Investigation on the Electrolysis Voltage of Electrocoagulation

Chemical Engineering Science 57 (2002) 2449 – 2455

www.elsevier.com/locate/ces

Investigation on the electrolysis voltage of electrocoagulation Xueming Chen, Guohua Chen ∗ , Po Lock Yue Department of Chemical Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong Received 29 August 2001; accepted 27 February 2002

Abstract The relation between electrolysis voltage and the other variables of an electrocoagulation process was analyzed. Theoretical models describing such a relation were established. Experiments were conducted to con4rm the theoretical analysis and to determine the constants in the models. Both the theoretical analysis and experiments demonstrated that water pH and 7ow rate had little e8ects on the electrolysis voltage within a large range. The electrolysis voltage depends primarily on the inter-electrode distance, conductivity, current density and the electrode surface state. The models obtained can be used to calculate the total required electrolysis voltage for an electrocoagulation process. ? 2002 Elsevier Science Ltd. All rights reserved. Keywords: Model; Overpotential; Aluminum electrode; Water; Wastewater

1. Introduction Electrocoagulation is an e8ective process to destabilize 4nely dispersed particles for water and wastewater treatment. It has been successfully used to treat potable water (Vik, Carlson, Eikum, & Gjessing, 1984), urban wastewater (Pouet & Grasmick, 1995) and a variety of industrial wastewaters (Dobolyi, 1978; Pazenko, Khalturina, Kolova, & Rubailo, 1985; Balmer & Foulds, 1986; Demmin & Uhrich, 1988; Renk, 1988; Do & Chen, 1994; McClung & Lemley, 1994; Lin & Peng, 1996; Chen, Chen, & Yue, 2000a, b). Usually, aluminum or iron plates are used as electrodes in the electrocoagulation process. When a DC voltage is applied, the anodes sacri4ce themselves to produce Al3+ or Fe2+ ions. These electrochemically generated metallic ions are good coagulants. They can hydrolyze near the anodes to produce a series of activated intermediates that are able to destabilize the 4nely dispersed particles present in the water=wastewater to treat. The destabilized particles then aggregate to form 7ocs. At the meantime, the tiny hydrogen bubbles produced at the cathode can 7oat most 7ocs formed, reaching e8ective separation of particles from water=wastewater. Compared with conventional coagulation, electrocoagulation has many advantages. Firstly, it is more e8ective in destabilizing small colloidal particles. ∗

Corresponding author. Tel.: +852-2358-7138; fax: +852-2358-0054. E-mail address: [email protected] (Guohua Chen).

Secondly, it is able to ful4ll simultaneous coagulation and 7otation, with less sludge produced. Thirdly, the electrocoagulation equipment is very compact and thus suitable for installation where the available space is rather limited. Furthermore, the convenience of dosing control only by adjusting current makes automation quite easy. Although electrocoagulation has been available for more than a century, nowadays the design of an industrial electrocoagulation unit is still mainly based on empirical knowledge due to the lack of available models. The electrolysis voltage is one of the most important variables. It is strongly dependent on the current density, the conductivity of the water=wastewater to treat, the inter-electrode distance, and the surface state of electrodes. A model involving terms of activation overpotential, concentration overpotential and ohmic drop of the solution resistance has been proposed by Vik et al. (1984). However, that model cannot directly predict the electrolysis voltage because it still contains unknown terms including the activation overpotential and the concentration overpotential. In fact, these unknown overpotentials can be related to other simple variables according to the Tafel equation and Nernst equation. Therefore, it is possible to obtain simpli4ed models for the estimation of electrolysis voltage. The objectives of the present study are to establish the theoretical models regarding the electrolysis voltage required in the electrocoagulation process and to verify them experimentally. Since the dispersed particles in water=wastewater do not take part in the electrochemical reactions during electrocoagulation, the electrochemical behavior performed in

0009-2509/02/$ - see front matter ? 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 0 9 - 2 5 0 9 ( 0 2 ) 0 0 1 4 7 - 1

2450

X. Chen et al. / Chemical Engineering Science 57 (2002) 2449–2455

real water=wastewater should be similar to that carried out in aqueous solutions. Therefore, in this work, all experiments were conducted in the synthesized Na2 SO4 solutions for the purpose of easy control of conductivity. In addition, aluminum electrodes were used because they are the most common electrodes found in industrial applications.

Anode surface

Al 3+

2. Theoretical analysis When current passes through an electrochemical reactor, it must overcome the equilibrium potential di8erence, anode overpotential, cathode overpotential and ohmic potential drop of the solution (Scott, 1995). The anode overpotential includes the activation overpotential and concentration overpotential, as well as the possible passive overpotential resulted from the passive 4lm at the anode surface, while the cathode overpotential is principally composed of the activation overpotential and concentration overpotential. Therefore, d U0 = Eeq + a; a + a; c + a; p + |c; a | + |c; c | + j: (1)

When aluminum is used as electrode material, there are three major reactions in the electrochemical reactor as follows: (i) The oxidation reaction at the anode, Al − 3e = Al3+ ; RT ∗ ’Al3+ =Al = + ln CAl 3+ : 3F (ii) The reduction reaction at the cathode, 2H+ + 2e = H2 ;

_ x

0

Diffusion layer Fig. 1. Concentration variation of Al3+ , H+ and OH− near the anode.

In an electrochemical reaction, the mass transport includes di8usion, convection and electric migration, and can be calculated by Nernst–Plank equation (Bard & Faulkner, 1980) @Cj (x) Cj (x) @(x) − zj FDj + Cj (x)v(x) @x RT @x

or Jj (x) = −Dj

RT CH∗2+ : ln 2F PH 2

@Cj (x) @(x) + uj Cj (x) + Cj (x)v(x): @x @x

The total current is contributed by all ions present in the water=wastewater

(iii) The hydrolysis reaction,

j = Fzj Jj

Al3+ + 3H2 O = Al(OH)3 + 3H+ ; C ∗3+ Kh = ∗H CAl3+

OH

Jj (x) = −Dj

’oAl3+ =Al

’H+ =H2 = ’oH+ =H2 +

H+

(2)

The equilibrium potential di8erence between the anode and the cathode is Eeq = ’Al3+ =Al − ’H+ =H2 ; i.e. RT (pH2 )3=2 Eeq = ’oAl3+ =Al − ’oH+ =H2 + ln : (3) 3F Kh Eq. (3) suggests that Eeq is not a8ected by pH. The activation overpotential can be calculated from Tafel equation when the current density is relatively large as is the case for most industrial installations a; a = aa + ba ln j;

(4)

|c; a | = ac + bc ln j:

(5)


 @Cj (x) @(x) + uj Cj (x) = Fzj −Dj + Cj (x)v(x) @x @x or


   @Cj (x) j =  zj F −Dj + Cj (x)v(x) + tj j : @x Near the electrode surface, the convective 7ux term is eliminated (Scott, 1995). Moreover, within the di8usion layer adjacent to the anode surface, except for electrochemically and chemically reactive ions including Al3+ , H+ and OH− , non-reactive ions such as Na+ , SO2+ 4 , and so on, can produce a gradient of concentration and thus cause a diffusion current which is equal but opposite to the migration current at steady state. Consequently, the net transport and the net current of ions except for Al3+ , H+ , OH− through the anode di8usion layer is zero. Fig. 1 illustrates the concentration variation of Al3+ , H+ and OH− near the

X. Chen et al. / Chemical Engineering Science 57 (2002) 2449–2455

anode. In the practical application, although the original pH of water=wastewater may be low or high, the in7uent pH is usually controlled in a mediate range as the best electrocoagulation eMciency is commonly achieved in that pH range. Because of this, concentrations of H+ and OH− near the anode are relatively low and can be neglected. The total current in the circuit is then composed primarily of the current from migration and di8usion of Al3+ , that is j = −3FDAl3+

2451

Cathode surface

@CAl3+ |x=0 + tAl3+ j: @x

OH

_

Suppose that Al3+ concentration varies linearly across the whole di8usion layer, that is ∗ CAl3+ |x=0 − CAl @CAl3+ |x=0 3+ = ; @x a

H+ 0

then j = 3FDAl3+

∗ CAl3+ |x=0 − CAl 3+ a

+ tAl3+ j:

∗ Usually, CAl 3+ CAl3+ |x=0 . Thus,

CAl3+ |x=0 =

(1 − tAl3+ )j 3FDAl3+

a

Diffusion layer Fig. 2. Concentration variation of H+ and OH− near the cathode.

:

from the di8usion and migration of OH−

a depends on the turbulence of water or wastewater 7ow. In the electrocoagulation process, the turbulence is caused mainly from the agitation of the hydrogen gas generated at the cathode because the 7ow rate through an electrochemical reactor is slow. The larger the current density, the more the generated hydrogen gas and thus the thinner the a . Suppose a power-law relationship exists between a and j, that is −p ; a = k1 j

then

j = −FDOH−

@COH− |x=0 + tOH− j: @x

An analogous approximation is to assume a linear variation of OH− concentration across the whole di8usion layer, which is valid when bulk pH is not far away from neutral. Then C ∗ − − COH− |x=0 @COH− |x=0 = OH : @x c Thus,

CAl3+ |x=0 =

k1 (1 − tAl3+ )j 3FDAl3+

1−p

:

(6)

∗ CAl 3+ =

CH∗3+ : Kh

(7)

∗ COH − − COH− |x=0 c

+ tOH− j:

COH− |x=0 =

(1 − tOH− )j c : FDOH−

Similarly, suppose c = k2 j −q , then

Therefore, RT CAl3+ |x=0 RT k1 Kh (1 − tAl3+ )j 1−p = : ln ln ∗ 3F CAl 3F 3FDAl3+ CH∗3+ 3+

j = −FDOH−

∗ Usually, COH − COH− |x=0 , thus

∗ CAl 3+ can be calculated from Eq. (2) as

a; c =

x

(8)

Similarly, the net transport and thus the net current of ions except for H+ , OH− through the cathode di8usion layer is zero. Fig. 2 illustrates the concentration variation of H+ and OH− near the cathode. Because H+ is reduced to produce hydrogen gas, pH near the cathode is alkaline even if bulk pH is acidic but not extremely strong. In other words, the concentration of OH− near the cathode is much higher than that of H+ and hence the current there comes predominantly

COH− |x=0 =

k2 (1 − tOH− )j 1−q : FDOH−

The corresponding concentration of H+ at the cathode surface can be calculated according to the ion product of water CH+ |x=0 =

Kw COH− |x=0

or CH+ |x=0 =

Kw FDOH− : k2 (1 − tOH− )j 1−q

(9)

2452

X. Chen et al. / Chemical Engineering Science 57 (2002) 2449–2455

Thus,

tOH− , it approaches constant when is large. This is because tAl3+ and tOH− approach zero at large . Eqs. (13) and (14) indicate that U0 is independent on pH. The values of A, K1 , K2 , m, n, need to be determined experimentally.

CH∗ + RT ln |c; c | = F CH+ |x=0 =

RT k2 (1 − tOH− )j 1−q CH∗ + ln : F Kw FDOH−

(10)

Combining Eqs. (3) – (5), (8) and (10), Eq. (1) can be rewritten as RT (pH2 )3=2 ln + aa + ac U0 = ’oAl3+ =Al − ’oH+ =H2 + 3F Kh +

RT k1 Kh (1 − tAl3+ ) ln 3F 3FDAl3+

RT k2 (1 − tOH− ) d ln + a; p + j F Kw FDOH−


 RT (1 − p) RT (1 − q) + ba + bc + + ln j: 3F F

+

(11)

Let A = ’oAl3+ =Al − ’oH+ =H2 + +

RT (pH2 )3=2 ln + aa + ac 3F Kh

RT k1 Kh (1 − tAl3+ ) RT k2 (1 − tOH− ) ln + ln 3F 3FDAl3+ F Kw FDOH−

and RT (1 − p) RT (1 − q) + ; 3F F then Eq. (11) becomes d U0 = A + a; p + j + K1 ln j: (12)

It should be noted that the passive overpotential highly depends on the electrode surface state. For the new non-passivated aluminum electrodes, the passive overpotential can be neglected and Eq. (12) simpli4es to d U0 = A + j + K1 ln j: (13)

For the old passivated aluminum electrodes, the passive overpotential is usually signi4cant. It is related to many factors including pH, conductivity and current density. However, taking into account the fact that pH close to the anode is always acidic as long as bulk pH is not overly alkaline, it can be generally believed that a; p depends mainly on the conductivity and current density. Usually, a; p increases with the current density and decreases with the conductivity. Assuming a power-law relation of a; p with j and , then K2 j n a; p = m :

Therefore, for old passivated aluminum electrodes d K2 j n U0 = A + j + K1 ln j + m : (14)



On the right-hand side of Eqs. (13) and (14), both K1 and K2 are constants. Although A is related to tAl3+ and K1 = b a + b c +

3. Experimental verication To con4rm the theoretical analysis, a series of experiments was conducted at di8erent pH, 7ow rate, current density, conductivity and anode surface state. The experimental setup is schematically shown in Fig. 3. It consists of an electrocoagulation system, a DC power supply (PD 110 −5 AD, Kenwood TMI Corporation, Japan), a feed tank and a microprocessor pump (Model 7518-12, Master7ex, Cole-Parmer Instrument Co., USA). The electrocoagulation system has an electrochemical reactor of 0:30 L and a separator of 1:2 l. The electrochemical reactor contains 4ve aluminum electrodes connected in a bipolar mode. The original dimension of each electrode is 140 mm × 44 mm × 3 mm. Water 7ows through the electrochemical reactor upward, perpendicular to the electric current. Deionized water was used in all the experimental runs. The conductivity and pH were adjusted by adding Na2 SO4 (100 g=L) solution and H2 SO4 (0:1 M) or NaOH (0:1 M) solution. Water pH and conductivity were measured using pH meter (Model 420A, Orion Research Inc, USA) and conductivity meter (Checkmate 90, Corning Incorporated Scienti4c Products Division, USA), respectively.

6

_

+

Sludge Effluent

5 3 4

1

2

Influent

1. Tank 2. Microprocessor Pump 3. Electrocoagulation System 4. Electrochemical Reactor 5. Separator 6. D.C. Power Supply Fig. 3. Schematic diagram of experimental setup.

X. Chen et al. / Chemical Engineering Science 57 (2002) 2449–2455 25 κ = 355 µs/cm κ = 578 µs/cm

20

κ = 908 µs/cm κ = 1210 µs/cm

15 U0 /V

The experiments were carried out in two stages. The 4rst stage experiments were conducted after the electrodes had been used for about a month. At this stage, the anode surface looked still relatively smooth, thus could be considered as not passivated. The second stage experiments were done after the electrodes had been used for more than 3 months. At this stage, the anode surface was full of pits and covered with metal oxide, i.e. the anodes have been passivated.

2453

κ = 2090 µs/cm 10

5

4. Results and discussion 0

4.1. E.ect of pH

0

4.2. E.ect of water /ow rate

30

40

50

60

70

80

90

100 110 120 130 140

Fig. 4. Dependence of electrolysis voltage on conductivity and current density for non-passivated electrodes. Inter-electrode distance 6:4 mm, pH ◦ 7.20 –7.37, temperature 22.5 –23:8 C, 7ow rate 6 l=h.

30

κ = 445 µs/cm κ = 849 µs/cm

25

κ = 1390 µs/cm κ = 2120 µs/cm

U0 /V

20

κ = 3130 µs/cm 15

10

In the theoretical analysis, the water 7ow rate was assumed not to a8ect the electrolysis voltage signi4cantly. This needs to be veri4ed experimentally. Although a slight decrease in U0 was found as 7ow rate increased from 3 L=h to 15 L=h using new electrodes, the extent of such a decrease was always less than 10%. In contrast, when the current density varied from 20 to 100 A=m2 , almost 4vefold increase in electrolysis voltage was measured. For passivated electrodes, it was found that the electrolysis voltage was almost the same within the investigated water 7ow rate range of 1.8–21:6 L=h. This phenomenon is expected because the rough surface of passivated electrodes makes the e8ect of 7ow rate on turbulence insigni4cant. Such results support the assumption made earlier regarding the e8ect of 7ow rate. 4.3. Dependence of U0 on j and

Fig. 4 demonstrates the electrolysis voltage between electrodes as a function of the conductivity and current density for non-passivated electrodes. Through nonlinear regression of the data in Fig. 4, the constant and coeMcient in Eq. (13) were obtained as A = −0:76, K1 = 0:20. Hence d j + 0:20 ln j:

20

Current density/A m−2

The theoretical analysis has demonstrated that the electrolysis voltage between electrodes is independent of pH as long as water is not far away from neutral. In order to con4rm it, electrolysis voltages were measured at di8erent pH values. It was found that the e8ect of pH on U0 was really insigni4cant for both new and passivated electrodes. For the new electrodes, an increase in pH from 3.75 to 10.41 resulted in only an increase in U0 from 13.2 to 13:8 V even at a current density as high as 137 A=m2 . For the passivated electrodes, when pH varied from 3.38 to 10.79, the maximum di8erence of U0 measured at a constant current density was only 7.7%. Therefore, both results from new and passivated electrodes support the theoretical analysis well.

U0 = −0:76 +

10

(15)

5

0

0

10

20

30

40

50

60

70

80

90

100

Current density/A m−2

Fig. 5. Dependence of electrolysis voltage on conductivity and current density for passivated electrodes. Inter-electrode distance 7:0 mm, pH ◦ 6.95, temperature 21.9 –22:8 C, 7ow rate 6 l=h.

It shows in Fig. 4 that good agreement exists between the measured and predicted U0 for large solution. This is expected because A is a constant only when is large. Although there is considerable di8erence in for di8erent industrial operation, it is usually within the range investigated in this study. Thus, the present model can be applied without much error. The constant A has a negative value, which is attributed to the large but negative standard electrode potential di8erence between the anode and the cathode, −1:706 V ◦ at 25 C. Better prediction was obtained for passivated electrodes as shown in Fig. 5. This may be because the power-law term with regard to j accommodates partially the variation of A with . Again the constants in Eq. (14) were obtained

2454

X. Chen et al. / Chemical Engineering Science 57 (2002) 2449–2455

through nonlinear regression, and the equation became 0:016 d U0 = −0:43 + 0:47 j 0:75 + j + 0:20 ln j:



(16)

With U0 obtained, the total required electrolysis voltage U of an electrocoagulation process can be calculated easily. In general, there are two basic electrode connection modes: the monopolar mode and the bipolar mode. For the monopolar mode, the total required electrolysis voltage is the same as the electrolysis voltage between electrodes, that is U = U0 :

(17)

For the bipolar mode, the total required electrolysis voltage is U0 times the number of total cell which is the number of electrodes minus one. Thus U = (N − 1)U0 :

(18)

5. Conclusions

p H2 R T tj U uj U0 v(x) zj

fractional pressure of hydrogen at the cathode, atm gas constant, J=mol K absolute temperature, K transport number of species j total required electrolysis voltage of an electrocoagulation process, V mobility of species j, m2 =V s electrolysis voltage between electrodes, V convective velocity of water 7ow in the current direction at distance x, m=s charge number of species j

Greek letters a c

a; a a; c a; p c; a c; c

(x)

di8usion layer thickness near an anode, m di8usion layer thickness near an cathode, m anode activation overpotential, V anode concentration overpotential, V anode passive overpotential, V cathode activation overpotential, V cathode concentration overpotential, V conductivity of water=wastewater treated, mho=m potential at distance x, V

Theoretical analysis and experiments demonstrated that water pH and 7ow rate had little e8ects on the electrolysis voltage of electrocoagulation process. Two mathematical models, one applicable to non-passivated aluminum electrodes and the other to passivated aluminum electrodes were established and veri4ed. With these models the total required electrolysis voltage of an electrocoagulation process can be calculated.

Acknowledgements

Notation

References

aa ac ba bc Cj∗ Cj (x) Cj |x=0 d Dj Eeq F j Jj (x) Kh Kw m n N p q

constant of Tafel equation at the anode, V constant of Tafel equation at the cathode, V coeMcient of Tafel equation at the anode, V coeMcient of Tafel equation at the cathode, V bulk concentration of species j, mol=l concentration of species j at distance x, mol=l concentration of species j at the electrode surface, mol=l net distance between electrodes, m di8usion coeMcient of species j, m2 =s equilibrium potential di8erence between an anode and a cathode, V Faraday constant, C=mol current density, A=m2 7ux of species j at distance x, mol=m2 s hydrolysis constant ion product of water constant constant total electrode number of an electrocoagulation unit constant constant

The authors wish to acknowledge the Environment and Conservation Fund=WooWheelock Green Fund for the 4nancial support of this project.

Balmer, L. M., & Foulds, A. W. (1986). Separation oil from oil-in-water emulsions by electro7occulation=electro7otation. Filtration and Separation, 23(11=12), 366–369. Bard, A. J., & Faulkner, L. R. (1980). Electrochemical methods. New York: Wiley. Chen, G. H., Chen, X. M., & Yue, P. L. (2000a). Electrocoagulation and electro7otation of restaurant wastewater. Journal of Environmental Engineering, 126, 858–863. Chen, X. M., Chen, G. H., & Yue, P. L. (2000b). Separation of pollutants from restaurant wastewater by electrocoagulation. Separation and Puri8cation Technology, 19, 65–76. Demmin, T. R., & Uhrich, K. D. (1988). Improving carpet wastewater treatment. American Dyestu. Reports, 77, 13–18, 32. Do, J. S., & Chen, M. L. (1994). Decolorization of dye-containing solutions by electrocoagulation. Journal of Applied Electrochemistry, 24, 785–790. Dobolyi, E. (1978). Experiments aimed at the removal of phosphate by electrochemical methods. Water Research, 12, 1113–1116. Lin, S. H., & Peng, C. F. (1996). Continuous treatment of textile wastewater by combined coagulation, electrochemical oxidation and activated sludge. Water Research, 30, 587–592. McClung, S. M., & Lemley, A. T. (1994). Electrochemical treatment and HPLC analysis of wastewater containing acid dyes. Wastewater Treatment, 26, 17–22; methods. Water Research, 12, 1113–1116. Pazenko, T. Ya., Khalturina, T. I., Kolova, A. F., & Rubailo, I. S. (1985). Electrocoagulation treatment of oil-containing wastewaters. Journal of Applied USSR, 58, 2383–2387.

X. Chen et al. / Chemical Engineering Science 57 (2002) 2449–2455 Pouet, M. F., & Grasmick, A. (1995). Urban wastewater treatment by electrocoagulation and 7otation. Water Science and Technology, 31, 275–283. Renk, R. R. (1988). Electrocoagulation of tar sand and oil shale wastewaters. Energy Progress, 8, 205–208.

2455

Scott, K. (1995). Electrochemical processes for clean technology (pp. 12– 62). Cambridge, UK: The Royal Society of Chemistry. Vik, E. A., Carlson, D. A., Eikum, A. S., & Gjessing, E. T. (1984). Electrocoagulation of potable water. Water Research, 18, 1355–1360.