Photodegradation of chlorinated hydrocarbons in the presence and absence of dissolved oxygen in water

Subterranean carbon flows from source to stygofauna: a case study on the atyid shrimp Stygiocaris stylifera (Holthuis, 1960) from Barrow Island (WA)

Groundwater biota are crucial for the ecological functioning of subterranean ecosystems. However, while knowledge of the taxonomic diversity of groundwater invertebrates (stygofauna) is increasing, functional ecological information is still limited. Here, we investigate seldom empirically tested assumptions around stygofaunal trophic plasticity in coping with oligotrophic habitats. We focus on Barrow Island (Western Australia), an ideal natural laboratory due to the occurrence of natural oil seeps in association with aquifers. The trophic position and food source use of the endemic atyid shrimp Stygiocaris stylifera (Holthuis, 1960) were assessed via δ¹³C and δ¹⁵N stable isotope analysis (SIA). Background information on the environmental conditions was gathered through hydrochemical data and δ¹³C SIA combined with ¹⁴C data from dissolved inorganic/organic carbon and particulate organic carbon from groundwater samples. Our results indicate carbon enrichment in proximity to the natural oil seepage coupled with changes in trophic positions of S. stylifera from higher consumers/predators to biofilm grazers/decomposers. These results are consistent with an increased involvement of hydrocarbon seeps and associated microbial communities in the carbon flows and confirm potential for the trophic flexibility in stygofauna. Further investigations involving other trophic groups will help elucidate the functioning of the ecosystems at a community level.

Understanding and modeling the formation and transformation of hydrogen peroxide in water irradiated by 254 nm ultraviolet (UV) and 185 nm vacuum UV (VUV): Effects of pH and oxygen

Understanding ultraviolet photolysis induced by low pressure mercury lamp that emits both 254 nm ultraviolet (UV₂₅₄) and 185 nm vacuum UV (VUV₁₈₅) is currently challenging due to the copresence of multiple direct and indirect photochemical processes involving a series of highly-reactive radicals. Herein we examined the formation and transformation of H₂O₂ in water, which is both a precursor and a product of radicals, under various pH and dissolved oxygen (DO) conditions. The trends show that H₂O₂ increased rapidly at early stage and then remained steady in DO-rich water or declined somewhat in DO-poor water, ultimately leading to higher steady-state H₂O₂ in DO-rich water. The maximum H₂O₂ contents nonetheless were similar among waters with different DO, suggesting that H₂O₂ in this system was mostly generated by hydroxyl radical (OH) recombination, which is an oxygen-independent H₂O₂ formation pathway, rather than by reduced oxygen via hydrogen atom (H) or hydrated electron (e_aq^-), which is an oxygen-dependent pathway. Increasing pH (from 6.3 to 10.0) or bicarbonate dosage dramatically decreased H₂O₂ formation too. Mathematically, the fates of H₂O₂ as a function of pH, DO, and time were well modeled (R² ≥ 0.92), in which the rates of H₂O₂ formation and destruction were greater in DO-poor water than those in DO-rich water. In addition, we found that the steady-state concentrations of OH used for degradation of p-chlorobenzoic acid, an OH probe, correlated well with the OH levels used for H₂O₂ formation (R² = 0.98). These results hence may help better understand the UV/VUV process via H₂O₂ evolutions.

Sono-coprecipitation synthesis of ZnO/CuO nanophotocatalyst for removal of parathion from wastewater

Semiconductor photocatalysis is an effective method used to degrade organophosphorus compounds. Here, the potential of a commonly mixed oxide semiconductor, ZnO/CuO, has been examined to degrade methyl parathion. Sono-coprecipitation method was used to provide ZnO/CuO nanocomposites, and it was applied to photocatalytic and sono-photocatalytic degradation of methyl parathion under solar light irradiation. Powder x-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), the Brunauer-Emmett-Teller (BET) surface area, field emission scanning electron microscopy (FE-SEM), and transmission electron microscopy (TEM) were used to characterize the synthesized samples. The optimal experimental conditions such as ZnO/CuO photocatalyst 90:10 M ratios, the initial concentration of 20 mg/L parathion, 1 g/L photocatalyst loading, no compressed air sparging, pH of 8, and ultrasonic power (60 W and 80 kHz) were used to degrade the parathion effectively. The parathion was fully (100% removal) degraded after 60 min sono-photoirradiation in the optimal experimental conditions. A real water sample was used to examine the ability of the ZnO/CuO photocatalyst 90:10 to remove the parathion in the water-soluble ions. Graphical abstract.

Prediction of 1,4-dioxane decomposition during VUV treatment by model simulation taking into account effects of coexisting inorganic ions

1,4-Dioxane is one of the most persistent organic micropollutants and is quite difficult to remove via conventional drinking water treatment consisting of coagulation, sedimentation, and sand filtration. Vacuum ultraviolet (VUV) treatment has recently been found to show promise as a treatment method for 1,4-dioxane removal, but the associated decomposition rate of 1,4-dioxane is known to be very sensitive to water quality characteristics. Some computational models have been proposed to predict the decomposition rate of micropollutants during VUV treatment, but the effects of only bicarbonate and natural organic matter have been considered in the models. In the present study, we attempted to develop a versatile computational model for predicting the behavior of 1,4-dioxane during VUV treatment that took into account the effects of other coexisting inorganic ions commonly found in natural waters. We first conducted 1,4-dioxane decomposition experiments with low-pressure mercury lamps and test waters that had been prepared by adding various inorganic ions to an aqueous phosphate buffer. The apparent decomposition rate of 1,4-dioxane was suppressed when bicarbonate, chloride, and nitrate were added to the test waters. Whereas bicarbonate and chloride directly suppressed the apparent decomposition rate by consuming HO•, nitrate became influential only after being transformed into nitrite by concomitant UV light (λ = 254 nm) irradiation. Cl-related radicals (Cl• and Cl₂•^-) did not react with 1,4-dioxane directly. A computational model consisting of 31 ordinary differential equations with respect to time that had been translated from 84 reactions (10 photochemical and 74 chemical reactions) among 31 chemical species was then developed for predicting the behavior of 1,4-dioxane during VUV treatment. Nine of the parameters in the ordinary differential equations were determined by least squares fitting to an experimental dataset that included different concentrations of bicarbonate, chloride, nitrate, and nitrite. Without further parameter adjustments, the model successfully predicted the behavior of 1,4-dioxane during VUV treatment of three groundwaters naturally contaminated with 1,4-dioxane as well as one dechlorinated tap water sample supplemented with 1,4-dioxane.

Effect of dissolved oxygen on efficiency of TOC reduction by UV at 185 nm in an ultrapure water production system

The effect of dissolved oxygen (DO) on the degradation of methanol by ultraviolet (UV) light at 185 nm (UV-185) was examined in this study. The experiments were conducted using a bench-scale experimental apparatus that was operated under continuous conditions. In the control tests with various DO conditions, it was confirmed that UV irradiation of water without methanol produced approximately 54 μg/L of H₂O₂ and removed 15%-90% of the DO. The production of H₂O₂ was affected by the mixing conditions within the UV reactor and the dose of UV. The degradation efficiency of methanol by UV-185 irradiation improved linearly with increasing DO concentration, with more than 90% DO consumption. The maximum total organic carbon (TOC) removal rate was observed at 100 μg/L DO and 433 mW·sec/cm², the highest DO and dosage conditions in this study. The production of H₂O₂ was also affected by DO concentration in the feed water, especially at the high UV dosage, in which lower H₂O₂ production was observed at higher DO.

Photochemical behavior of benzophenone sunscreens induced by nitrate in aquatic environments

Benzophenones (BPs), which are widely used UV filters, have aroused considerable public concern owing to their potential endocrine-disrupting activities. Herein, we systematically investigated their photochemical behavior and fate, which is mediated by nitrate in aquatic environments. The results showed that 10 μM of 3 BPs can be completely degraded within 4 h of simulated sunlight irradiation in a 10 mM nitrate solution at pH 8.0, and 2,4-dihydroxybenzophenone (BP-1) has a 31.6% mineralization rate after 12 h irradiation. Their photolytic rates (k_obs) presented a significant linear correlation with the logarithmic values of the nitrate concentration for 0.1-10 mM (R² > 0.98), and in three actual waters, the rates of BP-1 were also positively related to the intrinsic nitrate concentration. Furthermore, higher transformation rates under alkaline condition were observed, especially for BP-1, with its k_obs at pH 10 being 8.3-fold higher than that at pH 6.0. Moreover, dissolved oxygen (DO) also has an impact on the reaction kinetics to some degree. According to the quenching experiments, we found that three reactive oxygen species (ROS), namely, •OH, •NO, and •NO₂, participated in this photolysis of BPs, and the contribution of •OH accounted for 32.1%. Furthermore, we selected BP-1 as the model molecule to study the transformation pathways and toxicity changes in this system. Four main transformation pathways including hydroxylation, nitrosylation, nitration, and dimerization were proposed, based on liquid chromatography quadrupole time-of-flight mass spectrometry (LC-Q-TOF-MS) analysis and density functional theory (DFT). According to the toxicity test, the formed intermediates were more toxic to Photobacterium phosphoreum than the parent BP-1. Therefore, these results can help reveal primary phototransformation mechanisms and evaluate the potential ecological risks of BPs in aquatic environments.

Use of trichloroacetonitrile as a hydrogen chloride generator for ring-opening reactions of aziridines

Regioselective ring-opening reactions of 2-aryl-N-tosylaziridines are described, in which hydrogen chloride is generated by photodegradation of trichloroacetonitrile. HCl adducts are obtained in high yields in 1,4-dioxane, whereas methanol adducts are predominantly obtained in methanol. Trichloroacetonitrile can serve as a photoresponsive molecular storage generator for hydrogen chloride.

Comparison of various advanced oxidation processes for the degradation of phenylurea herbicides

Various types of advanced oxidation processes (AOPs), such as UV photolysis, ozonation, heterogeneous photocatalysis and their combinations were comparatively examined at the same energy input in a home-made reactor. The oxidative transformations of the phenylurea herbicides fenuron, monuron and diuron were investigated. The initial rates of transformation demonstrated that UV photolysis was highly efficient in the cases of diuron and monuron. Ozonation proved to be much more effective in the transformation of fenuron than in those of the chlorine containing monuron and diuron. In heterogeneous photocatalysis, the rate of decomposition decreased with increase of the number of chlorine atoms in the target molecule. Addition of ozone to UV-irradiated solutions and/or TiO2-containing suspensions markedly increased the initial rates of degradation. Dehalogenation of monuron and diuron showed that each of these procedures is suitable for the simultaneous removal of chlorinated pesticides and their chlorinated intermediates. Heterogeneous photocatalysis was found to be effective in the mineralization.

Factors driving epilithic algal colonization in show caves and new insights into combating biofilm development with UV-C treatments

The proliferation of epilithic algae that form biofilms in subterranean environments, such as show caves, is a major problem for conservators. In an effort to reduce the use of chemical cleansers when addressing this problem, we proposed investigating the effects of UV-C on combating algal biofilm expansion in a cave located in northeastern France (Moidons Cave). First, the biofilms and cavity were studied in terms of their algal growth-influencing factors to understand the dynamics of colonization in these very harsh environments. Next, colorimetric measurements were used both to diagnose the initial colonization state and monitor the UV-C-treated biofilms for several months after irradiation. The results indicated that passive dispersal vectors of the viable spores and cells were the primary factors involved in the cave's algae repartition. The illumination time during visits appeared to be responsible for greater colonization in some parts of the cave. We also showed that colorimetric measurements could be used for the detection of both thin and thick biofilms, regardless of the type of colonized surface. Finally, our results showed that UV-C treatment led to bleaching of the treated biofilm due to chlorophyll degradation even one year after UV-C treatment. However, a re-colonization phenomenon was colorimetrically and visually detected 16months later, suggesting that the colonization dynamics had not been fully halted.

Vacuum-UV radiation at 185 nm in water treatment--a review

The vacuum-UV radiation of water results in the in situ generation of hydroxyl radicals. Low-pressure mercury vapor lamps which emit at 185 nm are potential sources of VUV radiation. The scope of this article is to give an overview of the application of VUV radiation at 185 nm for water treatment including the transformation of inorganic and organic water constituents, and the disinfection efficiency. Another focus is on the generation of ozone by VUV radiation from oxygen or air and the application of the produced ozone in combination with VUV irradiation of water in the VUV/O3 process. The advantages and limitation of the VUV process at 185 nm as well as possible applications in water treatment are outlined.

Photodegradation of sulfapyridine under simulated sunlight irradiation: kinetics, mechanism and toxicity evolvement

In this study, the photoinduced degradation of sulfapyridine (SPY) was investigated under simulated light irradiation (λ>200nm). The effect of pH and main water constituents including nitrate ion, bicarbonate, dissolved organic matter (DOM) and iron(III) on the photodegradation was explored. SPY was effectively removed in aqueous solution at pH 8 under UV-vis irradiation, with removal efficiency of 100% within 120min. DOM and iron(III) had retarding influence on the SPY removal, whereas nitrate ion and bicarbonate did not show any obvious effect. Under UV-vis irradiation, the formation of singlet oxygen ((1)O2) accelerated the SPY photodegradation and the contribution of indirect photolysis due to reaction with (1)O2 was up to 42%. The transformation products of SPY were identified by HPLC-MS and the possible photoreaction pathways were proposed. It showed that photoinduced hydrolysis, photo-oxidation via (1)O2 and desulfonation were the main degradation ways for SPY decomposition. Toxicity assays by Vibrio fischeri proved that the transformation products were more toxic than the parent compound.

Modeling perchloroethylene degradation under ultrasonic irradiation and photochemical oxidation in aqueous solution

Sonolysis and photochemical degradation of different compounds such as chlorinated aliphatic hydrocarbons are among the recent advanced oxidation processes. Perchloroethylene is one of these compounds that has been mainly used as a solvent and degreaser. In this work, elimination of perchloroethylene in aqueous solution by ultrasonic irradiation, andphotochemical oxidation by ultra violet ray and hydrogen peroxide were investigated. Three different initial concentrations of perchloroethylene at different pH values, detention periods, and concentrations of hydrogen peroxide were investigated. Head space gas chromatography with FID detector was used for analyses of perchloroethylene. This research was performed in 9 months from April through December 2011.

Results showed that perchloroethylene could be effectively and rapidly degraded by ultrasonic irradiation, photochemical oxidation by ultra violet ray, hydrogen peroxide and a combination of these methods. Kinetics of perchloroethylene was strongly influenced by time, initial concentration and pH value. Degradation of Perchloroethylene increased with decrease in the initial concentration of perchloroethylene from 0.3 to 10 mg/L at all initial pH. The results showed an optimum degradation condition achieved at pH = 5 but did not affect significantly the perchloroethylene destruction in the various pH values. Kinetic modeling applied for the obtained results showed that the degradation of perchloroethylene by ultrasound and photo-oxidation followed first order and second order model. The percentage of removal in the hybrids reactor was higher than each of the reactors alone, the reason being the role of hydroxyl radical induced by ultrasound and photochemical reaction.

Heterogeneous photocatalytic oxidation of cyprodinil and fludioxonil in leaching water under solar irradiation

The efficiency of ZnO and TiO(2) suspensions in the photocatalytic degradation of two fungicides (cyprodinil and fludioxonil) in leaching water was investigated. The experiments were carried out at pilot plant scale using compound parabolic collectors under natural sunlight. The blank experiments for both irradiated compounds solutions showed that both oxides strongly enhanced the removal of the fungicides. The addition of an oxidant (Na(2)S(2)O(8)) to the ZnO or TiO(2) increased the rate of photooxidation. The degradation of cyprodinil and fludioxonil followed first order kinetics according to the Langmuir-Hinshelwood model. Complete degradation of both fungicides was achieved within 4 h (t(30W)=18 min) when treated with illuminated ZnO. The disappearance time (DT(75)), when referred to the normalized illumination time (t(30W)), was lower than 40 and 550 min (t(30W)=2 and 40 min) for both fungicides using ZnO or TiO(2), respectively. ZnO appeared to be more effective in cyprodinil and fludioxonil oxidation than TiO(2) probably due to its nonstoichiometry.

Recent developments in photocatalytic water treatment technology: a review

In recent years, semiconductor photocatalytic process has shown a great potential as a low-cost, environmental friendly and sustainable treatment technology to align with the "zero" waste scheme in the water/wastewater industry. The ability of this advanced oxidation technology has been widely demonstrated to remove persistent organic compounds and microorganisms in water. At present, the main technical barriers that impede its commercialisation remained on the post-recovery of the catalyst particles after water treatment. This paper reviews the recent R&D progresses of engineered-photocatalysts, photoreactor systems, and the process optimizations and modellings of the photooxidation processes for water treatment. A number of potential and commercial photocatalytic reactor configurations are discussed, in particular the photocatalytic membrane reactors. The effects of key photoreactor operation parameters and water quality on the photo-process performances in terms of the mineralization and disinfection are assessed. For the first time, we describe how to utilize a multi-variables optimization approach to determine the optimum operation parameters so as to enhance process performance and photooxidation efficiency. Both photomineralization and photo-disinfection kinetics and their modellings associated with the photocatalytic water treatment process are detailed. A brief discussion on the life cycle assessment for retrofitting the photocatalytic technology as an alternative waste treatment process is presented. This paper will deliver a scientific and technical overview and useful information to scientists and engineers who work in this field.

Photocatalytic degradation of eight pesticides in leaching water by use of ZnO under natural sunlight

Photodegradation of eight pesticides in leaching water at pilot plant scale using the tandem ZnO/Na(2)S(2)O(8) as photosensitizer/oxidant and compound parabolic collectors under natural sunlight is reported. The pesticides, habitually used on pepper culture and belonging to different chemical groups were azoxyxtrobin, kresoxim-methyl, hexaconazole, tebuconazole, triadimenol, and pyrimethanil (fungicides), primicarb (insecticide), and propyzamide (herbicide). As expected, the influence of the semiconductor used at 150 mg L(-1) on the degradation of pesticides was very significant in all cases. Photocatalytic experiments show that the addition of photosensitizer strongly improves the elimination of pesticides in comparison with photolytic tests; significantly increasing the reaction rates. The use of Na(2)S(2)O(8) implies a significant reduction in treatment time showing a quicker reaction time than ZnO alone. On the contrary, the addition of H(2)O(2) into illuminated ZnO suspensions does not improve the rate of photooxidation. The disappearance of the pesticides followed first-order kinetics according to Langmuir-Hinshelwood model and complete degradation occurs from 60 to 120 min. The disappearance time (DT(75)), referred to the normalized illumination time (t(30 W)) was lower than 3 min in all cases.

Degradation of methyl orange using short-wavelength UV irradiation with oxygen microbubbles

A novel wastewater treatment technique using 8 W low-pressure mercury lamps in the presence of uniform-sized microbubbles (diameter = 5.79 microm) was investigated for the decomposition of methyl orange as a model compound in aqueous solution. Photodegradation experiments were conducted with a BLB black light blue lamp (365 nm), a UV-C germicidal lamp (254 nm) and an ozone lamp (185 nm+254 nm) both with and without oxygen microbubbles. The results show that the oxygen microbubbles accelerated the decolorization rate of methyl orange under 185+254 nm irradiation. In contrast, the microbubbles under 365 and 254 nm irradiation were unaffected on the decolorization of methyl orange. It was found that the pseudo-zero order decolorization reaction constant in microbubble system is 2.1 times higher than that in conventional large bubble system. Total organic carbon (TOC) reduction rate of methyl orange was greatly enhanced by oxygen microbubble under 185+254 nm irradiation, however, TOC reduction rate by nitrogen microbubble was much slower than that with 185+254 nm irradiation only. Possible reaction mechanisms for the decolorization and mineralization of methyl orange both with oxygen and nitrogen mirobubbles were proposed in this study.

Kinetics of acetamiprid photolysis in solution

Photolysis of acetamiprid was investigated in detail under different reaction conditions. The photolysis of acetamiprid in solution followed pseudo-first-order kinetics under the experimental conditions. Acetamiprid kept stable under irradiation of high-pressure xenon lamp but degraded relatively fast when exposure to medium-pressure mercury lamp irradiation. The experimental results indicated that the degradation rate of acetamiprid in acetone was the largest among the used media and acetamiprid was more stable in basic media. The addition of oxygen, increase of light intensity and temperature will enhance the photolysis.

Photolysis behavior of herbicide propisochlor in water media and preliminary analysis of photoproducts under different light sources

Photolysis behavior of a new herbicide propisochlor in water media as well as the effects of light sources, initial concentration of propisochlor, pH value, dissolved oxygen (DO) level, and salinity on the photolysis process was investigated. It was found that the relationship between initial concentration of propisochlor and its photodegradation rate was negatively correlated. The changes in acidity and alkalinity of the reaction medium influenced the photoreaction rate evidently. In the alkaline solution the degradation was accelerated. In the reaction media with different pH values, the photolysis followed the first-order kinetics. The presence of dissolved oxygen may promote the photolysis and there existed an optimum of dissolved oxygen concentrations. Increasing the DO level can weaken the promotion and even have an adverse effect. It was demonstrated that with dissolved oxygen the photodegradation of propisochlor followed the first-order kinetics equation. The addition of salt ions Ca2+ and Mg2+ changed the ionic strength and solvent polarity, resulting in the effect on propisochlor photolysis. The photoproducts were detected by both HPLC and GC-MS methods. It was found that photolysis products varied under different light sources. Conclusions may be reached that in the photodegradation of propisochlor, the benzene ring remained intact under irradiation of both solar light and high-pressure mercury lamp, and the amido link was relatively stable, while dechlorination was liable to take place; moreover, alpha-hydrogen at the substituent of benzene ring was active.

Photocatalysis of p-chlorobenzoic acid in aqueous solution under irradiation of 254 nm and 185 nm UV light

The photolytic and photocatalytic degradation of p-chlorobenzoic acid (p-CBA) in aqueous solution were investigated using two kinds of low-pressure mercury lamps: one emitted at 254 nm and the other emitted at 254 nm and 185 nm. Both the photolytic and photocatalytic degradations of p-CBA followed pseudo first-order reaction rate form. The rate constants decreased with increase of initial concentration in VUV and TiO2/VUV. The TiO2/VUV was the most efficient process, in which rate constants was 3.0-6.5 times as that in TiO2/UV depending on the initial concentration. The gas bubbling efficiently improved the mass transfer and the kind of bubbling gas had evident influence on the reaction rate except in UV photolysis. TOC reduction rate was greatly reduced in VUV and TiO2/VUV processes when bubbled with nitrogen gas, it was no more than one third of that when bubbled with oxygen, however, the degradation rate of p-CBA was not affected by bubbled nitrogen so greatly.