No abstract

Gamma-irradiation of deaerated aqueous solutions containing FeSO(4) leads to the formation of uniform-sized colloidal particles of γ-FeOOH. At short irradiation times, or in solutions with a low initial [Fe(2+)](0), spherical particles with a size less than 10 nm are formed. These primary particles grow to form a dendritic structure upon longer irradiation, and the final size of the large particles is ∼60 nm with a very narrow size distribution. Further prolonged irradiation does not change the final particle size. The narrow size distribution is attributed to rapid homogeneous radiolytic oxidation of soluble Fe(2+) to relatively insoluble Fe(3+) hydroxides [Fe(H(2)O)(6-n)(OH)(n)](3-n) leading to particle nucleation by spontaneous condensation. These primary particles then grow into γ-FeOOH particles with a dendritic structure. The final size reached at long times is regulated by the steady-state redox conditions established during long-term irradiation at the aqueous-solid interface.

Radiolysis kinetics in NO(3)(-) and NO(2)(-) solutions during γ-irradiation were studied at an absorbed dose rate of 2.1 Gy·s(-1) at room temperature. Air- or argon-saturated nitrate or nitrite solutions at pH 6.0 and 10.6 were irradiated, and the aqueous concentrations of molecular water decomposition products, H(2) and H(2)O(2), and the variation in the concentrations of NO(3)(-) and NO(2)(-) were measured as a function of irradiation time. The experimental data were compared with computer simulations using a comprehensive radiolysis kinetic model to aid in interpretation of the experimental results. The effect of nitrate and nitrite, present at concentrations below 10(-3) M, on water radiolysis processes occurs through reactions with the radical species generated by water radiolysis, (•)e(aq)(-), (•)O(2)(-), and (•)OH. The changes in H(2) and H(2)O(2) concentrations observed in the presence of nitrate and nitrite under a variety of conditions can be explained by a reduction in the radical concentrations. The kinetic analysis shows that the main loss pathway for H(2) is the reaction with (•)OH and the main loss pathways for H(2)O(2) are reactions with (•)e(aq)(-) and (•)OH. Nitrate and nitrite compete for the radicals leading to an increase in the concentrations of H(2) and H(2)O(2). Post-irradiation measurements of H(2), H(2)O(2), NO(2)(-) and NO(3)(-) concentrations can be used to calculate the radical concentrations and provide information on the redox conditions of the irradiated aqueous solutions.

Low Linear Energy Transfer (LET) radiolysis escape yields (G values) are reported for the sum (G(H) + G(e-) aq) and for G(H 2) in subcritical water up to 350 o C. The scavenger system 1-10 mM acetate/0.001M hydroxide/0.00048M N 2 O was used with simultaneous mass spectroscopic detection of H 2 and N 2 product. Temperature-dependent measurements were carried out with 2.5MeV electrons from a van de Graaff accelerator, while room temperature calibration measurements were done with a 60 Co gamma source. The concentrations and dose range were carefully chosen so that initial spur chemistry is not perturbed and the N 2 product yield corresponds to those reducing radicals that escape recombination in pure water. In comparison with a recent review recommendation of Elliot and Bartels (AECL report 153-127160-450-001, 2009), the measured reducing radical yield is seven percent smaller at room temperature but in fairly good agreement above 150 o C. The H 2 escape yield is in good agreement throughout the temperature range with several previous studies that used much larger radical scavenging rates. Previous analysis of earlier high temperature measurements of G esc (OH) is shown to be flawed, although the actual G values may be nearly correct. The methodology used in the present report greatly reduces the range of possible error and puts the high temperature escape yields for low-LET radiation on a much firmer quantitative foundation than was previously available.