Efficient Reduction of Bromate by Iodide-Assisted UV / Sulfite Process

Bromate (BrO− 3 ) residue in drinking water poses a great health risk. Ultra-fast reduction of BrO− 3 , under aerobic conditions, was realized using an ultraviolet (UV)/sulfite process in the presence of iodide (UV/sulfite/iodide). The UV/sulfite/iodide process produced BrO− 3 removal efficiency of 100% at about 5 min with complete conversion to bromide, while UV/sulfite induced 13.1% BrO− 3 reduction under the same conditions. Hydrated electrons, generated from the photolysis of sulfite and iodide, was confirmed as the main contributor to BrO− 3 degradation (77.4% of the total contribution). As the concentration of iodide was kept constant, its presence remarkably enhancing the generation of hydrated electrons led to its consideration as a homogeneous catalyst in the UV/sulfite/iodide system. Sulfite played a role not only as a hydrated electron precursor, but also as a reactive iodine species shielding agent and a regenerant of iodide. Results surrounding the effects on common water quality parameters (pH, bicarbonate, nitrate, natural organic matter, and solution temperature) indicated that preferred degradation of BrO− 3 occurred in an environment of alkaline pH, low-content natural organic matter/bicarbonate/nitrate, and high natural temperature.


Bromate (BrO −
3 ) is considered as a carcinogen, and is produced during bromide-containing water treatment processes including chlorination, ozonation, and advanced oxidation [1].It is of great importance to remove the formed BrO − Table 1 summarizes the BrO − 3 degradation methods previously reported.One can find that the UV/sulfite/I − process is not inferior to any of the previously reported processes.Although the degradation efficiency is dependent on many factors such as reactor geometry and reaction conditions, the crude comparison still indicates the attractive prospect of the UV/sulfite/I − process.Table 2 lists the related reactions in the UV/sulfite/I − system.A preliminary judgment can be made that such a superior degradation efficiency may be attributed to the enhanced formation of e − aq /H • (No. ( 1)-( 45) in Table 2).UV-M Yes 0.60 pH = 6.8; 22-23 °C; 2400 μW•cm −2 100% (120 min) [20] UV-L Yes 0.50 pH = 6.8; 4900 μW•cm −2 100% (250 min) [21] UV254/TiO2 No 12.80 pH = 1.5-13.5;[TiO2] = 0.5 g•L −1 ; 1255 mW•cm −2 100% (90 min) [22] UV365/ TiO2 No 12.80 pH = 1.5−13.5;[TiO2] = 0.5 g•L −1 ; 1150 mW•cm −2 78% (180 min) [22] Catalyst (4% Pt/SBA-15) Yes 100.00 pH = 6.5; 25 °C; catalyst = 30 mg•L −1 80% (13 min) [23] Electrochemic 2.1.2.Validation of UV/Sulfite/I − Process with Real Water To evaluate the potential for practical water treatment, we also investigated the degradation efficiency of BrO − 3 with four different tap waters (TP1, TP2, TP3, and TP4) using UV/sulfite/I − process (Figure 2).The water quality parameters of four real waters are listed in Table S1 (Supplementary Materials).These four tap waters were collected from four different cities of Eastern China.Their pH values were nearly the same, but the other parameters (dissolved organic carbon (DOC), HCO − 3 , Cl − , NO − 3 , and SO 2− 4 ) were quite different.The degradation efficiency of BrO − 3 with the four authentic waters suffered some degree of decrease compared to that in pure water.For a UV dosage of 231.38 mJ•cm −2 (typical UV dosage for virus inactivation is  [40]), the degradation rates of BrO − 3 for the four tap waters were 91%, 88%, 85%, and 67%.Such results indicated that the degradation efficiency varied with changes in the water matrix.Over all, the degradation process of BrO − 3 in real water using UV/sulfite/I − was also satisfactory.These results powerfully confirm that BrO − 3 reduction with the UV/sulfite/I − process in real water is still efficient.
To evaluate the potential for practical water treatment, we also investigated the degradation efficiency of BrO with four different tap waters (TP1, TP2, TP3, and TP4) using UV/sulfite/I process (Figure 2).The water quality parameters of four real waters are listed in Table S1 (Supplementary Materials).These four tap waters were collected from four different cities of Eastern China.Their pH values were nearly the same, but the other parameters (dissolved organic carbon (DOC), HCO , Cl , NO , and SO ) were quite different.The degradation efficiency of BrO with the four authentic waters suffered some degree of decrease compared to that in pure water.For a UV dosage of 231.38 mJ•cm −2 (typical UV dosage for virus inactivation is 40-100 mJ•cm −2 [40]), the degradation rates of BrO for the four tap waters were 91%, 88%, 85%, and 67%.Such results indicated that the degradation efficiency varied with changes in the water matrix.Over all, the degradation process of BrO in real water using UV/sulfite/I was also satisfactory.These results powerfully confirm that BrO reduction with the UV/sulfite/I process in real water is still efficient.2).As shown in Figure 3, the presence of NO (scavenger for e and H • ) yielded a degradation rate of 0.31 μM•min −1 , while presence of MCAA (scavenger for e ) generated a degradation rate of 0.59 μM•min −1 .Based on the rate constant for the case without scavenger addition (2.61 μM•min −1 ), contributions for e and H • were calculated to be 77.4% and 10.7%, respectively, under current conditions.2).As shown in Figure 3, the presence of (scavenger for e − aq and H • ) yielded a degradation rate of 0.31 µM•min −1 , while presence of MCAA (scavenger for e − aq ) generated a degradation rate of 0.59 µM•min −1 .Based on the rate constant for the case without scavenger addition (2.61 µM•min −1 ), contributions for e − aq and H • were calculated to be 77.4% and 10.7%, respectively, under current conditions.

The Role of I and Sulfite
By fitting the degradation curves of BrO using the UV/sulfite/I process, pseudo zero-order kinetics was found to be followed.Figure 4a shows that BrO degradation improved with increased [sulfite]0.Figure 4b reveals three distinct phases for the influence of sulfite dosage.The degradation

The Role of I − and Sulfite
By fitting the degradation curves of BrO − 3 using the UV/sulfite/I − process, pseudo zero-order kinetics was found to be followed.Figure 4a shows that BrO − 3 degradation improved with increased [sulfite] 0 .Figure 4b reveals three distinct phases for the influence of sulfite dosage.The degradation rate of BrO − 3 (r) accelerated slightly from ~0.10 µM•min −1 to 0.12 µM•min −1 (phase I) by 0.5 mM sulfite, and a further increase of sulfite concentration caused a remarkable linear enhancement (phase II).However, much higher sulfite dosage (1.5-2.0 mM) only brought subtle increases of rate constants (phase III), which may be explained by the fact that incident photons were in short supply and obvious self-quenching of radicals occurred.Regarding the role of I − , one can find that a concentration change at the 10 µM level would result in a significant variation in degradation efficiency of BrO − 3 (Figure 4c).Comparing the two linear relationships in Figure 4b,d, the degradation efficiency was about 5.6 times (normalized to molar concentration) more dependent on than on sulfite.Thus, I − may play an important role in the process.Figure 5 presents the simultaneous evolution of sulfite and I − during the degradation of BrO − 3 .Sulfite decreased rapidly within the first 2 min and then slowly depleted, while I − almost stayed constant as long as sulfite was available.The UV photolysis mechanism of I − is shown in No. ( 6)-( 12) in Table 2. Iodide was a stronger UV-absorber at 254 nm compared to sulfite and had a higher quantum yield than sulfite [17].However, the UV/I − process failed to show superior reduction efficiency compared to the UV/sulfite process due to the scavenging effect of e − aq by the photogenerated RISs and self-consumption of I − (No. ( 5)-( 14) in Table 2).The nearly constant concentration of I − indicated that the presence of sulfite may inhibit negative effects of RISs and promote the regeneration of I − .Based on the above discussion, one can conclude that I − mainly served as an e − aq precursor, while sulfite not only played a role as an e − aq precursor, but also as an RIS shielding agent and a regenerant of I − .Considering the negligible chemical change of I − during the whole reaction, I − acted similarly to a catalyst.

Transformation Products of BrO
Transformation products of BrO (10 μM) treated by combining 1 mM sulfite and 100 μM I at pH 9.2 under 254-nm UV irradiation were monitored, and the results are shown in Figure 6.For the prepared water, all BrO converted to Br during degradation without other brominecontaining substances.No other inorganic bromine-containing substances, such as BrO /HBrO, were detected.This could be proven by the Br yield per μM BrO degradation (k0 = 1.0).We also tested the BrO degradation products in four tap waters (Figure 6), and a similar phenomenon was observed.These results declared that e -based reduction initiated with the UV/sulfite/I process was an efficient method of removing BrO .3 degradation (k 0 = 1.0).We also tested the BrO − 3 degradation products in four tap waters (Figure 6), and a similar phenomenon was observed.These results declared that e − aq -based reduction initiated with the UV/sulfite/I − process was an efficient method of removing BrO − 3 .

Influence of Water Quality Parameters
As stated above, the water matrix presented non-negligible effects on BrO degradation using the UV/sulfite/I process.How the water matrix influences the degradation process is of great concern.Thus, the effects on individual common water quality parameters were investigated one by one.
Taking into consideration the temperature fluctuation of drinking water, the degradation efficiency under different temperatures ranging from 9-38 °C was investigated (Figure 7a).When the solution temperature changed from 8.9 to 38 °C, the degradation rate increased about fivefold.It should be noted that, even at a relatively low temperature (8.9 °C), BrO could still be degraded thoroughly within 10 min.According to van 't Hoff's law, a temperature increase of 30 °C will lead to an 8-64-fold increase in reaction rate.Thus, the degradation process was not entirely thermodynamically controlled.In addition, based on the degradation data under different

Influence of Water Quality Parameters
As stated above, the water matrix presented non-negligible effects on BrO − 3 degradation using the UV/sulfite/I − process.How the water matrix influences the degradation process is of great concern.Thus, the effects on individual common water quality parameters were investigated one by one.
Taking into consideration the temperature fluctuation of drinking water, the degradation efficiency under different temperatures ranging from 9-38 • C was investigated (Figure 7a).When the solution temperature changed from 8.9 to 38 • C, the degradation rate increased about fivefold.It should be noted that, even at a relatively low temperature (8.9 • C), BrO − 3 could still be degraded thoroughly within 10 min.According to van 't Hoff's law, a temperature increase of 30 • C will lead to an 8-64-fold increase in reaction rate.Thus, the degradation process was not entirely thermodynamically controlled.
In addition, based on the degradation data under different temperatures, the apparent activation energy of BrO − 3 in the UV/sulfite/I − process was calculated (E a = 34.55kJ•mol −1 ; Figure S1, Supplementary Materials).This low apparent activation energy reasonably explained the efficient degradation of BrO − 3 .
Catalysts 2018, 8, x FOR PEER REVIEW 9 of 13   48)), and the formed CO • will compete for sulfite with RIS (No. ( 48)).Such a competitive effect undoubtedly interfered with the cycle of I .Given the important role of I , a deteriorating degradation performance was expected.Hence, unlike the case using the UV/sulfite system, the effect Figure 7b shows the effect of pH on BrO − 3 reduction.Increased pH improved the degradation of BrO − 3 and the improvement became especially remarkable when the pH was raised above 7. Photolysis of I − was almost not influenced by pH change [30].Solution pH plays a significant role in the distribution ratio of SO 2−  3 /S(IV) and the interconversion between H • and e − aq [14,18].As regards S(IV) species, HSO − 3 is dominant at pH 3-7, while SO 2− 3 becomes dominant at pH > 7 [42,43].Given that e − aq was mainly produced due to the UV activation of SO 2− 3 rather than HSO − 3 (No.( 1) and ( 2)) and that e − aq was the main contributor to BrO − 3 , one can easily understand why the alkaline environment

Figure 7d displays the
Figure 7d displays the influence of bicarbonate (HCO ) on the degradation process.Bicarbonate at an environmentally relevant concentration level (1-4 mM) caused obvious inhibition effects.Specifically, 1 mM and 2 mM HCO resulted in a degradation rate decrease by about 20% and 50%, respectively.Nevertheless, a further increase in HCO concentration (3-4 mM) only brought about an extra 3-5% inhibition.The rate constant of HCO with e (<1.0 × 10 6 M −1 •s −1 [45]) is much lower than that of BrO with e (3.0 × 10 9 M −1 •s −1 [10]).Thus, 1.0-4.0mM HCO produced an e scavenging rate of <(1.0-4.0)× 10 3 s −1 , while 10 μM BrO produced an e scavenging rate of 3.0 × 10 4 s −1 .From the point of view of competitive kinetics, 1.0-4.0mM HCO would confer a subtle influence on BrO degradation.On the other hand, e can react with HCO to generate oxidative CO • (No. (48)), and the formed CO • will compete for sulfite with RIS (No. (48)).Such a competitive

Table 2 .
Summary of related reaction equations during the degradation.