Highly Selective Adsorption of Antimonite by Novel Imprinted Polymer

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

pubs.acs.org/jced

Highly Selective Adsorption of Antimonite by Novel Imprinted Polymer with Microdomain Confinement Effect Lili Fang,† Xiao Xiao,† Renfei Kang,† Zhong Ren,† Haiyan Yu,† Spyros G. Pavlostathis,‡ Jinming Luo,‡ and Xubiao Luo*,† †

Key Laboratory of Jiangxi Province for Persistent Pollutants Control and Resources Recycle, Nanchang Hangkong University, Nanchang 330063, P.R. China ‡ School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0512, United States S Supporting Information *

ABSTRACT: The design and synthesis of metalloid imprinted materials is a challenge due to lack of a feasible functional monomer. A novel cyclic functional monomer (CFM) was used to develop Sb(III)-ion imprinted polymer (CFM-IIP) for efficient and selective removal Sb(III). CFM possesses positively charged imidazolium moiety and specific cyclic size matching with Sb(III). CFM-IIP has a maximum Sb(III) adsorption capacity of 79.1 mg·g−1, while that of a noncyclic functional monomer imprinted polymer (NCFMIIP) is only 30.9 mg Sb(III)·g−1. The relative selectivity coefficients of CFM-IIP compared to NCFM-IIP for Sb(III)/Cl−, Sb(III)/NO3−, Sb(III)/PO43−, Sb(III)/SO42−, and Sb(III)/ Cr2O72− were 6.6, 78.4, 5.9, 3.0, and 3.2, respectively. Kinetic data fitted well with pseudo-second-order model. The adsorption between Sb(III) and CFM-IIP was identified to be feasible, spontaneous, and endothermic. ζ-Potential, X-ray photoelectron spectroscopic analysis, and density functional theory calculations revealed the mechanism of Sb(III) adsorption on CFM-IIP is as follows: the microdomain confinement effect, derived from the nanoscale imprinted cavities of CFM-IIP, facilitates the hydrolysis of Sb(OH)3 to SbO45−, which is subsequently sequestered within the imprinted cavities of CFM-IIP due to the strong electrostatic attraction and size matching of CFM-IIP to SbO45−. Therefore, CFM-IIP is very promising adsorbent for the efficient and selective removal of Sb(III) from aqueous solutions.

1. INTRODUCTION In recent decades, antimony (Sb) has been extensively used in chemical production and medicine fields such as in flame retardants, catalysts in the synthesis of plastics, and alloys for ammunition.1 About 187 000 tonnes of Sb, approximately 90% of the total world production, are produced per year in China, where many active Sb mines are located.2 Mining sites in China have been severely contaminated with Sb, and the reported concentration of dissolved Sb in water ranges from 4.6 to 29.4 mg·L−1.3 Sb is highly toxic, carcinogenic, and even lethal at a high concentration.4 Furthermore, the toxicity of Sb depends strongly on its oxidation state; for example, Sb(III) is 10 times more toxic than Sb(V).5 Given potential health risk and threat to human health, the United States Environmental Protection Agency (U.S. EPA) has classified Sb and its compounds as priority pollutants, establishing a maximum contaminant level for Sb in drinking water at 6 μg·L−1.6 Consequently, it is urgent to develop an effective method for the removal of Sb from drinking water. Conventional techniques for the removal of Sb from water, such as precipitation/coagulation,7 electrodeposition,8 and reverse osmosis,9 are costly and/or ineffective for removing Sb from very dilute solutions. Moreover, these methods usually lack selectivity and cannot recover the Sb, resulting in a © XXXX American Chemical Society

secondary pollution. Adsorption is the most available treatment technique owing to its simplicity, speed, safety, and effectiveness, especially for removing Sb from very dilute solutions. As a result, Sb removal by adsorption is both technically and economically attractive. At present, various adsorbents have been investigated for the removal of Sb(III), including Fe−Mn binary oxide,10 graphene,11 and multiwalled carbon nanotubes.12 Although several studies have indicated that the above adsorbents can remove Sb(III), these adsorbents are not selective. Therefore, it is imperative to develop an adsorbent which is highly selective and has a high capacity to completely remove Sb(III) from aqueous solutions. Imprinted polymers can selectively remove target contaminants and be used repeatedly after the template is removed. In general, special methods are used to prepare imprinted polymers. Typically, functional monomers and cross-linkers copolymerize in the presence of target molecules or ions as template, and subsequently, the template molecules or ions are extracted.13 Thus, imprinted polymers promise to be a new generation of novel adsorbents that can be reused. Although Received: December 11, 2017 Accepted: March 20, 2018

A

DOI: 10.1021/acs.jced.7b01074 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

2. MATERIALS AND METHODS Details on reagents (Text S1), the synthesis route and characterization of CFM (Figures S1−S7; Text S2), preparation of the ion imprinted polymers (CFM-IIP/NIP, NCFM-IIP/ NIP) (Text S3), and characterization of adsorbents (Text S4) are provided in the Supporting Information. 2.1. Batch Adsorption Experiments. Potassium antimony tartrate (0.548 g) was dissolved in 200 mL of 0.1 M HAc−NaAc buffer solution (pH 4.75 ± 0.2), which was then diluted to different concentrations. The buffer solutions of different pH values were prepared according to different ratios of 0.1 M HAc to 0.1 NaAc. A mass of 20 mg of adsorbent was equilibrated with 20 mL of antimonious aqueous solutions at different initial concentrations (20−350 mg·L−1) and pH (2− 6) in 100 mL Erlenmeyer flasks, shaking at 180 rpm and constant temperature 25 °C for 12 h. Kinetic experiments were carried out by mixing 250 mg of adsorbent with 250 mL of antimonious aqueous solutions at 200 mg Sb(III)·g−1 and stirring at 25 °C; aliquots of 0.5 mL of the solution were removed at different contact times, and the adsorbents were separated by syringe filtration. The filtrates were then used to determine the Sb(III) concentration by atomic absorption spectroscopy (AAS, Analytik Jena, Germany). Competitive adsorption experiments were performed for CFM-IIP, CFM-NIP, NCFM-IIP, and NCFM-IIP by preparing binary mixtures of Sb(III) and Cl−, PO43−, NO3−, SO42−, and Cr2O72− with each ion at an initial concentration of 300 mg· L−1. All anions were measured by ion chromatography (IC, Dionex, Beijing, China), except for Sb(III) and Cr2O72−, which were measured by AAS (Analytik Jena, Germany). Competitive sorption of Cu(II), Ni(II), Pb(II), and Cd(II) in the presence of Sb(III) was also investigated for CFM-IIP and NCFM-IIP. 2.2. Data Analysis. The adsorption capacity Qe (mg·g−1) at equilibrium and the adsorbed amount at time t (min) Qt (mg· g−1) were calculated as follows:

different kinds of imprinted polymer adsorbents have been synthesized and applied for the selective removal of heavy metal cations, such as Cu,14 Cd,15 Ni16 and Pb,17 there are few reports concerning metalloid (i.e., arsenic and antimony) imprinted adsorbents. Metalloids such as arsenic and antimony generally exist in natural waters as oxyanions (AsO2−)18 or neutral molecules (Sb(OH)3)19 rather than as metal cations. As the charge and ionic radius ratio of metalloids are 3−5 times smaller than those for metal cations,20 the electrostatic effect between metalloids and the functional monomer or ligand is significantly weaker than in the case of metal cations.13 As a result, it is difficult to prepare metalloid imprinted polymers using coordination/complexation routes due to the weaker valence bond strength between metalloids and ligand compared to metal cations and ligand. The appropriate choice of functional monomer or ligand is crucial for preparing high performance metalloid imprinted sorbents. Noncyclic functional monomers (NCFM) such as 1vinyl imidazole21 and 3-iodopropyltrimethoxysilane22 were used to synthesize As(V) and Sb(III) imprinted polymers, respectively. However, the adsorption capacities of these noncyclic functional monomer-ion imprinted polymers (NCFM-IIPs) were limited for practical applications. In our previous work, a macrocyclic effect was observed when cyclic functional monomers (CFM) such as 2-(allyloxy) methyl-12crown-423 and 4-vinylbenzo-18-crown-624 were used to develop Li(I) and Pb(II) ion-imprinted polymer, respectively. The macrocyclic effect refers to the ability of cyclic compounds to selectively recognize target molecules or ions due to the appropriate ring size matching with target molecules or ions. Thus, if the CFM possesses an appropriate ring size matching the size of the metalloids, CFM-based sorbents may have a better performance compared to that of NCFM-based sorbents. Yousuf et al. designed three different water-soluble imidazolium-based probes to selectively recognize biomolecular anions.25 The electrostatic effect between the positively charged moiety of the imidazolium and the biomolecular anion species is responsible for the selective recognition. Similar to the biomolecular anions, some metalloids are also present as anions. Accordingly, introducing a positively charged moiety to the CFM may also enhance the performance of CFM-based metalloid sorbent. To the best of our knowledge, there are no reports using the CFM with a positively charged moiety to prepare a metalloid imprinted sorbent. The objective of this work was to evaluate the macrocyclic and electrostatic dual effects on the performance of a metalloid imprinted sorbent. A novel CFM with a positively charged moiety, tetra-bromine-bi-4,5-2(methylene biimidazole) acridine, was synthesized and used to prepare Sb(III)-ion imprinted polymer (CFM-IIP). Subsequently, Fourier transform infrared spectroscopy (FT-IR), scanning electron microscope (SEM), thermogravimetric analysis (TGA), and N2 adsorption−desorption (BET) were used to characterize the CFM-IIP. Batch adsorption experiments were carried out to investigate the adsorption performance of the CFM-IIP. The mechanism of Sb(III) adsorption on CFM-IIP was probed by using ζ-potential, XPS analysis, and density functional theory (DFT) calculations. The adsorption capacity and selectivity of CFM-IIP for Sb(III) were greatly improved, thus offering the possibility to achieve resourceful treatment of Sb(III)-bearing waste streams.

Qe =

(C0 − Ce)v m

Qt =

(C0 − Ct )v m

(1)

(2) −1

where C0, Ce, and Ct are concentrations (mg·L ) of Sb(III) initially, at equilibrium, and time t, respectively. V is the solution volume (mL), and m is the adsorbent mass (mg). The distribution coefficient K D (L·mg −1), selectivity coefficient α, and the relative selectivity ratios β and γ are as follows: KD = α=

β= γ=

Qe Ce

(3)

KD(Sb(III)) KD(X)

(4)

αIIP αNIP

(5)

αCFM ‐ IIP αNCFM ‐ IIP

(6)

where Qe represents the adsorption capacity and Ce is the equilibrium concentration of Sb(III) and the competitive ions; KD(Sb(III)) and KD(X) represent the distribution coefficient of B



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Sb(III) and Cl−, PO43−, NO3−, SO42−, Cr2O72−, Cu(II), Ni(II), Pb(II), or Cd(II); αIIP and αNIP are the selectivity coefficients of CFM/NCFM-IIP and CFM/NCFM-NIP, respectively; αCFM‑IIP and αNCFM‑IIP are the selectivity coefficients of CFM-IIP and NCFM-IIP, respectively; β are the relative selectivity ratio of CFM-IIP/CFM-NIP and NCFM-IIP/NCFM-NIP; and γ is the relative selectivity ratio of CFM-IIP/NCFM-IIP. The equilibrium adsorption isotherm data and related isotherm parameters were estimated using both the Langmuir (eq 7) and Freundlich (eq 8) adsorption models: Langmuir model: Q e =

where R is the gas constant (8.314 J mol−1 K−1), T is the absolute temperature, and K is the thermodynamic equilibrium constant in the adsorption process, determined using the method of Khan and Singh35 by plotting ln(Qe/Ce) versus Qe and extrapolating to zero Qe. Meanwhile, ΔS0 and ΔH0 are the intercept and slope, respectively, of the straight line of the plot ln K versus 103/T.

3. RESULTS AND DISCUSSION 3.1. Polymer Characteristics. FT-IR spectra of CFM-NIP and CFM-IIP were examined to verify the successful polymerization of the cyclic functional monomer with the template ion and cross-linking agent. As shown in Figure 1(a), the observed features around 1792 and 1393 cm−1 indicate a CO adsorption band of saturated aliphatic esters and C−H bending vibration of hydrocarbons, respectively. These findings demonstrate the presence of the cross-linking agent EGDMA in the resulting polymer. Furthermore, two stretch vibration absorption spectra of C−N are shown at wavenumbers of 1253 and 1158 cm−1, while two characteristic peaks at 1458 and 759 cm−1 indicate the skeletal vibration of CC in aromatic hydrocarbon and out-of-plane bending of C−H on the benzene ring. These results accounted for the presence of the functional monomer tetra-bromine-bi-4,5-2(methylene bi-imidazole) acridine in polymer particles. CFM-IIP and CFM-NIP had almost the same location and appearance of the major bands because both of them had the same composition after the removal of the template ion from the imprinted sorbent. Based on the results of the thermogravimetric analysis, shown in Figure 1(b), the CFM-IIP and CFM-NIP exhibited a weight loss of 78.3 and 96.7% at approximately 230−430 and 220−450 °C, respectively, attributed to the thermal decomposition of the polymers. Moreover, CFM-IIP had a second thermal decomposition with a weight loss of 10.8% in the temperature range of 430 to 800 °C. It is noteworthy that the residual mass of CFM-IIP was higher than that of CFM-NIP. These results indicate that a small amount of template molecule in CFM-IIP was not removed completely. Besides, the decomposition temperature of IIP is also higher than that of NIP. The cavities of CFM-IIP may be the reason for its superior thermal stability based on the experimental data and analysis shown below. First, the BET results (Figures 1(c) and S8) showed the adsorption average pore width of CFM-IIP is 14.6 nm, while that of CFM-NIP is only 5.2 nm. Second, the size of cavities in these polymers is larger, which would have a poor thermal conductivity of the air in the cavities; thus, decomposition temperature of the polymers would be higher than that of a polymer with smaller size cavities. The BET surface area measurements were performed based on N2 adsorption−desorption isotherms at 77 K.36,37 The CFM-IIP and CFM-NIP BET surface areas are 43.3 and 44.6 m2·g−1, respectively. The N2 adsorption−desorption isotherms of the CFM-IIP sample match that of a typical mesoporous material. As shown in Figure 1(c), CFM-IIP shows welldeveloped Type H3 hysteresis loop. Isotherms with Type H3 loop, which do not level off at relative pressures close to the saturation vapor pressure, were reported for materials comprised of aggregates (loose assemblages) of platelike particles forming slitlike pores.38 Slitlike pores were also discovered by scanning electron microscopy (Figure 1(d)), corresponding to the results of the N2 adsorption−desorption isotherms. On the basis of the SEM images, the size of the

Q mKLCe 1 + KLCe

Freundlich model: Q e = KFCe1/ n

(7) (8)

where Qe is the amount (mg·g−1) of Sb adsorbed at equilibrium, and Ce is the equilibrium Sb concentration (mg· L−1) in solution. According to the Langmuir model in eq 7, Qm is the maximum adsorption capacity (mg·g−1) and KL (L·mg−1) is the adsorption equilibrium constant. The essential characteristics of the Langmuir isotherm can be expressed as the dimensionless constant RL: RL =

1 1 + KLC0

(9)

where RL is the equilibrium constant that indicates the type of adsorption. Co is the concentration of Sb(III) in the solution. The R L values between 0 and 1 indicate favorable adsorption.26−29 According to the Freundlich model in eq 8, KF is the adsorption equilibrium constant (mg(1−(1/n))·L(1/n)·g−1); n is the heterogeneity factor, which represents the bond distribution, and values of n in the range of 1 < n < 10 indicate favorable adsorption. The pseudo-first-order, pseudo-second-order,30 and Weber’s intraparticle diffusion rate equations,31−34 used to investigate the adsorption kinetics, are as follows: Q t = Q e(1 − e−k1t ) Qt =

(10)

Q e2k 2t 1 + Q ek 2t

(11)

h0 = k 2Q e2

(12)

Q t = kit 1/2 + c

(13)

where Qe and Qt are as defined above; t, time (min); k1 (min−1), k2 (g mg·min−1), and ki (mg·g−1 min0.5) are the rate constants for the pseudo first- and second-order sorption and Weber’s intraparticle diffusion model, respectively; and h0 (mg· g−1·min−1) is the initial adsorption rate. c gives the boundary layer thickness. The thermodynamic parameters, including standard free energy change ΔG0 (kJ mol−1), enthalpy change ΔH0 (kJ mol−1), and entropy change ΔS0 (kJ K−1 mol−1), can be calculated from the following equations: ΔG° = −RT ln K ln K = −

ΔH ° ΔS ° + RT R

(14)

(15) C



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Figure 2. Influence of soaking time on the leakage of organic compound in CFM-IIP.

The pH is known to not only determine the adsorbate Sb(III) speciation in the solution but also have an impact on the surface charge of sorbents. Sb(III) precipitates at a solution pH of 7 or higher in experiment. Therefore, the Sb(III) adsorption on CFM-IIPs and CFM-NIPs under the pH range of 2−6 was investigated, and results are shown in Figure 3. No

Figure 3. Effect of pH on Sb(III) binding [conditions: concentration of Sb(III) aqueous solution, 200 mg·L−1; 20 mg of polymers; contact time, 12 h; temperature, 25 °C].

matter under which tested pH value, the adsorbed quantity of Sb(III) on CFM-IIP was always above 39.4 mg·g−1 (pH 2). The maximum adsorption quantity was 59.7 mg·g−1, which was observed at pH of 4. In the pH range 2−10, Sb(III) exists in the form of the most common species, Sb(OH)3,41 which may explain the above-stated lack of pH influence on Sb(III) adsorption. However, the surface charge of the sorbent was greatly influenced by pH. ζ-Potential measurements (Figure 4) show that the untreated CFM-IIP surfaces have a net positive Figure 1. (a) FT-IR of CFM-IIP and CFM-NIP; (b) TGA curves of CFM-IIP and CFM-NIP; (c) N2 adsorption/desorption isotherms of CFM-IIP; (d) SEM images of CFM-IIP.

CFM-IIP particles was estimated to be between 200 and 800 nm. 3.2. Sb(III) Adsorption on Imprinted Polymers. The influence of soaking time on the leakage of organic compound39,40 in CFM-IIP is shown in Figure 2. Although the COD content of the solution increased rapidly with time, it reached a limit value (18.5 mg O2/L) on the third day. This indicates that only very little organic compound in CFM-IIP leaked out. Therefore, CFM-IIP has a good stability in acidic medium.

Figure 4. ζ-Potential of CFM-IIP and CFM-NIP before (CFM-IIPbad and CFM-NIPbad) and after (CFM-IIPaad and CFM-NIPaad) Sb(III) adsorption. D



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correlation coefficient (R2) values, the Langmuir model (R2 = 0.992) is more suitable than the Freundlich model (R2 = 0.962) to describe the adsorption process, implying monolayer adsorption. The dimensionless constant RL was calculated from eq 9. The RL values were found to be between 0 and 1 (0.992−0.999) for the concentration range (20−350 mg·L−1) studied, which indicates favorable adsorption.29 When CFMNIP and NCFM-NIP are compared, the maximum adsorption capacities are 19.3 and 15.0 mg·g−1, respectively. The presence of CFM did not significantly improve the NIPs’ efficiency. However, the maximum adsorption capacity of CFM-IIP (79.1 mg·g−1; Table 1) is nearly 2.5 times higher than that of NCFMIIP (30.9 mg·g−1). This difference may be attributed to the introduction of the cyclic functional monomer. On one hand, the CFM possesses an appropriate ring size matching the size of Sb(III) and contains multiple positively charged nitrogen atoms, which efficiently improve the bonding between CFM and Sb(III) during the polymer synthesis and thus facilitate the formation of imprinted cavities. On the other hand, the introduction of CFM significantly improves the electrostatic attraction of the polymer particles with Sb(III). Thus, the adsorption capacity of CFM-IIP is significantly enhanced based on the interaction during the synthesis and not just the presence of the cyclic functional monomer. Moreover, the maximum adsorption capacity of the CFM-IIP for Sb(III) was compared with other adsorbents reported in the literature. As shown in Table 2, the CFM-IIP has the highest maximum adsorption capacity among all adsorbents reported in the literature, which suggests that the CFM-IIP composite could be a good candidate for the removal of Sb(III) from water. 3.3. Adsorption Kinetics. To further investigate the adsorption kinetics, the pseudo-first-order and pseudosecond-order rate eqs (eqs 10 and 11) were used to describe the kinetics of Sb(III) adsorption on CFM-IIP/NIP and NCFM-IIP/NIP. All estimated parameter values are listed in Table 3. Comparatively, the pseudo-second-order model fitted the experimental data better than the pseudo-first-order model. The adsorbed amount of Sb(III) on CFM-IIP/NIP and NCFM-IIP/NIP over time and the pseudo-second-order model fit are presented in Figure 6. The adsorption kinetics were divided into two stages: a rapid initial stage in which adsorption contributed significantly to the equilibrium adsorption, and a slower second stage whose contribution to the total adsorption was relatively small.43 According to Figure 6, the amount of Sb(III) adsorbed on CFM-IIP increased most rapidly during the first stage (about 20 min) among the 4 adsorbents. Subsequently, the adsorption rate was much lower until about 80 min when the adsorption equilibrium was reached. The adsorption pattern could be further explained by the initial adsorption rate (h0) values presented in Table 3, in which CFM-IIP had the highest h0 value (5.99 mg·g−1·min−1).

charge at pH values lower than the point of zero charge (pHpzc), which is 6.0. This suggested that positive charge on CFM-IIP might relate to the Sb(III) adsorption, actively. It was also noticed that the pH did not have a significant effect on the adsorbed quantity of Sb(III) on CFM-NIP, which is attributed to the nonspecific adsorption of Sb(III) on CFM-NIP. The ζ-potential of CFM-IIPs and CFM-NIPs before and after adsorption was measured, and results are shown in Figure 4. Before adsorption, the two sorbents were electrically neutral near pH 6, i.e., they had the same isoelectric point of 6; after adsorption, the isoelectric point of both sorbents shifted to a lower pH of 4. These results suggest that adsorption of Sb(III) on CFM-IIP and CFM-NIP changed the charge of the sorbents. CFM contains multiple positively charged nitrogen atoms, which leads to a positively charged surface of CFM-IIP and CFM-NIP particles. Therefore, a fraction of the positive charge on the surface of the sorbent was neutralized by Sb(III) compound after the adsorption. This indicated that the positively charged nitrogen atoms of the cyclic functional monomer played a vital role on the adsorptive properties of the adsorbents. Adsorption equilibrium experiments were carried out to determine the adsorption capacity of CFM-IIP/NIP and NCFM-IIP/NIP for Sb(III) at an initial Sb concentration range of 20−350 mg·L−1. The Langmuir model was fitted to the experimental data, as shown in Figure 5. At equilibrium, the

Figure 5. Sb(III) adsorption isotherms to different adsorbents. Initial Sb(III) concentration range, 20−350 mg·L−1; adsorbent dose, 1 g·L−1; solution volume, 20 mL.

adsorbed Sb(III) quantity increased as the equilibrium aqueous Sb(III) concentration increased. Moreover, it can be distinctly seen that the adsorption capacity of CFM-IIP/NCFM-IIP is significantly higher than that of CFM-NIP/NCFM-NIP at similar equilibrium concentrations, suggesting that CFM-IIP and NCFM-IIP possess a remarkable imprinting effect.42 The estimated parameter values based on the Langmuir and Freundlich models are presented in Table 1. According to the

Table 1. Langmuir and Freundlich Isotherm Parameter Values for Sb(III) Adsorption on CFM-IIPs/NIPs and NCFM-IIPs/ NIPs Langmuir modela

a

adsorbent

Qm

CFM-IIP CFM-NIP NCFM-IIP NCFM-NIP

± ± ± ±

79.1 19.3 30.9 15.0

Freundlich modela

KL 8.2 3.0 9.3 2.8

b

(2.3 (3.1 (8.0 (3.7

± ± ± ±

3.3) 9.6) 7.7) 4.9)

R × × × ×

10−5 10−6 10−4 10−3

2

0.992 0.962 0.923 0.940

n 1.15 0.95 1.39 1.83

± ± ± ±

R2

KF 0.14 0.17 0.26 0.30

0.45 0.05 0.57 1.83

± ± ± ±

0.26 0.05 0.38 0.33

0.962 0.910 0.871 0.889

For notation and units, see Section 2.2. bMean ± standard error (n = 8). E



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Table 2. Adsorption Performance of the CFM-IIP and Other Recently-Developed Adsorbents

a

adsorbents

maximum adsorption capacity (mg·g−1)

selectivity

ref

graphene graphene oxide sodium montmorillonite MNP@hematite Sb(III)-imprinteda Sb(III)-imprintedb CFM-IIP

8.05 4.75 38.9 40 25 32.4 72.5

no no no no yes yes yes

11 45 46 47 48 22 this study

Chlorine-functionalized organic−inorganic hybrid sorbent. bIodole-functionalized organic−inorganic hybrid sorbent.

Table 3. Kinetic Constant Values for Sb(III) Adsorption on CFM-IIPs/NIPs and NCFM-IIPs/NIPs pseudo-first-ordera

a

adsorbent

k1

CFM-IIP CFM-NIP NCFM-IIP NCFM-NIP

± ± ± ±

0.072 0.038 0.083 0.094

R2

Qe 0.007 0.003 0.011 0.002

b

67.4 27.3 22.2 17.4

± ± ± ±

pseudo-second-ordera

1.7 0.7 0.7 0.1

k2

0.987 0.994 0.958 0.998

0.0010 0.0012 0.0041 0.0059

± ± ± ±

1.92 1.66 8.23 6.32

Qe × × × ×

−4

10 10−4 10−4 10−4

77.4 33.3 24.3 19.5

± ± ± ±

2.8 1.2 0.7 0.4

h0

R2

5.99 1.33 2.42 2.24

0.987 0.995 0.980 0.990

For notation and units, see Section 2.2. bMean ± standard error (n = 10).

temperature. Figure 7 depicts Sb(III) adsorption on CFM-IIP at different temperatures. The fitting plots of ln K vs 103/T with

Figure 6. Kinetics of Sb(III) adsorption on CFM-IIPs/NIPs and NCFM-IIPs/NIPs Figure 7. Adsorption isotherms for CFM-IIP adsorption at different temperatures.

The high initial adsorption rate is attributed to a higher number of adsorption sites, higher affinity, and geometric matching between the target molecules and the imprinted cavities and rapid electrostatic interaction between CFM-IIP and Sb(III) compound. These results indicate that the mechanism represented by the pseudo-second-order model is predominant.30 The adsorption capacity of an adsorbent is proportional to the number of active sites on its surface, and chemisorption may be the rate-limiting step controlling the overall adsorption process. The kinetic results were also analyzed using the intraparticle diffusion model (eq 13). Figure S9 is the fitting plots of intraparticle diffusion model for Sb(III) adsorption on CFMIIPs/NIPs and NCFM-IIPs/NIPs. The fitting parameters are listed in Table S1. The value of c reflects the boundary layer effect; the larger the c value, the greater the contribution of surface adsorption in the rate controlling step. If the regression of Qt vs t1/2 is linear and passes through the origin, then intraparticle diffusion is the sole rate-limiting step.33 It can be clearly observed that the regression of Qt vs t1/2 is not linear and does not pass through the origin, so intraparticle diffusion is not the sole rate-limiting step. 3.4. Adsorption Thermodynamics. Batch adsorption experiments of Sb(III) on CFM-IIP were performed at different temperatures (298, 308, and 318 K) to evaluate the effect of

the CFM-IIP are presented in Figure S10. Thermodynamic parameters such as free energy change (ΔG0), enthalpy change (ΔH0), and entropy change (ΔS0) were calculated according to eqs 14 and 15. The thermodynamic parameters for Sb(III) adsorption on CFM-IIP are given in Table 4. For Sb(III) adsorption on CFM-IIP, the positive ΔH0 (31.5 kJ·mol−1) shows that the adsorption of Sb(III) onto CFM-IIP is an endothermic reaction at 298, 308, and 318 K. The positive value (0.15 kJ·mol−1·K−1) of entropy change (ΔS0) shows the increased randomness of the solution interface during the adsorption of Sb(III) on CFM-IIP. ΔG0 values were also Table 4. Thermodynamic Parameters for Sb(III) Adsorption on the CFM-IIP at Different Temperatures temperature (K) 298 308 318 ΔH0 (kJ·mol−1) ΔS0 (kJ·mol−1·K−1) F

ln K

ΔG0 (kJ·mol−1) −12.88 −14.34 −15.86

5.2 5.6 6.0 ΔH0 and ΔS0 31.5 0.15



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Table 5. Distribution Coefficient, Selectivity Coefficient, and Relative Selectivity Ratio Values of CFM-IIP/NIP and NCFM-IIP/ NIP distribution coefficient anion (X) Cl−

NO3−

PO43−

SO42−

CrO72−

relative selectivity ratios

adsorbent

KD (Sb, mL·g−1)

KD (X, mL·g−1)

selectivity coefficient α

Β (IIP/NIP)

γ (CFM-IIP/NCFM-IIP)

CFM-IIP CFM-NIP NCFM-IIP NCFM-NIP CFM-IIP CFM-NIP NCFM-IIP NCFM-NIP CFM-IIP CFM-NIP NCFM-IIP NCFM-NIP CFM-IIP CFM-NIP NCFM-IIP NCFM-NIP CFM-IIP CFM-NIP NCFM-IIP NCFM-NIP

258.3 91.0 68.8 37.6 266.9 91.2 20.8 19.3 74.7 69.5 86.8 22.3 216.6 70.7 72.1 29.0 168.3 79.8 94.1 29.2

88.8 172.3 154.7 44.4 20.0 27.0 122.2 152.6 9.2 36.7 63.7 52.5 7.4 36.4 74.5 23.0 4.3 30.3 77.6 243.8

2.91 0.53 0.44 0.85 13.32 3.38 0.17 0.13 8.08 1.89 1.36 0.42 2.92 1.94 0.97 1.26 3.92 2.63 1.21 1.48

5.49

6.6

calculated to be −12.88, −14.34, and −15.86 kJ·mol−1 at 298, 308, and 318 K, respectively. The negative values show the spontaneous adsorption of Sb(III) adsorption on CFM-IIP. 3.5. Adsorption Selectivity. The selectivity of CFM-IIP and NCFM-IIP toward Sb(III) was investigated by competitive adsorption of Sb(III)/Cl−, Sb(III)/PO43−, Sb(III)/NO3−, Sb(III)/SO42−, and Sb(III)/Cr2O72− in their binary mixtures. It should be noted that, with the exception of Cl−, the selected competitor ions are oxyanions. The common species of Sb(III) is Sb(OH)3 (or SbO(OH)), sometimes called meta-antimonious acid (HSbO2). Cl− is the most common anion in natural waters. Figure S11 depicts the adsorption of competitor ions PO43−, NO3−, SO42−, Cr2O72−, Cl−, and Sb(III) by CFM-IIP/ NIP and NCFM-IIP/NIP. CFM-IIP exhibits a much higher adsorption capacity for Sb(III) than for the other competitor ions, whereas CFM-NIP and NCFM-IIP/NIP did not exhibit a preferential adsorption for Sb(III). The values of the distribution and selectivity coefficients as well as the relative selectivity ratios are summarized in Table 5. The selectivity coefficients (α) of CFM-IIP for Sb(III) with respect to PO43−, NO3−, SO42−, Cr2O72−, and Cl− are all greater than 2.91, while the relative selectivity ratios of CFM-IIP/ CFM-NIP (β) and CFM-IIP/NCFM-IIP (γ) are more than 1.49 and 3.0, respectively. However, most of the relative selectivity ratios (β) of NCFM-IIP/NCFM-NIP are less than 1. These results illustrate that the imprinted cavities and specific binding sites were efficiently created after the removal of the template in the case of CFM-IIP. They were complementary to the template in terms of size, shape, and coordination geometries, which further validates that the CFM played an important role in the formation of specific imprinted cavities. Therefore, CFM-IIP particles can be used to selectively remove Sb(III) from solutions containing other oxyanions. The influence of different metal cations was also investigated to illustrate the specificity of CFM-IIP. Common heavy metal cations in water such as Cu(II), Ni(II), Pb(II), and Cd(II) were chosen as the competitor ions. Figure S12 shows the uptake of

0.52 3.94

78.4

1.35 4.27

5.9

3.21 1.50

3.0

0.77 1.49

3.2

0.82

the selected cations and Sb(III) by CFM-IIP and NCFM-IIP. CFM-IIP exhibited a much higher adsorption capacity for Sb(III) than for other cations. The values of the distribution and selectivity coefficients as well as the relative selectivity ratios are summarized in Table 6. The selectivity coefficients of Table 6. Distribution Coefficient, Selectivity Coefficient, and Relative Selectivity Ratio Values of CFM-IIP and NCFM-IIP CFM-IIP cation

KD (mL·g−1)

Sb(III) Cu(II) Ni(II) Pb(II) Cd(II)

533 114 70 97 96

NCFM-NIP α 4.68 7.61 5.49 5.55

relative selectivity ratio γ

KD (mL·g−1)

α

9.18 10.4 9.8 8.81

19 37 26 34 30

0.51 0.73 0.56 0.63

CFM-IIP for Sb(III) with respect to Cu(II), Ni(II), Pb(II), and Cd(II) are all greater than 4.68, while the relative selectivity ratios of CFM-IIP/NCFM-IIP are all greater than 8.81. These results demonstrate that CFM-IIP exhibited a selectivity for Sb(III) significantly higher than that for the metal cations tested. 3.6. Mechanism. The chemical composition of CFM-IIP and CFM-NIP before as well as after Sb adsorption was measured by XPS. As shown in Figures 8(a) and (e), the peaks corresponding to N 1s, O 1s, C 1s, and Sb 3d are clearly identified in the scan spectra. The peak of N 1s indicated that CFM was successfully grafted on the polymers, which agrees with the results of the FT-IR analysis (Figure 1(b)). In addition, the presence of Sb in the resultant polymer further confirmed the adsorption of Sb onto CFM-IIP and CFM-NIP. Two peaks at binding energy values of 530.5 and 539.8 eV shown in Figures 8(b) and (f) are attributed to Sb 3d5/2 and Sb 3d3/2, which indicated the adsorption of Sb(III). Moreover, the relative peak intensity of Sb adsorbed on CFM-IIP was G



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Figure 8. XPS characteristics of CFM-IIP and CFM-NIP before and after Sb(III) adsorption. (a) XPS spectra of CFM-IIP along with the spectra of (b) O 1s, (c) N 1s, (d) C 1s; and (e) XPS spectra of CFM-NIP and (f) O 1s, (g) N 1s, and (h) C 1s.

much stronger than in the case of CFM-NIP, which confirmed that the adsorption capacity of CFM-IIP was higher than that of CFM-NIP for the removal of Sb(III). As displayed in Figures 8(c) and (g), the N 1s peak can be decomposed into two peaks. The peak at approximately 399.3 eV corresponds to the protonated N in the imidazole moiety (Nmz), and the peak at

approximately 401.3 eV corresponds to N in the acridine moiety (Nad). In addition, the binding energy of Nmz is shifted to higher values after the adsorption (Figure 8(c)). This finding indicates that the protonated N in the imidazole (Nmz) plays an indispensable role in Sb adsorption. The peak at a binding H



Article

Sb(OH)3 + H 2O ⇌ SbO54‐ + 5H+

energy of 284.9 eV is assigned to C 1s (Figures 8(d) and (h)); no shift of C 1s occurred after Sb adsorption. DFT calculations are used to ascertain Sb(III) adsorption mechanism of CFM-IIP. It has been illustrated that the protonated N in the imidazole of CFM plays an indispensable role in Sb adsorption according to ζ-potential and XPS analysis. Therefore, four different Sb(III) species (HSbO32−, Sb(OH)3, SbO45−, and Sb(OH)4−) were hypothesized to evaluate their possibilities of coordination with the protonated N in the imidazole moiety of CFM (Figures 9 and S13). The adsorption

(16)

This assumption was attested by an interesting and abnormal observation. When the CFM-IIP was added to the water (180 rpm, 25 °C), the pH of the solution increased from 5.69 to 6.68 after 12 h of incubation. Moreover, the pH of the solution mixture of CFM-IIP and Sb(III) increased by about 0.56−0.86 units after 12 h of adsorption. According to the above-discussed results, the process for Sb(III) chemical adsorption on CFMIIP might be (a) Sb(OH)3 enters the nanoscale imprinted cavities of CFM-IIP; (b) Sb(OH)3 is hydrolyzed to SbO45− (eq 16) when it meets the released hydroxyl radicals of CFM-IIP at the specific microdomain confinement condition, and (c) the negatively charged O of SbO45− is rapidly captured by the positively charged N of CFM-IIP.

4. CONCLUSIONS Designing a functional monomer with strong affinity is a critical factor for preparing metalloid/anion imprinted polymers. In this work, a positively charged imidazolium-based CFM with strong affinity to metalloid/anion was used to synthesize a novel imprinted polymer (CFM-IIP) for the efficient and selective removal of Sb(III) from aqueous solutions. Batch adsorption experiments demonstrated the superior performance of CFM-IIP compared to NCFM-IIP for the selective adsorption of Sb(III). DFT calculations, in combination with ζpotential and XPS analysis, revealed that the microdomain confinement effect plays a crucial role in the process of Sb(III) adsorption. This effect derives from the nanoscale imprinted cavities of CFM-IIP and could provide (a) a local alkaline condition for the hydrolysis of Sb(OH)3 to SbO45−; (b) a strong electrostatic interaction between the positively charged N of CFM-IIP and the negatively charged O of SbO45−; and (c) size-matching imprinted cavities for further sequestering the adsorbed SbO45−. Therefore, CFM-IIP is very promising for the efficient and selective removal of Sb(III) from aqueous solutions. Assessment of the positively charged imidazoliumbased cyclic functional monomer for the adsorption of other metalloids/anions as well as their recovery from dilute aqueous solutions will further expand the potential application of CFMIIP developed in this work.

Figure 9. Two specific coordination modes of Sb(III) and CFM in the Sb(III) adsorption process (a is major, b is secondary) and their adsorption energy (Ead). (Sb atoms are purple, O atoms are red, N atoms are blue, and H atoms are white.)

energies (Ead) of four possible modes were calculated by the GGA-BLYP method.44 The positive Ead means this coordination mode cannot occur due to the high energy requirement. As a result, the two coordination modes, with Ead equal to 0.4 eV (HSbO 32−) and 2.01 eV (Sb(OH)3), respectively, are impossible (Figure S13). Figures 9(a) and (b) show the specific coordination modes with Ead equal to −7.39 eV (SbO45−) and −0.19 eV (Sb(OH)4−). These results suggest that Sb(III) might coordinate with CFM by negatively charged O bonding with positively charged N (Figure 9(a)) or N−H hydrogen-bond interaction (Figure 9(b)). In addition, when Ead is negative, the lower value of Ead is the more likely to occur, i.e., the positively charged N of CFM-IIP interacts with the negatively charged O of SbO45−. However, it seems impossible for Sb(III) to exist in the form of SbO45− in an acidic circumstance because Sb(III) usually exists in the form of Sb(OH)3 at pH of 2−10, and it cannot be hydrolyzed to SbO45− under acidic conditions. This conflict can be explained by considering the microdomain confinement effect. This effect refers to the specific microdomain confinement condition, which is provided by the nanoscale imprinted cavities of CFMIIP that facilitate hydrolysis of Sb(OH)3 to SbO45− at the local alkaline condition in an acidic bulk solution. It is assumed that if the 4 hydroxyl radicals of CFM-IIP, which result from the previous replacement of the Sb(III) template after alkaline elution, could be easily released into the solution, the reaction for the hydrolysis of Sb(OH)3 to SbO45− might be driven (eq 16).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.7b01074. Reagents list, details on preparations and syntheses, characterization of adsorbents; 13C NMR spectra, 1H NMR spectra, N2 adsorption/desorption isotherms of CFM-NIP, fitting plots, selective adsorption data, hypothesized coordination modes, and table of kinetic parameters of Weber’s intraparticle diffusion model for Sb(III) adsorption on CFM-IIPs/NIPs and NCFM-IIPs/ NIPs (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86 73183953371; Fax: +86 73183953373; E-mail: [email protected]. ORCID

Spyros G. Pavlostathis: 0000-0001-9731-3836 Xubiao Luo: 0000-0002-3935-1268 I



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This study was financially supported by the National Science Foundation of China (Grants 51238002 and 51678285), the National Science Fund for Excellent Young Scholars (Grant 51422807), the Key Project of Science and Technology Department of Jiangxi Province (Grant 20143ACG70006), and the Cultivating Project for Academic and Technical Leader of Key Discipline of Jiangxi Province (Grant 20153BCB22005). Notes

The authors declare no competing financial interest.

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Highly Selective Adsorption of Antimonite by Novel Imprinted Polymer

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