Transport of in Situ Mobilized Colloidal Particles in Packed Soil Columns

A systematic investigation of the transport behavior of in situ mobilized soil colloidal particles in their parent soil matrix medium is presented. Particle advection, dispersion, and deposition kinetics were studied by analysis of particle breakthrough curves as a response to short-pulse particle injections to the inlet of packed soil columns. The transport of the heterogeneous soil particles was compared to the transport 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 excluded from small pores within the soil medium. Dispersivity of the natural and latex particles was compared to that of the conservative tracer, and the results indicate that particle dispersivity is greater than the tracer dispersivity. Dispersivity of colloidal particles was shown to be essentially independent of pore water velocity, whereas tracer 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 counterions on particle deposition kinetics was also investigated by comparing the results to deposition kinetics with monovalent counterions. Particle deposition rate with Ca2+ was shown to be higher than with Na+, and the critical deposition concentrations with Na+ were greater than those with Ca2+. In contrast to the marked effect of ionic strength and divalent cations, changes in proton activity over more than 1 order 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 natural colloidal particles in the packed soil columns can be adequately described by the advection-dispersion equation with a first-order, irreversible deposition kinetics term.