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Knowledge of the rates of dissolution of rock-forming minerals close to equilibrium and their dependence on Δ G r , the Gibbs free energy of reaction, is essential for describing the temporal and spatial evolution of many environmental and engineering processes involving solid/fluid interactions. In the present study we used the isotope-doping method to quantify albite forward and net dissolution rates. Batch experiments were performed at 150, 180 °C and pH = 9 at near equilibrium and equilibrium conditions (-25 ≤ Δ G r ≤ + 8.2 kJ/mol) in 29Si spiked solutions which were undersaturated with the secondary Al-Si solid phases likely to precipitate. The forward dissolution rates normalized to the initial BET surface area were found to decrease continuously and monotonically with increasing time and Δ G r , achieving values 1.3 to 2.4 orders of magnitude lower at chemical equilibrium than initial far from equilibrium values, with the highest rate decrease for the highest solid to fluid ratio. These rate drops were accompanied by an increase in BET specific surface areas associated with the formation of widespread dissolution features. A decrease of albite forward dissolution rate together with an increase of its BET surface area was also observed as a function of reaction time at far from equilibrium conditions (Δ G r ≤ − 65 kJ/mol). These observations are consistent with a continuous decrease with time and increasing Δ G r of albite effective surface area linked to the formation of unreactive negative crystal facets, in addition to etch pits stopping nucleating above a critical value of Δ G r. Thus, the observed decrease in albite dissolution rate as equilibrium is approached is not linked primarily to the increase in the contribution of the backward reaction but rather to the decrease of the forward reaction rate in conjunction with the decrease in the density of reactive sites at the mineral surface. The results of this study show that the characterization of the effective surface area and its evolution with reaction time and chemical affinity is crucial to deriving robust dissolution rates of well-crystallized minerals like feldspars. Dissolution experiments in 29Si spiked solutions of samples with different history and well-characterized effective surface area and surface morphology should help to better quantify the rate laws controlling silicate minerals weathering. [ABSTRACT FROM AUTHOR]

Here, we demonstrate the potential advantages of using isotope tracers to test hypotheses of reaction mechanisms near-equilibrium. Using non-traditional stable Si isotopes as tracers, we measured albite unidirectional dissolution rates (r +) across a range of Gibbs free energy of reaction (Δ r G) close to equilibrium (−26 to −2 kJ/mol). Thirteen batch experiment series were conducted at 50 °C and pH ∼ 8 ± 0.25. Different distances from equilibrium were achieved by a stepwise increase of concentrations of Si (0–600 μM), Al (0–10 μM), and Na (0–1000 μM). The temperature, pH, sample preparation, and reaction duration were kept identical to isolate the Δ r G effect. Secondary phase precipitation, which is difficult to avoid in near-equilibrium, near-neutral pH experiments renders the rate measurement method based on changes in Si and Al concentration unworkable, but it should not impact the Si isotope ratios-based rates. The resulting r + values were nearly constant in the experimental Δ r G range, signaling no major Δ r G -related switch of reaction mechanisms. Our results suggest that the switch from etch pit opening at far-from-equilibrium to step retreat at near-equilibrium does not operate under circum-neutral pH in low-temperature systems; this mechanism switch was proposed based on experimental data in alkaline solutions at hydrothermal temperatures. The nearly constant r + values at pH 5–8 also suggest that an H 2 O-catalyzed reaction mechanism dominant at circumneutral pH, in addition to the H+- and OH–-catalyzed reaction mechanisms dominant at acidic and alkaline pH, respectively. The experimental results have implications for geochemical modeling of low-temperature geological and environmental processes. The results suggest that a term of H 2 O-catalyzed reaction mechanism should be included in rate laws and that the parallel rate law with a mechanism-switch is not applicable in the pH range of 5–8. [ABSTRACT FROM AUTHOR]

• Novel Si isotope tracer method deconvoluted feldspar dissolution and precipitation. • Feldspar backward reaction is insignificant at low temperatures. • Recommend forward dissolution rates only in low-temperature kinetics models. • This approach for weathering, geological carbon sequestration, and diagenesis. Here we show that the isotope tracer experimental method for kinetic studies, aided by the recent advance and accessibility of multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) analysis for non-traditional stable isotopes, can provide unidirectional dissolution rates at near-equilibrium conditions. For a long time, the only rates available at near-equilibrium were net reaction rates—dissolution rates minus precipitation rates. This is because the conventional experimental method of kinetic studies is based on element concentrations and can only provide net rates. The availability of unidirectional rates allows us to re-examine some fundamental concepts and practices of modeling weathering in geochemistry. In this study, we used the 29Si isotope tracer to conduct albite and K-feldspar dissolution experiments at near-equilibrium conditions in near-neutral pH solutions at 50 °C. Results show that the saturation indices (SI) of solutions approached zero with respect to albite and K-feldspar after ∼240–360 h (h), but 29Si/28Si ratios of the experimental solutions indicated continual dissolution for another 720–1440 h. The rates of total Si precipitation were much smaller than the rates of Si dissolution. The experimental solutions were supersaturated with respect to amorphous Al(OH) 3 , gibbsite, quartz, allophane, imogolite, and kaolinite. The SI of the solutions remained constant with respect to these phases while Al concentrations slightly decreased and Si concentrations slightly increased, indicating the coupled feldspar dissolution and precipitation of secondary phases, such as albite → amorphous Al(OH) 3 + quartz or albite → solution + Al-Si phase(s), instead of significant albite and K-feldspar precipitation (the reverse reaction) at 50 °C. Reaction path modeling of the temporal evolution of Si, Al, Na, and pH revealed that albite dissolution (without significant backward reaction) coupled with the precipitation of a secondary phase with a Si:Al ratio of ∼2:1 can successfully match the experimental data. Given the negligible feldspar precipitation reactions in low-temperature systems (e.g., T < 100 °C), we recommend modeling feldspar weathering using unidirectional forward rates together with secondary phase precipitation rates in near-equilibrium, feldspar-undersaturated systems. This can be accomplished with minor modifications in geochemical modeling software or input files. The coupled feldspar dissolution with secondary phase precipitation arrests the system in a near-equilibrium steady state. Using affinity-based rate equations such the classical linear Transition State Theory rate law or the Burch empirical relation together with far-from-equilibrium rate data will predict significant feldspar precipitation in solutions undersaturated but close to equilibrium with respect to feldspars, which is unlikely at near ambient temperatures. [ABSTRACT FROM AUTHOR]

Abstract: Nesquehonite crystallization experiments were conducted in H2O and water/dimethylformamide (DMF) mixture to examine solvent effect on the nucleation kinetics. Results show a reduction of nucleation induction time in the presence of DMF and a positive correlation between nucleation rate and DMF concentration. Analysis in the context of the classical nucleation theory reveals an unexpected increase in the surface energy of nesquehonite upon the solvent change, implying that kinetic factors, rather than surface energetics, is the driving force behind the resultant enhancement in nucleation rate. Further analyses suggest that kinetic acceleration under higher surface energy conditions is possible if nucleation proceeds through a cluster aggregation mechanism instead of critical nucleus formation. Fitting the Smoluchowski’s coagulation theory to the experimental data seems to provide support to the operation of the non-classical nucleation mechanism as it shows that the cluster association rate is independent of the sizes of the precursors but positively correlated to the DMF concentration, consistent with the scenario of growing population of sub-critical nuclei and ensuing higher probability of cluster collision/aggregation leading to an accelerated nucleation. [Copyright &y& Elsevier]

Abstract: Phase equilibria and thermodynamic properties of the quaternary H2S–CO2–H2O–NaCl system were studied using a statistical associating fluid theory (SAFT)-based equation of state (EOS) at temperatures from 0 to 200°C (373.15–473.15K), pressures up to 600bar (60MPa) and concentrations of NaCl up to 6mol/kgH2O. The understanding of the physical–chemical properties of this system is critical for predicting the consequences of co-injection of CO2 and H2S into geological formations (geological carbon sequestration) as an option for mitigating the global warming trend. Equation of state parameters were generated from regression of available and reliable experimental data and incorporation of existing parameters for some subsystems. Densities were predicted and compared with available experimental results. Using the EOS developed in this study, we predicted equilibrium compositions in both liquid and vapor phases, fugacity coefficients of components, the equilibrium pressures at a given composition of the H2O-rich phase in electrolyte solutions with NaCl varying from 0 to 4mol/kgH2O, and the aqueous solution densities. These predicted values are tabulated and available as supplementary data in the electronic version online. These predictions provide information and guidance for future experiments regarding the thermodynamic properties and phase behaviors in the H2S–CO2–H2O–NaCl system. [Copyright &y& Elsevier]

Abstract: This paper explores how dissolution and precipitation reactions are coupled in batch reactor experimental systems at elevated temperatures. This is the fourth paper in our series of “Coupled Alkali Feldspar Dissolution and Secondary Mineral Precipitation in Batch Systems”. In our third paper, we demonstrated via speciation–solubility modeling that partial equilibrium between secondary minerals and aqueous solutions was not attained in feldspar hydrolysis batch reactors at 90–300°C and that a strong coupling between dissolution and precipitation reactions follows as a consequence of the slower precipitation of secondary minerals (). Here, we develop this concept further by using numerical reaction path models to elucidate how the dissolution and precipitation reactions are coupled. Modeling results show that a quasi-steady state was reached. At the quasi-steady state, dissolution reactions proceeded at rates that are orders of magnitude slower than the rates measured at far from equilibrium. The quasi-steady state is determined by the relative rate constants, and strongly influenced by the function of Gibbs free energy of reaction () in the rate laws. To explore the potential effects of fluid flow rates on the coupling of reactions, we extrapolate a batch system () to open systems and simulated one-dimensional reactive mass transport for oligoclase dissolution and kaolinite precipitation in homogeneous porous media. Different steady states were achieved at different locations along the one-dimensional domain. The time-space distribution and saturation indices (SI) at the steady states were a function of flow rates for a given kinetic model. Regardless of the differences in SI, the ratio between oligoclase dissolution rates and kaolinite precipitation rates remained 1.626, as in the batch system case (). Therefore, our simulation results demonstrated coupling among dissolution, precipitation, and flow rates. Results reported in this communication lend support to our hypothesis that slow secondary mineral precipitation explains part of the well-known apparent discrepancy between lab measured and field estimated feldspar dissolution rates (). Here we show how the slow secondary mineral precipitation provides a regulator to explain why the systems are held close to equilibrium and show how the most often-quoted “near equilibrium” explanation for an apparent field-lab discrepancy can work quantitatively. The substantiated hypothesis now offers the promise of reconciling part of the apparent field-lab discrepancy. [Copyright &y& Elsevier]

Abstract: In order to evaluate the complex interplay between dissolution and precipitation reaction kinetics, we examined the hypothesis of partial equilibria between secondary mineral products and aqueous solutions in feldspar–water systems. Speciation and solubility geochemical modeling was used to compute the saturation indices (SI) for product minerals in batch feldspar dissolution experiments at elevated temperatures and pressures and to trace the reaction paths on activity–activity diagrams. The modeling results demonstrated: (1) the experimental aqueous solutions were supersaturated with respect to product minerals for almost the entire duration of the experiments; (2) the aqueous solution chemistry did not evolve along the phase boundaries but crossed the phase boundaries at oblique angles; and (3) the earlier precipitated product minerals did not dissolve but continued to precipitate even after the solution chemistry had evolved into the stability fields of minerals lower in the paragenesis sequence. These three lines of evidence signify that product mineral precipitation is a slow kinetic process and partial equilibria between aqueous solution and product minerals were not held. In contrast, the experimental evidences are consistent with the hypothesis of strong coupling of mineral dissolution/precipitation kinetics [e.g., Zhu C., Blum A. E. and Veblen D. R. (2004a) Feldspar dissolution rates and clay precipitation in the Navajo aquifer at Black Mesa, Arizona, USA. In Water–Rock Interaction (eds. R. B. Wanty and R. R. I. Seal). A.A. Balkema, Saratoga Springs, New York. pp. 895–899]. In all batch experiments examined, the time of congruent feldspar dissolution was short and supersaturation with respect to the product minerals was reached within a short period of time. The experimental system progressed from a dissolution driven regime to a precipitation limited regime in a short order. The results of this study suggest a complex feedback between dissolution and precipitation reaction kinetics, which needs to be considered in the interpretation of field based dissolution rates. [Copyright &y& Elsevier]

Abstract: Naturally weathered feldspar surfaces in the Jurassic Navajo Sandstone at Black Mesa, Arizona, was characterized with high-resolution transmission and analytical electron microscope (HRTEM-AEM) and field emission gun scanning electron microscope (FEG-SEM). Here, we report the first HRTEM observation of a 10-nm thick amorphous layer on naturally weathered K-feldspar in currently slightly alkaline groundwater. The amorphous layer is probably deficient in K and enriched in Si. In addition to the amorphous layer, the feldspar surfaces are also partially coated with tightly adhered kaolin platelets. Outside of the kaolin coatings, feldspar grains are covered with a continuous 3–5μm thick layer of authigenic smectite, which also coats quartz and other sediment grains. Authigenic K-feldspar overgrowth and etch pits were also found on feldspar grains. These characteristics of the aged feldspar surfaces accentuate the differences in reactivity between the freshly ground feldspar powders used in laboratory experiments and feldspar grains in natural systems, and may partially contribute to the commonly observed apparent laboratory-field dissolution rate discrepancy. At Black Mesa, feldspars in the Navajo Sandstone are dissolving at ∼105 times slower than laboratory rate at comparable temperature and pH under far from equilibrium condition. The tightly adhered kaolin platelets reduce the feldspar reactive surface area, and the authigenic K-feldspar overgrowth reduces the feldspar reactivity. However, the continuous smectite coating layer does not appear to constitute a diffusion barrier. The exact role of the amorphous layer on feldspar dissolution kinetics depends on the origin of the layer (leached layer versus re-precipitated silica), which is uncertain at present. However, the nanometer thin layer can be detected only with HRTEM, and thus our study raises the possibility of its wide occurrence in geological systems. Rate laws and proposed mechanisms should consider the possibility of this amorphous layer on feldspar surface. [Copyright &y& Elsevier]

Abstract: Experiments of Zn2+ and Fe3+ coprecipitation as a function of pH were conducted in the laboratory at ambient temperature and pressure. X-ray diffraction patterns of the coprecipitates show two broad peaks at 0.149 and 0.258 nm, which is consistent with published patterns for pure 2-line ferrihydrite. Zn2+ uptake occurred at pH ≥5 while Fe3+ precipitation occurred between pH 3 and 4, although both Zn2+ and Fe3+ were present in the same solution during the entire range of pH titration. High-resolution transmission electron microscopy shows that the coprecipitates are 2 to 6 nm sized single crystalline particles but aggregated to 50 to 400 nm sized clusters. Analytical electron microscopy indicated that the 5% atomic Zn with respect to Fe was homogeneously distributed. No segregated phases were found in the clusters or at single crystal edges, which is consistent with published extended X-ray absorption fine structure (EXAFS) results at similar Zn/(Zn + Fe) ratios. Hence, occlusion and surface precipitation may be excluded as possible coprecipitation mechanisms. The bulk solution Zn2+ sorption edge was fitted to both solid solution and generalized diffuse layer surface complexation models. However, a solid solution model is inconsistent with published EXAFS results that show tetrahedral polydentate Zn2+ complexes sharing apices with Fe3+octahedra. [Copyright &y& Elsevier]

Abstract: In situ silicate dissolution rates within the saturated Navajo sandstone, at Black Mesa, Arizona were determined from elemental fluxes in the aquifer. The mass transfer between groundwater and mineral matrix along flow paths was calculated from inverse mass balance modeling. The reaction time is bound by 14C-based travel time. BET surface areas were measured with N2 gas adsorption. Dissolution rates for K-feldspar and plagioclase are 10−19 and 10−16 mol (feldspar) m−2 s−1, respectively, which are ∼105 times slower than laboratory experiment-derived rates under similar pH and temperature but at far from equilibrium conditions. The rates obtained in this study are consistent with the slower field rates found in numerous watershed and soil profile studies. However, these rates are from saturated aquifers, overcoming some concerns on estimated rates from unsaturated systems. The Navajo sandstone is a quartz-sandstone with a relatively simple and well-studied hydrogeology, groundwater geochemistry, and lithology, a large number of groundwater analyses and 14C groundwater ages, groundwater residence times up to ∼37 ky, groundwater pH from ∼8 to 10, and temperature from ∼15 to 35°C. [Copyright &y& Elsevier]

This study attempts to provide a theoretical evaluation of coprecipitation and fundamental data of binary mixing properties in the barite isostructural family. Mixing properties of binary solid solutions in the barite isostructural family were derived from evaluation of coprecipitation experiments and partitioning coefficients reported in the literature. The Margules parameters, W, for these binary systems correlate well through the relationship, where Δ&Gmacr;n° denotes the non-solvation contribution to the standard partial molal Gibbs free energy of formation for the major (M2+) and trace (Tr2+) aqueous ions, and ΔV is the molar volume mismatch between the two substituting end-members , and V° is the molar volume of end-member components. From this correlation, Margules parameters for the BaSO4-TrSO4 and SrSO4-TrSO4 binary systems, for which no experimental data are available, were estimated. Tr here stands for trace divalent metals and includes Cu2+, Co2+, Zn2+, Fe2+, Mn2+, Eu2+, Cd2+, Ca2+, Sr2+, Pb2+, and Ra2+. These estimated mixing properties allow predictions of miscibility gaps for the BaSO4-TrSO4 and SrSO4-TrSO4 binaries, and partitioning coefficients for Tr2+ between barite or celestine and aqueous solutions. [Copyright &y& Elsevier]

Coprecipitation of barite with trace constituents was simulated with consideration of aqueous speciation and complexation, mixing properties for the binary solid solutions (Zhu, this issue), precipitation and dissolution kinetics, and advective-dispersive transport. Speciation-solubility modeling was used to reproduce BaSO4-RaSO4 coprecipitation experimental results, and to calculate CrO42- aqueous concentrations in equilibrium with a Ba(SO4,CrO4) solid solution. Kinetic reaction path modeling was used to simulate the coprecipitation of barite with RaSO4 to form an onion-like chemically zoned solid upon the cooling of oil field brine.A one-dimensional coupled reactive mass transport model shows a strikingly different transport pattern for the tracer Ra2+, when the dominant attenuation reaction is with solid solution (Ba, Ra) SO4 as compared to the case when it is controlled by pure RaSO4 and barite solids under local equilibrium conditions. A self-enrichment of Ra2+ in the groundwater and aquifer solid matrix—higher concentrations of Ra2+ downstream from the reaction front—results from the coprecipitation reaction and advective-dispersive transport. This self-enrichment process generates a secondary tracer source, which has tracer concentrations higher than that of the original source. On the other hand, coprecipitation reactions can reduce Ra2+ concentrations in groundwater to a much lower level (below ppb) than that of pure RaSO4(c) solubility (near ppm), which has been used to establish the Ra2+ concentration limits in groundwater, soil, and nuclear waste repositories. [Copyright &y& Elsevier]

Newly measured amorphous silica dissolution rate data were combined with data from the literature to produce an equation that predicts the dissolution flux ( J , mol/m 2 s) of amorphous silica as a function of temperature ( T , K), sodium concentration ( m Na+ , molal), and hydrogen ion activity ( a H+ ). J = 14.62 10 - 77400 2.303 R 1 T + 8.95 10 - 77700 2.303 R 1 T m Na + 0.490 a H + 0.377 The same data fit equally well to an equation that expresses the dissolution flux as a function of temperature, sodium concentration, and hydroxide ion activity ( a OH− ). J = 14.62 10 - 77400 2.303 R 1 T + 1.73 × 10 6 10 - 77700 2.303 R 1 T m Na + 0.490 a OH - 0.377 These equations have the same form as those for quartz ( Rimstidt, 2015 ) and show that sodium ions have the same catalytic effect on amorphous silica dissolution as they do for quartz. [ABSTRACT FROM AUTHOR]

We measured barium attachment and detachment fluxes from and to barite crystal surfaces in solutions with [Ba]/[SO 4 ] ratios from 0.06 to 52. The doping of 137Ba in the initial experimental solutions enabled us to measure unidirectional fluxes near solubility equilibrium (Q / K ≈ 1.0) in batch reactors. Both attachment and detachment fluxes increase with [Ba]/[SO 4 ] ratios. As expected, since the solutions were near solubility equilibrium ( S I ≈ 0.0 n), the attachment and detachment fluxes were nearly equal. The experimental data can be fit accurately using the Zhang and Nancollas (1998) (ZN98) process-based AB crystal growth model, which describes crystal growth and dissolution through nucleation and propagation of kink sites. Simultaneous fitting step velocity data of Kowacz et al. (2007) significantly reduced the number of non-uniqueness solutions. The remarkable fit implies that barite recrystallization at room temperature near equilibrium and moderate supersaturation occurs via ion-by-ion addition and removal, rather than the formation of amorphous precursors or aggregation of particles. The model is best fit using different detachment frequencies for Ba and SO 4 , suggesting the ZN98 assumption that detachment frequencies should be the same for the constituent ions does not apply to barite. Based on experimental data and model fitting, Ba attachment and detachment fluxes at a fixed saturation state increase with [Ba]/[SO 4 ] ratios monotonically. This has potentially significant implications for interpreting isotope and trace element cycling in natural environments. [ABSTRACT FROM AUTHOR]

Abstract: The two major problems in measuring dissolution rates under close-to-natural conditions in laboratory experiments are: (1) our inability to measure small differences in concentration between solutions with relatively high concentrations and (2) the inherent problem that the change in solution concentration is affected by both the dissolution of the primary mineral and the precipitation of secondary minerals. The present manuscript proposes and tests a novel method, “the isotope ratio method”, for measuring slow dissolution rates of silicate minerals by measuring the change in the ratios between stable isotopes of silicon of a spiked solution. Based on mass balance calculations, two equations that describe the dissolution rate of a silicate mineral in a batch reactor and in a flow-through reactor at steady-state are developed. The precipitation rate of the secondary mineral may be calculated by subtracting the release rate of Si that was calculated using isotope dilution from the rate that was calculated using the proposed isotope ratio method. Numerical simulations of flow-through and batch experiments demonstrate that the “isotope ratio method” is significantly more precise than conventional methods. The analytical uncertainty for the determination of dissolution rates was found to be low for the entire range of reported field-based dissolution rates. The calculation showed that even relatively large isotopic fractionations (up to ε values of 20‰), introduce insignificant uncertainties. Preliminary flow-through experiments support the above conclusion that dissolution rate may be obtained accurately and with small uncertainty using the proposed “isotope ratio method”. [Copyright &y& Elsevier]

Abstract: Pb2+ and Fe3+ coprecipitation was studied with sorption edge measurements, desorption experiments, sorbent aging, High Resolution Transmission and Analytical Electron Microscopy (HR TEM–AEM), and geochemical modeling. Companion adsorption experiments were also conducted for comparison. The macroscopic chemical and near atomic scale HRTEM data supplemented our molecule scale analysis with EXAFS (). Coprecipitation of Pb2+ with ferric oxyhydroxides occurred at ∼pH 4 and is more efficient than adsorption in removing Pb2+ from aqueous solutions at similar sorbate/sorbent ratios and pH. X-ray Diffraction (XRD) shows peaks of lepidocrocite and two additional broad peaks similar to fine particles of 2-line ferrihydrite (2LFh). HRTEM of the Pb–Fe coprecipitates shows a mixture of 2–6nm diameter spheres and 8–20 by 200–300nm needles, both uniformly distributed with Pb2+. Geochemical modeling shows that surface complexation models fit the experimental data of low Pb:Fe ratios when a high site density is used. Desorption experiments show that more Pb2+ was released from loaded sorbents collected from adsorption experiments than from Pb to Fe coprecipitates at dilute EDTA concentrations. Desorbed Pb2+ versus dissolved Fe3+ data show a linear relationship for coprecipitation (CPT) desorption experiments but a parabolic relationship for adsorption (ADS) experiments. Based on these results, we hypothesize that Pb2+ was first adsorbed onto the nanometer-sized, metastable, iron oxyhydroxide polymers of 2LFh with domain size of 2–3nm. As these nano-particles assembled into larger particles, some Pb2+ was trapped in the iron oxyhydroxide structure and re-arranged to form solid solutions. Therefore, the CPT contact method produced more efficient removal of Pb2+ than the adsorption contact method, and Pb2+ bound in CPT solids represent a more stable sequestration of Pb2+ in the environment than Pb2+ adsorbed on iron oxyhydroxide surfaces. [Copyright &y& Elsevier]