High-resolution mass spectrometry for molybdenum speciation in sulfidic waters

High resolution electrospray ionization mass spectrometry (ESI-HRMS) was used to study the speciation of molybdenum in interaction with sulfide and chloride. While reactions between molybdate and sulfide lead to creation of four conventional thiomolybdate species (MoO₃S^2-, MoO₂S₂^2-, MoOS₃^2-, MoS₄^2-), chemical formula assignment of recorded mass spectra confirmed the presence of intermediate thiomolybdate compounds (MoO₃S₂^2-, MoO₂S₃^2-, MoOS₄^2-, MoS₅^2-, MoS₆^2-) and thio-chloro-molybdate compounds (MoO₃Cl^-, MoO₂SCl^-, MoOS₂Cl^-, MoOS₃Cl^-). Two of the intermediate thiomolybdate compounds were previously suggested theoretically, and this study provides analytical support for the existence of these compounds. Fast ESI-HRMS analysis has allowed us to conduct a highly-resolved short-term kinetic study of these reactions, and we suggest that the reactivity between molybdate and sulfide is more complex than previously thought, particularly during the first 24 h of interaction. Also, the solution composition will impact reaction pathways, and different outcomes found in the literature may arise from choices of ionic strength and pH adjusting agents in previous studies . The occurrence of the thio-chloro-molybdate species detected in this study should be implemented in future Mo speciation models to better assess Mo reactivity in sulfidic waters and reducing environments.