In situ measurements of calcium carbonate dissolution under rising CO2 pressure using underwater laser-induced breakdown spectroscopy

Abstract

In the present study, we applied underwater laser-induced breakdown spectroscopy (underwater LIBS) for rapid in situ measurements of calcium carbonate (CaCO₃) dissolution as a function of CO₂ pressure (pCO₂). A pulsed Nd:YAG laser at 1064 nm was used to produce gaseous plasma in the fluid surrounding a pressed pellet of CaCO₃ powder. The ensuing plasma emission was spectrally analyzed, and the intensity of the calcium emission line at 422.67 nm was used to monitor Ca²⁺ 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 Ca²⁺ and Ba²⁺ cations can be obtained in CO₂-saturated water. More importantly, the results show that underwater LIBS is capable of performing quantitative analysis at elevated pCO₂, with an estimated Ca²⁺ detection limit of about 9 ppm over 50–350 bar. In the solution with the initially added CaCO₃ pellet, the concentration of Ca²⁺ increases by a factor of 2 as pCO₂ increases from 50 to 150 bar and remains nearly constant when pCO₂ 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.