On the critical salt concentrations for particle detachment in homogeneous sand and heterogeneous Hanford sediments

One of the mechanisms for sudden particle release is a decrease in groundwater salt concentration to below the critical salt concentration (CSC), where repulsion forces between fine particles and matrix surfaces exceed binding forces. In this paper, an attempt was made to determine the CSC with both batch and column experiments. Two types of sediments were tested: (a) homogeneous quartz sand and (b) mineralogically heterogeneous sediment, taken from the Hanford formation in southeast Washington. Stepwise decreasing concentrations of NaNO3 solution were applied until fine particles were released from the sediments and the CSC was determined. Two methods were used to minimize the interference of particle release due to physical forces (shear stress) in the batch experiments: (a) postexperimental correction for mechanical effects, and (b) minimization of shear stress on the sediments during the experiment. CSCs from batch experiments were compared to those obtained from column experiments. It was found that both the amount of particles released and the CSC were an order of magnitude higher for the Hanford sediment than for the Sand. Moreover, particle detachment above the CSC was observed for the Hanford sediment. This suggests that the concept of sharp CSCs could be problematic in natural heterogeneous sediments where fine particles may mobilize at salt concentrations significantly above the CSC, thus unexpectedly enhancing colloid-facilitated transport of contaminants.