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

Laboratory batch and column studies with two subsurface media demonstrated 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 isotherm was observed at concentrations > 100 mug/l. In column studies, the breakthrough curve for RWT was observed to exhibit a plateau at values of C/Co around 0.43 to 0.49. Titration curves for RWT indicated a single 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 elevated values of pH (e.g., 11.3); both observations are consistent with the ionogenic nature of RWT and the surface charge of the media. Chromatographic separation of RWT resulted in two UV spectrophotoscopically distinct peaks indicating two structural isomers. Combination of the individual breakthrough curves for each of the two RWT isomers resulted in a two-step breakthrough curve similar to that observed for the original RWT solution. The distinct sorptive characteristic of the different isomers is explained by the relative positions of two carboxyl groups (COO-) attached to the phenyl ring of RWT.