Mechanistic study on the combination of ultrasound and peroxymonosulfate for the decomposition of endocrine disrupting compounds

Ultrasonics - Sonochemistry 60 (2020) 104749

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Mechanistic study on the combination of ultrasound and peroxymonosulfate for the decomposition of endocrine disrupting compounds

T

⁎

Lijie Xua, , Xiaotian Wanga, Yang Suna, Han Gongb, Mingzhi Guoc, Xiaomeng Zhanga, ⁎ Liang Menga, Lu Gand, a

College of Biology and the Environment, Nanjing Forestry University, Nanjing 210037, Jiangsu, People’s Republic of China College of Marine Sciences, South China Agricultural University, Guangzhou 510642, Guangdong, People’s Republic of China c College of Mechanics and Materials, Hohai University, Nanjing 210037, Jiangsu, People’s Republic of China d College of Materials Science and Engineering, Nanjing Forestry University, Nanjing 210037, Jiangsu, People’s Republic of China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Ultrasound Peroxymonosulfate Dimethyl phthalate Bisphenol A Synergy

The effectiveness and synergistic mechanisms of combining ultrasonic process (US) with peroxymonosulfate (PMS) were investigated using Bisphenol A (BPA) and Dimethyl Phthalate (DMP) as the model pollutants. Synergy between US and PMS improved the degradation of target pollutants, and PMS was found to play a dual role. The optimum dosage of PMS and the extent of efficiency promotion were found to depend on not only the ultrasonic frequency but also on the hydrophobicity of target pollutants. The scavenger quenching experiments and electron paramagnetic resonance analysis indicated that %OH was responsible for DMP degradation in both US and US/PMS processes. The chemical probe experiments also proved that activation of PMS could increase the production of %OH while excess PMS consumed the available radicals. Furthermore, it was found for the first time that the constituent salts of KHSO4 and K2SO4 in the commercial Oxone also made considerable influence on US/PMS process. It was also found that the combination of US and PMS showed more pronounced synergistic effect for treating DMP at lower concentrations. Higher efficiency was achieved at more acidic condition and similar efficiencies were obtained at pH range of 5.1 ~ 8.12. DMP degradation pathways were found to be the % OH addition to the aromatic ring and hydrogen absorption at the aliphatic chains with and without the presence of PMS, but much better mineralization capability was obtained in the presence of PMS than ultrasonic degradation alone.

1. Introduction The advanced oxidation technologies (AOTs) have been known to be effective to oxidize refractory organic contaminants (e.g., photocatalysis [1], catalytic oxidation [2]). Recently, the sulfate radical based AOTs have drawn increasing attention mainly because of the higher redox potential (2.5 ~ 3.1 V) and stronger selectivity of sulfate radical anion (SO4%−) in comparison with the conventionally dominant %OH radical [3]. SO4%− is usually generated by various activation means to decompose the O-O bond in peroxydisulfate (PDS) or peroxymonosulfate (PMS). PDS and PMS can be activated by thermal, alkaline, ultraviolet light, transition metal (e.g. Fe2+, Co2+, Cu2+), carbonaceous materials, etc. The peroxysulfate (PMS or PDS) related processes attract the attention also due to the potential to serve as the in situ chemical oxidation (ISCO) technologies for the removal of organic contaminants from soil and groundwater [4].

⁎

The ultrasonic process (US) has been recently proposed and investigated as a promising technology for the degradation of bio-recalcitrant organic pollutants mainly due to its special advantages compared with other AOTs, such as safety, cleanness, chemical-free feature, lower requirement of the turbidity of the aqueous medium [5,6]. The most widely accepted mechanism of the ultrasonic process as an AOT is based on the “hot spot” theory. The propagation of ultrasound wave through liquid medium induces acoustic cavitation including the processes of formation, growth and adiabatically implosive collapse of bubbles in the liquid [7,8]. Extreme conditions (> 5000 °C, > 100 MPa) form during the final collapse of the bubbles, and reactive species can be produced after the homolysis of H2O (e.g·H2O + ))) → %OH + %H, %OH + %H → H2O, 2%OH → H2O2, 2% OH → H2O + %O, 2%H → H2) [7]. Previous studies of wastewater treatment involving ultrasound alone have questioned its economical viability based on calculations of power consumption by direct scale-up

Corresponding authors. E-mail addresses: [email protected] (L. Xu), [email protected] (L. Gan).

https://doi.org/10.1016/j.ultsonch.2019.104749 Received 12 February 2019; Received in revised form 5 July 2019; Accepted 22 August 2019 Available online 24 August 2019 1350-4177/ © 2019 Elsevier B.V. All rights reserved.

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purchased from Shanghai Aladdin Biochemical Co., Ltd. All the chemicals and solvents were of analytical grade and HPLC grade, respectively, and used without further purification. For pH adjustment, 0.1 M sulfuric acid and 0.1 M sodium hydroxide were used. Ultra-pure water used exclusively in this study was obtained from a Milli-Q system.

from laboratory conditions. Attitudes have changed with the installation of ultrasonic devices in water or wastewater treatment plants. Studies also demonstrate that the combination of US with different AOTs is economically more attractive than using US alone for wastewater treatment [9]. More recently, ultrasound has also been tried as an activation approach to decompose PDS or PMS [10–14] due to two primary considerations. On the one hand, the extreme conditions produced during the cavitation process are the direct energy source for O–O bond breaking. On the other hand, the avoidance of any exogenous catalysts or chemicals of the US process minimizes probable secondary pollution. Moreover, the successful implementation of ultrasound technique in the subsurface field (e.g., oil recovery from failing wells [15]) promotes the real application of US/PDS or US/PMS as an ISCO way. However, among those very limited studies, most have focused on sono-activation of PDS [10–13], in which the roles of PDS were proposed to be dual, i.e., radicals provider and scavenger depending on PDS concentration and the operating parameters [12]. Very limited studies reported on the probability and performance to activate PMS for pollutants removal. Yin et al. (2018) have reported recently on the generation of SO4%− and %OH when adopting US to activate PMS based on electron spin resonance (ESR) analysis [14]. The use of PMS has acquired more popularity and is becoming an alternative for H2O2 and PDS. Since there exist differences between PMS and PDS, such as the structural symmetry, O-O bond distance and the available commercial products, the capability of US as a means to activate PMS needs more investigation. Two endocrine disrupting compounds (EDCs) with different physicochemical properties were involved as the model pollutants in this study, which were dimethyl phthalate (DMP) and bisphenol A (BPA). DMP is the simplest member of the group of phthalate acid esters, which are widely used as plasticizers in industrial field. The relatively higher solubility of DMP (4000 mg/L at 25 °C [16]) increases the probability of being in the water phase. BPA is widely used as the raw material in the production of polycarbonate, epoxy resins, paper coatings, etc. China is one of the largest producer and consumer of BPA in the world, which increases the risk of water pollution by BPA and its derivatives. In this study, effects of combining ultrasound with PMS on the kinetics of DMP and BPA degradation were investigated in detail. Influencing factors including ultrasonic frequency, the properties of target pollutants, dosage of PMS, solution pH, and initial concentration of DMP were evaluated. Mechanisms of interactions between ultrasound and PMS were analyzed based on identification of reactive radicals and analysis of the degradation intermediates.

2.2. Apparatus and experimental conditions Three independent ultrasonic generators were applied in this study. The high frequency sonicators (200 kHz and 400 kHz) were tailor-made by Ning Bo Scientz Biotechnology Co., China, and the two frequencies were provided by independent basins with identical reactor size and an approximate effective volume of 1.5 L. The power densities applied for 200 kHz and 400 kHz ultrasound were calorimetrically determined as 0.09 and 0.11 W·mL−1, respectively [17]. The ultrasonic wave of either 200 kHz or 400 kHz was provided by one big piezoelectric ceramic attached centrally on the back of the basin bottom and the ultrasonic wave was therefore introduced into the solution by penetrating through the stainless steel bottom. A probe type sonicator with submersible plate (Nanjing Jiequan Microwave Development Co., Ltd.) provided the 20 kHz frequency ultrasound wave, and an input power of 100 W gave the absorbed power density of 0.14 W·mL−1. A glass beaker with interlayer cooling water for the 20 kHz process and a convolute cooling tube (same material with the basin) submerged in the tested water for the 200 and 400 kHz processes were used to keep the solution temperature constant at 25 ± 2 °C in order to minimize direct thermal activation of PMS. The cooling system is always at the same position for the different experiments. DMP and BPA solutions at volumes of 250 mL were used throughout including calorimetric measurement, and the initial concentrations of target compounds were 0.01 mM unless otherwise stated. The initial solution pH was not artificially adjusted or buffered, which was mainly determined by the concentration of PMS. Exceptionally, for the experiments examining the pH influence, exact volumes of sulfuric acid or sodium hydroxide were added with predetermined amount by several trials without the presence of organic compounds in order to obtain different pH levels. Experiments were all carried out under air-equilibrated conditions without artificial aeration. All the selected tests were duplicated with an observed deviation of less than 5%. 2.3. Analytical methods The concentrations of DMP and BPA were quantified by high performance liquid chromatography (HPLC) (Dionex Ultimate 3000). The mobile phase of methanol/0.1% phosphoric acid water solution (70/30, vol/vol) and a Thermo C18 column (5 μm particle size, 250 × 4.6 mm) were used. The flow rate of mobile phase was 1.0 mL min−1. The detection wavelengths for DMP and BPA were 230 nm and 225 nm, respectively. Coumarin was used as a probe for %OH quantification (Scheme S1) [18]. The %OH capture product, 7-hydroxycoumarin, was quantified by fluorescent spectrometry (PerkinElmer LS 55). Fluorescence emission was detected at 456 nm under excitation at 332 nm. Electron spin resonance (ESR) signals were detected by Bruker EMX10/12 equipment with X-band field sweep. The settings were: center

2. Materials and methods 2.1. Chemicals DMP (99.6%), BPA (≥99%) and Oxone (KHSO5·0.5KHSO4·0.5K2SO4) were all purchased from Sigma Aldrich Inc., USA. Table 1 shows the chemical structures and property parameters of DMP and BPA. Coumarin (> 99%) was obtained from Energy Chemical. The 5, 5-Dimethyl-1-pyrroline N-oxide (DMPO) (≥97%) was Table 1 Detailed information of the selected target pollutants. Name

Bisphenol A (BPA) Dimethyl Phthalate (DMP)

Chemical Structure

HO

OH O

CAS No.

Formula

Molecular Weight (mg/L)

LogKow

Water Solubility at 25 °C (mg/L)

pKa

Vapor Pressure at 25 °C (mm Hg)

80-05-7

C15H16O2

228.29

3.32

300

9.6

4.0 × 10−8

131-11-3

C10H10O4

194.18

1.60

4000

–

3.08 × 10−3

O O O

2

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field = 3480 G, sweep width = 200 G, micro frequency = 9.736 GHz, power = 19.92 mW. DMPO (70 mM) was used as the trapping agent, and all samples were tested after 10 min reaction. Total organic carbon (TOC) was quantified by Analytik Jena multi N/C 3100 TOC. Intermediates identification was performed using Agilent 6540 Q-TOF LC/ MS. The mobile phase was a mixture of methanol (A) and water (B) with a gradient programme described as below. Constant ratio of 3%A:97%B was kept for the initial 2 min, which was linearly increased to 80%A:20%B within 20 min. The ratio kept constant at 80%A:20%B for another 5 min, which turned back to the initial composition and maintained for 5 min before the next inject. The flow rate was 0.2 mL min−1 for LC/MS analysis. Both positive and negative modes were applied for intermediates identification.

pyrolysis, which was confirmed by the complete inhibition of BPA degradation when adding 100 mM tert-butanol (TBA) as the scavenger of %OH (discussed in 3.3.2). In addition, the vapor pressure of BPA (see Table 1) is extremely low, and therefore the probability of BPA molecules evaporating into the cavitation bubble inside is negligible. Thus, BPA degradation is assumed to be ascribed to %OH oxidation at the gasliquid interface of the cavitation bubbles. Overall, the addition of PMS ranging from 0.1 mM to 2.0 mM all increased BPA degradation compared to sonolysis alone. The sonodegradation of BPA with and without the presence of PMS was in good agreement with the pseudo first-order kinetics. Since direct oxidation of BPA by PMS at 25 °C was precluded by control experiment (data not shown), the promoting effect induced by PMS was due to the synergy between ultrasound and PMS. It was more clearly shown in Fig. 1(d) that the enhancement by PMS was concentration-dependent, and an optimum PMS concentration ([PMS]opm) was obtained, implying the dual role of PMS in improving BPA sonolysis. However, the experimentally determined [PMS]opm varied for different ultrasonic systems, which increased with the increase of ultrasonic frequency. The 400 kHz process demonstrating the best BPA degradation capability showed the highest [PMS]opm (1.2 mM), while the 20 kHz process demonstrating the lowest BPA removal efficiency showed the lowest [PMS]opm (0.35 mM). Additionally, the increase of rate constant by adding optimum amount of PMS in comparison with sonolysis-only (i.e. Δk = k opm − kUS) also followed the order of 400 kHz (Δk = 0.077 min−1) > 200 kHz −1 (Δk = 0.034 min ) > 20 kHz (Δk = 0.011 min−1). This variability may arise from the different properties of different ultrasonic systems. As reported previously [7,21], the critical size of the cavitation bubbles have shown to be inversely proportional to the ultrasonic frequency. The collapse of larger cavitation bubbles in 20 kHz process is more turbulent which can disturb the heterogeneous distribution of hydrophobic BPA molecules and hydrophilic PMS salts, i.e., more easily mixing of different solutes. In comparison, the 400 kHz ultrasonic process is characterized as having a well-defined heterogeneous environment, and relatively small gas bubbles with mild collapse allow the respective distribution of different solutes according to their hydrophobicity. Based on the above findings, PMS salt is more probably located in the bulk solution during the formation of cavitation bubbles in higher frequency ultrasonic environment, especially for the 400 kHz process. Therefore, higher PMS concentration is needed to push itself to the interface region to participate any reactions. It was also assumed that, possible activation of PMS most likely occurred near the liquid sheath surrounding the cavitation bubbles.

3. Results and discussion 3.1. Effect of PMS concentration on BPA degradation The BPA degradation performance at different ultrasonic frequencies (20, 200, 400 kHz) with and without the addition of PMS was investigated. The 20 kHz frequency is the most widely applied and investigated low-frequency in ultrasound-related reactions. Both 200 kHz and 400 kHz are the often studied high-frequency, and the 400 kHz frequency has been found to show good performance from previous studies done by others (e.g., [19]) and also by our own (e.g., [20]). The normalized remaining concentration of BPA ([BPA]/[BPA]0) versus reaction time is given in Fig. 1(a)–(c), and the overall profiles of relationship between pseudo first-order rate constants (k) and the initial concentration of PMS are shown in Fig. 1(d). It was found that, BPA could be degraded by sonolysis (without PMS) at all three involved frequencies (20, 200 and 400 kHz). However, higher frequency (200 and 400 kHz) was found more effective in decomposing BPA than the lower frequency condition (20 kHz). The rate constants for direct sonodegradation of BPA were determined as 0.004, 0.051, 0.063 min−1 for 20, 200 and 400 kHz processes, respectively. Since the power densities of three systems were maintained close, the difference in efficiency was primarily caused by frequency. Decomposition of BPA in the ultrasonic processes was found mainly caused by %OH oxidation rather than

3.2. Effect of PMS concentration on DMP degradation Another model pollutant (DMP) with much smaller logKow (see Table 1) was used to further elucidate the effects of PMS concentration (Fig. 2). It was found that, DMP degradation by sonolysis alone turned much slower in all three ultrasonic processes than BPA. The rate constants for direct sono-degradation of DMP were determined as 0.003, 0.023, 0.030 min−1 for 20, 200 and 400 kHz processes, respectively. This result was expectable since organics with lower hydrophobicity stayed further from the gas–liquid interface, which brought less opportunity to contact the concentrated radicals near the interface [6]. Fig. S1 showed kUS of both BPA and DMP in different ultrasonic processes, which indicated clearly that the removal efficiency of organic compounds in higher-frequency system showed stronger dependency on compound’s hydrophobicity. This also confirmed the aforementioned viewpoint that better-defined heterogeneous local environment was formed in higher frequency ultrasound. Overall, the effect of PMS on DMP degradation was similar to that on BPA. However, it was found that the [PMS]opm was obtained at much higher level in each ultrasonic system compared with that shown in Fig. 1(d), especially for the 200 and 400 kHz processes. For the

Fig. 1. Effect of initial PMS concentration on BPA degradation in the ultrasonic processes at different frequencies, (a) 20 kHz, (b) 200 kHz, (c) 400 kHz, (d) variation of pseudo first order rate constants along with the initial concentration of PMS ([BPA]0 = 0.01 mM, pH0 = 4.8 ~ 3.0 for [PMS]0 ranging from 0.1 to 2.0 mM). 3

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(Fig. 2(d)). Because the 400 kHz ultrasonic process was most efficient in degrading DMP and it was also more susceptible to the addition of PMS, subsequent studies were concentrated on the 400 kHz process. HSO5− + ))) → %OH + SO4%− H2O

+ SO4%− → −

+

H

+ SO42− + %OH

%OH + HSO5 → SO5

%−

+ H2O

SO4%− + HSO5− → HSO4− + SO5%−

(1) (2) (3) (4)

3.3. Study of the combinative effects

3.3.2. Quenching experiments The contributing radicals were examined using ethanol (EtOH) and TBA as radical scavengers, and results are shown in Fig. 4. TBA is more effective in quenching %OH (k = 3.8 ~ 7.6 × 108 M−1s−1) and less sensitive to SO4%− (k = 4.0 ~ 9.1 × 105 M−1s−1). However, EtOH is capable of quenching both %OH (k = 1.2 ~ 2.8 × 109 M−1s−1) and SO4%− (k = 1.6 ~ 7.7 × 107 M−1s−1) [23,24]. In our previous study using transition metal to activate PMS [25], both %OH and SO4%− existed and contributed to the degradation of DMP. However, in the present study, it can be seen from Fig. 4 that, application of 5 mM scavenger in the US/PMS process, TBA and EtOH achieved similar inhibition effect. This suggested that, the dominant radical responsible for DMP degradation in US/PMS process was %OH, and SO4%− if existed played minor effect on DMP removal. According to the results shown in Fig. 3, there should be extra source of %OH production except for the dissociation of water molecules when PMS was present. Based on reaction (1), extra source of %OH and SO4%− produced by PMS dissociation should be stoichiometrically balanced, while the undetected contribution of SO4%− may be due to the hydrolysis of SO4%−. Wei et al. (2017) investigated the US/PDS system and proposed that, the yield of %OH was much higher compared to that of SO4%− most likely resulting from the accelerated hydrolysis of SO4%− (reaction (3)) at the warm (340 K) interfacial region of cavitation bubbles [26]. This may explain the results of the quenching experiments obtained in the present study. Control experiments were also conducted in US-only process. TBA and EtOH also demonstrated similar quenching abilities, while slightly higher inhibition ratio was achieved in US-only process than in US/PMS process, which likely reflected higher amount of %OH prevailing in the US/PMS process. When the concentration of TBA was increased to 100 mM, DMP degradation was completely inhibited in both US and US/PMS processes, further confirming the dominant contribution of % OH in both processes.

3.3.1. Quantification of %OH by chemical probe As reported [14], PMS could be activated by US cavitation activity to produce sulfate radicals, which may further abstract hydrogen atoms from H2O molecules to generate %OH (reactions (1) ~ 2). Since it was found that overdose of PMS was unproductive, the mechanism was examined. Coumarin (1 mM) was applied as a chemical probe for %OH and the formation kinetics of 7-hydroxycoumarin in three ultrasonic processes is shown in Fig. 3. For each process, the production of 7hydroxycoumarin is compared under conditions of US-only, US combined optimum concentration of PMS (see Fig. 2(d)) and US combined overdose of PMS. It was found that, the 400 kHz ultrasound demonstrated the fastest generation of 7-hydroxycoumarin, suggesting the strongest capability in producing %OH. The addition of PMS could promote the generation of %OH compared to US-only. Obviously, 7hydroxycoumarin was accumulated most rapidly when adding the optimum amount of PMS in all processes, but overdose of PMS could decrease the available quantity of %OH. These results explained well the dual role of PMS on the degradation of target pollutants. The overall effect of PMS was a combinative outcome by balancing between %OH provider and consumer (reactions (3) ~ 4) [22]. Since SO5%− was a less capable radical, DMP degradation was slowed down with excess PMS

3.3.3. ESR detection ESR experiments were performed to further identify the oxidation mechanisms of the US/PMS process. As shown in Fig. 5, no signals were identified by mixing PMS alone with DMPO (Fig. 5(a)), precluding any interference caused by PMS in ESR detection. Characteristic signals for DMPO-%OH (Fig. 5(b)) with the intensity pattern of 1:2:2:1 and hyperfine coupling constants of αN = αH = 14.9 G were detected after 10 min reaction in US-only process, suggesting %OH as the dominant radical species in the ultrasonic process. However, in the US/PMS process, it was found that at the initial phase (3 min), signals for DMPO%OH (Fig. 5(d)) were detected, but a seven-line spectra (αH = 4.0 G, αN = 7.25 G) in accordance with the signals of 5,5-dimethyl-2-pyrrolidone-N-oxyl (DMPOX) (Fig. 5(c)) was obtained when sampling after 10 min reaction. These implied that, %OH was the prevailing radical species initially in the US/PMS system, while as the reaction proceeded some oxidizing agent reacted with DMPO to form the DMPOX. The origin for DMPOX formation was relatively complex. Chen et al. (2017) found that the air plasma-produced aqueous 1O2 led to the formation of DMPOX [27]. Hypochlorous acid in the electrolyzed oxidizing system was also found to produce DMPOX. The formation of DMPOX often occurred at highly oxidative conditions, and the DMPOX formation in

Fig. 2. Effect of initial PMS concentration on the degradation of DMP in the ultrasonic processes at different frequencies, (a) 20 kHz, (b) 200 kHz, (c) 400 kHz, (d) variation of pseudo first order rate constants along with initial concentration of PMS ([DMP]0 = 0.01 mM, , pH0 = 4.8 ~ 2.8 for [PMS]0 ranging from 0.1 to 5.0 mM).

400 kHz process, there was a clear, systematic enhancement with [PMS]0 < 5.0 mM. The [PMS]opm of the 200 kHz process was 2.0 mM for degrading DMP compared to 0.5 mM for degrading BPA. These results indicated that increasing PMS concentration was more advantageous to improve DMP degradation. The mechanisms were that (discussed below), apart from sono-activation of PMS, the “salting out” effect was also a remarkable origin, which made more significant influence on improving the sono-degradation of those less hydrophobic organics, such as DMP, by pushing them closer to the gas-liquid interface region. It should be noted, adding PMS also introduced other potassium salts (i.e., KHSO4 and K2SO4) into the ultrasonic system, which would change the local environment during ultrasonic cavitation. Thus, DMP was chosen as the model pollutant for further studying the combinative mechanisms of ultrasound and PMS.

4

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Fig. 3. Production of 7-hydroxycoumarin (coumarin-%OH adduct) with or without the addition of PMS in 20 kHz (a), 200 kHz (b) and 400 kHz (c) processes ([coumarin]0 = 1 mM, [DMP]0 = 0.01 mM).

3.3.4. Effect of KHSO4 and K2SO4 The commercial product of Oxone also consists of two sulfates, i.e., KHSO4 and K2SO4, except PMS. Therefore, effect of these two salts on the sonolytic degradation of DMP was also evaluated in this study, and results are given in Fig. 6. It was found that, adding a mixture of 2.5 mM KHSO4 and 2.5 mM K2SO4 that was equal to the respective concentration provided by 5 mM PMS (i.e., 5 mM Oxone) also promoted DMP degradation even though KHSO5 was absent. DMP removal rate within 30 min increased from 56% to 69%, suggesting that the constituent salts of KHSO4 and K2SO4 also contributed to the improvement of DMP sonodegradation. This may be explained by the “salting out” effect on DMP molecules, whereby DMP molecules were pushed toward the cavitation bubble interfaces exposed to the high radical concentrations. Chen et al. (2012) found that the addition of Na2SO4 benefited the US(20 kHz)/PDS process with better mineralization performance, and inhibition of coalescence of cavitation bubbles was assumed to be the dominant reason [11]. However, in the present study, when BPA was tried as the model pollutant, the presence of the mixture (2.5 mM KHSO4 and 2.5 mM K2SO4) slowed down BPA sono-degradation instead with the k value decreasing from 0.063 min−1 to 0.059 min−1 (data not shown). The contrasting results obtained from DMP and BPA in the same ultrasonic process was likely due to their different hydrophobicity. The strong hydrophobic BPA was less influenced by the “salting out” effect since it was close enough to the interface region, while the less hydrophobic DMP was more easily influenced by the

Fig. 4. DMP degradation performance in the presence of different radical scavengers in US and US/PMS processes ([DMP]0 = 0.01 mM, 400 kHz ultrasound, [PMS]0 = 5.0 mM).

Fig. 5. ESR spectra detected in different processes ([DMPO] = 70 mM, [PMS]0 = 5.0 mM, 400 kHz ultrasound, [DMP]0 = 0.01 mM).

the US/PMS process was an indicator of a highly oxidative environment induced by the interaction between US and PMS during the cavitation process. Signal for SO4%− was not detected, however, in good agreement with the results obtained by quenching experiments, which may be because SO4%− was quickly transformed to other radicals (e.g., %OH) and escaped capture by DMPO.

Fig. 6. Effect of constituent salts in Oxone (KHSO4, K2SO4) on the sonolytic degradation of DMP (400 kHz ultrasound, [DMP]0 = 0.01 mM, pH0 = 2.8 ± 0.1). 5

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ionic concentration in the bulk solution. Additionally, the presence of salts can also increase the surface tension of the solution, making the formation and retention of bubbles more difficult [28], which may be the origin for the inhibition of BPA degradation. The inhibitive results obtained from BPA also implied that preventing bubble coalescence by sulfate was not dominant in the 400 kHz process, while the “salting out” effect and surface tension may be more important for the high frequency ultrasonic process. Single sulfate with the same ionic strength as 5 mM of Oxone (i.e., 6.67 mM KHSO4 or 6.67 mM K2SO4) was applied to the US system to further examine the effect on DMP sono-degradation (Fig. 6). Similar improvement was achieved by adding KHSO4 or K2SO4 with the same ionic strength, and DMP degradation proceeded even faster initially compared to US/PMS(5 mM) process but was slowed afterwards as the residue [DMP] declined. These results implied that both KHSO4 and K2SO4 play the similar role in enhancing DMP degradation, most likely via increasing the ionic strength, which was more effective for improving the degradation of DMP at high concentrations. The observed different kinetics of DMP degradation obtained by adding KHSO4 (or K2SO4) and Oxone was a good indication of the different mechanisms induced by constituent salts and the active PMS. Fig. 8. Effect of initial solution pH on DMP degradation in the process of US/ PMS (400 kHz ultrasound, [PMS]0 = 5.0 mM, [DMP]0 = 0.01 mM).

3.4. Effect of initial DMP concentration

combination of US and PMS showed significant synergistic effect for treating organic compound at low concentration.

Since DMP degradation and PMS activation were all assumed to occur near the interface region during the cavitation process, increasing DMP concentration may strengthen the competition between DMP and PMS for the active sites. Therefore, experiments were carried out to examine the effect of initial DMP concentration on its degradation in US and US + PMS processes, respectively (see Fig. 7). In both processes, the DMP removal ratio ([DMP]/[DMP]0) decreased as [DMP]0 was raised with the first-order rate constant inversely related to the initial compound concentration. It can be seen from the inset of Fig. 7(b) that, for both processes, linear correlation was found between Lnk and Ln [DMP]0, and the equations were best fitted as Lnk = −4.89 to 0.41 × Ln[DMP]0 (R2 = 0.9941) and Lnk = −4.53 to 0.21 × Ln [DMP]0 (R2 = 0.9724) for US + PMS and US processes, respectively. For single-US process, similar relationship has been reported and analyzed by previous studies (e.g., [20]), which is mainly due to the less opportunity for each DMP molecule to contact with %OH when [DMP]0 is higher. However, it was found that the efficiency of US + PMS varied more widely as DMP concentration changed, and more significant difference was obtained between the two processes when [DMP]0 was lower. The ratio of k(US+PMS)/kUS gradually dropped from 2.09 to 1.14 when [DMP]0 was varied from 0.005 mM to 0.1 mM as shown in Fig. 7(b). This result is in good agreement with the hypothesis that more DMP molecules compete for the interface region so that PMS is less effective to be activated. Hence, it was concluded that the

3.5. Effect of initial solution pH on DMP degradation in US/PMS process Activation of PMS by the most efficient transition metal Co2+ was well known for its wide functional pH range of 3 ~ 8 [25,29], while the applicable pH range for US/PMS process was rarely reported. Herein, the influence of initial solution pH on DMP degradation was examined in the range of pH 2.5 ~ 9.3; results are presented in Fig. 8. It can be seen that the rate constant was not substantially influenced around pH 5 ~ 8, while higher removal rate was obtained at pH < 3 and evident deceleration was observed at pH > 9. Since DMP was a non-dissociating compound, the influence of pH was most likely related to the activity of radicals and the local environment of ultrasonic cavitation. The most dominant %OH could be scavenged by OH− (k = 1.3 × 1010 M−1s−1 [30]) when pH > 9 due to the comparable amount of OH− (> 0.01 mM) to the amount of DMP (0.01 mM). In addition, the oxidation potential of %OH was decreased at higher pH [31], and the HSO5− also shows minimum stability to be dissociated to SO52− near pH 9 (pKa2 = 9.4 [32]). Therefore, the efficiency of US/ PMS process decreased remarkably at pH > 9. However, the enhancement of DMP degradation obtained at more acidic pH condition was not observed in US-only process as reported by our previous study

Fig. 7. Effect of initial DMP concentration on DMP degradation in the processes of US and US + PMS (400 kHz ultrasound, [PMS]0 = 5.0 mM, [DMP]0 = 0.01 mM). 6

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DMP but also mono-, di-hydroxylated DMP were hardly detected in US/ PMS process. It can be seen from Fig. S2 that, intermediates with retention times in the range of 5 ~ 8 min accumulated significantly for the US process. Unlike the US process, accumulated intermediates in US/ PMS process demonstrated much shorter retention time (< 4 min), which was in good agreement with the better mineralization efficiency that primary intermediates could be more easily transformed to secondary intermediates with stronger polarity. 4. Conclusions This study demonstrated that synergistic effects were obtained by combining US with PMS. The mechanisms were found to be the activation of PMS by US, and the constituent salts in Oxone made contribution. Hydroxyl radical (%OH) rather than SO4%− was responsible for DMP degradation in US/PMS process. The ultrasonic frequency, PMS concentration and the hydrophobicity and concentration of the target compound all could influence the degree of synergy. More acidic condition was beneficial to DMP degradation in the US/PMS process. DMP degradation was dominated by two pathways, i.e., the %OH addition to the aromatic ring and hydrogen atom transfer by %OH. Stronger mineralization capability was also obtained with the presence of PMS.

Fig. 9. Degradation pathways of DMP degradation in both the US process and the US/PMS combined process.

[20]. Thus, such enhancement is believed to be related to PMS. The cavity bubbles were assumed to be negatively charged [33], the presence of high concentration of H+ may decrease the negative electric potential at the surface of cavitation bubbles so that to lower the repulsive forces to PMS. Therefore, PMS was more easily activated at strong acidic conditions so that to facilitate DMP degradation.

3.6. DMP degradation pathway and mineralization performance

Acknowledgements

The ultrasonic degradation of DMP has been reported [20] and mineralization was not the advantage of ultrasonic process since the degradation intermediates with more hydrophilic characteristics may have difficulties in contacting %OH in the heterogeneous local environment during cavitation. Herein, both US and US/PMS(5 mM) processes were evaluated to assess their mineralization capabilities, and DMP with concentration of 0.1 mM was applied to determine its degradation pathway. Identification of DMP degradation intermediates by LC/MS was carried out. The changes of HPLC chromatograms for DMP and its degradation intermediates with shorter retention time than DMP (13.5 min) in both processes are given in Fig. S2. The extracted ion chromatography of DMP and detected intermediates are also given in Fig. S2. Even though as discussed in Section 3.4, at higher DMP concentrations, the processes of US and US/PMS demonstrated insignificant difference of efficiency in degrading DMP, it can be seen clearly that much less product peaks were observed in US/PMS process compared to US-only. The variation of TOC after 4 h treatment (see Fig. S3) showed 26% mineralization (3.18 mg/L) of DMP in US/PMS process in comparison with only 3% (0.35 mg/L) obtained in US process. These results suggested that the combination of US and PMS was more efficient in DMP mineralization. Wei et al. (2017) [26] proposed a reactive warm interfacial zone with much bigger volume than the bubble-water interface for the activation of PDS during the cavitation. This may also explain the origin for better mineralization efficiency of the combination of US and PMS. The broader reactive region can provide more opportunities for those intermediates with higher polarity and less hydrophobicity to contact with the generated radicals, so that to improve the mineralization capability. Based on LC/MS analysis, the identified intermediates of DMP sonolysis with and without the presence of PMS were almost the same but with different intensities. The degradation pathways of DMP are proposed in Fig. 9. Generally, two mechanisms are involved for DMP degradation, namely %OH addition to the aromatic ring and hydrogen atom transfer by %OH. The same pathways were also reported when using other %OH-dominated AOTs for DMP degradation [34] and also for the degradation of other members of phthalate acid esters (e.g., di-nbutyl phthalate [35]). By comparing the temporal variation of the primary intermediates (see Fig. S2), it was found that further degradation of the primary products (e.g., mono-, di-hydroxylated DMP) proceeded faster in the presence of PMS. After 4 h reaction, not only

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