Experimental and simulation investigations of UV/persulfate treatment in presence of bromide: Effects on degradation kinetics, formation of brominated disinfection byproducts and bromate
Graphical abstract
Introduction
Water detoxification is an important process for ensuring the recovery of organic matter contaminated waters and thus achieving water conservation. Advanced oxidation processes (AOPs) (e.g., ozonation and the Fenton process) are considered as an effective means of removing organic pollutants that are refractory to biological degradation [1]. Therefore, these technologies are being studied extensively. Recently, sulfate radical ()-based AOPs have attracted a lot of interest for use as alternative oxidative treatments for organics in water and wastewater treatment, as well as for groundwater and soil remediation [2], [3], [4]. , which is highly oxidative and exhibits a high standard reduction potential (2.5–3.1 V), is capable of degrading a wide range of contaminants, including phenols, BTEXs (benzene, toluene, xylenes, and ethylbenzene), perfluorinated compounds, halogenated olefins, pharmaceuticals, pesticides, and various inorganics [4]. can be formed by activating persulfate (PS) or peroxymonosulfate (PMS) by increasing the temperature, exposure to ultraviolet (UV) radiation, or treatment with alkaline, transition metals, or carbon materials, among others. [5]. -based AOPs can also generate secondary oxidants, such as hydroxyl radicals (HO), carbonate radicals, and superoxide radicals (), which may affect the process itself [4]. In contrast to the case for HO-based AOPs, the high aqueous solubility and relative stability of the precursors (i.e., PS and PMS) for -based AOPs [4], the strong reactivity and selectivity of and its relatively high stability [6], as well as the wider operational pH range [7], make -based AOPs good alternatives for controlling recalcitrant micropollutants. However, one major shortcoming of these AOPs is the possibility of the formation of oxidation byproducts owing to reactions between the oxidants and the compounds in the water matrix or the micropollutants, some of which can have detrimental effects on the human body and the environment [8], [9], [10].
The elevated concentrations of halides in aquatic environments have become a matter of increasing concern, with bromide (Br−), which can be released during seawater intrusion, prolonged droughts, and human activities, including coal-fired power plants and industrial wastewater effluents, and seawater desalination, attracting particular attention [11], [12], [13], [14]. Br− is present in aquatic environments all over the world, and its concentration is approximately 837.5 μM in seawater and 0.1 μM to more than 10 μM in fresh water [8], [15]. It has been suggested that the presence of Br− can have a notable effect on the outcome of oxidation treatments. For instance, it can affect the degradation of the target organics by competing for the oxidants with these organics [16], [17] and cause the formation of undesired compounds [18], [19]. Thus, the fate of this ion during the water treatment process is a topic of active research [8], [19], [20]. Similar to other oxidative species, such as HO and ozone, the highly reactive can also react with Br− and quickly oxidize it to free bromine (HOBr/BrO− and ) and bromine radicals (such as Br, , and BrOH−) (some of these reactions are given as Eqs. (1), (2), (3), (4), (5), (6), (7), (8)) [18]. These reactive bromine species (RBSs), i.e., bromine radicals and free bromine, are electrophilic and can selectively react with electron-rich species, causing the bromination of these species [10]. The conversion of Br− and the reactivity of these RBSs have been confirmed by a number of studies [10], [18], [19].
The aforementioned RBSs show high reactivities towards phenolic compounds, exhibiting apparent second-order rate constants (k) that range from 103 to 105 M−1 s−1 [8]. Natural organic matter (NOM) is known to generate disinfection byproducts (DBPs) when it reacts with free chlorine during water chlorination [19], [21]. More precisely, the phenolic groups in NOM are the chief reactive sites involved in the halogenation process and result in the formation of DBPs [22]. Because both Br− and NOM are inevitable in aquatic environments, it is likely that brominated byproducts (Br-DBPs) form when -based AOPs are employed during water treatment. In addition, the target organic contaminants and their degraded intermediates, which contain phenolic moieties, can also be the precursors during bromination [10], [23].
RBSs can further be transformed into bromate () through reactions with oxidants such as ozone, HO, and . These oxidative processes involve two steps: the conversion of Br− into HOBr/BrO− which is a critical intermediate, and then into as the final product [9], [24], [25]. is a possible human carcinogen and its drinking water standard is only 10 µg L−1 [26], [27], [28]. However, the concentration of formed can exceed its drinking water regulation level in Br−-containing water treated with AOPs [24], [25].
We had shown recently that the UV/PS treatment can efficiently degrade phthalate esters [29], [30]. However, the effects of Br− on this degradation system and the transformation of Br– during this UV/PS process remain unknown. In this treatment, UV radiation was used to activate the PS because it is one of the most efficient and simplest ways of generating [9], [31]. Thus, the objectives of the present research were: (1) to reveal the influences of Br− on the degradation kinetics of a phthalate ester, diethyl phthalate (DEP), in the UV/PS system; (2) to investigate the kinetics and possible formation pathways of and Br-DBPs from Br− during this DEP degradation process, along with the impacts of the changes in the reaction conditions; and (3) to simulate the formation of and elucidate the contributions of the radicals to the formation process.
Section snippets
Chemicals
Sodium persulfate (Na2S2O8), sodium hydroxide (NaOH), and sulfuric acid (H2SO4) were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). The trihalomethane (THM) calibration mixtures were bought from Sigma-Aldrich (USA). Sodium bromate (NaBrO3), sodium bromide (NaBr), methyl tert-butyl ether, methanol, DEP and humic acid (HA) (used as the NOM in this study) were bought from Aladdin (China). The chemicals used were of analytical grade or higher. All the solutions were prepared from
Effects of Br− on DEP degradation and formation of TBM and bromate in UV/PS system under different PS dosages
Fig. 1 displays the degradation of DEP in the UV/PS system with Br− coexisting in the solution; the process followed pseudo-first-order kinetics. Br− retarded the degradation significantly, with its effect becoming more pronounced with an increase in its concentration. As the concentration of Br− increased from 0 to 1000 µM, the pseudo-first-order rate constant for the degradation of 2 µM DEP in UV/PS system (500 µM PS) declined from 0.0551 to 0.0004 min−1 (Fig. 1). Because Br− exhibits higher k
Conclusions
Br− is widely present in aquatic environments and its concentration ranges from approximately 0.1 μM to more than 10 μM in fresh water and approximately 837.5 μM in seawater. Thus, the interactions of Br− with , the effects of Br− on contaminant degradation, and the formation of hazardous byproducts, including Br-DBPs and , should be taken into consideration when -based AOPs are used for water treatment.
In this study, it was observed that the degradation efficiency of DEP by the
CRediT authorship contribution statement
Ziying Wang: Formal analysis, Investigation, Visualization, Writing - original draft. Na An: Supervision. Yisheng Shao: Conceptualization. Naiyun Gao: Writing - review & editing. Erdeng Du: Writing - review & editing. Bin Xu: Resources.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This research was supported by the National Natural Science Foundation of China (No. 51608372) and the National Major Science and Technology Project of China (No. 2017ZX07207004 and 2014ZX07406002).
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