Stable solid and aqueous H2CO3 from CO2 and H2O at high pressure and high temperature

Abstract

Carbonic acid (H2CO3) forms in small amounts when CO2 dissolves in H2O, yet decomposes rapidly under ambient conditions of temperature and pressure. Despite its fleeting existence, H2CO3 plays an important role in the global carbon cycle and in biological carbonate-containing systems. The short lifetime in water and presumed low concentration under all terrestrial conditions has stifled study of this fundamental species. Here, we have examined CO2/H2O mixtures under conditions of high pressure and high temperature to explore the potential for reaction to H2CO3 inside celestial bodies. We present a novel method to prepare solid H2CO3 by heating CO2/H2O mixtures at high pressure with a CO2 laser. Furthermore, we found that, contrary to present understanding, neutral H2CO3 is a significant component in aqueous CO2 solutions above 2.4 GPa and 110 °C as identified by IR-absorption and Raman spectroscopy. This is highly significant for speciation of deep C–O–H fluids with potential consequences for fluid-carbonate-bearing rock interactions. As conditions inside subduction zones on Earth appear to be most favorable for production of aqueous H2CO3, a role in subduction related phenomena is inferred.

Carbonic acid monohydrate

“…Recently, a new CO 2 -H 2 O compound was observed above 2.4 GPa and asserted to be H 2 CO 3 based on IR and Raman lines similar to those of the low pressure H 2 CO 3 solid (Wang et al 2016). Following this initial report, we (Abramson et al 2017) demonstrated that this new phase can exist in equilibrium with an aqueous fluid (starting from a quadruple point with ices H 2 O(VII) and CO 2 (I), at 4.4 GPa and 165 °C), which allowed the growth of single crystals for purposes of X-ray diffraction.…”

“…However, owing to unfavorable kinetics and equilibrium constants at ambient conditions (Loerting et al 2000), molecular carbonic acid has 3 long eluded detailed experimental observation and, as described below, a crystalline structure had not previously been determined. Recently, Wang et al (2016) claimed to have observed the formation of solid carbonic acid at high pressures, and Abramson et al (2017) demonstrated that the observed phase can exist in thermodynamic equilibrium with the aqueous fluid for pressures beyond a quadruple point at 4.4 GPa. Preliminary X-ray results indicated that the new phase was triclinic, but their quality did not allow a refinement of the crystal structure.…”

Vacuum ultraviolet photoabsorption spectroscopy of space-related ices: formation and destruction of solid carbonic acid upon 1 keV electron irradiation

“…On Earth, H 2 CO 3 is found in a solution with water (H 2 O) and carbon dioxide (CO 2 ). It plays an important role in the global carbon cycle and in biological carbonate-containing systems (Wang et al 2016). For instance, about 30 − 40% of human-induced emissions of atmospheric Send offprint requests to: S. Ioppolo; e-mail: s.ioppolo@qmul.ac.uk CO 2 dissolve into the oceans to form carbonic acid (Caldeira & Wickett 2003).…”

High Pressure:One of the many Tools to Study Material Properties at Extreme Conditions

“…This information can be used to model and understand the interiors of giant exoplanets 22 . Wang et al 23 have shown that contrary to earlier understanding, carbonic acid could be a stable species in the aqueous fluids found in the subduction zone of the earth (P > 2.4 GPa). Even simple oxides of iron like hematite and magnetite decompose at 60-70 GPa and 2500 K to release large amounts of oxygen and form novel oxides of iron.…”