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Title: Effect of solution saturation state and temperature on diopside dissolution

Abstract

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.

Authors:
;
Publication Date:
Research Org.:
Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
940871
Report Number(s):
UCRL-JRNL-229477
TRN: US0807232
DOE Contract Number:
W-7405-ENG-48
Resource Type:
Journal Article
Resource Relation:
Journal Name: Geochemical Transactions, vol. 8, N/A, March 26, 2007, pp. 1-14; Journal Volume: 8
Country of Publication:
United States
Language:
English
Subject:
58 GEOSCIENCES; CALCIUM; DIOPSIDE; DISSOLUTION; ION EXCHANGE; MAGNESIUM IONS; NUCLEATION; PRECURSOR; PROTONS; SATURATION; TITANIUM

Citation Formats

Dixit, S, and Carroll, S A. Effect of solution saturation state and temperature on diopside dissolution. United States: N. p., 2007. Web. doi:10.1186/1467-4866-8-3.
Dixit, S, & Carroll, S A. Effect of solution saturation state and temperature on diopside dissolution. United States. doi:10.1186/1467-4866-8-3.
Dixit, S, and Carroll, S A. Fri . "Effect of solution saturation state and temperature on diopside dissolution". United States. doi:10.1186/1467-4866-8-3. https://www.osti.gov/servlets/purl/940871.
@article{osti_940871,
title = {Effect of solution saturation state and temperature on diopside dissolution},
author = {Dixit, S and Carroll, S A},
abstractNote = {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.},
doi = {10.1186/1467-4866-8-3},
journal = {Geochemical Transactions, vol. 8, N/A, March 26, 2007, pp. 1-14},
number = ,
volume = 8,
place = {United States},
year = {Fri Mar 23 00:00:00 EDT 2007},
month = {Fri Mar 23 00:00:00 EDT 2007}
}
  • No abstract prepared.
  • To evaluate the release of uranium from natural ore deposits, spent nuclear fuel repositories, and REDOX permeable reactive barriers, knowledge of the fundamental reaction kinetics associated with the dissolution of uranium dioxide is necessary. Dissolution of crystalline uranium (IV) dioxide under environmental conditions has been studied for four decades but a cardinal gap in the published literature is the effect of pH and solution saturation state on UO2(cr) dissolution. To resolve these inconsistencies, UO2 dissolution experiments have been conducted under oxic conditions using the single-pass flow-through system. Experiments were conducted as a function of total dissolved carbonate ([CO3-2]T) from 0.001more » to 0.1 M; pH from 7.5 to 11.1; ratio of flow-through rate (q) to specific surface area (S), constant ionic strength (I) = 0.1 M, and temperatures (T) from 23 to 60 C utilizing both powder and monolithic specimens. The results show that UO2 dissolution varies as a function of the ratio q/S and temperature. At values of log10 q/S > -7.0, UO2 dissolution becomes invariant with respect to q/S, which can be interpreted as evidence for dissolution at the forward rate of reaction. The data collected in these experiments show the rate of UO2 dissolution increased by an order of magnitude with a 30? increase in temperature. The results also show the overall dissolution rate will increase with an increase in pH and decrease as the dissolved uranium concentration approaches saturation with respect to secondary reaction products. Thus, as the value of the reaction quotient, Q, approaches equilibrium, K, (with respect to a potential secondary phase) the dissolution rate decreases. This decrease in dissolution rate was also observed when comparing measured UO2 dissolution rates from static tests where r = 1.7 ?0.14 ? 10-8 mol m 2 s-1 to the rate for flow-through reactors where r = 3.1 ?1.2 ? 10-7 mol m-2 s-1. Thus, using traditional static test methods can result in an underestimation of the true forward rate of UO2(cr) dissolution. These results illustrate the release of uranium from UO2 in the natural environment will be controlled by pH, solution saturation state, and the concentration of dissolved carbonate.« less
  • The authors have measured the dissolution rate of diopside in dilute solutions (far from equilibrium) at 25, 50, and 70[degrees]C from pH 2 through pH 12 using a flow-through reactor. Reducing the CO[sub 2] concentration tenfold produced little, if any, effect on dissolution rate at alkaline pH (pH 8 through pH 12) at 25 and 70[degrees]C. Linear dissolution kinetics (i.e., time-invariant rates) were eventually observed in all runs. The overall trend with increasing pH is decreasing diopside dissolution rate based on the release rate of all constituents. The authors fit these rates by regression to a general rate law ofmore » the form r = Ak[[alpha][sub H[sup +]]][sup n], where A is the surface area, k is the rate constant, and n is the order with respect to hydrogen ion activity. At 70[degrees]C over the range pH 2 through pH 10 in solutions equilibrated with atmospheric CO[sub 2], the rate of diopside dissolution based on Si release is rate (mol/cm[sup 2]-s) = 2.45 ([plus minus]0.96)[times]10[sup [minus]13] [alpha][sub H[sup +]][sup 0.18[plus minus]0.03]. At 50[degrees]C the rate based on Si release is rate (mol/cm[sup 2]-s) = 1.10([plus minus]0.61)[times]10[sup [minus]13] [alpha][sub H[sup +]][sup 0.21[plus minus]]0.04. At 25[degrees]C the rate based on Si release is rate (mol/cm[sup 2]-s) = 2.88([plus minus]1.33)[times]10[sup [minus]14] [alpha][sub H[sup +]][sup 0.18[plus minus]0.03]. Based on a regression of the rate constants, over the temperature interval 25 to 70[degrees]C, the activation energy for the dissolution of diopside is 9.7 [plus minus] 0.4 kcal/mol. This energy is indicative of a surface-reaction controlled dissolution process, as is the observation of crystallographically controlled etch pits. 31 refs., 5 figs., 5 tabs.« less
  • Laboratory observations of aragonite dissolution in seawater, under temperature and pressure conditions that approximate the natural oceanic environment, indicate that R = k{prime}((CO{sup 2{minus}}{sub 3}){sub s} - (CO{sup 2{minus}}{sub 3})){sup n} is an appropriate expression describing the dependence of dissolution rate on seawater saturation, state, where R is aragonite dissolution rate in percent per day, (CO{sup 2{minus}}{sub 3}){sub s} is the carbonate ion concentration at saturation, and (CO{sup 2{minus}}{sub 3}) is the in-situ carbonate ion concentration. Under conditions in which the influence of surface alteration is minimized, plots of dissolution rate, R, versus ((CO{sup 2{minus}}{sub 3}){sub s} - (CO{sup 2{minus}}{submore » 3})) approach linearity. Consequently, in the absence of surface alteration effects, our results suggest that the reaction order, n, in the above expression should be equal to one. Use of single pteropod shells in extended experimental sequences indicates that progressive roughening of the shell surface by dissolution can substantially enhance shell dissolution rates. Surface alteration leads to variable values of the rate constant, k{prime}, in the expression above. For rate measurements obtained at increasing degrees of both undersaturation and shell roughness, the multiplicative factors k{prime} and ((CO{sup 2{minus}}{sub 3}){sub s} - (CO{sup 2{minus}}{sub 3})) give rise to curvature in plots of rate versus ((CO{sup 2{minus}}{sub 3}){sub s} - (CO{sup 2{minus}}{sub 3})). For models in which variations in k{prime} are not explicitly acknowledged, dissolution rates are generally described successfully with reaction orders (n) greater than one. Our experiments, performed at variable pressure, were modeled using several realistic partial molar volume changes ({delta} V) for aragonite dissolution in seawater.« less