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. 2015 May 26;15(6):12377-88.
doi: 10.3390/s150612377.

Investigation of two novel approaches for detection of sulfate ion and methane dissolved in sediment pore water using Raman spectroscopy

Zengfeng Du  1 , Jing Chen  2 , Wangquan Ye  3 , Jinjia Guo  4 , Xin Zhang  5 , Ronger Zheng  6
Affiliations

Investigation of two novel approaches for detection of sulfate ion and methane dissolved in sediment pore water using Raman spectroscopy

Zengfeng Du et al. Sensors (Basel). .

Abstract

The levels of dissolved sulfate and methane are crucial indicators in the geochemical analysis of pore water. Compositional analysis of pore water samples obtained from sea trials was conducted using Raman spectroscopy. It was found that the concentration of SO42- in pore water samples decreases as the depth increases, while the expected Raman signal of methane has not been observed. A possible reason for this is that the methane escaped after sampling and the remaining concentration of methane is too low to be detected. To find more effective ways to analyze the composition of pore water, two novel approaches are proposed. One is based on Liquid Core Optical Fiber (LCOF) for detection of SO42-. The other one is an enrichment process for the detection of CH4. With the aid of LCOF, the Raman signal of SO42- is found to be enhanced over 10 times compared to that obtained by a conventional Raman setup. The enrichment process is also found to be effective in the investigation to the prepared sample of methane dissolved in water. By CCl4 extraction, methane at a concentration below 1.14 mmol/L has been detected by conventional Raman spectroscopy. All the obtained results suggest that the approach proposed in this paper has great potential to be developed as a sensor for SO42- and CH4 detection in pore water.

Keywords: CCl4 extraction; LCOF; Raman spectroscopy; methane; sulfate ion.

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Figures

Figure 1
Figure 1
Schematic diagram of the LCOF-Raman experimental setup (R, dichroic mirror; L, optical lens; HPF, high pass filter).
Figure 2
Figure 2
Typical Raman spectrum of the pore water samples.
Figure 3
Figure 3
The calibration curve of sulfate concentration with conventional Raman spectroscopy.
Figure 4
Figure 4
Profiles of SO42− concentrations in pore water measured by Raman spectroscopy and LC.
Figure 5
Figure 5
The calibration curve of sulfate concentration with LCOF-Raman spectroscopy.
Figure 6
Figure 6
Raman spectra of 30 mmol/L sulfate solution using the LCOF-Raman experimental (LWCC-2100) setup and conventional experimental setup, respectively. The Raman peak of SO42− is located at 981 cm−1, and the Raman peak of H2O is located at 1640 cm−1.
Figure 7
Figure 7
The calibration curve of sulfate concentration with (a) conventional Raman spectroscopy and (b) LCOF-Raman spectroscopy.
Figure 8
Figure 8
Raman spectra of 30 mmol/L sulfate solution using the LCOF-Raman experimental (LWCC-3050) setup and conventional experimental setup respectively.
Figure 9
Figure 9
Raman spectrum of CCl4 after extraction after conventional experimental setup.
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