Research Article pubs.acs.org/journal/ascecg
Cite This: ACS Sustainable Chem. Eng. 2017, 5, 10011-10018
Biomass trans-Anethole-Based Hollow Polymer Particles: Preparation and Application as Sustainable Absorbent Saleem Raza,†,‡ Xueyong Yong,†,‡ Bowen Yang,†,‡ Riwei Xu,*,‡ and Jianping Deng*,†,‡ †
State Key Laboratory of Chemical Resource Engineering and ‡College of Materials Science and Engineering, Beijing University of Chemical Technology, Beisanhuan East Road 15#, Beijing 100029, China S Supporting Information *
ABSTRACT: A novel type of hollow polymer particles containing carboxyl groups was prepared from a widely available biophenylpropene trans-anethole (ANE). To prepare the hollow particles, we first prepared polymeric particles using vinyl acetate and maleic anhydride (MAH), and then such particles were taken as sacrificial templates for the subsequent formation of core/shell particles, which were synthesized by using ANE and MAH as comonomers and divinylbenzene (DVB) as cross-linking agent through precipitation polymerization. After removing the core in the prepared core/shell particles, we obtained hollow particles and then hydrolyzed the anhydride groups into carboxyl functional groups. The hollow particles were characterized by FT-IR, SEM, and TEM, and further used as absorbents. The maximum adsorption toward Cu2+ and methylene blue reached 270 and 940 mg/g, respectively. The recycling study showed that the biobased hollow particles can be easily restored and reused. The hollow particles may find practical applications as sustainable absorbents. The established methodology for preparing hollow polymer particles is expected to be applicable for other biophenylpropenes. KEYWORDS: Biomass, Hollow particles, Green absorbents, trans-Anethole
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INTRODUCTION Water pollution and resource shortage have become serious concerns nowadays. Waste water significantly comes from industrial processes such as mining, metallurgy, and chemical manufacturing containing abundant metallic ions and organic dyes; spillage and some accidental leakage also contribute to water pollution, which has been proven to be very toxic and carcinogenic to human and other organisms.1−3 In order to solve the problem of environmental pollution and provide a sustainable water resource, various treatment processes such as chemical precipitation,4 membrane-based filtrations,5 biological treatments,6 and adsorption7 have been explored to eliminate the contaminants in waste water. Among these methods proposed, adsorption technology is considered to be highly promising to achieve efficient water purification, because of its simplicity, high efficacy, flexibility, and insensitivity to toxic substances.8−13 A variety of materials such as clays,14 activated carbon,15 silica gel,16 fly ash,17 and polymers18 have been widely employed as absorbents for the removal of pollutants. However, most of these conventional absorbents suffer from either low adsorption effectiveness or limited recyclability. In particular, importantly, most of these materials are nonrenewable, which means they will be used up someday. Therefore, researchers have begun to make efforts to develop new high-efficiency absorbents from renewable biomass such as chitosan19 and vanillin.20 © 2017 American Chemical Society
As a category of plant-derived vinyl compounds, transanethole (ANE) is abundant in anise,21 fennel,22 and Chinese star anise,23 and has been broadly used as a seasoning.24 We hypothesize that the polymers derived from ANE shall provide various novel materials due to the unique molecular structure. Regrettably, ANE is difficult to homopolymerize through free radical polymerization because the 1,2-substituted vinyl structure leads to high steric hindrance.25,26 However, as a typical electron donor, ANE can be easily copolymerized with electron acceptors (e.g., maleic anhydride, MAH) through free radical copolymerization. 26 In our previous work, we successfully prepared novel biobased particles20 derived from vanillin and particles27 consisting of ANE and MAH via precipitation polymerization. The resulting polymeric particles show potential as green absorbents.27 Compared with common solid particles, hollow particles have drawn increasing attention because of their spherical morphology and attractive properties including encapsulation, controllable surface permeability, and surface functionality.28−31 In addition, as a unique type of absorbents, hollow polymeric absorbents also have excellent an mechanical property and facile regeneration and recycling properties for repeated uses.32,33 Accordingly, we in the present Received: June 15, 2017 Revised: September 4, 2017 Published: September 25, 2017 10011
DOI: 10.1021/acssuschemeng.7b01956 ACS Sustainable Chem. Eng. 2017, 5, 10011−10018
Research Article
ACS Sustainable Chemistry & Engineering Scheme 1. Schematic Strategy for Preparing ANE-Based Hollow Polymeric Particles (BHPPs)
(BHHPs) was observed with scanning electron microscopy (SEM) (Hitachi S-4800), and the hollow surface was observed with transmission electron microscope (TEM) (Hitachi H-800). FT-IR spectra were recorded with a Nicolet NEXUS 870 infrared spectrometer (KBr pellets). Atomic absorption spectrophotometer (AAS) (Varian Spectra AA55B) was used for determining Cu2+ concentrations which were fitted with copper hollow cathode lamp. The methylene blue absorption measurements were carried out by a UV−vis spectrophotometer (Shanghai Jinghua756MC). Synthesis of PMV Template. The hard PMV template particles (Scheme 1) were prepared by a method reported earlier.35 The basic procedure is stated briefly below. Monomer MAH (7.35 g, 0.075 mol) was added to a 100 mL three-neck flask containing solvent n-butyl acetate (42 mL). The flask was subjected to an ultrasonic process to dissolve the MAH. Then, 6.45 g (0.075 mol) of VAC and initiator BPO (0.048 g, 0.2 mmol) were added to the reaction solution. Polymerization proceeded for 6 h at 80 °C under N2 protection. After 6 h, the produced PMV template was centrifuged for 10 min at 10000 rpm, thoroughly washed with ethanol 3 times, and dried in a vacuum oven at 60 °C overnight. Preparation of Core/Shell Particles. The prepared PMV template (1.5 g) was dispersed in a three-neck flask containing a solution of monomer MAH (1.18 g, 12 mmol) and initiator AIBN (0.03 g, 0.18 mmol) in n-butyl acetate (28 mL). After complete dissolution, n-heptane (12 mL), monomer ANE (1.78 g,12 mmol) and cross-linker DVB (0.78 g, 6 mmol) were added in the flask. The flask containing the mixture was placed in a water bath at 75 °C. The polymerization was carried out for 6 h at the temperature under mechanical stirring at a speed of 250 rpm. The whole procedure was conducted under N2 atmosphere. After 6 h, the resulting product was separated by centrifugation, washed with ethanol three times, and dried in a vacuum oven at 60 °C for 24 h. Formation of Hollow Particles. After polymerization, the core/ shell particles were taken from the vacuum oven and added in a beaker with 20 mL of acetone. Then, the beaker was ultrasonicated for 30 min to remove the whole template and then centrifuged to isolate the particles. This treatment of ultrasonication and isolation was repeated three times. The remaining products were washed with ethanol three times, and the hollow particles were obtained. Surface Hydrolyzation of Hollow Particles To Prepare BHHPs. The hollow particles (0.2 g) were put in a small bottle, in which 10 mL of 0.2 M NH3·H2O aqueous solution was added. Hydrolyzation lasted for 6 h with constant stirring. Then, the pH of
work prepared hollow particles starting from ANE and MAH. Herein, it should be pointed out that MAH can also be regarded as “green” because nowadays it has been successfully prepared from biomass.34 In the present contribution, we report the first successful preparation of ANE-based hollow absorbents by using the hardtemplating method as illustrated in Scheme 1. Following the strategy, we first synthesized narrowly dispersed nanoparticles (PMV) by copolymerizing maleic anhydride (MAH) and vinyl acetate (VAC) through a free radical precipitation copolymerization technique. Taking the PMV as template, we further prepared core/shell particles by using ANE, MAH, cross-linker (DVB), and the template through precipitation polymerization. After removal of the template, the resulting hollow copolymeric particles were successively subjected to a hydrolyzation process converting the anhydride groups to carboxyl groups, followed by an exploration of their adsorption property toward metallic ions and organic dyes, respectively, taking Cu2+ and methylene blue (MB) as examples of adsorbates. The adsorption kinetics study and isotherm model were investigated in detail. Furthermore, the particulate absorbents can be easily recycled, and still maintained their high adsorption capability toward Cu2+ and MB. The methodology established in the present study is expected to provide various biobased absorbents with high efficiency.
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EXPERIMENTAL SECTION
Materials. Vinyl acetate (VAC) was purchased from Tianjin Chemical Reagents Company and purified by distillation under reduced pressure. Maleic anhydride (MAH) and trans-anethole (ANE) were purchased from Tokyo Chemical Industry (TCI) and used without further purification. Divinylbenzene (DVB) was purchased from Alfa Aesar and used directly. 2,2′-Azobis(isobutyronitrile) (AIBN) from Aldrich was purified by recrystallization from methanol. Copper sulfate (CuSO4·5H2O) was obtained from Sinopharm Chemical Reagent Co. and used as received. Methylene blue (MB) was obtained from Ciba and used directly. The solvents n-heptane, acetone, and n-butyl acetate were all distilled by standard methods. Characterizations. The surface morphology of the templates, core/shell particles, and the biobased hydrolyzed hollow particles 10012
DOI: 10.1021/acssuschemeng.7b01956 ACS Sustainable Chem. Eng. 2017, 5, 10011−10018
Research Article
ACS Sustainable Chemistry & Engineering
Figure 1. SEM images of (a) the PMV template, (b) core/shell particles, (c) hollow particles, and (d) BHHPs. The insets in parts c and d indicate the TEM images of the corresponding particles. Scale bar: 500 nm. the solution was adjusted to 5−6 by adding 0.2 mol L−1 HCl aqueous solution. Subsequently, the particles were isolated by centrifugation, washed with ethanol three times, and dried in a vacuum at 60 °C. Finally, we obtained the biobased hydrolyzed hollow particles (BHHPs). Adsorption of Pollutants. We took Cu2+ as a model for toxic metal ions and methylene blue (MB) as a model for organic dyes because they are typical pollutants causing serious environmental problems, especially water pollution. Adsorption of pollutants was performed by mixing the BHHPs (10 mg) separately with 100, 250, and 500 mg/L pollutant aqueous solution (10 mL) in glass bottles. The dispersions were then stirred at room temperature for a determined time. After the adsorption was complete, the supernatant was extracted, and the pollutant concentration was determined by AAS or UV−vis absorption spectra measurement. The removal percentage of adsorbate (R) is calculated by the following equation:
R(%) =
(C0 − Ce) × 100% C0
qt =
(C0 − Ce)V m
log(qe − qt) = log qe −
k1 t 2.303
(5)
where qe and qt represent the amount of pollutant adsorbed (mg/g) at the equilibrium stage and at time t (min), respectively. Also, in this equation k1 (1/min) is the rate constant of first-order kinetic adsorption. The pseudo-second-order kinetic model is used in the linear form of the equation
t 1 1 = + t 2 qt qe k 2qe
(1)
(6)
where k2 (g/mg min) is the rate constant of second-order adsorption, and the other parameters keep the meaning as stated above. Dye Desorption and BHHPs Recycling. The desorption and regeneration ability of BHHPs were examined by performing adsorption and desorption cycles toward MB. During the adsorption experiment, 10 mg of BHHPs was dissolved in 10 mL of MB aqueous solution with an initial concentration of 500 mg/L. The adsorption was carried out under stirring for 9 h. Then, the BHHPs were separated by centrifugation and washed by mixed solvent composed of 2 M HCl aqueous solution and THF (1:1, 20 mL in total) until no MB can be detected. Then, the recovered BHHPs were washed with ethanol and dried under vacuum at 60 °C overnight. The restored absorbent was reused for dye adsorption in the next cycle.
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RESULTS AND DISCUSSION As shown in Scheme 1, the narrowly dispersed PMV template particles were obtained by precipitation polymerization. Then, the template, comonomers (ANE and MAH), and cross-link agent DVB were mixed together to prepare the core/shell particles. After the core/shell particles were treated with acetone, the PMV template cores were removed, thereby providing hollow particles. In the next step, consecutive reactions of the hollow particle with NH3·H2O and HCl were performed, and the anhydride groups in the shells were easily converted to carboxyl groups. In theory, there are abundant carboxyl groups in the shell, which can serve as effective binding sites for absorbing metallic ions and organic dyes through electrostatic interaction. In addition, both the high surface area of particles and the mesopores in the cross-linked shells can strongly facilitate the rapid permeation and mass transport of metallic ions and dye molecules, which enable the hollow particles to show excellent adsorption performance toward both metallic ions and dye molecules. Characterization of the Particles. SEM and TEM were used to investigate the morphological feature of PMV particles, core/shell particles, hollow particles, and BHHPs. As presented in Figure 1a, the number-average diameter of the PMV
(2)
Herein, V is the volume of pollutant solution, and m is the mass of hollow particles. The Langmuir isotherm applied in this study is expressed by the following equation: Ce C 1 = e + qe qm KLqm
(4)
where Ct is the concentration of a pollutant in solution at the specified time (mg/L).The pseudo-first-order kinetic and pseudo-second-order kinetic model equations were followed to examine the adsorption mechanism. The pseudo-first-order kinetic model was examined, as expressed in the equation below:34
where C0 and Ce are the initial and equilibrium concentration (mg/L) of the pollutant. Effects of Pollutant Concentration. The effects of incremental pollutant concentration on the adsorption behavior were investigated next. The hollow particles of constant weight (10 mg) were immersed into pollutant solution with varied concentrations (10 mL, 50−2500 mg/L) at pH = 5−6. After adsorption, the supernatant was extracted, and the pollutant concentration was measured by AAS or UV−vis absorption technique. Equilibrium Isotherm. The equilibrium isotherm model examined in the study is the Langmuir isotherm, which has been widely applied to adsorption tests. The study was performed by using a certain amount of hollow particles (10 mg) in the solution of pollutant at varied initial concentration for 9 h. After the adsorption reached equilibrium, a proper measurement (AAS or UV−vis absorption) was taken to determine the pollutant concentration in the residual aqueous solution. The amount of pollutant adsorbed by the hollow particles (qe, mg/g) was calculated according to the following equation:
qe =
(C0 − Ct )V m
(3)
where Ce is the equilibrium concentration (mg/L) of pollutant, qe is the amount of pollutant adsorbed at equilibrium (mg/g), qm is the maximum adsorption capacity corresponding to the complete monolayer coverage (mg/g), and KL is the Langmuir constant related to the adsorption energy. Kinetics Study. To perform kinetics studies, the pollutant solution (10 mL) of certain concentration was mixed with 10 mg of BHHPs for a predetermined time. The removal rate of pollutant was determined by measuring the pollutant concentration at predetermined time intervals, t. For time t, the amount of absorbed pollutant, qt (mg/g), was calculated by the following equation: 10013
DOI: 10.1021/acssuschemeng.7b01956 ACS Sustainable Chem. Eng. 2017, 5, 10011−10018
Research Article
ACS Sustainable Chemistry & Engineering
peak at 1725 cm−1 in the spectrum of BHHPs, reflecting the carbonyl in carboxyl groups. These, together with the attenuation of the peak at 1780 cm−1 in the spectrum of BHHPs, demonstrate that the anhydride groups in hollow particles were transferred into carboxyl groups successfully. Adsorption Behavior of BHHPs. After hydrolyzation, the carboxyl groups on the shell hopefully enable the hollow particles to become an effective cation exchange absorbent. To prove this hypothesis, adsorption performance tests were carried out by using the BHHPs toward Cu2+ as a model for metallic ions and methylene blue (MB) as a model for organic dyes. Detailed results including an adsorption isotherm model and adsorption kinetics graphs toward Cu2+ are shown in Figure 3. The adsorption isotherm (Figure 3a) shows the relationship between the amount of adsorbed Cu2+ on the BHHPs versus Cu2+ concentration at equilibrium in solution, while the adsorption−time curve (Figure 3b) shows the relationship between the adsorption amount versus adsorption time. No matter whether we examine the adsorption isotherm or the adsorption−time curve, we can find that the amount of adsorbed Cu2+ rapidly increased at lower Cu2+ concentrations and within the first 2 h; then, the increase of adsorbed Cu2+ became much slower, finally reaching the maximum adsorption amount. We excitedly found that the maximum adsorption amount of Cu2+ by the BHHPs can be up to 270 mg g−1 and the adsorption equilibrium can be reached within 180 min. The Langmuir adsorption isotherm model was taken to further analyze the adsorption, based on the consideration that all the adsorption sites in BHHPs have an identical affinity to adsorbates and the adsorption at one site does not influence the adjacent sites. Hence it is clear that the Langmuir isotherm is valid for monolayer adsorption in which the adsorption behavior completely takes place on surfaces containing identical active sites. The fitting results (Figure 3c) present a good linearity and a high correlation coefficient value over 0.99, meaning that the adsorption of Cu2+ on the BHHPs was a single molecular layer chemical adsorption. In addition, the fitting of experimental data to Freundlich isotherm models is
template is nearly 550 nm. After the copolymerization of the template, comonomers, and cross-link agent, the core/shell particles were formed with a total average diameter in the range 750−800 nm (Figure 1b), indicating the efficient formation of shells. From Figure 1c,d, it is clear that there is no change in the size and morphology of the particles. TEM images indicate that the hollow structure was successfully formed. In order to analyze the chemical composition of the hollow particles for the purpose of acquiring more information about the structure of BHHPs, we further carried out FT-IR measurements, as presented in Figure 2. In the spectrum,
Figure 2. FT-IR spectra of (a) ANE, (b) MAH, (c) hollow particles, and (d) BHHPs (KBr tablet).
clear peaks at 1510 and 1249 cm−1 reflect the benzene ring and methoxyl group in ANE, respectively. The peaks at 1855 and 1780 cm−1 show us the anhydride groups only present in monomer MAH, while in the hollow particles and BHHPs, methoxyl and anhydride groups are both present, which indicates that the monomers underwent copolymerization sufficiently. The most important point found in the spectrum of BHHPs is the characteristic peaks of hydrogen bonding appearing at 3000−3500 cm−1. Besides, we can also find a new
Figure 3. (a) Adsorption isotherm of Cu2+ by the particles, adsorption time, 9 h; (b) time−adsorption quantity profile, initial concentration, 100 mg/mL; (c) fitting result using Langmuir model; and (d) fitting results using a pseudo-first-order model and a pseudo-second-order model. 10014
DOI: 10.1021/acssuschemeng.7b01956 ACS Sustainable Chem. Eng. 2017, 5, 10011−10018
Research Article
ACS Sustainable Chemistry & Engineering
Figure 4. (a) Effects of initial dye concentration and adsorption time on the adsorption of dye onto the particles; (b) equilibrium adsorption isotherm of MB onto the particles, adsorption time, 9 h; (c) Langmuir isotherm for the adsorption of MB; and (d) fitting results using a pseudo-firstorder model and a pseudo-second-order model.
first 30 min was found to be 420 mg/g, and then the adsorption rate decreased; finally, adsorption equilibrium was realized at approximately 9 h with an adsorption capacity of 480 mg/g. The equilibrium adsorption isotherm of MB at room temperature is presented in Figure 4b. When the initial dye concentration remained in a low range (
