Decomposition of SF6 in an RF plasma environment

Sulfur hexafluoride (SFd)-contained gas is a common pollutant emitted during the etching process used in the semiconductor industry. This study demonstrated the application of radio-frequency (RF) plasma in the decomposition of SF6. The decomposition fraction of SF6 [etaSF6 (C(in)-C(out))/C(in) x 100%] and the mole fraction profile of the products were investigated as functions of input power and feed O2/SF6 ratio in an SiO2 reactor. The species detected in both SF6/Ar and SF6/O2/ Ar RF plasmas were SiF4, SO2, Fe2, SO2F2, SOF2, SOF4, S2F10, S2OF10, S2O2F10, and SF4. The results revealed that at 40 W, etaSF6 exceeded 99%, and the reaction products were almost all converted into stable compounds such as SiF4, SO2, and F2 with or without the addition of oxygen. Sulfur oxyfluorides such as SO2F2, SOF2, SOF4, S2OF10, and S2O2F10 were produced only below 40 W. The results of this work can be used to design a plasma/chemical system for online use in a series of a manufacturing process to treat SF6-containing exhaust gases.     

Abatement of sulfur hexafluoride emissions from the semiconductor manufacturing process by atmospheric-pressure plasmas

Sulfur hexafluoride (SF6) is an important gas for plasma etching processes in the semiconductor industry. SF6 intensely absorbs infrared radiation and, consequently, aggravates global warming. This study investigates SF6 abatement by nonthermal plasma technologies under atmospheric pressure. Two kinds of nonthermal plasma processes--dielectric barrier discharge (DBD) and combined plasma catalysis (CPC)--were employed and evaluated. Experimental results indicated that as much as 91% of SF6 was removed with DBDs at 20 kV of applied voltage and 150 Hz of discharge frequency for the gas stream containing 300 ppm SF6, 12% oxygen (O2), and 40% argon (Ar), with nitrogen (N2) as the carrier gas. Four additives, including Ar, O2, ethylene (C2H4), and H2O(g), are effective in enhancing SF6 abatement in the range of conditions studied. DBD achieves a higher SF6 removal efficiency than does CPC at the same operation condition. But CPC achieves a higher electrical energy utilization compared with DBD. However, poisoning of catalysts by sulfur (S)-containing species needs further investigation. SF6 is mainly converted to SOF2, SO2F4, sulfur dioxide (SO2), oxygen difluoride (OF2), and fluoride (F2). They do not cause global warming and can be captured by either wet scrubbing or adsorption. This study indicates that DBD and CPC are feasible control technologies for reducing SF6 emissions.     

Catalytic oxidation of gaseous PCDD/Fs with ozone over iron oxide catalysts

Catalytic oxidation of PCDD/Fs (polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans) with ozone (catalytic ozonation) over nano-sized iron oxides (denoted as FexOy) was carried out at temperature of 120-180 degrees C. The effects of operating temperature, ozone concentration, space velocity (SV) and water vapor contents on PCDD/F removal and destruction efficiencies via catalytic ozonation were investigated. High activity of the iron oxide catalyst towards PCDD/F decomposition was observed even at low temperatures with the aid of ozone. The PCDD/F removal and destruction efficiencies achieved with FexOy/O3 at 180 degrees C reach 94% and 91%, respectively. In the absence of ozone, the destruction efficiencies of all PCDD/F congeners are below 20% and decrease with increasing chlorination level of PCDD/F congener at lower temperature (120 degrees C). However, in the presence of ozone, the destruction efficiencies of all PCDD/F congeners are over 80% on FexOy/O3 at 180 degrees C. Higher temperature and ozone addition increase the activity of iron oxide for the decomposition of PCDD/Fs. Additionally, in the presence of 5% water vapor, the destruction efficiency of the PCDD/Fs is above 90% even at lower operating temperature (150 degrees C). It indicates that the presence of appropriate amount of water vapor enhances the catalytic activity for the decomposition of gas-phase PCDD/Fs.     

Total toxicity equivalents emissions of SF6, CHF3, and CCl2F2 decomposed in a RF plasma environment

Sulfur hexafluorine compound (SF6), trifluoromethane (CHF3) and diclorodifluoromethane (CCl2F2) are extensively used in the semiconductor industry. They are global warming gases. Most studies have addressed the effective decomposition of fluorine compounds, rather than the toxicity of decomposed by-products. Hence, the concepts of toxicity equivalents (TEQs) were applied in this work. The results indicated that HF and SiF4 were the two greatest contributors of TEQ to the SF6/H2/Ar plasma system, while F2 and SiF4 were the two greatest contributors to the SF6/O2/Ar system. Additionally, SiF4 and HF were the two greatest contributors of TEQ to both the CHF3/H2/Ar and CHF3/O2/Ar plasma systems. HF and HCl were the two greatest contributors of TEQ to the CCl2F2/H2/Ar plasma system, and Cl2 and COCl2 were the two greatest contributors to the CCl2F2/O2/Ar system. HCl and HF can be recovered using wet scrubbing, which reduces the toxicity of these emission gases. Consequently, the hydrogen-based plasma system was a better alternative for treating gases that contained SF6, CHF3 and CCl2F2 from the TEQs point of view.     

Yield and nutritive quality of sericea lespedeza (Lespedeza cuneata) and little bluestem (Schizachyrium scoparium) exposed to ground-level ozone

Sericea lespedeza (Lespedeza cuneata cv. Interstate 76) and little bluestem (Schizachyrium scoparium cv. Aldous) were raised from seed in a glasshouse, transplanted into 5.7-l pots and placed into open-top chambers (OTC) on 6 June 1999. Following a 7-day adjustment period, each of six OTCs (duplicate OTCs per treatment) was ventilated with either air that had been carbon-filtered (CF) to remove ambient ozone (O3); non-filtered (NF), representative of ambient air; or enriched to twice-ambient O3 concentration (2X). Primary-growth forage was harvested on days 7, 32, 46, 59 and 72 following the start of fumigation, and regrowth forage from the first primary-growth harvest was harvested on days 36, 54 and 72 following the start of fumigation. Dry matter (DM) yield of either forage species did not differ among treatments except in the final regrowth period when yield of sericea lespedeza was greater for the NF than 2X O3 treatment. In vitro DM digestibility (IVDMD) and concentrations of crude protein (CP), soluble phenolics (SP) and condensed tannins (CT) in primary-growth sericea lespedeza did not differ between treatments, but NF primary-growth forage had higher concentration of protein-precipitating tannins (PPT) than did 2X primary-growth forage. Concentrations of neutral detergent fiber (NDF), acid detergent fiber (ADF) and acid detergent lignin (ADL) were higher in NF and 2X than in CF primary-growth sericea lespedeza. Similarly, concentrations of NDF and ADL were higher, and IVDMD was lower for NF and 2X than for CF regrowth sericea lespedeza. Concentrations of ADF and ADL were lower, whereas IVDMD, in vitro NDF digestibility (IVNDFD) and concentrations of CP and SP were greater, in CF than in NF and 2X primary-growth little bluestem. Percentages IVDMD and IVNDFD and concentrations of CP and SP in NF primary-growth little bluestem were greater than those in forage exposed to 2X O3 treatment. No significant differences were observed among treatments in percentages IVDMD and IVNDFD, or concentrations of cell wall constituents or SP in little bluestem regrowth. Nutritive quality of little bluestem was decreased by < 2%, and that of sericea lespedeza by approximately 7% as a result of increased concentrations of cell wall constituents and decreased in vitro digestibility of NF and 2X compared with CF forages. Results indicate that existing and projected O3 levels can drive alterations in forage quality of select warm-season forages sufficient to have nutritional and economic implications for their utilization by ruminant herbivores.     

Relationship of Destruction Parameters to the Destruction/Removal Efficiency of PCBs

The destruction/removal efficiency (DRE), and the ability to accurately measure it, is a function of the concentration of the chemical compound in the input waste, the incinerator design and operation, sampling methods, and the analytical procedures. All of these are interrelated. This paper discusses the basic DRE equation [DRE = W_In W_out)/W_in × 100] and how it relates to some of the other destruction parameters. Some example data from the literature are presented. While PCBs have been used as the example, the equations and graphs are equally valid for some other hazardous compounds (POHCs), with the substitution of the 99.99% DRE requirement in lieu of the 99.9999% DRE for PCBs. The use of the relationships discussed in this paper should allow incinerator operators to more efficiently plan demonstration test burns which will adequately demonstrate the DRE.     

Solar TiO2-assisted photocatalytic degradation of IGCC power station effluents using a Fresnel lens

The heterogeneous TiO2 assisted photocatalytic degradation of wastewater from a thermoelectric power station under concentrated solar light irradiation using a Fresnel lens has been studied. The efficiency of photocatalytic degradation was determined from the analysis of cyanide and formate removal. Firstly, the influence of the initial concentration of H2O2 and TiO2 on the degradation kinetics of cyanides and formates was studied based on a factorial experimental design. Experimental kinetic constants were fitted using neural networks. Results showed that the photocatalytic process was effective for cyanides destruction (mainly following a molecular mechanism), whereas most of formates (degraded mainly via a radical path) remained unaffected. Finally, to improve formates degradation, the effect of lowering pH on their degradation rate was evaluated after complete cyanide destruction. The photooxidation efficiency of formates reaches a maximum at pH around 5-6. Above pH 6, formate anion is subjected to electrostatic repulsion with the negative surface of TiO2. At pH<4.5, formate adsorption and photon absorption are reduced due to some catalyst agglomeration.     

Simultaneous conversion of CHClF(2) and CH(3)Br to CH(2)CF(2)

Gas phase reaction of CHClF(2) with CH(3)Br in an alumina tube reactor at 773-1123 K as a function of various input ratios of CH(3)Br to CHClF(2) is presented. The major products detected include C(2)F(4), CH(2)CF(2), and CH(4). Minor products include CH(3)Cl, CHF(3), C(2)H(4), C(2)H(2), CH(2)CF-CF(3), and C(2)H(3)F. The reaction produces a high yield of CH(2)CF(2) (53% based on CHClF(2) feed) at 1123 K and an input molar ratio of CH(3)Br to CHClF(2) of 1.8, suggesting that the reaction potentially can be developed as a process to convert two ozone depleting substances (CHClF(2) and CH(3)Br) to a highly valuable chemical, CH(2)CF(2). The reaction of CHClF(2) with CH(3)Cl and CH(3)I was also investigated under similar reaction conditions, to assist in understanding the reaction chemistry involved in the reaction of CHClF(2) with CH(3)Br.     

水泥窑共处置污染土壤的污染排放研究

以被DDT和六六六污染的土壤为研究对象，开展了利用水泥窑进行共处置的工程试验研究，分析了共处置过程对尾气排放的影响。结果表明，水泥窑共处置对农药污染物的焚毁去除率很高，DDT达到99.99991%，六六六达到99.99964%，均高于我国危险废物焚烧炉规定99.99％的焚烧效率；与空白处理相比，尾气排放中有机污染物（包括二恶英/呋喃，简称PCDD/F、多氯联苯PCB、六氯苯HCB、挥发性有机物VOC）、酸性气体和重金属的排放在共处置过程中没有显著增加，且都低于相应的规定限值。试验结果表明，水泥窑共处置固体废物不会导致污染负荷的增加。     

Sulfur hexafluoride (SF6): global environmental effects and toxic byproduct formation

This work provides information concerning possible global environmental implications and personnel safety aspects that should be considered during the commercial uses of sulfur hexafluoride (SF6). SF6 is an anthropogenically produced compound, mainly used as a gaseous dielectric in gas insulated switchgear power installations. It is a potent greenhouse gas with a high global warming potential, and its concentration in the earth atmosphere is rapidly increasing. During its working cycle, SF6 decomposes under electrical stress, forming toxic byproducts that are a health threat for working personnel in the event of exposure. Several precautions are recommended to avoid personnel exposure to toxic byproducts: oxyfluoride levels or other byproduct concentrations in the operating gas matrix should be traced to predetermine the overall gas toxicity; contaminants should be systematically considered during maintenance, chamber evacuation and system opening process; small SF6 quantities leaking into air or stagnated pollutant concentrations in the operating field should be analyzed and compared to the threshold limit values and permissible exposure levels. New system design rules (i.e., hermetically sealed gas compartments, gas recycling or disposal in the field area) and different handling policies--both during maintenance and final disposal--now should be considered globally to provide for environmental and personnel safety.     

Arsenic(V) removal with a Ce(IV)-doped iron oxide adsorbent

The removal of arsenic(V) by a new Ce-Fe adsorbent was evaluated under various conditions. Under an initial As(V) of 1.0 mg l(-1), the adsorption capacity of the Ce-Fe absorbent was constant around a value of 16 mgg(-1) over a wide pH range (3-7), while a maximum adsorption capacity of 8.3 mgg(-1) was obtained over a narrow pH range around 5.5 for activated alumina, a conventional adsorbent. Kinetics of adsorption obeys a pseudo-first-order rate equation with the rate constant K(ad) as 1.84 x 10(-3) min(-1). The pattern of adsorption of As(V) by the adsorbent fitted well both the Langmuir and Freundlich models. A Langmuir Q(0) of 70.4 mgg(-1) was obtained at an initial pH of 5.0 and temperature of 20 degrees C, significantly higher than those of other adsorbents reported. Phosphate seriously inhibited the removal of As(V) while fluoride did not compete with As(V) even at an F/As molar ratio as high as 30, suggesting that the adsorption sites for As(V) and fluoride were different. Salinity, hardness, and other inorganic anions such as Cl(-), NO(3)(-), and SO(4)(2-) had no apparent effect on As(V) adsorption. Fourier transform infrared spectra of Ce-Fe adsorbent before and after As(V) adsorption demonstrated that M-OH groups plays an important role for As(V) ions removal in the adsorption mechanisms of Ce-Fe adsorbent.     

Photocatalytic removal of organic pollutants in aqueous solution by Bi(4)Nb(x)Ta((1-x))O(8)I

In this work, Bi₄Nb_xTa_(1−x)O₈I photocatalysts have been synthesized by solid state reaction method and characterized by powder X-ray diffraction, scanning electron microscope and UV-Vis near infrared diffuse reflectance spectroscopy. The photocatalytic activity of these photocatalysts was evaluated by the degradation of methyl orange (MO) in aqueous solutions under visible light, UV light and solar irradiation. The effects of catalyst dosage, initial pH and MO concentration on the removal efficiency were studied, and the photocatalytic reaction kinetics of MO degradation as well. The results indicated that Bi₄Nb_xTa_(1−x)O₈I exhibited high photocatalytic activity for the removal of MO in aqueous solutions. For example, the removal efficiency of MO by Bi₄Nb_0.1Ta_0.9O₈I was as high as 92% within 12 h visible light irradiation under the optimal conditions: initial MO concentration of 5-10 mg L⁻¹, catalyst dosage of 6 g L⁻¹ and natural pH (6-8), the MO molecules could be completely degradated by Bi₄Nb_0.1Ta_0.9O₈I within 40 min under UV light irradiation, and the photodegradation efficiency reaches to 60% after 7 h solar irradiation. Furthermore, the photocatalytic degradation of Bisphenol A (BPA) was also investigated under visible light irradiation. It is found that 99% BPA could be mineralized by Bi₄Nb_0.1Ta_0.9O₈I after 16 h visible light irradiation. Through HPLC/MS, BOD, TOC, UV-Vis measurements, we determined possible degradation products of MO and BPA. The results indicated that MO was degradated into products which are easier to be biodegradable and innocuous treated, and BPA could be mineralized completely. Furthermore, the possibility for the photosensitization effect in the degradation process of MO under visible light irradiation has been excluded.     

Effect of various reaction parameters on THMs aqueous sonolysis

Ultrasonic irradiation was investigated for destruction of the following THMs: CHCl(3), CHBrCl(2), CHBr(2)Cl, CHBr(3), and CHI(3). The effect of pH, temperature, and the organics initial concentration on the THMs sonodegradation at acoustic frequency of 20 kHz was studied. An increase of the solution temperature resulted in a faster sonodegradation rates. Initial aqueous solution pH, in the range from 3 to 10, was found to have little effect on the degradation of the THMs. The THMs sonolysis efficiency was reduced when the initial organic compounds concentration was increased from 10 mg l(-1) to 300 mg l(-1).     

Iron-induced decomposition of perfluorohexanesulfonate in sub- and supercritical water

Decomposition of perfluorohexanesulfonate (PFHS), a bioaccumulative analogue of perfluorooctanesulfonate (PFOS), in sub- and supercritical water was investigated. Although PFHS was only slightly reactive in pure subcritical water at 350 degrees C, it decomposed to F(-) and SO(4)(2-) ions when the temperature was increased to 380 degrees C, at which temperature the water became supercritical state. Addition of zerovalent iron to the reaction system dramatically accelerated PFHS decomposition to F(-) ions in both sub- and supercritical water: for example, when the initial PFHS concentration was 741microM, the F(-) yields at 350 degrees C were 4.13-16.0 times as high as those in the absence of iron, depending on the amount and the particle size of the iron powder. After the reactions, small amounts of CO(2) and CF(3)H were also detected in the gas phase; these increased with temperature, and the amount of CF(3)H increased markedly when the reaction was carried out in supercritical water. Increasing the specific surface area of the iron powder markedly increased PFHS consumption and F(-) formation in the aqueous phase, which indicates that the reactions occurred on the iron surface and that the increased specific surface area was a key factor in accelerating the decomposition of PFHS to F(-) ions.     

Photochemical decomposition of environmentally persistent short-chain perfluorocarboxylic acids in water mediated by iron(II)/(III) redox reactions

The photochemical decomposition of short-chain (C(3)-C(5)) perfluorocarboxylic acids (PFCAs) was investigated. Direct photolysis in water proceeded slowly with the 220- to 460-nm light emission from a xenon-mercury lamp to form F(-), CO(2), and shorter-chain PFCAs. Addition of a small amount of Fe(3+) to the aqueous solutions of the PFCAs dramatically enhanced their photochemical decomposition under an oxygen atmosphere: when the (initial PFCA)/(initial Fe(3+)) molar ratio was 13.5 (initial PFCA concentration=67.3mM), the pseudo-first-order rate constants for the PFCA decomposition were 3.6-5.3 times those with photolysis alone, and the turnover number for the catalytic PFCA decomposition [i.e., (moles of decomposed PFCA)/(moles of initial Fe(3+))] reached 6.71-8.68 after 24h of irradiation. The catalysis can be explained by photoredox reactions between PFCA, Fe(3+)/Fe(2+) and oxygen via photo-induced complexation of Fe(3+) with the PFCAs.     

Waste Fe(III)/Cr(III) hydroxide as adsorbent for the removal of Cr(VI) from aqueous solution and chromium plating industry wastewater

Fe(III)/Cr(III) hydroxide, a waste material from the fertilizer industry, has been used for the adsorption of Cr(VI) from aqueous solution, over a range of initial metal ion concentrations (5-30 mg litre(-1)), agitation times (1-180 min), adsorbent dosages (100-1200 mg per 50 ml), temperatures (24, 29 and 38 degrees C) and pH values (4.5-10). The adsorption of Cr(VI) increased with the initial concentration of Cr(VI) and with temperature. The process of uptake follows both the Langmuir and the Freundlich isotherm models. The applicability of Lagergren and empirical kinetic models has also been investigated. Almost quantitative removal of Cr(VI) at 10 mg litre(-1) in a 50-ml solution by 500 mg of adsorbent was found at an equilibrium pH of 5.6. The efficiency of chromium removal was also tested using wastewater from the chromium plating industry.     

高比表面积活性炭的制备及对Cr(Ⅵ)吸附的研究

以椰树壳为原料,运用水蒸汽和二氧化碳复合物理活化法在4 h内制备了2 162.84 m2/g的高比表面积活性炭,其孔径分布范围为1.1~2.5 nm。应用此吸附剂考察了溶液pH、吸附剂用量、接触时间和初始浓度对Cr(VI)离子吸附效果的影响,并讨论了固定吸附床中不同溶液流量对Cr(VI)去除效果的影响。结果表明:在温度为298 K、溶液pH为1.96、吸附剂用量为0.10 g、铬离子初始浓度为100 mg/L与接触时间为70 min的条件下,活性炭对铬离子具有较高的吸附容量,去除率高达99.32%;铬离子在吸附床中的穿透曲线具有陡峭的传质锋面,但随着铬离子溶液流量的增大脱除果率降低。     

Dissociation of sulfur hexafluoride tracer gas in the presence of an indoor combustion source

As an odorless, nontoxic, and inert compound, sulfur hexafluoride (SF6) is one of the most widely used tracer gases in indoor air quality studies in both controlled and uncontrolled environments. This compound may be subject to reactions with water vapor under elevated temperature to form acidic inorganic compounds such as HF and H2SO4. Thus, in the presence of unvented combustion sources such as kerosene heaters, natural gas heaters, gas log fireplaces, candles, and lamps, the SF6 dissociation may interfere with measurements of the emissions from these sources. Tests were conducted in a research house with a vent-free natural gas heater to investigate these potential interferences. It was observed that the heater operation caused about a 5% reduction of SF6 concentration, which can be an error source for the ventilation rate measurement and consequently the estimated pollutant emission rates. Further analysis indicates that this error can be much greater than the observed 5% under certain test conditions because it is a function of the ventilation flow rate. Reducing the tracer gas concentration has no effect on this error. A simple theoretical model is proposed to estimate the magnitude of this error. The second type of interference comes from the primary and secondary products of the SF6 dissociation, mainly H2SO4, SO2, HF, and fine particulate matter (PM). In the presence of approximately 5 ppm SF6, the total airborne concentrations of these species increased by a factor of 4-10. The tests were performed at relatively high SF6 concentrations, which is necessary to determine the interferences quantitatively. The second type of interference can be significantly reduced if the SF6 concentration is kept at a low ppb level.     

Hydrogen peroxide-assisted UV photodegradation of Lindane

Aqueous solutions of gamma-hexachlorocyclohexane (Lindane) were photolyzed (lambda=254 nm) under a variety of solution conditions. The initial concentrations of hydrogen peroxide (H(2)O(2)) and Lindane varied from 0 to 20 mM and 0.21 to 0.22 microM, respectively, the pH ranged from 3 to 11, and several concentration ratios of Suwannee River humic acid and fulvic acid were dissolved in the irradiated solutions. Lindane rapidly reacted, and the maximum reaction rate constant (9.7 x 10(-3) s(-1)) was observed at pH 7 and initial [H(2)O(2)]=1 mM. Thus, 90% of the Lindane is destroyed in approximately 4 min under these conditions. In addition, within 15 min, all chlorine atoms were converted to chloride ion, indicating that chlorinated organic by-products do not accumulate. The reactor was characterized by measuring the photon flux (7.04 x 10(-6) E s(-1)) and the cumulative production of *OH during irradiation. The cumulative *OH production during irradiation was fastest at an initial [H(2)O(2)]=5 mM (k=0.77 micro M s(-1)).     

Innovative PCDD/F-containing gas stream generating system applied in catalytic decomposition of gaseous dioxins over V2O5-WO3/TiO2-based catalysts

Development of effective PCDD/F (polychlorinated dibenzo-p-dioxin and dibenzofuran) control technologies is essential for environmental engineers and researchers. In this study, a PCDD/F-containing gas stream generating system was developed to investigate the efficiency and effectiveness of innovative PCDD/F control technologies. The system designed and constructed can stably generate the gas stream with the PCDD/F concentration ranging from 1.0 to 100ng TEQ Nm(-3) while reproducibility test indicates that the PCDD/F recovery efficiencies are between 93% and 112%. This new PCDD/F-containing gas stream generating device is first applied in the investigation of the catalytic PCDD/F control technology. The catalytic decomposition of PCDD/Fs was evaluated with two types of commercial V(2)O(5)-WO(3)/TiO(2)-based catalysts (catalyst A and catalyst B) at controlled temperature, water vapor content, and space velocity. 84% and 91% PCDD/F destruction efficiencies are achieved with catalysts A and B, respectively, at 280 degrees C with the space velocity of 5000h(-1). The results also indicate that the presence of water vapor inhibits PCDD/F decomposition due to its competition with PCDD/F molecules for adsorption on the active vanadia sites for both catalysts. In addition, this study combined integral reaction and Mars-Van Krevelen model to calculate the activation energies of OCDD and OCDF decomposition. The activation energies of OCDD and OCDF decomposition via catalysis are calculated as 24.8kJmol(-1) and 25.2kJmol(-1), respectively.