Accelerated Weathering of Limestone for CO₂ Mitigation

Abstract

Rock weathering is a natural phenomenon which brings about several changes on the Earth’s landscape but it has one more useful function. It controls the CO₂ concentration in the atmosphere by precipitating the magnesium and calcium carbonates. The process is slow and takes place over a long period of time. However, if this process is accelerated, the atmospheric CO₂ can be removed in sufficient volumes at a faster rate and converted to bicarbonates thus mitigating the greenhouse effect. A geochemistry-based capture and sequestration process that reacts CO₂ with water to produce a carbonic acid solution is one of the methods to understand this phenomenon. This carbonic acid solution is then reacted with carbonate rocks to precipitate bicarbonates. The rate of reaction depends upon the temperature, flow rate and particle size. In the present study, the accelerated weathering of limestone was carried out using different sizes of limestone cuttings (4 mm, 500, 250, 100 and 50 µm) under different flow rates of carbonic acid (0.5, 1.0 and 1.5 L/min) for a duration of 2 h at a constant temperature of 80 °C. SEM and elemental analysis were done before and after the experiment. Results from the experiment showed that the highest flow rate (1.5 L/min) exhibits the greatest weight loss. This weight loss is brought about by the dissolution of calcium to release bicarbonate and carbonate ions. EDX analysis shows a reduction in both calcite and dolomite indicating dissolution of these two minerals. Acid attack on the samples forms dissolution patterns which are visible in SEM images. The principle mode of sequestration in limestone formation is ionic trapping. CO₂ trapped in this way in the form of bicarbonate solution can be safely discharged in a water body or reservoir for storage.