In situ measurements of calcium carbonate dissolution under rising CO sub(2) pressure using underwater laser-induced breakdown spectroscopy

In the present study, we applied underwater laser-induced breakdown spectroscopy (underwater LIBS) for rapid in situ measurements of calcium carbonate (CaCO sub(3)) dissolution as a function of CO sub(2) pressure (pCO sub(2)). A pulsed Nd:YAG laser at 1064 nm was used to produce gaseous plasma in the fluid surrounding a pressed pellet of CaCO sub(3) powder. The ensuing plasma emission was spectrally analyzed, and the intensity of the calcium emission line at 422.67 nm was used to monitor Ca super(2+) cation released to the water. Barium emission line at 455.40 nm was simultaneously recorded as an internal standard to calibrate calcium signal intensity. The study shows that relatively strong and well-resolved spectral lines of both Ca super(2+) and Ba super(2+) cations can be obtained in CO sub(2)-saturated water. More importantly, the results show that underwater LIBS is capable of performing quantitative analysis at elevated pCO sub(2), with an estimated Ca super(2+) detection limit of about 9 ppm over 50-350 bar. In the solution with the initially added CaCO sub(3) pellet, the concentration of Ca super(2+) increases by a factor of 2 as pCO sub(2) increases from 50 to 150 bar and remains nearly constant when pCO sub(2) is further increased up to 350 bar. Finally, our study provides evidence that underwater LIBS could be a useful tool to investigate/monitor carbonate dissolution (at low ppm levels) in various geochemical applications.