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Ind. Eng. Chem. Res. 2009, 48, 4312–4315
Dissolution and Diffusion of Oxygen in Deaerated Water and Escape of Oxygen to the Atmosphere from an Oxygen Saturated Aqueous Solution: An Investigation by a Pulse Radiolysis Technique Surajdevprakash B. Dhiman and Devidas B. Naik* Radiation and Photochemistry DiVision, Bhabha Atomic Research Centre, Trombay, Mumbai-400 085, India
Fast reaction of hydrated electrons (eaq-) with oxygen has been used to determine the dissolution and diffusion of oxygen in water by monitoring the decay of eaq- at 700 nm. Hydrated electrons in the irradiated volume (7.5 cm below the surface) were generated by pulse radiolysis technique. Water was purged with high purity nitrogen to remove dissolved oxygen, and subsequently, the lifetime of the eaq- formed on pulse radiolysis was determined at various times after exposure to atmosphere. The lifetime of the eaq- measured in the irradiated volume was negligibly affected until ∼60 min after the water was exposed to atmosphere. Then onward, the lifetime of eaq- decreased as a result of oxygen reaching the irradiated volume. In another set of experiments, reaction of methyl viologen radical cations (generated by the reaction of eaq- with methyl viologen) with oxygen (k ) 4.3 × 108 dm3 mol-1 s-1) was investigated to find out how the dissolved oxygen escapes from oxygen-saturated solution to the atmosphere by observing the increase in the lifetime of methyl viologen radical cations. In 140 min, concentration of oxygen comes down to the value present in the ambient aerated solution. Introduction Oxygen concentration in water and aqueous solutions plays a very important role in a variety of chemical processes and applications. Aeration is commonly done in wastewater treatment and water purification. Many researchers1-4 have studied the effect of oxygen on degradation of contaminants using oxygen as a carrier gas in Fenton and photo-Fenton systems. The studies of Du et al.5 have shown that the presence of oxygen not only enhanced the degradation process of p-chlorophenol, a common chlorinated phenolic organic contaminant, but also lead to different reaction mechanism and hence different final products compared to that in the absence of oxygen. The formation of organic peroxyl radicals in water systems mostly depends on the concentration of dissolved oxygen in the reaction system.3,6,7 Recently, quite a number of photochemical studies on mineralization of organic compounds have been carried out.8,9 In the mineralization process, organic carbon is converted into carbon dioxide, and in this process, dissolved oxygen gets consumed. Concentration of oxygen in the solution can become a limiting factor in deciding complete oxidations of organic compounds. Although oxygen gas or air can be passed through the solution during photoirradiation, it is not always advisible depending on the nature of organic solute. If the solute is volatile or if any volatile intermediate is formed in the photodegradation process, purging can have undesired effects. Under ambient conditions, in solar photodegradation reactions oxygen from solution gets consumed and replenishment can be time dependent. In this connection, it was thought that reaction of hydrated electron (eaq-) with oxygen10 can be made use of to get an idea about the real time diffusion of oxygen in deaerated water. Pulse radiolysis has proved to be a versatile technique for the study of a variety of transient species generated by radiolysis of water as well as organic solutions.11 Water radiolysis generates both reducing as well as oxidizing species. To study the reactions of reducing species viz. eaq- and H-atoms, it is essential to remove * To whom correspondence should be addressed. Fax: +91-2225505151. Tel.: +91-22-25595101. E-mail:
[email protected].
dissolved oxygen from the solution as both eaq- and H-atoms react quite fast with O2.12 For this reason, the solutions are purged with an inert gas such as nitrogen or argon, or sometimes, oxygen is removed by freeze and thaw cycles. Recently Janata et al.10 have used a pulse radiolysis technique to investigate the solubility of oxygen and nitrous oxide gases in concentrated NaCl solutions. There is no report on the pulse radiolysis technique being used to investigate oxygen diffusion in deaerated water from air. Along with this, it was also interesting to investigate the escape of oxygen from oxygen saturated solution to the atmosphere. To study this, the reaction of oxygen with methyl viologen radical cation (MV•+) was monitored by following the decay of MV•+ absorption at 600 nm. In the absence of oxygen, MV•+ radical is very stable. In this paper, the results of lifetime measurements of eaq- when the deaerated water is exposed to air as well as decay behavior of methyl viologen radical cation at different time intervals on exposure of oxygen saturated solution to atmosphere are discussed. Experimental Details The pulse radiolysis experimental setup has been described earlier.13,14 Pulses of 7 MeV electrons of 50 ns duration from a linear electron accelerator were used. The absorbed dose per pulse was measured using potassium thiocyanate dosimetry in which absorption of (SCN)2•- species at 500 nm formed on reaction of •OH radicals with SCN- was monitored. To avoid accumulation of H2O2 in the solution, the pulse dose used was kept to 8 Gy (J kg-1). Water from a Millipore A-10 system was taken in 1 cm × 1 cm suprasil quartz cuvette up to the brim and was purged with high purity N2 gas for 15 min to remove dissolved oxygen and was closed with a stopper. The height of the total water column was 9.5 cms. The centre of the irradiated volume was 7.5 cms below the top surface. In the kinetic spectrophotometric detection, the analyzing light beam from a 450 W xenon arc lamp, after passing through the electron beam irradiated sample, falls on the entrance slit of a Kratos monochromator. An R-955 photomultiplier tube coupled to a
10.1021/ie801928u CCC: $40.75 2009 American Chemical Society Published on Web 03/26/2009
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200 MHz digital oscilloscope measured the output intensity. Experiments were carried out in two sets. In the first set, after observing the lifetime of eaq- in deaerated solution, the stopper of the cuvette was removed and electron pulse was given after suitable time interval. The corresponding lifetime of eaq- at different time intervals was monitored. In another set of experiments, methyl viologen radical cations (MV•+) were generated by the reaction eaq- with methyl viologen (MV2+) in oxygen saturated solutions and on exposure to the atmosphere the escape of dissolved oxygen from the solution was studied. All the experiments were carried out at room temperature (∼25 °C). Results and Discussions In the radiolysis of water, highly reactive species are generated. H2O ' eaq-, H, •OH, H2, H2O2, H3O+
(1)
ThehydratedelectronshavestrongabsorptionintheUV-visible-NIR region with an absorption maximum at 715 nm ( ) 1.85 × 104 dm3 mol-1 cm-1),15 and it is quite convenient to follow its reaction with solutes by observing the decay of its absorption by pulse radiolysis technique using optical detection. Hydrated electrons react rapidly with oxygen, eaq- + O2 f O2•-
Figure 1. Oscilloscope traces showing hydrated electron decay at 700 nm in deaerated water at different times: (a) 18, (b) 82, (c) 118, and (d) 148 min of exposure to air.
(2)
The rate constant for this reaction is 1.9 × 1010 dm3 mol-1 s-1.11 The superoxide radical ion has a weak absorption band with a maximum at 245 nm ( ) 2.35 × 103 dm3 mol-1 cm-1).16 Hydrated electrons also undergo the following reactions eaq- + eaq- f H2+2OHk ) 1.1 × 1010 dm3 mol-1 s-1 (ref 12)
(3)
eaq- + H2O f H + OHk ) 1.9 × 101 dm3 mol-1 s-1 (ref 12)
(4)
eaq- + H f H2+OHk ) 2.5 × 1010 dm3 mol-1 s-1 (ref 12)
(5)
eaq- + •OH f OHk ) 3.0 × 1010 dm3 mol-1 s-1 (ref 12)
(6)
eaq- + H2O2 f •OH + OHk ) 1.1 × 1010 dm3 mol-1 s-1 (ref 12)
(7)
eaq- + H3O+ f H + H2O k ) 2.3 × 1010 dm3 mol-1 s-1 (ref 12)
(8)
In the absence of added reactive solutes, hydrated electrons undergo decay by above reactions and its observed lifetime is dependent on the radical concentration generated and hence the radiation dose absorbed. Additionally, concentration of H3O+ present at ambient pH of nanopure water used also contribute by reaction 8. At reasonably low absorbed doses, when the free radical concentrations are quite low, oxygen concentrations above a few µM becomes quite important in determining decay rate of hydrated electrons. At an electron pulse dose of 8 Gy used, the concentration of hydrated electrons generated is about 2.2 × 10-6 mol dm-3 (the G value for hydrated electron is 0.28 µmol/joule). On giving
Figure 2. Plot of measured first lifetime of hydrated electrons formed in electron beam pulsed deaerated water exposed to the atmosphere as a function of time.
the electron pulse to closed quartz cuvette containing deaerated water up to the brim, initial lifetime measured from the decay trace of hydrated electron was 7.6 µs which corresponds to halflife of 5.27 µs. Subsequently, the stopper of the quartz cuvette was removed without disturbing the cuvette and electron pulses were delivered after definite time intervals. In the preliminary experiments, electron pulses were given every 2 min and it was found that, after 10-12 min, the electron lifetime starts decreasing. In the radiolysis of water with high energy electrons, the G-value for H2O2 is 0.08 µmol/joule15 which has high reactivity with eaq-. (k ) 1.1 × 1010 dm3 mol-1 s-1).12 On giving a number of pulses, accumulation of H2O2 in the irradiated volume is expected which can also contribute in the decrease of lifetime of eaq- as observed after 10-12 min in our preliminary experiments. For this reason, in subsequent experiments, time interval between the pulses was increased so that there is less contribution from H2O2. Absorption decay traces of eaq- at various times after exposure of deaerated water to the atmosphere are given in Figure 1. It can be seen that with increase in the exposure time, eaq-, decay becomes successively faster. From the absorption trace, initial lifetimes of eaq- were measured. In Figure 2, the lifetimes of hydrated electrons so determined at 18, 33, 62, 82, 100, 118, 130, 140, and 148 min after exposure of the deaerated water to the atmosphere are plotted as a function of time of exposure. It can be seen that,
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Figure 3. Plot of calculated concentration of oxygen in the electron beam irradiated volume of initially deaerated water as a function of exposure time.
up to 18 min, there is hardly any change in hydrated electron lifetime implying that oxygen does not reach the irradiated sample volume, which is 7.5 cm from the surface, in this time of exposure. In 18 min, no effect of oxygen diffusion was observed. After 18 min, there is slow decrease in lifetimes of hydrated electron, and after 62 min, there is about an 8% decrease in the lifetime of hydrated electrons which indicates that less than 5 × 10-7 mol dm-3 oxygen reaches the irradiated volume. From then onward, lifetimes of hydrated electrons decrease at a noticeable rate. After 80 min, it was found that oxygen concentration increases more or less linearly with time. Using the rate constant for reaction 2, the oxygen concentrations were calculated by applying pseudo-first-order kinetics. Concentrations of the oxygen in the irradiated volume after 80 min of exposure are given in Figure 3. It is seen that concentrations of the oxygen increases linearly. The slope of this linear plot is found to be 2.39 × 10-7 mol dm-3 min-1 indicating that, after 80 min, increase in the concentration of oxygen in the irradiated volume per minute is 2.39 × 10-7 mol dm-3. To evaluate any contribution from radiolytically generated H2O2 and H3O+, a similar set of experiments were carried out at identical time duration without removing the stopper. It was observed that, after 140 min, the decay rate of eaq- does not become faster by more than 10%. This is due to the fact that radiolytically generated H3O+ will get neutralized by OH- ions formed in reactions 3 and 5-7 as well as partly by reaction 8. In the second set of experiments, by making use of a pulse radiolysis technique, it was investigated, how the dissolved oxygen from oxygen-saturated solution escapes on exposure to atmosphere. For this, the reaction of methyl viologen radical cation with oxygen was monitored. In the solution, initially methyl viologen radical cation was generated by reaction of hydrated electron with methyl viologen (k ) 5.5 × 1010 dm3 mol-1 s-1).17 MV2+ + eaq- f MV•+
(9)
MV•+ exhibits two absorption maxima at 392.5 and 600 nm.18,19 In our experiments, we have followed absorption of MV•+ at 600 nm. At 25 °C, concentration of oxygen in the ambient aerated aqueous solution is 2.5 × 10-4 mol dm-3 which becomes nearly five times higher when it is saturated with oxygen ([O2] ) 1.25 × 10-3 mol dm-3). The rate constant for the reaction of MV•+ with oxygen was determined by following the pseudofirst-order decay of MV•+ at 600 nm. The MV•+ radical is stable in the absence of oxygen. In the presence of oxygen either in
Figure 4. Oscilloscope traces showing methyl viologen radical cation decay at 600 nm in oxygen-saturated 4 × 10-4 mol dm-3 methyl viologen solution at different times: (a) 0, (b) 20, (c) 60, (d) 80, and (e) 110 min on exposure to air and (f) in aerated conditions.
ambient aerated conditions or in oxygen saturated conditions, MV•+ decays by pseudo-first-order kinetics as the MV•+ concentration is much lower than that of oxygen. MV•+ + O2 f MV•+ + O2•-
(10)
Following the pseudo-first-order decay of MV•+ in aerated and oxygen-saturated conditions, the rate constant for the reaction of MV•+ with oxygen was determined to be 4.3 × 108 dm3 mol-1 s-1. It was expected that if solutions of methyl viologen saturated with oxygen are exposed to air, the decay of MV•+ would become slower as oxygen will escape from the solution. As the concentration of methyl viologen cation radical (3 × 10-6 mol dm-3) generated is much below that of oxygen in aerated solution (2.5 × 10-4 mol dm-3), the decay of MV•+ would still be pseudo-first-order when the system reaches the ambient aerated condition. The oscilloscope traces obtained at 600 nm on pulse radiolysis of oxygen-saturated 4 × 10-4 mol dm-3 MV2+ solution left open to atmosphere at different times are given in Figure 4. With time of exposure, initial absorbance of MV•+ increases as a higher fraction of eaq- generated on radiolysis react with MV2+ due to a decrease in the oxygen concentration. It is clearly seen that, with time, the decay of MV•+ becomes successively slower indicating that the concentration of oxygen in the irradiated volume decreases with time of exposure to atmosphere. From the decay traces, the firstorder decay constant for the reaction of MV•+ with oxygen present at different times after exposure were determined. Using this first order decay constant and knowing the bimolecular rate constant (4.3 × 108 dm3 mol-1 s-1), we have estimated the concentration of oxygen present in the irradiated volume at different times of exposure, and the same are given in Figure 5. Once the oxygen-saturated solution is exposed to the atmosphere, oxygen will start escaping from the top layer. Thus, a concentration gradient will be established in the cuvette due to which oxygen from the lower layers will start diffusing up. The plot in Figure 5 reflects that initially in the irradiated volume the oxygen concentration comes down more or less linearly up to 30 min. Then onward the rate of decrease in oxygen concentration is lower. This seems to be due to some sort of equilibrium between oxygen from irradiated volume moving up and at the same time oxygen getting replenished from layers below. After 70 min of exposure, it is found that oxygen
Ind. Eng. Chem. Res., Vol. 48, No. 9, 2009 4315
Literature Cited
Figure 5. Plot of estimated oxygen concentration present in the irradiated volume of oxygen-saturated 4 × 10-4 mol dm-3 methyl viologen solution at different time intervals on exposure to atmosphere.
concentration is decreasing linearly and by 140 min it reaches the value expected in ambient aerated aqueous solution. Conclusions In the deaerated water, the dissolution of oxygen from air and diffusion of oxygen in the water takes a sufficiently long time. Thus in the pulse radiolysis experiments in aqueous solutions conducted after removing of oxygen from system, if there is slight leak of air due to improper stoppering of the cuvette, oxygen diffusion to the irradiated volume will take substantial time. Until that time, radiation chemical events in pulse radiolysis experiments may not get affected if the irradiated volume is well below the surface. These results also show that in the experiments carried out in aerated solutions, if there is depletion of oxygen in the chemical processes, it takes a reasonable time for equilibration to take place with oxygen in the air. The second set of experiments concerning the decay of methyl viologen radical cations suggests that when oxygen saturated aqueous solution is exposed to the atmosphere, it takes 140 min for the oxygen concentration in the irradiated volume, 7.5 cm from the top surface of the solution, to reach the concentration level that is present in an ambient aerated aqueous system at 25 °C. Note added After ASAP Publication: The version of this paper that was published on the Web March 26, 2009 had errors in the caption for Figure 1. The correct version of this paper was reposted to the Web April 1, 2009.
(1) Kim, S. M.; Vogelpohl, A. Degradation of organic pollutants by the photo-Fenton process. Chem. Eng. Technol. 1998, 21, 187. (2) Kim, S. M.; Geissen, S. U.; Vogelpohl, A. Landfill leachate treatment by a photoassisted Fenton reaction. Water Sci. Technol. 1997, 35, 239. (3) Sun, Y.; Pignatello, J. J. Photochemical reactions involved in the total mineralization of 2,4-D by Fe3+/H2O2/UV. EnViron. Sci. Technol. 1993, 27, 304. (4) Utset, B.; Garcia, J.; Casado, J.; Domenech, X.; Peral, J. Replacement of H2O2 by O2 in Fenton and photo-Fenton reactions. Chemosphere 2000, 41, 1187. (5) Du, Y.; Zhou, M.; Lei, L. The role of oxygen in the degradation of p-chlorophenol by Fenton system. J. Hazardous Mater. B 2007, 139, 108. (6) Legrini, O.; Oliveros, E.; Braun, A. M. Photochemical processed for water treatment. Chem. ReV. 1993, 93, 671. (7) Lukes, P.; Locke, B. R. Degradation of substituted phenols in a hybrid gas-liquid electrical discharge reactor. Ind. Eng. Chem. Res. 2005, 44, 2921. (8) Ollis, D. F.; Al-Ekabi, H. Photocatalytic purification and treatment of water and air; Elsevier Science Publishers: Amsterdam, 1993. (9) Parson, S. AdVanced oxidation processes for water and wastewater treatment; IWA Publishing: London, 2004. (10) Janata, E.; Kelm, M.; Ershov, B. G. Solubility of oxygen and nitrous oxide in aqueous solutions of NaCl: A pulse radiolysis study. Radiat. Phys. Chem. 2002, 63, 157. (11) Matheson, M. S.; Dorfman, L. M. Pulse Radiolysis; MIT Press: Cambridge, 1969. (12) Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. Critical review of rate constants for reaction of hydrated electron, hydrogen atoms and hydroxyl radicals in aqueous solution. J. Phys. Chem. 1988, 17, 518. (13) Guha, S. N.; Moorthy, P. N.; Kishore, K.; Naik, D. B.; Rao, K. N. One electron reduction of thionine studied by pulse radiolysis. Proc. Indian Acad. Sci. (Chem. Sci.) 1987, 99, 261. (14) Mukherjee, T. Some recent studies of molecular dynamics at BARC. In Atomic, Molecular and Cluster Physics; Ahmad, S. A., Ed.; Narosa: New Delhi, 1997; pp 299-311. (15) Spinks, J. W. T.; Woods, R. J. An Introduction to Radiation Chemistry, 3rd ed.; Wiley: New York, 1990. (16) Hug, G. L. Optical spectra of non metallic inorganic transient species in aqueous solution. NSRDS-NBS 1981, 69, 12. (17) Elliot, A. J.; McCracken, D. R.; Buxton, G. V.; Wood, N. D. Estimation of rate constants for near-diffusion-controlled reactions in water at high temperatures. J. Chem. Soc., Faraday Trans. 1990, 86, 1539. (18) Farrington, J. A.; Ebert, M.; Land, E. J. Bipyridylium quaternary salts and related compounds. Part 6.sPulse radiolysis studies of the reaction of paraquat radical analogues with oxygen. J. Chem. Soc., Faraday Trans. 1 1978, 74, 665. (19) Solar, S.; Solar, W.; Getoff, N. Pulse radiolysis of methyl viologen in aqueous solutions. J. Chem. Soc., Faraday Trans. 1 1982, 78, 2467.
ReceiVed for reView December 15, 2008 ReVised manuscript receiVed February 12, 2009 Accepted March 10, 2009 IE801928U
