# Efficient Removal Of U(VI) Ions from Aqueous Solutions by Tannic Acid/Graphene Oxide Composites
## 1 Introduction  
With the rapid development of modern industries, the generation of pollutants (organic pollutants, heavy metals, and radionuclides) is posing major threats to environmental ecosystems and human health [1; 2; 3]. To solve the problem of the global energy crisis, nuclear energy is widely adopted as a new energy source [4]; however, it also causes a certain side effect, nuclear wastes. Radioactive nuclides in nuclear wastes, such as \({}^{60}\)Co, \({}^{154}\)Eu, \({}^{232}\)Th, \({}^{235}\)U, \({}^{235}\)Np, \({}^{239}\)Pu, \({}^{241}\)Am, and \({}^{247}\)Cm, directly pollute surface water and groundwater resources [5; 6]. These radionuclides accumulate through food chains and cause radiation and biochemical damages to human organs due to their long-term radioactivity [7]. Hence, the safe disposal of nuclear wastes is a public concern [8]. Numerous methods, such as chemical precipitation, ion exchange, membrane separation, and adsorption, have been used to separate and preconcentrate radionuclides from contaminated wastewater [9; 10; 11; 12; 13; 14]. In comparison to other methods, the adsorption technology is often used to eliminate radionuclides due to its high efficiency, low cost, and ease of operation [15]. A variety of artificial adsorbents, such as metal hydroxides [16; 17], clay minerals [18; 19], metal-organic framework material [20; 21], nanoparticles [22; 23], and carbon nanotubes [18], has been developed to process radionuclides. The influences of different environmental factors (pH, ionic strength, and humus) on the adsorption of radionuclides by artificial adsorbents have been extensively studied [15; 16; 17; 18]. However, the adsorption capacity of these adsorbents can only be used under higher pH conditions, thus hindering their practical applications [8]. To preconcentrate and solidify radionuclides from wastewater, efficient and environment-friendly adsorbents are urgently needed [24].  
Graphene oxide (GO) has the advantages of large specific surface area, high acid and alkali resistance, and excellent irradiation resistance [4; 15; 16]. However, the low content of characteristic functional groups on the surface of graphene oxide leads to an adsorption capacity of only 97.5 mg/g at pH = 5.0 [25]. Therefore, the limited adsorption capacity and layer aggregation of GO in aqueous solutions limit its practical application. To improve the above deficiency, some suitable characteristic functional groups are generally grafted on the graphene oxide surface.  
Tannic acid has a large number of reactive oxygen-containing functional groups (hydroxyl, carboxyl); thus, it can efficiently capture organic and inorganic pollutants through \(\pi\)-\(\pi\) or electrostatic interactions and hydrogen bonding [25; 26; 27]. In comparison to polystyrene or silica, tannic acid has an easy-to-control synthesis scheme and possesses natural high-reactive surface functional groups (catechol) [8].  
In the present study, by combining the high specific surface area of graphene oxide and the high-level of active sites of tannic acid, tannic acid/graphene oxide composites as high selectivity and good recyclability adsorbents were prepared to increase the adsorption capacity of graphene oxide on radionuclides from wastewater at low pH, providing a certain basis for the safe treatment and disposal of radioactive wastewater.