Effectiveness,of,coal,mine,dust,control:A,new,technique,for,preparation,and,efficacy,of,self-adaptive,microcapsule,suppressant

Bo Ren ,Ling Yun ,Gng Zhou ,Shuilong Li,e,* ,Qunzhi Meng ,Ki Wng ,Bingyou Jing,Guofeng Yub,

a School of Emergency Management and Safety Engineering,China University of Mining and Technology -Beijing,Beijing 100083,China

b State Key Laboratory of Deep Coal Mining &Environment Protection,Huainan Mining Group Co.,LTD.,Huainan 232001,China

c Key Laboratory of Industrial Dust Control and Occupational Health,Ministry of Education,Anhui University of Science and Technology,Huainan 232001,China

d College of Safety and Environmental Engineering,Shandong University of Science and Technology,Qingdao 266590,China

e Department of Civil and Environmental Engineering,University of Alberta,Edmonton T6G 2R3,Canada

ABSTRACT This study aims to make full use of the agricultural waste peanut shells to lower material costs and achieve cleaner production at the same time.Cellulose nanofibrils (CNF) extracted from peanut shells were mixed with acrylic acid (AA) and dimethyl diallyl ammonium chloride (DMDAAC) to prepare a new type of capsule core(dust suppressant).Then,the self-adaptive AA-DM-CNF/CA microcapsules were prepared under the action of calcium alginate.The infrared spectroscopy and X-ray diffraction analysis results suggest that AA,DMDAAC and CNF have experienced graft copolymerization which leads to the formation of an amorphous structure.The scanning electron microscopy analysis results demonstrate that the internal dust suppressant can expand and break the wall after absorbing water,featuring a self-adaptive function.Meanwhile,the laser particle size analysis results show that the microcapsules,inside which the encapsulated dust suppressant can be observed clearly,maintain a good shape.The product performance experimental results reveal that the capsule core and the capsule wall achieve synergistic dust suppression,thus lengthening the dust suppression time.The product boasts good dust suppression,weather resistance,degradation and synergistic combustion performances.Moreover,this study,as the first report on the development and analysis of dust-suppressing microcapsules,fills in the research gap on the reaction mechanism between dust-suppressing microcapsules and coal by MS simulation.The proposed AA-DM-CNF/CA dust-suppressing microcapsules can effectively lower the dust concentration in the space and protect the physical and mental health of coal workers.In general,this research provides a new insight into the structure control and performance enhancement of dust suppressants.Expanding the application range of microcapsules is of crucial economic and social benefits.

Keywords:Waste peanut shell AA-DM-CNF/CA Self-adaptive Dust suppression microcapsule Molecular dynamics simulation

The quality of ecological environment plays a vital role in the survival and development of human beings and other organisms[1].A large amount of coal dust is generated during the production of open-pit mines.Coal dust,which is called an “invisible killer”,has become a major factor restricting the safe production of coal mines in recent years.It poses considerable hazards,including pneumoconiosis,environmental pollution,equipment wearing and coal dust explosions [2].According to incomplete statistics,in the past decade,272000 cases of occupational diseases have been reported in China,among which 212000 cases,i.e.78%,belong to occupational pneumoconiosis.Moreover,the number of pneumoconiosis patients in the coal industry accounts for 58% of the total number of occupational pneumoconiosis patients(Fig.1) [3].Relevant data show that the death rate of pneumoconiosis reaches 22.04% which is much higher than that of safety accidents [4].According to the data from the environmental protection department,dust pollution caused by open-pit coal production,storage and transportation has resulted in serious ecological damage.The above appalling statistics reveal that coal dust has seriously threatened the health of the surrounding population and the ecological environment [5].Hence,reducing the spread of coal dust is not only meaningful for the health and life safety of workers,but also crucial for the overall situation of national economic development and social stability [6,7].The control of coal dust in open-pit coal mines is urgent,and the research and development of new dust suppression materials is imperative.

Fig.1.Statistics of new cases of occupational diseases in the past decade.

The current methods for treating dust pollution mainly fall into two types,namely physical methods and chemical methods [8].Physical methods are relatively simple.They suppress flying dust by covering it with hood cloth,dust control net,watering,etc.[9].Chemical methods reduce coal dust by means of clean and environmentally friendly dust suppressants.Due to the suspension and hydrophobicity properties of coal dust,chemical methods are more efficient than physical ones in dealing with respirable dust[10].Chemical dust suppressants,which capture dust particles in air through the application of chemical substances,can eliminate or reduce dust pollution to environment.Considering their good dust suppression effect,they have been widely used and developed.Researchers all over the world have made fruitful achievements in the development of chemical suppressants.Representative ones include the Conherex bonding chemical dust suppressant and the DCL-1803 bonding chemical dust suppressant in the United States,the patented SS suppressant and the TH-C suppressant in Japan,the Mon-tan bonding chemical dust suppressant in Germany,and the ANT-1 wet dust suppressant in South Africa.The main components of these chemical dust suppressants are organic emulsions such as heavy oil residues,natural petroleum resins and highly aromatic compounds,as well as inorganic salt solutions such as calcium chloride,magnesium chloride,sodium silicate and lime [11].Despite the various types of chemical dust suppressants,they generally face some problems such as single function and high price,and even a series of disadvantages such as toxicity,corrosiveness,refractory degradation and secondary pollution.Moreover,their application in open air is rather limited [12].In practical application,conventional dust suppressants play the role of moisture absorption and water retention in the early stage,but the action time is short,which is not conducive to cost control [13].Thus,it is imperative to prepare an environmentally friendly dust suppressant boasting long action time and high dust suppression efficiency.

The slowed/controlled release technology enhances the utilization of active substances by controlling their release rate and prolonging their action time.The technology is widely adopted in the fields of daily chemicals,medicine,chemicals and agriculture[14].Microcapsules are tiny particles with a “core-shell” structure.It belongs to a micro-packaging technology in which natural or synthetic polymer materials are used to embed and seal solid particles,liquid droplets and even gases.Zhang et al.conducted experimental and numerical studies on the mechanical properties of single microcapsules with different shell types (PUF,silica and nickel)and corresponding polymers with modified microcapsules under quasi-static and dynamic compression,respectively [15].Based on the experimental results,they concluded that the strength of microcapsules with nickel shells was two orders of magnitude higher than those of the other two types.Chong et al.prepared a kind of multifunctional microcapsule by taking 8-HQ as the corrosion inhibitor and clove oil as the natural antibacterial compound[16].Such a microcapsule boasted good antibacterial and antiseptic properties.With the aid of a microfluidic platform,Md Danish Eqbal et al.prepared single-liquid core and double-liquid core alginate microcapsules,which marked some innovations in the liquid core microcapsule generation technology [17].Małgorzata Stanisz et al.reviewed the latest development in the preparation of biobased nanostructures,summarized their applications to the new drug delivery system and environmental protection,and comprehensively described the structure,raw materials,preparation methods,performances and application directions of microcapsules.Cao et al.designed a self-temperature-adjusting enzyme carrier system based on phase change microcapsules to fix laccase,which expanded the application of biocatalysis in a wider temperature range [18].Du et al.prepared a kind of concrete self-healing microcapsule with toluene diisocyanate (TDI) being the core and paraffin wax being the shell by adopting the melt polycondensation method [19].In addition,they investigated the influences of preparation temperature,mixing rate and paraffin/TDI mass ratio on the core ratio of the microcapsule.In summary,the microcapsule technology has been widely used in many fields such as medicine,pesticide,fertilizer,food,cosmetics,flame retardant and textile,but its application to coal dust suppression is rarely reported.Current dust suppressants on the market are of a single function and are not specially designed for coal mines.The preparation of efficient and environmentally friendly self-adaptive dustsuppressing microcapsules with good water absorption and retention performance can not only overcome these shortcomings,but also provide a research method and idea for the development of dust suppression materials in the future.

Peanut shell fiber is a kind of fruit fiber.China is the largest producer of peanuts in the world.Its annual output of peanuts reaches as high as 10 million tons,accounting for 42% of the world’s total output.About 5 million tons of peanut shells,the residue of peanut processing,are produced every year.A small number of them are processed into feed and adsorption materials or treated as the source of functional substance extraction,but the most of them are discarded or incinerated,which results in a great waste of resources [20].In order to realize the reintegration and utilization of crop waste resources,cellulose nanofibrils (CNF) are separated from fruit fibers to provide the matrix material for the preparation of dust-suppressing microcapsules [21].This not only realizes the secondary utilization of agricultural waste and lowers the product cost,but also fulfills the interdisciplinary concept and provides new ideas for the subsequent scientific research.

In this study,the microcapsule technology was introduced to coal dust prevention and control.First,the plant extraction technology was adopted to extract CNF from peanut shells through the chemical mechanical method.CNF,which is an emerging nanocellulose material,has become an important research direction in the field of biomass materials in recent years,because of its excellent properties including high aspect ratio,high strength and low thermal expansion,as well as its wide range of sources,low cost and friendliness to environment.With CNF as the matrix material and acrylic acid (AA) and dimethyl diallyl ammonium chloride (DMDAAC) as the monomer materials,a bio-based dust suppressant was prepared through the water bath-graft copolymerization method.On this basis,the microcapsule technology was used to encapsulate the bio-based dust suppressant for the first time.Promoted by the cross-linking reaction between sodium alginate and calcium chloride,the wall particles were selfassembled and deposited on the surface of the bio-based dust suppressant.In this way,a kind of self-adaptive dust-suppressing microcapsule was formed.The control of reaction speed and the improvement of suspension emulsion system stability were realized by the adhesiveness of sodium alginate and the hydrophilic and lipophilic properties of emulsifier.In the actual application process,the conventional dust suppressant is responsible for moisture absorption in the early stage,while the dust-suppressing microcapsule plays the role of water retention in the later stage.Each performs its own duties to maximize the water retention time of the dust suppressant and achieve synergistic dust suppression.Furthermore,the microstructure and reaction mechanism of the product were analyzed by Fourier transform infrared spectroscopy(FTIR),X-ray diffraction (XRD),thermogravimetry (TG) and scanning electron microscopy (SEM),and the performance parameters such as product particle size,dust suppression efficiency,weather resistance,and degradation properties were discussed by performing related experiments.The microscopic mechanism between dust suppressants and coal is rarely reported in previous researches,and there is a lack of macroscopic and microscopic analyses on the product.In this study,the migration and combination of functional groups in the action system were analyzed with the aid of molecular dynamics simulation software.By doing so,the microscopic action mechanism between dust-suppressing microcapsules and coal was revealed from a molecular perspective.The simulation results are consistent with the experimental phenomena,which verifies the feasibility and accuracy of molecular dynamics simulation.This study provides a new insight into the preparation and performance optimization of efficient waterretaining dust suppressants and expands the application range of the microcapsule technology.Moreover,this study is also an innovative simulation study,that is,it combines the utilization of agricultural waste resources with dust suppression,presenting one of the best solutions in the world.Besides,this study enables policy makers or industry operators to make more informed decisions.

2.1.Experimental reagents

Peanut shells were purchased from Mingcun town,Qingdao city,Shandong province,China.Reagents,including benzene,ethanol,glacial acetic acid,sodium chlorite,potassium hydroxide,DMDAAC,AA,ammonium persulfate,potassium thiosulfate,N,N’-methylenebisacrylamide (MBA),sodium alginate,Span-85,paraffin,Tween80 and CaCl2,were all analytical reagent (A.R.) purchased from Qingdao Jinke Chemical Reagent Co.,Ltd.

2.2.Experimental procedure

2.2.1.Extraction of nanocelluloses from peanut shells

First,10 g of dried peanut shell powder was accurately weighed and placed in the Soxhlet extractor to be treated with the benzene ethanol solution (volume ratio 2:1) at 90 °C for 6 h for removing the extract.Then,add 100 mL(volume ratio 1:1)of mixed solution of 31 mL/L acetic acid and 15 g/L sodium chlorite to the dried powder,and take a water bath at 75 ℃for 1 h.The treatment was repeated for 5 times to remove lignin.In this way,the holocellulose sampleS1was prepared.Next,300 mL of a potassium hydroxide mass fraction of 5% solution was prepared,after which the sample was immersed in the solution at room temperature for 24 h to remove hemicellulose.The treated sample was marked asS2.Finally,in order to further remove the residual lignin,the sample was treated with sodium chlorite repeatedly.In this way,the purified cellulose sampleS3was prepared [22].Furthermore,the peanut shell cellulose was defibrated by means of mechanical stepby-step refinement to realize nanocrystallization.To be specific,the sampleS3was configured as a suspension with the mass fraction of 0.8% and then poured into a grinder adjusted at zero(MKCA6-2,Masuko,Japan),with the grinding stone gap set as-9.It was ground at a speed of 1500 r/min for three times to prepare the ground sampleA1.After that,part ofA1was placed in the high-pressure homogenizer(Emulsi Flex-C3,Avestin,Canada)to be treated at an average pressure of 1×107Pa for 5 times.In this way,the ground-homogeneous sampleA2was obtained.At last,the peanut shell CNF was prepared after drying treatment [23].

2.2.2.Preparation of the dust suppressant

The preparation process of the product in this study is described as follows.First,a certain amount of DMDAAC and CNF was accurately weighed and placed into a 300 mL beaker,and a metered amount of deionized water was added into the beaker.The mixture was stirred until it was dissolved.Next,AA was neutralized with sodium hydroxide with the mass fraction of 20% under the condition of ice water bath,after which they were transferred into the flask.Next,an appropriate amount of initiator ammonium persulfate and potassium thiosulfate was added into the flask.After being stirred and mixed well,they were allowed to react for 1 h.Finally,the MBA solution was slowly poured into the beaker.After the reaction proceeded for 2 h,the white liquidC1,i.e.,the core of the dust-suppressing microcapsule,was obtained.

2.2.3.Preparation of self-adaptive dust-suppressing microcapsule

First,10 mL of sodium alginate solution was fully mixed with nano calcium chloride.Afterwards,the solution comprising 5 g of the abovementioned dust suppressantC1and water was evenly mixed with an ultrasonic cell grinder as the water phase,while 3% Span-85 (3% refers to the volume ratio of Span-85 to paraffin,the same below) was mixed with 50 mL of liquid paraffin as the oil phase.After the water phase and the oil phase was mixed at the ratio of 1:5 (volume),they were stirred at the rate of 400 r/min to form a stable emulsion.Subsequently,glacial acetic acid was dripped into it to dissociate Ca2+ions,thus inducing the gelation reaction.Finally,it was cleaned with the 1% Tween80 aqueous solution,filtered,washed and freeze-dried to obtain the dust-suppressing microcapsule sample (AA-DM-CNF/CA).The preparation procedure is illustrated in Fig.2.

2.3.Research on the optimal ratio of capsule core

The response surface-central combination design can predict the optimal test conditions by means of nonlinear model fitting.Specifically,the amount of CNF in the matrix was fixed;the AA neutralization degree,the amount of crosslinking agent and the reaction temperature were set to appropriate values;the acrylic acid concentration,the DMDAAC concentration and the initiator concentration in the reaction system were set as independent variables;the water absorption rate of the product was set as the response amount.Under the above setting,the factors and level values were encoded by using the response surface-central combination design module in EXPERT DESIGN 8.0.6 (Table 1) [24].

Table 1 Parameters and their levels to be examined.

The number of test points can be determined in accordance with the following formula.

wherenis the total number of test points;kthe number of independent variables;mcthe number of test points arranged on the spherical surface with the radius of;andm0the number of test points arranged in the center of the factor area.In this study,k=4,so there are 16 factor points,8 axial points,and 6 central points.

2.4.Measurement and characterization

With the aid of SEM,AA-DM-CNF/CA was fixed on the conductive adhesive.The morphological changes and microstructure of the product before and after preparation were observed after vacuum metal spraying.The particle size of the microcapsules was measured through a laser particle size analyzer.In addition,FTIR and XRD tests were performed on the microcapsules.According to the infrared spectra obtained,the changes in functional groups could be investigated.On this basis,the chemical reaction that the product experienced and the spatial structure of the product molecule were deduced.Besides,AA-DM-CNF/CA was placed in a DSC analyzer (Mettler Toledo,Switzerland) to probe into their thermal stability.During the TG analysis,the temperature of the product was gradually raised to 600 °C at the heating rate of 10 °C/min,so as to analyze its thermal stability.

2.5.Product performance tests

2.5.1.Adaptability test

After absorbing water to some extent,the AA-DM-CNF/CA microcapsules will expand and undergo wall breakage,thereby exerting an effect of secondary dust reduction.To accelerate the experimental process,a certain number of dried AA-DM-CNF/CA microcapsules were placed in a paper bag and then soaked in a large amount of distilled water to test the mass of the product at different moments.Meanwhile,the water absorption state of the dust suppressantC1was also tested.The water absorption and wall breakage law of the microcapsules was concluded through comparative analysis.

Fig.2.Schematic diagram of product preparation procedure.

2.5.2.Dust suppression rate test

The test was conducted on the roadway simulation device of the State Key Laboratory of Shandong University of Science and Technology(Fig.3).The size(length×width×height)of the wind tunnel test bench was 1.5 m×0.8 m×0.8 m,and the device was equipped with a TDI8000-0750G-4T stepless frequency converter and a SZ-11.2 axial flow fan.The prepared dust-suppressing microcapsules were sprayed on both the coal dust in the petri dish and the surface of the soil around the open-pit coal mine.Meanwhile,the same mass of water was sprayed as a control.With the wind speed set to 10 m/s,the variation of dust suppression rate with the passage of coal sample storage time was tested through a wind resistance experiment.The dust suppression rate is expressed byw:

whereMbis the mass of coal dust before wind below;Mathe mass of coal dust after wind below;andMtthe total mass of flying dust.

2.5.3.Weather resistance test

2.5.4.Degradability test

An appropriate amount of high-quality fertile soil 30 cm below the turf was taken and screened by a 20-mesh grading sieve to remove impurities for later use.The AA-DM-CNF/CA microcapsules dried at 50°C were mixed with soil and cinder at the ratio of 1:1:1 and then spread in a petri dish.The petri dish was stored under natural conditions,during which the mass change was recorded every day for calculating and analyzing the degradation situation.

2.5.5.Synergistic combustion test

Synergistic combustion avoids the post-treatment of products and enhances the utilization of resources.The specific test process is elaborated as follows.The dried product was broken into pieces(i.e.,particle diameter approximately 1.5 cm),and the products with different masses were placed in a sample box(100 mm×100 mm×3 mm) wrapped in aluminum foil paper for testing.The product should be evenly spread over the sample box.Given the increase in heating time,extensive data were recorded.The thermal radiation intensity of the cone calorimeter(R-S/FTT0007) was set to 35 kW/m2.

2.6.Microscopic action mechanism of AA-DM-CNF/CA dustsuppressing microcapsules

Due to the existence of associated minerals and the changes in the organic structure,coal is a mixture with complex composition.To promote reliability of the simulation results,the N element was added on the basis of the Wender model to construct the coal molecular model(Fig.4a).According to the analysis on experimental results in Section 3.2,the calcium alginate molecular model(Fig.4b) and the AA-DM-CNF/CA molecular model (Fig.4c) were constructed.The Forcite module was adopted for molecular dynamics simulation,and all calculations were performed in the Compass force field.First,the Amorphous Cell module was used to construct a coal cell model containing 4 optimized coal molecules (Fig.4d),after which the model was annealed.Next,to simplify the model,the same method was used to construct the calcium alginate unit model (Fig.4e) and the AA-DM-CNF/CAwater unit model (Fig.4f) whose bottom areas were the same as that of the coal unit model.Finally,the Nose thermostat was used for simulation in the 298 K constant volume and temperature(NVT) ensemble.With the time step set to 1.0 fs,the equation of motion was integrated.The simulation data in the final 300 ps were employed for dynamic analysis [25].

Fig.3.Roadway simulation device.

Fig.4.Model building.

3.1.Analysis of chemical composition of peanut shell

In order to extract highly purified cellulose from peanut shell,lignin and hemicellulose in peanut shell were removed by acidbase treatment with sodium chlorite and potassium hydroxide respectively.The chemical components of peanut shell raw powder and products in each stage of chemical treatment are shown in Table 2.It can be seen from Table 2 that after treatment,the cellulose content of peanut shell is significantly increased and the hemicellulose and lignin are significantly reduced.This is because the alkali solution can weaken the hydrogen bond between cellulose and hemicellulose and the ester bond between hemicellulose and lignin,and the benzene ring and double bond in the lignin structure are oxidized by sodium chlorite,which leads to the reduction of hemicellulose and lignin content,improves the relative content of cellulose,and provides a high-purity cellulose raw material for the preparation of peanut shell nano cellulose.

Table 2 Mass fraction of chemical components in peanut shells at each stage of chemical treatment,%.

3.2.Analysis on the optimal ratio of capsule core

The core materialC1can absorb a large number of water molecules.Theoretically,it can hydrate with hydrophilic groups andadsorb the water molecules around the polymer.Its thickness is no more than 2-3 layers.The first layer of water molecules is hydrated water with coordination bond or hydrogen bond formed by hydrophilic groups and water molecules;the second or third layer is the hydrogen bond and van der Waals force binding layer formed by water molecules and hydrated water,and the force decreases with the increase of the number of layers.The reason whyC1can absorb more water also depends on its cross-linked grid structure.This structure is wrapped,and the plane type is replaced by the three-dimensional type.Moreover,the complex interweaving of hydrophilic groups on the chain provides a good environment for containing water.

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Considering that the amounts of AA and DMDAAC have a great influence on the water absorption rate of the dust suppressantC1,the water absorption rates ofC1at different concentrations of AA,DMDAAC and initiator were analyzed under the AA neutralization degree of 60%,the crosslinking agent amount of 1.8% and the reaction system temperature of 60 °C.

Fig.5a shows the influences of AA and DMDAAC concentrations on the water absorption rate.Through analysis,it is found that the AA concentration has a greater influence on the water absorption rate than the DMDAAC concentration,and the water absorption rate rises with the increases in AA and DMDAAC amounts.The reason is that as the amount of monomer increases,the grafting rate goes up and the graft chain lengthens.As a result,the product boasts a more stable structure and exhibits better water absorption performance.The maximum water absorption rate is up to 325 times when the AA and DMDAAC amounts are 16 mL and 3.0 g,respectively.However,when their amounts increase further,the water absorption rate gradually declines.This is because excessive amounts of AA and DMDAAC will lead to the occurrence of monomer self-polymerization and destroy the stability of its three-dimensional structure,which is reflected by the reduction of water absorption performance.Fig.5b shows the influences of AA and initiator concentrations on the water absorption rate.It can be seen that the initiator and AA amounts significantly influence the water absorption rate.The best water absorption performance is exhibited when the initiator/AA percentage is 2.5% and the AA amount is 16 mL.When the initiator and AA amounts further grow,the water absorption rate gradually decreases.The decrease can be explained by two factors.First,the number of matrix groups is limited.Second,the initiator causes excess acrylic acid to self-polymerize,which destroys the spatial structure of the product.Fig.5c presents the influences of DMDAAC and initiator concentrations on water absorption rate.The experimental results reveal that the initiator and DMDAAC amounts have a slight influence on the water absorption rate.The best water absorption performance is reached when the initiator/AA percentage is 2.5% and the DMDAAC amount is 3.0 g.When the initiator and DMDAAC amounts further increase,the water absorption rate goes downward.This is due to the fact that an excessive amount of initiator will cause DMDAAC to generate many free radicals.Consequently,the grafting rate falls and the grafting chain shortens.The product can hardly form a spatial network structure,so the water absorption rate declines.

Fig.5.Influences of AA,DMDAAC and initiator concentrations on water absorption rate.

3.3.Experimental process component analysis

FTIR analysis is an important means to investigate the chemical components and functional groups of samples in the preparation process.It is also a necessary tool for determining the composition and molecular structure of materials.The FTIR spectra of peanut shell samples during acid-base treatment are given in Fig.6.Fig.6a is the spectrum of untreated original peanut shell powder.The characteristic peak of lignin can be clearly observed from the spectrum.The peak at 1422 cm-1corresponds to the C-H bending vibration of -CH2in lignin,and the one at 1601 cm-1represents the carbon skeleton vibration of benzene ring in lignin.Fig.6b shows the spectrum of the sample treated with sodium hypochlorite.As can be observed from the spectrum,the peak at 1601 cm-1is attributed to the skeleton vibration of benzene ring,and the one in the vicinity of 1630 cm-1corresponds to the stretching vibration of side chain carbonyl C=O in lignin.The two absorption peaks both weaken notably,suggesting that most of lignin in peanut shells has been removed [26].Fig.6c exhibits the absorption peak of purified cellulose after the removal of the entire matrix.In the spectrum,the absorption peak at 1601 cm-1,which represents the skeleton vibration of benzene ring,has disappeared completely.This indicates that after being immersed in potassium hydroxide,part of alkali soluble lignin in peanut shell powder has been removed.The C=O stretching vibration peak corresponding to the xylan acetyl group CH3C=O in the hemicellulose basically disappears at 1598 cm-1,but a C=O absorption peak of medium width and moderate intensity still appears at 1630 cm-1,which demonstrates that most hemicellulose has been removed during potassium hydroxide treatment[27].Fig.6d is the spectrum of the final product AA-DM-CNF/CA.It can be seen that a C=O stretching vibration peaks appear at 1700 cm-1;a RCOOcharacteristic peak appears at 1550 and 1401 cm-1,respectively;and a N+bonded methyl bending vibration absorption peak appears at 1452 cm-1.These peaks suggest that AA,DMDAAC and CNF have experienced graft copolymerization.

Fig.6.Infrared spectra.

To further analyze the chemical changes and crystallinity in different stages of the experimental process,the crystal forms and crystallinities of peanut shells,peanut shells treated with sodium chlorite,purified nanocellulose and final product were characterized by means of XRD (Fig.7).As can be observed from Fig.6a-c,the diffraction peaks of peanut shell nanocellulose and lignocellulosic material lie at basically the same position.Specifically,the two strong diffraction peaks are located at 2θ=16.5° and 2θ=22.7°,corresponding to the 110 and 200 crystal planes.They belong to a typical Type I cellulose structure.The internal crystalline structure of cellulose is not destroyed during removal of the matrix with sodium chlorite and potassium hydroxide.According to the intensities of XRD diffraction peaks,the original peanut shell powder has the lowest crystallinity,and the crystallinity is improved after removal of the matrix.This is because the removal of lignin and hemicellulose leads to the complete exposure of cellulose,and meanwhile part of the amorphous area is removed during removal of the matrix,thus increasing the proportion of the crystalline area.This indicates the occurrence of graft copolymerization reaction between the reagents.Resultantly,the structural strength of the final product is enhanced,and an amorphous structure is formed [28].

Fig.7.XRD spectra.

The above experimental results disclose that the capsule core undergoes a graft copolymerization reaction under the action of the initiator and the crosslinking agent.The specific reaction process is presented in Fig.8a.In the emulsification/inner gelation technology,calcium carbonate particles are dispersed evenly in sodium alginate droplets in advance.Then,when the oil soluble acetic acid diffuses through the oil-water interface,sodium alginate is gelled in situ by inducing the release of Ca2+(Fig.8b).For all the contact points on the surface of sodium alginate droplets,the acid will disperse rapidly in the water phase.Ca2+gets released in situ simultaneously and induces gelation reaction at a low pH.Thus,the structure of calcium alginate beads is uniform and symmetrical in all directions.

3.4.Analysis on the morphological characteristics of microcapsules

Peanut shells have an obvious porous network structure.As illustrated in Fig.9a,the natural fibers of peanut shells,which are in the shape of a round rod,are wrapped by lignin and hemicellulose.Because peanut shells contain much (up to 1/3) lignin and little hemicellulose,the fibers and matrix of peanut shells aggregate into a discontinuous tile-like structure with a size of nearly 50 μm.After the removal of lignin by sodium chlorite,the round rod-shaped structure of the peanut shell fibers changes into a slender hollow rod structure(Fig.9b).The inner and outer sides of the cell wall are both exposed,and the fiber diameter increases remarkably.The reason is that after the removal of a large amount of lignin,part of the fibrils become connected by hydrogen bonds in the drying process,resulting in an increase in their width.The microstructure of the sample treated with potassium hydroxide and sodium chlorite is given in Fig.9c.As shown in Fig.9c,attachments on the surface have been completely removed,and the cellulose is completely exposed.A large number of hydroxyl groups exposed on the surface of the molecular chain gather fibers into bundles as a result of hydrogen bonding,so that the fiber diameter grows further.The morphological structure of the dust suppressantC1is displayed in Fig.9d-f.On the whole,the product has an orderly-three-dimensional structure containing voids,a large number of folds and spatial pores.Such a structure greatly promotes its water absorption performance.After absorbing water,the product experiences volume expansion until the capsule wall breaks,thereby exerting the effect of secondary dust suppression.The structure of AA-DM-CNF/CA microcapsules is shown in Fig.9g and h.After the reaction,the microcapsules with a rough surface and a diameter of about 40 μm are formed.By observing the broken microcapsules,it can be found that the capsule wall is about 510 nm thick.This thickness not only ensures good water permeability,but also conduces to the formation of good water retention performance.With this thickness,the internal dust suppressant can expand and break the wall after absorbing water,featuring a self-adaptive function.In Fig.9i,the dust suppressant has been successfully wrapped.The results of EDS spectral characterization of the broken microcapsules are shown in Fig.9g.In the spectrum,a series of signals from elements such as N,O,Si and Na can be observed clearly,and the atomic fractions of C,O and N elements are relatively high.Besides,the element mapping maps of C,O and N elements disclose that the elements in the product are distributed evenly and densely,which proves the stability of the product structure [29].

Fig.8.Mechanism of microcapsule gel reaction.

To further explore changes in the particle size of microcapsules in the preparation process,the particle size was tested through a laser particle size analyzer.Based on the test results,the effect of calcium alginate concentration on the particle size was analyzed.The specific test results are displayed in Fig.10.With the rise of calcium alginate concentration,the particle size increases and the particle size distribution range expands gradually.The reason for this phenomenon is as follows.The rise of calcium alginate concentration promotes the viscosity of the water phase.As a result,the dust suppressant fails to be distributed evenly in the solution,which enables the released Ca2+to cross-link it sharply,so that large calcium alginate hydrogels will be formed locally [30].It can be seen through an optical microscope that the microcapsules,inside which the encapsulated dust suppressant can be observed clearly,maintain a good shape,which further verifies the success of the test.

3.5.Analysis on microcapsule stability

It can be found from the TG-DTG curve that the heating process of the product mainly falls into three stages,as shown in Fig.11.In Stage 1(＜220°C),the mass loss(about 7%)is mainly caused by the decomposition of calcium alginate on the capsule wall and the volatilization of free water and crystal water in the molecular structure.In Stage 2 (221-474 °C),the mass loss (about 22.6%) is induced by the dehydration and breakage of glycoside chains and cellulose chains.In Stage 3 (>475 °C),the mass loss (about 16.9%)is caused by the continuous carbonization of components in the product as a result of high temperature.The above curves and data disclose the high thermal stability of the microcapsules,because it is not until about 220 °C that the structure of the product undergoes light damages.The obvious improvement of thermal stability of the product is attributed to the highly stable cellulose chain and the stable NH structure.It can be observed from the DSC curve that the product always stays in an endothermic state throughout the heating process,which proves the good heat absorption capacity of AA-DM-CNF/CA.Its internal structure will not be damaged in an extreme environment.

3.6.Analysis on product performances

3.6.1.Analysis on self-adsorption performance

Fig.12 shows results of the swelling experiment on the dust suppressantC1and the AA-DM-CNF/CA microcapsules.According to the histogram in Fig.12,the water absorption rate of the dust suppressantC1increases with the rise of temperature within the same period of time,and the time for reaching the swelling equilibrium is shorter at a higher temperature.As can be known from the curves in Fig.12,at different temperatures,the water absorption rate of the dust-suppressing microcapsules surges suddenly in two stages.In Stage 1 (0-0.2 h),the sudden surge is caused by the joint absorption of water by calcium alginate on the capsule wall and the dust suppression agentC1in the capsule core.As time goes by,the microcapsules reach swelling equilibrium on the whole in the period of 2.0-3.2 h.In Stage 2 (4.4-4.6,3.8-4.0 and 3.2-3.4 h at 15,25 and 35°C respectively),the sudden surge occurs because the water-absorption-induced expansion of the capsule core is constrained by the capsule wall.When the capsule wall reaches the expansion limit and breaks,the capsule core is exposed and continues to absorb water.After conventional dust suppressants come into play,they gradually lose water as well as their ability of dust suppression,adhesion and capturing.In contrast,the AA-DM-CNF/CA dust-suppressing microcapsules boast selfadsorption performance.When they act on the surface of coal dust,the capsule wall and the capsule core absorb water slowly.In the early stage,the microcapsule particles mainly play the role of sticking and fixing dust.In the later stage,the capsule wall breaks,the dust suppressant in the capsule core plays the role of dust suppression again.In this way,synergistic dust suppression is achieved and the dust suppression time is prolonged [31].The specific mechanism is shown in Fig.13.

3.6.2.Analysis on dust suppression performance

Fig.14a shows the spraying situation of water and AA-DM-CNF/CA dust-suppressing microcapsules on the surface of coal powder and the surrounding soil of the coal mine.When water and dustsuppressing microcapsules of the same volume are sprayed,within the same period of time,water permeates and dries fast on the surface of the sample,while the dust-suppressing microcapsules stay on the surface for a long time.It can be inferred that the product has a certain moisturizing and dust suppression performance.Moreover,the dust suppression rates of the samples are tested under the wind speed of 10 m/s.The test results in Fig.14b demonstrate that although water has a certain dust suppression effect within 20 h,its dust suppression rate falls notably in a short time.Gradually,it loses its dust suppression effect.In contrast,the spraying of dust-suppressing microcapsules achieves a much higher dust suppression rate (over 95% within 16 h).The test is conducted under a wind scale of 5,i.e.,the wind speed of 8.0-10.7 m/s.In fact,there are not many strong winds that can reach such a wind scale under actual climate conditions.This indicates that the dust-suppressing microcapsules can be applied to longterm stacked coal yards.

Fig.9.SEM images.

To further explore the effect of dust-suppressing microcapsules,AA-DM-CNF/CA is sprayed on the surface of coal dust and allowed to stand for a period of time until the surface dries.Then,the surface is observed with the aid of SEM(Fig.15).As can be seen from the image,when the dust-suppressing microcapsules act on the surface of coal dust,the capsule wall breaks after water absorption and swelling.Meanwhile,the dust suppressantC1inside flows out and continues to stick,capture and control coal dust.Ultimately,the capsule wall and the capsule core become intertwined and solidified into a film,thus prolonging the water retention time ofC1to the greatest extent and achieving synergistic dust suppression[32].Besides,the research findings provide new ideas for dust suppression technology.For example,conventional dust suppressants can be combined with dust-suppressing microcapsules to suppress coal dust in the whole cycle.The combination is advantageous,because conventional dust suppressants serve to absorb and retain water in the early stage while the dust suppressant that is released slowly from the microcapsules is responsible for retaining water in the later stage.

Fig.10.Particle size distribution of microcapsules.

Fig.11.TG-DTG-DSC curves of AA-DM-CNF/CA.

3.6.3.Analysis on weather resistance performance

As can be seen from the weather resistance curve of AA-DMCNF/CA in Fig.16,the compressive strength of coal cake does not change much after first two high-temperature dryings,but it drops significantly after the third one.The reason is that after two cyclic dryings,much free water and binding water still exists in the product,and the spatial structure of microcapsules is not damaged greatly.After the third drying,water has evaporated completely,and the structure of microcapsules is damaged to some extent.After three freeze-thaw cycles,the compressive strength of coal cake goes up,because the freeze-thaw cycles enable the microcapsules to infiltrate into the cavities and grooves inside the coal cake.Resultantly,the combination between the product and coal powder are combined more densely.The compressive strength falls obviously after the fourth freeze-thaw cycle,because the multiple limit cycles damage the structure of microcapsules.Compared with the compressive strength at room temperature,the compressive strength under multiple limit cycles changes in the range of 46%-63%,which demonstrates that the dust-suppressing microcapsules boast good freeze-thaw resistance and hightemperature resistance and can be applied to many complex environments [33].

Fig.12.Water absorption and swelling performance of the product.

Fig.13.Mechanism of dust-suppressing microcapsules.

3.6.4.Analysis on degradation performance

Considering that people’s requirements for materials are becoming increasingly stringent,only safe,environmentally friendly and degradable materials have the value of promotion and use.AA-DM-CNF/CA is actually a high-molecular polymer whose good degradability is the premise of avoiding secondary pollution.Since this degradation experiment is performed in an open and natural state,the dust-suppressing microcapsules are gradually decomposed under the joint action of moisture,air,microorganisms,etc.As shown by the mass loss curve in Fig.17,the mass increases slightly in the first 6 d,because the product will absorb a small amount of water when initially exposed to the air.The mass loss is limited (about 5.67%) before 18 d.The mass loss becomes notable (about 32.97%) after 18 d,and it gradually levels off(2.98%)after 39 d.The reason is as follows.In the period 6-18 d,a colony begins to grow on the surface and inside of the sample.In the period 19-38 d,it grows in large numbers and gradually decomposes the dust-suppressing microcapsules.The colony formed is shown in Fig.17a.The colony surface is in a filamentous shape,according to its SEM image in Fig.17b.Judging by a comparison between its SEM image in Fig.17b,and the SEM image of actinomycetes in Fig.17c,the colony is mainly composed of actinomycetes [34].In summary,the dust-suppressing microcapsules are degradable and friendly to the surrounding environment.

Fig.14.Dust suppression performance of dust-suppressing microcapsules.

3.6.5.Analysis on synergistic combustion

In this study,a synergistic combustion method that regards coal as fossil energy is proposed.The combustion performances of the raw coal sample,the product and the coal sample-AA-DM-CNF/CA mixture are investigated,respectively.The investigation conduces to improving the strategies for environmental protection and resource utilization.The HRR curve during combustion test measurement is exhibited in Fig.18.A short time to ignition(TTI) of about 40 s exists for the product,and TTI and total heat release (THR) are proportional to the mass of the product.The HRR of AA-DM-CNF/CA reaches 35 kW/m2at about 178 s,which is probably attributed to the dehydration and rupture of glycoside chain and cellulose chain.In addition,compared with that of the raw coal sample,the combustion of the mixture lasts for about 70 s longer and the peak HRR is about 15 kw/m2.The peak HRR of AA-DM-CNF/CA will last for a long time,which is also a proof of its combustion inhibition effect.Hence,the results indicate that the prepared product can combust together with coal powder without requiring complicated post-treatment steps.Such a method is beneficial for the protection of the environment and the conservation of resources.Besides,this type of dustsuppressing microcapsules will not reduce the calorific value of coal.Instead,it will shorten the ignition time and raise the HRR.In short,the mixture of the product and coal can promote the combustion efficiency of coal.

Fig.15.Effect of AA-DM-CNF/CA.

Fig.16.Test curves of weather resistance performance.

Fig.17.Degradation performance of AA-DM-CNF/CA.

3.6.6.Current research status of dust suppressants

The current research status of dust suppressants is shown in Table 3.They are mainly classified into three categories,i.e.,inorganic dust suppressants,organic dust suppressants and microbial dust suppressants.Inorganic dust suppressants have good wettability,but they are cohesionless and corrosive and require a long curing time.Meanwhile,the combustion value of coal will change under the influence of halide and other additives.Organic dust suppressants use crude oil,asphalt,halide and sulfonate as raw materials.They have good hygroscopicity,evaporation resistance and bonding performance,but they are difficult to degrade,easy to cause secondary pollution to the environment,corrosive and toxic.Microbial dust suppressants boast remarkable water retention,film-forming performance and biocompatibility.However,they take effect slowly,and their dust suppression effect depends on the growth process of microorganisms,which has high requirements for the surrounding environment.In contrast,the product proposed in this study has the functions of wetting,water retention,bonding and curing,and it can be released gradually.Through the synergistic effect of capsule core and capsule wall,it can achieve the purpose of efficient dust suppression.In addition,it is worth noting that the central idea of this research is“controlling dust with waste”,which can solve the problem of coal mine dust while solving the waste and meaningless use of peanut shells.

Fig.18.Heat release rate (HRR) curve.

Table 3 Types of common dust suppressants.

The process in which water molecules wet the coal surface under the addition of dust-suppressing microcapsules is analyzed through molecular dynamics simulation software.The analysis results are given in Fig.19.The following phenomenon can be found from the front view of the entire reaction process and the coal surface wetting process.In the initial stage of the reaction,the AA-DM-CNF/CA is integrated with water molecules and isolated from calcium alginate.As the reaction proceeds,it gradually extends to the inside of the coal seam[35].At the end of the reaction,the end extending to the inside of the coal seam is in a hemispherical shape,and the other end remaining outside the coal seam is more dispersed,because abundant hydrophilic groups such as-OH,-COOH and C=C exist in the product after the occurrence of graft copolymerization.The dispersion at the end toward the water phase can increase the contact probability and area between hydrophilic groups and water molecules and attract water molecules to migrate and diffuse in the direction of the coal seam.The aggregation at the end toward the coal seam can effectively cover the hydrophobic sites on the coal surface and reduce the hydrophobicity of the coal surface.Such a phenomenon agrees with the analysis results in Section 3.5 [36].

Fig.19.Snapshot of molecular dynamics simulation of low-rank coal/(AA-DM-CNF/CA)/water system.

To further explore the effect of the product,the relative concentration distributions of components along thez-axis under the equilibrium state of the above system are analyzed (Fig.20).In the absence of AA-DM-CNF/CA,water,calcium alginate and dust suppressantC1on the coal surface are of a limited wetting degree,and the adsorption layer thickness at the coal/water interface is 0,indicating that water molecules can hardly enter the inside of the coal seam.After AA-DM-CNF/CA is added,the product is detected at the coal/water interface,suggesting that it mainly acts at the coal/water interface.Moreover,the adsorption layer thickness at the coal/water interface grows to 26 Å (26-52 Å),and the adsorption layer concentration is relatively high,demonstrating that AADM-CNF/CA promotes the adsorption of water molecules on the coal surface and improves the wettability of the coal surface.On the one hand,the improvement of coal surface wettability can increase the mass of coal dust so that it tends to fall rather than float.On the other hand,it increases the probability of collision and bonding between coal dust particles.The above is an explanation of the dust suppression mechanism of the product from a physical point of view [37].

Fig.20.Relative concentration profiles of the low-rank coal/(AA-DM-CNF/CA)/water system along the z-axis.

In this study,the raw materials,including peanut shells,DMDAAC and AA,were combined with sodium alginate and calcium chloride to prepare a kind of self-adaptive dust-suppressing microcapsule through the emulsification/inner gelation technology.Furthermore,the swelling,dust suppression,weather resistance,degradation and synergistic combustion performances of AA-DM-CNF/CA were comparatively analyzed.The following conclusions were drawn.

(1) When the ratio CNF:DMDAAC:AA equals 1:5:1,and the water phase to the oil phase is mixed at the ratio of 1:5 and stirred at the rate of 400 r/min,the product obtained boasts satisfactory swelling performance and selfadaptability.The maximum water absorption rate of the capsule core is up to 325 times,and the capsule wall reaches the expansion limit and breaks when exposed to distilled water for 2-4 h.Together,they achieve a synergistic dust suppression effect.

(2) The FTIR and XRD test results confirm that the product has experienced graft copolymerization,and the reaction process of the product is deduced based on the changes in functional groups.According to the SEM test,the product is about 40 μm in diameter and has a rough surface.Besides,the elements in the product are distributed evenly and densely,which proves the stability of the product structure.The TG test results disclose that AA-DM-CNF/CA has a good heat absorption capacity,and its internal structure will not be damaged in an extreme environment.

(3) The product performance test results indicate that AA-DMCNF/CA can achieve a dust suppression rate of above 95% within 160 h at a wind speed of 10 m/s.Under multiple limit cycles,the compressive strength of AA-DM-CNF/CA changes in the range of 46%-63%,which proves its good freeze-thaw resistance and high-temperature resistance.The degradation test results disclose that in the period 19-38 d,the colony grows in large numbers and gradually decomposes the dust-suppressing microcapsules.In addition,compared with that of the raw coal sample,the combustion of the mixture lasts about 70 s longer.Such changes are in line with the strategic significance of coal revolution.

(4) Through molecular dynamics simulation,the dust suppression mechanism and wetting effect of AA-DM-CNF/CA are further explained from the physical point of view.After its addition,the adsorption layer thickness at the coal/water interface grows to 26 Å.

(5) The AA-DM-CNF/CA dust-suppressing microcapsules prepared from peanut shells are advantageous for their low cost,good performance,and environmental friendliness.They can not only realize the recycling of waste,but also effectively control coal mine dust.This process ensures the clean production of coal mining enterprises,which is consistent with the requirements of coal revolution in the new era.

This study enables policy makers or industry operators to make more informed decisions,so that waste emissions and dust pollution can be minimized and the sustainable development of coal mines and local environment can be improved.In addition,AADM-CNF/CA dust-suppressing microcapsules also provide new insights into chemical dust suppression.

Acknowledgements

This work was financially supported by the National Key R&D Program of China (No.2022YFC2503201),the National Natural Science Foundation of China (Nos.52274215,52004150 and 52074012),the Qingchuang Science and Technology Project of Universities in Shandong Province,China (No.2019KJH005),the Outstanding Young Talents Project of Shandong University of Science and Technology (No.SKR22-5-01) and the China Scholarship Council (No.202108370223).

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