EVALUATION OF RHODAMINE WT AS AN ADSORBED TRACER IN AN AGRICULTURAL SOIL

Two column breakthrough tests and a series of 88 batch-equilibrium iso therms were conducted to quantify adsorption parameters for the fluore scent dye Rhodamine WT (Acid Red 388), in contact with a Nicollet loam soil. Results from batch-equilibrium isotherms performed at a 2:1 (wa ter:soil) ratio showed that dye adsorption reached an equilibrium afte r 10 min on a shaker table. Regression equations were developed to pre dict distribution coefficients (K(d)) for Rhodamine WT (RWT) adsorptio n to the soil based on soil organic carbon and the ionic strength of t he RWT solution. A linear isotherm fitted the batch-equilibrium data f or RWT in the concentration range evaluated (25-8000 mug l-1). K(d) va lues obtained from the regression equation generated from the batch-eq uilibrium isotherms were input to a one-dimensional convection-dispers ion model. The predicted results were compared with the experimental r esults obtained from two repacked column breakthrough curves. RWT adso rption was less than predicted in the first column (in which the break through curve was terminated after 45 pore volumes), but greater than predicted for the second column (which was terminated after 420 pore v olumes). The reason for the discrepancy was attributed to the plateau- shaped breakthrough curve observed for RWT and differences in the ioni c compositions of the two RWT solutions used to conduct the two column breakthrough curves. RWT did not appear to be a reliable adsorbed tra cer if quantitative prediction is needed. Breakthrough curves for atra zine and trifluralin, two herbicides present in the second column, sho wed atrazine adsorption to be less and trifluralin absorption greater as compared with RWT, as was predicted from their respective organic c arbon partitioning coefficients (K(oc)).