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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: 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]

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]

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]