Methanogenesis-induced pH-Eh shifts drives aqueous metal(loid) mobility in sulfide mineral systems under CO2 enriched conditions

To better understand the role of CO2-utilizing processes in determining geochemical outcomes in CO2-impacted, reduced subsurface environments; we studied hydrogenotrophic methanogenesis (a key process across a range of subsurface systems) and associated changes in CO2-CH4, pH-Eh and metal(loid) dynamics in CO2-enriched pyrite-, arsenopyrite- and galena-containing batch reactors. Hydrogenotrophic methanogenesis proceeded via two first-order, microbially-mediated steps. The first involved microbially-enhanced gas-to-solution transfer of CO2 with a rate constant of 0.95 ± 0.20 d−1 (compared to 0.17 ± 0.34 d−1 in non-methanogenic reactors). The second step involved the pseudo-first order reduction of HCO3−-to-CH4 with a rate constant of 0.46 ± 0.10 d−1. Bicarbonate reduction accounted for 76% of the CO2-C removed from the reactors' headspace and triggered a decrease in Eh (−0.215 to −0.332 V) and an increase in pH (6.93 to 8.03). No significant effect on dissolved Pb (or its release from galena) was observed but dissolved Fe decreased and dissolved As increased during methanogenesis. Changes in dissolved Fe and As were proportional to methanogenesis-induced pH-Eh shifts and attributable to an initial CO2-induced release of As and Fe from arsenopyrite (and Fe from pyrite) with subsequent methanogenesis-induced increase in pH and electron activity triggering the precipitation of Fe, as amorphous FeCO3·6H2O. In addition to the role of methanogenesis in enhancing, (1) aqueous dissolution of CO2, (2) CO2 to carbonate mineralization and (3) immobilization of some metals, our findings suggested that due to its occurrence after CO2 dissolution and before carbonate precipitation, extensive HCO3−-to-CH4 reduction would lower solubility- and mineral-trapping of CO2. Such considerations are central to assessments of how CO2-utilizing processes may alter long-term outcomes at geologic storage sites.