Elsevier

Applied Catalysis B: Environmental

Volumes 140–141, August–September 2013, Pages 457-467
Applied Catalysis B: Environmental

Degradation of sunscreen agent 2-phenylbenzimidazole-5-sulfonic acid by TiO2 photocatalysis: Kinetics, photoproducts and comparison to structurally related compounds

https://doi.org/10.1016/j.apcatb.2013.04.046Get rights and content

Highlights

  • TiO2-induced photocatalysis of 2-phenylbenzimidazole-5-sulfonic acid.

  • Hydroxyl radical played a key role during photocatalysis.

  • Photodegradation pathways included hydroxylation and ring cleavage.

  • Frontier electron densities calculation predicted the sites for HOradical dot attacking.

  • 2-Phenyl substituent stabilized the benzimidazole ring to photocatalysis.

Abstract

The wide occurrence of sunscreen agent micropollutants in natural environment received extensive attention in recent years due to their potential endocrine disrupting effect. The present study focuses on the kinetics and mechanism of photocatalytic degradation of sunscreen agent 2-phenylbenzimidazole-5-sulfonic acid (PBSA) in illuminated TiO2 suspensions. Photocatalysis of PBSA was 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 (HOradical dot) is the predominant reactive species responsible for an appreciable degradation of PBSA. Second-order rate constant of PBSA-HOradical dot reaction was determined to be 5.8 x 109 M−1 s−1 by competition kinetics method. Major intermediates included hydroxylated products, benzamide, hydroxylated benzamidine, hydroxylated 2-pheny-1H-benzimidazole as well as phenylimidazolecarboxylic derivatives which were elucidated by means of high performance liquid chromatograph–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 to a lesser extent as nitrate. The reduction of TOC processed much more slowly compared to PBSA degradation, however, approximately 80% TOC was removed after 720 min irradiation. A comparison of photocatalytic degradation of PBSA and structurally related compounds revealed that the 5-sulfonic moiety in PBSA had negligible effect on the photocatalysis of 2-pheny-1H-benzimidazole while 2-phenyl substituent stabilized the benzimidazole ring system to photocatalytic degradation.

Introduction

Sunscreens are widely used in personal care products (PCPs) in recent years due to the public awareness of protection against ultraviolet (UV) irradiation and skin cancer [1]. Sunscreens enter the aquatic environment directly as consequence of recreational water activities (e.g., bathing and swimming) or indirectly via wastewater treatment plants (WWTPs) as result of the use of cosmetics, showering, washing, rubbing off and excretion after dermal application [2], [3]. The extensive use of sunscreens already led to frequent detection in many aquatic ecosystems with a considerably high concentration [2], [3], [4]. Indeed, sunscreens are regarded as a new kind of environmental contaminants according to the US Environmental Protection Agency [5]. Recent studies have provided evidence that organic sunscreens possess estrogenic properties and behavior as endocrine disrupting chemicals (EDCs) in the environment [6], [7], [8], [9], [10], [11], [12]. Furthermore, photo-induced transformation products of some kinds of sunscreens under natural solar irradiation were found to be more toxic than their parent species, which could be a further danger to the biota in environment [9]. Therefore, the long-term and extensive use of sunscreens may cause irreversible adversity on the ecology system.

2-Phenylbenzimidazole-5-sulfonic acid (PBSA), as one kind of sunscreen, is widely used in sunscreen formulations and cosmetics because of its strong absorption in the UVB region (see Fig. 1). It has also been approved by the U.S. Food and Drug Administration as an effective sunscreen ingredient based on its ability to prevent erythema [1]. Due to its high polarity and continued input into the environment through personal care applications and incomplete elimination during wastewater treatment, PBSA was frequently detected in natural surface waters with a concentration ranging from 109 to 2679 ng L−1 [10]. PBSA has been known to photochemically generate reactive oxygen species (i.e., 1O2 and O2radical dot) under UV irradiation and cause DNA damage. For example, Johnson Inbaraj et al. reported that PBSA showed strong oxidizing properties when UV irradiated in neutral aqueous solution in the presence of cysteine, glutathione and azide [13]. Zhang et al. developed a computational method based on the density functional theory (DFT) to predict and evaluate the photodegradation behavior of PBSA and found that energy and electron transfer reactions of excited state PBSA (PBSA*) could photogenerate 1O2 and O2radical dot [14], [15]. Therefore, it is essential to develop advanced treatment technologies for eliminating PBSA in aqueous solutions for the sake of reducing potential risk to ecological system.

During the past years, various advanced oxidation processes (AOPs) have been reported for the decomposition of organic pollutants in aqueous matrices. Among them, heterogeneous semiconductor photocatalysis using TiO2 as the photocatalyst has been found to be a promising treatment technology for eliminating organic pollutants, including sunscreens and other PCPs. When irradiating with photons of energy equal to or exceeding the band gap energy of TiO2 (for anatase, 3.2 eV band gap), valence band holes (hvb+) and reductive conduction band electrons (ecb) are generated [16], [17]. The photogenerated holes can: (i) directly oxidize the adsorbed chemical substance; or (ii) produce adsorbed hydroxyl radical (HOradical dot) via the surface-bound OH and/or the adsorbed water molecules. As a highly reactive oxidant, HOradical dot is capable of unselectively reacting with most recalcitrant organic contaminants at near-diffusion-controlled rates in water (108–1010 M−1 s−1). Therefore, TiO2 is considered to be the most suitable semiconductor for widespread environmental photocatalytic applications because it is photoactive, non-toxic, inexpensive, and relatively biologically and chemically stable [16], [17].

This study reports on the photocatalytic degradation of PBSA in aqueous suspensions of TiO2. The main purpose of this study is to (i) study the influence of process condition and water matrix on photocatalytic degradation; (ii) clarify the dominant reactive species involved in photocatalysis process; (iii) characterize intermediates and photoproducts; (iv) elucidate the mechanism and transformation pathways leading to PBSA mineralization; (v) compare with structurally related compounds in photocatalytic degradation. To the authors’ knowledge, this is the first study that includes a systematic exploration of kinetics, intermediates and degradation pathways of PBSA photocatalytic degradation.

Section snippets

Chemicals and materials

PBSA, 2-phenyl-1H-benzimidazole and benzimidazole were purchased from Sigma–Aldrich and used as received. Commercial humic acid (sodium salt) was purchased from Aldrich. HPLC grade methanol and acetonitrile were obtained from Fisher Scientific. Other reagents were at least of analytical grade and used as received without further purification. Non-porous titanium dioxide (Degussa P25 TiO2, Germany) with primary particle diameter of 30 nm, specific surface area of 50 m2 g−1 and crystal distribution

Preliminary experiments of PBSA photocatalytic degradation

Preliminary experiments of hydrolysis (without TiO2 in dark), direct photolysis (without TiO2 under irradiation) and adsorption (with TiO2 in dark) were carried out to assess their contribution to photocatalytic degradation of PBSA by Degussa P25 TiO2 as presented in Fig. 2A. As seen, no loss of PBSA was observed due to hydrolysis, suggesting PBSA was stable in Milli-Q water under dark condition. Dark controls also indicated that PBSA was stable in aqueous solution with pH ranging from 3 to 13

Conclusions

The aqueous photocatalytic degradation of sunscreen agent 2-phenylbenzimidazole-5-sulfonic acid (PBSA) has been studied in illuminated TiO2 suspensions. TiO2-induced photocatalysis resulted in a pronounced degradation of PBSA. Photocatalytic reactions followed pseudo-first-order kinetics. Radical scavenger experiments indicated that HOradical dot was the predominant radical responsible for an appreciable degradation of PBSA. Second-order rate constant of PBSA-HOradical dot reaction was determined to be 5.8 x 109 M−1 s−1

Acknowledgements

This work was supported by National Natural Science Foundation of China (Nos. 20977045 and 21177056) and Fundamental Research Funds for the Central Universities (1112021101) and Yuefei Ji gratefully acknowledges the China Scholarship Council (CSC) for the financial support. The authors also would like to thank the scientific services of Institut de recherches sur la catalyse et l’environnement de Lyon (IRCELYON).

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