title: The role of aggregation in the fate and transport of nanoparticles in the environment: building a robust continuum modelling platform
Creators: name: Babakhani, P
Publisher: name: University of Liverpool Repository
Year: 2019

description: Nanoparticles (NP) can be released into the environment either purposefully e.g., for groundwater remediation of hazardous contaminants such as radionuclide or inadvertently from various sources such as consumer products. Crucially, models are required that can predict the fate and transport of NP in the environment. Continuum models have been pioneering in describing most mechanisms for NP transport in porous media. This study aims at addressing challenges that hinder the efficient application of such models. These can be broken down into six main areas, diversity in basis, assumptions made, ability to predict, transport mechanisms included, the efficiency of models, and finally not taking all natural environmental conditions into account. Among transport mechanisms, aggregation phenomenon is the most important one while it has less been considered within continuum models. In this study, extensive literature data analysis has been conducted using artificial neural network (ANN), Monte Carlo, and a sensitivity analysis technique to develop empirical correlations for predicting five unknown continuum model parameters based on 20 experimental factors. Comprehensive experiments and modelling were performed on the aggregation of NP in quiescent aqueous environments to find the best modelling approaches describing NP aggregation and removal. The selected model set was combined with continuum models of NP transport in porous media to predict the transport and aggregation of NP at environmentally-relevant scales, i.e., aquifer. This, however, revealed a need for a more efficient aggregation model. Further work was committed to proposing an efficient mass-concentration-based aggregation model and to investigating the impacts of system dynamics on aggregation. The results suggest that ANN can be a useful way of developing empirical correlations for predicting continuum model parameters, and many insights were obtained from its sensitivity analyses. For instance, porous media heterogeneity, which was considered as a parameter for the first time, showed sensitivities higher than those of dispersivity. Investigations in aqueous media showed that considering both early and late stages of aggregation and sedimentation in the model yielded highest sedimentation rate at an intermediate ionic strength rather than the highest ionic strength. A mass concentration-based chain-reaction model, which was more efficient than the standard population balance model with a comparable or better accuracy in describing NP aggregation was proposed for future combinations with continuum models. System dynamics can enhance aggregation rate and lead to relatively compact aggregates over different stages of aggregation, sedimentation, and resuspension. Although several challenges for using continuum models were mitigated in this study, incorporating aggregation, which is the most important mechanism that triggers other transport mechanisms and controls the final fate of NP in the environment, remains an issue due to complications identified in this study. This issue is mainly brought about because of dynamics of porous media. Nevertheless, continuum models are currently pioneering among various modelling approaches for simulating the fate and transport of NP in subsurface environments.