Applied Geochemistry (v.67, #C)
Developing the scientific framework for urban geochemistry by Lisa G. Chambers; Yu-Ping Chin; Gabriel M. Filippelli; Christopher B. Gardner; Elizabeth M. Herndon; David T. Long; W. Berry Lyons; G.L. Macpherson; Shawn P. McElmurry; Colleen E. McLean; Joel Moore; Ryan P. Moyer; Klaus Neumann; Carmen A. Nezat; Keir Soderberg; Nadya Teutsch; Elisabeth Widom (1-20).
Urban geochemistry is a unique discipline that is distinguished from general geochemistry by the complex infrastructure and intense human activities associated with concentrated population centers. As stated by Thornton (1991) “This subject is concerned with the complex interactions and relationships between chemical elements and their compounds in the urban environment, the influence of past and present human and industrial activities on these, and the impacts or effects of geochemical parameters in urban areas on plant, animal and human health.” Urban areas present special challenges to geochemists attempting to understand geochemical states and fluxes. On the 5–6 of August, 2014, the first meeting of the reorganized Urban Geochemistry Working Group of the International Association of GeoChemistry (IAGC) was held in Columbus, Ohio, United States. Two goals of the meeting were to develop the overall scope, and a general definition of urban geochemistry. Five grand themes were developed: 1) recognizing the urban geochemical signature; 2) recognizing the legacy of altered hydrologic and geochemical cycles in urban environments; 3) measuring the urban geochemical signature; 4) understanding the urban influence on geochemical cycles from the continuous development and erosion of physical infrastructure and episodic perturbations; and 5) relating urban geochemistry to human and environmental health and policy. After synthesizing the discussion of these themes we offer the following perspective on the science of urban geochemistry building on the work of Thornton (1991): Urban geochemistry as a scientiﬁc discipline provides valuable information on the chemical composition of environments that support large populations and are critical to human health and well-being. Research into urban geochemistry seeks to 1) elucidate and quantify the sources, transport, transformations, and fate of chemicals in the urban environment, 2) recognize the spatial and temporal (including legacies) variability in these processes, and 3) integrate urban studies into global perspectives on climate change, biogeochemical cycles, and human and ecosystem health. We hope that this discussion will encourage other geochemists to engage in challenges unique to urban systems, as well as provide a framework for the future of urban geochemistry research.
Keywords: Urban geochemistry; Geochemical change; Urbanization; Sustainable cities; Population growth; Urban hydrology;
Aqueous mineral carbonation of serpentinite on a pilot scale: The effect of liquid recirculation on CO2 sequestration and carbonate precipitation by Nassima Kemache; Louis-César Pasquier; Ikbel Mouedhen; Emmanuelle Cecchi; Jean-François Blais; Guy Mercier (21-29).
Following the promising results obtained on the laboratory scale, an aqueous mineral carbonation process was tested under industrial conditions as part of a pilot project conducted in a cement plant in Quebec. Experiments were conducted using a Parr 18.7 L reactor with cement plant flue gas (14–18 vol.%CO2) and serpentinite tailings as a source of magnesium. The gas was not concentrated or separated before use. The reactions occurred at a solid/liquid ratio of 150 g/L, 22 ± 3 °C and a total pressure between 2 and 10 bar. To decrease water consumption, the effect of liquid recirculation on the rates of CO2 sequestration, Mg leaching and carbonate precipitation were studied. The solid reacted with 6 successive batches of gas (15 min each), and the liquid was recovered for the carbonate precipitation after every two batches. For the recirculation assays, after carbonate filtration, the liquid was reused with subsequent batches.The results showed that the dissolution of CO2 was not affected by the liquid recirculation since 72.5% of the CO2 introduced was dissolved; in comparison to 77% when fresh liquid was used. The captured CO2 resulted in 0.215 and 0.211 g CO2/g of residue in the experiments with and without liquid recirculation, respectively. This result corresponds to approximately 45% of serpentinite's total capacity for CO2 sequestration, which is 0.47 g CO2/g of residue. The carbonate precipitation experiments were conducted in a separate system at low temperatures (32–40 °C) and included 2 h of stirring. When the liquid was recirculated, supersaturation was reached more quickly because of the accumulation of Mg2+ and HCO3 −/CO2− 3 ions. Therefore, the rate of precipitation and the amount of carbonate formed were significantly more important when the liquid was recirculated. However, the overall efficiency corresponding to the captured CO2 under carbonate form does not exceed 9% even with liquid recirculation.
Keywords: Mineral carbonation; Pilot scale; Industrial gas; Liquid recirculation; Carbonates precipitation;
Biogenic processes in crystalline bedrock fractures indicated by carbon isotope signatures of secondary calcite by Elina Sahlstedt; Juha A. Karhu; Petteri Pitkänen; Martin Whitehouse (30-41).
Variation in 13C/12C-isotope ratios of fracture filling calcite was analyzed in situ to investigate carbon sources and cycling in fractured bedrock. The study was conducted by separating sections of fracture fillings, and analyzing the 13C/12C-ratios with secondary ion mass spectrometry (SIMS). Specifically, the study was aimed at fillings where previously published sulfur isotope data indicated the occurrence of bacterial sulfate reduction. The results showed that the δ13C values of calcite were highly variable, ranging from −53.8‰ to +31.6‰ (VPDB). The analysis also showed high variations within single fillings of up to 39‰. The analyzed calcite fillings were mostly associated with two calcite groups, of which Group 3 represents possible Paleozoic fluid circulation, based on comparison with similar dated coatings within the Baltic Shield and the succeeding Group 1–2 fillings represent late-stage, low temperature mineralization and are possibly late Paleozoic to Quaternary in age. Both generations were associated with pyrite with δ34S values indicative of bacterial sulfate reduction. The δ13C values of calcite, however, were indicative of geochemical environments which were distinct for these generations. The δ13C values of Group 3 calcite varied from −22.1‰ to +11‰, with a distinct peak at −16‰ to −12‰. Furthermore, there were no observable depth dependent trends in the δ13C values of Group 3 calcite. The δ13C values of Group 3 calcite were indicative of organic matter degradation and methanogenesis. In contrast to the Group 3 fillings, the δ13C values of Group 1–2 calcite were highly variable, ranging from −53.8‰ to +31.6‰ and they showed systematic variation with depth. The near surface environment of <30 m (bsl) was characterized by δ13C values indicative of degradation of surface derived organic matter, with δ13C values ranging from −30.3‰ to −5.5‰. The intermediate depth of 34–54 m showed evidence of localized methanotrophic activity seen as anomalously 13C depleted calcite, having δ13C values as low as −53.8‰. At depths of ∼60–400 m, positive δ13C values of up to +31.6‰ in late-stage calcite of Group 1–2 indicated methanogenesis. In comparison, high CH4 concentrations in present day groundwaters are found at depths of >300 m. One sample at a depth of 111 m showed a transition from methanogenetic conditions (calcite bearing methanogenetic signature) to sulfate reducing (precipitation of pyrite on calcite surface), however, the timing of this transition is so far unclear. The results from this study gives indications of the complex nature of sulfur and carbon cycling in fractured crystalline environments and highlights the usefulness of in situ stable isotope analysis.
Keywords: Carbon-13; SIMS; Fracture calcite;
Reactive transport modelling of the long-term interactions of corrosion products and compacted bentonite in a HLW repository in granite: Uncertainties and relevance for performance assessment by Javier Samper; Acacia Naves; Luis Montenegro; Alba Mon (42-51).
Here we present a long-term nonisothermal reactive transport model for the interactions of the corrosion products of a carbon-steel canister and the compacted bentonite of the engineered barrier of a high-level radioactive waste repository in granite. Canister corrosion causes an increase in the pH and the concentration of dissolved Fe2+ of the bentonite porewater. Iron precipitates as magnetite and siderite and sorbs via cation exchange and surface complexation on weak sites. Magnetite precipitation reduces significantly the porosity of the bentonite near the canister. The thickness of the zone of reduced porosity is 7 cm at t = 1 Ma. This thickness increases significantly when the dependence of the corrosion rate on the chemical conditions is considered and decreases 3 cm when smectite dissolution and analcime precipitation are taken into account. Model results are not significantly sensitive to the thermal transient and the effect of temperature on the corrosion rate. The conclusions of our simulations are consistent for the most part with those reported by others for engineered barrier systems at similar chemical conditions.
Keywords: Radioactive waste; Reactive transport; Corrosion products; Compacted bentonite;
Contamination, bioaccessibility and human health risk of heavy metals in exposed-lawn soils from 28 urban parks in southern China's largest city, Guangzhou by Yang-Guang Gu; Yan-Peng Gao; Qin Lin (52-58).
The total concentrations and oral bioaccessibility of heavy metals in surface-exposed lawn soils from 28 urban parks in Guangzhou were investigated, and the health risks posed to humans were evaluated. The descending order of total heavy metal concentrations was Fe > Mn > Pb > Zn > Cu > Cr > Ni > Cd, but Cd showed the highest percentage bioaccessibility (75.96%). Principal component analysis showed that Grouped Cd, Pb, Cr, Ni, Cu and Zn, and grouped Cr and Mn could be controlled two different types of human sources. Whereas, Ni and Fe were controlled by both anthropogenic and natural sources. The carcinogenic risk probabilities for Pb and Cr to children and adults were under the acceptable level (<1 × 10−4). Hazard Quotient value for each metal and Hazard Index values for all metals studied indicated no significant risk of non-carcinogenic effects to children and adults in Guangzhou urban park soils.
Keywords: Urban park soils; Oral bioaccessibility; Health risk; Simple bioavailability extraction test (SBET); Heavy metals;
CO2 solubility in aqueous solutions containing Na+, Ca2+, Cl−, SO4 2− and HCO3 -: The effects of electrostricted water and ion hydration thermodynamics by Kimberly Gilbert; Philip C. Bennett; Will Wolfe; Tongwei Zhang; Katherine D. Romanak (59-67).
Dissolution of CO2 into deep subsurface brines for carbon sequestration is regarded as one of the few viable means of reducing the amount of CO2 entering the atmosphere. Ions in solution partially control the amount of CO2 that dissolves, but the mechanisms of the ion's influence are not clearly understood and thus CO2 solubility is difficult to predict. In this study, CO2 solubility was experimentally determined in water, NaCl, CaCl2, Na2SO4, and NaHCO3 solutions and a mixed brine similar to the Bravo Dome natural CO2 reservoir; ionic strengths ranged up to 3.4 molal, temperatures to 140 °C, and CO2 pressures to 35.5 MPa. Increasing ionic strength decreased CO2 solubility for all solutions when the salt type remained unchanged, but ionic strength was a poor predictor of CO2 solubility in solutions with different salts. A new equation was developed to use ion hydration number to calculate the concentration of electrostricted water molecules in solution. Dissolved CO2 was strongly correlated (R2 = 0.96) to electrostricted water concentration. Strong correlations were also identified between CO2 solubility and hydration enthalpy and hydration entropy. These linear correlation equations predicted CO2 solubility within 1% of the Bravo Dome brine and within 10% of two mixed brines from literature (a 10 wt % NaCl + KCl + CaCl2 brine and a natural Na+, Ca2+, Cl− type brine with minor amounts of Mg2+, K+, Sr2+ and Br−).
Keywords: Carbon sequestration; Hydration energy; Hydration number; Dissolved CO2; Bravo Dome;
Mechanism of Hg(II) immobilization in sediments by sulfate-cement amendment by Susana Serrano; Dimitri Vlassopoulos; Peggy A. O'Day (68-80).
Reactive amendments such as Portland and super-sulfate cements offer a promising technology for immobilizing metalloid contaminants such as mercury (Hg) in soils and sediments through sequestration in less bioavailable solid forms. Tidal marsh sediments were reacted with dissolved Hg(II) in synthetic seawater and fresh water solutions, treated with Portland cement and FeSO4 amendment, and aged for up to 90 days. Reacted solids were analyzed with bulk sequential extraction methods and characterized by powder X-ray diffraction (XRD), electron microscopy, and synchrotron X-ray absorption spectroscopy at the Hg LIII- and S K-edge. In amended sediments, XRD, SEM and sulfur K-edge XANES indicated formation of gypsum in seawater experiments or ettringite-type (Ca6Al2(SO4)3(OH)12 .26H2O) phases in fresh water experiments, depending on the final solution pH (seawater ∼8.5; freshwater ∼10.5). Analysis of Hg EXAFS spectra showed Cl and Hg ligands in the first- and second-coordination shells at distances characteristic of a polynuclear chloromercury(II) salt, perhaps as a nanoparticulate phase, in both seawater and fresh water experiments. In addition to the chloromercury species, a smaller fraction (∼20–25%) of Hg was bonded to O atoms in fresh water sample spectra, suggesting the presence of a minor sorbed Hg fraction. In the absence of amendment treatment, Hg sorption and resistance to extraction can be accounted for by relatively strong binding by reduced S species present in the marsh sediment detected by S XANES. Thermodynamic calculations predict stable aqueous Hg–Cl species at seawater final pH, but higher final pH in fresh water favors aqueous Hg-hydroxide species. The difference in Hg coordination between aqueous and solid phases suggests that the initial Hg–Cl coordination was stabilized in the cement hydration products and did not re-equilibrate with the bulk solution with aging. Collectively, results suggest physical encapsulation of Hg as a polynuclear chloromercury(II) salt as the primary immobilization mechanism.Display Omitted
Keywords: Mercury (Hg); Portland cement; Remediation; X-ray absorption spectroscopy; Sulfur XANES;
Uncertainties of geochemical codes and thermodynamic databases for predicting the impact of carbon dioxide on geologic formations by Christoph Haase; Markus Ebert; Frank Dethlefsen (81-92).
Numerical codes are applied to calculate chemical reactions following geologic carbon sequestration in deep formations and CO2 leakage in shallow formations. However, using different thermodynamic databases generates variations in the simulation results, which are referred to as the model uncertainty. The PHREEQC and The Geochemist's Workbench codes were used to simulate anorthite dissolution for storage, retention, transfer, and near-surface formation waters in the respective geological units. For each of the formation waters, a simple one-dimensional scenario was simulated using eight different thermodynamic databases. Groundwaters in shallow aquifers commonly exhibit low ionic strengths (<0.5 mol/kgw) and low temperatures, whereas storage formation waters are characterized by high ionic strength (>1.0 mol/kgw) and high temperatures. In storage formations, mineral trapping is the most efficient process for long-term CO2 storage. However, with respect to the geological formations and the time needed for anorthite dissolution, the model uncertainties associated with using different combinations of numerical codes and thermodynamic databases were largest (∼90%) for the storage formation waters at 58 °C and I = 6.5 mol/l. Conversely, in near-surface formation waters, the model uncertainty was less than 1%. Due to CO2 dissolution, the calculated pH of the formation waters decreased to a range between pH 4.0 and 5.5. In this pH range, the dissolution mechanism of anorthite switches from the slow neutral mechanism to the faster acid mechanism, causing dissolution time length variations. The calculated pH variation further increased with rising ionic strength. A detailed examination of the reasons revealed the activity coefficient calculation method of the main aquatic species to have the largest impact on the simulated model results. The calculation method of the CO2 activity coefficient had the second largest impact. Via calibration with the experimental data, a specific thermodynamic database can be chosen to represent these experimental results. However, the calibration of thermodynamic databases is not possible for all potential reactions in more complex geological systems at large ranges of temperature, ionic strength and pressure conditions. The uncertainties associated with using thermodynamic databases quantified in this study for CO2 storage systems will therefore persist independently from previously conducted calibrations of thermodynamic databases with experimental or field data. In view of these model uncertainties, the modeller is encouraged to include a routine in the simulations for quantification of the model uncertainty depending on the specific scenario or to assess the simulation results as a range of values that represent a soft outcome.Display Omitted
Keywords: Geochemical codes; Thermodynamic databases; CO2 storage; Mineral trapping; Kinetics; Anorthite; Phreeqc; The Geochemist's workbench; Uncertainty analysis;
Do mature hydrocarbons have an influence on acid rock drainage generation? by Martha E. Jiménez-Castañeda; Christopher Boothman; Jonathan R. Lloyd; David J. Vaughan; Bart E. van Dongen (93-100).
The generation of acid rock drainage (ARD) is a biogeochemical process that causes severe ecological impacts, threatening human health worldwide. Microbes involved in acid drainage reactions are generally considered autotrophic but heterotrophic and mixotrophic microorganisms have often been identified at ARD sites. This raises questions about the role of organic matter naturally present at these sites, such as mature hydrocarbons, in promoting the microbial processes underpinning ARD generation. To investigate this, aerobic microcosm experiments were carried out using ARD samples collected at a well-characterised site in northern England (Mam Tor, Derbyshire). Organic analyses indicated the presence of substantial amounts of mature, petroleum-derived hydrocarbons and microbial analyses indicated that the sediment hosts acidophilic bacteria with the capability of degrading petroleum-derived compounds. However, the aerobic microcosm experiments indicated that these petroleum-derived hydrocarbons were not used by the bacterial community and, therefore, are not involved in the reactions that ultimately lead to ARD generation. These observations support a primary role for autotrophs in ARD generation.
Keywords: Hydrocarbons; Acid rock drainage; Mam Tor land slide; Acidophilic bacteria; Jarosite; Goethite;
Sorption and diffusion studies with low molecular weight organic compounds in cementitious systems by E. Wieland; A. Jakob; J. Tits; B. Lothenbach; D. Kunz (101-117).
The uptake of methanol, ethanol, formaldehyde, acetaldehyde, formic acid and acetic acid by cement paste was determined in hydrating cement after 1 h, 28 d and 390 d hydration. The sorption values determined for formate and acetate were critically assessed by investigating through- and out-diffusion of these compounds in fully hydrated cement paste and their uptake by individual cement phases. Diffusion studies included inverse modelling of four data sets for each compound and an uncertainty analysis based on a Latin hypercube sampling procedure. Solid–liquid distribution ratios determined from the hydration experiments are on the order of 10−4 m3 kg−1 in the case of alcohols and aldehydes indicating non-specific (very weak) bonding onto the surface of the cement phases, e.g. through hydrogen bonding. Hydration and diffusion studies reveal slightly higher distribution ratios and reversible uptake by cement paste and cement phases in the case of acetate indicating specific adsorption (electrostatic interaction) onto partially positively charged surface sites of the cement phases. Selective binding of a small fraction of formate is evidenced from both sorption and out-diffusion experiments suggesting the presence of sorption sites capable of strongly bonding the anion, presumably by SO4 2−/HCOO− replacement in the ettringite structure.
Keywords: Cement; Diffusion; Inverse modelling; Sorption; Alcohols; Aldehydes; Carboxylic acids; Carbon-14;
Distribution and origin of dissolved methane, ethane and propane in shallow groundwater of Lower Saxony, Germany by S. Schloemer; J. Elbracht; M. Blumenberg; C.J. Illing (118-132).
More than 90% of Germany's domestic natural gas production and reserves are located in Lower Saxony, North Germany. Recently, research has been intensified with respect to unconventional shale gas, revealing a large additional resource potential in northern Germany. However, many concerns arise within the general public and government/political institutions over potential groundwater contamination from additional gas wells through hydraulic fracturing operations.In order to determine the naturally occurring background methane concentrations, ∼1000 groundwater wells, covering ∼48 000 km2, have been sampled and subsequently analyzed for dissolved methane, ethane and propane and the isotopic composition of methane (δ13C).Dissolved methane concentrations cover a range of ∼7 orders of magnitude between the limit of quantification at ∼20 nl/l and 60 ml/l. The majority of groundwater wells exhibit low concentrations (<1 μl/l), a small number of samples (65) reveal concentration in the range >10 ml/l. In 27% of all samples ethane and in 8% ethane and propane was detected. The median concentration of both components is generally very low (ethane 50 nl/l, propane 23 nl/l).Concentrations reveal a bimodal distribution of the dissolved gas, which might mirror a regional trend due to different hydrogeological settings. The isotopic composition of methane is normally distributed (mean ∼ −70‰ vs PDB), but shows a large variation between −110‰ and +20‰. Samples with δ13C values lower than −55‰ vs PDB (66%) are indicative for methanogenic biogenic processes. 5% of the samples are unusually enriched in 13C (≥25‰ vs PDB) and can best be explained by microbial methane oxidation.According to a standard diagnostic diagram based on methane δ13C values and the ratio of methane over the sum over ethane plus propane (“Bernard”-diagram) less than 4% of the samples plot into the diagnostic field of typical thermogenic natural gases. However, in most cases only ethane has been detected and the remaining less than 15 samples demonstrate an uncommon ratio of ethane to propane compared to typical thermogenic hydrocarbons. These data do not suggest a migration of deeper sourced gases, but a thermogenic source cannot be excluded entirely for some samples. However, ethane and propane can also be generated by microbial processes and might therefore represent ubiquitous background groundwater abundances of these gases. Nevertheless, our data suggest that due to the exceedingly low concentration of ethane and propane, respective concentration changes might prove to be a more sensitive parameter than methane to detect possible migration of deeper sourced (thermally generated) hydrocarbons into a groundwater aquifer.
Keywords: Dissolved hydrocarbon concentration; Groundwater; Baseline; Lower Saxony (Germany); Carbon isotopes;
Solid phase characterization and metal deportment in a mussel shell bioreactor for the treatment of AMD, Stockton Coal Mine, New Zealand by Z.A. DiLoreto; P.A. Weber; C.G. Weisener (133-143).
The Stockton Coal Mine, located on the West Coast of New Zealand, is evaluating the use of a mussel shell bioreactor (MSB) to treat acidic metalliferous runoff from acid forming overburden. This novel approach is similar in concept to vertical flow wetlands (VFWs) and successive alkalinity producing systems (SAPS). The MSB system is a trapezoidal pit 2 m deep, 35 m long, 3–10 m wide with 60° angle sides. During operation it contained 160 tonnes (240 m3) of mussel shell material and was saturated with a 100–200 mm water cap. Influent flowed through the reactor at a mean rate of 0.3 L s−1 resulting in a hydraulic retention time (HRT) of ≈6 days. The prototype MSB was in operation for a total of 1027 days, from June 2009 through March 2012, and effectively sequestered 99.7% of Al, 99.3% of Fe, 98.8% of Ni, 98.4% Tl and 99.3% of Zn, as determined from a previous evaluation of MSB performance. The MSB also effectively neutralized acidity, which resulted in an increase in influent pH from 2.8 to 6.9 in the effluent. Based on an examination of several excavated pits, five distinct reaction zones developed within the MSB. The reaction zones consisted of an allochthonous sediment layer (0–330 mm), an oxidized iron-rich ocherous layer (at 330–350 mm depth), an aluminum layer (at 350–600 mm depth) with geochemical variations throughout (350–500 mm and 500–600 mm); and a chemically reduced bottom shell layer (at 600–1100 mm). Representative samples were collected from each layer and analyzed using a combination of geochemical and physical methods to assess the stability of the secondary minerals and trace metal deportment within the MSB. Major elements Fe, Al, Ni, Tl, and Zn where preferentially associated with particular layers within the MSB. Elevated concentrations of Fe (110,000 mg kg−1) were observed in the allochthonous sediment and ocherous precipitate layers, while Al (27,816 mg kg−1), Ni (55 mg kg−1), and Zn (655 mg kg−1) were elevated within the aluminum and lower reduced depths within the MSB. Trace Tl (21 mg kg−1) showed varying concentrations throughout the MSB, but was strongly correlated to lower layers of the system. Microbial biofilms were observed within the reduced portions of the shell layers often proximal to bacterial shaped sulfides. The geochemical assessment of the MSB presented in this study is the first of its kind for a MSB, and supports the argument that this system is another viable option for passive treatment of AMD.
Keywords: Acid Mine Drainage (AMD); Passive treatment systems; Bioreactors; Bioremediation; Mussel shells; Adsorption; Precipitation; Metal removal;
Immobilization of Cu and As in two contaminated soils with zero-valent iron – Long-term performance and mechanisms by Charlotta Tiberg; Jurate Kumpiene; Jon Petter Gustafsson; Aleksandra Marsz; Ingmar Persson; Michel Mench; Dan B. Kleja (144-152).
Immobilization of trace elements in contaminated soils by zero-valent iron (ZVI) is a promising remediation method, but questions about its long-term performance remain unanswered. To quantify immobilization and predict possible contaminant remobilization on long timescales detailed knowledge about immobilization mechanisms is needed. This study aimed at assessing the long-term effect of ZVI amendments on dissolved copper and arsenic in contaminated soils, at exploring the immobilization mechanism(s), and at setting up a geochemical model able to estimate dissolved copper and arsenic under different scenarios. Samples from untreated and ZVI-treated plots in two field experiments where ZVI had been added 6 and 15 years ago were investigated by a combination of batch experiments, geochemical modeling and extended X-ray absorption fine structure (EXAFS) spectroscopy. Dissolved copper and arsenic concentrations were described by a multisurface geochemical model with surface complexation reactions, verified by EXAFS. The ZVI remained “reactive” after 6–15 years, i.e. the dissolved concentrations of copper and arsenic were lower in the ZVI-treated than in the untreated soils. There was a shift in copper speciation from organic matter complexes in the untreated soil to surface complexes with iron (hydr)oxides in the ZVI-treated soil. The pH value was important for copper immobilization and ZVI did not have a stabilizing effect if pH was lower than about 6. Immobilization of arsenic was slightly pH-dependent and sensitive to the competition with phosphate. If phosphate was ignored in the modeling, the dissolution of arsenate was greatly underestimated.Display Omitted
Keywords: Copper; Arsenic; ZVI; Zero-valent iron; Iron (hydr)oxide; Immobilization; Stabilization; Contaminated soil; Remediation; Geochemical modeling; Surface complexation model; EXAFS spectroscopy; X-ray absorption spectroscopy;
Trace element mobility in mine waters from granitic pegmatite U–Th–REE deposits, Bancroft area, Ontario by A.J. Desbarats; J.B. Percival; K.E. Venance (153-167).
Small, low-grade, granitic pegmatite U–Th–REE deposits are found throughout the Grenville geological province of eastern Canada. Groundwater quality at historical mining properties in the Bancroft area was investigated in order to better understand the mobility of trace elements that may pose health risks if there is renewed development of this class of mineral deposit. Groundwater samples were obtained from diamond drill holes, flowing adits and flooded mine shafts. Uranium occurs almost entirely in the dissolved (<0.45 μm) phase and is found at concentrations reaching 2579 μg/L. The Canadian maximum acceptable concentration for U in drinking water (0.02 mg/L) was exceeded in 70% of samples. Regulatory limits for 226Ra (0.5 Bq/L) and for 210Pb (0.2 Bq/L) were generally exceeded in these samples as well. Speciation modeling indicates that over 98% of dissolved U is in the form of highly mobile uranyl-Ca–carbonate complexes known to inhibit U adsorption. Uranium concentrations in groundwater appear to be correlated with the uranothorite content of the deposits rather than with their U grade. Uranothorite may be more soluble than uraninite, the other ore mineral, because of its non-ideal composition and metamict structure. Thorium, released concomitantly with U during the dissolution of uranothorite and thorian uraninite, exhibits median and maximum total concentrations of only 0.1 and 11 μg/L, respectively. Mass balance and stoichiometric considerations indicate that almost all Th is immobilized very close to its source. The sums of total light REE (La–Gd) concentrations have median and maximum values of 6 and 117 μg/L, respectively. The sums of total heavy REE (Tb–Lu) concentrations have median and maximum values of 0.8 and 21 μg/L, respectively. Light REE are derived mainly from the dissolution of metamict allanite whereas the sources of heavy REE are widely dispersed among accessory minerals. Fractionation patterns of REE in the dissolved phase are flat or concave, with negative Ce anomalies associated with more oxic groundwaters. The data suggest preferential LREE and HREE complexation with organic and carbonate ligands in the dissolved phase, respectively. Fractionation patterns in the suspended particulate phase exhibit decreasing enrichment with atomic number from La to Gd and a flat profile from Tb to Lu. This is explained by preferential sorption of LREE and uniform sorption of HREE. Manganese particulates are the most likely sorbents. Potential health risks from Th or REE in mine waters are unlikely due to the very low mobility of these elements. Uranium, on the other hand, exhibits high mobility in shallow, oxic groundwaters and drainage from some mine adits may require mitigation.
Keywords: Uranium; Thorium; Radionuclides; REE; Groundwater;
Carbonate precipitation in the alkaline lake Specchio di Venere (Pantelleria Island, Italy) and the possible role of microbial mats by Marianna Cangemi; Paolo Censi; Andreas Reimer; Walter D'Alessandro; Dorothea Hause-Reitner; Paolo Madonia; Ygor Oliveri; Giovannella Pecoraino; Joachim Reitner (168-176).
Alkaline lakes like the hydrothermally affected lake Specchio di Venere (Pantelleria Island, Central Mediterranean) are typical geological settings harbouring calcified microbial mats. The present work is focused on the discrimination between biotic and abiotic processes driving carbonate precipitation in this lake, using hydrochemical, mineralogical and isotopic data. Hydrochemical analyses demonstrate that the lake is nearly 10−fold supersaturated with regard to aragonite and seasonally reaches hydromagnesite supersaturation. Microscopic observations depict organosedimentary laminated structures consisting of microbial communities and aragonitic precipitates, which are rather disseminated in pores than directly linked to microorganisms. Oxygen isotopic data indicate that authigenic carbonate crystallisation from evaporating water is the dominant precipitation process, further suggested by the absence of textural evidence of diagenetic processes. Conversely, the observed δ13C values reflect an influence of extracellular polymeric substances (EPS) on carbon fractionation during the precipitation process, due to the selective sequestration of 12C in the biomass. The above considerations suggest that at lake Specchio di Venere the carbonate precipitation is mainly of inorganic nature, but a minor role played by biologically influenced processes in microbial mats is not excluded.
Keywords: Alkaline lake; Carbonate precipitation; Hydrochemistry; Pantelleria;
Leaching potential and redox transformations of arsenic and selenium in sediment microcosms with fly ash by Grace E. Schwartz; Nelson Rivera; Sung-Woo Lee; James M. Harrington; James C. Hower; Keith E. Levine; Avner Vengosh; Heileen Hsu-Kim (177-185).
The unintended release of coal ash to the environment is a concern due to the enrichment of contaminants such as arsenic (As) and selenium (Se) in this solid waste material. Current risk assessments of coal ash disposal focus on pH as the primary driver of leaching from coal ash. However, redox speciation of As and Se is a major factor for their mobilization potential and has received much less attention for risk assessments, particularly in disposal scenarios where coal ash will likely be exposed to microbially-driven redox gradients. The aim of this study was to demonstrate the differences of aerobic and anaerobic conditions for the leaching of As and Se from coal ash. Batch sediment-ash slurry microcosms were performed to mimic an ash spill scenario and were monitored for changes in As and Se speciation and mobilization potential. The results showed that the dissolved As concentrations were up to 50 times greater in the anaerobic microcosms relative to the aerobic microcosms during the two week incubation. This trend was consistent with As redox speciation determined by X-ray absorption spectroscopy, which indicated that 55% of the As in the solid phase at the end of the experiment was present as As(III) (a more leachable form of arsenic relative to As(V)). In the aerobic microcosms, only 13% of the As was As(III) and the rest was As(V). More than half of the Se was present as Se(IV) in the original fly ash and in the aerobic microcosms, while in the anaerobic microcosms Se was gradually transformed to less soluble Se(0) species. Likewise, dissolved Se concentrations were up to 25 times greater in the aerobic microcosms relative to anaerobic conditions. While the overall observations of As and Se mobilization potential from coal ash were consistent with expectations for aqueous and solid phase speciation of these elements, the findings directly show the relevance of these processes for coal ash disposal. These results highlight the need to select appropriate environmental parameters to include in risk assessments as well as provide potential geochemical monitoring tools through the use of dissolved Se/As ratios to determine the redox conditions of ash storage and spill sites.Display Omitted
Keywords: Coal combustion residuals; Water quality; Solid waste disposal;
Corrigendum to “Thermodynamic modelling of alkali-activated slag-based cements” [Appl. Geochem. 61 (2015) 233–247] by Rupert J. Myers; Barbara Lothenbach; Susan A. Bernal; John L. Provis (186).