Efficient Removal of Zn(II), Pb(II), and Cd(II) in Waste Water Based on

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

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Efficient Removal of Zn(II), Pb(II), and Cd(II) in Waste Water Based on Magnetic Graphitic Carbon Nitride Materials with Enhanced Adsorption Capacity Shuangzhen Guo,†,§ Kaili Wu,† Yan Gao,‡ Lihua Liu,† Xixi Zhu,*,† Xianlong Li,§ and Fan Zhang§ College of Chemical and Environmental Engineering and ‡College of Electrical Engineering and Automation, Shandong University of Science and Technology, Qingdao 266590, People’s Republic of China § School of Environmental Science and Engineering, Tianjin University, Tianjin, 300072, People’s Republic of China Downloaded via UNIV OF SUNDERLAND on September 9, 2018 at 03:20:54 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

S Supporting Information *

ABSTRACT: A novel magnetic nanomaterial (Fe3O4-gC3N4) based on Fe3O4 nanoparticles anchored on g-C3N4 was synthesized by a facile ultrasonic method and characterized by various means, including scanning electron microscopy, X-ray diffraction, vibrating sample magnetometer (VSM), Fourier transform infrared, thermogravimetric analysis, and particle size analyzer. Fe3O4-g-C3N4 was used as an magnetic adsorbent to remove Cd(II), Pb(II), and Zn(II) from aqueous solutions. Langmuir model was investigated to be the best one for fitting isothermal adsorption equilibrium data, and the corresponding adsorption capacities were predicted to be 168.93, 189.36, and 281.28 mg/g for Cd(II), Pb(II), and Zn(II), respectively. The X-ray photoelectron spectroscopy results revealed that the good adsorption ability of Fe3O4-g-C3N4 originated from the conjugation effect between Fe3O4-g-C3N4 and metal ions. All these results show the Fe3O4-gC3N4 as one of potential adsorbents in removing Cd(II), Pb(II), and Zn(II) from wastewater.

1. INTRODUCTION Environmental pollution induced by heavy metal has drawn wide concern to scientists due to its biology accumulation, highly toxic and carcinogenic.1 Many technologies were designed to remove heavy metal ion in wastewater including membrane processes,2 extractant impregnated resins,3 ionexchange resins,4 solvent extraction,5 and precipitation.6 However, no technology can simultaneously satisfy environmental friendliness, economic austerity, and technical compactness.7 Adsorption method was considered to be a feasible and a potential approach to remove heavy metal ion because of its merits including its low-cost, high-efficiency, and easy to operate.8−10 For the past few years, carbon−nitrogen materials have drawn much the attention of researchers because of their excellent electronic properties and catalytic properties.11−13 In theory, there are five different crystalline structures of carbon nitride in nature. Therein, graphitic carbon nitride (g-C3N4) was the most studied one due to its high stability.14,15 g-C3N4 had a planar π-conjugated structure, and it was constituted by tris-triazine units which had a large number of surplus electrons. It was the covalent bonds between g-C3N4 and heavy metal ions that played the role of getting rid of heavy metals ions from water.16 However, the adsorption capacity of the nanoabsorbent was inhibited seriously because the particles were too small and they easily aggregated together. Introducing © XXXX American Chemical Society

a carrier for nanoparticles was an potential way to solve this problem.17,18 Compared with sedimentation, filtration, and centrifugation separation methods, magnetic separation has the advantage of being cheap, clean, fast and efficient.19,20 Recently, magnetic adsorbent with short magnetic response time and large adsorption capacity have been developed by researchers.21 However, easy synthesis was also an important factor needed to be considered in real application. To the best of our knowledge, preparing magnetic g-C3N4 by one-step has not been reported. In this work, magnetic g-C3N4 was first synthesized by onestep method and used to adsorb Cd(II), Pb(II), and Zn(II) from wastewater. A series of assays were executed to assess the adsorption performance of as-prepared material. The optimum adsorption condition and adsorption mechanisms were also investigated.

2. EXPERIMENTAL SECTION 2.1. Reagents. Pb(NO3)2, Na2S2O3, NaOH, FeCl3, FeSO4, Cd(NO3)2, NaNO3, EDTA, melamine, Zn(NO3)2, and HCl were obtained from Guoyao Chemical Reagents Corp, (Shanghai, China). Received: June 27, 2018 Accepted: August 23, 2018

A

DOI: 10.1021/acs.jced.8b00526 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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2.2. Synthesis of Fe3O4-g-C3N4. g-C3N4 was prepared as follows: First, dicyandiamide was heated to 550 °C with a heating rate of 20 °C per minutes and was maintained for 4 h. Then, 2 g of the obtained material was dispersed in 50 mL of HCl (12 M) for 2 h. Finally, the g-C3N4 was collected by filtrating, washing, and drying at 60 °C for 24 h.22 Fe3O4 nanoparticles was prepared by precipitation method.23 A total of 1.622 g of FeCl3·6H2O and 1.39 g of FeSO4· 7H2O were added in 40 mL of deionized water and were maintained by stirring at 90 °C for 10 min. Five milliliters of ammonia solution (NH3·H2O, 28%) was added into the mixture slowly to precipitate and was continuously stirred for 10 min. Afterward, 4.4 g of risodium citrate dihydrate were added, and stirred at this temperature for 30 min. When the temperature cooled to room temperature, the as-prepared Fe3O4 was collected after being separated by a magnet, washed with water and ethanol several times, and dryed at 65 °C for 12 h in a vacuum condition. g-C3N4 (2.0 g) and Fe3O4 (0.5 g) were added in deionized water (100 mL) and subjected to ultraphonic for 48 h. Finally, the product was collected and this was followed by washing with deionized water and drying at 60 °C for 24 h.24 The proposed structural diagram of Fe3O4-g-C3N4 was displayed in Figure 1.

q=

(c0 − ce) ·V m

(1)

where C0 (mg/L), Ce (mg/L), V (L), and m (g) signified the initial concentration of metal ions equilibrium of metal ions, the volume of solution, and the weight of adsorbents, respectively. The effect of temperature was investigated with the range from 25 to 55 °C. Reusability of the Fe3O4-g-C3N4 was investigated by uses of EDTA (0.1 M) as the eluent. 2.5. Measurement of Point of Zero Charge (PZC). Salt addition method was used to measure the PZC of Fe3O4-gC3N4. First, Fe3O4-g-C3N4 (100 mg) and 0.01 M NaNO3 solution (100 mL) were mixed together. Second, the pH of the suspension was changed to different values by using NaOH (0.1 M) and HNO3 (0.1 M). Then, The mixed solution was kept continuous vibration for 24 h. Finally, the pH values were determined and the changes of the pH (ΔpH) was plotted against the initial pH. The point where the plot bisects the xaxis was corresponding to the PZC of Fe3O4-g-C3N4.25 2.6. Adsorption Kinetics. Three kinds of models were employed to investigate adsorption rate in this study. Intraparticle diffusion model qt = K it 0.5 + C

(2)

Pseudo-first-order kinetic model log(qe − qt) = log qe −

K1t 2.303

(3)

Pseudo-second-order kinetic model t 1 t = + 2 qt qe K 2(qe)

(4)

where qe and qt (mg/g) represented the adsorption capacity at equilibrium and time t, respectively. Ki K1, and K2 were the rate constant of each model. 2.7. Adsorption Isotherms. Two models were adopted to analyze adsorption behavior of the Fe3O4-g-C3N4. Langmuir model Ce C 1 = + e qe qm ·KL qm Figure 1. Schematic diagram of the as-prepared Fe3O4-g-C3N4.

(5)

Freundlich model log qe = log KF +

2.3. Characterization. Scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDAX) results were given by IGMA (Carl Zeiss, Germany). Rigaku (RigakuIV, Japan) provided X-ray diffraction (XRD) results. Fourier transform infrared (FT-IR) spectrometer (PerkinElmer, U.S.A.), vibrating sample magnetometer (LDJ 9600, U.S.A.), thermogravimetric analyzer (SDTA851, Swit), laser particle analyzer (Agilent Technologies, U.S.A.) and X-ray fluorescence (SEIKO 890, Japan) were also adopted to characterize the product. Inductively coupled plasma-mass spectrometer (ICP-MS) (Agilent 7500, U.S.A.) was adopted to determinate the content of metal ions. 2.4. Adsorption Experiments. The experimental process was operated at 298 K. In general, 100 mg of Fe3O4-g-C3N4 was dispersed into 100 mL of metal ions solution, followed by shaking for a certain time. Afterward, the Fe3O4-g-C3N4 was separated under external magnetic field. The eq 1 was adopted to calculate the adsorption capacity q (mg/g) of adsorbent

log Ce n

(6)

where qe (mg/g), Ce (mg/L), and qm (mg/g) represented the adsorption capacity, concentration at equilibrium and the maximum adsorption capacity. Equilibrium constant of Langmuir and Freundlich model were represented by KL and KF, respectively. n in eq 6 was the heterogeneity factor. RL (separation factor constant) was achieved by eq 7 RL =

1 1 + KLC0

(7)

RL reflects the favorable degree of adsorbing metal ion by Fe3O4-g-C3N4. RL = 0, irreversible; 0 < RL < 1, favorable; RL = 1, linear; and RL > 1, unfavorable.26

3. RESULTS AND DISCUSSION 3.1. Characterization. Figure 2 showed morphology and the particle diameter of the materials. From Figure 2, it can be B



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Figure 2. Morphology and the particle diameter of Fe3O4-g-C3N4 (a) and Fe3O4 (b).

Figure 3. EDAX of Fe3O4-g-C3N4 (a) and g-C3N4 (b).

found that Fe3O4-g-C3N4 had lamellar structure with spacing about 20 nm. The particle diameter of Fe3O4-g-C3N4 was about 500 nm to 2 μm, similar to the size of g-C3N4.27 However, the particle size of Fe3O4 was 20−40 nm, suggesting that magnetic Fe3O4 had been anchored in g-C3N4.28 The EDAX results of g-C3N4 and Fe3O4-g-C3N4 were exhibited in Figure 3. It can be concluded that the content ratio of C to N in g-C3N4 is approximately 2:3, which is consistent with the Xray fluorescence (XRF) results of g-C3N4 (Table 1). The Fe and O element are also shown in Figure 3a, suggesting Fe3O4 has been anchored in g-C3N4. Figure 4 showed the thermogravimetric analysis results of materials. As for Fe3O4, two decomposing intervals were discovered, corresponding to 4.23% from 70 to 130 °C, and 5.01% from 590 to 750 °C, relating to the elimination of adsorbed water, as well as decomposition of −OH in Fe3O4 Figure 4. Thermogravimetric curve of Fe3O4, Fe3O4-g-C3N4, and gC3N4.

Table 1. XRF Results of g-C3N4 and Fe3O4-g-C3N4 element

C

N

O

Fe

g-C3N4 Fe3O4-g-C3N4

39.3% 28.4%

60.7% 45.2%

16.1%

10.3%

surface. There were three decomposing stages observed for gC3N4. The first phase with a loss of 12.56 % from 40 to 90 °C stemmed from the dissociation of absorbed water. The second C



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XRD pattern of g-C3N4 has only one characteristic peak at 2θ = 27.8°. The characteristic peak of Fe3O4-g-C3N4 became weaker compared with single g-C3N4, which is due to the introduction of Fe3O4 reduces the crystallinity of g-C3N4. Six characteristic diffraction peaks of Fe3O4 are found in Fe3O4-gC3N4, suggesting Fe3O4 has been introduced into g-C3N4. In addition, there were two new peaks existed in Fe3O4-g-C3N4, belonging to the characteristic peak of FeO(OH). It can be explained that part of the Fe3O4 crystal form has been transformed into FeO(OH) in water solution under ultrasonic energy conditions. This crystal form change was beneficial to the increase of adsorption performance.31 The FTIR of materials were exhibited in Figure 7. The peak of Fe3O4 at 568 cm−1 was stemmed from bonding between Fe

phase corresponded to decomposition of the polymer g-C3N4 with a loss of 30.6% from 90 to 600 °C. The third weight loss stage was attributed to the decomposition of the triazine units with a loss of 29.8% from 600 to 750 °C.29 At 750 °C, g-C3N4 completely decomposed into gas with only about 5% carbon residue. Compared with Fe3O4, Fe3O4-g-C3N4 showed an additional loss of 75.5%. The physically adsorbed water content of Fe3O4-g-C3N4 was about 4%. Excluding the effects of moisture and carbon residue, at least 66.5% of g-C3N4 was contained in Fe3O4-g-C3N4. The magnetic results of the materials are shown in Figure 5. It can be found that the magnetization saturation of Fe3O4-g-

Figure 5. Magnetization curves of the materials.

C3N4 was 8.42 emu/g, much lower than that of Fe3O4. This was because Fe3O4 nanopaiticle was already embedded in gC3N4, whereas g-C3N4 with no magnetism would mask the magnetism of Fe3O4.30 The sharp drop in magnetization saturation values was also consistent with the TGA results, which suggested high content of g-C3N4 in composites. The XRD patterns of the materials were exhibited in Figure 6. The diffraction peaks of Fe3O4 at 30.1°, 35.5°, 43.1°, 53.4°, 57.0°, and 62.6° were corresponded to the (220), (311), (400), (422), (511), and (440) planes (JCPDS: 85-1436). The

Figure 7. FTIR of Fe3O4, Fe3O4-g-C3N4, and g-C3N4.

and O. The peak of g-C3N4 at 2150 cm−1 corresponded to C N,32 a batch of peaks ranging from 1100 to 1700 cm−1 were contributed from s-triazine derivatives, and the peak at 806 cm−1 was stemmed from heptazine ring.33 Bonds of N−H and O−H contributed to peaks ranged from 3100 to 3500 cm−1.34 As for Fe3O4-g-C3N4, it contained all the peaks above except for two peaks at 2200−2000 cm−1 and 3200−3500 cm−1. This may be because the high energy of the ultrasound destroys the conjugate structure of g-C3N4, leading to the disappearance of the CN and the blue shift of the N−H. In addition, the intensity peak of Fe3O4-g-C3N4 at 1650 cm−1 increased compared with single g-C3N4, indicating an increase in carboxyl content in Fe3O4-g-C3N4. This may mean that the damage of conjugate structure by ultrasound was more focused on the edge of g-C3N4. Under ultrasound conditions, the −C2N−H located at the boundary of g-C3N4 was transformed into −CNH2 and the C−N was transformed into −COOH. 3.2. Adsorption Experiments. Influence of pH. pH was an non-negligible factor in the heavy metal ion adsorbing process as it can affect surface charge, binding sites of the particles, and complexation.35 It can be concluded from Figure 8 that the adsorption capacity of the Fe3O4-g-C3N4 increased with the increase of pH on the whole. A slow increase of the adsorption ability was observed when pH value was lower than 3, followed by a rapid increase when pH values were higher than 3, and then reached its maximum at pH 6.0. Hence, 6.0 was selected as the optimal pH in the following tests.

Figure 6. XRD patterns of Fe3O4, Fe3O4-g-C3N4, and g-C3N4. D



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and accomplished fundamental equilibrium state at 20 min. The fitted curves with pseudo-first-order model and pseudosecond-order model were exhibited in Figure S1 (Supporting Information) and the fitting data were displayed in Table 2. As can be seen, it was the pseudo-second-order model that fitted better than the other, indicating the chemical process was the decisive process when adsorbing metal ions.34 The rate constant (K) of Zn(II) was lower than that of Cd(II) and Pb(II). It maybe explained from the viewpoint of the exist form of metal ions in water and the ion radius.39 The ionic radius of Cd(II), Pb(II), and Zn(II) was 0.95, 1.19, and 0.74 A°, respectively, while the hydrated ionic radius was Zn(II) (4.30 A°) > Cd(II) (4.26 A°) > Pb(II) (4.01 A°). Therefore, the smaller ionic radius of Pb(II) and Cd(II) in aqueous solution led to a faster rate of adsorption than that of Zn(II). The results of fitting intraparticle diffusion model40 were exhibited in Figure 10 by plotting qt versus t1/2. It can be seen that the fitted linear can be divided into three stages. From the diffusion constants which summarized in Table 3, we can see the order of adsorption rate was Kid,1 > Kid,2 > Kid,3. The first stage was attributed to metal ions adsorbed on surface of Fe3O4-g-C3N4, in which nearly 90% metal ions were adsorbed and the adsorption process was finished in a short time. When the exterior surface of Fe3O4-g-C3N4 basic saturated, metal ions gradually got into the pores of Fe3O4-g-C3N4. As the pore size became smaller, the metal ion diffusion resistance becomes larger, leading to the decline of diffusion rates. The final stage was equilibrium period, which the adsorption and desorption reached equilibrium.41 Adsorption Isotherms. Figure 11 showed the changes of adsorption capacity with initial concentration of metal ions. The adsorption capacity of three kinks of metal ions increased with the initial concentration increasing. However, the increasing rates dropped gradually. Figure S2 (Supporting Information) displayed the fitting results of adsorption isotherms with Freundlich and Langmuir model, and the fitting data was presented in Table 4. The Langmuir model seemed to be more appropriate than Freundlich model, indicating monolayer and homogeneous adsorption were advocated in adsorption process.41 The maximum adsorption capacity for Cd(II), Pb(II), and Zn(II) were calculated to be 168.93, 189.36, and 281.28 mg/g, respectively, higher than some magnetic adsorbents (Table 5). Furthermore, the dimensionless constant (RL) with the range of 0.4−0.8 suggested that the adsorption process was identified as favorable.49 Thermodynamics. Influence of temperature on adsorption was illustrated in Figure 12 with the range from 298 to 328 K. The adsorption capacity of the three kinds of metal ions were all increased as temperature increasing. There were two ways of affecting adsorption capacity by temperature. The one was that solution viscosity and metal ion diffusion rate were influenced by temperature. The other one was that coordination constants between metal ions and active site were also affected by temperature.50 The thermodynamic parameters including changes of entropy, enthalpy, and free energy (corresponding to ΔS0, ΔH0, and ΔG0) can be obtained by eqs 8, 9, and 10.

Figure 8. Influence of pH on the adsorption ability. The vertical short line: influence of pH without adsorbent. The transverse line: changes of pH after adsorption (metal ion, 200 mg/L; adsorption time, 30 min; Fe3O4-g-C3N4, 1 mg·mL−1; T, 298 K; repeated time, 3).

PZC of the adsorbent can reflect the influence of the pH on adsorption process. If the pH of the surroundings were below PZC, the surface of the absorbent was positively charged, and then the adsorption would be difficult due to charge repulsion.36 Because the PZC of Fe3O4-g-C3N4 was found to be around 3.4, the Fe3O4-g-C3N4 was positively charged when the pH of the solution was under 3.4, resulting in the lower adsorption capacity. When the pH of the solution exceeded 3.4, Fe3O4-g-C3N4 was negatively charged, leading to the approach of the metal ions to being adsorbent became easier, thus resulting in adsorption capacity increasing, which can explain why the adsorption capacity increased quickly with the pH increasing from 4 to 6.37 This phenomenon also appeared in previous reports.38 Kinetics. Figure 9 exhibited the changes of the adsorption capacity with time. The adsorption capacity of Cd(II), Pb(II), and Zn(II) reached 90% of the maximum value within 10 min

Figure 9. Changes of the adsorption capacity with the time (metal ion concentration, 200 mg/L; pH = 6; Fe3O4-g-C3N4, 1 mg·mL−1; T, 298 K; repeated time, 3).

Kd = E

qe Ce

(8) DOI: 10.1021/acs.jced.8b00526 J. Chem. Eng. Data XXXX, XXX, XXX−XXX


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Table 2. Dynamic Parameters for Adsorbing Metal Ions pseudo-first-order

pseudo-second-order

metal ion

K1

qe

R2

K2

qe

R2

Zn(II) Pb(II) Cd(II)

0.1157 0.2123 0.1412

73.46 74.12 43.27

0.8786 0.8823 0.9124

3.72 × 10−3 6.76 × 10−3 6.72 × 10−3

130.54 103.47 93.16

0.9995 0.9996 0.9996

Figure 10. Intraparticle diffusion model for adsorbing metal ions.

ΔS 0 ΔH 0 − R RT

(9)

ΔG 0 = ΔH 0 − T ·ΔS 0

(10)

ln Kd =

Figure 11. Influence of initial concentration on adsorption process of Fe3O4-g-C3N4. (pH, 6; contact time, 30 min; Fe3O4-g-C3N4, 1 mg· mL−1; T, 298 K, repeated time, 3).

Effect of Ionic Strength. Series of assays were carried out to assess the affect of ionic strength on adsorbing metal ions. It can be found from Figure 14 that the adsorption capacity slightly declined with ionic strength increasing. Theoretically speaking, the increasing of ionic strength would induce negative effects on adsorption process on the condition that electrostatic interaction was the main driving force. Thus, it was not the electrostatic interaction that played the main role in adsorbing metal ions by Fe3O4-g-C3N4. Moreover, the different kinds of coexisting ions would cause different effects on adsorption process, which can be represented as Mg(II) > Ca(II) > Na(I). This was because the charge numbers of Mg(II) and Ca(II) were more than that of Na(I), so they were more easily adsorbed by adsorbent. Meanwhile, the ionic radius of Ca was larger than that of Mg so it was easier to be affected by steric hindrance. Adsorption Performance under Mixture of Heavy Metal Ions. Figure 15 exhibited the adsorption performance of the asprepared material in ternary metal solutions. As we can see, the adsorption capacity of Cd(II) and Pb(II) increased at the beginning and followed in decline, while that of Zn(II) increased all the time with an increase of the mixture metal ions concentration. It was worthy to note that the maximum adsorption capacity in ternary metal solutions was lower than

The fitting straight line by ln Kd −1/T is shown in Figure 13, and the corresponding data is displayed in Table 6. It can be seen that ΔH0 and ΔS0 were positive values, and ΔG0 was negative values, suggesting the adsorption was spontaneous as well as endothermic. Thus, it can draw conclusion the adsorption was driven by increasing of entropy and the adsorption process was benefited by increasing temperature. The Arrhenius equation (eq 11) can be used to describe the activation energy (Ea) of this process ln K 2 = −

Ea + ln K 0 RT

(11)

where K2, K0, T, and R represent the rate constant, temperature influence factor, temperature, and gas constant, respectively. The fitting straight line plotted by ln K2 −1/T is displayed in Figure 13. The activation energy of Cd(II), Pb(II), and Zn(II) was calculated to be 40.56, 40.42, and 35.92 kJ/mol, respectively, indicating that chemical and physical adsorption both existed in the adsorption process of Fe3O4-g-C3N4. The chemical adsorption played a dominant role, whereas physical adsorption was secondary.51

Table 3. Intraparticle Diffusion Model Parameters for Adsorbing Metal Ions first linear portion

second linear portion

metal ion

Kd1

C1

R12

Kd2

C2

R22


38.4671 31.1124 28.2346

−5.4645 −1.1674 −1.1626

0.9938 0.9826 0.9717

2.5664 3.0136 3.8289

110.1947 85.2836 71.9746

0.9914 0.9658 0.9907

F



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Table 4. Isotherm Data for Adsorbing Metal Ions Langmuir isotherm model

Freundlich isotherm model

metal ion

q (mg/g)

KL

RL

R2

KF

n

R2


281.28 189.36 168.93

3.92 × 10−3 5.86 × 10−3 5.91 × 10−3

0.52−0.86 0.43−0.82 0.42−0.81

0.996 0.998 0.999

2.82 3.66 3.35

1.38 1.57 1.59

0.986 0.982 0.983

Table 5. Comparison of Fe3O4-g-C3N4 with Other Adsorbents adsorption capacity adsorbent

Zn(II)

Cd(II)

Pb(II)

references

Fe3O4-CS-L EDTA-modified chitosan/SiO2/Fe3O4 L-arginine modified magnetic Fe3O4 magnetic chrysotile nanotubes magnetic polyacrylamide microcomposite magnetite Si-Schiffbase complex polythiophene/TiO2 composite β-cyclodextrin polymers Fe3O4-g-C3N4

256.4

156.9 63.06 120.2 35.26 223.02 136.43 189.36

128.6 123.37

42 26 43 44 45 46 47 48 this work

150.4

281.28

29.58 174.88 107.2 213.22 196.42 168.93

189.36

Table 6. Thermodynamic Parameters for the Adsorption metal ion

ΔH0 (kJ/mol)

ΔS0 (J/k·mol)

Zn(II)

6.71

28.93

Pb(II)

7.16

25.36

Cd(II)

4.23

13.68

T (K)

ΔG0 (kJ/mol)

298 308 318 328 298 308 318 328 298 308 318 328

−1.91 −2.20 −2.48 −2.80 −0.39 −0.65 −0.59 −1.16 −0.15 −0.23 −0.28 −0.32

Figure 12. Changes of adsorption capacity with temperature. (Metal ion concentration, 200 mg/L; pH, 6; contact time, 30 min; Fe3O4-gC3N4, 1 mg·mL−1; T, 298 K; repeated time, 3).

Figure 14. Changes of absorption capacity with ionic strength. (metal ion concentration, 200 mg/L; pH, 6; contact time, 30 min; Fe3O4-gC3N4, 1 mg·mL−1; T, 298 K; repeated time, 3).

that of single metal solution. Therefore, the order of adsorption by Fe3O4-g-C3N4 was Zn(II) > Pb(II) > Cd(II). There was no unified mechanism to explain the competitive adsorption. The current study shows that there are several

Figure 13. Fitting of Arrhenius equation.

G



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was attributed to the sp2 N of striazine ring units or heptazine units, the peak at 400.2 eV was attributed to tertiary nitrogen, and the peaks at 401.6 and 404.1 eV were attributed to amino functions and charging effects, respectively.55 Figure 16b shows peaks at 284.6, 286.7, and 288.1 eV that constituted the C 1s spectrum. NC−N2 of striazine ring (C3N3) units or heptazine (C6N7) units corresponded to peaks at 288.1 eV, and CN corresponded to the peak at 286.7 eV.56 Moreover, according to the peak area, the peak of C 1s at 284.6 eV was considered to be the chiefly binding energy.57 The changes in peaks before and after adsorbing are shown in Figure 16c,d. The binding energy of C 1s and N 1s shifted toward metal ions with 0.20 and 0.25 eV, respectively, suggesting the local bonding environments had changed.58 On the basis of the results above, it can be derived that two probable interaction forms existed in the adsorption process. One was that metal ions can combine with N−H or CN of Fe3O4-g-C3N4 by forming complexes. The other was that the conjugated π-electron pairs of triazine ring (C3N3) units or heptazine (C6N7) units and sp2 C−N can be regarded as Lewis base, whereas the metal ions were considered to be Lewis acid. From the point of Lewis acid−base theory, the powerful complexation was easily formed between metal ions and gC3N4 during the adsorption process. In addition, by comparing the peak area, it can be deduced that the second interaction acted as the major contributor in adsorbing metal ions by Fe3O4-g-C3N4.

Figure 15. Influence of metal competition on the adsorption process (metal ion concentration, 200 mg/L; pH, 6; contact time, 30 min; Fe3O4-g-C3N4, 1 mg·mL−1; T, 298 K; repeated time, 3).

factors that affect the results of competitive adsorption, including ionic radius, antimagnetic electronegativity, and coordination constants.52−54 3.3. Adsorption Mechanism. The XPS changes of Fe3O4g-C3N4 before and after adsorption can further reflect the adsorption process. From Figure 16a, it can be seen that there are four peaks in N 1s spectrum in which the peak at 398.6 eV

Figure 16. High-resolution XPS N 1s (a), C 1s (b), N 1s (c), and C 1s (d) before and after adsorption. H



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In addition, the adsorption mechanism can be reflected from the extreme difference (ED) (ED = qmax − qmin) in adsorption capacity. The ED of Cd(II) Pb(II), and Zn(II) were calculated to be 14, 13, and 19 mg/g, respectively, with ionic strength increasing from 0 to 2, suggesting electrostatic attraction was not the major interaction force. From the results discussed above, it can be found that the chemical adsorption was the rate controlling step. Coordinate bond is the main form between metal ions and adsorbent.59 TG results indicated that the percentage of g-C3N4 in the absorbent can reach 66.5%. The active groups of g-C3N4 can combine with metal ions to form complexes. Moreover, the coordination of metal ions with functional groups was greatly influenced by solution pH.60 The ED of Cd(II) Pb(II), and Zn(II) were calculated to be 36, 33, and 45 mg/g, respectively, with the pH increasing from 1 to 6. From what had been discussed above, we may safely draw the conclusion that the chemical interaction played a major role in adsorbing metal ions. 3.4. Adsorption in Real Samples. Adsorption performance of Fe3O4-g-C3N4 was evaluated by the experiments performed in electrolytic zinc residue percolate. The wastewater was pretreated by filtering with nylon membrane before adsorption experiments were performed. It can be found from the results displayed in Table 7 that the adsorption capacity of Fe3O4-g-C3N4 for Cd(II), Pb(II), and Zn(II) reached 50.41, 69.38, and 82.31 mg/g, respectively.

Figure 17. Cycle adsorption experiments of of Zn(II), Cd(II), and Pb(II) by Fe3O4-g-C3N4.

above, we can draw the conclusion that the Fe3O4-g-C3N4 demonstrated to be a promising adsorbent material with enhanced performance in removing heavy metal ions from wastewater.

■

Table 7. Adsorption Ability in Real SamplesaHunan, China. samples electrolytic zinc residue percolate (pH = 2.11)

metal ions

Cfound (mg/L)

adsorption capacity (mg/g)

Zn(II) Cd(II)

236.37 123.75

82.31 50.41

Pb(II) Mg(II) Cu(II)

173.21 35.93 6.99

69.38 2.61 1.13

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.8b00526. The pseudo-first-order and pseudo-second-order adsorption kinetics fitting model; Langmuir and Freundlich adsorption isotherm fitting model (PDF)

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AUTHOR INFORMATION

Corresponding Author

a

The samples were obtained from Taifeng Company.

*E-mail: [email protected]. Tel.: +86 0532-86057104. Fax: +86 0532-80681197.

3.5. Reusability of Fe3O4-g-C3N4. The reusability of product was an nonnegligible factor in real applications. In this work, EDTA was used in regenerating Fe3O4-g-C3N4. From the results exhibited in Figure 17, the adsorption capacity of Fe3O4-g-C3N4 for Cd(II), Pb(II), and Zn(II) could still maintain 90.1%, 89.8%, and 90.4%, respectively, after reusing five times, demonstrating the Fe3O4-g-C3N4 possessed good reliability in real applications.

ORCID

Kaili Wu: 0000-0001-5415-7327 Xixi Zhu: 0000-0002-2060-654X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of Shandong Province (Grant ZR2018PEE006), Scientific Research Foundation of Shandong University of Science and Technology for Recruited Talents (Grant 2015RCJJ018), and Qingdao Postdoctoral Application Research Project Funding.

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CONCLUSION Herein, Fe3O4-g-C3N4 was successfully synthesized by a facile ultrasonic route and adopted as an enhanced adsorbent in adsorbing heavy metal ions from waster water. The pseudo second order model was studied to be the most suitable model for fitting kinetics data. The adsorption process was best described through Langmuir isothermal adsorption model, and the predicated adsorption capacities of Cd(II), Pb(II), and Zn(II) were 168.93, 189.36, and 281.28 mg/g, respectively. The order of interfering ion affecting the adsorption process was Mg(II) > Ca(II) > Na(I). Competitive adsorption assays demonstrated that Fe3O4-g-C3N4 preferably absorbed Zn(II), then Pb(II), and finally Cd(II). XPS results suggested that the adsorption performance was attributed to conjugation between Fe3O4-g-C3N4 and metal ions. From the results discussed

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Efficient Removal of Zn(II), Pb(II), and Cd(II) in Waste Water Based on

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