Research Article www.acsami.org
Biomass-Derived Porous Carbonaceous Aerogel as Sorbent for OilSpill Remediation Zhuqing Wang,†,‡ Pengxiang Jin,‡ Min Wang,‡ Genhua Wu,‡ Chen Dong,† and Aiguo Wu*,† †
Key Laboratory of Magnetic Materials and Devices, CAS & Key Laboratory of Additive Manufacturing Materials of Zhejiang Province, & Division of Functional Materials and Nanodevices, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China ‡ Anhui Key Laboratory of Functional Coordination Compounds, Anqing Normal University, Anqing 246011, China S Supporting Information *
ABSTRACT: We prepared a cost-effective, environmentally friendly carbonaceuous oil sorbent with a lotus effect structure using a simple one-pot hydrothermal reaction and a mild modification process. The carbonaceous oil sorbent can rapidly, efficiently, and continuously collect oil in situ from a water surface. This sorbent was unlike traditional sorbents because it was not dependent on the weight and volume of the sorption material. The sorbent was also successfully used to separate and collect crude oil from the water surface and can collect organic solvents underwater. This novel oil sorbent and oil-collection device can be used in case of emergency for organic solvent leakages, as well as leakages in tankers and offshore drilling platforms. KEYWORDS: carbonaceous aerogel, adsorption, oil-spill remediation, crude oil, oil collection fibrous mats, silicon aerogels, and modified activated carbons are traditionally used to clean oil spills because they are highly porous, low in density, and oleophilic and hydrophobic.15−20 Among these synthetic sorbents, aerogel attracts the most attention because of its ultralow density, three-dimensional porous network, and adjustable surface chemistry. Aerogel is normally prepared from a wet gel through vacuum freezedrying or supercritical drying technology, in which the liquid component of the gel is replaced by a gas. Typically, the sorbent is spread over the leakage, the absorbed sorbent is recollected, and the oil is removed from the sorbent by distilling or squeezing. However, this process is inefficient and complicated. Recently, Yu et al. developed a device for oil collection from a water surface using a simple mixture of hydrophobic and oleophilic materials with pipes and a selfpriming pump.21 This device was small, portable, unlimited in its oil-sorption capacity, and suitable for use in emergency oil
1. INTRODUCTION Fuel oil is an energy source crucial to production and human activity. However, oil leakage in the process of transportation and use causes economic losses and seriously affects the ecological environment, affecting wildlife, navigation, and fishing, etc.1 Research is needed to develop efficient and green methods for oil collection and recovery. Many current methods, such as in situ burning, physical collection, chemical dispersant, adsorption, and biodegradation, have been used to treat oil spills.2−4 Among these methods, adsorption has attracted more attention for its simple operation, recyclability, and less pollution. The sorbent for oil sorption primarily contains minerals, natural materials, and synthetic materials. The natural sorbents such as wool, cotton, hay, feather, straw, kenaf, sisal, and carbon-based products are inexpensive and available in large quantities. 5−10 The disadvantages of natural sorbents are low oil capacity, easy fatigue, and poor recycling performance. Mineral sorbents, such as perlite, vermiculite, clay, and peat moss have insufficient buoyancy and low oil absorbency.11−14 Synthetic sorbents, polymeric sponges, such as polyurethane, polypropylene, and polyethylene, and rubber sponges, nonwoven polypropylene © XXXX American Chemical Society
Received: September 13, 2016 Accepted: November 7, 2016 Published: November 7, 2016 A
DOI: 10.1021/acsami.6b11648 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 1. Preparation process of PDMS-CA: (a) image of carbonaceous hydrogel; (b) image of CA and water contact angle of CA; (c) image of PDMS-CA and water contact angle of PDMS-CA; (d, e) SEM photographs of PDMS-CA at different magnifications. The autoclave was maintained at 180 °C for 10 h in an oven and then cooled to room temperature. The black carbonaceous hydrogel monolith was produced by decantation and then was submerged in 200 mL of water and 200 mL of ethanol in sequence to remove soluble impurities. The resulting carbonaceous hydrogel was immersed in 40 mL of water and frozen in an ultralow-temperature freezer. The corresponding carbonaceous aerogel (named as CA) was obtained by lyophilized hydrogel for 12 h using a vacuum freeze-drying device (LBJ-10, Four-Ring Science Instrument Plant, Beijing). 2.4. Preparation of PDMS-Modified Carbonaceous Aerogel. The coating solution was prepared by dissolving 4 g of PDMS and 0.4 g of the curing agent in 100 mL of n-hexane. CA was immersed in the coating solution for 1 min, then removed, and allowed to cool at room temperature for 10 h, followed by curing at 120 °C for 2 h, yielding the PDMS-modified carbonaceous aerogel (named as PDMS-CA). 2.5. Oil Sorption Test. PDMS-CA (approximately 3 cm × 2 cm × 1 cm) was first placed on top of the oil. After 10 min of adsorption, PDMS-CA was taken out, left to stand for 1 min, and then weighed. The sorption capacity of PDMS-CA was calculated as follows:
spills, but cost prohibitive because it required expensive raw materials, such as silica nanoparticles. In this study, we developed a low-cost and environmentally friendly carbonaceous aerogel by a simple one-pot hydrothermal reaction and then modified it under mild conditions. On the basis of its macroporosity and rough surface structure, we designed an oil-collection device that can collect organic solvent and oil rapidly, efficiently, and continuously from a water surface. Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and infrared (IR) spectra were used to identify the carbonaceous aerogel and PDMSmodified carbonaceous aerogel. We also studied the adsorption properties and mechanism of the self-controlled oil-collection method, as well as its practical applications.
2. EXPERIMENTAL SECTION 2.1. Materials. Analytical-grade n-hexane and oil red O were acquired from the Aladdin Reagent Co. (Shanghai, China). Diesel oil and crude oil were provided by Sinopec Anqing Co. Engine oil was supplied by Castrol Ltd. (SPT 5W-40; Shenzhen, China). Sylgard 184 silicone elastomer base (PDMS) and curing agent were obtained from Dow Corning (Midland, MI, USA). The HeLa cells were provided by J&K Scientific Ltd. Lettuce (scientific name, Lactuca sativa) was purchased from Anqing Carrefour supermarket. 2.2. Apparatus. SEM images were obtained on a Hitachi S-4800 field-emission microscope. Digital photographs and videos were recorded on a Cannon 600D camera. The IR spectra were produced utilizing a Nicolet 6700 FT-IR spectrometer with KBr pellets. XPS analyses were carried out on a Thermo ESCALAB 250XI X-ray photoelectron spectrometer. The BET surface area analysis was carried out on a Micromeritics ASAP 2020 nitrogen adsorption apparatus at 77 K. A self-priming pump and Teflon pipe (3 mm inner diameter and 4 mm outer diameter) were purchased from Yuhang Tech Studio in Taobao mall, China. Adjustable digital DC power (TXN-1502D) was provided by Shenzhen Zhaoxin Company. A Data Physics OCA20 instrument was used to measure the static contact angles at room temperature. The average contact angle was calculated using measurements from three positions on the same sample. 2.3. Preparation of Carbonaceous Hydrogel and Aerogel. Lettuce was first peeled, cut into an appropriate volume (about 3.5 × 2.5 × 1.5 cm3), and placed in a Teflon-lined stainless steel autoclave.
Q=
m f − m0 m0
where Q is the mass sorption capacity of the PDMS-CA (g g−1), m0 is the initial weight of the PDMS-CA (g), and mf is the final weight of PDMS-CA after sorption (g). 2.6. Fabrication of the Oil-Collection Device. A self-priming pump, a piece of PDMS-CA (3.2 × 2.2 × 1.2 cm3), and two Teflon pipes were used to assemble the oil-collection device. First, one end of the pipe was inserted into the PDMS-CA and the other end was connected to the inlet of the pump. Then another Teflon pipe was connected to the outlet of the pump, and the other end of this pipe was extended into an oil-collection container. 2.7. Cell Viability Assay. A 50 mg amount of CA (or PDMS-CA) was separately immersed in water, PBS buffer, 1 mM HCl, and 1 mM NaOH solutions for 12 h at 50 °C and then filtered through a 0.2 μm filter. The filtrate was adjusted to neutral with NaOH or HCl, and osmotic pressure was adjusted with a PBS buffer. Then it was mixed with cell culture medium at a volume ratio of 1:1 (the mixture solution named as leachate). HeLa cells were cultured for 24 h and then incubated with the various leachates for 48 h. Cell viability under different treatments was assayed by a MTT assay.22 B
DOI: 10.1021/acsami.6b11648 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
3. RESULTS AND DISCUSSION 3.1. Materials Characterization. As shown in Figure 1a,b, CA (Figure 1b) maintained its original morphology, although it shrank slightly after vacuum freeze-drying. This is attributed to the sublimation of the solid water under vacuum conditions, so that it was not affected by the surface tension. Unlike the CA, the water contact angle of PDMS-CA increased sharply from 0 to 144.2°, which indicated that the PDMS-CA repelled water and absorbed oil. This may be attributed to coating of the lowfree-energy PDMS. The enlarged SEM images showed that the skeleton surface of PDMS-CA was rough with many peaks and troughs, similar to that of a lotus leaf (Figure S1). Moreover, the prepared PDMS-CA pore properties were also studied by N2 adsorption−desorption isotherm combined with an artificial method (Figure 2 and Supporting Information). The results
an oil spill. The sorption capacity (Figure 3c) of the PDMS-CA for crude oil, diesel, n-hexane, peanut oil, and engine oil were determined to be 10.6, 8.4, 3.3, and 11.4 g g−1, respectively, which is comparable to that of the activated carbon, functionalized alumina, and iron oxide nanoparticles sorbent reported recently.23−25 FT-IR spectroscopy was used to identify the surface functional groups of CA and PDMS-CA (Figure 4). The
Figure 4. IR spectra of (a) CA and (b) PDMS-CA.
broad band at 3421 cm−1 indicated O−H stretching vibrations. The peaks at 1621 and 1693 cm−1 were vibrations from CC and CO stretching, which indicated the presence of aromatic and furanic groups. The bands at 1450 cm−1 resulted from carboxylic O−H deformation vibrations.26,27 The bands at 1106 cm−1 indicated C−O−C stretching vibrations. The IR spectrum of PDMS-CA showed new absorption peaks at 1255 cm−1 (C− H in Si−CH3) and 797 cm−1 (Si−O), which were attributed to PDMS grafted onto the surface of the CA.28 XPS analysis further verified the aforementioned oxygen-containing groups. The high contents of O (24.9%) and C (71.3%) in the CA were shown by the photoelectron lines at binding energies of 530.5 and 283.1 eV, which indicated O 1s and C 1s, as observed in the XPS survey scan in Figure 5. Figure 5 also shows the deconvoluted C 1s spectra, indicating the presence of the following four carbon groups: the carboxyl or ester groups at 288.3 eV (CO−C), carbonyl carbon at 287.5 eV (CO), the alcohol, ether, and phenolic groups at 286.4 eV (C−O), and graphitic carbon at 284.6 eV (CC).29,30 The oxygen functional groups resulted from the incomplete carbonization of carbohydrates, such as glucose at 180 °C, during the
Figure 2. Nitrogen adsorption−desorption isotherm of the PDMSCA.
showed that the BET surface area and pore volume of PDMSCA were 16.02 m2 g−1 and 4.13 cm3 g−1, respectively. The smaller surface area and larger pore volume indicated that PDMS-CA has a macroporous structure, which facilitates oil storage and transport. Figure 3a shows that when PDMS-CA was immersed in water by an external force, its hydrophilic properties allowed air bubbles trapped around it to from a silver, mirror-like surface. Additionally, when it was introduced to a mixture of water and diesel oil (Figure 3b), PDMS-CA adsorbed oil while repelling water. Although it was saturated with oil, the PDMS-CA continued to float on the water surface by itself, showing its suitability for treating oil floating on a water surface following
Figure 3. Photographs of PDMS-CA (a) immersed in water and (b) on a water surface with a layer of floating diesel oil. (c) Sorption capacity of the PDMS-CA for various oils and organic solvent. C
DOI: 10.1021/acsami.6b11648 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 7. Effect of viscosity on the rate of oil recovery.
hexane-3 (126.2 CP) were respectively 17, 63, 127, and 217 s, remaining in accord with Darcy’s law.31
Q=
−kAΔP μh
where Q represents the oil recovery rate, μ represents oil viscosity, h represents the distance between the water surface and the nozzle of the pipe, k represents the permeability of PDMS-CA, A represents the cross-sectional area of PDMS-CA, and ΔP represents the pressure decrease at the nozzle of pipe. ΔP increased as the voltage of pump increased; hence, Q increased as the voltage of the pump increased. Conversely, Q decreased as the oil viscosity increased. 3.3. Effect of Height. Figure 8 shows that the nozzle position influenced the rate of oil recovery. Oil recovery rates
Figure 5. (A) XPS spectra of (a) CA and (b) PDMS-CA. (B) C 1s of CA.
hydrothermal treatment process. Moreover, the presence of Si 2p in PDMS-CA also clearly confirmed PDMS was successfully grafted onto the surface of CA, in accord with the results of our IR analysis. 3.2. Effect of Voltage and Viscosity. Figure 6 shows that the flux of diesel oil rapidly increased as the pump voltage
Figure 8. Recovery of diesel oil for various nozzle heights.
decreased as the distance between the water surface and the nozzle of the pipe increased, in accord with Darcy’s law. To obtain the highest rate of oil recovery using the least amount of energy, the altitude between the water surface and the nozzle of the pipe (h) was kept as low as possible, and the voltage of the pump was increased until air bubbles appeared in the collected oil. 3.4. Applications. In order to demonstrate the practicability of the prepared material, PDMS-CA was used to collect diesel oil and crude oil from the water surface with a selfpriming pump. The designed oil-collection device easily collected floating diesel oil (Figure 9a,b and Movie S1) and crude oil (Figure 9c,d and Movie S2). It can also collect a large area of floating n-hexane (Figure 9e,f; 1400 mL, dyed red) with
Figure 6. Effect of voltage on the initial flux of diesel oil.
increased from 0 to 2.2 V, but when the voltage exceeded 2.4 V, the flux was stable, and air bubbles emerged in the collected oil. We also studied the rate of oil recovery with respect to its viscosity. Figure 7 shows that the oil recovery rate reduced as the oil viscosity grew. Under similar conditions, recovery time for 40 mL of diesel oil (5.6 CP), engine oil/n-hexane-1 (20.7 CP), engine oil/n-hexane-2 (50.5 CP), and engine oil/nD
DOI: 10.1021/acsami.6b11648 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 9. Photographs of PDMS-CA collecting (a, b) diesel oil, (c, d) crude oil, and (e, f) n-hexane from a water surface with a self-priming pump. (g−i) Photographs of the PDMS-CA adsorbing CCl4 (dyed by oil red O) underwater.
the self-controlled oil-collection process, PDMS-CA was modeled as a bundle of capillary tubes (Figure 11). Before applying suction force to PDMS-CA, the capillary pressure of the oil−air interfaces at the top of PDMS-CA can be expressed as
a longer working time and can continuously collect diesel oil (Figure 10) from the water surface for over 18 h with little loss
Ps =
2γOA cos θ1 R
where Ps represents the capillary pressure, R represents the radius of the tube, γOA represents the oil−air interfacial tension, and θ1 represents the contact angle of oil in the tubes. Thus, after applying suction force, the capillary pressure (Ps) at plain A increases by decreasing the contact angle from θ1 to θ3, obtaining a new pressure equilibrium between plains A and C. As a result, air does not break through the oil−air interfaces of PDMS-CA. Similarly, the capillary pressure at the bottom of PDMS-CA increases with decreasing the contact angle from θ2 to θ4 after applying a suction force. The increased capillary pressure on the oil−water interfaces and hydrophobic surface prevent water permeation into PDMS-CA. However, as there is no capillary pressure in plain B, the pressure equilibrium between plains B and C is broken after applying a suction force to PDMS-CA. The pressure difference drives oil from plain B to plain C. A possible mechanism of the self-controlled oil collection is proposed in Figure 11c. The oil−water and oil−air interfaces act as sealed films, which repel water and air after application of a suction force. Only floating oil penetrates PDMS-CA and
Figure 10. Effect of working time on the flux of diesel oil under a circulating system.
of performance, respectively. PDMS-CA also rapidly collected organic solvents (such as CCl4) underwater (Figure 9g−i and Movie S3). Considering the high efficiency of PDMS-CA, our designed oil-collection device can be used as a response to oil leakage emergencies, including tanker oil spills, offshore drilling platform oil spills, and organic solvent leakage. 3.5. Mechanism of the Self-Controlled Oil Collection. In order to provide a simple explanation of the mechanism of
Figure 11. Simulation schematic representations of the PDMS-CA before (a) and after (b) application of suction force. (c) Plausible mechanism of the self-controlled oil collection. E
DOI: 10.1021/acsami.6b11648 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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flows from plain B to plain C under a pressure difference, and is then removed by the self-priming pump. 3.6. Cell Viability. Figure 12 shows the rate of cell viability remained stable when incubated in various leachates of PDMS-
ACKNOWLEDGMENTS We gratefully acknowledge financial support from the Chinese Academy of Sciences (Grants KFJ-EW-STS-016, KFJ-SW-STS172, and 2010-735), the National Natural Science Foundation of China (Grant 21407004), the Science and Technology Department of Zhejiang Province (Grants 2015B11002, 2014B82010, and R5110230), and the National Postdoctoral Committee (Grant 2015M570532).
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CA, indicating there were few toxic ingredients in the leaching solution, and further illustrating that PDMS-CA is a kind of environmentally friendly material, which can be used in different media, whether acidic, alkaline, or neutral.
4. CONCLUSIONS We developed a new porous hydrophobic/oleophilic material (PDMS-CA) for in situ separation and collection of oil or organic solvent from a water surface. With an external selfpriming pump, PDMS-CA rapidly, efficiently, and continuously separated and collected oil, including diesel oil, crude oil, engine oil, and n-hexane. This type of oil-collection device is small, lightweight, and portable and can be used in emergency response to organic solvent leakage, tanker spills, or offshore drilling platform oil leakage. ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b11648. Description of pore volume measurement method and figures of the surface of the lotus leaf and images of CA (PDF) Movie S1 of designed oil-collection device collecting floating diesel oil from the water surface (AVI) Movie S2 of designed oil-collection device collection floating crude oil from the water surface (AVI) Movie S3 of designed oil-collection device collecting CCl4 under water (AVI)
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Figure 12. Effect of leachate of (a) CA and (b) PDMS-CA on cell viability.
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Research Article
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Aiguo Wu: 0000-0001-7200-8923 Notes
The authors declare no competing financial interest. F
DOI: 10.1021/acsami.6b11648 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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DOI: 10.1021/acsami.6b11648 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
