A Low-cost, Sustainable and Environmentally Sound Cellulose

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A Low-cost, Sustainable and Environmentally Sound Cellulose Absorbent with High Efficiency for Collecting Methane Bubbles from Seawater Nan Li, Wei Chen, Guangxue Chen, Xiaofang Wan, and Junfei Tian ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00146 • Publication Date (Web): 11 Apr 2018 Downloaded from http://pubs.acs.org on April 11, 2018

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ACS Sustainable Chemistry & Engineering

A Low-cost, Sustainable and Environmentally Sound Cellulose Absorbent with High Efficiency for Collecting Methane Bubbles from Seawater

Nan Li a,1, Wei Chen b,1, Guangxue Chen a, Xiaofang Wan a, Junfei Tian a, *

a

State Key Laboratory of Pulp and Paper Engineering, South China University of Technology,

Guangzhou 510640, China b

College of Engineering, Qufu Normal University, RiZhao 276826, China

1

Nan Li and Wei Chen contribute equally to this work as first authors.

* Corresponding Author E-mail: [email protected] (J. Tian)

KEYWORDS: hydrophobic; aerogel; cellulose; methane absorption

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ABSTRACT:

The use of natural materials to solve environmental problems will not

increase extra burden on global environment. In this paper, we describe a natural, recyclable and low-cost method for terminating methane bubbles under water before it releasing into atmosphere. The cellulose aerogel with porous structure and large surface area was fabricated via combining dry pulp disintegration process and the following freeze drying. The aerogel became superhydrophobic after modified with methyltrimethoxysilane (MTMS), therefore it was able to effectively collect and store transport methane bubbles under water. The hydrophobic cellulose aerogel (HCA) exhibited an exceptional performance for collecting methane bubbles under both static condition and dynamic absorption-release condition. We hope this work could inspire alternative methods for the exploitation of methane with the potential to alleviate global warming.

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INTRODUCTION Cellulose is one of the most abundant naturally-occurring polysaccharides, which exists in plants, certain bacteria, algae, sea animals and fungi

1-2

. Due to its low cost,

biodegradability, and non-toxicity, cellulose is thereby considered as a promising environment-friendly material and one of the main sources of renewable material 3. Based on these advantages, various forms of cellulose and its derivatives have been considered as desirable candidates to solve environmental and ecological problems

4-7

. A great example is

cellulose aerogel (CA) which has attracted much interest and been extensively studied from many scientists. Cellulose aerogel is defined as a class of light-weight porous materials based on cellulosic polymer with low density, large specific surface area and porosity

8-10

.

Compared to other kinds of aerogels (e.g., silica aerogels, carbon-based aerogels), cellulose-based aerogel is more natural and has significant advantages from an economic and environmental perspective. Besides that, the aerogel fabrication proposed in this paper is a green process, which requires neither hazardous solvent nor complex process. Many attempts have proven that cellulose aerogels are sustainable and inexpensive sorbent materials for eliminating environmental problems. Various examples can be found in the fields of cleaning up of oil spills and removing of hazardous ions from aqueous mixtures 11-14

. However, as far as we know, cellulose-based aerogel has rarely been applied to solve

other major environmental problems such as the emission of greenhouse gases. In fact, the large surface area and porosity of cellulose-based aerogel are desirable properties for absorbing and storing greenhouse gases such as marine methane bubbles. Climate change is one of the most significant and timely issues challenging the world today. Over the past century, the mean surface temperature of our global has increased by 0.3–0.6°C due to the emissions of greenhouse gases. The changes leaded many unexpected global problems including the thawing of permafrost and sea ice, retreat of glaciers, which

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caused the river discharge increases, terrestrial and water ecosystems alteration. In the twenty-first century, the continuous rising concentrations of greenhouse gas could trigger even greater warming. Methane (CH4) is arguably the most dynamic and important greenhouse gas. It contributes about 20% to global warming, and the potential of global warming of CH4 is about 25 times greater than that of CO2 15. Thus, CH4 would be considered as an important climate forcing agent today and in the future. Aquatic systems (Ocean, lakes, reservoirs, streams, and rivers) are potentially significant sources of atmospheric CH4

16-17

. Generally, methane

produced in sediments under these aquatic systems could reach the atmosphere directly via rising bubbles. This situation is likely to become more severe in the future because of the supersaturation of dissolved gas, and the continuous decomposition of methane hydrate in the deep ocean and lakes. Considering a greater number of methane-bubbling seep sites have been found in both deep and shallow waters worldwide, it is therefore imperative that we strive to find a high efficiency, selectively, elastic, low-cost, sustainable and environmentally sound absorbent to directly terminate methane bubbles in waters to mitigate future changes. Several methods have been used to eliminate methane, including photochemical elimination and microbial oxidation

18-21

.

Nevertheless, these approaches could only

eliminate methane from atmosphere. The use of sorbents like zeolites, activated carbonaceous materials, porous coordination polymers (PCPs), microporous/mesoporous silica or metal-organic frameworks (MOFs) have been considered as the most effective approaches for methane absorbing due to its porous structures

22-23

. Nevertheless, the major limitations of

these traditional porous sorbents are insufficient CH4 adsorption capacity, hydrophilic properties and poor recyclability. More recently, Xiao Chen et al. reported the synthesis of superhydrophobic porous foaming polyurethane (PU) sponges by a simple dipping process, which provides the possibility of trapping methane bubbles as well as preventing water

4


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permeation

24

. Although, the modified PU exhibited excellent methane absorption, their

non-biodegradable properties may create extra waste to marine environment. Inspired by their work, we use cellulose aerogel, which has the similar porous structure as polyurethane but more environmental friendly, to absorb methane bubbles. The combination of a simple fabrication method and functionalized cellulose aerogels have potential in replacing environmental unfriendly polymer-based materials and could be a useful absorbent for terminating methane bubbles under water. In this study, we proposed an environmental friendly and efficient approach for collecting and transporting methane bubbles via innovative using cellulose-based aerogels. These light-weight, hydrophobic and porous aerogels were prepared via freezing-drying process followed by the modification with MTMS by using thermal chemical vapor deposition (CVD) method (Scheme 1a). As mentioned above, cellulose based aerogels show lots of advantages of using as methane absorbent material, since it not only has high absorption capacity but is nontoxic, sustainable, bio-degradability and low-cost (Scheme 1b). We have intensively studied the interaction between the cellulose aerogels and methane bubbles in aqueous solution and investigated the influence of porosity and submerged depth of cellulose aerogels to the methane absorption capacity. Notably, two different kinds of dynamic processes which consist of methane absorption-release cycles were conducted and compared in this study for the first time.

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Scheme 1.

(a) Schematic illustrations of the design of the hydrophobic cellulose aerogel. (b)

Multivariant aspects of HCA for terminating methane bubbles.

EXPERIMENTAL SECTION Materials.

Cellulose fibers were obtained from softwood dry pulp, which was supplied

by Chen Hui Co. Ltd (China). Methyltrimethoxysilane (MTMS), and oil red, were purchased from Aladdin Industrial Corporation. Sodium chloride, calcium chloride and potassium chloride were supplied by Sinopharm Chemical Reagent Co. Ltd (China). Edible oil was purchased from Jingke Trading Co. Ltd (China). All the solutions were prepared with deionized water.

Preparation of cellulose aerogels.

A piece of softwood dry pulp board was

immersed in deionized water to fully wetted and then teared into pieces. After that, the wetted pulp was soaked the pulp specimen overnight. Next, the pulp specimen was diluted to 2 L, and treated by a pulp disintegrator (Lorentzen & Wettre, Sweden) for 12,000 revolutions to separate into individual cellulose fibers. The separated cellulose fibers were dried in an oven. Subsequently, 30g distilled water and a certain amount of dried cellulose sample with concentrations of 0.5 wt%, 0.8 wt%, 1.0 wt%, and 1.5 wt% were prepared and acutely agitated under vigorous magnetic stirring. Next, the uniform cellulose fibers suspensions were frozen(-20°C) and freeze-dried (-50 °C, 2 days) to produce cellulose aerogels. The cellulose aerogels with different compositions were designated as CAx, where x designates the cellulose concentration (wt%，Table S1).

Preparation of HCAs.

Thermal chemical vapor deposition (CVD) method was

employed to modify CA and change its surface property from the hydrophilicity to the hydrophobicity via silanization. This experiment was conducted in a set of three bottles: two small open bottles, which contained water and methyltrimethoxysilane (MTMS) respectively, 6


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were placed in a closed big bottle which also contained several pieces of CAs. The silanation process was conducted at 70 °C for 2 h. After the unreacted silane was removed by a vacuum oven, the modified CAs were kept in a desiccator before use.

Fourier transform infrared (FTIR) spectroscopy.

About 0.5-1.0 mg CAs and HCAs

samples were ground into powder form and mixed with KBr, then the mixtures were pressed into ultra-thin pellets. The FTIR spectra of CAs and HCAs were obtained by a FTIR spectrometer (Bruker, Germany). The scan range was 400-4000 cm-1.

Scanning electron microscopy (SEM).

SEM analysis was performed by a Zeiss

EVO 18 SEM (Germany) operating at 10 kV. For the analysis of the samples, CAs and HCAs were coated with gold by a plasma sputtering apparatus.

Energy-dispersive X-ray (EDX) spectra.

The relative elements contents of CAs and

HCAs were determined by EDX spectra. The conditions of the EDX tests are almost the same as those used for SEM characterization, except that the samples were thicker and not sputtered with gold.

X-ray photoelectron spectroscopy (XPS).

The elemental compositions of CAs and

HCAs were determined by an X-ray photoelectron spectrometer (Axis Ultra DLD, Kratos). The chamber pressure was kept at ~5×10-9 torr.

Contact angle measurements.

The surface wettability of the MTMS-modified

aerogels was evaluated by contact angle value. which was measured by an OCA40 Micro (Data Physics Instruments), combined with a camera. For each test, a droplet (3µL) of deionized water was applied. The contact angle values were obtained by using the software of contact angle meter analyzing the shape of water droplet in the image. For each sample, contact angle measurements were conducted at 5 times in order to calculated the average value.

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Calculation of the density and porosity.

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The volumetric mass density of the

aerogels can be calculated according to the following equation:

=

(1)

Where is the weight (g) and is the volume (cm3) of the aerogel. Porosity of the cellulose-based aerogel was calculated using the following equation: !

% = 1 −

"!!#!$

% × 100

(2)

Where and ()* are the density of the aerogel and crystalline cellulose (1.528 g/cm3), respectively.

Static methane absorption of HCA.

The absorption capacity of submerged HCA to

methane bubbles under static condition was determined in this experiment. The artificial sea water and methane gas was prepared and collected according to previous literature

24

. The

dimension of each HCA was measured at least five times to calculate the volume of the HCA. The absorption capacities of HCAs with various cellulose concentrations were conducted by dipping a series of HCAs into the target sub-merged depth (3.3cm), the dependence of absorption capacity and the submerged depth was evaluated by dipping HCA1.0 at different depths to test the maximum absorption volume of methane. Each test was repeated five times to obtain the average value of the absorbed methane volume. In order to record the absorbed volume, a graduated manual syringe with a certain amount of methane was employed to continuously release methane bubbles. When the HCA reached the saturation status, the extra methane bubble escaping from the HCA could be observed and the absorbed volume was recorded. The volumetric sorption capacity was obtained by the following formula:

+, = ,

- -$

(3)

Where . is the volume of the absorbed methane and .* is the volume of cellulose based aerogel. 8


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Dynamic methane absorption of HCA.

In the first dynamic absorption test, a manual

syringe was used to release methane gas until the HCA became saturated with bubbles. Then, we manually compressed the HCA with a certain strain to ensure that most absorbed methane was released. After we removed the stain, the HCA might slowly and partially rebound due to its elastic property and was able to absorb methane bubbles in another circle. This dynamic compression-recovery procedure was repeated 10 times. After each cycle, the methane absorbency was measured. In the second dynamic absorption test, an external force was given by modified tweezers after compression to gradually recover the shape of the HCA during the absorption process (Figure S1). In this test, the HCA was sandwiched between two squares of cover glass which were stuck on the two legs of tweezers. After the HCA was full of methane, it was compressed to about 75% of its original volume. During the absorption process, the original shape of HCA was recovered by the tension of the tweezers and a rubber band. This dynamic test was also repeated 10 times.

RESULTS AND DISCUSSION Hydrophobic modification of CA. Figure 1a shows the FT-IR spectra of the pristine cellulose aerogel and MTMS modified cellulose aerogel. Compared with the spectrum of pristine cellulose aerogel, two new peaks at 2973 cm−1 and 1273 cm−1 were found in the FT-IR spectrum of MTMS modified cellulose aerogel, which stemmed from C-H stretching vibrations and C-H in-plane bending of methyl group, respectively. Moreover, small peaks at 854 cm−1 and 779 cm−1 were observed which correspond to the bending of Si-O-Si and C-Si of methyltrimethoxysilane

25

. These results confirmed the exits of the covalent bonds and

strong interface interaction between cellulose and organosilane. This emerged strong interface interaction could be beneficial for the maintenance of the hydrophobicity of HCA in practical applications.

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Figure 1.

(a) FT-IR spectra of the pristine cellulose aerogel and MTMS modified cellulose

aerogel. (b) XPS spectra of MTMS modified and unmodified cellulose aerogels. EDX spectra of CAs: (c) CA and (d) HCA. SEM images of (e) CA0.8, (f) HCA0.8, (g) CA1.5 and (h) HCA1.5 (scale bar of 100 µm).

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X-ray photoelectron spectroscopy (XPS) was applied to analyze chemical compositions of cellulose based-aerogels before and after MTMS modification. From figure 1b, shows the XPS spectra of MTMS modified cellulose aerogels in which new peaks at 154 eV and 102 eV corresponding to Si 2s and Si 2p, respectively, indicating the presence of silicon. From figure 1c and 1d, we can find the successful salinization on the porous surface of the CA, which is also proved by the EDX analysis. The EDX spectrum of CA shows oxygen and carbon peaks, but no silicon peak. After salinization, the EDX spectrum shows a small peak for silicon at a 2.08 at%. The morphology of hydrophobic CAs with cellulose concentration of 0.8 wt % and 1.5 wt % were examined and compared with neat 0.8 wt % and 1.5 wt % CAs using SEM and their microstructures are shown in figure 1e-h. According to these micrographs, aerogels with higher cellulose concentration (1.5 wt %) had lower porosity and forms a more compact network comparing with the aerogels with lower cellulose concentration (0.8 wt %). On a micro level, a continuously three-dimensional (3D) porous structure of CA consists of dispersive and disorderly cellulose fibers (Figure 1e and g). As shown in figure 1f and h, after hydrophobic modification the three-dimensional (3D) porous structure of HCAs were well maintained. Figure 2 shows the surface wettability of CAs before and after saline treatment. As shown in figure 2a, when a 3 µL water droplet contacted with the CAs, it immediately penetrated into the CA within 1s because of the presence of abundant hydroxyl groups in CAs. In contrast, the modified CAs became hydrophobic, on which the droplet of water maintained its round shape and its primary contact angle even after 120 s (Figure 2b). The water contact angle of the droplets was 149°± 3°. It is well known that the contact angle of the water droplets mainly depends on the functional groups on the surfaces of the cellulose-based aerogels. Hence, this variation of water contact angle is likely to be a result of the functional groups of -Si-O-CH3-, which were induced by MTMS modifying.

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Figure 2.

Water contact angles of (a) CA and (b) HCA. Edible oil and water were colored

by oil red and blue ink, respectively, and deposited on the surface of (c) CA and (d) HCA. Photographs of (e) CA0.5 and the HCA0.5 being placed on the surface of water and (f) HCA0.5 being immersed into water with an external force. (g) Optical pictures of a 4µL bubble toward a hydrophobic CA1.0 surface under water. As shown in figure 2c, both water droplet and oil droplet could penetrate into unmodified CA once they were applied onto the cellulose aerogel. After treated with MTMS, the HCA was only wetted by the oil droplet but repelled the water droplet (Figure 2d). These phenomena indicate the hydrophobic and oleophilic property of HCA. Figure 2e shows different behaviors between CA and HCA when they were placed onto water. After being placed on water surface, the modified cellulose aerogels showed a great water repellent behavior and could float on the surface of water. When external force was applied, HCA could be totally immersed in water. A mirror-like interface between the surrounding water and air trapped by HCA was formed (Figure 2f). After released the external force, the HCA rose to the surface immediately without absorbing any water, indicating its excellent water repellency (Figure 2e). However, different phenomena could be observed that the unmodified cellulose aerogel was quickly immersed and deformed in the water when it was deposited into

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the baker (Figure 2e). This hydrophobic-hydrophilic contrast clearly indicates the different surface groups of the two kinds of the aerogels due to the previous effective modification. It is also pointed out that the HCA could be broken down and gradually degrades in water after the loss of hydrophobicity due to the long-term aging. Comparing with the reported polymer sponge, this desirable property of HCA enables itself an environmentally friendly material with special potential for solving environmental problems. Figure 2g-f shows the superaerophilic property of the HCA underwater. When a 4 µL methane bubble contacted with HCA underwater, it quickly burst and penetrated into the inner part of the HCA within 50 ms showing the excellent methane bubble capture ability of the HCA. Since the absorption time was so short, the detailed performance was not able to be recorded by our camera. It can be seen that an interesting conjunction emerges between underwater superaerophilicity and in-air superhydrophobicity26-27. When HCA was dipped into water, the water could not penetrate into the HCA with low surface energy, and the air in the HCA matrix was enveloped by surrounding water (Figure 2f). Once methane bubbles contacted the HCA surface, it can coalesce with the blocked air phase as well as be absorbed into the HCA easily.

Properties of HCA. To determine the relationship between cellulose concentration and the microstructure of the cellulose-based aerogels, the densities and porosities of the cellulose-based aerogels were estimated and showed in figure 3a. The densities of the aerogel before and after the silane exhibited very high porosity (>98%) and low densities (< 21.27 mg/cm3), at high linearity with R2 of 0.9826 for unmodified CA and R2 of 0.9993 for HCA. The density of the modified aerogel increased about 10%.

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Figure 3.

(a) Densities and porosities of cellulose aerogels of different cellulose

concentration; (b) Height recovery of HCA0.8 as function of number of recycles at about 75% compression strain. The insets were the compressing-releasing process of the HCA0.8 and the weight putting on the hydrophobic cellulose absorbent was 10g. It can be seen that the density of unmodified cellulose-based aerogel of 0.5 wt % cellulose concentration could reach as low as 7.6 mg/cm3, while the porosity could be as high as 99.53%. the densities of cellulose-based aerogels gradually increased with the variation of cellulose concentration from 0.5 to 1.5 wt%, which also led the decrease of the porosity. Moreover, the MTMS modified aerogels possessed high porosities of more than 98%, confirming that there was no appreciable change of the original morphology and microstructure of the cellulose-based aerogel after the modification (Figure 3a). To evaluate the elasticity property, we employed HCA0.8 as a model candidate for the compression test. As shown in the inset of figure 3b, our HCA could mostly recover to its initial height after an extensive press exhibiting a surprising flexible property. We further conducted the cyclic compression on the HCA with about 75% compression strain. As can be seen from figure 3, the HCA still could survive with 83% of original height even after 6 repeated compression-recovery cycles. These results showed the excellent mechanical flexibility and robustness of HCA. Incorporating with its superhydrophobic property, the HCA exhibits great potential to be integrated into lightweight devices for various applications under water. 14


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Methane absorption capacity.

Figure 4.

The dependence of absorbed methane volume on (a) Cellulose concentration and

(b) Submerge depth; (c) Dynamic methane absorption capacity of HCA1.0 at a depth of ca.4 cm during cycles of forced compression and free recovery; (d) Dynamic methane absorption capacity of HCA1.0 at a depth of ca.4 cm during cycles of forced compression and forced recovery. The compression strain of HCA was about 75%. Despite the unique structure and excellent mechanical property, our HCAs showed a striking methane capture property under both static condition and dynamic condition in artificial sea water (Figure 4). Figure 4a and b show the methane capture ability of HCA has a close relation with the cellulose concentration and submerge depth. The amount of methane gas filled in the matrix of HCA can be calculated using the following equation: n=

0-

(4)

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Where n is the amount of substance of gas, P is the absolute pressure of atmospheres, V is the volume of porous matrix of the HCA, R is the gas constant and T is the Kelvin temperature. The gas volume trapped in HCA depends on the total pore volume with the assumption of constant volume of HCA in either air or water. V = Porosity

(5)

According to Pascal’s law, the pressure of water can be described by equation: P = = gh

(6)

Where ρw is the water density, h is the immersed height and g is the gravitational acceleration. According to Equation (4), (5) and (6), the amount of absorbed methane bubbles inside the HCA porous structure (after immerging the HCA in water) were evaluated as: @=

A B0*CDE

(7)

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Where T, R, ρw and g are constant values. When the immersed height h is constant, the relationship between the amount of absorbed methane bubbles n and porosity of the HCA is linear, which is accord with our experimental data (Figure 4a). According to figure 1: Porosity ∝ Cellulose concentration

(8)

A series of HCAs with different cellulose concentration were employed to study their potential capability for collecting methane bubbles (Figure 4 a). According to equation (7) and (8), the amount of gas absorbed in the HCA varies inversely with the decrease of cellulose concentration of HCA. This trend has been confirmed by our experimental result in figure 4a, which shows that the collected gas volume increases linearly with the decreasing cellulose concentration. According to figure 4a, when HCA1.5 was immersed at the depth of 3.3 cm, it was able to absorb 33.5 dm3/m3of methane, and the decreasing cellulose concentration from 1.5 to 0.5 wt% resulted in the trapped methane volume reaching to about 70 dm3/m3. The submerge depth of HCA also influences the capacity of methane absorption. 16


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We fixed the cellulose concentration of HCA (1.0 wt %), then research the effect of HCA submersion depth on the collected volume of methane. It can be seen from figure 4bthat the volume of absorbed methane increases linearly with the increasing submergence depth. This finding is consistent with the theoretical prediction calculated by Equation (7). According to figure 4b, when placed the HCA1.0 was in a depth of 3.3 cm, the trapped gas volume was about 50 dm3/m3. After increasing the submerged depth of HCA1.0 from 3.3 to 33.7, the trapped gas volume could reach as high as 95.9 dm3/m3. Although the results presented above show the great performance of HCA for capturing methane bubble underwater, the following investigation is more important and challengeable because the methane capability of HCA was further studied under two kinds of dynamic compression-release processes. Unlike the aforementioned static absorption test, in the dynamic absorption tests, once the HCAs were saturated with methane, they were compressed by an external force to release the methane out in order for another absorption circle. The major difference between the two dynamic absorption tests is that whether or not an external opposite force was applied on the HCA after the compression step so as to recover the original shape of the HCA. Figure 4c shows the absorption capability of HCA1.0 during successive forced-compression & free-recovery circles under water. Interestingly, after the first cycle, the absorption capacity of the HCA1.0 for methane bubbles significantly increased from about 50 dm3/m3 to 365 dm3/m3, researching about 7 times of its initial absorption capacity. It can be observed that the compression deformed the structure of the aerogel under water and resulted in the escape of the air from the aerogel. Since the water pressure is much higher than the air, the compressed HCA was hard to recover to its original shape by itself. However, due to the elastic property, the HCA resiled slightly which created a certain space with low vacuum in this aqueous environment. As most of the air was emptied, the appeared space was, therefore, able to absorb more methane bubbles in the next circle. After 10 times, the HCA1.0 still maintains high methane

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absorption efficiency (about 365 dm3/m3) as well high-water contact angle, suggesting that the HCA is highly suitable for the methane bubble absorption in practical applications. The methane absorption performance of HCA was further evaluated in another dynamic adsorption test, in which the HCA experienced to forced-compression & forced-recovery circles under water (Figure 4d). Due to the added external force during the recovery step, the compressed structure of HCA was almost fully released, which provided an extra space for storing methane bubbles. Upon ten dynamic methane absorption cycles, the aerogel appeared almost the same absorption capacity for methane (about 595 dm3/m3) in each circle, showing excellent ability for collecting methane gas from water even after multiple compression recovery cycles.

Figure 5.

Schematic of continuous methane bubbles transported through a plastic pipe.

Inspired by the previous work, the HCA was connected with a pipe in order to continuously capture and transport methane bubbles from the shallow water surfaces. The HCA was placed underwater, connected with a rigid pipe and used as a collector to collect the methane bubbles. Water was injected into the rigid plastic pipe for the purpose of easy observation of bubble transport. Then, a manual syringe was used to release the continuous methane bubbles (Figure 5). It can be seen that methane bubbles could be easily absorbed by

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the cellulose-based aerogel and transported through the plastic pipe. The process of continuous bubble transport is presented in Movie S1. CH4 bubbles that were generated at different positions under water could be absorbed by the HCA. Notably, the pressure outside the pipe is greater than that inside the pipe. Pressure difference between inside and outside of the pipe contributes to the saturated methane bubbles escaping and being transported through the pipe. This different pressure-controlled methane collection system will not only save a large amount of absorbents but will also make the collection of methane bubbles easier and faster. We also found that HCA can also efficiently collect bubbles of various kinds of greenhouse gases under water at different salinities. Therefore, they could terminate methane bubbles not only in oceans for terminating methane bubbles but also in various kinds of aquatic systems in terminating greenhouse gases like nitrous oxide and carbon dioxide to the atmosphere. We believe that this green material used for collecting methane bubbles intelligently from aquatics systems will bring the hydrophobic cellulose based-aerogels to applications in environmental and energy related areas. CONCLUSIONS In conclusion, a facile, environmental-friendly and cost-effectively approach for collecting methane bubbles using hydrophobic cellulose aerogel was established. The interaction between HCA and methane bubbles in artificial marine environment was systematically investigated. We found the cellulose concentration and submersion depth of the HCA impacted on the collection efficiency of methane bubble. With a decrease of cellulose concentration or an increase of submersion depth, the capacity of HCA to trap methane volume increased linearly. We, for the first time, studied the methane capture ability of hydrophobic aerogel in two kinds of dynamic absorption-release processes underwater. In each dynamic process, our HCA1.0 maintained a high methane collection efficiency which is more than 10 times higher than the efficiency of methane collection under static condition, as

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well as high contact angel even after 10 cycles. By integrating the HCA with a pipe, a simple model device was achieved which was able to continuously collect and transport methane bubbles from aqueous circumstance. We believe this green approach is extendable with great potential to reduce the methane emission from water environment and ultimately alleviate the global warming.

ASSOCIATED CONTENT Supporting Information Composition details of CAx aerogels, the figure of the dynamic absorption process with forced compression and forced recovery , and the movie of continuous collection and transportation of methane bubbles

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (J. Tian) Notes The authors declare no competing ﬁnancial interest.

ACKNOWLEDGEMENTS The authors are grateful to financial support from Natural Science Foundation of China (No.81671780), State Key Laboratory of Pulp and Paper Engineering (No. 2015TS01), Fundamental Research Funds for the Central Universities (No.2017MS076), the Science and Technology Project of Guangzhou City in China (No.2016070220045).

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TOC

Synopsis: We describe a facile, environmental-friendly and cost-effectively method for collecting marine methane bubbles based on the employment of hydrophobic cellulose aerogels.

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A Low-cost, Sustainable and Environmentally Sound Cellulose

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