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Enhanced disinfection efficiency of mechanically mixed oxidants with free chlorine
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Water Research 39 (2005) 721–727 www.elsevier.com/locate/watres
Enhanced disinfection efficiency of mechanically mixed oxidants with free chlorine Hyunju Sona, Min Choa, Jaeeun Kima, Byungtaek Oha, Hyenmi Chungb, Jeyong Yoona, a
School of Chemical Engineering, College of Engineering, Seoul National University, San 56-1, Sillim-dong, Gwanak-gu, Seoul 151-742, Republic of Korea b Water Microbiology Division, National Institute of Environmental Research, Gyungseo-dong, Seo-gu, Inchon 404-170, Republic of Korea Received 19 June 2004; received in revised form 13 October 2004; accepted 24 October 2004
Abstract To the best of our knowledge, this study is the first investigation to be performed into the potential benefits of mechanically mixed disinfectants in controlling bacterial inactivation. The purpose of this study was to evaluate the disinfection efficiency of mechanically mixed oxidants with identical oxidant concentrations, which were made by adding small amounts of subsidiary oxidants, namely ozone (O3), chlorine dioxide (ClO2), hydrogen peroxide (H2O2) and chlorite (ClO 2 ), to free available chlorine (Cl2), using Bacillus subtilis spores as the indicator microorganisms. The mechanically mixed oxidants containing Cl2/O3, Cl2/ClO2 and Cl2/ClO 2 showed enhanced efficiencies (of up to 52%) in comparison with Cl2 alone, whereas no significant difference was observed between the mixed oxidant, Cl2/H2O2, and Cl2 alone. This enhanced disinfection efficiency can be explained by the synergistic effect of the mixed oxidant itself and the effect of intermediates such as ClO 2 /ClO2, which are generated from the reaction between an excess of Cl2 and a small amount of O3/ClO 2 . Overall, this study suggests that mechanically mixed oxidants incorporating excess chlorine can constitute a new and moderately efficient method of disinfection. r 2004 Elsevier Ltd. All rights reserved. Keywords: Mixed oxidants; Chlorine; Ozone; Chlorine dioxide; Chlorite; B. subtilis spore
1. Introduction The elimination of pathogenic microorganisms (e.g., bacteria, viruses and protozoa) in water treatment systems is of great concern (White, 1992). One of the most widely used disinfectants for removing these pathogens is free available chlorine (Cl2) (Sayer et al., 1994). However, alternatives to chlorine have been constantly pursued due to the appearance of harsh Corresponding author. Tel.: +82 2 880 8927; +82 2 876 8911. E-mail address: [email protected] (J. Yoon).
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microorganisms as well as the formation of chlorinated disinfection by-products. Ozone (Bashtan et al., 1999), ultraviolet irradiation (Chizuko et al., 2000), and electrochemically mixed oxidants (Harrington, 1999; Kraft et al., 1999) have been proposed as alternative disinfectants. In addition, sequential disinfection with ozone or chlorine dioxide followed by chlorine has proven to be an effective approach to treating pathogens (Cho et al., 2003; Corona-Vasquez et al., 2002; Driedger et al., 2000). Sequential treatments are based on the idea of using a powerful oxidant such as ozone as the primary disinfectant, followed by chlorine as the secondary disinfectant. Each disinfection agent has its
0043-1354/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2004.10.018
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H. Son et al. / Water Research 39 (2005) 721–727
own specific reactivity toward cell structure materials, so that the use of multiple agents means that different mechanisms are brought to bear to inactivate the microorganisms. The synergistic effect of sequential treatments is likely caused by the activity of the various disinfection agents reacting with specific chemical groups of the cell wall. Consequently, it can be hypothesized that the secondary disinfectant is allowed to permeate through partially reacted cell wall layers at a faster rate (Corona-Vasquez et al., 2002). However, this putative synergism could be due to the use of mixed disinfectants as well as to the effect of sequential disinfection. In the case of mixed disinfectants, however, we would expect to observe additional synergistic effects other than that described above. The coincident reaction of two disinfectants (viz. chlorine and other oxidants) with microbial cells and the contribution of the reaction intermediates might provide additional synergistic effects. These intermediates might increase the inactivation efficiency or have no effect on the inactivation. There have been some studies which showed that electrochemically mixed oxidants achieved considerable disinfection efficiency for selected microorganisms (Casteel et al., 2000; MIOX literature, 1995), while the level of enhanced disinfection efficiency remains unclear, depending upon the type of tested microorganisms (Son, 2004). Due to the lack of effective analytical methods, however, the exact composition of the intermediates produced from these electrochemically mixed oxidants is not well understood. In this study, the microbial inactivation efficiency of mechanically mixed oxidants, which were made by adding small amounts of various oxidants (ozone, chlorine dioxide, hydrogen peroxide and chlorite) to a high concentration of chlorine, was investigated. In addition, we attempted to provide an indirect explanation for the presence of the chemical species observed in the mechanically mixed oxidants and their contribution to the enhanced disinfection efficiency.
2. Materials and methods 2.1. Preparation of mixed oxidants All reaction solution and reagents were prepared with Milli-Q-treated water (Millipore Co., USA), and analytical grade chemicals were used (Fisher Scientific, USA). A chlorine stock solution was prepared by dilution with sodium hypochlorite solution (5%, Junsei Co., Japan). Ozone (440 mg/L) was made by means of an ozone generator CFS-1 (Ozonia Co., Switzerland). Chlorine dioxide was prepared by oxidizing technical grade sodium chlorite (NaClO2) with hydrogen sulfate (H2SO4) (Christian et al., 2001). A stock solution
Table 1 Mechanically mixed oxidants as concentration of the stock solutiona Type
Contents
Subsidiary oxidant conc.
I
Cl2 þ O3
O3 0.021 mM (1 mg/L)
II
Cl2 þ ClO2
III
Cl2 þ H2 O2
ClO2 0.027 mM (1.8 mg/L) and 0.27 mM (18 mg/L) H2O2 0.27 mM (9.2 mg/L)
IV
Cl2 þ ClO2 2
ClO–2 0.59 mM (40 mg/L)
a
pH ¼ 2.5, Cl2 ¼ 2.82 mM (200 mg/L).
(100 mg/L) of chlorine dioxide was stored in the dark at 4 1C before use. Chlorite was prepared by diluting NaClO2. Hydrogen peroxide was used in the form of an analytical grade reagent (30%, Junsei Co., Japan). Table 1 shows the specific contents of the four mechanically mixed oxidants used in the experiment. The stock solution of each mixed oxidant was made by the addition of ozone (1 mg/L), chlorine dioxide (1.8 and 18 mg/L), hydrogen peroxide (9.2 mg/L) and chlorite (40 mg/L), respectively, to a chlorine stock solution with a concentration of 200 mg/L at pH 2.5, since chlorine dioxide and ozone are stable under acidic conditions (Csordas et al., 2001). A mixed oxidant containing chlorite was made, since chlorite is expected to be produced as a reaction intermediate between chlorine and ozone (Haag and Hoigne´, 1983). The pH was adjusted using phosphoric acid (20 mM) in order to have the required degree of acidity. In each case, the oxidant was fully mixed with chlorine for 30 min. The ratio of the concentration of chlorine to that of the subsidiary oxidant (ozone, chlorine dioxide, hydrogen peroxide and chlorite) was chosen based on the minimum subsidiary oxidant concentrations that can be measured by UV spectroscopy. In spectroscopic study, the higher concentration ratio of the ozone to the chlorine was chosen than that used in disinfection experiments, in order to observe the change in UV absorbance with time. The concentrations of chlorine and ozone were varied from 200 to 38 mg/L and from 1 to 2 mg/L, respectively. 2.2. Culture of bacteria Bacillus subtilis (B. subtilis) (ATCC 6633) spores were chosen as the representative indicator microorganisms to estimate the inactivation efficiency of the mixed oxidants. B. subtilis spores were prepared by following the procedure described by Nakayama et al. (1996), except for certain minor modifications involving the use of 1/10 nutrient agar with an extended incubation time
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of 5–7 days, the cells were harvested by centrifugation at 4000 g for 20 min and washed three times with 20 mM phosphate buffer solution (PBS, pH 8.2). The stock solution of B. subtilis spores was prepared by resuspending the cell pellets in 10 mL of PBS and by treating the cells at 80 1C for 20 min just before each experiment. The cell population was determined by plating the cells on nutrient agar and counting the number of colony forming units (CFU) per mL. Triplicate disinfection experiments showed a good reproducibility within the 95% confidence interval. 2.3. Disinfection experiments All of the materials for the disinfection experiments were autoclaved at 121 1C for 15 min. For the disinfection experiments, the mechanically mixed oxidants listed in Table 1 were diluted so as to have a total oxidant concentration of 2 mg/L, using 20 mM PBS (pH 8.2) at 20 1C, because the inactivation procedure was generally carried out in neutral or slightly basic conditions in water treatment facilities. The experiments were conducted using 50 mL of the solution in 60 mL Pyrex flasks for 8 h with an initial microbial population of 105 CFU/ mL. In order to minimize chlorine loss by volatilization, the contents were mixed slowly using a magnetic stirrer, and the reactors were capped during stirring. One milliliter of the solution was withdrawn at each sampling and diluted 1/1, 1/10 and 1/100 with PBS. Finally, 0.1 mL samples of the undiluted and diluted solutions were spread on nutrient agar. The CFU values were determined after 24 h incubation at 37 1C. The level of inactivation was expressed as the log10 reduction of the microbial survival ratio during the disinfection experiments. The disinfection efficiency in this study was ¯ values (C: ¯ the time-averaged condetermined as CT centration of disinfectant (mg/L); T: contact time (min)). To explain the kinetics of B. subtilis spore inactivation, the modified (or delayed) Chick–Watson model that includes the lag phase in the inactivation and ozone decomposition was used (Rennecker et al., 1999; Cho et al., 2002). N N0 8 > <0 ¼ > ¯ þ kCT lag : kCT
log
if
¯ CTpCT lag
if
¯ CTXCT lag
¼ k1 log NN0 ; ¼ k1 log NN0 ;
where N0 N C
initial cell population (CFU/mL) remaining cell population at time T (CFU/mL) disinfectant concentration (mg/L)
C¼
Rt
T K CTlag
0
C dt=t
723
time-averaged oxidants concentration (mg/L) reaction time (min) inactivation rate constant with disinfectant (L/(mg min)) intercept of the inactivation curve with the x-axis
2.4. Chemical analysis The change in the total concentration of the oxidants in the mixed oxidants solution and the residual chlorine were measured by DPD (N,N-diethyl-p-phenylelenediamine) methods using DR/2010 (HACH Co., USA). The concentrations of ozone and hydrogen peroxide were determined by measuring the difference in UV absorbance after reacting them with indigo thiosulfate and titanium sulfate, respectively. After mixing the oxidants, the spectra of the mixed oxidants were obtained between 240 nm and 450 (or 340) nm at different times using a UV (Model HP8452) spectrophotometer, in order to investigate the formation of unknown intermediates during the reaction. The concentration of chlorine dioxide was determined by measuring the change in UV absorbance with time at 359 nm, which was a unique peak for chlorine dioxide. Chlorite was determined by measuring the change in UV absorbance with time at 260 nm, corresponding to the characteristic absorbance of chlorite. Sodium thiosulfate (0.002 M) was added to neutralize the residual oxidants or residual chlorine (American Public Health Association, 1989). In addition, further experiments were conducted to confirm the identity of the unknown compounds using an ion chromatograph (IC, Dionex DX-120, Sunnyvale, CA, USA). After the disinfection experiments had been conducted for 8 h, an analysis was performed to identify the intermediates. Separation was achieved with an AS9HC column (Dionex) using 9 mM carbonate eluent with anion self-regenerating suppressor (ASRS-ULTRA, 4 mm) and conductivity detection. The eluent was pumped at a rate of 1.0 mL/min.
3. Results and discussion 3.1. Disinfection efficiency by mechanically mixed oxidants The inactivation of the Bacillus subtilis spores by the mechanically mixed oxidants containing chlorine and subsidiary oxidants (Cl2/O3, Cl2/ClO2, Cl2/H2O2 and Cl2/ClO 2 ) at pH 8.2 shows a lag phase, due to the difficulty of permeating the thick cell wall (Fig. 1). The inactivation curve for chlorine alone, with the same oxidant concentration, was included as a base line to
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that of chlorine alone. Fig. 1(d) shows that the mechanically mixed oxidant containing chlorine and chlorite is 52% more efficient than chlorine alone in inactivating B. subtilis spores. It is noted that the level of concentration of ozone, chlorine dioxide, hydrogen peroxide and chlorite alone presumably contained in tested mixed oxidants of 2 mg/L did not have any measurable disinfection ability for B. subtilis spores.
evaluate the enhanced disinfection efficiency afforded by the mechanically mixed oxidants. Fig. 1(a) shows the inactivation of B. subtilis spores by the mixed oxidant containing chlorine and ozone. The mechanically mixed ¯ value than chlorine alone oxidant needed a 21% less CT in achieving the 2 log inactivation of B. subtilis spores. The enhanced disinfection efficiency afforded by the mechanically mixed oxidant containing chlorine and ozone was attributed to the shortening of the lag phase of the inactivation curves. The inactivation of the B. subtilis spores afforded by the mechanically mixed oxidant containing chlorine and chlorine dioxide is shown in Fig. 1(b). The synergistic effect of chlorine and chlorine dioxide at the concentration ratio of 2.82/ 0.027 mM and 2.82/0.27 mM (as a concentration of the stock solution) in achieving 2 log inactivation of B. subtilis spores was 26% and 45%, respectively, more effective than chlorine alone. The disinfection efficiency at a constant chlorine concentration increased with increasing chlorine dioxide concentration. As shown in Fig. 1(c), the disinfection efficiency of the mechanically mixed oxidant containing chlorine and hydrogen peroxide for the inactivation of the B. subtilis spores was not significantly different from
3.2. Possible intermediates from oxidants reaction Based on the results shown in Fig. 1, three mixed oxidants (ozone, chlorine dioxide and chlorite) combined with chlorine among the four different mechanically mixed oxidants showed significantly enhanced ¯ disinfection efficiency (21–52% less CT values were needed to achieve 2 log inactivation) compared with chlorine alone. This significantly enhanced inactivation might be due to the synergistic effects of the mixed oxidant itself and/or the formation of unknown intermediates from the oxidants, which also have disinfection ability. Thus, further experiments were conducted in order to investigate the possible formation of transient
0
log(N/No)
-1 -2 -3 -4 (a)
(b)
(c)
(d)
-5 0
log(N/No)
-1 -2 -3 -4 -5 0
100
200
300
400
CT(mg.min/L)
500
600
0
100
200
300
400
500
600
CT(mg.min/L)
Fig. 1. Comparison of B. subtilis spores inactivation between mechanically mixed oxidants and chlorine (2.82 mM) alone (-3-) at pH 8.2: (a) Cl2/O3 (0.021 mM), -d-; (b) Cl2/ClO2 (0.027 mM), -’-, Cl2/ClO2 (0.27 mM), -&-; (c) Cl2/H2O2 (0.27 mM), -m-; (d) Cl2/ClO 2 (0.59 mM), -~-. The concentration of each mixed oxidant is that of the stock solution given in Table 1, and not that of the solution used in the disinfection experiment. For the disinfection studies, the mechanically mixed oxidants in Table 1 were diluted to 2 mg/L as the total oxidant concentration.
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unknown primary intermediates to unknown secondary intermediates. As a result of the IC analysis, chlorite was identified in the reaction with chlorine and ozone (data not shown), which is consistent with the observation of Haag and Hoigne´ (1983) (Table 2). However, measuring the quantitative amount of chlorite was impossible, because the concentration of chlorite was below the detection limit. For the mixed oxidants containing chlorine and chlorine dioxide, no significant change in UV absorbance was observed with time (data not shown). This phenomenon is plausible, since chlorine slowly reacts with chlorine dioxide at acidic pH, and the change in UV absorbance is negligible. This result is consistent with previous works which indicated that chlorine dioxide is stable in concentrated chlorine solution under
0.30 Cl2 (38 mg/L) O3(2 mg/L)
0.25
Cl2 + O3 (0 min) Cl2 + O3 (30 min) Cl2 + O3 (60 min)
0.20
Cl2 + O3 (theoretical)
Abs.
intermediates from the reaction between chlorine and one of the subsidiary oxidants. Table 2 summarizes the important reactions that can occur between chlorine and each subsidiary oxidant under acidic conditions. It is well known that the reaction of chlorine with a small amount of ozone, chlorite and chlorine dioxide can produce chlorite, chlorine dioxide and chlorate, respectively. Thus, similar reactions could occur in the disinfectant solution generated by the electrolysis of brine. The primary oxidant generated from the brine electrolysis system is chlorine (Son et al., 2004; Patermarakis and Fountoukidis, 1990; Gordon et al., 1998). In addition to chlorine, it was suggested that ozone and chlorine dioxide or hydrogen peroxide could be generated (United States Patent, 1988, 1995), although the amounts produced were very small. However, the exact composition of the agents produced by the electrolysis of brine is not well understood, due to analytical deficiencies. Evaluating the disinfection efficiency and chemical characteristics of mechanically mixed oxidants would be helpful, in order to provide an indirect understanding of the properties of the mixed oxidants generated by the electrolysis of brine. Fig. 2 shows the change in UV absorbance of the mixed oxidant produced when chlorine (38 mg/L) and ozone (2 mg/L) were mixed together. This experimental condition was followed as described in Emmert’s work (Emmert, 1999). As shown in Fig. 2, the absorbance of the mechanically mixed oxidants at 260 nm, which indicates the maximum absorbance of ozone, increased in comparison with the theoretical combined absorbance of chlorine and ozone, and decreased with time. The absorbance peak decreased to the level of chlorine after 1 h. This observation suggests that unknown intermediates might have been generated from the fast reaction between chlorine and ozone, and were subsequently degraded with time. Decreased absorbance at 260 nm would correspond to the conversion of ozone and the
725
0.15
0.10
0.05
0.00 240
260
280 300 Wavelength (nm)
320
340
Fig. 2. UV absorbance spectra of mechanically mixed oxidants containing Cl2 (38 mg/L) and O3 (2 mg/L) at pH 2.5.
Table 2 Mixed oxidants chemistry Reaction
Rate constant (M1 s1)
References
HOCl þ O3 ! ClO2 2
ko2 103 M1s1
Haag and Hoigne´ (1983)
HOCl þ H2 O2 ! H2 O þ O2 þ Cl2 þ Hþ
k ¼ 0.3–2 M1 s1
Connick (1947)
2 þ HOCl þ 2ClO2 þ H2 O ! 2ClO2 3 þ Cl þ 3 H
k ¼ (1.570.3) 103 M1 s1
Csordas et al. (2001)
þ 2 HOCl þ 2ClO2 2 þ H ! 2ClO2 þ H2 O þ Cl
k ¼ (1.1070.03) 106 M2 s1
Peintler et al. (1990)
2 2HOCl þ ClO2 2 ! ClO3 þ Cl2 þ H2 O
k ¼ (2.170.1) 103 M2 s1
Peintler et al. (1990)
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acidic conditions (Emmert, 1999; Taube and Dodgen, 1949; Gauw et al., 1999). Therefore, the enhanced disinfection efficiency afforded by this mechanically mixed oxidant may be due to the synergistic effect of the mechanically mixed oxidant itself. In the case of the mixed oxidants containing chlorine and hydrogen peroxide, no significant change in UV absorbance was observed (data not shown). This result was probably due to the hydrogen peroxide being consumed quickly as a result of its reacting with chlorine (Held et al., 1978). Therefore, in effect, this solution corresponds to the case of chlorine alone. This result implies that hydrogen peroxide does not increase the inactivation rate of B. subtilis spores, because hydrogen peroxide is oxidized by a high concentration of chlorine. This UV absorbance result is consistent with the observation shown in Fig. 1(c). A high concentration of chlorite (40 mg/L as a concentration of the stock solution) was used in making the mechanically mixed oxidants, in order to measure the UV absorbance at 359 nm in the spectroscopic study. Fig. 3 shows the UV absorbance of the mechanically mixed oxidants when chlorine (200 mg/L) was mixed with chlorite (40 mg/L). It was clearly observed that a high concentration of chlorine dioxide, which has a fingerprint absorbance at 359 nm, was generated as a result of mixing chlorine and chlorite, and the chlorine dioxide concentration decayed slowly with time. Peintler et al. (1990) reported that chlorine dioxide was produced from the reaction of chlorine and chlorite (Table 2). The maximum peak concentration of chlorine dioxide, which
0.4 ClO2- (40 mg/L)
Cl2 + ClO2- (0 min)
Cl2 (200 mg/L)
Cl2 + ClO2- (30 min) Cl2 + ClO2- (120 min) Cl2 + ClO2- (240 min)
Abs.
0.3
0.2
4. Conclusion In this study, the disinfection efficiency afforded by mechanically mixed oxidants, combining excess chlorine with small amounts of subsidiary oxidants (O3, ClO2, ClO 2 and H2O2), was first reported in comparison with Cl2 alone as B. subtilis spore was used as an indicator microorganism. The chlorine-based mixed oxidants containing O3, ClO2 and ClO 2 showed significantly enhanced efficiencies in comparison with Cl2 alone at the condition that the subsidiary oxidant itself without chlorine showed negligible disinfection ability. This enhanced disinfection efficiency was explained by the combined effect of the mixed oxidant itself and intermediates which may be generated from the reaction between an excess Cl2 and a small amount of subsidiary oxidants. The formation of intermediates was demonstrated by the measurement of UV absorbance at the wavelengths specific with O3 , ClO2 and ClO 2 along with the limited result of IC measurements. In actual drinking water treatment systems, chlorine, ozone and chlorine dioxide are popular disinfectants and the sequential disinfection with ozone or chlorine dioxide followed by chlorine has proven to be an effective process. This study reports that mixed oxidants with chlorine which contain even a negligible amount of ozone or chlorine dioxide can be a moderately viable alternative for the inactivation of pathogens in water treatment systems, although their disinfection efficiencies are insufficient to inactivate harsh microorganisms such as Cryptosporidium parvum.
Acknowledgements This research was partially supported by the Brain Korea 21 Program (the Ministry of Education). This support was greatly appreciated.
0.1
0.0 270
was calculated using the molar absorbance coefficient at 359 nm, was approximately 17 mg/L. To summarize the results, based on Figs. 2 and 3, chlorite was generated as an intermediate from the reaction of excess chlorine and ozone, and finally chlorine dioxide was generated from the reaction of chlorine and chlorite.
References 300
330 360 390 Wavelength (nm)
420
450
Fig. 3. UV absorbance spectra of mechanically mixed oxidants solution containing Cl2 (200 mg/L) and ClO 2 (40 mg/L) at pH 2.5.
American Public Health Association, 1989. Standard Methods for the Examination of Water and Wastewater, 17th ed. Washington, DC, USA. Bashtan, S.Y., Goncharuk, V.V., Chebotareva, R.D., Belyakov, V.N., Linkov, V.M., 1999. Production of sodium
ARTICLE IN PRESS H. Son et al. / Water Research 39 (2005) 721–727 hypochlorite in an electrolyser equipped with a ceramic membrane. Desalination 126 (1), 77–82. Casteel, M.J., Sobsey, M.D., Arrowood, M.J., 2000. Inactivation of Cryptosporidium parvum Oocysts and other microbes in water and wastewater by eletrolytically generated mixed oxidants. Water Sci. Technol. 41 (7), 127–134. Chizuko, M., Kouichi, S., Shinichi, M., Hiromasa, K., Toshiyuki, G., Takehiro, K., Wu, H., Hitofumi, M., Atsuo, I., Yoshiko, N., Masami, T., Osamu, Y., 2000. Disinfection potential of electrolyzed solution containing sodium chlorite at low concentrations. J. Virological Meth. 85 (2), 163–174. Cho, M., Chung, H., Yoon, J., 2002. Effect of pH and importance of ozone initiated radical reactions in inactivating Bacillus subtilis spore. Ozone Sci. Eng. 24 (2), 145–150. Cho, M., Chung, H., Yoon, J., 2003. Quantitative evaluation of the synergistic sequential inactivation of Bacillus subtilis spores with ozone followed by chlorine. Environ. Sci. Technol. 37 (10), 2134–2138. Christian, P.C., Radziminski, Z.R., Michael, L., Robin, C., Andrews, C.A., 2001. Chlorine Dioxide inactivation of Cryptosporidium parvum oocysts and bacterial spore indicators. Appl. Environ. Microbiol. 67 (7), 2993–3001. Connick, R.E., 1947. The interaction of hydrogen peroxide and hypochlorous acid in acidic solutions containing chloride ion. J. Am. Chem. Soc. 69 (6), 1509–1514. Corona-Vasquez, B., Rennecker, J.L., Driedger, A.M., Marin˜as, B.J., 2002. Sequential inactivation of Cryptosporidium pavum oocysts with chlorine dioxide followed by free chlorine or monochloramine. Water Res. 36 (1), 178–188. Csordas, V., Bubnis, B., Fabian, I., Gordon, G., 2001. Kinetics and mechanism of catalytic decomposition and oxidation of chlorine dioxide by the hypochlorite ion. Inorg. Chem. 40 (8), 1833–1836. Driedger, A.M., Rennecker, J.L., Marin˜as, B.J., 2000. Sequential inactivation of Cryptosporidium pavum oocysts with ozone and free chlorine. Water Res. 34 (14), 3591–3597. Emmert, G.L., 1999. Chemical Aspects of Water Treatment: Part 1. Chemical Characteristics of Oxidants Present in Liquors Following Electrolysis of Brine. Ph. D. Dissertation, Miami University, Oxford, OH, USA. Gauw, R.D., Emmert, G.L., Bubnis, B., Gorden, G., 1999. High resolution spectrophotometry for identification of chlorine dioxide in concentrated chlorine solution. Talanta 50 (5), 1073–1078. Gordon, G., Emmert, G., Gauw, R., Bubnis, B., 1998. Can ozone and ozone oxidative by by-products be formed during the electrolysis of salt brine? Ozone Sci. Eng. 20 (4), 239–249.
727
Haag, W.R., Hoigne´, J., 1983. Ozonation of water containing chlorine or chloramines. Water Res. 17 (10), 1397–1402. Harrington, R.E., 1999. Disinfection by mixed oxidant technology in a portable package plant. In: Contruvo, J.A., Graun, G.F., Hearne, N. (Eds.), Providing Safe Drinking Water in Small System. Lewis Publishers, USA. Held, A.M., Halko, D.J., Hurst, J.K., 1978. Mechanisms of chlorine oxidation of hydrogen peroxide. J. Am. Chem. Soc. 100 (18), 5732–5740. Kraft, A., Blaschke, M., Kreysig, D., Sandt, B., Schroder, F., Rennau, J., 1999. Electrochemical water disinfection. Part II. Hypochlorite production from potable water, chlorine consumption and the problem of calcareous deposits. J. Appl. Electrochem. 29 (8), 895–902. MIOX literature, 1995. Alternative Comparison for Important Features in Municipal Plants and Wells. MIOX Corporation, Los Alamos, NM, USA. Nakayama, A., Yano, Y., Kobayashi, S., Ishikawa, M., Sakai, K., 1996. Comparison of pressure resistances of spores of six Bacillus strains with their heat resistances. Appl. Environ. Microbiol. 62 (10), 3897–3900. Patermarakis, G., Fountoukidis, E., 1990. Disinfection of water by electrochemical treatment. Water Res. 24 (12), 1491–1496. Peintler, G., Nagypal, I., Epstein, I.R., 1990. Systematic design of chemical oscillators. 60. Kinetics and mechanism of the reaction between chlorite ion and hypochlorous acid. J. Phys. Chem. 94 (2), 954–958. Rennecker, J.L., Marin˜as, B.J., Owens, J.H., Rice, E.W., 1999. Inactivation of Cryptosporidium parvum oocysts with Ozone. Water Res. 33 (11), 2481–2488. Sayer, C.N., McCarty, P.L., Parkin, L., Parkin, G.F., 1994. Chemistry for Environmental Engineering, fourth ed. McGraw-Hill, New York, NY, USA. Son, H., Cho, M., Chung, H., Choi, S., Yoon, J., 2004. Bactericidal Activity of Mixed Oxidants: Comparison with free chlorine. J. Ind. Eng. Chem. 10 (5), 705–709. Taube, H., Dodgen, H., 1949. Applications of radioactive chlorine to the study of the mechanisms of reactions involving changes in the oxidation state of chlorine. J. Am. Chem. Soc. 71 (10), 3330–3336. United States Patent No. 4,761,208. 1988. Electrolytic Method and Cell for Sterilizing Water. 2 August. United States Patent No. 5,385,711. 1995. Electrolytic Cell for Generating Sterilization Solutions Having Increased Ozone Content. 31 January. White, G.C., 1992. The Handbook of Chlorination and Alternative Disinfections, third ed. Van Nostrand Reinhold, New York, NY, USA.
Water Research 39 (2005) 721–727 www.elsevier.com/locate/watres
Enhanced disinfection efficiency of mechanically mixed oxidants with free chlorine Hyunju Sona, Min Choa, Jaeeun Kima, Byungtaek Oha, Hyenmi Chungb, Jeyong Yoona, a
School of Chemical Engineering, College of Engineering, Seoul National University, San 56-1, Sillim-dong, Gwanak-gu, Seoul 151-742, Republic of Korea b Water Microbiology Division, National Institute of Environmental Research, Gyungseo-dong, Seo-gu, Inchon 404-170, Republic of Korea Received 19 June 2004; received in revised form 13 October 2004; accepted 24 October 2004
Abstract To the best of our knowledge, this study is the first investigation to be performed into the potential benefits of mechanically mixed disinfectants in controlling bacterial inactivation. The purpose of this study was to evaluate the disinfection efficiency of mechanically mixed oxidants with identical oxidant concentrations, which were made by adding small amounts of subsidiary oxidants, namely ozone (O3), chlorine dioxide (ClO2), hydrogen peroxide (H2O2) and chlorite (ClO 2 ), to free available chlorine (Cl2), using Bacillus subtilis spores as the indicator microorganisms. The mechanically mixed oxidants containing Cl2/O3, Cl2/ClO2 and Cl2/ClO 2 showed enhanced efficiencies (of up to 52%) in comparison with Cl2 alone, whereas no significant difference was observed between the mixed oxidant, Cl2/H2O2, and Cl2 alone. This enhanced disinfection efficiency can be explained by the synergistic effect of the mixed oxidant itself and the effect of intermediates such as ClO 2 /ClO2, which are generated from the reaction between an excess of Cl2 and a small amount of O3/ClO 2 . Overall, this study suggests that mechanically mixed oxidants incorporating excess chlorine can constitute a new and moderately efficient method of disinfection. r 2004 Elsevier Ltd. All rights reserved. Keywords: Mixed oxidants; Chlorine; Ozone; Chlorine dioxide; Chlorite; B. subtilis spore
1. Introduction The elimination of pathogenic microorganisms (e.g., bacteria, viruses and protozoa) in water treatment systems is of great concern (White, 1992). One of the most widely used disinfectants for removing these pathogens is free available chlorine (Cl2) (Sayer et al., 1994). However, alternatives to chlorine have been constantly pursued due to the appearance of harsh Corresponding author. Tel.: +82 2 880 8927; +82 2 876 8911. E-mail address: [email protected] (J. Yoon).
fax:
microorganisms as well as the formation of chlorinated disinfection by-products. Ozone (Bashtan et al., 1999), ultraviolet irradiation (Chizuko et al., 2000), and electrochemically mixed oxidants (Harrington, 1999; Kraft et al., 1999) have been proposed as alternative disinfectants. In addition, sequential disinfection with ozone or chlorine dioxide followed by chlorine has proven to be an effective approach to treating pathogens (Cho et al., 2003; Corona-Vasquez et al., 2002; Driedger et al., 2000). Sequential treatments are based on the idea of using a powerful oxidant such as ozone as the primary disinfectant, followed by chlorine as the secondary disinfectant. Each disinfection agent has its
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own specific reactivity toward cell structure materials, so that the use of multiple agents means that different mechanisms are brought to bear to inactivate the microorganisms. The synergistic effect of sequential treatments is likely caused by the activity of the various disinfection agents reacting with specific chemical groups of the cell wall. Consequently, it can be hypothesized that the secondary disinfectant is allowed to permeate through partially reacted cell wall layers at a faster rate (Corona-Vasquez et al., 2002). However, this putative synergism could be due to the use of mixed disinfectants as well as to the effect of sequential disinfection. In the case of mixed disinfectants, however, we would expect to observe additional synergistic effects other than that described above. The coincident reaction of two disinfectants (viz. chlorine and other oxidants) with microbial cells and the contribution of the reaction intermediates might provide additional synergistic effects. These intermediates might increase the inactivation efficiency or have no effect on the inactivation. There have been some studies which showed that electrochemically mixed oxidants achieved considerable disinfection efficiency for selected microorganisms (Casteel et al., 2000; MIOX literature, 1995), while the level of enhanced disinfection efficiency remains unclear, depending upon the type of tested microorganisms (Son, 2004). Due to the lack of effective analytical methods, however, the exact composition of the intermediates produced from these electrochemically mixed oxidants is not well understood. In this study, the microbial inactivation efficiency of mechanically mixed oxidants, which were made by adding small amounts of various oxidants (ozone, chlorine dioxide, hydrogen peroxide and chlorite) to a high concentration of chlorine, was investigated. In addition, we attempted to provide an indirect explanation for the presence of the chemical species observed in the mechanically mixed oxidants and their contribution to the enhanced disinfection efficiency.
2. Materials and methods 2.1. Preparation of mixed oxidants All reaction solution and reagents were prepared with Milli-Q-treated water (Millipore Co., USA), and analytical grade chemicals were used (Fisher Scientific, USA). A chlorine stock solution was prepared by dilution with sodium hypochlorite solution (5%, Junsei Co., Japan). Ozone (440 mg/L) was made by means of an ozone generator CFS-1 (Ozonia Co., Switzerland). Chlorine dioxide was prepared by oxidizing technical grade sodium chlorite (NaClO2) with hydrogen sulfate (H2SO4) (Christian et al., 2001). A stock solution
Table 1 Mechanically mixed oxidants as concentration of the stock solutiona Type
Contents
Subsidiary oxidant conc.
I
Cl2 þ O3
O3 0.021 mM (1 mg/L)
II
Cl2 þ ClO2
III
Cl2 þ H2 O2
ClO2 0.027 mM (1.8 mg/L) and 0.27 mM (18 mg/L) H2O2 0.27 mM (9.2 mg/L)
IV
Cl2 þ ClO2 2
ClO–2 0.59 mM (40 mg/L)
a
pH ¼ 2.5, Cl2 ¼ 2.82 mM (200 mg/L).
(100 mg/L) of chlorine dioxide was stored in the dark at 4 1C before use. Chlorite was prepared by diluting NaClO2. Hydrogen peroxide was used in the form of an analytical grade reagent (30%, Junsei Co., Japan). Table 1 shows the specific contents of the four mechanically mixed oxidants used in the experiment. The stock solution of each mixed oxidant was made by the addition of ozone (1 mg/L), chlorine dioxide (1.8 and 18 mg/L), hydrogen peroxide (9.2 mg/L) and chlorite (40 mg/L), respectively, to a chlorine stock solution with a concentration of 200 mg/L at pH 2.5, since chlorine dioxide and ozone are stable under acidic conditions (Csordas et al., 2001). A mixed oxidant containing chlorite was made, since chlorite is expected to be produced as a reaction intermediate between chlorine and ozone (Haag and Hoigne´, 1983). The pH was adjusted using phosphoric acid (20 mM) in order to have the required degree of acidity. In each case, the oxidant was fully mixed with chlorine for 30 min. The ratio of the concentration of chlorine to that of the subsidiary oxidant (ozone, chlorine dioxide, hydrogen peroxide and chlorite) was chosen based on the minimum subsidiary oxidant concentrations that can be measured by UV spectroscopy. In spectroscopic study, the higher concentration ratio of the ozone to the chlorine was chosen than that used in disinfection experiments, in order to observe the change in UV absorbance with time. The concentrations of chlorine and ozone were varied from 200 to 38 mg/L and from 1 to 2 mg/L, respectively. 2.2. Culture of bacteria Bacillus subtilis (B. subtilis) (ATCC 6633) spores were chosen as the representative indicator microorganisms to estimate the inactivation efficiency of the mixed oxidants. B. subtilis spores were prepared by following the procedure described by Nakayama et al. (1996), except for certain minor modifications involving the use of 1/10 nutrient agar with an extended incubation time
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of 5–7 days, the cells were harvested by centrifugation at 4000 g for 20 min and washed three times with 20 mM phosphate buffer solution (PBS, pH 8.2). The stock solution of B. subtilis spores was prepared by resuspending the cell pellets in 10 mL of PBS and by treating the cells at 80 1C for 20 min just before each experiment. The cell population was determined by plating the cells on nutrient agar and counting the number of colony forming units (CFU) per mL. Triplicate disinfection experiments showed a good reproducibility within the 95% confidence interval. 2.3. Disinfection experiments All of the materials for the disinfection experiments were autoclaved at 121 1C for 15 min. For the disinfection experiments, the mechanically mixed oxidants listed in Table 1 were diluted so as to have a total oxidant concentration of 2 mg/L, using 20 mM PBS (pH 8.2) at 20 1C, because the inactivation procedure was generally carried out in neutral or slightly basic conditions in water treatment facilities. The experiments were conducted using 50 mL of the solution in 60 mL Pyrex flasks for 8 h with an initial microbial population of 105 CFU/ mL. In order to minimize chlorine loss by volatilization, the contents were mixed slowly using a magnetic stirrer, and the reactors were capped during stirring. One milliliter of the solution was withdrawn at each sampling and diluted 1/1, 1/10 and 1/100 with PBS. Finally, 0.1 mL samples of the undiluted and diluted solutions were spread on nutrient agar. The CFU values were determined after 24 h incubation at 37 1C. The level of inactivation was expressed as the log10 reduction of the microbial survival ratio during the disinfection experiments. The disinfection efficiency in this study was ¯ values (C: ¯ the time-averaged condetermined as CT centration of disinfectant (mg/L); T: contact time (min)). To explain the kinetics of B. subtilis spore inactivation, the modified (or delayed) Chick–Watson model that includes the lag phase in the inactivation and ozone decomposition was used (Rennecker et al., 1999; Cho et al., 2002). N N0 8 > <0 ¼ > ¯ þ kCT lag : kCT
log
if
¯ CTpCT lag
if
¯ CTXCT lag
¼ k1 log NN0 ; ¼ k1 log NN0 ;
where N0 N C
initial cell population (CFU/mL) remaining cell population at time T (CFU/mL) disinfectant concentration (mg/L)
C¼
Rt
T K CTlag
0
C dt=t
723
time-averaged oxidants concentration (mg/L) reaction time (min) inactivation rate constant with disinfectant (L/(mg min)) intercept of the inactivation curve with the x-axis
2.4. Chemical analysis The change in the total concentration of the oxidants in the mixed oxidants solution and the residual chlorine were measured by DPD (N,N-diethyl-p-phenylelenediamine) methods using DR/2010 (HACH Co., USA). The concentrations of ozone and hydrogen peroxide were determined by measuring the difference in UV absorbance after reacting them with indigo thiosulfate and titanium sulfate, respectively. After mixing the oxidants, the spectra of the mixed oxidants were obtained between 240 nm and 450 (or 340) nm at different times using a UV (Model HP8452) spectrophotometer, in order to investigate the formation of unknown intermediates during the reaction. The concentration of chlorine dioxide was determined by measuring the change in UV absorbance with time at 359 nm, which was a unique peak for chlorine dioxide. Chlorite was determined by measuring the change in UV absorbance with time at 260 nm, corresponding to the characteristic absorbance of chlorite. Sodium thiosulfate (0.002 M) was added to neutralize the residual oxidants or residual chlorine (American Public Health Association, 1989). In addition, further experiments were conducted to confirm the identity of the unknown compounds using an ion chromatograph (IC, Dionex DX-120, Sunnyvale, CA, USA). After the disinfection experiments had been conducted for 8 h, an analysis was performed to identify the intermediates. Separation was achieved with an AS9HC column (Dionex) using 9 mM carbonate eluent with anion self-regenerating suppressor (ASRS-ULTRA, 4 mm) and conductivity detection. The eluent was pumped at a rate of 1.0 mL/min.
3. Results and discussion 3.1. Disinfection efficiency by mechanically mixed oxidants The inactivation of the Bacillus subtilis spores by the mechanically mixed oxidants containing chlorine and subsidiary oxidants (Cl2/O3, Cl2/ClO2, Cl2/H2O2 and Cl2/ClO 2 ) at pH 8.2 shows a lag phase, due to the difficulty of permeating the thick cell wall (Fig. 1). The inactivation curve for chlorine alone, with the same oxidant concentration, was included as a base line to
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that of chlorine alone. Fig. 1(d) shows that the mechanically mixed oxidant containing chlorine and chlorite is 52% more efficient than chlorine alone in inactivating B. subtilis spores. It is noted that the level of concentration of ozone, chlorine dioxide, hydrogen peroxide and chlorite alone presumably contained in tested mixed oxidants of 2 mg/L did not have any measurable disinfection ability for B. subtilis spores.
evaluate the enhanced disinfection efficiency afforded by the mechanically mixed oxidants. Fig. 1(a) shows the inactivation of B. subtilis spores by the mixed oxidant containing chlorine and ozone. The mechanically mixed ¯ value than chlorine alone oxidant needed a 21% less CT in achieving the 2 log inactivation of B. subtilis spores. The enhanced disinfection efficiency afforded by the mechanically mixed oxidant containing chlorine and ozone was attributed to the shortening of the lag phase of the inactivation curves. The inactivation of the B. subtilis spores afforded by the mechanically mixed oxidant containing chlorine and chlorine dioxide is shown in Fig. 1(b). The synergistic effect of chlorine and chlorine dioxide at the concentration ratio of 2.82/ 0.027 mM and 2.82/0.27 mM (as a concentration of the stock solution) in achieving 2 log inactivation of B. subtilis spores was 26% and 45%, respectively, more effective than chlorine alone. The disinfection efficiency at a constant chlorine concentration increased with increasing chlorine dioxide concentration. As shown in Fig. 1(c), the disinfection efficiency of the mechanically mixed oxidant containing chlorine and hydrogen peroxide for the inactivation of the B. subtilis spores was not significantly different from
3.2. Possible intermediates from oxidants reaction Based on the results shown in Fig. 1, three mixed oxidants (ozone, chlorine dioxide and chlorite) combined with chlorine among the four different mechanically mixed oxidants showed significantly enhanced ¯ disinfection efficiency (21–52% less CT values were needed to achieve 2 log inactivation) compared with chlorine alone. This significantly enhanced inactivation might be due to the synergistic effects of the mixed oxidant itself and/or the formation of unknown intermediates from the oxidants, which also have disinfection ability. Thus, further experiments were conducted in order to investigate the possible formation of transient
0
log(N/No)
-1 -2 -3 -4 (a)
(b)
(c)
(d)
-5 0
log(N/No)
-1 -2 -3 -4 -5 0
100
200
300
400
CT(mg.min/L)
500
600
0
100
200
300
400
500
600
CT(mg.min/L)
Fig. 1. Comparison of B. subtilis spores inactivation between mechanically mixed oxidants and chlorine (2.82 mM) alone (-3-) at pH 8.2: (a) Cl2/O3 (0.021 mM), -d-; (b) Cl2/ClO2 (0.027 mM), -’-, Cl2/ClO2 (0.27 mM), -&-; (c) Cl2/H2O2 (0.27 mM), -m-; (d) Cl2/ClO 2 (0.59 mM), -~-. The concentration of each mixed oxidant is that of the stock solution given in Table 1, and not that of the solution used in the disinfection experiment. For the disinfection studies, the mechanically mixed oxidants in Table 1 were diluted to 2 mg/L as the total oxidant concentration.
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unknown primary intermediates to unknown secondary intermediates. As a result of the IC analysis, chlorite was identified in the reaction with chlorine and ozone (data not shown), which is consistent with the observation of Haag and Hoigne´ (1983) (Table 2). However, measuring the quantitative amount of chlorite was impossible, because the concentration of chlorite was below the detection limit. For the mixed oxidants containing chlorine and chlorine dioxide, no significant change in UV absorbance was observed with time (data not shown). This phenomenon is plausible, since chlorine slowly reacts with chlorine dioxide at acidic pH, and the change in UV absorbance is negligible. This result is consistent with previous works which indicated that chlorine dioxide is stable in concentrated chlorine solution under
0.30 Cl2 (38 mg/L) O3(2 mg/L)
0.25
Cl2 + O3 (0 min) Cl2 + O3 (30 min) Cl2 + O3 (60 min)
0.20
Cl2 + O3 (theoretical)
Abs.
intermediates from the reaction between chlorine and one of the subsidiary oxidants. Table 2 summarizes the important reactions that can occur between chlorine and each subsidiary oxidant under acidic conditions. It is well known that the reaction of chlorine with a small amount of ozone, chlorite and chlorine dioxide can produce chlorite, chlorine dioxide and chlorate, respectively. Thus, similar reactions could occur in the disinfectant solution generated by the electrolysis of brine. The primary oxidant generated from the brine electrolysis system is chlorine (Son et al., 2004; Patermarakis and Fountoukidis, 1990; Gordon et al., 1998). In addition to chlorine, it was suggested that ozone and chlorine dioxide or hydrogen peroxide could be generated (United States Patent, 1988, 1995), although the amounts produced were very small. However, the exact composition of the agents produced by the electrolysis of brine is not well understood, due to analytical deficiencies. Evaluating the disinfection efficiency and chemical characteristics of mechanically mixed oxidants would be helpful, in order to provide an indirect understanding of the properties of the mixed oxidants generated by the electrolysis of brine. Fig. 2 shows the change in UV absorbance of the mixed oxidant produced when chlorine (38 mg/L) and ozone (2 mg/L) were mixed together. This experimental condition was followed as described in Emmert’s work (Emmert, 1999). As shown in Fig. 2, the absorbance of the mechanically mixed oxidants at 260 nm, which indicates the maximum absorbance of ozone, increased in comparison with the theoretical combined absorbance of chlorine and ozone, and decreased with time. The absorbance peak decreased to the level of chlorine after 1 h. This observation suggests that unknown intermediates might have been generated from the fast reaction between chlorine and ozone, and were subsequently degraded with time. Decreased absorbance at 260 nm would correspond to the conversion of ozone and the
725
0.15
0.10
0.05
0.00 240
260
280 300 Wavelength (nm)
320
340
Fig. 2. UV absorbance spectra of mechanically mixed oxidants containing Cl2 (38 mg/L) and O3 (2 mg/L) at pH 2.5.
Table 2 Mixed oxidants chemistry Reaction
Rate constant (M1 s1)
References
HOCl þ O3 ! ClO2 2
ko2 103 M1s1
Haag and Hoigne´ (1983)
HOCl þ H2 O2 ! H2 O þ O2 þ Cl2 þ Hþ
k ¼ 0.3–2 M1 s1
Connick (1947)
2 þ HOCl þ 2ClO2 þ H2 O ! 2ClO2 3 þ Cl þ 3 H
k ¼ (1.570.3) 103 M1 s1
Csordas et al. (2001)
þ 2 HOCl þ 2ClO2 2 þ H ! 2ClO2 þ H2 O þ Cl
k ¼ (1.1070.03) 106 M2 s1
Peintler et al. (1990)
2 2HOCl þ ClO2 2 ! ClO3 þ Cl2 þ H2 O
k ¼ (2.170.1) 103 M2 s1
Peintler et al. (1990)
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acidic conditions (Emmert, 1999; Taube and Dodgen, 1949; Gauw et al., 1999). Therefore, the enhanced disinfection efficiency afforded by this mechanically mixed oxidant may be due to the synergistic effect of the mechanically mixed oxidant itself. In the case of the mixed oxidants containing chlorine and hydrogen peroxide, no significant change in UV absorbance was observed (data not shown). This result was probably due to the hydrogen peroxide being consumed quickly as a result of its reacting with chlorine (Held et al., 1978). Therefore, in effect, this solution corresponds to the case of chlorine alone. This result implies that hydrogen peroxide does not increase the inactivation rate of B. subtilis spores, because hydrogen peroxide is oxidized by a high concentration of chlorine. This UV absorbance result is consistent with the observation shown in Fig. 1(c). A high concentration of chlorite (40 mg/L as a concentration of the stock solution) was used in making the mechanically mixed oxidants, in order to measure the UV absorbance at 359 nm in the spectroscopic study. Fig. 3 shows the UV absorbance of the mechanically mixed oxidants when chlorine (200 mg/L) was mixed with chlorite (40 mg/L). It was clearly observed that a high concentration of chlorine dioxide, which has a fingerprint absorbance at 359 nm, was generated as a result of mixing chlorine and chlorite, and the chlorine dioxide concentration decayed slowly with time. Peintler et al. (1990) reported that chlorine dioxide was produced from the reaction of chlorine and chlorite (Table 2). The maximum peak concentration of chlorine dioxide, which
0.4 ClO2- (40 mg/L)
Cl2 + ClO2- (0 min)
Cl2 (200 mg/L)
Cl2 + ClO2- (30 min) Cl2 + ClO2- (120 min) Cl2 + ClO2- (240 min)
Abs.
0.3
0.2
4. Conclusion In this study, the disinfection efficiency afforded by mechanically mixed oxidants, combining excess chlorine with small amounts of subsidiary oxidants (O3, ClO2, ClO 2 and H2O2), was first reported in comparison with Cl2 alone as B. subtilis spore was used as an indicator microorganism. The chlorine-based mixed oxidants containing O3, ClO2 and ClO 2 showed significantly enhanced efficiencies in comparison with Cl2 alone at the condition that the subsidiary oxidant itself without chlorine showed negligible disinfection ability. This enhanced disinfection efficiency was explained by the combined effect of the mixed oxidant itself and intermediates which may be generated from the reaction between an excess Cl2 and a small amount of subsidiary oxidants. The formation of intermediates was demonstrated by the measurement of UV absorbance at the wavelengths specific with O3 , ClO2 and ClO 2 along with the limited result of IC measurements. In actual drinking water treatment systems, chlorine, ozone and chlorine dioxide are popular disinfectants and the sequential disinfection with ozone or chlorine dioxide followed by chlorine has proven to be an effective process. This study reports that mixed oxidants with chlorine which contain even a negligible amount of ozone or chlorine dioxide can be a moderately viable alternative for the inactivation of pathogens in water treatment systems, although their disinfection efficiencies are insufficient to inactivate harsh microorganisms such as Cryptosporidium parvum.
Acknowledgements This research was partially supported by the Brain Korea 21 Program (the Ministry of Education). This support was greatly appreciated.
0.1
0.0 270
was calculated using the molar absorbance coefficient at 359 nm, was approximately 17 mg/L. To summarize the results, based on Figs. 2 and 3, chlorite was generated as an intermediate from the reaction of excess chlorine and ozone, and finally chlorine dioxide was generated from the reaction of chlorine and chlorite.
References 300
330 360 390 Wavelength (nm)
420
450
Fig. 3. UV absorbance spectra of mechanically mixed oxidants solution containing Cl2 (200 mg/L) and ClO 2 (40 mg/L) at pH 2.5.
American Public Health Association, 1989. Standard Methods for the Examination of Water and Wastewater, 17th ed. Washington, DC, USA. Bashtan, S.Y., Goncharuk, V.V., Chebotareva, R.D., Belyakov, V.N., Linkov, V.M., 1999. Production of sodium
ARTICLE IN PRESS H. Son et al. / Water Research 39 (2005) 721–727 hypochlorite in an electrolyser equipped with a ceramic membrane. Desalination 126 (1), 77–82. Casteel, M.J., Sobsey, M.D., Arrowood, M.J., 2000. Inactivation of Cryptosporidium parvum Oocysts and other microbes in water and wastewater by eletrolytically generated mixed oxidants. Water Sci. Technol. 41 (7), 127–134. Chizuko, M., Kouichi, S., Shinichi, M., Hiromasa, K., Toshiyuki, G., Takehiro, K., Wu, H., Hitofumi, M., Atsuo, I., Yoshiko, N., Masami, T., Osamu, Y., 2000. Disinfection potential of electrolyzed solution containing sodium chlorite at low concentrations. J. Virological Meth. 85 (2), 163–174. Cho, M., Chung, H., Yoon, J., 2002. Effect of pH and importance of ozone initiated radical reactions in inactivating Bacillus subtilis spore. Ozone Sci. Eng. 24 (2), 145–150. Cho, M., Chung, H., Yoon, J., 2003. Quantitative evaluation of the synergistic sequential inactivation of Bacillus subtilis spores with ozone followed by chlorine. Environ. Sci. Technol. 37 (10), 2134–2138. Christian, P.C., Radziminski, Z.R., Michael, L., Robin, C., Andrews, C.A., 2001. Chlorine Dioxide inactivation of Cryptosporidium parvum oocysts and bacterial spore indicators. Appl. Environ. Microbiol. 67 (7), 2993–3001. Connick, R.E., 1947. The interaction of hydrogen peroxide and hypochlorous acid in acidic solutions containing chloride ion. J. Am. Chem. Soc. 69 (6), 1509–1514. Corona-Vasquez, B., Rennecker, J.L., Driedger, A.M., Marin˜as, B.J., 2002. Sequential inactivation of Cryptosporidium pavum oocysts with chlorine dioxide followed by free chlorine or monochloramine. Water Res. 36 (1), 178–188. Csordas, V., Bubnis, B., Fabian, I., Gordon, G., 2001. Kinetics and mechanism of catalytic decomposition and oxidation of chlorine dioxide by the hypochlorite ion. Inorg. Chem. 40 (8), 1833–1836. Driedger, A.M., Rennecker, J.L., Marin˜as, B.J., 2000. Sequential inactivation of Cryptosporidium pavum oocysts with ozone and free chlorine. Water Res. 34 (14), 3591–3597. Emmert, G.L., 1999. Chemical Aspects of Water Treatment: Part 1. Chemical Characteristics of Oxidants Present in Liquors Following Electrolysis of Brine. Ph. D. Dissertation, Miami University, Oxford, OH, USA. Gauw, R.D., Emmert, G.L., Bubnis, B., Gorden, G., 1999. High resolution spectrophotometry for identification of chlorine dioxide in concentrated chlorine solution. Talanta 50 (5), 1073–1078. Gordon, G., Emmert, G., Gauw, R., Bubnis, B., 1998. Can ozone and ozone oxidative by by-products be formed during the electrolysis of salt brine? Ozone Sci. Eng. 20 (4), 239–249.
727
Haag, W.R., Hoigne´, J., 1983. Ozonation of water containing chlorine or chloramines. Water Res. 17 (10), 1397–1402. Harrington, R.E., 1999. Disinfection by mixed oxidant technology in a portable package plant. In: Contruvo, J.A., Graun, G.F., Hearne, N. (Eds.), Providing Safe Drinking Water in Small System. Lewis Publishers, USA. Held, A.M., Halko, D.J., Hurst, J.K., 1978. Mechanisms of chlorine oxidation of hydrogen peroxide. J. Am. Chem. Soc. 100 (18), 5732–5740. Kraft, A., Blaschke, M., Kreysig, D., Sandt, B., Schroder, F., Rennau, J., 1999. Electrochemical water disinfection. Part II. Hypochlorite production from potable water, chlorine consumption and the problem of calcareous deposits. J. Appl. Electrochem. 29 (8), 895–902. MIOX literature, 1995. Alternative Comparison for Important Features in Municipal Plants and Wells. MIOX Corporation, Los Alamos, NM, USA. Nakayama, A., Yano, Y., Kobayashi, S., Ishikawa, M., Sakai, K., 1996. Comparison of pressure resistances of spores of six Bacillus strains with their heat resistances. Appl. Environ. Microbiol. 62 (10), 3897–3900. Patermarakis, G., Fountoukidis, E., 1990. Disinfection of water by electrochemical treatment. Water Res. 24 (12), 1491–1496. Peintler, G., Nagypal, I., Epstein, I.R., 1990. Systematic design of chemical oscillators. 60. Kinetics and mechanism of the reaction between chlorite ion and hypochlorous acid. J. Phys. Chem. 94 (2), 954–958. Rennecker, J.L., Marin˜as, B.J., Owens, J.H., Rice, E.W., 1999. Inactivation of Cryptosporidium parvum oocysts with Ozone. Water Res. 33 (11), 2481–2488. Sayer, C.N., McCarty, P.L., Parkin, L., Parkin, G.F., 1994. Chemistry for Environmental Engineering, fourth ed. McGraw-Hill, New York, NY, USA. Son, H., Cho, M., Chung, H., Choi, S., Yoon, J., 2004. Bactericidal Activity of Mixed Oxidants: Comparison with free chlorine. J. Ind. Eng. Chem. 10 (5), 705–709. Taube, H., Dodgen, H., 1949. Applications of radioactive chlorine to the study of the mechanisms of reactions involving changes in the oxidation state of chlorine. J. Am. Chem. Soc. 71 (10), 3330–3336. United States Patent No. 4,761,208. 1988. Electrolytic Method and Cell for Sterilizing Water. 2 August. United States Patent No. 5,385,711. 1995. Electrolytic Cell for Generating Sterilization Solutions Having Increased Ozone Content. 31 January. White, G.C., 1992. The Handbook of Chlorination and Alternative Disinfections, third ed. Van Nostrand Reinhold, New York, NY, USA.