EXPERIMENTAL-DETERMINATION OF COLLOID DEPOSITION RATES AND COLLISION EFFICIENCIES IN NATURAL POROUS-MEDIA

Mobile colloids in groundwater aquifers and Soils can serve as carrier s for strongly sorbing contaminants and thereby facilitate contaminant transport. Therefore mobile colloids may have to be considered in mod eling the fate of strongly sorbing contaminants in subsurface environm ents. In this study we present a chromatographic short-pulse technique for measuring colloid deposition rate coefficients and experimental c ollision efficiencies in natural porous media. The method was evaluate d using four different experimental systems of increasing complexity. Short pulses (equivalent to 0.002 to 0.03 pore volumes) of latex or hu mic-coated hematite suspensions were injected under saturated flow con ditions into laboratory columns packed with glass beads, soil, or aqui fer materials. Colloid breakthrough curves were measured on-line using fluorescence and UV-VIS spectrophotometers. Deposition rate coefficie nts determined with the short-pulse method were in excellent agreement with results from step-input experiments. Experiments with different flow rates and column dimensions showed that colloid deposition genera lly followed a first-order kinetic rate law. On the basis of experimen tal fast deposition rates, collision efficiencies for colloid depositi on can be calculated. The results demonstrate that the short-pulse met hod can be used very efficiently for studying the effects of solution chemistry and flow velocity on the kinetics of colloid deposition in n atural porous media. The short-pulse method has several advantages ove r the more traditionally used step-input experiment, and allows runnin g several experiments on a single column without significant blocking or filter ripening effects.