The preparation of dodecyl trimethyl ammonium bromide modified titanium dioxide for the removal of uranium

Radioactive contamination, especially the uranium pollution, is threatening the ecological environment. How to efficiently and quickly remove uranium from the environment is a problem to be solved. Herein, the dodecyl trimethyl ammonium bromide embellished titanium dioxide (DTAB/TiO2) was prepared as an adsorbent to adsorb uranium (U) from water. The introduction of dodecyl trimethyl ammonium bromide can improve the adsorption capacity of titanium dioxide for U(VI). Besides, the excellent chemical stability of DTAB/TiO2 would not result in secondary pollution, which was the novelty of this work. The DTAB/TiO2 composite was composed of nanoparticles and presented a spherical morphology with a rough surface. The radius of DTAB/TiO2 was 0.45 μm, and the specific surface area reached 144.0 m2/g. The removal of U(VI) on DTAB/TiO2 was a monolayer adsorption process, and the removal process was dependent on the solution pH. The capture of U(VI) improved with the temperature increase, indicating an endothermic process. The adsorption process can reach equilibrium within 240 min. Based on the Langmuir model, the adsorption capacity of DTAB/TiO2 for U(VI) reached 108.4 mg/g. The surface oxygen-containing functional groups, especially hydroxyl groups, played a crucial role in removing U(VI). This work can provide useful information for the cleanup of uranium and expand the application of surfactants.


Introduction
Uranium (U) is one of the most widespread radioactive wastes in the ecological environment (Feng et al. 2022). Uranium and its decay products can be found in the environment through a series of anthropogenic activities, including the nuclear power industry and mining . Because of the long half-life, high chemical toxicity, and high level of mobility in the environment, uranium has been considered one of the main environmental pollutants, which poses a severe, long-term ecological and public health threat Chen et al. 2022). Unfortunately, it is not easy for uranium to degrade into the lower toxic substances by reason of radioactivity and bioaccumulation in the environment (Das et al. 2022). To protect ecosystem stability and public health, it is required to eliminate uranium before being discharged into the environment, as well as its recovery as an energy resource. In the past decades, many methods have been developed for uranium removal from aqueous solutions, such as chemical precipitation, membrane dialysis, bio-concentration, ionic exchange, adsorption, and extraction (Jana et al. 2022;Liu et al. 2022). Among these methods, adsorption is an essential approach to removing uranium from wastewater due to its low cost, high efficiency, and environmental friendliness, and adsorbent is a key factor in determining the final effect of the removal process (Ma et al. 2019a;Liu et al. 2019a). Various adsorbents, such as kaolinite, diatomite, goethite, sepiolite, Responsible Editor: Tito Roberto Cadaval Jr zeolite, mesoporous silica, montmorillonite, hydrogel, and apatite, have been widely produced and applied for the removal of uranium Wang et al. 2019a). However, most existing adsorbents can not meet current demand due to low removal efficiency, high cost, or poor stability (Table 1). It is necessary to explore new adsorbents to remove U(VI).
For the past few years, titanium dioxide (TiO 2 ) has obtained wide attention due to its activity, photostability, non-toxicity, and commercial availability (Yusoff et al. 2018;Yuenyongsuwan et al. 2021). Because of its excellent physicochemical properties, titanium dioxide has been widely applied in catalysts, cosmetics, pharmaceutical industries, etc. (Wu et al. 2021;Tan et al. 2018). Besides, titanium dioxide is also the most important white pigment used in the coating industry. More importantly, titanium dioxide can be used to study the adsorption of pollutants on positively and negatively charged surfaces over a broad range of pH values due to high chemical stability and negligible solubility over a wide pH range, which would not result in secondary pollution Ma et al. 2019b). For instance, Wang et al.  found that the adsorption of U(VI) on titanium dioxide was dependent on pH and ionic strength. Lots of recent publications have focused on the application of surfactants in environmental pollution control. Among these surfactants, dodecyl trimethyl ammonium bromide (DTAB) is the most widely studied in cosmetic, paint, and paper production due to its low cost and biological safety (Lopez-Diaz et al. 2006). Besides, dodecyl trimethyl ammonium bromide also can be used in wastewater treatment and purification of drinking water (Niranjan and Upadhyay 2014). Through the adjustment of pH, temperature, ionic strength, and hydrophilic/hydrophobic balance, various new surfactant-based materials can be created (Kotsmar et al. 2009).
Herein, the DTAB/TiO 2 composite was prepared to remove uranium from water. The introduction of dodecyl trimethyl ammonium bromide can improve the removal capacity of titanium dioxide for uranium. Besides, the distinguished chemical stability of DTAB/TiO 2 would not result in secondary pollution, which was the novelty of this work. The conceptual diagram of this study is shown in Fig. 1. In this work, the characteristics of DTAB/TiO 2 were tested by X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), and Brunauer-Emmett-Teller (BET). The environmental conditions, including pH, temperature, and contact time, were researched in detail. Besides, the removal mechanism of U(VI) on the DTAB/TiO 2 composite was investigated. This work may provide useful information for the cleanup of uranium and expand the application of surfactants.

Experimental section
Synthetic procedure for DTAB/TiO 2 Typically, 5.0 g urea, 1.0 g Ti(SO 4 ) 2 , and 0.2 g DTAB were moved to distilled water and stirred for 30 min (Wang et al. 2020). The mixture was kept at 473 K for 12 h (autoclave). Afterwards, the products were washed and dried at 343 K for 24 h. Finally, the products were calcined at 773 K under a continuous N 2 flow for 3 h and then used for the adsorption experiment.

Instruments
The crystal structure of DTAB/TiO 2 was tested on the Bruker D8 Advance (Bruker, Germany) with 2θ range from 5 to 80 degrees. The XPS spectra before and after adsorption were collected on an X-ray photoelectron spectrometer (ESCALAB 250Xi, ThermoFisher Scientific, United States) with an Al Kα X-ray source. The specific surface area was measured using Tristar-3020 volumetric adsorption analyzers at 77 K (Quantachrome, United States). The composite morphology was detected by the field emission scanning electron microscope (S-4800, Hitachi, Japan). The TEM of DTAB/TiO 2 was performed with the JEM-2100F microscope (JEOL, Japan) operated at 200 kV. The FT-IR of DTAB/TiO 2 was detected by the Tensor 27 equipment (Bruker, Germany).

Batch experiments
In this study, the U(VI) stock solution was prepared by dissolving UO 2 (NO 3 ) 2 ·6H 2 O into Milli-Q water. In a typical adsorption process, a certain amount of DTAB/TiO 2 suspension, U(VI) stock solution, and NaCl solution were added into a tube. The pH in the tube was maintained at 5.0 ± 0.1 by 0.01-0.1 M HCl or NaOH solutions. After shaking for 10 h (150 rpm), the tube was separated by centrifugation. The concentration of U(VI) in the supernatant was measured by inductively coupled plasma-optical emission spectrometry (ICP-OES). The adsorption capacity can be expressed as follows: where V (L) is the solution volume, C 0 and C e (mg/L) are the original and equilibrium concentrations of U(VI), q e (mg/g) is the removal capacity, and m (g) is the mass of DTAB/TiO 2 .

Characterization of DTAB/TiO 2
The surface structure of DTAB/TiO 2 is given in Fig. 2. As illustrated in Fig. 2a, the composite revealed a spherical morphology with a rough surface. The high-magnification SEM image proved that the composite was composed of thousands of nanoparticles (Fig. 2b). The spherical structure and nanoparticles of DTAB/TiO 2 can provide a larger specific surface area and more functional groups to remove U(VI). As can be seen from Fig. 2c, the TEM image also showed the circular structure of DTAB/TiO 2 , which was consistent with the SEM image. The radius of DTAB/ TiO 2 was about 0.45 μm. Figure 2d shows the high-magnification TEM image of DTAB/TiO 2 , which contributed to observing the micro nanostructure of the composite. As illustrated in Fig. 2e and f, the elemental mapping images of DTAB/TiO 2 confirmed the successful preparation of the composite. The crystal structure of DTAB/TiO 2 was investigated by the XRD pattern. As can be observed from Fig. 3a, the diffraction peaks at (101)

Effect of pH
The solution pH can significantly affect the speciation of U(VI) and the surface binding sites of DTAB/TiO 2 . The adsorption of U(VI) on the composite as a function of pH was investigated. As shown in Fig. 4a, the adsorption of U(VI) on DTAB/TiO 2 increased obviously from pH 2.0 to 6.0 and then reached the maximum at pH 6.0, whereas the decreased removal of U(VI) was observed at pH > 6.0. This phenomenon can be explained by the existent form of U(VI) at different solution pH values. The distribution of uranium in aqueous solutions was primarily as positively charged species (i.e., UO 2 2+ , (UO 2 ) 3 (OH) 5 + ) at pH < 6.0, and the increased removal of U(VI) on DTAB/TiO 2 from pH 2.0 to 6.0 can be assigned to strong surface complexation (Zhang et al. 2015a). Because of the occurrence of dissolved inorganic carbon at high pH values, the predominance of carbonate-uranyl complexes (UO 2 (CO 3 ) 2 2− and UO 2 (CO 3 ) 3 4− ) appeared in alkaline conditions, which caused the decreased removal of U(VI) at pH > 6.0 (Zhang et al. 2015b). Therefore, dissolved carbonate is also important in the removal process. As can be seen from Fig. 4b, the adsorption of U(VI) was conducted in 0.01 M of C 2 H 3 NaO 2 , MnCl 2 , Na 2 CO 3 , MgCl 2 , NaClO 4 , NaCl, NaNO 3 , KCl, ZnCl 2 , and Na 2 SO 4 solutions, respectively. The results showed that these coexisting ions hardly affect the adsorption of U(VI), indicating a huge advantage of the composite.

Influence of contact time
The impact of adsorption time on U(VI) removal is shown in Fig. 4c. The adsorption of U(VI) was composed of three stages. The adsorption of U(VI) on DTAB/TiO 2 increased sharply at the first contact time of 0-120 min and then increased slowly at 120-240 min. Finally, the removal process reached equilibrium after 240 min. The fast adsorption rate at the first contact time can be attributed to the larger active sites on the surface of DTAB/TiO 2 and the higher concentration gradient of U(VI) in the solution. The results indicated that the contact time of 240 min was sufficient to achieve adsorption Fig. 2 The SEM image (a) and high-magnification SEM image (b) of DTAB/TiO 2 , TEM image (c) and high-magnification TEM image (d) of DTAB/TiO 2 , and elemental mapping images (e and f) of DTAB/TiO 2 equilibrium. Besides, the removal process was fitted by different models. Compared to the pseudo-second-order kinetic model (R 2 = 0.927), the capture of U(VI) was better simulated by the pseudo-first-order kinetic model (R 2 = 0.959).
The pseudo-first-order kinetic model can be expressed as follows ): The pseudo-second-order kinetic model can be expressed as follows  where k 1 and k 2 are the equilibrium rate constants; q e and q t (mg/g) are the removal capacity of U(VI) at equilibrium and time t (min), respectively.

Adsorption isotherms
The adsorption temperature is an important factor that can affect the removal efficiency. The adsorption of U(VI) was performed at 298 K, 313 K, and 328 K, respectively. From Fig. 4d, one can see that the U(VI) removal increased with the increase in temperature, indicating an endothermic process. To study the removal mechanism, the adsorption process was fitted by the Freundlich model, Sips model, and Langmuir model. As shown in Table 2, the uptake of U(VI) was better fitted by the Langmuir model rather than the other two models, implying a monolayer removal process. The adsorption capacity of DTAB/TiO 2 for U(VI) reached 108.4 mg/g. As given in Table 3, although a direct comparison of DTAB/TiO 2 with other adsorbents is difficult due to the various experimental conditions, it can still be argued that the adsorption capacity of DTAB/TiO 2 for U(VI) is larger than other materials, showing a great advantage in practical applications. The Freundlich model can be expressed by the following (Yu et al. 2013): The Langmuir model can be described by the following (Yu et al. 2013):   where q e (mg/g) is the removal capacity; C e (mg/L) is the equilibrium concentration of U(VI) in solution; b (L/mg) is the Langmuir constant; Q 0 (mg/g) denotes the theoretical saturated adsorption capacity; K f (mg/g) and n are the Freundlich adsorption coefficients; n s is the Sips model exponent; and k s (L/mg) is the Sips isotherm constant.

Adsorption mechanism
To study the elimination mechanism of U(VI), the DTAB/ TiO 2 composites were investigated by XPS spectra. Figure 5a shows the wide scan XPS spectra of DTAB/TiO 2 . The peaks of Ti 2p and O 1 s can be seen on the surface of DTAB/ TiO 2 . Furthermore, the obvious peak of U 4f can also be seen (6) q e = q max C e k s 1∕n s 1 + C e k s 1∕n s after removal. As given in Fig. 5b, the high U 4f deconvolution of DTAB/TiO 2 after adsorption can be divided into two peaks at 380.33 eV (U 4f 7/2 ) and 391.54 eV (U 4f 5/2 ), respectively, confirming the capture of uranium on the surface of the composite (Haldorai et al. 2014). As shown in Fig. 5c, the high-resolution O 1 s was divided into two peaks at 530.79 eV (O-H) and 530.09 eV (Ti-O), respectively. Figure 5d showed that the intensity of O 1 s changed significantly after U(VI) adsorption. Compared to the composite before adsorption, the relative area ratio of O-H after adsorption decreased from  34.35 to 32.12%, implying that the hydroxyl groups participated in the removal process (Table 4). In the meantime, the relative area ratio of Ti-O increased from 65.65 to 67.88% after adsorption. The XPS results showed that the surface oxygen-containing functional groups, particularly hydroxyl groups, played a significant role in removing U(VI).

Conclusion
In this study, the DTAB/TiO 2 composite was composed of tens of thousands of nanoparticles and revealed a spherical morphology with a rough surface. The capture of U(VI) on DTAB/TiO 2 improved with the temperature increase. The removal of U(VI) was a monolayer adsorption process, and the removal process was affected by solution pH. Furthermore, the adsorption of U(VI) was better simulated by the Langmuir model, and the removal capacity of DTAB/TiO 2 for U(VI) was 108.4 mg/g. The XPS results proved that the surface oxygencontaining functional groups, particularly hydroxyl groups, played a significant role in eliminating U(VI). These findings can offer new ideas for the purification of radioactive waste. In summary, the DTAB/TiO 2 composite showed higher removal capacity for U(VI) in aqueous solutions, revealing a promising material to remove U(VI). The results might facilitate a better understanding of the migratory behaviors of U(VI), which is crucial for the elimination of U(VI) in aqueous solutions and reduces the environmental toxicity of U(VI) in the natural environment. This study not only provides a simple method for efficiently removing U(VI) but also offers an example of other kinds of pollutants removal in the natural aquatic environment.

Data availability
The datasets used and/or analyzed under the current study are available from the corresponding author upon the reasonable request.

Declarations
Ethics approval and consent to participate This section is "not applicable" for this study as the study does not involve any human participants nor their data or biological material.

Consent for publication
This section is "not applicable" for this study as the study does not involve any human participants nor their data or biological material.

Competing interests
The authors declare no competing interests.