Hollow carbon fiber sponges from crude catkins: an ultralow cost absorbent for oils and organic solvents

Abstract

In recent years, three-dimensional (3D) carbon-based frameworks have shown great potential as absorbents for oils or organic solvents, but the fabrication of carbon-based materials with low-cost and high-performance is still needed. In this paper, hollow carbon fiber sponges (HCFSs) have been synthesized through the pyrolysis of bulk crude catkins which are ultralow-cost, have a hollow structure and are a carbon-based fibrous material. Owing to their 3D framework and special structure, the HCFSs exhibited a high absorption capacity for organic solvents and oils (55–190 times their own weight) and excellent recyclability. Coupled with the convenient, economical and environmentally-friendly treatment process, HCFSs will be a promising candidate as a kind of absorbent for the removal of pollutants.

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Introduction

The shortage of clean water has become not only a serious problem in human society, but also a threat to the ecological balance of the earth.¹ While there is a lot of water on earth, only 14% can be utilized by humans and the vast majority of animals as a source of drinking water.² Pollution and the expansion of pollution are caused a great deal of pressure to the present and future supply of clean water. It is said that by the 2030s a third of people will suffer from a lack of fresh water resources, and most of those people will be present in developing countries, especially with the rapid development of industry and deterioration of the water environment.³ According to the report of United Nations, there are several hundred million tons waste water containing of heavy metal ions, dyes, oils and other organic contaminants, are released into the natural environment annually.⁴ Meanwhile, massive oil spills during drilling and transportation of crude oils have also bring water resources greatly anthropogenic disaster, especially marine, rivers and lakes.⁵

In this grim situation of water pollution, a number of effective measures have been taken by many people of insight to protect and remediate water. Among those measures, water purification technologies have been recognized as one of the most effective ways to address the urgent situation related to clean water. Many methods for water remediation have been developed in the last few decades, including ion exchange, electrolysis, and sorption, to remove pollutants from aquatic ecosystems. Among these methods, sorption is one of the most widely used techniques for water remediation due to its outstanding characteristics, such as cost-effectiveness, eco-friendliness, and fast performance.^6–10 Recently, many sorbents have been developed such as activated carbon,^11,12 expanded graphite,¹³ minerals,^14,15 metal oxides,^16–18 agriculture wastes,^19–23 and so on. As sorbents, carbon based materials have attracted wide attention for water remediation for its stability and recyclability.^11,12 To date, shortage of resources drives people keen to choose a low value renewable materials to prepare carbon based sorbents for treatment of water pollution such as winter melon,²⁴ raw cotton,^25,26 kapok wadding,²⁷ bacterial cellulose,²⁸ bamboo chopsticks,²⁹ and so on. However, developing low-cost and high-performance carbon-based 3D frameworks is still a great challenge and highly desired. Among them, only three types of biomass (cellulose, cotton and kapok wadding) which are composed of interconnected 3D networks of fibers have been utilized to synthesize carbon-based 3D frameworks for efficient sorption of oils and organic solvents.^25–28 Particularly, the synthesis process of carbon-based 3D frameworks from bacterial cellulose seems to be very complicated and expensive; as well, raw cotton is an economical plant, which may not be suitable for fabricating carbon-based 3D frameworks from the viewpoint of economics. Although kapok wadding is considered as the cheapest raw materials for absorbents, but it still has some economic value, such as filler for bedding, pillows and cushions, blending with other fibers for life jackets, knitted underwear, fleece, wool sweater and so on. Fortunately for us, catkins, an ultralight fibrous product, deriving from the willow trees, offer a low-cost and environment-friendly alternative for synthesizing carbon-based 3D frameworks, since they are widely distributed in the world and considered as solid wastes.

Herein, through utilizing catkins as raw materials, we have synthesized hollow carbon fiber sponges (HCFSs) with several advantages of low cost, simple fabrication, excellent superhydrophobicity and oleophilicity. And the HCFSs can absorb a variety of oils and organic solvents with a maximal absorption capacity up to 190 times their own weight. In addition, the HCFSs exhibited excellent recyclability and maintained a high absorption capacity even after five cycles through squeezing and combustion. We believe that HCFSs deriving from natural catkins will show great potential for industrial applications in environmental protection.

Experimental

The preparation of HCFSs

The crude catkins were collected from willow trees in Harbin, China. Before using them, the catkins were removed seeds and impurity, and dried in 80 °C for 12 h. 2 g white crude catkins were pressed in three same ceramic vessels, respectively. Subsequently, they were heated to 500, 700 and 900 °C at the rate of 5 °C min⁻¹ and kept for 2 h under nitrogen atmosphere. Finally, the products were cooled down to room temperature naturally to obtain HCFSs. The sample names were called as HCFSs-5, HCFSs-7 and HCFSs-9. The preparation and application of the HCFSs were showed in Fig. 1.

Fig. 1 The preparation and application of HCFSs.

Characterization

The morphology and structure of crude catkins and HCFSs were characterized by scanning electron microscope (SEM, S-4800), X-ray diffraction (XRD) patterns (D8 ADVANCE), Raman microscope (Jobin Yvon, HR800), X-ray photoelectron spectroscopy (XPS, AXIS Ultra DLD, Kratos) and Fourier transform infrared (FTIR) spectra (ADVANCE III). Thermo gravimetric analyse (TGA) was conducted on PE TGA-7 (PE) under N2 atmosphere at a heating rate of 10 °C min⁻¹ to 800 °C. Brunauer–Emmett–Teller (BET) surface area measurement (ASAP 2020) was carried out by nitrogen adsorption and desorption. Contact angle measurements for samples were performed using a contact angle meter (OCA20, Dataphysics) by placing a water droplet (∼5 μL) on the surface of samples.

Oil absorption and recyclability of HCFSs

The absorption capacity for various oils and organic solvents was evaluated at room temperature. In a typical test, the HCFSs were placed in contact with oils or organic solvents until the sample was filled with them completely. The weight measurements of the HCFSs with absorbed organic liquid were done quickly to avoid evaporation of the absorbed material. The absorption capacity is calculated as = (m − m_o)/m_o × 100 wt%, where m is the weight of sample after absorption and m_o is the initial weight of the sample.

The recyclability of the HCFSs was characterized by burning and squeezing operations. After absorption to saturation, ethanol in the HCFSs was burned off in air and reused for subsequent absorption experiments. Each sample was tested for 5 cycles of absorption/burning. The squeezing method was used for octadecylene absorption experiment. After saturation absorption, the HCFSs were squeezed for 5 s, and then weighed and reused for subsequent absorption tests.

Results and discussion

Morphology and properties of HFCSs

Crude catkins exhibited microtubular structure in Fig. 2a. From Fig. 2b–d, all of fibers became pipes after carbonization treatment, indicating that calcination process retained the tubular structure. And the nitrogen adsorption–desorption isotherm showed that the HCFSs-9 had a high BET surface area of ∼438 m² g⁻¹ (Fig. 3a). It can be seen from Fig. S1† that shrinkage ratios of the sponges changed along with increasing the pyrolysis temperature. In addition, the shape and size of obtained sponge can be controlled by using various shapes of containers.

Fig. 2 SEM images of willow catkins (a) and their carbonized samples at 500 °C (b), 700 °C (c) and 900 °C (d).

Fig. 3 (a) Nitrogen adsorption–desorption isotherm of HCFSs (900 °C) and (b) XRD spectra of the HCFSs prepared by pyrolysis at different temperatures.

Due to the pyrolysis treatment, the HCFSs lost part of oxygen-containing functional groups. Compared with the FT-IR spectrum of crude catkins, the main absorption peaks of them became weak and even disappeared in Fig. S2† suggesting the removal of hydrophilic moieties. The XPS spectrum in Fig. S3 and Table S1† also draw the corresponding conclusion that the relative content values of C/O and C/N trended to increase along with the pyrolysis temperature. Moreover, the existence of N element was conducive to improve the fire-resistance of carbon materials.^30,31 As shown in Fig. S4,† the HCFSs lost little weight and possessed outstanding thermostability when they were performed even at 800 °C under air atmosphere. XRD patterns of the crude catkins and the HCFSs were showed in Fig. 3b. For the crude catkins, there was a diffraction peak around 21.7° which was similar to that of cotton fibers.³² After pyrolysis treatment, a broadened and low peak centred at about 23.4° corresponding to the (002) plane of graphite.³³ There were also two characteristic peaks related to the D and G bands located around 1375 and 1570 cm⁻¹ in Raman spectra of Fig. S5,† and the relative intensity ratio of the D to G bands (I_D/I_G) represented disorder or defects in the carbon structure.³⁴

The above results revealed that the conversion of carbon structure after thermal treatment can make carbon materials possess hydrophobicity and lipophilicity, which was further confirmed by the tests of Fig. 4. It was clearly seen in Fig. 4b and d that spherical water droplets formed on the surface of the HCFSs-9, and the measured contact angle was 152.5°. When immersing the HCFSs-9 into water, a uniform mirror-reflection was observed on their surface due to the air bubbles entrapped at the interface between the HCFSs and the surrounding water (Fig. 4e). In addition, water droplets did not fall out of the HCFSs when they were placed upside down (Fig. 4a and c). The phenomenon demonstrated the sponges had excellent adhesion property, which was conducive to absorb oils or organic solvents underwater.

Fig. 4 Photographs of adhesion property (a) and hydrophobicity property (b) when water droplets were placed on the surface of thin HCFSs-9. (c) and (d) were their contact angles, respectively. (e) Mirror-reflection between HCFSs and water after immersing into water.

Absorption behavior of HCFSs for oils and organic solvents

The HCFSs possessed 3D porous structure and surface superhydrophobicity property, which made it an ideal candidate for the removal of pollutants such as oils and organic solvents. The absorption capability of HCFSs-9 was demonstrated in Fig. 5. When a piece of HCFSs-9 was brought into contact with a pump oil layer (stained with C. I. pigment yellow 12) on a water surface, it absorbed the oil completely within 30 seconds (Fig. 5a). Owing to its low density and hydrophobicity, the HCFSs-9 can float on the water surface after absorbing the oil, indicating the ease for recycling. In addition, the HCFSs-9 can also be used to absorb organic solvents denser than water, such as chloroform (stained with Sudan red III) (Fig. 5b). The specific time for absorbing other oils and organic solvents was provided in Fig. S6.†

Fig. 5 Removal process of pump oil from the water surface (a) and chloroform from underwater (b) by a piece of HCFSs-9.

To acquire the absorption capacity quantitatively, the weight gain (wt%) is defined as the weight of absorbed substance per unit weight of the pristine HCFSs materials. The absorption capacity of various kinds of oils and organic solvents was investigated such as petroleum products (pump oil, etc.), water-immiscible solvents (hexane, toluene, etc.). These materials are common pollutants in our daily life and industries. The HCFSs-900 showed a very high absorption capacity for all of the aforementioned oils and organic liquids ranging from 55 to 190 times of its original weight (Fig. 6). Importantly, our materials showed higher absorption capacity than many previously reported sorbents (Table 1),^{13,24,29,35–40} such as wool-based nonwoven (9–15 times),³⁵ winter melon aerogel (WCA) materials (16–50 times),²⁴ exfoliated graphite (60–90 times),¹³ magnetic CNT sponge (49–56 times),³⁶ vegetable fiber (1–100 times).³⁷ Additionally, the absorption capacity of HCFSs materials is also comparable to that of materials with high absorption capacity, for example, multi-functional carbon fiber (MCF) aerogel (30–129 times),²⁹ carbon microbelt (CMB) aerogel (56–188 times),³⁸ graphene sponges (60–160 times)³⁹ and CNT sponges (80–180 times).⁴⁰ Although the absorption capacity of HCFSs materials is still lower than that of MMC materials (87–273 times),²⁷ CNF aerogel (106–312 times)⁴¹ and nitrogen-doped graphene foam (200–600 times),⁴² the fabrication method of HCFSs is simpler and its precursor material (natural catkins) is cheapest among all these sorbents. Therefore, our HCFSs material is a cost-effective and promising sorbent for the removal of pollutants. In addition, absorption capacity of HCFSs-5 and HCFSs-7 for oil and organic solvents was shown in Fig. S7.† It can be seen that the absorption capacity of HCFSs trended to decrease slightly with the calcination temperature increasing.

Fig. 6 Absorption capacity of HCFSs-9 for oils and organic solvents.

Table 1 Comparison of various sorbent materials

Sorbent materials	Absorbed substances	Absorption capacity (g g^−1)	Cost	Ref.
Wool-based nonwoven	Diesel, crude oil	9–15	Low	35
WCA materials	Oils and organic solvents	16–50	Low	24
Exfoliated graphite	Heavy oil	60–90	Low	13
Magnetic CNT sponge	Oils and organic solvents	49–56	High	36
Vegetable fiber	Crude oil	1–100	Low	37
MCF aerogel	Oils and organic solvents	30–129	Low	29
CMB aerogel	Oils and organic solvents	56–188	Low	38
Graphene sponge	Oils and organic solvents	60–160	High	39
CNT sponges	Oils and organic solvents	80–180	Low	40
MMC materials	Oils and organic solvents	87–273	Low	27
CNF aerogel	Oils and organic solvents	106–312	Low	41
Nitrogen doped graphene foam	Oils and organic solvents	200–600	High	42
HCFSs	Oils and organic solvents	55–190	Low	Present work

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The recyclability of absorbent and the removal efficiency of pollutants also play important roles in pollution control and environmental protection because most pollutants are either precious raw materials or toxic, e.g., crude oil and toluene.^26,43 For the further recycle tests, combustion and squeeze methods were performed as shown in Fig. 7a and c. To demonstrate combustion test, ethanol was absorbed by HCFSs-9 materials. After 5 cycles of absorption–combustion process, the absorption capacity of the sponge dropped by 5.8% compared to the capacity in the first cycle (Fig. 7b), most likely due to the deposition of residues on the surface of fibers after combustion of ethanol. For the absorption of those pollutants with high boiling point, squeezing was a simple and easy-applied method. The squeezing recycle of HCFSs-9 was studied using octadecylene as a probe liquid. For the procedure of the squeezing operation, we first used a sponge to absorb 670 mg octadecylene. Subsequently, the sponge was squeezed by a tweezer to remove octadecylene in the greatest degree. As a result of the incomplete compression, about 103 mg solvent was remained in the sponge after the first cycle. We continued to use the above sponge to absorb octadecylene (about 505 mg) until saturation condition. And then the same procedure was repeated until five cycles. It can be observed from Fig. 7d, about 20.4% of absorbed octadecylene was left in the second cycle and the absorption capacity of sponge possessed 55% of the original value after five cycles due to the incomplete compression of sponge. Therefore, we found as the recycle times were raised, the absorption capacity of sponge decreased while the remnant mass of solvent gradually increased. Although the squeezing was less effective as compared with the combustion, the energy consumption and operation convenience of this method make it very competitive in practice.

Fig. 7 (a) Photographs showing the process of recycling HCFSs-9 via combustion. (b) Photographs showing the process of recycling HCFSs-9 via squeezing. (c) Combustion was applied to recycle the HCFSs-9 for absorption of ethanol and (d) squeezing was used to recycle the HCFSs-9 for absorption of octadecylene.

Conclusions

In conclusion, HCFSs from natural fibers have been prepared by a simple method. The raw materials possessed the characteristic economy, environmentally friend and recyclability. The HCFSs materials had a high absorption capacity of 55–190 times its own weight. The materials can be recycled and repeatedly used via a simple method of combustion or squeezing. Importantly, the natural source and simple preparation method make it cost-effective for possible industrial application. Therefore, the HCFSs can be used as an economic, efficient and safe absorbent for removal pollutants.

Acknowledgements

The present study has been supported by NSFC (51372072 and 21372067), and China postdoctoral science foundation (2014M561545).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra08183e

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