Clay Dissolution Research Articles

Terra Rossa Genesis by Replacement of Limestone by Kaolinite. III. Dynamic Quantitative Model

We model quantitatively the chemical dynamics of terra rossa formation that we deduced earlier from new field and petrographic evidence. The key phenomenon is that authigenic kaolinite (a proxy for kaolinite solid solutions and other clays) replaces limestone at a downward-moving reaction front at the base of the existing terra rossa. The chemical elements needed to make the authigenic clay are assumed to come from dissolution of eolian clay dust deposited at the surface. The model consists of equations of mass conservation (in one spatial dimension) that incorporate infiltration, diffusion, and replacement and dissolution reactions. Distinctive elements of the model are that (1) the kinetics of the kaolinite-for-calcite replacement is dominated by the crystallization stress generated by the kaolinite growth, in accordance with the view that replacement happens not by dissolution-precipitation but by guest-mineral-growth-driven pressure solution of the host; (2) the field observation that the terra rossa–forming front is narrow indicates that the growth of kaolinite at the front proceeds roughly at the pace at which the appropriate aqueous reactants reach the front, which in turn warrants scaling time, length, and concentrations such that transport and reaction terms are of the same order of magnitude; and (3) the huge contrast between aqueous concentrations and mineral densities, in effect, makes the scaled equation of mass conservation steady state. Numerical simulations correctly reproduce the formation of two adjacent zones in the moving reaction front—replacement and leaching—and of considerable secondary porosity in the leaching zone. This leaching porosity maximum constitutes a solitary wave, or soliton, that moves with the front. The simulations produce a range of predicted front widths that bracket the observed front width. Similarly, model-predicted front velocities of a few meters per million years agree well, within an order of magnitude, with both (1) a paleomagnetically derived rate of terra rossa formation at Bloomington and (2) the rate of authigenic clay formation that could be expected to form—by clay dissolution at the surface, downward infiltration of aqueous solutes, and precipitation of new clay at the front—from average clay dust deposition rates as measured today.

Soil chemistry and shale weathering on a hillslope influenced by convergent hydrologic flow regime at the Susquehanna/Shale Hills Critical Zone Observatory

Soil chemistry data (major and REEs) are presented from a swale transect for comparison to similar measurements on a planar transect published previously for the Susquehanna/Shale Hills Critical Zone Observatory. Similar reaction fronts are observed: plagioclase dissolution is indicated by Na and Ca depletion and a negative Eu anomaly; clay dissolution followed by particle loss is accompanied by depletion of Mg, K, Fe, Al and Si. However, in contrast to the planar transect, soils along the swale transect, especially in the topographically depressed site, do not show smooth elemental profiles. This documents both residuum soils and accumulation of colluvium sediments. The soils in the swale transect are thicker and on average wetter than those along the planar transect; however, the Ce anomaly observed in the swale soils is consistent with a generally oxic environment. Thus, preferential flowpaths are an important mechanism for water transport, preventing swale soils from water saturation.

Testing geochemical models of bentonite pore water evolution against laboratory experimental data

The determination of a bentonite pore water composition and understanding its evolution with time underpins many radioactive waste disposal issues, such as buffer erosion, canister corrosion, and radionuclide solubility, sorption, and diffusion, inter alia. Previous modelling approaches have tended to ignore clay dissolution–precipitation reactions, a consequence of which is that montmorillonite is theoretically preserved indefinitely in the repository system. Here, we investigate the applicability of an alternative clay pore fluid evolution model, that incorporates clay dissolution–precipitation reactions as an integral component and test it against well-characterised laboratory experimental data, where key geochemical parameters, Eh and pH, have been measured directly in compacted bentonite. Simulations have been conducted using different computer codes (Geochemist’s Workbench, PHREEQC, and QPAC) to test the applicability of this model. Thermodynamic data for the Gibb’s free energy of formation of MX-80 smectite used in the calculations were estimated using two different methods (‘Polymer’ and ‘Vieillard’ Models). Simulations of ‘end-point’ pH measurements in batch bentonite–water slurry experiments showed different pH values according to the complexity of the system studied. The most complete system investigated revealed pH values were a strong function of partial pressure of carbon dioxide, with pH increasing with decreasing PCO 2 (with log PCO 2 values ranging from −3.5 to −7.5 bars produced pH values ranging from 7.9 to 9.6). A second set of calculations investigated disequilibrium between clay and pore fluid in laboratory squeezing cell tests involving pure water (pH = 9.0) or a 1 M NaOH solution (pH = 12.1). Simulations carried out for 100 days (the same timescale as the experiments) showed that smectite remained far from equilibrium throughout, and that the lowering of pH due to smectite hydrolysis was trivial. However, extending the duration of the simulations to that required for clay–fluid equilibrium, necessitated timescales of 7 and 65 years for pure water and 1 M NaOH, respectively, but again produced relatively minor reduction in pH (in the order of 0.1–0.2 pH units). If the (equilibrium) precipitation of secondary minerals was included in the simulations, then not only was the clay–fluid equilibration period extended dramatically (from 7 to 360 years for pure water, and from 65 to 2600 years for 1 M NaOH), but concomitant changes in pH were significant, decreasing from 9.0 to 8.6 (pure water) and from 12.1 to 9.0 (1 M NaOH). Repetition of these latter calculations using an alternative method for Δ G f 0 smectite produced an increase in equilibration time for reaction with 1 M NaOH from 2600 to 5000 years, highlighting the potential effects of the uncertainty in thermodynamic data for smectite. A final set of calculations was carried out to investigate both the time- and space-dependent variations in pore fluid composition in laboratory in-diffusion experiments conducted for over 1200 days, initially with pure water and ‘spiked’ after 271 days with a Na–Ca–OH–Cl solution (pH = 11.7). Here, the sensitivity of the results to both variations in a number of parameters/conditions (porosity, reaction rate of secondary minerals, the degree of mixing of the external fluid reservoirs in the experiments, the effective diffusion coefficient) and the inclusion/exclusion of key processes (clay hydrolysis, secondary mineral precipitation, ion exchange, clay edge protonation–deprotonation reactions) was investigated. These calculations confirmed that smectite dissolution–precipitation reactions alone have an insignificant impact upon pH buffering over laboratory timescales and that the pH buffering observed is most likely controlled by clay protonation–deprotonation reactions, and kinetic secondary mineral (brucite + tobermorite) precipitation. Ion exchange reactions were found to have little effect on pH. Alternative data for the kinetic dissolution of smectite produced no observable differences, and the adoption of a reduced diffusion coefficient produced a poorer fit to experiment results. In conclusion, modelling predicts that the effects of smectite dissolution on the chemistry of bentonite pore waters would be essentially undetectable over experimental time scales, but when the model is combined with plausible constraints on the precipitation of secondary minerals, significant changes in solution chemistry and mineralogy are predicted to occur over time scales that are relevant to repository near-field evolution (hundreds to thousands of years). There are remaining fundamental uncertainties related to the variable chemistry of the smectite clays, the nature of porosity in highly compacted buffer materials, the reactive surface area of smectite, and the thermodynamic properties of these clay minerals.

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.

Glauconite formation in lacustrine/palaeosol sediments, Isle of Wight (Hampshire Basin), UK

AbstractThe clay mineralogy and chemistry of a green lacustrine marl that has been pedogenically modified in the upper part was investigated in order to better understand the formation of low-temperature Fe-rich 10 Å clay. Twelve samples in a vertical sequence have been investigated using X-ray diffraction (XRD), chemical analysis, scanning electron microscopy (SEM) and laser particle size analysis. The clay assemblage has a range of overall illite-smectite (I-S) compositions (64–100%) resulting from several I-S phases that, for the sake of modelling, have been simplified to one to three I-S phases of increasing illitic content. Where the lacustrine marl has been pedogenically modified, the smectite-rich I-S is much reduced in abundance or absent and the 10 Å -rich component is both more abundant and more illitic. These assemblages are a consequence of illitization of detrital I-S in the lake and soil, and dissolution of other clays (kaolinite and chlorite) in the hypersaline lake. Interlayer K, octahedral Fe and octahedral + interlayer Mg increase with intensity of illitization (increase range 0.32–0.63, 0.68–1.67, 0.18–0.24 per O10(OH)2, respectively), first in the increasingly saline lake, and latterly as a result of wetting and drying in a gley soil. In the soil environment, reduction of Fe(III) to Fe(II) resulted in increased layer charge but, as by this stage very few smectite interlayers remained, this did not result in an equivalent increase in illite. Laser particle-size analysis, supported by SEM observation, shows the existence of a bimodal distribution of clay particle size (maxima at 0.2 and 1.5–1.8 μm) in which the finer fraction increases largely in the pedogenically affected samples, probably due to particle break-up caused by seasonal wetting and drying. This ‘dual action’ illitization, first in a hypersaline lake and latterly through wetting and drying, may be responsible for both the intensity of illitization and exceptionally high (for the Solent Group) Fe content of the authigenic illite. The chemical characteristics of the illitic I-S and the illite end-member correspond to glauconite. Hence, this is an example of onshore, non-pelletal glauconite formation.

Natural systems evidence for the alteration of clay under alkaline conditions: An example from Searles Lake, California

Mudstones containing smectite have been altered under mildly alkaline conditions (9 < pH < 10) at Searles Lake, California over a 3 million-year time period. Published mineralogical investigations have shown that up to 70% of detrital clay may be replaced by authigenic minerals such as Fe-illite, analcime, and K-feldspar over timescales < 100 ka. This natural alteration has been simulated incorporating time-dependent boundary conditions of sedimentation and fluid composition, a Pitzer model for activities of aqueous species, and a coupled hydrogeological model for time-dependent flow in the sediment layers. Kinetic dissolution of detrital smectite under alkaline conditions was described using one of two models based on departure from thermodynamic equilibrium or by an empirical rate dependent upon aqueous Si concentrations. Secondary mineral growth was modelled with rates estimated using mineralogical data from the Searles Lake drillcore. The zonal pattern of smectite dissolution observed at Searles Lake was reproduced reasonably well by the ‘Cama TST’ model of montmorillonite dissolution; the other models strongly overestimated clay dissolution. However, the rate of primary montmorillonite dissolution was also a strong function of the assumed rate of growth of secondary minerals. The amount of K-feldspar growth was overestimated in each of the models, whereas analcime (comprising up to 10% of the Searles Lake drillcore composition in some zones) was absent from the results of all the model simulations. This assessment provides a test of the accuracy and reliability of published data in the application of models of smectite dissolution in the long-term and demonstrates that smectite will be reactive under mildly alkaline conditions under conditions of advective fluid transport.

Investigation of acidic dissolution of mixed clays between pH 1.0 and −3.0 using Si and Al X-ray absorption near edge structure

Although widely investigated in relation to acid mine drainage systems at pH > 1.0, we know little about the impact of sulfuric acid (H 2SO 4) on the geochemistry and mineralogy of clays at pH < 1.0 (including negative pH values). Thus, laboratory batch experiments were conducted on three mixed clay samples with different mass ratios of phyllosilicates (smectite, illite, and kaolinite) to investigate the impact of H 2SO 4 from pH 1.0 to −3.0 for exposure periods of 14, 90, 180, and 365 days. Si and Al K- and L 2,3-edge X-ray absorption near edge structure (XANES) spectroscopy were employed on these samples to determine the chemical and structural changes that occur during acidic dissolution of phyllosilicates that cannot be distinguished using X-ray diffraction analyses. A series of silicate, phyllosilicate, and Al-bearing standard compounds were also studied to provide an explanation for the observed changes in the clay samples. The Si XANES results indicated the preferential dissolution of the phyllosilicates (pH ⩽ 1.0, t ⩾ 14 d), the persistence of quartz even at pH ⩾ −3.0 and t ⩾ 365 d, and the formation of an amorphous silica-like phase that was confined to the surface layer of the altered clay samples at pH ⩽ 0.0 and t ⩾ 90 d). Al XANES results demonstrated dissolution of Al-octahedral layers (pH ⩽ 1.0, t ⩾ 14 d), the persistence of four-fold relative to six-fold coordinated Al, and the precipitation of an Al-SO 4-rich phase (pH ⩽ −1.0, t ⩾ 90 d). An existing conceptual model of phyllosilicate dissolution under extremely acidic conditions was modified to include the results of this study.

Effect of partial removal of adsorbed humus on kinetics of potassium and silica release by tartaric acid from clay - humus complex from two dissimilar soil profiles

The aim of this investigation is to study how adsorbed humus on clay minerals affects kinetics of dissolution of clay mineral by an organic acid in terms of K and Si release. Clay–humus complexes were isolated from soil samples collected from different depths of 2 soil profiles from an Inceptisol of North India under long-term rice–wheat and maize–wheat cropping systems. Clay–humus complexes were subjected to repeat extraction with 100 mg/L of tartaric acid solutions before and after removal of humus with 30% hydrogen peroxide and the resultant release of potassium and silica was measured. Before removal of humus, K release followed Elovich kinetics for the maize–wheat cropping system.For the rice–wheat cropping system, except surface soil, which followed Elovich kinetics, other subsurface soils followed the power form of kinetic equations. But when humus was partially removed, K release increased exponentially, therelease rate increasing with time. This indicated that for humus-depleted clay, initial release triggered further release of K. Silica was released at a constant rate. The dissolution reaction was incongruent, i.e. the amount of constituents released in solution was not proportional to their mole fraction present in the mineral. X-ray diffraction analysis showed that micaceous mineral was partially dissolved and vermiculite was completely dissolved when humus was partially removed from clay before extraction. This observation indicates that humus adsorbed on clay plays an important role in preventing clay dissolution by organic acids.

Stepwise effects of the BCR sequential chemical extraction procedure on dissolution and metal release from common ferromagnesian clay minerals: A combined solution chemistry and X-ray powder diffraction study

Sequential extraction procedures (SEPs) are commonly used to determine speciation of trace metals in soils and sediments. However, the non-selectivity of reagents for targeted phases has remained a lingering concern. Furthermore, potentially reactive phases such as phyllosilicate clay minerals often contain trace metals in structural sites, and their reactivity has not been quantified. Accordingly, the objective of this study is to analyze the behavior of trace metal-bearing clay minerals exposed to the revised BCR 3-step plus aqua regia SEP. Mineral quantification based on stoichiometric analysis and quantitative powder X-ray diffraction (XRD) documents progressive dissolution of chlorite (CCa-2 ripidolite) and two varieties of smectite (SapCa-2 saponite and SWa-1 nontronite) during steps 1–3 of the BCR procedure. In total, 8 (± 1) % of ripidolite, 19 (± 1) % of saponite, and 19 (± 3) % of nontronite (% mineral mass) dissolved during extractions assumed by many researchers to release trace metals from exchange sites, carbonates, hydroxides, sulfides and organic matter. For all three reference clays, release of Ni into solution is correlated with clay dissolution. Hydrolysis of relatively weak Mg–O bonds (362 kJ/mol) during all stages, reduction of Fe(III) during hydroxylamine hydrochloride extraction and oxidation of Fe(II) during hydrogen peroxide extraction are the main reasons for clay mineral dissolution. These findings underscore the need for precise mineral quantification when using SEPs to understand the origin/partitioning of trace metals with solid phases.

Kaolinite formation from palygorskite and sepiolite in rhizosphere soils

AbstractPalygorskite and sepiolite are fibrous clays that occur mostly in agricultural soils of arid regions. Although many investigations have examined the environmental conditions for the formation and stability of these clays, information on the transformation of these clays in the root zone (or rhizosphere) of agricultural crops is limited. In this study, changes in palygorskite and sepiolite within the rhizosphere of selected agricultural crops were determined and the ability of plants to extract Mg from these minerals compared. Alfalfa, barley, and canola were cultivated in pots under controlled conditions in a growth chamber using growth media that consisted of a mixture of Ottawa sand and clay-sized Florida palygorskite (PFl-1) or Spanish sepiolite (SepSp-1). After 100 days of cultivation, the biomass of plant roots and shoots were determined and Mg uptake measured by inductively coupled plasma analysis of the plant biomass after microwave oven digestion. The clay fraction in each pot was separated from the sand and analyzed using X-ray diffraction (XRD) and examined using transmission electron microscopy (TEM). The XRD reflection at 0.718 nm clearly indicated kaolinite in the rhizosphere after growth of the three crops. Furthermore, hexagonal kaolinite particles were observed, using TEM, and the amount of Mg extracted by the three crops was significantly greater for sepiolite than for palygorskite. Palygorskite and sepiolite kaolinization in the rhizosphere was apparently due: (1) to high acidity in the rhizosphere caused by root activity and organic matter decomposition; and (2) to fibrous clay destabilization caused by Mg uptake by plants. This study shows that kaolinite in agricultural soils of arid and semi-arid regions might be partly due to neoformation after fibrous clay dissolution and not entirely inherited from parent materials, as has been suggested in earlier literature.

Metal corrosion and argillite transformation at the water-saturated, high-temperature iron–clay interface: A microscopic-scale study

Abstract In this study, microscopic and spectroscopic techniques (scanning electron microscopy coupled with energy-dispersive X-ray analysis, Raman microspectroscopy, micro X-ray fluorescence spectroscopy, micro X-ray fine structure adsorption spectroscopy, and micro laser-induced breakdown spectroscopy) were combined to decipher the chemical and mineralogical properties of a saturated Fe–clay interface reacted at 90 °C and 50 bar for 8 months. The results collectively confirm the presence of a corrosion layer and a clay transformation layer. The corrosion layer is made of a magnetite-containing internal sublayer and a Fe-phyllosilicate external sublayer enriched in Na, with traces of goethite presumably resulting from sample reaction with air. The clay transformation layer is made of predominantly Ca-rich siderite (FeCO 3 ). It is depleted in Al and K, suggesting dissolution of rock-forming minerals. The corroded thickness determined from the amount of Fe in corrosion and transformation layers and assuming zero porosity equals 19 ± 9 μm. These data indicate that the interfacial clay was transformed by dissolution of calcite and clay minerals and precipitation of siderite close to the original surface. Silica released upon clay dissolution diffused into the corrosion layer and coprecipitated with oxidized Fe to form Fe-phyllosilicate.

Mineralogy as a critical factor of dust iron solubility

To understand the impact of dust deposition on ocean primary productivity, the evaluation of dust input is not sufficient, and the soluble iron supply is also needed. Measurements of iron in aerosols suggest a much higher solubility than in soil, implying substantial atmospheric processing of chemical or physical nature. Here, we have investigated the effect of the mineralogy on iron solubility. We have measured iron solubility from some minerals commonly found in dust (clay, feldspar and iron (hydr‐)oxides). Firstly, we observe a greater solubility of iron resulting from clays (~4%) in comparison to iron (hydr‐)oxides (&lt;1%), independently of specific surface of minerals. Secondly, by considering amount of dissolved iron and the mineralogical composition of dust, our results indicate that dissolved iron fraction mainly comes from clay dissolution in contrast to that assumed in some biogeochemical models. In consequence, it appears that iron solubility is closely linked to the mineralogical composition of aerosol.

Measurement of adsorbed and total 232Th/ 230Th ratios from marine sediments

The predictable in situ production of 230Th from the decay of uranium in seawater, and its subsequent removal by scavenging onto falling particles, provides a valuable tool for normalizing fluxes to the seafloor. We describe a new application, determination of the 232Th that dissolves in the water column and is removed to the seafloor. 232Th is supplied to the ocean in continental minerals, dissolution of which leads to a measurable standing stock in the water column. Sedimentary adsorbed 232Th/ 230Th ratios have the potential to provide a proxy for estimating the amount of dissolved material that enters the ocean, both today and in the past. Ten core top samples were treated with up to eight different leaching techniques in order to determine the best method for the separating adsorbed from lattice bound thorium. In addition, separate components of the sediments were analyzed to test whether clay dissolution was an important contribution to the final measurement. There was no systematic correlation between the strength of acid used in the leach and the measured 232Th/ 230Th ratios. In all cases clean foraminifera produced the same ratio as leaches on bulk sediment. In three out of five samples leaches performed on non-carbonate detritus in the < 63 μm size fraction were also identical. Without additional water column data it is not yet clear whether there is a simple one to one correlation between the expected deep-water 232Th/ 230Th and that produced by leaching, especially in carbonate-rich sediments. However, higher ratios, and associated high 232Th adsorbed fluxes, were observed in areas with high expected detrital inputs. The adsorbed fraction was ∼ 35–50% of the total 232Th in seven out of ten samples. Our 230Th normalized 232Th fluxes are reasonable by comparison to global estimates of detrital inputs to the ocean. In nine cases out of ten, the total 230Th-normalized 232Th flux is greater than predicted from the annual dust fall at each specific location, but lower than the average global detrital input from all sources.

Characterization of aqueous lead removal by phosphatic clay: Equilibrium and kinetic studies

Immobilization of heavy metals from contaminated environments is an emerging field of interest from both resource conservation and environmental remediation points of view. This study investigated the feasibility of using phosphatic clay, a waste by-product of the phosphate mining industry, as an effective sorbent for Pb from aqueous effluents. The major parameters controlling aqueous Pb removal, viz. initial metal ion concentrations, solution pH, sorbent amounts, ionic strength and presence of both inorganic and organic ligands were evaluated using batch experiments. Results demonstrated that aqueous Pb removal efficiency of phosphatic clay is controlled mainly by dissolution of phosphatic clay associated fluoroapatite [Ca 10(PO 4) 5CaCO 3(F,Cl,OH) 2], followed by subsequent precipitation of geochemically stable pyromorphite [Pb 10(PO 4) 6(F,Cl,OH) 2], which was confirmed by both X-ray diffraction (XRD) and scanning electron microscopic (SEM) analysis. Lead removal efficiency of phosphatic clay increased with increasing pH, sorbent amount and decreasing ionic strength. It also depends on the nature of complexing ligands. Formation of insoluble calcium oxalate and lead oxalate in the presence of oxalic acid explained high uptake of Pb by phosphatic clay from aqueous solution. However, Pb sorption kinetics onto phosphatic clay were biphasic, with initially fast reactions followed by slow and continuous Pb removal reactions. The slow reactions may include surface sorption, co-precipitation and diffusion. The exceptional capability of phosphatic clay to remove aqueous Pb demonstrated its potential as a cost effective way to remediate Pb-contaminated water, soils and sediments.

Cement/clay interactions – A review: Experiments, natural analogues, and modeling

The concept of storing radioactive waste in geological formations calls for large quantities of concrete that will be in contact with the clay material of the engineered barriers as well as with the geological formation. France, Switzerland and Belgium are studying the option of clayey geological formations. The clay and cement media have very contrasted chemistries that will interact and lead to a degradation of both types of material. The purpose of this review is to establish an exhaustive list of laboratory experiments so as to identify the reaction sequences in the evolution of both the clay minerals and accessory minerals during their alteration in an alkaline environment. We review the data on clay dissolution kinetics in this environment, and include an invaluable study of natural analogues that allow one to correlate the phenomena in time. The available data and experiments make it possible to construct predictive numerical models. However, as the quality of the data is inhomogeneous, we recommend a continuation of the thermodynamic and kinetic data acquisition. It is obvious that the numerical modeling of the alkaline disturbance will be more relevant if it can combine the advantages of the different detailed models: mineralogical completeness, combined modeling of the clay and cement media, evolution of the porosity, consideration of the pCO 2 and all the surface reactions.

Clay mineral weathering and contaminant dynamics in a caustic aqueous system: II. Mineral transformation and microscale partitioning

Microscopic and spectroscopic studies were conducted to assess mineral transformation processes in aqueous suspensions of illite (Il), vermiculite (Vm) and montmorillonite (Mt) that were subjected to weathering in a simulated high-level radioactive tank waste leachate (0.05 m Al T, 2 m Na +, 1 m NO 3 −, pH ∼14, Cs + and Sr 2+ 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 − mol kg −. Incongruent clay dissolution resulted in an accumulation of secondary aluminosilicate precipitates identified as nitrate-sodalite, nitrate-cancrinite and zeolite X, by molecular spectroscopy and electron microscopy (XRD, IR, NMR, SEM-EDS and TEM-EDS). Contaminant fate was dependent on competing uptake to parent clays and weathering products. TEM-EDS results indicated that high Il affinity for Cs was due to adsorption at frayed edge sites. The Il system also comprised Sr-rich aluminous precipitates after 369 d reaction time. In Mt systems, Cs and Sr were co-precipitated into increasingly recalcitrant spheroidal precipitates over the course of the experiment, whereas contaminant association with montmorillonite platelets was less prevalent. In contrast, Cs and Sr were found in association with weathered Vm particles despite the formation of spheroidal aluminosilicate precipitates that were comparable to those formed from Mt dissolution.

The titration of clay minerals: I. Discontinuous backtitration technique combined with CEC measurements

Previous experimental studies on clay potentiometric titration have been unable to distinguish inorganic cation exchange in the interlayer and on basal plane surfaces from specific pH-dependent sorption of cations and anions on the edges. In this study, we refined a titration technique, combining discontinuous backtitration and cation exchange capacity (CEC) measurements, and applied it to the potentiometric titration of Na- and Ca-conditioned montmorillonites. This technique can be used to accurately measure cation exchange, edge surface proton charge, dissolution of clay, and precipitation of new phases. Thus, a precise measurement of the variations of net proton surface charge is possible. This has important implications for clay surface modeling (see part II of this article) and for processes that depend on the clay surface charge, e.g., alteration, rheological processes, and contamination retention applications. In addition, this study confirms the adsorption of ionic pairs such as CaCl + in exchange site positions and shows that CaOH + could behave like CaCl +. This result, together with the evidence of precipitation of a CaSi phase over a short time-scale (1 week) at high pH and low temperature, can be used to model clay–concrete interactions more accurately. We confirmed and quantified the H +/Na + exchange reaction at low pH. Finally, we demonstrate that both the edge surface charge and the permanent structural charge are compensated for by the nonspecific sorption of cations from solution across the entire pH range from 4 to 11. Under these conditions, the surface potential is fully screened and does not need to be invoked in modeling sorption processes on clay particles in dilute suspensions.

Natural organic matter (NOM)-clay association and impact on Callovo-Oxfordian clay stability in high alkaline solution: Spectromicroscopic evidence

The understanding of chemical association between natural organic matter (NUM) and clay minerals is of paramount importance to predict the long term stability of host rock formations for deep geological nuclear waste repositories. Synchrotron-based soft X-ray spectromicroscopy demonstrates a strong association between K-rich clay phases (illite) and NOM in the Callovo-Oxfordian argillite (Meuse-Haute Marne, or MHM site, France) and a weaker association in the Opalinus clay (Benken, Switzerland). C(ls) spectra show no significant depth dependent variation in the MHM site 1447-516m). Alteration experiments under oxidizing conditions and high pH of the Callovo-Oxfordian clay indicate a passivation of chemically reactive sites by NOM that is responsible for the kinetic hindered clay dissolution/ transformation. These experiments lead to a significant release of humic/fulvic acid colloids in the alkaline solution with time dependent variation in size and functional group content.

Cations extraction of sandy-clay soils from cavado valley, portugal, using sodium salts solutions

Cases of contamination by metals in the water wells of the Cavado Valley in north-west Portugal can be attributed to the heavy leaching of clay soils due to an excess of nitrogen resulting from the intensive use of fertilisers in agricultural areas. This work focuses on the natural weathering characteristics of soils, particularly the clay material, through the study of samples collected near the River Cavado. Samples taken from various sites, after physico-chemical characterisation, were subjected to clay dissolution tests, using sodium salts of different ionic forces, to detect the relationship between certain physico-chemical parameters of water, such as pH, nitrate, chloride and sulphate content, in the dissolution of clay and the subsequent extraction of such cations as Al, Fe and K. In acidic sandy clay soils, the mineralogical composition of which was characterised by a predominance of quartz, micas, kaolinite and K-feldspars, decreases of the clay material/water pH ratio increases dissolution of the micaceous and K-feldspars phases. The presence of nitrates in the aqueous solution apparently advanced the extraction of all three cations Al, Fe and K. The specific surface area of the clay material showed a significant correlation with the main kinetic parameters of cation extraction.

Determining Biogenic Silica in Marine Samples by Tracking Silicate and Aluminium Concentrations in Alkaline Leaching Solutions

This study introduces an alkaline leaching technique for the simultaneous analysis of biogenic silica and aluminium in sediments. Measuring aluminium facilitates the discrimination between silica from the biogenic (BSiO2) and the non-biogenic fraction, because it originates almost solely from the lithogenic phase. The method was tested using fine-grained silicagel, standard clay minerals, artificial sediments, and natural samples ranging from fresh diatoms to aged sediment from different depositional settings. To determine the BSiO2 content, four different models each describing the dissolution curves, but of increasing complexity, were applied and for each different type of sample the optimum model was selected on the basis of F-test statistics. For mixtures of silicagel and clay minerals, the contribution of Si from the dissolution of clay was negligible compared to Si originating from silicagel. For natural samples with high clay content, complex dissolution curves were observed and single-phase first order dissolution was the exception. This deviation from `ideal' behavior could only be recognized because of high-resolution sampling, especially in the first 20 minutes of the experiment. For most of the samples, the distinction between the biogenic silica fraction and the silica originating from dissolution of clays could be made on the basis of the Si/Al ratios and reactivity constants of the dissolving phases calculated with the models. Clay minerals typically dissolve slowly at a Si/Al ratio close to 1–2, depending on the type of clay mineral. In contrast, biogenic silica displays a wide range of reactivities and Si/Al ratios. Fresh biogenic silica from the water column usually has a high reactivity and a low Al content. Aged biogenic silica from the sediments has a lower reactivity, but Si/Al ratios as low as 5 were found. The method as described here therefore presents an accurate method to analyze biogenic silica in marine sediments with a relatively high clay mineral content.