Adsorption behavior and mechanism of chloramphenicols, sulfonamides, and non-antibiotic pharmaceuticals on multi-walled carbon nanotubes

Accepted Manuscript Title: Adsorption behavior and mechanism of chloramphenicols, sulfonamides, and non-antibiotic pharmaceuticals on multi-walled carbon nanotubes Author: Heng Zhao Xue Liu Zhen Cao Yi Zhan Xiaodong Shi Yi Yang Junliang Zhou Jiang Xu PII: DOI: Reference:

S0304-3894(16)30172-8 http://dx.doi.org/doi:10.1016/j.jhazmat.2016.02.045 HAZMAT 17478

To appear in:

Journal of Hazardous Materials

Received date: Revised date: Accepted date:

29-12-2015 17-2-2016 21-2-2016

Please cite this article as: Heng Zhao, Xue Liu, Zhen Cao, Yi Zhan, Xiaodong Shi, Yi Yang, Junliang Zhou, Jiang Xu, Adsorption behavior and mechanism of chloramphenicols, sulfonamides, and non-antibiotic pharmaceuticals on multi-walled carbon nanotubes, Journal of Hazardous Materials http://dx.doi.org/10.1016/j.jhazmat.2016.02.045 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Adsorption behavior and mechanism of chloramphenicols, sulfonamides, and non-antibiotic pharmaceuticals on multi-walled carbon nanotubes Heng Zhaoa, Xue Liua, Zhen Caoa, Yi Zhana, Xiaodong Shia, Yi Yangb, Junliang Zhoua, Jiang Xua,* a

State Key Laboratory of Estuarine and Coastal Research, East China Normal University, 3663 North Zhongshan Road, Shanghai 200062, China

b

Department of Geosciences, East China Normal University, 3663 North Zhongshan Road, Shanghai 200062, China * Corresponding author: Dr. Jiang Xu Tel/Fax: +86-21-62238962; E-mail address: [email protected]

Graphic abstract

L-MWCNTs

Adsorption mechanism S-MWCNTs π-π and n-π interaction Hydrophobic interaction Lewis acid-base interaction Hydrogen bonding

Various emerging contaminants

1  

Highlights 

Adsorption of different emerging pharmaceuticals on MWCNTs was studied.



Pharmaceuticals adsorption was rapid and increased with the MWCNTs surface area.



Adsorption isotherms, kinetics and affinity were comprehensively investigated.



Electrostatic interaction, polarity, and substituent effects were discussed.



Adsorption was affected by environmental conditions, such as pH and ionic strength.

 

Abstract The adsorption behavior of different emerging contaminants (3 chloramphenicols, 7 sulfonamides, and 3 non-antibiotic pharmaceuticals) on five types of multi-walled carbon nanotubes (MWCNTs), and the underlying factors were studied. Adsorption equilibriums were reached within 12 h for all compounds, and well fitted by the Freundlich isotherm model. The adsorption affinity of pharmaceuticals was positively related to the specific surface area of MWCNTs. The solution pH was an important parameter of pharmaceutical adsorption on MWCNTs, due to its impacts on the chemical speciation of pharmaceuticals and the surface electrical property of MWCNTs. The adsorption of ionizable pharmaceuticals decreased in varying degrees with the increased ionic strength. MWCNT-10 was found to be the strongest adsorbent in this study, and the Freundlich constant (KF) values were 353-2814 mmol1-n·Ln/kg, 571-618 mmol1-n·Ln/kg, and 317-1522 mmol1-n·Ln/kg for sulfonamides, chloramphenicols, and non-antibiotic pharmaceuticals, respectively. The different adsorption affinity of sulfonamides might contribute to the different hydrophobic of heterocyclic substituents, while chloramphenicols adsorption was affected by the charge distribution in aromatic rings via substituent effects. 2  

Keywords: Carbon nanotubes; Adsorption; Emerging pharmaceuticals; Isotherm; Mechanism

1. Introduction Pharmaceuticals are widely used in human and veterinary medicines, and their occurrence in the natural environment is attracting attention due to potential long-term adverse effects, such as endocrine disrupting effects in aquatic organisms and antibiotic resistance genes in pathogenic bacteria [1,2]. Many pharmaceuticals are difficult to be metabolized by humans or animals, and will be excreted with feces and urine as initial compounds [3]. In addition, such pharmaceutical compounds are not effectively removed in sewage treatment plants (STPs), which are still geared towards traditional pollutants, such as heavy metals and chemical oxygen demand [4,5]. As a result, more and more pharmaceuticals residues from sewage effluents, hospital effluents, and other untreated wastewater will seep into the surface water and groundwater, then migrate and transform in the aquatic environment [6-8]. Carbon nanomaterials possess excellent chemical and thermal stability, such as carbon nanotubes (CNTs) are increasingly used in consumer products, engineering and medical devices [9]. Compared with other adsorbents, CNTs have attracted great interest because of their one-dimensional macromolecules, unique chemical structure, and high specific surface area. Due to their special properties, CNTs can act as potential adsorbents of high binding affinity and capacity for organic contaminants, such as pharmaceuticals from water [10]. Significant adsorption of pharmaceuticals by CNTs may drastically affect their migration and transformation in the environment. The adsorption of pharmaceuticals by CNTs is dependent on their surface morphology, physical and chemical properties of pharmaceuticals, and 3  

environmental conditions. Multi-walled carbon nanotubes (MWCNTs) contain several concentrically nested layers of highly ordered rolled graphite sheets [11], the space between the layers of MWCNTs is not enough for any organic molecule to access [12]. The inner cavities of MWCNTs are also inaccessible to adsorbate molecules due to the blocking of impurities (e.g.  metal catalyst,  functional groups and amorphous carbons) [13,14]. Moreover, MWCNTs monomers will tend to aggregate together as bundles by Van der Waals interactions [15], and the interstitial areas among bundles may be too small to be fit the organic molecules [16]. Thus, although there are four types of adsorption sites on carbon nanotube bundles in an ideal case, including the external surface area, the interstitial and groove areas between tubes, and the inner cavities of tubes, only the external surface and groove areas were available for adsorption [17]. The adsorption of pharmaceuticals on carbon nanomaterials has been widely studied in recent years [18-21]. Unfortunately, recent studies of adsorption of pharmaceuticals by CNTs focused primarily on the adsorption of one or two pharmaceuticals by individual CNTs, such as carbamazepine [19] and sulfamethoxazole [21]. Limited research has been done on the adsorption of a variety of pharmaceuticals. There is little information about the adsorption comparison among different pharmaceuticals on various MWCNTs, including sulfonamides, chloramphenicols, and non-antibiotic pharmaceuticals. The aim of this study was to obtain a comprehensive understanding of the adsorption of different pharmaceuticals on MWCNTs. Since different structures and dimensions would exhibit different adsorption affinities, five types of MWCNTs were selected as adsorbents and compared. Three chloramphenicols, seven sulfonamides, and three non-antibiotic pharmaceuticals (ibuprofen, carbamazepine and diclofenac) were chosen as representative pharmaceuticals. Adsorption kinetics and isotherms 4  

were evaluated via batch experiments. The adsorption under different pH and ionic strengths was also investigated by analyzing the changes of the adsorption coefficients. At last, the possible interaction mechanism between different pharmaceuticals and MWCNTs was discussed. 2. Experimental Section 2.1. Materials The standard reagents of 13 target pharmaceuticals including chloramphenicol, thiamphenicol, florfenicol, sulfadiazine, sulfapyridine, sulfamethoxazole, sulfathiazole, sulfamerazine, sulfamethzine, sulfaquinoxaline, ibuprofen, carbamazepine and diclofenac were purchased from Sigma Aldrich, St. Louis, MO, USA. The physicochemical properties of these target pharmaceuticals are shown in Table S1. Standard solution (1 g/L) was prepared via pharmaceuticals dissolution in methanol. Five types of MWCNTs purchased from Shenzhen Nanotech Port Co., China, which were used without further purification. The specific surface area was determined by N2 adsorption at 77 K using the Brunauer-Emmett-Teller (BET) method (BELSORP-max, BEL Japan Inc.). Their physicochemical properties are shown in Table S2. 2.2. Batch experiments Different amounts (0.5-10 mg) of MWCNTs were accurately weighed into centrifuge tubes (20 mL), which contained 18 mL of a pharmaceutical solution (0.00022-0.037 mmol/L) with 200 mg/L of sodium azide. The amount of methanol added was less than 1% of the total volume, hence, it was insignificant  for the cosolvent effect. The tubes were then capped and agitated on a continuous oscillator at 25±1

o

C in the dark to minimize potential

photodegradation. Adsorption kinetics were determined by taking samples at regular intervals, 5  

followed by high-speed centrifugation (Sigma 3K15, Germany) at 10,000 rpm for 10 min. The supernatant solutions were then analyzed by a high performance liquid chromatography (HPLC) system. Adsorption isotherm experiments were conducted by shaking the mixture of MWCNTs and pharmaceuticals for 72 h to ensure complete equilibrium. The control experiments without MWCNTs were also performed as the blanks. The samples were then centrifuged followed by HPLC analysis. The effects of pH values (1, 3, 5, 7, 9, and 11) and ionic strength (0, 0.2, 0.4, 0.6, 0.8, and 1.0 mol/L NaCl) on adsorption were also studied. 2.3. Method of analysis Transmission electron microscopy (TEM, JEOL, Japan, 200 kV) analysis with a point resolution of 0.19 nm was performed to characterize the morphology of MWCNTs. The potential changes in MWCNT-10 before and after pharmaceutical adsorption were investigated by Fourier transform infrared spectrum analysis (FT-IR, Nexus 670, Thermo Nicolet, USA). The pharmaceutical concentrations in the supernatant were measured by HPLC (Agilent 1260, USA) with either fluorescence or UV detector. The compounds were separated on a ZORBAX Eclipse Plus C18 column (4.6 × 100 mm). The chromatographic conditions and detector parameters were shown in Tables 1. 3. Results and Discussion 3.1. Characteristics of MWCNTs The physicochemical properties of MWCNTs used in the study were shown in Table S2. Each MWCNTs was different in the outer diameters and specific surface areas, and had similar length except the shorter S-MWCNT-2040. Generally, the specific surface areas decreased with the increased outer diameters. However, the specific surface area of 6  

L-MWCNT-2040 was larger than S-MWCNT-2040, which shown more agglomeration in the TEM images (Figure 1). Besides, FT-IR analysis was performed to detect the functional groups on the adsorbent before and after adsorption of 5 representative compounds (ibuprofen, carbamazepine, and diclofenac for non-antibiotic pharmaceuticals; sulfamethoxazole for sulfonamides; and thiamphenicol for chloramphenicols). As shown in Figure 2, the obvious adsorption peak at around 1620 cm-1 was assigned to the stretching vibration of C=O [22], and the adsorption peak around 1150 cm-1 and 3400 cm-1 was attributed to the stretching vibrations of C-O and -OH, respectively [22,23]. The stretching vibrations of -OH and C=O were weakened after the adsorption of sulfamethoxazole and carbamazepine, respectively [24,25], which was probably attributed to the Lewis acid-base interaction between the Lewis bases (-NH- and -NH2) on target molecules and Lewis acids (O-containing groups) on CNTs [26]. But this phenomenon did not occur in the adsorption of thiamphenicol and diclofenac, because both the CNTs and these pharmaceuticals only had Lewis acids groups (hydroxyl and carboxyl). Moreover, the obviously enhanced stretching and bending vibration of C-H after the adsorption of ibuprofen were mainly attributed to the three methyl groups in its molecule [22,27]. 3.2. Adsorption isotherms and kinetics of different pharmaceuticals on MWCNTs The adsorption isotherms of the target compounds were found to be non-linear, and two non-linear isotherm models (Freundlich, Langmuir) were employed to fit the experimental data [28,29]. Freundlich isotherm model: Qe =KF Cne



7  

(1)

Langmuir isotherm model: Qe =

Qmax bCe 1+bCe

(2)

where Qe (mmol/kg) is the equilibrium concentration of pharmaceutical adsorbed on MWCNTs, Ce (mmol/L) is the equilibrium concentration of pharmaceutical in solution, KF (mmol1-n·Ln/kg) is the Freundlich affinity coefficient, n (dimensionless) is the Freundlich linearity index, Qmax is the maximum adsorption capacity at saturation (mmol/kg), b is a constant. The fitted results were shown in Tables S3 and S4. Higher correlation coefficients and better fitting were observed with the Freundlich model, which was chosen as the preferred model for further investigation. Besides, the adsorption kinetics of 5 representative compounds (sulfamethoxazole, thiamphenicol, diclofenac, carbamazepine, ibuprofen) on MWCNTs was also investigated. As shown in Figure 3, rapid initial adsorption of 5 representative compounds was observed within 1.0 h, which indicated a fast transfer into the near surface boundary layers of CNTs [30]. Finally, the adsorption reached equilibrium within 12 h, and the pseudo-second-order model was used to fit the experimental data [31]: 1 1 1 1 t = + t= + t 2 Qt k2 Q Qe υo Qe e

(3)

where Qe and Qt (mmol/kg) is the amount of targets adsorbed on the adsorbents at equilibrium and time t (h), respectively. k2 is the adsorption rate constant (kg/mmol/h). υ0 is the initial adsorption rate (mmol/h/kg).

8  

The kinetics followed pseudo second-order as shown in Table S5 with perfect fitting for the pharmaceuticals (R2 up to 1.000), which indicated that chemical adsorption was involved [31]. 3.3. Adsorption affinity comparison of different MWCNTs types The adsorption affinity of pharmaceuticals between different adsorbents could be compared through the adsorbent-to-solution distribution coefficients (Kd) and its normalized species via specific surface areas (Kd'): Qe Ce Qe K'd = Ce ·SA Kd =

(4) (5)

The Kd values were selected at both high and low equilibrium concentrations (10-3 and 10-4 mmol/L) based on the data scope fitted by Freundlich model. As shown in Figure 4, the Kd values for all the target compounds at the selected high and low equilibrium concentrations were observed in a descending order: MWCNT-10 > Aligned-MWCNT > L-MWCNT-2040 > S-MWCNT-2040  L-MWCNT-60100, which was consistent with the order of the specific surface area in Table S2. The results could be explained by the structure of MWCNTs, as it was observed that there existed a significant positive correlation between Kd values and specific surface areas of MWCNTs (Figure 5). Notably, although the specific surface area of S-MWCNT-2040 was higher than that of L-MWCNT-60100, their Kd values and adsorption affinities were very similar. The adsorption phenomenon of these two types MWCNTs was probably attributed to the difference of adsorption affinity per surface area, which could be compared via the surface area-normalized adsorption data (Kd') [32]. As shown in Figures 4b and 4d, the Kd' values of different MWCNTs was close, indicated the MWCNTs used in this study had similar 9  

adsorption affinity per surface area. However, the significantly low adsorption affinity of S-MWCNT-2040 might be attributed to the more agglomeration and the smaller interstices between the tubes on S-MWCNT-2040 than the other types of MWCNTs (Figure 1), reducing the actual adsorption sites for the pharmaceutical molecules. 3.4. Adsorption affinity comparison of different pharmaceuticals Various adsorption affinities for the different target compounds were investigated by comparing different Kd values of pharmaceutical compounds on the same type of CNTs (Figures 4a and 4c). In the case of MWCNT-10, the Kd values were relatively concentrated except sulfaquinoxaline. In the group of chloramphenicol antibiotics, the adsorption affinities of thiamphenicol and florfenicol were very close, while the adsorption affinity of chloramphenicol was about 1.7 times the former two. In the group of sulfonamides, the adsorption affinities followed an order of sulfaquinoxaline >> sulfamethzine  sulfamerazine  sulfapyridine > sulfathiazole > sulfamethoxazole  sulfadiazine. Sulfaquinoxaline with the highest adsorption affinity was approximately 3.6 times the second one, and an order of magnitude higher than the last one (sulfadiazine). Notably, at the lower equilibrium concentration (Ce = 10-4 mmol/L), the adsorption affinity of sulfadiazine was slightly lower than sulfamethoxazole, but it was opposite at the higher equilibrium concentration (Ce = 10-3 mmol/L). It was similar between sulfamerazine and sulfapyridine, illustrated the adsorption of sulfadiazine and sulfamerazine with lower non-linearity levels, which was further demonstrated in Table S3. The linearity indexes (n) of sulfadiazine (0.55±0.03) and sulfamerazine (0.40±0.02) were higher than the others. For the non-antibiotic pharmaceuticals, the adsorption affinities was increased in the order of ibuprofen < diclofenac < carbamazepine. 10  

The different adsorption behavior of sulfonamides might contribute to the different substituents linked -NH- groups. For example, sulfaquinoxalin had a quinoxalinyl containing two coplane ring structures, and had much stronger π-electron-conjugating potential than other sulfonamides with single-ringed substituent. The more enhanced adsorption of sulfaquinoxalin might be due to the stronger electron-donor-acceptor (EDA) interactions between the substituent and MWCNTs surface, while other sulfonamides had only one heterocyclic ring substituent. Because the heterocyclic ring had more polarity than the p-amino-sulfonamide ring at the other end of sulfonamide molecules, and these two ring could not both contact the surface of CNTs due to the non-coplanar geometric shape. The heterocyclic ring was more inclined to the direction of water, while the p-amino-sulfonamide ring was the contact point with CNTs [33]. Moreover, in order to measure the effect of water on these heterocyclic rings, the n-octanol-water partition coefficient (Kow) of each sulfonamide corresponding heterocyclic amine was calculated using EPI SuiteTM. As shown in Table 2, it was observed that the order of hydrophobic for the heterocyclic amines was consistent with adsorption affinities of the sulfonamides, which indicated that the adsorption difference of sulfonamides was affected by the hydrophobic of different substituents. For the chloramphenicols, the charge distribution in aromatic ring was changed by the resonance effects of substituents, which affected the interaction between chloramphenicol molecules and CNTs. Nitro group on chloramphenicol was a stronger electron-withdrawing substituent and π-acceptor than the methylsulfonyl group on thiamphenicol and florfenicol [34], resulted in the higher adsorption affinity of chloramphenicol.

11  

3.5. Effects of pH on pharmaceutical adsorption on MWCNTs Solution pH could affect the chemical speciation (ionized or neutral) of organic compounds, resulted the change of their adsorption characteristics on MWCNTs. Figure 6 presents the effects of pH on adsorption of various pharmaceuticals. The pH effects on adsorption for sulfamethoxazole, ibuprofen and diclofenac were similar. Their adsorption increased with the pH value from 1 to 3, then decreased over the pH range of 3 to 11, which was consistent with the zeta potential of MWCNTs. In comparison, the adsorption of thiamphenicol was increased over the pH range of 1 to 9, but its ultraviolet spectrum was lost at pH 11. For carbamazepine, a similar trend of the pH effect with thiamphenicol was observed when the pH was less than 7. However, the effect was insignificant with the pH above 7. The changes of solution pH would affect the forms and properties of functional groups on the target pharmaceutical compounds and MWCNTs [35,36]. In general, the ionization, solubility and hydrophilic of organic chemicals would be increased and their adsorptions on CNTs would be reduced with the increase of pH [37-41], because neutral form of these ionizable organic chemicals contributed much to the adsorption of CNTs [18,42]. However, it was reported that some adsorption would be enhanced with the increased pH, due to the ionized amine groups at low pH and the strengthening of electron-donor-acceptor (EDA) interactions [41,43,44]. Obviously, the effects of pH on the organic compound adsorption depended on the balance between attractive forces (e.g., EDA) and repulsive forces (e.g., electrostatic repulsion). Comparing the solution pH, the pKa of organic compounds and the point of zero charge (PZC) of CNTs might help to explain the pH effect. Sulfamethoxazole has two pKa values 12  

with amino and carboxyl groups in the molecule, indicating that its molecular species would be significantly affected by the pH. The adsorption of sulfamethoxazole raised one order of magnitude in the lower pH range (1-3), which was due to the less hydrophobic in ionized form and the inhibited Lewis acid-base interaction at pH < pKa (1.97). In addition, almost all of the sulfamethoxazole existed in anionic species at high pH (9 and 11) > pKa (6.16), while some of the Lewis acid sites (e.g., -COOH, -OH) on CNTs were also ionized and the zeta potential of CNTs was negatively charged. Strong electrostatic repulsion between the negative sulfamethoxazole and the CNTs surface would significantly reduce the adsorption affinity. The trends of ibuprofen and diclofenac were relatively similar. Their predominant fractions were the non-ionized form when the pH was below their pKa values (4.85 for ibuprofen and 4.00 for diclofenac). However, the largest adsorption affinity appeared at pH 3 rather than pH 1, in other words, the slightly protonation-deprotonation transition of these compounds enhanced

the

adsorption

affinity,

which

indicated

that

the 

electrostatic

adsorption-enhancement interaction could  overwhelm the increase in the hydrophilia at this pH. The interactions of hydrogen bonding and π-hydrogen bonding also played a role in the adsorption of carbamazepine and thiamphenicol in a non-ionized form. The -OH, -NH2 and -NH- groups could interact with oxygen-containing functional groups on the surface of CNTs through hydrogen bonding [45]. At a lower pH value, hydrogen bonding acceptors on the CNTs and hydrogen bonding donors on the carbamazepine molecules could integrate with H+ in the aqueous solution more easily, resulted in the decrease of hydrogen bonding between carbamazepine and CNTs. On the contrary, at a higher pH value, hydrogen bonding donors on the surface of CNTs would be ionized, while carbamazepine molecules would maintain 13  

non-ionized.  Consequently, hydrogen bonding donors of carbamazepine molecules would interact with hydrogen bonding acceptors or π-donors on the CNTs and then the adsorption will be promoted [46]. Analogously, because of hydrogen bonding, thiamphenicol had a similar rising trend as carbamazepine at pH from 1 to 9. When pH was 9, although most of thiamphenicol molecules (pKa = 7.65) existed in ionized form, the zeta potential of the CNTs approached 0, resulting in an insignificant electrostatic repulsion. 3.6. Effects of ionic strength on pharmaceutical adsorption on MWCNTs Adsorption was a key process not only in freshwater environment, but also in estuarine and marine environment. Hence, it was important to study the effects of ionic strengte (IS) on the interactions between pharmaceutical and CNTs. As shown in Figure 7, the adsorption of sulfamethoxazole, thiamphenicol and ibuprofen was decreased with the increase of IS. However, the effect of IS was relatively weaker on diclofenac adsorption, and the highest adsorption affinity appeared at IS = 0. The ionic strength showed very limited influence on the adsorption of carbamazepine on CNTs. It was reported that the adsorption of ionic compounds on carbonaceous adsorbents could be enhanced with the increase of IS, due to a screening effect of the surface charge caused by the salt [47]. Some studies had shown that high ionic strength could promote the aggregation of carbon nanomaterials [48,49], which would decreased adsorption sites and adsorption affinity of the adsorbent. A portion of the sulfamethoxazole, thiamphenicol, and ibuprofen molecules, and most of diclofenac molecules were existed in the anion species under neutral condition in this study, while the surface of CNTs was partly positive charged. The promotion of the attraction between these negative and positive charge would be inhibited by the screening effect. Moreover, these partly ionized compounds could be affected by the salting-in effect, which 14  

were more prominent at higher salinity, and resulted in the increased concentration of ionized form and the decreased adsorption. A small amount of diclofenac molecules were existed in non-ionized form, which resulted the less salting-in effect than that for sulfamethoxazole, thiamphenicol, and ibuprofen. Besides, the salting-in effect on carbamazepine adsorption was negligible, which was an almost complete non-ionized compound. 3.7. Mechanism of pharmaceutical adsorption on MWCNTs The results of adsorption isotherm suggested that the MWCNTs very strong adsorption affinity for the target compounds. While the affinities of different pharmaceuticals on MWCNTs varied between compounds and adsorbents, possible mechanism could be inferred (Figure 8): (1) EDA interactions, including π-π EDA interaction and n-π EDA interaction, were considered as the dominated mechanism in the adsorption process on account of the literatures and our study. The π-π EDA interaction mainly occurred between the π-electron-depleted aromatic ring(s) (or regions) and the π-electron-rich regions (or aromatic rings) of the graphene surface of CNTs and the adsorbates. The role of π-donor and π-receptor was not fixed. CNTs could act as both π-donors and π-acceptors [44,50]. Increasing electron density of aromatic structures could create larger quadrupoles, which would make the adsorbates as electron-rich π-donors, while the regular graphene surface of CNTs was expected to act as a π-acceptor. On the contrary, reducing electron density of aromatic rings in the adsorbates could reverse the quadrupole moments, in which electron-deficient π-system served as a π-acceptor, while CNTs could act as π-donors in two forms. One was that the highly polarizable graphene surface of CNTs served as π-donors, while the other was that the surface defects of CNTs developed polarized electron-rich sites as π-donors [51,52]. Due to 15  

high electron density in these surface defect sites, the electron-withdrawing adsorbate would bind to these sites first. Another possible n-π EDA interaction was that the unshared pair of electrons of nitrogen and oxygen (e.g., amino group, hydroxyl group, -O- or N heteroatoms) as n-electron donors has directly interacted with the π-electron deficient sites of CNTs as π-acceptors. The n-π EDA interaction will be enhanced by ionized amino and hydroxyl groups at high pH, which were even stronger electron-donating groups [44]. This was probably one of the reasons why adsorption of thiamphenicol to CNTs increased when pKa < pH < 9. (2) The hydrophobic interaction. The hydrophobic interaction contributed to adsorption affinity despite the stronger non-hydrophobicity of the target pharmaceutical compounds containing some non-hydrophobic functional groups such as hydroxy, carboxyl and amino groups. Based on some research results [43,53], it could be indicated that the more hydrophobic

or

non-hydrophobic

of

compounds,

the

greater

the

effect

using

n-hexadecane-water partition coefficient normalizing aqueous phase concentrations for hydrophobic effect. The hydrophobic interaction was not a direct cause for the adsorption on CNTs, but its impact could not be neglected. (3) Hydrogen bonding and π-hydrogen bonding. Hydrogen bonding widely existed in the adsorption process of polar organic compounds on CNTs [54]. As the result of FT-IR analysis in previous discussion, there were both hydrogen bonding acceptors and donors on the surface of CNTs, while the target molecules had hydrogen bonding acceptor or donor groups. Likewise, both CNTs and target molecules contained π-donors. Therefore, hydrogen bonding interaction might exist at the same time with π-hydrogen bonding interaction. The effects of

16  

hydrogen bonding and π-hydrogen interaction were similar, and both of them would be affected by pH. (4) The Lewis acid-base interaction. Previous studies and our research both indicated that the Lewis acid-base interaction was an important extra interaction for the adsorption of molecules with amino group [43,44]. The Lewis acid-base interaction was regarded as another important mechanism controlling the adsorption of pharmaceutical molecules. All of the interactions mentioned above could be interpreted as electronic interactions between organic molecules and surface of CNTs. Electronic properties of bonded sites in organic molecules could play important roles in adsorption. In addition, the geometric shape of molecules might also affect adsorption affinity. 4. Conclusions To assess the role of MWCNTs as a carrier of emerging pharmaceutical pollutants, the adsorption of a range of pharmaceuticals on different MWCNTs was examined. TEM analysis showed that different CNTs had varying morphology, structure and properties, which determined their capacity for the adsorption of different pharmaceutical compounds. Overall, it was observed that smaller diameter and longer CNTs had a stronger adsorption ability. For adsorption isotherms, it was found that the Freundlich model provided a better fitting than the Langmuir model. The adsorption behavior of different pharmaceuticals on CNTs was affected by multiple mechanisms collaboratively, according to the different geometric shapes, functional groups, and substituents of molecules. Due to the change of charge on molecules and CNTs surface with different pH, the electrostatic interaction between them would also be changed, which caused the change of adsorption behaviors. The adsorption of ionizable

17  

compounds was greatly inhibited by the increasing ionic strength via the salting-in effect, which was negligible for a non-ionized compound like carbamazepine. Acknowledgements The authors would like to acknowledge the State Key Laboratory of Estuarine and Coastal Research (2014RCDW03, 44KZ001L/023), Shanghai Pujiang Program (No. 15PJD014), and China Postdoctoral Science Foundation (No. 2015M571523) for their financial support.

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Captions Figure 1. The TEM images of the different MWCNTs. Figure 2. FT-IR spectrum of MWCNT-10 before and after adsorption of the pharmaceuticals. Figure 3. Kinetics of pharmaceutical adsorption on MWCNTs in neutral condition. The mass of MWCNT: 8.55–31.11 mg; the initial pharmaceutical concentrations: 0.20 mg/L; temperature: 25±1 °C; neutral pH. Figure 4. Comparing the adsorbent-to-solution distribution coefficients (Kd) and its normalized species via specific surface areas (Kd') of different pharmaceuticals at equilibrium concentration (Ce) of 10-3 mmol/L (a and b) and of 10-4 mmol/L (c and d), respectively. Temperature: 25±1 °C; neutral pH; contact time 72 h. Figure 5. Linear relationship (y=ax+b) between Kd and specific surface area of MWCNTs at the equilibrium concentrations of 10-3 mmol/L and 10-4 mmol/L. Figure 6. Kd values of five pharmaceuticals and zeta potential on MWCNT-10 as a function of solution pH. The mass of MWCNTs: 1.00 mg; the initial pharmaceutical concentration: 0.0016-0.0046 mmol/L; temperature: 25±1 °C; contact time: 72 h. Figure 7. Kd values of five pharmaceuticals on MWCNT-10 under different ionic strengths. The mass of MWCNTs: 1.00 mg; the initial pharmaceutical concentration: 0.0016-0.0046 mmol/L; temperature: 25±1 °C; neutral pH; contact time: 72 h. Figure 8. Mechanism schematic of pharmaceuticals adsorption by MWCNTs. Table 1. Chromatographic conditions and detector parameters for the pharmaceutical compounds. Table 2. n-octanol-water partition coefficient of each sulfa antibiotic corresponding heterocyclic amine.

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MWCNT-10

Aligned-MWCNT

S-MWCNT-2040

L-MWCNT-2040

L-MWCNT-60100

Figure 1. The TEM images of the different MWCNTs.

23  

 (O-H) C-H C=OC-O C-H

0.03 0.02

MWCNT-10

0.01 0.00 0.03

MWCNT-10 + Sulfamethoxazole

0.02 0.01 0.00 0.03

MWCNT-10 + Thiamphenicol

Absorbance

0.02 0.01 0.00 0.03

MWCNT-10 + Ibuprofen

0.02 0.01 0.00 0.03

MWCNT-10 + Diclofenac

0.02 0.01 0.00 0.03

MWCNT-10 + Carbamazepine

0.02 0.01 0.00 4000

3500

3000

2500

2000

1500 -1

Wavenumbers (cm )

1000

500

 

Figure 2. FT-IR spectrum of MWCNT-10 before and after adsorption of the pharmaceuticals.  

24  

70 60

Qt (mmol/kg)

50

Diclofenac Carbamazepine Ibuprofen Thiamphenicol Sulfamethoxazole

40 30 20 10 0 0

2

4

6

8

10

12

t (h)

24

48

72  

Figure 3. Kinetics of pharmaceutical adsorption on MWCNT in neutral condition. The mass of MWCNT: 8.55–31.11 mg; the initial pharmaceutical concentrations: 0.20 mg/L; temperature: 25±1 °C; neutral pH.

25  

2

Kd (L/kg)

300000

200000

b

2

400000

Chloramphenicol Thiamphenicol Florfenicol Sulfadiazine Sulfapyridine Sulfamethoxazole Sulfathiazole Sulfamerazine Sulfamethazine Sulfaquinoxaline Ibuprofen Diclofenac Carbamazepine

Kd' (L/m )

a

1

100000

0

M W CN T-10

0

Aligned-M WCNT S-M W CN T-2040 L-M W CN T-2040 L-M WCNT-60100

3000000

M W CNT-10

Aligned-M W CNT S-M WCNT-2040 L-M WCNT-2040 L-M W CN T-60100

8

c

d

2500000 6

2

Kd' (L/m )

Kd (L/kg)

2000000

1500000

4

1000000 2 500000

0

M W CN T-10

0

Aligned-M WCNT S-M W CN T-2040 L-M W CN T-2040 L-M WCNT-60100

M W CNT-10

Aligned-M W CNT S-M WCNT-2040 L-M WCNT-2040 L-M W CN T-60100

 

Figure 4. Comparing the adsorbent-to-solution distribution coefficients (Kd) and its normalized species via specific surface areas (Kd') of different pharmaceuticals at equilibrium concentration (Ce) of 10-3 mmol/L (a and b) and of 10-4 mmol/L (c and d), respectively. Temperature: 25±1 °C; neutral pH; contact time 72 h.

26  

1000000

b

a

-4

-3

C e =10 m m ol/L

C e =10 m m ol/L

1000000

Kd (L/kg)

Kd (L/kg)

100000

100000

10000

10000 0

50

100

150

200

250

300

350

400

0

2

50

100

150

200

250

300

350

400

2

Specific surface area (m /g)

Specific surface area (m /g)

 

Figure 5. Linear relationship (y=ax+b) between Kd and specific surface area of MWCNTs at the equilibrium concentrations of 10-3 mmol/L and 10-4 mmol/L.

27  

Sulfamethoxazole Carbamazepine

Thiamphenicol Diclofenac

Ibuprofen Zeta potential

10000

1000 0 100

Zeta potential (mV)

Kd (L/g)

30

-30 10 1

3

5

7

9

11

pH  

Figure 6. Kd values of five pharmaceuticals and zeta potential on MWCNT-10 as a function of solution pH. The mass of MWCNT: 1.00 mg; the initial pharmaceutical concentration: 0.0016-0.0046 mmol/L; temperature: 25±1 °C; contact time: 72 h.  

28  

Sulfamethoxazole Carbamazepine

Thiamphenicol Diclofenac

Ibuprofen

Kd (L/g)

10000

1000

100 0.0

0.2

0.4 0.6 NaCl (mmol/L)

0.8

1.0  

Figure 7. Kd values of five pharmaceuticals on MWCNT-10 under different ionic strengths. The mass of MWCNT: 1.00 mg; the initial pharmaceutical concentration: 0.0016-0.0046 mmol/L; temperature: 25±1 °C; neutral pH; contact time: 72 h.  

 

29  

 

Hydrophobic interaction Hydrophilic 

 

δ-

:

δ-

δ-

δ+

δDefects

π−π

n−π

EDA interactions

Lewis acid−base Hydrogen bonding & π-hydrogen bonding interaction

 

Figure 8. Mechanism schematic of pharmaceuticals adsorption on MWCNTs.

30  

Table 1. Chromatographic conditions and detector parameters for the pharmaceutical compounds.

Group

Chloramphenicols

Sulfonamides

Non-antibiotic Pharmaceuticals a

Compound

Mobile phasea (V:V)

Retention time (min)

UV wavelength (nm)

Chloramphenicol

50:50

2.48

278

Thiamphenicol

40:60

1.64

225

225/304

Florfenicol

40:60

2.40

225

225/304

Sulfadiazine

30:70

1.66

270

Sulfapyridine

30:70

1.91

266

Sulfamethoxazole

40:60

2.13

270

Sulfathiazole

30:70

1.69

285

Sulfamerazine

30:70

2.11

266

Sulfamethazine

40:60

1.82

270

Sulfaquinoxaline

50:50

2.34

248

Ibuprofen

80:20

2.95

225

Diclofenac

80:20

2.66

276

Carbamazepine

75:25

1.63

285

methanol: 0.01M formic acid aqueous solution, isocratic elution at 1 mL/min.

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Fluorescence wavelength (Ex/Em) (nm)

Table 2. n-octanol-water partition coefficient of each sulfa antibiotic corresponding heterocyclic amine.

Compound

Corresponding heterocyclic amine

LogKow

Sulfadiazine

-0.22a

Sulfapyridine

0.65a

Sulfamethoxazo

-0.16b

le

 

0.38a

Sulfathiazole  

0.43b

Sulfamerazine  

0.97b

Sulfamethazine   a

Experimental database match (HANSCH,C ET AL. 1995); bValues calculated by EPI Suite

(USEPA)  

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