title: Influence of oxygenation on chromium redox reactions with manganese sulfide (MnS(s)).
abstract: 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 CrVI reduction under anaerobic conditions occurred primarily on the surface of MnS(s) displaying a biphasic behavior- the initial rapid removal of CrVI from solution was followed by a slow decline due to surface passivation by reaction products, mainly sorbed or precipitated CrIII. The reaction progress increased with MnS(s) surface area loading but decreased on increasing CrVI 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 CrVI reduction by reduced sulfur species in solution, whereas increased CrIII 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 MnIII(hydr)oxide that was composed of hausmanite and manganite. CrVI reoccurrence was observed on aeration of CrVI-spiked MnS(s) from the oxidation of product CrIII. The reoccurrence at pH≥7 was attributed to the oxidation of product CrIII by MnIII(hydr)oxide, whereas the reoccurrence at pH<7 was hypothesized from the oxidation of product CrIII by intermediate aqueous MnIII and/or sulfur species. Just as with Cr, MnS(s) may play an important role in speciation, fate, and transport of other environmental contaminants.
authors: †Department of Geography and Environmental Engineering, Johns Hopkins University, 3400 N. Charles Street, 313 Ames Hall, Baltimore, Maryland 21218, United States.|‡Geosyntec Consultants, 10220 Old Columbia Road, Suite A, Columbia, Maryland 21046, United States.
authors: Amar R
authors: AR
authors: Wadhawan
authors: §Earth and Planetary Science, Johns Hopkins University, St. Martin Drive, Olin Hall, Baltimore, Maryland 21218, United States.
authors: Kenneth J
authors: KJ
authors: Livi
authors: †Department of Geography and Environmental Engineering, Johns Hopkins University, 3400 N. Charles Street, 313 Ames Hall, Baltimore, Maryland 21218, United States.
authors: Alan T
authors: AT
authors: Stone
authors: †Department of Geography and Environmental Engineering, Johns Hopkins University, 3400 N. Charles Street, 313 Ames Hall, Baltimore, Maryland 21218, United States.
authors: Edward J
authors: EJ
authors: Bouwer
mesh_terms: D002857:Chromium / Q000737:chemistry*; D017895:Manganese Compounds / Q000737:chemistry*; D010084:Oxidation-Reduction; D010087:Oxides / Q000737:chemistry; D010100:Oxygen / Q000737:chemistry*; D013440:Sulfides / Q000737:chemistry*; D013455:Sulfur / Q000737:chemistry; D014874:Water Pollutants, Chemical / Q000737:chemistry*
chemical_list: D017895:Manganese Compounds; D010087:Oxides; D013440:Sulfides; D014874:Water Pollutants, Chemical; C494384:manganite; D002857:Chromium; C027424:manganese oxide; D013455:Sulfur; C041368:manganese sulfide; D010100:Oxygen