The optical properties of dissolved organic matter (DOM) are of great interest to scientists as a significant part of sunlight arriving on earth is absorbed by DOM driving photochemical reactions, but also for engineers for applications such as UV based water treatment processes. Two models currently attempt to explain the light absorption and fluorescence properties of DOM: a superposition model that assumes that DOM absorbance is the sum of the absorbance of DOM individual components, and a charge-transfer (CT) model that assumes that DOM absorbance is the sum of the individual components and a term that corresponds to CT interactions in-between DOM components. These CT interactions are assumed to arise from interactions between DOM electron donor moieties (e.g., phenols) and electron acceptor moieties (e.g., quinones).  We present here investigations on the two models using chemical modifications of DOM (ozone oxidation and sodium borohydride reduction) and computational absorbance calculations. The two chemical methods were chosen as they should have the effects of transforming electron donor into electron acceptor moieties for ozone and the opposite for borohydride, allowing us to poke into the CT interactions. Experimental results indicate that both ozone and borohydride induce an important decrease in light absorbance properties, the borohydride results were further investigated by computational calculation of DOM model compounds absorbance that successfully simulated the observed effect that can be attributed to the light absorbance properties of the reduced products absorbing less light than the parent compounds. The fluorescence presented either for both chemical modifications a decrease (autochthonous aquatic DOM) or an increase (terrestrial DOM), while for an aquatic allochthonous DOM an increase was observed for borohydride and a decrease for ozone. These results were explained by considering the transformation induced by either ozone oxidation or borohydride reduction and considering the fluorescence of the parents and products of reaction. Overall, this presentation will discuss the strengths and weaknesses of both theories and potential pathways to improve the understanding of DOM optical properties.

Chemical and computational investigations on the optical properties of dissolved organic matter

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