Collaborative Research: Nano- and micro-particle transport prediction in subsurface media: The role of heterogeneity and structure

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Many water quality contexts exist in which particle transport and retention in saturated sands and gravels is a critical process; e.g., streambed removal of particle-bound contaminants, low energy drinking water treatment using riverbank filtration, engineered subsurface delivery of novel nanoparticles or bacteria for contaminant cleanup, and protection of drinking water supplies from disease-causing pathogen sources. There is yet insufficient capability to predict the observed complex transport behaviors of these particles under environmental conditions. Consequently, the theory to support optimized design of the above environmental systems is lacking. Mathematical models currently can describe but not predict these behaviors because, as yet, the models do not represent the underlying mechanisms and processes for particle attachment to surfaces under environmental conditions. The proposed research aims to determine whether observed complex colloid transport behaviors will emerge from pore-scale representation of the surface heterogeneity responsible for particle attachment. The proposed investigations involve parallel experiments and simulations at pore (micromodel) and network (packed sand column) scales. The research will provide for a transformative platform for researchers and practitioners to perform mechanistic prediction of particle transport for design of solutions to environmental problems. Additional broader impacts include engagement of middle and high school biology, chemistry, and earth science teachers in six-week long summer internships where they undertake field and laboratory experiences examining the role of particles in trace element transport and transformation.The capability to predict the observed complex transport behaviors of colloids under environmental conditions (e.g., non log-linear profiles of retained colloids, extended tailing of low concentrations, blocking, and ripening) is currently lacking. Empirically based continuum-scale rate constants and scaling factors are employed in the advection-dispersion equation to describe, and to a limited extent predict, the observed complex transport behaviors. Whereas these descriptions are extremely useful indicators of mechanisms, true predictive capability will be possible only if the underlying physicochemical mechanisms/processes are identified and parameterized at a more fundamental level. Pore scale (nanoscale) colloid-surface interactions are well-demonstrated to exert profound influences on colloid transport behaviors at the continuum scale (column and field). This research aims to determine whether the continuum-scale rate constants and scaling factors can be predicted, and the whether the observed complex continuum-scale behavior will emerge, from pore-scale representation of surface heterogeneity and network-scale representation of packing structure. This investigation involves parallel experiments and simulations at pore (micromodel) and continuum (column) scales. Coupled pore scale force/torque balance simulations will be conducted to pore/grain network simulations in order to develop mechanistic prediction of continuum scale rate constants and scaling factors. New approaches will be used to represent surface heterogeneity responsible for colloid attachment to bulk repulsive surfaces at the pore scale. The proposed research will also capitalize on, and extend, recent understanding of influences of topology at the continuum (network) scale where the transition between molecular (diffusion-driven) and particle (trajectory-driven) transport behaviors will be explored.