Radiolytic degradation of 4-hydroxybenzoate in aerated and deoxygenated aqueous solutions

The radiolytic degradation of 4-hydroxybenzoate (4-HBA^-) in aerated, oxygen-free and N₂O-saturated aqueous solutions at concentrations of 0.10 and 0.25 mmol/dm³ were gamma irradiated at different doses in a source of Co-60. The results show that ·OH adds predominantly to the 3 position of the aromatic ring, and elimination of the acid group leads to the degradation of 4-HBA^-. With an N₂O-saturated 0.10 mmol/dm³ 4-HBA^- solution, total degradation occurred at 1.6 kGy, and with a 0.25 mmol/dm³ solution, total degradation occurred at 3.5 kGy. In the aerated and oxygen-free 0.25 mmol/dm³ 4-HBA^- solutions, the behavior was similar, degradation occurring at a dose of 13.1 kGy. At the concentration of 0.10 mmol/dm³, total degradation occurred at 7.0 kGy, with small amounts of radiolytic products and byproducts. We propose a mechanism for the degradation of 4-HBA^- caused by water radicals produced in the three environments, leading to formation of the identified stable products. Oxidation was followed by chemical oxygen demand (COD), which decreased as the 4-HBA^- concentration increased. The kinetics showed a pseudo-first-order behavior. The rate constant of degradation was similar for the solutions with and without oxygen.