Applied Geochemistry (v.52, #C)

The kinetics of the olivine dissolution under the extreme conditions of nano-silica production by A. Lazaro; L. Benac-Vegas; H.J.H. Brouwers; J.W. Geus; J. Bastida (1-15).
Display OmittedThis article addresses the kinetics of the dissolution of olivine for nano-silica production at extreme conditions. The extreme conditions are pH values between −0.7 and 1, temperature between 50 and 90 °C, solid content around 250 g/L and percentage dissolved between 80% and 99%. This work is structured in 3 parts: (1) chemical and mineralogical characterization of the dunites employed; (2) mechanism of the olivine dissolution focusing on the possible resistances to the transport; and (3) determination of the kinetic parameters kT and n.The results shown here demonstrate that: (1) the limiting step of this process is not the diffusion through a silica layer but the surface reaction; and (2) the dissolution of olivine under the olivine nano-silica production conditions is well described by: r = A · exp - E a R · T · a H + n = 856 · exp - 78.5 R · T · a H + 0.5 , where r is the dissolution rate (mol cm−2  s−1), A the pre-exponential factor (mol cm−2  s−1), Ea the activation energy (kJ/mol), R the gas constant (8.314 · 10−3  kJ mol−1  K−1), T the temperature (K), a H + the hydrogen ion activity and n the reaction order. The average error of the reaction rate calculated using these parameters is 5.5% for dunite CRS-US. In addition, this model is successfully applied to the dissolution of other commercial dunites and for bigger reactor volumes. Therefore, this model can be considered to be robust, and it can be used in the industrial production of olivine nano-silica.

A revised method for determining existing acidity in re-flooded acid sulfate soils by Scott G. Johnston; Edward D. Burton; Roslyn Hagan; Thor Aaso; Gerard Tuckerman (16-22).
Titratable actual acidity (TAA) is a technique commonly used to estimate the existing pool of exchangeable H+ in acid sulfate soils (ASS). A widely adopted version of the TAA method involves titrating a 1M KCl suspension of oven-dry soil (1:40) with NaOH to a known pH endpoint. However, when ASS are subject to long term re-flooding during wetland remediation, former sulfuric horizons can develop substantial quantities of porewater Fe2+, non-sulfidic solid-phase Fe(II) and a variety of reduced inorganic sulfur (RIS) species (e.g. pyrite, mackinawite, greigite and elemental sulfur). For these sediments, an oven-drying approach may induce oxidation of the abundant Fe(II) and/or reactive RIS species, thereby generating H+ and leading to overestimation of existing in situ exchangeable H+. In this study, we compare TAA via the standard approach (1M KCl; 1:40; oven-dry soil, 4 hr extract; TAAD) with an identical O2-free extraction approach using wet-sediment (TAAW). We apply both methods to former sulfuric horizon sediments from freshwater re-flooded ASS wetlands. There are significant (α  = 0.01) differences (up to 12×) between TAA measured by the two methods, with the oven-dried standard approach overestimating TAA relative to the wet, O2-free approach in 85% of cases. Despite the fact that all AVS-S and some S(0) was oxidised during the oven-drying process, the increases in TAA (TAAD–TAAW) show very weak correlation(s) with corresponding losses in RIS species or increases in water soluble sulfate and KCl extractable sulfate. However, oven-drying caused substantial loss of 1M KCl exchangeable Fe(II) and 1 M HCl-extractable Fe(II) and led to large increases in 1 M HCl-extractable Fe(III). These changes in Fe fractions displayed strong positive linear correlation (α  = 0.01) with increases in TAA. Although this is not evidence of causality, it suggests that oxidation of Fe(II) species are playing an important role in the development of additional exchangeable H+ and may be largely responsible for the contrasting TAA derived by the two methods. The differences in TAA between the two methods are greatest in organic-rich surface sediments and are significantly positively correlated with total organic carbon content. These findings have major implications for accurately assessing TAA in re-flooded ASS wetlands.

Hydrogeochemical classification of deep formation waters by Elke Bozau; Carl-Diedrich Sattler; Wolfgang van Berk (23-30).
Deep formation waters from Western Europe, Russia and North America are classified by the major water components (Ca2+, Cl, Mg2+, Na+) and barium/sulphate ratios. The data are used to identify important hydrogeochemical processes (e.g., halite dissolution and albitisation) that lead to the different composition of formation waters. Two significant water types are identified: Na–Cl water and Na–Ca–Cl water. Furthermore, differences in formation water according to stratigraphical units are shown for deep reservoirs in the North German Basin and the North Sea. Based on the collected hydrogeochemical data, development trends are stated for the formation waters, and albitisation is favoured as the main process causing Ca enrichment.

BET surface area distributions in polar stream sediments: Implications for silicate weathering in a cold-arid environment by Kristen R. Marra; Megan E. Elwood Madden; Gerilyn S. Soreghan; Brenda L. Hall (31-42).
BET surface area values are critical for quantifying the amount of potentially reactive sediments available for chemical weathering and ultimately, prediction of silicate weathering fluxes. BET surface area values of fine-grained (<62.5 μm) sediment from the hyporheic zone of polar glacial streams in the McMurdo Dry Valleys, Antarctica (Wright and Taylor Valleys) exhibit a wide range (2.5–70.6 m2/g) of surface area values. Samples from one (Delta Stream, Taylor Valley) of the four sampled stream transects exhibit high values (up to 70.6 m2/g), which greatly exceed surface area values from three temperate proglacial streams (0.3–12.1 m2/g). Only Clark stream in Wright Valley exhibits a robust trend with distance, wherein surface area systematically decreases (and particle size increases) in the mud fraction downstream, interpreted to reflect rapid dissolution processes in the weathering environment. The remaining transects exhibit a range in variability in surface area distributions along the length of the channel, likely related to variations in eolian input to exposed channel beds, adjacent snow drifts, and to glacier surfaces, where dust is trapped and subsequently liberated during summer melting. Additionally, variations in stream discharge rate, which mobilizes sediment in pulses and influences water:rock ratios, the origin and nature of the underlying drift material, and the contribution of organic acids may play significant roles in the production and mobilization of high-surface area sediment. This study highlights the presence of sediments with high surface area in cold-based glacier systems, which influences models of chemical denudation rates and the impact of glacial systems on the global carbon cycle.

This paper investigated the sources and behaviors of sulfate in groundwater of the western North China Plain using sulfur and oxygen isotopic ratios. The groundwaters can be categorized into karst groundwater (KGW), coal mine drainage (CMD) and pore water (subsurface saturated water in interstices of unconsolidated sediment). Pore water in alluvial plain sediments could be further classified into unconfined groundwater (UGW) with depth of less than 30 m and confined groundwater (CGW) with depth of more than 60 m. The isotopic compositions of KGW varied from 9.3‰ to 11.3‰ for δ34SSO4 with the median value of 10.3‰ (n  = 4) and 7.9‰ to 15.6‰ for δ18OSO4 with the median value of 14.3‰ (n  = 4) respectively, indicating gypsum dissolution in karst aquifers. δ34SSO4 and δ18OSO4 values of sulfate in CMD ranged from 10.8‰ to 12.4‰ and 4.8‰ to 8.7‰ respectively. On the basis of groundwater flow path and geomorphological setting, the pore water samples were divided as three groups: (1) alluvial–proluvial fan (II1) group with high sulfate concentration (median values of 2.37 mM and 1.95 mM for UGW and CGW, respectively) and positive δ34SSO4 and δ18OSO4 values (median values of 8.8‰ and 6.9‰ for UGW, 12.0‰ and 8.0‰ for CGW); (2) proluvial slope (II2) group with low sulfate concentration (median values of 1.56 mM and 0.84 mM for UGW and CGW, respectively) and similar δ34SSO4 and δ18OSO4 values (median values of 9.0‰ and 7.4‰ for UGW, 10.2‰ and 7.7‰ for CGW); and (3) low-lying zone (II3) group with moderate sulfate concentration (median values of 2.13 mM and 1.17 mM for UGW and CGW, respectively) and more positive δ34SSO4 and δ18OSO4 values (median values of 10.7‰ and 7.7‰ for UGW, 20.1‰ and 8.8‰ for CGW). In the present study, three major sources of sulfate could be differentiated as following: sulfate dissolved from Ordovician to Permian rocks (δ34SSO4  = 10–35‰ and δ18OSO4  = 7–20‰), soil sulfate (δ34SSO4  = 5.9‰ and δ18OSO4  = 5.8‰) and sewage water (δ34SSO4  = 10.0‰ and δ18OSO4  = 7.6‰). Kinetic fractionations of sulfur and oxygen isotopes as a result of bacterial sulfate reduction (BSR) were found to be evident in the confined aquifer in stagnant zone (II3), and enrichment factors of sulfate–sulfur and sulfate–oxygen isotopes calculated by Rayleigh equation were −12.1‰ and −4.7‰ respectively along the flow direction of groundwater at depths of 60–100 m. The results obtained in this study confirm that detailed hydrogeological settings and identification of anthropogenic sources are critical for elucidating evolution of δ34SSO4 and δ18OSO4 values along with groundwater flow path, and this work also provides a useful framework for understanding sulfur cycling in alluvial plain aquifers.

Geochemical reactive transport modeling was coupled to bench-scale leaching experiments to investigate and verify the mobilization of geogenic arsenic (As) under a range of redox conditions from an arsenic-rich pyrite bearing limestone aquifer. Modeling and experimental observations showed similar results and confirmed the following: (1) native groundwater and aquifer matrix, including pyrite, were in chemical equilibrium, thus preventing the release of As due to pyrite dissolution under ambient conditions; (2) mixing of oxygen- and nitrate-rich surface water with oxygen-depleted native groundwater changed the redox conditions and promoted the dissolution of pyrite, and (3) the behavior of As along a flow path was controlled by a complex series of interconnected reactions. This included the oxidative dissolution of pyrite and simultaneous sorption of As onto neo-formed hydrous ferric oxides (HFO), followed by the reductive dissolution of HFO and secondary release of adsorbed As under reducing conditions. Arsenic contamination of drinking water in these systems is thus controlled by the re-equilibration of the system to more reducing conditions rather than a purely oxidative process.

Terrestrial organic matter plays an important role in determining the chemical composition of headwater stream dissolved organic matter (DOM). We used excitation–emission matrix fluorescence spectroscopy (EEMs) and parallel factor analysis (PARAFAC) to detect the chemical composition of headwater stream DOM and riparian water-extractable soil organic matter (WSOM) among four different vegetation types, including evergreen broadleaf forest (EBF), coniferous forest (CF), subalpine dwarf forest (SDF) and alpine meadow (AM), along an altitudinal gradient (690–2060 m above sea level) in the Wuyi Mountains of subtropical China. Fluorescence index (FI) and biological/autochthonous index (BIX) values indicated that stream DOM was from terrestrial sources. Despite this link to terrestrial sources, the composition of stream DOM differed remarkably from that of riparian WSOM. Humic-like PARAFAC component C2 was observed in headwater streams, but not in riparian WSOM. Conversely, humic-like C3 compound was identified in riparian WSOM, but not in headwater streams. The relative abundance of humic-like C1 and C2 compounds did not differ among the headwater streams under four different vegetation types across the altitudinal gradient. In contrast, the relative abundance of C1 compounds in riparian WSOM was substantially higher in EBF than in other vegetation types. The relative abundance of tryptophan-like C4 compounds decreased from 26.47–54.26% in riparian WSOM to 5.71–14.24% in headwater streams. The relative abundance of tryptophan-like C4 compounds was higher in AM and EBF streams than in SDF and CF streams, and decreased in the order of AM > CF > SDF > EBF in the riparian WSOM. Our data suggest that variation in chemical composition of WSOM among four different vegetation types along an altitudinal gradient in the Wuyi Mountains does not induce corresponding changes in the chemical composition of stream DOM. Biogeochemical processes of plant-litter input directly into streams and bio-degradation and selective adsorption of DOM along a generalized hydrologic flow path from soil to stream may cause these qualitative differences between stream DOM and riparian WSOM composition across four different vegetation types along an altitudinal gradient in the Wuyi Mountains.

The effect of temperature on carbon steel corrosion under geological conditions by Y. El Mendili; A. Abdelouas; G. Karakurt; A. Aït Chaou; R. Essehli; J.-F. Bardeau; J.-M. Grenèche (76-85).
The carbon steel corrosion under simulated geological conditions has been investigated and the results show the formation of iron sulphide on steel surface due to microbial corrosion at 30 °C and to the reduction by hydrogen of pyrite originating from claystone into iron monosulphide and hydrogen sulphide at 90 °C.Display OmittedWe investigated the role of temperature on the carbon steel corrosion under simulated geological conditions. To simulate the effect of temperature increase due to radioactive decay, we conducted batch experiments using Callovo-Oxfordian (COx) claystone and synthetic water formation with steel coupons at 30 °C and 90 °C for 6 months. The corrosion products have been studied by scanning electron microscope/energy dispersive X-ray spectroscopy, X-ray diffraction and micro-Raman spectroscopy. At 30 °C, experiments showed the formation of magnetite and iron sulphide, indicating the activation of sulphate-reducing bacteria. At 90 °C a continuous iron sulphide layer was identified on steel surface due to the reduction by hydrogen of pyrite originating from claystone into pyrrhotite and hydrogen sulphide. Thus, sulphide production may occur even in the absence of microbial activity at high temperature and must be taken into consideration regarding the near-field geochemical evolution.

Spatial patterns of metal contamination and magnetic susceptibility of soils at an urban bonfire site by Nessa Golden; Liam Morrison; Paul J. Gibson; Aaron P. Potito; Chaosheng Zhang (86-96).
Bonfires are a major pollution source in urban soils, but there is a lack of knowledge about the impacts and spatial extent of bonfires on soil metal concentration and magnetic properties. In this study, a total of 379 soil samples were collected from a traditional bonfire site on a 1 × 1 m2 grid system and analysed for total metal concentration and low frequency magnetic susceptibility (MSχlf). High resolution maps of the spatial distribution of Cu, Fe, Mn, Pb, Sr, Ti, Zn and MSχlf were created and a significant relationship between each of the metals and MSχlf was revealed. Elevated levels of each metal were observed, with median and maximum values of 68 and 1117 mg kg 1 for Cu, 114 and 985 mg kg 1 for Pb and 561 and 21 681 mg kg 1 for Zn in particular, indicating the site may pose a significant health risk. The spatial patterns were generally consistent, with Zn and Fe in particular, encompassing the position of bonfires. The spatial extent of influence of bonfires was estimated at approximately 10 m, in line with the extent of bonfire materials. In addition, laboratory based experiments involving soil colour and the effect of temperature on MSχlf indicated that bonfires only raise soil temperatures to a maximum of 300 °C, having little effect on MSχlf. The results of this study indicate the importance of metal contamination associated with bonfires in urban soils.

Radiolytic alteration of biopolymers in the Mulga Rock (Australia) uranium deposit by Caroline M.B. Jaraula; Lorenz Schwark; Xavier Moreau; Walter Pickel; Leon Bagas; Kliti Grice (97-108).
Display OmittedWe investigated the effect of ionizing radiation on organic matter (OM) in the carbonaceous uranium (U) mineralization at the Mulga Rock deposit, Western Australia. Samples were collected from mineralized layers between 53 and 58.5 m depths in the Ambassador prospect, containing <5300 ppm U. Uranium bears a close spatial relationship with OM, mostly finely interspersed in the attrinite matrix and via enrichments within liptinitic phytoclasts (mainly sporinite and liptodetrinite). Geochemical analyses were conducted to: (i) identify the natural sources of molecular markers, (ii) recognize relationships between molecular markers and U concentrations and (iii) detect radiolysis effects on molecular marker distributions. Carbon to nitrogen ratios between 82 and 153, and Rock–Eval pyrolysis yields of 316–577 mg hydrocarbon/g TOC (HI) and 70–102 mg CO2/g TOC (OI) indicate a predominantly lipid-rich terrigenous plant OM source deposited in a complex shallow swampy wetland or lacustrine environment. Saturated hydrocarbon and ketone fractions reveal molecular distributions co-varying with U concentration. In samples with <1700 ppm U concentrations, long-chain n-alkanes and alkanones (C27–C31) reveal an odd/even carbon preference indicative of extant lipids. Samples with ⩾1700 ppm concentrations contain intermediate-length n-alkanes and alkanones, bearing a keto-group in position 2–10, with no carbon number preference. Such changes in molecular distributions are inconsistent with diagenetic degradation of terrigenous OM in oxic depositional environments and cannot be associated with thermal breakdown due to the relatively low thermal maturity of the deposits (R r  = 0.26%). It is assumed that the intimate spatial association of high U concentrations resulted in breakdown via radiolytic cracking of recalcitrant polyaliphatic macromolecules (spores, pollen, cuticles, or algal cysts) yielding medium chain length n-alkanes (C13–C24). Reactions of n-alkenes with OH radicals from water hydrolysis produced alcohols that dehydrogenated to alkanones or through carbonylation formed alkanones. Rapid reactions with hydroxyl radicals likely decreased the isomerization of n-alkenes and decreased alkanone diversity, such that the alkan-2-one isomer is predominant. This specific distribution of components generated by natural radiolysis enables their application as “radiolytic molecular markers”. Breaking of C–C bonds through radiolytic cracking at temperatures much lower than the oil window (<50 °C) can have profound implications on initiation of petroleum formation, paleoenvironmental reconstructions, mineral exploration and in tracking radiolysis of OM.

Concentrations of dissolved gold and silver species in hydrothermal fluids equilibrated with Au–Ag solid solutions have been calculated at wide conditions on the well known fO2–pH spaces. Ratios of the total concentrations of dissolved gold and silver species (∑Au/∑Ag) are higher as pH higher and fO2 lower. The ratios are constant at very low and high pH conditions where major dissolved species of both gold and silver are chloride complexes and thio complexes, respectively, while the ratios practically depend only on pH at intermediate pH conditions where Au(HS)2 and AgCl2 are major.The calculated results indicate that the solid solutions of high gold finenesses may precipitate from the fluids of low ratios of the total concentrations of dissolved gold and silver species when the conditions are (1) low pH’s and/or (2) high concentration ratios of dissolved chlorine and sulfur and/or (3) high temperatures.

Understanding groundwater inflows to rivers is important in managing connected groundwater and surface water systems and for protecting groundwater-dependant ecosystems. This study defines the distribution of gaining reaches and estimates groundwater inflows to a 62 km long section of Deep Creek (Maribyrnong catchment, Australia) using 222Rn. During summer months, Deep Creek ceases to flow and comprises a chain of ponds that δ18O and δ2H values, major ion concentrations, and 222Rn activities imply are groundwater fed. During the period where the river flows, the relative contribution of groundwater inflows to total river discharge ranges from ∼14% at high flow conditions to ∼100% at low flows. That the predicted groundwater inflows account for all of the increase in discharge at low flow conditions lends confidence to the mass balance calculations. Near-continuous 27 week 222Rn monitoring at one location in the middle of the catchment confirms the inverse correlation between river discharge and relative groundwater inflows, and also implies that there are limited bank return flows. Variations in groundwater inflows are related to geology and topography. High groundwater inflows occur where the river is at the edge of its floodplain, adjacent to hills composed of basement rocks, or flowing through steep incised valleys. Understanding the distribution of groundwater inflows and quantifying the contribution of groundwater to Deep Creek is important for managing and protecting the surface water resources, which support the endangered Yarra pygmy perch.

Use of multi-proxy approaches to determine the origin and depositional processes in modern lacustrine sediments: Carajás Plateau, Southeastern Amazon, Brazil by Prafulla Kumar Sahoo; Pedro Walfir Martins Souza-Filho; José Tasso Felix Guimarães; Marcio Sousa da Silva; Francisco Ribeiro Costa; Carmem-Lara de Oliveira Manes; Douglas Oti; Renato Oliveira Silva Júnior; Roberto Dall’Agnol (130-146).
Display OmittedGeochemical and isotopic compositions of surficial sediments from a plateau lake in Carajás, Southeastern Amazon region, were investigated to understand the spatial distribution of major and trace elements, δ13C, δ15N, and C/N ratio, depositional processes, and the origin of inorganic and organic fractions. The δ13C, δ15N, and C/N ratio indicate mainly an autochthonous source, with siliceous sponge spicules and algae being the major source of organic matter in the center of the lake (Sector 3), while an allochthonous source, mainly derived from C3 vascular forest plants dominates in the shallowest portion of the lake (Sector 1). Consequently, there was an apparent dilution of C4 plants (montane savanna) in the sediment/water interface. Among major elements, Fe2O3 is highly enriched in Sector 3, which is controlled by the erosion of catchment laterites and underwater topography, while Al2O3 and P2O5 enrichment near the northern border of the lake is controlled by the weathering of mafic rocks. Similar spatial distribution of SiO2 with total organic carbon (TOC) and isotopic evidence indicate that Si distribution is partially controlled by organic components such as siliceous sponge spicules and algae. The occurrence of most of the trace and rare earth elements (REE) is independent of Fe2O3 and TOC, but controlled by detrital aluminum silicates and heavy minerals, indicating a lack of post-depositional diagenetic control on their distribution. The distribution of As and Mo are possibly controlled by organic matter mineralization during early diagenesis and subsequently precipitation of Fe phases. The values of the chemical index of alteration (CIA) in the sediments are very high (94–99) and similar to those of the source rocks, suggesting that sediment composition is mainly controlled by mechanical weathering, rather than chemical weathering. Geochemical indices (Al/K, Ti/K, Al/Ti, La/Th, Ti/Zr, Zr/Hf, Hf/Nb, La/Al, Co/Th, Ba/Sr, and Th/Sc), together with the A–CN–K plot, suggest that the provenance of the inorganic sediments remained relatively uniform or constant during the depositional period, and that they were mainly derived from laterite crusts and subordinately from mafic soils. This inference was further substantiated by the chondrite-normalized REE patterns and discrimination plots. Geochemical indices such as U/Th, authigenic-U, Mo/Al, and V/Cr indicate that the sediments were deposited under an oxic environment.

Tracing sources of pollution in soils from the Jinding Pb–Zn mining district in China using cadmium and lead isotopes by Hanjie Wen; Yuxu Zhang; Christophe Cloquet; Chuanwei Zhu; Haifeng Fan; Chongguang Luo (147-154).
Systematic variations in the Cd and Pb isotope ratios in polluted topsoils surrounding the Jinding Pb–Zn mine in China were measured so that the sources of the metals could be traced. The average δ114 / 110Cd value and 206Pb/207Pb isotope ratio in background soils from the region were +0.41‰ and 1.1902, respectively, whereas the contaminated soil samples had different values, with the δ114 / 110Cd values varying between −0.59‰ and +0.33‰ and the 206Pb/207Pb isotope ratios varying between 1.1764 and 1.1896. We also measured the Cd and Pb isotopic compositions in oxide ores, sulfide ores, and slags, and found that binary mixing between ores and background soils could explain almost all of the variations in the Cd and Pb isotope ratios in the contaminated soils. This suggests that Cd and Pb pollution in the soils was mainly caused by the deposition of dust emitted during anthropogenic activities (mining and refining). The Pb and Cd isotope ratios clearly showed that contamination in soils in the northeastern part of the area was caused by surface mines and zinc smelters and their slagheaps, while contamination in soils in the southwestern part of the area also came from tailing ponds and underground mines. The main area of soil polluted by dust from Pb–Zn mining processes roughly extended for up to 5 km from the mine itself.

The influence of natural trace element distribution on the mobility of radionuclides. The exemple of nickel in a clay-rock by Sylvain Grangeon; Agnès Vinsot; Christophe Tournassat; Catherine Lerouge; Eric Giffaut; Stephanie Heck; Nora Groschopf; Melissa A. Denecke; Stefan Wechner; Thorsten Schäfer (155-173).
The natural distribution of nickel (Ni) in the Callovian–Oxfordian clay-rich rock of Bure (France) was investigated, together with that of cobalt (Co), zinc (Zn) and lead (Pb). The most Ni-enriched phases are pyrite (∼400 × 10−6  g g−1 Ni), sphalerite (∼300 × 10−6  g g−1), chlorite (∼285 × 10−6  g g−1), organic matter (∼300 × 10−6  g g−1), muscovite (100–200 × 10−6  g g−1) and possibly carbonate minerals (mainly calcite and minor dolomite, ∼10 × 10−6  g g−1). Despite their high abundance (up to ∼80% in the upper part of the formation), carbonate minerals are quantitatively a minor Ni reservoir; most of the Ni is borne by chlorite and pyrite, which are minor mineral phases. Co and Ni have a similar distribution, whereas Pb and Zn have partly different reservoirs (e.g. sphalerite – ZnS).In parallel, the equilibrium between rock and pore water was investigated, and importance of pyrite (or reduced sulfur minerals) as Ni reservoir in the formation was confirmed from an in situ experiment perturbed by oxidation processes. Under unperturbed conditions (laboratory kinetics experiments and in situ pore water sampling), Ni solubility ranges from ∼0.2 to 1 × 10−6  mol L−1. The rock sorption capacities with regards to radioactive Ni (in the range of ∼1.5 × 10−10  mol L−1 to ∼1.5 × 10−9  mol L−1 of spiked 63Ni) were also tested using batch experiments. Sorption kinetics of 63Ni on the Callovian–Oxfordian clay-rich rock was explained by two mechanisms: fast and reversible sorption onto clay minerals and slow incorporation in another phase. Indirect evidences point out the likely major role of calcite in this latter process albeit pyrite and organic matter may also be involved. Reversible sorption of Ni onto clay minerals surfaces is shown to be hindered by competition processes with other naturally occurring bivalent metals (e.g. naturally-occurring Ni and Zn), thus decreasing the 63Ni Rd value as compared to the value predicted from a bottom-up approach considering only 63Ni sorption on clay minerals. At longer time scale, irreversible 63Ni trapping is observed and compensates the competition processes. These two mechanisms were implemented in a reactive transport model to illustrate their influence on the prediction of long-term behavior of radioactive Ni. Migration distances are increased by the effect of cation competition, and reduced in a comparable extent by irreversible trapping in the time frame investigated.

CO2 geological storage in olivine rich basaltic aquifers: New insights from reactive-percolation experiments by Steve Peuble; Marguerite Godard; Linda Luquot; Muriel Andreani; Isabelle Martinez; Philippe Gouze (174-190).
To test the impact of fluid flow on the reactivity of porous (ultra-)mafic rocks, reactive percolation experiments were realized during which CO2-enriched water was injected at two different injection rates (Q  = 0.1 and 1 mL h−1) through sintered analogues of olivine-accumulation zones in basaltic flows at temperature and fluid composition conditions (T  = 180 °C; NaHCO3 buffered solution) favorable for CO2-mineralization (carbonation).All experiments resulted in silicate dissolution, carbonate precipitation upstream and (proto-)serpentine formation downstream indicating a decrease in the fluid reactivity along flow paths. The measured bulk carbonation rates ranged from 4 to 7 × 10−8  s−1; these values were significantly lower than previously published values of olivine carbonation rate obtained on powders in closed batch and flow-through reactors. Our study show complex couplings, at pore scale, between fluid flow, localization of reaction zones, and chemical reaction kinetics which in turn control hydrodynamic properties, carbonation rate and efficiency and fluid reactivity. This results in carbonation rates being higher when injection rates are high and permeability ultimately controlling carbonation reactions by limiting fluid input. During experiments, notable changes in permeability occurred for only minor changes in porosity indicating a control by the geometry of the porous network: heterogeneities in the distribution of flow paths favored the localization of precipitated minerals which in turn resulted in the closure of flow paths. This mechanism was particularly efficient at low injection rates. These results imply that controlling the injection rate could allow enhancing/limiting the efficiency of in situ carbonation.