INFLUENCE OF RHODAMINE WT PROPERTIES ON SORPTION AND TRANSPORT IN SUBSURFACE MEDIA

Laboratory batch and column studies with two subsurface media demonstr ated that structural isomers of rhodamine WT (RWT) are responsible for the observed two-step sigmoidal breakthrough curve. A linear isotherm was observed at RWT concentrations < 100 mug/l and a Freundlich isoth erm was observed at concentrations > 100 mug/l. In column studies, the breakthrough curve for RWT was observed to exhibit a plateau at value s of C/Co around 0.43 to 0.49. Titration curves for RWT indicated a si ngle pK(a) value of 5.1. The pH effects on the breakthrough curve for RWT were evaluated and showed no evidences related to the observed two -step breakthrough curve. Sorption of RWT was observed to be much more significant at low values of pH (e.g., 3.0), and to decrease at eleva ted values of pH (e.g., 11.3); both observations are consistent with t he ionogenic nature of RWT and the surface charge of the media. Chroma tographic separation of RWT resulted in two UV spectrophotoscopically distinct peaks indicating two structural isomers. Combination of the i ndividual breakthrough curves for each of the two RWT isomers resulted in a two-step breakthrough curve similar to that observed for the ori ginal RWT solution. The distinct sorptive characteristic of the differ ent isomers is explained by the relative positions of two carboxyl gro ups (COO-) attached to the phenyl ring of RWT.