Clay mineral weathering and contaminant dynamics in a caustic aqueous system: I. Wet chemistry and aging effects

Caustic high level radioactive waste induces mineral weathering reactions that can influence the fate of radionuclides released in the vicinity of leaking storage tanks. The uptake and release of Cs^I and Sr^II were studied in batch reactors of 2:1 layer-type silicates—illite (Il), vermiculite (Vm) and montmorillonite (Mt)—under geochemical conditions characteristic of leaking tank waste at the Hanford Site in WA (0.05 m Al_T, 2 m Na⁺, 1 m NO₃⁻, pH ∼14, Cs and Sr present as co-contaminants). Time series (0 to 369 d) experiments were conducted at 298 K, with initial [Cs]₀ and [Sr]₀ concentrations from 10⁻⁵ to 10⁻³ mol kg⁻¹. Clay mineral type affected the rates of (i) hydroxide promoted dissolution of Si, Al and Fe, (ii) precipitation of secondary solids and (iii) uptake of Cs and Sr. Initial Si release to solution followed the order Mt > Vm > Il. An abrupt decrease in soluble Si and/or Al after 33 d for Mt and Vm systems, and after 190 d for Il suspensions was concurrent with accumulation of secondary aluminosilicate precipitates. Strontium uptake exceeded that of Cs in both rate and extent, although sorbed Cs was generally more recalcitrant to subsequent desorption and dissolution. After 369 d reaction time, reacted Il, Vm and Mt solids retained up to 17, 47 and 14 mmol kg⁻¹ (0.18, 0.24 and 0.02 μmol m⁻²) of Cs, and 0, 27 and 22 mmol kg⁻¹ (0, 0.14 and 0.03 μmol m⁻²) Sr, respectively, which were not removed in subsequent Mg exchange or oxalic acid dissolution reactions. Solubility of Al and Si decreased with initial Cs and Sr concentration in Mt and Il, but not in Vm. High co-contaminant sorption to the Vm clay, therefore, appears to diminish the influence of those ions on mineral transformation rates.     

Weathering of the Rio Blanco quartz diorite, Luquillo Mountains, Puerto Rico: Coupling oxidation, dissolution, and fracturing

In the mountainous Rio Icacos watershed in northeastern Puerto Rico, quartz diorite bedrock weathers spheroidally, producing a 0.2-2 m thick zone of partially weathered rock layers (∼2.5 cm thickness each) called rindlets, which form concentric layers around corestones. Spheroidal fracturing has been modeled to occur when a weathering reaction with a positive ΔV of reaction builds up elastic strain energy. The rates of spheroidal fracturing and saprolite formation are therefore controlled by the rate of the weathering reaction.Chemical, petrographic, and spectroscopic evidence demonstrates that biotite oxidation is the most likely fracture-inducing reaction. This reaction occurs with an expansion in d (0 0 1) from 10.0 to 10.5 Å, forming “altered biotite”. Progressive biotite oxidation across the rindlet zone was inferred from thin sections and gradients in K and Fe(II). Using the gradient in Fe(II) and constraints based on cosmogenic age dates, we calculated a biotite oxidation reaction rate of 8.2 × 10⁻¹⁴ mol biotite m⁻² s⁻¹. Biotite oxidation was documented within the bedrock corestone by synchrotron X-ray microprobe fluorescence imaging and XANES. X-ray microprobe images of Fe(II) and Fe(III) at 2 μm resolution revealed that oxidized zones within individual biotite crystals are the first evidence of alteration of the otherwise unaltered corestone.Fluids entering along fractures lead to the dissolution of plagioclase within the rindlet zone. Within 7 cm surrounding the rindlet-saprolite interface, hornblende dissolves to completion at a rate of 6.3 × 10⁻¹³ mol hornblende m⁻² s⁻¹: the fastest reported rate of hornblende weathering in the field. This rate is consistent with laboratory-derived hornblende dissolution rates. By revealing the coupling of these mineral weathering reactions to fracturing and porosity formation we are able to describe the process by which the quartz diorite bedrock disaggregates and forms saprolite. In the corestone, biotite oxidation induces spheroidal fracturing, facilitating the influx of fluids that react with other minerals, dissolving plagioclase and chlorite, creating additional porosity, and eventually dissolving hornblende and precipitating secondary minerals. The thickness of the resultant saprolite is maintained at steady state by a positive feedback between the denudation rate and the weathering advance rate driven by the concentration of pore water O₂ at the bedrock-saprolite interface.     

Reconciling the elemental and Sr isotope composition of Himalayan weathering fluxes: insights from the carbonate geochemistry of stream waters

Determining the relative proportions of silicate vs. carbonate weathering in the Himalaya is important for understanding atmospheric CO₂ consumption rates and the temporal evolution of seawater Sr. However, recent studies have shown that major element mass-balance equations attribute less CO₂ consumption to silicate weathering than methods utilizing Ca/Sr and ⁸⁷Sr/⁸⁶Sr mixing equations. To investigate this problem, we compiled literature data providing elemental and ⁸⁷Sr/⁸⁶Sr analyses for stream waters and bedrock from tributary watersheds throughout the Himalaya Mountains. In addition, carbonate system parameters (P_CO2, mineral saturation states) were evaluated for a selected suite of stream waters. The apparent discrepancy between the dominant weathering source of dissolved major elements vs. Sr can be reconciled in terms of carbonate mineral equilibria. Himalayan streams are predominantly Ca²⁺-Mg²⁺-HCO₃⁻ waters derived from calcite and dolomite dissolution, and mass-balance calculations demonstrate that carbonate weathering contributes ∼87% and ∼76% of the dissolved Ca²⁺ and Sr²⁺, respectively. However, calculated Ca/Sr ratios for the carbonate weathering flux are much lower than values observed in carbonate bedrock, suggesting that these divalent cations do not behave conservatively during stream mixing over large temperature and P_CO2 gradients in the Himalaya.The state of calcite and dolomite saturation was evaluated across these gradients, and the data show that upon descending through the Himalaya, ∼50% of the streams evaluated become highly supersaturated with respect to calcite as waters warm and degas CO₂. Stream water Ca/Mg and Ca/Sr ratios decrease as the degree of supersaturation with respect to calcite increases, and Mg²⁺, Ca²⁺, and HCO₃⁻ mass balances support interpretations of preferential Ca²⁺ removal by calcite precipitation. On the basis of patterns of saturation state and P_CO2 changes, calcite precipitation was estimated to remove up to ∼70% of the Ca²⁺ originally derived from carbonate weathering. Accounting for the nonconservative behavior of Ca²⁺ during riverine transport brings the Ca/Sr and ⁸⁷Sr/⁸⁶Sr composition of the carbonate weathering flux into agreement with the composition of carbonate bedrock, thereby permitting consistency between elemental and Sr isotope approaches to partitioning stream water solute sources. These results resolve the dissolved Sr²⁺ budget and suggest that the conventional application of two-component Ca/Sr and ⁸⁷Sr/⁸⁶Sr mixing equations has overestimated silicate-derived Sr²⁺ and HCO₃⁻ fluxes from the Himalaya. In addition, these findings demonstrate that integrating stream water carbonate mineral equilibria, divalent cation compositional trends, and Sr isotope inventories provides a powerful approach for examining weathering fluxes.     

Connections between surface complexation and geometric models of mineral dissolution investigated for rhodochrosite

Mineral dissolution rates have been rationalized in the literature by surface complexation models (SCM) and morphological and geometric models (GM), and reconciliation of these conceptually different yet separately highly successful models is an important goal. In the current work, morphological alterations of the surface are observed in real time at the microscopic level by atomic force microscopy (AFM) while dissolution rates are simultaneously measured at the macroscopic level by utilizing the AFM fluid cell as a classic flow-through reactor. Rhodochrosite dissolution is studied from pH = 2 to 11 at 298 K, and quantitative agreement is found between the dissolution rates determined from microscopic and macroscopic observations. Application of a SCM model for the interpretation of the kinetic data indicates that the surface concentration of >CO₃H regulates dissolution for pH < 7 while the surface concentration of >MnOH₂⁺ regulates dissolution for pH > 7. A GM model explains well the microscopic observations, from which it is apparent that dissolution occurs at steps associated with anisotropic pit expansion. On the basis of the observations, we combine the SCM and GM models to propose a step-site surface complexation model (SSCM), in which the dissolution rates are quantitatively related to the surface chemical speciation of steps. The governing SSCM equation is as follows: R = χ_1/2(k_co + k_ca)[>CO₃H] + χ_1/2(k_mo + k_ma)[>MnOH₂⁺ ], where R is the dissolution rate (mol m⁻² s⁻¹), 2χ_1/2 is the fraction of surface sites located at steps, [>CO₃H] and [>MnOH₂⁺ ] are surface concentrations (mol m⁻²), and k_co, k_ca, k_mo, and k_ma are the respective dissolution rate coefficients (s⁻¹) for the >CO₃H and the >MnOH₂⁺ surface species on obtuse and acute steps. We find k_co = 2.7 s⁻¹, k_ca = 2.1 × 10⁻¹ s⁻¹, k_mo = 4.1 × 10⁻² s⁻¹, k_ma = 3.7 × 10⁻² s⁻¹, and χ_1/2 = 0.015 ± 0.005. The rate coefficients quantify the net result of complex surface step processes, including double-kink initiation and single-kink propagation. We propose that the SSCM model may have general applicability for dissolution far from equilibrium of flat mineral surfaces of ionic crystals, at least those that dissolve by step retreat.     

Kinetics of biotite dissolution and Fe behavior under low O2 conditions and their implications for Precambrian weathering

Biotite dissolution experiments were carried out to better understand the dissolution kinetics and Fe behavior under low O₂ conditions, and to give an insight into the Precambrian weathering. Mineral dissolution with a continuous flow-through reactor was employed at 25 °C for up to 65 days varying partial pressure of atmospheric oxygen (PO₂), pH (6.86 and 3.01) and Fe content in mineral (1.06 and 0.11 mol of Fe per O₁₀(OH,F)₂ for biotite and phlogopite, respectively) independently for the examination of their effects on biotite dissolution. Low PO₂ conditions were achieved in a newly developed glove box (PO₂ ? 6 × 10⁻⁴ atm; referred to as anoxic conditions), which was compared to the present, ambient air conditions (0.2 atm of PO₂; oxic conditions). The biotite dissolution rate was slightly faster under anoxic conditions at pH 6.86 while it was not affected by PO₂ at pH 3.01. There was no direct effect of Fe content on dissolution rate at pH 6.86 while there was a small difference in dissolution rate between biotite and phlogopite at pH 3.01. The 1.5 order-of-magnitude faster release rate of Fe under anoxic conditions for biotite dissolution at pH 6.86 resulted from the difference in ratio of Fe³⁺ precipitates remaining in the reactor to Fe dissolved (about 60% and 100% under anoxic and oxic conditions, respectively), which is caused mainly by the difference in PO₂. The results infer that the Fe²⁺ and Fe³⁺ contents in the Paleoproterozoic paleosols, fossil weathering profiles, are reflected by atmospheric oxygen levels at the time of weathering.     

The role of sulfur in chemical weathering and atmospheric CO2 fluxes: Evidence from major ions, δCDIC, and δSSO4 in rivers of the Canadian Cordillera

Water samples from the Fraser, Skeena and Nass River basins of the Canadian Cordillera were analyzed for dissolved major element concentrations (HCO₃⁻, SO₄²⁻, Cl⁻, Ca²⁺, Mg²⁺, K⁺, Na⁺), δ¹³C of dissolved inorganic carbon (δ¹³C_DIC), and δ³⁴S of dissolved sulfate (δ³⁴S_SO4) to quantify chemical weathering rates and exchanges of CO₂ between the atmosphere, hydrosphere, and lithosphere. Weathering rates of silicates and carbonates were determined from major element mass balance. Combining the major element mass balance with δ³⁴S_SO4 (−8.9 to 14.1‰_CDT) indicates sulfide oxidation (sulfuric acid production) and subsequent weathering of carbonate and to a lesser degree silicate minerals are important processes in the study area. We determine that on average, 81% of the riverine sulfate can be attributed to sulfide oxidation in the Cordilleran rivers, and that 25% of the total weathering cation flux can be attributed to carbonate and silicate dissolution by sulfuric acid. This result is validated by δ¹³C_DIC values (−9.8 to −3.7‰ _VPDB) which represents a mixture of DIC produced by the following weathering pathways: (i) carbonate dissolution by carbonic acid (−8.25‰) > (ii) silicate dissolution by carbonic acid (−17‰) ≈ (iii) carbonate dissolution by sulfuric acid derived from the oxidation of sulfides (coupled sulfide-carbonate weathering) (+0.5‰).δ³⁴S_SO4 is negatively correlated with δ¹³C_DIC in the Cordilleran rivers, which further supports the hypothesis that sulfuric acid produced by sulfide oxidation is primarily neutralized by carbonates, and that sulfide-carbonate weathering impacts the δ¹³C_DIC of rivers. The negative correlation between δ³⁴S_SO4 and δ¹³C_DIC is not observed in the Ottawa and St. Lawrence River basins. This suggests other factors such as landscape age (governed by tectonic uplift) and bedrock geology are important controls on regional sulfide oxidation rates, and therefore also on the magnitude of sulfide-carbonate weathering—i.e., it is more significant in tectonically active areas.Calculated DIC fluxes due to Ca and Mg silicate weathering by carbonic acid (38.3 × 10³ mol C · km⁻² · yr⁻¹) are similar in magnitude to DIC fluxes due to sulfide-carbonate weathering (18.5 × 10³ mol C · km⁻² · yr⁻¹). While Ca and Mg silicate weathering facilitates a transfer of atmospheric CO₂ to carbonate rocks, sulfide-carbonate weathering can liberate CO₂ from carbonate rocks to the atmosphere when sulfide oxidation exceeds sulfide deposition. This implies that in the Canadian Cordillera, sulfide-carbonate weathering can offset up to 48% of the current CO₂ drawdown by silicate weathering in the region.     

Does reactive surface area depend on grain size? Results from pH 3, 25 °C far-from-equilibrium flow-through dissolution experiments on anorthite and biotite

Laboratory determined mineral weathering rates need to be normalised to allow their extrapolation to natural systems. The principle normalisation terms used in the literature are mass, and geometric- and BET specific surface area (SSA). The purpose of this study was to determine how dissolution rates normalised to these terms vary with grain size. Different size fractions of anorthite and biotite ranging from 180-150 to 20-10 μm were dissolved in pH 3, HCl at 25 °C in flow through reactors under far from equilibrium conditions. Steady state dissolution rates after 5376 h (anorthite) and 4992 h (biotite) were calculated from Si concentrations and were normalised to initial- and final- mass and geometric-, geometric edge- (biotite), and BET SSA. For anorthite, rates normalised to initial- and final-BET SSA ranged from 0.33 to 2.77 × 10⁻¹⁰ mol_feldspar m⁻² s⁻¹, rates normalised to initial- and final-geometric SSA ranged from 5.74 to 8.88 × 10⁻¹⁰ mol_feldspar m⁻² s⁻¹ and rates normalised to initial- and final-mass ranged from 0.11 to 1.65 mol_feldspar g⁻¹ s⁻¹. For biotite, rates normalised to initial- and final-BET SSA ranged from 1.02 to 2.03 × 10⁻¹² mol_biotite m⁻² s⁻¹, rates normalised to initial- and final-geometric SSA ranged from 3.26 to 16.21 × 10⁻¹² mol_biotite m⁻² s⁻¹, rates normalised to initial- and final-geometric edge SSA ranged from 59.46 to 111.32 × 10⁻¹² mol_biotite m⁻² s⁻¹ and rates normalised to initial- and final-mass ranged from 0.81 to 6.93 × 10⁻¹² mol_biotite g⁻¹ s⁻¹. For all normalising terms rates varied significantly (p ? 0.05) with grain size. The normalising terms which gave least variation in dissolution rate between grain sizes for anorthite were initial BET SSA and initial- and final-geometric SSA. This is consistent with: (1) dissolution being dominated by the slower dissolving but area dominant non-etched surfaces of the grains and, (2) the walls of etch pits and other dissolution features being relatively unreactive. These steady state normalised dissolution rates are likely to be constant with time. Normalisation to final BET SSA did not give constant ratios across grain size due to a non-uniform distribution of dissolution features. After dissolution coarser grains had a greater density of dissolution features with BET-measurable but unreactive wall surface area than the finer grains. The normalising term which gave the least variation in dissolution rates between grain sizes for biotite was initial BET SSA. Initial- and final-geometric edge SSA and final BET SSA gave the next least varied rates. The basal surfaces dissolved sufficiently rapidly to influence bulk dissolution rate and prevent geometric edge SSA normalised dissolution rates showing the least variation. Simple modelling indicated that biotite grain edges dissolved 71-132 times faster than basal surfaces. In this experiment, initial BET SSA best integrated the different areas and reactivities of the edge and basal surfaces of biotite. Steady state dissolution rates are likely to vary with time as dissolution alters the ratio of edge to basal surface area. Therefore they would be more properly termed pseudo-steady state rates, only appearing constant because the time period over which they were measured (1512 h) was less than the time period over which they would change significantly.     

Rates of weathering rind formation on Costa Rican basalt

Weathering rind thicknesses were measured on ∼ 200 basaltic clasts collected from three regionally extensive alluvial fill terraces (Qt 1, Qt 2, and Qt 3) preserved along the Pacific coast of Costa Rica. Mass balance calculations suggest that conversion of unweathered basaltic core minerals (plagioclase and augite) to authigenic minerals in the porous rind (kaolinite, allophane, gibbsite, Fe oxyhydroxides) is iso-volumetric and Ti and Zr are relatively immobile. The hierarchy of cation mobility (Ca ≈ Na > K ≈ Mg > Si > Al > Fe ≈ P) is similar to other tropical weathering profiles and is indicative of differential rates of mineral weathering (anorthite > albite ≈ hypersthene > orthoclase ? apatite). Alteration profiles across the cm-thick rinds document dissolution of plagioclase and augite and the growth of kaolinite, with subsequent dissolution of kaolinite and precipitation of gibbsite as weathering rinds age. The rate of weathering rind advance is evaluated using a diffusion-limited model which predicts a parabolic rate law for weathering rind thickness, r_r, as a function of time, t(r_r =), and an interface-limited model which predicts a linear rate law for weathering rind thickness as a function of time (r_r = k_appt). In these rate laws, κ is a diffusion parameter and k_app is an apparent rate constant. The rate of advance is best fit by the interface model.Terrace exposures are confined to the lower reaches of streams draining the Pacific slope near the coast where the stream gradient is less than ∼3 m/km, and terrace deposition is influenced by eustatic sea level fluctuations. Geomorphological evidence is consistent with terrace deposition coincident with sea level maxima when the stream gradient would be lowest. Assigning the most weathered regionally extensive terrace Qt 1 (mean rind thickness 6.9 ± 0. 6cm) to oxygen isotope stage (OIS) 7 (ca. 240 ka), and assuming that at time = 0 rind thickness = 0, it is inferred that terrace Qt 2 (r_r = 2.9 ± 0.1 cm) is coincident with stage 5e (ca. 125 ka) and that Qt 3 (r_r = 0.9 ± 0.1 cm) is consistent with OIS 3 (ca. 37 ka). These assignments yield a value of k_app of 8.6 × 10⁻¹³ cm s⁻¹ (R² = 0.99). Only this value satisfies both the existing age controls and yields ages coincident with sea level maxima. Using this value, elemental weathering release fluxes across a weathering rind from Qt 2 range from 6.0 × 10⁻⁹ mol Si m⁻² s⁻¹ to 2.5 × 10⁻¹¹ mol K m⁻² s⁻¹. The rate of rind advance for the Costa Rican terraces is 2.8 × 10⁻⁷ m yr⁻¹. Basalt rind formation rates in lower temperature settings described in the literature are also consistent with interface-controlled weathering with an apparent activation energy of about 50 kJ mol⁻¹. Rates of rind formation in Costa Rica are an order of magnitude slower than reported for global averages of soil formation rates.     

Role of molecular oxygen in the dissolution of siderite and rhodochrosite

The dissolution of siderite (FeCO₃) and rhodochrosite (MnCO₃) under oxic and anoxic conditions is investigated at 298 K. The anoxic dissolution rate of siderite is 10^−8.65 mol m⁻² s⁻¹ for 5.5 < pH < 12 and increases as [H⁺]^0.75 for pH < 5.5. The pH dependence is consistent with parallel proton-promoted and water hydrolysis dissolution pathways. Atomic force microscopy (AFM) reveals a change in pit morphology from rhombohedral pits for pH > 4 to pits elongated at one vertex for pH < 4. Under oxic conditions the dissolution rate decreases to below the detection limit of 10⁻¹⁰ mol m⁻² s⁻¹ for 6.0 < pH < 10.3, and hillock precipitation preferential to steps is observed in concurrent AFM micrographs. X-ray photoelectron spectroscopy (XPS) and thermodynamic analysis identify the precipitate as ferrihydrite. At pH > 10.3, the oxic dissolution rate is as high as 10^−7.5 mol m⁻² s⁻¹, which is greater than under the corresponding anoxic conditions. A fast electron transfer reaction between solution O₂ or [Fe³⁺(OH)₄]⁻ species and surficial >Fe^II hydroxyl groups is hypothesized to explain the dissolution kinetics. AFM micrographs do not show precipitation under these conditions. Anoxic dissolution of rhodochrosite is physically observed as rhombohedral pit expansion for 3.7 < pH < 10.3 and is chemically explained by parallel proton- and water-promoted pathways. The dissolution rate law is 10^−4.93[H⁺] + 10^−8.45 mol m⁻² s⁻¹. For 5.8 < pH < 7.7 under oxic conditions, the AFM micrographs show a tabular precipitate growing by preferential expansion along the a-axis, though the macroscopic dissolution rate is apparently unaffected. For pH > 7.7 under oxic conditions, the dissolution rate decreases from 10^−8.45 to 10^−9.0 mol m⁻² s⁻¹. Flattened hillock precipitates grow across the entire surface without apparent morphological influence by the underlying rhodochrosite surface. XPS spectra and thermodynamic calculations implicate the precipitate as bixbyite for 5.8 < pH < 7.7 and MnOOH (possibly feitnkechtite) for pH >7.7.     

Biotite weathering and nutrient uptake by ectomycorrhizal fungus, Suillus tomentosus, in liquid-culture experiments

Ectomycorrhiza-forming fungi (EMF) alter the nutrient-acquisition capabilities of vascular plants, and may play an important role in mineral weathering and the partitioning of products of weathering in soils under nutrient-limited conditions. In this study, we isolated the weathering function of Suillus tomentosus in liquid-cultures with biotite micas incubated at room temperature. We hypothesized that the fungus would accelerate weathering by hyphal attachment to biotite surfaces and transmission of nutrient cations via direct exchange into the fungal biomass. We combined a mass-balance approach with scanning electron microscopy (SEM) and atomic force microscopy (AFM) to estimate weathering rates and study dissolution features on biotite surfaces. Weathering of biotite flakes was about 2-3 orders of magnitude faster in shaken liquid-cultures with fungus compared to shaken controls without fungus, but with added inorganic acids. Adding fungus in nonshaken cultures caused a higher dissolution rate than in inorganic pH controls without fungus, but it was not significantly faster than organic pH controls without fungus. The K⁺, Mg²⁺ and Fe²⁺ from biotite were preferentially partitioned into fungal biomass in the shaken cultures, while in the nonshaken cultures, K⁺ and Mg²⁺ was lost from biomass and Fe²⁺ bioaccumulated much less. Fungal hyphae attached to biotite surfaces, but no significant surface changes were detected by SEM. When cultures were shaken, the AFM images of basal planes appeared to be rougher and had abundant dissolution channels, but such channel development was minor in nonshaken conditions. Even under shaken conditions the channels only accounted for only 1/100 of the total dissolution rate of 2.7 × 10⁻¹⁰ mol of biotite m⁻² s⁻¹. The results suggest that fungal weathering predominantly occurred not by attachment and direct transfer of nutrients via hyphae, but because of the acidification of the bulk liquid by organic acids, fungal respiration (CO₂), and complexation of cations which accelerated dissolution of biotite. Results further suggest that both carbohydrate source (abundant here) and a host with which nutrients are exchanged (missing here) may be required for EMF to exert an important weathering effect in soils. Unsaturated conditions and physical dispersal of nutrient-rich minerals in soils may also confer a benefit for hyphal growth and attachment, and promote the attachment-mediated weathering which has been observed elsewhere on soil mineral surfaces.     

Dissolution of lead- and lead-arsenic-jarosites at pH 2 and 8 and 20 °C: Insights from batch experiments

Lead- and Pb-As-jarosites are minerals common to acidic, sulphate-rich environments, including weathering zones of sulphide ore deposits and acid rock or acid mine drainage (ARD/AMD) sites, and often form on or near galena. The structures of these jarosites are based on linear tetrahedral-octahedral-tetrahedral (T-O-T) sheets, comprised of slightly distorted FeO₆ octahedra and SO₄²⁻ (-AsO₄³⁻ in Pb-As-jarosites) tetrahedra. To better understand the dissolution mechanisms and products of the break down of Pb- and Pb-As-jarosite, preliminary batch dissolution experiments were conducted on synthetic Pb- and Pb-As-jarosite at pH 2 and 20 °C, to mimic environments affected by ARD/AMD, and at pH 8 and 20 °C, to simulate ARD/AMD environments recently remediated with slaked lime (Ca(OH)₂). All four dissolutions are incongruent. Dissolution of Pb-jarosite at pH 2 yields aqueous Pb, Fe and SO₄²⁻. The pH 8 Pb-jarosite dissolution yields aqueous Pb, SO₄²⁻ and poorly crystalline Fe(OH)₃, which does not appear to resorb Pb or SO₄²⁻, possibly due to the low solution pH (3.44-3.54) at the end of the experiment. The pH 2 and 8 dissolutions of Pb-As-jarosite result in the formation of secondary compounds (poorly crystalline PbSO₄ for pH 2 dissolution; poorly crystalline PbSO₄ and Fe(OH)₃ for pH 8 dissolution), which may act as dissolution inhibitors after 250 to 300 h of dissolution. In the pH 2 dissolution, aqueous Fe, SO₄²⁻ and AsO₄³⁻ also form, and in the pH 8 dissolution, Fe(OH)₃ precipitates then subsequently resorbs aqueous AsO₄³⁻. The dissolutions probably proceed by preferred dissolution of the A- and T-sites, which contain Pb, and SO₄²⁻ and AsO₄³⁻, respectively, rather than Fe, which is sterically remote, within the T-O-T Pb- and Pb-As-jarosite structures. These data provide the foundation necessary for further, more detailed investigations into the dissolution of Pb- and Pb-As-jarosites.     

Hydration of rhyolitic glass during weathering as characterized by IR microspectroscopy

The mechanism and rate of hydration of rhyolitic glass during weathering were studied. Doubly polished thin sections of two rhyolites with different duration of weathering (Ohsawa lava: 26,000 yr, Awanomikoto lava: 52,000 yr) were prepared. Optical microscope observation showed that altered layers had developed along the glass surfaces. IR spectral line profile analysis was conducted on the glass sections from the surface to the interior for a length of 250 μm and the contents of molecular H₂O (H₂O_m), OH species (OH) and total water (H₂O_t) were determined. The diffusion profile of H₂O_m in Ohsawa lava extends beyond the layer observed by optical microscope. The content of H₂O_m in the hydrated region is much higher than that of OH species. Thus, the reaction from H₂O_m to OH appears to be little and H₂O_m is the dominant water species moving into the glass during weathering. Based on the concentration profiles, the diffusion coefficients of H₂O_m(D_H2Om) and H₂O_t(D_H2Ot) were determined to be 2.8 × 10⁻¹⁰ and 3.4 × 10⁻¹⁰ μm² s⁻¹ for Ohsawa lava, and 5.2 × 10⁻¹¹ and 4.1 × 10⁻¹¹ μm² s⁻¹ for Awanomikoto lava, respectively. The obtained D_H2Om during weathering are more than 2-3 orders of magnitude larger than the diffusion coefficient at ∼20 °C that is extrapolated from the diffusivity data for >400 °C. This might suggest that the mechanism of water transport is different at weathering conditions and >400 °C.     

Particulate air pollution in a small settlement: The effect of local heating

Mass concentrations of PM₁₀, PM_2.5, and black smoke (BS) were measured in April 2003 during a 3-week campaign in a small village and at a nearby background location in the central part of the Czech Republic. In a pilot analysis, concentrations of selected trace elements (Al, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, As, Se, Sr, Cd, Sb, Cs, Pb) in the collected aerosol were determined by means of ICP-MS. Average concentrations of both PM fractions and BS were higher in the village (37, 26 and 26 μg m⁻³) than at the background location (26, 19 and 11 μg m⁻³) for PM₁₀, PM_2.5 and BS, respectively. Both PM₁₀ and PM_2.5 were reasonably correlated in the village (r = 0.80) and also at the background location (r = 0.79). Correlation between same fractions from the village and from the background site were even higher (r = 0.97 and r = 0.95 for the PM₁₀ and PM_2.5, respectively) suggesting that most of the aerosol in both locations may be influenced by similar sources. The ratio between PM₁₀ and PM_2.5 showed that sources in the village contributed about 33% and 35% to local aerosol concentration for PM₁₀ and PM_2.5, respectively. When the data from the two rural locations were compared with corresponding 24-h averages of PM₁₀ concentrations obtained for the period of the campaign from fixed site monitors situated near larger towns, the highest concentration was found in Prague the Czech capital (49 μg m⁻³) followed by a district town Beroun (41 μg m⁻³) and the village (37 μg m⁻³). The lowest PM₁₀ concentration was found in the village background (26 μg m⁻³). Elemental analysis revealed higher concentrations for most of the elements characteristic of combustion aerosol (namely Zn, Pb, As, Mn and Ti) in the PM collected in the village. The results support the idea that traditional heating in villages may contribute a great extent to local air pollution and may represent an important problem.     

Equilibrium and kinetic aspects of soddyite dissolution and secondary phase precipitation in aqueous suspension

The dissolution and transformation of soddyite ([UO₂]₂SiO₄ · 2H₂O) have been examined in aqueous suspension at pH 6 and 0.01 M NaNO₃. Soddyite is an important component of the paragenetic sequence of secondary minerals that arises from the weathering of uraninite ore deposits and corrosion of spent nuclear fuel. A soddyite of high purity and crystallinity was synthesized in the laboratory for use in dissolution experiments. In batch experiments, rapid dissolution occurred over an initial period of several hours followed by continuing steady-state dissolution for up to 700 h. Up to 200 h, U and Si were released into solution at their stoichiometric 2:1 ratio in soddyite. A decrease in the dissolved U concentration was observed at longer times, indicating the precipitation of a new phase. Even after precipitation of the secondary phase, the continuing dissolution of soddyite could be inferred from increasing dissolved Si concentrations. Through the use of X-ray diffraction, Raman spectroscopy, and scanning electron microscopy, the precipitated phase was identified as a clarkeite-like sodium uranyl oxide hydrate. The sodium uranyl oxide hydrate was ultimately the solubility-controlling solid, despite being only a minor component. Soddyite dissolution rates were quantified in flow-through experiments, in which reaction products were flushed from the reactors, thereby avoiding reprecipitation of U. The measured dissolution rate at pH 6 was 0.71 μmol U m⁻² h⁻¹. A slower dissolution rate of 0.44 μmol U m⁻² h⁻¹ was observed when 100 μM dissolved Si was added to the reactor influent.     

The reactivity of iron oxides towards reductive dissolution with ascorbic acid in a shallow sandy aquifer (Rømø, Denmark)

The pool of iron oxides, available in sediments for reductive dissolution, is usually estimated by wet chemical extraction methods. Such methods are basically empirically defined and calibrated against various synthetic iron oxides. However, in natural sediments, iron oxides are present as part of a complex mixture of iron oxides with variable crystallinity, clays and organics etc. Such a mixture is more accurately described by a reactive continuum covering a range from highly reactive iron oxides to non-reactive iron oxide. The reactivity of the pool of iron oxides in sediment can be determined by reductive dissolution in 10 mM ascorbic acid at pH 3. Parallel dissolution experiments in HCl at pH 3 reveal the release of Fe(II) by proton assisted dissolution. The difference in Fe(II)-release between the two experiments is attributed to reductive dissolution of iron oxides and can be quantified using the rate equation J/m₀ = k′(m/m₀)^γ, where J is the overall rate of dissolution (mol s⁻¹), m₀ the initial amount of iron oxide, k′ a rate constant (s⁻¹), m/m₀ the proportion of undissolved mineral and γ a parameter describing the change in reaction rate over time. In the Rømø aquifer, Denmark, the reduction of iron oxides is an important electron accepting process for organic matter degradation and is reflected by the steep increase in aqueous Fe²⁺ over depth. Sediment from the Rømø aquifer was used for reductive dissolution experiments with ascorbic acid. The rate parameters describing the reactivity of iron oxides in the sediment are in the range k′ = 7·10⁻⁶ to 1·10⁻³ s⁻¹ and γ = 1 to 2.4. These values are intermediate between a synthetic 2-line ferrihydrite and a goethite. The rate constant increases by two orders of magnitude over depth suggesting an increase in iron oxide reactivity with depth. This increase was not captured by traditional oxalate and dithionite extractions.     

Kinetics of brucite dissolution at 25°C in the presence of organic and inorganic ligands and divalent metals

Brucite (Mg(OH)₂) dissolution rate was measured at 25°C in a mixed-flow reactor at various pH (5 to 11) and ionic strengths (0.01 to 0.03 M) as a function of the concentration of 15 organic and 5 inorganic ligands and 8 divalent metals. At neutral and weakly alkaline pH, the dissolution is promoted by the addition of the following ligands ranked by decreasing effectiveness: EDTA ≥ H₂PO₄⁻ > catechol ≥ HCO₃⁻ > ascorbate > citrate > oxalate > acetate ∼ lactate and it is inhibited by boric acid. At pH >10.5, it decreases in the presence of PO₄³⁻, CO₃²⁻, F⁻, oxine, salicylate, lactate, acetate, 4-hydroxybenzoate, SO₄²⁻ and B(OH)₄⁻ with orthophosphate and borate being the strongest and the weakest inhibitor, respectively. Xylose (up to 0.1 M), glycine (up to 0.05 M), formate (up to 0.3 M) and fulvic and humic acids (up to 40 mg/L DOC) have no effect on brucite dissolution kinetics. Fluorine inhibits dissolution both in neutral and alkaline solutions. From F sorption experiments in batch and flow-through reactors and the analysis of reacted surfaces using X-ray Photoelectron Spectroscopy (XPS), it is shown that fluorine adsorption is followed by its incorporation in brucite lattice likely via isomorphic substitution with OH. The effect of eight divalent metals (Sr, Ba, Ca, Pb, Mn, Fe, Co and Ni) studied at pH 4.9 and 0.01 M concentration revealed brucite dissolution rates to be correlated with the water molecule exchange rates in the first hydration sphere of the corresponding cation.The effect of investigated ligands on brucite dissolution rate can be modelled within the framework of the surface coordination approach taking into account the adsorption of ligands on dissolution-active sites and the molecular structure of the surface complexes they form. The higher the value of the ligand sorption constant, the stronger will be its catalyzing or inhibiting effect. As for Fe and Al oxides, bi- or multidentate mononuclear surface complexes, that labilize Mg-O bonds and water coordination to Mg atoms at the surface, enhance brucite dissolution whereas bi- or polynuclear surface complexes tend to inhibit dissolution by bridging two or more metal centers and extending the cross-linking at the solid surface. Overall, results of this study demonstrate that very high concentrations of organic ligands (0.01-0.1 M) are necessary to enhance or inhibit brucite dissolution. As a result, the effect of extracellular organic products on the weathering rate of Mg-bearing minerals is expected to be weak.     

Silicate weathering in anoxic marine sediments

Two sediment cores retrieved at the northern slope of Sakhalin Island, Sea of Okhotsk, were analyzed for biogenic opal, organic carbon, carbonate, sulfur, major element concentrations, mineral contents, and dissolved substances including nutrients, sulfate, methane, major cations, humic substances, and total alkalinity. Down-core trends in mineral abundance suggest that plagioclase feldspars and other reactive silicate phases (olivine, pyroxene, volcanic ash) are transformed into smectite in the methanogenic sediment sections. The element ratios Na/Al, Mg/Al, and Ca/Al in the solid phase decrease with sediment depth indicating a loss of mobile cations with depth and producing a significant down-core increase in the chemical index of alteration. Pore waters separated from the sediment cores are highly enriched in dissolved magnesium, total alkalinity, humic substances, and boron. The high contents of dissolved organic carbon in the deeper methanogenic sediment sections (50-150 mg dm⁻³) may promote the dissolution of silicate phases through complexation of Al³⁺ and other structure-building cations. A non-steady state transport-reaction model was developed and applied to evaluate the down-core trends observed in the solid and dissolved phases. Dissolved Mg and total alkalinity were used to track the in-situ rates of marine silicate weathering since thermodynamic equilibrium calculations showed that these tracers are not affected by ion exchange processes with sediment surfaces. The modeling showed that silicate weathering is limited to the deeper methanogenic sediment section whereas reverse weathering was the dominant process in the overlying surface sediments. Depth-integrated rates of marine silicate weathering in methanogenic sediments derived from the model (81.4-99.2 mmol CO₂ m⁻² year⁻¹) are lower than the marine weathering rates calculated from the solid phase data (198-245 mmol CO₂ m⁻² year⁻¹) suggesting a decrease in marine weathering over time. The production of CO₂ through reverse weathering in surface sediments (4.22-15.0 mmol CO₂ m⁻² year⁻¹) is about one order of magnitude smaller than the weathering-induced CO₂ consumption in the underlying sediments. The evaluation of pore water data from other continental margin sites shows that silicate weathering is a common process in methanogenic sediments. The global rate of CO₂ consumption through marine silicate weathering estimated here as 5-20 Tmol CO₂ year⁻¹ is as high as the global rate of continental silicate weathering.     

Chemical weathering in the Three Rivers region of Eastern Tibet

Three large rivers - the Chang Jiang (Yangtze), Mekong (Lancang Jiang) and Salween (Nu Jiang) - originate in eastern Tibet and run in close parallel over 300 km near the eastern Himalayan syntaxis. Seventy-four river water samples were collected mostly during the summer season from 1999 to 2004. Their major element compositions vary widely, with total dissolved solids (TDS) ranging from 31 to 3037 mg/l, reflecting the complex geologic makeup of the vast drainage basins. The major ion distribution of the main channel samples primarily reflects the weathering of carbonates. Evaporite dissolution prevails in the headwater samples of the Chang Jiang in the Tibetan Plateau interior, as evidenced by the high TDS (928 and 3037 mg/l) and the Na-Cl dominant major element composition. Local tributary samples of the Mekong and Salween, draining the Lincang Batholith and the Tengchong Volcano, show distinctive silicate weathering signatures. We used five reservoirs - rain, halite, sulfate, carbonate, and silicate - in a forward model to calculate the contribution from silicate weathering to the total dissolved load and to estimate the consumption rate of atmospheric CO₂ by silicate weathering. Carbonate weathering accounts for about 50% of the total cationic charge (TZ⁺) in the samples of the Mekong and the Salween exiting the Tibetan Plateau. In the “exit” sample of the Chang Jiang, 45% of TZ⁺ is from halite dissolution inherited from the extreme headwater tributaries in the interior of the plateau, and carbonates contribute only 26% to the TZ⁺. The net rate of CO₂ consumption by silicate weathering is (103-121) × 10³ mol km⁻² year⁻¹, lower than the rivers draining the Himalayan front. GIS-based analyses indicate that runoff and relief can explain 52% of the spread in the rate of atmospheric CO₂ drawdown by silicate weathering, but other climatic (temperature, precipitation, potential evapotranspiration) and geomorphic (elevation, slope) factors also show collinearity. Only qualitative conclusions can be drawn for the significance of lithology due to lack of digitized lithologic information. The effect of the peculiar drainage pattern due to tectonic forcing is not readily apparent in the major element composition or in increased chemical weathering rates. The ⁸⁷Sr/⁸⁶Sr ratios and the silicate weathering rates are in general lower in the Three Rivers than in the rivers draining the Himalayan front.     

An atomic force microscopy study of calcite dissolution in saline solutions: The role of magnesium ions

In situ Atomic Force Microscopy, AFM, experiments have been carried out using calcite cleavage surfaces in contact with solutions of MgSO₄, MgCl₂, Na₂SO₄ and NaCl in order to attempt to understand the role of Mg²⁺ during calcite dissolution. Although previous work has indicated that magnesium inhibits calcite dissolution, quantitative AFM analyses show that despite the fact that Mg²⁺ inhibits etch pit spreading, it increases the density and depth of etch pits nucleated on calcite surfaces and, subsequently, the overall dissolution rates: i.e., from 10^−11.75 mol cm⁻² s⁻¹ (in deionized water) up to 10^−10.54 mol cm⁻² s⁻¹ (in 2.8 M MgSO₄). Such an effect is concentration-dependent and it is most evident in concentrated solutions ([Mg²⁺] >> 50 mM). These results show that common soluble salts (especially Mg sulfates) may play a critical role in the chemical weathering of carbonate rocks in nature as well as in the decay of carbonate stone in buildings and statuary.     

Atmospheric CO2 consumption by chemical weathering in North America

CO₂ consumption by chemical weathering is an integral part of the boundless carbon cycle, whose spatial patterns and controlling factors on continental scale are still not fully understood. A dataset of 338 river catchments throughout North America was used to empirically identify predictors of bicarbonate fluxes by chemical weathering and interpret the underlying controlling factors. Detailed analysis of major ion ratios enables distinction of the contributions of silicate and carbonate weathering and thus quantifying CO₂ consumption. Extrapolation of the identified empirical model equations to North America allows the analysis of the spatial patterns of the CO₂ consumption by chemical weathering.Runoff, lithology and land cover were identified as the major predictors of the riverine bicarbonate fluxes and the associated CO₂ consumption. Other influence factors, e.g. temperature, could not be established in the models. Of the distinguished land cover classes, artificial surfaces, dominated by urban areas, increase bicarbonate fluxes most, followed by shrubs, grasslands, managed lands, and forests. The extrapolation results in an average specific bicarbonate flux of 0.3 Mmol km⁻² a⁻¹ by chemical weathering in North America, of which 64% originates from atmospheric CO₂, and 36% from carbonate mineral dissolution. Chemical weathering in North America thus consumes 50 Mt atmospheric CO₂-C per year. About half of that originates from 10% of the area of North America.The estimated strength of individual predictors differs from previous studies. This highlights the need for a globally representative set of regionally calibrated models of CO₂ consumption by chemical weathering, which apply very detailed spatial data to resolve the heterogeneity of earth surface processes.