Enhancement of Photo-bromination of Phenol by Anthraquinone-2-sulphonate and Benzophenone: Implication for Photo-production of Organic Brominated Compounds by Dissolved Organic Matter in Marine Environment

Organobromine compounds are of great ecological risks due to their high toxicity on organisms. Photochemical halogenation reaction may represent an important natural formation process of natural organobromine compounds in marine environment. Here we reported the enhanced formation of bromophenols from phenol by sunlit anthraquinone-2-sulphonate (AQ2S) and benzophenone (BP) in aqueous bromide solutions. Quinones and aromatic ketones are ubiquitous components of dissolved organic matter (DOM) in surface waters, and AQ2S and BP were adopted here as proxies of DOM. Bromophenols’ formation increased with the increasing of the concentrations of AQ2S and BP, and the promotion effect was in the order AQ2S > BP, indicating that sunlit DOM plays an important role for the formation of reactive bromine species. Chloride was found to promote the formation of bromophenols obviously, suggesting a possible role of the mixed reactive halogen species. Finally, the natural DOM from Suwannee River was found to enhance photobromoination at a low concentration (1 mg L -1 ) in aqueous bromide solution. Our results demonstrated the importance of reactive halogen species generation from sunlit DOM, which possibly contributes to the abiotic source of organohalogen compounds in marine environment.


Introduction
Halogenated compounds, such as halophenols, are usually environmentally harmful, with a potential to act as or transfer to persistent organic pollutants. In recent years, more and more attention has been paid to the natural formation of halogenated organic compounds in marine environment, especially the photochemical synthesis of organobromine compounds, such as polybrominated diphenyl ether, bromophenols and brominated alkanes (Tamtam and Chiron 2012; Hao et al. 2017; Liu et al. 2020). One reason is that organobromine compounds can be produced through both natural biotic and abiotic processes in marine environment, which plays an important role in the biogeochemical cycling of bromine (Gribble 2003;Méndez-Díaz et al. 2014). More importantly, organobromine compounds can potentially cause more serious adverse health effects, due to their higher cytotoxicity and genotoxicity as compared to their chlorinated analogues (Komaki et al. 2009).
The photochemical formation of organobromine compounds is started by the production of brominated agents, involving radical and non-radical reactive bromine species (RBS), upon photolysis of bromide in the presence of some oxidants or photocatalysts (Calza et al. 2008 In this study, anthraquinone-2-sulphonate (AQ2S) and benzophenone (BP) were selected as two DOM sample models to illustrate the ability of quinones-and aromatic ketones-like components to generate RHS. It is highly possible that quinone and aromatic ketone structures exist in natural water, since they could be formed through the partial oxidation of lignin precursors (Sharpless and Blough, 2014). formation was also assessed in this work.

Standards and reagents
Phenol, 2-bromophenol, 4-bromophenol, AQ2S and BP were purchased from Sigma-Aldrich, and 2hydroxy-5-chlorobiphenyl was purchased from AccuStandard. SRNOM was obtained from International Humic Substances Society (IHSS, Darmstadt, Germany). The UV-vis spectra of AQ2S, BP and SRNOM were in Fig. S1. All other chemicals were reagent grade and used as received. Ultrapure water was obtained with a Millipore water puri cation water unit for preparing all aqueous solutions.

Photochemical experiments
The photochemical experiments were conducted in the simulated sunlight source (Phchem III, Beijing Newbit Technology Co., Ltd) which contains a 500 W xenon arc lamp and lters to cut off the light with wavelengths below 290 nm, and the light intensity of the solar simulator was 15 mW cm -2 . Aqueous samples (100 mL) were held in quartz tubes and cooled by a fan during irradiation; the dark control tubes wrapped in Al foil were also placed in the solar simulator. Two parallel samples were set up in experiments.

Chemical analysis
The amounts of bromophenols were extracted by dichloromethane and then analyzed by gas chromatography mass spectrometry (Agilent 7890 GC and 5975C MSD) using 2-hydroxy-5-chlorobiphenyl as the internal standard. The details are in Text S1 of the supporting information.
Phenol bromination takes place in two steps, involving a phenoxyl radical formation and bromine radical The rate constants for eq. 6 and eq.7 follows the order AQ2S > BP, which is consistent with the order of their reduction potential. The reduction potentials, E 3S/S-, for BP and AQ2S are 1.67 V NHE and 2.28 V NHE , respectively. Therefore, the promotion effect of AQ2S on phenol bromination was more obvious than that of BP.
In addition, although there ensued a long-lasting controversy about the possibility for excited AQ2S to generate • OH upon oxidation of water, some studies have indicated the possibility that free • OH was being formed with signi cant amounts (Alegía et al. 1999; Maurino et al. 2008). Therefore, AQ2S promoted phenol bromination via generating • OH as well as 3 AQ2S * , which are helpful for bromine radicals and phenoxyl radical production.  Fig. S4. The ternary exciplex has a lower tendency than the binary exciplex to decay to the ground state since it has weaker spin-orbit coupling of the incipient radical. Consequently, ternary exciplex dissociates to the radical products, Br 2 •− , more favorably than the binary exciplex. High concentrations of halide could favor ternary exciplex formation, thus increase the generation of bromophenols.

Effect of bromide and chloride concentrations
Considering chloride is the main anion in seawater, the effect of chloride on phenol bromination was studied next. Fig. 4 shows addition of 0.5 mol L -1 chloride enhanced the formation of bromophenols obviously, especially in the presence of AQ2S (Fig. 4C and D). As mentioned above, high concentration of chloride helped to generate ternary exciplex, 3  12) and subsequently generate Br 2 •− (eq. 9) (Zhang and Parker 2018). Therefore, the promotion effect of chloride on the bromophenols' formation was really signi cant with AQ2S. In contrast, the promotion effect of chloride in the presence of BP was not so obvious ( Fig. 4 A and B), which should be attributed to the lower oxidizing ability of 3 BP * . Although no data are available for the reduction potential of BrCl •− , it is typically assumed to present reduction potential between or similar to Br 2 •− (E Br2•−/Br-= 1.63 V NHE ) and Cl 2 •− (E Cl2•−/Cl-= 2.20 V NHE ). E 3S/S-for BP is 1.67 V NHE , so 3 BP * did not e ciently produce BrCl •− , consequently the promotion effect of chloride in the presence of BP was much lower than that of AQ2S.
It is noticeable that the concentrations of 4-bromophenol were higher that 2-bromophenol in almost all cases, especially in Fig. 4C As far as eq. 13 and 14 are concerned, it is decided that the halogen radicals generation depends on the oxidation capability of 3 DOM* which is determined by DOM structure. Although the relationship between the oxidation capability and the structure of DOM needs further investigation, it is undeniable that organic compounds with quinone and aromatic ketone structure can accelerate the bromination reaction. As we know, river DOM mainly contains humic-or fulvic-like components while marine DOM mainly contains protein-or amino acid-like component, whereas river DOM exists in estuarine and coastal aqueous environment due to the mixture of river and sea water. Nevertheless, this result justi ed the enhancement of DOM for the photochemical bromination reaction in the saline water that is relevant to the marine environment.
Although this research focused on aqueous conditions, RHS formation from direct halide oxidation by 3 DOM * likely also applies to marine aerosols, where halide concentrations can exceed those in seawater (Finlayson-Pitts 2003). Since non-radical dihalogen species form as products of radical RHS reactions, e.g., 2 ClBr •− → ClBr + Br − + Cl − , this pathway may contribute to the release of halogens to the atmosphere, with implications for tropospheric ozone degradation.

Conclusion
Here it was shown that AQ2S and BP enhanced the photochemical bromination of phenol, with the promotion effects of AQ2S > BP. These results indicated that DOM with quinone and aromatic ketone structures could promote photochemical bromination reaction. Chloride obviously promoted the formation of bromophenols, suggesting an important role of the mixed reactive halogen species. These results demonstrated the importance of direct halide oxidation of 3 DOM * for RHS generation in marine environment.

Con icts of interest
There are no con icts to declare.