Introduction In recent years, the pollution of radionuclides to water is becoming more and more serious with the rapid development of nuclear technology. The expansion of activities, such as uranium mining, nuclear research, weapon manufacturing, and nuclear power generation will produce many radioactive isotopes. Among these radionuclides, uranium (VI) (U(VI)) poses a great threat to the biosphere due to the enhanced chemical affinity with organic ligands, radioactive toxicity and long half-life. Therefore, it is necessary to develop a technology to reduce or recover U(VI) in wastewater for decreasing the risk of U(VI) pollution to the environment and realizing the resource utilization of U(VI). The adsorption method has been widely studied due to the relatively mature technology, good repeatability of adsorbent, high adsorption efficiency, low cost and easy operation. Titanium dioxide (TiO2) is considered one of the best adsorbents due to the low cost, non toxicity and high chemical activity. However, the agglomeration of TiO2 particles will reduce the number of active sites per unit area, leading to a weakened adsorption ability for U(VI). An effective strategy to improve the adsorption ability is to prevent the aggregation of TiO2 particles and improve the surface activity of TiO2. Therefore, in this work, iron doped nano-TiO2 materials (Ti–Fe composites) were prepared by the sol gel method, aiming to prevent the aggregation of TiO2 particles in use and improve the adsorption performance.
Methods Tetrabutyl titanate (C16H36O4Ti, 98%), ethanol (C2H6O, 99%), ethylene glycol ((CH2OH)2, 99%), hydrochloric acid (HCl, 36%), anhydrous ferric chloride (FeCl3, ≥ 99.9%), triazo arsine (III)(C22H18As2N4O14S2, 99.9%) and uranyl nitrate (UO2(NO3)2·6H2O, 98%) were purchased from Aladdin's reagent.
TiO2 and Ti–Fe composites were synthesized by sol gel freeze drying technology. Firstly, 3 mL of C16H36O4Ti was added to 15?mL of C2H6O to obtain a transparent mixture. Secondly, FeCl3 (100 mg) was added to the transparent mixture with rapid stirring for 30 min to evenly disperse FeCl3 in the solution. In the third step, 5 mL (CH2OH)2 was added to the mixture with stirring for 7 min to obtain a uniform sol. The sol was quickly transferred into the glass bottle, let stand to form a uniform Ti–Fe composite gel, which was aged at room temperature for 24 h. Put the aging Ti–Fe composite gel in distilled water to remove the excess C2H6O and (CH2OH)2 through solvent exchange. Finally, Ti–Fe composite was obtained by freeze-drying.
The effects of pH, ionic strength, ion species, contact time, initial U(VI) concentration and dosage of adsorbents on the adsorption of TiO2 and Ti–Fe composite for U(VI) were investigated through batch experiments. 10 mg of adsorbent was added to a glass container having 100 mL of U(VI) solution. The container was then placed in a water bath at a constant temperature with stirring at 300 r/min for 3 h. After adsorption, the solid phase was separated by polyethersulfone mem-brane (0.45 μm). The UV–Vis spectrophotometer was applied for U(VI) concentration measurement using arsenazo (III) as the colorant.
Results and discussion XPS spectra indicated that Ti–Fe composite was successfully prepared due to the presence of iron element.The anatase crystallinity of TiO2 and Ti–Fe composite was poor, which was mainly related to the amorphous structure of Ti—O—Ti.SEM images and the analysis of specific surface area further demonstrated that doping iron enhanced the dispersibility of TiO2 materials, which could provide more available active sites for U(VI) adsorption.
The adsorption properties of TiO2 and Ti–Fe composite for U(VI) were compared. At the optimum pH value, the adsorption behavior of Ti–Fe composite for U(VI) reached the equilibrium within 60 min, the adsorption efficiency was 94.5% and the maximum adsorption capacity was 672.9 mg/g. Moreover, the remarkable adsorption efficiency of Ti–Fe composite was above 90% even after five cycles and the adsorption efficiency was relatively high in the solution with different coexisting ions. The adsorption isotherm and kinetic models showed that the adsorption of U(VI) on Ti–Fe composite was a single homogeneous chemisorption. It was worth noting that the adsorption performance of TiO2 for U(VI) was significantly enhanced, which was mainly due to the significantly enhanced dispersion, oxygen vacancy filling, oxidation-reduction effect and inner-sphere surface complexation of TiO2 after doping with iron.
Conclusions Ti–Fe composite with excellent U(VI) removal ability were prepared by sol?gel freeze-drying technique. Under the condition of pH=4, T=298 K, the adsorption efficiency for U(VI) by Ti–Fe composite was 94.5% (m/V=0.1g /L, c0=10 mg/L). The experimental adsorption capacity was 672.9 mg/g (m/V=0.1 g/L, c0=180 mg/L). After 5 cycles, the adsorption efficiency of U(VI) by Ti–Fe composite remained above 90%. Ti–Fe composite exhibit excellent adsorption properties even in complex water environments, which might be due to the synergistic effect of oxygen vacancy filling, oxidation-reduction effect and inner-sphere surface complexation. In general, Ti–Fe composite possessed a promising application in U(VI)-containing wastewater treatment.