Abstract
Deep-sea carbon dioxide (CO2) plays a significant role in the global carbon cycle and directly affects the living environment of marine organisms. In situ Raman detection technology is an effective approach to study the behavior of deep-sea CO2. However, the Raman spectral characteristics of CO2 can be affected by the environment, thus restricting the phase identification and quantitative analysis of CO2. In order to study the Raman spectral characteristics of CO2 in extreme environments (up to 300 ℃ and 30 MPa), which cover most regions of hydrothermal vents and cold seeps around the world, a deep-sea extreme environment simulator was developed. The Raman spectra of CO2 in different phases were obtained with Raman insertion probe (RiP) system, which was also used in in situ Raman detection in the deep sea carried by remotely operated vehicle (ROV) “Faxian”. The Raman frequency shifts and bandwidths of gaseous, liquid, solid, and supercritical CO2 and the CO2–H2O system were determined with the simulator. In our experiments (0–300 ℃ and 0–30 MPa), the peak positions of the symmetric stretching modes of gaseous CO2, liquid CO2, and supercritical CO2 shift approximately 0.6 cm–1 (1387.8–1388.4 cm–1), 0.7 cm–1 (1385.5–1386.2 cm–1), and 2.5 cm–1 (1385.7–1388.2 cm–1), and those of the bending modes shift about 1.0 cm–1 (1284.7–1285.7 cm–1), 1.9 cm–1 (1280.1–1282.0 cm–1), and 4.4 cm–1 (1281.0–1285.4 cm–1), respectively. The Raman spectral characteristics of the CO2–H2O system were also studied under the same conditions. The peak positions of dissolved CO2 varied approximately 4.5 cm–1 (1282.5–1287.0 cm–1) and 2.4 cm–1 (1274.4–1276.8 cm–1) for each peak. In comparison with our experiment results, the phases of CO2 in extreme conditions (0–3000 m and 0–300 ℃) can be identified with the Raman spectra collected in situ. This qualitative research on CO2 can also support the further quantitative analysis of dissolved CO2 in extreme conditions.
© 2017 The Author(s)
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