Dissolution kinetics of dolomite: Effects of lithology and fluid flow velocity

The dissolution kinetics of three stoichiometric dolomite specimens (hydrothermal single crystal, microcrystalline sedimentary rock, coarse-grained marble) were studied in aqueous carbonate solutions. Hydrodynamic conditions were controlled through use of a rotating dolomite disk in which one face was exposed to solution and fluid flow regime was defined by spinning rate. The resulting mass transfer properties were uniform across the disk surface. The dissolution experiments were begun at an initially undersaturated condition set by CO₂ at ~ 1 atm dissolved in deionized water. The reaction was followed by measuring concentrations of Ca²⁺, Mg²⁺, HCO₃^?, and pH over time in a free-drift type of experiment at 0, 15, and 25°C.Dissolution rates for all three samples were similar in form and value; grain size effects were insignificant. Ca/Mg was constant throughout each run at 0.81–0.96. From initial conditions, the dissolution rate decreased as the solution became more saturated. At solution conditions still far from equilibrium (ion activity product = 10^?19), rate dropped off sharply to a very low value. Surface morphology, determined by SEM, showed deep narrow holes in the single crystal, while the rocks dissolved along grain boundaries. These features suggested preferential dissolution of energetically favored sites and surface reaction rate control. Initial rates were used to calculate an apparent activation energy of 32 kJ mol^?1 (sedimentary dolomite) and 27 kJ mol^?1 (single crystal).Initial dissolution rates at 25°C and pH ~ 4 for all samples varied with spinning speed and ranged from 1–3 μmol m^?2 s^?1 for laminar flow conditions to almost 3–6 μmol m^?2 s^?1 as the transition to turbulence began. At lower temperatures, the rate was lower, and increasing spinning velocity had less effect. The strongest spinning rate dependence occurred far from equilibrium, and it became a less important factor as the saturation state increased.     

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.     

Mechanism of pyroxene and amphibole weathering—I. Experimental studies of iron-free minerals

The short term (2–40 days) dissolution of enstatite, diopside, and tremolite in aqueous solution at low temperatures (20–60°C) and pH 1–6 has been studied in the laboratory by means of chemical analyses of reacting solutions for Ca²⁺, Mg²⁺, and Si(OH)₄ and by the use of X-ray photoelectron spectroscopy (XPS) for detecting changes in surface chemistry of the minerals. All three minerals were found to release silica at a constant rate (linear kinetics) providing that ultrafine particles, produced by grinding, were removed initially by HF treatment. All three also underwent incongruent dissolution with preferential release of Ca and/or Mg relative to Si from their outermost surfaces. The preferential release of Ca, but not Mg for diopside at pH 6 was found by both XPS and solution chemistry verifying the theoretical prediction of greater mobility of cations located in M₂ structural sites. Loss mainly from M₂ sites also explains the degree of preferential loss of Mg from enstatite at pH 6; similar structural arguments apply to the loss of Ca and Mg from the surface of tremolite. In the case of diopside and tremolite initial incongruency was followed by essentially congruent cation-plus-silica dissolution indicating rapid formation of a constant-thickness, cation-depleted surface layer. Cation depletion at elevated temperature and low pH (~ 1) for enstatite and diopside was much greater than at low temperature and neutral pH, and continued reaction resulted in the formation of a surface precipitate of pure silica as indicated by solubility calculations, XPS analyses, and scanning electron microscopy.From XPS results at pH 6, model calculations indicate a cation-depleted altered surface layer of only a few atoms thickness in all three minerals. Also, lack of shifts in XPS peak energies for Si, Ca, and Mg, along with undersaturation of solutions with respect to all known Mg and Ca silicate minerals, suggest that cation depletion results from the substitution of hydrogen ion for Ca²⁺ and/or Mg²⁺ in a modified silicate structure and not from the precipitation of a new, radically different surface phase. These results, combined with findings of high activation energies for dissolution, a non-linear dependence on a_H⁺ for silica release from enstatite and diopside, and the occurrence of etch pitting, all point to surface chemical reaction and not bulk diffusion (either in solution or through altered surface layers) as the rate controlling mechanism of iron-free pyroxene and amphibole dissolution at earth surface temperatures.     

Transient and quasi-steady-state dissolution of biotite at 22-25°C in high pH, sodium, nitrate, and aluminate solutions

Biotite dissolution under conditions of high pH and high aluminum, sodium, and nitrate concentrations analogous to those found in tank wastes at the Hanford Site was investigated using continuously stirred flow-through reactors at 22 to 25 °C. Experiments were designed to simulate tank leaks into the Hanford vadose zone where Fe(II) from biotite is the dominant reducing agent available to immobilize certain contaminants. Both non-steady-state and steady-state dissolution kinetics were quantified; interest in non-steady-state kinetics derives from the inherently transitory nature of tank leaks. Biotite was conditioned in pH 8 solutions to simulate the alkaline environment of the Hanford sediment, and then reacted in pH 10-14 solutions, some including 0.055 M Al(NO₃)₃ and/or 2 M or 6 M NaNO₃. Initial dissolution transients (intervals of rapid release rates that decay to slower steady-state rates) showed fast preferential release of K followed by near-stoichiometric release of Si, Al, and Mg, and slower release of Fe. Each increase in pH resulted in a second transient with the greatest amounts of Si, Al, and K released at pH 14, followed by pHs 13, 12, 11, and 10. Fe release also was highest at pH 14, but unchanging at pHs 10-13 within experimental error. Transient releases at high pH are attributed to dissolution of amphoteric secondary phases such as ferrihydrite that are inferred from saturation calculations and solid analyses to form during the conditioning interval. Transient release of Si was inhibited by the presence of 0.055 M Al(NO₃)₃; the effects of Al(NO₃)₃ and NaNO₃ on the release rates of Al, Fe, Mg, and K were variable and generally outweighed by the effect of pH. Quasi-steady-state release rates were slowest at pH 11-12 (10^−12.2 mol biotite m⁻² s⁻¹ for Si) and increased in either direction in pH away from this minimum (to 10^−11.5 at pHs 8 and 14 for Si). Fe release rates at high pH were sufficient to account for observed Cr(VI) reduction at Hanford. The net release rates of the major framework cations, from which the biotite dissolution rate is inferred, may reflect the precipitation of secondary phases or the alteration of biotite to vermiculite. The most extensive solid-phase alterations were observed in Na-enriched solutions.     

Oxygen isotope exchange rate between dissolved sulfate and water at hydrothermal temperatures

Oxygen isotope exchange rate between dissolved sulfate and water was experimentally determined at 100, 200 and 300°C. The isotope exchange rate is strongly dependent on temperature and pH of the solution. Combining the temperature and pH dependence of the reaction rate, the exchange reaction was estimated to be first-order with respect to sulfate. The logarithm of apparent rate constant of exchange reaction at a given temperature is a function of the pH calculated at the experimental temperatures. From the pH dependence of the apparent rate constant, it was deduced that the isotope exchange reaction between dissolved sulfate and water proceeds through collision between H₂SO⁰₄ and H₂O at low pH, and between HSO^?₄ and H₂O at intermediate pH. The isotope exchange rate obtained indicates that oxygen isotope geothermometry utilizing the studied isotope exchange is suitable for temperature estimation of geothermal reservoirs. The extrapolated half-life of this reaction to oceanic temperature is about 10⁹ years, implying that exchange between oceanic sulfate and water cannot control the oxygen isotope ratio of oceanic sulfates.     

An evaluation of rate equations for calcite precipitation kinetics at pCO2 less than 0.01 atm and pH greater than 8

We used a reproducible seeded growth technique with a pH-stat to study the kinetics of calcite precipitation at 25°C. We performed different experiments at initial Ca²⁺ and HCO₃^? concentrations ranging from 0.7–2 and 4–7 mmol L^?1, pH values ranging from 8.25 to 8.70, $\text{pCO}{}_2$ values ranging from 0.0006 to 0.01 atm, and ionic strengths ranging from 0.015 to 0.10 mol L^?1. With this experimental data set, we used initial rate measurements and integral methods to test several precipitation rate equations. Rate equations that possess a disequilibrium functional dependence, such as the BURTON et al. (1951) dislocation model, forms of the Davies and Jones (1955) model, and the model used by Reddy and Nancollas (1973), did not adequately describe the kinetics of calcite precipitation at pH greater than 8 and $\text{pCO}{}_2$ less than 0.01 atm. Rate equations that describe independent dissolution and precipitation mechanisms with elementary reactions, such as the equation presented by Plummeret al. (1978), and nancollas and Reddy (1971) were more successful. However, Plummer's model did not adequately describe the rate of all experiments due to the presence of an OH^? surface term in the precipitation rate equation. The elementary reaction of the Nancollas and Reddy model is written in terms of bulk Ca²⁺ and CO₃^? concentrations, and appears to be the most successful model which describes calcite precipitation at pH > 8 and $\text{pCO}{}_2\text{< 0.01}$ atm. The Nancollas and Reddy model, altered to account for varying ionic strengths, adequately described the rate of all experiments and yielded a precipitation rate constant of $\text{118.2 ± 13.9}$ dm⁶ mol^?1 m^?2 s^?1, with an apparent Arrhenius activation energy of 48.1 kJ mol^?1.     

Assessment of groundwater quality with special reference to arsenic in Nawalparasi district,Nepal using multivariate statistical techniques

Groundwater is a precious resource for humankind not only in Nepal but also across the globe due to its diverse functions. A total of 48 groundwater samples were collected from three villages of Nawalparasi district, Nepal, during pre-monsoon and monsoon to estimate the overall groundwater quality and to identify the sources of contamination with emphasis on arsenic (As). The average concentrations of all tested groundwater quality parameters (temp., pH, EC, ORP, Ca²⁺, Mg²⁺, Na⁺, K⁺, Cl^?, F^?,SO₄ ^2?, PO₄ ^3?, HCO₃ ^?, NO₃ ^?, Cu, Ni, Mn, Cd, Pb, Fe, Zn, Cr, and As) were well within permissible limits of WHO for drinking water, except for Ni, Cd, Pb, Cr, and As. Concentration of As ranged from 60 to 3,100 μg L^?1 and 155 to 1,338 μg L^?1 in pre-monsoon and monsoon, respectively. The Piper diagram of the groundwater chemistry showed groundwater of Nawalparasi belongs to Ca–Mg–HCO₃ and Mg–HCO₃ water type with HCO₃ ^? as dominant ions. As content in the study area was negatively correlated with Fe in pre-monsoon, while it was positively correlated in monsoon. Furthermore, As was negatively correlated with oxidation reduction potential suggesting reducing condition of groundwater. Principal component analysis revealed seven major factors that explained 81.996 and 83.763 % of total variance in water quality in pre-monsoon and monsoon, respectively. The variance of water quality was related mainly with the degree of water–rock interaction, mineralization, and anthropogenic inputs.     

Preliminary groundwater quality assessment in the central region of Ghana

Insufficient knowledge of the hydrogeochemistry of aquifers in the Central Region of Ghana has necessitated a preliminary water quality assessment in some parts of the region. Major and minor ions, and trace metal compositions of groundwater have been studied with the aim of evaluating hydrogeochemical processes that are likely to impair the quality of water in the study area. The results show that groundwater in the area is weakly acidic with mean acidity being 5.83 pH units. The dominant cation in the area is Na, followed by K, Ca, and Mg, and the dominant anion is Cl^?, followed by HCO₃ ^? and SO₄ ^2?. Two major hydrochemical facies have been identified as Na–Cl and Na–HCO_3, water types. Multivariate statistical techniques such as cluster analysis (CA) and factor analysis/principal component analysis (PCA), in R mode, were employed to examine the chemical compositions of groundwater and to identify factors that influenced each. Q-mode CA analysis resulted in two distinct water types as established by the hydrochemical facies. Cluster 1 waters contain predominantly Na–Cl. Cluster 2 waters contain Na–HCO₃ and Na–Cl. Cluster 2 waters are fresher and of good quality than cluster 1. Factor analysis yielded five significant factors, explaining 86.56% of the total variance. PC1 explains 41.95% of the variance and is contributed by temperature, electrical conductivity, TDS, turbidity, SO₄ ^2?, Cl^?, Na, K, Ca, Mg, and Mn and influenced by geochemical processes such as weathering, mineral dissolution, cation exchange, and oxidation–reduction reactions. PC2 explains 16.43% of the total variance and is characterized by high positive loadings of pH and HCO₃ ^?. This results from biogenic activities taking place to generate gaseous carbon dioxide that reacts with infiltrating water to generate HCO₃ ^?, which intend affect the pH. PC3 explains 11.17% of the total variance and is negatively loaded on PO₄ ^3? and NO₃ ^? indicating anthropogenic influence. The R-mode PCA, supported by R-mode CA, have revealed hydrogeochemical processes as the major sources of ions in the groundwater. Factor score plot revealed a possible flow direction from the northern sections of the study area, marked by higher topography, to the south. Compositional relations confirmed the predominant geochemical process responsible for the various ions in the groundwater as mineral dissolution and thus agree with the multivariate analysis.     

Reactivity of organic-rich sediment in seawater at 350°c, 500 bars: experimental and theoretical constraints and implications for the guaymas basin hydrothermal system

Hydrothermal alteration of organic-rich diatomaceous sediment by seawater was modelled experimentally at 350°C, 500 bars and seawater/sediment mass ratio of 3. The experiment was performed to assess the effect of organic matter reactivity on solution speciation and sediment alteration processes at an elevated temperature and pressure and provide requisite data to better understand the chemistry of hydrothermal fluids issuing from vents in the Guaymas Basin, Gulf of California.Seawater chemistry changed greatly during the experiment. In particular, Na, Mg and SO₄ decreased, while ∑ CO₂, ∑ NH₃, ∑ H₂S, SiO₂, Ca, K, H₂, CH₄ and heavy and base metals increased. Moreover, owing to the thermal alteration of sediment organic matter, organic acids, phenolic derivatives and phthlate were released to solution. Examination of solid alteration products revealed the effects of extensive dissolution and precipitation processes characterized by total elimination of diatoms and formation of cristobalite, quartz (?), pyrite, pyrrhotite, mixed layer chlorite/smectite and calcite. Plagioclase feldspar (An₄₀) recrystallized to a more albitic form owing to Na fixation and Ca cycling to calcite. A graphitic residue was also present in the products of the experiment.Mg and Na fixation reactions during the experiment generated significant H⁺, although the pH measured at 25°C was approximately 6.2. SO₄ reduction and thermal alteration and dissolution of organics, however, consume H⁺ and are chiefly responsible for the near neutral pH for the overall reaction. Speciation calculations including ammine and acetate protonation reactions give a pH at experimental conditions of approximately 5.1, while mineral solubility relations involving virtually all alteration phases require a pH of 5.57 to 5.94. A near neutral pH at experimental conditions constrains the mobility of Fe, Mn, Zn, Cu and Ni, which existed in solution as chloro-complexes. Dissolved concentrations of Pb and Al, in contrast, covaried with dissolved organics, especially acetate, suggesting organo-metallic complex formation.     

The interaction of cobalt with hydrous manganese dioxide

Adsorption of cobalt on synthetic hydrous manganese dioxide was studied as a function of pH and surface area in NaCl solutions and solutions containing sea water concentrations of Na, Ca and Mg. The amount of cobalt adsorbed increased sharply at pH 6, a significantly lower pH than that required for significant hydrolysis of Co(II) or precipitation of Co(OH)₂(S) in bulk solution. Sea water concentrations of Na, Ca and Mg have little effect on adsorption until the cobalt concentration is less than 10^?7 M.Micro-electrophoresis experiments from 1 × 10^?3 M to 1 × 10^?5 M to Co(II) show three charge reversals. The first is the pH of zero point charge of hydrous manganese dioxide. The second correlates well with the abrupt increase in adsorption at pH 6 and may reflect both specific adsorption of Co(II) and precipitation of Co(OH)₂ on the surface. The third agrees well with literature values for the pH of zero point of charge of Co(OH)₂.An adsorption isotherm was constructed for cobalt and these data were used to test the hypothesis that the enrichment of cobalt in the suspended matter of the Black Sea is due to adsorption of cobalt from sea water by manganese dioxide. The calculations indicate that adsorption is a feasible explanation for this example.     

Hydrochemical characteristics of groundwater and surface water for domestic and irrigation purposes in Vea catchment,Northern Ghana

The Vea catchment, mainly underlain by crystalline basement rocks, is located in Northern Ghana. Hydrogeochemical studies were carried out in this area with the objective of identifying the geochemical processes influencing water quality and suitability of surface and groundwater for agricultural and domestic uses. Sixty-one groundwater and four surface water samples were collected from boreholes, dams and rivers and analysed for Ca²⁺, Mg²⁺, Na⁺, K⁺, HCO₃ ^?, Cl^?, and SO₄ ^2?, Fe_tot, PO₄ ^3?, Mn_tot, NH₄ ⁺, NO₃ ^?, NO₂ ^?. In addition, pH, total dissolved solids, electrical conductivity, total hardness, turbidity, colour, salinity and dissolved oxygen were analysed. Chloro-alkaline indices 1 and 2, and characterization of weathering processes suggest that the chemistry of groundwater is dominated by the interaction between water and rocks. Cation exchange and silicate weathering are the dominant processes controlling the chemical composition of the groundwater in the area studied. Mineral saturation indices indicate the presence of at least three groups of groundwater in the Vea catchment with respect to residence time. The meteoric genesis index suggests that 86% of the water samples belong to the shallow meteoric water percolation type. The findings further suggest that the groundwater and surface water in the basin studied are mainly Ca–Mg–HCO₃ water type, regardless of the geology. Compared to the water quality guidelines of WHO, the study results on sodium absorption ratio, sodium percentage, magnesium hazard, permeability index and residual sodium carbonate indicate that groundwater and surface water in the Vea catchment are generally suitable for drinking and irrigation purposes.     

Iron release in aqueous environment by fresh volcanic ash from Mount Etna (Italy) and Popocatépetl (Mexico) volcanoes

In this study, we performed leaching experiments for timescales of hours-to-months in deionized water on fresh volcanic ash from Mt. Etna (Italy) and Popocatépetl (Mexico) volcanos to monitor Fe release as a function of ash mineral chemistry and size, with the aim of clarifying Fe release mechanisms and eventually evaluating the impact of volcanic ash on marine and lacustrine environments. To define sample mineralogy and Fe speciation, inclusive characterization was obtained by means of XRF, SEM, XRPD, EELS and Mössbauer spectroscopies. For Etna and Popocatépetl samples, glass proportions were quantified at 73 and 40%, Fe₂O₃ total contents at 11.6–13.2 and 5.8 wt%, and Fe³⁺/Fe_Tot ratios at 0.33 and 0.23, respectively. Leaching experiments showed that significant amounts of iron, ~?30 to 150 and ~?750 nmol g^?1 l^?1 for pristine Etna and Popocatépetl samples, respectively, are released within the first 30 min as a function of decreasing particle size (from 1 to 0.125 mm). The Popocatépetl sample showed a very sustained Fe release (up to 10 times Etna samples) all along the first week, with lowest values never below 400 nmol g^?1 l^?1 and a maximum of 1672 nmol g^?1 l^?1 recorded after 5 days. This sample, being composed of very small particles (average particle size 0.125 mm) with large surface area, likely accumulated large quantities of Fe-bearing sublimates that quickly dissolved during leaching tests, determining high Fe release and local pH decrease (that contributed to release more Fe from the glass) at short timescale (hours-to-days). The fractional Fe solubility (Fe_S) was 0.004–0.011 and 0.23% for Etna and Popocatépetl samples, respectively, but no correlation was found between Fe released in solution and either ash Fe content, glass/mineral ratio or mineral assemblage. Results obtained suggest that volcanic ash chemistry, mineralogy and particle size assume a relevant role on Fe release mostly in the medium-to-long timescale, while Fe release in the short timescale is dominated by dissolution of surface sublimates (formed by physicochemical processes occurring within the eruption plume and volcanic cloud) and the effects of such a dissolution on the local pH conditions. For all samples, a moderate to sustained Fe release occurred for leaching times comparable with their residence time within the euphotic zone of marine and lacustrine environments (variable from few minutes to few hours), revealing their possible contribution to increase Fe bioavailability.     

The molar volume of FeO–MgO–Fe2O3–Cr2O3–Al2O3–TiO2 spinels

We define and calibrate a new model of molar volume as a function of pressure, temperature, ordering state, and composition for spinels in the supersystem (Mg, Fe²⁺)(Al, Cr, Fe³⁺)₂O₄ ? (Mg, Fe²⁺)₂TiO₄. We use 832 X-ray and neutron diffraction measurements performed on spinels at ambient and in situ high-P, T conditions to calibrate end-member equations of state and an excess volume model for this system. The effect on molar volume of cation ordering over the octahedral and tetrahedral sites is captured with linear dependence on Mg²⁺, Al³⁺, and Fe³⁺ site occupancy terms. We allow standard-state volumes and coefficients of thermal expansion of the end members to vary within their uncertainties during extraction of the mixing properties, in order to achieve the best fit. Published equations of state of the various spinel end members are analyzed to obtain optimal values of the bulk modulus and its pressure derivative, for each explicit end member. For any spinel composition in the supersystem, the model molar volume is obtained by adding excess volume and cation order-dependent terms to a linear combination of the five end-member volumes, estimated at pressure and temperature using the high-T Vinet equation of state. The preferred model has a total of 9 excess volume and order-dependent parameters and fits nearly all experiments to within 0.02 J/bar/mol, or better than 0.5 % in volume. The model is compared to the current MELTS spinel model with a demonstration of the impact of the model difference on the estimated spinel-garnet lherzolite transition pressure.     

Growth of ringwoodite reaction rims from MgSiO3 perovskite and periclase at 22.5 GPa and 1,800 °C

The growth rate of ringwoodite reaction rims between MgSiO₃ perovskite and periclase was investigated at 22.5 GPa and 1,800 °C for 1–24 h using the Kawai-type high-pressure apparatus. The reaction was likely to proceed by a diffusion-controlled mechanism in which the dominant diffusion mechanism was grain-boundary diffusion. The reaction constant (the width of the ringwoodite reaction rim squared divided by time) determined from these experiments was between 1.3 × 10^?15 and 5.6 × 10^?15 m²/s. A Pt inert marker experiment indicated that the MgO component migrated faster than the SiO₂ component in ringwoodite. Thus, either Mg or O having the slower diffusion rate controlled the reaction. Because previous diffusion studies have shown that diffusion rates of O are slower than those of Mg, O would be a rate-controlling element for ringwoodite formation from MgSiO₃ perovskite and periclase. The growth rate appeared to be too fast to explain the observed topographic rise (~10 km) inside mantle plumes at the 660-km discontinuity.     

Metal sorption to dolomite surfaces

Potential human intrusion into the Waste Isolation Pilot Plant (WIPP) might release actinides into the Culebra Dolomite where sorption reactions will affect of radiotoxicity from the repository. Using a limited residence time reactor the authors have measured Ca, Mg, Nd adsorption/exchange as a function of ionic strength, P_CO2, and pH at 25°C. By the same approach, but using as input radioactive tracers, adsorption/exchange of Am, Pu, U, and Np on dolomite were measured as a function of ionic strength, P_CO2, and pH at 25°C. Metal adsorption is typically favored at high pH. Calcium and Mg adsorb in near-stoichiometric proportions except at high pH. Adsorption of Ca and Mg is diminished at high ionic strengths (e.g., 0.5M NaCl) pointing to association of Na⁺ with the dolomite surface, and the possibility that Ca and Mg sorb as hydrated, outer-sphere complexes. Sulfate amplifies sorption of Ca and Mg, and possibly Nd as well. Exchange of Nd for surface Ca is favored at high pH, and when Ca levels are low. Exchange for Ca appears to control attachment of actinides to dolomite as well, and high levels of Ca²⁺ in solution will decrease K_ds. At the same time, to the extent that high P_CO2s increase Ca²⁺ levels, K_ds will decrease with CO₂ levels as well, but only if sorbing actinide-carbonate complexes are not observed to form (Am-carbonate complexes appear to sorb; Pu-complexes might sorb as well. U-carbonate complexation leads to desorption). This indirect CO₂ effect is observed primarily at, and above, neutral pH. High NaCl levels do not appear to affect to actinide K_ds.     

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

Far from equilibrium enstatite dissolution rates both open to atmospheric CO₂ and CO₂ 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 CO₂ 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 CO₂ was purged by performing the experiments in a N₂ 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 CO₃²⁻ above pH 10.33. It is argued that CO₃²⁻ forms a >Mg₂-CO₃ 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 CO₂-purged enstatite dissolution in moles per cm² 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.     

Nutrient removal from activated sludge mixed liquor by wastewater protozoa in a laboratory scale batch reactor

The aim of the study was to investigate the nutrient removal rate of three wastewater protozoan isolates. The study was carried out in a laboratory-scale batch reactor for a period of 120 h. in a four batch study. Aliquot samples were withdrawn from the reactor every 24 h. for the analysis of phosphate, nitrate, nitrite, ammonia, chemical oxygen demand, dissolved oxygen and pH, using standard methods. The results obtained in the different batches among the three isolates showed PO₄ ^2? removal rate ranging from 0.04 to 0.52 mg-PO₄ ^2?/L/h. while NO₃ ^? nitrate removal rates ranged from 0.08 to 0.16 mg-NO₃ ^?/L /h. Also NO₂- and NH₃ rates were observed to range between 0.022 and 0.087 mg-NO₂ ^?/L /h. 0.05 and 0.16 mg-NH₃ ^?/L /h, respectively. For the physicochemical parameters, there was no observed COD decrease; rather there was an increase and this was irrespective of isolates and experimental batches. However, dissolved oxygen concentration decreased drastically (below 1 mg/L) at the end of each batch while pH show a decrease after an initial 24 h. period and thereafter increased. This trend was also irrespective of isolates and experimental batches. Overall, the study has been able to show the effect of the test isolates on nutrient removal rates and other physicochemical parameters (COD, DO and pH) in activated sludge mixed liquor.