Experimental determination of colloid deposition rates and collision efficiencies in natural porous media

Mobile colloids in groundwater aquifers and Soils can serve as carriers for strongly sorbing contaminants and thereby facilitate contaminant transport. Therefore mobile colloids may have to be considered in modeling the fate of strongly sorbing contaminants in subsurface environments. In this study we present a chromatographic short-pulse technique for measuring colloid deposition rate coefficients and experimental collision efficiencies in natural porous media. The method was evaluated using four different experimental systems of increasing complexity. Short pulses (equivalent to 0.002 to 0.03 pore volumes) of latex or humic-coated hematite suspensions were injected under saturated flow conditions into laboratory columns packed with glass beads, soil, or aquifer materials. Colloid breakthrough curves were measured on-line using fluorescence and UV-VIS spectrophotometers. Deposition rate coefficients 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 generally followed a first-order kinetic rate law. On the basis of experimental fast deposition rates, collision efficiencies for colloid deposition can be calculated. The results demonstrate that the short-pulse method can be used very efficiently for studying the effects of solution chemistry and flow velocity on the kinetics of colloid deposition in natural porous media. The short-pulse method has several advantages over the more traditionally used step-input experiment, and allows running several experiments on a single column without significant blocking or filter ripening effects.