351-3 Effects of Dissolved Fe(II) On As(III) Oxidation and As(V) Sequestration by Hydrous Manganese Oxide.



Wednesday, October 19, 2011
Henry Gonzalez Convention Center, Hall C, Street Level

Yun Wu and Donald Sparks, Plant & Soil Sciences, University of Delaware, Newark, DE
The oxidation of arsenite (AsIII) by manganese (Mn) oxides is an important process in the natural cycling of arsenic (As) in the environment. Previous studies have demonstrated rapid oxidation of AsIII by Mn oxides which produces the less toxic and less mobile arsenate (AsV) that can be sorbed on mineral surfaces. Under natural conditions, the presence of other ions (i.e., FeII) can influence the behavior of AsIII on Mn oxides. However, very few studies have focused on the effects of FeII on the redox and sorption processes. In this study, AsIII oxidation by hydrous manganese oxide (HMO) in the presence and absence of dissolved FeII was investigated using stirred-flow experiments. Chemically synthetic HMO was reacted with three influent solutions, containing the same AsIII concentrations but different FeII concentrations, at pH 4 for 24 hours. The results show an initially rapid AsIII oxidation by HMO, which is followed by an appreciably slow reaction after 4 hours. In the presence of FeII, AsIII oxidation is slightly decreased by 10%. This is due to FeII reacting with the HMO surface, decreasing HMO reactivity. Evidence for this was an increase in dissolved MnII concentrations as well as a decrease in dissolved FeII concentrations in the effluent solutions when FeII was present. However, when AsIII is oxidized to AsV, the retention of AsV increases greatly in the presence of FeII and also increases with increasing FeII influent concentrations. This could be attributed to the formation of FeIII hydroxides on the HMO surface after FeII is oxidized, which could provide additional sorption sites for AsV. This study suggests that the oxidative precipitation of FeIII compounds on HMO surface may play a very important role in AsIII oxidation and AsV sequestration. Molecular scale studies are in progress to address the reaction mechanisms.
See more from this Division: S02 Soil Chemistry
See more from this Session: Redox and Surficial Reactions In Soils: II