Unexpected Radiation Chemistry of Borate Buffer: I. First-Order Scavenging of OH Radicals

Although boric acid has long been considered to be a nonreactive buffer under ionizing radiation conditions in nuclear reactors, here for the first time, we report the reaction of borate anions with hydroxyl radicals (^•OH). Nanosecond pulse radiolysis combined with transient absorption spectroscopy gives evidence of the product borate radical formed with transient absorption peaks near 260, 360, and 550 nm. At 358 nm, the prompt absorption has extinction coefficient in the range 460-330 M^-1 cm^-1 for temperatures up to 300 °C. Ab initio Gaussian 4 theory calculations show that H atom abstraction from borate anions by ^•OH should be thermodynamically feasible, whereas electron transfer to form a neutral borate radical is energetically unlikely. The H atom abstraction rate constant at 25 °C is 2.6 × 10⁶ M^-1 s^-1. Arrhenius behavior up to 300 °C is given by log(k_r/M^-1 s^-1) = (11.12 ± 0.05) - (26.6 ± 0.4 kJ/mol)/2.303RT. pH variation experiments show that the reaction rate of borate with either ^•OH or ^•O^- has similar activation energy, but the ^•O^- reaction is ca. 40% slower. Evidence demonstrates that at least two radical product species are being formed, one of which absorbs more strongly in the visible and is more prominent at high temperature. Ab initio calculations suggest the absorbing species could be isomeric borate dimer radicals of formula B₂O₂(OH)₄^•-, formed via dehydration of a common transient intermediate B₂O(OH)₆^•-.