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Bioelectrochemistty and Bioenergetics, 21 (1989) 41-54 A section of J. Electroanal. Chem., and constituting Vol. 275 (1989) Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands

Bactericidal effect of an electrodialysis system on E. coli cells

Toshio Sato and Tatsuo Tanaka

Department of Analytical ChemistT, Showa Pharmaceutical College, 5-I -8 Turumaki Setagaya-ku, Tokyo I54 (Japan)

Haruhiko Ohya

Material Science and Chemical Engineering, Yokohama Unioersity, Tokiwadai Yokohama-shi, Kanagawa-ken 240 (Japan)

(Received 15 June 1988; in revised form 15 September 1988)

ABSTRACT

Using a new electrodialysis system with both cation- and anion-exchange membranes, the bactericidal effect on Escherichia cofi has been investigated in detail from the standpoint of electrochemistry. Various electrolyte solutions containing E. coli (10’ cells/cm3) were passed through a desalting chamber at a flow rate of 3 cm3/min under varying current densities, and the viability of the cell (a) and the pH changes in the effluents were measured. When a 0.1 M NaCl aqueous suspension was used, a disinfection effect emerged in the vicinity of the limiting current density (LCD 0.81 A/dm2) and increased with an increase in the current density. The pH value of the suspensions decreased owing to the dissociation of water to H+ and OH- ions by the well-known “neutrality disturbance phenomenon” in the region beyond the LCD. These tendencies were also observed when other electrolyte suspensions were used. Concerning the effect of the various species on the disinfection of E. coli cells, ionic systems in which a LCD was easily attained were found to have a strong effect.

The germicidal effect may be due to a synergistic effect of acidic H+ and basic OH- ions which are produced on the anion-exchange membrane and cation-exchange membrane, respectively, of the desalt- ing chamber.

INTRODUCTION

Electrodialysis systems with ion-exchange membranes have been generally used for sea-water desalting, concentration of the electrolyte solution, etc. During an investigation of the membrane characteristics, we have discovered a bactericidal effect in the desalting chamber surrounded by the cation- and anion-exchange membranes [1.2]. We have therefore begun to examine the feasibility of employing the electrodialysis system as a new disinfectant method for water treatment instead of chlorine, which has recently been found to react with a variety of organic impurities in water and convert them into carcinogens [3,4].

0302-4598/89/$03.50 0 1989 Elsevier Sequoia S.A.

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In this paper, we report the bactericidal effect of this system from the standpoint of electrochemistry.

EXPERIMENTAL

The electrodialysis apparatus used, consisting of five chambers, is shown in Fig. 1. Both cation- and anion-exchange membranes (CMV: a commercial strong acid cation-exchange membrane with SO,-Na + ionic groups from Asahi Glass Co.; AMV: a commercial strong base anion-exchange membrane with N+(CH,),Cl- ionic groups from Asahi Glass Co.) were placed alternatively as shown in the diagram of each chamber. The effective area of the membranes was 18.4 cm’ and the distance between them was 1 cm. Aqueous solutions of 0.1 M NaCl were passed through electrode chambers I and V from bottom to top at a flow rate of 25 cm3/min; 0.01 M NaCl aqueous solution was fixed in the concentrating chambers

(II and Iv). Various E. cofi (K-12 W3110) suspended electrolyte solutions (lo* cells/cm3) were passed through the central desalting chamber (III) from bottom to top at a flow rate of 3 cm3/min and at 20°C. A constant current power supply

0 5 10 15 20 25

Voltage/V

Fig. 1. Schematic diagram of the electrodialytic disinfection system. (Cl, C2) Cation-exchange mem- branes (Selemion CMV); (Al, A2) anion-exchange membranes (Selemion AMV). The area of the membranes used was 18.4 cm’. The distance between the membranes was 1 cm. (PS) Polar solution (0.1

M NaCI aqueous solution); (FS) fixed solution (0.01 M NaCl aqueous solution); (ES) Various E. coli

suspended electrolyte solutions. The cell concentration was 10’ cells/cm3.

Fig. 2. Current vs. voltage curve in the system of 0.1 M NaCl aqueous suspension. The limiting current density of the system was determined from the deviation of the curve. The limiting current density was given by

Current value at deviating point 0.150 A zz~=

Area of membrane 0.184 dm2 0.81 A/dm*.

(Kikusui Densi, Model PAD 25OL) was used. The viability (%) of and the pH value of the effluent were measured upon changing the The viability of the cells (%) is given by

No. of viable cells in effluent No. of viable cells in original suspension

x 100

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the E. coli cells current density.

The current vs. voltage curve in the system using 0.1 M NaCl aqueous suspen- sion is shown in Fig. 2. From the figure, it can be seen that the curve deviates around 0.15 A; and a limiting current density of 0.81 A/dm2 is obtained from this value.

RESULTS AND DISCUSSION

Table 1 and Fig. 3 show the changes of the viability (%) of E. cofi cells and of the pH of the effluent solutions under various current densities (0.27-1.63 A/dm*) and

TABLE 1

Viability changes (I%) of E. coli cells under various current densities and time intervals (f, is the

electrodialysis time) a

Current density/A dm-* Viability of cells %

r_=7min r-=15 min t, = 30 mitt t, = 45 min t, = 60 mitt

0.27 82.0 83.9 80.0 79.3 80.6

0.54 77.4 78.1 79.0 77.6 78.0

0.81 20.0 21.6 18.9 20.1 16.1

1.08 7.5 7.8 6.3 7.2 6.0

1.35 0 0 0 0 0

1.63 0 0 0 0 0

* E. coli suspended physiological saline suspension was passed through chamber III at a flow rate of 3 cm3/min and at 20 o C. The original cell concentration of the physiological saline suspension was lo8

ceIls/cm3.

7 15 30 45 60

Time/min

Fig. 3. pH changes of the effluent solution under the conditions of Table 1. Current density/A dm-*: (w) 0.27; (A) 0.54; (0) 0.81; (0) 1.08; (A) 1.35; (0) 1.63.

1 i

01 /I

0 2'0 4b 60 120

Time-course/min

Fig. 4. Disinfection effect of 0.1 M NaCl aqueous suspensions of various pHs on E. coli cells. pH: (0)

7.0; (0) 5.0; (0) 4.0; (A) 3.0; (A) pH 2.0.

elapsed times (7-60 min). From Table 1, it can be seen that in the regions of low current densities (0.27 and 0.54 A/dm*), there were no changes of the viability or the pH value. However, changes occurred with a current density of 0.81 A/dm’, which was the limiting current density of the system determined by measuring the current-voltage curve (cf. Fig. 2), and they increased with an increase in the current density. At current densities greater than 1.35 A/dm*, the E. coli cells were completely devitalized (0%) and the pH values of the solution decreased to 5.5-4.6 only 7 min after the beginning of the electrodialysis. In this manner, a remarkable bactericidal effect was found in the region of current densities greater than the limiting current density. This result suggests that the effect is strongly connected with the “neutrality disturbance phenomenon” [5,6], i.e. dissociation of water to H+ and OH- ions in the region of the limiting current density.

Figure 4 shows the simple pH effects on E. cofi cells by adjusting the pH values (2.0-7.0) by adding 0.1 M HCl aqueous solution to the 0.1 M NaCl aqueous suspensions. From this figure, it can be seen that the viability of the cells (W) in the pH 4.0 and pH 5.0 solutions is almost equal to that of the pH 7.0 one and moreover, in the solutions with lower pH values of 2.0 and 3.0. E. coli cells were alive to almost the same degree as in the systems of pH 4 and 5. This result means that the pH values do not parallel the disinfection effect shown in Table 1, i.e. the pH value itself without an electric current does not affect the devitalization of E. cob.

Figure 5 shows a comparison of the cell viability (%) between the ion-exchange membrane system and the covered system with cellophane on the ion-exchange membrane under the same conditions as in Table 1. It is well known that in the

a- 0 0.54 0.81 1.08 1 .35 1.63 0 0.54 0.81 1.08 1.35 1 .63

Current density/ A dm-* Current density/~ dme2

Fig. 5. Comparison of the cell viability (%) between the ion-exchange membrane system and the covered system with cellophane on the ion-exchange membrane under the same conditions as in Table 1. The

comparison was done using data 60 mitt after the beginning of the electrodialysis. (0) Ion-exchange

membrane system: (0) covered system with cellophane at a distance of 1 mm.

Fig. 6. Disinfection effect of the current densities on the system with an ion-exchange membrane, a

cellophane membrane, and the current itself. The comparison after the beginning of the electrodialysis.

(0) Ion-exchange membrane system; (A) cellophane membrane system; (0) only current.

region of current density greater than the limiting current density, H+ and OH- ions are produced near the surfaces of the cation- and anion-exchange membranes of the desalting chamber, owing to dissociation of water caused by the “neutrality disturbance phenomenon” [7,8] and they carry the current.

From the figure, it can be seen that the devitalization effects in the covered system are almost equal to those of the ion-exchange membrane system, and thus it is clear that E. co/i cells are not killed upon touching the surfaces of the ion-ex- change membranes.

Figure 6 shows the disinfection effects of the current densities on systems with an ion-exchange membrane and without any membrane. In the figure, no disinfection effect can be observed in the system with a cellophane membrane or that without any membrane. Note that there is no devitalization effect in the system without ion-exchange membranes, even at high current densities (1.35-1.63 dA/dm*) greater than the limiting current density. The results show that a high current itself does not devitalize E. coli cells.

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TABLE 2

Limiting current densities of various anionic suspensions

Anionic Limiting current

suspension density/A dm-’ a

0.1 M NaCl 0.79

0.1 M NaNO, 0.76

0.1 M Na,SO, 1.59

0.1 M Na,HPO, 1.71

a Limiting current densities were determined by measuring current vs. voltage curves.

Figure 7 shows the effect of various anionic species in the solution on the disinfection of E. coli under the conditions of Table 1. Table 2 gives the limiting

current densities of the electrolyte solutions determined by measuring current vs. voltage curves. As clearly shown in the figure, SO:- and HPOi- ions were not at all effective; NO; ions were found to have the same strong effect as that of Cl- ions. The permeability of the NO; and Cl- ions to the anion-exchange membrane was greater than that of the SOi- and HPOi- ions, and the limiting current

0 0.54 0.81 1.08 1.35 1.63

Current density / A dme2

0 0.54 0.81 1.08 1.35 1.63

Current density / A dme2

Fig. 7. Effect of various anionic species in the solution on the disinfection of E. coli under the conditions

of Table 1. The comparison was done using data 60 min after the beginning of the electrodialysis. (0) 0.1

M NaCI; (0) 0.1 M NaNO,; (A) 0.1 M Na,SO.,; (A) 0.1 M Na,HPO,.

Fig. 8. Effect of various cationic species in the solution on the disinfection of E. coli under the conditions

of Table 1. The comparison was done using data 60 mm after the beginning of the electrodialysis. (0) 0.1 M NaCI; (0) 0.1 M KCI; (A) 0.1 M CaCI,; (A) 0.1 M MgCI,.

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TABLE 3

Limiting current densities of various cationic suspensions

Cationic Limiting current

suspension density/A dme2 a

0.1 M NaCl 0.79

0.1 A4 KC1 0.72

0.1 M CaCl, 1.42

0.1 M MgCl, 1.49

a Limiting current densities were determined by measuring current vs. voltage curves.

densities of the NO; and Cl- ions were fairly low in comparison with those of the SO:- and HPOt- ions, as shown in Table 2. These results suggest that the disinfection effect is closely related to the ease with which the limiting current densities are attained, i.e. that of the occurrence of the “neutrality disturbance phenomenon”. The disinfection effect of cationic species was also investigated under the same conditions and is shown in Fig. 8 and Table 3. From the figure, a similar tendensy can be seen in the case of the cationic species. Namely, Kf and Na+ ions, which have a large permeability to the cation-exchange membranes, were found to have a remarkable devitalization effect, and the limiting current densities of the systems containing these two ions were lower than those of the Ca2+ and

Mg 2+ ion systems, as shown in Table 3. Moreover, the solution containing K+ and NO, ions was the most effective in the devitalization of E. coli cells. This is considered to be closely related to the largest permeability of the ions through the ion-exchange membranes, namely the lowest limiting current density as a result of the highest permeability. In this manner, the solution systems with lower limiting current densities were found to be more effective in the disinfection of E. coli cells.

CONCLUSIONS

Taking the results obtained into account, we can conclude that the disinfection effect of an electrodialysis system is due to a co-operative effect of the current density, the ionic species and the characteristics ofthe ion-exchange membranes. That is, as shown schematically in Fig. 9, it is caused by the synergistic effect of H+ and OH- ions which are produced on the anion-exchange membrane and cation-ex- change membrane of the desalting chamber, respectively, in the region of current densities greater than the limiting current density where the neutrality disturbance phenomenon occurs and water is dissociated into H+ and OH- ions. A similar synergistic phenomenon has been discovered by one of us in the case of a mixed bed of H+ and OH- from ion-exchange resins [9]. The mechanism of the disinfection effect will be discussed in detail in another paper. The electrodialysis system has the

Al C2 cathode

side 0

III chamber

Fig. 9. Disinfection mechanism of E. co/i with both H+ and OH- ions produced

membranes. (Al) Anion-exchange membrane; (C2) cation-exchange membrane. by the ion-exchange

potential to be a new and handy disinfectant for water treatment in place of

chlorine.

REFERENCES

1 T. Sato, T. Tanaka and T. Suzuki, De&i Kagaku (J. Electrochem. Sot., Jpn), 52 (1984) 239.

2 T. Sato, T. Tanaka and T. Suzuki, IONICS (Ion-sci. Technol.), 120 (1985) 199.

3 Y. Baba, Kagaku To Kogyo (Chem. Chem. Ind.), 31 (1978) 492.

4 J.J. Rook, Water Treat. Exam., 23 (1974) 234.

5 T. Yamabe, Koubunshi, 17 (1968) 306.

6 Y. Tanaka, Kaisuigakkaishi (Bull. Sot. Sea Water Sci.), 45 (1985) 259.

7 Y. Tanaka and M. Seno, J. Chem. Sot., Faraday Trans. 1, 82 (1986) 2065.

8 N.W. Rosenberg, Ind. Eng. Chem., 49 (1967) 780.

9 T. Suzuki, S. Goto and T. Tanaka, Denki Kagaku (J. Electrochem. Sot., Jpn.), 52 (1984) 272.