András Dombi

Investigation of the photodecomposition of phenol in near-UV-irradiated aqueous TiO2 suspensions. II. Effect of charge-trapping species on product distribution

Abstract The photodegradation of phenol was investigated in the presence of TiO 2 (anatase) as photocatalyst in near-UV-irradiated aqueous unbuffered suspensions. The distribution of degradation products and a reduction in total organic carbon are reported, focusing on the influence of charge-trapping species (O 2 , Ag + and H 2 O 2 ). In the presence of dissolved O 2 , the degradation proceeds predominantly via OH ⋅ , where hydroxylated aromatics were detected: catechol, hydroquinone and 1,2,4-trihydroxybenzene; in the presence of Ag + , the direct hole oxidation dominates, with p -benzoquinone as the only identified transient product; in the presence of H 2 O 2 , both OH ⋅ and direct oxidation by positive holes contribute to the degradation of phenol. Besides the identified compounds, the formation of presumed ring-opening products occurs simultaneously. In contrast with the general view, it is pointed out that the appearance of aliphatic products in the early stage of the oxidation process is not unambiguous proof of the participation of direct hole oxidation in the degradation mechanism.

Removal of 2-chlorophenol from water by adsorption combined with TiO2 photocatalysis

Abstract The results of a laboratory-scale study of an environmentally friendly water treatment method is presented, where the organic model pollutant 2-chlorophenol (2-CP) was first adsorbed and then removed by a direct photolytic or heterogeneous photocatalytic process. The adsorbent was an organically treated (with hexadecylpyridinium chloride) clay mineral (montmorillonite), and the photocatalyst was Degussa P25. The total organic carbon and total inorganic chloride contents were measured to monitor the mineralization process, while the degradation of 2-CP and the formation of intermediates were followed by HPLC. No negative effect of the suspended adsorbent was observed either in the direct photolytic process or in the heterogeneous photocatalytic degradation process. Although direct photolysis allows the highest degree of 2-CP removal, in this case the degradation of the organic pollutant is accompanied by destruction of the adsorbent. The removal of 2-CP through the combination of adsorption and heterogeneous photocatalysis was achieved in a reasonable acceptable time interval, and can therefore be suggested as an efficient, cost-effective and environment-friendly water treatment facility.

Investigation of the photodecomposition of phenol in near-UV-irradiated aqueous TiO2 suspensions. I: Effect of charge-trapping species on the degradation kinetics

Abstract The effects of charge-trapping species on the kinetics of phenol decomposition were studied in near-UV-irradiated aqueous TiO 2 (anatase) suspensions in a batch photoreactor. The influence of catalyst loading, initial phenol concentration, dissolved O 2 concentration, Ag + content and H 2 O 2 concentration on the rate of phenol degradation is reported. The observed heterogeneous degradation of phenol followed apparently zero-order kinetics up to ca. 70% conversion. The Langmuir–Hinshelwood kinetic model successfully described the influence of the initial phenol concentration and dissolved O 2 concentration on the rate of heterogeneous photooxidation of phenol. The data obtained by applying the Langmuir–Hinshelwood treatment are consistent with the available kinetic parameters. The results of the experiments in the presence of Ag + indicated that the phototransformation of phenol can proceed via direct electron transfer, neither dissolved O 2 nor its reduction forms playing a significant role in the degradation mechanism.

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 1h of treatment and this suggests the formation of stable by-products which were identified using GC-MS and HPLC-MS, respectively.

Degradation of naproxen by UV, VUV photolysis and their combination

Naproxen is a widely used nonsteroidal anti-inflammatory drug. Recently, this medicine was detected both in natural waters (up to 1.5 μg L(-1)) and in sewage treatment plant effluents (up to 5.2 μg L(-1)). Moreover, naproxen is only partly eliminated by classical processes used in sewage treatment plants. Therefore, its degradation is of utmost interest. Advanced oxidation processes proved to be the most suitable methods for the elimination of persistent organic contaminants. In this work ultraviolet (UV, 254 nm), vacuum ultraviolet photolysis (VUV, 172 nm) and their combination (UV/VUV, 254/185 nm) were investigated. The efficiency of the methods increased in the following order: UV < VUV < UV/VUV photolysis. However, VUV irradiation was found to mineralize the contaminant molecule most effectively. The chemical structures of three out of four aromatic by-products and of some aliphatic carboxylic acids were presumed. The effects of dissolved O2 and the initial concentration of naproxen on the degradation were also investigated.

Ketoprofen removal by O3 and O3/UV processes: Kinetics, transformation products and ecotoxicity

Ozonation (O3) and its combination with ultraviolet radiation (O3/UV) were used to decompose ketoprofen (KET). Depending on the initial KET concentration, fourteen to fifty times faster KET degradation was achieved using combined O3/UV method compared to simple ozonation. Using both methods, formation of four major aromatic transformation products were observed: 3-(1-hydroxyethyl)benzophenone, 3-(1-hydroperoxyethyl) benzophenone, 1-(3-benzoylphenyl) ethanone and 3-ethylbenzophenone. In the combined treatment the degradation was mainly due to the direct effect of UV light, however, towards the end of the treatment, O3 highly contributed to the mineralization of small carboxylic acids. High (~90%) mineralization degree was achieved using the O3/UV method. Toxicity tests performed using representatives of three trophic levels of the aquatic ecosystems (producers, consumers and decomposers) Pseudokirchneriella subcapitata green algae, Daphnia magna zooplanktons and Vibrio fischeri bacteria showed that under the used experimental conditions the transformation products have significantly higher toxicity towards all the test organisms, than KET itself. The bacteria and the zooplanktons showed higher tolerance to the formed products than algae. The measured toxicity correlates well with the concentration of the aromatic transformation products, therefore longer treatments than needed for complete degradation of KET are strongly suggested, in order to avoid possible impact of aromatic transformation products on the aquatic ecosystem.

The photochemical behavior of hydrogen peroxide in near UV-irradiated aqueous TiO2 suspensions

Abstract The kinetic behavior of H 2 O 2 decomposition in near UV-irradiated aqueous TiO 2 suspensions was investigated. The effects of the H 2 O 2 concentration, the catalyst loading, the dissolved oxygen concentration, and the light intensity on the initial photodecomposition rate were studied. The observed kinetics were treated by both the Langmuir–Hinshelwood and the Freundlich model; the latter proved more successful for characterization of the system. The H 2 O 2 photodegradation rate was found to be directly proportional to the light intensity and it increased with the catalyst concentration up to a saturation limit. The concentration of dissolved oxygen had no significant effect on the photodecomposition rate.

TiO2-based heterogeneous photocatalytic water treatment combined with ozonation

ABSTRACT The degradations of aliphatic carboxylic acids (formic acid, acetic acid, and propionic acid) were investigated by a combination of TiO2-based photocatalysis and ozonation at pH = 2. The carboxylic acids were analyzed by high-performance liquid chromatography (HPLC), while the mineralization process was characterized by measuring the dissolved organic carbon (DOC) content. The efficiency of heterogeneous photocatalysis combined with ozonation was compared with that of ozonation and that of heterogeneous photocatalysis alone. In all cases, the combined system proved most effective in the oxidation of carboxylic acids. The degradation of carboxylic acids was accompanied by a continuous decrease in the DOC content, and aldehydes and carboxylic acids containing one carbon atom less than the starting material were formed as intermediates.

VUV intensity measurement of a 172 nm Xe excimer lamp by means of oxygen actinometry

Abstract The light power of a xenon excimer VUV source was determined by means of oxygen actinometiy. It was found that the method is not applicable for direct measurement of the light power mainly because of the side-reactions of ozone with oxygen atoms. Model calculations led to a simple equation between the quantum yield and the concentrations of ozone and oxygen. The correlation between the overall quantum yield and the ozone and oxygen concentration at room temperature is Φ = 2 - 1.l0[O3]/(O2]1.94, and at atmospheric oxygen pressure Φ = 2 - 520 [O3].

Oxidation of [Fe(CN)6]4- and reduction of [Fe(CN)6]3- in VUV-irradiated aqueous solutions

Vacuum ultraviolet (VUV) photolysis is one of the straightforward alternatives method among the advanced oxidation processes (AOPs) for the elimination of pollutants from water and air. The VUV photolysis of water produces hydroxyl radicals and hydrogen atoms, which have widely different oxidation and reduction abilities. In this work the oxidation and reduction properties of VUV-irradiated solutions were compared by investigating the reduction of [Fe(CN)6]3- and the oxidation of [Fe(CN)6]4-. The rate of oxidation of [Fe(CN)6]3- was found to be practically the same as the reduction rate of [Fe(CN)6]4- in the irradiated oxygen-free solutions under identical conditions. Dissolved oxygen strongly influences the redox properties of this system.