Effect of Particle Aggregation on the Exchange of Kaolinite Clay Colloids Between Streams and Streambeds

Particles, such as silt and clay, can interact with each other and coagulate to form larger particles under appropriate environmental conditions. Once these particles aggregate, their deposition to the bed sediment could also be modified. These particle aggregates may sorb contaminants to their surfaces or trap contaminants within coagulated particles, which further complicate the contaminant removal mechanisms in water systems. Stream-subsurface exchange process has a significant role in the fate and transport of natural particles and contaminants due to their interactions with the bed sediments. In this study, the effects of particle aggregation on exchanging kaolinite colloids between streams and streambeds were investigated using batch and laboratory flume experiments. The concentration of the kaolinite colloids and salinity were varied. Batch experiments were used to determine the particle size changes due to aggregation process, aggregation rate, and the time to reach an equilibrium state, i.e., when the colloids no longer aggregate to form larger stable particles. The particle size distribution of the samples collected from both the flume and batch experiments was analyzed using a particle size analyzer. The concentrations of the particles sampled during the flume experiments were determined using a spectrophotometer. The results show that when the salinity of the system and the initial particle concentration were increased, the kaolinite particle aggregation also increased, which then in turn increased the stream-subsurface exchange of the kaolinite colloids. These results were interpreted using a theoretical colloid exchange model which was developed based on the self-preserving size distribution concept, the classical aggregation theory, colloid filtration, and advective pumping exchange. Results from study will contribute to thorough understanding of the particle aggregation processes under different environmental conditions and their effects on particle and contaminant transport.