Applied Geochemistry (v.25, #10)

With the improvement of analytical methods and the development of multiple-collector inductively coupled plasma-mass spectrometry (MC-ICP/MS), research on non-traditional stable isotope (Cu, Zn, Fe, Se, Mo, Cr, Hg) in geochemistry has made tremendous progress in the past decade. Recent studies have demonstrated that both organic and inorganic reactions may cause Hg isotope fractionation, and variations of Hg isotopic composition in the environment have been successfully employed to explain Hg pollution history, Hg sources and tracking Hg pathways in nature. Furthermore, Hg isotopic fractionation studies can be a powerful tool in the calibration of global Hg cycling models. Stable isotope geochemistry of Hg is therefore becoming a new frontier subject in earth sciences. Based on summarizing previous research, this paper outlines the main advances in the study of Hg stable isotopes with particular emphasis placed on a brief explanation of Hg isotope analytical techniques, possible Hg isotope fractionation mechanisms observed in both natural and experimental processes, Hg isotope composition variations in different environmental matrices, and the application prospects of the Hg stable isotopes in environmental geosciences.

Predominance of even carbon-numbered n-alkanes from lacustrine sediments in Linxia Basin, NE Tibetan Plateau: Implications for climate change by Yongli Wang; Xiaomin Fang; Tongwei Zhang; Yuanmao Li; Yingqin Wu; Daxiang He; Youxiao Wang (1478-1486).
► This study reports the first observation of predominant even carbon-numbered n-alkanes of sediments in the continuous lacustrine-sedimentary section (Maogou) from the Late Miocene to the Early Pliocene (13–4.4 Ma) in the Linxia Basin, NE Tibetan Plateau. ► Certain types of special autochthonous bacteria are a possible source for the special distribution of even carbon-numbered n-alkanes in lacustrine sediments. ► These bacteria may have a high production rate in weak oxic–anoxic and arid depositional environments, in which a variety of geochemical parameters have recorded palaeoclimate change. ► A close correspondence among the low ratio of n-C27/n-C31, the heavy δ13C values of TOC and a strong even carbon-number predominance (low OEP16–20 values) from approximately 6.5 to 4.4 Ma and at approximately 8 Ma in the studied section suggests that n-alkanes with a high predominance of even carbon-numbers may be treated as geochemical proxies for arid climate.This study reports the first observation of predominant even C-numbered n-alkanes from sediments in the continuous lacustrine-sedimentary section (Maogou) from the Late Miocene to the Early Pliocene (13–4.4 Ma) in the Linxia Basin, NE Tibetan Plateau. The n-alkanes showed a bimodal distribution that is characterised by a centre at n-C16n-C20 with maximum values at n-C18 and n-C27n-C31 as well as at n-C29. The first mode shows a strong even C-number predominance (OEP16–20 0.34–0.66). In contrast, the second mode has a strong odd C-number predominance (OEP27–31 1.20–2.45). Certain types of special autochthonous bacteria are a possible source for this distribution of even C-numbered n-alkanes in lacustrine sediments. These bacteria may have a high production rate in weak oxic–anoxic and arid depositional environments, in which a variety of geochemical parameters have recorded palaeoclimate change.

Arsenic in an alkaline AMD treatment sludge: Characterization and stability under prolonged anoxic conditions by Suzanne Beauchemin; Jean-François Fiset; Glenn Poirier; James Ablett (1487-1499).
Lime treatment of acid mine drainage (AMD) generates large volumes of neutralization sludge that are often stored under water covers. The sludge consists mainly of calcite, gypsum and a widespread ferrihydrite-like Fe phase with several associated species of metal(loid) contaminants. The long-term stability of metal(loid)s in this chemically ill-defined material remains unknown. In this study, the stability and speciation of As in AMD sludge subjected to prolonged anoxic conditions is determined. The total As concentration in the sludge is 300 mg kg−1. In the laboratory, three distinct water cover treatments were imposed on the sludge to induce different redox conditions (100%N2, 100%N2  + glucose, 95%N2:5%H2). These treatments were compared against a control of oxidized, water-saturated sludge. Electron micro-probe (EMP) analysis and spatially resolved synchrotron X-ray fluorescence (SXRF) results indicate that As is dominantly associated with Fe in the sludge. In all treatments and throughout the experiment, measured concentrations of dissolved As were less than 5 μg L−1. Dissolved Mn concentration in the N2  + glucose treatment increased significantly compared to other treatments. Manganese and As K-edge X-ray absorption near edge structure spectroscopy (XANES) analyses showed that Mn was the redox-active element in the solid-phase, while As was stable. Arsenic(V) was still the dominant species in all water-covered sludges after 9 months of anoxic treatments. In contrast, Mn(IV) in the original sludge was partially reduced into Mn(II) in all water-covered sludges. The effect was most pronounced in the N2  + glucose treatment, suggesting microbial reduction. Micro-scale SXRF and XANES analysis of the treated sludge showed that Mn(II) accumulated in areas already enriched in Fe and As. Overall, the study shows that AMD sludges remain stable under prolonged anoxic conditions. External sources of chemical reductants or soluble C were needed to induce lower redox state in the systems, and even under these imposed treatments, only weakly reducing conditions (Mn threshold) developed. The results suggest that As(V) in AMD sludge will remain stable under prolonged anoxic conditions as long as Mn(IV) is present and organic matter accumulation is negligible.

Chemical treatments for mobilizing arsenic from contaminated aquifer solids to accelerate remediation by Karen Wovkulich; Brian J. Mailloux; Allison Lacko; Alison R. Keimowitz; Martin Stute; H. James Simpson; Steven N. Chillrud (1500-1509).
► Phosphate and oxalic acid can accelerate release of As from contaminated solids. ► Oxalic acid is more effective at mobilizing As at lower amendment concentrations. ► Chemical amendments could improve As clean up efficacy at sites using pump & treat.Arsenic is a prevalent contaminant at US Superfund sites where remediation by pump and treat systems is often complicated by slow desorption of As from Fe and Al (hydr)oxides in aquifer solids. Chemical amendments that either compete with As for sorption sites or dissolve Fe and Al (hydr)oxides can increase As mobility and improve pump and treat remediation efficiency. The goal of this work was to determine optimal amendments for improving pump and treat at As contaminated sites such as the Vineland Chemical Co. Superfund site in southern New Jersey. Extraction and column experiments were performed using As contaminated aquifer solids (81 ± 1 mg/kg), site groundwater, and either phosphate (NaH2PO4·H2O) or oxalic acid (C2H2O4·2H2O). In extraction experiments, phosphate mobilized between 11% and 94% of As from the aquifer solids depending on phosphate concentration and extraction time (1 mM–1 M; 1–24 h) and oxalic acid mobilized between 38% and 102% depending on oxalic acid concentration and extraction time (1–400 mM; 1–24 h). In column experiments, phosphate additions induced more As mobilization in the first few pore volumes but oxalic acid was more effective at mobilizing As overall and at lower amendment concentrations. At the end of the laboratory column experiments, 48% of As had been mobilized from the aquifer sediments with 100 mM phosphate and 88% had been mobilized with 10 mM oxalic acid compared with 5% with ambient groundwater alone. Furthermore, simple extrapolations based on pore volumes suggest that chemical treatments could lower the time necessary for clean up at the Vineland site from 600 a with ambient groundwater alone to potentially as little as 4 a with 10 mM oxalic acid.

Multi-isotopic composition (δ7Li–δ11B–δD–δ18O) of rainwaters in France: Origin and spatio-temporal characterization by Romain Millot; Emmanuelle Petelet-Giraud; Catherine Guerrot; Philippe Négrel (1510-1524).
► A contour map of France for δ18O was drawn after compiling data that included more than 400 values from all of France. ► The seasonal effect (i.e. the month or rainfall amount) is not the main controlling factor for the Li and B isotopic variations. ► Most Li and B in rainwaters does not have a marine origin. ► Finally, this work also adds to the potential for use of Li and B isotopes as environmental tracers.In the present work, the first results are reported for both Li and B isotope ratios in rainwater samples collected over a long time period (i.e. monthly rainfall events over 1 a) at a national scale (from coastal and inland locations). In addition, the stable isotopes of the water molecule (δD and δ18O) are also reported here for the same locations so that the Li and B isotope data can be discussed in the same context. The range of Li and B isotopic variations in these rainwaters were measured to enable the determination of the origin of these elements in rainwaters and the characterization of both the seasonal and spatio-temporal effects for δ7Li and δ11B signatures in rainwaters. Lithium and B concentrations are low in rainwater samples, ranging from 0.004 to 0.292 μmol/L and from 0.029 to 6.184 μmol/L, respectively. δ7Li and δ11B values in rainwaters also show a great range of variation between +3.2‰ and +95.6‰ and between −3.3‰ and +40.6‰ over a period of 1 a, respectively, clearly different from the signature of seawater. Seasonal effects (i.e. rainfall amount and month) are not the main factors controlling element concentrations and isotopic variations. δ7Li and δ11B values in rainwaters are clearly different from one site to another, indicating the variable contribution of sea salts in the rainwater depending on the sampling site (coastal vs. inland: also called the distance-from-the-coast-effect). This is well illustrated when wind direction data (origin of air masses) is included. It was found that seawater is not the main supplier of dissolved atmospheric Li and B, and non-sea-salt sources (i.e. crustal, anthropogenic, biogenic) should also be taken into account when Li and B isotopes are considered in hydrogeochemistry as an input to surface waters and groundwater bodies as recharge. In parallel, the isotopic variations of the water molecule, vector of the dissolved B and Li, are also investigated and reported as a contour map for δ18O values based on compiled data including more than 400 δ18O values from throughout France. This δ18O map could be used as a reference for future studies dealing with δ18O recharge signature in relation to the characterization of surface waters and/or groundwater bodies.

Formation, fate and leaching of chloroform in coniferous forest soils by Christian N. Albers; Troels Laier; Ole S. Jacobsen (1525-1535).
► Chloroform may be formed in coniferous forest soil. ► The formed chloroform may enter the groundwater in μg/L concentrations. ► Clear seasonal patterns in chloroform formation in soil are observed. ► Sorption and degradation affects the fate of chloroform in forest soil.Chloroform is a common groundwater pollutant but also a natural compound in forest ecosystems. Leaching of natural chloroform from forest soil to groundwater was followed by regular analysis of soil air and groundwater from multilevel wells at four different sites in Denmark for a period of up to 4 a. Significant seasonal variation in chloroform was observed in soil air 0.5 m below surface ranging at one site from 120 ppb by volume in summer to 20 ppb during winter. With depth, the seasonal variation diminished gradually, ranging from 30 ppb in summer to 20 ppb during winter, near the groundwater table. Chloroform in the shallowest groundwater ranged from 0.5–1.5 μg L−1 at one site to 2–5 μg L−1 at another site showing no clear correlation with season. Comparing changes in chloroform in soil air versus depth with on-site recorded meteorological data indicated that a clear relationship appears between rain events and leaching of chloroform. Chloroform in top soil air co-varied with CO2 given a delay of 3–4 weeks providing evidence for its biological origin. This was confirmed during laboratory incubation experiments which further located the fermentation layer as the most chloroform producing soil horizon. Sorption of chloroform to soils, examined using 14C–CHCl3, correlated with organic matter content, being high in the upper organic rich soils and low in the deeper more minerogenic soils. The marked decrease in chloroform in soil with depth may in part be due to microbial degradation which was shown to occur at all depths by laboratory tests using 14C–CHCl3.

Sources and flux of trace elements in river water collected from the Lake Qinghai catchment, NE Tibetan Plateau by Zhangdong Jin; Chen-Feng You; Tsai-Luen Yu; Bo-Shian Wang (1536-1546).
► First trace element analyses of weekly river waters from semi-arid area. ► Assessing seasonal atmospheric and anthropogenic contributions to river water. ► Understanding of the solute budget of rivers and Lake Qinghai. ► Natural processes and anthropogenic contribution in the environment.River waters play a significant role in supplying naturally- and anthropogenically-derived materials to Lake Qinghai, northeastern Tibetan Plateau. To define the sources and controlling processes for river water chemistry within the Lake Qinghai catchment, high precision ICP-MS trace element concentrations were measured in water samples collected from the Buha River weekly in 2007, and from other major rivers in the post-monsoon (late October 2006) and monsoon (late July 2007) seasons. The distributions of trace elements vary in time and space with distinct seasonal patterns. The primary flux in the Buha River is higher TDS and dissolved Al, B, Cr, Li, Mo, Rb, Sr and U during springtime than those during other seasons and is attributed to the inputs derived from both rock weathering and atmospheric processes. Among these elements, the fluxes of dissolved Cr, B and Rb are strongly influenced by eolian dust input. The fluxes of dissolved Li, Mo, Sr and U are also influenced by weathering processes, reflecting the sensitivity of chemical weathering to monsoon conditions. The anthropogenic sources appear to be the dominant contribution to potentially harmful metals (Ni, Cu, Co, Zn and Pb), with high fluxes at onset of the main discharge pulses due, at least partially, to a runoff washout effect. For other major rivers, except for Ba, concentrations of trace elements are higher in the monsoon than in the post-monsoon season. A total of 38.5 ± 3.1 tons of potentially harmful elements are transported into the lake annually, despite human activities within the catchment being limited. Nearly all river water samples contain dissolved trace elements below the World Health Organization guidelines for drinking water, with the exception of As and B in the Daotang River water samples collected in late July probably mobilized from underlying lacustrine sediments.

Investigating dissolution of mechanically activated olivine for carbonation purposes by Tove Anette Haug; Rolf Arne Kleiv; Ingrid Anne Munz (1547-1563).
► Dissolution of mechanically activated olivine increased with 3 orders of magnitude. ► Crystallinity changes of olivine is important for the observed dissolution rates. ► Activation probably decreases with the degree of dissolution of each particle.Mineral carbonation is one of several alternatives for CO2 sequestration and storage. The reaction rates of appropriate minerals with CO2, for instance olivine and serpentine with vast resources, are relatively slow in a CO2 sequestration context and the rates have to be increased to make mineral carbonation a good storage alternative. Increasing the dissolution rate of olivine has been the focus of this paper. Olivine was milled with very high energy intensity using a laboratory planetary mill to investigate the effect of mechanical activation on the Mg extraction potential of olivine in 0.01 M HCl solution at room temperature and pressure. Approximately 30–40% of each sample was dissolved and water samples were taken at the end of each experiment. The pH change was used to calculate time series of the Mg concentrations, which also were compared to the final Mg concentrations in the water samples. Percentage dissolved and the specific reaction rates were estimated from the Mg concentration time series. The measured particle size distributions could not explain the rate constants found, but the specific surface area gave a good trend versus dissolution for samples milled wet and the samples milled with a small addition of water. The samples milled dry had the lowest measured specific surface areas (<4 m2/g), but had the highest rate constants. The crystallinity calculated from X-ray diffractograms, was the material parameter with the best fit for the observed differences in the rate constants. Geochemical modelling of mechanically activated materials indicated that factors describing the changes in the material properties related to the activation must be included. The mechanically activated samples in general reacted faster than predicted by the theoretical models. Mechanical activation as a pre-treatment method was found to enhance the initial specific reaction rates by approximately three orders of magnitude for a sample milled dry for 60 min in a planetary mono mill compared to an unactivated sample. Wet milling in the planetary mill did not produce samples with the same maximum reaction rate as dry milling, but wet milling in general might be easier to implement into a wet carbonation process. Mechanical activation in a planetary mill is likely to consume too much energy for CO2 sequestration purposes, but the increase in obtained olivine rate constants illustrates a potential for using milling as a pre-treatment method.

► Modelling of water–carbonate interactions using solid solutions. ► The stoichiometries of solid solutions are defined from Chalk geochemistry. ► The initial mineralisation of groundwater is calculated. ► Chemical evolution of groundwater along the flow lines is calculated. ► Modelling of incongruent carbonate dissolution is achieved from experimental data.The Chalk aquifer of Champagne (France) baseline geochemistry has been determined using a solid solution approach for the modelling of calcite dissolution. The water–rock interactions are modelled by the speciation code CHESS from field data and Ca, Mg and Sr aqueous concentrations in groundwater. The stoichiometries of solid solutions are defined in each stratigraphic unit of the Chalk aquifer from bulk geochemistry and Chalk mineralogy of samples taken from boreholes. The initial mineralisation of water at the bottom of the unsaturated zone and the characterisation of the theoretical evolution of groundwater chemistry along the flow lines associated with incongruent calcite dissolution are calculated from this approach.

The dissolution rates of dawsonite at pH 0.9 to 5 and temperatures of 22, 60 and 77 °C by H. Hellevang; J. Declercq; B. Kvamme; P. Aagaard (1575-1586).
► Obtained the dissolution rates of dawsonite at acidic conditions (pH 0.9–5). ► Obtained apparent activation energies for the dissolution. ► Compared to earlier published data on dawsonite dissolution (circumneutral pH, ). ► Because numerical simulations on CO2 storage have used proxy minerals for dawsonite as data on the dissolution rates have not existed, the implications of the fast dissolution on the potential for long-term mineral storage is given.Occurrences of the mineral dawsonite (NaAl(OH)2CO3) after natural CO2 accumulation support suggestions that this mineral may contribute to long-term storage of CO2 in geological formations. Knowledge of the reactivity of dawsonite is crucial to better understand its role as a CO2 storage host. Here the results of free-drift batch dissolution rate experiments at 22, 60 and 77 °C performed on synthesized dawsonite are presented. Based on previously reported dawsonite dissolution rate data at circumneutral conditions and the present experiments, it is suggested that the dissolution rates can be expressed by two parallel mechanisms according to r + = k 1 a H + v + k 2 , where a H + denotes the proton activity. The rate coefficient k 1 (22 °C) and order with respect to the proton activity v were estimated from the 22 °C dataset to be 10−4.48 ± 0.48 and 0.982 ± 0.15, respectively, at the 95% confidence level. The rate coefficient for the pH-independent region k 2 (77 °C) was found to be approximately 10−6.89 from the maximum R 2  = 0.95 for the rate equation using the 77 °C dataset. The k 2 value was however uncertain because of the few data points in the transition into the pH-independent region. The apparent activation energy Ea for the proton-promoted mechanism was estimated to 49.43 kJ/mol, increasing to 63.82 kJ/mol for the data points approaching pH independence. From the trends of data points for the 22 and 77 °C data series, it is likely that the difference in rates between 22 and 77 °C increases further at higher pH.

Trace element characterisation of Cretaceous Orange Basin hydrocarbon source rocks by A. Akinlua; S.A. Adekola; O. Swakamisa; O.A. Fadipe; S.A. Akinyemi (1587-1595).
► Vanadium and nickel contents indicate that the rock samples from the Orange Basin have marine organic matter input. ► The organic matter of the Orange Basin source rocks were deposited in reducing conditions. ► Despite the similarities in the organic matter source input and depositional environment of the samples from the two well, cross plots of Co/Ni versus V/Ni and Mo/Ni versus Co/Ni were able to reveal their subtle differences. ► Cluster analysis classified the samples into three groups based on subtle differences in their .thermal maturity.Trace elements in the kerogen fraction of hydrocarbon source rock samples from two wells obtained from the Cretaceous units of the Orange Basin, South Africa were determined using X-ray fluorescence spectrometry, in order to determine their distribution and geochemical significances. The concentrations of the elements (As, Ce, Co, Cu, Fe, Mo, Ni, Pb and V) determined ranged from 0.64 to 47,300 ppm for the samples analysed. The total organic carbon (TOC) values indicate that the samples are organic rich but did not show any trend with the distribution of the trace metals except Ce, Mo and Pb. Dendrogram cluster analysis discriminated the samples into three groups on the basis of their level of thermal maturity. Thermal maturity has a significant effect on the distribution of the trace metals. Cobalt/Ni and V/Ni ratios and cross plots of the absolute values of V and Ni indicate that the samples had significant marine organic matter input. The V and Ni contents and V/(V + Ni) ratio indicate that the organic matter of the source rocks had been deposited in reducing conditions. Despite the similarities in the organic matter source input and depositional environment of the organic matter of the samples from the two well, cross plots of Co/Ni versus V/Ni and Mo/Ni versus Co/Ni were able to reveal subtle differences. Cluster analysis of the samples was also able to reveal the subtle thermal maturity differences of the samples.

Adsorptive removal of nickel from water using volcanic rocks by Esayas Alemayehu; Bernd Lennartz (1596-1602).
► Basic process characteristics were determined under batch conditions. ► Adsorption was found to be highly dependent on pH. ► Pseudo-second order equation best described the kinetics mechanisms of Ni(II) adsorption. ► VPum had a higher affinity than VSco towards Ni ions. ► Given that enough VPum or VSco are provided, Ni(II) could be removed even from a mixture matrix.This paper presents the results of a study on Ni(II) removal from water by adsorption using abundant and low-cost volcanic rock grains: Scoria (VSco) and Pumice (VPum), which could be used as an alternative approach to remove potentially harmful metals from contaminated water. Basic process characteristics were determined under batch conditions. The maximum adsorption capacities for Ni(II) on VSco and VPum were found to be 980 and 1187 mg kg−1, respectively. These results were obtained at the optimized conditions of pH (5.0), temperature (24.9 °C), contact time (24 h), adsorbent/solution ratio (1:20), particle size (fine) and with the variation of initial concentrations between 0.5 and 50 mg L−1. Competitive adsorption of Ni(II), Cd(II) and Cu(II) on the adsorbents present in binary as well as ternary mixtures were also compared with the single metal solution. Thus, given that enough volcanic rock grains are provided, Ni(II) ions could be removed even from a metal ion bearing matrix. A number of available models like Lagergren pseudo-first order kinetics, second-order kinetics, intra-particle diffusion and liquid film diffusion were utilized to evaluate the kinetics and the mechanism of the sorption interactions. The results revealed that the pseudo-second order equation best described the kinetics mechanisms of Ni(II) adsorption although the removal process was found to be complex. Moreover, three adsorption models have been evaluated in order to attempt to fit the experimental data, namely the Langmuir, the Freundlich and the Redlich–Peterson isotherm models. It was found that the first two isotherms most closely described the adsorption parameters.

► The Xijiang River is the second largest river in China and flows through a large carbonate rock region in South China. ► Sulfuric acid, which emanate from acid precipitation and the oxidation of sulfide minerals, is involved as a proton donor in weathering reactions in the Xijiang basin. ► Calculated results show that the contribution of cations from rock weathering induced by sulfuric acid accounts for approximately 11.2%. ► The flux of CO2 released into the atmosphere is approximately 0.41 × 1012  gC yr−1 produced by sulfuric acid-induced carbonate weathering in the Xijiang basin. ► Sulfuric acid-induced carbonate weathering could counterbalance a significant part of the CO2 consumed by silicate weathering.The Xijiang River, the mainstream of the Zhujiang (Pearl) River, which is the second largest river in China in terms of discharge, flows through a large carbonate rock region in South China. The chemical and Sr isotopic compositions of the Xijiang waters were determined during the high-flow season in order to understand the chemical weathering processes, associated CO2 consumption and anthropogenic influences within the carbonate-dominated basin. The major ion compositions of the river waters are characterized by the dominance of Ca2+, Mg2+, HCO 3 - and are significantly rich in SO 4 2 - . The SO 4 2 - is mainly derived from the oxidation of sulfide minerals and acid precipitation caused by coal combustion. Chemical and Sr isotopic compositions of the river waters indicate that four reservoirs (carbonates, silicates, evaporites and anthropogenic inputs) contribute to the total dissolved loads. The chemical weathering rates of carbonates and silicates for the Xijiang basin are estimated to be approximately 78.5 and 7.45 ton km−2  a−1, respectively. The total chemical weathering rate of rocks for the Xijiang basin is approximately 86.1 ton km−2  a−1 or 42 mm ka−1, which is much higher than global mean values. The budgets of CO2 consumption by carbonate and silicate weathering are estimated to be 284 × 109 and 54.3 × 109  mol a−1, respectively. It would appear that H2SO4 is involved as a proton donor in weathering reactions in the Xijiang basin; calculated results show that the contribution of cations from rock weathering induced by H2SO4 accounts for approximately 11.2%. Results from this study show that the flux of CO2 released into the atmosphere is approximately 0.1 × 106  mol C km−2  a−1 or 0.41 × 1012  g C a−1 produced by H2SO4-induced carbonate weathering in the basin. When extrapolated to the entire surface area of carbonate in SW China, the flux of CO2 released to the atmosphere by H2SO4-induced carbonate weathering is about 1.41 × 1012  g C a−1.