Sunlight Polymerization of Poly(amidoxime) Hydrogel Membrane for Enhanced Uranium Extraction from Seawater

Abstract The uranium level in seawater is ≈1000 times as high as terrestrial ores and can provide potential near‐infinite fuel for the nuclear energy industry. However, it is still a significant challenge to develop high‐efficiency and low‐cost adsorbents for massively extracting uranium from seawater. Herein, a simple and fast method through low‐energy consumption sunlight polymerization to direct fabrication of a poly(amidoxime) (PAO) hydrogel membrane, which exhibits high uranium adsorption capacity, is reported. This PAO hydrogel owns semi‐interpenetrating structure and a hydrophilic poly(acrylamide) 3D network of hydrogel which can disperse and fix PAOs well. As a result, the amidoxime groups of PAOs exhibit an outstanding uranium adsorption efficiency (718 ± 16.6 and 1279 ± 14.5 mg g−1 of m uranium/m PAO in 8 and 32 ppm uranium‐spiked seawater, respectively) among reported hydrogel‐based adsorbents. Most importantly, U‐uptake capacity of this hydrogel can achieve 4.87 ± 0.38 mg g−1 of m uranium/m dry gel just after four weeks within natural seawater. Furthermore, this hydrogel can be massively produced through low‐energy consumption and environmentally‐friendly sunlight polymerization. This work will provide a high‐efficiency and low‐cost adsorbent for massive uranium extraction from seawater.

Method of detecting uranium element concentration 5 Test of uranium adsorption capacity 5 Test of uranium desorption rate 6 Synthesis of PAO 6 Fabrication of PAO Semi-IPN hydrogel membranes 6 Test of hydrogel adsorption selectivity on different ions 7 Test of hydrogel uranium adsorption from natural seawater 7 Data Analysis 8 Figure S1. The curvilinear regression of uranium concentration-absorbance in the uranium-spiked ultrapure water. 8 Figure S2. The curvilinear regression of uranium concentration-absorbance in uranium-spiked seawater. 8

Method of detecting uranium element concentration
Arsenazo (III) is commonly used as a uranium reagent, which can coordinate with the uranyl ion in aqueous solution. When the complex was detected by UV-Vis spectra, a specific absorption peak appears at 652 nm, and the absorbance is linear with the concentration of uranyl ion in a certain range.
Pre-configured standard uranium-spiked ultrapure-water solutions, with uranium concentrations of 0  Laboratory. The uranium adsorption capacity of PAO hydrogel in our 8 ppm U-spiked seawater is slightly lower but close to the uranium adsorption capacity in ORNL simulated seawater ( Figure S4).

Test of uranium adsorption capacity
A hydrogel membrane adsorbent containing 10 mg dry matter was put into a 1.0 L uranium-spiked ultrapure-water or seawater solutions. After shaking for some time with a table concentrator, we can measure the change of uranium concentration in the solution. The uranium adsorption content of the adsorbent is obtained according to the following formula (1): Where m u is the mass of adsorbed uranium, C o is the original concentration of uranium, C t is the concentration of uranium at specific time, V is the volume of uranium-containing solution. The uranium adsorption capacity of the hydrogel membrane and PAO can be calculated through the following formula (2) and formula (3) respectively:

Test of uranium desorption rate
The method used to elute uranium is based on published literature. [3] Eluent formulation: 500 ml ultrapure water, 5.7 ml 30 % aqueous hydrogen peroxide solution, 53 g sodium bicarbonate powder.
Next, soak the U-uptake hydrogel membrane in the eluent at room temperature, magnetic stirring for 35 min. After the elution is complete, the hydrogel membrane is removed from the eluent and immersed in pure water, and the water is changed several times until the pH is approximately equal to 7. Finally, the hydrogel was soaked in the uranium-spiked seawater solution (32 ppm) for the next cycle. Repeat this adsorption-elution process for five times.

Synthesis of PAO
The PAO was synthesized on the basis of the reported literature. [1,2] After the NH 2 OH·HCl (5.56 g, 80 mmol) was dissolved in DMF (60 mL) within a round-bottom flask at 45 ℃, the Na 2 CO 3 (3.82 g, 36 mmol) and NaOH (0.96 g, 24 mmol) was added and stirred by magnetic force for at least 3 h; dissolved PAN (4.24 g, 80 mmol) at 45 ℃, and then reacted at 65 ℃ for 24 h; replenished Na 2 CO 3 (1.91 g, 18 mmol) and NaOH (0.48 g, 12 mol), and then reacted at 65 ℃ for 12 h. The reaction mixture was dropped into deionized water to get a white flocculent precipitate. After filtered by a bush funnel, the precipitate was dried in a vacuum at 60 ℃ for 8 h to achieve as-prepared PAO.

Fabrication of PAO Semi-IPN hydrogels
Seven PAO semi-IPN hydrogels were prepared with different mass ratios of AAM and PAO ( Figure S9

Supporting Movies
Movie S1. Fabricating process of PAO hydrogel membranes through sunlight polymerization.
Movie S2. U-adsorption process of PAO hydrogel membrane in 100 ppm U-spiked ultrapure water.
Movie S3. U-desorption process of U-uptake PAO hydrogel membrane in elution.