Biomass Vanillin-Derived Polymeric Microspheres Containing

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Biomass Vanillin-Derived Polymeric Microspheres Containing Functional Aldehyde Groups: Preparation, Characterization, and Application as Adsorbent Huanyu Zhang,†,‡,§ Xueyong Yong,†,‡ Jinyong Zhou,†,‡,§ Jianping Deng,*,†,§ and Youping Wu*,‡,§ †

State Key Laboratory of Chemical Resource Engineering, ‡State Key Laboratory of Organic−Inorganic Composites, and §College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China S Supporting Information *

ABSTRACT: The contribution reports the first polymeric microspheres derived from a biomass, vanillin. It reacted with methacryloyl chloride, providing monomer vanillin methacrylate (VMA), which underwent suspension polymerization in aqueous media and yielded microspheres in high yield (>90 wt %). By controlling the N2 bubbling mode and by optimizing the cosolvent for dissolving the solid monomer, the microspheres were endowed with surface pores, demonstrated by SEM images and mercury intrusion porosimetry measurement. Taking advantage of the reactive aldehyde groups, the microspheres further reacted with glycine, thereby leading to a novel type of Schiff-base chelating material. The functionalized microspheres demonstrated remarkable adsorption toward Cu2+ (maximum, 135 mg/g) which was taken as representative for metal ions. The present study provides an unprecedented class of biobased polymeric microspheres showing large potentials as adsorbents in wastewater treatment. Also importantly, the reactive aldehyde groups may enable the microspheres to be used as novel materials for immobilizing biomacromolecules, e.g. enzymes. KEYWORDS: adsorbent, biomass, polymer microspheres, Schiff-base, vanillin

1. INTRODUCTION Biomass-derived polymers have been attracting rapidly increasing attention in the last years. To date, remarkable advancements have been and are being made in this significant research area, for which many comprehensive review articles can be found in literature.1−4 Among the biobased polymers, those derived from vanillin may be the most extensively explored ones due to the abundant sources and easy availability of the biomass.5−8 Vanillin-based polymers are widely used for preparing renewable benzoxazine and polybenzoxazines,9−11 polyesters,12−14 composites,15 etc. Novel polymers with distinctive structures and properties have been established based on vanillin.16−18 Also interestingly, the polymers were found to possess antibacterial ability19,20 and to work as antioxidant,21 adsorbent,22 and Schiff-base.23,24 In the aspect of polymer chemistry, impressive progress also has been achieved. Electrochemical polymerization25 and in particular living polymerization26,27 processes succeeded in polymerizing vanillin derivatives. Despite the significant progress as mentioned above, to our knowledge, the construction of polymeric microspheres derived from vanillin has not been exploited yet. The study along this direction may provide an important category of novel polymeric materials. As a proof of concept, we have prepared the anticipated polymer microspheres by making use of vanillin as raw material, as to be reported in this article. © XXXX American Chemical Society

Polymers, especially polymeric microspheres containing reactive aldehyde groups, constituted an unique class of polymeric materials.28−31 The reactive aldehyde groups provide a powerful platform for developing functional polymer materials, e.g. for immobilizing enzymes32,33 and for developing Schiff-bases,34 adsorbents,35 and sensors.36 Nonetheless, the materials containing aldehyde groups in literature are primarily limited to the use of acrolein37 and glutaraldehyde.38 The routine practices show some intractable problems. For acrolein, it polymerizes under strict conditions.37 Especially, the lowmolecular compounds (acrolein, glutaraldehyde) show toxicity.39 To solve these problems, biomass vanillin shall be an ideal candidate. Herein it should be noted that vanillin has been widely used as fragrant agent in diverse areas with a long history, powerfully demonstrating the nontoxicity of vanillin. In the present work, we took vanillin as a raw material to prepare a new type of polymeric microsphere. Delightfully, we not only constructed the designed microspheres but also demonstrated their potential use as adsorbent toward Cu2+ (as a model for metal ions). This is the first study of the kind. We are also convinced that the as-prepared polymeric microspheres containing reactive aldehyde groups will find various other Received: November 16, 2015 Accepted: January 11, 2016

A

DOI: 10.1021/acsami.5b11042 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. Schematic strategy for preparing polymeric microspheres starting from vanillin (A and B), for preparing Schiff-base chelating microspheres by reaction with glycine (C) and for performing adsorption by the microspheres toward Cu2+ ions (D). recrystallization in ethanol/deionized water to obtain pure product (white crystals, 72.6% yield). 2.4. Preparation of PVMA Microspheres. The microspheres of PVMA (the polymer derived from monomer VMA) were prepared via a suspension polymerization approach (Figure 1B). Since monomer VMA is solid at room temperature, we employed CHCl3 (or CHCl3/ toluene mixture) as a cosolvent to dissolve VMA. The monomer solution was then added in water to perform suspension polymerization. A typical procedure for preparing the microspheres is briefly stated as follows. First, 0.3 g VMA, 0.081 g TMPTA, and 0.012 g AIBN were dissolved in a mixed solvent (1 mL) composed of chloroform and toluene (volume ratio = 8/2). The solution was added in a 100 mL three-necked flask, which was equipped with a mechanical stirrer (stirring speed, 350 rpm). Then a PVA aqueous solution (5 wt %, 50 mL) was added in the flask under nitrogen atmosphere. After being vigorously stirred at a speed of 350 rpm for 30 min, the oil phase droplets were well dispersed, and the nitrogen ventilation was stopped. Afterward, the reaction system was heated to 65 °C and retained at the temperature for 6 h. The use of cross-linking agent (TMPTA) enabled the as-prepared microspheres to be sufficiently cross-linked. The yielded product was filtered and thoroughly washed with acetone and deionized water. Finally, the microspheres were dried in a vacuum oven at 50 °C until constant weight. The yield of the white microspheres was 92 wt % under optimal conditions: PVA, 5.0 wt %; AIBN, 4.0 wt %; and stirring speed, 350 rpm. 2.5. Preparation of Schiff-Base Chelating Microspheres. Due to the unique molecular structure of VMA, the resulting PVMA microspheres contained reactive aldehyde groups for conveniently performing postfunctionalization. In this work, we grafted glycine molecules onto the microspheres by taking advantage of the aldehyde groups, directly providing a Schiff-base chelating resin, as shown in Figure 1C. A typical procedure for the formation of Schiff-base is briefly described below. First, PVMA microspheres (500 mg) were sufficiently swelled in DMSO for 24 h and washed by this solvent three times to remove the soluble fraction. Glycine (5 mmol) and KOH (5 mmol) were dissolved in a mixed solvent (50 mL, VDMSO:Vwater = 1:1). Then the mixture was poured into a 250 mL three-necked flask, in which another 50 mL DMSO and the microspheres were added. After the reaction system was heated to 60 °C for 12 h under continuous stirring, the yielded product was filtered and washed with deionized water. Finally, the microspheres were dried in a vacuum oven at 60 °C until constant weight. 2.6. Determination of the Content of Accessible Aldehyde Groups in the Microspheres. The content of aldehyde groups in the microspheres was determined indirectly. Since each monomer VMA unit contains an aldehyde group, in theory we can approximately determine the total aldehyde groups in the microspheres. In synthesizing the Schiff-base, glycine was intentionally exceeded. We can track the residual glycine concentration in the reaction system to indirectly determine the accessible aldehyde moieties in the micro-

practical applications, apart from being used as adsorbent as in the present work.

2. EXPERIMENTAL SECTION 2.1. Materials. All the chemicals, unless otherwise noted, were obtained from Aldrich. Glycine and copper sulfate were used as received without further purification. Methacryloyl chloride (Alfa Aesar), pyridine, and trimethylolpropane triacrylate (TMPTA) were purified by distillation under reduced pressure. Vanillin and ninhydrin were recrystallized from water. 2,2-Azobis(isobutyronitrile) (AIBN) was recrystallized from methanol, dried under vacuum at room temperature, and stored in an amber bottle. Poly(vinyl alcohol) (PVA, polymerization degree 1750 ± 50) was obtained from Sinopharm Chemical Reagent Co. All the solvents were distilled by standard methods. Water was freshly deionized before use. 2.2. Measurements. FT-IR spectra (KBr pellet) were measured using a Nicolet NEXUS 670 spectrophotometer. 1H and 13C NMR (nuclear magnetic resonance, 400 and 100 MHz, respectively) spectra were measured in CDCl3 by using a Bruker AV400 spectrometer at 20 °C, with tetramethylsilane (TMS) as internal standard. The morphology of the microspheres was observed by S-4800 electron microscope (SEM, Hitachi). Pore structures of the microspheres were measured by mercury porosimetry (PoreMasterGT 60, USA). Elemental analysis was performed on a Vario EL cube (Elementar Analysensysteme GmbH). Differential scanning calorimetry (DSC) was carried out with a WATERWSLLC Q100 under 50 mL/min of flowing N2 at a heating rate of 10 °C/min. Thermogravimetric analysis (TGA) was performed with a Q50 TGA at a scanning rate of 10 °C/ min under N2. Glycine content was determined by ninhydrin−amino acid method through UV−vis absorption measurement on 756MC UV−visible spectrophotometer (Shanghai Jinghua Technology Instrument, λ = 570 nm). An atomic absorption spectrophotometer (AAS) (Varian SpectraAA55B) fitted with a copper hollow cathode lamp was used for determining Cu2+ concentrations, the measurement was performed in 3 replications, and the % RSD did not exceed 5%. 2.3. Synthesis of Monomer, Vanillin Methacrylate (VMA). Monomer vanillin methacrylate (VMA, as structurally presented in Figure 1A) was synthesized according to the method in the literature.40 Vanillin (3.04 g, 20 mmol) and pyridine (1.90 g, 24 mmol) were dissolved in methylene chloride (50 mL) and then introduced into a three-necked, round-bottomed flask. Methacryloyl chloride (2.51 g, 24 mmol) dissolved in methylene chloride (10 mL) was added dropwise into the flask at room temperature under continuous stirring. The reaction mixture was heated for 2−3 h for reflux under a nitrogen atmosphere. The solvent was distilled off on a water bath. The residue was dissolved in 50 mL chloroform, successively washed with water and saturated NaHCO3 three times, respectively, and then dried over magnesium sulfate. Crude VMA was obtained after evaporating the solvent and further purified by B


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ACS Applied Materials & Interfaces spheres. The ninhydrin−amino acid method was employed to track the residual glycine concentration.41 A 0.1 mL portion of the reaction solution was taken from the reaction system for preparing chelating resin at the time of 0, 0.5, 1, 1.5, 3, 6, 9, and 12 h. After diluting to a fixed volume, the residual glycine reacted with ninhydrin, for which we measured UV−vis absorption (λ = 570 nm). By referring to the standard curve of UV−vis absorption vs glycine concentration, we could calculate the residual glycine amount. This practice enabled us to calculate the content of the accessible aldehyde groups in the microspheres. 2.7. Adsorption Experiments. Adsorption studies were performed toward Cu2+ in the present study (Figure 1D). Each adsorption experiment was repeated 3 times to acquire an average value. In a typical adsorption test, 10 mg of the microspheres (chelating resin as prepared above) was dispersed in CuSO4 aqueous solution (10 mL). The adsorption capacity was measured at ten Cu2+ concentrations (10, 50, 100, 200, 300, 400, 500, 1000, 2000, and 3000 mg/L). The aqueous dispersions containing Cu2+ ions and microspheres were adjusted to pH 5−6 and then stirred for 24 h at about 25 °C. The initial Cu2+ concentration (C0, mg/L) and its concentration at equilibration state (Ceq, mg/L) were measured by atomic absorption spectrophotometry (AAS). The amount of Cu2+ adsorbed (qeq, mg/g) at equilibration state was calculated by the following equation: qeq =

Figure 2. FT-IR spectra of vanillin (a), monomer VMA (b), and polymer microspheres (c). The spectra were measured in KBr tablet.

(−(CO)H), 1640 (−C(CH3)C), and 1600 cm−1 (phenyl). Furthermore, the disappearance of the peak around 3520 cm−1 in VMA indicates the transformation of − OH groups originally in vanillin to the ester groups in VMA. This, together with the new peak at 1640 cm−1, strongly demonstrates the successful synthesis of VMA. The elemental analysis and the NMR spectra provide further evidence for this consideration (Figure S1, detailed analysis is also presented therein). Considering the targeted applications of the microspheres in this study, we directly prepared cross-linked microspheres by using cross-linking agent TMPTA, as shown in Figure 1B. In the process of forming microspheres, PVA aqueous solution acted as the continuous phase, while the CHCl3 (or CHCl3/ toluene, see below) solution containing monomer (VMA), initiator (AIBN), and TMPTA formed the disperse phase. CHCl3 played an important role as cosolvent since both the monomer and initiator are water-insoluble solids. CHCl3 was selected for its suitable boiling point, since in the course of suspension polymerization for forming the microspheres, CHCl3 could evaporate gradually. We further found that the surface pore structures in the resulting microspheres can be adjusted by controlling the evaporation of cosolvent and we then used mixed cosolvent (CHCl3/toluene). This phenomenon will be discussed in more detail later on. The resulting microspheres were characterized by FT-IR spectroscopy (Figure 2). For PVMA microspheres (spectrum c), the spectrum is analyzed as 2940 (methyl and methylene), 2840 and 2750 (−(CO)H), 1760 and 1135 (−(CO)O−), 1700 (−(CO)H), and 1600 (phenyl). The disappearance of the peak around 1640 cm−1 in PVMA indicates the transformation of −C(CH3)C groups in VMA to the saturated polymer main chains in PVMA. Also due to the transformation, the peak of ester group moved from 1735 to 1760 cm−1. Accordingly, FT-IR measurement strongly demonstrates the successful polymerization of VMA. Together with SEM images (to be discussed later on), polymer microspheres were convincingly formed as presented in Figure 1. We found that stirring speed in the suspension polymerizations played significant roles in forming regular microspheres. SEM images of the microspheres are presented in Figure 3. A stirring speed of 350 rpm led to satisfactory microspheres. In contrast, a lower stirring speed (250 rpm) resulted in microspheres showing a wide size distribution, while a higher stirring speed (450 rpm) led to microspheres with

(C0 − Ceq)V m

(1)

where m is the mass of microspheres (g) and V the volume of aqueous dispersion (L). For adsorption kinetic studies, 10 mg of chelating microspheres was added in 10 mL of CuSO4 aqueous solution and thoroughly mixed. The pH of the dispersion was adjusted to 5−6. In total, three initial concentrations of Cu2+, 50, 100, and 200 mg/L were taken to carry out adsorption under stirring at approximately 25 °C. The aqueous dispersion samples were subjected to AAS measurement at preset time intervals to determine the concentration of Cu2+. The amount of adsorbed Cu2+ at time t, qt (mg/g), was calculated by the equation: qt =

(C0 − Ct )V m

(2)

where m is the mass of microspheres (0.01 g), C0 (mg/L) is the initial Cu2+ concentration, Ct (mg/L) is the Cu2+ concentration at time t, and V the volume of aqueous dispersion (L). To compare the adsorption ability between the chelating microspheres and the microspheres without grafting glycine, they were separately subjected to the adsorption test as stated above.

3. RESULTS AND DISCUSSION 3.1. Synthesis of PVMA Microspheres. The microspheres were prepared according to the process illustrated in Figure 1 (A and B). First, vanillin reacted with methacryloyl chloride to form monomer VMA (Figure 1A), in which biomass vanillin was utilized for providing aldehyde groups while methacryloyl chloride provided CC structures for the subsequent radical polymerization. The produced monomer (VMA) was characterized by FT-IR spectroscopy (the spectra are presented in Figure 2) and the NMR technique (the spectra are presented in Supporting Information, Figure S1, the same below). For a comparison, the raw material vanillin was also measured. In Figure 2, compared to the spectrum of vanillin (spectrum a), new characteristic peaks of VMA can be clearly observed in spectrum b. In vanillin (spectrum a), the peaks are assigned as follows: 3200 (−OH), 2940 (methyl and methylene), 2840 and 2750 (−(CO)H), 1700 (−(CO) H), and 1600 cm−1 (phenyl). For VMA (spectrum b), the peaks are analyzed as 2940 (methyl and methylene), 2840 and 2750 (−(CO)H), 1735 and 1135 (−(CO)−O−), 1700 C


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Figure 3. SEM images of the microspheres prepared under different conditions (PVA, 5.0 wt %; AIBN, 4.0 wt %; TMPTA, 2.7 wt %; N2, continuously bubbled in the whole process; cosolvent, chloroform; stirring speed: 250 rpm (both A-1 and A-2), 350 rpm (both B-1 and B-2), 450 rpm (both C-1 and C-2)). A-2, B-2, and C-2 are partially enlarged images for A-1, B-1, and C-1.

Figure 4. SEM images of microspheres prepared under conditions: PVA, 5.0 wt %; AIBN, 4.0 wt %; TMPTA 2.7 wt %; N2 bubbling stopped before heating; stirring speed 350 rpm, and chloroform/toluene ratio (V/V) 10/0 (both A-1 and A-2), 8/2 (both B-1 and B-2), 6/4 (both C-1 and C-2). A2, B-2, and C-2 are partially enlarged images for A-1, B-1, and C-1.

discussed next. The related SEM images are presented in Figure 4. A comparison between the SEM images in Figure 4A (the first N2 bubbling mode: stopped before the beginning of polymerization) and Figure 3B (the second N2 bubbling mode: continued through the whole polymerization process) shows that the first mode dramatically increased the number of pores on the surface of the microspheres. This situation should be ascribed to the change of the N2 bubbling mode, the cosolvent finally aggregated on the surface of the microspheres rather than evaporating from the polymerization system. On the contrary, the second mode for N2 bubbling (Figure 3B) promoted the volatilization of cosolvent and inhibited the formation of voids. On the other hand, in order to elucidate the effects of cosolvent consisting of chloroform and toluene, we designed three varied solvent ratio: chloroform/toluene = 10/0, 8/2, and 6/4 (V/V). The related SEM images are presented in Figure 4. By comparing Figure 4A-2 and B-2, we can find that

irregular morphology. Furthermore, the stirring speed also affected the surface morphology of the microspheres (Figure 3A-2, B-2, and C-2). At the lower stirring speed, much more voids can be clearly observed in them. Increasing stirring speed resulted in the disappearance of the pores. So we propose that the evaporation of cosolvent affected largely the formation of the surface voids in the microspheres. At the higher stirring speed, chloroform evaporated faster, which is unfavorable for forming voids in the microspheres surface. From Figure 3 and the above analysis, the surface pore structures of the microspheres seem to be possibly tuned by controlling the evaporation of the cosolvent. Accordingly, two methods were further taken to intentionally slow down the volatilization of cosolvent. The first method is to stop N2 gas at the beginning of polymerization. The second method is by using toluene as a second cosolvent. Toluene was utilized for its higher boiling point. The two approaches were investigated to prepare regular microspheres bearing surface voids, as to be D


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to slow down the rate of cosolvent volatilization and further led to the surface porous structures. Mercury intrusion porosimetry were used to quantitatively compare the porous structure of the three groups of microspheres, as presented in Figure 6. Mercury volume

when chloroform was partially replaced by toluene, the pores became larger and the surface of the microspheres became less smooth. Both the two phenomena are favorable to increase the specific surface area of the microspheres. But a further increase in toluene content caused the microspheres surface to be much more rough (Figure 4C-2). The unique morphology of the microspheres in Figure 4C can be explained as follows. The polymerization of VMA in each monomer droplet (the dispersed oil phase) was transformed, to some degree, from the initial solution polymerization to precipitation polymerization because of the mixed cosolvent. The little particles obtained through precipitation polymerization aggregated together to form larger spheres. Based on the investigations, we choose the solvent ratio of chloroform/toluene being 8:2 (V/V) as the best one for the subsequent investigations. Next, to understand their detail structures, the microspheres obtained under optimal conditions (PVA, 5.0 wt %; AIBN, 4.0 wt %; TMPTA 2.7 wt %; N2 stopped after the beginning of heat; stirring speed 350 rpm, mixed solvent chloroform/toluene 8/2, V/V) were crushed and the fracture surfaces were observed by SEM. As shown in Figure 5, the pores were

Figure 6. Microspheres’ pore size distribution measured by using mercury porosimetry. (A) Chloroform as cosolvent, N2 bubbling continued. (B) Chloroform as cosolvent, N2 stopped before heating. (C) Chloroform/toluene as cosolvent (8/2, V/V), N2 stopped before heating.

intruded can be used to compare the volume of different pore sizes, as shown in the figure. For each method, the pores smaller than 0.1 μm amounted to the largest content. The total volume of the larger pores (>0.1 μm) increased in the microspheres in the order of Figure 6A, B, and C. This observation is in good agreement with the corresponding SEM images (A, Figure 3B-2; B, Figure 4A-2; C, Figure 4B-2). The specific surface area of the microspheres also increased in the same order: (A) 8.32, (B) 12.16, and (C) 15.52 m2/g. It thus indicates that we can control the pore structures of the microspheres by tuning the N2 bubbling mode and by varying the cosolvent ratio. We subsequently measured the thermal stability of the microspheres by thermogravimetric analysis (TGA) technique. TGA measurement was performed from room temperature to 800 °C at a heating rate of 10 °C/min under nitrogen. The relevant result is presented in Figure S2. The polymer chains were found to decompose in the temperature range of 250− 480 °C. DSC analysis was also performed on the microspheres, as presented in Figure S3. It shows that the polymer chains in the microspheres possessed a glass transition temperature (Tg) of about 102 °C. TGA and DSC measurements demonstrate that the microspheres can be used at moderately high temperatures. 3.2. Preparation of Chelating Resin and Adsorption of Cu2+. For all the separating techniques like selective adsorption, separation and preconcentration used in wastewater treatment, removal and recovery of metal ions, accurate analysis of trace metal ions, and even collection of nuclear fuel from seawater,42,43 chelating materials can be highly recommended due to their multiple functions, namely ion exchange and chelating functions. These functions are derived from the multidentate ligands which usually contain electron donor atoms like O, N, S, and P and can be easily coordinated with transition metals. Among various chelating resins, the Schiffbased chelating resins especially those containing multiple

Figure 5. Surface pore structures of cross-linked microspheres observed by SEM. The fragments were obtained by crushing the microspheres in a mortar. (A) Cross section of the microspheres. (B) Greater detail.

located in a style of gradient mode, i.e. from the microspheres’ center to the outer surface, the density of the pores increased gradually. This phenomenon can be explained by the fact that the oil droplets were rotated under stirring in the course of polymerization. In the early stage of microsphere formation, residual solvent inside the microspheres existed in the form of small droplets. The rotation force drove the solvent droplets to be primarily located near the surface of the droplets, finally leading to the pores mainly placed near the microspheres surface. The distinctive porous structures facilitate the microspheres to be used as efficient adsorbents, as to be reported later on. According to the investigations and discussion above, we put forward the mechanism of pore formation. In the initial stage of the polymerization, the cosolvent served to maintain the liquid drop. Along with the polymerization, most of the chloroform evaporated from the liquid drop and the morphology of the microspheres were fixed by the polymerization of monomer, so the residual cosolvent inside the microspheres gathered in the form of small droplets and formed the porous structure in the microspheres. Due to the rotation force from mechanical stirring, the pores were mainly placed near the microspheres surface. Therefore, we consider the surface pore structure of the microspheres was determined by two factors, i.e. the rate of polymerization and the rate of cosolvent volatilization. Stopping N2 gas at the beginning of polymerization and using toluene as a second cosolvent were proved to be effective E


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ACS Applied Materials & Interfaces donor atoms (e.g., N and O atoms) are well-known to have high adsorption capacity and adsorption selectivity toward transition metal ions. Due to the special structure of monomer VMA, the resulting microspheres contained intriguing active aldehyde moieties, which can undergo Schiff-base reaction with amino groups mildly and easily. Hence in this work, a novel type of Schiff-based chelating resin was prepared by the reaction of aldehyde groups and glycine. Glycine was choose as the amino ligand because of its simple chemical structure, nontoxicity, and containing both N and O donor atoms. In our experiments, the content of the accessible aldehyde groups in the microspheres was indirectly characterized by measuring the consumed glycine in the reaction system, as detailed in the Experimental Section. The reaction between aldehyde groups and glycine provided Schiff-base chelating microspheres, as illustrated in Figure 1C. Cu2+ was employed as a representative of heavy metal ions and the adsorption capacity of the Schiff-base chelating resin was characterized by the adsorption toward Cu2+ (Figure 1D). Figure 7 presents the

Figure 8. FT-IR spectra of PVMA microspheres (a), chelating microspheres (b), and chelating microspheres after adsorbing Cu2+ (c).

base was formed between aldehyde group and glycine. The characteristic peaks of aldehyde groups (2840, 2750, and 1700 cm−1) still can be clearly observed, showing that the aldehyde groups in the microspheres were not completely consumed. In spectrum c, the peak at 1640 cm−1 disappeared. It indicates that the CN double bond and Cu2+ complexed to make the peak at 1640 cm−1 shift to lower wavenumber (about 1600 cm−1, overlapped by other peaks). This, together with the characteristic peak of metal−carbon bond appearing at 785 and 640 cm−1, clearly demonstrated the successful synthesis of Schiffbase chelating microspheres and the efficient absorption of Cu2+. The process of preparing the Schiff-base chelating microspheres by the reaction between glycine and aldehyde groups can be utilized further for determining the accessible aldehyde groups on the microspheres. For this purpose, glycine was deliberately added in excess for performing the reaction. The residual concentration of glycine after reaction was characterized by UV−vis spectrophotometer with the help of ninhydrin (chromogenic reagent). The glycine grafted amount is mathematically expressed as

Figure 7. Typical photographs of pure PVMA microspheres, chelating microspheres, and the microspheres after adsorbing Cu2+.

typical photographs of pure PVMA microspheres, the chelating microspheres (Schiff-base) and the chelating microspheres after adsorbing Cu2+. The color of PVMA microspheres was white. After reacting with glycine, the microspheres changed to yellow. After absorbing Cu2+, they turned to green. This phenomenon can be explained by subtractive color process. The chelating microspheres appeared orange, meaning that they absorbed visible light of 380−500 nm in wavelength, so the other unabsorbed light made the microspheres seem orange. For the microspheres with adsorbed Cu2+, the Cu2+ ions absorbed the visible light of 600−750 nm, which enabled the microspheres to be green. The observation in remarkable color change preliminarily demonstrates the efficient adsorption ability of the microspheres toward Cu2+ ions. Figure 8 shows the FT-IR spectra of the chelating microspheres before and after absorbing Cu2+. For the original chelating microspheres (spectrum b): 3500 (carboxylic group), 2940 (methyl and methylene), 2840 and 2750 (−(CO)H), 1760 and 1135 (−(CO)O−), 1700 (−(CO)H), 1640(−(CN)−), and 1600 cm−1 (phenyl). For chelating microspheres after absorption (spectrum c): 3500 (carboxylic group), 2940 (methyl and methylene), 2840 and 2750 (−(C O)H), 1760 and 1135 (−(CO)O−), 1700 (−(CO)H), 1640(−(CN)−), 1600 (phenyl), 785 and 640 cm−1 (−C− M−). The peak at 3500 cm−1 means the hydrogen bond formed by the grafted glycine. The characteristic peak of CN double bond appeared at 1640 cm−1, proving that the Schiff-

grafted amount =

(C0 − Ct )V × 100% M nm

(3)

where m is the mass of microspheres (0.5 g), C0 (mg/L) is the initial glycine concentration, Ct (mg/L) is the glycine concentration at time t (both C0 and Ct were characterized by UV−vis spectrophotometer with the help of ninhydrin as chromogenic reagent), V is the volume of reaction mixture taken out from the reaction system (0.1 mL, diluted to 0.01 L), and Mn is the molecular weight of glycine (75.07 g/mol). We found that the maximum amount of glycine grafted on microspheres reached 4.3 mmol/g (Figure S4). Therefore, the content of the accessible aldehyde groups in the chelating microspheres was determined to be about 4.3 mmol/g. According to the molecular structure of monomer VMA forming the microspheres, the molecular weight of the repeating units is 220 g/mol, indicating that the theoretical maximum aldehyde content can reach 4.55 mmol/g. To explore the adsorption capacity of the Schiff-base chelating microspheres, we used a typical heavy metal ion, Cu2+ as the absorbate. Both the adsorption isotherms and F


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calculation of Langmuir constants from the intercept and slope of the linear plot. The calculated KL, qm, and correlation coefficients (RL2) values are given in Table 1. The Freundlich adsorption isotherm45,46 is mathematically expressed as

adsorption kinetics were subsequently explored in detail. First, we compared the pure PVMA microspheres and the chelating microspheres in terms of adsorption toward Cu2+. As shown in Figure 9, compared to the chelating microspheres, the pure

qeq = K f Ceq1/ n log qeq = log K f +

microspheres did not show any adsorption capacity. This encourages us to consider that the adsorption capacity is derived from the newly generated Schiff-base groups and carboxylic groups. The chelating microspheres are considered forming complexes with Cu2+ ions through Schiff-base groups and free carboxyl groups simultaneously, as illustrated in Figure 10. Figure 11 shows the relationships between the amount of Cu2+ adsorbed on the chelating resin vs Cu2+ concentration at equilibrium in the solution. The adsorption rapidly increased at low Cu2+ concentrations and then slowly increased at high concentrations. We excitedly found that under the specific conditions, the maximum amount of Cu2+ adsorbed by the microspheres can be up to 135 mg/g. In order to evaluate the adsorption characteristics, both Langmuir and Freundlich adsorption isotherm models were taken to fit in the experimental data. The Langmuir adsorption isotherm44 is mathematically expressed as qeq

=

Ceq qm

+

1 KLqm

1 log Ceq n

(6)

where Kf and n represent the Freundlich constants, which can be calculated from the slope and intercept of the linear plot of log qeq versus log Ceq as presented in Figure 11C. Ceq (mg/L) is the equilibrium concentration of the ions, and qeq (mg/g) is the amount of ions bonded at the concentration Ceq. The calculated Kf, 1/n, and correlation coefficients (RF2) values are given in Table 1. By fitting experimental data to Langmuir isotherm model, we found that the qm of the chelating resin in theory can be over 150 mg/g. A simple comparison was further made between the present microspheres with some typical adsorbents in the literature (see Table 2). Herein, we point out that a direct comparison is not practical due to the different experimental conditions. Nonetheless, the present microspheres show a remarkable higher adsorption ability than polymes,47,48 ion exchange resins,49 silica-based products,50 and inorganic materials.51 Next, according to relating literature, the Langmuir isotherm model assumes the adsorption process as a monolayer adsorption on completely homogeneous surfaces where all sites have identical affinity to the adsorbate, whereas the Freundlich adsorption model assumes nonideal adsorption by the formation of a multilayered adsorbate on a heterogeneous surface.45,46 As can be seen from the correlation coefficient values (Table 1, Figure 11B and C), although they are similar, Freundlich model exhibited a better fit to the experimental data than the Langmuir one. This result can be explained by the fact that the chelating microspheres have two kinds of chelating structures, as shown in Figure 10. The two kinds of chelating structures make the microspheres surface heterogeneous for absorbing metal ions. This consideration can be supported by the parameter of 1/n, since the 1/n value located in the range 0−1 indicates a heterogeneous absorption, whereas 1/n values above 1 imply a cooperative adsorption.52 So the 1/n value 0.544 obtained in the present work indicates the heterogeneous absorption toward Cu2+, as presented in Figure 10. Therefore, the adsorption process was demonstrated to be a chemical adsorption; in more detail, both the Schiff-base structures and the carboxylic groups simultaneously contributed to the adsorption. For practical uses, it is necessary to know which

Figure 9. Adsorption quantity of pure microspheres and chelating microspheres toward Cu2+ against time: Initial concentration of Cu2+, 100 mg/L; microspheres, 1 g/L; pH, 5−6; temperature, 25 °C.

Ceq

(5)

(4)

where KL (L/mg) is the binding equilibrium constant, qm (mg/ g) is the maximum amount of ions bonded, Ceq (mg/L) is the equilibrium concentration of ions, and qeq (mg/g) is the amount of ions bonded at the concentration Ceq. Figure 11B illustrates the Langmuir plot of Ce/qe vs Ce, enabling the

Figure 10. Adsorption of Cu2+ ions on the chelating microspheres: (1) by Schiff-base groups; (2) by free carboxyl groups. G


Research Article


Figure 11. (A) Adsorption isotherms of Cu2+ by chelating microspheres and fitting results using (B) Langmuir and (C) Freundlich models: initial Cu2+ concentration 10, 50, 100, 200, 300, 400, 500, 1000, 2000, and 3000 mg/L; microspheres 1 g/L; pH 5−6; stirring for 24 h; temperature 25 °C.

For conducting absorption kinetic study, the amount of Cu2+ adsorbed was examined as a function of time and the results are shown in Figure 12. As can be seen, the absorption of Cu2+ exhibited an initial fast rate and reached equilibrium within 2.5 h. Two kinetic models were considered for examining the mechanism of adsorption process. The first model is the pseudo-first-order model47 which can be expressed as

Table 1. Parameters of Cu2+ Adsorption by Chelating Microspheres Langmuir Adsorption Isotherm qm (mg g−1) 156.49

KL (L mg−1)

RL2

0.002132 Freundlich Adsorption Isotherm

0.9693

1/n

KF (mg g−1(L mg−1)1/n)

RF2

0.5438

1.368

0.9836

Table 2. Comparison among Adsorbents in Terms of Cu Adsorption

log(qe − qt ) = log qe − 2+

adsorbents

qm (mg/g)

refs

pyridine−pyrazole ligands chelating polymer Schiff-based ligands porous chelating polymer ion exchange resin Schiff-based immobilized silica gel Fe3O4 decorated graphene oxide nanosheets Shiff-based chelating microspheres

94 81.3 71.12 0.889 17.78 156

47 48 49 50 51 present study

k1 t 2.303

(7)

where qe and qt are the amounts of metal ions (mmol g−1) adsorbed at equilibrium and at time t (h), respectively, and k1 (h−1) is the rate constant of adsorption. Values of k1 were calculated from the plots of log(qe − qt) vs t (Figure 12B). The k1 and R2 values are given in Table 3. The second model applied is the pseudo-second-order equation based on the equilibrium adsorption53 which can be expressed as t 1 1 = + t qt qe k 2qe 2

(8)

where k2 (g mg−1 h−1) is the rate constant of second-order adsorption. The linear plot of t/qt vs t is shown in Figure 12C. The k2 and R2 values are given in Table 3.

factor plays a prevailing role. We will make it clear in the further studies.

Figure 12. (A) Time−adsorption quantity profiles of Cu2+ by the chelating microspheres and fitting results using a (B) pseudo-first-order model and (C) pseudo-second-order model: initial Cu2+ concentration 50, 100, 200 mg/L; microspheres 1 g/L; pH 5−6; stirring for 0, 1, 1.5, 2, 2.5, 3, 6, 9, 12, 24 h; temperature 25 °C. H


Research Article

ACS Applied Materials & Interfaces Table 3. Parameters of Cu2+ Adsorption Kinetic Models pseudo-first-order

a

pseudo-second-order

initial conca

qe, exp (mg/g)

k1 (h−1)

qe, cal (mg g−1)

R2

k2 (h−1 g mg−1)

qe, cal (mg g−1)

R2

50 100 200

17.25 24.85 34.22

0.3806 0.4586 0.5279

12.23 12.41 25.78

0.9794 0.9368 0.9550

0.06816 0.10860 0.06350

17.95 25.34 35.06

0.9995 0.9998 0.9996

Unit, mg/L.

Table 3 shows that all the obtained regression coefficient (R2) values from the pseudo-second-order kinetic model are above 0.999. Moreover, the calculated qe values are in good agreement with experimental values qe. Hence, the adsorption kinetics could be approximated more favorably by pseudosecond-order kinetic model for Cu2+ adsorbed by the chelating microspheres. This phenomenon further reflects that the adsorption is a kind of chemical adsorption. Although the reaction rate constant was not high enough (as found in Table 3 and Figure 12; the adsorption of Cu2+ reached equilibrium after 2.5 h), the chelating microspheres have been proved to be an efficient adsorbing material with a remarkable adsorption capacity toward Cu2+ (>135 mg/g). We hope to mention that, in theory the adsorption capacity of the microspheres can be further improved for instance by (1) increasing the surface area (e.g., by preparing smaller particle size microspheres or by increasing the pore structures) and (2) increasing the hydrophilicity of the microspheres. Furthermore, apart from Cu2+ ions, the microspheres are expected to show adsorption ability toward other transition metals. The study will be continued to further optimize the surface structure of the microspheres and the optimal adsorption conditions (e.g., pH and temperature), aim at improving their adsorption property (including selective adsorption), and finding out more potential applications in heavy metal ion adsorption.

can also be used for preparing functional microspheres by the same strategy. We are currently continuing our study along the important research directions.

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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/acsami.5b11042. Elemental analysis, 1H and 13C NMR spectra of monomer VMA; TGA and DTG thermograms of PVMA microspheres; typical DSC curve of the PVMA microspheres; time-change curve of glycine grafted on the PVMA microspheres (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86-10-6443-5128. Fax: +86-10-6443-5128. E-mail: [email protected] (J.D.) *E-mail: [email protected] (Y.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Funds for Creative Research Groups of China (51221002) and the “Specialized Research Fund for the Doctoral Program of Higher Education” (SRFDP 20120010130002).

4. CONCLUSION A novel class of biobased aldehyde-containing microspheres was prepared via suspension polymerization by using vanillin methacrylate (VMA) as monomer. By controlling the cosolvent evaporation, the microspheres were afforded with surface porous structures. This is the first polymeric microspheres derived from vanillin, thanks to its unique molecular structure the microspheres can be easily functionalized by using the highly reactive aldehyde groups. Thus, we subsequently prepared a novel type of Schiff-base chelating resin by the reaction of the microspheres’ aldehyde groups and glycine. SEM and other techniques demonstrated the porous structure of the PVMA microspheres. Remarkably, Cu2+ adsorption tests demonstrated the excellent absorption ability of the chelating microspheres toward heavy metal ions (over 130 mg/g). The adsorption process was demonstrated to be a chemical adsorption, i.e. both the Schiff-base structures and the carboxylic groups simultaneously contributed to the adsorption. We highlight that such novel polymeric microspheres are highly interesting not only because the monomer is derived from renewable biomass vanillin, but also because the microspheres contain a large number of active aldehyde groups. Accordingly, they will find significant practical applications in developing functional polymer materials, e.g. as green adsorbents toward metal ions in wastewater treatment and as biomaterials for immobilizing enzymes. Apart from vanillin, other lignin derivatives, e.g. syringaldehyde and p-hydroxy benzaldehyde,

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REFERENCES

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Biomass Vanillin-Derived Polymeric Microspheres Containing

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