How g-C3N4 Works and Is Different from TiO2 as an Environmental Photocatalyst: Mechanistic View.

Graphitic carbon nitride (CN) as a popular visible light photocatalyst needs to be better understood for environmental applications. The behaviors of CN as an environmental photocatalyst were systematically studied in comparison with a well-known TiO2 photocatalyst. The two photocatalysts exhibit different photocatalytic oxidation (PCO) behaviors and dependences on the experimental conditions (e.g., pH, Pt loading, and the kind of organic substrate and scavenger). The PCO of organic substrates was significantly enhanced by loading Pt on TiO2 under UV light (λ > 320 nm), whereas Pt-CN exhibited a lower PCO activity than bare CN under visible light (λ > 420 nm). While the presence of Pt enhances the charge separation in both TiO2/UV and CN/visible light systems (confirmed by transient IR absorption spectroscopic analysis), the opposite effects of Pt are ascribed to the different mechanisms of •OH generation in the two photocatalytic systems. The negative effect of Pt on CN is ascribed to the fact that Pt catalytically decomposes in situ-generated H2O2 (a main precursor of OH radical), which hinders •OH production. The production of OH radicals on CN is favored only at acidic pH but 1O2 generation is dominant in alkaline pH. The pH-dependent behaviors of reactive oxygen species generation on CN were confirmed by electron paramagnetic resonance spin trap measurements.

In Situ-Generated Reactive Oxygen Species in Precharged Titania and Tungsten Trioxide Composite Catalyst Membrane Filters: Application to As(III) Oxidation in the Absence of Irradiation.

This study demonstrates that in situ-generated reactive oxygen species (ROSs) in prephotocharged TiO2 and WO3 (TW) composite particle-embedded inorganic membrane filters oxidize arsenite (As(III)) into arsenate (As(V)) without any auxiliary chemical oxidants under ambient conditions in the dark. TW membrane filters have been charged with UV or simulated sunlight and subsequently transferred to a once-through flow-type system. The charged TW filters can transfer the stored electrons to dissolved O2, producing ROSs that mediate As(III) oxidation in the dark. Dramatic inhibition of As(V) production with O2 removal or addition of ROS quenchers indicates an ROS-mediated As(III) oxidation mechanism. Electron paramagnetic spectroscopic analysis has confirmed the formation of the HO2•/O2•- pair in the dark. The WO3 fraction in the TW filter significantly influences the performance of the As(III) oxidation, while As(V) production is enhanced with increasing charging time and solution pH. The As(III) oxidation is terminated when the singly charged TW filter is fully discharged; however, recharging of TW recovers the catalytic activity for As(III) oxidation. The proposed oxidation process using charged TW membrane filters is practical and environmentally benign for the continuous treatment of As(III)-contaminated water during periods of unavailability of sunlight.

Activation of Peroxymonosulfate by Oxygen Vacancies-Enriched Cobalt-Doped Black TiO2 Nanotubes for the Removal of Organic Pollutants.

Cobalt-mediated activation of peroxymonosulfate (PMS) has been widely investigated for the oxidation of organic pollutants. Herein, we employ cobalt-doped Black TiO2 nanotubes (Co-Black TNT) for the efficient, stable, and reusable activator of PMS for the degradation of organic pollutants. Co-Black TNTs induce the activation of PMS by itself and stabilized oxygen vacancies that enhance the bonding with PMS and provide catalytic active sites for PMS activation. A relatively high electronic conductivity associated with the coexistence of Ti4+ and Ti3+ in Co-Black TNT enables an efficient electron transfer between PMS and the catalyst. As a result, Co-Black TNT is an effective catalyst for PMS activation, leading to the degradation of selected organic pollutants when compared to other TNTs (TNT, Co-TNT, and Black TNT) and other Co-based materials (Co3O4, Co-TiO2, CoFe2O4, and Co3O4/rGO). The observed organic compound degradation kinetics are retarded in the presence of methanol and natural organic matter as sulfate radical scavengers. These results demonstrate that sulfate radical is the primary oxidant generated via PMS activation on Co-Black TNT. The strong interaction between Co and TiO2 through Co-O-Ti bonds and rapid redox cycle of Co2+/Co3+ in Co-Black TNT prevents cobalt leaching and enhances catalyst stability over a wide pH range and repetitive uses of the catalyst. Electrode-supported Co-Black TNT facilitates the recovery of the catalyst from the treated water.

Spontaneous Generation of H2O2 and Hydroxyl Radical through O2 Reduction on Copper Phosphide under Ambient Aqueous Condition.

Copper phosphide (Cu xP) was synthesized and tested for its reactivity for generating H2O2 through spontaneous reduction of dioxygen under ambient aqueous condition. The in situ generated H2O2 was subsequently decomposed to generate OH radicals, which enabled the degradation of organic compounds in water. The oxygen reduction reaction proceeded along with the concurrent oxidation of phosphide to phosphate, then copper ions and phosphate ions were dissolved out during the reaction. The reactivity of Cu xP was gradually reduced during 10 cycles with consuming 8.7 mg of Cu xP for the successive removal of 17 µmol 4-chlorophenol. CoP which was compared as a control sample under the same experimental condition also produced H2O2 through activating dioxygen but did not degrade organic compounds at all. The electrochemical analysis for the electron transfers on Cu xP and CoP showed that the number of electrons transferred to O2 is 3 and 2, respectively, which explains why OH radical is generated on Cu xP, not on CoP. The Cu+ species generated on the Cu xP surface can participate in Fenton-like reaction with in situ generated H2O2. Cu xP is proposed as a solid reagent that can activate dioxygen to generate reactive oxygen species in ambient aqueous condition, which is more facile to handle and store than liquid/gas reagents (e.g., H2O2, Cl2, O3).

Substrate oxidation enhances the electrochemical production of hydrogen peroxide.

Hydrogen peroxide (H2O2) is electrochemically produced via oxygen (O2) reduction on a carbon cathode surface. In order to enhance the production of H2O2, anodic loss pathways, which significantly reduce the overall H2O2 production rate, should be inhibited. In this study, we investigate the effects of organic electron donors (i.e., typical chemical contaminants) on the anodic loss pathways of H2O2 in a single-cell electrochemical reactor that employs an anode composed of TiO2 over-coated on a mixed-metal oxide ohmic contact catalyst, Ir0.7Ta0.3O2, deposited on a Ti-metal that is coupled with a graphite rod cathode in a sodium sulfate (Na2SO4) electrolyte that is saturated with oxygen (O2). Organic electron donors are shown to enhance the electrochemical production of H2O2, while simultaneously undergoing oxidative degradation. The observed positive effect of organic electron donors on the electrochemical production of H2O2 is due in part to a preferential adsorption of organic substrates on the TiO2 outer layer of the anode. The sorption of the organic electron donors inhibits the formation of surficial titanium hydroperoxo species ([bond, triple bond]Ti-OOH) on the anode surface. The organic sorbates also act as scavengers of surface-bound hydroxyl radical [bond, triple bond]Ti-OH. As a result, the decomposition of H2O2 on the anode surface is significantly reduced. The cathodic production rate of H2O2 at low pH is enhanced due to proton coupled electron transfer (PCET) to O2, while the anodic decomposition of H2O2 is inhibited due to electrostatic interactions between negatively-charged organic substrates and a positively-charged outer surface of the anode (TiO2 pHzpc = 5.8) at low pH.

Visible Light Sensitized Production of Hydroxyl Radicals Using Fullerol as an Electron-Transfer Mediator.

Fullerenes and their derivatives are known to photosensitize the production of singlet oxygen (1O2), but their role in generating hydroxyl radical (•OH) under visible light has not been reported. Here, we demonstrate that fullerol can mediate the electron transfer from Rhodamine B dye to O2 under visible light irradiation, achieving simultaneous dye decolorization and •OH-induced degradation of 4-chlorophenol. The hydroxyl radical is proposed to be produced via a consecutive reduction of molecular oxygen by fullerol anion radical, which is formed through the electron transfer from the dye to the triplet state of fullerol. Mechanistic investigations using various probe reagents such as superoxide dismutase (superoxide quencher), t-butanol (•OH quencher), and coumarin (•OH probe) provided indirect evidence for the generation of •OH under visible light. Furthermore, spin trapping technique directly detected the oxidizing species such as •OH, HO2•, and 1O2 in the visible light irradiated solution of RhB/fullerol mixture. It was proposed that the photochemical oxidation mechanism depends on pH: •OH production is favored at acidic pH through fullerol-mediated sequential electron transfer while 1O2 is generated as a main oxidant at neutral and alkaline condition through the energy-transfer process. Therefore, the photochemical oxidation can be switchable between •OH-driven and 1O2-driven mechanism by a simple pH adjustment.

Design and Economic Assessment of Alternative Evaporation Processes for Poly-Lactic Acid Production.

In this work, alternative evaporation processes for PLA production were designed with economic assessment. The suggested processes are the multiple-effect evaporation (MEE) process and thermal vapor recompression (TVR)-assisted evaporation process. First, the MEE process can efficiently reuse waste heat by additional column installation, thereby reducing the steam energy consumption. The proposed MEE process involves five columns, and after the evaporation in each column, the waste heat of the emitted vapor is reused to heat steam in the reboiler of the next column. Second, the suggested TVR-assisted evaporation process utilizes an additional steam ejector and recovers waste heat from the emitted vapor by increasing the pressure using high-pressure driving steam at the steam ejector. Each alternative process was modeled to predict the steam energy consumption, and to determine the cost-optimal process; the total annualized cost (TAC) of each alternative process was calculated as evaluation criteria. In the simulation results, the alternative processes using MEE and TVR reduced the steam consumption by 71.36% and 89.97%, respectively, compared to the conventional process. As a result of economic assessment, the cost-optimal process is the alternative process using TVR and the TAC can be decreased by approximately 90%.

Peroxymonosulfate activation by black TiO2 nanotube arrays under solar light: Switching the activation mechanism and enhancing catalytic activity and stability.

Black TiO2 nanotube arrays (black TNAs) suffer from the low activity and deactivation for peroxymonosulfate (PMS) activation, which limit their application in the oxidative destruction of organic pollutants in water. Here, we report an efficient, environmentally benign, and cost-effective method to enhance the catalytic activity and prevent the deactivation of black TNAs in PMS activation by utilizing solar energy. Optical absorption and electrochemical analysis and density functional theory calculations demonstrated that abundant oxygen vacancies (estimated to be 26%) on the black TNAs surface markedly improved solar light absorption and electrical conductivity and played a critical role as a catalytic active site for PMS activation. As a result, the solar light-irradiated black TNAs/PMS system exhibited the higher phenol degradation rate (k = 0.0488 min-1) and total organic carbon (TOC) removal efficiency (~70%) compared to other TNAs systems. These results were ascribed to the switching of the reaction mechanism from non-radical mechanism to radical-involved. Black TNAs oxidized organic pollutants by mediating electron transfer from organics to PMS in the dark (i.e., a non-radical pathway). On the other hand, PMS activation under solar light irradiation involved the production of highly reactive sulfate and hydroxyl radicals (i.e., radical pathway), markedly improving the degradation and mineralization of organics. Additionally, the solar light-irradiated black TNAs showed relative pH-independence for PMS activation and durable catalytic performance without the loss of activity during the repetitive reaction cycles.

Computational fluid dynamics-based optimal installation strategy of air purification system to minimize NOX exposure inside a public bus stop.

At public bus stops, NOX pollutants discharged by regularly stopping buses quickly accumulate, exposing waiting passengers to high levels of air pollutants, which creates a threat to public health. The environmental protection agency (EPA) presents air quality standards for NOX, a significant pollutant that causes lung diseases such as asthma when exposed to the human body. To handle this problem, air purification systems are installed inside bus stops in many public places. However, it is challenging to maintain a low concentration of NOX inside public bus stops due to the persistent inflow of bus exhaust gas. Therefore, it is crucial to design an optimal location for an air purification system to meet air environment standards for respiratory areas. This study proposed a computational fluid dynamics (CFD)-based optimal installation strategy for an air purification system to minimize NOX exposure inside a public bus stop. The CFD model was developed to numerically analyze NO2 exposure with the actual design value for a public bus stop in Ulsan, South Korea. The local NO2 concentration was evaluated in the human breathing zone. The case study was performed according to the locations of the inlet and outlet of the air purification system. A transient CFD simulation was performed to analyze the effect of the air purification system on pollutants generated from the stationary bus by time flow in various cases. NO2 concentration and exposure reduction effectiveness (ERE) were analyzed and compared for each case in the breathing zone. In the optimal case, the ERE of NO2 was confirmed to be 35.9 %, and the NO2 concentration according to the air quality standards of EPA could be maintained at 0.1 ppm or less. The theoretical framework proposed in this study can be generalized to design air purification systems for general external facilities.

Investigating the Unrevealed Photocatalytic Activity and Stability of Nanostructured Brookite TiO2 Film as an Environmental Photocatalyst.

Among three polymorphs of TiO2, the brookite is the least known phase in many aspects of its properties and photoactivities (especially comparable to anatase and rutile) because it is the rarest phase to be synthesized in the standard environment among the TiO2 polymorphs. In this study, we address the unrevealed photocatalytic properties of pure brookite TiO2 film as an environmental photocatalyst. Highly crystalline brookite nanostructures were synthesized on titanium foil using a well-designed hydrothermal reaction, without harmful precursors and selective etching of anatase, to afford pure brookite. The photocatalytic degradation of rhodamine B, tetramethylammonium chloride, and 4-chlorophenol on UV-illuminated pure brookite were investigated and compared with those on anatase and rutile TiO2. The present research explores the generation of OH radicals as main oxidants on brookite. In addition, tetramethylammonium, as a mobile OH radical indicator, was degraded over both pure anatase and brookite phases, but not rutile. The brookite phase showed much higher photoactivity among TiO2 polymorphs, despite its smaller surface area compared with anatase. This result can be ascribed to the following properties of the brookite TiO2 film: (i) the higher driving force with more negative flat-band potential, (ii) the efficient charge transfer kinetics with low resistance, and (iii) the generation of more hydroxyl radicals, including mobile OH radicals. The brookite-nanostructured TiO2 electrode facilitates photocatalyst collection and recycling with excellent stability, and readily controls photocatalytic degradation rates with facile input of additional potential.