Removal of uranium (VI) from aqueous solution by amidoxime functionalized superparamagnetic polymer microspheres prepared by a controlled radical polymerization in the presence of DPE

Highlights

• Fe₃O₄/P(GMA-AA-MMA) was prepared by a novel living radical polymerization technology.

• An novel amidoxime functionalized Fe₃O₄/P(GMA-AA-MMA) is synthesized.

• The sorbent exhibits high sorption capacity and distinct selectivity for uranium.

• The magnetic sorbent could be easily separated from aqueous solutions with a magnet.

• The kinetic and thermodynamic parameters of the adsorption process were estimated.

Abstract

A novel magnetic sorbent (AO-Fe₃O₄/P(GMA-AA-MMA) was prepared by grafting amidoxime groups onto the surface of superparamagnetic polymer microspheres prepared by a novel controlled radical polymerization technology named DPE method based on 1,1-diphenylethylene (DPE) as radical controlling agent, and characterized by Fourier transform infrared (FTIR), transmission electron microscopy (TEM), X-ray photoelectron spectra (XPS) and vibrating sample magnetometer (VSM). The synthesized magnetic sorbent was applied to adsorb uranium (VI) from aqueous solutions and could be easily separated by an external magnetic field. Effect of pH, contact time, temperature, and initial U(VI) concentration on adsorption of U(VI) were investigated. An optimum sorption capacity of 200.5 mg g⁻¹ was obtained under the current experimental condition. The sorption of U(VI) on the magnetic sorbent obeyed the Langmuir model, and was mainly attributed to surface complexation via the coordination of U(VI) ions with amidoxime groups. Meanwhile, AO-Fe₃O₄/P(GMA-AA-MMA) could also selectively adsorb U(VI) in aqueous solution containing co-existing ions efficiently. Furthermore, the desorption studies showed AO-Fe₃O₄/P(GMA-AA-MMA) could be used repeatedly and sorption capacity did not have any noticeable loss after five cycles.

Introduction

Uranium is the most important element for nuclear industry, which has significant commercial use as a fuel for electricity generation. In the last few decades, with the rapid development of the global nuclear industry, the demand for uranium is on the increase worldwide. Meanwhile, as a toxic and weakly radioactive heavy mental, the uranium has been excessively disposed into the environment. On the one hand, the shortage of uranium resources will be a serious problem for the development of nuclear power plants. On the other hand, the uptake of uranium by human beings can cause irreparable damage, such as severe liver damage, kidney damage and eventually death. Therefore, the effective removal, concentration and recovery of uranium has attracted more and more attentions [1], [2]. Several technologies such as chemical precipitation [3], solvent extraction [4], co-precipitation [5], nanofiltration [6], ultrafiltration or polymer assisted ultrafiltration [7], have been applied to remove uranium from aqueous solution. Compared with these technologies, adsorption was proven to be the most effective method due to its wide range of material sources, low cost, high adsorption selectivity and capacity. Recently, various types of adsorbents such as silica based materials [8], [9], carbon based materials [10], [11], synthetic polymer [12], [13], biosorbent [14], [15], Metal–organic frameworks (MOFs) [16], [17], have been developed for uranium removal. However, these adsorbents are limited due to the difficulty in separating the suspension from aqueous solution after the adsorption process, which may increase the cost of industrial application.

Magnetic sorbents can overcome the shortcomings of the traditional adsorbents mentioned above, due to which can be easily separated from the solution with an external magnet. The most commonly used magnetic sorbents are based on Fe₃O₄ nanoparticles, since they can be prepared conveniently and have strong magnetic responsiveness. In 2010, Fe₃O₄ nanoparticles have been used to remove uranium from aqueous solution by Das and his coworkers [18]. However, the sorption capacity is very low. Meanwhile, magnetic nanoparticles Fe₃O₄ are susceptible to leaching under acidic conditions and are prone to aggregate, due to high ratio of surface to volume and magnetization, which will also limit their applications in the field of nuclear industry to a large extent. Thus, in order to compensate for these disadvantages, surface modification of Fe₃O₄ nanoparticles with various small molecules [19], [20], silicon materials [21], [22], [23], carbon materials [24], [25], [26], polymers [27], [28], [29] and other materials [30], [31], [32] have been extensively studied in this field in 2013, Li and his workers [24] reported the synthesis of amidoximated magnetite/graphene oxide (AOMGO) composites for removal of uranium. Effects of pH, ionic strength and coexisted ions on the sorption of U(VI) on AOMGO were investigated. The results indicated that AOMGO had a maximum sorption capacity of 1.197 mmol g⁻¹ at pH = 5.0 ± 0.1 and T = 298 K. Subsequently in 2014, carbon-encapsulated Fe/Fe₃C nanoparticles embedded in porous carbon sheets (Fe/Fe₃@PCS) synthesized by a one step carbothermic reduction using natural abundant biomass derivatives were used as the magnetic sorbent to remove uranium in aqueous solution [25]. The prepared Fe/Fe₃@PCS can be effective in the removal of U(VI) in the presence of carbonate or calcium. Compared with activated carbon (AC), Fe/Fe₃@PCS is more efficient, and can remove U(VI) quantitatively at an initial concentration of up to 140 mg L⁻¹. Most recently, Hua and his coworkers [23] reported a novel method to synthesize surface ion-imprinted magnetic microspheres (SII-MM) for efficient removal of uranium (VI). Compared with non-imprinted composites, SII-MM showed higher selectivity, faster kinetics, and larger capacity for uranyl adsorption. Encouraged by the excellent sorption properties of magnetic adsorbents for removal of uranium in aqueous solutions, it is thus desirable to develop more efficient magnetic nanoparticles to adsorb uranium from aqueous solution in this field.

In the last two decades, superparamagnetic polymer composite microspheres with high magnetic content and high density of functional groups prepared by a novel controlled radical polymerization technology named DPE method based on 1,1-diphenylethylene (DPE) as radical controlling agent have received much attention in the fields such as magnetic biosperation, nucleic acid and protein purification, and catalyst immobilization [33], [34], [35], due to the DPE method has several advantages. First, it is not limited to certain monomers and many monomers can be used to this method; second, it can be used as a monomer, and the process is simple; Third, it has excellent control ability of molecular structure. Generally, the DPE method is a two-step procedure. Firstly, a DPE-containing amphiphilic precursor polymer could be prepared by copolymerization of methyl methacrylate and acrylic acid in presence of DPE. The DPE-containing amphiphilic precursor polymer can modify magnetic nanoparticles (Fe₃O₄). Secondly, the residual and fresh functional monomers would be initiated by the activated precursor polymer on the surface of magnetic nanoparticles, forming superparamagnetic polymer composite microspheres with high magnetic content. Meanwhile, the surface of superparamagnetic polymer composite microspheres is usually clean, due to no addition of a emulsifier to the polymerization system. Thus, superparamagnetic polymer composite microspheres with strong magnetic response, clean surface and high density of functional groups prepared by DPE method may be ideal sorbents for removal of uranium from aqueous solutions, However, superparamagnetic polymer composite microspheres prepared by DPE method, to the best of our knowledge, have not been used as supports for removing uranium from aqueous solution.

Recently, as an excellent amphoteric functional group, amidoxime has been grafted onto the surfaces of various substrates for recovery and removal of U(VI) from seawater and aqueous media, due to its fast sorption rate, high sorption capacity for uranium and safety of the environment [36], [37], [38], [39], [40]. For example, in 2014, Liu and his coworkers investigated the selective separation of uranium ions from an amidoxime functionalized multiwalled carbon nanotubes by plasma techniques [37]. In addition, a lot of researches on amidoxime modified polymers and magnetic nanoparticles employed for selective enrichment of U(VI) from aqueous solution have also been reported [38], [39], [40]. Taking into account of these, it would be desirable to anchor amidoxime groups onto superparamagnetic polymer composite microspheres to offer an efficient magnetic sorbent for selective separation of U(VI) from aqueous solution conveniently.

In this work, the amidoxime group was covalently immobilized onto superparamagnetic polymer composite microspheres through the reaction of magnetic microspheres with 3,3′-iminodipropionitrile (IDPN) followed by the treatment with hydroxylamine in methanol–water mixture to obtain the final product (AO-Fe₃O₄/P(GMA-AA-MMA)). The prepared AO-Fe₃O₄/P(GMA-AA-MMA) was characterized carefully and its sorption behavior of U(VI) ions was investigated in detail using batch equilibrium methods under varying operating conditions. Sorption results showed that AO-Fe₃O₄/P(GMA-AA-MMA) composites are competitive candidates for the removal and recovery of U(VI) from aqueous solution.