Removal of U(VI) from Aqueous Solution by Amino Functionalized

Research Article pubs.acs.org/journal/ascecg

Removal of U(VI) from Aqueous Solution by Amino Functionalized Flake Graphite Prepared by Plasma Treatment Shengxia Duan,†,‡ Yanan Wang,†,‡ Xia Liu,†,‡ Dadong Shao,† Tawwar Hayat,§ Ahmed Alsaedi,§ and Jiaxing Li*,†,∥ †

Key Laboratory of Novel Thin Film Solar Cells, Institute of Plasma Physics, Chinese Academy of Sciences, P.O. Box 1126, Hefei, 230031, P.R. China ‡ University of Science and Technology of China, Hefei, 230026, P.R. China § Faculty of Science, King Abdulaziz University, Jeddah, 21589, Saudi Arabia ∥ Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, Suzhou, P.R. China S Supporting Information *

ABSTRACT: Flake graphite (FG) with high uranium(VI) entrapment efficiency was successfully fabricated via a simple and efficient nonthermal plasma treatment method. FG was modified with −NH2 functional groups through nonthermal plasma with different treatment times under vacuum conditions. The modified FG samples were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transformed infrared spectroscopy (FT-IR spectra), thermogravimetric analysis (TGA), BET surface area measurements, and zeta potential. FG samples with different treatment times are used as contrast adsorbents for U(VI) entrapment. The adsorption experiments show that the modified FG has higher high U(VI) entrapment efficiency than others, and longer treatment time results in higher efficiency, demonstrating that the plasma treatment can greatly increase the active sites of FG samples and lead to the successful grafting of −NH2 on FG surface. The −NH2 modified FG with 2 h treatment time shows the highest adsorption capacity with 140.68 mg·g−1 among the five samples at 333.15 K. Thermodynamic studies reveal that the U(VI) entrapment process is spontaneous and entropy-driven endothermic. XPS studies reveal that the adsorption mechanism for U(VI) entrapment is achieved through the complexation of U(VI) with both −NH2 and phenolic hydroxyl group on the surface of modified FG. Moreover, desorption studies exhibit that PTFG-4 can be used repeatedly and adsorption capacity only shows slight decrease after five cycles. Thus, it can be concluded that the nonthermal plasma treatment can be used as an effective method for the fabrication of adsorbents with great adsorption performance for heavy metals entrapment. KEYWORDS: Flake graphite, Plasma treatment, Adsorption, U(VI)

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INTRODUCTION Uranium(VI) has been a key element of nuclear industry development, which also plays a critical role in commercial use as a fuel for electricity production.1 Moreover, according to the IEA, the global nuclear industry capacity could expand by more than 40% by 2030, leading to the rising demand for U(VI) consumption. As a result, an increasing amount of toxic and weakly radioactive U(VI) has been excessively released into environment. This phenomenon can bring about two main adverse effects. One side, the scarcity of U(VI) resources could be detrimental to the progress of nuclear industry. On the other side, the large amount of U(VI) disposing into the environment could cause serious damages to human health, including kidney damage, liver damage, and eventually death.2,3 Thus, it is imperative to achieve the highly efficient separation and recovery of U(VI) from aqueous solutions, which is not only beneficial to the sustainable development of nuclear power but also beneficial to resource recycling, environmental protection, and human health.4 A series of treatment methods have been © 2017 American Chemical Society

applied to remove U(VI) from aqueous solutions, including adsorption,5−7 coprecipitation,8 chemical precipitation,9 nanofiltration,10 solvent extraction, and ultrafiltration.11 Compared with all these treatment methods, adsorption has been confirmed to be the most effective method due to its large amount of advantages, including high efficiency, ease of operation, low-cost, and wide range of material sources. This method can not only applied in centralized water treatment plants but also in decentralized water treatment scenarios of remote areas and disaster zones.12,13 Many low-cost materials have been extensively applied in the removal of U(VI) from aqueous solutions, such as silica,14 sepiolites,15 clay,16 chitosan,17 and biosorbents.18 However, these low-cost adsorbents usually suffer low removal efficiency or low removal capacity, which prevents their large scale applications in the Received: January 8, 2017 Revised: March 3, 2017 Published: March 29, 2017 4073

DOI: 10.1021/acssuschemeng.7b00069 ACS Sustainable Chem. Eng. 2017, 5, 4073−4085

ACS Sustainable Chemistry & Engineering

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field of wastewater treatment. Meanwhile, these disadvantages also promote the further investigations on the fabrication of new kind of adsorbents for more efficient U(VI) removal. In most case, the removal efficiency of the adsorbents can heavily depend on the functional groups on the adsorbent surface, which can absorb the positively charged metal ions by chemical complex interaction or electrostatic attraction. Materials functionalized amine groups have been utilized in the removal of U(VI) from aqueous solutions, which exhibit great adsorption performance in U(VI) separation.19−21 Nevertheless, conventional modification method to immobilize the amine groups is usually through wet-chemistry methods,19−21 which have some defects, such as complexity and substrate dependency, hampering their wide applications.22,23 Moreover, this wet-chemistry method also owns a disadvantage of a high rate of waste production, further preventing it from wide application of modifying adsorbents.12 Thus, it is imperative to utilize a rapid, easy, and substrate-independent method for the modification of adsorbents, synchronously, to minimize the effects on the environment through keeping waste production low. Plasma treatment is regarded as a promising and an environmental friendly method technique, which can be utilized as a novel alternative to traditional modification methods since it can produce chemically active species enhancing their adsorption abilities.24,25 Virtually all solid materials can be modified via this plasma treatment, while minimal or no surface preparation is required.15 Moreover, since no solvent is involved in this modification process, almost no waste is produced. In view of these advantages, plasma treatment can be used to modify specific functional groups on adsorbent surfaces, which can greatly enhance its chemical functionalities and adsorption properties. Flake graphite (FG) is considered as the most stable allotrope carbon, which consists of carbon layers with covalent and metallic bonding within each layer and are linked by a weak van der Waals interaction produced by a delocalized π-orbital.26 Because of the special sandwich structure and long distance between the carbon layers, it is much easier to insert atoms or molecules between the carbon layers.27 In addition to its special sandwich structure, FG also has other great advantages, such as high intrinsic thermal conductivity, large specific surface area, commercially available, and low-cost,28,29 making it acquire numerous attentions in lithium storage,28 catalyst,30 adsorbent,31 and hydrogen adsorption and storage.32 Although FG has attracted great attentions in the various fields as mentioned above, it is still rare to apply it for wastewater treatment. Hence, in this study, FG was selected as the substrate and plasma treatment was applied to modify the amine groups onto FG. In this process, O-phosphorylethanolamine (O-PEA) was selected as the −NH2 group source given that it is not only nonpoisonous but also hydrosoluble, which can be easily washed off if unreacted. The plasma treatment time was varied from 30 to 120 min to investigate its effect on surface chemistry, zeta potential, and U(VI) entrapment efficiency. Meanwhile, the contact time, solution acidity, ionic strength, and reacting temperature were also studied to characterize the adsorption performance of the plasma treated flake graphite (PTFG). Additionally, possible adsorption mechanism was also put forward according to the XPS studies. Moreover, PTFG exhibits great entrapment performance for U(VI) removal when comparing with other adsorbents, suggesting that this plasma treatment method can be applied as a new approach in modifying materials for entrapment of toxic metal ions.

Research Article

EXPERIMENTAL SECTION

Materials. Flake graphite (FG) (99.99% purity) was purchased from Alfa Aesar. O-Phosphorylethanolamine (O-PEA) was obtained from J&K Chemical Reagent Co., Ltd. (Shanghai, China) without further purification. The U(VI) stock solution (1.0 mmol/L) was prepared by dissolving UO2(NO3)2·6H2O (99.99%, Sigma Aldrich) in deionized water. Plasma Treatment. To characterize the effects of plasma treatment on FG, FG was first treated as nonthermal plasma for 2 h (PTFG-1). In a typical method, FG was transferred to a glass flask and the HV pulsed dc voltage with 100 W was applied to the plasma coil until vacuum degree of the glass flask reached a relative stable state about 3.9 Pa. Additionally, to better understand the plasma treatment time on U(VI) entrapment, three sets of FG and O-PEA mixtures were treated under the processing conditions as mentioned above. First, FG was mixed with O-PEA by grinding with their mass ratio of 1:1. Subsequently, three sets of these mixtures were transferred to a glass flask, which were then treated under this condition for about 30 min (named as PTFG-2), 60 min (PTFG-3), and 120 min (PTFG-4), respectively. Finally, these three sets of mixtures were washed with water and ethanol, and then dried in a vacuum oven at 60 °C for 12 h. Characterizations. To characterize the purity of FG, X-ray diffraction (XRD) analysis was carried out on a Philips X’Pert Pro Super diffractometer with Cu Kα radiation. The Raman spectra were recorded at ambient temperature on a SPEX 1403 spectrometer with an argon-ion laser at an excitation wavelength of 514.5 nm. FTIR spectra were obtained on a Nicolet-5700 FT-IR spectrophotometer. The morphologies and nanostructures of these samples were performed by using field emission SEM (Hitachi S-4800) equipped with a GENESIS4000 energy dispersive X-ray spectroscope (EDS) and TEM (Hitachi H7700). TGA measurements were performed on a TGA-50/50H analyzer. BET surface areas were characterized by N2 adsorption−desorption measurements at 77 K using Quantachrom Autosorb IQ-C. XPS spectrum was conducted at 10 kV and 5 mA under 10−8 Pa residual pressure with the peak energy corrected with C 1s peak at 284.6 eV. The zeta potential was obtained by using a Malvern Zetasizer Nano ZS (England). Batch Adsorption Studies. Triplicate batch adsorption experiments of U(VI) adsorption onto these samples were performed in 10.0 mL polyethylene tubes. First, solutions containing 60 mg·L−1 of U(VI) were prepared, which were then regulated to specific concentrations to achieve different adsorption experiments. The pH-dependent adsorption of U(VI) was carried out under ambient conditions. The different pH values of suspension were adjusted by adding a negligible volume of 0.01−1.0 mol/L HNO3 or NaOH solution. Then, the suspension was agitated on a shaker for a reaction time of 24 h to achieve adsorption equilibrium. Additionally, the kinetic adsorption studies were carried out by using different polyethylene tubes containing 0.25 g·L−1 of these samples at pH 6. Adsorption equilibrium isotherms were determined at 293.15, 313.15, and 333.15 K using various initial U(VI) concentrations at pH 6 with 0.25 g·L−1 adsorbent concentration, consistently. The U(VI) concentration in suspensions were analyzed by using a kinetic phosphorescence analyzer (KPA-11, Richland, USA). The adsorption capacities (qe, mg·g−1) of U(VI) absorbed onto the adsorbents can be expressed as following:

qe =

(C0 − Ce)V m

(1)

where C0 (mg·L−1) and Ce (mg·L−1) are the initial concentration and equilibrated concentration of U(VI) in the solution, respectively. qe (mg·g−1) denotes the amount of ions adhered to the solid phase. V (L) denotes the volume of the suspension, and m (g) denotes the mass of the sorbents. Desorption and Reusability Studies. To characterize the reusability of PTFG-4, adsorption−desorption experiments were carried out for 5 times with CU(VI)initial of 30 mg·L−1 at 293.15 K (m/V = 0.25 g·L−1, pH = 6.0 ± 0.1). After reaching adsorption equilibrium, the U(VI) loaded PTFG-4 were separated by 4074


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Figure 1. XRD patterns (a) and Raman spectra of the obtained samples. centrifugation, and then 0.1 mol·L−1 HCl was added into the solution desorb the absorbed U(VI) ions. After each cycle of recovery, regenerated adsorbent was washed with deionized water and ethyl alcohol several times and dried for reuse.

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RESULTS AND DISCUSSION Characterization of the Materials. Figure 1a gives the XRD patterns of the obtained samples. It can be seen that no other peaks appeared in the XRD patterns of the plasma treatment samples, indicating this method brings no changes to the FG structure. Additionally, a significant diffraction peak occurred in XRD spectrum of graphite at 2θ = 24.28, corresponding to the (002) plane of the hexagonal phased graphite.33 The other diffraction peak centered at 2θ = 24.28 can be ascribed to the reflection of the (100) plane of the hexagonal phased graphite. Furthermore, the corresponding Raman spectra are shown in Figure 1b. All the five samples displayed the same peak locations in the Raman spectra, indicating that this method brings no changes to the FG structure, which agrees well with the result obtained from the XRD patterns. The Raman spectra showed two distinctive peaks at 1343 and 1594 cm−1, respectively. The peaks at 1343 cm−1 are labeled as the D band, which are associated with the A1g vibrational modes of carbon atoms related to the dangling bonds for the in-plane terminated disordered graphite due to the lattice distortion of the graphite crystals. The peaks at 1594 cm−1 are ascribed to the G band, which are corresponded to the E2g mode for the vibration of graphitic sp2-bonded carbon atoms in the two-dimensional (2D) hexagonal structure, such as in a graphene layer.34 Additionally, the relative intensity ratio of the D−G band (ID/IG), can give reliable information about the graphitization degree.35 It can be found that ID/IG for the three samples is about 1, implying a highly disordered graphitic structure, which is consistent with the XRD patterns. The surface morphologies of these samples are characterized by using SEM and TEM, shown in Figures 2 and 3, respectively. It can be seen that the raw FG generally has large particle size with flaky morphological structure with uneven lengths and thickness, shown in Figure 2a. Since the selected FG has nonuniform sizes of lengths and thickness, it is farfetched to compare the morphology size of the samples before and after plasma treatment. However, it still can be seen that the raw FG displays relatively smooth surface and relatively large morphology size, in comparison with the FG samples treated with plasma. As plasma treatment time increases, the FG surface becomes much rougher and its morphology size including the length and thickness becomes smaller, which can be ascribed to the exfoliated flakes of graphite edges during the

Figure 2. SEM images of the obtained samples: (a) raw FG, (b) PTFG-1, (c) PTFG-2, (d) PTFG-3, (e) PTFG-4.

Figure 3. TEM images of the obtained samples: (a) raw FG, (b) PTFG-1, (c) PTFG-2, (d) PTFG-3, (e) PTFG-4.

plasma treatment (Figure 2b−e). Meanwhile, a large number of tiny cracks as well as appended fine particles appear on the FG surface. This phenomenon obviously indicates that the plasma treatment can have a great impact on the micron scale of the outer surface structure. Therefore, it can be anticipated that air plasma treatment can lead to microetch formations on FG surfaces.36 Moreover, the excited species produced during the plasma processing can bring about some changes to the composites surface.37 Additionally, the effect of grafted functional groups onto FG can be observed from the TEM micrograph of FG. Comparing with raw FG and PTGF-1 materials, the particles of PTFG-2, PTFG-3, and PTFG-4 are 4075


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Figure 4. (a) FT-IR spectra of the prepared samples. (b) Zeta potential curves of the prepared samples as a function of pH.

result in higher vibration intensities at 1201 cm−1, which may result from that more air molecules are ionized with long treatment time. Furthermore, no signal of the integration of the phosphate group can be found at about 1090 cm−1, indicating that the whole O-PEA was not grafted onto FG surface during the plasma treatment.43 It can be inferred that the possible mechanism of the plasma treatment can be plasma induced etching and functionalization. Additionally, zeta potential was carried out to measure the sequential binding of the proton on the prepared samples surface. It is found that there are some differences among the pHPZC values of the five prepared samples, shown in Figure 4b. This phenomenon can be ascribed to the coexistence of both −NH2 and phenolic hydroxyl group. The PTFG-1 sample owns the pHPZC value of 2.54, which can be ascribed to the large amount of phenolic hydroxyl group produced during the plasma treatment. The PTFG-4 sample has a pHPZC value of 2.96, higher than that of PTFG-1. This phenomenon can be ascribed to the existence of both −NH2 and phenolic hydroxyl group on the sample surface. The sample surface charge intends to be positive when its surface is loaded alkaline groups (−NH2). Thus, the pHPZC value of PTFG-4 sample is higher than that of PTFG-1. The results are in agreement with the previous studies.44,45 To characterize the degree of functionalization of PTFG-2, PTFG-3, and PTFG-4, samples were analyzed by means of TGA as depicted in Figure S3. According to the results, FG shows two weight loss stages. The first stage around 70 °C can be ascribed to the loss of adsorbed water molecules (5.94 wt %). The FG shows no significant weight loss up to 456 °C. Then the second stage weight loss can be ascribed to the carbon decomposition. As for PTFG-2, PTFG-3, and PTFG-4, the modified samples show two weight loss stages. A major weight loss between 0 and 456 °C for all modified samples is assigned to the loss of adsorbed water molecules and degradation of grafted −NH2 and corresponds to the weight loss of 15.67, 17.71, and 19.94 wt % for PTFG-2, PTFG-3, and PTFG-4, respectively. Then the second stage weight loss can also be ascribed to the carbon decomposition. Thus, it can be concluded that the weight percent of −NH2 in FG sample is calculated to be 9.73, 11.77, and 14.40 wt % for PTFG-2, PTFG-3, and PTFG-4, respectively. Additionally, since the PTFG-1 was not modified with −NH2, which shows similar weight loss with FG, TGA curve of PTFG-1 was not presented. The specific BET surface area, pore volume, and average pore size of these samples were listed in Table 1, and the corresponding adsorption isotherms are shown in Figures S4−S6. It can be seen that the BET surface area of FG suffers a decrease after the plasma treatment and longer treatment time

getting agglomerated, probably due to the cohesive force originated from newly formed O-PEA functional groups. Similar phenomena have been reported in previous studies.38−40 For example, Borah et al. reported that the agglomeration of XC-72 carbon after acid treatment because of effect of cohesive attraction forces from the introduction of acidic functional groups on XC-72 carbon surfaces.38 Similarly, Wang et al. have also reported a very similar microstructure transformation of XC-72 carbon modified by SDBS (sodium dodecylbenzenesulfonate), which also resulted from the cohesive force generated from the introduction of SDBS functional groups.39 Additionally, Wang et al. also found that MWCNTs (multiwalled carbon nanotubes) modified by CMC (carboxymethyl cellulose) had a more compact stacking morphology compared with as-prepared MWCNTs, which was also ascribed to the strong interactions among the functional groups of CMC on MWCNT surfaces.40 The functional groups of the prepared samples were characterized by FT-IR, shown in Figure 4a. For FG and PTFG-1, the band at around of 1579 cm−1 can be related to CC bonds stretching vibration in unsaturated aromatic structure. However, in PTFG-2, PTFG-3, and PTFG-4, stretching vibration of CC bonds red-shifted to 1573 cm−1,41 which can be attributed to the −NH2 modified onto the FG surface because of the −NH2 bending modes. Generally, −NH2 groups have their bending vibrations at 1568 cm−1 while CC bonds have their stretching vibration at 1568 cm−1. The −NH2 bending vibrations can be covered up by the vibrations of aromatic nucleus within the FG samples, further leading to the red shift in the absorption of the CC bond. Additionally, two weak stretching peaks can be found at 2962 and 2906 cm−1 in PTFG-2, PTFG-3, and PTFG-4, which can be ascribed to the −NH2 antisymmetric and symmetric modes, respectively.42 This result is consistent with the FT-IR of pure O-PEA, shown in Figure S1. Furthermore, the EDS measurements also confirmed the existence of N in PTFG-4, shown in Figure S2. It can also be observed that PTFG-4 owns the highest intensity of stretching vibration at 1573 cm−1 among the five samples while FG and PTFG-1 own the same minimum vibration intensities, which also indicates the successful grafting of −NH2 on PTFG-2, PTFG-3, and PTFG-4 samples. Additionally, the absorption peak appeared at 1201 and 1720 cm−1, which relates to the C−O (phenolic hydroxyl group) and CO symmetric stretching vibration. Both the appearances of C−O and CO symmetric stretching vibration can be related to the oxidized hydrogen atom on FG surface during both the acid and plasma treatment. It can be seen that the samples treated by longer plasma treatment time 4076


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including dynamics, thermodynamics, and adsorption mechanism. Since the adsorption equilibrium time is found to be within 2 h, it is considered as the contact time for our further research. Since the adsorption kinetics reflect the uptake rate of contaminant during the adsorption process, three kinetic models, including the pseudo-first-order model,49 pseudosecond-order model,50 and intraparticle diffusion model,51 are utilized to understand the underlying adsorption mechanisms. These equations and their nomenclatures are listed in Table S1. For these equations listed as above, the calculated parameters are obtained from the fitted curves shown in Figure 5 and the results are listed in Table 2. A higher correlation coefficient (R2 = 0.9995) and a value of qe,cal (98.04 mg·g−1) of the pseudosecond order model that is close to qe,exp (96.42 mg·g−1) indicates that the adsorption process follows the pseudo-second order model better than the pseudo-first-order model. Additionally, intraparticle diffusion gives poor R2 values and multilinearity, which suggests that several stages may consist in the whole U(VI) entrapment process. The initial stage can donate surface diffusion, which derives from the U(VI) diffusion from the solution to the adsorbent surface. The second stage is a slow diffusion stage, ascribing to the U(VI) diffusion from adsorbent surface to intraparticle active sites (particle diffusion). The final stage donates the adsorbate retention on the active sites, which is considered as an instantaneous entrapment stage and can be regarded as negligible. Additionally, from the fitted curves, it can be clearly observed that all the three fitted straight lines do not pass through the origin, suggesting a two-step rate controlling adsorption process, i.e. film diffusion as well as intraparticle diffusion. Effect of pH and Ionic Strength. The liquid solution pH values and ion strength are important parameters for U(VI) entrapment in aqueous solution since they can not only affect the adsorbent surface charge but also affect the speciation of the adsorbate.52 Figure 6 displays the pH effect on U(VI) entrapment by PTFG-4 in three NaNO3 concentrations. The solution pH values have a great influence on U(VI) entrapment by PTFG-4 and adsorption efficiency of U(VI) onto the PTFG4 increases drastically with increasing pH (pH 2−7), indicating that the adsorption process is clearly pH-dependent. This phenomenon suggests that U(VI) entrapment by PTFG-4 is dominated by surface complexation.1,53 The pHpzc of PTFG-4 is measured to be ∼2.96 from the zeta potential measurement (Figure 4b). This means that when the

Table 1. Pore Structure Parameters of FG, PTFG-1, and PTFG-4 sample

BET surface area (m ·g )

pore volume (cm ·g )

average pore size (nm)

FG PTFG-1 PTFG-4

632.18 575.62 522.14

0.4116 0.3755 0.3589

2.60 2.61 2.75

2

−1

3

−1

results in smaller BET surface area of the samples. Additionally, it can also be seen that pore volume slightly decreased. All of these may be due to the increase of functional groups and surface defects on the surface of FG introduced by plasma treatment.46 And similar results have been found in previous reports.47,48 Moreover, based on the adsorption experiments, PTFG-4 owns the highest adsorption capacity for U(VI) entrapment even with minimum specific BET surface area while FG owns the lowest adsorption capacity for U(VI) entrapment with maximum specific surface area. Thus, it can be concluded that adsorption capacity for U(VI) entrapment is related with the functional groups on its surface instead of its specific BET surface area in this study. Contact Time Effect and Kinetics Study. The adsorption rate is commonly considered as a crucial criterion to evaluate the potential applications of the adsorbent. Thus, to investigate the effect of plasma treated time and reacting time on U(VI) entrapment by the samples, the adsorption kinetics of U(VI) onto the five different samples were compared at pH 6 with its concentration of 0.25 g·L−1 at 293.15 K, shown in Figure 5. It displays that the adsorbents treated by air nonthermal plasma have much better U(VI) removal performance than FG without treatment. Meanwhile, the FG samples treated with O-PEA shows higher removal capacity for U(VI) entrapment and longer treatment time resulted in higher efficiency. The results shows that the plasma etching can greatly increase the active sites of FG samples and lead to the successful grafting of −NH2 on FG surface. The PTFG-4 has the highest removal capacity for U(VI) entrapment, and it reaches the adsorption equilibrium in 120 min, indicating a very rapid process. The high adsorption efficiency and capacity indicate that PTFG can be applied in practical U(VI) entrapment from large volumes of wastewater solution, which also suggests that the plasma treated method is an effective route to modify adsorbent for the removal of contaminants. Besides, since PTFG-4 owns highest removal efficiency among the five samples, the adsorption data obtained from this adsorbent were selected for further studies,

Figure 5. Different kinetic models for U(VI) adsorption onto the adsorbents. (a) Pseudo-first-order. (inset) Pseudo-second-order. (b) Intraparticle diffusion, pH = 6.0 ± 0.1, CU(VI)initial = 30 mg·L−1, m/V = 0.25 g·L−1. 4077


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ACS Sustainable Chemistry & Engineering Table 2. Parameters for Pseudo-First-Order and Pseudo-Second-Order Models adsorbate FG

PTFG-1

PTFG-2

PTFG-3

PTFG-4

pseudo-first-order k1 (min−1) qe (mg·g−1) R2 k1 (min−1) qe (mmol·g−1) R2 k1 (min−1) qe (mmol·g−1) R2 k1 (min−1) qe (mmol·g−1) R2 k1 (min−1) qe (mmol·g−1) R2

pseudo-second-order 0.2031 69.40 0.9644 0.2438 77.47 0.9317 0.3170 86.99 0.8968 0.3475 91.10 0.8905 0.3054 95.18 0.8570

k2 (g·mg·min−1) qe (mg·g−1) R2 k2 (g·mg·min−1) qe (mg·g−1) R2 k2 (g·mg·min−1) qe (mg·g−1) R2 k2 (g·mg·min−1) qe (mg·g−1) R2 k2 (g·mg·min−1) qe (mg·g−1) R2

2.467 × 72.57 0.9994 3.050 × 80.13 0.9994 3.975 × 89.77 0.9996 4.511 × 93.72 0.9997 3.563 × 98.04 0.9995

10−3

10−3

10−3

10−3

10−3

Figure 6. (a) Effect of pH and the ionic strength on U(VI) entrapment by PTFG-4, T = 293.15 K, CU(VI)initial = 30 mg·L−1, m/V = 0.25 g·L−1. (b) Distribution of aqueous U(VI) species as a function of the pH values.

repulsion between U(VI) species and PTFG-4, and enhance the adsorption capacity for U(VI) entrapment, consequently. Since the concentration of U(VI) adsorbed onto the solid surfaces is comparatively large, it is possible for U(VI) to form uranyl hydroxides on PTFG-4 surface. Therefore, the hydrolysis of U(VI) occurring on the PTFG-4 surface can account for the elevated entrapment performance beginning at pH 5. When pH > 7.5, a decreased positive uranium species such as (UO2)3(OH)5+ and an increased negatively charged uranium species like (UO2)3(OH)7− were observed, inducing electrostatic repulsive force between PTFG-4 and (UO2)3(OH)7− due to the same negatively charged surface, causing the undesired adsorption decline. This electrostatic repulsion forces can explain the decreased U(VI) entrapment by PTFG-4. Thus, it can be concluded that the U(VI) entrapment process can be achieved via electrostatic attraction, which is identical to that of other materials for U(VI) entrapment, such as, AO-Fe3O4/ P(GMA-AA-MMA),1 graphene oxide,5 AOGONRs,52 graphene oxide nanosheets,54 and P(IA/MAA)-g-NC/NB.55 The effect of ionic strength on U(VI) entrapment by PTFG-4 in 0.001, 0.01, and 0.1 mol·L−1 NaNO3 is also shown in Figure 6a. It can be clearly seen that weak or even no connection is found between the U(VI) entrapment and ionic strength within the pH investigated, indicating that inner-sphere surface complexation is the primary adsorption mechanism in U(VI) entrapment rather than outer-sphere surface complexation or ion exchange.54 Effect of Initial U(VI) Concentration and Adsorption Isotherms. Generally, equilibrium experiments are considered

solution pH < pHpzc, the surface charge of PTFG-4 is positive due to the protonation reaction (−RSH(sur) + H+(aq) ⇌ −RSH2+(sur), where −R represents the PTFG-4 surface and −SH represents the surface functional groups). Under this circumstances, it will be difficult for the positive U(VI) ions to be adsorbed onto the positively charged surface PTFG-4 because of the electrostatic repulsive force. When the solution pH > pHpzc, the surface charge of PTFG-4 becomes negative, which can be ascribed to the deprotonation reaction (−RSH(sur) + OH−(aq) ⇌ RSH−(sur)+ H2O). Additionally, as can be seen from the species distribution of U(VI) under different pH values from Figure 6b and Table S2, the dominant U(VI) species is UO22+ at pH < 5. When the pH solution was adjusted from 5 to 8, the dominant U(VI) species in aqueous solution are UO2(OH)+, (UO2)3(OH)5+, and (UO2)4(OH)7+. Furthermore, when pH > 8, (UO2)3(OH)7−, UO2(OH)3−, and UO 2 (OH)4 2− would become the main U(VI) species distribution in solution. It can be seen that PTFG-4 still owns the removal efficiency of in excess of 50% at low solution pH values, which can be attributed to the binding interactions between UO22+ and the surface functional groups with considerate amount. When solution pH values increase from 5−7, the amount of negative charges on the PTFG-4 surface increases and the electrostatic attractions among the U(VI) species (UO2(OH)+, (UO2)3(OH)5+ and (UO2)4(OH)7+), and PTFG-4 become stronger, which results in the increase of adsorption capacity for U(VI) entrapment by PTFG-4. Besides, the protons produced during the complexation reaction can also help neutralize the solution pH, decrease the electrostatic 4078


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Figure 7. (a) Experimental equilibrium data of the five samples at 293.15 K. (b) Langmuir model (solid line) and Freundlich model (dash line) of PTFG-4. (c) D-R model and (d) Temkin model of PTFG-4.

listed in Table S3. And, the detailed information on each equation is described in the Supporting Information. On the basis of the equations as mentioned above, correlation coefficient (R2) coupled with relevant parameters originated from the fitted curves are shown in Table 3. It can be

as an effective way to understand various adsorption mechanisms in adsorption processes. In this study, equilibrium experiments were carried out with an adsorbent concentration of 0.25 g·L −1 accompanied by various initial U(VI) concentration. Meanwhile, the experiments were conducted under pH 6.0 ± 0.1 to avoid the possibility of U(VI) removal other than adsorption, precipitation for example. Figure 7a displays the equilibrium adsorption capacity of U(VI) onto the five samples as a function of different initial U(VI) concentrations. The equilibrium adsorption capacity for U(VI) entrapment increased significantly with the increase of initial U(VI) concentration increased until equilibrium was reached. This phenomenon indicates that the mass transfer resistance of U(VI) between the liquid and solid phase is impaired to large extent by a key driving force generated by initial U(VI) concentration. Additionally, it can also be observed that the PTFG-4 has the highest removal capacity among these five samples for U(VI) entrapment, which can be ascribed to the massive active and amino sites onto PTFG-4 produced during plasma treatment. Meanwhile, the higher adsorption capacity of PTFG-1 over that of FG can be attributed to the large number of active and hydroxyl on FG surface generated during plasma treatment. However, PTFG-1 and PTFG-4 were dealt with the same plasma treatment time, there still exists a difference in their adsorption capacities for U(VI) entrapment. This experimental result can be attributed to the grafted −NH2 onto PTFG-4, further demonstrating that the grafted −NH2 plays a main role in U(VI) entrapment comparing with that of −OH. Furthermore, to quantify the adsorption capacity and understand the specific adsorption characteristics of the modified adsorbent more adequately, Langmuir,56 Freundlich,57 Dubinin and Radushkevich (D− R),58 and Temkin,59 the four most commonly used models, are adopted to analyze the adsorption isotherms, which are also

Table 3. Isotherms Parameters for Langmuir, Freundlich, D−R, and Temkin Models parameters −1

KL (L·mg−1)

models

T (K)

Q0 (mg·g )

Langmuir

293.15 313.15 333.15

112.64 1.0371 120.66 1.2951 140.68 1.6079 KF (mg1−n·Ln·g−1) n

Freundlich

293.15 313.15 333.15 β (mol2·J−2)

D−R

293.15 313.15 333.15

69.99 78.68 95.55 Q0 (mg·g−1)

1.293 × 10−9 1.200 × 10−9 1.153 × 10−9

227.85 236.53 276.07 B

Tempkin

293.15 313.15 333.15

15.59 15.84 18.15

R2 0.9996 0.9990 0.9988 R2

6.066 0.9180 6.435 0.9231 6.583 0.9254 E (kJ·mol−1) R2

19.66 20.41 20.82 KT (L·g−1) 71.98 122.50 171.84

0.9353 0.9385 0.9402 R2 0.9485 0.9490 0.9500

clearly seen that the adsorption data fit the Langmuir isotherm model better than Freundlich isotherm model, demonstrating that the PTFG-4 provided the U(VI) entrapment with specific homogeneous sites. Moreover, the Qmax value calculated by the Langmuir model, increased with the rise of environmental temperature, demonstrating an endothermic adsorption process, which is in accordance with the better adsorption performance under escalated temperature condition. Addition4079


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ACS Sustainable Chemistry & Engineering ally, the Langmuir dimensionless separation factor RL is able to predict the affinity between adsorbate U(VI) and adsorbent: 1 RL = 1 + bC0

entrapment are larger than that of PTFG-4 here. For instance, polyaniline modified GO70 shows high adsorption capacity for U(VI) entrapment with 242.52 mg·g−1, which is also significantly larger than that of PTFG-4, which can also be ascribed to the amino groups on its surface. Additionally, the maximum adsorption capacity for U(VI) entrapment onto amidoximated magnetite/GO is up to 284.9 mg·g−1, which can be ascribed to numerous amino groups on its surface.70 Therefore, grafting adsorbents with some specific functional groups is an effective approach to enhance adsorption performance of adsorbents toward U(VI) entrapment. Besides, in some earlier reported investigations, other carbon materials have been applied as adsorbents to absorb U(VI) from aqueous solution as well. These adsorbents as mentioned above exhibit much lower adsorption capacity for U(VI) entrapment than that of PTFG-4. Although it is farfetched compare the adsorption performance of PTFG-4 with other adsorbents for U(VI) entrapment directly regardless of the different operating experimental conditions, it is still conspicuous that the adsorption capacity for U(VI) entrapment of PTFG-4 is higher than most of the other carbon materials. Furthermore, although some other adsorbents shows higher adsorption capacity for U(VI) entrapment, their modification method is not environmental friendly and complicated in most cases. Thus, given its environmental significance, cost-effectiveness and accessibility, PTFG-4 possesses huge potential to be an appropriate adsorbent for U(VI) entrapment from wastewater in practical application. Adsorption Thermodynamics. Since U(VI) entrapment is closely related to operating temperature, thermodynamic analysis was made to study the influence of temperature on U(VI) entrapment by PTFG-4 adsorbent. It is well-known that during the adsorption process, there are two predominant effects brought by temperature. Decreased solution viscosity and increased metal ion mobility can both be result from increasing temperature. And Qmax, the maximum amount of adsorbed U(VI) calculated from the Langmuir isotherm model, escalated gradually as the environmental temperature rises (Figure 8a), suggesting a favorable U(VI) entrapment at higher temperature. Moreover, thermodynamic parameters (ΔHo, ΔSo, and ΔGo) of U(VI) adsorption onto PTFG-4 are calculated from the slope and intercept of the ln Kd vs 1/T curve (Figure 8b), which is on the basis of the approach previously reported by Lyubchik et al.71 The values of ΔHo and ΔSo are calculated based on the following equations:72

The RL value can be used to estimate the reversibility of the adsorption process, i.e., is RL = 0 (irreversible), 0 < RL < 1 (favorable), RL = 1 (linear), or RL > 1 (unfavorable).60 Since the b value is positive, all the RL values are in the range of 0−1 for all tested U(VI) concentrations, indicating that the adsorption process for U(VI) entrapment is favorable. Furthermore, it can be seen from Table 3 that the E (E = (2β)−1/2) values obtained from the D−R isotherm model are 19.66 (T = 293.15 K), 20.41 (T = 313.15 K), and 20.82 kJ· mol−1 (T = 333.15 K). It is known that if E > 8 kJ·mol−1, the adsorption process is mainly adopted by chemical ion exchange, whereas in the case of E < 8 kJ·mol−1 physical forces may dominate. All the E values obtained are larger than 8 kJ·mol−1, indicating that chemical ion exchange is the predominant adsorption mechanism. Additionally, the distinct disparity of the maximum adsorption capacities Qmax based on Langmuir model and D−R model obtained from three different temperatures may be arising from the differences of their basic assumptions of the two models. Additionally, the R2 values obtained from the fitted curve of Tempkin isotherm are comparatively larger, giving another indication of uniform binding energy distribution of the adsorption process, which is totally in agreement with the conclusion reached from the Langmuir model. The potential practical applications were also evaluated, as listed in Table 4 with maximum capacity (Qmax) for U(VI) Table 4. Consolidated List of Adsorbents Applied for U(VI) Entrapment from Aqueous Solutions adsorbent sample

adsorption capacity (mg·g−1)

activated carbon multiwalled carbon nanotubes activated charcoal CMC grafted MWCNTs chitosan grafted MWCNT’s hydrazine reduced GO mesoporous carbon CMK-5 NH3-GO graphene oxide nanosheets PAM/GO composites polyaniline modified GO amidoximated magnetite/GO PTFG-4

10.47 24.9 28.8 39.2 39.2 47 65.4 80.13 97.5 166.2 242.52 284.9 140.68

ref 61 62 63 47 64 65 66 67 59 68 69 70 present study

ln Kd =

ΔS o ΔH o − R RT

ΔGo = ΔH o − T ΔS o Where R (8.314 J·mol−1·K−1) is the ideal gas constant, and T (K) is the temperature in Kelvin. The thermodynamic parameters are listed in Table 5. It can be clearly seen that the ΔGo values are all negative for U(VI) entrapment, suggesting that the U(VI) entrapment onto PTFG-4 is favorably spontaneous. Moreover, the value of ΔGo becomes more negative as the temperature increases, suggesting energetic favorable adsorption in high temperature condition. Besides, a positive value of ΔHo manifests that the adsorption is an endothermic process in nature. This phenomenon can be ascribed to that the endothermic energy of dehydration exceeds the exothermic energy of the ions attaching to the solid surface. Additionally, positive ΔSo value

entrapment onto PTFG-4 and other adsorbents reported in previous studies. The Qmax for U(VI) entrapment onto PTFG-4 is calculated to be 140.68 mg·g−1 at 333.15 K, which is above the moderate level of adsorption capacity for U(VI) entrapment. Generally, the specific surface functional groups of adsorbents can be beneficial to improve adsorption capacity for U(VI) entrapment. For example, without surface functional groups of raw MWCNTs40 and GO nanosheets,54 their maximum adsorption capacity for U(VI) entrapment is only 16.2 and 97.5 mg·g−1, respectively, which is smaller than that of PTFG-4 in this study. However, for the other modified adsorbents, their maximum adsorption capacities for U(VI) 4080


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Figure 8. (a) Temperature effect on the U(VI) entrapment onto PTFG-4. (b) Variation of ln Kd with T−1 (K−1).

Table 5. Thermodynamic Parameters for U(VI) Entrapment onto PTFG-4 temperature (K)

ln Kd (L·g−1)

ΔGo (kJ·mol−1)

ΔHo (kJ·mol−1)

ΔSo (J·mol−1·K−1)

293.15 313.15 333.15

7.463 8.071 8.571

−18.204 −20.981 −23.758

22.496

138.84

only one peak at 399.7 eV,74 which can be ascribed to the nitrogen of the NH2 species (C−NH2).1 Additionally, compared N 1s XPS peak of PTFG-4 with the corresponding peak of PTFG-4-U (Figure 10b), it can be seen that N 1s peak of C−NH2 moves toward higher binding energy, indicating that the nitrogen atoms suffer from decrease of electron density. The result demonstrates that U(VI) can interact with the −NH2 during the adsorption process. The O 1s peak fitting spectra of both PTFG-4 and PTFG-4-U are also shown in Figure 10c and d, respectively. The O 1s spectra could be resolved into three main peaks at 532.1, 531.2, and 533.2 eV, assigned to CO, absorbed O2−, and phenolic hydroxyl groups, respectively.73 Similarly, making a contrast with O 1s peak position of PTFG-4 with the corresponding peak of PTFG-4-U, the O 1s peak originated from terminal phenolic hydroxyl group moves toward higher binding energy (533.5 ev) as well while the binding energy of absorbed O2− and CO remained unchanged, suggesting that U(VI) can not only interact with −NH2 but also with a phenolic hydroxyl group. Thus, it can be concluded that the adsorption mechanism for U(VI) entrapment is achieved through the complexation of U(VI) with both −NH2 and phenolic hydroxyl group. And the adsorption mechanism of U(VI) entrapment by PTFG-4 is described in Scheme 1. Moreover, compared with the adsorption capacity of FG and PTFG-1 for U(VI) entrapment, the higher adsorption capacity of PTFG-1 can be ascribed to the large amount of phenolic hydroxyl group generated during the plasma treatment. Thus, a plausible adsorption mechanism can also be devoted to the phenolic hydroxyl group during the acid treatment. The adsorption mechanism of FG can be described as follows. The first step involves the electrostatic forces between FG and U(VI). This electrostatic attraction would promote the migration of U(VI) to the surface of FG. The second step includes U(VI) diffusion from boundary layer to the FG surface, also named film diffusion. Moreover, a pore diffusion occurred by migrating U(VI) from FG surface into the pores of FG, which is also considered as a rate-determining step. Finally, a combination of U(VI) with phenolic hydroxyl group of FG completed the adsorption process. Desorption and Reusability Studies. Desorption is a critical process in adsorption studies for an appropriate

manifests that the adsorption process is entropy-driven, suggesting an improved freedom degree at the solid−liquid interface during the process of U(VI) entrapment onto PTFG4. Thus, it can be concluded that the adsorption process for U(VI) entrapment onto PTFG-4 is naturally endothermic and spontaneous. Possible Adsorption Mechanism. Generally, XPS analysis is applied to provide oxidation states, chemical compositions, relative concentrations and bonding relationships for all surface and near-surface elements, further exploring the interaction mechanism between adsorbate and adsorbent. In this study, the XPS spectra of survey and high resolution scans for C 1s, O 1s, N 1s, and U 4f were recorded.1 The XPS spectra of the PTFG-4 before and after U(VI) entrapment (donated as PTFG-4-U) are depicted in Figure 9. Two U 4f peaks (U 4f5/2,

Figure 9. Typical XPS survey spectra of PTFG-4 and PTFG-U.

393.5 eV; U 4f7/2, 382.6 eV) can be clearly observed in PTFG4-U. The peak at 382.6 eV can be ascribed to the free U(VI) adsorbed on PTFG-4, and the peak at 381.1 eV can be attributed to covalent bond of amido-U(VI),73 indicating that U(VI) is successfully absorbed onto the PTFG-4 surface. Additionally, compared to the raw PTFG-4, the intensity of N 1s decreased after U(VI) entrapment, further demonstrating the binding interaction between −NH2 and U(VI). The XPS N 1s spectra of PTFG-4 are shown in Figure 10a. It can be clearly seen that the N 1s spectrum of PTFG-4 shows 4081


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Figure 10. High resolution XPS spectra of N 1s and O 1s of PTFG-4 before and after U(VI) entrapment.

Scheme 1. Probable Adsorption Mechanism of U(VI) Entrapment by PTFG-4

Figure 11. Recycling of PTFG-4 in the removal of U(VI) from aqueous solution (T = 293.15 K, pH = 6.0 ± 0.1, CU(VI)initial = 30 mg· L−1, m/V = 0.25 g·L−1).

FG samples with different treatment time are used as contrast adsorbents for U(VI) entrapment. The adsorption experiments show that the modified FG has higher U(VI) entrapment efficiency than others, and longer treatment time results in higher efficiency, demonstrating that the plasma treatment can greatly increase the active sites of FG samples and lead to the successful grafting of −NH2 on the FG surface. The −NH2 modified FG with 2 h treatment time shows the highest adsorption capacity among the five samples at 333.15 K. Additionally, the adsorption process is dependent on the solution pH values rather than ionic strength, indicating the inner-sphere surface complexation dominates the adsorption of U(VI) onto PTFG-4. The macroscopic experiments can be satisfactorily fitted by pseudo-second-order kinetics model and Langmuir isotherm model, indicating a monolayer adsorption process. Thermodynamic studies reveal that the U(VI) entrapment process is spontaneous and entropy-driven endothermic. XPS studies reveal that the adsorption mecha-

adsorbent considering its enhancement of the economic value. As illustrated in Figure 6, the PTFG-4 shows poor adsorption capacity at lower solution pH, implying that the absorbed U(VI) ions might be desorbed from PTFG-4 by acid medium. Thus, HCl with concentration of 0.1 mol·L−1 was chosen as the desorbing agent to characterize the regeneration−reusability property. The corresponding results are shown in Figure 11. It can be seen that PTFG-4 shows slight decrease in adsorption capacity, which can be attributed to the incomplete desorption of U(VI) from PTFG-4. The high regenerated availability suggested that PTFG-4 could be applied repeatedly as an effective material for entrapment of U(VI) from aqueous solutions.

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CONCLUSIONS In this study, FG modified with −NH2 functional groups was successfully fabricated via nonthermal plasma treatment. The 4082


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(9) Mellah, A.; Chegrouche, S.; Barkat, M. The precipitation of ammonium uranyl carbonate (AUC): Thermodynamic and kinetic investigations. Hydrometallurgy 2007, 85, 163−171. (10) Raff, O.; Wilken, R. D. Removal of dissolved uranium by nanofiltration. Desalination 1999, 122, 147−150. (11) Kryvoruchko, A. P.; Yurlova, L. Y.; Atamanenko, L. D.; Kornilovich, B. Y. Ultrafiltration removal of U(VI) from contaminated water. Desalination 2004, 162, 229−236. (12) Akhavan, B.; Jarvis, K.; Majewski, P. Plasma polymerfunctionalized silica particles for heavy metals removal. ACS Appl. Mater. Interfaces 2015, 7, 4265−4274. (13) Jarvis, K. L.; Majewski, P. Plasma polymerized allylamine coated quartz particles for humic acid removal. J. Colloid Interface Sci. 2012, 380, 150−158. (14) Gao, J.-K.; Hou, L.-A.; Zhang, G.-H.; Gu, P. Facile functionalized of SBA-15 via a biomimetic coating and its application in efficient removal of uranium ions from aqueous solution. J. Hazard. Mater. 2015, 286, 325−333. (15) Kilislioglu, A.; Aras, G. Adsorption of uranium from aqueous solution on heat and acid treated sepiolites. Appl. Radiat. Isot. 2010, 68, 2016−2019. (16) Anirudhan, T.-S.; Bringle, C.-D.; Rijith, S. Removal of uranium(VI) from aqueous solutions and nuclear industry effluents using humic acid-immobilized zirconium-pillared clay. J. Environ. Radioact. 2010, 101, 267−276. (17) Wang, J.-S.; Bao, Z.-L.; Chen, S. G.; Yang, J. H. Removal of uranium from aqueous solution by chitosan and ferrous ions. J. Eng. Gas Turbines Power 2011, 133, 84502−84054. (18) Al-Masri, M. S.; Amin, Y.; Al-Akel, B.; Al-Naama, T. Biosorption of cadmium, lead, and uranium by powder of poplar leaves and branches. Appl. Biochem. Biotechnol. 2010, 160, 976−987. (19) Song, W.-C.; Liu, M.-C.; Hu, R.; Tan, X.-L.; Li, J.-X. Watersoluble polyacrylamide coated-Fe3O4 magnetic composites for highefficient enrichment of U(VI) from radioactive wastewater. Chem. Eng. J. 2014, 246, 268−276. (20) Xin, X.-D.; Wei, Q.; Yang, J.; Yan, L.-G.; Feng, R.; Chen, G.-D.; Du, B.; Li, H. Highly efficient removal of heavy metal ions by aminefunctionalized mesoporous Fe3O4 nanoparticles. Chem. Eng. J. 2012, 184, 132−140. (21) Das, S.; Pandey, A. K.; Athawale, A. A.; Manchanda, V. K. Exchanges of Uranium(VI) species in amidoxime-functionalized sorbents. J. Phys. Chem. B 2009, 113, 6328−6335. (22) Akhavan, B.; Jarvis, K.; Majewski, P. Hydrophobic plasma polymer coated silica particles for petroleum hydrocarbon removal. ACS Appl. Mater. Interfaces 2013, 5, 8563−8571. (23) Yasuda, H.; Matsuzawa, Y. Economical Advantages of LowPressure Plasma Polymerization Coating. Plasma Processes Polym. 2005, 2, 507−512. (24) Olander, B.; Wirsén, A.; Albertsson, A. C. Argon microwave plasma treatment and subsequent hydrosilylation grafting as a way to obtain silicone biomaterials with well-defined surface structures. Biomacromolecules 2002, 3, 505−510. (25) Lewis, T.; Nowling, G. R.; Hicks, R. F.; Cohen, Y. Inorganic surface nanostructuring by atmospheric pressure plasma-induced graft polymerization. Langmuir 2007, 23, 10756−10746. (26) Chung, D. D. L. Review: graphite. J. Mater. Sci. 2002, 37, 1475− 1489. (27) Zhao, M.-F.; Liu, P. Adsorption of methylene blue from aqueous solutions by modified expanded graphite powder. Desalination 2009, 249, 331−336. (28) Zhao, Y.-H.; Zhang, Y.-F.; Bai, S.-L. High thermal conductivity of flexible polymer composites due to synergistic effect of multilayer graphene flakes and graphene foam. Composites, Part A 2016, 85, 148− 155. (29) Chen, J.; Zou, G.-Q.; Zhang, Y.; Song, W.-X.; Hou, H.-S.; Huang, Z.-D.; Liao, H.-X.; Li, S.-M.; Ji, X.-B. Activated flake graphite coated with pyrolysis carbon as promising anode for lithium storage. Electrochim. Acta 2016, 196, 405−412.

nism for U(VI) entrapment is achieved through the complexation of U(VI) with both −NH2 and phenolic hydroxyl group on the surface of modified FG. The nonthermal plasma treatment is a successful method to fabricate efficient heavy metals adsorbents with high regenerated availability. Meanwhile, this novel substrate independent approach could be utilized to convert other inexpensive natural materials to highvalue adsorbents.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00069. Information as mentioned in the text (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel./Fax: +86-551-65596617. E-mail: [email protected]. ORCID

Jiaxing Li: 0000-0002-7683-2482 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (21677146, 21272236, U1530131), the special scientific research fund of public welfare profession of China (201509074), the Jiangsu Provincial Key Laboratory of Radiation Medicine and Protection, and the Priority Academic Program Development of Jiangsu Higher Education Institutions is acknowledged.

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Removal of U(VI) from Aqueous Solution by Amino Functionalized

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