effect is the main reason for the acceleration phenomenon. Because the freezing phenomenon is spontaneous at high latitudes and at mid-latitudes in winter, and the chloride is ubiquitous elsewhere, the frozen chloride-PMS system has potential as a method for energy-free and eco-friendly technology for the degradation of organic pollutants in cold environments.

Photodegradation of bisphenol A in water and ice: identification of products using three photosensitizers.

Bisphenol A (BPA) is a widespread organic micro-pollutant, found in most environments, including alpine and Arctic regions, and several matrices such as waters and aerosols. Polar regions are characterized by periods of intense irradiation with no sunset due to the continuous sunlight, while alpine areas, despite following the day-night cycle of mid-latitudes, also undergo strong irradiation. For such conditions, it is possible that a fraction of the BPA present in snow may degrade through direct photolysis, producing other unknown species with different environmental mobility and possible ecotoxic effects. Furthermore, the snowpack is rich in species (known as photosensitizers) that facilitate indirect photodegradation processes through reactions involving hydroxyl radicals  · OH , singlet oxygen (1O2), excited triplet states of the organic fraction (3CDOM*), and nitrite/nitrate. In this study, we investigated both direct and indirect photodegradation of BPA in the presence of specific photosensitizers producing  · OH , 1O2, 3CDOM*, and NO2- to specifically explore the products of the reaction. The study was conducted in both liquid water and ice, under light and dark conditions. Results, obtained by HPLC-HRMS, revealed that the matrix in which the reaction takes place, in addition to the photosensitizer used, may influence the degradation by-products. This allows for the possibility of distinguishing the reaction environment based on the identified product.

Structural and functional characterization of sulfurtransferase from Frondihabitans sp. PAMC28461.

Sulfurtransferases transfer of sulfur atoms from thiols to acceptors like cyanide. They are categorized as thiosulfate sulfurtransferases (TSTs) and 3-mercaptopyruvate sulfurtransferases (MSTs). TSTs transfer sulfur from thiosulfate to cyanide, producing thiocyanate. MSTs transfer sulfur from 3-mercaptopyruvate to cyanide, yielding pyruvate and thiocyanate. The present study aimed to isolate and characterize the sulfurtransferase FrST from Frondihabitans sp. PAMC28461 using biochemical and structural analyses. FrST exists as a dimer and can be classified as a TST rather than an MST according to sequence-based clustering and enzyme activity. Furthermore, the discovery of activity over a wide temperature range and the broad substrate specificity exhibited by FrST suggest promising prospects for its utilization in industrial applications, such as the detoxification of cyanide.

Detoxification of arsenite by iodide in frozen solution.

The oxidation of arsenite (As(III)) to arsenate (As(V)) has received significant attention because it helps mitigate the hazardous and adverse effects of As(III) and subsequently improves the effectiveness of arsenic removal. This study developed an efficient freezing technology for the oxidative transformation of As(III) based on iodide (I-). For a sample containing a very low concentration of 20 µM As(III) and 200 µM I- frozen at -20 °C, approximately 19 µM As(V) was formed after reaction for 0.5 h at pH 3. This rapid conversion has never been achieved in previous studies. However, As(V) was not generated in water at 25 °C. The acceleration of the oxidation of As(III) by I- in ice may be attributed to the freeze-concentration effect. During freezing, all components (i.e., As(III), I-, and protons) are highly concentrated in the ice grain boundary regions, resulting in thermodynamically and kinetically favorable conditions for the redox reaction between As(III) and I-. The efficiency of the oxidation of As(III) using I- increased at high I- concentrations and low pH values. The low freezing temperature (below -20 °C) hindered the oxidative transformation of As(III) by I-. The efficiency of the oxidation of As(III) significantly increased using a fixed initial concentration of I- by subjecting the system to six freezing-melting cycles. The outcomes of this study suggest the possibility of the self-detoxification of As(III) in the natural environment, indicating the potential for developing an eco-friendly method for the treatment of As(III)-contaminated areas in regions with a cold climate. It also demonstrates radical remediation to almost completely remove a very small amount of As(III) that was input in As(III)-contaminated wastewater detoxification, a benchmark that existing methods have been unable to achieve.

Catalytic behavior of nitrous acid for acetaminophen transformation during the freezing process.

This study demonstrates the transformation of acetaminophen by reactive nitrous acid in a frozen solution and its abnormal stoichiometry. The chemical reaction between acetaminophen and nitrous acid (AAP/NO2- system) was negligible in the aqueous solution; however, the reaction rapidly progressed if the solution started to freeze. The ultrahigh performance liquid chromatography-electrospray ionization tandem mass spectrometry measurements showed that polymerized acetaminophen and nitrated acetaminophen were formed in the proceeding reaction. Electron paramagnetic resonance spectroscopy measurements showed that nitrous acid oxidized acetaminophen via a one-electron transfer reaction producing acetaminophen-derived radical species, which is the cause of acetaminophen polymerization. We demonstrated that a relatively smaller dose of nitrite than acetaminophen caused significant acetaminophen degradation in the frozen AAP/NO2- system and revealed that the dissolved oxygen content notably affected acetaminophen degradation. We showed that the reaction occurs in a natural Arctic lake matrix (nitrite and acetaminophen spiked). Considering that the freezing phenomenon is common in the natural environment, our research provides a possible scenario for the freezing chemistry of nitrite and pharmaceuticals in environmental chemistry.

Production of Molecular Iodine via a Redox Reaction between Iodate and Organic Compounds in Ice.

The abiotic mechanism of molecular iodine (I2) production from iodate (IO3-) remains largely unknown. Here, we demonstrate the production of I2 in the presence of IO3- and organic compounds in ice. When the solution containing IO3- (100 µM) and furfuryl alcohol (100 µM) at pH 3.0 was frozen at -20 °C, 13.1 µM of I2 was produced with complete degradation of furfuryl alcohol after 20 min. However, there was little change in the IO3- and furfuryl alcohol concentrations in water at 25 °C. The production of I2 in ice is due to the freeze concentration effect, which induces the accumulation of IO3-, furfuryl alcohol, and protons in the ice grain boundaries. This behavior facilitated the production of I2 via a redox reaction between IO3- and organic compounds. The production of I2 increased with increasing furfuryl alcohol concentration and decreasing pH. However, freezing temperature had a minor effect on the maximum production of I2. The production of I2 is highly dependent on the type of organic compounds. It was higher for organic compounds with higher electron-donating properties. This study suggests a new mechanism for I2 production, which is helpful for predicting precisely the atmospheric I2 budget in cold regions.

Ice-templated synthesis of tungsten oxide nanosheets and their application in arsenite oxidation.

Tungsten oxide (WO3) nanosheets were prepared as catalysts to activate hydrogen peroxide (H2O2) in arsenite (As(III)) oxidation. Ice particles were employed as templates to synthesize the WO3 nanosheets, enabling easy template removal via melting. Transmission electron microscopy and atomic force microscopy revealed that the obtained WO3 nanosheets were plate-like, with lateral sizes ranging from dozens of nanometers to hundreds of nanometers and thicknesses of <10 nm. Compared to that of the WO3 nanoparticle/H2O2 system, a higher efficiency of As(III) oxidation was observed in the WO3 nanosheet/H2O2 system. Electron spin resonance spectroscopy, radical quenching studies, and As(III) oxidation experiments under anoxic conditions suggested that the hydroperoxyl radical (HO2●) acted as the primary oxidant. The WO3 nanosheets possessed numerous surface hydroxyl groups and electrophilic metal centers, enhancing the production of HO2● via H2O2 activation. Various anions commonly present in As(III)-contaminated water exhibited little effect on As(III) oxidation in the WO3 nanosheet/H2O2 system. The high oxidation efficiency was maintained by adding H2O2 when it was depleted, suggesting that the catalytic activity of the WO3 nanosheets did not deteriorate after multiple catalytic cycles.

Frozen Hydrogen Peroxide and Nitrite Solution: The Acceleration of Benzoic Acid Oxidation via the Decreased pH in Ice.

We investigated benzoic acid oxidation via the reaction of hydrogen peroxide (H2O2) and nitrite (NO2-). The oxidation of benzoic acid by reactive nitrous acid (HONO) was negligible, and the reactivity of the H2O2/NO2- system decreased with a decrease in temperature under aqueous conditions. However, freezing markedly accelerated the chemical reaction. Based on Raman microscope measurements, concentrated species were confirmed in certain regions of the ice. We proposed that the change in nitrite speciation (accordingly, a decrease in the pH below pKa), derived from the freezing concentration effect, was the reason for the accelerated reactions. The oxidation characteristics of the system were monitored under varying conditions, such as initial pH, dosage ratio, benzoic acid concentration, and reaction with various benzene derivatives. The ultrahigh-performance liquid chromatography/electrospray ionization/mass spectrometry (UHPLC/ESI/MS) measurement showed that peroxynitrous acid (HOONO)-mediated oxidation generated hydroxylated and nitrated byproducts. Additionally, decarboxylated products were detected, indicating direct electron transfer from the organic compounds to HOONO. As freezing is a global phenomenon, and H2O2 and NO2- are ubiquitous in the environment, the transformation of aromatic compounds with H2O2/NO2- in cold environments must be considered in environmental chemistry.

Activation of peroxymonosulfate by bicarbonate and acceleration of the reaction by freezing.

This study demonstrates the positive effects of dissolved bicarbonate and carbonate anions on peroxymonosulfate (PMS) induced oxidation and the remarkable acceleration of the reaction by freezing. More than 90% of the initial 4-chlorophenol (4-CP) decomposed in the frozen case, whereas only less than 20% of the 4-CP was removed in the aqueous case in the same time period. This accelerated reaction is attributed to the freeze-concentration of the dissolved substrates (i.e., PMS, bicarbonate, and pollutants) in the quasi-liquid layer at the ice grain boundaries between ice crystals. The reaction between bicarbonate and PMS was found to be unique because none of the effects were observed in the phosphate and hydroxide cooperated system with freezing, although the base activation of PMS could participate under basic conditions (pH > 9). Based on electron paramagnetic resonance spectroscopy measurements and comparison with the photo-excited Rose Bengal system as a reference system for singlet oxygen (1O2) generation, 1O2 was found to have a minor effect on the oxidation of 4-CP in the frozen bicarbonate-PMS system. While, direct electron transfer from the target organic substrate to the PMS was suggested as a major mechanism of 4-CP oxidation, because the selected target organic substrates were decomposed with different tendencies, and the consumption of PMS was accelerated by the presence of an electron donating compound. The results show the potential applicability of the freezing phenomenon, which occurs naturally in the mid-latitude and polar area, to help a decomposition of water dissolved organic pollutants by the imitation of the natural purification process.

Chloride-Mediated Enhancement in Heat-Induced Activation of Peroxymonosulfate: New Reaction Pathways for Oxidizing Radical Production.

This study is the first to demonstrate the capability of Cl- to markedly accelerate organic oxidation using thermally activated peroxymonosulfate (PMS) under acidic conditions. The treatment efficiency gain allowed heat-activated PMS to surpass heat-activated peroxydisulfate (PDS). During thermal PMS activation at excess Cl-, accelerated oxidation of 4-chlorophenol (susceptible to oxidation by hypochlorous acid (HOCl)) was observed along with significant degradation of benzoic acid and ClO3- occurrence, which involved oxidants with low substrate specificity. This indicated that heat facilitated HOCl formation via nucleophilic Cl- addition to PMS and enabled free chlorine conversion into less selective oxidizing radicals. HOCl acted as a key intermediate in the major oxidant transition based on temperature-dependent variation in HOCl concentration profiles, kinetically retarded organic oxidation upon NH4+ addition, and enabled rapid organic oxidation in heated PMS/HOCl mixtures. Chlorine atom that formed via the one-electron oxidation of Cl- by the sulfate radical served as the primary oxidant and was involved in hydroxyl radical production. This was corroborated by the quenching effects of alcohols and bicarbonates, reactivity toward multiple organics, and electron paramagnetic resonance spectral features. PMS outperformed PDS in degrading benzoic acid during thermal activation operated in reverse osmosis concentrate, which was in conflict with the well-established superiority of heat-activated PDS.

Oxygen vacancy engineering of cerium oxide for the selective photocatalytic oxidation of aromatic pollutants.

The engineering of oxygen vacancies in CeO2 nanoparticles (NPs) allows the specific fine-tuning of their oxidation power, and this can be used to rationally control their activity and selectivity in the photocatalytic oxidation (PCO) of aromatic pollutants. In the current study, a facile strategy for generating exceptionally stable oxygen vacancies in CeO2 NPs through simple acid (CeO2-A) or base (CeO2-B) treatment was developed. The selective (or mild) PCO activities of CeO2-A and CeO2-B in the degradation of a variety of aromatic substrates in water were successfully demonstrated. CeO2-B has more oxygen vacancies and exhibits superior photocatalytic performance compared to CeO2-A. Control of oxygen vacancies in CeO2 facilitates the adsorption and reduction of dissolved O2 due to their high oxygen-storage ability. The oxygen vacancies in CeO2-B as active sites for oxygen-mediated reactions act as (i) adsorption and reduction reaction sites for dissolved O2, and (ii) photogenerated electron scavenging sites that promote the formation of H2O2 by multi-electron transfer. The oxygen vacancies in CeO2-B are particularly stable and can be used repeatedly over 30 h without losing activity. The selective PCOs of organic substrates were studied systematically, revealing that the operating mechanisms for UV-illuminated CeO2-B are very different from those for conventional TiO2 photocatalysts. Thus, the present study provides new insights into the design of defect-engineered metal oxides for the development of novel photocatalysts.

Activation of Peroxymonosulfate by Surface-Loaded Noble Metal Nanoparticles for Oxidative Degradation of Organic Compounds.

This study demonstrates the capability of noble metal nanoparticles immobilized on Al2O3 or TiO2 support to effectively activate peroxymonosulfate (PMS) and degrade select organic compounds in water. The noble metals outperformed a benchmark PMS activator such as Co(2+) (water-soluble) for PMS activation and organic compound degradation at acidic pH and showed the comparable activation capacity at neutral pH. The efficiency was found to depend on the type of noble metal (following the order of Pd > Pt ≈ Au ≫ Ag), the amount of noble metal deposited onto the support, solution pH, and the type of target organic substrate. In contrast to common PMS-activated oxidation processes that involve sulfate radical as a main oxidant, the organic compound degradation kinetics were not affected by sulfate radical scavengers and exhibited substrate dependency that resembled the PMS activated by carbon nanotubes. The results presented herein suggest that noble metals can mediate electron transfer from organic compounds to PMS to achieve persulfate-driven oxidation, rather than through reductive conversion of PMS to reactive sulfate radical.

Photoelectrochemical performance of multi-layered BiOx-TiO2/Ti electrodes for degradation of phenol and production of molecular hydrogen in water.

Multi-layered BiO(x)-TiO(2) electrodes were used for the oxidation of chemical contaminants coupled with the production of H(2) characterized by a synergistic enhancement. The BiO(x)-TiO(2) electrodes were composed of a mixed-metal oxide array involving an under layer of TaO(x)-IrO(x), a middle layer of BiO(x)-SnO(2), and a top layer of BiO(x)-TiO(2) deposited in a series on both sides of Ti foil. Cyclic voltammograms showed that the BiO(x)-TiO(2) electrodes had an electrocatalytic activity for oxidation of phenol that was enhanced by 70% under illumination with AM 1.5 light. When the BiO(x)-TiO(2) anode was coupled with a stainless steel cathode in a Na(2)SO(4) electrolyte with phenol and irradiated with UV light at an applied DC voltage, the anodic phenol oxidation rate and the cathodic H(2) production rates were enhanced by factors of four and three, respectively, as compared to the sum of each light irradiation and direct DC electrolysis. These synergistic effects depend on the specific electrode composition and decrease on TaO(x)-IrO(x) and BiO(x)-SnO(2) anodes in the absence of a top layer of BiO(x)-TiO(2). These results indicate that the BiO(x)-TiO(2) layer functions as the key photo-electrocatalyst. The heavy doping level of Bi (25 mol%) in TiO(2) increases the electric conductivity of the parent TiO(2).