Sorption of NOM-analog organic acids to goethite

A systematic study of the role of structural features of NOM-analog organic acids in influencing mineral-organic acid interactions was performed. Goethite, a crystalline form of iron oxide important in geochemical systems, was employed as a model mineral. Sorption experiments with a series of small molecular weight, aromatic organic acids were performed to assess the effects of particular structural features on sorption to goethite and to obtain insight into the kinds of surface reactions responsible for organic acid sorption. Important functional groups of the humic acid component of NOM were examined, both singularly and in various combinations. Experiments were also performed using Aldrich humic acid to allow comparison between sorption behavior of simple organic acids and NOM, and to determine suitable analogs for modeling NOM sorption. Sorption characteristics for most organic acids were typical for anionic sorbates, with high sorption at low pH and decreasing sorption as pH increased. Compounds having particular structural features in common exhibited similar sorption behavior. The important structural characteristics governing sorption of simple aromatic organic acids appeared to be: adjacent carboxylic groups, adjacent phenolic groups, phenolic groups in the ortho position relative to a carboxylic group, and overall acidity of the compound. Organic acid sorption also appeared to be influenced by hydrophobic interactions for some of the compounds.; Surface complexation modeling was performed using the Generalized Two Layer Model. Here, the ability to describe the observed sorption behavior for compounds with similar structural features using surface complexation modeling was investigated. A set of surface reactions and equilibrium constants yielding optimal data fits was obtained for each organic acid. Surface complexation modeling successfully described sorption of a number of the simple organic acids, but an additional hydrophobic component was needed to describe sorption behavior of some compounds with significant hydrophobic character. Comparison of optimized equilibrium constants for similar surface species showed that model results were consistent with this observed sorption behavior.; NOM sorption was best mimicked by 1,2,4,5-tetracarboxylic acid (pyromellitic acid) and 2,3-dihydroxybenzoic acid. Model results indicated that humic acid sorption could be described with surface complexation modeling, and that a simple organic acid having multiple carboxylic and phenolic functional groups might better represent NOM sorption.