Jianqiao Zhang‡
,
Huan Chen‡,
Zi Chen,
Jiaojie He,
Wenxin Shi,
Dongmei Liu,
Huizhong Chi,
Fuyi Cui and
Wei Wang*
State Key Laboratory of Urban Water Resource and Environment (SKLUWRE), School of Municipal and Environmental Engineering, Harbin Institute of Technology, Harbin 150090, P. R. China. E-mail: wangweirs@hit.edu.cn
First published on 15th June 2016
A high-performance, cost-effective and spongeous adsorbent was rationally designed for Cr(VI) removal from aqueous solution based on PPy modified natural quasi two-dimensional (2D) structures. Core of corncobs, a kind of agricultural waste composed of quasi 2D sub-micro sheets, has a unique macroporous spongeous microstructure. In consideration of the unique chemical structure of polypyrrole (PPy) and microstructure of the natural sponge, we constructed the microstructured spongeous adsorbents. The PPy active layer provided reactive sizes to detoxifies Cr(VI) ions by ion-exchange. While the spongeous microstructure provided by the microsheets could enhance the adsorption performance by increasing active areas and facilitating the access of pollutants inside the adsorbents. Compared with artificial nano-powder adsorbents, CC–PPy displayed larger advantage in separation process and fabrication/operation cost. Compared with other bulk-style PPy composites, the adsorption capacity of CC–PPy was several times higher. In addition, the regeneration and stability of CC–PPy was outstanding with no loss in the adsorption to Cr(VI) even after 3 adsorption–desorption cycles. The present development provides a solution to design highly efficient and cost-effective water treatment agents.
Biomass materials, cheap and easily available in quantity, have drawn increasing research attention in the development of bulk adsorbents,27–29 but the adsorption efficiency of these adsorbent is earnestly desired to be improved. Actually, the specific microstructure of the biomass materials is often overlooked. For example, the core of the corncobs, an agricultural waste available at very low costs, is observed to be composed of sub-micro sheets (Fig. 1a and S1†) and hence possesses a unique macroporous spongy microstructure. Due to the unique structure and properties, the 2D structures have exhibited outstanding performances in various field in recent years and even be honoured with Nobel Prize.30–32 Therefore, it is envisioned that this natural sponge composed of quasi two-dimensional structure was used as a cost-effective and efficient pollutant adsorbent.
In the family of the widely investigated conducting polymers,33–35 polypyrrole (PPy) was valued for its high electrical conductivity, facile synthesis, long-term environment stability and excellent redox property.36–39 In addition, PPy exhibited its potentiality in high-performance adsorption for its positively charged nitrogen atoms in the polymer chains.40–43
Considering the unique chemical structure of PPy and the microstructure of corncob-core, we rationally designed a microstructured macroporous adsorbent based on PPy modified corncob-core micro-sheets (CC–PPy) by a solution polymerization. The as-prepared CC–PPy adsorbent exhibits high performance for Cr(VI) removal with high capacities and selectivity, as well as easy separation and excellent reproducibility for water treatment. The present development may provide an efficient solution to treat Cr(VI) polluted water and a new angle for waste utilization.
qt = (C0 − C)V/m | (1) |
Removal percentage = (C0 − Ce/C0) × 100 | (2) |
(ii) In order to analyse the kinetic mechanism of the adsorption process, the experimental data were fitted in the pseudo-first-order and pseudo-second-order models, which are described as the following equations:17
Pseudo first-order equation: ln(qe − qt) = ln![]() | (3) |
![]() | (4) |
(iii) The reclaimed CC–PPy adsorbents were first washed with distilled water, then added into 0.10 M NaOH for 60 min, followed by washing with 0.10 M HCl solution for 60 min. The above procedure was repeated for 3 times to test the reusability of the adsorbent.
(iv) To study the pH effect on Cr(VI) removal, CC–PPy were added into 100 mg L−1 Cr(VI) solution at 25 ± 2 °C. The initial pH, ranging from 2.0 to 8.0, was adjusted by NaOH and/or HCl solutions. The effect of coexisting anions on Cr(VI) adsorption was evaluated by dissolving 100 mg L−1 Cl−, SO42−, NO3− and PO43− into 100 mg L−1 Cr(VI) solution. The adsorbent doses of the above two experiments were both 3 g L−1.
To investigate the effect of the mass ratio of pyrrole to corncob-core (CC) on adsorption performances, CC–PPy fabricated under different polymerization conditions with various Py to CC ratio was employed. For studying the influence of the size of CC–PPy granule, adsorbents synthesized according to 2.2 with different sizes were adopted. The above two experiments were conducted in Cr(VI) solutions with concentration of 117.85 mg L−1 an initial pH of 3.5 for 3 h. The doses of adsorbent were both 2.5 g L−1.
The Fourier-transform infrared (FTIR) spectra of the pure corncob-core and the as-prepared CC–PPy are presented in Fig. 2. For the pure corncob-core, the peaks related to the composition of cellulose or hemicellulose were appeared at 1734, 1631, 1514, 1374, 1246, 1039, 898 and 606 cm−1.29 After polymerization of PPy, several main peaks ascribed to pure corncob-core could also be observed. Besides, two new characteristic bands centered at 1545 cm−1, 1454 cm−1 were appeared, which are respectively assigned to the CC and C–N stretching vibrations of PPy. Moreover the peaks attributed to the C–H out-of-plane vibration and C–H in-plane deformation vibrations emerged at 780 cm−1 and 1300 cm−1 respectively, as well.44,45
The chemical compositions of the CC–PPy sponge were further characterized by X-ray photoelectron spectroscopy (XPS). In the wide-scan spectra, the peaks corresponding to C, N, O, Cl, Fe elements can be clearly detected, as shown in Fig. 3a. The N 1s peak shown in Fig. 3b can be reasonably decomposed into three Gaussian peaks with the binding energy of 397.8, 399.7 and 400.8 eV, which related to the imine-like structure (–CN–), the amine-like structure (–N–H–) and the positively charged nitrogen atoms (–NH+–) of PPy, respectively.46 The appearance of Cl 2p and Fe 2p spectrum is attributed to the doping state of the PPy.47 The XPS result together with the FTIR data confirmed the successfully modification of PPy along the corncob-core sponges.
The feasibility of the as-prepared CC–PPy macroporous sponges as a Cr(VI) scavenger from water was then explored. The size of the CC–PPy granules may influent the mass transfer inside of the adsorbents and consequently the adsorption performance. Therefore the impact of the granule size was studied first and shown in Fig. 4a. It was found that along with the decrease of the a.v. granule size from ∼20 mm to ∼5 mm, the adsorption efficiency increased gradually. When the granule size of CC–PPy changed from ∼5 mm to ∼2 mm, the removal efficiency slightly increased. Thus the optimized sized was selected at 5 mm. The effect of the mass ratio between pyrrole monomers (Py) to corncob-cores on the adsorption performance was also studied. As shown in Fig. 4b, the pure corncob-core exhibited poor adsorption property to remove Cr(VI) from water, which is similar to the results reported in literature.28,29 While for CC–PPy, the removal performance was enhanced with the increase of the pyrrole ratio. When the ratio increased to 2:
1, the removal efficiency was quite similar to that with 1
:
1 ratio. The loading of PPy in the above three composite CC–PPy samples was determined by weighing method and provided in Table S1.† The mass ratio of 2
:
1 was chosen in the following experiment. The effect of the initial solution pH on the Cr(VI) adsorption was presented in Fig. 4c. 3g L−1 of the as-prepared adsorbents were added into various Cr(VI) solutions with the concentration of 100 mg L−1 at 25 ± 2 °C under shaking. After 3 h, all the adsorption experiment reached equilibrium. It can be seen from Fig. 4c, the adsorption efficiency remained at a high level at a broad pH range of 2.0 to 5.0. The optimum pH range is pH 3–4. When the pH was higher than 6.0, the Cr(VI) removal efficiency was obviously decreased, which is due to the competitive interaction between the hydroxyl (OH−) ions and CrO42− ion on the adsorbent surface.48 Therefore the following adsorption experiments were performed at the condition of initial pH 3.5.
![]() | ||
Fig. 4 Effect of the size of CC–PPy granule (a), the mass ratio of pyrrole to corncob-core during polymerization (b) and the initial solution pH (c) on the Cr(VI) removal by the CC–PPy. |
The time-dependent adsorption at different initial Cr(VI) concentration was explored and shown in Fig. 5. The initial solution pH was 3.5, the dose was 3 g-adsorbent per L. It was found that the equilibrium was reached in 120 min when the initial concentration of Cr(VI) was 50 mg L−1. With increasing the Cr(VI) initial concentration to 200 mg L−1, it required about 180 min to reach equilibrium. In order to better understand the adsorption behaviors, adsorption kinetic data were then analysed using two common kinetic models, the pseudo-first-order and the pseudo-second-order. The kinetic parameters as well as corresponding correlation coefficients are summarized in Table S2.† The values of R2 clearly suggested that the adsorption kinetics closely followed the pseudo-second-order model rather than the pseudo-first-order model. From Fig. 5b, the rate constant of pseudo-second-order (k2) decreased from 0.0023 to 0.0002 g mg−1 min−1, when the initial solution concentration changed from 50 mg L−1 to 200 mg L−1. The calculated qe values were also close to the theoretical ones, and all the correlation coefficients (R2) were around 0.99. The above data indicates a chemisorption process of the Cr(VI) adsorption on CC–PPy.17
![]() | ||
Fig. 5 (a) Effect of contact time and initial concentration on the adsorption of Cr(VI) by CC–PPy. (b) The pseudo-second-order model for adsorption of Cr(VI) by the CC–PPy sponge. |
Moreover, we used the Langmuir model to describe the adsorption isotherms of Cr(VI) on the CC–PPy. The model can be written as follows:49
![]() | (5) |
Adsorbents | Type | Adsorption capacity (mg g−1) | Ref. |
---|---|---|---|
(1) Fe3O4/PPy composite microspheres | Around 500 nm, powder | 209.2 (qm) | 42 |
(2) Polypyrrole nanoclusters | Nanoparticle, powder | 180 for 3.4 mmol L−1 | 41 |
(3) PAN/PPy nanofiber films | Nanofiber mat | 61.8 (qm) | 40 |
(4) Polypyrrole/wood sawdust | PPy/biomass composite, bulk | 3.4 (qm) | 27 |
(5) Polypyrrole/cellulose fiber | PPy/biomass composite, bulk | ∼13 for 200 mg L−1 | 28 |
(6) Maize corncobs | Biomass, bulk | 3.0 (qm) | 29 |
(7) CC–PPy | PPy/biomass composite, bulk | 76.69 for 200 mg L−1 | Present work |
84.7 for qm |
To better understand the adsorption behaviour of Cr(VI) on the CC–PPy sponge, the XPS spectra of CC–PPy after adsorption were presented in Fig. 7. Compared with the as-prepared CC–PPy before adsorption experiment (Fig. 3), two new bands centered at ∼577.3 eV and ∼586.9 eV were appeared in the spectrum (Fig. 7a), which corresponded to the orbitals of Cr 2p3/2 and Cr 2p1/2, respectively.50 Additionally, the Cr 2p3/2 spectrum could be decomposed into two Gaussian peaks with binding energy of 576.9 and 578.0 ascribe to Cr(III) and Cr(VI), respectively.51 This result suggests that both Cr(III) and Cr(VI) were presented on the surface of the CC–PPy adsorbents after the adsorption of Cr(VI). The existence of Cr(VI) species was due to the adsorption of Cr(VI) ions through the anion exchange of PPy by replacing the doped Cl− ions.52 The presence of Cr(III) on the surface of adsorbents indicated that some fraction of the adsorbed Cr(VI) was reduced to Cr(III) during the adsorption process. It might be due to the presence of electron rich polypyrrole parts in the CC–PPy sponges.50
Selectivity is also important for practical adsorption in water treatment. For the adsorption mechanism of CC–PPy was mainly based on ion exchange, thus Cr(VI) competitive adsorption studies of the CC–PPy were carried out in the presence of common anions in waste water, e.g. Cl−, SO42−, NO3− and PO43−. The dose of the adsorbents was 3 g L−1, and the initial pH of Cr(VI) solution was 3.5. Form Fig. 8a, there was no obvious influences on the adsorption capacity (qe) by adding 100 mg L−1 Cl−, SO42−, NO3− coexisting anions into 100 mg L−1 Cr(VI) solution. The insignificant effect of chloride (Cl−) and nitrate (NO3−) are mainly due to that they are low-affinity ligands and generally form relative weaker bonds with the active sites through outer-sphere complexation.53 The reason for the negligible adsorption of sulfate (SO42−) was likely because of the higher hydration energy (−1080 kJ mol−1) of SO42− than HCrO4− (−180 kJ mol−1) and CrO42− (−950 kJ mol−1).54 Only phosphate ions exhibited a slight influence on the Cr(VI) removal, which was mainly owing to the addition of phosphate increased the solution pH.53 The above data suggested a selective Cr(VI) removal capacity of CC–PPy with strong anti-interference ability.
In the case of nano-powder water-treatment agents, filtration, centrifugation and sedimentation are usually employed to recover highly dispersed and suspended materials from the treated water. However, as the particle size diminished, smaller particles tend to suspend in water, penetrate through filtration materials or clog filter membrane.55 Our product of CC–PPy exhibited the advantage of easy solid–liquid separation and could be conveniently reclaimed with sifters or screens. For reusing, the reclaimed CC–PPy adsorbents were first washed with distilled water and then added into 0.10 M NaOH for 60 min, followed by washing with 0.10 M HCl solution. From Fig. 8b, it is clearly seen that after 3 cycles, the composite paper shows insignificant loss in the adsorption activity to Cr(VI).
The good regeneration property of CC–PPy, on one hand, is related to the inherent feature of PPy. The regeneration of CC–PPy was characterized by XPS in the revision, and the results were presented in Fig. S3 in the ESI.† Cr and Cl elements appeared both in the CC–PPy sample after adsorption and the regenerated sample (Fig. S3a and b†). The initial mole ratio of Cl/Cr in the sample after adsorption was 0.47, but the value increased to 1.05 in the sample regenerated by NaOH and HCl successively, suggesting an obvious decrease of Cr and an increase of Cl element in the adsorbents. Additionally, from the Cr 2p3/2 spectrum in Fig. 7b, the mole ratio of Cr(VI) was determined to be around 52% in the sample after adsorption, but after regeneration there was no Cr(VI) detected (Fig. S3c†), indicating the Cr(VI) ions adsorbed were nearly all desorbed. The existence of Cr(III) in the sample after regeneration was due to that the desorption of Cr(III) is more difficult than Cr(VI), but it exhibited insignificant influence on the adsorption properties of PPy based adsorbents, which was similar to the results reported in literature.25,53 Based on the above observation, the adsorption–desorption process was proposed as follows. The CC–PPy sponges were fabricated with FeCl3 as the polymerization in initiator, therefore Cl− ions were doped into the protonated PPy chains. During the adsorption process, the doped anions in PPy chains were replaced by Cr(VI) ions through ion exchange. When for regeneration process, NaOH solution was first employed, OH− would neutralize the positively charged PPy chain due to the strong affinity, and then replace the adsorbed Cr(VI) ions. When the sample was soaked in HCl solution, the PPy chain was re-charged by protons and meanwhile Cl− was doped into PPy again to stabilize the protonated chain. Thus the adsorption property was reproduced.
On the other hand, the stable natural spongy structure and good mechanical properties are beneficial to the good regeneration. The morphology of the CC–PPy after adsorption experiment is shown in Fig. S4.† The sponge materials kept their original 3D open structure, which was composed of CC–PPy micro-sheets. The mechanical properties of the adsorbent was measured and provided in Fig. 8c and d. For the samples were used in water, thus the mechanical properties of the CC–PPy adsorbents were measured after they were soaked in water. As shown in Fig. 8c, a CC–PPy sponge column was obtained. It is with a length of about 2.5 cm and a width of 0.5 cm. It can be easily bent at around 90°, indicating the flexibility. Moreover, the mechanical property of the CC–PPy was also measured under tensile mode on a universal materials testing machine. The diameter of the CC–PPy sponge column was around 0.5 cm. Fig. 8d presented the typical stress–strain curves of the sample. The sample is stretchable, and the ultimate strain was larger than 25%. Additionally, the ultimate tensile force of sample was around 5.8 N. The above experiments suggest that our CC–PPy is qualified in mechanical properties to serve as a water treatment agent. Therefore, the present CC–PPy composite adsorbents can be effectively reused several times and have the potential to meet practical application.
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra07687d |
‡ The authors contributed equally to this work. |
This journal is © The Royal Society of Chemistry 2016 |