CALORIMETRIC STUDY OF THE INFLUENCE OF VEGETATIVE FIRES ON THE ADSORPTION CHARACTERISTICS OF DOM

Adsorption of dissolved organic matter by iron oxide phases is critical to the long-term storage and preservation of organic carbon in soils and sediments. Previous studies have shown that iron oxide phases have a high affinity for carboxylated aromatic compounds. However, the adsorption mechanism remains unclear. We conducted flow-adsorption microcalorimetry experiments to investigate the adsorption mechanism of water-extractable organic matter (WEOM) from (un)charred plant feedstock onto ferrihydrite and how it is affected by the highly energetic process of pyrolysis (feedstock pyrolyzed at 450 oC) that occurs during wildfires. Adsorption of grass leachates was exothermic and more energetic for uncharred relative to charred feedstock. Charring of grass material decreased the total energy of adsorption from 267 mJ to 162 mJ and doubled the adsorption time from 25 to 50 min. Contrastingly, adsorption of WEOM from wood chars was endothermic with total energy of 1271 mJ over 50 min. Concurrent fluorescence spectral mapping of unsorbed WEOM revealed sorption-induced changes to a humic-like and protein-like component peak that were consistent with a shift in the mechanism of adsorption due to charring of feedstock. For example, contrary to the progressive increase in peak fluorescence intensity for uncharred grass leachates, fluorescence intensity of charred leachates, reached a maximum at around 30 min and decreased thereafter suggesting a change in the chemical composition of the adsorbed fluorophores. Additionally, three components were revealed from the deconvolution of adsorption thermograms for leachates from charred plant material. The third component had the least energy—for wood (140 mJ) and grass (17 mJ)—and was 2-4 times lower than the first component. Results point to the progression of discrete bonding environments of decreasing bond-formation energy with loading. This presentation will focus on charring-induced changes to adsorption enthalpy of WEOM onto ferrihydrite, the associated reaction mechanisms and implications for biogeochemical cycling of mineral-associated organic matter in terrestrial and aquatic environments.