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New insight into wastewater treatment by activation of sulfite with photosensitive organic dyes under visible light irradiation
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New insight into wastewater treatment by activation of sulfite with photosensitive organic dyes under visible light irradiation Gang Nie, Ling Xiao
⁎
Key Laboratory for Biomass-Resource Chemistry and Environmental Biotechnology of Hubei Province, School of Resource and Environmental Science, Wuhan University, Wuhan 430072, PR China
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
advanced oxidation process for • An contaminated water stream treatment is proposed.
proposed catalyst-free AOP was • The efficient for micropollutants degradation.
and O were the major ROS • inSOthe investigated photocatalytic %− 3
%
2
−
system.
compounds showed little • Co-existing influence on the degradation reaction.
A R T I C LE I N FO
A B S T R A C T
Keywords: Photosensitive organic dyes Visible light Activation of sulfite SO3%− Degradation pathway
In this work, an advanced oxidation process relying on sulfite radical (SO3%−) and superoxide anion radical (%O2−) for contaminated water stream treatment was proposed. SO3%− was generated from NaHSO3 which was activated by photosensitive organic dye (POD) under visible light irradiation. The organic pollutants such as organic dyes and carbamazepine (CBZ) in aqueous solution could be degraded in this vis/POD/HSO3− system. SO3%− and %O2− were identified as the major reactive oxygen species (ROS) in the investigated system for the degradation of organic pollutants by the results of quenching experiments and electron spin resonance (ESR) analysis. Moreover, the co-existing inorganic anions (such as SO42−, NO3−, and Cl−) and inorganic cations (such as K+, Mg2+, and Ca2+) and humic acid in water body showed little influence on the degradation reaction. The degradation pathway of rhodamine B (RhB) induced by SO3%− and %O2− in the vis/RhB/HSO3− system was clarified on the basis of the identification of intermediates by high performance liquid chromatography-mass spectrometry (HPLC-MS). This study is the first report on the activation of sulfite by photosensitive organic dyes under visible light irradiation for the abatement of micropollutants in water treatment, which may lead to a new advanced oxidation process relying on sulfite activation for wastewater treatment.
1. Introduction Advanced oxidation processes (AOPs) based on highly reactive species such as hydroxyl radicals are commonly used in treatment of organic contaminants [1]. In the last few years, as one of the most
⁎
promising new chemical oxidation technologies for the degradation of organic contaminants in groundwater and wastewaters, sulfate radical (SO4%−) based AOPs (SR-AOPs) have attracted worldwide attention in both research and application areas [2–7]. So far, the most frequently employed SO4%− precursors are exclusively persulfates, that is, either
Corresponding author at: The Luojia Mountain of Wuchang, Wuhan City, Hubei Province 430072, PR China. E-mail address: [email protected] (L. Xiao).
https://doi.org/10.1016/j.cej.2019.123446 Received 11 August 2019; Received in revised form 6 November 2019; Accepted 8 November 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Gang Nie and Ling Xiao, Chemical Engineering Journal, https://doi.org/10.1016/j.cej.2019.123446
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peroxydisulfate (PDS) or peroxymonosulfate (PMS). An expanding range of persulfate activation methods has been consequently established, such as transition metal catalysis [8–10], base activation [11], phenol activation [12], and input of high-energy for dissociation [13,14]. But all of the above require a massive utilization of high-cost oxidants or intensive energy consumption. In addition, the use of chemical activators such as transition metal catalysts (Co2+, Ni2+, Ag+) may pose a hazard for secondary contamination [15,16]. Furthermore, persulfates face the drawbacks of prohibitively high cost and inherent chronic toxicity brought by their stability [17]. Hence, increasing attention has been paid to develop new approaches for SO4%−generation with low cost chemicals instead of PMS and PDS. Sulfite is a reductive substrate that can be found as a residual product from flue gas desulfurization. The catalytic oxidation of sulfite by oxygen to sulfate has received much attention during the last 50 years [18]. Most recently, novel approaches by using sulfite as the radical source are proposed. Generally, sulfite radicals (SO3%−) can be generated from the one-electron oxidation of sulfites or bisulfites [19], and the reported methods include photolysis of sulfite at moderate UV light [20–22], the photocatalytic activation of sulfite by appropriate photocatalyst [23–26], the auto-oxidation of sulfite catalyzed by transition metal [27–29] and the oxidation of sulfite by other radicals such as hydroxyl radicals [26]. It seems that the activation of sulfites is closely related to an oxidation step. Researchers found that both oxysulfur radicals (including SO3%−, SO5%−, and SO4%−) and %OH work in such sulfite catalyzed systems. The high efficiency in generating radicals and the low cost of the sulfite source make the replacement of PDS or PMS by sulfite considerable. Various studies [19,20] confirm that the photolysis of dilute aqueous solutions of sulfite leads to the production of SO3%− and eaq−. Chawla et al. [30] found that this decomposition reaction can be sensitized by a number of ketones. Chu et al. [31] also utilized acetone as a photosensitizer to promote the degradation of disperse dyes with sulfite. The triplet state acetone, excited by UV light, has high energy (79–82 kcal mol−1), which makes the photosensitization process possible. The energy transfer from the triplet state acetone (acetone*) to SO32− produces SO3%− and eaq−, which result in the decay of dye. However, the use of acetone as a co-solvent and the needs of UV light make the process expensive and unsafe. In most natural aquatic systems, chromophoric dissolved organic matter (chromophoric DOM or CDOM) is the dominant light absorber [32–35]. Upon absorption of light of sufficient energy, ground-state CDOM is initially promoted to its excited singlet-state and then a small portion (i.e., ~5 − 10%) undergoes intersystem crossing to the excited triplet state (3CDOM*) [36]. 3CDOM* is a highly reactive species that plays a central role in sunlit natural waters through the generation of other reactive intermediates (e.g., singlet oxygen, 1O2) via energy transfer, and also through the degradation of environmental contaminants via direct oxidation [36,37]. Based on the above discussion, we assume that CDOM such as the photosensitive organic dye (POD), once excited by visible light, may have the same function as ketones to convert sulfite to SO3%−. In this work, photosensitive organic dyes were excited under visible light irradiation and then subsequently activated NaHSO3 to generate SO3%− and %O2−. The generated SO3%− and %O2− in turn attacked the dye molecules and other additional organic pollutants to degrade them. The proposed catalyst-free AOP not only saves costs but also avoids secondary pollution and has great significance in water treatment process for further research.
Fig. 1. Removal of rhodamine B, methylene blue, eosin Y, alizarin red, orange G, methyl orange, AO7 and Amaranth in vis/dye/HSO3− processes. Reaction L−1, [RhB] = 5 mg L−1, conditions: [NaHSO3] = 0.3 g [MB] = [EY] = [AR] = [MO] = [AO7] = [OG] = [Amaranth] = 20 mg L−1, 25 °C, pH = 5. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
organic pollutant used in this study were also described in Supplementary information (SI Fig. S1). 2.2. Experimental procedures Photo-reactivity of the vis/dye/HSO3− system was measured through observation of the decolorization of dye solutions in the visible light region (≥420 nm) without pH correction (natural pH was 5). The tests were carried out with a quartz reactor at room temperature. The reactor was placed in a circulating water bath to eliminate the concomitant heat during irradiation. Visible light irradiation was provided by a 300 W Xe light source (15A) (Beijing perfectlight technology co. LTD, China). The light source was equipped with a UV cutoff filter to provide visible light (λ ≥ 420 nm), which was located at distance of 150 mm above the surface of the dye solution. Typically, after the lamp was turned on for 30 min until the light source was stable, a certain amount of NaHSO3 was dissolved in 100 mL of dye solution. Subsequently, the reactor was placed directly beneath the light source. At given time intervals of irradiation, about 2 mL of the solution was collected. The concentrations of the dyes in single and two-dye solutions were separately evaluated. The derivative spectrophotometric method was used for the analysis of dye concentration in binary dyes solution and was described in Supplementary information (SI Text S2, S3). All measurements were repeated three times and the results were reproducible within an experimental error of 5%. 2.3. Analytical methods The concentration of residual pollutant species in degradation solution was analyzed by high performance liquid chromatography (HPLC) system (Agilent 1200 infinity series, USA) with a G1315D 12,600 DAD detector at a wavelength of 210 nm. An amethyst C18-P column (5 μm, 4.6 × 150 mm) was used as a separation column and a mixture of methanol and water (50: 50, v/v) was used as the mobile phase with a flow rate of 1 mL min−1. The injection volume was 20 μL. Electron spin resonance (ESR) signals were recorded on a Bruker ESR EMX nano spectrometer (Bruker Corporation, USA) at room temperature (298 K) and 5,5-dimethyl-1-pyrroline N-oxide (DMPO) was used as the spin trapping reagent. To identify the degradation intermediates, solution samples (5.0 mL) were taken out from the vis/RhB/HSO3−
2. Materials and methods 2.1. Materials All chemicals used in this work were described in Supplementary information (SI Text S1). Chemical structural formula of dyes and 2
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Fig. 2. (a) RhB degradation with different systems, (b) time-dependent UV–vis absorption spectra changes during RhB degradation by the vis/RhB/HSO3− system, (c) effect of NaHSO3 concentration and (d) three stage pseudo first-order reaction kinetics of RhB degradation by the vis/RhB/HSO3− system with different NaHSO3 concentrations. Number 1–7 in (c) and (d) represent 0、0.1、0.2、0.3、0.4、0.5、0.6 g L−1 NaHSO3 respectively. Reaction conditions: [RhB] = 5 mg L−1, 25 °C, pH = 5.
Table 1 Three-stage first order kinetic rate constants of RhB degradation in vis/RhB/HSO3− process. NaHSO3 (g L−1)
First stage a
kobs /10 0.1 0.2 0.3 0.4 0.5 0.6 a
Second stage
−3
min
4.36 ± 0.04 7.96 ± 0.12 13.34 ± 0.04 14.40 ± 0.22 13.28 ± 0.08 14.98 ± 0.04
−1
R
2
0.9999 0.9988 0.9997 0.9994 0.9999 0.9998
−3
a
kobs /10 10.81 26.62 44.48 43.40 42.91 40.51
± ± ± ± ± ±
min
0.08 0.22 0.01 0.06 0.02 0.01
Third stage −1
R
2
0.9998 0.9999 0.9994 0.9997 0.9966 0.9987
kobs a/10−3min−1
R2
10.81 18.96 18.92 21.33 21.27 19.98
0.9998 0.9995 0.9999 0.9998 0.9998 0.9999
± ± ± ± ± ±
0.09 0.06 0.01 0.04 0.04 0.01
The figures following the “ ± ” signs are 95% confidence intervals.
were kept as follows: spray shield voltage = 600 V, capillary voltage = 80 V, needle voltage = 5000 V, drying gas temperature = 400 ◦C, nebulizer pressure = 10 psi, and the electron multiplier voltage = 1360 V. The peak patterns were recorded in the m/z range of 50–500.
system after a reaction time of 60 min. The degradation intermediates were detected by mass spectrometry on Bruker micrOTOF II LC/MS (Bruker Corporation, USA) in electrospray positive ion (ESI+) mode with methanol and water (70: 30, v/v) as the mobile phase with a flow rate of 1 mL min−1 and an injection volume of 10 μL. The parameters 3
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Fig. 3. Effects of pH on the RhB degradation in vis/RhB/HSO3− system. (a) time profiles of RhB degradation in vis/RhB/HSO3− system with different pH; (b) the plot of kinetic rate constant (kobs) of RhB degradation with different pH. Reaction conditions: NaHSO3 concentration 0.3 g L−1, [RhB] = 5 mg L−1, 25 °C, pH = 3 ~ 9.
3. Results and discussion
indicating primary destruction of the molecular structure of RhB. These results indicate that the RhB is highly efficient for HSO3− activation for dye degradation under visible light irradiation. The effect of NaHSO3 concentration in the range of 0.1–0.6 g L−1 on the degradation activity of vis/RhB/HSO3− process was investigated in the presence of 5 mg L−1 RhB. As displayed in Fig. 2c, only 2% of RhB removal is obtained in 90 min without the addition of NaHSO3. The removal of RhB is promoted to 60% and 82% when the concentration of NaHSO3 is 0.1 and 0.2 g L−1, respectively. 95% removal of RhB is achieved by increasing the concentration of NaHSO3 to 0.3 g L−1. When the concentration of NaHSO3 was further increased, the removal efficiency for the degradation of RhB changed little. As shown in Fig. 2d, the RhB degradation process under visible light irradiation and NaHSO3 addition follow three stages, which consist of a short induction period (about 10 min) (first stage), then a rapid degradation stage (second stage), and at last a slower degradation period (third stage). The corresponding first-order kinetic rate constants can be calculated according to Eq. (1).
3.1. Degradation of organic dyes in the photo process The degradations of eight kinds of dyes were measured under the similar conditions. As shown in Fig. 1, in the presence of 0.3 g L−1 NaHSO3 and under visible light irradiation without pH correction (pH = 5), only four photosensitive organic dyes including rhodamine B (RhB), methylene blue (MB)、eosin Y (EY) and alizarin red (AR), were nearly completely removed, while the degradations of other four kinds of dyes such as orange G (OG), methyl orange (MO), acid orange 7 (AO7) and Amaranth, were negligible. However, for almost all tested organics, only less than 3% removal rates were obtained in the absence of NaHSO3, and no detectable reactions were observed with sulfite alone in dark condition (not shown). The experimental results confirm the existence of reactions between photosensitive organic dyes and NaHSO3 under visible light irradiation, which results in the degradation of organic dyes. Interestingly, the four kinds of dyes which degrade in the presence of NaHSO3 under visible light irradiation are cationic (RhB, MB) and neutral (EY, AR) dyes, and the other four which are not degradable are anionic dyes. We suggest part of the mechanism may be due to electrostatic interaction. In aqueous solution, both sulfite and anionic dyes are negatively charged, and anionic dyes do not easily interact with sulfite due to electrostatic repulsion. Cai et al. [38] reported that peroxydisulfate (PS) can be activated by dyes such as RhB, EY and MB under visible light irradiation. They found that due to the electrostatic repulsion between the anion dye and PS, the activation efficiency of anion dye toward PS is inferior to that of cationic dye.
ln(c / c0) = −kt
(1)
where t is the reaction time (min), k is the pseudo first-order kinetic rate constant (min−1), and c0 and c are pollutant concentrations (mol L−1) at t = 0 and t = t, respectively. As shown in Fig. 2d and Table 1, the first-order kinetic rate constant in the first stage is usually the lowest. During the initial stage of reaction, the reactive oxygen species (ROS) in the solution are very low, and the RhB dye itself is the initiator of a series of chain reactions. Therefore, RhB is stimulated by visible light to form a triplet state with high energy, and then the energy is transferred to HSO3−, and the resulting SO3%− and %O2− in turn decomposes RhB. It also can be clearly seen from the figure that the induction period will be shortened with the increase of NaHSO3 addition, because more NaHSO3 can produce more SO3%− free radicals. The degradation rate is the fastest in the second stage. In this stage, the produced free radicals in the solution enable the rapid degradation of dyes. In the third stage, the excited triplet state dye decreases as the concentration of rhodamine B in the solution becomes lower, so the reaction rate decreases for the free active species are reduced. It should be noted that at a low concentration of NaHSO3, such as 0.1 g L−1, the degradation rate of rhodamine B at the second stage is slow, the concentration of rhodamine B at the third stage is still high, and so the reaction rate at the third stage is nearly consistent with that at the second stage.
3.2. Degradation of RhB The photodegradation of rhodamine B (RhB) with sulfite addition under visible light irradiation was used as the model reaction system for detailed research (Fig. 2). In the absence of NaHSO3, only 2% of RhB is removed under visible light irradiation in 90 min, while 95% removal rate is achieved with the addition of 0.3 g L−1 NaHSO3 (Fig. 2a). When the light source was removed, the degradation of RhB was completely inhibited. Representative UV–Vis spectra changes during RhB decolorization by the vis/RhB/HSO3− system are depicted in Fig. 2b. The main absorption band at 554 nm is diminished after visible light illumination, 4
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Fig. 4. (a) Effect of scavengers on photocatalytic degradation of RhB in vis/RhB/HSO3− system and (b) kinetic fitting of the RhB degradation of the second stage in different quenching systems. (c) and (d) ESR spectra trapped by DMPO after 5 min of visible light irradiation in the vis/dye/HSO3− system. Reaction conditions: scavenger concentration: TBA = 200 mM, Ethanol = 200, 400 mM. BQ = 1 g L−1. NaHSO3 concentration 0.3 g L−1, DMPO concentration 25 mmol L−1. [RhB] = 5 mg L−1, [CBZ] = 5 mg L−1, [EY] = [MO] = [AO7] = 20 mg L−1, 25 °C, pH = 5.
quencher for %OH. As illustrated in Fig. 4a, the addition of TBA with a high dose of 400 mM has no significant effect on RhB degradation, indicating that the generation of %OH was very small in the vis/RhB/ HSO3− system. EtOH was added into the reaction solution to check whether there were other free radicals being involved in the degradation of RhB. As shown in Fig. 4a, the degradation performance was decreased with the increasing concentration of the added EtOH. Fig. 4b shows the kinetic fitting of the RhB degradation of second stage in different quenching systems. The pseudo-first order constant in the presence of 200 mM EtOH was 0.0075 min−1, which was 5.93 times lower than the corresponding value in the control experiment. High dose of EtOH (400 mM) almost completely inhibited the degradation of RhB, the pseudo-first order constant was 0.0028 min−1 in the presence of 400 mM EtOH, which was 15.89 times lower than the corresponding value in the control experiment, indicating the important contribution of SO3%− radicals in the system. As the formation of %O2− is quite common in many chemical oxidation based reactions, 1,4-benzoquinone (BQ) was also used as a quencher for %O2− detection [42,43]. As shown in Fig. 4a and b, the degradation of RhB was obviously inhibited with the addition of 1 g L−1 BQ, indicating the contribution of %O2− radical to the reaction. The pseudo-first order constant in the presence
3.3. Effects of pH on RhB degradation Effects of pH on sulfite activation were also investigated. The reactions of sulfite with RhB were carried out at various pH value (Fig. 3a), and the kobs of RhB decomposition at each pH were calculated (Fig. 3b). The RhB degradation was inhibited as the pH value increased from 3 to 9. RhB exists in two main forms in aqueous media, namely, cationic (RhB+) or zwitterionic form (RhB ± ). When the pH is lower than its pKa (3.7), it exists mainly in the cationic form. Nevertheless, when the pH value is higher than its pKa, it exists chiefly in the zwitterionic form [39]. RhB was deprotonated above pH 9 because of the deprotonation of its carboxyl group [40]. Therefore, the weaker electrostatic attraction between RhB and sulfite would be detrimental to the degradation of RhB. 3.4. Identification of main reactive oxidized species To identify the reactive oxidized species (ROS) in the photo-reaction self-degradation process, several types of scavengers were added into the vis/RhB/HSO3− system. Because the reaction between tert-butanol (TBA) and %OH is rapid (6.0 × 108 M−1 s−1) [41], TBA was used as a 5
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Fig. 5. (a) Zero order absorption spectra of RhB and AO7 in single and binary solutions. (b) first order derivative spectra of RhB and AO7 in single and binary solutions. Degradation of (c) mixed binary RhB and AO7 solution and (d) mixed RhB and CBZ solution in vis/RhB/HSO3− processes. Reaction conditions: [NaHSO3] = 0.3 g L−1, [RhB] = 5 mg L−1, [AO7] = 20 mg L−1, [CBZ] = 5 mg L−1, 25 °C, pH = 5.
of 1 g L−1 BQ was 0.0109 min−1, which was 4.08 times lower than the corresponding value in the control experiment. These results of quenching experiments verify that free SO3%− and %O2− are generated from the photo reaction of system, being mainly responsible for the degradation of RhB. In order to gain direct evidence for the involvement and the formation process of reactive species, electron spin resonance (ESR) analysis was employed using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a trapping agent. The significant signals of DMPO-SO3%− adducts are observed not only in the vis/RhB/HSO3− and vis/EY/HSO3− system (Fig. 4c) but also in the binary dye solutions (Fig. 4d). In comparison, there is no obvious ESR signal of DMPO-SO3%− in vis/MO/HSO3− and vis/AO7/HSO3− or vis/HSO3− system, which is in agreement with the results of degradation experiments; besides no DMPO-1O2 adducts were detected in any of the measurements (data are not shown). These results indicate that SO3%− is the main ROS which is generated from the dissociation of the excited HSO3− which gets energy from the excited state of photosensitive organic dyes.
3.5. Degradation of organic pollutants in vis/RhB/HSO3− system In order to further prove the important role of RhB dye in vis/RhB/ HSO3− system and exploring its potential application, we selected a mixture of RhB with AO7 dye solution and a mixture of RhB with CBZ for degradation experiments. As shown in Fig. 5a, the absorption spectrum of RhB and AO7 dyes in their binary mixture overlapped while the maximum absorbances of RhB and AO7 dyes in their single solutions were obtained at 554 nm and 484 nm, respectively (Δλmax = 70 nm). The overlapping of the RhB and AO7 dyes spectra shows the interference between the zero order spectra of RhB and AO7 dyes, and thus simultaneous determination of each dye in binary solutions was not accurate by direct absorption measurements. Hence, it was not possible to determine the amount of RhB or AO7 dye in binary mixtures. The method of derivation of the spectra is an analytical technique extensively used to solve this problem in the case of mixed solutions [44]. By measuring the derivative values at the wavelengths where one of the components has a zero or near zero value, the best linear responses are obtainable, and the calibration 6
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Scheme 1. Degradation pathway of RhB in vis/dye/HSO3− system.
degradation of both two dyes. The degradation rate of rhodamine B is slowed down compared to that in one dye solution, but AO7 is degraded at the same time. Under the degradation conditions, both dyes degraded by about 90%. As shown in Fig. 5d, when RhB was mixed with CBZ (5 mg L−1) for degradation experiment, in the same condition, 34% removal of CBZ was achieved in 90 min while CBZ alone was not photodegradable.
curves are less affected by concentration of other components [45]. Therefore, derivative spectrophotometric method was applied for the simultaneous determination of each individual dye in the case of mixed dye solutions. The first-order derivative absorption spectra of RhB and AO7 in single and mixed dye solutions are presented in Fig. 5b. The calibration curves for RhB and AO7 dyes at 573 and 451 nm (where the derivative absorbance of one of two dyes is zero) were separately obtained respectively to determine the concentration of each dye in mixture (SI Fig. S2). According to Fig. 1, AO7 alone was almost non-degradable under visible light even after NaHSO3 is added, indicating that AO7 which without photosensitive properties cannot activate NaHSO3 under visible light. But as seen from Fig. 5c, when RhB was mixed with AO7, the degradation of both RhB and AO7 at the same time in the mixed dye solution are obtained, indicating that the active species (SO3%−/%O2−) produced in vis/RhB/HSO3− system attack AO7 and lead to the
3.6. Degradation pathway of RhB Identification of possible intermediate products during the photoreaction is the best way to understand the photo degradation reaction mechanism. To study the photo degradation pathway of RhB dye under visible light irradiation, the possible intermediate products during the photo degradation were identified with electro spray ionization mass spectra (ESI-MS). The fragmented molecular ions were detected in 7
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Fig. 6. Effects of (a) inorganic ions and (b) humic acid (HA) on the removal of RhB. Conditions: [NaHSO3] = 0.3 g L−1, [RhB] = 5 mg L−1, inorganic ions = 1 mmol L−1, HA = 0.5 g L−1, 25 °C, pH = 5.
3.8. Degradation mechanism of pollutants in vis/POD/HSO3− processes
positive mode in the m/z range of 50–500. ESI mass spectra of the irradiated aqueous solutions of RhB at different time intervals were observed. The degradation of xanthene type dyes can occur via N-deethylation, de-alkylation, de-amination, de-carboxylation, de-hydration, cleavage of chromophore, and the breakage of ring structure. Major intermediates during the degradation process were proposed by using m/z values of the mass spectra (SI Table S1). Electrospray ionization analysis spectra reveal the formation of several aromatic intermediate, demonstrating the degradation of RhB dye. According to the earlier reports [46,47], most of the N-de-ethylation processes were taken place by the formation of nitrogen centered radical during the destruction of RhB dye chromopore structure. The photo degradation of RhB by the photogenerated active species such as %OH and hole could attack the central carbon of RhB to decolorize the dye and further degraded via N-de-ethylation process. The mass peaks at m/z 443, 415, 359, and 331 are intermediates of RhB and its N-de-ethylated process reported in earlier reports [48]. Several other peaks of fragments, m/ z = 437, 320, 304 and 192, were also observed at different time intervals. On the basis of mass results, the fragmentation pathway for the visible light induced photo degradation of RhB dye is proposed as shown in Scheme 1.
Previous studies [19,20] confirm that the triplet state acetone with high energy can be obtained by exciting acetone in solution using UV light, the energy transfer from the triplet state acetone to SO32− produces SO3%− and eaq−, and the energy transfer to dye molecule results in the decay of dye. Zepp et al. [50] indicate the excited triplet states of chromophoric dissolved organic matter can transfer energy to selected substrates and O2 and produce 1O2. In this work, photosensitive organic dyes are used as CDOM or acetone and excited by visible light. However, in our ESR experiment, no DMPO-1O2 adducts were detected in any of the measurements while significant signals of DMPO-SO3%− adducts were observed in the vis/POD/HSO3− system, which indicated that the triplet energy was transferred to HSO3−, not oxygen. Compared to oxygen, HSO3− is more reductive and more receptive to triplet energy. As the cleavage of (HSO3−)* produces sulfite radicals, the simultaneously generated electrons are trapped by oxygen, and the resulting %O2− also contribute greatly to organic pollutants degradation. Based on the above results, the degradation mechanism of pollutants in vis/POD/HSO3− system is proposed and shown in Eqs. (2)–(5). POD + vis → POD* POD* +
3.7. Influence of co-existing compounds
HSO3−
−
(HSO3 )* →
From the point of view of practical water treatment, the effects of possibly co-existing inorganic anions (such as SO42−, NO3−, and Cl−), inorganic cations (such as K+, Mg2+, and Ca2+) and natural organic matter (NOM) were further investigated. As shown in Fig. 6a, the influence of tested inorganic ion species was not considerable on the removal rate of RhB. RhB remove efficiency was not affected by the addition of 2.5, 5, 10, 25, 50 mmol L−1 inorganic ions (SO42−, NO3−, Cl−, K+, Mg2+, and Ca2+) (SI Fig. S5). Natural organic matter, a ubiquitous complex constituent existed in natural water and soil, can compete with contaminants for oxidants generated in AOPs. Humic acid (HA), as a significant constituent of NOM, was selected as a surrogate to assess the influence of NOM on RhB degradation in the vis/RhB/HSO3− process. HA can act as free radical scavengers, photosensitizer and light filter [49]. The different results on the degradation of organics have been reported in literature, depending on the quality and the type of the HA. In the vis/RhB/HSO3− system, RhB degradation rate was not affected by the addition of 0.5 g L−1 HA (Fig. 6b).
−
O2 + eaq →
(2)
→ POD
SO3%−+
+ (HSO3−)* +
H +
eaq−
%
O2−
SO3%−/%O2− + Pollutant
(3) (4) (5)
→ intermediates
(6)
The photosensitive organic dyes (POD) are at first excited to triplet state POD (POD*) by visible light, then, the energy is transferred from triplet state POD to HSO3−, SO3%− and eaq− are produced. The reaction of SO3%− with the pollutants results in the degradation of dye and CBZ in this system. 4. Conclusion In this work, the photochemical activation of HSO3− by photosensitive organic dyes/Vis system was employed as an AOP for organic pollutants degradation. Photosensitive organic dyes were excited under visible light irradiation, the transfer of triplet energy from excited organic dyes to HSO3− followed by dissociation of the excited HSO3− and generation of SO3%−. We confirmed that SO3%− and %O2− were the 8
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main ROS in our system. The reaction between SO3%−/%O2− and the dye molecules made the dye to degrade. The catalyst-free reaction system can degrade organic dyes and other additional organic pollutants. Unlike chemical activation, this vis/dye/HSO3−system uses only cheaper HSO3− as additional material, and does not require any expensive oxidants such as PMS or PDS, and can be operated under visible light irradiation. Thus, this catalyst-free AOP may provide a promising approach for the treatment of highly recalcitrant organic pollutants, such as organic dyes and CBZ complicated wastewater, and has the great potential of further research.
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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. Acknowledgements This work was supported by the National Natural Science Foundation of China (91647204) and the Basic Research Plan of Hubei Province (2017HB02). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.123446. References [1] E. Neyens, J. Baeyens, A review of classic Fenton's peroxidation as an advanced oxidation technique, J. Hazard. Mater. 98 (2003) 33–50. [2] N. Zrinyi, A.L.T. Pham, Oxidation of benzoic acid by heat-activated persulfate: effect of temperature on transformation pathway and product distribution, Water Res. 120 (2017) 43–51. [3] C.D. Qi, X.T. Liu, J. Ma, C.Y. Lin, X.W. Li, H.J. Zhang, Activation of peroxymonosulfate by base: implications for the degradation of organic pollutants, Chemosphere 151 (2016) 280–288. [4] Y.J. Qian, X. Guo, Y.L. Zhang, Y. Peng, P.Z. Sun, C.H. Huang, J.F. Niu, X.F. Zhou, J.C. Crittenden, Perfluorooctanoic acid degradation using UV-persulfate process: modeling of the degradation and chlorate formation, Environ. Sci. Technol. 50 (2016) 772–781. [5] C. Cai, H. Zhang, X. Zhong, L.W. Hou, Ultrasound enhanced heterogeneous activation of peroxymonosulfate by a bimetallic Fe-Co/SBA-15 catalyst for the degradation of Orange II in water, J. Hazard. Mater. 283 (2015) 70–79. [6] X.G. Duan, Z.M. Ao, H.Y. Zhang, M. Saunders, H.Q. Sun, Z.P. Shao, S.B. Wang, Nanodiamonds in sp(2)/sp(3) configuration for radical to nonradical oxidation: Core-shell layer dependence, Appl. Catal. B-Environ. 222 (2018) 176–181. [7] W.S. Chen, C.P. Huang, Mineralization of aniline in aqueous solution by electroactivated persulfate oxidation enhanced with ultrasound, Chem. Eng. J. 266 (2015) 279–288. [8] G.P. Anipsitakis, D.D. Dionysiou, Degradation of organic contaminants in water with sulfate radicals generated by the conjunction of peroxymonosulfate with cobalt, Environ. Sci. Technol. 37 (2003) 4790–4797. [9] G.P. Anipsitakis, D.D. Dionysiou, Radical generation by the interaction of transition metals with common oxidants, Environ. Sci. Technol. 38 (2004) 3705–3712. [10] G.P. Anipsitakis, D.D. Dionysiou, M.A. Gonzalez, Cobalt-mediated activation of peroxymonosulfate and sulfate radical attack on phenolic compounds. Implications of chloride ions, Environ. Sci. Technol. 40 (2006) 1000–1007. [11] O.S. Furman, A.L. Teel, R.J. Watts, Mechanism of base activation of persulfate, Environ. Sci. Technol. 44 (2010) 6423–6428. [12] M. Ahmad, A.L. Teel, R.J. Watts, Mechanism of persulfate activation by phenols, Environ. Sci. Technol. 47 (2013) 5864–5871. [13] C.Q. Tan, N.Y. Gao, Y. Deng, N. An, J. Deng, Heat-activated persulfate oxidation of diuron in water, Chem. Eng. J. 203 (2012) 294–300. [14] C.Z. Cui, L. Jin, L. Jiang, Q. Han, K.F. Lin, S.G. Lu, D. Zhang, G.M. Cao, Removal of trace level amounts of twelve sulfonamides from drinking water by UV-activated peroxymonosulfate, Sci. Total. Environ. 572 (2016) 244–251. [15] Y.G. Guo, X.Y. Lou, C.L. Fang, D.X. Xiao, Z.H. Wang, J.S. Liu, Novel photo-sulfite system: Toward simultaneous transformations of inorganic and organic pollutants, Environ. Sci. Technol. 47 (2013) 11174–11181. [16] Y.W. Gao, Y.X. Li, L.Y. Yao, S.M. Li, J. Liu, H. Zhang, Catalyst-free activation of peroxides under visible LED light irradiation through photoexcitation pathway, J. Hazard. Mater. 329 (2017) 272–279. [17] K.S. Sra, N.R. Thomson, J.F. Barker, Persistence of persulfate in uncontaminated aquifer materials, Environ. Sci. Technol. 44 (2010) 3098–3104. [18] L.D. Wang, Y.L. Ma, J.M. Hao, Y. Zhao, Mechanism and kinetics of sulfite oxidation in the presence of ethanol, Ind. Eng. Chem. Res. 48 (2009) 4307–4311.
9
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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
New insight into wastewater treatment by activation of sulfite with photosensitive organic dyes under visible light irradiation Gang Nie, Ling Xiao
⁎
Key Laboratory for Biomass-Resource Chemistry and Environmental Biotechnology of Hubei Province, School of Resource and Environmental Science, Wuhan University, Wuhan 430072, PR China
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
advanced oxidation process for • An contaminated water stream treatment is proposed.
proposed catalyst-free AOP was • The efficient for micropollutants degradation.
and O were the major ROS • inSOthe investigated photocatalytic %− 3
%
2
−
system.
compounds showed little • Co-existing influence on the degradation reaction.
A R T I C LE I N FO
A B S T R A C T
Keywords: Photosensitive organic dyes Visible light Activation of sulfite SO3%− Degradation pathway
In this work, an advanced oxidation process relying on sulfite radical (SO3%−) and superoxide anion radical (%O2−) for contaminated water stream treatment was proposed. SO3%− was generated from NaHSO3 which was activated by photosensitive organic dye (POD) under visible light irradiation. The organic pollutants such as organic dyes and carbamazepine (CBZ) in aqueous solution could be degraded in this vis/POD/HSO3− system. SO3%− and %O2− were identified as the major reactive oxygen species (ROS) in the investigated system for the degradation of organic pollutants by the results of quenching experiments and electron spin resonance (ESR) analysis. Moreover, the co-existing inorganic anions (such as SO42−, NO3−, and Cl−) and inorganic cations (such as K+, Mg2+, and Ca2+) and humic acid in water body showed little influence on the degradation reaction. The degradation pathway of rhodamine B (RhB) induced by SO3%− and %O2− in the vis/RhB/HSO3− system was clarified on the basis of the identification of intermediates by high performance liquid chromatography-mass spectrometry (HPLC-MS). This study is the first report on the activation of sulfite by photosensitive organic dyes under visible light irradiation for the abatement of micropollutants in water treatment, which may lead to a new advanced oxidation process relying on sulfite activation for wastewater treatment.
1. Introduction Advanced oxidation processes (AOPs) based on highly reactive species such as hydroxyl radicals are commonly used in treatment of organic contaminants [1]. In the last few years, as one of the most
⁎
promising new chemical oxidation technologies for the degradation of organic contaminants in groundwater and wastewaters, sulfate radical (SO4%−) based AOPs (SR-AOPs) have attracted worldwide attention in both research and application areas [2–7]. So far, the most frequently employed SO4%− precursors are exclusively persulfates, that is, either
Corresponding author at: The Luojia Mountain of Wuchang, Wuhan City, Hubei Province 430072, PR China. E-mail address: [email protected] (L. Xiao).
https://doi.org/10.1016/j.cej.2019.123446 Received 11 August 2019; Received in revised form 6 November 2019; Accepted 8 November 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Gang Nie and Ling Xiao, Chemical Engineering Journal, https://doi.org/10.1016/j.cej.2019.123446
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peroxydisulfate (PDS) or peroxymonosulfate (PMS). An expanding range of persulfate activation methods has been consequently established, such as transition metal catalysis [8–10], base activation [11], phenol activation [12], and input of high-energy for dissociation [13,14]. But all of the above require a massive utilization of high-cost oxidants or intensive energy consumption. In addition, the use of chemical activators such as transition metal catalysts (Co2+, Ni2+, Ag+) may pose a hazard for secondary contamination [15,16]. Furthermore, persulfates face the drawbacks of prohibitively high cost and inherent chronic toxicity brought by their stability [17]. Hence, increasing attention has been paid to develop new approaches for SO4%−generation with low cost chemicals instead of PMS and PDS. Sulfite is a reductive substrate that can be found as a residual product from flue gas desulfurization. The catalytic oxidation of sulfite by oxygen to sulfate has received much attention during the last 50 years [18]. Most recently, novel approaches by using sulfite as the radical source are proposed. Generally, sulfite radicals (SO3%−) can be generated from the one-electron oxidation of sulfites or bisulfites [19], and the reported methods include photolysis of sulfite at moderate UV light [20–22], the photocatalytic activation of sulfite by appropriate photocatalyst [23–26], the auto-oxidation of sulfite catalyzed by transition metal [27–29] and the oxidation of sulfite by other radicals such as hydroxyl radicals [26]. It seems that the activation of sulfites is closely related to an oxidation step. Researchers found that both oxysulfur radicals (including SO3%−, SO5%−, and SO4%−) and %OH work in such sulfite catalyzed systems. The high efficiency in generating radicals and the low cost of the sulfite source make the replacement of PDS or PMS by sulfite considerable. Various studies [19,20] confirm that the photolysis of dilute aqueous solutions of sulfite leads to the production of SO3%− and eaq−. Chawla et al. [30] found that this decomposition reaction can be sensitized by a number of ketones. Chu et al. [31] also utilized acetone as a photosensitizer to promote the degradation of disperse dyes with sulfite. The triplet state acetone, excited by UV light, has high energy (79–82 kcal mol−1), which makes the photosensitization process possible. The energy transfer from the triplet state acetone (acetone*) to SO32− produces SO3%− and eaq−, which result in the decay of dye. However, the use of acetone as a co-solvent and the needs of UV light make the process expensive and unsafe. In most natural aquatic systems, chromophoric dissolved organic matter (chromophoric DOM or CDOM) is the dominant light absorber [32–35]. Upon absorption of light of sufficient energy, ground-state CDOM is initially promoted to its excited singlet-state and then a small portion (i.e., ~5 − 10%) undergoes intersystem crossing to the excited triplet state (3CDOM*) [36]. 3CDOM* is a highly reactive species that plays a central role in sunlit natural waters through the generation of other reactive intermediates (e.g., singlet oxygen, 1O2) via energy transfer, and also through the degradation of environmental contaminants via direct oxidation [36,37]. Based on the above discussion, we assume that CDOM such as the photosensitive organic dye (POD), once excited by visible light, may have the same function as ketones to convert sulfite to SO3%−. In this work, photosensitive organic dyes were excited under visible light irradiation and then subsequently activated NaHSO3 to generate SO3%− and %O2−. The generated SO3%− and %O2− in turn attacked the dye molecules and other additional organic pollutants to degrade them. The proposed catalyst-free AOP not only saves costs but also avoids secondary pollution and has great significance in water treatment process for further research.
Fig. 1. Removal of rhodamine B, methylene blue, eosin Y, alizarin red, orange G, methyl orange, AO7 and Amaranth in vis/dye/HSO3− processes. Reaction L−1, [RhB] = 5 mg L−1, conditions: [NaHSO3] = 0.3 g [MB] = [EY] = [AR] = [MO] = [AO7] = [OG] = [Amaranth] = 20 mg L−1, 25 °C, pH = 5. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
organic pollutant used in this study were also described in Supplementary information (SI Fig. S1). 2.2. Experimental procedures Photo-reactivity of the vis/dye/HSO3− system was measured through observation of the decolorization of dye solutions in the visible light region (≥420 nm) without pH correction (natural pH was 5). The tests were carried out with a quartz reactor at room temperature. The reactor was placed in a circulating water bath to eliminate the concomitant heat during irradiation. Visible light irradiation was provided by a 300 W Xe light source (15A) (Beijing perfectlight technology co. LTD, China). The light source was equipped with a UV cutoff filter to provide visible light (λ ≥ 420 nm), which was located at distance of 150 mm above the surface of the dye solution. Typically, after the lamp was turned on for 30 min until the light source was stable, a certain amount of NaHSO3 was dissolved in 100 mL of dye solution. Subsequently, the reactor was placed directly beneath the light source. At given time intervals of irradiation, about 2 mL of the solution was collected. The concentrations of the dyes in single and two-dye solutions were separately evaluated. The derivative spectrophotometric method was used for the analysis of dye concentration in binary dyes solution and was described in Supplementary information (SI Text S2, S3). All measurements were repeated three times and the results were reproducible within an experimental error of 5%. 2.3. Analytical methods The concentration of residual pollutant species in degradation solution was analyzed by high performance liquid chromatography (HPLC) system (Agilent 1200 infinity series, USA) with a G1315D 12,600 DAD detector at a wavelength of 210 nm. An amethyst C18-P column (5 μm, 4.6 × 150 mm) was used as a separation column and a mixture of methanol and water (50: 50, v/v) was used as the mobile phase with a flow rate of 1 mL min−1. The injection volume was 20 μL. Electron spin resonance (ESR) signals were recorded on a Bruker ESR EMX nano spectrometer (Bruker Corporation, USA) at room temperature (298 K) and 5,5-dimethyl-1-pyrroline N-oxide (DMPO) was used as the spin trapping reagent. To identify the degradation intermediates, solution samples (5.0 mL) were taken out from the vis/RhB/HSO3−
2. Materials and methods 2.1. Materials All chemicals used in this work were described in Supplementary information (SI Text S1). Chemical structural formula of dyes and 2
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Fig. 2. (a) RhB degradation with different systems, (b) time-dependent UV–vis absorption spectra changes during RhB degradation by the vis/RhB/HSO3− system, (c) effect of NaHSO3 concentration and (d) three stage pseudo first-order reaction kinetics of RhB degradation by the vis/RhB/HSO3− system with different NaHSO3 concentrations. Number 1–7 in (c) and (d) represent 0、0.1、0.2、0.3、0.4、0.5、0.6 g L−1 NaHSO3 respectively. Reaction conditions: [RhB] = 5 mg L−1, 25 °C, pH = 5.
Table 1 Three-stage first order kinetic rate constants of RhB degradation in vis/RhB/HSO3− process. NaHSO3 (g L−1)
First stage a
kobs /10 0.1 0.2 0.3 0.4 0.5 0.6 a
Second stage
−3
min
4.36 ± 0.04 7.96 ± 0.12 13.34 ± 0.04 14.40 ± 0.22 13.28 ± 0.08 14.98 ± 0.04
−1
R
2
0.9999 0.9988 0.9997 0.9994 0.9999 0.9998
−3
a
kobs /10 10.81 26.62 44.48 43.40 42.91 40.51
± ± ± ± ± ±
min
0.08 0.22 0.01 0.06 0.02 0.01
Third stage −1
R
2
0.9998 0.9999 0.9994 0.9997 0.9966 0.9987
kobs a/10−3min−1
R2
10.81 18.96 18.92 21.33 21.27 19.98
0.9998 0.9995 0.9999 0.9998 0.9998 0.9999
± ± ± ± ± ±
0.09 0.06 0.01 0.04 0.04 0.01
The figures following the “ ± ” signs are 95% confidence intervals.
were kept as follows: spray shield voltage = 600 V, capillary voltage = 80 V, needle voltage = 5000 V, drying gas temperature = 400 ◦C, nebulizer pressure = 10 psi, and the electron multiplier voltage = 1360 V. The peak patterns were recorded in the m/z range of 50–500.
system after a reaction time of 60 min. The degradation intermediates were detected by mass spectrometry on Bruker micrOTOF II LC/MS (Bruker Corporation, USA) in electrospray positive ion (ESI+) mode with methanol and water (70: 30, v/v) as the mobile phase with a flow rate of 1 mL min−1 and an injection volume of 10 μL. The parameters 3
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Fig. 3. Effects of pH on the RhB degradation in vis/RhB/HSO3− system. (a) time profiles of RhB degradation in vis/RhB/HSO3− system with different pH; (b) the plot of kinetic rate constant (kobs) of RhB degradation with different pH. Reaction conditions: NaHSO3 concentration 0.3 g L−1, [RhB] = 5 mg L−1, 25 °C, pH = 3 ~ 9.
3. Results and discussion
indicating primary destruction of the molecular structure of RhB. These results indicate that the RhB is highly efficient for HSO3− activation for dye degradation under visible light irradiation. The effect of NaHSO3 concentration in the range of 0.1–0.6 g L−1 on the degradation activity of vis/RhB/HSO3− process was investigated in the presence of 5 mg L−1 RhB. As displayed in Fig. 2c, only 2% of RhB removal is obtained in 90 min without the addition of NaHSO3. The removal of RhB is promoted to 60% and 82% when the concentration of NaHSO3 is 0.1 and 0.2 g L−1, respectively. 95% removal of RhB is achieved by increasing the concentration of NaHSO3 to 0.3 g L−1. When the concentration of NaHSO3 was further increased, the removal efficiency for the degradation of RhB changed little. As shown in Fig. 2d, the RhB degradation process under visible light irradiation and NaHSO3 addition follow three stages, which consist of a short induction period (about 10 min) (first stage), then a rapid degradation stage (second stage), and at last a slower degradation period (third stage). The corresponding first-order kinetic rate constants can be calculated according to Eq. (1).
3.1. Degradation of organic dyes in the photo process The degradations of eight kinds of dyes were measured under the similar conditions. As shown in Fig. 1, in the presence of 0.3 g L−1 NaHSO3 and under visible light irradiation without pH correction (pH = 5), only four photosensitive organic dyes including rhodamine B (RhB), methylene blue (MB)、eosin Y (EY) and alizarin red (AR), were nearly completely removed, while the degradations of other four kinds of dyes such as orange G (OG), methyl orange (MO), acid orange 7 (AO7) and Amaranth, were negligible. However, for almost all tested organics, only less than 3% removal rates were obtained in the absence of NaHSO3, and no detectable reactions were observed with sulfite alone in dark condition (not shown). The experimental results confirm the existence of reactions between photosensitive organic dyes and NaHSO3 under visible light irradiation, which results in the degradation of organic dyes. Interestingly, the four kinds of dyes which degrade in the presence of NaHSO3 under visible light irradiation are cationic (RhB, MB) and neutral (EY, AR) dyes, and the other four which are not degradable are anionic dyes. We suggest part of the mechanism may be due to electrostatic interaction. In aqueous solution, both sulfite and anionic dyes are negatively charged, and anionic dyes do not easily interact with sulfite due to electrostatic repulsion. Cai et al. [38] reported that peroxydisulfate (PS) can be activated by dyes such as RhB, EY and MB under visible light irradiation. They found that due to the electrostatic repulsion between the anion dye and PS, the activation efficiency of anion dye toward PS is inferior to that of cationic dye.
ln(c / c0) = −kt
(1)
where t is the reaction time (min), k is the pseudo first-order kinetic rate constant (min−1), and c0 and c are pollutant concentrations (mol L−1) at t = 0 and t = t, respectively. As shown in Fig. 2d and Table 1, the first-order kinetic rate constant in the first stage is usually the lowest. During the initial stage of reaction, the reactive oxygen species (ROS) in the solution are very low, and the RhB dye itself is the initiator of a series of chain reactions. Therefore, RhB is stimulated by visible light to form a triplet state with high energy, and then the energy is transferred to HSO3−, and the resulting SO3%− and %O2− in turn decomposes RhB. It also can be clearly seen from the figure that the induction period will be shortened with the increase of NaHSO3 addition, because more NaHSO3 can produce more SO3%− free radicals. The degradation rate is the fastest in the second stage. In this stage, the produced free radicals in the solution enable the rapid degradation of dyes. In the third stage, the excited triplet state dye decreases as the concentration of rhodamine B in the solution becomes lower, so the reaction rate decreases for the free active species are reduced. It should be noted that at a low concentration of NaHSO3, such as 0.1 g L−1, the degradation rate of rhodamine B at the second stage is slow, the concentration of rhodamine B at the third stage is still high, and so the reaction rate at the third stage is nearly consistent with that at the second stage.
3.2. Degradation of RhB The photodegradation of rhodamine B (RhB) with sulfite addition under visible light irradiation was used as the model reaction system for detailed research (Fig. 2). In the absence of NaHSO3, only 2% of RhB is removed under visible light irradiation in 90 min, while 95% removal rate is achieved with the addition of 0.3 g L−1 NaHSO3 (Fig. 2a). When the light source was removed, the degradation of RhB was completely inhibited. Representative UV–Vis spectra changes during RhB decolorization by the vis/RhB/HSO3− system are depicted in Fig. 2b. The main absorption band at 554 nm is diminished after visible light illumination, 4
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Fig. 4. (a) Effect of scavengers on photocatalytic degradation of RhB in vis/RhB/HSO3− system and (b) kinetic fitting of the RhB degradation of the second stage in different quenching systems. (c) and (d) ESR spectra trapped by DMPO after 5 min of visible light irradiation in the vis/dye/HSO3− system. Reaction conditions: scavenger concentration: TBA = 200 mM, Ethanol = 200, 400 mM. BQ = 1 g L−1. NaHSO3 concentration 0.3 g L−1, DMPO concentration 25 mmol L−1. [RhB] = 5 mg L−1, [CBZ] = 5 mg L−1, [EY] = [MO] = [AO7] = 20 mg L−1, 25 °C, pH = 5.
quencher for %OH. As illustrated in Fig. 4a, the addition of TBA with a high dose of 400 mM has no significant effect on RhB degradation, indicating that the generation of %OH was very small in the vis/RhB/ HSO3− system. EtOH was added into the reaction solution to check whether there were other free radicals being involved in the degradation of RhB. As shown in Fig. 4a, the degradation performance was decreased with the increasing concentration of the added EtOH. Fig. 4b shows the kinetic fitting of the RhB degradation of second stage in different quenching systems. The pseudo-first order constant in the presence of 200 mM EtOH was 0.0075 min−1, which was 5.93 times lower than the corresponding value in the control experiment. High dose of EtOH (400 mM) almost completely inhibited the degradation of RhB, the pseudo-first order constant was 0.0028 min−1 in the presence of 400 mM EtOH, which was 15.89 times lower than the corresponding value in the control experiment, indicating the important contribution of SO3%− radicals in the system. As the formation of %O2− is quite common in many chemical oxidation based reactions, 1,4-benzoquinone (BQ) was also used as a quencher for %O2− detection [42,43]. As shown in Fig. 4a and b, the degradation of RhB was obviously inhibited with the addition of 1 g L−1 BQ, indicating the contribution of %O2− radical to the reaction. The pseudo-first order constant in the presence
3.3. Effects of pH on RhB degradation Effects of pH on sulfite activation were also investigated. The reactions of sulfite with RhB were carried out at various pH value (Fig. 3a), and the kobs of RhB decomposition at each pH were calculated (Fig. 3b). The RhB degradation was inhibited as the pH value increased from 3 to 9. RhB exists in two main forms in aqueous media, namely, cationic (RhB+) or zwitterionic form (RhB ± ). When the pH is lower than its pKa (3.7), it exists mainly in the cationic form. Nevertheless, when the pH value is higher than its pKa, it exists chiefly in the zwitterionic form [39]. RhB was deprotonated above pH 9 because of the deprotonation of its carboxyl group [40]. Therefore, the weaker electrostatic attraction between RhB and sulfite would be detrimental to the degradation of RhB. 3.4. Identification of main reactive oxidized species To identify the reactive oxidized species (ROS) in the photo-reaction self-degradation process, several types of scavengers were added into the vis/RhB/HSO3− system. Because the reaction between tert-butanol (TBA) and %OH is rapid (6.0 × 108 M−1 s−1) [41], TBA was used as a 5
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Fig. 5. (a) Zero order absorption spectra of RhB and AO7 in single and binary solutions. (b) first order derivative spectra of RhB and AO7 in single and binary solutions. Degradation of (c) mixed binary RhB and AO7 solution and (d) mixed RhB and CBZ solution in vis/RhB/HSO3− processes. Reaction conditions: [NaHSO3] = 0.3 g L−1, [RhB] = 5 mg L−1, [AO7] = 20 mg L−1, [CBZ] = 5 mg L−1, 25 °C, pH = 5.
of 1 g L−1 BQ was 0.0109 min−1, which was 4.08 times lower than the corresponding value in the control experiment. These results of quenching experiments verify that free SO3%− and %O2− are generated from the photo reaction of system, being mainly responsible for the degradation of RhB. In order to gain direct evidence for the involvement and the formation process of reactive species, electron spin resonance (ESR) analysis was employed using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a trapping agent. The significant signals of DMPO-SO3%− adducts are observed not only in the vis/RhB/HSO3− and vis/EY/HSO3− system (Fig. 4c) but also in the binary dye solutions (Fig. 4d). In comparison, there is no obvious ESR signal of DMPO-SO3%− in vis/MO/HSO3− and vis/AO7/HSO3− or vis/HSO3− system, which is in agreement with the results of degradation experiments; besides no DMPO-1O2 adducts were detected in any of the measurements (data are not shown). These results indicate that SO3%− is the main ROS which is generated from the dissociation of the excited HSO3− which gets energy from the excited state of photosensitive organic dyes.
3.5. Degradation of organic pollutants in vis/RhB/HSO3− system In order to further prove the important role of RhB dye in vis/RhB/ HSO3− system and exploring its potential application, we selected a mixture of RhB with AO7 dye solution and a mixture of RhB with CBZ for degradation experiments. As shown in Fig. 5a, the absorption spectrum of RhB and AO7 dyes in their binary mixture overlapped while the maximum absorbances of RhB and AO7 dyes in their single solutions were obtained at 554 nm and 484 nm, respectively (Δλmax = 70 nm). The overlapping of the RhB and AO7 dyes spectra shows the interference between the zero order spectra of RhB and AO7 dyes, and thus simultaneous determination of each dye in binary solutions was not accurate by direct absorption measurements. Hence, it was not possible to determine the amount of RhB or AO7 dye in binary mixtures. The method of derivation of the spectra is an analytical technique extensively used to solve this problem in the case of mixed solutions [44]. By measuring the derivative values at the wavelengths where one of the components has a zero or near zero value, the best linear responses are obtainable, and the calibration 6
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Scheme 1. Degradation pathway of RhB in vis/dye/HSO3− system.
degradation of both two dyes. The degradation rate of rhodamine B is slowed down compared to that in one dye solution, but AO7 is degraded at the same time. Under the degradation conditions, both dyes degraded by about 90%. As shown in Fig. 5d, when RhB was mixed with CBZ (5 mg L−1) for degradation experiment, in the same condition, 34% removal of CBZ was achieved in 90 min while CBZ alone was not photodegradable.
curves are less affected by concentration of other components [45]. Therefore, derivative spectrophotometric method was applied for the simultaneous determination of each individual dye in the case of mixed dye solutions. The first-order derivative absorption spectra of RhB and AO7 in single and mixed dye solutions are presented in Fig. 5b. The calibration curves for RhB and AO7 dyes at 573 and 451 nm (where the derivative absorbance of one of two dyes is zero) were separately obtained respectively to determine the concentration of each dye in mixture (SI Fig. S2). According to Fig. 1, AO7 alone was almost non-degradable under visible light even after NaHSO3 is added, indicating that AO7 which without photosensitive properties cannot activate NaHSO3 under visible light. But as seen from Fig. 5c, when RhB was mixed with AO7, the degradation of both RhB and AO7 at the same time in the mixed dye solution are obtained, indicating that the active species (SO3%−/%O2−) produced in vis/RhB/HSO3− system attack AO7 and lead to the
3.6. Degradation pathway of RhB Identification of possible intermediate products during the photoreaction is the best way to understand the photo degradation reaction mechanism. To study the photo degradation pathway of RhB dye under visible light irradiation, the possible intermediate products during the photo degradation were identified with electro spray ionization mass spectra (ESI-MS). The fragmented molecular ions were detected in 7
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Fig. 6. Effects of (a) inorganic ions and (b) humic acid (HA) on the removal of RhB. Conditions: [NaHSO3] = 0.3 g L−1, [RhB] = 5 mg L−1, inorganic ions = 1 mmol L−1, HA = 0.5 g L−1, 25 °C, pH = 5.
3.8. Degradation mechanism of pollutants in vis/POD/HSO3− processes
positive mode in the m/z range of 50–500. ESI mass spectra of the irradiated aqueous solutions of RhB at different time intervals were observed. The degradation of xanthene type dyes can occur via N-deethylation, de-alkylation, de-amination, de-carboxylation, de-hydration, cleavage of chromophore, and the breakage of ring structure. Major intermediates during the degradation process were proposed by using m/z values of the mass spectra (SI Table S1). Electrospray ionization analysis spectra reveal the formation of several aromatic intermediate, demonstrating the degradation of RhB dye. According to the earlier reports [46,47], most of the N-de-ethylation processes were taken place by the formation of nitrogen centered radical during the destruction of RhB dye chromopore structure. The photo degradation of RhB by the photogenerated active species such as %OH and hole could attack the central carbon of RhB to decolorize the dye and further degraded via N-de-ethylation process. The mass peaks at m/z 443, 415, 359, and 331 are intermediates of RhB and its N-de-ethylated process reported in earlier reports [48]. Several other peaks of fragments, m/ z = 437, 320, 304 and 192, were also observed at different time intervals. On the basis of mass results, the fragmentation pathway for the visible light induced photo degradation of RhB dye is proposed as shown in Scheme 1.
Previous studies [19,20] confirm that the triplet state acetone with high energy can be obtained by exciting acetone in solution using UV light, the energy transfer from the triplet state acetone to SO32− produces SO3%− and eaq−, and the energy transfer to dye molecule results in the decay of dye. Zepp et al. [50] indicate the excited triplet states of chromophoric dissolved organic matter can transfer energy to selected substrates and O2 and produce 1O2. In this work, photosensitive organic dyes are used as CDOM or acetone and excited by visible light. However, in our ESR experiment, no DMPO-1O2 adducts were detected in any of the measurements while significant signals of DMPO-SO3%− adducts were observed in the vis/POD/HSO3− system, which indicated that the triplet energy was transferred to HSO3−, not oxygen. Compared to oxygen, HSO3− is more reductive and more receptive to triplet energy. As the cleavage of (HSO3−)* produces sulfite radicals, the simultaneously generated electrons are trapped by oxygen, and the resulting %O2− also contribute greatly to organic pollutants degradation. Based on the above results, the degradation mechanism of pollutants in vis/POD/HSO3− system is proposed and shown in Eqs. (2)–(5). POD + vis → POD* POD* +
3.7. Influence of co-existing compounds
HSO3−
−
(HSO3 )* →
From the point of view of practical water treatment, the effects of possibly co-existing inorganic anions (such as SO42−, NO3−, and Cl−), inorganic cations (such as K+, Mg2+, and Ca2+) and natural organic matter (NOM) were further investigated. As shown in Fig. 6a, the influence of tested inorganic ion species was not considerable on the removal rate of RhB. RhB remove efficiency was not affected by the addition of 2.5, 5, 10, 25, 50 mmol L−1 inorganic ions (SO42−, NO3−, Cl−, K+, Mg2+, and Ca2+) (SI Fig. S5). Natural organic matter, a ubiquitous complex constituent existed in natural water and soil, can compete with contaminants for oxidants generated in AOPs. Humic acid (HA), as a significant constituent of NOM, was selected as a surrogate to assess the influence of NOM on RhB degradation in the vis/RhB/HSO3− process. HA can act as free radical scavengers, photosensitizer and light filter [49]. The different results on the degradation of organics have been reported in literature, depending on the quality and the type of the HA. In the vis/RhB/HSO3− system, RhB degradation rate was not affected by the addition of 0.5 g L−1 HA (Fig. 6b).
−
O2 + eaq →
(2)
→ POD
SO3%−+
+ (HSO3−)* +
H +
eaq−
%
O2−
SO3%−/%O2− + Pollutant
(3) (4) (5)
→ intermediates
(6)
The photosensitive organic dyes (POD) are at first excited to triplet state POD (POD*) by visible light, then, the energy is transferred from triplet state POD to HSO3−, SO3%− and eaq− are produced. The reaction of SO3%− with the pollutants results in the degradation of dye and CBZ in this system. 4. Conclusion In this work, the photochemical activation of HSO3− by photosensitive organic dyes/Vis system was employed as an AOP for organic pollutants degradation. Photosensitive organic dyes were excited under visible light irradiation, the transfer of triplet energy from excited organic dyes to HSO3− followed by dissociation of the excited HSO3− and generation of SO3%−. We confirmed that SO3%− and %O2− were the 8
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main ROS in our system. The reaction between SO3%−/%O2− and the dye molecules made the dye to degrade. The catalyst-free reaction system can degrade organic dyes and other additional organic pollutants. Unlike chemical activation, this vis/dye/HSO3−system uses only cheaper HSO3− as additional material, and does not require any expensive oxidants such as PMS or PDS, and can be operated under visible light irradiation. Thus, this catalyst-free AOP may provide a promising approach for the treatment of highly recalcitrant organic pollutants, such as organic dyes and CBZ complicated wastewater, and has the great potential of further research.
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