Phytic Acid Doped Polyaniline Nanofibers for Enhanced Aqueous

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Research Article pubs.acs.org/journal/ascecg

Phytic Acid Doped Polyaniline Nanofibers for Enhanced Aqueous Copper(II) Adsorption Capability Hyeong Jin Kim,†,‡ Sungjin Im,‡,§ Jong Chan Kim,∥ Won G. Hong,⊥ Koo Shin,‡,§ Hu Young Jeong,∥ and Young Joon Hong*,† †

Department of Nanotechnology and Advanced Materials Engineering, ‡Graphene Research Institute, and §Department of Chemistry, Sejong University, Seoul 05006, Republic of Korea ∥ UNIST Central Research Facilities (UCRF) & School or Materials Science and Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea ⊥ Division of Electron Microscopy Research, Korea Basic Science Institute (KBSI), Daejeon 34133, Republic of Korea S Supporting Information *

ABSTRACT: This study demonstrates the enhanced Cu2+ adsorption capability of polyaniline nanofibers (PAni NFs) by doping of phytic acid. The PAni NFs were synthesized by radical polymerization process using acidic solutions of hydrochloric and phytic acid, yielding chlorinated (Cl-) and phytic acid-doped (Ph-) PAni NFs. The Ph-PAni NFs showed remarkably higher Cu2+-adsorption efficiency than Cl-PAni NFs, presumably owing to high capacity and/or high ionic affinity of the doped phytic acid in Ph-PAni NFs. The pH-dependent adsorption capability exhibited increasing Cu2+ adsorption trend as increasing aqueous pH because of spontaneous deprotonation of the doped phytic acid in a basic environment. Furthermore, Ph-PAni NFs showed stable, high Cu2+ adsorption capability, irrespective of Co2+ concentration in the bimetallic Cu and Co aqueous solution. Surface morphologies of PAni NFs were investigated using electron microscopy, and molecular structures were identified using X-ray photoemission and Fourier transform infrared spectroscopies. The ability of PAni NFs to capture aqueous Cu2+ is discussed in terms of surface functional groups doped to NFs. Surface modification and/or doping to enhance the adsorption capability of Cu(II) introduced in this study will provide a great venue for expanding the use of many other polymeric nanostructures for reclamation in metal mining as well as the conventional environmental applications such as water purification. KEYWORDS: Polyaniline, Doping, Phytic acid, Polymeric nanofiber, Aqueous metal adsorption, Water purification, Cu2+ ion adsorption

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INTRODUCTION Scavenging metal pollutants (or resources) in aquatic environments has been of note in both wastewater purification1 and mineral mining,2−5 which have been usually processed using various methods, such as adsorption,6 membrane filtration,7 electrochemical precipitation,8 biological treatment,9 and so on. Among the diverse methods, adsorption/complexation by ion exchange has gained a great deal of interest for costeffective, energy-saving, and greener chemical technology because of the ability of the technique to reuse the adsorbent and recover the metal adsorbate.10,11 As of late, many types of mesoporous and nanostructured adsorbents (e.g., graphene,12 carbon nanotubes,13,14 zeolites,15,16 polymeric nanofibers (NFs),17−19 organo−silica,20,21 metal−organic frameworks,22 etc.) have been developed for high-efficiency collection of © 2017 American Chemical Society

aqueous metal ions, due to the high surface-to-volume (S/V) ratio. Especially, polymer has a structural advantage of being viable to modify surface chemical properties via doping chemical functional groups to backbone, which enables it to substantially raise the aqueous metal adsorption capability. Also, many polymers are deformable such that they can be structurally designed to form desirable shapes for diverse applications in demand. In addition, considering the significance of recovery of adsorbed metals,23 use of polymeric adsorbate deserves attention because the metals are separated by dissolving or ashing the polymers. Received: March 24, 2017 Revised: June 16, 2017 Published: July 3, 2017 6654

DOI: 10.1021/acssuschemeng.7b00898 ACS Sustainable Chem. Eng. 2017, 5, 6654−6664

Research Article

ACS Sustainable Chemistry & Engineering Polyaniline (PAni) is one of the well-known conducting polymers, having versatile strengths in many aspects, such as simple, inexpensive synthesis method, ease of nanostructure synthesis, good environmental stability, facile doping− dedoping process, tunable electrical properties, and physicochemical properties depending on oxidation states and doping in wide range, and so on.24,25 Recently, methods for fabricating pure, uniform, and template-free PAni NFs with small diameters ( ∼11) conditions, the intensity of absorption peaks was more reduced than that of the control solution. For such cases, CuCl2·2H2O was added to adjust the peak intensities to those of control solution, and the pH was also repeatedly measured using pH meter.19 To remove the salts or precipitates, the solutions were filtered using a PVDF syringe filter and were kept overnight. Then, we once again measured and compared the UV−vis absorption spectra with those of control solution. After confirming that the absorption peak did not change, the 0.1 mM Cu2+ aqueous solutions with desired pH were used for pH-dependent adsorption experiments.

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RESULTS AND DISCUSSION PAni NFs were synthesized by a radical polymerization process using APS as an oxidation agent. As a radical initiator, APS removes one electron from each aniline monomer by forming a radical sulfate, which consequently yields three canonical resonance forms of aniline with a radical cation, as depicted in Figure 1a.39 Among the resonance structures in Figure 1a, structures (i) and (ii) preferably grow to be a chain of PAni (Figure 1b) via head-to-tail coupling reaction, owing to less steric hindrance.40 In the synthesis process, secondary amine group in a chain of PAni possibly provides doping sites (marked with X− in Figure 1c); thus, dopants such as ions, molecules, and functional groups can be incorporated into the amine for surface functionalization. Since we used acidic solution of hydrochloric and phytic acid for PAni NF synthesis, Cl− and phytic acidic molecule are expected to be doped to PAni, which we name Cl- and Ph-PAni (Figure 1b,c), respectively. According to chemical formulas shown in Figure 1c, Ph-PAni has a functional group, which is a phosphoric acid group terminated with oxygen and hydroxyl (−OH), capable of capturing more aqueous metal ions than Cl-PAni; thus, it is expected that the Ph-PAni NFs exhibit greater capacity for collecting aqueous Cu2+ ions than Cl-PAni NFs. It should be noted that pristine PAni was named Cl-PAni in this paper because pristine PAni is typically synthesized in HCl solution using oxidizing agent of APS.41 The left panels of Figure 2a,b are FE-SEM images of the as-synthesized Cl- and Ph-PAni NFs, respectively, exhibiting that both the PAni NFs have a similar morphology of densely networked NFs with a typical diameter of 80−100 nm. Such similar surface morphology of PAni NFs is attributed to unidirectional monomer head-to-tail coupling via radical polymerization in almost the same, low pH condition ( Mg(II) > Co(II) > Ni(II).55 This clearly explains why Ph-PAni NFs preferably adsorb Cu(II) ions rather than Co(II) ions. Hence, our result validates that the Cu(II) removal capability of PAni NFs was not suppressed by adding Co(II) cations in the adsorption experiment batches, which implies that the Ph-PAni can be adopted for selectively capturing Cu from metal wastewater.

First, HPO42− in Ph-PAni NFs has a higher adsorption affinity than that of OH− to attract Cu2+ or H+ in high pH aqueous solution.52 In comparison, pKa (= −log10 Ka, referred as acid dissociation constant) of OH− and HPO42− are known to be 11.8 and 12.67, respectively, signifying that the phosphate ion attracts more Cu2+ than OH− does. Second, overall stability constant (= [ML]/[M][L], where [M], [L], and [ML] are molar concentrations of metal ion, ligand, and metal−ligand complex, respectively) of Cu−phosphate complex is greater than that of Cu(OH)2. According to the literature,53,54 log Kc = 2.14 for Cu−phosphate complex is greater than log β2 = −13.7 for Cu(OH)2(aq), where Kc and β2 are stability constants for single and double bonds, respectively. This indicates that the form of Cu−phosphate complex was stable rather than Cu(OH)2. According to the above two reasons, the precipitates were not thought to be tangibly formed for the adsorption batches. The adsorption experiments were further carried out using Cu(II) and Co(II) coexistent aqueous solution in order to investigate the influence of other aqueous metal ions on the removal capability of Ph-PAni NFs for Cu(II) ions. For the adsorption experiments, we fixed the initial concentration of aqueous Cu2+ ions at 0.1 mM, and varied Co2+ concentration from 0.2 to 1.0 mM for the bimetal aqueous solution. Figure 11 shows the Cu2+ removal percentage of Ph-PAni NFs plotted as a function of Co2+ concentration. Note that the initial Cu2+ concentration (0.1 mM) was several times lower than that of Co2+ (0.2−1.0 mM). In spite of high concentration of coexistent Co(II) ions, the adsorption capability of Ph-PAni for capturing Cu(II) ions was not seriously degraded. This indicates that the adsorption affinity between Cu ions and Ph-PAni was much stronger than that between Co ions and Ph-PAni. The combining affinity, defined as a measure how stably the complex (i.e., bivalent metal−phytic acid) existed in a given environment, is known to be higher for complexes between Cu2+ and ligands (including phosphoric acid group) than that for complexes between Co2+ and ligands. This trend commonly appears between phytic acid and transition metals as well: The combining affinity of metal ions for phytic acid

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CONCLUSION We investigated the adsorption of aqueous Cu2+ using Cl- and Ph-PAni NFs. The Cl- and Ph-PAni NFs with a high S/V ratio were prepared by a radical polymerization process using HCl and phytic acid solution, respectively, under similar low pH conditions. The XPS, EDS, FTIR, and TGA spectroscopic analyses evidenced doping of chlorine and phytic acid for Cl- and Ph-PAni NFs. The Ph-PAni NFs showed markedly improved Cu2+ adsorption capacity, owing to the phosphate functional group with high ionic attraction in doped phytic acid. Furthermore, the Ph-PAni NFs exhibited greater Cu2+ adsorption capability under higher pH conditions, irrespective of additional dedoping treatment, because spontaneous deprotonation of phosphate functional group in basic solution resulted in anionic feature for capturing aqueous Cu2+ cation. More intriguingly, Ph-PAni NFs showed high Cu2+ adsorption capability, regardless of Co2+ concentration in the bimetallic Cu and Co aqueous solution, due to high combining affinity between Cu2+ and phytic acid of Ph-PAni NFs. This study showed the feasibility of (i) how surface functionalization (or doping) increases the metal adsorption capability of polymer nanostructures, (ii) which environment maximizes the adsorption affinity for capturing more metal ions, and (iii) what specific aqueous metal adsorbate is preferentially adsorbed to the adsorbent. We believe that our comprehensive method of improving the adsorption capability and affinity via molecule doping can be applied to other polymeric or organic nanostructures to promote aqueous metal collection efficiency.

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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/acssuschemeng.7b00898. 6662

DOI: 10.1021/acssuschemeng.7b00898 ACS Sustainable Chem. Eng. 2017, 5, 6654−6664

Research Article

ACS Sustainable Chemistry & Engineering

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Surface morphology and Cu(II) adsorption performance of Ph-PAni NFs synthesized under different phytic acid concentration conditions; atomic groups and chemical bonds of Ph-PAni NFs investigated by spectroscopies; Langmuir and Freundlich isotherm fitting of PAni NFs; Cu(II) adsorption capability of Ph-PAni NFs in bimetal Co and Cu aqueous solution (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +82-2-3408-4382. ORCID

Young Joon Hong: 0000-0002-1831-8004 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Education (NRF-2016R1D1A1B03931518). H.J.K. and S.I. acknowledge financial support by the Human Resources Development of the Korea Institute of Energy Technology Evaluation and Planning grant funded by the Korean government Ministry of Trade, Industry & Energy (No. 20164030201340).

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DOI: 10.1021/acssuschemeng.7b00898 ACS Sustainable Chem. Eng. 2017, 5, 6654−6664