Capture of colloidal fine suspended particle by aquatic vegetation under rainfall.

The capture of colloidal fine suspended particles by vegetation plays an important role in water quality of the shallow aquatic system under rainfall. Quantifying impact of rainfall intensity and vegetation condition on this process remains poorly characterized. In this study, the colloidal particle capture rates under three rainfall intensities, four vegetation densities and with submerged or emergent vegetation were investigated in different travel distance in a laboratory flume. Considering vegetation as porous media, non-Darcy's law with rainfall as a source term, was coupled with colloid first-order deposition model, to simulate the particle concentration changes with time, determining the particle deposition rate coefficient (kd), representing capture rate. We found that the kd increased linearly with rainfall intensity; but increased and then decreased with vegetation density, suggesting the existence of optimum vegetation density. The kd of submerged vegetation is slightly higher than emergent vegetation. The single collector efficiency (η) showed the same trend as kd, suggesting colloid filtration theory well explained the impact of rainfall intensity and vegetation condition. Flow hydrodynamic enhanced the kd trend, e.g., the theoretical strongest flow eddy structure represented in the optimum vegetation density. This study is helpful for the design of wetland under rainfall, to remove colloidal suspended particles and the hazardous material, for the protection of the downstream water quality.

Colloidal transport and deposition through dense vegetation.

The effectiveness of submerged synthetic aquatic vegetation on removal of colloids from flowing water was investigated to explore retention of particulate nonpoint source pollutants in aquatic systems. In colloid transport experiments, the deposition rate coefficient of colloids in dense vegetation is often taken as spatially constant. This assumption was tested by experiments and modeling aimed at quantifying changes in colloid retention with travel distance in submerged synthetic aquatic vegetation. Experiments were performed in a 10-m long, 0.6-m wide flume with a 5-cm water depth under different fluid velocities, initial colloid concentrations, and solution pH values. A model accounting for advection, dispersion and first-order kinetic deposition described the experimental data. The colloid deposition rate coefficient showed a power-law decrease with travel distance, and reached a steady state value before the end of the flume. Measured changes in colloid properties with transport distance (ζ potential and size) could not explain the observed decrease. While gravity was shown to contribute to the decrease, its impact was too weak to explain the decreasing power law trend, suggesting that processes operating in granular media to produce similar outcomes may also apply to submerged vegetation.

Quantifying colloid fate and transport through dense vegetation and soil systems using a particle-plugging tempered fractional-derivative model.

Colloid contaminants are widely distributed in surface runoff from crop land and can be effectively removed by vegetative filter strips (VFS), whose quantification however proves difficult. Standard mechanism-based models contain many unknown parameters with intrinsic uncertainty, limiting their applicability and potential extension for other environmental conditions and colloid contaminant types. To remedy this limitation and capture the complex dynamics of colloids through the soil-vegetation system, this study proposes a parsimonious, particle-plugging tempered fractional advection-dispersion eq. (P-TFADE) with a few empirical parameters, which is built upon the promising fractional calculus engine. The P-TFADE model extends the promising tempered fractional derivative model by incorporating a plugging term, which is then proved to be able to capture both the plugging dynamics and tailing behavior of colloids under various hydrologic and geochemical conditions. Applications also show that the two critical parameters in the P-TFADE model, the time index (α) and plugging coefficient (Kp), can efficiently characterize the impact of the flowrate and ionic condition on transport of different sized colloids observed in our laboratory. In addition, the vegetation type determines the overall structure of the soil-vegetation system, whose impact on the colloid removal efficiency can be quantified by adding a parameter λ in the physical model. Therefore, the novel P-TFADE model can reduce the model uncertainty and help us further understand the nature of colloid dynamics through dense vegetation and soil systems.

Concurrent aggregation and transport of graphene oxide in saturated porous media: Roles of temperature, cation type, and electrolyte concentration.

Simultaneous aggregation and retention of nanoparticles can occur during their transport in porous media. In this work, the concurrent aggregation and transport of GO in saturated porous media were investigated under the conditions of different combinations of temperature, cation type (valence), and electrolyte concentration. Increasing temperature (6-24 °C) at a relatively high electrolyte concentration (i.e., 50 mM for Na+, 1 mM for Ca2+, 1.75 mM for Mg2+, and 0.03 and 0.05 mM for Al3+) resulted in enhanced GO retention in the porous media. For instance, when the temperature increased from 6 to 24 °C, GO recovery rate decreased from 31.08% to 6.53% for 0.03 mM Al3+ and from 27.11% to 0 for 0.05 mM Al3+. At the same temperature, increasing cation valence and electrolyte concentration also promoted GO retention. Although GO aggregation occurred in the electrolytes during the transport, the deposition mechanisms of GO retention in the media depended on cation type (valence). For 50 mM Na+, surface deposition via secondary minima was the dominant GO retention mechanism. For multivalent cation electrolytes, GO aggregation was rapid and thus other mechanisms such as physical straining and sedimentation also played important roles in controlling GO retention in the media. After passing through the columns, the GO particles in the effluents showed better stability with lower initial aggregation rates. This was probably because less stable GO particles with lower surface charge densities in the porewater were filtered by the porous media, resulting in more stable GO particle with higher surface charge densities in the effluents. An advection-dispersion-reaction model was applied to simulate GO breakthrough curves and the simulations matched all the experimental data well.

Effects of temperature on graphene oxide deposition and transport in saturated porous media.

Laboratory batch sorption and sand column experiments were conducted to examine the effects of temperature (6 and 24°C) on the retention and transport of GO in water-saturated porous media with different combination of solution ionic strength (IS, 1 and 10mM), sand type (natural and acid-cleaned), and grain size (coarse and fine). Although results from batch sorption experiment showed that temperature affected the sorption of GO onto the sand grains at the low IS, the interactions between GO and the sand were relatively weak, which did make the temperature effect prominent. When the IS was 1mM, experimental temperature showed little effect on GO retention and transport regardless of the medium properties. GO was highly mobile in the sand columns with mass recovery rates ranged from 77.3% to 92.4%. When the IS increased to 10mM, temperature showed notable effects on GO retention and transport in saturated porous media. For all the combinations of sand type and grain size, the higher the temperature was, the less mobile GO particles were. The effects of temperature on GO retention and transport in saturated porous media were further verified though simulations from an advection-dispersion-reaction model.