Effects of matrix heterogeneity and aqueous humic acid on transport and deposition of mineral colloids in sandy sediments

Cellulosic materials, such as paper, cardboard and wood, are commonly co-disposed with low-level radioactive waste. Cellulosic materials may not only enhance subsurface contaminant transport through complexation, but also promote colloid dispersion. The objective of this study was to evaluate the effect of humic acid (HA; as a surrogate for cellulosic materials and natural organic matters) and porous media heterogeneity on mineral colloid migration through a coarse-textured Savannah River Site (SRS) sediment or a more uniform Ottawa sand. Positively charged mineral colloids isolated from the SRS sediment were retained by the Ottawa sand columns. SRS colloids became increasingly more negatively charged and more stable in suspension with increasing concentrations of HA treatments. There was a critical HA and mineral colloid concentration ratio of ∼0.025 above which the transport of colloids was greatly increased. No quartz or SRS mineral colloids, even those coated with HA, were transported through the natural SRS sediment due to its heterogeneous surface coatings of goethite/kaolinite and surface charge properties. Additionally, much fewer mobile colloids were transported through columns of Ottawa sand with pre-deposited mobile colloids (resembling an aged sediment/mobile colloid system) than the clean Ottawa sand system. As such, even when conditions favored colloid dispersion (e.g., elevated pH or HA concentrations), subsequent transport may be limited due to the sediment's natural colloid removal processes caused by coherent particle-sediment attraction and straining.