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Degradation of triclosan in aqueous solution by dielectric barrier discharge plasma combined with activated carbon fibers
Chemosphere 144 (2016) 855–863
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Degradation of triclosan in aqueous solution by dielectric barrier discharge plasma combined with activated carbon fibers Lu Xin a, Yabing Sun a,∗, Jingwei Feng b,c, Jian Wang a, Dong He a a
State Key Laboratory of Pollution Control & Resources Reuse, School of the Environment, Nanjing University, Nanjing 210046, PR China School of Civil and Hydraulic Engineering, Hefei University of Technology, Hefei 230009, PR China c State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering, Hohai University, PR China b
h i g h l i g h t s • • • •
Degradation of aqueous triclosan by using DBD plasma and ACFs was investigated. The ACFs showed efficient synergistic effect with DBD plasma for various processing parameters. The ACFs achieved modification and in situ regeneration during triclosan degradation. Main intermediates and the pathway of triclosan degradation by DBD plasma were proposed.
a r t i c l e
i n f o
Article history: Received 10 June 2015 Received in revised form 9 September 2015 Accepted 13 September 2015 Available online 28 September 2015 Handling editor: Enric Brillas Keywords: Dielectric barrier discharge plasma Activated carbon fibers Triclosan Modification Intermediates Degradation pathway
a b s t r a c t The degradation of triclosan (TCS) in aqueous solution by dielectric barrier discharge (DBD) plasma with activated carbon fibers (ACFs) was investigated. In this study, ACFs and DBD plasma coexisted in a planar DBD plasma reactor, which could synchronously achieve degradation of TCS, modification and in situ regeneration of ACFs, enhancing the effect of recycling of ACFs. The properties of ACFs before and after modification by DBD plasma were characterized by BET and XPS. Various processing parameters affecting the synergetic degradation of TCS were also investigated. The results exhibited excellent synergetic effects in DBD plasma-ACFs system on TCS degradation. The degradation efficiency of 120 mL TCS with initial concentration of 10 mg L−1 could reach 93% with 1 mm thick ACFs in 18 min at input power of 80 W, compared with 85% by single DBD plasma. Meanwhile, the removal rate of total organic carbon increased from 12% at pH 6.26–24% at pH 3.50. ACFs could ameliorate the degradation efficiency for planar DBD plasma when treating TCS solution at high flow rates or at low initial concentrations. A possible degradation pathway of TCS was investigated according to the detected intermediates, which were identified by liquid chromatography-hybrid quadrupole time-of-flight mass spectrometry (LC–QTOF-MS) combined with theoretical calculation of Gaussian 09 program. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Pharmaceuticals and personal care products (PPCPs) which contain diverse organic compounds, such as antibiotics, hormones, antimicrobial agents, sunscreen products and so on, have raised significant concerns in recent years for their potential threats to ecological environment and human health (Kasprzyk-Hordern et al., 2009; Yang et al., 2013; Rudd et al., 2014). As a kind of PPCPs, triclosan (TCS) is a broad-spectrum antimicrobial agent in many household and personal care products, including soaps, deodorants,
∗
Corresponding author. E-mail address: [email protected] (Y. Sun).
http://dx.doi.org/10.1016/j.chemosphere.2015.09.054 0045-6535/© 2015 Elsevier Ltd. All rights reserved.
toothpastes and various plastic products. With the increasing utilization of TCS year by year, TCS is found to be ubiquitous in surface water and sediment of many major rivers in both northern and southern China, as well as in the sewage and the sludge of almost all the sewage treatment plants (STPs) (Lee do and Chu, 2013; Zhao et al., 2013). As an emerging environmental contaminant, TCS has raised a great concern because it can affect microorganisms in environmental systems as a biologically active compound that specifically targets bacteria (Svenningsen et al., 2011; McNamara et al., 2014). TCS is also a typical endocrine disruptor that can cause adverse effects in animals, such as inducing oxidative stress and decreasing cellular thiol content in rat thymocytes through increasing intracellular Zn2+ concentrations (Delorenzo et al., 2008; Tamura et al., 2012).
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Furthermore, when discharged into surface waters via wastewater, TCS and chlorinated TCS derivatives (CTDs, forming during disinfection with chlorine) will react photochemically to form polychlorinated dioxins, which threaten human health and survival of other organisms (Anger et al., 2013). Due to the long degradation time, the easy enrichment and the harmfulness for ecological system, TCS is ineffectively degraded by most commonly used water treatment technologies and the ecosystem of natural environment (Suarez et al., 2007; Butler et al., 2012; Lee do et al., 2012). Thus, an effective technology attempted for TCS degradation has been on the agenda. Non-thermal plasma (NTP), which has proved to be an efficient advanced oxidation process (Hao et al., 2007; Marouf-Khelifa et al., 2008; Jiang et al., 2014), presents one possible method for TCS degradation. NTP generated in electrical discharges is comprised of many reactive oxidizing species such as radicals (e.g. • H, • O, • OH, etc.), ozone, electrons and ultraviolet light, which are effective for the decomposition of target molecules and can lead to the high degradation efficiency of organic substance (Kim et al., 2013; Jiang et al., 2014). Dielectric barrier discharge (DBD) plasma technology, as one of NTP technologies, has received great attention in the field of wastewater treatment (Manoj Kumar Reddy et al., 2013; Rong et al., 2014). DBD can produce more reactive oxidizing species, form more uniform discharge and cause less electrode loss than gas corona discharge (GCD) (Feng et al., 2008, 2009; Hu et al., 2013). However, there are some shortcomings in single DBD plasma technology: low mineralization rates and low energy yields (Feng et al., 2008; Tang et al., 2009; Rong and Sun, 2015). Activated carbon fibers (ACFs), a promising carbon material, are widely applied to remove pollutants in wastewater treatment (Yi and Chen, 2007; Li et al., 2010). As we know, ACFs have high adsorption–desorption kinetics, low resistance to bulk flows and not particularly high manufacturing cost. Recently, it has been verified that DBD plasma can modify ACFs and enhance the adsorption capacity of ACFs, which has been applied to the treatment of harmful gas (Huang et al., 2007; Zhou et al., 2012; Chen and Xie, 2013), but little attention has been paid on the use of the DBD plasma-ACFs technology for wastewater treatment. In addition, taking account of the very short existence time of NTP (especially • OH, the vital oxidant) (Jiang et al., 2014), it is more suitable to employ synchronous processing technology to investigate synergistic effect of DBD plasma and ACFs in wastewater treatment rather than post-processing technology. In current study, a circular planar DBD plasma reactor was chose to make ACFs contact with plasma and solution simultaneously. TCS was selected as the target compound for researching the synergistic effect of DBD plasma and ACFs. Input power, flow rate, initial concentration and thickness of ACFs that probably affected the synergistic degradation efficiency of TCS was examined. Meanwhile, the basic characteristics of ACFs modified by DBD plasma and the recycling of ACFs were investigated carefully. In order to clarify the degradation pathway of TCS by DBD plasma, the changes of pH, TOC and UV–Vis absorption spectra were analyzed. Furthermore, the degradation intermediates of TCS were identified using liquid chromatography-hybrid quadrupole time-offlight mass spectrometry (LC-QTOF-MS) combined with theoretical calculation of Gaussian 09 program. 2. Experimental 2.1. Chemicals and materials pretreatment TCS (analytical standard, purity grade 99.9%), was purchased from Dr. Ehrenstorfer Gmbh. Methanol used in the analysis were chromatography grade. Other chemicals were all analytical grade. Fiber felt ACFs were purchased from Sigma–Aldrich. Before syn-
ergistic reaction, the ACFs were sheared into circular ACFs plates with diameter of 80 mm (about 0.36 g for 1 mm thickness), then washed by deionized water several times to remove the adsorbed ultrafine particles and organic molecules and dried in a vacuum oven at 383 K for 10 h. Afterwards, the dried ACFs were stored in an airtight container (named as ACF0 ). 2.2. Experimental process The schematic diagram of experimental apparatus, which included a plasma reactor, a high voltage AC power source and a liquid circulation system, was shown in Fig. 1. The plasma reactor was comprised of a quartz plate (2 mm in thickness and 85 mm in diameter) and a circular quartz reaction still (80 mm in inner diameter and 100 mm in outer diameter). The quartz plate and the circular quartz reaction still were placed on a trestle table. In the middle of the trestle table, there was an aluminum electrode of which the bottom was connected to the quartz plate, and the top of the aluminum electrode was connected to the negative electrode of the high voltage AC power source (CTP-2000K, Nanjing Suman Electronics Co., Ltd., China, with a frequency ranges of 5–35 kHz). The high voltage AC power source could be operated at an adjustable amplitude voltage and the intensity of discharge could be denoted by the input power, which was calculated by the average voltage (0–250 V) and current (0–1.2 A) of the AC power source. The pretreated circular ACFs plate was put into the inner of the quartz reaction still. The liquid circulation system consisted of two peristaltic pumps and a vessel which was laid onto a magnetic stirrer. A TCS solution of 120 mL was circulated by the two peristaltic pumps and was made to flow through the ACFs in the circular quartz reaction still in the form of liquid film. The thickness of the liquid film was maintained at 1 mm. Then the high voltage AC power source was opened and kept reacting for 18 min. The discharge areas of which the thickness was 3 mm, were formed between the ACFs surface and the quartz plate. Each sample was taken for every 3 min and the quantity of each sample was about 0.5 mL so that the deviation would be small. 2.3. Processing of ACFs after plasma modification For ACFs characterization, the modified ACFs by plasma were washed by methanol for several times and then cleaned by deionized water for several times. Subsequently, the ACFs were dried at 383 K in vacuum until constant weight and stored in an airtight container. Meanwhile, the ACFs used under different input powers of 60 W (100 V × 0.6 A), 80 W (100 V × 0.8 A) and 100 W (100 V × 1.0 A), were named as ACF-60, ACF-80 and ACF-100, respectively. To study the effect of recycling, the ACFs modified by plasma were treated by centrifuging at 6000 rpm for 5 min, which was repeated at least three times to remove extra moisture. After centrifugation, the dried ACFs were used for next experiment of repeatability. To investigate the TCS recovery (the adsorption quantity after synergistic reaction) of ACF-60, ACF-80 and ACF-100, the ACFs were extracted by 10 mL methanol and 10 mL dichloromethane in series, which was enhanced by ultrasonic treatment successively for 30 min. The extract of each sample were mixed together and evaporated by a rotary evaporation apparatus (TVE-1000, EYELA, Tokyo, Japan) in a thermostatic bath. Then the extracts were blown to dry under gentle nitrogen flow and reconstituted in 1 mL methanol. Finally, the samples were passed through a polycarbonate syringe filter (25 mm, 0.45 μm, Whatman) and then collected for chemical analysis.
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Fig. 1. Schematic diagram of the experimental apparatus.
2.4. ACFs characterization The textural properties of ACFs were calculated from the nitrogen adsorption at 77 K using a BET instrument (ASAP 2020, Micromeritics, USA). Prior to the N2 sorption analysis, the samples were degassed at 373 K for 10 h and degassed at 473 K for 5 h again. X-ray photoelectron spectrometer (XPS) technique was employed to characterize the surface functionalities by PHI 5000 Versa Probe, with the surface excitation by a Mg Kα X-ray source (hv = 1253 eV). 2.5. Analysis methods and evaluation The concentration of TCS was analyzed by an Agilent 1200 High Performance Liquid Chromatography (HPLC) system with a Thermo C18 column (4.6 × 150 mm, 5 μm, Akzo Nobel, Netherlands) which was maintained at 25 °C, and the detection wavelength of UV detector was set at 282 nm. The mobile phase consisted of 80% methanol and 20% water, the flow rate was 1.0 mL min−1 , and the injection volume was 20 μL. The degradation and adsorption efficiency of TCS sample was calculated from the following Eq. (1):
η=
C0 − Ct × 100% C0
(1)
Where η was the degradation and adsorption efficiency of TCS (%); Ct was the residual concentration of TCS after treatment (mg L−1 ); C0 was the initial concentration of TCS before treatment (mg L−1 ). Total organic carbon (TOC) was monitored by a Shimadzu TOC5000A total organic carbon analyzer. pH value was recorded by a pH monitor (Shanghai Kangyi Instrument Co., Ltd. China, PHS-2C). UV–Vis absorption spectra of the TCS solutions were measured by using a UV–Vis spectrophotometer (Shanghai the spectrum instrument co., LTD, 754NPC). The degradation efficiency of TCS was better illustrated by the energy yield, defined as the amount of TCS degraded per unit of energy consumed in the discharge Eq. (2):
−1 −1
Y gkW
h
=
C0 Vη Pt
(2)
where C0 was the initial concentration of TCS (g L−1 ), V was the solution volume (L), η was the degradation efficiency of TCS (%), P was the average power dissipated in the discharge (kW) and t was the treatment time (h).
The conformation and molecular orbital of TCS were optimized by Gaussian 09 program at B3LYP/6-311 + G∗ level in order to obtain the optimal conformation having a minimum energy. Meanwhile, in the Gaussian 09, Natural Bond Orbital (NBO) theory was used to compute Wiberg bond index matrix in the NBO basis (Wiberg bond order) and distribution of atomic natural charge respectively, which were helpful to estimate the possible position of bond breaking and the probable degradation intermediates. The identification of TCS and its degradation intermediates was performed by a liquid-chromatography-mass spectrometer (LC-MS) API5600 Triple TOFTM (AB Sciex, MA, USA) with C18 HPLC column (100 mm × 2.1 mm i.d, 3 μm, Thermo, USA). 5 μL of treated TCS solution were injected automatically into the liquid chromatography-hybrid quadrupole time-of-flight mass spectrometry (LC-QTOF-MS) system. The eluent consisted of 80% methanol and 20% water, and the flow rate was 0.4 mL min−1 . The mass spectrometer equipped with an electrospray ionization interface. The ion source parameters followed the suggested operating parameters: nebulizer (GS1), heater (GS2) and curtain gas flow rates of 55, 55, and 35 psi, respectively; ion-spray needle voltage 5500 V and −4500 V; and heater gas temperature (TEM) 550 °C. The spectra were acquired both in the negative ion scan mode and positive ion scan mode, over the m/z range from 60 to 800. The software tool Peak View (version 1.2) was used for spectra interpretations. 3. Results and discussion 3.1. Characterization of the ACFs before and after plasma modification The nitrogen adsorption isotherm and detailed information on the textural properties of ACF0 , ACF-60, ACF-80 and ACF-100 were displayed in Fig. SM-1 and Table SM-1 respectively. It was obvious that, as seen in Fig. SM-1, most of the pore volume of different samples were filled below a relative pressure of about 0.1, indicating that the samples were highly microporous; after a sharp increase up to relative pressure 0.1, the isotherms showed very small growth in further adsorption. Furthermore, it was found from Fig. SM-1 and Table SM-1 that the BET surface area and pore volume significantly decreased with plasma treatment, and higher input power treatment could generate more obvious effect; in contrast, average pore size increased with increasing input power. These were the same as other researchers’ findings on the modification of ACFs by DBD plasma in the pure gaseous phase
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(Park and Kim, 2004; Huang et al., 2007; Che et al., 2013). The reasons might be that the plasma discharge treatment could destroy micropores and mesoporous of ACFs to enlarge the pore size, and simultaneously bring about some new oxygen-containing functional groups which might be located at the entrance of pores to make the surface and pore volume decrease (Park and Kim, 2004; Huang et al., 2007). The surface atomic compositions of ACFs obtained by XPS analysis were presented in Table SM-2. It could be seen that the O1s /C1s ratio of ACFs increased with the increasing input power due to the formation of oxygen-containing functional groups on the carbon surfaces. Meanwhile, the C1s spectra for ACFs were shown in Fig. SM-2 and the optimum fitting was obtained by deconvolution of four peaks for each C1s spectrum (Puziy et al., 2008). Table SM-3 gave the electron binding energy (eV) and relative content of each surface functional group. It was found that the oxygen-containing functional groups, including lactone groups and carbonyl groups, increased after DBD plasma treatment. These new functional groups could improve the adsorption capacity of ACFs by increasing active sites (Park and Kim, 2004; Zhou et al., 2012). Due to the considerable oxygen-containing functional groups and the strong specific surface area, the adsorption capacity of ACFs towards TCS before and after modification should be investigated. Fig. SM-3 showed the TCS adsorption efficiency of ACFs before and after modification in the planar DBD plasma reactor without discharge. As the input power gradually enhanced, the adsorption efficiency of modified ACFs increased from 90% (ACF-60) to 95% (ACF-100) after 18 min, while the ACF0 could only adsorb 86%. The results also indicated that the adsorption capacity of ACFs after modification in this planar DBD plasma reactor expressed significant improvement. 3.2. Synergetic degradation 3.2.1. Effects of input power on the degradation of TCS The variation trends of TCS degradation at different input powers were portrayed in Fig. 2a. As shown in Fig. 2a, the degradation efficiency of TCS improved with increasing input power in the plasma alone, which was due to the generation of more reactive oxidizing species (Feng et al., 2008). It could also be seen that the degradation efficiency of aqueous TCS significantly increased in the DBD plasma-ACFs system. When the input power was 60 W, the degradation efficiency of TCS was 93% after 18 min in the exist of ACFs, while only 84% in the single DBD process. With the enhancement of input power, the degradation efficiency of TCS reached 93% and 95% at 80 W and 100 W respectively, whereas only 85% and 86% in single plasma condition. Meanwhile, the TCS recovery of ACF-60, ACF-80 and ACF-100 only reached 10%, 8.4% and 7.9% respectively; that was, the concentration of TCS absorbed on the ACFs could be very little after the synergistic reaction. This meant that the reduction of TCS in solution mostly attributed to the DBD plasma instead of the simple adsorption of ACFs. In consequence, there existed synergistic effects in the DBD plasma-ACFs system and TCS could be decomposed efficiently by DBD plasma-ACFs system. 3.2.2. Effects of flow rate on the degradation of TCS The effects of flow rate on the degradation of TCS were shown in Fig. 2b. In Fig. 2b, we observed that the degradation efficiency of TCS at flow rate of 45 mL min−1 was clearly higher than that at flow rate of 30 mL min−1 , but the degradation efficiency of TCS at flow rate of 60 mL min−1 sharply descended, which was similar to the research of Ye et al. (2013). This could be attributed to the following three reasons: firstly, relatively low flow rate made less TCS molecules pass the discharge area, and short-circuiting would be developed at the same time so that a number of TCS molecules
departed from the plasma reactor rapidly without degradation reaction; secondly, high flow rate made the retention time of the TCS molecules in the reactor reduce, which could cause the decline of the chance of contact between TCS molecules and plasma; finally, we found that the peristaltic pumps could induce fluctuation of the liquid level of TCS solution in the planar DBD plasma reactor, and the discharge in the plasma reactor could be extremely unstable with excessive flow rate, resulting in that the DBD plasma generated was sporadic. However, as was shown in Fig. 2b, the DBD plasma-ACFs system displayed superior degradation efficiency at different flow rates, particularly at flow rate of 60 mL min−1 . The degradation efficiency of TCS in synergistic action at flow rate of 60 mL min−1 increased by nearly 20%, far more than other degradation efficiencies in synergistic action. Obviously, in the planar plasma reactor, the ACFs were extremely useful to reduce fluctuation of the TCS solution. As we observed in the experiment, when the ACFs were added into the reactor, the liquid level would become smooth and the discharge would be very stable even if at a high flow rate. Meanwhile, because of the strong adsorbability, the ACFs could adsorb the TCS molecules and seemed to immobilize the TCS molecules in the planar plasma reactor even if flow rate was very high. Hence, DBD plasma combined with ACFs had the potential for wastewater treatment at high flow rates. 3.2.3. Effects of initial concentration on the degradation of TCS The degradation efficiencies of TCS at different initial concentrations were presented in Fig. 2c. It could be seen from Fig. 2c that the degradation efficiency of TCS increased with initial concentration decreasing from 10 to 8 mg L−1 . Certainly, the phenomenon that the low initial concentration could cause the high degradation efficiency in DBD plasma, had been verified by other researchers (Rong et al., 2014; Zhu et al., 2014). This could be explained that weaker competition for reaction with the active species existed between TCS molecules and its intermediates when the initial concentration was lower (Feng et al., 2008). While, it was interestingly noted in this research that, at the initial concentration of 5 mg L−1 , the degradation efficiency of TCS initially showed higher but subsequently expressed lower than that at 10 or 8 mg L−1 . It could be attributed to the thickness of liquid membrane which restricted the TCS molecules at the bottom of the reactor to contact with reactive oxidizing species generated by DBD. So there always had some TCS molecules flowing through the reactor which could not be degraded by DBD plasma, and the TCS molecules at the top of liquid membrane gradually reduced as reaction proceeding, causing the weakening of effect of DBD plasma degradation. In other words, it was difficult for TCS to decompose in the planar DBD reactor when TCS solution reached a certain low concentration. Nevertheless, in the presence of ACFs, the degradation efficiencies of TCS reached 93%, 94% and 94% respectively at the initial concentrations of 10, 8 and 5 mg L−1 , which almost eliminated the influence of low concentration. This indicated that ACFs could drastically improve the degradation efficiency of TCS at low concentrations in the planar DBD reactor due to the strong adsorption for pollutants at low concentrations (Chen and Xie, 2013). Hence, the DBD plasma-ACFs system could well enhance degradation efficiency of DBD plasma no matter whether the concentration of solution was high or low. Meanwhile, the synergistic effect provided a method to treat wastewater at a low initial concentration. 3.2.4. Effects of thickness of ACFs on the degradation of TCS The effects of thickness of ACFs on the degradation of TCS in the reaction system were presented in Fig. 2d. It showed that the degradation efficiency of TCS manifested significantly improvement with increasing thickness of ACFs. Especially when the thickness of ACFs reached 2 mm, the TCS degradation efficiencies attained 71%
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Fig. 2. Processing parameters affecting the synergetic degradation of TCS (η: degradation efficiency). (a) Different input powers on the degradation, conditions: initial TCS concentration = 10 mg L−1 , flow rate = 45 mL min−1 , thickness of ACFs = 1 mm. (b) Different flow rates on the degradation, conditions: initial TCS concentration = 10 mg L−1 , input power = 80 W, thickness of ACFs = 1 mm. (c) Different initial concentrations on the degradation, conditions: input power = 80 W, flow rate = 45 mL min−1 , thickness of ACFs = 1 mm. (d) Different thicknesses of ACFs on the degradation, conditions: initial TCS concentration = 10 mg L−1 , input power = 80 W, flow rate = 45 mL min−1 .
in 3 min and 97% in 18 min which were far beyond TCS degradation efficiencies with other thickness of ACFs at any reaction time. It was indicated that ACFs had the ability to offer strong adsorbability and stable discharge, which could enhance the degradation efficiency of DBD plasma. What was more, the discharge gap decreased with the increase of thickness of ACFs, which could produce more DBD plasma and enhance the TCS degradation efficiency (Hu et al., 2013). However, the ACFs could not be too thick because sufficient interspace must be left for DBD; if ACFs excessively approach negative electrode, the ACFs would be cremated by high temperature which was generated by DBD. Thus, there was a moderate thickness for ACFs on the degradation of TCS in the DBD plasma-ACFs system. 3.2.5. Effects of pH value on the removal of TOC Fig. 3 showed the TOC removal rates of TCS solution under different pH values. Obviously, only 12% TOC was removed by single DBD plasma at initial pH value of 6.26 in 18 min, and simultaneously the pH value in TCS solution gradually dropped to 3.84. But in the DBD plasma-ACFs system, TOC removal rate reached 19% after 18 min of treatment at the same pH. Furthermore, at initial pH values of 9.50 and 3.50, the removal rates of TOC were 21% and 24% respectively in 18 min. These results showed that the TOC removal rate was improved in the DBD plasma-ACFs system, and alkaline or acid initial condition could enhance the TOC removal rate. It could be attributed to the adsorption of degradation intermediates by ACFs, which could reduce the TOC in the reaction solution. The dissolved ozone was more easily decomposed to ·OH in alkaline condition and more ·OH radicals were produced in acid environment, which accelerated the decomposition of TCS and its intermediates (Feng et al., 2008). Furthermore, the pH value grad-
Fig. 3. Effects of pH value on the removal of TOC (initial TCS concentration = 10 mg L−1 , input power = 80 W, flow rate = 45 mL min−1 , thickness of ACFs = 1 mm).
ually declined as the reaction proceeding, demonstrating the formation of carboxylic acid. 3.3. Energy yield for the TCS degradation The degradation efficiency of TCS was better illustrated by the energy yield, defined as the amount of TCS degraded per unit of
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energy consumed in the discharge. The energy yield (g kW−1 h−1 ) after 18 min degradation of TCS for different input power, flow rate and initial concentration was shown in Table SM-4. It might be clear that the energy yield decreased with increasing input power and increased with increasing initial concentration. Moreover, the energy yield in the condition of high or low flow rate both showed lower values than that of middle flow rate. But in DBD plasmaACFs system, the energy yields were all improved under the condition of various processing parameters, especially flow rate. The best performance obtained in this study was 6.2 g kW−1 h−1 for 10 mg L−1 TCS solution (120 mL) at 60 W and 45 mL min−1 flow rate in the DBD plasma-ACFs system. 3.4. Recycling of ACFs In synergetic degradation processes, recycling of ACFs were of crucial importance to investigate for evaluating its applicability for wastewater treatment. Fig. 4 displayed the degradation efficiencies of TCS by recycling of five times of ACFs. As shown in Fig. 4, the degradation efficiency of TCS slightly increased from 93% to 96% after five cycles. The case that the degradation efficiency of TCS increased during five cycles could be owing to the modification of ACFs and the degradation of TCS adsorbed on ACFs by DBD plasma. After each cycle, the oxygen functional groups of ACFs increased, such as carbonyl groups (C=O) and lactone groups (O–C=O), which enhanced the adsorption of ACFs, and produced better synergy effect in the next cycle (Zhou et al., 2012). Meanwhile, DBD plasma could decompose the TCS molecules adsorbed on ACFs (had been confirmed by the TCS recovery in 3.2.1.), which could realize in situ regeneration. Of course, the degradation efficiency tended to be the same with the increasing number of cycles, because the modification effects gradually weakened. Therefore, in this DBD plasmaACFs system, degradation, modification and regeneration could be carried out simultaneously and the ACFs could be gradually regenerated so that they could be recycled without any treatment. 3.5. UV–Vis absorption spectra analysis of TCS In order to better understand the changes of TCS solution during reaction processes, the UV–Vis absorption spectra of TCS solu-
Fig. 4. Effects of recycling of ACFs in DBD plasma-ACFs system (initial TCS concentration = 10 mg L−1 , input power = 80 W, flow rate = 45 mL min−1 , thickness of ACFs = 1 mm).
tion were shown in Fig. SM-4. It was shown that, the absorbance spectrum at 282 nm that was the characteristic absorption peak of TCS, gradually decreased as the reaction proceeding, which indicated the rapid degradation of TCS molecules. However, the absorbance at around 200–250 nm displayed a rapid increase and redshift, which demonstrated a growing number of intermediates and aromatic base with reaction time. In a word, as the reaction progress, TCS molecules could be quickly decomposed by DBD plasma. 3.6. Possible degradation pathway of TCS by DBD plasma 3.6.1. Optimization and theoretical calculation It is well-known that Natural Bond Orbital (NBO) theory was usually used to analyze structure geometry and interaction among bonds of organic molecules (Appell and Bosma, 2015; Karthick et al., 2015). For TCS molecule, NBO analysis were calculated on structure geometry optimized at B3LYP/6-311 + G∗ level. The optimized conformation of TCS was shown in Fig. SM-5. The Wiberg bond order and atomic natural charge of TCS molecule were shown in Table SM-5. From Table SM-5, it was indicated that C–O on the ether group was the weakest site and C–Cl was also liable to be attacked by active species (i.e. • OH, O3 ); in contrast, C–C on the benzene ring was hard to be broken at the beginning of reaction, but as degradation proceeding, organic macromolecules (TCS and its intermediates) were decomposed into the ramification of aromatic compounds and subsequently the aromatic ring could be opened. 3.6.2. Identification of intermediates and degradation mechanism for TCS TCS solution at different reaction time (3, 9 and 15 min) was measured by LC-QTOF-MS to identify TCS and its degradation intermediates. Taking into account the above Wiberg bond order and atomic natural charge, the chromatographic and mass spectra characteristics of TCS and 10 main intermediates were summarized in Table 1. Firstly, it should be pointed out that, although ultraviolet light was generated by DBD, no polychlorinated dioxins were detected through carefully comparing mass-to-charge ratio with the mass spectrum. The reasons might be that most of TCS molecules were decomposed by • OH, which had stronger electrophilic property, higher redox potential and hundreds of thousands of times reaction rate than ultraviolet light (Haag and Yao, 1992; Son et al., 2009). Simultaneously ACFs could also block the penetration of the ultraviolet light. Secondly, some isomerides were found from Fig. SM-10 and Fig. SM-11, such as intermediates Ⅴ and Ⅵ, and distinguished by Octanol-water partition coefficient (Kow ) and molecular polarity. From Fig. SM-6, it could be seen that the response intensity of TCS decreased gradually, demonstrating that TCS molecules were increasingly decomposed as the reaction proceeding. At reaction time of 3 min, intermediates Ⅱ, Ⅳ, Ⅴ and Ⅶ showed response intensity in Fig. SM-7, SM-9, SM-10 and SM-11 respectively, which indicated that they were four uppermost degradation intermediates of TCS. As the reaction taking on to 9 min, other intermediates (Ⅲ, Ⅹ and Ⅺ) with a higher molecule weight were observed successively in Fig. SM-8, SM-13 and SM-14 respectively. Meanwhile, some micromolecules appeared, such as intermediates Ⅵ, Ⅷ and Ⅸ, which were shown in Fig. SM-10, SM-11 and SM-12 respectively. Therefore, there existed three different degradation patterns involving in TCS degradation. The identification of probable degradation pathway was depicted in Fig. 5. The first route (a), which was also the main route, initiated by ·OH acting on the ether bond of TCS, resulting in the formation of 2,4-Dichlorophenol (intermediate Ⅱ), m-Dichlorobenzene (intermediate Ⅳ), 3-Chlorophenol (intermediate Ⅴ), and 4-Chlorobenzene-1,2-diol (intermediate Ⅶ)
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Table 1 Retention time (based on the LC-MS spectra of TCS solution at reaction time of 15 min), structures of TCS and its main degradation intermediates. Compound
Molecular formula
Triclosan (Ⅰ)
Structure
RT (min)
Molecular ions (m/z)
C12 H7 Cl3 O2
5.602
286.94 ([M−H]− )
2,4-Dichlorophenol (Ⅱ)
C6 H4 Cl2 O
1.605
160.96 ([M−H]− )
2-Chloro-5-(2-dichlorophenoxy) benzene-1,4-diol (Ⅲ)
C12 H8 Cl2 O3
0.538
268.98 ([M−H]− )
m-Dichlorobenzene (Ⅳ)
C6 H4 Cl2
0.532
146.98 ([M+H]+ )
3-Chlorophenol (Ⅴ)
C6 H5 ClO
0.646
129.01 ([M+H]+ )
2-Chlorophenol (Ⅵ)
C6 H5 ClO
0.543
129.01 ([M+H]+ )
4-Chlorobenzene-1,2-diol (Ⅶ)
C6 H5 ClO2
0.659
145.01 ([M+H]+ )
Chlorohydroquinone (Ⅷ)
C6 H5 ClO2
0.526
145.01 ([M+H]+ )
2-Chloro-1,4-benzoquinone (Ⅸ)
C6 H3 ClO2
0.533
142.99 ([M+H]+ )
2-Chloro-5-(2,4-dichlorophenoxy) benzene-1,4-diol (Ⅹ)
C12 H7 Cl3 O3
0.552
304.95 ([M+H]+ )
2-Chloro-5-(2,4-dichlodichloro- phenoxy)-[1,4]benzoquinone (Ⅺ)
C12 H5 Cl3 O3
0.544
302.94 ([M+H]+ )
862
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Fig. 5. The proposed degradation pathway of TCS by DBD plasma.
(Chen et al., 2012). The other route (b) proceeded with attacking by ·OH on the para-position of phenol ring of TCS, causing the generation of intermediates Ⅹ and Ⅺ. This was because ·OH had strong electrophilic character and preferentially attacked the carbon atoms with the highest electron density, which was also facilitated by the ortho orientation of the ring chlorine (Yu et al., 2006). Then, ·OH broke the ether bond ring of intermediates Ⅹ and Ⅺ to form intermediates Ⅱ, Ⅷ and Ⅸ. Meanwhile, when excess ·OH radicals were produced, 2, 4-Dichlorophenol could be oxidized to intermediate Ⅷ and even intermediate Ⅸ (Song et al., 2012). Ultimately, ring-opening reaction occurred for intermediates Ⅳ, Ⅴ, Ⅵ, Ⅶ and Ⅸ to form carboxylic acid that being decomposed to CO2 , H2 O and Cl− at last. Certainly, the direct dechlorination– hydroxylation reaction (c) of TCS also occurred and primarily produced intermediate Ⅲ, which had been confirmed by Ferrer et al. (Ferrer et al., 2004). Subsequently, intermediate Ⅲ also undergone bond breaking and ring-opening and came into being carboxylic acid.
from 0 to 2 mm in 18 min at input power of 80 W. Meanwhile, the removal rate of TOC was evidently improved in the DBD plasma-ACFs system and TOC could be faster removed at low pH value. In addition, ACFs could ameliorate the degradation efficiency for planar DBD plasma when treating TCS solution at high flow rates or at low initial concentrations. In this DBD plasma-ACFs system, degradation of TCS and modification of ACFs could be carried out simultaneously and ACFs could realize in situ regeneration, enhancing the effect of recycling of ACFs. The possible degradation pathway of TCS by DBD plasma was inferred based on 10 intermediates detected by LC-QTOFMS combined with theoretical calculation of Gaussian 09 program. The main degradation mechanism of TCS by DBD plasma involved a series of processes, such as bond breaking, dechlorinationhydroxylation, hydroxylation and oxidative opening of the aromatic ring; ultimately, carboxylic acid and inorganic species were also formed.
Acknowledgments 4. Conclusions In the present study, aqueous TCS was degraded by DBD plasma combined with ACFs in the planar DBD plasma reactor. The synchronous processing technology expressed efficient degradation efficiency for TCS. The degradation efficiency of TCS improved from 85% to 97% with the thickness of ACFs increased
This work was supported by the National Science and Technology Major Project on Water Pollution Control and Treatment (No. 2014ZX07204-008), the National Natural Science Foundation of China (No. 51208163), and Open Fund of State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering, Hohai University (2013491211).
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Degradation of triclosan in aqueous solution by dielectric barrier discharge plasma combined with activated carbon fibers Lu Xin a, Yabing Sun a,∗, Jingwei Feng b,c, Jian Wang a, Dong He a a
State Key Laboratory of Pollution Control & Resources Reuse, School of the Environment, Nanjing University, Nanjing 210046, PR China School of Civil and Hydraulic Engineering, Hefei University of Technology, Hefei 230009, PR China c State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering, Hohai University, PR China b
h i g h l i g h t s • • • •
Degradation of aqueous triclosan by using DBD plasma and ACFs was investigated. The ACFs showed efficient synergistic effect with DBD plasma for various processing parameters. The ACFs achieved modification and in situ regeneration during triclosan degradation. Main intermediates and the pathway of triclosan degradation by DBD plasma were proposed.
a r t i c l e
i n f o
Article history: Received 10 June 2015 Received in revised form 9 September 2015 Accepted 13 September 2015 Available online 28 September 2015 Handling editor: Enric Brillas Keywords: Dielectric barrier discharge plasma Activated carbon fibers Triclosan Modification Intermediates Degradation pathway
a b s t r a c t The degradation of triclosan (TCS) in aqueous solution by dielectric barrier discharge (DBD) plasma with activated carbon fibers (ACFs) was investigated. In this study, ACFs and DBD plasma coexisted in a planar DBD plasma reactor, which could synchronously achieve degradation of TCS, modification and in situ regeneration of ACFs, enhancing the effect of recycling of ACFs. The properties of ACFs before and after modification by DBD plasma were characterized by BET and XPS. Various processing parameters affecting the synergetic degradation of TCS were also investigated. The results exhibited excellent synergetic effects in DBD plasma-ACFs system on TCS degradation. The degradation efficiency of 120 mL TCS with initial concentration of 10 mg L−1 could reach 93% with 1 mm thick ACFs in 18 min at input power of 80 W, compared with 85% by single DBD plasma. Meanwhile, the removal rate of total organic carbon increased from 12% at pH 6.26–24% at pH 3.50. ACFs could ameliorate the degradation efficiency for planar DBD plasma when treating TCS solution at high flow rates or at low initial concentrations. A possible degradation pathway of TCS was investigated according to the detected intermediates, which were identified by liquid chromatography-hybrid quadrupole time-of-flight mass spectrometry (LC–QTOF-MS) combined with theoretical calculation of Gaussian 09 program. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Pharmaceuticals and personal care products (PPCPs) which contain diverse organic compounds, such as antibiotics, hormones, antimicrobial agents, sunscreen products and so on, have raised significant concerns in recent years for their potential threats to ecological environment and human health (Kasprzyk-Hordern et al., 2009; Yang et al., 2013; Rudd et al., 2014). As a kind of PPCPs, triclosan (TCS) is a broad-spectrum antimicrobial agent in many household and personal care products, including soaps, deodorants,
∗
Corresponding author. E-mail address: [email protected] (Y. Sun).
http://dx.doi.org/10.1016/j.chemosphere.2015.09.054 0045-6535/© 2015 Elsevier Ltd. All rights reserved.
toothpastes and various plastic products. With the increasing utilization of TCS year by year, TCS is found to be ubiquitous in surface water and sediment of many major rivers in both northern and southern China, as well as in the sewage and the sludge of almost all the sewage treatment plants (STPs) (Lee do and Chu, 2013; Zhao et al., 2013). As an emerging environmental contaminant, TCS has raised a great concern because it can affect microorganisms in environmental systems as a biologically active compound that specifically targets bacteria (Svenningsen et al., 2011; McNamara et al., 2014). TCS is also a typical endocrine disruptor that can cause adverse effects in animals, such as inducing oxidative stress and decreasing cellular thiol content in rat thymocytes through increasing intracellular Zn2+ concentrations (Delorenzo et al., 2008; Tamura et al., 2012).
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Furthermore, when discharged into surface waters via wastewater, TCS and chlorinated TCS derivatives (CTDs, forming during disinfection with chlorine) will react photochemically to form polychlorinated dioxins, which threaten human health and survival of other organisms (Anger et al., 2013). Due to the long degradation time, the easy enrichment and the harmfulness for ecological system, TCS is ineffectively degraded by most commonly used water treatment technologies and the ecosystem of natural environment (Suarez et al., 2007; Butler et al., 2012; Lee do et al., 2012). Thus, an effective technology attempted for TCS degradation has been on the agenda. Non-thermal plasma (NTP), which has proved to be an efficient advanced oxidation process (Hao et al., 2007; Marouf-Khelifa et al., 2008; Jiang et al., 2014), presents one possible method for TCS degradation. NTP generated in electrical discharges is comprised of many reactive oxidizing species such as radicals (e.g. • H, • O, • OH, etc.), ozone, electrons and ultraviolet light, which are effective for the decomposition of target molecules and can lead to the high degradation efficiency of organic substance (Kim et al., 2013; Jiang et al., 2014). Dielectric barrier discharge (DBD) plasma technology, as one of NTP technologies, has received great attention in the field of wastewater treatment (Manoj Kumar Reddy et al., 2013; Rong et al., 2014). DBD can produce more reactive oxidizing species, form more uniform discharge and cause less electrode loss than gas corona discharge (GCD) (Feng et al., 2008, 2009; Hu et al., 2013). However, there are some shortcomings in single DBD plasma technology: low mineralization rates and low energy yields (Feng et al., 2008; Tang et al., 2009; Rong and Sun, 2015). Activated carbon fibers (ACFs), a promising carbon material, are widely applied to remove pollutants in wastewater treatment (Yi and Chen, 2007; Li et al., 2010). As we know, ACFs have high adsorption–desorption kinetics, low resistance to bulk flows and not particularly high manufacturing cost. Recently, it has been verified that DBD plasma can modify ACFs and enhance the adsorption capacity of ACFs, which has been applied to the treatment of harmful gas (Huang et al., 2007; Zhou et al., 2012; Chen and Xie, 2013), but little attention has been paid on the use of the DBD plasma-ACFs technology for wastewater treatment. In addition, taking account of the very short existence time of NTP (especially • OH, the vital oxidant) (Jiang et al., 2014), it is more suitable to employ synchronous processing technology to investigate synergistic effect of DBD plasma and ACFs in wastewater treatment rather than post-processing technology. In current study, a circular planar DBD plasma reactor was chose to make ACFs contact with plasma and solution simultaneously. TCS was selected as the target compound for researching the synergistic effect of DBD plasma and ACFs. Input power, flow rate, initial concentration and thickness of ACFs that probably affected the synergistic degradation efficiency of TCS was examined. Meanwhile, the basic characteristics of ACFs modified by DBD plasma and the recycling of ACFs were investigated carefully. In order to clarify the degradation pathway of TCS by DBD plasma, the changes of pH, TOC and UV–Vis absorption spectra were analyzed. Furthermore, the degradation intermediates of TCS were identified using liquid chromatography-hybrid quadrupole time-offlight mass spectrometry (LC-QTOF-MS) combined with theoretical calculation of Gaussian 09 program. 2. Experimental 2.1. Chemicals and materials pretreatment TCS (analytical standard, purity grade 99.9%), was purchased from Dr. Ehrenstorfer Gmbh. Methanol used in the analysis were chromatography grade. Other chemicals were all analytical grade. Fiber felt ACFs were purchased from Sigma–Aldrich. Before syn-
ergistic reaction, the ACFs were sheared into circular ACFs plates with diameter of 80 mm (about 0.36 g for 1 mm thickness), then washed by deionized water several times to remove the adsorbed ultrafine particles and organic molecules and dried in a vacuum oven at 383 K for 10 h. Afterwards, the dried ACFs were stored in an airtight container (named as ACF0 ). 2.2. Experimental process The schematic diagram of experimental apparatus, which included a plasma reactor, a high voltage AC power source and a liquid circulation system, was shown in Fig. 1. The plasma reactor was comprised of a quartz plate (2 mm in thickness and 85 mm in diameter) and a circular quartz reaction still (80 mm in inner diameter and 100 mm in outer diameter). The quartz plate and the circular quartz reaction still were placed on a trestle table. In the middle of the trestle table, there was an aluminum electrode of which the bottom was connected to the quartz plate, and the top of the aluminum electrode was connected to the negative electrode of the high voltage AC power source (CTP-2000K, Nanjing Suman Electronics Co., Ltd., China, with a frequency ranges of 5–35 kHz). The high voltage AC power source could be operated at an adjustable amplitude voltage and the intensity of discharge could be denoted by the input power, which was calculated by the average voltage (0–250 V) and current (0–1.2 A) of the AC power source. The pretreated circular ACFs plate was put into the inner of the quartz reaction still. The liquid circulation system consisted of two peristaltic pumps and a vessel which was laid onto a magnetic stirrer. A TCS solution of 120 mL was circulated by the two peristaltic pumps and was made to flow through the ACFs in the circular quartz reaction still in the form of liquid film. The thickness of the liquid film was maintained at 1 mm. Then the high voltage AC power source was opened and kept reacting for 18 min. The discharge areas of which the thickness was 3 mm, were formed between the ACFs surface and the quartz plate. Each sample was taken for every 3 min and the quantity of each sample was about 0.5 mL so that the deviation would be small. 2.3. Processing of ACFs after plasma modification For ACFs characterization, the modified ACFs by plasma were washed by methanol for several times and then cleaned by deionized water for several times. Subsequently, the ACFs were dried at 383 K in vacuum until constant weight and stored in an airtight container. Meanwhile, the ACFs used under different input powers of 60 W (100 V × 0.6 A), 80 W (100 V × 0.8 A) and 100 W (100 V × 1.0 A), were named as ACF-60, ACF-80 and ACF-100, respectively. To study the effect of recycling, the ACFs modified by plasma were treated by centrifuging at 6000 rpm for 5 min, which was repeated at least three times to remove extra moisture. After centrifugation, the dried ACFs were used for next experiment of repeatability. To investigate the TCS recovery (the adsorption quantity after synergistic reaction) of ACF-60, ACF-80 and ACF-100, the ACFs were extracted by 10 mL methanol and 10 mL dichloromethane in series, which was enhanced by ultrasonic treatment successively for 30 min. The extract of each sample were mixed together and evaporated by a rotary evaporation apparatus (TVE-1000, EYELA, Tokyo, Japan) in a thermostatic bath. Then the extracts were blown to dry under gentle nitrogen flow and reconstituted in 1 mL methanol. Finally, the samples were passed through a polycarbonate syringe filter (25 mm, 0.45 μm, Whatman) and then collected for chemical analysis.
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Fig. 1. Schematic diagram of the experimental apparatus.
2.4. ACFs characterization The textural properties of ACFs were calculated from the nitrogen adsorption at 77 K using a BET instrument (ASAP 2020, Micromeritics, USA). Prior to the N2 sorption analysis, the samples were degassed at 373 K for 10 h and degassed at 473 K for 5 h again. X-ray photoelectron spectrometer (XPS) technique was employed to characterize the surface functionalities by PHI 5000 Versa Probe, with the surface excitation by a Mg Kα X-ray source (hv = 1253 eV). 2.5. Analysis methods and evaluation The concentration of TCS was analyzed by an Agilent 1200 High Performance Liquid Chromatography (HPLC) system with a Thermo C18 column (4.6 × 150 mm, 5 μm, Akzo Nobel, Netherlands) which was maintained at 25 °C, and the detection wavelength of UV detector was set at 282 nm. The mobile phase consisted of 80% methanol and 20% water, the flow rate was 1.0 mL min−1 , and the injection volume was 20 μL. The degradation and adsorption efficiency of TCS sample was calculated from the following Eq. (1):
η=
C0 − Ct × 100% C0
(1)
Where η was the degradation and adsorption efficiency of TCS (%); Ct was the residual concentration of TCS after treatment (mg L−1 ); C0 was the initial concentration of TCS before treatment (mg L−1 ). Total organic carbon (TOC) was monitored by a Shimadzu TOC5000A total organic carbon analyzer. pH value was recorded by a pH monitor (Shanghai Kangyi Instrument Co., Ltd. China, PHS-2C). UV–Vis absorption spectra of the TCS solutions were measured by using a UV–Vis spectrophotometer (Shanghai the spectrum instrument co., LTD, 754NPC). The degradation efficiency of TCS was better illustrated by the energy yield, defined as the amount of TCS degraded per unit of energy consumed in the discharge Eq. (2):
−1 −1
Y gkW
h
=
C0 Vη Pt
(2)
where C0 was the initial concentration of TCS (g L−1 ), V was the solution volume (L), η was the degradation efficiency of TCS (%), P was the average power dissipated in the discharge (kW) and t was the treatment time (h).
The conformation and molecular orbital of TCS were optimized by Gaussian 09 program at B3LYP/6-311 + G∗ level in order to obtain the optimal conformation having a minimum energy. Meanwhile, in the Gaussian 09, Natural Bond Orbital (NBO) theory was used to compute Wiberg bond index matrix in the NBO basis (Wiberg bond order) and distribution of atomic natural charge respectively, which were helpful to estimate the possible position of bond breaking and the probable degradation intermediates. The identification of TCS and its degradation intermediates was performed by a liquid-chromatography-mass spectrometer (LC-MS) API5600 Triple TOFTM (AB Sciex, MA, USA) with C18 HPLC column (100 mm × 2.1 mm i.d, 3 μm, Thermo, USA). 5 μL of treated TCS solution were injected automatically into the liquid chromatography-hybrid quadrupole time-of-flight mass spectrometry (LC-QTOF-MS) system. The eluent consisted of 80% methanol and 20% water, and the flow rate was 0.4 mL min−1 . The mass spectrometer equipped with an electrospray ionization interface. The ion source parameters followed the suggested operating parameters: nebulizer (GS1), heater (GS2) and curtain gas flow rates of 55, 55, and 35 psi, respectively; ion-spray needle voltage 5500 V and −4500 V; and heater gas temperature (TEM) 550 °C. The spectra were acquired both in the negative ion scan mode and positive ion scan mode, over the m/z range from 60 to 800. The software tool Peak View (version 1.2) was used for spectra interpretations. 3. Results and discussion 3.1. Characterization of the ACFs before and after plasma modification The nitrogen adsorption isotherm and detailed information on the textural properties of ACF0 , ACF-60, ACF-80 and ACF-100 were displayed in Fig. SM-1 and Table SM-1 respectively. It was obvious that, as seen in Fig. SM-1, most of the pore volume of different samples were filled below a relative pressure of about 0.1, indicating that the samples were highly microporous; after a sharp increase up to relative pressure 0.1, the isotherms showed very small growth in further adsorption. Furthermore, it was found from Fig. SM-1 and Table SM-1 that the BET surface area and pore volume significantly decreased with plasma treatment, and higher input power treatment could generate more obvious effect; in contrast, average pore size increased with increasing input power. These were the same as other researchers’ findings on the modification of ACFs by DBD plasma in the pure gaseous phase
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(Park and Kim, 2004; Huang et al., 2007; Che et al., 2013). The reasons might be that the plasma discharge treatment could destroy micropores and mesoporous of ACFs to enlarge the pore size, and simultaneously bring about some new oxygen-containing functional groups which might be located at the entrance of pores to make the surface and pore volume decrease (Park and Kim, 2004; Huang et al., 2007). The surface atomic compositions of ACFs obtained by XPS analysis were presented in Table SM-2. It could be seen that the O1s /C1s ratio of ACFs increased with the increasing input power due to the formation of oxygen-containing functional groups on the carbon surfaces. Meanwhile, the C1s spectra for ACFs were shown in Fig. SM-2 and the optimum fitting was obtained by deconvolution of four peaks for each C1s spectrum (Puziy et al., 2008). Table SM-3 gave the electron binding energy (eV) and relative content of each surface functional group. It was found that the oxygen-containing functional groups, including lactone groups and carbonyl groups, increased after DBD plasma treatment. These new functional groups could improve the adsorption capacity of ACFs by increasing active sites (Park and Kim, 2004; Zhou et al., 2012). Due to the considerable oxygen-containing functional groups and the strong specific surface area, the adsorption capacity of ACFs towards TCS before and after modification should be investigated. Fig. SM-3 showed the TCS adsorption efficiency of ACFs before and after modification in the planar DBD plasma reactor without discharge. As the input power gradually enhanced, the adsorption efficiency of modified ACFs increased from 90% (ACF-60) to 95% (ACF-100) after 18 min, while the ACF0 could only adsorb 86%. The results also indicated that the adsorption capacity of ACFs after modification in this planar DBD plasma reactor expressed significant improvement. 3.2. Synergetic degradation 3.2.1. Effects of input power on the degradation of TCS The variation trends of TCS degradation at different input powers were portrayed in Fig. 2a. As shown in Fig. 2a, the degradation efficiency of TCS improved with increasing input power in the plasma alone, which was due to the generation of more reactive oxidizing species (Feng et al., 2008). It could also be seen that the degradation efficiency of aqueous TCS significantly increased in the DBD plasma-ACFs system. When the input power was 60 W, the degradation efficiency of TCS was 93% after 18 min in the exist of ACFs, while only 84% in the single DBD process. With the enhancement of input power, the degradation efficiency of TCS reached 93% and 95% at 80 W and 100 W respectively, whereas only 85% and 86% in single plasma condition. Meanwhile, the TCS recovery of ACF-60, ACF-80 and ACF-100 only reached 10%, 8.4% and 7.9% respectively; that was, the concentration of TCS absorbed on the ACFs could be very little after the synergistic reaction. This meant that the reduction of TCS in solution mostly attributed to the DBD plasma instead of the simple adsorption of ACFs. In consequence, there existed synergistic effects in the DBD plasma-ACFs system and TCS could be decomposed efficiently by DBD plasma-ACFs system. 3.2.2. Effects of flow rate on the degradation of TCS The effects of flow rate on the degradation of TCS were shown in Fig. 2b. In Fig. 2b, we observed that the degradation efficiency of TCS at flow rate of 45 mL min−1 was clearly higher than that at flow rate of 30 mL min−1 , but the degradation efficiency of TCS at flow rate of 60 mL min−1 sharply descended, which was similar to the research of Ye et al. (2013). This could be attributed to the following three reasons: firstly, relatively low flow rate made less TCS molecules pass the discharge area, and short-circuiting would be developed at the same time so that a number of TCS molecules
departed from the plasma reactor rapidly without degradation reaction; secondly, high flow rate made the retention time of the TCS molecules in the reactor reduce, which could cause the decline of the chance of contact between TCS molecules and plasma; finally, we found that the peristaltic pumps could induce fluctuation of the liquid level of TCS solution in the planar DBD plasma reactor, and the discharge in the plasma reactor could be extremely unstable with excessive flow rate, resulting in that the DBD plasma generated was sporadic. However, as was shown in Fig. 2b, the DBD plasma-ACFs system displayed superior degradation efficiency at different flow rates, particularly at flow rate of 60 mL min−1 . The degradation efficiency of TCS in synergistic action at flow rate of 60 mL min−1 increased by nearly 20%, far more than other degradation efficiencies in synergistic action. Obviously, in the planar plasma reactor, the ACFs were extremely useful to reduce fluctuation of the TCS solution. As we observed in the experiment, when the ACFs were added into the reactor, the liquid level would become smooth and the discharge would be very stable even if at a high flow rate. Meanwhile, because of the strong adsorbability, the ACFs could adsorb the TCS molecules and seemed to immobilize the TCS molecules in the planar plasma reactor even if flow rate was very high. Hence, DBD plasma combined with ACFs had the potential for wastewater treatment at high flow rates. 3.2.3. Effects of initial concentration on the degradation of TCS The degradation efficiencies of TCS at different initial concentrations were presented in Fig. 2c. It could be seen from Fig. 2c that the degradation efficiency of TCS increased with initial concentration decreasing from 10 to 8 mg L−1 . Certainly, the phenomenon that the low initial concentration could cause the high degradation efficiency in DBD plasma, had been verified by other researchers (Rong et al., 2014; Zhu et al., 2014). This could be explained that weaker competition for reaction with the active species existed between TCS molecules and its intermediates when the initial concentration was lower (Feng et al., 2008). While, it was interestingly noted in this research that, at the initial concentration of 5 mg L−1 , the degradation efficiency of TCS initially showed higher but subsequently expressed lower than that at 10 or 8 mg L−1 . It could be attributed to the thickness of liquid membrane which restricted the TCS molecules at the bottom of the reactor to contact with reactive oxidizing species generated by DBD. So there always had some TCS molecules flowing through the reactor which could not be degraded by DBD plasma, and the TCS molecules at the top of liquid membrane gradually reduced as reaction proceeding, causing the weakening of effect of DBD plasma degradation. In other words, it was difficult for TCS to decompose in the planar DBD reactor when TCS solution reached a certain low concentration. Nevertheless, in the presence of ACFs, the degradation efficiencies of TCS reached 93%, 94% and 94% respectively at the initial concentrations of 10, 8 and 5 mg L−1 , which almost eliminated the influence of low concentration. This indicated that ACFs could drastically improve the degradation efficiency of TCS at low concentrations in the planar DBD reactor due to the strong adsorption for pollutants at low concentrations (Chen and Xie, 2013). Hence, the DBD plasma-ACFs system could well enhance degradation efficiency of DBD plasma no matter whether the concentration of solution was high or low. Meanwhile, the synergistic effect provided a method to treat wastewater at a low initial concentration. 3.2.4. Effects of thickness of ACFs on the degradation of TCS The effects of thickness of ACFs on the degradation of TCS in the reaction system were presented in Fig. 2d. It showed that the degradation efficiency of TCS manifested significantly improvement with increasing thickness of ACFs. Especially when the thickness of ACFs reached 2 mm, the TCS degradation efficiencies attained 71%
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Fig. 2. Processing parameters affecting the synergetic degradation of TCS (η: degradation efficiency). (a) Different input powers on the degradation, conditions: initial TCS concentration = 10 mg L−1 , flow rate = 45 mL min−1 , thickness of ACFs = 1 mm. (b) Different flow rates on the degradation, conditions: initial TCS concentration = 10 mg L−1 , input power = 80 W, thickness of ACFs = 1 mm. (c) Different initial concentrations on the degradation, conditions: input power = 80 W, flow rate = 45 mL min−1 , thickness of ACFs = 1 mm. (d) Different thicknesses of ACFs on the degradation, conditions: initial TCS concentration = 10 mg L−1 , input power = 80 W, flow rate = 45 mL min−1 .
in 3 min and 97% in 18 min which were far beyond TCS degradation efficiencies with other thickness of ACFs at any reaction time. It was indicated that ACFs had the ability to offer strong adsorbability and stable discharge, which could enhance the degradation efficiency of DBD plasma. What was more, the discharge gap decreased with the increase of thickness of ACFs, which could produce more DBD plasma and enhance the TCS degradation efficiency (Hu et al., 2013). However, the ACFs could not be too thick because sufficient interspace must be left for DBD; if ACFs excessively approach negative electrode, the ACFs would be cremated by high temperature which was generated by DBD. Thus, there was a moderate thickness for ACFs on the degradation of TCS in the DBD plasma-ACFs system. 3.2.5. Effects of pH value on the removal of TOC Fig. 3 showed the TOC removal rates of TCS solution under different pH values. Obviously, only 12% TOC was removed by single DBD plasma at initial pH value of 6.26 in 18 min, and simultaneously the pH value in TCS solution gradually dropped to 3.84. But in the DBD plasma-ACFs system, TOC removal rate reached 19% after 18 min of treatment at the same pH. Furthermore, at initial pH values of 9.50 and 3.50, the removal rates of TOC were 21% and 24% respectively in 18 min. These results showed that the TOC removal rate was improved in the DBD plasma-ACFs system, and alkaline or acid initial condition could enhance the TOC removal rate. It could be attributed to the adsorption of degradation intermediates by ACFs, which could reduce the TOC in the reaction solution. The dissolved ozone was more easily decomposed to ·OH in alkaline condition and more ·OH radicals were produced in acid environment, which accelerated the decomposition of TCS and its intermediates (Feng et al., 2008). Furthermore, the pH value grad-
Fig. 3. Effects of pH value on the removal of TOC (initial TCS concentration = 10 mg L−1 , input power = 80 W, flow rate = 45 mL min−1 , thickness of ACFs = 1 mm).
ually declined as the reaction proceeding, demonstrating the formation of carboxylic acid. 3.3. Energy yield for the TCS degradation The degradation efficiency of TCS was better illustrated by the energy yield, defined as the amount of TCS degraded per unit of
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energy consumed in the discharge. The energy yield (g kW−1 h−1 ) after 18 min degradation of TCS for different input power, flow rate and initial concentration was shown in Table SM-4. It might be clear that the energy yield decreased with increasing input power and increased with increasing initial concentration. Moreover, the energy yield in the condition of high or low flow rate both showed lower values than that of middle flow rate. But in DBD plasmaACFs system, the energy yields were all improved under the condition of various processing parameters, especially flow rate. The best performance obtained in this study was 6.2 g kW−1 h−1 for 10 mg L−1 TCS solution (120 mL) at 60 W and 45 mL min−1 flow rate in the DBD plasma-ACFs system. 3.4. Recycling of ACFs In synergetic degradation processes, recycling of ACFs were of crucial importance to investigate for evaluating its applicability for wastewater treatment. Fig. 4 displayed the degradation efficiencies of TCS by recycling of five times of ACFs. As shown in Fig. 4, the degradation efficiency of TCS slightly increased from 93% to 96% after five cycles. The case that the degradation efficiency of TCS increased during five cycles could be owing to the modification of ACFs and the degradation of TCS adsorbed on ACFs by DBD plasma. After each cycle, the oxygen functional groups of ACFs increased, such as carbonyl groups (C=O) and lactone groups (O–C=O), which enhanced the adsorption of ACFs, and produced better synergy effect in the next cycle (Zhou et al., 2012). Meanwhile, DBD plasma could decompose the TCS molecules adsorbed on ACFs (had been confirmed by the TCS recovery in 3.2.1.), which could realize in situ regeneration. Of course, the degradation efficiency tended to be the same with the increasing number of cycles, because the modification effects gradually weakened. Therefore, in this DBD plasmaACFs system, degradation, modification and regeneration could be carried out simultaneously and the ACFs could be gradually regenerated so that they could be recycled without any treatment. 3.5. UV–Vis absorption spectra analysis of TCS In order to better understand the changes of TCS solution during reaction processes, the UV–Vis absorption spectra of TCS solu-
Fig. 4. Effects of recycling of ACFs in DBD plasma-ACFs system (initial TCS concentration = 10 mg L−1 , input power = 80 W, flow rate = 45 mL min−1 , thickness of ACFs = 1 mm).
tion were shown in Fig. SM-4. It was shown that, the absorbance spectrum at 282 nm that was the characteristic absorption peak of TCS, gradually decreased as the reaction proceeding, which indicated the rapid degradation of TCS molecules. However, the absorbance at around 200–250 nm displayed a rapid increase and redshift, which demonstrated a growing number of intermediates and aromatic base with reaction time. In a word, as the reaction progress, TCS molecules could be quickly decomposed by DBD plasma. 3.6. Possible degradation pathway of TCS by DBD plasma 3.6.1. Optimization and theoretical calculation It is well-known that Natural Bond Orbital (NBO) theory was usually used to analyze structure geometry and interaction among bonds of organic molecules (Appell and Bosma, 2015; Karthick et al., 2015). For TCS molecule, NBO analysis were calculated on structure geometry optimized at B3LYP/6-311 + G∗ level. The optimized conformation of TCS was shown in Fig. SM-5. The Wiberg bond order and atomic natural charge of TCS molecule were shown in Table SM-5. From Table SM-5, it was indicated that C–O on the ether group was the weakest site and C–Cl was also liable to be attacked by active species (i.e. • OH, O3 ); in contrast, C–C on the benzene ring was hard to be broken at the beginning of reaction, but as degradation proceeding, organic macromolecules (TCS and its intermediates) were decomposed into the ramification of aromatic compounds and subsequently the aromatic ring could be opened. 3.6.2. Identification of intermediates and degradation mechanism for TCS TCS solution at different reaction time (3, 9 and 15 min) was measured by LC-QTOF-MS to identify TCS and its degradation intermediates. Taking into account the above Wiberg bond order and atomic natural charge, the chromatographic and mass spectra characteristics of TCS and 10 main intermediates were summarized in Table 1. Firstly, it should be pointed out that, although ultraviolet light was generated by DBD, no polychlorinated dioxins were detected through carefully comparing mass-to-charge ratio with the mass spectrum. The reasons might be that most of TCS molecules were decomposed by • OH, which had stronger electrophilic property, higher redox potential and hundreds of thousands of times reaction rate than ultraviolet light (Haag and Yao, 1992; Son et al., 2009). Simultaneously ACFs could also block the penetration of the ultraviolet light. Secondly, some isomerides were found from Fig. SM-10 and Fig. SM-11, such as intermediates Ⅴ and Ⅵ, and distinguished by Octanol-water partition coefficient (Kow ) and molecular polarity. From Fig. SM-6, it could be seen that the response intensity of TCS decreased gradually, demonstrating that TCS molecules were increasingly decomposed as the reaction proceeding. At reaction time of 3 min, intermediates Ⅱ, Ⅳ, Ⅴ and Ⅶ showed response intensity in Fig. SM-7, SM-9, SM-10 and SM-11 respectively, which indicated that they were four uppermost degradation intermediates of TCS. As the reaction taking on to 9 min, other intermediates (Ⅲ, Ⅹ and Ⅺ) with a higher molecule weight were observed successively in Fig. SM-8, SM-13 and SM-14 respectively. Meanwhile, some micromolecules appeared, such as intermediates Ⅵ, Ⅷ and Ⅸ, which were shown in Fig. SM-10, SM-11 and SM-12 respectively. Therefore, there existed three different degradation patterns involving in TCS degradation. The identification of probable degradation pathway was depicted in Fig. 5. The first route (a), which was also the main route, initiated by ·OH acting on the ether bond of TCS, resulting in the formation of 2,4-Dichlorophenol (intermediate Ⅱ), m-Dichlorobenzene (intermediate Ⅳ), 3-Chlorophenol (intermediate Ⅴ), and 4-Chlorobenzene-1,2-diol (intermediate Ⅶ)
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Table 1 Retention time (based on the LC-MS spectra of TCS solution at reaction time of 15 min), structures of TCS and its main degradation intermediates. Compound
Molecular formula
Triclosan (Ⅰ)
Structure
RT (min)
Molecular ions (m/z)
C12 H7 Cl3 O2
5.602
286.94 ([M−H]− )
2,4-Dichlorophenol (Ⅱ)
C6 H4 Cl2 O
1.605
160.96 ([M−H]− )
2-Chloro-5-(2-dichlorophenoxy) benzene-1,4-diol (Ⅲ)
C12 H8 Cl2 O3
0.538
268.98 ([M−H]− )
m-Dichlorobenzene (Ⅳ)
C6 H4 Cl2
0.532
146.98 ([M+H]+ )
3-Chlorophenol (Ⅴ)
C6 H5 ClO
0.646
129.01 ([M+H]+ )
2-Chlorophenol (Ⅵ)
C6 H5 ClO
0.543
129.01 ([M+H]+ )
4-Chlorobenzene-1,2-diol (Ⅶ)
C6 H5 ClO2
0.659
145.01 ([M+H]+ )
Chlorohydroquinone (Ⅷ)
C6 H5 ClO2
0.526
145.01 ([M+H]+ )
2-Chloro-1,4-benzoquinone (Ⅸ)
C6 H3 ClO2
0.533
142.99 ([M+H]+ )
2-Chloro-5-(2,4-dichlorophenoxy) benzene-1,4-diol (Ⅹ)
C12 H7 Cl3 O3
0.552
304.95 ([M+H]+ )
2-Chloro-5-(2,4-dichlodichloro- phenoxy)-[1,4]benzoquinone (Ⅺ)
C12 H5 Cl3 O3
0.544
302.94 ([M+H]+ )
862
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Fig. 5. The proposed degradation pathway of TCS by DBD plasma.
(Chen et al., 2012). The other route (b) proceeded with attacking by ·OH on the para-position of phenol ring of TCS, causing the generation of intermediates Ⅹ and Ⅺ. This was because ·OH had strong electrophilic character and preferentially attacked the carbon atoms with the highest electron density, which was also facilitated by the ortho orientation of the ring chlorine (Yu et al., 2006). Then, ·OH broke the ether bond ring of intermediates Ⅹ and Ⅺ to form intermediates Ⅱ, Ⅷ and Ⅸ. Meanwhile, when excess ·OH radicals were produced, 2, 4-Dichlorophenol could be oxidized to intermediate Ⅷ and even intermediate Ⅸ (Song et al., 2012). Ultimately, ring-opening reaction occurred for intermediates Ⅳ, Ⅴ, Ⅵ, Ⅶ and Ⅸ to form carboxylic acid that being decomposed to CO2 , H2 O and Cl− at last. Certainly, the direct dechlorination– hydroxylation reaction (c) of TCS also occurred and primarily produced intermediate Ⅲ, which had been confirmed by Ferrer et al. (Ferrer et al., 2004). Subsequently, intermediate Ⅲ also undergone bond breaking and ring-opening and came into being carboxylic acid.
from 0 to 2 mm in 18 min at input power of 80 W. Meanwhile, the removal rate of TOC was evidently improved in the DBD plasma-ACFs system and TOC could be faster removed at low pH value. In addition, ACFs could ameliorate the degradation efficiency for planar DBD plasma when treating TCS solution at high flow rates or at low initial concentrations. In this DBD plasma-ACFs system, degradation of TCS and modification of ACFs could be carried out simultaneously and ACFs could realize in situ regeneration, enhancing the effect of recycling of ACFs. The possible degradation pathway of TCS by DBD plasma was inferred based on 10 intermediates detected by LC-QTOFMS combined with theoretical calculation of Gaussian 09 program. The main degradation mechanism of TCS by DBD plasma involved a series of processes, such as bond breaking, dechlorinationhydroxylation, hydroxylation and oxidative opening of the aromatic ring; ultimately, carboxylic acid and inorganic species were also formed.
Acknowledgments 4. Conclusions In the present study, aqueous TCS was degraded by DBD plasma combined with ACFs in the planar DBD plasma reactor. The synchronous processing technology expressed efficient degradation efficiency for TCS. The degradation efficiency of TCS improved from 85% to 97% with the thickness of ACFs increased
This work was supported by the National Science and Technology Major Project on Water Pollution Control and Treatment (No. 2014ZX07204-008), the National Natural Science Foundation of China (No. 51208163), and Open Fund of State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering, Hohai University (2013491211).
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