Abiotic interactions of natural dissolved organic matter and carbonate aquifer rock

Abiotic interactions between natural dissolved organic matter (NDOM) and carbonate aquifer rock may be controlling factors of biogeochemical processes and contaminant fate in carbonate aquifer systems. The importance and effects of these interactions were examined using batch adsorption experiments of soil NDOM and representative carbonate sorbents from the Floridan Aquifer. Adsorption of NDOM carbon to aquifer rocks was well-described using a modified linear model and was mostly reversible. Significant adsorption was observed at higher NDOM concentrations, while the release of indigenous organic matter from the rocks occurred at lower concentrations. Longer interaction periods led to more adsorption, indicating that adsorption equilibrium was not achieved. For relatively pure carbonate rock samples, sorbent surface area was found to be the most important controlling factor of adsorption, whereas the presence of indigenous organic matter and subdominant mineral phases were more important, when they occurred. Preferential adsorption of a high over low molecular weight and humic over fulvic components of NDOM onto carbonate sorbents was detected using liquid size exclusion chromatography and excitation–emission fluorometry, respectively. The presence of NDOM inhibited mineral dissolution, though this inhibition was not proportional to NDOM concentration as surface area and mineralogy of carbonate sorbents played additional roles. Though the NDOM–carbonate rock adsorption mechanism could not be completely determined due to the heterogeneity and complexity of NDOM and sorbent surfaces, it is speculated that both rapid and weak outer-sphere bonding and stronger but slower hydrophobic interaction occur. These results have important implications for groundwater quality and hydrogeologic projects such as aquifer storage and recovery.