Uranium(IV) Complexation by Natural Organic Matter Controls Speciation in the Subsurface

Abstract:

Uranium contaminates groundwater at many sites throughout the United States. At the aquifer in Rifle, CO, U(IV) has been found to accumulate in natural organic matter (NOM)-rich sediments comprising buried alluvial material. We expect that NOM, which is composed of detrital plant material and microbial biomass and necromass, profoundly influences the speciation of U(IV). Specifically, we hypothesize that NOM forms stable complexes with U(IV) (i.e., “noncrystalline” U(IV)), particularly through organic phosphorus moieties associated with bacteria and exopolymeric substances (EPS). Complexation with NOM can help to explain why noncrystalline U(IV) is more abundant in the subsurface than the mineral uraninite (UO₂). The abundance and relative reactivity of non-crystalline U(IV) suggests that it drives U fate and transport in the subsurface.

W are examining the reduction of U(VI) and subsequent complexation of U(IV) in model NOM systems comprising homogenized, partially degraded plant material, which is analogous to the detrital plant material abundant in Rifle sediments, and its associated microbial consortia. We employ a suite of spectroscopic (X-ray absorption spectroscopies) and microscopic (scanning transmission X-ray microscopy, scanning electron microscopy, and nano-scale secondary ion mass spectrometry) tools that allow us to identify the number and types of coordinating ligands and the distribution of U with respect to NOM components, including bacterial cells, plant matter and entrained minerals. In preliminary experiments we found that 50 - 100 % of U(VI) was reduced to U(IV) within several days. Furthermore, NOM was observed to sorb both U(VI), as a carbonyl complex, and U(IV), possibly as a phosphoryl complex. Further microscopic analyses are designed to elucidate whether U(IV)-P complexes are associated with bacteria or EPS. Our research suggests a new, more complicated model for U(IV) speciation in subsurface sediments, in which complexation by NOM, as opposed to mineral formation, controls U reactivity.