Geochemical and mineralogical impacts of H2SO4 on clays between pH 5.0 and −3.0

Natural and constructed clay liners are routinely used to contain waste and wastewater. The impact of acidic solutions on the geochemistry and mineralogy of clays has been widely investigated in relation to acid mine drainage systems at pH > 1.0. The impact of H₂SO₄ leachate characterized by pH < 1.0 and potentially negative pH values on the geochemistry and mineralogy of clays is, however, not clear. Thus, laboratory batch experiments were conducted on three natural clay samples with different mass ratios of smectite, illite and kaolinite to investigate the impact of H₂SO₄ on the geochemistry and mineralogy of aluminosilicates from pH 5.0 to −3.0. Batch testing was conducted at seven pH treatments (5.0, 3.0, 1.0, 0.0, −1.0, −2.0 and −3.0) using standardized H₂SO₄ solutions for four exposure periods (14, 90, 180, and 365 d). Aqueous geochemical and XRD analyses showed: increased dissolution of aluminosilicates with decreasing pH and increasing exposure period, that smectite was more susceptible to dissolution than illite and kaolinite, precipitation of an amorphous silica phase occurred at pH ? 0.0, and anhydrite precipitated in Ca-rich clays at pH ? −1.0. In addition, global dissolution rates were calculated for the clays and showed good agreement to literature smectite, illite and kaolinite dissolution rates, which suggests global dissolution rates for complex clays could be determined from monomineralic studies. A stepwise conceptual model of the impact of H₂SO₄ on aluminosilicate geochemistry and mineralogy between pH 5.0 and −3.0 is proposed.     

Experimental investigation of main controls to methane adsorption in clay-rich rocks

In this study a series of CH₄ adsorption experiments on clay-rich rocks were conducted at 35 °C, 50 °C and 65 °C and at CH₄ pressure up to 15 MPa under dry conditions. The clay-dominated rock samples used are fresh samples from quarries and mines. Samples are individually dominated by montmorillonite, kaolinite, illite, chlorite, and interstratified illite/smectite. The experimental results show that clay mineral type greatly affects CH₄ sorption capacity under the experimental conditions. In terms of relative CH₄ sorption capacity: montmorillonite ? illite/smectite mixed layer > kaolinite > chlorite > illite. Physisorption is the dominant process for CH₄ absorption on clay minerals, as a result, there is a linear correlation between CH₄ sorption capacity and BET surface area in these clay-mineral dominated rocks. The abundance of micro-mesopores in the size range of a few to a few 10 s of nanometers in montmorillonite clay and illite–smectite interstratified clay results in large BET surface area values for these mineral species.     

Towards a link between the energetic heterogeneities of the edge faces of smectites and their stability in the context of metallic corrosion

In the context of the potential confinement of high-level radioactive wastes (HLW) within the clay engineered barrier system (EBS) in deep geological formations, the evolution of the retention properties of smectite when interacting with Fe(0) needs to be assessed. If some potential natural analogues describing iron-clay reactivity are easily found, metallic iron-clay interactions are poorly described in studies regarding the Earth. Therefore, experimental investigations are needed. Several parameters influence Fe(0)-clay interactions, such as temperature, the interlayer composition of swelling clays, and the presence of octahedral Fe³⁺, etc. From a geometrical point of view, it is thought that clay destabilization is mainly controlled by phenomena starting at the edge faces of the particles. In the present work, the rates of the smectite-Fe(0) reaction at 80 °C were assessed by XRD, Mössbauer, and CEC analyses for three smectites. The investigations show marked differences in the degree of stability, which cannot be explained by the crystal-chemistry rules established in previous studies. Therefore, the Fe(0)-smectite interactions were studied in view of textural and energetic surface quantitative analyses. The studied smectites have equivalent nitrogen BET-specific surface areas, equivalent argon edge surface areas and slightly different basal surface areas. This similarity in particle shape indicates that the edge surface area cannot be accounted for when explaining the observed differences in reactivity. However, a correlation is obtained between smectite reactivity and the energetic heterogeneity of its edge faces. This is interpreted in terms of a multiplication of the number of sites on the edge faces, where the electron transfer between Fe(0) and the smectite structure can occur.     

The spatial distribution of potassium status and clay mineralogy in relation to different land-use types in a calcareous Mediterranean environment

In order to investigate changes caused in clay mineralogy and potassium (K) status by different land-use types, 42 soils samples (0–30 cm) were monitored and analyzed. Soil samples belonging to Reference Soil Groups of Cambisols and Vertisols were collected from three neighboring land uses involving cropland (under long-term continuous cultivation), grassland, and forestland. The soils reflected an alkaline and calcareous aspect as were characterized by high pH (mean of 7.1 to 7.5) and calcium carbonate equivalent (mean of 35 to 97 g?kg^?1) in the three land-use types. X-ray diffraction patterns of the clay fraction showed that the soils were mainly composed of illite, smectite, chlorite, and kaolinite. Chlorite and kaolinite remained unweathered irrespective of land use and soil types, soil processes, and physicochemical attributes assessed. Some changes in the XRD diffractograms of illite and smectite (the intensity or the position of peaks) were observed in the cultivated soils compared to those of the adjoining grassland that may explain the dynamics of the K trapped in illite interlayer sites. Potassium issues reflected a heterogeneous response to changes in land-use types. In light of this, a pronounced variation in soluble K (4–22 mg?kg^?1), exchangeable K (140–558 mg?kg^?1), and non-exchangeable K (135–742 mg?kg^?1) appeared among the land-use types for both Cambisols and Vertisols, corresponding to variability in clay content, nature and type of clay mineral (mainly illite and smectite), cation exchange capacity (CEC), and soil organic carbon (SOC). In general, the largest amounts of soluble K and exchangeable K were recorded in the forestland, whereas the highest contents of non-exchangeable K were found in the grassland for both Cambisols and Vertisols. Exchangeable K, available K, CEC, and clay contents in the soils with higher smectite values (25–50 %) were significantly different (P?≤?0.05) compared to those of the lower smectite values (10–25 %). This suggests that smectite is a major source for surface sorption of K in the studied soils.     

Dissolution of illite in saline-acidic solutions at 25 °C

The dissolution rate of illite, a common clay mineral in Australian soils, was studied in saline-acidic solutions under far from equilibrium conditions. The clay fraction of Na-saturated Silver Hill illite (K_1.38Na_0.05)(Al_2.87Mg_0.46Fe³⁺_0.39Fe²⁺_0.28Ti_0.07)[Si_7.02Al_0.98]O₂₀(OH)₄ was used for this study. The dissolution rates were measured using flow-through reactors at 25 ± 1 °C, solution pH range of 1.0-4.25 (H₂SO₄) and at two ionic strengths (0.01 and 0.25 M) maintained using NaCl solution. Illite dissolution rates were calculated from the steady state release rates of Al and Si. The dissolution stoichiometry was determined from Al/Si, K/Si, Mg/Si and Fe/Si ratios. The release rates of cations were highly incongruent during the initial stage of experiments, with a preferential release of Al and K over Si in majority of the experiments. An Al/Si ratio >1 was observed at pH 2 and 3 while a ratio close to the stoichiometric composition was observed at pH 1 and 4 at the higher ionic strength. A relatively higher K⁺ release rate was observed at I = 0.25 in 2-4 pH range than at I = 0.01, possibly due to ion exchange reaction between Na⁺ from the solution and K⁺ from interlayer sites of illite. The steady state release rates of K, Fe and Mg were higher than Si over the entire pH range investigated in the study. From the point of view of the dominant structural cations (Si and Al), stoichiometric dissolution of illite occurred at pH 1-4 in the higher ionic strength experiments and at pH ?3 for the lower ionic strength experiments. The experiment at pH 4.25 and at the lower ionic strength exhibited lower R_Al (dissolution rate calculated from steady state Al release) than R_Si (dissolution rate calculated from steady state Si release), possibly due to the adsorption of dissolved Al as the output solutions were undersaturated with respect to gibbsite. The dissolution of illite appears to proceed with the removal of interlayer K followed by the dissolution of octahedral cations (Fe, Mg and Al), the dissolution of Si is the limiting step in the illite dissolution process. A dissolution rate law showing the dependence of illite dissolution rate on proton concentration in the acid-sulfate solutions was derived from the steady state dissolution rates and can be used in predicting the impact of illite dissolution in saline acid-sulfate environments. The fractional reaction orders of 0.32 (I = 0.25) and 0.36 (I = 0.01) obtained in the study for illite dissolution are similar to the values reported for smectite. The dissolution rate of illite is mainly controlled by solution pH and no effect of ionic strength was observed on the dissolution rates.     

Mobilization of aluminium and magnesium by roots of banana (Musa spp.) from kaolinite and smectite clay minerals

Banana plants (Musa spp.) are very sensitive to Al, which is mobilized in acid soil conditions. These plants may, however, contribute to their own intoxication because their roots can excrete protons in large quantities. The authors studied the mobilization of Al by banana roots from clay minerals in experimental designs exacerbating the root–mineral contact. The plants were grown on agarose-gel or sand substrates previously mixed with smectite (montmorillonite) and kaolinite as sole source of Al. The pH and the ion concentrations in the aqueous and exchangeable phases of the substrates were determined as concentrations of Al, Ca, Mg and K in plants. In both agarose and sand substrates, pH significantly decreased in the close vicinity of roots, relative to the bulk substrate. This root-induced acidification involved a preferential mobilization of Al in kaolinite substrates and of Mg in smectite substrates, and thereby a significant plant uptake of Al and Mg from these respective substrates. Root-induced weathering of kaolinite and montmorillonite thus suggests that the mobilization of, respectively, Al and Mg are the limiting steps in the dissolution of these respective minerals, just as demonstrated in previous chemical weathering studies.     

Surface chemistry of kaolinite and Na-montmorillonite in aqueous electrolyte solutions at 25 and 60 °C: Experimental and modeling study

The aqueous interfacial chemistry of kaolinite and Na-montmorillonite samples was investigated by potentiometric measurements using acid/base continuous titrations and batch experiments at 25 and 60 °C. Using the batch experimental method, a continuous drift of pH was observed reflecting the mineral dissolution. Consequently, the continuous titration method appears to be the best way of studying solid surface reactions. For each clay mineral, the net proton surface excess/consumption was calculated as a function of pH and ionic strength (0.025, 0.1 and 0.5 M). At 25 °C, and according to the literature data, the pH corresponding to zero net proton consumption for montmorillonite appears to depend on ionic strength, whereas the value for kaolinite is constant and close to 5. Similar results are obtained at 60 °C, which suggests that the point of zero net proton consumption for clay minerals does not depend on temperature, at least up to 60 °C. On the other hand, the temperature rise induces a slight increase of the net proton surface excess. Finally, the diffuse double layer formalism (DDLM) is used to model the experimental data. The model involves two processes: the protonation/deprotonation of two types of edge sites (aluminol and silanol) and H⁺/Na⁺ exchange reactions on basal surfaces, while a tiny proportion of the negative structural charge remains uncompensated. This last process maintains a negative surface potential whatever the pH of the solution, which is in agreement with electrokinetic data.     

Europium retention onto clay minerals from 25 to 150 °C: Experimental measurements, spectroscopic features and sorption modelling

The sorption of Eu(III) onto kaolinite and montmorillonite was investigated up to 150 °C. The clays were purified samples, saturated with Na in the case of montmorillonite. Batch experiments were conducted at 25, 40, 80 and 150 °C in 0.5 M NaClO₄ solutions to measure the distribution coefficients (Kd) of Eu as a trace element (<10⁻⁶ mol/L) between the solution and kaolinite. For the Na-montmorillonite, we used Kd results from a previous study [Tertre, E., Berger, G., Castet, S., Loubet, M., Giffaut, E., 2005. Experimental study of adsorption of Ni²⁺, Cs⁺ and Ln³⁺ onto Na-montmorillonite up to 150 °C. Geochim. Cosmochim. Acta69, 4937-4948] obtained under exactly the same conditions. The number and nature of the Eu species sorbed onto both clay minerals were investigated by time resolved laser fluorescence spectroscopy (TRLFS) in specific experiments in the same temperature range. We identified a unique inner-sphere complex linked to the aluminol sites in both clays, assumed to be AlOEu²⁺ at the edge of the particles, and a second exchangeable outer-sphere complex for montmorillonite, probably in an interlayer position. The Kd values were used to adjust the parameters of a surface complexation model (DLM: diffuse layer model) from 25 to 150 °C. The number of Eu complexes and the stoichiometry of reactions were constrained by TRLFS. The acidity constants of the amphoteric aluminol sites were taken from another study [Tertre, E., Castet, S., Berger, G., Loubet, M., Giffaut, E. Acid/base surface chemistry of kaolinite and Na-montmorillonite at 25 and 60 °C: experimental study and modelling. Geochim. Cosmochim. Acta, in press], which integrates the influence of the negative structural charge of clays on the acid/base properties of edge sites as a function of temperature and ionic strength. The results of the modelling show that the observed shift of the sorption edge towards low pH with increasing temperature results solely from the contribution of the AlOEu²⁺ edge complexes. Finally, we successfully tested the performance of our model by confronting the predictions with experimental Kd data. We used our own data obtained at lower ionic strength (previous study) or higher suspension density and higher starting concentration (TRLFS runs, this study), as well as published data from other experimental studies [Bradbury, M.H., Baeyens, B., 2002. Sorption of Eu on Na and Ca-montmorillonite: experimental investigations and modeling with cation exchange and surface complexation. Geochim. Cosmochim. Acta66, 2325-2334; Kowal-Fouchard, A., 2002. Etude des mécanismes de rétention des ions U(IV) et Eu(III) sur les argiles: influence des silicates. Ph.D. Thesis, Université Paris Sud, France, 330p].     

Fe sorption onto nontronite (NAu-2)

The sorption of ferrous iron to a clay mineral, nontronite (NAu-2, a ferruginous smectite), was investigated under strictly anoxic conditions as a function of pH (3-10), Fe²⁺ concentration (0.01-50 mM), equilibration time (1-35 days), and ionic strength (0.01-0.5 M NaClO₄). The surface properties of NAu-2 were independently characterized to determine its fixed charge and amphoteric site density in order to interpret the Fe²⁺ sorption data. Fe²⁺ sorption to NAu-2 was strongly dependent on pH and ionic strength, reflecting the coupled effects of Fe²⁺ sorption through ion exchange and surface complexation reactions. Fe²⁺ sorption to NAu-2 increased with increasing pH from pH 2.5 to 4.5, remained constant from pH 4.5 to 7.0, increased again with further increase of pH from pH 7.0 to 8.5, and reached a maximum above pH 8.5. The Fe²⁺ sorption below pH 7.0 increased with decreasing ionic strength. The differences of Fe²⁺ sorption at different ionic strengths, however, diminished with increasing equilibration time. The Fe²⁺ sorption from pH 4.5 to 7.0 increased with increasing equilibration time up to 35 days and showed stronger kinetic behavior in higher ionic strength solutions. The kinetic uptake of Fe²⁺ onto NAu-2 is consistent with a surface precipitation mechanism although our measurements were not able to identify secondary precipitates. An equilibrium model that integrates ion exchange, surface complexation and aqueous speciation reactions reasonably well describes the Fe²⁺ sorption data as a function of pH, ionic strength, and Fe²⁺ concentration measured at 24 h of equilibration. Model calculations show that the species Fe(OH)⁺ was required to describe Fe²⁺ sorption above pH 8.0 satisfactorily. Overall, this study demonstrated that Fe²⁺ sorption to NAu-2 is affected by complex equilibrium and kinetic processes, likely caused by surface precipitation reactions.     

Computer simulations of the interactions of the (0 1 2) and (0 0 1) surfaces of jarosite with Al, Cd, Cu and Zn

Jarosite is an important mineral on Earth, and possibly on Mars, where it controls the mobility of iron, sulfate and potentially toxic metals. Atomistic simulations have been used to study the incorporation of Al³⁺, and the M²⁺ impurities Cd, Cu and Zn, in the (0 1 2) and (0 0 1) surfaces of jarosite. The calculations show that the incorporation of Al on an Fe site is favorable on all surfaces in which terminal Fe ions are exposed, and especially on the (0 0 1) [Fe₃(OH)₃]⁶⁺ surface. Incorporation of Cd, Cu or Zn on a K site balanced by a K vacancy is predicted to stabilize the surfaces, but calculated endothermic solution energies and the high degree of distortion of the surfaces following incorporation suggest that these substitutions will be limited. The calculations also suggest that incorporation of Cd, Cu and Zn on an Fe site balanced by an OH vacancy, or by coupled substitution on both K and Fe sites, is unfavorable, although this might be compensated for by growth of a new layer of jarosite or goethite, as predicted for bulk jarosite. The results of the simulations show that surface structure will exert an influence on uptake of impurities in the order Cu > Cd > Zn, with the most favorable surfaces for incorporation being (0 1 2) [KFe(OH)₄]⁰ and (0 0 1) [Fe₃(OH)₃]⁶⁺.     

Microstructures and compositional variation in the intravolcanic bole clays from the eastern Deccan volcanic province: Palaeoenvironmental implications and duration of volcanism

Clay minerals associated with intra-volcanic bole horizons of varied colours and thicknesses contain montmorillonite, halloysite and kaolinite, show distinct microstructures and microaggregates. In kaolinite, Fe³⁺ ions substitute for Al³⁺ at octahedral sites. Most of these clays are dioctahedral type, show balance between net layer and interlayer charges. The interstratified illite — smectite (I/S) mixed layers containing variable proportions of montmorillonite. Illite contains sheet-like, well oriented microaggregates. The parallel stacks of chlorite sheets show chlorite/smectite (C/S) mixed layers. Progressive enrichment of Fe and depletion of Al ions with the advancement of kaolinization process is observed. High order of structural and compositional maturity observed in these bole clays, indicate long hiatus between the two volcanic episodes.     

Reversible surface-sorption-induced electron-transfer oxidation of Fe(II) at reactive sites on a synthetic clay mineral

The sorption of ⁵⁷Fe(II) onto an Fe-free, mineralogically pure and Ca-saturated synthetic montmorillonite sample (structural formula: Ca_0.15(Al_1.4Mg_0.6)(Si₄)O₁₀(OH,F)₂), was studied as a function of pH under strictly anoxic conditions (N₂ glove box atmosphere, O₂ content <1 ppm), using wet chemistry and cryogenic (T = 77 K) ⁵⁷Fe Mössbauer spectrometry. No Fe(III) was detected in solution at any pH. However, in pH conditions where Fe(II) is removed from solution, a significant amount of surface-bound Fe(III) was produced, which increased with pH from 0% to 3% of total Fe in a pre-sorption edge region (i.e. at pH < 7.5 where about 15% of total Fe is sorbed) to 7% of total Fe when all Fe is sorbed. At low pH, where the pre-sorption edge plateau occurs (2 < pH < 7.5), the total sorbed-Fe amount remained constant but, within this sorbed-Fe pool, the Fe(III)/Fe(II) ratio increased with pH, from 0.14 at pH 2 up to 0.74 at pH 7. The pre-sorption edge plateau is interpreted as cation exchange on interlayer surfaces together with a sorption phenomenon occurring on highly reactive (i.e. high affinity) surface sites. As pH increases and protons are removed from the clay edge surface, we propose that more and more of these highly reactive sites acquire a steric configuration that stabilizes Fe(III) relative to Fe(II), thereby inducing a Fe to clay particle electron transfer. A sorption model based on cation exchange combined with surface complexation and electron transfers reproduces both wet chemical as well as the Mössbauer spectrometric results. The mechanism is fully reversible: sorbed-Fe is reduced as pH decreases (Mössbauer solid-state analyses) and all Fe returned to solution is returned as Fe(II) (solution analyses). This would not be the case if the observed oxidations were due to contaminant oxidizing agents in solution. The present work shows that alternating pH may induce surface redox phenomena in the absence of an electron acceptor in solution other than H₂O.     

Sorption of Np(V) and Np(IV) onto kaolinite: Effects of pH,ionic strength,carbonate and humic acid

The sorption of Np(V) and Np(IV) onto kaolinite has been studied in the absence and presence of humic acid (HA) in a series of batch equilibrium experiments under different experimental conditions: [Np]₀: 1.0 × 10^-6 or 1.0 × 10^-5 M, [HA]₀: 0 or 50 mg/L, I: 0.01 or 0.1 M NaClO₄, solid to liquid ratio: 4 g/L, pH: 6–11, anaerobic or aerobic conditions, without or with carbonate. The results showed that the Np(V) sorption onto kaolinite is affected by solution pH, ionic strength, Np concentration, presence of carbonate and HA. In the absence of carbonate, the Np(V) uptake increased with pH up to ∼96% at pH 11. HA further increased the Np(V) sorption between pH 6 and 9 but decreased the Np(V) sorption between pH 9 and 11. In the presence of carbonate, the Np(V) sorption increased with pH and reached a maximum of 54% between pH 8.5 and 9. At higher pH values, the Np(V) sorption decreased due to the presence of dissolved neptunyl carbonate species with a higher negative charge that were not sorbed onto the kaolinite surface which is negatively charged in this pH range. HA again decreased the Np(V) uptake in the near-neutral to alkaline pH range due to formation of aqueous neptunyl humate complexes. The decrease of the initial Np(V) concentration from 1.0 × 10⁻⁵ M to 1.0 × 10⁻⁶ M led to a shift of the Np(V) adsorption edge to lower pH values. A higher ionic strength increased the Np(V) uptake onto kaolinite in the presence of carbonate but had no effect on Np(V) uptake in the absence of carbonate.     

Surface complexation model for multisite adsorption of copper(II) onto kaolinite

We measured the adsorption of Cu(II) onto kaolinite from pH 3-7 at constant ionic strength. EXAFS spectra show that Cu(II) adsorbs as (CuO₄H_n)ⁿ⁻⁶ and binuclear (Cu₂O₆H_n)ⁿ⁻⁸ inner-sphere complexes on variable-charge ≡AlOH sites and as Cu²⁺ on ion exchangeable ≡X--H⁺ sites. Sorption isotherms and EXAFS spectra show that surface precipitates have not formed at least up to pH 6.5. Inner-sphere complexes are bound to the kaolinite surface by corner-sharing with two or three edge-sharing Al(O,OH)₆ polyhedra. Our interpretation of the EXAFS data are supported by ab initio (density functional theory) geometries of analog clusters simulating Cu complexes on the {110} and {010} crystal edges and at the ditrigonal cavity sites on the {001}. Having identified the bidentate (≡AlOH)₂Cu(OH)₂⁰, tridentate (≡Al₃O(OH)₂)Cu₂(OH)₃⁰ and ≡X--Cu²⁺ surface complexes, the experimental copper(II) adsorption data can be fit to the reactions

     

The effect of high pH alkaline solutions on the mineral stability of the Boom Clay – Batch experiments at 60 °C

Boom Clay is currently viewed as a reference host formation for studies on deep geological disposal of radioactive waste in Belgium. The interactions between bulk rock Boom Clay and 0.1 M KOH, 0.1 M NaOH, 0.1 M Ca(OH)₂, young cement water and evolved cement water solutions, ranging in pH from 12.5 to 13.2, were examined as static batch experiments at 60 °C to simulate alkaline plume perturbations, which are expected to occur in the repository due to the presence of concrete. Both liquids and solids were investigated at specific times between 90 and 510 days in order to control the elemental budget and to search for potential mineralogical alterations. Also, the clay fraction was separated from the whole-rock Boom Clay at the end of each run and characterized for its mineralogical composition. Thereby, the importance of the mineral matrix to buffer the alkaline attack and the role of organic matter to protect clay minerals were also addressed. The results indicate that the degree of geochemical perturbation in Boom Clay is dependent on the initial pH of the applied solution together with the nature of the major cation in the reactant fluids. The higher the initial pH of the media, the stronger its interaction with Boom Clay. No major non-clay mineralogical alteration of the Boom Clay was detected, but dissolution of kaolinite, smectite and illite occurred within the studied experimental conditions. The dissolution of clays is accompanied by the decrease in the layer charge, followed by a decrease in the cation-exchange capacity. The highest TOC values coincide with the highest total elemental concentrations in the leachates, and correspondingly, the highest dissolution degree. However, no quantitative link could be established between the degree of organic matter decomposition and clay dissolution.     

Mechanism of kaolinite dissolution at room temperature and pressure Part II: kinetic study

Studies on the dissolution kinetics of kaolinite were performed using batch reactors at 25°C and in the pH range from 1 to 13. A rapid initial dissolution step was first observed, followed by a linear kinetic stage reached after approximately 600 hr of reaction during which the kaolinite dissolves congruently at pH < 4 and pH > 11. The apparent incongruency between pH 5 and 10 was due to the precipitation of an Al–hydroxide phase. The true dissolution rates were computed from the amount of Si released into solution. The rate dependence on pH can be described by: r = 10−12.19aH+0.55 + 10−14.36 + 10−10.71aOH−0.75Between pH 5 and 10, the rate is approximately constant, although a smooth minimum was observed at pH close to 9. mAn attempt was made to obtain a general rate law based on the coordination theory, which was first applied to the mineral dissolution studies by Stumm and co-workers. The kinetic data were combined with the results obtained for the surface speciation by Huertas et al. (1998). It is possible to express the linear dissolution rate as a simple power function of the concentration of the surface sites active in various pH ranges: r = 10−8.25 [>Al2OH2+] + 10−10.82 [>AlOH2+]0.5 + 10−9.1 [>Al2OH + >AlOH + >SiOH] + 103.78 [>Al2O− + >AlO−]3This equation assumes that the dissolution mechanism is mainly controlled by the two Al surface sites (external and internal structural hydroxyls, and aluminol at the crystal edges) under both acidic and alkaline conditions. The model reflects well the important contribution of the crystal basal planes to the dissolution of kaolinite.     

Rare earth element sorption by basaltic rock: Experimental data and modeling results using the “Generalised Composite approach”

Sorption of the 14 rare earth elements (REE) by basaltic rock is investigated as a function of pH, ionic strength and aqueous REE concentrations. The rock sample, originating from a terrestrial basalt flow (Rio Grande do Sul State, Brazil), is composed of plagioclase, pyroxene and cryptocrystalline phases. Small amounts of clay minerals are present, due to rock weathering. Batch sorption experiments are carried out under controlled temperature conditions of 20 °C with the <125 μm fraction of the ground rock in solutions of 0.025 M and 0.5 M NaCl and at pH ranging from 2.7 to 8. All 14 REEs are investigated simultaneously with initial concentrations varying from 10⁻⁷ to 10⁻⁴ mol/L. Some experiments are repeated with only europium present to evaluate possible competitive effects between REE. Experimental results show the preferential retention of the heavy REEs at high ionic strength and circumneutral pH conditions. Moreover, results show that REE sorption increases strongly with decreasing ionic strength, indicating two types of sorption sites: exchange and specific sites. Sorption data are described by a Generalised Composite (GC) non-electrostatic model: two kinds of surface reactions are treated, i.e. cation exchange at >XNa sites, and surface complexation at >SOH sites. Total site density (>XNa + >SOH) is determined by measuring the cation exchange capacity (CEC = 52 μmol/m²). Specific concentrations of exchange sites and complexation sites are determined by fitting the Langmuir equation to sorption isotherms of REE and phosphate ions. Site densities of 22 ± 5 and 30 ± 5 μmol/m² are obtained for [>XNa] and [>SOH], respectively. The entire set of REE experimental data is modeled using a single exchange constant (log K_ex = 9.7) and a surface complexation constant that progressively increases from log K = −1.15 for La(III) to −0.4 for Lu(III).The model proves to be fairly robust in describing other aluminosilicate systems. Maintaining the same set of sorption constants and only adjusting the site densities, we obtain good agreement with the literature data on REE/kaolinite and REE/smectite sorption. The Generalised Composite non-electrostatic model appears as an easy and efficient tool for describing sorption by complex aluminosilicate mineral assemblages.     

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.     

Provenance of Clay Minerals in the Amami Sankaku Basin and Their Paleoclimate Implications Since Late Pleistocene

We analyzed the clay mineral assemblages, content and mineralogical characteristics of Hole U1438A sediment recovered from Amami Sankaku Basin during International Ocean Discovery Program (IODP) expedition 351. The results show that the clay minerals are mainly composed of illite (average 57%), smectite (average 26%), chlorite (average 14%) and minor kaolinite(average 3%). The crystallinity of illite in all samples are good (<0.4 Δ° 2θ), and the chemical indexes of illite in all samples are low (<0.4). Both indicate that illite in Hole U1438A formed in cold and dry climate. By comparing clay mineral assemblages of hole U1438A and the potential sediment sources, we suggest that smectite be mainly derived from the volcanic materials around Amami Sankaku Basin. Illite, chlorite and kaolinite are mainly derived from the Asian dust. The ratios of (illite+chlorite)/smectite show a phased increase over the last 350 ka, which is consistent with the cold and drying trend of the Asian continent since late Pleistocene. The high ratios of (illite+chlorite)/smectite and (illite+chlorite)/kaolinite during glacial period indicate that much more Asian dust was input into the Amami Sankaku Basin, which are responded to the aridity of Asian continent and strengthened east Asian Monsoon during glacial period.     

Aqueous dissolution,solubilities and thermodynamic stabilities of common aluminosilicate clay minerals: Kaolinite and smectites

Determinations of the aqueous solubilities of kaolinite at pH 4, and of five smectite minerals in suspensions set between pH 5 and 8, were undertaken with mineral suspensions adjusted to approach equilibrium from over- and undersaturation. After 1,237 days, Dry Branch, Georgia kaolinite suspensions attained equilibrium solubility with respect to the kaolinite, for which K_eq = (2.72 ± 0.35) × 10⁷. The experimentally determined Gibbs free energy of formation (ΔG_f,298⁰) for the kaolinite is −3,789.51 ± 6.60 kj mol⁻¹. Equilibrium solubilities could not be determined for the smectites because the composition of the solution phase in the smectite suspensions appeared to be controlled by the formation of gibbsite or amorphous aluminum hydroxide and not by the smectites, preventing attempts to determine valid ΔG_f⁰ values for these complex aluminosilicate clay minerals. Reported solubility-based ΔG_f⁰ determinations for smectites and other variable composition aluminosilicate clay minerals are shown to be invalid because of experimental deficiencies and of conceptual flaws arising from the nature of the minerals themselves. Because of the variable composition of smectites and similar minerals, it is concluded that reliable equilibrium solubilities and solubility-derived ΔG_f⁰ values can neither be rigorously determined by conventional experimental procedures, nor applied in equilibriabased models of smectite-water interactions.