Magnetic Porous Polymers Prepared via High Internal Phase

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Magnetic Porous Polymers Prepared Via High Internal Phase Emulsions for Efficient Removal of Pb2+ and Cd2+ Hongshan Zhu, Xiaoli Tan, Liqiang Tan, Huifang Zhang, Haining Liu, Ming Fang, Tasawar Hayat, and Xiangke Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04868 • Publication Date (Web): 02 Mar 2018 Downloaded from http://pubs.acs.org on March 4, 2018

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ACS Sustainable Chemistry & Engineering

Magnetic Porous Polymers Prepared Via High Internal Phase Emulsions for Efficient Removal of Pb2+ and Cd2+ Hongshan Zhu,a,b Xiaoli Tan*,a,b,e Liqiang Tan,a,b Huifang Zhang,d Haining Liu,d Ming Fang*,a,c Tasawar Hayat,f Xiangke Wanga,e a

School of Environment and Chemical Engineering, North China Electric Power

University, Beinong Road 2, Changping District, Beijing 102206, P.R. China b

Institute of Plasma Physics, Chinese Academy of Sciences, Shushanhu Road 350,

Shushan District, Hefei, 230031, P.R. China c

Institute of Solid State Physics, Chinese Academy of Sciences, Shushanhu Road

350, Shushan District, Hefei 230031, P.R. China d

Key Laboratory of Salt Lake Resources and Chemistry, Qinghai Institute of Salt

lakes, Chinese Academy of Sciences, Xinning Road 18, Xining 810008, China e

Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher

Education Institutions, Soochow University, Shizi Street 1, Suzhou 215123, P.R. China f

NAAM Research Group, King Abdulaziz University, Al Jamiaa District 80200,

Jeddah 21589, Saudi Arabia * Corresponding author. Email: [email protected]. or [email protected] (X. Tan); [email protected] (M. Fang). ABSTRACT Magnetic porous polymers (MPPs) were successfully fabricated by a facile strategy of high internal phase emulsions (HIPEs) technique. The microstructure, 1

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chemical composition, and magnetic properties of the MPPs were characterized. Impregnated

with

poly(styrene-divinylbenzene),

stabilized

by

the

amine-functionalized Fe3O4 nanoparticles (Fe3O4-NH2), the as-prepared MPPs with rich pore hierarchy were employed to removal Pb2+ and Cd2+ from aqueous solution. The MPPs display outstanding removal capacities toward Pb2+ (257 mg/g) and Cd2+ (129 mg/g) within 15 min, and the encapsulated Fe3O4-NH2 nanoparticles endow the MPPs with the ability of magnetic separation (30.15 emu/g). Additionally, the results indicate that the adsorption of Pb2+ and Cd2+ are strongly dependent on pH and ionic strength, demonstrating that the interaction of Pb2+ and Cd2+ were mainly dominated by outer-sphere surface complexation and electrostatic attraction. The adsorption process is revealed by thermodynamic parameters to be spontaneous and endothermic. Further study demonstrates the adsorption is involved ion exchange and cation-π interactions (between heavy metals and aromatic ring) on the surface of MPPs. Thus, feasible preparation of the MPPs with high adsorption capacities, excellent regeneration and easy separation properties open a new expectation in the potential application for engineering. KEYWORDS:

magnetic

porous

polymers,

high

internal

phase

emulsion

polymerization, Pb2+, Cd2+, adsorption

INTRODUCTION The recovery and reuse of wastewater have attracted considerable interest

2


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because of the ever-increasing demand for water.1,2 More than 2000 chemical contaminants have been found in wastewater, and among the toxic pollutants, heavy metal ions are hazardous to human beings, which can cause disease in central nervous, immune, and reproductive systems, etc., by accumulating in the human body.3,4 Thus, the remediation of heavy metal ions in water has become an increasingly public and environmental concern. Lead (Pb) and cadmium (Cd) have been accepted as the most two concerned heavy metal ions.2,5 Conventional methods, such as chemical (chemical precipitation and ion-exchange), physical (adsorption and membrane filtration), electrochemical and biological treatment for heavy metal ions removal from contaminated water have been extensively investigated and applied, by which lots of polluted water bodies have had an effective control or remediation to some extent.4,6-8 Among these, adsorption is the most frequently studied method, proving advantages over other methods due to its high efficiency, low operating cost, easy handling and without production of chemical or biological sludge.9-11 Over the last few decades, various adsorbents (i.e. nanosized metal oxides, polymer and polymer-based hybrids, zeolites, metal organic frameworks and natural materials porous metal-oxides) have been employed to remove Pb2+ or Cd2+ from the aqueous solutions.12-15 However, these materials are still far beyond satisfactory for treating Pb2+ and Cd2+ from contaminated water due to their low adsorption capacity, long time to achieve adsorption equilibrium, or poor regeneration possibility. In fact, the aggregation of the nanomaterials or the difficulty of separation,12,16 the nonrenewable of the functional groups or the destroyed structure of adsorbents after 3



adsorption have fairly limited their recycle performance.12-16 To overcome these challenges, advanced materials need to be designed with excellent adsorption efficiency, simple operation performance, and fast removal of Pb2+ and Cd2+ from polluted water. Porous polymers have attracted significant attention due to their specific surface area, abundant permanent porous structure and surface functional groups in addition to the good physicochemical stability,17-19 which have been used for various applications, such as the supports for organic synthesis, separation, catalysis, gas storage and as tissue engineering matrices.20-22 In addition to these attractive applications, it is particularly interest in utilizing porous polymers as active materials for the removal of heavy metal contaminants. Most polymers have a lot of cyclic aromatic π-system, which could be used as the π-donor to interact with heavy metal ions.23-25 As we know, the six (four) Cδ--Hδ- bond dipoles of a molecule like benzene (ethylene) combine to produce a region of negative electrostatic potential on the face of the π-system.26-29 Simple electrostatic attraction provides an approach for cations to the surface of adsorbents. Furthermore, cation-π interaction is very strong compared with hydrogen bonding and van der Waals terms.30-32 It should be noted that heavy metal ions removal using the cation-π interaction have been increasingly utilized as potent methods. However, the investigation on Pb2+ and Cd2+ removal as cation-π interaction of polymers seldom concern.29,33 Several template methods have been used so far to produce thermoset porous polymers. High internal phase emulsions (HIPEs) technique is a versatile approach 4


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to produce polymers with connected pores and tunable pore size, which has been investigated extensively for the fabrication of porous materials, with good moisture resistance, low cost, and simple preparation process. 34-36 HIPEs are concentrated in emulsion systems possessing more than 74% volume of the internal phase, and the spherical droplets of dispersed phase are deformed into polyhedra and separated by thin film of continuous phase, yielding a highly interconnected mesoporous/macroporous polymeric network.37 The obtained porous polymers with interconnected and well-defined micropores (50 nm), are particularly useful for applications in adsorption due to diffusion rates can be increased inside large pores (at micron level) and the interconnected structure enhanced adsorption activity of the pores to heavy metals. Although polyHIPE materials are highly porous, their surface area is usually tested to be in the range of 3-20 m2/g.22 This is most probably due to the large void size (approximate 0.5-600 µm) and the smaller windows (approximate 0.1-300 µm) present in their structure, which is induced by the droplets of the internal phase.38 The large pores allow fast and efficient mass transport, which can be fast and efficient adsorption of heavy metal ions from aqueous medium.17 Nevertheless, real industrial applications are limited because of the poor mechanical properties of polyHIPEs that may lead to the collapse of the monolithic structure when it is subjected to the flow-through of liquids.39 Thus, in the previous study, numerous attempts have been focused on improving the mechanical properties of polyHIPEs, for example incorporation of inorganic components.21,40 A novel and facile route was 5



made by varying particles and surfactant concentrations as dual stabilizer system to synergistic stabilize and modulate the structures of HIPEs.21,40 In addition, the polyHIPEs is usually difficult to be separated from water while used as adsorbents. A recent solution has induced great concerns is to introduce magnetic nanoparticles into some similar systems. The magnetic particles are easily to be functionalized, and have high trapping capacity, most importantly it can be easily isolated under an external magnetic field. Therefore in this study, we report a facile synthetic approach to prepare magnetic porous polymers (MPPs) by the use of styrene (St) as monomer, divinylbenzene (DVB) as cross-linker, azobisisobutyronitrile (AIBN) as initiator, sorbitane monooleate (Span 80) as emulsifier and amine-functionalized magnetic Fe3O4 nanoparticles as particle stabilizer and magnetic nanoparticles via high internal phase emulsion polymerization. The surface of the MPPs with a large number of benzene rings is negatively charged and is peculiarly prone to attach with the cation by π bond. The main objective of this study was to report a new MPP, and to demonstrate the adsorption performance towards Pb2+ and Cd2+ from the aqueous solutions by determining the adsorption capacity, kinetics rate, and stability under the interference of various environmental factors including pH, and ionic strength (Scheme 1). This work offers an alternative route to synthesize MPPs, as a selective adsorbent for Pb2+ and Cd2+ removal and provide insights of the interaction between heavy metal ions and MPPs. EXPERIMENTAL SECTION 6


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Materials. All chemicals in this work were of reagent grade and used without any further purification. Sodium citrate (NaC6H5O7), anhydrous sodium acetate (CH3COONa), ferric chloride hexahydrate (FeCl3·6H2O), ethylene glycol (HOCH2)2, sodium

persulfate

(Na2S2O8),

1,6-hexanediamine,

divinylbenzene

(DVB),

azobisisobutyronitrile (AIBN) and styrene (St) were purchased in analytical reagent from Sinopharm Chemical Reagent Co. Ltd. Preparation

of

Fe3O4-NH2

and MPPs.

Amine-functionalized

Fe3O4

nanoparticles (Fe3O4-NH2) with uniform particle size of approximately a mean diameter of 50 nm were prepared by a solvent thermal method according to our previous study.41 The oil phase containing 7 mL St and 6.8 mL DVB was charged into a 500 mL glass reactor at room temperature and mechanically stirred at 400 rpm (15 min). Then 4 mL Span 80, 0.5 g initiator of thermoset AIBN and 0.572g Fe3O4-NH2 particles was dispersed into the oil phase by ultrasonication. 20 min later, 210 mL water was added drop-wise to the oil phase through a feeder at a constant rate of 1.3 mL/min

to

form

a

concentrated

emulsion.

After

that,

the

as-prepared

poly(styrene-divinylbenzene) emulsion was put in an oven at 70 oC for 7 h. The obtained monolithic foam was dried by vacuum. Then the residual surfactant and monomer in the foam was removed by Soxhlet extraction with ethanol. Finally, the poly(styrene-divinylbenzene) foam was dried by vacuum. The as-prepared MPPs were ground into particles and screened by mesh 40-80 to study the batch adsorption experiments. 7



Characterization. The morphology and pore sizes of the samples were characterized by field emission scanning electron microscopy (SEM, JSM-6700F) and transmission electron microscopy (TEM, JEOL-2010). Powder X-ray diffraction (XRD) was performed by using a (Philips X'Pert Pro) diffractometer with Kα source (λ=1.54178 Å). Fourier transform infrared spectroscopy (FT-IR, Nicolet 8700) was used to identify the surface organic functional groups of MPPs. The samples were also investigated by Raman spectroscopy (RAMANLOG 6) at room temperature. Thermal gravity analysis (TGA) was implemented by the TGA-60/60H thermal analyzer (Shimadzu) under a N2 atmosphere at a heating speed of 20 oC/min. Vibrating sample magnetometer (VSM) was employed to test the magnetism of as-prepared nanocomposites, and the range of magnetic field was between -30000 and 30000 Oe. The zeta potentials (ZP) of the samples were obtained as a function of pH using a Nanosizer ZS instrument (Malvern) at 25 oC. The X-ray photoelectron spectroscopy (XPS) measurements were recorded on an ESCALAB 250 (Thermo-VG Scientific). Batch Adsorption Experiments. The adsorption performance of MPPs was studied by batch adsorption experiments in 10 mL polythene centrifuge tubes. The stock suspensions of MPPs (0.9 g/L) and Pb2+ or Cd2+ solution (360 mg/L) were prepared by Milli-Q water. The desired experiments were performed by adding NaCl to achieve the concentration of 0.001, 0.01, and 0.1 mol/L, respectively, and the mixture was adjusted to desired pH value with adding negligible 0.1 mol/L NaOH or HCl, and then shaken for 24 h. Finally, the supernatant was taken off for 8


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measurements after centrifugation. The

reusability

of

MPPs

was

evaluated

via

sequential

cycles

of

adsorption-desorption experiments. After adsorption, the MPPs were collected and washed thoroughly with deionized water to remove unadsorbed metal ions on the surface. Then it was regenerated in 0.5 mol/L of HCl solution or 0.5 mol/L EDTA, and shaken for 2 h. The desorbed adsorbent was further used in the next adsorption cycle. Each experiment was carried out for five times under the same conditions. The concentration of Pb2+ or Cd2+ adsorbed on MPPs was obtained by the difference between the original concentration (C0, mg/L) and the equilibrium concentration (Ce, mg/L). The adsorption rate (%), distribution coefficient (Kd) and adsorption capacity (Cs, mg/g) were expressed by follows:25-27 Adsorption (%) =

C0 - Ce ×100% C0

(1)

Kd =

C0 - C e V × Ce m

(2)

Cs =

C0 - Ce ×V m

(3)

where V (mL) is the volume of suspension, and m (g) represents the mass of the adsorbents. Experimental data were the average value after triplicate measurements, and the relative errors were less than 5%.

RESULTS AND DISCUSSION Characterization of Fe3O4-NH2 and MPPs. TEM and HRTEM images of Fe3O4-NH2 prepared by the solvent thermal method are shown in Figure S1. The

9



as-prepared nanoparticles are remarkably uniform with diameter of approximate 50 nm and the spacing of lattice fringes is 0.484 nm, which is corresponded to the (111) plane of Fe3O4 (cubic shape crystal). Furthermore, the transparent materials on the surface of the Fe3O4-NH2 are ascribed to the presence of a specific ligand (1,6-hexanediamine) which can control the size of the Fe3O4-NH2 nanoparticles and against aggregation. Figure 1A, B, and C show the SEM images of the MPPs under different magnification. It can be clearly seen that it is uniform in a large scale, and there are lots of voids with diameters about 0.5-20 µm dispersed on the surface. The surface area of the porous structures of PPMs is as large as 5.532 m2/g (Figure S2).22 From the enlarged view of Figure 1D, lots of nanoparticles with diameters of about 100 nm can be clearly seen. This could also be proved by the TEM images (Figure 1E) that lots of solid particles bounded together by polymers having weak contrast can be seen. Further investigation was performed by EDS mapping (Figure 1F-H), from which one can see that the distribution area of C is obviously larger than that of O and Fe, indicating the Fe3O4 nanoparticles are wrapped by the polymers. Moreover, the EDS spectrum of MPPs, only show C, O and Fe elements (Figure S3). Figure 2A shows the XRD patterns of MPPs and Fe3O4-NH2 nanoparticles. The XRD peaks of Fe3O4-NH2 and MPPs at 18.4, 30.1, 35.5, 43.1, 53.4, 57.1, and 62.7o are specified to the lattice plane of (111), (220), (311), (400), (422), (511), and (440), respectively, of the magnetic face-centered cubic Fe3O4 structure.42 After polymerization, the intensity and width of the diffraction peak at 18.35o increased due to the dispersed peaks of the amorphous porous polymers is also at the same 10


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position.43 The structural information and chemical components in Fe3O4-NH2 and MPPs were performed by FT-IR (Figure 2B). As for the Fe3O4-NH2, the band at 582 cm-1 is attributed to the Fe-O lattice model of Fe3O4, and the bands at 874, 1626, and 3436 cm-1 correspond to the N-H stretching vibration.11 The bands at 2925, 2850, and 1448 cm-1 are attributed to -CH2- stretching and bending vibrations.44,45 The bands at 698 and 759 cm-1 can be assigned to flexural vibrations (C-H) of a benzene ring.45 The absorption bands due to the aromatic stretching vibrations of styrene unites are observed at around 1491 and 3030 cm-1.44,45 Moreover, the band at 582 cm-1 also indicates the existence of Fe3O4 in the synthesized MPPs.46 Figure 2C shows the Raman spectra of the synthesized Fe3O4-NH2 and MPPs. The peaks at 331, 524, and 670 cm-1 confirm the magnetic Fe3O4 phase.11 The peaks from 1300 to 1700 cm-1 of Fe3O4-NH2 are ascribed to the bond-stretching of sp2 hybridized carbon atoms, defects, and the specific surface ligand (-NH2).11 As for the MPPs, the characteristic peaks at 3059, 2915, 2855, 1602, 1448, 1181, and 1003 cm-1 correspond to the conventionally observed benzene ring Raman frequency, -CH2antisymmetic stretching, C-H stretching, ring antisymmetic stretching deformation, the C-H deformation, ring antisymmetic bend, and aromatic ring vibration, respectively.47 Thermal gravity analysis was performed to determine the stability and degradation profiles of all the synthesized Fe3O4-NH2 and MPPs (Figure 2D). The curve of the Fe3O4-NH2 nanoparticles presents the first weight loss (1.1%) as the 11



temperature is lower than 150 oC, which is ascribed to the elimination of moisture and ethanol on the surface; after that, in the range from 150 to 320 oC, the second weight loss (2.5%) is ascribed to the phase transformation of Fe3O4-NH2;11 and finally the third weight loss (9 %) is found in the range from 320 to 750 oC, which is ascribed to the decomposition of the abundant amino groups on Fe3O4-NH2. For the MPPs, the polymers are highly stable up to 200 oC which is within the working temperature range of MPPs for the removal of heavy metals from contaminated water. The major decomposition temperatures are found above 300 oC, especially 350-450 oC. Small decreases at 200-350 oC are due to the random chain scission, followed by the surface decomposition at higher temperatures (350-450 oC). After 450 oC, the porous polymers have been decomposed mostly. The saturation magnetization of the adsorbents was investigated by magnetization hysteresis loops at 20 oC (Figure 2E). The saturation magnetizations of Fe3O4-NH2 and MPPs are 80.02 and 30.15 emu/g, respectively, meaning the adsorbents have a satisfactory magnetic property. Therefore, the MPPs can be separated easily by magnetic adsorption from the contaminated solutions.48 The zeta potential of MPPs was measured as a function of pH shown in Figure 2F. It can be seen that the zeta potentials of MPPs decrease with pH increasing and all the zeta potentials are negative, which are mainly attributed to the deprotonated H+ on the surface of the MPPs at higher pH values. Generally, the negatively charged surface of adsorbents can be easily used to remove the cations due to the electrostatic attraction and it is difficult to dispose the anion for the electrostatic repulsion. 12


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Furthermore, the thickness of the double layer also depends upon the concentration of ions in solution, and the zeta potential increases with the ionic strength increasing.

Effect of pH on Pb2+ and Cd2+ Removal. The removal capacities of heavy metal ions in solutions are highly dependent on the pH of solution due to the pH has effect on the solubility and chemical properties of metal ions as well as the activity of the metal binding sites of adsorbents.49 Therefore, a series of experiments were conducted at pH range of 3.0-7.0 for Pb2+ and Cd2+ (Figure 3).13 The metal loading increases with increasing the pH up to 7.0 mainly due to the driving force of electrostatic attraction between the metal ions and MPPs. The relative low adsorption capacity in the strongly acidic conditions is attributed to the excess H+ ions competing with Pb2+ and Cd2+ ions for adsorption sites. High pH (> 7.0) will result in the formation of precipitation due to the hydrolysis of Pb2+ and Cd2+. No further experiments were carried out in the following adsorption under alkaline conditions. Therefore, to avoid the interference of hydroxide precipitation and ensure the only ionic species presenting in the solution are Pb2+ and Cd2+, all of the following adsorption experiments were carried out at pH 5.5.

Effect of Ionic Strength on Adsorption. The ionic strength can influence the electric double layer thickness and interface potential of the adsorbents, thus affecting the electrostatic attraction between heavy metals and MPPs.50 The effect could assist in making a distinction between the inner-sphere and outer-sphere surface complexes. The inner-sphere complexes are hardly affected by the ionic strength, whereas outer-sphere complexation is significantly decreased by increasing 13



ionic strength.51 Figure 3 shows the influence of ionic strength on the adsorption process. With the increase of ionic strength from 0.001 to 0.1 mol/L NaCl, the electrostatic interaction active sites decrease, which means that the more and more foreign positive ions (such as: Na+) can be adsorbed on the surface of MPPs as ionic strength increasing. Moreover, the ionic strength influences the activity coefficients of Pb2+ and Cd2+, which limits metal ions transferring to the MPPs surface. Thus, less metal ions can be adsorbed. This can be explained by the nature of electrostatic interactions attraction between the MPPs and the metal ions at low ionic strength, and demonstrate that outer-sphere surface complexation mainly exists in the adsorption process. Furthermore, it implies that Pb2+ has higher affinity or stronger attraction to negatively charged MPPs surface than Cd2+. Because of the large ion radius and unique electronic configuration ([Xe]4f14 5d10 6s2 6p2), the Pb2+ is hard to be polarized, and the formed Pb(II) is insoluble (Ksp = 1.7×10-5). Meanwhile, due to the relatively small ion radius, the Cd2+ is easily to be polarized, and their chlorides usually have high solubility.52

Effect of Adsorbent Contents. The contents of MPPs range from 75 to 825 mg/L with keeping other variables constant, and the results are shown in Figure 4A and B. The adsorption percentage is found to increase obviously with increasing the content of MPPs. This is expected due to the increase of the adsorbent content may present more adsorption active sites for Pb2+ and Cd2+. The additional vital parameter of distribution coefficient (Kd) is important to understand the affinity of the adsorbate and the adsorbent.53 As shown in Figure 4, the Kd values are 14


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independent on the solid/liquid ratio when it is relative small due to the physicochemical properties of Kd.53 Moreover, the Kd values for Pb2+ are slightly higher than that for Cd2+.

Adsorption Kinetics. Figure 5A exhibits the effect of contact time on Pb2+ and Cd2+ adsorption on MPPs. The adsorption capacities for Pb2+ and Cd2+ increase rapidly within the first 10 min, because of a large number of adsorption sites supplied by MPPs. Then, the adsorption capacities increase slowly with the increase of contact time until reaching equilibrium, due to the diffusion of metal ions into pores and the adsorption by interior surface are slow processes after almost all facial adsorption sites of MPPs were occupied. To explore adsorption behavior, the pseudo-first-order, pseudo-second-order rate models and intraparticle diffusion model were used to fit the adsorption data, as shown in Figure 5A and B, and the kinetic parameters of adsorption for Pb2+ and Cd2+ on MPPs are exhibited in Table S1.54 Compared with pseudo-first-order model, the higher correlation coefficient (R2 = 0.999) of pseudo-second-order model means the pseudo-second-order kinetic model could provide a good interpretation of the Pb2+ and Cd2+ adsorption behaviors on MPPs, suggesting that there is strong chemical forces caused by exchanging electrons at the solid/solution interface.55 Figure 5B depicts the relationship between Ct versus t1/2 for intraparticle diffusion model.11,45 All plots exhibit a segmented type and the excursion of the curves from the origin points implies that the intraparticle diffusion is not the sole rate-limiting step.11 The adsorption process can be divided into four stages with kid. 15



Higher diffusion rates in first step could be ascribed to the diffusion of Pb2+ and Cd2+ through the boundary layer to the external surface of the adsorbents. As contact time increased, the adsorption process is governed by intraparticle, pore diffusion and the relatively low residual of Pb2+ and Cd2+ due to kid decelerates in the second stage and third stage. A plateau is found at the fourth stage, implying the adsorption equilibrium has been achieved as a result of the exhaustion of the available active sites for Pb2+ and Cd2+ adsorption. Values of intercept (A) are proportional to the extent of the boundary layer thickness, in other words, the larger A values are, the greater boundary layer effect would be.9 The results suggest that apart from the intraparticle diffusion, the transfer of Pb2+ and Cd2+ from fluid phase to MPPs across its boundary layer could also affect the adsorption.

Adsorption Isotherms and Thermodynamics. The solution temperature plays a significant role in reaction rate and removal efficiency. The adsorption isotherms of Pb2+ and Cd2+ at 298, 308, and 318 K onto MPPs are displayed in the Figure 6. The adsorption capacity increases gradually with increasing the initial Pb2+ and Cd2+ concentration. Furthermore, the adsorption performance is improved as the temperature increased. To better investigate the adsorption mechanism and quantify the adsorption data, the Langmuir and Freundlich isotherm models (see Supporting Information) were employed to quantitatively analyze the adsorption thermodynamic behavior.56 The fitting of Langmuir and Freundlich isotherm models and the thermodynamic parameters are shown in Figure 6 and Table S2. Compared with the Freundlich model, the adsorption data of Pb2+ and Cd2+ on MPPs fit with Langmuir 16


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model well, proving the adsorption of Pb2+ and Cd2+ on MPPs is mainly affected by the active sites on the surface area of MPPs. The applicability of the Langmuir model also indicates that the surface of MPPs is uniform and homogeneous for Pb2+ and Cd2+ removal. Based on the Langmuir model, the maximum adsorption capacities of Pb2+ and Cd2+ on MPPs at pH 5.5 are 257 and 129 mg/g, which are higher than previous report (Table 1). Although, the adsorption capacities can be affected by the reaction conditions, the adsorption capacities of Pb2+ and Cd2+ on MPPs are much higher in regardless of lower pH and the adsorbent content. This observation highlights the potential application of MPPs as adsorbent for the uptake of Pb2+ and Cd2+ from aqueous solutions in contaminated water. Traditional thermodynamic parameters (∆H0, ∆S0, and ∆G0) for Pb2+ and Cd2+ adsorption onto MPPs were calculated from the van’t Hoff isothermal equation (see Supporting Information) and are presented in Figure 6C and D and Table S3. The positive values of ∆H0 prove the endothermic process, which can be interpreted by the high solvated Pb2+ and Cd2+ ions in aqueous solutions. To adsorb Pb2+ and Cd2+ on the MPPs, solvated Pb2+ and Cd2+ are required to denude their hydration sheath to some extent, and this dehydration energy exceeds the exothermicity of the ion attaching to the MPPs. The negative ∆G0 values and the positive ∆S0 values reveal that the process is spontaneous with high affinity, and more active sites interact with Pb2+ and Cd2+ with the increasing of adsorption temperature.41

Adsorption Mechanism. The surface chemical elements of MPPs before and after adsorption were determined by XPS (Figure 7A). From the XPS survey spectra, 17



the main elements of MPPs are C and O (mostly from H3O+). After the Pb2+ and Cd2+adsorbed (MPPs-Pb and MPPs-Cd), the new peaks for Pb and Cd clearly confirm the adsorption of Pb2+ and Cd2+. The Figure 7B and C show the spectra of the Pb4f and Cd3d, respectively. Furthermore, the high-resolution XPS spectra of C1s for MPPs before adsorption were deconvoluted into three peaks at 286.69, 285.23 and 284.71 eV, corresponding to the binding energies of -Cπ-H3O+, C-H/C-C and aromatic ring, respectively (Figure 7D).57-60 As shown in Figure 7E and F, the binding energy of C1s changes in some extent after adsorption. The peaks for -Cπ-H3O+ for MPPs-Pb and MPPs-Cd decrease, indicating the -Cπ-H3O+ also contributes to the adsorption (ion exchange interaction that the ionic exchange of protons in the MPPs-H3O+ interaction for Pb2+ and Cd2+).60 The peaks at 285.21 and 285.22 eV correspond to the C-H/C-C, and the peaks at 284.70 and 284.72 eV correspond to the aromatic ring. The intensity of aromatic ring decreases due to the new peaks at 284.63 and 284.61 eV appeared. Furthermore, the new peaks can be attributed to the cation (Pb or Cd)-π (aromatic ring) interactions (the sharing of electrons by Pb2+ and Cd2+ with benzene ring).

Recycle and Desorption Performance. To investigate the recoverable potential of MPPs, HCl and EDTA were applied as eluent. The Pb2+ and Cd2+ removal efficiency remained >70% after the fifth recycle and desorption efficiency is evaluated to be >90% of each cycle, as shown in Figure S4. The desorption of adsorbed Pb2+ and Cd2+ is achieved by ion exchange selectivity reversal between the protons and Pb2+ and Cd2+ during the acid treatment. As a strong ligand and in its 18


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sodium form, EDTA can effectively eluent Pb2+ and Cd2+ from the surface of MPPs. The stability of the MPPs adsorbent after fifth cycle of regeneration were also investigated (Figure S5-7), and the results imply the MPPs still maintain a high degree of structural stability. Furthermore, the magnetic property is favored for the separation and can significantly reduce the loss of adsorbent while repetitive usage. Therefore, the advantageous magnetic property together with the excellent reusability and removal efficiency to Pb2+ and Cd2+ makes MPPs as a potential candidate in wastewater treatment.

CONCLUSIONS With the development of industry, heavy metal pollution is threatening the human health. In this study, the MPPs were prepared via high internal phase emulsions for the efficient elimination of Pb2+ and Cd2+. The results imply that the removal of Pb2+ and Cd2+ on MPPs is mainly relied on ionic strength and solution pH. The interaction mechanism of Pb2+ and Cd2+ with the composites is mainly ascribed to electrostatic attraction and cation-π (aromatic ring) interactions. The removal capacities of Pb2+ and Cd2+ on MPPs are 257 and 129 mg/g at pH 5.5, respectively, and the removal process is a spontaneous endothermic process. The findings in this study can improve the application of MPPs composites in the current understanding of Pb2+ and Cd2+ removal or other kinds of metal ions in environmental pollution cleanup.

ASSOCIATED CONTENT Supporting Information 19



The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng. xxxxx. Descriptions of Langmuir, Freundlich models, van’t Hoff isothermal equation were constructed. TEM and HRTEM images for Fe3O4-NH2 (Figure S1); EDS for MPPs (Figure S2); BET of MPPs (Figure S3); recycling of the MPPs using HCl and EDTA (Figure S4), the TEM (Figure S5), XRD (Figure S6), and TGA (Figure S7) of MPPs after regeneration, and the parameters obtained from kinetic model (Table S1), isotherm models (Table S2) and thermodynamics (Table S3) (PDF)

AUTHOR INFORMATION Hongshan Zhu: [email protected] Xiaoli Tan: [email protected] or [email protected] Liqiang Tan: [email protected] Huifang Zhang: [email protected] Haining Liu: [email protected] Ming Fang: [email protected] Tasawar Hayat: [email protected] Xiangke Wang: [email protected] or [email protected] Corresponding Author *Tel.: 86-10-61772890. Fax: 86-10-61772890. E-mail: [email protected]. or [email protected] (X. Tan); [email protected] (M. Fang).

Notes

20


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The authors declare no competing financial interest.

ACKNOWLEDGEMENTS

Financial supports from National Natural Science Foundation of China (U1607102, U1504107, 11675210), 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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31



Figure Captions Scheme 1. The scheme of the whole experimental process. Figure 1. SEM images of MPPs (A), (B), (C), and (D) (the magnification of the red frame of C); TEM image of MPPs (E); and C (F), O (G) and Fe (H) elemental mapping for MPPs.

Figure 2. XRD (A), FT-IR spectra (B), Raman spectra (C), TGA (D), and VSM (E) of Fe3O4-NH2 and MPPs; Zeta potential of MPPs (F).

Figure 3. Effect of solution pH and ion strength on Pb2+ (A) and Cd2+ removal (B). m/V = 0.15 g/L, [Pb2+] or [Cd2+] = 90 mg/L, time = 24 h, T = 298 K.

Figure 4. The adsorption of Pb2+ (A) and Cd2+ (B) on MPPs as a function of adsorbent content. pH = 5.5, [Pb2+] or [Cd2+] = 90 mg/L, I = 0.001 mol/L NaCl, time = 24 h, T = 298 K.

Figure 5. The effect of contact time on adsorption capacity of MPPs. Fitting of the pseudo-first-order (solid line) and pseudo-second-order (dot line) models (A), intraparticle diffusion model (B). pH = 5.5, [Pb2+] or [Cd2+] = 90 mg/L, I = 0.001 mol/L NaCl, T = 298 K.

Figure 6. Langmuir (solid line) and Freundlich (dash line) isotherms for the Pb2+ (A) and Cd2+ (B) adsorption onto MPPs; linear plots of ln Kd versus Ce for the adsorption of Pb2+ and Cd2+ adsorption on the MPPs (C), and liner plot of ln Ko versus 1/T of the Pb2+ and Cd2+ adsorption onto MPPs (D). pH = 5.5, m/V = 0.15 g/L, I = 0.001 mol/L NaCl, time = 24 h.

32


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Figure 7. XPS spectra of MPPs, MPPs-Pb and MPPs-Cd(A); Pb4f spectrum of MPPs-Pb (B); Cd3d spectrum of MPPs-Cd (C); C1s spectra of MPPs (D), MPPs-Pb (E) and MPPs-Cd (F).

33



Scheme 1.

Figure 1

34


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Figure 2

Figure 3

35



Figure 4

Figure 5

36


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Figure 6

37



Figure 7

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Table 1 Comparison of the maximum adsorption capacity of Pb2+ and Cd2+ on MPPs with those of other adsorbents m/V

C0

(g/L)

(mg/L)

Porous tablet ceramic

1

10-800

6.0

215.52(Pb2+)

3

Chitosan nanoparticle

1

40-140

5.0

94.34(Pb2+)

8

~

~

5.0

21.92(Pb2+)

15

Fe3O4/TiO2 /polypyrrole

0.45

0.2-40

6.0

126(Pb2+)

51

MPPs

0.15

30-300

5.5

257(Pb2+)

1,2,4-triazole-3-thiol modified lignin-based

1

25-300

6.0

87.4(Cd2+)

4

0.57

3.6-35.7

5.5

35.26(Cd2+)

12

L-arginine modified magnetic

0.5

~

6.0

120.2(Cd2+)

48

Porous attapulgite/polymer beads

10

100-500

5.2

32.7(Cd2+)

53

MPPs

0.15

30-300

5.5

129(Cd2+)

Adsorbents

PEG-functionalized bis-prolinium chloride bridged mesoporous organosilica

Silica particle modified by conjugated β-ketoenol furan adsorbent

39


pH

Cmax (mg/g)

Ref.

This study

This study


For Table of Contents Use Only

MPPs show fast and efficient adsorption of Pb2+ and Cd2+ from aqueous solutions.

40


Page 40 of 40

Magnetic Porous Polymers Prepared via High Internal Phase

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