The experimental determination of hydromagnesite precipitation rates at 22.5–75ºC

Abstract

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 75ºC 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+ + 4HCO 3 À + 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 33.7Ô0.9 , 10 30.5Ô0.5 and 10 26.5Ô0.5at 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.

The temporal evolution of magnesium isotope fractionation during hydromagnesite dissolution, precipitation, and at equilibrium

“…This pH increase was due to the consumption of protons by the hydromagnesite dissolution reaction (3); the pH in the other experiments remained closer to constant because the pH of their initial reactive fluids was higher leading to aqueous CO3 2formation in the fluid phase as hydromagnesite dissolved. Chemical equilibrium between the fluid and the hydromagnesite is rapidly attained in all experiments consistent with the results reported by Berninger et al (2014). For example, chemical equilibrium was attained after ~5 hours during experiment Hmgs-1 and after ~2 days during experiment Hmgs-2.…”

“…Close matches between calculated and measured reactive fluid concentrations are evident. Although there is some uncertainty in fitting these fast reaction rates, they are consistent with the conclusions of Berninger et al (2014), as precipitation rate constant was found to be substantially smaller than that of the dissolution rate constants.…”

“…Notably, field observations have shown that passive CO2 storage may occur in ultramafic mine tailings through the weathering of Mg-silicates and precipitation of hydrous Mg-carbonates (Wilson et al, 2009(Wilson et al, , 2014Pronost et al, 2011). Such observations have motivated a number of experimental studies characterizing the formation conditions, reactivity, and thermal stability of the hydrous Mg-carbonates (Hänchen et al, 2008;Frost et al, 2008;Ballirano et al, 2010;Gautier et al, 2014;Berninger et al, 2014).…”

Solubility of the hydrated Mg-carbonates nesquehonite and dypingite from 5 to 35 °C: Implications for CO2 storage and the relative stability of Mg-carbonates

“…Estimation of the CO 2 storage security of these approaches requires knowledge of the stability, solubility, and formation pathways of the carbonate mineral products that store the CO 2 . The relative stability and solubility of several Mg-carbonates has been investigated (Langmuir, 1965;Canterford et al, 1984;Königsberger et al, 1999;Zhang et al, 2006;Hopkinson et al, 2008Hopkinson et al, , 2012Hänchen et al, 2008;Xiong and Lord, 2008;Bénézeth et al, 2011;Xiong, 2011;Ballirano et al, 2013;Berninger et al, 2014;Kristova et al, 2014;Gautier et al, 2014). In general, the stability of Mg-carbonates increases from more to less hydrated phases, in the order: lansfordite < nesquehonite < dypingite < hydromagnesite < magnesite (Langmuir, 1965;Canterford et al, 1984).…”

Comment on “The experimental determination of hydromagnesite precipitation rates at 22.5– 75°C” by Berninger, U.-N., Jordan, G., Schott, J. and Oelkers, E.H.

“…If they are inconsistent with field observations, those laboratory measurements should be considered invalid. To this end, we tested the equilibrium constants of hydromagnesite (5424) suggested by Berninger et al (2014) against field observations to determine if they are consistent with the field observations. This work focuses on hydromagnesite (5424) which for simplicity, is abbreviated as hydromagnesite.…”