An anti-algae adsorbent for uranium extraction: l -Arginine functionalized graphene hydrogel loaded with Ag nanoparticles

Accepted Manuscript An anti-algae adsorbent for uranium extraction: L-Arginine functionalized graphene hydrogel loaded with Ag nanoparticles Jiahui Zhu, Hongsen Zhang, Rongrong Chen, Qi Liu, Jingyuan Liu, Jing Yu, Rumin Li, Milin Zhang, Jun Wang PII: DOI: Reference:

S0021-9797(19)30219-X https://doi.org/10.1016/j.jcis.2019.02.045 YJCIS 24669

To appear in:

Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

3 December 2018 13 February 2019 14 February 2019

Please cite this article as: J. Zhu, H. Zhang, R. Chen, Q. Liu, J. Liu, J. Yu, R. Li, M. Zhang, J. Wang, An anti-algae adsorbent for uranium extraction: L-Arginine functionalized graphene hydrogel loaded with Ag nanoparticles, Journal of Colloid and Interface Science (2019), doi: https://doi.org/10.1016/j.jcis.2019.02.045

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An anti-algae adsorbent for uranium extraction: L-Arginine functionalized graphene hydrogel loaded with Ag nanoparticles Jiahui Zhu,a,b Hongsen Zhang,*a Rongrong Chen, a,d Qi Liu,a,b,c Jingyuan Liu,a,b Jing Yu,a,b Rumin Li, a,b,c Milin Zhanga,e and Jun Wang,*a,b, c, e

a. Key Laboratory of Superlight Material and Surface Technology, Ministry of Education, Harbin Engineering University,

Harbin 150001, China

b. College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China

c. Harbin Engineering University Capital Management Co. Ltd, Harbin 150001, China.

d. Institute of Advanced Marine Materials, Harbin Engineering University, 150001, China.

e. College of Science, Heihe University, Heihe 164300, China.

＊Corresponding author: E-mail: [email protected]; [email protected]; [email protected]

Abstract

Uranium (VI) is very essential element in nuclear technique and the enrichment uranium has attracted lots of

attention. In this work, L-Arginine and Ag nanoparticles (AgNPs) functionalized reduced graphene oxide

ternary hydrogel composites (Ag-L-Arg-rGH) were successfully synthesized, which combined the insertion of

AgNPs with one-step thermal reduction and an assembly of graphene oxide nanosheets, using L-Arginine

(L-Arg) as both a functional and cross-linking agents. The Ag-L-Arg-rGH composites exhibited great enhanced

sorption capacity. Kinetic data best fitted the pseudo-second-order model. The thermodynamic data fitted the

Langmuir isotherm model and the calculated maximum adsorption capacity is 434.78 mg/g. In addition, the

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anti-algae experimental results indicated adsorbent showed marked algal inhibition with the presence of AgNPs

in the Ag-L-Arg-rGH composites. In the simulated seawater experiments, The distribution coefficient (Kd) value of uranium(VI) with other competing ions was 2.41 × 104 mL g−1. Thereby, the Ag-L-Arg-rGH composites

possessed a promising potential for the enrichment uranium (VI) from nature seawater.

Keywords: uranium (VI); Ag nanoparticles; L-Arginine; graphene hydrogel

1. Introduction

Nuclear power is considered as the main energy sources for the future[1], and the availability of uranium as a

nuclear fuel is of paramount importance. However, the exhaustion of land uranium resources is predicted in less

than a century[2]. As an alternative resource, there are an estimated 4 billion metric tons of uranium in seawater

which is about one thousand times more than the total land reserves[3, 4]. Therefore, the extraction of uranium

from seawater has attracted the interest of many researchers. Physical and chemical methods had been developed

to extract uranium, including electrochemical[5], solvent extraction[6], co-precipitation[7] and sorption[8-12].

One of the more promising strategies is the adsorption of uranium onto a suitable sorbent with associated

advantages of low cost, wide adaptability and availability[13-15]. Up to now, more efficient adsorbents for the

removal of uranium include titanate nanomaterials[16, 17],

metal−organic framework (MOF)[18, 19],

nanometer carbon material[20-22], layered double hydroxide[23] and polyethylene fibers[24]. It is desirable to

develop a kind of adsorbent with a low-cost, high absorption efficiency and high adsorption selectivity for

uranium (VI).

A three-dimensional (3D) interconnected porous self-assembled graphene based adsorbent has recently

become one of the most appealing materials in heavy metal absorption. It has abundant oxygen-containing

groups, large specific surface area, good mechanical properties and the unique network structure of

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interconnected micro-, meso- and macro-pores, which provided favorable pathways for the numerous ions

dispersed into the 3D structure[25, 26]. At the same time, 3D interconnected porous structure can overcome the

disadvantage that two-dimensional graphene material is easy accumulation in water. Meanwhile, most 3D

graphene based materials are formed by physical cross-linking without the dangerous chemicals. Importantly, a

monolithic morphology is beneficial for operation and collection, which avoid the shortcomings of conventional

solid adsorbents. Based on the above potential advantages, Sun et. al[27] synthesized graphene hydrogel modified with lignosulfonate for the enrichment of Pb2+ in wastewater. Ivan et. al[28] have employed FeOOH

nanoparticles functionalized graphene aerogels for removal of As. These researches reveal the advantageous of

3D graphene as the adsorbent for the removal of heavy metals.

Furthermore, in order to improve adsorption and selectivity performance, it is necessary for modifying 3D

graphene. Generally, nitrogen and oxygen functional groups are available to extract uranium (VI) and exhibit

superior adsorption capacity, which is due to the coordination effect between them and uranium (VI).

L-Arginine is a natural amino acid and possesses multiple amino groups and a carboxyl group. Meanwhile,

L-Arg can also be used as a promoter to combine with the -COOH or -OH groups on graphene oxide through the

electrostatic attraction or hydrogen bonding, which can promote the assembly of graphene oxide sheets into

hydrogels[29]. Thus, L-Arg can be modified on rGH surfaces as functional reagent to improve the uranium

adsorption performance.

In practical applications for the enrichment of uranium (VI) from the nature seawater, adsorbents might be

subjected to microorganism adherence during the adsorption process, which could significantly reduce

adsorption capacity. Therefore, the design of efficient and sustainable uranium adsorbent materials is necessary.

Different types of nano-materials like gold, copper and its oxide, titanium oxide, and silver have been studied to

explore their biological resistance[30-32]. Based on previous studies, we know that silver has effective

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anti-algae adhesion properties. However, the reducing agents were required during the synthesis of silver

nanoparticles. For this study, we use the one-step thermal reduction to avoid the complicated synthesis route.

Herein, we prepared AgNPs and L-Arg modified 3D porous reduced graphene oxide hydrogel composites

(Ag-L-Arg-rGH), by one-step thermal reduction with the assembly of graphene oxide nanosheets and metal ions, using L-Arginine（L-Arg）as a functional reagent, and taking advantage of the negative charge of graphene oxide to promote the loading of AgNPs, which avoided the side effect of hazardous chemicals involving the solvents

and surfactants. The morphology and structure of the composites were characterized and the adsorption

performance for uranium (VI) was devaluated. Importantly, the Ag-L-Arg-rGH adsorbent exhibited good

anti-algae performance. Our experimental results reveal that AgNPs and L-Arg loaded ternary reduced grphene

oxide hydrogel composites (Ag-L-Arg-rGH) possess promising potential for the enrichment of uranium (VI)

from simulated seawater.

2. Experimental

2.1 Preparation of Ag-L-Arg-rGH

The traditional Hummer method was used for preparing the Graphene oxide (GO)[33]. GO was added to

deionized water (100 mL) and a uniform dispersion obtained after ultrasonic treatment for 20 min.

Ag-L-Arg-rGH ternary hydrogel composites were prepared by a one-step hydrothermal process. First, L-Arg

and silver nitrate with concentrations each of 4.5 mg/mL was dispersed in homogeneous GO aqueous solution at

a mass ratio of 1:2:5 for Ag: L-Arg: GO. Sonication was continued for 1 h and the solution was then transferred

to a Teflon-autoclave and heated for 12 h at 180 ºC. After cooling to room temperature, the product was washed

with deionized water. Finally, we obtained 3D AgNPs and L-Arg modified hydrogel composites by a

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freeze-dried process(Scheme. 1). rGH samples were prepared by the same procedure in the absence of L-Arg

and silver nitrate.

2.2 Algae inhibition test

Algae Nitzschia closterium was used as a test organism. After the algae cell number reached 1×105, 100 mL, the

algae solution was transferred to individual conical flasks containing 0.02g Ag-L-Arg-rGH adsorbent for 3 days

soaking. The microalgae cell concentration was counted on a cell count chamber haemocytometer with an

optical microscope (Leica DML 300B, Germany). The anti-algae properties of the Ag-L-Arg-rGH adsorbents were studied by comparing the number of algae before and after soaking.

3. Result and Discussion

3.1 Characterization of Ag-L-Arg-rGH composites SEM and TEM were carried out to ascertain the morphology and structural details of the prepared

Ag-L-Arg-rGH composites (Fig. 1). Comparing with the image of rGH sheets (Fig. 1a), the prepared

Ag-L-Arg-rGH showed a unique 3D honeycomb network with large interconnecting channels (Fig. 1b and c),

formed by the introduction of L-Arg and silver ions to promote the assembly of graphene oxide layers[34]. We

clearly saw that AgNPs existed in the 3D network structure, demonstrating the successful loading of AgNPs.

The TEM image (Fig. 1d) further indicated that AgNPs of several nanometers in size were loaded onto the

surface of the rGH sheets. From the typical SAED pattern (inset Fig. S1), it indicated that the polycrystalline

nature of AgNPs. Fig. S1(b) showed that the visible set of lattice fringes with a lattice spacing of 0.22 nm was

characteristic of the (111) lattice plane of AgNPs[35].

The integrity of the Ag-L-Arg-rGH composites was characterized by Fourier transform infrared (FT-IR) spectroscopy as shown in Fig 2 (a). The stretching vibration peak of O−H appeared at 3416[9]. The peak at 1731

5

and 1628 cm−1 correspond to C=O stretching[36, 37]. The observed bands at 1394 and 1085 cm-1 could be

ascribed to the C-C and C-O, respectively[38, 39]. For the Ag-L-Arg-rGH composites, the new bands appeared at 3135, 1557 and 1188 cm-1 corresponding to the vibrations of N-H, C=N and C-N[40], which illustrated the

successful modification of L-Arg onto rGH.

The X-ray diffraction (XRD) patterns of the prepared rGH and Ag-L-Arg-rGH (Fig 2b) showed the

characteristic diffraction peaks of reduced graphene oxide at around 26.4°and 43.4°, corresponding to structure

of the graphitic layered structure and relating to the (0 0 2) and (1 0 0) reflection, respectively[41, 42]. The

appearance of other peaks at the diffraction angle of 38.1º, 44.2º, 64.5º, 77.5º were assigned to the four strongest

reflection peaks (111), (200), (220) and (311), which matched well with the structure of face-centered cubic

silver[43]. The XRD patterns illustrates that silver nanoparticles formed in this present synthesis.

In addition, X-ray photoelectron spectroscopy (XPS) measurements were performed to determine the

composition of as-prepared composites. As shown in Fig.2 (c), the survey XPS spectra of rGH demonstrated

main characteristic peaks of C (1s) and O (1s). At the same time, the XPS survey spectra of the Ag-L-Arg-rGH

composites were investigated, which showed the new peaks of Ag (1s) and N (1s). This confirmed that the

Ag-L-Arg-rGH had been successful synthesized.

Nitrogen adsorption-desorption measurements were performed to determine the porous features. As shown in

Fig. 2 (d), the hysteresis present in the adsorption and desorption isotherms indicated the presence of typical

type-IV curves for Ag-L-Arg-rGH, which was due to the mesoporous structure. We obtained the BET specific surface area was 71.09 cm2/g. The pore size distributions by Barrett-Joyner-Halenda (BJH) model were shown in Fig. 2d[44]. The total pore volume and pore size were 0.103 cm3/g and 5.95 nm, respectively.

3.2 Effect of pH and ionic strength

The effect of pH and ionic strength for the adsorption of uranium by Ag-L-Arg-rGH composites were shown in 6

Fig. 3 (a), which was studied for the range of 2-9. The adsorption performance strongly depends on the solution

pH, the adsorption capacity was obviously increased at pH values from 2 to 6, then the high-level removal was

observed at pH = 6. The removal of uranium (VI) decreased with a pH higher than 6. With a low pH, the

adsorbing active sites of Ag-L-Arg-rGH composites were protonated, and there was the ion competition for surface active sites between uranium (VI) and H+. With further increased of pH, we noted that de-protonation on

the surface of Ag-L-Arg-rGH composites was associated with a gradual increase of adsorption capacity with the

exposure of more active sites. At pH 6.0, approximately 75 and 95% of uranium (VI) were removed by rGH and

Ag-L-Arg-rGH. When the pH was higher than 6, electrostatic repulsion between the negatively charged uranium (VI) species (such as (UO2)3(OH)7- and UO2(OH)3-) and the negatively charged Ag-L-Arg-rGH composites, leaded to a reduction in the adsorption capacity[44]. In Fig. 3(b), the increase of ionic strength (0.001 to 0.1

mol/L NaClO4) had little effect on the adsorption of uranyl ions on Ag-L-Arg-rGH composites, which indicated that inner-sphere surface complexation mainly dominated the enrichment of uranium (VI)[45].

3.3 Removal kinetic studies

Adsorption time was one of the important factors for the assessment of uranium (VI) adsorption process by

Ag-L-Arg-rGH. The result presented in Fig. 3c and Fig. S2 showed that the rate of uranium (VI) adsorption was

initially relatively high and we note the maximum amount of uranium (VI) adsorbed onto Ag-L-Arg-rGH at

about 240 min, and a much slower subsequent adsorption had leaded gradually to an equilibrium condition.

After four hours, little further increase of uranium (VI) adsorption capacity occurs.

To further explain the sorption process of the Ag-L-Arg-rGH composites. The kinetics models

(pseudo-first-order and pseudo-second-order) were used to simulate the adsorption kinetics data as shown in Fig.

3c (inset). The kinetic fitted data were presented in Table S1. The results showed that the pseudo-second-order model fitted the sorption kinetic data well with higher correlation coefficient values (R2 > 0.99) contrasted to the 7

pseudo-first-order model (R2 < 0.96), which suggested chemical adsorption contributed to the rate determining

step between uranium (VI) and the active sites of Ag-L-Arg-rGH composites[46].

3.4 Adsorption isotherm studies

To describe the relationship between the uranium (VI) adsorption capacity of Ag-L-Arg-rGH composites and its

equilibrium concentration in solution, Langmuir, Freundlich and Temkin models were used. A detailed

description of three models was presented in support information section. The adsorption mechanism and

calculation maximum adsorption capacity of Ag-L-Arg-rGH were also studied. As shown in Fig. 3(d), the

adsorption amount increased with the increase of initial uranium (VI) concentration. The isotherm models were

applied to fit the experimental data in Fig. 4 a, b and c, the values of the corresponding isotherm constants for

uranium (VI) adsorption on Ag-L-Arg-rGH composites are obtained from the fitted straight line and are summarized in Table S2 and S3. According to the correlation coefficient (R2), the fitting results indicated that Langmuir model (R2 > 0.99) better fitted the isotherm experimental data of uranium (VI). This suggested that

the adsorption onto Ag-L-Arg-rGH composites was a monolayer adsorption and the adsorbent provides specific

homogeneous sites[47]. The value of theoretical adsorption capacity qe was 434.78 mg/g at 298K. The maximum uranium adsorption capacity of Ag-L-Arg-rGH composites and the other sorbents reported in the

literatures were shown in Table 1. The graphene oxide nanosheets, organic functional carbon spheres, waste

paper derived carbon, amidoxime modified inorganic material and fibers and metal organic frameworks

materials were included. The adsorption amount for Ag-L-Arg-rGH composites was higher than most of other

adsorbent materials, which suggested that Ag-L-Arg-rGH composites exhibited good adsorption performance

for uranium.

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Furthermore, in order to determine the effect of temperature on the enrichment of uranium (VI) by

Ag-L-Arg-rGH composites, the thermodynamic parameters of standard entropy change standard enthalpy change H 0 (kJ mol-1), S 0 (J mol-1 K-1) and standard Gibbs free energy G 0 (kJ mol-1) for the adsorption process were calculated. The values of entropy ( S 0 )and enthalpy ( H 0 ) were obtained from the intercept and slope of Van’t Hoff plots[17]. The thermodynamic parameters for uranium (VI) adsorption by Ag-L-Arg-rGH composites were summarized in Fig.4 (d) and Table. S4, we noted that the G 0 values were negative in the temperature range of 298-328.15 K, indicating that the uranium (VI) adsorption onto Ag-L-Arg-rGH composites was spontaneous. H 0 and S 0 were found to be 22.74 kJ mol-1 and 90.04 J mol-1 K-1, respectively. The positive value of H 0 indicated that the adsorption was an endothermic process. The positive value of S 0 suggested that randomness at the solid−liquid interface during the process of uranium(VI) adsorption onto

Ag-L-Arg-rGH composites was increased[48]. In summary, the thermodynamic parameters confirmed that

uranium(VI) adsorption was a spontaneous and endothermic processes on Ag-L-Arg-rGH composites.

3.5 Effect of competitive ions

In order to evaluate the potential performance of the Ag-L-Arg-rGH composites for selective adsorption of

uranium (VI), the adsorption experiments were carried out in the aqueous solutions with competitive metal ions (Ba2+, K+, Mg2+, Co3+, Ni2+, Sr2+, Cu2+)[49]. The molar ratios of competing ions and uranyl ions were 1:1, the

initial concentration (C0), equilibrium concentration (Ce) and removal rate of each ion were shown in Fig. 5 (a). The results showed that the removal rate for uranium (VI) onto the Ag-L-Arg-rGH composites was as much as

90% revealing a large adsorption capacity and high removal rate. At the same time, we calculated the

distribution coefficient Kd in Fig.5 (b). The kd value of Ag-L-Arg-rGH composites for uranium (VI) was 1.06× 104 mg L-1 and greater than that for other ions, which indicates that Ag-L-Arg-rGH composites could selectively

adsorb uranium (VI) and show promising potential as candidates for removing uranium (VI). 9

3.6 Uranium (VI) adsorption in simulated seawater

Algae are the main components of bio-film, which were the basis for subsequent large-scale fouling from

organism adhesion. Therefore, a study of the anti-algae properties of the adsorbent was beneficial to improve its

antifouling organisms adhesion ability, and improve the viability in future practical applications. The algae

inhibition performance for prepared Ag-L-Arg-rGH adsorbents was shown in Fig.5 (c) and (d). The number of initial algae, calculated with the aid of a cell count chamber haemocytometer, was 3.55×105, and the number of algae after soaking for three days was 1.27×105. The result indicated that the prepared Ag-L-Arg-rGH

adsorbents demonstrated algal inhibition, which was attributed to the reduced graphene oxide loaded AgNPs

inhibiting the growth of algae. Therefore, Ag-L-Arg-rGH adsorbent had a fascinating application prospect for

the removal of uranium (VI) from seawater.

From the above research, Ag-L-Arg-rGH composites showed superior selectivity and high adsorption

capacity for uranium (VI) for high concentration of uranium in solution (mg/L). However, we need to explore

the removal of uranium (VI) at a trace concentration for its application in seawater. The simulated seawater with the different uranium (VI) concentration, including 3 μg L−1 10 μg L−1, 50 μg L−1, 100 μg L−1 and 200 μg L−1,

were prepared according to the previous report[50]. In Fig. 6 (a) and Table S5, the results showed that the

removal rate was greater than 85% on Ag-L-Arg-rGH composites from simulated seawater. To further

investigate the effect of competitive ions for the enrichment of uranium (VI) from simulated seawater, the adsorption experiments were performed in simulated seawater with a concentration of about 3.3μg L−1 and the

results were shown in Fig. 6 (b). The distribution coefficient (Kd) value of uranium(VI) with other competing ions was 2.41 × 104 mL g−1. It is widely accepted in literatures that the nitrogen atoms of amino functional

groups are responsible for the adsorption capabilities, which is believed to have strong affinity with trace U(VI)

ions in aqueous solutions[51]. The results demonstrated that Ag-L-Arg-rGH composites had great potential for 10

the extraction of uranium (VI) from seawater.

3.7 Recyclability of the Ag-L-Arg-rGH adsorbents In order to enhance commercial application, the reusability of Ag-L-Arg-rGH composites was also investigated.

0.1M NaOH, Na2EDTA, NaHCO3, CH3COOH, HCl and Na2SO4 solution were selected as elution solvent. The results were shown in Fig.7 (a). The most effective desorption agent was found to be HCl. The desorption efficiency was 91.6% for the first time. Furthermore, five adsorption−desorption cycles were carried out and the

results were shown in Fig.7 (b). It could be found that desorption efficiency of Ag-L-Arg-rGH composites for

uranium (VI) had slight reduction with the increase of cycle numbers. The desorption efficiency was still reach

to 80.21% after the fifth cycle. Thus, the obtained Ag-L-Arg-rGH composites could be a promising material for

highly efficient enrichment of uranium (VI) ions.

3.7 Removal mechanism

To further investigate the removal mechanism between the Ag-L-Arg-rGH composites and uranium (VI), XRD

spectra were studied, and the results were shown in Fig. S3(a). The XRD patterns of Ag-L-Arg-rGH and

Ag-L-Arg-rGH-U had no obvious changes, which indicated that the structures of composites were preserved

after adsorption. FTIR was used to investigate the surface functional groups of Ag-L-Arg-rGH composites after

U (VI) adsorption in Fig. S3 (b). Some peaks in Ag-L-Arg-rGH-U had shifted compared to Ag-L-Arg-rGH. The bands at 3473, 3135 and 1557 cm−1 in Ag-L-Arg-rGH correspond to the stretching vibrations of the –OH, N-H and C=N, which shifted to 3411, 3137 and 1570 cm−1 for Ag-L-Arg-rGH-U, respectively. It demonstrated the

presence of strong affinities between the functional group and uranium (VI). Moreover, there was a new band at 904 cm−1 for Ag-L-Arg-rGH-, which may be attributed to the antisymmetric stretching vibration of O=U=O[52].

The XPS survey spectrum of Ag-L-Arg-rGH composites consists of carbon, nitrogen, oxygen and silver in

Fig. 8 (a). The new peaks appeared on the Ag-L-Arg-rGH/U composites after uranium (VI) sorption. In addition, 11

the U 4f XPS spectrum of Ag-L-Arg-rGH/U is presented in Fig. 8 (b). The results indicated the uptake of

uranium (VI) on the surface of the Ag-L-Arg-rGH composites through chemical bonding[53].

Fig. 8 (c) displayed the O 1s peaks of Ag-L-Arg-rGH and Ag-L-Arg-rGH/U. The presence of O 1 s at 532.18

eV corresponded to the C–O type oxygen in COOR groups and C–OH[54]. The characteristic peak with 531.18

eV was assigned to the C=O type oxygen[55, 56]. The binding energies around 533.57 eV was assigned to

chemisorbed oxygen or strongly adsorb water molecules[57, 58].After the uranium was adsorbed, the binding

energy shifted to 531.87 eV, 531.06 eV and 533.47 eV, respectively, which demonstrated that

oxygen-containing functional groups in Ag-L-Arg-rGH composites could act as active sites for the enrichment

of uranium (VI).

The N 1 s spectrum of the Ag-L-Arg-rGH exhibits two peaks at 399.00 eV and 400.22 eV in Fig. 8 (d), which were ascribed to C-N and protonated amine (-NH3+)[59, 60]. For Ag-L-Arg-rGH/U composites, the peaks were shifted to the higher binding energy (399.81 and 400.39 eV), which were attributed to the introduction of

cross-linking agent L-Arg, making available nitrogen containing functional groups to facilitate uranium (VI)

adsorption[61]. Based on the above results, the adsorbate–adsorbent interactions most likely involved

complexation between oxygen and nitrogen-containing functional groups with uranium (VI)[20].

4. Conclusion

In summary, to improve the absorption efficiency and selectivity for uranium (VI), and avoid the adhesion of

algae on the adsorbent, we employed a rapid one-step thermal process to synthesize Ag-L-Arg-rGH composites.

The Ag nanoparticles combined with 3D network structure rGO, the as-prepared Ag-L-Arg-rGH composites

exhibited excellent anti-algae property. Adsorption followed the pseudo-second-order kinetics model.

Specifically, Ag-L-Arg-rGH composites showed excellent adsorption performance compared with others

reported and the maximum adsorption capacity of Ag-L-Arg-rGH composite was 434.78 mg/g, which depended 12

on the availability of numerous adsorption sites and the data fitted well with the Langmuir model. Furthermore,

it also showed outstanding selectivity for uranium (VI) in the simulated seawater. The results indicated that

Ag-L-Arg-rGH was a potential adsorbent and demonstrated a significant enhancement of uranium (VI)

adsorption efficiency from natural seawater.

Acknowledgments This work was supported by National Science Foundation of China (NSFC 51872057), the Application

Technology Research and Development Plan of Heilongjiang Province (GX16A009), Fundamental

Research Funds of the Central University and Natural Science Foundation of Heilongjiang Province

(QC2018010), Ph.D. Student Research and Innovation Fund of the Fundamental Research Funds for the

Central Universities (HEUGIP201817).

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15

Figure Captions

Scheme. 1 Illustration of the synthetic procedure of Ag-L-Arg-rGH composites.

Fig. 1 (a) SEM images of rGH sheets; (b) and (c) Ag-L-Arg-rGH composites with different magnification; (d) TEM image of Ag-L-Arg-rGH composites.

Fig. 2 (a) IR of rGH and Ag-L-Arg-rGH before and after adsorption uranium (VI); (b) XRD patterns of rGH and Ag-L-Arg-rGH; (c) XPS of rGH and Ag-L-Arg-rGH before and after adsorption uranium (VI); (d) N2 adsorption-desorption curve and pore size distribution of Ag-L-Arg-rGH samples.

Fig.3 (a) Effect of initial pH of rGH and Ag-L-Arg-rGH; (b) Ionic strength on adsorption property of Ag-L-Arg-rGH. (c) Effect of contact time on the adsorption of uranium (VI) and pseudo-second-order (inset)

by rGH and Ag-L-Arg-rGH; (d) Effect of temperature on uranium (VI) adsorption by Ag-L-Arg-rGH.

Fig. 4 (a) Langmuir model; (b) Freundlich model; (c) Temkin model; (d) Vant Hoff curve on uranium (VI) adsorption by Ag-L-Arg-rGH.

Table 1. The maximum adsorption capacity of different adsorbents for uranium (VI).

Fig. 5 (a) and (b) Effect of co-existing ions on the removal of uranium by Ag-L-Arg-rGH; (c) and (d) The algae inhibition test of Ag-L-Arg-rGH adsorbent.

Fig. 6 (a) Effect of initial trace concentration of uranium on its adsorption capacity from simulated seawater and the removal rate, (b) selected results of Ag-L-Arg-rGH composites for the extraction of uranium from simulated

seawater. Fig. 7 (a) Desorption efficiency of different desorption agents, (b) cycle number of Ag-L-Arg-rGH composites. Fig. 8 (a) Typical XPS survey spectrum; (b) The high resolution spectra of U 4f; (c) O 1s for the raw Ag-L-Arg-rGH; (d) N 1s for the Ag-L-Arg-rGH before and after uranium (VI) adsorption.

16

Scheme. 1

17

Fig. 1

18

(a)

(111)

(b)

3000

2500

2000

(200)

1000

500

10

20

30

40

50

60

(311)

70

80

90

80

(d)

3

Quantity adsorbed (cm /g)

O 1s

rGH

N 1s Ag 1s

rGH

2-Theta

C 1s

O 1s

Intensity (cps)

(100)

(002)

(220)

Intensity (a.u.)

1368

1500

Wavenumeber (cm-1) (c)

Ag/L-Arg-rGH

1188

1557

3135

1726 1666

3416

Ag/L-Arg-rGH

3473

3500

1085

1394

1731 1628

Transmittance (a.u)

rGH

Ag-L-Arg-rGH

60

40

20 adsorption desorption 0

700

600

500

400

300

200

100

0

0.0

0.2

0.4

0.6

0.8

Relative pressure (p/p0)

Binding energy (eV)

Fig. 2

19

1.0

Fig. 3

20

Fig. 4

21

Table. 1 The maximum adsorption capacity of different adsorbents for uranium (VI).

Adsorbents

Adsorption Capacity mg-U/g-adsorbent

Conditions

Ref.

Graphene oxide nanosheets

97.5

T = 298 K, pH = 5.0

[62]

Imine-functionalized carbon spheres

113.16

T = 298 K, pH = 5.0

[63]

Poly(dopamine) functionalized waste paper derived carbon

384.6

T = 298 K, pH = 7.0

[64]

Amidoxime functionalized wool fibers

56.05

T = 303 K, pH = 6.0

[65]

117 65 

T = 298 K, pH = 6.0

[66]

Amidoxime modified Fe3O4/SiO2

105.5

T = 298 K, pH=5.0

[67]

Fe3O4@AMCA-MIL53(Al)

227.3

T = 298 K, pH = 6.0

[68]

Polypyrrole/ZIF-8

534.0

T = 298 K, pH = 3.5

[69]

Ag-L-Arg-rGH

434.78

T = 298 K, pH = 6.0

This work

Magnetic amidoxime-functionalized chitosan

22

Fig. 5

23

Fig. 6

24

Fig. 7

25

(a)

(b) Ag-L-Arg-rGH

Intensity (cps)

N 1s Ag 1s

Intensity (cps)

O 1s

C 1s

U 4f

U 4f

Ag-L-Arg-rGH

600

500

400

300

200

100

Ag-L-Arg-rGH/U

394

0

392

(531.87 eV) (533.47 eV) (531.06 eV)

Ag-L-Arg-rGH/U

536

386

384

382

380

Ag-L-Arg-rGH

(400.39 eV) (399.81 eV)

Ag-L-Arg-rGH/U

534

532

530

378

N 1s

(400.22 eV)

(399.00 eV)

Intensity (cps)

Ag-L-Arg-rGH

538

388

(531.18 eV)

(533.57 eV)

Intensity (cps)

(d)

O 1s

(532.19 eV)

390

Binding energy (eV)

Binding energy (eV) (c)

(382.31 eV)

(392.63 eV)

Ag-L-Arg-rGH/U

700

(381.99 eV)

(393.19 eV)

404

528

Binding energy (eV)

403

402

401

400

399

Binding energy (eV) Fig. 8

26

398

Graphical abstract

An anti-algae adsorbent Ag-L-Arg-rGH composites were synthesized for removing uranium (VI) from simulated seawater

27