Assessment of Impacts of Dissolved Organic Matter and Dissolved Iron on the Performance of Amidoxime-Based Adsorbents for Seawater Uranium Extraction

Abstract

One critical challenge of the development of seawater uranium extraction is to make uranium adsorbents perform well in the complex real seawater matrix, which is comprised of many competing ions and natural organic substances. Here, we conducted a systematic study using a continuous-flow seawater flume system to assess the potential impacts of dissolved organic matter (DOM) and dissolved iron on the uranium uptake performance, including adsorbent reusability, of amidoxime-based adsorbents. In the 28-day exposure, the adsorbent exposed in dissolved Fe-spiked seawater (low DOM/high Fe) and humic acid-spiked seawater (high DOM/high Fe) showed lower uranium adsorption loadings (73% and 56% of adsorption loading, respectively) than the same adsorbent exposed in seawater without spiking (low DOM/low Fe). The uranium adsorption loading of the reused adsorbent (after uranium stripping by a mild bicarbonate elution) in the dissolved Fe-spiked seawater dropped substantially to only 24% of the loading in the unspiked clean seawater counterpart, while not much change was observed in the performance of the adsorbent exposed to the humic acid-spiked seawater. Fourier transform infrared signatures of adsorbents suggest that the amidoxime ligands in the adsorbent exposed to dissolved Fe-spiked seawater had more severe degradation than the adsorbents exposed to humic acid-spiked seawater and unspiked clean seawater. Unlike the adsorbent exposed to dissolved Fe-spiked seawater, the adsorbent exposed to humic acidspiked seawater did not adsorb an elevated level of Fe compared to the adsorbent in the unspiked clean seawater. This suggests that the species of Fe in the humic acid-spiked seawater (primarily humic acid bound Fe) did not interact with the amidoximebased adsorbent, while the Fe species in the Fe-spiked seawater strongly interacted with the adsorbent and caused significant degradation of amidoxime ligands. On the other hand, highly variable uranium adsorption loadings were observed in the adsorbent in contact with humic acid-spiked seawater, but not in the adsorbents exposed to dissolved Fe-spiked seawater and seawater without spiking. Our observations indicate that seawater receiving high inputs of DOM and dissolved Fe, such as coastal waters, is not ideal for efficient extraction of uranium.

Heterometallic Uranyl (Hydroxyethyl)iminodiacetic Acid (Heidi) Complexes: Molecular Models for U(VI) Uptake in Complex Media

“…[5a,27] In the presence of the HEIDI ligand at pH 3, U(VI) uptake decreases to 63.6 � 0.1 %. A decrease in the U(VI) loading in the presence of organic ligands (3.51 mg/L dissolved organic carbon) was also reported by Kuo et al, [8] where they observed a reduction of approximately 40 % compared to the U(VI) control at pH 7. In a separate study, Ahmad et al [28] reported a reduction of 11.1 % U(VI) in the presence of 25 mg/L of humic acid at pH 5.5.…”

“…However, there is no significant differences in the overall U(VI) uptake in any of these systems, which also indicates that these binary phases do not influence the ability of the AO‐PAN to effectively remove U(VI) from the system at a pH of 2–4. Under more alkaline conditions, Kuo et al [8] . did note that in the presence of Fe(III) the U(VI) capacity decreased to 43 % compared to the control.…”

“…[9] These humic acid species can increase solubility of the U(VI) in solution by changing the overall charge on the complex or hindering the ability of the U(VI) to further bind to reactive surface sites. [10] Teterin et al observed that Fe(III) will preferentially bind to humic acid over U(VI) and the reaction is heterogeneous, [11] which likely contributes to the variability observed by Kuo et al [8] However, Dublet et al reported that U(VI) in the presence of both Fe(III) minerals (ferrihydrite) nanoparticles and natural organic matter forms ternary phases under acidic and circumneutral pH values, but the exact nature of this complex was unclear. [12] Dodge and Francis provide additional clarity of the possible ternary complexes using Extended X-ray Absorption Fine Structure (EXAFS) analysis and identified a soluble molecular cluster that contains a 1 : 1 : 2 Fe : U:citric acid ratio.…”

“…Kuo et al. explored the performance of amidoxime‐based adsorbents in the presence of Fe(III) and humic acid spiked seawater and found that the U loadings were variable and decreased by 73 % and 56 % when compared to the control [8] . U(VI) in the presence of dissolved organic matter, such as humic acid, has previously been shown to form metal‐organic complexes through carboxylate functional groups [9] .…”

Sequestering Rare Earth Elements and Precious Metals from Seawater Using a Highly Efficient Polymer Adsorbent Derived from Acrylic Fiber

“…We employed a 63 L flume (length 1.22 m, width 0.20 m) and the target linear velocity was 2 cm/s in this study. The pH, salinity, temperature, trace elements, and dissolved organic carbon (DOC) of flume seawater were monitored throughout the experiment as described in previous literature [10,[14][15][16][17]. Three braided adsorbent materials were mounted in the circulating seawater flume system and flowed smoothly for 56 days at 20 • C. According to our published data [7,17], the uranium adsorption capacity of the amidoxime-carboxylate containing fiber reaches near equilibrium after 56 days of exposure to seawater.…”