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Applied and Environmental Microbiology, February 2004, p. 1129-1134, Vol. 70, No. 2
0099-2240/04/$08.00+0     DOI: 10.1128/AEM.70.2.1129-1134.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

Inactivation of Escherichia coli by Photochemical Reaction of Ferrioxalate at Slightly Acidic and Near-Neutral pHs

Min Cho,¹ Yunho Lee,¹ Hyenmi Chung,² and Jeyong Yoon^1*

School of Chemical Engineering, College of Engineering, Seoul National University, Sillim-dong, Gwanak-gu, Seoul 151-742,¹ Water Microbiology Division, National Institute of Environmental Research, Kyungseo-dong, Seo-gu, Inchon 404-170, South Korea²

Received 28 July 2003/ Accepted 10 November 2003

Top Abstract Introduction Photoferrioxalate chemistry. Materials and Methods Results Discussion References

 
Fenton chemistry, which is known to play an effective role in degrading toxic chemicals, is difficult to apply to disinfection in water treatment, since its reaction is effective only at the acidic pH of 3. The presence of oxalate ions and UV-visible light, which is known as a photoferrioxalate system, allows the Fe(III) to be dissolved at slightly acidic and near-neutral pHs and maintains the catalytic reaction of iron. This study indicates that the main oxidizing species in the photoferrioxalate system responsible for microorganism inactivation is OH radical. Escherichia coli was used as an indicator microorganism. The CT value (OH radical concentration x contact time; used to indicate the effect of the combination of the concentration of the disinfectant and the contact time on inactivation) for a 2-log inactivation of E. coli was approximately 1.5 x 10^-5 mg/liter/min, which is approximately 2,700 times lower than that of ozone as estimated by the delayed Chick-Watson model. Since the light emitted by the black light blue lamp is similar to sunlight in the specific wavelength range of 300 to 420 nm, the photoferrioxalate system, which can have a dual function, treating water for both organic pollutants and microorganisms simultaneously, shows promise for the treatment of water or wastewater in remote or rural sites. However, the photoferrioxalate disinfection system is slower in inactivating microorganisms than conventional disinfectants are.

Top Abstract Introduction Photoferrioxalate chemistry. Materials and Methods Results Discussion References

 
The Fenton reaction (resulting from Fe²⁺ plus H₂O₂) and Fenton-like reactions (resulting from Fe³⁺ plus excess H₂O₂) have been widely applied as advanced oxidation processes in treating nonbiodegradable wastewater (7, 11, 13, 14, 27, 28). However, Fenton systems (the Fenton reaction and Fenton-like reactions) have limitations, such as the formation of large amounts of iron sludge when ferrous ions need to be continuously supplied. Illumination of the system by UV-visible light was suggested as a means to overcome these limitations. Depending on the presence of the ligands combined with the ferric ions, the photoreduction of various ferric species contributes to the production of ferrous ions and radical species (3, 6, 25).

These photochemical reactions, involving iron species, are known to be a major source of various oxidants (OH radical, H₂O₂, and HO₂· or O₂·^-) and a sink for refractory synthetic and natural organic compounds that are resistant to degradation by other processes (28). Of the iron species, the ferrioxalate complex is known to play an important role in the production of oxidants because of its high molar absorptivity and quantum yield (1.0 to 1.2 mol/einstein at 254 to 442 nm [3]). Several attempts have been made to use ferrioxalate in photochemical methods that use artificial or solar light for treating polluted water (13, 16, 18, 20). However, there has been no report of the application of this system as a disinfection technology.

The photoferrioxalate system was studied with the aim of evaluating its disinfection ability in water. Escherichia coli, a well-known indicator bacterium, was chosen as the experimental microorganism for this inactivation study. The effects of three important reaction parameters, light intensity, hydrogen peroxide, and ferric ion concentration, in the photoferrioxalate system were examined. Finally, the disinfection ability of OH radical (·OH) produced in the photoferrioxalate system was evaluated quantitatively and compared with those of other disinfectants (ozone, chlorine, and chlorine dioxide).

Top Abstract Introduction Photoferrioxalate chemistry. Materials and Methods Results Discussion References

 
Table 1 shows the kinetic formulation of the photoferrioxalate system. Ferrioxalate produces the Fe(II) ion and the oxalyl radical anion (C₂O₄·^-) through a ligand-to-metal charge transfer process as a result of light absorption (reaction I). The rate of initiation for this reaction is linearly proportional to the primary quantum yield (_p,) and the rate of light absorption (I_a). Most C₂O₄·^- that is produced from reaction II decomposes to the carbon dioxide radical anion (CO₂·^-) and carbon dioxide (reaction III) as a result of rapid decarboxylation.

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TABLE 1. Kinetic formulation of the photoferrioxalate system

The fate of CO₂·^- depends on the competitive reactions between dissolved oxygen (O₂) (reaction IV) and the ferrioxalate ion itself (reaction V). Reaction IV produces superoxide radicals (O₂·^-), and reaction V produces another Fe(II) ion. Fe(II) ions formed in this system can react with the H₂O₂ present in solution and generates OH radical (reaction I, well known as Fenton's reaction). Therefore, ferrioxalate photolysis in the presence of H₂O₂ provides a continuous source of OH radical through the Fenton reaction.

Top Abstract Introduction Photoferrioxalate chemistry. Materials and Methods Results Discussion References

 
Preparation and analysis of chemicals.
All solutions and reagents were prepared with deionized distilled water (Barnstead NANO pure system), and analytical-grade chemicals (Aldrich Co.) were used. All glassware used was washed with distilled water and then autoclaved at 121°C for 15 min. A stock solution of Fe(III) (5 mM) was prepared from ferric perchlorate in 0.1 M perchloric acid as required. A stock solution of oxalate (300 mM) was prepared from oxalic acid or potassium oxalate. H₂O₂ (35%) was standardized by its UV absorption at 240 nm ( = 40.0 M^-1 cm^-1 [2]) and used directly. tert-Butanol (t-BuOH; 20 mM) was used as the OH radical scavenger (4, 23, 27).

Fe(III) was standardized by the phenanthroline method [₅₁₀ = 11,050 M^-1 cm^-1 for Fe(II)-phenanthroline complex (22)], which involves the reduction of Fe(III) to Fe(II) by hydroxyl amine (1 M). H₂O₂ was measured by the titanium sulfate method (₄₀₅ = 730 M^-1 cm^-1 [5]). The pH and dissolved oxygen concentration were measured by a 710 A pH meter (Orion Co.) and a model 52 DO meter (YSI Co.), respectively. The UV-visible spectra were obtained with a Hewlett-Packard 8452A diode array spectrophotometer. The oxalic acid and para-chlorobenzoic acid (pCBA) concentrations were analyzed by high-performance liquid chromatography (Waters Co.). A SUPELCOGEL C-610H carbohydrate column (300 by 7.8 mm, 9-µm inside diameter; Supelco Co.) with a UV detector at 210 nm was used for the oxalic acid analysis. The mobile phase was water containing 0.1% (vol/vol) H₃PO₄. An XTerra Rp-18 C₁₈ reverse-phase column (150 by 2.1 mm, 5-µm inside diameter; Supelco Co.) with UV detection at 230 nm was used for the pCBA analysis. A solvent mixture of 35% acetonitrile and 65% water containing 40 mM phosphate buffer was employed as the mobile phase.

Preparation and analysis of E. coli.
The E. coli (ATCC 8739) bacteria were cultured in tryptic soy broth for 18 h at 37°C. The bacteria were harvested by centrifuging the broth in a 50-ml conical tube at 1,000 x g for 10 min and washing the bacteria twice with 50 ml of phosphate-buffered saline (pH 7.2). The stock solution of E. coli was prepared by resuspending the final pellets in 50 ml of phosphate-buffered saline. The number of viable cells on nutrient agar was measured by the spread plate method (BD-0003-17; Difco Co.) with incubation at 37°C for 24 h. This nonselective nutrient agar medium was chosen for E. coli counting, since it allowed a more conservative estimate of the population than could be obtained with selective media such as m-Endo, m-FC, deoxycholate lactose agar, and mT7. The initial population of E. coli in each disinfection experiment, which was designed to measure the maximum 3.38- to 4.92-log inactivation, ranged from approximately 2.4 x 10⁴ to 8.4 x 10⁵ CFU/ml. During the experiment, 1 ml of solution was withdrawn at each sampling time and diluted 1/10 and 1/100. Three replicate 0.1-ml samples of both the diluted and the undiluted solutions were spread to determine the number of cells, and these counts showed good reproducibility, with a 10% standard deviation.

Experimental procedures.
The reactions were conducted by using 50 ml of the solution in 60-ml Pyrex flasks (UV cutoff, 300 nm) open to the atmosphere. The solution was irradiated with one or four black light blue (BLB) lamps (18 W; Philips Co., Eindhoven, The Netherlands) at a distance of 2 cm, stored at 21 ± 1°C with air cooling, and stirred vigorously. The BLB lamps, which emit light in the wavelength range of 300 to 420 nm (15, 16, 26), were turned on for 10 min prior to the reaction to obtain a constant light intensity output. The light intensity, which was measured by ferrioxalate actinometry (10), was mainly 7.9 x 10^-6 einsteins/liter/s when the four lamps were turned on. The light intensity was varied by changing the number of lamps.

The general experimental procedures were as follows. The photolysis solution was prepared by adding the appropriate amounts of Fe(III), oxalate, the phosphate buffer (pH 5.8), and the E. coli stocks or any other related compounds (except H₂O₂) to the flask and diluting the solution to the desired volume with autoclaved distilled water. Finally, the desired volume of the H₂O₂ (35%) stock solution was injected into the mixture and the flask was immediately inserted in front of the lamps. All of the ferrioxalate solutions had been kept from light with aluminum foil prior to photolysis to prevent any photochemical reactions. Because the oxalate and H₂O₂ were continuously decomposed, during all experiments, the appropriate amounts of oxalate (300 mM) and H₂O₂ stock solution were injected into the reaction solution at each time interval with an automatic injection system so that the levels of these components were maintained at averages of 75 and 68% of their initial concentrations, respectively. The sample reactions were quickly quenched with the appropriate amounts of sodium sulfite (20 mM), which consumed any ferrioxalate-initiated oxidants and residual H₂O₂. An anoxic environment was obtained by sparging with nitrogen gas.

Table 2 shows all of the details of the experimental conditions used in this study. As shown in Table 2, 15 sets of experiments (experiments 1 to 15) were conducted under different conditions. Experiments 1 to 8, 9 to 12, and 12 to 14 concerned the effects of hydrogen peroxide concentration, light intensity, and ferric ion concentration, respectively, on pCBA degradation. Experiment 15 was performed to study the role of CO₂·^- and O₂·^-. The pCBA degradation constant (k_exp) was obtained at the initial stage of pCBA degradation. Parts of the disinfection experiments (Table 2, experiments 4, 7, 11, and 14) were repeated three times, and the deviations of the experimental measurements are presented in the figures as error bars. Twelve of the 15 experiments were conducted to determine the extent of E. coli inactivation. Four to nine samples were taken to measure the E. coli population for reaction times of 50 to 360 min.

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TABLE 2. Experimental conditions in this studya

Determination of OH radical concentration.
The OH radical concentration was quantified by the method reported by Haag and Hoigné (8). This method is based on the competitive reactions between the OH radical probe compound (pCBA in this study) and all organic and inorganic OH radical scavengers present (S_i) in a photoferrioxalate system, including H₂O₂, oxalate, Fe(II), etc. If pCBA as the OH radical probe compound is present only as a trace component (at 1.9 µM in this study) and the S_i of the photoferrioxalate system remains constant (it was kept constant by the continuous injection of H₂O₂ and oxalate in this study), the decay of pCBA in the photoferrioxalate system can be expressed as

(1)

where t is time and the pCBA degradation constant (k_exp) is equal to the rate constant with ·OH and pCBA (k_OH,pCBA) multiplied by the steady-state OH radical concentration ([·OH]_ss) if the pseudo-steady-state assumption for OH radical concentration is valid.

The integration of equation 1 yields

(2)

where [pCBA]₀ is the initial concentration of pCBA.

k_exp, obtained only from the early stage of the disinfection experiment, is summarized in Table 2. In addition, [·OH]_ss can be calculated from equation 1 if the pseudo-steady-state assumption is valid in the disinfection experiments. Therefore, the measurements of k_exp were done under the conditions under which the extent of E. coli inactivation was measured. These measurements were conducted at three H₂O₂ concentrations (the highest, middle, and lowest concentrations). It should be noted that H₂O₂ was added to maintain its concentration within a certain range. A finding that k_exp was the same at the three different reaction times indicated that the pseudo-steady-state OH radical concentration had been maintained (experiments 7, 10, 11, and 14). This pseudo-steady-state concentration (Table 2) was used to determine the kinetics models for OH radical. The typical duration of pCBA degradation was 30 to 140 s.

Disinfection experiments with chlorine and chlorine dioxide.
The disinfection experiments were performed by using a 50-ml batch reactor to compare the disinfection ability of OH radical for E. coli inactivation with those of free chlorine and chlorine dioxide. A free-chlorine stock solution (300 mg/liter) was prepared by dilution of the sodium hypochlorite solution (5%; Junsei Co., Tokyo, Japan). The chlorine (Cl₂) dose ranged from 0.05 to 0.1 mg/liter. The residual free-chlorine levels were assayed with a DPD (N,N-dimethyl-p-phenylenediamine) reagent (DR/2010; Hach Co.). In general, three to five samples were taken during the disinfection experiments (from 5 to 120 s) to measure the microorganism population.

Chlorine dioxide was prepared by oxidizing sodium chlorite (NaClO₂) with sulfuric acid (H₂SO₄) by the procedure reported by Radziminski et al. (19). The chlorine dioxide dose ranged from 0.05 to 0.1 mg/liter. Chlorine dioxide was measured by KI titration or by using the direct UV absorbance at 385 nm, which is a unique peak of chlorine dioxide. Three to five samples were taken during the disinfection experiments (from 5 to 90 s) to measure the microorganism population. Na₂S₂O₃ (0.002 N) was used to quench the residual free chlorine or chlorine dioxide. The range of the initial E. coli concentrations in the experiments involving free chlorine and chlorine dioxide was the same as that employed in the photochemical ferrioxalate disinfection experiments.

Application of disinfection kinetics model.
A disinfection kinetics model is usually used to compare results obtained under different experimental conditions. One of the most fundamental and simple models reflecting the principle of disinfection kinetics is the Chick-Watson model, log(N/N₀) = -kCT). This model assumes that the rate of inactivation of the microorganisms by the disinfectants is primarily affected by the combination of the concentration of the disinfectants (C) and the contact time (T) (C x T, which is expressed as the CT value). The CT value is widely used as a measure of compliance with disinfection requirements in drinking water treatment. However, since the shoulder or the tailing off which frequently appears in the log expression of microorganism inactivation cannot be explained by this model, we used the delayed Chick-Watson model (4), whose capacity to explain the inactivation kinetics, including the shoulder, has been successfully demonstrated, as described below.

(3)

where N₀ is the initial E. coli population (in CFU per milliliter), N is the E. coli population remaining at time T (in CFU per milliliter), C is the disinfectant concentration (in milligrams per liter), T is the reaction time (in minutes), k is the inactivation-by-disinfectant rate constant (in liters per milligram per minute), and CT_lag is the intercept of the inactivation curve on the x axis.

Other, more sophisticated models, such as the Hom and the rational models, are better at explaining the inactivation kinetics, including the shoulder or tailing off (9). However, they do not allow the calculation of the CT value, and consequently, it is difficult to compare the efficacies of different disinfectants, such as ozone, free chlorine, and chlorine dioxide, by using these models, which were therefore not considered in this study. The statistical estimation was performed by using the Solver add-in function of Microsoft Excel XP software.

Top Abstract Introduction Photoferrioxalate chemistry. Materials and Methods Results Discussion References

 
E. coli inactivation in the photoferrioxalate system.
Figure 1 clearly shows that E. coli inactivation occurred in the photoferrioxalate system even at slightly acidic and near-neutral pHs (5.8 to 6.5). In the control experiments, it was confirmed that without either light irradiation, Fe(III), or oxalate, E. coli inactivation did not occur within the experimental time scale of this study (data not shown). The results shown in Fig. 1 represent one of the first presentations of Fenton chemistry applied to microbial inactivation under slightly acidic and near-neutral pH conditions. E. coli inactivation, as shown in Fig. 1, appears to have a slight initial lag phase (shoulder). The times required for 2- and 3-log inactivations were 55 and 75 min, respectively (Fig. 1). On the other hand, no E. coli inactivation was observed in the presence of an excessive OH radical scavenger (20 mM t-BuOH) (Fig. 1). This observation suggests the possibility that OH radical is the main species responsible for inactivating E. coli in the photoferrioxalate system (1, 4, 7, 21). However, the possibility of the involvement of C₂O₄·^-, CO₂·^-, and O₂·^- in inactivating E. coli cannot be excluded, since all of these radical species are also generated in the photoferrioxalate system.

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FIG. 1. Inactivation of E. coli in the photoferrioxalate system. [Fe3+], 0.1 mM, [H2O2]0, 2.0 mM; initial oxalate concentration, 5.0 mM; light intensity, 7.9 x 10-6 einsteins/liter/s; oxalate concentration/Fe3+ concentration, 25; H2O2 concentration/Fe3+ concentration, 6; pH, 5.8 to 6.5. Symbols: , experiment 8; •, experiment 15; , experiment 7.

Effect of H₂O₂ dose on E. coli inactivation.
The influence of the H₂O₂ dose on E. coli inactivation is presented in Fig. 2; the H₂O₂ doses ranged from 0 to 2.0 mM. Figure 2 shows that in the absence of an H₂O₂ dose, only slight E. coli inactivation occurred. During 60 min of reaction time, only a 0.5-log inactivation was achieved. With increasing H₂O₂ doses, the time required for the inactivation of E. coli was shortened. Although the rate of pCBA degradation at the initial stage with an initial H₂O₂ concentration of 0.5 mM was the same as that with initial H₂O₂ concentrations of 1.0 and 2.0 mM (Table 2), the k_exp value with an H₂O₂ concentration of 0.5 mM at later stages of the disinfection experiments became smaller (data not shown). This decrease in the k_exp value explains the lower rate of inactivation of E. coli, as shown in Fig. 2.

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FIG. 2. Effect of initial H2O2 dose on E. coli inactivation in the photoferrioxalate system. [Fe3+], 0.1 mM; initial oxalate concentration, 5.0 mM; light intensity, 7.9 x 10-6 einsteins/liter/s; oxalate concentration/Fe3+ concentration, 25; H2O2 concentration/Fe3+ concentration, 6; pH, 5.8 to 6.5. Symbols: , experiment 1; •, experiment 4; , experiment 5; , experiment 6; , experiment 7.

On the other hand, the pCBA degradation rate constants (k_exp) at initial H₂O₂ concentrations of 1.0 and 2.0 mM remained the same throughout the disinfection experiment (the value of k_exp during the later period of the experiment is not presented). This finding is consistent with the levels of inactivation of E. coli being the same in both cases (Fig. 2) and indicates that OH radical is the main species responsible for inactivating E. coli in the photoferrioxalate system.

Effect of light intensity on E. coli inactivation.
The effect of light intensity on E. coli inactivation is presented in Fig. 3; light intensity ranged from 0 to 7.9 x 10^-6 einsteins/liter/s. Figure 3 clearly shows that a higher level of E. coli inactivation was achieved when a higher light intensity was applied. The times required to achieve a 2-log inactivation of the E. coli population decreased from about 270 min at a light intensity of 1.5 x 10^-6 einsteins/liter/s to about 140 and 58 min at 3.4 x 10^-6 and 7.9 x 10^-6 einsteins/liter/s, respectively. In the absence of light, no E. coli inactivation was observed during the reaction period.

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FIG. 3. Effect of light intensity on E. coli inactivation in the photoferrioxalate system. [Fe3+], 0.1 mM; [H2O2]0, 2.0 mM; initial oxalate concentration, 5.0 mM; oxalate concentration/Fe3+ concentration, 25; H2O2 concentration/Fe3+ concentration, 6; pH, 5.8 to 6.5; Symbols: , experiment 9; •, experiment 10; , experiment 11; , experiment 7.

Effect of ferric ion dose on E. coli inactivation.
The effect of the ferric ion concentration on E. coli inactivation was investigated; the initial ferric ion concentrations ranged from 0 to 0.5 mM. Figure 4 shows that the rate of E. coli inactivation increased with increasing ferric ion doses. The times required for a 2-log inactivation of E. coli populations were about 180, 55, and 40 min with initial ferric ion doses of 0.02, 0.1, and 0.5 mM, respectively. As shown in reaction II, a higher level of OH radical generation was expected at a higher Fe³⁺ concentration due to the enhanced absorption of light by Fe(III) complexes.

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FIG. 4. Effect of ferric ion dose on E. coli inactivation in the photoferrioxalate system. [H2O2]0, 2.0 mM; initial oxalate concentration, 5.0 mM; light intensity, 7.9 x 10-6 einsteins/liter/s; oxalate concentration/Fe3+ concentration, 25; H2O2 concentration/Fe3+ concentration, 6; pH, 5.8 to 6.5. Symbols: , experiment 12; •, experiment 13; experiment 7; , experiment 14.

Top Abstract Introduction Photoferrioxalate chemistry. Materials and Methods Results Discussion References

 
Role of OH radical in E. coli inactivation.
As mentioned in "Photoferrioxalate chemistry" above, there are four radical species (OH radical, C₂O₄·^-, CO₂·^-, and O₂·^-) in the photoferrioxalate system that have the potential to cause E. coli inactivation. Due to the instantaneous decomposition of C₂O₄·^- into CO₂·^- (reaction II), the chance of C₂O₄·^- inactivating E. coli is minimal. Two additional experiments were performed to investigate the roles of CO₂·^- and O₂·^- in E. coli inactivation. First, in order to examine the role of CO₂·^- in inactivating E. coli, the disinfection experiment was performed with OH radical and O₂·^- formation being blocked. This specific reaction environment was achieved in the absence of H₂O₂ and oxygen (Table 2, experiment 15) in the photoferrioxalate system, as shown in reactions III and V. The proportional production of CO₂·^- with light was confirmed by the continuous formation of Fe(II) (data not shown) in the first stage of the reaction, since CO₂·^- is involved in the reduction of Fe(III) (reaction VI). No E. coli inactivation was observed in the absence of H₂O₂ and oxygen during the reaction period. Second, another disinfection experiment was performed in the presence of t-BuOH (Table 2, experiment 8), used as an OH radical scavenger, and oxygen in order to examine the role of O₂·^-. Although more O₂·^- would be produced as a result of oxygen saturation (reaction IV), E. coli inactivation was not observed. These results suggest that CO₂·^- and O₂·^- do not cause any significant E. coli inactivation under the given experimental conditions.

Therefore, it can be concluded that of the four radical species, OH radical is the main species responsible for E. coli inactivation in the photoferrioxalate system (Fig. 1). OH radical, which has a high oxidation potential (2.70 V), is able to kill bacteria and viruses mainly by destroying their cell membranes or walls (17, 24, 26). The slight initial lag phases in the E. coli inactivation curve shown in Fig. 1 to 4 might also be hypothesized to be the times necessary for the destruction of the E. coli cell membrane by OH radical (4, 17, 24, 26).

Determination of CT values for OH radical.
In order to improve the understanding of the correlation between E. coli inactivation and the quantity of OH radical generated from the photoferrioxalate system, an attempt was made to determine the CT values of OH radical. The conditions of the experiments in which the pseudo-steady-state OH radical concentration was confirmed to be maintained (Table 2, experiments 7, 10, 11, and 14) were used to determine the OH radical CT values for E. coli inactivation. The fact that the semilog plots of pCBA degradation were linear for all experimental times (R² 0.96; data not shown) means that the steady-state assumption for OH radical (see "Determination of OH radical concentration" above) was well satisfied under these experimental conditions.

Based on both the OH radical concentration and the extent of E. coli inactivation under the selected conditions, the adequacy of the delayed Chick-Watson model (equation 3) was evaluated as shown in Fig. 5a. The OH radical CT values were found by using the model fitness shown in Fig. 5b. The suitability of the delayed Chick-Watson model for photoferrioxalate disinfection was supported by the high correlation coefficients (R² = 0.90). Figure 5b also shows the OH radical CT values constructed from the E. coli inactivation results. The OH radical CT value for a 2-log inactivation was found from the results shown in Fig. 5 to be approximately 1.5 x 10^-5 mg/liter/min. In Table 3, the CT values of several important disinfectants for a 2-log inactivation of E. coli are compared with that of OH radical. The CT value for ozone was obtained from literature (12), and separate experiments were conducted in our laboratory to determine the CT values for free chlorine and chlorine dioxide. The measured OH radical CT value for a 2-log inactivation in this study was 2,700, 8,700, and 5,300 times smaller than those of ozone, free chlorine, and chlorine dioxide, respectively, which shows the proportional relationship between the inactivating abilities and the oxidation potentials of the disinfectants (Table 3).

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FIG. 5. Determination of the CT values of OH radical from the delayed Chick-Watson model fitness. (a) Evaluation of fit; (b) model fitness.

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TABLE 3. Comparison of CT values of several important disinfectants for a 2-log inactivation of E. coli

Conclusions.
This study reports the first application of photoferrioxalate chemistry for the purpose of inactivating E. coli under slightly acidic and near-neutral pH conditions. The application of Fenton chemistry under slightly acidic and near-neutral pH conditions was possible because the amount of the oxalate ion, which allows the dissolution of Fe(III), is quite large and its quantum yield is quite high. Although this system lacks any residual disinfection capacity like that of chlorine and cannot achieve the rapid inactivation of microorganisms like UV irradiation can, it has great potential in terms of simultaneously treating organic pollutants and microorganisms in water. The main oxidizing species of the photoferrioxalate system was found to be OH radical, which can effectively kill microorganisms. With the delayed Chick-Watson model, the OH radical CT value for a 2-log inactivation of E. coli was approximately 1.5 x 10^-5 mg/liter/min, which is approximately 2,700 times smaller than that of ozone.

Although the spectrum of the light emitted by the BLB lamps employed in this study is not identical to that of sunlight, it is sufficiently similar in the specific wavelength range of 300 to 420 nm (16, 26). Another promising application of this system in this regard is related to the treatment of wastewaters at remote or rural sites where surface water or groundwater is frequently contaminated by agricultural chemicals and microorganisms. This type of application is promising, considering the simplicity of the hardware and procedures, the ability to use the sun as the light source, and the safety of all of the chemicals in this system. Studies investigating larger-scale and field applications are highly recommended.

 
This research was partially supported by the Brain Korea 21 Program (of the Ministry of Education). This support was greatly appreciated.

 
* Corresponding author. Mailing address: School of Chemical Engineering, College of Engineering, Seoul National University, San 56-1, Sillim-dong, Gwanak-gu, Seoul 151-742, Korea. Phone: 82-2-880-8927. Fax: 82-2-876-8911. E-mail: jeyong{at}snu.ac.kr.

Top Abstract Introduction Photoferrioxalate chemistry. Materials and Methods Results Discussion References

 

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Applied and Environmental Microbiology, February 2004, p. 1129-1134, Vol. 70, No. 2
0099-2240/04/$08.00+0     DOI: 10.1128/AEM.70.2.1129-1134.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

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