Visible-Light Photocatalytic CO2-to-CO and H2O-to-H2O2 by g-C3N4/Cu2O-Pd S-Scheme Heterojunctions.

Visible-light photocatalytic conversion of CO2-to-fuels for green electricity is sustainably attractive for alleviating carbon emissions. Photocatalytic CO2-to-CO frequently suffered from relatively low yields, mainly due to ineffective charge transfer rates. A new approach for photocatalytic CO2-to-CO enhanced with effective H+ from H2O-to-H2O2 through the water oxidation reaction (WOR) has been studied in the present work. Here, the nano palladium (9 wt %), serving as a cocatalyst, dispersed on the g-C3N4/Cu2O heterojunctions (i.e., g-C3N4/Cu2O-Pd) has been prepared to facilitate charge separation for the two-electron reduction of CO2 to CO. Experimentally, the g-C3N4/Cu2O-Pd heterojunctions have a higher photocatalytic H2O-to-H2O2 yield than the g-C3N4/Cu2O heterojunction by 5.3 times. The photocatalytic WOR provides sufficient electrons (e-) and H+ (2H2O → H2O2 + 2H+) for CO2-to-CO (CO2(aq) + 2H+ + 2e- → CO(g) + H2O(l)). Relatively high photocatalytic yields of H2O2 (34.0 µmol/mg) and CO (14.6 µmol/mg) affected by the heterojunctions can be achieved. Also, the heterojunctions have a high photostability with a photocatalytic generated CO/H2 ratio of 1.75 approximately. This visible-light photocatalytic CO2-to-CO and H2O-to-H2O2 by the new g-C3N4/Cu2O-Pd S-scheme heterojunctions demonstrates the feasibility of the zero carbon emission approach with additional green oxidant (H2O2) generation.

Pseudocapacitive Deionization of Saltwater by Mn3O4@C/Activated Carbon.

Capacitive deionization (CDI), a m ethod with notable advantages of relatively low energy consumption and environmental friendliness, has been widely used in desalination of saltwater. However, due to the weak electrical double-layer electrosorption of ions from water, CDI has suffered from low throughput capacity that may limit its commercial applications. Thus, it is of importance to develop a high-efficiency and engineering-feasible CDI process. Manganese and cobalt and their oxides, being faradic materials, have a relatively high pseudocapacitance, which can cause an increase of positive and negative charges on opposing electrodes. However, their low conductivity properties limit their electrochemical applications. Pseudocapacitive Mn3O4 nanoparticles encapsulated within a conducting carbon shell (Mn3O4@C) were prepared to enhance charge transfer and capacitance for CDI. Desalination performances of the Mn3O4@C (5-15 wt %) core-shell nanoparticles on activated carbon (AC) (Mn3O4@C/AC) serving as CDI electrodes have thus been investigated. The pseudocapacitive Mn3O4@C/AC electrodes with relatively low diffusion resistances have much greater capacitance (240-1300 F/g) than the pristine AC electrode (120 F/g). In situ synchrotron X-ray absorption near-edge structure spectra of the Mn3O4@C/AC electrodes during CDI (under 1.2 and -1.2 V for electrosorption and regeneration, respectively) demonstrate that reversible faradic redox reactions cause more negative charges on the negative electrode and more positive charges on the positive electrode. Consequently, the pseudocapacitive electrodes for CDI of saltwater ([NaCl] = 1000 ppm) show much better desalination performances with a high optimized salt removal (600 mg/g·day), electrosorption efficiency (48%), and electrosorption capacity (EC) (25 mg/g) than the AC electrodes (288 mg/g·day, 23%, and 12 mg/g, respectively). The Mn3O4@C/AC electrode has a maximum EC of 29 mg/g for CDI under +1.2 V. Also, the Mn3O4@C/AC electrodes have much higher pseudocapacitive electrosorption rate constants (0.0049-0.0087 h-1) than the AC electrode (0.0016 h-1). This work demonstrates the feasibility of high-efficiency CDI of saltwater for water recycling and reuse using the low-cost and easily fabricated pseudocapacitive Mn3O4@C/AC electrodes.

High-Temperature Syngas Desulfurization and Particulate Filtration by ZnO/Ceramic Filters.

Combustible gas (e.g., gasification syngas) cleaning at high temperatures can obtain further gains in energy efficiency for power generation and importantly leads to a simplified process and lower cost as a commercially viable source of clean energy. Thus, a feasibility study for high-temperature desulfurization (HTDS) and additional high-temperature particulate filtration (HTPF) of a raw syngas using ZnO sorbent-dispersed Raney CuO (ZnO/R-CuO) and ceramic filter (ZnO/CF) has been carried out. By synchrotron X-ray absorption near-edge structure (XANES) spectroscopy, mainly Zn(II) and Cu(II) are found in the ZnO/R-CuO sorbents. Both ZnO and R-CuO in the sorbents are involved in HTDS (1% H2S) at 873 K to form ZnS, Cu2S, and a small amount of CuS and reach relatively high HTDS efficiencies (82-90%). In addition, regeneration of the sulfurized sorbent by oxidation with O2 at 873 K (HTRG) for 1 h can restore ZnO and CuO for continuous and repetitive HTDS-HTRG cycles. To facilitate the HTDS engineering applications by the ZnO/R-CuO sorbents, their reaction rate constant (8.35 × 104 cm3/g/min) and activation energy (114.8 kJ/mol) at 873 K have also been determined. Furthermore, the ZnO/CF sorbent/filter can perform HTDS and additional HTPF at 873 K with very high particulate removal efficiencies (>98%). This demonstrates the feasibility for hot-syngas cleaning with a much better energy efficiency and lesser cost for cleaner power generation.

XANES/EXAFS and quantum chemical study of the speciation of arsenic in the condensate formed in landfill gas processing: Evidence of the dominance of As-S species.

The XANES/EXAFS data and quantum chemical simulations presented in this study demonstrate several features of the chemistry of arsenic compounds found in the condensates and solids generated in landfill gas (LFG) processing carried out for renewable natural gas (RNG) production. The XANES data show the decrease in the position of the absorption edge of As atoms, similar to that characteristic for sulfur-containing As solutes and solids. The EXAFS data show that the As-O and As-S distances in these matrixes are similar to those in thioarsenates. Quantum-chemical calculations demonstrated the close agreement between the experimental and modeled As-S and As-O distances determined for a range of methylated and thiolated arsenic solutes. These calculations also showed that the increase of the number of the As-S bonds in the coordination shell of arsenic is accompanied by a consistent decrease of the charges of As atoms. This decrease is correlated with the number of the As-S bonds, in agreement with the trend observed in the XANES data. These results provide insight into the intrinsic chemistry and reactivity of As species present in LFG matrixes; they may be helpful for the development of treatment methods to control arsenic in these systems.

Sulfonated GO coated carbon electrodes with cation-selective functions for enhanced capacitive deionization of saltwater.

Deionization of salt, contaminated underground and inorganic waste waters for water recycling and reuse is of increasing importance mainly due to the shortage of freshwater worldwide. Membrane capacitive deionization (MCDI) possessing a high electrosorption capacity and energy efficiency has been considered a promising method for desalination. However, the MCDI reaction system has limited applications because of the high interfacial resistance during operation. In the present work, the novel sulfonated graphene oxide (SGO) serving as a hydrophilic cation exchange membrane that was coated directly on the activated carbon (AC) electrode was prepared to enhance capacitive deionization of saltwater. Experimentally, the electrosorption capacity and charge efficiency of the AC/SGO (negative)||AC (positive) electrode pair using the coated SGO thin film increased from 12.8 to 19.8 mg/g and 56.7 to 89.3%, respectively. The enhancements were associated with the reduction of the co-ion effect during electrosorption. The strong negative PhSO3- group grafted on the SGO thin film could selectively accelerate the transport rate of cations during CDI. The increase of the charge efficiency also led to lower implemented current. This work demonstrates a simple, low-cost and effective desalination method that will likely have many new applications especially in water recycling and reuse.

Visible-Light Driven H2O-to-H2O2 Reaction by Nitrogen-Enriched Resins for Photocatalytic Oxidation of an Organic Pollutant in Wastewater.

A photocatalytic H2O-to-H2O2 reaction for sustainable organic wastewater treatment is environmentally attractive. Phenolic resins, inexpensive metal-free photocatalysts, are capable of harvesting visible light. Herein, novel nitrogen-enriched resin photocatalysts with a desired band-gap energy (1.83-1.98 eV) for harvesting visible light were prepared by copolymerization of resorcinol and melem for simultaneous photocatalytic H2O-to-H2O2 and oxidation of methylene blue. Under visible light irradiation for 5 h, very high yields of H2O2 (870-975 µM of H2O2/g/h) by RFM resin photocatalysts could be achieved. The photocatalytic H2O2 for reactive oxygen species (•OH) and photogenerated h+ could account for high conversion (40% conversion under visible light irradiation within 3 h) in oxidation of methylene blue. Such unique low-cost metal-free resins demonstrate the visible light photocatalytic H2O-to-H2O2 reaction which can synergize with the oxidation of organic pollutants in wastewater.

In situ X-ray absorption spectroscopic studies of TiO2 photocatalytic active sites for degradation of trace CHCl3 in drinking water.

Toxic disinfection byproducts such as trihalomethanes (e.g. CHCl3) are often found after chlorination of drinking water. It has been found that photocatalytic degradation of trace CHCl3 in drinking water generally lacks an expected relationship with the crystalline phase, band-gap energy or the particle sizes of the TiO2-based photocatalysts used such as nano TiO2 on SBA-15 (Santa Barbara amorphous-15), TiO2 clusters (TiO2-SiO2) and atomic dispersed Ti [Ti-MCM-41 (Mobil Composition of Matter)]. To engineer capable TiO2 photocatalysts, a better understanding of their photoactive sites is of great importance and interest. Using in situ X-ray absorption near-edge structure (XANES) spectroscopy, the A1 (4969 eV), A2 (4971 eV) and A3 (4972 eV) sites in TiO2 can be distinguished as four-, five- and six- coordinated Ti species, respectively. Notably, the A2 Ti sites that are the main photocatalytic species of TiO2 are shown to be accountable for about 95% of the photocatalytic degradation of trace CHCl3 in drinking water (7.2 p.p.m. CHCl3 gTiO2-1 h-1). This work reveals that the A2 Ti species of a TiO2-based photocatalyst are mainly responsible for the photocatalytic reactivity, especially in photocatalytic degradation of CHCl3 in drinking water.

In situ X-ray absorption spectroscopic studies of photocatalytic oxidation of As(III) to less toxic As(V) by TiO2 nanotubes.

Arsenic in groundwater caused the black-foot disease (BFD) in many countries in the 1950-1960s. It is of great importance to develop a feasible method for removal of arsenic from contaminated groundwater in BFD endemic areas. Photocatalytic oxidation of As(III) to less toxic As(V) is, therefore, of significance for preventing any arsenic-related disease that may occur. By in situ synchrotron X-ray absorption spectroscopy, the formation of As(V) is related to the expense of As(III) disappearance during photocatalysis by TiO2 nanotubes (TNTs). Under UV/Vis light irradiation, the apparent first-order rate constant for the photocatalytic oxidation of As(III) to As(V) is 0.0148 min-1. It seems that As(III) can be oxidized with photo-excited holes while the not-recombined electrons may be scavenged with O2 in the channels of the well defined TNTs (an opening of 7 nm in diameter). In the absence of O2, on the contrary, As(III) can be reduced to As(0), to some extent. Cu(II) (CuO), as an electron acceptor, was impregnated on the TNTs surfaces in order to gain a better understanding of electron transfer during photocatalysis. It appears that As(III) can be oxidized to As(V) while Cu(II) is reduced to Cu(I) and Cu(0). The molecular-scale data are very useful in revealing the oxidation states and interconversions of arsenic during the photocatalytic reactions. This work has implications in that the toxicity of arsenic in contaminated groundwater or wastewater can be effectively decreased via solar-driven photocatalysis, which may facilitate further treatments by coagulation.

Capacitive deionization of a RO brackish water by AC/graphene composite electrodes.

A feasibility study for water recycling and reuse of a reverse osmosis (RO) brackish wastewater by capacitive deionization (CDI) was carried out in the present work. Palm-shell wastes enriched in carbon was recycled to yield valuable activated carbon (AC) that has advantages of high surface area, high specific capacitance, and low electrical resistance as the CDI electrodes. The GAC prepared by dispersion of AC in the graphene (rGO) layers has a high surface area and electrical conductivity for CDI. The GAC electrodes have increasing electrosorption efficiencies from 1.6 to 3.0% during the repeated electrosorption-regeneration cycles under +1.2 â†’ 0 → +1.2 V while the efficiencies the AC electrodes decrease from 2.7 to 1.6%. It is clear that the GAC-based electrodes have a better electrosorption efficiency and stability in, for example, the three repeated electrosoption-regeneration cycles for CDI of the wastewater. This work also exemplifies that the AC recycled from biomass such as palm-shell wastes can be used in CDI electrodes for recycling and reuse of wastewater.

Dual Alkali Solvent System for CO2 Capture from Flue Gas.

A novel two-aqueous-phase CO2 capture system, namely the dual alkali solvent (DAS) system, has been developed. Unlike traditional solvent-based CO2 capture systems in which the same solvent is used for both CO2 absorption and stripping, the solvent of the DAS system consists of two aqueous phases. The upper phase, which contains an organic alkali 1-(2-hydroxyethyl) piperazine (HEP), is used for CO2 absorption. The lower phase, which consists of a mixture of K2CO3/KHCO3 aqueous solution and KHCO3 precipitate, is used for CO2 stripping. Only a certain kind of amine (such as HEP) is able to ensure the phase separation, satisfactory absorption efficiency, effective CO2 transfer from the upper phase to the lower phase, and regeneration of the upper phase. In the meantime, due to the presence of K2CO3/KHCO3 in the lower phase, HEP in the upper phase is capable of being regenerated from its sulfite/sulfate heat stable salt, which enables the simultaneous absorption of CO2 and SO2/SO3 from the flue gas. Preliminary experiments and simulations indicate that the implementation of the DAS system can lead to 24.0% stripping energy savings compared to the Econamine process, without significantly lowering the CO2 absorption efficiency (∼90%).

Lithium recovery with LiTi2O4 ion-sieves.

A feasibility study for the recovery of lithium from salt water with the protonated lithium titanium oxide ion-sieves was carried out in this work. Lithium ions (Li+) in LiTi2O4 having a similar ion density with H+ allow repeated exchanges and regeneration with high selectivity. By Li7 magic angle spinning solid-state magnetic resonance, it is apparent that chemical structure of lithium in the ion-sieves is not perturbed during the repeated Li+/H+ exchange processes. As the dissolution of titanium is negligible (<0.1%), the secondary contamination during the capture process can be minimized. The ion-sieves exhibit lithium capture capacities of up to 9.5mg/g during the repeated Li+/H+ exchanges with H0.23Li0.77Ti2O4/LiTi2O4 for 24h, and the captured Li+ may be recovered in the form of Li2CO3. Accordingly, the lithium capture method developed in this work could be integrated with current desalination processes for valuable lithium recovery.

Photocatalytic reduction of CO2with SiC recovered from silicon sludge wastes.

In the present study, silicon carbide (SiC) recovered from silicon sludge wastes is used as catalysts for photocatalytic reduction of CO2. By X-ray diffraction, it is clear that the main components in the silicon sludge wastes are silicon and SiC. The grain size of the SiC separated from the sludge waste is in the range of 10-20 µm in diameter (observed by scanning electron microscopy). By solid state nuclear magnetic resonance, it is found that α-SiC is the main crystallite in the purified SiC. The α-SiC has the band-gap of 3.0 eV. To yield C1-C2chemicals from photocatalytic reduction of CO2, hydrogen is provided by simultaneous photocatalytic splitting of H2O. Under the light (253-2000 nm) illumination, 12.03 and 1.22 µmol/h g cat of formic and acetic acids, respectively, can be yielded.

Selective leaching process for the recovery of copper and zinc oxide from copper-containing dust.

The purpose of this study was to develop a resource recovery procedure for recovering copper and zinc from dust produced by copper smelting furnaces during the manufacturing of copper-alloy wires. The concentrations of copper in copper-containing dust do not meet the regulation standards defined by the Taiwan Environmental Protection Administration; therefore, such waste is classified as hazardous. In this study, the percentages of zinc and copper in the dust samples were approximately 38.4% and 2.6%, respectively. To reduce environmental damage and recover metal resources for industrial reuse, acid leaching was used to recover metals from these inorganic wastes. In the first stage, 2 N of sulphuric acid was used to leach the dust, with pH values controlled at 2.0-3.0, and a solid-to-liquid ratio of 1:10. The results indicated that zinc extraction efficiency was higher than 95%. A selective acid leaching process was then used to recover the copper content of the residue after filtration. In the second stage, an additional 1 N of sulphuric acid was added to the suspension in the selective leaching process, and the pH value was controlled at 1.5-2.0. The reagent sodium hydroxide (2 N) was used as leachate at a pH greater than 7. A zinc hydroxide compound formed during the process and was recovered after drying. The yields for zinc and copper were 86.9-93.5% and 97.0-98.9%, respectively.

Conducting glasses recovered from thin film transistor liquid crystal display wastes for dye-sensitized solar cell cathodes.

Transparent conductive glasses such as thin film transistor (TFT) array and colour filter glasses were recovered from the TFT-liquid crystal display panel wastes by dismantling and sonic cleaning. Noble metals (i.e. platinum (Pt)) and indium tin oxide (ITO) are generally used in the cathode of a dye-sensitized solar cell (DSSC). To reduce the DSSC cost, Pt was replaced with nano nickel-encapsulated carbon-shell (Ni@C) nanoparticles, which were prepared by carbonization of Ni²âº-ß-cyclodextrin at 673 K for 2 h. The recovered conductive glasses were used in the DSSC electrodes in the substitution of relatively expensive ITO. Interestingly, the efficiency of the DSSC having the Ni@C-coated cathode is as high as 2.54%. Moreover, the cost of the DSSC using the recovered materials can be reduced by at least 24%.

Capacitive deionization of seawater effected by nano Ag and Ag@C on graphene.

Drinking water shortage has become worse in recent decades. A new capacitive deionization (CDI) method for increasing water supplies through the effective desalination of seawater has been developed. Silver as nano Ag and Ag@C which was prepared by carbonization of the Ag(+)-ß-cyclodextrin complex at 573 K for 30 min can add the antimicrobial function into the CDI process. The Ag@C and Ag nanoparticles dispersed on reduced graphene oxide (Ag@C/rGO and nano Ag/rGO) were used as the CDI electrodes. The nano Ag/rGO and Ag@C/rGO electrodes can reduce the charging resistant, and enhance the electrosorption capability. Better CDI efficiencies with the nano Ag/rGO and Ag@C/rGO electrodes can therefore be obtained. When reversed the voltage, the electrodes can be recovered up to 90% within 5 min. This work presents the feasibility for the nano Ag and Ag@C on rGO electrodes applied in CDI process to produce drinking water from seawater or saline water.

Photocatalytic splitting of seawater effected by (Ni-ZnO)@C nanoreactors.

Novel photocatalysts i.e., metallic nickel and zinc oxide nanoparticles embedded in the carbon-shell ((Ni-ZnO)@C) have been used for photocatalytic splitting of seawater to generate H2. The (Ni-ZnO)@C core-shell nanoparticles having the Zn/Ni ratios of 0-3 were prepared by carbonization of Ni(2+)- and Zn(2+)-ß-cyclodextrin at 673 K for 2 h. To increase the collision frequency of water and photoactive sites within the carbon-shell, Ni and ZnO are partially etched from the (Ni-ZnO)@C core-shell to form yolk-shell nanoparticles with a H2SO4 solution (2N). By X-ray diffraction spectroscopy, mainly Ni and ZnO crystallites are observed in the core- and yolk-shell nanoparticles. The sizes of the Ni and ZnO in the (Ni-ZnO)@C nanoreactors are between 7 and 23 nm in diameters determined by TEM and small angel scattering spectroscopy. Under a 5-h UV-Vis light irradiation, 5.01 µmol/hgcat of H2 are yielded from photocatalytic splitting of seawater effected by (Ni-ZnO)@C nanoreactors.

Detoxification of hazardous dust with marine sediment.

Hazardous electric arc furnace dust containing dioxins/furans and heavy metals is blended with harbor sediment, fired at 950-1100 °C to prepare lightweight aggregates. Dust addition can lower the sintering temperature by about 100 °C, as compared to a typical industrial process. After firing at 950 °C and 1050 °C, more than 99.85% of dioxins/furans originally present in the dust have been removed and/or destructed in the mix containing a dust/sediment ratio of 50:100. The heavy metals leached from all fired mixes are far below Taiwan EPA legal limits. The particle density of the lightweight aggregates always decreases with increasing firing temperature. Greater addition of the dust results in a considerably lower particle density (mostly <2.0 g cm(-3)) fired at 1050 °C and 1100 °C. However, firing at temperatures lower than 1050 °C produces no successful bloating, leading to a denser particle density (>2.0 g cm(-3)) that is typical of bricks.

Recycling steel-manufacturing slag and harbor sediment into construction materials.

Mixtures consisting of harbor sediment and slag waste from steel industry containing toxic components are fired to produce non-hazardous construction materials. The fired pellets become lighter as firing temperature increases. At a sintering temperature of ≦1050°C, the fired pellets are in a form of brick-like product, while at 1100°C, they become lightweight aggregates. Calcium silicate, kyanite, and cristobalite are newly formed in the pellets after firing, demonstrating that calcium oxide acts as a flux component and chemically reacted with Si- and/or Al-containing components to promote sintering. Dioxin/furan content present in the pure slag is 0.003ng I-TEQg(-1) and, for the fired pellet consisting of slag and sediment, the content appears to be destructed and diminishes to 0.0003ng I-TEQg(-1) after 950°C-firing; while it is 0.002ng I-TEQg(-1) after firing at 1100°C, suggesting that dioxins/furans in the 950°C-fired pellets have a greater chance to escape to atmosphere due to a slower sintering reaction and/or that construction of dioxins/furans from molten chloride salts co-exists with their destruction. Multiple toxicity characteristic leaching procedure extracts Cu, Cr, Zn, Se, Cd, Pb, Ba, As, and Hg from all fired products at negligible levels.

Tracking of arsenics during the thermal stabilization of an AsH3 scrubber sludge.

Toxic arsenics in an AsH(3) scrubber sludge were thermally stabilized in the temperature range of 973-1,373 K. To better understand how the high-temperature treatments can stabilize arsenics in the sludge, their synchrotron X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra of arsenics were determined. It is found that the reduced arsenic leachability may be associated with the formation of As(2)O(5) (51-59%) and embedded As(V) within the Ca(3)(PO(4))(2) matrix (41-49%) in the stabilized sludge. In addition, the As-O bond distances in the stabilized As(2)O(5) are much less than that of normal As(2)O(5) by 0.05-0.07 Å. The shorter As-O bond distances accompanied with the higher bonding energy also have a contribution to the thermal stabilization of arsenics.

Preparation of magnetic recoverable nanosize Cu-Fe2O3/Fe photocatalysts.

Iron based catalysts generally have the advantage of the easily operated magnetically recovery from application sites. In the present work, paramagnetic iron and copper core-shell nanoparticles having the iron fractions (X(Fe) = Fe/(Cu+Fe)) of 0.33-1.0 were prepared and characterized by in situ synchrotron X-ray absorption and scattering spectroscopy. During the temperature-programmed carbonization (TPC) of Cu(2+)- and Fe(3+)-ß-cyclodextrin (CD) complexes, a rapid reduction of Cu(II) occurs at about 453 K together with a growth of the metallic copper (Cu). Iron proceeds in the distinct growth path. At 453-513 K, the Fe(III) → Fe(II) → Fe consecutive reduction is observed. The unreduced Fe(III) (7-13%) is coated on the surfaces of the Fe nanoparticles (as Fe2O3/Fe). Growth of the Fe nanoparticle is inhibited by the surface Fe2O3, while the steady growth in Cu is observed. The Cu has a size range of 14-18 nm in diameter, compared to the small Fe2O3/Fe ones (3-6 nm). Under the UV-visible light irradiation for four hours, methylene blue can be photocatalytically degraded (>90%) by the (Cu-Fe2O3/Fe)@C. The (Cu-Fe2O3/Fe)@C photocatalysts can effectively oxidize dye molecules, providing a promising alternative for dye degradation using solar energy. Recovery of the (Cu-Fe2O3/Fe)@C photocatalysts can be attained by applying external magnetic field to trap the ferromagnetic Cu-Fe2O3/Fe nanoparticles, which suggests an economically attractive process, especially applied in photocatalytic degradation of dye-contaminated wastewater.