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CO 2 flooding is a proven enhanced oil recovery (EOR) technique and is also considered as a potential method for CO 2 sequestration. Despite having successful field trials on CO 2 EOR, the effects of reservoir wettability and permeability heterogeneity on the efficiency of miscible CO 2 flooding are not well understood. In this work, laboratory investigations have been carried out to evaluate the influence of these properties on the miscible CO 2 EOR performance. The wettability of hydrophilic Berea core samples was altered to be oil-wet by vacuum saturation of the clean and dry core samples with n-hexadecane. The permeability heterogeneity was obtained by combining two half pieces of axially split water-wet core samples of different permeabilities. Core flooding experiments were conducted for n-hexadecane – synthetic brine – CO 2 systems at 1400 psig backpressure to achieve minimum miscibility pressure (MMP) of CO 2 in n-hexadecane at the test temperature (24 ± 1 °C). It was found that wettability strongly influences CO 2 EOR. For the alternate cases of previously brine flooded (to remaining oil saturation) oil-wet and water-wet core samples, five pore volumes (PVs) of CO 2 recovered 100% and only 43% of remaining oil in place (ROIP) respectively. Three PVs of CO 2 could recover only about 0–5% ROIP from the split core samples. The mechanisms underlying these results are discussed. This study sheds light on the significant influence of reservoir wettability and permeability heterogeneity on the performance of miscible CO 2 EOR.

Recharge of alluvial aquifers is a key component in understanding the interaction between floodplain vadose zone biogeochemistry and groundwater quality. The Rifle Site (a former U-mill tailings site) adjacent to the Colorado River is a well-established field laboratory that has been used for over a decade for the study of biogeochemical processes in the vadose zone and aquifer. This site is considered an exemplar of both a riparian floodplain in a semiarid region and a post-remediation U-tailings site. In this paper we present Sr isotopic data for groundwater and vadose zone porewater samples collected in May and July 2013 to build a mixing model for the fractional contribution of vadose zone porewater (i.e. recharge) to the aquifer and its variation across the site. The vadose zone porewater contribution to the aquifer ranged systematically from 0% to 38% and appears to be controlled largely by the microtopography of the site. The area-weighted average contribution across the site was 8% corresponding to a net recharge of 7.5 cm. Given a groundwater transport time across the site of ~1.5 to 3 years, this translates to a recharge rate between 5 and 2.5 cm/yr, and with the average precipitation to the site implies a loss from the vadose zone due to evapotranspiration of 83% to 92%, both ranges are in good agreement with previously published results by independent methods. A uranium isotopic (234U/238U activity ratios) mixing model for groundwater and surface water samples indicates that a ditch across the site is hydraulically connected to the aquifer and locally significantly affects groundwater. Groundwater samples with high U concentrations attributed to natural bio-reduced zones have 234U/238U activity ratios near 1, suggesting that the U currently being released to the aquifer originated from the former U-mill tailings.

Author(s): Tokunaga, TK; Kim, Y; Wan, J; Bill, M; Conrad, M; Dong, W | Abstract: Transport from the soil surface to groundwater is commonly mediated through deeper portions of the vadose zone and capillary fringe, where variations in temperature and water saturation strongly influence biogeochemical processes. This technical note describes a sediment column design that allows laboratory simulation of thermal and hydrologic conditions found in many field settings. Temperature control is particularly important because room temperature is not representative of most subsurface environments. A 2.0-m-tall column was capable of simulating profiles with temperatures ranging from 3 to 22°C, encompassing the full range of seasonal temperature variation observed in the deep vadose zone and capillary fringe of a semiarid floodplain in western Colorado. The water table was varied within the lower 0.8-m section of the column, and profiles of water content and matric potential were measured. Vadose zone CO2 collected from depth-distributed gas samplers under representative seasonal conditions reflected the influences of temperature and water table depth on microbial respiration. Thus, realistic subsurface biogeochemical dynamics can be simulated in the laboratory through establishing column profiles that represent seasonal thermal and hydrologic conditions.

Author(s): Hu, R; Wan, J; Yang, Z; Chen, YF; Tokunaga, T | Abstract: When a more viscous fluid displaces a less viscous one in porous media, viscous pressure drop stabilizes the displacement front against capillary pressure fluctuation. For this favorable viscous ratio conditions, previous studies focused on the front instability under slow flow conditions but did not address competing effects of wettability and flow rate. Here we study how this competition controls displacement patterns. We propose a theoretical model that describes the crossover from fingering to stable flow as a function of invading fluid contact angle θ and capillary number Ca. The phase diagram predicted by the model shows that decreasing θ stabilizes the displacement for θ≥45° and the critical contact angle θc increases with Ca. The boundary between corner flow and cooperative filling for θ l 45° is also described. This work extends the classic phase diagram and has potential applications in predicting CO2 capillary trapping and manipulating wettability to enhance gas/oil displacement efficiency.

Significance Reliable estimates of geologic carbon storage capacities (needed for policymaking) in both saline aquifers and unconventional gas/oil shales rely on understanding trapping mechanisms. We found that CO2 uptake by muscovite (a common mineral and a conservative proxy for illite) far exceeds the maximum adsorption capacity of its external surface area. Our measurements using different methods collectively suggest that CO2 enters muscovite interlayers without bulk interlayer expansion, contrary to the conventional wisdom that only swelling clays take up CO2 into interlayers. Because the nonswelling illitic clay is the major clay mineral in deep subsurface tight rocks, their excess uptake of CO2 may significantly contribute to CO2 storage capacity and warrants further in-depth studies., Interactions between supercritical (sc) CO2 and minerals are important when CO2 is injected into geologic formations for storage and as working fluids for enhanced oil recovery, hydraulic fracturing, and geothermal energy extraction. It has previously been shown that at the elevated pressures and temperatures of the deep subsurface, scCO2 alters smectites (typical swelling phyllosilicates). However, less is known about the effects of scCO2 on nonswelling phyllosilicates (illite and muscovite), despite the fact that the latter are the dominant clay minerals in deep subsurface shales and mudstones. Our studies conducted by using single crystals, combining reaction (incubation with scCO2), visualization [atomic force microscopy (AFM)], and quantifications (AFM, X-ray photoelectron spectroscopy, X-ray diffraction, and off-gassing measurements) revealed unexpectedly high CO2 uptake that far exceeded its macroscopic surface area. Results from different methods collectively suggest that CO2 partially entered the muscovite interlayers, although the pathways remain to be determined. We hypothesize that preferential dissolution at weaker surface defects and frayed edges allows CO2 to enter the interlayers under elevated pressure and temperature, rather than by diffusing solely from edges deeply into interlayers. This unexpected uptake of CO2, can increase CO2 storage capacity by up to ∼30% relative to the capacity associated with residual trapping in a 0.2-porosity sandstone reservoir containing up to 18 mass % of illite/muscovite. This excess CO2 uptake constitutes a previously unrecognized potential trapping mechanism.

It is commonly true that a diluted colloidal suspension is more stable over time than a concentrated one because dilution reduces collision rates of the particles and therefore delays the formation of aggregates. However, this generalization does not apply for some engineered ligand-coated nanoparticles (NPs). We observed the opposite relationship between stability and concentration of NPs. We tested 4 different types of NPs: CdSe-11-mercaptoundecanoic acid, CdTe-polyelectrolytes, Ag-citrate, and Ag-polyvinylpyrrolidone. The results showed that dilution alone induced aggregation and subsequent sedimentation of the NPs that were originally monodispersed at very high concentrations. Increased dilution caused NPs to progressively become unstable in the suspensions. The extent of the dilution impact on the stability of NPs is different for different types of NPs. We hypothesize that the unavoidable decrease in free ligand concentration in the aqueous phase following dilution causes detachment of ligands from the suspended NP cores. The ligands attached to NP core surfaces must generally approach exchange equilibrium with free ligands in the aqueous phase; therefore, ligand detachment and destabilization are expected consequences of dilution. More studies are necessary to test this hypothesis. Because the stability of NPs determines their physicochemical and kinetic behavior including toxicity, dilution-induced instability needs to be understood to realistically predict the behavior of engineered ligand-coated NPs in aqueous systems. Environ Toxicol Chem 2018;37:1301-1308. © 2018 SETAC.

© 2017 Elsevier Inc. When a wetting liquid is displaced by air in a capillary tube, a wetting film develops between the tube wall and the air that is responsible for the snap-off mechanism of the gas phase. By dissolving a dye in the wetting phase it is possible to relate a measure of the absorbance in the capillary to the thickness of liquid films. These data could be used to compare with cutting edge numerical simulations of the dynamics of snap-off for which experimental and numerical data are lacking. Drainage experiments in constricted capillary tubes were performed where a dyed wetting liquid is displaced by air for varying flow rates. We developed an optical method to measure liquid film thicknesses that range from 3 to 1000 μm. The optical measures are validated by comparison with both theory and direct numerical simulations. In a constricted capillary tube we observed, both experimentally and numerically, a phenomenon of snap-off coalescence events in the vicinity of the constriction that bring new insights into our understanding and modeling of two-phase flows. In addition, the good agreement between experiments and numerical simulations gives confidence to use the numerical method for more complex geometries in the future.

The current ambiguity on wettability of minerals in CO2–brine systems under the geological CO2 sequestration (GCS) reservoir conditions imparts the greatest uncertainty in predicting capillary behavior controlling safe-storage of CO2. To address this issue we conducted a series of experiments using muscovite as a representative of common aluminosilicate minerals. Based on new experimental results we identified several possible causes of the ambiguity problem in contact angle (CA) measurements. We also found that reaction with water-saturated supercritical (sc) CO2 (but not with scN2) phase severely roughened the muscovite surfaces, largely increased CA hysteresis and CO2 adhesion. Although some methodological influences on contact angle uncertainty can be reduced, the high surface-energy of clean and pristine aluminosilicate minerals have strong tendency to adsorb oppositely changed molecules and particles to reduce their surface energy, resulting in less reproducible CA values. Giving the fact that such clean and pristine mineral surfaces do not exist in real reservoirs, our future investigations shall focus on improving understanding of the effect of long-term CO2–mineral–brine reactions on reservoir wettability under realistic reservoir geochemical conditions.

© 2017 American Chemical Society. Environmentally benign, economical, and effective surfactants and additives are needed in engineering subsurface energy recovery processes. Biosurfactants have some advantages over chemically synthesized surfactants, but the high costs of microbial-biosynthesis limit their applications in subsurface engineering. Here we propose to use naturally occurring biosurfactants contained within Earth's readily available and inexpensive humus deposits for subsurface engineering applications. We collected humus samples of different types from four different regions, and developed a simple method for extracting natural biosurfactant (NBS) using only four common chemicals. The average NBS extraction yields are 16 ± 3% of the raw humus. No significant differences in elemental composition and functional group chemistry were found among the NBS extracted from humus of different origins, suggesting that any humus deposit can be used for NBS extraction. Measurements of interfacial tensions between air-water and supercritical (sc) CO2-water interfaces indicate that the NBS is a highly effective surfactant. NBS has good foaming ability. Preliminary tests with only 0.5 mass % NBS in the aqueous phase (no other additives) yielded scCO2-in-water foams of 83% foam quality. The apparent viscosity of 13 cP measured at a shear rate of 2320 s-1indicates that much higher viscosities are achievable at lower shear rates. These results suggest that NBS merits further research and development for potential applications in subsurface energy production.

© 2017 the Owner Societies. Brine-mica interfacial interactions affect both the caprock integrity and the fate and transport of reactive fluids at deep subsurface sites. Phosphate naturally exists at low concentration in subsurface brines, and its concentration can be increased significantly during energy-related engineered subsurface processes. However, our understanding of the influence of phosphate on brine-mica interactions is limited, especially under subsurface conditions. Here, biotite dissolution experiments were conducted without and with phosphate (0.1, 1, and 10 mM) at 95 °C and 102 atm CO2. Compared to the control, 0.1 mM, and 1 mM phosphate systems, biotite dissolution was four times higher with 10 mM phosphate. Despite the dissolution differences, in all the phosphate systems, phosphate interacted with Al and Fe sites in biotite, forming surface complexation and precipitating as Fe- or Al-bearing minerals on surfaces and in solutions. Consequently, aqueous Fe and Al concentrations became lower with phosphate than in the control experiments. In addition, the biotite basal surfaces became more hydrophilic after reaction with phosphate, even at 0.1 mM, mainly from phosphate adsorption. This study offers new information on how phosphate-containing brine interacts with caprocks and on the consequent wettability changes, results that can benefit current and future energy-related subsurface engineering processes.

Author(s): Wan, Jiamin | Abstract: There are several types of hydraulic fracturing methods that have been investigated in the past decades that use little or no water. The criteria for selecting an optimal hydraulic fracturing fluids are many, and no single fluid can meet all the desired characteristics. Developing a set of technologies by using different fracturing fluids can better meet individual geologic, economic, and environmental needs. CO2 as an alternative of water for hydraulic fracturing has its unique place among all other fracturing fluids, and can meet both economic and environmental objectives under certain circumstances. There are three main categories of CO2-based fracturing fluids, simple liquid CO2 (LCO2), directly thickened LCO2, and CO2-foams/emulsions. Although LCO2 has many advantages, critical disadvantages arise due to its high operating pressure, low viscosity, poor proppant-carrying capacity, and generation of shallower fracture networks. To date, techniques for direct thickening of LCO2 to overcome these limitations have proven ineffective. To increase the apparent viscosity of CO2, we propose to further develop CO2-foam (and emulsion) fracturing techniques. While CO2-foams have long been used in EOR, the required properties for hydraulic fracturing are different, and relatively few peer-reviewed publications and field test reports are available regarding their applications in fracturing, and the surfactants and additives used are often proprietary This report describes a brief state-of-the-art analysis of CO2-based hydraulic fracturing methods and then discusses a joint plan for a long-term RaD collaboration to advance the most promising CO2-based hydraulic fracturing techniques as part of the U.S.-China Clean Energy Research Center, Water Energy Technology (CERC-WET). The objective of this U.S.-China joint research project is to design and laboratory test a CO2-foam fracturing fluid system, and then conduct pilot field-tests on them in China’s Jilin Oilfield. This joint research project will benefit both U.S. and China for advancing their shared goal of developing better technologies for CO2-based fracturing. The Chinese CERC-WET has two teams joining the project, one from RIPED, responsible for aboveground infrastructure design and construction. Another team from the Jilin Oilfield will be responsible for the field test design and conducting the test. The U.S.CERC-WET team at LBNL will be responsible for providing the CO2-foam formulations and their rheology parameters to be tested in the field (a challenging task and a key to success); participating in the equipment design and field test design; assisting in analyzing field data; and reporting to U.S. DOE and CERC-WET.

© 2017 American Chemical Society. Diffusion is important in controlling local solute transport and reactions in unsaturated soils and geologic formations. Although it is commonly assumed that thinning of water films controls solute diffusion at low water contents, transport under these conditions is not well understood. We conducted experiments in quartz sands at low volumetric water contents (θ) to quantify ion diffusion within adsorbed films. At the lowest water contents, we employed fixed relative humidities to control water films at nm thicknesses. Diffusion profiles for Rb+ and Br- in unsaturated sand packs were measured with a synchrotron X-ray microprobe, and inverse modeling was used to determine effective diffusion coefficients, De, as low as ∼9 × 10-15 m2 s-1 at θ = 1.0 × 10-4 m3 m-3, where the film thickness = 0.9 nm. Given that the diffusion coefficients (Do) of Rb+ and Br- in bulk water (30 °C) are both ∼2.4 × 10-9 m2 s-1, we found the impedance factor f = De/(θDo) is equal to 0.03 ± 0.02 at this very low saturation, in agreement with the predicted influence of interface tortuosity (τa) for diffusion along grain surfaces. Thus, reduced cross-sectional area (θ) and tortuosity largely accounted for the more than 5 orders of magnitude decrease in De relative to Do as desaturation progressed down to nanoscale films.

Many aquifers contaminated by U(VI)-containing acidic plumes are composed predominantly of quartz-sand sediments. The F-Area of the Savannah River Site (SRS) in South Carolina (USA) is an example. To predict U(VI) mobility and natural attenuation, we conducted U(VI) adsorption experiments using the F-Area plume sediments and reference quartz, goethite, and kaolinite. The sediments are composed of ∼96% quartz-sand and 3-4% fine fractions of kaolinite and goethite. We developed a new humic acid adsorption method for determining the relative surface area abundances of goethite and kaolinite in the fine fractions. This method is expected to be applicable to many other binary mineral pairs, and allows successful application of the component additivity (CA) approach based surface complexation modeling (SCM) at the SRS F-Area and other similar aquifers. Our experimental results indicate that quartz has stronger U(VI) adsorption ability per unit surface area than goethite and kaolinite at pH ≤ 4.0. Our modeling results indicate that the binary (goethite/kaolinite) CA-SCM under-predicts U(VI) adsorption to the quartz-sand dominated sediments at pH ≤ 4.0. The new ternary (quartz/goethite/kaolinite) CA-SCM provides excellent predictions. The contributions of quartz-sand, kaolinite, and goethite to U(VI) adsorption and the potential influences of dissolved Al, Si, and Fe are also discussed.

Increasing methane recovery rates from coal mines reduces greenhouse gas emissions and benefits mining safety. In recent years, raising the temperature of coal seams has been recognized as a potential method of improving methane recovery. However, the effect of elevated temperature (especially higher than 80 °C) on methane adsorption on coals is rarely studied. In this study, we investigated the effects of elevated temperature and pressure on methane adsorption on different types (ranking) of coal, and their thermal stability. We found that for all six coal samples measured, maximum methane adsorption capacities sharply decrease with increasing temperature, showing a linear relationship (r2 = 0.705–0.975), and that coals began to pyrolyze at 267–500 °C depending on the coal type. We defined the temperature-dependent decrease in methane adsorption of coal as its thermal desorption capacity, and found that coal types with higher fixed carbon content have a larger magnitude thermal desorption capacity. These results indicate that raising the coal seam temperature while capturing methane is a promising method for obtaining higher coalbed methane recovery rates, reducing greenhouse gas emissions, and improving mining safety.

Understanding the fate and transport of engineered nanoparticles (ENPs) in subsurface environments is required for developing the best strategy for waste management and disposal of these materials. In this study, the deposition and release of quantum dot (QD) nanoparticles were studied in saturated sand columns. The QDs were first deposited in columns using 100 mM NaCl or 2 mM CaC12 solutions. Deposited QDs were then contacted with deionized (DI) water and/or varying Na(+) concentrations to induce release. QDs deposited in 100 mM Na(+) were easily reversible when the column was rinsed with DI water. Conversely, QDs deposited in the presence of Ca(2+) exhibited resistance to release with DI water. However, significant release occurred when the columns were flushed with NaCl solutions. This release behavior was explained by cation exchange (Ca(2+) in exchange sites were replaced by Na(+)) which resulted in the breakdown of calcium bridging. We also studied the effect of aging time on the QD release. As the aging time increased, smaller amounts of QDs were released following cation exchange. However, deposited QDs were subsequently released when the column was flushed with DI water. The release behavior was modeled using a single first-order kinetic release process and changes in the maximum solid phase concentration of deposited QDs with transition in solution chemistry. The results of this study demonstrate that the presence of carboxyl groups on ENPs and divalent ions in the solution plays a key role in controlling ENP mobility in the subsurface environment.

Capillary pressure and saturation relations for supercritical CO 2 and brine in sand: High-pressure P c (S w ) controller/meter measurements and capillary scaling predictions Tetsu K. Tokunaga, 1 Jiamin Wan, 1 Jong-Won Jung, 1,2 Tae Wook Kim, 1,3 Yongman Kim, 1 and Wenming Dong 1 [ 1 ] In geologic carbon sequestration, reliable predictions of CO 2 storage require understanding the capillary behavior of supercritical (sc) CO 2 . Given the limited availability of measurements of the capillary pressure (P c ) dependence on water saturation (S w ) with scCO 2 as the displacing fluid, simulations of CO 2 sequestration commonly rely on modifying more familiar air/H 2 O and oil/H 2 O P c (S w ) relations, adjusted to account for differences in interfacial tensions. In order to test such capillary scaling-based predictions, we developed a high-pressure P c (S w ) controller/meter, allowing accurate P c and S w measurements. Drainage and imbibition processes were measured on quartz sand with scCO 2 -brine at pressures of 8.5 and 12.0 MPa (45 o C), and air-brine at 21 o C and 0.1 MPa. Drainage and rewetting at intermediate S w levels shifted to P c values that were from 30% to 90% lower than predicted based on interfacial tension changes. Augmenting interfacial tension-based predictions with differences in independently measured contact angles from different sources led to more similar scaled P c (S w ) relations but still did not converge onto universal drainage and imbibition curves. Equilibrium capillary trapping of the nonwetting phases was determined for P c ¼ 0 during rewetting. The capillary-trapped volumes for scCO 2 were significantly greater than for air. Given that the experiments were all conducted on a system with well-defined pore geometry (homogeneous sand), and that scCO 2 -brine interfacial tensions are fairly well constrained, we conclude that the observed deviations from scaling predictions resulted from scCO 2 -induced decreased wettability. Wettability alteration by scCO 2 makes predicting hydraulic behavior more challenging than for less reactive fluids. and Bachu, 2008 ; Benson and Cole, 2008]. The depend- ence of capillary pressure (P c ) on water saturation (S w ) under reservoir conditions is a basic constitutive relation needed to predict CO 2 flow and capillary trapping during sequestration. Indeed, with only knowledge of how (rela- tive) permeability depends on S w , neither fluid flow nor equilibrium saturation conditions are predictable. At later stages of geologic carbon sequestration, rewetting of reser- voirs with native brine results in capillary trapping (resid- ual trapping) of CO 2 , a main storage mechanism [Bachu et al., 2007; IPCC, 2005]. The capillary trapping capacity of reservoir materials has complex dependence on porosity, pore-size distribution, nonwetting phase displacement extent, and the rewetting process [Iglauer et al., 2011b ; Tanino and Blunt, 2012]. Although capillary trapping of CO 2 at its residual saturation depends on P c (S w ) relations, few investigations to date have directly measured these ba- sic relations with CO 2 /H 2 O at reservoir pressures and tem- peratures [Krevor et al., 2011 ; Pentland et al., 2011 ; Pini et al., 2012 ; Plug and Bruining, 2007]. The study by Plug and Bruining [2007] appears to be the only one in which capillary-trapped scCO 2 saturation was determined while 1. Introduction [ 2 ] Geologic carbon sequestration may become an im- portant technology for mitigating CO 2 buildup in the atmosphere from fossil fuel combustion and for moderating climate change [Intergovernmental Panel on Climate Change (IPCC), 2005]. However, predicting the perform- ance of geologic CO 2 sequestration requires reliable under- standing of CO 2 mobility, and its saturation and pressure distribution within reservoirs [Bachu et al., 2007 ; Bennion Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, USA. Now at Civil and Environmental Engineering Department, Louisiana State University, Baton Rouge, Louisiana, USA. Now at Department of Energy Resources Engineering, Stanford Uni- versity, Stanford, California, USA. Corresponding author : T. K. Tokunaga, Earth Sciences Division, Law- rence Berkeley National Laboratory, 1 Cyclotron Rd., Berkeley, CA 94720, USA. (tktokunaga@lbl.gov)

The movement of injected CO2 through permeable pore networks determines its distribution and stability within reservoirs used for carbon sequestration (Fig. 1), and this process is dependent on capillary interactions with the displaced brine (IPCC 2005; Benson and Cole 2008). Capillary phenomena controlling the distribution of CO2 and reservoir brine depend on pore size (from mm to nm), wetting and interfacial properties. The pressure of the usually nonwetting CO2 phase relative to that of the native brine is the capillary pressure, P c , which in combination with pore size, wettability, and interfacial properties determines the saturation of each fluid phase. In typically reservoirs used for geologic carbon sequestration, CO2 exists as a supercritical (sc) fluid because pressures and temperatures associated with storage depths exceed the critical point values for CO2 (7.38 MPa, 31.1 °C). Thus, predicting carbon sequestration in reservoirs require understanding of interfacial tension, mineral surface wettability, and the capillary pressure dependence on saturation, for interactions between scCO2, brine, and reservoir mineral surfaces. Investigations into these aspects of scCO2 behavior, especially at elevated pressures and temperatures characteristic of geologic carbon sequestration, have largely only recently begun. Given the fairly early stage of research, many models for CO2 transport in reservoirs rely heavily on the better understood capillary characteristics of other immiscible fluid pairs such as air-water, oil-water, and oil-gas, supplemented with properties of scCO2. However, a growing number of experimental studies are being published on the behavior of scCO2 under reservoir conditions, leading to better understanding of C sequestration while also giving rise to new questions. In this chapter, we review aspects of the current (mostly laboratory-based) understanding of equilibrium capillary interactions between scCO2 and brines, and their implications for scCO2 behavior in …

© 2016 In order to understand the transport and humification processes of dissolved organic matter (DOM) within sediments of a semi-arid floodplain at Rifle, Colorado, fluorescence excitation–emission matrix (EEM) spectroscopy, humification index (HIX) and specific UV absorbance (SUVA) at 254 nm were applied for characterizing depth and seasonal variations of DOM composition. Results revealed that late spring snowmelt leached relatively fresh DOM from plant residue and soil organic matter down into the deeper vadose zone (VZ). More humified DOM is preferentially adsorbed by upper VZ sediments, while non- or less-humified DOM was transported into the deeper VZ. Interestingly, DOM at all depths undergoes rapid biological humification process evidenced by the products of microbial by-product-like (i.e., tyrosine-like and tryptophan-like) matter in late spring and early summer, particularly in the deeper VZ, resulting in more humified DOM (e.g., fulvic-acid-like and humic-acid-like substances) at the end of year. This indicates that DOM transport is dominated by spring snowmelt, and DOM humification is controlled by microbial degradation, with seasonal variations. It is expected that these relatively simple spectroscopic measurements (e.g., EEM spectroscopy, HIX and SUVA) applied to depth- and temporally-distributed pore-water samples can provide useful insights into transport and humification of DOM in other subsurface environments as well.

© Soil Science Society of America. Although CO2fluxes from soils are often assumed to originate within shallow soil horizons (

© 2016 American Chemical Society. The wettability of rocks and minerals significantly affects the safety and efficiency of energy-related subsurface operations. Salinity is an important controlling factor in terms of wettability but has received limited attention. We studied the effects of salinity-induced chemical reactions on biotite's wettability changes under relevant subsurface conditions. Biotite was reacted at 95 °C and 102 atm of CO2for 70 h in solutions with salinities of 0, 0.1, 0.5, and 1.0 M NaCl. Then, static and dynamic water contact angles on reacted biotite basal surfaces were measured using a captive drop method. As a result of enhanced biotite dissolution at higher salinities, increased roughness, more negatively charged surfaces, and higher densities of hydroxyl groups on the biotite surfaces made biotite basal surface more hydrophilic. These results provide new information about the interplay of chemical reactions and wettability alterations of minerals, providing a better understanding of CO2transport in subsurface environments.

© 2016, US Government. Flood plains play a potentially important role in the global carbon cycle. The accumulation of organic matter in flood plains often induces the formation of chemically reduced groundwater and sediments along riverbanks. In this study, our objective is to evaluate the cumulative impact of such reduced zones, water table fluctuations, and temperature gradients on subsurface carbon fluxes in a flood plain at Rifle, Colorado located along the Colorado River. 2-D coupled variably-saturated, non-isothermal flow and biogeochemical reactive transport modeling was applied to improve our understanding of the abiotic and microbially mediated reactions controlling carbon dynamics at the Rifle site. Model simulations considering only abiotic reactions (thus ignoring microbial reactions) underestimated CO2partial pressures observed in the unsaturated zone and severely underestimated inorganic (and overestimated organic) carbon fluxes to the river compared to simulations with biotic pathways. Both model simulations and field observations highlighted the need to include microbial contributions from chemolithoautotrophic processes (e.g., Fe+2and S−2oxidation) to match locally-observed high CO2concentrations above reduced zones. Observed seasonal variations in CO2concentrations in the unsaturated zone could not be reproduced without incorporating temperature gradients in the simulations. Incorporating temperature fluctuations resulted in an increase in the annual groundwater carbon fluxes to the river by 170 % to 3.3 g m−2d−1, while including water table variations resulted in an overall decrease in the simulated fluxes. We conclude that spatial microbial and redox zonation as well as temporal fluctuations of temperature and water table depth contribute significantly to subsurface carbon fluxes in flood plains and need to be represented appropriately in model simulations.

Wettability of reservoir mineral surfaces is a critical factor controlling CO2 mobility, trapping, and safe-storage in geological carbon sequestration. Although recent studies have begun to show that wettability of some minerals can change in the presence of supercritical CO2 (scCO2), different laboratories have reported significantly different wetting behavior. We studied wettability alteration of silica in CO2–brine systems through measuring equilibrium water contact angles under wide ranges of pressures (0.1 to 25 MPa) and ionic strengths (0 to 5.0 M NaCl), at 45 °C. Using two independent approaches for each of the experiments, we found the following: (1) Equilibrium water contact angles on silica increased up to 17.6° ± 2.0° as a result of reactions with scCO2. This increase occurred primarily within the pressure range 7–10 MPa, and the contact angles remain nearly constant at pressure greater than 10 MPa. (2) The contact angle increased with ionic strength nearly linearly, with a net increase of 19.6...

At the Savannah River Site's F-Area, wastewaters containing radionuclides were disposed into seepage basins for decades. After closure and capping in 1991, the U.S. Department of Energy (DOE) has being monitoring and remediating the groundwater plume. Despite numerous studies of the plume, its persistence for over 20 years has not been well understood. To better understand the plume dynamics, a limited number of deep boreholes were drilled to determine the current plume characteristics. A mixing model was developed to predict plume tritium and nitrate concentrations. We found that the plume trailing edges have emerged for some contaminants, and that contaminant recharge from the basin's vadose zone is still important. The model's estimated time-dependent basin drainage rates combined with dilution from natural recharge successfully predicted plume tritium and nitrate concentrations. This new understanding of source zone influences can help guide science-based remediation, and improve predictions of the natural attenuation timeframes.

Comprehensive understanding of the transport and deposition of engineered nanoparticles (NPs) in subsurface is required to assess their potential negative impact on the environment. We studied the deposition behavior of functionalized quantum dot (QD) NPs (CdTe) in different types of sands (Accusand, ultrapure quartz, and iron-coated sand) at various solution ionic strengths (IS). The observed transport behavior in ultrapure quartz and iron-coated sand was consistent with conventional colloid deposition theories. However, our results from the Accusand column showed that deposition was minimal at the lowest IS (1 mM) and increased significantly as the IS increased. The effluent breakthrough occurred with a delay, followed by a rapid rise to the maximum normalized concentration of unity. Negligible deposition in the column packed with ultrapure quartz sand (100 mM) and Accusand (1 mM) rules out the effect of straining and suggests the importance of surface charge heterogeneity in QD deposition in Accusand at higher IS. Data analyses further show that only a small fraction of sand surface area contributed in QD deposition even at the highest IS (100 mM) tested. The observed delay in breakthrough curves of QDs was attributed to the fast diffusive mass transfer rate of QDs from bulk solution to the sand surface and QD mass transfer on the solid phase. Scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) analysis were used to examine the morphology and elemental composition of sand grains. It was observed that there were regions on the sand covered with layers of clay particles. EDX spectra collected from these regions revealed that Si and Al were the major elements suggesting that the clay particles were kaolinite. Additional batch experiments using gold NPs and SEM analysis were performed and it was observed that the gold NPs were only deposited on clay particles originally on the Accusand surface. After removing the clays from the sand surface, we observed negligible QD deposition even at 100 mM IS. We proposed that nanoscale charge heterogeneities on clay particles on Accusand surface played a key role in QD deposition. It was shown that the value of solution IS determined the extent to which the local heterogeneities participated in particle deposition.

In response to the rapid development and emerging commercialization of nanoparticles, fundamental studies concerning the fate of nanoparticles in the environment are needed. Precise control over the nanoparticle size, shape, and surface coating of cadmium selenide particles modified with thiolate ligands has been used to analyze the effects of nanoparticle design on their stability in aqueous environments. Nanoparticle stability was quantified using the concept of critical coagulation concentration (CCC) in solutions of sodium chloride. These investigations characterized the instability of the ligand coatings, which varied directly with chain length of the capping ligands. The stability of the ligand coatings were characterized as a function of time, pH, and ionic strength. Ligand dissociation has been shown to be a primary mechanism for nanoparticle aggregation when short-chain (C2−C6) ligands are used in the ligand shell. Stable nanoparticle suspensions prepared with long chain ligands (C11) were used t...

Bioreduction-based strategies for remediating uranium (U)-contaminated sediments face the challenge of maintaining the reduced status of U for long times. Because groundwater influxes continuously bring in oxidizing terminal electron acceptors (O2, NO3(-)), it is necessary to continue supplying organic carbon (OC) to maintain the reducing environment after U bioreduction is achieved. We tested the influence of OC supply rates on mobility of previously microbial reduced uranium U(IV) in contaminated sediments. We found that high degrees of U mobilization occurred when OC supply rates were high, and when the sediment still contained abundant Fe(III). Although 900 days with low levels of OC supply minimized U mobilization, the sediment redox potential increased with time as did extractable U(VI) fractions. Molecular analyses of total microbial activity demonstrated a positive correlation with OC supply and analyses of Geobacteraceae activity (RT-qPCR of 16S rRNA) indicated continued activity even when the effluent Fe(II) became undetectable. These data support our hypothesis on the mechanisms responsible for remobilization of U under reducing conditions; that microbial respiration caused increased (bi)carbonate concentration and formation of stable uranyl carbonate complexes, thereby shifted U(IV)/U(VI) equilibrium to more reducing potentials. The data also suggested that low OC concentrations could not sustain the reducing condition of the sediment for much longer time. Bioreduced U(IV) is not sustainable in an oxidizing environment for a very long time.

Leakage of saline-alkaline tank waste solutions often creates a serious environmental contamination problem. To better understand the mechanisms controlling the fate of such waste solutions in the Hanford vadose zone, we simulated reactive transport in columns designed to represent local site conditions. The Pitzer ion interaction module was used, with principal geochemical processes considered in the simulation including quartz dissolution, precipitation of brucite, calcite, and portlandite, multi-component cation exchange, and aqueous complexation reactions. Good matches were observed between the simulated and measured column data at ambient temperature ( approximately 21 degrees C). Relatively good agreement was also obtained at high temperature ( approximately 70 degrees C). The decrease of pH at the plume front is examined through formation of secondary mineral phases and/or quartz dissolution. Substantial formation of secondary mineral phases resulting from multi-component cation exchange suggests that these phases are responsible for a decrease in pH within the plume front. In addition, a sensitivity analysis was conducted with respect to cation exchange capacity, selectivity coefficient, mineral assemblage, temperature, and ionic strength. This study could serve as a useful guide to subsequent experimental work, to thermodynamic models developed for the concentrated solutions at high ionic strength and to other types of waste plume studies.

The oxidation status of uranium in sediments is important because the solubility of this toxic and radioactive element is much greater for U(VI) than for U(IV) species. Thus, redox manipulation to promote precipitation of UO2 is receiving interest as a method to remediate U-contaminated sediments. Presence of Fe and Mn oxides in sediments at much higher concentrations than U requires an understanding of their redox status as well. This study was conducted to determine changes in oxidation states of U, Fe, and Mn in U-contaminated sediments from Oak Ridge National Laboratory. Oxidation states of these elements were measured in real-time and nondestructively using X-ray absorption spectroscopy on sediment columns supplied with synthetic groundwater containing organic carbon (OC, 0, 3, 10, 30, and 100 mM OC as lactate) for over 400 days. In sediments supplied with OCor = 30 mM, 80% of the U was reduced to U(IV), with transient reoxidation at about 150 days. Mn(III,IV) oxides were completely reduced to Mn(II) in sediments infused with OCor = 3 mM. However, Fe remained largely unreduced in all sediment columns, showing that Fe(III) can persist as an electron acceptor in reducing sediments over long times. This result in combination with the complete reduction of all other potential electron acceptors supports the hypothesis that the reactive Fe(III) fraction was responsible for reoxidizing U(IV).

Soils contain the largest inventory of organic carbon on the Earth’s surface. Therefore, it is important to understand how soil organic carbon (SOC) is distributed in soils. This study directly measured SOC distributions within soil microaggregates and its associations with major soil elements from three soil groups (Phaeozem, Cambisol, and Ultisol), using scanning transmission X-ray microscopy (STXM) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy at a spatial resolution of 30 nm. Unlike previous studies, small intact soil microaggregates were examined directly in order to avoid preparatory procedures that might alter C speciation. We found that SOC exists as distinct particles (10s to 100s of nm) and as ubiquitous thin coatings on clay minerals and iron-oxides coatings. The distinct SOC particles have higher fractions of aromatic C than the coatings. NEXAFS spectra of the C coatings within individual microaggregates were relatively similar. In the Phaeozem soil, the pervasive spectral features were those of phenolic and carboxylic C, while in the Cambisol soil the most common spectral feature was the carboxyl peak. The Ultisol soil displayed a diffuse distribution of aromatic, phenolic, and carboxylic C peaks over all surfaces. In general, a wide range of C functional groups coexist within individual microaggregates. In this work we were able to, for the first time, directly quantify the major mineral elemental (Si, Al, Ca, Fe, K, Ti) compositions simultaneously with C distribution and speciation at the nm to μm scale. These direct microscale measurements will help improve understanding on SOC–mineral associations in soil environments.

Long-term historic spills of uranium at the 300 Area fuel fabrication site (58,000 kg of disposed uranium over 32 yr) and at the 200 East Area BX tank farm (7000 kg of spilled uranium in one event), both within the Hanford formation in the Hanford Site, Washington State, were investigated by subsurface sampling and subsequent microscale investigations of excavated samples. The 200 Area sediments contained uranyl silicate mineralization (sodium boltwoodite) in restrictive microfractures in granitic clasts, in the vadose zone over a narrow range in depth. Well logging and column experiments indicated that tank wastes migrated deeper than observed in core samples. The 300 Area sediments included metatorbernite and uranium at low concentrations associated with detrital aluminosilicates, along with other mineral phases that could accommodate uranyl, such as uranophane and calcium carbonate. The association of contaminant uranyl with Hanford formation sediments provided a persistent source of uranium to groundwater. The results of both studies suggest that the formation of secondary solid uranyl-bearing phases infl uences the subsequent release of uranium to the environment and that our understanding of these processes and individual waste sites is incomplete.

Nanoparticles are ubiquitous in environment and are potentially important in many environmental processes such as sorption, coprecipitation, redox reactions, and dissolution. To investigate particle size effects on nanoparticle aggregation and stability, this study tested aggregation behavior of 12(±2), 32(±3), and 65(±3) nm (hydrated radius) hematite particles under environmental relevant pH and ionic strength conditions. The results showed that at the same ionic strength and pH conditions, different particle sizes show different tendency to aggregate. At the same ionic strength, aggregation rates are higher for smaller particles. The critical coagulation concentration also depends on particle size, and decreases as particle size decreases. As the particle size decreases, fast aggregation shifted to lower pH. This may be related to a dependence of PZC on particle size originating from change of structure and surface energy characteristics as particle size decreases. Under the same conditions, aggregation occurs faster as particle concentration increases. Even though the nanoparticles of different sizes show different response to the same pH and ionic strength, DLVO theory can be used to qualitatively understand hematite nanoparticle aggregation behavior.

The ambiguity of contact angle experimental measurements due to surface chemistry changes resulted from sample contamination and/or the degrees of reaction with supercritical CO2 has resulted in great difficulties to precisely understand the wetting behavior of CO2 under the geological carbon sequestration (GCS) conditions. In this study, water contact angles on quartz surface under GCS conditions were investigated through the combined experimental and molecular dynamics simulation (MDS) methods. The experimental results show that water contact angles increases as ionic strength increases. The effects of pressure and temperature are very weak. The dependence of ionic strength, pressure and temperature is same for monovalent and divalent ions solutions. In the MDS, a hydroxylated quartz surface was used as the base point. A good agreement between the MDS and experimental results were obtained. Using the MDS method, a clean mineral surface with a desired surface chemistry can be constructed, which is difficult in experiments. So by comparing MDS and experimental results, the mechanisms of the reservoir wettability can be better understood. Further investigation can be made on quartz surface with different functional groups to better understand wettability alteration caused by contamination and/or CO2 reaction.

Waste forms of uranium (U), such as those in the U.S. Department of Energy's Hanford Site, often contain high concentrations of sodium and phosphorus. Low solubility sodium uranyl phosphates such as sodium meta-autunite have the potential to form mobile colloids that can facilitate transport of this radionuclide. To understand the stability behavior of uranyl phosphate colloids, we synthesized sodium meta-autunite colloids and characterized their morphology, chemical composition, structure, dehydration, and surface charge. The stability of these synthetic, plate-shaped colloids was tested with respect to time and pH. We observed the highest aggregation rate to be at pH 3, with the rate decreasing as pH increases, indicating a higher stability of colloid dispersion under neutral and alkaline pH conditions. The synthetic colloids are all negatively charged, and no isoelectric points were found over a pH range of 3–9. The zeta potentials (ζ) of the colloids in the phosphate solution show a strong pH-dependence in the more acidic range over time, but are relatively constant in the neutral and alkaline pH range. Using Derjaguin–Landau–Verwey–Overbeek theory, the stability behavior of these synthetic colloids was interpreted. The high stability of sodium meta-autunite colloids under neutral–alkaline conditions suggests that formation of sodium meta-autunite colloids can enhance the transport of U in U-contaminated sediments.

Costly disposal of uranium (U) contaminated sediments is motivating research on in situ U(VI) reduction to insoluble U(IV) via directly or indirectly microbially mediated pathways. Delivery of organic carbon (OC) into sediments for stimulating U bioreduction is diffusion-limited in less permeable regions of the subsurface. To study OC-based U reduction in diffusion-limited regions, one slightly acidic and another calcareous sediment were treated with uranyl nitrate, packed into columns, then hydrostatically contacted with tryptic soy broth solutions. Redox potentials, U oxidation state, and microbial communities were well correlated. At average supply rates of 0.9 micromol OC (g sediment)(-1) day(-1), the U reduction zone extended to only about35-45 mm into sediments. The underlying unreduced U(VI) zone persisted over 600 days because the supply of OC was diffusion-limited and metabolized within a short distance. These results also suggestthat low U concentrations in groundwater samples from OC-treated sediments are not necessarily indicative of pervasive U reduction because interior and exterior regions of such sediment blocks can contain primarily U(VI) and U(IV), respectively.

Nuclear weapons and fuel production have left many soils and sediments contaminated with toxic levels of uranium (U). Although previous short-term experiments on microbially mediated U(VI) reduction have supported the prospect of immobilizing the toxic metal through formation of insoluble U(IV) minerals, our longer-term (17 months) laboratory study showed that microbial reduction of U can be transient, even under sustained reducing conditions. Uranium was reduced during the first 80 days, but later (100-500 days) reoxidized and solubilized, even though a microbial community capable of reducing U(VI) was sustained. Microbial respiration caused increases in (bi)-carbonate concentrations and formation of very stable uranyl carbonate complexes, thereby increasing the thermodynamic favorability of U(IV) oxidation. We propose that kinetic limitations including restricted mass transfer allowed Fe-(III) and possibly Mn(IV) to persist as terminal electron acceptors (TEAs) for U reoxidation. These results show that in-situ U remediation by organic carbon-based reductive precipitation can be problematic in sediments and groundwaters with neutral to alkaline pH, where uranyl carbonates are most stable.

Summary The retention, mobility, and bio-availability of U(VI) in contaminated soils depend strongly on release of U(VI). Laboratory batch experiments were performed to evaluate the factors (ionic strength of the extraction solution and solution volume to solid mass ratio) controlling the release of U(VI) from contaminated soil at Oak Ridge, Tennessee. Increase in ionic strength shows an evident effect on U(VI) release as indicated by the increase in release rates and associated release mass of U(VI). The ratio of solution volume to solid mass (V/M) has a significant impact on the release of U(VI). Increase in the V/M ratio shows a negligible effect on the U(VI) release over a 4-day period. However, at Day 30 and Day 120, larger V/M ratios cause greater U(VI) release. The maximum U(VI) concentrations observed in the release experiments are in the range of schoepite solubility estimated under conditions relevant to the experiments, suggesting that schoepite solubility primarily controls the U(VI) release, but that solubilization and desorption effects cannot be distinguished using macroscopic methods.

Uranium contamination of soils and sediments often originates from acidic or alkaline waste sources, with diffusion being a major transport mechanism. Measurements of U(VI) diffusion from initially pH 2 and pH 11 solutions into a slightly alkaline Altamont soil and a neutral Oak Ridge soil were obtained through monitoring uptake from boundary reservoirs and from U concentration profiles within soil columns. The soils provided pH buffering, resulting in diffusion at nearly constant pH. Micro X-ray absorption near edge structure spectra confirmed that U remained in U(VI) forms in all soils. Time trends of U(VI) depletion from reservoirs and U(VI) concentration profiles within soil columns yielded Kdvalues consistent with those determined in batch tests at similar concentrations (approximately 1 mM) and much lower than values for sorption at much lower concentrations (nM to microM). These results show that U(VI) transport at high concentrations can be relatively fast at non-neutral pH, with negligible surface diffusion, because of weak sorption.

Leakage of highly saline and alkaline radioactive waste from storage tanks into underlying sediments is a serious environmental problem at the Hanford Site in Washington State. This study focuses on geochemical evolution of tank waste plumes resulting from interactions between the waste solution and sediment. A synthetic tank waste solution was infused into unsaturated Hanford sediment columns (0.2, 0.6, and 2 m) maintained at 70°C to simulate the field contamination process. Spatially and temporally resolved geochemical profiles of the waste plume were obtained. Thorough OH − neutralization (from an initial pH 14 down to 6.3) was observed. Three broad zones of pore solutions were identified to categorize the dominant geochemical reactions: the silicate dissolution zone (pH > 10), pH-neutralized zone (pH 10 to 6.5), and displaced native sediment pore water (pH 6.5 to 8). Elevated concentrations of Si, Fe, and K in plume fluids and their depleted concentrations in plume sediments reflected dissolution of primary minerals within the silicate dissolution zone. The very high Na concentrations in the waste solution resulted in rapid and complete cation exchange, reflected in high concentrations of Ca and Mg at the plume front. The plume-sediment profiles also showed deposition of hydrated solids and carbonates. Fair correspondence was obtained between these results and analyses of field borehole samples from a waste plume at the Hanford Site. Results of this study provide a well-defined framework for understanding waste plumes in the more complex field setting and for understanding geochemical factors controlling transport of contaminant species carried in waste solutions that leaked from single-shell storage tanks in the past.