The speciation of Cr either as relatively non-toxic CrIII or as toxic CrVI governs its risk to aquatic organisms and human health. In this study, we elucidate the role of MnS(s), a naturally occurring mineral phase, in controlling Cr speciation with implications on its fate and toxicity in sulfidic environments. Mechanisms of Cr redox interactions with MnS(s) were investigated in batch reaction experiments. MnS(s) suspensions reduced aqueous CrVI under anaerobic conditions via a biphasic reaction on MnS(s) surfaces. The initial rapid removal of CrVI under adsorption- and/or electron transfer- controlled conditions preceded a slow reaction under conditions limited by product-CrIII desorption and surface-site regeneration. Increased MnS(s) and product- CrIII solubility on lowering pH increased CrVI reduction rates. MnIII(hydr)oxide formation on aerating MnS(s) suspensions at pH = 7 caused CrVI reoccurrence from the oxidation of product-CrIII. The reaction proceeded under conditions limited by MnIII(hydr)oxide surface-site renewal due to product-MnII oxidation.

Role of manganese sulfide (MnS(s)) minerals in chromium (Cr) speciation and fate

244th National Meeting (2012)

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