Acid–base properties of dissolved organic matter from pristine and oil-impacted marshes of Barataria Bay, Louisiana

Five samples of dissolved organic matter (DOM) were isolated from Barataria Bay, Louisiana using the coupled reverse osmosis/electrodialysis method. Two of the samples (BU) were isolated from relatively pristine waters and three samples (BC) were isolated from waters that were strongly impacted by the Deepwater Horizon oil spill in April–July, 2010. The acid–base properties of these five samples were investigated by potentiometric titration. Carboxyl contents were estimated using an empirical pH-based method, and carboxyl contents, average pKaʹs, and the range of pKa in each sample were determined by fitting two numerical models (the Gaussian distribution model and the modified Henderson–Hasselbalch model) to the titration data. The carboxyl contents of the titrated samples range from 6.0 to 6.9 meq gC− 1 and the average pKaʹs for carboxyl groups range from 4.0 to 4.6. The distribution of pKa values is somewhat broader for the BU samples than for the BC samples, but all other acid–base properties of the five samples were comparable. Phenolic hydroxyl groups could not be detected in any of the samples. These samples of coastal DOM are much less acidic than fulvic acids from soils and fresh waters.