Modular Polymer Biosensors by Solvent Immersion Imprint Lithography.

We recently demonstrated Solvent Immersion Imprint Lithography (SIIL), a rapid benchtop microsystem prototyping technique, including polymer functionalization, imprinting and bonding. Here, we focus on the realization of planar polymer sensors using SIIL through simple solvent immersion without imprinting. We describe SIIL's impregnation characteristics, including an inherent mechanism that not only achieves practical doping concentrations, but their unexpected 2-fold enhancement compared to the immersion solution. Subsequently, we developed and characterized optical sensors for detecting molecular O2. To this end, a substantially high dynamic range is reported, including its control through the immersion duration, a manifestation of SIIL's modularity. Overall, SIIL exhibits the potential of improving the operating characteristics of polymer sensors, while significantly accelerating their prototyping, as it requires a few seconds of processing and no need for substrates or dedicated instrumentation. These are critical for O2 sensing as probed by way of example here, as well as any polymer permeable reactant.

Demonstration of combined zero-valent iron and electrical resistance heating for in situ trichloroethene remediation.

The effectiveness of in situ treatment using zero-valent iron (ZVI) for nonaqueous phase or significant sediment-associated contaminant mass can be limited by relatively low rates of mass transfer to bring contaminants in contact with the reactive media. For a field test in a trichloroethene (TCE) source area, combining moderate-temperature subsurface electrical resistance heating with in situ ZVI treatment was shown to accelerate TCE treatment by a factor of about 4 based on organic daughter products and a factor about 8 based on chloride concentrations. A mass-discharge-based analysis was used to evaluate reaction, dissolution, and volatilization processes at ambient groundwater temperature (~10 °C) and as temperature was increased up to about 50 °C. Increased reaction and contaminant dissolution were observed with increased temperature, but vapor- or aqueous-phase migration of TCE out of the treatment zone was minimal during the test because reactions maintained low aqueous-phase TCE concentrations.

Enhanced remedial amendment delivery through fluid viscosity modifications: experiments and numerical simulations.

Low-permeability zones are typically bypassed when remedial fluids are injected into subsurface heterogeneous aquifer systems. Therefore, contaminants in the bypassed areas may not be contacted by the amendments in the remedial fluid, which may significantly prolong remediation operations. Laboratory experiments and numerical studies have been conducted to investigate the use of a shear-thinning polymer (Xanthan gum) to improve access to low-permeability zones in heterogeneous systems. The chemicals sodium mono-phosphate and the surfactant MA-80 were used as the remedial amendments. The impact of polymer concentration, fluid injection rate, and permeability contrast in the heterogeneous systems has been studied in a series of eleven two-dimensional flow cell experiments. The Subsurface Transport over Multiple Phases (STOMP) simulator was modified to include polymer-induced shear-thinning effects. The experimental and simulation results clearly show that using the polymer leads to an enhanced delivery of remedial amendments to lower-permeability zones and an increased sweeping efficiency. An added benefit of using the polymer is the stabilization of the displacing front when density differences exist between displaced and displacing fluids. The modified STOMP simulator was able to predict the experimental observed fluid displacing behavior well and might be used to predict subsurface remediation performance when a shear-thinning fluid is used to remediate a heterogeneous system at larger scales.

An experimental investigation of nitrogen gas produced during denitrification.

In situ denitrification relies on indigenous microorganisms to reduce nitrate to N(2) gas. However, when initial nitrate concentrations are large, produced gas volumes also can be very large, potentially resulting in reduced water saturation and hydraulic conductivity in the treatment zone. In this study, we investigated the fate of N(2) and other gases produced during denitrification in a laboratory flow cell containing packed sediment. Denitrifying activity was stimulated by additions of nitrate and ethanol. Microbial activity was monitored by measuring nitrate, nitrite, and ethanol concentrations; gas saturations were measured during the experiment using a gamma imaging system. Biomass was measured using phospholipid fatty acid analysis of sediment samples. Bioenergetic calculations calibrated to measured nitrate consumed and biomass produced predicted that 1.2 L N(2) gas/L water should have been produced following the addition of 100 mM nitrate. However, the maximum measured gas saturation was only 23%, indicating substantial gas loss from the sediment pack. Temporal gamma images and visual observations confirm that small gas bubbles formed in the sediment pack coalesced into larger bubbles and migrated upward through gas-filled channels to the sediment pack surface. Although gas saturations increased, there was no significant change in sediment pack hydraulic conductivity. These results suggest that in permeable reactive barriers used for in situ denitrification, gas production will not necessarily lead to unlimited gas accumulation in the pore space and that the effects of gas production on water saturation and hydraulic conductivity may be relatively minor.

Flow behavior and residual saturation formation of liquid carbon tetrachloride in unsaturated heterogeneous porous media.

The formation of residual, discontinuous nonaqueous phase liquids (NAPLs) in the vadose zone is a process that is not well understood. To obtain data that can be used to study the development of a residual NAPL saturation in the vadose zone and to test current corresponding models, detailed transient experiments were conducted in intermediate-scale columns and flow cell. The column experiments were conducted to determine residual carbon tetrachloride (CCl(4)) saturations of two sands and to evaluate the effect of CCl(4) vapors on the water distribution. In the intermediate-scale flow cell experiment, a rectangular zone of the fine-grained sand was packed in an otherwise medium-grained matrix. A limited amount of CCl(4) was injected from a small source and allowed to redistribute until a pseudo steady state situation had developed. A dual-energy gamma radiation system was used to determine fluid saturations at numerous locations. The experiments clearly demonstrated the formation of residual CCl(4) saturations in both sands. Simulations with an established multifluid flow simulator show the shortcomings of current relative permeability-saturation-capillary pressure (k-S-P) models. The results indicate that nonspreading behavior of NAPLs should be implemented in simulators to account for the formation of residual saturations.

In-situ oxidation of trichloroethene by permanganate: effects on porous medium hydraulic properties.

In-situ oxidation of dense nonaqueous-phase liquids (DNAPLs) by strong oxidants such as potassium permanganate (KMnO4) has been proposed as a possible DNAPL remediation strategy. In this study, we investigated the effects of in-situ trichloroethene (TCE) oxidation by KMnO4 on porous medium hydraulic properties. In particular, we wanted to determine the overall effects of concurrent solid phase (MnO2) precipitation, gas (CO2) evolution and TCE dissolution resulting from the oxidation reaction on the porous medium's aqueous-phase relative permeability, krw. Three TCE removal experiments were conducted in a 95-cm long, 5.1-cm i.d. glass column, which was homogeneously packed with well-characterized 30/40-mesh silica sand. TCE was emplaced in the sand-pack in residual, entrapped form through a sequence of water/TCE imbibition and drainage steps. The column was then flushed under constant aqueous flux conditions for up to 104 h with either deionized water (reference experiment), deionized water containing 5 mM KMnO4 or deionized water containing 5 mM KMnO4 and 300 mM Na2HPO4. Aqueous-phase relative permeabilities were computed from measured flow rates and measurements of aqueous-phase pressure head, h obtained using pressure transducers connected to tensiometers distributed along the column length. A dual-energy gamma radiation system was used to monitor changes in fluid saturation that occurred during each experiment. In addition, column effluent samples were collected for chemical analyses. Dissolution of TCE during deionized water flushing led to an increase in krw by approximately 22% and a local reduction in h. On the other hand, vigorous CO2 gas production and precipitation of MnO2 was visually observed during flushing with deionized water that contained 5 mM KMnO4. As a consequence, krw declined by approximately 96% and h increased locally by more than 1000 cm H2O during the first 24 h of the experiment, causing sand-pack ruptures and pump failure. Conversely, less CO2 gas production and MnO2 precipitation was visually observed during flushing with deionized water that contained 5 mM KMnO4 and 300 mM Na2HPO4. Consequently, only small increases in h (< 15 cm H2O) were observed in this experiment due to a reduction in krw of approximately 53%. While we must attribute changes in h due to variations in krw to our specific experimental design (constant aqueous flux, one-dimensional flow experiments), these experiments nevertheless confirm that successful application of in situ chemical oxidation of TCE requires consideration of detrimental processes such as MnO2 precipitation and CO2 gas formation. In addition, our results indicate that utilization of a buffered oxidant solution may improve the effectiveness of in-situ oxidation of TCE by KMnO4 in otherwise weakly buffered porous media.