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CoFe2O4 composites
Chemical Engineering Journal 373 (2019) 995–1002
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Molecular insights into the effects of Cu(II) on sulfamethoxazole and 17βestradiol adsorption by carbon nanotubes/CoFe2O4 composites
T
Si Lia, Fei Wangb, Weiyi Panb, Xi Yangc, Qiang Gaoc, Weiling Sunc, , Jinren Nic ⁎
a
College of Environmental Sciences and Engineering, Peking University, The Key Laboratory of Water and Sediment Sciences, Ministry of Education, Beijing 100871, China Shenzhen Key Laboratory for Heavy Metal Pollution Control and Reutilization, School of Environment and Energy, Peking University Shenzhen Graduate School, Shenzhen 518055, China c State Key Laboratory of Plateau Ecology and Agriculture, Qinghai University, Xining 810016, China b
HIGHLIGHTS
GRAPHICAL ABSTRACT
slightly inhibited E2 adsorption • Cu(II) onto CNTs/CoFe O composites. promoted SMX adsorption onto • Cu(II) CNTs/CoFe O composites. influencing mechanisms of Cu(II) • The were revealed by DFT calculation. adsorption can be enhanced • SMX through the bridging effects of Cu(II). binding between Cu(II) and SMX • The enhanced π-π interactions between 2
2
4
4
SMX and CNTs.
ARTICLE INFO
ABSTRACT
Keywords: Adsorption Sulfamethoxazole 17β-estradiol Copper Carbon nanotubes/CoFe2O4
Effects of metal ions on the adsorption of organic pollutants have drawn great interests considering their coexistence and interactions in aqueous solution. However, the potential influencing mechanisms of metal ions on the adsorption of organic pollutants at a molecular level were rarely studied. Here, we investigated the effects of Cu(II) on the adsorption of 17β-estradiol (E2) and sulfamethoxazole (SMX) by carboxylic and amino CNTs/CoFe2O4 composites, and the mechanisms were revealed by spectroscopy analyses and density functional theory (DFT) calculations. The results showed distinct effects of Cu(II) on SMX and E2 adsorption by CNTs/CoFe2O4. For E2, a nonionic chemical, Cu(II) slightly inhibited E2 adsorption by CNTs/CoFe2O4 due to the increased aggregation of the composites induced by Cu(II). For SMX, an ionic chemical, Cu(II) enhanced SMX adsorption significantly mainly due to Cu(II) bridging and stronger π-π interactions between SMX and Cu(II) complexes and CNTs. The complexation of SMX with Cu(II) was confirmed by the fluorescence, ultraviolet (UV), and attenuated total reflection fourier transform infrared spectroscopy (ATR-FTIR) analysis. DFT calculation further revealed that isoxazole ring nitrogen, sulfonamide nitrogen, amino nitrogen, and sulfonyl oxygen were the four possible binding sites in SMX with Cu(II). The adsorbed Cu (II) on CNTs/CoFe2O4 could form complexes with SMX, and the binding between SMX and Cu(II) enhanced the π-π interactions between SMX and CNTs. These two aspects led to the enhanced adsorption of SMX. This study provides new insights into the molecular interactions among Cu(II), SMX, and CNTs, and it sheds lights on the adsorption mechanisms of organic pollutants by CNTs in the presence of metal ions.
⁎
Corresponding author. E-mail address: [email protected] (W. Sun).
https://doi.org/10.1016/j.cej.2019.05.111 Received 10 April 2019; Received in revised form 15 May 2019; Accepted 17 May 2019 Available online 18 May 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.
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1. Introduction
hormone, is listed in the “Drinking Water Contaminant Candidate List 3” by USEPA [21,31]. Cu(II), as a common heavy metal, was selected due to its strong complexation ability with organic chemicals [16,19]. Carboxylized CNTs/CoFe2O4 (CNTs-C/CoFe2O4) and amino CNTs/ CoFe2O4 (CNTs-N/CoFe2O4) composites were prepared. The adsorption behaviors of SMX/E2 by CNTs/CoFe2O4 composites were compared with and without addition of Cu(II). The potential influencing mechanisms of Cu(II) on SMX/E2 adsorption were revealed through spectroscopic analysis and DFT calculation.
Carbon nanotubes (CNTs), since their discovery by Iijima [1], are one of the most widely used nanomaterials [2,3]. Particularly, CNTs exhibit great adsorption capacity to varieties of contaminants, including metals [4,5] and organic compounds [6,7] due to their large surface area, abundant surface functional groups, and high hydrophobicity. Nonetheless, one main practical challenge in CNTs’ application is to recover the nanosized CNTs from the treated water in regards to economic and safety concerns. Magnetic materials modified CNTs may provide a feasible solution to overcome this limitation. Among diverse magnetic materials, ferrite (MIIFe2O4, M = Ni, Co, Mn, etc) exhibits excellent magnetic property and good stability in acidic solution [8,9]. Especially, CoFe2O4 decorated CNTs (CNTs/CoFe2O4) had the highest saturation magnetization [8] and they have been widely applied in the removal of dyes [9,10], uranium(VI) ions [11], aniline [12], and emerging contaminants of sulfamethoxazole (SMX), 17β-estradiol (E2) [8], tetrabromobisphenol A [13], and bezafibrate [14] from aqueous solution. Considering the coexistence and interactions of organic contaminants and metals in the environment, the effects of metals on adsorption of organic contaminants by CNTs have aroused growing interests [15–17]. Metal ions could affect the adsorption of organic pollutants through competitive adsorption [18,19], electrostatic counteracting interactions [15], cation bridging [16,19], and/or salting out effect [20,21]. Therefore, the presence of metals may increase, decrease, or have no apparent effects on the adsorption of organic pollutants, and these effects could alter depending on the pH range, metal types, and metal concentrations [15,16,19,22]. However, most of these previous studies were investigated using batch experiments and spectroscopy analysis, little work has been conducted to examine the effect of metals at a molecular level. Density functional theory (DFT) calculation is a powerful tool for providing molecular insights into the adsorption mechanisms of metals [23–25] and organic chemicals [26–28] to various nanomaterials. For example, DFT calculation revealed that Eu(III) and 243Am(III) could form surface complexes with oxygen-containing functional groups of CNTs [23]. Wei et al. [26] reported that SMX was adsorbed by CNTs mainly through π-π interactions, and the oxygen-containing functional groups on CNTs inhibited SMX adsorption through weakening or breaking the π-π interactions between SMX and CNTs. However, the adsorption mechanisms of organic compounds in the presence of metal ions by CNTs were rarely elucidated by DFT calculations. In this study, SMX and E2 were chosen as model emerging contaminants because of their widespread occurrence and potential risks to the ecosystem [21,29]. SMX is one of the most frequently detected antibiotic in China’s major rivers [29] and it is a potential indicator for monitoring and control of antibiotic pollution [30]. E2, as an estrogenic
2. Materials and methods 2.1. Materials and reagents Sources of materials and reagents are provided in Text S1. 2.2. Synthesis and characterization of CNTs/CoFe2O4 composites CNTs-C/CoFe2O4 and CNTs-N/CoFe2O4 were synthesized by a hydrothermal method as described in our previous work [8]. Details on synthesis procedure of CNTs/CoFe2O4 composites are shown in Text S2. Zeta potentials of CNTs/CoFe2O4 were measured using a Malvern Zetasizer Nano ZS90 (Worcestershire, UK) at 25 ± 1 °C. Chemical compositions of CNTs/CoFe2O4 were measured by X-ray photoelectron spectroscopy (XPS; Kratos Analytical, UK). 2.3. Effect of Cu(II) on adsorption Batch experiments were conducted using 100 mL glass vials. CNTs/ CoFe2O4 (5 mg) were added to 50 mL 0.05 M NaNO3 solution. The initial concentration of SMX or E2 was 2.0 mg/L. The Cu(II) concentrations varied from 0 to 10 mg/L at pH 6.0 ± 0.2. Solution pH ranged from 2.0 to 11.0 with HNO3 or NaOH, when the Cu(II) concentration was kept at 5 mg/L. The solutions were equilibrated for 24 h at 25 ± 1 °C. Final pH values were measured after adsorption equilibrium. The suspensions were filtrated via filter membrane (0.22 μm) before analysis. 2.4. SMX, E2, and Cu(II) analysis The concentrations of SMX and E2 in the supernatant were measured by a HP 1100 LC-MSn Trap SL System (Agilent Technologies, USA), as described in our previous study [8] and Text S3. The concentration of Cu(II) was measured by ICP-OES (Prodigy, Leeman, USA). 2.5. Interactions between Cu(II) and SMX Interactions between Cu(II) and SMX were investigated by
Fig. 1. Adsorption of SMX (a) and E2 (b) by CNTs/CoFe2O4 at different Cu(II) concentrations (SMX/E2 = 2.0 mg/L, CNTs/CoFe2O4 = 100 mg/L, I = 0.05 M NaNO3, T = 25 ± 1 °C, pH = 6.0). 996
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ultraviolet (UV), fluorescence, and attenuated total reflection fourier transform infrared spectroscopy (ATR-FTIR) analysis. The details are given in Text S4.
Cu(II) could reduce the negative charges on the surface of CNTs/ CoFe2O4 [19]. 3.2. Effects of Cu(II) at different pH
2.6. DFT calculation
Great effects of pH on SMX adsorption by CNTs/CoFe2O4 were observed with and without addition of Cu(II) (Fig. 2a and b). In absence of Cu(II), the adsorption amounts of SMX decreased sharply from ∼8 mg/ g at pH 2.0 to almost 0 mg/g at pH 11.0. This is in agreement with that found by Yu et al. [32] for SMX adsorption by CNTs. The presence of Cu (II) promoted SMX adsorption by CNTs/CoFe2O4. Moreover, a sharp increase in adsorption amounts of SMX was observed around pH 6.5 in presence of Cu(II) (Fig. 2a and b). It was reported that hydrophobic interaction was the predominant adsorption mechanism for SMX onto CNTs [32,35]. At pH of 2.0–5.5, the main species of SMX is SMX0, and SMX− became dominant at pH > 5.5. The hydrophobicity of SMX− (log Kow− = −2.14) is much lower than that of SMX0 (log Kow0 = 0.85) [36], so a decreasing tend of SMX adsorption was found with increasing pH. On the other hand, the surface of CNTs/CoFe2O4 was more negatively charged at higher pH (Fig. 3). This induced the stronger electrostatic repulsion between CNTs/CoFe2O4 and SMX−, and then decreased SMX adsorption with rising pH. Compared with CNTs-C/CoFe2O4 (Fig. 3a), more positive charges were observed on the surface of CNTs-N/CoFe2O4 (Fig. 3b). This may explain the relatively higher adsorption amounts of SMX by CNTs-N/CoFe2O4 than those by CNTs-C/CoFe2O4. The main species of Cu(II) is Cu2+ at pH < 7.0 (Fig. S1). The adsorbed Cu(II) could reduce the negative charges on the surface of CNTs/ CoFe2O4 (Fig. 3), and then weaken the electrostatic repulsion between CNTs/CoFe2O4 and SMX−. In addition, the zeta potentials of the CNTs/ CoFe2O4 composites showed a peak at pH 6.5–7.0 (Fig. 3), because of the formation of Cu(OH)2 precipitate (Fig. S1). As a result, a sharp increase in adsorption amounts of SMX was observed at pH 6.5–7.0 (Fig. 2). This indicates Cu(OH)2 may also act as adsorbents for SMX adsorption [19]. It was noted that pH had little effect on E2 adsorption by CNTs/ CoFe2O4 (Fig. 2a and b), and the E2 adsorption amounts declined from 20 mg/g to 17 mg/g in the pH range of 2.0–11.0. Because the neutral form of E2 is always the main species of E2 at pH < 10.4 [8], the change of pH had little effect on E2 adsorption by CNTs/CoFe2O4. This finding proves the hydrophobic interaction plays a dominant role in E2 adsorption by CNTs/CoFe2O4.
The interactions between CNTs/CoFe2O4 composites, Cu(II), and organic chemicals were calculated by DFT using DMOL3 code in Materials Studio (v7.0) [26,27]. The detailed description of DFT calculation is provided in Text S5. 3. Results and discussion 3.1. Adsorption of SMX and E2 at different Cu(II) concentrations The adsorption amounts of SMX by CNTs/CoFe2O4 increased with increasing Cu(II) concentrations (Fig. 1a), while the adsorption amounts of E2 decreased slightly with rising Cu(II) concentrations up to 4 mg/L and then nearly did not change with further increase in Cu(II) concentrations (Fig. 1b). Moreover, E2 showed greater adsorption than SMX due to the its higher hydrophobicity (logKow = 3.94 for E2 and 0.85 for SMX) [8], because both SMX and E2 were adsorbed by CNTs/ CoFe2O4 mainly through π-π interactions [26,31]. In addition, CNTs-N/ CoFe2O4 showed a higher adsorption capacity to SMX and E2 than CNTs-C/CoFe2O4, which is consistent with that found by Wang et al. [8]. It was speculated that the different effects of Cu(II) on SMX and E2 adsorption were mainly resulted from their distinct species in aqueous solution. SMX is a typical amphoteric compound, with pKa values of 1.80 and 5.46, respectively [32]. At the experimental pH of 6.0, SMX exists in neutral (SMX0, 22.4%) and anionic (SMX−, 77.6%) forms (Table S1) [8], while Cu2+ is the predominant species of Cu(II), as revealed by Visual Minteq (v3.0) calculation (Fig. S1). Cu(II) may promote the adsorption of SMX through the following two aspects: (1) Cu(II) adsorbed onto the composites, and then promote SMX adsorption through cation bridging effect [16,22]; (2) the complexation of Cu(II) with SMX could decrease electrostatic repulsion between negatively charged SMX and CNTs/CoFe2O4 [15,16]. Consequently, the SMX adsorption by CNTs/CoFe2O4 was increased. Wu et al. [16] also found that the ternary complexes of CNTs-SMX-Cu and CNTs-Cu-SMX may occur at pH 6.5 depending on Cu(II) concentrations. Morel et al. [33] reported that the surface adsorbed Cu(II) significantly enhanced SMX adsorption by soil because of the formation of SMX-Cu-soil complexes. At pH 6.0, E2 is not ionized (Table S1), because its pKa is 10.4 [8,34]. Therefore, the effects of Cu(II) on E2 adsorption by CNTs/ CoFe2O4 were minor since hydrophobic interactions dominate E2 adsorption at low metal concentrations. The weak inhibition effect of Cu (II) on E2 adsorption may be attributed to the strong aggregation of CNTs/CoFe2O4 particles in presence of Cu(II), because the adsorption of
3.3. Spectroscopic investigation on influencing mechanisms As shown by the above results, Cu(II) posed stronger effects on SMX adsorption than on E2 adsorption by CNTs/CoFe2O4. The adsorption of SMX and Cu(II) onto CNTs/CoFe2O4 was proven by XPS analysis (Text S6, Fig. S2, and Table S2). Moreover, it was speculated that the binding
Fig. 2. SMX and E2 adsorption by CNTs-C/CoFe2O4 (a) and CNTs-N/CoFe2O4 (b) at different pH in the absence (open symbol) and presence (solid symbol) of Cu(II) (SMX/E2 = 2.0 mg/L, Cu(II) = 5 mg/L, CNTs/CoFe2O4 = 100 mg/L, I = 0.05 M NaNO3, T = 25 ± 1 °C). 997
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Fig. 3. Effects of Cu(II) on zeta potential of CNTs-C/CoFe2O4 (a) and CNTs-N/CoFe2O4 (b) after adsorption of SMX and E2 at different pH (SMX/E2 = 2.0 mg/L, Cu (II) = 5 mg/L, CNTs/CoFe2O4 = 100 mg/L, I = 0.05 M NaNO3, T = 25 ± 1 °C).
between Cu(II) and SMX promoted SMX adsorption. To confirm the binding of SMX and Cu(II), fluorescence, UV, and ATR-FTIR analysis were conducted at two selected pH values of 6.0 and 9.6.
the fluorescence of NOMs [38,39]. 3.3.2. UV analysis The UV absorption spectra of SMX solutions are shown in Fig. 5a. An absorption band at 262.5 nm was observed for SMX at pH 6.0, and it moved to 256.5 nm at pH 9.6. Blue shift of the peak location of SMX with rising pH was reported previously [35,40]. This shift was due to changes in the dissociation species of SMX (SMX0/SMX−) at pH 6.0 and pH 9.6 (Table S1). With addition of Cu(II), the absorption band moved to lower wavelength, from 262.5 to 257.0 nm at pH 6.0 and from 256.5 nm to 253.0 nm at pH 9.6. This indicates the complexation of SMX with Cu(II).
3.3.1. Fluorescence analysis The excitation/emission matrix (EEM) spectra showed that SMX had one main peak centered at Ex/Em = 260 nm/345 nm (Fig. 4a and c and Table S3). Although the positions of the peak did not change, its intensity increased significantly at pH 9.6 (Fig. 4c) compared with that at pH 6.0 (Fig. 4a and Table S3). This indicates that SMX− demonstrates stronger fluorescence intensities than SMX0. The addition of Cu(II) decreased the intensities of the peak at both pH (Fig. 4b and d), suggesting the complexation of Cu(II) with SMX. Similar results were reported previously, in which decreased fluorescence intensities were observed for E2, 17α-ethynylestradiol, and bisphenol-A due to their interactions with Ca2+ [37]. In addition, the binding of metals to natural organic matters (NOMs) could also quench
3.3.3. ATR-FTIR analysis ATR-FTIR was employed to uncover the binding sites of SMX with Cu(II). Similar ATR-FTIR spectra were observed for SMX at pH 6.0 and 9.6 (Fig. 5b), and the assignment of the peaks is shown in Table S4. The
Fig. 4. EEM fluorescence spectra of SMX solutions in absence and presence of Cu(II) at pH 6.0 (a and b) and 9.6 (c and d). 998
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Fig. 5. UV (a) and ATR-FTIR (b) spectra of SMX solutions in absence and presence of Cu(II) at pH 6.0 and 9.6.
bands at 3478 and 3401 cm−1 (pH 6.0) or 3468 and 3401 cm−1 (pH 9.6) were ascribed to the antisymmetric and symmetric vibrations of aniline NH2 group [41,42], respectively. The band at 3281 cm−1 (pH 6.0) or 3323 cm−1 (pH 9.6) was assigned to the vibration of sulfonamide NH group [41,42]. In presence of Cu(II), these three bands disappeared at both pH 6.0 and pH 9.6, suggesting the binding of Cu(II) with aniline NH2 and sulfonamide NH groups. Isoxazole ring vibrations were observed at 1460 and 1408 cm−1 in the spectra of SMX [41,42]. The band at 1460 cm−1 shifted to 1466 cm−1, while the band at 1408 cm−1 disappeared in presence of Cu(II), indicating interactions between isoxazole ring nitrogen and Cu(II). For the complexation of SMX with metals, different binding sites were proposed depending on the metal types. Kanagaraj and Rao [42] found that SMX could bind with Cr, Mn, and Ni through deprotonated sulfonamide nitrogen and sulfonyl oxygen, while bind with Cu through deprotonated isoxazole ring nitrogen and sulfonamide nitrogen. Kesimli
and Topacli [41] reported that Cd bound with SMX through sulfonamide nitrogen and sulfonic oxygen, while Co bound with SMX through sulfonamide and amino nitrogens. In the present study, it was found that Cu(II) combined with SMX through sulfonamide, amino, and isoxazole ring nitrogens. 3.4. DFT calculation 3.4.1. Binding of SMX and Cu(II) Spectroscopic analysis demonstrated that SMX can bind with Cu(II). To further reveal the binding mechanisms of SMX with Cu(II), the quantum mechanical calculations were carried out using Materials Studio (v7.0) [43]. Cu(II) surrounded by four water molecules ([Cu (H2O)4]2+) was employed as the form of Cu(II) in aqueous solution [44]. For the binding of SMX and Cu(II), four possible binding sites in the deprotonated SMX (SMX−) molecular (sulfonamide nitrogen, amino
Table 1 DFT calculation results for the binding of SMX− and Cu(II). Total energy (E, Hartree) −
Binding energy(kcal/mol)
Structure (bond length, Å)
Binding site
−
SMX
Cu(II)
SMX -Cu(II)
−1174.581
−502.678
−1677.326
−42.20
Sulfonamide nitrogen
−1174.581
−502.678
−1677.317
−36.69
Sulfonyl oxygen
−1174.581
−502.678
−1677.308
−30.88
Amino nitrogen
−1174.581
−502.678
−1677.331
−45.45
Isoxazole ring nitrogen
999
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nitrogen, isoxazole ring nitrogen, and sulfonyl oxygen) [41,42] were considered during DFT calculation. Therefore, a total 4 scenarios were selected to characterize the metal–ligand bonds between SMX− and Cu (II) (Table 1). The optimized structures for [Cu(H2O)4]2+ and SMX− are presented in Fig. S3a and b. The bond length between Cu(II) and H2O molecules changed from 1.976 Å to 1.992 Å (Fig. S3a), which were comparable to that (2.186 ± 0.215 Å) reported in CRC Handbook of Chemistry and Physics [45]. Similar to our previous study [26], V-shape SMX− was observed in aqueous solution (Fig. S3b). The binding energies between SMX− and Cu(II) varied from −30.88 kcal/mol to −45.45 kcal/mol (Table 1), indicating all the four binding sites in SMX− molecular can bind with Cu(II). The combination between amino/sulfonamide/isoxazole ring nitrogen and Cu(II) was identified by ATR-FTIR (Fig. 4b). Moreover, the binding energies between Cu(II) and various binding sites of SMX− decreased in the order of isoxazole ring nitrogen > sulfonamide nitrogen > sulfonyl oxygen > amino nitrogen. This displays that isoxazole ring nitrogen and sulfonamide nitrogen in SMX− molecular are the most favorable binding sites with Cu(II). Our previous study showed that SMX can be adsorbed by CNTs mainly through π-π interactions, and the functional groups on CNTs surface can weaken and break the π-π interactions between SMX and CNTs [26]. In presence of Cu(II), two mechanisms may be responsible for the adsorption of SMX by CNTs/CoFe2O4: (1) SMX was adsorbed through the bridging effect of Cu(II), since Cu(II) can simultaneously bind with the functional groups on CNTs/CoFe2O4 (e.g. carboxyl and amino groups) and SMX−; (2) the SMX-Cu(II) complex was adsorbed through π-π interactions between SMX and CNTs. These two
hypothetical mechanisms were considered in DFT calculation. 3.4.2. Adsorption of SMX through bridging effect of Cu(II) Deprotonated carboxyl group for carboxylized CNTs and undeprotonated amino group for amino CNTs were considered during DFT caclulation [26]. Two positions of functional groups (basal plane and edge) on the surface of CNTs were included in calculation. Consequently, 4 scenarios were calculated to investigate the adsorption of Cu (II) by CNTs (Table S5). Considering the four binding sites in SMX molecular (Table 1), a total of 16 scenarios were chosen to elucidate the adsorption mechanisms of SMX− onto CNTs through bridging effect of Cu(II) (Table S6). The optimized structures of four CNTs models are shown in Fig. S3c–f. In binary systems of CNTs/CoFe2O4 and Cu(II) (Table S5), Cu(II) can be adsorbed by CNTs through binding with the carboxyl and amino groups. The bindings between eCOO− and Cu(II) (binding energies of −116.07 and −106.67 kcal/mol) were a little stronger than those between eNH2 and Cu(II) (binding energies of −105.35 and −92.49 kcal/mol). The bond lengths between CNTs and Cu(II) ranged from 1.953 Å to 2.161 Å. In ternary system, the CNTs−Cu(II)−SMX− complexes (Fig. 6, Tables 2 and S6) formed, and the binding energies of CNTs−Cu (II)−SMX− varied from −12.64 kcal/mol to −44.56 kcal/mol. For functional groups on the edge of CNTs, the binding energies of CNTs−Cu(II)−SMX− for different binding sites of SMX− followed the order of isoxazole ring nitrogen > sulfonamide nitrogen > sulfonyl oxygen > amino nitrogen. This order is the same to that of SMX−-Cu (II) (Table 1). For functional groups on the basal plane of CNTs, the
Fig. 6. Optimized molecular structures for adsorption of SMX− through bridging effect of Cu(II) (a–d) and adsorption of SMX−-Cu(II) complexes through π-π interaction (e–g) by CNTs with carboxyl group on the edge. 1000
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Table 2 Binding energies (kcal/mol) between SMX− with CNTs in absence and presence of Cu(II). Adsorption mechanism
Bridging effect of Cu(II)
Functional group
−
eCOO
eNH2
π-π interaction
eCOO−
eNH2
Binding site between SMX− and Cu(II)
Position of functional group Basal plane
Edge
−30.12 −22.96 −13.67 −28.59 −43.50 −30.40 −26.74 −40.10
−32.28 −31.42 −20.17 −41.74 −34.82 −30.42 −12.64 −44.56
Sulfonamide nitrogen Sulfonyl oxygen Amino nitrogen Isoxazole ring nitrogen Sulfonamide nitrogen Sulfonyl oxygen Amino nitrogen Isoxazole ring nitrogen
−74.63 −88.95 −88.04 −105.72 −93.50 −72.66 −82.30 −77.69 −87.60 −77.65
−76.61 −88.73 −90.56 −99.83 −91.24 −74.02 −78.91 −86.92 −90.22 −84.77
− (without Cu(II)) Sulfonamide nitrogen Sulfonyl oxygen Amino nitrogen Isoxazole ring nitrogen − (without Cu(II)) Sulfonamide nitrogen Sulfonyl oxygen Amino nitrogen Isoxazole ring nitrogen
binding energies of CNTs−Cu(II) − SMX− for different binding sites of SMX− followed the order of sulfonamide nitrogen > isoxazole ring nitrogen > sulfonyl oxygen > amino nitrogen. This order is a little different from that of SMX−-Cu(II) (Table 1), which may be ascribed to the steric hindrance effects of CNTs. In general, the more stable binding sites, sulfonamide nitrogen and isoxazole ring nitrogen, displayed relatively shorter bond lengths between SMX− and Cu(II) (Table 1) or between SMX− and the adsorbed Cu(II) on CNTs (Fig. 6 and Table S6). Moreover, analogous results were obtained for CNTs with both carboxyl and amino functional groups. For these two types of functional groups, CNTs with amino group demonstrated a little stronger binding than CNTs with carboxyl group, especially for the stable binding sites of SMX− (Table 2). This could explain the relatively greater adsorption of SMX onto CNTs-N/CoFe2O4 than those onto CNTs-C/CoFe2O4 (Fig. 1).
adsorption of SMX− without Cu(II) (Table 2). This indicates the binding between SMX− and Cu(II) enhanced the π-π interactions between SMX− and CNTs. This is mainly ascribed to the changes in the electron donating/accepting abilities of SMX− after binding with Cu(II). HOMO and LUMO characterize the electron donating and withdrawing ability, respectively [46]. As shown in Fig. 7a, the LUMO of SMX− was mainly concentrated on its benzene ring and amino group. After binding with Cu(II), the LUMO of SMX− was mainly concentrated on Cu(II) ion and the segment of SMX− complexing with Cu(II) (Fig. 7b–e). In contrast, the HOMO always mainly concentrated on the π-orbitals of benzene ring and p-orbitals of amino group before and after binding with Cu(II) [47], although the sulfonyl nitrogen and isoxazole ring nitrogen/oxygen of SMX− also showed stable HUMO before complexing with Cu(II). Moreover, both the LUMO and HOMO became more stable after complexing with Cu(II), which indicates stronger electron donating and withdrawing abilities. This can explain the stronger π-π interactions between SMX−-Cu(II) complexes and CNTs compared with that between SMX− and CNTs.
3.4.3. Adsorption of SMX−-Cu(II) complexes through π-π interactions In absence of Cu(II), the binding energies for adsorption of SMX− by CNTs through π-π interactions changed from −72.66 kcal/mol to −76.61 kcal/mol (Tables 2 and S7). In presence of Cu(II), the binding energies for the adsorption of SMX−−Cu(II) complexes by CNTs through π-π interactions varied from −88.04 to −105.72 kcal/mol for CNTs with carboxyl group, and changed from −77.65 to −90.22 kcal/ mol for CNTs with amino group (Tables 2 and S8 and Fig. 6e–g). Moreover, the SMX−-Cu(II) complex at the binding site of amino nitrogen demonstrated the strongest binding (the most negative binding energies) with CNTs among the four binding sites of SMX−. Notably, the binding energies for the adsorption of SMX−−Cu(II) complexes by CNTs through π-π interactions were higher than those for the
4. Conclusions The influences of Cu(II) on the adsorption of E2 and SMX by CNTsC/CoFe2O4 and CNTs-N/CoFe2O4 composites were investigated, and the influencing mechanisms were revealed at a molecular level by DFT calculations. E2 adsorption amounts decreased slightly with rising Cu (II) concentrations due to the enhanced aggregation of CNTs/CoFe2O4. In contrast, the adsorption amounts of SMX by CNTs/CoFe2O4 increased with increasing concentrations of Cu(II). The increased SMX adsorption with Cu(II) was ascribed to two reasons. First, the
Fig. 7. HOMO and LUMO orbitals and energy levels of SMX− (a) and its complexes with Cu(II) (b–e). 1001
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adsorption of SMX was enhanced through the bridging effect of Cu(II). DFT calculation confirmed the formation of CNTs-Cu(II)-SMX−. Moreover, Cu(II) could complex with SMX at four binding sites including isoxazole ring nitrogen, sulfonamide nitrogen, amino nitrogen, and sulfonyl oxygen. Among them, isoxazole ring and sulfonamide nitrogens are the most favorable binding sites in SMX with Cu(II), as revealed by their higher binding energies. Second, the enhanced SMX adsorption was resulted from the stronger π-π interactions between SMX−−Cu(II) complexes and CNTs than those between SMX and CNTs. The formation of SMX−−Cu(II) complex was also confirmed by fluorescence, UV, and ATR-FTIR analysis.
[20] [21] [22] [23]
[24]
Acknowledgements
[25]
This research was funded by Qinghai Natural Science Foundation (Grant No. 2019-ZJ-923) and the National Natural Science Foundation of China (Grant No. 51879001).
[26]
Appendix A. Supplementary data
[27]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.05.111.
[28] [29]
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Molecular insights into the effects of Cu(II) on sulfamethoxazole and 17βestradiol adsorption by carbon nanotubes/CoFe2O4 composites
T
Si Lia, Fei Wangb, Weiyi Panb, Xi Yangc, Qiang Gaoc, Weiling Sunc, , Jinren Nic ⁎
a
College of Environmental Sciences and Engineering, Peking University, The Key Laboratory of Water and Sediment Sciences, Ministry of Education, Beijing 100871, China Shenzhen Key Laboratory for Heavy Metal Pollution Control and Reutilization, School of Environment and Energy, Peking University Shenzhen Graduate School, Shenzhen 518055, China c State Key Laboratory of Plateau Ecology and Agriculture, Qinghai University, Xining 810016, China b
HIGHLIGHTS
GRAPHICAL ABSTRACT
slightly inhibited E2 adsorption • Cu(II) onto CNTs/CoFe O composites. promoted SMX adsorption onto • Cu(II) CNTs/CoFe O composites. influencing mechanisms of Cu(II) • The were revealed by DFT calculation. adsorption can be enhanced • SMX through the bridging effects of Cu(II). binding between Cu(II) and SMX • The enhanced π-π interactions between 2
2
4
4
SMX and CNTs.
ARTICLE INFO
ABSTRACT
Keywords: Adsorption Sulfamethoxazole 17β-estradiol Copper Carbon nanotubes/CoFe2O4
Effects of metal ions on the adsorption of organic pollutants have drawn great interests considering their coexistence and interactions in aqueous solution. However, the potential influencing mechanisms of metal ions on the adsorption of organic pollutants at a molecular level were rarely studied. Here, we investigated the effects of Cu(II) on the adsorption of 17β-estradiol (E2) and sulfamethoxazole (SMX) by carboxylic and amino CNTs/CoFe2O4 composites, and the mechanisms were revealed by spectroscopy analyses and density functional theory (DFT) calculations. The results showed distinct effects of Cu(II) on SMX and E2 adsorption by CNTs/CoFe2O4. For E2, a nonionic chemical, Cu(II) slightly inhibited E2 adsorption by CNTs/CoFe2O4 due to the increased aggregation of the composites induced by Cu(II). For SMX, an ionic chemical, Cu(II) enhanced SMX adsorption significantly mainly due to Cu(II) bridging and stronger π-π interactions between SMX and Cu(II) complexes and CNTs. The complexation of SMX with Cu(II) was confirmed by the fluorescence, ultraviolet (UV), and attenuated total reflection fourier transform infrared spectroscopy (ATR-FTIR) analysis. DFT calculation further revealed that isoxazole ring nitrogen, sulfonamide nitrogen, amino nitrogen, and sulfonyl oxygen were the four possible binding sites in SMX with Cu(II). The adsorbed Cu (II) on CNTs/CoFe2O4 could form complexes with SMX, and the binding between SMX and Cu(II) enhanced the π-π interactions between SMX and CNTs. These two aspects led to the enhanced adsorption of SMX. This study provides new insights into the molecular interactions among Cu(II), SMX, and CNTs, and it sheds lights on the adsorption mechanisms of organic pollutants by CNTs in the presence of metal ions.
⁎
Corresponding author. E-mail address: [email protected] (W. Sun).
https://doi.org/10.1016/j.cej.2019.05.111 Received 10 April 2019; Received in revised form 15 May 2019; Accepted 17 May 2019 Available online 18 May 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.
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1. Introduction
hormone, is listed in the “Drinking Water Contaminant Candidate List 3” by USEPA [21,31]. Cu(II), as a common heavy metal, was selected due to its strong complexation ability with organic chemicals [16,19]. Carboxylized CNTs/CoFe2O4 (CNTs-C/CoFe2O4) and amino CNTs/ CoFe2O4 (CNTs-N/CoFe2O4) composites were prepared. The adsorption behaviors of SMX/E2 by CNTs/CoFe2O4 composites were compared with and without addition of Cu(II). The potential influencing mechanisms of Cu(II) on SMX/E2 adsorption were revealed through spectroscopic analysis and DFT calculation.
Carbon nanotubes (CNTs), since their discovery by Iijima [1], are one of the most widely used nanomaterials [2,3]. Particularly, CNTs exhibit great adsorption capacity to varieties of contaminants, including metals [4,5] and organic compounds [6,7] due to their large surface area, abundant surface functional groups, and high hydrophobicity. Nonetheless, one main practical challenge in CNTs’ application is to recover the nanosized CNTs from the treated water in regards to economic and safety concerns. Magnetic materials modified CNTs may provide a feasible solution to overcome this limitation. Among diverse magnetic materials, ferrite (MIIFe2O4, M = Ni, Co, Mn, etc) exhibits excellent magnetic property and good stability in acidic solution [8,9]. Especially, CoFe2O4 decorated CNTs (CNTs/CoFe2O4) had the highest saturation magnetization [8] and they have been widely applied in the removal of dyes [9,10], uranium(VI) ions [11], aniline [12], and emerging contaminants of sulfamethoxazole (SMX), 17β-estradiol (E2) [8], tetrabromobisphenol A [13], and bezafibrate [14] from aqueous solution. Considering the coexistence and interactions of organic contaminants and metals in the environment, the effects of metals on adsorption of organic contaminants by CNTs have aroused growing interests [15–17]. Metal ions could affect the adsorption of organic pollutants through competitive adsorption [18,19], electrostatic counteracting interactions [15], cation bridging [16,19], and/or salting out effect [20,21]. Therefore, the presence of metals may increase, decrease, or have no apparent effects on the adsorption of organic pollutants, and these effects could alter depending on the pH range, metal types, and metal concentrations [15,16,19,22]. However, most of these previous studies were investigated using batch experiments and spectroscopy analysis, little work has been conducted to examine the effect of metals at a molecular level. Density functional theory (DFT) calculation is a powerful tool for providing molecular insights into the adsorption mechanisms of metals [23–25] and organic chemicals [26–28] to various nanomaterials. For example, DFT calculation revealed that Eu(III) and 243Am(III) could form surface complexes with oxygen-containing functional groups of CNTs [23]. Wei et al. [26] reported that SMX was adsorbed by CNTs mainly through π-π interactions, and the oxygen-containing functional groups on CNTs inhibited SMX adsorption through weakening or breaking the π-π interactions between SMX and CNTs. However, the adsorption mechanisms of organic compounds in the presence of metal ions by CNTs were rarely elucidated by DFT calculations. In this study, SMX and E2 were chosen as model emerging contaminants because of their widespread occurrence and potential risks to the ecosystem [21,29]. SMX is one of the most frequently detected antibiotic in China’s major rivers [29] and it is a potential indicator for monitoring and control of antibiotic pollution [30]. E2, as an estrogenic
2. Materials and methods 2.1. Materials and reagents Sources of materials and reagents are provided in Text S1. 2.2. Synthesis and characterization of CNTs/CoFe2O4 composites CNTs-C/CoFe2O4 and CNTs-N/CoFe2O4 were synthesized by a hydrothermal method as described in our previous work [8]. Details on synthesis procedure of CNTs/CoFe2O4 composites are shown in Text S2. Zeta potentials of CNTs/CoFe2O4 were measured using a Malvern Zetasizer Nano ZS90 (Worcestershire, UK) at 25 ± 1 °C. Chemical compositions of CNTs/CoFe2O4 were measured by X-ray photoelectron spectroscopy (XPS; Kratos Analytical, UK). 2.3. Effect of Cu(II) on adsorption Batch experiments were conducted using 100 mL glass vials. CNTs/ CoFe2O4 (5 mg) were added to 50 mL 0.05 M NaNO3 solution. The initial concentration of SMX or E2 was 2.0 mg/L. The Cu(II) concentrations varied from 0 to 10 mg/L at pH 6.0 ± 0.2. Solution pH ranged from 2.0 to 11.0 with HNO3 or NaOH, when the Cu(II) concentration was kept at 5 mg/L. The solutions were equilibrated for 24 h at 25 ± 1 °C. Final pH values were measured after adsorption equilibrium. The suspensions were filtrated via filter membrane (0.22 μm) before analysis. 2.4. SMX, E2, and Cu(II) analysis The concentrations of SMX and E2 in the supernatant were measured by a HP 1100 LC-MSn Trap SL System (Agilent Technologies, USA), as described in our previous study [8] and Text S3. The concentration of Cu(II) was measured by ICP-OES (Prodigy, Leeman, USA). 2.5. Interactions between Cu(II) and SMX Interactions between Cu(II) and SMX were investigated by
Fig. 1. Adsorption of SMX (a) and E2 (b) by CNTs/CoFe2O4 at different Cu(II) concentrations (SMX/E2 = 2.0 mg/L, CNTs/CoFe2O4 = 100 mg/L, I = 0.05 M NaNO3, T = 25 ± 1 °C, pH = 6.0). 996
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ultraviolet (UV), fluorescence, and attenuated total reflection fourier transform infrared spectroscopy (ATR-FTIR) analysis. The details are given in Text S4.
Cu(II) could reduce the negative charges on the surface of CNTs/ CoFe2O4 [19]. 3.2. Effects of Cu(II) at different pH
2.6. DFT calculation
Great effects of pH on SMX adsorption by CNTs/CoFe2O4 were observed with and without addition of Cu(II) (Fig. 2a and b). In absence of Cu(II), the adsorption amounts of SMX decreased sharply from ∼8 mg/ g at pH 2.0 to almost 0 mg/g at pH 11.0. This is in agreement with that found by Yu et al. [32] for SMX adsorption by CNTs. The presence of Cu (II) promoted SMX adsorption by CNTs/CoFe2O4. Moreover, a sharp increase in adsorption amounts of SMX was observed around pH 6.5 in presence of Cu(II) (Fig. 2a and b). It was reported that hydrophobic interaction was the predominant adsorption mechanism for SMX onto CNTs [32,35]. At pH of 2.0–5.5, the main species of SMX is SMX0, and SMX− became dominant at pH > 5.5. The hydrophobicity of SMX− (log Kow− = −2.14) is much lower than that of SMX0 (log Kow0 = 0.85) [36], so a decreasing tend of SMX adsorption was found with increasing pH. On the other hand, the surface of CNTs/CoFe2O4 was more negatively charged at higher pH (Fig. 3). This induced the stronger electrostatic repulsion between CNTs/CoFe2O4 and SMX−, and then decreased SMX adsorption with rising pH. Compared with CNTs-C/CoFe2O4 (Fig. 3a), more positive charges were observed on the surface of CNTs-N/CoFe2O4 (Fig. 3b). This may explain the relatively higher adsorption amounts of SMX by CNTs-N/CoFe2O4 than those by CNTs-C/CoFe2O4. The main species of Cu(II) is Cu2+ at pH < 7.0 (Fig. S1). The adsorbed Cu(II) could reduce the negative charges on the surface of CNTs/ CoFe2O4 (Fig. 3), and then weaken the electrostatic repulsion between CNTs/CoFe2O4 and SMX−. In addition, the zeta potentials of the CNTs/ CoFe2O4 composites showed a peak at pH 6.5–7.0 (Fig. 3), because of the formation of Cu(OH)2 precipitate (Fig. S1). As a result, a sharp increase in adsorption amounts of SMX was observed at pH 6.5–7.0 (Fig. 2). This indicates Cu(OH)2 may also act as adsorbents for SMX adsorption [19]. It was noted that pH had little effect on E2 adsorption by CNTs/ CoFe2O4 (Fig. 2a and b), and the E2 adsorption amounts declined from 20 mg/g to 17 mg/g in the pH range of 2.0–11.0. Because the neutral form of E2 is always the main species of E2 at pH < 10.4 [8], the change of pH had little effect on E2 adsorption by CNTs/CoFe2O4. This finding proves the hydrophobic interaction plays a dominant role in E2 adsorption by CNTs/CoFe2O4.
The interactions between CNTs/CoFe2O4 composites, Cu(II), and organic chemicals were calculated by DFT using DMOL3 code in Materials Studio (v7.0) [26,27]. The detailed description of DFT calculation is provided in Text S5. 3. Results and discussion 3.1. Adsorption of SMX and E2 at different Cu(II) concentrations The adsorption amounts of SMX by CNTs/CoFe2O4 increased with increasing Cu(II) concentrations (Fig. 1a), while the adsorption amounts of E2 decreased slightly with rising Cu(II) concentrations up to 4 mg/L and then nearly did not change with further increase in Cu(II) concentrations (Fig. 1b). Moreover, E2 showed greater adsorption than SMX due to the its higher hydrophobicity (logKow = 3.94 for E2 and 0.85 for SMX) [8], because both SMX and E2 were adsorbed by CNTs/ CoFe2O4 mainly through π-π interactions [26,31]. In addition, CNTs-N/ CoFe2O4 showed a higher adsorption capacity to SMX and E2 than CNTs-C/CoFe2O4, which is consistent with that found by Wang et al. [8]. It was speculated that the different effects of Cu(II) on SMX and E2 adsorption were mainly resulted from their distinct species in aqueous solution. SMX is a typical amphoteric compound, with pKa values of 1.80 and 5.46, respectively [32]. At the experimental pH of 6.0, SMX exists in neutral (SMX0, 22.4%) and anionic (SMX−, 77.6%) forms (Table S1) [8], while Cu2+ is the predominant species of Cu(II), as revealed by Visual Minteq (v3.0) calculation (Fig. S1). Cu(II) may promote the adsorption of SMX through the following two aspects: (1) Cu(II) adsorbed onto the composites, and then promote SMX adsorption through cation bridging effect [16,22]; (2) the complexation of Cu(II) with SMX could decrease electrostatic repulsion between negatively charged SMX and CNTs/CoFe2O4 [15,16]. Consequently, the SMX adsorption by CNTs/CoFe2O4 was increased. Wu et al. [16] also found that the ternary complexes of CNTs-SMX-Cu and CNTs-Cu-SMX may occur at pH 6.5 depending on Cu(II) concentrations. Morel et al. [33] reported that the surface adsorbed Cu(II) significantly enhanced SMX adsorption by soil because of the formation of SMX-Cu-soil complexes. At pH 6.0, E2 is not ionized (Table S1), because its pKa is 10.4 [8,34]. Therefore, the effects of Cu(II) on E2 adsorption by CNTs/ CoFe2O4 were minor since hydrophobic interactions dominate E2 adsorption at low metal concentrations. The weak inhibition effect of Cu (II) on E2 adsorption may be attributed to the strong aggregation of CNTs/CoFe2O4 particles in presence of Cu(II), because the adsorption of
3.3. Spectroscopic investigation on influencing mechanisms As shown by the above results, Cu(II) posed stronger effects on SMX adsorption than on E2 adsorption by CNTs/CoFe2O4. The adsorption of SMX and Cu(II) onto CNTs/CoFe2O4 was proven by XPS analysis (Text S6, Fig. S2, and Table S2). Moreover, it was speculated that the binding
Fig. 2. SMX and E2 adsorption by CNTs-C/CoFe2O4 (a) and CNTs-N/CoFe2O4 (b) at different pH in the absence (open symbol) and presence (solid symbol) of Cu(II) (SMX/E2 = 2.0 mg/L, Cu(II) = 5 mg/L, CNTs/CoFe2O4 = 100 mg/L, I = 0.05 M NaNO3, T = 25 ± 1 °C). 997
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Fig. 3. Effects of Cu(II) on zeta potential of CNTs-C/CoFe2O4 (a) and CNTs-N/CoFe2O4 (b) after adsorption of SMX and E2 at different pH (SMX/E2 = 2.0 mg/L, Cu (II) = 5 mg/L, CNTs/CoFe2O4 = 100 mg/L, I = 0.05 M NaNO3, T = 25 ± 1 °C).
between Cu(II) and SMX promoted SMX adsorption. To confirm the binding of SMX and Cu(II), fluorescence, UV, and ATR-FTIR analysis were conducted at two selected pH values of 6.0 and 9.6.
the fluorescence of NOMs [38,39]. 3.3.2. UV analysis The UV absorption spectra of SMX solutions are shown in Fig. 5a. An absorption band at 262.5 nm was observed for SMX at pH 6.0, and it moved to 256.5 nm at pH 9.6. Blue shift of the peak location of SMX with rising pH was reported previously [35,40]. This shift was due to changes in the dissociation species of SMX (SMX0/SMX−) at pH 6.0 and pH 9.6 (Table S1). With addition of Cu(II), the absorption band moved to lower wavelength, from 262.5 to 257.0 nm at pH 6.0 and from 256.5 nm to 253.0 nm at pH 9.6. This indicates the complexation of SMX with Cu(II).
3.3.1. Fluorescence analysis The excitation/emission matrix (EEM) spectra showed that SMX had one main peak centered at Ex/Em = 260 nm/345 nm (Fig. 4a and c and Table S3). Although the positions of the peak did not change, its intensity increased significantly at pH 9.6 (Fig. 4c) compared with that at pH 6.0 (Fig. 4a and Table S3). This indicates that SMX− demonstrates stronger fluorescence intensities than SMX0. The addition of Cu(II) decreased the intensities of the peak at both pH (Fig. 4b and d), suggesting the complexation of Cu(II) with SMX. Similar results were reported previously, in which decreased fluorescence intensities were observed for E2, 17α-ethynylestradiol, and bisphenol-A due to their interactions with Ca2+ [37]. In addition, the binding of metals to natural organic matters (NOMs) could also quench
3.3.3. ATR-FTIR analysis ATR-FTIR was employed to uncover the binding sites of SMX with Cu(II). Similar ATR-FTIR spectra were observed for SMX at pH 6.0 and 9.6 (Fig. 5b), and the assignment of the peaks is shown in Table S4. The
Fig. 4. EEM fluorescence spectra of SMX solutions in absence and presence of Cu(II) at pH 6.0 (a and b) and 9.6 (c and d). 998
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Fig. 5. UV (a) and ATR-FTIR (b) spectra of SMX solutions in absence and presence of Cu(II) at pH 6.0 and 9.6.
bands at 3478 and 3401 cm−1 (pH 6.0) or 3468 and 3401 cm−1 (pH 9.6) were ascribed to the antisymmetric and symmetric vibrations of aniline NH2 group [41,42], respectively. The band at 3281 cm−1 (pH 6.0) or 3323 cm−1 (pH 9.6) was assigned to the vibration of sulfonamide NH group [41,42]. In presence of Cu(II), these three bands disappeared at both pH 6.0 and pH 9.6, suggesting the binding of Cu(II) with aniline NH2 and sulfonamide NH groups. Isoxazole ring vibrations were observed at 1460 and 1408 cm−1 in the spectra of SMX [41,42]. The band at 1460 cm−1 shifted to 1466 cm−1, while the band at 1408 cm−1 disappeared in presence of Cu(II), indicating interactions between isoxazole ring nitrogen and Cu(II). For the complexation of SMX with metals, different binding sites were proposed depending on the metal types. Kanagaraj and Rao [42] found that SMX could bind with Cr, Mn, and Ni through deprotonated sulfonamide nitrogen and sulfonyl oxygen, while bind with Cu through deprotonated isoxazole ring nitrogen and sulfonamide nitrogen. Kesimli
and Topacli [41] reported that Cd bound with SMX through sulfonamide nitrogen and sulfonic oxygen, while Co bound with SMX through sulfonamide and amino nitrogens. In the present study, it was found that Cu(II) combined with SMX through sulfonamide, amino, and isoxazole ring nitrogens. 3.4. DFT calculation 3.4.1. Binding of SMX and Cu(II) Spectroscopic analysis demonstrated that SMX can bind with Cu(II). To further reveal the binding mechanisms of SMX with Cu(II), the quantum mechanical calculations were carried out using Materials Studio (v7.0) [43]. Cu(II) surrounded by four water molecules ([Cu (H2O)4]2+) was employed as the form of Cu(II) in aqueous solution [44]. For the binding of SMX and Cu(II), four possible binding sites in the deprotonated SMX (SMX−) molecular (sulfonamide nitrogen, amino
Table 1 DFT calculation results for the binding of SMX− and Cu(II). Total energy (E, Hartree) −
Binding energy(kcal/mol)
Structure (bond length, Å)
Binding site
−
SMX
Cu(II)
SMX -Cu(II)
−1174.581
−502.678
−1677.326
−42.20
Sulfonamide nitrogen
−1174.581
−502.678
−1677.317
−36.69
Sulfonyl oxygen
−1174.581
−502.678
−1677.308
−30.88
Amino nitrogen
−1174.581
−502.678
−1677.331
−45.45
Isoxazole ring nitrogen
999
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nitrogen, isoxazole ring nitrogen, and sulfonyl oxygen) [41,42] were considered during DFT calculation. Therefore, a total 4 scenarios were selected to characterize the metal–ligand bonds between SMX− and Cu (II) (Table 1). The optimized structures for [Cu(H2O)4]2+ and SMX− are presented in Fig. S3a and b. The bond length between Cu(II) and H2O molecules changed from 1.976 Å to 1.992 Å (Fig. S3a), which were comparable to that (2.186 ± 0.215 Å) reported in CRC Handbook of Chemistry and Physics [45]. Similar to our previous study [26], V-shape SMX− was observed in aqueous solution (Fig. S3b). The binding energies between SMX− and Cu(II) varied from −30.88 kcal/mol to −45.45 kcal/mol (Table 1), indicating all the four binding sites in SMX− molecular can bind with Cu(II). The combination between amino/sulfonamide/isoxazole ring nitrogen and Cu(II) was identified by ATR-FTIR (Fig. 4b). Moreover, the binding energies between Cu(II) and various binding sites of SMX− decreased in the order of isoxazole ring nitrogen > sulfonamide nitrogen > sulfonyl oxygen > amino nitrogen. This displays that isoxazole ring nitrogen and sulfonamide nitrogen in SMX− molecular are the most favorable binding sites with Cu(II). Our previous study showed that SMX can be adsorbed by CNTs mainly through π-π interactions, and the functional groups on CNTs surface can weaken and break the π-π interactions between SMX and CNTs [26]. In presence of Cu(II), two mechanisms may be responsible for the adsorption of SMX by CNTs/CoFe2O4: (1) SMX was adsorbed through the bridging effect of Cu(II), since Cu(II) can simultaneously bind with the functional groups on CNTs/CoFe2O4 (e.g. carboxyl and amino groups) and SMX−; (2) the SMX-Cu(II) complex was adsorbed through π-π interactions between SMX and CNTs. These two
hypothetical mechanisms were considered in DFT calculation. 3.4.2. Adsorption of SMX through bridging effect of Cu(II) Deprotonated carboxyl group for carboxylized CNTs and undeprotonated amino group for amino CNTs were considered during DFT caclulation [26]. Two positions of functional groups (basal plane and edge) on the surface of CNTs were included in calculation. Consequently, 4 scenarios were calculated to investigate the adsorption of Cu (II) by CNTs (Table S5). Considering the four binding sites in SMX molecular (Table 1), a total of 16 scenarios were chosen to elucidate the adsorption mechanisms of SMX− onto CNTs through bridging effect of Cu(II) (Table S6). The optimized structures of four CNTs models are shown in Fig. S3c–f. In binary systems of CNTs/CoFe2O4 and Cu(II) (Table S5), Cu(II) can be adsorbed by CNTs through binding with the carboxyl and amino groups. The bindings between eCOO− and Cu(II) (binding energies of −116.07 and −106.67 kcal/mol) were a little stronger than those between eNH2 and Cu(II) (binding energies of −105.35 and −92.49 kcal/mol). The bond lengths between CNTs and Cu(II) ranged from 1.953 Å to 2.161 Å. In ternary system, the CNTs−Cu(II)−SMX− complexes (Fig. 6, Tables 2 and S6) formed, and the binding energies of CNTs−Cu (II)−SMX− varied from −12.64 kcal/mol to −44.56 kcal/mol. For functional groups on the edge of CNTs, the binding energies of CNTs−Cu(II)−SMX− for different binding sites of SMX− followed the order of isoxazole ring nitrogen > sulfonamide nitrogen > sulfonyl oxygen > amino nitrogen. This order is the same to that of SMX−-Cu (II) (Table 1). For functional groups on the basal plane of CNTs, the
Fig. 6. Optimized molecular structures for adsorption of SMX− through bridging effect of Cu(II) (a–d) and adsorption of SMX−-Cu(II) complexes through π-π interaction (e–g) by CNTs with carboxyl group on the edge. 1000
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Table 2 Binding energies (kcal/mol) between SMX− with CNTs in absence and presence of Cu(II). Adsorption mechanism
Bridging effect of Cu(II)
Functional group
−
eCOO
eNH2
π-π interaction
eCOO−
eNH2
Binding site between SMX− and Cu(II)
Position of functional group Basal plane
Edge
−30.12 −22.96 −13.67 −28.59 −43.50 −30.40 −26.74 −40.10
−32.28 −31.42 −20.17 −41.74 −34.82 −30.42 −12.64 −44.56
Sulfonamide nitrogen Sulfonyl oxygen Amino nitrogen Isoxazole ring nitrogen Sulfonamide nitrogen Sulfonyl oxygen Amino nitrogen Isoxazole ring nitrogen
−74.63 −88.95 −88.04 −105.72 −93.50 −72.66 −82.30 −77.69 −87.60 −77.65
−76.61 −88.73 −90.56 −99.83 −91.24 −74.02 −78.91 −86.92 −90.22 −84.77
− (without Cu(II)) Sulfonamide nitrogen Sulfonyl oxygen Amino nitrogen Isoxazole ring nitrogen − (without Cu(II)) Sulfonamide nitrogen Sulfonyl oxygen Amino nitrogen Isoxazole ring nitrogen
binding energies of CNTs−Cu(II) − SMX− for different binding sites of SMX− followed the order of sulfonamide nitrogen > isoxazole ring nitrogen > sulfonyl oxygen > amino nitrogen. This order is a little different from that of SMX−-Cu(II) (Table 1), which may be ascribed to the steric hindrance effects of CNTs. In general, the more stable binding sites, sulfonamide nitrogen and isoxazole ring nitrogen, displayed relatively shorter bond lengths between SMX− and Cu(II) (Table 1) or between SMX− and the adsorbed Cu(II) on CNTs (Fig. 6 and Table S6). Moreover, analogous results were obtained for CNTs with both carboxyl and amino functional groups. For these two types of functional groups, CNTs with amino group demonstrated a little stronger binding than CNTs with carboxyl group, especially for the stable binding sites of SMX− (Table 2). This could explain the relatively greater adsorption of SMX onto CNTs-N/CoFe2O4 than those onto CNTs-C/CoFe2O4 (Fig. 1).
adsorption of SMX− without Cu(II) (Table 2). This indicates the binding between SMX− and Cu(II) enhanced the π-π interactions between SMX− and CNTs. This is mainly ascribed to the changes in the electron donating/accepting abilities of SMX− after binding with Cu(II). HOMO and LUMO characterize the electron donating and withdrawing ability, respectively [46]. As shown in Fig. 7a, the LUMO of SMX− was mainly concentrated on its benzene ring and amino group. After binding with Cu(II), the LUMO of SMX− was mainly concentrated on Cu(II) ion and the segment of SMX− complexing with Cu(II) (Fig. 7b–e). In contrast, the HOMO always mainly concentrated on the π-orbitals of benzene ring and p-orbitals of amino group before and after binding with Cu(II) [47], although the sulfonyl nitrogen and isoxazole ring nitrogen/oxygen of SMX− also showed stable HUMO before complexing with Cu(II). Moreover, both the LUMO and HOMO became more stable after complexing with Cu(II), which indicates stronger electron donating and withdrawing abilities. This can explain the stronger π-π interactions between SMX−-Cu(II) complexes and CNTs compared with that between SMX− and CNTs.
3.4.3. Adsorption of SMX−-Cu(II) complexes through π-π interactions In absence of Cu(II), the binding energies for adsorption of SMX− by CNTs through π-π interactions changed from −72.66 kcal/mol to −76.61 kcal/mol (Tables 2 and S7). In presence of Cu(II), the binding energies for the adsorption of SMX−−Cu(II) complexes by CNTs through π-π interactions varied from −88.04 to −105.72 kcal/mol for CNTs with carboxyl group, and changed from −77.65 to −90.22 kcal/ mol for CNTs with amino group (Tables 2 and S8 and Fig. 6e–g). Moreover, the SMX−-Cu(II) complex at the binding site of amino nitrogen demonstrated the strongest binding (the most negative binding energies) with CNTs among the four binding sites of SMX−. Notably, the binding energies for the adsorption of SMX−−Cu(II) complexes by CNTs through π-π interactions were higher than those for the
4. Conclusions The influences of Cu(II) on the adsorption of E2 and SMX by CNTsC/CoFe2O4 and CNTs-N/CoFe2O4 composites were investigated, and the influencing mechanisms were revealed at a molecular level by DFT calculations. E2 adsorption amounts decreased slightly with rising Cu (II) concentrations due to the enhanced aggregation of CNTs/CoFe2O4. In contrast, the adsorption amounts of SMX by CNTs/CoFe2O4 increased with increasing concentrations of Cu(II). The increased SMX adsorption with Cu(II) was ascribed to two reasons. First, the
Fig. 7. HOMO and LUMO orbitals and energy levels of SMX− (a) and its complexes with Cu(II) (b–e). 1001
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adsorption of SMX was enhanced through the bridging effect of Cu(II). DFT calculation confirmed the formation of CNTs-Cu(II)-SMX−. Moreover, Cu(II) could complex with SMX at four binding sites including isoxazole ring nitrogen, sulfonamide nitrogen, amino nitrogen, and sulfonyl oxygen. Among them, isoxazole ring and sulfonamide nitrogens are the most favorable binding sites in SMX with Cu(II), as revealed by their higher binding energies. Second, the enhanced SMX adsorption was resulted from the stronger π-π interactions between SMX−−Cu(II) complexes and CNTs than those between SMX and CNTs. The formation of SMX−−Cu(II) complex was also confirmed by fluorescence, UV, and ATR-FTIR analysis.
[20] [21] [22] [23]
[24]
Acknowledgements
[25]
This research was funded by Qinghai Natural Science Foundation (Grant No. 2019-ZJ-923) and the National Natural Science Foundation of China (Grant No. 51879001).
[26]
Appendix A. Supplementary data
[27]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.05.111.
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