Further Insights on the Chemical Structure of Humic Substances (HS) and Chromophoric Dissolved Organic Matter (CDOM) in Relation to their Optical/Chemical Properties

HS have been studied extensively over the last decades, yet the structural basis of their optical properties is still highly debated. Aromatic ketones, aldehydes and quinones along with carboxylic groups and phenolic moieties are significant constituents of HS, however their contribution to the optical properties has only recently been investigated. Chemical manipulation of selected functional groups thus represents an extremely promising approach to highlight the contribution of such groups to the HS (and CDOM) optical properties. Chemical reduction (and re-oxidation) along with pH titrations are employed herein to assess the relative contribution of aromatic ketones/aldehydes/quinones and carboxylic groups/phenolic moieties, respectively to the optical properties of HS (and CDOM). Results indicate that (a) the contribution of quinones to HS absorption and fluorescence is minor (or nil), while that of aromatic ketones (and aldehydes) is significant; (b) phenolic groups contribute more than carboxylic acids to the HS optical properties; (c) the effects of borohydride reduction and pH on the long-wavelength absorption and fluorescence is consistent with charge-transfer interactions between carbonyl and phenolic groups (as well as aromatic carboxylic acids, but to a smaller extent). Results will be presented within the context of our proposed charge-transfer model.