Applied Geochemistry (v.25, #2)

Antimony in the environment: Lessons from geochemical mapping by Clemens Reimann; Jörg Matschullat; Manfred Birke; Reijo Salminen (175-198).
The distribution of Sb in a variety of sample materials, including soils, plants and surface water, was studied at different scales, from continental to local, combining published data sets with the aim of delineating the impact and relative importance of geogenic vs. anthropogenic Sb sources. Geochemical mapping demonstrates that variation is high at all scales – from the detailed scale with sample densities of many sites per km2 to the continental-scale with densities of 1 site per 5000 km2. Different processes govern the Sb distribution at different scales. A high sample density of several samples per km2 is needed to reliably detect mineralisation or contamination in soil samples. Median concentrations are so low for Sb in most sample materials (below 1 mg/kg in rocks and soils, below 0.1 mg/kg in plants, below 0.1 μg/L in surface water) that contamination is easier to detect than for many other elements. Distribution patterns on the sub-continental to continental-scale are, however, still dominated by natural variation. Given that the geochemical background is characterised by a high variation at all scales, it appears impossible to establish a reliable single value for “good soil quality” or a “natural background concentration” for Sb for any sizeable area, e.g., for Europe. For such a differentiation, geochemical maps at a variety of scales are needed.Different sample materials can reflect different geochemical sources and processes, even when collected from the same survey area. Weathering (soil formation) leads to an increased Sb concentration in soils compared to rocks. Organic soils are highly enriched (factor 5–10 compared to mineral soils) in Sb. Soils and stream sediments return comparable median Sb concentrations. Plants are usually well protected against Sb uptake. There exist, however, plant species that can accumulate Sb to values of more than 1000 mg/kg. Antimony concentrations in the marine environment are not sufficiently well-documented. High Sb concentrations, related to hydrothermal and volcanic processes may have been previously underestimated.
Keywords: Exploration; Contamination; Background; Soil; Sediment; Biogeochemistry;

Lacustrine sediments, submerged tailings, and their pore waters have been collected at several sites in Yellowknife Bay, Great Slave Lake, Canada, in order to investigate the biogeochemical controls on the remobilization of As from mining-impacted materials under different depositional conditions. Radiometric dating confirms that a mid-core enrichment of Pb, Zn, Cu and Sb corresponds to the opening of a large Au mine 60 a ago. This was evident even in a relatively remote site. Arsenic was enriched at mid-core, coincident with mining activity, but clearly exhibited post-depositional mobility, migrating upwards towards the sediment water interface (SWI) as well as down-core. Deep-water (15 m) Yellowknife Bay sediments that contain buried mine waste are suboxic, relatively organic-rich and abundant in microbes with As in pore waters and sediments reaching 585 μg/L and 1310 mg/kg, respectively. Late summer pore waters show equal proportions of As(III) and As(V) (16–415 μg/L) whereas late winter pore waters are dominated by As(III) (284–947 μg/L). This can be explained by As(III) desorption mechanisms associated with the conversion of FeS to FeS2 and the reduction of As(V) to As(III) through the oxidation of dissolved sulfide, both microbially-mediated processes. Processes affecting As cycling involve the attenuating efficiency of the oxic zone at the SWI, sediment redox heterogeneity and the reductive dissolution of Fe(hydr)oxides by labile organic matter, temporarily and spatially variable.

In order to ameliorate acidic discharge, the inactive Omega Coal Mine, West Virginia was partially filled by injection of a grout consisting of 98% coal utilization byproducts (CUB), including fluidized bed combustion ash and fly ash, and 2% Portland cement. In this study, discharge chemistry and Sr isotope ratios were determined to identify and quantify the extent of interaction between mine waters and the CUB–cement grout. Eight sampling sites were monitored around the downdip perimeter of the mine. The major and trace element chemistry of the discharges was generally not sufficient to distinguish between discharges that interacted with grout and those that did not. Elements that showed the most separation include K and As, which were elevated in some waters that interacted with CUB–cement grout. In contrast, the Sr isotope ratios clearly distinguished discharges from grouted and non-grouted areas. Discharges that bypassed the grouted portions had 87Sr/86Sr ratios ranging from 0.71510 to 0.71594, while two discharges that interacted with grout had ratios in the range of 0.71401–0.71456. The Treatment Inlet, which includes both grouted and ungrouted discharges, yielded intermediate isotopic ratios. Leaching experiments on CUB–cement grout, coal and surrounding rocks are consistent with the isotopic trends observed in the discharges. Based on these results, waters that interacted with grout received 30–40% of their Sr from the CUB–cement grout material. These results suggest that the grout material is chemically eroding at a rate of approximately 0.04% per year. This novel application of the Sr isotope system illustrates its ability to sensitively track and quantify fluid interaction with coal and CUB-based grout.

Cellulosic materials, such as wood, paper products and cardboard that have been co-disposed with low-level nuclear waste have been shown to produce leachate with natural organic matter (NOM) concentrations of hundreds of mg/L C and, as such, have the potential to influence the fate and transport of radionuclides in the subsurface environment. The objective of this study was to examine the influence of NOM on the sorption of Eu (an analogue for trivalent radionuclides) to two coastal plain sediments from the US Department of Energy’s Savannah River Site. Particular attention was directed at quantifying Eu interactions with NOM sorbed to sediments (NOMsed) in laboratory experiments and developing conditional stability constants for that interaction using the thermodynamic equilibrium speciation model MINTEQA2. Europium sorption to the two sediments systematically increased as pH increased from 3.9 to 6.7. With increasing additions of NOM to the aqueous phase from 0 to 222 mg/L C, Eu sorption initially increased to a maximum at 10 mg/L C NOMaq and then decreased with increasing NOMaq concentrations. Increases in Eu sorption at low NOM additions was attributed to the sorption of NOM to the sediment surface increasing the number of sorption sites on the low cation-exchange capacity sediments and/or increasing the association constant (log  K) for the Eu-sediment surface reaction. Decreases in Eu sorption at higher NOM levels was attributed to Euaq complexation to NOMaq being more favored than Eu sorption to the solid phase. A component additivity model was developed to describe the Eu–NOM-sediment system by the additive effects of the three binary system models: Eu–NOM, Eu-sediment and NOM-sediment. The model generally captured the data trends in the ternary system. Conditional stability constants developed from the experimental data for the complexation of Eu to NOMsed were as much as four orders of magnitude greater than Eu complexation with NOMaq, presumably due to the NOMsed deriving additional negative (attractive) charge from the sediment surface. At high initial NOMaq levels, >99 mg/L C, the model captured the trend of reduced Eu sorption but tended to over-estimate Eu sorption. The additivity approach of combining binary models to form a ternary model was only successful when the unique complexation properties of the NOMsed were properly calculated.

The fate of technetium in reduced estuarine sediments: Combining direct and indirect analyses by Ian T. Burke; Francis R. Livens; Jonathan R. Lloyd; Andrew P. Brown; Gareth T.W. Law; Joyce M. McBeth; Beverley L. Ellis; Richard S. Lawson; Katherine Morris (233-241).
Technetium-99 is an important fission product in radioactive wastes. As Tc ( VII ) O 4 - , Tc is highly mobile in oxic environments but, under reducing conditions, Tc becomes strongly associated with sediments as hydrous Tc(IV)O2 like phases. In order to further examine the behaviour of Tc over a range of concentrations in estuarine sediments, anoxic incubation experiments were combined with a range of direct (transmission electron microscopy and gamma camera imaging) and indirect (incubation experiments and chemical extractions) experimental techniques. When TcO4 was incubated in sediment microcosms at micro-molar (10−6  mol L−1) concentrations, >99% Tc O 4 - was removed from solution over the course of 36 days in systems undergoing active microbial Fe(III)-reduction. By contrast, when spiked into pre-reduced estuarine sediments that were predominantly Fe(III)-reducing (incubated for 60 days) or SO 4 2 − -reducing (incubated for 270 days), >99% Tc O 4 - was removed from solution in under 10 min in both microbially active and heat sterilised systems. Chemical extraction techniques showed that 70 ± 3% of Tc bound to sediments was remobilised when sediments were exposed to the first strong oxidant (H2O2) in the extraction scheme. At higher Tc concentrations (∼0.05 mol kg−1 of sediment) scanning transmission electron microscopy, combined with energy dispersive X-ray mapping, was used to examine the associations of Tc in sediments. At these concentrations, Tc was localised and co-associated with nanometre size Fe(II)-rich particles, consistent with the hypothesis that removal of Tc may be controlled by reduction of Tc(VII) to Tc(IV) by biogenic Fe(II) in sediments. In addition, gamma camera imaging with the γ-emitting 99 m Tc O 4 - (half-life 6 h) at pico-molar (10−12  mol L−1) concentrations, was used to visualise the interaction of Tc in sediments at very low concentrations. Here, over the course of 24 h the scavenging of Tc to SO 4 2 − -reducing sediments was observed. As the Tc concentrations used in the 99mTc experiments were below the solubility limits for hydrous Tc(IV)O2 (ca. 10−9  mol L−1 at pH 7–9), sorption of Tc(IV) species is likely to be a significant control on Tc behaviour in these sediments even at very low concentrations. Overall, the results of this study show that multiple approaches are essential to understanding Tc speciation in complex heterogeneous sediments over the wide range of concentrations relevant to contaminated natural and engineered environments.

Log–log correlation plots between the dissociation constants of known metal–chromate complexes and those of corresponding metal–sulfate complexes at 25 °C, 1 bar were used to derive the standard partial molal Gibbs free energies of formation of unknown metal–chromate complexes involving either (i) monovalent cations, divalent cations, and trivalent lanthanides or (ii) trivalent cations (excluding those of rare earth elements, REE) and tetravalent cations. For each of these two classes of ionic associations, empirical relationships between the standard partial molal volumes, isobaric heat capacities and entropies of known metal–chromate complexes and the corresponding thermodynamic properties of metal ions have been found. These data were utilized to evaluate the solute-characteristic parameters of the revised Helgeson–Kirkham–Flowers equation of state and to compute the thermodynamic properties of the dissociation reactions of metal–chromate complexes at high temperatures and pressures.Speciation of Cr(VI) in seawater and in shallow groundwaters interacting with serpentinites and ultramafites shows the importance of the NaCrO 4 - , MgCrO 4 ° , and CaCrO 4 ° aqueous complexes.

Geochemical controls on sediment reactivity and buffering processes in a heterogeneous aquifer by Carlos Descourvières; Niels Hartog; Bradley M. Patterson; Carolyn Oldham; Henning Prommer (261-275).
The injection and recovery of oxic water into deep anoxic aquifers may help to alleviate short- and long-term imbalance between freshwater supply and demand. The extent and structure of physical and geochemical heterogeneity of the aquifer will impact the water quality evolution during injection, storage and recovery. Water–sediment interactions within the most permeable parts of the aquifer, where the bulk of the injectant will penetrate, may dominate, however, water quality may also be impacted by interactions within the finer-grained, less permeable but potentially highly reactive media. In this study, the heterogeneity of the reductive capacity of an aquifer selected for water reuse projects was characterised, the amount, type and reactivity of the sedimentary reductants present determined, and the relationship between reductive capacity and sedimentary lithologies quantified. The average potential reductive capacities (PRCTOT), based on total organic C and pyrite concentrations of the sediment, were quantified for sands (382 μmol O2 g−1), clays (1522 μmol O2 g−1), and silts (1957 μmol O2 g−1). Twenty-seven samples, spanning the three different lithologies, were then incubated for 50 days and the measured reductive capacities (MRC) determined for the sands (29.2 μmol O2 g−1), silts (136 μmol O2 g−1), and clays (143 μmol O2 g−1). On average, the MRC were 10% of the PRCTOT. The main consumers of O2 were pyrite (20–100%), sedimentary organic matter (SOM; 3–56%), siderite (3–28%) and Fe(II)-aluminosilicates (8–55%). The incubation data plus hydrogeochemical modelling, indicated that pH-buffering was controlled firstly by dissolution of trace level carbonates, followed by dissolution of feldspars. Zinc, Co, Ni, Cd and Pb were readily mobilized during incubation.

The parameter S 1  +  S 2 (genetic potential) of Rock-Eval analysis is widely used as an evaluation of the genetic potential for the source rocks. Oligocene–Miocene saline lacustrine source rocks in the western Qaidam basin have low total organic C contents (TOC), most around 0.5% with a few exceptions >1.0%. Mineral matrix effects are substantial for source rocks with low TOC, resulting in relatively low S 1 and S 2 peaks. Based on the results of confined pyrolyses (sealed Au capsules) on 6 Oligocene–Miocene source rocks from the western Qaidam basin, with TOC ranging between 0.48% and 2.22%, the relationship between the S 1  +  S 2 parameter and the maximum amount of extracted bitumen or saturated and aromatic hydrocarbons (SA) after the confined pyrolysis has been established as follows: bitumen (mg/g rock) = 1.4924 × (S 1  +  S 2) + 0.3201 (r  = 0.987), or SA (saturates + aromatics) (mg/g rock) = 0.7083 × (S 1  +  S 2) + 0.4045 (r  = 0.992). Based on these formulas, the amounts of hydrocarbons generated from source rocks can be reasonably estimated. The typical crude oils with low biomarker maturities in this region appear substantially different to the pyrolysates of these six rocks at 180–300 °C but comparable to the pyrolysates at 320 °C and higher temperatures based on molecular parameters. This result, in combination with the physical and gross compositions of the crude oils, suggests that the majority of these crude oils were generated from the source rocks during the main oil-generative stage, possibly at a maturity higher than R o 0.74%.

This work deals with the wash-out processes of evaporitic salts in the Tinto river in October 2005, with the arrival of the first rainfall after the summer. In order to monitor water levels and electrical conductivity, a datalogger was set up in the river, while sampling was performed by a portable autosampler. Thirty-two samples were selected for analysis for a wide range of elements by ICP-AES. Three different flood events, with a maximum discharge of 8.1 m3/s, were monitored. River waters suffered from a dilution effect at the beginning of the first event, recording a concentration decrease of most elements, just before the wash-out of soluble salts precipitated during the summer took place. Wash-out processes provoked a sharp increase in most element concentrations coinciding with an intense decrease in Na and Sr. After the first event, there was strong enrichment of As, and to a lesser extent in Fe, Cr and Pb, due probably to the redissolution/transformation of Fe oxyhydroxysulfates. During the third event, evaporitic sulfate salts were depleted from riverbanks ending wash-out processes, and a decrease in most element concentrations was observed. Barium exhibited different behaviour to the rest of elements owing to the solubility control exerted by barite. Lead also showed a different pattern throughout the study period, its concentration decreasing due probably to its great affinity for coprecipitate on jarosite-group minerals, and increasing when schwertmannite precipitation is thermodynamically favoured. In October 2005, the Tinto river carried around 8100 t SO4, 1300 t Fe, 400 t Al, 100 t Zn and Cu, etc., highlighting the importance of wash-out processes of soluble salts in the Ría de Huelva ecosystem and metal fluxes into the Atlantic Ocean.

Slag from the former Hegeler Zn-smelting facility in Illinois (USA) is mainly composed of spinifex Ca-rich plagioclase, fine-grained dendritic or coarse-grained subhedral to anhedral clinopyroxenes, euhedral to subhedral spinels, spherical blebs of Fe sulfides, silicate glass, and less commonly fayalitic olivine. Mullite and quartz were also identified in one sample as representing remnants of the furnace lining. Secondary phases such as goethite, hematite and gypsum are significant in some samples and reflect surficial weathering of the dump piles or represent byproducts of roasting. A relatively rare Zn-rich material contains anhedral willemite, subhedral gahnite, massive zincite, hardystonite and a Zn sulfate (brianyoungite), among other phases, and likely represents the molten content of the smelting furnace before Zn extraction. The bulk major-element chemistry of most slag samples is dominated by SiO2, Al2O3, Fe2O3 and CaO. The bulk composition of the slag suggests a high viscosity of the melt and the mineralogy suggests a high silica content of the melt. Bulk slag trace-element chemistry shows that the dominant metal is Zn with >28.4 wt.% in the Zn-rich material and between 212 and 14,900 mg/kg in the other slags. The concentrations of other trace elements reach the following: 45 mg/kg As, 1170 mg/kg Ba, 191 mg/kg Cd, 242 mg/kg Co, 103 mg/kg Cr, 6360 mg/kg Cu, 107 mg/kg Ni, and 711 mg/kg Pb.Zinc, as the dominant metal in the slags, is likely the most environmentally significant metal in these samples; Cd, Cu, and Pb are also of concern and their concentrations exceed US Environmental Protection Agency preliminary remediation goals for residential soils. Spinel was found to be the dominant concentrator of Zn for samples containing significant Zn (>1 wt.%); the silicate glass also contained relatively high concentrations of Zn compared to other phases. Zinc partitioned into the silicates and oxides in these samples is generally more resistant to weathering and therefore less leached when compared to the slag samples with lower bulk Zn concentrations where Zn is likely partitioned into volumetrically minor sulfides. This is confirmed by leachate tests that resulted in low leachate Zn concentrations for samples with Zn partitioned into spinel. In contrast, the concentrations of Zn and SO4 are close to those expected from the dissolution of stoichiometric ZnS in leachates from samples in which the dominant host of Zn is suspected to be sulfides. The fact that Zn and other metals occur commonly as sulfides, which are more reactive than the silicates and oxides into which they dominantly partition according to other slag studies, indicates the Hegeler slag pile may be more of an environmental concern than other slag piles.