Impact of Temperature and Adsorptive Fractionation on Transport of DOC in Iron-Rich Porous Media.

Natural organic matter (NOM) makes up the largest terrestrial pool of carbon and is twice the size of the atmospheric carbon pool. Understanding the processes that govern NOM stabilization and degradation are crucial for accurate modeling of carbon cycling and for predicting the impacts of climate change. Sorption to mineral surfaces has been implicated as an important mechanism for long-term preservation of NOM. Iron, the fourth most abundant element in Earth’s crust, creates oxide minerals and surface coatings that act as effective sorbents of NOM. While NOM sorption to iron oxides has been extensively investigated, less is known about how changes in climate, such as an increase in temperature, will affect sorption behavior.

            In this study, we assessed the impact of temperature on the sorption and desorption behavior of Suwannee River (SR) NOM, fulvic acid (FA), and humic acid (HA); Pahokee Peat fulvic acid (PPFA) and humic acid (PPHA); and Elliott soil humic acid (ESHA) within packed columns of ferrihydrite-coated sand at neutral pH. At 25˚C, the HA solutions broke through their respective columns prior to the FA and SRNOM solutions, meaning more FA and SRNOM sorbed to the ferrihydrite coated sand. Immediately following breakthrough, the concentrations of NOM in the effluent solutions increased rapidly for several pore volumes, then gradually slowed as the NOM concentrations approached the influent solution concentrations. At 25˚C, the HAs also desorbed more rapidly and to a greater extent than the FAs and SRNOM. Some key compositional differences between the HA and FA fractions may explain these differences in behavior: SRFA contains more aliphatic carbon and carboxylic acids while SRHA and ESHA contain more carbohydrates, aromatic carbon, and amino acids. These results are consistent with numerous studies that have demonstrated the importance of acidic moieties in metal oxide sorption. In addition, both pH and OM concentration were found to influence the temperature sensitivity of NOM sorption. We are currently using Fourier transform ion cyclotron resonance mass spectrometry (FTICRMS) in order to determine the extent to which chemical fractionation of NOM is taking place at the mineral-water interface. These findings suggest that NOM sorption and transport are temperature sensitive and that adsorptive fractionation is taking place at the iron mineral-water interface.