ALBERT

Description: 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 150°C. 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 \[ {r}_{+}=\left({A}_{A}{\left(\frac{{a}_{H+}^{2}}{{a}_{{\mathrm{Mg}}^{2+}}}\right)}^{1/8}\right)\mathrm{exp}({E}_{\mathrm{A}}/\mathrm{R}T) \] where r + signifies the BET surface area-normalized forward tremolite steady-state dissolution rate, A A refers to a pre-exponential factor = 6 x 10 –3 mol cm –2 s –1 , E A designates an activation energy equal to 80 kJ mol –1 , R represents the gas constant, T denotes absolute temperature, and a i 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 Mg 2+ 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.

Description: Natural hydromagnesite (Mg 5 (CO 3 ) 4 (OH) 2 ·4H 2 O) dissolution and precipitation experiments were performed in closed-system reactors as a function of temperature from 22.5 to 75°C and at 8.6 〈 pH 〈 10.7. The equilibrium constants for the reaction Mg 5 (CO 3 ) 4 (OH) 2 ·4H 2 O + 6H + = 5Mg 2+ + $$4{\mathrm{HCO}}_{3}^{-}$$ + 6H 2 O were determined by bracketing the final fluid compositions obtained from the dissolution and precipitation experiments. The resulting constants were found to be 10 33.7±0.9 , 10 30.5±0.5 and 10 26.5±0.5 at 22.5, 50 and 75°C, respectively. Whereas dissolution rates were too fast to be determined from the experiments, precipitation rates were slower and quantified. The resulting BET surface area-normalized hydromagnesite precipitation rates increase by a factor of ~2 with pH decreasing from 10.7 to 8.6. Measured rates are approximately two orders of magnitude faster than corresponding forsterite dissolution rates, suggesting that the overall rates of the low-temperature carbonation of olivine are controlled by the relatively sluggish dissolution of the magnesium silicate mineral.

Description: Natural hydromagnesite (Mg5(CO3)4(OH)2·4H2O) dissolution and precipitation experiments were performed in closed-system reactors as a function of temperature from 22.5 to 75ºC and at 8.6 〈 pH 〈 10.7. The equilibrium constants for the reaction Mg5(CO3)4(OH)2·4H2O + 6H+ = 5Mg2+ + 4HCO3– + 6H2O were determined by bracketing the final fluid compositions obtained from the dissolution and precipitation experiments. The resulting constants were found to be 1033.7±0.9, 1030.5±0.5 and 1026.5±0.5 at 22.5, 50 and 75ºC, respectively. Whereas dissolution rates were too fast to be determined from the experiments, precipitation rates were slower and quantified. The resulting BET surface areanormalized hydromagnesite precipitation rates increase by a factor of ~2 with pH decreasing from 10.7 to 8.6. Measured rates are approximately two orders of magnitude faster than corresponding forsterite dissolution rates, suggesting that the overall rates of the low-temperature carbonation of olivine are controlled by the relatively sluggish dissolution of the magnesium silicate mineral.