An Assessment of U(VI) Adsorption at Multiple Scales

Abstract

Iron (III) (oxyhydr)oxide coatings on soils and sediments are one of the most important factors in controlling the adsorption and transport of U(VI) in the subsurface. In this study, iron-coated sands were prepared via two common protocols, a precipitation method, where Fe was precipitated directly onto the sand in a single step, and an adsorption method, where pure goethite was prepared in the first step and then adsorbed onto the sand in a second step (Chapter 2). The coated sands from both the systems were characterized using scanning electron microscopy, energy dispersive spectroscopy, x-ray diffraction, and selective Fe extraction. Although neither of the methods produced a completely crystalline Fe coating, the precipitation method produced sands with larger portions of amorphous Fe than the adsorption method, with the fraction of amorphous Fe decreasing with increasing Fe content. U(VI) adsorption isotherms and pH adsorption edges were measured on three coated sands with Fe contents ranging from 0.04 to 0.3% (Chapter 3). Experimentally, the adsorption of U(VI) onto the three sands was more comparable when normalized to surface area than when normalized to Fe content. A surface complexation model, although originally developed for U(VI) adsorption onto amorphous Fe oxide, captured the differences in adsorption when adjusted for the surface area of the coated sand. One of the most perplexing and yet unresolved problems is the discrepancy observed between batch derived and column derived adsorption capacities. Several studies indicate the existence of the above phenomena, especially when U(VI) has been used as an adsorbate. One of the goals of this study was to understand the causatives of the above problem using both natural heterogeneous geomedia and a homogeneous synthetic adsorbent (Chapter 4). The uncontaminated natural geomedia was obtained from the Oak Ridge Reservation (OR) and the synthetic media, iron oxide-coated sand (IOCS), was meticulously prepared in the laboratory. The OR soil and IOCS were used to perform adsorption experiments with U(VI) as a solute in both batch and column modes. In the case of batch scale, adsorption experiments were conducted by varying the solid-to-solution ratio (SSR) over an order of magnitude at a fixed pH. The results indicated that the adsorption isotherms were scalable in iron oxide-coated sand system but not in OR soil system. Based on our theoretical analysis, and also supported by recent literature, the observed phenomenon occurring in natural soils could be due to a competing solute (e.g. Phosphate) that could affect the speciation of surface complexes by forming ternary species with U(VI). Hence, our study underscores the serious implications of transferring adsorption data obtained from experiments performed at different scales. Although the interactions of U(VI) and Fe-coated sands were used as representative adsorbate and adsorbent, the general principles may be applicable to other adsorbate-adsorbent systems as well. The ever increasing growth of biorefineries is expected to produce huge amounts of lignocellulosic biochar as a byproduct. The hydrothermal carbonization (HTC) process to produce biochar from lignocellulosic biomass is getting more attention due to its inherent advantage of using wet biomass. In the present study, biochar was produced from switchgrass at 300°C in subcritical water (Chapter 5). The physiochemical properties indicated that biochar could serve as an excellent adsorbent to remove uranium from groundwater. A batch adsorption experiment at the natural pH (~ 3.8) of biochar indicated an H-type isotherm with a maximum sorption capacity of 2 mg/g. The adsorption process was highly dependent on the pH of the system. An increase towards circumneutral pH resulted in the maximum U(VI) adsorption of ca. 4 mg g-1. The results indicated a strong relationship between the speciation of U(VI) and its adsorption onto biochar. Our study demonstrates that biochar could be used as an effective adsorbent medium for U(VI). Overall, the biochar produced via HTC is environmentally benign, carbon neutral, and efficient in removing U(VI) from groundwater.