The Role of Nanopores on U(VI) Sorption and Redox Behavior in U(VI)-Contaminated Subsurface Sediments

Most reactive surfaces in clay-dominated sediments are present within nanopores (pores of nm dimension). The behavior of geological fluids and minerals in nanopores is significantly different from those in normal non-nanoporous environments. The effect of nanopore surfaces on U(VI) sorption/desorption and reduction is likely to be significant in clay-rich subsurface environments. Recent studies of ethanol stimulated reduction of U(VI) associated with weathered shale saprolite sediments from the Oak Ridge National Laboratory Field Research Center (ORFRC) indicate that only ca. 50% of solid-associated U(VI) is subject biotic or abiotic reduction. The redox speciation of the residual, non-reducible U(VI) cannot be explained based on standard models of U(VI) redox speciation. We propose that nanopores in these clay-rich sediments coordinate U(VI) with very strong physical-chemical affinity, and thereby control the reactivity of a large fraction of solid-associated U(VI). Our FTIR study shows that vibration modes at the range of ~1620 to ~1640 cm-1 from nanopore water (1625 cm-1 for water in 2 nm pores, and 1635 cm-1 for water in 3 nm pores) are different from that from bulk water (1643 cm-1). Recent molecular dynamics simulations show that the dielectric constant of water confined in a nanodimensional spherical cavity is much lower than that of bulk water. The dielectric constant of water will affect Born solvation energy of both cations and anions. We propose that solute water in nanopores can reduce uranyl and/or uranyl-carbonate solvation and therefore increase U(VI) sorption and chemical affinity (i.e., significantly lower effective redox potential). The energy difference between the surface with negative curvature and the surface positive curvature can further and limit/decrease desorption of U(VI) from the nanopore surfaces. The influence of nanopore surfaces on coupled uranium sorption/desorption and reduction processes is likely to be significant in virtually all subsurface environments.