Stacking polymer microspheres matrix: a facile, practical, and energy-saving strategy for suppression of acid mist

ABSTRACT The electroplating, electrolysis, and pickling industrial processes would generate numerous gas pollutes, acid mist, which could not be essentially diminished due to its synthesis mechanism and cause gaseous environmental pollution, equipment corrosion, and endanger workers’ health. In this study, a facile, practical, and energy-saving acid mist suppression system was constructed by introducing a stacking microsphere matrix as a floating porous phase on the acid solution and not causing secondary pollution. The mechanism of this green acid mist suppression strategy mainly focused on size-selective blocking of acid mist droplets by dense stacking microsphere layer and dissipation of floating kinetic energy of bubbles in the acid mist. The factors relating to the matrix's microstructure, the particle size of microspheres, the combination of the complex particles with a wide range of particle sizes, and the thickness of the matrix on the acid mist suppression were explored. It found that the matrix constituted of a medium-sized polymer sphere (1.075 ± 0.175 mm) presents a better appearance in the acid mist suppression. When the thickness of this matrix reached 15 mm, its acid mist efficiency also came up to 100%, totally blocking the acid mist. Meanwhile, complex particles with different particle sizes and PMMA porous blocks are beneficial for suppressing acid mist. Herein, this research opened up a green and effective strategy for regulating this hazardous gas pollute, acid mist. GRAPHICAL ABSTRACT


Introduction
As one gaseous pollutant, the acid mist [1] can cause gaseous environmental pollution, equipment corrosion and endanger workers' health [2,3]. Meanwhile, this air pollute could not be essentially diminished in the electroplating, electrolysis, and pickling process due to its synthesis mechanism [4,5]. Since the acid mist originated from the oxidation-reduction reaction on the surface of the metal parts in the above-mentioned process, which could generate H 2 or O 2 gas in the acidic solvent, and then the bubble filled with H 2 or O 2 gas could float on the surface of the solvent and disrupt into micro-size acidic droplets, formation of acid mist [6,7]. Therefore, the acid mist is an inevitable environmental issue attracting much attention from ecological and material scientists worldwide. The versatile methods for suppressing acid mist are introduced in recent years, mainly divided into two kinds of strategy: physical and chemical processes. The physical treatment for the acid mist was primarily focusing on collecting the acid mist by the mechanical facility, such as ventilation and electrostatic demisting, which need huge upfront investment and consume much energy in operation [8]. The core mechanism of chemical treatment for acid mist is to employ small molecules, such as perfluoro surfactant, to change the interfacial surface tension of the bubble [9], decrease the size of the floating bubbles, and then minish the volume of acid mist [10,11]. However, this small molecule appearance high efficiency in the suppression of acid mist, it also introduces secondary pollution to the environment. Another chemical method is to build a dense foam above the solvent, acting as a barrier to prevent the diffuse of the acid mist. Still, this foam covering method could result in the accumulation of flammable gas. Nowadays, concerning the term of carbon neutrality globally, it is necessary to suppress acid mist in a green and energy-saving strategy.
As we know, the acid mist is micro-size droplets, which come from the collapse of H 2 or O 2 bubbles, and their initial diffusion velocity was primarily dependent on two factors, the bubble size and floating kinetic energy [12]. Based on this mechanism of acid mist generation, suppression or minimisation of acid mist by the no-chemical reagent strategy should be focused on the dissipation of the floating kinetic energy and buliding a size-controlled porous structure to present the diffusion of the micro-size droplets and drain out of the flammable gas.
According to this hypothesis, the porous floating barriers, such as balls and beads, could be a great potential 'green' strategy for suppressing acid mist. This study attempts to employ the polystyrene (PS) beads stacking matrix as a floating barrier to suppress acid mist. The schematic mechanism for minimisation of the acid mist of floating beads could be described as follows and described in Figure 1. Firstly, the designed size gap between the spherical beads could be tuned by the size of microspheres and prevent the transition of acid mist droplets with a diameter bigger than the gap. On the other hand, the floating kinetic energy of the bubble raised from the bottom of the solvent could be dissipated by the friction between the adjacent microspheres in the multilayer stacking matrix. Furthermore, the physical properties of the microsphere could be crucial factors, affecting the stacking polymer beads matrix structure and its appearance in acid mist suppression. In this research, we introduced a novel, practical, and energy-saving strategy for suppression of acid mist, which could open up a new avenue for the industry waste gas government.

Materials
The zinc sulfate electrolyte solution (The ingredients: 17 g/L MgSO 4 , 39 g/L ZnSO 4 , 160 g/L H 2 SO 4 , 4 g/L Mn 2 + , 45 mg/L F − and 450 g/L Cl − ); Polystyrene microspheres, which was fabricated by our reference. All the reagents were purchased from Aladdin Biochemical Technology Co., Ltd.

Equipment setup for acid mist simulation
As Figure 2 shown, the transparent tank is filled with the acid solution. The acid solution was covered with a polymer microspheres matrix. The nitrogen flow is introduced to mimic the generation of O 2 during the zinc sulfate electrolytic process. The exposure time time is 60 min. The N 2 emerged from the pore of the gas wire adhered to the bottom of the tank. The method of acid mist stimulation system was revealed as video 1 shown (The nitrogen flow rate in the experiment is 1 L/min).

Determination of acid mist concentration at a certain height above the acid solution
Determination of acid mist concentration is demonstrated as follows: When the colour of the solution changes from red to yellow, it is the endpoint of the titration. (1) Place wetting filter paper [13] on a specific height above the acid solution, as Figure S1 shown. The area and pore size of the filter paper are 0.025447 m 2 and 30 ∼ 50 micrometers, respectively. (2) After exposed to the artificial acid mist for a certain time, the filter paper was removed into a beaker containing 80 mL of deionised water and stirring slightly for 15 min. (3) Take the filter paper out of the beaker and add 0.2 mL 0.1% methyl orange solution into the beaker. (4) Add 0.01 mol/L NaOH solution or 0.1 mol/L of NaOH solution into the conical flask. Record the amount of sodium hydroxide at this time, repeat the above operation three times, and take the average value. Calculation of acid mist amount per unit area as follows: A is the amount of acid mist per unit area, mg/m 2 . S is the area of the filter paper, m 2 , V NaOH is The volume of NaOH solution. C NaOH is the concentration of NaOH solution. Calculation formula of acid mist suppression efficiency: B is the blank value, e is the mist suppression efficiency.

Results and discussion
Based on our previous research, the distribution of acid mist in the atmosphere was mainly demonstrated in Figure 3. There are two critical points (10 and 25 cm) at which acid mist concentration could change rapidly. Therefore, the mist suppression efficiency of stacking polymer microspheres matrix was detected at these two heights (10 cm, 25 cm).
3.1. The effect of the particle size of the microspheres of the matrix on the acid mist suppression efficiency The five kinds of the matrix constituted of the PS microspheres with the same density and particle size in different ranges (0.3 ± 0.15 mm, 0.675 ± 0.225 mm, 1.075 ± 0.175 mm, 1.425 ± 0.175 mm, 1.8 ± 0.15 mm) was selected to investigate the effect of the particle size of the microspheres in the matrix on the efficiency of mist suppression. (Figure 4). It could be found that the acid mist suppression efficiency increase, and then decrease with the rise of the particle size. More specifically, the matrix constituted of the microspheres with a range of the 1.075 ± 0.175 mm particle size exhibited higher acid mist suppression efficiency and lower acid mist concentration  at the height of 10 and 25 cm than other matrices. The relationship between the suppression efficiency and particle size is mainly related to two factors, the gap between the stacking microspheres and the number of voids in the matrix. It should be noted that there will be liquid bridge forces in Figure 5(B) between floating particles, which could be ignored when the particle size is much larger than the liquid bridge [14]. The PFC software was used to stimulate the matrix of the stacking of microspheres. The physcial properties of the gap in the matrix were calculated, as Table 1 shown. Based on the hypothes that the gap constructed by the stacking of microspheres could be recognised, all the gaps could be constituted by the stacking of three microspheres. Therefore, the volume of the gap in the matrix can be approximately calculated as follows: V t is the total volume of gaps, N is the total number of other types of voids converted into triangular voids constituted of three microspheres stacking, and V tr is the triangular void volume.
The cross-sectional area of the gap decrease with diminish of particle size. According to the previous research revealed, the diameter of the bubbles at the air/water surface is 1.88 mm, the cross-sectional area of acid mist droplet is 0.2328 mm 2 . To suppress the diffusion of droplets, S (area of the gap in the matirx) shoud be less than S0 (S0 is the cross-sectional area of the droplet). Table 1 showed that the size of the gap formed under microspheres (particle size: 1.075 ± 0.175 mm) is 0.187 mm 2 . Therefore, the matrix constituted of polymer microspheres with a particle size in 0.25∼1.25 mm can block the diffusion of acid mist droplets. The number of the gap in the matrix is also an essential factor in suppressing acid mist. The fewer gaps would impart fewer entrances for acid mist droplets, revealing a better acid mist suppression appearance. The number of the gap in the matrix constituted of the microspheres with the particle size 1.075 ± 0.175 mm was 8986, which was smaller than 0.3 ± 0.15 mm and 0.675 ± 0.225 mm. Considering the gap size between the stacking microspheres and the number of the gaps, the matrix constituted of the microspheres with the particle size 1.075 ± 0.175 mm should have a better appearance in the acid mist suppression,  which was also corresponding to the result of the experiment.
On the other hand, the coordination of PS microsphere matrix and liquid bridge force between the adjacent microspheres is responsible for the dissipation of the kinetic energy of the bubble rise. As Figure 6 shown, when F b (buoyancy force)> F lbsinθ (liquid bridge force), the bubble will enter the liquid bridge and then break through the liquid bridge layer. Otherwise, the bubbles will be blocked. In addition, the liquid bridge force has an adhesion effect on the particles [15]. The liquid bridge force between the adjacent microspheres was related to the particle size. The larger the particle size or larger the radius of curvature of the liquid bridge would both result in a greater liquid bridge force [16]. Therefore, the microspheres (1.075 ± 0.175 mm) have the largest liquid bridge than other microspheres with the particle size 0.3 ± 0.15 mm and 0.675 ± 0.225 mm.

The matrix constructed of the complex microspheres with broad particle size distribution on the acid mist suppression efficiency
The four kinds of microspheres with different particle sizes (PZ), 0.3 ± 0.15 mm 0.675 ± 0.225 mm 1.075 ± 0.175 mm 1.425 ± 0.175 mm, were selected as elemental agent for the matrix constituted of complex microspheres. (Table 2; Figure 7).
As Figure 8 shown, the acid mist suppression efficiency under the matrix constituted of mixed microspheres with broad particle size distribution was significantly higher than with narrow particle size distribution. This result should be related to the Furnas theory, which insists that the matrix consisted of the mixed microspheres with wide particle size distribution presented a denser stacking structure than microspheres with narrow particle size distribution. More specially, the microspheres with a smaller particle size was prone to be embedded in the gaps between large particles, forming a denser packing structure. The packing density will increase, and the gaps between particles will be smaller, and the acid mist suppression efficiency will be improved [17].

The effect of the density of spheres in the floating matrix on the acid mist suppression efficiency
The three kinds of PS microspheres with the same PZ but with different densities are selected to construct the floating matrix exploring the influence of polymer sphere ′ s density on the acid mist suppression efficiency. It can be found in Figure 9 that as the density of the spheres increases, the mist suppression .93 D is the particle size range of polymer microspheres, D 1 is the average particle size, S is the cross-sectional area of the void, V is the gap volume, n 1 is the number of microsphere particles in a certain volume, n 2 is the number of voids in a certain volume. Figure 6. The influence of liquid bridge force between particles on air bubbles. Table 2. Constitution of the matrix constitued of complex microspheres. efficiency gradually decreases. The low-density polymer sphere matrix presented higher mist suppression efficiency. The density of acid solution was higher than the stacking microspheres floating on the air/water surface could be divided into immersed and floating layers [18]. (The density of the acid solution was around 1.24). When the bubble rises from the bottom of the acid solution, gravity from the floating microspheres could dissipate its kinetic energy. Therefore, the floating microspheres could be responsive for acid mist suppression. The sphere with lower density could result in more floating polymer sphere, consuming more floating kinetic energy [19]. (Figure 10).

3.4.
The effect of the thickness of the floating matrix on the acid mist suppression efficiency Figure 9 shown that with the increase of the matrix's thickness, acid mist suppression efficiency correspondingly rise. When the thickness reached 15 mm, the acid mist suppression efficiency at the height of 10 and 25 cm both came 100%, completely blocking the dispersion of the acid mist. The effect of matrix's thickness on the acid mist suppression efficiency was the   same as the density of microspheres. Larger thickness means more floating microspheres could participate in the dissipation of floating kinetic energy of bubbles. Meanwhile, the gaps from horizontal stacking microspheres, as Figure 5 shown, could overlap to fabricate the combined gap with smaller size, blocking smaller-size acid mist droplets as Figure 8S revealed. As Figures 4B, 8B, 9B, and 10B shown, the acid mist suppression efficiency of the same matrix detected at 10 cm was mainly higher than at 25 cm. As Figure 5 shown, the floating PS microsphere matrix is principally a size-selective barrier for the acid mist droplets. It could be indicated that under the same matrix, the difference between the acid mist suppression efficiency detected at 10 cm and at 25 cm could be attributed to the original acid mist droplet's particle size. The PFC software was used to stimulate the relationship between the acid mist droplet's particle size and their location in the atmosphere, height above the acid mist solution in Figure 11 [20]. It could be found that the particle size of acid mist droplets at 25 cm is much smaller than at 10 cm. That's the reason for the difference in acid mist suppression efficiencies detected at different heights and under the same experimental condition.
3.5. The porous polymethyl methacrylate (PMMA) board faciliates suppressing the acid mist by microsphere matrix According to the result mentioned above and discussion, it could be found that size-selective blocking of acid mist droplets and dissipation of floating kinetic energy of bubbles are two curial factors for suppressing acid mist by microsphere matrix [7,21]. However, the flow rate of H 2 or O 2 increased during the electrolysis or electroplating as Video 1S shown. The stacking structure of the microsphere could be destroyed under the strong gas flow. A porous polymethyl methacrylate (PMMA) board, as Figure 12(B) shown, was employed as a barrier for dissipation of floating kinetic energy of bubbles. Figure 12 demonstrated that the signal PMMA block rarely had an effect on the acid mist suppression. Figure 12(C-F) revealed that the matrix combined with PMMA block presents better acid mist suppression efficiency than the matrix alone. For example, the suppression efficiency of the matrix constituted of the microspheres with 1.075 mm mean particle size detected at 10 cm was 81.5%, which rise to 97.2% after combined with the PMMA block.

Conclusions
In this study, a self-made laboratory's acid mist simulation and detection device were used to generate acid mist and test the acid mist concentration at any specific height in the atmosphere. A facile and  effective acid mist suppression system was constructed by employing a stacking microspheres matrix as a floating porous phase for the acid solution. The factors relating to the matrix's microstructure, the mean particle size of spheres, the combination of the complex particles with different mean particle sizes, and the thickness of the matrix on the acid mist suppression were explored. It could be found that the matrix constituted of medium-sized microspheres (1.075 ± 0.175 mm) present a better appearance in the acid mist suppression. When the thickness of this matrix reached 15 mm, its acid mist efficiency also came up to 100%, totally blocking the acid mist. Meanwhile, the combination of complex particles with different particle sizes and PMMA porous block all benefit from suppressing acid mist. Meanwhile, it should be noted that this green strategy would not bring other pollutes to the environment, such as the perfluoro surfactant.