Fabrication of CoreâShell CMNP@PmPD Nanocomposite for Efficient

Research Article pubs.acs.org/journal/ascecg

Fabrication of Core−Shell CMNP@PmPD Nanocomposite for Efficient As(V) Adsorption and Reduction Jin Wu,†,‡,∇ Hongshan Zhu,‡,∇ Ge Liu,‡ Liqiang Tan,‡ Xiaoye Hu,§ Changlun Chen,‡,# Njud S. Alharbi,# Tasawar Hayat,∥ and Xiaoli Tan*,‡,⊥ †

School of Resources & Environmental Engineering, Hefei University of Technology, No. 193 Tunxi Road, Hefei 230009, Anhui, People’s Republic of China ‡ Institute of Plasma Physics, Chinese Academy of Sciences, P.O. Box 1126, Hefei 230031, Anhui, People’s Republic of China § Institute of Solid States Physics, Chinese Academy of Sciences, P.O. Box 1129, 350 Shushanhu Road, Hefei, 230031, Anhui, People’s Republic of China ⊥ Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, Soochow University, Suzhou, Jiangsu 215123, People’s Republic of China # Department of Biological Sciences, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia ∥ NAAM Research Group, King Abdulaziz University, Jeddah 21589, Saudi Arabia S Supporting Information *

ABSTRACT: Here, we prepared novel carboxyl-functionalized Fe3O4 nanoparticles (CMNPs) coated with poly(mphenylenediamine) (CMNP@PmPDs) without complicated premodification procedures. The CMNP@PmPDs show welldefined core−shell structures and combine both the facile separation properties of magnetic particles and the extraordinary adsorption performance of polymers. The CMNP@ PmPDs were employed to investigate the influence of various environmental factors (initial pH, ionic strength, etc.) on the removal of As(V) through batch experiments. The CMNP@ PmPDs display much better As(V) adsorption performance than the CMNPs, and the adsorption capacity is enhanced from 51.2 mg g−1 to 95.2 mg g−1. The CMNP@PmPDs exhibit high magnetization (∼46.7 emu g−1), indicating their easy separation under an external magnetic field in practical applications. The major reaction pathway involving the reduction of As(V) to As(III) was identified by X-ray photoelectron spectroscopy (XPS) analysis. The removal mechanisms can be explained by the adsorption of As(V) on protonated imino and carboxyl groups via electrostatic attraction, which is then reduced to As(III) by amine groups. This study demonstrates the potential application of CMNP@PmPDs as a low-cost and effective remediation strategy for the removal of As(V) from wastewater. KEYWORDS: Core−shell nanocomposite, Poly(m-phenylenediamine), Fe3O4 microspheres, As(V), Adsorption and reduction

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INTRODUCTION

thought to be one of the most important methods for the removal of As(V) from most aquatic systems, because of its low cost, simplicity, and environmental friendliness.8 Many types of absorbents, such as iron oxide nanoparticles, zerovalent iron, and modified cellulose have been widely applied to remove As(V).5,9,10 Magnetic nanoparticles (MNPs), such as Fe3O4, have aroused a great deal of interest for their advantages of easy separation and large available adsorptive areas.11,12 However, the anticipated adsorption performance is not achieved by MNPs, because these materials, such as porous Fe3O4 and yeast

Arsenic is a typical pollutant both in natural water and industrial wastewater, which greatly threatens water resources and human health.1,2 Arsenic pollution arises from both anthropogenic activities and natural geogenic processes, such as industrial production, metallurgy, and natural weathering of arsenic-bearing minerals.1,3 Pentavalent arsenic (As(V)) is highly toxic to biological systems, because of its long-term retention in living organisms.4 Chronic exposure to arsenic can lead to cardiovascular system problems, keratosis, and many other diseases, as it can inhibit the glycolytic pathway.5,6 Arsenic is regarded as one of the most hazardous substances by the World Health Organization, and arsenic removal has gained increasing attention over the years.7 It is urgent to find appropriate treatments to reduce its pollution. Adsorption is © XXXX American Chemical Society

Received: February 13, 2017 Revised: March 19, 2017 Published: March 27, 2017 A

DOI: 10.1021/acssuschemeng.7b00468 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

demonstrate their tremendous potential for the removal of As(V) from water.

cross-linked Fe3O4 composites, suffer from a high tendency for self-aggregation, limited active units, and a poor affinity to pollutants.11,13 To overcome these limitations, widespread attention has been directed toward the modification of MNPs with functional groups to extend their application.14 Nevertheless, the limited functional groups of these modified MNPs cannot attain high affinity for contaminants.15,16 On the other hand, organic polymers involving polyaniline, polyacrylamide, or poly(p-phenylenediamine) may be desirable absorbents, because of their rich functional groups, which benefits the removal of heavy metal ions, such as As(V).17−19 Therefore, in view of this point, grafting MNPs to organic structures has been investigated in order to promote their adsorption ability by combining the properties of distinctively different components.20 A considerable number of materials modified by organic polymers have been applied for the removal of pollutants.21−23 Some organic−inorganic hybrid materials have been developed using two or more steps to make polymers grafted on the surface of composites.24,25 In previous studies, magnetic core−shell adsorbents including Fe3O4@ polydopamine nanoparticles, nanostructured metal oxide aerogels, Fe3O4@C, AgNO3-functionalized Fe3O4@mesoporous SiO2, and polyaniline-coated protonic titanate nanobelt have been applied for the removal of pollutants.21,26−29 In fact, the complicated fabrication procedures and difficulty of recycling these composites have greatly limited their practical application in the removal of contaminants. Note that pollutant removal processes involving redox reactions have been increasingly utilized as potent methods. However, existing studies on As(V) removal seldom concern simultaneous reduction and adsorption, because the reduction of As(V) to As(III) is difficult to achieve.1 Poly(m-phenylenediamine) (PmPD) has gained considerable interest, because of its superior redox properties and chelation abilities, which are favorable for its adsorption behavior.30 Another superior property that PmPD exhibits is extraordinary chemoresistance, as this material has been proven to be insoluble in many types of solvents, including oleum, formic acid, and N-methylpyrrolidone, which greatly extends its practical application in water purification.31 In addition, the preparation of PmPD particles can be controlled in terms of morphology and molecular structure.32 In particular, PmPD nanoparticles are readily prepared via oxidation polymerization in the presence of an available matrix.20 Moreover, PmPD exhibits superior performance in sensors and as a water purification agent. Motivated by the superior advantages of MNPs and PmPD polymers, this work aims to fabricate PmPD-coated MNPs. Here, carboxyl-functionalized Fe3O4 nanoparticles (CMNPs) were prepared using sodium citrate, because it can highly stabilize the nanoparticles and its special chemical structure facilitates attachment to negative nanoparticles, such as Fe3O4,33 and then the PmPD-coated CMNPs were synthesized in an ice bath. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared (FT-IR) spectroscopy, and X-ray photoelectron spectroscopy (XPS) were conducted to characterize the physicochemical properties of the CMNP@PmPDs. As(V) removal with the as-synthesized CMNP@PmPDs was investigated under different solution chemistry conditions (pH, ionic strength, contact time, etc.). The detailed removal of As(V) by CMNP@PmPD materials was discussed, and the adsorption and reduction mechanism was reported. Our results

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EXPERIMENTAL SECTION

Chemicals. All chemicals were employed in the experiments without any further treatment. A stock solution containing 254.4 mg L−1 As(V) was prepared by dissolving Na2HAsO4·7H2O. Milli-Q water (Millipore, Bedford, MA) was employed in all batch experiments. Preparation of CMNPs and CMNP@PmPD Composites. The synthetic route is illustrated in Figure 1. CMNPs were first synthesized

Figure 1. Schematic illustration of the formation of CMNP@PmPDs and As(V) adsorption mechanisms on CMNP@PmPDs. by a solvothermal reaction. FeCl3·6H2O (2.9 g), sodium acetate (6 g), and sodium citrate (0.6 g) were added in ethylene glycol (60 mL), and then the mixture was homogenized by magnetic stirring for several hours. After that, a Teflon-lined autoclave was used to seal and heat the obtained reaction mixture at a temperature of 200 °C for 10 h, and the obtained composites were cooled to room temperature for further use. Afterward, the CMNP particles were separated using a bar magnet and then ultrasonically washed by deionized (DI) water and ethanol. The synthesis method for CMNP@PmPDs was developed according to the method reported in previous research.20 In brief, 0.05 g of CMNPs were fully dispersed in 100 mL of aqueous solution under ultrasonication, followed by the addition of PmPD (0.05 g) and Na2S2O8 (0.1102 g). The reaction was conducted under continuous, vigorous stirring in an ice−water bath at 0 °C for 5 h. Subsequently, the obtained black particles were ultrasonically washed by water and ethanol several times, isolated via a magnet and finally dried under vacuum freeze-drying. Characterization. The as-prepared CMNP and CMNP@PmPD samples were characterized using SEM (JEOL, Model JSM-6700F) and TEM (TECNAI, Model G2). The XRD patterns were obtained using a X’Pert PRO diffractometer (in the range of 2θ = 10°−70°). A Fourier transform spectrophotometer (JASCO, Model FT-IR 410 spectrophotometer) was used to obtain related information about the samples. Thermogravimetric analysis (TGA) was conducted using thermoanalytical equipment (SDTQ600, USA) from 25 to 800 °C. A vibrating sample magnetometry (VSM) device was used to investigate the magnetic properties at room temperature under an applied magnetic field of 30 kOe. XPS measurements were taken using an ESCALab 220I-XL system. Batch Adsorption Experiments. All adsorption experiments were conducted under ambient conditions. Solutions containing the desired concentrations of the different components were achieved in a series of 10 mL polyethylene tubes using Na2HAsO4·7H2O and NaCl aqueous solutions, and 0.5 mol L−1 HCl and NaOH solutions were used to control the initial pH of the suspensions. After the As(V) removal reaction was complete, the absorbents were isolated by a conventional magnet. An inductively coupled plasma−atomic emission spectrometry (ICP-AES) system (Model CAP6300, Thermo Scientific) was used to ascertain the concentration of As(V). The adsorption isotherms were measured by adjusting the pH to 2.3 with As(V) concentrations ranging from ∼8 mg L−1 to 150 mg L−1. The desorption of the absorbent was conducted using 0.5 M NaOH solution as an eluent. The absorbent was first ultrasonicated for 30 min in 0.5 M NaOH solution and further shaken for 10 h. The absorbent B


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ACS Sustainable Chemistry & Engineering then was collected by magnetic separation and washed with water several times. Finally, the absorbent was applied in recycle adsorption. The adsorption of As(V) on the absorbents was determined from the variation between the initial concentration and the concentration after adsorption. The adsorption percentage (%), distribution coefficient (Kd), and adsorption capacity at equilibrium (qe) can be expressed by the following equations: adsorption (%) =

C0 − Ce × 100 C0

(1)

Kd =

C0 − Ce V × Ce m

(2)

qe =

C0 − Ce ×V m

(3)

−1

where C0 (mg L ) represents the initial concentration of As(V) and Ce (mg L−1) represents the equilibrium concentration of As(V). V (mL) represents the suspension volume, and m (g) is the mass of the absorbents. The relative errors of all the experimental data were not more than 5%.

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Figure 3. (A) X-ray diffraction (XRD) patterns, (B) Fourier transform infrared (FT-IR) spectra, (C) thermogravimetric analysis (TGA) curves, and (D) room-temperature magnetization curves.

RESULTS AND DISCUSSION Characterization of CMNPs and CMNP@PmPDs. The SEM and TEM images in Figure 2 exhibit the size and

Fe3O4, and no significant structural changes occurred after the polymer was coated on the Fe3O4 surface.8,35 To verify the structural information and chemical components in the CMNP@PmPDs and CMNPs, FT-IR spectroscopy was performed. Figure 3B shows that the peak in the CMNP spectrum at ∼590 cm−1 (belonging to the Fe−O bond) is characteristic of Fe3O4, and the same peak also appears in the spectrum of the CMNP@PmPD composite.36,37 Meanwhile, for the CMNPs, the spectrum shows two absorption bands at 1400 and 1060 cm−1, which are due to CO symmetric stretching and C−O stretching of the COO− group, confirming the carboxyl modification on the surface of Fe3O4.11,35,38 There are two broad and strong peaks at 3430 and 1630 cm−1, which are assignable to the adsorption of the −OH stretching vibration and twisting vibration belonging to physically adsorbed H2O, respectively.4,39 In addition to the same typical CMNP peaks, the CMNP@PmPD spectrum exhibits an absorption band at 1120 cm−1 correlated to C−N groups.37 The peaks at 1620, 1540, and 1270 cm−1 may be ascribed to the stretching vibrations of quinoid imines and benzenoid amines and the C−N stretching mode.20,34,40 Moreover, the peaks at 1060 and 3430 cm−1 shifted to 1040 and 3420 cm−1, respectively, which may be the results of overlaying the PmPD shell and the interaction between PmPD and carboxyl groups on the surface of the CMNP@PmPDs. From the above results, we can conclude that CMNP@PmPDs with well-defined core−shell structures were successfully developed. Thermogravimetric measurement was also taken to evaluate the contents of the CMNPs and CMNP@PmPDs, as shown in Figure 3C. The initial weight loss stage below 150 °C should result from the release of absorbed water. The second weight loss from 150 °C to 330 °C should be attributed to the decomposition of carboxyl on the surface of the composites. For the CMNP@PmPDs, it can be found that a sharp weight loss appears from 330 °C to 560 °C, because of the presence of PmPD particles. This result reveals that PmPD was grafted onto the CMNPs, with a content of ∼30%.

Figure 2. SEM images of (A) CMNPs and (B) CMNP@PmPDs; TEM images of (C) CMNPs and (D) CMNP@PmPDs.

morphology of the as-prepared CMNPs and CMNP@PmPDs. It can be seen that well-distributed microsphere composites with rough surfaces are formed. These spherelike composites appear to be microspheres with diameters of ∼300 nm, and PmPD is uniformly polymerized on the CMNPs. For the TEM images, one can see that a well-defined core−shell structure can be confirmed. By comparison to the as-synthesized CMNPs, the light-colored and uniform layers of PmPD are successfully coated on the CMNP core, and the thickness of the PmPD layer is ∼50 nm. XRD patterns were collected to distinguish the phase purity and crystal structure of the composites. All exhibited peaks correspond to the indices (111), (220), (311), (400), (422), (511), and (440) (see Figure 3A), which are in accordance with the typical pattern for cubic-phase Fe3O4.34 Compared with Fe3O4, CMNPs and CMNP@PmPDs exhibit the characteristic peak pattern of magnetic Fe3O4, suggesting that the obtained composites retain the magnetic properties of Fe3O4, and no other peaks appear. The results suggest the high purity of C


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representative of benzenoid and quinoid imine species). At lower pH, the comparably higher H+ concentration promotes the occurrence of the redox reaction. In contrast, the higher OH− concentration with increasing pH has a tendency to inhibit the above reaction from proceeding. As(V) removal at various ionic strengths is shown in Figure 4A. Generally, outer-sphere surface complexation is sensitive to increased ionic strength, while variation in the ionic strength exhibits little influence on inner-sphere surface complexation.43 The increased concentration of NaCl induces a competition effect with As(V) removal, indicating that electrostatic interaction is also a mechanism for As(V) adsorption on the CMNP@PmPDs. Isotherm and Adsorption Thermodynamics. The As(V) adsorption isotherms for the CMNPs and CMNP@ PmPDs at pH 2.3 are present in Figure 5. The amount of

Figure 3D shows the magnetization curves of the CMNP and CMNP@PmPD microspheres. From the magnetization curves, the saturated magnetization values of the CMNPs and CMNP@PmPDs decrease from 68.9 emu g−1 to 46.7 emu g−1, because of the PmPD shell. PmPD evidently has a negative effect on the Fe3O4 magnetic properties. However, the saturation magnetization value achieved with the CMNP@ PmPD material reveals that an external magnetic field and slight ultrasonic agitation can make the material separate and redisperse rapidly, which renders its separation and purification very convenient in water. The great magnetic characteristics guarantee rapid separation in practical applications. Effect of pH and Ionic Strength on As(V) Removal. The as-prepared CMNP@PmPDs were employed to study As(V) removal at various pH values. One can see from Figure 4A that

Figure 4. (A) Effect of pH and ionic strength on As(V) adsorption on the CMNP@PmPDs (mv−1 = 0.1 g L−1, C0(As(V)) = 59.3 mg L−1, T = 25 °C); (B) zeta potentials of CMNPs and CMNP@PmPDs at pH of 2.0 to 10.0.

as the pH increases from 2.0 to 10.0, the As(V) adsorption amount on the CMNP@PmPDs decreases from ∼40 mg g−1 to 20 mg g−1 at C0 = 59.3 mg L−1, and its adsorption versus pH trend is consistent with previously reported research.1 The observed As(V) adsorption can be explained by the physicochemical properties of the CMNP@PmPDs and the speciation distribution of As(V) in aqueous solution over the pH range. It is well-known that As(V) mainly exists as H2AsO4− in a pH range of 3.0−7.0, and HAsO42− species exist in a pH range of 7.0−10.0 in solution.41 Amine and carboxyl functional groups on the surface of the CMNP@PmPDs can be either protonated to form −N+ and −COOH2+ at low pH (R−N + H+ ↔ R−N+, R−COOH + H+ ↔ −COOH2+) or deprotonated at high pH (R−N + OH− ↔ R−N···OH−, R−COOH + OH− ↔ R−COO− + H2O). The zeta potential of the CMNP@PmPDs and CMNPs are shown in Figure 4, and the point of zero charge (pHzpc) values of CMNP@PmPD and CMNPs are ∼5.8 and ∼4.1, respectively. That means the surface charge of the CMNP@PmPDs is positive at pH 5.8, which indicates that increasing the pH contributes to stronger electrostatic repulsion between As(V) species and CMNP@PmPDs, which can explain the poorer adsorption of As(V) on the CMNP@ PmPDs. In addition, the removal capacity at different pH values can result from the reduction reaction proceeding between the As(V) species (H2AsO4−, HAsO42−) and absorbents (under acidic conditions: H2AsO4− + 4H+ + 2−NH− → As3+ + 2− N + 4H2O; under alkaline conditions: HAsO42− + H2O + 2− NH− → 2As3+ + 2−N + 5OH−; −NH− and −N are

Figure 5. (A) Adsorption isotherms of the As(V) on CMNP@PmPDs at 55, 40, and 25 °C and on CMNPs at 25 °C (mv−1 = 0.1 g L−1, pH 2.3); (B) Langmuir isotherm model; (C) Freundlich isotherm model; and (D) plot of ln(K0) vs 1/T.

As(V) adsorbed on the absorbents increases as the As(V) concentration increases, because higher initial concentrations possess higher driving forces, which are favorable for adsorbate transportation from the solution to the absorbents, resulting in an increase in collisions between As(V) and the active sites on the absorbents.9 The CMNP@PmPDs show enhanced As(V) adsorption capability, compared with individual CMNPs, because of their richer functional groups. The results indicate that additional functional groups of PmPDs exhibit excellent As(V) adsorption performance, which compensates for the reduced adsorption capacity, which is due to the decreased BET surface area (see Figure S1 in the Supporting Information). To measure the adsorption capacity of the CMNP@PmPDs, the experimental data were fitted by both the Langmuir and Freundlich models.44 The two models are expressed by the following equations:

D

Ce C 1 = + e qe bqm qm

(4)

ln qe = ln KF + n ln Ce

(5) DOI: 10.1021/acssuschemeng.7b00468 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering where the Langmuir parameter b (L mg−1) is correlated to the heat of adsorption, while the parameter qm (mg g−1) represents the maximum As(V) removal capacity of the absorbents. KF and n are correlated to the adsorption capacity and energetic heterogeneity, respectively. The relative parameters of the two isotherm models were calculated and are listed in Table 1. From the correlation

ln k0 =

Langmuir Model qmax (mg g−1)

25

51.2

25 40 55

95.2 138.9 151.5

b (L mg−1)

where ΔG represents the change in Gibbs free energy; R represents the gas constant (R = 8.314 J mol−1 K−1; and T represents the reaction temperature (in Kelvin). ΔH0 (the standard enthalpy change) and ΔS0 (the standard entropy change) can be calculated by plotting ln k0 (kJ mol−1) vs T−1 (see Figure 5D). The related thermodynamic parameters (Table 3) show negative ΔG0 values, positive ΔH0 values,

Freundlich Model R2

KF

CMNP Sample 0.0431 0.999 10.11 CMNP@PmPD Sample 0.0170 0.997 5.92 0.0120 0.995 4.24 0.0160 0.988 6.54

n

Table 3. Values of Thermodynamic Parameters for As(V) Adsorption on CMNP@PmPDs

R2

0.31

0.981

0.50 0.62 0.59

0.994 0.992 0.984

(9)

0

Table 1. Summary of the Langmuir and Freundlich for As(V) adsorption temperature, T (°C)

ΔS 0 ΔH 0 − R RT

temperature, T (°C)

ΔH0 (kJ mol−1)

ΔS0 (J mol−1 K−1)

ΔG0(kJ mol−1)

25 40 55

2.239 2.239 2.239

64.58 64.58 64.58

−0.757 −1.724 −2.713

and positive ΔS 0 values, indicating spontaneous and endothermic reaction under the conditions applied and increased disorderliness of the solid-solution system during the adsorption and reduction process.20 Adsorption Kinetics of As(V). The kinetics of As(V) removal by the CMNP@PmPDs is presented in Figure 6. One

coefficients (R2), the experimental data clearly show better fit to the Langmuir model than the Freundlich model (see Table 1), indicating that the binding energy on the entire surface of the adsorbents is uniform, and chemisorption is involved in this adsorption process.45 To prove the feasibility of using CMNP@ PmPDs as a desirable absorbent for As(V) removal, the maximum As(V) adsorption capacity of the CMNP@PmPDs is compared with that of other reported absorbents (see Table 2). Table 2. Comparison of the Adsorption Capacity of As(V) on CMNP@PmPDs with Other Different Absorbents absorbent Fe3O4 particles goethite iron oxideloaded slag Cu-loaded PmPD chestnut-like Fe3O4 CMNP@ PmPDs

optimum pH

temperature, T (°C)

adsorption capacity (mg g−1)

ref

5.0 5.0 2.5

25 37 20

7.2 5.0 78.5

11 46 47

5.0

35

25.0

48

4.0

25

2.3

25

6.07 95.2

49 this work

Figure 6. (A) Effect of contact time on As(V) adsorption onto CMNP@PmPDs, (B) fitting of the pseudo-first-order kinetic model, (C) pseudo-second-order kinetic model, and (D) intraparticle diffusion model (C0(As) = 59.3 mg L−1, mv−1 = 0.1 g L−1, pH 2.3).

Although the removal capacity of As(V) can be affected by pH, the adsorption capacity of 95.2 mg g−1 demonstrates that the adsorption capacity of the CMNP@PmPDs is much higher than that of reported Fe 3 O4 particles and Cu-loaded PmPD.11,48 Meanwhile, note that the better As(V) adsorption also due to the poorer coagulation of CMNP@PmPDs (see Figure S2 in the Supporting Information).50,51 The above results suggest that the novel absorbent (CMNP@PmPDs) is expected to be applied in wastewater, especially industrial water, because of its good adsorption behavior. The As(V) adsorption performance of the CMNP@PmPDs under different temperatures is shown in Table 1. The removal capacity increased from 95.2 mg g−1 to 151.5 mg g−1 as the adsorption temperature increased from 25 °C to 55 °C, which suggests that the adsorption performance is promoted at high temperature. The values of the thermodynamic parameters were obtained according to eqs 8 and 9. ΔG 0 = −RT ln k0

can see that the amount of adsorbed As(V) increases rapidly during the initial contact time of 60 min and then increases slowly until it reaches a steady state. The kinetic process implies that 5 h is sufficient to make the As(V) removal by CMNP@ PmPDs to achieve complete equilibrium in our experiments. For detailed investigation of As(V) removal, the pseudo-firstorder and pseudo-second-order rate models were used to simulate the experimental data, which are expressed by the following equations:52 pseudo-first-order model: ln(qe − qt) = ln qe − k1t

(8) E

(10) DOI: 10.1021/acssuschemeng.7b00468 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering pseudo-second-order model: t 1 t = + 2 qt qe k 2qe

(11)

where qt (mg g−1) represents the As(V) removal capacity at time t (min) and k1 (min−1) and k2 (g mg−1 min−1) represent the rate constant related to each model. Plotting ln(qe − qt) vs t can give the value of k1 and qe. The fitting curve of tqt−1 vs t can give the value of the constant (k2) and the adsorption capacity at equilibrium (qe). The kinetic fitting curves for the adsorption of As(V) onto CMNP@PmPDs are shown in Figure 6. The model fitting parameters are tabulated in Table 4. According to Table 4. Kinetic Parameters for As(V) Adsorption parameter

value of parameter

Pseudo-First-Order Model K1 0.02 min−1 qe 46.1 mg g−1 R2 0.9306 Pseudo-Second-Order Model K2 0.00089 g mg−1 min−1) qe 52.6 mg g−1) R2 0.9959 Initial Phase of Intraparticle Diffusion Model Kp,1 5.5275 mg g−1 min−0.5 C1 −0.8994 mg g−1 min−0.5 R2 0.9735 Secondary Phase of Intraparticle Diffusion Model Kp,2 0.58307 mg g−1 min−0.5 C2 37.852 mg g−1 min−0.5 2 R 0.9305

Figure 7. (A) XPS wide spectra of CMNP@PmPDs before and after adsorption of As(V); also shown are high-resolution spectra of (B) As 3d, (C) N 1s before adsorption, and (D) N 1s after adsorption.

1s can be clearly identified. The high-resolution XPS spectrum of As 3d (Figure 7B) shows two peaks located at 45.1 and 45.8 eV, indicating that As existed on the surface of the CMNP@ PmPDs in two forms: 59.2% As(V) and 40.8% As(III).48,53 This means that some As(V) on the composites was reduced to As(III).48,54 These results reveal that the main mechanism of As(V) removal should be from the combined effects of the reduction reactions of As(V) to As(III) and electrostatic interactions. Based on the N 1s data before adsorption, the peak in Figure 7C is deconvoluted into two peaks69.8% −NH− at 399.8 eV and 30.2% −N at 399.1 eVwhich are attributed to the benzenoid amine units and quinoid imine units, respectively. After adsorption, it can be found that the molar ratio of benzenoid amine units decreased from 69.8% to 49.7%, whereas the content of quinoid imine units increased from 30.2% to 44.3%. This result demonstrates that −NH− in the conjugated polymers should be oxidized by As(V) and transformed to −N.39,55 Meanwhile, a new peak appeared at 400.6 eV, assigned to protonated quinoid imine units (−N+).55 One reason for the presence of −N+ should be the doping of H+ on the quinoid imine units.22 Another explanation for the appearance of −N+ can be the chelation of As(III) during adsorption, produced by the reduction of As(V).56 Hence, the possible adsorption mechanism is as follows: (i) abundant As(V) species are adsorbed rapidly to interact with the protonated imino and carboxyl functional groups on the surface of the CMNP@PmPDs via electrostatic attraction; (ii) a portion of As(V) then is reduced to As(III) by the benzenoid amine units, and then As(III) readily interacts with imino groups through coordination; and (iii) the decrease in As(V) and proton contents in the reaction process has a tendency to retard and terminate the redox reaction in the end. Recycle Performance. To investigate the recycle performance of CMNP@PmPDs, 0.5 M NaOH was used as an eluent. Figure 8 shows that As(V) adsorption capacity decreases from ∼41.7 mg g−1 to 30.1 mg g−1 and remains ∼72% of the initial adsorption capacity after the sixth recycle. The decrease in adsorption amount can be explained by the incomplete

the related kinetic parameters, the Langmuir model exhibits higher regression constants than the Freundlich model, which reveals that the rate-controlling mechanism for As(V) adsorption on CMNP@PmPDs is chemisorption.45 In addition, an intraparticle diffusion model was employed to further investigate As(V) removal. The model is written as

qt = k pt 0.5 + c

(12)

where kp represents the rate constant and c is a parameter correlated with the boundary layer effect. The intraparticle diffusion model parameters are tabulated in Table 4. One can see from Figure 6D that the fitting curves exhibit a piecewise line with two slopes, indicating that the entire adsorption process involves two steps with different limiting processes. The first rapid section may be ascribed to As(V) diffusion to the external surface of the absorbents and occupation the active sites of the absorbents. The subsequent slow adsorption process may result from the relatively low amount of residual As(V) in the suspension. Meanwhile, the whole of the fitting curves (Figure 6D) do not pass through the origin. This result confirms that the rate-limiting steps do not only include intraparticle diffusion, and other reactions (i.e., chemical redox) may be involved.5,30 Adsorption Mechanism of As(V) onto CMNP@PmPD Particles. To gain insight into the adsorption mechanism of As(V), X-ray photoelectron spectroscopy (XPS) was applied to characterize the CMNP@PmPD composites before and after As(V) adsorption. In the survey spectrum (Figure 7A), it can be seen that peaks corresponding to C 1s, O 1s, As 3d, and N F


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21577032), the Priority Academic Program Development of Jiangsu Higher Education Institutions, and the Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions are acknowledged.

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Figure 8. Recycle property of CMNP@PmPDs (mv−1 = 0.1 g L−1, C0(As(V)) = 59.3 mg L−1, pH 2.3, T = 25 °C).

recovery of the adsorbent and that amine groups cannot be reduced to imine groups. The inset of Figure 8 shows the digital image of separation of CMNP@PmPDs from solution using a conventional magnet, indicating its convenient separation or efficient enrichment in practical application.

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CONCLUSIONS In the present work, we have developed a facile method for preparing CMNP@PmPD microspheres with uniform welldefined core−shell structures, which were demonstrated to be effective adsorbents through batch experiments. The asprepared CMNP@PmPDs show a saturated magnetization value of 46.7 emu g−1 at room temperature, enabling rapid separation under an external magnetic field. The CMNP@ PmPDs exhibit a much higher adsorption capacity toward As(V) (95.2 mg g−1) than other absorbents, and the adsorption is spontaneous and endothermic. The adsorption of As(V) on protonated imino and carboxyl groups and the reduction of As(V) to As(III) by amine groups are suitable to explain the interaction between As(V) and the CMNP@PmPDs. These results confirm that the CMNP@PmPDs could be promising candidates for the removal of As(V) in practical applications.

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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.7b00468. N2 adsorption−desorption isotherm plot of CMNPs and CMNP@PmPDs (Figure S1); Figure S2 aggregation of CMNP@PmPDs and CMNPs (Figure S2); descriptions of how Figures S1 and S2 were constructed (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Tel.: 86-551-65593308. Fax: 86-551-65591310. E-mail: [email protected]. ORCID

Xiaoli Tan: 0000-0003-4427-3396 Author Contributions ∇

These authors contributed equally to this paper.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support of the National Natural Science Foundation of China (Nos. 21377132, U1607102, G


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