# Lagrangian Modeling of Marine Microplastics Fate and Transport: The State of the Science
## 6 Conclusions and Future Research Directions
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \begin{tabular}{c} **Particle-Tracking** \\ **Model** \\ \end{tabular} & \begin{tabular}{c} **Open Access** \\ **(Free to Use)** \\ \end{tabular} & \begin{tabular}{c} **Application (Particle** \\ **Tracking of)** \\ \end{tabular} & \begin{tabular}{c} **Shape and Type of** \\ **the Simulated** \\ **Particles** \\ \end{tabular} & \begin{tabular}{c} **Main Mechanism Can** \\ **Be Simulated** \\ \end{tabular} & \begin{tabular}{c} **Initial** \\ **Year of** \\ **Release*** \\ \end{tabular} & **Ref.** \\ \hline \multirow{3}{*}{D-WAQ PART} & \multirow{3}{*}{No} & \begin{tabular}{c} Microplastics, Sediments \\ (first-order \\ decaying substances) \\ \end{tabular} & Different shapes & \begin{tabular}{c} Advection (and \\ windage), Diffusion, \\ Beaching and \\ washing-off, Shinking \\ \end{tabular} & \multirow{3}{*}{2019} & \multirow{3}{*}{[75,77]} \\  & & & & & \\ \hline Ichthyop & Yes & Microplastics & Prolate spheroids & Advection, Diffusion, \\  & & & & & \\ \hline \multirow{3}{*}{TrackMPD} & \multirow{3}{*}{Yes} & \multirow{3}{*}{Microplastics} & \
\begin{tabular}{c} Advection (and \\ windage), Diffusion, \\ Beaching and \\ washing-off, Shinking, \\ Biofouling, Degradation \\ \end{tabular} & \multirow{3}{*}{2008} & \multirow{3}{*}{[76,97]} \\  & & & & & \\ \hline \multirow{3}{*}{CaMPSim-3D} & \multirow{3}{*}{Yes} & \multirow{3}{*}{Microplastics} &
\begin{tabular}{c} Advection (and \\ windage), Diffusion, \\ Beaching and \\ washing-off, Shinking, \\ Biofouling, Degradation \\ \end{tabular} & \multirow{3}{*}{2019} & \multirow{3}{*}{[61]} \\  & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Summary of the main information of the reviewed models. induced drift as an additional term of advection. In addition, all models are capable of meeting horizontal and vertical diffusion (dispersion). All reviewed models use spatio-temporally varying diffusion coefficients. D-WAQ PART, TrackMPD, and CaMPSim-3D can predict beaching and washing-off using some probability-based relationships. * D-WAQ PART, Ichthyop, and TrackMPD can read the extracted hydrodynamic data and simulate particle behavior and particle trajectories only on a structured mesh, while CaMPSim-3D reads hydrodynamic data from an unstructured mesh system. Moreover, the hydrodynamic data are only interpolated linearly at each time step and prepared as inputs for Ichthyop. D-WAQ PART and TrackMPD utilize the spatio-temporal interpolation of the current characteristics, while CaMPSim-3D can use only spatially interpolated data. Given the aforesaid descriptions, there are some physical transport and transformation processes that some of the studied models are unable to simulate or exhibit less accuracy for in modeling. In fact, in order to understand the impacts of microplastics on the sustainability of water resources comprehensively, with higher levels of accuracy, we need to fully understand their behavior, which can be elucidated by using a versatile and accurate modeling system. None of the reviewed models can simulate homo- and hetero-aggregation or agglomeration. None of the four models can predict photodegradation or chemical and biological degradation as important processes in the fate of plastic particles. Wave-induced drift as an additional advection is observed only in CaMPSim-3D. It is suggested that further studies be conducted in order to expand the current knowledge of microplastics' fate and transport. There are some important processes that need to be further studied with regard to the behavior of microplastics in the real condition (e.g., hetero- and homo-aggregation, agglomeration, weathering, photodegradation, chemical and biological degradation, defouling, resuspension, Stokes drift, and remobilization). More in-depth studies, as well as more accurate numerical models in capturing the microplastics' behavior, along with significant modifications in the numerical simulation structure of the reviewed models, are recommended for the future. Conceptualization, A.M. and A.P.