Preparation of a novel polyacrylic acid and chitosan interpenetrating network hydrogel for removal of U(vi) from aqueous solutions

A clean and simple method has been developed for preparation of interpenetrating polymer networks using polyacrylic acid (PAA) and chitosan (CS) for extraction of uranium from polluted water. The peak of Fourier transform infrared spectroscopy (FTIR) occurred at 928 cm−1 indicating combination of uranium and PAA/CS. The energy dispersive X-ray (EDX) and the scanning electron microscope (SEM) studies illustrated the formation of a crosslinking structure and excellent binding ability of uranium on PAA/CS. The maximum adsorption capacity was 289.6 mg g−1 calculated using the equation of the Langmuir model. The adsorption capacity reached a plateau at pH 4 and the sorption process fits the pseudo-second-order model well. The PAA/CS composite has stability of reuse, with the adsorbent capacity decreasing slowly with increasing usage frequency. The experimental results confirm that the PAA/CS hydrogel could be a novel alternative for highly efficient removal of uranium from wastewater.


Introduction
Conventional energy sources from the degradation of coal and petroleum cause serious environmental and human health problems. The high speed development of nuclear energy as an alternative means that various radionuclides are being released into the environment from the legacy of mining, nuclear reactors, and activities of nuclear weapons. 1 The uranium ion is one of the most common contaminating ions, with high radiation and chemical toxicity. Therefore, focus in recent years has been on how to reduce the uranium ion content in drinking water. Although various methods are available to reduce uptake of radioactive elements in various elds, efficient techniques are still required.
There are many physical and chemical methods for removal of uranium from wastewater including instance ion-exchange processes, 2 solvent extraction, 3 surface complexation, and adsorption. [4][5][6] Adsorption has been used by many plants and enterprises because of its excellent performance, easy operation, low consumption, high efficiency, and environmental friendliness. 7-10 A variety of materials have been extensively studied for adsorption. Traditional materials for extraction of uranium ions from aqueous solution, such as carbon, 11,12 zeolite, 13 clay, 14 composites, 15 natural polymers, 16 and synthetic polymers, 17 have abundant pores or functional groups. Among them, the binding capacity of chitosan is higher than that of commercial adsorbents, it is cheap and rich in nature, so it is of great interest to researchers for remediation of metal ions. 18,19 However, the raw material of particle chitosan is usually difficult to recycle aer adsorption reaction. Hydrogel, one of the most widely used polymers, consists of a three-dimensional network of hydrophilic polymers which allows diffusion of solutes into the interior network, and can be prepared using a covalent or a non-covalent approach with different functional groups such as carboxylic acid, hydroxyl and amine groups. 18,19 PAA hydrogel has a specic adsorption capacity based on the mutual attraction between carboxyl and metal cations, and its synthetic steps are relatively timely, convenient, and exible. 20 Compared with chitosan, PAA has a stable and water insoluble structure. Furthermore, the bulk gel prepared from acrylic monomer holds recycle performance, hence research into CS/ PAA composites is desirable.
In this work, a PAA-chitosan interpenetrating network is fabricated for sorption of uranium in potable water. The pH, contact time, and initial concentration were assessed by batch adsorption tests. The adsorption performance and mechanism were investigated by kinetic and isotherm models. PAA/CS composites were reused ve times to predict properties of sustainable utilization. The experimental results indicate that the PAA/CS hydrogel has much potential in disposal of sewage.
Xilong Chemical Co., Ltd. Ammonium persulfate (APS) was supplied by Shanghai SANGON Biological Engineering Co., Ltd, China. A uranium stock solution (1000 mg L À1 ) was prepared by adding UO 2 (NO 3 ) 2 $6H 2 O to deionized water. All chemicals used in this study were of analytical grade.

Preparation of PAA/CS
The chitosan (2 g) was dissolved in a beaker of acetic acid solution with a mass fraction of 20% by stirring for about 20 min at room temperature, and about 0.20 g APS and 0.20 g SHS was added to the solution with slow stirring. Then the acrylic monomer (14 mL) and 0.48 g MBA were put into a beaker, with ultrasound for 10 minutes. One of the solutions was poured into the other aer both solutes were welldistributed. Aer the hydrogel formed, the product was washed with deionized water several times. Finally, the obtained hydrogel was dried in a vacuum freeze-drying machine for 12 h and stored in a vacuum drying oven at 55 C.

Instrumentation
FTIR (Fourier transform infrared spectroscopy) of the samples was analyzed using a FTIR spectrophotometer in the range of 4000-400 cm À1 . SEM (scanning electron microscope) analysis of PAA/CS and PAA/CS aer adsorption were recorded using Nova Nano 230 apparatus.
EDX (energy dispersive X-ray) measurements were carried out employing a scanning electron microscope as mentioned before.

Adsorption experiments
Batch adsorption experiments were performed by adding 0.01 g PAA/CS to a conical ask with 10 mL standard solution, of concentration 100 mg L À1 diluted by the stock solution and shaking at 150 rpm, 28 C. Aer that, the concentration of supernatant in the system was analyzed by UV-vis spectrophotometer with a wavelength of 650 nm using Arsenazo-III as the complexing agent.
The U(VI) ion removal amounts on PAA/CS hydrogel were calculated as follows: where q e is the adsorption capacity of the PAA/CS (mg g À1 ), C 0 is the initial concentration of U(VI) ions before adsorption (mg  L À1 ), C e is the nal concentration of U(VI) ions aer adsorption (mg L À1 ), V is the volume of U(VI) ion solution (mL), and m is the absorber mass supplied (g).
To investigate the effect of pH on adsorption, the pH value of solution before adsorption was adjusted to 1-5.5 with the diluted HNO 3 (0.1 M) or NaOH (0.1 M) at initial U(VI) ions of 50, 100, and 150 mg L À1 .
To assess the effect of initial ion concentrations on adsorption, the PAA/CS of 0.01 g was added to several asks containing 10 mL solutions in which the initial U(VI) ions concentrations were 20-500 mg L À1 and pH 4.0 at 28 C.
The isotherms were studied by adding 0.01 g PAA/CS in eight equal volume asks containing 10 mL U(VI) ion solution with initial ion concentrations of 20, 50, 100, 150, 200, 300, 400, and 500 mg L À1 , at 28 C, with shaking for 300 min. The effect of contact time and the adsorption kinetic experiments were conducted using initial uranium concentrations of 50, 100, and 150 mg L À1 , at 28 C, with shaking for 350 min. Finally, the supernatants were taken at different time intervals (10, 20,   temperature and washed with distilled water to neutral. The regenerative adsorbent was reused ve times.

Characterization of PAA/CS
The infrared spectra of AA, CS, PAA/CS, and PAA/CS aer adsorption in the region of 400-4000 cm À1 are depicted in Fig. 1. Strong absorption peaks appeared at 1694 cm À1 , which indicated the stretching of the carboxylic group from AA, PAA/ CS, and U-PAA/CS complexes. CS shows characteristic peaks at 3251, 1630, 1515, and 1060 cm À1 corresponding to the stretching of O-H, amide-I, amide-II, and the stretching of -C-O-C, respectively. 21,22 The peak that disappeared at 1635 cm À1 from -C]Cis attributed to polymerization of AA monomer, and the emerging peaks at 1525 cm À1 (amide characteristic absorption bands) are assigned to the addition of CS, con-rming the PAA/CS product formed. Aer adsorption, there was a new peak at 928 cm À1 from the stretching vibration of O]U] O, suggesting that uranium ions can be absorbed by PAA/CS composite.
Scanning electron microscopy (SEM) images of PAA/CS and U-PAA/CS are shown in Fig. 2. As depicted in Fig. 2(c) and (d), the overall frame structure of composite was not changed aer adsorption, which suggested that the gel aer adsorption of uranium was no trail of destruction. The surface of the materials became denser and roughened during the diffusion of UO 2 2+ in many pores of hydrogels. The surface stripe of the adsorbent changed to a spiral, which could be attributed to the swelling of the uranium solution into the interior of the absorbent. The results showed the PAA/CS hydrogels had a strong binding capacity with UO 2 2+ .
EDX analysis was conducted, and is depicted in Fig. 3. It is worth noting that the absorbent contains three elements, C, O, and N (Table 1), and aer adherence of uranium onto the surface of PAA/CS, a new peak of U appeared in the EDX spectrum, conrming that the UO 2 2+ is successfully captured by PAA/CS. 3.1.1 Effect of initial concentration of the U(IV) ions. The effects of initial U(IV) ions concentrations on the adsorption process by PAA/CS at 28 C with pH ¼ 4 are presented in Fig. 4, in which it can be seen that adsorption of U(IV) on PAA/CS increased with increasing concentrations of the U(VI) ions. This may be because the increase of concentration decreases mass transfer resistance between the solid and liquid phases. 23 At the same time, with the concentration increased, the particular number of active binding sites determined that adsorption no longer occurred.
3.1.2 Effect of initial pH. The pH of aqueous solution affects the adsorption process as it can inuence interaction between different ions in the system, and the adsorption of UO 2 2+ ions depends on distribution of the uranium species in solution. 24 Fig. 5 shows that the adsorption capacities of PAA/CS enhanced sharply because of deprotonation of the amino groups and carboxyl group with increase of pH from 2 to 4, and then the adsorption capacity showed independent on further increasing of pH and reached a plateau at pH 4, attributed to the xed amount of -COO À , -OH, and -NH 2 on absorber in aqueous solution. The decreased concentration of uranium in the aqueous phase suggests that U(IV) ions are adsorbed through ion-exchange with hydrogen ions on the binding sites formed by amino groups and through electrostatic attraction with carboxyl groups on PAA/CS hydrogel. 3.1.3 Effect of contact time and kinetic studies. Fig. 6(a) shows the trend of adsorption versus contact time by PAA/CS. The rate of adsorption at the early stage was faster than that at the latter period, and the maximum adsorption capacity was   Kinetic studies revealed the dynamic process and the mechanism of sorption. Pseudo-rst-order, pseudo-secondorder kinetic models and intra-particle diffusion model are mainly used to t the experimental data.
The pseudo-rst-order model is expressed in the following equation: The pseudo-second-order model is expressed in the following equation: where q e and q t represent the absorbed amounts on PAA/CS at equilibrium and time t (min), respectively. k 1 and k 2 (min À1 ) are the rate constant of the pseudo-rst-order model and second-order kinetic equation. t is the contact time of the adsorption process.
In addition, the model of intra-particle diffusion can be expressed as: where k i (mg g À1 min À1/2 ) is the rate constant of intra-particle diffusion, and C is thickness of the boundary layer. As shown in Table 2, the k 1 , k 2 , and q e values in each formula are calculated from the pseudo-rst and the pseudo-secondorder models, which showed that the pseudo-second-order model ts the experiment data better as the values of q e,cal are closer to the experimental results (q e,exp ) and the R 2 of 0.977-0.996 obtained from the pseudo-second-order model were much higher than that calculated from the pseudo-rst-order model.
The plots in Fig. 6(d) based on the intra-particle diffusion models provide an explanation of the adsorption process, which emphasizes that adsorption in aqueous solution can be viewed as three steps, instantaneous adsorption or external surface adsorption, diffusion into mesopores, and equilibrium stage. The rate of each step follows the order of k 1 > k 2 > k 3 .

Adsorption isotherm studies.
The adsorption isotherm is a model to investigate how the adsorbed ions are distributed over the adsorbent. From the research, Langmuir and Freundlich isotherm models are expressed as in eqn (5) and (6), respectively.
where C e is the equilibrium concentration of the absorbate (mg L À1 ), q e is the amount of absorbed ions at equilibrium (mg g À1 ). q max represents the maximum adsorption capacity of the absorber (mg g À1 ). K L and K F are the Langmuir adsorption constant and Freundlich constant related to the energy of adsorption (L mg À1 ). n is the Freundlich constant related to the intensity.
The Temkin isotherm provides an assumption that the free energy of sorption is a function of surface coverage. The interaction of adsorbents and adsorbates has a signicant effect on adsorption. The Temkin isotherm is written as: where T is the absolute temperature (K), R is the ideal gas constant (8.314 J mol À1 K À1 ), a T is the equilibrium binding constant correlating to the maximum binding energy, and b T is the Temkin isotherm constant. The Langmuir isotherm model states that adsorption is a simple monolayer adsorption process on a homogeneous surface, while the Freundlich isotherm model predicts the forces between ions cannot be neglected and the surface of the absorbent is not homogeneous. Fig. 7 shows the tting plot of the three isotherm models. As can be seen, the Langmuir isotherm model is found to be more in line with the experimental data, with the highest correlation value (R 2 ¼ 0.964) revealing a single layer adsorption process. The K L value from the calculations of the Langmuir isotherm model is 0.02714 and the separation factor constant (0.0686 < R L < 0.4243) suggests that the adsorption between the adsorbent (PAA/CS) and UO 2 2+ is favorable.
Based on the kinetic and isotherm studies, the mechanism of adsorption is probably a combination of the unpaired electrons of oxygen and nitrogen atoms with uranium ions through coordination bonds. 23 3.1.4 Recovery experiments and comparison with other adsorbents. The PAA/CS hydrogel was rinsed with 0.1 M HNO 3 solution for investigating the reusable performance of adsorbent. According to Fig. 8, the capacity of composite in absorbing radioactive ions in water only slightly declined (by 4.04%). The outcome shows that the adsorbent in this study could be used at least ve times.   As summarized in Table 3, comparison of organic, inorganic materials, and composites applied to extraction of contaminated ions in wastewater conducts that the PAA/CS hydrogel has more advantages including high efficiency of adsorption and simplicity of synthesis process than other adsorption materials.

Conclusions
A PAA/CS interpenetrating network hydrogel was prepared successfully in aqueous solution by free radical polymerization, and this hydrogel can be used in a simple and effective method for removal of the uranium from wastewater. The adsorption kinetics of PAA/CS were well described by a pseudo-secondorder kinetic model. The experimental data t the Langmuir model well and the maximum sorption capacity of PAA/CS was 289.6 mg g À1 . Analysis of the hydrogel structure aer adsorption gave the indication that PAA/CS composites have excellent prospects in management of polluted water.

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