Removal of scale forming species from cooling tower blowdown water by electrocoagulation using different electrodes

Removal of scale forming species from cooling tower blowdown water by electrocoagulation using different electrodes

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Accepted Manuscript Title: Removal of Scale Forming Species from Cooling Tower Blowdown Water by Electrocoagulation Using Different Electrodes Authors: Omar M. Hafez, Madiha A. Shoeib, Mohamed A. El-Khateeb, Hussein I. Abdel-Shafy, Ahmed O. Youssef PII: DOI: Reference:

S0263-8762(18)30284-3 https://doi.org/10.1016/j.cherd.2018.05.043 CHERD 3207

To appear in: Received date: Revised date: Accepted date:

13-3-2018 13-5-2018 30-5-2018

Please cite this article as: Hafez, Omar M., Shoeib, Madiha A., El-Khateeb, Mohamed A., Abdel-Shafy, Hussein I., Youssef, Ahmed O., Removal of Scale Forming Species from Cooling Tower Blowdown Water by Electrocoagulation Using Different Electrodes.Chemical Engineering Research and Design https://doi.org/10.1016/j.cherd.2018.05.043 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Removal of Scale Forming Species from Cooling Tower Blowdown Water by Electrocoagulation Using Different Electrodes Omar M. Hafeza*, Madiha A. Shoeibb, Mohamed A. El-Khateebc, Hussein I. Abdel-Shafyc and Ahmed O. Youssefd

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a*: Chemical Laboratories Section, Helwan Fertilizer Company (HFC), P.O. Box1081 El-Tabbin, Cairo, Egypt. E-mail: [email protected]

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b: Surface Coating Department, Central Metallurgical Research and Development Institute (CMRDI), P.O. Box12422 Helwan, Cairo, Egypt. E-mail: [email protected]

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c: Water Research & Pollution Control Department, National Research Centre, P.O. Box 12622

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Doki, Cairo, Egypt. E-mail: [email protected], E-mail: [email protected]

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Egypt. E-mail: [email protected]

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d: Department of Chemistry, Faculty of Science, Ain-Shams University, P.O. Box 11566 Cairo,

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Graphical abstract

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Highlights:  Removal of scale forming species from cooling tower blowdown water was investigated by EC.

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 The effect of a various operating parameter on EC has been optimized to achieve promised removal efficiencies.

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 the EC generated sludge was characterized by SEM-EDX, XRD and FTIR.

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 The maximum removal efficiency of 55.36 % and 99.54 % was achieved for hardness and silica ions respectively using Al-electrode.

Abstract

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This work investigates the effect of electrocoagulation (EC) using Al, Fe, and Zn electrodes for removing hardness ions and dissolved silica from cooling tower blowdown (CTB) water. The real

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samples were collected from urea fertilizer factory (Helwan Fertilizer Company), Cairo, Egypt. The

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effect of operational parameters, such as current density, electrolysis time, inter-electrode distance and stirring rate were studied and evaluated for the maximum efficiency. At the optimum operational conditions, Al-electrode removed the scale forming species from CTB water more efficiently than Zn

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and Fe electrodes. Al, Fe, and Zn electrodes removed 55.36% and 99.54%, 36.99% and 98.93% as well as 38.63% and 95.62% for the total hardness and silica ions, respectively. In order to rationalize the removal mechanism, the EC generated sludge was characterized by SEM-EDX, XRD, and FTIR. The present investigation inferred that Al-EC generates amorphous nature crystalline and other anode materials (Fe and Zn) forms definite crystalline particles. EDX showed the presence of Ca2+, Mg2+, 2

and silica ions in the sludge which proved the removal of these scale species from CTB water. As a conclusion, this study revealed that EC process using Al-electrode is a promising technology for the removal of scale forming species from CTB water.

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Keywords Cooling tower blowdown, Electrocoagulation, Al, Fe and Zn electrodes, Hardness removal, Silica

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removal, Sludge characterization.

Nomenclature and units: Name

EC

Electrocoagulation

CTB

Cooling Tower Blowdown

V

Volt

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Ampere

s

second

Ci

initial ions’ concentration

Cf

final ions’ concentration

min

minutes

rpm

rotation per minutes

SEM

Scanning Electron Microscope

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Symbol

EDX

Energy-Dispersive X-ray spectrometer

XRD

X-Ray Diffractometer

FTIR

Fourier Transform Infrared Spectrophotometer

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1. Introduction Cooling towers are constructions used for exchanging heat as well as preventing the discharge of excess heat into the air. Due to evaporation, the processed heat removed and the water temperature dropped down to a value of the wet-bulb air temperature. Their application is widespread in several industries (e.g. petrochemical, fertilizer plants, oil refineries, chemical plants, steel mills, electronics

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works, power plants, food industries and textile plants) (John 2009).

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Water sources have different concentrations of both suspended and dissolved solids (Abdel-Shafy, Salem et al. 2016). When water vaporizes from the cooling tower, these solids remain, causing the cooling water body to become more concentrated which may then produce scale deposit on heat

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transfer surfaces. The hardness and silica ions are the knotty scale forming species in cooling tower

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water (Li, Hsieh et al. 2011).

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To enhance the heat transfer efficiency, the concentrations of both hardness and silica ions should be

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under the levels that result in scale formed on heat transfer surfaces. This requires the continuous drain

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and commutation of fresh water for a small portion of the water circulating in the cooling tower loop. This water is usually referred to as blowdown water and is routinely rejected into the sanitary sewer

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system (Mathie 1998). The blowdown rate controls the level of suspended and dissolved solids in the system. Decreasing the blowdown rate increases the solids and vice versa. Calculations of the

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blowdown can be estimated from the relationship between cycles of concentration in relation to the

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evaporation as shown in the following formula (Flynn 2009).

Blowdown = Evaporation/ (Cycles - 1)

(1)

The volume of blowdown water needed for cooling tower operation can be decreased by removing or decreasing the scale forming species from water.

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The usual scale forming species removal methods applied in industrial water include: adding antiscalant, which always contaminates the water source during preventing scaling (Hakizimana, Gourich et al. 2016, Neveux, Bretaud et al. 2016), reverse osmosis and ion exchange, which generates large volumes of brine streams. This requires disposal or further treatment by evaporation or crystallization (Jiang, Li et al. 2017, Tong, Zhao et al. 2017) and chemical precipitation

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(typically, lime softening). The later emits large volumes of lime sludge stream (Stewart, Nyman et al. 2011). One of the established methods to overcome the disadvantages of conventional water and

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wastewater treatment is EC. This EC is a green technology that could be applied in water and wastewater treatment, which collects the tasks and advantages of electrochemistry, beside

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conventional chemical techniques (Demirbas and Kobya 2017). In comparison from both economic and technical perspective to conventional treatment methods, EC process has drawn attention of

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researchers due to its high removal efficiency without adding any chemicals. This makes EC as “a

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green technology”. Unlike membrane and ion exchange and other conventional treatment methods

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EC has the advantages of producing less sludge and it doesn’t generate any brine streams that require

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disposal or further treatment. Thus, minimizes the cost sludge disposal, as well as the operating costs than the conventional technologies. Since the EC process can be started by turning on the switch, it

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requires minimal startup time (Khandegar and Saroha 2013, Hakizimana, Gourich et al. 2017,

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Naje, Chelliapan et al. 2017).

There are some deficiencies in EC process that were reported by several authors (Franco, Lee et al. 2017, Garcia-Segura, Eiband et al. 2017, Gönder, Balcıoğlu et al. 2017). This includes the use of

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electricity is considered one of the main limitations, due to the increasing cost of energy costs, the possible anode passivation or/and sludge deposition on the electrodes that can constrain the electrolytic process in continuous operation mode, the sacrificial anodes are consumed and must be replaced periodically, a resistant oxide film may be grown on the cathode which may provide resistance to the

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flow of electric current, moreover, high conductivity of the solutions is required and occasionally the gelatinous hydroxide may have a tendency to solubilize. The primary reactions taking place in the EC cell with different anode materials are as follows (Kamaraj and Vasudevan 2015):

2H2O + 2e- →

H2(g) + 2OH-

(2)

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At the anode: When, aluminum as anode

Al3+ (aq) + 3 H2O →

Al (OH) 3↓ + 3H+

(3)

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Al3+(aq) + 3e−

(4)

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Al (s) →

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At the cathode:

When, zinc as anode

(5)

4Fe(OH)3 ↓+ 4H2

(6)

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4Fe + 10H2O + O2→

4Fe2+ + 8e−

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4Fe →

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When, iron as anode

Zn2+(aq) + 2e−

(7)

Zn2+(aq) + 2H2O →

Zn (OH) 2↓ + 2H+

(8)

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Zn (s) →

Several studies using EC to remove scale forming species from different types of water, including drinking water (Malakootian, Mansoorian et al. 2010), industrial process water (Zhao, Huang et al.

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2014), reverse osmosis reject water (Chen, Baygents et al. 2017), and seawater (Hakizimana, Gourich et al. 2016). However, few studies have planned for the treatment of potentially scale forming species, such as Ca2+, Mg2+, and silica ions in the CTB water by EC. Liao et al (Liao, Gu et al. 2009) checked the effect of pH values as well as different polymeric components on the performance of EC using Al and Fe electrodes for the treatment of CTB water having dissolved silica, Ca2+,and Mg2+.

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Gelover et al (Gelover-Santiago, Pérez-Castrejón et al. 2012) established three electrochemical systems to remove silica in make-up water for cooling tower. They found that the most suitable one with only Al-electrodes. Zhi et al (Zhi, Zhang et al. 2013) made a systematic study of some parameters which affect the EC process to remove silica from cooling water using response surface methods. Schulz et al (Schulz, Baygents et al. 2009) studied the efficiency of EC using Al and Fe electrodes

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for removing Ca2+, Mg2+, and silica from CTB.

However, the comparison between different three anodes (namely Al, Fe, and Zn electrodes) for

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removing hardness and silica ions from CTB water was never studied previously. In the meantime, the characteristics of the produced sludge was also not studied before.

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The aim of the current study is to reduce the disposal of CTB water. This could be achieved by

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optimizing the experimental conditions of hardness ions (Ca2+, Mg2+) and dissolved silica removal

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from cooling loop water by EC. In individual, the goals of this research work are to study extensively

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the functioning of Al, Fe, and Zn electrodes for removing hardness and silica ions from CTB water.

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2. Materials and methods

2.1. Wastewater characteristics

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The wastewater used in this research was the CTB water that discharged from the urea fertilizer factory (Helwan Fertilizer Company), El-Tabbin, Cairo, Egypt. The factory is producing about 2000 m3 of

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blowdown water daily. Table 1 shows the characteristics of the CTB.

2.2. EC experimental apparatus Experiments were carried out in a Plexiglas reactor having dimensions of (11 cm × 8.0 cm × 8.0 cm). Electrodes of Al, Fe, and Zn represented as the anode and stainless-steel sheets of the same size as the 7

cathode. A pair of parallel electrodes with dimensions of 5.0 cm × 12 cm × 0.2 cm operated in the monopolar mode was vertically positioned in the reactor with a total effective surface area of 70 cm 2 to a digital DC power supply (PsP Vari plus MUNK; 25 V/ 50 A) as established in Fig.1.

2.3. Experimental procedure

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Prior to each run, the electrodes were polished mechanically with sandpaper and rinsed with 10 % HCl solution for 30 s to remove the oxide and/or passivation layer from the electrodes. Finally, the

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electrodes were rinsed with deionized water, dried and weighted. Theses electrodes were placed into the EC reactor and the inter-electrode gap between anode and cathode was maintained at 1.0 cm,

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corresponding to an inner cell volume of 0.70 L. In each run, 500 ml of the CTB were placed into the

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batch EC reactor. Current held constant at desired values for each run and the experiment were started.

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The solution in the electrochemical cell was stirred gently with magnetic stirrer at 300 rpm using a

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digital magnetic stirrer (Wisd Wise Stir MSH-20D) to guarantee good mixing conditions. After settling, the supernatant samples were filtered using 4 μm filter paper and then the filtrate was then

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analyzed. Finally, the electrodes were washed thoroughly with deionized water to remove any residue

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on the surface, dried and re-weighed.

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2.4. Analytical procedures

Hardness (Ca+2, Mg+2) and silica ions concentration were conducted according to the standard methods

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for the examination of water and wastewater, 22nd edition (Rice, Bridgewater et al. 2012). The silica ions analyzed using Varian Cary 50-Conc UV-visible spectrophotometer. The pH was measured using WTW InoLab pH 720 pH meter, and the conductivity was determined with EUTECH CON 510 conductivity meters. The removal efficiency of hardness and silica ions concentration calculated using the following equation: 8

𝐑𝐞𝐦𝐨𝐯𝐚𝐥 𝐞𝐟𝐟𝐢𝐜𝐢𝐞𝐧𝐜𝐲 (%) =

𝑪𝒊 −𝑪𝒇 𝑪𝒊

× 𝟏𝟎𝟎

(9)

Where 𝐶𝑖 is initial ions’ concentration (mg/L) and 𝐶𝑓 is the final ions’ concentration (mg/L).

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2.5. Sludge characterization The evolution of surface morphology of generated sludge was determined by using scanning electron microscope (JEOL-JSM-5410, Japan) attached with an energy-dispersive spectrometer (EDX-Oxford)

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which was used for the determination of the elemental compositions. The phase and crystal structure of the generated sludge were investigated by using X-ray diffractometer (Bruker AXS-D8, Advance,

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Germany; operated at 35 kV and 45 mA with CuKα radiation λ = 0.1540 nm), XRD diffractions drew

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using DIFFRAC.SUITE™ software. Information about surface chemistry and presence of functional

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groups were studied by using Fourier Transform Infrared Spectrophotometer (FTIR-BUCK M500,

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USA), FTIR spectra were drawn by using GRAMS Version 7.0 software.

3. Results and discussion

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3.1. Effect of applied current density

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The optimization of the current density improve the rate of EC reaction (Al-Ghoul 2017). Meanwhile, the current density controls the rate of coagulant production, adjusts bubble production and hence affects the growth of flocculate (Mollah, Schennach et al. 2001, Aouni, Fersi et al. 2009) . As

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illustrated in Fig. 2, that the removal efficiency of the hardness and silica ions increased by increasing the current density, then it was slightly decreased above 14.29 mA/cm2. By increasing the current density from 1.43 to 14.29 mA/cm2, at the electrolysis time of 60 min, the removal efficiency of the total hardness increased from 19.4 to 55.36%, 11.62 to 36.99 %, and 18.0 to 38.63 % for Al, Fe, and Zn electrodes, respectively. In addition, the removal efficiency of silica ion increased from 67.74 to 9

99.54 %, 50.22 to 98.93 % and 65.25 to 95.62 % for Al, Fe, and Zn electrodes, successively. This result may be attributed to the increase in the current density, where anode dissolution rate increases simultaneously (according to Faraday's law). This leads to an increase in the number of metal hydroxide flocs that induce the increase the efficiency of scale removal (Akyol, Can et al. 2015). On the contrary, the removal efficiency of hardness and silica ions slightly decreased at a higher current

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density (namely: 18.57 mA/cm2). Beyond optimum current density, Al, Fe, and Zn ions were in the form of Al(OH)4- or Fe(OH)4- or Zn(OH)42- flocs remaining in dissolved form, thereby, reducing the

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removal efficiency of hardness and silica ions (Garg and Prasad 2016).

The effect of current density on electrode consumptions is shown in Fig. 5. The results indicated that

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for all anode materials (Al, Fe, and Zn), the electrode consumptions increased by increasing the current

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density. By increasing the current density from 1.43 to 14.29 mA/cm2, at constant electrolysis time 60

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min, electrode consumptions increased from 0.065 to 0.787 kg/m3, 0.142 to 2.08 kg/m3 and 0.187 to

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2.152 kg/m3 for Al, Fe, and Zn electrode, respectively.

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3.2. Effect of electrolysis time

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Anodic dissolution and the release of coagulating ion species occurred during the electrolysis process. The performance of hardness and silica ions removal depends mainly on the concentration of metal

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ions produced from the electrode materials. As the electrolysis process proceeds, anodic dissolution takes place leading to the release of ion species (act as coagulants). By extending the electrolysis time, the level of metal ions and their hydroxide flocs increases (Thakur and Mondal 2017). The effect of

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electrolysis time on Al, Fe, and Zn electrodes was examined at a constant current density (14.29 mA/cm2). As depicted in Fig. 4, an increase in the electrolysis time from 15 to 60 min, yields an increase in the removal efficiency of the total hardness from 25.0 to 55.36%, 12.33 to 36.99% and 23.29 to 38.63% for Al, Fe, and Zn electrodes, respectively. The corresponding removal efficiency of silica ions, thus, increased from 88.87 to 99.54%, 81.67 to 98.93 % and 80.84 to 95.62 % for Al, Fe, 10

and Zn electrodes, respectively. Beyond optimum electrolysis time (namely: 60 min), Al, Fe, and Zn hydroxide flocs were found in dissolved form, therefore, less number of scale anions were neutralized at flocs surface resulting in removal of hardness and silica ions not further improved (Garg and Prasad 2016).The effect of electrolysis time on electrode consumptions is shown in Fig. 5. Electrodes (Al, Fe, and Zn) consumptions were found to increase, upon increasing electrolysis time. At constant

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current density of 14.29 mA/cm2, and by increasing the electrolysis time from 15 to 60 min, the electrode consumptions increased from 0.167 to 0.786 kg/m3, 0.586 to 2.08 kg/m3 and 0.512 to 2.152

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kg/m3 for Al, Fe, and Zn electrodes, respectively.

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3.3. Effect of inter-electrode distance

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The effect of inter-electrode distance on removal efficiency of hardness and silica ions is shown in

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Figs. 6 & 7. The electrodes were fixed at a different space of 0.5, 1.0, 2.0 and 3.0 cm and at a current

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density of 14.29 mA/cm2. By increasing the electrode distance from 0.5 to 1.0 cm, the removal

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percentage of hardness and silica ions increases. Further increase of the electrode distance from 1.0 to 3.0 cm decreases the removal rate of hardness and silica ions. The shorter inter-electrode distance (0.5

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cm) makes it difficult to circulate the solution between the electrodes, hence the removal efficiency was decreased and increases gradually with increasing the inter-electrode distance up to 1 cm. Removal

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of both hardness and silica ions reduces with the increase in electrode distance from 1.0 to 3.0 cm is attributed to less interaction of scale forming species with the hydroxyl polymers (Modirshahla,

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Behnajady et al. 2008).

3.4. Effect of stirring rate The effect of stirring rate on removal efficiency of hardness and silica ions are shown in Figs. 8 & 9 by using different stirring speeds (from 80 to 600 rpm). As the stirring speed increased from 80 to 300 11

rpm, the removal efficiency of scale forming species increased. However, when the stirring rate was increased from 300 to 600 rpm, removal efficiency of hardness and silica ions decreased. Increasing the stirring speed up to the optimum at 300 rpm, there was an increase in the pollutant removal performance. This may be attributed to the increase in the motion of the generated ions, the flocs are formed much earlier resulting in an increase in the pollutant removal efficiency up to limited stirring

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speed. But with further increase in the stirring speed beyond the optimum value (300 rpm), there was a decrease in the removal efficiency. At higher stirring speed, the flocs were broken up due to a high

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turbulence in the reaction media and accordingly, the particles responsible for coagulation do not have enough time to be agglomerate to remove the scale forming species. In addition, the flocs were

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degraded by collision with each other, as indication of the higher stirring speed results in lower removal

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efficiency. (Fajardo, Rodrigues et al. 2015).

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3.5. Sludge characteristics

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The EC generated sludge was characterized by SEM-EDX, XRD and FTIR. Such characterization could help to identify the structure and physicochemical properties of the generated sludge to

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investigate the proposed mechanism for the removal of scale forming species through EC process as

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well as identifying the required properties to evaluate any further utilization/ management.

3.5.1. SEM-EDX characterization

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SEM and EDX spectra of sludge obtained at optimum operating conditions using Al, Fe, and Zn electrodes are given in Figs. 10 (a–c). SEM images of samples showed lumpy particles with an average size of several micrometers for Al, Fe, and Zn electrodes. The flocs and sludge material obtained using Zn-electrode under similar conditions was found to contain much smaller particles (Fig. 10 c). EDX spectra of sludge generated by Al, Fe, and Zn electrodes showed the presence of Ca2+, Mg2+, and silica

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ions in the sludge (Table 2). EDX analysis suggests that this procedure was successful to captured a substantial part of the scale forming species from CTB water.

3.5.2. XRD studies

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XRD spectra of Al, Fe, and Zn electrodes generated sludge are shown in Fig. 11. XRD spectra of Alelectrode sludge showed a very broad and shallow diffraction peaks (Fig. 11 a). So that the analyzed

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phase does not have long-range order required to give sharp Bragg peaks. Broad and shallow diffraction peaks of Bragg reflection showed that Al-electrode sludge produced amorphous and poorly crystalline phases. The obtained results were in a good agreement of that obtained by Dixon et al

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(Dixon, Weed et al. 1977). XRD pattern of Fe-electrode sludge showed a mixture of the well and

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poorly crystalline phases such as magnetite and lepidocrocite (Fig. 11 b). Zn-electrode produced

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sludge of well crystalline phases, such as Zincite and Diopside (Fig. 11 c). Table 3 lists the phases

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numbers, and their most likely nature.

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identified in EC sludge by using Al, Fe, and Zn electrodes via XRD, with their corresponding PDF

3.5.3. FTIR analysis

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Fig. 12 represents the FTIR spectra of different types of sludge. This figure exhibited 20 to 30 discernible peaks at frequencies of 600 - 4000 cm-1 related to the standard in the FTIR library (Rayner,

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Cushing et al. 2009). The presence of various functional groups in the generated sludge of Al, Fe, and Zn electrodes showed electrolyte interaction between cations and flocs. FTIR spectrum of Alelectrode sludge (Fig. 12 a) appeared, OH stretching, hydroxyl bending, Al–O–H bending on aluminum hydroxide/oxyhydroxides at ca. 3405, 1680, 959, and 794 cm−1, respectively.

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(Fig. 12 b) shows the characteristics of Fe-electrode sludge. There is OH stretching at 3830 and 3113 cm−1, hydroxyl bending and -OH water bending vibration or overtones of hydroxyl bending around 1566 cm−1 (Goldberg and Johnston 2001, Ruan, Frost et al. 2002). Bands for lepidocrocite phase showed up at 1227, 1023, and 794 cm−1 (Balasubramaniam and Kumar 2000). FTIR spectrum of Zn-electrode sludge showed band located near 600 cm-1 that can be attributed to the Zn-O stretching

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mode (Fig. 12 c). The band at 3200 to 3600 cm-1 corresponds to the stretching vibration of –OH bond and a weak band at ~ 1640 cm -1, arising from the H-O-H bending vibration attributed to the water

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molecule.

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The results indicate that EC process effective for removing hardness ions (Ca2+, Mg2+) and

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4. Conclusion

The maximum removal efficiency of 55.36 % and 99.54 % was achieved for hardness and silica

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dissolved silica responsible for scale formation on cooling water systems.

ions respectively using Al-electrode at a current density of 14.29 mA/cm2; electrolysis time of

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60 min; inter-electrode distance of 1.0 cm and stirring rate of 300 rpm.



The electrode consumptions for treating of CTB water with different electrodes at optimal

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conditions were calculated as 0.787, 2.08 and 2.152 kg/m3 for Al, Fe, and Zn electrodes, respectively.



EC sludge was characterized by SEM-EDX, XRD, and FTIR, the results indicated that scale forming species could be removed by the hydroxide flocs generated in the EC cell.

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5. Acknowledgments The authors express their sincere thanks to Helwan Fertilizer Company (HFC), Cairo, Egypt for supporting and providing the lab facilities to bring about this work.

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Khandegar, V. and A. K. Saroha (2013). "Electrocoagulation for the treatment of textile industry effluent–a review." Journal of environmental management 128: 949-963.

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ED

M

A

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SC R

Chemistry 25(16): 9309. https://doi.org/10.14233/ajchem.2013.15468

21

SC R

IP T

22

T.H Ca.H Mg.H SiO2

N Removal Efficiency (%)

A

60

D

40 20

0

2

4

6

8

10

12

14

16

18

80 60 40 20

(b)

20

Current Density ( mA/cm2)

A

CC

EP

(a)

100

M

80

TE

Removal Efficiency (%)

100

T.H Ca.H Mg.H SiO2

U

Fig. 1. Experimental setup used in the EC process.

22

0

2

4

6

8

10

12

Current Density

14

16

18

( mA/cm2)

20

23

T.H Ca.H Mg.H SiO2

IP T

80 60 40

0

(c)

2

4

6

8

10

12

14

16

18

SC R

20

20

Current Density ( mA/cm2)

U

Fig. 2. Effect of current density on the efficiency of EC process using (a) Al-electrode, (b) Fe-electrode

N A

2.5

Al Fe Zn

M

2.0

D

1.5

TE

1.0 0.5 0.0 0

2

4

6

8

10

12

Current Density

CC

Electrode Concumption, (kg /m3)

and (c) Zn-electrode (pH:8, Inter-electrode distance:1cm and t:60 min)

EP

Removal Efficiency (%)

100

14

16

18

20

(mA/cm2)

Fig.3. Effect of current density on the electrode consumptions using Al, Fe, and Zn electrodes (pH:8,

A

Inter-electrode distance:1cm and t: 60 min) .

23

24

. T.H Ca.H Mg.H SiO2

100

60 40 20

80

IP T

80

60 40

SC R

Removal Efficiency (%)

20 0

20

(a)

40

60

80

100

(b)

40

60

80

100

Electrolysis Time ( minutes)

M

A

N

Electrolysis Time ( min )

20

U

Removal Efficiency (%)

100

T.H Ca.H Mg.H SiO2

40

D TE EP

80

CC

Removal Efficiency (%)

100

60

T.H Ca.H Mg.H SiO2

A

20

(c)

20

40

60

80

100

Electrolysis Time ( min )

Fig. 4. Effect of electrolysis time on the efficiency of EC process using (a) Al-electrode, (b) Feelectrode and (c) Zn-electrode (pH:8, Inter-electrode distance: 1cm and current density: 14.29 mA/cm2)

24

Al Fe Zn

2.5 2.0

IP T

1.5

SC R

1.0 0.5 0.0 10

20

30

40

50

60

Electrolysis Time (min)

U

Electrode Concumption, (kg /m3)

25

N

Fig.5. Effect of electrolysis time on the electrode consumptions using Al, Fe, and Zn electrodes (pH:8,

A

current density:14.29 mA/cm2 and Inter-electrode distance:1cm)

M D

TE

Removal Efficiency (%)

60

50

Al Fe Zn

A

CC

EP

40

30

20 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Inter-electrode Distance ( cm)

Fig. 6. Effect of inter-electrode distance on the removal efficiency of total hardness using Al, Fe, and Zn electrodes (pH:8, current density:14.29 mA/cm2 and t: 60 min)

25

26

Al Fe Zn

IP T

95 90

SC R

85 80 75 0.0

0.5

1.0

1.5

2.0

2.5

3.0

U

Removal Efficiency (%)

100

3.5

N

Inter-electrode Distance (cm)

A

Fig.7. Effect of inter-electrode distance on the removal efficiency of silica ions using Al, Fe, and Zn

M

electrodes (pH:8, current density :14.29 mA/cm2 and t:60 min).

D TE

CC

40

EP

50

35

A

Removal Efficiency (%)

55

45

Al Fe Zn

30

25 0

100

200

300

400

500

Stirring Rate (rpm)

600

Fig. 8. Effect of stirring rate (rpm) on the removal efficiency of total hardness ions 26

27

using Al, Fe, and Zn electrodes (pH: 8, current density :14.29 mA/cm2 and t:60 min).

Al Fe Zn

IP T

98

SC R

96

U

94

0

100

200

300

400

500

M

Stirring Rate (rpm)

600

N

92

A

Removal Efficiency (%)

100

Fig. 9. Effect of stirring rate (rpm) on the removal efficiency of silica ions using Al, Fe, and Zn

A

CC

EP

TE

D

electrodes (pH:8, current density :14.29 mA/cm2 and t:60 min).

.

(a)

27

D

TE

EP

CC

A

SC R

(b)

U

N

A

M

IP T

28

(c)

28

29

Fig.10. SEM images and EDX analysis of EC generated sludge (pH:8, current density :14.29 mA/cm2

N

U

SC R

IP T

and t:60 min) using (a) Al-electrode, (b) Fe-electrode and (c) Zn-electrode.

A

CC

EP

TE

D

M

A

(a)

(b)

29

SC R

IP T

30

TE

D

M

A

N

U

(c)

Fig.11. X-ray diffraction patterns of EC generated sludge (pH:8, current density :14.29 mA/cm2 and

A

CC

EP

t:60 min) using (a) Al-electrode, (b) Fe-electrode and (c) Zn-electrode.

(a) 30

31

IP T

.

A

N

U

SC R

(

TE

D

M

(c

EP

Fig.12. FTIR spectrum of EC generated sludge (pH:8, current density :14.29 mA/cm2 and t:60 min)

CC

using (a) Al-electrode, (b) Fe-electrode and (c) Zn-electrode.

Table 1. Characteristic of cooling tower blowdown water were collect from Helwan Fertilizer

A

Company, El- Tabbin, Cairo, Egypt.

31

32

Parameter

Unit

Value

7.93

TDS

mg/l

1670

Total-Alkalinity (as CaCO3)

mg/l

150

Total-Hardness (as CaCO3)

mg/l

765

Ca-Hardness (as CaCO3)

mg/l

485

Mg-Hardness (as CaCO3)

mg/l

280

Silicates (as SiO2)

mg/l

27

Chloride

mg/l

250

Zn

mg/l

N

Total-phosphate (as PO4)

mg/l

5.80

Fe

mg/l

0.1

mg/l

733

mg/l

0.59

mg/l

6.00

mg/l

0.92

A

A

CC

EP

NO2

TE

NO3 (as N)

D

NH3

1.2

M

Sulphate (as SO4)

32

SC R

NTU

U

Turbidity

IP T

8.10

pH

33

Table 2. Elemental composition of sludge based on weight (%) using Al, Fe, and Zn electrodes. Element (wt.%) Al (K)

Fe (K)

Zn (K)

Mg (K)

Ca (K)

Aluminum

30.20

---

---

1.70

1.33

Iron

---

50.37

---

0.58

Zinc

-----

---

71.61

0.46

Si (K)

O (K)

IP T

Electrode Types

SC R

0.48

66.30

0.15

48.79

1.76

0.25

25.93

U

0.11

A

N

Table 3. Phases identified in Al, Fe and Zn electrodes’ EC sludges via XRD, their corresponding PDF

Phases identified or most likely

TE

Type of electrode(s)

D

M

numbers, and most likely nature of the identified phases.

EP

to be appear in the sludge

CC

Aluminum

A

Iron

Aluminum hydroxide Aluminum oxyhydroxide Iron hydroxide oxide

JCPDS-ICDD PDF# ----------70–0713

Lepidocrocite Magnetite 44–1415

79–0416

33

34

070-2551

Zincite Diopside

Zinc

SC R

IP T

082-0599

-The identification of all compounds was established by computer- assisted search of the PDF Database taken from The Joint Committee on Powder Diffraction Standards-International Centre for Diffraction

A

CC

EP

TE

D

M

A

N

U

Data (JCPDS-ICDD).

34