Research Article pubs.acs.org/journal/ascecg
Extracellular Polymeric Substances Induced Porous Polyaniline for Enhanced Cr(VI) Removal from Wastewater Qian Hu,† Chenxi Guo,† Dezhi Sun,† Yong Ma,‡ Bin Qiu,*,† and Zhanhu Guo*,‡ †
Beijing Key Laboratory for Source Control Technology of Water Pollution, College of Environmental Science and Engineering, Beijing Forestry University, 35 Qinghua East Road, Haidian District, Beijing, 100083 China ‡ Department of Chemical and Biomolecular Engineering, University of Tennessee, 1512 Middle Drive, Knoxville, Tennessee 37996 United States S Supporting Information *
ABSTRACT: Extracellular polymeric substances (EPS) of bacteria were used as templates for synthesizing unique polyaniline nanocomposites, i.e., porous EPS-modified polyaniline (EPS@PANI). The proteins were responsible for forming porous structure, while polysaccharides for the fiber morphology of EPS@PANI. The specific surface area (53.2 m2/g) of these unique EPS@PANI with an optimal EPS loading of 2 wt % was ∼2 times larger than that of pristine PANI. The PANI in EPS@PANI stayed as the emeraldine form and acted as the electron donor for reduction of Cr(VI) to Cr(III). Herein, 1.0 mg/L Cr(VI) was completely reduced to Cr(III) by 600 mg/L of EPS@PANI within 10 min, which was much faster than the pristine PANI (1 h). A maximum Cr(VI) removal capacity of 913.2 mg/g was achieved by these unique EPS@PANI nanocomposites and was ∼4.7 times higher than the pristine PANI (193.8 mg/g). Moreover, the isoelectric point (pI) was decreased from pH 7.5 for pure PANI to ∼4.5 for these porous EPS@PANI nanocomposites due to the low pI of polysaccharides remained in the composites. This lowered pI facilitated further Cr(III) removal on the surface of EPS@PANI from the wastewater. KEYWORDS: Porous polyaniline, Extracellular polymeric substances, Template, Cr(VI) removal
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INTRODUCTION Cr(VI) has been classified as one of the most toxic substances due to its high toxicity and mobility in the aqueous environment.1 Recently, reducing Cr(VI) anions to nontoxic and immobile Cr(III) ions has been demonstrated to be an effective approach for detoxication of the Cr(VI) containing wastewater.2−5 Among various electron donors used for this reduction, polynialine (PANI), a conductive polymer, has attracted more attention due to its advantages of easy synthesis, stable chemical structure, low cost and good environment stability.6 PANI has three oxidation states, i.e., pernigraniline, emeraldine, and leucoemeraldine forms.7,8 The pernigraniline and emeraldine PANI contains abundant amine groups, which can easily give electrons for Cr(VI) reduction.9−11 However, PANI has a poor porosity, which leads to low Cr(VI) removal capacity.12 Moreover, PANI has an isoelectric point (pI) as high as 10 and is positively charged when the pH is lower than ∼10.13 This facilitates Cr(VI) anion attraction from wastewater. However, Cr(III) existing in the form of Cr3+ ion makes it © 2017 American Chemical Society
difficult to further remove due to the electrostatic repulsion. Therefore, improving the porosity and decreasing the pI of the PANI is urgently needed. To improve the porosity of PANI, templates were commonly used due to the good control of structure. The templates extensively used include two main categories, i.e., organic copolymers,14−16 and inorganic particles,17−20 polystyrene nanoparticle14,21,22 and block copolymer15 were commonly used as organic templates. After polymerization, the copolymer composites were further immersed in solvents, such as toluene14 and chloroform15 to remove the organic copolymers templates and form porous structure. Moreover, some soluble organics, such as Acid Red 8,20 were also employed for synthesizing porous PANI. Meanwhile, various inorganic nanoparticles such as SiO223 and zeolites24 were used as Received: October 4, 2017 Revised: October 24, 2017 Published: October 30, 2017 11788
DOI: 10.1021/acssuschemeng.7b03564 ACS Sustainable Chem. Eng. 2017, 5, 11788−11796
Research Article
ACS Sustainable Chemistry & Engineering
In this study, EPS of bacteria were used as templates for synthesizing porous PANI nanocomposites. The EPS, excreted by the microbial population, is nontoxic and safe to the environment. Moreover, no additional chemicals were used for synthesizing the porous PANI fiber. All of the above indicated that EPS is a green template for synthesis the porous PANI. The effects of EPS loading on the morphology and the porous property of PANI as well as on the redox state and the isoelectric point were investigated. Meanwhile, the obtained porous PANI was used for Cr(VI) removal from the wastewater. The Cr(VI) removal rate and capacity were determined by the kinetics and thermodynamic studies. The mechanism involved in the Cr(VI) removal was also disclosed.
templates for enhancing the porosity of PANI composite as well. After polymerization, the templates are also normally removed to obtain the micron/nanoporous structures. This post-treatment also results in disorder or destruction of the micro-/nanoporous structures. Other inorganics such as Cu2O25 and MnO217−19 were used for the synthesis of conductive polymers because no further removal of templates was needed. For example, MnO2 can be used as an oxidant to oxidize the monomer of conductive polymers because of its high redox potential in an acidic environment. Meanwhile, MnO2 nanoparticles will be dissolved to Mn4+ in the acidic reaction solution, which will be reduced to Mn2+ and removed completely from the matrices of PANI by deionized water through centrifugation. The porosity of the PANI was significantly improved by using these templates. For example, 44.24 m2/g surface area and 0.3673 cm3/g pore volume was obtained for the porous PANI fiber synthesized by using the MnO2 as the template.17 As discussed above, in order to get the porous structure of the PANI, the acid had to be used to further remove the inorganic template after polymerization,25 which will generate wastewater containing heavy metals and may make secondary pollution. Therefore, a suitable template is required, which will allow the monomer to polymerize around its surface and will not cause pollution during the synthesis. In order to decrease the pI of the PANI, it has been modified by various substrates with low pI, such as sawdust,26 fibers,27 cellulose,6 and carbon materials.28 However, porous structure cannot be obtained by this modification. The extracellular polymeric substances (EPS), as a biomaterial, are excreted by the microbial population. EPS mainly consist of spherical proteins and linear polysaccharides,29 which contain abundant −NH2 and −OH groups. These groups can act as active sites for growing the PANI chains, forming EPS−PANI composite. More hydrophilic groups were exposed in the outer of the protein, which makes the protein contain a large amount of water and form a hydrophilic sphere. However, the proteins can be easily denatured when heated and dried.30 Then, more hydrophobic groups were exposed in the outer of the protein, leading to the dewater and shrivel of the protein sphere. This may facilitate to form porous structures in EPS−PANI composite. The EPS accounts for 20−50 wt % of the cell weight.31,32 As documented, more than 6.25 million tons of excess sludge was generated from the urban wastewater treatment plant in China every year,33 which was an abundant source of EPS. Moreover, EPS of the excess sludge has been reported to be easily obtained by chemical methods (such as the cation exchange resin, EDTA, and the NaOH methods) and physical methods (such as heating, sonication, and centrifugation methods).34,35 Therefore, EPS of the microbial population was considered a good candidate of the template for synthesis of porous PANI. However, its application as a template has not yet been reported. When the organic copolymers14,15 and inorganic colloidal particles17−20,25 were used as templates for synthesizing the porous PANI, the templates were dissolved and removed after synthesis. This lead to a high pI of the synthesized porous PANI, which resulted to the electrostatic repulsion to the positive charged Cr(III) ions. The pI of polysaccharides is as low as ∼3.36 Therefore, the zeta potential of PANI−EPS was envisioned to be decreased significantly. Therefore, both porous structure and low pI of PANI composites were supposed to be achieved simultaneously by using EPS as the template and thus to improve the further removal of Cr(III) ions.
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MATERIALS AND METHODS
Materials. Aniline (C 6H7N), ammonium persulfate (APS, (NH4)2S2O8), and p-toluene sulfonic acid (PTSA, C7H8O3S) used for PANI synthesis were purchased from Sigma-Aldrich company. Acetone, potassium dichromate (K2Cr2O7), H3PO4 (85 wt %), and 1,5-diphenylcarbazide were purchased from Sigma-Aldrich as well. All the chemicals were used as-received without any further purification. EPS was lab-extracted from the excess sludge form the urban wastewater treatment plant by a heating−high-speed centrifugation method.32 The proteins and polysaccharides was determined as the main composition of EPS (Figure S1), and the concentrations of proteins and polysaccharides were 492 and 325 mg/L, respectively. After extraction, EPS solution was stored at 4 °C and was used directly for synthesis of EPS@PANI composites without any purification. Preparation of EPS@PANI Nanocomposites. Porous structured EPS@PANI composites were fabricated by a surface polymerization method. Briefly, 1.6 mL of aniline was added into the desired amount of EPS solution (20, 40, and 80 mL) and was stirred at 300 rpm for 30 min in an ice-water bath. Then, 8.22 g of APS and 2.58 g of PTSA was mixed in 50 mL of water, getting a solution with a APS and PTSA concentrations of 36.0 and 15.0 mM. Then, this solution was added dropwise to EPS solution. The aniline was polymerized in the icewater bath until the product was deep dark green. The product was vacuum filtered and washed by acetone to remove any oligomers, then further washed with deionized water until the pH of filtrate was ∼7. The final EPS@PANI composites were dried at 80 °C overnight for further usage. Cr(VI) Removal Experiments. The effects of pH, reaction time, and initial concentration on Cr(VI) removal performance of EPS@ PANI composites were investigated in this study. The effect of initial pH on the Cr(VI) removal performance was investigated with the pH range from 1.0 to 11.0 by using 10.0 mg of EPS@PANI to treat 25.0 mL of Cr(VI) solutions (1.0 mg/L) for 24 h. The initial pH of Cr(VI) solutions was adjusted by NaOH (1.0 M) and H2SO4 (1.0 M). The effect of initial Cr(VI) concentration on the Cr(VI) removal was investigated by using EPS@PANI (10.0 mg) to treat 25.0 mL of Cr(VI) solutions with an initial Cr(VI) concentration varying from 1.0 to 1500 mg/L for 30 min. To investigate the effect of anions on the Cr(VI) removal performance, 0.5 mM Cl−, NO3−, and SO42− were assessed in the 1.0 mg/L Cr(VI) solutions (25.0 mL), and 10 mg of EPS@PANI was added to treat this Cr(VI) solution. All the experiments for Cr(VI) removal were done in triplicate. For kinetic study, EPS@PANI was used to treat 20.0 mL of Cr(VI) solution with an initial Cr(VI) concentration of 20.0 mg/L at a pH of 1.0. The Cr(VI) concentration was determined at different time intervals. The Cr(VI) concentration in the solution was determined by the colorimetric method at 540 nm obtained from the UV−vis test.1 The total Cr concentration was detected by using inductively coupled plasma-mass spectrometry (ICP-MS, Agilent 7500a, Agilent Technologies, Palo Alto, CA), and Cr(III) concentration was calculated by the total Cr concentration minus Cr(VI) concentration. Characterizations of EPS@PANI. The morphology and EDS mapping of EPS@PANI were characterized by a scanning electron microscopy (SEM, Hitachi S-4800) and transmission electron 11789
DOI: 10.1021/acssuschemeng.7b03564 ACS Sustainable Chem. Eng. 2017, 5, 11788−11796
Research Article
ACS Sustainable Chemistry & Engineering
morphology of EPS@PANI. The EPS@PANI fibers has ∼500 nm of length and were surrounded by the PANI nanoparticles (Figure 1D). The IV type isotherm (Figure S3A) indicated the mesoporous structure of these synthesized EPS@PANI composites. The calculated BET specific surface area, pore size and pore volume of EPS@PANI with different EPS loadings were listed in Table 1. The biggest surface area of
microscopy (TEM, JEM-2100). Before measurement, the samples were sputter-coated with a thin layer of gold (about 5 nm) to ensure good conductivity. The specific Brunauer−Emmett−Teller (BET) surface area and pore size distribution were measured on a Quanta Chrome Nova 2200e by nitrogen adsorption at 77.4 K. Prior to the measurement, the samples were degassed at 100 °C for 12 h under high vacuum condition (
