Electrogeochemical Conversion of Air CO2 to Dissolved Mineral Bicarbonate and the Production of C-Negative H2 

Carbonate and silicate minerals dissolve in strong acids, and such acids are generated at the anode of a conventional saline water electrolysis cell.  It was therefore reasoned that encasing such an anode with base minerals would lead to enhanced mineral dissolution and hence increased hydroxide (base) generation at the cathode, formed in the course of splitting water to generate H₂ and OH^-. Subsequent exposure of the alkalized solution to CO₂ (e.g., in air or waste streams) would lead to absorption of the CO₂ and formation of stable dissolved or solid (bi)carbonates for carbon sequestration. Previously, it has been demonstrated that mineral carbonate encasement of a seawater electrolysis cell anode indeed generated basic solutions in excess of pH 9 that were subsequently neutralized via contact with air CO₂, increasing the carbon content of the initial seawater by 30% (ref. 1). Relatedly, House et al. (ref. 2) proposed accelerating silicate weathering via mineral reaction with HCl derived by combining the H₂ and Cl₂ produced from NaCl electrolysis, the co-produced NaOH being used for CO₂ capture and conversion to NaHCO_3aq. 

To test the efficacy of the more direct electro-weathering/CO₂ capture scheme (ref. 1) using silicate minerals, either powdered wollastonite or ultramafic rock standard (UM-4) was encased around the anode of an electrolysis cell composed of graphite electrodes and a 0.25M Na₂SO₄ electrolyte solution (ref. 3). After 0.5 to 1.5 hrs of electricity application (3.5Vdc, 5-10mA), the electrolyte pH rose to as much as 11.1 (initial and blank solution pH’s <6.6). Subsequent bubbling of these basic solutions with air lowered pH by at least 2 units and increased dissolve carbon content (primarily bicarbonate) by as much as 50X that of the blanks.  While Ca²⁺ and Mg²⁺ concentrations were elevated, these were insufficient to balance the majority of the bicarbonate anions formed in solution.  This suggests that in these experiments the silicate minerals acted as a neutralizer of the anolyte acid, H₂SO₄, forming mostly CaSO₄ and MgSO₄ at the anode.  This then allowed NaOH normally produced at the cathode to accumulate in solution, in turn reacting with air CO₂  to form dissolved NaHCO₃.  In order to avoid the consumption of the NaSO₄ electrolyte, the use of an alternative salt such as MgSO₄ or CaSO₄might allow formation, precipitation and harvesting of Ca or Mg carbonates while conserving the electrolyte. 

The implications are that such electro-geochemistry might ultimately provide a safe, efficient, high-capacity method of  harnessing the planet’s: i) large, off-peak or off-grid renewable electricity potential (e.g., ref. 4), ii) abundant base minerals, and iii) vast natural brine electrolytes for large-scale CO₂ mitigation (ref. 5). The value of the co-produced, carbon-negative H₂ would help to significantly offset the cost of the process. Our preliminary cost estimate for air CO₂ capture and storage is <$100/tonne, competitive with that of point source CO₂ mitigation using CCS, and less than 1/6 the previously estimated cost of direct air CO₂ capture (ref. 6). The generation of large quantities of dissolved mineral bicarbonates and their addition to the ocean would not only provide massive, safe C storage, but this alkalinity would also help offset the chemical and biological effects of ongoing, CO₂-induced ocean acidification. Further research is needed to better determine the economics, environmental benefits, and potential scale of this technology.

1) Rau, G.H. 2008. Environ. Sci. Technol. 42: 8935–.

2) House, C.Z. et al. 2007. Environ. Sci. Technol. 41: 8464-.

3) Rau, G.H. et al. 2013. PNAS 110: 1009-.

4) Lu, L. et al. 2015. Environ. Sci. Technol. 49: 8193–.

5) http://climatecolab.org/web/guest/plans/-/plans/contestId/20/planId/1304119

6) Socolow R. et al. 2011. Direct Air Capture of CO₂ with Chemicals.  Amer. Phys. Soc.