Preparation of Sulfur-Free Exfoliated Graphite by a Two-Step Intercalation Process and Its Application for Adsorption of Oils

The sulfur-free exfoliated graphite (EG) was prepared by a two-step chemical oxidation process, using natural flake graphite (NFG) as the precursor. The first chemical intercalation process was carried out at a temperature of 30C for 50min, with the optimum addition ofNFG, potassiumpermanganate, and perchloric acid in aweight ratio of 1 : 0.4 : 10.56.Then, in the secondary intercalation step, dipotassium phosphate was employed as the intercalating agent to further increase the exfoliated volume (EV) of EG. NFG, graphite intercalation compound (GIC), and EG were characterized by scanning electron microscope (SEM), energy dispersive spectrometer (EDS), X-ray diffractometer (XRD), Fourier transform infrared spectrometer (FTIR), BET surface area, and porosity analyzer. Also, the uptakes of crude oil, diesel oil, and gasoline by EG were determined. Results show that perchloric acid and hydrogen phosphate are validated to enter into the interlayer of graphite flake.The obtained EG possesses a large exfoliated volume (EV) and has an excellent affinity to oils; thus, the material has rapid adsorption rates and high adsorption capacities for crude oil, diesel oil, and gasoline.


Introduction
Although the petroleum consumption has resulted in a great deal of damage to natural environment [1], it is still extensively employed as one of the major fossil fuels.The increasing need of petroleum promotes the transport of this fossil; consequently, oil spill accidents have occurred and posed a serious threat to organisms and human beings [2].Thus, for the purpose of alleviating the pollution for ocean, rivers, and lakes in the emergency accident caused by oil spill, it is urgent to dispose oil pollutants using the high efficient technology.Generally, the adsorption technique can be exactly competent for the disposal of spilled oils, so suction felt, suction linoleum, foam materials, and some porous polymers have always been employed as the adsorbents.Unfortunately, these adsorbents have an undesirable affinity to oils, thereby weakening the absorption efficiencies of them [3,4].In order to alleviate the pollution caused by spilled oils, the exploitation of novel adsorbents with a high affinity to oil is very imperative.Compared with the above-mentioned adsorbents, exfoliated graphite (EG) has received tremendous attention in dealing with marine oil pollution due to its large adsorption capacity of oil and the noticeable disposal efficiency [5][6][7][8].
EG as a sort of versatile material is applied in various fields such as packing [9], sealing material [10], fire retardant [11], oil absorbing material [8], and electrode [12], due to its peculiar properties of flexibility, lubricity, and adsorption.There are three techniques applied to prepare the precursor of EG, namely, graphite intercalation compound (GIC)-gas phase intercalation, chemical oxidation, and electrochemical intercalation [13].Among these three techniques, chemical oxidation [14] and electrochemical intercalation methods [15] are commonly used.Compared with electrochemical intercalation method, chemical oxidation method owns the merits of more convenience, lower cost, and higher stability of the products.EG with an exfoliated volume (EV) of 200-300 mL/g can be obtained via the conventionally chemical oxidation process using concentrated sulfuric acid and concentrated nitric acid as the oxidants and intercalating reagents [16,17].However, this process always releases sulfur dioxide and nitrogen oxide into the air.In this regard, the fabrication process of EG with the friendly environmental characteristics should be developed [18,19].
The aim of this research is to develop a fabrication process of EG without using concentrated sulfuric acid and nitric acid as oxidants and intercalating reagents and then evaluate its adsorption efficiency of oils.In this study, a two-step chemical oxidation process was proposed; herein, reagents of perchloric acid (HClO 4 ) and dipotassium phosphate (KH 2 PO 4 ) were employed as intercalating agents in the first and second steps, respectively.Structures, crystallinities, components, and chemical groups of natural flake graphite (NFG), GIC, and EG were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive X-ray analysis (EDS), and Fourier transform infrared spectroscopy (FTIR) techniques.Influences of intercalation temperature, reaction time, and additions of potassium permanganate (KMnO 4 ) and HClO 4 on EV of EG were analyzed.Also, the uptakes of crude oil, diesel oil, and gasoline by EG were determined.

Preparations of GIC and EG.
GIC was prepared by a chemical oxidation process as follows.Under the continuous stirring condition, 3 g of NFG and the amount of KMnO 4 (0.3-2.1 g) were added slowly into a glass beaker, where the amount of HClO 4 (9-27 mL) was previously added in this beaker.The addition of KMnO 4 should be controlled discreetly to prevent the temperature of the mixture exceeding 80 ∘ C.After the one-step intercalation reaction, 50 mL of saturated KH 2 PO 4 solution was added to the mixture, and the oxidation process continued for 1 h at a temperature of 40 ∘ C.Then, the mixed solution was filtered and the collected GIC was cleaned with distilled water until the pH value of the washing effluent was neutral.Lastly, the cleaned GIC was dewatered by a suction filtration process followed by drying in an oven at 65 ∘ C for 24 h.
The preparation process of EG was as follows: 1 g of GIC was first added to a crucible, and then the crucible was placed into a muffle furnace and heated with a temperature of 950 ∘ C for 15 s.During the heating treatment, the laminar structure of GIC sample was exfoliated, and EG sample was finally obtained.

Characterization.
The morphologies of NFG, GIC, and EG were characterized by a Hitachi S-4800 scanning electron microscope (SEM, Hitachi, Japan); the chemical components of GIC and EG were determined by an energy dispersive spectrometer (EDS) equipped into the above-mentioned microscope.The crystalline structures of NFG, GIC, and EG were measured using a Rigaku D/max 2500 PC X-ray diffractometer (XRD, Rigaku, Japan) with Cu K radiation ( = 1.5418Å) at 40 KV and 30 mA.The X-ray diffraction patterns were recorded in the range of 2 = 10-90 ∘ .After being blended with KBr, a Nicolet IS10 Fourier transform infrared (FTIR) spectrometer (Nicolet, America) was employed to detect chemical groups of the obtained GIC and EG samples; FTIR data were recorded in the range of 4000-500 cm −1 .BET surface area, pore volume, and average pore diameter of the fabricated EG were measured by mercury intrusion porosimetry using an Autopore IV 9500 analyzer (Micromeritics, USA).
The exfoliated volume (EV) was determined by exfoliating 1.0 g of GIC in a temperature of 950 ∘ C for 15 s and its volume was measured using a graduated cylinder.Then the volume was recorded, and this datum was considered to be EV value.In order to ensure the accuracy, measurements were carried out in triplicate, and the average value was adopted.

Adsorption Rate and Capacity of EG for Oils.
Crude oil, diesel oil, and gasoline as the oil candidates were selected as the target pollutants to measure the adsorption rate and capacity of EG.Three portions of EG samples with the same weight were placed in different beakers; then the excessive target oil was added and the solution was stirred at a temperature of 25 ∘ C. The mixture of EG and oil was separated by 35-numbered mesh net.After the different adsorption time, the collected EG sample was statically placed to make the excessive oil completely dropping down.The adsorption time was set at 1, 2, 4, 5, 6, 8, 10, 20, and 30 min.The adsorption capacity in different time was obtained from the weight increase before and after adsorption.The adsorption capacity of oil at certain time was calculated via where  is the adsorption capacity of EG at certain time (g/g);  and  (g) are the weights of EG before and after the oil adsorption.In order to ensure the accuracy, measurements were carried out in triplicate, and the average value was adopted.
The regeneration ratio of EG (the ratio between the readsorbed oil amount and the initial adsorbed value) was assessed with 1 g addition of EG.EG and oil were separated through air pump filtration process; then the regenerated EG was used for oil adsorption; this process was repeated three times.The regeneration ratio is calculated by regeneration ratio () where  is the times of cyclic regeneration.in intercalation temperature, and then it reduces when the temperature was higher than 30 ∘ C. Thus, the temperature of 30 ∘ C can be identified as the optimum intercalation temperature.As the reaction time was 60 min and the mass ratio of NFG, KMnO 4 , and HClO 4 was 1 : 0.4 : 10.56, the value of EV reaches the maximum value (420 mL/g).

Results and Discussion
It is well known that the preparation of GIC via the chemical oxidation technique is an exothermic process, so the low temperature can be favorable to the intercalating reaction.But a high reaction temperature (no more than 30 ∘ C) might be needed at the beginning, which will be valuable for the intercalating reagent entering into the laminar spacing of NFG.However, the intercalating temperature higher than 30 ∘ C will accelerate the volatilization of HClO 4 .As a result, the amount of this agent entering the graphite interlayer reduces, thus resulting in a low EV.

Influence of Reaction Time.
When the intercalating temperature and the mass ratio of NFG, KMnO 4 , and HClO 4 were kept at 30 ∘ C and 1 : 0.4 : 10.56, respectively, the effect of reaction time on EV was analyzed and the results are shown in Figure 2. The value of EV increases from 310 to 420 mL/g as the reaction time elapsed from 10 to 50 min, and then it slightly decreases as time extends.When the reaction time was less than 50 min, the intercalating reaction cannot complete and the extended time will be helpful in destroying the edge layer of graphite by the oxidant.Thus, the prolonged time will guarantee more intercalating reagents entering into the laminar layer of graphite, which is beneficial to the achievement of high EV.This oxidation process is almost finished as the intercalating time is 50 min; then EV will decrease when the time was further prolonged because intercalating reagents will run away as time goes on.Therefore, the optimum reaction time of 50 min was ascertained for the preparation of EG.KMnO 4 Addition.EV of EG increases significantly with the increase in KMnO 4 addition (from 0.1 to 0.4 g/g NFG) (Figure 3); then the EV change can be ignored as the addition of KMnO 4 ranges from 0.5 to 0.7 g/g NFG.The maximum EV with a value of 420 mL/g can be obtained with KMnO 4 addition of 0.4 g/g NFG.During the intercalating reaction, the concentration of KMnO 4 in HClO 4 could hardly maintain saturated when its addition is below 0.4; as a consequence, NFG cannot be oxidized sufficiently, and the value of EV will be low.However, the excessive dosage of KMnO 4 (>0.4 g/g NFG) will erode the edge layers of NFG and destroy its lamellar structure.The intercalating agents will not be readily inserted into interlayer spacing, thereby resulting in the decrease in EV (<420 mL/g).The optical addition in weight ratio of NFG and KMnO 4 is validated as 1 : 0.4. the lamellar structure of NFG, the amount of HClO 4 was not enough to guarantee that the interlayer spacing of graphite is filled as most possible.On the contrary, at the higher addition of HClO 4 (>10.56g/g NFG), the excessive acid will dilute the concentration of KMnO 4 , thereby weakening the oxidation potential of this oxidant; this is unfavorable for the enhancement in EV.The optimal weight ratio between NFG and HClO 4 is 1 : 10.56; with these additions of the above two reagents, the measured EV value is 420 mL/g.In summary, the optimal conditions for the preparation of EG with EV value of 420 mL/g are as follows: mass ratio of NFG : KMnO 4 : HClO 4 = 1 : 0.4 : 10.56; the intercalating temperature 30 ∘ C, and the reaction time 50 min.At the aforementioned preparation condition, the EV value of the prepared EG is significantly higher than those reported in literatures [20][21][22].

SEM Analysis.
Morphologies of NFG, GIC, and EG under the previously described conditions were characterized, and SEM micrographs of them are presented in Figure 5.It is clear that the interlayer spacing of NFG is compact and much smaller than that of GIC (Figure 5(a)).The change in interlayer spacing between these two samples can be due to the intercalation and exfoliation of NFG.After the oxidation treatment, the interlayer spacing of NFG is enlarged and its boundary layers are crimped (Figure 5(b)).The worm-like porous structure can be seen from the surface morphology of EG (Figures 5(c)-5(f)), and the pore size ranges from several microns to hundreds of microns.The slit-shaped gaps between the graphite platelets can also be identified; this can be due to the fact that the compounds in interlayers of GIC were decomposed and some micropores are formed during the exfoliation process.The presence of the micropores will be of benefit for guaranteeing an excellent adsorption property of the prepared EG.  6.The chloride (Cl) and phosphorus (P) elements can be identified on the surfaces of tested samples.For both GIC and EG, the determined elements (atomic weight) are shown in Table 1.Atomic weights of Cl and P in GIC are 8.03% and 0.95%, respectively.The existence of Cl and P may derive from the additions of HClO 4 and KH 2 PO 4 .Thus, it can be deduced that both HClO 4 and KH 2 PO 4 were inserted into the interlayer of NFG; this process is a prerequisite for the preparation of EG.But the content of Cl in EG decreases to 0.05% and P cannot be detected.After the exfoliation treatment at 950 ∘ C, the inserted agents were decomposed and the worm-like EG was obtained.

XRD Analysis.
The tested XRD patterns of NFG, GIC, and EG are shown in Figure 7.The peak appearing at 2 = 26.42∘ (Figure 7(a)) with a -spacing of 3.37 Å for NFG is observed.For the sample of GIC, this peak with a -spacing of 3.50 Å shifts to 25.40 ∘ (Figure 7(b)).This can be due to the fact that KMnO 4 can easily oxidize the graphite layers, and the repulsion interactions between the layers increase, thereby resulting in the increase in the interlayer spacing.As a result, HClO 4 migrates into the graphite layers and expands the lamellar spacing along the -axis direction, which leads to the peak intensity of GIC lower than that of NFG.The similarity of these two peaks mentioned above evidences the insufficient oxidation reaction.As for the XRD pattern of GIC, another peak at 2 = 30.02∘ (Figure 7(b)) can be observed, suggesting that NFG is successfully intercalated by HClO 4 .Also, for the XRD pattern of GIC, it is easy to find that the reflection peak at 2 = 54.56 ∘ is divided into two peaks (2 = 51.88 ∘ and 56.94 ∘ ).This change can also be attributed to the oxidation reaction.Two diffraction peaks (2 = 26.44 ∘ and 54.54 ∘ ) for EG are observed in Figure 7(c), but no significant difference in their locations can be identified between EG and NFG [21].
The crystallinity of EG is still analogous to that of NFG; between them, however, the discernable difference in diffraction intensities of the above peaks can be attested.After the exfoliation treatment at 950 ∘ C, the inserted agents will be decomposed to gaseous compounds and they escape from the edges of the particles, leaving spaces between the layers.Thus, the crystal structure of EG was partially destroyed, so the decrease in crystallinity for EG samples is validated [23].

Analysis of FTIR Spectra.
The determined FTIR spectra of GIC and EG are shown in Figure 8.The peaks at 3450  and 1630 cm −1 (for GIC) and those at 3444 and 1624 −1 cm (for EG) are derived from adsorbed water [22].The peak of GIC at 1397 cm −1 is ascribed to hydroxyl groups because of the oxidation of NFG [24].The presence of two peaks of GIC (625 and 1112 cm −1 ) suggests the existence of ClO 4 − , and other peaks (1121-1136 cm −1 ) can be assigned to H 2 PO 4 − .Therefore, reagents of HClO 4 and KH 2 PO 4 are further ascertained to enter into graphite layers, being helpful in the preparation of EG.

Analysis of BET and Pore
Structure.The surface area of NFG and GIC is low because they are bulk materials with an average size of 500 m and less of porosity.BET surface area, pore volume, and pore diameter for EG sample were measured, and results are summarized in Table 2.The results show the feature of EG sample, that is, high surface area and micron pore structure.The surface area of the prepared EG was calculated to be 245 m 2 /g, which is much higher than that of exfoliated graphite synthesized by microwave irradiation [8].It is worth mentioning that the micrometer level of average pore diameter of EG will be advantageous to significantly enhance the adsorption performance toward oils [25,26].

Adsorption Capacity and Adsorption Rate of EG for Crude
Oil, Diesel Oil, and Gasoline.Figure 9 displays dynamic adsorption behaviors of EG for crude oil, diesel oil, and gasoline.The maximum adsorption capacities are 123.3g/g for crude oil, 76.5 g/g for diesel oil, and 61.4 g/g for gasoline.The adsorption rate is another important factor during adsorption process.The time to reach the equilibrium sorption capacity for each oil is 5, 2, and 2 min, respectively, reflecting rapid adsorption rates for these oils.These results indicate that the as-prepared EG will be competent for the application in oil spill accidents.Herein, the adsorption characteristics of EG toward the oil will be comprehensively investigated in the further research.3 exhibits cyclic regeneration abilities of EG after adsorbing the aforesaid three oils.The adsorption capacity of EG for each oil decreases slightly after regeneration, confirming that EG can be repeatedly applied as an effective adsorbent after the separation process by air pump filtration.

Mechanism of Adsorption.
In the process of GIC sample preparation, HClO 4 is used as both an intercalation agent and oxidizing agent.The strong acidity, volatility, and oxidability of HClO 4 can lead to a high-temperature expansion and the formation of microns pores for EG sample, guaranteeing its  excellent adsorption properties toward oils.In addition, high BET surface area and large pore volume of EG will also ensure rapid adsorption rates and high adsorption capacities.

Conclusions
In this study, a facile two-step intercalation method is proposed to prepare the sulfur-free EG, using HClO

Figure 2 :Figure 3 :
Figure 2: Effect of reaction time on EV.

Figure 9 :
Figure 9: Dynamic adsorption curves of EG for crude oil, diesel oil, and gasoline.

4 and KH 2
PO 4 as the inserting agent and KMnO 4 employed as the oxidant.The oxidant can easily open the graphite layers and the inserting agent will move into the interlayer spacing to obtain the GIC sample.The optimum conditions for the preparation of GIC are as follows: the mass ratio of NFG, KMnO 4 , and HClO 4 is 1 : 0.4 : 10.56; the reaction temperature and time are kept at 30 ∘ C and 50 min.EV of the prepared EG is 420 mL/g.The worm-like porous morphology, high BET surface area, and large pore volume of EG guarantee the excellent adsorption property toward oils.The maximum adsorption capacities of EG for crude oil, diesel oil, and gasoline are 123.3,76.5, and 61.4 g/g, respectively.

Table 1 :
The atomic ratios of GIC and EG.

Table 2 :
BET results of EG.

Table 3 :
Cyclic regeneration of EG for the three kinds of oils.