Microstructured macroporous adsorbent composed of polypyrrole modified natural corncob-core sponge for Cr(VI) removal

Abstract

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.

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1 Introduction

Pollution of heavy metal ions in water constitutes one of the greatest global challenges facing both environmental sustainability and public healthcare today.^1–3 Cr(VI) ion is regarded as one of the most toxic ions due to its carcinogenicity and teratogenicity.^4–6 The Cr(VI) ion containing water was usually discharged into surrounding environment by some polluted industries, like steel manufacturing, metal plating, military purposes, leather tanning, and so on, and therefore threatens people's health.^7,8 So far, many technologies have been developed to reduce/remove Cr(VI) ion from aqueous solutions such as adsorption, photocatalysis, ion exchange, membrane process, chemical coagulation, etc.^9–16 Among these methods, adsorption is regarded as one of the most simple, effective and economically favourable ways.^17,18 By far, kinds of outstanding adsorbents for Cr(VI) removal with high performance were developed. Particularly, intensive attentions were focused on nanostructured adsorbents, including carbon based nanostructures, oxides based nanostructures, polymer based nanostructures, etc.^19–24 Some functional groups were decorated onto nanostructured supports to capture the heavy metal ions, where the nanostructures provide more reactive sites to enhance the adsorption capacities. However, those nanostructured adsorbents generally suffered from complex fabrication procedures, higher costs, as well as complicated operating methods in real water treatment, which seriously hinder the practical application.²⁵ Although, some magnetic powder based nano-adsorbents can resolve the separation problem in a certain extent, they will still increase the operation cost. Additionally, magnetic powders tend to agglomerate during application, thus would suppress the removal efficiency when for recycling.^24,26

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.

Fig. 1 (The inset of a) A digital photograph of a corncob. It comprises a hard shell and a soft core. (a) and (b) Pictures of corncob-cores and corncob-cores coated by PPy (CC–PPy), respectively. (c) FE-SEM of the as-prepared CC–PPy, displaying a blooming flower-like sponge microstructure. (d) Cross-section of a PPy coated corncob-core micro-sheet with a thickness of ∼850 nm.

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.

2 Experimental

2.1 Chemicals

Ferric chloride hexahydrate (FeCl₃·6H₂O), pyrrole monomer (Py), ethanol and potassium dichromate (K₂Cr₂O₇) were provided by Aladdin Chemical Reagent Co., Ltd. (Shanghai, China). Sodium hydroxide (NaOH) and hydrochloric acid were purchased from Sinopharm Chemical Reagent Co., Ltd. (Beijing, China). All chemicals were used as received without any further purification. The corncobs were got from local farm. After removing the hard shell of the corncobs, the inner part like sponge (corncob-core) was cut into small granules. The corncob-core granules' diameter was in the range of around 2 mm–2 cm, which were selected by sieving.

2.2 Preparation of the adsorbents

0.4 g of corncob-core granules were washed with ethanol and deionized water in turn both for three times, and then dried in air at 60 °C for 12 h. Then the corncob-core granules were immersed in an aqueous solution of pyrrole (0.8 g of pyrrole in 50 mL of distilled water) under magnetic stirring of 6 h. 3.2 g of ferric chloride hexahydrate (FeCl₃·6H₂O) was dissolved in 15 mL of deionized water. After adequate dissolution, the Fe(III) ions solution was dripped into the solution with corncob-core inside slowly under magnetic stirring. Then the PPy layer gradually formed on the corncob-cores with the FeCl₃ as the polymerization initiator.³⁶ After 1.5 h polymerization, the as-prepared products were rinsed with ethanol and deionized water in turn both for three times and then dried in a dry oven at 60 °C for 12 h.

2.3 Adsorption experiments

(i) The Cr(VI) solution was prepared by dissolving potassium dichromate (K₂Cr₂O₇) in aqueous solution. In order to determine the concentration of Cr(VI) ions, a calibration curve of Cr(VI) ions was made by UV-vis spectrophotometer. For the effect of Cr(VI) concentration on the adsorption capacity experiment, the as-prepared CC–PPy adsorbents was added into Cr(VI) solution with different (the adsorbent dose: 3 g L⁻¹) concentrations, and the adsorption was conducted at pH = 3.5 at 25 ± 2 °C. The solution was taken out for ultraviolet analysis, after a certain reaction time under magnetic stirring, to determine the Cr(VI) concentration. Based on calibration curve, the removal amount of Cr(VI) ions could be calculated. The adsorption capacity of Cr(VI) could be calculated by the following formula,

q_t = (C₀ − C)V/m (1)

where V (L) the initial volume of Cr(VI) solution, m (g) the mass of the adsorbents, C₀ (mg L⁻¹) the initial concentration of Cr(VI) ions, C (mg g⁻¹) the concentration of solution and q_t (mg g⁻¹) the adsorption capacity at time of t, respectively. The removal percentage of Cr(VI) can be determined by the following equation:

Removal percentage = (C₀ − C_e/C₀) × 100 (2)

where C₀ is the initial concentration of Cr(VI) in solution (mg L⁻¹) and C_e is the equilibrium concentration (mg L⁻¹).

(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(q_e − q_t) = ln q_e − k₁t (3)

where q_t and q_e (mg g⁻¹) are the amount of Cr(VI) adsorbed over a given period of time t and at equilibrium, respectively; t is the adsorption time (min); and k₁ (1/min) and k₂ (g mg⁻¹ min⁻¹) are the adsorption rate constant of the pseudo first-order adsorption and the pseudo-second-order adsorption, respectively.

(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.

2.4 Characterization

The microstructures of the products were observed on a Helios Nanolab600i field emission scanning electron microscope (FE-SEM). FT-IR spectra of the samples were recorded on a PerkinElmer Spectrum One B spectrometer with KBr as the reference. X-ray photoelectron spectroscopy (XPS) was recorded with a Kratos ASIS-HS X-ray photoelectron spectroscope equipped with a standard and monochromatic source (AlKR) operated at 150 W (15 kV, 10 mA). The absorption properties of the samples were characterized by UV-vis spectrophotometer (Shanghai Metash Instruments Co., Ltd., China). The pH of the solution was analysed by Sartorius PB-10 (Sartorius, Germany). The mechanical property of the CC–PPy was measured under tensile mode on a universal materials testing machine (SHIMADZU, AG-I 20KN).

3 Results and discussion

A corncob can be divide into two parts, as shown in Fig. 1a, i.e. the hard shell and the soft and low-density core. The core of a corncob presents a unique macroporous spongeous microstructure, which is composed of sub-microsheets (Fig. S1†). As depicted in the experimental part, the CC–PPy composite microstructured sponges were prepared through a solution-polymerization by using corncob-core as the substrate. The colour of the materials can be clearly observed from milk white to black after the polymerization, indicating the PPy layer has been formed in the paper structures (Fig. 1b). The morphologies of the as-prepared CC–PPy were then characterized by SEM, which display a blooming flower-like microstructure, with macropores inside the sponges (Fig. 1c and S2†). From Fig. S2,† there is no PPy particles emerged in the sponge pores, indicating that PPy grew along the corncob-core micro-sheets. Fig. 1d presents a SEM image of a single CC–PPy micro-sheet with a thickness of ∼850 nm.

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 C=C 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

Fig. 2 The FT-IR spectra of pure corncob-core and CC–PPy.

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 (–C=N–), 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.

Fig. 3 XPS spectra of CC–PPy. (a) Full survey spectrum, (b) N 1s.

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 q_e 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

where q_m stands for the maximal adsorption capacity of the metal ions on the adsorbent (mg g⁻¹) and K_L denotes the adsorption equilibrium constant. The value of q_m is taken as the inverse value of the slope of the plot of C_e/q_e versus C_e. The Langmuir isotherm was presented in Fig. 6. From the inset, the adsorption data of the Cr(VI) ions were fitted well with the Langmuir model with a correlation coefficient of R² = 0.999, indicating a chemical homogeneity of the adsorbents.¹⁷ The maximal adsorption capacity of Cr(VI) ions on CC–PPy could be determined to be around 84.7 mg g⁻¹ at the temperature of 25 °C. The maximum adsorption capacity (q_m) is an important figure of merit to assess an adsorbent. A comparison of q_m between CC–PPy and other PPy based and corncob based Cr(VI) adsorbents reported in literature was presented in Table 1. It was observed that the adsorption capacity of the bulk-style adsorbents of CC–PPy was lower than that of some PPy based nano-powder adsorbents. However, our CC–PPy adsorbents exhibited larger superiority at the aspects of separation process and fabrication/operation cost than the artificial nanostructures. On the other hand, it can be seen from Table 1 that the adsorption capacity of the CC–PPy was several times higher than other bulk-style PPy/biomass composites, which should be due to the unique microstructure of CC–PPy. The macroporous spongeous structures could promote the mass transfer of pollutants in the adsorbents, meanwhile quasi-2D microsheets would enhance reactive sizes. Additionally the performance of CC–PPy surpasses other corncob based materials, obviously.

Fig. 6 Fit of equilibrium data to Langmuir isotherm model.

Table 1 Comparison of Cr(VI) adsorption performance of the CC–PPy with other adsorbents

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

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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 2p_3/2 and Cr 2p_1/2, respectively.⁵⁰ Additionally, the Cr 2p_3/2 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

Fig. 7 XPS spectra of CC–PPy after adsorption, (a) wide scan of Cr 2p and (b) Cr 2p_3/2.

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 (q_e) 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.

Fig. 8 (a) Effect of coexisting anions on the Cr(VI) adsorption performance. (b) The reusability of the CC–PPy adsorbent. (c) Pictures of a flexible CC–PPy column. The bending test was conducted after the sample was saturated with water. (d) Typical stress–strain curve of a CC–PPy sponge column.

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 2p_3/2 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.

4 Conclusion

A high-performance and cost-effective adsorbent was rationally designed for Cr(VI) removal from aqueous solution based on PPy modified corncob-cores via a simple solution polymerization. The PPy active layer provided reactive sizes to detoxifies Cr(VI) ions by ion-exchange. Meanwhile the core of corncobs exhibited a unique macroporous spongy microstructure, which is composed of quasi 2D sub-microsheets, thus could enhance the adsorption performance by increasing active areas and facilitating the access of pollutants inside the bulk-style adsorbents. Compared with artificial nano-powder adsorbents, CC–PPy displayed large 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 selectivity of CC–PPy adsorbents were also outstanding. The present development provides a solution to efficiently treat with Cr(VI) polluted water, and demonstrated a new route for agricultural waste utilization. It may also offers a useful route for high-performance adsorbents and catalysts in other fields.

Acknowledgements

The authors gratefully acknowledge National Natural Science Foundation of China (Grant no. 21304024, 51573034), State Key Laboratory of Urban Water Resource and Environment in HIT of China (2014TS02), Fundamental Research Funds for the Central Universities of China (HIT.BRETIII.201417), Postdoctoral Science Foundation of China (2014T70324) and Heilongjiang Prov. (LBH-TZ0606).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra07687d

‡ The authors contributed equally to this work.

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