Sonocatalytic degradation coupled with single-walled carbon nanotubes for removal of ibuprofen and sulfamethoxazole

Accepted Manuscript Sonocatalytic degradation coupled with single-walled carbon nanotubes for removal of ibuprofen and sulfamethoxazole Yasir A.J. Al-Hamadani, Chanil Jung, Jong-Kwon Im, Linkel K. Boateng, Joseph R.V. Flora, Min Jang, Jiyong Heo, Chang Min Park, Yeomin Yoon PII: DOI: Reference:

S0009-2509(17)30030-1 http://dx.doi.org/10.1016/j.ces.2017.01.011 CES 13357

To appear in:

Chemical Engineering Science

Received Date: Revised Date: Accepted Date:

24 October 2016 3 January 2017 7 January 2017

Please cite this article as: Y.A.J. Al-Hamadani, C. Jung, J-K. Im, L.K. Boateng, J.R.V. Flora, M. Jang, J. Heo, C. Min Park, Y. Yoon, Sonocatalytic degradation coupled with single-walled carbon nanotubes for removal of ibuprofen and sulfamethoxazole, Chemical Engineering Science (2017), doi: http://dx.doi.org/10.1016/j.ces. 2017.01.011

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Sonocatalytic degradation coupled with single-walled carbon nanotubes for removal of ibuprofen and sulfamethoxazole

Yasir A. J. Al-Hamadania, Chanil Junga, Jong-Kwon Imb, Linkel K. Boatenga, Joseph R.V. Floraa, Min Jangc, Jiyong Heod, Chang Min Parka, Yeomin Yoona,*,

a

Department of Civil and Environmental Engineering, University of South Carolina, Columbia, SC 29208, USA

b

National Institute of Environmental Research, Water Environmental Engineering Research Division, Hwangyeong-ro, Seo-gu, Incheon 404-708, Republic of Korea c

Department of Environmental Engineering, Kwangwoon University, 20 Kwangwoon-ro, Nowon-Gu, Seoul 01897, Republic of Korea

d

Department of Civil and Environmental Engineering, Korea Army Academy at Young-Cheon, 495 Hogook-ro, Kokyungmeon, Young-Cheon, Gyeongbuk 38900, South Korea

*Corresponding author phone: 1-803-777-8952; fax: 1-803-777-0670; e-mail: [email protected] (Y. Yoon)

ABSTRACT

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This study examined the degradation of pharmaceuticals ((PhACs), ibuprofen (IBP) and sulfamethoxazole (SMX)) using an ultrasonic (US) reactor at a 1000 kHz frequency in the absence and presence of single walled carbon nanotubes (SWNTs). In the absence of SWNTs, maximum degradation of PhACs were achieved under high temperature; 55 > 35 > 25 > 15°C. In addition, the relatively higher degradation of IBP and SMX was obtained under acidic condition at pH 3.5 than pH 7 and 9.5; >99%, 79%, and 72% for IBP and >99%, 75%, and 65% for SMX, respectively. However, H2O2 production increased from 77 µM (no SWNTs) to 115 µM in the presence of SWNTs (45 mg/L) at pH 7. In addition, the removal of IBP and SMX significantly increased under US/SWNTs reaction conditions than US and SWNTs only reactions. The removal of IBP and SMX was 57% and 48% under SWNTs (adsorption) reactions, 77% and 70% under US reactions, and 97% and 92% under US/SWNTs reactions, respectively. This study evaluated the effect of temperature, pH, SWNTs, and physiochemical properties of selected PhACs under US process. In addition, the adsorption molecular modeling was validated with the experimental results.

Keywords: ibuprofen; sulfamethoxazole; sonocatalytical degradation; single-walled carbon nanotubes; molecular modeling

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1. Introduction Since the prevalent use of pharmaceutical (PhACs) products was identified as an environmental problem, numerous studies have focused on the sources, occurrence, and impact of these compounds in the aquatic environment (Kolpin et al., 2002; Snyder et al., 2003). Previous studies have reported that the concentrations of PhACs in surface water and wastewater effluent vary, with ranges from ng/L to g/L, creating unique challenges in water treatment processes that typically can remove only 10-20% of those compounds (Lishman et al., 2006; Rivera-Utrilla et al., 2013). Ibuprofen (IBP), a non-steroidal anti-inflammatory analgesic, has been detected between 0.002 and 24.6 µg/L in the effluent of several sewage treatment plants (Buser et al., 1999; Méndez-Arriaga et al., 2008). Sulfamethoxazole (SMX) has also been found with many other PhACs in wastewater treatment plant effluents (0.01-2 µg/L) and in surface waters (0.03-0.48 µg/L) (Hirsch et al., 1999; Méndez-Arriaga et al., 2008). Previous studies of selected group of micropollutants, such as pesticides and herbicides, have shown that coagulation, sedimentation, and filtration achieve only minimal levels of removal (Adams et al., 2002; Westerhoff et al., 2005). However, addition of common disinfectants (e.g., chlorine or ozone) can result in the reaction and transformation of these compounds (Lei and Snyder, 2007; Snyder et al., 2003; Westerhoff et al., 2005). The rate constants and oxidation mechanisms that accompany the use of chlorine (Huerta-Fontela et al., 2007; Westerhoff et al., 2005) and ozone (Beltran et al., 2010; Westerhoff et al., 2005) have been quantified for quite a large number of PhACs. Advanced oxidation processes (AOPs) are the most appropriate way of treatment to deal with these complex compounds (Huber et al., 2003; Klavarioti et al., 2009). Generally, among AOPs, the use of sonocatalysis in combination with TiO2, ZnO, glass bead, sand, steel beads, and

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Al2O3 represents a relatively new and effective technique for degrading contaminants (BejaranoPerez and Suarez-Herrera, 2008; Kaur and Singh, 2007; Shimizu et al., 2007; Wang et al., 2008). Ultrasonic (US) treatment is one of the recent AOPs technologies that can oxidize various complex organic pollutants (Koseoglu-Imer et al., 2013). The closest technology to the sonocatalytic degradation, is the photocatalytic degradation, which was used in the past to degrade organic and coloring pollutants (Sivakumar et al., 2010; Turki et al., 2015; Zhang et al., 2016). However, main differences between the photocatalytic degradation and the sonocatalytic degradation is that the ultrasonic waves have stronger penetrating power - resulting of the sonoluminescence and hot spot which are very effective in degrading complex contaminants than the photocatalytic waves, which made the photocatalytic process unsuitable for the treatment of complex compounds (Harichandran and Prasad, 2016; Zhang et al., 2016; Zhou et al., 2015). In addition, advantages of using US technology include ease of use and safety, short contact time, that it works without any additives to oxidize the contaminants, and there are minimal by-products generated after treatment (Hao et al., 2003; Mahvi, 2009; Teo et al., 2001). However, Gogate et al. (Gogate et al., 2004) indicated that the presence of solid particles in the ultrasound system has the potential to increase the intensity of cavitation and decrease the energy transmitted, which increase the ultrasonic efficiency. Because an increase in the intensity will result in a decrease in the collapse pressure for single cavity and increase the number of cavitational bubbles lead to an enhancement in the sonochemical activity (Sivakumar and Pandit, 2001). Carbon nanotubes (CNTs) are considered as attractive adsorbents for the removal of heavy metals, organic and inorganic matter, pharmaceuticals, personal care products, and endocrinedisrupting compounds (Jung et al., 2013). Their ability to adsorb various types of contaminant is

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due to the unique properties of CNTs, including their electrical conductivity, optical activity, and mechanical strength. Additionally, two studies have shown that nanoparticle-CNT hybrid materials and CNTs can be used as catalysts (Al-Hamadani et al., 2015; Wang et al., 2012; Yang et al., 2006). Other solid particles such as loquat seeds (Hamdaoui, 2011), corn-cob-activated carbon (Milenković et al., 2013), and granular activated carbon are commonly applied during US reactions to improve the generation of OH • via increasing the presence of cavitation bubbles but also physically adsorb the pollutant onto the surface of the particles (Zhao et al., 2011). Recent studies show very attractive results using sonocatalycal removal for dye in aqueous system such as; Soltani et al., (2016) who used ultrasonic with ZnO and found great removals of decolorization of methylene blue (MB) dye in the aqueous phase (Soltani et al., 2016). Also, a separated study has reported that SonoFenton methods effectively decolorize DR81 dye in waste water (Harichandran and Prasad, 2016). Despite the significant growth in CNT use and sonocatalysis in water and wastewater treatment, much is still unknown, such as, for example, how sonocatalysis coupled with CNTs influences the transport of PhACs, while removal mechanisms (e.g., sorption of PhACs onto CNTs and thermal degradation/oxidation during sonocatalytic degradation) are relatively well known. Molecular-level simulations can also provide unique insight into the molecular interactions among PhACs and CNTs. Previous studies have clarified the adsorption process of contaminants onto carbon nanomaterials by applying the quantum chemistry and molecular dynamics simulations (Arsawang et al., 2011; Mucksch and Urbassek, 2011; Zhao and Johnson, 2007). The main objectives of this study were to evaluate the removal efficiency of two PhACs (IBP and SMX) under US irradiation, to estimate the effect of US coupled with single-walled carbon

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nanotubes (SWNTs) on the removal of IBP and SMX, and to illustrate the mechanism of US/SWNTs via molecular modeling. Sonocatalytic novelty of a sonodegradation process combined with CNT adsorption is, at one level, a relatively simple approach that involves the combination of two existing technologies (i.e., oxidation and adsorption). In addition, limited work has been reported on the adsorption behavior of PhACs on SWNT at the molecular level. Therefore, the adsorption of IBP/SMX on SWNTs was simulated in this study in order to validate the modeling with the experiment data to better understand the mechanism of dispersed SWNT particles. The processes were carried out as a function of temperature (15, 25, 35, and 55°C) and pH (3.5, 7, and 9.5) at a frequency of 1000 kHz. We hypothesized that sonocatalytic degradation coupled with SWNTs for the target PhACs would be enhanced for two reasons: (i) IBP and SMX removal due to adsorption will be enhanced, because ultrasonication greatly enhances the dispersion and debundling of SWNTs, providing more adsorption sites for IBP and SMX, and (ii) SWNTs will act as a catalyst to enhance and promote sonochemical reactions.

2. Materials and methods 2.1. Chemicals High-purity IBP (C13H18O2, > 98%) and SMX (C10H11N3O3S, > 98%) were obtained from Sigma-Aldrich (St. Louis, MO, USA). The target PhAC characteristics are summarized in Table 1, from the SRC PhysProp Database (SRC, 2006). Potassium hydrogen phthalate (C8H5KO4, 99.95%), potassium iodide (KI, 99%), ammonium molybdate tetrahydrate (H 24Mo7N6O24·4H2O), and H2O2 (30% w/v used to measure H2O2 production, were also purchased from Sigma-Aldrich. High purity SWNTs having a length of 5-30 µm and an outer diameter of 1-4 nm and were

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obtained from Cheap Tubes, Inc. (Brattleboro, VT, USA). Stock solutions of IBP, SMX, and SWNTs were prepared in ultrapure deionized (DI) water.

2.2. Apparatus In this study, US and US/SWNTs experiments were conducted in a US generator (Ultech, Dalseo, Daegu, South Korea) having a double-jacketed stainless steel reservoir (L×W ×H, 15×10×20 cm) with a water-cooled (Fisher Scientific Inc., Pittsburgh, PA, USA) at a fixed frequency of 1000 kHz with applied power of 180±3 W at various temperatures. The contact time of 60 min was employed to determine the effect SWNTs (0, 5, 15, and 45 mg/L (ppm)) on sonodegradation, as well as in the absence and presence of PhACs at 10 µM. Fig. S1 shows a diagram of the experimental set-up. Batch adsorption experiments were conducted to assess the adsorption capacity of SWNTs without sonication. SWNTs were hydrated for 24 hours in DI water prior to and added as a slurry to the sample reactor. Both US/SWNTs and adsorption experiments were conducted using 1000 mL of this initial solution. Samples taken periodically were filtered with 0.22 µm glass microfiber filters (Whatman, Little Chalfont, Buckinghamshire, UK) for additional analysis.

2.3. Analysis To perform a complex and immediate analysis, high-performance liquid chromatography (HPLC, 1200 series; Agilent Technologies, Santa Clara, CA, USA) was employed to measure IBP and SMX. The mobile phase solvent profileof 40:60 (DI water:acetonitrile) for IBP and 50:50 (DI water:acetonitrile) for SMX was used. Separation was achieved under the following conditions: a LiChrosorb RP-18 analytical column (4.6 mm × 100 mm i.d., 5 µm particles,

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Atlantis; Waters, Milford, MA, USA); a flow rate of 1.0 mL/min for IBP and 0.75 mL/min for SMX with a 100 µL sample loop; a detection wavelength of 210 nm for both compounds. The KI dosimetry method was employed to determine the H 2O2 concentration using an ultraviolet-visible (UV-vis) spectrophotometer (Agilent Technologies) at 350 nm during US reactions (Kormann et al., 1988). A Zeta potential analyzer (ZetaPLAS, Brookhaven Instruments Corporation, Holtsville, NY, USA) was employed to determine zeta potentials of SWNTs before and after sonication . The zeta was calculatedwith the electrophoretic mobility (µ) values using the Smoluchowski equation: (1) where ξ is the zeta potential, ε is the dielectric constant of the medium (water), V is the applied voltage, η is the viscosity of the suspension, and d is the electrode separation (Saleh et al., 2008). The average hydrodynamic radii of the SWNT clusters and their size distribution were determined with a robust dynamic light scattering and static light scattering DLS/SLS instrument (ALV/CGS-3, Langen, Germany), equipped with a 22 mW He-Ne laser at 632 nm (equivalent to an 800 mW laser at 532 nm) and a sensitive high-QE APD detector with photomultipliers. The average cluster size of a SWNT suspension before and after sonication was determined for a 090° scattering angle. The measured distribution data was used to calculate the average hydrodynamic diameter of SWNT particles.

2.4. Molecular modeling for solutes and adsorbents The original molecular structures of each SWNTs and the PhACs were produced using Gaussview (Dennington et al., 2003) and improved using dispersion-corrected density functional theory (DFT-D) (Grimme et al., 2010; Grimme et al., 2011) with the B3LYP5 functional and the

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6-31G basis set in TeraChem (Kastner et al., 2009; Ufimtsev and Martinez, 2009). The geometries of the SWNTs-PhACs complexes (SWNTs-IBP and SWNTs-SMX) were improved by following geometry optimization processes described in previous studies (Zaib et al., 2012). The adsorption of IBP and SMX onto SWNTs in aqueous solution was measured using the SMD continuum solvation model applied in GAMESS (Gordon and Schmidt, 2005; Gordon et al., 2007; Schmidt et al., 1993; Smith et al., 2008). Aqueous phase energy calculations were conducted at the DFT-D/B3LYP5/6-31++G(d,p) level, and the binding energies (∆E) between the SWNTs and PhACs were calculated as:

A negative value of the binding energy shows a positive interaction and a constant SWNTsPhACs system. Aqueous phase binding energies were determined by considering the dissociated (pH > pKa) and un-dissociated (pH < pKa ) forms of the PhACs to demonstrate the effect of pH on the adsorption mechanism.

3. Results and discussion 3.1. Effect of temperature on PhAC degradation and H2O2 formation 3.1.1. PhAC degradation Temperature is an important factor in the US process because sonication produces cavitational bubbles that have high temperatures and vapor pressures over time. Thus, the effects of aqueous temperature on sonochemical reaction rate of selected PhACs were investigated at a 1000 kHz frequency with an US power of 180 ± 3 W. The pseudo-first-order rate (k) of IBP and SMX increased when the temperature increased from 15 to 55°C (Fig. 1). The increase in temperature affects the cavitational intensity due to the change in the physicochemical properties

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of the compound and the type of cavities formed, which can affect the kinetic rate constant for the degradation reaction (Golash and Gogate, 2012). Four important parameters were affected when the temperature of the solution increased: (i) Cavitational energy decreased, (ii) the threshold limit of cavitational energy, required to produce cavitation, decreased, (iii) the quantity of dissolved gas was reduced, leading to the transfer of organic molecules from bulk solution to the gas-liquid interface region, and (iv) the vapor pressure increased, causing cavitation bubbles to comprise more water vapor (Im et al., 2014; Jiang et al., 2006). Previous studies have estimated the effect of temperature on different pharmaceuticals, such as acetaminophen, naproxen (Im et al., 2014) and diclofenac (Naddeo et al., 2010), and dyes, such as rhodamine B (Behnajady et al., 2008), and found a proportional relationship of the degradation rate to the temperature. Because the degradation rate is proportional to the temperature, the reaction can be assumed to follow the Arrhenius equation (Eq. (3)),  ln k   ln A 

Ea RT

(3)

where k = pseudo-first-order rate constant (min-1), A = Arrhenius coefficient, Ea = apparent activation energy (kJ/mol), R = the gas constant (8.314 J/mol.K), and T = temperature (K). Fig. 1a shows the correlation between -lnk and 1/T, where the apparent activation energy was 17.49 kJ/mol (R2 = 0.961) for IBP and 7.28 kJ/mol (R2 = 0.977) for SMX at pH 7. These low apparent activation energy values indicate that the degradation of PhACs is influenced by diffusion (Im et al., 2014). This is presumably because the degradation rate apparently reflects IBP and SMX molecules in bulk solution moving to the gas-liquid interface region, where the temperatures and OH• concentrations are high (Im et al., 2014; Kim et al., 2001).

3.1.2. H2O2 formation 9

During US treatment, high temperatures (5000 K) and pressures (1000 atm) in the cavities created are reached in a very short time, leading to the dissociation of water molecules to H• and OH• (Guo et al., 2015; Méndez-Arriaga et al., 2008). In this study, different temperatures (15, 25, 35, and 55°C) were studied to understand the effects of temperature on PhACs degradation and H2O2 production. By increasing the temperature of the solution, from 15 to 55°C, the degradation of IBP and SMX increased (Table S1). This could be explained by the properties of US process in generating hydroxyl radicals. However, the relationship between temperature and H2O2 production can be used as an indirect method of measuring OH• concentration. When the temperature increased, the cavitation threshold decreased, leading to increased numbers of cavitation bubbles on sonolysis and thus increased OH• production (Jiang et al., 2006). When the temperature increased, the free OH• increased for a particular limit, as shown in Figs. 1b for IBP and 1c for SMX due to the high collapse of cavitation bubbles, resulting in increased destruction of the compounds. In contrast, other studies found that the increase in the temperature has an adverse effect on the degradation rate because as temperature increased, the surface tension and viscosity of the solution increase, which make it easy to generate cavitational bubbles but lower cavitational intensity due to the increase in the vapour pressure of the liquid (Sivakumar et al., 2002).

3.2. Effects of pH on PhAC degradation and H2O2 formation 3.2.1. PhAC degradation In US processes, pH is also an important factor, affecting the degradation of compounds (Lin and Ma, 1999; Sivakumar and Muthukumar, 2011). Consequently, experiments were performed at pH values of 3.5, 7, and 9.5 to better understand the effects of pH on the selected PhACs

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degradation, The pH values chosen were acidic conditions, pH 3.5, below the pKa values of IBP and SMX (4.52 and 5.81, respectively), above the pKa values at neutral conditions (pH 7), and clearly alkaline conditions at pH 9.5. Under acidic conditions at pH 3.5, complete degradation of both IBP and SMX was achieved within 50 and 60 min, respectively. However, the degradation decreased with increasing the pH to 7 and then 9.5, as shown in Figs. 2a for IBP and 3b for SMX. This could be explained by the acid-basic properties of each compound with its pKa value; below the pKa values of IBP and SMX, the compounds would be in their molecular forms, while above the pKa values, they would be in their ionic forms. Thus, at pH 3.5 the selected compounds have greater hydrophobic characteristics when their structures are in the molecular form and would accumulate at the boundary of cavitation bubbles, where the OH• concentration is higher, leading to higher degradation. However, at pH values higher than their pKa values, the hydrophilicity and solubility would be superior because the compounds dissociate to their ionic forms and thus degradation will occur in the bulk liquid region where the OH• concentration is lower; as a result degradation was slower (Im et al., 2013; Méndez-Arriaga et al., 2008; Soltani et al., 2016). Moreover, Fig. 2c clearly shows that higher IBP and SMX degradation rate constants were achieved with decreasing pH value: pH 3.5 > pH 7 > pH 9.5. This is presumably because larger amounts of OH• interact with IBP and SMX under lower pH conditions. In contrast, the lower oxidation potential was due to the smaller amounts of free OH•, which tend to recombine to form H2O2 at higher pH values. The difference in the removal efficiencies of IBP and SMX is due to differences in their physicochemical properties (Her et al., 2011), such as logKOW, water solubility (SW), vapor pressure, and Henry’s law constant. IBP has higher logKOW (3.84) and lower solubility (0.049 g/L at 25°C) than SMX (logKOW = 0.79, SW = 0.459 g/L at 25°C) in water, as described in Table 1. Thus, higher degradation of IBP was achieved because

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OH• induced reactions are likely the major degradation mechanism and the reaction occurs at the boundary of the cavitational bubbles where the more OH• presents (Manickam et al., 2014). However, for the more hydrophilic and nonvolatile SMX, due its low logKOW and high SW, the reaction may occur more at the gas-liquid interface and the bulk liquid region where smaller amounts of free OH• are present (Im et al., 2013).

3.2.2. H2O2 formation As previously described, H2O2 is likely the key parameter to understand the degradation mechanism of US. Thus, the concentrations of H2O2 for the selected pH values were measured to examine the degradation of IBP and SMX in the US process. The generation of H 2O2 increased with increasing pH in the absence and presence of the compounds (Fig. 2d): pH 9.5 > pH 7 > pH 3.5. At the lower pH condition, where the uncombined OH• concentration is maximal, the generated OH• is more likely to react with PhACs than OH • to produce H2O2. The generation of OH• increases due to the decomposition of H2O2 at pH 3.5, which leads to more free radicals enhancing the oxidation of the IBP and SMX in the system (Harichandran and Prasad, 2016). In contrast, the highest accretion of H2O2 was observed at the highest pH due to the more rapid recombination of OH• being than attacking PhACs. Because at pH > pKa, high concentration of OH- occurs that enhance the production of H2O2 , also, less degradation achieved because the hydrophilicity of the compounds dominate and reactions are likely to be carried out in the bulk liquid where the OH• concentration is lower (Im et al., 2014). To calculate the H2O2 consumption required for the complete degradation of 10 µM IBP and 10 µM SMX, the following stoichiometric calculations were used (Eq. (4)) for IBP and (Eq. (5)) for SMX:

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C13H18O2 + 33H2O2 → 13CO2 + 42H2O

(4)

C10H11N3O3S + 33H2O2 → 10CO2 + 37H2O + 3HNO3 + SO3

(5)

Thus, from Eqs. (4) and (5), 165 µM H2O2 for each reaction was required theoretically to fully degrade IBP and SMX in the system. However, H2O2 generated via ultrasonication at the different pH values in the absence of PhACs was considerably less than 165 µM. As shown in Fig. 2d, a maximum 87 µM H2O2 was achieved at pH 9.5 in the absence of PhACs. Although the US did not provide sufficient amounts of H2O2 to completely remove the selected PhACs based on Eqs. (4) and (5), complete degradation of IBP and SMX was achieved at pH 3.5 within 50 and 60 min, respectively. This could be explained by H2O2 acting as both a hydroxyl scavenger (Eq. (6)) and as a hydroxyl source (Eq. (7)) under sonochemical conditions. Thus, the remaining H 2O2 concentration was not quantified exactly based on the experimentally determined H 2O2 concentration during the reactions. H2O2 + OH• → H2O + HO2• [k = 2.7×107 1/M.s]

(6)

H2O2 + H• → OH• + H2O [k = 9.0×107 1/M.s]

(7)

3.3. Effect of SWNTs on PhAC removal and H2O2 formation 3.3.1. H2O2 formation To evaluate the effect of solid surfaces on the US process, SWNTs at various initial concentrations were added to aqueous samples at pH 7. Under 1000 kHz ultrasonication, H 2O2 production was determined in the presence of 0, 5, 15, and 45 mg/L SWNTs to determine the optimum concentration that generates the greatest quantity of free OH •. Figs. 3a and 3b show that H2O2 production increased with increased initial concentrations of SWNTs. The increase in H 2O2 production was because the dispersed SWNTs particles tended to act as additional nuclei for the

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pyrolysis of water molecules and formation of OH• and can thus be used to quantify the effectiveness of reactors in generating the desired cavitational intensity (Her et al., 2011; Im et al., 2013). Therefore, 45 mg/L was used for PhACs removal due to the higher oxidation activities caused by the higher number of free OH• in the system. In the absence of PhACs, higher concentrations of H2O2 were achieved with increasing SWNTs in the US/SWNTs reaction than with US alone and the concentration of H2O2 in the presence of PhACs decreased. This could explain the difference in H2O2 concentrations in the absence and presence of PhACs; it is due to the amount of free OH• that interact with IBP and SMX.

3.3.2. PhAC removal Fig. 3c shows higher removal of IBP and SMX under US/SWNTs reaction conditions versus US and SWNTs only reactions. Moreover, the removal rate constants of IBP and SMX were higher under US/SWNTs reactions, followed by US and SWNTs only reactions, as shown in Fig. 3d. This could be explained in two ways: (i) The removal increased in the presence of SWNTs due to an increase in the number of free OH• because the dispersed particles acted as additional nuclei for the pyrolysis of water molecules and formation of OH •, leading to an increase in the oxidation activities; and (ii) the dispersed particles increased the adsorption capacity of SWNTs because ultrasonication of CNT dispersions is used to break up CNT agglomerates in solution, which would also lead to increased adsorption activities for the compounds (Im et al., 2013; Krause et al., 2010; Zhang et al., 2016). This indicates that SWNTs play a major role due to their interaction with PhACs during US processes. To understand the removal of PhACs and H2O2 production mechanism, the mechanisms of US and US/SWNT processes were proposed in Fig. S3. Fig. S3 shows the possible mechanisms

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and the interaction of OH•/PhACs and SWNTs/PhACs in the aqueous system. The adsorption was enhanced due to the dispersion of SWNTs, resulting in more adsorption sites due to the ultrasonication activity. Therefore, the combinations of these oxidation and adsorption in the US/SWNTs process are the main focus of this study. Furthermore, to understand the effect of SWNTs during US reaction, synergism was assessed based on Eq. (8), which was adapted from Madhavan et al. (Madhavan et al., 2010). The synergy index is determined by assessing the difference between the rate constants obtained under US/SWNT and the sum of those obtained under separate SWNT adsorption and US reactions, as summarized in Table 2.

Synergy 

k (US  SWNTs)

(8)

k (US )  k ( SWNTs)

A synergy index > 1 indicates that the combined US/SWNT process exceeds the sum of the separate US and SWNT adsorption processes. The deceasing synergy index values for IBP from 1.71 at 10 min to 1.17 at 60 min and for SMX from 1.47 at 10 min to 1.12 at 60 min indicated that SWNTs played a more important role at the beginning of the process in terms of adsorption. The results in Figs. 3c and 3d suggest higher removal of IBP than SMX in all reactions (US, SWNTs, and US/SWNT). As described previously, under US reaction conditions, higher removal of IBP was obtained due to the high logKOW and low solubility versus SMX. In the adsorption reaction, the removal of IBP also was higher due to the strong hydrophobicity and lower water solubility compared to the logKOW and water solubility of SMX. The hydrophobicity of compounds can be expressed by their octanol–water partition coefficients (logKOW), an important factor in evaluating adsorption capacity (Yu et al., 2008). A higher logKOW value indicates higher sorption affinity to the adsorbent material: the SWNTs in this study. However, in the adsorption reaction, the removal rate of SMX was faster than IBP in the 15

first 30 min (Fig. 3c). In the first 30 min, the repulsion between SMX and the SWNTs decreased due to the complexation or iron pair of SMX with SWNTs ions at pH 7, which results in an increase in the adsorption of SMX onto the SWNTs (Zhang et al., 2010). Thus, SMX adsorption rate on SWNTs increased until reaching equilibrium. In addition, the adsorbed cations may make sites available for cation association, leading to an increased removal rate for the first 30 min (Zhang et al., 2011). However, the size exclusion (pore size) of SWNTs and the volume of SMX (204.6 Å3), which is less than the IBP volume (211.8 Å3), could be another factor enhancing the affinity of SMX molecules than IBP (Pan et al., 2013). Taken together, these results indicate that the removal of PhACs was dependent on their physicochemical properties.

3.3.3. Effect of US on SWNTs To understand the effects of US on SWNTs, the hydrodynamic radii of the SWNTs were measured by DLS before ultrasonication, after 10 min, and after 60 min of ultrasonication. Fig. 4a shows a significant decrease in the hydrodynamic radius of SWNTs from 140 nm before ultrasonication to 80 and 70 nm after 10- and 60-min ultrasonication, respectively. Moreover, Fig. S2 shows the effects of ultrasonication on the dispersion and stabilization on SWNTs (45 mg/L). Clearly, SWNTs after treatment were found in small agglomerates or bundles versus before treatment, indicating that ultrasonication played a major role in dispersing the SWNTs. The dispersion of SWNTs takes place, which reduces the negatively charged state of SWNTs, as shown in Fig. 4b. Dreyer et al. (Dreyer and Bielawski, 2012) reported that sonication can enhance the exfoliation of graphite oxide (GO), causing ‘destruction’ of GO platelets. Thus, GO may undergo greater graphite exfoliation during sonication, losing more surface functional groups and negative charges due to the breakup of aggregated GO clusters. Bai et al. (Bai et al.,

16

2017) also reported that due to the harsh condition provided by the ultrasonic irradiation, a mechanical shear stress would be introduced to the individual layers of GO due to Van der Waals interactions and π-π bonding, which simplifies the exfoliation GO sheets. This result is consistent with other reports of CNTs by Krause et al. (Krause et al., 2010)and with SWNTs by Niyogi et al. (Niyogi et al., 2003). These findings may support the idea of high removal of IBP and SMX in the presence of SWNTs during US treatment due to the high dispersion of SWNTs. This leads to an increased SWNT surface area, which enhances the adsorption process and increases OH• generation, thus enhancing the oxidation process. Furthermore, the zeta potential of SWNTs before sonication, and after 10 and 60 min ultrasonication was determined to estimate the effect of ultrasonication on the surface charge of SWNTs. SWNTs were examined for surface charge using zeta potential measurements over the pH range of 3.5-9.5. Fig. 4b shows the decrease in surface charge of the SWNTs particles with increasing ultrasonication time. For example, at pH 7, the zeta potential negatively decreased from -18 mV before sonication to 5 and 22 mV after sonication for 10 and 60 min, respectively. This can be explained by the behavior of surface functionalization; the presence of oxygencontaining functional groups, such as carboxyl, hydroxyl, and carbonyl, on the surface of SWNTs negatively increases the zeta potential (Li et al., 2008). Due to the sonication, the SWNTs’ loss of negatively charged functional groups over a wide range of pH values indicates a decrease in the amount of functional groups, because OH‫ ־‬on the surface of SWNTs is protonated and becomes OH2+. Thus, a positively charged surface is created due to the high concentration of protonated functional groups produced during sonication (Jung et al., 2013).

3.4. Binding energies of PhACs on the SWNTs

17

The adsorption of un-dissociated PhACs onto SWNTs is mainly due to π–π and van der Waals interactions between the adsorbent and the PhACs. In its un-dissociated form, SMX interacts favorably with SWNTs compared to IBP (–23.1 vs. –14.5 kcal/mol) because of its larger molecular structure and greater surface area exposure to the SWNTs. As shown in Fig. 5, both rings in SMX are oriented towards the surface of the SWNTs, thus maximizing π–π interactions between SMX and the adsorbent. However, considering that IBP has a higher logKOW (3.84) and lower solubility (0.049 g/L at 25°C) than SMX (logKOW = 0.79, SW = 0.459 g/L at 25°C) in water, which are the main factors affecting the adsorption mechanism, the hydrophobicity effect could play a major role in the adsorption of PhACs onto SWNTs (Jung et al., 2015). In the dissociated form, the binding of IBP onto SWNTs was greater than that of SMX (–11.2 vs. –5.8 kcal/mol), which was consistent with the trend of the adsorption experiment where IBP removal was higher than SMX at pH 7 (see Fig.3d). In the dissociated form, the negative charge on the SMX is on the N atom in the middle of the SMX molecule, causing both rings to be oriented away from the surface of the SWNTs and reducing the π–π interactions between SMX and the adsorbent. In contrast, the negative charge on IBP is on the carboxylic group at the edge of IBP molecule, which still allowed interaction of the IBP ring with the SWNTs to be intact. Thus, the net reduction in binding energy is larger for SMX than for IBP. The binding energies are still favorable because the binding strength is dominated by π–π and van der Waals interactions between the molecules and the adsorbent.

4. Conclusions

18

Sonocatalytical degradation of two target PhACs (IBP and SMX) having different physicochemical properties was carried out in the absence and presence of SWNTs at a frequency of 1000 kHz. While the degradation of IBP and SMX depended on temperature and pH, the maximum degradation efficiencies of IBP and SMX were achieved under optimum pH of 3.5 and temperature 35°C in the absence of SWNTs. However, the removal of IBP and SMX was enhanced when SWNTs were added to the system. Higher removal was obtained under US/SWNT than the sum of those obtained under SWNTs and US- only reactions. The role of SWNTs in this study approved our hypothesis referring to the enhancement of the oxidation and adsorption activities when SWNTs are added to the system due to the dispersion of SWNTs under US irradiation. In addition, H2O2 formation significantly increased in the presence of SWNTs, indicating that the SWNTs dispersed particles performed as additional nuclei for the pyrolysis of water molecules and formation of OH•. Higher removal of IBP was achieved than that of SMX under US reaction, SWNTs adsorption, and US/SWNTs reactions due to their chemical properties. Furthermore, results of DFT-D calculations were consistent with the experimental results and provided insight on the adsorption of IBP and SMX onto SWNTs in aqueous system at different pH levels.

Acknowledgments This research was supported by the Korea Ministry of Environment, ‘GAIA Project, 2015000540003’. This research was also supported by the Ministry of Higher Education and Scientific Research of Iraq and the Iraqi Cultural Office in Washington D.C.

19

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29

4.0

IBP SMX

-lnk = 876.4/T –1.148 R2 = 0.977

-lnK (1/min)

3.5

-lnk = 2105/T –4.128 R2 = 0.961

3.0 2.5 2.0

(a)

1.5 0.0030 0.0031 0.0032 0.0033 0.0034 0.0035 0.0036

1/T (K) 100

o

15 C 25o C

80

35o C

H2O2 (µM)

55o C

60 40 20

(b) 0 0

100

10

20

30

40

50

60

15o C 25o C 35o C

H2O2 (µM)

80

55o C

60 40 20

(c) 0 0

10

20

30

40

50

60

Time (min)

Fig. 1. Effect of temperature on (a) the degradation of IBP and SMX. H2O2 production in the presence of (b) IBP and (c) SMX at various temperatures, pH 7, 0.18 W/mL and 1000 kHz. Error bars are smaller than the symbols in most cases. 30

100 80 60 40 20

40

(a)

(b) 0

0

10

20

30 40 Time (min)

IBP SMX

50

60

0

100

(c)

10

(d)

10µM IBP 10µM SMX

0.06 0.04

20

30

40

50

60

30 40 Time (min)

50

60

W/o (pH 3.5) W/o (pH 7) W/o (pH 9.5) IBP (pH 3.5) IBP (pH 7) IBP (pH 9.5) SMX (pH 3.5) SMX (pH 7) SMX (pH 9.5)

80

0.08 H2O2 (µM)

k1 rate constant (1/min)

60

20

0

0.10

SMX (pH 3.5) SMX (pH 7) SMX (pH 9.5)

80 C/Co %

C/Co %

100

IBP (pH 3.5) IBP (pH 7) IBP (pH 9.5)

60 40 20

0.02

0

0.00 pH 3.5

pH 7

pH 9.5

0

10

20

Fig. 2. Effect of pH on the degradation of (a) IBP and (b) SMX, (c) degradation rate constants of IBP and SMX, and (d) H2O2 production for different reactions at 15 ± 1°C, 0.18 W/mL, and 1000 kHz. Error bars are smaller than the symbols.

31

120

5 ppm SWNTs 45 ppm SWNTs

No SWNTs 15 ppm SWNTs

H2O2 (µM)

100 80 60 40 20 (a)

(b)

0

100

IBP (SWNTs) IBP (US) IBP (SWNTs/US)

C/Co (%)

80

SMX (SWNTs) (c) SMX (US) SMX (SWNTs/US)

60 40

1

20

k rate constant (1/min)

0

0 0

10

20

30

40

50

2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0

10

30 40 Time (min)

50

60 (d)

IBP SMX

SWNTs

60

20

US

SWNTs/US

Time (min)

Fig. 3. (a) Effect of SWNTs on H2O2 production in the absence and presence of IBP and SMX, (b) effect of SWNTs concentration on H2O2 production, (c) degradation of IBP and SMX under different reaction conditions, and (d) degradation rate constants of IBP and SMX of each reaction at 15 ± 1°C, pH 7, 0.18 W/mL, and 1000 kHz. Error bars are smaller than the symbols in most cases.

32

Hydrodynamic radius (nm)

200

(a)

150

100

50

0 0

Zeta potential (mV)

60

10 US Time (min)

60

(b)

US 0 min US 10 min US 60 min

40 20 0 -20 -40 3

4

5

6

7

8

9

10

pH Fig. 4. Effect of ultrasonication on (a) hydrodynamic radius of SWNTs and (b) zeta potential of SWNTs at 15 ± 1°C, pH 7, 0.18 W/mL, and 1000 kHz.

33

(a.1)

(a.2)

(b.1)

(b.2)

Fig. 5. Molecular modeling of adsorption mechanisms of (a.1) IBP un-dissociated form, (a.2) IBP dissociated form, (b.1) SMX un-dissociation form, and (b.2) SMX dissociated form.

34

Table 1. Properties of ibuprofen and sulfamethoxazole studied in this paper. Pharmaceuticals

Ibuprofen (IBP)

Sulfamethoxazole (SMX)

Classification

Analgesic

Analgesic

206.3

253.3

pKa

4.52

5.81

LogKOW

3.84

0.79

0.049

0.459

Molecular weight (g/mol)

Water solubility (g/L at 25ºC) Henry’s law constant 3

(atm·m /mol)

1.50×10

-7

6.42×10

Vapor pressure (mmHg at 25 ºC)

0.000139

Chemical structure

35

NA

-13

Table 2. Pseudo-first order (k1) rate constants, coefficient of determination (R2), and synergistic index values within 10 and 60 min in different reactions (15 ± 1°C, pH 7, and 1000 kHz). Within 10 min IBP Processes SWNTs (mg/L)

US only

SWNTs (adsorption)

US/SWNTs

Synergistic index values

Within 60 min

SMX

IBP

k1

k1

-2

-2

(×10 1/min)

(×10 1/min)

SMX R2

k1 -2

(×10 1/min)

R2

-

3.73

2.53

1.85

0.939

1.35

0.980

45

3.11

2.62

0.99

0.985

1.03

0.940

45

11.7

7.58

3.25

0.953

2.68

0.962

IBP

SMX

IBP

SMX

1.71

1.47

1.17

1.12

45

36

Supplementary Data

Fig. S1. Schematic diagram of the ultrasonic system.

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(a)

(b)

Fig. S2.Visual examination of SWNTs solution with and without sonication: (a) SWNTs without ultrasonication and (b) SWNTs with ultrasonication. (SWNTs = 45 mg/L, pH = 7, and 1000 kHz).

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US/SWNT process OH•

H2O2

SWNT process PhACs

US

Cavitation bubbles

US process H2O PhACs

+ H• +

HO2 •

SWNTs PhACs

O2 + H•

Dispersed SWNTs

Reaction

By-products CO2 +H2O

Adsorption

PhACs By-products

US Desorption

By-products

PhACs

Fig. S3. Proposed mechanisms of US and US/SWNT processes.

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Table S1. Degradation efficiencies of PhACs and coefficient of determination (R2) at different solution temperatures (pH 7, contact time = 60 min, and 1000 kHz).

IBP degradation

SMX degradation

2

2

ºC

%

R

%

R

15

77.2

0.925

72.2

0.963

25

86.4

0.889

80.8

0.952

35

98.5

0.899

87.7

0.924

55

>99

0.916

90.34

0.952

40

Graphical abstract

Gaseous region

HO·

e-

HO·

H·

: Micropollutants

H· H·

HO2· HO·

HO2·

HO•

US

H·

Hydrophobic & volatile

H2O

Moderate hydrophobic & moderate volatile

HO·

Bulk liquid region HO· HO·

H·

H· H·

HO· Hydrophilic & non-volatile

:CNTs

Gas-liquid interface region

28 kHz

HO• 1000 kHz

HO•

HO•

HO•

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Research Highlights 

Sonocatalyical degradation of ibuprofen and sulfamethoxazole was investigated.



Degradation of PhACs was enhanced in the presence of SWNTs.



Adsorption binding energies between of PhACs and SWNTs were calculated.

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