Influence of Oxygenation on Chromium Redox Reactions with Manganese Sulfide (MnS(s))

Manganese sulfide (MnS(s)) minerals exist in sulfidic environments and can have unique reactive abilities because of sulfide, which is a known reductant, and Mn, the oxyhydroxides of which are known oxidants. This study elucidated the role of MnS(s) in controlling Cr speciation with implications on its fate and toxicity in the natural environment, specifically sulfidic sediments that undergo biogeochemical changes due to sediment resuspension during dredging, bioturbation, and flood events. In continuously mixed batch reaction experiments, aqueous Cr-VI reduction under anaerobic conditions occurred primarily on the surface of MnS(s) displaying a biphasic behavior- the initial rapid removal of Cr-VI from solution was followed by a slow decline due to surface passivation by reaction products, mainly sorbed or precipitated Cr-III. The reaction progress increased with MnS(s) surface area loading but decreased on increasing Cr-VI concentration and pH, suggesting that surface site regeneration through product desorption was the rate-controlling mechanism. Below circum-neutral pH, higher solubility of MnS(s) resulted in additional Cr-VI reduction by reduced sulfur species in solution, whereas increased Cr-III solubility lowered surface passivation allowing for more reactive sites to participate in the reaction. Aeration of MnS(s) at pH >= 7 caused the formation of a heterogeneous Mn-III(hydr)oxide that was composed of hausmanite and manganite. Cr-VI reoccurrence was observed on aeration of Cr-VI-spiked MnS(s) from the oxidation of product Cr-III. The reoccurrence at pH >= 7 was attributed to the oxidation of product Cr-III by Mn-III(hydr)oxide, whereas the reoccurrence at pH < 7 was hypothesized from the oxidation of product CrIII by intermediate aqueous Mn-III and/or sulfur species. Just as with Cr, MnS(s) may play an important role in speciation, fate, and transport of other environmental contaminants