Feasibility Study on Quantitative Analysis of Sulfide Concentration and pH of Marine Sediment Pore Water via Raman...

Marine sediment pore water is one of the important objects in the study of global environmental change, marine geology and biogeochemistry. Anoxic pore water in highly reducing deep-sea sediments commonly contains a large amount of dissolved sulfide (H2S and HS-). The sulfide species within sediment pore water are significant not only because the importance of themselves, but also because they exist as a function of pH which is another key parameter in pore water study. As degassing and chemical equilibrium altering are both inevitable, concentrations of sulfide species and pH value of marine sediment pore water acquired with traditional non-in situ technologies are of great uncertainty, and cannot represent the real geochemistry information. However, the recent deployment of an in situ laser Raman pore water sampler allows us to observe spectral sulfide signals of marine sediments in situ and in real time, which provide us a new technique to solve this problem. Sulfide species in water have a relatively strong Raman signal, which often appears in the form of a characteristic overlapping peak between 2 550 - 2 620 cm(-1) and can be decomposed into HS- at 2 572 cm(-1) and H2S at 2 592 cm(-1). In the present paper, quantitative analysis of H2S and HS- with Raman spectroscopy is proved practicable and the accuracy is good. The pH of pore water is an important influencing factor of the diagenetic processes. As H2S and HS- are conjugate acid-base pairs, sulfide species within pore water exist as a function of pH and their concentration ratio depend on pH. This relationship is also shown in the Raman spectrum. To formulate the pore water pH calculation, sulfide solutions with pH range from 6.11 to 13.05 were prepared and their Raman spectra were observed. It is verified that the morphology of overlapping peaks change regularly with pH values. This phenomenon provides us the possibility of measuring the pH of pore water in situ via Raman spectroscopy. Based on peaks decomposition and correlativity analysis, we propose here a novel in situ pH measuring method for sediment pore water containing sulfide. This method can be used to measure the pH of pore water when the overlapping peak of sulfide is resolvable. The application scope of this pH measuring method in this study is 6.11 - 8.32, which covers almost all pH value of marine sediment pore water already known. The study provides additional technical reference for obtaining high-fidelity information of marine sediment pore water.