Preparation of modified expanded graphite and research on emulsified oil adsorption performance

Modified expanded graphite was prepared from expanded graphite and characterized by FT-IR, XRD and SEM. The results showed that the modified linear alkyl was successfully grafted onto the surface of expanded graphite, and the crystal structure and overall morphology did not change significantly after modification. Expanded graphite and modified expanded graphite adsorption columns were prepared, the adsorption experiment of emulsified oil was carried out. When the initial concentration of emulsion oil solution is 100 mg·L−1, the filling density is 3.5 g·L−1, and the flow rate is 3 L·h−1, the emulsion oil concentration decreases to 93 mg·L−1 and 6.5 mg·L−1, respectively, after adsorption by expanded graphite and modified expanded graphite, indicating that the adsorption performance of modified expanded graphite on emulsified oil is obviously improved. The saturated adsorbed modified expanded graphite was recycled by vacuum filtration. The results show that the modified expanded graphite still has good adsorption performance for emulsified oil after 1-3 times of regeneration, and can be used as adsorption material or recycled into sealing material.


Experiment 2.1 Materials
EG(purity 99%, expansion volume 250 mL•g -1 ) was purchased from QingDao Teng Sheng Co., Ltd,Shandong,China.Oil (5w-40) was purchased from Mobil.All other chemicals used in this study were commercially analytical reagents grade and purchased from Tianjin FuYu Co.Ltd,China.

Preparation
At room temperature, mix 1g expanded graphite with NaOH solution(1mol•L -1 ) for 4 hours, then filter, clean, and dry.Then soak for 2 hours in a specific amount of H2O2 solution(1mol•L -1 ) , filter and clean.Next, N. N-dimethylformamide(DMF) was added into the expanded graphite and ultrasound for 30 min.After mixed evenly, add 5g dodecyl isocyanate (C13H25NO) and 0.5 g triethylenediamine (TEDA) and stir at 50 ℃ for 8 h.Finally, the obtained mixture was centrifuged with dichloromethane to remove unreacted C13H25NO and then dried in a vacuum drying oven at 40 ℃ for 4h to yield modified expanded graphite(M-EG).After treatment with NaOH and H2O2, hydroxyl and carboxyl functional groups were grafted onto the surface of expanded graphite, which reacted with isocyanate groups to successfully graft linear alkanes onto the expanded graphite to obtain M-EG.The synthesis route of M-EG is shown in figure 1.

Figure 1. Synthesis route of M-EG
Drip a certain amount of engine oil into the distilled water, mix and put it into the cell crusher.And then set parameters power to 45%, 30 min,40 ℃.After the procedure is completed, the oil emulsion with corresponding concentration can be obtained.

2.3Characterization
The crystal structures of the samples were analyzed by X-ray diffraction(XRD, PANalytical X'Pert Powder) using Cu Kα radiation(λ=0.154187nm)operating at 40 kV and 40 mA.The surface functional groups of EG were nanlyzed by Fourier transform infrared spectroscopy (FT-IR ,Nicolet iS50 ， Thermo Fisher Scientific) from 4000 to 400 cm -1 by means of the KBr tablet method.The micro-morphologies were viewed by field emission scanning electron microscopy (SEM, Sigma 300 field,Zeiss Instruments, Germany).The adsorption capacity of expanded graphite on emulsified oil was determined by UV-Vis spectrophotometer (UV-6000, Mepida).

Adsorption performance test
The dynamic adsorption is carried out by using a customized glass adsorption column with an inner diameter of 20 mm and a length of 300 mm.First, prepare the adsorption column by inserting a little piece of filter screen into the bottom of the adsorption column to prevent EG from dropping into the filtrate during the adsorption experiment.Insert a small amount of EG or M-EG into the adsorption column multiple times, record the weight and filling height of EG, calculate the filling density, and finally insert a piece of filter screen to press the top of EG.The emulsified oil is placed in a funnel with a pear form.The opening of the funnel valve is modified during the experiment to control the flow of emulsified oil.Fix the opening of the funnel to allow the emulsified oil to flow out for a period of time.Then measure the volume of liquid that is dripping out and determine the ralated flow rate.
Change the opening of the funnel to get different flow values and apply them in the experiment.The experimental setup is shown in figure 2. The concentration of oil in water was analyzed by ultraviolet spectrophotometer and the self-made emulsified oil solution was used as standard oil in the experiment.The sample has obvious absorption at wavelength 218 nm, which is selected as the detection wavelength.Standard oil was used as solute and diluted with water successively to the emulsion with concentrations of 80 mg•L -1 , 75 mg•L -1 , 50 mg•L -1 , 25 mg•L -1 , 20 mg•L -1 and 10 mg•L -1 .The absorbance of the reference solution and the emulsion with calibrated concentration were measured respectively .The standard curve's regression equation is as follows: Y=0.1025X+0.00026,where X represents absorbance in percent and Y represents oil concentration in milligrams per liter.With a regression coefficient of R 2 of 0.9998, it demonstrated a good linear relationship between the assayed concentrations .

Regeneration performance test
The Brinell funnel was filled with oil-absorbing saturated M-EG,and the vacuum degree of the circulating water vacuum pump was set as 0.009 MPa.The vacuum filtration was carried out until no oil dripped out.The weight loss of the regenertated treatment was calculated by weighting the regenerated M-EG, and the removal efficiency of M-EG was estimated.The regenerated M-EG was utilized to continue the adsorption experiment on emulsified oil and to repeat the regeneration procedure three times according to the above steps.The removal efficiency and regeneration efficiency, as calculated by equation ( 1) and ( 2), reflect the amount of oil removed by adsorbed saturated M-EG and the adsorption of regenerated M-EG.

3.1Material characterization and analysis
Figure 3 shows the FTIR of the M-EG and EG synthesized for this experiment.It is clear that the peak at 3400 cm-1 may have resulted from air moisture during the KBr compression process or from traces of moisture in the sample.The stretching vibration peaks of C=O, C-O, and C-H are represented by the wave numbers 1635 cm -1 , 1105 cm -1 , and 619 cm -1 , respectively, whereas the denaturation vibration peak of C-H is represented by the wave number 619 cm -1 .In M-EG, new methyl and methylene vibration absorption peaks developed between 2900 cm -1 and 2800 cm -1 , demonstrating that linear alkyl was successfully grafted onto the surface of EG [13] .  4 shows the XRD patterns of EG and M-EG.It can be seen that both EG and M-EG exhibit diffraction peaks of EG at 26 °with strong peaks.The M-EG has no significant difference when compared to the unmodified expanded graphite, showing that the crystal structure of EG has not changed much during the modification of linear chain alkyl, and the crystal structure of graphite has been retained.5 shows a SEM image of EG.As can be seen, there has been little difference between the morphology of M-EG and EG.There are many honeycomb pores inside of EG, however the size of M-EG increases slightly compared with that before modification, which may be due to the etching effect of sodium hydroxide on EG during the preparation of M-EG.Since the EG utilized in this experiment has a purity of above 99%, it contains certain oxide components, such as SiO2, Al2O3, CaO, etc., which could cause unreacted oxides to enter the EG.These components make up the expanded graphite ash, which has an immediate impact on the material's quality.These oxides are removed from the expanded graphite under the action of sodium hydroxide.As a result, the space that it once filled will become the expanded graphite's new pore structure, greatly contributing to the material's specific surface area.

Study on adsorption properties
The effects of different flow rates on the adsorption effect were studied.EG and M-EG of the same mass were weighed respectively, and the adsorption column was filled.The filling density was set to 5 g•L -1 , the initial concentration of emulsified oil solution was set to 100 mg•L -1 , and the flow rates were set to 1.
, respectively.The experimental results are shown in figure 6.It can be seen from the figure that the adsorption properties of EG and M-EG change in the same trend.The smaller the flow rate is, the better the adsorption effect is.The oil particles accumulated on the surface of the EG are easy to be taken away.Therefore, the adsorption of the oil particles by the EG is a dynamic process, that is, the oil particles are captured and the oil particles are affected by the water from the surface in a two-way process.The relative saturation state under this dynamic will be obtained when the dynamic balance is reached.The adsorption effect is poor when the EG is used to treat oil-bearing wastewater because the flow rate is too high and the reaction time is both excessively long and short.As a result, the appropriate flow rate should be managed in accordance with the actual demand.The filtered oil concentration of M-EG is substantially lower than that of EG at the same flow rate.EG has a low adsorption effect on emulsified oil as compared to the adsorbed oil concentration of M-EG.The influence of filling density on the adsorption was investigated.According to the effect of flow rate on adsorption performance, a high flow rate will result in an unsatisfactory adsorption effect, while a low flow rate would prolong the experimental procedure.In order to analyze the filling denstiy, a fixed flow rate of 3 L•h -1 is selected to research the filling density.The initial emulsified oil solution concentration was 100 mg•L -1 , and the filling density was 3.5 g•L -1 , 5 g•L -1 , 6.5 g•L -1 , 8 g•L -1 and 10 g•L -1 , respectively.The influence of filling density on EG and M-EG adsorption is shown in Fig. 7.The figure shows that the oil content of EG and M-EG falls as filling density increases, indicating that the higher the filling density, the better the adsorption effect.As the packing density increases, the entanglement space formed between the expanded graphite particles decreases, and the area between the non-polar surface of the expanded graphite and the oil particles in the water increases, so the oil content after adsorption decreases.However, excessive filling density may increase the resistance to the flow of emulsified oil to a certain extent, that is, blockage occurs, which affects the adsorption efficiency.Therefore, a reasonable filling density should be controlled to meet the actual needs.The filtered oil concentration of M-EG is substantially lower than that of EG at the same filling density.The adsorbed oil content of EG is 93 g•L -1 when the filling density is g•L -1 , and it falls to g•L -1 when the filling density is increased to 10 g•L -1 .However, there is still a considerable disparity when compared to the oil concentration of 2.8 g•L -1 after M-EG adsorption, indicating that EG has a limited adsorption impact on emulsified oil, which is consistent with the results reported in Figure 5.

Study on regeneration performance
Table 1 and 2 illustrate the adsorption properties of M-EG for emulsified oil after the first, second, and third regenerations.The initial concentration of emulsified oil is 100 mg•L -1 , the filling density is 3.5 g•L -1 , and the flow rate is 3 L•h -1 under the parameters of the adsorption experiment.As can be seen from the table, the regeneration adsorption capacity of expanded graphite decreases with the increase of regeneration times.After the first, second, and third regenerations, the adsorption action of M-EG on the emulsified oil was still effective but dramatically diminished by the time the concentration of emulsified oil had increased to 36.6 mg•L -1 .Because the structure of expanded graphite is damaged to some extent by pressure, and the pore volume is reduced, the adsorption capacity of expanded graphite to oil substances is affected, and the amount of residual oil in the reclaimed expanded graphite will also affect the regeneration adsorption performance of expanded graphite.The oil substances removed by vacuum filtration method are mainly the oil existing in the oil storage space inside the wormlike structure and the inside of the large pores.The oil that has been absorbed by the micropores and a portion of the mesoporous pores cannot be released with the air flow, and the remaining oil fills the pores, preventing the regenerated expanded graphite from regaining its initial adsorption capabilities.
After regeneration, M-EG has a removal effectiveness of more than 70% and a regeneration efficiency of more than 40%, indicating that after regeneration, M-EG can remove more than half of its oil each time.It is clear that even if oil absorption has decreased, M-EG can still be employed as an adsorption medium following regeneration treatment.Because expanded graphite naturally has strong lubricity and can be processed again with the addition of grease and other materials, it can be utilized as a sealing material even after regeneration treatment when the adsorption of oil cannot be entirely eliminated, leaving some oil in the interior.

Study on adsorption mechanism
The oil absorption mechanism of EG was further studied using thermodynamics and surface chemistry, after which the oil absorption procedure and the cause of its extraordinarily high oil absorption capacity were examined.Expansion is a process of disorder increase in the EG preparation process, natural flake graphite after puffed treatment, entropy value increases, that is, S>0, expanded graphite in a more stable state, on the other hand, due to a large number of new surface generated in expanded graphite, surface energy increases, so that it is in a new unstable state.The adsorption of other molecules or atoms can increase the entropy value and reduce the surface energy, which increases the stability of the system.In addition, since the adsorption of oil in EG is an exothermy process, i.e. △ H<0, the Gibbs free energy of the system △ G = △ H-t △S, △G <0, so EG can spontaneously adsorb other molecules or atoms.Additionally, as a result of lattice distortion and the creation of new surfaces during the expansion process, EG's lattice defects, suspended bonds, and surface activity all rise, making it simpler for EG to form surface compounds with other molecules or atoms, resulting in the formation of chemisorption layers.At the same time, a physical adsorption layer is created concurrently as a result of the vander Waals force's adsorption of additional molecules.Therefore, both monomolecular layer adsorption and multimolecular layer adsorption should be included in the adsorption of oil onto expanded graphite.Due to the small size and low concentration of individual oil molecules under the dynamic adsorption condition, some oil particles are easily carried away by the water flow when the water contains only a small amount of oil.After the water has flowed for a while, the oil particles on the surface of the expanded graphite gradually enrich, increasing the likelihood that they will collide.Coupled with the easy with which the oil itself nucleates, the oil particles on the surface of the expanded graphite are constantly adsorbed, and the size increases [14,15] .These oil particles gradually permeate inward along the pore surface of the expanded graphite.Under the capillary force, the permeability is enhanced to form a stronger adsorption force on the oil particles.As a result, the oil absorption grows gradually.The ability of oil particles enriched on the pore surface to capture oil particles flowing through gradually decreases with increasing adsorption time, leading to a downward trend in oil absorption and lower adsorption efficiency when the pores are filled with oil [16] .

Conclusion
Linear alkyl modified EG was prepared.The modified expanded graphite (EG) had new vibration absorption peaks of methyl and methylene near 2900 cm -1 and 2800 cm -1 , which demonstrated that linear alkyl was successfully grafted to the surface of EG.This was in contrast to the infrared spectra of unmodified expanded graphite.On emulsified oil, EG and M-EG adsorption tests were conducted, and the adsorption mechanism was analyzed.The results demonstrate that the adsorption trends of EG on emulsified oil are identical.In other words, when the filling density is constant, the lower the flow rate, the greater the adsorption impact.The greater the filling density is, the better the adsorption effect will be.when the initial concentration of emulsion oil solution is 100 mg•L -1 , the filling density is 3.5 g•L -1 , and the flow rate is 3 L•h -1 ,the emulsion oil concentration decreases to 93 mg•L -1 and 6.5 mg•L -1 after EG and M-EG adsorption, respectively.This shows that the adsorption performance of the emulsion oil is obviously improved after the modification of EG.M-EG retains good adsorption property for emulsified oil after 1-3 times of vacuum pumping and filter regeneration, and can be utilized as adsorption material or recycled and reused as sealing material.

Figure 2 .
Figure 2. Dynaamic tester with EG or M-EG for oil floating on the water The concentration of oil in water was analyzed by ultraviolet spectrophotometer and the self-made emulsified oil solution was used as standard oil in the experiment.The sample has obvious absorption at wavelength 218 nm, which is selected as the detection wavelength.Standard oil was used as solute and diluted with water successively to the emulsion with concentrations of 80 mg•L -1 , 75 mg•L -1 , 50 mg•L -1 , 25 mg•L -1 , 20 mg•L -1 and 10 mg•L -1 .The absorbance of the reference solution and the emulsion with calibrated concentration were measured respectively .The standard curve's regression equation is as follows: Y=0.1025X+0.00026,where X represents absorbance in percent and Y represents oil concentration in milligrams per liter.With a regression coefficient of R 2 of 0.9998, it demonstrated a good linear relationship between the assayed concentrations .

Figure 3 .
Figure 3. FT-IR diagram of EG and M-EG Figure4shows the XRD patterns of EG and M-EG.It can be seen that both EG and M-EG exhibit diffraction peaks of EG at 26 °with strong peaks.The M-EG has no significant difference when compared to the unmodified expanded graphite, showing that the crystal structure of EG has not changed much during the modification of linear chain alkyl, and the crystal structure of graphite has been retained.

Figure 4 .
Figure 4. XRD patterns of EG and M-EG Figure5shows a SEM image of EG.As can be seen, there has been little difference between the morphology of M-EG and EG.There are many honeycomb pores inside of EG, however the size of M-EG increases slightly compared with that before modification, which may be due to the etching effect of sodium hydroxide on EG during the preparation of M-EG.Since the EG utilized in this experiment has a purity of above 99%, it contains certain oxide components, such as SiO2, Al2O3, CaO, etc., which could cause unreacted oxides to enter the EG.These components make up the expanded graphite ash, which has an immediate impact on the material's quality.These oxides are removed from the expanded graphite under the action of sodium hydroxide.As a result, the space

Figure 5 .
Figure 5. SEM images of EG and M-EG.(a)EG (b)M-EG

Figure 6 .
Figure 6.Effect of flow on the adsorption of EG and M-EG.(a)EG (b)M-EGThe influence of filling density on the adsorption was investigated.According to the effect of flow rate on adsorption performance, a high flow rate will result in an unsatisfactory adsorption effect, while a low flow rate would prolong the experimental procedure.In order to analyze the filling denstiy, a fixed flow rate of 3 L•h -1 is selected to research the filling density.The initial emulsified oil solution concentration was 100 mg•L -1 , and the filling density was 3.5 g•L -1 , 5 g•L -1 , 6.5 g•L -1 , 8 g•L -1 and 10 g•L -1 , respectively.The influence of filling density on EG and M-EG adsorption is shown in Fig.7.The figure shows that the oil content of EG and M-EG falls as filling density increases, indicating that the higher the filling density, the better the adsorption effect.As the

Figure 7 .
Figure 7. Effect of filling density on the adsorption of EG and M-EG.(a)EG (b)M-EG

Table 1 .
Emulsified oil concentration after adsorption of regenerated M-EG.

Table 2 .
EG removal efficiency and regeneration efficiency.