Adsorption Behavior and Mechanism of Antibiotic Sulfamethoxazole

Subscriber access provided by CORNELL UNIVERSITY LIBRARY

Article

Adsorption behavior and mechanism of antibiotic sulfamethoxazole on carboxylic-functionalized carbon nanofibers encapsulated Ni magnetic nanoparticles Yi-Kang Lan, Tse-Ching Chen, Hsing-Jui Tsai, Hung-Chi Wu, Jarrn-Horng Lin, I-Kuan Lin, Jyh-Fu Lee, and Ching-Shiun Chen Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b02904 • Publication Date (Web): 31 Aug 2016 Downloaded from http://pubs.acs.org on September 6, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Langmuir is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Adsorption behavior and mechanism of antibiotic sulfamethoxazole on carboxylic-functionalized carbon nanofibers encapsulated Ni magnetic nanoparticles Yi K. Lana, Tse C. Chenb, Hsing J. Tsaic, Hung C. Wuc, Jarrn H. Lind, I K. Lind, Jyh F. Leee and Ching S. Chenb,c* a

Department of Chemical Engineering, National Tsing Hua University, Hsinchu 300, Taiwan, Republic of China b

Department of Pathology, Chang Gung Memorial Hospital, 5 Fusing St., Guishan Dist., Taoyuan City 333, Taiwan, Republic of China

c

Center for General Education, Chang Gung University, 259 Wen-Hwa 1st Road, Guishan Dist., Taoyuan City 333, Taiwan, Republic of China

d

Department of Materials Science, National University of Tainan, 33, Sec. 2, Shu-Lin St., Tainan 700, Taiwan, Republic of China e

National Synchrotron Radiation Research Center, Hsinchu 300, Taiwan, Republic of China

KEYWORDS: antibiotics, sulfamethoxazole, carbon-encapsulated magnetic Ni nanoparticles, CO2 hydrogenation, wastewater treatment

ACS Paragon Plus Environment

1

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 36

ABSTRACT

In this work, we have developed an one-step process for synthesizing carboxylic-functionalized carbon nanofibers (CNFs)-encapsulated Ni magnetic nanoparticles (Ni@CNFs) that exhibit an excellent magnetic response and a large content of hydrophilic carboxylate groups with a negative charge (RCOO-) on the carbon surface. The carbon-encapsulated magnetic Ni nanoparticles could be rapidly separated from water, and they showed high efficiency for adsorption of the antibiotic sulfamethoxazole (SMX) in aqueous solution. The adsorption of SMX on Ni@CNFs as a function of pH was investigated, and the greatest adsorption occurred at pH 7.0. The adsorption isotherms for SMX on Ni@CNFs depended on different pH values. A Monte Carlo simulation was used to probe the relationship between molecular conformation and π-π interaction. The high adsorption of SMX on Ni@CNFs at pH 7.0 could be ascribed to deprotonated SMX being easily converted to a planar-like conformation, thereby resulting in the formation of π rings that were approximately parallel to the graphite surface and that enhanced strong π-π interaction. Electrostatic and π-π interactions both contributed to deprotonated SMX adsorption at pH 7.0, and they influenced the adsorption isotherm toward the Freundlich model. However, in weakly acidic environments (pH 2.0 and 4.0), the electrostatic interaction alone could induce an adsorption pattern that was similar to the Langmuir model.


2

Page 3 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

1. Introduction Pharmaceutical antibiotics are produced in large quantities, and they are used widely in human therapy and in industrial agriculture. Residues from antibiotic compounds and metabolites discharged from municipal wastewater treatment plants and from agricultural runoff inevitably enter into aquatic and soil environments.1-3 These antibiotics are considered to be potentially toxic to humans through the food chain and in drinking water. For example, growth-inhibitory effects are intensified by the simple combination of binary mixtures of antibiotics. Conversely, antibiotic resistance is promoted by prolonged exposure of bacteria to antibiotics in the environment, which then leads to the failure of antibiotics in clinical applications. Thus, removal of residual pharmaceutical antibiotics from water has become an important challenge for contemporary society, as the long-term effects of antibiotics on human health and on aqueous organisms can be detrimental, even at very low concentrations on the order of a few parts per billion (ppb). Hence, there has been increased interest in the development of more effective treatment technologies that remove pharmaceutical antibiotics from water.4-17 Recently, antibiotic degradation by treatment with ozone or UV/H2O2 has been used as an alternative method to remove antibiotic micropollutants from water, but such methods may lead to undesired oxidation-derived products.4-8 Therefore, it is generally accepted that adsorption to high-binding sorbents provides a simple and efficient way to eliminate contaminant substances from waste water and drinking water. Carbon materials, such as nanotubes and active carbon, have shown great potential as effective adsorbents for the removal of pharmaceutical antibiotics in water treatment because of their large surface areas.9-17 However, the traditional filtration method for separating the adsorbents from water can lead to filter blockage or loss of adsorbent. The application of magnetic separation


3

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 36

technology can be rapid and effective, avoiding time-consuming filtration operations.18 The high surface hydrophobicity of carbon materials allows them to adsorb many hydrophobic organic compounds. However, carbon materials dispersed in water are typically difficult to rapidly separate from the liquid phase if the solid powder is highly dispersed in the liquid phase. Thus, carbon-encapsulated magnetic metal nanoparticles hold great potential due to their magnetic properties, which facilitate rapid separation of carbon and liquid. Carbon-encapsulated magnetic metal nanoparticles (NPs) currently attract considerable attention for several applications, including clinical applications and water pollution-treatment applications.19-21 Several magnetic transition metals, such as Ni or Fe, have been shown to be efficient catalysts for the production of carbon nanotubes (CNTs) in the high-pressure decomposition of carbon monoxide (HiPco) process.22-25 The synthesis of Co, Ni, and Fe nanoparticles encapsulated in onion-like carbon shells (Co@C, Ni@C, and Fe@C) occurs when a pulsed-plasma technique is used in liquid.26 However, raw carbon materials prepared from hydrocarbons are extremely hydrophobic; thus, they must be suitably oxidized to enable their convenient use in aqueous solution. Generally, the carbon surface is treated with concentrated acids (HNO3 or H2SO4) to generate carboxylic acid and/or carboxylate groups for high dispersion in water. In reality, treatment of carbon materials with concentrated acids may lead to the removal of metal NPs from the surface. Thus, it is assumed that it is inherently difficult to prepare combinations of hydrophilic carbon and magnetic NPs. Wang et al. prepared carbon nanochain encapsulated magnetic iron using ferrocene as a single reactant that contained a hydrophilic surface, but the carbon material with iron oxide required a multi-step process.19,20 Recently, we have developed a novel Ni-Na/Al2O3 catalyst for synthesizing magnetic Ni NPs that are encapsulated in CNFs material (Ni@CNFs) by using the CO2 hydrogenation reaction.


4

Page 5 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

The Ni-Na/Al2O3 catalyst not only effectively produced solid carbon from CO2, but it also led to high reaction rates for the formation of CO and CH4 in a long-term test.27 Ni@CNFs could be directly obtained from the CO2 hydrogenation reaction on the Ni-Na/Al2O3 catalyst in a one-step process that uniquely yielded abundant hydrophilic oxygen-containing functional groups. On the other hand, the graphite structures with hydrophobic property were also present on Ni@CNFs surface. In this study, sulfamethoxazole (SMX), which are bacteriostatic antibiotics that are often used to treat urinary infections, were chosen as the target pharmaceuticals. The SMX is not easy to dissolve in water because its low solubility is ~610 ppm at pH 7.0 and ~1193 ppm at pH 1.0.28 However, the highly hydrophilic and hydrophobic properties for Ni@CNFs can effectively disperse the carbon adsorbent in water and increase the efficiency of antibiotic adsorption. In comparison to traditional antibiotic molecules, SMX adsorption is relatively more complicated, as SMX can exist as three different species with different pH values, namely SMX+, SMX, and SMX-, as shown in Scheme 1. The pKa values for SMX are 1.7 and 5.6.29

Scheme 1 Equilibrium of acid dissociation for the SMX antibiotic Under biological conditions, the different SMX species at different pH values can lead to large differences in adsorption mechanisms. To date, several studies have investigated the pH dependence of SMX antibiotic adsorbtion on carbon materials, such as active carbon and carbon nanotubes, but the adsorption mechanism of SMX on carbon is still unclear.14-17 Although the adsorption isotherm of SMX on carbon materials is commonly discussed in terms of electrostatic


5

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 36

and π-π stacking interactions, the molecular-level relationship between the molecular conformation of SMX and graphite sheets has not attracted much attention. In our work, a Monte Carlo simulation was combined with an analysis of pH-dependent adsorption to investigate the mechanism by which SMX bound to the surface of Ni@CNFs based on their molecular structures. 2. Experimental 2.1 Preparation of catalyst and CNFs The catalyst used in this study was a commercially available 12 wt% Ni/Al2O3 catalyst manufactured by Süd-Chemie Catalysts, Inc. (catalyst # FCR-42). A Ni/Al2O3 sample containing 3 wt% Na was prepared by adding the required volume of aqueous NaNO3 to the Ni/Al2O3 catalyst without any pretreatment. The samples were subsequently air-dried at 80°C for 10 h. The catalysts were then calcined in air and reduced under H2 at 500°C for 5 h prior to use. All CNF syntheses were performed in a fixed-bed reactor at atmospheric pressure. A total of 50 mg of catalyst was used for each carbon deposition reaction. The CNFs were generated at 500°C by feeding a stream of H2/CO2 (1:1) into the reactor at 100 mL/min.27 The Ni@CNFs samples that were synthesized with reaction times of 3, 9 and 15 h are indicated by Ni@CNFs-3h, Ni@CNFs9h and Ni@CNFs-15h, respectively. 2.2 Characterization of CNFs The chemical composition and oxidation state of the catalyst surface were examined by X-ray photoelectron spectroscopy (XPS). XPS data were obtained using a Thermo VG-Scientific Sigma Probe spectrometer at the Precision Instrument Center of the College of Engineering at the National Central University, Taiwan. The spectrometer was equipped with an Al-Kα X-ray source (1486.6 eV; 1 eV=1.602×10–19 J) operated at 108 W and a hemispherical analyzer


6

Page 7 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

operated at a pass energy of 50 eV. The instrument was typically operated with an analysis chamber pressure of approximately 1 × 10–9 Torr. N2 physisorption isotherms were measured at 77 K using a Quantachrome Autosorb-1-MP instrument. Brunauer–Emmett–Teller (BET) surface areas were calculated from the adsorption branches in the relative pressure range of 0.05-0.30. The isotherms were analyzed by the nonlocal density functional theory (NLDFT) method to evaluate the pore sizes of the samples using the kernel of NLDFT equilibrium capillary condensation isotherms of nitrogen at 77 K on silica (adsorption branch, assuming cylindrical pore geometry), and total pore volumes were evaluated at a relative pressure of 0.95. 2.3 SMX adsorption SMX loading onto the Ni@CNFs was achieved by mixing varying quantities of SMX with 25 mg of Ni@CNFs in 40 mL of PBS-buffered aqueous solutions at various pH values at room temperature and then stirring for 6 h. UV-Vis spectroscopy was used to confirm the concentration of free SMX in the aqueous phase over the course of the adsorption. UV-Vis spectra were acquired with a Shimadzu UV-1800 spectrophotometer. 2.4 In situ X-ray absorption spectroscopy (XAS) XAS spectra were recorded at the BL17C1 beam line at the National Synchrotron Radiation Research Center (NSRRC), Taiwan; the electron storage ring was operated at 1.5 GeV. All the XAS powder studies of the NPs were conducted in a custom stainless steel cell. Two holes were present in the cell, one on the top of the cell and the other on one side. After the solid samples were placed inside the cell, the holes were closed with Kapton film to avoid sample exposure to the ambient atmosphere. All spectra were recorded at room temperature in the transmission mode. The higher harmonics were eliminated via detuning of the double Si(111) crystal monochromator.


7

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 36

Three gas-filled ionization chambers were used in series to measure the intensities of the incident beam (I0), the beam transmitted by the sample (It) and the beam subsequently transmitted by the reference foil (Ir). The third ion chamber was used in conjunction with a Ni foil reference sample for Ni K-edge measurements. The parameters for the extended X-ray absorption fine structure (EXAFS) measurements, data collection modes and error calculations were all controlled according to the guidelines established by the International XAFS Society Standards and Criteria Committee. The EXAFS data were reduced using standard procedures. We obtained the EXAFS function χ by subtracting the post-edge background from the overall absorption and normalizing with respect to the edge jump step. The normalized χ(E) was transformed from energy space to k-space, where k is the photoelectron wave vector. The χ(k) data were multiplied by k3 for the Ni K-edge experiments to compensate for the dampening of the EXAFS oscillations in the high k region. Subsequently, the χ(k) data in k-space, which ranged from 2.8 to 14.6 Å–1 for the Ni Kedge, were Fourier transformed (FT) to r-space to separate the EXAFS contributions of the different coordination shells. All computer programs were implemented using the UWXAFS 3.0 software package, and the backscattering amplitude and the phase shift for specific atom pairs were theoretically calculated using the FEFF7 code. Structural parameters such as the coordination number (N) and the bond distance (R) were successfully calculated from these analyses. 2.5 Monte Carlo simulation Possible adsorption configurations for SMX were identified by conducting canonical Monte Carlo searches of the configurational space of the substrate-adsorbate systems as the temperature was slowly decreased according to a simulated annealing schedule. Each step of the Monte Carlo sampling attempts to randomly perturb/change the current state/conformation of the system into


8

Page 9 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

a neighboring state followed by a decision (depending on the potential energy and thermalkinetic distribution) whether to accept the selected neighboring state or to use the current state as the starting point for the next step. In practice, a graphite (001) surface with a=b=12.3 Å and c=30.2 Å (corresponding to a 5 × 5 × 2 repeating unit cell with a 20-Å vacuum space along the caxis) was generated as the substrate system. Initially, the selected adsorbate conformation was placed onto the substrate with structural constraints applied, and then the constrained adsorbate was perturbed randomly by translation and rotation relative to the substrate to determine the minimized configuration. The thermally stable conformations of the SMX+, SMX and SMX- adsorbates in the vacuum state were determined by combining the grid scan of specified torsions with the geometry optimization processes. A series of conformations was generated by applying specified torsions to the initial conformations of the three adsorbates. Each specified torsion angle, indicated as τ1 – τ3 in Figure 1, varied with 45° spatial increments from 0° to 359°, resulting in 512 conformations generated per adsorbate. By applying geometry optimization to each conformation and comparing the resulting potential energy, we obtained the most stable conformation for each adsorbate. All the conformation searches, geometry optimization and Monte Carlo simulations were performed with the Dreiding Forcefield30, which includes a good description of π-π interactions and is suitable for organic systems. The partial charge of systems was calculated by the QEq method.31 The graphite surface was generated via the Visualizer interface of the Material Studio commercial package. The rigid adsorption energy denoted by E1 can be obtained via the minimized total system potential energy of a pure substrate under vacuum. After obtaining the rigid adsorption configuration, the adsorbate conformation was randomly perturbed 250,000 times to generate an


9

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 36

adequate sampling space, and the geometry optimization was applied to each sampling to determine the stable structures. By comparing all of the minimized samplings/conformations, the structure with the lowest adsorption potential was selected, and the energy change between the original and final structure was set as the deformation energy E2.

Fig. 1 The pathway of an SMX molecule adsorbed on a graphite surface. 3. Results and Discussion 3.1 Characterization of Ni NPs before and after CNFs formation XRD spectroscopy was used to characterize Ni particles from Ni-Na/Al2O3 with and without carbon deposition, as shown in Figure S1. The metallic Ni NPs showed peaks at 2θ=44.3° and 51.7°, corresponding to the (111) and (200) facets, respectively. The average particle size of the metallic Ni NPs was calculated from the full-width at half maximum (FWHM) values of the Ni(111) peak using the Scherrer equation. The average size of Ni NPs from Ni-Na/Al2O3 before carbon deposition was determined to be ~ 42 nm; however, the particle size of Ni was markedly reduced, to ~ 34, 28 and 24 nm for 3 h, 9 h and 15 h of deposition time, respectively. The typical


10

Page 11 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

XRD pattern for the (002) graphitic basal plane was approximately 2θ=26.1° across multiple carbon deposition periods, and the intensity increased with longer reaction times. All of the diffraction angles of the (002) graphite planes were slightly lower than is typically observed for graphite (2θ=26.5°). Figure S2 shows the Ni K-edge X-ray absorption near-edge structure (XANES) spectra of the Ni-Na/Al2O3 when undergoing various treatments. The spectrum of the calcined Ni-Na/Al2O3 sample corresponded to the typical Ni oxide phase. As expected, the reducing treatment in H2 at 500oC led to the formation of reduced metallic Ni, as evidenced by the decreased intensity of the white line and low pre-edge energy.32-34 The intensity of the white line was slightly enhanced with the formation of CNFs in the short 3-h period of CNFs deposition (Ni@CNFs-3h). It was suggested that the reduced Ni NPs might be partially oxidized by the mild oxidant (CO2) during the course of the hydrogenation reaction. Notably, the extent of oxidation on Ni NPs gradually decreased with the formation of Ni@CNFs-9h and Ni@CNFs-15h samples, accompanying the decrease in the white-line intensity. These results may be associated with redox behavior occurring on the Ni surface, with the H2 and CNFs potentially functioning as reductants.35 The local structures of the Ni atoms in the samples shown in Figure S3 were investigated via K-edge extended X-ray absorption fine structure (EXAFS). The Fourier transforms of the k3weighted EXAFS results at the Ni K-edge with phase correlation are shown in Figure S3. The k3x(k) spectra were obtained via a comparison of the FEFF theoretical fit with the backtransformed experimental EXAFS data. The structural parameters extracted from the best-fit EXAFS data are listed in Table S1. The calcined Ni-Na/Al2O3 was assumed to generate typical Ni oxides that led to Ni–O bonding with a coordination number NNi-O of 5.7 and a Ni–O distance of 2.09 Å. The Ni-Ni bonding data could be determined from the EXAFS spectra, to obtain a


11

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 36

coordination number NNi-Ni of 10.2 and a bond distance of 2.95 Å. The Ni oxides became more metallic, with a coordination number NNi-Ni of 9.2 and an Ni-Ni bond distance of 2.48 Å, as the reduction of the reduced Ni-Na/Al2O3 progressed at 500oC. The coordination number NNi-Ni markedly decreased with increased carbon deposition time, implying that the formation of CNFs induced a decreased Ni particle size. However, the Ni-O bonding data could not be effectively analyzed from the EXAFS spectra for all of the Ni@CNFs samples, even if the slightly oxidized Ni atoms of Ni@CNFs were observed in the XANES spectra. These results indicate that the observed change in the oxidized effect was likely due to surface adsorption of atomic oxygen on Ni rather than significant structural change by the insertion of oxygen atoms into the Ni structure. Interestingly, the CNFs species were rapidly generated, but no comparable Ni-C bonding could be obtained from the EXAFS analysis. This result suggests that the CNFs did not directly form on the Ni surface but that their formation depended on the addition of Na species. Figure 2 shows the magnetization curves and saturation magnetization values of Ni-Na/Al2O3 with and without CNFs synthesis, as a function of the magnetic field at 25°C on a SQUID magnetometer. Ni-Na/Al2O3 exhibited saturation of magnetization (Ms) at 4.97 emu/g. The Ms value decreased to ca. 2.41 and 2.26 emu/g as the synthesis time for the CNFs increased to 3 h (Ni@CNFs-3h) and 9 h (Ni@CNFs-9h), respectively, and the Ms of Ni@CNFs-15h decreased to 1.09 emu/g.


12

Page 13 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Fig. 2 (A) Magnetic hysteresis loops of reduced Ni-Na/Al2O3, Ni@CNFs-3h, Ni@CNFs-9h and Ni@CNFs-15h samples; (B) saturation of magnetization versus reaction time. 3.2 Characterization of CNFs Figure S4 shows the rate of CNFs formation on the Ni-Na/Al2O3 catalysts at 500oC over a long period of time. It can be seen that the amount of CNFs formed rapidly increased with increasing time on the Ni-Na/Al2O3 catalyst before 15 h and then almost remained constant for longer reaction time. Figure 3 shows transmission electron microscopy (TEM) images of the NiNa/Al2O3 before and after carbon deposition. The original Ni NPs show an irregular shape before the reaction (image a). Image b shows the large quantity of CNFs formed from H2/CO2 fed over the Ni-Na/Al2O3 catalyst for 15 h at 500°C. Isolated CNFs are shown in images c and d, demonstrating that the Ni NPs remained at the tip of the carbon structure, deeply encapsulated in carbon film and appearing to have a rectangle-like shape. The parallel graphene layers that were stacked around the Ni encapsulated in carbon could be observed after CNFs growth. On the other hand, the energy-dispersive X-ray spectroscopy (EDS) was further used to analyze the chemical composition of the magnetic NPs covered by carbon film, as shown in Figure S5. The EDS spectrum reveals obvious C and O signals, but the intensity of peaks for Ni, Na and Al elements


13

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 36

were too weak to observe. The EDS spectrum gave a strong evidence that Ni-Na/Al2O3 NPs should be completely covered by CNFs. (a)

(b)

(c)

(d)

Fig. 3 High-resolution TEM images of (a) reduced Ni-Na/Al2O3 and (b)-(d) CNFs growth on the Ni-Na/Al2O3 catalyst for 15 h.


14

Page 15 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Figure S6 compares the C1s XPS spectra of the time-dependent CNFs formed on the NiNa/Al2O3 catalysts to identify the carbon functional groups. The XPS peaks of these carbon samples could be fitted into five overlapping peaks at 284.4, 284.7, 285.9, 287.9 and 289.2 eV. The assignments and relative ratios for all of the fitting peaks are listed in Table 1.36 We observed that the characteristic graphitic (sp2 C=C) and carboxylic groups (-COOH) increased, and the relative ratio of sp3 C-C and –C=C-O (keto-enol equilibria structure) was inhibited as the carbon deposition time increased. Oxygen-containing functional groups could be formed directly on the CNFs surface, increasing the likelihood of hydrophilicity. Conversely, increased carbon deposition time could also lead to the high graphitization of CNFs. The ratio of peak intensity for sp2 C=C to carboxylic groups (-COOH) ( I(C=C)/I(-COOH)) was also listed in Table 1, showing that the Ni@CNFs-15h sample possessed higher hydrophobic property. However, the presence of abundant sp2 C=C and –COOH groups on the carbon surface indicated that π-π stacking and electrostatic interactions could support adsorption of the SMX molecule. XPS spectroscopy was also used to directly probe Ni and Na species on the Ni-Na surface before and after carbon deposition, as shown in Figure S7. The Ni 2p and Na 1s signals for Ni@CNFs-9h and Ni@CNFs-15h were too weak to be discriminated from the background, suggesting that surface Ni and Na species were not present on the Ni@CNFs synthesized over periods of carbon deposition of 9 h or longer.


15

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 36

Table 1 Relative intensities of C1s XPS spectra for all Ni@CNF samples relative intensity ratio sample

sp2 C=C

sp3 C-C

-C=C-O

-COOH

(284.4 eV) (284.7 eV) (285.9 eV) (287.9 eV)

π-π* satellite

I(C=C)/I(COOH)

(289.1 eV)

Ni@CNF-3h

0.12

0.57

0.22

0.04

0.06

3

Ni@CNF-9h

0.18

0.51

0.12

0.06

0.09

3

Ni@CNF-15h

0.41

0.31

0.14

0.08

0.15

5.1

The N2 adsorption-desorption isotherms of the Ni@CNFs materials synthesized with different reaction times are shown in Figure S8. These isotherms can be classified as type II isotherms, because inflection point occurs near the completion of the first adsorbed monolayer.37 All Ni@CNFs samples are typical of nonporous materials with diameters exceeding micropores. The specific surface areas were calculated using the Brunauer–Emmett–Teller (BET) method in the relative pressure range of p/p0=0.05-0.3. The surface areas of the CNFs samples increased with reaction time, from 56 to 113 m2/g. Figure 4 shows the titration curves of the CNFs samples obtained by using a 0.1 M HCl(aq) solution. An 0.1-g sample of CNFs initially dispersed in 10 mL of pure water (pH 7.0) could generate a weakly basic solution with pH values of 7.2, 7.3 and 7.7 for Ni@CNFs-3h, Ni@CNFs-9h and Ni@CNFs-15h, respectively. The Kb values were calculated from these titration curves to be 5.8×10-8, 7.9×10-8 and 1.7×10-7 for Ni@CNFs-3h, Ni@CNFs-9h and Ni@CNFs-15h, respectively. From the XPS analysis, we determined that the abundance of carboxylic groups on CNFs increased with the duration of carbon deposition; thus,


16

Page 17 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

the formation of a weak base likely depended on the R-COO－ group on CNFs, following the reaction R-COO－+H2O→R-COOH+OH－.

Fig. 4 Titration curves of Ni@CNFs-3h, Ni@CNFs-9h and Ni@CNFs-15h. The Ni@CNFs samples (0.1 g) were initially dispersed in 10 mL pure water (pH 7.0) followed by titration using a 0.1 M HCl(aq) solution. 3.2 Adsorption of SMX antibiotic on CNFs Figure 5 shows the amount of SMX antibiotics at saturated adsorption on Ni@CNFs-3h, Ni@CNFs-9h and Ni@CNFs-15h samples by comparing unit weight- and surface area-based adsorption data at pH 7.0. However, no detectable SMX adsorption could be found on the NiNa/Al2O3 without CNFs deposition. Thus, it was suggested that the SMX adsorbed on all Ni@CNFs samples should completely result from the contribution of CNFs surface. We observed that increased deposition time could enhance the magnitude of SMX antibiotic adsorption. The Ni@CNFs-15h with saturated SMX adsorption at pH 7.0 could be dispersed in water by vigorous shaking or sonication, resulting in a dark-colored suspension. Fast aggregation


17

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 36

of the Ni@CNFs-15h with SMX from a homogeneous dispersion was observed in the presence of a magnet within 1 min (inset images in Figure 5). These results demonstrate that Ni@CNFs15h possessed a good magnetic response and strong hydrophilicity, even if the material was saturated SMX adsorption has been achieved. The XPS spectra showed that the relative intensity of the sp2 C=C and carboxylic group (-COOH) signals increased with surface area, indicating that specific sites associated with π-π interaction and electrostatic attraction might play essential roles in the adsorption process. The related results reported in literature for adsorption of SMX antibiotic on the carboxylized carbon nanotubes has been also discussed in term of electrostatic and π-π interactions.14,15,17 The magnetization curves for all CNFs samples with and without SMX-saturated adsorption are compared in Figure S9. Having SMX molecules covering the CNFs surface did not appear to influence the magnetization of the Ni particles. The adsorption data for SMX on the Ni@CNFs was further compared with the adsorption affinity among different several carbon materials reported in literatures as listed in Table S2. It can see that the Ni@CNFs materials provided good efficiency for SMX adsorption, comparing the other carbon materials.


18

Page 19 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Fig. 5 Comparison of the unit weight- and surface area-based adsorption for the SMX antibiotic with different deposition times. The Ni@CNFs-15h sample was used to further investigate the effect of pH changes on SMX adsorption, as it yielded the optimal adsorption result. Figure 6 is the amount of SMX adsorbed on Ni@CNFs-15h at saturation as a function of pH, showing a volcano-type dependence of pH value on SMX adsorption. The greatest adsorption occurred at pH 7.0. However, a basic environment at pH 10.0 strongly and negatively affected SMX adsorption.


19

Langmuir

50

40

SMX loading (mg/g CNFs)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 36

30

20

10

0 0

2

4

6

8

10

pH

Fig. 6 Saturated SMX adsorption on Ni@CNFs-15h at different pH values. Adsorption isotherms of the SMX antibiotic on Ni@CNFs-15h at different pH values are shown in Figure 7A. Different adsorption patterns were obtained in different pH conditions. At pH 7.0 and 5.6, the isotherm fitted well by using the Freundlich sorption model,

, where Qeq

and Ce are the adsorbed concentration and the aqueous concentration at the adsorption equilibrium, respectively. KF is the Freundlich affinity coefficient, and n is the Freundlich linearity index. The linear form for the Freundlich equation can be expressed as ln(Qeq)=ln(KF)+(n)ln(Ce). The adsorption isotherms at pH 7.0 and 5.6 shown in Figure 7A were further treated with the linear form, as shown in Figure 7B. The model-fitting parameters are listed in Table S3. In this case, the result suggested that SMX adsorption occurred on the heterogeneous surface at pH higher than 5.6. The very small n value of less than 1 exhibited much higher nonlinearity for SMX adsorption, implying that the Ni@CNFs-15h displayed a heterogeneous distribution of multiple binding sites for SMX.15 Notably, weakly acidic environments (pH 2.0 and 4.0) exhibited very different SMX adsorption patterns and caused


20

Page 21 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

lower adsorption than at pH 7.0, as shown in Figure 7A. In these cases, the Langmuir model provided a better fit for the experimental data (Figure 7C). The Langmuir isotherm follows the model

, where Ceq (mg/L) is the equilibrium concentration of SMX in the

solution, Qeq (mg/g) denotes the amount of SMX adsorbed at equilibrium, Qmax (mg/g) is the maximum adsorption capacity of the adsorbent for SMX and KL (L/mg) is the Langmuir constant. The plots of Ceq/Qeq versus Ceq in Figure 7C indicate good linear correlations for adsorption at pH 2.0 and 4.0. The Langmuir model suggests that SMX can be loaded on homogeneous single binding sites on Ni@CNFs-15h, with weak interactions between SMX molecules. All of the fitting parameters are indicated in Table S4. For a viewpoint of adsorption efficiency, a comparison of apparent adsorption kinetic on the different Ni@CNFs materials should be essential for practical operation. Figure S10 reveals the time-dependent adsorption of SMX on Ni@CNFs samples at pH 7.0. It can see that all Ni@CNFs samples could lead to the rapid increase in the adsorption amount with increasing time and almost approach to equilibrium adsorption within 1 min. Sample of Ni@CNFs-15h with saturated SMX adsorption at pH 7.0 was used to probe the SMX release at different pH values, as shown in Figure S11. The release environment at pH 7.0 could give better efficiency than that at pH 2.0 and 10.0. However, merely 11% of the SMX could be released from Ni@CNFs-15h at pH 7.0 within 8 h. These results provided the low likelihood of secondary pollution in water that SMX escaped from the absorbent because the SMX strongly bonded to the surface of Ni@CNFs-15h.


21

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 36

Fig. 7 (A) Adsorption isotherms for SMX adsorption on Ni@CNFs-15h at pH 7.0, 5.6, 4.0 and 2.0; (B) the linear dependence of Ln(Qeq) on Ln(Ceq) based on the Freundlich sorption model and (C) the linear dependence of Ceq/Qeq on Ceq based on the Langmuir isotherm model. 3.3 Adsorption mechanism of SMX antibiotic The large differences among the pH-dependent adsorption isotherms might depend on changes in the chemical structure of SMX at different pH values. The SMX antibiotic is an amphoteric molecule that has multiple charged/polar groups. Over a wide pH range, the SMX antibiotic is predominated by anionic (SMX-), zwitterionic (SMX) or cationic (SMX+) forms that exhibit two acid dissociation constants, the first involving protonation of the aniline N (pKa=1.7) and the other entailing deprotonation of the sulfonamide NH (pKa=5.6), as shown in Scheme 1. At a pH of 2.0, the ratio of the concentration of the protonated form (SMX+) to that of the neutral form (SMX) could be calculated as ~2.0, based on calculations using the Henderson-Hasselbalch equation (pH=pKa+log([SMX]/[SMX+])). When the pH increased to the range of pH 4.0-7.0, the dominant species became the neutral (SMX) or deprotonated (SMX-) forms; the ratios of [SMX]/[SMX] were ~0.025 for pH 4.0, 1 for pH 5.6 and 25.1 for pH 7.0, respectively. The adsorption of SMX might be greatly affected by pH because the adsorption of different SMX species is


22

Page 23 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

governed by different mechanisms at different pH values. It was reported that various mechanisms could simultaneously dominate with respect to electrostatic and π-π interactions. However, the π-π interaction between different SMX conformations and the CNFs surface might be the essential factor influencing pH-dependent SMX adsorption. Thus, a Monte Carlo simulation was used to comprehensively investigate the energy between the stereo structures of SMX-, SMX and SMX+ and the graphite surface during the adsorption process. The adsorption energy was considered to include two stages: (1) the rigid adsorption that the thermally stabilized form can directly adopt on the surface and (2) the conformational change from the rigid form to a planar-like form, as shown in Figure 1. The presence of planarlike adsorption induced by the graphite surface interaction could lead to a greater likelihood of the π rings being in chemical structures parallel to the graphite surface. The adsorption energies of rigid adsorption (E1) and planar-like adsorption (E2) calculated by the Monte Carlo simulation are listed in Table 2. The molecular structures in the rigid and planar-like forms for the three SMX species are shown in Figure 8. Table 2 Adsorption energy of SMX species on graphite surface

E1 (rigid form)

E2 (planar-like form)

E2-E1

(kcal/mol)

(kcal/mol)

(kcal/mol)

SMX+

-40.47

-38.19

2.28

SMX

-35.80

-33.84

1.97

SMX-

-47.38

-46.87

0.51


23

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 36

Fig. 8 The change of molecular structures between rigid and planar-like forms for the three SMX species. The rigid adsorption mainly represents the interaction between the average charge of the molecule and the pure graphite surface. The rigid adsorption conversion to a planar-like adsorption appeared to be an endothermic process (E2-E1>0), as shown in Table 2. It can be seen that the adsorption energy of the adsorbates for E1 and E2 increased in the following order: SMX

Adsorption Behavior and Mechanism of Antibiotic Sulfamethoxazole

Recommend Documents