Cobalt catalyzed peroxymonosulfate oxidation of tetrabromobisphenol A: Kinetics, reaction pathways, and formation of brominated by-products

Journal of Hazardous Materials 313 (2016) 229–237

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Cobalt catalyzed peroxymonosulfate oxidation of tetrabromobisphenol A: Kinetics, reaction pathways, and formation of brominated by-products Yuefei Ji a,1 , Deyang Kong b,1 , Junhe Lu a,∗ , Hao Jin a , Fuxing Kang a , Xiaoming Yin a , Quansuo Zhou a a b

Department of Environmental Science and Engineering, Nanjing Agricultural University, Nanjing 210095, China Nanjing Institute of Environmental Science, Ministry of Environmental Protection of PRC, Nanjing 210042, China

g r a p h i c a l

a b s t r a c t

h i g h l i g h t s • • • • •

Cobalt catalyzed peroxymonosulfate oxidation of tetrabromobisphenol A. Phenolic moiety was the reactive site for sulfate radical attack. Pathways include ␤-scission, oxidation, debromination and coupling reactions. Brominated disinfection by-products were found during TBBPA degradation. Humic acid inhibited TBBPA degradation but promoted DBPs formation.

a r t i c l e

i n f o

Article history: Received 12 February 2016 Received in revised form 6 April 2016 Accepted 13 April 2016 Available online 14 April 2016 Keywords: Cobalt catalyzed peroxymonosulfate oxidation Sulfate radical Tetrabromobisphenol A Brominated disinfection by-products http://dx.doi.org/10.1016/j.jhazmat.2016.04.033 0304-3894/© 2016 Published by Elsevier B.V.

a b s t r a c t Degradation of tetrabromobisphenol A (TBBPA), a flame retardant widely spread in the environment, in Co(II) catalyzed peroxymonosulfate (PMS) oxidation process was systematically • explored. The second-order-rate constant for reaction of sulfate radical (SO4 − ) with TBBPA was determined to be 5.27 × 1010 M−1 s−1 . Apparently, degradation of TBBPA showed firstorder kinetics to the concentrations of both Co(II) and PMS. The presence of humic acid (HA) and bicarbonate inhibited TBBPA degradation, most likely due to their competition for • SO4 − . Degradation of TBBPA was initiated by an electron abstraction from one of the phenolic rings. Detailed transformation pathways were proposed, including ␤-scission of isopropyl bridge, phenolic ring oxidation, debromination and coupling reactions. Further oxidative degradation of intermediates in Co(II)/PMS process yielded brominated disinfection by-products (Br-DBPs) such as bromoform and brominated acetic acids. Evolution profile of Br-DBPs showed an initially increasing and then decreasing pattern with maximum concentrations occurring around 6–10 h. The presence of HA enhanced the formation of Br-DBPs significantly. These findings reveal potentially important, but previously unrecognized, formation of Br-DBPs during sulfate radical-based oxidation of bromide-containing organic compounds that may pose toxicological risks to human health. © 2016 Published by Elsevier B.V.

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1. Introduction Tetrabromobisphenol A (TBBPA) is an important brominated flame retardant with nearly 60% of the total market share [1]. TBBPA is primarily used in the synthesis of brominated epoxy and polycarbonate resins which are the raw material of printed circuit boards [2]. This substance is now ubiquitous in the environment due to the widespread use of related products [3,4]. TBBPA is highly lipophilic and persistent [1,2]. It is demonstrated to have hepatotoxicity, cytotoxicity, and immunotoxicity [5]. It can also disrupt thyroid homeostasis and estrogen signaling by acting as both a thyroid hormone agonist and an estrogen agonist [6]. TBBPA is refractory to microbial degradation with half-life times of 65 d in activated sludge and 430 d in anaerobic soil [7]. Advanced oxidation processes (AOPs) are relatively effective to remove TBBPA. Degradation of TBBPA by photolysis [8–10], photocatalysis [11,12] and photo-Fenton [13,14] technologies was reported. Other oxidants, such as manganese dioxide[15], permanganate [16], and ferrate [17] were also found to be effective in the removal of TBBPA. In most of these processes, hydroxylation/debromination and ␤-scission of isopropyl bridge were the principal degradation pathways, resulting in the formation of various transformation products with reduced hormonal activities [15–17]. • Sulfate radical (SO4 − ) is one of the strongest oxidants with a standard reduction potential (E◦ ) of 2.5–3.1 V (depending on pH) which is comparable to hydroxyl radical (OH• ) [18]. It reacts with a wide spectrum of compounds via electron transfer, H-atom abstraction, and addition-elimination mechanisms with second• order rate constants ranging from 106 to 109 M−1 s−1 [19]. SO4 − can be generated by activation of peroxydisulfate (PDS) or peroxymonosulfate (PMS) with heat, UV light, chelated or unchelated transition metals, etc [20–26]. Compared to H2 O2 and ozone which serve the precursors of OH• , PDS and PMS are more stable and read• ily delivered to subsurface [27]. In addition, SO4 − based advanced oxidation process (SR-AOPs) have a wider operational pH range than OH• based advanced oxidation processes (OH-AOPs) which usually maintain high efficiency at acidic conditions [28]. Giving these advantages, SR-AOPs are ideal in situ remediation technologies for contaminated soils and groundwaters although PDS and PS are more expensive than H2 O2 on a molar basis [20,29]. Of various activation approaches, Co(II) catalyzed PMS is attractive due to its high efficiency, low catalyst demand and no requirement for post sludge treatment (Eqs. (1)–(6)) [22,30]. 3+ − Co2+ + HSO− + SO• − 5 → Co 4 + HO 3+

+ HSO− 5

2+

+ SO• − 4

Co Co

2SO• − 5

↔

2+

→ Co →

SO2− 4

+ SO• − 5

+H

+

3+

+ Co

− O3 SOOOOSO− 3

→

(1) (2) (3)

•− {SO• − 4 O2 SO 4 }

(4)

2− •− {SO• − 4 O2 SO 4 } → O2 + S2 O8

(5)

•− {SO• − 4 O2 SO 4 }

(6)

→

O2 + 2SO• − 4

Removal of TBBPA in SR-AOPs has been explored. For example, • Ding et al. reported the degradation of TBBPA by SO4 − produced by CuFe2 O4 catalyzed PMS decomposition [31]. Guo et al. studied the degradation of TBBPA in a UV/base/PS system [32]. Both of the studies found that TBBPA degradation involved ␤-scission and debromination reactions. Although how TBBPA molecules are • broken down by SO4 − is relatively understood, little attention has

∗ Corresponding author. E-mail address: [email protected] (J. Lu). 1 Co-first author.

been paid to the transformation and fate of bromine during this • process. Release of Br− due to debromination in SO4 − oxidation processes is of great concern because halides can be converted • • to reactive halogen species by SO4 − (e.g., HOBr, Br• , and Br2 − ) [17]. These reactive halogen species are electrophiles and can react with dissolved electron-rich compounds, leading to the formation of halogenated by-products [33,34]. Prior studies found the forma• tion of CCl4 in SO4 − -mediated degradation of 2,4-dichlorophenol and 2,4,6-trichlorophenol [33,34]. Several studies demonstrated the formation of hazardous brominated disinfection by-products (Br-DBPs, including bromoform and bromoacetic acids) and bromate (BrO3 − ) in SR-AOPs in the presence of Br− and natural organic matter (NOM) [35–38]. In the present study we investigated the oxidation of TBBPA in Co(II)/PMS process. Specific objectives were to (i) determine the reaction kinetics and factors affecting the degradation of TBBPA by • SO4 − ; (ii) identify the intermediates/products and elucidate the pathways of TBBPA degradation; (iii) explore the fate of bromine • during the degradation of TBBPA by SO4 − . 2. Material and methods 2.1. Chemicals and reagents Chemicals, suppliers, and purities are listed in the Supplementary data, Text S1. 2.2. Experimental procedures 2.2.1. TBBPA oxidation by Co(II)/PMS Batch reactions were conducted in 33 mL screw-cap cylindrical • borosilicate vials at room temperature (20 ± 1 ◦ C) with SO4 − generated by Co(II) activated PMS. An appropriate volume of PMS stock solution was transferred into a vial containing specific aliquots of Co(II) and TBBPA to achieve a total 10 mL solution with predetermined molar ratios of TBBPA, Co(II), and PMS. Control experiments with TBBPA only, TBBPA + Co(II), and TBBPA + PMS were run concurrently under the same conditions. The initial solution pH was adjusted by 0.01 M H2 SO4 or NaOH to 8.0 unless otherwise stated. No buffer was used to rule out any complexities related to potential reactions between radicals and buffer species. Change of pH was within 0.2 unit in 1 h. Aliquots (0.5 mL) were withdrawn at predetermined time points and quenched immediately with 20 ␮L 1 M Na2 S2 O3 before analysis by HPLC. The quenching agent Na2 S2 O3 was demonstrated to have no interference with the analysis of TBBPA. All the experiments were carried out in duplicates or triplicates, and the data were averaged. The standard deviations were usually within 5–10% unless otherwise stated. 2.2.2. Measurement of second-order rate constant for TBBPA• SO4 − reaction The second-order rate constant for the reaction of TBBPA and • SO4 − was determined by competition kinetics approach (Eq. (7)) following a procedure described in literature [39]. Atrazine was used as a reference compound with a known rate constant for • the reaction with SO4 − (3.0 × 109 M−1 s−1 ). Detailed experimental procedures are provided in Text S2 in Supplementary data. ln

 [TBBPA]  t

[TBBPA]0

=

kSR,TBBPA ln kSR,ATZ

 [ATZ]  t

[ATZ]0

(7)

2.2.3. Br-DBPs formation Formation of Br-DBPs was assessed under conditions identical to those of kinetics studies except a series of Teflon-capped EPA vials (43 mL in volume) were used as batch reactors. The vials were headspace free during the entire reaction period. At predetermined

Y. Ji et al. / Journal of Hazardous Materials 313 (2016) 229–237

time points, an aliquot (500 ␮L) of reaction solution was withdrawn and equivalent volume of 1 M Na2 S2 O3 was spiked into the reactor immediately to quench the reaction and maintain the total volume unchanged. The quenched reaction solutions were subjected to further treatment and analysis within 24 h. 2.3. Analytical methods Concentrations of TBBPA was analyzed by a Hitachi L-2000 high performance liquid chromatography equipped with an L-2455 diode array detector (HPLC-DAD). Separation was performed on a Hitachi La Chrom C18 column (5 ␮m, 125 mm × 4.6 mm, I.D.). Details are provided in Supplementary data, Text S3. Transformation products of TBBPA were identified by a liquid chromatography-tandem mass spectrometry, consisting of an Agilent 1200 series HPLC (Agilent, Palo Alto, CA) coupled to an Agilent 6410B triple quadrupole mass spectrometer. The separation was carried out on a waters Symmetry C18 column (3.5 ␮m, 150 mm × 2.1 mm, I.D.). Detailed procedures are provided in Supplementary data, Text S4. DBPs in the reaction solutions were extracted with MTBE according to EPA method 551.1 and 552.2, and quantified by an Agilent 7890 gas chromatograph equipped with an electron capture detector (GC-EDC) and HP-5 fused silica capillary column (30 m × 0.53 mm I.D., 1.5 ␮m film thickness). Detailed procedures and instrumental setup are specified in Text S5, Supplementary data. Br− and BrO3 − were analyzed using a PerkinElmer Nex300X ICP-mass spectrometer equipped with High performance liquid chromatography (HPLC, PerkinElmer). The separation was performed in an ion chromatography column (Hamilton PRP-X100, 5 ␮m × 250 mm × 4.6 mm, Peek). An isocratic elution consisting of 20 mM NH4 NO3 -NH4 H2 PO4 (pH = 6) at a flow rate of 1.5 mL min−1 was used as mobile phase. 2.4. Quantum chemistry computation Molecular orbital calculations were carried out at the single determinant (B3LYP/6-311G**) level with the optimal conformation having a minimum energy obtained at the same level in a Gaussian 09 program. The bulk solvent effect of water was considered using the integral equation formalism polarized continuum model (IEFPCM) within the self-consistent reaction field (SCRF) theory. The frontier electron densities (FEDs) of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) were determined from Gaussian output files. Values of 2FED2 HOMO and (FED2 HOMO + FED2 LUMO ) were calculated to predict the reaction sites for electron extraction and radical addition, respectively [40,41]. 3. Results and discussion 3.1. Reaction kinetics Experimental results demonstrated that TBBPA could be effectively removed in Co(II)/PMS oxidation process. Controls with either Co(II) or PMS absent gave negligible attenuation of TBBPA. • Co(II) activating PMS generates SO4 − which can further react with water and/or hydroxide to generate OH• . TBBPA degradation was significantly inhibited in the presence of methanol which • could scavenge both SO4 − and OH• . But only slightly inhibition was observed in the presence of tetra-butanol which selectively quenched OH• [42] (Fig. S1 in Supplementary data). Such results • demonstrated SO4 − was the primary radical species responsible for TBBPA degradation. A competition kinetics approach was employed to measure the second-order rate constant for

231

•

SO4 − reaction with TBBPA using atrazine as the reference compound (kSR,atrazine = 2.59 × 109 M−1 s−1 ) [43]. It was revealed that • the reaction between SO4 − and TBBPA approached diffusioncontrolled limit with a second-order rate constant (kSR,TBBPA ) of 3.49 × 1010 M−1 s−1 (Fig. S2 in Supplementary data). TBBPA was • more reactive with SO4 − than other organic compounds such as halobenzenes [44], atrazine [45], and sulfamethoxazole [39], which was most likely due to TBBPA’s unique molecular structure with two phenyl rings being connected by isopropyl moiety. Such structure is reported to favor the formation and stabilization of radical intermediates through resonance [46]. Fig. 1(a) shows the effect of Co(II) concentration on TBBPA oxidation in the presence of 0.2 mM PMS at pH 8.0. It is evident that increasing the concentration of Co(II) accelerated the degradation of TBBPA, consistent with previous reports of contaminants degradation by Co(II)/PMS [39,47]. As seen, more than 96% of TBBPA was degraded in the presence of 0.5 ␮M Co(II) after 25 min, while less than 50% of TBBPA was eliminated in the presence of 0.05 ␮M Co(II). At any given Co(II) concentration, TBBPA degradation showed pseudo first-order kinetics. The rate constant kobs at each Co(II) level was determined by exponential regression of the data in Fig. 1(a). Fig. 1(b) presents the relationship between kobs and Co(II) concentration. It is evident that increasing Co(II) concentration resulted in linear increase of kobs , suggesting TBBPA removal rate was also first-order with respect to Co(II) concentration. Increasing the initial PMS concentration favored TBBPA oxidation in Co(II)/PMS process as well (Fig. 1(c)). At a fixed concentration of Co(II) (i.e., 0.5 ␮M), the measured kobs also linearly correlated with the initial PMS concentration (Fig. 1(d)), suggesting a firstorder dependence of the reaction rate on PMS concentration. Further experimental results revealed that TBBPA oxidation in the Co(II)/PMS system with Co(II) concentration ranging from 0 to 0.5 ␮M and PMS concentration ranging from 0.025 to 0.3 mM all followed pseudo-first-order reaction kinetics. The measured kobs values corresponding to different Co(II) and PMS concentrations were tabulated in Table S1, Supplementary data. Taking these data together, the removal rate of the TBBPA in Co(II)/PMS oxidation process can be described by Eq. (8). d(TBBPA) = −kobs (PMS)(Co2+ )(TBBPA) dt

(8)

•

Considering it was SO4 − that attacks TBBPA and causes its degradation, it can be reasonably postulated that the steady-state • concentration of SO4 − in the reaction solution was proportional to the concentrations of both PMS and Co(II). As a result, the reaction apparently showed first order to the concentrations of both species. 3.2. Effects of bicarbonate and humic acid Humic acid (HA) and bicarbonate (HCO3 − ) ubiquitous in natural waters may influence the degradation of contaminants in SR-AOPs [17,48]. Fig. 2 presents the effects of HA and HCO3 − on TBBPA oxidation. As it can be seen, both HA and HCO3 − inhibited TBBPA degradation, and such inhibition was enhanced with increasing HA or HCO3 − concentration. The inhibitory effect of HA can be attributed to the scavenging of radicals by HA molecules [48]. HA • could serve as a sink of HO• and SO4 − because some moieties in NOM molecules react with these radicals rapidly [48]. In addition, hydroxyl and carboxyl groups in NOM molecules can form complex with Co(II) thus diminishing its catalytic performance [47]. Similarly, HCO3 − also inhibited TBBPA oxidation by scaveng• ing HO• and SO4 − [48]. The second-order rate constants for the • oxidation of bicarbonate by HO• and SO4 − were 8.5 × 106 and 6 −1 −1 1.6 × 10 M s , respectively. Although the values were orders of magnitude less than that of the oxidation of TBBPA, the overall reaction rates could be offset by the high concentration of bicarbonate

232

Y. Ji et al. / Journal of Hazardous Materials 313 (2016) 229–237

Fig. 1. (a) Effects of initial Co(II) concentration on TBBPA oxidation by Co(II)/PMS, (b) Relationship between observed pseudo-firt-order rate constant for TBBPA oxidation (kobs ) and corresponding Co(II) concentration, (c) Effects of initial PMS concentration on TBBPA oxidation by Co(II)/PMS, and (d) Relationship between kobs and corresponding PMS concentration. Experimental conditions: [TBBPA]0 = 9.2 ␮M, [PMS]0 = 0.2 mM, [Co(II)]0 = 0.05–0.5 ␮M, pH0 = 8.0, T = 20 ◦ C, V = 10 mL for (a) and (b); [TBBPA]0 = 8.8 ␮M, [PMS]0 = 0.2–0.8 mM, [Co(II)]0 = 0.5 ␮M, pH0 = 8.0, T = 20 ◦ C, V = 10 mL for (c) and (d).

(e.g., 5 mM) in relative to TBBPA (e.g., 8.8 ␮M). Thus, the scavenging effect could still be pronounced. Oxidation of bicarbonate by • • SO4 − generates carbonate radical (CO3 − ) (Eqs. (9)–(13)) which shows considerable reactivity toward electron-rich compounds such as anilines, phenols and S-containing compounds [49–51]. However, TBBPA molecule has two electron-negative bromine atoms attached on each of the aromatic rings. The attachment of bromine atoms makes the phenolic moiety electron-poor, and • hence reaction with CO3 − was insignificant. − 8 −1 −1 •− HO• + CO2− 3 → CO 3 + OH , k = 3.9 × 10 M s

HO• + HCO− 3

→

CO• − 3

6

−1 −1

+ H2 O,k = 8.5 × 10 M

s

(9) (10)

2− 2− 6 −1 −1 •− SO• − 4 + CO3 → SO4 + CO 3 , k = 6.1 × 10 M s (pH ≥ 11) (11) − 2− 6 −1 −1 • SO• − 4 + HCO3 → SO4 + HCO 3 , k = 1.6 × 10 M s (pH = 8.4)(12)

HCO• 3 ↔ H+ + CO• − 3 , pKa = 9.5

(13)

3.3. Degradation products Degradation of TBBPA yielded a series of intermediates and products which were analyzed by HPLC–MS. Due to the lack of authentic standards, structure elucidations were based on the molecular weights determined by MS, characteristic bromine isotope patterns, and reported degradation pathways in various oxidative processes for TBBPA in the literature [15,16,46]. A total of 9 intermediates were tentatively identified using this approach and the molecular structures were presented in Fig. S3, Supplementary data. Of these intermediates, P2 (2,6-dibromo4-(2-hydroxyisopropyl)-phenol), P4, and P9 have been found in

TBBPA transformation by manganese dioxide, permanganate, and laccase mediated processes [15,16,46]. Intermediates P2, P3 (2,6dibromo-4-isopropyl-phenol), P6 (2,4,6-tribromophenol), and P8 (monobromophenol) have aromatic structures but lower molecular weights than the parent compound. They were probably formed via the cleavage of TBBPA, most likely ␤-scission of the isopropyl bridge [15,16,46]. For intermediates with higher molecular weights than parent TBBPA, such as P4, P5, P7 and P9, radical coupling of intermediates was likely responsible for their formation [15,16,46]. Fig. 3 presents the time-dependent evolution of the intermediates in Co(II)/PMS oxidation process. As seen from Fig. 3(a), P1 was formed only transiently after the reaction started. Formation of P2 increased first and reached a maximum in 35–45 min then decreased rapidly. The concentration of P3 increased monotonically as the reaction proceeded. A comparison of the molecular structures of P4 and P5 indicates they should be formed sequentially, which was corroborated by their evolution profiles presented in Fig. 3(b), in which P5 appeared after 25 min and its formation was accompanied by the gradual decay of P4. Fig. 3(c) shows that there was a lag period before P6, P7, and P8 were formed, indicating they were secondary products derived from other intermediates (see transformation pathways proposed later). It should be noted that except P3 and P7, the concentrations of all the intermediates decreased after 60 min, indicating their further degradation in Co(II)/PMS oxidation process. Such degradation probably resulted in the cleavage of the aromatic ring and formation of products with lower molecular weights.

Y. Ji et al. / Journal of Hazardous Materials 313 (2016) 229–237

233

Fig. 2. (a) Effects of (a) humic aicd (HA), and (b) bicarbonate (HCO3 − ) on TBBPA oxidation by Co(II)/PMS. Experimental conditions: [TBBPA]0 = 8.8 ␮M, [PMS]0 = 1.0 mM, [Co(II)]0 = 0.1 ␮M, pH0 = 7.0 for (a) and 8.5 for (b), T = 20 ◦ C, V = 10 mL.

3.4. Reactive sites in TBBPA molecule •

Freely diffusing SO4 − is believed to be the primacy reactive oxidizing species responsible for the degradation of TBBPA • in Co(II)/PMS process. Unlike HO• , SO4 − is more selective and prone to attack electron rich compounds [19]. In order to deter• mine the sites in TBBPA susceptible to SO4 − attack, frontier electron densities (FEDs) of TBBPA were calculated. The threedimensional isosurfaces of the HOMO and LUMO frontier orbitals were displayed to visualize the location of electron density. Here the TBBPA-H form, the predominant species in the reaction solution due to the deprotonation at one of the hydroxyl groups in TBBPA molecule (pKa1 = 7.4 and pKa2 = 8.5), was adopted for computations. According to the frontier orbital theory, reactions preferentially occur at the position where mutual overlap of frontier orbitals is most effective. Electrons can be easily extracted at positions with high 2FED2 HOMO values in an electrophilic reaction, while the addition of radicals usually takes place at positions with high FED2 HOMO + FED2 LUMO values. It can be seen from Fig. S4(a) (Supplementary data) that O16 has the highest 2FED2 HOMO value, indicating this site was susceptible to electron abstraction • by SO4 − . The oxidized TBBPA may lose a proton to form the corresponding TBBPA radical, which was found to have the highest FED2 HOMO + FED2 LUMO values at O16 and C10 atoms (Fig. S4(b)). These observations suggest that phenolic moieties in the TBBPA • structure are reactive sites for SO4 − reaction, and TBBPA transformation is most likely initialized by electron abstraction. This hypothesis is consistent with previous reports of TBBPA oxidation by manganese dioxide, permanganate, and laccase-catalyzed reactions [15,16,46]. 3.5. Reaction mechanisms and transformation pathways •

On the basis of FEDs calculation, TBBPA oxidation by SO4 − • was assumed to be initiated by electron abstraction. That is, SO4 − abstracted an electron, most likely from one of the phenolic oxygens, generating a phenoxy radical R1, which could be stabilized by resonance to form R2 intermediate. The resonance between R1 and R2 was previously justified by charge and spin densities calculations using a molecular modeling approach [46]. R2 underwent ␤-scission to produce dibromo-4-isopropylphenol carbocation (R3) and a corresponding carbon-center radical (R4). Further oxidation, debromination, and coupling of these intermediates gave rise to a variety of products with different structures. For instance, oxidation followed by debromination of either R1 or R2 could yield P1. P9 could be formed by radical coupling between R1 and R4. R3 could undergo a series of substitution and elimination reactions to produce several secondary prod-

ucts, i.e., 2,6-dibromo-4-(2-hydroxyisopropyl)-phenol (P2) and 2,6-dibromo-4-isopropyl-phenol (P3). The formation of P2 and P3 during TBBPA transformation was also found in CuFe2 O4 catalyzed PMS oxidation [31]. R3 could obtain a hydroxyl anion to yield P2 or eliminate a proton to form P3 [15,16,46]. On the other hand, R4 would be further oxidized to 2,6-dibromo-1,4-hydroquinone (R5), which would undergo coupling reaction with R3, leading to the formation of P4. Oxidation of P4 followed by debromination yielded P5. Intermediate P6 was presumed to be 2,4,6-tribromophenol (TBP) and this was verified using an authentic standard. This compound was further quantified by comparison with the standard in HPLC and its formation along with the degradation of TBBPA is shown in Fig. S5. Formation of TBP has not been documented in the degradation of TBBPA in other oxidation processes [15,16,46] • and is assumed to be SO4 − oxidation specific. Because TBP has 3 bromine atoms on the phenolic ring while there is only 2 in the phenolic moiety of the parent TBBPA molecule and there was no additional bromine sources, the third bromine atom should also be originated from the parent TBBPA. This suggests both debromination and bromination reactions occurred during the degradation of TBBPA in Co(II)/PMS process. Debromination of TBBPA upon • SO4 − attack probably resulted in the release of inorganic bromide • − (Br ). Br− could be oxidized by SO4 − to form bromine radical • (Br ) with a second-order kinetic constant of 3.5 × 109 M−1 s−1 . • • Br• reacted with additional Br− or OH− to form Br2 − or BrOH − , respectively (Eqs. (14)–(20)) [35,36]. All these reactions would proceed at rates close to diffusion-controlled limit. Kinetic modeling • demonstrated that SO4 − could be mostly scavenged and concentrations of bromine radical species could be orders of magnitude • higher than that of SO4 − or OH• when a trace level of Br− was added [17]. These radical bromine species would couple with each other to form free bromine (Br2 and HBrO). Both free bromine and bromine radical species are reactive bromine species (RBS). RBS are electrophilic agents and can selectively react with phenolic intermediates of TBBPA degradation such as R4 and P8, leading to the • formation of TBP. Meanwhile, scavenging of SO4 − by Br− would suppress the degradation of TBBPA intermediates, making them more available for the reactions with RBS. Similar formation of • highly halogenated compounds in SO4 − based oxidation processes has been reported previously; examples included the degradation of 2,4-dichlorophenol, 2,4,6-trichlorophenol, and chlorinated ethenes [21,33,34]. However, these studies focused on the chlorinated organic compounds. So far, this is the first report on de • novo formation of highly brominated phenol by SO4 − oxidation of a bromide-containing compound. Coupling reaction between P6 and 2,6-dibromo-1,4hydroquinone (R5) could produce P7. Further debromination of R4 would lead to 2-bromophenol and/or 4-bromophenol (P9).

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Y. Ji et al. / Journal of Hazardous Materials 313 (2016) 229–237

Fig. 3. Time-dependent evolution of TBBPA transformation products during Co(II)/PMS oxidation process: (a) brominated phenols (2,4,6-tribromophenol and 4bromophenol), and (b) bromide ion (Br− ) and bromate (BrO3 − ). Experimental conditions: [TBBPA]0 = 183.8 ␮M, [PMS]0 = 2.0 mM, [Co(II)]0 = 0.2 ␮M, pH0 = 8.1 T = 20 ◦ C.

However, MS was not able to distinguish the isomers. Based on the above analysis, a comprehensive transformation scheme of TBBPA in Co(II)/PMS oxidation process is presented in Fig. 4. 2− 9 −1 −1 − • SO• − 4 + Br → SO4 + Br , k = 3.5 × 10 M s

(14)

Br• + Br− ↔ Br• − , kf = 1 × 1010 M−1 s−1 ; kr = 1 × 105 s−1 2

(15)

−

Br • + OH → HOBr•− , k = 1.06 × 10 M−1 s−1

(16)

Br• + Br• → Br2 , k = 1.0 × 109 M−1 s−1

(17)

Br• − 2

10

+ Br• − 2

Br• + Br• − 2

−

9

−1 −1

→ Br2 + 2Br , k = 1.9 × 10 M −

9

s

(18)

−1 −1

(19)

→ Br2 + Br , k = 2.0 × 10 M

Br2 + H2 O ↔ HOBr + H+ + Br−

s

(20)

3.6. Formation of Br-DBPs Formation of 2,4,6-tribromophenol (P6) during the degradation of TBBPA in Co(II)/PMS process implies the formation of RBS and subsequent bromination reactions. It is widely recognized that halogenation of phenolic compounds leads to the formation of halogenated DBPs such as trihalomethanes (THMs) and haloacetic acids (HAAs) in water disinfection processes, which causes worldwide concern [52,53]. Because halogen substituted phenols are key intermediates during the formation of DBPs, it is very likely that Br-DBPs could be generated when P6 was further oxidized or attacked by additional RBS. Thus, formation of THMs and HAAs were assessed and the data are presented in Fig. 5. As it can be seen, bromoform and monobromoacetic acid (MBAA) were formed gradually. No appreciable dibromoacetic acid (DBAA) and tribromoacetic acid (TBAA) were found during the time course of investigation. It is noted that formation of Br-DBPs was remarkably slower than the degradation of TBBPA. DBPs were formed predominantly after TBBPA was completely removed in the solution. For instance, the yield of bromoform and MBAA was 89.3 and 7.0 ␮g L−1 after 1 h, respectively, when no TBBPA could be detected in the solution.

Br-DBPs concentrations increased as the reaction progressed and reached maximum at 6 h for bromoform (177.8 ␮g L−1 ) and at 10 h for MBAA (38.4 ␮g L−1 ). The slow DBP formation compared to TBBPA degradation indicates complicated reaction processes were involved during their formation. Obviously, Br-DBPs were not the ultimate form of Br transformation in Co(II)/PMS oxidation process. After reaching the temporal maximums, the concentration of Br-DBPs decreased and only 6.0 and 5.8 ␮g L−1 were found for bromoform and MBAA, respectively, after 48 h. Decrease of Br-DBPs • implies their degradation by SO4 − , which has been verified in sev• eral earlier studies [37,38]. Formation of Br-DBPs in SO4 − based oxidation processes has been reported recently [37,38]. Such formation was ascribed to the generation of RBS and presence of DBP precursors prone to RBS attack. In previous studies, reactions were carried out in the presence of Br− and NOM (or model compounds) which served as RBS source and DBP precursors, respectively. Nonetheless, both RBS and DBP precursors were originated from TBBPA per se in this research, which makes it distinct from previous works. To the best of our knowledge, this is the first study that • demonstrates the de novo formation of Br-DBPs in SO4 − mediated degradation of brominated organic pollutants. In addition to the differences regarding the sources of RBS and DBP precursors with the previous researches, different DBP distribution pattern was found, i.e. MBAA was predominant over DBAA and TBAA. Such distribution is significantly different from that in ordinary bromination process • where TBAA is the dominant HAA species or in SO4 − induced halo− genation in the presence of Br where DBAA is more dominant [53]. The underlying mechanisms leading to such distribution is unclear. We presume it is related to the limitation of bromine supply in the reaction system.

3.7. Formation of Br-DBPs in the presence of HA Formation of Br-DBPs during the degradation of TBBPA in Co(II)/PMS process underlines the potential risks of SR-AOPs in the treatment of organobromine pollutants. Such risks are presumed

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Fig. 4. Proposed transformation pathways of TBBPA in Co(II)/PMS oxidation process. Below the transformation products were their corresponding molecular ion masses shown in the LC–MS.

to be even more prominent in real practice because natural organic matter (NOM), which is ubiquitous in the environment, can provide additional substrates for RBS attack thus increasing DBP formation. To test this hypothesis, formation of DBPs was examined in the presence of HA and the data are also illustrated in Fig. 5. It was found that the same Br-DBPs species were formed as in the absence of HA, i.e., bromoform and MBAA. However, compared to the DBP formation in the absence of HA, Br-DBP concentrations did not show an increase and decrease trend in the presence of HA. For instance, bromoform formation kept increasing in the solution with 10 mg L−1 HA, while its concentration reached the maximum at 12 h and remained almost constant after that in the solution with 5 mg L−1 HA (Fig. 5(a)). MBAA formation kept increasing during the investigated period in the presence of HA (Fig. 5(b)). Consequently, more DBPs were generated ultimately in the presence of HA. Elevated DBP yields in the presence of HA were presumably attributed to 2 reasons. First, HA provided additional DBP precursors readily reacting with RBS generated during the oxidative degradation of TBBPA. Second, HA inhibited the further degradation of DBPs by • scavenging SO4 − in the solution.

•

oxide radical anion (O2 − ) [35,36]. As such, formation of Br− and BrO3 − during oxidative degradation of TBBPA was quantified. No BrO3 − was detected, possibly due to the low initial concentration of TBBPA (i.e., 9.2 ␮M) and the yield rate of BrO3 − . The reduction of BrO3 − to Br− in the presence of Co(II) was also possible since transition metals (e.g., Fe(II)) could suppress the production of BrO3 − [13,29]. However, our recently studies demonstrated the formation of BrO3 − in both thermo and Co(II) activated PS processes in the presence of Br− [38,54]. Fig. 6 shows the time-dependent evolution of Br− . As seen, concentration of Br− increased as the reaction proceeded both in the presence and absence of HA, suggesting debromination of TBBPA occurred. In the absence of HA, after 48 h reaction, the Br− concentration accounted for approximately half of the total bromide. Presence of HA decreased the yield of Br− , which could be attributed to the fact that HA competed with TBBPA as well • as its degradation intermediates for reactions with SO4 − and RBS. This result was further evidenced by the finding that the presence of HA inhibited TBBPA degradation (Fig. 2(b)) while promoted the Br-DBPs formation (Fig. 5). 4. Conclusions

3.8. Evolution of inorganic bromine In activated persulfate system, Br− could ultimately be transformed to bromate (BrO3 − ), a potent carcinogen that subjected to strict regulation globally, via HOBr as critical intermediate [18,19]. It was found that the presence of NOM could inhibit the formation of BrO3 − by quenching HOBr and/or reducing RBS through super-

The above data and interpretation lead to the following conclusions. (1) TBBPA could be effectively degraded in Co(II)/PMS process. Such removal was primarily attributed to the oxidation by SO4 − . The reaction apparently showed first order kinetics to the concen-

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(3) TBP as an intermediate was found during TBBPA degradation, suggesting both debromination and bromination reactions occurred. Formation of Br-DBPs was a result of either further transformation of bromine-containing intermediates or reactions of intermediates with RBS. (4) Formation of Br-DBPs during the degradation of TBBPA in Co(II)/PMS process underlines the potential risks of SR-AOPs in the treatment of halogenated pollutants. Acknowledgments This research was supported by the Natural Science Foundation of China (51578294), the Fundamental Research Funds for the Central Universities (KYZ201407), the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institute. The content of the paper does not necessarily represent the views of the funding agencies. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jhazmat.2016.04. 033. References

Fig. 5. Formation of brominated disinfection by-products (Br-DBPs) in Co(II)/PMS oxidation of TBBPA in the presence and absence of humic acid (HA): (a) bromoform, (b) MBAA. Experimental conditions: [TBBPA]0 = 5 mg L−1 (9.2 ␮M), [PMS]0 = 2 mM, [Co(II)]0 = 0.2 ␮M, [HA]0 = 0–10 mg L−1 (as TOC), pH0 = 8.0.

Fig. 6. Time-dependent evolution of bromide during Co(II) catalyzed PMS oxidation of TBBPA in the absence and presence of humic acid. Experimental conditions: [TBBPA]0 = 5 mg L−1 (9.2 ␮M), [PMS]0 = 5 mM, [Co(II)]0 = 0.2 ␮M, [HA]0 = 0 mg L−1 (blank) or 5 mg L−1 (as TOC) (black), pH0 = 8.0.

trations of both PMS and Co(II). Reaction was inhibited in the presence of HA and bicarbonate. (2) The phenolic moieties in TBBPA were confirmed to be the reactive sites for SO4 − attack. Degradation was proposed to be initialized by electron abstraction followed by ␤-scission, oxidation, debromination and coupling reactions, leading to the formation of a series of intermediates.

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