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Graphene Quantum Dots in Two-Dimensional Confined and Hydrophobic Space for Enhanced Adsorption of Nonionic Organic Adsorbates Qingfeng Yao,†,‡ Siming Wang,† Wenying Shi,*,† Chao Lu,*,† and Guangqing Liu‡ †

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, China

‡

S Supporting Information *

ABSTRACT: Graphene quantum dots (GQDs) should be expected to become an alternative material for removal of several pollutants from water due to their lager specific surface areas, abundant functional groups, and excellent biocompatibility. However, very little effort focused on adsorption behavior as higher water solubility of GQDs. Here we not only showed highly efficient adsorption of GQDs confined in twodimensional (2D) hydrophobic space for nonionic organic adsorbates but also systematically explored the interaction between adsorbents and adsorbates. The cointercalation of citrate and dodecyl sulfate (SDS) into interlayer of layered double hydroxides (LDHs) was prepared by hydrothermal method to obtain GQDs confined in 2D hydrophobic space. The adsorption efficiency of (GQDs+SDS)-LDHs for 2,4,6-trichlorophenol is 80%, which is higher than that of GQDs-LDHs (15%) and SDS-LDHs (40%). Adsorption mechanism showed the synergistic effects of hydrophobic, hydrogen bond, and π−π interaction were responsible for adsorption of nonionic organic adsorbates by (GQDs+SDS)-LDHs. As a result, this work not only solved the problems of high water solubility and aggregation of GQDs by immobilizing and dispersing GQDs in 2D confined and hydrophobic interlayer of LDHs but realized highly efficient adsorption of GQDs-based complex for nonionic organic adsorbates. Therefore, this strategy might be expanded to adsorb a wide variety of adsorbates by employing the structure designable ability of GQDs-LDHs-based composites. are composed primarily of amorphous carbon and sp2hybridized graphitic carbon,17 which can be regarded as unrolled CNTs. In view of the special structure and properties, GQDs could be expected to be an alternative material for removal of several pollutants from water. However, GQDs were barely used as adsorbents mainly due to their high solubility in aqueous solutions.18,19 Thus, some researchers concentrated on immobilization of GQDs on the following matrix surfaces such as gold20 and layered double hydroxides (LDHs)21 to realize GQDs as an adsorbent. Although these resultant complexes showed good adsorption ability, they were only restricted in ionic adsorbates. As a result, it is indispensable to construct a GQDs-based composite material for the extended adsorption for nonionic organic adsorbates, such as 2,4,6-trichlorophenol, the most common organic pollutant used widely in agriculture, industry, and public health.22

1. INTRODUCTION Carbon nanotubes (CNTs), due to their large specific surface areas (calculated 3000 m2/g) and excellent stability, have been regarded as one of most attractive adsorbents in water treatment and environmental remediation.1−4 However, the adsorption sites of CNTs are far less than theoretical value, which are caused by the following aspects: (1) inner cavities would be inaccessible for adsorbates as its blocked entrance by impurities (metal catalysts and/or amorphous carbons) or too narrow;5,6 (2) increased surface curvature with decease of diameter leads to lower adsorptive capacity and weaker interaction;7,8 and (3) aggregation of CNT monomers results in decease of effective surfaces to be exposed.9 Therefore, it is reasonable to increase the adsorption sites through cutting and spreading CNTs into small, dispersed, and planar plates, so as to realize their improved adsorption capacity. Graphene quantum dots (GQDs) are a relatively new class of carbon nanomaterials which have received a high concern in the field of catalysts,10 bioimaging,11 sensing,12 and photoemissive devices13 due to their biocompatibility,14 O-rich functional groups (−OH, −COOH, and −CO), low cost, and highly efficient luminescence.15,16 GQDs with 2−10 nm in diameter © XXXX American Chemical Society

Received: Revised: Accepted: Published: A

June 21, 2016 December 12, 2016 December 16, 2016 December 20, 2016 DOI: 10.1021/acs.iecr.6b02389 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

higher than 99%. Deionized and decarbonated water were used in all the experimental processes. 2.2. Preparation of Cointercalation of Citrate and SDS. (Citrate+SDS)-LDHs were prepared by a coprecipitation method with a constant pH value. A mixed salt solution containing 0.045 mol of Mg(NO3)2·6H2O and 0.015 mol of Al(NO3)3·9H2O in 60.00 mL of water was added dropwise to a 60.00 mL solution containing 0.0025 mol of citrate and 0.0075 mol of SDS at 45 °C with vigorous stirring. Then, 0.12 M NaOH solution was added to keep pH = 10.5. After that, the suspension was kept at 45 °C for 24 h under N2 atmosphere. Finally, the precipitate was filtered, washed with deionized water three times, and dispersed in deionized water to obtain suspension. Other control samples (citrate-LDHs and SDSLDHs) were prepared by a similar process. 2.3. Preparation of (GQDs+SDS)-LDHs. (GQDs+SDS)LDHs were prepared by a hydrothermal method. Volumes of 25.00 mL of (citrate+SDS)-LDHs (0.045 g/mL) and 2.25 mL of ammonia were added into a Teflon-equipped stainless steel autoclave with hydrothermal treatment at 180 °C for 8 h. The resulting precipitate was washed thoroughly with deionized water by centrifugation (10 000 rpm/min, 5 min), followed by drying in vacuum at 60 °C for 24 h. GQD-LDHs were prepared by a similar process. 2.4. Preparation of GQDs@SDS-LDHs. GQDs@SDSLDHs were prepared by hydrothermal treatment for citrate adsorbed on the SDS-LDH surface. In total, 0.0045 mol of SDS-LDHs were added into 25.00 mL of sodium citrate solution (0.0139 M) with stirring for 2 h. The mixed solution was concentrated to 12.75 mL, and 2.25 mL of ammonia− water was added. Then the mixed solution was transferred to a Teflon-equipped stainless steel autoclave at 180 °C for 8 h. The resulting white suspension was cooled to room temperature, washed with deionized water three times by centrifugation (10 000 rpm/min, 5 min), followed by drying in vacuum at 60 °C for 24 h. 2.5. Adsorption Experiments. Dynamics Process. (GQD +SDS)-LDHs (20.00 mg) were added into the 2,4,6trichlorophenol solution with 60.00 and 100.00 mg/L, respectively. The adsorption performance was monitored by taking 2.00 mL of sample solution at 5 min intervals (from 5 to 90 min) during the whole adsorption process, and the equilibrium time was 60 min. The absorption intensity at 290 nm was used to determine residual content of 2,4,6trichlorophenol in supernatant solution by centrifugation (10 000 rpm/min, 5 min). The adsorbent performance for 2,4,6-trichlorophenol was calculated by eq 1, and the amount of 2,4,6-trichlorophenol adsorbed at equilibrium qe (mg/g) was calculated by eq 2, where C0 and Ce are the initial and equilibrium concentrations of adsorbate solution (mg/L), V is the volume of 2,4,6-trichlorophenol solution used (L), and W is the mass of adsorbent (g). The results are the mean of three measurements. The influence of inorganic electrolytes and humic acid on adsorption was followed by a similar process, except adding 50.00 mg/L NaCl and 50.00 mg/L humic acid to 2,4,6-trichlorophenol solution, respectively.

In our previous work, a hydrophobic microenvironment in the interlayer of LDHs was constructed by cointercalating surfactants and optical molecules, which demonstrated enhanced and tunable optical performances.23−25 Inspired by this, we considered to simultaneously cointroduce GQDs and surfactant into interlayer of LDHs, which would show the following advantages for the potential adsorption in nonionic organic species: (1) the aggregation of GQDs can be effectively inhibited by host−guest interactions (e.g., electrostatic attraction, hydrogen bonding), therefore leading to the highly exposed active site of GQDs; (2) the intercalated surfactants provide GQDs a hydrophobic microenvironment in the interlayer of LDHs, which is beneficial to entrance of nonionic organic adsorbates by hydrophobic interaction; (3) the surfactants protect the functional groups of GQDs from disturbance of water molecule, therefore facilitating the formation of hydrogen bond between GQDs and nonionic organic adsorbates. The above advantages represent the efficient interlayer microenvironment for the potentially adsorb nonionic organic adsorbates. Herein, GQDs confined in twodimensional (2D) hydrophobic space were obtained by hydrothermal treatment for the cointercalation of citrate and dodecyl sulfate (SDS) into the interlayer of LDHs (denoted as (GQDs+SDS)-LDHs). The adsorption efficiency of (GQDs +SDS)-LDHs to 2,4,6-trichlorophenol is 80%, which is much higher than those of GQDs-intercalated LDHs (15%) and SDSintercalated LDHs (40%). Mechanism gave the evidence that the synergistic effects of hydrophobic, hydrogen bond and π−π interaction were responsible for the efficient adsorption of nonionic organic adsorbates by (GQDs+SDS)-LDHs (Scheme 1). This work provided the solution to the issues of high water Scheme 1. Representation of 2,4,6-Trichlorophenol Adsorption in the Interlayer of (GQDs+SDS)-LDH Composite

solubility and aggregation of GQDs through immobilization and dispersion of GQDs in the 2D hydrophobic interlayer of LDHs and realized highly efficient adsorption for nonionic organic species. Thus, the strategy is expected to be widely used in water treatment for nonionic organic adsorbates by taking advantage of structure designable ability of GQDs-LDHs-based composites.

C0 − Ce × 100% C0

2. EXPERIMENTAL SECTION 2.1. Materials. Analytical grade chemicals including Mg(NO3)2·6H2O, Al(NO3)3·9H2O, NH3·H2O, NaOH, HCl, sodium citrate, 2,4,6-trichlorophenol, and sodium dodecyl sulfate (SDS) were purchased from Sigma-Aldrich and used without further purification. The purity of all chemicals is

qe = B

(C0 − Ce) × V W

(1)

(2) DOI: 10.1021/acs.iecr.6b02389 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 1. (A) XRD patterns of (citrate+SDS)-LDHs and (GQDs+SDS)-LDHs, (B) fluorescence spectra of (GQDs+SDS)-LDHs and GQDs@SDSLDHs, (C) TEM, and (D) HRTEM images of GQDs.

Figure 2. (A) Removal efficiency of 2,4,6-trichlorophenol by different adsorbents including (GQDs+SDS)-LDHs, GQDs-LDHs, and SDS-LDHs, (B) adsorption isotherm of (GQDs+SDS)-LDHs for 2,4,6-trichlorophenol, (C) Langmuir isotherm model, and (D) Freundlich isotherm model (experimental conditions: 2,4,6-trichlorophenol concentration = 100.00 mg/L, pH = 3.0, (GQDs+SDS)-LDH concentration = 1.00 g/L, T = 25 °C).

Adsorption Process. A series of 2,4,6-trichlorophenol solutions with different concentrations (40.00−160.00 mg/L) were prepared as the adsorbates. The 1.00 M HCl solution was added to keep pH = 3.0. Then 20.00 mg (GQD+SDS)-LDHs was added into the above 2,4,6-trichlorophenol solutions with stirring until the adsorption equilibrium was reached. The absorption intensity at 290 nm was used to determine the residual amount of 2,4,6-trichlorophenol in supernatant solution by centrifugation (10 000 rpm/min, 5 min). 2.6. Characterizations. X-ray powder diffraction (XRD) patterns were measured using D8 ADVANCE (Bruker) under the conditions: 40 kV, 50 mA, Cu Kα radiation (λ = 0.154 nm) step-scanned with a scanning rate of 0.5°/min and a 2θ angle ranging from 2 to 70°. TGA curves were recorded by using differential thermal analyzer with 1/1100 SF (Mettler,

America). The morphology was investigated by using scanning electron microscope (SEM, S-4700, Hitachi), transmission electron microscope (TEM, Tecnai G220, FEI), and highresolution TEM (JEM-2100, JEOL). Fluorescence spectra were measured with a Hitachi F-7000 fluorescence spectrophotometer (Hitachi, Japan). UV−vis absorption spectra were recorded by using a UV-2401 ultraviolet spectrophotometer (Shimadzu). FTIR spectra were recorded on a Nicolet 6700 FTIR spectrometer with 4.0 cm−1 resolution (Thermo, America).

3. RESULTS AND DISCUSSION 3.1. Structure and Morphology. The XRD pattern of MgAl-NO3-LDH powder sample gives an obvious (003) reflection of 2θ = 9.95°, with an interlayer space of 0.88 nm C

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Adsorption Isotherms. The study of adsorption isotherm is very important to determine the equilibrium distribution of adsorbent in solid and liquid phases through detecting the maximum adsorption capacity.30 Figure 2B shows the adsorbed amount increases with increase of the equilibrium concentrations of 2,4,6-trichlorophenol upon 65.00 mg/L. The maximum adsorption capacity of 2,4,6-trichlorophenol from aqueous solutions achieves ∼119.00 mg/g, which is superior to the previous reported adsorbents (Table S1).31−33 Langmuir (eq 3) and Freundlich (eq 4) models were used to describe the equilibrium adsorption, where Ce (mg/L) is the equilibrium concentration of adsorbate, qe (mg/g) is the equilibrium adsorbate amount on adsorbent, KL (L/mg) is the Langmuir constant related to the adsorption energy, qm (mg/g) is the maximum adsorption capacity of the adsorbent corresponding to the complete monolayer coverage, KF and 1/n are Freundlich constants related to adsorption capacity and intensity, respectively. Moreover, RL is a separation factor represented by eq 5, which can be used to predict the adsorption nature to be either irreversible (RL = 0), favorable (0 < RL < 1), linear (RL = 1), or unfavorable (RL > 1).34 For (GQDs+SDS)-LDHs, the calculated RL value is between 0 and 1 for different initial concentrations of 2,4,6-trichlorophenol, indicating the adsorption of 2,4,6-trichlorophenol on (GQDs +SDS)-LDHs is favorable. On the other hand, the 1/n is equal to 0.15, further confirming highly efficient adsorption of 2,4,6trichlorophenol on (GQDs+SDS)-LDHs. The isotherm parameters were obtained by fitting experimental data (Figure 2C,D). It is observed that the value of correlation coefficient (R2) for Langmuir model is higher than that of Freundlich models. The equilibrium adsorption capacity calculated by the Langmuir model is 119.00 mg/g, which is consistent with experimental results, demonstrating the adsorption isotherm of 2,4,6-trichlorophenol on (GQDs+SDS)-LDHs is suitable for Langmuir model (Table S2). These results prove the confined and hydrophobic interlayer of (GQDs+SDS)-LDHs shows a monolayer uptake.35

ascribed to hydrotalcite (JCPDS card 22-700, Figure S1). After cointercalating citrate and SDS into LDH layer, the interlayer space of (citrate+SDS)-LDHs increases to 2.56 nm (Figure 1A), which is almost equal to that of SDS-LDHs (Figure S2). After hydrothermal treatment, the interlayer space of (GQD +SDS)-LDHs shows no obvious change. Without intercalating SDS into LDH, the interlayer space of GQD-LDHs is similar to that of MgAl-NO3-LDH (Figure S2). From calculation, the length of SDS (1.83 nm), citrate (0.73 nm) and thickness of LDH layer (0.48 nm, Figure S3),26 it can be inferred that SDS serves as a support for determining the interlayer space of (GQD+SDS)-LDHs and citrate can provide carbon resource to form GQDs. This is further proved by TGA, in which (GQD +SDS)-LDHs show less weight loss from water molecules of LDHs below 180 °C (Figure S4). In addition, in order to further clarify, the exact position of GQDs, the control sample of GQDs loaded SDS-LDHs (denoted as GQDs@SDS-LDHs) were prepared by hydrothermal treatment through adsorbing citrate on the surface of SDS-LDHs. The obtained GQDs@ SDS-LDHs show weaker fluorescence emission intensity compared to (GQDs+SDS)-LDHs (Figure 1B). The fluorescence emission peaks of (GQDs+SDS)-LDHs remain at 425 nm regardless of varied excitation wavelengths from 325 to 395 nm (Figure S5). An excitation-independent emission behavior highlights a high size uniformity of GQDs,27 demonstrating the 2D interlayer space is benefit for the intercalated GQDs with a homogeneous dispersion for the improved fluorescence, relative to the adsorbed GQDs on the surface with severe aggregation. This result shows the small and uniform particle size of GQDs results from immobilization of LDH layer and dispersion of SDS for citrate molecules. To monitor the size of GQDs, (GQDs+SDS)-LDHs were then etched by hydrochloric acid.28 TEM image shows that the GQDs own a narrow size distribution of 2.00 ± 0.20 nm (Figure 1C), consistent with results from fluorescence spectra. The lattice spacing of GQDs imaged by HRTEM is ∼0.20 nm (Figure 1D), which is very close to the hexagonal pattern of graphene with d1100.29 3.2. Adsorption Behavior of (GQDs+SDS)-LDHs. 2,4,6Trichlorophenol was employed as a nonionic organic adsorbate to evaluate the adsorption capacity of GQDs-LDHs, SDSLDHs, and (GQDs+SDS)-LDHs. As shown in Figure 2A, 80% of 100.00 mg/L 2,4,6-trichlorophenol was adsorbed by (GQDs +SDS)-LDHs within 10 min. In contrast, GQDs-LDHs and SDS-LDHs can only adsorb 15% and 40% of 2,4,6trichlorophenol under the same conditions, respectively. These results demonstrate the adsorption capacity of (GQDs +SDS)-LDHs are higher than those of GQDs-LDHs and SDSLDHs, which originates from synergistic effect of GQDs and SDS in the interlayer of LDHs. Moreover, the adsorption capacity of SDS-LDHs is better than GQDs-LDHs, indicating hydrophobic interaction of SDS plays a leading role to improve the adsorption capacity of (GQDs+SDS)-LDHs. This conclusion can also be confirmed by insignificant effect of inorganic electrolyte (NaCl) on the adsorption capacity of (GQDs +SDS)-LDHs. Note that with prolonging adsorption time to 50 min, the removal efficiency begins to decline, which may be caused by departure of SDS and GQDs from interlayer of LDHs due to the competition of Cl− (Figure S6). Moreover, humic acid, often present in water, has a positive effect on removal efficiency of 2,4,6-trichlorophenol (Figure S7). This results from adsorption of humic acid on LDH layer by electrostatic interaction due to its rich functional groups (COO−, OH−, etc.), realizing its property as a solid adsorbent.

Ce C 1 = + e qe KLqm qm

log qe = log KF + RL =

1 1 + KLC0

(3)

1 n log Ce

(4)

(5)

Adsorption Kinetics. To investigate the adsorption rate and the adsorption mechanism, the adsorption kinetics of 2,4,6trichlorophenol on (GQDs+SDS)-LDHs were analyzed by using the pseudo-first-order (eq 6),36 the pseudo-second-order (eq 7),37 and the intraparticle diffusion kinetic model (eq 8),38 respectively, where qt (mg/g) and qe (mg/g) represent the amount of 2,4,6-trichlorophenol adsorbed at time t and at equilibrium time, respectively; k1 (min−1), k2 (g/mg min), and ki (mg/g min0.5) represent the rate constant of the pseudo-firstorder model, the pseudo-second-order model, and the intraparticle diffusion model, respectively; C is the intercept. Concentrations of 60.00 and 100.00 mg/L were selected as two initial 2,4,6-trichlorophenol concentrations to study adsorption kinetics. The pseudo-second-order model best describes the kinetic data of 2,4,6-trichlorophenol adsorption on the surface of (GQDs+SDS)-LDHs with a higher R2 value (Figure 3 and D

DOI: 10.1021/acs.iecr.6b02389 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 3. Plots of (A) pseudo-first-order kinetics, (B) pseudo-second-order kinetics, and (C) intraparticle diffusion models of 2,4,6-trichlorophenol adsorption on the surface of (GQDs+SDS)-LDHs (Experimental conditions: pH = 3.0, (GQDs+SDS)-LDH concentration = 1.00 g/L, T = 25 °C).

Figure 4. (A) UV−visible absorption spectra and (B) XRD patterns of (GQDs+SDS)-LDHs, SEM images of (GQDs+SDS)-LDHs (C) before and (D) after adsorption of 2,4,6-trichlorophenol (experimental conditions: 2,4,6-trichlorophenol concentration = 100.00 mg/L, (GQDs+SDS)-LDH concentration = 1.00 g/L, T = 25 °C).

Figure 5. (A) Effect of initial pH values on removal efficiency of 2,4,6-trichlorophenol and equilibrium pH values (experimental conditions: 2,4,6trichlorophenol concentration = 100.00 mg/L, (GQDs+SDS)-LDH concentration = 1.00 g/L, T = 25 °C), (B) fluorescence spectra of (GQDs +SDS)-LDHs before and after adsorbing 2,4,6-trichlorophenol (λex = 365 nm, inset: the photographs under 365 nm UV light).

qt = kit 0.5 + C

Table S3), indicating the rate-limiting step is chemical adsorption.39 k ×t log(qe − qt ) = log qe − 1 2.303

(6)

t 1 t = + qt qe k 2qe 2

(7)

(8)

3.3. Possible Adsorption Mechanism of (GQDs+SDS)LDHs. The interaction of adsorbates, solvents, and adsorbents are responsible for the adsorption property.40,41 In general, several interactions simultaneously exist in an adsorption system. To clarify contribution of each interaction to the overall adsorption, we systematically studied the relationship between the interactions and adsorption capacity. E

DOI: 10.1021/acs.iecr.6b02389 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 6. FTIR spectra of (A) (GQDs+SDS)-LDHs and (B) SDS-LDHs before and after adsorbing 2,4,6-trichlorophenol, (C) effect of temperatures on adsorption of 2,4,6-trichlorophenol and (D) van’t Hoff plot for adsorption of 2,4,6-trichlorophenol by (GQDs+SDS)-LDHs (experimental conditions: 2,4,6-trichlorophenol concentration = 100.00 mg/L, pH = 3.0, (GQDs+SDS)-LDH concentration = 1.00 g/L).

Change of Adsorption, Structure, and Morphology. UV− vis absorption spectra of 2,4,6-trichlorophenol before and after adsorption are shown in Figure 4A. After adsorption, the decreased adsorption peak of 2,4,6-trichlorophenol indicates the adsorption ability of (GQDs+SDS)-LDHs. The XRD patterns of (GQDs+SDS)-LDHs before and after adsorption show no changes, which demonstrates the composite structure is stable (Figure 4B). SEM and TEM were used to observe the morphological features of (GQDs+SDS)-LDHs. After adsorption, the decreased particle size and irregular fragments of the composite can be observed, which was caused by mechanical stirring for long time during adsorption process. However, insignificant change can be found on the surface of (GQDs +SDS)-LDHs, indicating the adsorption mainly appears in the interlayer of LDHs (Figure 4C,D and Figure S8). Hydrophobic Interaction. In order to clarify the hydrophobic interaction between 2,4,6-trichlorophenol and (GQDs +SDS)-LDHs, we controlled the ionization state of 2,4,6trichlorophenol by adjusting its initial pH from 2.0 to 12.0. Figure 5A shows the initial pH value of 2,4,6-trichlorophenol solution is less than 3.0, and the pH of adsorption equilibrium is less than pKa of 2,4,6-trichlorophenol (6.23) due to buffer capacity of LDHs;42,43 when the initial pH value is greater than 3.0, the pH of adsorption equilibrium is greater than pKa. The relationship between adsorption efficiency and initial pH shows the adsorption efficiency first increases to a maximum then decreases with a further increase of the initial pH value. The optimum adsorption occurs with initial pH of 3.0 (Figure 5A). This phenomenon can be explained that the layered structure of (GQDs+SDS)-LDHs may be destroyed when the initial pH value is less than 3.0. When the initial pH is larger than 3.0, 2,4,6-trichlorophenol with negative charge is difficult to diffuse into the hydrophobic interlayer of LDHs, while prefering to adsorb on the surface of LDH layer due to electrostatic

interaction. When the initial pH is equal to 3.0, hydrophobic interaction between 2,4,6-trichlorophenol and (GQDs+SDS)LDHs provided by SDS molecules in the LDH interlayer plays a major role, and maximum adsorption capacity is obtained. π−π Interaction. The change of π−π interaction will lead to fluorescence signal change, which reflects the microenvironment of dispersed QCDs in the LDH interlayer. Figure 5B shows the fluorescence intensity of (GQDs+SDS)-LDHs decreases after adsorbing 2,4,6-trichlorophenol. In addition, the photographs of adsorbents under 365 nm UV light also show a decreased brightness after adsorbing 2,4,6-trichlorophenol (inset in Figure 5B). These results indicate the appearance of π−π interaction in the interlayer of LDHs between the C atom of GQDs with sp2 hybridization, and the benzene ring of 2,4,6-trichlorophenol causes fluorescence quenching.44 Hydrogen Bond. Fourier transform infrared (FTIR) spectrum is an efficient method to confirm the existence of a hydrogen bond. After adsorbing, the IR spectrum of (GQDs +SDS)-LDH sample shows the appearance of the C−H stretching vibrations at 1243 cm−1 attributed to 2,4,6trichlorophenol,45 confirming successful adsorption of 2,4,6trichlorophenol. After adsorbing, the IR signal of O−H stretching vibration for (GQDs+SDS)-LDHs shifts to higher wavelength from 3483 to 3507 cm−1 compared to before adsorbing (Figure 6A). In contrast, the O−H stretching vibration of SDS-LDHs after adsorbing shows no change. These results indicate that the hydrogen bond exists between OH of GQDs and 2,4,6-trichlorophenol, rather than between OH of LDH layer and 2,4,6-trichlorophenol.46 To further confirm the existence of hydrogen bond between CQD and 2,4,6-trichlorophenol, the relationship between adsorption capacity and temperature was explored because hydrogen bond length and intensity are sensitive to temperF

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ature.47,48 Figure 6C shows the adsorption capacity of (GQDs +SDS)-LDHs decreases with increase of temperature from 308 to 318 K, which is caused by two reasons: (1) hydrogen bonding declines with increase of temperature and (2) increased mobility is not beneficial to the entrance of 2,4,6trichlorophenol into the LDH interlayer at high temperature. Moreover, the adsorption enthalpy (ΔH) was also calculated by using van’t Hoff eq 9 for obtaining the adsorption types, where KL (L/mg) is Langmuir constant, ΔS is entropy change, and R is the gas constant (8.3145 J/mol K). Figure 6D shows ΔH is less than zero (−11.7 kJ/mol), in the range of the hydrogen bond force (0 to −20 kJ/mol), indicating that the hydrogen bond has a major role in the adsorption process.49 ln KL =

ΔS ΔH − R RT

AUTHOR INFORMATION

Corresponding Authors

*Phone: +86-10-64411957. E-mail: [email protected]. *E-mail: [email protected]. ORCID

Wenying Shi: 0000-0002-2109-2069 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (973 Program, Grant 2014CB932103), the National Natural Science Foundation of China (Grants 21521005, 21375006, 21575010, and 21571014), and Innovation and Promotion Project of Beijing University of Chemical Technology.

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4. CONCLUSIONS In summary, (GQDs+SDS)-LDH composite was fabricated by hydrothermal treatment for the cointercalation of citrate and SDS into LDHs, which showed excellent adsorption capacity for nonionic organic adsorbate (2,4,6-trichlorophenol). The adsorption efficiency of the obtained (GQDs+SDS)-LDH composite reaches 80%, which is higher than those of control samples of GQDs-LDHs (15%) and SDS-LDHs (40%). The adsorption isotherm conforms to the Langmuir model, displaying the confined and hydrophobic interlayer of (GQDs +SDS)-LDHs is a monolayer adsorption for 2,4,6-trichlorophenol. The pseudo-second-order model best describes the kinetic data of 2,4,6-trichlorophenol adsorption by (GQDs +SDS)-LDHs with a higher R2 value, indicating the ratelimiting step is chemical adsorption. Adsorption mechanism demonstrates that the synergistic effects of hydrophobic, π−π interaction and hydrogen bond between (GQDs+SDS)-LDHs and 2,4,6-trichlorophenol contribute to the excellent adsorption performance, which was confirmed by controlling the ionization state of 2,4,6-trichlorophenol, fluorescence spectra and FTIR spectra, respectively. Especially, SDS surfactant stretches the interlayer space of LDHs and provides a hydrophobic microenvironment for GQDs, which are beneficial to the entrance of nonionic organic adsorbate into confined interlayer of LDHs through hydrophobic interaction. Therefore, a new class of adsorbents for nonionic organic adsorbates can be achieved based on GQDs intercalated LDH composites, which can be potentially used to remove nonionic organic pollutants.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b02389. XRD patterns of MgAl-NO3-LDHs, GQDs-LDHs, and SDS-LDHs; molecular structure scheme of 2,4,6trichlorophenol, citrate, dodecyl sulfate; TGA curves of (GQDs+SDS)-LDHs, GQDs-LDHs, and SDS-LDHs; fluorescence spectra of (GQDs+SDS)-LDHs at different excitation wavelengths; maximum adsorption capacities of adsorbents for 2,4,6-trichlorophenol; Langmuir and Freundlich parameters of (GQDs+SDS)-LDHs; parameters of kinetic model for the adsorption of 2,4,6trichlorophenol on (GQDs+SDS)-LDHs; and TEM images (PDF) G

DOI: 10.1021/acs.iecr.6b02389 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.iecr.6b02389 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX