Characterization and Testing of Amidoxime-Based Adsorbent Materials to Extract Uranium from Natural Seawater

Abstract

Extraction of uranium (U) from seawater for use as a nuclear fuel is a significant challenge due to the low concentration of U in seawater (∼3.3 ppb) and difficulties to selectively extract U from the background of major and trace elements in seawater. The Pacific Northwest National Laboratory (PNNL)’s Marine Sciences Laboratory (MSL) has been serving as a marine test site for determining performance characteristics (adsorption capacity, adsorption kinetics, and selectivity) of novel amidoxime-based polymeric adsorbents developed at Oak Ridge National Laboratory (ORNL) under natural seawater exposure conditions. This manuscript describes the performance of three formulations (38H, AF1, AI8) of amidoxime-based polymeric adsorbents produced at ORNL in MSL’s ambient seawater testing facility. The adsorbents were produced in two forms, fibrous material (40–100 mg samples) and braided material (5–10 g samples), and exposed to natural seawater using flow-through columns and recirculating flumes. All three formulations demonstrated high 56 day uranium adsorption capacity (>3 g U/kg adsorbent). The AF1 formulation had the best uranium adsorption performance, with a 56 day capacity of 3.9 g U/kg adsorbent, a saturation capacity of 5.4 g U/kg adsorbent, and ∼25 days half-saturation time. The two exposure methods, flow-through columns and flumes, were demonstrated to produce similar performance results, providing confidence that the test methods were reliable, that scaling up from 10’s of mg quantities of exposure in flow-through columns to gram quantities in flumes produced similar results, and confirm that the manufacturing process produces a homogeneous adsorbent. Adsorption kinetics appear to be element specific, with half-saturation times ranging from minutes for the major cations in seawater, to 8–10 weeks for V and Fe. Reducing the exposure time provides a potential pathway to improve the adsorption capacity of U by reducing the V/U ratio on the adsorbent.

Bicarbonate Elution of Uranium from Amidoxime‐Based Polymer Adsorbents for Sequestering Uranium from Seawater

“…About 33 % of vanadium can be eluted under our experimental conditions shown in Figure . Acid leaching (0.5 M HCl) is known to remove a significant portion of most elements, except vanadium but the reusability of adsorbent is a problem , . The high‐surface‐area amidoxime‐based adsorbents synthesized by radiation induced grafting method degrades after acid leaching.…”

Temperature Dependence of Uranium and Vanadium Adsorption on Amidoxime‐Based Adsorbents in Natural Seawater

“…The amidoxime ligand, which has a high stability constant for the uranyl ion, also has a strong affinity for several other metal ions in seawater, including V, Fe, Zn, Cu, and Ni ,,,. This multi‐element affinity indicates that a competition exists between seawater cations and binding sites of the amidoxime‐based adsorbents.…”

“…This multi‐element affinity indicates that a competition exists between seawater cations and binding sites of the amidoxime‐based adsorbents. Of particular interest is V adsorption, as this element exists at higher molar concentrations than U in seawater (U ∼ 14 nM; V ∼ 34 nM) and also V adsorbs at a higher degree than U on amidoxime‐based adsorbents in seawater ,,. To illustrate V vs. U adsorption at 20 °C, the V capacity of a saturated amidoxime‐based adsorbent was 5 times higher than the U saturation capacity .…”

“…Of particular interest is V adsorption, as this element exists at higher molar concentrations than U in seawater (U ∼ 14 nM; V ∼ 34 nM) and also V adsorbs at a higher degree than U on amidoxime‐based adsorbents in seawater ,,. To illustrate V vs. U adsorption at 20 °C, the V capacity of a saturated amidoxime‐based adsorbent was 5 times higher than the U saturation capacity . Because U and V have different temperature sensitivities to adsorption, however, the mass ratio of adsorbed V to U (V/U, w/w) decreases significantly from 8 to 31 °C; there is a 3‐fold drop in the V/U mass ratio for the AF1 adsorbent and a 4.5‐fold drop in the V/U mass ratio for the AI8 adsorbent from 56‐d loading data (Table ).…”

“…Recently, significant improvements in adsorption capacity have been achieved for extraction of U from seawater using amidoxime‐based polymeric adsorbents ,,. Reported U adsorption loading in real seawater tests (20 °C, salinity 35 psu, 56 days of exposure) have reached 5.96 ± 0.29 g U (kg adsorbent) −1 , with a saturation capacity of 9.84 ± 0.48 g U (kg adsorbent) −1 .…”