Isotopic Disequilibrium during Uptake of Atmospheric CO2 into Mine Process Waters: Implications for CO2 Sequestration

Dypingite, a hydrated Mg-carbonate mineral, was precipitated from high-pH, high salinity solutions to investigate controls on carbon fixation and to identify the isotopic characteristics of mineral sequestration in mine tailings. delta C-13 values of dissolved inorganic carbon content and synthetic dypingite are significantly more negative than those predicted for equilibrium exchange of CO2 gas between the atmosphere and solution. The measured delta C-13 of aqueous carbonate species is consistent with a kinetic fractionation that results from a slow diffusion of atmospheric CO2 into solution. During dypingite precipitation, dissolved inorganic carbon concentrations decrease and delta C-13 values become more negative, indicating that the rate of CO2 uptake into solution was outpaced by the rate of carbon fixation within the precipitate. This implies that CO2 gas uptake is rate-limiting to CO2 fixation. PC of carbonate mineral precipitates in mine tailings and of DIC in mine process waters display similar C-13-depletions that are inconsistent with equilibrium fractionation. Thus, the rate of carbon fixation in mine tailings may also be limited by supply of CO2. Carbon sequestration could be accelerated by increasing the partial pressure of CO2 in tailings ponds or by using chemicals that enhance the uptake of gaseous CO2 into aqueous solution.