An Experimental Investigation of the Potential Role of Zeolite in CO2 Mineralization

Abstract

Although there are many parts of the world where there are saline aquifers or depleted hydrocarbon reservoirs available for large-scale subsurface carbon storage, these may not be present in other areas near major point sources. In such cases, the mineralization of carbon injected into mafic or ultramafic rocks may provide an alternative. Carbon mineralization has been successful in fresh basalts. More common are basalts that have been hydrothermally altered over time by their interaction with warm waters. The most common Ca-bearing silicate alteration phase of basalts altered at temperatures less than 200 oC are zeolite phases. Zeolites are noted for the rapid exchange of their interstitial cations to the aqueous solution. This process suggests zeolite-bearing altered basalts may be a favorable target for subsurface mineral carbonation efforts. This study was designed to assess this possibility. The dissolution rate and Ca release rate of the Ca-bearing zeolite stilbite (NaCa4(Si27Al9)O72·28(H2O)) have been measured in mixed flow experimental reactors at 60 oC and a function of pH. The reactor containing the ground stilbite has been placed inside a shaking incubator held at a constant rotational speed. The reacting fluid with varying pH levels is injected into the reactor using a peristaltic pump at a constant flow rate. Fluid samples were frequently collected to monitor the fluid pH and the elemental concentration analysis. This mineral's BET-surface-area normalized dissolution rate at pH 2 and 60oC was determined at 14.3x10-9 mol m-2 s-1. Calcium was preferentially released at all pH, and this release rate is essentially independent of solution pH. Corresponding closed system experiments in the presence of sodium carbonate solutions demonstrate the rapid and efficient carbonation of this mineral. Measured reaction rates have been used in reactive transport calculations to assess the efficiency of zeolite carbonation in the subsurface. Results of these calculations indicate that zeolite-bearing basalts strongly favor the fixation of injected CO2 through the formation of calcite and aragonite. This result suggests the targeting of subsurface carbon mineral storage efforts into zeolite-bearing altered basaltic rocks.