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. 2021 Sep 27;6(39):25762–25771. doi: 10.1021/acsomega.1c04182

Effect of Na2CO3, HF, and CO2 Treatment on the Regeneration of Exhausted Activated Carbon Used in Sintering Flue Gas

Jianlu Zheng †,‡, Xiangdong Xing †,‡,*, Zhuogang Pang †,‡, Sunxuan Wang §, Yueli Du †,‡, Ming Lv †,‡,*
PMCID: PMC8495852  PMID: 34632232

Abstract

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The method of continuous treatment with Na2CO3 solution, HF solution, and CO2 was proposed for the regeneration of the exhausted activated carbon (EAC) produced in the sintering flue gas purification process. In order to obtain the optimal operation conditions, the effect of key parameters such as Na2CO3 solution concentration, HF solution concentration, and CO2 activation temperature on the sulfur conversion rate and regeneration efficiency was analyzed. Also, the N2 adsorption, Brunauer–Emmett–Teller analysis, scanning electron microscopy-energy dispersive spectrometry, X-ray diffraction, X-ray fluorescence, and Fourier transform infrared spectroscopy were adopted to investigate the deactivation reason and the change of the physical–chemical properties. The results showed that the deactivated EAC was mainly due to the deposition of inorganic compounds such as CaSO4, SiO2, and KCl to block the pores. Continuous treatment with Na2CO3 solution and HF solution could remove the inorganic compounds effectively. CO2 activation treatment further developed the blocked porosity and decreased the surface acidity. The optimal conditions for the regeneration of EAC were a Na2CO3 concentration of 0.5 mol/L, an HF concentration of 0.8 mol/L, and a CO2 activation temperature of 1073 K with the activation time of 1 h, corresponding to the specific surface area of 607.91m2/g. In the fourth regeneration cycle, the adsorption performance during the successive adsorption–regeneration process could still maintain a high level and the regeneration efficiency was 95.31%.

1. Introduction

Activated carbon (AC) has good adsorption performance due to having a large specific surface area, well-developed pore structure, and abundant functional groups.1−3 In recent years, it is increasingly applied in the sintering flue gas purification industry, such as desulfurization. After a period of industrial application, the adsorption capacity of AC could not be recovered by desorption and it is difficult to continue to use, which produces a lot of exhausted activated carbon (EAC). The EAC is usually treated as hazardous waste due to that the adsorbed substance contains sulfur dioxide (SO2) and dioxins as well as other dangerous pollutants from sintering flue gas.4 At present, the main way to dispose EAC is incineration or dumping in landfills, which often leads to serious secondary environmental pollution and resource waste.5,6 Therefore, the regeneration of EAC has attracted increasing attention recently because of its economic, environmental, and energy benefits.

The regeneration mainly removes the adsorbed pollutants through desorption and decomposition to restore the adsorption capacity of activated carbon.7,8 The choice of regeneration technology is closely related to the type and characteristics of adsorbed substances.9−11 Research by Guilane and Hamdaoui reported that low-frequency ultrasonication could desorb 4-chlorophenol with a stable structure in EAC from petroleum and pharmaceutical wastewater and the regenerated activated carbon do not differ significantly with the virgin carbon.4 Liu et al. investigated that the magnesium sulfate with a low melting point deposited on Mn-based EAC from coal-fired flue gas is decomposed under 650 °C thermal regeneration and the specific surface area is 800 m2/g.12 Ma et al. found that the Fenton oxidation can degrade hydrocarbon compounds toluene in saturated activated carbon produced in chemical industry waste gas and the regeneration rate of activated carbon is 94.52%.13 A work by Guo et al. reported that the amount of organic pollutant phenol and its derivatives in EAC from coking wastewater can be extracted by the low boiling point organic solvents and the regeneration efficiency is 98.27%.14 However, the EAC produced in the sintering flue gas purification process contains a variety of high thermal stability pollutants such as sulfate, alkali metal, and dust,15−17 which makes it is difficult to regenerate by traditional technology. Up to now, the regeneration technology of EAC from sintering flue gas has not been systematically studied. In order to achieve sustainable development of the steel industry, it is necessary to develop a feasible regeneration method for EAC produced in the sintering flue gas purification process.

In this paper, the continuous treatment method with Na2CO3 solution, HF solution, and CO2 are proposed to regenerate EAC. The deposited sulfate is converted into carbonate and then removed. Also, the texture properties are also changed to enhance the adsorption capacity by CO2 treatment. The effects of the adsorption–regeneration cycles and regeneration conditions such as Na2CO3 solution concentration, HF solution concentration, and CO2 activation temperature on the sulfur conversion rate and regeneration efficiency are investigated. In addition, the EAC properties are analyzed by N2 adsorption, Brunauer–Emmett–Teller (BET) analysis, scanning electron microscopy-energy dispersive spectrometry (SEM-EDS), X-ray diffraction (XRD), X-ray fluorescence spectroscopy (XRF), and Fourier transform infrared spectroscopy (FTIR). The regeneration efficiency is evaluated at a fixed bed reactor under simulating sintering flue gas.

2. Results and Discussion

2.1. Characterization of EAC

The content of C of the FAC and EAC were obtained by ultimate analysis and the other chemical elements contents by XRF analysis, as summarized in Table 1. It could be seen that the C content of EAC was less than that of FAC, as FAC was 93.62 wt %, while the EAC was reduced to 83.06 wt %. The content of S was increased from 0.21 to 4.96 wt %, and the content of Ca, Si, and Cl were 2.21, 1.85, and 1.77 wt %, respectively. This was because the ions adsorbed on the AC were not eliminated during the sintering flue gas purification process but reacted to form inorganic compounds to deposit on the activated carbon.

Table 1. Chemical Elements of EAC and FAC Analyzed (wt %).

sample C S Si Ca Fe Cl K others
FAC 93.62 0.21 0.14 0.16 0.07 0.11 0.06 5.63
EAC 83.06 4.96 1.85 2.21 1.54 1.77 1.15 3.46
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Figure 1 presents the XRD pattern of EAC and RAC. The characteristic peaks of both RAC and EAC could be observed at 2θ = 26° and 2θ = 43°, representing the graphite crystallite structure (002) and graphite-like structure (100) or (101) of the carbon materials,3,18,19 suggesting that the amorphous and graphite carbon structure of activated carbon were not affected. For FAC, no other characteristic peaks were shown in the XRD pattern, which confirmed its purity. In contrast, the characteristic peaks CaSO4, SiO2, and a small amount of KCl appear in the XRD pattern of EAC, which was due to the inorganic compounds deposited during the sintering flue gas purification process. The formation of CaSO4 might be due to the reaction of SO2 and Ca-based substances such as CaO in the sintering flue gas purification process. The existence of SiO2 and KCl was attributed to the deposition of particulates or dust in the sintering flue gas. Also, the CaSO4, SiO2, and KCl were difficult to desorb because of their good thermal stability.

Figure 1.

Figure 1

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XRD pattern of EAC and FAC.

To understand the microstructure and the distribution of inorganic compounds deposited on EAC, SEM images of EAC and FAC are shown in Figure 2. It can be seen from Figure 2a–c that the surface of FAC was smooth and the pores were clearly visible. There were also many circular pores with different pore diameters. The surface of EAC was rough, and the pore channel was completely covered by the acicular, lump, and graininess compounds. Combining with the SEM image and EDS analysis, the acicular compounds were CaSO4, which was mainly distributed in two parts. Most of the CaSO4 were around the pores, and a small amount was attached to the lump compounds. The phases marked “2” represent the gray lumps embedded in the pores of the activated carbon, which were SiO2. In addition, a small number of angular particles existed on the surface of the activated carbon, which were identified as KCl. The CaSO4, SiO2, and KCl were deposited into the pores, resulting in the increased in mass transfer resistance and decreased in desulfurization capacity.

Figure 2.

Figure 2

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SEM images and EDS results of EAC and FAC. (a): FAC image; (b, c): EAC images; (1): CaSO4; (2) SiO2; (3) KCl.

The N2 adsorption isotherms of FAC and EAC are illustrated in Figure 3. The shape of the isotherm of FAC was type I according to the IUPAC classification, which was characteristic of microporous materials. When the relative pressure was less than 0.1, the adsorption volume increased rapidly. In contrast, the EAC exhibited a type IV isotherm, indicating the main presence of mesopores. Also, the adsorption capacity of EAC was very low. When the relative pressure was up to 1.0, the adsorption volume of EAC was less than 5 cm3/g, suggesting the loss of adsorption capacity.

Figure 3.

Figure 3

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N2 adsorption isotherms of FAC and EAC.

The physical properties of FAC and EAC are listed in Table 2. It could be found that the specific surface area seriously reduced from 537 to 2.03 m2/g, while total pore volume dropped from 0.364 to 0.006 cm3/g, implying that the physical properties of EAC significantly deteriorated. The micropore volume was dropped from 0.195 to 0.001 cm3/g, and the average pore size increased from 2.84 nm to 11.65 nm, showing that the number of micropores sharply declined, which led to the increase in the number of mesopores. This had an adverse effect on activated carbon desulfurization because the micropores were the main site of the desulfurization reaction.

Table 2. Physical Properties of FAC and EAC.

sample specific surface area (m2/g) total pore volume (cm3/g) micropore volume (cm3/g) average pore size (nm)
FAC 537 0.364 0.195 2.84
EAC 2.03 0.006 0.001 11.65
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2.2. Effect of Na2CO3 Concentration on Sulfur Conversion Rate of EAC

Na2CO3 solution was used to convert the CaSO4 into CaCO3 and removed the water-soluble compounds such as KCl. The change of sulfur content on the EAC was defined as the sulfur conversion rate, which was calculated through eq 1,

2.2. 1

where M0 refers to the mass of EAC, g; W0 refers to the mass percentage of S on EAC, wt %; M1 and W1 refer to the mass of EAC treated by Na2CO3 solution, g; and W1 refer to the mass percentage of S on EAC treated by Na2CO3 solution, wt %.

The relationship between the Na2CO3 solution concentration and sulfur conversion rate is shown in Figure 4. It could be concluded that at low solution concentration of Na2CO3, the increased in solution concentration also increased the sulfur conversion rate. When the solution concentration of Na2CO3 was 0.1 mol/L, the sulfur conversion rate was only 75.02%. As the solution concentration continued to rise to 0.5 mol/L, the sulfur conversion rate was as high as 94.60%. This was because the diffusion rate of CO32– in the solution was accelerated with the increase of solution concentration to better contact with sulfate, leading to an enhanced conversion effect, as expressed in eq 2. However, When the Na2CO3 concentration exceeded 0.5 mol/L, the change of sulfur conversion rate slowed down, which was determined as the optimum Na2CO3 concentration.

2.2. 2

Figure 4.

Figure 4

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Relationship between Na2CO3 solution concentration and sulfur conversion rate of EAC (regeneration time 1 h, L/S = 10).

The XRD pattern of EAC after 0.5 mol/L Na2CO3 solution treatment is shown in Figure 5. It could be seen that the phases were mainly CaCO3 and SiO2. The characterization peaks of CaSO4 and KCl disappeared, which indicated that the sulfur conversion efficiency was high. The CaCO3 was identified as conversion product, and the KCl was washed by deionized water. SiO2 had no change in the reaction during the conversion process.

Figure 5.

Figure 5

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XRD pattern of EAC after 0.5 mol/L Na2CO3 solution treatment.

The surface morphology and EDS analysis of EAC after Na2CO3 solution treatment under different magnifications are shown in Figure 6. The pore structure of EAC could not be observed clearly, and the shapes of impurity compounds were lumps and flakes corresponding to the CaCO3 and SiO2, respectively. The CaCO3 was latched onto the pores, and the SiO2 was distributed around the pore.

Figure 6.

Figure 6

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SEM images and EDS results of RAC-Y under different magnifications: (a) 2.0 k× and (b) 5.0 k×; EDS results of phases marked “1” to “2,” where “1” indicates CaCO3 and “2” SiO2.

2.3. Effect of HF Concentration on Regeneration Efficiency of EAC

HF solution treatment was carried out to improve the desulfurization capacity by removing the inorganic compounds deposited on activated carbon. However, the degree of regeneration was greatly dependent on the treatment conditions. The effect of HF solution concentration on the regeneration efficiency is shown in Figure 7. It could be found that the regeneration efficiency first increased significantly. The regeneration efficiency reached the maximum value of 81.55% when the HF concentration was 0.8 mol/L due to the reaction shown in eqs 3 and 4:

2.3. 3
2.3. 4

Figure 7.

Figure 7

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Influence of HF concentration on the regeneration efficiency of EAC (regeneration time 30 min, L/S = 10).

However, the regeneration efficiency decreased gradually as the HF solution concentration continued to rise. When the solution concentration of HF was 1.4 mol/L, the regeneration efficiency was reduced to 75.02%. This was attributed to two possible features of HF solution treatment. The main HF solution could effectively remove the inorganic compounds. On the other hand, the HF solution had an etching to the surface pore morphology of material.20 As the HF solution concentration continued to increase, the activated carbon framework was corroded, which caused the original pore structure to collapse. This was because when the concentration increased to a certain extent, HF could react with ash from the attached wall of the activated carbon such as Al, Ca, K, and other metal oxides, causing the change in the pore structure. In addition, the SiF4 gas generated by the reaction of HF and SiO2 was not easy to diffuse in the micropores, leading to the collapses of the micropores, which made the micropores expand into mesopores or macropores of the activated carbon.

Figure 8a shows the XRD pattern of RAC-H-0.8. The characterization peaks of CaCO3 and SiO2 disappeared. It indicated that the HF solution could effectively remove CaCO3 and SiO2, which was conducive to the recovery of the adsorption capacity.

Figure 8.

Figure 8

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Structure characteristics of RAC-H-0.8: (a) XRD pattern; (b) N2 adsorption isotherm.

The N2 adsorption isotherm of RAC-H-0.8 is presented in Figure 8b. The adsorption capacity increased rapidly when the relative pressure was low and the isotherm type was the same as that of FAC. It means that the RAC micropores were well restored after HF treatment. Table 3 lists the physical properties of RAC-H-0.8, showing that the specific surface area, total pore volume, and micropore volume of EAC were 432.14m2/g, 0.274cm3/g, and 0.114cm3/g, respectively, which were close to 80% of the FAC. HF treatment effectively improved the physical properties of EAC.

Table 3. Physical Properties of RAC-H-0.8.

sample specific surface area (m2/g) total pore volume (cm3/g) micropore volume (cm3/g) average pore size (nm)
RAC-H-0.8 432.14 0.274 0.114 4.56
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Figure 9 shows the microstructure of RAC-H-0.8. It was noticed that the pore was irregular and visible. The impurities on the surface of activated carbon after HF solution treatment were not observed.

Figure 9.

Figure 9

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SEM images of RAC-H-0.8 under different magnifications: (a) 1.0 k× and (b) 3.0 k×.

2.4. Effect of Activation Temperature on Regeneration Efficiency of EAC

The RAC had good adsorption performance after Na2CO3 solution and HF solution treatment. However, the recovery of texture properties was not significant, and the percentages of microporosity for the total porosity of the EAC were less than that of the FAC. CO2 activation treatment could improve the texture characteristics of RAC and increase the desulfurization capacity. The effect of activation temperature on the regeneration efficiency and yield of RAC are presented in Figure 10. As the activation temperature increased from 773 to 1073 K, the regeneration efficiency increased from 82.15 to 110.5%, and the yield gradually decreased. This was because the activated carbon reacted with CO2 at high temperature to reactivate the pristine carbon, which further developed the blocked porosity for EAC and produced new pores. However, the regeneration efficiency reduced seriously to 97.35% when the temperature was 1173 K. It might be due to the fact that the continued enhancement of activation reaction would lead to the original pore structure being destroyed, and a part of micropores collapsed into mesopores.

Figure 10.

Figure 10

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Effect of activation temperature on regeneration efficiency and yield.

The N2 adsorption isotherm measurements of RAC-C are given in Figure 11a. For the sample after CO2 activation treatment, the shape of isotherms belonged to type I. The adsorption isotherm of RAC tended to be saturated when the relative pressure was lower than 0.1, which belonged to the characteristics of microporous materials. In addition, the isotherms had a larger slope, indicating that there was a certain amount of mesopores in the RAC when the relative pressure was higher than 0.1.

Figure 11.

Figure 11

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Structure characteristics of RAC-C: (a) N2 adsorption isotherms of RAC-C; (b) FT-IR spectra of RAC-C.

Table 4 summarizes the physical properties of RAC. The specific surface, total pore volume, and micropore volume continuously increased, but the average pore size gradually reduced with the increase in activation temperature. When the activation temperature was 1073 K, the specific surface area, the micropore volume, and the total pore volume reached the maximum values of 607.91 m2/g, 0.211 cm3/g, and 0.406cm3/g, respectively. Also, the RAC-C-1073 exhibited better adsorption ability than that of other RAC, even higher than that of FAC. However, the micropore volume reduced rapidly to 0.189cm3/g, but the total pore increased to 0.433cm3/g as the temperature increased to 1173 K. It indicated that the RAC contained a large number of mesopores, and the decrease of micropores lead to the relative increase in the average pore size. The CO2 treatment could effectively improve the physical properties, and the optimal regeneration temperature of EAC was 1073 K.

Table 4. Physical Properties of RAC.

sample specific surface area (m2/g) total pore volume (cm3/g) micropore volume (cm3/g) average pore size (nm)
RAC-H-0.8 432.14 0.274 0.114 4.56
RAC-C-773 447.36 0.323 0.165 3.98
RAC-C-873 464.69 0.328 0.186 3.95
RAC-C-973 517.32 0.345 0.190 3.94
RAC-C-1073 607.91 0.406 0.211 3.91
RAC-C-1173 529.45 0.433 0.189 4.12
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The FT-IR spectra of RAC after CO2 activation treatment is provided in Figure 11b. It could be found from Figure 11b that the positions of the stretching vibration peaks were similar except the intensities, which indicated that the type of surface functional groups was consistent. In all the recorded spectra, the adsorption band around 1640 cm–1 could be assigned to the stretching vibration of the C=C bond on the surface of olefins and the C=O stretching vibration in carbonyl groups almost did not change after CO2 activation treatment.21,22 Also, peaks in the 2250–2400 cm–1 range were observed due to the fact that the band of C–O stretching vibration and the C–O bands became a little weaker as the temperature increased.23 Moreover, the stretching vibration of the O–H bands in the 3300–3600 cm–1 range decreased significantly, which was associated with the hydroxyl groups, and carboxyl acid groups as well as chemisorbed water on the surface of RAC decreased.24,25 The functional groups such as C–O bands and O–H bands decreased, which led to the attenuated surface acidity and the release of activating site, which further enhanced the SO2 adsorption capacity.26 It was also an important reason for the increase of regeneration efficiency after CO2 activation treatment.

The surface morphology of RAC and RAC-H are shown in Figure 12. It could be seen that there were no obvious differences between the morphology of RAC-H and RAC-C. The surface of RAC was smooth, and the pores were present as a regular circle. As the activation temperature reached 1173 K, the pore size of RAC was obviously larger than that treated under other temperatures.

Figure 12.

Figure 12

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SEM images of the (a) RAC-H-0.8, (b) RAC-C-773, (c) RAC-C-873, (d) RAC-C-973, (e) RAC-C-1073, and (f) RAC-C-1173.

In order to study the change in regeneration performance with adsorption–regeneration cycles, the effects of the adsorption–regeneration cycle on the regeneration efficiency of activated carbon were carried, and the results are shown in Figure 13. It was obvious that under the optimal experimental conditions used, the adsorption performance during the successive adsorption–regeneration process could still maintain a high level. In the fourth regeneration cycle, the regeneration efficiency was 95.31%. The results showed that the regenerative effect of continuous treatment with Na2CO3 solution, HF solution, and CO2 regeneration was remarkable.

Figure 13.

Figure 13

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Effect of regeneration efficiency in successive adsorption–regeneration cycles.

3. Conclusions

A novel continuous treatment method with Na2CO3 solution, HF solution, and CO2 could effectively regenerate EAC from the sintering flue gas purification process. When the Na2CO3 concentration was 0.5 mol/L, HF concentration was 0.8 mol/L, and CO2 activation temperature was 1073 K, the regeneration efficiency was up to 110.5%. The deposition of inorganic compounds such as CaSO4 and SiO2 as well as KCl leads to the deactivation of activated carbon. Na2CO3 solution could convert the CaSO4 into CaCO3 and remove the water-soluble compounds such as KCl. The sulfur conversion rate first increased significantly, and then the change slowed down with the increase in Na2CO3 concentration. When the Na2CO3 concentration was 0.5 mol/L, the sulfur conversion rate radical was up to 94.6%. Also, the CaCO3 was identified as the conversion product. Secondary chemical regeneration using HF solution after the Na2CO3 solution treatment removed residual inorganic compounds, and the specific surface area of the activated carbon recovered to 80% of FAC. The regeneration efficiency increased at first and then decreased with the increase in HF solution concentration. Finally, CO2 activation treatment developed the blocked porosity and reduced the surface acidity, further improving the adsorption performance of activated carbon. In the four adsorption–regeneration processes, the adsorption performance could still maintain a high level.

4. Experimental Section

4.1. Materials and Equipment

The EAC was derived from the steel plant in China. The solutions used in the experiment were prepared with analytical grade Na2CO3 (Damao Chemical Factory, Tianjin, China) and HF solution (Damao Chemical Factory, Tianjin, China, 40%). All aqueous solutions were prepared with deionized water. The numerical control ultrasonic instrument (model no. KQ5200DE, Ultrasonic instrument Co., Ltd., Kunshan, China) was used to produce ultrasound for the regeneration of EAC. CO2 activation treatment was carried out in a vacuum tubular furnace (Jinbi Electrical Equipment Co., Ltd., Chongqing, China).

4.2. Experimental Methods

The schematic diagram of the experimental flow chart is shown in Figure 14. The EAC was washed in deionized water and dried at 378 K for 24 h, which would be used in the following regeneration process. First, 10 g of EAC was added to the beaker containing 100 mL Na2CO3 solution with different concentrations of 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6 mol/L for pretreatment and then placed in the ultrasonic instrument with the ultrasound power of 100 W and reaction temperature of 333 K. Once the reaction time was up to 1 h, the EAC was taken out to be washed with deionized water until the washing solution reached neutral and part of sample was used to analyze the properties after drying for 24 h at 378 K.

Figure 14.

Figure 14

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Schematic diagram of the experimental flow chart.

Second, 5 g of the EAC after the Na2CO3 solution treatment was immersed in the beaker containing 50 mL HF solution with different concentrations of 0.6, 0.8, 1.0, 1.2, and 1.4 mol/L and stirred at room temperature. After the HF solution treatment for 30 min, the samples were handed out to be washed with deionized water for 1 h and part of sample was used to analyze the properties after drying for 24 h at 378 K.

Finally, 5 g of the EAC treated by Na2CO3 solution and HF solution was loaded into the vacuum tubular furnace with the temperature increasing at a constant rate of 7 K/min to the set target temperature under a N2 atmosphere, which was shifted to CO2 and reacted for 1 h when the set temperature was reached with the gas flow rate of 40 mL/min. The nomenclature for the applied regeneration methods is summarized in Table 5.

Table 5. Nomenclature of RACa.

sample name Na2CO3 solution treatment HF solution treatment CO2 treatment
RAC-Y-S yes no no
RAC-H-S yes yes no
RAC-C-T yes yes yes
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a

Y refers to the pretreatment method of Na2CO3 solution; S refers to the solution concentration; H refers to the treatment method with HF solution; C refers to the CO2 activation treatment; and T refers to the regeneration temperature.

4.3. Regeneration Efficiency Testing

The regeneration efficiency of activated carbon was evaluated in the fixed bed reactor, and the equipment apparatus is shown in Figure 15. The equipment was mainly consisted of a gas mixing system, adsorption reaction system, and gas analysis system. The reactor had a 10 mm inner diameter and was 200 mm high. A total of 1 g activated carbon was put into the reactor before testing. The composition of the mixture gas based on the actual sintering flue gas contained 1200 ppm SO2, 10 vol % H2O, 15 vol % O2, and N2 as balance. The flue gas analyzer was used to monitor the SO2 concentration, and the reaction temperature was 393 K. The regeneration efficiency of RAC was calculated by the SO2 adsorption capacity of RAC and FAC, as shown in eq 5:

4.3. 5

where Re refers to the regeneration efficiency; Rn and R0 refer to the adsorption capacity of RAC and fresh activated carbon (FAC), respectively.

Figure 15.

Figure 15

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Schematic diagram of the experimental apparatus.

4.4. Characterization

N2 adsorption–desorption experiment was conducted to characterize the pore structure of activated carbon on an aperture analyzer (V-Sorb2800P, Kine Spectrum technology, China) at 77 K. The specific surface area and total pore volume were calculated by the Brunauer–Emmett–Teller (BET) equation, and the micropore volume was obtained by the T-plot method.

The phase composition of the inorganic compounds deposited was analyzed by X-ray diffraction (XRD, D8 ADVANCE A25, Bruker AXS, Germany) using Cu Kα radiation (λ = 1.5406 Å), and the measuring method was stated in our previous study.27

Fourier transform infrared spectroscopy (FT-IR, Nicolet 6700, Thermo Fisher Scientific, USA) was used to characterize the surface functional groups. The 2.0 mg RAC samples with 400 mg potassium bromide (KBr) were mixed and pressured into the thin film. The wavenumber range of RAC was recorded at a scope of 400–4000 cm–1, and the scanning time and resolution were set as 32 s and 4 cm–1, respectively.

X-ray fluorescence (XRF, S4 Explorer, Bruker AXS, Germany) was used to analyze the content of the chemical elements of the EAC and FAC. The samples were ground to −0.074 μm.

The surface morphology of samples was observed by using a scanning electron microscope (SEM, Quanta200, FEI Company, USA) with an acceleration voltage of 5 kV. The elemental composition was analyzed by energy-dispersive spectroscopy (EDS, Oxford instruments company, U.K.).

Acknowledgments

The present work was financially supported by the Natural Science Basic Foundation of China (program no. 51874058) and China Postdoctoral Science Foundation (grant no. 2019M663932XB). The authors gratefully acknowledge their support.

The authors declare no competing financial interest.

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