Adsorption of Uranium(VI) from Aqueous Solution by …...lutions of 10mgL−1 uranium(VI), where the...

Research ArticleAdsorption of Uranium(VI) from Aqueous Solution byModified Rice Stem

Zhang Xiao-teng, Jiang Dong-mei, Xiao Yi-qun, Chen Jun-chang, Hao Shuai,and Xia Liang-shu

School of Nuclear Science and Technology, University of South China, Hengyang 421001, China

Correspondence should be addressed to Xia Liang-shu; [email protected]

Received 13 November 2018; Revised 20 January 2019; Accepted 27 February 2019; Published 23 April 2019

Academic Editor: Doina Humelnicu

Copyright © 2019 Zhang Xiao-teng et al. /is is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work isproperly cited.

/e biosorption is an effective and economical method to deal with the wastewater with low concentrations of uranium. In thisstudy, we present a systematic investigation of the adsorption properties, such as the kinetics, thermodynamics, and mechanisms,of modified rice stems. /e rice stems treated with 0.5mol/L NaOH solutions show higher removal percentage of uranium thanthose unmodified under the conditions of initial pH (pH� 4.0), absorbent dosage (5–8 g/L), temperature (T� 298K), andadsorption equilibrium time (t� 180min)./e removal percentage of uranium(VI) decreases with increasing initial concentrationof uranium(VI)./e Langmuir isothermmodel, which suggests predominant monolayered sorption, is better than Freundlich andTemkin 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./e pseudo-second-order kinetic model, with thecorrelation coefficient of R2 � 0.9992, fits the adsorption process much better than other kinetic models (e.g., pseudo-second-orderkinetic model, Elovich kinetic model, and intraparticle diffusion model). /e thermodynamic parameters ΔG0, ΔH0, and ΔS0demonstrate that the adsorption of uranium(VI) is an endothermic and spontaneous process, which can be promoted bytemperature. /e adsorption of uranium can change the morphology and the structure characteristics of the modified rice stemthrough interaction with the adsorption sites, such as O-H, C�O, Si�O, and P-O on the surface.

1. Introduction

Uranium is a heavy metal with significant chemical toxicityand radioactivity. Since the leakage of radio nuclides in theJapan’s Fukushima Daiichi nuclear plant, the uraniumcontamination has attracted more and more attentions.Large amounts of uranium released from aqueous solutionsof natural deposits, uranium mill tailings, and ammunitionshave migrated into the environment [1, 2]. If uranium-containing wastewater is drained directly without appro-priate treatment, the uranium would be absorbed by thehumic and mineral materials and gradually enter thebiological-geochemical circulation, finally breaking the or-ganism’s normal metabolism and even endangering humanhealth and the ecosystem’s safety. For example, the uraniumleaking into the environment can cause extensive human

diseases (e.g., skin erythema, trichomadesis, nephritis, leu-kaemia, and bone cancer) and even deaths [3]. /erefore, itis critically important to prevent the uranium of wastewaterfrom transporting into the environment.

Many methods, such as chemical precipitation-crystallizing, ion exchange, solvent extraction, and filmseparation [4–9], have so far been developed to deal with theuranium-containing wastewater. However, these methodsare not effective and economical, especially when treatinglarge amounts of wastewater containing the low contents ofthe radioactive ionic material [10]. /erefore, it is urgent todevelop low-cost and easily obtained uranium sorbents forefficient treatment of uranium-containing wastewater.Biosorption has been considered to be a useful method as itutilizes various natural materials of biological origin, in-cluding bacteria, fungi yeast and algae, to remove metal ions

HindawiJournal of ChemistryVolume 2019, Article ID 6409504, 10 pageshttps://doi.org/10.1155/2019/6409504

mailto:[email protected]://orcid.org/0000-0002-1314-3372https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://doi.org/10.1155/2019/6409504

from solutions [11–14]. Compared with the traditionalmethods, the biosorption has the following advantages: (1)the fast sorption kinetic and high selectivity for removinglow concentrations of heavy metals (e.g., uranium); (2) lowenergy consumption; (3) abundant sources of biosorbents;(4) simple operation; (5) wide operation ranges of pH andtemperature; and (6) good reproduction of adsorbents.

/e treatment of heavy metals through biosorption hasbeen extensively studied in the past decades. /e researchesmainly focus on the selectivity of the biosorbents, the bio-sorption kinetics and thermodynamics, the biosorptionmechanism, and large-scale experiments [15–18]. Currently,some potential biomaterials with higher adsorption capacity ofheavy metals have been adopted to treat uranium-containingwastewater. From an economic point of view, researchersmainly use various disposed products from industries oragricultural farms as adsorbents. /us, it is necessary to seekthe most promising biosorbents from an extremely large poolof easily available and inexpensive biomaterials. Disposedagricultural products such as biosorbents are considered to beone of the most potential adsorbents [19]. Until now, variousdisposed agricultural biosorbents, such as wood powder,wheat straw, grapefruit peel, coir pith, and banyan leaves[19–22], have been investigated for the adsorption of ura-nium(VI). /ese biosorbents have been identified as an effi-cient kind of sorbent for adsorption of uranium.

/ere are abundant low-cost rice stems from agriculturalwastes. At present, the world annual output amounts of therice stem are over 17 billion tons. Large amounts of rice stemare burnt as fertilizer, resulting in environmental pollutionand waste of resources. /e rice stem, which is composed ofrice hemicellulose, cellulose, lignin, and protein, can be used toefficiently solve the environmental pollution caused by metalsin an economical and eco-friendly way because it has uniquecharacteristics to remove themetal ion from aqueous solution.Some researchers have recognized the feasibility of rice stemfor the treatment of wastewater. For example, Liu [23] andAmin et al. [24] suggested that rice stem had a great ability toenrich organic contaminants (e.g., methylene blue and phe-nol). However, while Sugashini [25] studied the sorption ofheavymetal ions (Hg2+, Cr3+, andMo3+) by rice stem, very fewstudies have reported the rice stem as a biosorbent [26] fortreatment of the wastewater with low contents of uranium.

In this study, the rice stem is chosen to deal with thewastewater with low contents of uranium. /e main aim ofthis study is to investigate the effects of different experi-mental conditions, such asmodifiedmethods, pH, adsorbentdose, initial concentration, sorption equilibrium time, andtemperature. In addition, the thermodynamics and mech-anism of sorption were also systematically investigated basedon the batch experiments of uranium(VI) ions adsorption.Our study suggests that the modified rice stem is a kind ofpromising biosorbent material to efficiently and economi-cally remove uranium(VI) from the wastewater.

2. Materials and Methods

2.1. Materials. All the chemicals and reagents used in thisstudy were analytically pure. /e rice stem was supplied by

Hengyang County, Hunan Province. Uranium stock solu-tion was prepared by dissolving an certain amount ofUO2(NO3)2·6H2O in distilled water, and the initial pH of theuranium(VI) solution was adjusted by addition of 10% HCland 10% NaOH.

2.2. Preparation of Rice Stem. Rice stems were cut into 5 cmsegments and then washed in running water twice and indistilled water once. /e washed rice stems were dried in theoven at 80°C for 24 h. /en, the solids were ground into60mesh.

2.3. Batch Sorption Experiments. 40mL uranium(VI) ionsolution was put into a 250mL Erlenmeyer flask. /e pH ofsolution was adjusted by addition of HCl and NaOH in eachexperiment. A certain amount of rice stem was added intothe Erlenmeyer flask placed in the constant temperaturewater bath oscillator. /e experiment was carried out for120min. /e suspension was separated by centrifuging at4000 rpm for 10min. /en, the supernatant liquid was fil-tered, and the filtrate was used to analyze the concentrationof uranium by 5-Br-PADAP (used as a chromogenic agent inthe spectrophotometric method). /e uranium(VI) removalpercentage R (%) and adsorption capacity Qt (mg/g) weredetermined using the following equations, respectively:

R �C0 −Ct(

C0× 100%,

Qt �C0 −Ct(

m× V,

(1)

where C0 and Ct represent the initial concentration andconcentration at time t, respectively, with the unit of mg/L;V is the volume of uranium solution (mL); and m is the massof the sorbent (g).

3. Results and Discussion

3.1. Effect of the Modified Materials. 4 g of rice stem wasadded into 50mL mixture of NaOH, HCl, H2O2, ammo-nium hydroxide, and citric acid solutions and then stirredfor 24 h. /e resulted mixture was filtered and washed withdistilled water at the same time, until the pH of the ricestem was close to neutral. /en, the modified rice stem wasdried in the oven at 80°C for 24 h. /e color changes ofmodified rice stem are shown in Figure 1. In this study, theeffect of the modified materials on the adsorption capacityof rice stem is investigated using ion solutions of 10mg·L−1

uranium(VI) at pH � 5. 0.2 g of different modified ricestems was added into aqueous solution containing 10mg/Lof uranium(VI) at room temperature, where the reactionlasted for 120min. /e experimental results are given inTable 1.

Figure 1 shows the difference in color and texture ofmodified rice stem after modification through differentways. /e NaOH-modified rice stems show lighter colorand slimmer and softer texture. /e rice stem treated with0.5mol/L NaOH has the highest uranium removal rate

2 Journal of Chemistry

(94.55%) because of many SiO2 and nonaromatic esters inthe surface of modi�ed rice stem [27], which were dis-solved out by NaOH. In this kind of modi�ed rice stem, thecross-linked network structure among cellulose, hemi-cellulose, and lignin were broken and then exposed to thesurface of sorbents [28]. us, the uranium removalpercentage increases as a result of increasing speci�csurface area.

3.2. E�ect of the Initial pH. e pH of solution has beenconsidered as one of the most important parameters aectinguranium ion sorption because it can change the ionic forms ofthe uranyl ions. e eect of the initial pH on the adsorptioncapacity of modi�ed rice stem is investigated using ion so-lutions of 10mgL−1 uranium(VI), where the pH values(pH� 1.4–7.0) are adjusted using HCl and NaOH at roomtemperature. e solutions were mixed with 0.2 g modi�edrice stem, where the reaction lasted for 120min. e ex-perimental results of the initial pH are shown in Figure 2,which indicates that uranium(VI) removal percentage andadsorption capacity reaches the maximum value (97.6% and

1.95mg/g at pH� 4.0, respectively) with pH values increasingfrom 3.0 to 4.0. But the uranium removal percentage andadsorption capacity decrease when pH values increase from4.0 to 7.0. e results demonstrate that pH� 4 is the mostecient pH value for removal of U(VI) by modi�ed rice stem.It could explain the dierences between the existence speciesof the uranium ion and the cell’s surface activity sites atdierent pH values [29]. At lower pH values, H+ (or H3O+)competed with UO22+ for the active sites on the surface of themodi�ed rice stem [30]. Uranium can exist in many forms indierent acid environments, such as UO22+, [UO2(OH)]+,[(UO2)2(OH)2]2+, and [(UO2)3(OH)5]+ [2]. e rice stemmodi�ed by 0.5mol/L NaOH has a high sorption capacity inthe pH range of pH� 3-4. e adsorption capacity decreaseswith the increasing pH value (pH> 4), due to the formation ofother forms or precipitation [22]. e main form of UO22+ in

(a) (b) (c) (d)

(e) (f) (g) (h)

Figure 1: e hue changes of rice stem before and after modi�cation by the modi�ed materials: (a) 0.25mol/L NaOH; (b) 0.5mol/L NaOH;(c) 1.0mol/L NaOH; (d) 1.5mol/L NaOH; (e) HCl; (f ) H2O2; (g) ammonium hydroxide; (h) citric acid.

Table 1: Eect of modi�ed materials on adsorption ofuranium(VI).

Modi�ed methods Removal rate(%)Adsorption capacity

(mg/g)— 76.11 1.520.25mol/L NaOH 92.00 1.840.5mol/L NaOH 94.55 1.891mol/L NaOH 93.84 1.881.5mol/L NaOH 87.68 1.75HCl 89.46 1.79H2O2 92.74 1.85Ammonium hydroxide 91.98 1.84Citric acid 79.09 1.58 1 2 3 4 5 6 7

60

65

70

75

80

85

90

95

100

Rem

oval

rate

(%)

pH

Removal rateAdsorption amount

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2.0

Ads

orpt

ion

amou

nt (m

g/g)

Figure 2: e eect of pH on adsorption of uranium(VI) bymodi�ed rice stem.

Journal of Chemistry 3

solution is UO2(OH)2, which deposited on the surface ofbanyan leaf cells and interfered with the enrichment of cellsthat cause the losing of absorbent ecacy.erefore, pH� 4 issuggested as the best pH value for further sorptionexperiments.

3.3. E�ect of the Contact Time. e eect of the contact timeis investigated in the time range of 20–200min. 0.2 g ofmodi�ed rice stem was added into 10mg·L−1 uranium(VI)solution. e results are shown in Figure 3, which indicatesan increase in the removal of U(VI) by modi�ed rice stemwith the time up to 200min. is increase is most pro-nounced during the �rst 180min but becomes smaller withfurther increase in time, demonstrating that the sorptionprocess almost reaches equilibrium at about 180min. us,all the mixtures of the sorbent and the solution were shakenfor at least 180min to ensure full equilibrium in subsequentexperiments.

3.4. E�ect of theAdsorbentDose. e eect of adsorbent doseis investigated in the dose range of 1.25–8.75mg/L at roomtemperature (Figure 4), where the solution was shaken for180min, and the initial concentration of the solution was10mg·L−1 at pH� 4.

Figure 4 shows that the removal percentage increases,but the adsorption capacity of uranium(VI) decreases withthe increasing adsorbent dose. It is worth noting that theremoval percentage and the adsorption capacity of ura-nium(VI) are nearly constant when the adsorbent dosereaches 7.5mg·L−1. is may be because the adsorptioncapacity of modi�ed rice stem per unit area decreases as aresult of the aggregation of excessive adsorbent [21].

3.5. Adsorption Kinetics. To assess the kinetic characteristicsof uranium(VI) ions of the adsorption process, we adoptedseveral kinetic models (i.e., pseudo-�rst-order kinetic model,pseudo-second-order kinetic model, Elovich kinetic model,and intraparticle diusion model) to calculate the param-eters [31, 32]. e expressions of models are shown in thefollowing equations:

pseudo first order kineticmodel: ln qe − qt( ) � ln qe − k1t,

pseudo second order kineticmodel:t

qt�

1k2q2e

+t

qe,

Elovich kineticmodel: qt � b + kE · ln t,

intraparticle diffusionmodel: qt � kit0.5 + C,

(2)

where k1 (min−1), k2 (mg/(g·min)), kE (mg/(g·min)), ki(min−1), and kd (mg/(g·minn)) are the adsorption rateconstant; qt (mg/g) is the amount of adsorbed uranium attime t; qe (mg/g) is the maximum adsorption capacity; and band C are the constant.

e results of each kinetic model are given in Table 2,which shows that the correlative parameter R2 of the

pseudo-second-order kinetic model is the best among the�ve kinetic models mentioned above, and the calculatedequilibrium adsorption amount was 1.9467mg/g, whichwas close to the experimental result of 1.8903mg/g. isdemonstrates that the pseudo-second-order kinetic modelbest �ts the adsorption process of uranium(VI) ions. Inaddition to the physical and chemical processes, there maybe other adsorption mechanisms of uranium(VI) bymodi�ed rice stem because the �tted curve of the intra-particle diusion model does not pass across the origin ofcoordinate [28, 33].

Rem

oval

rate

(%)

0 20 40 60 80 100 120 140 160 180 200 22080828486889092949698

100


Time (min)

1.60

1.65

1.70

1.75

1.80

1.85

1.90

Ads

orpt

ion

amou

nt (m

g/g)

Figure 3: Eect of time on uranium adsorption by modi�ed ricestem.

Rem

oval

rate

(%)

1 2 3 4 5 6 7 8 970

75

80

85

90

95

100

Adsorbent dose (mg/L)

1

2

3

4

5

6

7


Ads

orpt

ion

amou

nt (m

g/g)

Figure 4: Eect of the adsorbent dose by modi�ed rice stem.

Table 2: Adsorption kinetic parameters of uranium(VI) adsorptionby modi�ed rice stem.

Adsorption kinetic parametersR2

k q C bPseudo-�rst-order 0.00094 16.679 — — 0.9673Pseudo-second-order 0.0721 1.9467 — — 0.9992Intraparticle diusion 0.0302 — 1.4779 — 0.9885Elovich 0.1308 — — 1.1923 0.9610


3.6. E�ect of the Initial Uranium(VI) Concentration. eeect of initial uranium ion concentration is investigated atroom temperature under the conditions of the initial pH(pH� 4.0) and uranium contents (5–60mg·L−1), using 0.2 gmodi�ed rice stem, which was shaken for 180min. eresults about the eect of the initial uranium concentrationare shown in Figure 5.

As shown in Figure 5, the uranium(VI) removal per-centage slightly decreases from 97.8% to 94.7% with theincreasing initial uranium concentration from 5mg/L to60mg/L, where the adsorption capacity, however, increasesfrom 0.97mg/g to 11.36mg/g. is may be because mosturanium(VI) ions interact with the sucient surface activesites 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 solutionsincrease with further increase in the uranium concentration,which would cause the increase of adsorption amount athigher uranium(VI) concentrations.

3.7. Equilibrium Adsorption Isotherms. Langmuir, Freund-lich, and Temkin isotherms models are generally used tostudy the adsorption equilibrium [20]. e Langmuir iso-therm model assumes that the sorption is monolayered, andthe attractive force of the intermolecular sharply depresseswith the increasing distance [34]. e Freundlich isothermmodel is robust to elucidate the surface heterogeneity andthe exponential distribution of active sites [35]. On thecontrary, the Temkin isotherm model suggests that theadsorption heat decreases with the reduction of adsorptionquantity [36]. e expressions of the Langmuir, Freundlich,and Temkin models are given by the following equations:

Langmuir isothermsmodel:ceqe�ceqm+

1bqm

,

Freundlich isothermsmodel: ln qe �1nln ce + lnKf,

Temkin isothermsmodel: qe � a lnKT + a lnCe,

(3)

where qe is the amount of adsorbed metal ions, mg/g; ce isthe equilibrium concentration in solution, mg/L; qm is theamount of maximum adsorbed metal ions, mg/g; b is theLangmuir adsorption model constant; Kf and n are theFreundlich adsorption model constants; and KT and a arethe Temkin adsorption model constants. With ce/qe as theordinate and ce as the abscissa, a straight line can be ob-tained, the slope of the line is 1/qm, and the vertical interceptof the line is 1/bqm. e parameters of the remaining twoequations can be calculated in the same way. e parametersof the Langmuir, Freundlich, and Temkin isotherm modelsfor the adsorption of uranium ions by rice stem are shown inTable 3.

Table 3 shows that the model parameters (e.g., qm in theLangmuir model,Kf in the Freundlich model, and a andKTin the Temkin model) increases slightly with increasingtemperature, demonstrating that the uranium adsorptionprocess by rice stem is endothermic. On the contrary, the

comparison between the correlation parameters R2 of thethree isotherm models (Langmuir, Freundlich, and Temkin)(Table 3) shows that the Langmuir model is better thanFreundlich and Temkin models to elucidate the adsorptionisotherm of adsorbed uranium. erefore, it can be con-cluded that the predominant adsorption of uranium bymodi�ed rice stem is monolayered.

3.8. E�ect of Temperature. e eect of temperature onadsorption properties of uranium is shown in Figure 6. Ascan be seen from Figure 6, the removal percentage ofuranium increases slightly with temperature in the Trange of298–318K, demonstrating that the adsorption reaction ismore pronounced at higher temperature. is may be be-cause (1) some new active sites are produced with heatingand (2) the metal ions have stridden across the energybarrier to endure diusion transmission or resist concen-tration gradient [28]. us, T� 298K is suggested as themost appropriate temperature for adsorption experiments.

3.9. Adsorption ermodynamics. e thermodynamic pa-rameters of the sorption process are calculated according tothe following equations at dierent temperatures [37]. evalues of thermodynamic parameters are showed in Table 4:

ΔG0 � −RT lnKd,

lnKd �ΔS0

R−ΔHRT

,

ΔG0 � ΔH0 −TΔS0,

(4)

where ΔG0 is the Gibbs free energy, kJ/mol; R is the ideal gasconstant, with the value of 8.314 kJ/(mol·K); T is the absolutetemperature;Kd is the thermodynamic stability constant; Ceis the equilibrium concentration, mg·L−1; K0 is the distri-bution coecient; ΔS0 is the standard entropy, kJ/(mol·K);and ΔH0 is the enthalpy, kJ/mol.

Rem

oval

rate

(%)

0 10 20 30 40 50 60

94.0

94.5

95.0

95.5

96.0

96.5

97.0

97.5

98.0


Initial concentration (mg/L)

0

2

4

6

8

10

12

Ads

orpt

ion

amou

nt (m

g/g)

Figure 5: Eect of the initial uranium concentration by modi�edrice stem.


For the initial mass concentration, with ln Kd as theordinate and 1000/T as the abscissa, a straight line can beobtained, the slope of the line is−ΔH/R, and the verticalintercept of the line is S0/R.

As showed in Table 4, the negative free energy ΔG0 is lessthan zero, andΔG0 decreases with the increasing temperature,indicating that the uranium adsorption process by rice stem isspontaneous, where the reaction is promoted by temperature.e enthalpy change (ΔH0 � 9.4533) indicates an endo-thermic uranium adsorption.e positive entropyΔS0 re«ectsthe inconvertible process of adsorption uranium and littlepossibility of desorption. e three parameters illustrate thatthe adsorption process is a physical and chemical reaction.

3.10. Adsorption Mechanism

3.10.1. FT-IR Analysis. FT-IR spectra of samples are mea-sured by Fourier infrared spectrum instrument which collectsand analyzes scattered IR energy [38] (Figure 7). FT-IR spectrarecord the presence of dierent typical peaks which

correspond to functional groups and surface properties. ecurve a in Figure 7 shows the FT-IR spectrum of modi�ed ricestem before adsorption of uranium. e widely strong peakobserved 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 asymmetricstretching vibration of methyl and methylene groups in cel-lulose [4], whereas that at 2312 cm−1 is produced due toovertone or combination band of amino acids. e peak lo-cated at 1635 cm−1 indicates the stretching vibration of car-bonyl groups from carboxylates and ketones [5]. e peak at1433 cm−1 is due to C-Hbending vibration in lignin [39], whilethat at about 1026 cm−1 is attributed to C-O-C and asymmetricstretching vibrations of phosphate and silicate groups [39].

e curve b in Figure 7 illustrates the FT-IR spectrum ofmodi�ed rice stem after adsorption of uranium, whichshows a slight shift. None of new bands appear, but thetransmittance is reduced. It can be inferred that the structureof rice stem does not change after adsorption of uranium.e peaks at 3431, 2912, 2312, 1635, 1433, and 675 cm−1 areshifted to 3429, 2900, 2310, 1633, 1381, and 686 cm−1 afterthe adsorption of uranium, respectively, implying the re-placement of some H+ by UO22+, which causes the decreaseof the vibration strength and the absorption peak wave-number [40, 41].e observed FT-IR spectra show that O-H,C�O, Si�O, and P-O are the main adsorption sites in theuranium adsorption process.

3.10.2. EDS Analysis. e EDS can detect the dierence inexcited X-ray energies when the surface atoms of sample arebombard by the electron beam. e EDS images of modi�edrice stem before and after adsorption are shown in Figure 8.

Table 3: Parameters of dierent isothermal models of uranium(VI) adsorption by rice stem.

T (K)Langmuir model Freundlich model Temkin model

qm b R2 Kf n R2 a KT R2

298 18.0857 0.5200 0.9854 5.0343 1.3550 0.9934 3.0980 7.8284 0.9309308 18.2883 0.5107 0.9899 5.5823 1.3651 0.9844 3.11 8.991 0.9450318 18.9753 0.5012 0.9903 6.3818 1.3254 0.9767 3.2576 9.818 0.9583

295 300 305 310 315 32093.5

94.0

94.5

95.0

95.5

96.0

96.5

97.0

97.5

98.0

Rem

oval

rate

(%)

T (K)

405060

Concentration (mg/L)

30

51020

Figure 6: Eect of temperature on adsorption properties.

Table 4: ermodynamic parameters of uranium(VI) adsorptionby modi�ed rice stem.

T (K) ΔG0 (kJ/mol) ΔH0 (kJ/mol) ΔS0 [J/(mol·K)]298 −1.4883 9.4533 49.9505308 −1.5383318 −1.5882

100

95

90

85

80

75

703500

a

b

3000 2500 2000Wavenumber (cm–1)

T (%

)1500 1000 500

Figure 7: FT-IR spectra of modi�ed rice stem: (a) before ad-sorption of uranium(VI); (b) after adsorption of uranium(VI).


e EDS images before adsorption of uranium (Figure 8(a))show that the modi�ed stem is composed of C, O, Ca, Si, andMg elements. On the contrary, the EDS images after ad-sorption of uranium (Figure 8(b)) show the appearance ofthe U element and the decrease in the amounts of Si and Caelements. ese results indicate that the interaction of activegroups of modi�ed rice stem with uranium(VI) can cause agood adsorption of uranium(VI).

3.10.3. SEMAnalysis. e SEM images of modi�ed rice stembefore and after adsorption of UO22+ are shown in Figure 9,which indicates the variation in morphology of modi�ed ricestem. For example, the images before adsorption show arough surface of modi�ed rice stem, with many voids in thedistribution (Figure 9(a)). However, the distributed in-terstices and voids in the surface obviously decrease afteradsorption, indicating a signi�cant change in the surfacemorphology of rice stem (Figure 9(b)). is means that alarge amount of UO22+ is adsorbed by the rice stem in theadsorption process. e reason for this phenomenon may bebecause of (1) the interaction of UO22+ with the functionalorganic groups (e.g., the hydroxyl group, carbonyl group,silicon oxygen bond (Si-O), and P-O) of the cell walls of ricestem [28, 37]; (2) the exchange of H+ on the surface withUO22+; and (3) diusion of free UO22+ into the void of thestem.

3.10.4. XRD Analysis. e XRD images of the modi�ed ricestem before and after adsorption of uranium(VI) are shownin Figure 10. e XRD result of the rice stem before ad-sorption represented by the curve a shows diraction peaksat 2θ �15.64, 22.08, and 29.17, whereas that after adsorptionshown in curve b illustrates the weaker diraction peak at2θ�15.64, and 22.08, and even disappearance of the peak at2θ� 29.17°, implying that new crystals are not formed, andthat the number of the crystal structure decreases in theadsorption process. is may be due to the adsorption ofuranium(VI) by modi�ed rice stem, which causes part ofthe molecule structures change from crystal to amorphous[21, 42].

3.11. Adsorption Process. Based on the results of FT-IR,SEM, EDS, and XRD mentioned above, we suggest thatthe adsorption process may be divided into the following 3stages (Figure 11):

(1) UO22+ migrates to the cell wall surface in solution(2) UO22+ reacts with active sites of the cell wall of rice

stem(3) UO22+ deposites on the surface of the rice stem cell

wall

4. Conclusions

is study presents a systematic investigation of the ad-sorption properties, such as the kinetics, thermodynamics,and mechanisms of modi�ed rice stem. e followingconclusions can be drawn:

(1) e rice stem treated with 0.5mol/L NaOH solutionscan eectively remove uranium(VI) from aqueoussolution. pH� 4.0 is the best pH condition to removeuranium(VI). e removal percentage of ura-nium(VI) decreases with increasing initial concen-tration of uranium(VI).

(2) e sorption mode calculations using Langmuir,Freundlich, and Temkin isotherm models show thatthe maximum adsorption capacity of uranium bymodi�ed rice stem increases with temperature.Langmuir model, which indicates predominantmonolayered adsorption of uranium, is better thanFreundlich and Temkin models to describe the ad-sorption isotherm of uranium.

(3) e thermodynamic parameters ΔG0, ΔH0, and ΔS0show that the adsorption of uranium(VI) is an en-dothermic and spontaneous process, where thetemperature promotes the process. It is also sug-gested that the adsorption process is a physical andchemical reaction.

(4) e pseudo-second-order kinetic model, with thecorrelation coecient of R2� 0.9992, �ts the ad-sorption process of uranium(VI) much better thanother kinetic models (e.g., pseudo-�rst-order kinetic

Ca

Ca

CO

Au

Au Au

Au

Mg

0 1 2 3 4 5keV

6 7 8 9 10

Si

(a)

C

O

U U UAu Au

Au Au

0 1 2 3 4 5keV

6 7 8 9 10

Si

(b)

Figure 8: EDS analyses of modi�ed rice stem: (a) before adsorption of uranium(VI); (b) after adsorption of uranium(VI).


model, pseudo-second-order kinetic model, Elovichkinetic model, and intraparticle diusion model). (5)FT-IR and EDS analyses show that the interaction of

the main adsorption sites (O-H, C�O, Si�O, andP-O) of modi�ed rice stem with uranium(VI) cancause a good adsorption of uranium(VI). SEM and

(a) (b)

Figure 9: SEM images of modi�ed rice stem: (a) before adsorption of uranium(VI) (2000x); (b) after adsorption of uranium(VI) (2000x).

1200

1000

800

600

400

200

010 20

a

b

Inte

nsity

(cps

)

30 40 50 60 70 80

Figure 10: XRD images of modi�ed rice stem: (a) before adsorption of uranium(VI); (b) after adsorption of uranium(VI).

Solution Solution

Cell wall Cell wallIon exchange

Diffusion

Cells

UO22+

Cells

Figure 11: Adsorption process of uranium(VI) by modi�ed rice stem.


XRD analyses show that the surface morphology andthe crystal structure of rice stem treated with NaOHsolutions have significantly changed during the ad-sorption process.

Data Availability

/is is a research article, and the data are from the exper-iment. All the data used to support the findings of this studyare included within the article.

Conflicts of Interest

/e authors declare that they have no conflicts of interest.

Acknowledgments

/is project was supported by the National Natural ScienceFoundation of China (Grant no. 11575078).

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Adsorption of Uranium(VI) from Aqueous Solution by …...lutions of 10mgL−1 uranium(VI), where the...

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