Current pulsated electrochemical precipitation for water softening

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Current pulsated electrochemical precipitation for water softening Yang Yu, Huachang Jin, Xindi Jin, Runxin Yan, Li Zhang, and Xueming Chen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00448 • Publication Date (Web): 24 Apr 2018 Downloaded from http://pubs.acs.org on April 24, 2018

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Industrial & Engineering Chemistry Research

Current pulsated electrochemical precipitation for water softening

Yang Yu, Huachang Jin, Xindi Jin, Runxin Yan, Li Zhang, Xueming Chen*

Environmental Engineering Department, Zhejiang University, 866 Yuhangtang Road, Hangzhou 310058, China

*

Corresponding author phone:(86)57187951239; fax:(86)57187952771; email: [email protected]

1

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT Electrochemical precipitation has obtained great attentions for water softening in recent years. In order to avoid cathode deactivation, periodical scale detachment is essential. In general, the scale adhered to the cathode is detached by mechanical scraping. Although mechanical scraping is easy operated, elastic scraper must be installed between electrodes, leading to the low softening efficiency and high energy consumption. To overcome such a problem, a new current pulsated electrochemical precipitation process was proposed in this paper. Scale detachment was accomplished by increasing the current density significantly. Experimental results showed that the both high softening and detachment performance were achieved. The precipitation rate was as high as 40.47 g/h/m2. The energy consumption and the total hardness removal efficiency were 8.9-13.2 kWh/kg CaCO3 and 17.8-22.8 %, respectively. Repetitive experimental results indicated the current pulsated electrochemical precipitation process could run steadily without any performance decay detected after repetitive operation. Keywords: Water softening; Electrochemical precipitation; Scale detachment; Current pulsation 1. Introduction Evaporative cooling is widely used in many industrial applications, such as electric power generation, petrochemical industry and steel manufacturing

1, 2

. With the rapid

development of relevant industries, recirculate cooling water is becoming a major source of industrial water use nowadays. Generally, recirculate cooling water contains high concentration of hardness ions including calcium and magnesium ions which are the most 2


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problematic scale-forming species 3. Scale deposits often lead to numerous technical and economical problems in industrial plants by limiting heat transfer efficiency in heat exchangers and blocking the water flow in pipelines 4. Therefore, effective removal of hardness ions in recirculate cooling water is very important. Many technologies have been developed and used for water softening. Chemical precipitation is the most commonly utilized method for water softening. However, it demands plenty of chemical agents such as calcium oxide and sodium carbonate, and produces large volume of sludge 5. Furthermore, the softened water needs to be neutralized because the strong alkalinity of the chemical agents added. An important technique for water softening is ion exchange (IE). In spite of high softening efficiency, the ion exchange process still faces many drawbacks. The periodical regeneration of resins consumes corrosive chemicals, produces secondary wastewater that with strong acidity or alkalinity, and demands a large number of auxiliary facilities for chemicals storage and wastewater treatment, which is laborious, costly and non-environmentally friendly

6-10

. Relatively, reverse osmosis is more

attractive due to its high softening efficiency. Nevertheless, the membrane used is prone to scale fouling because hardness ions are chemically inclined to precipitate on the surface of the membrane

11, 12

. This not only leads to a decrease in softening efficiency, but also gives

rise to an increase in the energy consumption 13, 14. In recent years, electrochemical precipitation has been the focus of much research attention in water softening due to its environmental compatibility and versatility

15-23

. The

water softening process relies on the generation of high concentration of hydroxide ions in the vicinity of the cathode by water electrolysis, which can be expressed as follows: 3



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O2 + 2H2O + 4e− → 4OH −

(1)

2H2O + 2e− → 2OH − + H2 ↑

(2)

The hydroxide ions are believed to play crucial roles in harness ions removal by the following reactions:

OH − + HCO3− =CO32- +H2O

(3)

Ca2+ + CO32− =CaCO3 ↓

(4)

Mg 2+ + 2OH − =Mg ( OH )2 ↓

(5)

The chemical reaction products, which are recognized as scale deposits, are precipitated on the cathode

24-26

. In general, the scale must be detached from the cathode before the

cathode surface is totally covered. Otherwise, cathode deactivation would occur

27-29

.

Therefore, scale detachment method is necessary for such electrochemical reactor. In addition, the softening efficiency is highly dependent on the detachment efficacy. Mechanical scraping, which detaches scale by moving the elastic scraper periodically from top to the bottom of the circular shaped cathode surface, is the most widespread used scale detachment method 30. It has been used for decades in scale detachment due to its easy operation. However, the elastic scrapers installed between anodes and cathodes would significantly increase the electrodes distance. This not only lowers the effective cathode area leading to a decrease in water softening efficiency, but also causes overly high energy consumption

31

. Moreover,

mechanical scraping is known to be inefficient in long term use, hence affecting the softening efficiency. Despite the technical availability of such equipment, the industrial application prospect is limited by the shortcomings described above. If scale detachment can be more convenient and efficient, the technical and economic benefits of electrochemical precipitation 4


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will be bettered substantially. Another important scale detachment method is polarity reversal. Nevertheless, it leads to a reduction in electrode lifetime dramatically. In this study, we proposed and tested a current pulsated electrochemical precipitation process for water softening. It was different from the conventional polarity reversal electrochemical process. In this work, scale detachment was accomplished by increasing the current density significantly instead of reversing the electrodes polarity. Therefore, the electrodes lifetime reduction problem occurred in the polarity reversal could be avoided. Compared with the conventional mechanical scraping electrochemical precipitation process, the elastic scraper is no longer needed. Accordingly, the inter-electrode gap of the reactor can be reduced remarkably, leading to a higher softening efficiency and lower energy consumption. The major objectives of this study are to investigate the softening performance, to demonstrate the high efficiency of scale detachment by current increase, and to evaluate the operational stability of the reactor. 2. Materials and method 2.1. Experimental setup The experimental setup is schematically shown in Figure 1. The reactor has a working volume of 100 mL. A DSA sheet anode with dimension of 21x7x2 cm and a mirror stainless steel cathode with the same area were fixed in the reactor. Mirror stainless steel was chosen as the cathode because its good scale detachment ability, which has been proved in our previous work 31. An insulated silicone spacer was placed between the electrodes to keep an electrode gap of 5 mm. The electrochemical reactor was operated in a batch mode, including a water softening 5



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step and a scale detachment step. In the water softening step, feed water passed the electrochemical reactor upward. At the same time, low density electricity was supplied to the electrochemical reactor. The scale detachment step started when the time interval reached a certain value. In the scale detachment step, feed water passed the electrochemical reactor upward and meanwhile high density electricity was supplied to the electrochemical reactor. DC

Ef f l uent +Gas

Anode

1

2

Cat hode

I nf l uent

Figure 1. Experimental setup. 2.2. Feed solution The synthetic solution was prepared on the basis of the chemical composition of the recirculate cooling water. The dominant ions in recirculate cooling water were Ca2+, Mg2+, Na+, HCO3- and Cl-. Furthermore, the anti-scalant was considered as one of the common composition elements in recirculate cooling water. The solution was prepared by dissolving 333 mg/L CaCl2 (AR, 96.0%, Aladdin, Shanghai, China), 504 mg/L NaHCO3 (AR, 99.5%, Aladdin, Shanghai, China), 101.5 mg/L MgCl2·6H2O ((AR, 98.0%, Aladdin, Shanghai, China) and 2 mg/L 804anti-scalant in deionized water, corresponding to 300 mg/L calcium hardness, 50 mg/L magnesium hardness and 300 mg/L alkalinity in terms of CaCO3, respectively. Its conductivity was nearly 1400 µS/cm and the pH value was around 8.5, consistent with the 6


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typical values of recirculate cooling water 32-34. 2.3. Analysis methods Ca2+ and Mg2+ concentrations were determined by EDTA titration. The morphology of the scale deposits was examined using field emission scanning electron microscopy (FE-SEM: TM3000, Hitachi, Japan). Turbidity, conductivity and pH values were measured using a turbidimeter (1900C, Hach, America), a conductivity meter (Sension 5, Hach, OH, resolution: 0.01 µS/cm) and a pH/ISE meter (Orion Dual StarTM, Thermo Scientific, Singapore). The properties of the cathode before and after softening period were investigated by cyclic voltammetry using an electrochemical workstation (CHI660 Chenhua Instruction Co., Shanghai, China) and a three-electrode cell. Pt was used as a counter-electrode, and Ag/AgCl (3 M KCl, 0.205 V vs standard hydrogen electrode, SHE) served as a reference electrode. Cyclic voltammograms were obtained in feed solution at a scan rate of 5 mV/s. 3. Results and discussion 3.1 Water softening performance In an attempt to better understand the softening performance of the electrochemical precipitation process, the total hardness and cation ions concentrations in the effluent, the turbidity value of the effluent and the voltage required were examined systematically during softening. Figure 2 shows the total hardness and cation ions concentrations variations during softening. It was found that the Ca2+ concentration in the effluent dropped rapidly from initial 330 mg/L to 284 mg/L after 20 min. Then, it decreased gradually, and reached the minimum value of 259 mg/L after 6 h 20 min. Subsequently, it increased gradually, and reached to 305 7



mg/L after 12 h. In contrast, Mg2+ shows a different removal trend from Ca2+. Its initial concentration in the effluent was 53 mg/L; the concentration of Mg2+ then decreased gradually, and maintained around 41 mg/L after 8 h 20 min. The total hardness varied with time similar to that of Ca2+. It was found that the total hardness in the effluent decreased rapidly from initial 382 mg/L to 335 mg/L after 20 min. The total hardness then decreased gradually and reached about 304 mg/L after 6 h 40 min. The total hardness increased gradually thereafter, and finally, it reached 347 mg/L.

400

Total hardness and cation ions concentrations, mg/L CaCO3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Ca2+ Mg2+ Total hardness

380 360 340 320 300 280 260

40 20 0

2

4

6

8

10

12

14

Time, h Figure 2. Total hardness and cation ions concentrations variations in the effluent during softening at a current density of 100 A/m2 and a flow rate of 10 L/h. The Ca2+ removal process could be illustrated in Figure 3. This process was mainly constituted of two stages: a nucleation growth stage and a cathode blocking stage. The reactions in the nucleation growth stage are shown in Figure 3(a). The reaction zone was located on the cathode surface first, accompanied by the formation of a thin CaCO3 layer, 8


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which served as crystal nucleus. This explained why the Ca2+ concentration decreased in the initial period of time. The reasons why the Ca2+ concentration increased gradually in the later period of the softening were complex. On important reason was cathode blocking. With the proceeding of the precipitation reactions, the Ca2+ removal process was gradually converted into the cathode blocking stage. As demonstrated in Figure 3(b), the rapid growth of the CaCO3 deposits would block the most of the cathode surface, which caused a reduction in the effective precipitation area. Another important reason causing the gradual increase in the Ca2+ concentration was mass transfer limitation. As the scale layer became thicker, correspondingly, the decline in OH-, Ca2+ and HCO3- convective diffusion rate occurred due to slowly upward shifting the reaction zone and simultaneously shrinking the ion transport channels between the deposition aggregates. Furthermore, the gradual increase in the Ca2+ concentration was also associated with H2 evolution reaction inhibition. Figure 4 displays the cyclic voltammograms (CVs) obtained on the new cathode and the scaled cathode in feed solution. It can be seen that both cathodes presented typical voltammetric behaviors. The increased currents observed below 0.2 V vs NHE and -0.3 V vs NHE were attributed to O2 reduction and H2 evolution, respectively. However, at potentials below -0.3 V vs NHE, the cathodic current density of the scaled cathode was much lower than that of the new cathode, indicating that the scale seriously inhibited the H2 evolution. The reason why Mg2+ concentration had no rising trend was that the Mg2+ in the solution could react directly with OH-. This one step precipitation reaction was much easier than the two steps Ca2+ precipitation reactions, in which HCO3- served as an intermediate. Furthermore, the radius of Mg2+ was smaller than that of Ca2+, revealing that the diffusion was enhanced. Therefore, the 9



interfacial charge transfer was the rate determining step for Mg2+ precipitation instead of the mass transfer. Because Ca2+ was the major hardness ion in the feed solution, the total hardness variation with time was similar to that of Ca2+.

Ca2+

HCO3-

Hydrogen bubbles

Mg2+ HCO3- Channel

Mg2+

Ca2+ Channel

Reaction zone

-

OH

OH-

Scale layer Cathode

(a)

(b)

Figure 3. Ca2+ removal process of water softening. (a) The nucleation growth stage; (b) The cathode blocking stage.

6

New cathode Scaled cathode

5

Current density, A/m2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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4 3 2 1 0 -1 0.4

0.3

0.2

0.1

0.0

-0.1

-0.2

-0.3

-0.4

-0.5

-0.6

Potential, V vs. NHE Figure 4. Cyclic voltammograms of the new and scaled cathodes obtained in feed solution at a scan rate of 5 mV/s. Figure 5 shows the effluent turbidity variation during softening. It can be seen clearly 10


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that the turbidity of the effluent increased slowly from 0.08 NTU to 15 NTU in the initial 8 h, then increased rapidly, and reached about 50 NTU after 10 h. The turbidity increased slowly thereafter, and finally, it reached 63 NTU. With the accumulation of the scale on the cathode, the rapid increase in turbidity was clearly related to the accelerated scale breaking under the effect of H2 evolution. In our previous study 31, a sedimentation tank was used to separate the suspended solids in the effluent, and it was efficient. However, it is uneconomical to build auxiliary separation facilities in industrial application. The suspended solids in the effluent could be controlled by detaching the scale before the rapid increase of the turbidity.

70 60 50

Turbididy, NTU

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


40 30 20 10 0 0

2

4

6

8

10

12

14

Time, h Figure 5. Effluent turbidity variation during softening at a current density of 100 A/m2 and a flow rate of 10 L/h. The total voltage imposed on the electrochemical cell is the summation of equilibrium potential difference, anode overpotential, cathode overpotential, and the ohmic potential drop of the solution resistance, which could be illustrated below 3, 35, 36: 11



U = Eeq +ηa + ηc + I（ ⋅ Rs +Rsolution） where Eeq is the equilibrium potential difference;

ηa

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(6)

is the anode overpotential, V;

ηc

is

the cathode overpotential, V; Rs is the total resistances of the wires, anode and the cathode, respectively, Ω; Rsolution is the solution resistance, Ω; and I is the current density , A/m2. The variation of the voltage is shown in Figure 6. It can be found that the voltage increased consecutively from 5.3 V to 7 V during the whole softening step. In general, the voltage required is mainly determined by the ohmic potential drop, especially at high current density and low conductivity. But in this study, it is worth mention that the cathode overpotential was found to be another critical factor affecting the voltage. When the reactions were at the nucleation growth stage, obviously, the decrease in total hardness concentration would result in the decrease in solution conductivity, causing an increase in voltage. As water softening proceeded to the cathode blocking stage, on the one hand, the conductivity of the solution increased gradually, leading to a decrease in solution resistance. On the other hand, the progressive occupancy of the cathode surface by the scale caused an increase in the local current density. Therefore, an increase in cathodic polarization would occur and subsequently leaded to an increase in cathode overpotential. As a whole, the slight solution resistance decrease would be offset by the significant cathode overpotential increase, and hence caused an increase in voltage.

12


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10

9

8

Voltage, V

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


7

6

5

4 0

2

4

6

8

10

12

14

Time, h

Figure 6. Voltage variation during softening at a current density of 100 A/m2 and a flow rate of 10 L/h. 3.2 Effects of current density and flow rate Current density and flow rate are both important operating factors influencing the performance of electrochemical process

37-43

. In this study, investigation of these effects on

precipitation rate, energy consumption and current efficiency were carried out, and the results are shown in Figure 7. The CaCO3 precipitation rate, energy consumption and current efficiency could be calculated according to Eq. 7, Eq. 8 and Eq. 9, respectively. mCaCO3 =

E= CE =

5 ( Ca 2 + in − Ca 2 + out ) Q 2 Ac

UIt m(CaCO3removed )

F × ( n ( C a 2 + rem oved ) + 2 n ( M g 2 + rem oved )) It

(7)

(8) (9)

where mCaCO3 is the CaCO3 precipitation rate, g/h/m2; Q is the feed solution flow rate, L/h; Ac is the cathode area, m2; (Ca2+)in and (Ca2+)out are the concentration of Ca2+ in the 13



influent and the effluent, g/L; E is the energy consumption, kWh/kg; U is the average voltage, V; t is the treatment time; I is the current, A; m(CaCO3removed) is the total mass of removed hardness ions in terms of CaCO3, g; CE is the current efficiency, %; n(Ca2+removed) and n(Mg2+removed) are the Ca2+ and Mg2+ removed moles number; F is the Faraday constant. Figure 7(a) shows the precipitation rate variation with current density under different flow rates. As expected, the precipitation rate depended strongly on the flow rate and the current density. At the same current density, the precipitation rate increased when the flow rate increased. That was understandable because the volume of feed solution treated increased when the flow rate increased. Three main regions appeared in each precipitation rate-current density curve. Region

, whose current densities were lower than that of the first turning

point, presented linear variation, implying that the simultaneous increased OH- concentration and the accelerated ion electrical migration were responsible for the accelerated precipitation reactions. Nonetheless, Region

, whose current densities were higher than that of the

second turning point, had an asymptotic precipitation tendency, indicating that the reactions (3) and (4) were restrained by the limiting mass transfer of Ca2+ and HCO3- from the bulk solution to the reaction zone and the OH- from the cathode vicinity to the reaction zone, oppositely. A curvature region (Region

) existed between these two linear regions

demonstrated that the mass transfer limitation took place and became predominant gradually. The critical current densities were estimated to be 60 A/m2, 100 A/m2, 120 A/m2 and 130 A/m2 at flow rates of 5 L/h, 10 L/h, 15 L/h and 20 L/h, respectively. It can be found from Figure 7(b) that as the flow rate increased, the energy consumption decreased; while the current efficiency increased. For example, when the current density was 100 A/m2, the energy 14


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consumption and the current efficiency were 26 kWh/kg CaCO3 and 8.3 %, respectively, at a flow rate of 5 L/h, while the energy consumption and the current efficiency were 11 kWh/kg CaCO3 and 19 %, respectively, at a flow rate of 20 L/h. It can also be found that as the current density increased, the energy consumption increased; while the current efficiency decreased initially, and then reached an asymptotic limit. For example, when the flow rate was 10 L/h, the energy consumption and the current efficiency were 2.7 kWh/kg CaCO3 and 38 %, respectively, at a current density of 20 A/m2, while the energy consumption and the current efficiency were 70 kWh/kg CaCO3 and 6.2 %, respectively, at a current density of 300 A/m2. In order to obtain high precipitation rate and high softening efficiency, an adequate mass transfer limitation was required, which indicated that a relatively high current density should be imposed to the reactor. However, this lowered the current efficiency and increased the energy consumption. Based on such considerations, current densities in region considered to be appropriate in this work.

100

(a)

90

Precipitation rate, g/h/m2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80

5L/h 10L/h 15L/h 20L/h

Asymptotic limit

Critical current density

70 60 50 40 30 20

Region III

Region II

10

Region I

0 0

50

100

150

200

250

Current density, A/m 15


2

300

350

are


(b) 40%

5L/h 10L/h 15L/h 20L/h

30%

20%

120 EC, KWh/kg CaCO3

50%

Current efficiency, %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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90 60 30 0

0

100 200 300 Current density, A/m2

10%

0% 0

100

200

300

Current density, A/m2 Figure 7. Effects of current density and flow rate on precipitation rate, energy consumption and current efficiency. (a) Precipitation rate-current density relationship curves under different flow rates; (b) Energy consumption–current density and current efficiency–current density relationship curves under different flow rates. Table 1 presents the performance comparison between this current pulsated electrochemical precipitation process and conventional systems

3-5, 15

. It can be seen clearly

that the current pulsated electrochemical precipitation process achieved much higher precipitation rate and required much shorter retention time than conventional ones. This is because electrochemical precipitation is a heterogeneous reaction occurred at the cathode surface, and the compact structure and short inter-electrode gap of the electrochemical precipitation reactor increased the effective electrode area remarkably, producing higher softening efficiency. Moreover, the energy consumption was much less than those in conventional systems. This was also contributed by the short inter-electrode gap, which could 16


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enhance the mass transfer and reduce the voltage. A previous novel seeds system achieved high precipitation rates by placing a cationic ion exchange membrane between the electrodes 4, 19, 23

. Table 1 shows the softening comparison between the current pulsated electrochemical

precipitation process and the seeds system. Obviously, we achieved a lower precipitation rate than the seeds system. However, due to the relatively short inter-electrode gap, we had much lower energy consumption than that of the seeds system. In addition, the current pulsated electrochemical precipitation has many other advantages including simple configuration, easy maintenance and long service life. Table 1. Water softening performance comparison of current pulsated electrochemical precipitation with conventional systems. Hardness (mg/L as CaCO3) 250 295 300 254 305 300 240 350 1180 300 350 600 600

Current density (A/m2) 20 22 10.9 ~30 10.9 20 ~20 20 108 100 100 100 100

Precipitation rate (g/h/m2) 2.6 3.2-5.5 3 2.5 2.5-5.0 2-4 3.4-6.9 19.89 22.8 4-6 40.47 200 174.1

Energy consumption (kWh/kg CaCO3) 20 ~20 9 21 8-16 72 22.4 2.75 16 153 14.8 7.6 2.34

Retention time (min)

Ref

2.1 2.6 2.6 18 6.3 5.8 16.8 0.6 2.2 5.8 0.6 — 1.9

[3] [4] [4] [4] [4] [5] [15] This work [3] [5] This work [4, 19, 23] This work

3.3 Scale detachment performance After the softening process, the scale should be detached. In order to know if scale could be effectively detached by current density change, the change in the scale layer was evaluated under different current densities. Figure 8 shows the FESEM images of scale layer variation 17



with current density. Irregularly shaped crystals can be clearly observed under all current densities. However, the distribution and size of the crystallites were different. Apparently, it was observed that at a current density of 100 A/m2, the cathode surface was totally blocked by the compact insulating layer that with different crystal morphologies. At a current density of 200 A/m2, some cracks and gaps were observed in the big crystal aggregates. But the cathode surface was still covered by the plenty of the tightly coupled crystals. However, with the further increase of current density, the detachment efficacy was enhanced rapidly. For example, at a current density of 300 A/m2, big crystal aggregates on the superficial layer were broke up and detached. But there was another scale layer, which consisted of small crystals, was observed. At a current density of 400 A/m2, it can be found that only a part of cathode surface was covered by some small crystals, whereas another area of the cathode was regenerated by the departure of the scale layer. At a current density of 500 A/m2, crystals adhered to the cathode became smaller and more dispersive. In the meantime, the cathode bare area became larger. When the current density increased to 600 A/m2, most scale has been removed. The average effluent turbidity and the detachment voltage during 5 min examination under different current densities were investigated, and the results are shown in Table 2. It was found that the average turbidity increased slowly, below the current density of 200 A/m2. With the increase of current density, the increase in average turbidity became rapid, and reached about 1270.1 NTU at current density of 500 A/m2, indicating that the detachment was enhanced. When the current density increased to 600 A/m2, the average turbidity was almost the same as that of the former case, indicating that the scale had been detached efficiently. Good detachment effects could be obtained under the high current density 18


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conditions. Nevertheless, this increased the detachment voltage. Based on such consideration, current density was chosen to be 500 A/m2 in the subsequent work.

Figure 8. Typical FESEM images of scale layer after 5 min examination under different current densities. (a) 100 A/m2; (b) 200 A/m2; (c) 300 A/m2; (d) 400 A/m2; (e) 500 A/m2; (f) 600 A/m2. Table 2. Average turbidity and detachment voltage during 5 min examination under different 19



current densities. Current density, A/m 100 200 300 400 500 600

2

Turbidity, NTU

Voltage, V

29.4 155.7 561.7 986.2 1270.1 1305.6

6.3 9.3 12.4 16.8 19.8 23.1

Scale detachment process could be illustrated in Figure 9. The H2, produced by water electrolysis between the gaps of the crystal aggregates, created pressure on the scale; while the scale had strong adhesion force to the cathode. Under low current densities, the pressure created by H2 was less than the adhesion force between the scale layer and cathode, thus only a small percentage of scale deposits were detached. This explained why the effluent turbidity was low under low current densities. As the current density increased, the amount of H2 produced increased significantly due to the enhanced water electrolysis, which could impose tremendous pressure on scale, leading to an enhancement in scale breaking and detachment. This explained why further increase of current density could increase the effluent turbidity amazingly. Cathode Hydrogen bubbles

Adhesion force

Pressure

Scale

20


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Figure 9. Scale detachment process. Figure 10 shows the variations of effluent turbidity and detachment voltage with detachment time under different flow rates. It could be found from Figure 10(a) that change trends of turbidities were similar under different flow rates investigated. As expected, the turbidity decreased when the flow rate increased. This was mainly attributed to the dilution effect. When the flow rate was 15 L/h, it was found that the turbidity increased slowly with the detachment time initially, and reached 2490 NTU after 2 min. The turbidity then decreased gradually. At the end of detachment, the turbidity of the effluent reached 686 NTU. The initial turbidity increase was attributed to the gradual H2 accumulation between the gaps of the crystal aggregates, which could create gradually enhanced pressure on scale. As the detachment proceeded, the scale adhered to the cathode decreased, and therefore, the turbidity decreased gradually. It could be found from Figure 10(b) that the detachment voltage decreased when the flow rate increased. This was probably attributed to the accelerated release of bubbles generated in the electrodes at the high flow rates. It was also found that the voltage decreased gradually with the detachment time initially, and then maintained a steady value at the end of the detachment. For example, when the flow rate was 15 L/h, the detachment voltage was 20.7 V initially, then decreased gradually, and reached about 19.8 V after 4 min. After that, the detachment voltage maintained around 19.7 V. The initial decrease in voltage was mainly attributed to a gradual decrease in cathodic polarization as the detachment proceeded, which leaded to a gradual decrease in cathode overpotential. At the end of the detachment, most of scale had been detached, which meant that the current density was uniformly distributed on the cathode. Therefore, the cathode overpotential would 21



maintain a constant value, leading to a steady voltage.

3000

(a)

15L/h 20L/h 25L/h

Turbidity, NTU

2300

1600

900

200 0

1

2

3

4

5

6

Time, min

21.0

(b)

15L/h 20L/h 25L/h

20.5

Voltage, V

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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20.0

19.5

19.0

18.5 0

1

2

3

4

5

6

Time, min Figure 10. Detachment performance under a current density of 500 A/m2 and various flow rates. (a) Turbidity variation; (b) Voltage variation. In our previous work, a new scale detachment method, named as air-scoured washing, 22


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was investigated

31

. It showed many advantages such as simple configuration, convenient

operation and environmental friendliness. Nevertheless, its detachment efficiency was rather unsatisfactory, especially in long-term operation, hence lowering the softening efficiency in the subsequent operational cycle. To know if the scale detachment was enhanced, a detachment performance comparison between air-scoured washing and current increase was conducted, and the result is shown in Figure 11. It can be seen from the figure that the cathode detached by current increase had smaller and more dispersive crystals than that detached by air-scoured washing. Meanwhile, the bare area of the cathode detached by current increase was larger than that of the cathode detached by air-scoured washing. In other words, the current increase had greater detachment efficiency than air-scoured washing.

Figure 11. Detachment performance comparison between air-scoured washing and current increase. (a) Air-scoured washing; (b) Current increase. 3.4 Operational stability In order to know if frequent detachment would induce a softening performance deterioration, the total hardness removal efficiency and the average energy consumption variations were investigated for 30 operational cycles, and the results are shown in Figure 12. The total hardness removal efficiency fluctuated in the range of 17.8-22.8 %. There was no 23



deterioration tendency in softening after 30 operational cycles, demonstrating that the electrochemical precipitation process was still effective in softening after repeated detachment. Moreover, it was noticed that the average energy consumption was found to be 8.9-13.2 kWh/kg CaCO3, and yet no energy consumption increase tendency appeared in these operational cycles. The results above showed that the electrochemical precipitation process

40

30

Total hardness removal efficiency Energy consumption 25 30 20

20

15

10 10 5

0 0

4

8

12

16

20

24

28

0 32

Energy consumption, KWh/kg CaCO3

had good softening performance and high stability.

Total hardness removal efficiency， %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Operational Cycles Figure 12. Total hardness removal efficiency and energy consumption variations in 30 operational cycles. 4. Conclusions A current pulsated electrochemical precipitation process was tested successfully for hardness removal from recirculate cooling water. Good softening performance and effective scale detachment were achieved. The precipitation rate was as high as 40.47 g/h/m2. The energy consumption and the total hardness removal efficiency were 8.9-13.2 kWh/kg CaCO3 24


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and 17.8-22.8 %, respectively. After repetitive operation, the current pulsated electrochemical precipitation process could run steadily without any performance decay.

References: 1. Lee, J.; Park, K.; Eum, H.; Lee, C. Desalination of a thermal power plant wastewater by membrane capacitive deionization. Desalination 2006, 196, 125-134. 2. Jiang, J. Q.; Graham, N.; Andre, C.; Kelsall, G. H.; Brandon, N. Laboratory study of electro-coagulation-flotation for water treatment. Water Res. 2002, 36, 4064-4078. 3. Hasson, D.; Lumelsky, V.; Greenberg, G.; Pinhas, Y.; Semiat, R. Development of the electrochemical scale removal technique for desalination applications. Desalination 2008, 230, 329-342. 4. Hasson, D.; Sidorenko, G.; Semiat, R. Low electrode area electrochemical scale removal system. Desalin. Water Treat. 2011, 31, 35-41. 5. Zhi, S.; Zhang, S. A novel combined electrochemical system for hardness removal. Desalination 2014, 349, 68-72. 6. Hu, J.; Chen, Y.; Guo, L.; Chen, X. Chemical-free ion exchange and its application for desalination. Desalination 2015, 365, 144-150. 7. Hu, J.; Fang, Z.; Jiang, X.; Li, T.; Chen, X. Membrane-free electrodeionization using strong-type resins for high purity water production. Sep. Purif. Technol. 2015, 144, 90-96. 8. Shen, X.; Li, T.; Jiang, X.; Chen, X. Desalination of water with high conductivity using membrane-free electrodeionization. Sep. Purif. Technol. 2014, 128, 39-44. 9. Shen, X.; Fang, Z.; Hu, J.; Chen, X. Membrane-free electrodeionization for purification of wastewater containing low concentration of nickel ions. Chem. Eng. J. 2015, 280, 711-719. 10. Xing, Y.; Chen, X.; Wang, D. Electrically Regenerated Ion Exchange for Removal and Recovery of Cr(VI) from Wastewater. Environ. Sci. Technol. 2007, 41, 1439-1443. 11. Lee, H.; Song, J.; Moon, S. Comparison of electrodialysis reversal (EDR) and electrodeionization reversal (EDIR) for water softening. Desalination 2013, 314, 43-49. 12. Lee, H.; Hong, M.; Moon, S. A feasibility study on water softening by electrodeionization with the periodic polarity change. Desalination 2012, 284, 221-227. 13. Su, W.; Pan, R.; Xiao, Y.; Chen, X. Membrane-free electrodeionization for high purity water production. Desalination 2013, 329, 86-92. 14. Su, W.; Li, T.; Jiang, X.; Chen, X. Membrane-free electrodeionization without electrode polarity reversal for high purity water production. Desalination 2014, 345, 50-55. 25



15. Gabrielli, C.; Maurin, G.; Francy-Chausson, H.; Thery, P.; Tran, T. T. M.; Tlili, M. Electrochemical water softening: principle and application. Desalination 2006, 201, 150-163. 16. Tlili, M. M.; Benamor, M.; Gabrielli, C.; Perrot, H.; Tribollet, B. Influence of the interfacial pH on electrochemical CaCO3 precipitation. J. Electrochem. Soc. 2003, 150, C765-C771. 17. Tlili, M. M.; Ben Amor, M.; Gabrielli, C.; Joiret, S.; Maurin, G.; Rousseau, P. Study of electrochemical deposition of CaCO3 by in situ Raman spectroscopy - II. Influence of the solution composition. J. Electrochem. Soc. 2003, 150, C485-C493. 18. Gabrielli, C.; Jaouhari, R.; Joiret, S.; Maurin, G.; Rousseau, P. Study of the electrochemical deposition of CaCO3 by in situ Raman spectroscopy - I. Influence of the substrate. J. Electrochem. Soc. 2003, 150, C478-C484. 19. Zaslavschi, I.; Shemer, H.; Hasson, D.; Semiat, R. Electrochemical CaCO3 scale removal with a bipolar membrane system. J. Membrane Sci. 2013, 445, 88-95. 20. Liao, Z.; Gu, Z.; Schulz, M. C.; Davis, J. R.; Baygents, J. C.; Farrell, J. Treatment of cooling tower blowdown water containing silica, calcium and magnesium by electrocoagulation. Water Sci. Technol. 2009, 60, 2345-2352. 21. Zhi, S.; Zhang, K. Hardness removal by a novel electrochemical method. Desalination 2016, 381, 8-14. 22. Zeppenfeld, K. Electrochemical removal of calcium and magnesium ions from aqueous solutions. Desalination 2011, 277, 99-105. 23. Gorni-Pinkesfeld, O.; Shemer, F.; Hasson, D.; Semiat, R. Electrochemical Removal of Phosphate Ions from Treated Wastewater. Ind. Eng. Chem. Res. 2013, 52, 13795-13800. 24. Tlili, M. M.; Rousseau, P.; Ben Amor, M.; Gabrielli, C. An electrochemical method to study scaling by calcium sulphate of a heat transfer surface. Chem. Eng. Sci. 2008, 63, 559-566. 25. Jaouhari, R.; Benbachir, A.; Guenbour, A.; Gabrielli, C.; Garcia-Jareno, J.; Maurin, G. Influence of Water Composition and Substrate on Electrochemical Scaling. J. Electrochem. Soc. 2000, 147, 2151. 26. Ketrane, R.; Leleyter, L.; Baraud, F.; Jeannin, M.; Gil, O.; Saidani, B. Characterization of natural scale deposits formed in southern Algeria groundwater. Effect of its major ions on calcium carbonate precipitation. Desalination 2010, 262, 21-30. 27. Gabrielli, C.; Maurin, G.; Poindessous, G.; Rosset, R. Nucleation and growth of calcium carbonate by an electrochemical scaling process. J. Cryst. Growth 1999, 200, 236-250. 28. Deslouis, C.; Gabrielli, C.; Keddam, M.; Khalil, A.; Rosset, R.; Tribollet, B.; Zidoune, M. Impedance techniques at partially blocked electrodes by scale deposition. Electrochim. Acta 1997, 42, 1219-1233. 29. Gabrielli, C.; Keddam, M.; Khalil, A.; Rosset, R.; Zidoune, M. Study of calcium carbonate scales by electrochemical impedance spectroscopy. Electrochim. Acta 1997, 42, 1207-1218. 30. Kraft, A.; Blaschke, M.; Kreysig, D. Electrochemical water disinfection Part III: Hypochlorite production from potable water with ultrasound assisted cathode cleaning. J. Appl. Electrochem. 2002, 32, 597-601. 31. Yu, Y.; Jin, H.; Meng, P.; Guan, Y.; Shao, S.; Chen, X. Electrochemical water softening using air-scoured washing for scale detachment. Sep. Purif. Technol. 2018, 191, 216-224. 26


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32. BF, C. Water Hardness Removal for Industrial Use: Application of the Electrolysis Process. Journal of Cancer Science & Therapy 2012, 01. 33. Deziani, M.; Rahmani, K.; Mirrezaei Roudaki, S. J.; Kordloo, M. Feasibility study for reduce water evaporative loss in a power plant cooling tower by using air to Air heat exchanger with auxiliary Fan. Desalination 2017, 406, 119-124. 34. Neveux, T.; Bretaud, M.; Chhim, N.; Shakourzadeh, K.; Rapenne, S. Pilot plant experiments and modeling of CaCO3 growth inhibition by the use of antiscalant polymers in recirculating cooling circuits. Desalination 2016, 397, 43-52. 35. Ling, Y.; Xu, H.; Chen, X. Continuous multi-cell electrochemical reactor for pollutant oxidation. Chem. Eng. Sci. 2015, 122, 630-636. 36. Chen, X. M.; Chen, G. H.; Yue, P. L. Investigation on the electrolysis voltage of electrocoagulation. Chem. Eng. Sci. 2002, 57, 2449-2455. 37. Karoui, H.; Riffault, B.; Jeannin, M.; Kahoul, A.; Gil, O.; Ben Amor, M.; Tlili, M. M. Electrochemical scaling of stainless steel in artificial seawater: Role of experimental conditions on CaCO3 and Mg(OH)2 formation. Desalination 2013, 311, 234-240. 38. Zuo, Q.; Chen, X.; Li, W.; Chen, G. Combined electrocoagulation and electroflotation for removal of fluoride from drinking water. J. Hazard. Mater. 2008, 159, 452-457. 39. Shen, F.; Chen, X.; Gao, P.; Chen, G. Electrochemical removal of fluoride ions from industrial wastewater. Chem. Eng. Sci. 2003, 58, 987-993. 40. Yi, S.; Ma, Y.; Wang, X.; Ha, Y. Green chemistry: Pretreatment of seawater by a one-step electrochemical method. Desalination 2009, 239, 247-256. 41. Ling, Y.; Hu, J.; Qian, Z.; Zhu, L.; Chen, X. Continuous treatment of biologically treated textile effluent using a multi-cell electrochemical reactor. Chem. Eng. J. 2016, 286, 571-577. 42. Sáez, V.; Esclapez, M. D.; Tudela, I.; Bonete, P.; González-García, J. Electrochemical Degradation of Perchloroethylene in Aqueous Media: Influence of the Electrochemical Operational Variables in the Viability of the Process. Ind. Eng. Chem. Res. 2010, 49, 4123-4131. 43. Boudreau, J.; Bejan, D.; Li, S.; Bunce, N. J. Competition between Electrochemical Advanced Oxidation and Electrochemical Hypochlorination of Sulfamethoxazole at a Boron-Doped Diamond Anode. Ind. Eng. Chem. Res. 2010, 49, 2537-2542.

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Page 28 of 28

Table of contents

Ca2+

Hydrogen bubbles

Mg2+ HCO3- Channel

HCO3-

Mg2+

Ca2+ Channel

Reaction zone

-

OH

Scale layer Cathode

softening step (low current density)

Pressure

Hydrogen bubbles Scale layer Cathode

Adhesion force

detachment step (high current density)

28


OH-

Current pulsated electrochemical precipitation for water softening

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