Assessment of Impacts of Dissolved Organic Matter and Dissolved Iron

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Assessment of Impacts of Dissolved Organic Matter and Dissolved Iron on the Performance of AmidoximeBased Adsorbents for Seawater Uranium Extraction Li-Jung Kuo, Horng-Bin Pan, Jonathan E. Strivens, Nicholas J Schlafer, Christopher J. Janke, Jordana R. Wood, Chien M. Wai, and Gary A. Gill Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b00670 • Publication Date (Web): 15 Apr 2019 Downloaded from http://pubs.acs.org on April 15, 2019

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Industrial & Engineering Chemistry Research

Assessment of Impacts of Dissolved Organic Matter and Dissolved Iron on the Performance of Amidoxime-Based Adsorbents for Seawater Uranium Extraction Li-Jung Kuo1*, Horng-Bin Pan2, Jonathan E. Strivens1, Nicholas Schlafer1, Christopher J. Janke3, Jordana R. Wood1, Chien M. Wai2, and Gary A. Gill1

1.

Marine Sciences Laboratory, Pacific Northwest National Laboratory, Sequim, WA 98382, USA

2.

Department of Chemistry, University of Idaho, Moscow, ID 83844, USA

3.

Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA

* Corresponding author: Li-Jung Kuo: phone 1 (360) 681-4589; e-mail [email protected]

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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 adsorbent exposed to the humic acid-spiked seawater. FTIR signatures of adsorbents suggest that the amidoxime ligands in the adsorbent exposed to dissolved Fe-spiked seawater had severer 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 acid-spiked seawater didn’t adsorb 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) didn’t interact with amidoxime-based adsorbent; while the Fe species in the Fe-spiked seawater strongly interacted with adsorbent and caused significant degradation of amidoxime ligands. On the other hand, highly variable uranium adsorption loadings were observed in adsorbent in contact with humic acid-spiked

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seawater, but not in the adsorbents exposed to dissolved Fe-spiked seawater and seawater without spike. 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.

Keywords: Seawater; uranium; dissolved organic matter; iron; amidoxime

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1. Introduction Extraction of uranium from seawater has gained significant attention in recent years due to increasing demands around the world and the desire to establish energy supply security.1 Interest in seawater uranium stems from the current understanding that terrestrial ore reserves for land mining contain ~7.6 million tons of uranium; sufficient only to power current reactors for 100 years. Eminently, the oceans contain 4.5 billion tons of uranium; nearly 1000 times more than terrestrial sources.2-4 Toward capture of this resource, great advances on novel amidoximebased adsorbent development have recently been achieved; allowing more efficient sequestration of uranium from seawater.5, 6 To advance these achievements, a continued effort toward comprehensive testing, reflective of in situ conditions, is fundamental. There are multiple challenges, which hinder deployment of the technology for seawater uranium extraction, driven by the low (~3 ppb) concentration in seawater and the stability of the uranyl tris-carbonato complex UO2(CO3)34−.7, 8 Recent efforts within the scientific community have begun to address cardinal environmental effects on the performance of amidoxime-based adsorbents. These efforts include understanding of ion-competition for binding sites,8-10 temperature dependence of uranium adsorption,11-15 the effect of current velocity on passive adsorption,16 and the effects of pH on the uranyl ion-amidoxime interaction.17-19 An unexplored factor, the effects of dissolved organic matter (DOM) with amidoxime-based adsorbents, is the next pragmatic area of consideration due to ubiquitous presence of DOM in natural waters. DOM is a heterogeneous mixture of organic molecules with a broad range of molecular weights (typically from 100 Da to >10000 Da). Primary biochemical components of oceanic DOM include proteins, lipids, carbohydrates, and abundant uncharacterized fractions.2022

The sources of DOM can be from living organisms and the decomposition residues of dead

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organisms, as well as the terrestrial inputs. Because DOM is ubiquitous in natural waters, adsorption of DOM to adsorbent surfaces is unavoidable. Coastal waters, in particular, often have higher DOM concentration than open ocean due to significant terrestrial inputs. Thus, the impacts of DOM on uranium-extraction from seawater must be considered in deployment strategy and economic assessments. Indeed, preliminary testing found that stripping of DOM from used adsorbent material could improve the reusability of the adsorbent, warranting a more in-depth investigation of DOM’s effect on adsorbent endurance.23 Commensurate to DOM, dissolved Fe is abundant in coastal waters, as the majority of which exists as organic complexes.24 A recent field study in coastal seawater found potential for significant Fe adsorption to amidoxime-based polyethylene adsorbents under polluted conditions.25 It has been reported that the complexation of Fe(III) with glutarimidedioxime, a small molecule analogues of the cyclic form of amidoxime, possesses a higher stability constant than the complexation of U(VI) with glutarimidedioxime.26 In light of site selection for largescale deployment, the effects of dissolved Fe on the performance of amidoxime-based adsorbents are of interest. In the present study, we conducted an in-depth evaluation of the effect of DOM, dissolved Fe, and a synergistic combination on extraction of uranium from seawater using amidoxime-based polymeric adsorbents. Specifically designed flume systems with continuous flow of natural seawater acted as mesocosms for testing adsorbents,8, 10 modified by the addition of DOM and dissolved Fe which was achieved by constant addition of humic acid (a surrogate of terrestrial DOM) and Fe solutions. The use of natural seawater for evaluation of adsorbents’ performance on uranium extraction allowed the testing to mimic the complex organic and inorganic compositions found in natural seawater.8, 27-29 The goals of this study were to gain

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better understanding about how DOM and dissolved Fe can affect uranium adsorption performance, and the subsequent adsorbent re-use performance of amidoxime-based adsorbents. The observations from this study also provide important information on site selection for ocean deployments.

2. Materials and Methods 2.1. Adsorbent materials Hollow-gear-shaped, high surface area polyethylene (PE) fibers were prepared for adsorbent synthesis by using a radiation-induced graft polymerization (RIGP) technique. Detailed methods of adsorbent synthesis can be found in our earlier work.28 In brief, the PE fibers were grafted with acrylonitrile and the comonomer vinylphosphonic acid. The grafted nitriles on the fibers were converted into amidoxime groups by treatment with 10% hydroxylamine hydrochloride in 50/50 (w/w) water/methanol (previously neutralized with KOH) at 80 °C for 72 hrs. After amidoximation, the adsorbent was washed with deionized water followed by a methanol rinse and allowed to dry at 40 °C under a vacuum. The adsorbent was coded as the ORNL AI8 adsorbent and has been evaluated for performance in seawater uranium extraction in our earlier work.10, 29 Before seawater testing, the adsorbents were pre-treated by immersing in a 0.44 M KOH solution at 80 °C for 1 hour. This alkaline pre-treatment step enhances the hydrophilicity of the adsorbents, and also induces swelling of the fibers which is beneficial for facilitating uranium adsorption.30, 31 After the KOH pre-treatment, the adsorbents were immediately washed with deionized water until the pH became neutral.

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2.2. Natural seawater exposure experiment Flow-through channel (flume) systems were used for seawater exposure of braided adsorbent materials under controlled temperature and flow-rate conditions.8, 10 Ambient seawater from Sequim Bay, WA was heated in a head tank to the desired testing temperature (20 °C) using a titanium immersion heater, filtered sequentially (40, 5, 1, 0.45 µm) through nonmetallic filtration, fed into the testing flumes at a flow rate at 1.5 L/min, and recirculated at linear velocity held to 2 cm/s. The volume of flume is 50 L. The seawater turnover rate in the flumes were ~30 min. Braided adsorbent materials with dry weights of 8 - 10 g were mounted in the flumes for seawater exposure. Over the course of seawater exposure (28 days), small portions (~100 mg) were taken from braided adsorbents using titanium-coated scissors at a weekly interval. Flumes were constructed from opaque Plexiglas to prevent exposure to photosynthetically active radiation. Seawater salinity and pH were determined daily using a hand-held salinometer (YSI, Model 30), and pH meter calibrated with NIST buffers, respectively. Water temperature was logged every 10 minutes using a flexible hermetically sealed RTD sensor probe (OMEGA Engineering, Stamford, CT, USA). Classification of the flumes was as follows, (1) the “HA-flume” was spiked with a concentrated humic acid solution (2.4 g/L in 0.001 M NaOH, ACROS Organics, technical grade), a DOM surrogate, to elevate the concentration of dissolved organic carbon (DOC); (2) the “Fe-flume” was spiked with a concentrated Fe solution (2.5 mg/L) prepared from diluting a commercially available Fe standard solution (10 g Fe/L in 10% HCl, High-Purity Standards) with deionized water; and (3) the “control-flume”, a flume fed with fresh seawater only, was with adsorption data reported earlier.29 The Fe-flume was intended a quasi-control; intended to mimic elevated Fe concentration resulting from humic acid bound Fe in the HA-flume.

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Concentrates were continuously added to flumes using Masterflex diaphragm pumps equipped with PTFE pump heads and perfluoroalkoxy (PFA) tubing at a flow rate at 10 mL/min. For both HA- and Fe-flumes, the inlet carrying spike solution introduced into the recirculation line with seawater for optimal mixing. The concentrations of humic acid and Fe concentrates were determined in a 2-week preliminary seawater flume test to ensure that desired DOC (> 3 mg/L) and dissolved Fe concentrations (= dissolved Fe level in HA-flume) in HA- and Fe-flumes, respectively, can be consistent. After confirmation of stable seawater compositions in HA- and Fe-flumes, adsorbents were then deployed to flumes for seawater exposure. Seawater flumes were sampled weekly and filtered (0.45 µm) for trace element and DOC analysis. These routine measurements of flume seawater (pH, salinity, temperature, trace elements, and DOC) help monitor seawater compositions and condition throughout the experiment. All obtained uranium adsorption capacity data were normalized to a salinity of 35 psu in order to correct the varying salinity of natural seawater during the adsorption experiments. Adsorption kinetics and saturation capacity were determined by fitting time course measurements of adsorption capacity using a one-site ligand saturation model (Eq. 1):10, 29 𝑢=

𝛽𝑚𝑎𝑥𝑡

(1)

𝐾𝑑 + 𝑡

where u is uranium capacity (g U/kg adsorbent), t is exposure time (days), βmax is the adsorption capacity at saturation (g U/kg adsorbent), and Kd is the half-saturation time (days).

2.3. Reuse of Adsorbent After 28-day seawater exposure, the braided adsorbents were rinsed with deionized water to remove entrained seawater. Uranium was stripped off the adsorbents using a bicarbonate elution approach, which is selective to uranium removal from amidoxime-based adsorbents.32

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Briefly, adsorbents were soaked in 3 M KHCO3 at 40°C for 24 hrs to strip off uranium. The adsorbents were then further soaked in 0.5 M NaOH at room temperature for 3 hrs to remove adsorbed natural organic matter. After the elution, the adsorbents were rinsed with deionized water and redeployed to flumes for the next cycle. For re-used adsorbent, its true adsorption loadings of measured elements (Ax) is the observed capacity (Ax,obs) minus the residual contents (A(x-1),R) from the previous uranium stripping process, which are the contents of these elements on adsorbents before being redeployed to the seawater flume (Eq 2). Ax = Ax,obs - A(x-1),R

X1

(2)

2.4. Analysis Analysis of uranium and other trace elements adsorbed to the AI8 adsorbent was conducted using a Perkin-Elmer 7300 inductively coupled plasma optical emission spectrometer (ICP-OES). Determination of elements in natural seawater samples was conducted using a Thermo Scientific ICapTM Q inductively coupled plasma mass spectrometer (ICP-MS) equipped with an online pre-concentration system (seaFAST S2TM automated sample introduction system, Elemental Scientific).33 DOC content in seawater was measured by high-temperature catalytic oxidation (HTCO) with a Shimadzu TOC-LCSH TOC analyzer. Fourier transform infrared spectroscopy (FTIR) was acquired using a ThermoNicolet 6700 ATR-FTIR spectrometer equipped with a deuterated triglycine sulfate (DTGS) detector. FTIR measurements were made with a SplitPea attenuated total reflection accessory (Harrick Scientific Corporation) along with a silicon internal reflection element used as a reflection

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medium. High resolution FTIR spectra in the range of 4000−700 cm-1 were acquired using 500 co-added scans at 2 cm-1 resolution with the Norton–Beer “medium” apodization function.

3. Results and Discussion 3.1. Water quality monitoring for flumes Water quality and trace element concentrations in flumes’ seawater were monitored throughout the course of experiment. As shown in Table 1, seawater from the HA- and Feflumes have comparable pH and salinity to the control-flume, demonstrating that spiking of concentrated humic acid and Fe solutions did not alter these water quality parameters. The HAflume’s seawater was modified to a DOC concentration of 3.51 ± 0.40 mg/L, while the Fe-flume remained consistent with the control (0.82 ± 0.04 and 0.85 ± 0.05 mg/L DOC, respectively). A DOC concentration of this magnitude is not unusual in coastal waters receiving significant inputs from terrestrial sources/human activities.34 Trace element concentrations are shown in Table 2. Compared to the control-flume, the HA-flume contained more than 10-fold the Fe (10.2 ± 1.31 μg/L) and slightly higher V, Cu, Zn. The relatively high concentrations of trace elements in the HA-flume’s seawater result from humic acid complexed trace metals in the humic acid standard; the uranium concentration was unaffected. Conversely, in the Fe-flume, with Fe (12.5 ± 2.2 μg/L) similar to the magnitude of Fe in the HA-flume, ancillary metals remained comparable to the non-spiked fresh seawater. This indicates that the major differences in seawater between the three seawater conditions were: HA-flume (high DOC, high Fe), Fe-flume (low DOC, high Fe), and control-flume (low DOC, low Fe).

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Table 1. Summary of water quality parameters from seawater during the testing period Seawater

pH

HA-flume Fe-flume control-flume

7.89±0.05 7.90±0.04 7.89±0.05

Salinity (psu) 30.1±3.53 30.4±0.39 30.6±0.45

DOC (mg/L)

3.51±0.40 0.82±0.04 0.85±0.05

Table 2. Summary of trace element concentrations (ug/L) in seawater during the testing period Seawater

V

Mn1

Fe

Ni1

Cu1

Zn1

HA-flume Fe-flume control1.59±0.094 1.173±0.124 0.813±0.244 0.482±0.030 0.228±0.020 3.74±2.91 flume 1. These trace elements were only monitored in the initial 28-day seawater exposure. 2.06±0.066 1.57±0.052

0.844±0.195 0.775±0.184

10.2±1.31 12.5±2.23

0.652±0.064 0.496±0.044

0.429±0.045 0.264±0.029

6.00±5.74 2.75±0.443

U 2.79±0.064 2.82±0.074 2.79±0.110

3.2. Initial 28-day seawater exposure Uranium adsorption kinetics of the AI8 adsorbents in three tested seawater conditions is shown in Figure 1. In the initial 28-day seawater exposure, the adsorbents in the HA- and Feflumes had 56% and 73% adsorption loading in comparison to the control, respectively (Table 3). The difference in uranium saturation adsorption capacities resulting from different seawater conditions was even larger, as the adsorbent in the Fe flume had only 43% of the capacity in respect to the control. The uranium adsorption kinetic curve of AI8 in the HA flume was highly variable (as shown in the 28-d time point replications), and a one-site ligand saturation model could not be used for capacity determination due to the near linear increase with time. The uranium adsorption performance of AI8 in the initial seawater exposure was normal seawater (without spike) > Fe-spiked seawater > HA-spiked seawater.

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Figure 1. Comparison of uranium adsorption kinetics of ORNL AI8 braids during initial seawater exposure (28 days) in the control-flume, the Fe-flume, and the HA-flume. Adsorption loadings were normalized to a salinity of 35 psu. Triplicate sampling was conducted at the 28-day time point.

Table 3. The 28-day uranium adsorption loading, saturation capacity, and half-saturation time of ORNL AI8 braids in exposures of seawater without spike, Fe-spiked seawater, and humic acidspiked seawater. Adsorption data were normalized to a salinity of 35 psu. Seawater HA-flume2 Fe-flume control-flume

28-day adsorption loading (g/kg) 1.64±0.44 2.16±0.10 2.94±0.08

Saturation capacity1 (g/kg)

Half-saturation time1 (days)

3.23±0.23 7.41±1.14

13.9±7.39 41.8±10.0

1 predicted 2 one-site

from one-site ligand saturation modeling (Eq. 1). ligand saturation model is not suitable for modeling of this data set.

Adsorption loadings of seven elements (Mg, Ca, Fe, V, U, Zn, Cu) on AI8 adsorbents from three different seawater conditions are shown in Figure 2. While most elements have

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similar adsorption loadings on AI8 in the three seawater conditions, the Fe content of the adsorbent in Fe-flume was strikingly different from the adsorbents in HA- and control-flumes. Notably, the adsorbent in Fe-flume has Fe content ~25 times higher than the adsorbent in HAflume, although the seawater in the two flumes has comparable Fe concentrations (Table 2). Such drastically different Fe levels on the same type of adsorbent indicates that Fe speciation in the HA-flume remained as organic matter bound Fe, while the Fe-flume represents the kinetic window of Fe (II) input to seawater resulting in a heterogeneous mixture of Fe (II) and Fe (III) as free ion, organic and inorganic ligands bound, and colloidal Fe. Organic matter bound Fe in the Fe-flume should be minor as the DOC content in its seawater is very low (Table 1). In the HAflume, Fe species were dominated by organic matter bound Fe, as the excess Fe in this flume was introduced from humic acid spiking. Data in Figure 2 indicates that organic matter bound Fe did not interact with the AI8 adsorbent. Since Fe content of the AI8 adsorbent in the HA-flume is lower than the AI8 in the control-flume, humic acid likely competes with the adsorbent for Fe adsorption. 35

AI8, Initial exposure Adsorption loading (g/kg)

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Background seawater

Fe flume

HA flume

Zn

Cu

30 25 20 15 10 5 0

Mg

Ca

Fe

V

13

U



Figure 2. Comparison of adsorption loadings of seven elements (Mg, Ca, Fe, V, U, Zn, Cu) on ORNL AI8 braids in the control-flume, Fe-flume, and HA-flume after 28-day seawater exposure. All data were normalized to a salinity of 35 psu.

3.3. Bicarbonate elution for adsorbent regeneration After the initial 28-day seawater exposure, the AI8 braids were treated with 3 M KHCO3 and 0.5 M NaOH soaking for uranium stripping and organic matter removal before being redeployed for a re-use test.32 Figure 3 shows percentage removal of seven selected elements from AI8 braids in the three seawater conditions. It is demonstrated that the KHCO3+NaOH elution can strip ~95% of Mg and ~90% of U from adsorbents, regardless the exposed seawater conditions. The highly consistent uranium removal rate from the AI8 adsorbent suggests that a KHCO3+NaOH elution is effective in adsorbents exposed to seawater with high Fe or high organic matter content. The percentage removal rates of other elements are not as high as Mg and U. This observation confirms that the KHCO3+NaOH elution is a method with higher selectivity for uranium stripping than the often-used mineral acid elution.35 However, vanadium can be removed from 30 – 45% under the presented method; much higher than reported % V removal rate of humic acid-spiked seawater > Fe-spiked seawater. Notably, in comparison to the 28-day U loading in the initial HA-flume exposure (1.64±0.44

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g/kg), the reuse (1.59±0.40 g/kg) showed very high recovery (97%). In contrast, the 28-day U loading of reused AI8 adsorbent in the Fe-flume (0.73±0.05 g/kg) dropped substantially; as the recovery of uranium loading was just 34% of that observed from the initial 28-day seawater exposure (2.16±0.10 g/kg). Such contrast in performance of AI8 in Fe flume and HA flume suggests that Fe adsorption may play a unique role in adsorbent reusability, as the major difference in distribution of adsorbed trace elements on AI8 from these two seawater environments is Fe content (Figure 2). FTIR signatures of the adsorbents from these tested seawater environments can provide some insights into the potential causes of observed change of adsorbent performance. FTIR spectra of ORNL AI8 from different steps along the course of the experiment are shown in Figure 5. In general, both the 928 cm-1 peak (N−O stretching) and the 1643 cm-1 peak (C=N stretching) became smaller after seawater exposure; while the 1559 cm-1 peak (-COO- stretching) increased. This is partly due to amidoxime degradation under seawater exposure (i.e degradation of amidoxime ligands can form carboxylate groups).29, 30, 35 Two indices, the intensity of the 928 cm-1 peak (I928) and the relative ratio of the 1643 cm-1/1559 cm-1 peaks (I1643/I1559), from FTIR signatures were calculated to quantitatively assess the abundance of the amidoxime ligand and its degradation.29, 32, 35 The decrease of I928 suggests degradation of amidoxime. The ratio of I1643/I1559 indicates the change of relative intensities between amidoxime and carboxylate groups. The decrease of I1643/I1559 is a combined result of decrease of C=N and increase of the −COO- peak that indicates the degradation of the amidoxime ligand and its subsequent conversion to a carboxylate group.30 The % decrease of the two indices of AI8 adsorbents at three stages of the experiment (after the initial 28-day seawater exposure, after KHCO3+NaOH elution, and after the 28-day seawater re-exposure [reuse])

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relative to the fresh, unexposed AI8 are shown in Figure 6. After the initial 28-day seawater exposure, the % decrease of I928 in the Fe-flume’s AI8 (45%) was greater than two times higher than those exposed in the HA- and control-flumes (15-20%), suggesting that AI8 in Fe-spiked seawater had accelerated amidoxime degradation. Only minimal change was observed after the KHCO3+NaOH elution. This confirms that the KHCO3+NaOH elution is a mild uranium stripping process toward the functional groups.29, 32 In the following seawater re-exposure, I928 further decreased in all seawater conditions. I928 of reused AI8 in Fe-flume still had the highest decrease (51%) than others (20-29%). The significant decrease in abundance of amidoxime on the Fe-flume’s AI8 provides additional explanation for the poor reusability under increased Fe conditions (Figure 4). Our data thus suggests that high Fe adsorption (Figure 2) may cause much more pronounced amidoxime degradation. A recent report indicated that transition metals such as Cu(II) and Ni(II) can convert glutardiamidoxime, a single molecule surrogate of branch-chain amidoxime, to diiminopoperidin-1-ol under ambient conditions and may influence uranium adsorption by amidoxime ligand.36 The mechanism responsible for the Fe-induced amidoxime degradation and the subsequent poor adsorbent reusability are not clear and warrant further investigation. On the other hand, the % decreases of I928 and I1643/I1559 of AI8 adsorbent from humic acid-spiked seawater are only slightly higher than those from the un-spiked seawater; the observed minor change in amidoxime abundance is reflected by the high reusability of AI8 under high HA conditions.

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Figure 5. FTIR spectra of ORNL AI8 adsorbent before seawater exposure, after 28-day seawater exposure, after KHCO3+NaOH elution, and after 28-day seawater re-exposure in control-flume, Fe-flume, and HA-flume.

% decrease vs. unexposed adsorbent

60% 28-d exposure After KHCO3+NaOH 28-d re-exposure

Peak intensity I928 50%

40%

30%

20%

10%

0%

Control

Fe flume

HA flume

60%

% decrease vs. unexposed adsorbent

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


28-d exposure After KHCO3+NaOH 28-d re-exposure

Peak ratio I1643 / I1559 50%

40%

30%

20%

10%

0%

Control

Fe flume

HA flume

Figure 6. Percentage decrease of FTIR signals (upper panel: I928; lower panel: ratio of I1643/I1559) of ORNL AI8 at different stages of seawater exposure (after 28-day seawater exposure, after

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KHCO3+NaOH elution, and after 28-day seawater re-exposure) in comparison to unexposed AI8. AI8 were sampled from control-flume, Fe-flume, and HA-flume.

Finally, a comparison of the change of adsorption loadings of seven selected elements between initial seawater exposure and the reuse was assessed (Figure 7). AI8 adsorbents from three different seawater conditions all show increased adsorptions of Mg and Ca, reflecting the increase of carboxylate groups observed by FTIR (Figure 5).10, 29 Adsorptions of Fe, V, U, Zn, Cu decreased in AI8 from Fe-spiked and humic acid-spiked seawater, but not in the control. The decreases of Fe, V, U, and Cu adsorptions on AI8 are greater in Fe-flume than in the HA-flume, pointing to significantly weakened adsorption capability of the adsorbent under high Fe conditions. Being that Fe(III), V(V), and Cu(II) are strong competing ions for uranium adsorption to amidoxime ligands,2, 10, 26, 27 the overall drop of adsorption performance of the AI8 adsorbent in the Fe flume is also supported by the significant decreased I928 and ratio of I1643/I1559 from FTIR characterization of the adsorbent (Figure 6).

100%

Change in adsorption (%)

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Control flume

Fe flume

HA flume

80% 60% 40% 20% 0% -20% -40% -60% -80%

Mg

Ca

Fe

V

20

U


Zn

Cu

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Figure 7. Percentage change of 28-day adsorption loadings of seven elements (Mg, Ca, Fe, V, U, Zn, Cu) between initial seawater exposure and the reuse on ORNL AI8 braids in background seawater, Fe-spiked seawater, and humic acid-spiked seawater.

4. Conclusions In the present study, the effects of high Fe and high DOC concentrations on adsorption performance and reusability of amidoxime-based polymeric adsorbents for extraction of uranium from seawater were systematically investigated. The HA- and Fe-flumes represented high Fe/high DOC, and high Fe/low DOC seawater conditions in comparison to ambient Sequim Bay seawater (low Fe/low DOC). The uranium adsorption loadings of ORNL AI8 adsorbents in the HA- and Fe-flumes were 56% and 73% of the loading observed in the control-flume, respectively, in the initial 28-day exposure. Highly variable uranium adsorption loading was observed in AI8 under high DOC condition, suggesting that humic acid may adsorb on the adsorbent and interfere uranium adsorption. In the adsorbent reuse experiment, the AI8 in HA-flume showed great reusability (97% recovery of uranium adsorption loadings of initial exposure), while in the Fe-flume the adsorbent had very poor recovery of uranium adsorption loading (34%). Evidence from FTIR showed that the AI8 under high Fe/low DOC conditions had accelerated degradation of amidoxime ligands. Overall, the data indicate that in high DOC and high Fe seawater environments amidoxime-based adsorbents could suffer from inhibited adsorption performance and reusability. To achieve optimal uranium extraction from seawater, an open ocean deployment site where seawater contains low Fe and low DOC is preferable over coastal waters that receive significant terrestrial/anthropogenic inputs.

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Acknowledgement This work was funded by the U.S. Department of Energy, Office of Nuclear Energy, Fuel Cycle Research and Development Program, Fuel Resources Program (Contract DE-AC0576RL01830). We thank Brett A. Romano for help with construction, maintenance, and operation of the marine testing facility.

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Abstract graphic 253x142mm (96 x 96 DPI)


Assessment of Impacts of Dissolved Organic Matter and Dissolved Iron

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