This study characterizes the physical and chemical properties and the oxidative kinetics of the manganese soils found in Graskop, South Africa. Despite their importance in cycling redox-sensitive compounds in natural systems, much remains unknown about the reactivity of manganese-oxides formed under environmental conditions. In order to study the kinetics of geogenic manganese-oxides, soils were collected from Graskop, South Africa. Three soil profiles were excavated with a range of manganese concentrations. Each profile was further separated based on horizons, with some of the profiles containing over 200 g kg<sup>-1</sup> manganese. Manganese nodules of various sizes are ubiquitous in these soils. The soil in each horizon was analyzed to determine the cation exchange capacity (CEC), point of zero charge (PZC), surface area, and pH. X-ray powder diffraction (XRD) and scanning electron microscopy (SEM) were used to characterize the mineralogy of the crystalline material found in the clay fraction. Finally, a series of batch reactions were used to determine the capacity of these soils and nodules to oxidize arsenite into arsenate. The conditions of the reaction were varied in order to elucidate how pH and temperature influenced the oxidation reaction. Aliquots collected from these experiments were analyzed by liquid chromatography-inductively coupled plasma-mass spectrometry (LC-ICP-MS). Solid samples from these reactions were taken to Stanford Synchrotron Radiation Lightsource and analyzed on beamline 4-3 to determine the changes in manganese oxidation state after reacting with arsenite under varying conditions. Thin sections of native and reacted soils were also taken to beamline 4 BM (XFM) at the National Synchrotron Light Source II to determine the distribution of manganese and arsenic in these soils. This study provides key insights to more fully understand the role of manganese-oxides in controlling redox-sensitive reactions in the environment.

Kinetics of arsenic oxidation by naturally formed manganese-oxides

ACS Spring Meeting 2021

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