Cellulose Fiber Composite

Jul 15, 2012 - Thus, the PPy/CF composite gave very good recyclability, although its ... The present study showed that a polypyrrole/cellulose fiber c...
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Integrated Reductive/Adsorptive Detoxification of Cr(VI)Contaminated Water by Polypyrrole/Cellulose Fiber Composite Yu Lei, Xueren Qian,* Jing Shen, and Xianhui An Key Laboratory of Bio-based Material Science and Technology of Ministry of Education, Northeast Forestry University, Harbin, Heilongjiang Province, 150040, People's Republic of China ABSTRACT: For detoxification of Cr(VI)-contaminated water, a new process concept of using conductive polypyrrole/ cellulose fiber composite (prepared by in situ polymerization of pyrrole in the presence of cellulose fibers) for water treatment was proposed and demonstrated. The effects of preparation conditions of the composite, as well as the water treatment conditions, on the detoxification efficiency were studied. Under the optimized conditions, the composite was highly effective in Cr(VI) detoxification. The desorption results and XPS analyses showed that the highly toxic Cr(VI) was reduced to less toxic Cr(III) and then adsorbed onto the composite. At least 3/4 of the Cr adsorbed to the composite was Cr(III). ATR-FTIR spectra and SEM images also proved that redox reaction occurred during the water treatment process. The integrated reductive/ adsorptive Cr(VI) detoxification by polypyrrole-engineered cellulose fibers would provide new possibilities for the commercial application of conductive fibers.

1. INTRODUCTION Intrinsically conducting polymers containing repeating units of oxidized or reduced monomers have a number of promising applications such as electrostatic dissipation. With the development of polymer science and other related disciplines, the interesting performances/attributes of these polymers are now receiving increasing attention. Poor processability of the intrinsically conducting polymers can act as an inhibitive barrier for their large-scale industrial applications. Fortunately, the engineering of cellulose fibers with these conducting polymers has been demonstrated to be one of the efficient ways that can improve their processability and applicability through versatile formability of the fiber substrates,1−6 which is expected to lead to the development of value-added and performance-tailored functional products with acceptable biodegradability, opening the door for more flexible possibilities. The in situ synthesis of conducting polymers, e.g., polyaniline and polypyrrole, in the presence of cellulose fibers has been reported to be an efficient method for the preparation of conductively engineered cellulose fibers with varied conductivities.1,6 In such a process, the conducting polymers are coated onto the cellulose fiber substrates, forming conductive polymer/cellulose fiber composites. This chemical/conductive modification of cellulose fibers fits exactly into the so-called “fiber engineering”7 concept, which involves any possible strategy for improving the traditional use of cellulose fibers or providing new functions and capabilities. Another interesting and noteworthy fact is that conducting polymers can be used for the reduction of highly toxic hexavalent chromium (Cr(VI)) to less toxic trivalent chromium (Cr(III)), which significantly facilitates the treatment of contaminated water.8,9 After Cr(VI) reduction, the water can be further treated by precipitation or adsorption, so that the water quality can meet the requirements. A possible disadvantage of the commonly used reductants (e.g., sulfur dioxide and sodium metabisulfite) in comparison to conducting polymers is that they cannot be reused at all.9 In addition to the © 2012 American Chemical Society

use of conducting polymers, other treatment methods of Cr(VI)-contaminated water include but are not limited to the use of ion exchange resins,10 reverse osmosis,11 electrochemical deposition,12 solvent extraction, 13 and adsorption. 14−17 Although there are already a very large number of scientific publications on treatment of Cr(VI)-contaminated water, there is still an ongoing need for further explorations, so that more cost-effective and/or environmentally friendly technologies can be developed and practiced. In this study, a process concept of using conducting-polymerengineered cellulose fibers derived from papermaking pulp, i.e., polypyrrole/cellulose fiber (PPy/CF for short) composite, for treatment of Cr(VI)-contaminated water, was proposed and explored. It was hypothesized that the use of conductively engineered fibers may serve two purposes simultaneously: (1) reduction of Cr(VI) to Cr(III); (2) adsorption of Cr(III) onto the fiber matrixes. The negatively charged hydroxyl/carboxyl groups in the cellulose fiber matrixes are expected to be favorable for metal ion adsorption. In some sense, the integration of these two purposes as a result of the use of the engineered fibers may provide obvious advantages over existing technologies for treatment of Cr(VI)-contaminated water. The process conditions for the preparation of engineered fibers as well as the water treatment were optimized, and the effectiveness of the concept was evaluated.

2. EXPERIMENTAL SECTION 2.1. Materials. Pyrrole of reagent grade was freshly distilled under reduced pressure and was then stored in a refrigerator at 4 °C before use. The fully bleached softwood kraft pulp (cellulose fibers) imported from Canada was obtained from Received: Revised: Accepted: Published: 10408

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desorption ratio is the desorbed amount divided by the adsorbed amount. For recycling of PPy/CF composite, 0.25 mol/L H2SO4 solution was used for desorption. 2.6. X-ray Photoelectron Spectroscopy (XPS), Attenuated Total Reflection Fourier Transform Infrared (ATRFTIR), and Scanning Electron Microscopy (SEM) Analyses. The samples for XPS and ATR-FTIR analyses were the handsheets with a grammage of 60 g/m2 made from PPy/CF composites before and after treatment with Cr(VI) solution, and those for SEM observation were PPy/CF composites before and after the treatment. XPS spectra were obtained using a PHI5700 X-ray photoelectron spectroscopy (XPS) system. An Al Kα X-ray source was used. The analyzer was operated at 29.35 eV pass energy for survey spectra. Elemental atomic concentrations were calculated from the XPS peak areas. The calibration of binding energy of the spectra was performed with the C 1s peak of the aliphatic carbons at 285 eV. ATR-FTIR (attenuated total reflection Fourier transform infrared) spectra in the range 600−4000 cm−1 were recorded on an FTIR spectrometer equipped with an InspectIR microscope (MagnaIR 560 ESP, Nicolet Corp.). The crystal used in the ATR cell was Si. The resolution was 4 cm−1, and 40 scans were averaged. SEM observations were performed using an FEI Quanta 200 environmental scanning electron microscope, and the specimens were coated with gold before observation.

Mudanjiang Hengfeng Paper Co. Ltd., People's Republic of China, and its beating degree was 16.5 °SR. All other chemicals, including ferric chloride, sodium hydroxide, hydrochloric acid, sulfuric acid, potassium bichromate, and chromium trichloride hexahydrate, were of analytical grade, and were used as received. 2.2. Preparation of PPy/CF Composite. Six grams of cellulose fibers and 250 mL of distilled water were added to a three-neck round-bottom flask with a 1000 mL capacity. The mixture was stirred at 200 rpm for 5 min. Certain amounts of ferric chloride solution (containing 70 mL of deionized water) and distilled water were then added, and the consistency of fibers was adjusted to 1%. The mixture was restirred at 350 rpm for 10 min. Subsequently, under stirring, a given amount of pyrrole was added using a liquid injector to initiate the in situ polymerization reaction. The reaction was stopped after a given time; the mixture was filtered and then sufficiently washed/ rinsed using distilled water to remove any unreacted chemicals. The amount of PPy coated on cellulose fibers was calculated as PPy coated (%) = ((W2 − W1)/W1) ·100

(1)

where W1 and W2 are oven-dry weights of the fibers before and after PPy deposition, respectively. 2.3. Treatment of Cr(VI)-Contaminated Water. The water treatment experiments were performed in a 250 mL three-necked flask equipped with a mechanical stirrer. A desired amount of PPy/CF composite (oven-dry basis) was added to the flask containing 100 mL of potassium bichromate solution (a model solution for Cr(VI)-contaminated water), and the mixture was kept at room temperature for the desired time under stirring at 350 rpm. After treatment, the aqueous mixture was filtered through a medium-speed filter paper, and the filtrate was then analyzed for residual Cr(VI) and total Cr concentrations. 2.4. Determination of Chromium Concentration. The concentration of Cr(VI) in the filtrate, i.e., the treated water, was determined by spectrophotometry.18,19 A UV−vis spectrophotometer (TU-1901, Beijing Purkinje General Instrument, China) was used for the analyses. The pH of the filtrate was adjusted to about 13 with 1 mol/L NaOH solution, and then the concentration of Cr(VI) (on the basis of chromate) was measured using the UV−vis spectrophotometer at 372 nm. The standard working curve (y = 0.0934x + 0.0094) was obtained in the concentration range 0.2−12 mg/L, with a correlation coefficient of 0.9993. The total Cr concentration of the filtrate was also determined by spectrophotometry. The filtrate was oxidized by ammonium persulfate for 10 min under acidic and boiling conditions, and then the total chromium concentration was determined according to the same procedure as the Cr(VI) determination described above. The Cr(III) concentration was calculated based on the difference between the total Cr and Cr(VI) concentrations. According to the analyses in the current study, the recovery of Cr(III) from chromium trichloride solution was above 99%. 2.5. Desorption Experiments. After water treatment, the PPy/CF composite was treated with 100 mL of 0.25 mol/L NaOH and H2SO4 solutions at room temperature for the desired time under stirring at 350 rpm to remove Cr(VI) and Cr(III), respectively. After desorption, the filtrates were analyzed for Cr(VI) and Cr(III) concentrations, and the desorption ratios of Cr(VI) and Cr(III) were calculated. The

3. RESULTS AND DISCUSSION 3.1. Effects of Preparation Conditions of PPy/CF Composite. 3.1.1. Effects of Pyrrole Concentration and FeCl3/Pyrrole Molar Ratio. In this work, it was found that treatment of 100 mg/L Cr(VI) solution (100 mL) with 1 g of unmodified cellulose fibers only resulted in a very minor change of Cr(VI) concentration, i.e., from 100 to 98.19 mg/L. In the pH range 2−6, Cr(VI) is mainly in the form of Cr2O72− and HCrO4− ions.18,19 As is well-known in the papermaking discipline, natural cellulose fibers (i.e., pulp fibers) are negatively charged mainly due to the presence of carboxyl and hydroxyl groups. Therefore, at about pH 5 (system pH of the unmodified-fiber-containing Cr(VI) solution), electrostatic repulsion can occur between cellulose fibers and Cr(VI), hurdling the adsorption of Cr(VI) on fibers. With the temperature for in situ polymerization, the reaction time for in situ polymerization, the time for water treatment, the initial Cr(VI) concentration of contaminated water, and the dosage of PPy/CF composite fixed at room temperature, 1 h, 1 h, 100 mg/L, and 1 g/100 mL Cr(VI) solution, respectively, the effects of pyrrole concentration and FeCl3/pyrrole molar ratio in the preparation process of the PPy/CF composite on the coated amount of PPy relative to cellulose fibers and the Cr(VI) concentration of the treated water are shown in Figure 1. For Figure 1, the initial pH of the Cr(VI)-contaminated water was 4.8 (original pH without pH adjustment), and the time for treatment of the contaminated water with the PPy/CF composite was 1 h. It should also be noted that the FeCl3/ pyrrole molar ratio for Figure 1a is 2 (mol/mol). It can be seen from Figure 1a that with the increase of pyrrole concentration in the range 0−1.5 g/L, the amount of PPy coated on cellulose fibers increased gradually. For the concentration of the highly toxic Cr(VI) in treated water, it decreased gradually in the pyrrole concentration range 0−1 g/ L, and notably it decreased by almost 100% at a pyrrole concentration of 1 g/L; further increase of the pyrrole concentration had only a negligible effect on the Cr(VI) 10409

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decreased gradually, and it decreased by almost 100% at the FeCl3/pyrrole molar ratio of 2 mol/mol; further increase of the FeCl3/pyrrole molar ratio did not have a significant effect on the Cr(VI) concentration of the treated water. The FeCl3/ pyrrole ratio of 2 mol/mol is close to the theoretical value of 2.33 mol/mol, which is equivalent to converting 1 mol of pyrrole to its polymer form.20,21 3.1.2. Effects of Reaction Time and Temperature. With the pyrrole concentration, the FeCl3/pyrrole molar ratio, the initial Cr(VI) concentration of contaminated water, and the dosage of the PPy/CF composite fixed at 1 g/L, 2 mol/mol, 100 mg/L, and 1 g/100 mL Cr(VI) solution, respectively, the effects of reaction time and temperature during the preparation process of the PPy/CF composite on a coated amount of PPy and the Cr(VI) concentration of the treated water are shown in Figure 2. As shown in Figure 2a, regardless of different temperatures (i.e., in ice bath or at room temperature) for the preparation of the PPy/CF composite, with the increase of reaction time, the

Figure 1. Effect of (a) pyrrole concentration and (b) FeCl3/pyrrole molar ratio on coated amount of PPy on cellulose fibers and Cr(VI) concentration of treated water. Temperature for in situ polymerization, room temperature; reaction time for in situ polymerization, 1 h; time for water treatment, 1 h; initial Cr(VI) concentration of contaminated water, 100 mg/L; dosage of PPy/CF composite, 1 g/100 mL Cr(VI) solution.

concentration. It is clear that, under the conditions studied, the use of the conductive composite efficiently minimized the toxicity of Cr(VI)-contaminated water. Due to the general trend that the Cr(VI) concentration of the treated water decreased significantly with the increase of the amount of coated PPy (on cellulose fibers), it is also evident that PPy can play an important role in lowering the concentration of the Cr(VI). The polymerization of pyrrole can be initiated by using ferric chloride (FeCl3) as an oxidant, and Cl− ions can exhibit a doping effect. As a result, FeCl3 dosage may influence the extent of oxidation and the doping of PPy coated on cellulose fibers. Thus, it is desirable to investigate the effect of FeCl3 dosage on the performance of the PPy/CF composite in terms of lowering the Cr(VI) concentration of the contaminated water. As shown in Figure 1b (pyrrole concentration, 1 g/L), with the increase of FeCl3 dosage (i.e., in the FeCl3/pyrrole molar ratio range 0−3 mol/mol), the coated amount of PPy on cellulose fibers increased gradually. On the other hand, as the FeCl3/pyrrole molar ratio increased in the range 0−2 mol/mol, the concentration of highly toxic Cr(VI) in treated water

Figure 2. Effects of reaction time and temperature on (a) PPy coating and (b) Cr(VI) concentration of treated water. Pyrrole concentration, 1 g/L; FeCl3/pyrrole molar ratio, 2 mol/mol; initial Cr(VI) concentration of contaminated water, 100 mg/L; dosage of PPy/CF composite, 1 g/100 mL Cr(VI) solution. 10410

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coated amount of PPy increased gradually. Compared with the ice bath (around 0 °C), at room temperature (20−25 °C) it took much less time for the coated amount to reach the plateau value. The plateau values of the coated amount around 0 °C and room temperature were generally close to each other, i.e., around 8.5−9%, although at room temperature the polymerization of pyrrole and the coating of the polymer on cellulose fibers was a faster process. As shown in Figure 2b, regardless of the temperatures during the preparation process of the PPy/CF composite, with the increase of the reaction time, the Cr(VI) concentration of the treated water decreased, which is similar to the trend regarding the coated amount of PPy (Figure 2a). When in situ polymerization of pyrrole in the presence of cellulose fibers was conducted in an ice bath, it took 6 h for the decreased percentage of Cr(VI) concentration to reach almost 100%. Interestingly, when in situ polymerization was conducted at room temperature, it took only 0.5 h for Cr(VI) concentration to reach about 0. Thus, at room temperature the as-prepared PPy/CF composite was more effective in terms of Cr(VI) detoxification, i.e., lowering the concentration of the highly toxic Cr(VI). From Figure 2, it can also be seen that the performance of the PPy/CF composite in terms of lowering the concentration of Cr(VI) was not merely dependent upon the coated amount of PPy. The structure and morphology of the coating may also be influencing factors. 3.2. Effects of Process Conditions for Treatment of Cr(VI)-Contaminated Water with PPy/CF Composite. From what has been discussed above, for in situ polymerization of pyrrole in the presence of cellulose fibers, when the pyrrole concentration, the FeCl3/pyrrole ratio, the reaction temperature, and the reaction time were 1 g/L, 2 mol/mol, room temperature, and 1 h, respectively, the PPy/CF composite exhibited excellent performance in terms of lowering the concentration of Cr(VI). With an attempt to optimize the process conditions, the influencing factors of water treatment with the PPy/CF composite were investigated. 3.2.1. Effect of Initial pH of Cr(VI)-Contaminated Water. The chemistry of Cr(VI) in aqueous systems is very pHdependent.22 Consequently, for the treatment of Cr(VI)contaminated water with PPy/CF composite, its initial pH may influence the performance of the composite. Figure 3 shows the effect of initial pH on Cr(VI), Cr(III), and total Cr concentrations of the treated water. Here, the total Cr concentration refers to the concentration of Cr in the form of both Cr(VI) and Cr(III). In addition to Cr(VI) concentration, the evaluation of the concentrations of Cr(III) and total Cr may be useful for better understanding the performance of the PPy/CF composite. At a pH of lower than about 5, treatment of Cr(VI)-contaminated water resulted in a significant decrease of Cr(VI) concentration, i.e., by almost 100%; however, further increase of pH resulted in increased Cr(VI) concentration. In another study, Ansari and Fahim19 found that when PPy/sawdust composite was used for treatment of Cr(VI)-contaminated water, the Cr(VI) concentration of the treated water was not affected by the initial pH of the test solution up to pH 10, which is different from what is reported here. This difference in the effect of pH may be due to the use of different substrates onto which the PPy is coated; for example, the lignin and extractive contents of sawdust are substantially higher than those of the cellulose fibers used in the present study. At a pH of lower than about 3, the Cr(III)

Figure 3. Effect of initial pH of Cr(VI)-contaminated water on Cr(VI), Cr(III), and total Cr concentrations of treated water. Initial Cr(VI) concentration of contaminated water, 100 mg/L; dosage of PPy/CF composite, 1 g/100 mL Cr(VI) solution; time for water treatment, 1 h.

concentration in the treated water increased significantly with the decrease of pH, indicating the chemical conversion of Cr(VI) to Cr(III). The reduction of Cr(VI) to Cr(III) by the PPy/CF composite agrees well with the published works on the direct use of conducting polymers in treatment of Cr(VI)contaminated water.8,9 This reduction reaction can be shown as Cr2O7 2 − + PPy + 14H+ → 2Cr 3 + + PPyox + 7H 2O

(2)

The conversion of Cr(VI) to Cr(III) can be understood by considering the oxidizing power of Cr(VI) to be a function of pH. As the pH decreases, Cr(VI) is more inclined to oxidize PPy. The redox potential of Cr(VI) decreases with the increase of pH, and it is less reactive at relatively high pH. The enhanced Cr(VI) reduction at low pH is due to the high redox potential of Cr(VI) as a result of the presence of a large amount of hydrogen ions in the aqueous system. It should be noted that when 1 g of unmodified cellulose fibers or PPy/CF composite was used for treatment of 100 mL of 100 mg/L Cr(III) solution (chromium trichloride solution), the Cr(III) concentration only decreased by 9.00 or 9.39%, respectively. This indicates that the oxidation of PPy by Cr(VI) can play an decisive role in the adsorption of Cr(III) onto the composite. The reduction of Cr(VI) (due to the presence of PPy/CF composite) to Cr(III) well supports the first part of the integrated reductive/adsorptive hypothesis (see the Introduction). An interesting phenomenon is that, in the pH range ∼3 to ∼5, the total Cr concentration and the Cr(III) concentration of the treated water were all very low, and the contaminated water was substantially detoxified. Further change of pH (increase or decrease) impaired the effectiveness of the PPy/CF composite. The original pH of 100 mg/L Cr(III) solution was tested and was found to be 4.8, which falls well within the pH range of ∼3 to ∼5. 3.2.2. Effect of Time for Treatment of Cr(VI)-Contaminated Water. Figure 4 shows the effect of the time for treatment of Cr(VI)-contaminated water on the Cr(VI) concentration of the treated water. It should be noted that in this work water treatment was conducted without any pH adjustment of the contaminated water, and the original pH of 10411

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Figure 6. Effect of initial Cr(VI) concentration on Cr(VI) concentration of treated water and decreased amount of Cr(VI) relative to the weight of PPy/CF composite (q). Time for water treatment, 1 h; dosage of PPy/CF composite, 1 g/100 mL Cr(VI) solution.

Figure 4. Effect of treatment time on Cr(VI) concentration of treated water. Initial Cr(VI) concentration of contaminated water, 100 mg/L; dosage of PPy/CF composite, 1 g/100 mL Cr(VI) solution.

the Cr(VI) solution was 4.83. Under the conditions studied, with the increase of treatment time, the Cr(VI) concentration decreased relatively slightly. In general, the reductive/ adsorptive detoxification was a fast process. Nevertheless, a relatively long treatment time (e.g., 1 h) resulted in a very low Cr(VI) concentration of the treated water. 3.2.3. Effect of Dosage of PPy/CF Composite. Figure 5 shows the Cr(VI) concentration of the treated water as a

(q) as a function of the initial Cr(VI) concentration of Cr(VI)contaminated water. When 1 g of PPy/CF composite was used for the treatment of 100 mL of Cr(VI) solution, with the increase of initial Cr(VI) concentration in the range 100−700 mg/L, the Cr(VI) concentration of the treated water increased almost linearly. For the decreased amount of Cr(VI) in water relative to the weight of PPy/CF composite, it increased gradually in the initial Cr(VI) concentration range 100−700 mg/L. For the PPy/CF composite prepared under optimized conditions, it was re-collected after completion of the water treatment under the following conditions: treatment time of 1 h; composite dosage of 1 g/100 mL Cr(VI) solution; initial Cr(VI) concentration of 100 mg/L. The re-collected PPy/CF composite or its paper sheet (see Experimental Section) was used in the below-mentioned desorption study, as well as in XPS, ATR-FTIR, and SEM characterization. 3.3. Desorption Study. Table 1 shows the results on the desorption Cr(III) and Cr(VI) from the PPy/CF composite Table 1. Results of Desorption Experiments desorption ratio by 0.25 mol/L NaOH solution

Figure 5. Effect of dosage of PPy/CF composite on Cr(VI) concentration of treated water. Initial Cr(VI) concentration of contaminated water, 100 mg/L; time for water treatment, 1 h.

desorption ratio by 0.25 mol/L H2SO4 solution

desorption time (h)

Cr(VI) (%)

total Cr (%)

Cr(III) (%)

total Cr (%)

1 2 6 12

10.13 11.31 20.00 30.58

9.16 9.21 15.59 20.94

46.41 68.31 72.97 71.26

47.05 69.34 73.94 72.01

with which Cr(VI)-contaminated water was treated. There was almost no difference in desorption ratios between total Cr and Cr(III) when 0.25 mol/L H2SO4 solution was used for desorption of Cr from Cr(VI)-treated PPy/CF composite, indicating that the desorbed Cr almost exclusively was Cr(III). As the desorption time increased, the desorption ratio of Cr(III) increased, but basically was unchanged after 6 h, showing that complete desorption was achieved. However, the desorption ratio of total Cr was actually lower than that of Cr(VI) when 0.25 mol/L NaOH solution was used for desorption of Cr from the re-collected PPy/CF composite;

function of the dosage of PPy/CF composite. For water treatment, various amounts of the composite (in the range 0−1 g) were added to 100 mL of 100 mg/L Cr(VI)-contaminated water followed by filtration. With the increase of the dosage of the PPy/CF composite, the Cr(VI) concentration of the treated water decreased almost linearly. At a dosage of 1 g, the Cr(VI) concentration of the treated water was almost 0. 3.2.4. Effect of Initial Cr(VI) Concentration of Cr(VI)Contaminated Water. Figure 6 shows the Cr(VI) concentration of the treated water as well as the decreased amount of Cr(VI) in water relative to the weight of PPy/CF composite 10412

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and after water treatment are shown in Table 3. The area percentages of the peaks at 288.1 and 533.0 eV (associated with CO and C−OH, respectively) for the composite after water treatment were higher than those for the composite before water treatment, which can be due to oxidation of the PPy/CF composite by Cr(VI). 3.4.2. ATR-FTIR Analyses. As seen in Figure 8, the obvious change in the ATR-FTIR spectrum of the PPy/CF composite after water treatment was the enhancement of intensity at 3600−3000 cm−1, indicating an increase of the free hydroxyl groups on the PPy/CF composite as a result of Cr(VI) reduction. The band around 1700 cm−1 ascribed to CO stretching vibrations became more intense after water treatment. The band around 1600 cm−1 attributed to skeletal CC aromatic vibrations became more intense after water treatment, probably reflecting a dehydrogenation process accompanying the redox process. An increase in intensity of the bands in the range 1300−900 cm−1 possibly ascribed to C−O vibrations (associated with hydroxyl groups and ether type structures) may reflect the increase of carbon−oxygen groups on the surface of the PPy/CF composite, as a result of the oxidation reaction. In general, the ATR-FTIR results agree well with those obtained from XPS analyses (section 3.4.1). 3.4.3. SEM Analyses. As indicated from our scanning electron microscopy (SEM) analyses, the surface morphologies of the PPy/CF composites before and after water treatment should be noted (SEM pictures not shown here). However, the gray levels of the SEM image of the composite after water treatment were different from those of the original composite, possibly indicating the change in surface chemical composition24 of the composite after water treatment. This change can be due to the oxidation of the PPy coating by Cr(VI) and the deposition of Cr(III)-based compounds on the surface of the composite. Other evidence of PPy oxidation is that the conductivity of the composite after water treatment was changed. The surface resistivity of the handsheets with a grammage of 60 g/cm2 made from the original composite was 522 kΩ, whereas the value of the handsheets made from the composite after water treatment was out of the full range of the instrument (1000 kΩ); i.e., there was at least a 2-fold increase after water treatment. 3.4.4. Preliminary Evaluation of Recyclability of PPy/CF Composite. The recyclability of the PPy/CF composite is very important in terms of practical applications of water treatment. When 1 g of the PPy/CF composite was used for the detoxification of 100 mL of 100 mg/L Cr(VI)-contaminated water, the filter cake of the composite was collected by filtration, followed by desorption/recycling using 100 mL of 0.25 mol/L H2SO4 solution. When the recycled composite was used for the treatment of 100 mL of 100 mg/L Cr(VI)contaminated water under the above-mentioned optimized condition, the Cr(VI) and Cr(III) concentrations of the treated water were 16.98 and 2.36 mg/L, respectively. When the double-used composite was recycled and reused for the second time, the Cr(VI) and Cr(III) concentrations of the treated water were 28.01 and 5.35 mg/L, respectively. Thus, the PPy/ CF composite gave very good recyclability, although its performance in terms of detoxification decreased slightly upon reuse. In this regard, the PPy/CF composite is superior to the common reductants, such as sulfur dioxide and sodium metabisulfite. On the other hand, the use of these reductants

the desorption time was longer, and the difference in desorption ratios between total Cr and Cr(VI) was greater, as compared with the use of 0.25 mol/L H2SO4 solution for desorption. This somewhat anomalous experimental result may be explained by the phenomenon that, during the desorption process using NaOH solution, the oxidative degradation products of PPy were dissolved in alkaline solution, leading to a higher measured concentration of Cr(VI). During total Cr determination, the oxidative degradation products of PPy were further degraded to colorless substances by ammonium persulfate; thus the negative effect of the oxidative degradation products of PPy on Cr determination was basically eliminated. Therefore, the results on total Cr determination can be well reflected by desorption of Cr using 0.25 mol/L NaOH solution. Theoretically, the Cr desorbed by NaOH solution should mainly be Cr(VI). It can be concluded from these desorption results that at least 3/4 of the Cr adsorbed on PPy/CF composite was Cr(III), and less than 1/4 of the Cr may be adsorbed through ion exchange between Cr(VI) and Cl−. This also confirmed that the hypothesis of the integrated reductive/ adsorptive removal is self-explanatory. 3.4. XPS, ATR-FTIR, and SEM Characterization. 3.4.1. XPS Analyses. The PPy/CF composites before and after water treatment were characterized using XPS. The calibration of the binding energy of the spectra was performed with the C 1s peak of the aliphatic carbons, i.e., 285 eV. As seen in Table 2, more oxygen atoms were incorporated into the Table 2. Elemental Atomic Concentrations of PPy/CF Composites before (a) and after (b) Water Treatment sample

C 1s (%)

N 1s (%)

O 1s (%)

Cr (%)

a b

56.46 54.08

3.27 2.06

40.28 42.86

0 1

PPy/CF composite after water treatment due to oxidation of PPy by Cr(VI). Also, it was found that after water treatment 1% of the Cr was bound to the PPy/CF composite (Table 2). The high resolution spectrum collected from the Cr 2p core region is shown in Figure 7. The bands appeared at 577.0−578.0 and 586.0−588.0 eV, indicating that Cr(VI) was reduced to Cr(III), and then Cr(III) was adsorbed onto the PPy/CF composite.23 The distributions of C 1s, N 1s, and O 1s peaks measured by XPS deconvolution analyses of the PPy/CF composites before

Figure 7. High resolution Cr 2p spectrum of the PPy/CF composite after water treatment. 10413

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Table 3. C 1s, N 1s, and O 1s Peak Distributions (%) in PPy/CF Composites before (a) and after (b) Water Treatment C 1s

N 1s

O 1s

sample

284.8 eV (C−C, C−H, C−N)

286.6 eV (C−O, C−N)

288.1 eV (CO)

398.1−400.0 eV (−N= , −N−H)

401.8 eV (−N+−)

531.0 eV (C−OH···N)

533.0 eV (C−OH, C−OC)

a b

45.98 48.51

42.07 38.73

11.95 12.76

88.40 91.77

11.16 8.23

11.64 10.27

88.36 89.73

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(1) Huang, B.; Kang, G.; Ni, Y. Electrically Conductive Fiber Composites Prepared from Polypyrrole-Engineered Pulp Fibres. Can. J. Chem. Eng. 2005, 83, 896. (2) Huang, B.; Kang, G.; Ni, Y. Preparation of Conductive Paper by In-situ Polymerization of Pyrrole in a Pulp Fibre System. Pulp Pap. Can. 2006, 107, 38. (3) Chen, Y.; Qian, X.; An, X. Preparation and Characterization of Conductive Paper via In Situ Polymerization of 3,4-Ethylenedioxythiophene. BioResources 2011, 6, 3410. (4) Qian, X.; Chen, J.; An, X. Polypyrrole-coated Conductive Paper Prepared by Vapor-phase Deposition Method. Appita J. 2010, 63, 102. (5) Ding, C.; Qian, X.; Yu, G.; An, X. Dopant Effect and Characterization of Polypyrrole-Cellulose Composites Prepared by In-situ Polymerization Process. Cellulose 2010, 17, 1067. (6) Li, J.; Qian, X.; Chen, J.; Ding, C.; An, X. Conductivity Decay of Cellulose−Polypyrrole Conductive Paper Composite Prepared by In Situ Polymerization Method. Carbohydr. Polym. 2010, 82, 504. (7) Baum, G. A. Fiber Engineering: The Key to Industry Change. Solutions 2002, 85, 42. (8) Olad, A.; Nabavi, R. Application of Polyaniline for the Reduction of Toxic Cr(VI) in Water. J. Hazard. Mater. 2007, 147, 845. (9) Ruotolo, L. A. M.; Gubulin, J. C. Chromium (VI) Reduction Using Conducting Polymer Films. React. Funct. Polym. 2005, 62, 141. (10) Rengaraj, S.; Joo, C. K.; Kim, Y.; Yi, J. Kinetics of Removal of Chromium from Water and Electronic Process Wastewater by Ion Exchange Resins: 1200H, 1500H and IRN97H. J. Hazard. Mater. 2003, 102, 257. (11) Padilla, A. P.; Tavani, E. L. Treatment of an Industrial Effluent by Reverse Osmosis. Desalination 1999, 129, 219. (12) Carrington, N. A.; Li, Y.; Xue, Z.-L. Electrochemical Deposition of Sol-gel Films for Enhanced Chromium(VI) Determination in Aqueous Solutions. Anal. Chim. Acta 2006, 572, 17. (13) Mauri, R.; Shinnar, R.; Amore, M. D.; Giordano, P.; Volpe, A. Solvent Extraction of Chromium and Cadmium from Contaminated Soils. AIChE J. 2001, 47, 509. (14) He, S.; Zhao, Y.; Wei, M.; Evans, D. G.; Duan, X. Fabrication of Hierarchical Layered Double Hydroxide Framework on Aluminum Foam as a Structured Adsorbent for Water Treatment. Ind. Eng. Chem. Res. 2012, 51 (1), 285. (15) Liu, S.; Sun, J.; Huang, Z. Carbon Spheres/Activated Carbon Composite Materials with High Cr(VI) Adsorption Capacity Prepared by a Hydrothermal Method. J. Hazard. Mater. 2010, 173, 377. (16) Luo, P.; Zhang, J.; Zhang, B.; Wang, J.; Zhao, Y.; Liu, J. Preparation and Characterization of Silane Coupling Agent Modified Halloysite for Cr(VI) Removal. Ind. Eng. Chem. Res. 2011, 50 (17), 10246. (17) Oladoja, N. A.; Ololade, I. A.; Olaseni, S. E.; Olatujoye, V. O.; Jegede, O. S.; Agunloye, A. O. Synthesis of Nano Calcium Oxide from a Gastropod Shell and the Performance Evaluation for Cr (VI) Removal from Aqua System. Ind. Eng. Chem. Res. 2012, 51 (2), 639. (18) Ansari, R. Application of Polyaniline and Its Composites for Desorption/Recovery of Chromium(VI) from Aqueous Solutions. Acta Chim. Slov. 2006, 53, 88. (19) Ansari, R.; Fahim, N. K. Application of Polypyrrole Coated on Wood Sawdust for Removal of Cr(VI) Ion from Aqueous Solutions. React. Funct. Polym. 2007, 67, 367. (20) Ansari, R. Polypyrrole Conducting Electroactive Polymers: Synthesis and Stability Studies. Eur. J. Chem 2006, 3, 186.

Figure 8. ATR-FTIR spectra of the PPy/CF composites before (a) and after (b) water treatment.

needs to be combined with other treatment steps to remove Cr(III), which is formed by reduction of Cr(VI)). There is still another thing that is noteworthy. That is, in the current study, only the batch process was applied to treat Cr(VI)-contaminated water using the PPy/CF composite; however, in practice, a continuous process may be adopted for the treatment of flowing water. In such a case, a fixed packed bed formed by the composite may be used.

4. CONCLUSIONS The present study showed that a polypyrrole/cellulose fiber composite was quite effective in Cr(VI) detoxification of contaminated water. The process conditions for the preparation of the composite as well as the water treatment were optimized mainly based on the efficiency of Cr(VI) detoxification. The detoxification of the contaminated water was a fast process. The hypothesis of integrated reductive/adsorptive detoxification by the composite was confirmed by the research findings; specifically, Cr(VI) was reduced to Cr(III) and then the Cr(III) was adsorbed onto the composite. The engineering of regenerable cellulose fibers with polypyrrole will provide interesting possibilities for the treatment of Cr(VI)-contaminated water.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 451 82192081. Fax: +86 451 82192081. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the National Natural Science Foundation of China (Grant 31170552) for financial support of this work. 10414

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Industrial & Engineering Chemistry Research

Article

(21) Pecher, J.; Mecking, S. Nanoparticles of Conjugated Polymers. Chem. Rev. 2010, 110, 6260. (22) Ansari, R.; Delavar, A. F. Removal of Cr(VI) Ions from Aqueous Solutions Using Poly 3-methyl Thiophene Conducting Electroactive Polymers. J. Polym. Environ. 2010, 18, 202. (23) Park, D.; Yun, Y.-S.; Park, J. M. XAS and XPS Studies on Chromium-binding Groups of Biomaterial During Cr(VI) Biosorption. J. Colloid Interface Sci. 2008, 317, 54. (24) Khor, K. A.; Dong, Z. L.; Gu, Y. W. Influence of Oxide Mixtures on Mechanical Properties of Plasma Sprayed Functionally Graded Coating. Thin Solid Films 2000, 368, 86.

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dx.doi.org/10.1021/ie301136g | Ind. Eng. Chem. Res. 2012, 51, 10408−10415