Direct electrolytic dissolution of silicate minerals for air CO ₂ mitigation and carbon-negative H ₂ production

Abstract

We experimentally demonstrate the direct coupling of silicate mineral dissolution with saline water electrolysis and H 2 production to effect significant air CO 2 absorption, chemical conversion, and storage in solution. In particular, we observed as much as a 10 5 -fold increase in OH − concentration (pH increase of up to 5.3 units) relative to experimental controls following the electrolysis of 0.25 M Na 2 SO 4 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 CO 2 capture, chemical conversion, and storage as a bicarbonate, predominantly as NaHCO 3 . The excess OH − initially formed in these experiments apparently resulted via neutralization of the anolyte acid, H 2 SO 4 , 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 CO 2 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 CO 2 captured), and inexpensive (<$100 per tonne of CO 2 mitigated) removal of excess air CO 2 with production of carbon-negative H 2 . Furthermore, when added to the ocean, the produced hydroxide and/or (bi)carbonate could be useful in reducing sea-to-air CO 2 emissions and in neutralizing or offsetting the effects of ongoing ocean acidification.T he abundance of silicate minerals and their ability to react with CO 2 to form stable carbonates and bicarbonates make them relevant to CO 2 mitigation efforts (e.g., refs. 1-4). The global capacity of these reactions to moderate atmospheric CO 2 is evident in the central role silicate mineral weathering plays in naturally consuming excess atmospheric CO 2 on geological time scales (5). Indeed, various methods have been proposed to accelerate this natural geochemical air CO 2 mitigation (6-9).Although silicate weathering is extremely slow under ambient conditions, silicate mineral dissolution and subsequent reaction with CO 2 can be significantly increased in strong acids and/or bases (ref. 10 and references therein). Because very large pH gradients are produced in saline water electrolysis cells [anolyte pH < 2, catholyte pH > 12 (11)], it was reasoned that placing a silicate mineral mass in direct contact with such solutions would facilitate their dissolution to metal and silicate ions. Once formed, the positively charged metal ions could migrate to the negatively charged catholyte to form metal hydroxide, whereas the negatively charged silicate ions would react with the H + -rich anolyte to form silicic acid, silica, and/or other silicon compounds (Fig. 1A).Alternatively ...

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