Tracer-grade rhodamine WT: Structure of constituent isomers and their sorption behavior

Rhodamine WT (RWT), a fluorescent xanthene dye, is often used as a conserva tive tracer in aquifer characterization and as a surrogate or sorbing trace r for contaminant fate and transport. Quantitative tracing employing RWT is confounded by the presence of two major fluorescent constituents in the tr acer-grade mixture. Here, we have confirmed that the two constituents are i somers of RWT, elucidated their molecular structures and their percent mass distribution in the tracer-grade mixture, and examined their individual so rption behavior onto soil solids. The energy-minimized geometry of the meta isomer indicates that it possesses a greater potential for (i) hydrophobic exclusion from bulk solution, (ii) electrostatic attraction to the solid p hase, and (iii) surface complexation with surface-bound Al and Fe ions as c ompared with the para isomer. Hence, the meta isomer consistently sorbs to a higher extent onto the mineral phases examined. The para isomer has the p otential to be a nearly conservative tracer, and the meta isomer has the po tential to be a nonconservative tracer. To facilitate RWT use as a conserva tive tracer and comparison of tracer tests at different locations, we recom mend modification of the RWT manufacturing process toward production of 100 % of the para isomer. Alternatively, appropriately designed tests with trac er-grade RWT have the potential for simultaneous estimation of hydraulic pa rameters and contaminant fate and transport.