Evaluation of Rhodamine WT as an adsorbed tracer in an agricultural soil

Two column breakthrough tests and a series of 88 batch-equilibrium isotherms were conducted to quantify adsorption parameters for the fluorescent dye Rhodamine WT (Acid Red 388), in contact with a Nicollet loam soil. Results from batch-equilibrium isotherms performed at a 2:1 (water:soil) ratio showed that dye adsorption reached an equilibrium after 10 min on a shaker table. Regression equations were developed to predict distribution coefficients (Kd) for Rhodamine WT (RWT) adsorption to the soil based on soil organic carbon and the ionic strength of the RWT solution. A linear isotherm fitted the batch-equilibrium data for RWT in the concentration range evaluated (25-8000 microgram 1(-1)). Kd values obtained from the regression equation generated from the batch-equilibrium isotherms were input to a one-dimensional convection dispersion model. The predicted results were compared with the experimental results obtained from two repacked column breakthrough curves. RWT adsorption was less than predicted in the first column (in which the breakthrough curve was terminated after 45 pore volumes), but greater than predicted for the second column (which was terminated after 420 pore volumes). The reason for the discrepancy was attributed to the plateau-shaped breakthrough curve observed for RWT and differences in the ionic compositions of the two RWT solutions used to conduct the two column breakthrough curves. RWT did not appear to be a reliable adsorbed tracer if quantitative prediction is needed. Breakthrough curves for atrazine and trifluralin, two herbicides present in the second column, showed atrazine adsorption to be less and trifluralin absorption greater as compared with RWT, as was predicted from their respective organic carbon partitioning coefficients (Koc).