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ABSTRACT: This study demonstrates that the pattern assembly and attachment strength of colloids in an evaporating sessile droplet resting on a smooth substrate can be controlled by adding non-ionic solutes (surfactant) to the solution. As expected, increasing the surfactant concentration leads to a decrease in initial surface tension of the drop, σ(0). For the range of initial surface tensions investigated (39-72 mN m(-1)), three distinct deposition patterns were produced: amorphous stains (σ(0) = 63-72 mN m(-1)), coffee-ring stains (σ(0) = 45-53 mN m(-1)), and concentric rings (σ(0) = 39-45 mN m(-1)). A flow-displacement system was used to measure the attachment strength of the dried colloids. Characteristic drying regimes associated with the three unique pattern formations are attributed to abrupt transitions of contact line dynamics during evaporation. The first transition from slipping- to pinned-contact line was found to be a direct result of the competition between mechanical instability of the droplet and the friction generated by pinned colloids at the contact line. The second transition from pinned- to recurrent stick-rip-slip-contact line was caused by repeated liquid film rupturing from evaporation-intensified surfactant concentration. Data from flow-displacement tests indicate that attachment strength of dried particles is strongest for amorphous stains (lowest surfactant concentration) and weakest for concentric rings (highest surfactant concentration). The mechanism behind these observations was ascribed to the formation and adsorption of micelles onto colloid and substrate surfaces as the droplet solution evaporates. The range of attachment forces observed between the colloids and the solid substrate were well captured by extended-DLVO interactions accounting for van der Waals attraction, electric double layer repulsion and micelle-protrusion repulsion. Both empirical and theoretical results suggest that an increasingly dense layer of adsorbed micellar-protrusions on colloid and substrate surfaces acts as a physical barrier that hinders strong van der Waals attractive interactions at close proximity. Thereby, colloid stains dried at higher surfactant concentrations are more easily detached from the substrate when dislodging forces are applied than stains dried at lower surfactant concentrations.
Langmuir 01/2013; · 4.19 Impact Factor
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ABSTRACT: Colloid retention mechanisms in partially saturated porous media are currently being researched with an array of visualization techniques. These visualization techniques have refined our understanding of colloid movement and retention at the pore scale beyond what can be obtained from breakthrough experiments. One of the remaining questions is what mechanisms are responsible for colloid immobilization at the triple point where air, water, and soil grain meet. The objective of this study was to investigate how colloids are transported to the air-water-solid (AWS) contact line in an open triangular microchannel, and then retained as a function of meniscus contact angle with the wall and solution ionic strength. Colloid flow path, meniscus shape and meniscus-wall contact angle, and colloid retention at the AWS contact line were visualized and quantified with a confocal microscope. Experimental results demonstrated that colloid retention at the AWS contact line was significant when the meniscus-wall contact angle was less than 16°, but was minimal for the meniscus-wall contact angles exceeding 20°. Tracking of individual colloids and computational hydrodynamic simulation both revealed that for small contact angles (e.g., 12.5°), counter flow and flow vortices formed near the AWS contact line, but not for large contact angles (e.g., 28°). This counter flow helped deliver the colloids to the wall surface just below the contact line. In accordance with DLVO and hydrodynamic torque calculations, colloid movement may be stopped when the colloid reached the secondary minimum at the wall near the contact line. However, contradictory to the prediction of the torque analysis, colloid retention at the AWS contact line decreased with increasing ionic strength for contact angles of 10-20°, indicating that the air-water interface was involved through both counter flow and capillary force. We hypothesized that capillary force pushed the colloid through the primary energy barrier to the primary minimum to become immobilized, when small fluctuations in water level stretched the meniscus over the colloid. For large meniscus-wall contact angles counter flow was not observed, resulting in less colloid retention, because a smaller number of colloids were transported to the contact line.
Water Research 10/2011; 46(2):295-306. · 4.86 Impact Factor
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ABSTRACT: Although numerous studies have been conducted to discern colloid transport and stability processes, the mechanistic understanding of how dissolved organic matter (DOM) affects colloid fate in unsaturated soils (i.e., the vadose zone) remains unclear. This study aims to bridge the gap between the physicochemical responses of colloid complexes and porous media interfaces to solution chemistry, and the effect these changes have on colloid transport and fate. Measurements of adsorbed layer thickness, density, and charge of DOM-colloid complexes and transport experiments with tandem internal process visualization were conducted for key constituents of DOM, humic (HA) and fulvic acids (FA), at acidic, neutral and basic pH and two CaCl(2) concentrations. Polymeric characteristics reveal that, of the two tested DOM constituents, only HA electrosterically stabilizes colloids. This stabilization is highly dependent on solution pH which controls DOM polymer adsorption affinity, and on the presence of Ca(+2) which promotes charge neutralization and inter-particle bridging. Transport experiments indicate that HA improved colloid transport significantly, while FA only marginally affected transport despite having a large effect on particle charge. A transport model with deposition and pore-exclusion parameters fit experimental breakthrough curves well. Trends in deposition coefficients are correlated to the changes in colloid surface potential for bare colloids, but must include adsorbed layer thickness and density for sterically stabilized colloids. Additionally, internal process observations with bright field microscopy reveal that, under optimal conditions for retention, experiments with FA or no DOM promoted colloid retention at solid-water interfaces, while experiments with HA enhanced colloid retention at air-water interfaces, presumably due to partitioning of HA at the air-water interface and/or increased hydrophobic characteristics of HA-colloid complexes.
Water Research 02/2011; 45(4):1691-701. · 4.86 Impact Factor
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ABSTRACT: Silage bunker runoff can be a very polluting substance and is increasingly being treated by vegetative treatment areas (VTAs), but little information exists regarding nutrient removal performance of systems receiving this wastewater. Nutrient transport through the shallow subsurface of three VTAs (i.e. one VTA at Farm WNY and two VTAs at Farm CNY) in glaciated soils containing a restrictive layer (i.e., fragipan) was assessed using a mass balance approach. At Farm WNY, the mass removal of ammonium was 63%, nitrate was 0%, and soluble reactive phosphorus (SRP) was 39%. At Farm CNY, the mass removal of ammonium was 79% in the West VTA, but nitrate and SRP increased by 200% and 533%, respectively. Mass removal of ammonium was 67% in the East VTA at Farm CNY; nitrate removal was 86% and SRP removal was 88%. The East VTA received a much higher nutrient loading, which was attributed to a malfunctioning low-flow collection apparatus within the settling basin. Results demonstrate that nutrient reduction mechanisms other than vegetative uptake can be significant within VTAs. Even though increases in nitrate mass were observed, concentrations in 1.65m deep wells indicated that groundwater impairment from leaching of nitrate was not likely. These results offer one of the first evaluations of VTAs treating silage bunker runoff, and highlight the importance of capturing concentrated low flows in VTA systems.
Journal of Environmental Management 10/2010; 92(3):587-95. · 3.24 Impact Factor
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ABSTRACT: Biochar land application can potentially be used for carbon sequestration, improving soil quality, and reducing non-point source pollution. Understanding biochar mobility is important because its transport in soil greatly influences its stability, the dynamics of soil microbial communities and organic matter, and the movement of biochar-associated contaminants. Here, the transport of biochar particles was studied in saturated and unsaturated sand columns by measuring breakthroughs of biochar pulse under three pH and two ionic strength (IS) levels. Breakthrough curves (BTCs) were fitted to a convection–dispersion model with kinetic and equilibrium deposition sites to estimate the key transport parameters (e.g. biochar deposition rate coefficients). Biochar retention was enhanced by lowering pH and increasing IS, corroborating the trends of fitted deposition rate coefficients. Under both saturated and unsaturated conditions, effluent mass recoveries decreased, respectively, by a factor of 6·6 or 15 when pH decreased from 10 to 4 at 10 mM IS, and by a factor of 1·4 or 3·9 when IS increased from 10 to 100 mM at pH 7. Biochar retention was greater in unsaturated media, implying that saturated flow elutes more biochar particles. The particles larger than 5·4% of median grain diameter were filtered out of suspension during passage through the media; whereas, the retention of smaller particles was clearly dependent on solution chemistry. Similar to other types of colloids, this study highlights the importance of pH, IS, particle size, and soil water saturation in controlling biochar movement by soil matrix flow. Copyright © 2010 John Wiley & Sons, Ltd.
Ecohydrology 09/2010; 3(4):497 - 508. · 2.13 Impact Factor
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ABSTRACT: Vegetative treatment areas (VTAs) are commonly being used as an alternative method of agricultural process wastewater treatment. However, it is also apparent that to completely prevent discharge of pollutants to the surrounding environment, settling of particulates and bound constituents from overland flow through VTAs is not sufficient. For effective remediation of dissolved agricultural pollutants, VTAs must infiltrate incoming wastewater. A simple water balance model for predicting VTA soil saturation and surface discharge in landscapes characterized by sloping terrain and a shallow restrictive layer is presented and discussed. The model accounts for the cumulative effect of successive rainfall events and wastewater input on soil moisture status and depth to water table. Nash-Sutcliffe efficiencies ranged from 0.65 to 0.81 for modeled and observed water table elevations after calibration of saturated hydraulic conductivity. Precipitation data from relatively low, average, and high annual rainfall years were used with soil, site, and contributing area data from an example VTA for simulations and comparisons. Model sensitivity to VTA width and contributing area (i.e. barnyard, feedlot, silage bunker, etc.) curve number was also investigated. Results of this analysis indicate that VTAs should be located on steeper slopes with deeper, more-permeable soils, which effectively lowers the shallow water table. In sloping landscapes (>2%), this model provides practitioners an easy-to-use VTA design and/or risk assessment tool that is more hydrological process-based than current methods.
Journal of Environmental Management 08/2010; 91(8):1794-801. · 3.24 Impact Factor
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ABSTRACT: Colloids play an important role in facilitating transport of adsorbed contaminants in soils. Recent studies showed that under saturated conditions colloid retention was a function of its concentration. It is unknown if this is the case under unsaturated conditions. In this study, the effect of colloid concentration on colloid retention was investigated in unsaturated columns by increasing concentrations of colloid influents with varying ionic strength. Colloid retention was observed in situ by bright field microscopy and quantified by measuring colloid breakthrough curves. In our unsaturated experiments, greater input concentrations resulted in increased colloid retention at ionic strength above 0.1 mM, but not in deionized water (i.e., 0 mM ionic strength). Bright field microscope images showed that colloid retention mainly occurred at the solid-water interface and wedge-shaped air-water-solid interfaces, whereas the retention at the grain-grain contacts was minor. Some colloids at the air-water-solid interfaces were rotating and oscillating and thus trapped. Computational hydrodynamic simulation confirmed that the wedge-shaped air-water-solid interface could form a "hydrodynamic trap" by retaining colloids in its low velocity vortices. Direct visualization also revealed that colloids once retained acted as new retention sites for other suspended colloids at ionic strength greater than 0.1 mM and thereby could explain the greater retention with increased input concentrations. Derjaguin-Landau-Verwey-Overbeek (DLVO) energy calculations support this concept. Finally, the results of unsaturated experiments were in agreement with limited saturated experiments under otherwise the same conditions.
Environmental Science and Technology 07/2010; 44(13):4965-72. · 5.23 Impact Factor
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ABSTRACT: The phosphorus (P) sorption isotherm experiment is a widely used tool in environmental applications for assessing soil's vulnerability to P loss to runoff or drainage. The sorbed legacy P (S0) (i.e., the P retained in soils from previous P applications) participates in sorption processes but cannot readily be determined in a sorption experiment. Thus, it is important to accurately estimate S0 for P-enriched soils (e.g., the soils that heavily receive fertilizer, manure, farm wastewater, or sewage sludge). Two curve-fitting procedures (i.e., one-step method and two-step method) with Langmuir models have been used to estimate S0 and other sorption parameters, including the P sorption maxima (Smax), the bonding energy constant (k), and the zero-sorption equilibrium concentration (EPC0). This study evaluated these two methods on 16 samples of Langford, Volusia, and Mardin channery silt loam soils at surface (0-8 cm) and subsurface (61-91 cm) in New York. The results indicate that the two methods agreed well in estimating P sorption maxima, and the estimates of k were close. The S0 estimates by the two methods had a good agreement for surface soils but a poor agreement for subsurface soils, which may be of little concern because of small S0 of subsurface soils. Although the one-step method yielded greater EPC0 estimates, the EPC0 estimates by the two methods had an excellent linear correlation for P-enriched surface soils, suggesting that both methods could work equally if only the relative magnitudes of EPC0 among soils are needed. Overall, both methods are acceptable to fit the Langmuir isotherms.
Soil Science 09/2009; 174(10):523-530. · 1.14 Impact Factor
