3.1 Adsorption models of different crystal planes on kaolinite
The usual crystal planes of kaolinite are (001), (110), (002), (010), (111), etc[1, 21]. To simplify adsorption models and analyze the occurrence state differences with the organic matters, several crystal planes of kaolinite were discussed as shown in Fig. 4.
As(001)surface corresponding to the maximum characteristic peak of kaolinite, taking the crystal face༈001༉as an example, the calculations result of the adsorption energy between the organic molecules and kaolinite was shown in Table 3.
Different functional groups of organic matter in coal-series kaolin may present diverse adsorption characteristics when adsorbed on kaolinite (001) surface. Taking alkane as an example, with the increasing of carbon chain length, the adsorption energy between alkanes and kaolinite (001) surface may be slightly increased (C30H62 > C24H50), suggesting that the adsorption may be more difficult to occur on the long-chain alkane, such as C30H62 discussed in this article, the adsorption energy may turn to be positive. For the structure containing olefins (C = C), the unsaturated double bonds make the molecular structure possess relatively strong activity, which are easier to adsorb on kaolinite. The adsorption energy of olefin (C18H36, C20H40) may be lower when compared with alkane, and the energy on C20H40 is the least (-7.3271729eV) among all the compositions. While the adsorption of alkanes and oxygenated-compounds (C15H24O) with kaolinite may be more difficult to occur for their relatively stable structure [29, 30, 31].
As the adsorption on C30H62 is relatively hard, we will further discuss the rest of the functional groups (C24H50, C18H36, C20H40, C15H24O), so as to compare the adsorption energy between the functional groups and different crystal planes of kaolinite. The comparison of the adsorption energy was shown in Table 3.
The adsorption energy on different crystal planes is rarely identical for the same functional group. For the kaolinite (001), the adsorption energy on C20H40, C18H36, C24H50 is much lower when compared with the rest of crystal planes, while kaolinite (200) may be easier to adsorb on C15H24O. The adsorption energy between C20H40 and kaolinite (001) is the least among all the structure models (-7.3271729eV), suggesting that the (001) surface may possess higher adsorption characteristic on the organic molecules, which may be consistent with the crystal face index of the kaolinite [28, 31, 32].
3.2 Adsorption models of different minerals
As discussed above, the coal-series kaolin often contains pyrite, quartz, anatase and other impurities, which may also interact with the organic matters. To fully understand the interaction mechanism and stability differences with the minerals in coal-series kaolin, several crystal planes of such minerals were presented to explore the adsorption between the organic matters and the impurities. To simplify the adsorption models, taking the maximum characteristic peaks of such minerals as shown in Fig. 5, we will further investigate the adsorption characteristic between the organic molecules and different impurities of coal-series kaolin, The calculations result was shown in Table 5.
The adsorption energy between the organic molecules and the impurities in coal-series kaolin was positive all the time, suggesting that the organic matter may be very hard to adsorb on pyrite, quartz. While the absolute values of adsorption energy on anatase were always the maximum compared with the rest of minerals, indicated that the organic matter was least likely to interact with the anatase[26, 27, 33].
3.3 Calculation of State density
State density is always used for the visual analysis of electronic structure, and the adsorption process, variation of electron orbits could also be differentiated by density of states (DOS) and partial density of states (PDOS) [23, 26, 34].
Taking olefin as an example, DOS for kaolinite when adsorbed with different organic molecules was shown in Fig. 6. The initial value of energy is -20.88eV before adsorption, while on C20H40 and C18H36, the energy is decreased to -22.51, -22.11eV, respectively. The displacement of the energy band may shift to the lower state after adsorption. New characteristic peaks were present in the energy band nearly − 14~-8eV, and the curves on C20H40 and C18H36 may shift to the lower energy when compared with kaolinite, especially in the energy band − 10 ~ 0eV, which also confirmed that a rearrangement of the electron clouds and strong chemical bonding between kaolinite and the organic molecules. The peak values of state density after adsorption may also increase dramatically, the value on C20H40 may reach to 70.4 electrons/eV, higher than that on C18H36 and kaolinite before adsorption, verifying that the effect on adsorption between kaolinite and C20H40 can be more significant, which may be consistent with the adsorption energy shown in Table.4[24, 35, 36].