Nanoparticles (NP) offer special characteristics for various applications, including environmental remediation, due to their fine sizes. Their size also controls their fate and transport behaviours in the environment. Once introduced into environmental media, the initial particle size distribution (PSD) changes spatiotemporally due mainly to NP aggregation and selective removal of certain size fractions. Considering the aggregation of NP in current fate and transport models is crucial whereas it is hindered by high computational expenses, mathematical complexity, and the variable used in current population balance models of aggregation, i.e., particle number concentration being different from that of fate and transport models, i.e., mass concentration.  In this study an accurate population balance model, known as the fixed pivot technique, is combined with an accurate 3-D groundwater numerical model, MT3D-USGS, to investigate the impact of NP aggregation on their transport behaviours in groundwater environments. Special attention is devoted to investigate concurrent occurrence of aggregation and size exclusion mechanism, which has long been known as being responsible for unexpectedly high mobility of colloidal particles in subsurface environments and carrying hazardous contaminants over long distances.  Model simulations at a field-scale shows that aggregation increased NP mobility at regions close to the injection point and retains them at greater distances through alteration of their diffusivities, secondary interaction-energy minima, and settling behaviour. This results in a change of residual concentration profiles from exponential for non-aggregating dispersions to non-monotonic for aggregating dispersions. High computation expenses encountered in these simulations led us to develop a new paradigm for modelling aggregation phenomenon in aquatic environment. This model which is based on a chain-reaction expression uses mass concentration, and is comparable or better than the current population balance model in terms of accuracy and efficiency; offering the potential for being combined with various NP fate, transport, and reaction models.

Size dynamics control nanoparticle fate and transport in the environment: Toward an accurate and efficient model

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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