Characterization of the Acid-Base Properties of Selected Humic Substances and Model Compounds
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Kang, June Y
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Abstract
Natural organic matter (NOM) such as humic substances (HS) found in soil, sediment and water have diverse composition and molecular structural complexity. HS has significant influence on the heavy metals in terrestrial and aquatic environments due to its ability to bind the metal ions. Among various functional groups on the surface of HS, carboxylic acids and phenols are the most important ionizable sites present in HS. Determination of the surface density of acidic functional groups and the magnitude of pKa of carboxylic and phenolic functional groups is a first step in characterizing acidity, ion exchange capacity, and charge accumulation properties in humic substances (HS). Potentiometric titration was used to determine the fundamental acidic properties of natural HS and selected model HS to better assess the role structure plays in carboxyl and phenol group dissociation. Acid-base titration data from the HSs were fitted to the fully optimized continuous pKa spectral (FOCUS) model to determine the surface charge excess, Ka and pKa values of carboxylic and phenolic group on each selected HS. FOCUS model is an empirical model and provides the mean pKa value determined from a Gaussian model for the peak shapes. Each HS was assumed as a mixture of monoprotic acids in the model. In this study, the pKa values found to be in the range of 3.890 ± 0.018 to 4.881 ± 0.036 for carboxylic group and 8.712 ± 0.017 to 9.738 ± 0.071 for phenolic group. These results are in good agreement with literatures. In order to evaluate the influence of structural variations in carboxylic and phenolic group dissociation, simple benzofuran (C8H6O) derivatives, a common structural motif in HS were titrated as well. In this work, it was observed that pKa value increases as the carboxylic group is located close to the oxygen on furan ring. Temperature effect on carboxylic group on a model compound was also observed that pKa value went up as the temperature increased.
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Keywords
Humic substance, Fulvic acid, Benzofuran model compound, Humic acid, FOCUS model, Acid dissociation constant