Structure-reactivity mechanisms of organic matter sorption and photochemical transformation in aqueous environments.
Black carbon produced from vegetation fires accounts for 15% of soil organic carbon and the dissolved component makes up 10% of organic carbon transported by rivers to the oceans annually. The association of organic carbon with metal oxides and its photochemical transformation are critical factors controlling C turnover in soils and aquatic systems. However, the influence of chemical structure on photodegradation of dissolved organic matter and its sorption to metal oxides remains questionable. The original research in this dissertation is aimed towards better understanding structure-reactivity mechanisms that control organic matter sorption to metal oxides and its photodegradation in aquatic environments. The molar heat of sorption of lignin monomers onto ferrihydrite was 1.17 kJ mol-1 and occurred via an outer-sphere mechanism involving multiple points of attachment. Sorption of lignin monomers was characterized by an exchangeable fraction that was bound to surface sites and within unblocked intraparticle pores and an unexchangeable fraction that was not accessible and/or removable by simple ion exchange. Plant biomass type and its processing via charring were significant drivers of dissolved organic matter (DOM) photolability and its sorption to nano-crystalline boehmite. The photo-bleaching behavior of DOM was captured by a three-component energy-based model. The first component was resistant to photobleaching, whereas the second and third components required a mean energy flux of 42-204 kJ m-2 and 168-1540 kJ m-2, respectively. Model predictions showed that an increase in vegetative fires would decrease photodegradative contributions to DOM cycling in the conifer-dominated east Texas, increase contributions in the hardwood shrub-dominated west, but have no effect in the grass-dominated central regions. The sorption of DOM to nano-crystalline boehmite was related to the preponderance of alkyl attachments to the aromatic backbone. Photolysis of DOM reduced the complexity and total energy of sorption for DOM thereby suggesting that solar-exposed DOM will be less prone to being sorbed, sorbed strongly and hence preserved via sorptive preservation on metal oxide colloids.