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Functionalized natural cellulose fibres for the recovery of uranium from seawater

Simple organophosphonate functionalization of sisal fibres yields materials that extract uranium from sea water with high efficiency.

Nuclear ssion is a dense form of energy generation that is generally considered to produce few carbon emissions especially if the uranium utilized derives from high grade ore. 1 It has been suggested that a signicant build-out of ssion nuclear energy would represent an effective measure to combat climate change. 2 A sustainable expansion of low carbon-emitting nuclear ssion energy based on the 238 U-239 Pu fuel cycle would require a plentiful supply of high-grade uranium ore the availability of which is already limited.
Considering that the world's oceans contain about 4 billion tonnes of uranium at a concentration of about 3-4 ppb predominantly in the form of the UO 2 (CO 3 ) 3 4À ion, seawater represents a potentially limitless uranium resource if it can be recovered economically. A variety of separation technologies have been explored for uranium recovery from seawater including nanoltration membranes, 3,4 nanostructured metal oxides, 5 biomass, 6,7 biopolymers, micro-organisms 8 and synthetic polymers. Most recently electrolytic processes have also been proposed. 9 As ostensibly attractive as adsorption technologies are, each material class has its advantages and disadvantages. 10 Of the synthetic polymers, amidoxime bres have garnered by far the most interest as they offer a powerful combination of materials properties including selectivity, capacity and stability in marine conditions. Amidoxime-functionalized polymers have been studied since the early eighties 11 and can be deployed at small scale in uidized beds 12 and in granular and brous forms. 13,14 Gram-scale production of UO 2 has been demonstrated in oating pontoons that support submerged cages containing weaves. 15 Recently a new generation of amidoxime bres with high capacity have been reported using Atom-Transfer Radical Polymerization (ATRP) from poly(vinyl chloride)-co-chlorinated poly(vinyl chloride) (PVC-co-CPVC) 16 and high surface area amidoxime-based polymer bres co-graed with various acids. 17 However, phosphonic acid-functionalized polymers prepared by radiation-induced graing techniques have also been of interest for some time. 18,19 Das et al. 20 reported the use polymers prepared by the Radiation-Induced Gra Polymerization (RIGP) of acrylonitrile (AN) and vinylphosphonic acid (VPA) as well as carboxylate functionalized polymers using AN and itaconic acid. 21 Such bres, as well as those prepared by atom-transfer polymerization, are unlikely to be inexpensive considering the cost of precursors and the number of synthetic steps involved. 16 The latest generation of gra polymers prepared by either ATRP or RIGP have maximum capacity for uranium in the range 4-5 mg g À1 measured in spiked seawater. 22 New generation bres having amidoxime functionality attached to sophisticated high surface area substrates have now reached capacities in excess of 1000 mg U g À1 in spiked seawater and 10 mg U g À1 in natural seawater. [23][24][25] Compared to petrochemical-based polymers, natural bres such as Sisal and Hemp can be mass produced, are likely to have a signicant cost advantage, and most importantly, they are renewable materials. Methods for improving the surface properties of cellulose-based renewable materials involving radiation-induced graing and direct functionalization have been extensively explored. 26 In the present communication we report on the functionalization of common Sisal bres using a simple one-step hydrothermal process utilizing aminotrismethylenephosphonic (ATMP) acid and N-(phosphonomethyl)glycine as the uranium selective functional molecule. The latter is most commonly known as the herbicide glyphosate (GLY). Sisal is one of a number of renewable and inexpensive lignocellulosic bres that have been in widespread use for more than century with current worldwide production of about 375 000 tonnes. ATMP and GLY were chosen for functionalization because they are also inexpensive and due to the well-known affinity that phosphonates and carboxylates have for actinide elements, and the ability of phosphonate groups to gra to hydroxylated surfaces as well as their cost effectiveness. The present functionalization method is extremely simple relative to previously reported methods 26 and yields materials with excellent efficiency for the recovery of uranium from seawater.
Commonly available Sisal twine was utilized in this work. The two phosphonic acids ATMP and GLY with structures as shown below were supplied by Sigma-Aldrich and Gleba S. A. respectively. See ESI † section for the molecular structures.
The functionalization procedure involved hydrothermal treatment of the bres in dilute solutions of the phosphonate at various temperatures. See ESI † section for synthetic details. The products are referred to as ATMP-Sisal-x and GLY-Sisal-x where x is the hydrothermal treatment temperature.
Phosphorus was never observed in any of the seawater supernatants indicating that the phosphonic acids were rmly anchored under the given conditions. X-ray diffraction patterns comparing ATMP-Sisal-150 and GLY-Sisal-150 with unreacted Sisal bres are shown in Fig. 1a-c. Following hydrothermal treatment with GLY and ATMP the linewidth (FWHM) of the (002) reection of cellulose shows a dramatic decrease from 3.22 to 1.85 and 2.17 2q for ATMP-Sisal-150 and GLY-Sisal-150 respectively indicating an improvement in the crystalline ordering of the cellulose matrix. The crystallinity index for all three samples falls in the range 70-90%.
The FTIR spectrum of the untreated Sisal ( Fig. 1d) is typical of that reported in the literature. 28 Functionalization with either phosphonate (Fig. 1e and f) results in the elimination of the intense 1250 cm À1 band assigned to d(C-OH) vibrations of the glucosidic rings 28 and the appearance of a weak band at 1204 cm À1 consistent with vibrations of the -PO(OH) 2 group. Additionally, it can be observed that the intensity of the n(C]O) ester vibration (1720 cm À1 ) is being reduced and a new ester band arises at 1710 cm À1 . The intensity of the former band is reduced proportionately with the P content of the bres (Fig. S2 †). These changes are consistent with graing of the phosphonates and are supported by the fact that P cannot be detected in solution aer the sorption experiments.
SAXS measurements of the unfunctionalized and functionalized bres showed Porod law behaviour in the high q region indicating a sharp phase boundary for both samples. Large differences were observed in the Porod constant aer hydrothermal reactions consistent with sharpening of this boundary (Fig. S3 †).
The degree of uranium recovery from the 4 ppm spiked seawater by the variously treated Sisal bres is shown in Fig. 2.
Untreated sisal bres that had been treated hydrothermally in water or water-ethanol solutions for the same period of time as the samples treated with phosphonate solution did not sorb uranium from the 4 ppm U seawater solution. Hydrothermal treatment of Sisal bres with deionized water resulted in increasing extraction of uranium as a function of temperature reaching 50% for W-Sisal-180. Hydrothermal treatment in the water-ethanol mixture was relatively ineffective at increasing extraction efficiency over the entire temperature range. The extraction efficiency of the ATMP-Sisal-150 was quantitative while the GLY-Sisal-150 bres extracted about 90% of the uranium. The GLY-Sisal-180 bres extracted uranium quantitatively.
Sisal bres treated hydrothermally at 150 C in 0.1 M solutions of H 2 SO 4 and H 3 PO 4 were also tested using the same protocol. Uranium uptake on these acid-treated bres never exceeded 30-50% for these materials conrming that the enhanced performance is due to phosphonate functionalization. The materials show strong sorption of uranium in the pH range 3-9 (Fig. S4 †) but little adsorption below about 2 suggesting that sorbed uranium could be easily eluted. A rough measure of the sorption capacity of the phosphonate-functionalized bres to extract uranium from seawater was obtained using a 10 ppm uranium seawater solution and volume/mass (v/m) ratios of between 300 and 3000 mL g À1 . These results all indicated a saturation capacity in the range 2-3 mg U g À1 .
Detailed measurements of the sorption isotherms were performed for ATMP-Sisal-150 and GLY-Sisal-150 using the spiked 10 ppm U seawater solution and varying the v/m ratio (Fig. S5 †). Langmuir ts of these data gave capacities of 3.2 and 2.3 mg g À1 for ATMP-Sisal-150 and GLY-Sisal-150 respectively. These capacity values are not substantially lower than the best performing synthetic amidoxime polymers reported until 2017 that have capacity values in the range 4-5 mg g À1 . 22 We have observed however that simply increasing the phosphonate solution concentration during functionalization can yield materials having capacities as high as 16 mg g À1 in spiked seawater (10 ppm). In natural seawater we have obtained values of capacity of the order of 0.1 mg g À1 .
The kinetics of adsorption of the best performing materials were measured (Fig. 3a) and show that maximum adsorption is achieved within the rst hour for both materials for U feed concentrations of a few ppm. These are relatively slow kinetics for an adsorbent material but are fast compared with synthetic amidoxime bres that typically take around 20 days to reach maximum uptake.
Although the extraction of U from seawater would not employ xed-bed columns, the testing of the functionalized bres under dynamic conditions in xed bed columns is relevant to other applications. Therefore column breakthrough curves were measured for the functionalized Sisal bres and a typical curve is presented for GLY-Sisal-150 (Fig. 3b). Breakthrough was observed aer reaching a loading of about 0.15 mg U g À1 sorbent. The early breakthrough and slow decline in the C/C 0 value beyond the peak is probably due to the residence time being relatively short compared to the adsorption kinetics of the functionalized Sisal and the fact that column packing was not optimized. Since as previously mentioned U sorption falls off dramatically at low pH, elution with strong and weak acids is possible (Fig. S4 †). Both HCl (1 M) and citric acid (0.20 M, pH 2.0) were tested and were shown to be effective in eluting uranium from the both GLY-Sisal-150 and ATMP-Sisal-150. The use of 1.0 M HCl appeared to be particularly efficient with the bulk of the U being eluted in only 15 mL of eluent (Fig. 3b). Multiple loading and elution of the column was also found to be possible. This work has demonstrated that an exceedingly simple hydrothermal treatment of extremely cost-effect Sisal twine with similarly inexpensive relatively dilute phosphonic acid solutions results in delignication and functionalization of the bres. Phosphonate functionalization results in a substantial increase in the capacity of the Sisal bres to sorb uranium from seawater. Of the two phosphonic acids investigated hydrothermal reaction above 120 C using dilute ATMP solution consistently resulted in materials with slightly superior capacity at the expense of bre cleavage.
It is hypothesized that both phosphonic acids become grafted to the bres through condensation reactions between the phosphonate groups and hydroxyl groups of the cellulose. Similar reactions are observed between organophosphonates and the hydroxylated surfaces of group (IV) metal oxides 29,30 and layered double hydroxides (LDH). The thermal and hydrothermal stability of glyphosate graed to the hydroxyl surfaces of LDH has been demonstrated at temperatures in excess of 180 C. 31 If it is the phosphonate group that gras to the cellulose surface then it must be the carboxylate groups of the glyphosate that bind uranium. In support of this hypothesis we have observed that commercial aminodicarboxylate resins can also extract uranium efficiently from seawater at the  concentrations investigated in this study. However, these materials certainly cannot compete with the present materials. The present one-step functionalization is extremely simple to implement compared to more sophisticated multi-step methods reported in the literature such as the classical TEMPO-mediated oxidation.
This preliminary work opens the way to a potentially new and vast class of renewable and extremely cost-effective materials for applications to uranium recovery from seawater provided capacity can be signicantly optimized. The materials could also have applications in the decontamination of surface and ground waters as well as the efficient extraction of other heavy metals.

Conflicts of interest
There are no conicts to declare.