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Multi-meshes coupled cathodes enhanced performance of electrochemical water softening system
Separation and Purification Technology 217 (2019) 128–136
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Multi-meshes coupled cathodes enhanced performance of electrochemical water softening system
T
Jinxin Luana, Lida Wanga, Wen Suna, Xinhao Lia, Tianzhen Zhua, Yingzheng Zhoub, Haitao Dengb, ⁎ Shuai Chenb, Shaohui Heb, Guichang Liua, a b
Department of Chemical Engineering, Dalian University of Technology, No. 2 Linggong Road, Dalian 116024, China Chambroad Chemical Industry Research Institute Co., Ltd., Economic Development Zone, Boxing County, Binzhou 256500, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Cathode Electrochemical water softening Descaling rate Energy consumption Circulating cooling water
The high electrode area requirement limited the applications of electrochemical water softening technology, despite of conspicuous advantages. This work introduces an electrochemical water softening system with multimeshes coupled cathodes to address fouling in industrial circulating cooling water systems. Experimental data indicate that the coupled cathode increases the descaling rate of electrochemical water softening system due to the produced self-synergy effect based on its unique multilayer structure. The synergistic effect is constructed by separating and positioning the chemical reactions (alkalinity production and scale deposition) to different regions of the coupled cathode, thus a large amount of OH− ions from the internal layers and the preferentially deposited scale on external layers accelerate the subsequent scale deposition in later stage of the process. This system displays the reduced electrode area requirement and energy consumption depending on the increased descaling rate. This work provides a facile strategy for the development and applications of electrochemical water softening technology.
1. Introduction Scaling poses a big technical challenge and financial burden for the chemical industrial operators, especially in recirculating cooling water systems. It can reduce boiler power output by 10–20% and thermal efficiency by 10% [1–4], plug the pipeline [5,6], and even make more shut-downs [7]. Various water treatment methods have been utilized to stabilize water quality, such as the addition of chemicals or desalination. At present, many methods including acidification of the water, chemical precipitation for calcium carbonate and addition of chemical inhibitors have been applied in fouling prevention [8–10]. Besides, desalination techniques are also adopted frequently to stabilize water quality including exchange resins [11,12], membrane separation (reverse osmosis, nanofiltration, electrodialysis etc.) [13–16], electrodeionization [17], electrocoagulation [18–20] and electrochemical water softening techniques [21,22]. Among these methods, electrochemical water softening techniques have attracted great attentions in recent years due to its environmental compatibility and versatility [23,24]. However, the large demand for cathode surface area seriously hinders their applications [25,26]. Many efforts have been made to solve this problem. One is increasing the specific surface area of water
⁎
softening reaction. For example, Hasson et al. developed the novel electrochemical seeds system which greatly increased the water softening reaction area [25]. This was achieved by directing the precipitation to occur on seeds surfaces in the crystallizer rather than on the cathode. Zhi et al. proposed a novel electrochemical system by combining the conventional electrocoagulation and electrochemical precipitation processes [27,28]. The polymer formed by electroflocculation provided a deposition surface for electrochemical precipitation. Rinat et al. used a porous aerogel carbon electrode to remove CaCO3 in water, and proposed to use inexpensive materials with high surface area in an electrochemical device [29]. The other is improving the descaling rate to reduce the electrode area requirement of the system. For example, Euvrard et al. showed that the nature of the metallic substrate had a strong influence on the descaling rate, and scale had a higher nucleation and growth rate on copper substrate than steel substrate [30]. Zaslavschi et al. proposed a higher descaling rate system with a bipolar membrane [31]. The system was divided into an acidic water chamber and an alkaline one. The highly alkaline environment in the alkaline chamber accelerated the deposition of scale. In addition, Yang et al. demonstrated that mirror stainless steel cathodes had higher descaling rate by delaying cathode deactivation,
Corresponding author. E-mail address: [email protected] (G. Liu).
https://doi.org/10.1016/j.seppur.2019.01.054 Received 24 December 2018; Received in revised form 22 January 2019; Accepted 22 January 2019 Available online 23 January 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.
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because smooth surface reduced the scale adhesion and caused a spontaneous scale detachment [32]. All in all, although the electrochemical water softening technology had made a certain progress, a series works on this technology are still in progress focusing on strategies that how to increase the specific surface area or the descaling rate of cathode and clean the deactivated cathode. Electrochemical water softening technology is based on a heterogeneous reaction occurred on the surface of the cathode. The reaction processes can be divided into two categories mainly including alkalinity generation and scale deposition. The two processes compete with each other when the cathode is working. Once the cathode surface is totally covered by the scale, cathode deactivation occurs [32]. This work aims at describing a multi-meshes coupled cathode which greatly weaken the competition of the above two processes. A novel electrochemical water softening system with these cathodes achieves a higher descaling rate and lower energy consumption. The system overcomes the limitations of traditional electrochemical systems and provides a facile strategy for the development and applications of electrochemical water softening technology.
Fig. 2. Experimental setup.
2.3. Feed solutions There were two kinds of feed solutions. One was calcium solution whose total hardness was almost calcium hardness. This solution was prepared by dissolving analytical grade salts of CaCl2 and NaHCO3 in deionized water. Both the total hardness and the alkalinity were around 350 mg/L as CaCO3. The other was simulated circulating cooling water which was prepared according to the composition of the circulating water (Chinese Shan Dong Chambroad Holding Group Co., Ltd.). The solution was prepared by dissolving analytical grade CaCl2, NaCl, MgSO4 and NaHCO3 in deionized water. The total hardness was around 870 mg/L as CaCO3, and the alkalinity was only about 450 mg/L as CaCO3.
2. Materials and method 2.1. Multi-meshes coupled cathode In this work, multi-meshes coupled cathode was used as alternative one of stainless steel plate cathode. The cathode consisted of 7 layers of SS 304 woven nets with different mesh sizes. The mesh numbers were 8, 12, 20 and 50 respectively. The 20 and 12 mesh nets were mainly used for the deposition of scales. The 50 mesh net was used to generate alkalinity, and 8 mesh nets were used to provide pathway for the diffusion of ions. The entire cathode was centered on the 50 mesh net. The 8, 12 and 20 mesh nets were sequentially arranged on both sides of 50 mesh net symmetrically. The seven layers with different mesh were tightly held together to build this cathode, as shown in Fig. 1.
2.4. Analysis methods The descaling rate was evaluated by measuring the hardness of water in the reservoir. The total hardness as calcium carbonate was determined by EDTA titrimetric method. The total alkalinity of the solution sample was determined according to ASTM standard method D1067 by HCl titration to the end point of pH 4.3. Conductivity and pH values were measured using a conductivity meter (FiveEasy FE30, Mettler Toledo, resolution: 0.01 μS/cm) and a pH/ISE meter (PXSJ-216, Inesa, China). Repeat experiments were carried out so as to ascertain the accuracy of the results. Ryznar stability index (RSI) was used to evaluate the scaling potential of the water in the reservoir simultaneously [33,34].
2.2. Experimental setup The experimental setup is schematically shown in Fig. 2. The reactor had a working volume of 3.2L. Dimensional stable anodes (DSA) served as the anodes of the system. Seven anodes with dimension of 13.6 × 10 cm and six cathodes in same size were arranged in parallel in the reactor. The spacing between each adjacent cathode and anode was kept at 0.8 cm. Water to be treated entered the reactor from the lower part of the reactor, flowed parallel to the surface of the cathodes and anodes, and exited from the upper part. A reservoir was used to store 50 L circulation solutions, and the solution temperature was maintained at 25 °C using an electric heater. The current density is obtained though dividing the current by the apparent area of the cathode. The apparent area of the cathode (not the specific surface area of all layers) is obtained by the length and width of the single mesh layer. The current density was selected in the range of 24.5 to 61.3 A/m2, and the feed flow rate was selected in the range of 50 to 500 L/h.
3. Results and discussion 3.1. Effect of the flow rate on electrochemical water softening process Electrochemical water softening experiments were carried out at 18.4 A/m2 and 25 °C. Fig. 3a-b shows the changes in calcium solution composition over time at different flow rates. It can be seen that both the hardness and alkalinity dropped from 350 mg/l to below 100 after 480 min. RSI rose from 5.2 to above 8.5 due to the decrease in hardness and alkalinity (Fig. 3c), which means that the water quality changed from high sedimentation to high corrosion. An important result is observed in Fig. 3, showing for the descaling rate increased firstly and then decreased with increasing flow rate, and the highest descaling rate was obtained at a flow rate of 300 L/h. This phenomenon could be explained as follows. The higher flow rate improved the descaling rate by increasing the mass transfer rate of hardness and alkalinity when the flow rate was lower than 300 L/h. However, the alkaline environment on cathode surface tended to be disturbed after the feed rate exceeded 300 L/h and the scale could not be deposited quickly. This work also examined the descaling rate in simulated circulating
Fig. 1. Schematic diagram of an expanded view of the cathode (a) 20 mesh net; (b) 12 mesh net; (c) 8 mesh net; (d) 50 mesh net. 129
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Fig. 3. Effects of flow rate on changes in total hardness (a), alkalinity (b) and RSI (C) of calcium solution.
Fig. 4. Effects of flow rate on changes in total hardness (a), alkalinity (b) and RSI (C) of simulated circulating cooling water.
decreased from 450 mg/L to 0, and RSI rose from 4.8 to above 14. The decline rate of hardness and alkalinity also increased firstly and then decreased with the increase of flow rate. The descaling rate reached maximum at a flow rate of 400 L/h. Moreover, the descaling rate in the simulated circulating water was less affected by the flow rate. This also
cooling water at the same current density and temperature. Fig. 4 shows the change of the solution composition over time at different flow rates. It can be clearly seen that the hardness decreased from 850 mg/L to about 400 after 480 min. The descaling rate in the simulated circulating water was higher than that in the calcium solution. The alkalinity 130
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Fig. 5. Effects of current density on changes in total hardness (a), alkalinity (b) and RSI (C) of calcium solution.
proves that a wider range of flow rates could be selected in high concentration solutions.
3.3. Descaling rate and energy consumption of electrochemical water softening system The required electrode area and energy consumption (kWh/kg CaCO3) are major parameters for characterizing the performance of electrochemical water softening system. Electrode area requirement is closely related to the descaling rate (gCaCO3/m2h). In order to explore the performance parameters of this system, 50 L of calcium solution and simulated circulating water were treated to non-sedimentation and noncorrosion state. The descaling rate and energy consumption were also characterized, as shown in Fig. 7 and Fig. 8. Fig. 7 shows the descaling rate and energy consumption versus flow rate for calcium solution and simulated circulating water. As the flow rate increased, the descaling rate increased firstly, and then decreased after reaching the maximum value. The descaling rate was up to 25.9 g/ m2h in the calcium solution, and the energy consumption was only 3.25 kWh/kg CaCO3 when the flow rate was 300 L/h. The descaling rate reached a maximum of 39.1 g/m2h at 400 L/h, and the minimum energy consumption approached to 1.45 kWh/kg CaCO3 in simulated circulating water. It was feasible to set the flow rate to 300–400 L/h to treat water with a hardness of 350 to 850 mg/L CaCO3. Fig. 8 shows the descaling rate and energy consumption versus current density for calcium solution and simulated circulating water. It can be clearly seen that the descaling rate and energy consumption were substantially linear with increasing current density. As anticipated, the increase of current density increased the descaling rate and reduced the required cathode area. However, the energy consumption was gradually augmented as the current density increased. More importantly, the rate of increase in descaling rate and energy consumption with increasing current density was not consistent. The slope of the descaling rate - current density curve was less than that of the energy consumption - current density curve in both calcium solution and simulated circulating water. This is because the current efficiency decreased as the current density increased, which means that more and more electricity was wasted [26,35].
3.2. Effect of current density on electrochemical water softening process It is well known that current density is a crucial factor that affects the treatment efficiency of electrochemical water softening system. Experiments were carried out at 300 L/h and 25 °C. Fig. 5 shows the change of the total hardness and alkalinity with respect to time in the calcium solution. It can be seen that the higher the current density, the faster the decline rate of hardness and alkalinity. However, the change in hardness was gradually reduced as the current density increased, especially at a current density between 18.4 and 30.6 A/m2. The excess of OH− ions induced by increased current density did not contribute to the scale formation, because the descaling rates were limited by the convective diffusion kinetics of calcium and hydrogen carbonate ions towards the reaction zone. Therefore, the scaling tendency was practically suppressed in treating water with a low hardness and alkalinity. Meanwhile, a higher current intensity would not improve the cost-effectiveness of the system significantly. These experiments were also carried out in simulated circulating water. As shown in Fig. 6 the decline rate in hardness and alkalinity in simulated circulating water was faster than that in calcium solution. It is important to note that at a current density of 18.4, 24.5 and 30.6 A/ m2, the hardness dropped to 360 mg/L as CaCO3 after 480 min. The alkalinity dropped below 50 mg/L as CaCO3 after 350 min and dropped to 0 at the end of the experiments. Since the alkalinity determined the max treatment capacity of this system in simulated circulating water, prolonging the electrolysis time could not increase the total amount of deposited scale. Therefore, the appropriate current density and running time should be selected depending on the hardness and alkalinity of the water to be treated.
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Fig. 6. Effects of current density on changes in total hardness (a), alkalinity (b) and RSI (C) of simulated circulating cooling water.
reached 1000 g/m2h. Therefore, membrane-free water softening systems also need to be further optimized to achieve better performances.
Usually, there is a limiting current density in conventional water softening systems beyond which the descaling rate remains constant [21]. However, the limiting current density did not appear in this system. It can be attributed to the fact that the limiting current density of the water softening system was greatly improved thanks to the large specific surface area of the cathode. Table 1 presents the performance comparison between this work and other published papers [21,26,28,31,32,35]. It can be seen that a better water softening performance was achieved by this work in the membrane-free water softening systems. For example, when the calcium solution was treated, the highest descaling rate reached 29.16 g/ m2h, and the energy consumption was down to 6.0 kWh/kg CaCO3. The energy consumption was much less than those reported in other works at the same descaling rate. From the above results, this system displayed a higher descaling rate under the same energy consumption, which demonstrates that the electrode area required for the system was greatly reduced under the same amount of water treatment. However, water softening systems with ion exchange membranes (Ref. [31] and Ref. [35]) achieved better performance, and the highest descaling rates
3.4. Cathodic working principle The core of the electrochemical water softening process is to create a high pH environment on the cathode surface by water and oxygen reduction reactions. At the same time, the scale precipitation reactions also occur on the boundary layer of the cathode surface. The reactions for generating alkalinity are: 2H2O + O2 + 4e− → 4OH− −
−
2H2O + 2e → H2↑ + 2OH (main cathode reaction)
(1) (2)
The scale precipitation reactions are: Ca
2+
Mg
+ HCO3− + OH− → CaCO3↓ + H2O
2+
−
+ 2OH → Mg(OH)2 ↓
Fig. 7. Effect of flow rate on descaling rate and energy consumption (a) Calcium solution; (b) Simulated circulating cooling water. 132
(3) (4)
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Fig. 8. Effect of current density on descaling rate and energy consumption (a) Calcium solution; (b) Simulated circulating cooling water.
cathode dominated the alkalinity production. From the above analysis, the working principle of the multi-meshes coupled cathode can be described as follows: as shown in Fig. 12, at the initial stage of water softening process, the reactions for generating alkalinity occurred mainly on external layers of the coupled cathode due to the lower potential. The highest OH− ions concentration was produced on the surfaces of external layers, and these OH− ions simultaneously diffused toward the bulk solution outside of the cathode and the internal layers of the cathode. The calcium and magnesium ions in the bulk solution migrated to the surfaces of external layers of the coupled cathode under the action of an electric field. The scale precipitation reactions occurred preferentially on these layers. However, the external layers were gradually covered by scale as the water softening process proceeds. The reactions for generating alkalinity were gradually shifted from the external layers to the internal layers of the cathode. The highest OH− ions concentration was produced on internal layers of the cathode at later stage of water softening process. A large amount of OH− ions diffused from the internal layers toward the bulk solution outside of the cathode, and calcium and magnesium ions migrated toward the internal layers of the cathode from the bulk solution outside. These ions contacted and reacted near the external layers of the cathode. More importantly, the preferentially deposited scale on external layers provided a nucleation surface which accelerated the subsequent scale deposition [21]. Overall, the coupled cathode greatly weakened the competition between the reactions for generating alkalinity and scale precipitation reactions, thus built a self-synergistic effect through their different functions of external layers and internal layers of the cathode. That is to say, the synergistic effect was constructed by separating and positioning the chemical reactions (alkalinity production and scale deposition) to different regions of the coupled cathode. Once the external layers of the cathode were covered by scale, the alkalinity was generated on the clean internal layers. This process also ensured a high alkaline environment in the cathode region thus reduced the rate of cathode deactivation and energy consumption. The deposition of scale occurred mainly on the external layers of the
Fig. 9 and Fig. 10 show series of images of the multi-meshes coupled cathode and corresponding wire meshes before and after electrochemical deposition. It can be seen that a large amount of white scale layer deposited on the cathode surface after the softening process. The amount of scale on each layer of cathode was also different. It appears that a large amount of scale deposited on external layers of the coupled cathode (20 mesh and 12 mesh nets, Fig. 10b.d), and almost no scale appeared on internal layers of the cathode (8 mesh and 50 mesh nets, Fig. 10f.h). It indicates that there existed apparent differences in the roles played by each layer of the cathode during the water softening process. In order to illustrate the role of the cathode, the surface potentials of each layer were monitored during the water softening process. Potentials of each layer of the cathode are shown in Fig. 11. The potentials of external layers of the coupled cathode were lower than those of internal layers at the beginning of the water softening process. Particularly, the potentials of 20 mesh nets were the lowest (−1.992 V), and the potentials of 50 mesh nets were the highest (−1.530 V). A lower potential represents a higher current density together with a higher rate of cathodic reactions that produce alkalinity. Thus, the rate of generating alkalinity on 20 mesh nets was the fastest, indicating that the concentration of OH− ions centered on these layers was the highest, and the scale precipitation reactions mainly occurred in the vicinity of these layers. What’s more, the potentials together with current densities of the 20 mesh and 12 mesh nets (line 1 and line 2) were almost constant during the electrolysis process. However, the surfaces of external layers (20 mesh and 12 mesh nets) were gradually covered with scale, and the area for generating alkalinity was reduced, and the current through the external layers was reduced. This means that the rate of producing OH− ions on external layers of the cathode decreased. In addition, the potentials of the 8 and 50 mesh nets (Line 3 and Line 4) gradually decreased. This shows that the current densities of these layers gradually increased. As can be seen from Fig. 10, almost no scale deposited on the surfaces of internal layers (8 mesh and 50 mesh nets). So, the increased current densities indicate that the internal layer of the Table 1 Water softening performance comparison of this work with other papers. Hardness (mg/L as CaCO3)
Current density (A/m2)
Descaling rate (g/m2h)
Energy consumption (kWh/kg CaCO3)
Membrane electrolysis
Ref.
250 240 350 1180 300 350 350 870 6540 1600 1500
20 ∼20 20 108 100 100 18.3 30.6 250 100 600
2.6 3.4–6.9 8–15.09 22.8 10 25.5–34.3 29.16 42.32 530–570 460 1000–1280
20 ∼25 3.6 16 118 17.6–22.3 6 4.1 1.7 2.8–3.0 6.6
No No No No No No No No Yes Yes Yes
[26] [21] [32] [26] [28] [32] This work This work [31] [31] [35]
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Fig. 9. Typical images of the cathode before (a) and after (b) water softening process.
Fig. 10. Typical images of each net in the cathode before (a.c.e.g) and after (b.d.f.h) water softening process. (a.b) 20 mesh net; (c.d) 12 mesh net; (e.f) 8 mesh net; (g.h) 50 mesh net.
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Fig. 11. Potentials of each layer of the cathode.
Fig. 13. Voltage changes during water softening.
cathode throughout the water softening process. A large amount of OH− ions from the internal layers and the preferentially deposited scale on external layers accelerated the subsequent scale deposition in later stage of the process. For conventional electrochemical water softening systems, a higher descaling rate resulted in a faster cathode deactivation rate and more frequent electrode cleaning. While the system with multi-meshes coupled cathodes displayed a higher descaling rate, the cathode deactivation was not accelerated. In order to verify this effect, water softening experiments were performed at the same constant current using a system with multi-meshes coupled cathodes and a system with stainless steel plate cathodes of the same area. A certain proportion of calcium chloride and sodium bicarbonate were continuously added to ensure the conductivity of the water unchanged. Fig. 13 shows the change in voltage during the experiment. It can be clearly seen that the voltage of the system using multi-meshes coupled cathodes was significantly lower than that of conventional system. This was one of the reasons why the system displayed lower energy consumption. In addition, the voltage in the system gradually increased as the cathodes deactivated due to the deposition of scale on the surface. The rise of these two system’s voltage was almost the same which illustrated that the rate of cathode deactivation was not faster in the system with the coupled cathodes compared to that with stainless steel plate cathodes despite the higher descaling rate. This also indicates that the coupled cathode
could carry more scale and avoid frequent cathode cleaning. However, the efficient cleaning and regeneration of inactivated cathodes is also major technical challenge for industrial applications. There are many methods for regeneration of stainless steel plate cathode, such as mechanical scraping, polarity reversal, ultrasonic, air-scoured washing and acid washing [32,36–40]. This work describes multi-meshes coupled cathode that greatly increased the descaling rate and reduced energy consumption of the electrochemical water softening system. Suitable cathode regeneration technology is under investigation to match with this system. 4. Conclusions This work describes an electrochemical water softening system with multi-meshes coupled cathode. The coupled cathode greatly improves the performance of the system, which is manifested by a significant increase in the descaling rate and a great reduction in energy consumption. The improved performance is attributed to the self-synergistic effect of the coupled cathode which is constructed by separating and positioning the chemical reactions (alkalinity production and scale deposition) to different regions of the coupled cathode, namely, a large amount of OH− ions from the internal layers and the preferentially deposited scale on external layers accelerates the subsequent scale deposition in later stage of the process. This work provides a facile
Fig. 12. Schematic illustration of working principle of the coupled cathode. 135
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strategy for the development and applications of electrochemical water softening technology.
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Acknowledgements This study has been partially supported by Chinese Shan Dong Chambroad Holding Group Co., Ltd.. The authors thank the referee for his valuable comments. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.seppur.2019.01.054. References [1] Y. Pan, F. Si, Z. Xu, C.E. Romero, An integrated theoretical fouling model for convective heating surfaces in coal-fired boilers, Powder Technol. 210 (2011) 150–156. [2] Y. Zarga, H. Ben Boubaker, N. Ghaffour, H. Elfil, Study of calcium carbonate and sulfate co-precipitation, Chem. Eng. Sci. 96 (2013) 33–41. [3] E. Mavredaki, E. Neofotistou, K.D. Demadis, Inhibition and dissolution as dual mitigation approaches for colloidal silica fouling and deposition in process water systems: functional synergies, Ind. Eng. Chem. Res. 44 (2005) 7019–7026. [4] I. Nishida, Y. Shimada, T. Saito, Y. Okaue, T. Yokoyama, Effect of aluminum on the deposition of silica scales in cooling water systems, J. Colloid Interf. Sci. 335 (2009) 18–23. [5] K.S. Brastad, Z. He, Water softening using microbial desalination cell technology, Desalination 309 (2013) 32–37. [6] Y. Zeng, C. Yang, W. Pu, X. Zhang, Removal of silica from heavy oil wastewater to be reused in a boiler by combining magnesium and zinc compounds with coagulation, Desalination 216 (2007) 147–159. [7] C.H. Koo, A.W. Mohammad, F. Suja’, Recycling of oleochemical wastewater for boiler feed water using reverse osmosis membranes - A case study, Desalination 271 (2011) 178–186. [8] J. MacAdam, S.A. Parsons, Calcium carbonate scale formation and control, Rev. Environ. Sci. Bio/Technology 3 (2004) 159–169. [9] H. Li, M.K. Hsieh, S.H. Chien, J.D. Monnell, D.A. Dzombak, R.D. Vidic, Control of mineral scale deposition in cooling systems using secondary-treated municipal wastewater, Water Res 45 (2011) 748–760. [10] T. Neveux, M. Bretaud, N. Chhim, K. Shakourzadeh, S. Rapenne, Pilot plant experiments and modeling of CaCO3growth inhibition by the use of antiscalant polymers in recirculating cooling circuits, Desalination 397 (2016) 43–52. [11] A. Dąbrowski, Z. Hubicki, P. Podkościelny, E. Robens, Selective removal of the heavy metal ions from waters and industrial wastewaters by ion-exchange method, Chemosphere 56 (2004) 91–106. [12] K. Vaaramaa, J. Lehto, Removal of metals and anions from drinking water by ion exchange, Desalination 155 (2003) 157–170. [13] J.B. Lee, K.K. Park, H.M. Eum, C.W. Lee, Desalination of a thermal power plant wastewater by membrane capacitive deionization, Desalination 196 (2006) 125–134. [14] L.F. Greenlee, D.F. Lawler, B.D. Freeman, B. Marrot, P. Moulin, Reverse osmosis desalination: water sources, technology, and today’s challenges, Water Res 43 (2009) 2317–2348. [15] J.S. Park, J.H. Song, K.H. Yeon, S.H. Moon, Removal of hardness ions from tap water using electromembrane processes, Desalination 202 (2007) 1–8. [16] H. Strathmann, Electrodialysis, a mature technology with a multitude of new applications, Desalination 264 (2010) 268–288.
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Multi-meshes coupled cathodes enhanced performance of electrochemical water softening system
T
Jinxin Luana, Lida Wanga, Wen Suna, Xinhao Lia, Tianzhen Zhua, Yingzheng Zhoub, Haitao Dengb, ⁎ Shuai Chenb, Shaohui Heb, Guichang Liua, a b
Department of Chemical Engineering, Dalian University of Technology, No. 2 Linggong Road, Dalian 116024, China Chambroad Chemical Industry Research Institute Co., Ltd., Economic Development Zone, Boxing County, Binzhou 256500, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Cathode Electrochemical water softening Descaling rate Energy consumption Circulating cooling water
The high electrode area requirement limited the applications of electrochemical water softening technology, despite of conspicuous advantages. This work introduces an electrochemical water softening system with multimeshes coupled cathodes to address fouling in industrial circulating cooling water systems. Experimental data indicate that the coupled cathode increases the descaling rate of electrochemical water softening system due to the produced self-synergy effect based on its unique multilayer structure. The synergistic effect is constructed by separating and positioning the chemical reactions (alkalinity production and scale deposition) to different regions of the coupled cathode, thus a large amount of OH− ions from the internal layers and the preferentially deposited scale on external layers accelerate the subsequent scale deposition in later stage of the process. This system displays the reduced electrode area requirement and energy consumption depending on the increased descaling rate. This work provides a facile strategy for the development and applications of electrochemical water softening technology.
1. Introduction Scaling poses a big technical challenge and financial burden for the chemical industrial operators, especially in recirculating cooling water systems. It can reduce boiler power output by 10–20% and thermal efficiency by 10% [1–4], plug the pipeline [5,6], and even make more shut-downs [7]. Various water treatment methods have been utilized to stabilize water quality, such as the addition of chemicals or desalination. At present, many methods including acidification of the water, chemical precipitation for calcium carbonate and addition of chemical inhibitors have been applied in fouling prevention [8–10]. Besides, desalination techniques are also adopted frequently to stabilize water quality including exchange resins [11,12], membrane separation (reverse osmosis, nanofiltration, electrodialysis etc.) [13–16], electrodeionization [17], electrocoagulation [18–20] and electrochemical water softening techniques [21,22]. Among these methods, electrochemical water softening techniques have attracted great attentions in recent years due to its environmental compatibility and versatility [23,24]. However, the large demand for cathode surface area seriously hinders their applications [25,26]. Many efforts have been made to solve this problem. One is increasing the specific surface area of water
⁎
softening reaction. For example, Hasson et al. developed the novel electrochemical seeds system which greatly increased the water softening reaction area [25]. This was achieved by directing the precipitation to occur on seeds surfaces in the crystallizer rather than on the cathode. Zhi et al. proposed a novel electrochemical system by combining the conventional electrocoagulation and electrochemical precipitation processes [27,28]. The polymer formed by electroflocculation provided a deposition surface for electrochemical precipitation. Rinat et al. used a porous aerogel carbon electrode to remove CaCO3 in water, and proposed to use inexpensive materials with high surface area in an electrochemical device [29]. The other is improving the descaling rate to reduce the electrode area requirement of the system. For example, Euvrard et al. showed that the nature of the metallic substrate had a strong influence on the descaling rate, and scale had a higher nucleation and growth rate on copper substrate than steel substrate [30]. Zaslavschi et al. proposed a higher descaling rate system with a bipolar membrane [31]. The system was divided into an acidic water chamber and an alkaline one. The highly alkaline environment in the alkaline chamber accelerated the deposition of scale. In addition, Yang et al. demonstrated that mirror stainless steel cathodes had higher descaling rate by delaying cathode deactivation,
Corresponding author. E-mail address: [email protected] (G. Liu).
https://doi.org/10.1016/j.seppur.2019.01.054 Received 24 December 2018; Received in revised form 22 January 2019; Accepted 22 January 2019 Available online 23 January 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.
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because smooth surface reduced the scale adhesion and caused a spontaneous scale detachment [32]. All in all, although the electrochemical water softening technology had made a certain progress, a series works on this technology are still in progress focusing on strategies that how to increase the specific surface area or the descaling rate of cathode and clean the deactivated cathode. Electrochemical water softening technology is based on a heterogeneous reaction occurred on the surface of the cathode. The reaction processes can be divided into two categories mainly including alkalinity generation and scale deposition. The two processes compete with each other when the cathode is working. Once the cathode surface is totally covered by the scale, cathode deactivation occurs [32]. This work aims at describing a multi-meshes coupled cathode which greatly weaken the competition of the above two processes. A novel electrochemical water softening system with these cathodes achieves a higher descaling rate and lower energy consumption. The system overcomes the limitations of traditional electrochemical systems and provides a facile strategy for the development and applications of electrochemical water softening technology.
Fig. 2. Experimental setup.
2.3. Feed solutions There were two kinds of feed solutions. One was calcium solution whose total hardness was almost calcium hardness. This solution was prepared by dissolving analytical grade salts of CaCl2 and NaHCO3 in deionized water. Both the total hardness and the alkalinity were around 350 mg/L as CaCO3. The other was simulated circulating cooling water which was prepared according to the composition of the circulating water (Chinese Shan Dong Chambroad Holding Group Co., Ltd.). The solution was prepared by dissolving analytical grade CaCl2, NaCl, MgSO4 and NaHCO3 in deionized water. The total hardness was around 870 mg/L as CaCO3, and the alkalinity was only about 450 mg/L as CaCO3.
2. Materials and method 2.1. Multi-meshes coupled cathode In this work, multi-meshes coupled cathode was used as alternative one of stainless steel plate cathode. The cathode consisted of 7 layers of SS 304 woven nets with different mesh sizes. The mesh numbers were 8, 12, 20 and 50 respectively. The 20 and 12 mesh nets were mainly used for the deposition of scales. The 50 mesh net was used to generate alkalinity, and 8 mesh nets were used to provide pathway for the diffusion of ions. The entire cathode was centered on the 50 mesh net. The 8, 12 and 20 mesh nets were sequentially arranged on both sides of 50 mesh net symmetrically. The seven layers with different mesh were tightly held together to build this cathode, as shown in Fig. 1.
2.4. Analysis methods The descaling rate was evaluated by measuring the hardness of water in the reservoir. The total hardness as calcium carbonate was determined by EDTA titrimetric method. The total alkalinity of the solution sample was determined according to ASTM standard method D1067 by HCl titration to the end point of pH 4.3. Conductivity and pH values were measured using a conductivity meter (FiveEasy FE30, Mettler Toledo, resolution: 0.01 μS/cm) and a pH/ISE meter (PXSJ-216, Inesa, China). Repeat experiments were carried out so as to ascertain the accuracy of the results. Ryznar stability index (RSI) was used to evaluate the scaling potential of the water in the reservoir simultaneously [33,34].
2.2. Experimental setup The experimental setup is schematically shown in Fig. 2. The reactor had a working volume of 3.2L. Dimensional stable anodes (DSA) served as the anodes of the system. Seven anodes with dimension of 13.6 × 10 cm and six cathodes in same size were arranged in parallel in the reactor. The spacing between each adjacent cathode and anode was kept at 0.8 cm. Water to be treated entered the reactor from the lower part of the reactor, flowed parallel to the surface of the cathodes and anodes, and exited from the upper part. A reservoir was used to store 50 L circulation solutions, and the solution temperature was maintained at 25 °C using an electric heater. The current density is obtained though dividing the current by the apparent area of the cathode. The apparent area of the cathode (not the specific surface area of all layers) is obtained by the length and width of the single mesh layer. The current density was selected in the range of 24.5 to 61.3 A/m2, and the feed flow rate was selected in the range of 50 to 500 L/h.
3. Results and discussion 3.1. Effect of the flow rate on electrochemical water softening process Electrochemical water softening experiments were carried out at 18.4 A/m2 and 25 °C. Fig. 3a-b shows the changes in calcium solution composition over time at different flow rates. It can be seen that both the hardness and alkalinity dropped from 350 mg/l to below 100 after 480 min. RSI rose from 5.2 to above 8.5 due to the decrease in hardness and alkalinity (Fig. 3c), which means that the water quality changed from high sedimentation to high corrosion. An important result is observed in Fig. 3, showing for the descaling rate increased firstly and then decreased with increasing flow rate, and the highest descaling rate was obtained at a flow rate of 300 L/h. This phenomenon could be explained as follows. The higher flow rate improved the descaling rate by increasing the mass transfer rate of hardness and alkalinity when the flow rate was lower than 300 L/h. However, the alkaline environment on cathode surface tended to be disturbed after the feed rate exceeded 300 L/h and the scale could not be deposited quickly. This work also examined the descaling rate in simulated circulating
Fig. 1. Schematic diagram of an expanded view of the cathode (a) 20 mesh net; (b) 12 mesh net; (c) 8 mesh net; (d) 50 mesh net. 129
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Fig. 3. Effects of flow rate on changes in total hardness (a), alkalinity (b) and RSI (C) of calcium solution.
Fig. 4. Effects of flow rate on changes in total hardness (a), alkalinity (b) and RSI (C) of simulated circulating cooling water.
decreased from 450 mg/L to 0, and RSI rose from 4.8 to above 14. The decline rate of hardness and alkalinity also increased firstly and then decreased with the increase of flow rate. The descaling rate reached maximum at a flow rate of 400 L/h. Moreover, the descaling rate in the simulated circulating water was less affected by the flow rate. This also
cooling water at the same current density and temperature. Fig. 4 shows the change of the solution composition over time at different flow rates. It can be clearly seen that the hardness decreased from 850 mg/L to about 400 after 480 min. The descaling rate in the simulated circulating water was higher than that in the calcium solution. The alkalinity 130
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Fig. 5. Effects of current density on changes in total hardness (a), alkalinity (b) and RSI (C) of calcium solution.
proves that a wider range of flow rates could be selected in high concentration solutions.
3.3. Descaling rate and energy consumption of electrochemical water softening system The required electrode area and energy consumption (kWh/kg CaCO3) are major parameters for characterizing the performance of electrochemical water softening system. Electrode area requirement is closely related to the descaling rate (gCaCO3/m2h). In order to explore the performance parameters of this system, 50 L of calcium solution and simulated circulating water were treated to non-sedimentation and noncorrosion state. The descaling rate and energy consumption were also characterized, as shown in Fig. 7 and Fig. 8. Fig. 7 shows the descaling rate and energy consumption versus flow rate for calcium solution and simulated circulating water. As the flow rate increased, the descaling rate increased firstly, and then decreased after reaching the maximum value. The descaling rate was up to 25.9 g/ m2h in the calcium solution, and the energy consumption was only 3.25 kWh/kg CaCO3 when the flow rate was 300 L/h. The descaling rate reached a maximum of 39.1 g/m2h at 400 L/h, and the minimum energy consumption approached to 1.45 kWh/kg CaCO3 in simulated circulating water. It was feasible to set the flow rate to 300–400 L/h to treat water with a hardness of 350 to 850 mg/L CaCO3. Fig. 8 shows the descaling rate and energy consumption versus current density for calcium solution and simulated circulating water. It can be clearly seen that the descaling rate and energy consumption were substantially linear with increasing current density. As anticipated, the increase of current density increased the descaling rate and reduced the required cathode area. However, the energy consumption was gradually augmented as the current density increased. More importantly, the rate of increase in descaling rate and energy consumption with increasing current density was not consistent. The slope of the descaling rate - current density curve was less than that of the energy consumption - current density curve in both calcium solution and simulated circulating water. This is because the current efficiency decreased as the current density increased, which means that more and more electricity was wasted [26,35].
3.2. Effect of current density on electrochemical water softening process It is well known that current density is a crucial factor that affects the treatment efficiency of electrochemical water softening system. Experiments were carried out at 300 L/h and 25 °C. Fig. 5 shows the change of the total hardness and alkalinity with respect to time in the calcium solution. It can be seen that the higher the current density, the faster the decline rate of hardness and alkalinity. However, the change in hardness was gradually reduced as the current density increased, especially at a current density between 18.4 and 30.6 A/m2. The excess of OH− ions induced by increased current density did not contribute to the scale formation, because the descaling rates were limited by the convective diffusion kinetics of calcium and hydrogen carbonate ions towards the reaction zone. Therefore, the scaling tendency was practically suppressed in treating water with a low hardness and alkalinity. Meanwhile, a higher current intensity would not improve the cost-effectiveness of the system significantly. These experiments were also carried out in simulated circulating water. As shown in Fig. 6 the decline rate in hardness and alkalinity in simulated circulating water was faster than that in calcium solution. It is important to note that at a current density of 18.4, 24.5 and 30.6 A/ m2, the hardness dropped to 360 mg/L as CaCO3 after 480 min. The alkalinity dropped below 50 mg/L as CaCO3 after 350 min and dropped to 0 at the end of the experiments. Since the alkalinity determined the max treatment capacity of this system in simulated circulating water, prolonging the electrolysis time could not increase the total amount of deposited scale. Therefore, the appropriate current density and running time should be selected depending on the hardness and alkalinity of the water to be treated.
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Fig. 6. Effects of current density on changes in total hardness (a), alkalinity (b) and RSI (C) of simulated circulating cooling water.
reached 1000 g/m2h. Therefore, membrane-free water softening systems also need to be further optimized to achieve better performances.
Usually, there is a limiting current density in conventional water softening systems beyond which the descaling rate remains constant [21]. However, the limiting current density did not appear in this system. It can be attributed to the fact that the limiting current density of the water softening system was greatly improved thanks to the large specific surface area of the cathode. Table 1 presents the performance comparison between this work and other published papers [21,26,28,31,32,35]. It can be seen that a better water softening performance was achieved by this work in the membrane-free water softening systems. For example, when the calcium solution was treated, the highest descaling rate reached 29.16 g/ m2h, and the energy consumption was down to 6.0 kWh/kg CaCO3. The energy consumption was much less than those reported in other works at the same descaling rate. From the above results, this system displayed a higher descaling rate under the same energy consumption, which demonstrates that the electrode area required for the system was greatly reduced under the same amount of water treatment. However, water softening systems with ion exchange membranes (Ref. [31] and Ref. [35]) achieved better performance, and the highest descaling rates
3.4. Cathodic working principle The core of the electrochemical water softening process is to create a high pH environment on the cathode surface by water and oxygen reduction reactions. At the same time, the scale precipitation reactions also occur on the boundary layer of the cathode surface. The reactions for generating alkalinity are: 2H2O + O2 + 4e− → 4OH− −
−
2H2O + 2e → H2↑ + 2OH (main cathode reaction)
(1) (2)
The scale precipitation reactions are: Ca
2+
Mg
+ HCO3− + OH− → CaCO3↓ + H2O
2+
−
+ 2OH → Mg(OH)2 ↓
Fig. 7. Effect of flow rate on descaling rate and energy consumption (a) Calcium solution; (b) Simulated circulating cooling water. 132
(3) (4)
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Fig. 8. Effect of current density on descaling rate and energy consumption (a) Calcium solution; (b) Simulated circulating cooling water.
cathode dominated the alkalinity production. From the above analysis, the working principle of the multi-meshes coupled cathode can be described as follows: as shown in Fig. 12, at the initial stage of water softening process, the reactions for generating alkalinity occurred mainly on external layers of the coupled cathode due to the lower potential. The highest OH− ions concentration was produced on the surfaces of external layers, and these OH− ions simultaneously diffused toward the bulk solution outside of the cathode and the internal layers of the cathode. The calcium and magnesium ions in the bulk solution migrated to the surfaces of external layers of the coupled cathode under the action of an electric field. The scale precipitation reactions occurred preferentially on these layers. However, the external layers were gradually covered by scale as the water softening process proceeds. The reactions for generating alkalinity were gradually shifted from the external layers to the internal layers of the cathode. The highest OH− ions concentration was produced on internal layers of the cathode at later stage of water softening process. A large amount of OH− ions diffused from the internal layers toward the bulk solution outside of the cathode, and calcium and magnesium ions migrated toward the internal layers of the cathode from the bulk solution outside. These ions contacted and reacted near the external layers of the cathode. More importantly, the preferentially deposited scale on external layers provided a nucleation surface which accelerated the subsequent scale deposition [21]. Overall, the coupled cathode greatly weakened the competition between the reactions for generating alkalinity and scale precipitation reactions, thus built a self-synergistic effect through their different functions of external layers and internal layers of the cathode. That is to say, the synergistic effect was constructed by separating and positioning the chemical reactions (alkalinity production and scale deposition) to different regions of the coupled cathode. Once the external layers of the cathode were covered by scale, the alkalinity was generated on the clean internal layers. This process also ensured a high alkaline environment in the cathode region thus reduced the rate of cathode deactivation and energy consumption. The deposition of scale occurred mainly on the external layers of the
Fig. 9 and Fig. 10 show series of images of the multi-meshes coupled cathode and corresponding wire meshes before and after electrochemical deposition. It can be seen that a large amount of white scale layer deposited on the cathode surface after the softening process. The amount of scale on each layer of cathode was also different. It appears that a large amount of scale deposited on external layers of the coupled cathode (20 mesh and 12 mesh nets, Fig. 10b.d), and almost no scale appeared on internal layers of the cathode (8 mesh and 50 mesh nets, Fig. 10f.h). It indicates that there existed apparent differences in the roles played by each layer of the cathode during the water softening process. In order to illustrate the role of the cathode, the surface potentials of each layer were monitored during the water softening process. Potentials of each layer of the cathode are shown in Fig. 11. The potentials of external layers of the coupled cathode were lower than those of internal layers at the beginning of the water softening process. Particularly, the potentials of 20 mesh nets were the lowest (−1.992 V), and the potentials of 50 mesh nets were the highest (−1.530 V). A lower potential represents a higher current density together with a higher rate of cathodic reactions that produce alkalinity. Thus, the rate of generating alkalinity on 20 mesh nets was the fastest, indicating that the concentration of OH− ions centered on these layers was the highest, and the scale precipitation reactions mainly occurred in the vicinity of these layers. What’s more, the potentials together with current densities of the 20 mesh and 12 mesh nets (line 1 and line 2) were almost constant during the electrolysis process. However, the surfaces of external layers (20 mesh and 12 mesh nets) were gradually covered with scale, and the area for generating alkalinity was reduced, and the current through the external layers was reduced. This means that the rate of producing OH− ions on external layers of the cathode decreased. In addition, the potentials of the 8 and 50 mesh nets (Line 3 and Line 4) gradually decreased. This shows that the current densities of these layers gradually increased. As can be seen from Fig. 10, almost no scale deposited on the surfaces of internal layers (8 mesh and 50 mesh nets). So, the increased current densities indicate that the internal layer of the Table 1 Water softening performance comparison of this work with other papers. Hardness (mg/L as CaCO3)
Current density (A/m2)
Descaling rate (g/m2h)
Energy consumption (kWh/kg CaCO3)
Membrane electrolysis
Ref.
250 240 350 1180 300 350 350 870 6540 1600 1500
20 ∼20 20 108 100 100 18.3 30.6 250 100 600
2.6 3.4–6.9 8–15.09 22.8 10 25.5–34.3 29.16 42.32 530–570 460 1000–1280
20 ∼25 3.6 16 118 17.6–22.3 6 4.1 1.7 2.8–3.0 6.6
No No No No No No No No Yes Yes Yes
[26] [21] [32] [26] [28] [32] This work This work [31] [31] [35]
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Fig. 9. Typical images of the cathode before (a) and after (b) water softening process.
Fig. 10. Typical images of each net in the cathode before (a.c.e.g) and after (b.d.f.h) water softening process. (a.b) 20 mesh net; (c.d) 12 mesh net; (e.f) 8 mesh net; (g.h) 50 mesh net.
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Fig. 11. Potentials of each layer of the cathode.
Fig. 13. Voltage changes during water softening.
cathode throughout the water softening process. A large amount of OH− ions from the internal layers and the preferentially deposited scale on external layers accelerated the subsequent scale deposition in later stage of the process. For conventional electrochemical water softening systems, a higher descaling rate resulted in a faster cathode deactivation rate and more frequent electrode cleaning. While the system with multi-meshes coupled cathodes displayed a higher descaling rate, the cathode deactivation was not accelerated. In order to verify this effect, water softening experiments were performed at the same constant current using a system with multi-meshes coupled cathodes and a system with stainless steel plate cathodes of the same area. A certain proportion of calcium chloride and sodium bicarbonate were continuously added to ensure the conductivity of the water unchanged. Fig. 13 shows the change in voltage during the experiment. It can be clearly seen that the voltage of the system using multi-meshes coupled cathodes was significantly lower than that of conventional system. This was one of the reasons why the system displayed lower energy consumption. In addition, the voltage in the system gradually increased as the cathodes deactivated due to the deposition of scale on the surface. The rise of these two system’s voltage was almost the same which illustrated that the rate of cathode deactivation was not faster in the system with the coupled cathodes compared to that with stainless steel plate cathodes despite the higher descaling rate. This also indicates that the coupled cathode
could carry more scale and avoid frequent cathode cleaning. However, the efficient cleaning and regeneration of inactivated cathodes is also major technical challenge for industrial applications. There are many methods for regeneration of stainless steel plate cathode, such as mechanical scraping, polarity reversal, ultrasonic, air-scoured washing and acid washing [32,36–40]. This work describes multi-meshes coupled cathode that greatly increased the descaling rate and reduced energy consumption of the electrochemical water softening system. Suitable cathode regeneration technology is under investigation to match with this system. 4. Conclusions This work describes an electrochemical water softening system with multi-meshes coupled cathode. The coupled cathode greatly improves the performance of the system, which is manifested by a significant increase in the descaling rate and a great reduction in energy consumption. The improved performance is attributed to the self-synergistic effect of the coupled cathode which is constructed by separating and positioning the chemical reactions (alkalinity production and scale deposition) to different regions of the coupled cathode, namely, a large amount of OH− ions from the internal layers and the preferentially deposited scale on external layers accelerates the subsequent scale deposition in later stage of the process. This work provides a facile
Fig. 12. Schematic illustration of working principle of the coupled cathode. 135
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strategy for the development and applications of electrochemical water softening technology.
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