Insights into effect of chloride ion on the degradation of 4-bromo-2-chlorophenol by sulphate radical-based oxidation process

ABSTRACT Degradation efficiency of 4-bromo-2-chlorophenol (BCP) containing both chlorine and bromine substituents in Co/PMS process was investigated in the light of a wide range of substrate, oxidant, catalyst concentrations and pH value. The effects of chloride ion (Cl−) on degradation kinetics, total organic carbon (TOC) removal and intermediates formation during BCP depletion in Co/PMS system were studied. The kinetics results demonstrated that the dual effect of Cl− on BCP depletion in Co/PMS system due to different mechanisms involved. High concentrations of Cl− (>5 mM) can significantly promote the degradation of BCP, but did inhibit BCP mineralisation to a certain extent which was closely related to Cl− content. High degradation rates but lower mineralisation rates were found in the laboratory experiments, owing to the fact that BCP was mainly transformed to new halogenated intermediates instead of complete mineralisation. Gas chromatograph-mass spectrometer (GC-MS) data verified that a series of chlorinated by-products were formed during BCP decomposition process involving of the participation of Cl−. The proposed degradation pathways of BCP and its derivatives in presence of Cl− were discussed on the basis of intermediate products including the undesirable halogenated by-products recognised by GC-MS. These results might offer some new perspectives on the transformation fates of BCP by utilising Co/PMS regent.


Introduction
As an important compound, halophenols (HPs) have attracted extensive attention with their ubiquitous distribution, high toxicity and environmental persistence [1][2][3].They are extensively applied in different fields, such as in disinfectants and preservatives (in wood and dye industries), and as precursors for some pesticides, herbicides, disinfectants personal care agents, pharmaceuticals, flame retardants and polymer intermediates, resulting in the discharge of HPs-based organic wastewater [4][5][6][7].Previous references have confirmed that HPs in the ambient water could easily accumulate in living organisms, which might disturb the free radical metabolism and further damage to the endocrine system [8][9][10][11].It is known that the properties of HPs were closely associated with the number and type of halogen substituents on a benzene ring, and HPs containing multiple different halo-groups are more harmful to the environment [12][13][14][15].Thus, elimination of HPs from contaminated environments has attracted increasing public attention.In particular, compared to its parent compound 4-chlorophenol, 4-bromo-2-chlorophenol (BCP), as a typical chlorobromoaromatic compound has a higher toxicity [16].Dadson et al. (2013) presented that the BCP levels in urine of Egyptian agriculture workers were 34.5-3566 μg/g creatinine and higher than application (3.3-30.0μg/g creatinine) when these workers were spraying profenofos in cotton fields [17].Therefore, special attention is paid to the role of BCP in regard to its widespread distribution, and it is very essential to greatly reduce the toxicity of BCP in environment by developing appropriate and effective methods.
In recent decades, some methodologies, including biological process [18,19] and advanced oxidation processes (AOPs) [20][21][22][23], have been applied to eliminate HPs from the contaminated environments.Various methods based on AOPs were explored and successfully applied to deplete refractory pollutants and alleviate their environment implication [24][25][26].Peroxymonosulfate (PMS) as an eco-friendly oxidant has caught the world-wide attention for removing environmental contaminants [27,28].The dominant source of sulphate radicals (SO 4 •-, E 0 = 2.5-3.1 V vs NHE) is the activated process of PMS by the ways of ultraviolet light [29], base [30], transition metal catalysts [31], ozone [28], carbon materials [32].References have reported that oxidation by Co(II)-mediated decomposition of PMS is included in these methods, which has been confirmed to be the most efficient system to generate SO 4 •- [20,33,34].The successful removal of phenolic compounds, such as phenol, 4-chloro-2-nitrophenol and bisphenol A, makes PMS-based AOPs become a good option for the treatment of HPs [32,35].However, Guan et al. (2018) found that the carbon nanotubes (CNT)/PMS system could effectively degrade the selected bromophenols, but undesired polybrominated products were also identified during the oxidation process [36].The degradation efficiency of PMS-based AOPs was affected by many factors, such as chloride ion (Cl − ), PMS dosage, natural organic matter, pH and temperature [36,37].Our previous studies have reported that different halo-groups (such as chloro and bromo group) on the benzene ring will affect the migration and transformation of halophenols [38].The release of Cl − from the benzene ring of chlorophenols is more likely to happen than the release of bromide ion (Br − ) from bromphenols during oxidation reaction.Up to now, limited data are available regarding the degradation and transformation of chlorobromoaromatic compounds whether the Cl − is present.
To evaluate to the efficiency of BCP degradation through Co/PMS treatment, different factors (e.g.BCP, PMS and Co(II) concentrations, pH and Cl − concentrations) were investigated in this work.Meanwhile, the effect of different Cl − concentrations on BCP mineralisation was also explored.The transformation products of BCP in the absence and presence of Cl − were identified by the gas chromatograph-mass spectrometer (GC-MS) measurement, and then the reaction mechanism of BCP transformation in Co/PMS system was systematically proposed.

Experimental procedures
The BCP degradation experiments were carried out in open quartz tubes of 50 mL capacity.A magnetic stirrer was used to fully fuse the solutions.BCP, CoSO 4 • 7H 2 O, Cl − (if necessary) and PMS were added in turn to the quartz tubes, and then the pH was not adjusted while it was only considered when its influence was studied.Followed, take samples periodically and divided into two parts.One of them was promptly extinguished with methanol before analysis by HPLC involving BCP measurement.The other was quenched with NaNO 2 for TOC and GC-MS analysis.Besides, NaCl was used to adjust the salinity.For pH influence, the pH of PMS and pre-mixed Co(II)/BCP solution were changed by adding NaOH or H 2 SO 4 , respectively.

Analytical methods
The concentrations of BCP residues were analysed by an Agilent 1100 HPLC, in which a GL Inertsil ODS-SP column (5 μm, 250 mm × 4.6 mm) was applied and the methanol/Milli-Q water aqueous solution (70:30, v/v) as mobile phase was employed at 1.0 mL/min.The wavelength of 284 nm was used for UV detector.The total BCP mineralisation rates were examined by the TOC-V CPH analyser.A GC-MS (Agilent 7890A-5975C, USA) was employed to measure and investigate the organic intermediates in the degradation process of BCP.The detailed analytical procedures were presented in Text S1 in the Supplementary Material.
The percentage amount of BCP degraded was obtained through Eq. 1.
Where C 0 and C t are, respectively, the initial concentration and instant concentration at t (min) of BCP (mM).

Effect of BCP concentration
The results (Figure 1a,b) show that the BCP degradation was significantly decreased with BCP concentrations increment.As illustrated in Figure 1a, when the substrate concentrations varied from 0.1 to 0.3 mM, the degradation rate of BCP within 10 min was 95.2%, 53.2% and 30.5%, respectively.Regarding the concentration of substrate, BCP depletion followed the pseudo-first-order kinetic model exhibited as Eq. 2.
Where C 0 and C t are, respectively, the BCP concentrations (mM) at 0 and t (min); k represents the reaction rate constant (min −1 ). Figure 1b has shown that values of rate constant were obtained by calculating pseudofirst-order kinetics versus substrate concentration.The rate constants were 0.24425, 0.10753, and 0.05467 min −1 corresponding to 0.1, 0.2 and 0.3 mM of BCP, respectively.The reason for this phenomenon might result from the fixed yielding of SO 4 •-, when catalyst and oxidant (PMS) concentrations remain constant.The higher the substrate concentration is, however, the less free radical attack the sample itself gets, thus inhibiting the degradation of BCP.

Effect of PMS dosage
The BCP decomposition rates under different initial PMS concentrations (0.2-10 mM) were measured to explore the impact of oxidant.Experimental results displayed in Figures 1 c & d demonstrate that the degradation rates of BCP were accelerated significantly with the increasing PMS concentration, when other conditions remain constant.Increasing PMS concentration from 0.2 to 10 mM, 24.7%, 46.2%, 57.2%, 92.2% and 95.4% of BCP degradation rates were obtained after 10 min, respectively.Then, the rate constants were 0.01159, 0.07402, 0.10038, 0.23820 and 0.23882 min −1 for BCP, correspondingly.The production of HSO 5 − would increase with the increasing of PMS dosage, then more active sulphate radicals (SO 4 •-) were yielded through the reactions between HSO 5 − and Co 2+ , thus increasing the number of attacks on the benzene ring and resulting in higher BCP decomposition rates.Further increasing PMS concentration from 5 to 10 mM, the enhancement of the rate constants improved can be negligible.This could be explained by the unpleasant consumption of SO 4 •-by the excessive HSO 5 − , resulting in the scavenging of SO 4 •-and formation of less reactive SO 5 − (Eq.3) [39].Therefore, in Co/PMS system, the effective degradation of organics mainly depends on the oxidation of PMS itself or the function of a substance produced by decomposition.The aforementioned effect of PMS concentration on organics is in agreement compared to the activation of PMS system in the published literature [37].large quantities in practical application, since cobalt salt will cause serious pollution to the environmental water and soil.BCP can be completely degraded within 30 min at the ratio of 1:100 shown in Figure 1e.Therefore, this ratio of Co 2+ to PMS 1:100 can be considered in the actual treatment process.

Initial pH
The sequences of pH adjustment could influence the degradation efficiency of target pollutants, but the reactions could not be triggered by pH adjustment alone in the absence of Co 2+ .Herein, we followed the sequence of PMS, adjusting pH, BCP and Co 2+ during exploring the effect of pH. Figure 2a exhibits the oxidation kinetics and the time-dependent degradation of BCP in Co/PMS system when pH is set to 3.0, 5.0 and 7.0.At pH 3.0, 55.8% of BCP degradation was observed at 60 min.When pH values are added to 5.0 and 7.0, the degradation rates, respectively, reached 82.0% and 95.2% at 60 min.BCP degraded slowly with k values of 0.01372 min −1 at pH 3.0, but k increased to 0.03221 and 0.06996 min −1 at pH 5.0 and 7.0, respectively, indicating the favourable impact of neutral conditions on reaction rate.The effect of pH in the presence of Cl − was also explored as shown in Figure 2b.The presence of 50 mM Cl − promoted the degradation reaction, since BCP were completely degraded after 60 min reaction at pH 5.0 and 7.0.As pH value increased from 3.0 to 7.0, the k value is improved from 0.0305 to 0.1247 min −1 , which is consistent with the above results.
The positive influences of pH on BCP degradation phenomenon can be explained by the following reasons.The neutral pH is favourable for the formation of CoOH + through Co (II) hydrolysis (Eq.4).Increasing pH up to 7.0, the yield of CoOH + is presumably accelerated due to the combination of H + and OH − , which promotes Eq. 5 and increases the production of SO 4 •-gradually, thus leading to a higher degradation efficiency at pH 7.0 [20,40].Also, the joint action of SO 4 •-and • OH under neutral conditions (Equations.6 and 7) accelerate the depletion of the target pollutant as previous studies have shown [41,42].
In addition, another possible explanation was that pH could affect the existence species of halophenols, which further influence the degree of the overall reaction.The dissociation of phenolic compounds increase with pH value, and deprotonated phenolates have been recognised to be more readily oxidised than the phenol molecule [43].

Effect of Cl −
The BCP depletion in the Co/PMS system was investigated in saline water (Figure 3a,b).
Figure 3b shows that depletion rate of BCP declined with soaring Cl − concentration from 0 to 1 mM.Surprisingly, BCP degradation was significantly accelerated with further addition of sodium chloride.Similar consistent results also can be found in other studies [44,45].
For instance, Wang et al. ( 2011) suggested that Cl − has dual effect on azo dye-AO7 bleaching kinetics.When Cl − concentration is 100 mM, the half time of BCP of 1 min and the degradation kinetics of 0.7098 min −1 are significantly better than that of 5 mM Cl − (~40 min, 0.0197 min −1 ).The BCP depletion was also ascribed to the contribution of two kinds of oxidation species in Co/PMS/Cl − system, such as sulphate radicals (SO 4 •-, Eq. 5) and active chlorine species (Cl • , Cl 2 •− , HOCl, Cl 2 , etc., Equations.8-13).The presence of Cl − is not conductive to the BCP degradation, and the phenomenon can be explained by the fact that active chlorine species may be reduced with the reaction between sulphate radicals and Cl − [46].With the presence of PMS, new chlorine species could be formed and then BCP degradation rate is improved with further addition of Cl − (>5 mM).Thus, the BCP depletion can be enhanced at higher chloride concentration.

TOC analysis
As the comprehensive index of total organic pollutants in water, total organic carbon (TOC) is commonly used to represent the total amount of organic matters.The mineralisation effect of BCP during Co/PMS process was estimated based on TOC variation.Figure 4 illustrates the influences of different Cl − concentrations on BCP mineralisation in Co/PMS system after 2 h and 4 h of reaction, respectively.Visibly, the TOC removal rates of BCP were positively correlated with reaction times.However, the mineralisation rates were not consistent with the variation trend of degradation rates when chloride ions involved in the reactions.When Cl − concentration is 100 mM, the mineralisation rate was 4.32% after 2 h of reaction, which was much lower than the depletion efficiency of BCP (the depletion efficiency was 100%), as can be seen from Figures 3 and 4. The higher degradation rate and lower mineralisation rate of BCP in Co/PMS system may be attributed to the conversion of BCP into other halogenated pollutants rather than complete dehalogenation or degradation.The BCP mineralisation rates were 10.5% and 22.9% after 2 h and 4 h reaction, when the chloride ion was not added into the system.With increasing Cl − concentration (<5 mM), the mineralisation rates were relatively accelerated compared to the absence of Cl − .But further increasing Cl − concentration (5-50 mM), the mineralisation rates were retarded by high dosage of Cl − .Results from Figure 4 show that high concentration of Cl − exhibited obviously inhibitory effect on BCP mineralisation.It is noteworthy that BCP completely disappeared within 30 min in the presence of 50 mM Cl − ; however, 4.32% and 18.5% of TOC removal were achieved after 2 h and 4 h reaction, respectively.The reason might be that much lower active chlorine species were produced with increasing of Cl − concentration, which would react with BCP to form other halogenated byproducts instead of complete mineralisation.The hypothesis is proven by investigating the degradation intermediates of BCP in Co/PMS process with the presence of Cl − .
As shown in Table 1, Table S1 and Figure S1~ S13, a large number of chlorinated intermediates were identified in Co/PMS system.Importantly, more intermediates were obtained in a chloride-rich environment (100 mM) than that in absence of Cl − .Polychlorinated organics were indeed formed during BCP degradation, resulting in the lower mineralisation rates.At the same time, one bromide product, 4-bromo-2, 5-dichlorophenol, was also identified during BCP degradation.And, other polybrominated products were not detected by GC-MS, which may result from the different electronegativity of chlorinated and brominated groups.Previous literatures have confirmed that dehalogenation is positively correlated with the electronegativity [46,47,49].Therefore, the oxidative debromination on bromophenols is more difficult than the dechlorination on chlorophenols.The lower polybrominated products formed during BCP degradation are due to the fact that less bromine is removed from the benzene ring and without the absence of additional Br − in Co/PMS system.Therefore, the bromination process was unlikely occurred during the depletion of BCP by Co/PMS.
According to TOC and GC analysis, BCP was mainly transformed into highly chlorinated intermediate compounds instead of complete mineralisation, especially in the presence of chlorine ions.It is known that these polychlorinated compounds (e.g.multichlorophenols) exhibit low biodegradability and high toxicity, which may be more toxic than the mother compound [12,16].The other previous studies have reported that the toxicity of these chlorinated aromatic compounds changes depending on the position and number of Cl atoms associated with the hydroxyl group [12,48,49].

Degradation pathways
The proposed mechanisms of BCP oxidation by Co/PMS/Cl − (Figure 5) are presented based on the products identification and previous literature [16,40,46].Free radical reactions and non-free radical reactions as two different reaction mechanisms are involved in the formation of polyhalogenated products during BCP degradation by Co/PMS/Cl − process.

Free radical reactions
As the dominant active species of Co/PMS, the sulphate radical (SO 4 •-) might attack the aromatic ring of BCP and form the carbon-centred radicals under electron transfer.The chloric or bromic substituents were released from the benzene ring along with the dehalogenation reaction, which can react with SO 4 •-and then involved in the chain radical  opening.Some small molecules of acids like 2-hydroxyacetic acid and oxalic acid were then detected followed by the oxidation of free radicals, which were eventually mineralised into CO 2 and H 2 O.

Non-free radical reactions
In a chlorine-rich environment, PMS could be directly activated by chloride ions through electron transfer reaction, leading to the formation of chlorine species (Cl 2 /HOCl, Equations.11-13) [53,54].Both Cl 2 in liquid or gaseous form and HOCl in protonated ion form can react with phenols and its intermediate products to generate polychlorinated products (Eq.14).
Chlorine reacts with aromatic compounds mostly by electrophilic substitutions and the reaction scheme for the chlorination of phenoxide ion was given by Deborde and von Gunten [55].The electrophilic substitution reactions occur mainly in ortho or para position to a substituent.Narender et al. [56] have reported the para selective oxychlorination of aromatic compounds using KCl as a chlorine source and Oxone® as an oxidant.The presence of the electron donating group on the aromatic ring can accelerate the rate of chlorination reaction while on electron withdrawing group restrains it.HOCl possesses lower stability because of its pronounced ionic nature and stronger reactivity towards the aromatic nucleus.Several previous studies also confirmed that chloride could directly react with PMS via a non-radical, leading to a rapid degradation of 2,4,6-tribromophenol [38,57].In addition, the identified chlorinated intermediates by Cl 2 /HOCl chlorination were found to be similar to those detected in Co/PMS systems [44].

Conclusions
This work describes the effect of halides on BCP destruction efficiency and the evolution of halogenated intermediates for Co/PMS treatment of saline wastewater.The degradation rates of BCP obviously reduced with the initial BCP concentration increment, however, which was positively correlated with the concentration of PMS and Co 2+ .The degradation efficiency of BCP was accelerated with the pH increasing (3.0-7.0).BCP degradation efficiency can be influenced dramatically by the addition of Cl − , displaying a dual effect (inhibiting and accelerating effect) in the Co/PMS system.An obvious negative correlation between BCP mineralisation and Cl − concentration was observed.The potentially toxic halogenated byproducts including 4-bromo-2, 5-dichlorophenol were confirmed in the presence of Cl − using GC-MS analysis.More importantly, the above results also provide primary evidence for the adverse effect of Cl − on PMS-based treatments with unexpected halogenated compounds formation, which advances the understanding of saline wastewater treatment using PMS-based systems.

Figure
Figure 1e,f illustrates the BCP degradation with the different Co 2+ concentrations.The influence of Co 2+ on BCP reaction rate constant was investigated via changing the amount of cobalt salt added, keeping the concentration of PMS and substrate constant in the solution.The rate constants were 0.1348, 0.1813, 0.2101 and 0.4758 min −1 , respectively, when the ratio of Co 2+ to PMS was 1:100, 1:80, 1:60 and 1:20.The degradation rate of BCP increases with Co 2+ concentration increment.However, it is not suitable to add in

Table 1 .
[41]intermediate products of BCP degradation in Co/PMS and Co/PMS/Cl − system (Figs.S1-S14).Because of its electrophilic nature owed, SO 4 •-tends to attack on para positions and ortho positions of hydroxyl groups.Subsequently, the aromatic cation radicals were generated, which can be hydrolysed to form hydroxylated products.Copious literature confirmed that Cl − in BCP solution could react with SO 4•-in a series of free radical chain reactions, leading to the generation of Cl • , Cl 2 •− and ClOH •−[41].