Composition,nucleation, and growth of iron oxide concretions

Iron oxide concretions are formed from post depositional, paleogroundwater chemical interaction with iron minerals in porous sedimentary rocks. The concretions record a history of iron mobilization and precipitation caused by changes in pH, oxidation conditions, and activity of bacteria. Transport limited growth rates may be used to estimate the duration of fluid flow events. The Jurassic Navajo Sandstone, an important hydrocarbon reservoir and aquifer on the Colorado Plateau, USA, is an ideal stratum to study concretions because it is widely distributed, well exposed and is the host for a variety of iron oxide concretions.Many of the concretions are nearly spherical and some consist of a rind of goethite that nearly completely fills the sandstone porosity and surrounds a central sandstone core. The interior and exterior host-rock sandstones are similar in detrital minerals, but kaolinite and interstratified illite–smectite are less abundant in the interior. Lepidocrocite is present as sand-grain rims in the exterior sandstone, but not present in the interior of the concretions.Widespread sandstone bleaching resulted from dissolution of early diagenetic hematite grain coatings by chemically reducing water that gained access to the sandstone through fault conduits. The iron was transported in solution and precipitated as iron oxide concretions by oxidation and increasing pH. Iron diffusion and advection growth time models place limits on minimum duration of the diagenetic, fluid flow events that formed the concretions. Concretion rinds 2 mm thick and 25 mm in radius would take place in 2000 years from transport by diffusion and advection and in 3600 years if transport was by diffusion only. Solid concretions 10 mm in radius would grow in 3800 years by diffusion or 2800 years with diffusion and advection.Goethite (α-FeO (OH)) and lepidocrocite (γ-FeO (OH)) nucleated on K-feldspar grains, on illite coatings on sand grains, and on pore-filling illite, but not on clean quartz grains. Model results show that regions of detrital K-feldspar in the sandstone that consume H⁺ more rapidly than diffusion to the reaction site determine concretion size, and spacing is related to diffusion and advection rates of supply of reactants Fe²⁺, O₂, and H⁺.     

Do organic ligands affect forsterite dissolution rates?

Far-from equilibrium, steady state forsterite dissolution rates were measured at pH ∼3 and 25 °C in aqueous solutions containing 0.1 m/kg NaCl and up to 0.1 mol/kg of 13 distinct dissolved organic ligands in mixed-flow reactors. The organic ligands considered in this study include those common in Earth surface environments and those considered as potential catalysts for use in CO₂ sequestration efforts: acetate, oxalate, citrate, EDTA⁴⁻, glutamate, gluconate, malonate, aspartate, tartrate, malate, alginate, salycilate and humate. The presence of up to 0.1 mol/kg of each organic ligand altered forsterite dissolution rates less than 0.2 log units, which is the estimated uncertainty of the measured rates. Results obtained in this study, therefore, suggest that the presence of aqueous organic anions negligibly affects forsterite far-from equilibrium dissolution rates in most natural environments, and indicate that forsterite carbonation may not be appreciably accelerated by organic ligand catalysis.     

Mechanisms controlling acid neutralization and metal mobility within a Ni-rich tailings impoundment

Low-quality pore waters containing high concentrations of dissolved H⁺, SO₄, and metals have been generated in the East Tailings Management Area at Lynn Lake, Manitoba, as a result of sulfide-mineral oxidation. To assess the abundance, distribution, and solid-phase associations of S, Fe, and trace metals, the tailings pore water was analyzed, and investigations of the geochemical and mineralogical characteristics of the tailings solids were completed. The results were used to delineate the mechanisms that control acid neutralization, metal release, and metal attenuation. Migration of the low-pH conditions through the vadose zone is limited by acid-neutralization reactions, resulting in the development of distinct pore-water pH zones at depth; the neutralization reactions involve carbonate (pH ⩾ 5.7), Al-hydroxide (pH ⩾ 4.0), and aluminosilicate solids. As the zone of low-pH pore water expands, the pH will then be primarily controlled by less soluble solids, such as Fe(III) oxyhydroxides (pH < 3.5) and the relatively more recalcitrant aluminosilicates (pH ⩾ 1.3). Precipitation/dissolution reactions involving secondary Fe(III) oxyhydroxides and hydroxysulfates control the concentrations of dissolved Fe(III). Concentrations of dissolved SO₄ are principally controlled by the formation of gypsum and jarosite. Geochemical extractions indicate that the solid-phase concentrations of Ni, Co, and Zn are associated predominantly with reducible and acid-soluble fractions. The concentrations of dissolved trace metals are therefore primarily controlled by adsorption/complexation and (or) co-precipitation/dissolution reactions involving secondary Fe(III) oxyhydroxide and hydroxysulfate minerals. Concentrations of dissolved metals with relatively low mobility, such as Cu, are also controlled by the precipitation of discrete minerals. Because the major proportion of metals is sequestered through adsorption and (or) co-precipitation, the metals are susceptible to remobilization if low-pH or reducing conditions develop within the tailings.     

Kaolinite and smectite dissolution rate in high molar KOH solutions at 35° and 80°C

Experiments measuring kaolinite and smectite dissolution rates were carried out using batch reactors at 35° and 80°C. No potential catalysts or inhibitors were present in solution. Each reactor was charged with 1 g of clay of the ≤2 μm fraction and 80, 160 or 240 ml of 0.1–4 M KOH solution. An untreated but sized kaolinite from St. Austell and two treated industrial smectites were used in the experiments. One smectite is a nearly pure montmorillonite, while the second has a significant component of beidellitic charge (35%). The change in solution composition and mineralogy was monitored as a function of time. Initially, the 3 clays dissolved congruently. No new formed phases were observed by XRD and SEM during the pure dissolution stage. The kaolinite dissolution is characterized by a linear release of silica and Al as a function of the log of time. This relationship can be explained by a reaction affinity effect which is controlled by the octahedral layer dissolution. Far from equilibrium, dissolution rates are proportional to a^0.56±0.12_OH⁻ at 35°C and to a^0.81±0.12_OH⁻ at 80°C. The activation energy of kaolinite dissolution increases from 33±8 kJ/mol in 0.1 M KOH solutions to 51±8 kJ/mol in 3 M KOH solutions. In contrast to kaolinite, the smectites dissolve at much lower rates and independently of the aqueous silica or Al concentrations. The proportionality of the smectite dissolution rate constant at 35 and 80°C was a^0.15±0.06_OH⁻. The activation energy of dissolution appears to be independent of pH for smectite and is found to be 52±4 kJ/mol. The differences in behavior between the two kinds of minerals can be explained by structural differences. The hydrolysis of the tetrahedral and the octahedral layer appears as parallel reactions for kaolinite dissolution and as serial reactions for smectite dissolution. The rate limiting step is the dissolution of the octahedral layer in the case of kaolinite, and the tetrahedral layer in the case of smectite.     

Dissolution kinetics of soil clays in sulfuric acid solutions: Ionic strength and temperature effects

Significant amounts of sulfuric acid (H₂SO₄) rich saline water can be produced by the oxidation of sulfide minerals contained in inland acid sulfate soils (IASS). In the absence of carbonate minerals, the dissolution of phyllosilicate minerals is one of very few processes that can provide long-term acid neutralisation. It is therefore important to understand the acid dissolution behavior of naturally occurring clay minerals from IASS under saline–acidic solutions. The objective of this study was to investigate the dissolution of a natural clay-rich sample under saline–acidic conditions (pH 1–4; ionic strengths = 0.01 and 0.25 M; 25 °C) and over a range of temperatures (25–45 °C; pH 1 and pH 4). The clay-rich sample referred to as Bottle Bend clay (BB clay) used was from an IASS (Bottle Bend lagoon) in south-western New South Wales (Australia) and contained smectite (40%), illite (27%), kaolinite (26%) and quartz (6%). Acid dissolution of the BB clay was initially rapid, as indicated by the fast release of cations (Si, Al, K, Fe, Mg). Relatively higher Al (pH 4) and K (pH 2–4) release was obtained from BB clay dissolution in higher ionic strength solutions compared to the lower ionic strength solutions. The steady state dissolution rate (as determined from Si, Al and Fe release rates; R_Si, R_Al, R_Fe) increased with decreasing solution pH and increasing temperature. For example, the highest log R_Si value was obtained at pH 1 and 45 °C (−9.07 mol g⁻¹ s⁻¹), while the lowest log R_Si value was obtained at pH 4 and 25 °C (−11.20 mol g⁻¹ s⁻¹). A comparison of these results with pure mineral dissolution rates from the literature suggests that the BB clay dissolved at a much faster rate compared to the pure mineral samples. Apparent activation energies calculated for the clay sample varied over the range 76.6 kJ mol⁻¹ (pH 1) to 37.7 kJ mol⁻¹ (pH 4) which compare very well with the activation energy values for acidic dissolution of monomineralic samples e.g. montmorillonite from previous studies. The acid neutralisation capacity (ANC) of the clay sample was calculated from the release of all structural cations except Si (i.e. Al, Fe, K, Mg). According to these calculations an ANC of 1.11 kg H₂SO₄/tonne clay/day was provided by clay dissolution at pH 1 (I = 0.25 M, 25 °C) compared to an ANC of 0.21 kg H₂SO₄/tonne clay/day at pH 4 (I = 0.25 M, 25 °C). The highest ANC of 6.91 kg H₂SO₄/tonne clay/day was provided by clay dissolution at pH 1 and at 45 °C (I = 0.25 M), which is more than three times higher than the ANC provided under the similar solution conditions at 25 °C. In wetlands with little solid phase buffering available apart from clay minerals, it is imperative to consider the potential ANC provided by the dissolution of abundantly occurring phyllosilicate minerals in devising rehabilitation schemes.     

Dissolution kinetics and biodurability of tremolite particles in mimicked lung fluids: Effect of citrate and oxalate

The effect of citrate and oxalate on tremolite dissolution rate was measured at 37 °C in non-stirred flow-through reactors, using modified Gamble’s solutions at pH 4 (macrophages), 7.4 (interstitial fluids) and 5.5 (intermediate check point) containing 0, 0.15, 1.5 and 15 mmol L⁻¹ of citrate or oxalate. The dissolution rates calculated from Si concentration in the output solutions without organic ligands depend on pH, decreasing when the pH increases from −13.00 (pH 4) to −13.35 (pH 7.4) mol g⁻¹ s⁻¹ and following a proton-promoted mechanism. The presence of both ligands enhances dissolution rates at every pH, increasing this effect when the ligand concentration increases. Citrate produces a stronger effect as a catalyst than oxalate, mainly at more acidic pHs and enhances dissolution rates until 20 times for solutions with 15 mmol L⁻¹ citrate. However, at pH 7.4 the effect is lighter and oxalate solutions (15 mmol L⁻¹) only enhances dissolution rates eight times respect to free organic ligand solutions. Dissolution is promoted by the attack to protons and organic ligands to the tremolite surface. Magnesium speciation in oxalate and citrate solutions shows that Mg citrate complexes are more effective than oxalate ones during the alteration of tremolite in magrophages, but this tendency is the opposite for interstitial fluids, being oxalate magnesium complexes stronger. The biodurability estimations show that the destruction of the fibers is faster in acidic conditions (macrophages) than in the neutral solutions (interstitial fluid). At pH 4, both ligands oxalate and citrate reduce the residence time of the fibers with respect to that calculated in absence of ligands. Nevertheless, at pH 7.4 the presence of ligands does not reduce significantly the lifetime of the fibers.     

Diffusion kinetics of Cr in olivine and 53Mn–53Cr thermochronology of early solar system objects

We have determined the diffusion coefficient of Cr in olivine as function of temperature, oxygen fugacity (fO₂), and crystallographic orientation and used these data to develop a quantitative understanding of the resetting of the short-lived ⁵³Mn–⁵³Cr decay system in olivine during cooling within meteorite parent body. The diffusion of Cr in olivine was found to be anisotropic, and effectively independent of fO₂ between wüstite–iron buffer and two orders of magnitude above this buffer. The diffusion data were used to calculate the spatially averaged mean closure temperature of the ⁵³Mn–⁵³Cr decay system in olivine as function of the initial temperature, cooling rate and grain size, and also the closure age profile of this system in olivine single crystal as function of radial distance and a dimensionless parameter that incorporates the effects of various parameters that affect the closure age. We also present a thermochronolgic formulation that permits retrieval of cooling rates from the extent of resetting of the bulk ⁵³Mn–⁵³Cr closure age of olivine during cooling. This method was applied to determine the cooling rate of the pallasite Omolon, which showed ⁵³Mn–⁵³Cr bulk age of olivine that is 10 Myr younger than the age of the solar system. The calculated cooling rate, which is 20–40 °C/Myr at ∼985–1000 °C, is in good agreement with the metallographic cooling rate at ∼500 °C, when the two results are considered in terms of a cooling model in which the reciprocal temperature increases linearly with time. The inferred cooling rate of Omolon, which seems to be a sample from the core-mantle boundary, yields a burial depth of ∼30 km in a parent body of at least ∼100 km radius.     

Factors affecting the dissolution kinetics of volcanic ash soils: dependencies on pH,CO2, and oxalate

Laboratory experiments were conducted with volcanic ash soils from Mammoth Mountain, California to examine the dependence of soil dissolution rates on pH and CO₂ (in batch experiments) and on oxalate (in flow-through experiments). In all experiments, an initial period of rapid dissolution was observed followed by steady-state dissolution. A decrease in the specific surface area of the soil samples, ranging from 50% to 80%, was observed; this decrease occurred during the period of rapid, initial dissolution. Steady-state dissolution rates, normalized to specific surface areas determined at the conclusion of the batch experiments, ranged from 0.03 μmol Si m⁻² h⁻¹ at pH 2.78 in the batch experiments to 0.009 μmol Si m⁻² h⁻¹ at pH 4 in the flow-through experiments. Over the pH range of 2.78–4.0, the dissolution rates exhibited a fractional order dependence on pH of 0.47 for rates determined from H⁺ consumption data and 0.27 for rates determined from Si release data. Experiments at ambient and 1 atm CO₂ demonstrated that dissolution rates were independent of CO₂ within experimental error at both pH 2.78 and 4.0. Dissolution at pH 4.0 was enhanced by addition of 1 mM oxalate. These observations provide insight into how the rates of soil weathering may be changing in areas on the flanks of Mammoth Mountain where concentrations of soil CO₂ have been elevated over the last decade. This release of magmatic CO₂ has depressed the soil pH and killed all vegetation (thus possibly changing the organic acid composition). These indirect effects of CO₂ may be enhancing the weathering of these volcanic ash soils but a strong direct effect of CO₂ can be excluded.     

Laboratory study of calcite–gypsum sludge–water interactions in a flooded tailings impoundment at the Kristineberg Zn–Cu mine,northern Sweden

Due to liming of acid mine drainage, a calcite–gypsum sludge with high concentrations of Zn (24,400 ± 6900 μg g⁻¹), Cu (2840 ± 680 μg g⁻¹) and Cd (59 ± 20 μg g⁻¹) has formed in a flooded tailings impoundment at the Kristineberg mine site. The potential metal release from the sludge during resuspension events and in a long-term perspective was investigated by performing a shake flask test and sequential extraction of the sludge. The sequentially extracted carbonate and oxide fractions together contained ⩾97% of the total amount of Cd, Co, Cu, Ni, Pb and Zn in the sludge. The association of these metals with carbonates and oxides appears to result from sorption and/or coprecipitation reactions at the surfaces of calcite and Fe, Al and Mn oxyhydroxides forming in the impoundment. If stream water is diverted into the flooded impoundment, dissolution of calcite, gypsum and presumably also Al oxyhydroxides can be expected during resuspension events. In the shake flask test (performed at a pH of 7–9), remobilisation of Zn, Cu, Cd and Co from the sludge resulted in dissolved concentrations of these metals that were significantly lower than those predicted to result from dissolution of the carbonate fraction of the sludge. This may suggest that cationic Zn, Cu, Cd and Co remobilised from dissolving calcite, gypsum and Al oxyhydroxides were readsorbed onto Fe oxyhydroxides remaining stable under oxic conditions. In a long-term perspective (≳10² a), ⩾97% of the Cd, Co, Cu, Ni, Pb and Zn content of the sludge potentially is available for release by dissolution of calcite and reductive dissolution of Fe oxyhydroxides if the sludge is subject to a soil environment with lower dissolved Ca concentrations, pH and redox than in the impoundment.     

The Diavik waste rock project: Initial geochemical response from a low sulfide waste rock pile

Three large-scale experimental waste rock piles (test piles) were constructed and instrumented at the Diavik Diamond Mine in the Northwest Territories, Canada, as part of an integrated field and laboratory study to measure and compare physical and geochemical characteristics of experimental, low sulfide waste rock piles at various scales. This paper describes the geochemical response during the first season from a test pile containing 0.053 wt.% S. Bulk drainage chemistry was measured at two sampling points for pH, Eh, alkalinity, dissolved cations and anions, and nutrients. The geochemical equilibrium model MINTEQA2 was used to interpret potential mineral solubility controls on water chemistry. The geochemical response characterizes the initial flushing response of blasting residues and oxidation products derived from sulfides in waste rock exposed to the atmosphere for less than 1 year. Sulfate concentrations reached 2000 mg L⁻¹ when ambient temperatures were >10 °C, and decreased as ambient temperatures declined to <0 °C. The pH decreased to <5, concomitant with an alkalinity minimum of <1 mg L⁻¹ (as total CaCO₃), suggesting all available alkalinity is consumed by acid-neutralizing reactions. Concentrations of Al and Fe were <0.36 and <0.11 mg L⁻¹, respectively. Trends of pH and alkalinity and the calculated saturation indices for Al and Fe (oxy)hydroxides suggest that dissolution of Al and Fe (oxy)hydroxide phases buffers the pH. The effluent water showed increased concentrations of dissolved Mn (<13 mg L⁻¹), Ni (<7.0 mg L⁻¹), Co (<1.5 mg L⁻¹), Zn (<0.5 mg L⁻¹), Cd (<0.008 mg L⁻¹) and Cu (<0.05 mg L⁻¹) as ambient temperatures increased. Manganese is released by aluminosilicate weathering, Ni and Co by pyrrhotite [Fe_1−xS] oxidation, Zn and Cd by sphalerite oxidation, and Cu by chalcopyrite [CuFeS₂] oxidation. No dissolved metals appear to have discrete secondary mineral controls. Changes in SO₄, pH and metal concentrations indicate sulfide oxidation is occurring and effluent concentrations are influenced by ambient temperatures and, possibly, increasing flow path lengths that transport reaction products from previously unflushed waste rock.     

Rock alteration in alkaline cement waters over 15 years and its relevance to the geological disposal of nuclear waste

The interaction of groundwater with cement in a geological disposal facility (GDF) for intermediate level radioactive waste will produce a high pH leachate plume. Such a plume may alter the physical and chemical properties of the GDF host rock. However, the geochemical and mineralogical processes which may occur in such systems over timescales relevant for geological disposal remain unclear. This study has extended the timescale for laboratory experiments and shown that, after 15 years two distinct phases of reaction may occur during alteration of a dolomite-rich rock at high pH. In these experiments the dissolution of primary silicate minerals and the formation of secondary calcium silicate hydrate (C–S–H) phases containing varying amounts of aluminium and potassium (C–(A)–(K)–S–H) during the early stages of reaction (up to 15 months) have been superseded as the systems have evolved. After 15 years significant dedolomitisation (MgCa(CO₃)₂ + 2OH⁻ → Mg(OH)₂ + CaCO₃ + CO₃²⁻_(aq)) has led to the formation of magnesium silicates, such as saponite and talc, containing variable amounts of aluminium and potassium (Mg–(Al)–(K)–silicates), and calcite at the expense of the early-formed C–(A)–(K)–S–H phases. This occured in high pH solutions representative of two different periods of cement leachate evolution with little difference in the alteration processes in either a KOH and NaOH or a Ca(OH)₂ dominated solution but a greater extent of alteration in the higher pH KOH/NaOH leachate. The high pH alteration of the rock over 15 years also increased the rock’s sorption capacity for U(VI). The results of this study provide a detailed insight into the longer term reactions occurring during the interaction of cement leachate and dolomite-rich rock in the geosphere. These processes have the potential to impact on radionuclide transport from a geodisposal facility and are therefore important in underpinning any safety case for geological disposal.     

Phosphate removal from water by naturally occurring shale,sandstone, and laterite: The role of iron oxides and of soluble species

Aqueous phosphate removal by three geomaterials from Ivory Coast was evaluated to determine their potential application as low-cost phosphate adsorbents in wastewater treatment. Batch experiments showed that phosphate uptake strongly depended on pH. Laterite and sandstone dissolution was less pronounced compared to shale. A correlation between concentrations of aqueous cation species released from shale and phosphate uptake was observed. The kinetics were well described using the pseudo-second-order model. Isotherms displayed a saturation level on shale, while phosphate uptake continuously increased for laterite and sandstone. The removal efficiency decreased in the following ranking order: laterite > sandstone > shale. Laterite was also the most efficient adsorbent in column experiments. The high phosphate removal efficiency of laterite (8.3 mg PO₄ g^?1) was attributed to the presence of superparamagnetic low grain sizes of goethite. Laterite is a particularly promising material for further investigation in wastewater treatment technology such as constructed wetlands.     

Effects of citrate on the dissolution and transformation of biotite,analyzed by chemical and atomic force microscopy

The aim of this study was to investigate the dissolution and transformation characteristics of phyllosilicate under low molecular weight organic acids in the farmland environment (pH 4.0–8.0). Changes of dissolution and morphology of biotite were evaluated using chemical extraction experiments and in situ/ex situ atomic force microscopy (AFM) with fluids of citric acid (CA) solution at pH 4.0, 6.0, and 8.0. Results of extracting experiments show that CA solutions contributed to the release rate of potassium (K), silicon (Si), and aluminum (Al) from biotite relative to a control aqueous solution. In situ AFM observations indicate that the dissolution of biotite from the biotite (0 0 1) surface occurred on the terrace, segment, and fringe of pits, while new etch pits did not readily form on biotite (0 0 1) surfaces in aqueous solutions. However, dissolution rates of terraces can be greatly accelerated with the help of citrate. In pH 4.0 CA solution, 70 min dissolution reactions of biotite (0 0 1) surfaces result in more etch pits than in pH 6.0 and 8.0 solutions. In addition, the transformation of biotite occurred simultaneously with the dissolution process. Secondary coating was observed on the biotite (0 0 1) surface after 140 h of immersion in a weak acid environment. Thus, the protons have a dominant role in the dissolution process of biotite with organic (carboxyl) acting as a catalyst under acidic condition. Based on the theory of interactions on a water–mineral interface in a weak acid environment, dissolution of biotite starts from defect/kink sites on the surface, one layer by one layer, and develops along the [h k 0] direction. A secondary coating that forms on the biotite (0 0 1) surface may restrain the formation and growth of etch pits, whereas this process may have a positive role on the stability of soil structure during long-term soil management.     

The dependence of albite feldspar dissolution kinetics on fluid saturation state at acid and basic pH: Progress towards a universal relation

Here we report on two separate ongoing, multi-year investigations on the dependence of the dissolution rate (R) of albite feldspar on fluid saturation state, as defined by the Gibbs free energy of reaction (ΔG_r) for dissolution. The investigations are based on dissolution at pH 9.2, 150 °C and pH 3.3, 100 °C. Both studies reveal that the R–ΔG_r relation is highly non-linear and sigmoidal. The kinetic data from the first study, being the most complete, can be fitted with a sigmoidal rate curve that is composed of two separate, parallel rate laws that represent distinct mechanisms of dissolution. The switch between one dominant mechanism and the other may be controlled by a critical free energy. The fact that in both studies the same type of sigmoidal R–ΔG_r relation exists for dissolution at different pH and temperature condition suggests that this behavior may be universal for albite and other feldspars. Moreover, the experimental data contradict the commonly used R–ΔG_r relation that is loosely based on transition state theory (TST). This has important implications with respect to the accuracy of geochemical codes that model water–rock interactions at near-equilibrium conditions.     

Kinetics of Au (III) extraction by DBC from hydrochloric solution using Lewis cell

The kinetics of forward extraction [AuCl₄]^? from aqua regia medium by diethylene glycol dibutyl ether (DBC) have been investigated by the Lewis cell (LC) technique. At first gold extraction has been carried out under different experimental conditions for achieving the stoichiometry coefficients and the value of the extraction equilibrium constant (K = 0.1). For kinetic data treatment, flux ‘F’ method has been applied. Reaction order with respect to DBC, pH and [AuCl₄]^? was determined and then the rate constant was calculated. The rate of gold extraction from 2 M chloride medium can be expressed as F = 10^0.88[AuCl₄^?]^1.25 [DBC]^0.4 [H⁺]^?0.22. Kinetics data were treated by EVIEWS software and coefficients were obtained. The comparison of manual and software results indicated that the results had good conformity. Influence of temperature was studied and then activation energy, E_a, (11.17 kJ/mol), activation enthalpy (11.66 kJ/mol) and entropy (?187 J/mol K) were calculated by using Arrhenius and activation complex theory respectively. E_a value (< 20.9 kJ/mol) indicates that, the extraction of gold (III) in the investigated system is controlled by diffusion process.     

Activation energies of the self-diffusion of HTO, 22Na+ and 36Cl− in a highly compacted argillaceous rock (Opalinus Clay)

The temperature dependence of the self-diffusion of HTO, ²²Na⁺ and ³⁶Cl⁻ in Opalinus Clay (OPA) was studied using a through-diffusion technique, in which the temperature was gradually increased in the steady state phase of the diffusion. The measurements were done on samples from two different geological locations. The dependence of the effective diffusion coefficient on temperature was found to be of an Arrhenius type in the temperature range between 0 and 70 °C. A slight difference between the two locations could be observed. The average value of the activation energy of the self-diffusion of HTO in OPA was 21.1 ± 1.6 kJ mol⁻¹, and 21.0 ± 3.5 and 19.4 ± 1.5 kJ mol⁻¹ for ²²Na⁺ and ³⁶Cl⁻, respectively. The measured values for HTO are slightly higher than the values found for the bulk liquid water (HTO: 18.8 ± 0.4 kJ mol⁻¹). This indicates that the structure of the confined water in OPA might be slightly different from that of bulk liquid water. Also for Na⁺ and Cl⁻, slightly higher values than in bulk liquid water (Na⁺: 18.4 kJ mol⁻¹; Cl⁻: 17.4 kJ mol⁻¹) were observed.The Stokes–Einstein relationship, based on the temperature dependency of the viscosity of bulk water, could not be used to describe the temperature dependence of the diffusion of HTO in OPA. This additionally indicates the slightly different structure of the pore water in OPA.     

Pressure solution of quartz aggregates under low effective stress (0.42–0.61 MPa) at 25–45 °C

Dissolution rates of pressure solution (PS) for quartz aggregates in 0.002 M NaHCO₃ solution were experimentally determined under low effective stress conditions of 0.42–0.61 MPa, and low temperatures of 25–45 °C. At temperatures of 25 °C, 35 °C, and 45 °C, the resultant silicon dissolution rates are 4.2 ± 1.2 × 10⁻¹⁵, 6.0 ± 1.0 × 10⁻¹⁵ and 7.8 ± 1.9 × 10⁻¹⁵ mol/cm²/s, respectively. Ratios between these dissolution rates and those of quartz sand at zero effective stress are 4.1 ± 1.2 at 25 °C, 3.0 ± 0.5 at 35 °C, and 2.4 ± 0.6 at 45 °C. As the uniaxial pressure was increased, the dissolution rate of PS also increased, though gradually decreased when the effective stress was kept constant. After the removal of stress, the dissolution rate was observed to increase once again. The activation energy of our PS experiments was determined to be approximately 24 kJ/mol, lower than the amount required for quartz sand dissolution to commence at zero effective stress. Our results clearly show that, even at such low temperature and effective stress, Si released into solution as a result of PS can be detected. This implies that experimental compaction of quartz aggregates can be measured even under such condition.     

Trace element source terms for mineral dissolution

We present an approach for determining source terms for modeling trace element release from minerals, using arsenic (As) as an example. The source term function uses laboratory-measured mineral dissolution rates to predict the time rate of change of As concentrations (mol/L s) released to water by the dissolving mineral. Application of this function to As-bearing minerals (realgar, orpiment, arsenopyrite, scorodite, pyrite, and jarosite) in air saturated water at 25 °C shows that mineralogy, grain size and pH are important factors affecting the As source term while DO concentration and temperature are relatively unimportant for conditions found in typical aquifers. The derived function shows that the source term decreases as a function of (1 − t/t_L)², where t_L is the grain lifetime, due to the shrinkage of the mineral grains as they dissolve. For some models, either a constant or an instantaneous term might be used, provided that certain time constraints are met. The methods outlined in this paper are intended to help bridge the gap between laboratory measurements and field-based models. Although this paper uses As as an example, the methods are general and can be used to predict source terms for other mineral-derived trace elements to groundwater.     

Dissolution kinetics of selected natural minerals relevant to potential CO2-injection sites – Part 2: Dissolution and alteration of carbonates and feldspars in CO2-bearing brines

Chemical interaction processes among injected CO₂, saline fluids and potential reservoir materials are experimentally simulated to derive dissolution rates of natural materials (minerals) that can be used as input parameters for modeling of CO₂ storage in deep saline formations and risk analyses. In order to study dissolution processes, mineral aliquots were exposed to CO₂-bearing brines at elevated temperature (60, 100, 150 °C) and pressure (85 bar) and at various run durations. Several potential reservoir rocks include carbonates as cement. Calcite and dolomite grains were therefore mainly used as solid starting material. Experiments with the two feldspar varieties alkali feldspar and almost pure anorthite were performed in addition. Grain sizes of the mineral starting materials varied between <63 μm and 500 μm with most experiments performed at grain size fractions of 160 – 250 μm and 250 – 500 μm. All experiments run with a complex synthetic brine (total dissolved solids: ∼156 g/l) according to a natural upper cretaceous formation water. Dry ice was used as CO₂-source. All experiments were done in closed batch reactors. These reactors allow mimicking reservoir conditions far from the injection site as well as reservoir conditions after finishing the CO₂ injection. The concentration changes during the experiment were monitored by ICP-OES measurements of the initial and the post-run fluids. Dissolution rates were derived based on the concentration changes of the brine.Most of the studied experimental variables and parameters (temperature, run duration, grain size, brine composition – expressed as pH-value and ionic strength) impact alteration of the reacting agents, i.e. they change the chemical composition of the brine, change the surfaces of the mineral aliquots exposed to the CO₂-bearing brine, and induce formation of secondary minerals. Hence, all influencing parameters on dissolution processes have to be considered and time-resolved changes of the dissolution behavior have to be implemented in numerical simulations of processes at CO₂ injection sites and CO₂ storage reservoirs.