3.1 Selection of surfactants
3.1.1 The effect of different surfactants on urease activity
The determination of surfactant type is the key of urease to play its role. Figure 3 shows the effects of different surfactants on urease activity. It can be seen from the figure that CTAB has a strong inhibitory effect on urease activity, that is, the amount of urea decomposition is small, SDBS and SDS have a smaller inhibitory effect on urease activity than CTAB, and there is little difference between SDBS and SDS. While APG hardly affects urease activity, the amount of urea decomposition at this time is close to 25 mmol/L.
The effect of surfactant on urease activity is attributed to the electrostatic interaction between surfactant and urease and the electrostatic potential generated by surfactant. Anionic/cationic surfactants ionize ions in aqueous solutions. If the ionic strength is too high, its charge will change the spatial structure of the protein, thereby reducing the urease activity. In addition, the hydrophobic groups of anionic/cationic surfactants may enter the active center of the urease molecule to form a complex, which will deactivate the urease. Non-ionic surfactants have almost no effect on urease activity because they cannot ionize charges in the solution.
3.1.2 Contact angle
Figure 4 shows the contact angles of biological dust suppressants with different concentrations of surfactants. It can be seen from the figure that for biological dust suppressants containing the same surfactant, as the concentration of the surfactant increases, the contact angle shows a gradual decrease trend. And when the concentration is lower than 0.1%, the contact angle decreases faster, and when the concentration is greater than 0.1%, the contact angle decreases gradually. This is due to the existence of various adsorption sites in coal. When the surfactant concentration is low, the surfactant is not enough to occupy all the adsorption sites in the coal (Guo et al., 2018). Therefore, the contact angle drops faster. When the concentration of the surfactant in the biological dust suppressant gradually increases, the adsorption sites in the coal are gradually occupied, resulting in a smaller decrease in the contact angle. For different types of surfactants, when the concentration of APG, CTAB, SDBS, and SDS increases from 0 to 0.5%, their contact angles on the briquettes surface decrease by 51.8 °, 39.9 °, 47.2 °, and 43.7 °, respectively. Among the four types of surfactants, CTAB has no obvious wetting effect on coal. This is because cationic surfactants will form an arrangement of hydrophobic groups facing outwards and hydrophilic groups facing inward on the surface of the liquid, resulting in poor wettability.
3.1.3 Settling rate of coal dust
The sedimentation process of coal dust is shown in Figure S4. It can be obviously seen in Figure 5 that as the concentration of APG, SDS, and SDBS in the biological dust suppressant increases, the sedimentation rate of coal dust gradually increases. The sedimentation rate of coal dust in the dust suppressant containing CTAB is the slowest. Only when its concentration is 0.3% can more obvious sedimentation be observed. This shows that the wettability of cationic surfactants to coal dust is the worst. When the SDBS concentration is 0.02%, it can be seen that the sedimentation rate of coal dust is relatively fast, indicating its excellent wetting performance. In addition, it can also be seen from the figure that the wetting effect of biological dust suppressants containing SDS and APG on coal dust is relatively small.
Considering the influence of surfactants on urease activity, contact angle, and sedimentation rate of coal dust, APG was selected for research in the following tests.
3.2 Evaluation of the dust-fixing performance of biological dust suppressants
3.2.1 Wind erosion resistance
When the biological dust suppressant is sprayed, the mineralization products will be deposited in the coal dust, and then the coal dust will be consolidated. Figure 6 shows the surface morphology of coal dust after spraying 1, 5, and 10 times respectively. It can be seen from the figure that there is no obvious change on the surface of the coal dust after spraying once. After spraying 5 times, there are some white substances on the surface of samples A-1, A-2, and A-3, and these white substances become more obvious as the number of spraying increases. After spraying 10 times, the surface of the coal powder was almost covered with a layer of white material. On the surface of samples R and A-0, no white matter appeared.
The weight loss of the sample at different wind speeds indicates the resistance to wind erosion. It can be seen from Figure 7 that as the wind speed increases, the weight loss of all samples shows an increasing trend. The weight loss of sample R is the largest under different wind speeds (7.83, 15.33, and 31.03g respectively). When the wind speed of sample A-1 is 15 m/s, the weight loss even exceeds that of sample A-0. The reason for this phenomenon is that as the wind erosion time increases, the consolidation layer on the surface of sample A-1 is gradually eroded, while the coal dust under the consolidation layer is not effectively consolidated. At this time, the unconsolidated coal dust is exposed to the wind current, which causes the weight loss of the sample to increase. Although urease was not added to sample A-0, with the extension of wind erosion time, CaCl2 in coal dust absorbed moisture in the air, leading to moisture on the surface of coal dust and increased resistance between pulverized coal particles, resulting in a smaller final weight loss than sample R.
The weight loss of samples A-2 and A-3 is small (both are lower than 5g), which shows that both can be effectively consolidated. However, some samples in A-2 also appeared similar to A-1 when the wind speed was 15 m/s. That is, a small amount of coal powder at the bottom of the sample (far less than A-1) was not effectively consolidated. When the wind speed is 15m/s, the wind erosion resistance of samples A-1, A-2, and A-3 are increased by 51.66, 86.69, and 94.10%, respectively. Taking into consideration the consolidation effect and the cost of the dust suppressant, the APG concentration is 0.3%.
3.2.2 Evaporation resistance
Figure 8 shows the evaporation resistance results of the samples after different treatments. It can be observed from Figure 8 that when the temperature is 25°C and the humidity is 40%, the water content in the sample gradually decreases with time. In particular, the moisture content of sample R dropped sharply, and it decreased by 16.44% within 12 h. Although water containing APG can penetrate into the coal dust after spraying, the molecular structure of coal has a certain degree of hydrophobicity. When the environmental humidity is low, a large amount of water evaporates. The water content of samples S and T reduced by 8.38% and 9.52% in 12h, respectively. This is because CaCl2 has a certain degree of water retention, and part of the CaCl2 in the samples sprayed with biological dust suppressants is mineralized, resulting in a slight decrease in water retention. When the humidity is 60 and 80%, the moisture content of sample R hardly changes. While the samples S and T can absorb the moisture in the environment under the action of CaCl2 when the environmental humidity increases, resulting in an increase in the moisture content. Even at the end of the test, the moisture content in the sample is greater than the initial moisture content (48.18 and 47.36%, respectively), which indicates that the biological dust suppressant has good anti-evaporation properties.
3.2.3 XRD characterization
Figure 9 shows the XRD patterns of mineralized products, samples R and A-2. It can be seen from the FTIR spectrum of sample R that the coal powder contains a large amount of non-crystalline substances, which also contains a small number of crystalline substances such as quartz, kaolinite and other minerals. The XRD pattern of the mineralized product shows that the 2θ are 29.3 °, 36.1 °, 39.4 °, 43.1 °, and 47.4 °, which are characteristic diffraction peaks of CaCO3, and they are of calcite type. In addition to a large number of amorphous substances in sample A-2, there are characteristic diffraction peaks of CaCO3 (2θ are 29.3 °, 39.4 °, 43.1 °, and 47.4 °), which indicates that sample A-2 contains mineralized products.
3.2.4 FTIR characterization
Figure 10 shows the FTIR spectra of mineralized products, samples R and A-2. Figure 10 shows the FTIR spectra of mineralized products, samples R and A-2. In sample R, 797.69 cm-1 is the stretching vibration peak of -CH outside the aromatic ring. At 1031.46 cm-1 is the stretching vibration peak of minerals such as quartz and kaolinite. The stretching vibration peaks of –CH2 and –CH3 are at 2862.23 and 2915.95 cm-1, respectively. The stretching vibration peak of -OH is at 3438.75 cm-1. In the mineralized product samples, at 1416.80 and 872.81 cm-1 are the antisymmetric and symmetrical stretching vibration peaks of the C–O bond in CaCO3, respectively. In sample A-2, the antisymmetric and symmetric stretching vibration peaks of the C–O bond in CaCO3 are at 1405.99 and 871.70 cm-1, respectively. The reason for this difference is that the –OH bond in the coal dust and the C–O bond in the mineralized product form an intermolecular hydrogen bond O–H···O, which makes the density of electron cloud between C–O move towards O atom, resulting in the decrease of electron cloud density and the weakening of vibration force. Therefore, the antisymmetric stretching vibration peak and the symmetric stretching vibration peak move to the low-frequency direction (Rong et al., 2015). In the same way, the –CH (776.85 cm-1) and –OH (3440.35 cm-1) groups in sample A-2 also shifted. From the above analysis, it can be seen that the mineralization product can consolidate coal dust because the intermolecular hydrogen bond is formed between the coal dust and the mineralization product.
3.3 Molecular dynamics simulation
3.3.1 Distribution of water molecules in the system
The initial state and the equilibrium state of the APG-water-coal system are shown in Figure 11. It can be observed from the figure that the water molecules are stratified with coal molecules in the initial state, and the water molecules move to the coal surface and interact with it in the equilibrium state. It can be seen from the equilibrium configuration that surfactant molecules cover the surface of coal seam. This is because the surface of coal molecules contains a large number of oxygen-containing groups, which makes it have a strong affinity with the hydrophilic groups of the surfactant. When Van der Waals force, Coulomb force, and solid-liquid surface tension reach equilibrium, the droplet will no longer spread. As the molecular weight of the surfactant increases, more surfactant molecules are adsorbed on the surface of coal molecules, and more water molecules enter the interstices of coal molecules, which indicates that the degree of diffusion of water molecules on the coal surface becomes stronger.
3.3.2 Relative concentration distribution
Figure 12 shows the relative concentration distribution of the APG-water-coal system along the Z-axis perpendicular to the coal surface. The relative concentration distribution peaks of different substances represent the locations where the molecules are more concentrated (Meng et al., 2019). It can be seen from Figure 12 that the relative concentration distribution of coal is not affected by the number of water molecules and APG molecular chains. The overlapping part of the concentration distribution of surfactant or water molecule and coal indicates that the surfactant or water penetrates into the coal, which to some extent indicates the wetting effect of water on the coal. When the surfactant is added, the location of the surfactant is closer to coal than water. This is because the hydrophobic group in the surfactant is in contact with coal, and the hydrophilic group is in contact with water. The surfactant acts as an "insulation layer" between water and coal and connects water and coal powder together. When the amount of surfactant added is different, the positions of surfactant appearing at 44.359, 39.288,38.529, and 47.551 Å, respectively (Figure 12b-d). As the amount of surfactant increases, the more "forward" it appears, which means that the surfactant is more permeable in coal. Although the surfactant dose added in Figure 12e is the largest, its appearance is "behind" than System II-IV. This shows that when the added amount of surfactant exceeds a certain concentration, the wettability of the system cannot play a positive role.
3.3.3 Interaction energy between surfactant and coal dust
The interaction energy in the APG-water-coal system is calculated using the following formula (Meng et al., 2019).
Etotal=Etotal (APG-water-coal)-Etotal (APG)-Etotal (water)-Etotal (coal) （5）
EVDW=EVDW (APG-water-coal)-EVDW (APG)-EVDW (water)-EVDW (coal) （6）
EL=EL (APG-water-coal)-EL (APG)-EL (water)-EL (coal) （7）
Among them, Etotal is the total energy of the APG-water-coal system, kcal/mol; EVDW is the van der Waals energy, kcal/mol; EL is the electrostatic interaction energy, kcal/mol.
From the energy point of view, the more negative the Etotal, the more favorable the adsorption of the surfactant APG with coal. The total energy of the system, van der Waals interaction energy and electrostatic interaction energy are shown in Table 4. It can be seen from Table 4 that the system (I) without surfactant molecules has the largest interaction energy, which indicates that the adsorption stability between pure water and coal is poor. As the number of surfactant molecules increases, the interaction energy between coal molecules and surfactant molecules increases gradually. and the system becomes increasingly stable. In addition, the results of this study are consistent with the study of Vatanparast et al. (2018), that is, as the number of surfactant molecules increases, the proportion of van der Waals action energy gradually decreases, while the proportion of electrostatic action energy gradually increases. In all the systems, the proportion of electrostatic interaction is very high, indicating that electrostatic interaction plays a dominant role in the adsorption process (Rai et al., 2011).
3.4 SEM-EDS characterization
Figure 13a-j shows the microscopic morphology of different samples. It can be seen from Figure 13a that the mineralized product is spherical and has a smooth surface. Figure 13b shows the microscopic morphology of coal powder. Its shape is irregular, there is no connection between the coal particles, and they are loosely piled together. Figure 13c is a sample sprayed with APG and urea-CaCl2 solution, and its morphology is almost the same as Figure 13b.It can be seen in Figure 13d, f and i that there are no obvious gaps between the coal dust particles, almost becoming a whole, and the surface of the coal dust is relatively rough compared to Figure 13b. The reason for this phenomenon is presumed to be that the mineralized product wraps the coal powder during the crystallization process, and then connects the coal particles together. At the same time, spherical mineralization products were also found in these samples. Further observation in Figure 13e, h and j, it can be found that there are still larger gaps between the coal dust at the bottom of Figure 13e, and most of the coal dust particles have flat surfaces. In the coal dust at the bottom of Figure 13h, the surface of coal dust is rough, and the gap between coal dust particles is small, and there is a tendency to connect as a whole, which is consistent with the phenomenon observed in the anti-wind erosion test. The difference between the bottom and the surface of the coal dust in Figure 13i and j is small, and both can be consolidated. Spherical mineralization products are also observed in the coal dust in Figure 13j. The reason for the difference in the samples selected at the bottom of the coal dust in Figure 13e, h, and j is that the concentration of APG added is different. When the APG concentration is small, the coal dust is not easy to wet, and the urea-CaCl2 and urease solutions are difficult to penetrate into the coal dust, resulting in limited consolidation of the coaldust at the bottom. As the concentration of APG increases, the wettability of the solution increases, and the solution of urea-CaCl2 and urease penetrates into the coal dust, consolidating the coal dust particles together.
Figure 13k and l show the EDS analysis at the mark in Figure 13g. It can be seen from the figure that the coal dust contains elements such as C, O, N, S, Si, and Al, and the agglomerated spherical material contains C, O, and Ca. It can be seen from the mass ratio of the three elements that the substance is CaCO3.At the same time, from the changes in the microscopic morphology of the bottom sample of the coal dust in Figure 13e, h, and j, it can be inferred that the consolidation process of the biological dust suppressant on the coal dust is as follows. The mineralization product CaCO3 first appeared on the surface of the coal dust and gradually wrapped the coal dust. With the further precipitation of CaCO3, the coal dust particles were connected together to form a consolidated body, thereby achieving the purpose of dust retention.
In summary, the consolidation process of coal dust by biological dust suppressants is divided into the following two steps: (Ⅰ) The hydrophobic group in the surfactant (APG) combines with the hydrophobic group in the coal dust to form a distribution of hydrophobic groups facing inward and hydrophilic groups facing outward, thereby improving the wettability of coal dust (Figure 14a). At this time, the urea, Ca2+ and urease in the biological dust suppressant solution can enter the coal dust and be distributed in the pores between the coal dust (Figure 14b). (Ⅱ) Urease in coal dust pore decomposes urea into CO32-, CO32- reacts with Ca2+ to produce CaCO3, in which C–O bond forms intermolecular hydrogen bond O–H···O with -OH in coal dust. With the increase of CaCO3, coal dust particles are consolidated into a whole (Figure 14c).
3.7 Environmental protection of biological dust suppressant
Urease, urea, CaCl2 and APG are the main components of dust suppressant in this study. Urease is essentially a protein, urea is easy to decompose, CaCl2 is an inorganic salt, APG is essentially a degradable polysaccharide, and the above substances will hardly have a bad impact on the ecological environment. In addition, its mineralization product CaCO3 is a widely existing substance in nature. Through previous research (Wu et al., 2020), it was found that the highest pH value of the dust suppressant during the mineralization process was 8.04, and after the reaction was stable at about 7.86, which was non-toxic. This shows that the prepared dust suppressant is neutral and is an environmentally friendly dust suppressant.