Adsorption of Uranium(VI) from Aqueous Solution by Modified Rice Stem

The biosorption is an effective and economical method to deal with the wastewater with low concentrations of uranium. In this study, we present a systematic investigation of the adsorption properties, such as the kinetics, thermodynamics, and mechanisms, of modified rice stems. The rice stems treated with 0.5 mol/L NaOH solutions show higher removal percentage of uranium than those unmodified under the conditions of initial pH (pH = 4.0), absorbent dosage (5–8 g/L), temperature (T = 298 K), and adsorption equilibrium time (t = 180 min). The removal percentage of uranium(VI) decreases with increasing initial concentration of uranium(VI). The Langmuir isotherm model, which suggests predominant monolayered sorption, is better than Freundlich and Temkin models to elucidate the adsorption isotherm of adsorbed uranium. Kinetic analyses indicate that the uranium(VI) adsorption of the modified rice stem is mainly controlled by surface adsorption. The pseudo-second-order kinetic model, with the correlation coefficient of R2 = 0.9992, fits the adsorption process much better than other kinetic models (e.g., pseudo-second-order kinetic model, Elovich kinetic model, and intraparticle diffusion model). The thermodynamic parameters ΔG0, ΔH0, and ΔS0 demonstrate that the adsorption of uranium(VI) is an endothermic and spontaneous process, which can be promoted by temperature. The adsorption of uranium can change the morphology and the structure characteristics of the modified rice stem through interaction with the adsorption sites, such as O-H, C=O, Si=O, and P-O on the surface.


Introduction
Uranium is a heavy metal with significant chemical toxicity and radioactivity. Since the leakage of radio nuclides in the Japan's Fukushima Daiichi nuclear plant, the uranium contamination has attracted more and more attentions. Large amounts of uranium released from aqueous solutions of natural deposits, uranium mill tailings, and ammunition shave migrated into the environment [1,2]. If uraniumcontaining wastewater is drained directly without appropriate treatment, the uranium would be absorbed by the humic and mineral materials and gradually enter the biological-geochemical circulation, finally breaking the organism's normal metabolism and even endangering human health and the ecosystem's safety. For example, the uranium leaking into the environment can cause extensive human diseases (e.g., skin erythema, trichomadesis, nephritis, leukaemia, and bone cancer) and even deaths [3]. erefore, it is critically important to prevent the uranium of wastewater from transporting into the environment.
Many methods, such as chemical precipitationcrystallizing, ion exchange, solvent extraction, and film separation [4][5][6][7][8][9], have so far been developed to deal with the uranium-containing wastewater. However, these methods are not effective and economical, especially when treating large amounts of wastewater containing the low contents of the radioactive ionic material [10]. erefore, it is urgent to develop low-cost and easily obtained uranium sorbents for efficient treatment of uranium-containing wastewater. Biosorption has been considered to be a useful method as it utilizes various natural materials of biological origin, including bacteria, fungi yeast and algae, to remove metal ions from solutions [11][12][13][14]. Compared with the traditional methods, the biosorption has the following advantages: (1) the fast sorption kinetic and high selectivity for removing low concentrations of heavy metals (e.g., uranium); (2) low energy consumption; (3) abundant sources of biosorbents; (4) simple operation; (5) wide operation ranges of pH and temperature; and (6) good reproduction of adsorbents. e treatment of heavy metals through biosorption has been extensively studied in the past decades. e researches mainly focus on the selectivity of the biosorbents, the biosorption kinetics and thermodynamics, the biosorption mechanism, and large-scale experiments [15][16][17][18]. Currently, some potential biomaterials with higher adsorption capacity of heavy metals have been adopted to treat uranium-containing wastewater. From an economic point of view, researchers mainly use various disposed products from industries or agricultural farms as adsorbents. us, it is necessary to seek the most promising biosorbents from an extremely large pool of easily available and inexpensive biomaterials. Disposed agricultural products such as biosorbents are considered to be one of the most potential adsorbents [19]. Until now, various disposed agricultural biosorbents, such as wood powder, wheat straw, grapefruit peel, coir pith, and banyan leaves [19][20][21][22], have been investigated for the adsorption of uranium(VI). ese biosorbents have been identified as an efficient kind of sorbent for adsorption of uranium.
ere are abundant low-cost rice stems from agricultural wastes. At present, the world annual output amounts of the rice stem are over 17 billion tons. Large amounts of rice stem are burnt as fertilizer, resulting in environmental pollution and waste of resources. e rice stem, which is composed of rice hemicellulose, cellulose, lignin, and protein, can be used to efficiently solve the environmental pollution caused by metals in an economical and eco-friendly way because it has unique characteristics to remove the metal ion from aqueous solution. Some researchers have recognized the feasibility of rice stem for the treatment of wastewater. For example, Liu [23] and Amin et al. [24] suggested that rice stem had a great ability to enrich organic contaminants (e.g., methylene blue and phenol). However, while Sugashini [25] studied the sorption of heavy metal ions (Hg 2+ , Cr 3+ , and Mo 3+ ) by rice stem, very few studies have reported the rice stem as a biosorbent [26] for treatment of the wastewater with low contents of uranium.
In this study, the rice stem is chosen to deal with the wastewater with low contents of uranium. e main aim of this study is to investigate the effects of different experimental conditions, such as modified methods, pH, adsorbent dose, initial concentration, sorption equilibrium time, and temperature. In addition, the thermodynamics and mechanism of sorption were also systematically investigated based on the batch experiments of uranium(VI) ions adsorption. Our study suggests that the modified rice stem is a kind of promising biosorbent material to efficiently and economically remove uranium(VI) from the wastewater.

Materials.
All the chemicals and reagents used in this study were analytically pure. e rice stem was supplied by Hengyang County, Hunan Province. Uranium stock solution was prepared by dissolving an certain amount of UO 2 (NO 3 ) 2 ·6H 2 O in distilled water, and the initial pH of the uranium(VI) solution was adjusted by addition of 10% HCl and 10% NaOH.

Preparation of Rice Stem.
Rice stems were cut into 5 cm segments and then washed in running water twice and in distilled water once. e washed rice stems were dried in the oven at 80°C for 24 h. en, the solids were ground into 60 mesh.

Batch Sorption
Experiments. 40 mL uranium(VI) ion solution was put into a 250 mL Erlenmeyer flask. e pH of solution was adjusted by addition of HCl and NaOH in each experiment. A certain amount of rice stem was added into the Erlenmeyer flask placed in the constant temperature water bath oscillator. e experiment was carried out for 120 min. e suspension was separated by centrifuging at 4000 rpm for 10 min. en, the supernatant liquid was filtered, and the filtrate was used to analyze the concentration of uranium by 5-Br-PADAP (used as a chromogenic agent in the spectrophotometric method). e uranium(VI) removal percentage R (%) and adsorption capacity Q t (mg/g) were determined using the following equations, respectively: where C 0 and C t represent the initial concentration and concentration at time t, respectively, with the unit of mg/L; V is the volume of uranium solution (mL); and m is the mass of the sorbent (g).

Results and Discussion
3.1. Effect of the Modified Materials. 4 g of rice stem was added into 50 mL mixture of NaOH, HCl, H 2 O 2 , ammonium hydroxide, and citric acid solutions and then stirred for 24 h. e resulted mixture was filtered and washed with distilled water at the same time, until the pH of the rice stem was close to neutral. en, the modified rice stem was dried in the oven at 80°C for 24 h. e color changes of modified rice stem are shown in Figure 1. In this study, the effect of the modified materials on the adsorption capacity of rice stem is investigated using ion solutions of 10 mg·L −1 uranium(VI) at pH � 5. 0.2 g of different modified rice stems was added into aqueous solution containing 10 mg/L of uranium(VI) at room temperature, where the reaction lasted for 120 min. e experimental results are given in Table 1. Figure 1 shows the difference in color and texture of modified rice stem after modification through different ways. e NaOH-modified rice stems show lighter color and slimmer and softer texture. e rice stem treated with 0.5 mol/L NaOH has the highest uranium removal rate (94.55%) because of many SiO 2 and nonaromatic esters in the surface of modi ed rice stem [27], which were dissolved out by NaOH. In this kind of modi ed rice stem, the cross-linked network structure among cellulose, hemicellulose, and lignin were broken and then exposed to the surface of sorbents [28].
us, the uranium removal percentage increases as a result of increasing speci c surface area.

E ect of the Initial pH.
e pH of solution has been considered as one of the most important parameters a ecting uranium ion sorption because it can change the ionic forms of the uranyl ions. e e ect of the initial pH on the adsorption capacity of modi ed rice stem is investigated using ion solutions of 10 mgL −1 uranium(VI), where the pH values (pH 1.4-7.0) are adjusted using HCl and NaOH at room temperature. e solutions were mixed with 0.2 g modi ed rice stem, where the reaction lasted for 120 min. e experimental results of the initial pH are shown in Figure 2, which indicates that uranium(VI) removal percentage and adsorption capacity reaches the maximum value (97.6% and 1.95 mg/g at pH 4.0, respectively) with pH values increasing from 3.0 to 4.0. But the uranium removal percentage and adsorption capacity decrease when pH values increase from 4.0 to 7.0. e results demonstrate that pH 4 is the most e cient pH value for removal of U(VI) by modi ed rice stem. It could explain the di erences between the existence species of the uranium ion and the cell's surface activity sites at di erent pH values [29]. At lower pH values, H + (or H 3 O + ) competed with UO 2 2+ for the active sites on the surface of the modi ed rice stem [30]. Uranium can exist in many forms in di erent acid environments, such as UO 2 2+ , [UO 2 (OH)] + , [(UO 2 ) 2 (OH) 2 ] 2+ , and [(UO 2 ) 3 (OH) 5 ] + [2]. e rice stem modi ed by 0.5 mol/L NaOH has a high sorption capacity in the pH range of pH 3-4. e adsorption capacity decreases with the increasing pH value (pH > 4), due to the formation of other forms or precipitation [22]. e main form of   solution is UO 2 (OH) 2 , which deposited on the surface of banyan leaf cells and interfered with the enrichment of cells that cause the losing of absorbent e cacy. erefore, pH 4 is suggested as the best pH value for further sorption experiments.

E ect of the Contact Time.
e e ect of the contact time is investigated in the time range of 20-200 min. 0.2 g of modi ed rice stem was added into 10 mg·L −1 uranium(VI) solution. e results are shown in Figure 3, which indicates an increase in the removal of U(VI) by modi ed rice stem with the time up to 200 min.
is increase is most pronounced during the rst 180 min but becomes smaller with further increase in time, demonstrating that the sorption process almost reaches equilibrium at about 180 min. us, all the mixtures of the sorbent and the solution were shaken for at least 180 min to ensure full equilibrium in subsequent experiments.

E ect of the Adsorbent Dose.
e e ect of adsorbent dose is investigated in the dose range of 1.25-8.75 mg/L at room temperature (Figure 4), where the solution was shaken for 180 min, and the initial concentration of the solution was 10 mg·L −1 at pH 4. Figure 4 shows that the removal percentage increases, but the adsorption capacity of uranium(VI) decreases with the increasing adsorbent dose. It is worth noting that the removal percentage and the adsorption capacity of uranium(VI) are nearly constant when the adsorbent dose reaches 7.5 mg·L −1 .
is may be because the adsorption capacity of modi ed rice stem per unit area decreases as a result of the aggregation of excessive adsorbent [21].

Adsorption Kinetics.
To assess the kinetic characteristics of uranium(VI) ions of the adsorption process, we adopted several kinetic models (i.e., pseudo-rst-order kinetic model, pseudo-second-order kinetic model, Elovich kinetic model, and intraparticle di usion model) to calculate the parameters [31,32]. e expressions of models are shown in the following equations: pseudo first order kinetic model: ln q e − q t ln q e − k 1 t, pseudo second order kinetic model: Elovich kinetic model: where k 1 (min −1 ), k 2 (mg/(g·min)), k E (mg/(g·min)), k i (min −1 ), and k d (mg/(g·min n )) are the adsorption rate constant; q t (mg/g) is the amount of adsorbed uranium at time t; q e (mg/g) is the maximum adsorption capacity; and b and C are the constant. e results of each kinetic model are given in Table 2, which shows that the correlative parameter R 2 of the pseudo-second-order kinetic model is the best among the ve kinetic models mentioned above, and the calculated equilibrium adsorption amount was 1.9467 mg/g, which was close to the experimental result of 1.8903 mg/g. is demonstrates that the pseudo-second-order kinetic model best ts the adsorption process of uranium(VI) ions. In addition to the physical and chemical processes, there may be other adsorption mechanisms of uranium(VI) by modi ed rice stem because the tted curve of the intraparticle di usion model does not pass across the origin of coordinate [28,33].

E ect of the Initial Uranium(VI) Concentration.
e e ect of initial uranium ion concentration is investigated at room temperature under the conditions of the initial pH (pH 4.0) and uranium contents (5-60 mg·L −1 ), using 0.2 g modi ed rice stem, which was shaken for 180 min. e results about the e ect of the initial uranium concentration are shown in Figure 5.
As shown in Figure 5, the uranium(VI) removal percentage slightly decreases from 97.8% to 94.7% with the increasing initial uranium concentration from 5 mg/L to 60 mg/L, where the adsorption capacity, however, increases from 0.97 mg/g to 11.36 mg/g. is may be because most uranium(VI) ions interact with the su cient surface active sites of modi ed rice stem, resulting in higher uranium(VI) removal percentages at lower uranium(VI) concentrations; however, the excessive uranium(VI) ions in the solutions increase with further increase in the uranium concentration, which would cause the increase of adsorption amount at higher uranium(VI) concentrations.

Equilibrium Adsorption Isotherms.
Langmuir, Freundlich, and Temkin isotherms models are generally used to study the adsorption equilibrium [20]. e Langmuir isotherm model assumes that the sorption is monolayered, and the attractive force of the intermolecular sharply depresses with the increasing distance [34]. e Freundlich isotherm model is robust to elucidate the surface heterogeneity and the exponential distribution of active sites [35]. On the contrary, the Temkin isotherm model suggests that the adsorption heat decreases with the reduction of adsorption quantity [36]. e expressions of the Langmuir, Freundlich, and Temkin models are given by the following equations: where q e is the amount of adsorbed metal ions, mg/g; c e is the equilibrium concentration in solution, mg/L; q m is the amount of maximum adsorbed metal ions, mg/g; b is the Langmuir adsorption model constant; K f and n are the Freundlich adsorption model constants; and K T and a are the Temkin adsorption model constants. With c e /q e as the ordinate and c e as the abscissa, a straight line can be obtained, the slope of the line is 1/q m , and the vertical intercept of the line is 1/bq m . e parameters of the remaining two equations can be calculated in the same way. e parameters of the Langmuir, Freundlich, and Temkin isotherm models for the adsorption of uranium ions by rice stem are shown in Table 3. Table 3 shows that the model parameters (e.g., q m in the Langmuir model, K f in the Freundlich model, and a and K T in the Temkin model) increases slightly with increasing temperature, demonstrating that the uranium adsorption process by rice stem is endothermic. On the contrary, the comparison between the correlation parameters R 2 of the three isotherm models (Langmuir, Freundlich, and Temkin) ( Table 3) shows that the Langmuir model is better than Freundlich and Temkin models to elucidate the adsorption isotherm of adsorbed uranium. erefore, it can be concluded that the predominant adsorption of uranium by modi ed rice stem is monolayered.

E ect of Temperature.
e e ect of temperature on adsorption properties of uranium is shown in Figure 6. As can be seen from Figure 6, the removal percentage of uranium increases slightly with temperature in the Trange of 298-318 K, demonstrating that the adsorption reaction is more pronounced at higher temperature. is may be because (1) some new active sites are produced with heating and (2) the metal ions have stridden across the energy barrier to endure di usion transmission or resist concentration gradient [28]. us, T 298 K is suggested as the most appropriate temperature for adsorption experiments.

Adsorption ermodynamics.
e thermodynamic parameters of the sorption process are calculated according to the following equations at di erent temperatures [37]. e values of thermodynamic parameters are showed in Table 4: where ΔG 0 is the Gibbs free energy, kJ/mol; R is the ideal gas constant, with the value of 8.314 kJ/(mol·K); T is the absolute temperature; K d is the thermodynamic stability constant; C e is the equilibrium concentration, mg·L −1 ; K 0 is the distribution coe cient; ΔS 0 is the standard entropy, kJ/(mol·K); and ΔH 0 is the enthalpy, kJ/mol. Adsorption amount (mg/g) Figure 5: E ect of the initial uranium concentration by modi ed rice stem.

Journal of Chemistry
For the initial mass concentration, with ln K d as the ordinate and 1000/T as the abscissa, a straight line can be obtained, the slope of the line is−ΔH/R, and the vertical intercept of the line is S 0 /R.
As showed in Table 4, the negative free energy ΔG 0 is less than zero, and ΔG 0 decreases with the increasing temperature, indicating that the uranium adsorption process by rice stem is spontaneous, where the reaction is promoted by temperature. e enthalpy change (ΔH 0 9.4533) indicates an endothermic uranium adsorption. e positive entropy ΔS 0 re ects the inconvertible process of adsorption uranium and little possibility of desorption. e three parameters illustrate that the adsorption process is a physical and chemical reaction.

FT-IR Analysis.
FT-IR spectra of samples are measured by Fourier infrared spectrum instrument which collects and analyzes scattered IR energy [38] (Figure 7). FT-IR spectra record the presence of di erent typical peaks which correspond to functional groups and surface properties. e curve a in Figure 7 shows the FT-IR spectrum of modi ed rice stem before adsorption of uranium. e widely strong peak observed at 3431 cm −1 is due to O-H stretching vibration [28]. e band at 2912 cm −1 is formed as a result of C-H asymmetric stretching vibration of methyl and methylene groups in cellulose [4], whereas that at 2312 cm −1 is produced due to overtone or combination band of amino acids. e peak located at 1635 cm −1 indicates the stretching vibration of carbonyl groups from carboxylates and ketones [5]. e peak at 1433 cm −1 is due to C-H bending vibration in lignin [39], while that at about 1026 cm −1 is attributed to C-O-C and asymmetric stretching vibrations of phosphate and silicate groups [39]. e curve b in Figure 7 illustrates the FT-IR spectrum of modi ed rice stem after adsorption of uranium, which shows a slight shift. None of new bands appear, but the transmittance is reduced. It can be inferred that the structure of rice stem does not change after adsorption of uranium. e peaks at 3431, 2912, 2312, 1635, 1433, and 675 cm −1 are shifted to 3429, 2900, 2310, 1633, 1381, and 686 cm −1 after the adsorption of uranium, respectively, implying the replacement of some H + by UO 2 2+ , which causes the decrease of the vibration strength and the absorption peak wavenumber [40,41]. e observed FT-IR spectra show that O-H, C O, Si O, and P-O are the main adsorption sites in the uranium adsorption process.

EDS Analysis.
e EDS can detect the di erence in excited X-ray energies when the surface atoms of sample are bombard by the electron beam. e EDS images of modi ed rice stem before and after adsorption are shown in Figure 8.    e EDS images before adsorption of uranium (Figure 8(a)) show that the modi ed stem is composed of C, O, Ca, Si, and Mg elements. On the contrary, the EDS images after adsorption of uranium (Figure 8(b)) show the appearance of the U element and the decrease in the amounts of Si and Ca elements. ese results indicate that the interaction of active groups of modi ed rice stem with uranium(VI) can cause a good adsorption of uranium(VI).

SEM Analysis.
e SEM images of modi ed rice stem before and after adsorption of UO 2 2+ are shown in Figure 9, which indicates the variation in morphology of modi ed rice stem. For example, the images before adsorption show a rough surface of modi ed rice stem, with many voids in the distribution (Figure 9(a)). However, the distributed interstices and voids in the surface obviously decrease after adsorption, indicating a signi cant change in the surface morphology of rice stem (Figure 9(b)). is means that a large amount of UO 2 2+ is adsorbed by the rice stem in the adsorption process. e reason for this phenomenon may be because of (1) the interaction of UO 2 2+ with the functional organic groups (e.g., the hydroxyl group, carbonyl group, silicon oxygen bond (Si-O), and P-O) of the cell walls of rice stem [28,37]; (2) the exchange of H + on the surface with UO 2 2+ ; and (3) di usion of free UO 2 2+ into the void of the stem.

XRD Analysis.
e XRD images of the modi ed rice stem before and after adsorption of uranium(VI) are shown in Figure 10. e XRD result of the rice stem before adsorption represented by the curve a shows di raction peaks at 2θ 15.64, 22.08, and 29.17, whereas that after adsorption shown in curve b illustrates the weaker di raction peak at 2θ 15.64, and 22.08, and even disappearance of the peak at 2θ 29.17°, implying that new crystals are not formed, and that the number of the crystal structure decreases in the adsorption process. is may be due to the adsorption of uranium(VI) by modi ed rice stem, which causes part of the molecule structures change from crystal to amorphous [21,42].

Adsorption
Process. Based on the results of FT-IR, SEM, EDS, and XRD mentioned above, we suggest that the adsorption process may be divided into the following 3 stages ( Figure 11): (1) UO 2 2+ migrates to the cell wall surface in solution (2) UO 2 2+ reacts with active sites of the cell wall of rice stem (3) UO 2 2+ deposites on the surface of the rice stem cell wall

Conclusions
is study presents a systematic investigation of the adsorption properties, such as the kinetics, thermodynamics, and mechanisms of modi ed rice stem. e following conclusions can be drawn: (1) e rice stem treated with 0.5 mol/L NaOH solutions can e ectively remove uranium(VI) from aqueous solution. pH 4.0 is the best pH condition to remove uranium(VI). e removal percentage of uranium(VI) decreases with increasing initial concentration of uranium(VI).
(2) e sorption mode calculations using Langmuir, Freundlich, and Temkin isotherm models show that the maximum adsorption capacity of uranium by modi ed rice stem increases with temperature. Langmuir model, which indicates predominant monolayered adsorption of uranium, is better than Freundlich and Temkin models to describe the adsorption isotherm of uranium. (3) e thermodynamic parameters ΔG 0 , ΔH 0 , and ΔS 0 show that the adsorption of uranium(VI) is an endothermic and spontaneous process, where the temperature promotes the process. It is also suggested that the adsorption process is a physical and chemical reaction. (4) e pseudo-second-order kinetic model, with the correlation coe cient of R 2 0.9992, ts the adsorption process of uranium(VI) much better than other kinetic models (e.g., pseudo-rst-order kinetic   Journal of Chemistry XRD analyses show that the surface morphology and the crystal structure of rice stem treated with NaOH solutions have significantly changed during the adsorption process.

Data Availability
is is a research article, and the data are from the experiment. All the data used to support the findings of this study are included within the article.

Conflicts of Interest
e authors declare that they have no conflicts of interest.