INFLUENCE OF STRUCTURAL FEATURES ON SORPTION OF NOM-ANALOG ORGANIC-ACIDS TO GOETHITE

A systematic study of the role of structural features of NOM-analogue organic acids in influencing mineral-organic acid interactions was per formed. Goethite, a crystalline form of iron oxide important in geoche mical systems, was employed as a model mineral. Sorption experiments w ith a series of small molecular weight, aromatic organic acids were pe rformed to assess the effects of particular features of compound struc ture on sorption to goethite and to obtain insight into the kinds of s urface reactions responsible for organic acid sorption, important func tional groups of the humic acid component of NOM were examined. Experi ments were also performed with Aldrich humic acid to allow comparison between sorption behavior of simple organic acids and NOM and to deter mine suitable analogues for modeling NOM sorption. Sorption characteri stics far most organic acids were typical for anionic sorbates, with h igh sorption at low pH and decreasing sorption as pH increased. Sorpti on 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 b ehavior. The important structural characteristics governing sorption o f simple aromatic organic acids appeared to be adjacent carboxylic gro ups, adjacent phenolic groups, phenolic groups in the ortho position r elative to a carboxylic group, and overall acidity of the compound. Or ganic acid sorption also appeared to be influenced by hydrophobic inte ractions for some of the compounds. NOM sorption was best mimicked by 1,2,4,5-benzenetetracarboxylic acid (pyromellitic acid) and 2,3-dihydr oxybenzoic acid. Compounds having a combination of the structural feat ures present on these organic acids might better represent NOM sorptio n.