Manganese Valence in Oxides Formed from in Situ Chemical Oxidation of TCE by KMnO4

Abstract

Batch and column experiments designed to simulate in situ chemical oxidation (ISCO) in a sand aquifer were conducted to create Mn-oxides (MnOx) by oxidation of trichloroethylene (TCE) with permanganate (MnO4-). Electron energy-loss spectroscopy (EELS) and X-ray photoelectron spectroscopy (XPS) were used to quantify Mn valence in the oxides. The valence of Mn in the MnOx generated in near-source ISCO conditions was 2.2 and 2.3 when formed at low (<3) and neutral (6-7) pH conditions, respectively. There is no significant difference between these values. Valence was found to be sensitive to the preparation method and to aging. When formed in the presence of excess MnO4-, or aged for 3 months, Mn valence ranged from 2.5 to 3.6. Aging in a lower pH environment inhibited Mn oxidation. The EELS and XPS methods provided similar results, but there was a slight bias to higher values for XPS. This work demonstrates that MnO2(s) may not be the main product of MnO4- reaction with chlorinated solvents as is commonly assumed and that the efficiency of ISCO treatment may be greater than previously known.