COLLOIDS A AND

Colloids and Surfaces SURFACES

ELSEVIER A: Physicochemical and Engineering Aspects 104 (1995) 101-109

Electrocoagulation and separation of aqueous suspensions of ultrafine particles

Michael J. Matteson *, Regina L. Dobson, Robert W. Glenn, Jr., Nagesh S. Kukunoor, William H. Waits III, Eric J. Clayfield

School of Chemical Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0100, USA

Received 28 December 1994: accepted 3 May 1995

Abstract

Conventional methods for the removal of suspended ultrafine particulates in industrial effluents and wastewater treatment frequently involve the bulk addition of inorganic coagulants (e.g. aluminum or ferric salts). This electrolytic dosing is followed by sedimentation to obtain a clarified supernatant liquid. The increased size of the coagulated material facilitates subsequent solid-liquid separation processes such as filtration.

An alternative separation treatment for ultrafine particles is electrocoagulation, which involves the in situ formation of ions by electrolysis. This generation of ions is succeeded by the electrophoretic concentration of the particulates in the immediate region surrounding the electrode. The addition of the electrochemically generated reagent can be controlled by adjusting the supplied power, thereby enabling optimization of the process.

The effective performance of the electrocoagulation technique was evaluated with kaolinite suspensions, using a batch, stirred cell system, and a continuously flowing suspension through a series of stirred cells. Particular attention was focused on determining the rate constants for the kinetics of the particulate coagulation process. The parameters examined were electrode voltage, residence time, particle concentration, and suspension flow rate. The results showed that the electrocoagulation rate follows a second order relationship, accounting for the electrophoretic movement of the particles toward the anode.

Keywords: Electrocoagulation; Enhanced sedimentation; Particle removal; Ultrafines; Water treatment

1. Introduction

The concept of electrochemically treating water for purification using electrodes is thought to have been first proposed in 1889 in England [1] . In the United States, a patent for the purification of wastewater by electrolysis using aluminum and iron corroding electrodes was received in 1909 [2]. On a larger scale, a method for the purification of drinking water by electrocoagulation was first

* Corresponding author.

0927-7757/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0927-7757(95 )03259-2

applied in the U.S. in 1946 [3,4]. This method utilized aluminum anodes to produce aluminum hydroxide flocs by reaction at the electrode fol- lowed by hydrolysis. The electrogenerated flocs sedimented rapidly, removing color from the drink- ing water. In 1956, Holden [5] treated river water in Britain using iron electrodes with a similar type of system. Both the investigations in 1946 and in 1956 showed promising results, e.g. high water quality as measured by turbidity and color removal, but the methods were not widely accepted because of some expectations of high initial capital costs as compared to chemical dosing.

102 M.J. Matteson et aL/Colloids Surjhces A: Physicochem. Eng. Aspects 104 (1995) 101-109

Recently, however, there has been renewed inter- est in the use of electrocoagulation owing to the increase in environmental restrictions on effluent wastewater. In 1972, electrocoagulation was studied in the treatment of wastewater from a food industry [6] . This report compared electrocoagu- lation to chemical dosing followed by dissolved air flotation. Floc formation was rapid in both cases, with the flocs formed from electrocoagulation being compacted more quickly. The concept of electrocoagulation has also been applied to micro- organism harvesting from cultures [7a,b] and the removal of protein and ether soluble substances from wastewater [8,9]. In the 1980s, there was a great deal of work performed by Russian scientists employing electrocoagulation in a variety of ways in the treatment of wastewater [10], and in the removal of oil from oil-in-water emulsions [ 11 ].

One of the problems with some of the studies discussed above is that the use of aluminum electrodes can lead to the formation of undesirable sediments. Sediments formed during electrocoagu- lation often contain large amounts of metallic hydroxides [12]. These hydroxides are formed from the dissolving electrode material. If aluminum is utilized for the electrode, then the sediments formed may be dumped in landfills. Therefore, some attention has been directed towards establish- ing a larger body of knowledge concerning the preferred application of iron as the electrode mate- rial [13]. Also, the use of alternating current in place of direct current has gained some interest [14].

For the efficient use of this process, the design requirements must be carefully determined. A thor- ough study includes determining the amount of power needed for a given amount of coagulation, and the specification of the transport properties. These transport properties must ensure the appro- priate residence time, voltage, and electrode char- acteristics given the type of suspension, the zeta potential, and its concentration.

2. Theory

Electrocoagulation involves the in situ formation of ions by electrolysis. In general, dissolving metal

anodes are used to continuously produce metal ions close to the anodic surface. This generation of ions is followed by the electrophoretic concen- tration of particulates in the region around the anode. The ions there are attracted by the particu- late, thereby neutralizing their charge and allowing them to coagulate.

At stainless steel electrodes, the following reac- tions are expected:

anode (a)

(b)

cathode

Fe--+ Fe 2+ + 2 e -

H20--+2H + + 1/202 + 2e

2 H 2 0 + 2 e --+2OH + H 2

In practice, anode reaction (b) must be domi- nant, as bubbles of gas are seen at both electrodes and the concentration of iron in the supernatant (measured by atomic absorption) does not increase during the course of the experiment. Further, the pH of the supernatant is also constant throughout the experiment, as would be expected from the addition of anode reaction (b) and the cathode reaction. The presence of Fe 3 + ions in equilibrium with Fe 2+ is quite small [15].

2.1. Batch system

A model that expresses both the electrophoretic concentration of particles near the anode, and their subsequent coagulation can be constructed if we consider a stirred suspension between two oppos- ing flat steel wire mesh electrodes, as shown in Fig. 1. The electrophoretic velocity of the particles toward the collector electrode may be expressed as the Henry equation for non-conducting particles [16]:

E~e u = 1.5 I tf(tca) (1)

where E=field strength between electrodes, ~= zeta potential, e=dielectric constant of the con- tinuous phase, // = viscosity of the continuous phase and

f (Ka)= 1.2 for /ca= 10.

Now, the number of particles, dnp, approaching

M.J. Matteson et al./Colloids Surfitces A. Physicochem. Eng. Aspects 104 (1995) 101 109 103

It 12 cm .I " I

ELECTRODE

COVER

NALGENE

CONTAINER

WATER L~NET 2°s Cm

/

I

17crnOD

STIR BAR

II 4 cm "I

Fig. 1. E l e c t r o c o a g u l a t i o n cell wi th electrode.

- - g -

23 5 Cm

the collector in time, dt, is

d n v = - uCA~dt (2)

where C is the number of particles per unit volume and A~ is the facial area of the collecting electrode. Also

n p = C V s o dnp=VdC (3)

where V is the volume of the suspension. Combining Eqs. (2) and (3), we obtain

dC uA~dt - ( 4 )

C V

which may be further simplified, since A, /V= l /x , a geometric characteristic of the electrode size and suspension volume. If u is independent of time,

position and concentration, then

C - - = e -k2' ( 5 ) Co

where Co is the concentration of particulate at time zero, C is the concentration that is changing with time, and k 2 = u/x s- 1. Further, since u is only slightly dependent on particle size, we may apply this to a suspension such as kaolin that has a polydisperse size distribution. Now, the fraction of concentration that is susceptible to coagulation is that which has migrated to the anode and whose particles have come into contact with ferrous ions:

C 1 - - - = l - e k2, ( 6 )

Co

Hence, we write the loss of particles due to coagula- tion as the second order relation

dC - - klC2( 1 - e - k2,)2 (7)

dt

4 k T - - - w h e r e k = For Brownian coagulation, kt - 3 ~t

Boltzmann's constant and T = temperature. The assumption of Brownian coagulation between par- ticles of the same size in representing k~ introduces a certain amount of error. Coagulation between particles of different size within a polydisperse suspension proceeds faster than that between par- ticles of the same size, since the tendency is for the very small particles to attach to the larger ones. In contrast, coagulation between particles of the same size gradually leads to a polydispersity, with some increasing in size relative to others. For logarithmic normal distributions with geometric standard devi- ations of less than 2.0 and limited coagulation, it is considered reasonable to assume a constant geometric standard deviation and that coagulation proceeds as if it were monodisperse for particles having a diameter of average mass [ 17]. Another assumption inherent in using the Brownian coagu- lation rate constant is that we are neglecting coagu- lation of particles resulting from collisions due to velocity gradients in the bulk of the suspension. The model is based on the particles becoming discharged in the vicinity of the anode, and then circulating throughout the bulk where they

104 M.,L Matteson et al./Colloids Surfitces A: Physicochem. Eng Aspects 104 (1995) 101-109

encounter other discharged particles and coagulate. We will assess the relative significance of these two forms of coagulation in Section 4.

We can solve Eq. (7) for the fractional concen- tration remaining at any time, t:

klCo C _ l + k l C o t + - - Co k2

(-- 1.5+2e_k~,_0.5e_2k2,)] 1 18)

If all particles were electrostatically neutral at time zero, then we could expect coagulation to proceed as

C oC-- = [ 1 +klCot ] 1 (9)

and we use this ideal case to define the minimum half-time, rm.o.5, as the time required to reduce the concentration of neutrally charged particles by one half, or rm.o.5 = (klCo) -1

Another parameter to emerge from Eq. (8) is what we refer to as the electrocoagulation parame- ter, ¢,:

klCo (10)

This parameter incorporates both electrophoretic and coagulation rate constants, and serves as a dimensionless modulus for predicting the overall efficiency of a process.

If we set C/Co=0.5 in Eq. (8), we can then derive an expression for the half-time of the electro- coagulation process, to.5:

("gm/'C)0.5 = I1 -- qb(- 1.5+2e I~m/~)o._~

__ 0.5e- 21rm/r)o._s)l 1 (11)

2.2. Continuously flowing model

Now, if we consider a system in which the suspension is continuously flowing through a series of stirred cells (Fig. 2) where each cell is provided with a set of electrodes, then from a material balance over the first cell:

QoCo = Q1C1 + rV1 (12)

where Qo = Q1 is the volumetric flow rate, and C1 is the number concentration of particles in the first cell with volume of suspension V1. The particle coagulation rate is

r= klC2( 1 - e-k2~l) z (13)

Combining Eqs. (12) and (13), we obtain

V1 C O -- C 1 Q ~ - r l - ~1~1 ~ ( 1 - e k2rt) 2 (14)

where rl is the residence time of the suspension in the first cell. Eq. (14) may be rewritten in terms of the relative concentration change with time as

[ ]' C1 C1 "cl (1--e-k:~l) 2 (15) Coo - 1 + Corm.2.5

and for a series of n cells, where rl = r2 = rn

Cn _ 1+ - - ( 1 - e k2~)2 Co Corm,o.5

I C2 T x 1+ - - ( 1 - e k2~)2

Co fm.O.5

x ... 1+ - - ( 1 - e k~)2 (16) CO'L'm,0. 5

To indicate the degree of completion of the process, we define the fraction coagulated as

Cn j ~ = l - - - ( 1 7 )

Co

3. E x p e r i m e n t a l procedure

so that, for a given value of ~, there is a specific value of (Z'm/'E)0.5 < 1.0, which expresses the ratio of the minimum possible half-time to that for the electrocoagulation process.

The electrocoagulation tests were performed with suspensions of kaolinite, A14(Si4Oao)(OH)s, obtained from ECC International of Sandersville, GA. The clay was logarithmically normally distri-

M.J. Matteson et al./Colloids Surfaces A: Physicochem. Eng. Aspects 104 (1995) 101-109

Fee~

105

Supernatant = i Product to Settling Tank

Sludge :

Fig. 2. Series of electrocoagulation cells with continuously flowing suspension.

buted with a mass median particle size of 1.2 lam and a geometric standard deviation of 2.5. The kaolinite was dispersed in 3 1 of distilled water with the aid of 2 ml of 1.0 N NaOH. The latter was used as a peptizer to ensure complete defloc- culation of the clay before beginning the tests. The zeta potential was determined with a Rank Particle Micro-Electrophoresis Apparatus Mark II. A dis- persion of particles was placed in a 0.26 mm inner diameter glass cell in contact with platinum blacked electrodes across which an electric field generated the electrophoretic effect. Particle veloci- ties were measured by timing their movement over a calibrated distance. This was carried out in alternate directions by reversing the polarity in order to negate the effect of drift. An average of 20 timings ( 10 in each direction) on different sized particles was used to calculate the electrophoretic velocity. From these measurements, the zeta poten- tial was calculated according to Eq. (1). At a pH of 10.8, the zeta potential was found to be -33 .9 mV.

The cell in which the batch tests were conducted was a 5 1 Nalgene jar (Fig. 1 ). A special cover was constructed from Plexiglas, with overhang and o-ring to ensure a secure fit. A small hole was cut into the center of the cover to serve as a sampling port. Three different sets of electrodes were fabri- cated from stainless steel mesh, with openings of 5, 2.5 and 1.5 mm and nominal wire diameters of

1.82, 0.81 and 0.65 mm respectively. The electrode pairs were placed through 2 cm × 12 cm openings in the cover, such that they were suspended a distance of 8 cm from each other and there was a 3.5cm distance between the bot tom of the electrodes and the bot tom of the cell, which allowed for a stirring zone.

A VIZ Model WP-705A DC power supply pro- vided voltage in the 0-60 V range. Two different Fisher stirring plates were used to provide stirring speeds in the 50-450 r.p.m, range. A Milton Roy spectronic 21MV Spectrophotometer was used to measure the coagulation rate by monitoring changes in concentration. In preliminary testing, the dispersion was stirred for 30 rain before an initial sample was taken and the power applied. The stirring was continued for 30 min and the power for 60 min. Samples of 10 ml were taken at 15 min intervals for up to 2 h, three aliquots were placed in separate cuvettes and the absorbance was measured. The absorbance data were con- verted to concentrations via a calibration curve. The temperature and pH were also monitored.

The set-up for the tests on the continuously flowing suspension consisted of a 20 1 mixing tank from which a peristaltic pump (Masterflex) deliv- ered an aqueous kaolin slurry to three cells (2.5 1) in series, as shown in Fig. 2. The cells were arranged such that when the level of slurry reached an outlet port, the mixture would fall by gravity through

106 M.J. Matteson et al./Colloids Surjaces A." Physicochem. Eng. Aspects 104 (1995) 101 109

1/2-inch tygon connections down to the next cell, and finally to a collection reservoir. Flow rates were chosen such that residence times in each of the cells could be set at 20, 40, 54, 86, or 160 min. The flow within a cell was always from a position 2 inches from the bottom, upward and through both electrodes in the direction of the electropho- retic flow, and out of the outlet port. After all three cells had filled, a pipette was used for sampling as described above.

The electrical network was arranged in parallel, so that each cell received the same potential, with 1/3 of the current. The voltages tested were 15, 25 and 50 V.

4. Results and discussion

4.1. Batch tests

1.0"

o. o- e O

z~ 50 Volts

• 25 Volts

o 15 Volts

0.60" ~,, al~oo o

ClCo ,,, ,,~^~ 0.40"

t~ ° ,Ik~o ~i,"~' Zh * o

• ,. ,~; . 0 - s o o ~ _ 0.20" at, * * ~ ' ~ e

0.0

0 50 1 O0 150 200 250 300

T i m e , rain

Fig. 3. E lec t rocoagula t ion rate normal ized concent ra t ion of kaol in suspension as a function of t ime at three different voltages.

The concentration of an aqueous suspension of kaolin particles, when subjected to an electrical field, decreased because of the electrophoretic migration of particles toward the anode, where their electrical charge was neutralized by ferrous ions, and the subsequent coagulation of those particles in the bulk. The results of measurements in a single cell, batch arrangement, where the voltage was varied from 15 to 50 V, are shown in Fig. 3. These results follow quite closely the theo- retical derivation described by Eqs. (1)-(8). The reverse S-shaped curve of Fig. 3 is characteristic of a differential equation such as Eq. (7), wherein the fraction of particles neutralized (1--e -k2t) is increasing with time, while the overall concen- tration, C, is decreasing. Initially, none of the particles are neutralized, and they must migrate toward the anode. After a certain time, a rather sharp drop in concentration occurs because coagu- lation has begun, and finally the rate begins to slow as most of the particles have become neutralized.

We found that an overly vigorous stirring rate will hinder the coagulation, because particles are removed from the region near the anode before they have a chance to interact. Also, the ferrous ion concentration is decreased by too strong an agitation. However, if there is no convection at all,

the flocs tend to build up on the anode mesh and, in time, will reduce the effectiveness of the anode to generate ferrous ions.

The relative motion created by stirring in the batch process, and by the flow of the suspension in the continuous process can take the form of velocity gradients leading to collisions between adjacent particles. Probstein [18] compares the coagulation rate of shear induced collisions with that from Brownian motion:

U.)shea r 4a3} ' - (18)

(OB . . . . kT/l~

where a=part ic le radius and 7=shear rate, s ~. For the stirred cell, with a stirring rate of 0.5 s t, the shear induced coagulation is about 10 % of that due to diffusion. In the case of the continu- ously flowing suspension, using the reciprocal of the residence time to represent the shear rate, the coagulation rate resulting from velocity gradients is less than 0.02 % of that from the thermal induced diffusion for the fastest flowing suspension.

In order to collapse the data of Fig. 3 into one characteristic curve, we rewrote Eq. (8) in the form

1 (~-1)=k2t+(-l.5+2e k2'--0.5e 2k2r)

(19)

M.J. Matteson et aL/Colloids Surfaces A: Physicochem. Eng. Aspects 104 (1995) 101 109 107

and have plotted the left hand side of Eq. (19) vs. kzt in Fig. 4. The experimental data show very good fit to the theoretical curve at low values of kzt, but seem to scatter somewhat at the larger values. We suspect that for longer residence times, the floc build-up at the anode may have some influence here; also, the assumption of monodisper- sity in the Brownian coagulation constant, kl, becomes weaker at longer times.

4.2. Rate determining step

We have assumed throughout the analysis that the generation of ferrous ions is not a rate deter- mining factor in the electrocoagulation process. An order of magnitude calculation can verify this assumption. If we take the concentration of ferrous ions near the anode to be [19]

2i Cve+ ~ zF (t/~D)°5 (20)

where i is the current density (10 A m 2 at 50 V), z is the ferrous ion valence, F is the Faraday constant (96 500 C mol 1), and D is the diffusivity ( ~ 1 0 9 m 2 s - l ) ; further, if we assume the magni- tude of the concentration of ferrous ions necessary

I

l J

O

w 7

3°I 50vo,,s, / [ o 25 vo,t~ I / E o 1 5 Volts I A a /

2.0

1.0

0.50

0 . 0 ' ~ ' . . . . = • , , • , . . . . , , , , , I

0 1 2 3 4 5

k t z

Fig. 4. Consolidated electrocoagulation rate comparison of theoretical curve from Eq. (19) with experimental data at three voltages.

to initiate coagulation to be the same as the hydroxyl ion concentration, then the characteristic times to reach the necessary ferrous ion concen- tration are 0.1, 0.5 and 1.5 s at 50, 25 and 15 V respectively. Therefore, we feel safe in assuming the formation of ferrous ions to be rapid in compar- ison to the actual coagulation process, and not rate determining. Further support for this claim was found in comparing tests with three different electrode mesh sizes. We would expect that the mesh with the greater surface area would provide the greatest concentration of ferrous ions for a given time. However, no discernable differences in coagulation rates were observed.

4.3. Initial concentration

Another parameter that we studied was the initial concentration of the suspension, which was varied from 0.278 x 1014 t o 1.39 x 1014 m -3. This allowed us to vary q~ in Eq. (11) and measure (TIn/T)0. 5 (see Fig. 5). This is a useful parameter in comparing the relative efficiencies of processes, since the ratio of half-times can never reach 1.0, but the closer the value is to 1.0, the more efficient the electrocoagulation in separating the suspen- sion. However, we can see that if g' 1 is to be increased much past a value of 2.0, by increasing the voltage for example, then the corresponding gains in (TIn/T)0.5 become increasingly smaller. The solid line in Fig. 5 is the theoretical value as

1.000-

5 "

0.I000 ........ ' . . . . . . . . ' I I

0.1 1 ~-, 10'

Fig. 5. Ratio of minimum possible half-time to that obtained in electrocoagulation tests, using initial concentrations ranging from 0.278 x 10 '4 to 1.39 × ]014 m 3, and at 15, 25 and 50 V. Curve is from Eq. (11).

108 M.J. Matteson et aL/Colloids Surjaces A." Physicochem. Eng. Aspects 104 (1995) 101 109

predicted by Eq. (11). This comparison is for batch tests only, and the relation between the half-times and the electrocoagulation parameter takes a different form for a continuous process.

4.4. Continuously flowing system

When suspensions were sampled from three cells in series with a continuously flowing stream of suspension, the fraction coagulated, f,, in each cell was measured and reported, as shown in Fig. 6. We notice a tendency for the experimental data to separate according to the cell number, with the results improving slightly as we move from cell 1 to cell 3. We would expect all the data to fall on a single line because of the form of the independent variable, derived from Eq. (16). However, this variation can be explained if we consider that the net motion of the suspension is in the direction of the electrophoretic velocity, so there is some assis- tance in driving the suspension toward the collector electrode. The considerably greater scatter at the lower end of the scale may arise from the fact that these represent shorter residence times, and the velocity of the suspension through the cell is greater than the electrophoretic velocity (~<x/u). This

1.0-

f 0.10- n

0.010 0.01

• &o

• I / • n = 2

/ • n=3 Theory

0.1 ~ 1'0 ' 1tO 2

n'c / 'c ( I - e -kZ ' ) z n m l l . 5

Fig. 6. Fractional coagulation for three cells in series. Theoretical curve from Eqs. (16) and (17).

results in greater tendencies for flocs to break up, or to dilute the concentration of ferrous ions in the vicinity of the anode. Nevertheless, the coagula- tion seems to be improved in cells 2 and 3 over that predicted, and there may be an opportunity in these cells to pick up particles neutralized, but not coagulated, in the first cell.

4.5. Potential and current

Placing the electrodes in parallel, rather than series, allowed us to maintain the same potential in each cell while the current was reduced to 1/3 of that for an equivalent potential drop in a batch cell. However, we did not notice any reduction in performance from the batch tests due to the reduc- tion in current, and the same rate constants were applied to both batch and continuous flow equa- tions. This is explained by the fact that k2 is based on electrophoretic velocity, which in turn is based on the potential difference between the electrodes. The current, however, determines the amount of ferrous ions going into solution, but as we have shown, the supply of ferrous ions is generated rapidly compared to the coagulation process. If the current were to be reduced by 1/3, then we would expect the time required to generate the required amount of Fe z+ to be tripled; however, the order of magnitude of this time is still less than 10 s. So, with the continuous flow process, as with the batch process, at the voltages tested, the current flow does not dominate the electrocoagulation rate.

5. Conclusions

The electrocoagulation of aqueous suspensions of ultrafine kaolin particles was measured in a single stirred cell apparatus and in a continuously flowing system consisting of three cells in series. The electrodes were fabricated of steel mesh through which the suspension freely circulated, and were set at a distance of 0.08 m in the cylindrical containers.

The coagulation rate can be described by a second order rate equation that incorporates the electrophoretic migration of particles toward the anode, and the subsequent release of discharged

M.J. Matteson et al./Colloids Surfaces A: Physicochem. Eng. A~peets 104 (1995) 101 109 109

particles into the bulk. The constants in this rate equation were derived from basic electrophoretic and coagulation phenomena, and depend on the zeta potential of the particles, the applied voltage, the initial particle concentration and the electrode geometry. It was possible, with the derived equa- tions, to describe the experimental reduction in concentration with time for the voltage and con- centration ranges tested.

An electrocoagulation parameter was defined, 1~)=[72m.O.5(U/X)] 1, where rm,0.5 is the time required to reduce a specific concentration of uncharged particles by one-half, u is the electro- phoretic velocity and x is a geometric parameter. With the electrocoagulation parameter as an inde- pendent variable, the half-time for the electrocoag- ulation of a series of concentrations of charged particles was measured, and compared to a theoret- ical derivation of the relationship between half- time and q~. Experimental results validated this relationship, and showed that it is a useful parame- ter in assessing the overall efficiency of an electro- coagulation process.

The electrocoagulation in a series of stirred cell reactors, with electrodes in parallel, was measured, and the results compared to a theoretically derived expression that incorporates the cell residence time, the number of cells, and the electrocoagulation parameter. Reasonable agreement was found between the experimental results and the theoreti- cally predicted fraction coagulated.

Acknowledgment

The authors wish to acknowledge support of this project by the Bureau of Mines, U.S. Department of the Interior, Allotment Grant No. G1194113. The contents do not necessarily reflect

the views and policies of the U.S. Bureau of Mines. Mention of commercially produced instruments and products does not constitute endorsement or recommendation for use by the U.S. Bureau of Mines.

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