Christophe J. G. Darnault

Aggregation and transport of nano-TiO2 in saturated porous media: effects of pH, surfactants and flow velocity.

Transport of manufactured nano-TiO(2) in saturated porous media was investigated as a function of morphology characteristics, pH of solutions, flow velocity, and the presence of anionic and non-ionic surfactants in different concentrations. Surfactants enhanced the transport of nano-TiO(2) in saturated porous media while a pH approaching the point of zero charge of nano-TiO(2) limited their transport. The deposition process, a retention mechanism of nano-TiO(2) in saturated porous media was impacted by surfactant and pH. In Dispersion 1 systems (pH 7), the size of the nano-TiO(2) aggregates was directly related to the presence of surfactants. The presence of non-ionic surfactant (Triton X-100) induced a size reduction of nano-TiO(2) aggregates that was dependent on the critical micelle concentration. In Dispersion 2 systems (pH 9), the stability provided by the pH had a significant effect on the size of nano-TiO(2) aggregates; the addition of surfactants did impact the size of the nano-TiO(2) aggregates but in less significance as compared to Dispersion 1 systems. The electrostatic and steric repulsion forces in connection with the size of nano-TiO(2) aggregates and flow velocity impacted the single-collector efficiency and attachment efficiency which dictated the maximum transport distance of nano-TiO(2) for the Dispersion 1 and Dispersion 2 systems. By doubling the flow velocity at pH 9, the No Surfactant, 50% CMC Triton X-100, 100% CMC Triton X-100 and 100% CMC SDBS dispersion systems allowed nano-TiO(2) to attain maximum transport distances of 0.898, 2.17, 2.29 and 1.12 m, respectively. Secondary energy minima played a critical role in the deposition mechanisms of nano-TiO(2). Nano-TiO(2) deposited in the secondary energy wells may be released because of changes in solution chemistry. The deposition of nano-TiO(2) in primary and secondary energy minima, the reversibility of their deposition should be characterized to analyze the transport of nanoparticles in porous media. This is necessary to assess the risk of nanoparticles to the environment and public health.

Visualization by light transmission of oil and water contents in transient two-phase flow fields

Abstract The difficulty of determining transient fluid contents in a soil–oil–water system is hampering an understanding of the systems flow characteristics. In this paper, we describe a light transmission method (LTM) which can rapidly obtain oil and water contents throughout a large two-dimensional flow field of silica sand. By appropriately coloring the water with 0.005% FD&C blue #1, the hue of the transmitted light is found to be directly related to the water content within the porous media. The hue provides a high resolution measurement of the water and oil contents in transient flow fields (such as unstable flow). Evaluation of the reliability of LTM was assessed by checking the mass balance for a known water injection and its utility in visualizing a whole flow field was exemplified for unstable fingered flow by comparing fluid contents to those obtained with synchrotron X-ray radiation.

Lateral expansion of preferential flow paths in sands

The stability and persistence of preferential flow paths in sands can determine the flow paths of subsequent infiltration events. We have measured the evolution of preferential flow paths in a slab of sandy soil using an array of tensiometers and light transmission. The pressure and water content measurements show that the nonuniform moisture content exists even when the potentials are equalized horizontally and that then are the result of hysteresis in the soils pressure-saturation relationship. The equalization of potential takes place over several days, if at all, and is consistent, initially, with estimates of vapor transport out of the finger cores. Once the soil is wet enough, the remainder of water movement takes place in liquid films. Hysteresis produces another interesting situation when the pack is drained. We find that the wetter portions of the soil can be at a lower potential than the drier portions, resulting in a horizontal driving force for a flow of water from the drier to the wetter soil.

Preferential Flow and Transport of Cryptosporidium parvum Oocysts through the Vadose Zone: Experiments and Modeling

in the form of 4- to 6-m-long ovoid-shaped oocysts, with a double wall that is resistant to most oxidation As a result of Cryptosporidium parvum in drinking water, several processes such as ozonation and chlorination (Current, outbreaks of cryptosporidiosis have occurred in the last 10 yr. Al1986; Atwill et al., 1997). though it is generally believed that movement of pathogens through the soil is minimal, recent research has shown that appreciable num- During the past two decades, the presence of C. parbers of C. parvum oocysts may be transported via preferential or vum in surface- and groundwaters in the United States fingered flow to groundwater. The objective of the present research and Great Britain (Galbraith et al., 1987; Rose et al., was to further investigate and model the transport of oocysts through 1991; Craun et al., 1998) has been associated with several preferential flow paths in the vadose zone under a “worst-case” sce- major outbreaks of cryptosporidiosis (Hayes et al., 1989; nario. This was studied by adding calves feces containing C. parvum MacKenzie et al., 1994). Among the different pathways oocysts with a Cl tracer to undisturbed silt loam columns and disfor the transport of oocysts to drinking water sources, turbed sand columns during a simulated steady-state rain. The sand columns exhibited preferential flow in the form of fingers whereas downward percolation is usually considered to be insigmacropore flow occurred in the undisturbed cores. In the columns nificant, because soils are generally assumed to be an with fingered flow, oocysts and Cl were transported rapidly with the effective filter for a wide range of pathogens. Studies same velocity through the columns. Although only 14 to 86% of the of packed columns with saturated flow by Brush et al. amount applied, the number of oocysts transported across the columns (1999) and Harter et al. (2000) and undisturbed columns was several orders of magnitude above an infective dose. The macwith unsaturated flow (Mawdsley et al., 1996), however, ropore columns had only a very limited breakthrough of oocysts, showed that C. parvum oocysts could be transported which appeared several pore volumes after the Cl broke through initially. A simulation model for the transport of oocysts via preferen- rapidly downward through the soil. Although transport tial flow was developed on the basis of an existing preferential flow of C. parvum oocysts in saturated flow has been studied model for nonadsorbing solutes, with addition of a first-order sink experimentally and described mathematically (Brush et term for adsorbance of the C. parvum to the air–water–solid (AWS) al., 1999; Harter et al., 2000), detailed observations of interfaces, and with velocity and dispersivity parameters derived from the transport and persistence of C. parvum oocysts in Cl transport. The breakthrough of C. parvum oocysts could be de- unsaturated soils with preferential flow are still lacking, scribed realistically for the sand columns. However, the model could particularly in the presence of preferential flow pronot describe oocyst transport in the columns with macropores. cesses.

Measurement of fluid contents by light transmission in transient three-phase oil-water-air systems in sand

Most three-phase flow models lack rigorous validation because very few methods exist that can measure transient fluid contents of the order of seconds of whole flow fields. The objective of this study was to develop a method by which fluid content can be measured rapidly in three-phase systems. The method uses the hue and intensity of light transmitted through a slab chamber to measure fluid contents. The water is colored blue with CuSO4. The light transmitted by high-frequency light bulbs is recorded with a color video camera in red, green, and blue and then converted to hue, saturation, and intensity. Calibration of hue and intensity with water, oil, and air is made using cells filled with different combinations of the three fluids. The results show that hue and water content are uniquely related over a large range of fluid contents. Total liquid content is a function of both hue and light intensity. The air content is obtained by subtracting the liquid content from the porosity. The method was tested with static and transient experiments. Measurements made with the light transmission method (LTM) and synchrotron X rays of the static experiment agreed well. In the transient experiments, fingers were formed by dripping water on the surface in a two-dimensional slab chamber with partially oil-saturated sand. The LTM is able to capture the spatial resolution of the fluid contents and can provide new insights in rapidly changing, three-phase flow systems.

Deposition and release kinetics of nano-TiO2 in saturated porous media: effects of solution ionic strength and surfactants.

The aggregation, transport and deposition kinetics (i.e. attachment and release) of TiO(2) nanoparticles (nano-TiO(2)) were investigated as a function of ionic strength and the presence of anionic (sodium dodecylbenzene sulfonate, SDBS) and non-ionic (Triton X-100) surfactants in 100% critical micelle concentration (CMC). The electrolyte concentration of the suspensions dictated the kinetic stability of nano-TiO(2) thus influencing the transport and retention of the nanoaggregates in the saturated porous medium. With increasing ionic strength, the interaction between approaching nano-TiO(2) and nano-TiO(2) already deposited onto collectors surfaces seemed to be more favorable than the interaction between approaching nano-TiO(2) and bare collectors surfaces. The abrupt and gradual reduction in electrolyte concentration during the flushing cycles of the column experiments induced the release of previously deposited nano-TiO(2) suggesting attachment of nano-TiO(2) through secondary energy minimum.

Coupled effects of solution chemistry and hydrodynamics on the mobility and transport of quantum dot nanomaterials in the vadose zone

To investigate the coupled effects of solution chemistry and hydrodynamics on the mobility of quantum dot (QD) nanoparticles in the vadose zone, laboratory scale transport experiments involving single and/or sequential infiltrations of QDs in unsaturated and saturated porous media, and computations of total interaction and capillary potential energies were performed. As ionic strength increased, QD retention in the unsaturated porous media increased; however, this retention was significantly suppressed in the presence of a non-ionic surfactant in the infiltration suspensions as indicated by surfactant enhanced transport of QDs. In the vadose zone, the non-ionic surfactant limited the formation of QD aggregates, enhanced QD mobility and transport, and lowered the solution surface tension, which resulted in a decrease in capillary forces that not only led to a reduction in the removal of QDs, but also impacted the vadose zone flow processes. When chemical transport conditions were favorable (ionic strength of 5 × 10(-4)M and 5 × 10(-3)M, or ionic strengths of 5 × 10(-2)M and 0.5M with surfactant), the dominating phenomena controlling the mobility and transport of QDs in the vadose zone were meso-scale processes, where infiltration by preferential flow results in the rapid transport of QDs. When chemical transport conditions were unfavorable (ionic strength of 5 × 10(-2)M and 0.5M) the dominating phenomena controlling the mobility and transport of QDs in the vadose zone were pore-scale processes governed by gas-water interfaces (GWI) that impact the mobility of QDs. The addition of surfactant enhanced the transport of QDs both in favorable and unfavorable chemical transport conditions. The mobility and retention of QDs was controlled by interaction and capillary forces, with the latter being the most influential. GWI were found to be the dominant mechanism and site for QD removal compared with solid-water interfaces (SWI) and pore straining. Additionally, ripening phenomena were demonstrated to enhance QDs removal or retention in porous media and to be attenuated by the presence of surfactant.

Dual-energy synchrotron X ray measurements of rapid soil density and water content changes in swelling soils during infiltration

Understanding soil swelling is hampered by the difficulty of simultaneously measuring water content and bulk density. A number of studies have used dual-energy gamma rays to investigate soil swelling. The long counting time of this technique makes it impracticable for studying the rapid changes in moisture content and soil swelling shortly after infiltration is initiated. In this paper, we use the dual-energy synchrotron X ray to measure, for the first time, the water content and bulk density changes during the fast, initial phase of the swelling process. Ponded infiltration experiments were performed with two soils: a bentonite-sand mixture and a vertisol. Swelling curves and hydraulic diffusivity were determined. Deformation was very rapid immediately after water application and then became progressively slower. The hydraulic diffusivity decreased with time, which can partially explain the very rapid decrease in infiltration rates observed in the field.

Quantification of Cryptosporidium parvum in natural soil matrices and soil solutions using qPCR

Traditional microscopy methods for the detection and quantification of Cryptosporidium parvum in soil matrices are time-consuming, labor-intensive, and lack sensitivity and specificity. This research focused on developing a qPCR protocol for the sensitive and specific detection and quantification of C. parvum in natural soil matrices and soil-water extracts. The physico-chemical parameters - lysis media, number of thermal shocks and thawing temperatures - controlling DNA extraction efficiency were investigated. Experimental results identified oocyst age as a critical parameter affecting oocyst disruption and quantification. The most efficient oocyst disruption method for C. parvum oocysts regardless of their age was established as 5 thermal shocks with thawing at 65°C in Tris-EDTA (TE) buffer. In addition to the purification columns used to remove PCR inhibitors present in environmental matrices, a combination of 3mM MgCl(2) and 600ng/μl BSA yielded the highest amplicon yield for both young and aged oocysts. Sucrose flotation was determined to be a better oocyst isolation method than two-phase flotation. The optimized parameters for DNA extraction and the qPCR assay resulted in very specific and sensitive detection of C. parvum. Minimum detection limits were 0.667 for young C. parvum oocysts and 6.67 for aged C. parvum oocysts per PCR reaction. The accuracy of the detections and quantifications was 0.999. Protocol performance was tested in contrasting soil samples and soil-water extract samples on the basis of percentage of recovery (PR) values. Depending on the number of oocysts used to inoculate the samples, the average PR values ranged from 7.2 to 43.5%, 29.3-52.5%, and 11.5-60.8% for Trenton, Greenson, and Sparta soil-water extracts, respectively, and 12.1-77% for DI water. PR values ranged from 4.3% to 107.8% for Trenton, Greenson and Sparta soil samples.

Mixing and transport

In order to truly understand the physics of environmental flow, one must understand the mixing and transport process. A better understanding of mixing and transport phenomena will in turn be crucial to predict the fate and transport of contaminants in environmental flows. The present review is intended to constitute and discuss the key points of the mixing and transport papers published in 2007. Many papers reviewed herein are grouped into general mixing and fluid dynamics, some active areas related to environmental fate and transport, riverine, lacustrine, estuarine and coastal systems and then into subfields within these categories.