The carbon flux between the earth and the atmosphere from the degradation of natural organic matter is enormous, and estimates used in modern climate models are poorly constrained due to the multitude of chemical, physical, and biological processes involved in this transformation. Natural organic matter can be degraded by direct photolysis, through the absorption of sunlight, and by indirect photolysis, through reaction with photochemically produced reactive intermediates (PPRIs), such as H2O2, the OH radical, singlet oxygen, and superoxide. Lignins and Tannins are two important classes of natural organic matter with similar polyphenolic structures, and recent studies have found that lignins photochemically degrade in aquatic ecosystems; however, the photochemistry of tannins remains largely unconstrained. My lab studied the direct and indirect photolysis of gallic acid, a tannin monomer abundant in natural waters. We isolated reactions with individual PPRI:  H2O2 and superoxide were added directly while singlet oxygen and OH required the use of an appropriate photosensitizer. Experiments were conducted in a UV photoreactor and the concentration of each proxy was measured through time via HPLC to determine the rate constant for the reaction. To understand the role of molecular structure in the degradation of gallic acid, we also examined the photochemical degradation of eight simple phenolic probe compounds. By better understanding the photochemical oxidation mechanism and product formation, we can begin to comprehend whether tannins are susceptible to biodegradation in a coupled photochemical-biochemical degradation pathway.

Investigating the role of direct and indirect photolysis in the degradation of polyphenolic tannin proxies

ACS Spring 2022

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