Photochemical formation rate

Abstract—The influence of the hydroxyl radical (OH) on the photodegradation of the estrogen-like compound, bisphenol A (BPA), was examined in this study. The formation rate of OH, normalized to the vernal equinox solar noon condition of Higashi-Hiroshima (34°N) was in the range 0.70-3.25 X 10 °M s in Kurose river water. The total consumption rate constant of OH in river water ranged from 1.66 to 3.89 X 10 s . Based on the photochemical formation rate and the total consumption rate constant of OH, steady-state OH concentrations on the order of 3.33-8.35 X 10 M were determined. The reaction rate constant for OH with BPA determined by competition kinetics was found to be 1.55 X 10 ° s in water containing nitrate ions that photochemically produced OH. 

The rate constant for the reaction of BPA with nitrate-mediated OH was determined using a competitive reaction with benzene. Initial concentrations of reagents used were 1 X 10 M of sodium nitrate, 2 X 10 " M of benzene, and 0-4.77 X 10 " M of BPA. These experimental solutions were irradiated for 0,15,30,45, and 60 min. After irradiation using the solar simulator, photochemical formation rates of phenol in the mixture samples were determined using HPLC-FL and the scavenging constant rate of BPA with OH was calculated. 

The lifetime of OH in the river water, based on the reciprocal of the consumption rate constant, was in the range 2.6-6.0 x 10 s (Table 4). This value is similar to OH lifetimes reported in dew (Arakaki et al., 1999b) and cloud waters (Arakaki and Faust, 1998), while those in polluted cloud waters based on a modeling study were in the range 3-66 X 10 s (Jakob, 1986). Based on the photochemical formation rate and the total consumption rate constant of OH, steady-state OH concentrations were calculated to be in the range 3.3-8.4 X 10 M (Table 4), which is similar to reported values in river water (Brezonik and Fulkerson-Brekken, 1998), and in rain and dew water (Arakaki et al., 1999b). 

The photochemical formation rate (0.70-3.25 X 10 Ms ), the total consumption rate constant (1.66-3.89 X 10 s ) and the steady-state concentration of OH (3.3-8.4 X 10 M) in river water samples collected in Higashi-Hiroshima, Japan were measured in this study. The measured values were similar to previous values reported for river, rain and dew water samples. In the investigation of production mechanisms of OH, it was found that OH production from nitrite photolysis was greater than that from nitrate photolysis in Kurose river water. The summation of consumption rate constants of OH for major anions occurring in river water was less than 25% of the total consumption rate constant. Based on the reaction rate constant of OH for DOM, it is estimated that DOM accounts for 12-56% of the total consumption rate constant of OH in river water. 

These assumptions lead, by the same method of calculation as that used in the case of the thermal reaction, to the correct velocity equation. It is to be noted that since (2) is followed by either (3) or (4) the actual formation of hydrobromic acid does not use up the supply of atomic bromine, and a stationary concentration of bromine atoms is established by the balancing of the rate of their photochemical formation (1) and the rate of their thermal recombination (5). ... 

The rate of production of bromine atoms by light is estimated on the basis of Einstein s law, which requires one molecule of bromine to be dissociated for each quantum of light absorbed. In the stationary state the number of bromine atoms recombining thermally in unit time is equal to this rate of photochemical formation. Thus the number of bromine atoms which recombine per second at a known atomic concentration is found. In this way Bodenstein and Liitkemeyer find that about one collision in a thousand between bromine atoms results in combination. This number is of the right order of magnitude only, since the estimation of the number of light quanta absorbed was not very certain, and a value based only on analogy had to be assumed for the diameter of the bromine atom. 

FIGURE 7. Kinetic profile for photophysical and photochemical processes of a phenylpentamethyld-isilane in alcohol-hexane mixtures kf, k f = fluorescence rates ka, k = radiationless decay rates k = rate for CT quenching kt, k t = reaction rates for 1,3-silyl migration and reaction with alcohol, respectively kc = formation rates of ICT state from LE. Modified from Reference 105... 

The photochemical formation of HCI from H2-CI2 mixtures has been studied by Chapman. The reaction was followed by dissolving the HCI formed in water which was present in the reaction vessel thereby reducing the total pressure in the system as time passed and permitting the rate to be calculated. Only qualitative significance can be attached to this data since the influence of the presence of water in the system is not accounted for. 

The accuracy of our understanding of photochemical mechanisms is an additional source of uncertainty. Known uncertainties in photochemical reaction rates and stoichiometries cause an uncertainty of 20% in calculated ozone formation rates (Gao et al., 1996). 

It is reasonable to expect that the same path is followed also in the present case of discharge excitation. Since the rate of formation of adduct II is independent of the concentration of maleic anhydride (experiments 2 and 4), it is concluded that the formation of the intermediate monoadduct is the rate determining step. The same conclusion has been reached in the photochemical formation of adduct II (6, 11). 

Photochemical Reactioii with CO2. The Ni(bpy)3 -TEA system produces CO from CO2 by irradiation at 313 nm with quantum yield 0.1%. Because Ni(bpy)3 has an absoiption band at 309 nm (e = 41,700 M" cm ), over 95% of light was absorbed by Ni(bpy)3 ". The CO produced reacts with the reduced Ni (bpy)2 and Ni (bpy)2 to form CO adducts therefore, photochemical reaction is stoichiometric and the CO production is 0.5 mole from 1.0 mole of Ni (bpy)3. The final spectrum of continuous photolysis (Figure 1) is similar to that observed in the addition of CO to the reduced nickel species, indicating the formation of a CO adduct. The addition of excess bpy (3 times that of Ni(bpy)3 ) accelerated the reaction rate however, no significant difference was observed for CO yield. Emission from Ni(bpy)3 in MeCN was not observed at room temperature or at 77 K. However flash photolysis, electrochemistry, and pulse radiolysis experiments provide evidence of the intermediate, Ni (bpy)2, in the photochemical Ni(bpy)3 -TEA system. The mechanism of the photochemical formation of Ni (bpy)2 has not yet been identified. The formation of Ni (bpy)2 could involve the direct excitation of an electron from a donor (TEA) to die solvent (30, 42, 43). This electron would be expected to react rapidly with Ni(bpy)3 to produce Ni (bpy)2. It should also be pointed out that Ni (bpy)2 seems unreactive toward CO2 addition. However, Ni (bpy)2 does react with CO2. The reduced Ni(bpy)3 solution contains various species such as Ni (bpy)2, Ni (bpy)2, and [Ni(bpy)2]2- Studies to determine the equilibrium constants between these species are in progress. 

The only observable transient species in the laser flash photolysis of a-nitro-naphthalene in solution at room temperature has been designated as the triplet state (tt 30 ms). The change of dipole moment accompanying the transition Tx - Tn, as well as rate constants for electron and proton transfer involving the Tx state, were determined, and the low reactivity of the 7i state in polar solvents was attributed to reduced mr and increased charge-transfer character of the triplet state.221 222 223 Heavy-atom perturbation experiments have implicated the triplet state of the nitro-compound (13) in the photochemical formation of (14).22a... 

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