Relating the evolution of the pH to the dissolution rate of calcite
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Abstract
With the increasing concentrations of carbon dioxide (CO2) in the atmosphere and its impact on climate, it is urgent to develop methods to mitigate CO2release from industrial processes.
One of the most promising methods is Geological Carbon Storage (GCS), where CO2is injected into a geologic formation for permanent storage. The geological formations are required to have certain characteristics, such as sufficient porosity and permeability to accept and hold large amounts of CO2. In Denmark, one of the suitable rock types is chalk, which is mainly composed of calcite.
This mineral reacts fast with water, in particular at low pH, and CO2 injection could cause substantial dissolution of the chalk. To estimate the location of this dissolution, quantification of the dissolution rate of calcite is required at variable saturation.
In this study, we wish to determine the calcite dissolution rate primarily through measurement of pH. For this purpose, we aim to improve in situ pH measurement using an optical probe and apply the probe to obtain calcite dissolution rates under representative GCS conditions.
Initial tests of the use of pH for determination of calcite dissolution rate have yielded promising results. Batch experiments have been performed with crushed Iceland spar powders in closed system and in open systems with CO2 partial pressures from the atmospheric to 30%.
During the 1 to 24 hours experiments, the evolution of calcium concentrations in the solution and the pH were measured. The results show that the measurement of pH coupled to simple PHREEQC calculations allows us to correctly quantify the amount of calcite dissolved. These results set the stage for future experiments under high temperature and pressure conditions.