Adsorption of sulfamethazine by multi-walled carbon nanotubes: effects of aqueous solution chemistry

Abstract

The adsorption of sulfamethazine (SMZ) by pristine and hydroxylated multi-walled carbon nanotubes (P-MWCNTs, H-MWCNTs) was studied under varied pH, ionic strength, cations and anions in solution. The results suggest that the SMZ adsorption onto MWCNTs can be depicted well by the pseudo-second-order and Langmuir models. The adsorption of SMZ onto MWCNTs was ionic strength and pH dependent, which indicated hydrophobic and electrostatic interactions, may be the main adsorption mechanisms. The presence of cations at 0.5 mM showed different effects on SMZ adsorption. Cu²⁺ slightly decreased SMZ adsorption by 10% to 20% at solution pH of 2.3 and 4.9, due to the competing effect of Cu²⁺ with SMZ⁺ and SMZ^±. But Cu²⁺ increased SMZ⁻ adsorption by 20% to 60% at solution pH of 7.4 and 10.0, due to the facilitating effect of the complex formation of Cu²⁺–SMZ. Al³⁺ promoted the SMZ adsorption onto P-MWCNTs, which can be attributed to the facilitating effect of Al³⁺ through a metal bridge, while inhibiting the SMZ adsorption by H-MWCNTs, which can be ascribed to the competing effect between Al³⁺ and SMZ for the negatively charged functional groups and the shielding effect of adsorbed Al³⁺ with a larger hydration radius at pH of 2.3, 4.9 and 7.4. SMZ adsorption was slightly decreased with the addition of 0.5 mM anions (Cl⁻, CO₃²⁻, SO₄²⁻, PO₄³⁻) due to the increase in density of negative charge on the MWCNTs' surface. The μ-FTIR results showed that π–π interaction may also play an important role in the adsorption process.

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1. Introduction

Antibiotics are widely applied all over the world. The fate, distribution, biodegradation and removal of antibiotic residues in the environment have drawn much research interest.¹ Sulfamethazine (SMZ), a major sulfonamide drug,² is extensively used as a veterinary medicine to control infectious diseases and promote the growth of farm animals. However, most of the SMZ fed to animals cannot be metabolized, up to 90% of SMZ is excreted into the environment via feces and urine.^3–5 SMZ has been frequently detected in surface water,⁶ groundwater,⁷ drinking water,⁸ soil⁹ and sediment¹⁰ in agricultural areas, with concentrations ranging from ng L⁻¹ to ug L⁻¹. Residues of SMZ may impose toxic effects on the soil ecosystem. It has been suggested that SMZ in the soil (53.6 mg kg⁻¹) has a dramatic short term detrimental effect on readily culturable bacteria, potential metabolic activity, and selected enzyme activities. Moreover, a shift of the microbial populations toward a lower bacterial/fungal ratio was observed after SMZ treatment.¹¹ In addition, it has been reported that 13 mg kg⁻¹ of SMZ had significant toxic effects on soil respiration and plant growth.¹² Furthermore, SMZ may be accumulated throughout the food chain contribute to acute and/or chronic disease. In sum continued research into the environmental behavior of SMZ is both important and necessary.

Carbon nanotubes (CNTs), first discovered in 1991,¹³ have drawn great attention due to their layered and hollow structures, their rich nano porous and high specific surface area,¹⁴ and their unique electrical and mechanical properties.^15,16 These unique structures and properties allow CNTs to have strong interaction with contaminants via non-covalent forces, such as van der Waals forces, electrostatic forces, hydrogen bonding, hydrophobic interactions and π–π interactions.^17–19 Hence, CNTs exhibit excellent adsorption capacity to contaminants, particularly to those containing benzene rings.^20,21

Antibiotics are a type of new emergent contaminant, whose adsorption behavior and mechanism in relation to CNTs have been recently studied.^22,23 Ji et al. (2009) compared the adsorption of tetracycline onto CNTs, activated carbon (AC) and graphite. The results showed the order of adsorption capacities of tetracycline based on unit mass was SWCNT > MWCNT > AC > graphite, while the adsorption affinity of tetracycline decreased in the order of graphite/SWNT > MWNT ≫ AC after the normalization for adsorbent surface area.²² Other research suggested sulfamethoxazole adsorption onto CNTs was controlled by hydrophobic interaction and the π–π donor–acceptor system.²³

Antibiotics are usually used together with metal salts as a growth promoter on livestock farms and both may continue to interact in the environment.²⁴ Previous studies showed cations Cs⁺ and Ca²⁺ may decrease sulfamethoxazole adsorption by CNTs, and the decrease range depended on the concentration of cations in the solution.²⁵ The suppress effect was also closely related with the solution pH. The addition of Cu²⁺ and Al³⁺ inhibited the adsorption of ionizable sulfathiazole and tylosin contaminants on peat and soil at low solution pH, while Cu²⁺ promoted the adsorption at high solution pH.²⁶ The effect of the suppress mechanism was ascribed to the large hydration shell of metal cations which shielded the hydrophobic sites occupied by organic chemicals.^27–29 The presence of anions, such as phosphate, enhanced the adsorption of sulfamethoxazole (SMX) by CNTs in a low pH condition; this increase was ascribed to the formation of ion pairs between phosphate and SMX⁺. Increasing the phosphate decreased the electrostatic repulsion between SMX⁺ and the positive CNTs' surface.²⁵ However, there is still a fundamental issue to be understood: what are the underlying solution chemistry characteristics that impact the adsorption of antibiotics by CNTs in aquatic environments?

The current study selected SMZ as the model antibiotic, and the pristine and functionalized multi-walled carbon nanotubes (P-MWCNTs, H-MWCNTs), as the adsorbents. The effects and mechanisms of solution chemistry, including pH, ionic strength, metal cations, and anions on the adsorption of SMZ by the MWCNTs were explored in detail.

2. Materials and methods

2.1. Materials

P-MWCNTs and H-MWCNTs (purity > 95%) with outer diameter of 10–20 nm, surface oxygen contents 0.85% and 7.07%, respectively, were purchased from Chengdu Organic Chemistry Co., Chinese Academy of Sciences. They were synthesized by the chemical vapor deposition method of methane in hydrogen mixture at 700 °C using Ni nanoparticles as the catalyst. The ζ-potential of MWCNTs at different pH was recorded by a Zeta potential analyzer (Nano-Z, Malvern Instruments, UK) after the suspension was rotated continuously for 24 h at 298 K. The detailed structural properties such as specific surface area and pore volume of the MWCNTs were determined and presented in Table S1,† which were firstly published in ref. 30.

Sulfamethazine (4-amino-N-[4,6-dimethyl-2-pyrimidinyl] benzenesulfonamide, in purity > 99%) was purchased from Sigma-Aldrich Trading Co., Ltd (Shanghai, China). The molecular structure and physicochemical properties of SMZ are listed in Table S2.† Metal cations (Al(NO₃)₃, Cd(NO₃)₂, Cu(NO₃)₂ and Pb(NO₃)₂) and anions (KCl, K₂CO₃, K₂SO₄ and K₃PO₄) were obtained from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Ultrapure grade water was used in all experiments. All other chemicals and solvents were of analytical reagent grade or better.

2.2. Batch adsorption

All batch adsorption experiments were performed in 40 mL glass centrifuge vials sealed with Teflon-lined screw-caps. MWCNTs were weighed in the amounts of 8 mg P-MWCNTs or 12 mg H-MWCNTs and 25 mL background solution (0.02 M NaNO₃ and 200 mg L⁻¹ NaN₃) containing different concentration of SMZ were filled into vials. The reaction solution was suspended in dark in a constant temperature shaker (HZQ-F160, Huamei Biochemistry Instrument, Soochow, China) with a revolving speed of 150 rpm at 298 K. The solution pH was adjusted by 0.1 M NaOH or 0.1 M HNO₃. The ionic strength was adjusted by NaNO₃. Adsorption kinetics and isotherm experiments were run in duplicate and the experiments of effects of aqueous solution chemistry were run in triplicate.

2.2.1. Adsorption kinetics. Adsorption kinetics of SMZ by MWCNTs was performed at an initial concentration of 20 mg L⁻¹ at pH 5.0 ± 0.1. 36 independent assays vials were conducted and two vials of them were sampled at predetermined time intervals, and a certain amount of the supernatant was filtered through a hydrophilic membrane filter of 0.45 μm after centrifugation at 1000g for 10 min. The sampling time was 10, 20, 30, 40, 60, 90 min and 2, 3, 4, 5, 6, 8, 10, 12, 24, 36, 48, 72 h (the centrifugation time (10 min) was included in sampling time).

2.2.2. Adsorption isotherms. The initial concentration of SMZ was 5, 10, 20, 30, 40, 60, 80 and 100 mg L⁻¹. Solution pH was adjusted to 5.0 ± 0.1. After reaction of 24 h, the MWCNTs and the supernatant were separated as described above.

2.2.3. The effects of aqueous solution chemistry. The initial solution pH ranging from 3.00 to 11.00 was adjusted to explore the effect of pH on SMZ adsorption. During the reaction, the pH was readjusted by adding 0.1 M NaOH or 0.1 M HNO₃ at intervals of 2, 12 and 22 h. The effect of ionic strength, varying from 0.02 to 0.2 M NaNO₃, was investigated at pH 5.0 ± 0.1. The effect of cations and anions on the adsorption of SMZ onto MWCNTs were investigated at concentration of 0.5 mM metal cations (Na⁺ (as a control test), Cu²⁺, Al³⁺, Cd²⁺ and Pb²⁺) or anions (NO₃⁻ (as a control test), Cl⁻, SO₄²⁻, PO₄³⁻ and CO₃²⁻) in solution containing SMZ at concentrations of 10, 20, 40 mg L⁻¹, at solution pH of 2.3, 4.9, 7.4 and 10.0. After the reaction of 24 h, the MWCNTs were separated as described above.

2.3. SMZ determination

In order to eliminate the interference of metal cations coexisted in solution on the quantitative determination of SMZ, the absorbance spectrum of SMZ was scanned with 1800PC ultraviolet-visible spectrophotometer (Shanghai Mapada Instrument CO., Ltd). The result showed that the presence of metal cations had only slight influence on the maximum absorbance and intensity of SMZ at 263 nm (Fig. S1†).

SMZ in the supernatant was determined by High Performance Liquid Chromatography (HPLC, Waters Alliance) equipped with a Waters 478 UV detector at 263 nm and reversed-phase C₁₈ column (Waters, 5 μm, 3.9 mm × 150 mm). The mobile phase was methanol and water with volume ratio of 70 : 30. The injection volume was 30 μL, the flow rate was 1 mL min⁻¹, and the retention time was 1.5 min.

2.4. μ-FTIR measurements

The micro-Fourier transform infrared (μ-FTIR) spectroscopic spectrum of adsorbents and adsorbate were measured by a FTIR spectrophotometer (Nicolet 6700, Thermo Nicolet). The samples for μ-FTIR analysis were prepared with identical condition to that used in the adsorption experiments. The MWCNTs, MWCNTs-SMZ and MWCNTs-SMZ-Cu were filtered through a hydrophilic membrane filter of 0.45 μm, and washed with ultrapure water and freeze-dried. The samples were placed on a diamond bracket and μ-FTIR spectra were measured. The resolution for μ-FTIR was 8 cm ⁻¹, and a total 64 scans were collected for each spectrum. The data of spectrum were analyzed with OMNIC software.

2.5. Data analysis

To account for possible SMZ losses during experiments the recoveries of batch equilibrium experiments were evaluated with vials containing solution without MWCNTs since SMZ losses were smaller than 1%. Therefore, the amount of SMZ adsorbed by MWCNTs was calculated by the decrease of the SMZ concentration in solution using the following eqn (1) (ref. 31)

In the equation, q_e (mg g⁻¹) is the amount of SMZ adsorbed onto MWCNTs; c_o (mg L⁻¹) and c_e (mg L⁻¹) are the initial and final concentrations of SMZ, respectively; and V (L) is the solution volume and m (g) is the mass of MWCNTs.

Below, the pseudo-first-order model (eqn (2)), pseudo-second-order model (eqn (3)),³² and intraparticle diffusion model³³ (eqn (4)) were used to fit the adsorption kinetics of SMZ onto MWCNTs.

ln(q_e − q_t) = ln q_e − k₁t (2)

q_t = k₃t^0.5 + C (4)

In all of the above equations, q_e [mg g⁻¹] and q_t [mg g⁻¹] are the amount of SMZ adsorbed on MWCNTs at equilibrium and various times t (h). The sorption rate constants are k₁ [h⁻¹], k₂ [g mg⁻¹ h⁻¹] and k₃ [mg g⁻¹ h^−0.5], respectively. In eqn 4, C (mg g⁻¹) is the intercept of the vertical axis.

The Langmuir³⁴ (eqn (5)) and Freundlich³⁵ (eqn (6)) models were employed to fit the equilibrium sorption data of SMZ by MWCNTs.

q_e = k_Fc_e^1/n (6)

where q_e (mg g⁻¹) is the adsorption at equilibrium; q_m (mg g⁻¹) is the maximum adsorption capacity; c_e (mg g⁻¹) is the equilibrium concentration in solution; b (L mg⁻¹) is the affinity parameter; k_F (mg^(1−(1/n)) L^(1/n) g⁻¹) is the adsorption coefficient; and n is the adsorption constant as an indicator of isotherm nonlinearity.

All statistical analysis was performed using Data Processing System (DPS 7.05, Zhejiang University, Hangzhou, China) and plotted with Microcal Origin 7.5 (Originlab Corporation, Northampton, MA, USA). A one-way analysis of variance with least significant difference (LSD) test was conducted at a significance level of 0.05.

3. Results and discussion

3.1. Adsorption kinetics

Adsorption kinetics governs the solute uptake rate and represents the adsorption efficiency of the adsorbents in aquatic solution which varies in solution chemistry characteristics. The C_e/C₀ decreased significantly in the first 2 h of reaction (Fig. 1a), indicated that the SMZ adsorption onto MWCNTs increased significantly in the first 2 h, which can be ascribed to the large number of vacant sites on the surface of the MWCNTs. The adsorption rate reduced significantly after 2 h that followed likewise reinforces the idea that the adsorption became increasingly difficult because of the decrease of vacant surface sites.³⁶ Still, the adsorption did take place even if at a slower pace. MWCNTs usually exhibit obvious aggregation in aqueous solution and form interstitial and groove sites,³⁷ which are available for SMZ adsorption during this second slow adsorption stage.

Fig. 1 Adsorption kinetics and linear regressions of SMZ by P-MWCNTs and H-MWCNTs (Initial SMZ 20 mg L⁻¹; temperature: 298 K; initial pH: 5.0 ± 0.1): (a) 0–72 h; (b) pseudo-first-order model; (c) pseudo-second-order model; (d) intraparticle diffusion model.

Among the three models, the pseudo second-order model, with the highest correlation coefficients (R²), fits the kinetics data best (Fig. 1b–d). Additionally, the comparisons of q_e,measured (experimentally measured equilibrium capacity) with q_e,calculated (models calculated equilibrium capacity) suggests that the pseudo second-order model is also the best model to fit the adsorption kinetics (Table 1). Thus, the adsorption rate is assumed to be controlled by chemical adsorption.³⁸

Table 1 The pseudo-first-order model, pseudo-second-order model and intraparticle diffusion model constants of SMZ adsorption by MWCNTs

1.Pseudo-first-order model
	qe,measured (mg g^−1)	qe,calculated (mg g^−1)	K1 (h^−1)	R^2	SD
a The 1, 2 and 3 indicated the K3 or C of 3 distinct linear regions, respectively.
P-MWCNTs	24.38	10.97	0.6590	0.780	0.704
H-MWCNTs	13.32	5.02	0.5514	0.851	0.429

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2.Pseudo-second-order model
	qe,measured (mg g^−1)	qe,calculated (mg g^−1)	K2 (g mg^−1 h^−1)	R^2	SD
P-MWCNTs	24.38	24.78	0.194	0.997	0.005
H-MWCNTs	13.32	13.31	0.486	0.997	0.008

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3.Intraparticle diffusion model
	K3 (mg g^−1 h^−0.5)			C (mg g^−1)
	1^a	2^a	3^a	1^a	2^a	3^a
P-MWCNTs	20.93	5.54	1.26	3.49	13.33	24.50
H-MWCNTs	10.10	1.67	0.96	3.61	9.31	10.83

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The intraparticle diffusion model assumes the existence of 3 distinct linear regions, which corresponded to the 3 steps on adsorption process, the external surface or instantaneous adsorption, the intraparticle diffusion and the equilibrium plateau, respectively.^39–41 It is observed that the adsorption rate was descending in the order of the first step > the second step > the third step (Table 1).

The kinetics results show that the H-MWCNTs had slower and smaller adsorption of SMZ than that of P-MWCNTs (Fig. 1, Table 1), despite the former one presenting the larger specific surface area, higher micro and mesopore volumes (Table S1†). Hence the surface chemistry such as hydrophobicity and zeta potential of MWCNTs is most probably playing an important role in the adsorption process, which will be investigated in the following section.

3.2. Adsorption isotherms

The adsorption isotherm of SMZ onto MWCNTs was highly nonlinear, with 1/n values of 0.371 and 0.454 for P-MWCNTs and H-MWCNTs, respectively (Fig. 2, Table 2), which suggests the distribution of adsorption energy of MWCNTs are highly heterogeneous.⁴² The Langmuir model fitted the isothermal data better than the Freundlich model with high R² values of 0.995 and 0.998 for P-MWCNTs and H-MWCNTs, respectively (Table 2).

Fig. 2 Adsorption isotherms of SMZ by P-MWCNTs and H-MWCNTs (temperature: 298 K; Initial pH: 5.0 ± 0.1): The solid line (—) is Langmuir model fitting; the dotted line ([dash dash, graph caption]) is Freundlich model fitting.

Table 2 The Langmuir and Freundlich model fitting adsorption isotherm parameters for adsorption of SMZ by P-MWCNTs and H-MWCNTs

Carbons	Langmuir			Freundlich
	qm (mg g^−1)	b (L mg^−1)	R^2	kF (mg^(1−(1/n)) L^(1/n) g^−1)	n^−1	R^2
P-MWCNTs	38.13 ± 0.64	0.072 ± 0.004	0.995	6.73 ± 0.76	0.371 ± 0.030	0.947
H-MWCNTs	27.29 ± 0.38	0.042 ± 0.002	0.998	2.99 ± 0.36	0.454 ± 0.031	0.963

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The maximum adsorption capacities of P-MWCNTs and H-MWCNTs were approximately 38.13 and 27.29 mg g⁻¹ (Table 2), respectively. The H-MWCNTs had larger specific surface area and pore volume (Table S1†), but the adsorption capacity was lower than that of P-MWCNTs, which suggests the functional groups of H-MWCNTs exerted negative effect on the SMZ adsorption. These functional groups decreased the hydrophobicity of MWCNTs in aqueous solution.⁴³ Consequently, hydrophobic interaction between SMZ and H-MWCNTs is weaker than that between SMZ and P-MWCNTs, making the adsorption of SMZ onto H-MWCHTs less than onto P-MWCNTs. Furthermore, the water molecules around the MWCNTs may be adsorbed on the functional groups and form water dense shell, which may decrease the surface available sites of MWCNT for the SMZ adsorption, and block the access of the SMZ to the active adsorption sites,⁴⁴ thus decreasing the SMZ adsorption onto H-MWCHTs more than onto P-MWCNTs. In addition, ionized functional groups can adsorb Na⁺ from background solution,²⁰ which may increase the diffusion resistance and steric hindrance, thus preventing SMZ from approaching and further interacting with MWCNTs.^17,45

3.3. Effect of solution pH

The SMZ adsorption by MWCNTs increased slowly with increasing pH from 2 to 7, then, decreased abruptly as pH values further increased (Fig. 3a). The adsorption of SMZ onto MWCNTs at pH of 7 is about 4 times that of pH at 10.

Fig. 3 Effect of solution pH on adsorption (a) of SMZ by MWCNTs and the Zeta potential (b) of MWCNTs and.

Solution pH has significant effect on both the ionic adsorbate speciation and the surface charge of MWCNTs.⁴³ SMZ has two pK_a values of 2.28 and 7.42 (Table S2†), which can exist as positively charged species (SMZ⁺) at pH < pK_a1 (2.28), negatively charged species (SMZ⁻) at pH > pK_a2 (7.42), and zwitterionic species (SMZ^±) at pH ranging from 2.28 to 7.42 (Fig. S2†).⁴⁶ The surface of the MWCNTs was positively charged at pH < pH_PZC (4.7 for P-MWCNTs and 1.7 for H-MWCNTs, Fig. 3b) and negatively charged at pH > pH_PZC.^17,47 Thus, electrostatic interaction between the SMZ molecules and the MWCNTs surface is expected to dominate the adsorption process. When pH < pK_a2 (7.42), the adsorption was enhanced by electrostatic attraction between the opposite charges of the SMZ and the MWCNT surface. The adsorption was suppressed when solution pH > pK_a2 (7.42) due to the repulsion of the SMZ molecules and the MWCNT surface with the same charges. Furthermore, the hydrophobic partitioning of anionic SMZ at pH > 7.42 is significantly less than in the non-ionized form of SMZ at pH 2.28–7.42.⁴⁸ Therefore, the hydrophobic interactions between MWCNTs and SMZ would decrease when pH increased.⁴⁶ Also, the solubility of SMZ increases rapidly as pH increases,⁴⁹ which helps to account for the decrease in adsorption force between the SMZ and the MWCNTs.

3.4. Effect of ionic strength

As ionic strength increased from 0.02 to 0.20 M (NaNO₃) in solution, the adsorption capacity of SMZ by P-MWCNTs and H-MWCNTs reduced 68% and 87%, respectively (Fig. 4). The increase of ionic strength compresses the electric double layers surrounding MWCNTs which leads to the aggregation of MWCNTs.^43,50,51 MWCNTs become more compact and unfavorable for SMZ adsorption due to less available surface adsorption sites.^43,51 The increase of ionic strength also weakens the electrostatic attraction between SMZ⁺/SMZ^± and MWCNTs because of the decreasing density of negative charge on MWCNTs.⁵¹ Increased ionic strength also enhances competition interaction of the salt ions and the SMZ on surface adsorption sites,⁴³ thus reducing SMZ adsorption.

Fig. 4 Effect of solution ionic strength on adsorption of SMZ by P-MWCNTs and H-MWCNTs (initial SMZ 20 mg L⁻¹; temperature: 298 K; initial pH: 5.0 ± 0.1).

3.5. Effect of metal cations

The introduction of metal cations showed different effects on the SMZ adsorption by MWCNTs, which are dependent of solution pH, cations, and carbon nanotubes (Fig. 5). The presence of Cu²⁺, Cd²⁺ and Al³⁺ significantly decreased or increased the SMZ adsorption, while Pb²⁺ showed little influence on the SMZ adsorption by MWCNTs. At three SMZ concentrations, the SMZ adsorption as affected by metal cations showed insignificant difference, which can be ascribed to the lower molar concentration of SMZ (40 mg L⁻¹ = 0.144 mM) than that of metal cations (0.5 mM).

Fig. 5 Effect of metal cations on adsorption of SMZ by P-MWCNTs and H-MWCNTs (temperature: 298 K. Different letters (a > b > c > d) means significant differences at a significance level of 0.05.).

At solution pH of 2.3 and 4.9, Cu²⁺ slightly decreased the adsorption of SMZ onto P-MWCNTs and H-MWCNTs by 10–20%. This can be attributed to the competition of Cu²⁺ with SMZ⁺ and SMZ^± for negatively charged adsorption sites on the surface of the MWCNTs.²⁶ It was reported that the surface complex of Cu²⁺ at hydrophilic defect sites of MWCNTs²⁸ forms sizable hydration shells of dense water which reduce available sites for SMZ⁺/SMZ^± adsorption.⁵² The adsorption of Cu²⁺ on MWCNTs may weaken the electrostatic attraction and van der Waals between SMZ⁺/SMZ^± and MWCNTs, and may decline the adsorption of SMZ onto MWCNTs at low pH. At pH of 7.4 and 10.0, the dominant SMZ⁻ forms the complexes of Cu²⁺–SMZ⁻ and Cd²⁺–SMZ⁻ with a positive charge, which are more easily adsorbed by MWCNTs than SMZ⁻/SMZ^±.⁵³ In addition, Cu²⁺–SMZ or Cd²⁺–SMZ are adsorbed on MWCNTs through metal ion bridges.^26,54 Consequently, the presence of Cu²⁺ and Cd²⁺ increased the adsorption of SMZ onto MWCNTs by 20 to 60% at this pH range.

For H-MWCNTs, Al³⁺ inhibited the SMZ adsorption at four selected pH values, and reduced the adsorption by 40 to 80% at pH of 10.0. Al³⁺ can compete for the negative charged functional groups of MWCNTs with SMZ. Furthermore, Al³⁺ has a larger hydrated radius⁵⁵ which can form dense water shell and significantly block the available sites for SMZ adsorption once Al³⁺ was adsorbed by MWCNTs. Both of these competing and shielding effects can decrease the SMZ adsorption onto MWCNTs. On the other hand, Al³⁺ can also form the complexes with SMZ, which may facilitate the SMZ adsorption through metal ion bridges. The facilitating or suppressing effects of Al³⁺ on the SMZ adsorption by MWCNTs depend on which force is stronger. The adsorption of metal ions including Al³⁺ by MWCNTs were strongly positively correlated with surface oxygen content,⁵⁶ suggesting that the H-MWCNTs can adsorb more Al³⁺ than that of P-MWCNTs, indicating the suppressing effect was dominant on the effect of Al³⁺ on the SMZ adsorption by H-MWCNTs. The facilitating effect of Al³⁺ on the SMZ adsorption by P-MWCNTs at pH 2.3, 4.9 and 7.4 can be attributed to the metal bridge of Al³⁺ and SMZ. At pH 10, Al(OH)₄⁻, the predominant species of Al³⁺, can interact with SMZ⁻, which may increase the electrostatic repulsion between the Al(OH)₄⁻–SMZ⁻ and the negative charged surface of MWCNTs, thus decreasing the SMZ adsorption onto both MWCNTs.²⁶ The specific mechanism of Al³⁺ on the SMZ adsorption needs to be further investigated.

3.6. Effect of anions

The anions such as Cl⁻, SO₄²⁻, PO₄³⁻ and CO₃²⁻ in surface water are usually less than 1 mM.⁵⁷ We selected 0.5 mM anion in this experiment to simulate an anion concentration that occurs in real aquatic environments. The results suggest that the presence of anions showed different effects on the SMZ adsorption by MWCNTs. The addition of Cl⁻ and SO₄²⁻ had little effect on the SMZ adsorption, while the presence of PO₄³⁻ and CO₃²⁻ inhibit the SMZ adsorption by MWCNTs (Fig. 6).

Fig. 6 Effect of anions on adsorption of SMZ by P-MWCNTs and H-MWCNTs (temperature: 298 K. Different letters (a > b > c > d) means significant differences at a significance level of 0.05.).

When PO₄³⁻ was present in solution, the SMZ adsorption decreased slightly by 3.0–13.9% and 2.1–15.1% for P-MWCNTs and H-MWCNTs, respectively. At pH 10, the suppression of the SMZ adsorption reached the maximum by 13.9% and 15.1% for P-MWCNTs and H-MWCNTs, respectively. The suppression effect of PO₄³⁻ on the SMZ adsorption can be attributed to the electrostatic repulsion between anion and the negatively charged surface of the MWCNTs.⁵⁸ PO₄³⁻ adsorption may block the available adsorption sites for SMZ through increasing surface negative charge density of the MWCNTs, hence suppressing the adsorption of SMZ by MWCNTs.⁵⁹ Another study found the sulfamethoxazole (SMX) adsorption, a similar antibiotic in structure, was enhanced by PO₄³⁻ at pH < 7,²⁵ however, the current study did not confirm the same effect of PO₄³⁻ on the SMZ adsorption.

At pH 10, CO₃²⁻ significantly decreased the SMZ adsorption onto P-MWCNTs by 2% to 20% and onto H-MWCNTs by 40% to 55%. The one carbon atom and three oxygen atoms of CO₃²⁻ may form a π-bond. The resulting carbonate adsorbs onto the MWCNTs surface by π–π interaction, reducing the adsorption of SMZ. At pH 2.3, 4.9 and 7.4, CO₃²⁻ generates HCO₃⁻ and H₂CO₃, which weakens π–π interaction. The inhibition effect of CO₃²⁻ on the SMZ adsorption by P-MWCNTs is weaker than that by H-MWCNTs. This is due to H-MWCNTs' oxygen containing functional groups which increase the adsorption density of CO₃²⁻ through enhancing π–π interaction.

3.7. μ-FTIR analysis

The μ-FTIR was performed to explore the adsorption sites of SMZ onto MWCNTs (Fig. 7 and 8). The important peaks and corresponding vibrations were listed in Table 3. The peaks at 1706/1708 cm⁻¹ and 1092/1100 cm⁻¹ refer to the C=O and the C–O stretching vibrations, respectively,³⁰ and 1706/1708 cm⁻¹ and 1372 cm⁻¹ all correspond to the carboxylic group (–COOH).^27,60 The bands at 1199/1194 cm⁻¹ were assigned to the –C=O stretch and to the –OH bonding for the carboxylic groups (–COOH)⁶¹ or symmetric stretching vibration of –COO⁻.²⁸

Fig. 7 Micro-Fourier transform infrared spectroscopy spectrum of MWCNTs and SMZ.

Fig. 8 The μ-FTIR spectra of adsorbed SMZ and SMZ-Cu complexes on P-MWCNTs (a) and H-MWCNTs (b).

Table 3 The important peaks and corresponding vibrations of FTIR spectra of MWCNTs

Wavenumber (cm^−1)	Bond	Functional group	References
1700–1720	C=O	COOH	62
1596, 1565, 1438	C=C	Benzene ring	63 and 64
1507, 1508	N–H	NH2	63 and 64
1385	C–C	Benzene ring	65
1190–1200	–C=O, –OH	COOH	28 and 61
1090–1100	C–O	COOH	30

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When SMZ was adsorbed on the MWCNTs, some new peaks appeared at bands 1380–1580 cm⁻¹ (Fig. 8). The peak at 1438 cm⁻¹ (Fig. 8a) was a C=C stretching vibration of the benzene ring of the SMZ; the peak at 1507/1508 cm⁻¹ (Fig. 8a and b) was a scissoring vibration of the NH₂ groups of the SMZ, both indicate the SMZ adsorbed onto the surface of the MWCNTs. At pH of 2.3 and 4.9, the peak at 1717 cm⁻¹ (Fig. 8a) was assigned to C=O stretching vibrations of the carboxylic groups (–COOH). As solution pH increased to 7.4 and 10.0, the 1717 cm⁻¹ peak disappeared, due to the deprotonation of –COOH at high solution pH.⁶² The presence of Cu²⁺ in solution also made the peak at 1717 cm⁻¹ (Fig. 8a) declined, indicating that Cu²⁺ was coordinated with the carboxyl group of MWCNTs at low pH.⁵⁴ For H-MWCNTs, the benzene ring C=C stretching vibration of SMZ shifted from 1596 cm⁻¹ (Fig. 7)^63,64 to 1585 cm⁻¹ (Fig. 8b), 1571 cm⁻¹ (Fig. 8b), 1571 cm⁻¹ (Fig. 8b) and 1588 cm⁻¹ (Fig. 8b), respectively, and the benzene ring C–C skeletal vibrations⁶⁵ of SMZ shifted from 1385 cm⁻¹ (Fig. 7) to 1399 cm⁻¹ (Fig. 8b), 1375 cm⁻¹ (Fig. 8b), 1380 cm⁻¹ (Fig. 8b) and 1382 cm⁻¹ (Fig. 8b). All of this suggests that the benzene ring of SMZ was partly adsorbed onto the two types of MWCNT by π–π interaction.^66,67

Similarly, for P-MWCNTs, the benzene ring C=C stretching vibrations of SMZ shifted from 1565 cm⁻¹ (Fig. 7)^63,64 to 1567 cm⁻¹ (Fig. 8a), 1573 cm⁻¹ (Fig. 8a), 1558 cm⁻¹ (Fig. 8a) and 1562 cm⁻¹ (Fig. 8a). The benzene ring C–C skeletal vibrations of SMZ shifted from 1385 cm⁻¹ (Fig. 7) to 1382 cm⁻¹ (Fig. 8a), 1386 cm⁻¹ (Fig. 8a), 1381 cm⁻¹ (Fig. 8a) and 1371 cm⁻¹ (Fig. 8a). The peaks at 1199/1194 cm⁻¹ of P/H-MWCNTs (Fig. 7) also shifted to 1201/1189 cm⁻¹ (Fig. 8), 1199/1199 cm⁻¹ (Fig. 8), 1195/1194 cm⁻¹ (Fig. 8) and 1197/1203 cm⁻¹ (Fig. 8), which may be ascribed to the interaction of SMZ and P/H-MWCNTs.

4. Conclusion

The results of ionic strength and solution pH on the adsorption suggest that hydrophobic and electrostatic interactions were the major adsorption mechanisms for SMZ onto MWCNTs. The analysis of μ-FTIR spectra indicates that the π–π interaction also plays an important role in the adsorption process. The presences of metal cations enhance or inhibit the adsorption of SMZ by MWCNTs, which depended on the type of metal cations, carbon nanotubes and the solution pH value. The selected anions slightly suppressed the SMZ adsorption by MWCNTs at different pH. The results suggest the solution chemistry play an important role in SMZ adsorption by MWCNTs.

Acknowledgements

We sincerely thank Mr Mark Eugene Reynolds, for English editing of the manuscript. This research was funded by the National Natural Science Foundation of China (21207157), and Shandong Provincial Higher Educational Science and Technology Program (J12LC02). Dr Guangcai Chen gratefully acknowledges the support from the China Scholarship Council (201303270005).

References

1. S. Managaki, A. Murata, H. Takada, B. C. Tuyen and N. H. Chiem, Environ. Sci. Technol., 2007, 41, 8004.
2. A. K. Sarmah, M. T. Meyer and A. Boxall, Chemosphere, 2006, 65, 725.
3. F. Ingerslev and B. Halling-Sørensen, Environ. Toxicol. Chem., 2000, 19, 2467.
4. K.-R. Kim, G. Owens, S.-I. Kwon, K.-H. So, D.-B. Lee and Y. S. Ok, Water, Air, Soil Pollut., 2011, 214, 163.
5. L.-J. Zhou, G.-G. Ying, S. Liu, R.-Q. Zhang, H.-J. Lai, Z.-F. Chen and C.-G. Pan, Sci. Total Environ., 2013, 444, 183.
6. F. Tamtam, F. Mercier, B. Le Bot, J. Eurin, Q. Tuc Dinh, M. Clément and M. Chevreuil, Sci. Total Environ., 2008, 393, 84.
7. G. Hamscher, H. T. Pawelzick, H. Höper and H. Nau, Environ. Toxicol. Chem., 2005, 24, 861.
8. A. Watkinson, E. Murby, D. Kolpin and S. Costanzo, Sci. Total Environ., 2009, 407, 2711.
9. A. Ostermann, J. Siemens, G. Welp, Q. Xue, X. Lin, X. Liu and W. Amelung, Chemosphere, 2013, 91, 928.
10. X. Liang, B. Chen, X. Nie, Z. Shi, X. Huang and X. Li, Chemosphere, 2013, 92, 1410.
11. M. Vittoria Pinna, P. Castaldi, P. Deiana, A. Pusino and G. Garau, Ecotoxicol. Environ. Saf., 2012, 84, 234.
12. F. Liu, G.-G. Ying, R. Tao, J.-L. Zhao, J.-F. Yang and L.-F. Zhao, Environ. Pollut., 2009, 157, 1636.
13. S. Iijima, Nature, 1991, 354, 56.
14. K. Yang and B. Xing, Environ. Pollut., 2007, 145, 529.
15. T. Ebbesen, H. Lezec, H. Hiura, J. Bennett, H. Ghaemi and T. Thio, Nature, 1996, 382, 54.
16. M. Treacy, T. Ebbesen and J. Gibson, Nature, 1996, 381, 678.
17. B. Pan and B. Xing, Environ. Sci. Technol., 2008, 42, 9005.
18. V. K. Gupta and T. A. Saleh, Environ. Sci. Pollut. Res., 2013, 20, 2828.
19. J. G. Yu, X. H. Zhao, H. Yang, X. H. Chen, Q. Yang, L. Y. Yu, J. H. Jiang and X. Q. Chen, Sci. Total Environ., 2014, 482–483, 241.
20. X. Wang, Y. Liu, S. Tao and B. Xing, Carbon, 2010, 48, 3721.
21. O. G. Apul, Q. Wang, Y. Zhou and T. Karanfil, Water Res., 2013, 47, 1648.
22. L. Ji, W. Chen, L. Duan and D. Zhu, Environ. Sci. Technol., 2009, 43, 2322.
23. D. Wu, B. Pan, M. Wu, H. Peng, D. Zhang and B. Xing, Sci. Total Environ., 2012, 427–428, 247.
24. J. Chen, W. Chen and D. Zhu, Environ. Sci. Technol., 2008, 42, 7225.
25. D. Zhang, B. Pan, M. Wu, B. Wang, H. Zhang, H. Peng, D. Wu and P. Ning, Environ. Pollut., 2011, 159, 2616.
26. Z. Pei, S. Yang, L. Li, C. Li, S. Zhang, X. Q. Shan, B. Wen and B. Guo, Environ. Pollut., 2014, 184, 579.
27. G. C. Chen, X. Q. Shan, Y. S. Wang, Z. G. Pei, X. E. Shen, B. Wen and G. Owens, Environ. Sci. Technol., 2008, 42, 8297.
28. G. C. Chen, X. Q. Shan, Y. S. Wang, B. Wen, Z. G. Pei, Y. N. Xie, T. Liu and J. J. Pignatello, Water Res., 2009, 43, 2409.
29. G. C. Chen, X. Q. Shan, Z. G. Pei, H. Wang, L. R. Zheng, J. Zhang and Y. N. Xie, J. Hazard. Mater., 2011, 188, 156.
30. G. C. Chen, X. Q. Shan, Y. Q. Zhou, X. E. Shen, H. L. Huang and S. U. Khan, J. Hazard. Mater., 2009, 169, 912.
31. A. S. Bhatt, P. L. Sakaria, M. Vasudevan, R. R. Pawar, N. Sudheesh, H. C. Bajaj and H. M. Mody, RSC Adv., 2012, 2, 8663.
32. Y.-S. Ho, J. Hazard. Mater., 2006, 136, 681.
33. W. J. Weber and J. C. Morris, J. Sanit. Eng. Div., Am. Soc. Civ. Eng., 1963, 89, 31.
34. I. Langmuir, J. Am. Chem. Soc., 1918, 40, 1361.
35. H. M. F. Freundlich, J. Phys. Chem., 1906, 57, 385.
36. X. Guo, C. Yang, Z. Dang, Q. Zhang, Y. Li and Q. Meng, Chem. Eng. J., 2013, 223, 59–67.
37. F. Wang, J. Yao, H. Chen, Z. Yi and B. Xing, Environ. Pollut., 2013, 180, 1.
38. Y. S. Ho and G. McKay, Process Biochem., 1999, 34, 451.
39. F. C. Wu, R. L. Tseng and R. S. Juang, Chem. Eng. J., 2009, 153, 1.
40. C. Valderrama, X. Gamisans, F. X. de las Heras, J. L. Cortina and A. Farran, React. Funct. Polym., 2007, 67, 1515.
41. A. E. Ofomaja, Bioresour. Technol., 2010, 101, 5868.
42. H. Li, D. Zhang, X. Han and B. Xing, Chemosphere, 2014, 95, 150.
43. X. Ma and S. Uddin, Nanomaterials, 2013, 3, 289.
44. K. Yang and B. Xing, Chem. Rev., 2010, 110, 5989.
45. T. Wang, W. Liu, L. Xiong, N. Xu and J. Ni, Chem. Eng. J., 2013, 215–216, 366.
46. S. T. Kurwadkar, C. D. Adams, M. T. Meyer and D. W. Kolpin, J. Agric. Food Chem., 2007, 55, 1370.
47. Z. Wang, X. Yu, B. Pan and B. Xing, Environ. Sci. Technol., 2009, 44, 978.
48. W. Lertpaitoonpan, S. K. Ong and T. B. Moorman, Chemosphere, 2009, 76, 558.
49. Chemical Index Database, Sulfamethazine, CAS Registry Number: 57-68-1 http://www.drugfuture.com/chemdata/Sulfamethazine.html.
50. D. Lin, N. Liu, K. Yang, L. Zhu, Y. Xu and B. Xing, Carbon, 2009, 47, 2875.
51. S. Zhang, T. Shao, S. S. Bekaroglu and T. Karanfil, Water Res., 2010, 44, 2067.
52. J. Chen, D. Zhu and C. Sun, Environ. Sci. Technol., 2007, 41, 2536.
53. Y. Sun, Q. Yue, B. Gao, Y. Gao, X. Xu, Q. Li and Y. Wang, J. Taiwan Inst. Chem. Eng., 2014, 45, 681.
54. Z. G. Pei, X. Q. Shan and J. J. Kong, Environ. Sci. Technol., 2009, 44, 915.
55. G. Chen, Y. Wang and Z. Pei, Environ. Sci. Pollut. Res., 2014, 21, 2002.
56. F. Yu, Y. Wu, J. Ma and C. Zhang, J. Environ. Sci., 2013, 25, 195.
57. J. D. Hem, Study and interpretation of the chemical characteristics of Natural Water, US Geological Survey Water Survey Water Supply Paper, 1985, p. 2254.
58. T. X. Bui and H. Choi, Chemosphere, 2010, 80, 681.
59. Y. Tao, W. Li, B. Xue, J. Zhong, S. Yao and Q. Wu, J. Hazard. Mater., 2013, 261, 21.
60. X.-k. OuYang, R.-N. Jin, L.-P. Yang, Y.-G. Wang and L.-Y. Yang, RSC Adv., 2014, 4, 28699.
61. C. Lu and F. Su, Sep. Purif. Technol., 2007, 58, 113.
62. D. G. Lumsdon and A. R. Fraser, Environ. Sci. Technol., 2005, 39, 6624.
63. A. M. Mansour, J. Coord. Chem., 2013, 66, 1118.
64. A. M. Mansour, Inorg. Chim. Acta, 2013, 394, 436.
65. V. Udayakumar, S. Periandy and S. Ramalingam, Spectrochim. Acta, Part A, 2011, 79, 920.
66. Z. Pei, L. Li, L. Sun, S. Zhang, X.-q. Shan, S. Yang and B. Wen, Carbon, 2013, 51, 156.
67. J. Wang, Z. Chen and B. Chen, Environ. Sci. Technol., 2014, 48, 4817.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra15056b

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