Desalination 263 (2010) 285–289

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Desalination

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Calcium carbonate hardness removal by a novel electrochemical seeds system

David Hasson ⁎, Georgiy Sidorenko, Raphael SemiatRabin Desalination Laboratory, Grand Water Research Institute, Department of Chemical Engineering, Technion–Israel Institute of Technology, Haifa 32000, Israel

⁎ Corresponding author. Tel.: +972 4 829 2936/2009E-mail address: hasson@tx.technion.ac.il (D. Hasson

0011-9164/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.desal.2010.06.036

a b s t r a c t

a r t i c l e i n f o

Article history:Received 31 May 2010Received in revised form 16 June 2010Accepted 17 June 2010

Keywords:Scale controlElectrochemical precipitationCaCO3 precipitation kineticsSeeds crystallizationElectrode area

Scale prevention is widely encountered in cooling water systems and is one of the main difficulties in boththermal and membrane water desalination processes. The usual scale control method applied in waterdesalination systems is based on the dosage of inhibiting compounds which are able to suppress scaleprecipitation up to a certain degree. Electrochemical scale control systems are beneficially used for hardnessabatement of cooling tower waters. The main drawback hindering their use in desalination applications is thevery high electrode area requirement. The novel electrochemical system developed in this study enables drasticreduction in the electrode area requirement. This is achieved by directing the precipitation to occur in a seedscrystallization vessel rather than on the cathode. Results obtained in preliminary experiments have alreadyyielded a reduction in the specific cathode area by a factor exceeding 10 without altering the specific energyrequirement. Furthermore, the seeds systemappears tobe free fromthe restrictionof an asymptotic precipitationrate limit. The outstanding advantages of the low electrode area seeds system opens possibilities for widespreadapplications of electrochemical hardness removal in diverse processes requiring scale prevention measures.

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1. Introduction

Scale deposition is a difficulty encountered in processing aqueoussolutions containing ions of sparingly soluble salts. Scale deposits canreadily form on flow surfaces when a solution is concentrated beyondthesolubility limit of a dissolved sparingly soluble salt orwhena solutioncontaining an inverse solubility salt is in contact with a hot surface. Suchconditions are met in both thermal and membrane desalinationprocesses. Scale deposition cannot be tolerated because of its highlydeleterious effects on production capacity and specific energy consump-tion. The usual scale control method applied in water desalination isbased on the dosage of inhibiting compoundswhich are able to suppressscale precipitation up to a certain degree. Themaximumwater recoverylevel that can be achieved in brackish water desalination is governedby the scale suppression capability of anti-scalants.

Many brackish water sources contain alkaline scale forming ionswhich are prone to precipitate CaCO3 and Mg(OH)2. One of thetechniques used to control the scaling potential of water circulating incooling towers is by electrochemical precipitation of the hardnesscomponents. The precipitation is induced by the generation of a high pHenvironment around the cathode by the following cathodic reactions:

O2 þ 2H2O þ 4e−→4OH

− ð1Þ

2H2O þ 2e−→H2 þ 2OH

−: ð2Þ

Thehighalkalineenvironmentacts to convert theHCO3− ion into theCO3

2−

form. The ensuing high supersaturation level of CaCO3 promotes itsprecipitation:

Ca2þ þ HCO

−3 þ OH

−→CaCO3 þ H2O: ð3ÞThe high pH conditions also promote precipitation of magnesiumhydroxide:

Mg2þ þ 2OH

−→MgðOHÞ2: ð4ÞElectrochemical scale removal offers many advantages: environ-

mental compatibility, no need to handle and dose chemicals,accessibility to automation and convenient process control [1,2]. Themain difficulty is disposal of the precipitated scale. Most of the depositadheres to the cathode leading to an increase in electrical resistance.Several techniques have been used for removing the scale depositingon the cathode including polarity reversal, periodic mechanicalscrapping and ultrasonic cleaning [3–5]. The prevalent technique ispolarity reversal. Its drawbacks are that it restricts the allowablecurrent density and shortens the lifetime of DSA electrodes [6].

The main factor prohibiting use of the current electrochemicaltechnology for scale control in desalination applications is the veryhigh specific electrode area requirement. For instance, in a brackishdesalination plant having a yearly output of one million cubic meter,the flow rate of the concentrate stream is of the order 20 m3/h.Assuming that the calcium content of the concentrate is around2000 ppm as CaCO3 and that it is desired to reduce this value by onehalf in order to extract additional permeate, it is necessary toprecipitate 20 kg/h CaCO3. A typical precipitation rate attained with

286 D. Hasson et al. / Desalination 263 (2010) 285–289

the current technology is around 50 g CaCO3/h/m2 cathode area. Thus,the required electrode area is as high as 400 m2.

The present paper describes a novel electrochemical precipitationconcept which has the potential for drastic reductions of the requiredelectrode area. The preliminary results presented below have alreadyyielded reduction of the specific electrode area by a factor exceeding10.

2. Seeded electrochemical precipitation

2.1. Basic concept

In the conventional equipment currently used for hardnessreduction in cooling tower systems, the water is in contact withboth the cathode and the anode electrodes. The cathode performs twofunctions: it generates alkalinity and serves as a scale depositionsurface. There is no medium separating the cathodic and anodicenvironments. High pH conditions prevail only in a thin boundarylayer near the cathodic surface while the bulk of the water is at thefeed pH level. Consequently, the precipitation reaction occurs only inthe water film adjacent to the cathodic surface. Periodic removal ofthe scale accumulating on the cathode is essential and the cleaningtechniques described above are rather cumbersome.

The basic concept of the novel process is separation of the anodeand cathode into two separate compartments using an appropriatemembrane (Fig. 1). In this case a high alkaline environment isgenerated throughout the whole volume of the cathodic compart-ment and not only in the boundary layer adjacent to the cathode. Bytransferring the alkaline solution to a separate reaction vesselcontaining calcium carbonate particles, the precipitation surface isnow the extensive area of the crystal seeds rather than the restrictedarea of the cathode. This concept also offers the advantage of flexibledesigns through control of retention time, suspension seeds concen-tration and seeds specific area.

Fig. 1. Electrochemical cell with separate compartments.

2.2. Factors governing seeded precipitation

The hydroxyl ion needed for precipitation of the alkaline scalecomponents (Eqs. (3) and (4)) is generated by the electric current.According to Faraday's law the rate of OH− generation, WOH mol/s,with a current of I Ampere is given by:

WOH =IF ⋅φ ð5Þ

where F is Faraday constant (96845 C/mol) and φ is the currentefficiency. The efficiency depends on the level of the current densityand on the leakage of hydroxyl ions through the separatingmembrane. Some literature data [7,8] suggest that the maximumcurrent efficiency is around 10 A/m2 and that the specific precipita-tion rate tends to be an asymptotic value at current densities around100 A/m2.

Design of a seeds electrochemical system can be guided byliterature data on the kinetics of calcium carbonate precipitation.Kinetic coefficients reported in the literature are of two types:fundamental coefficients kRS based on the actual crystallization areaand coefficients kRm based on seeds concentration. The most widelyadopted kinetic model was first proposed by Nancollas and Reddy [9].The equations for continuous flow precipitation in a mixed vesselaccording to this model are:

Ca½ �i− Ca½ �oτ

= kRS⋅SCaCO3 ⋅ Ca2+� �

⋅ CO2−3

� �−kSP

� �ð6Þ

Ca½ �i− Ca½ �oτ

= kRm⋅mCaCO3 ⋅ Ca2+� �

⋅ CO2−3

� �−kSP

� �: ð7Þ

An alternative kinetic model used by some researchers wasproposed by Davies and Jones [10]. According to this model,continuous flow precipitation in a mixed vessel is given by:

Ca½ �i− Ca½ �oτ

= kRS⋅SCaCO3 ⋅ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiCa2+� �

⋅ CO2−3

� �q−

ffiffiffiffiffiffiffikSP

q� �2ð8Þ

Ca½ �i− Ca½ �oτ

= kRm⋅mCaCO3 ⋅ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiCa2+� �

⋅ CO2−3

� �q−

ffiffiffiffiffiffiffikSP

q� �2: ð9Þ

The terms in brackets are activities, SCaCO3and mCaCO3

are crystalssurface area and seeds concentration respectively, τ is precipitationtime and superscripts i and o denote inlet and outlet conditionsrespectively. As pointed out by Inskeep and Bloom [11] there is littledifference in the values of the kinetic coefficients obtained by datareductions according to the two alternative kinetic expressions.

Table 1 summarizes literature values of the kinetic coefficients kRSand kRm at room temperatures and also provides values of theArrhenius activation energy E [7,11–19]. The kinetic coefficients ofreferences [15,16] were measured in a falling film flowing over avertical tube. In all other cases, the coefficients were determined inbatch seeded experiments. There is some scatter in the data but moststudies report kRS values in the range of 0.5 to 1.5 L2/minmol cm2.

3. Experimental

Electrochemical precipitation of CaCO3 was studied in thecontinuous flow system shown in Fig. 2. Flow of the feed solutionthrough the alkaline cathodic compartment was in the once-throughmode while flow of a solution in the acidic anodic compartment wasin recycling mode.

Two electrochemical cells were used. The first cell had a totalvolume of 900 mL and was separated into two compartments by acationic ion-exchange membrane (Nafion N-966, DuPont). The

Table 1Literature values of the kinetic coefficients in CaCO3 precipitation.

Reference Temp. kRS kRm SCaCO3mCaCO3

E

°C L2/minmol cm2 L2/minmol mg m2/L mg/L J/mol

Lisitsin et al., 2009 [12] 33 0.9 0.37 0.04–0.01 1000–2500Inskeep and Bloom, 1985 [11] 25 0.62–0.8 4.3–5.6 0.1–0.4 140–570 48,100Kazmierczak et al., 1982 [13] 1.5–3.0a 13.1–26.1a 0.09–0.17 100–200 39,200Reddy and Gaillard, 1981 [14] 25 0.8–1.9 0.004–0.456 20–2000Hasson et al., 1981 [15] 25–30 0.48–0.78 72,210Hasson et al., 1978 [16] 25–30 0.3–0.54 86,250Benjamin et al., 1977 [17] 20 5.7a 900–2000Sturrock et al., 1976 [18] 20 1.1–2.8 300–3000

3.0–4.5a

Wiechers et al., 1975 [19] 25 2.2–2.5 100–1000 43,100Nancollas and Reddy, 1971 [9] 25 0.1–0.16 0.3–0.5 0.1–0.6 350–2000 46,000

a Kinetic coefficients evaluated by the Davies and Jones model.

287D. Hasson et al. / Desalination 263 (2010) 285–289

solution leaving the cathodic compartment with an augmented pHflowed into a 1 L stirred vessel in which the main crystallizationprocess took place. The anode consisted of a 100×100 mm DSA platewhile the cathode consisted of stainless steel plate of the samedimensions. This cell enabled operation at current densities in therange of 40 to 120 A/m2. The second electrochemical cell wasdesigned to provide higher current densities. The cell had a totalvolume of 50 mL. The anode consisted of a 100×25 mm DSA platewhile the cathode consisted of a stainless steel plate of the samedimensions. This cell enabled operation at current densities up to600 A/m2. The experimental systems enabled feed flow rates throughthe cathodic compartment in the range of 50–150 mL/min. Flow in thecell was laminar with Reynolds numbers below 50.

Test solutions were prepared by dissolving technical grade saltsCaCl2 and NaHCO3 in a solution containing 50 mM of NaCl. Thesalinities of the solutions flowing in both cathodic and anodiccompartments were identical. Solution conductivity was around7.5 mS/cm. The pH of the solution in the feed tank was maintainedconstant by controlled bubbling of CO2 actuated by a pH controller(Mettler Toledo—pH 2050e). The calcium concentration was varied inthe range of 400 to 800 ppm as CaCO3, the total alkalinity in the rangeof 250 to 500 ppm as CaCO3 and the inlet pH was in the range of 6.8 to8.1.

The calcium removal rate was evaluated from the difference incalcium concentration between the feed and the solution leaving thecrystallizer. Calcium concentrations were determined by the EDTA

Fig. 2. Flowsheet of the continuous flow precipitation system.

titrimetric method (Standard method 3500-Ca). Alkalinity wasmeasured by potentiometric titration to the end point of pH=4.3(Standard method 2320). Each experiment lasted at least 6 residencetimes; steady state conditions were reached after 2–3 residence times.Each experiment was repeated several times and the reproducibilityof results was satisfactory.

4. Results

4.1. Electrode area

The improvement in electrolyzer performance obtained by shiftingthe role of precipitation surface to the seeds crystallizer wasinvestigated in a series of experiments carried out at varying currentdensities in the two seeds systems. The data obtained were comparedwith results measured in a conventional electrolyzer in whichprecipitation mainly occurs on the cathode [20,21].

Fig. 3 compares calcium carbonate removal rates per unit cathodearea measured in the 900 mL seeds electrolyzer with data obtained inconventional equipment. The figure displays a phenomenon observedin several studies [7,8,20,21] that increase in current density initiallyaugments the precipitation but that at sufficiently high currentdensities the precipitation rate tends to an asymptotic limit. There isno clear explanation for this phenomenon. According to Faimon et al.

Fig. 3. Rates of CaCO3 precipitation per unit cathode area in the 900 mL seeds systemcompared with data measured in conventional systems.

Fig. 5. Energy consumption in seeds systems compared with data measured inconventional systems.

288 D. Hasson et al. / Desalination 263 (2010) 285–289

[22], the decreased precipitation rate at high chlorine release rates isdue to CaCO3 dissolution by the following acidifying effect:

2CaCO3 þ Cl2 þ H2O→2Ca2þ þ 2HCO

−3 þ Cl

− þ ClO−: ð10Þ

Gabrielli et al. [7] suggest that limitations in the mass transfer rateof Ca2+ and HCO3

− from the solution bulk to the reaction zone areresponsible for the asymptotic precipitation tendency. This hypothe-sis finds support in a previous study [20] which provides theoreticaland experimental evidence that CaCO3 precipitation on a cathodicsurface is mass transfer controlled. Themost likely explanation for thelinearity relationship between precipitation rate and current densityin the seeds system is that the overwhelming proportion of theprecipitation reaction occurs on the seeds and the process is kineticrather than mass transfer limited.

The maximum precipitation rates achieved in conventionalsystems are seen in Fig. 3 to fall below 100 g CaCO3/h m2. The dataobtained with the seeds system showed no asymptotic limitation. Theprecipitation rate increased linearly with the current density and arate of 300 g CaCO3/h m2 was attained at the maximum allowablecurrent density of 120 A/m2 in the 900 mL seeds system.

Data measured with the improved 50 mL seeds system (Fig. 4)confirmed the linear increase of precipitation rate with currentdensity. At the maximum allowable current density of 600 A/m2 themeasured precipitation rate represents a reduction in specific cathodearea by a factor exceeding 10. Another outstanding advantage of theseeds system is that it appears to be free from the restriction of anasymptotic precipitation rate limit.

4.2. Energy consumption

An additional major parameter influencing the economics ofelectrochemical scale precipitation is the specific energy consump-tion. Analysis of the electrolytic CaCO3 precipitation system showsthat the main parameters governing the specific energy consumptionare the electrical resistances of the solution, of the electrodes and ofthe wiring connections [20]. The solution resistance depends on thesolution conductivity and on the distance between the electrodes.Fig. 5 displays specific energy results obtained in the electrolyticsystem described in references [20,21] and energy data measured inthe present study. It is seen that the energy consumption in bothelectrochemical systems is of the same order of magnitude and is

Fig. 4. Rates of CaCO3 precipitation per unit cathode area in the two seeds systemscompared with data measured in conventional systems.

typically in the range of 4 to 6 kWh/kg CaCO3. The reason for the lowenergy levels observed in the seeds system is that it allowednarrowing the electrodes gap, thus enabling low energy consump-tions at high current densities.

4.3. Kinetics coefficients

The design of an electrochemical seeds system is closely related tothe kinetics of the precipitation in the crystallization vessel. Kineticcoefficients based on seeds concentration in the 12 experimentscarried out in this study were in the range of 6 to 12 for Eqs. (7) and10 to 18 L2/minmol mg seeds, for Eq. (9). These values are somewhathigher than literature data andmight be due to the small particle sizesformed in the self nucleating crystallizer of the experimental system.

5. Conclusions

The major restriction barring application of the electrochemicalhardness reduction technology in desalination applications is therequirement for very high electrode area. The aim of the novel systemdeveloped in this study was to achieve a drastic reduction in therequired electrode area. The exploratory experiments carried out havealready shown the possibility of electrode area reduction by a factorexceeding 10. The new concept opens the possibility of integratingelectrochemical hardness removal in desalination processes andimplementing significant improvements in the electrochemicaltechnology currently used in cooling water practice. It could alsofind widespread use in diverse processes requiring scale controlmeasures.

Acknowledgement

Thanks are due to Albert Musafir, Project Manager of “Dead SeaBromineCo.”, for providing the ion-exchangemembraneNafionN-966used in this work.

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