Synthesis, Development, and Testing of High-Surface-Area Polymer

Article pubs.acs.org/IECR

Synthesis, Development, and Testing of High-Surface-Area PolymerBased Adsorbents for the Selective Recovery of Uranium from Seawater Yatsandra Oyola,*,† Christopher J. Janke,‡ and Sheng Dai† †

Chemical Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831-6181, United States Material Science and Technology Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831-6181, United States

‡

S Supporting Information *

ABSTRACT: The ocean contains uranium with an approximate concentration of 3.34 ppb, which can serve as an incredible supply source to sustain nuclear energy in the United States. Unfortunately, technology currently available to recover uranium from seawater is not efficient enough and mining uranium on land is still more economical. We have developed polymer-based adsorbents with high uranium adsorption capacities by grafting amidoxime onto high-surface-area polyethylene (PE) fibers. Various process conditions have been screened, in combination with developing a rapid testing protocol (12 μm, the fibers appear to lose their high-surface-area advantage and obtain an adsorption capacity that is equivalent to that of the TBR fibers (d = 23 μm). High-Surface-Area Shaped Polyethylene Fibers. In order to obtain a high uranium adsorption capacity while maintaining the mechanical integrity, the morphology of a thicker fiber can be adjusted. Non-round-shaped fibers can significantly increase the surface area:weight ratio, compared with round fibers having the same diameter while maintaining a higher mechanical strength. Polyethylene fibers obtained from Hills, Inc., also had the capability of producing nonround fibers using a unique aspect of their technology that involves a process of melt-spinning fibers with unique shapes. The fiber shapes studied include solid or hollow flower shapes, gear shapes, trilobal shapes, solid trilobal gear shapes, and others. Further information on each shaped fiber evaluated can be obtained from Table 2. Figure S6 in the Supporting Information also shows some additional microscopy images of some of the shaped fibers in this study (Fibers 3, 8, 12, 16, 17, and 18) coated in PLA. Just as in the case of small-diameter fibers, the irregularly shaped fibers offer more surface area per volume with an increased number of grafting sites, but without the loss of mechanical strength. Furthermore, the fibers that have hollow interiors can potentially provide greater access to the grafting sites in the interior. The shaped fibers were grafted using the optimized conditions for the TBR and small diameter round fibers with a 25% DMSO and 75% (70/30 AN/MAA) comonomer

Figure 3. Comparison of the adsorption capacity of round fibers with varying diameters in 25% or 50% DMSO grafting solution at 60 °C.

smaller-diameter fibers have a tendency to have a higher adsorption capacity than the larger-diameter fibers, although there is some variation between measurements due to experimental error. The smallest-diameter fiber (Fiber 8, d = 0.24 μm) clearly has the highest adsorption capacity, because of its high surface area:volume ratio. It is believed that not only do the comonomers graft on the surface of the PE fibers, but they also graft throughout the interior. With a diameter of only 0.24 μm, a higher percentage of grafting sites are at or near the surface per weight of the fiber, which makes the functional groups responsible for metal adsorption more accessible. The drawback to using such a small diameter is that the mechanical strength is weaker and may not survive ocean conditions. Fiber 4154

DOI: 10.1021/acs.iecr.5b03981 Ind. Eng. Chem. Res. 2016, 55, 4149−4160

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Industrial & Engineering Chemistry Research Table 2. Description of Polyethylene Fibers Shapes, Diameters, and Composition Used To Prepare the Adsorbents

solution at 60 °C in a closed reaction system for 6.3 h. Fibers with the gear structure (Fibers 12, 14, 17, 18, and 19), shown in Table 2, showed the highest adsorption, as shown in Figure 4,

the interior of the fiber. When the comonomers are grafted onto the polymer fibers, they graft onto the surface of the polymer and through the interior. Therefore, the hollow structure allows greater access to the metal binding sites and a higher adsorption capacity can be obtained. Fiber 12 was identified as having the greatest potential to produce the best adsorbent and was used as the standard for optimization studies. During the irradiation process, the radicals that are generated on the polymer surface are extremely reactive with oxygen and must be kept under an inert atmosphere. It was found that, as long as air was kept out of the system, there was no significant difference if the irradiation was performed under static nitrogen (133 gU/kgads), flowing nitrogen (124 gU/kgads), or even under vacuum (115 gU/kgads). The importance of the reaction timing was investigated more deeply by observing how the adsorption capacity was affected by the length of time to add the fibers to the grafting solution, in combination with the time needed to place the reaction flask into the oven and the initial temperature of the solution. As can be seen in Table S4 in the Supporting Information, a higher adsorption capacity was obtained if the irradiated fibers were immediately placed in the grafting solution at room temperature and the reaction flask was placed into the oven at 60 °C. Once the irradiated fibers are placed into the grafting solution, the highly reactive radicles have an option to react with the comonomers and graft to the surface of the fiber or recombine with another radical. Therefore, it is necessary to immediately start the grafting reaction after the irradiation process happens. It is also important to immerse the fibers into the grafting solution at room temperature rather than at a high temperature.

Figure 4. Comparison of the adsorption capacity of PE fibers with different diameters and shapes in 25% DMSO grafting solution at 60 °C.

which indicates that its shape had the highest surface area, compared to the other shapes. Fiber 12 was found to have the highest adsorption capacity out of all of the hollow gear shapes, and it is believed that the polymer grade may play a role. The hollow gear structures have a tendency to have the higher adsorption capacities, because there is increased surface area on the exterior of the fiber, but also the hollowness allows access to 4155

DOI: 10.1021/acs.iecr.5b03981 Ind. Eng. Chem. Res. 2016, 55, 4149−4160

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Industrial & Engineering Chemistry Research

50, and 60 psi, at room temperature, the capacity of uranium adsorption did not significantly change. On the other hand, the vapor pressure of the solvent was enough to increase the DOG, but additional pressure on top of the vapor pressure did not increase the DOG. High pressures can increase the propagation rates and favor the higher densities and less branching or termination.56 However, this will be also true for the copolymer byproduct formation; therefore, this can explain the fact that higher pressures do not have a significant effect on the final product. During the grafting process, an excess amount of the comonomer solution was used for optimization purposes, and it is important to reduce the amount of materials consumed in order to manufacture a commercially viable adsorbent. As shown by Figure S7 in the Supporting Information, different volumes (8, 24, 38, and 240 mL) of the comonomer solution were added to the grafting reaction. Even when decreasing the amount of the comonomer solution, the adsorption capacity did not decrease and the minimum amount of the comonomer solution has not been found at this time. However, the JAEA have estimated their cost analysis based on 1 mL of solution per gram of fiber to be sufficient to achieve an acceptable level of grafting. We have not attempted to graft comonomers (AN and MAA) at that low of a ratio, since, from our observations, for the 0.5 g of PE fibers used in the study, 8 mL of solution were needed to damp the fibers. In addition, we feel it was important to uphold the same volume to flask size ratio (pressure generated by the vapor pressure in the solution) to avoid compromising the reaction conditions. However, in the event of a scaleup, we might find that the relative excess can be reduced while maintaining enough solution to obtain a high DOG while preserving homogeneous wetness of the fibers and pressure. Commercial-Grade Adsorbents. Amidoxime-based polyethylene adsorbent fibers were compared to commercial-grade adsorbent materials in simulated seawater. The Metsorb series, manufactured by Graver Technologies, is specifically engineered to provide excellent arsenic, lead, and other heavy-metal containment removal for the purification of drinking water, process water, and other critical purification applications. The Metsorb adsorbents are composites of TiO2/hydroxide/vinyl alcohol. The DYNA AQUA product line, from Dynamic Adsorbents, involves specialized alumina oxides that have been designed for the removal of toxic metal from water and claim to be the awaited solution for the environmental cleanup of waste systems targeting uranium, lead, copper, and chloride cleanups. These commercially available adsorbents were compared with three of our polymer adsorbents, including the sample provided by the JAEA, our optimized amidoxime-grafted TBR polymer adsorbent, and the high-surface-area hollow gear-shaped adsorbent (Fiber 12); the results are shown in Figure 6. The Metsorb HMRA 50 commercial adsorbent was the strongest contender out of all the commercial samples and was comparable with the JAEA adsorbent. The high-surface-area hollow gear-shaped fiber (Fiber 12) outperformed all samples by at least 1 order of magnitude, with an adsorption capacity of 188.1 gU/kgads. The incredible improvement in adsorption capacity shown by Fiber 12 demonstrates how critical a high surface area is when grafting a metal adsorption functional group on a solid support, in combination with the selectivity of amidoxime groups. Furthermore, the astonishing outperformance of Fiber 12 demonstrates the potential for opportunities in adsorption applications, like the removal of toxic metals and

It is likely that the monomers can start to copolymerize if the initial grafting solution temperature is kept at 60 °C before the fibers are added. Another possibility is that there may be competing reactions between the comonomers grafting on the polymer and the copolymerization between the comonomers, which may be favored at 60 °C. By slowly heating the reaction system to 60 °C from room temperature, more time may be given for the comonomers to graft onto the polymer fiber. As Figure 5a shows, a higher adsorption capacity is obtained if the

Figure 5. Comparison of Fibers 11, 12, and 14, prepared according to standardized procedure, but immediately after the fibers were irradiated at a low temperature they were immersed into a grafting solution at (a) room temperature or (b) 60 °C in either 75% (70/30 AN/MAA) or 90% (80/20 AN/MAA) grafting mixtures.

initial grafting solution temperature is near 20 °C versus when the initial grafting solution temperature is conducted at 60 °C, as shown in Figure 5b. As was seen for the TBR fibers, a comonomer concentration of 75% (30/70 AN/MAA) is the optimal mixture required to provide the higher uranium adsorption capacities, as well as starting with a roomtemperature grafting solution Additional experiments were conducted to determine if different agitation methods could help improve the grafting of the polymer with an initial temperature at 60 °C. It was found that stirring the reaction at 60 °C slightly decreased the efficiency of grafting with the adsorption capacity decreasing from 79 gU/kgads to 68 gU/kgads. Sonication of the reaction while in a water bath at 60 °C was even more detrimental, with a capacity at 47 gU/kgads. Some of the best adsorption capacities were obtained from carrying out the grafting in a closed reaction system, and it was necessary to investigate how increased pressure could affect the reaction. For example, Table S5 in the Supporting Information shows that increasing the pressure in the reaction flask to 20, 4156

DOI: 10.1021/acs.iecr.5b03981 Ind. Eng. Chem. Res. 2016, 55, 4149−4160

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Industrial & Engineering Chemistry Research

coast of Savannah, GA, which contained an initial uranium concentration of 3.6 ppm. On the other hand, an adsorption capacity of 3.36 gU/kgads was found when Fiber 12 was immersed in seawater from near-surface seawater in Charleston, SC harbor with an initial uranium concentration of 3.2 ppm. In addition, Fiber 14 (solid gear, 17 μm, diameter) was tested and found to have an adsorption capacity of 2.6 gU/kgads for uranium in seawater off the coast of Charleston, SC. However, when it was tested in seawater from Savannah, GA, it obtained 3.55 gU/kgads. The maximum amount of uranium uptake from seawater in Washington was an average of 3.3 gU/kgads after 8 weeks of contact between the adsorbent and seawater in several independent tests performed by ORNL and PNNL.41 This uranium adsorption amount was ∼3 times higher than the maximum amount achieved by the JAEA’s leading adsorbent, which was 1.1 gU/kgads at equilibrium. Both adsorbents were tested using similar conditions in all independent studies, using filtered (0.45 μm) seawater from Sequim Bay, in Sequim, WA, at a temperature of 20 ± 0.5 °C and two different flow rates (250 and 500 mL/min). The temperature in the seawater reservoir was controlled with an all-titanium immersion-heating element. The adsorbents were dispersed and packed in the column and held in place using glass wool and/or 3 mm glass beads, placed either in series or in a parallel arrangement directly connected to a 12-port manifold system (composed of PVC). The previous studies also detailed the concentrations and uptake amounts of various elements in a field test for the seawater from Sequim Bay. As can be seen, a discrepancy in uranium adsorption capacity exists when the same adsorbent fibers (Fibers 12 and 14) are tested using different sources of seawater. These values suggest that the adsorbent capacity is dependent on the concentration of uranium, competing metal ions, and seawater conditions (e.g., salinity, temperature, and pH). Several of the best-performing amidoxime-based highsurface-area adsorbent fibers (Fibers 8, 11, 12, 14, and 16, as reported in Table 3) were tested for uranium adsorption in seawater and compared to the polymer-based adsorbent kindly donated by the JAEA in Japan. The fiber adsorbents were also placed in a “flow through column” system arranged in parallel and attached to a 12 port PVC manifold. Because of the

Figure 6. Uranium adsorption capacity of commercial adsorbents compared to the amidoxime grafted; JAEA, TBR, and high-surface-area polymer fiber adsorbents.

environmental cleanup. The performance of the Fiber 12 adsorbent in a solution containing metal ions, without the complications from the composition of seawater (excess of sodium chloride and the competition from the carbonate ions), can be extraordinary. We estimate the adsorption to be 1000 g of metal per kg of material and with a very fast adsorption, measured using 700 mL of a solution containing ∼60 ppm of metal ions at a of pH of ∼3.5. In this case, the pH is low, because the metal solution was prepared from the corresponding metal salt dissolved in deionized water without the addition of any other chemicals. Seawater Testing. Initial studies with natural seawater were performed in the laboratory with the best performing adsorbents, according to our laboratory screening protocol. These initial studies were useful (critical) to characterize the uranium uptake kinetics from natural seawater from batch and flow through experiments, as well as the development of proper field experiments, modeling, and the economic assessment after improving the adsorption capacity.40 When 5 gal seawater batch experiments were carried out, using Fiber 12, an adsorption capacity of 3.94 gU/kgads was obtained in coastal gulfstream seawater from a location 210 m deep, 75 miles east from the

Table 3. Comparison of Adsorption Capacity of Different Metals between the Best High Surface-Area Adsorbent and the JAEA Adsorbent after Either 8 or 11 Weeks of Exposure to Natural Seawater fiber samples exposed to seawater Fiber 12 Fiber 12 Fiber 12 Fiber 12 Fiber 12 Fiber 12 JAEA sample JAEA sample Fiber 8 (round) Fiber 11 (flower) Fiber 12 (hollow gear) Fiber 14 (solid gear) Fiber 16 (caterpillar) a

sample numbera Sample 1 Sample 2, batch Sample 2, batch Sample 3, batch Sample 3, batch Sample 4, batch Sample 1

1 2 1 2 1

Cu (gCu/kgads)

Fe (gFe/kgads)

After 8 Weeks of Exposure 0.5 2.5 0.4 1.3 0.5 1.8 0.5 2.1 0.6 2.3 0.6 2.9 0.1 0.6 After 11−12 Weeks of Exposure 0.1 0.5 1.0 3.1 1.4 4.5 0.5 2.8 1.1 4.3 1.2 4.0

V (gV/kgads)

U (gU/kgads)

Mn (gMn/kgads)

6.0 5.1 5.8 5.1 5.4 6.7 2.0

2.9 3.5 3.1 2.9 2.9 2.7 1.1

−0.05 −0.1 0.0 0.0 −0.1 −0.1 −0.03

2.0 7.7 7.3 7.3 8.3 8.3

1.1 2.9 2.3 3.1 2.6 2.3

−0.07 −0.13 −0.13 −0.14 −0.12 −0.12

Sample number used for samples of Fiber 12 adsorbent packed slightly different. 4157

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uranium adsorption capacity, using PE fibers from TBR, Inc. The most critical step of the synthesis is ensuring that the reaction is performed in a closed system, where the increase in vapor pressure pushes the equilibrium toward grafting amidoxime onto the polymer. The most optimal conditions to produce an amidoxime-based polymer adsorbent was found to be 25% dimethylsulfoxide (DMSO) with 75% (70/30 acrylonitrile (AN)/methacrylic acid (MAA)) at 60 °C for 6.5 h. When the optimized reaction conditions were determined, attention was focused on how the morphology of the PE fiber can affect the uranium adsorption capacity. It was found that, by decreasing the diameter of the PE fiber, to increase the surface area, the uranium adsorption capacity also increased significantly, because of the existence of more grafting sites and, thus, more metal adsorption sites. Fibers below a diameter of 20 μm did not show a significant improvement, compared to larger-diameter fibers, until the diameter went as low as 0.24 μm. However, even with such high uranium adsorption capacity, the low mechanical strength of the 0.24-μm-diameter fiber does not make it the most ideal choice. Thicker-diameter fibers with various shapes can be used to increase the surface area and offer increased access to grafted sites without sacrificing the mechanical strength. The most effective of these shaped polymer fibers is the hollow gear adsorbent (Fiber 12), which not only allowed for a higher degree of grafting on the surface, but the hollow interior allowed increased access to the amidoxime groups that grafted through the interior of the fiber which increased the adsorption sites. These shaped polyethylene amidoxime-based polymer adsorbents have an adsorption capacity that is at least 1 order of magnitude greater than that of commercially available inorganic adsorbents, when measured in simulated seawater. Several types of amidoximegrafted PE fiber adsorbents were tested in seawater for the extraction of uranium. Fiber 12, with the hollow gear shape, not only more than tripled the adsorption capacity of the JAEA adsorbent, but also was the most effective adsorbent among all of the shaped polymers. With this result, we can estimate that uranium production cost is approximately half that was estimated for the current state-of-the-art technology.

polymer fiber’s natural tendency to agglomerate together, they were spread apart and held in place inside the column using glass beads to ensure their maximum contact with seawater. All adsorbents were tested under similar conditions, as previously reported, in different batches and packed in different configurations using filtered seawater from Sequim Bay, at 20 ± 0.5 °C.41 The temperature in the seawater reservoir was controlled with an all-titanium immersion-heating element. In addition to uranium, the PE fiber adsorbents were also found to be selective for the adsorption of other metals in the following order: vanadium > iron > uranium > copper. The quality of seawater was continuously monitored for pH, temperature, salinity, and trace-metal concentrations over the experimental period to ensure that there were no drastic changes in the conditions.41 Adsorbent Fiber 12 was found to have the highest adsorption capacity for uranium when being exposed to seawater in the “flow through column” system, even though it had a relatively large diameter of ∼30 μm. The high adsorption capacity of Fiber 12 is attributed to the gear shape structure with a hollow interior that not only has increased surface area, but can provide greater access to the grafted active sites for metal adsorption (or grafted ligands). In our results using seawater from Sequim Bay, the uranium adsorption capacity ranged between 2.7 gU/kgads and 3.5 gU/kgads over four separate batches of material, as shown in Table 3, which demonstrates that the results are consistent and reproducible. The average is consistent with previous independent testing, and the small variation can be attributed to the differences in the concentrations of other metals, ions competing with uranium, variations in seawater conditions, and water testing temperatures.41 Other samples tested, such as adsorbent Fiber 8 (round, 0.24 μm in diameter), Fiber 11 (flower, 14 μm in diameter) and Fiber 16 (caterpillar, 17−20 μm in diameter) were each measured to have a uranium adsorption capacity of 2.9 gU/kgads and 2.3 gU/kgads, respectively, using our designed “flow through column” system and seawater from Sequim Bay. Overall, Fiber 12, with the hollow gear morphology, had the highest adsorption capacity, and one possible reason is due to the change it undergoes during KOH conditioning. For instance, the initial density (prior to grafting) of Fiber 12 was 0.941 g/cm3 and a diameter of 30 μm, prior to grafting, grew to ±153 μm after KOH conditioning, which facilitates the transfer of metal ions into the fiber. Furthermore, the hollow gear structure can also allow increased access to the sites grafted in the interior of the polymer fiber, allowing for an even higher increase in adsorption capacity. This demonstrates that the hollow morphology and porosity of the polymer can have significant influence on a higher grafting density and access to metal adsorption sites. On the other hand, higher density polymer fibers, with a lower surface area, had lower adsorption capacities such as Fiber 11 (flower-like) with 2.2 gU/kgads or the JAEA adsorbent with 1.1 gU/kgads. Based on the most modest results, it was concluded that the estimated uranium production cost was reduced to $610/kg of uranium, which is approximately half the cost estimated for the JAEA technology.41

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b03981. Structure of amidoxime binding to uranyl dioxide (Scheme S1); irradiation conditions for RIGP (Table S1); grafting conditions and adsorption capacity determined by our laboratory screening protocol (Table S2); grafting solution composition and quantities (Table S3); effect of time on the grafting of acrylonitrile onto the PE fiber (Table S4); effect of pressure on grafting (Table S5); uranium adsorption capacity monitored over time (Figure S1); effect of grafting at different temperatures on the adsorption capacity (Figure S2); different grafting solution compositions (Figure S3); effect of PE fiber diameter on the surface area (Figure S4); SEM images of the PE before and after being encased in PLA (Figure S5); optical micrographs of the different morphologies of the PE fibers (Figure S6); relationship between flask volume and grafting solution volume and how it influences grafting (Figure S7) (PDF)

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CONCLUSION High-capacity uranium adsorbents were made by electron-beam (e-beam) irradiation-induced grafting of amidoxime onto polyethylene (PE) fibers. Reaction conditions were optimized to achieve the highest degree of grafting possible, measured by 4158

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Research sponsored by the U.S. Department of Energy, Office of Nuclear Energy and performed at Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U.S. Department of Energy, under Contract No. DE-AC0500OR22725. This publication has been authored by a contractor of the U.S. government under Contract No. DEAC05-00OR22725. Accordingly, the U.S. Government retains a nonexclusive, royalty-free license to publish or reproduce the published form of this contribution, or allow others to do so, for U.S. government purposes. We thank Dr. Gary Gill for exposing our adsorbents to seawater at the Pacific Northwest National Laboratory in Sequim, WA. We also thank Dr. Seko and Dr. Tamada of JAEA, Takasaki, Japan for their insightful conversations and for kindly donating The Japan Atomic Energy Agency (JAEA) uranium adsorbent material for testing.

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DOI: 10.1021/acs.iecr.5b03981 Ind. Eng. Chem. Res. 2016, 55, 4149−4160

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DOI: 10.1021/acs.iecr.5b03981 Ind. Eng. Chem. Res. 2016, 55, 4149−4160