Application of poly(vinylphosphonic acid) modified poly(amidoxime) in uptake of uranium from seawater

To enhance the anti-biofouling properties and adsorption capability of poly(amidoxime) (PAO), vinylphosphonic acid (VPA, CH2 
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Created by potrace 1.16, written by Peter Selinger 2001-2019
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 CH-PO3H2) was polymerized on poly(acrylonitrile) (PAN) surface by plasma technique, followed by amidoximation treatment to convert the cyano group (–C 
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Created by potrace 1.16, written by Peter Selinger 2001-2019
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 N) into an amidoxime group (AO, –C(NH2)N–OH). The obtained poly(vinylphosphonic acid)/PAO (PVPA/PAO) was used as an adsorbent in the uptake of U(vi) from seawater. The effect of environmental conditions on the anti-biofouling property and adsorption capability of PVPA/PAO for U(vi) were studied. Results show that the modified PVPA enhances the anti-biofouling properties and adsorption capability of PAO for U(vi). The adsorption process is well described by the pseudo-second-order kinetic model and reached equilibrium in 24 h. Adsorption isotherms of U(vi) on PVPA/PAO can be well fitted by the Langmuir model, and the maximum adsorption capability was calculated to be 145 mg g−1 at pH 8.2 and 298 K. Experimental results highlight the application of PVPA/PAO in the extraction of U(vi) from seawater.


Introduction
Uranium is very important in the nuclear industry, but uranium ores could be exhausted in a few decades. Researchers have been seeking new uranium sources for half a century. Uranium (U(VI)) in seawater is the only alternative source on Earth, and could meet the growing demand for thousands of years. [1][2][3][4][5] However, the task of extracting U(VI) from seawater faces many practical problems, such as its very low concentration, unpredictable ocean weather, and destructive marine biofouling etc.
The development of specic materials with excellent antibiofouling properties, high adsorption capability and good reusability are critical for U(VI) recovery. PAO based materials are widely applied in U(VI) separation from seawater because of the strong coordination capability between the -C(NH 2 )]N-OH group and U(VI). [6][7][8] However, the adsorption capability of PAO based materials is strongly restrained by its poor anti-biofouling properties. Marine organisms are the foundation of the marine ecosystem, and marine biofouling refers to the undesired accumulation and growth of marine organisms on material surfaces when immersed in seawater. 9,10 A long operation process is necessary to achieve the target enrichment of U(VI) from seawater, and unwanted marine biofouling is inevitable. It can signicantly decrease the stability, adsorption capability, and reusability of an adsorbent. 11,12 Therefore, effectively solving the biofouling problem is crucial for PAO based materials when used in U(VI) extraction.
It is strategically important to design economical materials with sound anti-biofouling capability and good affinity towards U(VI) in seawater. 11,12 Researchers 12,13 have found that the modication of hydrophilic/acidic groups is a feasible method to enhance the hydrophilic, anti-biofouling properties and adsorption capability of PAO based materials. Phosphorylated reagents are widely used as extraction reagents for U(VI) separation 7,12,14,15 and are used as disinfectants 16 due to their high affinity for U(VI) and excellent broad-spectrum anti-microbial properties, respectively. Surface modication with phosphorylated reagents is an attractive method to improve the antibiofouling property and adsorption capability of PAO based materials. 17,18 Among the reported phosphorylated organic monomers, VPA is a simple structure, has low toxicity, and is an industrially available monomer. [19][20][21] The typical radical polymerization method of PVPA usually suffers from impurities, 19,22,23 very slow reaction rate, 23 underutilization of reagents, 24 and serious chemical pollutants.
To enhance the anti-biofouling properties and adsorption capability of PAO, PVPA was modied on the PAO surface. Briey, VPA was polymerized on the PAN surface by plasma technique, followed by amidoximation treatment. To evaluate the anti-biofouling properties and adsorption capability of PVPA/PAO, the well-known marine microorganism V. alginolyticus 11,25 was selected as a representative of marine microorganisms. The effects of environmental conditions were studied. We found that the modied PVPA enhances the anti-biofouling properties and adsorption capability of PAO, and PVPA/PAO has excellent properties in U(VI) recovery.

Results and discussion
Characterization Surface topology provides direct information about the formation of PVPA/PAO. The surface morphologies of PAN are spherical in shape with wrinkles and numerous supercial holes ( Fig. 1A and B). Aer modication with PVPA and amidoximation treatment, the surface morphologies of PVPA/PAO ( Fig. 1C and D) are still spherical in shape, but the holes can hardly be seen. Instead, due to the cohesive force generated from PVPA, smooth and agglomerated surfaces of PVPA/PAO are clearly recognized. All this indicates the successful formation of PVPA/PAO.
To evaluate the effect of plasma and amidoximation treatment on the PAN framework, PAN and PVPA/PAO are characterized by XRD. The XRD pattern ( Fig. 2A) of PAN shows typical peaks related to PAN at 2q ¼ 16.8 and 29.3 , which cannot be detected in the XRD patterns of PAO and PVPA/PAO. The latter shows a typical peak at 2q 21.8 related to PAO. PVPA/PAO also shows a new peak at 2q ¼ 11.2 related to PVPA, which conrms the successful synthesis of PVPA/PAO. PVPA/PAO was also characterized by TGA curves (Fig. 2B) to evaluate its thermal stability. Since AO and VPA are hydrophilic functional groups, the weight loss of moisture (before 115 C) in PAO and PVPA/PAO was $7.1% and $12.1%, respectively. The decomposition temperatures of PAN and PAO are $291-492 and $151-328 C, respectively. The TGA curve of PVPA/PAO depicts the typical decomposition of PVPA and PAO. The $31.0% weight loss at $151-328 C is related to the pyrolysis of PAO and the dehydration of PVPA. 24,26,27 The new $12.8% weight loss at $363-520 C as compared to PAO, could be due to the pyrolysis of PVPA. PAO and PVPA/PAO lose $27.9% and $32.5% at 800 C, respectively. Combined with the fact that PVPA typically loses $40% at 800 C when it degrades in nitrogen, 27 the PVPA weight percent in dry PVPA/PAO and PVPA/PAO mass ratio were roughly estimated to be 38% and 0.62 : 1, respectively. This result reveals the effective modication of PVPA using the plasma technique.
The disorder carbon structure (D band) materials resonate with adjacent atoms and then affect the graphite carbon structure (G band) materials, 28 which can be revealed by Raman spectroscopy. As depicted in Fig. 2C, PAN, PAO and PVPA/PAO  show typical Raman peaks at $1367 and $1591-1598 cm À1 , which relate to the D band and G band, respectively. The G bands of PAO and PVPA/PAO are shied to $1598 and $1594 cm À1 as compared to that of PAN at $1591 cm À1 . The graphitic degrees were roughly evaluated by the peak intensity ratio of the G and D bands (I D /I G ). The I D /I G values are 0.67, 0.72, and 0.78 for PAN, PAO and PVPA/PAO, respectively. Raman results indicate that amidoximation treatment and PVPA modication can decrease the graphitic degree of PVPA/PAO. XPS spectroscopy technique can be used to identify surface functional groups. The relative peak intensities of O 1s and N 1s are increased in PAO and PVPA/PAO as compared to that of PAN (Fig. 2D), indicating -C^N groups were successfully converted into -C(NH 2 )]N-OH groups. The new peak at $133 eV relates to P 2p and reveals the successful synthesis of PVPA/PAO. The N 1s spectra (Fig. 3A) were resolved into three species of -C^N, N-H, and -C]NOH (only for PVPA/PAO). The result in Table 1 indicates that most -C^N were converted into -C]NOH. The XPS C 1s spectra (Fig. 3B) further conrm this, and can be resolved into species -C^N, C-C, C-OH and -C]NOH (only for PVPA/PAO); C]O, -COOH and C-PO 3 H 2 (only for PVPA/PAO). The result in Table 2 conrms that most -C^N was converted into -C]NOH, and C-PO 3 H 2 is an important carbon species of PVPA/PAO. The related XPS O 1s spectra (Fig. 3C) were resolved into three species (Table 3)      -OH and -PO 3 H 2 . The decrease of -COOH and increase of -OH and -PO 3 H 2 conrm that PVPA/PAO was synthesized successfully. The P 2p spectrum of PVPA/PAO (Fig. 3D) is deconvoluted into two species of -PO 3 H 2 and polyphosphate (contains P-O-P bond), which were centered at 133.21 and 134.47 eV, respectively (Table 4). [28][29][30][31] Anti-biofouling properties and adsorption capability of PVPA/ PAO The existing forms of U(VI) are affected by solution pH, and mainly exist as U(VI)-CO 3 2À species in seawater due to the weak alkalinity (pH $8.2) of seawater and the presence of CO 2 . The uptake of U(VI) from seawater is difficult, and fairly limited by the extremely low concentration and the decomplexation of U(VI)-CO 3 2À species to free UO 2 2+ . Thereby, an adsorbent must be soaked in seawater for a long-time during applications. To estimate the essential operating time of PVPA/PAO in applications, the inuence of operation time on U(VI) extraction efficiency is evaluated. The recovery of U(VI) by PVPA/PAO rises fast with increasing reaction time upto $24 h, and then remains steady with further increasing reaction time (Fig. 4A). Faster kinetics need shorter operation times, which can reduce the effect of biofouling. To investigate the adsorption kinetics, the pseudo-rst order kinetic models (q t ¼ q e Â (1 À exp(Àk 1 t)), where k 1 (1 h À1 ) is the adsorption rate constant, q e (mg g À1 ) and q t (mg g À1 ) are the equilibrium and experimental adsorption capabilities, respectively); and pseudo-second-order kinetic models (q t ¼ q e Â t/(1/(K 0 Â q e ) + t), where K 0 (g mg À1 h À1 ) is the adsorption rate constant), are used to simulate the experimental data. Based on the correlation parameters (R 2 ) in Table  5, the adsorption of U(VI) on PVPA/PAO can be described with the pseudo-second-order kinetic model better than the pseudo-rst-order kinetic model. The result indicates that U(VI) adsorption on PVPA/PAO is a chemisorption process, and the modied PVPA groups formed stable complexation with U(VI) under experimental conditions.  Based on the fact that NaCl is the predominant salt in seawater, the effect of NaCl on the adsorption of U(VI) on the PVPA/PAO surface was studied. As shown in Fig. 4B, the increasing NaCl concentration just slightly reduces the recovery of U(VI) by PVPA/PAO, which suggests the good selectivity of PVPA/PAO for U(VI).
To assess the adsorption capability of PVPA/PAO for U(VI), the adsorption isotherms were studied and the results are shown in Fig. 4C. The modied PVPA groups on PVPA/PAO enhance the U(VI) concentrating ability of PVPA/PAO. The experimental data are simulated by the widely used Langmuir model (C s ¼ b Â C s,max Â C eq /(1 + b Â C eq ), C eq is the equilibrium concentration of U(VI) in supernatant aer centrifugation, while C s,max (mg g À1 ) and b (L mg À1 ) are the maximum adsorption capability of adsorbent and the Langmuir constant, respectively) and Freundlich model (q s ¼ K Â q e 1/n , K (mg g À1 ) and 1/n are the constants indicative of adsorption capability and intensity, respectively). According to the R 2 values in Table  6, the adsorption of U(VI) on PAO and on PVPA/PAO can be better described by the Langmuir model. The C s,max of U(VI) on PVPA/PAO (140 mg g À1 ) is $1.8 times that of PAO (77.0 mg g À1 ) at pH 8.2 and 298 K.
Adsorption isotherms of U(VI) on the PVPA/PAO surface were also performed at three different temperatures to evaluate its thermodynamic parameters. The increased reaction temperature can enhance the adsorption of U(VI) on PVPA/PAO (Fig. 4D). The thermodynamic parameters in Table 7 indicate that the recovery of U(VI) by PVPA/PAO is an endothermic and    3 ] 2À ) in seawater. Meanwhile, V(V) is believed to be a bigger obstacle because PAO has a stronger affinity for V(V) than U(VI). To evaluate the selectively of PVPA/PAO for U(VI), the competitive adsorption of U(VI) with Ca(II), Mg(II) and V(V) was measured at 50 mol L À1 , and the results are shown in Fig. 5A. The selectivity follows the order U(VI) > Ca(II) > Mg(II) > V(V) in this work, which indicates high selectivity of PVPA/PAO for U(VI). The selectively of PVPA/PAO for U(VI) was further conrmed by the XPS technique. The peaks at $381 and $516 eV in the XPS survey spectrum of PVPA/PAO adsorbed U(VI) and V(V) corresponding to the V 2p and U 4f spectra (Fig. 5B). XPS V 2p and U 4f spectra were deconvoluted into V(IV) (516.16 eV) and V(V) (517.05 eV), 32 and U(IV) (380.00 eV) and U(VI) (381.44 eV), 33 respectively. According to Fig. 5C and D, there is no redox reaction during the adsorption of V(V) and U(VI) on PVPA/PAO surface.
Biofouling in seawater is inevitable during the U(VI) extraction process. Modied PVPA signicantly inuences the biofouling of V. alginolyticus on PVPA/PAO, which was conrmed by SEM images. The enrichment of V. alginolyticus on PVPA/PAO ( Fig. 6C and D) is much lower than that on PAO ( Fig. 6A and B), which reveals the good anti-biofouling properties of PVPA/PAO. To further study the effects of biofouling on the recovery of U(VI) by PVPA/PAO, the adsorption of U(VI) on PVPA/PAO in the presence of V. alginolyticus under ambient conditions was evaluated. With increasing V. alginolyticus from 0 to 4.45 Â 10 4 CFU mL À1 , the recovery of U(VI) by PAO and by PVPA/PAO (Fig. 6E) were decreased $48.9% (from $22.5% to $11.5%) and $23.0% (from $27.0% to $20.8%), respectively. It conrms that biofouling severely restricts the recovery of U(VI) from seawater, and PVPA/PAO has excellent anti-biofouling properties.

Conclusions
PVPA/PAO has excellent anti-biofouling properties and high adsorption capability for U(VI). The modied PVPA on PVPA/ PAO enhances its anti-biofouling property and adsorption capability for U(VI) in seawater. The enrichment of U(VI) on the PVPA/PAO surface reaches equilibrium in 24 h and follows the pseudo-second-order model. Based on the Langmuir model, the adsorption capability of PVPA/PAO for U(VI) at pH 8.2 and 298 K reaches 145 mg g À1 .

Experimental section
PVPA/PAO preparation PVPA/PAO was prepared based on plasma induced polymerization and amidoximation treatment techniques. For typical reaction conditions, 5.0 g commercial PAN powder was activated by N 2 plasma (10 Pa, 20 W, and 940 V) for 20 min under continuous stirring, then 10 mL VPA was rapidly poured into the reactor. The polymerization of VPA on the PAN surface was performed for 24 h at room temperature under high purity N 2 and continuous stirring. Aer repeatedly rinsing with ethanol, the derived material was amidoximated in 5.0 wt% NH 2 OH ethanol/water (4 : 1, v/v) at 60 C for 3 h, and eluted with corresponding ethanol/water. The resulting PVPA/PAO was vacuum dried at 60 C. To assess the effect of modied PVPA, PAO was synthesized by the same method.

Characterization
To characterize the physicochemical properties of PVPA/PAO, scanning electron microscopy (SEM), Raman spectroscopy, Xray diffraction pattern (XRD), thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS) were used. SEM images were obtained by a JSM-6320F FE-SEM (JEOL). Raman spectroscopy analysis was performed using a LabRam HR Raman spectrometer. XRD pattern was collected using a Rigaku D/max 2550 X-ray diffractometer. The TGA curve was obtained using a Shimadzu TGA-50 thermogravimetric analyser, and the heating rate and ow rate were 10 C min À1 and 50 mL min À1 N 2 , respectively. XPS spectroscopy was obtained using a ESCALab220i-XL surface microanalysis system.

Enrichment of V. alginolyticus on PAO and on PVPA/PAO
To obtain the target solutions, adsorbents (PAO and PVPA/PAO) and NaCl are pre-reacted for 24 h at rst, and then deionized water and exponential growth phase V. alginolyticus were injected, the suspension pH was adjusted. Aer shaking for

Effect of biofouling on U(VI) adsorption
To obtain target compositions, adsorbents (PVPA/PAO and PAO) and NaCl were pre-reacted for 24 h at rst, and then deionized water, U(VI), and exponential growth phase V. alginolyticus were injected, and the suspension pH was adjusted. Aer reacting for 24 h, the suspensions were centrifuged at 9000 rpm for 30 min at 25 C, and then ltered. Except during their specic evaluation, the reaction temperature, time, U(VI) initial concentration, mV, pH, and ionic strength are 298 AE 1 K, 24 h, 50.0 mg L À1 , 0.20 g L À1 , 8.2 AE 0.1, and 0.10 mol L À1 NaCl, respectively. The nal U(VI) concentrations were analysed using Optima 2100 DV (PerkinElmer, for mg L À1 level) and X-Series II (Thermo Scientic, for mg L À1 level).

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