Experimental and simulation investigations of UV/persulfate treatment in presence of bromide: Effects on degradation kinetics, formation of brominated disinfection byproducts and bromate

https://doi.org/10.1016/j.seppur.2020.116767Get rights and content

Highlights

  • Br suppressed the degradation of diethyl phthalate in UV/PS system significantly.

  • Tribromomethane formed and was unstable during the conversion of Br in UV/PS.

  • Br would be eventually transformed to BrO3- in the UV/PS process.

  • Brradical dot contributed more to oxidizing HOBr/OBr to BrO3- than SO4- and HOradical dot did.

Abstract

Bromide (Br), which is omnipresent in water bodies, can not only affect the degradation kinetics of target pollutants but may also form undesired byproducts (such as bromate (BrO3-) and brominated disinfection byproducts (Br-DBPs)) through sulfate radical (SO4-)-based advanced oxidation processes (AOPs). This study found that Br significantly suppressed the degradation of diethyl phthalate (DEP) based on the ultraviolet (UV)/persulfate (PS) treatment and was efficiently converted into reactive species (radicals and free bromine) by the SO4- or hydroxyl radical (HOradical dot) generated in the UV/PS system. These reactive bromine species could, in turn, brominate the phenolic degradation intermediates of DEP as well as natural organic matter (NOM), yielding Br-DBPs, including tribromomethane (TBM), which was observed when Br, DEP and/or NOM coexisted in this UV/PS system. However, the Br-DBPs were a short-lived form of bromine during the transformation of Br and were degraded by the excessive oxidants (e.g., SO4- and HOradical dot). Instead, Br was eventually transformed into BrO3-, with free bromine acting as the requisite intermediate. The BrO3- formation initially showed a delay before increasing monotonically. In general, raising the PS dosage and the initial Br concentrations enhanced both the maximum concentration of TBM and the formation of BrO3-, while increasing the amounts of DEP and NOM facilitated the former and inhibited the latter. The maximum concentration of TBM increased while the formation of BrO3- was suppressed with increasing pH from 5.0 to 8.0. In addition, through simulating the steady-state concentration of radicals, it was found that the contribution of bromine atom radical (Brradical dot) towards oxidizing free bromine to BrO3- was far greater than those of SO4- and HOradical dot. The findings demonstrate the potential negative effects of Br on SO4--based AOPs, which need to be considered when this technology is applied in practice.

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 (SO4-)-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]. SO4-, 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]. SO4- 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]. SO4--based AOPs can also generate secondary oxidants, such as hydroxyl radicals (HOradical dot), carbonate radicals, and superoxide radicals (O2-), which may affect the process itself [4]. In contrast to the case for HOradical dot-based AOPs, the high aqueous solubility and relative stability of the precursors (i.e., PS and PMS) for SO4--based AOPs [4], the strong reactivity and selectivity of SO4- and its relatively high stability [6], as well as the wider operational pH range [7], make SO4--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 HOradical dot and ozone, the highly reactive SO4- can also react with Br and quickly oxidize it to free bromine (HOBr/BrO and Br2) and bromine radicals (such as Brradical dot, Br2-, and BrOHradical dot) (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].SO4-+Br-SO42-+Br,k=3.5×109M-1s-1OH-+BrBrOH-,k=1.1×1010M-1s-1,k-=4.2×106s-1Br+Br-Br2-,k=1.2×1010M-1s-1,k-=1.9×104s-1Br+BrBr2,k=1.0×109M-1s-1Br2-+BrBr2+Br-,k=2.0×109M-1s-1Br2-+Br2-Br2+2Br-,k=1.9×109M-1s-1Br2+H2OHOBr+H++Br-,k=97s-1,k-=1.6×1010M-2s-1HO+Br-BrOH-,k=1.1×1010M-1s-1,k-=3.3×107s-1

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 SO4--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 (BrO3-) through reactions with oxidants such as ozone, HOradical dot, and SO4-. These oxidative processes involve two steps: the conversion of Br into HOBr/BrO which is a critical intermediate, and then into BrO3- as the final product [9], [24], [25]. BrO3- is a possible human carcinogen and its drinking water standard is only 10 µg L−1 [26], [27], [28]. However, the concentration of formed BrO3- 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 SO4- [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 BrO3- 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 BrO3- 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 SO4-, the effects of Br on contaminant degradation, and the formation of hazardous byproducts, including Br-DBPs and BrO3-, should be taken into consideration when SO4--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).

References (54)

  • Z. Wang et al.

    Degradation kinetic of phthalate esters and the formation of brominated byproducts in heat-activated persulfate system

    Chem. Eng. J.

    (2019)
  • H.V. Lutze et al.

    Formation of bromate in sulfate radical based oxidation: Mechanistic aspects and suppression by dissolved organic matter

    Water Res.

    (2014)
  • J. Lu et al.

    Transformation of bromide in thermo activated persulfate oxidation processes

    Water Res.

    (2015)
  • G. Wen et al.

    Bromate formation during the oxidation of bromide-containing water by ozone/peroxymonosulfate process: Influencing factors and mechanisms

    Chem. Eng. J.

    (2018)
  • Z. Wang et al.

    Comprehensive study on the formation of brominated byproducts during heat-activated persulfate degradation

    Chem. Eng. J.

    (2020)
  • F. Soltermann et al.

    Options and limitations for bromate control during ozonation of wastewater

    Water Res.

    (2017)
  • Z. Wang et al.

    Degradation of diethyl phthalate (DEP) by UV/persulfate: An experiment and simulation study of contributions by hydroxyl and sulfate radicals

    Chemosphere

    (2018)
  • Z. Wang et al.

    Degradation kinetic of dibutyl phthalate (DBP) by sulfate radical- and hydroxyl radical-based advanced oxidation process in UV/persulfate system

    Sep. Purif. Technol.

    (2018)
  • P. Xie et al.

    Impact of UV/persulfate pretreatment on the formation of disinfection byproducts during subsequent chlorination of natural organic matter

    Chem. Eng. J.

    (2015)
  • A. Ghauch et al.

    Ibuprofen removal by heated persulfate in aqueous solution: A kinetics study

    Chem. Eng. J.

    (2012)
  • A. De Luca et al.

    Effects of bromide on the degradation of organic contaminants with UV and Fe2+ activated persulfate

    Chem. Eng. J.

    (2017)
  • K. Ichihashi et al.

    Brominated trihalomethane formation in halogenation of humic acid in the coexistence of hypochlorite and hypobromite ions

    Water Res.

    (1999)
  • K. Liu et al.

    Formation of brominated disinfection by-products and bromate in cobalt catalyzed peroxymonosulfate oxidation of phenol

    Water Res.

    (2015)
  • Z. Li et al.

    Bromate formation in bromide-containing water through the cobalt-mediated activation of peroxymonosulfate

    Water Res.

    (2015)
  • C. Luo et al.

    Simulation and comparative study on the oxidation kinetics of atrazine by UV/H2O2, UV/HSO5 and UV/S2O82

    Water Res.

    (2015)
  • Y. Liu et al.

    Mechanistic insight into suppression of bromate formation by dissolved organic matters in sulfate radical-based advanced oxidation processes

    Chem. Eng. J.

    (2018)
  • U. von Gunten

    Ozonation of drinking water: Part II. Disinfection and by-product formation in presence of bromide, iodide or chlorine

    Water Res.

    (2003)
  • Cited by (29)

    View all citing articles on Scopus
    View full text