EXAMINING QUINONE CONTRIBUTION TO THE OPTICAL PROPERTIES OF CHROMOPHORIC DISSOLVED ORGANIC MATTER

Abstract

Chromophoric Dissolved Organic Matter (CDOM) in natural waters is largely responsiblefor absorption of light and photochemistry in the water, impacting environmental reactions and aquatic life. The composition of CDOM is greatly varied based on source, photochemical reactions, and natural cycles. The impact of quinone moieties on this structure and photochemical and redox reactions involving CDOM remains the subject of controversy. To investigate the impact of quinone structure on optical properties, model quinone compounds were thoroughly characterized by their optical properties and reactions with sodium dithionite and sodium sulfite. A series of methyl-substituted p-benzoquinones, a methoxy p-benzoquinone, and a range of napthoquinones and anthraquinones were investigated. These model compounds were characterized according to their quinone and hydroquinone molar absorptivities and fluorescence quantum yields. Sodium dithionite reduction of quinones and the impact of structure on the products of this reaction was investigated by reducing the quinones with both sodium dithionite and sodium sulfite and comparing the optical properties of the products to those of the quinone and hydroquinone. The spectra of dithionite reduced p-benzoquinones and napthoquinones suggested the presence of products other than the hydroquinone. Sulfite is produced in solution as a result of dithionite reduction of quinones. Model quinones were therefore also reduced with sodium sulfite to investigate the impact of this side reaction on the dithionite reduction products. High performance liquid chromatography (HPLC) was used to further investigate and quantify the products of dithionite reduction of quinones and the importance of sulfite interference. Although some of the model quinones react with sulfite to form a proposed sulfonated hydroquinone product, based on the observed extent of this reaction in dithionite reductions, the structures of quinones likely to be found in CDOM, and their relatively small contribution to CDOM optical properties, the sulfite reaction was determined to not significantly impact the study of quinone moieties in CDOM. Dithionite selectively reduces quinones, while borohydride reduces ketones, aldehydes, and quinones. Therefore, in CDOM samples, dithionite can be used to isolate the effects of quinone moieties on the optical properties. Dithionite reduction was used to analyse CDOM standards and natural water extracts from the North Pacific Ocean and the Chesapeake Bay to investigate quinone contribution to their optical properties. These results are compared to borohydride reduction results from Cartisano and McDonnell to compare the contribution of quinones to that of ketones and aldehydes. (1, 2) Dithionite reduction showed small impacts on absorbance and fluorescence, whereas significant changes in both were observed for borohydride reduction. Therefore, the optical changes observed under borohydride reduction are attributed to primarily ketones and aldehydes rather than quinones. Model quinones showed significant changes in fluorescence intensity due to dithionite reduction, which are largely not observed for CDOM standards and natural water extracts, further supporting the conclusion that their role in CDOM optical properties is small.