Thomas W. Wietsma

Zero-valent iron for the in situ remediation of selected metals in groundwater

Abstract Zero-valent iron (Fe0), metallic iron, is being evaluated as a permeable reactive barrier material to mitigate the transport of a wide array of highly mobile contaminants in groundwater. Zero-valent iron has previously been shown to destroy effectively numerous chlorinated hydrocarbon compounds via reductive dehalogenation. No references could be found regarding the ability of zero-valent iron to reduce UO2+2, MoO2−4, or TcO−4. A series of kinetic-batch studies was conducted to determine the capability of particulate Fe0 to remove UO2+2, MoO2−4, TcO−4, and CrO2−4 from groundwater. Particulate Fe0 effectively removed each of these contaminants from solution; removal rates decreased as follows: CrO2−4 > TcO−4 > UO2+2 ⪢ MoO2−2. The removal mechanism appears to be reductive precipitation. Thermodynamic equilibrium calculations indicated that the rate of removal of the metals from solution increased as the difference in pe (Δpe) increased between the redox half reaction for the redox couple of interest and the Fe 0 Fe 2+ couple. Furthermore, the pe value for a redox couple provided a qualitative indication of the reduction rate by Fe0. These results indicate that the rate of removal of CrO2−4, TcO−4, and UO2+2 from groundwater is rapid, permitting an inexpensive barrier of practical dimensions to be used for in situ remediation purposes.

Interaction of Hydrophobic Organic Compounds with Mineral-Bound Humic Substances

The sorption of hydrophobic organic compounds (HOC) on mineral-associated peat humic acid (PHA) was evaluated under different pH and electrolyte regimes. Relative size distribution measurements indicated that PHA was [open quotes]coiled[close quotes] in solution at high ionic strength (I) and elongated at low I. The sorption of PHA to hematite and kaolinite varied with I and electrolyte cation, suggesting that the configuration of the humic acid in solution influenced its structure on the mineral surface. The sorption maxima for PHA on kaolinite indicated that PHA occupies twice the mineral surface area at low I (0.005) as that observed at high I (0.1). HOC sorption to mineral-bound PHA in Na[sup +] electrolyte was greater at lower I, indicating that humate structure was a plausible determinant of HOC sorption. Freundlich isotherms of dibenzothiophene on the PHA-coated kaolinite did not display unit slope, regardless of pH, I, or cation. Carbazole and anthracene displayed competitive behavior for sorption onto PHA-coated kaolinite. Collectively, the experimental observations indicate that hydrophobic adsorption rather than phase partitioning was the dominant mode of HOC binding. 70 refs., 8 figs., 1 tab.

The influence of physical heterogeneity on microbial degradation and distribution in porous media

Intermediate-scale experiments (meter-long, two-dimensional flow cell) were performed with aerobic biodegradation of benzoate substrate in physically heterogeneous (bimodal inclusive) media. Clastic heterogeneities were represented in a quasi-two-dimensional field, with low-conductivity inclusions embedded in a high-conductivity sandy matrix. The two media had similar pore-scale dispersivities but the conductivity ratio (∼1∶50) incurred macrodispersive spreading in the longitudinal direction. The high-conductivity sand was uniformly inoculated with Pseudomonas cepacia sp., and a pulse input of substrate and chloride ion tracer were evaluated. Degradation and growth were oxygen-limited under nonlinear dual-Monod kinetics and controlled by spatial and temporal variations in nutrient flux. The low-conductivity inclusions created regions of slow transport that prolonged the dual availability of both oxygen and substrate, which in turn enhanced microbial growth in these regions. Bacterial detachment was significant, and the fivefold increase in biomass due to growth was entirely accounted for in the aqueous effluent which displayed a complicated nonlinear breakthrough curve. High-resolution deterministic modeling was applied to simulate the intermediate-scale experiment, with parameters of the relevant constitutive relations calibrated independently through batch and small-scale column experiments. Parameter fitting to match flow cell data was avoided. This approach was taken in order both to test the predictive modeling capability as it would necessarily be used in a field application and to avoid the a priori assumption that all relevant processes were adequately represented in the respective constitutive theories. Analyses of the fit between the independently calibrated model and the flow cell data were then used to isolate processes for further experimental study. This iterative experimental/modeling approach identified processes that contributed (surprisingly) to biodegradation in heterogeneous media and yet are not currently incorporated in most mathematical models: (1) buoyancy effects associated with very small solution density variations, amplified in heterogeneous media, and (2) dynamic biological processes associated with growth, namely, endogenous respiration, cell division partitioning to the aqueous phase, and active adhesion/detachment that are strongly coupled to the transport of dissolved nutrients or microorganisms.

Heterogeneous Chemistry of Individual Mineral Dust Particles with Nitric Acid. A Combined CCSEM/EDX, ESEM AND ICP-MS Study

The heterogeneous chemistry of individual dust particles from four authentic dust samples with gas-phase nitric acid was investigated in this study. Morphology and compositional changes of individual particles as they react with nitric acid were observed using conventional scanning electron microscopy with energy dispersive analysis of X-rays (SEM/EDX) and computer controlled SEM/EDX. Environmental Scanning Electron Microscopy (ESEM) was utilized to investigate the hygroscopic behavior of mineral dust particles reacted with nitric acid. Differences in the reactivity of mineral dust particles from these four different dust source regions with nitric acid were observed. Mineral dust from source regions containing high levels of calcium, namely China loess dust and Saudi coastal dust, were found to react to the greatest extent.

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 mediums 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.

Pore-scale study of transverse mixing induced CaCO3 precipitation and permeability reduction in a model subsurface sedimentary system

A microfluidic pore structure etched into a silicon wafer was used as a two-dimensional model subsurface sedimentary system (i.e., micromodel) to study mineral precipitation and permeability reduction relevant to groundwater remediation and geological carbon sequestration. Solutions containing CaCl(2) and Na(2)CO(3) at four different saturation states (Ω = [Ca(2+)][CO(3)(2-)]/K(spCaCO(3))) were introduced through two separate inlets, and they mixed by diffusion transverse to the main flow direction along the center of the micromodel resulting in CaCO(3) precipitation. Precipitation rates increased and the total amount of precipitates decreased with increasing saturation state, and only vaterite and calcite crystals were formed (no aragonite). The relative amount of vaterite increased from 80% at the lowest saturation state (Ω(v) = 2.8 for vaterite) to 95% at the highest saturation state (Ω(v) = 4.5). Fluorescent tracer tests conducted before and after CaCO(3) precipitation indicate that pore spaces were occluded by CaCO(3) precipitates along the transverse mixing zone, thus substantially reducing porosity and permeability, and potentially limiting transformation from vaterite to the more stable calcite. The results suggest that mineral precipitation along plume margins can decrease both reactant mixing during groundwater remediation, and injection and storage efficiency during CO(2) sequestration.

Liquid CO2 displacement of water in a dual-permeability pore network micromodel.

Permeability contrasts exist in multilayer geological formations under consideration for carbon sequestration. To improve our understanding of heterogeneous pore-scale displacements, liquid CO(2) (LCO(2))-water displacement was evaluated in a pore network micromodel with two distinct permeability zones. Due to the low viscosity ratio (logM = -1.1), unstable displacement occurred at all injection rates over 2 orders of magnitude. LCO(2) displaced water only in the high permeability zone at low injection rates with the mechanism shifting from capillary fingering to viscous fingering with increasing flow rate. At high injection rates, LCO(2) displaced water in the low permeability zone with capillary fingering as the dominant mechanism. LCO(2) saturation (S(LCO2)) as a function of injection rate was quantified using fluorescent microscopy. In all experiments, more than 50% of LCO(2) resided in the active flowpaths, and this fraction increased as displacement transitioned from capillary to viscous fingering. A continuum-scale two-phase flow model with independently determined fluid and hydraulic parameters was used to predict S(LCO2) in the dual-permeability field. Agreement with the micromodel experiments was obtained for low injection rates. However, the numerical model does not account for the unstable viscous fingering processes observed experimentally at higher rates and hence overestimated S(LCO2).

Experimental study of crossover from capillary to viscous fingering for supercritical CO2-water displacement in a homogeneous pore network.

Carbon sequestration in saline aquifers involves displacing brine from the pore space by supercritical CO(2) (scCO(2)). The displacement process is considered unstable due to the unfavorable viscosity ratio between the invading scCO(2) and the resident brine. The mechanisms that affect scCO(2)-water displacement under reservoir conditions (41 °C, 9 MPa) were investigated in a homogeneous micromodel. A large range of injection rates, expressed as the dimensionless capillary number (Ca), was studied in two sets of experiments: discontinuous-rate injection, where the micromodel was saturated with water before each injection rate was imposed, and continuous-rate injection, where the rate was increased after quasi-steady conditions were reached for a certain rate. For the discontinuous-rate experiments, capillary fingering and viscous fingering are the dominant mechanisms for low (logCa ≤ -6.61) and high injection rates (logCa ≥ -5.21), respectively. Crossover from capillary to viscous fingering was observed for logCa = -5.91 to -5.21, resulting in a large decrease in scCO(2) saturation. The discontinuous-rate experimental results confirmed the decrease in nonwetting fluid saturation during crossover from capillary to viscous fingering predicted by numerical simulations by Lenormand et al. (J. Fluid Mech.1988, 189, 165-187). Capillary fingering was the dominant mechanism for all injection rates in the continuous-rate experiment, resulting in monotonic increase in scCO(2) saturation.

Correlation of Oil–Water and Air–Water Contact Angles of Diverse Silanized Surfaces and Relationship to Fluid Interfacial Tensions

The use of air-water, θ(wa), or air-liquid contact angles is customary in surface science, while oil-water contact angles, θ(ow), are of paramount importance in subsurface multiphase flow phenomena including petroleum recovery, nonaqueous phase liquid fate and transport, and geological carbon sequestration. In this paper we determine both the air-water and oil-water contact angles of silica surfaces modified with a diverse selection of silanes, using hexadecane as the oil. The silanes included alkylsilanes, alkylarylsilanes, and silanes with alkyl or aryl groups that are functionalized with heteroatoms such as N, O, and S. These silanes yielded surfaces with wettabilities from water wet to oil wet, including specific silanized surfaces functionalized with heteroatoms that yield intermediate wet surfaces. The oil-water contact angles for clean and silanized surfaces, excluding one partially fluorinated surface, correlate linearly with air-water contact angles with a slope of 1.41 (R = 0.981, n = 13). These data were used to examine a previously untested theoretical treatment relating air-water and oil-water contact angles in terms of fluid interfacial energies. Plotting the cosines of these contact angles against one another, we obtain the relationship cos θ(wa) = 0.667 cos θ(ow) + 0.384 (R = 0.981, n = 13), intercepting cos θ(ow) = -1 at -0.284, which is in excellent agreement with the linear assumption of the theory. The theoretical slope, based on the fluid interfacial tensions σ(wa), σ(ow), and σ(oa), is 0.67. We also demonstrate how silanes can be used to alter the wettability of the interior of a pore network micromodel device constructed in silicon/silica with a glass cover plate. Such micromodels are used to study multiphase flow phenomena. The contact angle of the resulting interior was determined in situ. An intermediate wet micromodel gave a contact angle in excellent agreement with that obtained on an open planar silica surface using the same silane.

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.