Microbially enhanced carbon capture and storage by mineral-trapping and solubility-trapping

The potential of microorganisms for enhancing carbon capture and storage (CCS) via mineral-trapping (where dissolved CO₂ is precipitated in carbonate minerals) and solubility trapping (as dissolved carbonate species in solution) was investigated. The bacterial hydrolysis of urea (ureolysis) was investigated in microcosms including synthetic brine (SB) mimicking a prospective deep subsurface CCS site with variable headspace pressures [p(CO₂)] of ¹³C-CO₂. Dissolved Ca²⁺ in the SB was completely precipitated as calcite during microbially induced hydrolysis of 5-20 g L^-1 urea. The incorporation of carbonate ions from ¹³C-CO₂ (¹³C-CO₃^2-) into calcite increased with increasing p(¹³CO₂) and increasing urea concentrations: from 8.3% of total carbon in CaCO₃ at 1 g L^-1 to 31% at 5 g L^-1, and 37% at 20 g L ^-1. This demonstrated that ureolysis was effective at precipitating initially gaseous [CO₂(g)] originating from the headspace over the brine. Modeling the change in brine chemistry and carbonate precipitation after equilibration with the initial p(CO₂) demonstrated that no net precipitation of CO₂(g) via mineral-trapping occurred, since urea hydrolysis results in the production of dissolved inorganic carbon. However, the pH increase induced by bacterial ureolysis generated a net flux of CO ₂(g) into the brine. This reduced the headspace concentration of CO₂ by up to 32 mM per 100 mM urea hydrolyzed because the capacity of the brine for carbonate ions was increased, thus enhancing the solubility-trapping capacity of the brine. Together with the previously demonstrated permeability reduction of rock cores at high pressure by microbial biofilms and resilience of biofilms to supercritical CO₂, this suggests that engineered biomineralizing biofilms may enhance CCS via solubility-trapping, mineral formation, and CO₂(g) leakage reduction.