X-ray fluorescence mapping of mercury on suspended mineral particles and diatoms in a contaminated freshwater system

Mercury (Hg) bioavailability and geochemical cy- cling is affected by its partitioning between the aqueous and particulate phases. We applied a synchrotron-based X- ray fluorescence (XRF) microprobe to visualize and quan- tify directly the spatial localization of Hg and its correlations with other elements of interest on suspended particles from a Hg-contaminated freshwater system. Up to 175 µg g 1 Hg is found on suspended particles, but less than 0.01 % is in the form of methylmercury. Mercury is heterogeneously dis- tributed among phytoplankton (e.g., diatoms) and mineral particles that are rich in iron oxides and natural organic mat- ter (NOM). The diatom-bound Hg is mostly found on outer surfaces of the cells, suggesting passive sorption of Hg on di- atoms. Our results indicate that localized sorption of Hg onto suspended particles, including diatoms and NOM-coated ox- ide minerals, may play an important role in affecting the partitioning, reactivity, and biogeochemical cycling of Hg in natural aquatic environments.