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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]

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]