Preparation and performance of a biological dust suppressant based on the synergistic effect of enzyme-induced carbonate precipitation and surfactant

To control the dust pollution caused by open-pit coal mining and reduce or avoid the secondary hazards of existing dust suppressants to the environment, a biological dust suppressant was prepared through the synergistic effect of a surfactant and an enzyme-induced carbonate precipitation. The optimal ratio of biological dust suppressant was determined, and the dust suppressive effect and dust consolidation mechanism of the biological dust suppressant were investigated. The results showed that the optimal biological dust suppressant had an alkyl polyglycoside (APG) concentration of 0.3 wt.%, a urea-CaCl2 concentration of 0.6 mol/L, and a urease to urea-CaCl2 volume ratio of 1:3. The wind erosion resistance of coal dust treated with this dust suppressant was enhanced by 86.69%. The adsorption of the biological dust suppressant by coal dust was mainly due to the electrostatic interaction between the surfactant and coal dust. The mineralization product of the dust suppressant was calcite-type CaCO3, which consolidated coal dust due to the formation of intermolecular hydrogen bonds between CaCO3 and coal dust.


Introduction
In recent years, with the progress of science and technology and the improvement of mechanical automation, the production efficiency of open-pit coal mines has rapidly improved (Dixon-Hardy et al. 2008). The concentration of dust in coal mining areas has also increased, and this dust has become the main pollution source in safe and efficient mining (Cui et al. 2011;Fan et al. 2018;Wang et al. 2019a, b). Dust affects the operation of mining facilities and pollutes the surrounding environment of coal mines . It also adversely affects the health of miners (Chang et al. 2019;Wang et al. 2019). Studies have shown that long-term exposure to high concentrations of coal dust has significant negative effects on the body, causing health issues such as chronic stomach disease and noise-induced deafness (Musselman and Chander 2002;Zhang et al. 2019;Wang et al. 2020;Fu et al. 2021). Therefore, the control of coal dust in open-pit coal mines is of utmost importance .
At present, commonly used coal dust control methods include spraying water and chemical dust suppressants (Han et al. 2020). However, due to the large difference in the degree of coal dust deterioration and particle size, water spraying is often ineffective, and the average dust reduction rate is less than 60% (Ayoglu et al. 2014;Zhou et al. 2016). This is because the surface tension of water is high, so water cannot quickly wet the coal dust. Adding surfactants to the water can change the wettability of coal dust. This makes coal dust more hydrophilic and improves dust reduction efficiency (Pollock et al. 2010;Cybulski et al. 2015;Yao et al. 2017). Although surfactants can be added to the water to achieve a good dust reduction effect, multiple sprays are required due to the lack of Responsible Editor: Philippe Garrigues long-lasting inhibition and poor wind erosion resistance, resulting in a significant waste of water resources (Bao et al. 2020;Xu et al. 2018;Northey et al. 2016). Researchers have developed a variety of chemical dust suppressants. For example, Medeiros et al. (2012) used the biodiesel by-product glycerin to prepare dust suppressants. Xiao et al. (2015) prepared a new type of dust suppressant with strong hydrophobicity and high viscosity by copolymerizing carboxymethyl cellulose, acrylamide, and polyvinyl alcohol in a specific proportion. Dang et al. (2017) developed a co-polymer (gelatin and oxidized corn starch mixture) dust suppressant and conducted dust removal experiments in a closed dust removal room. Although these prepared chemical dust suppressants can control coal dust pollution to a certain extent, they have disadvantages such as complicated preparation processes, poor degradation characteristics, and secondary pollution. Therefore, finding a dust suppressant that is simple to prepare and environmentally friendly would be of great significance.
Enzyme-induced calcite precipitation (EICP) is a green mineralization technology (Krajewska 2018In this process, urea (CO(NH 2 ) 2 ) is decomposed into carbonic acid (H 2 CO 3 ) and ammonia (NH 3 ) by urease. Ammonia forms ammonium (NH 4 + ) in the water system, increasing the pH value of the solution and creating favorable conditions for carbonate precipitation. Carbonate precipitation can be realized in the presence of appropriate divalent cations, such as calcium. The reaction formula of this process is as follows (Hamdan and Kavazanjian 2016): The CaCO 3 produced by EICP technology has good compatibility with the environment. The generated calcium carbonate can fill pores and has a certain degree of cohesion, so it can effectively consolidate the loose matrix. Therefore, this technology is favored by engineering experts. Hamdan and Kavazanjian (2016) carried out a wind tunnel test on sand soil treated by EICP technology and showed that its dust suppression effect is better than that of traditional methods. Kavazanjian et al. (2017) confirmed that EICP technology can strengthen soil. Song et al. (2019) evaluated the inhibitory effect of EICP on fugitive dust and its impact on the stability of near-surface soil. Wu et al. (2020) used EICP technology to effectively consolidate coal particles passed through a 40-mesh sieve. However, when the particle size of coal dust is small, particle wettability is poor. When coal dust is consolidated using EICP technology, the urease cannot penetrate into the coal dust. Because of this, the CaCO 3 is unable to bind the coal dust particles together. Therefore, the use of surfactants added to urease for enhanced consolidation of coal dust should be investigated. In addition, the types and amount of surfactant also need to be further researched.
In this work, a biological dust suppressant based on the synergistic effect of a surfactant and EICP technology was prepared. First, the effects of cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS), sodium dodecylbenzene sulfonate (SDBS), and alkyl polyglycoside (APG) on urease activity and coal wetting were studied. The dust-holding performance of the biological dust suppressant was then studied through anti-evaporation and wind erosion resistance experiments. Finally, through molecular dynamics simulation, scanning electron microscope analysis (SEM-EDS), and phase analysis (XRD and FTIR), the dust consolidation mechanism of the biological dust suppressant was studied. The environmental protection characteristics of the biological dust suppressant were also analyzed.

Materials
First, the coal was crushed to screen out small particles of 120 mesh pulverized coal. A fully automatic industrial analyzer (5E-MAG6600) was used to conduct an industrial analysis of the coal powder, as shown in Table S1. A Malvern particle size analyzer (Mastersizer 3000) was then used to analyze the particle size of the coal powder, as shown in Fig. S1.
The cationic surfactant CTAB, anionic surfactants SDS and SDBS, and non-ionic surfactant APG were purchased from Shanghai McLin Biochemical Technology Co., Ltd., China. Anhydrous calcium chloride and urea were purchased from Sinopharm Shanghai Reagent Co., Ltd., China.
Urease was extracted from yellow soybeans. The soybeans were purchased from Daxinganling Prefecture, Heilongjiang Province, China. The detailed extraction method for obtaining urease in this study is reported in the Supplementary Material.

Selection of surfactants
The effect of different surfactants on urease activity The activity of the soybean enzyme urease was determined by the change of conductivity in solution. The decomposition of urea produces NH 4 + and CO 3 2-, and conductivity increases with increasing ion concentration in solution. Therefore, urease activity is related to the amount of urea decomposition. The detailed test method used to determine urease activity is reported in the Supplementary Material. The relationship between the amount of decomposed urea and the change in conductivity is shown in Formula (1) (Wu et al. 2020): CTAB, SDS, SDBS, and APG were selected to study the effect of different surfactants on urease activity. The mass concentration of surfactant in the solution was 0, 0.02, 0.04, 0.06, 0.08, 0.10, 0.20, 0.30, 0.40, or 0.50 wt.%.

Contact angle
A FW-4A tablet press (Tianjin Tuopu Instrument Co., Ltd.) was used to press 0.5 ± 0.02 g of pulverized coal into briquettes with a pressure of 25 MPa and a pressing time of 3 min. A Kruss DSA30 optical contact angle measuring instrument (Kruss, Germany) was used to measure the contact angle of the urease-CaCl 2 solution (0.6 mol/L) containing different concentrations of surfactant in the briquettes. To prevent urease from decomposing urea, the solution did not contain urea. A camera was used to record a photograph 5 s after adding a drop of the solution on the briquette surface. The mass concentrations of different surfactants in the urease-CaCl 2 solution were 0, 0.02, 0.04, 0.06, 0.08, 0.10, 0.20, 0.30, 0.40, or 0.50 wt.%.

Settlement test
The device shown in Fig. S2 was used to determine the sedimentation rate of the pulverized coal. First, 250 mg of pulverized coal was weighed and formed into a conical coal pile on the surface of filter paper through a glass funnel. The filter paper was then placed at the bottom of a metal ring and slowly brought into contact with the surfactant solution in a beaker using a lifting device. When the filter paper was in contact with the solution, it absorbed water and sunk to the bottom of the urease-CaCl 2 solution (0.6 mol/L, with no urea). At this time, the time t necessary for the pulverized coal and the filter paper to completely settle in the liquid was recorded, and the sedimentation rate of the pulverized coal was calculated. When the sedimentation time exceeded 1500 s, the surfactant at the selected concentration was not considered to have a wetting effect , and no further investigation was performed. The volume of the beaker was 1000 mL, and the mass concentrations of different surfactants in the urease-CaCl 2 solution were 0, 0.02, 0.04, 0.06, 0.08, 0.10, 0.20, 0.30, 0.40, or 0.50 wt.%.
The settlement rate was calculated by Formula (2): where M 0 is the weight of coal dust (250 mg) and t is the time for the coal dust to completely settle (s).

Evaluation of biological dust suppressant properties
According to prior work done by our research group (Wu et al. 2020), the prepared biological dust suppressant contains urea, CaCl 2 , urease, and APG as the surfactant. A mixed solution of surfactant and urea-CaCl 2 was sprayed on the coal dust, followed by spraying of the urease solution as shown in Fig. 1.

Wind erosion resistance test
First, the coal sample (50 ± 0.5g) was weighed in a disk (152 mm in diameter and 18 mm in height). According to the formula in Table 1, the biological dust suppressant was then prepared and sprayed on the surface of the coal dust at room temperature. Sample R was only sprayed with water. Sample A-0 was sprayed with water and urea-CaCl 2 solution with an APG concentration of 0.3 wt.%. Samples A-1, A-2, and A-3 were sprayed with urease and urea-CaCl 2 solutions with APG mass concentrations of 0.1, 0.3, and 0.5 wt.%, respectively. After 6 h, the samples were placed in an oven at a temperature of 60°C to completely evaporate the water on their surface. They were then sprayed again with biological dust suppressant. This was repeated a total of 10 times. After spraying, the coal samples were placed in the oven (60°C) to dry to a constant weight and then tested for wind erosion resistance. Wind erosion resistance tests were conducted with wind speeds of 5 m/s, 10 m/s, or 15 m/s for 30 min. Formula (3) was used to calculate the weight loss of the coal samples in the wind erosion test: where M 0 is the weight of the sample before the wind erosion test (g) and M v is the weight of the sample after the wind erosion resistance test (g).

Evaporation resistance test
First, the coal sample (40 ± 0.5g) was weighed in a disk (100 mm in diameter and 20 mm in height). According to the formula in Table 2, the biological dust suppressor was then prepared and sprayed on the surface of the coal dust at room temperature. Sample R was sprayed with water with an APG mass concentration of 0.3 wt.%. Sample S was sprayed with water and urea-CaCl 2 solution with an APG concentration of 0.3 wt.%. Sample T was sprayed with urease and urea-CaCl 2 solutions with an APG concentration of 0.3 wt.%. After 6 h, the samples were placed in an oven at a temperature of 60°C to completely evaporate the water on their surface. They were then sprayed again with biological dust suppressant. This was repeated a total of 10 times. The samples were weighed after drying, with their weights denoted W 0 . All the samples were then completely wetted, and their weights after moisture absorption were recorded (W 1 ). Finally, the samples were placed in a constant temperature and humidity oven ( Fig. S3) with a temperature of 25°C and a relative humidity of 40, 60, or 80%. After a specific period of time, as the samples were removed and weighed (W 2 ). Evaporation resistance was expressed as a percentage of the moisture content in the sample.
The moisture content of the sample was calculated by Formula (4): where W 0 is the weight of the coal sample after drying, W 1 is the weight of the coal sample after moisture absorption, and W 2 is the weight of the coal sample at different stages of evaporation.

Molecular dynamics simulation
Materials Studio 2018 software was used for molecular dynamics (MD) simulation. All simulations were carried out under a compass II force field. In this paper, an APG molecular model (as shown in Fig. 2a) and a bituminous coal molecular model (Wiser model, as shown in Fig. 2b) were used for simulation in the APG-water-coal simulation system.
An APG-water-coal simulation system ( Fig. 2c) was constructed using the Amorphous cell module and Build Layers tools (Guo et al. 2018). First, the APG, water, and coal molecular models were optimized using the geometric optimization method in the Forcite module. Second, the amorphous unit cells of coal, APG, and water were established using the Amorphous cell module according to the density of APG, water, and coal molecules at normal temperature and pressure. The monomer (coal molecule, water molecule, and APG of different molecular chain lengths) was then randomly integrated into the rectangular simulation box through the "building layer" tool, which was set periodically to form the APGwater-coal simulation system with different APG concentrations, as shown in Table 3. A 20 Å vacuum layer was established on the top layer of the simulation box. Finally, in order to achieve structural relaxation, an annealing treatment was performed.
All MD simulations were performed in the Forcite module. First, the isothermal and pressure system (NPT) was selected to perform MD calculations at 50 ps. Then, 200 ps molecular dynamics calculations were performed on the system in a The interaction energy in the APG-water-coal system was calculated using the following formula (Meng et al. 2019): where E total is the total energy of the APG-water-coal system (kcal/mol), E VDW is the van der Waals energy (kcal/mol), and E L is the electrostatic interaction energy (kcal/mol).

Microscopic morphology and phase analysis
First, a graduated cylinder was used to measure urease solution (10 ± 0.1mL) and urea-CaCl 2 (30 ± 0.1mL, 0.6 mol/L) solution. The solutions were added to a beaker for mineralization. After the mineralization was complete, the mineralization product was centrifuged at 8000 rpm for 10 min. The centrifuged mineralization product was then washed three times each with deionized water and absolute ethanol. Finally, the cleaned mineralization product was dried in a drying oven at 60°C until the weight change was lower than 0.1%.
A Rigaku Ultima IV X-ray diffractometer (XRD) (Thermo Fisher Scientific, Japan) was used to test the mineralization  products and samples R and A-2 with a scanning range of 15-85°and a scanning speed of 10°/min. An APREO field emission scanning electron microscope (SEM) (FEI, USA) was used to observe the microscopic morphology and perform energy-dispersive X-ray spectroscopic (EDS) analysis of the mineralization products and the samples described in the "Wind erosion resistance test" section. Electron microscope samples were taken from the top surface and bottom of samples A-1, A-2, and A-3. A schematic diagram of the sampling points is shown in Fig. S4.
Fourier-transform infrared spectroscopy (FTIR) was conducted with a Nicolet iS50 FTIR Spectrometer (Thermo Fisher Scientific, USA) to test the mineralization products and samples R and A-2 in the wavenumber range of 4000-500 cm −1 .

Selection of surfactants
The effect of different surfactants on urease activity The determination of surfactant type is crucial for enabling urease to play its role in the fixing of coal dust. Fig. 3 shows the effects of different surfactants on urease activity. CTAB has a strong inhibitory effect on urease activity. When 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. APG only insignificantly affects urease activity, with the amount of urea decomposition almost 25 mmol/L.
The effect of the surfactants on urease activity is attributed to the electrostatic interaction between the surfactants and urease as well as the electrostatic potential generated by the surfactants. Anionic and cationic surfactants ionize ions in aqueous solutions. If the ionic strength is too high, their charges will change the spatial structure of the urease protein, reducing urease activity. In addition, the hydrophobic groups of anionic and cationic surfactants may enter the active center of the urease molecule to form a complex, leading to urease deactivation. The non-ionic surfactant APG has almost no effect on urease activity because it cannot ionize charges in the solution. Fig. 4 shows the contact angles of biological dust suppressants with different concentrations of surfactants. For biological dust suppressants containing the same surfactant, as the concentration of the surfactant increases, the contact angle shows a gradual decreasing trend. When the concentration is lower than 0.1 wt.%, the contact angle decreases faster, and when the concentration is higher than 0.1 wt.%, the contact angle decreases more gradually. This is due to the existence of various adsorption sites in coal. Low surfactant concentrations are not enough to occupy all the adsorption sites of the coal (Guo et al. 2018). Therefore, the contact angle drops more rapidly. As the concentration of the surfactant in the biological dust suppressant gradually increases, the adsorption sites in the coal are more fully occupied, resulting in a smaller decrease in the contact angle. When the concentrations of APG, CTAB, SDBS, and SDS increase from 0 to 0.5 wt.%, their contact angles on the surface of the coal briquettes decrease by 51.8°, 39.9°, 47.2°, and 43.7°, respectively. Among the four surfactants, CTAB has no obvious wetting effect on coal. This is because cationic surfactants will form an arrangement of hydrophobic groups facing outward and hydrophilic groups facing inward on the surface of the liquid, resulting in poor wettability.

Settling rate of coal dust
The coal dust sedimentation process is shown in Fig. S5. Fig.  5 shows that as the concentration of APG, SDS, and SDBS in the biological dust suppressant increases, the coal dust sedimentation rate gradually increases as well. The coal dust sedimentation rate is slowest in the dust suppressant containing CTAB. For this suppressant, obvious sedimentation can only be observed at concentrations of 0.3 wt.% or higher. This shows that cationic surfactants have poor wettability performance for coal dust. When the SDBS concentration is 0.02 wt.%, the sedimentation rate of coal dust is relatively high, indicating the excellent wetting performance of this surfactant. Fig. 5 also shows that biological dust suppressants containing SDS and APG have a relatively small wetting effect on coal dust. Considering the influence of these surfactants on urease activity, contact angle, and coal dust sedimentation rate, APG was selected for further investigation in the following experiments.

Wind erosion resistance
When the biological dust suppressant is sprayed, the mineralization products will be deposited in the coal dust, and the coal dust will be consolidated. Fig. 6 shows the surface morphology of coal dust after spraying the suppressant 1, 5, or 10 times. As can be seen, there is no obvious change to the surface of the coal dust after spraying once. Spraying 5 times results in the appearance of a white substance on the surface of samples A-1, A-2, and A-3. This white substance becomes more significant as the amount of spraying increases. Spraying the coal dust 10 times causes its surface to be almost completely covered with a layer of white material. No white layer appears on the surfaces of samples R and A-0.
The weight loss of the samples at different wind speeds indicates resistance to wind erosion. Fig. 7 shows that as the wind speed increases, the weight loss of all the samples also increases. The weight loss of sample R is the largest under different wind speeds (7.83, 15.33, and 31.03 g under wind speeds of 5, 10, and 15 m/s, respectively). The weight loss of sample A-1 under a wind speed of 15 m/s exceeds that of sample A-0. This is because as the wind erosion time increases, the consolidation layer on the surface of sample A-1 is gradually eroded. At the same time, the coal dust under the consolidation layer of sample A-1 is not effectively consolidated. Therefore, when the unconsolidated coal dust is exposed to the wind current, significant weight loss occurs. While urease was not added to sample A-0, CaCl 2 in the coal dust absorbs moisture in the air as the wind erosion time increases, leading to moisture on the surface of the coal dust and increased resistance between pulverized coal particles. This is why sample A-0 has a smaller final weight loss than sample R.
The weight losses of samples A-2 and A-3 are low (both lower than 5 g), showing that both can be effectively consolidated. However, some of the A-2 samples also appear similar to A-1 when the wind speed is 15 m/s. That is, a small amount of coal powder at the bottom of sample A-2 (but far less than A-1) is not effectively consolidated. When the wind speed is 15 m/s, the wind erosion resistance values 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 optimal APG concentration is 0.3 wt.%. Fig. 8 shows the evaporation resistance results of the samples after different treatments. When the temperature is 25°C and the humidity is 40%, the water content in the samples gradually decreases with time. In particular, the moisture content of sample R sharply drops, decreasing 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 environmental humidity is low, a large amount of water evaporates. The water content Fig. 6 Surface morphology of coal powder after spraying with suppressant 1, 5, or 10 times Fig. 7 Mass loss at different wind speeds of samples S and T are reduced by 8.38% and 9.52% in 12 h, respectively. This is because CaCl 2 can retain water. Some of the CaCl 2 in the samples sprayed with biological dust suppressants is mineralized, resulting in a slight decrease in water retention capacity. When the humidity is 60 or 80%, the moisture content of sample R does not significantly change. In contrast, samples S and T can absorb moisture from the environment under the action of CaCl 2 at high humidity levels, resulting in an increase in moisture content. Even at the end of the test, the moisture content in these samples is higher than the initial value (48.18 and 47.36%, respectively), indicating that the biological dust suppressant has good anti-evaporation properties. Fig. 9 shows the XRD patterns of the mineralized products and samples R and A-2. The XRD spectrum of sample R shows that the coal powder contains a large quantity of noncrystalline substances as well as a small number of crystalline substances such as quartz, kaolinite, and other minerals. The XRD pattern of the mineralized products shows 2θ peaks at 29.3°, 36.1°, 39.4°, 43.1°, and 47.4°, which are the characteristic diffraction peaks of calcite-type CaCO 3 . The XRD pattern of sample A-2 shows a large quantity of amorphous substances and the characteristic diffraction peaks of CaCO 3 (at 2θ values of 29.3°, 39.4°, 43.1°, and 47.4°), indicating that sample A-2 contains mineralized products. Fig. 10 shows the FTIR spectra of the mineralized products and samples R and A-2. In the spectrum of sample R, the peak at 797.69 cm −1 is the stretching vibration of -CH outside the aromatic ring. The peak at 1031.46 cm −1 is the stretching vibration of minerals such as quartz and kaolinite. The stretching vibration peaks of -CH 2 and -CH 3 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 sample, the peaks at 1416.80 and 872.81 cm −1 are the antisymmetric and symmetric stretching vibration peaks of the C-O bond in CaCO 3 , respectively. In sample A-2, the antisymmetric and symmetric stretching vibration peaks of the C-O bond in CaCO 3 are at 1405.99 and 871.70 cm −1 , respectively. This shift is because 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 causes the density of the electron cloud between C-O to move toward the O atom. This results in a decrease in electron cloud density and weakening of the vibration force. Therefore, the antisymmetric and symmetric stretching vibration peaks move in a low-frequency direction (Rong and Qian 2015). Similarly, the -CH (776.85 cm −1 ) and -OH (3440.35 cm −1 ) Fig. 8 Resistance of different samples to evaporation Fig. 9 XRD patterns of different samples Fig. 10 FTIR spectra of different samples peaks in sample A-2 also shift. This analysis shows that the mineralization product can consolidate coal dust because an intermolecular hydrogen bond is formed between the coal dust and the mineralization product.

Molecular dynamics simulation
Distribution of water molecules in the system The initial state and the equilibrium state of the APG-watercoal system are shown in Fig. 11. The water molecules are stratified with coal molecules in the initial state, while they move to the coal surface and interact with the coal in the equilibrium state. The equilibrium configuration shows that surfactant molecules cover the surface of the coal seam. This is because the surface of coal molecules contains a large number of oxygen-containing groups, creating a strong affinity with the hydrophilic groups of the surfactant. When the 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. More water molecules then enter the interstices of the coal molecules, indicating a higher degree of diffusion of water molecules on the coal surface.
Relative concentration distribution Fig. 12 shows the relative concentration distribution of the APGwater-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). Fig. 12 shows that the relative concentration distribution of coal is not affected by the number of water molecules and APG molecular chains. The overlapping concentration distributions of surfactant or water molecules and coal indicate that the surfactant or water penetrates into the coal. To a certain extent, this is indicative of the wetting effect of water on the coal. When the surfactant is added, the location of the surfactant is closer to the coal than it is to water. This is because the hydrophobic group in the surfactant is in contact with the coal and the hydrophilic group is in contact with the water. The surfactant acts as an "insulation layer" between the water and coal, binding them together. With a varying amount of surfactant, the surfactant appears at positions of 44.359, 39.288, 38.529, and 47.551 Å, respectively ( Fig. 12b-d). The surfactant molecules appear increasingly "forward" with increasing surfactant concentration, meaning that the surfactant is more permeable in coal. However, the largest surfactant dose (Fig. 12e) results in an appearance "behind" that of Systems II-IV. This shows that excessively high surfactant concentrations do not continue to positively enhance the wettability of the system.

Interaction energy between surfactant and coal dust
From an energy point of view, more negative E total values are more favorable for the adsorption of surfactant APG on coal. The total energy of the system, the van der Waals interaction  energy, and the electrostatic interaction energy are shown in Table 4. System I, without surfactant molecules, has the largest interaction energy. This 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 also gradually increases, and the system becomes increasingly stable. 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 systems, the proportion of electrostatic interaction is very high, indicating that electrostatic interaction plays a dominant role in this adsorption process (Rai et al. 2011).

SEM-EDS characterization
Fig. 13a-j shows the microscopic morphology of different samples. As shown in Fig. 13a, the mineralized product is spherical and has a smooth surface. Fig. 13 b shows the microscopic morphology of coal powder. Its shape is irregular and there is no connection between the coal particles, which are loosely piled together. Fig. 13 c shows a sample sprayed with APG and urea-CaCl 2 solution, and its morphology is very similar to that shown in Fig. 13b. Fig. 13 d, f, and i show that there are no significant gaps between the coal dust particles, which almost form a continuous structure. The surface of the coal dust in these images is relatively rough compared to that of the unsprayed coal powder sample shown in Fig. 13b. This is attributed to the mineralized product wrapping the coal powder during the crystallization process and connecting the coal particles together. At the same time, spherical mineralization products are also found in these samples. Further observation in Fig. 13 e, h, and j shows that there are still larger gaps between the coal dust particles at the bottom of Fig. 13e and that most of the coal dust particles have flat surfaces. In the coal dust sample at the bottom of Fig. 13h, the surface of the coal dust is rough, and the gap between the coal dust particles is small. Therefore, these particles form a more continuous structure, which is consistent with the phenomenon observed in the anti-wind erosion tests. The difference between the bottom and the top surface of the coal dust in Fig.  13 i and j is small, and both show consolidation. Spherical mineralization products are also observed in the coal dust in Fig. 13j. The differences between the samples selected from  Fig. 13g; l EDS analysis at spectrum 2 in Fig. 13g the bottom of the coal dust shown in Fig. 13 e, h, and j are due to the different concentrations of APG. When the APG concentration is low, the coal dust is not easily wetted, and the urea-CaCl 2 and urease solutions do not easily penetrate into the coal dust, resulting in limited consolidation of the coal dust at the bottom of the sample. As the concentration of APG increases, the wettability of the solution also increases and the urea-CaCl 2 and urease solutions penetrate into the coal dust, consolidating the coal dust particles together throughout the sample. Fig. 13 k and l show the EDS analysis of the position marked in Fig. 13g. The coal dust contains elements including C, O, N, S, Si, and Al, and the agglomerated spherical material contains C, O, and Ca. The mass ratio of C, O, and Ca show that the spherical substance is CaCO 3 . Changes in the microscopic morphology of the bottom sample of the coal dust in Fig. 13 e, h, and j show that the consolidation process of the biological dust suppressant on the coal dust can be described as follows: The mineralization product CaCO 3 first appears on the surface of the coal dust and gradually wraps the coal dust. With further precipitation of CaCO 3 , the coal dust particles join together to form a consolidated body, thereby achieving the purpose of dust retention.
In summary, the coal dust consolidation process by the biological dust suppressant can be 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. This improves the wettability of the coal dust (Fig. 14a). At this time, the urea, Ca 2+ , and urease in the biological dust suppressant solution can enter the coal dust and are distributed in the pores between the coal dust particles (Fig. 14b). (II) Urease in the coal dust pores decomposes urea into CO 3 2− . The CO 3 2− reacts with Ca 2+ to form CaCO 3 , whose C-O bonds form intermolecular hydrogen bonds O-H···O with -OH in the coal dust. With an increasing amount of generated CaCO 3 , the coal dust particles are consolidated into a continuous structure (Fig. 14c).

Environmental protection benefits of biological dust suppressant
Urease, urea, CaCl 2 , and APG are the main components of the biological dust suppressant investigated in this study. Urease is a protein, urea easily decomposes, CaCl 2 is an inorganic salt, and APG is a degradable polysaccharide. These substances are not expected to have a negative impact on the ecological environment. In addition, the mineralization product CaCO 3 is widely present in nature. Previous research (Wu et al. 2020) has shown that the highest pH value of a dust suppressant during the mineralization process was 8.04, and after the reaction was stable, the pH value was about 7.86. These pH values are non-toxic. This shows that the prepared dust suppressant is neutral and is an environmentally friendly dust suppressant.

Conclusion
In this paper, a biological dust suppressant was prepared to control coal dust based on the synergistic effect of a surfactant and enzyme-induced carbonate precipitation. The best type and dosage of the surfactant, the dust consolidation effect, and the dust consolidation mechanism of the biological dust suppressant were determined. APG had the lowest effect on urease activity among the tested surfactants (CTAB, SDS, SDBS, and APG), with urease activity close to 25 mmol/L. Contact angle and sedimentation rate tests showed that when the surfactant concentration exceeded 0.2 wt.%, the wetting effect of the biological dust suppressant was more significant. The non-ionic surfactant APG was selected to prepare the dust suppressant with a concentration of 0.3 wt.%. The optimal proportion of biological dust suppressant was 0.6 mol/L urea-CaCl 2 , 1:3 (V/V) ratio of urease to urea-CaCl 2 , and 0.3 wt.% APG. The wind erosion resistance of a coal dust sample increased by 86.69% after treatment with the biological dust suppressant. Evaporation resistance tests showed that the biological dust suppressant had good water retention properties. An investigation of the dust consolidation mechanism showed that the adsorption of biological dust suppressant on the coal dust was mainly due to the electrostatic force between the surfactant and coal powder. The mineralization product was calcite-type calcium carbonate. After the reaction, the mineralization product first adhered to the surface of the coal dust, with some of the mineralization product filling the coal dust pores. With an increase in the amount of mineralization product, the calcium carbonate adhered to the coal dust particles and gradually wrapped the coal dust, consolidating it into a continuous structure. The mineralization product was able to consolidate the coal powder because it formed intermolecular hydrogen bonds with the coal dust. The synergistic effect of the surfactant and urease in inducing carbonate precipitation was embodied by the fact that the surfactant wetted the coal dust. This allowed the urease and urea-CaCl 2 to enter the coal powder. At this point, the urease mineralized the CaCO 3 to consolidate the coal powder into a continuous structure. Furthermore, this biological dust suppressant is an environmentally friendly material.