1.Introduction

With the development of industry,heavy metal pollution caused serious environmental problem and ecological damage.Compared with organic pollutants,heavy metals could not be degraded by microbial and existed for a long time in the environment.Among all heavy metals,cadmium is a kind of toxic heavy metal and can pose a threat to plants,animals and human safety when the concentration of Cd(II)is high in the environment[1].The methods for treating wastewater containing cadmium include the precipitation method[2],adsorption[3],the ionexchange method[4]and membrane separation[5].Among these methods,adsorption has long been concerned because of its low cost and good removal effect.However,commonly used adsorbent has poor selectively for cadmium ions.Currently,ion imprinted polymer with high selectively and good adsorption capacity has greatly drawn people's attention[6–9].

Ion imprinted technology is developed based on molecular imprinted technology.Ion imprinted polymer can recognize target ion and retain the advantages of molecular imprinted polymer[6].Ion imprinted polymer has served as selective adsorbent for removing target ions[7].A variety of methods have been applied to synthesize Cd(II)imprinted polymer for removing Cd(II)in aqueous sample[8,9].Xi et al.[10]utilized precipitation polymerization method to synthesize cadmium imprinted polymers for removal of Cd(II)from aqueous solutions and environmental samples.Adsorption experiments suggested that the maximum adsorption capacity that could be achieved is46.5 mg·g−1.Li et al.[11]prepared a surface-grafted Cd(II)imprinted polymer using surface imprinting technology.The Cd(II)imprinted polymer had good thermal stability and selectivity and the maximum adsorption amount was 38.3 mg·g−1.Alizadeh[12]applied the copolymerization method to synthesize Cd(II)imprinted polymer and the maximum adsorption capacity for Cd(II)in water samples was 45 mg·g−1.Li et al.[13]employed a one-step hydrothermal process to prepare Cd(II)imprinted particles.The particles had good recognition performance and the adsorption capacity could reach 40 mg·g−1.

Among all of the synthesis methods of molecular or ion imprinted polymer,the emulsion polymerization method including emulsion,inverse emulsion and inverse microemulsion polymerization could be used to prepare molecularly or ion imprinted polymer[14,15].The inverse emulsion polymerization method is able to speed up the polymerization rate and the product has good performance[16].Luo et al.[17]used crown ether as functional monomer to synthesis novel Pb(II)ion imprinted polymer(IIP)by inverse emulsion polymerization.Compared with the non-imprinted polymer,IIP had high adsorption capacity(27.95 mg·g−1)and selectivity coefficients.Younis et al.[18]prepared ion imprinted polymer for removing Cu(II)by O/W/O emulsion polymerization.The superior adsorption capacity could reach 84 mg·g−1.The selectivity for Cu(II)is four times than that of other bivalent ions.

1.2.4 统计学分析 采用SPSS18.0软件进行数据处理与分析，计量资料用M（Q1，Q3）表示，组间比较采用Mann-Whitney U检验，P＜0.05表示有统计学意义，检验水准á=0.002 4。11项临床技能培训项目考核成绩的分布情况用箱图表示。

In this work,inverse emulsion polymerization wasemployed to prepare inverse emulsion Cd(II)imprinted polymers(IEIIP),with Cd(II)as template,acrylamide and β-cyclodextrin as functional monomers,epichlorohydrin as cross linking agent,ammonium persulfate as initiator and liquid paraf fin as oil phase.SEM,FTIR and TG were used for characterization of IEIIP.The adsorption properties of IEIIP were investigated under different adsorption conditions.The selectivity of polymers was studied in binary solutions.The adsorption property of IEIIP was evaluated by kinetics and thermodynamics.

where,R(8.314 J·mol−1·K−1)and K D represent the universal gas constant and adsorption equilibrium constant,respectively.Changes of ΔH0 and ΔS0 are calculated based on the slope and interceptby 1/T versus ln K D,respectively.E a and A0 are apparent activation energy(kJ·mol−1)and Gibbs free energy factors,respectively.Results of thermodynamic parameters for adsorption of Cd(II)are presented in Table 3.The values of ln K D increase first and then decrease with the temperature rising from293 Kto 313 K.The results show that303 K is most beneficial to the adsorption process.Under different temperatures,the values ofΔG0 are negative,suggesting the spontaneity of the adsorption process.The standard enthalpyΔH0 is positive,indicating the endothermic reaction of IEIIP for Cd(II).The changes of entropy ΔS0 are positive,suggesting that the adsorption process for Cd(II)in aqueous solution is an entropy increase process.An entropy increase process could cause the increase of disorder in an adsorption system,re flecting that the interaction between IEIIP and Cd(II)produces the structural changes.The reaction activation energy E a is 37.44 kJ·mol−1,showing the adsorption process could be performed easily.

2.Experimental

2.1.Materials and instruments

Acrylamide(AM), β-cyclodextrin(β-CD),Span 80,Tween 20,CdCl2·2.5H2O,epichlorohydrin(ECH),liquid paraf fin,ammonium persulfate,CuCl2·2H2O,Zn(NO3)2·6H2O,acetic acid,methyl alcohol and ethyl alcohol were supplied by Sinopharm Chemical Reagent Co.,Ltd.,China.Lead nitrate was purchased from Tianjin Damao Chemical Reagent Industry,China.NaOH and HNO3 for pH adjustment were supplied by Sinopharm Chemical Reagent Co.and Tian Jin Chemical Co.,Ltd.,China,respectively.

The static adsorption experiments were conducted in a SHAC constant-temperature shaker,which was purchased from Changzhou Guohua Instrument Co.,Ltd.,China.ATAS-990 flame atomic adsorption spectrometer(FAAS,Beijing Purkinje General Instrument Co.,Ltd.,China)was used to measure the concentrations of Cd(II),Zn(II),Cu(II)and Pb(II).A JSM-6700F Field Emission Scanning electron microscope(SEM)(Japan Electron Optics Laboratory Co.,Ltd.,Japan)was used to analyze the surface morphology of IEIIP.The Fourier transmission infrared spectra(FTIR,450 cm−1–4500 cm−1)with KBr pellets by a Bruker Tensor 27 FTIR spectrometer(Bruker,Germany)were used to analyze various functional groups of IEIIP.The TG 209 F3 thermogravimetric analyzer was applied to analyze the thermal stability of IEIIP as the temperature changes from 30 °C to 850 °C with the heating rate of 10 °C·min−1 under argon atmosphere.

2.2.Synthesis of IEIIP

IEIIP was synthesized by inverse emulsion polymerization.4 g β-cyclodextrin was dissolved in sodium hydroxide aqueous solution and 0.75 g CdCl2·2.5H2O was dissolved in distilled water.Two kinds of solution were mixed and stirred slowly for 30 min at 303 K.Subsequently,3.11 g AM was added into the solution and continue to stir for 30 min to mix uniformity.Then,a mixed emulsifier(Span 80 and Tween 20)solved in the liquid paraf fin was mixed with the above mixture and rapidly stirred for 30 min to form an inverse emulsion.After rising to 60°C,3.4 mlECH and 0.006 g ammonium persulfate were added to the inverse elution and stirred for 3 h.After the end of the reaction,the product was demulsified by ethanol.Cd(II)imprinted polymer was eluted with methanol and acetic acid(Vol:Vol=9:1)and distilled water to get IEIIP.Finally,IEIIP was dried in vacuum at 70°C for 24 h.

2.3.Adsorption experiments

Fig.1 presents the schematic of reaction for crosslinked polymer formation.Oil phase and water phase formed an inverse emulsion through an emulsifier.Then,β-cyclodextrin and AM as functional monomer reacted with Cd(II)to form an imprinted polymer under crosslinking ECH and initiator ammonium per sulfates.The chemical structure of the resulted polymer was showed in Fig.1.

where,C o,Ct and C e(mg·L−1)are the initial concentration of Cd(II),concentration of Cd(II)at t min and the equilibrium Cd(II)concentration,respectively.

The selective property of IEIIP was verified by competition adsorption experiments.Zn(II),Cu(II)and Pb(II)that have similar ionic radius and same charge with Cd(II)were used as competing ions.The study of selectivity was carried outin binary solutions ofCd2+/Pb2+,Cd2+/Cu2+and Cd2+/Zn2+under 303 K for 80 min.The concentrations of Cd(II)and other metal ions were measured using FAAS after adsorption reached equilibrium.The distribution coefficient k d and selective coefficient k were obtained according to the following formulas:

3.Results and Discussion

3.1.Synthesis and characterization

Fig.1.The schematics of reaction process.

Fig.2.SEM images of Cd(II)imprinted polymers.

The adsorption ability of IEIIP was studied by static adsorption experiments.0.01 g IEIIP was put into 50 ml Cd(II)solutions.The adsorption time was 0–80 min.The concentration ofCd(II)was 0–150 mg·L−1.The pH value of Cd(II)aqueous solution was adjusted to 5,6 and 7 by NaOH and HCl dilute solution.The reaction temperature was 293 K,303 K and 313 K.The adsorption experiments were conducted in the constant temperature oscillator under different conditions.IEIIP was filtered and the residual concentration was determined by FAAS.The adsorption capacity(mg·g−1)was calculated by the following equation:

综上所述，依达拉奉联合瑞舒伐他汀治疗急性脑出血的临床效果显著，可有效降低β-内啡肽、促性腺激素释放激素、N端脑钠肽前体、和肽素的表达水平，缓解脑水肿，减轻神经缺损程度，改善日常生活能力，提高预后。

The difficulty degree and spontaneity of the adsorption process could be analyzed by thermodynamic behavior.Thermodynamic parameters including changes of standard free energy ΔG0,changes of enthalpy Δ H0 and the changes of entropy Δ S0 could be calculated by Eqs.(7)and(8).The Arrhenius Equation(Eq.(9))is used to calculate the reaction activation energy E a.

The existence of functional groups in IEIIP could be confirmed by FTIR spectra.Fig.3 showed the FTIR spectra ofIEIIP before and after elution.The absorption peaks of IEIIP before and after elution have the same position.The N--H stretching vibration in functional monomer acrylamide and stretching vibration of rest O--H after the crosslinking reaction between β-cyclodextrin and epichlorohydrin are shown at 3409 cm−1–3330 cm−1.The anti-symmetric stretching vibration of--CH2--is re flected by the peak of 2923 cm−1.The absorption peak at 1641 cm−1 is C=O stretching vibration and N--H bending vibration in acrylamide.The peak at 1029 cm−1 indicates the presence of Cc--O group on the IEIIP.Results showed that IEIIP have synthesized successfully using the monomers of β-cyclodextrin and acrylamide.

Fig.3.FTIR spectra of Cd(II)imprinted polymers:(a)before and(b)after elution.

Fig.4 presented the thermal mass loss of IEIIP before and after elution.In the first stage,the mass loss of IEIIP before and after elution changed from 100%to 35.67%and 100%to 34.15%at 30 °C to 400 °C.The IEIIP decomposed at this stage.At 850°C,the mass of IEIIP before elution was 22.54%,while the mass of IEIIP after elution was only 21.34%.This is because the IEIIP before elution contains cadmium ion.Cadmium is a kind of heavy metal and does not decompose with increasing temperature.Therefore,the mass of IEIIP before elution is heavier than that of IEIIP after elution at 850°C.The results of the TGA thermogram showed that cadmium was successfully imprinted and eluted.

Fig.4.TG curves:(a)IEIIP before elution;(b)IEIIP after elution.

3.2.The in fluence of pH values on the adsorption process

Fig.5.Effect of pH value on the adsorption capacity.

The adsorption capacity of IEIIP is significantly affected by the pH value of the aqueous solution.Results in Fig.5 present the influence of pH values on the adsorption process of IEIIP.As shown in Fig.5,the adsorption capacity for IEIIP is increased with the rise of pH values of Cd(II)aqueous solution.The adsorption capacity is 53 mg·g−1 at pH=5.With pH=7,the adsorption capacity could reach 86.7 mg·g−1.The pHvalues of Cd(II)aqueous solution have a significant effect on the adsorption capacity of Cd(II).In acidic conditions,the hydroxyl groups in the functional monomer β-cyclodextrin could be protonated with H+,resulting in electrostatic repulsion with Cd(II)in aqueous solutions.Moreover,Cd(II)and H+could produce competitive adsorption on the surface of IEIIP.Therefore,metal ion is noteasy to beadsorbed on the surface of polymer materials with positive charge,leading to the decrease of adsorption capacity of IEIIP.With the increase of pH,the hydroxyl protonation gradually weakens and the negative charge on the surface of IEIIP increases.The competitive adsorption of Cd(II)with H+is also weakened.The adsorption capacity of IEIIP increases with the increase of pH value.However,Cd(II)and OH−are easy to form hydroxide under alkaline conditions,leading to the reduction of concentration of Cd(II)in Cd(II)aqueous solutions and the adsorption capacity of IEIIP begin to decrease.

3.3.The influence of initial concentration on the adsorption process

As shown in Fig.6,the initial concentration of Cd(II)greatly affects the uptake of IEIIP for Cd(II)in aqueous solution.Fig.6 presented that the maximum adsorption capacity of IEIIP in aqueous solution increased quickly at the low initial concentration.With the increase of the initial concentration of Cd(II),the adsorption capacity of IEIIP reached maximum adsorption capacity at the concentration of Cd(II)120 mg·L−1.Under high initial concentration,more and more Cd(II)gathered on the surface of IEIIP easily,causing the increase of metal ion uptake.When IEIIP reached adsorption saturation,the adsorption capacity kept equilibrium with increasing of initial concentration of Cd(II)in aqueous solutions.

Fig.6.Effect of the concentration of Cd(II)on adsorption capacity.

3.4.The influence of adsorption time and temperature on the adsorption process

Adsorption time and temperature could influence the adsorption behavior of IEIIP.Fig.7 presents the effects of adsorption time and temperature on the adsorption capacity of IEIIP for Cd(II)in the aqueous solutions.In the initial stage of 30 min,adsorption rate is fast.The adsorption reaches equilibrium when the adsorption time is 80 min.Cd(II)in the aqueous solution adsorbed on imprinted sites of IEIIP in the initial stage.As the reaction time goes on,the adsorption process reaches adsorption equilibrium.Fig.7 shows that the equilibrium adsorption capacity values are 69.8,86.7 and 51 mg·g−1 when the temperatures are 293 K,303 K and 313 K,respectively.Results suggest that the adsorption capacity for Cd(II)in aqueous solution is increased and then decreased with the increase of temperature.The adsorption capacity achieves maximum value under a temperature of 303 K.High or low temperature go against adsorption process.At low temperature,the mobility of ions is low,which lowers the ion's ability to reach the cavity.At higher temperature,due to high mobility,the stability of template/functional group interaction is lower,making the adsorption lower.Namely,303 K is the optimum temperature for adsorption conditions.

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Fig.7.Effect of time and temperature on the adsorption capacity.

3.5.Adsorption kinetics

The mechanism of adsorption is investigated by adsorption kinetic models.Pseudo first and second order kinetic models are used to evaluate the whole adsorption behavior of IEIIP.The regression coefficients and corresponding rate constants are obtained.The pseudo first order kinetic model and pseudo second order model are expressed as follows:

1）提升污水外调泵的机组运行效率。2台机泵平均机组运行效率提升了5.38%，平均机组单耗下降了0.02 kWh/m3，组机运行效率得到明显提升。

Fig.8.Pseudo first and pseudo second order kinetic models.

Inverse emulsion polymerization was employed to synthesize inverse emulsion Cd(II)imprinted polymers(IEII P).The morphology and functional groups of IEIIP were characterized by SEM,FTIR and TG.Static adsorption experiments and competitive adsorption test were used to evaluate the adsorption ability of IEIIP.The adsorption capacity of polymers could reach 86.7 mg·g−1 under the optimal adsorption conditions.The pseudo second order kinetic model and Langmuir isotherm model could be used to analyze the experimental data well.The adsorption process of IEIIP was chemical adsorption process and mono molecular type.Thermodynamic parameters showed that the adsorption process was endothermic and could occur spontaneously.The selectivity coefficients k of Cd2+/Pb2+,Cd2+/Zn2+and Cd2+/Cu2+were 2.4998,1.2437 and 4.6882,respectively.The proposed method provides a new thought for removing Cd(II)in water samples.

Table 1 Fitting results of Cd(II)adsorption kinetic equations

Temperature/K Pseudo- first-order kineticequation Pseudo-second-order kineticequation q cal/mg·g−1 k1/min−1 R2 q cal/mg·g−1 k2/min−1 R2 293 62.17 0.0690 0.9856 76.29 0.0021 0.9930 303 75.19 0.0646 0.9801 100 0.0016 0.9910 313 42.95 0.0649 0.9818 55.55 0.0033 0.9941

3.6.Adsorption isotherm

The Langmuir isotherm and Freundlich isotherm models are used to analyze the adsorption property.The relationship between the concentration and adsorption capacity at equilibrium moment could be analyzed by isotherm models.The models are described as follows:

where,C e is the equilibrium concentration of Cd(II),(mg·L−1);q e represents the adsorption capacity of IEIIP at equilibrium moment,(mg·g−1);q m is the maximum adsorption capacity,(mg·g−1);k and k F are the Langmuir and Freundlich constants,respectively;1/n is the adsorption index.The values of k and 1/n could be obtained by the slope and intercept of linear equations,respectively.The Langmuir and Freundlich isotherms are applied to describe the homogeneous and heterogeneous adsorption,respectively.The linear plot and isotherm constants of the Langmuir and Freundlich isotherms are showed in Fig.9 and Table 2.The R2 and k values of the Langmuir isotherm are 0.993 and 0.2319,respectively.The values of R2,k and 1/n of the Freundlich isotherm are 0.958,7.486 and 0.511,respectively.Asshown in Fig.9,linear fitting of the Freundlich isotherm presents a nonlinear relationship.1/n is 0.511,showing that adsorption reaction is easy to be performed.Therefore,the adsorption behavior could be described by the Langmuir isotherm.Results suggest that the adsorption of IEIIP is in mono molecular type and homogeneous.

Fig.9.Langmuir and Freundlich isotherm models.

Table 2 Fitting results of Cd(II)adsorption isotherm equations

Langmuir isotherm equation Freundlich isotherm equation R2 0.993 0.958 k 0.2319 7.486 1/n 0.511

3.7.Adsorption thermodynamics and activation energy

The morphology of IEIIP before and after elution was analyzed using a scanning electron microscope(SEM),as shown in Fig.2.The approximate thickness of IEIIP is about 1 μm.The surface of IEIIP before elution is smooth.While,the surface of IEIIP after elution is rough and has rich pore structure.Imprinted sites were left on the IEIIP after eluting template Cd(II).In the preparation process of IEIIP,eluting template ions could improve the adsorption surface area and binding sites to increase the adsorption capacity.

由于紫色很罕见，所以在很长一段时间内，它都是皇室和贵族独享的颜色，就像在古代中国，黄颜色象征着皇权一样。

Table 3 Thermodynamic parameters at different temperatures

Temperature/K ln K D ΔG0/kJ·mol−1 ΔH0/kJ·mol−1 ΔS0/kJ·mol−1 293 6.4902 −15.810 12.637 0.012 303 6.7455 −16.993 313 6.1492 −16.002

3.8.Adsorptive selectivity

The selective property of IEIIP was verified by competition adsorption experiments.Zn(II),Cu(II)and Pb(II)that had similar ionic radius and same charge with Cd(II)were used as competing ions.The study of selectivity was carried out in 50 ml binary solutions of Cd2+/Pb2+,Cd2+/Cu2+and Cd2+/Zn2+under 303 K for 80 min.The concentrations of Cd(II)and other metal ions were measured using FAAS after adsorption reached equilibrium.

where,k d is the distribution coefficient(ml·g−1);k is the selective coefficient;and k d(Cd(II))and k d(X(II))are the selective coefficients of Cd(II)and disturbing ions,respectively.The distribution coefficients and selective coefficients are presented in Table 4.The selectivity coefficients k of Cd2+/Pb2+,Cd2+/Zn2+and Cd2+/Cu2+are 2.4998,1.2437 and 4.6882,respectively.From the selectivity coefficient,IEIIP has better adsorption performance for Cd(II)than that of Pb(II),Cu(II)and Zn(II)in binary solutions.Namely,IEIIP has high selectivity for target ion.The recognition sites and specific binding sites are formed in the synthesis process of IEIIP.The size and shape of the imprinted sites could be matched with cadmium ions.While,the shape of competing ions who have similar ionic radius and same charge with Cd(II)could not match the recognition sites well.Hence,IEIIP has high selectivity for Cd(II)in binary solution(Fig.10).

Table 4 Distribution coefficient and selectivity coefficient of selective adsorption

Binary solutions Cd2+/Pb2+ Cd2+/Zn2+ Cd2+/Cu2+Cd2+ Pb2+ Cd2+ Zn2+ Cd2+ Cu2+k d 442.06 176.84 325.77 261.94 470.56 100.37 k 2.4998 1.2437 4.6882

Fig.10.The adsorption capacity of IEIIP for different ions.

4.Conclusions

In this paper,IEIIP using acrylamide and β-cyclodextrin as functional monomers,epichlorohydrin as cross linking agent and ammonium persulfate as initiator were prepared by inverse emulsion polymerization.The IEIIP were used to absorb Cd(II)in aqueous solutions selectively.The adsorption data suggested that the adsorption capacity of IEIIP could reach 86.7 mg·g−1 under the optimum adsorption conditions.The pseudo second order kinetic model(0.990–0.9941)and Langmuir isotherm model(R2＞0.99)could evaluate the adsorption process well.Thermodynamic parameters(ΔG0＜0,ΔH0＞0,ΔS0＞0)showed the adsorption reaction was spontaneous and endothermic.The selectivity coefficients k of Cd2+/Pb2+,Cd2+/Zn2+and Cd2+/Cu2+were 2.4998,1.2437 and 4.6882,respectively,showing the good selectivity for Cd(II)in aqueous solutions.Hence,the IEIIP could be used to treat Cd(II)in waste water.

总有一些细枝末节的痕迹还在不经意间、不动声色地带我穿越时空，回到当初我们在一起的时间亦无法将切封存，记忆不是固然存在的物，它是叶片中的风，是指尖的沙，总有些缝隙任由它自由穿梭。它终究无法打断时间的腿让它停下来，那些回不去的光阴流年终究会离我远去，但我永远记得有一个你在我的那段时光里。关于那段时光，直都被我安放在心底最柔软的地方，那年，那些栀子花般纯洁的过往，是关于青春最浪漫最陆美的画面，我将永远珍藏。那些曾经，其实早就在我心底生根发芽，成为不可替代的永恒。

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