Incongruent Dissolution Research Articles

Zircon solubility and zirconium complexation in H2O+Na2O+SiO2±Al2O3 fluids at high pressure and temperature

Zircon is an important host mineral for many high-field strength elements (HFSE), particularly Zr and Hf. Thus, its solubility in geologic fluids at high pressure and temperature plays an important role in terrestrial cycling of these elements during processes in the Earth's crust and mantle. We performed in-situ high-pressure, high-temperature measurements of zircon solubility in H2O–Na2Si3O7, H2O–Na2Si3O7+Al2O3, H2O–Na2Si2O5, H2O–NaAlSi3O8 fluids, as well as of baddeleyite solubility in H2O–NaOH fluids, by in-situ synchrotron radiation X-ray fluorescence analysis using hydrothermal diamond-anvil cells. Zirconium complexation in fluids in equilibrium with zircon was constrained by in-situ X-ray absorption near-edge structure (XANES) spectroscopy. Zircon solubility is strongly enhanced by addition of Na2Si3O7 to H2O. The degree of enhancement increases with Na2Si3O7 concentration. The Zr content of fluids containing 10wt% Na2Si3O7 reached up to 86±2ppm Zr at 550°C and 400MPa. At 30wt% Na2Si3O7, the maximum Zr concentration was 997±6ppm at 600°C and 440MPa. Zircon solubility in these fluids decreases considerably with pressure and increases slightly with temperature. Addition of Al2O3 decreases the zircon solubility. In H2O–NaAlSi3O8 fluids, the Zr concentrations are in the sub-ppm to ppm range. Zr concentrations in NaOH solutions in equilibrium with baddeleyite reached up to 390±2ppm at 600°C and 930MPa and increase with pressure and temperature. In-situ XANES spectra collected on Zr in H2O–Na2Si3O7, H2O–Na2Si3O7+Al2O3, H2O–NaOH, and H2O–HCl fluids in equilibrium with zircon provide evidence for strong differences in the Zr complexation between these fluids. Comparison of XANES spectra to those of model compounds and ab-initio simulation of XANES spectra revealed [8]Zr for the HCl solution, [7]Zr for the NaOH solution, and [6]Zr for the Na–Al-silicate-bearing solutions. For the latter solutions, formation of alkali zircono-silicate complexes is indicated by the strong dependence of zircon solubility on Na/Al and the similarity of the XANES spectra to spectra simulated based on the local structure around Zr in the alkali zircono-silicates vlasovite and catapleite. Alkali zircono-silicate complexes are responsible for the enhancement of Zr concentrations in Na–Al-silicate-bearing solutions and very likely play an important role for mobilization of HFSE during fluid–rock interaction. Because high alkali/Al can be expected in aqueous fluids at high pressure and temperature due to incongruent dissolution of feldspar and mica, the increase of zircon solubility along the NaAlSi3O8–Na2Si3O7 join points to potentially considerable Zr or HFSE transport by silicate-bearing aqueous fluids in the lower crust and upper mantle.

The impact of igneous bedrock weathering on the Mo isotopic composition of stream waters: Natural samples and laboratory experiments

River waters have been shown to be systematically enriched in the heavy molybdenum (Mo) isotopes when compared to typical granites and basalts, which generally possess Mo isotopic compositions (δ98/95Mo) of around 0‰. This inconsistency has been used to argue against weathering of crustal rocks as the cause for heavy riverine δ98/95Mo signatures. Incongruent dissolution of primary bedrock, however, may be an important process by which the anomalous Mo signatures of the river dissolved load are produced. This study therefore investigates the effect of igneous crustal rock weathering on the aquatic δ98/95Mo signal by comparing stream water and bedrock Mo isotope data to results of bulk rock leach experiments. For this purpose, stream water and bedrock (orthogneiss, granite, basalt), as well as soil and vegetation samples were collected in a small catchment in the French Massif Central. In accordance with the results of earlier studies on riverine Mo, both streams are isotopically heavier (δ98/95Mo=0.5–1.1‰) than the typical granites and basalts. The excellent agreement of these data with those of Mo released during experimental leaching of the basalt bedrock (0.6–1.0‰) identifies a predominance of basalt weathering over the stream water Mo geochemistry, while other processes (i.e. soil formation, secondary mineral precipitation and adsorption) are subordinate in this catchment. Given that the basalt bulk rock δ98/95Mo reflects a value typical for crustal magmatic rocks (ca. 0.1‰), Mo isotope fractionation during the incongruent dissolution of basalt can explain the observed isotopically heavy aquatic Mo signatures. Laser ablation analyses demonstrate that the volumetrically minor magmatic sulfides can be highly enriched in Mo and mass balance calculations identify the sulfide melt inclusions as the principal Mo source for the leach solutions. These data suggest that the magmatic sulfides possess a distinctly heavier δ98/95Mo signature than the coexisting silicate melt. In this case, Mo would behave like Fe by showing a detectable isotope fractionation at magmatic temperatures. Incongruent crustal bedrock weathering may thus cause a preferential release of heavy Mo isotopes. This effect, however, is highly dependent on the primary bedrock mineralogy. Consequently, the average continental runoff may have been significantly affected by incongruent weathering during periods when the Earth system was exceptionally far from steady state, e.g., large glaciations with enhanced physical weathering or large subaerial basalt eruptions such as the Deccan and the Siberian plateau.

Kinetics of incongruent dissolution of carbonates in a Chalk aquifer using reverse flow modelling

s u m m a r y Water–rock interactions are major processes that control the solute transfer (from natural or anthropo-genic sources) in aquifers. In carbonate aquifers, these water–rock interactions are mainly characterised by the incongruent dissolutions of carbonates. The in situ quantification of these reactions is few studied. In the present paper, we provide a new quantification of the kinetic rates of simultaneous incongruent dissolution of composite carbonate and dolomite in a Chalk aquifer. The approach is based on a mass-balance model used to calculate the rates of mineral dissolution and precipitation along a 500 m to 4 km flow path starting at the groundwater divide line. The groundwater transfer times between the groundwater divide line and the wells are calculated thanks to flow and transport modelling, used in a backward particle tracking mode. The uncertainties on porosity, dispersivity and transmissivity are taken into account. The rates are found ranging between 1.5 Â 10 A5 and 1.5 Â 10 A4 mol C5 chalk L A1 w yr A1 for the composite carbonate dissolution, between 1.0 Â 10 A7 and 3.0 Â 10 A6 mol dolomite L A1 w yr A1 for the dolomite dissolution and between 1.0 Â 10 A5 and 1.5 Â 10 A4 mol calcite L A1 w yr A1 for the calcite precipitation. The results highlight the importance of physical and geological properties of the aquifer.

Characterization and assessment of chemical modifications of metal-bearing sludges arising from unsuitable disposal

Ettringite–gypsum sludge, formed by neutralization of acid mine drainage with lime, has been stored temporarily in the open pit of a uranium mine that floods periodically. The present study characterized samples of this sludge, named according to the time of placement as Fresh, Intermediate, and Old. Standard leaching and sequential extraction procedures assessed the associations and stabilities of U, Zn, Fe, Mn, and other contaminants in the solid phases. Corresponding mineralogical transformations associated with sludge weathering were modeled using PHREEQC. The main crystalline phases were ettringite, gypsum and calcite; the minor constituents were fluorite and gibbsite. This mineral assemblage could be attributed to the incongruent dissolution of ettringite to form gibbsite, calcite, and gypsum. Sequential extractions indicated high contents of U, Ca, SO 4, and Zn in the water-soluble (exchangeable) and carbonate fractions. Thus, the analytical and modeling results indicated that despite being classified as non-toxic by standard leaching protocols, the minerals composing the sludge could be sources of dissolved F, SO 4, Fe, Zn, Mn, U, and Al under various environmental conditions. Decommissioning strategies intended to prevent contaminant migration will need to consider the chemical stability of the sludge in various environments.

Mafic and silica-rich glasses in mantle xenoliths from Wau-en-Namus, Libya: Textural and geochemical evidence for peridotite–melt reactions

Anhydrous spinel peridotite xenoliths in Quaternary nepheline-basanite and melilite- or sodalite-bearing lavas of the Wau-en-Namus volcano in S Libya range from lherzolites to harzburgites recording melt extraction in a shallow setting (≤ 2 GPa). Primary clinopyroxenes have distinct trace element characteristics documenting LILE (large ion lithophile element) depletion or enrichment events predating the formation of glass pockets and veins in the xenoliths. These glasses are aluminous and alkali-rich, range in composition from ultrabasic to silicic (43–67 wt.% SiO 2) and may contain empty vugs and micro-phenocrysts of olivine, clinopyroxene, spinel, plagioclase, sodalite, apatite that are similar in composition to phenocrysts in the host lavas. Reactions of infiltrating melt and xenolith minerals are documented by diffuse Fe–Ca-rich rims of olivine in contact with glass, and by spongy-textured reaction domains caused by incongruent dissolution of primary pyroxenes and spinel. Some glasses have trace element characteristics similar to that of the host Ne-basanite, suggesting they were derived from the same source during entrainment and transport to the surface. Incompatible element enrichment and Sr–Nd isotopic compositions of the analyzed host lava are similar to HIMU (high μ; μ = 238Pb/ 204Pb)-type magmas, but the Pb isotopic composition is less radiogenic compared to other intra-plate Neogene magmatic rocks from N Africa.

Far from equilibrium enstatite dissolution rates in alkaline solutions at earth surface conditions

Far from equilibrium enstatite dissolution rates both open to atmospheric CO 2 and CO 2 purged were measured as a function of solution pH from 8 to 13 in batch reactors at room temperature. Congruent dissolution was observed after an initial period of incongruent dissolution with preferential Si release from the enstatite. Steady-state dissolution rates in open to atmospheric CO 2 conditions decrease with increase in solution pH from 8 to 12 similar to the behavior reported by other investigators. Judging from the pH 13 dissolution rate, rates increase with pH above pH 12. This is thought to occur because of the increase in overall negative surface charges on enstatite as Mg surface sites become negative above pH 12.4, the pH of zero surface charge of MgO. Steady-state dissolution rates of enstatite increase above pH 10 when CO 2 was purged by performing the experiments in a N 2 atmosphere. This suggests inhibition of dissolution rates above pH 10 when experiments were open to the atmosphere. The dissolved carbonate in these solutions becomes dominantly CO 3 2− above pH 10.33. It is argued that CO 3 2− forms a >Mg 2–CO 3 complex at positively charged Mg surface sites on enstatite, resulting in stabilization of the surface Si–O bonds. Therefore, removal of solution carbonate results in an increase in dissolution rates of enstatite above pH 10. The log rate of CO 2-purged enstatite dissolution in moles per cm 2 per s as a function of increasing pH above pH 10 is equal to 0.35. This is consistent with the model of silicate mineral dissolution in the absence of surface carbonation in alkaline solutions proposed earlier in the literature.

Deciphering interaction of regional aquifers in Southern Tunisia using hydrochemistry and isotopic tools

► We examine the origin and chemical evolution of groundwater in southern Tunisia. ► Contribution of water from the deep CI aquifer to Turonian formation was demonstrated. ► Influence of geogenic carbon dioxide in the studied groundwater system was shown. ► The work aids management strategies of scarce groundwater resources in Tunisia. Groundwater is the most important source of water supply in southern Tunisia. Previous hydrogeologic and isotopic studies carried out in this region revealed the existence of two major aquifer systems: the “Complex Terminal” (CT) and the “Continental Intercalaire” (CI). Turonian carbonates constitute one of the major aquifer levels of the CT multilayered aquifer. It extends over most of southern Tunisia, and its hydrodynamic regime is largely influenced by tectonics, lithology and recharge conditions. Forty-eight groundwater samples from the CI and Turonian aquifers were collected between January and April 2004 for chemical and isotopic analyses. Hydrochemistry and isotopic tools were combined to get an insight into the processes controlling chemical composition of groundwater and wide-scale interaction of these two aquifer systems. Analysis of the dissolved constituents revealed that several processes control the observed chemical composition: (i) incongruent dissolution of carbonate minerals, (ii) dissolution of evaporitic minerals, and (iii) cation exchange. Dissolution alone cannot account for the observed high supersaturation states of groundwater with respect to calcite and dolomite. The observed supersaturation is most probably linked to geogenic CO 2 entering water-bearing horizons of the CT and CI aquifers via deep tectonic faults and discontinuities and subsequent degassing in the exploitation wells. Presence of geogenic CO 2 in the investigated region was confirmed by C isotope data of the DIC reservoir. The radiocarbon content of the Turonian samples varied between 9.5 and 43 pmc. For CI samples generally lower values were recorded, between 3.8 and 22.5 pmc. Stable isotope composition of Turonian groundwater samples varied from −8.3 to −5.3‰ for δ 18 O and from −60 to −25‰ for δ 2 H. The corresponding ranges of δ values for the Continental Intercalaire samples were from −8.9‰ to −6.9‰ for δ 18 O and from −68.2‰ to −45.7‰ for δ 2 H. Stable isotope composition of groundwater representing CT and CI aquifers provide strong evidence for regional interaction between both systems.

Acidic dissolution behavior of U containing ZrO 2–MgO ceramics

This study explores the possibility of dissolving zirconia–magnesia inert matrix fuel containing uranium oxide as a fissile material and plutonium homolog and erbium oxide as a burnable poison with nitric and sulfuric acid as a potential first step in a reprocessing scheme. The progress of the dissolution is followed by monitoring the amount of material in solution by inductively coupled plasma-atomic emission spectroscopy, assessing the speciation of the material by time resolved laser fluorescence spectroscopy, and determining and quantifying the crystalline phases present in the remaining residue by X-ray diffraction. This study has shown a linear incongruent dissolution of the cubic zirconia phase in concentrated nitric acid under certain chemical compositions, while the magnesium oxide phase is completely soluble. In sulfuric acid uranium, erbium, and magnesium are soluble to different extents while zirconium forms a colloidal suspension that conglomerates and settles out of solution. The feasibility of the dissolution of zirconia–magnesia inert matrix fuel with nitric and sulfuric acid for reprocessing is discussed.

Relative stability and significance of dawsonite and aluminum minerals in geologic carbon sequestration

[1] Computer simulations predict dawsonite, NaAlCO3(OH)2, will provide long-term mineral sequestration of anthropogenic CO2 whereas dawsonite rarely occurs in nature or in laboratory experiments that emulate a carbon repository. Resolving this discrepancy is important to determining the significance of dawsonite mineralization to the long-term security of geologic carbon sequestration. This study is an equilibrium-based experimental and modeling evaluation of underlying causes for inconsistencies between predicted and observed dawsonite stability. Using established hydrothermal methods, 0.05 molal NaHCO3 aqueous solution and synthetic dawsonite were reacted for 18.7 days (449.2 hours) at 50°C, 20 MPa. Temperature was increased to 75°C and the experiment continued for an additional 12.3 days (295.1 hours). Incongruent dissolution yielded a dawsonite-gibbsite-nordstrandite assemblage. Geochemical simulations using Geochemist's Workbench and the resident database thermo.com.V8.R6+ incorrectly predicted a dawsonite-diaspore assemblage and underestimated dissolved aluminum by roughly 100 times. Higher aqueous aluminum concentrations in the experiment suggest that dawsonite or diaspore is less stable than predicted. Simulations employing an alternate database, thermo.dat, correctly predict dawsonite and dawsonite-gibbsite assemblages at 50 and 75°C, respectively, although dissolved aluminum concentrations are still two to three times lower than experimentally measured values. Correctly reproducing dawsonite solubility in standard geochemical simulations requires an as yet undeveloped internally consistent thermodynamic database among dawsonite, gibbsite, boehmite, diaspore, aqueous aluminum complexes and other Al-phases such as albite and kaolinite. These discrepancies question the ability of performance assessment models to correctly predict dawsonite mineralization in a sequestration site.

Major-ion chemistry, δ13C and 87Sr/86Sr as indicators of hydrochemical evolution and sources of salinity in groundwater in the Yuncheng Basin, China

Processes controlling hydrogeochemistry in the Yuncheng Basin, China, were characterised using major-ion chemistry, 87Sr/86Sr ratios and δ13C values. Evapotranspiration during recharge increased solute concentrations by factors of ∼5–50 in deep palaeowaters, while higher degrees of evapotranspiration have occurred in shallow, modern groundwater. Aquifer sediments (loess) contain approximately 15 weight% calcite; trends in groundwater HCO3 concentrations and δ13C values (ranging from −16.4 to −8.2‰) indicate that carbonate weathering is a significant source of DIC. Groundwater 87Sr/86Sr ratios (0.7110–0.7162, median of 0.7116) are similar to those in both loess carbonate (0.7109–0.7116) and local rainfall (0.7112), and are significantly lower than Sr in aquifer silicates (0.7184–0.7251). Despite evidence for substantial carbonate dissolution, groundwater is generally Ca-poor (< 10% of total cations) and Na-rich, due to cation exchange. Saturation with respect to carbonate minerals occurs during or soon after recharge (all calcite and dolomite saturation indices are positive). Subsequent carbonate dissolution in the deep aquifer must occur as a second-stage process, in response to Ca loss (by ion exchange) and/or via incongruent dissolution of dolomite and impure calcite. The latter is consistent with positive correlations between δ13C values and Mg/Ca and Sr/Ca ratios (r2 = 0.32 and 0.34).

Mineralogy and geochemistry of Al-hydroxide/oxyhydroxide mineral-bearing coals of Late Paleozoic age from the Weibei coalfield, southeastern Ordos Basin, North China

Abstract This paper mainly describes mineralogy and geochemistry of coals from the Weibei coalfield in the southeastern Ordos Basin, North China. A number of Al-hydroxide/oxyhydroxide minerals were detected in the Late Carboniferous coals (Nos. 5, 10 and 11 coals), especially in the No. 10 coal. Aluminum-hydroxide/oxyhydroxide minerals (nordstrandite, boehmite and diaspore) in the No. 10 coal are associated with kaolinite, suggesting that these minerals are derived from the breakdown of kaolinite. A model in which Al-hydroxide/oxyhydroxide minerals form from the incongruent dissolution of kaolinite is presented. Nordstrandite and boehmite mainly occur as massive lenses ( n = 2) mainly occurs within Al-hydroxide/oxyhydroxide minerals (nordstrandite, boehmite, and diaspore), kaolinite and organic matter.

Stability of pyrochlores in alkaline matrices: Solubility of calcium antimonate

The stability of pyrochlores as a function of solution composition is relevant in the context of hazardous waste immobilisation as this mineral family comprises minerals that can control actinide solubility in ceramic formulations as well as romeites or calcium antimonates that possibly control the solubility of Sb in cement pastes. However, to date, no thermodynamic model exists that can be used for long-term risk assessment of pyrochlores as stabilising minerals. In this work synthetic romeites were precipitated at pH 6.5 and pH 12 and at varying molar Ca:Sb ratio and analysed by XRD and Rietveld refinement which showed that, like naturally occurring romeites, synthesized romeites have a Ca1+xSb2O6OH2−2x structure with less vacancies and a higher Ca-content as pH and Ca-availability were higher during synthesis. These romeites dissolve incongruently with preferential Ca leaching and antimonate is less soluble from romeites synthesized in alkaline conditions. Batch extractions of romeite in equilibrium with increasing Ca(NO3)2 additions up to 1 mol L−1 showed a minimum equilibrium Sb concentration at [Ca] = 0.01 mol L−1 that could only be explained by assuming the formation of a CaSb(OH)6+ complex. Increasing the Ca concentration in romeite suspensions also resulted in a pH decline in the equilibrium solution, which may be explained by the increasing concomitant uptake of Ca2+ and OH− in the romeite structure as a function of increasing Ca(NO3)2 additions. Thermodynamic modelling supported these assumptions that may explain the incongruent dissolution behaviour of romeite, but the fitting of two equilibrium constants was required. Although the validity of these constants needs more experimental confirmation, this study suggests that despite possible CaSb(OH)6+ formation, the solubility of Ca antimonate in alkaline conditions can be responsible for lower equilibrium Sb concentrations than previously assumed.

Study on mineral surface reacted with water at temperatures above 300°C and 23 MPa

In this work, we carried out experiments on silicate mineral dissolution using a flow-through reactor from 20 to 400°C at 23 MPa. The dissolution of silicate minerals such as actinolite, diopside, and albite in water may require the breaking of more than one metal–oxygen bond type. Different metal elements in silicate minerals have different release rates and the dissolution product is often non-stoichiometric. Na, Mg, Fe, and Ca dissolve faster than Si at T < 300°C. At T ≥ 300°C, the release rate of Si is higher than that of the other metals. The molar concentration ratios of the dissolving metal Mi versus Si such as Mg/Si, Ca/Si and Al/Si, which are the release ratios in the effluent solutions, are often different from the molar ratios of these elements in the minerals. The results show that the incongruent dissolution of minerals is related to surface chemical modifications. Non-stoichiometric dissolution is caused by the formation of a non-stoichiometric leaching layer at the surface or by the presence of a secondary mineral at the surface. Our experiments indicate that the dissolution of most silicate minerals is close to stoichiometric at 200 and 300°C, e.g., for actinolite and albite at 300°C. The surfaces after reaction with aqueous solutions were investigated using SEM and TEM. At T < 300°C, the mineral surfaces (e.g., for actinolite) after the reaction with water are slightly Si-rich and slightly Fe (and/or Mg, Ca) deficient. In contrast, at T ≥ 300°C, the surfaces after reaction with water are slightly Fe-rich and somewhat Si deficient.

Impact of nano-size weathering products on the dissolution rates of primary minerals

The natural weathering rates of primary minerals are often orders of magnitude lower than the rates of mineral dissolution measured in laboratory experiments. Primary dissolution rates are thought to be determined by the rate of secondary mineral precipitation, and in this paper we present a new approach to quantify the role played by interfacial energy, crystal size, and degree of supersaturation on precipitation kinetics in a population of crystals growing in a supersaturated fluid. We demonstrate that net mineral precipitation rates in systems that are close to equilibrium, and which possess a large number of micron and nanometer scale crystals, can be much lower than the rates predicted by standard kinetic equations. Moreover, when crystals are small enough, net dissolution dominates even when the system is supersaturated with respect to large crystals so that the standard reaction rate models used to describe bulk rates will no longer apply. Importantly, secondary minerals that form from the incongruent dissolution of primary phases are often submicron in size and field conditions are often far closer to equilibrium than those typically encountered in laboratory experiments. Thus, we propose that standard kinetic models – which ignore interfacial energy effects in small crystals – may be unsuitable to describe reaction kinetics in weathering systems.

Tracing the origin of water and solute transfers in deep groundwater from Oxfordian, Dogger and Trias formations in the east of the Paris Basin – France

In order to assess the feasibility for long lived radioactive wastes storage facilities in deep geological formation, solute transport processes must be investigated in the vicinity of the host formation. In France, the Oxfordian and Dogger limestone layers surrounding the Callovo-Oxfordian (CO x ) argillite in the east of the Paris Basin are investigated for this purpose. More than 60 samples of Oxfordian and Dogger formation groundwater and one sample of Triassic formation groundwater, located at the bottom section of the investigated sedimentary cover, were collected over a 250 km 2 area, and were analysed for major ions, δ 18O and δ 2H of water, 87Sr/ 86Sr, and δ 34S and δ 18O of dissolved sulphate. Oxfordian and Dogger formation water is from meteoric origin, and no direct water flow between Oxfordian, Dogger, nor Triassic formations was evidenced. Mineralization processes of the Dogger limestone groundwater were fully investigated. These processes correspond to a series of geochemical reactions including: calcite dissolution, incongruent dissolution of dolomite, ion addition from upward vertical diffusion through the sedimentary pile inducing cation exchange and further dissolution due to the increased ionic strength of the solutions. A PHREEQC model was developed to simulate inferred chemical processes. The model output is consistent with observed data.

Prolonged toxicity characteristic leaching procedure for nickel and copper aluminates

The toxicity characteristic leaching procedure (TCLP) is a regulatory testing method widely employed to evaluate the environmental friendliness of waste materials. TCLP analysis provides a fast, easy and economical way to determine the mobility of waste pollutants under simulated landfill conditions. Recent studies on metal stabilization have reported the potential for nickel and copper aluminates to form in thermal treatment conditions, and suggested a more reliable method of stabilizing hazardous metals, particularly when products are to be reused. There is thus an urgent need for a convenient and effective method of quantifying metal leachability and identifying the metal leaching behavior of sparingly soluble materials. In this study, standard TCLP analysis was modified into a prolonged leaching experiment to investigate the leaching behavior of nickel and copper oxides (NiO and CuO) and their aluminates (NiAl(2)O(4), CuAl(2)O(4) and CuAlO(2)). The results demonstrate the difficulty of differentiating the leachability of highly insoluble phases, such as NiO and NiAl(2)O(4), using the standard TCLP. The prolonged TCLP method, however, confirmed NiAl(2)O(4) to have a lower degree of intrinsic leachability than NiO and that it could be expected to undergo congruent dissolution under landfill conditions. For the more soluble copper system, the aluminates were still found to possess a much lower degree of leachability, and their leaching behavior to follow an incongruent dissolution pattern. The results of this study prove prolonged TCLP analysis to be a convenient and effective way to evaluate the environmental friendliness of metal waste and to identify the leaching behavior of waste materials.

Environmental Tritium ( 3H) and hydrochemical investigations to evaluate groundwater in Varahi and Markandeya river basins, Karnataka, India

The present study aimed at assessing the activity of natural radionuclides ( 3H) and hydrochemical parameters (viz., pH, EC, F −, NO 3 −, Cl −, Ca 2+, Mg 2+) in the groundwater used for domestic and irrigation purposes in the Varahi and Markandeya river basins to understand the levels of hydrochemical parameters in terms of the relative age(s) of the groundwater contained within the study area. The recorded environmental 3H content in Varahi and Markandeya river basins varied from 1.95 ± 0.25 T.U. to 11.35 ± 0.44 T.U. and 1.49 ± 0.75 T.U. to 9.17 ± 1.13 T.U. respectively. Majority of the samples in Varahi (93.34%) and Markandeya (93.75%) river basins being pre-modern water with modern recharge, significantly influenced by precipitation and river inflowing/sea water intrusion. The EC-Tritium and Tritium–Fluoride plots confirmed the existence of higher total dissolved solids (SEC > 500 μS/cm) and high fluoride (MAC > 1.5 mg/L) in groundwater of Markandeya river basin, attributed to relatively longer residence time of groundwater interacting with rock formations and vice versa in case of Varahi river basin. The tritium-EC and tritium–chloride plots indicated shallow and deep circulating groundwater types in Markandeya river basin and only shallow circulating groundwater type in Varahi river basin. Increasing Mg relative to Ca with decreasing tritium indicated the influence of incongruent dissolution of a dolomite phase. The samples with high nitrate (MAC > 45 mg/L) are waters that are actually mixtures of fresh water (containing very high nitrate, possibly from agricultural fertilizers) and older ‘unpolluted’ waters (containing low nitrate levels), strongly influenced by surface source.

Dissolution of nano-size Mg–Al–Cl hydrotalcite in aqueous media

Aqueous dissolution of a synthetic chloride-containing Mg2+–Al3+ layered double hydroxide was studied at 298K as a function of pH. Mild acidification (final pH 9 –5) results in incongruent dissolution of Mg2+, exclusively. Formation of an amorphous Al(OH)3 passive layer prevents further Mg2+ leaching, thus dissolved Mg2+ amounts for only 20% of the solid. At lower pH values, massive dissolution readily takes place in a congruent fashion. Congruent dissolution profiles are accounted for by a contracting volume kinetic law. The pH dependence of the observed rate constant shows that the kinetic order on proton concentration is 0.41, indicating that dissolution is surface controlled. The implications of these findings are discussed.

Groundwater origins and mixing pattern in the multilayer aquifer system of the Gafsa-south mining district: a chemical and isotopic approach

Major ion geochemistry and environmental isotopes were used to identify the origins and the mineralisation processes of groundwater flowing within the three aquifer levels of the multilayer system of the Gafsa-south mining district (Southwestern Tunisia). It has been demonstrated that groundwaters are characterised by a Ca–Mg–SO4 water type. Geochemical pattern is mainly controlled by the dissolution of halite, gypsum and/or anhydrite as well as by the incongruent dissolution of dolomite. δ18O and δ2H values are much lower than the isotopic signature of regional precipitation and fall close to the meteoric water lines, indicating that groundwaters have not been significantly affected by evaporation or mineral–water reactions. The distribution of stable and radiogenic isotopes (δ18O, δ2H, δ13C and 14C) within the aquifer levels suggests that the deep confined aquifer receives a significant modern recharge at higher altitudes, while, the shallow unconfined aquifer has been mainly recharged under cooler paleoclimatic condition, likely during Late Pleistocene and Early Holocene humid periods. However, waters from the intermediate confined/unconfined aquifer have composite isotopic signatures, highlighting that they are derived from a mixture of the two first end-members.

Geochemistry of surface waters associated with an undisturbed Zn–Pb massive sulfide deposit: Water–rock reactions, solute sources and the role of trace carbonate

We report results of a detailed geochemical study of surface waters from several streams in a small catchment in northern New Brunswick, Canada. Rocks in the catchment represent metamorphosed (greenschist facies) felsic volcanic rocks, metavolcaniclastic sediments, and mafic volcanic rocks; no massive carbonate or evaporite lithologies are present, thus providing an excellent opportunity to investigate the relative influence of silicate weathering compared to trace carbonate (vein and disseminated) dissolution and the influence of volcanogenic massive sulfide (VMS) mineralization on surface water chemistry. Surface waters, catchment lithologies and stream sediments were analyzed for a full suite of major and trace elements. Surface waters are dilute (typically < 60 mg/L total dissolved solids), and are of dominantly Ca–HCO 3-type. Most waters have Ca/Na molar > 1. The major ion chemistry of the waters is consistent with binary mixing between silicate weathering and dissolution of trace calcite; Si/Ca molar relationships suggest that trace calcic silicates are insignificant compared to calcite as a Ca source, and PO 4 concentrations of waters are too low for apatite to be a major source of Ca. Host rocks have highly variable Ca/Na, Mg/Na and K/Na values complicating the assignment of a silicate end-member. Geochemical modeling indicates that surface waters range from essentially 100% of the Na and Ca being derived by silicate weathering, to being dominantly controlled by trace calcite, also consistent with mass balance calculations. The Cl/Ca ratios are consistent with the host felsic and mafic metavolcanic rocks as being an important source of Cl in addition to precipitation. The host lithologies have much larger variations in Mg/Na, Ca/Na, K/Na, and Sr/Na than the stream sediments and waters and the sediments are shifted to higher Mg/Na and K/Na and lower Ca/Na than the waters. It is remarkable that the waters have such small variations in major ion ratios relative to the host rocks, indicating that the controls on solute loads of these streams is more a function of relative elemental solubility (Ca > Mg > Na), secondary mineral formation (e.g. Mg- and K-rich clays), incongruent dissolution, and water–rock reactions than end-member rock compositions. Oxidation of massive and disseminated sulfide mineralization accounts for on average 60% of the dissolved sulfate. Calculations indicate that sulfide oxidation (sulfuric acid) weathering accounts for around 20% of the cation flux in the watershed. On average 12% of the Ca and 72% of the Mg are derived from silicate weathering, although cationic silicate denudation rates are only 1.88 tonnes/km 2/year compared to 4.16 tonnes/km 2/year for trace carbonate dissolution. The total cation denudation rate and CO 2 consumption rate are similar to watersheds draining volcanic rocks in the Western Canadian Cordillera.