Study on the degradation mechanism of cathode in electrochemical water descaling

doi:10.21203/rs.3.rs-3431993/v1

This article studies the degradation mechanism of cathode descaling efficiency in the electrochemical water descaling process. The prolonged reaction causes many calcium carbonate crystals to adhere to the surface of the electrode plate. It results in significant degradation of the descaling efficiency and eventually approaches 0. This paper focuses on the interactive effects of multiple factors on the degradation of electrochemical water softening descaling efficiency. The results show that when the DC voltage exceeds 20V, the degradation process of descaling efficiency of the electrode plate is prolonged and self-cleaning occurs. The hardness removal amount is increased by 20%. Supported by the LSI and RSI indices, voltage current density and mesh size directly affect the degradation of descaling efficiency. The main reason for the increase of unit energy consumption is the thicker scale, which leads to the decrease of scale removal efficiency between cathode and anode.Combined energy consumption per unit and degradation of descaling efficiency, using a mesh size of 1 mm×3 mm, can reduce the unit energy consumption by 18.75%, effectively prolonging the degradation of descaling efficiency.

PIV

Flow field

Electrochemical water descaling

Degradation of descaling efficiency

Mesh cathode

High concentrations of hardness are usually present in industrial circulating cooling water systems. The presence of hardness tends to make it easy for scale to form on the surface of pipes (Zeppenfeld K et al. 2000). Not only does it reduce the heat transfer efficiency of the heat exchanger while increasing the energy consumption (Song W F et al. 2003;Nebot E et al. 2006), this causes blockages in the pipes and affects the production safety of the equipment. Therefore, the critical measure for the regular operation of the circulating cooling water system is to reduce or block scaling in the pipeline(Zeppenfeld K et al. 2010). The use of electrochemical descaling methods is crucial.

To overcome pipe scaling concerns, conventional water descaling currently uses chemicals such as descaling agents(Hasson D et al. 2011). However, this approach is prone to secondary contamination, while electrochemical water descaling is a typical active hardness removal and inhibition technology(M.M.Tlili et al.2013). Electrochemical water descaling systems produce OH-, CO₃^2- by electrolysis reaction. When Ca²⁺ is adsorbed onto the cathode in an electric field, the anions react with Ca²⁺at the cathode to form crystals such as calcium carbonate(Almarzooqi et al. 2014). Detach these crystals from the cathode to increase the descaling efficiency of the cathode.

The cathode, anode, and power supply systems are all components of electrochemical water descaling technology (D. Hasson et al. 2017; Y. Yu, et al. 2018; Sanjuán I et al. 2019). However, current electrochemical water descaling systems suffer from low efficiency of scale deposition per unit(Y. Yu et al. 2018). At the heart of electrochemical water descaling technology lies the role of the cathode in the system to establish a zone of high alkalinity and complete deposition descaling(Jin H et al. 2020; Clauwaert P et al. 2020; Xu H et al. 2020). Some scholars used to improve descaling efficiency by using composite mesh cathodes (Yu Y et al.2018). Luan J et al(2019) used a composite mesh cathode coupled with multiple layers of metal screens of different pore sizes to enhance the performance of electrochemical water descaling systems. The results show that separating and localizing chemical reactions (alkalinity generation and scale deposition) to different areas of the composite reticulated cathode increases the descaling rate. The above studies have mainly analyzed the structure of the cathode and process conditions to make electrochemical water descaling technology applicable to industrial production (Kim C et al. 2016). Lin W et al(2021) established five different levels of influencing factors for orthogonal tests. As the distance between cathode and anode increases, the number of scaling removed per unit energy consumption increases and decreases. Increasing the number of scaling released per unit of energy consumption, reducing energy consumption and lowering the voltage are important ways to improve the number of scaling removed per unit of energy consumption. Current density is one of the essential factors influencing the development of electrochemical water descaling technology (Sanjuán I et al. 2019), experiments by Zhang G et al(2015) showed that water descaling was most effective at a current density of 10.7 mA/cm². Studies have shown that stability can be seen in installations with pulsed power supplies. The deposition process can operate steadily with no trend towards increased energy consumption(Y. Lv et al. 2020; I. San juan et al. 2019; Li X et al.2020). It has also been found that the crystal structure and morphology of the precipitates on the surface of the cathode affect the water descaling efficiency by using electron microscopy (SEM), X-ray diffractometry (XRD), and infrared spectroscopy (IR)( Jin Het al. 2019). Wang Z et al(2021) investigated the CaCO₃ crystallization behavior of seven different structured cathodes under the effect of the pulsed electric field by examining the impact of cathode structure on the crystallization behavior of CaCO₃. The experiment found that CaCO₃ crystals are generally calcite, forming a more compact and thick scale. With the scale deposition covering the surface of the cathode gradually, the cathode is deactivated, which leads to a decrease in hardness scale removal efficiency and an increase in energy consumption. In summary, the deposition efficiency of hardness ions on the cathode surface and the growth behavior of scaling crystals affect the efficiency of electrochemical descaling (Zhu H et al.2021). This phenomenon is called the degradation of descaling efficiency.

This article studies the degradation mechanism of cathode descaling efficiency in the electrochemical water descaling process. It is found that different mesh-size structures of the cathode significantly affect the descaling efficiency of the cathode. In this paper, four types of cathode were selected, and experiments were carried out using a DC power supply to investigate the effect of cathode structure on the degradation of descaling efficiency under different voltages. The records the voltage, current, and hardness drop values. During the experiments, consider the overall efficiency of hardness removal and reflect the corresponding energy consumption. In this paper, the bubbles, flow field and crystal morphology on the electrode's surface are observed with a PIV, high-speed camera. The mainly degradation of descaling efficiency under different mesh-sizes and different voltages from macroscopic and microscopic phenomena to select a suitable mesh electrode cathode to improve the removal efficiency of water softening, aiming to enhance the performance of water descaling and regulating the surface calcium carbonate crystals. This study provides meaningful parameters for electrochemical water descaling in industrial circulating cooling water systems and explores the conditions of reticulated cathodes for optimal hardness removal efficiency.

Experimental principles

As shown in Fig. 1, electrochemical water descaling is the production of OH- on the cathode surface by cathodic reaction and ion exchange, forming an alkaline region in the vicinity of the cathode, converting HCO₃^- to CO₃^2- in solution, and the electric field induces Ca²⁺ to migrate towards the cathode and combine with CO₃^2- to form CaCO₃ crystals, this removes the hardness from the solution( Beaunier L et al. 2001).

According to the experimental principle in Fig. 1. First, near the cathode, the O₂ and H₂O in the water get electrons from each other to form OH^-:

$${\text{O}}_{\text{2}}\text{+2}{\text{H}}_{\text{2}}\text{O+4}{\text{e}}^{\text{-}}\text{→}\text{4O}{\text{H}}^{\text{-}}$$

1

Secondly, the decomposition of water molecules near the cathode into H₂ and OH-:

$$\text{2}{\text{H}}_{\text{2}}\text{O+2}{\text{e}}^{\text{-}}\text{→}{\text{H}}_{\text{2}}\text{↑+2O}{\text{H}}^{\text{-}}$$

2

Finally, the electric field forces Combine Ca²⁺ on the cathode with OH- and HCO₃- in solution to produce CaCO₃ crystals:

$$\text{C}{\text{a}}^{\text{2+}}\text{+HC}{\text{O}}_{\text{3}}^{\text{-}}\text{+O}{\text{H}}^{\text{-}}\text{→CaC}{\text{O}}_{\text{3}}\text{↓+}{\text{H}}_{\text{2}}\text{O}$$

3

Electrolytic system

The experimental setup is shown in Fig. 2. It consists of three major parts: reservoir (500mm×500mm×500mm), reaction pond (230mm×160mm×160mm) and collection pond (640mm×420mm× 450mm). Configure the hardness solution required for the experiment in the reservoir, which flows to the reaction tank via pumping and then into the collection tank after the reaction. A high-speed camera was used to observe the bubble flow phenomenon on the surface of the cathode and the phenomenon generated on the surface after degradation of descaling efficiency. Determine the Ca²⁺ concentration according to the hardness required for the experiment. Dispense pure anhydrous calcium chloride with sodium bicarbonate in proportion to 110L of the hardness solution needed was prepared for each experiment to simulate industrial circulating cooling water. The initial hardness was 720mg/L.Reaction cell with cathode fixing device, and one cathode and one anode, cathode spacing fixed at 50 mm, Anode size 100 mm×100 mm. The cathodes were selected of TM1×3,TM3×4.5,TM4×9(Mesh size) and a flat plate without mesh(TP). The power supply uses a DC power supply 20V-30V adjustable DC power supply.

Each set of experimental time was set for one set of 8 hours, current and hardness are recorded every hour. Samples were taken from the reservoir and the reaction tank to measure pH, and calculate the hardness drop and hardness removal efficiency, pH values were measured using a HACH water analyser.

In order to simulate actual working conditions, observation of changes in bubble migration in a water tank under degradation of descaling efficiency. As shown in Fig. 3, the platform is built with a frame rate of 800 fps flow field observation platform (PIV).

The different mesh cathodes are shown in Fig. 4.

Experimental reagents

The specifications and manufacturers of the reagents used in the experiments are shown in Table 1.

Table 1

Experimental reagents.
reagents	Specification	Manufacturer
NaHCO₃	500g/ Bottle	Tianjin Dingshengxin Chemical Co.
CaCl₂	500g/ Bottle	Tianjin Dingshengxin Chemical Co.
EDTA-2 Na	0.01mol/L	Tianjin Jinbei Fine Chemical Co.
C₆H₈O₇	0.975g/mL	Sinopharm Group Chemical Reagent Co.
pH buffer	pH = 10	Shanghai Yidian Scientific Instruments Co.

Test and data analysis

Under a DC electric field, the different meshes show different periods of degradation of descaling efficiency and different weights of CaCO₃ crystals attached to the surface. The longer the mesh works, the more CaCO₃ crystals adhere to the surface. When excessive adhering material clogs the mesh holes and adheres to the cathode's entire surface, cathode descaling efficiency is significantly reduced. Experimenting with different mesh sizes, Records the entire cathode efficiency cycle and effectively compares the cathode descaling efficiency performance. As shown in Table 2, the weight of the cathode was measured in advance to obtain scale deposition amount per unit area.

Table 2

mesh Cathode size data.
Mesh size/mm	Area /m²	Weight /g
TM1×3	0.007	16
TM3×4.5	0.006	15
TTM4.5×9	0.0055	14
TP	0.01	20

At the end of the reaction, the cathode were dried at 80°C for 2 h and then weighed and recorded as m₁. The cathode were removed and soaked in C₆H₈O₇ solution and cleaned, then dried at 60°C for 10 h. Remove and weigh and record as m₂. The whole experiment was worked continuously, with water changes and refilling of the solution every eight hours, the following equation expresses the increment per unit area (in g⋅h^{- 1}⋅m^{- 2}).

where m₁ and m₂ are in g; t(electrochemical reaction time)is in h.

The current and voltage values are recorded for each period during the experiment. Calculate the power Q (in kWh). The Q-t variation relationship was plotted. The reaction times for this experiment were all 60 min, and Q can be obtained from the following equation.

$$\text{Q=}{\int }_{\text{0}}^{\text{1}}\text{1}\text{f(P,t)dt}$$

5

The energy consumption per unit of weight (in kWh⋅g^-1) can be calculated from the electricity consumption and the weight of the scaled crystals using the following formula.

$$\eta =\frac{Q}{{{m_2} - {m_1}}}$$

(6)

In order to record hardness removal efficiency, the solutions in the reservoir and the reaction tank are sampled separately every 1h, and measuring the hardness of the solution(in mg/L).

$$y=\sum {\frac{{{Y_{Ii}} - {Y_i}}}{{{Y_{Ii}}}}}$$

(7)

Where Y represents the hardness removal efficiency; Y_Ii represents the hardness of the solution in the reservoir recorded at hour (I = 1, 2, 3, 4); and Y_i represents the hardness of the solution in the reaction tank recorded at hour (i = 1, 2, 3, 4).

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Comparison of cathodic descaling

Mesh cathodes have a more effective cathode area, and typically have higher scaling efficiency and lower energy performance in short-term experiments. However, in long-period experiments, the fouling rate on the cathode is usually continued and the use of a smooth cathode surface facilitates the deposition and stripping of fouling ions. This is because, as the proceeding of the precipitation reactions, the Ca²⁺ removal process was gradually converted to the cathode scale layer to block the scale removal. The rapid growth of the CaCO₃ deposits would block most of the cathode surface, which caused a reduction in the effective precipitation area.

Figure 5(a) shows the variation of scale removal efficiency decay with time. The final efficiency of all four cathodes under the degradation of the descaling efficiency stage was 0. The hardness removal was 23.22%, 22.98%, 18.59% and 17.62%, respectively. The continuous working time and scale deposition of the four types of cathode are in Fig. 5(b). When TM1×3, TM3×4.5, TM4.5×9 and TP worked at 20v in 36h,45h,30h, and 71h, the scale deposition was 23.64g, 32.78g, 22.93g, 42.74g, respectively; when worked in 38h, 55h,33h at 30v, the scale deposition was 30.28g,35.37g,24.22g. Based on the above data, it shows a longer reaction time in the degradation of the descaling efficiency phase, but a lower descaling efficiency.

Figure 5(c) indicates that the hardness of the water sample decreases continuously with increasing reaction time, which further leads to a decreasing pH value. The pH value decreases because the alkalinity at the cathode becomes scale precipitation and remove, and the alkalinity becomes carbon dioxide spillage at the anode. The drop in pH is because circulating cooling water usually contains high concentrations of chlorides, and chlorine precipitation becomes common. Due to the continuous heating of the electrochemical equipment, increasing temperature of the circulating water can weaken its heat exchange capacity. In practical applications, current density and energy consumption need to be taken into account.

High-speed camera observation of degradation of descaling efficiency

Crystal amplification morphology analysis was carried out for each experimental cathode at a degraded efficiency transition state. Figure 6 below shows the morphology of the different cathode meshes at 75 times magnification after the degradation of descaling efficiency during prolonged water descaling. The calcium and magnesium ions near the cathode are difficult to soluble in water, and the precipitate adheres to the cathode in large quantities. The tighter the mesh, the thinner the scale tissue attached to the cathode; the narrower the mesh, the easier the scale tissue builds up and affects efficiency. Calcium carbonate deposits block a large part of the cathode surface, reducing the area of cathode surface area action. The corresponding concentration is limited by mass transfer, leading to an increase in the scale layer. Due to the adhesion of the scale layer, the contact between the electrode and the electrolyte is obstructed, resulting in a reduced contact area between the electrode and the electrolyte, significantly reducing the electrolysis efficiency. Figure (a) is the appearance of TM1×3 during the descaling process. The outer layer is covered tightly, and the crevices have small holes. Figure (b) shows the pattern of scale formation of TM3×4.5, with a thick outer layer and more deposits in the cathode protuberance. Figure (c) is the pattern of the scaling formation of TM3×4.5. The outer layer is fully covered. Figure (d) shows the pattern of the scaling formation of the cathode after the degradation of descaling efficiency. Because of the meshless surface, the CaCO₃ Crystal is not easy to adhere to the meshless cathode and is easy to fall off. By comparison, it is found that the crystal on the surface of the meshes is becoming thicker and thicker because the meshes are getting smaller and smaller. At the same time, the growth rate of calcium carbonate increases gradually from the inside to the outside, and the deposition reaction of calcium carbonate occurs preferentially in the outer layer.

The entire descaling efficiency cycle is captured using a high-speed camera with dynamic tracking. The growth cycle of CaCO₃ at 60 times magnification of the experimental cathode is shown in Fig. 7, during the entire cathode descaling efficiency cycle, significant thickening of the scale layer after 8h, after 16h the small bubbles start to intermingle, after 20h the entire air curtain of the cathode was significantly reduced and the number of large bubbles increased significantly. A state of degradation in descaling efficiency begins to appear after 24h, 28-32h full-scale layer attached to the surface of the cathode, the mesh is completely covered at the holes, the air bubbles are sparse and do not appear to be rising in the air curtain layer. Indicating that the scale has a strong adhesion to the cathode, also under the action of the flow field, enhanced bubble bursting and cross-fertilisation, attachment to the surface of the scale layer leads to the formation of a protective film on the scaling crystals, which cannot be easily dislodged.

Throughout the experimental shoot, the voltage regulates the current density above the cathode, thus increasing the electrolysis efficiency. As shown in Fig. 8, higher current densities promote the nucleation and growth of bubbles and accelerate the detachment of precipitates from the surface of the cathode. In (a), it can be seen that air bubbles are clogged in the scale layer. As the bubble coverage increases, the bubbles grow faster at high current densities, and the nucleation to separation process is shorter, and the bubbles coming off the scale layer. The natural convection or micro-convection caused by the movement of the bubbles in the middle will result in a more rapid gas-liquid flow and consequently, a faster migration of particles, resulting in an accelerated detachment of deposits from the surface the cathodes, resulting in longer cycles of degradation of descaling efficiency.

In order to capture the detachment of deposits from the surface of the cathode caused by the gas-liquid flow, the front of the cathode is photographed by a high-speed camera. Figure (b) shows the detached surface of the scale crystals. A large number of bubbles float due to the violent hydrogen precipitation reaction near the cathode. At the same time, under the action of the electric field force, the ion velocity of the solution rises, and the bubbles generate uninterruptedly, causing the detachment of scale crystals on the surface of the cathode.

Figure (c) shows a microscopic-view cathode at rest. As the scaling crystals fall off in their entirety, the dislodged area comes back into direct contact with the solution, increasing the reaction area of the cathode. Scaling crystals fall out of the mesh gaps and onto the top edge of the lower mesh. The degradation of descaling efficiency is evident as the cathode tends to saturate.

Comparison of energy consumption

Electrochemical efficiency is judged using energy consumption and hardness removal efficiency. Energy consumption is related to current density, which is an essential indicator of electrochemical descaling, and an increase in current density raises energy consumption.

The energy consumption curves for different meshes are shown in Fig. 9. A comparison of Figure (a) with Figure (b) shows that TM1×3 has a lower overall energy consumption than the other three meshes. TM1×3 maintains a low energy consumption during the decreasing degradation phase of descaling efficiency. At 20V, and continues to decrease. At 30V, the three cathodes show a continuous increase in energy consumption.

Figure 10 shows the energy consumption of the TP. Figure (a) shows that the efficiency of the cathode under a DC field of 20V continues to decline throughout the degradation of the descaling efficiency phase, but the overall energy consumption stays up. The current density shown in Figure (b) continues to increase. However, the overall descaling efficiency is low, and the energy consumption per unit is high. The cathode efficiency decreases gradually with time. This is attributed to the higher operating voltage intensifying the hydrogen precipitation reaction on the surface of the flat plate cathode, and the resulting floating H₂ can seriously disturb the water column near the cathode, making it very unstable in the vicinity of the flat plate cathode. This situation directly affects the migration of scaling ions such as Ca²⁺ and HCO₃⁻ to the cathode, interfering with the mass transfer process and leading to the hardness removal rate of the effluent water cannot be improved even if the voltage is increased. In addition, with the production of large amounts of hydrogen, a thin gas film forms on the surface of the cathode, which prevents the deposition of Ca²⁺ on the flat cathode walls. At the same time, the higher voltage causes the reduction of Cl- and H⁺ in the anode area to generate Cl₂ and H₂, which also increases the corrosiveness of the water sample and directly affects its service life, causing the electrical power of the equipment to increase with time (Zeppenfeld K et al. 2011; Zaslavschi I et al. 2013).

The hardness and alkalinity of the water sample continue to decrease with increasing reaction time as the degradation of descaling efficiency. This further leads to decreasing pH values. Circulating cooling water usually contains high concentrations of chlorides, and chlorine precipitation will become common. In order to determine the precipitation and dissolution of calcium carbonate in water, by introducing the Langelier Index (LSI) and the Reznor Stability Index (RSI) ( Janssen L J J et al. 2002), the following equations are calculated:

$$LSI=pH - p{H_S}$$

(8)

$RSI=2p{H_S} - pH$ (9) $\text{p}{\text{H}}_{\text{S}}\text{=pH-9.3-A-B+C+D}$ ༈10༉

A represents total dissolved solids, B represents temperature, C represents calcium hardness, and D represents total alkalinity.

As shown in Table 3, the scale deposition per unit area is different for the four types of the cathode at different voltages, with RSI data exceeding 6 for all three types of mesh plates and LSI data exceeding 0.5 for all four types of mesh plates, which belong to the severe scaling and severe corrosion transition states, as can be seen from the comparison of energy consumption per unit weighing and scale deposition, the higher the scale deposition of the cathode, the higher the energy consumption per unit during the whole degradation of descaling efficiency process, but when retaining some residual of scale can facilitate rapid and efficient electrochemical scale deposition in the next stage.

Table 3

Changes of water quality indexes.
Mesh	Voltage/ V	LSI	RSI	Scale deposition per unit area		energy consumption per unit scale weight
Mesh	Voltage/ V	LSI	RSI	Data/g.m^− 2	Error	Data/kWh.g¹ Error
TM1×3	20	0.86	6.20	23.64	0.0133	1.099 0.042
TM1×3	30	0.89	6.12	30.28	0.0254	2.476 0.331
TM3×4.5	20	0.85	6.18	32.78	0.0282	0.793 0.031
TM3×4.5	30	0.88	6.14	35.37	0.0152	2.121 0.282
TM4.5×9	20	0.51	6.64	22.93	0.0417	1.133 0.043
TM4.5×9	30	0.63	6.48	24.22	0.0322	3.096 0.412
TP	20	1.34	5.51	42.74	0.0128	0.608 0.023

Effect of degradation of descaling efficiency on the flow field

Repeated fluctuations in cathode efficiency due to the deposition of scaling tissue on the cathode surface were observed with a high-speed camera during the entire experimental cycle. Due to the formation of a large number of bubbles in the water, the reaction of hydrogen resolution near the cathode is intensified when an applied voltage is applied, and many of the bubbles will float upwards in large numbers, causing their detachment from the scaling crystals attached to the surface of the cathode. As shown in Fig. 11, the frontal view shows that when bubbles are subjected to buoyancy forces that are greater than the tug forces on the surface of the scale tissue, the tiny bubbles are concentrated at high densities and collide with each other, bringing about rapid detachment and separating some of the scale crystals in the process(Hu J et al.2015; Guo Y et al. 2021).

However, this detachment property causes the cathode to self-clean itself, effectively prolonging the degradation of the descaling efficiency process. However, due to the concave surface of the mesh cathode, the detached scale-forming tissue will fall back onto the top and bottom edges of the mesh, causing thicker and thicker crystalline growth. The degradation in efficiency causes more excellent disturbance to the flow field and affects the migration between ions. In order to investigate the changes in the flow field near the cathode in the degraded efficiency condition, the PIV technique was used to analyse the changes in the flow field.

As shown in Fig. 12(a), The bubble flow formed by bubbles rising vertically from the cathode can be observed in the graph for the case of degraded cathode efficiency, where the electrolysis reaction rate is relatively weak. (b) In the diagram, with 20V applied, the flow field on the cathode surface is enhanced. In the process of rising, gradually diffuses and impinges on the water surface and the lateral moving distance on the water surface increases. Because of the high voltage applied in the tank with constant solution hardness, the electrochemical reaction is accelerated, and the number of detached bubbles gradually increases. As can be observed from the images of the PIV, the number of bubbles increases significantly as the applied electric field is gradually increased and the behaviour of the bubbles' movement gradually changes. (c) The diffusion reaction of the bubbles with a DC field of 30 V is enhanced, and the active area covering the surface of the electrode is significantly larger. (d) The pulsed electric field of 20V in the diagram shows that the bubbles rise in a spiral and move directly above the cathode. As seen in the image of the PIV, the flow rate increases significantly with the gradual increase of the applied electric field, and the gas-liquid flow gradually spreads.

The gas-liquid flow process causes vortices that interfere with the orderly flow of the surrounding liquid. The flow rate between the poles affects the descaling efficiency of the poles (Chen Q et al. 2022). As shown in Fig. 13, the gas-liquid flow rate in the extraction water tank at the height of H = 15 ~ 35mm, H = 35 ~ 65mm and H = 65 ~ 100mm in a non-degraded state. The straight lines in the figure indicate the anode and the cathode, respectively. As the outer side of the electrode is subjected to the rising effect of bubbles, the flow field is driven to rise along the outer side of the cathode in diffusion. Due to the buildup of scale layers between the electrode, resulting in a lower gap rate on the electrode's surface, the bubbles' natural rise leads to a lower flow rate in the state of decreasing efficiency, and the overall flow rate does not exceed 0.01 m/s. Figure (a) indicates the gradual increase in flow rate with increasing liquid surface height at 10V. The flow rate does not exceed 0.002 m/s at its highest point. Figure (b) shows a vortex between the two cathodes at 20V, with a flow velocity of up to 0.0025 m/s. Figure (c) shows the enhancement of the flow velocity up to 0.0035 m/s for a voltage of 30 V. Figure (d) shows a pulsed electric field with a significant weakening of the flow velocity up to a maximum of 0.0030 m/s. It can be found that the overall flow velocity increases with height as the gas-liquid gradually spreads at 65–95 mm height in the tank in the non-degraded state.

The above diagram demonstrates the trend of the flow field in the non-degraded state with a DC field and a pulsed field. Figure 14 shows the variation of the flow field for the degraded state of the cathode. In Figures (a), (b), (c) and (d), there is no significant vortex between the two cathodes due to the build-up of the scale layer, which prevents the flow field from migrating. As the voltage increases, the flow velocity between the cathode increases significantly, and the flow field rises and spreads along the surface of the cathode, forming vortices at the top. In Figure (d), under the influence of the pulsed electric field, the flow field between the cathode spreads rapidly, and the flow velocity increases along the top of the surface close to the liquid.

The degraded state has a disturbing effect on the flow field, as shown in Fig. 15 (a). At a voltage of 10 V, the flow velocity does not exceed 0.002 m/s. The flow velocity does not change significantly as the liquid surface height increases. Figure (b) shows that the flow velocity is up to 0.003 m/s at 20 V. At a liquid level of 35 mm, the flow velocity is 15 mm below the liquid level. Figure (c) shows that the flow velocity is enhanced to 0.004 m/s at a voltage of 30 V. Figure (d) indicates a pulsed electric field where the flow velocity remains at 0.0030 m/s. It can be noticed that the flow velocity is not stable in the degraded state, and there are no powerful vortices between the cathodes.

This paper addresses the problems of degradation of the efficiency of electrodes in electrochemical water treatment, the increase in energy consumption and the continuous decrease in efficiency. The phenomena during the degradation of the plating efficiency were observed for four types of cathode mesh and forms of the electric field, and their effects were analyzed. The following conclusions were obtained by observing the flow field changes and microstructural phenomena.

Under the DC electric field, the increased voltage will prolong the degradation of descaling efficiency. During the degradation phase of the descaling efficiency, the overall descaling efficiency maintains a decreasing trend and eventually approaches 0. The sparseness of the mesh determines the appearance of the self-cleaning phenomenon on the surface of the electrode and the crystals attached to the surface of the cathode. The highest weight of CaCO₃ crystals attached to the surface of the electrode was 42.74g, which prolonged the degradation phenomenon of the descaling efficiency by 56% compared to other electrodes.
When the DC voltage exceeds 20V, the descaling degradation of the descaling efficiency process is effectively prolonged, and the removal capacity is increased by 20%. The increase in voltage shortens the induction period for crystal formation and actively accelerates the redox reaction in the solution. At the same time, increasing the current density promotes the creation of highly alkaline regions while accelerating the cathodic reaction rate. The saturation index shows that the higher the scale deposition on the cathode, the higher the energy consumption per unit. Comparing the energy consumption data of the cathode, the TM1×3 consumes up to 10.9% less energy than other cathodes at 20V and up to 18.75% less energy at 30V.
Based on the PIV observation of the flow field, the non-degraded state will increase the velocity of gas-liquid flow and natural convection with increasing current density and the presence of vortices between the electrode. In contrast, the scale layer on the electrode cathode in the degraded efficiency state will impede the flow of liquid in the electrode gap and inhibit the enhancement of mass transfer convection, with vortices on both sides of the cathode.

Authors’ contributions Wei Lin: data curation, writing-review and editing. Ze Yu Gao: writing—review and editing. Wei Wang: writing—review and editing. Cheng gang Wang: writing—review and editing. Jin Xiang: writing—review and editing. Jian min Xu: writing—review and editing. Jiu yang Yu: writing—review and editing. All authors read and approved the fnal manuscript.

Funding Open access funding provided by Scientific Research Foundation of Wuhan Institute of Technology (50976080).

Data availability All data are available upon request.

Ethics approval The authors declare that their study is compliant with ethical standards.

Consent to participate All the authors listed consent to participate.

Consent for publication We confrm that this manuscript has not been published elsewhere and is not under consideration by another journal. All authors have approved the fnal manuscript and agreed with its submission to Electrocatalysis.

Competing interests The authors declare that they have no competing interests.

Almarzooqi, F. A., Al Ghaferi, A. A., Saadat, I. & Hilal, N. 2014 Application of capacitive deionisation in water desalination: a review. Desalination 342, 3–15.
Beaunier L, Gabrielli C, Poindessous G, et al. Investigation of electrochemical calcareous scaling: Nuclei counting and morphology[J]. Journal of Electroanalytical Chemistry, 2001, 501(1-2): 41-53.
Chen Q, Lin W, Wang Z, et al. Flow field characterization between vertical plate electrodes in a bench-scale cell of electrochemical water softening[J]. Water Science and Technology, 2022, 85(6): 1736-1753.
Clauwaert P, De Paepe J, Jiang F, et al. Electrochemical tap water softening: A zero chemical input approach[J]. Water research, 2020, 169: 115263.
D. Hasson, A. Cornel, Effect of residence time on the degree of CaCO₃ precipitation in the presence of an anti-scalant, Desalination 401 (2017) 64–67.
Guo Y, Xu Z, Guo S, et al. Selection of anode materials and optimization of operating parameters for electrochemical water descaling[J]. Separation and Purification Technology, 2021, 261: 118304.
Hasson D, Sidorenko G, Semiat R. Low electrode area electrochemical scale removal system[J]. Desalination and WaterTreatment, 2011, 31(1/2/3): 35-41.
Hu J, Fang Z, Jiang X, et al. Membrane-free electrodeionization using strong-type resins for high purity water production[J]. Separation and Purification Technology, 2015, 144: 90-96
I. Sanjuan,´ et al., Electrochemical softening of concentrates from an electrodialysis brackish water desalination plant: Efficiency enhancement using a three-dimensional cathode, Sep. Purif. Technol. 208 (2019) 217–226.
Janssen L J J, Koene L. The role of electrochemistry and electrochemical technology in environmental protection[J]. Chemical Engineering Journal, 2002, 85(2/3): 137-146.
Jin H, Yu Y, Chen X. Membrane-based electrochemical precipitation for water softening[J]. Journal of membrane science, 2020, 597: 117639.
Jin H, Yu Y, Zhang L, et al. Polarity reversal electrochemical process for water softening[J]. Separation and Purification Technology, 2019, 210: 943-949.
Kim C, Lee J, Kim S, et al. Electrochemical softening using capacitive deionization (CDI) with zeolite modified carbon electrode (ZMCE)[J]. Desalination and Water Treatment, 2016, 57(51): 24682-24687.
Li X, Wang L, Sun W, et al. Study on electrochemical water softening mechanism of high-efficient multi-layer mesh coupled cathode[J]. Separation and Purification Technology, 2020, 247: 117001.
Luan J , Wang L, Sun W, et al. Multi-meshes Coupled Cathodes Enhanced Performance of Electrochemical Water softening System[J]. Separation and Purification Technology, 2019. 217: 128-136.
Lin W, Wang Z, Wang W, et al. Comparative analysis the performance of electrochemical water softening between high frequency electric fields and direct current electric fields based on orthogonal experimental methods[J]. Water Science and Technology, 2021, 83(7): 1677-1690.
M.M. Tlili, et al., Influence of the Interfacial pH on Electrochemical CaCO₃[sub 3] Precipitation, J. Electrochem. Soc. 150 (11) (2003) C765.
Nebot E , Casanueva J F , Casanueva T , et al. In situ experimental study for the optimization of chlorine dosage in seawater cooling systems[J]. Applied Thermal Engineering, 2006, 26(16):1893-1900.
Sanjuán I, García-García V, Expósito E, et al. Paired electrolysis for simultaneous electrochemical water softening and production of weak acid solutions[J]. Electrochemistry Communications, 2019, 101: 88-92.
Song W F, Zhu Y C, Xiao Y K, et al. Experimental study on low pressure pulse scale inhibition technology[J]. Industrial Water Treatment, 2003, 023(005):21-23
Sanjuán I, Benavente D, Expósito E, et al. Electrochemical water softening: influence of water composition on the precipitation behaviour[J]. Separation and Purification Technology, 2019, 211: 857-865.
Wang Z, Lin W, Wang W, et al. Research on performance optimization and mechanism of electrochemical water softening applied by pulse power supply[J]. Water Science and Technology, 2021, 84(9): 2432-2445.
Xu H, Xu Z, Guo Y, et al. Research and application progress of electrochemical water quality stabilization technology for recirculating cooling water in China: a short review[J]. Journal of Water Process Engineering, 2020, 37: 101433.
Y. Yu, et al., Electrochemical water softening using air-scoured washing for scale detachment, Sep. Purif. Technol. 191 (2018) 216–224.
Y. Lv, K. Lu, Y. Ren, Composite crystallization fouling characteristics of normal solubility salt in double-pipe heat exchanger, Int. J. Heat Mass Transf. 156 (2020).
Y. Yu, et al., Continuous Multistage Electrochemical Precipitation Reactor for Water Softening, Ind. Eng. Chem. Res. 58 (1) (2018) 461–468.
Yu Y, Jin H, Jin X, et al. Current pulsated electrochemical precipitation for water softening[J]. Industrial & Engineering Chemistry Research, 2018, 57(18): 6585-6593.
Zeppenfeld K . Prevention of CaCO₃ scale formation by trace amounts of copper (II) in comparison to zinc (II)[J]. Desalination, 2010, 252(1-3):60-65.
Zhang G, Qiu Y, Yang X, et al. Electrolytic treatment of industrial circulating cooling water using titanium – ruthenium – iridium anode and stainless steel cathode[J]. Desalination and Water Treatment, 2015, 56(4): 905-911.
Zeppenfeld K. Electrochemical water softening as an alternative to cation exchangers[J]. Chemie Ingenieur Technik, 2000, 72(1-2): 101-106.
Zaslavschi I, Shemer H, Hasson D, et al. Electrochemical CaCO₃ scale removal with a bipolar membrane system[J]. Journal of Membrane Science, 2013, 445: 88-95.
Zeppenfeld K. Electrochemical removal of calcium and magnesium ions from aqueous solutions[J]. Desalination, 2011, 277(1-3): 99-105.
Zhu H, Zheng F, Lu S, et al. Effect of electrochemical pretreatment on the control of scaling and fouling caused by circulating cooling water on heat exchanger and side-stream reverse osmosis membrane[J]. Journal of Water Process Engineering, 2021, 43: 102261.

No competing interests reported.

Study on the degradation mechanism of cathode in electrochemical water descaling

Status:

Version 1

Abstract

Figures

Introduction

Materials and methods

Experimental principles

Electrolytic system

Experimental reagents

Test and data analysis

Experimental results and discussion

Comparison of cathodic descaling

High-speed camera observation of degradation of descaling efficiency

Comparison of energy consumption

Effect of degradation of descaling efficiency on the flow field

Conclusion

Declarations

References

Additional Declarations

Status:

Version 1