PROCESSES OF ZN ATTENUATION BY BIOGENIC MANGANESE OXIDES FORMING IN THE HYPORHEIC ZONE OF PINAL CREEK, ARIZONA, USA

Characterization of the molecular scale mechanisms of metal sorption by Fe, Al, and Mn oxides is needed to better understand conditions that affect cycling and natural attenuation of metal contaminants, to enable design of remediation strategies, and to assess the potential for remobilization of attenuated metal contaminants in aquatic systems. At Pinal Creek, Arizona, basin-scale attenuation of hard-rock mine metal contaminant loads was attributed to microbial Mn oxidation in the hyporheic zone. Based on field observations, the ongoing formation of Mn oxide sorbent phase was proposed to enhance metal attenuation. Speciation of sorbed Zn and the mineralogy of Mn oxide coatings of sediments from the active hyporheic zone of Pinal Creek were characterized using micro-focused X-ray diffraction (µ-XRD), X-ray fluorescence (µ-XRF), and extended X-ray absorption spectra (µ-EXAFS) collected at the Advanced Light Source, Berkeley, California, USA, on beam lines 7.3.3 and 10.3.2. Bulk EXAFS spectra were collected at beamlines 11-2 and 4-3 at the Stanford Synchrotron Radiation Laboratory.

SEM images and u-XRF maps indicate extensive grain coatings of Mn oxides of up to 50 µm thickness. The mineralogy of these coatings is dominated by nanoparticulate Ca-bearing c-disordered hexagonal birnessite (10-Å phyllomanganate) indicative of biogenic origin. Correlations between Zn and Mn in µ-XRF images demonstrate that Zn is primarily associated with these biogenic birnessite coatings and is distributed throughout the birnessite coatings. Analysis of bulk and µ-EXAFS indicate that coordination of hexagonal and tetrahedral Zn occurs at octahedral vacancies of the hexagonal birnessite. The combination of structurally bound Zn and burial within the Mn oxide coatings is consistent with the limited reversibility of Zn measured in laboratory desorption experiments designed to simulate changes in stream-water chemistry. These findings demonstrate that the ongoing formation of Mn oxides in the hyporheic zone is the dominant sequestration process contributing to the observed attenuation of the Zn contaminant load in Pinal Creek.