Adsorption, Aggregation, and Deposition Behaviors of Carbon Dots on

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The Adsorption, Aggregation and Deposition Behaviors of Carbon Dots on Minerals Xia Liu, Jia-Xing Li, Yong-Shun Huang, Xiangxue Wang, Xiaodong Zhang, and Xiangke Wang Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 02 May 2017 Downloaded from http://pubs.acs.org on May 2, 2017

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Environmental Science & Technology

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The Adsorption, Aggregation and Deposition Behaviors of Carbon Dots on

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Minerals

3

Xia Liua,b,c, Jiaxing Lia,b,d*, Yongshun Huangb, Xiangxue Wanga, Xiaodong Zhangb,

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Xiangke Wanga,b,d*

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a

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University, Beijing 102206, P. R. China

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b

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230031, P.R. China

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c

University of Science and Technology of China, Hefei, 230026, P.R. China

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d

Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education

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Institutions, P.R. China. Soochow University, Suzhou 215123, P. R. China

College of Environmental Science and Engineering, North China Electric Power

Institute of Plasma Physics, Chinese Academy of Sciences, P.O. Box 1126, Hefei,

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1

ACS Paragon Plus Environment


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ABSTRACT

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The increased production of CDs and the release and accumulation of CDs in both

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surface and groundwater has resulted in the increasing interest in research on carbon

17

dots (CDs). To assess the environmental behavior of CDs, the interaction between

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CDs and goethite was studied under different environmental conditions.

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Electrokinetic characterization of CDs suggested that the zeta potential and size

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distribution of CDs were affected by pH and electrolyte species, indicating that these

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factors influenced the stability of CDs in aqueous solutions. Traditional

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Derjaguin-Landau-Verwey-Overbeek (DLVO) theory did not fit well the aggregation

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process of CDs. Results of the effects of pH and ionic strength suggested that

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electronic attraction dominated CDs aggregation. Compared with other minerals,

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hydrogen-bonding interactions and Lewis acid-base interactions contributed to CDs

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aggregation, in addition to van der Waals and electrical double-layer forces.

27

Adsorption isotherms and microscopic Fourier transformed infrared (FTIR)

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spectroscopy indicated that chemical bonds were formed between CDs and goethite.

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These findings are useful to understand the interaction of CDs with minerals, as well

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as the potential fate and toxicity of CDs in the natural environment, especially in soils

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and sediments.

32 33

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■ INTRODUCTION

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Many types of carbon nanomaterials, such as carbon nanotubes, graphene and

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fullerene, have been produced and extensively used in a diverse array of

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applications.1-3 Carbon dots (CDs), especially graphene CDs, have shown great

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potential in many applications ranging from chemistry-related materials to

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biomedicines including bioimaging, printing ink, photocatalysis and sensors.4-7 The

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rapidly increasing production and use of CDs increases the possibility of their

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environmental release and transported between environmental media.7-12 For example,

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Havrdova et al. 7 presented a comprehensive cytotoxic study of CDs with different

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functional groups. They found that positively charged CDs can cause significant

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changes in cell morphology and exhibit the highest toxicity, as reflected in the lowest

45

half maximal inhibitory concentration value. The toxicity effect on the embryo

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development, and bio-distribution of CDs in zebrafish were clearly illustrated in

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Kang’s work.8 Therefore, understanding the fate, transport, and negative

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environmental effects of CDs is of significant importance.

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Recently, a few studies have examined the environmental behavior and toxicology

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of quantum dots.9,10 Li et al. 10 investigated the aggregation kinetics and self-assembly

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mechanisms of graphene CDs in aqueous solutions. The complex influences of pH

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and cation valence on graphene CDs aggregation were investigated, and a three-step

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mechanism of self-assembly that involved aggregation was proposed for the first

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time.9 Hardman gave a toxicologic review of quantum dots. Nurunnabi et al.11

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reported the biodistribution and toxicology of carboxylated graphene CDs. However, 3



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almost no reports focused on the environmental behavior (i.e., aggregation, deposition

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and transport) of CDs on natural mineral particles. As a novel quantum dot material, it

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is necessary to understand the environmental behavior of CDs.

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Minerals with different charges and geometric dimensions, such as goethite,

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kaolinite, montmorillonite, and attapulgite, are important components of sediments

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and soils.13-19 Goethite, as the most thermodynamically stable iron oxyhydroxide

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mineral, is the most widespread form of iron oxide in the natural environment.13-16

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Kaolinite is also one of the most abundant clay minerals in most soils and consists of

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silica tetrahedral sheets connected via apical oxygen atoms to aluminum dioctahedral

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sheets.17 Montmorillonite has a 2:1 layered structure that can incorporate many types

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of metal cations between its layers to offset the surplus negative charge and is also a

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typical mineral in the natural environment.18 Attapulgite is a hydrated magnesium

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aluminum silicate present in nature as a fibrillar mineral.19 Because of the ubiquity of

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these minerals in the environment, they have high potential to interact with other

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colloids (e.g., engineered nanoparticles, metal ions, or dissolved organic molecules)

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and could serve as carriers of colloids, affecting their distribution in the natural

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environment. In recent years, carbon nanomaterials have received increased attention

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in many fields and applications because of their unique structures and exceptional

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physical and chemical properties.20-22 As such, the aforementioned minerals have been

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reported to play significant roles in interaction of carbon nanomaterials in the

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environment. Van der Waals and electrostatic forces likely influence the association

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between multi-walled carbon nanotubes and three soil minerals, montmorillonite, 4


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kaolinite, and goethite suspension.20 The heteroaggregation of graphene oxide (GO)

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with different minerals, including montmorillonite, kaolinite, and goethite, in aqueous

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solution revealed that electrostatic attraction was critical in the heteroaggregation

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between GO and positively charged minerals.21 The size of nanomaterials greatly

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influences their environmental migration and their interactions with environmental

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media.5 With promising potential for many applications, CDs are smaller in size than

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other carbon materials, and thus, would be probably more easily released into the

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environment and transported between minerals.

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In this study, four common minerals (montmorillonite, kaolinite, attapulgite and

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goethite) with different surface charge properties and geometric dimensions were

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employed to investigate their interactions with CDs. A convenient, one-step

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hydrothermal method was used to synthesize CDs, and the stability of CDs in aqueous

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solution was examined by varying the storage time, pH and ionic strength. Batch

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experiments were used to study the interaction between CDs and goethite. The

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interaction mechanism was investigated by SEM-EDS and microscopic FT-IR.

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■ MATERIALS AND METHODS

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Materials. All solutions were prepared with Milli-Q water. Commercially available

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Montmorillonite (97% purity, Zhejiang Sanding Group Co., Ltd., China) was used.

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Kaolinite was obtained from Ward’s Natural Science (China clay, item 46-0005, d

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(0.5):0.377 µm). Attapulgite was purchased from Sinopharm Chemical Reagent Co.,

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Ltd. (Shanghai, China), and goethite was purchased from Sigma-Aldrich (America).

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Synthesis of CDs. CDs were synthesized from sodium citrate and ammonium 5



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bicarbonate using a simple, one-step hydrothermal method.22 Specifically, sodium

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citrate (0.2 g) and NH4HCO3 (1.5 g) (Sinopharm Chemical Reagent Co., Ltd.,

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Shanghai, China) in water (30 mL) were sealed in a Teflon-lined stainless-steel

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autoclave and then placed in a drying oven (DHG-9038, Shanghai Jinghong

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Laboratory Instrument Co., Ltd), followed by hydrothermal treatment at 180 °C for 5h.

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CDs were purified using a dialysis tube (500 Da molecular weight cutoffs) for 24h.

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The concentration of the purified CDs was measured by lyophilizing aliquots of the

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samples and was calculated to be 0.45 g/L.

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Characterization. The zeta potential and size of the samples were measured at 25 °C

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using a Zetasizer Nanosizer ZS instrument (Malvern Instrument Co., UK). The SEM

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images were recorded on a field-emission scanning electron microscope (FEI Quanta

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200 FEG SEM). Transmission electron microscopy (TEM) images were obtained on a

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JEOL-2010 transmission electron microscope operated at an acceleration voltage of

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200 kV. X-ray photoelectron spectra (XPS) of CDs were collected on an X-ray

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photoelectron spectrometer (ESCALAB250Xi, Thermo Scientific). The powder X-ray

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diffraction patterns (XRD) were taken on a Philips X’Pert X-ray diffractometer using

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Cu Kα radiation (λ=0.15406 nm). A Shimadzu UV-2550 spectrophotometer was used

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to obtain ultraviolet-visible (UV-vis) absorption spectra of CDs. Fourier transform

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infrared (FT-IR) spectroscopy was performed, acquiring spectra at 8 cm-1 resolution

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under N2 purge.

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Adsorption of CDs by Minerals. The early stage of heteroaggregation involves

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adsorption processes, which have a strong or determining influence on the 6


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aggregation efficiencies and properties. The adsorption of CDs on the surface of

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goethite was examined in batch experiments in which stock solutions of mineral. CDs

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suspensions were added to polyethylene test tubes to achieve the desired

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concentrations of the different components. A negligible amount of highly

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concentrated HCl or NaOH solution was added to the suspensions to adjust the pH to

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the desired value. Then, the tubes were placed in an oscillator and shaken for 24 h.

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Next, the minerals were separated completely from water by centrifugation (9000 rpm,

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10 min). The concentration of CDs was determined from a standard curve using

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UV-vis spectrophotometer. Three calibration curves at pH 3.00, 7.45 and 10.0 were

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used because pH influenced its spectrum absorption slightly (Figure S1).

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Mineral particles may release metal ions and alters the concentration detection of

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CDs,21 and thus the dissolution of mineral particles at pH 3.00, 7.45 and 10.0 was

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studied. The detailed methods and results were shown in the supporting information

135

(Figure S2 and S3). Concentrations of metal ions in filtrates were determined using

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inductively coupled plasma (ICP-6300, Thermo Fisher Scientific). The results

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indicated that the released metal ions of minerals did not disturb the concentration

138

detection of CDs.

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The adsorption of CDs on minerals can be calculated from the difference between

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the initial concentration C0 (mg/L) and the equilibrium concentration Ce (mg/L). The

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adsorption capacity was expressed in terms of adsorption percentage (%), which can

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be calculated from the equation: Adsorption% = (C0 − Ce ) / C0 × 100% .23 The adsorption

143

experiments at a fixed equilibrium pH were run in duplicate, while the zeta potential 7



144

analyses were run in triplicate. All the experimental data were the averages of

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duplicate or triplicate determinations. Error bars represent standard deviation.

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Deposition and Transport Experiments. Four kinds of minerals (kaolinite,

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montmorillonite, attapulgite and goethite) were used in the deposition and transport

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experiments. The intensity of CDs after deposition and transport experiments were

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determined by UV-vis spectrophotometer. The processes are detailed in the supporting

150

information (SI).

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■ RESULTS AND DISCUSSION

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Characterization of CDs and Minerals.

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Spectroscopic and Morphological Analysis of CDs and Minerals. The XRD

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pattern of CDs (Figure 1A) presented a broad peak centered at 22° (0.20 nm), which

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was attributed to highly disordered carbon atoms.24 The functional groups on CDs

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were examined using XPS. Three different types of carbon atoms, including aliphatic

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(C=C, C-C), oxygenated (C-O/C=O) and nitrogenated (C-N) (Figure 1B), were

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observed.25 Detailed information on the content is given in Figure S4. An absorption

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peak at 331 nm was detected in the UV-vis absorption spectrum (Figure 1C). The

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photoluminescent (PL) spectrum displayed an emission maximum at 440 nm for

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excitation at 371 nm, which is characteristic of CDs (Figure 1D).26 Morphological and

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structural analysis of CDs by TEM revealed a uniform dispersion without obvious

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aggregation and a mean diameter of 5.0 nm (1.9-7.5 nm) (Figure 1E),22 and a size

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distribution histogram for CDs was shown in Figure 1E.

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The morphologies of the four minerals were characterized by SEM, as different 8


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minerals have different structures, which may affect their adsorption capacity (Figure

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S5). Natural attapulgite presented long nanorods with uniform sizes of approximately

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2 µm and short rods with sizes of only 0.4–0.6 µm.27 Montmorillonite has a lamellar

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structure with partial aggregation. Kaolinite is rounded or pillow-like with a slightly

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hexagon-like structural arrangement and a non-regular, modular appearance.28

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Goethite possesses a rod-shaped structure with lengths of approximately 200–700 nm

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and aggregation.29

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Contact time, CDs concentration and pH were examined for the stabilities of

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CDs under environmental conditions (Figure S6A to D). Previous studies have shown

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that other carbon-based nanomaterials, such as carbon nanotubes and GO, have

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different stabilities at various pH values. Therefore, the stability of CDs in aqueous

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solution may also be dependent on the pH (Figure S6C and D). A red shift from 331

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nm to 346 nm in the characteristic absorption peak was found in the UV-vis spectrum

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from pH 2.26 to 10.52, due to the n–p* transition of the C=O band and the p–p*

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transition of the conjugate C=C band.30

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Electrokinetic Characterization.

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Zeta Potentials. The stability of suspended particles, determined from their

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deposition and aggregation behavior, is expected to increase with increasing absolute

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zeta potential.31-34 To study the influence of the physicochemical properties of CDs on

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their deposition and aggregation behavior, the zeta potentials of CDs were measured

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as a function of the electrolyte species and concentration.33 The zeta potentials of CDs

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were negative over the entire range of solution chemistries examined (Figure 2A). 9



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This negative surface charge was caused by the presence of -OH and -COOH

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functional groups on the surface of CDs.25,32 CDs particles became less negative with

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increasing concentration of either NaCl or MgCl2 because of compression effects

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from the diffuse double layer of ions at the particle surface. Notably, the absolute zeta

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potential value of the divalent cation (Mg2+) was less than that of the monovalent

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electrolyte (Na+) at the same pH and ion concentration. Thus, divalent cations affect

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CDs potential more strongly than the ionic strength, which may be related to charge

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neutralization of the divalent cations. This result is consistent with previous studies on

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related nanoparticls.32,35

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The zeta potentials of the minerals were also measured at different pH. Figure 2B

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graphically illustrates the measured zeta potentials of the different minerals at various

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pH values. The zeta potentials of both CDs and minerals decrease with increasing pH,

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suggesting that the suspended particles were more stabilized at increased pH.36

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Goethite, the point of zero charge (pHpzc) of approximately 6.3, was selected to

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undergo comprehensive investigation because of its positive surface charge under

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acidic and neutral conditions.

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Particle Size. The average size distributions of CDs in the different solution

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chemistries were determined from dynamic light scatter (DLS) measurements (Figure

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2C). The size of CDs increased with the electrolyte concentration, and the impact of

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the divalent cations (Ca2+) was higher than that of the monovalent electrolyte (Na+).

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When CDs are suspended in NaCl solution (pH 8.0), their hydrodynamic diameters

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(Dh) are measured between 170 and 230 nm at an ionic strength of 16 mM, and they 10


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can reach equilibrium within 70 min. However, for CDs suspended in CaCl2 solution,

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a clearly faster aggregation is observed with increasing CaCl2 concentration because

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the charge screening from divalent ions is higher than that from monovalent ions.33,34

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Dh are measured between 1200 and 1700 nm when CaCl2 concentration is 16 mM that

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may be over the critical coagulation concentration values according to Schulze−Hardy

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rule.36-38

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Adsorption of CDs on Minerals.

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Figure 3A presents Dh of goethite particles at three pH values (3.00, 7.45, and 10.0)

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as a function of time. Dh of the goethite particles remained almost constant (300 nm)

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over time at pH 10.0 and increased slowly from 350 to 400 nm at pH 7.45. However,

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the size of goethite particles increased obviously from 500 to 750 nm at pH 3.00 after

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20 min. Dh tended to remain constant at the three pH values after equilibrium. These

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results clearly indicate that the aggregation of goethite is strongly dependent on

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pH.39,40 Figure 3C (state I to II) vividly illustrates the transition from instability to

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stability for the goethite suspension. Since the zeta potential of the goethite

225

suspension is approximately +22 mV at pH 3.00, the surface of goethite was

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positively charged at this pH (Figure 2B). The commonly used threshold for the

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absolute zeta potential of a stable colloidal suspension is >30 mV.34-35 Aggregates may

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be formed because their surface potential cannot provide enough electrostatic

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repulsion between the particles to prevent aggregation.41-43 Therefore, the suspension

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is unstable and tends to be stabilized by goethite aggregations, leading to an increase

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in the Dh upon extension of the shaking time. At pH 7.45, the zeta potential of the 11



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goethite suspension was approximately -20 mV, and thus, the surface of goethite was

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negatively charged at this pH. Aggregates may also form because the existing

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attractive forces may be stronger than the repulsive forces, which is similar to the

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result obtained at pH 3.00. The difference in the Dh at pH 3.00 and 7.45 can be

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ascribed to the characteristic interaction forces. While at pH 10.0, the zeta potential of

237

the goethite suspension was -38 mV which absolute value was over 30 mV, the

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suspension was stable for the strong electrostatic repulsion between the particles

239

resulting in the nearly invariable Dh values. This process is also presented in Figure

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3C (state I to II). The aggregation process from state I to II can be called

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homoaggregation which was also observed in the literature for cerium oxide

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nanoparticles.44

243

Dh of goethite after interaction with CDs (CDs-Goethite) was also measured

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(Figure 3B). An obvious decrease in Dh from 1100 to 600 nm was observed at pH

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3.00, while Dh decreased slightly from 600 to 510 nm at pH 7.45 and 10.0. The results

246

indicated that the stability of goethite suspension was destroyed after CDs was added,

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as shown in Figure 3C (state II to III). From pH 3.00 to 10.0, all of the zeta potentials

248

of CDs were negative while the goethite surface was positively charged at pH 3.00,

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leading to form strong electrostatic attraction between CDs and goethite. Some CDs

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were adsorbed onto the goethite surface at the beginning of the process, and then

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heteroaggregates were formed between CDs and goethite, as illustrated in Figure 3C

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(state III to IV) at pH 3.00. The aggregates became larger over time and then began to

253

deposit on the bottom of the sample pool until an equilibrium state was achieved 12


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(state III to IV). Therefore Dh of particles left in the suspension decreased at the

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beginning and tended to remain constant finally. Although the interaction between

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goethite and CDs was due to electrostatic repulsion at pH 7.45 and 10.0, Dh still

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slightly decreased, indicating that both aggregation and deposition caused by other

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interaction forces occurred in this process.

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Figure 4A displays the improved adsorption capacity (qe) of CDs on goethite at

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pH 3.00 compared with that at pH 7.45 and 10.0. qe was calculated from

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qe=(C0-Ce)×V/m, where C0 (mg/L) is the initial concentration and Ce (mg/L) is CDs

262

concentration after sorption. V (mL) is the volume of the suspension, and m (g) is the

263

mass of minerals. According to the zeta potential, the surface of goethite is positively

264

charged at pHpHpzc, the surface charge becomes negative, and goethite

266

becomes repulsive to CDs via deprotonation of its surface hydroxyl groups. Therefore

267

the adsorption capacity of CDs on goethite decreased with the increase of pH due to

268

the electrostatic interactions.45-49 To further confirm that, the adsorption capacities of

269

CDs on attapulgite, montmorillonite and kaolinite at pH 3.00, 7.45 and 10.0 were also

270

investigated. Similar results were displayed to that of goethite, the hierarchical order

271

of adsorption performance was pH 3.00>7.45>10.0. Notably, some CDs can still be

272

adsorbed on the minerals though both CDs and the surface of minerals became

273

negative at pH 10.0.

274 275

In

Derjaguin-Landau-Verwey-Overbeek

(DLVO)

theory,

the

stability

of

nanoparticles suspended in an aqueous environment can be evaluated as the sum of 13



276

the van der Waals (VDW) and electrical double-layer (EDL) interactions.45-47 The

277

resultant interaction energy (VT), the sum of the VDW and EDL interactions,

278

determines the particle stability as two surfaces approach each other. Based on the

279

values of the VDW force and EDL force, it is clear that the EDL force is the dominate

280

factor in the adsorption of CDs on goethite. At pH>pHpzc, the surface of both CDs and

281

goethite are negatively charged, and the EDL force is electrostatic repulsion. However,

282

goethite can still adsorb CDs (5.1 and 0.45 mg/g at pH 7.45 and 10.0). Therefore,

283

traditional DLVO theory is not suitable for describing CDs aggregation on goethite.39

284

According to Dušak’s work, besides the electrostatic repulsion, the adsorption of

285

CDs onto goethite was also affected by the chemical interactions originating from

286

covalent bonding, hydrogen-bonding, or Lewis acid-base interactions.50 To investigate

287

this interaction force, the influence of ionic strength on CDs adsorption onto goethite

288

at different NaCl and MgCl2 concentrations were measured at three pH values (Figure

289

4B). The adsorption capacity increased with the increase of ionic strength because the

290

increase of cation concentration compressed the double-layer thickness and thereby

291

reduced the double-layer repulsion between the CDs and goethite surface. At higher

292

Na+ concentration, the more accumulation of hydrated Na+ ions on the mineral surface,

293

the more CDs adsorption on goethite.51,52 It is worth mentioning that the surfaces of

294

both CDs and goethite are negatively charged at pH>pHpzc and thereby shows

295

repulsive electrostatic interactions. However, a small amount of CDs adsorption on

296

goethite was still observed at pH 7.45 and 10.0, suggesting that hydrogen-bonding

297

interactions may be formed between the hydroxyl groups on the goethite particle 14


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surface and the oxygen-containing functional groups of CDs. 53,54 This result was

299

consistent with those obtained for other carbon materials, such as GO and CNTs.2,35

300

Moreover, Lewis acid-base interactions may also be formed.55 The hydroxyl groups

301

on the goethite surface can be viewed as Lewis bases, while carboxyl groups on CDs

302

can be regarded as Lewis acids. Both Lewis acid-base and hydrogen-bonding

303

interactions may overcome the charge repulsion, which leads to the goethite surface

304

still to adsorb some CDs at pH>pHpzc. In addition, overcoming the charge repulsion

305

becomes increasingly difficult as the pH increases. Therefore, the hierarchical order of

306

adsorption performance is pH 3.00>7.45>10.0.

307

Figure 4B shows that the adsorption performance was higher in Mg2+ solution than

308

in Na+ solution, which can be explained by the higher valence of the bivalent cation

309

(Mg2+) better neutralizing CDs surface charge compared with monovalent cation (Na+)

310

at the same concentration.56,57 It is consistent with the results that the presence of

311

Mg2+ was much more effective in increasing the zeta potential of CDs (Figure 2A).

312

The Langmuir model (qe=bqmaxCe/(1+bCe)) assumes monolayer attachment, and

313

linear models are frequently employed to approximate adsorption data for soils and

314

sediments. Freundlich isotherms can result from the overlapping of several Langmuir

315

isotherms (combination of Langmuir isotherms) and describe the adsorption onto

316

heterogeneous sorbents with surfaces that contain several different sites.49 The

317

experimental data from the study of CDs equilibrium attachment onto goethite at three

318

different pH values were fitted with the Freundlich isotherm and linear isotherm

319

models (Figure 5A). The corresponding isotherm parameters are listed in Table S1. 15



320

The Freundlich isotherm (qe=KFCen) gives a nonlinear relationship between the

321

aqueous-phase CDs concentration at equilibrium and CDs concentration adsorbed

322

onto the mineral at equilibrium,58 where Ce (mg/L) is the equilibrium concentration of

323

CDs in aqueous solution and qe (mg/g) is the amount of CDs adsorbed on the mineral.

324

The R2 values of the linear models were generally higher than those of the Freundlich

325

models, suggesting a more linear response. An apparent intercept was observed on the

326

vertical axis after fitting by the linear model, indicating that this fraction of CDs was

327

unable to desorb from goethite (irreversible adsorption).21 This result suggests the

328

existence of chemisorption during this process.

329

Characterization of CDs after Adsorption on Goethite.

330

The morphology of CDs−Goethite was characterized by SEM (Figure S7). Many

331

particles were observed on the surface, indicating the successful aggregation of CDs

332

on goethite. Moreover, carbon was detected by EDS, which further corroborates the

333

adsorption of CDs on goethite. It should be noted that Cu was the base material, Pt

334

was plated on the surface, and O, Fe, and Si were the elements in goethite.

335

CDs–Goethite complexes were further analyzed using microscopic FT-IR

336

spectroscopy (Figure 5B). Compared with pristine goethite, a new band appeared at

337

approximately 1400 cm-1 at all three pH values, which was caused by the

338

complexation between the COO- groups of CDs and the Fe in goethite, based on the

339

fact that a strong absorption band at 1400 cm-1 was observed to appear when organic

340

matter interacted with Fe to form complexes with goethite.42,43,59 Noticeably, after

341

CDs adsorption, new peaks appeared at 3036 cm-1 and 3160 cm-1, corresponding to 16


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aliphatic C-H stretching, at pH 3.00 and 7.45, suggesting the sorption of CDs on

343

goethite. At pH 10.0, weak peaks were detected by FT-IR probably due to the low

344

amount of detected CDs aggregation. Hence, we suggest that electrostatic attraction or

345

the outer-sphere adsorption of CDs on the goethite surface occurs first, followed by

346

chemical bonding and inner-sphere complexation between the carboxylic groups of

347

CDs on the surface. The peak at 2793 cm-1 was assigned to O-H vibrations, revealing

348

the complexation between the goethite surfaces and the C-O functional groups of CDs

349

and showing a contribution from hydrogen bonding.

350

■ ENVIRONMENTAL IMPLICATIONS

351

After studying the aggregation kinetics and mechanism, the fate of CDs

352

undergoing aggregation, deposition and transport in the subsurface environment were

353

simulated

354

montmorillonite, attapulgite and goethite) were used, and pH 7.45 was chosen

355

because most subsurface environments are approximately neutral. Figure 6A and B

356

illustrates the process of CDs aggregation and deposition on the minerals, where

357

sample 1, 2, 3 and 4 contained the same concentration of kaolinite, montmorillonite,

358

attapulgite and goethite, respectively, and as well as the same CDs concentration.

359

After settling for 48h, the mixed solution became clear, and the minerals and some of

360

CDs had deposited on the bottom of the bottle (Figure 6B). UV-vis absorption

361

spectrum was used to compare the different effects among the four kinds of minerals.

362

The results (Figure S8A) showed that goethite had the most influence on CDs

363

aggregation and deposition and that the difference among the other minerals was not

by

the

model

experiment.

Four

common

17


minerals

(kaolinite,


364

obvious. Figure 6C models the migration and penetration of CDs on the minerals

365

through a packed-bed column. The order from 1 to 4 refers to kaolinite,

366

montmorillonite, attapulgite and goethite, respectively. The color of CDs on top of the

367

minerals was close to orange, and after migration and penetration, the color lightened

368

(Figure 6D and E). The UV-vis absorption spectra of CDs were shown in Figure S8B.

369

The enhanced interaction between CDs and goethite compared to the other three

370

minerals is also shown by the aggregation and deposition experiments. According to

371

the above results, the aggregation kinetics and mechanism matches the behavior of

372

CDs in water environments. It will provide a guide to understand the interaction

373

between CDs particles and minerals in rivers and lakes.40

374

The extensive applications of CDs make their release into the environment

375

inevitable. Understanding the stability of CDs in the environment, as well as the

376

interactions between CDs and minerals, is important for predicting the fate of CDs in

377

environmental matrices. In this work, CDs were successfully synthesized using a

378

convenient, one-step hydrothermal method, followed by spectroscopic and

379

morphological analyses. Electrokinetic characterization of CDs suggested that pH,

380

electrolyte species and concentration influence the zeta potential and size of CDs,

381

thereby influences the stability of CDs. The adsorption capacities were studied by

382

examining the pH, ionic strength and adsorption isotherms. CDs were also

383

characterized after their aggregation on goethite. The results suggested that traditional

384

DLVO theory does not adequately describe the process of CDs aggregation and

385

deposition. The effect of pH and ionic strength suggested that electronic attraction 18


Page 18 of 34

Page 19 of 34


386

was dominant in these processes. In contrast to other mentioned minerals and oxides,

387

hydrogen-bonding and Lewis acid-base interactions can form, in addition to VDW

388

and EDL forces. Adsorption isotherms and microscopic FT-IR characterization

389

indicated that chemical bonds can form between CDs and goethite. These findings

390

may provide guidance to understand the interaction between CDs particles and

391

minerals in rivers and lakes. In addition, the results presented herein may be useful for

392

assessing the environmental exposure, risk, and ecological implications of CDs.

393

■ ASSOCIATED CONTENT

394

The available supporting information contains 8 figures (Figures S1–S8), 1 table and

395

detailed descriptions of the figures. This material is available free of charge via the

396

Internet at http://pubs.acs.org.

397

■ AUTHORS INFORMATION

398

Corresponding Authors:

399

*(J. Li) Email: [email protected]; Phone/fax: 86-551-65596617

400

*(X. Wang) Email: [email protected]; Phone/fax: 86-10-61772890

401

ORCID

402

Jiaxing Li: 0000-0002-7683-2482

403

Xiangke Wang: 0000-0002-3352-1617

404

Notes

405

The authors declare no competing financial interest.

406

ACKNOWLEDGEMENT

407

Financial supports from National Natural Science Foundation of China (21577032, 19



Page 20 of 34

408

21677146), the special scientific research fund of public welfare profession of China

409

(201509074),

410

2015ZX07204-007) and the Jiangsu Provincial Key Laboratory of Radiation

411

Medicine and Protection and the Priority Academic Program Development of Jiangsu

412

Higher Education Institutions are acknowledged. X. Wang acknowledged the CAS

413

Interdisciplinary Innovation Team of Chinese Academy of Sciences.

414

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583 27



584

Figure captions

585

Figure 1 XRD spectrum (A) and C1s spectrum of CDs (B). UV-vis spectrum of CDs

586

where the inset picture is photograph taken under 365 nm UV light (C).

587

Photoluminescence (PL) spectra of CDs at 371 nm excitation (D) TEM image of CDs,

588

the insert picture is the size distribution histogram (E).

589

Figure 2 Zeta potentials of CDs at various concentrations of NaCl, CaCl2 and MgCl2

590

at pH 7.45±0.05. CDs concentration (CCDs) was maintained at 78.5 mg/L. The pH was

591

controlled using NaOH or HCl. The solid lines are only to guide the eyes (A) Zeta

592

potentials of minerals and CDs at various pH values, where CCDs=78.5 mg/L and

593

Cminerals=0.25 g/L (B) Aggregation kinetics of CDs in the presence of Na+, Mg2+ and

594

Ca2+ (CIS=16.6 mM) at pH 7.45±0.05 (C).

595

Figure 3 Change in Dh with time for goethite, Cgoethite=1.0 g/L (A) and CDs-Goethite,

596

Cgoethite=1.0 g/L, CCDs=0.15g/L (B) Illustrative diagram of CDs adsorption,

597

aggregation and deposition on goethite (C) The Dh were measured by dynamic light

598

scattering (DLS) (ZetaSizer Nano, Malvern), PDI is between 0.2 and 0.5.

599

Figure 4 Adsorption capacity of CDs on various minerals at pH 3.00, 7.45 and 10.0,

600

where Cminerals=2.0 g/L and initial CDs concentration is 37.5 mg/L (A) Adsorption

601

percentage (%) of goethite with various concentrations of NaCl (dash line) and MgCl2

602

(solid line) at pH 3.00, 7.45 and 10.0, where CCDs=18.7 mg/L, Cgoethite=0.25 g/L,

603

time=12 h and T=298 K (B).

604

Figure 5 Isotherms fitted using a linear model and the Freundlich model, where the

605

symbols represent experimental data and the curves are the fitted model simulations. 28


Page 28 of 34

Page 29 of 34


606

The dashed and solid lines correspond to the linear and Freundlich models,

607

respectively, where Cgoethite=0.25 g/L, time=12 h, and T=298 K (A). The microscopic

608

FTIR spectra of goethite and goethite after CDs adsorption at pH 3.00, 7.45 and 10.0,

609

where CCDs=37.5 mg/L, Cgoethite=0.25 g/L, time=12 h, and T=298 K (B).

610

Figure 6 Model of a real water environment, where Cminerals=1.0 g/L, CCDs=0.125 g/L

611

(A) and after settling for 48 hours (B) Passing through borosilicate glass columns

612

(5.0cm×1.5 cm) with 5 µm filter membranes, where mass of minerals is 0.1 g,

613

VCDs=1.0 mL, and CCDs=0.125 g/L (C) 4 h later, the minerals left in columns (D) and

614

CDs after filtered (E) Sample numbers 1, 2, 3 and 4 represent kaolinite,

615

montmorillonite, attapulgite and goethite, respectively.

616

29



617 618

Figure 1

30


Page 30 of 34

Page 31 of 34


619 620

Figure 2

31



621 622

Figure 3

623

624 625

Figure 4

32


Page 32 of 34

Page 33 of 34


626 627

Figure 5

628 629

Figure 6

630

33



631

TOC

632

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Adsorption, Aggregation, and Deposition Behaviors of Carbon Dots on

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