Method development for chromatographic separation of thiolated arsenic species in iron- and sulfide-rich marine waters
Elena Hartner (08/2018-05/2019)
Support: Britta Planer-Friedrich
Thioarsenates play an important role for the geochemistry of arsenic (As) under sulfidic reducing conditions as they exist in submarine hydrothermal fluids. Besides sulfide, these fluids contain high amounts of iron, leading to difficulties during the ion chromatographic (IC) separation of arsenite, arsenate and thioarsenates and their subsequent detection by inductively coupled plasma mass spectrometry (ICP-MS).
The aim of this thesis therefore was to develop an improved method for arsenic speciation analysis by IC-ICP-MS to chemically characterize hydrothermal fluids. Furthermore, it aimed towards an evaluation of the newly developed method for samples with complex arsenic-sulfide-iron matrices from the Tonga arc and Lau back-arc basin in the southwest Pacific.
The presented chromatographic setup featured a so-called InGuard cartridge for in-line iron removal by means of cation exchange and employed gradient elution of 20-100 mM NaOH for species separation in the analytical column. The chromatograms could significantly be improved concerning peak shape and separation. The lowest detectable concentration for arsenite and arsenate was as low as 0.05 ppb. However, the huge range of recoveries when comparing the sum of the species to total arsenic concentrations showed that there still are some remaining challenges.
The presence of thioarsenates in hydrothermal fluids could be confirmed. They predominantly occurred under reducing conditions and at high sulfide concentrations, where they dominated the species distribution to up to 94%. The samples with high proportions of thioarsenates were also those with the lowest total arsenic concentrations. This suggests that thioarsenates play a crucial role during the transition from dissolved oxyarsenic species to arsenic-sulfide minerals, which precipitate on the seafloor. This limits the amount of total arsenic in solution and indicates the necessity to consider thioarsenates in marine arsenic cycling. With increasing distance from the venting source, transformation of thioarsenates to arsenate was observed, caused by mixing of the hydrothermal fluids with oxygenated seawater.
This thesis shows the great potential of in-line sample pretreatment during IC analyses of arsenic speciation for iron- and sulfide-rich waters and highlights the importance of suitable analyses techniques for the detection of thioarsenates in marine environments.