Direct electrolytic dissolution of silicate minerals for air CO₂ mitigation and carbon-negative H₂ production

Abstract

We experimentally demonstrate the direct coupling of silicate mineral dissolution with saline water electrolysis and H₂ production to effect significant air CO₂ absorption, chemical conversion, and storage in solution. In particular, we observed as much as a 10⁵-fold increase in OH⁻ concentration (pH increase of up to 5.3 units) relative to experimental controls following the electrolysis of 0.25 M Na₂SO₄ solutions when the anode was encased in powdered silicate mineral, either wollastonite or an ultramafic mineral. After electrolysis, full equilibration of the alkalized solution with air led to a significant pH reduction and as much as a 45-fold increase in dissolved inorganic carbon concentration. This demonstrated significant spontaneous air CO₂ capture, chemical conversion, and storage as a bicarbonate, predominantly as NaHCO₃. The excess OH⁻ initially formed in these experiments apparently resulted via neutralization of the anolyte acid, H₂SO₄, by reaction with the base mineral silicate at the anode, producing mineral sulfate and silica. This allowed the NaOH, normally generated at the cathode, to go unneutralized and to accumulate in the bulk electrolyte, ultimately reacting with atmospheric CO₂ to form dissolved bicarbonate. Using nongrid or nonpeak renewable electricity, optimized systems at large scale might allow relatively high-capacity, energy-efficient (<300 kJ/mol of CO₂ captured), and inexpensive (<$100 per tonne of CO₂ mitigated) removal of excess air CO₂ with production of carbon-negative H₂. Furthermore, when added to the ocean, the produced hydroxide and/or (bi)carbonate could be useful in reducing sea-to-air CO₂ emissions and in neutralizing or offsetting the effects of ongoing ocean acidification.