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Bioelectrochemistty and Bioenergetics, 9 (1982) 29-243

A section of J. Electroanal. Chem., and constituting Vol. 141 (1982)

Elsevier Sequoia S.A., Lausanne-Printed in The Netherlands

229

457-THE MECHANISM OF LOW FREQUENCY ax.ELECTROCHEMICAL DISINFECTION

G.E. STONER and G.L. CAHEN, Jr.

Department of Materi als Science, School of Engineeri ng and Applied Science, Uni versity of Virginia,

Char lottesvil le, Va (U.S.A.)

M. SACHYANI and E. GILBADI

Department of Chemistry, Tel-Aviv Uni versity, Ramat Aviv (I srael)

(Manuscript received April 23rd 1981)

SUMMARY

The mechanism of low frequency a.c. electrochemical disinfection was studied. A random oriented

graphite fiber-epoxy matrix composite material was employed to fabricate the electrodes. This material

was found to have very good electrochemical stability when cycled anodically and cathodically in

solutions containing NaCl or NaBr.

The lethal species was identified as HCIO and HBrO in solutions containing Cl- and Br-,

respectively, although the free halogen molecule may contribute to the observed disinfection activity

under certain experimental conditions.

The efficiency of disinfection increases with increasing concentration of Cl- (or Br- ) and with

decreasing pH. Deaerating the solution decreases the efficiency of disinfection significantly. NaBr was

found to be effective at much lower concentrations than NaCl at all pH values but the effect is enhanced

at pH >7 due to ionization of HClO.

The unique advantage of the method described in this paper is that a high transient concentration (in

time and space) of the lethal species is generated, enough to destroy the most resistant microorganisms,

while the average concentration of active halogen in the effluent liquid remains well below the

objectionable level.

INTRODUCTION

Disinfection of a fluid can be achieved by different means, some based on

chemical reactions, e.g., the use of a halogen or of ozone and others based on

physical methods such as application of ultrasonic or electromagnetic radiation such

as X-rays or y radiation. The choice of method to be used depends on the fluid

treated as well as on cost and safety considerations. In the final analysis, disinfection

is caused by the input of energy either directly, as in the power-supply of an

ultrasonic transducer or U.V. lamp, or indirectly, as the energy required to produce

0302-4598/82/0000-O/$02.75 0 1982 Elsevier Sequoia S.A.



230

(and in certain cases also to store and transport) the chemicals employed. Thus,

when large volumes of fluid are to be treated, as in municipal drinking water or

sewage plants, the cost of energy may become a major consideration. In smaller

systems initial cost, safety, reliability and convenience of operation are usually the

predominant considerations in the choice of the preferred disinfection method.Production of the chemical in situ, for example, by mixing sewage with sea-water

and electrolyzing to produce chlorine, has an obvious appeal in that the dangerous

chemical is not transported and stored. The low frequency U.C. electrochemical

disinfection method presented below is one such in situ method. It is unique in that

the active species is produced by a slow a.~. signal, at a frequency of 1 Hz or less,

instead of the commonly used d.c. or line frequency techniques. In a patent issued in

1973 to G.E. Stoner, it was shown [l] that application of a low frequency voltage

signal of 5- 15 V amplitude between two graphite electrodes could effectively destroy

a large variety of microorganisms contained in the fluid between the electrodes.

Approximately 40 species of microorganisms, ranging in size from viruses throughbacteria and algea to relatively large species such as Euglena were successfully

treated.

The lethal species was not identified but it was established [2] that it was formed

during the anodic half cycle on each electrode. Reversing the polarity helps to

maintain a clean electrode surface, but it also serves an additional purpose, as will be

discussed below. The exact waveform had little effect on the results as long as the

frequency was kept below ca. 10 Hz and preferably not above 1 Hz.

Graphite electrodes were employed because of their low cost and relatively high

stability during alternating voltage conditions in chloride-containing solutions. Much

better performance was later observed [3] with composite electrodes consisting ofgraphite fibers in an epoxy matrix. This type of electrode material was used in all

further experiments *.

In the present work, the lethal species formed in the anodic half-cycle was

identified and its activity under different experimental conditions was determined in

relation to situations which may occur in practical applications.

EXPERIMENTAL

The ceil and electrodes

Two types of cells were employed in this work and are shown in Fig. 1. The flow

cell consists of two compartments in a bipolar configuration. The liquid was allowed

to flow from one compartment to the other through 24 holes in the central electrode

(diameter of each hole 0.1 cm). The electrodes (surface area 54 cm*) were composed

l It was recently shown [4] that the epoxy matrix-graphite fiber composite electrodes maintained their

stability under conditions similar to those employed in the present work after more than 450 hours of

continuous operation.



231

of a random-oriented graphite fiber-epoxy composite with approximately 60 w/o

graphite loading. The back of the two outer electrodes was plated with a layer of

nickel from a commercial plating bath (Sel-Rex Lectro-Nit lo-035), to achieve good

Composite material

\ NI plated/

Flow cell

/ Ni plated \

U Composite

Stationary cell*

Fig. 1. Expanded view of the flow cell and the stationary cell.

electric contact and reduce voltage loss and heating due to the resistance of the

electrode l All experiments in this cell were performed at a flow rate of 2 cm3 s-’

with electrode separation of 0.075 cm, corresponding to a total residence time of 4 s

in the two compartments.

The stationary cell consisted of two electrodes of the same material, having an

area of 4.5 cm’ and a separation of 0.10 cm. The solutions were introduced into the

cell by means of a syringe and were treated for 120 s in all experiments. Once in a

few months, ‘the electrodes were cleaned by polishing with fine emery paper. This

was found to increase the activity to some extent, although there was no visible

damage to the electrode surface. Best reproducibility was obtained if the cell was

operated for about 20 min with a dummy solution before taking each set of

measurements.

* The specific resistance of this composite electrode was about 2 !J cm, and its thickness was 0.3 cm. This

gives a total resistance of R = 11 X lo-’ hl. The highest current applied was 1.0 A, leading to a negligible

voltage drop of

11mV across each electrode.



232

The test organism

All measurements were performed employing Escherichia co/i B as the test

organism. This bacterium was chosen because it is not pathogenic, is relatively

insensitive to changes in pH (it can grow in the range of 4.5-9.0 and survive in therange of 3.0-9.5), can withstand osmotic pressure changes (from distilled water to

sea water) and is relatively stable thermally (it can grow at temperatures up to

45’C). In addition, E. coli B can grow in the presence of oxygen or anaerobically.

A fresh overnight culture was used in each set of experiments. The cultures

contained lo8 cell/cm3 and were obtained by inoculating 5 cm3 of nutrient broth

(Difco) and incubating at 37°C without shaking.

Since this electrochemical disinfection method was tested earlier [l] with many

different organisms, it is reasonable to assume that the behavior reported below is

representative for many other microorganisms. It should be remembered, though,

that quantitatively these results are valid only for the specific organism tested.

Method of counting

For viable count 10 mm3 (p-11) amples were dispended on the surface of nutrient

agar plates with an Eppendorf micropipette. Different dilutions were used so that

the number of colonies counted would be large enough to be statistically meaningful,

yet small enough to be countable. The use of small samples limits the sensitivity, but

is very convenient and enables counting of a large number of samples easily and

inexpensively. This was considered advantageous since small changes in the viable

count, i.e. the number of living microorganisms per unit volume, are of little

practical significance. The number of colonies counted in a typical experiment was

lo-20 and each dilution was counted twice. This leads to a statistical error of ca.

20-30%, which amounts to half a unit or less on the logarithmic scale shown in Figs.

3-7.

The results are presented in the form of log( No/N), where N,, is the concentration

of bacteria in each experiment before treatment (usually (l-5) X lo5 cells/cm3) and

N the concentration after treatment in the electrochemical cell. Since one colony

counted in 10 mm3 solution represents lo2 cells/cm3, the limit of detection was

N,/N = lo3 or 99.9% destruction of the cells. Thus, the limiting value of log

N,/N = 3 shown in several of the figures below represents the detection limit of this

technique and not necessarily a saturation effect in the disinfection process. While

this limit of detection could be readily increased (e.g. by taking 100 mm3 samples), it

was not considered necessary; since all the significant features required to under-

stand the behavior of this system were already apparent with the method employed.

Determination of active chlorine or bromine

The concentration of active chlorine or bromine was determined by its color

reaction with o-tolidine. Comparison with commercial color standards yields a quick



233

determination of the active halogen concentration in the range of 0. l- 1 O ppm. A

“positive” o-tolidine was recorded when the concentration of active halogen was

estimated at 0.2-0.3 ppm or above [5].

Electric circuit

The cell was connected to a power supply (constructed at U.Va) through an 1:

9, 11 W power resistor. The voltage waveform produced by the power supply and

that observed at the cell terminals are shown in Fig. 2. This waveform was chosen

because it was considered to be economical in energy consumption compared to, say,

a triangular wave. It was shown earlier [l] that the results were not sensitive to the

form of the applied voltage but mainly to its frequency.

The current was determined by measuring the voltage across the power resistor in

series with the cell and the voltage was measured directly on the cell terminals, both

with a Tektronic model 564 storage oscilloscope.

Preparation of solutions

The following solutions were prepared:

normal saline :

artificial tap water:

0.85% NaCl(O.145 M);

0.05% NaCl(8.5 X low3 M);

a

0= f(s), , , , , , , , , , , ,

3 0.3 0.6 1.0 1. 3 1. 6LI_;o- 1

Y -

b

Fig. 2. The voltage waveform at the output of the power supply (a) and at the cell terminals (b).



234

bromide solution : 0.85% NaBr (0.082 M);

potassium sulfate solution: 0.15% K,SO, (8.6 X 1O-3 M);

potassium dihydrogen phosphate solution: 0.68% KH,PO, (0.050 M).

Buffer solutions were prepared by standard methods [6,7]. The solutions used,each in its usual pH range, were acetate buffer, phosphate buffer, tris buffer and

carbonate buffer.

The buffer capacity B, defined as the number of moles of strong base which has

to be added to 1.0 dm3 of the buffer solution to change its pH by one unit, was

calculated from the approximate formula [8]

p = 2.3K,cH+c

(K,+c,+jz +‘H+++OH-I

(1)

where K, is the acid dissociation constant of the weak acid and c the total molar

concentration of the acid and the salt.

Bacteriological preparations

The test organism was E. coli B obtained from Professor E.Z. Ron of the

Department of Microbiology, Faculty of Life Sciences, T.A.U. The bacteria were

grown in a nutrient broth (Difco). The colonies for viable count were grown on

nutrient agar plates. All solutions and test tubes were sterilized (15 min, 1.5 atm) in

an automatic vertical autoclave (Tuttnauer, model 13000 L). Pipettes and larger glass

containers were sterilized in an oven at 18O’C for 2 h. The bacteria on the agarplates were incubated overnight at 37OC in a water jacketed incubator (Tuttnauer,

model 12000).

RESULTS

Flow cell

The relative disinfection (N,,/N) is plotted in Fig. 3 on a semi-logarithmic scale a:

a function of the peak current density, for two concentrations of NaCl. Changing the

salt concentration from 0.05% to 0.85% had little effect. Complete disinfection (i.e.log N,/N 2 3) was observed when the current density reached about j = 7.5 mA

cm-*. The peak voltage across the cell at this current density was 7.OV in artificial

tap water and 5.5 V in normal’ saline. The difference in cell voltage is due to the

change in solution resistance and’has no effect on the electrode reaction proper. The

pH in both solutions was close to 5.

All experiments were performed at 25’C. Experiments at 10°C and 40°C showed

no significant change in the efficiency of disinfection with temperature.

The pH of the solution has a very significant effect on the disinfection efficiency

as seen in Fig. 4. Tap water was used in this experiment and the relative disinfection



235

Fig. 3. The relative disinfection, log(NO/N), as a function of the peak current density in normal saline

(NaCl 0.85%) (AAA) and in artificial tap water (NaCl 0.05%) (0 0 0). pH=5.0.

is seen to decrease from its maximum value of 99.9% (log NO/N = 3) at pH = 5 to

zero at pH 2 7. It should be noted that the two current densities used in this

experiment are higher than the value of 7.5 mA cmW2 required to cause maximum

disinfection at pH = 5, as seen in Fig. 3.

In further experiments, the effect of aeration was studied by bubbling oxygen,

nitrogen or argon through a solution of normal saline at pH = 5 and determining the

dependence of log( N,/N) on the peak current density. The results are shown in Fig.

5a. A very significant decrease in efficiency of disinfection is observed when oxygen

is removed from the system by bubbling an inert gas. It was also noted that a

positive o-tolidine test was obtained at j 2 5.5 mA cmP2 in the presence of oxygen

while in deaerated solutions a current density of 13- 15 rn4 cmW2 was needed. The

voltage developed across the cell at a given current density was also higher in

deaerated solutions.

Fig. 4. The effect of pH on the efficiency of disinfection in tap water (ca 0.05% NaCl) at two current

densities: (000) 9.3 mA cm-*; (AAA) 18.6 mA cmm2.



Fig. 5. The effect of oxygen on the efficiency of disinfection. (a) 0.85% NaCl solution; (0 0 0) 0s;

(000) Ar; (AAA) N,. (b) 0.85% NaBr solution; (vvv) no bubbling; (AAA) 0,; 000) N,.

pH = 5.0, unbuffered.

The effect of aeration was also tested in a solution of NaBr 0.85%. The results are

shown in Fig. 5b. Although the effect is not as large as in the case of NaCl of the

same concentration, the influence of deaeration is clearly seen. In the deaerated

solution, the results were somewhat irreproducible. The data given in Fig. 5b for this

solution are the average of eight independent experiments, while the other data

represent the average of only three experiments.

Stationary cell

The effect of pH on the disinfection efficiency inbuffered solutions of NaCl and

NaBr is shown in Fig. 6a and 6b respectively. In comparing Fig. 6a to Fig. 4, it

should be borne in mind that in the stationary cell the solution is treated for 120 s,

while in the flow cell the residence time is only ca. 4 s. Even so, it is observed that no

disinfection takes place when the concentration of NaCl is below 100 ppm (ca. 2

mM) at any pH.

10 20 30 40I I 1 I

[Na CL] (mW

c (NaCl)(ppm)

500 1000 1500 2000 2500

1.0 2.0 3.0

I ‘[NaBr] (mi) ’

Fig. 6. The effect of pH on the efficiency of disinfection in well buffered solutions. Peak current density:

12.2 mA cme2. (6a) NaCl pH=(VVV) 5.2; (AAA) 7.0; (mm=) 8.6. (6b) NaBr pH=(vvv) 5.0;

(AAA) 7.0; (000) 9.2; (000) 10.0.



237

500

1I

1000 1500 2000 2500I r I Ic NaCl)(ppm) ,

30 L 1

$

,Pp’~~-o\,,, -.-...

; :f /

3

2.0 ~

:! !

:iv/v

,o s /A

/-

.i

!;

1:

) Gh fi /’ ,c(NaClHppm),

1000 1500

Fig. 7. The effect of the buffer capacity B (phosphate buffer) on the efficiency of disinfection in NaCl

solutions at pH=7.0, jz12.2 mA cm-‘; p=(OOO) 2.7X10W5; (AAA) 2.7X10p4; (000) 2.7X

10-3; (VVV) 2.7X IO-*.

In a buffered solution at pH = 8.6, no disinfection was observed up to a

concentration of lo4 ppm (O.l7M), which was the highest concentration of NaCl

tested in this experiment.

Replacing NaCl by NaBr gives rise to two major changes, seen by comparison of

Fig. 6b to Fig. 6a. Disinfection starts at a much lower concentration of the salt and,

although a small decrease in efficiency is observed with increasing pH, efficient

disinfection has been observed throughout the pH range up to pH = 10.

The effect of buffer capacity on the disinfection efficiency in NaCl at pH = 7.0 is

shown in Fig. 7. Relatively high concentrations of NaCl are required to achieve

efficient disinfection at this pH in a well buffered solution, but this concentration

decreases (i.e. the efficiency of disinfection increases) as the buffer capacity of the

solution is decreased at constant pH.

DISCUSSION

I denti fi cation of the lethal species

It was shown earlier by direct observation under an optical microscope [2] that

the lethal species are formed at the anode. The electrochemical reactions which can

take place at the anode in this system are the formation of oxygen and chlorine and

the oxidation of molecular hydrogen *.

2 Cl- + Cl, + 2 e- (2)

2H,0-*0,+4H++4eP (3)

H,+2H++2eP (4) *

l Hydrogen can be present due to its formation in the previous half-cycle, when the electrode acted as the

cathode.



238

We shall proceed to show that all the experimental evidence is consistent with the

assumption that the lethal species are Cl, and HClO in solutions of NaCl and Br,

and HBrO in solutions of NaBr. Hypochlorous acid is formed in the hydrolysis of

Cl*:

Clz+H,O+H++C1-+HClO &,(25”(Z) = 4.5 X 1O-4 (5)

Its acid dissociation constant is K,(25”C) = 3.7 X lo-*, and the anion formed has

little or no activity for disinfection. In Fig. 8a the distribution of Cl,, HClO and

ClO- is shown as a function of pH in a normal saline solution (NaCl 0.85%) into

which 10 ppm Cl, was introduced. A similar plot for the corresponding bromine-

containing species is shown in Fig. 8b. The values of the two constants in this case

are &(25”C) = 5.8 X lop9 and K,(25’C) = 1.0 X 10e9 [9,10].

HCIO CIO-

2 4 6 8 10

Fig. 8. (a) The effect of pH on the distribution of Cl,, HCIO and ClO- in normal saline (0.85% NaCl) at

25OC. Initial concentration of Cl,: IO ppm. (b) The effect of pH on the distribution of Br,. HBrO and

BrW at 25°C. Initial concentration of Br,: 25 ppm, with no NaBr.



239

The current density

Considering Fig. 2b, it is evident that a great part of the current measured is

consumed to charge the double layer capacitors, and the potential across the cell

does not reach steady-state during the voltage pulse. Under these conditions, thecritical value of the peak current density (ca. 3.5 mA cm-*) required for the

disinfection process to commence (cf. Fig. 3) is related to the buildup of the potential

across the double layer to a value required for the faradaic reactions to occur. The

concentration of NaCl could effect the amount of chlorine formed during the anodic

half-cycle in the following ways:

(i) increasing the concentration lowers the reversible potential for chlorine evolu-

tion and increases the exchange current density, both leading to better disinfection;

(ii) more of the faradaic current is consumed for the formation of Cl, as the

concentration of NaCl increases and less for the two possible side reactions. While

this may at first seem advantageous it has a drawback in that the local decreases inpH near the anode due to these side reactions [equations (3) and (4)] will occur to a

lesser extent. This may decrease the efficiency for disinfection, since molecular

chlorine is more active than HClO and ClO- has very little activity compared to

either. Thus, lowering the pH favors the formation of the more active species, as seen

in Fig. 8a.

(iii) A change in capacitance due to the change in concentration of NaCl may

affect the way in which the potential across the double layer is built up during the

anodic half-cycle. This is expected to be a rather small effect, since saturation

adsorption of chloride ions probably occurs already in artificial tap water, and

increasing the concentration of NaCl further would hence have little effect on thedouble-layer capacitance.

The net effect of increasing the concentration of NaCl from 0.05% to 0.85% is

seen to increase the disinfection efficiency to a small extent.

The effect ofpH

The effect of pH on the efficiency of disinfection in tap water is shown in Fig. 4.

Comparison with Fig. 8a shows that the ratio &,/IV decreases as more and more of

the hypochlorous acid is ionized, showing that the anion ClO- is much less effective

in the destruction of the test organism.

A more detailed test of the pH effect is shown in Fig. 6a, where the efficiency of

disinfection is plotted as a function of the concentration of NaCl in well buffered

solutions at three pH values. The value of pK, for HClO is 7.4. Thus one would

expect that the concentration of NaCl required to reach a certain degree of

disinfection would increase somewhat as the pH is increased from 5.2 to 7.0. A

further increase of pH to 8.6 leaves only a few per cent undissociated acid.

Accordingly, no disinfection was observed up to a NaCl concentration of 1 X lo4

ppm.An obvious test of this interpretation of the experimental results is to replace



240

NaCl by NaBr. Since the acid dissociation constant of HBrO is pK, = 9 one would

not expect a drastic decrease in disinfection efficiency in NaBr solutions up to a pH

of 9. This is seen to be the case in Fig. 6b. Although the concentration of NaBr

required to reach a given value of (No/N) increases steadily with pH, there is no

drastic change between pH = 7.0 and 9.2 as in the case of NaCl. One might haveexpected that in a pH = 10 buffer the efficiency of disinfection would decrease

drastically. However, at this high pH the test organism can no longer survive. The

viable count was found [ 1 l] to decrease by one order of magnitude in 3 h and by at

least three orders of magnitude in 24 h without application of a current. The cells

which are not immediately destroyed are probably rendered more sensitive at this

high pH. Thus the disinfection efficiency is not very much decreased from pH 9.2 to

10.0, even though the concentration of undissociated HBrO is decreased about

fivefold.

The steady decrease in efficiency of disinfection with increasing pH seen in Fig.

6b may be caused by two factors:(i) it is generally accepted that the free halogen is more active than the hypoha-

lous acid formed by its hydrolysis. Considering Fig. 8b, it is seen that the concentra-

tion of molecular bromine is not negligible in the pH range shown in Fig. 6b. Thus,

some loss of activity may be expected as the pH is increased;

( i i ) the reversible potential for oxygen evolution decreases with increasing pH,

thus the relative rate of oxygen evolution would be expected to increase, leaving less

current for the production of bromine.

A further test of the model was performed by studying the effect of the buffer

capacity at a fixed pH of 7.0 on the efficiency of disinfection in NaCl solution. It

was shown above that this is not a very favorable pH value, since almost half of the

active material (HCIO) is lost by dissociation. On the other hand, the two plausible

side reactions [equations (3) and (4)] both lead to the formation of H30f ions and a

local decrease in pH, which coincides in t ime and space with the formation of Cl,

and HClO. Thus, one would expect to have a relatively high disinfection efficiency

in unbuffered solutions at pH = 7, decreasing as the buffer capacity increases. This

is seen to be the case in Fig. 7. The concentration of NaCl needed to reach 99.9%

disinfection (log N,/N = 3) increases steadily from ca. 250 ppm in an unbuffered

solution to about 2500 ppm in a well buffered solution at the same pH.

The eff ect of aerat i on

The effect of deaerating the solution on the efficiency of disinfection is shown in

Fig. 5a and 5b for NaCl and NaBr, respectively. A very substantial decrease in

efficiency of disinfection is caused by deaerating the solution in both cases. In

addition, the cell voltage is found to increase and the current density at which

formation of the active halogen is observed (positive o-tolidine) is roughly doubled.

On the other hand, experiments in which NaClO was added to a solution containing

the test organism revealed no effect of oxygen content on the rate of deactivation of

the cells. Thus; the possibility that deaeration influenced the stability of the

microorganism can be excluded.



241

The effect of deaeration on the disinfection efficiency in the electrochemical cell

can be understood in terms of the side reactions which take place during both the

anodic and the cathodic half-cycle.

Consider the electrochemical processes taking place in a deaerated solution,

starting with the cathodic cycle. Since there is no halogen or oxygen in the solution,the only reaction which can take place is hydrogen evolution. When the polarity is

reversed the first anodic process will be oxidation of molecular hydrogen [equation

(4)] followed by halogen formation [equation (2)] and possibly some oxygen evolu-

tion [equation (3)]. The reversible potential for chlorine and bromine evolution in

neutral solutions is about 1.8 V and 1.5 V respectively, more anodic than that for the

oxidation of molecular hydrogen. Thus, the latter may be expected to be essentially

completed before the former starts. In the next cathodic half-cycle, the first reaction

will be reduction of the halogen, followed by a shift of potential by ca. 1.8-2.0 V to

hydrogen evolution *.

Consider now the effect of aerating the solution. During the cathodic half-cycle,

oxygen reduction will take place first l, and the amount of hydrogen formed will be

significantly reduced. In the following anodic half-cycle, most of the faradaic current

will be consumed for the production of the halogen and oxygen, since there is little

or no molecular hydrogen near the electrode. Thus the formation of active halogen

(positive o-tolidine) and the consequent disinfection may be expected to occur in

aerated solutions at lower current densities than in deaerated solution, as shown in

Fig. 5a and 5b. It may be noted that the system is self regulating with respect to its

oxygen content. Thus, if one starts with a deaerated solution, the hydrogen formed

during the cathodic half-cycle will retard the formation of oxygen in the following

anodic half-cycle, leading again to formation of molecular hydrogen in the next

cathodic half-cycle and so on. If, .on the other hand, the solution contains oxygen to

start with, little or no hydrogen will be formed during the cathodic half-cycle, and

this will lead to the formation of more oxygen at the electrode surface during the

anodic half-cycle, preventing the formation of hydrogen in the next cathodic

half-cycle even more efficiently.

Aerating the solution has an effect of reducing the potential during the cathodic

half-cycle since oxygen reduction can occur instead of hydrogen evolution. During

the anodic half-cycle, it has no effect on the potential since chlorine evolution occurs

at the same potential irrespective of the presence of molecular hydrogen.

* The magnitude of the shift depends on the kinetics of the two reactions, which are not known on this

type of electrodes. However, on graphite the reduction of chlorine has a higher exchange current density

than hydrogen evolution, hence the shift in potential may be expected to be larger than the difference in

reversible potentials.

l * This may lead to the formation of hydrogen peroxide which could enhance the disinfection efficiency,

but we shall ignore this added effect for lack of direct experimental evidence.



242

The relat iv e acti vi t y of Cl, and Br,

Literature data on the relative activity of chlorine and bromine are rather

scattered. It seems to be generally accepted that the free halogen is more active than

the hypohalous acid. Studies with different organisms seem to indicate that Cl, and

HClO are more active than Br, and HBrO [ 12,131, although this is not universally

accepted [14]. When the pH of the solution is increased above 7 or 8, the activity of

bromine is found to be much higher than that of chlorine, due to ionization of

HClO, while HBrO is still undissociated.

In the present work, the disinfection efficiency was found to be much higher in

solutions of NaBr than solutions of NaCl, under otherwise identical conditions.

Comparison of Fig. 6a and 6b shows that in relatively acid solutions (pH = 5) the

concentration of NaCl required to reach 99.9% disinfection was about 17 mM while

the concentration of NaBr required to achieve the same effect was only about 0.5mM. Even at pH = 10, a concentration of 2.5 mM NaBr was sufficient to reach the

same extent of disinfection.

The enhanced efficiency found in solutions of NaBr is due to the lower reversible

potential of the BrJBr- couple. Thus, bromine evolution can start earlier in the

anodic half-cycle than chlorine evolution and the amount of bromine (and HBrO)

formed is larger than the amount of chlorine (and HClO) formed at the same current

density. Moreover, the part of the current consumed for side reactions is also smaller

and the beneficiary effect of the side reaction of reducing the local pH is of no

importance in NaBr solutions, due to the higher value of pK, for HBrO.

CONCLUSIONS

The lethal species in the electrochemical disinfection method was identified as

HClO or HBrO in NaCl and NaBr solutions. In acid solutions (pH 4-5) some of the

activity may also be due to the presence of free halogen. The acid dissociation

constant of HClO is pK , = 7.4 and since the anion is much less active as a

bactericide, the disinfection efficiency decreases rapidly with increasing pH beyond

the pK, value, particularly in buffered solutions. In unbuffered solution the genera-

tion of H,O+ ions associated with oxygen evolution or hydrogen oxidation, which

may occur as side reactions, can cause a decrease in local pH near the anode and

thus enhance the disinfection efficiency in solutions of NaCl.

The efficiency of disinfection is much higher in solutions of NaBr than in

solutions of NaCl, because of the lower anodic potential required for bromine

evolution. The difference is even larger at pH > 7 since HClO is ionized and the

anion ClO- is much less active, while HBrO remains mostly undissociated up to

pH = 9. This higher efficiency of disinfection may not be of practical importance in

many instances, since most natural fluids (e.g. drinking water, sewage etc.) contain

already enough NaCl for this electrochemical disinfection technique to operate

efficiently. However, where the addition of a salt is undesirable and the pH should

not be changed, the higher activity of NaBr will be invaluable.



243

One of the most important features of this electrochemical disinfection method is

that the local concentration of the active halogen during the anodic half-cycle can

exceed the average concentration found in the fluid emerging from the cell by one or

two orders of magnitude. A detailed calculation for the specific pulse shape

employed in this study may be rather involved, but an order-of-magnitude typeestimate can serve to elucidate this point. Assume that a current density of 6

mA/cm* is applied (cf. Fig. 3) for 0.3 s and the efficiency of chlorine evolution is

20% l The total amount of Cl, formed in a single pulse will then be 3.6 X lop9

es/cm*. If this amount will be in the diffusion layer of thickness 6 = (rDt)‘/* = 3 X

lo3 cm, the average concentration in this region will be 1.2 X 10e3 eq/dm3, or 42

ppm. If we assume that half of this will be reduced in the cathodic half-cycle and the

rest will be diluted in the cell, which has a separation of 0.1 cm between the

electrodes, the concentration of chlorine in the treated fluid will be about 016 ppm.

Thus, the local concentration could be high enough to destroy the hardiest micro-

organisms, yet the concentration of active halogen in the treated liquid would bebelow the objectionable level.

ACKNOWLEDGEMENTS

The authors wish to thank Professor E.Z. Ron of the Department of Microbiol-

ogy, Faculty of Life Sciences, Tel-Aviv University, for her most valuable assistance

in the microbiological aspects of this study.

Partial support of this work by the U.S.-Israel Binational Foundation is grate-

fully acknowledged.

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l This low efficiency is assumed because the current densities shown in Fig. 3 are peak current densities

(the average is much smaller) and because the faradaic current does not start as soon as the anodic voltageis applied.

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