AQUEOUS PHOTOCHEMISTRY OF 1,4-BENZOQUINONES AND THEIR POSSIBLE ROLE IN THE PHOTOCHEMISTRY OF NATURAL ORGANIC MATTER
Blough, Neil V
Past research implicated the presence of an organic species as a photochemical source of OH radical in natural waters. Quinones were postulated to be one of the possible OH sources due to 1) the ubiquitous occurence of quinonoid compounds in natural systems, and 2) prior work indicating that OH was produced in the photolysis of benzoquinones. However, more recent work indicated that photolysis of 1,4-benzoquinones does not give rise to OH, but instead proceeds through an oxidizing intermediate. To further examine these two possibilities, radical trapping experiments, electron paramagnetic resonance measurements, product analysis of the reactions of benzoic acid, and optical studies were performed for a series of quinones with differing substituents. OH was shown not to be a major product in the photolysis of methyl-1,4-benzoquinone in aqueous solution. Instead, an oxidizing intermediate arising from, but distinct from the triplet quinone was implicated in the photolysis of low concentrations of mBQ. This intermediate is believed to be a triplet quinone-H2O exciplex. In the absence of electron donors, the intermediate collapses to benzene-1,2,4-triol, ultimately to form hydroquinone and hydroxybenzoquinone. At high concentrations of mBQ, however, these products are formed through a reaction of triplet quinone with ground state quinone. A complete kinetic scheme consistent with experimental results is presented. The formation of an intermediate exciplex between triplet state quinone and water was consistent with the results obtained in the photolysis of aqueous solutions of dimethyl- and dichloro-1,4-benzoquinones. Leakage of small amounts of OH from this intermediate was observed for dichloro-1,4-benzoquinones, which are much better electron acceptors than dimethyl-1,4-benzoquinones. In the case of tetrachloro-1,4-benzoquinone, which is an extremely good electron acceptor, a substantial amount of OH was produced. The investigation of isolated natural organic matter (Suwannee River fulvic acid) revealed that either OH radical or a strong oxidant was produced upon UV irradiation. Several natural waters collected from Atlantic Ocean and Chesapeake Bay were studied. In these waters, nitrite and nitrate photolysis appears to be significant source of OH.