Reaction Scheme for the Oxidation of As(III) to As(V) by Birnessite

Johnnie N. Moore, Jeffrey R. Walker and Thomas H. Hayes
Department of Geology, University of Montana, Missoula, Montana 59812
Department of Geology, Vassar College, Poughkeepsie, New York 12601
Department of Chemistry, University of Montana, Missoula, Montana 59812

Abstract: The oxidation of As(III) to As(V) by K-birnessite was examined at different temperatures, pHs, and birnessite/As(III) ratios. Experiments ranged in duration from 5 to 64 hr, and solution and solid products were determined at several intervals. All experiments showed that the reaction produced large amounts of K+ to solution and very little Mn2+. As(V) was released to solution and incorporated into the K-birnessite. The oxidation was initially rapid and then slowed. The oxidation of As(III) was probably facilitated initially by autocatalytic Mn-As(V) reactions occurring mostly in the interlayer, in which large amounts of As(V) and K+ could be easily released to solution. The reaction also slowed when interlayer Mn was exhausted by forming Mn-As(V) complexes. Mn(IV) could only be acquired from the octahedral sheets of the birnessite. The two-stage reaction process proposed here depended on the layered structure of birnessite, the specific surface, and presence of exchangeable cations in K-birnessite.

Key Words: Arsenic • Birnessite • Oxidation • Potassium • Solution

Clays and Clay Minerals; October 1990 v. 38; no. 5; p. 549-555; DOI: 10.1346/CCMN.1990.0380512
© 1990, The Clay Minerals Society
Clay Minerals Society (www.clays.org)