Molybdenum scavenging by iron monosulfide

Molybdenum profiles in dated sediment cores provide useful historical information about anoxia in anthropogenically impacted natural waters but would be of greater service if Mo fixation mechanisms were better understood. Here, we explore Mo scavenging by precipitated FeS in a model system consisting of an Fe^III-bearing kaolinite (KGa-1B) dispersed in NaHS solutions. Test solutions contain 18 μM thiomolybdates (mainly MoOS₃ ^2-). Optically measuring dissolved polysulfides monitors the rate of FeS production from Fe^III minerals. Even though the exposed clay surface area is large (450 m²/L), the clay itself sorbs little Mo at pH 8.6. As FeS forms, Mo is taken up in initial Mo/Fe mole ratios of 0.04-0.06, irrespective of HS^- concentration (4-40 mM range). After about a day, Mo expulsion from the solids begins, accompanied by net polysulfide consumption. These changes reflect recrystallization of amorphous FeS to more ordered products such as greigite. FeS captures some MoO₄ ^2- but captures thiomolybdates more effectively. Kaolinite accelerates conversion of MoOS₃^2- to MoS₄ ^2-, as predicted previously, and thiomolybdates facilitate reduction of Fe^III minerals in the clay compared to Mo-free solutions. FeS is a potentially effective, transient scavenging agent for Mo in sulfidic environments, although FeS₂ and organic matter appear to be the ultimate sedimentary hosts.