Effect of solution saturation state and temperature on diopside dissolution

Steady-state dissolution rates of diopside are measured as a function of solution saturation state using a titanium flow-through reactor at pH 7.5 and temperature ranging from 125 to 175°C. Diopside dissolved stoichiometrically under all experimental conditions and rates were not dependent on sample history. At each temperature, rates continuously decreased by two orders of magnitude as equilibrium was approached and did not exhibit a dissolution plateau of constant rates at high degrees of undersaturation. The variation of diopside dissolution rates with solution saturation can be described equally well with a ion exchange model based on transition state theory or pit nucleation model based on crystal growth/dissolution theory from 125 to 175°C. At 175°C, both models over predict dissolution rates by two orders of magnitude indicating that a secondary phase precipitated in the experiments. The ion exchange model assumes the formation of a Si-rich, Mg-deficient precursor complex. Lack of dependence of rates on steady-state aqueous calcium concentration supports the formation of such a complex, which is formed by exchange of protons for magnesium ions at the surface. Fit to the experimental data yields [Formula: see text] where the Mg-H exchange coefficient, n = 1.39, the apparent activation energy, E(a )= 332 kJ mol(-1), and the apparent rate constant, k = 10(41.2 )mol diopside cm(-2 )s(-1). Fits to the data with the pit nucleation model suggest that diopside dissolution proceeds through retreat of steps developed by nucleation of pits created homogeneously at the mineral surface or at defect sites, where homogeneous nucleation occurs at lower degrees of saturation than defect-assisted nucleation. Rate expressions for each mechanism (i) were fit to [Formula: see text] where the step edge energy (α) for homogeneously nucleated pits were higher (275 to 65 mJ m(-2)) than the pits nucleated at defects (39 to 65 mJ m(-2)) and the activation energy associated with the temperature dependence of site density and the kinetic coefficient for homogeneously nucleated pits (E(b-homogeneous )= 2.59 × 10(-16 )mJ K(-1)) were lower than the pits nucleated at defects (E(b-defect assisted )= 8.44 × 10(-16 )mJ K(-1))

Contrasting Sorption Behavior of Arsenic (III) and Arsenic(V) in Suspensions of Iron and Aluminum Oxyhydroxides

United Kingdom It has been widely observed that the sorption of arsenic (As) as As(III) and As(V) on iron (Fe) oxyhydroxides is comparable as is the sorption of As(V) on Fe and aluminum (Al) oxyhydroxides. However, quite different sorption behavior has been observed for As(III) on Fe and Al oxyhydroxides. Here, surface complexation modeling is used to show that recent reports of As(III) sorption onto Al oxyhydroxides are consistent with previous observations of negligible sorption under conditions relevant to water treatment. This modeling exercise also demonstrates that the level of protonation of surface species is not well constrained by commonly-reported pH edges and that the possibility of distinguishing between mononuclear and binuclear surface complexes depends on the level of surface coverage. These issues complicate the integration of macroscopic sorption studies with spectroscopic studies and molecular modeling

Comparison of Arsenic(V) and Arsenic(III) Sorption onto Iron Oxide Minerals: Implications for Arsenic Mobility

Arsenic derived from natural sources occurs in groundwater in many countries, affecting the health of millions of people. The combined effects of As(V) reduction and diagenesis of iron oxide minerals on arsenic mobility are investigated in this study by comparing As(V) and As(III) sorption onto amorphous iron oxide (HFO), goethite, and magnetite at varying solution compositions. Experimental data are modeled with a diffuse double layer surface complexation model, and the extracted model parameters are used to examine the consistency of our results with those previously reported. Sorption of As(V) onto HFO and goethite is more favorable than that of As(III) below pH 5−6, whereas, above pH 7−8, As(III) has a higher affinity for the solids. The pH at which As(V) and As(III) are equally sorbed depends on the solid-to-solution ratio and type and specific surface area of the minerals and is shifted to lower pH values in the presence of phosphate, which competes for sorption sites. The sorption data indicate that, under most of the chemical conditions investigated in this study, reduction of As(V) in the presence of HFO or goethite would have only minor effects on or even decrease its mobility in the environment at near-neutral pH conditions. As(V) and As(III) sorption isotherms indicate similar surface site densities on the three oxides. Intrinsic surface complexation constants for As(V) are higher for goethite than HFO, whereas As(III) binding is similar for both of these oxides and also for magnetite. However, decrease in specific surface area and hence sorption site density that accompanies transformation of amorphous iron oxides to more crystalline phases could increase arsenic mobility

Sorption of Fe(II) and As(III) on goethite in single- and dual-sorbate systems

Experiments were conducted to quantify Fe(II) sorption onto goethite in the absence and presence of As(III). The experimental data obtained in single-sorbate experiments were modeled using a diffuse double layer surface complexation model and used to predict and compare sorption in dual-sorbate systems. The sorption process was shown to be reversible by the complete recovery of sorbed Fe(II) upon extraction with 0.5 N HCl. Sorption of Fe(II) increases with increasing pH, as observed previously for various iron oxides. Sorption isotherms obtained between pH 6.0 and 7.5 showed continuous increase in sorption density with increase in dissolved Fe(II) concentration; under these conditions, surface saturation was approached but not reached. Experiments conducted in the absence and presence of 500 and 1000 μM total As(III) did not show any significant difference in the Fe(II) sorption density. As(III) sorption density did not change with increasing sorbed Fe(II) concentration when the total arsenic concentration was 500 μM. However, when the total As(III) concentration was 1000 μM, As(III) sorption densities increased almost linearly with increasing sorbed Fe(II) concentrations. The model provided a good-to-adequate description of Fe(II) and As(III) sorption in single-sorbate systems over a range of experimental conditions but failed to predict the experimental observations in dual-sorbate systems. The predicted sorption densities for both As(III) and Fe(II) were lower than those observed. These discrepancies illustrate problems that may arise when model parameters obtained in single-sorbate systems are used to predict sorption in multi-sorbate systems where all sorbates are presumed to compete for the same sites. The lack of competition observed between As(III) and Fe (II) for sorption sites indicate that the concurrent release of Fe(II) and As(III) during reductive dissolution of iron oxides, inferred as the mechanism of arsenic mobilization in many reducing ground waters, may have relatively minor effects on the subsequent resorption of As(III) to residual iron oxides remaining in the sediment

Contrasting Sorption Behavior of Arsenic (III) and Arsenic(V) in Suspensions of Iron and Aluminum Oxyhydroxides

United Kingdom It has been widely observed that the sorption of arsenic (As) as As(III) and As(V) on iron (Fe) oxyhydroxides is comparable as is the sorption of As(V) on Fe and aluminum (Al) oxyhydroxides. However, quite different sorption behavior has been observed for As(III) on Fe and Al oxyhydroxides. Here, surface complexation modeling is used to show that recent reports of As(III) sorption onto Al oxyhydroxides are consistent with previous observations of negligible sorption under conditions relevant to water treatment. This modeling exercise also demonstrates that the level of protonation of surface species is not well constrained by commonly-reported pH edges and that the possibility of distinguishing between mononuclear and binuclear surface complexes depends on the level of surface coverage. These issues complicate the integration of macroscopic sorption studies with spectroscopic studies and molecular modeling

Additional file 12 of Effect of solution saturation state and temperature on diopside dissolution

Authors’ original file for figure 1

Additional file 1 of Effect of solution saturation state and temperature on diopside dissolution

Authors’ original file for figure