Influence of Structural Features on Sorption of NOM-Analogue Organic Acids to Goethite

Abstract

A systematic study of the role of structural features of NOM-analogue 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 features of compound structure 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. Experiments were also performed with Aldrich humic acid to allow comparison between sorption behavior of simple organic acids and NOM and to determine suitable analogues 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. Sorption of simple organic acids exhibited strong dependence on the number, type, and position of reactive functional groups, and 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. NOM sorption was best mimicked by 1,2,4,5-benzenetetracarboxylic acid (pyromellitic acid) and 2,3-dihydroxybenzoic acid. Compounds having a combination of the structural features present on these organic acids might better represent NOM sorption.