Preparation and optimization of the environmental dust suppressant with agricultural waste straw

In order to reduce the dust pollution caused by the coal mining process, a novel composite environmental dust suppressant for coal dust control was synthesized by corn straw, sodium carboxymethyl cellulose (CMC), and additives. This study focused on the preparation conditions of the dust suppressant, and the performances of which were investigated systematically. Response surface method (RSM) was used to optimize the raw material formulation and preparation parameters. The optimum mass ratio of straw, CMC, and alkali of the dust suppression was 65:20:15 (m/m), which was prepared under the conditions of the reaction time being 1.5 h and the rotation speed being 300 r/min. The pH of the dust suppressant was 8.0, and the state of which was suspension. Additives were benefited to enhance the suppressant performance, and the surface tension and the contact angle could decrease to 32.4 mN/m and 32.0°. The suppressant has a maximum viscosity of 363.6 mPa·s, and the compressive strength could be up to 200 kPa. The hygroscopic rate could reach more than 4%. The wind erosion resistance could be up to 99 % at the wind speed of 14 m/s. After spraying the dust suppressant, the gap between particles was filled with dust suppressant, and the adjacent particles were bound by strong mechanical action.


Introduction
As one of the major hazards produced in the coal mining process, dust is considered a severe threat to the safety production and health of miners. Coal dust is an important source of danger for coal worker's pneumoconiosis, which is common in major coal-producing countries in the world (Zhou et al. 2018a, b, c;Mo et al. 2014). According to statistics, occupational pneumoconiosis accounts for more than 90% of occupational diseases in China (Moreno et al. 2020). Pneumoconiosis has a high mortality rate, which seriously threatens the health and quality of life of workers (Liu et al. 2018a, b). Therefore, dust control is of great significance to the coal industry.
Previous measurements for dust control at coal industry have predominantly focused on water spraying and antidust nets in China. Although the conventional methods have played an important role in reducing the dust concentration, however, they still have obvious drawbacks. Taking water spraying as an example, periodic reapplications are necessary to achieve the dust-control effect due to the temporary control of dust, so frequent sprinkling is needed to maintain the dust suppression effect (Dariusz 2013;Wang et al. 2015).
Different from the previous two methods, another effective method is to apply chemical suppression. This method has much lower frequency reapplication requirements compared to water spraying. Among these methods, the use of composite chemical dust suppressant aimed at specific situations has attracted considerable attention because of its superior performance. Nowadays, the dust suppressant has been widely used for its excellent performance (Liu et al. 2018a, b). The development of dust suppressant technology is rapid, and the commonly chemical dust suppressants include wetting dust suppressant Gulia et al. 2018), adhesive dust suppressant (Ma et al. 2018), condensed dust suppressant (Fan et al. 2018;Xi et al. 2018), and compound dust suppressant (Zhou et al. 2018a, b, c). The graft copolymerization of guar gum and sodium alginate was used to produce a new kind of polymer dust inhibitor (Zhang et al. 2018;Zhou et al. 2018a, b, c;Bao et al. 2020;Jiang et al. 2021). Some progress has been made in the preparation of new materials for dust suppressant, but the preparation process is complicated, which requires the addition of polymer initiator and crosslinking agent to initiate the reaction. The wettability of water to material heap is poor, so adding surfactant can improve its wettability effectively Wang et al. 2018). However, these products may pose environmental hazards.
Considering the reuse of wastes and environmental protection, some researches focus on the extraction and reuse of materials. Li et al. (2020) extracted cellulose from waste paper to prepare an environmentally friendly dust inhibitor for surface mining. In addition, some studies have been carried out on preparation of biological dust suppressant using plant extract enzyme as raw material (Wu et al. 2020). Zhang synthesized a hydroxypropyl guar gum dust suppressant by a chemical modification method .
Environmental protection compound dust inhibitor has become the main trend of dust inhibitor (Yao et al. 2017). It can be observed that the raw material of a dust suppressant is from industrial waste, by-products, garbage, and so on, producing a cost-effective and environmental product. Wang synthesized a cellulose-based dust suppressant using straw as the raw material . At a concentration of 5%, the dust suppressant demonstrates some excellent functionality. Zhao extracted Sapindus mukorossi saponins and prepared a new and efficient saponin-based compounded wetting agent (Zhao et al. 2021).
At present, straw as a sustainable green material has attracted more and more attention in recent years (Zeng et al. 2019;Jiang and Xu 2016). Corn straw is considered to be one of the most abundant and renewable materials in the world that could be sustainably used in the biorefinery industry as raw materials (Yu et al. 2021;Ahring et al. 2016). The reuse of corn straw can contribute to the decrease of the waste and environmental pollution and the achievement of improving the added value of renewable resources Cai et al. 2017;Zhou et al. 2018a, b, c).
However, there are currently few papers that investigate the use of maize straw cellulose as a chemical dust suppressant. The aim of this study was to investigate a novel composite environmental dust suppressant for coal dust control and optimize the preparation condition. As the raw material is easy to obtain and pollution-free, this new dust suppressant has promising market prospects. The dust suppressant was synthesized by corn straw, sodium carboxymethyl cellulose, and additives. The formulation of raw materials and production process were optimized by response surface method (RSM). Furthermore, the performance of dust suppressant was evaluated and characterized comprehensively. An efficient dust suppressant compounding scheme was selected based on the parameters of viscosity, surface tension, hardness, sedimentation time, contact angle, wind erosion resistance, and water retention capacity. The mechanism of synergistic wetting of coal dust was explained from a microscopic perspective.

Preparation of corn straw
The corn straw was obtained from the suburb of Shijiazhuang, Hebei Province, and was crushed into pieces about 2 mm using a grinder, then washed, and dried for use.

Dust selection
Dust was taken from a coal field to simulate the real conditions. The collected samples were dried in a drying oven at 90°C for 3 h to remove excess water and then were sieved through a mesh to filter the dust particles. The dust particles which size being less than 0.6 mm were selected for the continuous experiment.

Dust suppressant preparation
The crushed corn straw (13g) was put into distilled water in a 500-mL beaker. This system was placed inside a thermostat water bath and then stirred with a precision electric mixer constantly for 30 min at 300 rpm (Ronghua Instrument, Jiangsu, China). Then, sodium carboxymethyl cellulose (4g) was added into this system and stirred for 1 h at 400 rpm. After this step, 3g NaOH was injected into the system as the initiator, which will increase the accessibility of cellulose. The sample was not corrosive because the pH of the dust suppressant was 8.0. The producing process of dust suppressant was shown in Fig.2. The sample was in the state of suspension and was sprayed with spraying equipment in application.
The response surface method (RSM) is an important technique for predictive design of experiments, for model development, and for finding complex process interactions for maximizing response (Ghafarzadeh et al. 2017). RSM was used to optimize the quality ratio of straw, CMC, and alkali to determine the formulation of dust suppressant. By adopting the Box-Behnken experimental design method, the qualities of the straw, CMC, and alkali were taken as three influencing factors, respectively, and three response surfaces were added, namely Y1 viscosity, Y2 surface tension, and Y3 dispersion (Table 1). For the optimization of the preparation process of dust suppressant, straw particle size, reaction time, and stirring speed were chosen as three influencing factors and added viscosity as the response surface ( Table 2).

Characterization of the dust suppressant
To investigate the influence of various factors on the dust suppressant performance, some performance indices were tested, including viscosity, surface tension, water retention rate at room temperature, hardness, and anti-wind erosion of consolidated soil.
The viscosity of the dust suppressant samples was measured using the Thermo Viscotester C viscosity tester made in Thermofisher technologies. Generally, viscosity is a physicochemical property that measures the degree of movement of layers of molecules moving over each other (Medeiros et al. 2012). High viscosity is a desired characteristic of a dust suppressant product because the wind cannot break through the layers of product on the particulate material with the higher the viscosity.
To measure the water evaporation properties of the dust suppressant, 20 g of dust suppressant was sprayed on the dish and was placed in the thermostat. The evaporation temperature was set as 40°C, 45°C, and 50°C, respectively. The water evaporation of dust suppressant was recorded every 10 min, and the mass of the dish was weighed, and the water evaporation capacity was calculated according to Eq. (1).
where m 1 is mass of the dust suppressant before evaporating and m 2 mass of the dust suppressant after evaporating.
To evaluate the wind erosion resistance of the dust suppressant, coal sample and gravimetric method was used to test the performance. The mass of the coal sample in the sampler was weighed, and the wind erosion resistance was calculated according to Eq. (2).
where w 1 is mass of the coal sample in the sampler without using dust suppressant and w 2 mass of the coal sample in the sampler with using dust suppressant. Surface tension, which is a very important attribute for a dust suppressant solution, can determine the wetting effect of a solution toward other materials. In this paper, the surface tension of the solution was measured using the platinum plate method on a JYW-200B surface tension meter made by Kecheng Instrument, China.
In our experiment, the dispersion capability of dust suppressant was investigated. Higher dispersion capability would  In order to test the penetration capability between the dust suppressant and the dust, the penetration rate test was developed. Thirty grams of dust suppressant was put into the tube, and 0.2 g coal powder was thrown into the tube. The settling time of the coal powder in the tube was recorded.
The sand column model was used to test the compressive strength and to characterize the firmness of the dust inhibitor shell. The consolidated dust samples were mixed with the dust suppressant evenly in the mold according to the mass ratio, and the pressure on the surface of the sand column was investigated with the stress testing.
The hardness of the consolidated dust samples was determined by the shore hardness tester. The hardness value was recorded when the number was stable, and the measurement of quality difference was taken before and after the blowing test.
The surface morphology of the dust suppressant was observed and analyzed by a Hitachi S4300 field emission SEM (Hitachi Co., Japan).

Effect of the material qualities on the viscosity of dust suppressant
The relevant factor that influences the viscosity of dust suppressant such as the material qualities was considered and optimized using Design Expert software. In this investigation, the effect of the qualities of the straw, CMC, and alkaline were discussed, just as shown in Fig.3. From Fig.3 a and b, we could see that the influence of CMC quality on the viscosity was the main factor. The viscosity of the dust suppressant increased with the increase of CMC quality, because CMC could achieve hydrolysis at room temperature and formed a viscous colloidal form without heating. In Fig.3 c and d, the effect of the straw and alkali mass on the viscosity was analyzed; it could be seen that these two factors had little influence on viscosity. The role of alkali in the system was contributed to the CMC hydrolysis, which was conducive to the formation of uniform and stable viscosity system, and alkali was also beneficial to the destruction of straw fibrous tissue (Jiang and Xu 2016). From Fig.3 e and f, the effect of the CMC and alkali quality on the viscosity was investigated, and we could conclude that the CMC also had the higher effect on the viscosity. Based on the above analysis, CMC was the main factor affecting the viscosity of dust suppressant. With the increase of the quality of CMC, the viscosity increased, presenting a nearly linear relationship. As could be seen in Fig.4, the dispersion performance increased firstly and then decreased with the increase of the viscosity. In practical applications, appropriate viscosity was conducive to increase the strength of crusting and enhance the effect of dust suppression. Too low or too high viscosity was not conducive to spray the dust suppressant. In our experiment, the dust suppressant with the viscosity being 250~350 mPa·s had the best performance. The best mass ratio of the straw, CMC, and alkali was 65:20:15 (m/m).

Effect of the preparation process on the viscosity of dust suppressant
In our experiment, the effect of preparation process on the viscosity of the dust suppressant was investigated, just as shown in Table 3. Through the results of the experiment, we could conclude that when the particle size of straw was 0.5~2 mm, and the viscosity of the dust suppressant decreased firstly and then increased with the particle size increasing, which mainly because when the particle size was less than 1 mm, the powdered straw had good dispersion in the liquid. Within the test range, the viscosity firstly increased and then decreased with the extension of preparation time, the reason of which was that little amount of CMC could quickly dissolve and gelatinized at room temperature, the long stirring time, and the shear force of the stirring paddle reduced the CMC viscosity (Wang et al. 2019a;Wang et al. 2019b). The results of response surface analysis showed that in the beaker test, the stirring speed had no significant effect on the viscosity of the dust suppressant, and the mixing of powder could be guaranteed within the test range. The optimal parameters of straw particle size, reaction time, and stirring speed could be obtained by optimization prediction of the process conditions, so in our experiment, when straw particle size was 2 mm, reaction time was 1.5 h, and rotation speed was 300 r/min, and the viscosity of the dust suppressant was 350 mPa·s.
According to the influence of feeding sequence on viscosity (Table 4), when three kinds of raw materials were added separately and successively to prepare dust suppressant, the viscosity were almost all the same. But when two of them were mixed firstly and then added with the third one, the viscosity was significantly reduced. The uneven hydrolysis inside the CMC surrounded by alkali led to the appearance of small CMC clumps. When the three were mixed, the viscosity was moderate. Based on the results, when CMC was mixed with other two, it was not conducive to the rapid hydrolysis of CMC, resulting in a low viscosity. So, these raw materials were added separately and successively were the chosen production process.  Generally, the contact angle of liquid on the surface of solid is used to assess the liquid's capability of wetting solid under laboratory conditions (Kollipara et al. 2014). The smaller the contact angle was, the higher the performance between liquid and particles was. In this study, contact angles of different additives were measured. Fig. 5a presents the relationship between contact angles and mass fractions of additives. We could see from Fig. 5a that the contact angle between all kinds of dust suppressants and coal decreased firstly and then increased with the increase of the mass fractions. Among the additives, the smallest contact angle of AOS was 40.8°when the mass fraction was of 0.2~0.3 %, which showed the best wetting ability. The addition of surfactant could significantly reduce the contact angle and enhance wettability. This was mainly due to the fact that anionic surfactants could dissociate negatively charged surfactant ions in aqueous solution and have good osmotic wetting and dispersion effects.

Effect of additive on the dust suppressant
The shorter the penetration time of coal dust is, the faster the permeation and the stronger the wettability of surfactant solution on coal dust are. In our experiment, the penetration time of coal dust for the same quality in dust suppressant was measured by colorimetric tube settling method to characterize its wetting ability. Fig. 5b presents the penetration time of coal dust in six kinds of additive dust suppressants of different mass fractions. From Fig. 5b, we could see that the penetration time of coal dust in the dust suppressant of anionic additives were better than the other additives. In the anionic additives, AOS had the shortest penetration time, which showed that the dust suppressant had the best performance of wettability. The result of penetration time was similar to the contact angle.
The premise of coal wetting is that the surface tension of liquid is lower than the critical surface tension of coal. The surface tension of pure water (about 72.8 mN/m) is much higher than the critical surface tension of coal wetting (about 45 mN/m), leading to its poor coal dust wetting effect (Shi    . In order to improve the wetting effect between the dust suppressant and the coal dust particles, which is benefit to improve the efficiency of dust suppression, it is necessary to reduce the contact angle between dust suppressant and the dust particles. Therefore, AOS was investigated in our experiment to achieve this goal. Fig. 6 showed the surface tensions and the contact angles of anionic surfactant (AOS) compound solutions under different mass fractions (0.01-0.5%). From Fig. 6, with the higher mass fraction of AOS, the surface tensions and the contact angles followed the same trends, that is, decreased firstly and then flattened out. When the mass fraction of AOS was 0.2%, the inflection point was reached, and the surface tension and the contact angle were 32.4 mN/m and 32.0°, respectively.

Effect of the materials of dust suppressant on the spraying dispersion
The performance and economics of a dust suppressant in practical applications are greatly affected by the performance of spraying. The spraying dispersion of the dust suppressant was investigated. Just as shown in Fig. 7, the dispersion performance increased firstly and then decreased with the increase of the straw qualities. Too little or too much straw is not conducive to spray the dust suppressant. In our experiment, 6 g of straw was the best quality. As mentioned above, with the increase of the CMC content, the viscosity of dust suppressant increased, the cross-linking effect of straw was strengthened, and the dust suppressant was sprayed evenly. However, as the content of CMC continued to increased, the viscosity was too high, which led to the difficulty of spraying. When the mass of the straw, CMC, and alkali was 6.0 g, 2.0 g, and 1.1 g, respectively, the dust suppressant had the best performance of spraying, just as shown in Fig. 8.

Performance of the water conservation and compressive strength
Nowadays, water scarcity is one of the most important challenges to the sustainable development in China. The dust suppressant must have the best performance of water conservation. In our experiment, the performance of water conservation had been investigated. In Fig. 9 a, the evaporation rate of 50°C was almost twice as fast as the  evaporation rate of 40°C after 120 min. The reason was that viscous CMC cross-links the hydrolyzed straw fragments to form a layer of colloidal straw fragments on the surface of the stack. This layer structure was conducive to retaining water and slowing down the rate of water evaporation, so that the stack surface could maintain a relatively wetted state for a long time, which was conducive to maintaining dust suppression effect for a long time, just shown as Fig. 9 b. Hygroscopicity is one of the important parameters of dust suppressant. Through absorbing the moisture in the air and decreasing the evaporation of moisture, the effect of dust suppression can be maintained for a long time. In our experiment, the hygroscopicity performance was investigated. From the results of the experiment, it could be seen that the dust suppressant could maintain the better hygroscopic capacity during the experimental period. The hygroscopic rate could reach more than 4%.
When the dust suppressant was mixed with coal particles with a mass ratio of 1:1, the compressive strength could reach 222.7 kPa (Fig. 10a). The reason was that the dust suppressant acted as an adhesive after stirring with coal particles, which promoted the adhesion between coal particles. When the straw mass in the dust suppressor was reduced and the proportion of other raw materials remained unchanged, the compressive strength of the sand column significantly decreased. When the straw content was reduced to 2.55 g, the compressive strength was reduced to 176.1 kPa, just as shown in Fig.  10b. For our dust suppressant, the straw acted as a skeleton inside the sand column, which strengthened the support of the sand column and increased the compressive strength.

Performance of the wind erosion resistance
The performance of the wind erosion resistance was carried out using a wind tunnel. The wind tunnel construction was composed of organic glass, which is 2 m in length, 1 m in width, and 1 m in height. The wind speed in tunnel reached 14   /s. Inside the wind tunnel, there was a platform to fix the samples. The measurement time of wind erosion resistance was 360 min. Coal sample (250 g) was weighed and evenly placed on a circular dish, 600 mm in diameter, and 300 mm in height. According to Equation (2), the performance of the wind erosion resistance was obtained, just as listed in Table 5. When the spraying thickness of dust suppressor was higher than 2 mm, the wind erosion resistance rate could be up to 99 %. After spraying, the sample could dry in the natural environment, then to form a dense shell, which prevent the coal sample to be blown up.

Dust suppression mechanism analysis
The microstructures of the samples were observed and analyzed by SEM. As shown in Fig. 11a, the coal dust particles were loosely stacked together, the space between adjacent After spraying the dust suppressant, the gap between particles was filled with dust suppressant, and the adjacent particles were bound by strong mechanical action. The sharp edges and corners of the particles are wrapped, and the particles have a certain degree of displacement under the action of flowing liquid dust suppressant. The viscous liquid will penetrate the coal pores and remain in the pore structure after drying, thereby enabling the coal dust particles to bond with each other and form a block of the consolidated layer. As shown in Fig.11b, the soil samples were relatively dense, adhesive, and flat, consistent with its best dust-suppressant capacity. The result showed that the entire dust surface was more uniform and stable. In the process of relative movement of particles after spraying, the particles first made contact with the surface of the product. Due to the certain viscosity of the product, the adhesive force was much greater than the van der Waals force caused by Brownian motion of particles, and the particle size increased due to the collision between particles and the product. At the same time, under the action of surfactant, the particles were effectively wetted by the product, the density of the bound particles increased, the gravity started to be greater than the buoyancy, and finally the dust particles settled, just as shown in Fig. 11c. The straw fragments undergo partial hydrolysis by alkali, and partial hydrolysis of starch and macromolecule sugars results in a large number of exposed active groups such as hydroxyl group and carbonyl group. After CMC went through gelatinization, its polymer chain was easy to absorb water molecule hydroxyl, hydrolyze, and at the same time polymerize with the active group exposed by straw. CMC connected each straw fragment like a net, just as shown in Fig.11d.

Conclusions
The aim of this study was to investigate a novel composite environmental dust suppressant for coal dust control, which was made successfully with corn straw, CMC, and additives.
1) The optimum mass ratio of straw, CMC, and alkali of the dust suppression was 65:20:15 (m/m), which was prepared under the conditions of the reaction time being 1.5 h and the rotation speed being 300 r/min. When the mass of the straw, CMC, and alkali was 6.0 g, 2.0 g, and 1.1 g, respectively, the dust suppressant had the best performance of spraying. 2) Additive contributed to the decrease of the surface tension effectively, which could improve the wettability. AOS had the shortest penetration time, which showed that the dust suppressant had the best performance of wettability. The result of penetration time was similar to the contact angle. With the higher mass fraction of AOS, the surface tensions and the contact angles followed the same trends, that is, decreased firstly and then flattened out. When the mass fraction of AOS was 0.2%, the inflection point was reached, and the surface tension and the contact angle were 32.4 mN/m and 32.0°, respectively.
3) The prepared dust suppressant had a maximum viscosity of 363.6 mPa·s, the compressive strength could be up to 200 kPa. The hygroscopic rate could reach more than 4%. The wind erosion resistance of dust suppressant was significant, which could be up to 99% at the wind speed of 14 m/s for 6 h. 4) After spraying the dust suppressant, the gap between particles was filled with dust suppressant, and the adjacent particles were bound by strong mechanical action. 5) Through the reuse of agricultural waste straw, it is benefit not only to achieve the reduction of agricultural waste, but also to play a positive role in air pollution control, which is of great significance for the realization of sustainable development.
Author contribution All authors contributed to the study conception and design. Material preparation, data collection, and analysis were performed by Wenjun Liang, Zhixue Zhang, Hao Chi, and Sida Ren. The first draft of the manuscript was written by Hao Chi, and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.
Funding This research was financially supported by the Beijing M u n i c i p a l S c i e n c e a n d T e c h n o l o g y P r o j e c t P r o g r a m (Z191100009119002).
Data availability Not applicable.

Declarations
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Competing interest The authors declare no competing interest.