title: Tracer-grade rhodamine WT: structure of constituent isomers and their sorption behavior.
abstract: Rhodamine WT (RWT), a fluorescent xanthene dye, is often used as a conservative tracer in aquifer characterization and as a surrogate or sorbing tracer for contaminant fate and transport. Quantitative tracing employing RWT is confounded by the presence of two major fluorescent constituents in the tracer-grade mixture. Here, we have confirmed that the two constituents are isomers of RWT, elucidated their molecular structures and their percent mass distribution in the tracer-grade mixture, and examined their individual sorption 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 phase, and (iii) surface complexation with surface-bound Al and Fe ions as compared with the para isomer. Hence, the meta isomer consistently sorbs to a higher extent onto the mineral phases examined. The para isomer has the potential to be a nearly conservative tracer, and the meta isomer has the potential to be a nonconservative tracer. To facilitate RWT use as a conservative tracer and comparison of tracer tests at different locations, we recommend modification of the RWT manufacturing process toward production of 100% of the para isomer. Alternatively, appropriately designed tests with tracer-grade RWT have the potential for simultaneous estimation of hydraulic parameters and contaminant fate and transport.
authors.affiliation: Nicholas School of the Environment, Duke University, Durham, North Carolina 27708-0328, USA. dharni@duke.edu
authors.forename: D
authors.initials: D
authors.lastname: Vasudevan
authors.forename: R L
authors.initials: RL
authors.lastname: Fimmen
authors.forename: A B
authors.initials: AB
authors.lastname: Francisco
mesh_terms: D000042:Absorption; D004396:Coloring Agents / Q000032:analysis / Q000737:chemistry*; D004563:Electrochemistry; D004784:Environmental Monitoring; D007536:Isomerism; D012235:Rhodamines / Q000032:analysis / Q000737:chemistry*; D014872:Water Movements; D014881:Water Supply
chemical_list: D004396:Coloring Agents; D012235:Rhodamines; C222011:rhodamine WT