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Article

Co-Extraction of Uranium and Mercury Using Ion Exchange from Cemented Radioactive Waste Sulfuric Leachate in Iodide Media

by
Maxime Courchesne
1,
Raoul-Marie Couture
2,
Justine Basque
1,2,
Nicolas Reynier
1,2,* and
Dominic Larivière
2,*
1
CanmetMINING, Natural Resources Canada, Ottawa, ON K1A 0E4, Canada
2
Department of Chemistry, Laval University, Quebec, QC G1V 0A6, Canada
*
Authors to whom correspondence should be addressed.
Minerals 2023, 13(3), 405; https://doi.org/10.3390/min13030405
Submission received: 3 February 2023 / Revised: 6 March 2023 / Accepted: 7 March 2023 / Published: 14 March 2023
(This article belongs to the Special Issue Geochemical Behavior and Evaluation for Radioactive Waste Disposal)
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Abstract

:
The production of medical isotopes in Chalk River Laboratories facilities (Chalk River, ON, Canada) has resulted in a large quantity of cemented radioactive waste (CRW) containing valuable elements such as uranium. From the perspective of recovering and ultimately recycling valuable elements from CRW, the solubilization of key constituents such as uranium, mercury, and cesium has been previously investigated using H2SO4/KI. However, to achieve recycling of these elements, separation must be performed as they are co-solubilized. In this study, the extraction of uranium and mercury by chelating resin Lewatit TP260 from surrogate cemented radioactive waste (SCRW) leaching solution in sulfuric media and in the presence of iodide was investigated. Extraction of U and Hg was assessed as a function of the concentration of KI (0.12 M to 0.24 M) used during the SCRW dissolution process. Continuous experiments showed that the Lewatit TP260 functional group, aminomethylphosphonic acid, had a high affinity for U. Mercury was also extracted onto the Lewatit TP260. However, the presence of iodide in the SCRW leaching solution increased the competition between the adsorbed mercury of the stationary phase and the iodide–mercury complexes of the mobile phase. Additionally, the reusability of the resin was tested through extraction and desorption cycles. Due to the presence of trivalent cation, the capacity of Lewatit TP260 for U and Hg decreases with the number of cycles.

1. Introduction

Medical imaging has become one of the most used techniques for cancer diagnosis. For decades, nuclear medicine has used various radioisotopes for this purpose. Amongst the medical isotopes available, over 80% of the diagnostic imaging is performed by 99mTc obtained by β− decay of 99Mo [1]. While new nuclear approaches are currently in development worldwide for the alternative production route of 99Mo sources, most of the 99Mo is currently produced through the nuclear fission of uranium sources. Until 2016, the facility operated by Chalk River Laboratories (Ontario, Canada) produced 99Mo from U target material. These targets, made of an aluminum-uranium alloy, were irradiated in the National Research Experimental (NRX) reactor for 99Mo production. Afterwards, the U targets were removed from the reactor when the conversion of U to fission products reached ca. 10% [2]. The U targets were then solubilized in nitric acid, and mercury was added to enhance solubilization [3]. Finally, the radioisotopes were separated from the solution by ion exchange. Following separation, isolated radioisotopes were shipped, and the liquid wastes were cemented. From the mid-1950s to 2016, Chalk River Laboratories produced medical radioisotopes for up to 40% of the worldwide demand, which generated an extensive amount of a unique cemented radioactive waste (CRW) containing uranium (U), mercury (Hg), and various fission products such as cesium (Cs) and strontium (Sr) [4]. Canadian Nuclear Laboratories (CNL) are now developing a long-term management strategy to minimize the potential environmental impact of the CRW. The presence of uranium in the CRW represents a great economic opportunity, whereas environmental concerns were raised regarding the long-term sealing ability of the concrete matrix to retain toxic Hg [4,5] and radioactive elements (Cs and Sr). To this end, Reynier et al. [6] have proposed a leaching procedure to solubilize U, Hg, and Cs from different surrogate CRW (SCRW). This would allow the extraction of U, Hg, and 137Cs from the concrete matrix and produce low hazard gypsum and a concentrated SCRW leaching solution. Sodium chloride (NaCl) and potassium iodide (KI) were added during the sulfuric leaching step to enhance the solubilization efficiency of mercury species, present as HgO, Hg0, and HgS. The use of iodide was found to be the best-suited strategy to reach significant dissolution of all key elements without compromising the U recovery.
Various separation methods can be used to recover key elements from acidic leachates, including selective precipitation (SP) [7,8], solvent extraction (SX) [9,10,11,12,13], and ion exchange (IX). SP aims to form insoluble salts with a targeted element to retrieve it from pregnant solutions. The presence of high concentrations other ions (Fe3+, Al3+, Mg2+, Ca2+) in the solution can reduce the efficiency of the precipitation of the targeted element and consume a part of the precipitating agents. For this reason, SP is considered less selective than SX [14]. SX is a liquid–liquid extraction method that uses highly selective organic ligands solubilized in organic diluents to extract metals from aqueous solutions. SX has proven its versatility in various aqueous media, such as nitric acid [15], sulfuric acid [16], and hydrochloric acid [17]. Popular ligands include diglycolamide (DGA) [18], N-donor [10,11,12,13,19], and acidic phosphorus compounds. The PUREX process is an SX process used to separate plutonium and uranium from spent nuclear fuel. SX is a good method used for solutions with a high concentration of analyte, whereas IX is used when the concentration of analyte is lower [20]. IX uses a chromatographic approach based on polymeric resins to extract analytes from pregnant leaching solutions. Resins are categorized as anionic [21,22,23], cationic [24,25], and chelating [26,27,28] based on their chemical functional groups, quaternary amine, carboxylic acid, and SX-inspired ligands, respectively. IX is a versatile and efficient technique used from nuclear medicine [29] to hydrometallurgy [30] to enrich and purify constituents of the mobile phase.
According to the Global Threat Reduction Initiative (GTRI), CNL needs to retrieve the waste and recover the uranium for its repatriation to the USA. Furthermore, the GTRI requires the recovery of U to be close to 100%. To recover U from the SCRW leaching solution, Reynier et al. [31] proposed a recovery strategy using IX. Column experiments based on using the three following resins, potassium cobalt hexacyanoferrate (KCFC), chelating resin Lewatit TP214, and chelating resin Lewatit TP260, resulted in good extraction yields for cesium, mercury, and uranium, respectively. The separation of Hg using Lewatit TP214 (thiourea resin) allows the production of a distinct Hg waste. However, the separation of Hg using Lewatit TP214 was found to reduce, to some extent, the recovery of U (5% loss) [31]. The recovery of U was tested in continuous mode using a SCRW leaching solution containing from 0.006 M to 0.60 M of iodide (as KI) on chelating resin Lewatit TP260. However, the co-extraction of U and Hg on Lewatit TP260 in iodide media was not investigated. In addition, speciation of U and Hg in the SCRW leaching solution was not evaluated. The present work focuses on the extraction of U and Hg on chelating resin Lewatit TP260 from SCRW leaching solution in iodide media with iodide concentrations ranging from 0.12 M to 0.24 M. The expected outcome is to determine the best parameters for the separation of U and Hg by ion exchange to achieve the extraction and recovery of U close to 100%. Computational modelling was applied to better understand the behavior of U and Hg in solution. Then, column experiments designed to assess the impact of some parameters (i.e., geometry, flow rate) were performed. Based on the optimal parameters found, the reusability of Lewatit TP260 was finally assessed using the SCRW leaching solution.

2. Materials and Methods

2.1. Reagents

Ion exchange was performed using the macroporous chelating resin Lewatit TP260, containing an amino methyl phosphonic acid (AMAP) (Lanxess, Cologne, Germany). This resin was selected based on the preliminary results obtained by Reynier et al. [31] concerning its extractive performances in KI for uranium extraction, which was also highlighted by the manufacturer as being effective for divalent cations [32]. KI, HNO3, HCl, H2SO4, and the Au standard used in this study were purchased from Fisher Chemical (Hampton, VA, USA). Sodium carbonate was purchased from Sigma Aldrich (Saint-Louis, MI, USA). All chemicals were ACS grade and used without further purification. Solutions were made using ultrapure water, resistivity of 18.2 MΩ.cm, from a Milli-Q purification system from Millipore Sigma (Oakville, ON, Canada).

2.2. Elemental Analysis

Elemental analyses performed during this investigation were carried out using either an ICP-OES (725 ICP-OES, Agilent Technologies, Mississauga, ON, Canada) or an ICP-MS (X-Series II ICP-MS, Thermo-Fisher Scientific, Nepean, ON, Canada). Samples were diluted in 4% (v/v) HNO3 and 2% (v/v) HCl for ICP-OES and ICP-MS, respectively. Rh, In, and Tl were used as internal standards (Fisher Chemical, Hampton, USA). Ar was also used as internal standard due to high TDS matrix during ICP-OES analysis (Linde Canada, Mississauga, Canada). To minimize the memory effect of mercury during analysis, 10 mg/L and 10 µg/L of Au was added to 4% HNO3 (ICP-OES) and 2% HCl (ICP-MS) rinsing solutions, respectively [33]. The certified reference materials used during analysis include IV-ICPMS-71A and CGHG1 from Inorganic Ventures (Mississauga, ON, Canada). The instrumental parameters for each instrument are presented in Table 1.

2.3. Samples

SCRW leaching solutions were prepared using the procedure described by Reynier et al. [6]. The procedure involves the dissolution of pulverized SCRW with concentrated H2SO4 in the presence of KI. To understand the impact of KI concentration on the retention of U and Hg on Lewatit TP260, different quantities of KI were added during the dissolution to obtain initial concentrations of KI ranging from 0.12 M to 0.24 M. While most of the iodine stays in the solution, a volatilization of iodine (I2 gas) can be observed during the SCRW dissolution process. A gravimetric determination of the final concentration of iodine in solution with silver nitrate is being developed but still needs to be validated. Table 2 presents the dissolved concentration in uranium and mercury measured in the various SCRW leaching solutions used throughout this investigation. Other elements, such as iron, aluminum, magnesium, and calcium, were also measured in the leaching solutions in concentrations ranging from 804 to 1235 mg/L, 2113 to 2535 mg/L, 933 to 1273 mg/L, and 649 to 731 mg/L, respectively. The variation of concentration is due to the leaching experiments and the type of SCRW used. To ensure the representativeness of the SCRW leaching solution samples, the post-leaching concentrations were not modified.

2.4. Distribution Coefficients (Kd) Experiments

To evaluate the distribution coefficients (Kd) of each analyte (Hg, U, Mg, Fe, Al) onto the chelating resin Lewatit TP260, 50 mg of solid sorbent were brought into contact, with stirring, with 10 mL of a 25 mg/L solution of individual element with the appropriate matrix for 24 h. The matrices were prepared using high purity water with a resistivity of 18.2 MΩ cm, obtained from a Milli-Q water purification system (Millipore, Etobicoke, ON, Canada), concentrated sulfuric acid (Anachemia Chemical, Montreal, QC, Canada), and salts such as KI (EMD Chemicals Inc., Darmstadt, Germany) and KNO3 (EMD Chemicals Inc., Darmstadt, Germany), depending on the experiment. The analytes were added by diluting stock solutions of 1000 mg/L of each element (SCP Science, Baie D’Urfé, QC, Canada). Subsequently, the pH of the solutions was adjusted to 1.82 with NaOH (Anachemia Chemical, Montreal, QC, Canada). After 24 h, the final concentration of the solution was measured by making the appropriate dilutions on the supernatant. Thus, by knowing the initial (C0) and final (Cf) concentration of the metal ions, the mass of sorbent (m) and the volume of solution used (V), it was possible to calculate the Kd (Equation (1)). The experiments were made in triplicate, except for those made with KNO3.
K d = V m C 0 − C f C f

2.5. Speciation of U and Hg-Computational Modelling

The speciation of uranium and mercury in SCRW leaching solution was evaluated by computational modelling of the SCRW leaching solution at different initial KI concentrations. An equilibrium model was implemented in PHREEQC v.3.1.2 using the SOLUTION keyword and the thermodynamic equilibrium constants provided in the Minteq.v4 database [34]. Measured pH and a redox at equilibrium with the atmosphere was imposed, as well as charge balance using Ca2+ as a counter-ion. Three simulations were performed to evaluate the effect of the concentration of KI in the SCRW leaching solution on the complexation of uranium and mercury. Remaining parameters of the simulations are presented in Table 3.

2.6. Ion Exchange Study of U and Hg

2.6.1. Kinetics Experiments

All batch mode experiments were performed in 250-mL Erlenmeyer flasks covered with a rubber stopper during agitation. Agitation was achieved using an orbital shaker (MaxQ 3000, Thermo Scientific, Nepean, ON, Canada) at a nominal speed of 180 rpm. Each experiment was performed on a SCRW leaching solution volume of 100 mL. To determine the adsorption kinetics, 3 g of resin was mixed with SCRW of leaching solution and shaken for 24 h. Supernatant aliquots (0.1 mL) were taken from the leaching solution after 1, 2, 4, 6, and 24 h, diluted, and analyzed.

2.6.2. Continuous Mode Experiments

For continuous mode experiments, Omnifit® labware glass chromatography columns with various bed volumes were used: 12 mL (15 mm id., 70 mm long), 39 mL (15 mm id., 220 mm long), 83 mL (15 mm id., 470 mm long), and 231 mL (25 mm id., 470 mm long). The SCRW leaching solution was introduced into the resin using a peristaltic pump (Masterflex, Cole Parmer) at flow rates ranging from 0.3 mL/min to 12.3 mL/min, based on the experiment performed. The resin eluate was collected using a fraction collector (Universal Fraction Collector 1243, Eldex) collecting fractions representing 3 resin bed volume (BV). Breakthrough of the column was considered for an analyte when the ratio of its concentration in the eluate (C) on the concentration in the eluent (C0) was equal to 0.05.
To evaluate the effect of mercury on the non-equilibrium extraction of uranium on Lewatit TP260, solutions mimicking the pH and the U/Hg concentrations typically measured in SCRW leaching solution, named SCRW’ leaching solution hereafter, were prepared. SCRW’ leaching solutions were prepared using 1 M H2SO4, in which calcium sulfate was added to achieve pH ≈ 1.8. Uranium and mercury were then added by pipetting volumes from two separate stock solutions (1000 mg/L) of uranyl nitrate and mercuric nitrate. The stock solutions were added to obtain concentrations of uranium and mercury, ca. 100 mg/L. KI salt was added directly to the solution to obtain the desired concentration. Table 4 shows the composition of the two SCRW’ leaching solutions produced.

2.6.3. Effect of Extraction Cycles

To assess the reusability of the resin, five adsorption/desorption cycles were performed on the SCRW leaching solution, containing an initial KI concentration of 0.15 M. The experiments were performed in batches, in triplicate. During the adsorption step, 30 BV of the SCRW leaching solution was mixed with the resin. As for the desorption phase, 5 BV of the eluent, Na2CO3 1 M, was mixed with the resin [35,36]. Rinsing was performed between each steps using 3 BV of water, and the resin was conditioned with a 1 M H2SO4/0.15 M KI solution between each cycle.

3. Results

3.1. Determination of Distribution Coefficients for Major Elements, Hg, and U on Lewatit TP260 in the Absence and Presence of KI

Based on the results from Reynier et al. [31], the chelating resin Lewatit TP260 was selected to study the extraction of uranium and the impact of mercury on uranium extraction at various KI concentrations. While the retention of many elements on Lewatit TP260 has been documented [27,37,38], there is little information regarding the distribution coefficient of elements such as Hg and U. Distribution coefficients (Kd) are useful to understand the partitioning of an analyte onto a solid adsorbent.
Figure 1 presents the Kd values and extraction yields obtained for three major constituents of the SCRW leaching solution (i.e., Fe, Al, and Mg) and the two analytes of interest.
From this figure it is possible to observe that, except for Mg, most elements are retained with an efficiency greater than 5% in the absence of KI on the resin, with uranium and iron been quantitatively (>99%) retained in the conditions tested. The statistical calculations carried out with the Student’s t-test with a confidence level of 95% showed that Mg, Al, and U distribution between the solid and the aqueous phase was not significantly affected when KI was added. For Fe, a significant reduction in Kd-value (from ca. 18,800 to approximately 1400) was observed in the presence of KI. On the contrary, the addition of KI enhances the retention of Hg by the resin. To determine if the increase was caused by the increase in the ionic strength of the solution, Kd values obtained with KNO3 (0.15 M) were also determined. As seen in the figure, the addition of KNO3 did not significantly affect the adsorption of U and Fe on the Lewatit TP260 when compared to the results obtained in the absence of salt, suggesting that, under the present conditions, ionic strength has a limited impact on the retention of those elements. Mg and Al showed little increase in Kd value upon the addition of KNO3 compared to KI.
To better understand the role of I− on the retention of elements on Lewatit TP260, Kd values for U, Hg, and Fe were determined as a function of the KI concentration. Results are presented in Figure 2.
The presence of KI had little impact on the retention of U, but higher Kd values were observed for KI concentrations higher than 0.15 M. For Fe, the retention on the resin decreased with the increase in KI concentration. For Hg, the addition of KI enhanced the retention on the resin, which is stable up to 0.15 M. Beyond 0.15 M, there seems to be a decrease in the adsorption of Hg on the resin. These findings tend to indicate that the speciation of the elements is a critical factor in the understanding of adsorption behavior on Lewatit TP260.

3.2. Predicted Complexes of U and Hg at Different Concentration of KI in SCRW Leaching Solution

To better understand the behavior observed experimentally during the Kd-values investigation in simple solution and to predict the extraction behavior of the Hg and U present in the SCRW leaching solution on Lewatit TP260, computational modelling was used to predict the formation of the aqueous complexes of both elements. Parameters were defined to represent the concentrations of the most common elements present in the SCRW leaching solution. As the results of the Kd-value investigation, indicating that KI molarity affects the extraction of Hg and, to a lower extent, U, simulations were performed at three distinct concentrations of KI: 0.015, 0.15, and 0.30 M. Thermodynamic modelling indicates that, in the SCWR leaching solution, uranium and mercury species are found to a large extent (>95%) as [UO2(SO4)2)]2− and [HgI4]2− (Kf [HgI4]2- = 2 × 1030 [39]).
As the chelating resin Lewatit TP260 extractive functional groups, AMPA, are considered strong extractive agent for divalent cations, interactions between the AMPA groups must be stronger than those complexing U and Hg with SO42− and I−, respectively, to observe adsorption onto the resin. Furthermore, because the SCRW leaching solution has a pH around 2, the AMPA groups of Lewatit TP260 are partially deprotonated, because pKa for phosphonic acid are reported to be ca. 2 and 5 [27], increasing their affinity for positively charged ions and complexes as the pH increases. Thus, it is expected that uranium will mostly be adsorbed onto Lewatit TP260 phosphonic groups as uranyl, UO22+. Uranyl phosphonate complexes [40] tend to exhibit much stronger stability constants than uranyl sulfate complexes [41]. This indicates that during the extraction of uranium on Lewatit TP260, a transition of the uranyl sulfate complexes [UO2(SO4)2)]2− in SCRW leaching solution to neutral uranyl phosphonate complexes onto the resin will occur. Assuming a similar log KML value for both UO22+ and Hg2+ and AMPA, it is expected to observe a weaker adsorption for mercury on the Lewatit TP260 because the complexation constants of [HgI4]2− are higher than those of [UO2(SO4)2)]2−. Kd-values that were measured in the presence of KI (Figure 2) for Lewatit TP260 were indeed lower for Hg compared to U. However, when KI was added, Hg extraction increased, suggesting the favorable adsorption of [HgI4]2− onto Lewatit TP260. This increase could be associated with a possible interaction between this anionic complex and the positively charged amine group. As the concentration of I− increases, competition for the amine groups of Lewatit TP260 between [HgI4]2− and uncomplexed I− becomes more apparent and could explain the decrease in Kd-values observed with a KI concentration higher than 0.15 M. While the theoretical capacity of the resin provided by the manufacturer is estimated at 2.3 eq/L (e.g., 380 mg/g for U), results from batch experiments have shown that the maximum capacity for U calculated by adsorption isotherm ranges from 10.5 to 12.5 mg/g. Previous work (continuous mode) reported a capacity for U of 6 mg/g [31]. Therefore, the capacity of the resin being used by uranium is very limited. Most of the resin capacity is likely being used by other metals (Fe, Al, Hg).

3.3. Retention of U and Hg from SCRW Leaching Solution by Lewatit TP260—Batch Mode Experimentation

As the SCRW leaching solution contains many ions, such as Al3+, Fe2+, Ca2+, and Mg2+, originating from the cemented waste matrix, the ionic composition of the leachate is very different from the simple solutions used to determine Kd values. Ion exchange chromatography is sensitive to the presence of competing ions; it is expected that the retention behavior observed previously might not be representative of the actual retention of U and Hg from the SCRW leaching solution onto the resin. Results from batch extraction kinetics experiments are presented in Figure 3. The extraction equilibrium of U and Hg for most of the SCRW leaching solutions was achieved within 4 h. No significant differences were observed for U extraction kinetics when the initial KI concentrations during the leaching process increase. For Hg, the extraction seems to follow the order 0.24 M > 0.18 M > 0.12 M. While the retention mechanism of Hg in the sulfate/iodide system appears very complex, Hg extraction seems to increase when the KI concentration increases, resulting in faster adsorption kinetics.
Additionally, adsorption kinetics models of uranium on Lewatit TP260 were compared. The pseudo-first order model from Lagergren [42] was used as the following equation:
log ⁡ q e − q t = − k 1 2303 ∗ t + l o g ⁡ ( q e )
where qe is the quantity of uranium adsorbed at equilibrium (mg/g), qt is the quantity of uranium adsorbed at time, t (mg/g), k1 is the 1st order rate constant (min−1), and t is the time (min).
Similarly, the pseudo-second order model from Ho was used as the following equation [43]:
t q t = − 1 q e ∗ t + 1 k 2 q e 2
where qe is the quantity of uranium adsorbed at equilibrium (mg/g), qt is the quantity of uranium adsorbed at time, t (mg/g), k2 is the 2nd order rate constant (g/mg min), and t is the time (min). The data of adsorption kinetics models are available in the electronic supplementary information (ESI, Tables S1 and S2).
Results have shown that the pseudo-second order was found to match more closely with the data as correlation factors higher than 0.998 were obtained for all of the SCRW leaching solution samples tested. This observation suggests that the adsorption process is likely chemisorption instead of physisorption. Furthermore, these results correlate with the previous hypothesis made that the uranyl sulfate complexes from the SCRW leaching solution will most likely be complexed with the phosphonate groups on the resin.
To confirm the adsorption of U on the resin, characterization by FTIR was performed before and after adsorption to establish the interaction of U with the adsorbent. The figure is available in the electronic supplementary information (ESI, Figure S1). In Figure S1, the IR bands are moving when U is added as follows: bands at 1055 are moving to 1063 cm−1, bands at 1157 are shifting to 1063 cm−1. The bands at 1100–1200 cm−1 are link to phosphate (O=P-(OH)3), whereas the bands at 900–1050 cm−1 are phosphonate ester (P-O-R). Because the TP260 functional group is the phosphonate, uranium adsorption as uranyl phosphonate complexes on the resin is confirmed.

3.4. Retention of U and Hg from SCRW Leaching Solution by Lewatit TP260—Continuous Mode Experimentation

3.4.1. Assessment Using SCRW’ Leaching Solution

To evaluate the effect of mercury on the extraction of uranium on Lewatit TP260, synthetic SCRW leaching solutions (SCRW’ leaching solution) were produced and used in continuous mode, with a column packed with a 12-mL bed volume (BV) of Lewatit TP260 resin. Based on previous studies, it was decided to perform this experiment with an initial KI concentration of 0.15 M. Figure 4 shows the chromatographic profile of (A) SCRWL’-U and (B) SCRWL’-U.Hg. The extraction of uranium in the conditions tested indicates the absence of breakthrough from the column within the first 60 BV, independent of the presence of Hg. This suggests that if U is eluted from the column during the loading phase, this is likely caused by the presence of other elements, such as iron, that have a stronger affinity with Lewatit TP260. Hg is released from the resin after 33 BV, an indicator consistent with the fact that Kd-values for Hg are much lower than for U. This result confirmed the previous assumption that the capacity of the resin being used by the ions of interest appears very limited. Much of the resin capacity is used by multivalent elements (Fe, Al, Mg, Ca) present in the leaching solution, which outnumbers U and Hg on a mole basis. While previous work (continuous mode) reported a capacity for U of 6 mg/g, the result in Figure 4 shows a capacity for U (at 60 BV) of 9 mg/g.

3.4.2. Retention of U and Hg by Lewatit TP260 in SCRW Leaching Solution

The extraction capacity of U and Hg in a continuous loading mode was investigated using the same BV column as in the previous section, but for the SCRW leaching solution. The various SCRW leaching solutions with different initial KI concentrations, from 0.12 M to 0.24 M, were introduced into the column at a flow rate of 3 BV/h. The element concentration of Hg and U in the solution coming out of the column (C) was measured and compared to the initial concentration in the leaching solution (C0). The resin uptake (C/C0) was calculated and is presented in Figure 5.
U breakthrough occurs at 12 BV for most tested initial KI concentrations, with the exception of KI 0.18 M, which occurred slightly earlier (Figure 5). For mercury, the C/C0 values above 1.0 could either indicate a desorption of Hg from the resin or a residual memory effect of Hg during analysis. The memory effect of Hg was considered negligible during analysis as the following precautions were taken: presence of iodide in samples to complex Hg, rinses with a solution containing Au were performed between each measurements, certified reference material were regularly measured, and quality control samples were added systematically to the analytical sequence [33]. This suggests that the Hg present in the leachate is adsorbed onto the resin early in the process and is slowly desorbed when the resin reaches saturation. The desorption of Hg is mostly caused by the substitution of the adsorbed Hg by the U present in the leachate. Based on the results presented in Figure 5, an initial KI concentration of 0.24 M would result in no Hg breakthrough and a good U retention. While a concentration of 0.15 M of KI is optimal for the dissolution of the cement matrix and the solubilization of metals, increased concentration of KI will be considered for the future pilot scale demonstration testing.
Additional extraction of U and Hg in a continuous loading mode (similar conditions) was investigated using the 0.15 M KI leaching solution in triplicate. The objective was to confirm the representativeness of the continuous mode experiment and further evaluate the Hg extraction. Results of U and Hg extraction and the standard deviation are presented in Table 5.
The results presented in Table 5 confirm the replicability in U and Hg extraction in continuous mode (standard deviation remains below 6%). The results also confirmed the Hg adsorption onto the resin early in the process and is subsequently desorbed when the resin reaches saturation. The presence of multivalent elements (especially Fe) may enhance the substitution of the adsorbed Hg by the salts present in the leachate. However, the mechanism for Hg extraction may not be primarily through chelation by the phosphonate groups of the resin, but through another mechanism. The high content of Ca in the solution (from cement dissolution) may suggest a complex ion extraction, such as Ca(HgI4). In addition to the high concentration of salt, the pH of the SCRW leaching solution affects the extractive properties of the functional group on Lewatit TP260, made of AMPA groups (R-NH-CH2-P(O)(OH)2) that possess a pK1 of 1.5 [44]. Because the average pH of the SCRW leaching solution is approximately 2, it is possible to postulate that most AMPA functional groups are found as R-NH2+-CH2-P(O)(OH)2 and R-NH2+-CH2-P(O)O2H− during extraction. Thus, there are two sites where chelation could occur. First, the negative charge on the phosphonate groups could chelate uranium ions, as proposed by Kadous et al. [26]. Second, negatively charged species such as [HgI4]2− could be chelated by the positively charged amine groups. It is shown from batch extraction capacity results that the phosphonate groups chelate U more efficiently than the amine groups for Hg.

3.4.3. Effect of the Flow Rate and the Geometry

Flow rate experiments were performed with a 12 mL Omnifit column filled with chelating resin Lewatit TP260. Flow rates of 1.5 BV/h, 3 BV/h, and 4.5 BV/h were used to evaluate the extraction performances at higher velocity. As expected, the breakthrough of the column for uranium occurs earlier as the flow rate increases, Figure 6A. At higher flow rates, the leachate circulates through the column faster, reducing its contact time with the resin, leading to an early breakthrough [45]. For example, at a 4.5 BV/h flow rate, breakthrough (C/C0 = 0.050) occurs between 18 BV (C/C0 = 0.027) and 21 BV (C/C0 = 0.073), resulting in a breakthrough slope, calculated from breakthrough to C/C0 = 0.70 of 0.0459 BV−1. On the other hand, a flow rate of 1.5 BV/h has a breakthrough near 30 BV (C/C0 = 0.066) and a breakthrough slope of 0.499 BV−1. The longer contact time between the leachate and the resin can explain these differences. It was shown that Lewatit TP260 can take up to 4 h in batch mode to achieve equilibrium. This means that a longer time of contact of Lewatit TP260 with the leachate would result in an increased extraction yield. In addition, the slope is sharper at a low flow rate because the resin can achieve equilibrium more easily. As the mean bead size (d50) of the Lewatit TP260 resin is 0.63 ± 0.05 mm, the observed kinetics may be primarily linked to the large particle size of the resin. In Figure 6A, a flow rate of 1.5 BV/h applied to a 12-mL column exhibits better performance (breakthrough near 30 BV). The kinetics are sufficient to construct processes as the flowrate will be increased along with the size of the column. For example, experiments are in progress to extract U in 15 L column at a 2.1 L/min flowrate (8 BV/h)
The effect of the column geometry on the retention of U and Hg were investigated using three Omnifit columns for their different geometric parameters (length; diameter): 39-mL (22 cm; 1.5 cm), 83-mL (47 cm; 1.5 cm), and 231-mL (47 cm; 2.5 cm). Extraction of uranium (Figure 6B) was investigated using 0.15 M KI at a flow rate of 3 BV/h.
Comparing columns with the same diameters of 1.5 cm, 39-mL, and 83-mL, it is evident that uranium breakthrough occurs at higher BV for the 83-mL column (12 BV) than for the 39-mL column (6 BV). However, the 39-mL column possess a higher extraction slope, C/C0 = 0.05 to 0.70, from breakthrough to achieving resin saturation faster. From breakthrough to saturation of the column, the 39-mL column requires 27 BV, whereas the 83-mL column has a value of C/C0 near 0.796 in 36 BV. The effect of the diameter of the column was also tested on 50 cm columns (83-mL and 231-mL). Increasing the diameter of the column from 1.5 cm to 2.5 cm resulted in similar trends than when the length was modulated. The 231-mL column has the same chromatographic pattern as the 39-mL column for U. Overall, the increase in the length of the column delays the breakthrough of the column, whereas the increase in diameter of the column tends to reduce losses of metal, observed through sharper extraction slopes.

3.5. Retention of U from SCRW Leaching Solution by Lewatit TP260—Extraction Cycles

To assess the reusability of Lewatit TP260 to recover uranium from SCRWL, five cycles of extraction and desorption were executed in batches, in triplicate. A volume of 30 BV of SCRW leaching solution (containing 57 mg/L of U, 57 mg/L of Hg, 674 mg/L of Fe, 1190 mg/L of Al, 635 mg/L of Mg, and 102 mg/L of Ca) were mixed with the resin Lewatit TP206 conditioned with a sulfuric (1 M) and iodide (0.15 M) solution. A previous study [31] shows that U is completely desorbed from the resin with 3 BV of 1 M Na2CO3 solution. Therefore, desorption was performed with 5 BV of 1 M Na2CO3 solution. The resin was rinsed with 3 BV of water between each step (extraction, desorption) and conditioned with the sulfuric (1 M) and iodide (0.15 M) solution. Table 6 presents the extraction and desorption efficiencies obtained during the five cycles.
The results (not shown) indicate that Mg and Ca were not retained on the resin. Only a limited amount of Ca (less than 5%) and Mg (less than 10%) were extracted from the leaching solution. The desorption of Ca (1%) and Mg (4%) over the five cycles was limited, which is consistent with the resin selectivity. However, Table 6 shows that Fe was strongly retained by the resin. While Fe adsorption reached approximately 18% at the first cycle, it increases to 44%–56% during the subsequent cycles. Almost no Fe was desorbed from the resin with the sodium carbonate solution, which is consistent with the manufacturer notice regarding the difficult desorption of trivalent cations from this resin [32]. Al (results not shown) was extracted from the solution to a lower extend (17%) and was then partially desorbed (17%) by the sodium carbonate solution.
The adsorption of U and Hg on the resin significantly decreased during the three first cycles (from 87 to 62% for U, from 64 to 29% for Hg) and then slowly decreased. The U desorption remained stable over the five cycles (80%–84%), while only 5%–6% of the Hg is desorbed. The decrease of U and Hg adsorption is likely due to the fact that much of the resin sites are progressively occupied by the trivalent cation (Fe and Al) present in the leaching solution and not eluted between each cycle. Because Fe and Al are not properly eluted from the resin in the conditions tested, an additional rinsing step should be included in order to desorb these elements prior to the next cycle. The use of strong ligands such as EDTA could enable the removal of Fe and Al and lead to a more replicable level of U and Hg adsorption over many cycles.

4. Conclusions

The use of Lewatit TP260 to co-extract uranium and mercury in sulfuric and iodine media was found conclusive. Furthermore, it was proven that the affinity of Lewatit TP260 was stronger for uranium than for mercury. Thermodynamic modeling predicts that >95% of the species of U and Hg is found as [UO2(SO4)2)]2− and [HgI4]2−. Continuous mode experiments have shown that quantitative extraction of uranium, in the conditions tested, is possible in approximately 8 h, whereas lower equilibrium extraction yields were obtained for Hg even after 24 h. The adsorption kinetics results for uranium were best fitted by the pseudo-second order model, suggesting chemisorption as the main sorption process. In continuous mode, breakthrough of uranium on the Lewatit TP260 is dependent on the flowrate and column geometry, but can be delayed to approximately 12 BV when using a 1.5 BV/h on a 83-mL (length: 47 cm; diameter: 1.5 cm) column. It was also found, through reusability experiments, that uranium and mercury have reduced absorption on the Lewatit TP260 after the first cycle due to the low desorption rate of trivalent cations, iron, and aluminum observed with the elution conditions tested. Because the selective elution of uranium and mercury from Lewatit TP260 has not been achieved in this study, removal of Hg ions prior to loading on the Lewatit TP260 should be considered throughout the development of a two-step process using a thiourea ion exchange resin. This would lead to a selective desorption of uranium ions from Lewatit TP260, a desired feature for the objective of this project.
© 2023 Her Majesty the Queen in Right of Canada, as represented by the Minister of Natural Resources.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/min13030405/s1, Figure S1: FTIR resin characterization with (top) or without (bottom) uranium; Table S1: Summary of kinetic parameters for the pseudo-first order models calculated for Hg and U on the Lewatit TP260; Table S2: Summary of kinetic parameters for the second order models calculated for Hg and U on the Lewatit TP260.

Author Contributions

Conceptualization, N.R., M.C. and D.L.; methodology, N.R., M.C., R.-M.C. and D.L.; investigation, N.R., M.C., J.B. and D.L.; writing—original draft preparation, N.R., M.C., R.-M.C., J.B. and D.L.; writing—review and editing, N.R., R.-M.C. and D.L.; supervision, N.R. and D.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Not applicable.

Acknowledgments

The authors would like to thank Cheryl Laviolette for the chemical analyses. The authors also thank the internal reviewers and the anonymous reviewers, who provided very constructive comments and greatly contributed to the improvement of this manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

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Minerals 13 00405 g001 550
Figure 1. Distribution coefficient values (A) and extraction yield (B) of selected analytes on Lewatit TP260 at pH 1.8 (H2SO4 media), without any salt added, with KNO3 (0.15 M) and KI (0.15 M) (equilibration time, 24 h).
Figure 1. Distribution coefficient values (A) and extraction yield (B) of selected analytes on Lewatit TP260 at pH 1.8 (H2SO4 media), without any salt added, with KNO3 (0.15 M) and KI (0.15 M) (equilibration time, 24 h).
Minerals 13 00405 g001
Minerals 13 00405 g002 550
Figure 2. Impact of the KI concentration on the distribution coefficient values (A) and extraction yield (B) for U, Hg, and Fe on Lewatit TP260 at pH 1.8 (H2SO4 media) (equilibration time, 24 h).
Figure 2. Impact of the KI concentration on the distribution coefficient values (A) and extraction yield (B) for U, Hg, and Fe on Lewatit TP260 at pH 1.8 (H2SO4 media) (equilibration time, 24 h).
Minerals 13 00405 g002
Minerals 13 00405 g003 550
Figure 3. Extraction kinetics of (A) uranium and (B) mercury on chelating resin Lewatit TP260 (mresin = 3 g; VSCRWL = 100 mL; Speed = 180 rpm).
Figure 3. Extraction kinetics of (A) uranium and (B) mercury on chelating resin Lewatit TP260 (mresin = 3 g; VSCRWL = 100 mL; Speed = 180 rpm).
Minerals 13 00405 g003
Minerals 13 00405 g004 550
Figure 4. Chromatographic extraction profile of uranium and mercury on Lewatit TP260 using (A) SCRWL’-U and (B) SCRWL’-U-Hg; BV = 12 mL; Flow rate = 3 BV/h.
Figure 4. Chromatographic extraction profile of uranium and mercury on Lewatit TP260 using (A) SCRWL’-U and (B) SCRWL’-U-Hg; BV = 12 mL; Flow rate = 3 BV/h.
Minerals 13 00405 g004
Minerals 13 00405 g005 550
Figure 5. Chromatographic extraction profile of (A) uranium and (B) mercury on Lewatit TP260. SCRWL = KI 0.12, 0.18, 0.24 M; BV = 12 mL; Flow rate = 3 BV/h.
Figure 5. Chromatographic extraction profile of (A) uranium and (B) mercury on Lewatit TP260. SCRWL = KI 0.12, 0.18, 0.24 M; BV = 12 mL; Flow rate = 3 BV/h.
Minerals 13 00405 g005
Minerals 13 00405 g006 550
Figure 6. Continuous extraction of uranium on chelating resin Lewatit TP260. Effect of (A) flow rate (SCRWL-[KI]int—0.15 M; BV = 12 mL; Flow rate = 1.5 BV/h, 3 BV/h, 4.5 BV/h) and (B) geometry (SCRWL-[KI]int—0.15 M; BV = 39 mL, 83 mL, 231 mL; Flow rate = 3 BV/h).
Figure 6. Continuous extraction of uranium on chelating resin Lewatit TP260. Effect of (A) flow rate (SCRWL-[KI]int—0.15 M; BV = 12 mL; Flow rate = 1.5 BV/h, 3 BV/h, 4.5 BV/h) and (B) geometry (SCRWL-[KI]int—0.15 M; BV = 39 mL, 83 mL, 231 mL; Flow rate = 3 BV/h).
Minerals 13 00405 g006
Table
Table 1. Instrumental parameters used for the elemental analyses.
Table 1. Instrumental parameters used for the elemental analyses.
Instrumental ParametersICP-OESICP-MS
RF Power (W)12501500
Plasma gas flow (L/min)1514
Auxiliary gas flow (L/min)1.50.8
Nebulizer gas flow (L/min)0.750.78
Replicates33
Replicate Time (s)1030
Stabilization Time (s)3010
Sample flow rate (rpm)1415
Wavelength (nm)/IsotopeMg(279.553), Al(237.312),
Ca(422.673), Fe(238.204),
In(190.794), Hg(184.887),
U(385.957), Tl(190.794),
In(230.606), Ar(737.212)		24Mg, 27Al, 44Ca, 56Fe,
200Hg, 202Hg
232Th, 238U
103Rh, 115In
Table
Table 2. Uranium and mercury concentrations (in mg/L) of the different SCRW leaching solutions used depending of the IX experiments performed.
Table 2. Uranium and mercury concentrations (in mg/L) of the different SCRW leaching solutions used depending of the IX experiments performed.
SCRW Leaching
Solution		KI a
(M)		KI a
(g/L)		U (mg/L)Hg (mg/L)
KineticsColumnKineticsColumn
SCRWL-[KI]int—0.120.12201371647274
SCRWL-[KI]int—0.150.1525-152-87
SCRWL-[KI]int—0.180.183013418611882
SCRWL-[KI]int—0.240.244010716810462
a—Initial concentration used for the dissolution step.
Table
Table 3. Parameters of the simulations used to evaluate the speciation of elemental constituents in SCRW leaching solution.
Table 3. Parameters of the simulations used to evaluate the speciation of elemental constituents in SCRW leaching solution.
ParameterConcentration
(mol/L)		Concentration
(mg/L)
U(VI)0.000495.2
Hg2+0.0005100
Fe3+0.013726
Al3+0.0561511
Mg2+0.0061148
K+0.015; 0.15; 0.30587; 5865; 11,730
I−0.015; 0.15; 0.301904; 19,036; 38,071
N(V)0.0721008.5
S(VI)132,660
Ca2+0.5 *20,039
pH1.821.82
* set as charge balance.
Table
Table 4. Composition of SCRW’ leaching solution.
Table 4. Composition of SCRW’ leaching solution.
Sample IdentificationConcentration
U (mg/L)Hg (mg/L)Ca (mg/L)KI (M)
SCRWL’-0.15-U115-3510.15
SCRWL’-0.15-U.Hg100973170.15
Table
Table 5. U and Hg extraction in a continuous mode (SCRWL = KI 0.15 M; BV = 12 mL; Flow rate = 3 BV/h) in triplicate (mean uptake and standard deviation).
Table 5. U and Hg extraction in a continuous mode (SCRWL = KI 0.15 M; BV = 12 mL; Flow rate = 3 BV/h) in triplicate (mean uptake and standard deviation).
U Extraction (%)Hg Extraction (%)
Bed VolumeMean Uptake (C/C0)Standard DeviationMean Uptake (C/C0)Standard Deviation
00.000.000.000.00
50.370.0910.321.39
100.750.1841.695.87
156.744.0690.512.23
2016.511.97105.023.20
2529.843.59107.621.28
3042.494.47104.975.88
3555.313.34102.764.45
4060.545.52107.425.64
4569.723.36101.762.83
5073.913.09100.864.10
Table
Table 6. Extraction and desorption of U, Hg, and Fe on chelating resin Lewatit TP260 through multiple cycles in triplicate (mean and standard deviation).
Table 6. Extraction and desorption of U, Hg, and Fe on chelating resin Lewatit TP260 through multiple cycles in triplicate (mean and standard deviation).
CyclesStepsU (% ± STD)Hg (% ± STD)Fe (% ± STD)
1Adsorption87 ± 1%64 ± 1%18 ± 2%
Desorption82 ± 5%5 ± 1%0%
2Adsorption75 ± 2%56 ± 4%54 ± 4%
Desorption80 ± 13%5 ± 1%0%
3Adsorption62 ± 2%29 ± 9%56 ± 1%
Desorption82 ± 14%6 ± 2%0%
4Adsorption58 ± 1%26 ± 1%44 ± 6%
Desorption84 ± 3%6 ± 1%0%
5Adsorption54 ± 1%22 ± 1%50 ± 1%
Desorption84 ± 7%5 ± 1%0%
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Courchesne, M.; Couture, R.-M.; Basque, J.; Reynier, N.; Larivière, D. Co-Extraction of Uranium and Mercury Using Ion Exchange from Cemented Radioactive Waste Sulfuric Leachate in Iodide Media. Minerals 2023, 13, 405. https://doi.org/10.3390/min13030405

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Courchesne M, Couture R-M, Basque J, Reynier N, Larivière D. Co-Extraction of Uranium and Mercury Using Ion Exchange from Cemented Radioactive Waste Sulfuric Leachate in Iodide Media. Minerals. 2023; 13(3):405. https://doi.org/10.3390/min13030405

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Courchesne, Maxime, Raoul-Marie Couture, Justine Basque, Nicolas Reynier, and Dominic Larivière. 2023. "Co-Extraction of Uranium and Mercury Using Ion Exchange from Cemented Radioactive Waste Sulfuric Leachate in Iodide Media" Minerals 13, no. 3: 405. https://doi.org/10.3390/min13030405

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