Role of nano-goethite in controlling U(VI) sorption-desorption in subsurface soil

Iron oxide nanoparticles in natural soils and sediments can be nanoporous due to the aggregation of nanoparticles. Nanopores can greatly affect the sorption-desorption as well as reduction-oxidation behavior of U(VI) because of their high internal surface area and nanopore confinement effects. Batch U(VI) sorption-desorption experiments were conducted to understand the role of nano-goethite in controlling U(VI) sorption-desorption behavior in saprolite subsoil from Oak Ridge Field Research Center (ORFRC), Tennessee, a premier field research site for environmental remediation. The subsoil samples from ~6.5–8.5m depth were nearly homogeneous in mineral composition (quartz, illite and smectite) and particle size distribution. The subsoil samples were characterized by a range of dithionite-citrate-bicarbonate (DCB) extractable Fe(III) oxide content (127–404μM/g), BET surface area (25–38m2/g), and nanoporosity (pore size<10nm; 16–30μL/g). Transmission electron microscopy (TEM) imaging and X-ray energy dispersive spectroscopy (EDS) analyses revealed that aggregated goethite nano-crystals were poorly crystalline and nanoporous, and U(VI) sorption was dominantly associated with those nanoporous goethite aggregates rather than clay minerals. U(VI) sorption to the saprolite subsoils at circumneutral pH increased with higher amount of nanoporous goethite in the presence of a range of Ca2+ and HCO3−. Sequential extraction of U(VI) pre-sorbed to saprolite subsoils with NaHCO3 (1, 10, and 100mM) exhibited more resistant U(VI) desorption from soils containing higher amount of nanoporous goethite. This study suggests that nano-crystalline goethite with nanoporous structure may exert a significant role in controlling the mobility and transport of U(VI) in subsurface environment where nanoporous minerals are ubiquitous and abundant.