Phenol Formation in Gamma Radiolysis of Aqueous Benzene Solution

Feb 26, 2013 - Ten micromolar sodium hypochlorite enhanced the formation of phenol up to a total dose of 6 Gy. Above 6 Gy in solutions containing sodi...
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Phenol Formation in Gamma Radiolysis of Aqueous Benzene Solution with Sodium Hypochlorite Kazuhiko Takeda*,† and Koki Nagano‡ †

Graduate School of Biosphere Science, Hiroshima University, 1-7-1, Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8521, Japan Graduate School of Engineering, Hiroshima University, 1-4-1, Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8521, Japan

‡

ABSTRACT: Phenol formation by gamma radiolysis of an aqueous benzene solution containing sodium hypochlorite is reported. The phenol formation in a benzene solution containing sodium hypochlorite irradiated with 60Co γ-rays is about six times higher than that without sodium hypochlorite. Ten micromolar sodium hypochlorite enhanced the formation of phenol up to a total dose of 6 Gy. Above 6 Gy in solutions containing sodium hypochlorite, the rate of phenol yield sharply decreased and was essentially the same as that without sodium hypochlorite. The yield of phenol with sodium hypochlorite is 0.89 μmol J−1 and is larger than the sum of yield for the radicals and reactive oxygen species by water radiolysis such as •OH, e−, H, H2, and H2O2. The formation of phenol with sodium hypochlorite was reduced by NaCl. Results suggest that the radiolytic formation of phenol in a benzene solution with sodium hypochlorite relates to the reaction process involving chlorine atoms. Sodium hypochlorite can be applied as a sensitizer for a benzene chemical dosimetry system. The lower limit of dose detection calculated from the detection limit of phenol and the G value of phenol was estimated to be 1 × 10−3 Gy.

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INTRODUCTION Chemical dosimetry systems, such as the Fricke dosimeter, based on the oxidation of Fe(II) to Fe(III),1 ethanolchlorobenzene dosimetry systems,2 and methods based on the reduction of Ce(IV) to Ce(III)1 are widely used for the determination of the dose of γ-rays, X-rays, and electron beams. In these methods, the total dose of the ionizing radiation is determined by the amount of substance formed by radiolysis. These chemical dosimetry systems are chemically traceable, and they provide a reliable and accurate method for the quantitative measurement of radioactive sources. However, their sensitivity is so low that they can only be used for quantitative measurements of radioactive sources at high total doses (more than 1 kGy). Recently, highly sensitive fluorescent dosimeters with coumarin derivatives have been reported.3,4 The benzene dosimetry system for γ-rays, which is based on the formation of phenol by the reaction of radiolytically produced hydroxyl radicals and benzene in aqueous solution, was proposed by Day and Stein.5 Johnson and Martin demonstrated the linear relationship between phenol formation and total dose of 60Co γ-rays for the range of 10 000−80 000 rep6 (rep = Rontgen equivalent physical7). They mentioned that a benzenewater dosimeter was easier to use and more reliable than a ferrous sulfate−sulfuric acid dosimeter. Although a large number of studies dealing with reactions between benzene and radiolytically formed hydroxyl radical in aqueous media have been reported,8−11 the benzene−water dosimeters are not as well-known as other chemical dosimeters, and the characteristic features of the method are not clear. The authors and co-workers have investigated a simple and highly sensitive determination technique for hydroxyl radicals photochemically generated in environmental water using benzene and fluorescence HPLC.12,13 We have applied the © 2013 American Chemical Society

technique to the benzene dosimetry system and demonstrated a linear increase of phenol in aerated benzene solutions to 100Gy.14 The lower limit of dose detection, calculated from the detection limit of phenol of the system, was estimated to be 0.007 Gy; this is two or more orders more sensitive than chemical dosimeters currently in use. The phenol formation rates are not affected by the oxygen concentration in the benzene solutions, and chemical stability of the system before and after irradiation was good. In a series of experiments using benzene dosimetry systems, the formation of phenol from benzene in tap water was found to be higher than that in Milli-Q water. Detailed experiments demonstrated that sodium hypochlorite, which is added to tap water as a disinfectant, accelerated the formation of phenol. In this paper, the characteristics of the formation of phenol by radiolysis of aqueous benzene solution with sodium hypochlorite are described.

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MATERIALS AND METHODS Reagents. A standard solution of phenol (1000 mg L−1) was obtained from the Sigma-Aldrich Japan Co. Ltd. (Tokyo, Japan). Commercial sodium hypochlorite solution (10% active chlorine) was purchased from the Sigma-Aldrich Japan Co. The concentrations of sodium hypochlorite were determined spectrophotometrically in alkaline solution using the absorption coefficient (365 M−1 cm−1 at 292 nm) reported by Feng et al.15 Other chemicals and solvents were of reagent grade or HPLC grade and were used without further purification. All solutions Received: December 26, 2012 Revised: February 12, 2013 Published: February 26, 2013 1941

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solution without sodium hypochlorite increased linearly with an increase in total dose, and no saturation was observed up to 12 Gy. In a benzene solution containing 10 μM sodium hypochlorite, the phenol formation up to a total dose of 6 Gy was about six times lager than that without sodium hypochlorite. However, the rate of phenol yield decreased sharply above 6 Gy, and the rate of phenol production was the same as that without sodium hypochlorite. In a benzene solution containing 5 μM sodium hypochlorite, the phenol formation rate decreased above a total dose of 3 Gy. The effects of sodium hypochlorite concentration on the phenol formation are shown in Figure 2. Phenol formation at a

were made using ultrapure water obtained from a Milli-Q Plus system (Millipore, Tokyo, Japan). Benzene aqueous solutions were prepared by adding benzene directly to Milli-Q water and shaking hard for 1−2 min. The solution was irradiated with 60Co γ-rays in an amber glass screw vial. Sample solutions saturated with N2O/O2 (= 4:1 (v/v)) were prepared by bubbling N2O/O2 (= 4:1 (v/v)) through Milli-Q water for 1 h just before the benzene was added. N2O/ O2 (= 4:1 (v/v)) gas was prepared from N2O gas (liquefied N2O, Showa Denko, Tokyo, Japan) and O2 in an in-house gas blender. The solutions saturated by N2O/O2 (= 4:1 (v/v)) were immediately transferred to an amber glass screw vial, and the headspace of the vial for γ-irradiation was purged with N2O/O2 (= 4:1 (v/v)). γ-Ray Irradiation. The γ-ray irradiation was carried out at the 60Co γ-ray irradiation source of the Faculty of Engineering, Hiroshima University. Irradiations were conducted at distances of 30−180 cm from the center of the source and a height of 15 cm from the irradiation table (dose rate: 0.14−2.8 Gy min−1). Irradiations were carried out at room temperature, and the γ-ray irradiations in this study did not change the solution temperature. The kinetic energy released in the water was obtained using the conversion factor 1 R h−1 = 0.00973 Gy h−1. In this paper, the radiation doses are given as the absorbed doses in water. These details have been reported previously.1416 Determination of Phenol. Phenol concentrations were determined using an isocratic HPLC system equipped with a fluorescence detector (LC-1500; Jasco, Tokyo, Japan). The fluorescence detector was operated at 270 nm for excitation and 300 nm for emission. A sample-injection valve with a 100 μL sample loop (7250i; Rheodyne, Rhonert Park, CA, USA) was used. A C-18 reversed-phase column (Kanto Kagaku, Tokyo, Japan, RP-18GP, 5 μm, 4.6 mm i.d. × 150 mm length) was used with an acetonitrile-water (55:45 v/v) mobile phase at a flow rate of 1.2 mL min−1. The detection limit of phenol in the HPLC system employed, defined as the equivalent concentration of three times the standard deviation of five measurements of a 10 nM phenol standard solution, was less than 1 nM.

Figure 2. Formation of phenol as a function of sodium hypochlorite concentration at a total dose of 4.0 Gy (○), 1.6 Gy (■), and 0.53 Gy (▲).

total dose of 4.0 Gy increased linearly with increasing sodium hypochlorite concentration and then reached a constant at 8 μM. At lower total doses, phenol formation reached a constant at lower concentrations of sodium hypochlorite. Excess sodium hypochlorite, however, prevented the formation of phenol. It has been reported that phenol reacts thermally with sodium hypochlorite and is converted to polychlorophenols and other chlorinated compounds.17 The decrease in phenol formation at higher concentrations of sodium hypochlorite could be the result of phenol decomposition. The maximum phenol yield was obtained at a sodium hypochlorite concentration of 10 μM up to a total dose of 6 Gy. Formation of Phenol with N2O. N2O reacts with hydrated electrons generated by water radiolysis to form hydroxyl radicals. The radiolytic production of hydroxyl radicals in aqueous solution is enhanced in the presence of N2O.

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RESULTS AND DISCUSSION Enhancement of Phenol Formation by Sodium Hypochlorite in Aerated Benzene Solution. The relationship between the formation of phenol and total dose is shown in Figure 1. The phenol formation in an aerated benzene

eaq − + N2O + H 2O → •OH + N2 + OH−

Phenol formations in benzene solutions saturated with N2O/ O2 (= 4:1 (v/v)) are shown in Figure 3. The formation of phenol in a benzene solution without sodium hypochlorite was approximately doubled by the presence of N2O/O2 (= 4:1 (v/ v)). In a benzene solution containing 10 μM sodium hypochlorite, however, N2O did not affect the formation of phenol up to a total dose of 6 Gy. Above 6 Gy, the rate of phenol yield with sodium hypochlorite was essentially the same as that without sodium hypochlorite. pH Dependences. The pH dependence of the phenol formation at a total dose of 1.5 Gy is shown in Figure 4. The formation of phenol without sodium hypochlorite was unchanged over the pH range 3−11. Phenol formation in the presence of 4.8 μM sodium hypochlorite, however, was strongly depended on the solution pH. Hypochlorous acid (HClO) is a weak acid with an acid dissociation constant pKa = 7.5. In Figure 4, the theoretical fractions of HClO are also shown. The pH-dependence curve of phenol formation in the presence of

Figure 1. Phenol concentrations in γ-ray irradiated aqueous benzene solutions (air-saturated) with and without sodium hypochlorite (○, 0 μM; Δ, 5 μM; and □, 10 μM). 1942

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acid (HClO), rather than the hypochlorite anion (ClO−), since phenol formation with sodium hypochlorite was enhanced in acidic pH ranges, lower than the pKa of HClO. The enhancement mechanism may be the generation of a hydroxyl radical by the reaction of HClO with a superoxide radical generated from a hydrated electron and oxygen eaq − + O2 → O2− HClO + O2− → •OH + Cl− + O2

Long and Bielski reported on the pH dependence of the reaction rate constants of the superoxide radicals and hypochlorite; the rate constants are high at low pH and decrease with increasing pH.18 At pH > 4.7, superoxide radicals are also generated from hydrogen atoms:19

Figure 3. Formation of phenol in N2O/O2 (= 4:1 (v/v))-saturated benzene solution with and without sodium hypochlorite (●, 0 μM and ■, 10 μM). Phenol formation in air-saturated solutions with and without sodium hypochlorite (○, 0 μM and □, 10 μM) shown in Figure 1 are also shown.

H + O2 → HO2 HO2 ⇔ H+ + O2−

k = 1.3 × 1010 M−1 s−1 pK a = 4.7

Zuo et al. reported a more direct reaction between HClO and hydrated electrons at pH 5.20 HClO + eaq − → •OH + Cl−

However, it is virtually impossible to explain the result obtained in this study based on a reaction involving hydrated electrons for the following two reasons. The first reason is a conflict with the results in the presence of N2O shown in Figure 3. The reaction rate of hydrated electrons with N2O and O2 has been reported to be 9.6 × 109 and 1.8 × 1010 M−1 s−1, respectively.19,21 The concentrations of N2O and O2 in an aqueous solution saturated with N2O/O2 (= 4:1 (v/v)) at room temperature are calculated to be 20 and 0.25 mM, respectively. These indicate that almost all of the hydrated electrons would react with N2O rather than O2 and/or HClO in the presence of N2O. In other words, the results in Figure 3 show that N2O, which has a high reaction rate with hydrated electrons, does not affect the enhancement of sodium hypochlorite, indicating that the reaction by sodium hypochlorite does not involve reaction with hydrated electron. The second reason is the G values of hydrated electrons and other radicals. The G values in water are 0.282 μmol J−1 (= μM Gy1−) for hydroxyl radicals and 0.273 μmol J−1 for hydrated electrons, 0.057 μmol J−1 for hydrogen atoms, 0.047 μmol J−1 for hydrogen molecules, and 0.071 μmol J−1 for hydrogen peroxide.22 As shown in Figure 3 and Table 1, phenol formation without sodium hypochlorite was doubled in the presence of N2O, since the G values of hydroxyl radicals and hydrated electrons in water are similar. Reactions using radiolytically generated hydrated electrons and other radicals should increase the formation 2- or 2.6-fold at best comparing with the phenol formation without sodium hypochlorite. The phenol formation with sodium hypochlorite is about six times higher than without sodium hypochlorite. The G values of phenol in the aqueous benzene solution containing sodium hypochlorite are larger than the sum of G values for the radicals and reactive oxygen species by water radiolysis (0.730 μmol J−1 for •OH, e−, H, H2, and H2O2). Oliver and Carey reported the photochemical reactions of ethanol with sodium hypochlorite, and their chlorination reactions.23 They mentioned the radical chain reaction.

Figure 4. pH dependence of phenol formation with 4.8 μM sodium hypochlorite (□, ○, and Δ) and without sodium hypochlorite (■, ●, and ▲). Total dose of γ-ray radiation was 1.54 Gy. The pH of each solution was adjusted using 10 mM phosphate buffer (○ and ●) or HNO3/NaOH (□ and ■). The pH of Milli-Q water was not controlled (Δ and ▲). The pH of each solution was measured just before γ-ray irradiation. The + marks show the theoretical fraction of HClO in acid dissociation at pKa = 7.5 (= [HClO]/([HClO] + [ClO−])).

sodium hypochlorite is roughly in agreement with the fraction curve for acid dissociation of HClO, although the pHdependence curve of phenol formation is slightly shifted to high pH. It is assumed that HClO was the active species increasing the phenol formation. At pH < 4, phenol formation in the presence of sodium hypochlorite decreased steeply, as a result of the evaporation of chlorine gas derived from sodium hypochlorite and acid. Reaction Mechanism of the Enhancement of Phenol Formation by Sodium Hypochlorite. The yield of phenol under different conditions is summarized in Table 1. The results show that sodium hypochlorite clearly enhanced the formation of phenol at lower doses. The active species which enhances the phenol formation is assumed to be hypochlorous Table 1. G Value of Phenol under Different Conditions G(phenol)/μmol J−1 conditions No-NaClO 5 μM NaClO 10 μM NaClO No-NaClO 10 μM NaClO

air-saturated air-saturated air-saturated N2O/O2 (= 4:1) N2O/O2 (= 4:1)

low dose

high dose

0.145 0.826 (6 Gy)

k = 6.5 × 108 M−1 s−1

RH + Cl• → R• + HCl 1943

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R• + HClO → ROH + Cl•

where [Cl•]ss is the steady-state concentration of chlorine atoms, kbenzene‑Cl is the reaction rate constant between benzene and chlorine atoms (kbenzene‑Cl = 1.2 × 1010 M−1 s−1),29 and kCl−Cl is the reaction rate constant between chloride ions and chlorine atoms (kCl−Cl = 6.5 × 109 M−1 s−1).28 R0(by-NaClO) is the phenol formation involving sodium hypochlorite without NaCl. It is estimated to be 2.5 μM at 3.45 Gy. The benzene concentration used in this study is 2 mM. Although the predicted line was slightly lower than the experimental results with sodium hypochlorite, these results indicated the radiolytic formation of phenol in benzene solution with sodium hypochlorite relates to the reaction process involving chlorine atoms.

A chlorine atom (chlorine radical) reacts with an organic compound to produce an organic radical, which reacts with HClO to generate another chlorine radical. Martire et al. reported on the formation of organic radicals by chlorine atoms with benzoic acid.24 The abstraction of a hydrogen atom from benzene by chlorine atom in gas phase was reported.25 The radical chain reaction may be a explanation for the high yields of phenol in an aqueous benzene solution containing sodium hypochlorite. The radiolytically generated benzene radical and/ or chlorine atom might work as the trigger for the chain reaction. Chloride ions (Cl−) react with chlorine atoms. The reaction rate between a chloride ion reacting with a chlorine atom is reported in the literature as 6.5−20 × 109 M−1 s−1,26−28 −

Cl + Cl• → Cl 2

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CONCLUSION In conclusion, we have shown that the formation of phenol in aqueous benzene solution was enhanced by sodium hypochlorite. Sodium hypochlorite can be applied as a sensitizer for a benzene chemical dosimetry system in lower doses. The lower limit of dose detection, which is calculated from the detection limit of phenol (DL = 1 nM) and the G value of phenol (0.894 μM Gy1−), was estimated to be 1 × 10−3 Gy. The outstanding characteristic of this system is its high sensitivity. The sensitivity of this dosimetry system may approach the sensitivity of luminescence systems, such as thermoluminescence and photoluminescence. This sensitivity is owing to high product yield rather than the analytical system. The yield of phenol with sodium hypochlorite exceeded the sum of the yield for the radicals and reactive oxygen species by water radiolysis such as •OH, e−, H, H2, and H2O2. The high product yield has never been reported in a chemical dosimetry system. There have been a large number of reports on the formation of phenol by radiolysis of aqueous benzene solutions; however, no report has mentioned increased phenol formation with sodium hypochlorite. This may be because the high product yield could be only observed at low total dose with a low concentration of sodium hypochlorite. The reaction mechanism for the enhancement of phenol formation in γ-ray irradiated benzene solutions with sodium hypochlorite involves chlorine atoms. A chain reaction including chlorine atoms and benzene radicals is one possible reaction to explain the high product yield. However, it is still difficult to understand the results shown in Figure 3, i.e., the effect of N2O with sodium hypochlorite up to a total dose of 6 Gy. The details mechanism of the enhancement mechanism of phenol formation by sodium hypochlorite in the aqueous benzene solution should involve multiple and complicated reactions.

−

Chloride ions would scavenge the chlorine atom to quench the formation of phenol. In Figure 5, the effect of chloride ion

Figure 5. Dependence of NaCl concentration for phenol formation in γ-ray irradiated aqueous benzene solutions with and without sodium hypochlorite (■, 0 μM and ○, 10 μM) at a total dose of 3.45 Gy. The solid line shows the predicted formation of phenol. Details are in the text.

being present during the formation of phenol in a γ-rayirradiated benzene solution is shown. Without sodium hypochlorite, the phenol formations were not effected by the presence of sodium chloride (chloride ions) in this concentration range. As mentioned before,14 interference by coexisting salts and solvents is understood in terms of quenching reactions of hydroxyl radicals by coexistences. The scavenging reactions of hydroxyl radical by chloride ion are negligible comparing with the reaction between benzene and the hydroxyl radical in the experimental conditions used ([benzene] = 2 mM and [Cl−] = 0.17−34 mM). On the other hand, formation of phenol with sodium hypochlorite decreased with increasing NaCl. The solid line in Figure 5 shows the dependence of phenol formation on NaCl concentration as predicted by the following equations:

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AUTHOR INFORMATION

Corresponding Author

R = R(by‐•OH) + R(by‐NaClO)

*E-mail: [email protected].

R(by-•OH) is the formation of phenol by direct reaction between benzene and hydroxyl radical. It is estimated to be 0.5 μM at 3.45 Gy, and is independent of NaCl concentration in this NaCl range. R(by-NaClO) is the formation of phenol involving sodium hypochlorite. It is expressed as

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Prof. Dr. K. Shizuma and the staff at the radiation facility of Graduate School of Engineering, Hiroshima University for their technical advice and support of γ-ray irradiation.

R(by‐NaClO) = R 0(by‐NaClO) k benzene − Cl[benzene][Cl•]ss k benzene − Cl[benzene][Cl•]ss + k Cl − Cl[Cl−][Cl•]ss 1944

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