Molecular to Micron-Scale Investigations of Floc and Colloidal Fractions of Wetland Groundwater and Surface Waters

Authors

Kenneth M. Kemner¹* (kemner@anl.gov), Maxim I. Boyanov^2,1, Dan Kaplan³, Kent Orlandini¹, Maggie Bowman⁴, Rosey Chu⁴, Alice Dohnalkova⁴, Libor Kovarik⁴, Ravi Kukkadapu⁴, Crystal Ng⁵, Anthony Boever⁶, Martial Taillefert⁶, Odeta Qafoku⁴, Cara Santelli⁵, Jason Toyoda⁴, Pamela Weisenhorn¹, Edward J. O’Loughlin¹

Institutions

¹Argonne National Laboratory, Lemont, IL; ²Bulgarian Academy of Science, Sofia, Bulgaria; ³University of Georgia, Aiken, SC; ⁴Pacific Northwest National Laboratory, Richland, WA; ⁵University of Minnesota, Minneapolis, MN; ⁶Georgia Institute of Technology, Atlanta, GA

URLs

Abstract

The mobility and bioavailability of nutrients and contaminants have a strong effect on wetland ecosystem function and are highly dependent upon their: (1) partitioning between solid and solution phases in environmental media; and (2) atomic scale associations with environmental solids. Abundant orange and reddish-brown flocs have been observed along gaining sections of the Argonne Wetland Hydrobiogeochemistry Science Focus Area (SFA) field site at Tims Branch, where anoxic groundwater containing iron(II) discharges into oxygenated stream water. Flocs contain high levels of iron (Fe) and are effective scavengers of phosphorous (P; 2 to 4 wt%), uranium (U; 32 to 600 ppm), and trace metals. The laboratory microcosm studies of these flocs show that the transition from oxic to anoxic conditions leads to the reduction of Fe(III) to Fe(II) and U(VI) to non-uraninite U(IV); following a return to oxic conditions, Fe(II) and U(IV) oxidize back to Fe(III) and U(VI). Analysis of material along the depth profile of a core of accumulated floc material shows a progressive reduction of Fe(III) to Fe(II) and U(VI) to U(IV) with depth, consistent with decreasing redox potential.

In addition to characterizing the molecular to micron-scale structure of naturally forming flocs at Tims Branch, researchers have also characterized colloids from groundwaters and surface waters. Researchers used tangential flow filtration approaches to size fractionate colloids from groundwater and stream water, under gaining stream conditions. Researchers used a combination of radiochemistry, inductively coupled plasma atomic emission spectroscopy (ICP-AES), Fe K-edge and U L-edge XAFS, ⁵⁷Fe Mössbauer spectroscopy, and scanning and scanning transmission electron microscopy (SEM, STEM) equipped with energy dispersive spectroscopy (EDS) to understand the physical and chemical character of the colloids. Molecular scale results indicated Fe-(hydr)oxide and Fe-OM associations within all colloid size ranges. SEM/EDS results consistently identified U associated with carbon- and Fe-rich colloids. U-XAFS analysis identified monomeric U(VI) species in the colloids. STEM analysis resolved individual and spatially separated U atoms associated with many of the colloid OM formations. These results indicate that the vast majority of U within the surface water and groundwater is not in a purely hydrated form but is associated with colloids. The studies of Fe floc and colloid biogeochemistry in Tims Branch and its potential impact on U speciation and transport expand the understanding of their role in the speciation and cycling of nutrients and trace elements in wetlands, which in turn can lead to more robust modeling of their behavior in aquatic and terrestrial environments.