| The wide occurrence of pharmaceutical and personal care products (PPCPs) in natural aquatic environment received extensive scientific interest as well as public awareness recently due to their potential hazardous effect on human beings and ecological system. The environmental fate of these PPCPs, including transportation and transformation, is still largely unknown up to date. Photochemical degradation is known to be an important abiotic transformation pathway for organic compounds depletion in natural environment. Photochemical reactions can fall into two categories, namely direct photolysis and indirect photolysis. Direct photolysis occurs when the absorption spectrum of one compound overlap with the solar emitting spectrum. Indirect photolysis takes place by reaction with reactive oxygen species (ROS) and/or excited triplet state generated from photosensitizers such as nitrate and dissolved organic matter (DOM). For those organic compounds unable to undergo direct photolysis, indirect photolysis may play a critical role in limiting their occurrence in natural environment. On the other hand, the extensive and frequent detection of PPCPs in aquatic environment requires the development of high efficient, economic and environment-friendly advanced oxidation processes (AOPs) for their elimination. Heterogeneous semi-conductor photocatalysis using TiO2as the photocatalyst has recently been found to be a promising treatment technology for destructing organic pollutants, including PPCPs. Valence band holes (hvb+) and reductive conduction band electrons (ecb-) are generated after the photocatalyst excited by the photons with energy equal to or exceeding the band gap energy. The photogenerated holes can directly oxidize the adsorbed chemical substance or produce adsorbed hydroxyl radical (HO) via the surface-bound OH-and/or the adsorbed water molecules. hvb+and HO-have been known to be highly reactive species capable of destructing a variety of organic compounds.In the present study, the photochemical and photocatalytic degradation of atenolol (ATL) and2-phenylbenzimidazole-5-sulfonic acid (PBSA) have been systematically investigated in aqueous solutions with the purpose of evaluating the photofate as a potential loss process and photocatalysis as a promising treatment technology for degrading these two compounds. ATL and PBSA have been chosen as two model compounds of PPCPs in the current work due to their wide occurrence and relatively high level in natural aquatic environment. The main conclusions are as follows:(1) nitrate-induced photodegradation of ATL followed pseudo-first-order kinetics upon simulated solar irradiation. The photodegradation was found to be dependent on nitrate concentration and increasing the nitrate from0.5mM to10mM led to the enhancement of rate constant from0.00101min-1to0.00716min-1. Hydroxyl radical (HO)was determined to play a key role in the photolysis process by using isopropanol as molecular probe. Increasing the solution pH from4.8to10.4, the photodegradation rate slightly decreased from0.00246min-1to0.00195min-1, probably due to pH-dependent effect of nitrate-induced-OH formation. Bicarbonate decreased the photodegradation of ATL in the presence of nitrate ions mainly through pH effect, while humic substance inhibited the photodegradation via both attenuating light and competing radicals. Upon irradiation for240min, only10%reduction of total organic carbon (TOC) can be achieved in spite of72%transformation rate of ATL, implying a majority of ATL transformed into intermediate products rather than complete mineralization. The main photoproducts of ATL were identified by using solid phase extraction-liquid chromatography-mass spectrometry (SPE-LC-MS) techniques and possible nitrate-induced photodegradation pathways were proposed. The toxicity of the photo-transformation products was evaluated using aquatic species Daphnia magnna, and the results revealed that photodegradation was an effective mechanism for ATL toxicity reduction in natural waters.(2) Photocatalytic degradation of ATL was investigated in aqueous suspensions using TiO2as photocatalyst. Complete degradation of37.6μM ATL was obtained after60min irradiation in pH6.8Milli-Q water in the presence of2.0g L-1Degussa P25TiO2. Degradation of ATL followed pseudo-first-order reaction kinetics. HO· was determined to be the predominant reactive species during photocatalysis by means of radical probes. Major transformation products were elucidated by high performance liquid chromatograph-mass spectrometry (HPLC-MS) technique. ATL photodegradation pathways included generation of3-(isopropylamino)propane-1,2-diol and p-hydroxyphenylacetamide through ether chain cleavage, hydroxylation and the formation of4-[2-hydroxy-3-(isopropylamino)propoxy] benzaldehyde. Frontier electron densities calculation verified the formation of mono-hydroxylation products with HO·primarily attacking on benzene ring, which is in agreement with LC-MS analysis. Five carboxylic acids, i.e., oxalic, glyoxylic, malonic, oxamic and formic acids were identified by ion exchange chromatography by comparison with authentic standards. Photocatalytic degradation efficiency of ATL was highly dependent on the properties of the water matrix, such as pH, the presence of organic and inorganic species (e.g., humic substance, HCO3-). River water matrix was found to play a detrimental effect on ATL photocatalytic degradation with a longer irradiation time required for complete elimination of mother compound and intermediate products. Degussa P25exhibited the highest photocatalytic activity for oxidizing ATL as well as intermediates compared to Aldrich rutile, Millennium PC500and Hombikat UV100.(3) Photochemical degradation mechanism and pathways of PBSA were investigated under artificial solar irradiation with the goal of assessing the potential of photolysis as a transformation mechanism in aquatic environments. The quantum yield of PBSA direct photolysis in pH6.8buffer solution under filtered mercury lamp irradiation was determined as2.70×10-4. Laser flash photolysis (LFP) experiments confirmed the involvement of PBSA radical cation (PBSA+) during direct photolysis. Acidic or basic condition facilitated PBSA direct photolysis in aqueous solution. Indirect photolysis out-competes direct photolysis as a major process for PBSA attenuation only at higher level of photosensitizers (e.g., NO3->2mM). Thus, direct photolysis is likely to be the major loss pathway responsible for the elimination of PBSA in natural sunlit surface waters, while indirect photolysis (e.g., mediated by HO·) appeared to be less important due to a general low level of steady-state concentration of HO·([HO·]ss) in natural surface waters. Direct photolysis pathways of PBSA includes desulfonation and benzimidazole ring cleavage, which are probably initiated by the excited triplet state (3PBSA*) and radical cation (PBSA·+). Conversely, hydroxylation products of PBSA and2-phenyl-lH-benzimidazole as well as their ring opening intermediates were found in nitrate-induced PBSA photolysis, suggesting the indirect photodegradation was primarily mediated by HO· and followed a different mechanism.(4) The kinetics and mechanism of photocatalytic degradation of PBSA were studied in illuminated TiO2suspensions. Photocatalysis of PBSA were systematically investigated under different process conditions and water matrices. Experimental results demonstrated that PBSA photocatalytic reactions followed pseudo-first-order kinetics. Radical scavenging experiments indicated that hydroxyl radical (HO·) is the predominant reactive species responsible for an appreciable degradation of PBSA. Secondary order rate constant of PBSA-HO· reaction was determined to be5.8x109M-s-1by competition kinetics method. Major intermediates included hydroxylated products, benzamide, hydroxylated benzamidine, hydroxylated2-pheny-lH-benzimidazole as well as phenylimidazolecarboxylic derivatives which were elucidated by means of high performance liquid chromatography-mass spectrometry (HPLC-MS) technique. Four carboxylic acids, oxalic, malonic, acetic and maleic acids were detected during PBSA photocatalysis by HPLC-UV analysis. Ion chromatography (IC) results revealed that the sulfonic group of PBSA was primarily converted to sulfate ion while nitrogen atoms were released predominantly as ammonium and a less extent as nitrate. The reduction of TOC processed much more slowly compared to PBSA degradation, however, approximately80%TOC was removed after720min irradiation. A comparison of photocatalytic degradation of PBSA and structurally related compounds revealed that the5-sulfonic moiety in PBSA had negligible effect on the photocatalysis of2-pheny-lH-benzimidazole while2-phenyl substituent stabilized the benzimidazole ring system to photocatalytic degradation. |
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