UV/ozone degradation of gaseous hexamethyldisilazane (HMDS)

As a carcinogen, hexamethyldisilazane (HMDS) is extensively adopted in life science microscopy, materials science and nanotechnology. However, no appropriate technology has been devised for treating HMDS in gas streams. This investigation evaluated the feasibility and effectiveness of the UV (185+254nm) and UV (254nm)/O(3) processes for degradation of gaseous HMDS. Tests were performed in two batch reactors with initial HMDS concentrations of 32-41mgm(-3) under various initial ozone dosages (O(3) (mg)/HMDS (mg)=1-5), atmospheres (N(2), O(2), and air), temperatures (28, 46, 65 and 80 degrees C), relative humilities (20%, 50%, 65%, 99%) and volumetric UV power inputs (0.87, 1.74, 4.07 and 8.16Wl(-1)) to assess their effects on the HMDS degradation rate. Results indicate that for all conditions, the decomposition rates for the UV (185+254nm) irradiation exceeded those for the UV (254nm)/O(3) process. UV (185+254nm) decompositions of HMDS displayed an apparent first-order kinetics. A process with irradiation of UV (185+254nm) to HMDS in air saturated with water at temperatures of 46-80 degrees C favors the HMDS degradation. With the condition as above and a P/V of around 8Wl(-1), k was approximately 0.20s(-1) and a reaction time of just 12s was required to degrade over 90% of the initial HMDS. The main mechanisms for the HMDS in wet air streams irradiated with UV (185+254nm) were found to be caused by OH free-radical oxidation produced from photolysis of water or O((1)D) produced from photolysis of oxygen. The economic evaluation factors of UV (185+254nm) and UV (254nm)/O(3) processes at different UV power inputs were also estimated.     

Evaluation of photolysis and hydrolysis of atrazine and its first degradation products in the presence of humic acids

Relative importance of hydrolysis and photolysis of atrazine and its degradation products in aqueous solutions with dissolved humic acids (HA) has been assessed under exposure to sunlight and under UV irradiation. Quantum yield for direct photolysis of atrazine at 254 nm was 0.037 mol photon(-1), the reaction order was 0.8. Atrazine, desethylatrazine and desisopropylatrazine converted to their 2-hydroxy analogs with rate constants 0.02-0.08 min(-1) in clear solutions, while addition of HA (300 mg L(-1)) caused a 10-fold increase in rate constants. Hydroxyatrazine was not degraded. No evidence of photo-Fenton reaction was found. Under exposure to solar light, atrazine, desethylatrazine and desisopropylatrazine were converted to 2-hydroxy analogs only at pH 2 because of acid hydrolysis and possible contribution of photolysis. At lower HA concentration, only their light-shielding effect was noticed, while at higher concentrations, HA-catalysed hydrolysis prevailed. Hydroxyatrazine concentration diminished at all pH values in solutions without HA exposed to sunlight.     

太阳能固定膜光催化去除双酚A的特性与效果

以太阳能固定膜光催化中试装置,研究了光解、初始浓度和平均光强等对双酚A(BPA)光催化去除的影响及BPA的矿化和在自来水中的处理效果.试验结果表明,BPA在日光照射下很难光解,其光催化降解呈表观一级反应,在平均光强介于5.7～23.5 W/m2时,表观反应速率常数和平均光强呈线性关系.太阳能光催化对BPA具有良好的矿化作用,但其降解与以UV254为光源的降解有不同的机理.太阳能光催化对自来水中BPA也具有较好的处理效果.     

Photochemical and photocatalytic degradation of gaseous toluene using short-wavelength UV irradiation with TiO2 catalyst: comparison of three UV sources

The feasibility of the use of short-wavelength UV (254+185 nm) irradiation and TiO2 catalyst for photodegradation of gaseous toluene was evaluated. It was clear that the use of TiO2 under 254+185 nm light irradiation significantly enhanced the photodegradation of toluene relative to UV alone, owed to the combined effect of photochemical oxidation in the gas phase and photocatalytic oxidation on TiO2. The photodegradation with 254+185 nm light irradiation was compared with other UV wavelengths (365 nm (black light blue lamp) and 254 nm (germicidal UV lamp)). The highest conversion and mineralization were obtained with the 254+185 nm light. Moreover, high conversions were achieved even at high initial concentrations of toluene. Catalyst deactivation was also prevented with the 254+185 nm light. Regeneration experiments with the deactivated catalyst under different conditions revealed that reactive oxygen species played an important role in preventing catalyst deactivation by decomposing effectively the less reactive carbon deposits on the TiO2 catalyst. Simultaneous elimination of photogenerated excess ozone and residual organic compounds was accomplished by using a MnO2 ozone-decomposition catalyst to form reactive species for destruction of the organic compounds.     

利用UVC去除低浓度苯的实验研究

探讨了不同实验参数对苯的UVC去除效果的影响，获得了苯的去除率与苯的初始浓度、气体流量、相对湿度和氧气含量等参数之间的关系。数据表明，在实验条件范围内，苯的去除率的倒数与苯初始浓度、气体流量之间为线性关系；苯的去除率随相对湿度的增加呈现先升高后缓慢降低的关系，最佳相对湿度值在30%～50%之间；苯的去除率随氧气量的增加而缓慢增加；波长为185+254 nm的UV与254 nm的UV相比净化效果更为理想。还分析了苯降解产生的中间产物，探讨了苯的降解机理。     

Photodegradation of the steroid hormones 17beta-estradiol (E2) and 17alpha-ethinylestradiol (EE2) in dilute aqueous solution

The photochemical transformation of natural estrogenic steroid 17beta-estradiol (E2) and the synthetic oral contraceptive 17alpha-ethinylestradiol (EE2) has been studied in dilute non buffered aqueous solution (pH 5.5-6.0) upon monochromatic (254 nm) and polychromatic (lambda>290 nm) irradiation. Upon irradiation at 254 nm, the quantum yields of E2 and EE2 photolysis were similar and evaluated to be 0.067+/-0.007 and 0.062+/-0.007, respectively. Upon polychromatic excitation, and by using phenol as chemical actinometer, the photolysis efficiencies have been determined to be 0.07+/-0.01 and 0.08+/-0.01 for E2 and EE2, respectively. For both estrogens, photodegradation by-products were identified with GC/MS and LC/MS. In a first step, a model compound--5,6,7,8-tetrahydro-2-naphthol (THN)--, which represents the photoactive phenolic group, was used to obtain basic photoproduct structural informations. Numerous primary and secondary products were observed, corresponding to hydroxylated phenolic- or quinone-type compounds.     

Phototransformation of ibuprofen and ketoprofen in aqueous solutions

The UV (254 nm) and UV/VUV (254/185 nm) photolysis of two anti-inflammatory drugs, ibuprofen and ketoprofen, have been studied in aqueous solutions as a possible process for the removal of non-biodegradable compounds.We have examined the effects of dissolved oxygen and initial target concentration. Upon irradiation at 254 nm, the decomposition rate of ketoprofen is almost forty times higher as it of ibuprofen whilst VUV irradiation only increased the ibuprofen decomposition rate. The presence of dissolved oxygen accelerated the photodegradation of ibuprofen, whereas no effect was observed on the degradation of ketoprofen. The maximum quantum yield for the phototransformation was 0.2. The rate of mineralization in both cases was ∼60%, even after 1 h of treatment and this suggests the formation of stable by-products which were identified using GC-MS and HPLC-MS, respectively.     

An investigation into reservoir NOM reduction by UV photolysis and advanced oxidation processes

A comparison of four treatment technologies for reduction of natural organic matter (NOM) in a reservoir water was made. The work presented here is a laboratory based evaluation of NOM treatment by UV-C photolysis, UV/H(2)O(2), Fenton's reagent (FR) and photo-Fenton's reagent (PFR). The work investigated ways of reducing the organic load on water treatment works (WTWs) with a view to treating 'in-reservoir' or 'in-pipe' before the water reaches the WTW. The efficiency of each process in terms of NOM removal was determined by measuring UV absorbance at 254 nm (UV(254)) and dissolved organic carbon (DOC). In terms of DOC reduction PFR was the most effective (88% removal after 1 min) however there were interferences when measuring UV(254) which was reduced to a lesser extent (31% after 1 min). In the literature, pH 3 is reported to be the optimal pH for oxidation with FR but here the reduction of UV(254) and DOC was found to be insensitive to pH in the range 3-7. The treatment that was identified as the most effective in terms of NOM reduction and cost effectiveness was PFR.     

Photodegradation of bisphenol Z by UV irradiation in the presence of beta-cyclodextrin

In this work, the formation of the inclusion complex of bisphenol Z (1,1-bis(4-hydroxyphenyl)cyclohexane, abbreviated as BPCH) with beta-cyclodextrin (beta-CD) has been studied, 1:1 inclusion complex can be obtained, the formation constant of the beta-CD/BPCH complex is 5.94x10(3)M(-1). The photodegradation behavior of BPCH was investigated under monochromatic UV irradiation (lambda=254 nm). The photodegradation rate constant of BPCH in aqueous solutions with beta-CD showed a 9.0-fold increase, and simultaneously the mineralization of BPCH can be enhanced by beta-CD. The influence factors on photodegradation of BPCH were also studied and described in details, such as concentration of beta-CD, initial concentration of BPCH, organic solvent and pH. The photodegradation of BPCH in the presence of beta-CD includes the direct photolysis and the photooxidation of BPCH during the photochemical process. Some predominant photodegradation products are 4-(2,4,5-trihydroxy-phenyl)-4-(4-hydroxyphenyl) butanoic acid, 5,5-bis(4-hydroxyphenyl)pentanoic acid, meta-hydroxylated BPCH, ortho-hydroxylated BPCH and 4-(1-(4-hydroxyphenyl)pentyl)phenol respectively. The enhancement of photodegradation of BPCH mainly results from moderate inclusion depth of BPCH molecule in the beta-CD cavity. This kind of inclusion structure allows BPCH molecule sufficient proximity to secondary hydroxyl groups of the beta-CD cavity, and these hydroxyl groups could be activated and converted to hydroxyl radicals under UV irradiation, which can enhance the photooxidation of BPCH.     

紫外光(UV)光解油田采出水中多环芳烃

为了探讨紫外光光解人工模拟油田采出水中多环芳烃的降解效率,利用自制反应装置对油田采出水中多环芳烃(PAHs)的紫外光光解做了一个初步研究。研究结果证明,紫外光光解对油田采出水中的多环芳烃萘和芴有显著的降解能力。实验室的测试表明,与紫外UVA(365 nm)、UVB(308 nm)的光照相比,紫外UVC(254 nm)在光照60 min的条件下,2种多环芳烃各自的去除率都近似达到了99%。可见,在光解效力和暴露时间两方面,紫外UVC对采出水中萘和芴的去除具有相对稳定和比较高的效率。     

Decomposition of gas-phase trichloroethene by the UV/TiO2 process in the presence of ozone.

The decomposition of gas-phase trichloroethene (TCE) in air streams by direct photolysis, the UV/TiO2 and UV/O3 processes was studied. The experiments were carried out under various UV light intensities and wavelengths, ozone dosages, and initial concentrations of TCE to investigate and compare the removal efficiency of the pollutant. For UV/TiO2 process, the individual contribution to the decomposition of TCE by direct photolysis and hydroxyl radicals destruction was differentiated to discuss the quantum efficiency with 254 and 365 nm UV lamps. The removal of gaseous TCE was found to reduce by UV/TiO2 process in the presence of ozone possibly because of the ozone molecules could scavenge hydroxyl radicals produced from the excitation of TiO2 by UV radiation to inhibit the decomposition of TCE. A photoreactor design equation for the decomposition of gaseous TCE by the UV/TiO2 process in air streams was developed by combining the continuity equation of the pollutant and the surface catalysis reaction rate expression. By the proposed design scheme, the temporal distribution of TCE at various operation conditions by the UV/TiO2 process can be well modeled.     

Kinetics of simazine advanced oxidation in water

Comparison of the effects and kinetics of UV photolysis and four advanced oxidation systems (ozone, ozone/hydrogen peroxide, ozone/UV radiation and UV radiation/hydrogen peroxide) for the removal of simazine from water has been investigated. At the conditions applied, the order of reactivity was ozone < ozone/hydrogen peroxide < UV radiation < ozone/UV radiation and UV radiation/hydrogen peroxide. Rate constants of the reactions between ozone and simazine and hydroxyl radical and simazine were found to be 8.7 M-1s-1 and 2.1 x 10(9) M-1s-1, respectively. Also, a quantum yield of 0.06 mol.photon-1 was found for simazine at 254 nm UV radiation. The high value of the quantum yield corroborated the importance of the direct photolysis process. Percentage contributions of direct reaction with ozone, reaction with hydroxyl radicals and direct photolysis were also quantified.     

Photolysis of low concentration H2S under UV/VUV irradiation emitted from microwave discharge electrodeless lamps

The photolysis of simulating low concentration of hydrogen sulfide malodorous gas was studied under UV irradiation emitted by self-made microwave discharge electrodeless lamps (i.e. microwave UV electrodeless mercury lamp (185/253.7 nm) and iodine lamp (178.3/180.1/183/184.4/187.6/206.2 nm)). Experiments results showed that the removal efficiency (eta H2S) of hydrogen sulfide was decreased with increasing initial H2S concentration and increased slightly with gas residence time; H2S removal efficiency was decreased dramatically with enlarged pipe diameter. Under the experimental conditions with pipe diameter of 36 mm, gas flow rate of 0.42 standard l s(-1), eta H2S was 52% with initial H2S concentration of 19.5 mg m(-3) by microwave mercury lamp, the absolute removal amount (ARA) was 4.30 microg s(-1), and energy yield (EY) was 77.3 mg kW h(-1); eta H2S was 56% with initial H2S concentration of 18.9 mg m(-3) by microwave iodine lamp, the ARA was 4.48 microg s(-1), and the EY was 80.5mg kW h(-1). The main photolysis product was confirmed to be SO4(2-) with IC.     

The direct photolysis and photocatalytic degradation of alachlor at different TiO2 and UV sources

Direct photolysis and photocatalytic degradations of alachlor, a widely used herbicide, were studied using three different monochromatic UV lamps (254, 300 and 350 nm) and two TiO(2) sources. Both the direct photolysis and photocatalytic degradations of alachlor follow pseudo-first-order decay kinetics. TiO(2)-P25 was found to be an effective photocatalyst compared to TiO(2)-BDH. The direct photolysis of alachlor was dominant at 254 nm even if TiO(2) was present in the solution. Among the three UV wavelengths used, the highest photocatalysis quantum yield was obtained at 300 nm. The photocatalytic degradation rate of alachlor increased with the dosages of TiO(2), but an overdose of TiO(2) would retard the reaction due to light attenuation. Photocatalytic reactions were slightly enhanced in an alkaline medium, and the different proton sources causing various degrees of rate retardation were due to the presence of the corresponding counter anions. This effect was diminished at a later stage after the reaction intermediates were formed.     

Photodegradation of sulfonamide antibiotics in simulated and natural sunlight: Implications for their environmental fate

The photochemical fate of seven sulfonamides was investigated in matrices representative of natural water bodies under various light sources. Fundamental photolysis parameters such as molar absorption coefficient, quantum yield (QY) and first-order rate constants were determined. The photolysis decay rate was dependent on the protonation state of the molecule, pH of the water sample and dissolved organic matter. Natural organic matter was the most significant factor in the indirect photolysis of sulfonamides. Half-lives were in the range of minutes at 254 nm to days under natural sunlight. Under natural sunlight, all sulfonamides showed higher removal rates in natural waters implying that indirect photolysis is the predominant mechanism.     

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)).     

Modeling the quantum yields of herbicide 2,4-D decay in UV/H2O2 process

The photodecay of herbicide 2,4-D in a hydrogen peroxide-aided photolysis process was studied and modeled. The decay rate of 2,4-D was known to be low in the natural environment, but rate improvement was achieved in an H2O2/UV system. The 2,4-D decay quantum yields under ultraviolet (UV) light at 253.7 nm increased from 4.86 x 10(-6) to 1.30 x 10(-4) as the ratio of [H2O2]/[2,4-D] increased from 0.05 to 12.5. Apparently, in the presence of UV light, the decay rate of 2,4-D could be greatly improved as the concentration of hydrogen peroxide increased. However, the efficiency of 2,4-D photodecay was retarded if the concentration of H2O2 was overdosed, because the excess hydrogen peroxide consumes the hydroxyl radicals (HO*) in the solution, resulting in a much weaker oxidant HO2*. The decay of 2,4-D was also pH dependent. A ranking of acid (highest), base (middle) and neutral (lowest) was observed owing to the property change of reactants and the shifting of dominant mechanisms among photolysis, photohydrolysis and chemical oxidation. Two mathematical models were proposed to predict the quantum yield for various [H2O2]/[2,4-D] ratios and initial pH levels, in which very good correlation was found for the ranges of regular application.     

Gas phase trichloroethylene (TCE) photooxidation and byproduct formation: photolysis vs. titania/silica based photocatalysis

Photooxidation of trichloroethylene (TCE) was examined in comparative study using photolysis and photocatalysis. Degussa P25 titania coated on reactor wall and deposited on silica based microporous support were used as photocatalyst. The destruction of TCE and formation of potential byproducts were investigated under steady state conditions using annular photoreactors. Experimental work involved passing polluted air containing TCE through the UV photoreactor at varying concentrations and residence times. Ultraviolet illumination was provided by low pressure mercury lamps with outputs at either 254 nm, 365 nm, or 185/254 nm. Silica supported photocatalyst yielded maximum removal capacity of up to about 6 kg TCE per m3 per hour, nearly twice that provided by the coated titania. Direct photolysis with ozone generating UV also provided very high TCE conversion of up to 6kg TCE per m3 per hour. However, major quantities of phosgene and dichloroacetyle chloride (DCAC) were produced as byproducts. TCE removal using silica based photocatalyst did not result in any detectable DCAC. Only phosgene along with trace amounts of chloroform and carbon tetrachloride were identified as oxidation byproducts with silica based photocatalyst.     

Kinetics of simazine advanced oxidation in water

Abstract

Comparison of the effects and kinetics of UV photolysis and four advanced oxidation systems (ozone, ozone/hydrogen peroxide, ozone/UV radiation and UV radiation/hydrogen peroxide) for the removal of simazine from water has been investigated. At the conditions applied, the order of reactivity was ozone < ozone/hydrogen peroxide < UV radiation < ozone/UV radiation and UV radiation/hydrogen peroxide. Rate constants of the reactions between ozone and simazine and hydroxyl radical and simazine were found to be 8.7 M^‐1s^‐1 and 2.1x10⁹ M^‐1s^‐1, respectively. Also, a quantum yield of 0.06 mol.photon^‐1 was found for simazine at 254 nm UV radiation. The high value of the quantum yield corroborated the importance of the direct photolysis process. Percentage contributions of direct reaction with ozone, reaction with hydroxyl radicals and direct photolysis were also quantified.     

Solar photocatalytic decolorization of methylene blue in water

In this study, a photocatalytic decolorization system equipped with immobilized TiO2 and illuminated by solar light was used to remove the color of wastewater. To examine the decoloring efficiency of this system, photocatalytic decolorization of an organic dye such as methylene blue was studied as an example. The effects of light source, pH, as well as the initial concentration of dye were also investigated. It was observed that the solution of methylene blue could be almost completely decolorized by the solar light/TiO2 film process while there was about 50% color remaining with solar irradiation only. In addition, it was found that the decoloring efficiency of solution was higher with solar light irradiation than with artificial UV light irradiation, even though the artificial UV light source supplied higher UV intensity at 254 nm. The color removal rate of methylene blue with solar light irradiation was almost twice that of artificial UV light irradiation. This phenomena was mainly attributed to that some visible light range of solar light was useful for exciting the methylene blue molecules adsorbed on TiO2 film, leading to a photosensitization process undergoing and decoloring efficiency promoted. This solar-assisted photocatalytic device showed potential application for decoloring organic dyes in wastewater.