In order to promote the application of electrochemical water softening technology in industrial circulating cooling water systems, electric field type, cathode structure and solution residence time are selected for optimization analysis of an electrochemical water softening device. The experimental results show that the water softening performance per unit area of mesh cathode is better than that of a plate cathode. In addition, the softening amount per unit area of the mesh cathode can be further increased when the high-frequency (HF) power supply is applied. When the HF power supply is applied, the softening amount per unit area is 158.58 g/m2·h−1 more than when the direct current power supply is applied. In order to explore the growth mechanism of calcium carbonate, micro-analysis technology and high-speed bubble photography technology are used. The results show that the bubbles escape along the longitudinal direction of the electrode plate, and the main growth direction of calcium carbonate growth is consistent with the escape direction of the bubble; that is, the bubbles grow along the longitudinal direction of the electrode plate. The special structure of the diamond-shaped mesh cathode facilitates the growth of calcium carbonate crystals.

  • The high-frequency power supply can further increase the descaling amount per unit area of the mesh cathode.

  • The special structure of the diamond-shaped mesh cathode facilitates the growth of calcium carbonate crystals.

  • The maximum desalination rate of this experimental device is 604.5 g/m2 h−1.

Circulating cooling water is an indispensable part of modern industry. Due to the large amount of Ca2+ and Mg2+ ions in the circulating cooling water (Rahmani et al. 2016; Yu et al. 2018), it is easy to form scale on the heat transfer surface, which leads to an increase in energy consumption and a decrease in heat transfer coefficient. It is very important to soften the circulating cooling water (Brastad & Zhen 2013; Georgiou et al. 2018).

A variety of water treatment technologies are applied to soften the circulating water, such as the addition of acidic detergents and other chemicals (Neveux et al. 2016), exchange resins, reverse osmosis, nanofiltration and electrodialysis etc. (Almarzooqi et al. 2014; Zhi & Zhang 2014). However, these technologies have drawbacks. The advantages of electrochemical water softening technology includes simple maintenance, environmental protection, and convenient process control (Hu et al. 2015; Sanjuán et al. 2019a). The electrochemical water softening technique produces OH ions by electrolytic reaction, which are enriched near the cathode to form a highly alkaline region, and promotes the formation of calcium carbonate crystals on the cathode surface (Zhang et al. 2015; Sanjuán et al. 2019b).

Although electrochemical water softening technology has broad market prospects, its application is still restricted by some problems. In recent years, scholars have optimized the structure and process conditions of the electrochemical water softening device to promote the application of this technology in medium and large circulation water treatment systems (Hasson et al. 2008, 2010; Chelladurai et al. 2020). Aiming at the problem of low cathode deposition efficiency of electrochemical water softening technology, Hasson et al. (2010) designed an electrochemical seed system to greatly increase the softening reaction area, but the precipitation occurred on the surface of the seed in the crystallizer instead of the surface of the crystallizer. Luan et al. (2019) also introduced an electrochemical water softening system with a multi-mesh coupling cathode to increase the deposition efficiency of electrochemical water softening technology . It is proved that the use of porous cathodes can significantly improve the water softening performance of the device.

High frequency power supply has significant advantages in reducing power consumption of electrochemical water softening technology. Lin et al. (2021) compared the performance of the electrochemical water softening device in HF electric fields with that of direct current (DC) electric fields, and found that HF electric fields can improve the energy utilization rate of the device. Yu et al. (Wu et al. 2018) used a pulsed power supply for electrochemical water softening treatment. After 30 operating cycles, the equipment operated stably, and energy consumption did not increase. The results show that the deposition rate of calcium carbonate can be increased by proper mass transfer restriction. Sayadi et al. (Ilhem et al. 2015) found that the use of a pulsed electric field accelerated the deposition of calcium carbonate. The energy consumption and water softening performance of electrochemical water softening can be promoted when pulsed power is applied, but the influence of the pulsed electric field on the performance of the electrochemical water softening device under different parameters has not been studied.

In order to maintain the performance of the electrochemical water softening device during operation, Zhang et al. (2020a) designed a large-scale automatic electrochemical water softening device and conducted a comprehensive study on the amount of circulating water, and found that the optimal circulating water flow rate of the equipment is 20 m3/h when the performance of the device reaches the optimal. Zhi et al. (Zhi & Zhang 2016) developed a different electrochemical water treatment method to optimize the electrochemical water softening. The results show that when the solution flow rate is 120 ml/min, the electrochemical water softening device has the optimum operating conditions. However, the overall impact of the circulating water volume on the softening performance after the equipment has been running for a long time has not been carried out.

Since the performance of electrochemical water softening is affected by the interaction of many factors, and the evaluation indicators are more complicated, the experiments are optimized in order to explore the interaction of multiple factors. The response surface method is a mathematical and statistical method for designing experiments (Box & Wilson 1951). Multiple quadratic regression equations are used to fit the functional relationship between factors and response values. Through the analysis of regression equations, the best parameter combination is obtained, and a statistical method to solve multivariate problems (Box & Wilson 1951; Chelladurai et al. 2019, 2020). Due to the advantages of the response surface method, the influence of the cathode plate structure, power supply duty ratio and solution residence time on the electrochemical water softening device is analyzed.

In order to study the internal formation mechanism of scale, scholars have adopted many technical methods, such as Electrochemical Impedance Spectroscopy (EIS) (Hu et al. 2020), X-ray Diffraction (XRD) (Sanjuán et al. 2019b), Fourier Transform Infrared spectroscopy (FTIR), Raman Spectra (RAMAN) spectroscopy (Simpson 1998), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) (Gabrielli et al. 2006; Tuan et al. 2015), the microscopic morphology and crystal structure of scale has been analyzed comprehensively in these studies. Under normal temperature and pressure, calcite is stable. Aragonite is a metastable crystal type. Vaterite is unstable and can be transformed into other two crystal forms. The hardness and density of calcite are higher than that of aragonite and vaterite, and it is easy to form a dense scale layer and thicker hard scale, which adhere to the electrode surface (Ranjbar 2010). However, the above research ignores the influence of the cathode structure on the growth of calcium carbonate crystals. There are many bubbles in the electrochemical reaction process, which causes the disturbance of the flow field, and there is a lack of research on the mass transfer process.

In the present study, the electrochemical water softening has been comprehensively optimized from three aspects, power supply type, cathode structure and solution circulation time. High-power electron microscope, SEM and XRD are used to study the precipitation and growth mechanism of calcium carbonate. High-speed industrial cameras are used to photograph and analyze the law of bubble movement in the electrochemical reaction process.

Materials

The analytical reagents (AR) used for the experiment solution mainly include anhydrous calcium chloride (CaCl2), sodium bicarbonate (NaHCO3), ethylenediaminetetraacetic acid disodium salt, dihydrate (C10H14N2O8Na2·2H2O), acetylacetone and Tris (hydroxymethyl) aminomethane (TRIS), and are purchased from Sinopharm Chemical Reagent Co., Ltd.

The power supply device used in the experiment is a high-frequency pulse power supply produced by Shenzhen Electronic Technology Co., Ltd. According to the flow rate of the experimental device, an electromagnetic diaphragm metering pump is WS-03-07-s, BeiJing WELL&OFF. Potentiometric method is used to determine the hardness of the test solution. The hardness analysis instrument was purchased from HACH, and its model is AT1000. The models of micro analysis equipment are shown in Table 1.

Table 1

Micro-analysis equipment model

Technical meansEquipment manufacturerDevice model
High-power electron microscope Shanghai Zhiqi Industry Co., Ltd ZQ-603 
XRD Bruker D8 ADVANCE 
SEM Japan Electronics Corporation JEOL-IT300 
Technical meansEquipment manufacturerDevice model
High-power electron microscope Shanghai Zhiqi Industry Co., Ltd ZQ-603 
XRD Bruker D8 ADVANCE 
SEM Japan Electronics Corporation JEOL-IT300 

Experimental device

In order to simulate the actual operating environment in the industry, one-way real-time replenishment of high-hardness solutions is adopted by this experimental platform. A stirring device is added to the make-up tank to ensure that insoluble substances in the solution will not deposit. The average residence time of the solution is calculated according to the rate of replenishing the solution in the reaction tank (Figure 1). The reaction tank volume is 1.6 L. The size of electrodes is 100 mm × 100 mm. The material of the electrode is titanium. The anode surface is coated with iridium and tantalum to prevent corrosion. In order to explore the influence of bubbles on the mass transfer process, an 800-frame high-speed camera bubble shooting platform is built as shown in Figure 2.

Figure 1

Experimental setup.

Figure 1

Experimental setup.

Figure 2

High-speed camera platform.

Figure 2

High-speed camera platform.

Experimental design

In this study, three independent parameters of the cathode plate structure (A), solution residence time (B) and power supply duty ratio (C) are chosen for optimization and the performance of the electrochemical water softening is chosen as the response item (Table 2). The titanium plate (TP) and the titanium mesh with 3 × 6 (3 × 6TM) and 4.5 × 6 (4.5 × 6TM) are selected as the cathode. According to the actual work conditions of an industrial circulating water system, the solution with hardness of 800 mg/L is configured. The power supply voltage and frequency are 36 V and 8 kHz, the solution residence time is 10 min, 20 min, and 30 min respectively, and the power supply duty cycle is 20, 50, and 80% respectively.

Table 2

Factor levels and symbols

FactorSymbolLevel
Low (-1)Central (0)High (+1)
Cathode plate structure (/) TP 3 × 6TM 4.5 × 6TM 
Power supply duty ratio (%) 20 50 80 
Solution residence time (min) 10 20 30 
FactorSymbolLevel
Low (-1)Central (0)High (+1)
Cathode plate structure (/) TP 3 × 6TM 4.5 × 6TM 
Power supply duty ratio (%) 20 50 80 
Solution residence time (min) 10 20 30 

The experiment duration of each group is 600 minutes. The solution is stored in the storage tank and continuously stirred. The flow rate of the pump is adjusted according to the duration of each experiment. The average residence time of the solution is calculated according to the flow rate and the size of the reaction tank. Finally, the solution softened overflows into the reservoir.

In order to explore the comprehensive influence of different factors on the device, the response surface method (RSM) is used to optimize the factors (A, cathode plate structure, B, power supply duty ratio, C, solution residence time). The experiment is designed in combination with the Box-Behnken design (BBD) model. The result value of the response is calculated according to the following formula.
formula
(1)
where Y is the response value, φ is constant, φi, φij, φii are the linear coefficients, interaction coefficients and quadratic coefficients, respectively. Xi (Xj) is the i th (j th) independent variable, and ε is random error.

Experimental results

During each experiment, the solution in the rehydration tank is sampled every 120 minutes to test the hardness in order to eliminate the influence of the natural precipitation of calcium carbonate on the experimental results. (Table 3) The treated solution is sampled every 2 h to measure the hardness and calculate the average hardness of the solution. The total amount of calcium carbonate removed in each group of experiments is calculated by the following formula (2) and recorded in Table 4.
formula
(2)
where HI, Hi and Tsr represent the rehydration tank solution hardness of the I-th record (I = 2,4,6,8,10), the solution hardness of the i-th record (i = 2,4,6,8,10, and I = i) and the residence time of the solution respectively.
Table 3

Solution hardness in rehydration tank

Time (min) 120 240 360 480 600 
HI (mg/L) 795.13 686.8 546.06 368.08 342.99 
Time (min) 120 240 360 480 600 
HI (mg/L) 795.13 686.8 546.06 368.08 342.99 
Table 4

Experimental results

GroupCathode plate structurePower supply duty ratioSolution residence timeCalcium carbonate removalAverage removal hardness
A (/)B (%)C (min)RC (g)RA (mg/L)
TP 50 30 7.02 219.42 
TP 50 10 21.32 222.15 
TP 80 20 13.94 290.35 
3 × 6TM 50 20 21.49 431.71 
3 × 6TM 80 10 31.64 329.57 
4.5 × 6TM 50 10 36.22 377.34 
3 × 6TM 20 10 33.33 347.23 
4.5 × 6TM 20 20 18.15 378.05 
3 × 6TM 50 20 18.03 375.67 
10 3 × 6TM 80 30 11.43 357.17 
11 4.5 × 6TM 50 30 10.78 336.79 
12 TP 20 20 14.68 305.80 
13 4.5 × 6TM 80 20 17.29 360.15 
14 3 × 6TM 50 20 20.71 431.42 
15 3 × 6TM 50 20 20.34 423.70 
16 3 × 6TM 20 30 13.68 427.57 
17 3 × 6TM 50 20 20.82 433.68 
GroupCathode plate structurePower supply duty ratioSolution residence timeCalcium carbonate removalAverage removal hardness
A (/)B (%)C (min)RC (g)RA (mg/L)
TP 50 30 7.02 219.42 
TP 50 10 21.32 222.15 
TP 80 20 13.94 290.35 
3 × 6TM 50 20 21.49 431.71 
3 × 6TM 80 10 31.64 329.57 
4.5 × 6TM 50 10 36.22 377.34 
3 × 6TM 20 10 33.33 347.23 
4.5 × 6TM 20 20 18.15 378.05 
3 × 6TM 50 20 18.03 375.67 
10 3 × 6TM 80 30 11.43 357.17 
11 4.5 × 6TM 50 30 10.78 336.79 
12 TP 20 20 14.68 305.80 
13 4.5 × 6TM 80 20 17.29 360.15 
14 3 × 6TM 50 20 20.71 431.42 
15 3 × 6TM 50 20 20.34 423.70 
16 3 × 6TM 20 30 13.68 427.57 
17 3 × 6TM 50 20 20.82 433.68 

Response surface analysis

The prediction model of design variables in response to the target calcium carbonate removal amount and average removal hardness is established according to the second-order polynomial model, the least square method is used to perform regression analysis on the data, and the response surface function relationship between RA and RC with the variable is obtained respectively.
formula
(3)
formula
(4)

In order to explore the influence of multiple factors on RA and RC, the three-dimensional response surface and contour map formed by the interaction of experimental factors are obtained respectively according to the second-order fitting model. The center value of one factor is fixed unchanged, and the interaction of the other two factors affects RA and RC.

It is found that the plate type, the residence time and the power supply duty cycle are 3 × 6TM, 20 min and 50% respectively, which are the best working conditions for the average hardness removal rate, as is shown in Figure 3. The steep slope of the response surface in the figure proves that changes in various factors have a greater impact on the average hardness removal rate of the device. However, the contour lines in Figure 3(a) and 3(c) are distributed in an elliptical shape. The interaction between the duty cycle and the selection of the electrode plate type and the residence time of the solution is significantly demonstrated. The circular contour in Figure 3(b) indicates that the residence time of the solution and the electrode selected cathode type without significant interaction.

Figure 3

The 3-D response surface plot of RA.

Figure 3

The 3-D response surface plot of RA.

The continuous increase in the residence time of the solution did not cause a continuous increase in the average hardness removal rate of the device. The residence time of the solution reaches its peak at 20 minutes. A longer residence time will cause the concentration of CO32− ions in the solution to decrease, the mass transfer process weakens, and the softening effect of the device is inhibited. The shorter residence time causes the ions in the solution to not be fully combined with the reaction, and the ideal softening performance cannot be achieved.

In Figure 4, the 3-D plots are used to illustrate the effects of different cathode structures, power supply duty ratio and solution residence time on removal efficiency of Ca2+ ions in the solution. The slope of the response surface in Figure 4(a) is relatively small. Obviously, the selection of cathode structure and the duty cycle of the power supply have little effect on the softening capacity of the device, but there is an interaction between the two factors. Figure 4(a) and 4(b) show that the residence time of the solution is a decisive factor for the calcium carbonate removal amount of the device. When the residence time is 10 minutes, the calcium carbonate removal amount of the device reaches the maximum value.

Figure 4

The 3-D response surface plot of RC.

Figure 4

The 3-D response surface plot of RC.

The amount of calcium carbonate removed by the device is negatively correlated with the residence time of the solution. The residence time is reduced, and the volume of the solution processed by the device increases. The higher concentration of CO32− and Ca2+ in the device is caused by the solution replenishment of the device for one-way circulation. Conducive to the softening rate of the device through a good mass transfer environment is ensured by one-way circulation.

Effect of cathode structure

As shown in Figure 5, the removal of calcium carbonate is sorted according to the retention time of the solution. The removal of calcium carbonate is the least for each group of medium plate cathodes. At the same time, in terms of average removal hardness, the result shows that the performance of the electrochemical water treatment unit that applied the 3 × 6TM cathode is the best among the cathode structures. In order to explore the influence of the cathode structure on the average hardness removal rate, a high-power electron microscope is used to observe three different structures of cathodes.

Figure 5

Rc with solution residence time.

Figure 5

Rc with solution residence time.

At the end of the experiment, the calcium carbonate crystal scales deposited on the three structures of cathode are magnified by 60 times to observe. It can be seen from Figure 6 that calcium carbonate deposited on the surface of the mesh cathode is thicker, while the calcium carbonate crystals deposited on the plate cathode are poorly aggregated and relatively dispersed. The reason is that the main growth direction of calcium carbonate is along the vertical direction of the electrode plate. The special structure of the mesh cathode is favorable for the growth of calcium carbonate, but the plate cathode has a certain inhibitory effect on the aggregation and growth of calcium carbonate crystals.

Figure 6

Scales on the cathode.

Figure 6

Scales on the cathode.

The related research on electrochemical water softening mainly focuses on the horizontal growth of scale, the vertical growth behavior of scale has been ignored. At the end of the experiment, the cathode covered with calcium carbonate crystals was observed under a high-power microscope, and it was found that the inside of the mesh of the cathode was filled with calcium carbonate precipitation. It proves that the calcium carbonate crystals are more likely to grow in the longitudinal direction of the plate. Since the cathode structure of the device provides a growth environment for the longitudinal growth of calcium carbonate, the device's water softening capacity per unit area is significantly increased. The comparison results of this device and related research are shown in Table 5.

Table 5

Water softening performance comparison of this work with other papers

Hardness (mg/L as CaCO3)Current density (A/m2)Softening rate (g/m2 h−1)Reference
250 20 2.6 Hasson et al. (2008)  
240 ∼20 3.4–6.9 Gabrielli et al. (2006)  
350 20 8–15.09 Yu et al. (2018)  
1,180 108 22.8 Hasson et al. (2008)  
300 100 10 Zhi & Zhang (2014)  
350 100 25.5–34.3 YU et al. (2018)  
350 18.3 29.16 Luan et al. (2019)  
870 30.6 42.32 Luan et al. (2019)  
6,540 250 530–570 Zaslavschi et al. (2013)  
1,600 100 460 Zaslavschi et al. (2013)  
1,150 30 19.95 Zhang et al. (2020b)  
800 10 ∼ 60 188.5–604.5 This work-HF 
800 10 ∼ 70 195–558.6 This work-DC 
Hardness (mg/L as CaCO3)Current density (A/m2)Softening rate (g/m2 h−1)Reference
250 20 2.6 Hasson et al. (2008)  
240 ∼20 3.4–6.9 Gabrielli et al. (2006)  
350 20 8–15.09 Yu et al. (2018)  
1,180 108 22.8 Hasson et al. (2008)  
300 100 10 Zhi & Zhang (2014)  
350 100 25.5–34.3 YU et al. (2018)  
350 18.3 29.16 Luan et al. (2019)  
870 30.6 42.32 Luan et al. (2019)  
6,540 250 530–570 Zaslavschi et al. (2013)  
1,600 100 460 Zaslavschi et al. (2013)  
1,150 30 19.95 Zhang et al. (2020b)  
800 10 ∼ 60 188.5–604.5 This work-HF 
800 10 ∼ 70 195–558.6 This work-DC 

As shown in Table 6, the performance of the electrochemical water softening device is quite different in the DC electric field and HF electric field environment due to the different structure of the cathode. When the plate cathode is used, the device performance is less affected by the electric field. A large number of bubbles are generated in the electrochemical softening process, and the bubbles have an impact on the mass transfer process. In order to explore the impact of bubbles on the mass transfer process, the high-speed camera platform is used for analyzing.

Table 6

Water softening performance of the device under different electric fields

Cathode structureAverage removal hardness(mg/L)
Calcium carbonate removal(g)
HFDCHFDC
TP 305.80 311.87 14.68 14.97 
3 × 6TM 431.42 363.52 20.71 17.45 
4.5 × 6TM 360.15 237.25 17.29 11.39 
Cathode structureAverage removal hardness(mg/L)
Calcium carbonate removal(g)
HFDCHFDC
TP 305.80 311.87 14.68 14.97 
3 × 6TM 431.42 363.52 20.71 17.45 
4.5 × 6TM 360.15 237.25 17.29 11.39 

More bubbles escape from the mesh cathode than from the plate cathode. The effect of intermittently generated bubbles on the mesh cathode under HF power supply is more significant. The current rise effect of the plate cathode under DC power supply is not as significant as that of the mesh cathode, which indicates that the increase of bubbles is less than that of the mesh cathode, so the softening effect of the device does not change significantly.

The bubble movement behavior applied by a DC electric field is shown in Figure 7(a)–7(c), and bubble movement behavior applied by an HF electric field is shown in Figure 7(d)–7(f). The bubble movement diagrams show that the escape and generation of bubbles mainly exist in the longitudinal direction of the cathode. Compared with the bubble motion diagram under the HF electric field and the DC electric field, the number of bubbles generated in the DC electric field is too large, which causes the accumulation of bubbles to produce larger-diameter bubbles, which adhere to the surface of the cathode and affect the mass transfer process. In the HF electric field, the current is generated intermittently, which is conducive to the escape of bubbles. At the same time, the intermittent property of the pulsating electric field is beneficial to the uniformity of bubble diameter and the enhancement of mass transfer rate, as shown in Figure 8.

Figure 7

Bubble movement behavior.

Figure 7

Bubble movement behavior.

Figure 8

Bubbles under high frequency and DC electric field.

Figure 8

Bubbles under high frequency and DC electric field.

It was found that the bubbles mainly generated in the longitudinal direction of the plate and the bubbles were generated at the edge of the mesh cathode and the edge of the plate by observing the bubble movement. At the same time, the escape of bubbles mainly exists at the outermost section of the upper part of the electrode plate. The calcium carbonate growth graph was compared with the bubble movement graph. It was found that the growth of calcium carbonate was mainly along the movement direction of the bubbles. Because the 3 × 6TM cathode has more bubble growth areas than the 4.5 × 6TM cathode, the water softening effect is more significant, as shown in Figure 9.

Figure 9

Bubble detachment motion diagram.

Figure 9

Bubble detachment motion diagram.

Solution residence time analysis

As shown in Figure 10, the groups are grouped according to the solution residence time. Each group is arranged in the order of plate type TP, 3 × 6TM and 4.5 × 6TM. When the solution retention time is 30 min or 20 min, the removal rate of calcium carbonate and the average hardness removal rate are higher when the 3 × 6TM cathode is used. But when the solution residence time is 10 min, 4.5 × 6TM cathode is used, the removal rate of calcium carbonate and the average hardness removal rate are significantly higher than that of TP and 3 × 6TM cathodes.

Figure 10

Trends of RA and RC.

Figure 10

Trends of RA and RC.

As shown in Figure 11, when the solution residence time is increased from 10 minutes to 20 minutes, the circulating water volume increases, the amount of CO32− and Ca2+ supplemented in the solution increases. The mass transfer process between electrodes is intensified, and the removal of calcium carbonate from the plate increases greatly. When the residence time of the solution is 20 minutes, the mesh of the 3 × 6TM cathode has been filled with calcium carbonate crystals, while the 4.5 × 6TM has larger pores. When the residence time is reduced to 10 minutes, the calcium carbonate adsorption of the 3 × 6 TM cathode without mesh is reduced, the pore size of 4.5 × 6 TM cathode can provide the longitudinal growth space of calcium carbonate and maintain the excellent hardness removal efficiency.

Figure 11

Scales on cathode under different working conditions.

Figure 11

Scales on cathode under different working conditions.

Microscopic analysis of cathode fouling

It can be seen from the table that the mesh cathode used has better water softening performance than the plate cathode. (Table 7) Compared with the plate cathode used, the maximum hardness removal rate and the calcium carbonate removal amount is 208.15 mg/L and 12.01 g respectively when the mesh cathode is used. High magnification electron microscope, SEM and XRD and other technical means are used to further explore the internal growth mechanism of calcium carbonate on the cathode.

Table 7

Difference of RA and RC with different cathode structures

Solution residence timeRA difference between TP and 3 × 6 TMRA difference between TP and 4.5 × 6 TMRC difference between TP and 3 × 6 TMRC difference between TP and 4.5 × 6 TM
(min)mg/Lmg/Lgg
10 125.08 155.19 12.01 14.9 
20 143.33 87.7 6.88 4.21 
30 208.15 117.37 6.66 3.76 
Solution residence timeRA difference between TP and 3 × 6 TMRA difference between TP and 4.5 × 6 TMRC difference between TP and 3 × 6 TMRC difference between TP and 4.5 × 6 TM
(min)mg/Lmg/Lgg
10 125.08 155.19 12.01 14.9 
20 143.33 87.7 6.88 4.21 
30 208.15 117.37 6.66 3.76 

The calcium carbonate crystals deposited on 3 × 6 TM, 4.5 × 6 TM and TP are shown in Figure 12(a)–12(c) respectively, under a high magnification electron microscope at 120 times. Area 1, 2 in the Figure 12 are the morphologies of scales in different regions on the cathode, respectively. Figure 12(a) and 12(b) show that the morphology of calcium carbonate crystals formed by the mesh cathode is like fine sand, and the accumulation of calcium carbonate crystals forms a dense scale layer. In Figure 12(c), the tribes of calcium carbonate crystals formed by the plate cathode are scattered and sparse, and the crystals pile up to form clusters of scale.

Figure 12

High magnification electron microscope images, SEM and XRD of scales on TP, 3× 6 TM and 4.5× 6 TM.

Figure 12

High magnification electron microscope images, SEM and XRD of scales on TP, 3× 6 TM and 4.5× 6 TM.

Observed by SEM of the microscopic morphology of calcium carbonate on different cathodes, the degree of aggregation of calcium carbonate deposited on mesh cathodes in Figure 12(d) and 12(e) are relatively high, forming large crystals, and the crystals have more pores. At the same time, the large crystal surface has a small regular shape of calcium carbonate crystals. In Figure 12(f) of the plate-shaped cathode, calcium carbonate crystals are formed by the union of small regular-shaped crystals.

X-ray diffraction technology is used to explore the composition of calcium carbonate crystals. Figure 12(g)–12(i) indicate that the calcium carbonate cathodes with different cathodes are all calcite. The results show that the structure of the calcium carbonate crystals is mainly calcite, and it is difficult to form aragonite. The calcite does not fall off easily and the structure is stable. Therefore, the aggregation of calcium carbonate by the cathode is the key to softening the device.

The special structure of the mesh cathode provides space for the growth of calcium carbonate. The calcium carbonate crystals can form large porous crystals, which is conducive to the aggregation of calcium carbonate crystals. The surface of the plate cathode is composed of small calcium carbonate. Crystal aggregation leads to insufficient crystal stability, and calcium carbonate crystal growth is difficult.

The Box-Behnken model in the response surface method was used to optimize the calcium carbonate removal and the average removal hardness under the influence of the duty ratio of the high-frequency pulse power supply, the residence time of the solution in the reactor and the structures of the cathode.

The experimental results show that the removal rate and average hardness of calcium carbonate are the highest when the cathode structure, power duty ratio and solution residence time are 3 × 6TM, 20% and 20 min, respectively. The results show that the average hardness removal and calcium carbonate removal of the mesh cathode are 208.15 mg/L and 12.01 g higher than those of a plate cathode. The optimized softening amount per unit area of this experimental device is as high as 604.5 g/m2·h−1.

High-speed cameras were used to analyze movement of bubbles. The results show that bubbles are mainly generated in the longitudinal direction of the cathode. The bubbles are generated on the upper edge of the mesh of the mesh cathode. At the same time, the escape phenomenon occurs concentratedly at the highest section of the cathode.

Microscopic analysis shows that in the electrochemical water softening device, the main growth direction of calcium carbonate is along the longitudinal direction of the cathode. It is consistent with the direction in which bubbles are generated and dispersed. The special structure of the mesh cathode provides space for the growth of calcium carbonate, which is conducive to the softening effect of the device.

This work supported by Scientific Research Foundation of Wuhan Institute of Technology (K2021020) and Scientific Research Project of Hubei Provincial Department of Education (B2020050).

All relevant data are included in the paper or its Supplementary Information.

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