Arsenite oxidation by a poorly crystalline manganese-oxide. 2. Results from X-ray absorption spectroscopy and X-ray diffraction.
Arsenite (As(III)) oxidation by manganese oxides (Mn-oxides) serves to detoxify and, under many conditions, immobilize arsenic (As) by forming arsenate (As(V)). As(III) oxidation by Mn(IV)-oxides can be quite complex, involving many simultaneous forward reactions and subsequent back reactions. During As(III) oxidation by Mn-oxides, a reduction in oxidation rate is often observed, which is attributed to Mn-oxide surface passivation. X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) data show that Mn(II) sorption on a poorly crystalline hexagonal birnessite (δ-MnO₂) is important in passivation early during reaction with As(III). Also, it appears that Mn(III) in the δ-MnO₂ structure is formed by conproportionation of sorbed Mn(II) and Mn(IV) in the mineral structure. The content of Mn(III) within the δ-MnO₂ structure appears to increase as the reaction proceeds. Binding of As(V) to δ-MnO₂ also changes as Mn(III) becomes more prominent in the δ-MnO ₂ structure. The data presented indicate that As(III) oxidation and As(V) sorption by poorly crystalline δ-MnO₂ is greatly affected by Mn oxidation state in the δ-MnO₂ structure.
Department of Plant and Soil Sciences, Delaware Environmental Institute, University of Delaware, Newark, Delaware 19716, USA. bjl@udel.edu
Brandon J
BJ
Lafferty
Matthew
M
Ginder-Vogel
Mengqiang
M
Zhu
Kenneth J T
KJ
Livi
Donald L
DL
Sparks
D000327:Adsorption; D018053:Arsenites / Q000737:chemistry*; D003460:Crystallization; D017895:Manganese Compounds / Q000737:chemistry*; D010084:Oxidation-Reduction; D010087:Oxides / Q000737:chemistry*; D056928:X-Ray Absorption Spectroscopy; D014961:X-Ray Diffraction
D018053:Arsenites; D017895:Manganese Compounds; D010087:Oxides; C505018:birnessite; C027424:manganese oxide; C015001:arsenite