Graphene Oxide-Based FeâMg (Hydr)oxide Nanocomposite as Heavy

Article pubs.acs.org/jced

Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Graphene Oxide-Based Fe−Mg (Hydr)oxide Nanocomposite as Heavy Metals Adsorbent Dayong Huang,†,‡ Boxuan Li,† Min Wu,*,† Shigenori Kuga,† and Yong Huang*,† †

National Engineering Research Center of Engineering Plastics, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, P.R. China ‡ University of Chinese Academy of Sciences, Beijing 100049, P.R. China S Supporting Information *

ABSTRACT: A novel adsorbent for divalent metal cations was prepared as nanocomposite of Fe−Mg (hydr)oxide with graphene oxide by one-step coprecipitation. This material showed adsorption selectivity of Pb2+ > Cu2+ > Ag+ > Zn2+ ≫ Co2+, Ni2+, Cd2+ with high adsorption capacity of 100−600 mg/g for Pb2+, Cu2+, Ag+, and Zn2+. Distribution coefficient (Kd) was as high as ∼107 mL/g for Pb2+ and Cu2+. The adsorption isotherms for Pb2+, Cu2+, Ag+, and Zn2+ followed the Langmuir model, indicating monolayer adsorption. The adsorption kinetics for Pb2+, Cu2+, Ag+, and Zn2+ followed pseudo-second-order model, suggesting chemisorption. Removal of 50 ppm Pb2+ or Cu2+ from 100 mL solution by 0.1 g of the nanocomposite was over 99.7%. The thermodynamics studies implied that the adsorption process toward heavy metals was spontaneous and endothermic. Together with recyclability through magnetic separation, this adsorbent would be useful in polluted water processing. for chemical modification.34,35 In addition, graphene oxide has been proven to be nontoxic and biodegradable.36 So far, graphene oxide has been examined as nanoparticle support for manganese dioxide,23 magnetite,37,38 and magnesium hydroxide.39 Moreover, graphene oxide-based nanocomposites have attracted more and more attention as adsorbents for heavy metals such as ethylenediaminetetraacetic acid-graphene oxide nanocomposite for Pb2+ removal;51 magnetite/graphene oxide composite for Co2+ removal;37 graphene oxide-chitosan composite for Pb2+ and Cu2+ removal;52 SnO2/reduced graphene oxide nanocomposite for Cd2+, Pb2+, Cu2+, and Hg2+ removal;53 graphene oxide nanosheets for U (VI) removal;54 EDTA functionalized magnetic graphene oxide for removal of Pb (II), Hg (II), and Cu (II); and so on.55 In this study, we examined suitability of graphene oxide as supporting medium of Fe−Mg (hydr)oxide nanoparticles. The double oxide was uniformly loaded on graphene oxide by one-step coprecipitation. We studied the adsorption behavior of the product Fe−Mg (hydr)oxide@GO for divalent metal cations and found this nanocomposite has very large adsorption capacity and strong selectivity for Pb2+ and Cu2+. These features would make this material highly useful in environmental remediation efforts.

1. INTRODUCTION Water pollution by toxic heavy metals such as Pb2+, Cu2+, Ag+, and Zn2+, either of natural or artificial origin, is a serious problem in many areas in the world.1−4 Lead is one of the most toxic heavy metals in the environment, which can destroy human brain and nervous systems.5−7 Copper is a crucial element for human health, but its excessive intake increases hepatolenticular degeneration risk.8−10 A large amount of wastewater containing silver and zinc ions occurs in industrial electroplating.11,12 Thus, the efficient removal of harmful heavy metals from water is intensely sought after. Various methods have been applied for removal of heavy metals from water, such as chemical precipitation,13 ion exchange,14 coagulation,15 adsorption,16 ultrafiltration,17 and photocatalysis.18 Of these techniques, adsorption is generally identified as a promising method due to its efficiency, reusability, and low cost.19 Various nanoparticles were examined for removal of heavy metals, such as ferric oxides, manganese oxides, aluminum oxides, titanium oxides, magnesium oxides, zinc oxides, and cerium oxides.8,15,20−24 However, nanoparticle adsorbents suffer from aggregation, leading to the decrease in the number of active sites; therefore, avoiding aggregation is crucial for the adsorption method. For that purpose, immobilization of nanoparticles onto certain substrates would be effective. This has been reported in research using graphene oxide (GO),25,26 cellulose,27,28 clay,29,30 carbon nanotube,31,32 microspheres,33 etc. as substrates. Graphene oxide has large theoretical surface area (∼2600 m2/g) and many oxygen-containing functional groups such as hydroxyl, carboxyl, and epoxy groups, which can be readily used © XXXX American Chemical Society

Received: February 1, 2018 Accepted: May 18, 2018

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DOI: 10.1021/acs.jced.8b00100 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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2. EXPERIMENTAL SECTION 2.1. Materials. Graphene oxide powder was purchased from XFNANO (Nanjing, China) as dry powder. Iron(III) chloride hexahydrate (FeCl3·6H2O), iron(II) sulfate heptahydrate (FeSO4·7H2O), magnesium nitrate hexahydrate (Mg(NO3)2· 6H2O), and ammonia−water (NH3·H2O, 25 wt %) were used for preparing Fe−Mg(hydr)oxide nanoparticles. Lead(II) acetate trihydrate (Pb(CH3COOH)2·3H2O), cupric sulfate (CuSO4), silver nitrate (AgNO3), and zinc sulfate (ZnSO4) were used for test solutions. All the reagents and solvents were purchased from Beijing Chemical Works (Beijing, China) with an analytical grade and used without further purification. All solutions were prepared using distilled water. 2.2. Synthesis of Graphene Oxide-Based Fe−Mg (Hydr)oxide Nanocomposite. About 50 mg of graphene oxide was dispersed by sonication in 100 mL of aqueous solution of FeCl3 (0.081 mol/L), FeSO4 (0.042 mol/L), and Mg(NO3)2 (0.125 mol/L) (total ion concentration 0.25 mol/ L). Under nitrogen atmosphere, 10 mL ammonia−water (25 wt %) was added dropwise into the solution with vigorous stirring at 343 K for 2 h and then kept 363 K for 10 h. The precipitate formed was collected by centrifugation, washed with water several times, and dialyzed against water (dialysis bag with 8000−14 000 molecular weight cutoff). The resulting suspension was freeze-dried from water. For comparison, Fe−Mg (hydr)oxide was synthesized in the same way without adding graphene oxide. 2.3. Characterization. The specific surface area was measured by nitrogen adsorption−Brunauer−Emmett−Teller (BET) method using an automated gas sorption analyzer (Quadrasorb SI-MP). The morphology of synthesized nanocomposite material was observed using a Hitachi S-4800 instrument equipped with energy-dispersive X-ray spectroscopy (EDS). Transmission electron microscopy (TEM) was done using a JEOL JEM-2100 instrument. Atomic force microscope (AFM) images were acquired using a Bruker Multimode 8 instrument. Fourier transform infrared (FTIR) spectra were recorded on an Excalibur 3100 instrument for 400−4000 cm−1. The magnetic properties were characterized using a vibratingsample magnetometer (SQUID-VSM, Quantum Design). 2.4. Adsorption Experiments. Batch adsorption experiments of heavy metals on the composite were carried out in aqueous solution with different initial concentrations of ions under stirring at room temperature for 24 h. The adsorbent dose was 1 g/L for all the experiments. Metal ions were quantified by an inductively coupled plasma emission spectrometer (ICP) (710-OES, Varian). The adsorption capacity qe (mg/g) was given by the following equation: qe =

(C0 − Ce)V m

720 min) at room temperature. The adsorbent dose was 1 g/L for all the experiments. For analyzing adsorption mechanism, the pseudo-first-order and pseudo-second-order kinetic models and intraparticle diffusion model were applied.56 Pseudo-first-order kinetic model: ln(qe − qt) = ln qe − k1t

Pseudo-second-order kinetic model: t 1 t = + 2 qt qe k 2qe

C0 − Ce C0

(4)

Intraparticle diffusion model: qt = kit 1/2 + C

(5)

where qe and qt (mg/g) are the adsorbed amount for heavy metals at equilibrium and at any time t (min), and k1 (min−1) and k2 (g/mg min−1) are the pseudo-first-order and pseudosecond-order rate constants, respectively. Additionally, ki is the rate constant of intraparticle diffusion. 2.6. Recycling. Continuous desorption−adsorption experiments were carried out at room temperature. A certain amount of the composite was first immersed in heavy metal ion solutions, then magnetically separated from the solution, and thoroughly washed with distilled water. Desorption of heavy metal ions was carried out in 100 mL of saturated ethylenediaminetetraacetic acid solution for 6 h. The adsorption− desorption cycles were repeated four times.

3. RESULTS AND DISCUSSION 3.1. Characterization of Nanocomposite. Figure 1 shows scanning electron microscopy (SEM) images of the

Figure 1. SEM images of (a−c) Fe−Mg (hydr)oxide, (d−f) graphene oxide, and (g−i) Fe−Mg (hydr)oxide@GO.

product. The Fe−Mg (hydr)oxide without GO is composed of dense aggregates at any magnification (Figures 1a−c). The graphene oxide as received consists of randomly curved thin flakes (atomic force microscopy (AFM) image of GO is shown in Figure S1), though not all of them are single-layer graphene (Figures 1d−f). The hybrid nanocomposite of these two maintained the sheet-like morphology of GO, which is supporting Fe−Mg (hydr)oxide nanoparticles on the surface (Figures 1g−i). The specific surface areas of Fe−Mg (hydr)oxide and Fe−Mg (hydr)oxide@GO are 27.1 and 113.5 m2/g (the nitrogen adsorption−desorption curves are shown in Figure S3), respectively, indicating that Fe−Mg (hydr)oxide nanoparticles can be dispersed effectively on the GO surfaces to provide adsorption sites. The morphology and element

(1)

The % removal was calculated with the following eq 2: % removal =

(3)

(2)

where C0 (mg/L) and Ce (mg/L) are initial and equilibrium concentrations of the heavy metals in the solution, respectively. V (L) is the solution volume, and m (g) is the adsorbent amount. 2.5. Adsorption Kinetics. Adsorption kinetic batch experiments of Pb2+, Cu2+, Ag+, and Zn2+ on Fe−Mg(hydr)oxide@GO were carried out in various adsorption times (90− B



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contrast, the neat Fe−Mg(hydr)oxide (Figure 3a) consisted of heavily aggregated particles that were not discernible as individual particles. Figure 3f shows the magnetic behavior of Fe−Mg (hydr)oxide@GO showing typical superparamagnetism with saturated magnetization of 25.42 emu/g. The Fe−Mg (hydr)oxide@GO sample maintained magnetism with 15.73 emu/g. This level of magnetism is enough to separate the hybrid nanocomposite by a magnet, as shown in Figure 3f inset. Figure 4 shows FTIR spectra of the samples. Graphene oxide has peaks at 3413, 1720, 1400, 1219, 1053, and 1624 cm−1

distribution of Fe−Mg (hydr)oxide@GO (Figure 2) shows that iron and magnesium are uniformly distributed on GO surface.

Figure 2. (a) SEM image of Fe−Mg (hydr)oxide and EDS elemental maps of the same area for (b) Fe and (c) Mg. (d) SEM image of Fe− Mg (hydr)oxide@GO and EDS maps of (e) Fe, (f) Mg, and (g) C.

Figure 3 shows the TEM images of the products confirming the features seen by SEM. Figure 3c shows good dispersion of Fe−Mg (hydr)oxide particles on the surface of GO. Its electron diffraction (Figure 3d) shows reflections from (220), (311), (400), (422), (511), and (440) of Fe3O438 and (201), (202), (113), and (104) of Mg(OH)2.6,39 Figure S4 shows the XRD patterns of the products; the diffraction peaks of (220), (311), and (440) can be assigned Fe3O4, and (101) and (110) can be assigned Mg(OH)2. The particle size distribution determined by image analysis (Figure 3e, for 400 particles) ranged 2−22 nm, but mostly 4−12 nm. This sharpness of size distribution is likely to result from the nucleation effect of GO when the hydr(oxide) particles are formed by alkaline precipitation. In

Figure 4. FTIR spectra of (a) Fe−Mg (hydr)oxide, (b) graphene oxide, and (c) Fe−Mg (hydr)oxide@GO.

ascribed to −OH, CO, carboxyl OC−O, epoxy C−O−C, C−O groups, and CC stretching, respectively.23,40 The Fe−

Figure 3. TEM images of (a) Fe−Mg (hydr)oxide, (b) graphene oxide, and (c) Fe−Mg (hydr)oxide@GO. (d) Electric diffraction pattern of Fe−Mg (hydr)oxide@GO. (e) Size distributions of Fe−Mg (hydr)oxide nanoparticles loaded on graphene oxide. (f) Magnetic hysteresis loops of Fe− Mg(hydr)oxide and Fe−Mg (hydr)oxide@GO. The inset shows the dispersion and magnetic separation of the Fe−Mg (hydr)oxide@GO in water. C



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Mg (hydr)oxides@GO has peaks at 1720 cm−1 (CO), 1400 cm−1 (carboxyl OC−O) and 1053 cm−1 (C−O) diminshed, and the peak at 1624 cm−1 increased. The OH stretching shifted from 3413 cm−1 of GO to 3433 cm−1 for Fe−Mg (hydr)oxides@GO, suggesting that the metal hydroxide particles were attached to the surface of graphene oxide through oxygen-containing functional groups.23,41 The new peaks at 3699, 1110, 578, and 432 cm−1 were introduced by the metal (hydr)oxides. The strong adsorption bands at 3699 and 432 cm−1 can be assigned to the Mg−O group.39,42 The 1110 cm−1 peak can be ascribed to Fe−OH group.43,44 The 578 cm−1 band might be related to the lattice vibrations of Fe−O−Mg or Mg−O−Mg.45,46 3.2. Heavy Metals Selectivity of the Nanocomposite Material. Selectivity of metal ion adsorption by Fe−Mg (hydr)oxide@GO was tested for a mixed solution of Pb2+, Cu2+, Ag+, Zn2+, Cd2+, Co2+, and Ni2+. Selectivity is evaluated by distribution coefficient Kd, defined as

Table 2. Adsorption of Fe−Mg (Hydr)oxide@GO toward the Seven Mixed Ionsa

Ce (mg/L)

removal (%)

20.52 21.32 19.89 20.13 21.60 18.97 22.56

0.00088 0.0011 0.010 0.023 19.67 17.54 20.85

99.99 99.99 99.95 99.89 8.94 7.54 7.58

107 107 106 105

a

Dose of Fe−Mg (hydr)oxide@GO = 1 g/L, contact time: 12 h. The experiments of adsorption were carried out in room temperature.

after 12 h contact, these species were decreased to less than 1 ppb, achieving virtually complete removal. The adsorbent also showed high adsorption ability for Ag+ and Zn2+, both achieving about 99.9% removal rate and >105 mL/g Kd values within 12 h. Thus, the Fe−Mg (hydr)oxide@GO is a highperformance adsorbent for Pb2+, Cu2+, Ag+, and Zn2+. On the other hand, Table 1 shows the adsorbent is not effective for Cd2+, Co2+, and Ni2+. Table 2 shows the adsorption behavior from a mixed solution of seven metal ions. The adsorbent showed selectivity as Pb2+ > Cu2+ > Ag+ > Zn2+ ≫ Co2+ > Cd2+ > Ni2+. The Kd values of Pb2+, Cu2+, and Ag+ are close to those in single ion adsorption, i.e. not affected by the presence of other ions. 3.3. Heavy Metals Adsorption Behaviors of the Nanocomposite. For understanding the adsorption mechanism of Fe−Mg (hydr)oxide@GO nanocomposite, Langmuir and Freundlich isotherm models (eqs 7 and 8) were used. Ce C 1 = + e qe bQ 0 Q0

99.99 99.31 86.41 56.08 0.52 3.05 1.67

Kd (mL/g) 1.7 × 1.4 × 6.4 × 1.3 × 5.24 31.48 17.01

107 105 104 103

where Kf (mg/g) and n are the adsorption capacity and adsorption intensity of Freundlich constants, respectively. The adsorption isotherms were fitted with eqs 7 and 8 as in Figure 5 and Table 3. It can be noted that the behavior of the cations can be fitted with the Langmuir isotherm with high correlation coefficient (R2 > 0.99), indicating that the adsorption is monolayer-type. On the basis of the Langmuir isotherm model, the theoretical maximum adsorption capacities (Q0) for Pb2+, Cu2+, Ag+, and Zn2+ of Fe−Mg (hydr)oxide@GO were 617.3, 432.9, 142.2, and 121.7 mg/g, which were close to the experimental values of 619.4, 441.5, 144.3, and 121.2 mg/g (Table 3 and Tables S1− S4), respectively. More importantly, ions of Pb2+ and Cu2+ could be effectively down to 8 and 6 ppb even with high initial concentrations of 99 and 61 ppm (Tables S1 and S2), respectively. This could satisfy the international standard for drinking water, so it has enormous potential for application of water treatment. In addition, the removal rate for Pb2+ and Cu2+ could be up to as high as 99.97 and 98.68%, even the initial concentrations of 601.7 and 358.8 ppm (Tables S1 and S2). For comparison, the theoretical maximum Pb2+, Cu2+, Ag+, and Zn2+ adsorption capacity (Q0) of Fe−Mg(hydr)oxide also obtained from Langmuir isotherm model were 210.1, 156.7, 85.5, and 40.7 mg/g (Table S9); these values are much lower than the values of Fe−Mg (hydr)oxide@GO. For comparison, the maximum adsorption capacity (Q0) values of heavy metals on other materials are listed in Table 4. Clearly, the Fe−Mg (hydr)oxide@GO shows an excellent adsorption capacity of 619.4, 441.5, 144.3, and 121.2 mg/g compared to those of other adsorbents for Pb2+, Cu2+, Ag+, and Zn2+, respectively. In addition, the Fe−Mg (hydr)oxide@GO could be separated easily under the external magnetic field and obtained easily by one-step coprecipitation method with low cost. 3.4. Heavy Metals Adsorption Kinetics of the Nanocomposite Material. Figure 6 shows the time course of adsorption and pseudo-second-order kinetic plots for the four metal cations. Under mild stirring for 90 min, the adsorbent removed 99.75 and 99.73% of Pb2+ and Cu2+, respectively. For the Ag+ and Zn2+, the adsorption was slightly slower but still achieved 86.56 and 80.60% removal in 90 min, respectively.

Kd (mL/g) 2.3 × 1.9 × 2.0 × 8.7 × 98.12 81.53 82.01

removal (%)

0.0013 0.15 2.78 8.78 22.89 21.28 19.99

where Ce (mg/L) is the equilibrium concentration and qe (mg/ g) is the adsorption capacity. Q0 (mg/g) and b (L/mg) are the maximum adsorption capacity and adsorption energy related to Langmuir constants, respectively. 1 lnqe = lnK f + lnCe (8) n

Table 1. Adsorption of Individual Seven Ions by Fe−Mg (Hydr)oxide@GOa C0 (mg/L)

Ce (mg/L)

22.38 21.76 20.45 19.99 23.01 21.95 20.33

Dose of Fe−Mg (hydr)oxide@GO = 1 g/L, contact time: 12 h. The experiments of adsorption were carried out in room temperature.

(V [(C0 − Ce)/Ce]) (6) m where C0 (mg/L) and Ce (mg/L) are the initial and equilibrium concentrations of the heavy metals in the solution, respectively. V (mL) is the solution volume, and m (g) is the adsorbent amount.2 In general, materials with Kd values range between 104−105 are regarded as excellent adsorbents.47 The adsorption data for single ions by Fe−Mg (hydr)oxide@GO are listed in Table 1. The Kd values for Pb2+ and Cu2+ are well above 106;

Pb2+ Cu2+ Ag+ Zn2+ Cd2+ Co2+ Ni2+

C0 (mg/L)

Pb2+ Cu2+ Ag+ Zn2+ Cd2+ Co2+ Ni2+ a

Kd =

single ions

mixed ions

(7) D



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Figure 5. Adsorption isotherm for the adsorption of (a) Pb2+, (b) Cu2+, (c) Ag+, and (d) Zn2+ by Fe−Mg (hydr)oxide@GO. (e) Langmuir model and (f) Freundlich model fittings of Pb2+, Cu2+, Ag+, and Zn2+ sorption by Fe−Mg (hydr)oxide@GO. All the experiments were carried out in room temperature, pH 7, Fe−Mg (hydr)oxide@GO dosage = 1 g/L.

The pseudo-second-order kinetic model parameters for the adsorptions toward the four metals are summarized in Table 5. From Figure 6d and Table 5, the excellent fit coefficient (R2) very close to 1.000, suggesting that the adsorption for Pb2+, Cu2+, Ag+, and Zn2+ by Fe−Mg (hydr)oxide@GO can be expressed with pseudo-second-order kinetic model and proved that the adsorption process is chemisorption. The intraparticle diffusion model gives nonlinear fittings of the experimental data (Figure S5), indicating multistage adsorption process.56 3.5. Adsorption Thermodynamics. To study the effect of temperature on the adsorption process, the thermodynamic parameters, Gibbs energy (ΔG), entropy (ΔS), and enthalpy (ΔH) were calculated from eqs 9 and 10.49,50

Table 3. Langmuir and Freundlich Parameters of Fe−Mg (Hydr)oxides@GO for Heavy Metals Adsorption Langmuir model parameters 2+

Pb Cu2+ Ag+ Zn2+

Pb2+ Cu2+ Ag+ Zn2+

R2

b (L/mg)

Q0 (mg/g)

617.284 2.115 432.900 0.951 142.247 0.571 121.654 0.119 Freundlich model parameters

0.9998 0.9965 0.9927 0.9959

Kf (mg/g)

n

R2

352.482 201.341 79.519 62.302

8.117 5.349 7.694 8.804

0.4325 0.9063 0.9616 0.9890

ΔG = −RT ln Kc

ln Kc =

Table 4. Comparison of the Maximum Adsorption Capacity of Different Adsorbents for Heavy Metals

layered double hydroxide intercalated with the MoS42− ion graphene/Co3O4 nanocpmposite functionalized graphene oxide magnetic porous Fe3O4− MnO2 lignosulfonate-graphene oxide-polyaniline ternary nanocomposite Fe−Mg (hydr)oxide Fe−Mg (hydr)oxide@GO

Pb2+

Cu2+

Ag+

Zn2+

288.9

180.9

452.4

58

77

16

310.1

282.3

9

208.2

111.9

ref 2

100.2

216.4

where qe (mg/g) is the adsorption capacity of heavy metals at equilibrium time and C e (mg/L) is the equilibrium concentration of heavy metals solution. ΔH and ΔS can be calculated from the slope and intercept of the linear plot according to ln Kc vs 1/T. The calculated data are presented in Table 6. As shown in Table 6, the ΔG values are all negative, which showed that the adsorption process was spontaneous. The ΔS values are all positive, suggesting that there was an increased randomness during the adsorption of heavy metals on Fe−Mg (hydr)oxide@GO. The positive values of ΔH implied that the adsorption process was endothermic.49,50 3.6. Adsorption Mechanism. The adsorption of heavy metal ions usually includes ion exchange, precipitation, and complexation. In the present case, it could be concluded that chemisorption played a major role in the adsorption process. Figure 7a shows the FTIR spectra of Fe−Mg (hydr)oxide@GO

48 7

159.1

148.5

85.3

34.7

619.4

441.5

144.3

121.2

(10)

where R is the gas constant (8.314 J/mol·k), T (K) is the absolute temperature, and Kc is the equilibrium partition coefficient and can be calculated from eq 11.49,50 q Kc = e Ce (11)

Q0 (mg/g) adsorbent

ΔS ΔH − R RT

(9)

this study this study

Near equilibrium was achieved in 90 min for Pb2+ and Cu2+ but took 450 min for Ag+ and Zn2+. At 720 min, the removal levels were 99.99, 99.99, 99.98, and 99.92% for Pb2+, Cu2+, Ag+, and Zn2+, respectively. E



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Figure 6. Adsorption kinetics curves for Pb2+, Cu2+, Ag+, and Zn2+ by Fe−Mg (hydr)oxide@GO: (a) ion concentration change following contact time, (b) adsorption capacity (qt) with contact time, (c) removal % as a function of contact time, and (d) pseudo-second-order kinetic plots for ion adsorption. All experiments were carried out in room temperature, pH 7, Fe−Mg (hydr)oxide@GO dosage = 1 g/L.

before and after mixing four heavy metal ions adsorption. A notable change after metal adsorption is the shifts of 3699− 3680 cm−1 (Mg−O), 1110−1122 cm−1 (Fe−OH), and 578− 590 cm−1 (Fe−O−Mg or Mg−O−Mg). These changes indicate that adsorption of heavy metal ions take place primarily by complexation.48 Figure 7b shows the changes in ion concentration after addition of adsorbent. Simultaneously with decrease in the adsorbates’ concentration, Mg2+ and Fe2+,3+ increased, representing ion exchange, but their total amount was about 1/100 of the total adsorption. The morphologies and element distribution of Fe−Mg (hydr)oxide@GO after adsorption of heavy metal ions are shown in Figure S2. It could be suggested that a fringe of ion change occurred between heavy metal ions and Fe−Mg (hydr)oxide@ GO. The major ion change reactions are listed as eqs 12−16.

Table 5. Pseudo-First-Order and Pseudo-Second-Order Kinetic Parameters for Adsorbing Heavy Metals onto Fe− Mg (Hydr)oxide@GO pseudo-first-order model qe,cal (mg/g) 2+

Pb Cu2+ Ag+ Zn2+

2+

Pb Cu2+ Ag+ Zn2+

46.11 46.83 43.01 44.99

qe,exp (mg/g)

k1 (min−1)

46.78 47.64 43.39 45.03 pseudo-second-order model

0.063 0.066 0.022 0.018

R2 0.9731 0.9676 0.9254 0.9878

qe,cal (mg/g)

qe,exp (mg/g)

k2 (g/mg min−1)

R2

47.79 47.66 44.60 47.44

46.78 47.64 43.39 45.03

0.141 0.132 1.277 × 10−3 5.617 × 10−4

1.000 1.000 0.9998 0.9991

Mg(OH)2 (s) ⇌ Mg 2 + + 2OH−

(12)

Table 6. Thermodynamic Data for Heavy Metals Adsorption on Fe−Mg (Hydr)oxide@GO

Pb2 + + 2OH− ⇌ Pb(OH)2 (s)

(13)

Cu 2 + + 2OH− ⇌ Cu(OH)2 (s)

(14)

2Ag + + 2OH− → Ag 2O(s) + H 2O

(15)

Zn 2 + + 2OH− ⇌ Zn(OH)2 (s)

(16)

heavy metals

temperature (K)

ΔG (kJ/mol)

ΔS (J/mol·K)

ΔH (kJ/mol)

Pb2+

288 298 308 288 298 308 288 298 308 288 298 308

−19.931

118.849

14.300

−13.315

134.878

25.627

−17.347

211.442

43.676

−16.481

173.023

33.471

Cu2+

Ag+

Zn2+

There are many oxygen-containing functional groups on the surface of graphene oxide, and they may participate to the complexation of heavy metal ions working synergistically with Fe−Mg (hydr)oxide. 3.7. Desorption and Regeneration. Recyclability is important for environment-related adsorbents. The adsorption−desorption cycle was repeated four times under the same conditions. Figure 8 shows the changes in adsorption capacity of Fe−Mg (hydr)oxide@GO with recycling. The decrease is moderate for all metal pieces after four cycles, which may have resulted from aggregation of the adsorbent particles. F



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Figure 7. (a) FTIR spectra of Fe−Mg (hydr)oxide@GO and Fe−Mg (hydr)oxide@GO adsorbed the four mixed heavy metal ions and (b) ion concentration change following contact time.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Min Wu: 0000-0003-0542-4235 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the National Natural Science Foundation of China (Grant 51472253), National Key Project of Research and Development Plan (Grant 2016YFC1402500), and Chinese Academy of Sciences Visiting Professorships.

Figure 8. Decrease in adsorption capacity of Fe−Mg (hydr)oxide@ GO for metal ions with cycle number. All experiments were carried out in room temperature, pH 7, Fe−Mg (hydr)oxide@GO dosage = 1 g/ L.

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4. CONCLUSIONS A novel adsorbent Fe−Mg (hydr)oxide supported by graphene oxide was prepared and applied to remove heavy metal ions Pb2+, Cu2+, Ag+, and Zn2+ in aqueous phase. The adsorbent is magnetic and easily separated from water by a magnet. The adsorbent shows selectivity to heavy metals in the order of: Pb2+ > Cu2+ > Ag+ > Zn2+ ≫ Co2+, Ni2+, Cd2+. The Langmuir isotherm model describes well the adsorption behavior, and the adsorption kinetics followed pseudo-second-order. The Fe−Mg (hydr)oxide@GO shows exceptionally high adsorption capacities of 619.4, 441.5, 144.3, and 121.2 mg/g for Pb2+, Cu2+, Ag+, and Zn2+, respectively. The adsorption is likely to be chemisorption primarily by complexation with certain ion change. The adsorbent could be recycled, maintaining high level of adsorption capacity for heavy metal ions. Thus, the material is potentially useful in treating contaminated water.

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REFERENCES

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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/acs.jced.8b00100. AFM image of graphene oxide; adsorption data of Fe− Mg (hydr)oxide and Fe−Mg (hydr)oxide@GO for Pb2+, Cu 2+ , Ag + , and Zn 2+ ; Langmuir and Freundlich parameters of Fe−Mg (hydr)oxide for Pb2+, Cu2+, Ag+, and Zn2+ adsorption; and morphology and element distribution of Fe−Mg (hydr)oxide@GO after adsorption of heavy metal ions (PDF) G



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