TRANSPORT OF IN-SITU MOBILIZED COLLOIDAL PARTICLES IN PACKED SOIL COLUMNS

A systematic investigation of the transport behavior of in situ mobili zed soil colloidal particles in their parent soil matrix medium is pre sented. Particle advection, dispersion, and deposition kinetics were s tudied by analysis of particle breakthrough curves as a response to sh ort-pulse particle injections to the inlet of packed soil columns. The transport of the heterogeneous soil particles was compared to the tra nsport of monodisperse carboxyl latex particles to further understand the various particle transport mechanisms. Results show that colloidal particles travel much faster than a conservative tracer (nitrate) due to size exclusion effects, whereby mobile colloidal particles are exc luded from small pores within the soil medium. Dispersivity of the nat ural and latex particles was compared to that of the conservative trac er, and the results indicate that particle dispersivity is greater tha n the tracer dispersivity. Dispersivity of colloidal particles was sho wn to he essentially independent of pore water velocity, whereas trace r dispersivity increased with increasing pore water velocity due to a combination of convective and diffusive transport of tracer molecules in small pores within the soil aggregates. The effect of divalent coun terions on particle deposition kinetics was also investigated by compa ring the results to deposition kinetics with monovalent counterions. P article deposition rate with Ca2+ was shown to be higher than with Na, and the critical deposition concentrations with Na+ were greater tha n those with Ca2+. In contrast to the marked effect of ionic strength and divalent cations, changes in proton activity over more than 1 orde r of magnitude (from pH 4.0 to 5.5) did not have a significant effect on particle deposition kinetics. Quantitative analysis of the observed particle transport results demonstrates that the transport of the nat ural colloidal particles in the packed soil columns can be adequately described by the advection-dispersion equation with a first-order, irr eversible deposition kinetics term.