Abstract
Steady-state tremolite dissolution rates, at far-from-equilibrium conditions, were measured as a function of aqueous silica and magnesium activity, pH from 1.9 to 6.7, and temperature from 25 to 150ºC. Calcium is released from tremolite faster than either Mg or Si throughout most of the experiments even after these latter elements attained steady-state release rates. The preferential removal of Ca releases fine Mg-Si rich needle-like fibres from the tremolite, probably promoting its toxicity. In contrast, Mg was released in stoichiometric or near to stoichiometric proportion to Si once steady-state was attained. Measured steady-state tremolite dissolution rates based on Si release can be described using where r+ signifies the BET surface area-normalized forward tremolite steady-state dissolution rate, AA refers to a pre-exponential factor = 6610–3 mol cm–2 s–1, EA designates an activation energy equal to 80 kJ mol–1, R represents the gas constant, T denotes absolute temperature, and ai refers to the activity of the subscripted aqueous species. This rate expression is consistent with tremolite dissolution rates at acidic pH being controlled by the detachment of partially liberated silica tetrahedra formed from the exchange of Mg2+ for two protons near the mineral surface after the near-surface Ca has been removed. Nevertheless, Mg release rates from tremolite are ~3 orders of magnitude slower than those from forsterite and enstatite suggesting that tremolite carbonation will be far less efficient than the carbonation of these other Mg-silicate minerals.
Highlights
THIS work is part of a systematic study aimed at the characterization of Mg-silicate mineral dissolution rates
The motivation for this study is several fold: (1) The dissolution rates of Ca- and Mg-bearing silicates, such as tremolite, are of particular interest as they are potential sources of the divalent metals required for the mineralogical storage of CO2
Tremolite dissolution rates based on Si or Mg release are orders of magnitude slower than corresponding dissolution rates of forsterite and enstatite
Summary
THIS work is part of a systematic study aimed at the characterization of Mg-silicate mineral dissolution rates. The motivation for this study is several fold: (1) The dissolution rates of Ca- and Mg-bearing silicates, such as tremolite, are of particular interest as they are potential sources of the divalent metals required for the mineralogical storage of CO2 (cf Oelkers and Schott, 2005; Hanchen et al, 2008; Oelkers and Cole, 2008; Oelkers et al, 2008a; King et al, 2010; Ryu et al, 2011; Saldi et al, 2012; Gislason and Oelkers, 2014). A particle’s ability to resist dissolution once submerged in lung fluid (i.e. its biodurability) may be a critical determinant in its toxicity (Addison and McConnell, 2008) This connection has motivated several previous studies aimed at quantifying tremolite biodurability (Oze and Solt, 2010; Rozalen et al, 2013). To date there have been relatively few studies of the dissolution rates of tremolite reported in the literature. Schott et al (1981) reported the dissolution rates of tremolite at 22oC at 1 4 pH 4 6 in aqueous HCl solutions measured in batch reactors. Mast and Drever (1987) reported the dissolution rates of tremolite at 22oC and 2 4 pH
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