Many previous studies suggest that colloids enhance transport of contaminants such as radionuclides and metals in field studies and laboratory experiments. However, it is hard to predict the transport of natural colloids from the prior results because natural colloids are never uniform in their sizes and surface charges and most of the previous studies were carried out based on homogeneous particles. In this study, several batch and column experiments are being undertaken using particles for α-Fe2O3 (~100 nm) and amorphous silica (~40 nm or ~ 80 nm), which are oppositely charged under a normal pH condition (α-Fe2O3, positive; amorphous silica, negative), with a column matrix of pure quartz (negatively charged). Preliminary batch experiments suggest that particles remain suspended in the solution under a homogeneous colloidal conditions but readily precipitate when the oppositely charged particles are mixed together with due to aggregation. However, when the population of one kind of particle in the solution is several times (~ 4) higher than that of the other, particles rarely precipitate. This suggests that the particles of higher population can effectively cover the surface of the oppositely charged particles; therefore, additional increase in particle size is prevented after an initial aggregation. This phenomenon further implicates that iron oxide particles can effectively transport through the negatively charged soils when their surfaces are coated with the negatively charged silica nano-particles. Our ongoing experiments are focused on this point. When our experiments are successfully completed, the results would provide more clear insights into the colloid-facilitated transport of contaminants in the subsurface.
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