Radiation Chemistry - Water Chemistry

Water Chemistry

Both the harmful effects of radiation upon biological systems (induction of cancer and acute radiation injuries) and the useful effects of radiotherapy involve the radiation chemistry of water. The vast majority of biological molecules are present in an aqueous medium; when water is exposed to radiation, the water absorbs energy, and as a result forms chemically reactive species that can interact with dissolved substances (solutes). Water is ionized to form a solvated electron and H₂O+, the H₂O+ cation can react with water to form a hydrated proton (H₃O+) and a hydroxyl radical (HO.). Furthermore, the solvated electron can recombine with the H₂O+ cation to form an excited state of the water, this excited state then decomposes to species such as hydroxyl radicals (HO.), hydrogen atoms (H.) and oxygen atoms (O.). Finally, the solvated electron can react with solutes such as solvated protons or oxygen molecules to form respectively hydrogen atoms and dioxygen radical anions. The fact that oxygen changes the radiation chemistry might be one reason why oxygenated tissues are more sensitive to irradiation than the deoxygenated tissue at the centre of a tumor. The free radicals, such as the hydroxyl radical, chemically modify biomolecules such as DNA, leading to damage such as breaks in the DNA strands. Some substances can protect again radiation-induced damage by reacting with the reactive species generated by the irradiation of the water.

It is important to note that the reactive species generated by the radiation can take part in following reactions, this is similar to the idea of the non-electrochemical reactions which follow the electrochemical event which is observed in cyclic voltammetry when a non-reversible event occurs. For example the SF₅ radical formed by the reaction of solvated electrons and SF₆ undergo further reactions which lead to the formation of hydrogen fluoride and sulfuric acid.

In water the dimerisation reaction of hydroxyl radicals can form hydrogen peroxide, in saline systems the reaction of the hydroxyl radicals with chloride anions form hypochlorite anions.

It has been suggested that the action of radiation upon underground water is responsible for the formation of hydrogen which was converted by bacteria into methane.. A series of papers on the subject of bacteria living under the surface of the earth which are fed by the hydrogen generated by the radiolysis of water can be read on line.