Although abiotic Mn(II) oxidation is one of the fundamental geochemical processes that controls the formation and the phase transformation of numerous types of Mn (oxyhydr)oxides, the importance of homogeneous Mn(II) oxidation has been neglected primarily due to its slow kinetics under the prevailing pH conditions of natural water. However, it has been recently reported that some chemical reactions could be accelerated in freezing aqueous solution. If the Mn(II) oxidation could be accelerated upon freezing, the role of homogenous Mn(II) oxidation would be far more important than previously understood. In this study, we systematically examined the effects of pH and Mn(II) concentration on the rate of Mn(II) oxidation under freezing conditions. Batch experiments were conducted with varying Mn(II) concentrations (10 – 100 mM) in a range of pH (7 – 9) with or without a buffer at the atmospheric PO2. Ice samples were prepared by placing amber conical tubes in an ethanol coolant at −20 °C for a range of reaction time and rapidly thawed in lukewarm water. Our preliminary results show that the extent and the rate of Mn(II) oxidation was negligible at pH &lt; 8.5 but substantially enhanced at the higher pH upon freezing. At pH 8.5, the overall rate of cryogenic Mn(II) oxidation apparently followed pseudo-1st order kinetics with respect to Mn(II) concentration.

Effects of pH and Mn(II) concentration on Mn(II) oxidation under freezing condition

2019 Fall National Meeting

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.