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Electrochemically activated PMS and PDS: Radical oxidation versus nonradical oxidation
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Electrochemically activated PMS and PDS: Radical oxidation versus nonradical oxidation ⁎
Haoran Songa, Linxia Yanb, Yuwei Wanga, Jin Jiangc, , Jun Mad, Changping Lia, Gang Wanga, Jia Gue, Peng Liua a
Research Center for Eco-environmental Engineering, School of Environment and Civil Engineering, Dongguan University of Technology, Dongguan, Guangdong 523808, PR China b Shenzhen Li Yuan Water Design and Consultation Co. Ltd., Shenzhen, Guangdong 518031, PR China c Guangdong University of Technology, Institute of Environmental & Ecological Engineering, Guangzhou, Guangdong 510006, PR China d State Key Laboratory of Urban Water Resource and Environment, School of Environment, Harbin Institute of Technology, Harbin, Heilongjiang 150090, PR China e School of Environmental and Biological Engineering, Nanjing University of Science and Technology, Nanjing, Jiangsu 210094, 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
activated PMS sig• Electrochemically nificantly enhanced organic pollutants degradation.
with PDS, electro• Compared chemically activated PMS was easier to generate SO4%−.
activated PMS ex• Electrochemically hibited strong resistance towards water background components.
A R T I C LE I N FO
A B S T R A C T
Keywords: Peroxomonosulfate Peroxydisulfate Sulfate radical Nonradical oxidation Carbon anodes
In this study, a comparison between electrochemically activated peroxomonosulfate (PMS) and peroxydisulfate (PDS) using carbon anodes was conducted for the first time. PMS activation was achieved using graphite (GR) and multi-walled carbon nanotube (MWCNT) anodes, which significantly promoted the degradation of organic pollutants sulfamethoxazole (SMX). The radical probing and scavenging experiments demonstrated that SO4%− was the dominant reactive species (64.93% relative contribution ratio). By contrast, nonradical oxidation accounted for 95.79% relative contribution ratio to organic pollutants degradation in electrochemically activated PDS process under the identical conditions. The structure difference between PDS (−O4S-SO4−) and PMS (HOSO4−) led to their various reactivities. The electrochemically activated PMS molecule (PMS*, acting as nonradical oxidation) had higher reactivity and lower stability than electrochemically activated PDS molecule (PDS*, acting as nonradical oxidation), thus to quickly decomposed to SO4%−. Interestingly, electrochemically activated PMS (radical oxidation system) exhibited stronger resistance towards water background components than PDS (nonradical oxidation system), being suitable to treat the complicated water and wastewater containing various ions and organic compounds.
⁎
Corresponding author. E-mail addresses: [email protected], [email protected] (J. Jiang).
https://doi.org/10.1016/j.cej.2019.123560 Received 8 May 2019; Received in revised form 14 November 2019; Accepted 18 November 2019 1385-8947/ © 2019 Published by Elsevier B.V.
Please cite this article as: Haoran Song, et al., Chemical Engineering Journal, https://doi.org/10.1016/j.cej.2019.123560
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1. Introduction
2.3. Experimental setup and procedure
Recently, extensive attention has been focused on the removal of recalcitrant organic pollutants in water and wastewater [1–3]. These recalcitrant organic pollutants such as pharmaceuticals, pesticides and industrial chemicals always have strong persistence towards natural water purification processes, which is beneficial to their accumulation in water and wastewater as well as organism [4,5]. Traditional water and wastewater treatment processes exhibited dissatisfactory removal efficiency towards recalcitrant organic pollutants, thus to form a potential pathogenic risk to humans and animals [6–8]. For instance, sulfamethoxazole (SMX) and trimethoprim (TMP) were detected in tomato fruits harvested from the wastewater irrigation soil [9]. Eating these contaminated foods (e.g., fishes and vegetables) may lead to the further accumulation of recalcitrant organic pollutants in the tissues of humans and animals. Therefore, it is highly desirable to develop efficient and feasible treatment processes for removal of these recalcitrant organic pollutants. Sulfate radical (SO4%−) oxidation has been proved to be an effective method for recalcitrant organic pollutants removal in water and wastewater [10–12]. Differed from physical removal by adsorption or membrane filtration, SO4%− can degrade or mineralize organic pollutants to carbon dioxide (CO2) and water (H2O) due to its ultrahigh redox potential. Activation of persulfates (including PMS (peroxomonosulfate) and PDS (peroxydisulfate)) by metal ions [13,14], UV radiation [14], carbon materials [15,16], quinones [17,18] and electrolysis [19–21] was the common method to produce SO4%−. Electrochemically activated persulfates is an emerging advanced oxidation process. In our previous study, degradation of antibiotic SMX was significantly improved by electrochemically activated PDS at carbon anode surface [19]. Compared with PDS, PMS is easier to activate owing to its asymmetric structure. Hitherto that date, little research has been reported on electrochemically activated PMS using carbon anodes. A comparison study between electrochemically activated PDS and PMS is necessary and meaningful. In this work, carbon electrodes prepared by polytetrafluoroethylene (PTFE) and a series of carbon, such as graphite (GR), multi-walled carbon nanotube (MWCNT), granular activated carbon (GAC) and black carbon (BC) used in our previous study (electrochemically activated PDS process), were used to activate PMS under the identical reaction conditions. The generated reactive species was discerned by radical probing, followed by radical scavenging. Simultaneously, the influence of operating parameter including PMS concentration, current density and water background components (such as chloride (Cl−), phosphate (PO43−), bicarbonate (HCO3−) and natural organic matter (NOM)) on organic pollutants degradation was evaluated and compared with PDS.
The experimental setup was similar to that we used before [22]. All electrochemical experiments were performed in an electrolysis cell that consists of two plate and frame chambers (100 mL) separated by a piece of Nafion 117 cation exchange membrane. Carbon electrode was used as working electrode (anode) and stainless steel mesh acted as counter electrode (cathode). Active areas of all electrode were controlled at 10 cm2. All electrodes were fixed with a PTFE electrode clamps, which were connected to a ITECH 6300 DC power. A magnetic stirring apparatus coupled with a PTFE-covered stirrer was used to mix the electrolyte. The temperature of the electrolyte was maintained at 25 ℃ with the assistance of a water bath. In a typical run, 50 mM NaClO4 solution was employed as the background electrolyte. The final net volume of electrolyte was 80 mL spiked with desired concentration of organic compounds and PMS. The solution pH buffer was not used. Followed by addition of organic compounds and PMS, the electrolyte was constantly stirred thoroughly and the initial sample was drawn from the anodic chamber. Then, the electrochemical reaction was initiated by applying individual direct current (10–200 A m−2). Samples were collected at predetermined time intervals from the anodic chamber, and then injected into 1 mL vials through a 0.22 μm PTFE membrane. Excess methanol and ascorbic acid were preadded in each vial. All experiments were performed for twice or three times and the average values were provided. 2.4. Analytical method The concentration of organic compounds was analyzed with a liquid chromatograph (Waters 2695, USA). Detection was performed using a photodiode array detector (Waters 2998, USA). Organic compounds separation was performed with a 4.6 × 150 mm symmetry C18 column (5 μm particle size, Waters, USA). The mobile phase was methanol and water in varying ratios. The flow rate was set as 1 mL min−1 for separation of all organic compounds. 3. Results and discussion 3.1. SMX degradation Carbon material always has high adsorption capacity towards organic pollutants. Adsorption control experiment was conducted to investigate the influence of adsorption using MWCNT, BC, GR and GAC anodes (without applying current) on SMX removal (Fig. S4). SMX concentration slowly decreased along time in adsorption control experiments. Compared with adsorption, electrolysis using four carbon anodes all promoted the degradation of SMX (Fig. 1). The promotion effect was attributed to DET reaction at anode surface. The pseudo-firstorder degradation rate of SMX in electrolysis process using various carbon electrode did not show much difference (from 0.0238 min−1 to 0.0292 min−1) (Table S1). Addition of PMS accelerated SMX degradation relative to electrolysis (Fig. 1). The removal rate of SMX in the combination process (PMS + electrolysis) was significantly higher than that of electrolysis or adsorption. For instance, the removal rate of SMX in the combination process (PMS + electrolysis with GR anode) was 0.1945 min−1, greatly higher than that of electrolysis (0.0259 min−1) or GR alone (0.0061 min−1). Differed from electrolysis (from 0.0238 min−1 to 0.0292 min−1), SMX degradation in the combination process varied greatly dependent on anodes. MWCNT and GR anodes produced remarkably higher degradation rate of SMX (0.1571 min−1 for MWCNT, 0.1945 min−1 for GR) than BC and GAC anodes (0.0475 min−1 for BC, 0.065 min−1 for GAC). The above results demonstrated that electrochemically activated PMS might be achieved at MWCNT, BC, GR and GAC anodes. However, previous studies reported that carbon material had catalytic ability for
2. Chemicals and methods 2.1. Chemicals SMX, PMS, atrazine (ATZ), nitrobenzene (NB) and humic acid (HA) were obtained from Sigma-Aldrich. PTFE dispersion solution (60 ± 2% w/w) was provided by Shenzhen Kejing Star Technology Co., Ltd.. Perchlorate (NaClO4), sodium bicarbonate (NaHCO3), sodium chloride (NaCl), sodium phosphate (Na3PO4), ascorbic acid, BC and GAC were obtained from Sinopharm Chemical Reagent Co., Ltd.. MWCNT and GR were provided by Aladdin. Methanol was obtained from Tedia. Ultrapure water (Millipore, Billerica, MA) was used to prepare all stock solution and electrolyte. 2.2. Preparation and characterization of carbon electrodes The preparation and characterization methods of carbon electrodes have been presented in our previous study [22]. The detailed process was provided in Text S1. 2
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Fig. 1. Degradation of SMX by electrolysis (□) and the combination of electrolysis and PMS ( ) using (a) MWCNT, (b) BC, (c) GR and (d) GAC anodes. Conditions: current density = 100 A m−2; [PMS]0 = 5 mM, [SMX]0 = 5 μM.
PMS activation [16,23]. Thus, activation of PMS by carbon anodes without applying current (carbon/PMS) was investigated. As shown in Fig. S4, SMX degradation by carbon/PMS was more effective compared with carbon alone or electrolysis, in consistent with previous studies [16,23]. Activation of PMS by MWCNT, BC, GR and GAC indeed existed in the combination process. Interestingly, applying current on MWCNT or GR in the presence of PMS produced synergistic effect. Nevertheless, applying current on BC or GAC in the presence of PMS generated negligible effect. SMX degradation rate in the combination process using BC and GAC anodes (0.0425 min−1 for BC, 0.0685 min−1 for GAC) was smaller than the sum of electrolysis (0.0275 min−1 for BC and 0.0238 min−1 for GAC) and carbon/PMS (0.0475 min−1 for BC, 0.0652 min−1 for GAC). From the result of control experiment, it can be concluded that the effect of PMS alone on SMX degradation was negligible (Fig. S5). Thus, electrochemically activated PMS using MWCNT and GR anodes was achieved. Electrochemically activated PDS at the same four anodes (MWCNT, BC, GR and GAC) had been investigated in our previous study [22]. Compared with carbon/PDS (0.0082–0.0321 min−1), higher degradation rate of SMX was observed in carbon/PMS (0.0473–0.0685 min−1), indicating that PMS was easier to activate than PDS. It is noteworthy that, electrochemically activated PDS (0.4568–0.9944 min−1) generated higher degradation rate of SMX than that of PMS (0.1571 min−1 for MWCNT and 0.1945 min−1 for GR) under the identical reaction conditions. Additionally, electrochemically activated PDS could be achieved at all four electrodes, while electrochemically activated PMS was just achieved at MWCNT and GR electrodes.
Fig. 2. Degradation of ATZ using GR anode. GR alone (□); electrolysis ( ); GR/PMS ( ) and electrochemically activated PMS ( ). Conditions: current density = 100 A m−2; [PMS]0 = 5 mM, [ATZ]0 = 5 μM.
GR anode (0.0202 min−1) was proved to be more effective towards ATZ degradation than GR alone because of DET reaction. ATZ degradation was improved by GR/PMS (0.0261 min−1), suggesting that some radicals were generated from GR/PMS. This result was in conformity with previous studies, in which HO% and SO4%− were generated from carbon/PMS system [16,27]. ATZ degradation was furtherly enhanced unexpectedly by applying current on GR/PMS (0.1060 min−1) relative to GR/PMS or electrolysis (Fig. 2, Table S2), demonstrating that electrochemically activated PMS at GR anode might produce HO% or SO4%−. The degradation of NB was explored to recognize the involvement of HO% (Fig. 3). GR/PMS (0.0198 min−1) exhibited higher NB degradation rate than GR alone (0.0128 min−1) or electrolysis (0.0098 min−1). However, NB degradation by electrochemically activated PMS (0.0169 min−1) was inferior to GR/PMS, suggesting no HO% was generated in this process.
3.2. Reactive species identification 3.2.1. Radical probing In advanced oxidation processes, ATZ and NB were often used as radical probes (ATZ for HO% and SO4%−, NB for HO%) to identify the generated reactive species [24–26]. GR anode was employed to examine the degradation of ATZ and NB. PMS alone could not enhance ATZ degradation (Fig. S6). GR alone could remove ATZ (0.0143 min−1) due to adsorption of ATZ at GR anode surface (Fig. 2). Electrolysis with 3
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Fig. 3. Degradation of NB using GR anode. GR alone (□); electrolysis ( ); GR/ PMS ( ) and electrochemically activated PMS ( ). Conditions: current density = 100 A m−2; [PMS]0 = 5 mM, [NB]0 = 5 μM.
Similarly, the above phenomena also occurred in electrochemically activated PMS using MWCNT anode (Figs. S7–S9, Table S2). In this process, ATZ degradation was obviously promoted, but NB degradation was suppressed compared with MWCNT/PMS. The above results suggest that SO4%− was produced in electrochemically activated PMS at MWCNT or GR anode. The inhibitory effect towards NB degradation was attributable to the oxygen evolution reaction at anode surface. Oxygen evolution hindered the adsorption of NB and PMS activation, thus to decrease the degradation rate of NB. Differed from PMS, electrochemically activated PDS using GR anode not only enhanced ATZ degradation but also improved NB degradation and ATZ degradation rate was significantly faster than that of NB. It can be concluded that both HO% and SO4%− were produced from electrochemically activated PDS. 3.2.2. Radical scavenging Nonradical oxidation played the key role for organic pollutants degradation in electrochemically activated PDS. It is necessary to clear whether nonradical oxidation is produced in electrochemically activated PMS. According to our previous study, organic pollutants degradation could be significantly inhibited by 10 M methanol in electrochemically activated persulfates process [19]. As can be concluded from Fig. 4, SMX removal by carbon alone was completely inhibited in the methanol-containing system. However, the presence of 10 M methanol failed to prevent SMX removal in electrolysis, carbon/PMS and electrochemically activated PMS processes. The rapid degradation of SMX by carbon/PMS in the methanol-containing system indicates that the occurrence of nonradical oxidation. In addition, the electrochemically activated PMS showed higher SMX degradation rate than electrolysis or carbon/PMS in the methanol-containing system, suggesting the generation of more nonradical oxidation compared with carbon/PMS.
Fig. 4. Effect of addition of 10 M methanol on SMX degradation using MWCNT anode (a) and GR anode (b). MWCNT or GR alone with addition of 10 M methanol (□); electrolysis with addition of 10 M methanol ( ); MWCNT/PMS or GR/PMS with addition of 10 M methanol ( ) and electrochemically activated PMS using MWCNT or GR with addition of 10 M methanol ( ). Conditions: current density = 100 A m−2; [PMS]0 = 5 mM, [SMX]0 = 5 μM.
to 0.3999 min−1 for 200 A m−2. An exponential relationship emerged between SMX degradation rate and current density (Fig. S11). Similarly, increase of PDS concentration from 0.1 mM to 5 mM and current density from 10 A m−2 to 200 A m−2 in electrochemically activated PDS process also significantly enhanced SMX degradation. Moreover, SMX degradation in PDS system also exhibited linear increase along with increasing PDS concentration and exponential increase along with increasing current density. Thus, the role of PDS/PMS or current might be the same in dual processes. PDS/PMS acted as the oxidants and current played as the driving force to generated reactive species.
3.3. Influence of operating parameter: PMS concentration and current density As shown in Fig. 5, increasing PMS concentration from 0.1 mM to 5 mM significantly enhanced SMX degradation. The degradation rate of SMX increased about 3 folds, from 0.0638 min−1 for 0.1 mM PMS to 0.1945 min−1 for 5 mM PMS. Simultaneously, a good linear relationship (R2 > 0.97) between SMX degradation rate and PMS concentration was observed (Fig. S10). Increasing current density also remarkably promoted SMX degradation from 0.0884 min−1 for 10 A m−2
3.4. Influence of water background components Addition of Cl−, HCO3−, PO43− and HA (the model for NOM) in the electrolysis cell was used to evaluate the influence of water background components on the degradation of SMX. As shown in Fig. 6, SMX degradation was significantly enhanced when addition of various 4
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(PDS*). Thus, PMS* was able to oxidize Cl− to generate some chlorinecontaining reactive species, which furtherly contributed to the quick degradation of SMX (Eq. (1)).
PMS* + Cl−→Chlorine - containing reactive species
(1) −
Nevertheless, PDS* had low reactivity towards Cl , being unable to oxidize Cl−. Addition of excess Cl− inhibited PDS adsorption at electrode surface, thus to inhibit PDS activation and SMX degradation. Interestingly, addition of various concentrations of HCO3− in electrochemical activation PMS process generated minor promotion effect on SMX degradation, while addition of HCO3− in electrochemically activated PDS process inhibited SMX degradation (Fig. S13). Similar to Cl−, addition of excess HCO3− can also inhibit PDS/PMS adsorption and their activation. The opposite effect HCO3− on SMX degradation in electrochemically activated PDS and PMS processes might originate from pH buffering effect. Excess HCO3− could increase the solution pH in the anodic chamber [24]. PMS activation could be achieved at high pH condition [30,31]. Similar to PDS, the presence of PO43− or HA all inhibited SMX degradation in electrochemically activated PMS process due to the competitive adsorption (PO43− and HA) towards persulfates and competitive oxidation (HA) towards to SMX [24]. Increase of PO43− or HA concentration from 5 mM to 10 mM and 20 mM generated greater inhibitory effect. However, the inhibitory effect of same concentration of PO43− or HA on SMX degradation in electrochemically activated PMS process was significantly smaller than that in electrochemically activated PDS process under the identical conditions. For instance, SMX degradation rate in PMS process with addition of 20 mgC L−1 HA decreased about 24% relative to that in the absence of HA, while SMX degradation rate in PDS process with addition of 20 mgC L−1 HA decreased about 81% relative to that in the absence of HA. The above results indicated that electrochemically activated PMS exhibited stronger resistance towards water background components than PDS under the identical conditions. 3.5. Involved mechanism Adsorption, electrolysis (DET), nonradical oxidation and SO4%− were demonstrated to be responsible for organic pollutants degradation in electrochemically activated PMS process (Scheme 1). For clearly understanding the difference between electrochemically activated PMS and PDS, the relative contribution ratio of various reactive species to SMX degradation were calculated based on its kinetic data (Table 1, detailed calculation methods were provided in Text S1).
Fig. 5. Effect of (a) PMS concentration and (b) current density on the degradation of SMX by electrochemically activated PMS using GR anode. Conditions: (a) current density = 100 A m−2, (b) [PMS]0 = 5 mM, [SMX]0 = 5 μM.
3.5.1. Adsorption and electrolysis As shown in Table 1, the relative contribution of adsorption in PDS and PMS processes was 0.61% and 3.14%, respectively. Electrochemical reaction belongs to interfacial reaction system, in which adsorption is the prerequisite step. Organic pollutants degradation and PDS/PMS activation all took place at electrode surface. Adsorption not only contributed to the removal of organic pollutants but also participated in PDS/PMS activation. Organic pollutants degradation in electrolysis process was attributed to direct electron transfer (DET) reaction. The adsorbed organic pollutants were directly oxidized to their products. The relative contribution of DET in PDS and PMS processes was about 1.99% and 10.18%, respectively. Moreover, electrolysis contributed to PMS/PDS activation, playing as the reaction driving force. Application of current on adsorbed PDS/PMS might change the structure of PDS/PMS molecule [19,24], leading to the generation of reactive species.
concentration of Cl− (5 mM to 20 mM). Increasing Cl− concentration could improve SMX degradation. The free chlorine generated from electrolysis of Cl− at anode might contribute to the SMX degradation [28,29]. As can be concluded from Fig. S12, 20 mM Cl− (in the absence of PMS) indeed improved SMX degradation, but SMX degradation rate (0.1455 min−1) in this process was still far smaller than that in electrochemically activated PMS with addition of 20 mM Cl− (1.5883 min−1). Thus, in addition to free chlorine generated from electrolysis of Cl−, electrochemically activated PMS in the presence of Cl− must produce other reactive species which improve SMX degradation. By contrast, SMX degradation was inhibited when addition of 5 mM, 10 mM and 20 mM Cl−in electrochemically activated PDS process and higher concentration of Cl− exhibited greater inhibition effect. The opposite effect of Cl− on SMX degradation in PDS and PMS processes could be attributable to the structure difference between PDS and PMS. The asymmetric structure of PMS (HO-SO4−) leads to that activation of PMS was easier than PDS. The electrochemically activated PMS (PMS*) might have higher reactivity than electrochemically activated PDS
3.5.2. Nonradical oxidation According to the results of radical scavenging experiments, both carbon/PMS and electrochemically activated PMS generated nonradical oxidation. A reactive complex formed between PMS and carbon 5
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Fig. 6. Effect of water background components (a) Cl−, (b) HCO3−, (c) PO43− and (d) HA on degradation of SMX by electrochemically activated PMS using GR anode. Conditions: current density = 100 A m−2; [PMS]0 = 5 mM, [SMX]0 = 5 μM.
photoelectron spectroscopy (XPS) verifies that electrolysis indeed increased carbonyl group proportion (Fig. S14). Furthermore, activation of PMS by the aged GR electrode (6 h with 100 A m−2 current) was investigated. As shown in Fig. S15, the aged GR electrode indeed promoted SMX degradation. Nevertheless, the SMX
material was proposed to play as the nonradical oxidation in carbon/ PMS process [16]. Compared with carbon/PMS, electrochemically activated PMS enhanced the generation of nonradical oxidation. Electrolysis might increase the activate sites of carbon material (e.g., carbonyl group), resulting in enhancing PMS activation [16]. The X-ray
Scheme 1. Proposed mechanism of electrochemically activated PMS using carbon anodes. 6
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in PMS system (3.83 × 10−13 M).
Table 1 Estimation of the relative contribution ratio of involved reactive species. Process
k (min−1)
Relative Contribution ratio (%)
PDS
Adsorption DET PDS Nonradical HO% SO4%−
0.0061 0.0259 0.0004 0.9524 0.0123 0.0038
0.61 1.99 Neglected 95.79 1.23 0.38
PMS
Adsorption DET PMS Nonradical HO% SO4%−
0.0061 0.0259 0.0009 0.0423 – 0.1263
3.14 10.18 Neglected 21.75 – 64.93
4. Conclusions In this work, we performed a comparative study between electrochemically activated PMS and PDS at carbon anodes. MWCNT and GR anodes could be used for electrochemically activated PMS. Both nonradical oxidation and SO4%− were generated in electrochemically activated PMS process. The electrochemically activated PMS molecule played as the nonradical oxidation and it further decomposed to SO4%−. The presence of various concentration of Cl− significantly improved SMX degradation. Addition of HCO3− produced minor promotion on degradation of SMX, while the presence of PO43− and HA all inhibited SMX degradation. After longtime aging treatment, the prepared electrode exhibited excellent stability (Fig. S15), which is beneficial to its practical application. By contrast, nonradical oxidation, SO4%− and HO% was produced in electrochemically activated PDS process. Differed from electrochemically activated PMS process in which SO4%− mainly contributed to organic pollutants degradation, nonradical oxidation was the main reactive species in electrochemically activated PDS process. Electrochemically activated PDS generated considerably higher SMX degradation rate than that of PMS under the identical conditions. However, electrochemically activated PDS was more susceptive to water background components compared with PMS. Employment of electrochemically activated PDS or PMS depends on the characters of organic pollutants and water background components. On condition that water and wastewater contains various organic pollutants that are resistant to nonradical oxidation (e.g., ATZ) or high concentration of background ions and NOM, application of electrochemically activated PMS is superior to electrochemically activated PDS. However, Cl− participated in organic pollutants degradation in electrochemically activated PMS process. The risk of chlorinated byproducts generation deserves further consideration in the future works.
k HO·,SMX = 5.5 × 109 M−1 s−1; k SO·4−,SMX = 5.5 × 109 M−1 s−1; k HO·,ATX = 2.6 × 109 M−1 s−1 k HO·,NB = 3.9 × 109 M−1 s−1; k SO·4−,ATZ = 2.6 × 109 M−1 s−1 The contribution of PDS/PMS alone oxidation was neglected.
degradation rate in aged GR/PMS process was still smaller than that in electrochemically activated PMS, verifying that nonradical oxidation mainly originated from electrochemically activated PMS rather than aged GR/PMS. The adsorbed PMS molecule at carbon electrode surface was induced to activated PMS molecule (PMS*) by applying current. The above mechanism was also suitable to carbon/PDS and electrochemically activated PDS [22,24]. Although electrochemically activated PDS and PMS all generated nonradical oxidation and the involved mechanism was identical to each other, the relative contribution of nonradical oxidation varied greatly. The relative contribution of nonradical oxidation in PDS system was 95.79% (Table 1), far higher than that of PMS (21.75%). Furthermore, electrochemically activated PDS exhibited remarkably higher SMX degradation rate (0.9944 min−1) than PMS (0.1945 min−1) under the identical reaction conditions even if PMS activation was easier than that of PDS. This is because that PDS* was more stable than PMS*, meaning that PDS* had longer lifetime and higher steady-state concentration than PMS*. Generation of a large amount of SO4%− in electrochemically activated PMS also verified this conclusion, as explained in the next Section 3.5.3.
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.
3.5.3. Sulfate radical oxidation Differed from PDS system, SO4%− was the dominant reactive species accounting for 64.93% relative contribution in electrochemically activated PMS system. As shown in Eq. (2), PMS* directly decomposed to SO4%−.
PMS* → SO·4−
Acknowledgement Thanks for the support from Research Start-up Funds of DGUT (GC300501-122).
(2)
Appendix A. Supplementary data
The steady-state concentration of SO4%− in electrochemically vated PMS process (exclusion of SO4%− from carbon/PMS) was −13
actiestimated to be 3.83 × 10 M, about eight times of the sum of that of HO% (3.73 × 10−14 M) and SO4%− (1.15 × 10−14 M) in electrochemically activated PDS process (calculation method listed in Text S1). The quick decomposition of PMS* (nonradical oxidation) reduced the contribution of nonradical oxidation. This mechanism also explained the reasons why electrochemically activated PDS exhibited remarkably higher SMX degradation rate (0.9944 min−1) than PMS (0.1945 min−1). Assuming that the reaction rate of nonradical oxidation towards SMX ranged from 108-109 M−1 s−1, then the stead-state concentration of nonradical oxidation in PDS and PMS processes was estimated to be 1.59 × 10−11–1.59 × 10−10 M (PDS) and 7.05 × 10−137.05 × 10−12 M (PMS), respectively. Although the reaction reactivity of nonradical oxidation with organic pollutants was always lower than that of SO4%− (> 109 M−1 s−1), the steady-state concentration of nonradical oxidation in PDS system (1.59 × 10−11–1.59 × 10−10 M) was much higher than that of SO4%−
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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Electrochemically activated PMS and PDS: Radical oxidation versus nonradical oxidation ⁎
Haoran Songa, Linxia Yanb, Yuwei Wanga, Jin Jiangc, , Jun Mad, Changping Lia, Gang Wanga, Jia Gue, Peng Liua a
Research Center for Eco-environmental Engineering, School of Environment and Civil Engineering, Dongguan University of Technology, Dongguan, Guangdong 523808, PR China b Shenzhen Li Yuan Water Design and Consultation Co. Ltd., Shenzhen, Guangdong 518031, PR China c Guangdong University of Technology, Institute of Environmental & Ecological Engineering, Guangzhou, Guangdong 510006, PR China d State Key Laboratory of Urban Water Resource and Environment, School of Environment, Harbin Institute of Technology, Harbin, Heilongjiang 150090, PR China e School of Environmental and Biological Engineering, Nanjing University of Science and Technology, Nanjing, Jiangsu 210094, 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
activated PMS sig• Electrochemically nificantly enhanced organic pollutants degradation.
with PDS, electro• Compared chemically activated PMS was easier to generate SO4%−.
activated PMS ex• Electrochemically hibited strong resistance towards water background components.
A R T I C LE I N FO
A B S T R A C T
Keywords: Peroxomonosulfate Peroxydisulfate Sulfate radical Nonradical oxidation Carbon anodes
In this study, a comparison between electrochemically activated peroxomonosulfate (PMS) and peroxydisulfate (PDS) using carbon anodes was conducted for the first time. PMS activation was achieved using graphite (GR) and multi-walled carbon nanotube (MWCNT) anodes, which significantly promoted the degradation of organic pollutants sulfamethoxazole (SMX). The radical probing and scavenging experiments demonstrated that SO4%− was the dominant reactive species (64.93% relative contribution ratio). By contrast, nonradical oxidation accounted for 95.79% relative contribution ratio to organic pollutants degradation in electrochemically activated PDS process under the identical conditions. The structure difference between PDS (−O4S-SO4−) and PMS (HOSO4−) led to their various reactivities. The electrochemically activated PMS molecule (PMS*, acting as nonradical oxidation) had higher reactivity and lower stability than electrochemically activated PDS molecule (PDS*, acting as nonradical oxidation), thus to quickly decomposed to SO4%−. Interestingly, electrochemically activated PMS (radical oxidation system) exhibited stronger resistance towards water background components than PDS (nonradical oxidation system), being suitable to treat the complicated water and wastewater containing various ions and organic compounds.
⁎
Corresponding author. E-mail addresses: [email protected], [email protected] (J. Jiang).
https://doi.org/10.1016/j.cej.2019.123560 Received 8 May 2019; Received in revised form 14 November 2019; Accepted 18 November 2019 1385-8947/ © 2019 Published by Elsevier B.V.
Please cite this article as: Haoran Song, et al., Chemical Engineering Journal, https://doi.org/10.1016/j.cej.2019.123560
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1. Introduction
2.3. Experimental setup and procedure
Recently, extensive attention has been focused on the removal of recalcitrant organic pollutants in water and wastewater [1–3]. These recalcitrant organic pollutants such as pharmaceuticals, pesticides and industrial chemicals always have strong persistence towards natural water purification processes, which is beneficial to their accumulation in water and wastewater as well as organism [4,5]. Traditional water and wastewater treatment processes exhibited dissatisfactory removal efficiency towards recalcitrant organic pollutants, thus to form a potential pathogenic risk to humans and animals [6–8]. For instance, sulfamethoxazole (SMX) and trimethoprim (TMP) were detected in tomato fruits harvested from the wastewater irrigation soil [9]. Eating these contaminated foods (e.g., fishes and vegetables) may lead to the further accumulation of recalcitrant organic pollutants in the tissues of humans and animals. Therefore, it is highly desirable to develop efficient and feasible treatment processes for removal of these recalcitrant organic pollutants. Sulfate radical (SO4%−) oxidation has been proved to be an effective method for recalcitrant organic pollutants removal in water and wastewater [10–12]. Differed from physical removal by adsorption or membrane filtration, SO4%− can degrade or mineralize organic pollutants to carbon dioxide (CO2) and water (H2O) due to its ultrahigh redox potential. Activation of persulfates (including PMS (peroxomonosulfate) and PDS (peroxydisulfate)) by metal ions [13,14], UV radiation [14], carbon materials [15,16], quinones [17,18] and electrolysis [19–21] was the common method to produce SO4%−. Electrochemically activated persulfates is an emerging advanced oxidation process. In our previous study, degradation of antibiotic SMX was significantly improved by electrochemically activated PDS at carbon anode surface [19]. Compared with PDS, PMS is easier to activate owing to its asymmetric structure. Hitherto that date, little research has been reported on electrochemically activated PMS using carbon anodes. A comparison study between electrochemically activated PDS and PMS is necessary and meaningful. In this work, carbon electrodes prepared by polytetrafluoroethylene (PTFE) and a series of carbon, such as graphite (GR), multi-walled carbon nanotube (MWCNT), granular activated carbon (GAC) and black carbon (BC) used in our previous study (electrochemically activated PDS process), were used to activate PMS under the identical reaction conditions. The generated reactive species was discerned by radical probing, followed by radical scavenging. Simultaneously, the influence of operating parameter including PMS concentration, current density and water background components (such as chloride (Cl−), phosphate (PO43−), bicarbonate (HCO3−) and natural organic matter (NOM)) on organic pollutants degradation was evaluated and compared with PDS.
The experimental setup was similar to that we used before [22]. All electrochemical experiments were performed in an electrolysis cell that consists of two plate and frame chambers (100 mL) separated by a piece of Nafion 117 cation exchange membrane. Carbon electrode was used as working electrode (anode) and stainless steel mesh acted as counter electrode (cathode). Active areas of all electrode were controlled at 10 cm2. All electrodes were fixed with a PTFE electrode clamps, which were connected to a ITECH 6300 DC power. A magnetic stirring apparatus coupled with a PTFE-covered stirrer was used to mix the electrolyte. The temperature of the electrolyte was maintained at 25 ℃ with the assistance of a water bath. In a typical run, 50 mM NaClO4 solution was employed as the background electrolyte. The final net volume of electrolyte was 80 mL spiked with desired concentration of organic compounds and PMS. The solution pH buffer was not used. Followed by addition of organic compounds and PMS, the electrolyte was constantly stirred thoroughly and the initial sample was drawn from the anodic chamber. Then, the electrochemical reaction was initiated by applying individual direct current (10–200 A m−2). Samples were collected at predetermined time intervals from the anodic chamber, and then injected into 1 mL vials through a 0.22 μm PTFE membrane. Excess methanol and ascorbic acid were preadded in each vial. All experiments were performed for twice or three times and the average values were provided. 2.4. Analytical method The concentration of organic compounds was analyzed with a liquid chromatograph (Waters 2695, USA). Detection was performed using a photodiode array detector (Waters 2998, USA). Organic compounds separation was performed with a 4.6 × 150 mm symmetry C18 column (5 μm particle size, Waters, USA). The mobile phase was methanol and water in varying ratios. The flow rate was set as 1 mL min−1 for separation of all organic compounds. 3. Results and discussion 3.1. SMX degradation Carbon material always has high adsorption capacity towards organic pollutants. Adsorption control experiment was conducted to investigate the influence of adsorption using MWCNT, BC, GR and GAC anodes (without applying current) on SMX removal (Fig. S4). SMX concentration slowly decreased along time in adsorption control experiments. Compared with adsorption, electrolysis using four carbon anodes all promoted the degradation of SMX (Fig. 1). The promotion effect was attributed to DET reaction at anode surface. The pseudo-firstorder degradation rate of SMX in electrolysis process using various carbon electrode did not show much difference (from 0.0238 min−1 to 0.0292 min−1) (Table S1). Addition of PMS accelerated SMX degradation relative to electrolysis (Fig. 1). The removal rate of SMX in the combination process (PMS + electrolysis) was significantly higher than that of electrolysis or adsorption. For instance, the removal rate of SMX in the combination process (PMS + electrolysis with GR anode) was 0.1945 min−1, greatly higher than that of electrolysis (0.0259 min−1) or GR alone (0.0061 min−1). Differed from electrolysis (from 0.0238 min−1 to 0.0292 min−1), SMX degradation in the combination process varied greatly dependent on anodes. MWCNT and GR anodes produced remarkably higher degradation rate of SMX (0.1571 min−1 for MWCNT, 0.1945 min−1 for GR) than BC and GAC anodes (0.0475 min−1 for BC, 0.065 min−1 for GAC). The above results demonstrated that electrochemically activated PMS might be achieved at MWCNT, BC, GR and GAC anodes. However, previous studies reported that carbon material had catalytic ability for
2. Chemicals and methods 2.1. Chemicals SMX, PMS, atrazine (ATZ), nitrobenzene (NB) and humic acid (HA) were obtained from Sigma-Aldrich. PTFE dispersion solution (60 ± 2% w/w) was provided by Shenzhen Kejing Star Technology Co., Ltd.. Perchlorate (NaClO4), sodium bicarbonate (NaHCO3), sodium chloride (NaCl), sodium phosphate (Na3PO4), ascorbic acid, BC and GAC were obtained from Sinopharm Chemical Reagent Co., Ltd.. MWCNT and GR were provided by Aladdin. Methanol was obtained from Tedia. Ultrapure water (Millipore, Billerica, MA) was used to prepare all stock solution and electrolyte. 2.2. Preparation and characterization of carbon electrodes The preparation and characterization methods of carbon electrodes have been presented in our previous study [22]. The detailed process was provided in Text S1. 2
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Fig. 1. Degradation of SMX by electrolysis (□) and the combination of electrolysis and PMS ( ) using (a) MWCNT, (b) BC, (c) GR and (d) GAC anodes. Conditions: current density = 100 A m−2; [PMS]0 = 5 mM, [SMX]0 = 5 μM.
PMS activation [16,23]. Thus, activation of PMS by carbon anodes without applying current (carbon/PMS) was investigated. As shown in Fig. S4, SMX degradation by carbon/PMS was more effective compared with carbon alone or electrolysis, in consistent with previous studies [16,23]. Activation of PMS by MWCNT, BC, GR and GAC indeed existed in the combination process. Interestingly, applying current on MWCNT or GR in the presence of PMS produced synergistic effect. Nevertheless, applying current on BC or GAC in the presence of PMS generated negligible effect. SMX degradation rate in the combination process using BC and GAC anodes (0.0425 min−1 for BC, 0.0685 min−1 for GAC) was smaller than the sum of electrolysis (0.0275 min−1 for BC and 0.0238 min−1 for GAC) and carbon/PMS (0.0475 min−1 for BC, 0.0652 min−1 for GAC). From the result of control experiment, it can be concluded that the effect of PMS alone on SMX degradation was negligible (Fig. S5). Thus, electrochemically activated PMS using MWCNT and GR anodes was achieved. Electrochemically activated PDS at the same four anodes (MWCNT, BC, GR and GAC) had been investigated in our previous study [22]. Compared with carbon/PDS (0.0082–0.0321 min−1), higher degradation rate of SMX was observed in carbon/PMS (0.0473–0.0685 min−1), indicating that PMS was easier to activate than PDS. It is noteworthy that, electrochemically activated PDS (0.4568–0.9944 min−1) generated higher degradation rate of SMX than that of PMS (0.1571 min−1 for MWCNT and 0.1945 min−1 for GR) under the identical reaction conditions. Additionally, electrochemically activated PDS could be achieved at all four electrodes, while electrochemically activated PMS was just achieved at MWCNT and GR electrodes.
Fig. 2. Degradation of ATZ using GR anode. GR alone (□); electrolysis ( ); GR/PMS ( ) and electrochemically activated PMS ( ). Conditions: current density = 100 A m−2; [PMS]0 = 5 mM, [ATZ]0 = 5 μM.
GR anode (0.0202 min−1) was proved to be more effective towards ATZ degradation than GR alone because of DET reaction. ATZ degradation was improved by GR/PMS (0.0261 min−1), suggesting that some radicals were generated from GR/PMS. This result was in conformity with previous studies, in which HO% and SO4%− were generated from carbon/PMS system [16,27]. ATZ degradation was furtherly enhanced unexpectedly by applying current on GR/PMS (0.1060 min−1) relative to GR/PMS or electrolysis (Fig. 2, Table S2), demonstrating that electrochemically activated PMS at GR anode might produce HO% or SO4%−. The degradation of NB was explored to recognize the involvement of HO% (Fig. 3). GR/PMS (0.0198 min−1) exhibited higher NB degradation rate than GR alone (0.0128 min−1) or electrolysis (0.0098 min−1). However, NB degradation by electrochemically activated PMS (0.0169 min−1) was inferior to GR/PMS, suggesting no HO% was generated in this process.
3.2. Reactive species identification 3.2.1. Radical probing In advanced oxidation processes, ATZ and NB were often used as radical probes (ATZ for HO% and SO4%−, NB for HO%) to identify the generated reactive species [24–26]. GR anode was employed to examine the degradation of ATZ and NB. PMS alone could not enhance ATZ degradation (Fig. S6). GR alone could remove ATZ (0.0143 min−1) due to adsorption of ATZ at GR anode surface (Fig. 2). Electrolysis with 3
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Fig. 3. Degradation of NB using GR anode. GR alone (□); electrolysis ( ); GR/ PMS ( ) and electrochemically activated PMS ( ). Conditions: current density = 100 A m−2; [PMS]0 = 5 mM, [NB]0 = 5 μM.
Similarly, the above phenomena also occurred in electrochemically activated PMS using MWCNT anode (Figs. S7–S9, Table S2). In this process, ATZ degradation was obviously promoted, but NB degradation was suppressed compared with MWCNT/PMS. The above results suggest that SO4%− was produced in electrochemically activated PMS at MWCNT or GR anode. The inhibitory effect towards NB degradation was attributable to the oxygen evolution reaction at anode surface. Oxygen evolution hindered the adsorption of NB and PMS activation, thus to decrease the degradation rate of NB. Differed from PMS, electrochemically activated PDS using GR anode not only enhanced ATZ degradation but also improved NB degradation and ATZ degradation rate was significantly faster than that of NB. It can be concluded that both HO% and SO4%− were produced from electrochemically activated PDS. 3.2.2. Radical scavenging Nonradical oxidation played the key role for organic pollutants degradation in electrochemically activated PDS. It is necessary to clear whether nonradical oxidation is produced in electrochemically activated PMS. According to our previous study, organic pollutants degradation could be significantly inhibited by 10 M methanol in electrochemically activated persulfates process [19]. As can be concluded from Fig. 4, SMX removal by carbon alone was completely inhibited in the methanol-containing system. However, the presence of 10 M methanol failed to prevent SMX removal in electrolysis, carbon/PMS and electrochemically activated PMS processes. The rapid degradation of SMX by carbon/PMS in the methanol-containing system indicates that the occurrence of nonradical oxidation. In addition, the electrochemically activated PMS showed higher SMX degradation rate than electrolysis or carbon/PMS in the methanol-containing system, suggesting the generation of more nonradical oxidation compared with carbon/PMS.
Fig. 4. Effect of addition of 10 M methanol on SMX degradation using MWCNT anode (a) and GR anode (b). MWCNT or GR alone with addition of 10 M methanol (□); electrolysis with addition of 10 M methanol ( ); MWCNT/PMS or GR/PMS with addition of 10 M methanol ( ) and electrochemically activated PMS using MWCNT or GR with addition of 10 M methanol ( ). Conditions: current density = 100 A m−2; [PMS]0 = 5 mM, [SMX]0 = 5 μM.
to 0.3999 min−1 for 200 A m−2. An exponential relationship emerged between SMX degradation rate and current density (Fig. S11). Similarly, increase of PDS concentration from 0.1 mM to 5 mM and current density from 10 A m−2 to 200 A m−2 in electrochemically activated PDS process also significantly enhanced SMX degradation. Moreover, SMX degradation in PDS system also exhibited linear increase along with increasing PDS concentration and exponential increase along with increasing current density. Thus, the role of PDS/PMS or current might be the same in dual processes. PDS/PMS acted as the oxidants and current played as the driving force to generated reactive species.
3.3. Influence of operating parameter: PMS concentration and current density As shown in Fig. 5, increasing PMS concentration from 0.1 mM to 5 mM significantly enhanced SMX degradation. The degradation rate of SMX increased about 3 folds, from 0.0638 min−1 for 0.1 mM PMS to 0.1945 min−1 for 5 mM PMS. Simultaneously, a good linear relationship (R2 > 0.97) between SMX degradation rate and PMS concentration was observed (Fig. S10). Increasing current density also remarkably promoted SMX degradation from 0.0884 min−1 for 10 A m−2
3.4. Influence of water background components Addition of Cl−, HCO3−, PO43− and HA (the model for NOM) in the electrolysis cell was used to evaluate the influence of water background components on the degradation of SMX. As shown in Fig. 6, SMX degradation was significantly enhanced when addition of various 4
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(PDS*). Thus, PMS* was able to oxidize Cl− to generate some chlorinecontaining reactive species, which furtherly contributed to the quick degradation of SMX (Eq. (1)).
PMS* + Cl−→Chlorine - containing reactive species
(1) −
Nevertheless, PDS* had low reactivity towards Cl , being unable to oxidize Cl−. Addition of excess Cl− inhibited PDS adsorption at electrode surface, thus to inhibit PDS activation and SMX degradation. Interestingly, addition of various concentrations of HCO3− in electrochemical activation PMS process generated minor promotion effect on SMX degradation, while addition of HCO3− in electrochemically activated PDS process inhibited SMX degradation (Fig. S13). Similar to Cl−, addition of excess HCO3− can also inhibit PDS/PMS adsorption and their activation. The opposite effect HCO3− on SMX degradation in electrochemically activated PDS and PMS processes might originate from pH buffering effect. Excess HCO3− could increase the solution pH in the anodic chamber [24]. PMS activation could be achieved at high pH condition [30,31]. Similar to PDS, the presence of PO43− or HA all inhibited SMX degradation in electrochemically activated PMS process due to the competitive adsorption (PO43− and HA) towards persulfates and competitive oxidation (HA) towards to SMX [24]. Increase of PO43− or HA concentration from 5 mM to 10 mM and 20 mM generated greater inhibitory effect. However, the inhibitory effect of same concentration of PO43− or HA on SMX degradation in electrochemically activated PMS process was significantly smaller than that in electrochemically activated PDS process under the identical conditions. For instance, SMX degradation rate in PMS process with addition of 20 mgC L−1 HA decreased about 24% relative to that in the absence of HA, while SMX degradation rate in PDS process with addition of 20 mgC L−1 HA decreased about 81% relative to that in the absence of HA. The above results indicated that electrochemically activated PMS exhibited stronger resistance towards water background components than PDS under the identical conditions. 3.5. Involved mechanism Adsorption, electrolysis (DET), nonradical oxidation and SO4%− were demonstrated to be responsible for organic pollutants degradation in electrochemically activated PMS process (Scheme 1). For clearly understanding the difference between electrochemically activated PMS and PDS, the relative contribution ratio of various reactive species to SMX degradation were calculated based on its kinetic data (Table 1, detailed calculation methods were provided in Text S1).
Fig. 5. Effect of (a) PMS concentration and (b) current density on the degradation of SMX by electrochemically activated PMS using GR anode. Conditions: (a) current density = 100 A m−2, (b) [PMS]0 = 5 mM, [SMX]0 = 5 μM.
3.5.1. Adsorption and electrolysis As shown in Table 1, the relative contribution of adsorption in PDS and PMS processes was 0.61% and 3.14%, respectively. Electrochemical reaction belongs to interfacial reaction system, in which adsorption is the prerequisite step. Organic pollutants degradation and PDS/PMS activation all took place at electrode surface. Adsorption not only contributed to the removal of organic pollutants but also participated in PDS/PMS activation. Organic pollutants degradation in electrolysis process was attributed to direct electron transfer (DET) reaction. The adsorbed organic pollutants were directly oxidized to their products. The relative contribution of DET in PDS and PMS processes was about 1.99% and 10.18%, respectively. Moreover, electrolysis contributed to PMS/PDS activation, playing as the reaction driving force. Application of current on adsorbed PDS/PMS might change the structure of PDS/PMS molecule [19,24], leading to the generation of reactive species.
concentration of Cl− (5 mM to 20 mM). Increasing Cl− concentration could improve SMX degradation. The free chlorine generated from electrolysis of Cl− at anode might contribute to the SMX degradation [28,29]. As can be concluded from Fig. S12, 20 mM Cl− (in the absence of PMS) indeed improved SMX degradation, but SMX degradation rate (0.1455 min−1) in this process was still far smaller than that in electrochemically activated PMS with addition of 20 mM Cl− (1.5883 min−1). Thus, in addition to free chlorine generated from electrolysis of Cl−, electrochemically activated PMS in the presence of Cl− must produce other reactive species which improve SMX degradation. By contrast, SMX degradation was inhibited when addition of 5 mM, 10 mM and 20 mM Cl−in electrochemically activated PDS process and higher concentration of Cl− exhibited greater inhibition effect. The opposite effect of Cl− on SMX degradation in PDS and PMS processes could be attributable to the structure difference between PDS and PMS. The asymmetric structure of PMS (HO-SO4−) leads to that activation of PMS was easier than PDS. The electrochemically activated PMS (PMS*) might have higher reactivity than electrochemically activated PDS
3.5.2. Nonradical oxidation According to the results of radical scavenging experiments, both carbon/PMS and electrochemically activated PMS generated nonradical oxidation. A reactive complex formed between PMS and carbon 5
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Fig. 6. Effect of water background components (a) Cl−, (b) HCO3−, (c) PO43− and (d) HA on degradation of SMX by electrochemically activated PMS using GR anode. Conditions: current density = 100 A m−2; [PMS]0 = 5 mM, [SMX]0 = 5 μM.
photoelectron spectroscopy (XPS) verifies that electrolysis indeed increased carbonyl group proportion (Fig. S14). Furthermore, activation of PMS by the aged GR electrode (6 h with 100 A m−2 current) was investigated. As shown in Fig. S15, the aged GR electrode indeed promoted SMX degradation. Nevertheless, the SMX
material was proposed to play as the nonradical oxidation in carbon/ PMS process [16]. Compared with carbon/PMS, electrochemically activated PMS enhanced the generation of nonradical oxidation. Electrolysis might increase the activate sites of carbon material (e.g., carbonyl group), resulting in enhancing PMS activation [16]. The X-ray
Scheme 1. Proposed mechanism of electrochemically activated PMS using carbon anodes. 6
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in PMS system (3.83 × 10−13 M).
Table 1 Estimation of the relative contribution ratio of involved reactive species. Process
k (min−1)
Relative Contribution ratio (%)
PDS
Adsorption DET PDS Nonradical HO% SO4%−
0.0061 0.0259 0.0004 0.9524 0.0123 0.0038
0.61 1.99 Neglected 95.79 1.23 0.38
PMS
Adsorption DET PMS Nonradical HO% SO4%−
0.0061 0.0259 0.0009 0.0423 – 0.1263
3.14 10.18 Neglected 21.75 – 64.93
4. Conclusions In this work, we performed a comparative study between electrochemically activated PMS and PDS at carbon anodes. MWCNT and GR anodes could be used for electrochemically activated PMS. Both nonradical oxidation and SO4%− were generated in electrochemically activated PMS process. The electrochemically activated PMS molecule played as the nonradical oxidation and it further decomposed to SO4%−. The presence of various concentration of Cl− significantly improved SMX degradation. Addition of HCO3− produced minor promotion on degradation of SMX, while the presence of PO43− and HA all inhibited SMX degradation. After longtime aging treatment, the prepared electrode exhibited excellent stability (Fig. S15), which is beneficial to its practical application. By contrast, nonradical oxidation, SO4%− and HO% was produced in electrochemically activated PDS process. Differed from electrochemically activated PMS process in which SO4%− mainly contributed to organic pollutants degradation, nonradical oxidation was the main reactive species in electrochemically activated PDS process. Electrochemically activated PDS generated considerably higher SMX degradation rate than that of PMS under the identical conditions. However, electrochemically activated PDS was more susceptive to water background components compared with PMS. Employment of electrochemically activated PDS or PMS depends on the characters of organic pollutants and water background components. On condition that water and wastewater contains various organic pollutants that are resistant to nonradical oxidation (e.g., ATZ) or high concentration of background ions and NOM, application of electrochemically activated PMS is superior to electrochemically activated PDS. However, Cl− participated in organic pollutants degradation in electrochemically activated PMS process. The risk of chlorinated byproducts generation deserves further consideration in the future works.
k HO·,SMX = 5.5 × 109 M−1 s−1; k SO·4−,SMX = 5.5 × 109 M−1 s−1; k HO·,ATX = 2.6 × 109 M−1 s−1 k HO·,NB = 3.9 × 109 M−1 s−1; k SO·4−,ATZ = 2.6 × 109 M−1 s−1 The contribution of PDS/PMS alone oxidation was neglected.
degradation rate in aged GR/PMS process was still smaller than that in electrochemically activated PMS, verifying that nonradical oxidation mainly originated from electrochemically activated PMS rather than aged GR/PMS. The adsorbed PMS molecule at carbon electrode surface was induced to activated PMS molecule (PMS*) by applying current. The above mechanism was also suitable to carbon/PDS and electrochemically activated PDS [22,24]. Although electrochemically activated PDS and PMS all generated nonradical oxidation and the involved mechanism was identical to each other, the relative contribution of nonradical oxidation varied greatly. The relative contribution of nonradical oxidation in PDS system was 95.79% (Table 1), far higher than that of PMS (21.75%). Furthermore, electrochemically activated PDS exhibited remarkably higher SMX degradation rate (0.9944 min−1) than PMS (0.1945 min−1) under the identical reaction conditions even if PMS activation was easier than that of PDS. This is because that PDS* was more stable than PMS*, meaning that PDS* had longer lifetime and higher steady-state concentration than PMS*. Generation of a large amount of SO4%− in electrochemically activated PMS also verified this conclusion, as explained in the next Section 3.5.3.
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.
3.5.3. Sulfate radical oxidation Differed from PDS system, SO4%− was the dominant reactive species accounting for 64.93% relative contribution in electrochemically activated PMS system. As shown in Eq. (2), PMS* directly decomposed to SO4%−.
PMS* → SO·4−
Acknowledgement Thanks for the support from Research Start-up Funds of DGUT (GC300501-122).
(2)
Appendix A. Supplementary data
The steady-state concentration of SO4%− in electrochemically vated PMS process (exclusion of SO4%− from carbon/PMS) was −13
actiestimated to be 3.83 × 10 M, about eight times of the sum of that of HO% (3.73 × 10−14 M) and SO4%− (1.15 × 10−14 M) in electrochemically activated PDS process (calculation method listed in Text S1). The quick decomposition of PMS* (nonradical oxidation) reduced the contribution of nonradical oxidation. This mechanism also explained the reasons why electrochemically activated PDS exhibited remarkably higher SMX degradation rate (0.9944 min−1) than PMS (0.1945 min−1). Assuming that the reaction rate of nonradical oxidation towards SMX ranged from 108-109 M−1 s−1, then the stead-state concentration of nonradical oxidation in PDS and PMS processes was estimated to be 1.59 × 10−11–1.59 × 10−10 M (PDS) and 7.05 × 10−137.05 × 10−12 M (PMS), respectively. Although the reaction reactivity of nonradical oxidation with organic pollutants was always lower than that of SO4%− (> 109 M−1 s−1), the steady-state concentration of nonradical oxidation in PDS system (1.59 × 10−11–1.59 × 10−10 M) was much higher than that of SO4%−
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