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Adsorption mechanism of oil by resilient graphene aerogels from oil-water emulsion Jiankun Huang, and Zifeng Yan Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03866 • Publication Date (Web): 06 Jan 2018 Downloaded from http://pubs.acs.org on January 6, 2018

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Adsorption mechanism of oil by resilient graphene aerogels from oil-water emulsion Jiankun Huang†, Zifeng Yan*† †State Key Laboratory for Heavy Oil Processing, PetroChina Key Laboratory of Catalysis, China University of Petroleum(east), Qingdao, Shandong 266580, People’s Republic of China Abstract：：A facile synthesis strategy was adopted to prepare resilient graphene aerogel(GA) with properties of high emulsified oil adsorption capacities, excellent rebounding performance, oil-water selectivity and recycling abilities. The maximum adsorption capacities of GA for emulsified diesel oil were 2.5×104 mg g -1. The microscopic kinetic and thermodynamic mutual reaction models of diesel oil emulsion adsorption onto graphene aerogel were investigated to describe the adsorption mechanism. The emulsified diesel oil was able to be separated efficiently from the oil-water emulsion by GA due to their high oil selectivity. Interestingly, both kinetics and thermodynamic experiments show emulsified oil adsorption on GA is a physical adsorption and spontaneous process. Besides, GA can be reused with a prominent repeatability for at least 10 cycles, demonstrating feasibility in practical applications of graphene aerogel-based oily water treatment. Keywords: graphene aerogel, adsorption, emulsified oil, resilience, regeneration. Introduction Water, an important resource in nature, has significant impacts on the living conditions of creatures on the earth. However, with the exploration of crude oil, products and by-products derived from petroleum have caused detrimental and long-term influences on human life and ecosystem [1-2]. Meanwhile, the small amount of oil dispersed in water always forms emulsion, i.e. emulsion oil, where tiny oil droplets are much more stable and difficult to be separated than immiscible oil-water mixture, and they do have the possibilities to cause further pollutions to water resource. For this reason, various techniques, including membrane separation [3], wet oxidation [4], photocatalysis degradation [5] and biological separation [6], have been carried out to remove oils from contaminated effluents, while considering the practical applications and economic efficiencies, it would be appreciated that the wasted oil can be rational collected and recycled. Among these methods, adsorption has been partially considered as one of the most efficient and promising solutions due to their possibilities of oil recovery, and caused less unfavorable damage to the environment [7-10]. An excellent oil-adsorbent should possess hydrophobicity, high oil-retention capacity and high oil-water selectivity. Nature materials, such as wood, zeolite, and cotton fiber have been widely used as conventional oil adsorbents [11-13], because of large specific area and multi-hole structure. However, the listed traditional porous materials exhibited numerous problems such as poor oil-water separation efficiency, weak recyclability and low adsorption ability. Poor integration of the listed weaknesses set limitation to the development of adsorbent-based oily water treatment process. Therefore, a surging demand for the fabrication of novel porous oil sorbent has arisen. As earth-abundant element, carbon can be founded in every aspect of life. With the discoveries of carbon allotropic substance, an ingenious two-dimensional structured substance, graphene [14], with high surface area and excellent hydrophobic properties have been

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incorporated into the synthesis of carbon-based adsorbents to deal with oil leakage problem [15,16]. According to the recent achievements on the preparation of hierarchical monolithic graphene-based aerogels using chemical vapor deposition, chemical reduction, dip coating and self-polymerization [17-20], these 3D structural and chemical functionalized materials have shown excellent oil adsorption capacities than the traditional natural porous materials. Besides, the quantities of aerogels used as oil sorbents are much less than that of conventional oil adsorbent. Moreover, the synthesized graphene aerogel with additional properties, such as mechanical robustness, fire-resistance, magnetism and so on [20,21], which may help to facilitate the oil contamination treatment to some extent. However, the adsorption abilities of graphene aerogels still hold potential improvements by controllable pores design. Rational pores distribution would endow graphene aerogel with sufficient oil capturing volume and excellent resilience to release oil from the pores under certain stress. Besides, though majorities of investigations on oil slick adsorption onto graphene aerogel were carried out during the past decades [22-24], it is worth considering that few studies were conducted for emulsion oil adsorption by graphene aerogel, especially shortage in systematic evaluation of adsorption behaviors in terms of thermodynamic and kinetic. In this work, a novel ethylenediamine-mediated and ammonia-immersed graphene hydrogel was prepared, followed by directional ice-templates growth as freezing method to obtain the resilient graphene aerogel (GA) and employed as emulsion oil adsorbent. Detailed characterizations and adsorption experiments were performed to verify the effect of pore orientation and pore volume on capturing behaviors of oil onto GA for the first time. Besides, four kinds of oils, gasoline, kerosene, straight-run diesel oil and diesel oil with different properties were selected as hypothetical models to explore effects of oil physical properties on adsorption performance of GA. Thermodynamic and kinetic experiments were carried out to confirm the adsorption mechanisms of emulsified oil adsorption onto graphene aerogel in details. Moreover, the as-made products also exhibit other prominent properties, such as rebounding performance, high selectivity of oil in oil-water mixture and favorable cycling performance. Experimental Section Materials Graphite powders (≤30 µm) and activated carbons were purchased from Sinophram Chemical Reagent Corporation. Hydrochloric acid (HCl, 36%wt), ammonia solution(NH3 H2O), sulphuric acid (H2SO4, 98%wt), ethylenediamine (EDA), NaNO3, KMnO4, and H2O2 aqueous solution (30%. wt) were purchased from Aladdin Industrial Corporation, and used directly without further purification. All the used oils were purchased from China Petrochemical Corporation (Qingdao), the physical properties were also acquired from the company and listed in Table 1. (298K). Ultrapure water (18 MΩ) was produced by a Millipore System (Millipore Q, USA). All the reagents and materials were used without purification. Table 1. Oil Properties of Four Kinds of Oils(298K) Oil type

Density (g cm-1)

Viscosity (mm2 s-1)

Gasoline Kerosene Automotive diesel oil Straight-run diesel oil

0.7432 0.8074 0.8270 0.8485

0.7652 1.3255 5.0342 7.2874


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Synthesis of graphene oxide and graphene aerogels Graphene oxide (GO) was prepared using modified hummers’ method [25]. EDA solution was added into GO dispersion (3mg mL-1,15ml) in autoclave, and the EDA-assisted GO hydrogel formed after hydrothermal process at 90℃ for 6h, then the hydrogel was immersed into the ammonia solution (14 vol%) at 90℃ for 1h, after that, the synthesized hydrogel was placed into a stainless steel mold, liquid nitrogen was introduced to trigger ice-templates growth. The freezed sample was placed in freeze dryer at-50℃ for 24h to get the graphene aerogel. In this work, bottom-up and diagonal-direction were chosen to manipulate ice-templates growth (Fig 1(a).), graphene aerogel synthesized by diagonal-direction freezing methods was denoted as GA-1, another sample fabricated by bottom-up strategy was denoted as GA-2. Preparation of emulsion and diameter distribution measurement The simulated oil-water emulsion was prepared by dispersing 0.40 g diesel oil into 500 mL ultrapure water homogeneously, then milk-like solution was obtained, which exhibited the characteristics of chemically stabilized solution. Diameter distribution of the emulsion was measured by laser particle size analyzer, before measurement, the sample was dispersed by ethanol solution in the sample cells. During the measurement, the refractive index was set as 1.54, the interval of measurement was 10 seconds, the scanning cycles was set as 3 times to ensure the accuracy. Emulsions of gasoline, kerosene and straight-run diesel were also prepared to evaluate the effect of oil properties on emulsion oil adsorption using graphene aerogels. Adsorption experiments All the experiments were conducted in closed 100ml glass tubes with lids. These 100ml tubes, containing certain amount of graphene aerogels and 60ml of simulated emulsion oil, which was placed in a water bath and shaken at 50 times min-1. In the kinetics experiment, the concentration of emulsion was measured by infrared petroleum determination instrument at 3,5,8,10,20,30,60,90 and 120min, respectively. To explore the effect of temperature on adsorption, the temperature was maintained at 298K, 308K, 318K and 328K. Each experiment was performed twice and experimental results are average values. The adsorbed amounts Qe was calculated from the following equation: =

0

×

(1)

In eq (1), where C0 and Ce are the initial and equilibrium concentrations, respectively, mg/l, and m is the amount of adsorbent mass, g. V is the volume of oil-water emulsion, ml. The equilibrium adsorption removal rates were calculated from the equation below: =

−

× 100%

(2)

Where C0 and Ce have the same meaning in eq(2). The method, selective adsorption of oil in emulsion, was summarized in the supplementary material, all the measurements and equations were also listed in it. Characterization Morphology and microstructure were observed via field emission scanning electron microscopy (FE-SEM, S4800, Japan). The X-ray photoelectron spectroscopy (XPS, PHI 5000 Versa Probe, ULVAC-PHI, Japan) was applied to analyze the detailed contents and surface properties of the as-made product. The nitrogen adsorption-desorption isotherms were obtained by


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Micrometrics equipment (ASAP 2020) at 77K, the pore size distribution (PSD) was calculated using N2 adsorption collected branch data. The concentration of oil contents in emulsion was measured by infrared petroleum determination instrument (OIL2000B, China). The dynamic droplets behavior of emulsions was recorded and measured using polarized light microscopy (Motic, USA). Rebounding performance of as-made product was tested with Zwick testing machine (Allround, German), the hammer falling to the surface of the aerogel at the rate of 10mm min-1. Hydrophobicity of as-made product was measured by the contact angle measuring device (Biolin, Finland). The characterization results of X-ray photoelectron spectroscopy, Raman spectra, N2 adsorption-desorption isotherms and contact angel measurements were listed in the supporting information.

Fig 1. Freeze-drying method and SEM images (a) method of ice-templates growth control. (b) GA-1(left) and GA-2(right). Results and Discussion Characterization Fig 1(b). shows the SEM photos of the as-made graphene aerogels, GA-1 and GA-2, through different freezing strategies. Clearly, both images show typical flask-shape graphene sheets with some folds, and the interconnected sheets are assembled to form three-dimensional nanostructure [26]. The XPS spectra(Fig S1) exhibits that the amount of oxygen-containing functional groups were reduced through hydrothermal reaction, and the increased ID/IG ratio in Raman spectra (Fig S3)also confirmed the reduction of graphene oxides. Interestingly, though the reduced graphene sheets assembled into hydrogel, the modifications of microstructure are verified by manipulating freezing method to control ice-templates growth direction, the results are evident for GA-1(Fig 1b. left) and GA-2(Fig 1b. right), the water in hydrogel of GA-1 was freezed, radially growing along the alignment. After freeze-drying, the ice-crystals disappeared and GA-1 with fan-like porous structure. While for GA-2, SEM observation reveals that ice-templates grew along vertical axial direction by adopting bottom-up strategy, then crosslinked honeycomb-like pores formed after freeze-drying. The above results verified the effectiveness of controlling ice-crystals growth to instruct graphene aerogel pores formation, besides, the pre-freezing using liquid nitrogen is time-saving than placing hydrogel directly under atmospheric temperature of-10 oC or -20oC for several hours. Effect of pore orientation of aerogels, oil types and temperatures on the adsorption


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ability of graphene aerogels Effect of pore orientation In order to confirm the effect of pore orientations of graphene aerogels on adsorption, the comparisons of oil adsorption abilities of GA-1 and GA-2 were made using diesel oil emulsion under 298K, besides, oil adsorption using active carbons (AC) were also adopted as reference. Figure 2. exhibits that the emulsion adsorption capacities are positively associated with the GA-1, GA-2 and activated carbons. GA-2 exhibits higher adsorption performance than GA-1, which might be explained by the pore structure that ice-templates grew along the vertical direction, the uniform pore channels were formed in GA-2, these channels endow as-made sample(GA-2) possess higher oil-capturing capacities than GA-1, describing vital influence of the pore designing on the synthesis of graphene aerogels as adsorbents. Therefore GA-2 was selected for further experiments. Moreover, both emulsion oil adsorption capacities of graphene aerogels are higher than that of commercial adsorbent activated carbon. The monolithic graphene aerogel exhibits better oil capturing abilities, and ease of recovery than other powder-like carbon-based materials, thereby rending the potential applications of graphene aerogel-based oily water treatment process.

Fig 2. Adsorption performance of GA-1, GA-2 and activate carbon. Effect of oil properties In order to verify the relationships between adsorption capacities and physical properties of emulsion oil, other three kinds of oil-water emulsions, gasoline, kerosene and straight-run diesel were chosen (properties were listed in the Table 1.), GA-2 was adopted due to the higher adsorption capacities. According to the former research, the equilibrium adsorption time was set around 60mins to ensure sorptive saturation. As it shown in Fig 3., with the increase of oil densities, the adsorption capacities of GA-2 increase positively with them, when plotting four data, the extrapolate line pass through original point (slope=8.727), which means the adsorption volume of GA-2 for four emulsions are almost the same, in this sense, different kinds of oils and their properties have slight influence on adsorption process. Therefore, the key point is to increase pore volume of the synthesized graphene aerogel through adjusting rational pore size distributions, which may help to improve oil adsorption performance of volume-based aerogels.


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Fig 3. The relationship between adsorption mass and oil densities for adsorption of four kinds of oils onto GA-2. Effect of temperature As it well known, adsorption temperature is the key parameter to evaluate the adsorption performance, as it shown in Fig 4., the equilibrium adsorption quantities of GA-2 under four temperatures (298K,308K,318K and 328K) are 768.33,769.23,769.69 and 769.87 mg g-1, respectively. The calculated corresponding equilibrium removal rates are 98.9,99.1,98.9 and 99.1%, respectively. Besides, adsorption rates associate positively with the increase of temperature. At the initial stage of the adsorption (0 ~ 10 min), GA-2 exhibits higher adsorption rates, which is reflected by the sharp increasing line. While at the intermediate stage (10~20min), the adsorption rates exhibited the tendency to slow down. When being around 60 min, the adsorption process reached equilibrium. The reason for this phenomenon is that adsorption volume formed by the interconnected graphene sheets are available, oil in water entered into the pores, thus resulting in the high separation efficiency for oil-water mixture. With adsorption proceeding, the available sites reduced, which may result in the lower adsorption rates. Therefore, optimized temperature should also be taken into consideration in the practical applications of graphene aerogel-based oily water treatment.

Fig 4. The relationship of Qt vs time of GA-2 under four kinds of temperatures. Adsorption isotherms Three commonly used isotherm models, Langmuir [27], Freundlich [28] and Temkin [29], were adopted to depict the solid-liquid adsorption system. Three kinds of isotherms have their own assumptions, Langmuir model assuming that the active sites from homogeneous surface share


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Langmuir

equal relations with the adsorbate. The hypothesis of Freundlich model describe heterogeneous adsorption surface with different affinities of adsorption sites. While for Temkin model, it assumed that adsorption heat of adsorbate decreases linearly with the increase of coverage. The equations of those models are expressed as followed [27-29]: Langmuir model: =

m 1

(3)

Where Qm (mg g-1) is the saturated adsorption capacity, Ce (mg L-1) is the equilibrium solute concentration, KL (L mg-1) is the Langmuir equilibrium constant. Freundlich model: 1

=

(4)

-1

Where Qe (mg g ) is the equilibrium adsorption capacity, KF and n stand for Freundlich constants and intensity factors, respectively. Temkin model: = ln ln =

(5) (6)

In eq.(5) and eq.(6), Qe (mg g-1) is the equilibrium adsorption capacity, Ce (mg L-1) is the equilibrium solute concentration, KT (L mg-1) is the equilibrium binding constant that corresponds to the maximum binding energy and B is related to the adsorption heat. T (K) in Eq. (6) is the absolute temperature. R is the universal gas constant, which equals to 8.314 J·mol-1⋅K-1.

Fig 5. Relationships between Qe and Ce ,(a)Langmuir model and Freundlich model. (b) Temkin model(c).Plots of InQe vs Qe for calculation of thermodynamics parameters (△G0) under 298K. As it shown in Fig 5. (a) and (b), these figures described the relationships between the oil adsorption equilibrium concentrations and adsorption capacities of GA-2 in aqueous solution, the fitting results exhibited variance(R2) of three models, remarkably, this parameter of Freundlich was higher than those of other two models (Table 2.), suggesting multi-layer adsorption occurred in this process. The reason can be illustrated by the complex composition and molecular size of diesel oil. Besides, the Freundlich model constant (1/n) is 0.8246 for emulsified diesel oil, revealing the adsorption process was favorable [28].


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Table 2. Isotherms parameters for three models for emulsified oil adsorption onto GA-2(298K) sample

Langmuir model KL

Emulsified

Qm -1

-1

（L mg ）

（mg g ）

0.0039

25000

Freundlich

Temkin

R2

KF

1/n

R2

0.9023

162.6184

0.8246

0.9861

KT （L mg-1）

B

R2

6089

0.7746

diesel oil 0.0041

Thermodynamics research can help to get insightful knowledge of driving force and reaction extent in the adsorption process. The relationships between thermodynamic parameter free energy changes (△G0) and thermodynamic equilibrium constant (K) can be calculated by the following formulas [30]: ∆! " = −#$%"

(7)

Where the values of K0 can be achieved by plot fitting In Qe/Ce vs Qe and extending Qe to origin. K0 is defined as follows: " =

& '( '

(8)

Where Qe and Ce are the same parameters in the calculation of Langmuir and Freundlich. Vs and Ve are the activity coefficients of the adsorbed solute and solute in solution. While for the first adsorption, the concentration of oil contents is closed to zero, so the equation can be deduced as &

lim&→ = "

(9)

Therefore, the value of K0 can be obtained through linear fitting of In (Qe/Ce) vs Qe by extrapolating Qe to zero, the intersection of Y axial and line is the result of K0. As it shown from the Fig 5., the data, In (Qe/Ce) and Qe , were fitted well with the line. The result of calculated △G0 is negative, -3.39kJ mol-1, thereby confirming the adsorption behavior of oil onto GA-2 was spontaneous and favorable in thermodynamics [31]. Besides, if this value is between -20 and 0 kJ mol-1, this adsorption can be considered as physical adsorption, and if this value is between -80 and -400 kJ mol-1, this process can be regarded as chemical adsorption [31]. The △G0 value of our work belongs to the former range, which means the emulsion oil adsorption process onto GA-2 is dominated by physical adsorption, and thus the release of adsorbed oil and recovery of aerogels is feasible than chemical adsorption.

Fig 6. Kinetics models of emulsion oil adsorption onto GA-2 under 298K (a) pseudo


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-first-order model, (b) pseudo-second-order model. Adsorption kinetics In order to provide insights into adsorption kinetics properties and mechanisms, pseudo-first-order and pseudo-second-order kinetic models were used for linear fitting of the experimental data, the linear equations are as followed [32]: ln+ − , - = ln − ./ 0 , &1

=2

/

(10)

,

3 3 &

&

(11)

Where Qe and Qt (mg g-1) are equilibrium adsorption capacities and adsorption capacities at time t, respectively. k1 (min-1) is the rate constant of the pseudo-first-order-kinetics. The values of k1 and Qe can be obtained from the slopes and intercepts of ln (Qe-Qt) vs. t plots. k2 (g mg-1 min-1) is the pseudo-second-order rate constant. The values of Qe and k2, can be obtained from the fitting line of t/Qt vs. t. In this work, coefficient (R2) and Standard deviation (S.D.) were used to evaluate the fitting results. As it shown in Fig 6., the experimental data fitted well with the pseudo-second-order kinetic model, which is reflected by the value of coefficient (R2=0.9982), and the corresponding standard error (S.D.) is below 6%, besides, the differences between experimental data Qe,cal and Qe,exp are very small(Table 3.), which means pseudo-second-order kinetic model is preferable to describe the adsorption behavior of oil in emulsion onto GA-2. Table 3. Isotherms parameters for three models for emulsified oil adsorption onto GA-2(298K) T/K 298

qe,cal1

k1 -1

-1

L·min

mg·g

0.0786

290.47

qe,cal2

S.D.

k2

%

-1

g·mg ·min

mg·g

0.9201

94.69

0.0005

768.33

R1 2

-1

-1

S.D.

qe,exp

%

mg·g-1

0.9997

6.28

745.69

R22

308

0.0800

220.79

0.9178

98.99

0.0010

769.23

0.9987

6.80

749.74

318

0.0814

267.15

0.9563

94.67

0.0011

766.23

0.9996

6.01

753.70

328

0.0969

444.66

0.9577

74.66

0.0013

769.23

0.9982

5.21

759.41

At the same time, in order to explore relationships between chemical reaction rate constant and temperature, Arrhenius equation was adopted in this work, the equation is listed as followed [33]: 7

Ink 6 = − 8 In+ -

(11)

Where, k2 is the pseudo-second-order kinetic constant, g mg-1 min-1, Ea is the Arrhenius activation energy, kJ mol-1.


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Fig 7. Arrhenius models of emulsion oil adsorption onto GA-2 under 298K. Fig7 describe the relationships between lnK and T-1. The calculated adsorption activated energy is 23.94kJ mol-1, this value can be used to distinguish the adsorption process between physical adsorption and chemical adsorption. The former is between 5~40 kJ mol-1, while the latter is between 40~800 kJ mol-1 [34]. Evidently, the Ea value for oil emulsion adsorption by GA-2 is in the range of physical adsorption, which is in consistent with the former result of evaluation of free energy changes(△G0). Besides, to confirm the rate control steps of this adsorption process, intra-particle diffusion model was adopted to analyze the kinetics experimental data, the equation is listed followed [32]: , = .9: 0 ".< C

(12)

In this equation, kdi is the corresponding intra-particle diffusion rate constant in stage i, mg g min -0.5. This value is the indicator to describe the degree of diffusion, and C can be used to evaluate the boundary layers around adsorbents, the greater value means greater effect of boundary layers on adsorption[33]. -1

Fig 8. Intra-particle diffusion models of emulsion oil adsorption onto GA-2 under 298K,308K,318K and 328K.


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Fig 8 shows the relation between Qt and t0.5, the fitted lines can be separated into three stages, which means adsorption process of emulsion onto GA-2 is composed of three stages. In stage ℃(i=1), the sharply increased line accounts for the adsorbate adsorption on the surface of adsorbents. In stage ℃, the adsorption rate shows the tendency to slow down, emulsion oil diffused into the porous space interconnected by the graphene sheets. While in stage ℃, the adsorption rate is the slowest, the whole process tends to reach equilibrium. Besides, the intra-particle diffusion experiments were also conducted under other three temperatures, 308K,318K,328K. The related parameters were listed in the Table 4. The result shows that kd of stage ℃ is associated positively with the increase of temperature, which means the adsorption rates are associated positively with the increasing temperature. As it also shown in Fig 8., the equilibrium removal rate increased with the increasing temperatures, the reason is that the viscosity of diesel oil in emulsion is associated negatively with the increasing temperatures, thus diminishing the resistance in adsorption. Besides, the value of kd1 is greater than kd2 (Table 4.), this mainly due to the emulsified oil formed a thick layer during adsorption, which may increase resistance in stage ℃. At the same time, the values of C are not same, which means other conditions may also have influences on adsorption rate control [34,35].

Table 4. Intra-particle diffusion parameters for emulsified oil adsorption onto GA-2 T/K

kd1 -1

C1 -0.5

mg·g ·min

kd2 -1

mg·g

-1

C2 -0.5

mg·g ·min

kd3 -1

mg·g

-1

C3 -0.5

mg·g ·min

mg·g-1

298

208.3260

97.0420

123.3233

202.3127

17.5562

626.9812

308

252.5321

226.3247

59.7319

468.8164

10.8255

663.5938

318

263.5845

173.3031

93.3410

395.8963

11.1630

710.4467

328

293.6968

345.5321

12.9769

644.1745

9.2546

735.8742

Fig 9. Selectivity research of oil adsorption in oil-water mixture by GA-2 under 298K(a) Oil adsorption mass (b) Oil and water adsorption volume. Selective adsorption of oil and water in oil-water emulsion As it shown in Fig 9(a)., for selective adsorption studies, the higher oil adsorption capacities increased with the increase of oil contents in oil-water mixture. On the contrary, water adsorption capacities decreased simultaneously, which suggested that oil had a suppression effect against


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water during the actual adsorbing condition. Two kinds of mechanisms could be adopted to explain this phenomenon, on the one hand, the immediate competition of oil and water for adsorption sites on external surfaces and inner pores of GA-2 happened. On the other hand, the hydrophobicity of GA-2 made graphene sheets serving as the micro-skimmer for separation of oil and water (Fig S4.). In this sense, it can be concluded that phenomenon of weak-adsorbing water existed in emulsion oil adsorption. Besides, with the variations of oil samples, fewer water was adsorbed into the pore space of graphene aerogel when the oil density is high in emulsion, which means that the bigger molecular size compounds of high-density oil have the superiority in squeezing, occupying and shielding the adsorption sites of GA-2. What’s more, constant oil-uptake volume was also evaluated in this experiment (Fig 9(b).), which indicated that tunable pore-fracture would lead to higher adsorption capacities, this conclusion confirmed the former result. Noticeably, though phenomenon of water adsorbing was existed during the adsorption of GA-2 in emulsion system, the faint capture of water can be ignored during the large-scale oily-water treatment due to the high-selectivity of graphene aerogel in oil adsorbing. Mechanism of the adsorption Through the above results, the adsorption of oil emulsion onto graphene aerogels(GA-2) are spontaneous and physical adsorption process. As it shown in Fig 10(a), when referring to the definition of aerogels, it is said that the pores of these materials were captured by air [36], thus for physical adsorption of oil, the oil would occupy the pore space by expelling air, then air-filled aerogels were transferred into oil-filled aerogels, and slight water adsorption in aqueous solution also occurred in this process [37,38]. The whole adsorption process of GA-2 took place from the bottom, because low-density graphene aerogels always floated on the water surface. As mentioned before, the recycling performance of physical adsorption is better than chemical adsorption in terms of recovery of adsorbents and adsorbates, this is an economical way for the practical applications of aerogel-based oily water treatments, especially resilient graphene aerogels.


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Fig 10. Adsorption-desorption process (a)a schematic procedure of emulsion oil adsorption and desorption. (b) optical images of the oil-water emulsion before and after adsorption. The droplet size distribution was also measured using the granulometer. Batch adsorption performance and regeneration Fig 10(b). shows the optical photos of oil-water emulsion before and after adsorption using GA-2 in both low- and high-magnification. The stabilized oil-water emulsion with a milk-like color, the measured oil droplets sizes were around 1µm. After first batch adsorption, the adsorption efficiency of GA-2 was very high, no evident oil droplets were observed in the windows, the droplet sizes were around 1nm, which indicated most of oil in water were adsorbed by GA-2 in the first adsorption. While in the 4th adsorption, the oil-water emulsion with faint milk-like color, and the oil droplets were observed in the windows with the average size around 600 nm, which suggested the reduction of adsorption efficiency. After 8th adsorption, the GA-2 is sorptive saturation. Combining with the excellent mechanical properties of graphene aerogel in terms of recycling performance (Fig S5. and Fig 11. (a)), squeezing was a feasible method to release the oil from the pores of aerogel due to the excellent rebounding performance of graphene aerogel. The compressive strain-stress test of GA-2 was evaluated in advance, the as-made aerogel exhibited an excellent rebounding performance under stress around 5kPa for 10 cycles, which endow the applicability of squeezing to realize the recovery of aerogel. As it shown in Fig 11. (b), after 10 cycles, the batch adsorption capacities of GA-2 decreased gradually, showing 20% reduction of adsorption capacities. The residual oil and the slight damage of aerogel during the regeneration process could be considered as the main reason for this shrinkage(Fig S6.). Therefore, mechanical strength of graphene aerogel was very crucial to maintain recycling performance, and squeezing is an effective way to recover graphene aerogel.

Fig 11. The compression tests of GA-2 and recycling performance. (a) The stress-strain curves of GA-2 at the maximum strain of 50% for 10 cycles. (b) Relationship between Qt vs. recycling times. Summary and Conclusions A resilient graphene aerogel was fabricated by vertical ice-templates growth through freeze drying and employed as adsorbent for oily water treatment. As an outstanding adsorbent, GA-2 with honeycomb-like pores possess higher emulsion removal abilities than GA-1 without directional ice-templates growth, and integrated recycling performance as well as selectivity of oil in emulsion. Four kinds of oils with different properties and sensitivity of adsorption temperatures were verified to explore their effects on adsorption. The microscopic kinetics and thermodynamics mutual reaction models of emulsion adsorption onto graphene aerogel were presented. Through confirmation of experiments, the adsorption of oil emulsion onto graphene aerogels (GA-2) are the spontaneous and physical adsorption process. The investigations of adsorption mechanism of


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