NORMA HACCP (PUNTOS CRÍTICOS DE CONTROL)

For initially deaerated water at a low pH with an available headspace, hydrogen is produced by radiolysis, and it is removed by its reaction with radiolytically-produced •OH and its transfer to the gas phase:

H O H a GH mol 1 e 4.1a

H2(a ) •OH → •H H2O kR4.2= 4.2 × 107 mol−1∙dm3∙s−1 4.2

H2(aq) ⇄ H2(g) 4.3

where is the apparent rate constant for the mass transfer of hydrogen from the aqueous to the gas phase, as defined in Chapter 3. Similarly, the primary production and removal reactions for non-volatile H2O2 in deaerated water were established previously:

H O H O GH O mol 1 e _4.1b

H2O2 •OH → •HO2 + H2O kR4.4= 2.7 × 107 mol−1∙dm3∙s−1 4.4

H2O2 •eaq− → •OH OH− kR4.5= 1.6 × 1010 mol−1∙dm3∙s−1 4.5

Except at short time scales (< 1 s), the net changes in the concentrations of H2 and

H2O2 are relatively small with time, as demonstrated in Figure 4.1a. A steady-state

approximation can thus be applied using only the main reactions listed above to obtain analytical solutions for the rate equations, given in Equations E4.1 and E4.2. The analytical solutions define the concentrations for the molecular species as a function of the radical species:

H (a ) •OH E4.1 H O •OH •ea E4.2

where kR4.2, kR4.4, and kR4.5 are bimolecular rate constants, CR is a unit conversion

constant (1.036 × 10− ), Gx is the G-value for production of species x at 25 °C, and DR is the gamma-radiation dose rate in units of Gy∙s−1 (2.5 in this work). For irradiation at a constant dose rate, CRGxDR is equivalent to a zeroth-order rate constant, .

As stated in Chapter 3, the effect of interfacial transfer on [H2(aq)] is small for the

experimental conditions used, and Equation E4.1 can be reduced to:

H (a )

•OH

E4.3

These analytical equations state that the concentrations of H2(aq) and H2O2 are

inversely proportional to the concentrations of •eaq– and •OH Since the molecular

radiolysis product behaviour is strongly influenced by changes in the radical species concentrations, it is important to examine the behaviour of these key radical products as well. Primary radiolysis (reaction 4.1) remains the main production pathway for the radicals, but they can be removed through other reaction pathways in addition to those discussed above:

For •OH:

•O2− •OH → O2(aq) + OH− kR4.6= 8 × 109 mol−1∙dm3∙s−1 4.6

And for •eaq−:

•eaq− + H+  •H pKa of •H 9 6 4.7

Figure 4.1 shows that even under deaerated conditions, interaction between the molecular and radical radiolysis products leads to the generation of the secondary radiolysis product O2 that can establish a pseudo-catalytic reaction cycle with the radical

species by reactions 4.8 and 4.6. As water radiolysis progresses, the accumulation of O2

changes the relative contributions of different radical removal pathways. Reactions 4.8 and 4.6 become more important as the concentration of O2 builds up, leading to lower

concentrations of •eaq– and •OH in the system. As a result the removal rates of H2 and

H2O2 via reactions with •OH and •eaq– decrease (E4.2 and E4.3) and this leads to higher

molecular product concentrations. Hence, at longer irradiation times (> 1 s), the concentrations of the primary radical species are lower than those of the molecular products.

Also observed in Figure 4.1 is that the molecular product concentrations reached at steady state are about three orders of magnitude larger in deaerated water at pH 10.6 than in deaerated water at pH 6.0. These dramatic changes in concentration occur in the pH range 9 to 11 [17]. At pH 10.6, the solution pH is higher than the pKa of •H (9.6).

This causes reaction 4.7 (the forward reaction as shown) to be very slow. The reaction of •eaq– with the secondary radiolysis product, O2 (reaction 4.8) then becomes an important

removal path for •eaq

–_{. However, the resulting product, •O}

2, reacts with •OH to reform

O2 (reaction 4.6). Once this pseudo-catalytic cycle is established, the secondary

radiolysis product O2 can continuously remove the radicals from the aqueous phase.

Consequently, at high pH, reactions 4.8 and 4.6 become the dominant removal pathways for the radical species. This leads to a reduction of the loss rates of the molecular products, thus increasing their concentrations.

A consequence of this pH dependence is that, for pH > 9, the concentrations of O2

and H2O2 must reach higher levels before the rate of removal of •eaq– matches its rate of

radiolytic production. While O2 is generated by reaction 4.6, H2O2 can also be

radiolytically produced by reaction 4.9 in this pH region:

The accumulation of molecular products, however, is fairly slow, and attainment of steady state is delayed at high pH compared to a lower pH (Figure 4.1). Furthermore, considerable net production of O2 leads to the generation of secondary and tertiary

oxygen products, such as •O2 and •O3, and these oxygen products can significantly alter

the behaviour of the molecular radiolysis products [10,16,17]

•O− + O2(a ) → •O3− kR4.10 = 3.8 × 109 mol−1∙dm3∙s−1 4.10

H2(a ) •O3− → O2(a ) •H OH− kR4.11 = 2.5 × 105 mol−1∙dm3∙s−1 4.11

H2O2 •O3− → O2(a ) •O2− + H2O− kR4.12 = 1.6 × 106 mol−1∙dm3∙s−1 4.12

Thus, at pH 10.6, more complex relationships are required to describe the steady-state concentrations of the molecular products:

H (a ) •OH 11 •O E4.4 H O 9 •HO _•O •OH •ea 1 •O E4.5

Although the molecular and radical products are produced continuously by water radiolysis at different rates and have different removal paths, their steady-state concentrations are still inversely related.