Synthesis and characterization of amidoxime modified chitosan/bentonite composite for the adsorptive removal and recovery of uranium from seawater

Abstract

A novel amidoxime functionalized adsorbent, poly(amidoxime)-grafted-chitosan/bentonite composite [P(AO)-g-CTS/BT] was prepared by in situ intercalative polymerization of acrylonitrile (AN) and 3-hexenedinitrile (3-HDN) onto chitosan/bentonite composite using ethylene glycol dimethacrylate (EGDMA) as cross linking agent and potassium peroxy disulphate (K₂S₂O₈) as free radical initiator. The adsorbent was characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), Scanning electron microscopy (SEM), Energy dispersive spectroscopy (EDS), BET surface area analyser and X-ray photoelectron spectroscopy (XPS). Nitrile groups from two monomers converted to amidoxime groups and therefore, increases the adsorption efficiency of uranium(VI) [U(VI)] from seawater. The optimum pH for U(VI) adsorption was found to be 8.0. The adsorbent dosage of 2.0 g/L was sufficient for the complete removal of U(VI) from seawater. The kinetic data fitted well with pseudo-second-order kinetic model which assumes the presence of chemisorption. The equilibrium attained within 60 min and well agreement of equilibrium data with Langmuir adsorption model confirms monolayer coverage of U(VI) onto P(AO)-g-CTS/BT. The maximum adsorption capacity was found to be 49.09 mg/g. Spent adsorbent was effectively regenerated using 0.1 N HCl. Six cycles of adsorption-desorption experiments were conducted to study the practical applicability and repeated use of the adsorbent. The feasibility of the adsorbent was also tested using natural seawater. The results show that P(AO)-g-CTS/BT is a promising adsorbent for the removal of U(VI) from seawater.

Introduction

Nowadays, some important nonrenewable fossil fuels are being gradually consumed, which causes several social and environmental problems [1]. The development of nuclear energy offers a promising solution of the energy crisis and shows a great advantage over fossil fuel due to less greenhouse gases emission, which can help to prevent the global climate change. U(VI) is used as a nuclear fuel for the generation of nuclear energy, and nuclear energy is considered as an alternative sustainable energy source. According to current assessment, terrestrial U(VI) reserves can only guarantee U(VI) supply for nuclear power generation for 100 years. For long term development of nuclear power generation, it is essential to exploit non-conventional energy resources, such as U(VI) in sea water. The main components of the dissolved elements in sea water are Mg, Ca, Br, B, Cl and Sr, which represents more than 99.9%. Similarly, a trace amount of U(VI) ions is present in sea water. Based on the standard salt content in sea water which is about 3.5%, the total salt concentration is 10⁶–10⁷ times larger than that of U(VI). But U(VI) has long half-life and stable oxidation state in an aqueous medium. The main dissolved form of U(VI) in sea water is the uranyl tricarbonate complex [UO₂(CO₃)₃]⁴⁻ with a hexavalent (U⁶⁺) ions [2]. CO₂ concentration and pH largely influence the state of soluble U(VI) species in sea water. Hence, a HCO³⁻/CO₃²⁻ buffer present in seawater helps the formation of a uranyl tricarbonate complex [UO₂ (CO₃)₃]⁴⁻. This complex is stable at pH 8.0 and eagerly decomposes into [UO₂(CO₃)₂]²⁻ and [UO₂(CO₃)] complexes in the pH range of 6.0–8.0 and 5.0–7.0, respectively. An enormous volume of all ocean contains about 4.5 billion tons of U(VI). It is nearly about 1000 times greater than terrestrial U(VI) reserves [1]. If only 50.0% of this resource is recovered, it would be enough to sustain the nuclear reactors worldwide for about 6500 years [3]. From the actinide elements, U is considered as an important and common fuel for nuclear reactors. Therefore, there is a great curiosity in extracting U(VI) from seawater. But, efficient and selective removal of U(VI) from seawater is mostly challenging because of high salinity, high carbonate concentration, basic pH (7.9–8.4), low U(VI) concentration is only about 3.3 ppb (∼3 mg of U(VI) per one cubic meter of seawater [4] and other metal ions at similar or higher concentrations in sea water [5].

A variety of approaches, such as solvent extraction, ion-exchange, membrane separation, nanofiltration, and adsorption have been developed to recover U(VI) from seawater [6]. Adsorption, due to its high efficiency, low cost and ease of handling, has been used for the removal of U from nuclear industrial effluents, mine water and seawater [7]. Several adsorbents for this purpose have recently been studied and special interest has been shown to polysaccharide-based adsorbents, which are biocompatible, biodegradable, and nontoxic [8]. Throughout the past two decades, chitosan (CTS) of abundant origin derived from chitin and a basic polysaccharide carrying amine group has been commonly utilized as drug carrier and tissue scaffold, and for enzyme encapsulation and the removal of heavy metal from aqueous medium [9], [10], [11]. In reality, CTS has been found to have a high tendency to chelate metal ions compared with other biopolymers [8]. This activity arises from the presence of primary amine and hydroxyl groups on CTS’s backbone, which makes this biopolymer a better chelation and adsorption agent for toxic metal ions [12]. Although the high tendency of CTS to remove metal ions from aqueous solutions, the solubility and poor mechanical properties of this biopolymer in acidic media limit its application as an insoluble adsorbent [13]. To overcome the said disadvantages, modified CTS-based adsorbents with high specific area, easy separation ability, and good chemical stability have been developed. For the recovery of U(VI) from seawater, various adsorbent have been studied, among these amidoxime grafted adsorbents have great attention since the 1960s because of the high selectivity and affinity to U [14]. Amidoxime can represent electron and form chelate with uranyl ions, showing great advantage over the usual adsorbent. In the recent twenty years, through in situ polymerization or radiation-induced graft-polymerization technique, amidoxime has been grafted onto various substrates such as resin [15], fibers [16], carbon nanotubes [17], and bacterial cellulose [18] and so on. In most cases, those Amidoxime functionalized material can be synthesized by introducing acrylonitrile groups (CH₂CHCN) into solid structures and then converting these groups to amidoxime groups (CH₂CHC(NH₂)NOH) [3]. Combination of CTS with cation exchanger clays such as rectorite, montmorillonite, bentonite, and zeolite can produce good adsorbents with high mechanical properties and high adsorption capacity for metal ions [19].

Bentonite (BT) is a kind of clay mineral containing essentially of montmorillonite (MMT). It has large surface area, high cation exchange capacity and negative surface charge. These outstanding physicochemical features contribute to its special adsorption properties for heavy metals and dyes [20]. So, BT has been extensively used as an adsorbent in wastewater treatment. Chitosan (CTS) is a good applicant as an intercalant for BT modification. Amine groups of CTS, in acidic solution, are converted to a cationic form (NH₃⁺) which is necessary for the cation exchange reaction between the BT and CTS. Based on the above discussion, we endeavored to prepare poly(amidoxime)-grafted-Chitosan/Bentonite [P(AO)-g-CTS/BT] composite as new adsorbent for U extraction from seawater. The graft copolymerization of acrylonitrile (AN) and 3-hexenedinitrile (3-HDN) on chitosan was performed in the presence of BT. In situ intercalative polymerization was used for the preparation of P(AN-co-3-HDN)-g-CTS/BT.