Molybdenum scavenging by iron monosulfide

Area of Science:

• Geochemistry
• Environmental Science
• Mineralogy

Background:

• Molybdenum (Mo) sedimentary records are valuable for reconstructing past anoxia.
• Understanding Mo fixation mechanisms is crucial for accurate geochemical interpretations.
• Anthropogenically impacted waters present complex biogeochemical cycling of Mo.

Purpose of the Study:

• To investigate the role of iron sulfide (FeS) precipitation in scavenging molybdenum (Mo) from aqueous solutions.
• To elucidate the mechanisms of Mo interaction with FeS in a model system.
• To assess the influence of kaolinite and sulfide concentrations on Mo scavenging by FeS.

Main Methods:

• A model system using Fe(III)-bearing kaolinite (KGa-1B) dispersed in sodium hydrosulfide (NaHS) solutions.
• Addition of thiomolybdates (MoOS3(2-)) to the system.
• Monitoring FeS production via dissolved polysulfide measurements.
• Quantifying Mo uptake and release using Mo/Fe mole ratios.

Main Results:

• Kaolinite itself showed minimal Mo sorption at pH 8.6.
• Newly formed FeS rapidly scavenged thiomolybdates, with initial Mo/Fe mole ratios of 0.04-0.06.
• Mo was expelled from FeS solids after approximately one day, coinciding with FeS recrystallization.
• FeS demonstrated higher affinity for thiomolybdates than molybdate (MoO4(2-)).
• Kaolinite accelerated thiomolybdate conversion and facilitated Fe(III) reduction.

Conclusions:

• Iron sulfide (FeS) acts as a transient but effective scavenger for Mo, particularly thiomolybdates, in sulfidic environments.
• The scavenging capacity of FeS is temporary due to its transformation into more stable mineral phases.
• While FeS plays a role, iron disulfide (FeS2) and organic matter are likely the ultimate sinks for sedimentary Mo.