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.