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Degradation of endocrine disruptor bisphenol A by ultrasound-assisted electrochemical oxidation in water
Accepted Manuscript Degradation of endocrine disruptor Bisphenol A by ultrasound-assisted electrochemical oxidation in water Matz Dietrich, Marcus Franke, Michael Stelter, Patrick Braeutigam PII: DOI: Reference:
S1350-4177(17)30260-2 http://dx.doi.org/10.1016/j.ultsonch.2017.05.038 ULTSON 3719
To appear in:
Ultrasonics Sonochemistry
Received Date: Revised Date: Accepted Date:
10 November 2016 18 May 2017 29 May 2017
Please cite this article as: M. Dietrich, M. Franke, M. Stelter, P. Braeutigam, Degradation of endocrine disruptor Bisphenol A by ultrasound-assisted electrochemical oxidation in water, Ultrasonics Sonochemistry (2017), doi: http://dx.doi.org/10.1016/j.ultsonch.2017.05.038
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Degradation of endocrine disruptor Bisphenol A by ultrasound-assisted electrochemical oxidation in water Dedicated to Professor Bernd Ondruschka on the occasion of his 70th birthday
Matz Dietricha, Marcus Frankea, Michael Steltera,b, Patrick Braeutigama,* a
b
Center for Energy and Environmental Chemistry, Institute of Technical Chemistry and Environmental Chemistry, Friedrich Schiller University Jena, Philosophenweg 7a, 07743 Jena, Germany
Fraunhofer IKTS, Fraunhofer Institute for Ceramic Technologies and Systems, Michael-Faraday-Straße 1, 07629 Hermsdorf, Germany
Abstract Micropollutants are becoming an increasing problem for the environment and wastewater treatment. One example is Bisphenol A (BPA), an endocrinic disruptor, which is widely used in plastic production. Due to its endocrine disrupting effects on aquatic (micro-)organisms and its ubiquity, in surface- and wastewater alike, adequate treatment techniques are necessary. In this study, the degradation of BPA by a sonoelectrochemical hybrid system was investigated, using a low frequency (24 kHz) ultrasound horn and two boron doped diamond electrodes. It was found that by the combination of the individual processes, i.e. ultrasound and electrochemical oxidation, more than 90 % of BPA could be removed within 30 min at an initial concentration of 1 mg L-1. Moreover, synergistic effects were discovered and a considerable improvement compared to the individual processes could be achieved by using a potential of 5 V , whereas synergistic effects were absent at a potential of 10 V. This study provides investigation of ultrasound amplitude, potential and electrode positioning on BPA degradation. The reaction was found to follow pseudo first order kinetics with a rate constant of 0.089 min-1. Samples were analysed by high pressure liquid chromatography (HPLC) using a diode array detector. Moreover, the presence and distribution of hydroxyl radicals within the reactor was visualized by using sonochemiluminescence.
Keywords: sonoelectrochemistry, Bisphenol A, advanced oxidation process, synergistic effect, sonochemiluminescence
1. Introduction In recent years, endocrine disrupting substances in natural waters have become an emerging issue. Numerous investigations have been carried out on the occurrence, distribution and fate of these substances [1-4]. Endocrine disruptors have been found in the aquatic environment around the world, and were detected in surface water, groundwater and even drinking water [5, 6]. In 2008, endocrine disrupting substances were first legally regarded as a potential risk for the aquatic environment in an amendment of the EU water framework directive which concerns water policy in Europe [7, 8]. This *
Corresponding author. Tel.: +49 3641 948458; fax: +49 3641 948402. E-mail address: [email protected] (Patrick Braeutigam).
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directive now includes a watch list for substances, which require monitoring and estimation of their environmental risks to allow their classification. If a substance presents a significant risk to the aquatic environment it is categorized as priority substance. The first watch list contains Bisphenol A (BPA), which is a diphenylmethane derivative with hormone-like properties and a worldwide annually consumption of more than 3.8 million tons [5]. BPA is used predominantly in plastic production, especially for the synthesis of polycarbonate, which is largely used in electronic components, constructing materials, phones and water bottles [5]. In the human organism, BPA possesses hormone-like effects by bounding on specific hormone receptors such as estrogen-, androgen- and hydrocarbon receptors, which leads to a restriction or complete inhibition of the gene-expression [9]. Possible phenotypic consequences are disruptions of the central nervous and sexual system. The entry of BPA into the environment takes place in different ways. One is the diffuse entry of BPA containing materials in the environment and another via sewage water treatment plants (SWTPs). In groundwater, BPA concentrations of 1.47 µg L-1 could be measured, in sediments up to 24 µg kg-1 [5]. Numerous investigations concerning the effect of BPA on algae, slugs, fish and invertebrates have shown, that even low doses (ng L-1 - µg L-1) causes harmful dysplasia [10-12]. The lowest known effect concentration of 8 ng L-1 causes superfeminization in Ampullariidae [13]. Based on this knowledge the predicted no effect concentration (PNEC) of 0.8 ng L-1 was determined. BPA could not be completely removed in classical SWTPs [5, 14]. According to different sludge types and biological treatment processes, BPA concentrations in SWTPs effluents in Germany range from 0.06 µg L-1 to 3 µg L-1 [5] and in industrial SWTPs the BPA concentration can rise up to 118 µg L-1 [6]. End-of-pipe strategies for BPA removal can be found in literature and divided in technologies based on processes without chemical transformation, like adsorption and filtration [15] and degradation processes based on (bio-)chemical reactions, mostly induced by reactive oxygen species (ROS). The latter include biological [16-18], photocatalytical [19-24], photoelectrochemical [25-27], electrochemical [28-30] and other advanced oxidation processes (AOPs), such as Fenton [31], photoFenton [32], ozonation [33] and ultrasound-induced degradation processes [34-36]. AOPs are defined by the in-situ generation of ROS, like hydroxyl or perhydroxyl radicals, which have a high oxidation potential and therefore show a high reactivity with organic pollutants [37]. In order to minimize the environmental impact, the most promising of the above-mentioned methods are those which do not rely on additional chemicals like sonolysis and electrochemical treatment. Sonolysis uses ultrasound with frequencies between 20 kHz and 2 MHz [38]. In this frequency range acoustic cavitation occurs, i.e. microbubbles are generated and expanded within the ultrasonic field until they collapse at a critical size, leading to hotspots with temperatures up to 5000 K and pressures of about 1000 bar [39-41]. At these extreme conditions, homolytic bond cleavage of water molecules takes place accompanied by the generation of hydroxyl and hydrogen radicals (equation 1) [40]. (1) The electrochemical formation of hydroxyl radicals takes place at the anode by an electron transfer (equation 2) and is associated with the formation of hydrogen peroxide, ozone and oxygen, as side reactions [42]. However, the main drawback in this process is oxygen evolution. In order to avoid this undesirable side reaction, the anode material should possess a high overpotential for O2 formation. For this purpose, materials like boron-doped diamond (BDD) have been used, having an oxygen overpotential up to 2.8 V, which is the potential for hydroxyl radical formation. Thus, BDD electrodes have a great potential for electrochemical water treatment [43]. 2
(2) The hydroxyl radicals that are generated by sonochemical and electrochemical processes can oxidize organic contaminants. Next to the formation of ROS, by combining ultrasonic irradiation and electrochemical oxidation in one reactor, synergistic effects could be expected [37]. Moreover, the physical effects of ultrasound such as streaming, microjets and shockwaves based on acoustic cavitation (AC) contribute to an increased mass transfer, a reduced electrode passivation and a minimization of gas bubble accumulation at the electrodes [44, 45]. In this study, a systematic investigation of an efficient reactor arrangement for BPA degradation in terms of interaction between ultrasound and electrochemistry is provided. Influencing factors on both individual processes and their interactions were identified and optimized. Experiments were carried out at low frequency ultrasound (24 kHz) since it is well known that physical effects of cavitation like electrode surface cleaning and improvement of mass transport are predominant at low frequencies [46, 47]. The fact that the initial BPA concentration of 1 mg L-1 used in this investigation is higher than in SWTPs mentioned above (10 times higher than in industrial SWTPs, 300 times higher than in normal SWTPs) is due to limitations in instrumental analysis. To the best of our knowledge, there are no studies carried out on sonoelectrochemical degradation of BPA. This work will focus on the investigation of parameters such as ultrasound amplitude, potential, positioning of sonotrode and electrodes, and the resulting synergistic effects that arise during mutual usage. Furthermore, kinetic studies were conducted. The quantitative investigation of reactor configuration and parameters is accompanied by the visualization of hydroxyl radical distribution within the reactor by the use of sonochemiluminescence of luminol in alkaline solution [48].
2. Experimental 2.1. Chemicals All chemicals were used as received without any further purification. Bisphenol A (4-[2-(4Hydroxyphenyl)propane-2-yl]phenol, 97.0 %) was purchased from Alfa Aesar and luminol (5-Amino2,3-dihydro-1,4-phthalazinedione, >95%) from AppliChem. Sodium sulphate (99.7%), methanol (99.9%), acetonitrile, hexane and acetone (all HPLC-grade) were supplied by VWR. Deionized water (3 μS cm-1) was used for the preparation of BPA-solutions and ultrapure water (0.056 μS cm-1) for sample preparation and analysis. 2.2.
Setup
The experiments were performed in a double-wall cylindrical glass reactor (500 mL) at a constant temperature of 20 ± 2 °C, controlled by a thermostat (FP50-ME, Julabo, Seelbach, Germany). An overview of the setup and the position of the electrodes and the sonotrode is presented in Figure 1. The low-frequency ultrasound was introduced through the cap of the glass reactor by an ultrasound generator (UP200S, 24 kHz, Hielscher Ultrasonics, Teltow, Germany) connected with a sonotrode (S14L2D, titanium, length: 200 mm, diameter: 14 mm, Hielscher Ultrasonics, Teltow, Germany). Two BDD-electrodes (Diachem®, 5 µm diamond layer on niobium, expanded metal grid, 50 mm x 30 mm x 1.5 mm, Condias, Itzehoe, Germany) were placed in the reactor trough the cap allowing the adjustment of immersion depth (ID) in the range of 3 – 8 cm. The electrode distance (ED) could be varied by electrode clamps between 2 – 3 cm. The electrodes were connected to a universal power supply (EA-3050B, EA-Elektro-Automatik, Viersen, Germany) working at a constant direct current 3
potential of 0 – 10 V. An energy monitor (Voltcraft Energie Monitor 3000, Conrad Electronic SE, Hirschau, Germany, accuracy: ±1%) was used to record the energy input of the setup.
Figure 1: Setup of the sonoelectrochemical reactor (double-wall glass reactor, variable electrode distance ED, immersion depth of the sonotrode ID, reaction volume: 500 mL, BDD-electrodes).
2.3. Procedure A stock solution of 100 mg L-1 BPA was prepared by dissolving 0.050 g BPA in 500 mL deionized water. For degradation experiments, 5 mL of the stock solution and 4.26 g of the electrolyte Na2SO4 were diluted in deionized water to give a volume of 500 mL BPA solution (1 mg L-1). The chosen electrolyte concentration of 30 mM relates to the conductivity range of real waste water (4.6 mS cm-1) [49]. The solution was filled in the glass reactor and was subjected to ultrasound irradiation and/or a constant potential was applied at the electrodes for a given reaction time. Samples (1.2 mL) were taken in 5 min intervals and measured by HPLC. For the investigation of the ultrasound (AC) as single method, two different ultrasound amplitudes, i.e. 25 µm and 125 µm were chosen and compared The amplitude is defined as the elongation of the sonotrode tip (peak-to-peak) and given in µm and For relation between amplitude and ultrasound power see fig. 3. Further investigations on immersion depth of the ultrasound horn (ID) and the interdependence of ultrasound and electrodes (without applied potential) on BPA degradation were undertaken. For the electrochemical (EC) degradation as single method, different electrode distances (2, 2.5, 3 cm) and potentials (5, 10 V) were examined. Finally, both systems (sonochemical and electrochemical system) were combined in a hybrid system (AC/EC) , the parameters potential (U), electrode distance (ED), immersion depth of the electrodes (ID) and ultrasound amplitude (UA) were evaluated and calculations concerning synergistic effects were conducted. Kinetics were studied based 4
on the experimental data, where the rate constants were determined by measured BPA concentrations during degradation experiments.
2.4. Analytical methods HPLC measurements were carried out on a HPLC system (LC-2000 series, CD-2060 Plus column oven, PU-2080 Plus eluent pumps, AS 2055 Plus autosampler, Jasco, Gross-Umstadt, Germany). The separation of BPA (injection volume: 50 µl) was performed on a Kromasil C18 (250 mm x 4.6 mm x 5 µm) column, at a flow rate of 1.5 ml min-1 (mobile phase (ultrapure water / methanol 65/35 (v/v)) and a column oven temperature of 30 °C. For the detection a diode array detector (MD-2010 Plus multi-wavelength detector, Jasco, Gross-Umstadt, Germany) was used working at 276 nm (BPA (6.4 min). A chromatogram of bisphenol A is shown in Figure 2. The integrated peak areas correlate with the BPA concentration. The quantitative analysis was done by external calibration with standards. Each analysis was repeated three times to ensure reliable results. The limit of quantification (LOQ) was found to be 34.00 µg L-1.
Figure 2: Example of chromatogram of Bisphenol A (c(BPA): 5 mg L-1)
2.5. Visualization of reaction zones The spatial distribution of hydroxyl radicals within the reactor was visualized by the use of chemiluminescence of luminol in alkaline solution. The radicals oxidize the luminol leading to an excited state, which further undergoes a relaxation emitting blue light at 430 nm [48]. The luminescence can be observed and is associated with the formation of an aminophthalat anion. For the experiments, 500 mL of a luminol alkaline solution (1.0 g L-1 luminol, 5.0 g L-1 Na2CO3) was filled in the glass reactor and ultrasound and/or a potential was applied. The sonochemiluminescence was captured in darkness with an SLR camera (Canon EOS 1100D, EF-S 18-55 mm IS II lens) at an exposure time of 119 s (ISO 800, 55 mm, F/5.6). The distance between reactor and lens was 46 cm. It should be considered that the spatial distribution of hydroxyl radicals visualized in the luminol set-up may be deviate from the distribution in the degradation experiments, as chemical factors such as pH, conductivity, chemical composition may affect cavitation activity [50].
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3. Results and discussion In this section, the results of Bisphenol A degradation in a sonochemical (sections 3.1 – 3.3), electrochemical (sections 3.4 and 3.5) and sonoelectrochemical (sections 3.6 and 3.7) reaction system are presented, discussed and compared with literature data (section 3.8).
3.1.
Effect of ultrasound amplitude on the sonochemical BPA degradation
To enable the comparison between the individual methods (AC and EC) and their combination (AC/EC) and to generate data for synergy calculations, the data for the single methods are measured. For investigations concerning the influence of sonochemical BPA degradation at 24 kHz, two different ultrasound amplitudes were considered, i.e. 25 µm and 125 µm, in the absence of the two BDD electrodes. The results are displayed in Figure 3.
Figure 3: Influence of ultrasound power input on the sonochemical BPA degradation (reactor without electrodes, c(BPA): 1 mg L-1, ultrasound amplitude: 25 µm and 125 µm, n(Na 2SO4): 30 mmol, 500 mL.
BPA degradation increased at higher ultrasound amplitude. After 30 min ultrasonic irradiation, 38% BPA could be removed at an amplitude of 25 µm, whereas, 61% BPA by using an ultrasound amplitude of 125 µm. This could be explained by an increasing hydroxyl radical production at higher amplitudes, as already described elsewhere for the sonochemical method using volatile and nonvolatile compounds [36, 51]. The results show that, even at low ultrasound frequencies, a considerable degradation degree for organic pollutants could be obtained.
3.2. Effect of sonotrode immersion depth on the sonochemical BPA degradation Ultrasonic wave propagation, as well as cavitation field distribution, substantially affected by the geometry and material of the sonochemical reactor, due to reflections, interference and coupling between the reaction liquid and the cooling liquid [52], to name only a few. The dependence of the 6
acoustic field propagation and transducer-electrode resp. electrode wall gaps was shown by I. Tudela et al. [53]. Therefore, an examination of the relative position of the sonotrode and their influence on the power consumption, and hence on the BPA degradation is necessary. By varying the immersion depth of the sonotrode, the BPA decay can be altered, as shown in Figure 4.
Figure 4: Influence of sonotrode immersion depth on the sonochemical BPA degradation (reactor without electrodes, c(BPA): 1 mg L-1, ultrasound amplitude: black - 25 µm, grey - 125 µm, n(Na2SO4): 30 mmol, 500 mL, reaction time t = 30 min. Power data above columns: electrical power consumption.
By reducing the immersion depth from 8.1 cm to 3.1 cm (see Figure 4), the BPA degradation decreases from 61% to 29% (125 µm) and 32% to 22% (25 µm), respectively. A possible explanation for that result are the vibration properties of the sonotrode, which include longitudinal and thickness oscillations. By decreasing the immersion depth of the sonotrode, the longitudinal transmission to the liquid still remains, whereas the transversal part diminishes gradually, because the active oscillating area of the sonotrode decreases. Since the amplitude is kept constant, it follows that the power consumption decreases (see Figure 4). With increasing ultrasound amplitude the transversal oscillating part increases as well as shown in Figure 5 where the visualization of hydroxyl radicals via luminol sonochemiluminescence is pictured. The same observation was made by Klima et al. [54].
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Figure 5: Visualization of longitudinal and thickness oscillations via chemiluminescence of luminol (left: ultrasound amplitude 25 µm, right: 125 µm, luminol: 1 g L-1, Na2CO3: 5.0 g L-1, 500 mL, exposure time: 119 s, ISO 800, focus: 55 mm, focal ratio: F/5.6; graph. White lines indicate the sonotrode and the reactor walls (not true to scale).
Hence, the effect of different immersion depths on BPA degradation is higher with increasing amplitude, which can be seen in Figure 5. The luminescence at an amplitude of 25 µm is much smaller compared to an amplitude of 125 µm (Figure 5),
3.3. Effect of electrode distance on the sonochemical BPA degradation The formation of cavitation events can be influenced by the sound field during sonication. The insertion of electrodes in the reactor could have a significant influence on the sound field and cavitation field distribution by means of interruption of ultrasound propagation (see Figures 4 and 5). Therefore, three different electrode distances were evaluated (2, 2.5, 3 cm) without applied potential and compared with the system without inserted electrodes (see section 3.1). Figure 6 shows the BPA degradation for 25 µm and 125 µm in dependence of the electrode distance.
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Figure 6: Influence of electrode distance on the sonochemical BPA degradation (ED: 2, 2.5, 3 cm and without electrodes, c(BPA): 1 mg L-1, ultrasound amplitude: 25 µm (triangles) and 125 µm (squares), n(Na2SO4): 30 mmol, 500 mL).
A considerably much faster BPA decomposition could be achieved by increasing the electrode distance from 2 cm to 3 cm. After 30 min ultrasound irradiation (25 µm) the degradation rises from 19% (ED = 2 cm) over 35% (ED = 2.5 cm) up to 41% (ED = 3 cm). At 125 µm ultrasound amplitude after 30 min irradiation, a BPA degradation of 71% could be obtained. In comparison with the BPA degradation degrees obtained without electrode presence (see Figure 2), the attendance of electrodes promotes the BPA degradation in case of 125 µm (60% without electrodes, 71% ED = 3 cm)), whereas the difference at 25 µm is marginal (38% without electrodes, 41% ED = 3 cm). With a decreasing electrode distance an increasing interaction with the electrode surface could occur (reflection / dispersion / absorption), leading to losses and/or interference (enhancement / cancellation) [55]. Hence, the reactor design as well as the position of electrodes have a substantial influence on the degradation efficiency and should be taken into account [56]. These experimental results support the FEM simulations made by I. Tudela et al. showing that with decreasing transducer-electrode gap the pressure amplitude increases and therefore optimizes mechanical phenomena on the electrode surface [53]. Adjustments with respect to the sonotrode geometry, power input, ultrasound amplitude and electrode size and design should be considered.
3.4.
Effect of applied potential on the electrochemical BPA degradation
Before the consideration of the synergy between the sonochemicaland electrochemical BPA degradation the individual potentiostatic electrochemical degradation was investigated. For this evaluation six different potentials (2, 3.5, 4, 5, 7.2 and 10 V) were considered. As shown in Figure 6 the BPA degradation is highly improved by increasing the applied potential at a constant electrode distance.
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Figure 7: Influence of the applied potential on the electrochemical BPA degradation (ED: 2.5 cm, c(BPA): 1 mg L-1, n(Na2SO4): 30 mmol, reaction time t = 5 min, 500 mL).
After 5 min reaction time with a potential of 2 V, 12% BPA could be degraded, whereas with an applied potential of 10 V the degree of degradation rises up to 93% with an ED of 2.5 cm. This is due to the fact, that higher potentials lead to higher current densities and therefore to an increasing electrochemical water decomposition on the electrode surface and to the concomitant generation of OH radicals [57].
3.5. Effect of electrode distance on the electrochemical BPA degradation The electrode distance has a significant effect on the sonochemical degradation (see section 3.3). Moreover, the electrochemical degradation can be affected by this parameter. Therefore, the electrochemical BPA degradation was investigated with electrode distances of 2, 2.5 and 3 cm at two potentials (5 V and 10 V). The results are presented in Figure 8.
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Figure 8: Influence of electrode distance on the electrochemical BPA degradation (c(BPA): 1 mg L-1, n(Na2SO4): 30 mmol L-1, reaction time t = 5 min, 500 mL.
It was found that, at low potential (5 V) the electrode distance has an influence on BPA degradation, as the degradation increased when decreasing electrode distance from 15% (ED = 3 cm), over 18% (ED = 2.5 cm) up to 27% (ED = 2 cm). At high potential (10 V) the electrode distance seems to be neglectable in the investigated range. For all investigated distances, BPA degradation ranges between 80% and 100%. The correlation between BPA degradation and electrode distance can be explained by the resistance between the two electrodes. At 10 V, the mentioned resistance caused by the electrode distance seems to have no effect.
3.6.
Sonoelectrochemical BPA degradation
The sonoelectrochemical BPA degradation was investigated using four parameters, i.e. immersion depth of sonotrode (ID = 3, 5.6, 8.1 cm), electrode distance (ED = 2, 2.5, 3 cm), ultrasound amplitude (UA = 25, 125 µm) and potential (V = 0, 5, 10 V). The results are shown in Figure 9 where the graphs are compiled pairwise. The left columns (a., c., e.) contain the results for low amplitude (25 µm), the right ones (b., d., f.) for high amplitude (125 µm). Each row considers a different applied potential from 0 V (a. and b.) over 5 V (c. and d.) to 10 V (e. and f.). Each of these graphs (a. – f.) display three different electrode distances (ED) combined with three different immersion depths (ID).
25 µm
125 µm
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0V
5V
10 V
Figure 9: Influence of electrode distance (ED), ultrasound amplitude (UA) and sonotrode immersion depth on BPA degradation ((ID) at 0 V (a. and b.), 5 V (c. and d.) and 10 V (e. and f.), c(BPA): 1 mg L-1, n(Na2SO4): 30 mmol L-1, reaction time t = 5 min, 500 mL).
It can be seen in Figure 8, an increase in ultrasound amplitude from 25 to 125 µm (ceteris paribus) leads to an increase in degradation degree for all investigated parameter combinations (comparison of a. with b., c. with d. and e. with f.). The amplitude increases wih invreasing power input accompanied with an increase in cavitation events per volume and an enhancement of cavitational effects [51]. With increasing applied potential under identical conditions, the degradation degree increases, too (comparison of a. with c. and e. or comparison of b. with d. and f.). Moreover, with increasing applied potential, a lower electrode distance results in higher degradation degrees (comparison of a. with e. or b. with f.). The optimal electrode distance in the sonoelectrochemical setup is dependent on the sound field distribution, which is more favourable at higher electrode distances (see section 3.3) and on the electrochemical resistance, which leads to better degradation at lower distance (see section 3.5). At higher applied potentials, the electrochemical oxidation and the effect of resistance become more important and lower electrode distances preferable. An increase in sonotrode immersion depth leads in nearly all cases to better results, due to the fact, that the active oscillating area is increased and more 12
cavitation events could be produce per volume. Moreover, with an increased oscillation area, the electrical power consumption ascends, too. 3.7.
Synergistic effects
Based on degradation experiments the synergy of the sonoelectrochemical method can be calculated. A synergistic effect defines an interaction between two or more agents that produce a greater effect than the sum of the effects of its individual parts. To determine the synergistic effect the rate constants of the BPA degradation of the sonoelectrochemical system and its components are compared. The synergy index S can be calculated as follows [58]:
(4) where kAC/EC, kAC and kEC are the rate constants of the sonoelectrochemical, sonochemical and electrochemical system, respectively. The rate constants and the calculated synergy indices S are summarized in Table 1.
Table 1: Overview on the rate constants k and synergy indices for the individual and combined methods at different reaction conditions. U [V]
Amplitude [µm] 25
5 125
EA [cm]
kAC [min-1]
kEC [min-1]
kAC/EC [min-1]
Synergy Index S
3 2.5 2 3 2.5
0.016 0.012 0.006 0.043 0.031
0.010 0.013 0.016 0.010 0.013
0.035 0.043 0.038 0.084 0.078
1.311 1.725 1.773 1.633 1.772
2 0.024 3 0.016 25 2.5 0.012 10 2 0.006 3 0.043 125 2.5 0.031 2 0.024 c(BPA): 1 mg L-1, ID = 8.1 cm, n(Na2SO4) = 30 mmol 500 mL; AC: degradation, AC/EC: sonoelectrochemical degradation.
0.016 0.090 2.268 0.120 0.111 0.811 0.150 0.124 0.765 0.170 0.163 0.931 0.120 0.152 0.933 0.150 0.175 0.973 0.170 0.193 1 Sonochemical degradation, EC: Electrochemical
A synergy index equal to 1 indicates identical BPA degradation properties of the combined and the sum of the individual systems, whereas a synergy index > 1 exhibits an improved degradation of the combined system. As can be seen in Table 1, the synergy indices at 5 V for both ultrasound amplitudes (25 and 125 µm) at all electrode distances (2, 2.5 and 3 cm) are higher than 1. This means that the interaction of the sonochemical and electrochemical system is beneficial and lead to an improvement of BPA degradation compared to the individual systems. The highest synergy index was observed at UA = 125 µm and ED = 2 cm. For the individual systems, the rate constants are 0.024 min-1 (AC) and 0.016 min1 (EC). In the combined system, the rate constant increases up to 0.090 min-1, which means that rate constant is more than 2 times higher than the sum of the rate constants of the individual systems. Based on this, the half life of BPA degradation could be reduced to 7.7 min within the sonoelectrochemical method, compared to 43.3 min and 28.9 min for the electrochemical and sonochemical method under the given conditions, respectively. The results can be explained by an ultrasound induced enhancement of the electrochemical reactions through increased mass transport 13
and decreased diffusion layer at the electrode surface and therefore to an increasing oxidation rate. This effect magnifies gradually with decreasing electrode distance, as can be seen in the increasing synergy index from maximum to minimum electrode distance. The smaller the distance between the sonotrode and electrodes, the greater the effect on BPA degradation. Contrary to the positive interaction effect at 5 V, synergy effects at 10 V are absent and even in some combinations of AC and EC using 10 V potential disturbances as indicated by synergy indices < 1 were observed. Consequently there is an interdependency between AC and EC which diminish the sum of the single methods (AC and EC) without combination. One possible explanation for decreasing synergy-indices could be the formation of scavenging species, like for instance hydrogen, that arises from the electrochemical decomposition of water at the potentials applied. The produced hydrogen can react with hydroxyl radicals to form water and therefore reduce the hydroxyl radical concentration as described amongst others by Christensen and Sehested [59]. Furthermore, through increased gas production the sound field could be altered in comparison to the individual sonochemical reaction, which could also reduce the degradation efficiency. In addition a variation of the acoustic modes within the reactor is possible which could also lead to a disruption of the cavitation field and therefore to a decreasing BPA Degradation [56]. Yet, the observed effect is not fully understood and needs further investigation. 3.8.
Comparison with existing methods
To the best of our knowledge, this is the first study of sonoelectrochemical degradation of BPA in water. In Table 2 a comparison with other methods for BPA degradation described in literature is displayed. Due to the fact, that the data of several papers was not completely given (e.g. required energy, rate constants), the comparison could only be done on base of the degradation degree D and reaction time tr. Moreover, a direct comparison is impossible due to the different reaction conditions (initial concentration, reaction volume), but basic trends could be derived. The best result obtained for the sonoelectrochemical system described in this article was a degradation degree of 93% at 1 mg L-1 initial BPA concentration and tr = 30 min. Among the listed treatments, the studied sonoelectrochemical treatment shows a high degradation degree within short reaction time. Only treatments containing auxiliaries, like ozone, Fe(VI) species and hydrogen peroxide (H2O2) based photochemical oxidation lead to better results. Until now, the best methods for BPA elimination in aquatic media seem to be photo-Fenton processes (Table 2, No 11) described by Felis et al. [32] and ozone oxidation processes described by Umar et al. [33]. Significant disadvantages of these methods are certainly the use of chemicals that often require (re-)activation of catalysts, pH control and/or neutralization steps. Moreover, ozone could cause the generation of many oxidation products, whose toxic potential can even be higher than BPA itself and whose hazardous properties have not been clarified yet [33]. In comparison, electrochemical methods for BPA degradation using different anode materials than BDD require long periods of degradation time and seem therefore unusable for application in wastewater treatment so far. Within the class of sonochemical treatments for BPA degradation described in literature, the system investigated in this research provides the best results. At low frequency (24 kHz) the rate constant for BPA degradation is 0.016 min-1 (UA = 25 µm, PI = 68 W) and 0.041 min-1 (UA = 125 µm, PI = 155 W). Compared to the results obtained by Lim et al. (Table 2, No 3,4,6), where rate constants of 0.006 min-1 (1 MHz) and 0.001 min-1 (300 kHz) were received [60]. Studies on biological degradation (Table 2, No 13) have shown the ability of these systems to remove even high BPA concentrations. Some bacterial strains like Sphingomonas bisphenolicum are capable of removing 115 mg L-1 BPA within 15 hours. Disadvantageous are on the one hand the required BPA degradation time and on the other hand, biological systems like Sphingomonas bisphenolicum are
14
highly substrate-specific and therefore not applicable for removing other micro pollutants, that can also occur in wastewater. Other methods like anodic polymerisation, where BPA is not removed by chemical elimination, but by polymerisation reaction on the electrode surface, have the disadvantage, that only small BPA concentrations could be removed (1.14 mg L-1 in 2 h). In addition, after a specific period of time, the electrode surface has to be cleaned and in this context additional chemicals are needed. For a better comparison, more aspects like energy requirements, technical implementation potentials, demand of chemicals and security aspects must be taken into account at comparable conditions (concentration of BPA, reaction volume, temperature). Table 2: Investigated sonoelectrochemical method for BPA degradation compared to methods described in literature No
Method
Source
c0 (BPA) (mgL-1)
V (mL)
Chemicals
tr (min)
D (%)
k (min-1)
1
AC/EC 24 kHz, 125 µm
This work
1
500
Na2SO4 [30mmol]
30
93
0.089
2
AC 300 kHz, 80 W
[34]
27
300
No chemicals added
90
100
n.a.
3
AC 1 MHz
[60]
10
1000
No chemicals added
120
98
0.006
4
AC 300 kHz
[60]
10
1000
No chemicals added
n.a.
n.a.
0.001
5
Sono-Fenton (300 kHz, 80 W)
[35]
27
300
FeSO4 (100 µmol L-1) H2O2 (35*10-3 mol L-1)
90
100
n.a.
6
AC/UV/TiO2
[60]
10
1000
TiO2
120
98
0.045
7
EC (Ti/BDD)
[30]
100
100
Na2SO4 (0.1 mol L-1)
390
100
n.a.
8
EC (BDD)
[61]
20
n.a.
Na2SO4 (0.1 molL-1)
240
98
n.a.
9
EC (Pt)
[30]
100
100
Na2SO4 (0.1 molL-1)
720
100
n.a.
8
28 kHz/stainless Steel [62] mesh 200 W, 0.2 W/L [63] Anodic Polym. [32] UV [32] Photo-Fenton
0.228
1000
No chemicals added
60
76
n.a.
1.14
50
Na2SO4 (0.1 mol L-1)
120
99
n.a.
10
n.a.
Hg-lamp (400 W)
30
100
n.a.
10
n.a.
Hg-lamp (400 W) H2O2 (100 mg L-1)
15
100
n.a.
40
100
TiO2 (1g L-1) Hg/Xe-Lampe (10mWcm-2)
15 h
99
n.a.
9 10 11
12
[64, 65] Photocatalytic
15
13
[17]
115
n.a.
Biological
14
720
100
n.a.
[33]
10
500
O3 (4.05 mg min-1)
10
100
n.a.
[66]
1.5
200
K2FeO4 (1 g L-1)
30
100
n.a.
O3 15
A01, S011 Sphingomonas bisphenolicum
K2FeO4
c0: initial concentration, V: reaction volume, t r: reaction time, D: degradation, k: rate constant, EC: electrochemical oxidation, AC: acoustic cavitation, n.a. data not available.
4. Conclusion Within this study it could be shown, that Bisphenol A could significantly be removed in an aquatic system via sonochemical, electrochemical and sonoelectrochemial methods using low frequency ultrasound (24 kHz, amplitude: 25 µm and 125 µm) and voltage of 5 V resp. 10 V. It could be shown that the combination of AC (24 kHz)/EC (5V) could degrade 90 % BPA of an initial concentration of 1 mg L-1 within 30 min. Furthermore synergistic effects were discovered for the combination of 24 kHz and 5 V. The best synergistic results were obtained for 2 cm electrode distance at high ultrasound amplitude (125 µm, PI = 155 W) (synergy-index: 2.268). Using higher potentials (10 V) the synergistic effects observed in the AC/EC system (24 kHz and 5 V) were absent and in some cases the synergy-index was even <1, which suggests a disturbing interdependence between AC and EC at high potentials (10 V). Possible causes like formation of scavenging species and/or processeses at the boundary layer liquid/electrode need to undergo further investigations. Finally could be shown, that the position of the sonotrode and the electrodes have substantial effect of the degradation results and therefore should be taken into account at the design of sonoelectrochemical reactors.
Acknowledgement The authors thank Wolfgang Faehndrich (FSU Jena) for the assistance in constructing the sonoelectrochemical systems.
16
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21
List of Figures Fig.1: Schematic representation of the 500 mL reactor; Electrodes are placed in the reactor with variable electrode distance ED. Ultrasound horn probe is placed with variable immersion depth ID. Cooling water CW. Reaction volume is 500 mL; BDD-electrodes (DIACHEM®).
Fig.2: Influence of ultrasound power input on BPA degradation; without electrodes, c(BPA): 1 mg L-1, ultrasound power densities: 26, 68.5, 104 and 155 W, n(Na 2SO4): 30 mmol, 500 mL.
Fig.3: Influence of ultrasound horn tip immersion depth on BPA degradation; without electrodes, c(BPA): 1 mg L-1, ultrasound power densities: blue 68.5 W, brown 155 W, n(Na 2SO4): 30 mmol, 500 mL, t = 30 min, ID = 3.1, 5.6, 8.1 cm.
Fig.4: Visualization of transversal oscillations via chemiluminescence of luminol; photographs of chemiluminescence, left: power input 68.5 W, right: 155.3 W luminol: 1 g L-1, Na2CO3: 5.0 g L-1, 500 mL, exposure time: 119 s, ISO 800, focus: 55 mm, focal ratio: F/5.6; graph: DCF degradation by ultrasound, Na2SO4: 30 mmol L-1, 5 min. White lines indicate the sonotrode and the reactor walls (not true to scale).
Fig.5: Influence of electrode distance on the sonochemical BPA degradation; (ED: 2, 2.5, 3 cm and without electrodes, c(BPA): 1 mg L-1, ultrasound amplitude: 25 µm (triangles) and 125 µm (squares), n(Na2SO4): 30 mmol, 500 mL).
Fig.6: Potential dependency on degradation of BPA; ED: 2.5 cmc(BPA): 1 mg L-1, n(Na2SO4): 30 mmol L-1, t = 5 min, 500 mL.
Fig.7: Influence of electrode distance (ED) on BPA degradation (with applied potential); ED: 2.5 cm, c(BPA): 1 mg L-1, n(Na2SO4): 30 mmol L-1, t = 5 min, 500 mL.
Fig.8: Influence of electrode distance (ED), ultrasound amplitude (UA) and ultrasound horn tip immersion depth (ID) on BPA degradation, at 0 V (a. and b.), 5 V (c. and d.) and 10 V (e. and f.); c(BPA): 1 mg L-1, n(Na2SO4): 30 mmol L-1, reaction tim t = 5 min, 500 mL.
22
Highlights
sonoelectrochemical degradation of bisphenol A yielded to a removal of more than 90% degradation process follows first-order kinetics in comparison to single systems, synergistic effects were found distribution of hydroxyl radicals in the reactor was pictured by chemiluminescence
23
S1350-4177(17)30260-2 http://dx.doi.org/10.1016/j.ultsonch.2017.05.038 ULTSON 3719
To appear in:
Ultrasonics Sonochemistry
Received Date: Revised Date: Accepted Date:
10 November 2016 18 May 2017 29 May 2017
Please cite this article as: M. Dietrich, M. Franke, M. Stelter, P. Braeutigam, Degradation of endocrine disruptor Bisphenol A by ultrasound-assisted electrochemical oxidation in water, Ultrasonics Sonochemistry (2017), doi: http://dx.doi.org/10.1016/j.ultsonch.2017.05.038
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Degradation of endocrine disruptor Bisphenol A by ultrasound-assisted electrochemical oxidation in water Dedicated to Professor Bernd Ondruschka on the occasion of his 70th birthday
Matz Dietricha, Marcus Frankea, Michael Steltera,b, Patrick Braeutigama,* a
b
Center for Energy and Environmental Chemistry, Institute of Technical Chemistry and Environmental Chemistry, Friedrich Schiller University Jena, Philosophenweg 7a, 07743 Jena, Germany
Fraunhofer IKTS, Fraunhofer Institute for Ceramic Technologies and Systems, Michael-Faraday-Straße 1, 07629 Hermsdorf, Germany
Abstract Micropollutants are becoming an increasing problem for the environment and wastewater treatment. One example is Bisphenol A (BPA), an endocrinic disruptor, which is widely used in plastic production. Due to its endocrine disrupting effects on aquatic (micro-)organisms and its ubiquity, in surface- and wastewater alike, adequate treatment techniques are necessary. In this study, the degradation of BPA by a sonoelectrochemical hybrid system was investigated, using a low frequency (24 kHz) ultrasound horn and two boron doped diamond electrodes. It was found that by the combination of the individual processes, i.e. ultrasound and electrochemical oxidation, more than 90 % of BPA could be removed within 30 min at an initial concentration of 1 mg L-1. Moreover, synergistic effects were discovered and a considerable improvement compared to the individual processes could be achieved by using a potential of 5 V , whereas synergistic effects were absent at a potential of 10 V. This study provides investigation of ultrasound amplitude, potential and electrode positioning on BPA degradation. The reaction was found to follow pseudo first order kinetics with a rate constant of 0.089 min-1. Samples were analysed by high pressure liquid chromatography (HPLC) using a diode array detector. Moreover, the presence and distribution of hydroxyl radicals within the reactor was visualized by using sonochemiluminescence.
Keywords: sonoelectrochemistry, Bisphenol A, advanced oxidation process, synergistic effect, sonochemiluminescence
1. Introduction In recent years, endocrine disrupting substances in natural waters have become an emerging issue. Numerous investigations have been carried out on the occurrence, distribution and fate of these substances [1-4]. Endocrine disruptors have been found in the aquatic environment around the world, and were detected in surface water, groundwater and even drinking water [5, 6]. In 2008, endocrine disrupting substances were first legally regarded as a potential risk for the aquatic environment in an amendment of the EU water framework directive which concerns water policy in Europe [7, 8]. This *
Corresponding author. Tel.: +49 3641 948458; fax: +49 3641 948402. E-mail address: [email protected] (Patrick Braeutigam).
1
directive now includes a watch list for substances, which require monitoring and estimation of their environmental risks to allow their classification. If a substance presents a significant risk to the aquatic environment it is categorized as priority substance. The first watch list contains Bisphenol A (BPA), which is a diphenylmethane derivative with hormone-like properties and a worldwide annually consumption of more than 3.8 million tons [5]. BPA is used predominantly in plastic production, especially for the synthesis of polycarbonate, which is largely used in electronic components, constructing materials, phones and water bottles [5]. In the human organism, BPA possesses hormone-like effects by bounding on specific hormone receptors such as estrogen-, androgen- and hydrocarbon receptors, which leads to a restriction or complete inhibition of the gene-expression [9]. Possible phenotypic consequences are disruptions of the central nervous and sexual system. The entry of BPA into the environment takes place in different ways. One is the diffuse entry of BPA containing materials in the environment and another via sewage water treatment plants (SWTPs). In groundwater, BPA concentrations of 1.47 µg L-1 could be measured, in sediments up to 24 µg kg-1 [5]. Numerous investigations concerning the effect of BPA on algae, slugs, fish and invertebrates have shown, that even low doses (ng L-1 - µg L-1) causes harmful dysplasia [10-12]. The lowest known effect concentration of 8 ng L-1 causes superfeminization in Ampullariidae [13]. Based on this knowledge the predicted no effect concentration (PNEC) of 0.8 ng L-1 was determined. BPA could not be completely removed in classical SWTPs [5, 14]. According to different sludge types and biological treatment processes, BPA concentrations in SWTPs effluents in Germany range from 0.06 µg L-1 to 3 µg L-1 [5] and in industrial SWTPs the BPA concentration can rise up to 118 µg L-1 [6]. End-of-pipe strategies for BPA removal can be found in literature and divided in technologies based on processes without chemical transformation, like adsorption and filtration [15] and degradation processes based on (bio-)chemical reactions, mostly induced by reactive oxygen species (ROS). The latter include biological [16-18], photocatalytical [19-24], photoelectrochemical [25-27], electrochemical [28-30] and other advanced oxidation processes (AOPs), such as Fenton [31], photoFenton [32], ozonation [33] and ultrasound-induced degradation processes [34-36]. AOPs are defined by the in-situ generation of ROS, like hydroxyl or perhydroxyl radicals, which have a high oxidation potential and therefore show a high reactivity with organic pollutants [37]. In order to minimize the environmental impact, the most promising of the above-mentioned methods are those which do not rely on additional chemicals like sonolysis and electrochemical treatment. Sonolysis uses ultrasound with frequencies between 20 kHz and 2 MHz [38]. In this frequency range acoustic cavitation occurs, i.e. microbubbles are generated and expanded within the ultrasonic field until they collapse at a critical size, leading to hotspots with temperatures up to 5000 K and pressures of about 1000 bar [39-41]. At these extreme conditions, homolytic bond cleavage of water molecules takes place accompanied by the generation of hydroxyl and hydrogen radicals (equation 1) [40]. (1) The electrochemical formation of hydroxyl radicals takes place at the anode by an electron transfer (equation 2) and is associated with the formation of hydrogen peroxide, ozone and oxygen, as side reactions [42]. However, the main drawback in this process is oxygen evolution. In order to avoid this undesirable side reaction, the anode material should possess a high overpotential for O2 formation. For this purpose, materials like boron-doped diamond (BDD) have been used, having an oxygen overpotential up to 2.8 V, which is the potential for hydroxyl radical formation. Thus, BDD electrodes have a great potential for electrochemical water treatment [43]. 2
(2) The hydroxyl radicals that are generated by sonochemical and electrochemical processes can oxidize organic contaminants. Next to the formation of ROS, by combining ultrasonic irradiation and electrochemical oxidation in one reactor, synergistic effects could be expected [37]. Moreover, the physical effects of ultrasound such as streaming, microjets and shockwaves based on acoustic cavitation (AC) contribute to an increased mass transfer, a reduced electrode passivation and a minimization of gas bubble accumulation at the electrodes [44, 45]. In this study, a systematic investigation of an efficient reactor arrangement for BPA degradation in terms of interaction between ultrasound and electrochemistry is provided. Influencing factors on both individual processes and their interactions were identified and optimized. Experiments were carried out at low frequency ultrasound (24 kHz) since it is well known that physical effects of cavitation like electrode surface cleaning and improvement of mass transport are predominant at low frequencies [46, 47]. The fact that the initial BPA concentration of 1 mg L-1 used in this investigation is higher than in SWTPs mentioned above (10 times higher than in industrial SWTPs, 300 times higher than in normal SWTPs) is due to limitations in instrumental analysis. To the best of our knowledge, there are no studies carried out on sonoelectrochemical degradation of BPA. This work will focus on the investigation of parameters such as ultrasound amplitude, potential, positioning of sonotrode and electrodes, and the resulting synergistic effects that arise during mutual usage. Furthermore, kinetic studies were conducted. The quantitative investigation of reactor configuration and parameters is accompanied by the visualization of hydroxyl radical distribution within the reactor by the use of sonochemiluminescence of luminol in alkaline solution [48].
2. Experimental 2.1. Chemicals All chemicals were used as received without any further purification. Bisphenol A (4-[2-(4Hydroxyphenyl)propane-2-yl]phenol, 97.0 %) was purchased from Alfa Aesar and luminol (5-Amino2,3-dihydro-1,4-phthalazinedione, >95%) from AppliChem. Sodium sulphate (99.7%), methanol (99.9%), acetonitrile, hexane and acetone (all HPLC-grade) were supplied by VWR. Deionized water (3 μS cm-1) was used for the preparation of BPA-solutions and ultrapure water (0.056 μS cm-1) for sample preparation and analysis. 2.2.
Setup
The experiments were performed in a double-wall cylindrical glass reactor (500 mL) at a constant temperature of 20 ± 2 °C, controlled by a thermostat (FP50-ME, Julabo, Seelbach, Germany). An overview of the setup and the position of the electrodes and the sonotrode is presented in Figure 1. The low-frequency ultrasound was introduced through the cap of the glass reactor by an ultrasound generator (UP200S, 24 kHz, Hielscher Ultrasonics, Teltow, Germany) connected with a sonotrode (S14L2D, titanium, length: 200 mm, diameter: 14 mm, Hielscher Ultrasonics, Teltow, Germany). Two BDD-electrodes (Diachem®, 5 µm diamond layer on niobium, expanded metal grid, 50 mm x 30 mm x 1.5 mm, Condias, Itzehoe, Germany) were placed in the reactor trough the cap allowing the adjustment of immersion depth (ID) in the range of 3 – 8 cm. The electrode distance (ED) could be varied by electrode clamps between 2 – 3 cm. The electrodes were connected to a universal power supply (EA-3050B, EA-Elektro-Automatik, Viersen, Germany) working at a constant direct current 3
potential of 0 – 10 V. An energy monitor (Voltcraft Energie Monitor 3000, Conrad Electronic SE, Hirschau, Germany, accuracy: ±1%) was used to record the energy input of the setup.
Figure 1: Setup of the sonoelectrochemical reactor (double-wall glass reactor, variable electrode distance ED, immersion depth of the sonotrode ID, reaction volume: 500 mL, BDD-electrodes).
2.3. Procedure A stock solution of 100 mg L-1 BPA was prepared by dissolving 0.050 g BPA in 500 mL deionized water. For degradation experiments, 5 mL of the stock solution and 4.26 g of the electrolyte Na2SO4 were diluted in deionized water to give a volume of 500 mL BPA solution (1 mg L-1). The chosen electrolyte concentration of 30 mM relates to the conductivity range of real waste water (4.6 mS cm-1) [49]. The solution was filled in the glass reactor and was subjected to ultrasound irradiation and/or a constant potential was applied at the electrodes for a given reaction time. Samples (1.2 mL) were taken in 5 min intervals and measured by HPLC. For the investigation of the ultrasound (AC) as single method, two different ultrasound amplitudes, i.e. 25 µm and 125 µm were chosen and compared The amplitude is defined as the elongation of the sonotrode tip (peak-to-peak) and given in µm and For relation between amplitude and ultrasound power see fig. 3. Further investigations on immersion depth of the ultrasound horn (ID) and the interdependence of ultrasound and electrodes (without applied potential) on BPA degradation were undertaken. For the electrochemical (EC) degradation as single method, different electrode distances (2, 2.5, 3 cm) and potentials (5, 10 V) were examined. Finally, both systems (sonochemical and electrochemical system) were combined in a hybrid system (AC/EC) , the parameters potential (U), electrode distance (ED), immersion depth of the electrodes (ID) and ultrasound amplitude (UA) were evaluated and calculations concerning synergistic effects were conducted. Kinetics were studied based 4
on the experimental data, where the rate constants were determined by measured BPA concentrations during degradation experiments.
2.4. Analytical methods HPLC measurements were carried out on a HPLC system (LC-2000 series, CD-2060 Plus column oven, PU-2080 Plus eluent pumps, AS 2055 Plus autosampler, Jasco, Gross-Umstadt, Germany). The separation of BPA (injection volume: 50 µl) was performed on a Kromasil C18 (250 mm x 4.6 mm x 5 µm) column, at a flow rate of 1.5 ml min-1 (mobile phase (ultrapure water / methanol 65/35 (v/v)) and a column oven temperature of 30 °C. For the detection a diode array detector (MD-2010 Plus multi-wavelength detector, Jasco, Gross-Umstadt, Germany) was used working at 276 nm (BPA (6.4 min). A chromatogram of bisphenol A is shown in Figure 2. The integrated peak areas correlate with the BPA concentration. The quantitative analysis was done by external calibration with standards. Each analysis was repeated three times to ensure reliable results. The limit of quantification (LOQ) was found to be 34.00 µg L-1.
Figure 2: Example of chromatogram of Bisphenol A (c(BPA): 5 mg L-1)
2.5. Visualization of reaction zones The spatial distribution of hydroxyl radicals within the reactor was visualized by the use of chemiluminescence of luminol in alkaline solution. The radicals oxidize the luminol leading to an excited state, which further undergoes a relaxation emitting blue light at 430 nm [48]. The luminescence can be observed and is associated with the formation of an aminophthalat anion. For the experiments, 500 mL of a luminol alkaline solution (1.0 g L-1 luminol, 5.0 g L-1 Na2CO3) was filled in the glass reactor and ultrasound and/or a potential was applied. The sonochemiluminescence was captured in darkness with an SLR camera (Canon EOS 1100D, EF-S 18-55 mm IS II lens) at an exposure time of 119 s (ISO 800, 55 mm, F/5.6). The distance between reactor and lens was 46 cm. It should be considered that the spatial distribution of hydroxyl radicals visualized in the luminol set-up may be deviate from the distribution in the degradation experiments, as chemical factors such as pH, conductivity, chemical composition may affect cavitation activity [50].
5
3. Results and discussion In this section, the results of Bisphenol A degradation in a sonochemical (sections 3.1 – 3.3), electrochemical (sections 3.4 and 3.5) and sonoelectrochemical (sections 3.6 and 3.7) reaction system are presented, discussed and compared with literature data (section 3.8).
3.1.
Effect of ultrasound amplitude on the sonochemical BPA degradation
To enable the comparison between the individual methods (AC and EC) and their combination (AC/EC) and to generate data for synergy calculations, the data for the single methods are measured. For investigations concerning the influence of sonochemical BPA degradation at 24 kHz, two different ultrasound amplitudes were considered, i.e. 25 µm and 125 µm, in the absence of the two BDD electrodes. The results are displayed in Figure 3.
Figure 3: Influence of ultrasound power input on the sonochemical BPA degradation (reactor without electrodes, c(BPA): 1 mg L-1, ultrasound amplitude: 25 µm and 125 µm, n(Na 2SO4): 30 mmol, 500 mL.
BPA degradation increased at higher ultrasound amplitude. After 30 min ultrasonic irradiation, 38% BPA could be removed at an amplitude of 25 µm, whereas, 61% BPA by using an ultrasound amplitude of 125 µm. This could be explained by an increasing hydroxyl radical production at higher amplitudes, as already described elsewhere for the sonochemical method using volatile and nonvolatile compounds [36, 51]. The results show that, even at low ultrasound frequencies, a considerable degradation degree for organic pollutants could be obtained.
3.2. Effect of sonotrode immersion depth on the sonochemical BPA degradation Ultrasonic wave propagation, as well as cavitation field distribution, substantially affected by the geometry and material of the sonochemical reactor, due to reflections, interference and coupling between the reaction liquid and the cooling liquid [52], to name only a few. The dependence of the 6
acoustic field propagation and transducer-electrode resp. electrode wall gaps was shown by I. Tudela et al. [53]. Therefore, an examination of the relative position of the sonotrode and their influence on the power consumption, and hence on the BPA degradation is necessary. By varying the immersion depth of the sonotrode, the BPA decay can be altered, as shown in Figure 4.
Figure 4: Influence of sonotrode immersion depth on the sonochemical BPA degradation (reactor without electrodes, c(BPA): 1 mg L-1, ultrasound amplitude: black - 25 µm, grey - 125 µm, n(Na2SO4): 30 mmol, 500 mL, reaction time t = 30 min. Power data above columns: electrical power consumption.
By reducing the immersion depth from 8.1 cm to 3.1 cm (see Figure 4), the BPA degradation decreases from 61% to 29% (125 µm) and 32% to 22% (25 µm), respectively. A possible explanation for that result are the vibration properties of the sonotrode, which include longitudinal and thickness oscillations. By decreasing the immersion depth of the sonotrode, the longitudinal transmission to the liquid still remains, whereas the transversal part diminishes gradually, because the active oscillating area of the sonotrode decreases. Since the amplitude is kept constant, it follows that the power consumption decreases (see Figure 4). With increasing ultrasound amplitude the transversal oscillating part increases as well as shown in Figure 5 where the visualization of hydroxyl radicals via luminol sonochemiluminescence is pictured. The same observation was made by Klima et al. [54].
7
Figure 5: Visualization of longitudinal and thickness oscillations via chemiluminescence of luminol (left: ultrasound amplitude 25 µm, right: 125 µm, luminol: 1 g L-1, Na2CO3: 5.0 g L-1, 500 mL, exposure time: 119 s, ISO 800, focus: 55 mm, focal ratio: F/5.6; graph. White lines indicate the sonotrode and the reactor walls (not true to scale).
Hence, the effect of different immersion depths on BPA degradation is higher with increasing amplitude, which can be seen in Figure 5. The luminescence at an amplitude of 25 µm is much smaller compared to an amplitude of 125 µm (Figure 5),
3.3. Effect of electrode distance on the sonochemical BPA degradation The formation of cavitation events can be influenced by the sound field during sonication. The insertion of electrodes in the reactor could have a significant influence on the sound field and cavitation field distribution by means of interruption of ultrasound propagation (see Figures 4 and 5). Therefore, three different electrode distances were evaluated (2, 2.5, 3 cm) without applied potential and compared with the system without inserted electrodes (see section 3.1). Figure 6 shows the BPA degradation for 25 µm and 125 µm in dependence of the electrode distance.
8
Figure 6: Influence of electrode distance on the sonochemical BPA degradation (ED: 2, 2.5, 3 cm and without electrodes, c(BPA): 1 mg L-1, ultrasound amplitude: 25 µm (triangles) and 125 µm (squares), n(Na2SO4): 30 mmol, 500 mL).
A considerably much faster BPA decomposition could be achieved by increasing the electrode distance from 2 cm to 3 cm. After 30 min ultrasound irradiation (25 µm) the degradation rises from 19% (ED = 2 cm) over 35% (ED = 2.5 cm) up to 41% (ED = 3 cm). At 125 µm ultrasound amplitude after 30 min irradiation, a BPA degradation of 71% could be obtained. In comparison with the BPA degradation degrees obtained without electrode presence (see Figure 2), the attendance of electrodes promotes the BPA degradation in case of 125 µm (60% without electrodes, 71% ED = 3 cm)), whereas the difference at 25 µm is marginal (38% without electrodes, 41% ED = 3 cm). With a decreasing electrode distance an increasing interaction with the electrode surface could occur (reflection / dispersion / absorption), leading to losses and/or interference (enhancement / cancellation) [55]. Hence, the reactor design as well as the position of electrodes have a substantial influence on the degradation efficiency and should be taken into account [56]. These experimental results support the FEM simulations made by I. Tudela et al. showing that with decreasing transducer-electrode gap the pressure amplitude increases and therefore optimizes mechanical phenomena on the electrode surface [53]. Adjustments with respect to the sonotrode geometry, power input, ultrasound amplitude and electrode size and design should be considered.
3.4.
Effect of applied potential on the electrochemical BPA degradation
Before the consideration of the synergy between the sonochemicaland electrochemical BPA degradation the individual potentiostatic electrochemical degradation was investigated. For this evaluation six different potentials (2, 3.5, 4, 5, 7.2 and 10 V) were considered. As shown in Figure 6 the BPA degradation is highly improved by increasing the applied potential at a constant electrode distance.
9
Figure 7: Influence of the applied potential on the electrochemical BPA degradation (ED: 2.5 cm, c(BPA): 1 mg L-1, n(Na2SO4): 30 mmol, reaction time t = 5 min, 500 mL).
After 5 min reaction time with a potential of 2 V, 12% BPA could be degraded, whereas with an applied potential of 10 V the degree of degradation rises up to 93% with an ED of 2.5 cm. This is due to the fact, that higher potentials lead to higher current densities and therefore to an increasing electrochemical water decomposition on the electrode surface and to the concomitant generation of OH radicals [57].
3.5. Effect of electrode distance on the electrochemical BPA degradation The electrode distance has a significant effect on the sonochemical degradation (see section 3.3). Moreover, the electrochemical degradation can be affected by this parameter. Therefore, the electrochemical BPA degradation was investigated with electrode distances of 2, 2.5 and 3 cm at two potentials (5 V and 10 V). The results are presented in Figure 8.
10
Figure 8: Influence of electrode distance on the electrochemical BPA degradation (c(BPA): 1 mg L-1, n(Na2SO4): 30 mmol L-1, reaction time t = 5 min, 500 mL.
It was found that, at low potential (5 V) the electrode distance has an influence on BPA degradation, as the degradation increased when decreasing electrode distance from 15% (ED = 3 cm), over 18% (ED = 2.5 cm) up to 27% (ED = 2 cm). At high potential (10 V) the electrode distance seems to be neglectable in the investigated range. For all investigated distances, BPA degradation ranges between 80% and 100%. The correlation between BPA degradation and electrode distance can be explained by the resistance between the two electrodes. At 10 V, the mentioned resistance caused by the electrode distance seems to have no effect.
3.6.
Sonoelectrochemical BPA degradation
The sonoelectrochemical BPA degradation was investigated using four parameters, i.e. immersion depth of sonotrode (ID = 3, 5.6, 8.1 cm), electrode distance (ED = 2, 2.5, 3 cm), ultrasound amplitude (UA = 25, 125 µm) and potential (V = 0, 5, 10 V). The results are shown in Figure 9 where the graphs are compiled pairwise. The left columns (a., c., e.) contain the results for low amplitude (25 µm), the right ones (b., d., f.) for high amplitude (125 µm). Each row considers a different applied potential from 0 V (a. and b.) over 5 V (c. and d.) to 10 V (e. and f.). Each of these graphs (a. – f.) display three different electrode distances (ED) combined with three different immersion depths (ID).
25 µm
125 µm
11
0V
5V
10 V
Figure 9: Influence of electrode distance (ED), ultrasound amplitude (UA) and sonotrode immersion depth on BPA degradation ((ID) at 0 V (a. and b.), 5 V (c. and d.) and 10 V (e. and f.), c(BPA): 1 mg L-1, n(Na2SO4): 30 mmol L-1, reaction time t = 5 min, 500 mL).
It can be seen in Figure 8, an increase in ultrasound amplitude from 25 to 125 µm (ceteris paribus) leads to an increase in degradation degree for all investigated parameter combinations (comparison of a. with b., c. with d. and e. with f.). The amplitude increases wih invreasing power input accompanied with an increase in cavitation events per volume and an enhancement of cavitational effects [51]. With increasing applied potential under identical conditions, the degradation degree increases, too (comparison of a. with c. and e. or comparison of b. with d. and f.). Moreover, with increasing applied potential, a lower electrode distance results in higher degradation degrees (comparison of a. with e. or b. with f.). The optimal electrode distance in the sonoelectrochemical setup is dependent on the sound field distribution, which is more favourable at higher electrode distances (see section 3.3) and on the electrochemical resistance, which leads to better degradation at lower distance (see section 3.5). At higher applied potentials, the electrochemical oxidation and the effect of resistance become more important and lower electrode distances preferable. An increase in sonotrode immersion depth leads in nearly all cases to better results, due to the fact, that the active oscillating area is increased and more 12
cavitation events could be produce per volume. Moreover, with an increased oscillation area, the electrical power consumption ascends, too. 3.7.
Synergistic effects
Based on degradation experiments the synergy of the sonoelectrochemical method can be calculated. A synergistic effect defines an interaction between two or more agents that produce a greater effect than the sum of the effects of its individual parts. To determine the synergistic effect the rate constants of the BPA degradation of the sonoelectrochemical system and its components are compared. The synergy index S can be calculated as follows [58]:
(4) where kAC/EC, kAC and kEC are the rate constants of the sonoelectrochemical, sonochemical and electrochemical system, respectively. The rate constants and the calculated synergy indices S are summarized in Table 1.
Table 1: Overview on the rate constants k and synergy indices for the individual and combined methods at different reaction conditions. U [V]
Amplitude [µm] 25
5 125
EA [cm]
kAC [min-1]
kEC [min-1]
kAC/EC [min-1]
Synergy Index S
3 2.5 2 3 2.5
0.016 0.012 0.006 0.043 0.031
0.010 0.013 0.016 0.010 0.013
0.035 0.043 0.038 0.084 0.078
1.311 1.725 1.773 1.633 1.772
2 0.024 3 0.016 25 2.5 0.012 10 2 0.006 3 0.043 125 2.5 0.031 2 0.024 c(BPA): 1 mg L-1, ID = 8.1 cm, n(Na2SO4) = 30 mmol 500 mL; AC: degradation, AC/EC: sonoelectrochemical degradation.
0.016 0.090 2.268 0.120 0.111 0.811 0.150 0.124 0.765 0.170 0.163 0.931 0.120 0.152 0.933 0.150 0.175 0.973 0.170 0.193 1 Sonochemical degradation, EC: Electrochemical
A synergy index equal to 1 indicates identical BPA degradation properties of the combined and the sum of the individual systems, whereas a synergy index > 1 exhibits an improved degradation of the combined system. As can be seen in Table 1, the synergy indices at 5 V for both ultrasound amplitudes (25 and 125 µm) at all electrode distances (2, 2.5 and 3 cm) are higher than 1. This means that the interaction of the sonochemical and electrochemical system is beneficial and lead to an improvement of BPA degradation compared to the individual systems. The highest synergy index was observed at UA = 125 µm and ED = 2 cm. For the individual systems, the rate constants are 0.024 min-1 (AC) and 0.016 min1 (EC). In the combined system, the rate constant increases up to 0.090 min-1, which means that rate constant is more than 2 times higher than the sum of the rate constants of the individual systems. Based on this, the half life of BPA degradation could be reduced to 7.7 min within the sonoelectrochemical method, compared to 43.3 min and 28.9 min for the electrochemical and sonochemical method under the given conditions, respectively. The results can be explained by an ultrasound induced enhancement of the electrochemical reactions through increased mass transport 13
and decreased diffusion layer at the electrode surface and therefore to an increasing oxidation rate. This effect magnifies gradually with decreasing electrode distance, as can be seen in the increasing synergy index from maximum to minimum electrode distance. The smaller the distance between the sonotrode and electrodes, the greater the effect on BPA degradation. Contrary to the positive interaction effect at 5 V, synergy effects at 10 V are absent and even in some combinations of AC and EC using 10 V potential disturbances as indicated by synergy indices < 1 were observed. Consequently there is an interdependency between AC and EC which diminish the sum of the single methods (AC and EC) without combination. One possible explanation for decreasing synergy-indices could be the formation of scavenging species, like for instance hydrogen, that arises from the electrochemical decomposition of water at the potentials applied. The produced hydrogen can react with hydroxyl radicals to form water and therefore reduce the hydroxyl radical concentration as described amongst others by Christensen and Sehested [59]. Furthermore, through increased gas production the sound field could be altered in comparison to the individual sonochemical reaction, which could also reduce the degradation efficiency. In addition a variation of the acoustic modes within the reactor is possible which could also lead to a disruption of the cavitation field and therefore to a decreasing BPA Degradation [56]. Yet, the observed effect is not fully understood and needs further investigation. 3.8.
Comparison with existing methods
To the best of our knowledge, this is the first study of sonoelectrochemical degradation of BPA in water. In Table 2 a comparison with other methods for BPA degradation described in literature is displayed. Due to the fact, that the data of several papers was not completely given (e.g. required energy, rate constants), the comparison could only be done on base of the degradation degree D and reaction time tr. Moreover, a direct comparison is impossible due to the different reaction conditions (initial concentration, reaction volume), but basic trends could be derived. The best result obtained for the sonoelectrochemical system described in this article was a degradation degree of 93% at 1 mg L-1 initial BPA concentration and tr = 30 min. Among the listed treatments, the studied sonoelectrochemical treatment shows a high degradation degree within short reaction time. Only treatments containing auxiliaries, like ozone, Fe(VI) species and hydrogen peroxide (H2O2) based photochemical oxidation lead to better results. Until now, the best methods for BPA elimination in aquatic media seem to be photo-Fenton processes (Table 2, No 11) described by Felis et al. [32] and ozone oxidation processes described by Umar et al. [33]. Significant disadvantages of these methods are certainly the use of chemicals that often require (re-)activation of catalysts, pH control and/or neutralization steps. Moreover, ozone could cause the generation of many oxidation products, whose toxic potential can even be higher than BPA itself and whose hazardous properties have not been clarified yet [33]. In comparison, electrochemical methods for BPA degradation using different anode materials than BDD require long periods of degradation time and seem therefore unusable for application in wastewater treatment so far. Within the class of sonochemical treatments for BPA degradation described in literature, the system investigated in this research provides the best results. At low frequency (24 kHz) the rate constant for BPA degradation is 0.016 min-1 (UA = 25 µm, PI = 68 W) and 0.041 min-1 (UA = 125 µm, PI = 155 W). Compared to the results obtained by Lim et al. (Table 2, No 3,4,6), where rate constants of 0.006 min-1 (1 MHz) and 0.001 min-1 (300 kHz) were received [60]. Studies on biological degradation (Table 2, No 13) have shown the ability of these systems to remove even high BPA concentrations. Some bacterial strains like Sphingomonas bisphenolicum are capable of removing 115 mg L-1 BPA within 15 hours. Disadvantageous are on the one hand the required BPA degradation time and on the other hand, biological systems like Sphingomonas bisphenolicum are
14
highly substrate-specific and therefore not applicable for removing other micro pollutants, that can also occur in wastewater. Other methods like anodic polymerisation, where BPA is not removed by chemical elimination, but by polymerisation reaction on the electrode surface, have the disadvantage, that only small BPA concentrations could be removed (1.14 mg L-1 in 2 h). In addition, after a specific period of time, the electrode surface has to be cleaned and in this context additional chemicals are needed. For a better comparison, more aspects like energy requirements, technical implementation potentials, demand of chemicals and security aspects must be taken into account at comparable conditions (concentration of BPA, reaction volume, temperature). Table 2: Investigated sonoelectrochemical method for BPA degradation compared to methods described in literature No
Method
Source
c0 (BPA) (mgL-1)
V (mL)
Chemicals
tr (min)
D (%)
k (min-1)
1
AC/EC 24 kHz, 125 µm
This work
1
500
Na2SO4 [30mmol]
30
93
0.089
2
AC 300 kHz, 80 W
[34]
27
300
No chemicals added
90
100
n.a.
3
AC 1 MHz
[60]
10
1000
No chemicals added
120
98
0.006
4
AC 300 kHz
[60]
10
1000
No chemicals added
n.a.
n.a.
0.001
5
Sono-Fenton (300 kHz, 80 W)
[35]
27
300
FeSO4 (100 µmol L-1) H2O2 (35*10-3 mol L-1)
90
100
n.a.
6
AC/UV/TiO2
[60]
10
1000
TiO2
120
98
0.045
7
EC (Ti/BDD)
[30]
100
100
Na2SO4 (0.1 mol L-1)
390
100
n.a.
8
EC (BDD)
[61]
20
n.a.
Na2SO4 (0.1 molL-1)
240
98
n.a.
9
EC (Pt)
[30]
100
100
Na2SO4 (0.1 molL-1)
720
100
n.a.
8
28 kHz/stainless Steel [62] mesh 200 W, 0.2 W/L [63] Anodic Polym. [32] UV [32] Photo-Fenton
0.228
1000
No chemicals added
60
76
n.a.
1.14
50
Na2SO4 (0.1 mol L-1)
120
99
n.a.
10
n.a.
Hg-lamp (400 W)
30
100
n.a.
10
n.a.
Hg-lamp (400 W) H2O2 (100 mg L-1)
15
100
n.a.
40
100
TiO2 (1g L-1) Hg/Xe-Lampe (10mWcm-2)
15 h
99
n.a.
9 10 11
12
[64, 65] Photocatalytic
15
13
[17]
115
n.a.
Biological
14
720
100
n.a.
[33]
10
500
O3 (4.05 mg min-1)
10
100
n.a.
[66]
1.5
200
K2FeO4 (1 g L-1)
30
100
n.a.
O3 15
A01, S011 Sphingomonas bisphenolicum
K2FeO4
c0: initial concentration, V: reaction volume, t r: reaction time, D: degradation, k: rate constant, EC: electrochemical oxidation, AC: acoustic cavitation, n.a. data not available.
4. Conclusion Within this study it could be shown, that Bisphenol A could significantly be removed in an aquatic system via sonochemical, electrochemical and sonoelectrochemial methods using low frequency ultrasound (24 kHz, amplitude: 25 µm and 125 µm) and voltage of 5 V resp. 10 V. It could be shown that the combination of AC (24 kHz)/EC (5V) could degrade 90 % BPA of an initial concentration of 1 mg L-1 within 30 min. Furthermore synergistic effects were discovered for the combination of 24 kHz and 5 V. The best synergistic results were obtained for 2 cm electrode distance at high ultrasound amplitude (125 µm, PI = 155 W) (synergy-index: 2.268). Using higher potentials (10 V) the synergistic effects observed in the AC/EC system (24 kHz and 5 V) were absent and in some cases the synergy-index was even <1, which suggests a disturbing interdependence between AC and EC at high potentials (10 V). Possible causes like formation of scavenging species and/or processeses at the boundary layer liquid/electrode need to undergo further investigations. Finally could be shown, that the position of the sonotrode and the electrodes have substantial effect of the degradation results and therefore should be taken into account at the design of sonoelectrochemical reactors.
Acknowledgement The authors thank Wolfgang Faehndrich (FSU Jena) for the assistance in constructing the sonoelectrochemical systems.
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List of Figures Fig.1: Schematic representation of the 500 mL reactor; Electrodes are placed in the reactor with variable electrode distance ED. Ultrasound horn probe is placed with variable immersion depth ID. Cooling water CW. Reaction volume is 500 mL; BDD-electrodes (DIACHEM®).
Fig.2: Influence of ultrasound power input on BPA degradation; without electrodes, c(BPA): 1 mg L-1, ultrasound power densities: 26, 68.5, 104 and 155 W, n(Na 2SO4): 30 mmol, 500 mL.
Fig.3: Influence of ultrasound horn tip immersion depth on BPA degradation; without electrodes, c(BPA): 1 mg L-1, ultrasound power densities: blue 68.5 W, brown 155 W, n(Na 2SO4): 30 mmol, 500 mL, t = 30 min, ID = 3.1, 5.6, 8.1 cm.
Fig.4: Visualization of transversal oscillations via chemiluminescence of luminol; photographs of chemiluminescence, left: power input 68.5 W, right: 155.3 W luminol: 1 g L-1, Na2CO3: 5.0 g L-1, 500 mL, exposure time: 119 s, ISO 800, focus: 55 mm, focal ratio: F/5.6; graph: DCF degradation by ultrasound, Na2SO4: 30 mmol L-1, 5 min. White lines indicate the sonotrode and the reactor walls (not true to scale).
Fig.5: Influence of electrode distance on the sonochemical BPA degradation; (ED: 2, 2.5, 3 cm and without electrodes, c(BPA): 1 mg L-1, ultrasound amplitude: 25 µm (triangles) and 125 µm (squares), n(Na2SO4): 30 mmol, 500 mL).
Fig.6: Potential dependency on degradation of BPA; ED: 2.5 cmc(BPA): 1 mg L-1, n(Na2SO4): 30 mmol L-1, t = 5 min, 500 mL.
Fig.7: Influence of electrode distance (ED) on BPA degradation (with applied potential); ED: 2.5 cm, c(BPA): 1 mg L-1, n(Na2SO4): 30 mmol L-1, t = 5 min, 500 mL.
Fig.8: Influence of electrode distance (ED), ultrasound amplitude (UA) and ultrasound horn tip immersion depth (ID) on BPA degradation, at 0 V (a. and b.), 5 V (c. and d.) and 10 V (e. and f.); c(BPA): 1 mg L-1, n(Na2SO4): 30 mmol L-1, reaction tim t = 5 min, 500 mL.
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Highlights
sonoelectrochemical degradation of bisphenol A yielded to a removal of more than 90% degradation process follows first-order kinetics in comparison to single systems, synergistic effects were found distribution of hydroxyl radicals in the reactor was pictured by chemiluminescence
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