Transport and retention of nanoparticles in porous media: Role of aggregation and hydrodynamics

With the increasing production and use of manufactured nanomaterials, increasing amounts of nanoparticles are likely to enter groundwater and soil. Understanding the transport and retention of nanoparticles through subsurface porous media is essential to evaluate and control its risks to ecosystems and human health. Current approaches using the classical colloid filtration theory applied in quantifying colloidal interaction and transport may not be applicable to nanomaterials as the mechanisms and dominant forces could differ from those of micro-colloids. Aggregation, which is a common phenomenon for nanoparticles, gives rise to considerable deposition due to pore straining. The objective of this study is to quantify the effect of aggregate and hydrodynamics on nanoparticle transport and retention. Fluorescent latex particles with the diameter of 20 nm and 500 nm were used in the experiments. Stability of 20 nm particles is evaluated using dynamic light scattering under different solution ionic strengths. Spatial and temporal characteristics of aggregation and deposition of aggregates on solid surfaces are directly observed in micro flow chamber under confocal scanning microscope. Column experiments are conducted to obtain breakthrough curves under different solution ionic strengths and flow velocities. Comparison of experimental results with 500 nm particles and nano-aggregates of similar size will be discussed in terms of shape, surface roughness and structure of the aggregates and the effects on their deposition.