A Modified HYDRUS Model for Simulating PFAS Transport in the Vadose Zone

The HYDRUS unsaturated flow and transport model was modified to simulate the effects of non-linear air-water interfacial (AWI) adsorption, solution surface tension-induced flow, and variable solution viscosity on the unsaturated transport of per- and polyfluoroalkyl substances (PFAS) within the vadose zone. These modifications were made and completed between March 2019 and May 2019, and were implemented into both the one-dimensional (1D) and two-dimensional (2D) versions of HYDRUS. Herein, the model modifications are described and validated against the available literature-derived PFAS transport data (i.e., 1D experimental column transport data). The results of both 1D and 2D example simulations are presented to highlight the function and utility of the model to capture the dynamic and transient nature of the temporally and spatially variable interfacial area of the AWI (Aaw) as it changes with soil moisture content (Θw) and how it affects PFAS unsaturated transport. Specifically, the simulated examples show that while AWI adsorption of PFAS can be a significant source of retention within the vadose zone, it is not always the dominant source of retention. The contribution of solid-phase sorption can be considerable in many PFAS-contaminated vadose zones. How the selection of an appropriate Aaw(Θw) function can impact PFAS transport and how both mechanisms contribute to PFAS mass flux to an underlying groundwater source is also demonstrated. Finally, the effects of soil textural heterogeneities on PFAS unsaturated transport are demonstrated in the results of both 1D and 2D example simulations.

Quantitative study of unsaturated transport of glycerol through aquaglyceroporin that has high affinity for glycerol

Graphical AbstractABSTRACTThe structures of several aquaglyceroporins have been resolved to atomic resolution showing two or more glycerols bound inside a channel and confirming a glycerol-facilitator’s affinity for its substrate glycerol. However, the kinetics data of glycerol transport experiments all point to unsaturated transport that is characteristic of low substrate affinity in terms of the Michaelis-Menten kinetics. In this article, we present an in silico-in vitro research focused on AQP3, one of the human aquaglyceroporins that is natively expressed in the abundantly available erythrocytes. We conducted 2.1 μs in silico simulations of AQP3 embedded in a model erythrocyte membrane with intracellular-extracellular asymmetries in leaflet lipid compositions and compartment salt ions. From the equilibrium molecular dynamics (MD), we elucidated the mechanism of glycerol transport at high substrate concentrations. From the steered MD simulations, we computed the Gibbs free-energy profile throughout the AQP3 channel. From the free-energy profile, we quantified the kinetics of glycerol transport that is unsaturated due to glycerol-glycerol interaction mediated by AQP3 resulting in the concerted movement of two glycerol molecules for the transport of one glycerol molecule across the cell membrane. We conducted in vitro experiments on glycerol uptake into human erythrocytes for a wide range of substrate concentrations and various temperatures. The experimental data quantitatively validated our theoretical-computational conclusions on the unsaturated glycerol transport through AQP3 that has high affinity for glycerol.

Quantitative study of unsaturated transport of glycerol through aquaglyceroporin that has high affinity for glycerol

In addition to the single-glycerol mechanism for saturable kinetics, a second transport pathway becomes more significant at higher substrate concentrations, resulting in unsaturable transport characteristics of an aquaglyceroporin.

Saturated and unsaturated salt transport in peat from a constructed fen

The underlying processes governing solute transport in peat from an experimentally constructed fen peatland were analyzed by performing saturated and unsaturated solute breakthrough experiments using Na+ and Cl− as reactive and non-reactive solutes, respectively. We tested the performance of three solute transport models, including the classical equilibrium convection–dispersion equation (CDE), a chemical non-equilibrium one-site adsorption model (OSA) and a model to account for physical non-equilibrium, the mobile–immobile (MIM) phases. The selection was motivated by the fact that the applicability of the MIM in peat soils finds a wide consensus. However, results from inverse modeling and a robust statistical evaluation of this peat provide evidence that the measured breakthrough of the conservative tracer, Cl−, could be simulated well using the CDE. Furthermore, the very high Damköhler number (which approaches infinity) suggests instantaneous equilibration between the mobile and immobile phases underscoring the redundancy of the MIM approach for this particular peat. Scanning electron microscope images of the peat show the typical multi-pore size distribution structures have been homogenized sufficiently by decomposition, such that physical non-equilibrium solute transport no longer governs the transport process. This result is corroborated by the fact the soil hydraulic properties were adequately described using a unimodal van Genuchten–Mualem model between saturation and a pressure head of ∼-1000 cm of water. Hence, MIM was not the most suitable choice, and the long tailing of the Na+ breakthrough curve was caused by chemical non-equilibrium. Successful description was possible using the OSA model. To test our results for the unsaturated case, we conducted an unsaturated steady-state evaporation experiment to drive Na+ and Cl− transport. Using the parameterized transport models from the saturated experiments, we could numerically simulate the unsaturated transport using Hydrus-1-D. The simulation showed a good prediction of observed values, confirming the suitability of the parameters for use in a slightly unsaturated transport simulation. The findings improve the understanding of solute redistribution in the constructed fen and imply that MIM should not be automatically assumed for solute transport in peat but rather should be evidence based.

Saturated and unsaturated salt transport in peat from a constructed fen

To determine the underlying processes of solute transport in peat an experimental constructed fen peatland, soil hydraulic properties were measured and saturated and unsaturated solute breakthrough experiments were performed using Na+ and Cl− as reactive and non-reactive solutes, respectively. We tested the performanceof three solute transport models, including the classical equilibrium Convection-Dispersion Equation (CDE), a chemical non equilibrium one-site adsorption model (OSA) and a model to account for physical non-equilibrium, the mobile-immobile phases (MIM). The selection was motivated by the fact that the applicability of the MIM in peat soils finds a wide consensus. However, results from inverse modelling and a robust statistical evaluation of this peat provide evidence that the measured breakthrough of the conservative tracer, Cl− could be simulated well using the CDE. This is demonstrated by a very high Damköhler number (→infinity) suggesting instantaneous equilibration between the mobile and immobile phases; this underscores the redundancy of the MIM approach for this particular peat. Scanning electron microscope images of the peat show the typical multi-pore size distributions structure have been homogenised sufficiently by decomposition, such that physical non-equilibrium solute transport no longer governs the transport process. This is corroborated by the fact the soil hydraulic properties were adequately described using a unimodal van Genuchten-Mualem model between saturation and a pressure head of ~ −1000 cm of water. Hence, MIM is not the most suitable choice, and the long tailing of the Na+ breakthrough curve is caused by chemical non-equilibrium. Successful description was possible using the OSA model. To test our results for the unsaturated case, we conducted an unsaturated steady state evaporation experiment to drive Na+ and Cl− transport. Using the parameterised transport models from the saturated experiments, we could numerically simulate the unsaturated transport using Hydrus-1D. The simulation showed a good prediction of observed values, confirming the suitability of the parameters for use in a slightly unsaturated transport simulation. The findings improve the understanding of solute redistribution in the constructed fen.