Radiolytic degradation of 2-methylisoborneol and geosmin in water: Reactive radical species and transformation pathways

Water radiolysis can serve as a useful tool to study the degradation of organic pollutants in water. Manipulation of the radiolytic system enables the selective production of reactive species (RS) with known yields. Our aim was to explore the effects of the radiolytically produced RS on commonly occuring water taste and odor compounds (T&O), 2-methylisoborneol (MIB) and geosmin (GSM). Observed degradation rate constants differ among experimental conditions/dominant RS and follow the order: HO•>H•>>eaq−>(O2−•/HO2•), ranging from 0.002 Gy−1 (O2−•/HO2•) to 0.083 Gy−1 (HO•) for MIB and from 0.006 Gy−1 (O2−•/HO2•) to 0.068 Gy−1 (HO•) for GSM. Degradation by HO• was very efficient, requiring 1.14 and 1.49 μmoles of HO• for each degraded μmole of MIB and GSM, respectively. The oxidative degradation of MIB by HO• proceeds with the production of carbonyl- and hydroxyl-containing transformation products (TPs), leading to linear structures, while for GSM degradation proceeds with ring opening followed by formation of carboxyl-groups. Fewer TPs are produced by HO2•, while degradation with H• led to numerous TPs, via dehydroxylation, dehydration and ring opening. Degradation with eaq−, yielded demethylated and rearranged TPs with formation of double bonds.