Hydroxyl radical-induced photochemical formation of dicarboxylic acids from unsaturated fatty acid (oleic acid) in aqueous solution

Abstract

In this study, we assess under laboratory controlled conditions the direct and hydroxyl radical ( OH)-induced photochemical production of low molecular weight (LMW) dicarboxylic acids and related compounds (C 2–C 9) (DCAs) from oleic acid ( cis-9-octadecenoic, Δ9C 18) in aqueous solution. Nitrate (NO 3 −)-amended and unamended oleate solutions were irradiated under ultraviolet-B radiation (UV-B, 313 nm) for 5 h, with NO 3 − being the source of OH. The OH-induced photochemical production of DCAs (C 2di–C 9di) (170 ± 26 nM h −1) was much higher than that induced by the direct effect of UV-B (33 ± 22 nM h −1), accounting for approximately 85% of the total (direct + OH-induced) photochemical production of DCAs (C 2di–C 9di) (198 ± 15 nM h −1). Azelaic acid (C 9di) was the dominant photoproduct (comprising 63 and 44% of DCAs in the direct and OH-induced photochemical production, respectively) followed by C 8di, C 7di and C 6di, whereas shorter chain compounds (C 2di–C 5di) were minor produced species. Using our estimate of OH photoproduction (P-OH in nM h −1), the production of C 9di from 50 μM of oleic acid was evaluated at ∼45 nM (nM OH) −1.