Carl D. Palmer

Reduction of Cr(VI) in the Presence of Excess Soil Fulvic Acid.

The rate of hexavalent chromium reduction by a soil fulvic acid (SFA) was measured in aqueous solutions where concentrations of Cr(VI), H + , and SFA were independently varied. Rates of reduction increase strongly with decreasing pH. Typical Cr(VI)-SFA reactions display a nonlinear reduction of Cr(VI) with timethat cannotadequately be modeled by either firstorder or second-order rate equations. An empirical rate equation that treats the SFA as a continuum of reactive groups which reduce Cr(VI) at varying rates adequately describes the effects of solution parameters on the rates of Cr(VI) reduction. The rate equation is R=k r [HCrO 4 - ][SFA][Cr(VI)] 0 P[H + ] q where [Cr(VI)] 0 is the initial Cr(VI) concentration

Solubility of jarosite at 4–35 °C

Abstract The solubility of jarosite (KFe 3 (SO 4 ) 2 (OH) 6 ) was studied in a series of dissolution experiments. The experiments were conducted at 4–35 °C and at pH values between 1.5 and 3.0 using a synthetic jarosite with a composition very close to ideal. The solids were left in the reaction vessel for up to 6 months. Equilibrium was established in the experiments after approximately 3 to 4 months. The log K SP for the jarosite dissolution reaction KFe 3 (SO 4 ) 2 (OH) 6 + 6H + K + + 3Fe 3+ + 2SO 4 2− + 6H 2 O at 25 °C is determined to be −11.0 ± 0.3. From this measured solubility product, the free energy of formation, Δ G f .298 o is calculated to be −3309.8 ± 1.7 kJ mol −1 . Based on the temperature dependence of the solubility product, the enthalpy of reaction at 25 °C, Δ H r .298 o is −45 ± 5 kJ mol −1 , the entropy of reaction, Δ S r .298 o , is −350 ± 40 J mol −1 K −1 , and the heat capacity of the reaction, ΔC p.r , over the temperature range of the experiments is determined to be −2.1 ± 0.2 kJ mol −1 K −1 . The rate of the dissolution reaction can be described by a first-order model.

Thermal energy storage in an unconfined aquifer: 2. Model development, validation, and application

A fully three-dimensional numerical model for simulating coupled density-dependent groundwater flow and thermal energy transport is developed and validated. The transport solution is based on a finite element time integration algorithm which generates a symmetric coefficient matrix while retaining second-order accuracy in time. The use of a symmetric conjugate gradient solver for both the flow and transport matrices results in a high degree of computational efficiency. Three-dimensional deformable block elements are used to allow the model to conform to domains with irregular geometry. The thermal transport model is validated against the results of the Borden thermal injection field experiment presented in the companion paper. The model simulations provide an excellent match with the observed temperature distribution over time, with the effects of thermal buoyancy and losses across the ground surface accurately reproduced by the model. The model is shown to be a practical tool for simulating the type of low-temperature thermal transport problems that arise in connection with seasonal aquifer thermal energy storage and ground source heat extraction systems.

Thermal energy storage in an unconfined aquifer: 1. Field Injection Experiment

A thermal injection and storage experiment was conducted to investigate the feasibility of storing thermal energy in shallow unconfined aquifers near the water table. Heated water was injected into a shallow aquifer and plume temperatures were monitored over a 141-day period by means of a dense array of bundle-type piezometers. The highly detailed data, which provide the three-dimensional temperature distribution within the aquifer, give good insight into the physical processes of aquifer thermal energy storage, and provide an excellent basis for the verification of simulation models. The experimental data also allow the physical processes of heat advection, dispersion, retardation, buoyancy and boundary heat loss to be quantified. In a companion paper, a three-dimensional density-dependent groundwater flow and thermal transport model is developed and validated using the results of the thermal injection experiment.

Solubility of chromate hydrocalumite (3CaO·Al2O3·CaCrO4·nH2O) 5–75°C

Abstract A secondary precipitate was consistently observed in a series of dissolution and precipitation experiments conducted on synthesized Ca 6 [Al(OH) 6 ] 2 (CrO 4 ) 3 ·26H 2 O. X-ray diffraction (XRD) patterns, scanning electron microscopy, thermogravimetry, and solid digest analyses demonstrate that d-spacings, crystal morphology, water content, and Ca/Cr/Al ratios for the secondary solid are consistent with chromate hydrocalumite (3CaO·Al 2 O 3 ·CaCrO 4 ·15H 2 O). Steady-state aqueous ion concentrations indicate that the experimental aqueous solutions were in apparent equilibrium with both the Cr(VI) analog of ettringite and chromate hydrocalumite. Over the pH range 10.9 to 12.2, the log of the solubility product (log K SP ) for the reaction 3 CaO · Al 2 O 3 · CaCrO 4 ·15 H 2 O ⇄4 Ca 2+ +2 Al ( OH ) 4 − + CrO 4 2− +4 OH − +9 H 2 O at 25°C is −30.38±0.28. The temperature dependence of the log K SP obtained from six additional temperatures from 5°C to 75°C is log K SP = −2042.1 T −23.480. Δ G r ° and Δ H r ° for the reaction are 173.1±3.7 and 39.1±3.2 kJ mol −1 and Δ S r ° is −450±10 J mol −1 K −1 . Using these values and published partial molal quantities for constituent ions, we calculate Δ G f,298 °=−9905±15.7 kJ mol −1 , Δ H f °=−11303±8.3 kJ mol −1 , and ΔS°=1439±89 J mol −1 K −1 for 3CaO·Al 2 O 3 ·CaCrO 4 ·15H 2 O.

Solubility of KFe3(CrO4)2(OH)6 at 4 to 35°C

Abstract The solubility of KFe3(CrO4)2(OH)6, the chromate analog of the sulfate mineral jarosite, was studied in a series of dissolution experiments. Experiments were conducted at 4 to 35°C and pH values between 1.5 and 3.0 using synthetic KFe3(CrO4)2(OH)6. The solids were kept in the reaction vessel for up to 6 months. Equilibrium was established in the experiments between 2 and 4 months. The log Ksp for the dissolution reaction of KFe3(CrO4)2(OH)6 KFe 3 ( CrO 4 ) 2 ( OH ) 6 + 6 H + ⇄ K + + 3 Fe 3 + + 2 CrO 4 2 + + 6 H 2 O at 25°C is −18.4 ± 0.6. Based on this measured solubility product, the free energy of formation, ΔGf,298°, is −3305.5 ± 3.4 kJ mol−1. The dissolution experiments at 25°C indicate the formation of a FeCrO4+ complex. The dissolution experiments combined with previously published spectroscopic data between 0 and 25°C yield a formation constant of the form 2.303log KFeCrO4+ = −ΔH°/RT + ΔS°/R, where ΔH° = 19.1 ± 2.2 kJ mol−1 and ΔS° = 214 ± 8 J K−1 mol−1. At 25°C, log KFeCrO4+ is 7.8 ± 0.5. The equilibrium ion activity products calculated from the experiments at 4, 15, 25, and 35°C do not show a statistically significant trend indicating a weak temperature dependence of the solubility product over the temperature range of the experiments. The rate of the dissolution reaction can be described by a first-order model. The measured solubility indicates that the chromate analog of jarosite is stable over a wide range of conditions and could form in large parts of a Cr(VI)-contaminated aquifer.

Solid-solution aqueous-solution reactions between jarosite (KFe3(SO4)2(OH)6) and its chromate analog

Abstract The sulfate mineral jarosite (KFe3(SO4)2(OH)6), its chromate analog (KFe3(CrO4)2(OH)6), and seven precipitates with intermediate compositions (KFe3(CrXS(1-X)O4)2(OH)6) were synthesized. The unit cell volume of the precipitates is closely represented by a linear function of composition, suggesting a continuous solid solution. This solid solution dissolves stoichiometrically according to KFe 3 (Cr X S (1−X) O 4 ) 2 (OH) 6 + 6H + → K + + 3Fe 3+ + 2X CrO 4 2− + (2 − 2X) SO 4 2− + 6H 2 O and reaches stoichiometric saturation after approximately 40 d. Log KSS values calculated from samples taken after 1090 d are consistently lower than what would be expected for an ideal solid solution, indicating that the excess free energy of mixing, GE, is negative. GE calculated from the log KSS values can be closely modeled by the one-parameter Guggenheim expansion G E = X CrJar X Jar RT a 0 where a0 is −4.9 ± 0.8, XCrJar and XJar are the mole fractions of KFe3(CrO4)2(OH)6 and KFe3(SO4)2(OH)6 in the solids, R is the gas constant, and T the absolute temperature. Based on the calculated excess free energy, a Lippmann diagram with a modified abscissa was constructed.

Borehole dilution tests in the vicinity of an extraction well

Abstract A mathematical solution is derived for a borehole dilution test conducted in the proximity of an extraction well under conditions where the Theis assumptions are applicable. The solution is similar to the steady-state velocity case except for a factor that accounts for the transient response of the aquifer to pumping. Curves calculated from the derived solution show a delay in response that increases with the specific storage of the aquifer and with the radius from the extraction well. The delay decreases with increasing values of hydraulic conductivity and aquifer thickness. Deviations of the response curves from the straight-line steady-state velocity case can be used to calculate the hydraulic diffusivity, while the limiting slope at late time can be used to calculate groundwater flux. When borehole dilution tests are conducted at several intervals over the thickness of the aquifer under steady-state velocity conditions, K ( z ) can, in principle, be calculated using several equations depending on the type of information available. Two existing methods to account for the distortion of the flow lines near the open test interval are related to well construction and development. Reasonable estimates of borehole factors can only be obtained for minimally developed wells.

Improved Geothermometry Through Multivariate Reaction-path Modeling and Evaluation of Geomicrobiological Influences on Geochemical Temperature Indicators: Final Report

The project was aimed at demonstrating that the geothermometric predictions can be improved through the application of multi-element reaction path modeling that accounts for lithologic and tectonic settings, while also accounting for biological influences on geochemical temperature indicators. The limited utilization of chemical signatures by individual traditional geothermometer in the development of reservoir temperature estimates may have been constraining their reliability for evaluation of potential geothermal resources. This project, however, was intended to build a geothermometry tool which can integrate multi-component reaction path modeling with process-optimization capability that can be applied to dilute, low-temperature water samples to consistently predict reservoir temperature within ±30 °C. The project was also intended to evaluate the extent to which microbiological processes can modulate the geochemical signals in some thermal waters and influence the geothermometric predictions.

Advancing Reactive Tracer Methods for Measurement of Thermal Evolution in Geothermal Reservoirs: Final Report

The injection of cold fluids into engineered geothermal system (EGS) and conventional geothermal reservoirs may be done to help extract heat from the subsurface or to maintain pressures within the reservoir (e.g., Rose et al., 2001). As these injected fluids move along fractures, they acquire heat from the rock matrix and remove it from the reservoir as they are extracted to the surface. A consequence of such injection is the migration of a cold-fluid front through the reservoir (Figure 1) that could eventually reach the production well and result in the lowering of the temperature of the produced fluids (thermal breakthrough). Efficient operation of an EGS as well as conventional geothermal systems involving cold-fluid injection requires accurate and timely information about thermal depletion of the reservoir in response to operation. In particular, accurate predictions of the time to thermal breakthrough and subsequent rate of thermal drawdown are necessary for reservoir management, design of fracture stimulation and well drilling programs, and forecasting of economic return. A potential method for estimating migration of a cold front between an injection well and a production well is through application of reactive tracer tests, using chemical whose rate of degradation is dependent on the reservoir temperature between the two wells (e.g., Robinson 1985). With repeated tests, the rate of migration of the thermal front can be determined, and the time to thermal breakthrough calculated. While the basic theory behind the concept of thermal tracers has been understood for some time, effective application of the method has yet to be demonstrated. This report describes results of a study that used several methods to investigate application of reactive tracers to monitoring the thermal evolution of a geothermal reservoir. These methods included (1) mathematical investigation of the sensitivity of known and hypothetical reactive tracers, (2) laboratory testing of novel tracers that would improve method sensitivity, (3) development of a software tool for design and interpretation of reactive tracer tests and (4) field testing of the reactive tracer temperature monitoring concept.