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Environmental Processes

Exploring the Aggregation Mechanism of Graphene Oxide in the Presence of Radioactive Elements: Experimental and Theoretical Studies Yang Gao, Ke Chen, Xue Mei Ren, Ahmed Alsaedi, Tasawar Hayat, and Changlun Chen Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b02234 • Publication Date (Web): 19 Oct 2018 Downloaded from http://pubs.acs.org on October 21, 2018

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Environmental Science & Technology

Graphic Abstract Aggregation mechanism of graphene oxide in the presence of radioactive elements

ACS Paragon Plus Environment


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Exploring the Aggregation Mechanism of Graphene Oxide in the

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Presence of Radioactive Elements: Experimental and Theoretical

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Studies

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Yang Gao,a,b Ke Chen,a,b Xuemei Ren,*ac Ahmed Alsaedi,d Tasawar Hayat,d and

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Changlun Chen*acd

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aCAS

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Plasma Physics, Chinese Academy of Sciences, P.O. Box 1126, Hefei 230031, PR China

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bUniversity

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cCollaborative

Key Laboratory of Photovoltaic and Energy Conservation Materials, Institute of

of Science and Technology of China, Hefei 230026, PR China Innovation Center of Radiation Medicine of Jiangsu Higher Education

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Institutions, Soochow University, Suzhou 215123, PR China

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dNAAM

Research Group, King Abdulaziz University, Jeddah 21589, Saudi Arabia

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*Corresponding author. Tel: 86–551–65592788, Fax: +86–551–65591310

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E–mail address: [email protected] (C.L. Chen); [email protected] (X.M. Ren)

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Word counts: Main text (4500 words) + 5 big figures (4 × 600 = 2400 words) +5

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small figure (1 × 300 = 300 words) +1 table (1 × 300 = 300 words) = 7500 words.

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ABSTRACT

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In this study, the aggregation kinetics, aggregate morphology and aggregation

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mechanisms of graphene oxide (GO) in the presence of Cs+, Sr2+, UO22+, Eu3+, or Th4+

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are characterized by using time-resolved dynamic light scattering, transmission electron

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microscopy (TEM)-element mapping, re-dispersion of GO aggregate, and the density

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functional theory (DFT) calculations. The destabilization capability of Cs+, Sr2+, UO22+,

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Eu3+, and Th4+ and the corresponding values of critical coagulation concentrations (CCC)

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are obtained for the first time. Polyacrylic acid is used as a dispersant to investigate the

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reversion of GO aggregates induced by various radioactive elements. The combined

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results of polyacrylic acid effect and TEM-element mapping show that Cs+ induces the

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aggregation of GO through electric double layer suppression and weak binding with

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oxygen containing functional groups. By employing DFT calculation, we find that the

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electrostatic potential distribution and the charge transfer rather than coordination with

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oxygen containing functional groups control the destabilizing ability of radioactive

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elements with higher valence. A comprehensive process of experimental-theoretical

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studies is considered to better elucidate the colloidal behavior, self-assembly process,

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application as a novel adsorbent and environmental risk of GO.

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2



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 INTRODUCTION

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The remarkable characteristics and special structure of graphene oxide (GO) increase its

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commercial manufacture and application greatly.1,2 The preparation of GO nanosheets

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from graphite through modified Hummers’ method introduces a great many oxygen

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containing functional groups in forms of −COOH, C−O−C, and −COH on GO surfaces.

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These oxygen containing functional groups are essential for the high adsorption of

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radioactive elements (e.g., Cs+, Sr2+, UO22+, Eu3+, and Th4+).3-6 Indeed, our previous

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study showed that the maximum adsorption capacity of GO was 0.684 mmol/g for Cs+,7

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0.620 mmol/g for Sr2+,8 0.875 mmol/g for UO22+,9 0.953 mmol/g for Eu3+,7 and 0.253

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mmol/g for Th4+,10 which are higher than those of most of today’s nanomaterials. This

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will open the possible application of GO in radionuclide pollution cleanup.11 With the

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continued increase in the large scale of production and application in various fields, it is

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inevitable that GO will be either intentionally or unintentionally discharged into the

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environment with negative impact on human health and ecological security.12 The

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colloidal properties of GO play a dominant role in its application, reactivity, fate,

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transport, bioavailability, and toxicity. Therefore, a comprehensive understanding

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colloidal properties of GO is important not only for promoting its application as a novel

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adsorbent but also for more accurately evaluating its environmental fate, mobility, impact,

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and risks.13-19

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It has been found that the stability and colloidal properties of GO are affected

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significantly by the solution chemistries such as pH, ionic strength, cation type, and 3


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natural organic matter in the aquatic system.20,21 Specially, there are some reports on the

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colloid properties of GO in the presence of metal ions (i.e., Na+, K+, Mg2+, Ca2+, Ag+,

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Pb2+, Cu2+, Cd2+, and Cr3+).15,16,22 Generally speaking, the agglomeration tendency of GO

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increases in the order of Me+ < Me2+ < Me3+.23,24 It is worth noting that the equivalent

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cations such as Na+ and K+ or Mg2+ and Ca2+ show the different destabilization capability,

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which is corporately controlled by hydration shell thickness and electronegativity of these

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metal cations. Meanwhile, Yang et al.,15 reported that the heavy metal cations (i.e., Cr3+,

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Pb2+, Cu2+, Cd2+ and Ag+) destabilized the GO suspension more intensively than the

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common cations (i.e., Ca2+, Mg2+, Na+ and K+) and that the destabilizing ability of

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bivalent heavy metal cations depended on their adsorption affinity with GO. For alkali

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metal ions and alkaline earth metal ions, their effects on the transport of GO and

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sulfide-reduced GO were found to obey the Hofmeister series.25 In a word, when GO

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comes into contact with metal ions, its physiochemical properties and subsequent

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aggregation and deposition behaviors will change significantly, depending on the nature of

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metal ions and the interaction of GO with metal ions. Therefore, the environmental behavior

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and toxicity of GO after being as an adsorbent especially radionuclide carrier cannot be

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extrapolated from the available findings. Additionally, most of the previous studies mainly

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focused on the electrokinetic and hydrodynamic properties, aggregation kinetics, and

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microstructure transformation of GO through Zetasizer Nano instrument, time-resolved

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dynamic light scattering (TR-DLS) and transmission electron microscopy (TEM),

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respectively. Density functional theory (DFT) calculation is a powerful tool for 4



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describing and predicting the change in chemical and physical properties of GO after

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combining with cations. However, the use of DFT calculation to assist in illustrating the

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aggregation mechanism of GO in the presence of metal ions is scarce. Moreover, very

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little is known about how lanthanides, actinides and other fission products may

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differently affect GO colloidal properties.

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In order to get valuable information for more accurately predicating the environmental

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fate and mobility and consequently assessing the environmental impact and risks after GO is

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employed as an adsorbent for radionuclide pollution cleanup, aggregation behavior of GO

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in the presence of Cs+, Sr2+, UO22+, Eu3+, or Th4+ is fist investigated systematically in this

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study. Polyacrylic acid (PAA) can compete with GO for combining metal ions and

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despoil/desorb the adsorbed metal ions from GO surface, so it is especially employed as a

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dispersant to study its impact on GO aggregation and the re-suspension of GO aggregates.

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Additionally, the microstructure of aggregate is characterized by TEM. More importantly,

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the underlying aggregation mechanisms of GO with Cs+, Sr2+, UO22+, Eu3+, and Th4+ are

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first analyzed by using DFT calculations. Through this work, we want to explore the

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environmental behavior and risk of GO after being as radionuclide carrier.

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 MATERIALS AND METHODS

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Materials and Characterization. All the chemicals used in these experiments were of

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analytical grade. GO was synthesized from the natural flake graphite with 99.95% purity

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and an average particle diameter of 20 mm (Tianhe Graphite Co. Ltd., China) by the

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modified Hummers’ method.26 GO was characterized by X-ray diffraction and Fourier 5


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transformed infrared spectra (Figure S1 in Supporting Information). The zeta potential

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and hydrodynamic diameter (Dh) of GO (Figure S1 in Supporting Information) were

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determined by a Zetasizer Nano instrument (Malvern Instrument Co. Worcestershire, UK)

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in aqueous solutions containing Cs+, Sr2+, UO22+, Eu3+, or Th4+ over a wide range of

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concentrations. Because of the nonspherical structure of initial GO nanosheets, DLS was

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mainly used to study the aggregation process of GO. The physicochemical properties of

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the tested Cs+, Sr2+, UO22+, Eu3+, and Th4+ were shown in the Table S1. The size and

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thickness distributions of GO were measured by scanning probe microscopy (SPM). The

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surface morphology of GO aggregates was examined by TEM.

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Batch Experiments. The initial GO concentration of 20 mg/L was used in all

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experiments. The GO colloidal stability and aggregation kinetics were investigated in

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well-controlled aqueous solutions by using a batch technique. The stock suspension of

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GO, the stock solution of radionuclide analogue (i.e., Cs+, Sr2+, UO22+, Eu3+, or Th4+),

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and Milli-Q water (18.2 MΩ·cm) were added into a glass vial (CNW Technologies) to

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obtain the desired concentration of each component. Then the glass vials were placed on

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a horizontal shaker. After shaking at a constant speed of 150 rpm for 24 h, those glass

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vials were still standing on a flat surface for another 24 h to assure the complete

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sedimentation of the large GO aggregates. Figure S2 showed that 16 h was enough to the

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complete sedimentation of the large GO aggregates under our experimental conditions, so

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24 h of standing time was employed in the following experiments.

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Aggregation Kinetics Experiments. The 20 mg/L GO provided a strong DLS signal 6



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and was therefore used in all aggregation studies. Meanwhile, there is a precedent in the

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previous literatures for using this concentration,21 facilitating comparisons between the

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results obtained by different investigators. The changes of Dh as a function of radioactive

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element concentration and type were investigated by the TR-DLS to obtain the

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destabilization ability of various radioactive elements. The aggregation kinetics of GO in

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the presence of Cs+, Sr2+, UO22+, Eu3+, or Th4+ were quantified by attachment efficiency

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(α), whose detailed determination procedures were provided in Supporting Information.

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The critical coagulation concentration (CCC) values of GO for Cs+, Sr2+, UO22+, Eu3+, or

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Th4+ were obtained via the intersection of diffusion-limited and reaction-limited fitting

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lines. The detailed coagulation kinetics analysis processes were summarized in

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Supporting Information.

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DFT Calculations. In the optimized calculation, the hybrid functional B3LYP27 in

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conjunction with 6-31G (d)28 basis set was used for H, C, and O. The quasi-relativistic

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pseudo-potential LANL2DZ basis set29 was employed for Cs as well as Sr. The

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Stuttgart-Dresden-Dunning basis set and the accompanying effective core potentials were

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used for metal atoms of Eu (MWB52),30 Th (MWB60),31 and U (MWB60).31 The integral

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equation formalism model (IEFPCM) was used to simulate the solvation effects.32 The

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involved solvent was water unless otherwise mentioned. During the optimization, all

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degrees of freedom were allowed to relax. Energy of all optimized configurations was

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evaluated by bigger Def2-TZVP basis set33 for H, C, and O atoms. To adequately

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characterize the binding, dispersion correction was included in the calculations by the 7


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DFT-D3 approach of Grimme.34,35 All optimizations of electronic and geometrical

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structures were performed by Gaussian 16 suite of program.36 The Hirshfeld charge37,38

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analysis was adopted to evaluate changes in the electron density after Cs+, Sr2+, UO22+,

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Eu3+, or Th4+ adsorption. To further clarify the interaction between selected cation and

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GO, electrostatic potential (ESP) was analyzed and reduced density gradient (RDG)

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method was carried out. The ESP mapped van der Waals (vdW) surface and the RDG

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isosurfaces were obtained by using VMD 1.9.1 program39 based on the outputs from the

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Multiwfn program.40

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 RESULTS AND DISCUSSION

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Effect of the Valence and Concentration of Radioactive Elements on GO

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Aggregation. Figure 1A directly presents a visual proof of the GO aggregation

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phenomenon after standing for 24 h as a function of the concentration and valence of

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radioactive elements. One can see that GO can be well-dispersed in the absence and

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presence of the low concentration of Cs+, Sr2+, UO22+, Eu3+, or Th4+. When their

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concentrations reach the critical values, i.e., 12.0 mmol/L of Cs+, 0.3 mmol/L of Sr2+, 0.2

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mmol/L of UO22+, 2.0×10−2 mmol/L of Eu3+, and 5.0×10−3 mmol/L of Th4+, visible

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aggregation appears. The bulk aggregation phenomena are accompanied by the

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corresponding variations of the zeta potential and Dh as a function of the concentration

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and type of Cs+, Sr2+, UO22+, Eu3+, or Th4+ (Figures S3 in Supporting Information).

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Figure S3A, C, E, G, and I shows that GO displays the typical electrokinetic behavior,

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namely, the values of zeta potential increase with increasing the concentration of 8



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radioactive elements. And the radioactive element with higher valence is more effective

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in changing the surface zeta potential of GO. While, the values of Dh almost keep

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unchanging until the nuclide concentration increases to a certain threshold value, which

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results in a very steep transition in this curve as the radioactive element concentration

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progressively increases (Figure S3B, D, F, H, and J). These observations prove that both

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the concentration and type of radioactive elements are pivotal factors controlling the

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aggregation process of GO, thus influencing application of GO as a novel adsorbent for

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radioactive element cleanup in aqueous solutions. As shown in Figure S3, the changes in

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particle size do not seem to correlate well to those in the surface zeta potential, indicating

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that the electrostatic interaction is not only mechanism controlling GO stability in the

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presence of Cs+, Sr2+, UO22+, Eu3+, or Th4+.41

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In order to further evaluate the stability of GO in the presence of radioactive elements

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quantitatively, the aggregation kinetics of GO are determined by TR-DLS. The typical

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aggregation kinetics profiles of GO as a function of the concentration of Cs+, Sr2+, UO22+,

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Eu3+, or Th4+ are presented in Figure S4A-E in Supporting Information. The aggregation

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rate accelerates with increasing the radioactive element concentration due to the

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enhancement of charge screening. The α and CCC values of GO as a function of

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radioactive element concentration are presented in Figure 1B. The distinct

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reaction-limited and diffusion-limited regimes can be observed, indicating that GO

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aggregation

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reaction-limited region, the values of α increase quickly from 10-3 to 1 as the

follows

the

Derjaguin−Landau−Verwey−Overbeek

9


theory.

At

the

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concentrations of radioactive elements increase. At the diffusion-limited region, where

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the values of α are equal to 1 and independent of the concentrations of radioactive

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elements, the electrostatic repulsion between GO is suppressed completely and the

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aggregation rate reaches the maximum. The concentration ranges before and after the

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CCC are reaction-limited and diffusion-limited regions, representing different

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aggregation processes. Once the concentration of radioactive element reaches the CCC,

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the thermal diffusion process is the primary restriction for GO aggregation rather than the

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energy-barrier repulsion. As shown in Figure 1B, the CCC values of GO are 21.2 mmol/L

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of Cs+, 0.6 mmol/L of Sr2+, 3.7×10−1 mmol/L of UO22+, 3.3×10−2 mmol/L of Eu3+, and

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6.3×10−3 mmol/L of Th4+. The CCC values of GO for radioactive elements are firstly

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reported in our current study. For cations with equal valence, the CCC values of GO for

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radioactive elements are obviously lower than those of GO for usual cations, i.e., Cs+

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(21.2 mmol/L) < K+ (28.0 mmol/L) < Na+ (36.0 mmol/L), UO22+ (3.7×10−1 mmol/L)
UO22+ > Sr2+ > Cs+). Therefore,

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charge screening contributes to the GO aggregation. Theoretically, the equivalent 10



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radioactive elements should induce the similar charge screening effect and have the

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similar destabilization capability.23,24 However, the CCC values of GO for Sr2+ and UO22+

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are distinct. Meanwhile, the overall Dh values of GO aggregates in the UO22+ solution are

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bigger than those in the Sr2+ solution (Figure S3D and F). UO22+ is more efficient in

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destabilizing GO than Sr2+, which indicates that the aggregation behavior of GO in the

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equivalent radioactive elements is not only charge screening but additional mechanisms

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are involved.15,24 The different binding energy of radioactive elements with oxygen

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containing functional groups of GO (10.2−50.5 kcal/mol for UO22+ and 3.3−14.6

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kcal/mol for Sr2+) may be the cause.8,42 Further evidences for the addition mechanisms

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are investigated in the following part.

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PAA Effect on the GO Aggregation and on the Re-dispersion of GO Aggregates.

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PAA rich in carboxylic groups (−COOH) can combine with metal cation strongly,

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forming stable compound.43,44 Accordingly, PAA is employed as a dispersant to study its

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impact on GO aggregation process and the re-suspension of GO aggregate for

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differentiating the aggregation mechanism. The effect of different concentration of PAA

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on the aggregation kinetics of GO is shown in the Figure S5A-E, where GO, radioactive

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elements and PAA are simultaneously added. The used concentration of radioactive

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elements is CCC at which the α value equals to 1 (22.0 mmol/L of Cs+, 0.6 mmol/L of

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Sr2+, 0.4 mmol/L of UO22+, 3.3×10−2 mmol/L of Eu3+, and 6.3×10−3 mmol/L of Th4+)

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(Figure 1B). The impact of PAA concentration on the value of α is shown in Figure 1C.

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We can see that the values of α decrease as the PAA concentration increases. Since PAA 11


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has specific interaction with radioactive elements and can compete with GO for binding

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with radioactive elements, the increase of PAA concentration decreases the effectiveness

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of radioactive elements in destabilizing GO. The concentration of PAA that makes α

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value of GO be 3.0×10−3 is 10 mg/L for Cs+, 117 mg/L for Th4+, 150 mg/L for UO22+,

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200 mg/L for Sr2+, and 250 mg/L for Eu3+. That is the effect of PAA on the destabilizing

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ability of radioactive elements decreases in the order of Cs+ > Th4+ > UO22+ > Sr2+ > Eu3+.

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The more PAA concentrations are needed, the stronger the binding affinities of

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radioactive elements with GO are. So we can infer that the binding affinities of

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radioactive elements with GO decrease in order of Cs+ < Th4+ < UO22+< Sr2+ < Eu3+,

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which is not accordance with the destabilizing ability of radioactive elements. This

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suggests coordination between radioactive elements with oxygen containing functional

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groups on GO surface does not play the dominant role in the aggregation of GO. In our

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previous study, we found that the impact of PAA on common environmental cation

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followed the order of Na+ ≈ K+ < Al3+ < Ca2+ < Mg2+ and the GO aggregates in the Na+

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and K+ solution are hard to be re-dispersed into solution.22 Accordingly, we deduce that

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Cs+ and Na+/K+ induce the GO aggregation via different mechanisms. Since there is no

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specific interactions between Na+/K+ with the functional groups of GO,20 Na+/K+ induces

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GO aggregation via electrostatic double layer (EDL) suppression.

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In order to further evaluate the effect of the binding affinity on the aggregation process,

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the long-term impact of PAA on GO sedimentation and on the re-dispersion of GO

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aggregates is also investigated. Figure 2A shows the impact of the concentration of PAA 12



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on the sedimentation process, where PAA, radioactive elements and GO are added at the

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same time. We can see that when the PAA concentration is higher than a critical

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threshold value, GO can be completely stabilized in solution containing radioactive

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elements even aging for one month. The amounts of PAA needed to keep the one month

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stability of GO in solution containing radioactive elements increase in the order of Th4+

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