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Previous studies have established the key reactions that determine the concentrations of the molecular water radiolysis products, H2 and H2O2 in pure water

systems, and this topic has been discussed in detail in the previous two Chapters [27,28]. Among the ~ 40 elementary reactions that occur in pure water upon irradiation, the main production pathway for the molecular products is primary radiolysis (reaction 5.19), while the main removal pathways for these species are by reaction with •OH and •eaq in

H₂O •OH, •e_a , H+, •H, H₂, H₂O₂ 5.19 where GH2 and G H2O2 are 0.45 and 0.70 molecules/100 eV, respectively.

H2(a ) + •OH → •H + H2O kR5.20= 4.2 × 107 mol−1∙dm3∙s−1 5.20

H2O2 + •OH → •HO2 + H2O kR5.21= 2.7 × 107 mol−1∙dm3∙s−1 5.21

H2O2 + •eaq− → •OH + OH− kR5.22= 1.6 × 1010 mol−1∙dm3∙s−1 5.22

Since the changes in the concentrations of the molecular and radical radiolysis products are relatively slow (except at very short time scales (< ms)), a steady-state approximation can be applied to the respective rate equations for reactions 5.19 to 5.22. The concentrations of the molecular products can then be described as a function of the key radical species:

H2(a ) 2 •OH E5.1 H₂O₂ 21 •OH 22  E5.2

where represents a zeroth-order radiolytic production rate that is determined by the G-value for species x at 25 °C (Gx) multiplied by the gamma-radiation dose rate (DR) in units of Gy∙s−1

(2.1 in this work) and the unit conversion constant (CR) (1.036 × 10−7). The other rate constants, kR5.20, kR5.21, and kR5.22 are second order rate constants.

The results of the previous Chapters have supported the validity of the inverse relationships shown in Equations E5.1 and E5.2.

For the radical radiolysis products •OH and •eaq, primary radiolysis (reaction

5.19) is also the main production pathway but the main removal paths are reactions with molecular species:

For •OH:

H2O2 + •OH → •HO2 + H2O kR5.21= 2.7 × 107 mol−1∙dm3∙s−1 5.21 •O2− + •OH → O2(aq) + OH− kR5.23= 8 × 109 mol−1∙dm3∙s−1 5.23 And for •eaq−:

H2O2 + •eaq− → •OH + OH− kR5.22= 1.6 × 1010 mol−1∙dm3∙s−1 5.22

H+ + •eaq−  •H pKaof •H = 9.6 5.24

O2(aq) + •eaq− →•O2− kR5.25 = 2.22 × 1010 mol−1∙dm3∙s−1 5.25

Note that although the reaction of •OH with H2 (reaction 5.20) is an important removal path for H2, it is only important for the removal of •OH if H2 is present at a high concentration prior to the build-up of H2O2and •O2–.

When present in water, some chemical species can establish pseudo-catalytic reaction cycles with the radical species. This changes the relative contributions of different reactions to the net removal of the radical species as water radiolysis progresses. Since the molecular radiolysis product concentrations are also a function of the radical concentrations in the solution, it is difficult to predict the net effect of cyclic reactions on radiolysis at longer times based on the rates of elementary reactions alone.

The concept of cycling reactions was first introduced in this thesis in Chapter 3, where differences in the kinetics of O2 production lead to a pH effect on radiolysis behaviour. This was further explored in Chapter 4, where O2 was added as an initially dissolved solute. In initially deaerated solutions, changes in the kinetics of the •H/•eaq− equilibrium (reaction 5.24) with pH leads to a shift in the relative contributions of the •eaq− removal reactions. While the reaction of •eaq− and H+ (reaction 5.24) is very slow at pH > pKa of •H (9.6), the reaction with the secondary radiolysis product O2 (reaction 5.25) becomes an important removal path for •eaq− at these pH conditions. The product of this reaction, •O2−, can then react with •OH (reaction 5.23) to regenerate O2. Once this pseudo-catalytic cycle is established, oxygen can build up to high levels in the system and the radical species can be effectively scavenged from the aqueous phase. This results in a reduction of the removal rates of the associated molecular products, thus, increasing the observed concentrations of H2 and H2O2 (E5.1 and E5.2). Hence, solutions

containing higher concentrations of dissolved oxygen, arising from either a change in pH (Chapter 3) or initial saturation with O2 (Chapter 4), yield higher concentrations of the molecular radiolysis products H2 and H2O2 upon irradiation.

The pH effect described above is easily observed for deaerated water in Figure 5.3. At pH 6.0, the molecular products could not be measured experimentally, and the model simulations predict concentrations below the detection limits of 1×10-5mol∙dm-3 and 3×10-6mol∙dm-3 for H2 and H2O2 respectively (Figure 5.3a). The molecular concentrations remain low for pH 8.5 solution (results not shown). However, for solutions at pH 10.6 (> 9.6, the pKa of •H), the molecular product concentrations are measurable, demonstrating an increase of 2-3 orders of magnitude in comparison to the model predicted concentrations at pH 6.0 (Figure 5.3b).

The introduction of other chemical species that can react with the radical species can upset the system chemistry by providing additional removal pathways for those radicals. If the added species can be partially regenerated by back reactions such that they can establish a (semi) pseudo-catalytic cycle with the radical species before they are totally consumed, their influence on radiolysis product concentrations can be significant and last for a long time. Aqueous phase reactions of a chemical additive with the molecular water radiolysis products are typically too slow to be important. However, dissolved additives can affect the molecular product concentrations indirectly, via their interactions with radical species.

5.6.2 Radiolysis of Aqueous Solutions Initially Containing Nitrate or Nitrite