Carl S. Kirby

Geochemical and Strontium Isotope Characterization of Produced Waters from Marcellus Shale Natural Gas Extraction

Extraction of natural gas by hydraulic fracturing of the Middle Devonian Marcellus Shale, a major gas-bearing unit in the Appalachian Basin, results in significant quantities of produced water containing high total dissolved solids (TDS). We carried out a strontium (Sr) isotope investigation to determine the utility of Sr isotopes in identifying and quantifying the interaction of Marcellus Formation produced waters with other waters in the Appalachian Basin in the event of an accidental release, and to provide information about the source of the dissolved solids. Strontium isotopic ratios of Marcellus produced waters collected over a geographic range of ~375 km from southwestern to northeastern Pennsylvania define a relatively narrow set of values (ε(Sr)(SW) = +13.8 to +41.6, where ε(Sr) (SW) is the deviation of the (87)Sr/(86)Sr ratio from that of seawater in parts per 10(4)); this isotopic range falls above that of Middle Devonian seawater, and is distinct from most western Pennsylvania acid mine drainage and Upper Devonian Venango Group oil and gas brines. The uniformity of the isotope ratios suggests a basin-wide source of dissolved solids with a component that is more radiogenic than seawater. Mixing models indicate that Sr isotope ratios can be used to sensitively differentiate between Marcellus Formation produced water and other potential sources of TDS into ground or surface waters.

Relative contributions of abiotic and biological factors in Fe(II) oxidation in mine drainage

The oxidation of Fe(II) is apparently the rate-limiting step in passive treatment of coal mine drainage. Little work has been done to determine the kinetics of oxidation in such field systems, and no models of passive treatment systems explicitly consider iron oxidation kinetics. A Stella II™ model using Fe(II)init concentration, pH, temperature, Thiobacillus ferrooxidans and O2 concentration, flow rate, and pond volume is used to predict Fe(II) oxidation rates and concentrations in seventeen ponds under a wide range of conditions (pH 2.8 to 6.8 with Fe(II) concentrations of less than 240 mg L−1) from 6 passive treatment facilities. The oxidation rate is modeled based on the combination of published abiotic and biological laboratory rate laws. Although many other variables have been observed to influence Fe(II) oxidation rates, the 7 variables above allow field systems to be modeled reasonably accurately for conditions in this study. Measured T. ferrooxidans concentrations were approximately 107 times lower than concentrations required in the model to accurately predict field Fe(II) concentrations. This result suggests that either 1) the most probable number enumeration method underestimated the bacterial concentrations, or 2) the biological rate law employed underestimated the influence of bacteria, or both. Due to this discrepancy, bacterial concentrations used in the model for pH values of less than 5 are treated as fit parameters rather than empirically measured values. Predicted Fe(II) concentrations in ponds agree well with measured Fe(II) concentrations, and predicted oxidation rates also agree well with field-measured rates. From pH 2.8 to approximately pH 5, Fe(II) oxidation rates are negatively correlated with pH and catalyzed by T. ferrooxidans. From pH 5 to 6.4, Fe(II) oxidation appears to be primarily abiotic and is positively correlated with pH. Above pH 6.4, oxidation appears to be independent of pH. Above pH 5, treatment efficiency is affected most by changing design parameters in the following order: pH>temperature≈influent Fe(II)>pond volume≈O2. Little to no increase in Fe(II) oxidation rate occurs due to pH increases above pH 6.4. Failure to consider Fe(II) oxidation rates in treatment system design may result in insufficient Fe removal.

Interaction of municipal solid waste ash with water

Municipal solid waste (MSW) incineration ash is composed mainly of glasses and common minerals. We examine the dominant chemical reactions occurring between water and MSW ash using batch reactors. The ash-water solutions are dominated by ions released by soluble salts. X-ray diffraction documents the dissolution of soluble salts and the precipitation of at least one secondary alteration mineral. Three types of reactions are identified. (1) After rapid exhaustion of soluble salts, sodium and potassium exhibit nearly steady-state behavior due to the slow release of ions from less soluble minerals and glasses. Approximately 10 kg of anhydrous salts could be precipitated from a solution contacting each metric ton of ash

Field determination of Fe2+ oxidation rates in acid mine drainage using a continuously-stirred tank reactor

A gravity-fed, battery-powered, portable continuously-stirred tank reactor has been developed to directly measure aqueous reaction rates in the field. Dye and tracer experiments indicate the reactor is well-mixed. Rates of Fe2+ oxidation at untreated and passively treated coal mine drainage sites in Pennsylvania were measured under ambient conditions and with the addition of either O2 gas or NaOH solutions. Rates at 5 sites ranged from below the detection limit for this technique (approximately 10−9 mol L−1 s−1) to 3.27±0.01×10−6 mol L−1 s−1. Uncertainties in rates ranged from 70% near the lower limit of measurement to as little as 1% at higher rates of reaction. Multiple linear regressions showed no universal correlations of rates to Fe2+, dissolved O2, and pH (Thiobacillus populations were not measured), although data for two more acidic sites were found to fit well for the model log rate=log K+a log [Fe2+]+b log [OH−]+c log [O2]. Field rates of Fe oxidation from this and other studies vary by 4 orders of magnitude. A model using the ambient field rate of Fe oxidation from this study successfully reproduced independently-measured Fe2+ concentrations observed in a passive wetland treatment facility.

IRON DYNAMICS IN ACID MINE DRAINAGE 1

The oxidation of iron sulfides in mine wastes is the main cause of acidic, sulfate, and trace element-rich acid mine drainage (AMD). However, the suite of reactions that transform iron from one species to another is quite complex. A reasonable strategy for controlling AMD production is to identify and further slow the slowest, rate-determining step (RDS) in the overall process. This paper provides an overall quantitative comparison of iron transformation rates in the AMD process using data from the literature and this comparison allows us to confirm that pyrite oxidation is the RDS for overall acid production over the entire pH range.

Corrections: mineralogy and surface properties of municipal solid waste ash.

The cation exchange capacities (CEC) estimated in this paper are too high and should not be used as estimates of the positive surface charge for the ash from this study. In the original experiments, Ca2+ was used as the saturating cation and was displaced subsequently by Mg2+. The measurement of the saturating cation was accurate, but the concentrations of Ca2+ reflect not only surface-bound species but also contain Ca2+ released by the dissolution of matrixminerals, especially gypsum and calcite, which were both present in the ash. Therefore, the CEC values reflect more than just surface processes and include a potentially large undesired contribution from the dissolution of the matrix. Anion exchange capacity values should be accurate.

AERATION TO DEGAS CO2, INCREASE PH AND IRON OXIDATION RATES, AND DECREASE TREATMENT POND SIZE IN TREATMENT OF NET ALKALINE MINE DRAINAGE 1

Flow-through reactor field experiments were conducted at two large net alkaline mine discharges in central Pennsylvania. The goal was to drive off CO2, increase pH, and document increased Fe(II) oxidation rates compared to passive treatment methods. Both discharges were low Mn, low Al, net alkaline discharges with pH of ≈ 5.7 and Fe(II) concentration of ≈ 16 mg/L. Flow rates were ≈ 3000 and 15000 L/min. Three-hour aeration experiments with flow rates scaled to a 14-L reactor resulted in pH increases from 5.7 to greater than 7, temperature increases from 12 to 22 oC, dissolved oxygen increases to saturation with respect to the atmosphere, and Fe(II) concentration decreases to less than 0.05 mg/L. The same experiment at one of the sites with a 13-hour run time and no active aeration had a pH change from 6.1 to 6.3 and decrease in Fe(II) concentration from 16.3 to 13.8 mg L -1 . Results from an Fe(II) oxidation model, using field-measured pH, temperature, dissolved oxygen, and initial Fe(II) concentration and written in a differential equation solver, were the same as the field experiments within analytical uncertainty. The maximum oxidation rate was 1.3 x 10 -4 mol L -1 sec -1 . The model was also modified to predict alkalinity, PCO2, and pH changes based on initial conditions and aeration rate. This modified model also matched the data within analytical uncertainty, is more predictive than the first model, and should serve as a tool for predicting pond size needed for aerated Fe(II) oxidation at the field scale without the need for field pilot studies. Using a published Fe removal rate of 20 g m -2 day -1 and Fe loading from field data, 3.6 x 10 3 and a 3.0 x 10 4 m 2 passive oxidation treatment ponds would be required for Site 21 and Packer 5 discharges, respectively. Fe(II) oxidation modeling of actively aerated systems predicted that a 1 m deep pond with 10 times less area would be adequate to lower Fe(II) concentrations to less than 1 mg L -1 at summer and winter temperatures for both sites. The use of active aeration for net alkaline discharges with high CO2 concentrations can result in considerably reduced treatment area for oxidation and may lower treatment costs, but settling of iron hydroxides was not considered in this study. The reduced capital cost for earthmoving will need to be compared to energy and maintenance costs for aeration.