Richard T. Wilkin

History of water-column anoxia in the Black Sea indicated by pyrite framboid size distributions

Abstract A detailed study of size distributions of framboidal pyrite in Holocene Black Sea sediments establishes the timing of a change from deposition under an oxic water column to deposition under an anoxic and sulfidic water column. In the most recent carbonate-rich sediments (Unit I) and in the organic carbon-rich sapropel (Unit II), framboid size distributions are remarkably uniform (mean diameter = 5 μm); over 95% of the framboids in Unit I and Unit II are

Geochemical impacts to groundwater from geologic carbon sequestration: controls on pH and inorganic carbon concentrations from reaction path and kinetic modeling.

Geologic carbon sequestration has the potential to cause long-term reductions in global emissions of carbon dioxide to the atmosphere. Safe and effective application of carbon sequestration technology requires an understanding of the potential risks to the quality of underground sources of drinking water. In particular, concern is warranted regarding the potential for CO(2) leakage through geological features and abandoned wells that may result in detrimental perturbations to subsurface geochemistry. Reaction path and kinetic models indicate that geochemical shifts caused by CO(2) leakage are closely linked to mineralogical properties of the receiving aquifer. CO(2) gas dissolution into groundwater and subsequent reaction with aquifer minerals will control the evolution of pH-bicarbonate envelopes. These parameters provide geochemical context for predicting how regulated contaminants associated with aquifer solids will respond via various mineral-water reaction processes. The distribution and abundance of carbonate, silicate, oxide, and phyllosilicate minerals are identified as key variables in controlling changes in groundwater geochemistry. Site-specific risk assessments may require characterization of aquifer geology, mineralogy, and groundwater chemistry prior to CO(2) injection. Model results also provide a frame of reference for developing indicative measurement, monitoring, and verification (MMV) protocols for groundwater protection.

Variations in pyrite texture, sulfur isotope composition, and iron systematics in the Black Sea: evidence for Late Pleistocene to Holocene excursions of the o2-h2s redox transition

—Time-dependent variations in the physicochemical properties of pyrite were investigated in four sediment cores from the Black Sea. In the laminated, deep-basin sediments of Unit I and Unit II, >86% of pyrite particles are present as fine-grained framboidal aggregates. In these sediments, the dominance of pyrite framboids, with a narrow size distribution and maximum size < 18 μm, is evidence of syngenetic (water column) pyrite formation subjacent to the O2-H2S boundary. Sediments from the basin margin collected below the impingement of the O2-H2S boundary contain an increased proportion of fine-grained euhedral grains of pyrite relative to framboidal aggregates, suggesting increased additions of diagenetic pyrite below the sediment-water interface. The more efficient reduction and sulfidation of iron in the water column of the central region of the basin implies a more reactive source of iron there compared to the basin margins, and is tied to increasing %pyrite as framboids and greater degrees of pyritization (DOP) in the basin center relative to the basin margins. An evaluation of Fe-S-C relations indicates that pyrite formation was limited by sulfate availability during deposition of the oldest lacustrine sediments of Unit III, C-limited for upper Unit III due to the input of sulfate-rich Mediterranean seawater, and Fe-limited in the laminated, organic carbon-rich sediments of Units I and II. These changes in pyrite-limiting factors occurred within 4 m of compacted burial representing <15 ka. Sediment fabric and framboid size distributions at a shallow, basin-margin site suggest that during two extended periods of Unit II deposition, the O2-H2S transition retreated to depths below 200 m. However, during these same intervals, the δ34S of pyrite remains uniform compared to overlying and underlying laminated sediments, implying that this chemical signature may not be uniquely tied to the position of the O2-H2S boundary relative to the sediment-water interface, but rather is likely related to biogeochemical processes.

Perchlorate Behavior in a Municipal Lake Following Fireworks Displays

Perchlorate salts of potassium and ammonium are the primary oxidants in pyrotechnic mixtures, yet insufficient information is available regarding the relationship between fireworks displays and the environmental occurrence of perchlorate. Here we document changes in perchlorate concentrations in surface water adjacent to a site of fireworks displays from 2004 to 2006. Preceding fireworks displays, perchlorate concentrations in surface water ranged from 0.005 to 0.081 microg/L, with a mean value of 0.043 microg/L. Within 14 h after the fireworks, perchlorate concentrations spiked to values ranging from 24 to 1028x the mean baseline value. A maximum perchlorate concentration of 44.2 microg/L was determined following the July 4th event in 2006. After the fireworks displays, perchlorate concentrations decreased toward the background level within 20 to 80 days, with the rate of attenuation correlating to surface water temperature. Adsorption tests indicate that sediments underlying the water column have limited (< 100 nmol/g) capacity to remove perchlorate via chemical adsorption. Microcosms showed comparatively rapid intrinsic perchlorate degradation in the absence of nitrate consistent with the observed disappearance of perchlorate from the study site. This suggests that at sites with appropriate biogeochemical conditions, natural attenuation may be an important factor affecting the fate of perchlorate following fireworks displays.

Solubility and stability of zeolites in aqueous solution; I, Analcime, Na-, and K-clinoptilolite

Abstract The solubilities of analcime and clinoptilolite were determined in dilute, weakly alkaline, aqueous solutions below 300 ℃ and at vapor-saturated pressures. Analcimes used in this study were from Mont St. Hilaire. Quebec (an1, Si/Al = 2.02) and Wikieup, Arizona (an2, Si/Al = 2.55): clinoptilolite samples were from Castle Creek. Idaho (Si/Al = 4.50). The effects of alkali content (Na,K) on clinoptilolite solubility were determined by using cation-exchanged varieties of the Castle Creek material (cp1, cp2). In neutral to weakly alkaline solutions, the dominant solubility-controlling reactions of these zeolites are NaAlSi2O6 · H2O(an1) + 5H2O(1) ↔ Na+ + Al(OH)-4 + 2Si(OH)4(aq), Na0.85Al0.85Si2.15O6 · H2O(an2) + 5H2O(1) ↔ 0.85Na+ + 0.85Al(OH)-4 + 2.15Si(OH)4(aq), Na1.1Al1.1Si4.9O12 · 3.5H2O(cp1) + 8.5H2O(1) ↔ 1.1Na+ + 1.1Al(OH)-4 + 4.9Si(OH)4(aq), and K1.1Al1.1Si49O12 · 2.7H2O(cp2) + 9.3H2O(1) ↔ 1.1K+ + 1.1Al(OH)-4 + 4.9Si(OH)4(aq). The logarithm of the equilibrium constants of these reactions were fit to the function: log K = A + BT + C/T + D log T. At 25 ℃, log K25 values for the Mont St. Hilaire analcime, Wikieup analcime, Na-clinoptilolite, and K-chnoptilolite are -16.1, -15.0, -26.5, and -28.1, respectively. These data were combined with the thermodynamic properties of the aqueous (aq) species Si(OH)4, Al(OH)-4, Na+, K+, and liquid water (1) to determine standard Gibbs free energies of formation as a function of temperature. Values of ΔG0f at 25 ℃ and 1 bar for the Mont St. Hilaire analcime and Wikieup analcime are -3089.2 and -3044.4 kJ/mol. respectively. The ΔG0f values for hydrous Na-clinoptilolite and K-clinoptilolite, respectively, are -6267.9 and -6107.4 kJ/mol at 25 ℃ and 1 bar. The solubility data reported here, and results obtained from previous calorimetric studies, indicate that the aluminosilicate frameworks of analcime and clinoptilolite are stabilized by an increase in Al content.

Black sea chemocline oscillations during the Holocene: Molecular and isotopic studies of marginal sediments

We measured d 13 C values of free and sulfur-bound lipids and framboidal pyrite-size distributions in three sediment cores from the southern margins of the Black Sea. The margin cores show a marked diAerence in the occurrence of biomarkers from green sulfur bacteria compared with the deep-basin cores, as a result of deepening of the chemocline resulting from enhanced mixing and/or decreased light-penetration as a consequence of high turbidity and productivity in shelf waters. Quantitation of biomarkers suggests that photic-zone anoxia along the shelf margin was generally absent during the deposition of unit I, although occurred during the deposition of Unit IIb at two sites. # 2000 Elsevier Science Ltd. All rights reserved.

Nucleation and growth kinetics of analcime from precursor Na-clinoptilolite

Abstract The kinetics and mechanism of analcime formation from precursor Na-clinoptilolite (Cpt-Na) and Na-mordenite (Mor-Na) were investigated from 125-225 °C, pressures up to 34.5 MPa, and pH = 9.2-10.7. By using batch and flow-through experimental methodologies, compositions of solids and solutions were monitored to track reaction progress, determine rates of analcime nucleation and growth, and evaluate the rate dependency of these processes on aqueous supersaturation. Analcime formation proceeds via a clinoptilolite (or mordenite) dissolution→analcime nucleation→analcime growth sequence. Synthetic analcime crystals are similar in morphology (trapezohedron to cubic trapezohedron) and composition (Si/Al = 2.1-2.7) to sedimentary analcimes formed during the lowgrade alteration of crustal rocks, evidence that the experimental reaction mechanism is similar to that in natural environments. Rates of analcime nucleation were approximated by evaluating the time-dependence of the size and number of particles and range between 109.80 and 1011.88 per h per cm3 at 150° and 225 °C, respectively. The nucleation rate is a function of temperature and degree of supersaturation: ln rate = 56.76 - 15978.9/T + 2.99 × 10-4⋅ΔGr, where ΔGr is the free energy change of analcime precipitation and T is temperature in Kelvins. This rate equation is consistent with an apparent activation energy of analcime nucleation (Ea,n) of 132.8 ± 8.3 kJ/mol. Although conditions were thermodynamically favorable for analcime formation at 125 °C, nucleation was not detected after 144 hours. These data suggest that diagenetic timing of the clinoptilolite to analcime transformation is principally controlled by kinetics rather than by thermodynamic equilibrium. Rates of analcime growth were estimated by measuring particle size distributions. Average growth rates ranged from 0.15 μm/h at 150 °C to 0.396 μm/h at 225 °C, and are consistent with an apparent activation energy of analcime growth (Ea,g) of 77.1 ± 9.4 kJ/mol. These nucleation and growth parameters are combined to successfully model the evolution of analcime particle size distributions. The experimental methods developed in this study demonstrate the use of hydrothermal flow-through methods in the study of zeolite transformations.

Solubility and stability of zeolites in aqueous solution: II. Calcic clinoptilolite and mordenite

Abstract The solubilities of Ca-exchanged clinoptilolite (Cpt-Ca) and Ca-exchanged mordenite (Mor-Ca) have been measured in aqueous solutions between 25 and 275 °C and at saturated water vapor pressures. Natural zeolites were cation exchanged to close to Ca end-member composition (90% for Cpt-Ca, and 98% for Mor-Ca). The controlling dissolution reactions may be written as: Cpt-Ca: Ca0.56(Al1.12Si4.88O12)(3.9H2O) + 8.1H2O = 0.56Ca2+ + 1.12Al(OH)4 - + 4.88Si(OH)04(aq) Mor-Ca: Ca0.515(Al1.03Si4.97O12)(3.1H2O) + 8.9H2O = 0.515Ca2+ + 1.03Al(OH)4 - + 4.97Si(OH)04(aq) These reactions are reversible as shown by equilibrium constants calculated for approach from under- and supersaturation. The log Ksp for Cpt-Ca increases from -26.9 at 25 °C to a maximum of -16.9 at 275 °C, whereas for Mor-Ca the equilibrium constant varies from -25.3 at 25 °C to -17.7 at 265 °C. The solubilities for both zeolites increase with increasing temperature showing a positive enthalpy for the dissolution reaction. At lower temperatures Cpt-Ca is slightly more soluble than Mor-Ca, which agrees with natural observations where mordenite and clinoptilolite commonly occur together spatially but mordenite is in general the higher-temperature phase. A comparison with other exchanged clinoptilolites indicates that Cpt-Ca is more stable than the Na, K, and Mg varieties. The results demonstrate that the exchanged cation has a large effect on the solubility behavior, and that divalently exchanged varieties are less soluble than monovalent varieties. From the solubility constants, the standard Gibbs free energies of formation for hydrous Cpt-Ca and Mor-Ca at 25 °C and 1 bar were determined to be -6387 ± 5 kJ/mol and -6275 ± 7 kJ/mol respectively. However, compared to the hydration states and the aluminosilicate structure, the effect of the cation on the Gibbs free energies of formation is small.

Evidence of Sulfate-Dependent Anaerobic Methane Oxidation within an Area Impacted by Coalbed Methane-Related Gas Migration

We evaluated water quality characteristics in the northern Raton Basin of Colorado and documented the response of the Poison Canyon aquifer system several years after upward migration of methane gas occurred from the deeper Vermejo Formation coalbed production zone. Results show persistent secondary water quality impacts related to the biodegradation of methane. We identify four distinct characteristics of groundwater-methane attenuation in the Poison Canyon aquifer: (i) consumption of methane and sulfate and production of sulfide and bicarbonate, (ii) methane loss coupled to production of higher molecular weight (C2+) gaseous hydrocarbons, (iii) patterns of 13C enrichment and depletion in methane and dissolved inorganic carbon, and (iv) a systematic shift in sulfur and oxygen isotope ratios of sulfate, indicative of microbial sulfate reduction. We also show that the biogeochemical response of the aquifer system has not mobilized naturally occurring trace metals, including arsenic, chromium, cobalt, nickel, and lead, likely due to the microbial production of hydrogen sulfide which favors stabilization of metals in aquifer solids.