Browsing by Author "Chermak, John A."

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    Aluminum hydroxide coatings in limestone drains
    Palomino-Ore, Sheyla B.; Rimstidt, J. Donald; Chermak, John A.; Schreiber, Madeline E.; Seal, Robert R. II (2019-04)
    This paper describes a mixed flow reactor experiment and associated data analysis scheme that are well suited for studying the chemical and physical processes that occur in limestone drains used to treat acid mine drainage (AMD). The experiment simulates the slowly evolving, near steady state, reactions that form coatings on limestone. The resulting coatings can be recovered for analysis of their structure and composition. Analysis of the time evolution of the composition of the effluent solutions is used to isolate and understand key factors that affect limestone drain performance. The experiment investigated reactions between acidic aluminum sulfate solutions and calcite. The aluminum sulfate feed solutions contained 0.002-0.01 molal (32-329 mg/kg) Al and had pH values ranging from 3.7 to 4.2. At the beginning each experiment, the rate of H+ consumption by reaction with the calcite was fast causing a distinct increase of the effluent pH. The pH increase caused some of the dissolved Al to precipitate as a coating on the calcite surfaces. The coating blocked the transfer of ions to and from the calcite causing the reaction rates to be limited by ion diffusion through the coating. The continued growth of the coating caused it to become an increasingly effective barrier to ion transport, which caused the neutralization rate to slow and the effluent solution pH to decline toward that of the feed solution. Powder X-ray diffraction (XRD) and scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS) suggested that the coatings were mostly poorly crystalline gibbsite. Effluent solutions were analyzed to determine pH along with Al, Ca and S concentrations. The coating thickness at each sample time was estimated from the amount of Al lost from the solution since the beginning of the experiment. This thickness and the Ca and H+ fluxes were used to find the apparent H+ diffusion coefficient in the coatings.
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    Arsenic Release from Chlorine Promoted Oxidation of Pyrite in the St. Peter Sandstone Aquifer, Eastern Wisconsin
    West, Nicole Renee (Virginia Tech, 2008-04-25)
    High arsenic concentrations (>100 ppb) have been measured in wells completed in the Ordovician St. Peter sandstone aquifer of eastern Wisconsin. The primary source of arsenic is As-bearing sulfide minerals within the aquifer. There is concern that periodic disinfection of wells by chlorination may facilitate arsenic release to groundwater by increasing the rate of sulfide mineral oxidation. Current guidance from the Wisconsin Department of Natural Resources recommends a "low-dose" treatment of 20% of the chlorine strength and 10% of the of the contact time of chlorine treatments used in non-arsenic impacted wells for well disinfection and biofilm removal. In order to provide information pertaining to WDNR's recommendations, St. Peter sulfide minerals were reacted with a range of chlorine "shock-treatments" similar to those occurring in wells. This study focuses on abiotic processes that mobilize arsenic from the solid phase during controlled exposure to chlorinated solutions. Thin sections were made from aquifer material collected at Leonard's Michael quarry, located in Winnebago County, Wisconsin. Bulk arsenic content of this material was measured as 674 ppm. Quantitative EPMA analysis shows As zoning in pyrite grains with concentrations up to 1 wt. % As. After mineral characterization, the thin sections were exposed to solutions of 60 mg/L "free chlorine," 1200 mg/L "free chlorine," and nanopure water (control) at pH 7.0 and pH 8.5 for 24 hours. Thin sections were then analyzed to measure changes in the pyrite surfaces. For solution experiments, aquifer material was crushed to between 250 μm and 355 μm mesh sizes (S.A. ~ 50 cm2/g – 60 cm2/g, Foust et al. 1980) and reacted under the same conditions as the thin sections in a batch reactor. Solution samples were collected periodically during the 24 hour exposure and analyzed for arsenic, iron, and sulfate ion. Pyrite oxidation is shown to dramatically increase with increasing chlorine concentrations as shown by measurements of released sulfate ion, used here as the reaction progress variable. EPMA maps also reveal complete oxidation of pyrite cements to Fe-oxyhydroxides at 1200 mg/L "free chlorine" and pH 7.0. This behavior does not occur at lower concentrations or higher pH. Arsenic release to solution does not appear to be directly correlated to increasing chlorine concentrations, but is governed by Fe-oxyhydroxide nucleation, which inhibits the release of dissolved arsenic at higher concentrations of chlorine.
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    Effect of Coatings on Mineral Reaction Rates in Acid Mine Drainage
    Huminicki, Danielle Marie Cecelia (Virginia Tech, 2006-07-24)
    This dissertation includes theoretical and applied components that address the effect of coatings on rates of mineral reactions that occur in acid mine drainage (AMD) environments. The two major projects investigated how diffusion-limited transport of reactants through pore spaces in coatings on mineral grains affects the reaction rate of the underlying mineral. The first project considered the growth of gypsum coatings on the surface of dissolving limestone in anoxic limestone drains (ALD), which reduces the neutralization rate of the dissolving limestone and the subsequent effectiveness of this treatment. The second project investigated the conditions where iron oxyhydroxide coatings form on oxidizing pyrite and the potential strategies to prevent "runaway" AMD by reducing the rate of acid production to the point that the acid can be neutralized by the surrounding rocks. In both studies, experiments were conducted to measure reaction rates for the underlying minerals, as coatings grew thicker. These experimental data were fit to a diffusion model to estimate diffusion coefficients of reactants through pore spaces in coatings. These models are extrapolated to longer times to predict the behavior of the coated grains under field conditions. The experimental results indicate that management practices can be improved for ALDs and mine waste piles. For example, supersaturation with respect to gypsum, leading to coating formation, can be avoided by diluting the ALD feed solution or by replacing limestone with dolomite. AMD can be prevented if the rate of alkalinity addition to mine waste piles is faster than acid is produced by pyrite oxidation. The diffusion model developed in this study predicts when iron oxyhydroxide coatings will become thick enough that alkalinity from the surroundings is sufficient to neutralize acid produced by coated pyrite oxidation and additional alkalinity is no longer required.
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    Silica dissolution at low pH in the presence and absence of fluoride
    Mitra, Arijit (Virginia Tech, 2008-04-29)
    SiO₂ is the most abundant oxide in the earth and its properties, behaviors and interactions are of immense scientific and technological importance. Of particular importance are the interactions of silica with aqueous fluids because these fluids are present in nearly every natural setting. The dissolution of silica and glass by HF plays a very important role in technology and is widely used for the etching of silica and silicate glasses in the glass industry, in the flint industry, in surface micromachining, in etching of glass fibers for near-field optical probes, in the creation of frosted surfaces for decorative applications like frosted glass and cosmetic vials. I performed 57 batch reactor experiments in acidic fluoride solutions to measure the dissolution rate of quartz. Quartz dissolution rate data from other published studies were combined with the rate data from my experiments and these 75 data were analyzed using multiple linear regression to produce an empirical rate law for quartz rqz = 10-4.53 (e-18932/RT) aHF1.18 aH⁺-0.39 where -5.13 < aHF < 1.60, -0.28 < pH < 7.18, and 25 < T < 100 °C. Similarly, 97 amorphous silica dissolution rate data from published studies were analyzed using multiple linear regression to develop an empirical rate law for amorphous silica ras = 100.48 (e-34243/RT) aHF1.50 aH⁺-0.46 where -5.13 < aHF < 1.60, -0.28 < pH < 7.18 and 25 < T < 70 °C. An examination of the empirical rate laws suggests that the rate-determining step in the reaction mechanism involves a coordinated attack of HF and H⁺ on the Si-O bond where the H⁺ ion, acting as a Lewis acid, attacks the bridging O atom, while the F end of a HF molecule, acting as a Lewis base, attacks the Si atom. This allows a redistribution of electrons from the Si-O bond to form a O-H and a Si-FH bond, thus "breaking" the Si-O bond. In order to quantify the effect of fluoride on the dissolution of silica, I also performed a series of 81 quartz dissolution and 20 amorphous silica dissolution experiments in batch reactors over a pH range of 0 to 7 to investigate the effect of H⁺ on silica dissolution rates. Between pH 3.5 and 7 silica dissolution rates are independent of pH, but they increase significantly below pH 3.5, so that the dissolution rate of both quartz and amorphous silica at pH 0 is more than an order magnitude faster than the dissolution rate at pH 3.5. I found that the empirical rate law for the dissolution of the"disturbed surface" of quartz in the pH range of 0 to 3.5 is rqz,pH = 10-0.23 (e-59392/RT) aH⁺0.28 where 0 < pH < 3.5 and 25 < T < 55°C. The empirical rate law for amorphous silica dissolution in the pH range 0 to 3.5 is rqz,pH = 100.56 (e-64754/RT) aH⁺0.40 where 0 < pH < 3.5 and 25 < T < 55°C. Based on the empirical rate laws I suggest that the rate-determining step in the reaction mechanism involves a coordinated attack of H₃O⁺, acting as a Lewis acid reacts, on a bridging O atom and the O end of a H₂O, acting as a Lewis base, on the Si atom. This results in a redistribution of electrons from the Si-O bridging bond to form two Si-OH surface species.