High frequency discharge plasma induced plasticizer elimination in water: Removal performance and residual toxicity

Journal of Hazardous Materials 383 (2020) 121185

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High frequency discharge plasma induced plasticizer elimination in water: Removal performance and residual toxicity

T

⁎

Hongshuai Kana,b, Tiecheng Wanga,b, , Zhengshuang Yanga,b, Renren Wuc,d, Jing Shene, Guangzhou Qua,b, Hanzhong Jiaa,b a

College of Natural Resources and Environment, Northwest A&F University, Yangling, Shaanxi Province, 712100, PR China Key Laboratory of Plant Nutrition and the Agri-environment in Northwest China, Ministry of Agriculture, Yangling, Shaanxi, 712100, PR China c The Key Laboratory of Water and Air Pollution Control of Guangdong Province, PR China d South China Institute of Environmental Science, MEE, Guangzhou, 510655, PR China e College of Biology and the Environment, Nanjing Forestry University, Nanjing, Jiangsu Province, 210037, PR China b

A R T I C LE I N FO

A B S T R A C T

Editor: Xiaohong Guan

Plasticizers are widely present in water and soil environment, and they can bring enormous threats to environmental safety and human health. A discharge plasma system driven by a high-frequency electric source was used to remove the plasticizer from wastewater; and dimethyl phthalate (DMP) was chosen as the representative of plasticizer. DMP elimination performance at various operating parameters, roles of active species in DMP degradation, DMP decomposition process, and its residual toxicity after decomposition were systematically investigated. The experimental results demonstrated that almost all of the DMP and 80.4% of the total organic carbon (TOC) were removed after 30 min of treatment. The DMP decomposition process fitted well with the firstorder kinetic model. Relatively higher applied voltage, lower initial concentration, and alkaline conditions favored its decomposition. •OH was the decisive species for DMP decomposition, in addition to •O2− and 1O2; while the role of hydrated electrons was negligible. The analysis of DMP decomposition process showed that the molecular structures of the DMP were destroyed, and 3-hydroxy-dimethyl phthalate, monomethyl phthalate, and phthalic acid were detected. Furthermore, the residual toxicity after DMP decomposition was analyzed via seed germination and photobacterium bioassay.

Keywords: High frequency discharge Plasticizer Elimination Dimethyl phthalate Residual toxicity

1. Introduction More than two million tons of plastic films are used in China per year (Department of Rural Survey National Bureau of Statistics of China, 2012). As a result, a large amount of plastic debris continues to enter water environment (Wang et al., 2015a). To improve the plasticity and flexibility of the plastic films, a large number of plasticizers have been added in the production of the plastic films. Phthalates are considered as the predominant plasticizer in the plastic films, and thus they would inevitably enter the water environment along with plastic fragments. Phthalates can interfere with endocrine systems, cause damage to the liver and kidneys, and even lead to infertility and cancer (Jia et al., 2017; Ferguson et al., 2015). It is necessary to develop some methods to control phthalate pollution in the water environment. Recently, non-thermal discharge plasma technique, an advanced oxidation process, has received extensive interest in the removal of contaminants (Wang et al., 2018a; Jiang et al., 2019; Wang et al.,

⁎

2016a; Liu et al., 2013). A variety of strong oxidizing chemically active substances, including O3, ·OH, 1O2, and ·O2−, could be generated in the discharge plasma process. And some physical effects (ultraviolet radiation, electric field, etc.) would also be induced simultaneously. These active substances and physical effects then synergistically acted on pollutants in wastewater, leading to rapid and efficient removal of them (Wang et al., 2018a; Jiang et al., 2019; Wang et al., 2016a; Liu et al., 2013; Malik et al., 2005; Jiang et al., 2018; Sano et al., 2002). To date, three types of plasma forms have emerged; direct liquid discharge (Malik et al., 2005), gas-liquid interface discharge (Jiang et al., 2018), and gas phase discharge (Sano et al., 2002). High voltage and ground electrodes were both or either in contact with the wastewater in the gas-liquid interface discharge or liquid discharge system, and electrode erosion affected the stability of the plasma; thus the active substance formation was negatively affected. For the traditional gas phase discharge, the solution electroconductivity could not affect the stability of the plasma. However, the diffusion of the active substances into the

Corresponding author at: College of Natural Resources and Environment, Northwest A&F University, Yangling, Shaanxi Province, 712100, PR China. E-mail address: [email protected] (T. Wang).

https://doi.org/10.1016/j.jhazmat.2019.121185 Received 1 May 2019; Received in revised form 3 August 2019; Accepted 7 September 2019 Available online 09 September 2019 0304-3894/ © 2019 Elsevier B.V. All rights reserved.

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wastewater was constrained by gas-liquid interfacial resistance. To overcome these problems, a surface discharge reaction system was developed in our previous study (Wang et al., 2016a). The formation of chlorinated disinfection byproducts after natural organic matter degradation was significantly inhibited in this system with a 50 Hz electric source as the driver (Wang et al., 2016a). That result suggested that this system might be used as a new approach for the control of precursors of the chlorinated disinfection byproducts. In the meantime, the active species formed in the gas atmosphere were infused into the water via microbubbles, which improved their mass transfer. The solution electroconductivity also did not affect the stability of the system. As one type of organic pollutants from human activities, phthalates were widely present in the wastewater. It was reported that the medial lethal concentrations of dimethyl phthalate, diethyl phthalate, and dibutyl phthalate to H. azteca were 28.1, 4.21, and 0.63 mg L-1, respectively (Call et al., 2001). Previous studies reported that phthalates could be degraded by glow discharge plasma and non-thermal air plasma, respectively (Shen et al., 2016; Qi et al., 2019). These studies were mainly focused on the decrease in the concentration of phthalates, but little was conducted on the residual toxicity of byproducts. In addition, high frequency (several kHz) discharge plasma has a higher energy density and larger amounts of active species formation than those of low frequency (50 Hz) plasma, and thus it might be more suitable for water treatment (Zhao et al., 2017; Ahmed et al., 2017; Krcma et al., 2015). Therefore, phthalate removal performance and residual toxicity were evaluated in a surface discharge reaction system with a high frequency electric source as the driver in this study. DMP was used as a representative plasticizer, which is a type of phthalate and is typically used in cellulose ester-based plastic films. DMP removal performance was first evaluated under various operating parameters, such as applied voltage, DMP initial concentration, and solution pH. Then, the DMP decomposition process was explored via a series of chromatography and spectral diagnostics. Finally, the residual toxicity after DMP decomposition was diagnosed via seed germination and photobacterium bioassay.

Fig. 1. Experimental system of high frequency discharge for DMP elimination.

P=

1 T

T

T

0

0

dt = fC ∮ UdUc = fCS ∫ UIdt = TC ∫ U dUc dt

(1)

where U is the applied voltage; P is the input energy; I is the current; f is the discharge frequency; C is the capacitance; and S is the Lissajous area. To evaluate the energy utilization efficiency (the amount of removed pollutant per unit of consumed energy) of this system for DMP degradation, the energy yield (G50) for DMP removal was calculated as follows,

G50 =

0.5 × mDMP P⋅t50

(2)

where mDMP is the amount of removed DMP, and t50 is the time when half of the DMP is decomposed. The UV–vis spectrum of the DMP solution was sampled by a UV–vis spectrophotometer (UV-2300, Shanghai) to describe the changes in the characteristic absorption peaks during its decomposition. The morphology of DMP sample was diagnosed using an Atomic force microscopy (AFM, Park NX20, Korea). The change in the characteristic functional groups in the DMP molecules during its decomposition was described using a FTIR (TENSOR-27, Bruker). The quantitative analysis on DMP and its degradation byproducts were measured using HPLC with a UV detector. The mobile phase was the mixture of methanol/water (v/v = 7:3) with a flow rate of 1.0 mL min−1. The detection wavelength was set as 228 nm. DMP degradation intermediates were extracted using dichloromethane, and they were qualitatively analyzed using GC–MS (7890-5975, Agilent, USA). For GC–MS analysis, the GC oven temperature was held at 60 0C for 2 min, increasing to 100 0C at a rate of 20 0C/min; and then it increased to 300 °C at a rate of 10 °C/min, and finally held at 300 °C for 10 min. Electron ionization mode (70 eV) was chosen as the ion source. The interface temperature was 300 0C. Helium was the carrier gas. To describe the DMP decomposition process, the first-order reaction kinetic model was used to fit the experimental data as follows,

2. Experimental 2.1. Reagents DMP (purity > 99%) and P.phosphoreum sp.-T3 were bought from the Sinopharm and the Chinese Academy of Sciences (Institute of Soil Science), respectively. Wheat seed was bought in a seed company in Yangling, Shaanxi province, China. Analytically pure reagents benzoquinone (BQ), 1,4-diazabicyclooctane triethylenediamine (DABCO), isopropanol (IPA), monomethyl phthalate, and phthalic acid were purchased from the Tianjin Fuyu Refinery Chemical Co., Ltd. 2.2. DMP removal experiment Fig. 1 shows the experimental setup for DMP removal. The reaction container, the configuration and sizes of the high voltage electrode and ground electrode were the same as previously reported (Wang et al., 2016a). Specifically, a high-frequency electric source (CTP-2000 K) bought from Nanjing Suman Electronics Co., Ltd. China was used as the driver, and its frequency was set at 7.0 kHz in this study. Natural air after dehydration by color changing silica gel was used as the source of active substance formation, and its gas flow rate was 2.5 L min−1. In a batch treatment, the total treatment volume was 500 mL.

ln(C0/C ) = kt

(3)

where C0 and C are DMP concentration at the time 0 min and t min, respectively. And k is the reaction rate constant. The toxicity of DMP solutions was analyzed by a photobacterium bioassay in an ELIASA (Infinite M200pro, Tecan) as well as via wheat seed germination experiment. For the seed germination test, at the discharge treatment times of 0, 5, 10, 15, 20, 25, and 30 min, 10 mL of plasma-treated DMP solutions was sampled and added to each petri dish (9 cm) containing 50 wheat seeds, which were marked as T0, T5, T10, T15, T20, T25, and T30, respectively. Then, they were cultivated

2.3. Analytical methods A Tektronix TDS2014 digital oscilloscope equipped with a voltage probe (Tektronix P6015A) and a current probe (Tektronix P6021) was used to sample the voltage and current signals; then, the input energy was calculated as follows (Wang et al., 2016b), 2

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in a constant temperature incubator at 20 °C and 70% air humidity. After cultivation for 4 d, germination characteristics and malonaldehyde (MDA) content were measured as reported previously (Guo et al., 2017). The wheat seed germination test was also conducted using deionized water as a control (TCK). Each test was repeated three times. For the photobacterium bioassay, at the discharge treatment times of 0, 10, 20, and 30 min, the P.phosphoreum sp.-T3 suspension was mixed with plasma-treated DMP solutions, which were marked as T0, T10, T20, and T30, respectively. Then, the luminescent intensity of the sample was measured, and the inhibition ratio for the P.phosphoreum sp.-T3 suspension was calculated as follows,

Inhibitionratio =

I0 − I × 100% I0

(4)

where I0 and I are the luminescent intensities of the control experiment and DMP samples, respectively. 3. Results and discussion 3.1. Elimination efficacy of DMP under various operating parameters 3.1.1. Impact of applied voltage Fig. 2(a) shows the impact of the applied voltage on DMP removal. The initial concentration of DMP solution was 100 mg L−1, and the solution pH was 6.0. Relatively higher applied voltage favored DMP decomposition. Only 31.9% of DMP was decomposed after 30 min of treatment at 5.0 kV, and there was an approximately 38.2% increase when the applied voltage was 8.0 kV. The DMP removal process fitted well with the pseudo first-order kinetics, as depicted in Fig. 2(a). Only 0.013 min−1 of the reaction rate (k) was obtained when the applied voltage was 5.0 kV, and it increased to 0.042 min−1 at 8.0 kV. More amounts of chemically active species would be generated at relatively higher applied voltage or energy input (Wang et al., 2018a; Jiang et al., 2019; Wang et al., 2016a; Liu et al., 2013; Cao et al., 2019), and thus greater DMP removal performance was observed at higher applied voltage in this study. Later experiments were all carried out at the applied voltage of 8.0 kV. 3.1.2. Impact of DMP concentration Fig. 2(b) depicts the impact of the initial DMP concentration. Relatively greater removal performance was observed at lower initial concentrations. After 30 min of treatment, the removal efficiency was improved from 33.7% to 98.0% as the DMP concentration decreased from 300 to 50 mg L−1, and the value of k also increased from 0.013 to 0.131 min-1. The generation amounts of the active substances were constant when the applied voltage was kept constant (Wang et al., 2018a). The DMP molecules would have more opportunities to react with the active species if their initial concentrations were relatively lower, resulting in relatively greater removal performance. The energy yield on DMP removal (initial concentration 50 mg L−1) was approximately 1.35 mg kJ−1. The DMP removal efficiency was about 30%˜95% after 50˜300 min of the electrochemical oxidation treatment with an energy yield of 0.45˜2.05 mg kJ-1 (Souza et al., 2014a, b). TiO2 photocatalytic oxidation could remove 43%˜88% of DMP after 60˜120 min with an energy yield of 0.006˜0.014 mg kJ-1 (Ding et al., 2008; Yuan et al., 2008a; Liao and Wang, 2009). Therefore, the present high frequency discharge system would have a significant advantage for DMP wastewater treatment.

Fig. 2. Effects of operation parameters on DMP degradation (a. applied voltage; DMP initial concentration; c. solution pH).

Ozone was easily decomposed to •O2- and •OH in an alkaline environment (Kasprzyk-Hordern et al., 2003). DMP could react rapidly with radicals such as •OH with a reaction rate of approximately 109 mol L-1 s1 , while the reaction rate of DMP with ozone was only 0.09 mol L-1 s-1 (Wen et al., 2011). Therefore, the dissatisfactory DMP removal performance under acidic conditions could be attributed to the slow reaction between the DMP molecules and ozone. Yan et al. (Yan et al. (2013)) also found that an alkaline environment benefited DMP removal in a heterogeneous ozonation catalyst system.

3.1.3. Impact of solution pH The DMP removal efficacy at different solution pH values is shown in Fig. 2(c). The initial DMP concentration was 50 mg L−1. Relatively greater DMP removal performance was observed under alkaline conditions. After 20 min of treatment, the removal efficiency was improved from 70.0% to 99.0% as the solution pH value increased from 2.0 to 10.0, and the value of k also increased from 0.068 to 0.208 min-1.

3.2. Identification of active species for DMP decomposition 3.2.1. Role of •OH radicals Isopropanol (IPA) is a commonly-used trapping agent of •OH, and the reaction rate of IPA with •OH is approximately 109 M−1s−1 (Li et al., 2019; Tang et al., 2018). DMP removal efficiency as a function of 3

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time in the presence of IPA is presented in Fig. 3a. DMP removal efficacy was inhibited in the presence of IPA. Approximately 98.0% of DMP was eliminated after 30 min of treatment in the absence of IPA, while it decreased to 9.6% when the IPA concentration was 4.0 mmol L−1; and the k value was dramatically reduced to 0.003 min-1 (97.7% decrease). These results indicated that the •OH radical played a decisive role in the DMP decomposition in this study. Previous research also found that the •OH radical was an important active species for some organic pollutant elimination, such as Cu-EDTA, in a discharge plasma system (Wang et al., 2018a). 3.2.2. Role of •O2− •O2− is a strong oxidizing active species that can be generated in the discharge plasma process via HO2• speciation dissociation (Bielskie et al., 1985). Benzoquinone (BQ) is a commonly-used trapping agent of •O2−, and the reaction rate of BQ with •O2− is approximately 109 M-1s-1 109 M-1s-1 (Zhang et al., 2006). Fig. 3b shows the DMP removal efficiency as a function of time in the presence of BQ. DMP removal efficacy was inhibited after some amounts of BQ were added. Approximately 98.0% of DMP was degraded after 30 min of treatment in the absence of BQ, while it decreased by 75.6% when the BQ concentration was 4.0 mmol L-1. Therefore, the •O2− was a significant active species for the DMP decomposition in this study. Our previous research also found that the •O2− played a significant role in Cu-EDTA decomplexation and removal in a discharge plasma system (Wang et al., 2018a). 3.2.3. Role of 1O2 1 O2 is a strong oxidizing active species that can be formed via reactions of •OH with •O2−, or •OH with •HO2; and the 1O2 can attack organic pollutant molecules through electrophilic reactions (Chen et al., 2010). Previous study reported that 1O2 played a crucial role in carbamazepine decomposition in a photocatalytic system (Wang et al., 2017). 1,4-Diazabicyclooctane triethylenediamine (DABCO) is a commonly-used trapping agent for 1O2 (Shikhova et al., 2007). Fig. 3c shows the DMP removal efficacy as a function of time. The addition of DABCO inhibited DMP elimination. The DMP removal efficacy decreased from 98.0% to 45.8% within 30 min after 4.0 mmol L-1 of DABCO was added, and the k value decreased by 84.7% (inset in Fig. 3c). Therefore, 1O2 played a crucial role in the DMP decomposition in this study. 3.2.4. Role of hydrated electrons Hydrated electron is an important species that can affect the formation of other active species. Previous study reported that hydrated electrons participated in the microcystin-LR and atrazine degradation processes (Zhang et al., 2012; Leitner et al., 2005). Phosphate was commonly selected to capture the hydrated electron, and the reaction rate between them was larger than 107 M−1s-1 (Shikhova et al., 2007; Zhang et al., 2012). Fig. 3d depicts the DMP degradation performance as a function of time in the presence of HPO42-. The DMP degradation performance was nearly unchanged after 4.0 mmol L−1 of HPO42- was added. Therefore, the role of hydrated electrons in the DMP decomposition in this study was negligible. Previous studies reported that the presence of phosphate could inhibit the formation of H2O2 and then disfavored nitrophenol decomposition in aqueous in the discharge process (Wang et al., 2015b). Leitner et al. (Leitner et al. (2005)) also found that the addition of phosphate to the pulsed discharge system exhibited some inhibition effects for atrazine decomposition. However, it should be noted that in the above mentioned studies, the discharge plasma was triggered directly in the liquid phase, which provided favorable conditions for the formation of the hydrated electrons. In this study, the discharge plasma was formed in an air atmosphere and then diffused into the liquid phase to set off a series of reactions; in this process, the hydrated electrons might hardly be generated.

Fig. 3. DMP degradation efficacy as a function of time in the presence of trapping agents (a. IPA addition; b. BQ addition; c. DABCO addition; d. HPO42− addition).

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Fig. 4. Changes of TOC removal with the treatment time.

In summary, for 4.0 mmol L−1 trapping agent addition, the decrease degree of the reaction rate ranked as IPA > DABCO > BQ, and thus the •OH radical played the most important roles in the DMP degradation. Certainly, the roles of 1O2 and •O2- were also quite significant. 3.3. DMP decomposition process The DMP decomposition process was analyzed using UV–vis, TOC, FTIR, AFM, HPLC, and GC–MS. Fig. 5. UV–vis and FTIR spectrum of DMP samples before and after treatment (a. UV–vis; b. FTIR).

3.3.1. Mineralization performance Fig. 4 depicts the changes in TOC removal efficiency with time. Satisfactory mineralization efficiency appeared, and 80.4% of TOC was removed after 30 min of treatment. A comparison of TOC removal during DMP degradation by different methods was conducted and listed in Table 1. Electrochemical oxidation and UV/ozonation both exhibited relatively lower TOC removal performance, 20% of TOC removal within 180 min of treatment and 37.6% of TOC removal within 30 min of treatment (Souza et al., 2014b; Chen et al., 2008). In total, 46.5% and 58% of TOC removal were gained by anaerobic biodegradation and Fenton oxidation, respectively; while they took a long treatment time (360–720 min) (Liang et al., 2007; Xiao et al., 2016). Satisfactory TOC removal performance (˜75%) was obtained by persulfate oxidation, but it took 1440 min (Wang et al., 2019a). Approximately 75% of TOC was removed within 90 min of treatment in a TiO2 photocatalytic system (Pulido Melián et al., 2016). Therefore, relatively greater mineralization performance was exhibited by the present discharge plasma compared with the above mentioned methods.

they disappeared. These changes preliminarily indicated that some functional groups of DMP molecules were broken after the discharge treatment. Fig. 5b depicts the FTIR of the DMP samples with/without treatment. Three typical characteristic peaks at approximately 3423, 1624, and 1090 cm−1 were both observed in DMP samples before and after treatment, which were assigned to the stretching vibrations of OeH, C]C, and C − OeC, respectively (Pulido Melián et al., 2016; Lu et al., 2016; Wang et al., 2018b, b). Their intensities were weakened after the discharge treatment. In addition, a band at 1138 cm−1 occurred after treatment, which was associated with the HCOOR group (Pulido Melián et al., 2016; Lu et al., 2016). These changes in the FTIR indicated that small pieces were formed during the DMP decomposition process. Fig. 6 depicts the morphology of the DMP samples as analyzed by AFM. DMP particles gathered together on the surface of the mica and they were indiscernible before the surface discharge treatment (Fig. 6a). However, the aggregation was gradually broken and some white deposit particles could be clearly observed after 15–30 min of processing (Fig. 6b and 6c). These changes in the morphology visually suggested that the surface discharge treatment led to the decomposition of the DMP molecules. 2,4-Dichlorophenol decomposition and inorganic fraction formation were also observed in a discharge plasma process as analyzed by AFM (Lu et al., 2006).

3.3.2. DMP molecular structure Fig. 5a depicts the UV–vis spectrum of DMP as a function of time. Two characteristic peaks at 277 nm and 230 nm were observed for the untreated DMP samples; the former represented the –C = O– groups in the DMP molecular structure, and the latter was assigned to the benzoic ring structure (Jiang et al., 2016). As the treatment time continued to increase, the intensities of these two peaks were both weakened until Table 1 TOC removal during DMP degradation by different methods. Techniques

CDMP (mg L−1)

TOC removal efficiency (%)

Time (min)

Reference

Electrochemical oxidation UV/ozonation Anaerobic biodegradation Fenton oxidation Persulfate oxidation TiO2 photocatalysis Discharge plasma

162 78 300 194 10 50 50

˜20 37.6 46.5 ˜58 ˜75 ˜75 80.4

180 30 720 360 1440 90 30

(Souza et al., 2014b) (Chen et al., 2008) (Liang et al., 2007) (Xiao et al., 2016) (Wang et al., 2019a) (Pulido Melián et al., 2016) This study

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Fig. 6. AFM spectrum of DMP as a function of time.

3.3.3. Decomposition byproducts GC–MS was further used to identify the decomposition byproducts. The typical ion chromatograph of DMP degradation intermediates and their information were shown in Fig. 7 and Table 2, respectively. 2,6Dihydroxybenzoic acid, dihydro-4-hydroxy-Furanone, phthalic acid, monomethyl phthalate, 3-hydroxy-dimethyl phthalate, and 2,3-dimethyl-hydroquinone were detected as the main intermediates in this study. 3-Hydroxy-dimethyl phthalate was also detected during DMP degradation in O3-TiO2-Al2O3 oxidation system, UV-TiO2 oxidation system, and persulfate oxidation system, respectively (Chen et al., 2011; Yuan et al., 2008b; Wang et al., 2014). The phthalic acid and monomethyl phthalate also appeared after DMP decomposition by other advanced oxidation techniques such as electrochemical oxidation, microwave-assisted photocatalytic oxidation, and electron pulse radiolysis (Souza et al., 2014b; Liao and Wang, 2009; An et al., 2014). HPLC

Table 2 Information on degradation byproducts in.GC–MS Serial number

Retention time (min)

Byproducts

m/z

1

7.046

2,6-dihydroxybenzoic acid

154

2

7.703

dihydro-4-hydroxyFuranone

102

3

9.365

phthalic acid

174

4

9.472

monomethyl phthalate

180

5

11.106

dimethyl phthalate

194

6

15.110

3-hydroxy-dimethyl phthalate

210

7

16.519

2,3-dimethylhydroquinone

138

Structural formula

was further used to evaluate the evolution of phthalic acid and monomethyl phthalate, and their concentrations as a function of time are depicted in Fig. 8. The contents of monomethyl phthalate and phthalic acid were both aggregated gradually with the time at the first treatment stage (0–20 min) and then subsequently decreased. These results suggested that the formed byproducts suffered from further decomposition.

Fig. 7. The typical ion chromatograph of DMP degradation intermediates using.GC–MS. 6

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Fig. 10. Changes of MDA content in seed at different cultivation solutions. Fig. 8. DMP degradation byproduct as a function of time.

it decreased gradually with the discharge plasma processing. The differences in MDA concentration between T30 and TCK were not significant. These results further demonstrated that the oxidative injury to the wheat seeds brought by the DMP samples was reduced gradually with the discharge treatment; and there was no oxidative injury at the end of treatment. Previous study also reported that discharge plasmainduced DMP degradation in soil decreased its toxicity to seeds (Jia et al., 2018). Mesdaghinia et al. (Mesdaghinia et al. (2017)) also found that zeolite/Fe3O4 induced DMP decomposition decreased its toxicity to Ceriodaphnia dubia and Daphnia magna. Fig. 11 depicts the inhibition ratio of P.phosphoreum sp.-T3 as a function of time. The inhibition ratio was approximately 68.0% in the T0 group, suggesting that the initial DMP solution without the discharge treatment exhibited a strong toxicity to the P.phosphoreum sp.T3. However, the inhibition ratio gradually decreased after the discharge treatment. Approximately 1.5% of the inhibition ratio was obtained in the T30 group. These results indicated that nearly no toxicity to P.phosphoreum sp.-T3 was observed in the DMP samples at the end of the treatment. Zhang et al. reported that the Fenton reaction-induced DMP decomposition decreased its toxicity to photobacterium (Zhang et al., 2016). Therefore, the high-frequency discharge plasma system would have significant advantages in removing DMP from wastewater and controlling its residual risks.

3.4. Residual toxicity The above analysis on the decomposition byproducts demonstrated that some small organic compounds were generated after the discharge treatment. To further explore their toxicity, Fig. 9 depicts the germination characteristics of the wheat seeds as a function of treatment time. The germination rate was approximately 47.8% in the T0 group, which was much lower than that in the TCK group (90.9%), suggesting that the initial DMP solution without the discharge treatment exhibited a strong toxicity to the wheat seed germination. However, the germination rate gradually increased after the discharge treatment. Approximately 88.3% of the germination rate was obtained in the T30 group, and the difference in the germination rate between the T30 and TCK groups was not significant. These results indicated that there was no toxicity from the DMP samples towards wheat seed germination at the end of treatment in this study. This deduction could also be further confirmed by the germination index. The germination index gradually increased with the treatment time; it significantly increased from 21.6 to 40.6 with the treatment time increasing from 0 to 30 min. The differences between T30 and TCK were not significant. In adverse conditions, large numbers of reactive oxidation species would be accumulated in the plants, which then led to oxidative injury to the membrane lipids of the plants. MDA is generally considered as a referent of membrane oxidative injury (Guo et al., 2017). Fig. 10 depicts the evolution of MDA concentration under different treatment conditions. The highest MDA content was obtained in the T0 group, and

4. Conclusions The performance of DMP removal from wastewater using a highfrequency discharge plasma system was investigated. Satisfactory DMP

Fig. 9. Changes of germination rate and germination index of seed at different cultivation solutions.

Fig. 11. Changes of photobacterium bioassay toxicity of DMP samples. 7

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removal performance and mineralization efficacy were achieved in a short treatment time. The reaction rate of DMP decomposition was faster at a higher applied voltage, lower initial DMP concentration, and alkaline environment. Radical capture experiments demonstrated that •OH, 1O2, and •O2− all participated in DMP decomposition, whereas hydrated electrons did not. UV–vis, TOC, FTIR, AFM, HPLC, and GC–MS analysis suggested that the discharge treatment significantly destroyed the DMP molecular structures. The generated monomethyl phthalate and phthalic acid were both aggregated gradually at the first treatment stage, and then they were decomposed subsequently. There was no toxicity to the wheat seed germination and P.phosphoreum sp.-T3 growth for the DMP samples after the plasma treatment.

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