Article pubs.acs.org/IECR
Persulfate Chemical Functionalization of Carbon Nanotubes and Associated Adsorption Behavior in Aqueous Phase Shengyi Huang, Chenju Liang,* and Yan-Jyun Chen Department of Environmental Engineering, National Chung Hsing University, 250 Kuo-kuang Road, Taichung 402, Taiwan S Supporting Information *
ABSTRACT: The chemical functionalization of carbon nanotubes (CNTs) using sodium persulfate (SPS) oxidation was designed to improve their dispersion stability in water. The test results indicated that base activated SPS oxidation of CNTs (BSPS/CNTs) adds a significant amount of oxygen functional groups to the surface of CNTs. The BSPS/CNTs dispersion is dependent on a solution within the pH range of 5−12. Experimental results obtained by Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy demonstrated that CNTs were successfully modified and the carboxylic functional groups (e.g., −COO−Na+ or −COO−H+) were created. The BSPS/CNTs, which carry negative charges, enhance the dispersion characteristics of CNTs. The BSPS/CNTs adsorption capacities of inorganic ions (e.g., copper ion) and organic compounds (e.g., benzene) were higher than those obtained by raw CNTs mainly due to enhanced CNTs dispersion. Furthermore, Langmuir and Freundlich adsorption models were applied to examine both raw CNTs and BSPS/CNTs adsorption behaviors. Copper and benzene sorption onto BSPS/CNTs fit the Freundlich isotherm model well, while raw CNTs adsorption did not fit any model. The findings of this study are of great significance for the base activated persulfate oxidation process, indicating that the functionalization of CNTs enhances CNTs dispersivity in water.
1. INTRODUCTION Carbon nanotubes (CNTs) are examples of nanomaterials with specific properties that have become a theme of basic and applied research. Nanomaterial studies have led to important applications in many fields of technology.1,2 CNTs are composed roughly of 97−99% elemental carbon, in the form of nearly spherical particles with diameters 10−100 nm, and possess a large surface/volume ratio and higher adsorption capacity than granular activated carbons.3 There are many potential uses for CNTs. However, a most important challenge in the practical application of CNTs is the development of their uniform and reproducible dispersion in aqueous solution. This requires the separation of the nanotube agglomerates into individual filaments in order to optimize their adsorptive capacity and attain their intriguing transport properties. Modification of CNTs surface chemistry (i.e., generally via physical or chemical treatments) can be adopted to improve dispersibility, stability, and hydrophilicity of CNTs in aqueous solution. Physical dispersion methods include ultrasonication, stirring, ball milling, grinding, and high speed shearing, etc. The use of physical methods does separate the CNTs agglomerates, but may also cause fragmentation of the nanotubes. For instance, Tchoul et al.4 indicated that ultrasonication makes CNTs shorter, and also thinner as a function of time owing to the expansion and peeling of the graphene layers. Thus, the efficient dispersion of CNTs, with limited fragmentation and damage of CNTs, is important for practical applications. An © 2016 American Chemical Society
alternative to chemical methods is to employ surfactants to change the surface energy of the nanotubes to improve their wettability and to reduce their tendency to agglomerate in the aqueous phase. But surfactants might not be able to generate a thermodynamically stable water-based dissolution of CNTs in solution because the surfactant modified CNTs solution is composed of chemical-colloidal suspensions, which is temporary, but the CNTs always reaggregate over time.5 Compared to surfactants, functionalization, defined as a chemical process that inserts functional groups on the sidewall of CNTs, is a superior modification processes, which produces a long lasting change to the CNTs surface characteristics, and is therefore more valuable for commercial applications.6 Concentrated acids such as nitric acid,6 sulfuric acid,7 tartaric acid8 or hydrogen chloride9 exhibit the capability to break down the carbon−carbon bonded network of the graphitic layers through acid-induced oxidation on the CNTs surface and allow the introduction of acidic functional groups. Additionally, chemical oxidative modification has also been reported as having the ability to improve the wettability of CNTs, by increasing the hydrophilicity. Specifically, Park et al. (2009) reported the preparation of water-soluble CNTs by potassium Received: Revised: Accepted: Published: 6060
March 17, 2016 May 5, 2016 May 10, 2016 May 11, 2016 DOI: 10.1021/acs.iecr.6b01065 Ind. Eng. Chem. Res. 2016, 55, 6060−6068
Article
Industrial & Engineering Chemistry Research
hydroxide (NaOH, min. 99.0%) and hydrochloric acid (HCl, min. 37%) from Sigma-Aldrich. The MWCNTs was purified using an acid washing procedure (2.5% HCl) to remove impurities present and then rinsed with reverse osmosis (RO) water in accordance with the procedure outlined by Liang et al.19 Thereafter, the MWCNTs were dried at 105 °C for 1 h prior to storage in a desiccator at ambient temperature.20,21 Note that the acronym “CNTs” was used to represent the original acronym “MWCNTs”, which represents CNTs that has already been acid-washed. 2.2. Methodology. 2.2.1. CNTs Modification. Two separate types of modified CNTs were prepared, SPS treated CNTs (denoted as SPS/CNTs) and base activated SPS/CNTs (denoted as BSPS/CNTs). SPS/CNTs was prepared by adding 1 g of CNTs to 100 mL of SPS solution (0.1 or 0.5 g L−1), sonicated for 30 min (Ultrasonic Cleaner), and then mixed on a reciprocating shaker (100 rpm, IKA) for 24 h at 20 °C. The SPS/CNTs was rinsed with RO water until the pH of the solution was stable and then filtered through 0.45 μm filter (Advantec Inc.). Thereafter, the SPS/CNTs was dried at 105 °C for 24 h prior to storage in a desiccator. In a similar manner, the BSPS/CNTs was prepared by base (NaOH 1 M) activated SPS solution at 80 °C. 2.2.2. Dispersion Behaviors of CNTs. A wide range of pH conditions (pH 1, 2, 3, 5, 7, 9, 12, and 13) was tested for dispersion behaviors of the BSPS/CNTs. The dosage of 0.1 g L−1 was prepared in a series of 20 mL bottles at 20 °C in a temperature-controlled chamber (KANSIN Low-Temp incubator, LT1603). 2.2.3. Cu2+ and Benzene Adsorption. Adsorption kinetic and isotherm experiments were carried out in a series of 40 mL amber EPA vials equipped with Teflon-lined screw caps. Duplicate samples were analyzed for all adsorption experiments. The benzene solution was prepared at 60 mg L−1, in RO water (pH 6) by adding the required amount of pure benzene and stirred for 12 h in a 2 L borosilicate reservoir (Schott Puran) equipped with a Teflon stopper and bottom outlet valve. The same general preparation procedure was used for the Cu2+ solution, at 30 mg L−1 in RO water (pH 5 with HCl adjusted) where the Cu2+ solution was prepared by diluting a 1000 mg L−1 Cu2+ in nitric acid standard solution. It should be noted that a pH 5 condition could result in a theoretical total dissolved Cu concentration of 1400 mg L−1 (see Figure S1, Supporting Information (SI), for total dissolved Cu concentration as a function of pH). Therefore, Cu precipitation in these adsorption experiments is likely to be or could be negligible. For determining the kinetics of the benzene and Cu2+ adsorption processes, 0.04 g of CNTs and BSPS/CNTs were added in a series of 40 mL EPA vials prior to addition of benzene or Cu2+ solution. At each sampling time, two bottles were sacrificed for benzene and Cu2+ analysis, which was done by withdrawing solution through septum using a 1 mL gastight syringe (SGE Analytical Science) fitted with a 0.2 μm PTFE filter in a stainless steel syringe holder (ADVANTEC, KS-13). Equilibrium isotherm experiments were conducted using the bottle-point method. The benzene (60 mg L−1) and Cu2+ (30 mg L−1) solutions were prepared in accordance with the procedure described earlier. The solution was then added to a series of 40 mL vials with no head space, in which different amounts of CNTs or BSPS/CNTs within a range of 0.1 to 5 g L−1 were initially added. When adsorption reached equilibrium, which can be determined based on the results of the adsorption
persulfate (KPS) oxidation of CNTs at pH 13 and a temperature of 85 °C, and a 3 h reaction time in water. The KPS-treated CNTs exhibited improvement of the CNTs water solubility due to the formation of potassium carboxylate (−COOK), carbonyl (−CO) or hydroxyl (−C−OH) groups on the CNTs surface. These ionizable negatively charged functional groups on the surface of the CNTs enable the nanotubes to repel each other, keeping them uniformly dispersed; thereby improving the water-solubility. The presence of functional groups on the sidewall of CNTs to establish π−π electrostatic interactions could facilitate adsorption of chemical pollutants such as aniline,10 phenol,11 dissolved organic matters,12 aromatic compounds,13 and several divalent metal ions14,15 from water. The persulfate anion (S2O82−, E0 = 2.01 V) is a strong oxidant with redox potential of 2.01 V and commonly used for in situ chemical oxidation remediation of subsurface contamination.16 Because sodium persulfate (SPS) has a higher water solubility than that of KPS, SPS proves to be advantageous for a broader range of applications. Moreover, S2O82− can be activated, e.g., thermal (eq 1)17 or base18 activations (eqs 2−5), to generate sulfate radicals (SO4−•, E0 = 2.4 V). Basic pH conditions could induce hydrolysis of persulfate to produce hydro-peroxide anions (HO2−) (eqs 2−3). The reduction of persulfate by HO2− can further generate SO4−• and superoxide radicals (O2−•) (eq 4). Furthermore, SO4−• (generated via eq 1 or eqs 2−4) can also react with OH− and undergo radical interconversion to form hydroxyl radicals (HO•) in accordance with eq 5. Thermal SPS activation S2 O82 − + heat → 2SO4 −•
(1)
Base SPS activation S2 O82 − + OH− → HSO4 − + SO52 −
(2)
SO52 − + H 2O → HO2− + SO4 2 − + H+
(3)
HO2− + S2 O82 − → SO4 − ·+SO4 2 − + H+ + O2−•
(4)
Radical interconversion SO4 −• +OH− → HO• +SO4 2 −
(5)
As illustrated above, this chemical functionalization involving oxidations by SPS and the accompanying production of radical oxidizing species may serve as an effective way to improve the dispersivity of CNTs in aqueous media. Therefore, this study examined the thermal persulfate oxidative treatment of CNTs with or without alkaline adjustment on the potential to improve their dispersion stability in water, and explored the optimal working pH conditions. Characterizations of chemical oxidation modified CNTs were carried out. Also, the comparison between functionalized and nonfunctionalized CNTs for the adsorption of inorganic ions, e.g., copper (Cu2+), and organic compounds, e.g., benzene (C6H6), was studied.
2. MATERIALS AND METHODS 2.1. Chemicals. The chemicals used were purchased from the following sources: multiwalled carbon nanotubes (MWCNTs, min. 95%) from Golden Innovation Business Co., Ltd.; both benzene (C6H6, min. 99.7%) and copper (Cu2+, 1000 mg L−1 in nitric acid) from Sigma-Aldrich; sodium persulfate (Na2S2O8, min. 99.0%) from Merck; sodium 6061
DOI: 10.1021/acs.iecr.6b01065 Ind. Eng. Chem. Res. 2016, 55, 6060−6068
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Industrial & Engineering Chemistry Research kinetic experiments, solution samples were collected using a gastight syringe and filtered for benzene and Cu2+ analysis. The amount of adsorbed benzene and Cu2+ was determined by calculating the difference between initial and equilibrium adsorbate concentrations in the solution in accordance with the following eq 6: qe =
V (C0 − Ce) m
Fx =
Ix /Sx ∑ (I / S )
(9)
where Fx is the molar fraction of x in the sample; Ix is the integration area of the peak of x; I is the peak area of several elements in the sample; Sx is the atomic sensitive factor of x element; and S is the atomic sensitive factor of elements in the sample.
(6)
3. RESULTS AND DISCUSSION 3.1. Characterization of Persulfate Modified CNTs. 3.1.1. UV−vis Spectroscopy Analysis. To evaluate the dispersion stability of persulfate modified CNTs, UV−vis spectroscopy was utilized as a quantitative tool for nanoscale dispersion characterization. The small graph inserted in Figure 1 shows in the UV−vis scan spectra that BSPS/CNTs dispersed
−1
where qe (mg g ) is the amount of adsorbed adsorbate; V (L) is the volume of the aqueous solution; m (g) is the amount of the adsorbent; C0 (mg L−1) is the initial concentration of adsorbate; and Ce (mg L−1) is the equilibrium concentration of adsorbate. Adsorption isotherms depict the relationship between the amount of adsorbate adsorbed by the adsorbent and the adsorbate concentration remaining in the solution after the adsorption process has reached the equilibrium. Langmuir (eq 7) and Freundlich (eq 8) equations are the most commonly used isotherms to describe the solid−liquid adsorption system.11 ⎛ 1 ⎞ 1 Ce C =⎜ ⎟ + e qe Cm ⎝ KL ⎠ Cm
(7)
where Cm (mg g−1) is monolayer adsorption capacity and KL (L mg−1) is the Langmuir adsorption constant which is related to the affinity between the adsorbent and adsorbate. log qe =
1 log Ce + log KF n −1
(8)
−1 1/n
where KF (mg g ) (L g ) and n, the Freundlich constants, are related to adsorption capacity and adsorption intensity, respectively. 2.3. Analysis. Cu2+ in the sample solution was acid digested according to the analytical method of the Taiwan National Institute of Environmental Analysis (Taiwan NIEA) S321.63B and measured using a PerkinElmer Analyst 100 flame atomic absorption spectrophotometer in accordance with the method of Taiwan NIEA W306.52A. The aqueous benzene concentration was measured using high performance liquid chromatography/UV (Agilent 1100) equipped with a ZORBOX Eclipse XDB-C18 column and the effluent monitored at 254 nm. Specific surface area, pore volume, and diameter of CNTs were determined using a Brunauer−Emmett−Teller (BET) sorptometer (Porous Materials, Inc., CBET-201A) via nitrogen adsorption/desorption isotherm at 77K as a function of relative pressure. Fourier Transform Infrared Spectroscopy (FTIR) (Jasco FT/IR-4100) was employed in the qualitative determination of the functional groups on the surface of CNTs. The angle of incidence of the IR beam was 45°, 100 scans were collected at a resolution of 0.9 cm−1, and the spectra were collected within the spectral range of 4000−400 cm−1 wavenumber. Zeta potential measurement for CNTs was conducted using a Nano ZS (Malvern Intrument). X-ray photoelectron spectroscopy (XPS) (PHI 5000 VersaProbe with Al (Ka = 1486.6 eV) X-ray source) was used to quantify the functional groups of O, C, Na, and S on the surface layer of CNTs. A survey of spectral region scans from 0 to 1100 eV was performed to determine the functional groups and/or elemental composition on CNTs surfaces and calculations were done using the following eq 9:22
Figure 1. Dispersion behaviors of BSPS/CNTs, SPS/CNTs, and CNTs measured with absorbance at 267 nm versus time at initial pH ∼6. The insert figures exhibit UV−vis scan spectra of BSPS/CNT and pictures of different CNTs in water (1 g L−1).
into aqueous suspension, and revealed a maximum absorption peak at 267 nm, coincident with the value reported in Kim et al.23 Therefore, the change of absorbance at 267 nm was measured over time for three different types of CNTs. As can be seen in Figure 1, dispersions for two dosages of each of the three CNTs reach equilibrium in half an hour. BSPS/CNTs appeared to be the most stable dispersion in water over an extended time period. Because of the superior dispersion behavior of BSPS/CNTs, the BSPS/CNTs dispersion over a wide range of pH was further tested and the results are shown in Figure 2. It can be seen that the working pHs ranged from 5 to 12, as observed over a time period of one month. The reason for this observation will be elucidated based on the results of FTIR and XPS analysis in later sections. 3.1.2. FTIR Measurements Analysis. Figure 3 shows an FTIR spectrum of the functional groups on the BSPS/CNTs, CNTs (acid washed), and raw CNTs (unwashed). In general, the strength of BSPS/CNTs in the FTIR spectrum shows more apparent absorbance than SPS/CNTs and raw CNTs. More functional groups were detected on the modified CNTs, than the raw CNTs, as noted in Figure 3 where the lines labeled “a” marking 3332, 1306, and 1043 cm−1 assigned for R-CH2−OH; lines “b” marking 1734 and 1232 cm−1 assigned for CC− 6062
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Table 1. Surface Characteristics of the CNTs and BSPS/ CNTs surface area (m2/g) sample CNTs BSPS/ CNTs
pore volume (cm3/g)
pore diameter (Å)
SBET
Smicro
Vtotal
Vmicro
davg
pHpzc
187.16 228.36
20.16 16.24
1.34 1.28
0.009 0.006
119.49 97.51
3.7 4.5
appeared nearly the same. In addition, BSPS/CNTs exhibits more oxygen functional groups than CNTs; hence the BSPS/ CNTs carrying negative charges shows a reduction in the tendency to aggregate into CNTs bundles. Again, these results suggest that base activated persulfate oxidation is effective in the introduction of oxygen functional groups onto the nanotubes and trigger repulsion between nanotubes. 3.1.4. XPS Analysis. The XPS was used to qualitate and quantitate the functional groups on CNTs. Comparing both CNTs and BSPS/CNTs, as shown from the survey spectra in Figure 4a, the C 1s and O 1s photoemission peaks are centered at 533.0 and 285.0 eV, respectively.29 There are mostly carbon atoms on the surface of CNTs; but the surface of CNTs oxidized by SPS under base conditions has may carbon atoms bonded to oxygen. Therefore, as can be seen in Figure 4a, there are more oxygen containing functional groups bonded to BSPS/CNTs with peak O 1s appearing at 533.0 eV. The CNTs C 1s XPS spectrum peaks located at 284.56, 285.66, 286.28, 287.13, and 292.77 eV and the BSPS/CNTs C 1s XPS spectrum peaks located at 284.45, 284.60, 285.84, 288.40, and 292.61 eV are associated with C−C, C−O, CO, O−CO, π−π* hybridized carbons as shown in Figure 4b,c, respectively.30 Therefore, the peak with the higher binding energy at 288.3 eV shown in Figure 4d can be associated with the carbons of carboxylic acid on CNTs, evidenced by the O 1s features at 531.7 and 533.1 eV, associated with the oxygen of − CO and −OH, respectively.31 The Na 1s peak located at 1072 eV in Figure 4e represents that CNTs was successfully modified and the ion bonding between −COO− and Na+ was created.32 Furthermore, for quantitative analysis, there are oxygen atoms (O 1s) from oxygen functional groups present on the surface of BSPS/CNTs. After modification, there is 0.31% of sodium observed over the surface of BSPS/CNTs; and the oxygen content ratio increased from 1.30% to 7.35%, as shown in Table S1 (SI). In addition, there is 71.23% of C−C structure in the CNTs, which is reduced to 30.03% in the BSPS/CNTs (Note that C 1s occupied 98.70% and 92.34% in the CNTs and BSPS/CNTs, respectively.). The C−O, CO and O−CO content ratios are also increased in the BSPS/CNTs. SPS is used as an oxidant and sulfate is a major degradation product, but no sulfur was detected on the surface of BSPS/CNTs. This is possibly due to a negative charge of sulfate and relatively large size of its molecule, which prevents its binding with CNTs. The carboxylic functional group (e.g., −COO−Na+ or −COO−H+) present in BSPS/CNTs can dissociate in the solution dependent on pH. The pKa of −COONa is 4.5333 and hence, the greater portion of dissociated negatively charged −COO− at pH greater than 4.53 would better repel CNTs from each other, resulting in better aqueous dispersion. However, the well dispersed CNTs gradually aggregate and precipitate to the bottom at the extreme alkaline condition (i.e., pH > 13), the stronger negatively charged carboxylic functional
Figure 2. Effect of pHs on BSPS/CNTs dispersion in water. [CNTs] = 0.1 g L−1; absorbance at wavelength = 267 nm.
Figure 3. FTIR spectra of the CNTs (a. 3332, 1306, 1043 cm−1 (R)2CH2−OH; b. 1734, 1232 cm−1 CC−CO−O−R; c. 1561 cm−1 CO−CC−OH).
CO−O−R; and line “c” marking 1561 cm−1 assigned for COCC−OH.24,25 The bands in the region 1600−1585 cm−1 are due to carbon−carbon stretching in the aromatic ring structure and hence the peak of 1561 cm−1 is associated with CNTs side wall effects. The peaks at around 1733.7 cm−1 (CO), and asymmetric 1232 cm−1 (C−O), apparently correspond to the stretching mode of the carboxylic acid group (−COOH). The peaks of 1306 and 1068 cm−1 spectrum peaks apparently correspond to the asymmetrical vibration of (−CH3) and asymmetrical stretch of C−O−C and the bands obtained at 907 cm−1 are related to out-of-plane C−H and C−C bending vibration.26,27 A peak at 3332 cm−1 can be related to the OH groups, and peaks at 1561 cm−1 and 1418 relate to (−COONa) groups, also 1561 cm−1 (CO of −COONa antisymmetric stretching) and 1418 cm−1 (CO of −COONa symmetric stretching).28 3.1.3. BET Sorptometer Analysis. Comparisons of variations of surface characteristics between CNTs and BSPS/CNTs are presented in Table 1. The specific surface area (SSA) of the BSPS/CNTs (228.36 m2/g) was slightly higher than the CNTs (187.16 m2/g). It was also seen that average pore diameter of BSPS/CNTs is smaller than that of CNTs, but pore volume 6063
DOI: 10.1021/acs.iecr.6b01065 Ind. Eng. Chem. Res. 2016, 55, 6060−6068
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Industrial & Engineering Chemistry Research
Figure 4. XPS spectra. (a) survey of the spectral region from 0 to 1100 eV of CNTs and BSPS/CNTs, (b) the narrow scan of carbon 1s of raw CNTs, (c) the narrow scan of carbon 1s of BSPS/CNTs, (d) the narrow scan of oxygen 1s of BSPS/CNTs, and (e) the narrow scan of sodium 1s of BSPS/CNTs. Note: O KLL is the oxygen peak of Auger electron spectroscopy. The notation KLL indicates that a high-energy electron knocks out an inner electron at the K shell of an atom. Then, a core level hole at K-level was occupied by a second L shell electron which in turn knocks out another L shell electron as Auger electron, leaving the atom.
groups (−COO−) on CNTs would be affected by the increased sodium cations via the electrostatic attractions between CNTs− COO− and Na+, resulting in the compression of the electrical double layer of the charged surface. According to the DLVO (Derjaguin, Landau, Verwey, and Overbeek) theory of colloidal systems, the double layer compression could induce the destabilization of dispersed particles.34 Additionally, the zeta potential (mV) of CNTs and BSPS/CNTs as a function of pH
are shown in Figure S2 (SI) and the point of zero charge (pHpzc) values are 3.7 and 4.5, respectively (presented in Table 1). Zeta potential measurements show that electronegativity decreased with increasing pH, and pHpzc indicates the hydrophilicity of CNTs. The pHpzc of CNTs is lower and possesses more strong acidic groups than BSPS/CNTs. This is possibly due to the presence of −COONa on the surface of BSPS/CNTs, instead of −COOH. Based on colloidal stability, 6064
DOI: 10.1021/acs.iecr.6b01065 Ind. Eng. Chem. Res. 2016, 55, 6060−6068
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Industrial & Engineering Chemistry Research Table 2. Summary of Modified CNT Benzene and Cu2+ Adsorption Capacitiesa adsorbate
CNT modification
Qe (mg/g)
C0 (mg/L)
CNT dose (g/L)
pH
temp. (°C)
benzene
base Na2S2O8 NaOCl HNO3 HCl H2SO4 NaOCl HNO3 base Na2S2O8 HNO3 HNO3/NaOH UV/HNO3 ultrasonic/HNO3 HNO3/H2SO4/80 °C HNO3/40 °C
61 212 8 30 43 213 106 40 0.9 7 143 44 30 28
60 200 15 200
1 0.6 0.18 0.6
6 7 7 7
20 25 20 25
this study Su et al.41 Chin et al.42 Lu et al.43
30 10
1 0.964
5 5.1
20 20
the study Rosenzweig et al.44
14
0.001
5
25
Bayazit and Iṅ ci20
12 8
0.5 2
4 5
25 25
Wang et al.45 Li et al.46
Cu2+
a
refs
Qe is the equilibrium adsorption capacity, C0 is the initial concentration of adsorbate solution.
Figure 5. Benzene (a and b) and Cu2+ (c and d) Langmuir and Freundlich adsorption isotherm plots of CNTs and BSPS/CNTs.
that the sorption process of the CNTs for adsorbing Cu2+ was rapid and reached equilibrium in 2 h. Also, Iman et al.36 reported a CNT adsorption capacity of 10 mg Cu2+ g−1 equilibrated in 40 min at 20 °C and Sahika and Ismail20 also reported that various CNTs with different oxidation treatments (i.e., ultrasonication/HNO3 and UV/HNO3) could generally reach over 40 mg g−1 of Cu2+ adsorption capacities. The BSPS/ CNTs with more dissociative functional groups (e.g., − COONa) compared to CNTs show better dispersion and have more effective specific surface area for adsorption to occur. Moreover, the adsorption kinetics of the uncharged organic molecules, benzene, are shown in Figure S4 (SI). It was seen that the adsorption capacity of CNTs and BSPS/CNTs gradually increased with time and reached equilibrium (i.e., 50.6 ± 1.6−60.8 ± 0.4 mg g−1) in 5 and 12 h, which are much longer than that required for Cu2+ adsorption. The bundled CNTs may inhibit available surface area, resulting in reduced
BSPS/CNTs are shown to have superior dispersion in water over an extended time period. After oxidation with base activated persulfate, the CNTs surface became acidic because of the formation of carboxylic functional groups (i.e.,-COONa present in BSPS/CNTs), which is consistent with the results of XPS analysis. 3.2. Adsorption Kinetics and Isotherms. Figure S3 (SI) shows the adsorption kinetics for Cu2+ reacting with CNTs and BSPS/CNTs. Note that Cu2+ (30 mg L−1) is well dissolved (see Figure S1, SI), and BSPS/CNTs are also well suspended in the solution at pH 5 (see Figure 2). It can be seen that that the time required to reach equilibrium is within 2 h and the maximum Cu2+ adsorption capacities are 11.4 ± 1.5 and 39.5 ± 1.0 mg g−1, for CNTs and BSPS/CNTs, respectively. Cu2+ adsorption capacities on CNTs oxidized by various chemical methods are summarized in Table 2. These results are comparable to those reported by Tang et al.35 who indicated 6065
DOI: 10.1021/acs.iecr.6b01065 Ind. Eng. Chem. Res. 2016, 55, 6060−6068
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Table 3. Benzene and Cu2+ Adsorption Parameters of CNTs and BSPS/CNTs Using Langmuir and Freundlich Models Langmuir
Freundlich
adsorbate
adsorbent
Cm
KL
R2
1/n
KF
R2
benzene
CNTs BSPS/CNTs CNTs BSPS/CNTs
0.870 −8.222 0.007 37.825
−0.023 −0.016 −0.035 0.010
0.192 0.828 0.438 0.900
−5.608 3.759 −66.221 1.255
2.94 × 1010 1.31 × 10−05 9.30 × 1096 0.216
0.086 0.825 0.451 0.903
Cu2+
the 1/n value of BSPS/CNTs adsorption for both benzene and Cu2+ are 3.759 and 1.255, which may indicate unfavorable adsorption and possible cooperative adsorption, which implies that carboxylic functional groups on BSPS/CNTs tend to help to hold the adsorbate to the surface, thereby supporting the adsorbent−adsorbate interaction.40
adsorption. In addition, since benzene is in the molecular form in solution, the adsorption occurs mainly by dispersive attraction between the π orbital on the carbon basal planes and the electronic density in the benzene aromatic rings. The more negatively charged functional groups on the surface of BSPS/CNTs would lead to adsorption sites blocked and a weakening of the carbon dispersive interaction with benzene.19 Benzene adsorption capacities on CNTs oxidized by various chemical methods are also summarized in Table 2. It can be seen that BSPS/CNTs exhibited higher benzene adsorption capacity than those treated using HCl and H2SO4 reported in other studies. However, even though other chemical methods of oxidizing CNTs have higher benzene adsorption capacities, their initial benzene concentrations are higher than those used in this study and this may tend to enhance the adsorption capacity. Furthermore, Cu2+ and benzene adsorption capacities were analyzed using Langmuir and Freundlich isotherm models. The basic assumption of Langmuir isotherm is based on the hypothesis of monolayer adsorption onto a homogeneous surface with a finite number of identical sites where the sorption takes place at specific homogeneous sites within the adsorbent.37 Langmuir constants KL and Cm are calculated based on the results presented in Figure 5 and in accordance with eq 7 (see Table 3 for data). It can be seen that the R2 values for CNTs adsorbing benzene and Cu2+ showed poor correlation. The CNTs have nonlinear adsorption isotherms, likely due to aggregation of CNTs creating heterogeneous sites with unstable adsorption behavior. Additionally, the Langmuir isotherm does not provide a good description of benzene adsorption onto BSPS/CNTs because it leads to a negative intercept on the 1/qe vs 1/Ce plot, which is an unexpected result and therefore should be rejected. The Freundlich isotherm is derived by assuming a heterogeneous surface with a nonuniform distribution of heat of adsorption over the surface.38 KF and n, the Freundlich constants, are related to adsorption capacity and adsorption intensity, respectively. Also, the constant n can be a measure of the deviation from linearity of the adsorption. If a value for n is equal to unity, the adsorption is linear. A value of n below unity implies that the adsorption process is a chemical process, but if a value for n is above unity, adsorption is favorable as a physical process.39 Values of KF and n were obtained from data presented in Figure 5 and in accordance with eq 8. As seen from the Freundlich isotherm parameters in Table 2, the n value of CNTs adsorptions of both benzene and Cu2+ are negative, which indicate that the Fruendlich model would not fit the experimental data. However, the BSPS/CNTs adsorptions of both benzene and Cu2+ show a better fit to the Freundlich isotherm model with an R2 value of 0.825 and 0.903, respectively. It should be noted that 1/n above one (i.e., 3.759 and 1.255 for benzene and Cu2+ adsorption onto BSPS/ CNT) could be indicative of a nonlinear adsorption mechanism and deficient adsorption strength.21 Therefore, it was seen that
4. CONCLUSION In order to increase the dispersion and associated adsorption capability of CNTs in solution, the surface of CNTs was modified by base activated SPS oxidation in this study. The introduction of oxygen functional groups (e.g., carboxylic functional group (e.g., −COO−Na+ or −COO−H+) onto the nanotubes (BSPS/CNTs) was demonstrated by the results of FTIR and XPS analysis and triggered repulsion between nanotubes, thereby resulting in better CNT aqueous dispersion. The BSPS/CNTs are well dispersed within 1 month under a pH range of 5 to 12 (due to pH > pKa = 4.53 of −COONa). However, at extreme alkaline conditions (e.g., pH > 13), the stronger negatively charged −COO− on CNTs would be affected by the increased sodium cations, resulting in compression of the electrical double layer of the charged surface and inducing the destabilization of dispersed particles. The comparison between functionalized and nonfunctionalized CNTs for the adsorption of inorganic copper ion and organic benzene compound was examined. The Cu2+ adsorption capacity of 39.5 mg g−1 for the BSPS/CNTs showed better adsorption than CNTs, because of greater dispersion in solution and a higher SSA of BSPS/CNTs (i.e., 187.16 m2 g−1 for CNTs vs 228.36 m2 g−1 for BSPS/CNTs). However, the BSPS/CNTs adsorption of benzene is comparable to the CNTs due to the presence of the oxygen functional groups on the surface of BSPS/CNTs, which weakens the carbon dispersive interaction with benzene. The analysis of Langmuir and Freundlich adsorption isotherms showed a better BSPS/ CNTs fit to the Freundlich isotherm model with higher R2 values for copper and benzene, and also indicated possible cooperative adsorption. These results observed in this work might have significant implications for the removal of environmental contaminants with base activated persulfate oxidation functionalized CNTs.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b01065. Table S1. The parameters of XPS survey scans of CNTs and BSPS/CNTs. Figure S1. Total dissolved Cu concentration as a function of pH. Figure S2. Zeta potentials of CNTs and BSPS/CNT under various pHs. Figure S3. Cu2+ adsorption kinetic profiles at pH 5. [Cu2+]0 = 30 mg/L; [CNTs] = 1 g/L. Figure S4. 6066
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Benzene adsorption kinetic profiles at pH 6. [benzene]0 = 60 mg/L; [CNTs] = 1 g/L. (PDF)
AUTHOR INFORMATION
Corresponding Author
*Tel.: +886-4-22856610. Fax: +886-4-22862587. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study was funded by the Ministry of Science and Technology of Taiwan under Project No. 103-2221-E-005005-MY3. The authors wish to thank graduate student Simon Jatta at the Department of Environmental Engineering at National Chung Hsing University (Taiwan) for conducting literature survey to initiate the preparation of this paper in support of this study.
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