Cooperation between Humic Acid and Graphene Oxide in

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Competition/Cooperation between Humic Acid and Graphene Oxide in Uranyl Adsorption Implicated by Molecular Dynamics Simulations Tu Lan, Jiali Liao, Yuanyou Yang, Zhifang Chai, Ning Liu, and Dongqi Wang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.9b00656 • Publication Date (Web): 04 Apr 2019 Downloaded from http://pubs.acs.org on April 6, 2019

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Environmental Science & Technology

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Competition/Cooperation between Humic Acid and Graphene Oxide in

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Uranyl Adsorption Implicated by Molecular Dynamics Simulations

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Tu Lan,1,2,3 Jiali Liao,1 Yuanyou Yang,1 Zhifang Chai,2,4 Ning Liu,*,1 and Dongqi Wang*,2

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1 Key

7

Institute of Nuclear Science and Technology, Sichuan University, Chengdu 610064, China

8

2 CAS

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Center, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China

Laboratory of Radiation Physics and Technology (Sichuan University), Ministry of Education;

Key Laboratory of Nuclear Radiation and Nuclear Techniques, Multidisciplinary Initiative

10

3 Department

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T6G 1H9, Canada

12

4

13

Radiation Medicine and Interdisciplinary Sciences (RAD-X), Soochow University, Suzhou 215123,

14

China

of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta,

State Key Laboratory of Radiation Medicine and Protection, Soochow University, and School of

15

1

ACS Paragon Plus Environment


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ABSTRACT: Molecular dynamics (MD) simulations were performed to investigate the

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influence of curvature and backbone rigidity of an oxygenated surface, here graphene

18

oxide (GO), on its adsorption of uranyl in collaboration with humic acid (HA). The

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planar curvature of GO was found to be beneficial in impeding the folding of HA. This,

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together with its rigidity that helps stabilize the extended conformation of HA, offered

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rich binding sites to interact with uranyl with only marginal loss of binding strength.

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According to our simulations, the interaction between uranyl and GO was mainly driven

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by electrostatic interactions. The presence of HA not only provided multiple sites to

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compete/cooperate with GO for adsorption of free uranyl, but also interacted with GO

25

acting as a “bridge” to connect uranyl and GO. The potential of mean force (PMF)

26

profiles implied that HA significantly enhanced the interaction strength between uranyl

27

and GO, and stabilized the uranyl-GO complex. Meanwhile, GO could reduce the

28

diffusion coefficients of uranyl and HA, and retard their migrations in aqueous solution.

29

This work provides theoretical hints on the GO-based remediation strategies for the

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sites contaminated by uranium or other heavy metal ions and oxygenated organic

31

pollutants.

32 33

TOC Graphic:

34 35

2


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 INTRODUCTION

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Uranium is usually present in the hexavalent oxidation state (uranyl) under aerobic

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condition in the environment. The high mobility of uranyl in the aqueous phase and its

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long half-life and high radiological and biological toxicity makes it a major concern to

40

environment and human health in the application of nuclear energy.1-3 Over the years

41

various methods have been developed, including chemical precipitation, solvent

42

extraction, ion exchange, adsorption, reduction and membrane processing,4-9 to strip it

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and other radionuclides from the aqueous phase, among which adsorption is considered

44

to be the most appropriate choice in industrial wastewater treatment due to its low cost,

45

easy operation and high efficiency.10, 11 Recent years’ development in nanoscience has

46

spotlighted graphene oxide (GO) as one of the most promising adsorbents.

47

GO is oxidized derivative of graphene with its basal plane modified mostly with

48

epoxyl and hydroxyl groups and its edges with carbonyl and carboxyl groups.12, 13 As a

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carbon-based sorbent, the high surface area and abundant oxygen-containing organic

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functional groups of GO could ensure its much higher adsorption capacity in the

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removal of uranyl from aqueous solutions, which is 299 mg/g,14 than those of carbon

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nanotubes (45.9 mg/g) and activated carbon (25 mg/g).15, 16 The influence of pH on the

53

adsorption of ionic species on GO was evaluated,17,

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carboxyl, hydroxyl on the surfaces of GO, were demonstrated to play important

55

roles.19-24 These oxygenated groups are also typical functional groups of humic acid

56

(HA), a complex mixture of organic compounds widely distributed in the environment25

57

and received considerable interest due to its relevance with environment and water

58

treatment.26-28

18

and oxygenated groups, e.g.

59

Owing to its hydrophilic and hydrophobic functional groups and high flexibility, HA

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is amphiphilic, and can potentially bind with toxic heavy metal ions such as

61

actinides,29-34 and organic matter such as polycyclic aromatic compounds (PAC).35-37

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Earlier studies on its interaction with uranyl often used HA as additive to investigate its

63

influence on the adsorption of uranyl on adsorbents,38-43 and few work focused on the

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cooperative adsorption.44-46 In the work of Song et al.,45 the mutual influence of uranyl

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and HA on their simultaneous removal from an aqueous system by the synthesized

66

cyclodextrin-modified graphene oxide nanosheets (CD/GO) has been investigated,

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which indicated that the presence of HA enhanced uranyl sorption at low pH and

68

reduced uranyl sorption at high pH, while the presence of uranyl enhanced HA sorption.

69

The surface adsorbed HA acted as a “bridge” between uranyl and CD/GO, and formed 3



70

strong inner-sphere surface complexes with uranyl. However, the molecular level of

71

understanding on their interaction mechanism and dynamics is hard to probe

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experimentally, and remains to study.

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In our previous studies,47, 48 by means of MD simulations, we have shown that the

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presence of HA enhances the hydrophilicity of GO or CNT via non-bonded surface

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functionalization on the hydrophobic surfaces by π-π stacking and hydrogen bonding,

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and proposed that in a HA-uranyl-CNT ternary system, they may mutually influence

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their migration behavior. Though both GO and CNT are carbon-based materials, they

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differ from each other in their surface curvature, hydrophilicity, and rigidity, and it

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remains to learn how these factors may affect the interaction between GO and

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HA/uranyl and the composite of them. In addition, as mentioned above, both GO and

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HA bare similar oxygenated groups but differ in their backbone rigidity. It is beneficial

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to understand how they mutually influence each other to interact with uranyl in

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co-existence. Motivated by this, the adsorption of uranyl on GO in presence of HA has

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been systematically investigated using MD simulations enhanced by umbrella sampling

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technique. Our results provide new insights to understand the dynamics of

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GO/uranyl/HA binary and ternary systems in aqueous solution, with implications on the

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GO-based remediation strategies for the sites contaminated by uranium or other heavy

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metal ions and oxygenated organic pollutants.

89 90

 SIMULATION METHODS

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Models and Computational Details. The model structures of HA and GO were

92

shown in Figure 1. The HA model proposed by Stevenson25 and the GO model based on

93

experimental characterization in our previous study48 were used to mimic HA and GO,

94

respectively. It is important to note that “real” HAs are a complex class of

95

macromolecules with large variations in their chemical compositions depending on the

96

environments.25 Consequently, it is impossible to expect a single type of model

97

compound to represent the full features of HAs. However, this model has the typical

98

hydrophobic and hydrophilic functional groups of “real” HAs, e.g. aromatic rings,

99

aromatic carboxyl, phenolic hydroxyl, quinone, oxygen-bridge, amino acid residue, and

100

carbohydrate groups, which enable it amphiphilic with flexible backbone,47 and was

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identified to show excellent agreement with experimental characterizations both in its

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chemical compositions and Fourier Transform Infrared (FT-IR) spectra.49 It has been

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widely employed in the literatures as a representative model to study HA properties.47, 50 4


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(a)

COOH

(b)

Aromatic Hydroxo Aromatic Carboxylic Acid

O OH

HO OH

Sugar CHO

O

O

106

O

OH

COOH

O

O C

HO

O

OH O

O

N NH2

HO

O

HN

O

105

O

HOOC

O

OH

O

O

HO

OH

OH HO

Quinone

Tyr Amino Acid

HO

HO

O

O

HOOC

OH

HO

O

O

OH

OH O

O

O

O

COOH

Figure 1. Model structures of (a) HA and (b) GO employed in this work.

107 108

To probe the adsorption behavior of uranyl and HA on GO, the binary systems with

109

the co-presence of GO and uranyl (GU) or HA (GH) and the ternary system with GO,

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uranyl and HA (GUH1: 1 GO, 1 uranyl, and 1 HA; and GUH2: 1 GO, 8 uranyl, and 2

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HA) were constructed. For comparison, three reference systems with one molecule of

112

uranyl, or GO, or HA in each box were also simulated. The compositions of these

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systems are summarized in Table S1 of Supporting Information (SI), and the details of

114

the simulated systems are described in Section S1 (SI). In the present study, all of the

115

carboxylate groups were deprotonated and the amine group was protonated to simulate

116

their presences in natural environment with a pH range of 6 ~ 8 according to their pKa

117

in aqueous solution (typical functional groups see Figure S1, SI). HA, GO, and uranyl

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were treated by the force field created and validated in our previous work.47, 48 Water

119

molecules were described by the extended simple point charge (SPC/E) model.51 The

120

excess negative or positive charges were neutralized by Na+ or Cl– ions.

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All simulations were performed using the MD package GROMACS 5.0.452 with

122

periodic boundary conditions applied in all three directions. In each simulation system,

123

the solutes were solvated in a cubic water box with box length of 5.0 nm. After an initial

124

steepest descent energy minimization to remove unphysical repulsive interactions, the

125

systems were first equilibrated at 300 K with a canonical (NVT) ensemble for 100 ps by

126

applying the Nose-Hoover thermostat,53 then with an isothermal-isobaric (NPT)

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ensemble at 1 bar for another 100 ps using the Parrinello-Rahman barostat.54 The

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systems were then sampled for 25 ns with NPT ensemble. The trajectories were saved

129

every 10 ps for the ﬁrst 20 ns and every 0.01 ps for the last 5 ns. Unless otherwise 5



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specified, for all systems, the first 20 ns was used to analyze the adsorption process and

131

mechanism, and the last 5 ns was used to analyze the thermodynamic properties of

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model systems. During the simulations, the LINCS algorithm was applied to constrain

133

all bond lengths,55 and a time step of 2 fs was used. Particle-mesh Ewald (PME) method

134

was used to handle long range electrostatic interactions.56 A cutoff scheme was used for

135

short-range electrostatics and van der Waals interactions with a cutoff value of 1.4 nm.

136 137

Umbrella Sampling. Umbrella sampling (US) is one of the most accurate methods

138

to estimate the free energy of systems by means of the potential of mean force (PMF).57

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During the US simulations, the distance between the center-of-mass (COM) of each

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entity (uranyl, GO, HA, or GO+HA in GUH1) was defined as the reaction coordinate

141

(RC), ξ, and varied in the range 0.0 nm ≤ ξ ≤ 2.5 nm. The RC was restrained by a

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Gaussian bias potential wi(ξ), which has the form of

143

wi   

2 Ki   ic   2

(1)

144

where ξic is the position at which the system is restrained with a force constant Ki. A

145

value of 1000 kJ mol-1 nm-2 was used for the force constant, and a Nose-Hoover

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thermostat was used to keep the temperature constant during the production run of 10 ns

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in each window. The simulation parameters are same as in the plain simulations. The

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PMF was calculated from unbiased probability distributions of the system using the

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Weighted Histogram Analysis Method (WHAM),58, 59 and statistical uncertainties were

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estimated by Bayesian bootstrapping of complete histograms.59

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Appropriate postprocessing programs available in GROMACS were employed for

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trajectory analysis. Multiwfn 3.3.9 program60 was used to localize and identify the

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noncovalent interactions (NCI), which is introduced by Johnson et al.,61 and VMD62

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was utilized for visualization.

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 RESULTS AND DISCUSSION

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Adsorption Process and Configuration. In Figure 2, representative snapshots

158

of GU, GH, GUH1 and GUH2 systems are shown to demonstrate the adsorption process

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and the binding of hydrated uranyl to GO or HA. As seen in Figure 2a, GO coordinated

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with uranyl by carboxyl group spontaneously. Owning to its amphiphilicity, the access

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of uranyl to GO may be assisted by HA crouching down or leaning over the GO (Figure

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2c and 2d). As seen in Figure 3, the presence of HA not only provided multiple sites to 6


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compete with GO for adsorption of free uranyl, but also interacted with GO acting as a

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“bridge” to connect uranyl and GO, which provided a straightforward evidence to the

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inference of previous experimental study.45 This indicated that HA could cooperate with

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GO to trap free uranyl. These results indicated that multiple interaction modes existed

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between the individual components in the GO-uranyl-HA ternary system.

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169 170

Figure 2. Representative snapshots of (a) GU, (b) GH, (c) GUH1 and (d) GUH2

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systems during the simulations. These snapshots are labeled as X-m, representing the

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snapshot of the model system X (X = GU, GH, GUH1, GUH2) at the m-th ns. C, H, O,

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N and U atoms are shown in cyan, white, red, blue and light pink, respectively. Water

174

molecules close to the polar groups of the solutes are shown while the others are

175

removed for clarity. Some hydrated uranyl ions lack of interaction with GO/HA in

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GUH2 system are not shown for simplicity. GO can coordinate with uranyl via its

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carboxyl groups and interact with HA spontaneously. The access of uranyl to GO may

178

be assisted by HA crouching down or leaning over the GO.

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Figure 3. Representative snapshot of GUH2 system at 20 ns to show the multiple

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interaction modes between the individual components: (a,c) side views of GO; (b) top

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view of GO. Color scheme: GO in blue, and C, H, O, U atoms of HA and uranyl in

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cyan, white, red and light pink, respectively. Water molecules are not shown for clarity.

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HA not only provided multiple sites to compete/cooperate with GO for adsorption of

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free uranyl, but also interacted with GO acting as a “bridge” to connect uranyl and GO.

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RMSD (nm)

1.0

1.4

HA (GH) HA (GUH1) HA1 (GUH2) HA2 (GUH2)

(a)

0.8 0.6 0.4

188

1.0 0.8

0.2 0

HA (GH) HA (GUH1) HA1 (GUH2) HA2 (GUH2)

(b)

1.2

Rg (nm)

1.2

5

10

Time (ns)

15

20

0.6 0

5

10

Time (ns)

15

20

189

Figure 4. (a) Atomic positional root mean square deviation (RMSD) and (b) radius of

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gyration (Rg) of HA in GH, GUH1, and GUH2 systems as a function of time.

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It is generally believed that HA has the ability to adapt to the chemical environment

193

by folding itself, and the knowledge on the folding of HA is crucial to understand the

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co-adsorption process of uranyl and HA on GO. In Figure 4, the positional root mean

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square deviation (RMSD) of non-hydrogen atoms of HA and its radius of gyration (Rg)

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in GH, GUH1 and GUH2 systems were plotted to analyze the folding of HA during the

197

simulations. For HA, its extended conformation at the initial stage, which was

198

energetically minimized, was used as the reference. In GUH1, the folding of HA is

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accompanied by an increase of its RMSD and a decrease of its Rg. Similar phenomena

200

were also observed in GUH2 system. We also note that, in Figure 4b, the time evolution

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of Rg of HA in GH system displayed distinct behavior from that in other ones. Analysis

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of the trajectory shows that it is due to the unfolding of HA after 1.5 ns, as seen in

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Figure 2b, when crouching down on GO in the absence of uranyl. In the presence of

204

uranyl, HA could interact with uranyl in a folding form, resulting in the smaller Rg

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values. This indicates that uranyl may enhance the folding of HA, which agrees with

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our previous study.47

207

Additionally, the solvent accessible surface area (SASA)63 was calculated to evaluate

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the folding of HA and solvation free energy (SFE) of solutes (Figures S2–S3, SI). At the

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initial stage when HA is far away from GO (red line), the SASA value reached up to

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about 35 nm2. It quickly decreased once HA interacts with GO and some water solvent

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molecules between them were expelled, and remained roughly constant during the rest

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simulation time (Figure S2, SI). Comparing the SASA values of GO+HA composite in

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the GH and GUH1 systems shows that the value in GH system (24.6 ± 0.5 nm2) is larger

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than that in GUH1 system (22.7 ± 0.6 nm2), indicating that HA folded itself more

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compact in GUH1 system owing to its binding with uranyl. This provides further

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evidence that uranyl could enhance the folding of HA. In Figure S3a, a positive SFE

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value of GO (10.9 ± 2.6 kJ/mol) and a negative one of HA (-2.2 ± 2.4 kJ/mol) show

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opposite solvent affinity of these two components. The marginal negative value (-0.3 ±

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3.0 kJ/mol) for the composite of GO+HA (red line) clearly indicates that HA benefits

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the dissolution of GO in water via noncovalent functionalization.

221 222

Interaction Mechanism. To understand the mechanism of uranyl adsorption on

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GO in the absence/presence of HA, the interaction energies between individual

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components (GO, HA, and uranyl) in GU, GH, GUH1 and GUH2 systems were

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calculated and plotted in Figure 5. The results clearly show that, in GU system, the van

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der Waals (vdW) interaction resists the close contact of uranyl to GO, whereas the

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electrostatic interaction extremely favors their binding. A similar case occurs between

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HA and uranyl in GUH1 system, which indicates that the electrostatics appears as the

229

driving force to dominate the interaction between GO/HA and uranyl. In contrast, in GH

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system, electrostatics only marginally favors the binding of HA to GO, and vdW

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interaction is the key player to govern the spontaneous assembly of the two entities.

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Similar phenomena were observed in GUH2 system, where the electrostatics

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dominates the interactions between GO/HA and uranyl while the vdW interaction

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dictates that between GO and HA. The interaction energies between individual

235

components in the four systems were calculated, and tabulated in Table S2 (SI). By 9



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comparing the interaction energies in GU and GUH1 systems, the addition of HA

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brought gain in electrostatic interaction energy between GO and uranyl by c.a. 16.5

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kJ/mol, with a negligible change in vdW interaction. This suggests that the coordination

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structure formed by GO and uranyl becomes more stable after HA is introduced, i.e.,

240

HA could stabilize the complex.

241 100

100

(a)

Energy (kJ/mol)

-100

0

EvdW: GO-Uranyl

-200

Energy (kJ/mol)

0

Eelec: GO-Uranyl

-300 -400 -500

242

0

5

100

Energy (kJ/mol)

0

10

Time (ns)

(c)

-100

EvdW: GO-HA

-200

15

-200 -300

EvdW: GO-HA

-400

-400

10

Time (ns)

15

10

Time (ns)

15

20

20

-1200

-2000

EvdW: HA-Uranyl EvdW: GO-Uranyl

Eelec: GO-Uranyl

-800

EvdW: GO-HA

Eelec: HA-Uranyl

-1600

Eelec: HA-Uranyl 5

5

Eelec: GO-HA

Eelec: GO-Uranyl

0

0

0

-400

-600

-600

20

EvdW: HA-Uranyl EvdW: GO-Uranyl Eelec: GO-HA

-300

-500

243

Eelec: GO-HA

-500

Energy (kJ/mol)

-600

(b)

-100

(d) 0

5

10

Time (ns)

15

20

244

Figure 5. Electrostatic and vdW interactions between individual components in (a) GU,

245

(b) GH, (c) GUH1, and (d) GUH2 systems as a function of time.

246 247

In order to quantitatively evaluate the binding strength of GO with uranyl, the free

248

energy changes (ΔG) were calculated by using US technique for the binding of uranyl

249

with GO in GU and GO+HA in GUH1 systems and the binding of HA with GO in GH

250

system. The PMF profiles of three systems are shown in Figure S4 and the ΔG are

251

collected in Table S3 (SI). The dissociation of uranyl from the GO-uranyl binary system

252

cost a free energy of 32.1 ± 0.6 kJ/mol. The presence of HA significantly enhanced the

253

interaction strength between GO and uranyl (81.1 ± 1.8 kJ/mol). This, on the other

254

hand, confirmed the above analysis that HA could strengthen the stability of the

255

complex. For the interaction between GO and HA in GH system, the ΔG was calculated

256

to be 247.7 ± 3.5 kJ/mol, indicating that rather strong noncovalent interaction could be 10


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established between GO and HA.

258

In earlier work,47 we evaluated the interaction strength of uranyl with HA/CNT, and

259

reported that the binding free energies of uranyl to HA were 88.3 ± 0.9 and 87.6 ± 1.2

260

kJ/mol in the HA-uranyl binary and HA-uranyl-CNT ternary systems, respectively.

261

These values are moderately larger than that in the HA-uranyl-GO ternary system by 6–

262

7 kJ/mol. This suggests that the co-existence of HA-GO moderately induces a decrease

263

in the binding strength of uranyl on HA. This is conceivable concerning the planar

264

curvature that hinders a best fit of HA to interact with uranyl, whereas the hyperbolic

265

surface of CNT allows a limit folding of HA to access uranyl. Note that, the HA-GO

266

system gains more binding sites, i.e. high adsorption capacity, during the unfolding of

267

HA at the cost of only marginal loss of binding strength of uranyl. This shows the minor

268

difference in the consequence due to the distinct curvature of GO and CNT. We also

269

observed that GO (247.7 ± 3.5 kJ/mol) displayed a much stronger affinity to HA than

270

CNT (138.1–143.0 kJ/mol). As mentioned above, GO differs from CNT not only in

271

their curvatures, but also in that GO is both edge- and surface-oxygenated. These polar

272

groups offers electrostatic interaction between GO and HA which is missing in the case

273

of CNT. Additionally, it is interesting to compare the affinity of HA and GO to uranyl.

274

As mentioned above, the binding free energy of GO with uranyl in the GO-uranyl

275

binary system was calculated to be about 32.1 kJ/mol, which is smaller than that of HA

276

with uranyl in the HA-uranyl binary system (88.3 kJ/mol). The stronger binding affinity

277

of HA with uranyl originates from the presence of two vicinal carboxyl groups in HA

278

(Figure 1) and its flexibility that allows the carboxyl groups far away each other to

279

cooperate to clamp the cations it captures, here uranyl, which is not possible for GO due

280

to its rigidity.

281

Here the potential influence of other charged species, e.g. CO32–, OH–, and Mg2+,

282

were not included to simply the understanding of the mutual interactions in the

283

HA-uranyl-GO ternary systems. These ions are commonly available in ground water,

284

and constitute an unavoidable aspect in environmental science in a more realistic view.

285

In earlier study,47 the effect of these ions was evaluated, and the presence of CO32– and

286

OH– was found to substantially weaken the binding strength of uranyl with HA (88.3,

287

31.2, 28.8 kJ/mol for UO22+(aq), UO2(CO3)(aq), UO2(OH)2(aq), respectively), which

288

was consistent with experimental data.40, 64-67 The Mg2+ dication may interact with the

289

carboxyl groups of HA with much weaker strength (13.6 kJ/mol) and can hardly

290

compete against uranyl. As these solvated charged species mainly exist as monomeric 11



291

hard spheres, they do not construct specific surfaces to complicate their interaction with

292

other species in the vicinity, and we expect the above-mentioned conclusions remains

293

hold in the uranyl-HA-GO systems.

294

To gain more insights into the adsorption mechanism and identify the interaction

295

characteristics between GO and HA, noncovalent interaction (NCI) analysis was carried

296

out based on the electron density and its derivatives, i.e. the reduced density gradient

297

(RDG).68,

298

Figure 6 (GUH2) according to the values of electron density (ρ) multiplied by the sign

299

of the second Hessian eigenvalue λ2 (sign(λ2)ρ) with blue for negative values (strong

300

attractive interactions, e.g. hydrogen bonds), red for positive values (strong repulsive

301

interactions, e.g. steric clashes), and green for values near zero (weak interactions, e.g.

302

van der Waals). It can be observed that there is weak interactions between GO and HAs

303

(green), corresponding to π-π interactions between the aromatic rings of HA and the

304

hexagonal rings of GO. Meanwhile, the epoxide and hydroxyl groups on the basal plane

305

of GO bring repulsive interactions (light red) to repel the aromatic rings of HA. Similar

306

results were also found in other systems (see Figure S5, SI).

69

The gradient isosurfaces are colored on a blue-green-red (RGB) scale in

307

308 309

Figure 6. Gradient isosurfaces (s = 0.2 a.u.) for the interactions between GO and HAs

310

in GUH2 system. The surfaces are colored on a blue-green-red (BGR) scale according

311

to the corresponding values of sign(λ2)ρ, ranging from –0.02 to +0.02 a.u. Color scheme:

312

blue: strong attractive interactions (e.g. hydrogen bonds); green: weak interactions (e.g.

313

van der Waals); red: strong repulsive interactions (e.g. steric clashes). 12


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314 315

In addition, from the gradient isosurfaces, hydrogen bonds were formed between GO

316

and HAs in GUH2 system (black circles in Figure S6, SI). The time evolution of the

317

formation of hydrogen bonds between GO and HA in GH, GUH1 and GUH2 systems

318

were plotted in Figure S7 (SI). In GH and GUH1 systems, the number of hydrogen

319

bonds between GO and HA(s) increased tardily during simulation and then fluctuated

320

between 1 and 2. In GUH2 system where there were two HA molecules, the number of

321

hydrogen bonds quickly increased to 6, then fluctuated with an average of ca. 4. The

322

much higher presence of hydrogen bonds in GUH2 than in GH and GUH1 was due to

323

the better adaption of HA on GO, indicating that higher concentration of HA may assist

324

the opportunistic formation of hydrogen bond between HA and GO. These results show

325

that hydrogen bonding also contributed to the interaction between GO and HA in

326

addition to van der Waals and π-π interactions.

327 328

Local Topology of Uranyl. The coordination structures of uranyl (Table 1) were

329

derived from the radial distribution functions (RDF) of O atoms (Ow of water, OGO of

330

GO, and OHA of HA) around U atom and their integrals (Figure S8, SI). According to

331

our calculations, the hydrated uranyl ion contained five Ow of water in its equatorial

332

plane with a U-O distance (dU-O) of 2.46 Å, which agrees with earlier experimental and

333

theoretical studies.70-78 The EXAFS measurements predict a coordination number (C.N.)

334

= 5.3 and dU-O = 2.41 Å,70 while X-ray scattering data predict a mixture of penta-

335

(dominant) and tetra-coordination and a U-O distance of 2.42 Å.71 This demonstrates

336

that MD simulations are able to predict the solvation structure of uranyl with high

337

accuracy.

338

13



Page 14 of 22

339

Table 1. Coordination Number (C.N.) in the Equatorial Plane of Uranyl, and Diffusion

340

Coefficients (D, in 10—5 cm2/s) of Uranyl, GO, HA and Water System Uranyl(aq)

C.N. C.N.

OW

5.0

5.0

D (10-5 cm2/s)

OGO

OHA

Uranyl

GO

0.69

GO(aq)

0.28

2.67 0.46

5.0

4.0

1.0

0.48

GH

H2O 2.69

HA(aq) GU

HA

0.43

2.59 2.65

0.24

0.31

2.55

GUH1

5.3

2.3

1.0

2.0

0.26

0.22

0.26

2.58

GUH2

5.0

3.6

0.6

0.8

0.32

0.12

0.10

2.42

341 342

As shown in Table 1, in GU system, one OGO and four by water molecules constituted

343

the first coordination shell of uranyl with dU-O = 2.48 Å. In the GUH1 and GUH2

344

systems, the maximum number density appeared at 2.44 and 2.46 Å respectively for the

345

first shell, similar to that of hydrated uranyl in aqueous solution. In the presence of HA

346

(GUH1), uranyl remained in a penta-coordinated conformation in its equatorial plane,

347

with two sites occupied by the carboxyl group of HA, and the average number of O

348

atoms in the first shell is 5.3. The slightly larger value might be due to the local

349

topology of HA. As seen in Figure S9 (SI), the rigid carboxyl group of HA, with limited

350

space between its two Ocarb atoms, could form a smaller Ocarb-U-Ocarb bond angle than

351

the case of water ligands. Meanwhile, it might rotate to leave more space for water to

352

access uranyl, leading to a little larger coordination number, which is consistent with

353

our previous study of HA-uranyl-CNT system.47

354

In GUH2 system, where multiple uranyl cations were present, two uranyl were fully

355

coordinated by water molecules while six were coordinated by GO and/or HA as well as

356

water molecules (see Table S4, SI). The total coordination number of each U atom of

357

uranyl remained 5.0 as in other systems. Note that, careful examination of each

358

configuration of uranyl (Figure S10, SI) indicated that, besides carboxyl oxygen, the

359

carbonyl oxygen atom of HA also accessed occasionally the first coordination shell of

360

uranyl to build direct interaction with U atom in GUH2 system, while the other polar

361

groups, i.e. phenolic hydroxyl, epoxy and aldehyde groups, were not observed to be

362

able to coordinate with uranyl during the whole simulation of 25 ns.

363 14


Page 15 of 22


364

Diffusion. The diffusion coefficients of uranyl, GO, HA, and water were calculated

365

according to the Einstein relation79 (eq.2) to understand their translational motion. The

366

data are collected in Table 1.

367 368

(2)

where

is the mean square displacement (MSD) of the molecules at time t.

369

As shown in Table 1, the diffusion coefficient of uranyl in Uranyl(aq) system was

370

calculated to be 0.69  10–5 cm2/s, which agrees with the previous studies where a value

371

of 0.6  10‑5 cm2/s was reported.77,

372

translational motion of uranyl is significantly slowed down, indicating that GO may

373

hinder the migration of uranyl in aqueous solution, which is mainly attributed to the

374

much smaller diffusion coefficient of GO (0.28  10‑5 cm2/s). In turn, the binding of

375

uranyl moderately accelerates the migration of GO, which was calculated to be 0.43 

376

10‑5 cm2/s. Comparing the HA(aq) and GH systems, the addition of GO also caused a

377

decrease of the diffusion coefficient of HA from 0.46  10‑5 to 0.31  10-5 cm2/s. As

378

mentioned above, both uranyl and HA favor to interact with GO, and they could form

379

larger aggregates that retard their respective migrations, resulting in smaller diffusion

380

coefficients of both. Similar phenomena were also observed in the GO-Uranyl-HA

381

ternary systems (GUH1 and GUH2). These results suggest that GO could significantly

382

reduce the diffusion coefficient of uranyl and HA, and hinder their migration in aqueous

383

solution.

78

In the presence of GO (GU system), the

384 385

Environmental Implications. As important carbon-based nanomaterial, GO

386

differs from graphene in its amphiphilicity and from carbon nanotubes in its roughly

387

planar curvature besides its amphiphilicity. These properties enable GO to either well

388

disperse in aqueous medium or aggregate into assemblies to adapt to the

389

microenvironment,80-83 both of which are expected to complicate the dynamics of HA

390

and uranyl in the environment. In earlier work,47 CNT was observed to affect the

391

migration of metal ions such as uranyl after being non-covalently functionalized by HA.

392

GO bares polar groups on its edges and basal plane. Its hydrophilicity may also be

393

modulated by binding with HA on its surface. Compared to CNT, according to our

394

simulations, GO may marginally decrease the binding strength of HA with uranyl due to

395

its planar curvature that unfolded HA more significantly to block the carboxylate groups 15



396

on different wings to cooperatively clamp uranyl, while offering more binding sites for

397

uranyl. This shows that the curvature of the surface may influence the binding

398

mechanism of HA with uranyl by impeding its folding, and HA/GO composites have

399

larger capacity in binding with uranyl than HA/CNT and HA systems.33, 34, 45

400

This work also has implication on the effect of rigidity of oxygenated hydrophobic

401

materials widely available in the environment on their affinity for actinides. The

402

materials with highly flexible backbone, e.g. HA, may adopt various conformations to

403

adapt to the thermodynamic conditions, e.g. pH, pressure, temperature, and the presence

404

of ions, whereas the rigid ones, e.g. GO, can hardly fold its backbone. These structure

405

features dictate a stronger binding strength of HA than GO for a single uranyl, while a

406

larger binding capacity of GO than HA. This suggests for GO and HA baring

407

comparable quantity of carboxylate groups, HA may guarantee a more complete

408

cleaning of uranyl from a dilute uranyl system; while for more concentrated uranyl

409

system, GO may achieve better performance.

410

In earlier experimental work,84 Wang et al. observed that the mixing order of the

411

HA-uranyl-GO ternary system affected the dispersity of GO in aqueous phase with

412

better dispersity for simultaneous mixing than for sequentially mixing HA and uranyl

413

with GO. According to our simulations, pre-mixing HA with GO may generate well

414

assembled HA-GO sandwich aggregates,48 which may survive from the perturbation of

415

the late-comer of uranyl. In these aggregates, the coordination of uranyl with adjacent

416

carboxylate groups of GO and HA also help to lock the layered structure. The assembly

417

of HA-GO buried not only the hydrophobic surface but also the oxygenated groups

418

distributed on the basal plane. On the other hand, the simultaneous addition of HA and

419

uranyl to GO causes the competition between HA and GO to capture uranyl, and the

420

binding with uranyl enhances the folding of HA and impedes the self-assembly of GO.

421 422

 ASSOCIATED CONTENT

423

Supporting Information

424

The Supporting Information is available free of charge on the ACS Publications website

425

at DOI:

426

Computational details, compositions of the simulated systems, interaction energy,

427

binding strength, coordination number, pKa, SASA, SFE, PMF, additional NCI

428

analysis, hydrogen bonds, RDF, representative snapshots and distributions (PDF)

429 16


Page 16 of 22

Page 17 of 22


430

 AUTHOR INFORMATION

431

Corresponding Authors

432

*E-mail: [email protected]. Phone: +86-28-85412613 (N.L.).

433

*E-mail: [email protected]. Phone: +86-10-88236606 (D.W.).

434 435

ORCID

436

Tu Lan: 0000-0002-9877-2636

437

Dongqi Wang: 0000-0001-9415-1173

438 439

Notes

440

The authors declare no competing financial interest.

441 442

 ACKNOWLEDGMENTS

443

This work was financially supported by the National Natural Science Foundation of

444

China (U1330125 to N.L.; 21876123 to J.L.; 91026000 to Z.C., 21473206 and

445

91226105 to D.W.), and the CAS Hundred Talents Program (to D.W., Y2291810S3),

446

which are gratefully acknowledged. We thank Prof. Chris Oostenbrink for instructive

447

comments. Calculations were done on the computational grids in the National

448

Supercomputing Center in Tianjin (NSCC-TJ).

449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467

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