Polym. Bull. DOI 10.1007/s00289-016-1769-1 ORIGINAL PAPER

Removal of chromium(VI) from aqueous solutions using polypyrrole-based magnetic composites Wanhong Sun1 • Yanqing Zhou2 • Qiong Su3 Lihua Chen1 • Yanbin Wang3 • Juanli Liu3 • Yu Sun1 • Huixia Ma3

•

Received: 17 December 2015 / Revised: 26 May 2016 / Accepted: 28 July 2016 Ó Springer-Verlag Berlin Heidelberg 2016

Abstract PPy/Fe3O4/AgCl composites were prepared via in situ polymerization for the removal of highly toxic Cr(VI). The structure and morphology of the prepared composites were characterized by the XRD, SEM, TEM, and VSM examinations. Up to 100 % removal was found with 1000 mg/L Cr(VI) aqueous solution at pH 2.0. The process of Cr(VI) ions’ adsorption was easy to reach equilibrium at higher temperatures. Adsorption results showed that Cr(VI) removal efficiency by the composites decreased with an increase in pH. Adsorption kinetics was described by the pseudo-second-order rate model. Isotherm data fitted well to the Langmuir isotherm model. Desorption experiment showed that the regenerated adsorption of PPy/Fe3O4/AgCl can be reused successfully for three times successive adsorption– desorption cycles without appreciable loss of its original capacity. Keywords Polypyrrole  Magnetic composites  Chromium removal

Introduction There is growing public concern over the contamination of wastewater by heavy metal (such as Cr6?, Cd2?, Cu2?, Pd2?, Hg2?, and Ni2?). The removal of heavymetal ions from wastewater has become a major research. This is due to the fact that the presence of heavy-metal ions in water, even at very low concentrations, is highly & Wanhong Sun [email protected] 1

Experiment Center of Northwest University for Nationalities, Lanzhou 730030, China

2

Department of Petrochemical Engineering, Lanzhou Petrolchemical College of Vocational Technology, Lanzhou 730060, China

3

College of Chemical Engineering, Northwest University for Nationalities, Lanzhou 730030, China

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undesirable [1]. Exposure to Cr(VI) in drinking water increases the risk of bladder, liver, kidney, and skin cancers [2–6]. Some conventional methods, such as reduction, reverse osmosis, ion exchange, and adsorption, have been used for the removal of heavy-metal ions from contaminated effluents [2–5]. Unfortunately, rapid global economic development is accompanied by the emission of large volumes of Cr-containing effluents. Hence, the efficient removal of Cr(VI) from water is a critical issue [5, 7]. The use of adsorption technology in solving the problem of heavy-metal wastewater pollution is considered promising, because the adsorbents are environmentally friendly and can be used repeatedly [6, 8]. Compared with traditional adsorbents, magnetic adsorbents can be manipulated or recovered rapidly by an external magnetic field. In addition, magnetic adsorbents possess an efficient highspecific surface area [8–10]. However, magnetic adsorbents also have many drawbacks, such as low selectivity and stability in strong acidic aqueous media and low dispersibility in various sample matrices. Hence, most previous works on functional modified magnetic adsorbents focused on further improvement of the adsorption capacity [11]. To acquire better sensitivity and selectivity, polymer coating is one of the alternatives to improve the stability of magnetic nano-adsorbents and allow the modification of surface with functionalities [5, 6, 12]. Polypyrrole (PPy) is one of the most widely investigated intrinsically conducting polymers [13–22]. PPy are particularly promising owing to good environmental stability, hydrophobicity, large p-conjugated structure, polar functional groups, and ion exchange characteristics [23–25]. The applicability of PPy-coated magnetic NPs (NPs) as an anion exchange magnetic adsorbent is demonstrated. Under the optimum extraction conditions for three nitrophenols, limits of detection ranging from 0.3 to 0.4 lg L-1 were obtained [11]. Lei et al. investigated that the PPy/cellulose fiber composite was quite effective and fast in Cr(VI) detoxification of contaminated water [26]. Qiu et al. pointed out that PANI/ethyl celluloses can be used for fast Cr(VI) removal with a PANI loading of 20.0 wt% [27]. Fe3O4/PPy magnetic composites were used to remove toxic Cr(VI) [28]. To the best of our knowledge, a method of using pyrrole, iron oxide, and silver nitrate to synthesize PPy/Fe3O4/AgCl composites has not yet been reported. Herein, our motivation is to use PPy as a subject for forming the network structure and magnetic NPs as a magnetic source for forming the sewage treatment of the composites. Different from the previous synthesis methods [11, 12, 23], the synthesis of composite was ultrasonic oxidative polymerization and application of composites was as efficient adsorbent for the removal of Cr(VI) from wastewater solution.

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Experiment Materials Pyrrole (Py, 99 %, Shanghai Chemical Company, China) was purified by vacuum distillation and stored in refrigerator prior to use. Iron (III) chloride hexahydrate, ferrous sulfate heptahydrate, ethyl alcohol, sodium hydroxide, hydrochloric acid, p-toluenesulfonic, and potassium dichromate (K2Cr2O7) were obtained from commercial suppliers and used as received. All chemicals were of reagent grade. Preparation of Fe3O4 The Fe3O4 magnetic NPs were synthesized through a coprecipitation method according to the previous reports [29]. The synthetic process was as follows: 0.1 mol L-1 FeCl36H2O and 0.05 mol L-1 FeSO47H2O with the same volume were kept sufficiently stirring after mixing in a round bottom flask. Then, 1.5 mol L-1 30 mL ammonia hydroxide was added slowly into the solution to adjust the pH value to 9–10. The solution was stirred at 80 °C for 2 h and washed for several times with deionized water and anhydrous ethanol, respectively. The powders were dried overnight in a 50 °C vacuum oven to obtain the magnetic NPs. Preparation of PPy/Fe3O4/AgCl As the preparation of PPy/Fe3O4/AgCl composites, 0.2 mg dodecyl benzene sulfonic acid sodium (SDBS) and 15 mL ethyl alcohol water (1:1) were mixed and sonicated for 15–20 min at room temperature. Then, 0.1 g of Fe3O4 was injected into the above mixture and sonicated for 60 min. 0.22 mL of pyrrole and 0.2 g of p-toluene sulfonic acid were added into the above mixture and sonicated for 15 min. 29 mL 2.86 9 10-3 mol/L AgNO3 were added into the above mixture and sonicated for 30 min. Finally, 30 mL 0.15 mol L-1 FeCl36H2O aqueous solution was added slowly into the reactor and a rapid oxidation occurred. The resulted suspension was sonicated at 0–5 °C for 2 h. The obtained product was filtered and washed with deionized water and ethanol, and dried at 50 °C in vacuum for 24 h. For comparison, the pure PPy was prepared without Fe3O4 and AgNO3 by the similar procedure. Characterization X-ray diffraction studies were performed using D/max-2400 diffraction X-ray diffractometer (Rigaku) with CuKa as radiation source. Scanning electron microscope (SEM, Hitachi, Japan, JEOL, JSM-6330F) was used to observe the morphologies of composite. Prior to the examination, the specimens were coated with a very thin layer of gold. Transmission electron microscopy (TEM) was performed with a TECNAI G2 model transmission electron microscope at 100 kV accelerated voltage, and the observations were carried out after retrieving the slices

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onto Cu grids. The room-temperature magnetization curves were obtained using a Quantum Design PPMS-6000 magnetometer. Isothermal magnetization curves were measured with several different points at external magnetic field of 0–9 kOe. Batch experimental procedures The Cr(VI) solution was prepared by dissolving K2Cr2O7 in distilled water and used as inlet solution. The pH values of solution in these experiments were approximately 4, 6, 7, and 9. The pH value was adjusted by H2SO4 and NaOH solution. For determining the amount of Cr(VI) removal by samples, 0.2 g adsorbent was added into 20 mL of Cr(VI) solution with scheduled concentration and strongly shaken at a rate of 160 rpm in an oscillator to ensure a complete mixing. At the end of predetermined time intervals, the samples were filtered and the concentration of Cr(VI) was determined using a UV–visible spectrometer at 540 nm according to the diphenylcarbazide method. All adsorption experiments were conducted at 20 ± 2 °C unless noted, and all of the adsorption results were corrected by blank tests in which no adsorbent was added into the Cr(VI) solution. All experiments were carried out twice, and the adsorbed concentrations given were the means of duplicate experimental results.

Results and discussion Structure and morphology The crystallinity and phase purity of the as-prepared PPy, Fe3O4, and PPy/Fe3O4/ AgCl composites were examined by XRD measurements. Figure 1a shows the XRD images of pure PPy. PPy in 2h = 20–30 has a broad peak, and the absorption peak located at 23.52 is obvious, which is the characteristic peaks of pure PPy [30]. As shown in Fig. 1b, the diffraction peaks at 2h values of 30.22, 35.56, 43.25, 53.53, 57.20, and 62.81 can be ascribed to the reflection of (220), (311), (400), (422), (511), and (440) planes of the Fe3O4, respectively [5, 28]. The observed peaks of Fig. 1 XRD spectra of PPy, Fe3O4, and PPy/Fe3O4/AgCl composites

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Fe3O4 are consistent with the database in JCPDS file (PDF no. 85–1436) and also reveal that the NPs are pure Fe3O4 phase with a spinal structure. The XRD image of synthesized PPy/Fe3O4/AgCl (Fig. 1c) not only can show the strong diffraction peak of Fe3O4 NPs, and also appear the strong diffraction peak of cubic-phase AgCl. The peaks at 27.81, 32.22, 46.21, 54.80, 57.46, and 67.49 (Fig. 1c) can be assigned to the reflection of (111), (200), (220), (311), (222), and (440) planes of the AgCl, respectively (PDF no. 31–1238) [31]. Thus, XRD results indicate that there is no visual change in the crystalline structure of Fe3O4 and AgCl during the polymerization of pyrrole monomer. Figure 2 shows the FT-IR spectra of PPy and Fe3O4, before and after the PPy/ Fe3O4/AgCl adsorption. Pure PPy shows the peak locations at 1523 and 1428 cm-1, which are due to C=C and C–N stretching, respectively (Fig. 2a). The band at 1385 cm-1 is attributed to C–H vibrations. The peak at 1170 cm-1 is attributed to C–C stretching. The band at 1042 cm-1 is due to in-plane deformation of C–H bond and N–H bond of pyrrole ring. The band at 925 cm-1 is assigned to the C–C out-ofphase deformation vibration [32]. In the sample Fe3O4, the peak at about 610 and 489 cm-1 can be attributed to Fe3O4. Compared with before and after Cr(VI) adsorption, it is clearly observed that the spectra of PPy/Fe3O4/AgCl have no obvious difference except that all peak positions shifted towards higher values after Cr(VI) ions adsorption. The new peaks at 570 cm-1 (Fig. 2b) are intrinsic vibration of the Cr–O bonds. This feature indicates that Cr(VI) ions adsorbed on the surface of the PPy/Fe3O4/AgCl composite. Microscopic morphology investigations of the as-prepared composites were performed using SEM. The morphology of the as-received PPy and Fe3O4, as shown in Fig. 3a, b, appeared nanospheres and NPs, respectively. The pure PPy prepared by ultrasonic oxidative polymerization showed uniform nanospheres with the size about 100–130 nm [33]. Figure 3b shows the SEM image of Fe3O4 NPs indicating the formation of slight agglomerates due to high-surface area and magneto dipole– dipole interactions between the NPs with average diameter of 10–15 nm. It is interesting to observe that nearly agglomerated composite (Fig. 3c) with larger size than Fe3O4 was formed via encapsulation of Fe3O4 NPs by PPy. Small NPs could be

Fig. 2 FT-IR of PPy and Fe3O4 (a) and before and after Cr(VI) adsorption (b)

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Fig. 3 SEM micrographs of PPy (a), Fe3O4 (b), and PPy/Fe3O4/AgCl (c)

seen in Fig. 3c, in which the light gray spots’ surface corresponds to Fe3O4 and AgCl NPs embedded in the surface of PPy or surrounded by PPy matrix and the gray and relatively large phases are the PPy shells. The TEM image of the Fe3O4 NPs is shown in Fig. 4a, in which sphere-like Fe3O4 NPs were obtained with 10–20 nm diameter. Spherical NPs could be seen in Fig. 4b, in which the dark spots inside the NPs correspond to Fe3O4 and AgCl surrounded by PPy matrix and light gray and relatively large phases are the PPy shells. However, from Fig. 4b, it is difficult to distinguish between Fe3O4 and AgCl NPs, and AgCl NPs can be seen the presence from the EDX spectra (Fig. 4c). It can see clearly from Fig. 4b that the dispersibility of Fe3O4 NPs in the PPy/Fe3O4/AgCl composite is better than pure Fe3O4. The atomic composition of Fe3O4 and PPy/ Fe3O4/AgCl (Fig. 4c) composites is determined from EDX spectra. According to the collected data from EDX, it is clear that the as-prepared Fe3O4 and PPy/Fe3O4/ AgCl composites does not contain other elements. These results demonstrate that PPy has a strong effect on the dispersity and morphology of Fe3O4 and AgCl NPs and adsorption performance of the PPy/Fe3O4/AgCl composite. In addition, these results are similar to those reported in [28, 30, 34, 35].

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Fig. 4 TEM images (a) Fe3O4 and (b) PPy/Fe3O4/AgCl, EDX spectra of (c) Fe3O4 and PPy/Fe3O4/AgCl composites

To study the magnetic properties of the composites, the room-temperature magnetization hysteresis curves of the as-synthesized composites were measured using vibrating sample magnetometry (VSM), as shown in Fig. 5a. The magnetic curve of the Fe3O4, PPy/Fe3O4, and PPy/Fe3O4/AgCl composites is measured by cycling the magnetic field between -9 and ?9 kOe. All the as-made samples

Fig. 5 M(H) curves of the Fe3O4 and PPy/Fe3O4/AgCl composites (a) and magnetic separability of composite (b)

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Scheme 1 A photographic representation of Cr(VI) removal from aqueous solution and magnetic separation of used PPy/Fe3O4/AgCl composite

displayed typical ferromagnetic behavior. As shown in Fig. 5a, the saturation magnetization (Ms) of Fe3O4, PPy/Fe3O4, and PPy/Fe3O4/AgCl is 78.50, 42.05, and 19.29 emu/g, respectively. Hc values of Fe3O4, PPy/Fe3O4, and PPy/Fe3O4/AgCl are 118.71, 131.07, and 113.98 Oe, respectively. The decrease in the saturation magnetization of PPy/Fe3O4 and PPy/Fe3O4/AgCl compared with Fe3O4 NPs is attributed to the introduction of PPy and AgCl and hence the decrease of the Fe3O4 component. The nonmagnetic PPy was coated on the surface of some Fe3O4 NPs, which negatively contributes to magnetization readings in the standard practice of normalizing magnetization by sample mass [36–38]. Despite the lowered magnetism, the PPy/Fe3O4/AgCl composite can be separated from the wastewatertreated systems with the help of a magnetic field (Fig. 5b), which will facilitate the recycling of the PPy/Fe3O4/AgCl composite in wastewater treatment [39]. The magnetic property of the PPy/Fe3O4/AgCl composite was also corroborated, as shown in Scheme 1 in which the composite particles after Cr(VI) adsorption are attracted by magnetic bar. This provides a good prospect for cost-effective application of the composite in magnetic adsorption for industrial wastewater treatment. Adsorption performance of PPy/Fe3O4/AgCl composites Adsorption equilibrium experiments were carried out in a temperature-controlled thermostatic shaker operated at 160 rpm. All the Cr(VI) solutions required for experiments were freshly prepared by diluting the stock solution. The removal percentage of Cr(VI) was calculated as C ¼ ½ ðc0  ct Þ =c0   100 %

ð1Þ

where c0 and ct are the initial and at time t concentrations (mg/L) of Cr(VI), respectively. At optimum pH, adsorption contact time, initial Cr(VI) concentration, and temperatures were studied with 20 mL of Cr(VI) solution. Adsorption isotherms at three different temperatures (20, 30, and 40 °C) were investigated at pH 2.0 by changing the initial concentration of Cr(VI) from 100 to 1000 mg/L. The procedure was similar to that described for the effect of pH. The equilibrium adsorption capacity (qe) of the adsorbent was calculated using

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qe ¼ ½V  ðc0  ce Þ =m

ð2Þ

where qe is the amount of metal adsorbed per specific amount of adsorbent (mg/g). c0 and ce are the initial and equilibrium concentrations (mg/L) of Cr(VI), respectively. V (L) is the volume, and m (g) is the weight of the adsorbent. Adsorption kinetics were also performed to assess the rate of Cr(VI) sorption. The experiments were conducted at five initial concentrations (100–1000 mg/L) of Cr(VI). In a typical kinetic experiment, 0.2 g of the PPy/Fe3O4/AgCl composites were added to 20 mL Cr(VI) solution at pH 2.0 and thermostatic shaker at 160 rpm. At a predetermined time interval, 10 mL of solution was collected, filtered, and analyzed for Cr(VI) concentration. PPy/Fe3O4/AgCl composites and its components of removal percentage for Cr(VI) The Cr(VI) removal of the PPy/Fe3O4/AgCl composite and its constituents (PPy and Fe3O4) is shown in Table 1. The initial concentration of Cr(VI) ions is 40 mg/L at pH 6.0 for Fe3O4. The Cr(VI) removal is 94.95 % for Fe3O4 nanoparticles. The initial concentration of Cr(VI) ions is 500 mg/L at pH 2.0 for PPy and PPy/Fe3O4. Compared with the Cr(VI) removal for PPy and PPy/Fe3O4 composites, it is easy to find that the discrepancy of removal efficiency for Cr(VI) ions is very small. However, the PPy/Fe3O4 composite with an Fe3O4 loading of 50 or 33 wt% can not only increase its specific surface area, but also enhance its magnetism, which is very easy to separate PPy/Fe3O4 composite from wastewater. The Cr(VI) removal increased with the decrease of Fe3O4 content from 1:1 to 2:1 for the both adsorbents of PPy/Fe3O4 composites. Therefore, this mass ratio (PPy:Fe3O4 = 2:1) is chosen in the synthesis of PPy/Fe3O4/AgCl. However, over 99 % removal efficiency is achieved on PPy/Fe3O4/AgCl for 1000 mg/L initial concentration of Cr(VI). Therefore, in the next study, we study the relative performance of PPy/Fe3O4/AgCl composites. Effect of pH and dose on Cr(VI) adsorption The pH values of the aqueous solution are an important controlling parameter in the adsorption process. These pH values affect the surface charge of adsorbent, the Table 1 Removal percentage of PPy/Fe3O4/AgCl composites and components for Cr(VI) wastewater (contact time 30 min) Type of adsorbent

Fe3O4

Initial conc. (mg/L) 40

Final conc. (mg/L)

Removal percentage (%, w/w)

2.02

94.95

PPy

500

1.72

99.66

PPy/Fe3O4 (1:1)

500

26.95

94.61

PPy/Fe3O4 (2:1)

500

10.89

97.82

1000

2.55

99.74

PPy/Fe3O4/AgCl(PPy:Fe3O4 = 2:1)

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degree of ionization, and the speciation of adsorbate during the adsorption process. The effect of the initial solution pH on Cr(VI) removal by the PPy/Fe3O4/AgCl composite is shown in Fig. 6a. The initial concentration of Cr(VI) is 1000 mg/L, and it was strongly shaken at a rate of 160 rpm in an oscillator after the PPy/Fe3O4/ AgCl adsorbent was added into its solution. It is evident that the Cr(VI) removal efficiency (99.74 %) was found for PPy/Fe3O4/AgCl composite at pH 2.0. Moreover, with the increase of pH, the Cr(VI) removal efficiency of PPy/Fe3O4/ AgCl has slight change. Therefore, the composite can be considered as a highly efficient adsorbent for the removal of Cr(VI). It reported that the speciation of Cr(VI) in aqueous solution is strongly pH-dependent [40]. In the range of pH 2.0–6.0, the predominant Cr(VI) species are monovalent bichromate (HCrO4-) and divalent dichromate (Cr2O72-), and above pH 6, the dominant species is chromate (HCrO4-) ions. The higher removal efficiency in the acidic pH (2.0–6.0) range is due to the anion exchange property of the PPy/Fe3O4/AgCl by replacing the doped Cl- ions with either HCrO4- or Cr2O72- ions. In particular, more than 98 % removal efficiency is achieved on PPy/Fe3O4/AgCl composite at pH 4.0–9.0. However, the amount of Cd (II) adsorption by PPy has not significant change with increasing pH 3.0–10.0 [1], which is very consistent with our findings. Figure 6b shows the effect of adsorbent dose on the removal efficiency of Cr(VI) from aqueous solution. It is observed that the removal efficiency increases from 53.81 to 99.74 % with an increase in adsorbent dose from 50 to 200 mg. This is due to an increase in the surface area and availability of more active sites for adsorbate. However, when continue to increase the amount of adsorbent (from 200 to 300 mg), the removal efficiency remains unchanged with increase of adsorbent dose, because the Cr(VI) ions become limiting in the system. This results further confirm the high affinity of PPy/Fe3O4/AgCl composites for Cr(VI) removal. In a recent study by Bhaumik et al. 100 % removal was observed with the initial Cr(VI) concentration 200 mg/L using 0.1 g PPy/Fe3O4 composites and it required 30–180 min for equilibrium to be reached [32]. The concentration of 100 mL of 100 mg/L Cr(VI)contaminated water was almost 0 using 1 g PPy/cellulose fiber composites. [26]. Jinhua et al. reported that 0.4 g of polyaniline doped with sulfuric acid are required

Fig. 6 Effect of pH (a) and dose (b) on the removal of Cr(VI) of the PPy/Fe3O4/AgCl composite

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to obtain 100 % of Cr(VI) removal of 100 mL of 50 mg/L Cr(VI) solution [41]. Thus, PPy/Fe3O4/AgCl composites developed in this study display a better efficiency in the removal of Cr(VI) from aqueous solution compared to materials reported in the literature. Effect of contact time and initial Cr(VI) concentration on adsorption of Cr(VI) The adsorption processes as a function of time to determine the point of equilibrium were studied from the adsorption experiments of Cr(VI) ions onto the PPy/Fe3O4/ AgCl composite. All experiments were run twice, and a good reproducibility of the procedures was obtained. The results are shown in Fig. 7a, where it is clear that the Cr(VI) adsorption was quite fast toward the beginning, followed by a much slower subsequent removal and at last leading to the complete adsorption equilibrium. About 99.79 % of Cr(VI) was removed by PPy/Fe3O4/AgCl composite during the first 5 min of the reaction, which almost reached equilibrium. The Cr(VI) removal is about 99.94 % during after 15 min of the reaction. The rapid adsorption of Cr(VI) using PPy/Fe3O4/AgCl as adsorbent may be attributed to a network structure of PPy/ Fe3O4/AgCl composite. This suggests that most of the adsorption sites of the PPy/ Fe3O4/AgCl composite existed in the exterior of PPy/Fe3O4/AgCl composites and were easily accessible by the Cr(VI) ions, resulting in a rapid equilibrium process. For PPy/Fe3O4/AgCl adsorbent, the equilibrium was achieved in 5 min. In comparison, this time length of Cr(VI) adsorption onto PPy/Fe3O4/AgCl (about 5 min, about 99.79 % removal) is close to that of magnetite NPs (about 2 min, about 90 % removal) [42]. The PANI nanostructure was used to remove Cr(VI) in aqueous medium and observed decrease from 0.8 to about 0.1 mmol/L in 5 min and then decrease slightly [43]. The experiments were carried out in the initial Cr(VI) anion concentration range 100–2000 mg/L. As shown in Fig. 7b, the removal decreased slightly with the increase of the initial concentration of Cr(VI) anion in aqueous solution. This is mainly because of the adsorption driving force caused by the concentration difference of the Cr(VI) anion between the bulk solution and the surface of PPy/

Fig. 7 a Effect of contact time on adsorption capacity. b Effect of the initial Cr(VI) concentration on adsorption capacity

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Fe3O4/AgCl. In the Cr(VI) concentration range 100–1000 mg/L, the adsorption driving force increased with the increase of the Cr(VI) concentration. When the Cr(VI) concentration in aqueous solution was higher than 1000 mg/L, the removal fell significantly. Adsorption kinetics To obtain the adsorption rate constants, the pseudo-first-order and pseudo-secondorder kinetic models were adopted to fit the experimental data, which can be defined by the following Eqs. (3) and (4):   k1 logðqe  qt Þ ¼ log qe  t ð3Þ 2:303 t 1 t ¼ þ qt k2 q2e qe

ð4Þ

where qt is the adsorption capacity at time t, mg/g, k1 (min-1) is the rate constant of the pseudo-first-order equation, and k2 (g/(mgmin)) is the rate constant of the pseudo-second-order equation. Figure 8a shows a plot of log(qe - qt) vs t for adsorption of Cr(VI) for the pseudo-first-order equation. The values of pseudo-first-order rate constants, k1, and equilibrium adsorption capacities, qe, were calculated from slope and intercept of straight lines in Fig. 8a. The values of the pseudo-first-order equation parameters together with correlation coefficients are given in Table 2. The correlation coefficients for the pseudo-first-order equation obtained at all the studied concentrations were low. Figure 8b shows the application of the pseudo-secondorder equation by plotting t/qt versus t. The calculated adsorption capacity from the pseudo-second-order equation was 100 mg/g, which was in accordance with the experimental value, 99.74 mg/g (Table 2). The correlation coefficient was 0.9999. The results indicated that these kinetic data agreed with the pseudo-second-order

Fig. 8 Kinetic fitting plots of pseudo-first order (a) and pseudo-second order (b) equations (20 mL of 1000 mg/L Cr(VI) solution, 0.2 g of PPy/Fe3O4/AgCl, 20 °C, pH 2.0)

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Polym. Bull. Table 2 Comparison of pseudo-first-order and pseudo-second-order adsorption rate constants and adsorption capacities

PPy/Fe3O4/AgCl

qe,expt (mg/g)

First-order rate constant K1 (min )

qe (mg/g)

R

K2 g/(mgmin)

qe (mg/g)

R2

99.74

0.048

2.81

0.8107

1.49

100

0.9999

-1

Second-order rate constant 2

equation, not the pseudo-first-order equation. The adsorption behavior of PPy/ Fe3O4/AgCl was similar to chemical adsorption. Kinetics models are used to find the potential rate-controlling step involved in the Cr(VI) removal process by PPy/Fe3O4/AgCl composite. Similar kinetics was also observed for the Cr(VI) adsorption for many adsorbents, such as magnetite NPs [42], PANI [43], PANI/ECs [29], and IIP [44]. In this study, two models, such as pseudo-first order and pseudo-second order, were used to evaluate the kinetics of Cr(VI) removal by PPy/Fe3O4/AgCl composite. The composites were used to adsorb Cr(VI) from artificial wastewater with an initial Cr(VI) concentration of 1000 mg/L. The adsorption reached equilibrium within 5 min. More importantly, *99.79 % of Cr(VI) could be removed within 5 min. Adsorption isotherm To evaluate the adsorption performance of PPy/Fe3O4/AgCl, adsorption isotherms of composite were investigated at different concentrations with 0.2 g of PPy/Fe3O4/ AgCl for 30 min at pH 2.0. Various isotherm models, including the Langmuir and Freundlich model, were used to fit the Cr(VI) removal behavior by PPy/Fe3O4/ AgCl. The Langmuir isotherm is described as ce 1 ce ¼ þ qe bqmax qmax

ð5Þ

where ce is the equilibrium concentration (mg/L) of Cr(VI), qe is the Cr(VI) amount adsorbed at equilibrium (mg/g), qmax is the adsorption capacity of PPy/Fe3O4/AgCl (mg/g), and b is a constant (L/mg). The Freundlich isotherm is an empirical model that considers heterogeneous adsorptive energies on the surface of absorbent, and it can be described as logqe ¼ logKf þ 1=nlogCe

ð6Þ

where qe is the amount of Cr(VI) adsorbed on the surface of PPy/Fe3O4/AgCl at equilibrium, ce is the equilibrium concentration, and kf and n are constants of the Freundlich model. The PPy/Fe3O4/AgCl composites were used to treat the Cr(VI) solution with Cr(VI) concentrations ranging from 100 to 1000 mg/L at three different temperatures (20, 30, and 40 °C). The adsorption isotherms are shown in Fig. 9a, b. Relative calculation data, including the Langmuir and Freundlich adsorption constants and the corresponding correlation coefficients, are listed in Table 3. The

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Fig. 9 a Langmuir and b Freudlich isotherms for Cr(VI) removal by PPy/Fe3O4/AgCl Table 3 Isotherm parameters for the adsorption of Cr(VI) on PPy/Fe3O4/AgCl

T (K)

Langmuir model qmax (mg/g)

b (L/mg)

Freundlich model 2

R

kf

n

R2

293

166.7

0.7

0.9900

65.3

2.0

0.9329

303

111.1

5.0

0.9938

89.1

4.3

0.9918

313

101.0

17.4

0.9856

96.5

6.7

0.9523

results showed that the adsorption process was more similar to the Langmuir isotherm adsorption than the Freundlich isotherm, suggesting a monolayer adsorption for the uptake of Cr(VI) on the surface of PPy/Fe3O4/AgCl. The correlation coefficients, R2, were 0.9900, 0.9938, and 0.9856, respectively. The calculated maximum adsorption capacity of PPy/Fe3O4/AgCl was up to 126.3 mg/g based on the Langmuir adsorption equation, which was very close to the experimental data. Then, it showed that the adsorption occurred in a monolayer and at a fixed number of identical adsorption sites [44–46]. Desorption and regeneration The stability and regeneration of adsorbents are important factors in the industrial application of composite. Regeneration of PPy/Fe3O4/AgCl was studied by performing an adsorption–desorption experiment for four consecutive cycles. The elute solution was 0.1 mol L-1 NaOH solution. A 0.2 g sample of PPy/Fe3O4/AgCl with saturation adsorption of Cr(VI) was eluted in a bottle containing 100 mL of elute solution under stirring. Until Cr(VI) could not be detected, the regenerated PPy/Fe3O4/AgCl composites were reused the next time. Figure 10 shows that only 3.18 % of the adsorbed Cr(VI) was not desorbed in the first cycle of desorption process. This is due to the reduction of adsorbed Cr(VI) to Cr(III) by PPy which could not be desorbed upon treatment with NaOH solution [39, 40]. It is observed that the removal efficiency (99.74 %) of PPy/Fe3O4/AgCl remained almost same for the three cycles and in the subsequent fourth cycle removal efficiency decreased to

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Polym. Bull. Fig. 10 Desorption and regeneration of PPy/Fe3O4/AgCl (20 mL of 1000 mg/L Cr(VI) anion solution, 0.2 g of PPy/ Fe3O4/AgCl, 30 min, pH 2.0)

89.8 %. Therefore, PPy/Fe3O4/AgCl can be successfully reused for three adsorption cycles without any loss of its removal efficiency. The results showed that PPy/ Fe3O4/AgCl had good stability and regeneration, which could be used in practical wastewater treatment. Moreover, as the adsorbent material, the components of PPy/ Fe3O4/AgCl composite are very environmentally friendly. PPy, Fe3O4, and AgCl have the good environmental stability within a relatively wide range of pH and nontoxic. In addition, PPy, Fe3O4, and AgCl NPs possess an efficient high-specific surface area.

Conclusion Magnetic composites are emerging materials for the removal of contaminants from water. PPy/Fe3O4/AgCl composites were synthesized, characterized, and used as an effective adsorbent for the removal of Cr(VI) form aqueous solution. The Cr(VI) adsorption behavior on the prepared PPy/Fe3O4/AgCl has been studied at different solution pH values, dose of PPy/Fe3O4/AgCl, initial concentration, adsorption contact time, and temperature. Adsorption equilibrium is attained within a short contact time of 5 min when the initial Cr(VI) concentration was 1000 mg/L. The Cr(VI) ions adsorption was favored at higher temperatures and at slightly initial acid pH values in the equilibrium under acidic conditions (pH = 2.0). Adsorption kinetic data agreed with the pseudo-second-order equation, not the pseudo-first-order equation. Experimental isotherms of Cr(VI) ions were successfully fit to the Langmuir isotherm model. The results indicate that the PPy/Fe3O4/AgCl composites are an effective adsorbent for the removal of Cr(VI) ions from aqueous solutions, and it could be useful in the treatment of Cr(VI) wastewaters. Acknowledgments The authors would like to thank the financial supports of the Science and Technology Plan of Gansu Province (145RJZA224), supported by the fund of the Education Department of Gansu Province (2014B-007), the financial supported by the fund Key Laboratory and Engineering Center of Independent Study of Northwest University for Nationalities (zyp2015016), supported by funds of Central Universities Fundamental Research (31920160011), supported by fund of the National natural Science Foundation of China (51563022), and supported by the fund of Gansu Science and Technology Support Projects (1504GKCA093).

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