2.1. Materials

HDPE fibers in average molecular weight of 1.5×10⁶ and an average diameter of 30 μm, made via a melt spinning method, were supplied by Donghua University and used as trunk polymer for grafting. Acrylic acid (AA), acrylonitrile (AN) and other chemical agents, all of analytical grade, were purchased from Sinopharm Chemical Reagent Co. Ltd. All agents were used without further purification.

2.2. Adsorbent preparation

The amidoxime-based polymeric adsorbents were prepared by RIGP of AA and AN onto HDPE fibers and subsequent amidoximation modifications as shown in Fig. 1. HDPE fibers were pre-irradiated by electron beam to 200 kGy in air at room temperature. The irradiated fibers were immersed into a 60wt% DMF solution of AN/AA co-monomer and deaerated by nitrogen. The mole ratio of AN/AA was set as 80/20. Co-grafting of AN and AA onto the irradiated HDPE fibers was performed at 70°C for several hours. After repeated rinsing by dimethylformamide and de-ionized water, the grafted fibers were vacuum-dried and weighed. The degree of grafting was calculated by DG = [(W₁− W₀)/ W₀]×100%, where W₀ and W₁ are the weights of HDPE fiber before and after grafting, respectively.

Fig.1.Full-screen View

Preparation of amidoxime-based polymeric adsorbents by RIGP of AA and AN onto HDPE fibers and subsequent amidoximation.

Various degrees of grafting of HDPE were prepared and screened, the grafted HDPE-g-PAA-co-PAN fibers with DG at 230wt%–250wt% were selected for amidoximation. They were treated by hydroxylamine hydrochloride (NH₂OH·HCl) solution of pH7.0 at 80°C for 2 h to covert the grafted cyano groups to AO groups. After reaction, the obtained HDPE-g- PAA-co-PAO adsorbents were washed by de-ionized water for several times and vacuum-dried.

2.3. Characterization.

Chemical structures of the pristine HDPE fiber, HDPE-g-PAA-co-PAN fiber and HDPE-g-PAA-co-PAO fiber were analyzed with Fourier transform infrared spectroscopy on a Bruker Tensor 27 FT-IR spectrometer (Bruker Optics, Germany) in the range of 600-4000 cm⁻¹.

2.4. Flow-through test.

The adsorbent performance in simulated seawater was carried out on a lab scale 1.5 m³/day simulated seawater adsorption system as shown in Fig. 2.

Fig. 2Full-screen View

Schematic diagram of continuous flow-through system.

Simulated seawater was prepared by sea salt in DI water followed by adjusting of salinity (35 psu), metallic ion concentrations (Table.1) and pH of solution. For characterization of kinetic adsorption data on the amount of uranium and other co-existing ions as a function of time, a series of adsorption experiment over 6 weeks was performed using this system. Seven columns of HDPE-g-PAA-co-PAO fiber adsorbents were placed in parallel in this continuous flow-through system. Amidoxime-based HDPE adsorbents (0.2 g) was freely dispersed and packed in each column, and the empty space was filled up with polyethylene terephthalate (PET) fibers. The simulated seawater was drawn from a reservoir and forced through the manifold using a pump with all fluorine plastic components in the pump head and PPH tubing feed lines. With a thermostatic water tank kept at 25±2°C, the simulated seawater adsorption was performed at a flow rate of 35 ± 2 mL·min⁻¹ using actively pumping systems. Adjustments were made when the temperature increased by 2°C and the flow rate increased by 10%. The adsorbent in each column was periodically removed at intervals of 6–8 days.

2.5. Adsorption in seawater

The seawater testing was performed in a seawater desalination plant (Zhejiang, China). The seawater with precipitation treatment was pumped through 4 g adsorbents fixed on the canal bed at the flow rate of 80 cm/s at 21±2°C. After 15 days of exposure, the adsorbents were taken out of canal. The trace metal concentrations in seawater are given in Table. 1.

2.6. Analysis process.

The exposed adsorbent fibers were collected from adsorbent columns, washed with DI water to remove salts, and vacuum-dried at 60°C. The dried adsorbents (100 mg) were digested with 10 mL high-purity concentrated nitric acid in an MARS6 Microwave Digestion System (CEM, USA). High-purity DI water (Optima, Fisher Scientific) was added to make a 50-mL dilute acid solution and to give desired concentration range of uranium for analysis. Inductively coupled plasma atomic emission spectrometry (ICP-AES, Thermo Scientific X-Series II) was used for quantitative analysis of uranium. Each sample was analyzed twice against a four-point calibration curve. The adsorbed amounts of uranium (Q) were calculated by Q = C V / W, where C is the uranium concentration in digested solution (V), and W is the weight of initial adsorbents.

3.1. Chemical structure of HDPE-g-PAA-co-PAO adsorbents

In order to prepare HDPE-g-PAA-co-PAO fiber, AA and AN were first grafted onto HDPE fibers through RIGP method. HDPE-g-PAA-co-PAN fiber was then amidoximated to give the amidoxime-based adsorbent. The changes in chemical structures of the grafted fiber were confirmed by FT-IR spectra.

Figure 3 shows FT-IR spectra of HDPE, HDPE-g-PAA-co-PAN and HDPE-g-PAA-co-PAO fibers. Compared with the FTIR-ATR spectrum of virgin HDPE fibers, two new peaks appeared in the spectrum of HDPE-g-PAA-co-PAN fibers at 2243 and 1731 cm⁻¹, corresponding to the C≡N groups of AN and C=O of AA grafted onto HDPE, respectively [17, 25]. After amidoximation, the C≡N band at 2243 cm⁻¹ disappeared completely, while a new and strong C=N stretching vibration at 1645 cm⁻¹ appeared, indicating the completely transformation of nitrile into amidoxime groups.

Fig. 3Full-screen View

FT-IR spectra of HDPE, HDPE-g-PAA-co-PAN and HDPE-g-PAA-co-PAO fibers.

3.2. Uranium adsorption in simulated seawater

Flow-through experiments in simulated seawater were performed to investigate the uranium adsorption behavior and the adsorbent with DG of 252% was employed for this experiments. The uranium adsorption kinetic data is shown in Fig. 4(a). The amount of uranium on adsorbents increased nonlinearly with the exposure time. After 42 days of exposure, the uranium uptake was about 2.51 mg/g-ads, but the adsorption equilibrium was still not reached. The initial uranium uptake rate of HDPE adsorbent was 0.25 mg·g⁻¹·day⁻¹ (Fig.4b), and decreased by 50% at Day 6, where the decrease rate began to slow down considerably. The rapid initial uranium uptake rate could be attributed to the chemical adsorption between uranium ions and AO groups on the surface of adsorbents. Upon contacting with simulated seawater, the hydrophilic AO groups on the adsorbent surface reacted immediately with uranium ions with the assistance of carboxylate groups [13], hence a rapid uptake rate. When adsorption on adsorbent surface reached saturation, the diffusion of metal ions to the inner part of adsorbents appeared. However, this process could be hampered by the metal ions adhered on surface of adsorbents, consequently, leading to the low uptake rate of uranium.

Fig. 4Full-screen View

Uranium adsorption kinetic data (a) and uranium uptake rate (b) from flow-through tests in simulated seawater.

The uptake capacities of metal ions for 42 days adsorption are summarized in Fig.5(a). The uranium adsorption capacity is the highest of all metal ions except lead ions (whose very high capacity could be attributed to the high Pb concentration in simulated seawater). This indicates that HDPE-g-PAA-co-PAO adsorbents possess good selectivity for the uranyl ion.

Fig. 5Full-screen View

Uptake capacity of metal ions in flow-through experiments (a) and marine test (b).

3.3. Uranium adsorption in natural seawater

To further conform the above results, and to evaluate the adsorption capacity of uranium in natural seawater, the adsorption experiment in real seawater were carried out. After 15 days of exposure, the adsorbents were taken out from the canal and analyzed. As shown in Fig. 5(b), the maximum uranium uptake for 15 days was 0.13 mg/ g-ads. The low uranium uptake could be attributed to the short adsorption time and the contamination of marine microorganisms. Furthermore, the concentration of iron in seawater is very high (Table. 1), indicating that the seawater used in this work might be polluted by iron or steel parts of facilities. Consequently, the iron uptake is the highest of all, which could affect the adsorption of uranium. It should be noticed that the uranium adsorption capacity is about 6 times higher than that of vanadium, consistent well with the result of flow-through experiment and suggesting high selectivity towards uranium against vanadium. The U/V selectivity of HDPE-g-PAA-co-PAO fiber in real seawater was far higher than that of JAEA and ORNL fibers [26]. This is meaningful for its use of harvesting uranyl ion onto adsorbents and improving the economic feasibility for uranium recovery from seawater.