Reduction of Cr(VI) Oxyanion by Halloysite

Jan 25, 2017 - Effective Adsorption/Reduction of Cr(VI) Oxyanion by Halloysite@Polyaniline Hybrid Nanotubes ... E-mail: [email protected]. ... The a...
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Effective Adsorption/Reduction of Cr(VI) Oxyanion by Halloysite@Polyaniline Hybrid Nanotubes Tianzhu Zhou, Cuiping Li,* Huiling Jin, Yangyang Lian, and Wenmei Han School of Chemical Science and Technology, Key Laboratory of Medicinal Chemistry for Natural Resource, Ministry of Education, Yunnan University, Kunming 650091, China S Supporting Information *

ABSTRACT: Halloysite@polyaniline (HA@PANI) hybrid nanotubes are synthesized by the in situ chemical polymerization of aniline on halloysite clay nanotubes. By facilely tuning the dopant acid, pH, and apparent weight proportion for aniline (ANI) and halloysite (HA) nanotubes in the synthesis process, PANI with tuned oxidation state, doping extent, and content are in situ growing on halloysite nanotubes. The reaction system’s acidity is tuned by dopant acid, such as HCl, H2SO4, HNO3, and H3PO4. The adsorption result shows the fabricated HA@PANI hybrid nanotubes can effectively adsorb Cr(VI) oxyanion and the adsorption ability changes according to the dopant acid, pH, and apparent weight proportion for ANI and HA in the synthesis process. Among them, the HA@PANI fabricated with HCl as dopant acid tuning the pH at 0.5 and 204% apparent weight proportion for ANI and HA (HP/0.5/204%-HCl) shows the highest adsorption capacity. The adsorption capacity is in accordance well with the doping extent of PANI in HA@PANI. Furthermore, when HP/0.5/204%-HCl is redoped with HNO3, H2SO4, and H3PO4, the adsorption capacity declines, implying the dopant acid in the process of redoping exhibits a marked effect on Cr(VI) oxyanion adsorption for the HA@PANI hybrid nanotubes. HP/0.5/204%-HCl and HP/0.5/204%-H3PO4 have demonstrated good regenerability with an above 80% removal ratio after four cycles. Moreover, the HA@PANI adsorbent has better sedimentation ability than that of pure PANI. The adsorption behavior is in good agreement with Langmuir and pseudo second-order equations, indicating the adsorption of HA@ PANI for Cr(VI) oxyanion is chemical adsorption. FT-IR and XPS of HA@PANI after Cr(VI) oxyanion adsorption indicate that the doped amine/imine groups (−NH+/N+− groups) are the main adsorption sites for the removal of Cr(VI) oxyanion by electrostatic adsorption and reduction of the adsorbed Cr (VI) oxyanion to Cr(III) simultaneously. KEYWORDS: halloysite nanotube, polyaniline, dopant acid, Cr(VI) oxyanion adsorption, Cr(VI) oxyanion reduction

1. INTRODUCTION Studies on Cr(VI) oxyanion removal with high efficiency are rather attractive because of their serious threat to ecosystems and human health due to high toxicity, potential carcinogenicity, and nondegradability.1,2 The maximum permissible concentrations of Cr(VI) oxyanion in drinking water and industrial wastewater are, respectively, 0.05 and 0.10 mg/L. Hence, Cr(VI) oxyanion-containing wastewater is required to be treated to achieve certain discharge standards.3 During the past decades, many kinds of methods have been developed to ameliorate and remove Cr(VI) oxyanion in wastewater, including electrocatatalysis,4 ion exchange,5 precipitation,6 photocatalytic reduction,7 and adsorption.8−13 Particularly, the adsorption method is widely studied for Cr(VI) oxyanion pollution due to its economy and extensive application. Magnetic composites,9 polymers,10 carbon,11 bioorganic materials,12 and agricultural waste13 have been selected as adsorbents for Cr(VI) oxyanion pollution. Nevertheless, secondary pollution resulting from the Cr(VI) oxyanion toxicity and adsorbate is still the major obstacle. Simultaneous © 2017 American Chemical Society

adsorption and reduction of toxic Cr(VI) oxyanion is considered as the most convenient and effective method. Different adsorbent-containing electron donors like zerovalent iron,14,15 amino-functionalized MCM-41,16 and polyacrylonitrile/ferrous chloride17 have been developed for simultaneous adsorption and reduction of Cr(VI) oxyanion. On the other hand, polyaniline (PANI) is the most widely studied conducting polymer due to its low cost, corrosion resistance, and special proton doping/dedoping mechanism.18 It can be a conductor, semiconductor, or insulator by tuning the protonation degree and the synthesis condition. Consequently, PANI has been extensively used in the fields of conductive adhesive/ink/paint,19 sensors,20 rechargeable batteries,21 corrosion resistance,22 and photosensitizer.23,24 Because there is benzenoid and quinonoid units in PANI, which has several oxidation state associated with the chain nitrogen, Received: November 15, 2016 Accepted: January 25, 2017 Published: January 25, 2017 6030

DOI: 10.1021/acsami.6b14079 ACS Appl. Mater. Interfaces 2017, 9, 6030−6043

Research Article

ACS Applied Materials & Interfaces

capacity of PANI, especially of halloysite nanotubes supported PANI for Cr(VI) oxyanion, has seldom been reported. In this study, the HA@PANI hybrid nanotube is synthesized in various dopant acids by the in situ chemical polymerization of aniline (ANI) on HA at room temperature. By tuning the dopant acid and the acidity in the synthesis process simply, HA@PANI hybrid nanotubes composed of different PANI oxidation state and protonation extent can be produced. The effect of pH, dopant acid, and the apparent weight proportion for ANI and HA in the synthesis process on the adsorption properties of HA@PANI for Cr(VI) oxyanion removal are systematically investigated. Furthermore, its adsorption capacity for Cr(VI) oxyanion can be conditioned simply by the dopant acid’s variety and concentration in the process of redoping. The regeneration, isothermal adsorption behavior, and adsorption kinetics of HA@PANI have also been further investigated. According to the results of XPS and FT-IR spectra, a mechanism of electrostatic adsorption/reduction of Cr(VI) oxyanion simultaneously is suggested for Cr(VI) oxyanion removal by HA@PANI.

PANI can act as the reducing agent. Furthermore, the innate cationic amine and imine functional groups in PANI are expected to electrostatically interact with the negatively charged ion. Thus, PANI should be able to remove Cr(VI) oxyanion efficiently because of its good reduction and adsorption behaviors. The bulk PANI has been applied in the Cr(VI) oxyanion removal with a high removal ratio.25,26 However, the difficulty in the regeneration and recycling of bulk PANI limits its further application. Although PANI film can be facilitated to separate from the solution, it is limited by the decreased removal efficiency resulting from the lower active adsorption sites and surface area.27 Therefore, it is a requisite to synthesize supported PANI with high Cr(VI) oxyanion adsorption/ reduction efficiency. All kinds of support have been used to support PANI, for example, magnetic composites,28,29 carbon,30 Mg/Al layered double hydroxide,31 biomass,32−34 MoS2,35 graphene,4,36 polymer,37,38 and clay.39−41 Especially, adsorbent supported with clay has been widely researched for the heavy metal removal due to its high cation exchange capacity, special surface area, and heavy metal ions enrichment ability.39−41 In the family of clay, halloysite (HA) nanotubes (chemical composition of Al2Si2O5(OH)4·2H2O) have gained wide attention as the support for metals, metal−oxide, and polymer nanoparticles in virtue of its anticorrosion for organic solvents and high special surface area.24,42−44 The difference of HA from carbon nanotubes is that it is an abundantly widespread natural clay with different outside/inside surface chemical properties and massive surface hydroxyl groups.44,45 The hydroxyl can induce the growth of PANI, organochlorosilanes, and alkoxides on the HA in situ.24,43−45 It has been demonstrated that PANI acts as photosensitizer, and the type/concentration of dopant acid can exert a significant effect on the photocatalytic activity of PANI−TiO2−HA and PANI/TiO2.23,46 Even the dopant acid is same, it shows a different trend: the photocatalytic activity of PANI/TiO2 fabricated in different dopant acids under UV follows the order of PANI/TiO2−H2SO4 > PANI/ TiO2−H3PO4 > PANI/TiO2−HCl > PANI/TiO2−HNO3 and decreases with HCl concentration increasing,46 whereas the visible light photocatalytic dye degradation activity of PANI− TiO2−HA fabricated in different dopant acid follows the order of PANI−TiO2−HA−HCl > PANI−TiO2−HA−H2SO4 > PANI−TiO2−HA−HNO3 > PANI−TiO2−HA−H3PO4, and the optimal HCl redoping concentration is 0.5 M.23 By that analogy, we guess the dopant acid and doping concentration in the process of synthesis or redoping should exercise a significant impact on the adsorption properties of halloysite@ polyaniline (HA@PANI) for Cr(VI) oxyanion. Recently, Wang et al. have studied the effect of dopant acids on the adsorption properties of PANI for Cr(VI) oxyanion.25 It demonstrates the PANI synthesized in hydrochloric acid (HCl) shows the maximum adsorption capacity, and then decreases with the order of PANI-surfulamic acid, PANI-citric acid, PANI-taurine, and PANI-neutral deionized water. However, as the pH of 0.2 M protonic acid is different, the effect of protonic acids on the adsorption properties of PANI for Cr(VI) oxyanion is largely rested with the acidity. Also, the authors have shown the adsorption capacity of PANI fabricated in various protonic acids is correlated well with its corresponding acidity of 0.2 M protonic acid, although the authors have ascribed the difference to the different oxidation state and protonation extent of PANI. For all we know, the effect of dopant acid with the same acidity in the process of synthesis and redoping on the adsorption

2. EXPERIMENTAL SECTION 2.1. Materials. Halloysite (HA) nanotubes were kindly afforded by Fenghui Minerals Trade Co., Ltd. Aniline (ANI, brown yellow) and ammonium persulfate ((NH4)2S2O8, APS) were acquired from Beijing Chemical Reagent Co., Ltd. All of the chemical reagents were used directly. 2.2. Fabrication of HA@PANI Hybrid Nanotubes under Various Dopant Acids. In the fabrication of HA@PANI hybrid nanotubes, the concentration of HA and the molar ratio of ANI to (NH4)2S2O8 were fixed at 2 g/L and 1:1, respectively. The pH of the reaction system was tuned by 2 M dopant acid. A typical procedure for fabricating HA@PANI hybrid nanotubes with HCl as dopant acid tuning the pH value was as follows: 63.2 mL of HCl solution (2 M) was added into a polypropylene container under stirring at room temperature. Next, 336.8 mL of deionized water was added. The final pH of the solution was 0.5. After the mixture was stirred for 0.5 h, 0.8 g of HA, 1.63 g of ANI, and 3.99 g of (NH4)2S2O8 were added to the above 400 mL solution. After being stirred at room temperature for 24 h, it was centrifuged, and the deposit was washed with water until the deposit was neutral to remove the oligomer/dopant acid and dried at 60 °C to obtain HP/0.5/204%-HCl (the HA@PANI nanotubes were fabricated with HCl as dopant acid tuning the pH at 0.5 and 204% weight proportion for ANI and HA in the synthesis process). The content of PANI in HA@PANI was controlled by the added amount of ANI. Note: The added amount of (NH4)2S2O8 should be adjusted according to the added amount of ANI, and the molar ratio of ANI to (NH4)2S2O8 was fixed at 1:1. Following a similar process, HA@PANI hybrid nanotubes with 204% apparent weight proportion for ANI and HA and with HCl, H2SO4, HNO3, and H3PO4 as dopant acid tuning the pH at 0.5 were fabricated; these were labeled, respectively, as HP/ 0.5/204%-H2SO4, HP/0.5/204%-HNO3, and HP/0.5/204%-H3PO4. Accordingly, HA@PANI hybrid nanotubes with 204% apparent weight proportion for ANI and HA and with H2SO4, HNO3, and H3PO4 as dopant acid tuning the pH at 1.5 were, respectively, denoted as HP/ 1.5/204%-HCl, HP/1.5/204%-H2SO4, HP/1.5/204%-HNO3, and HP/1.5/204%-H3PO4. For facilitating comparison, PANI with HCl tuning the pH at 1.5 and 0.5 (PANI/1.5-HCl, PANI/0.5-HCl) and HA treated by HCl at pH 0.5 were also fabricated. 2.3. Characterization. TEM was performed on a JEM-2100 at 200 kV accelerating voltage. FT-IR measurement was conducted on a Thermo Nicolet 8700 with 128 scans and 2 cm−1 spectral resolution. UV−vis diffuse reflectance spectra were conducted on a Shimadzu UV2550PC with wavelength of 200−800 nm. XPS characterization was conducted on a PHI5000 Versaprobe-II electron spectrometer with 50 W Al Kα radiation. The C 1s line at 284.8 eV was used to calibrate the 6031

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ACS Applied Materials & Interfaces binding energies. Impedance analyzer (Agilent 4294) was used to determine electrical conductivity with 40−110 MHz testing frequency. 2.4. Cr(VI) Oxyanion Adsorption by the HA@PANI Hybrid Nanotubes. Cr(VI) oxyanion adsorption test was conducted on a series of magnetically stirred quartz tube containing 50 mL of 2−60 mg/L parent Cr(VI) oxyanion solution (without acid or alkali tuning the pH, pH 5.07) and 25 mg of HA@PANI hybrid nanotubes (HP/A/ B−C) at room temperature in the dark. After periodic intervals, a certain amount of suspensions was taken out and centrifuged at 3000 rpm to remove the adsorbent. Cr(VI) oxyanion concentration in filtrates was determined according to the method of GB7466-87.32 The detailed determined step is as follows: first, 5 mL of filtrate was withdrawn into a colorimeter tube and diluted with deionized water to 50 mL; then 0.5 mL of H2SO4 solution (1:1), 0.5 mL of H3PO4 solution (1:1), and 2 mL of 1,5-diphenyl carbazide solution were added to the colorimeter tube; third, the mixture was shaken well and stood for 10 min (the color of the filtrates became purple-violet); finally, the absorbance of the purple-violet filtrates at 540 nm was recorded by a UV1800PC photometer for calculating the Cr(VI) oxyanion concentration in filtrates. 2.5. Regeneration of HA@PANI Hybrid Nanotubes. HA@ PANI was used to adsorb 50 mL of 20 mg/L Cr(VI) oxyanion solution for 1 h, and then was separated by centrifugation and regenerated by 50 mL of 0.23 M HNO3 solution.47 After 40 min, the HA@PANI was centrifuged and washed with deionized water to pH 6.57. The Cr concentration [Cr(III)/Cr(VI) oxyanion] in HNO3 solution was analyzed by an Agilent 7700X ICP-MS. The regenerated HA@PANI was again applied to adsorb 50 mL of 20 mg/L Cr(VI) oxyanion solution. The regeneration process was carried out four times. The removal ratio of the regenerated HA@PANI for Cr(VI) oxyanion was analyzed each time.

Figure 1. TEM images of the HA@PANI hybrid nanotubes fabricated at pH 1.5 with 204% apparent weight proportion for ANI and HA (HP/1.5/204%-C): the dopant acid and pH tuning agent in the synthesis process are (a) HCl, (b) H2SO4, (c) HNO3, and (d) H3PO4, respectively.

the surface of HA@PANI became rougher (Figure 1). When the pH is decreased to 0.5, a morphology similar to that fabricated at pH 1.5 is obtained (Figure 2). It is obvious that

3. RESULTS AND DISCUSSION 3.1. Characterization of HA@PANI Hybrid Nanotubes. A naturally occurring halloysite clay mineral is chosen as the support for fabricating HA@PANI hybrid nanotubes. It can be seen from SEM that the halloysite clay mineral is mainly composed of cylindrical tubes. The diameter of the cylindrical tube is 30−70 nm, and the length is 0.5−1.5 μm. TEM demonstrates the cavity structure, and the cavity diameter is 20−30 nm (see Figure S1). The spacing of the individual layer is 0.73 ± 0.02 nm, indicating the halloysite mineral is dehydrated halloysite (see Figure S2). Furthermore, the XRD pattern of halloysite does not match that of the standard calcite (JCPDS 05-0586), indicating there is no calcite in the halloysite. EDX and XPS demonstrate that halloysite is composed of 17.6 at. % Al, 17.5 at. % Si, and 64.9 at. % O (see Figures S3 and S4). The above results imply the halloysite clay mineral is a halloysite-rich tubular mineral. In our previous work, it has been proved that the negatively charged outer surface of the halloysite nanotubes when the pH is below 2 can induce PANI and inorganic TiO2 nanocrystals to in situ grow onto the halloysite (HA) nanotubes in different inorganic acids with FeCl3 or (NH4)2S2O8 as oxidant at low temperature.23,24 In this work, HA@PANI hybrid nanotubes are fabricated at room temperature under HCl, H2SO4, HNO3, and H3PO4 solution with (NH4)2S2O8 as oxidant. By tuning the pH of the reaction system, PANI oxidation state can be controlled. Also, the adsorption properties of HA@PANI hybrid nanotubes can be altered by changing the dopant acid in the synthesis process. The HA@PANI hybrid nanotubes fabricated under various dopant acids with 204% apparent weight proportion for ANI and HA (HP/A/204%-C) are studied first. When the initial pH of the reaction system is 1.5, the outer surface of the HA@PANI hybrid nanotubes is rough, which is marked as HP/1.5/204%-C (Figure 1). As compared to HA,

Figure 2. TEM images of the HA@PANI hybrid nanotubes fabricated at pH 0.5 with 204% apparent weight proportion for ANI and HA (HP/0.5/204%-C): the dopant acid and pH tuning agent in the synthesis process are (a) HCl, (b) H2SO4, (c) HNO3, and (d) H3PO4, respectively.

the morphologies of HA@PANI hybrid nanotubes fabricated under various dopant acids are alike and there is no PANI in the bulk, illustrating PANI can be induced to grow on the HA in situ. It can be explained by the in situ polymerization mechanism of ANI on solid substrates:37,48 at the initial polymerization stage, oligomer species are formed and prone to be adsorbed onto the surface of HA; the adsorbed oligomers act as nucleation sites for inducing the growth of PANI chains. Therein, once a PANI chain is grown on the surface, it will boost new PANI chains to grow perpendicularly to the axis of the original HA. 6032

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3A). Also, the quinoid unit to benzenoid unit ratio (Q/B) (meaning the oxidation state of PAN) in HP/0.5/204%-HCl is lower than that in HP/1.5/204%-HCl.18,19 Besides, as the peaks of 1143 (1113) cm−1 and 1240 (1240) cm−1 are, respectively, due to the doped quinonoid unit and benzenoid unit, thus by calculating the sum peak intensity of 1143 (1113) cm−1, 1240 (1240) cm−1 to those of 1581 (1553) cm−1, 1492 (1490) cm−1, the doping degree of PANI can be roughly determined. On the basis of the FT-IR data, the calculated doping degree of PANI in HA@PANI fabricated at pH 0.5 is higher than that in HA@ PANI fabricated at 1.5, signifying higher electrical conductivity in HP/0.5/204%-HCl. HA@PANI with H2SO4, HNO3, and H3PO4 adjusting the pH and as dopant acid in the synthesis process presents characteristic peaks similar to those of HP/A/ 204%-HCl fabricated under the same pH conditions (Figure 3c−e). However, there are still some obvious differences among the HA@PANI fabricated under various dopant acids: the doping degree of PANI in the HA@PANI nanotubes fabricated at pH 1.5 follows the order of HP/1.5/204%-HCl > HP/1.5/ 204%-HNO3 > HP/1.5/204%-H2SO4 > HP/1.5/204%-H3PO4, whereas that fabricated at pH 0.5 presents the order as HP/0.5/ 204%-HCl > HP/0.5/204%-H2SO4 > HP/0.5/204%-HNO3 > HP/0.5/204%-H3PO4. Particularly worth mentioning is that the doping degree of PANI in HP/A/204%-C calculated based on the FT-IR is the relative value, not the real or absolute value. To obtain the real value of the doping degree, HP/0.5/204%-C fabricated under various acids is further characterized by XPS. XPS provides a truly unique tool in the quantitative analysis of the various intrinsic redox states of PANI and its derivatives (including the doping degree).18 The N 1s high-resolution spectra of HP/0.5/ 204%-C are given in Figure 4. It shows three distinct peaks at

Figure 3A and B shows, respectively, the FT-IR spectra of HP/1.5/204%-C and HP/0.5/204%-C. In addition to the

Figure 3. FT-IR spectra of (A) HP/1.5/204%-C and (B) HP/0.5/ 204%-C: (a) HA; the dopant acid and pH tuning agent in the synthesis process are (b) HCl, (c) H2SO4, (d) HNO3, and (e) H3PO4, respectively.

characteristic peaks of HA (line a, Figure 3), new peaks ascribed to PANI appeared in HP/A/204%-HCl. The assignment of the new peaks is below: the peak of 3412 (3422) cm−1 is ascribed to N−H stretching;49 the peaks of 1581 (1553) and 1492 (1490) cm−1 correspond to CC stretching vibration in quinoid unit and CC stretching vibration in benzenoid unit, respectively; the peaks of 1240 (1240) cm−1 and 1300 (1295) cm−1 are, respectively, ascribed to C−N+ and C−N stretching vibration in benzenoid units; and the peak of 1143 (1113) cm−1 is assigned to N+− stretching vibration in the doped quinonoid unit.18,19,23,24,36 The values inside and outside of the bracket are, respectively, characteristic peaks of PANI fabricated at pH 0.5 and pH 1.5. PANI characteristic peaks of 1300 (1295), 1492 (1490), and 1581 (1553) cm−1 imply the leucoemeraldine and pernigraniline forms of PANI coexist in HP/1.5/204%-HCl and HP/0.5/204%-HCl. Furthermore, the characteristic peaks of PANI (1600−1100 cm−1) in HP/0.5/ 204%-HCl (line b, Figure 3B) are shifted to lower wavenumber when compared to those of HP/1.5/204%-HCl (line b, Figure

Figure 4. N 1s high-resolution XPS spectra of HP/0.5/204%-C: (a) HA; the dopant acid and pH tuning agent in the synthesis process are (b) HCl, (c) H2SO4, (d) HNO3, and (e) H3PO4, respectively.

398.6/399.0/398.7/398.9, 399.6/399.9/399.7/399.8, and 400.6/401.2/400.8/401.2 eV, respectively, corresponding to undoped imine (N−), undoped amine (−NH), and doped amine/imine group (−NH+/N+−) (Figure 4). So by comparing the peak area of −NH+ and N+− to the sum peak area of N−, −NH, and −NH+/N+−, the real doping degree of PANI in HP/0.5/204%-C can be calculated. The result shows the doping degrees of PANI in HP/0.5/204%HCl, HP/0.5/204%-H2SO4, HP/0.5/204%-HNO3, and HP/ 6033

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ACS Applied Materials & Interfaces 0.5/204%-H3PO4 are 33.61%, 28.97%, 27.97%, and 25.45%, respectively, which decrease in the order of HP/0.5/204%-HCl, HP/0.5/204%-H2SO4, HP/0.5/204%-HNO3, and HP/0.5/ 204%-H3PO4 (see Table S1). The tendency is in well accordance with the doping degree of PANI calculated on the basis of the FT-IR. So although the doping degree of PANI in HP/A/204%-C by analyzing FT-IR spectra is the relative value, it still can be used to evaluate the general trend of the doping degree of PANI in HP/A/204%-C. The above results demonstrate that, although the supported PANIs on the HA fabricated at the same pH have similar backbones, they differ obviously in specific molecular structures including oxidation state, degree of protonation, and electron delocalization extent, which are expected to influence their adsorption performance. To demonstrate the tunable PANI amount in HA@PANI by changing the apparent weight proportions for ANI and HA in the synthesis process, TEM and FT-IR of HP/0.5/B-HCl and HP/1.5/B-HCl (HA@PANI nanotubes fabricated at various apparent weight proportions for ANI and HA with HCl tuning the pH at 0.5 and 1.5) are given in Figures S5−7. The TEM shows that when the ANI/HA apparent weight proportion is 2.04%, HA@PANI bears a strong morphology resemblance to HA (see Figures S5a and S6a). When the ANI/HA apparent weight proportion is increased to 40.8% and 204%, a thin PANI layer is covered on HA (see Figures S5b,c and S6b,c). From the FT-IR, the increase of the PANI characteristic peaks intensity with the apparent weight proportion for ANI and HA increasing also illustrates the tunable PANI amount in HA@ PANI by changing the apparent weight proportion for ANI and HA in the synthesis process. The characteristic peaks of HA (3694, 3620 cm−1, 912 cm−1, 536 and 468 cm−1)24,43,45 also can be observed in HP/1.5/B−C and HP/0.5/B−C, indicating the structure of HA is not affected by the growing of PANI. UV−vis diffuse reflectance spectra are further used to study the protonated state of PANI in HA@PANI. Figure 5A and B shows the UV−vis diffuse reflectance spectra of HP/1.5/204%C and HP/0.5/204%-C, respectively. It shows HP/A/204%-C absorbs UV and visible light to result in two peaks around 350 and 420 nm, and a wide peak (460−800 nm), which are assigned to the π−π* transition in benzenod unit, π−polaron transition in benzenod unit, and polaron−π* transitions in quinoid unit, respectively,18,19,24 further confirming the leucoemeraldine and pernigraniline forms of PANI coexist in HP/A/204%-C. In addition, the beginning of the wide peak in HP/0.5/204%-C is red-shifted (dashed line, Figure 5B) when compared to that of HP/1.5/204%-C (dashed line, Figure 5A). Also, the equal peak intensity ratio of 460−800 to 350 nm, illustrating the PANI in HP/0.5/204%-C, is the emeraldine salt form.50 Even fabricated under the same pH conditions, the beginning of the wide peak and the peak intensity ratio of 460− 800 to 350 nm changes as well according to the dopant acid used in the synthesis process, which is in accordance with the results of FT-IR. The above data indicate that the HA@PANI with various dopant acids adjusting the pH in the synthesis process has varied oxidation state. The chemical composition of HP/A/204%-C is further confirmed by EDX. It can be observed from Table S1 that the mass ratio of PANI in HP/A/204%-C is around 35%, which is far lower than the feeding weight proportion for ANI and HA (204%). If the feeding ANI has been completely grown on HA, the mass ratio of PANI in HP/A/204%-C should be 67.10%. From the TEM of HP/A/204%-C (Figures 1 and 2), it does show there is no PANI in the bulk. The possible reason for the

Figure 5. UV−vis diffuse reflectance spectra of (A) HP/1.5/204%-C and (B) HP/0.5/204%-C: (a) HA; the dopant acid and pH tuning agent in the synthesis process are (b) HCl, (c) H2SO4, (d) HNO3, and (e) H3PO4, respectively.

lower PANI content than 67.10% is as follows: first, ANI is used directly without further purification. The directly used ANI was left at room temperature for 7 years without adding any antioxidants, and the color is brown yellow, indicating ANI has been partly oxidized and the purity of ANI is not 100%, so the real feeding weight proportion for ANI and HA is lower than 204%, which will result in the mass ratio of PANI in HP/ A/204%-C lower than 67.10%. Second, the fabrication of HP/ A/204%-C is conducted at room temperature (20 °C). It will cause a large number of oligomers. The oligomer can be removed in the following centrifugation and washing process. The later experiment demonstrates the PANI content in HP/ 0.5/204%-HCl fabricated with the distilled ANI is 34.79% (see Table S1), which is close to that in HP/0.5/204%-HCl fabricated with undistilled ANI, indicating that, although the parent ANI is unpurified, it has little impact on the PANI mass ratio in HP/A/204%-C. So the parent ANI (brown yellow) is still used to fabricate HA@PANI hybrid nanotubes. The above result indicates the lower mass ratio of PANI in HP/A/204%-C than 67.10% is mainly due to the removal of oligomer in the centrifugation and washing process. Meanwhile, the electrical conductivity of HP/A/204%-C decreases with the pH increasing in the synthesis process. Also, HP/0.5/204%-HCl shows the highest conductivity, followed by HP/0.5/204%6034

DOI: 10.1021/acsami.6b14079 ACS Appl. Mater. Interfaces 2017, 9, 6030−6043

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ACS Applied Materials & Interfaces H2SO4, HP/0.5/204%-HNO3, HP/0.5/204%-H3PO4, HP/1.5/ 204%-HCl, HP/1.5/204%-HNO3, HP/1.5/204%-H2SO4, and HP/1.5/204%-H3PO4 with the lowest conductivity, which is in keeping with the doping degree order of PANI in HA@PANI obtained by FT-IR. XRD shows the PANI in HA@PANI fabricated under various dopant acids and H2O is amorphous, not crystalline (see Figure S2). The above results illustrate that by adjusting the dopant acid in the synthesis process, HA@ PANI nanotubes with controllable conductivity are achieved. 3.2. Cr(VI) Oxyanion Adsorption by the HA@PANI Hybrid Nanotubes. To illustrate the influence of dopant acid on the adsorption properties of HA@PANI hybrid nanotubes, the HA@PANI hybrid nanotubes with HCl, H2SO4, HNO3, and H3PO4 as dopant acid adjusting the pH in the synthesis process are used to adsorb Cr(VI) oxyanion. Furthermore, parent halloysite nanotube is used as a comparative adsorbent to study the adsorption properties of the HA@PANI hybrid nanotubes. R (Cr(VI) removal ratio, %) and qt (adsorption capacity, mg/g) of HA@PANI hybrid nanotubes are determined on the basis of the following equations: R% =

qt =

C0 − Ct × 100% C0

(C0 − Ct )V m

Figure 6. Removal ratio of HP/A/204%-C and HA treated by various dopant acids at pH 0.5 (HA/0.5-C) for 20 mg/L Cr(VI) oxyanion solution.

C using various dopant acids to tune the pH in the synthesis process is different. HP/0.5/204%-HCl has the maximum removal ratio among all of the HP/A/204%-C adsorbents. The possible reason is the high doping degree of PANI in HP/0.5/ 204%-HCl, whereas the removal ratio of HP/1.5/204%-HCl is lower than that of HP/0.5/204%-HCl at the same apparent weight proportion for ANI and HA, implying the removal ratio will be influenced by the pH value in the synthesis process, which correlated well with the results of PANI 1D nanostructures fabricated in HCl, citric acid, surfulamic acid, taurine, and neutral media.25 The order of the removal ratio for the HA@PANI hybrid nanotubes is HP/0.5/204%-HCl > HP/ 0.5/204%-H2SO4 > HP/0.5/204%-H3PO4 > HP/0.5/204%HNO3 > HP/1.5/204%-HCl > HP/1.5/204%-HNO3 > HP/ 1.5/204%-H2SO4 > HP/1.5/204%-H3PO4 > HA. Obviously, the dopant acid in the synthesis process will exert an impact on the removal ratio of HP/A/204%-C. For HP/1.5/204%-C, the removal ratio follows this sequence: HP/1.5/204%-HCl > HP/ 1.5/204%-HNO3 > HP/1.5/204%-H2SO4 > HP/1.5/204%H3PO4, which is in accordance with the oxidation state and doping degree of PANI obtained by FT-IR. At pH 0.5, HP/0.5/ 204%-HCl shows the maximum removal ratio, followed in a descending order as HP/0.5/204%-H2SO4, HP/0.5/204%H3PO4, and HP/0.5/204%-HNO3, which corresponds to the doping degree of PANI, except for the sample HP/0.5/204%H3PO4. The reason for this phenomenon may be explained by the higher utilization efficiency of doped PANI in HP/0.5/ 204%-H3PO4 than that in HP/0.5/204%-HNO3 for Cr(VI) oxyanion adsorption. It is worthwhile to note that both HP/ 0.5/204%-HCl samples fabricated with distilled ANI or undistilled ANI are fairly similar in adsorption performance (95% removal ratio for 20 mg/L Cr(VI) solution) (see Figure S9). So considering the similar PANI content/Cr(VI) adsorption performances in HP/0.5/204%-HCl fabricated by the distilled ANI or undistilled ANI and the practical application, all of the HA@PANI hybrid nanotubes are still fabricated with the parent ANI and used for Cr(VI) removal. As shown in Figure S8, Figure 6, and Figure S10, the Cr(VI) oxyanion adsorption equilibrium of HA@PANI can be achieved within 30−60 min with an initial Cr(VI) concentration of 10, 20, and 40 mg/L, which is quicker than that of the active carbon (6 h), polyacrylonitrile/polyaniline core/shell nanofibers,51 and magnetic mesoporous carbon incorporated with

(1)

(2)

wherein C0 (mg/L), Ct (mg/L), V (L), and m (g), respectively, represent the Cr(VI) oxyanion concentration in solution before adsorption, the Cr(VI) oxyanion concentration in solution after adsorption, Cr(VI) oxyanion solution volume, and the dry mass of the HA@PANI. The removal ratio of the HA@PANI hybrid nanotubes fabricated under various dopant acids with 204% apparent weight proportion for PANI and HA (HP/A/204%-C) and HA are first investigated at lower concentrations of Cr(VI) oxyanion solution (10 mg/L) (see Figure S8). From Figure S8, it can be observed that Cr(VI) oxyanion removal by the parent HA is not obvious. Whereas with HP/A/204%-C as adsorbent, Cr(VI) oxyanion removal is striking, especially with HP/0.5/204%-C as adsorbent, the removal ratio is almost 100% within 60 min, indicating the introduction of PANI can enhance the adsorption capacity of HA. Although the contribution of HA to the Cr(VI) removal can be ignored, the role of HA in this reaction system focuses on two aspects: the first is to induce ANI to in situ chemical polymerization on the surface of HA. In this case, HA acts as the role of template and inducer. Due to the nanostructure and template of HA, the growing PANI is nanostructure, which will increase the surface area of PANI to achieve enhanced adsorption properties of HA@PANI when compared to the bulk PANI. The second one acted as a support for ANI to facilitate the separation of HA@ PANI after adsorption of Cr(VI). To distinguish the adsorption capacity of HP/A/204%-C, higher concentrations of Cr(VI) oxyanion solution are used, for example, 20 mg/L (Figure 6, Figure S9) and 40 mg/L (see Figure S10). For comparison, the removal ratio of HA, HA@PANI fabricated in the H2O without adding acid to tune pH (HP/6.57/204%-H2O), and HA treated by various dopant acid at pH 0.5 without adding ANI and (NH4)2S2O8 (HA/0.5-C) for 20 mg/L Cr(VI) oxyanion have also been presented in Figure 6. It shows that the Cr(VI) oxyanion removal ratio of HA and HA/0.5-C is nearly 0%. HP/ 6.57/204%-H2O has the lowest removal ratio among all of the HA@PANI samples. The adsorption capacity of HP/A/204%6035

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attraction mechanism between Cr(VI) oxyanions (HCrO4−, Cr2O72−, and CrO42−) and nitrogen-containing groups on PANI and Cr(VI) oxyanion reduction mechanism.25 The decrease of removal ratio significantly under alkaline Cr(VI) solution may result from the following three reasons: first, the mainly existing form of Cr(VI) oxyanion is CrO42−, which has lower redox potential; second, under alkaline Cr(VI) solution, it will lead to the dedoping of doped imine/amine groups; and third, the competitive adsorption of OH− with Cr(VI) oxyanions exists.25,30 Although the optimal pH of Cr(VI) oxyanion solution is 0, at such Cr(VI) solution acidity, the adsorbed and reduced Cr(VI) (Cr(III)) (which will be discussed later) will be desorbed and re-entered into the acidic solution (see Figure S13A). When the Cr(VI) adsorption is conducted on the parent Cr(VI) solution (pH 5.07), Cr(VI) and Cr(III) concentrations in the adsorption solution can be regarded unchanged, and the Cr(III) concentration in the adsorption solution during the adsorption process is considerably reduced from that conducted in pH 0 Cr(VI) solution, indicating the amount of the re-entered Cr(III) into the adsorption solution is little (Figure S13B). So considering the practical application and little desorption of the adsorbed/ reduced Cr(VI) (Cr(III)) into the adsorption solution, Cr(VI) oxyanion removal by HA@PANI hybrid nanotubes is still conducted on the parent pH of Cr(VI) oxyanion solution at 5.07. The impact of Cr(VI) oxyanion concentration on the removal ratio of HP/0.5/204%-HCl is also explored (see Figure S14). It demonstrates the removal ratio can achieve 100% with Cr(VI) concentration below 10 mg/L at the parent pH of Cr(VI) oxyanion solution (pH 5.07). The removal ratio then decreases with the Cr(VI) oxyanion concentration increasing due to the limited active adsorption sites of HA@PANI. However, 55.3% Cr(VI) oxyanion removal ratio can still be achieved even for 60 mg/L Cr(VI) oxyanion solution. As discussed above, the acidity in the synthesis process of HP/A/204%-HCl will affect its adsorption efficiency for Cr(VI) oxyanion. To find the optimal acidity, we prepare a series of HP/A/204%-HCl under different concentrations of HCl. The adsorption result is shown in Figure 7. It is demonstrable that when the acidity is between 0.0316 (pH 1.5) and 1 M, the removal ratio increases with the acidity increasing, and then when the acidity is between 1 and 2 M, the removal ratio decreases with the HCl concentration increasing. An optimum

polyaniline (3 h),28 but slower than that of magnetic nanopolyaniline composite (5 min).52 This can be understandable if considering 70 wt % PANI content in magnetic nanopolyaniline composite and 30−38 wt % PANI content in HA@PANI (see Table S1). It is interesting that 94.4, 88.4, and 54.9% Cr(VI) can be, respectively, removed by 0.5 g/L HP/ 0.5/204%-HCl within 10 min under the above Cr(VI) concentrations, demonstrating the HA@PANI is an effective adsorbent and can remove Cr(VI) oxyanion quickly. The removal efficiency is still higher than that of PANI/CF;30 even the dosage of HP/0.5/204%-HCl is lower than that of PANI/ CF (2.75 g/L), and the initial concentration and pH of Cr(VI) oxyanion solution are higher than those used in PANI/CF (24 mg/L Cr(VI) oxyanion solution, pH 1.0). Furthermore, the adsorption capacities of the HA@PANI hybrid nanotube fabricated at various apparent weight proportion for ANI and HA with HCl as dopant acid and pH tuning agent in the synthesis process (HP/A/B-HCl) for the Cr(VI) adsorption in the dark are also investigated. From Figure S11, it is obvious the adsorption capacity of HP/1.5/BHCl is lower than that of HP/0.5/B-HCl at the same apparent weight proportions for ANI and HA, which is in agreement with the adsorption result of HP/A/204%-C (Figure 6). When the apparent weight proportion for ANI and HA is lower than 204%, the removal ratio of HP/A/B-HCl increases with the apparent weight proportion for ANI and HA increasing, whereas when the apparent weight proportion for ANI and HA is higher than 204%, the removal ratio of HP/A/B-HCl decreases with the apparent weight proportion for ANI and HA increasing. An optimum of the ANI/HA apparent weight ratio is found at 204%. We attribute this to a synergy of PANI content and PANI dispersion on the HA. A similar result is also reported when PANI/carbon fiber fabrics is used for Cr(VI) oxyanion adsorption.30 Combined with the FT-IR result of the increase of PANI characteristic peaks with the apparent weight proportion for ANI and HA increasing (see Figure S7), the above result indicates the adsorption performance of HP/A/B− C will be affected by the PANI content in HP/A/B−C. On the basis of the above results, it can be concluded that the pH value, dopant acid, and the apparent weight proportion for ANI and HA in the synthesis process will affect the adsorption efficiency of HP/A/B−C, and the highest adsorption efficiency is achieved with HP/0.5/204%-HCl as adsorbent. Therefore, HP/0.5/204%-HCl is selected for further studies. As the Cr(VI) removal efficiency will be affected by the pH of Cr(VI) solution, in the following, Cr(VI) removal efficiency of HP/0.5/204%-HCl for 30 mg/L Cr(VI) solution with different pH values is further investigated, and the HP/0.5/ 204%-HCl dose is 0.5 g/L (see Figure S12A). The pH of Cr(VI) solution is tuned by 2 M HCl and 2 M NaOH solution. It can be seen from Figure S12A that Cr(VI) can be completely removed when the Cr(VI) solution pH is 0; however, the removal ratio decreases with the Cr(VI) solution pH increasing when the Cr(VI) solution is acidic. This phenomenon can also be observed when PANI/carbon fiber fabrics (55 mg) are used as Cr(VI) adsorbent.30 When the Cr(VI) oxyanion solution is basic, the removal ratio decreases significantly with the Cr(VI) solution pH increasing; for example, only 9.69% Cr(VI) oxyanion can be removed at pH 11.9. The Cr(VI) removal ratio of HP/1.5/204%-HNO3 for 20 mg/L Cr(VI) solution with different pH values also shows the same trend (see Figure S12B). The impact of Cr(VI) solution pH value on Cr(VI) oxyanion removal can be explained by the electrostatic

Figure 7. Removal ratio of HP/A/204%-HCl fabricated at different acidities for removal of 30 mg/L Cr(VI) oxyanion. 6036

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ACS Applied Materials & Interfaces Table 1. Cr(VI) Oxyanion Adsorption Kinetics by HP/A/204%-C, PANI/A-HCl, and PANI/CF

pseudo first-order equation

pseudo second-order equation adsorbent HP/0.5/204%-HCl HP/1.5/204%-HCl HP/0.5/204%-H2SO4 HP/1.5/204%-H2SO4 HP/0.5/204%-HNO3 HP/1.5/204%-HNO3 HP/0.5/204%-H3PO4 HP/1.5/204%-H3PO4 PANI/0.5-HCl PANI/1.5-HCl PANI/CF30 (pH 1.0) a

k2 (g/mg min)

R2

k1 (min−1)

R2

0.0033 (0.0014)a 0.0120 (0.0042)a 0.0040 (0.0014)a 0.0150 (0.0012)a 0.0086 (0.0031)a 0.008 (0.003)a 0.0035 (0.0013)a 0.0029 (0.0010)a 0.0018 0.0034 0.06

0.9998 0.9998 0.9995 0.9999 0.9999 0.9996 0.9995 0.9991 0.9993 0.9999 0.9973

0.0392 0.0502 0.0412 0.0257 0.0485 0.0452 0.0392 0.0288 0.0354 0.0408 0.057

0.9353 0.8957 0.9315 0.9269 0.9029 0.9096 0.9382 0.9573 0.9349 0.9271 0.939

qe (mg/g) 62.1 44.4 61.0 35.5 53.4 42.3 59.7 30.3 82.6 69.3 11.0

(173)a (128)a (173)a (108)a (147)a (111)a (165)a (83.9)a

Calculated according to the PANI content in HP/A/204%-C.

Table 2. Cr(VI) Oxyanion Adsorption Isotherm Data by HP/A/204%-C and PANI/CF Langmuir equation

Freundlich equation

adsorbent

qm (mg/g)

b (L/mg)

R2

K (L/g)

n

R2

HP/0.5/204%-HCl HP/1.5/204%-HCl HP/0.5/204%-H2SO4 HP/1.5/204%-H2SO4 HP/0.5/204%-HNO3 HP/1.5/204%-HNO3 HP/0.5/204%-H3PO4 HP/1.5/204%-H3PO4 PANI/CF30 (pH 1.0)

62.9 44.9 61.4 36.6 54.2 43.1 60.9 31.2 18.1

8.9 3.0 6.7 0.7 4.6 2.5 4.5 0.7 0.873

0.9999 0.9997 0.9992 0.9838 0.9999 0.9997 0.9998 0.9891 0.9890

42.8 30.1 46.1 26.3 35.1 27.6 38.7 21.6 7.24

5.2 6.5 7.0 21.1 5.4 6.2 4.8 13.6 1.08

0.9941 0.9861 0.8858 0.8305 0.97116 0.9875 0.9834 0.8427 0.8691

HCl concentration in the synthesis process is 1 M. For contrast, the adsorption efficiency of the PANI fabricated with the same HCl concentration is also included in Figure 7. It is still obvious the PANI fabricated in 1 M HCl solution (PANI/1 M-HCl) shows the maximum adsorption efficiency. The result coincides with the adsorption efficiency trend of HP/A/204%-HCl. Besides, what is worth mentioning is the adsorption efficiency of HP/A/B−C can be controlled by the type of dopant acid in the process of redoping. To prove this, comparable acidity of 1 M protonic acid (1 M HCl, 1 M HNO3, 0.5 M H2SO4, and 0.33 M H3PO4) solution is used to redope HP/0.5/204%-HCl and HP/1.5/204%-HCl. The redoped adsorbent is marked as HP/ A/204%-HCl-D. D represents dopant acid’s concentration. For instance, HP/0.5/204%-HCl-1 M HCl represents HP/0.5/ 204%-HCl redoped by 1 M HCl solution, whereas HP/0.5/ 204%-HCl-0.5 M H2SO4 stands for HP/0.5/204%-HCl redoped by 0.5 M H2SO4. It is demonstrable that, after redoping, the adsorption efficiency of HP/A/204%-HCl is changed, although the acidity of doping solution is roughly the same (see Figure S15), indicating the controllable adsorption properties of HA@PANI by adjusting the dopant acid in the process of redoping. 3.3. Adsorption Kinetics and Adsorption Isotherms. Figure 6, Figure S8, and Figure S10 indicate the Cr(VI) oxyanion adsorption rate of HP/A/204%-C is rapid for the initial 10 min, then gradually slows after 10 min. To better clarify the adsorption behavior, Cr(VI) oxyanion adsorption kinetics of HA@PANI are further evaluated by the following adsorption kinetic equations. Pseudo first-order equation:

log(qe − qt ) = log qe −

k1 t 2.303

(3)

Pseudo second-order equation: t 1 t = + 2 qt qe k 2qe

(4)

wherein qt and qe are, respectively, the Cr(VI) oxyanion adsorption capacity (mg/g) at time t and equilibrium; and k1 (min−1) and k2 (g/mg−1 min−1) are the adsorption rate constant. It is obvious that R2 (correlation coefficient) fitted with a pseudo second-order kinetic equation exceeds 0.999 (Table 1), indicating adsorption is governed by chemical adsorption.53 It is in well agreement with the result of adsorbed Cr(VI) oxyanion reduction by PANI/carbon nanofiber, PANI/ kapok fiber, and PANI/polystyrene fibers.30,32,51 It shows HP/ 0.5/204%-HCl has the highest equilibrium adsorption capacity (qe 62.1 mg/g), followed in the descending order by HP/0.5/ 204%-H2SO4 (61.0 mg/g), HP/0.5/204%-H3PO4 (59.7 mg/g), HP/0.5/204%-HNO3 (53.4 mg/g), HP/1.5/204%-HCl (44.4 mg/g), HP/1.5/204%-HNO3 (42.3 mg/g), HP/1.5/204%H2SO4 (35.5 mg/g), and HP/1.5/204%-H3PO4 (30.3 mg/g). It is noteworthy that the qe calculated on the basis of PANI content in HP/A/204%-C (the bracket value of line 2,3 in Table 1) is twice that of the pure PANI (line 10,11 in Table 1), and the adsorption rate constant for HA@PANI is greater than that for the bulk PANI fabricated with the same pH and dopant acid, meaning its higher adsorption capacity and adsorption rate. It is because of the hollow structure of HA and the homogeneous dispersion of PANI on HA, as observed from the TEM images (Figures 1 and 2). 6037

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ACS Applied Materials & Interfaces It is obvious in Figure S14 that the initial Cr(VI) oxyanion concentration will exert a significant impact on the Cr(VI) oxyanion adsorption for HA@PANI. For example, the adsorption capacity of HP/0.5/204%-HCl is, respectively, 4 and 62.1 mg/g for a initial Cr(VI) oxyanion concentration of 2 and 60 mg/L. In this work, Cr(VI) oxyanion adsorption efficiency of HA@PANI is also characterized by Langmuir (indicating adsorbent’s surface homogeneity) and Freundlich (hinting adsorbent’s surface heterogeneity) isotherms. Langmuir isotherm:

qe =

qmbCe 1 + bCe

(5)

Freundlich isotherm: qe = KCe1/ n

(6)

wherein qm and qe are, respectively, the maximum adsorption capacity and equilibrium adsorption capacity of HA@PANI; Ce is the Cr(VI) oxyanion concentration in the adsorption solution when adsorption has achieve equilibrium; and b, K, and n are model constants. The HA@PANI are applied to adsorb the parent Cr(VI) oxyanion solution (pH 5.07) with concentration in the range of 10.0−60.0 mg/L. The adsorption isotherm data are concluded in Table 2. It is signifying that R2 fitted with the Langmuir equation exceeds 0.98 (Table 2), indicating PANI is homogeneously distributed on HA. The determined maximum Cr(VI) oxyanion adsorption capacity for HP/0.5/204%-HCl is 62.9 mg/g, greater than that of PANI/CF (18.1 mg/g).30 It is also worth noting that as the negligible Cr(VI) oxyanion adsorption capacity of HA, the maximum adsorption capacity of HP/0.5/204%-HCl determined according to the mass ratio of PANI in HP/0.5/204%-HCl is up to 175.0 mg/g. 3.4. Regeneration of HA@PANI. Recycling and regeneration of the adsorbent are very important for wastewater treatment. To elaborate HA@PANI’s regeneration ability, HP/ 0.5/204%-C is chosen for the adsorption cycles, and the adsorption time of each cycle is 1 h. After each cycle, the adsorbent is separated and immersed in 50 mL of 0.23 M HNO3 solution at room temperature for 40 min. The HP/0.5/ 204%-C is then centrifuged and washed with deionized water several times to a pH of 6.57. The regenerated HP/0.5/204%-C is applied to remove 50 mL of 20 mg/L Cr(VI) oxyanion solution again. Most noteworthy is that a number of regeneration agents, such as deionized water, NH3·H2O, NaOH, HCl, and HNO3, are used to regenerate and recover the HA@PANI, and the adsorption efficiency of the HP/0.5/ 204%-HCl after regeneration for Cr(VI) occurred in the order: 1 M HCl (99.10%) > 0.23 M HNO3 (96.81%) > 0.23 M HCl (71.96%) > 0.23 M NaOH (44.96%) > H2O (32.53%) > 0.3 M NH3·H2O (5.86%). So considering the lower concentration and the high efficiency of 0.23 M HNO3, 0.23 M HNO3 is selected as the regeneration agent in the regeneration process. It presents the removal ratio of HP/0.5/204%-C deteriorates with the increase of regeneration cycles (Figure 8). However, the removal ratio of HP/0.5/204%-HCl still can reach 82.3% after four regeneration cycles, which demonstrates that HA@PANI can be recycled and regenerated effectively. Furthermore, all of the HP/0.5/204%-C samples exhibit good adsorption properties in the first three cycles, and there is a sharp decrease in removal efficiency in the fourth cycle, especially for HP/0.5/ 204%-H 2 SO 4 , HP/0.5/204%-HNO3 , and HP/0.5/204%-

Figure 8. Effect of adsorption cycles on Cr(VI) oxyanion removal ratio over the regenerated HP/0.5/204%-C.

H3PO4. The reason is discussed below. First, after adsorption, the HP/0.5/204%-C is treated with 0.23 M HNO3. At such acidity, the PANI and Al species may undergo dissolution from the HP/0.5/204%-C. To exclude this possibility and prove HP/ 0.5/204%-C is stable in the Cr(VI) oxyanion adsorption or regeneration process, the PANI content and Al/Si at. % of HP/ 0.5/204%-HCl after two cycles and four cycles are, respectively, measured by element analysis and EDX (see Table S2). It is demonstrable that the PANI content of HP/0.5/204%-HCl after two cycles and four cycles is, respectively, 35.31% and 35.20%, whereas Al/Si at. % is, respectively, 116% and 115%. Considering the experimental error, the loss of PANI and Al species of HP/0.5/204%-C in the regeneration process can be negligible. Second, a 20 mg/L Cr(VI) oxyanion solution is used to investigate the regeneration performances. At such a low Cr(VI) oxyanion concentration, the HP/0.5/204%-C may be unsaturated and could adsorb more Cr(VI) oxyanion in the first three cycles. To exclude this possibility, we randomly selected HP/0.5/204%-HCl and HP/0.5/204%-HNO3 to conduct the regeneration experiment at higher Cr(VI) oxyanion concentration, for example, 40 mg/L (Figure S16). HP/0.5/204%-HCl demonstrates an 82.6% removal ratio in the first cycle, indicating that after the first cycle, the HP/0.5/204%-C is saturated. The removal ratio trend along with the number of cycles is still similar to that shown for 20 mg/L Cr(VI) oxyanion solution, indicating the good adsorption properties of HP/0.5/204%-C in the second and third cycles have not resulted from the nonsaturation of PANI on HP/0.5/204%-C after the first cycle. To further explore the reason, the Cr concentrations (Cr(VI) oxyanion, Cr(III)) in the desorption solutions (0.23 M HNO3) are analyzed by ICP. The result is provided in Table S2. It is clear the Cr concentration in the desorption solution is lower than that on the HP/0.5/204%-C; for example, after the first cycle, the Cr concentration in the desorption solutions for HP/0.5/204%-HCl is 19.43 mg/L, while the removal ratio of HP/0.5/204%-HCl for 40 mg/L Cr(VI) oxyanion is 82.6%. If the Cr on the HP/0.5/204%-HCl has been completely desorbed, the Cr concentrations in desorption solution should be 33.04 mg/L; that is to say, 41.2% Cr has not been desorbed by the treatment of 0.23 M HNO3. The Cr concentration in the desorption solution of HP/0.5/204%-HNO3 also demonstrates a similar trend. The 6038

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oxyanion adsorption sites. Note that HP/0.5/204%-HCl, which presents the highest doping degree of PANI, shows the highest qm for Cr(VI) oxyanion removal, indicating the best adsorption properties of HP/0.5/204%-HCl may result from the highest doping degree of PANI in HP/0.5/204%-HCl. After Cr(VI) oxyanion adsorption, the peak of 1143 (1113) cm−1 almost disappears and shifts to 1173 cm−1 (CN stretching of quinoid unit) in HP/1.5/204%-C, while in HP/0.5/204%-C, this peak still exists, but its intensity greatly decreases. This implies that the different adsorption capacities of HP/A/204%C for Cr(VI) oxyanion adsorption corresponded to the different doped PANI contents and utilization of doped PANI. The higher utilization of −NH+− and N+− in HP/ 0.5/204%-HCl probably resulted from the small sizes of Cl− counteranion, which facilitates the anion exchange between Cr(VI) oxyanions and Cl− counteranion.28 Third, the Q/B ratio of HA@PANI increases after Cr(VI) oxyanion adsorption. For example, the Q/B ratio of HP/0.5/204%-HCl and HP/0.5/ 204%-HNO3 increases from 0.72 and 0.79 to 1.02 and 1.01, respectively, suggesting the benzenoid amines in HA@PANI have been partly oxidized to the pernigraniline form accompanied by Cr(VI) oxyanion adsorption. Moreover, XPS spectra of HP/0.5/204%-HCl after Cr(VI) oxyanion adsorption are further considered and compared to that before Cr(VI) oxyanion adsorption to explore the Cr(VI) oxyanion adsorption mechanism (Figure 10, Table S3). It demonstrates that the fabricated HP/0.5/204%-HCl is mainly composed of 1.61% Si, 1.86% Al, 16.57% O, 0.54% Cl, 73.05% C, and 6.37% N (atom %, lines a in Figure 10A, Table S3). An element such as Cr is not detected by XPS (lines a in Figure 10B). After immerging the HP/0.5/204%-HCl into 30 mg/L Cr(VI) oxyanion solutions for 1 h, the Cl content is greatly decreased from 0.54 to 0.25 atom % (Table S3). The decrease of Cl primarily resulted from the anion exchange between Cr(VI) oxyanion anions and Cl−. Moreover, the peak of Cr appeared, whereas the N and Cl contents decreased. The highresolution spectrum of Cr 2p (Figure 10B) shows that it deconvolutes to two major peaks of 577.1 and 586.9 eV, suggesting the adsorbed Cr(VI) oxyanion has been reduced to Cr(III) in the adsorption process.54 N 1s high-resolution spectra show that before Cr(VI) oxyanion adsorption, N 1s spectra present three peaks of 398.6, 399.6, and 400.6 eV, respectively, assigned to N−, −NH, and −NH+/N+− (line a in Figure 10C). After Cr(VI) oxyanion adsorption, the total N content decreases from 6.37 to 5.80 atom %. Besides, doped amine/imine (−NH+/N+−) proportion sharply decreases from 33.61% to 10.91%, undoped amine (−NH) decreases from 49.23% to 47.94%, and correspondingly undoped imine (=N−) increased from 17.16% to 41.15% (Figure 10C), which further demonstrates the redox reaction of −NH in HA@PANI and Cr(VI) oxyanion occurs. Analyses of N 1s spectra indicate the Cr(VI) oxyanion adsorption process associates closely with the nitrogen groups, especially the doped amine/imine group (−NH+/−N+), in concert with deprotonation of doped PANI. The C 1s of the HP/0.5/204%-HCl after and before interaction with Cr(VI) oxyanion is shown in Figure 10D. C 1s XPS spectra of the HP/0.5/204%-HCl are fitted into three peaks of 284.1, 285.4, and 287.1 eV, which are respectively assigned to the C−H/C−C, C−N/CN, and C−O/CO.55 As can be seen, there are no demonstrable differences for the C 1s spectra of HP/0.5/204%-HCl before (line a in Figure 10D) and after adsorption (line b in Figure 10D), indicating that the

above results imply a sharp decrease in removal efficiency in the fourth cycle, especially for HP/0.5/204%-H2SO4, HP/0.5/ 204%-HNO3, and HP/0.5/204%-H3PO4, which may result from the uncompleted desorption of Cr on the HP/0.5/204%C by treating with 0.23 M HNO3. In addition, the HA@PANI adsorbent has better sedimentation ability than that of bulk PANI: the HA@PANI hybrid nanotubes can settle within 4 min from the parent aqueous Cr(VI) oxyanion solution, whereas the bulk PANI fails to settle within 1 h. So the regeneration capacity and easy separation of HA@PANI hybrid nanotube indicate its potential application in the treatment of Cr(VI) oxyanion containing industrial wastewater. 3.5. Cr(VI) Oxyanion Adsorption Mechanism of HA@ PANI. To further follow through with the Cr(VI) oxyanion adsorption mechanism of HA@PANI, FT-IR and XPS spectra of HA@PANI after Cr(VI) oxyanion adsorption are investigated and compared to those before Cr(VI) oxyanion adsorption. As demonstrated in Figure 9 and Figure S17, the

Figure 9. FT-IR spectra of HP/A/204%-C fabricated at pH 1.5 (A) and 0.5 (B) after adsorption 30 ppm of Cr(VI) oxyanion solution: (a) HP/A/204%-HCl, (b) HP/A/204%-H2SO4, (c) HP/A/204%-HNO3, and (d) HP/A/204%-H3PO4.

FT-IR spectra of HA@PANI experience fundamental changes after Cr(VI) oxyanion adsorption. First, most of the PANI characteristic peaks shift to higher wavenumber, implying a stronger interaction between PANI and adsorbed Cr species. Second, the peak intensity of 1240 (1240) cm−1 is decreased in all of the HA@PANI hybrid nanotubes. The peak of 1143 (1113) cm−1 became lower in intensity, indicating that doped −NH+− and N+− in HA@PANI are the main Cr(VI) 6039

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ACS Applied Materials & Interfaces

Figure 10. XPS spectra of HP/0.5/204%-HCl before Cr(VI) oxyanion adsorption (a) and after Cr(VI) oxyanion adsorption (b): (A) XPS spectra and the corresponding high-resolution spectra of (B) Cr 2p, (C) N 1s, and (D) C 1s.

carbon in HP/0.5/204%-HCl is not related to Cr(VI) oxyanion adsorption. Experimental data on Cr(III) formation are further proved by chemical oxidation and ICP. The concentration of Cr(III) in adsorption solution is calculated by the deviation of Cr(VI) oxyanion concentration after and before oxidizing the solution by APS at 100 °C for 1 h based on the method of GB7466-87. The Cr(III)/Cr(VI) concentrations on HA@PANI are determined by the following method: after adsorption of 20 mg/L Cr(VI) oxyanion solution, 50 mL of 0.23 M HNO3 solution is applied to desorb the adsorbed Cr on HA@PANI. The concentration of Cr(VI) oxyanion in the desorption solution is determined by the method of GB7466-87. The concentration of Cr specials in the desorption solution is determined by ICP. Figure S18 demonstrates Cr(III)/Cr(VI) oxyanion concentrations on HP/0.5/204%-HCl and in the adsorption solution after adsorbing 50 mL of 20 mg/L Cr(VI) oxyanion over time ranging from 0 to 60 min. It shows that the Cr(VI) oxyanion concentration in the adsorption solution decreases with the adsorption time increasing, and 1−2 mg/L Cr(III) is detected in the adsorption solution (see Figure S18A). On the other hand, Cr(VI) oxyanion is not detected in

the desorption solution, and the concentration of Cr(III) in the desorption solution is high, accounting for 40−80% of the initial Cr(VI) oxyanion adsorption solution concentration (20 mg/L) (see Figure S18B). It is worth mentioning the total concentration of Cr(III) and Cr(VI) oxyanion in the desorption solution and the adsorption solution is lower than the initial concentration of Cr(VI) oxyanion in the adsorption solution (20 mg/L). As the Cr(III)/Cr(VI) concentrations on HA@PANI are determined by desorbing the adsorbed Cr from HA@PANI, the above results indicate there are still partial Cr species on HA@PANI after desorption. Although we are not sure whether the remaining Cr species on HA@PANI after desorption is Cr(III) or Cr(VI) oxyanion, in combination with the XPS results (Figure 10B) and the absence of Cr(VI) oxyanion in the desorbed solution, it can be concluded the adsorbed Cr(VI) oxyanion on HA@PANI has been totally reduced to Cr(III). According to the above analysis, the main adsorption mechanism between HA@PANI and Cr(VI) oxyanion is proposed as follows. First, when dispersing HA@PANI into Cr(VI) oxyanion solutions, the electrostatic interaction between doped amine/imine groups in HA@PANI and Cr(VI) 6040

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ACS Applied Materials & Interfaces oxyanions bring about Cr(VI) oxyanion adsorption on the surface of HA@PANI. As discussed above, the oxidation state and doping degree of PANI in HA@PANI will be affected by the dopant acids in the synthesis process; thus the dopant acids in the synthesis process will influence Cr(VI) oxyanion adsorption. Because the main adsorption sites and mechanism are doped amine/imine groups and electrostatic interaction, respectively, HP/0.5/204%-HCl demonstrates the highest adsorption capacity. The influence of Cr(VI) oxyanion solution pH on Cr(VI) oxyanion removal is the result of the competitive adsorption between OH− and Cr(VI) oxyanions and dedoping of −NH+/−N+ groups under alkaline condition. In a word, any factors (PANI fabrication condition, dopant acid concentration, or a variety in the redoping process) that influence the doping degree of PANI in HA@PANI would exert an impact on the Cr(VI) oxyanion adsorption. Second, due to the strong oxidability of Cr(VI) oxyanion, the adsorbed Cr(VI) oxyanion will react with leucoemeraldine (reduction state PANI) in HA@PANI. Consequently, a higher oxidation state PANI in HA@PANI and Cr(III) is achieved.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 871 65031567. E-mail: [email protected].

4. CONCLUSIONS A convenient method for fabricating HA@PANI hybrid nanotubes by the in situ chemical polymerization of aniline on halloysite clay nanotubes is proposed. By facilely adjusting the dopant acid and pH value in the synthesis process, HA@ PANI hybrid nanotubes composed of different PANI oxidation states and doping degrees can be produced. The HA@PANI fabricated with HCl as dopant acid tuning the pH at 0.5 and 204% apparent weight proportion for ANI and HA (HP/0.5/ 204%-HCl) presents the highest doping degree. The HA@ PANI hybrid nanotubes are used as adsorbent for Cr(VI) oxyanion removal in an aqueous solution. It demonstrates HA@PANI hybrid nanotubes (25 mg) can effectively remove 50 mL of 20 mg/L Cr(VI) oxyanion solution in 10 min. Also, the adsorption properties will be affected by the dopant acid, pH, and the apparent weight proportion for ANI and HA in the synthesis process, and the maximal adsorption efficiency is reached with HP/0.5/204%-HCl as adsorbent. This can be attributed to the highest doping degree of PANI. Furthermore, preparing a series of HP/A/204%-HCl under different concentrations of HCl and redoping HP/0.5/204%-HCl by H2SO4, HNO3, and H3PO4 demonstrate that an optimum HCl concentration in the synthesis process is 1 M, and the adsorption efficiency decreases when redoped by H2SO4, HNO3, and H3PO4, indicating the adsorption capacity of HA@PANI can be controlled by the concentration/type of dopant acid in the process of synthesis or redoping. The adsorption process is in well agreement with pseudo secondorder and Langmuir equations. The maximum adsorption capacity with HP/0.5/204%-HCl as adsorbent is 62.9 (175) mg/L. According to the FT-IR and XPS results, an adsorption mechanism for Cr(VI) oxyanion by HA@PANI about the electrostatic interaction between doped amine/imine groups in HA@PANI and Cr(VI) oxyanion and reduction of the adsorbed Cr(VI) oxyanion simultaneously is proposed.

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Data of HA; XRD of HA@PANI; TEM of HP/A/BHCl; FT-IR of HP/A/B-HCl before and after adsorption; mass ratio, doped degree of PANI, and conductivity of HP/A/B−C; the removal ratio of HP/A/ 204%-C and HP/A/B-HCl after and before being redoped; influence of the initial Cr(VI) oxyanion solution pH and concentration on removal efficiency; PANI content (wt %), Al/Si (at) of the regenerated HP/ 0.5/204%-HCl, and the Cr(III)/Cr(VI) oxyanion concentration in the desorption solutions over the regenerated HP/0.5/204%-HCl and HP/0.5/204%HNO3; the regeneration of HP/0.5/204%-HCl and HP/0.5/204%-HNO3; chemical composition of HP/ 0.5/204%-HCl after and before Cr(VI) oxyanion adsorption; and Cr(III)/Cr(VI) concentration in the adsorption solution and on HP/0.5/204%-C after adsorption (PDF)

ORCID

Cuiping Li: 0000-0002-2452-7320 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (nos. 51563023, 51003091) and the Natural Science Foundation of Yunnan Province (no. 2013FB002) for financial support.

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