Research Article pubs.acs.org/journal/ascecg
Hybrid Mesoporous Silica Based on Hyperbranch-Substrate Nanonetwork as Highly Efficient Adsorbent for Water Treatment Jin Tao, Jiaqing Xiong, Chenlu Jiao, Desuo Zhang, Hong Lin, and Yuyue Chen* College of Textile and Clothing Engineering, Soochow University, Suzhou, Jiangsu 215021, People’s Republic of China ABSTRACT: Hybrid mesoporous silica based on a hyperbranch-substrate nanonetwork as highly efficient adsorbent was explored by an efficient and facile approach combined with onepot condensation and grafting-to methodology, including a terminated amino hyperbranched polymer (HBP) modification. The specific synthesis procedure involves the following steps: (i) premodification of SBA-15 via a co-condensation-hydrolysis route, obtaining carboxyl functionalized SBA-15 (SBA-CAR), and (ii) grafting HBP onto SBA-CAR by a grafting-to method, obtaining hybrid mesoporous silica (SBA-HBP). The main structural characteristics of SBA-15 are preserved in the resultant SBA-HBP, which exhibits high surface area, large pore volume, and well-ordered porosity made up of uniform mesopores. Due to its reusability and with a three-dimensional hyperbranchsubstrate nanonetwork with substantial functional groups, the as-synthesized SBA-HBP is considered a versatile and sustainable adsorbent for dyes (i.e., methylene blue and congo red) and heavy metal ions (i.e., Cu2+ and Fe3+) from aqueous media with high adsorption capacity and quick adsorption rate. Also, the saturated adsorption capacities are 452.1 mg/g for MB, 593.4 mg/g for CR, 224.2 mg/g for Fe3+, and 158.7 mg/g for Cu2+, respectively. KEYWORDS: Mesoporous silica, Hyperbranched polymer, Adsorption network, Water treatment, High efficiency, Kinetics
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INTRODUCTION Treatment of industrial wastewater has become of increasing importance in recent years as industrial effluents, containing different kinds of toxic substances including organic and inorganic pollutants, cause severe environmental contamination and hinder the sustainable development of society.1,2 Synthetic dyes and heavy metal ions, as the major pollutants, are difficult to naturally degrade due to the complex structure and specific toxicity, which jeopardize the sustainability of the ecosystem.3−5 Heavy metal ions from wastewater, unlike organic pollutants, do not undergo physical, chemical, or microbial degradation, or even cannot be destroyed; they tend to bioaccumulate in living organisms, causing irreversible damage to biological chain due to their carcinogenic and mutagenic nature.6−8 As a consequence, with the increasingly rapid development of modern industry, there is a pressing need to treat industrial effluents containing dyes and heavy metal ion pollutants to preserve a virtuous cycle of aquatic ecosystems. So far, several approaches have been developed to treat pollutants in aqueous media, such as adsorption, coagulation, membrane filtration, chemical oxidation, photodegradation, and electrodialysis.9−11 Among them, adsorption is one of the most attractive technologies because of its high efficiency, low process cost, and ease of operation.12 As one of most widely used adsorbents, active carbon (AC) shows great properties and application value owing to its relatively efficient removal of adsorbates which benefits from its stable pore structure and © XXXX American Chemical Society
high surface area. Nevertheless, due to the disorder pore structure, lack of surface functional sites, and high production requirement, several drawbacks belong to the AC adsorbent that slows adsorption kinetics, lowers adsorption capacity, demonstrates inferior adsorption selectivity, and has comparatively high cost, which largely limit its application in adsorption.13−16 In addition, for adsorption in aqueous media, diversity of charges makes it difficult to efficiently adsorb different kinds of pollutants in the effluent by utilizing only one adsorbent; however, several adsorbents may not coexist with each other with stable characteristics and efficiency in aqueous media. Therefore, a versatile and high efficiency adsorbent for a variety of pollutants is urgently required. As far as we know, mesoporous silica (MS), a promising carrier owing to its large surface area, high pore volume, tunable nanoscale mesostructures, and high hydrothermal stability, has been widely investigated on applications of removal for organic and inorganic pollutants.17−19 However, MS also lacks surface functional groups, which greatly limited its adsorption capacity and selectivity. Due to the presence of abundant silanol groups on the surfaces of MS, various functional organic molecules, e.g., N,N-dimethyldodecylamine (DMDDA), dodecylamine (DDA), 3-aminopropyltrimethoxy (APTES) silane, tetrakis(4Received: July 8, 2015 Revised: September 7, 2015
A
DOI: 10.1021/acssuschemeng.5b00652 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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(EDTA), methyl acrylate, diethylene triamine, ethanol, methanol, cationic dye methylene blue (MB), anionic dye congo red (CR), copper chloride dihydrate (CuCl2·2H2O), and iron trichloride (FeCl3) were purchased from Sinopharm Chemical Reagent. All reagents used were analytically pure and without any treatment. Synthesis of SBA-CAR. In this study, P123 (2.0 g) was dissolved in 75 g of HCl solution (2.0 mol/L) under continuous stirring at 40 °C. Then 0.2 g of CTES was added dropwise to the aforementioned solution. After the mixture stirred for 30 min, 4.5 g of TEOS was added dropwise to the homogeneous solution, and the mixture was subjected to vigorous stirring at 40 °C for 20 h, followed by crystallization at 90 °C for 24 h under static condition in polytetrafluoroethlene bottles, when the hydrolysis of TEOS and cocondensation were performed. The molar composition of the mixture was 1TEOS:0.05CTES:6.9HCl:178.6H2O:0.016P123. Subsequently, the product was filtered, washed with ethanol for 4 times, and dried in vacuum oven at 80 °C overnight, obtaining cyanide modified SBA15 (SBA-CN). For hydrolysis of the −CN groups to −COOH and removal of template, 1.0 g of dried powder as-synthesized sample was dispersed in 150 mL of 48 wt % H2SO4 solution under stirring at 95 °C for 24 h.25,32 The treated sample was filtered and washed with deionized water until the eluent became neutral and then dried at 60 °C, obtaining the resultant SBA-CAR. Fabrication of SBA-HBP. The terminated amino hyperbranched polymer (HBP) was prepared by a method of polymerization according to the previous study of our research group.24,33 For grafting of HBP onto SBA-CAR by a grafting-to method, 0.05 g of SBA-CAR was dispersed in 50 mL of ethanol solution of HBP with the concentration of 20 g/L, and the mixture was then stirred under reflux in the atmosphere of nitrogen at 85 °C for 12 h.34−36 The resulting product was washed with deionized water and ethanol 6 times and dried at 60 °C, obtaining desired product SBA-HBP. Characterization. The apparent morphologies of SBA-15, SBACAR, and SBA-HBP were studied by field emission scanning electron microscopy (SEM; Hitachi S-4800). The crystal structures of them were determined by X-ray diffractometer (XRD; Bruker AXS D8 Advance). The sizes and internal features of them were observed by field emission transmission electron microscopy (TEM; Tecnai G2 F20 S-Twin). Fourier transform infrared (FTIR) spectroscopy was made by Nicolet 5700 (Thermo Nicolet) to identify the structures and chemical compositions of them. The N2 adsorption−desorption isotherms and pore characterization were measured at 77.34 K using Micromeritics ASAP 2020 analyzer (Micromeritics). The samples were degassed at 200 °C for several hours before measurement. The Brunauer−Emmett−Teller (BET) method was employed to calculate the specific surface area using adsorption data in a relation pressure ranging from 0.05 to 0.3. The pore volumes and pore size distributions were derived from the adsorption branches of isotherms using the Barrett−Joyner−Halenda (BJH) model. Thermogravimetric analysis (TGA) was carried out on a thermal analyzer (PerkinElmer DIAMOND 5700) with a heating rate of 10 °C min−1 in nitrogen flow. Zeta potential was measured using a zetasizer (Malvern ZEN 3600). X-ray photoelectron spectroscopy (XPS) measurements were conducted with an Axis Ultra HAS system. The concentrations of metal ions were measured using inductively coupled plasma atomic emission spectroscopy (ICP; Thermo Scientific ICAP 6000 DUO). The concentrations of CR and MB were estimated by UV−vis spectrophotometer (Purkinje General, TU-1810) at 495 and 664 nm, respectively. Adsorption and Desorption Experiments. SBA-15, SBA-CAR, and SBA-HBP were used as adsorbent for the removal of dyes and heavy metal ions. Typically, initial pH values of the pollutant solutions were calibrated ranging from 2 to 12 using HCl and NaOH solutions. Then, the adsorbents were added to the above pollutant solutions at a dosage of 1.5 g/L for dyes and 1.0 g/L for Cu2+ and Fe3+. Specifically, 150 and 100 mg portions of adsorbents were mixed with 100 mL portions of dyes and heavy metal ions solutions at desired concentrations (50−1500 mg/L), respectively. Adsorption took
carboxyphenyl) porphyrin (TCPP), and polymers, e.g., chitosan, can be decorated onto the interior and exterior surfaces of MS through either postsynthesis or co-condensation methods.20−23 Terminated amino hyperbranched polymer (HBP), designed and synthesized via our previous research, possesses a three-dimensional hyperbranch structure with abundant imino groups and terminal primary amino groups as reactive sites, which is believed to be an excellent candidate for organic functional modifier. The resultant functionalized MS materials proved superior to other adsorbents in terms of faster mass transport rates and higher adsorption capacities for different kinds of pollutants from aqueous media.24 In this study, we aimed to develop a novel hybrid SBA-15 as high efficiency adsorbent for dyes and heavy metal ions, via a facile and effective synthesis route based on a terminated amino hyperbranched polymer (HBP) modification. Specifically, a premodification of SBA-15 was carried out that synthesized carboxyl-modified SBA-15 (SBA-CAR) by a facile cocondensation method of CTES and TEOS, followed by hydrolysis of cyanide groups in sulfuric acid.25 HBP was prepared according to our previous research.26 Subsequently, as illustrated in Scheme 1, HBP was grafted onto SBA-CAR via Scheme 1. Schematic Synthesis and Adsorption Mechanism of SBA-HBP
the grafting-to method, obtaining SBA-HBP. Owing to the three-dimensional hyperbranch-substrate adsorption nanonetwork formed by considerable functional sites on the surface of SBA-HBP, dyes and heavy metal ions can be efficiently adsorbed from aqueous media and immobilized onto the SBAHBP by various interaction bonds. As a consequence, the saturated adsorption capacities of SBA-HBP are up to 452.1, 593.4, 224.2, and 158.7 mg/g for methylene blue (MB), congo red (CR), Fe3+, and Cu2+, respectively, while the corresponding adsorption capacities of AC for those pollutants are 270, 185, 29, and 35 mg/g, respectively.27−31
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MATERIALS AND METHODS
Materials. Triblock copolymer, poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (P123), average molecular weight 5800, was purchased from Sigma-Aldrich and used without further purification. 2-Cyanoethyltriethoxysilane (CTES), tetraethoxysilane (TEOS), sulfuric acid (H2SO4), sodium hydroxide (NaOH), hydrochloric acid (HCl), ethylenediaminetetraacetic acid B
DOI: 10.1021/acssuschemeng.5b00652 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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ACS Sustainable Chemistry & Engineering place during agitation of mixture solutions at 30 °C for a set time period (for 0−15 h, studied by adsorption kinetics experiments), by monolayer or multilayer adsorption mode which was determined by the undermentioned adsorption isotherm experiments. Then, the mixture was centrifuged, and a small amount of supernatant was taken for analysis using a UV−vis spectrophotometer by monitoring the absorbance changes at wavelength of maximum absorbance of adsorbates after being diluted. The adsorption capacity Qe (mg/g) is calculated according to the equation Qe =
(Co − Ce)V m
(1)
where Co and Ce are the initial and equilibrium adsorbate solutions concentration (mg/L), respectively. V is the volume of adsorbate solution (L), and m is the mass of adsorbent used for adsorption experiment (g).37 For desorption, the used SBA-HBP was separated by centrifugation and dispersed in eluent. The mixture was agitated at 120 °C for 4 h, followed by filtration and drying, obtaining the regenerated SBA-HBP. Particularly, the eluents for the SBA-HBP loaded with MB, CR, and heavy metal ions are 0.1 M HCl solution, 0.1 M NaOH solution, and EDAT, respectively.8
Figure 2. SEM images of (a) SBA-15 and (d) SBA-HBP, and TEM images of (b, c) SBA-15 and (e, f) SBA-HBP.
morphology altered and the radial pore strips became distorted compared with the ordered ones of SBA-15, which is ascribed to the co-condensation process. Notably, though the mesoporous structure of SBA-HBP has been altered to some extent, the general mesostucture is still preserved, which is also supported by the aforementioned XRD results. As shown in Figure 3, weight alteration of samples during modification procedure was monitored by TGA curves. Below
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RESULTS AND DISCUSSION Characterization of MS Materials. Figure 1 shows the mesopore structure of SBA-15, SBA-CAR, and SBA-HBP
Figure 3. TGA curves for SBA-15, SBA-CAR, and SBA-HBP.
Figure 1. Powder XRD patterns of SBA-15, SBA-CAR, and SBA-HBP.
about 200 °C, all the curves show a decrease to a small extent at a weight loss of 8.7%, 4.5%, and 5.8% for SBA-15, SBA-CAR, and SBA-HBP, respectively, which is considered to result from the reduction of water including physically adsorbed and chemically bonding water. It indicates that SBA-15 possesses excellent hydrophilicity due to its largest pore volume, highest surface area, and numerous silanol groups. For all samples, about 5 wt % thermogravity loss above 500 °C could be ascribed to the residual organic template P123. For SBA-CAR, between 200 and 800 °C, 16.5% weight loss occurred, of which about 11.5% is attributed to the loss of the organic component introduced by co-condensation. Furthermore, the weight loss of SBA-HBP between 200 and 800 °C increases to 26.8%, of which about 21.8% weight loss is attributed to the organic component, demonstrating that the graft of HBP has been successfully implemented. In addition, functionalization can be further confirmed by FTIR spectra, as shown in Figure 4. All curves display characteristic absorbance bands at 3436 and 1085 cm−1 attributed to the stretching vibration of Si−OH and the asymmetric stretching vibration of SiOSi, respectively, revealing the preserved general silica surface characters and frame structures. For SBA-CN, as a precursor of SBA-CAR, new bands at 2972 and 2931 cm−1 appeared, which are associated with the CH stretching vibration, and the new
determined by powder small-angle X-ray diffraction (XRD). Without modification, for SBA-15, three well-resolved characteristic diffraction peaks [2θ = 1.1° (100), 1.7° (110), 2.0° (200)] can be clearly observed, with associated 2D hexagonal (P6mm) symmetry, indicating its regular and integrated pore structure.3,25,38,39 After premodification was performed, however, the intensity of the three characteristic diffraction peaks decreased, demonstrating that the general mesostructure of SBA-CAR remained while structural ordering was altered to some extent owing to the addition of organosilane into the silica frame by co-condensation. As HBP functionalization is implemented, diffraction peaks in the range 0.7−2.0° further decreased so that (110) and (200) reflections became indistinct compared with those of SBA-15, specifically due to the presence of the amorphous HBP component grafted onto the interior and exterior surface of SBA-HBP. As shown in Figure 2, SEM images shows SBA-15 possesses a smooth surface with ordered pore strips, while the rougher surface of SBA-HBP coated with HBP organic component can be clearly observed. Besides, the TEM images show the structures of MS in the radial (b, e) and axial (c, f) direction, which confirms that the structure of SBA-15 is highly ordered as 2D hexagonal (P6mm) pore structure.32 After functionalization, it can be clearly observed by TEM images that the pore C
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of SBA-CAR (Figure 5c) contain three components: CC species (284.6 eV), CO species (286.3 eV), and CO species (287.5 eV) attributed to the carboxylic group. For SBAHBP (Figure 5d), the deconvoluted C 1s spectra has four peaks at 284.6, 286.3, 287.5, and 285.4 eV, corresponding to CC, CO, CO, and CN, respectively, indicating the presence of amino groups in HBP and amide bonds between HBP and SBA-CAR.43,45,46 The XPS results suggest that HBP is successfully grafted to SBA-CAR, which is consistent with that of FTIR. As shown in Figure 6a, the textural properties of SBA-15, SBA-CAR, and SBA-HBP were investigated by N2 adsorption and desorption isotherms, and pore size distribution curves are exhibited in Figure 6b. The corresponding BET surface area, pore volume, and pore diameter derived from the basis of the isotherms are summarized in Table 1. It can be obviously seen that all the samples synthesized in this study exhibited type IV adsorption profiles and showed H1-type hysteresis loops, which suggest well-ordered cylindrical mesoporous channels. Furthermore, the type IV adsorption profile observed in all the cases indicates distinct capillary condensation and also suggests a narrow pore size distribution.47,48 In a comparison with SBA-15, the BET surface area, pore volume, and pore size of SBA-CAR decreased to some extent because of the introduction of organosilane via co-condensation which brought a slight impact on its mesostructure. Importantly, the nitrogen sorption isotherms reveal that SBACAR is still fully accessible due to the almost parallel adsorption and desorption branches. This implies that no pore blocking occurs, which is of utmost importance in applications, allowing facile access for the chemical reagents or guest molecules. For SBA-HBP, the values of BET surface area, pore volume, and pore size further decreased to 80 m2/g, 0.15 cm3/g, and 5.8 nm from 354 m2/g, 0.61 m3/g, and 8.7 nm for SBA-CAR, respectively, whose reduction in textural properties is much greater than that of SBA-CAR. That is supposed to the result of substantial occupation of HBP grafted inside the mesopores,
Figure 4. FTIR spectra of SBA-15, SBA-CN, SBA-CAR, and SBAHBP.
band at 2258 cm−1 is assigned to the vibration of C≡N, which confirms the successful introduction of the organic component. After hydrolysis for cyanide-to-carboxyl, bands at 2972, 2931, and 2258 cm−1 declined, for the structure of introduced organic component had been influenced and the chemical environment also altered. At the same time, a new band at 1718 cm−1 associated with the stretching vibration of carboxylic acid group appeared. Finally, after grafting HBP, the band at 1718 cm−1 decreased, accompanied by the appearance of a new band at 1562 cm−1 attributed to the vibration of amide group. Along by TGA results, this phenomenon further reveals that HBP was covalently loaded to SBA-CAR by amide bonds.40,41 Further investigation about the chemical state of the elements was carried out by XPS. As shown in Figure 5a, the wide scan XPS spectrum of SBA-15, SBA-CAR, and SBA-HBP indicates the apparent peaks at binding energy around 103, 154, 284, 400, and 532 eV which were attributed to Si 2p, Si 2s, C 1s, and O 1s, respectively. Moreover, the details of the high resolution C 1s peak were used to further define the chemical properties of functional components.42−44 For SBA-15 (Figure 5b), the spectra can be deconvoluted into two contributions: carbon in aliphatic structure (284.6 eV) and carbon in CO (286.3 eV). Also, the deconvoluted high resolution C 1s spectra
Figure 5. XPS spectra of SBA-15, SBA-CAR, and SBA-HBP: (a) wide scan, (b) C 1s spectra of SBA-15, (c) C 1s spectra of SBA-CAR, (d) C 1s spectra of SBA-HBP. D
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SBA-15 exhibited a negative charge on the surface over almost the whole pH range. However, compared with SBA-15, the zeta potential values of SBA-HBP increased over the entire pH range, rendering its surface positively charged at pH lower than 8.0 which is shown to be electrically neutral (zero point charge). This phenomenon is owed to the presence of HBP on the surface of SBA-HBP with abundant imino groups and terminal primary amino groups, which can gain H+ ions in the aqueous media and then turn into cationic groups at pH lower than 8.0. Effect of pH Value on Adsorption. The initial pH value of adsorbate solution has a great influence on adsorption capacity of SBA-HBP for dyes and heavy metal ions, which is attributed to the alteration of surface charge of both adsorbent and adsorbate with different pH values.49 The effect of pH on the adsorption capacities of SBA-HBP for CR, MB, and Cu2+ is depicted in Figure 7b. Besides, the adjustable initial pH range of solution for CR and MB is limited, for acidic and alkalic conditions can easily affect the whole conjugate structure in molecules of CR and MB, respectively, which means that the concentration of their solutions cannot be precisely measured by a UV−vis spectrophotometer. Also, the calculated adsorption capacity from the experimental data affected by pH could be slightly lower than the theoretical one. Generally, the adsorption capacity for CR decreased from pH 6.0 to 12.0; it especially declined sharply at pH higher than 8.0. As far as we know, change of surface charge is due to protonation and deprotonation of the active functional groups of adsorbent and adsorbate. Consequently, it is supposed that the CR surface is negatively charged since the nitrogen atoms and sulfonate groups of CR molecule become deprotonated from pH 6.0 to 12.0 with a pKa value of 4.5−5.5, which makes it an anionic dye. In addition, SBA-HBP exhibits positive surface charge at pH lower than 8.0, leading to electrostatic attraction between SBAHBP and negatively surface charged CR, which gives rise to maximum adsorption capacity of nearly 600 mg/g at pH 6.0. However, as pH is increased to higher than 8.0, the surface charge of adsorbent became negative, hindering the adsorption caused by electrostatic repulsion, and the adsorption capacity for CR decreased dramatically. In the pH range 2.0−10.0, adsorption capacity for cationic dye MB kept increasing, reaching 425 mg/g, owing to the electronegativity of carboxylic groups on SBA-HBP continuing to increase as the pH is raised. Meanwhile, the deprotonation of amino groups is also of benefit to the adsorption for MB, for the amount of H+ in aqueous media kept decreasing, releasing more and more vacant adsorption sites on SBA-HBP for MB. Also, electrostatic attraction between MB and SBA-HBP contributed to adsorption capacity as pH increased higher than 8.0. For Cu2+, the similar adsorption trend is attributed to the same adsorption mechanism for MB. The results exhibit the excellent adsorption property of SBA-HBP for both anionic and cationic adsorbates. Adsorption Kinetics. The adsorption kinetic study plays a significant role in analysis of the adsorption process, and can depict the adsorption rate which in turn controls the residual time of the adsorption process at the solid−solution interface. The adsorption kinetics of dyes and heavy metal ions onto SBA-HBP was investigated with the Lagergren pseudo-firstorder model in Figure 8b and pseudo-second-order model in Figure 8c, which are given as the following equations. The pseudo-first-order model follows:
Figure 6. (a) N2 adsorption−desorption isotherms for SBA-15, SBACAR, and SBA-HBP. (b) Pore diameter distribution curves of SBA-15, SBA-CAR, and SBA-HBP.
Table 1. Surface Area, Pore Volume, and Pore Size of SBA15, SBA-CAR, and SBA-HBP material
BET surface area (m2/g)
total pore volume (cm3/g)
av pore diameter (nm)
SBA-15 SBA-CAR SBA-HBP
507 354 80
0.84 0.61 0.15
11.1 8.7 5.8
which is also supported by the aforementioned XRD results, suggesting a successful incorporation of the organic polymer. As shown in Figure 7a, the characteristic surface charge of SBA-15 and SBA-HBP was investigated by zeta potential measurement in the pH range 2.0−10.5. The results show that
Figure 7. (a) Zeta potential curves of SBA-15 and SBA-HBP. (b) pHdependent adsorption capacity of SBA-HBP for CR, MB, and Cu2+. E
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Figure 8. (a) Effect of contact time on the adsorption by SBA-HBP for MB, CR, Fe3+, and Cu2+. (b) Pseudo-first-order kinetic plots for adsorption by SBA-HBP. (c) Pseudo-second-order kinetic plots for adsorption by SBA-HBP. (d) Effect of initial concentration on the adsorption by SBA-HBP. (e) Langmuir adsorption isotherm plots of SBA-HBP. (f) Freundlich adsorption isotherm plots of SBA-HBP.
Table 2. Kinetic Parameters and Experimental Adsorption Capacities for the Removal of MB, CR, Fe3+, and Cu2+ by SBA-HBP pseudo-first-order model
pseudo-second-order model
adsorbent
Qexp (mg/g)
Q1cal (mg/g)
K1
R2
Q2cal (mg/g)
K2
R2
MB CR Fe3+ Cu2+
452.1 593.4 209 148
83.7 108.0 115.9 50.5
0.005 61 0.005 72 0.006 03 0.004 57
0.4022 0.4438 0.8328 0.5022
460.8 613.5 219.3 152.4
0.000 149 0.000 076 0.000 126 0.000 253
0.9991 0.9942 0.9967 0.9962
ln(Q 1e − Q t) = ln Q 1e − k1t
have higher value (>0.99) than those from the pseudo-firstorder kinetic model (Figure 8b), and the theoretical adsorption capacities (Q2e) calculated from pseudo-second-order kinetic equation are much closer to the experimental adsorption capacities (Qexp), demonstrating that the pseudo-second-order adsorption model is predominant to depict the adsorption process.50 Adsorption Isotherm. As shown in Figure 8d, the adsorption amount at equilibrium increases dramatically as the initial concentration raises, and then tends to level off, which is attributed to the increasing driving force from concentration gradient. Adsorption isotherm and relevant parameters are used to determine the adsorption behavior. In this study, Langmuir and Freundlich adsorption isotherm, as two kinds of isotherm equations, are employed to investigate the adsorption processes. It presumes by the Langmuir isotherm model that adsorption is based on the formation of adsorbate as monolayer coverage onto the homogeneous adsorbent surface. The linearized Langmuir equation is given as follows:
(3)
The pseudo-second-order model follows t t 1 = + Qt Q 2e k 2Q 2e 2
(4)
Here Q1e (mg/g) and Q2e (mg/g) represent the calculated adsorption capacity of SBA-HBP at equilibrium. Qt is the adsorption amount at t (min); k1 (min−1) and k2 [g/(mg min)] are the rate constants of pseudo-first-order and pseudo-secondorder kinetics equations, respectively. As shown in Figure 8a, it can be observed from the adsorption kinetic curves that the adsorption rate during the initial period is very high, which is owing to electrostatic attraction between adsorbates and adsorbent that possesses mesostructure with high surface area and substantial functional sites. Besides, the considerable adsorption capacity can be attributed to other chemical interactions and physics forces such as hydrogen bonding for dyes and complexation for metal ions, which make the adsorbates immobilized. Also, these interactions cost more time than the initial adsorption period did, leading to the flattening curves. Specifically, the adsorption equilibrium was reached within 2 h for MB and CR, 4 h for Fe3+, and 3 h for Cu2+, respectively. The corresponding parameters and experimental adsorption capacities are displayed in Table 2. It is obvious that the correlation coefficients (R2) for the pseudo-second-order kinetic (Figure 8c) model
Ce Ce 1 = + Qe Q max kLQ max
(5)
Here Ce (mg/L) is the equilibrium concentration; Qe (mg/g) is the adsorption amount at equilibrium; Qmax (mg/g) denotes the maximum monolayer capacity of adsorbent; kL (L/mg) is F
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ACS Sustainable Chemistry & Engineering Table 3. Langmuir and Freundlich Parameters for Adsorption of SBA-HBP Langmuir
Freundlich
adsorbent
Qmax (mg/g)
KL (L/mg)
R2
RL
KF (L/mg)
n
R2
MB CR Fe3+ Cu2+
537.6 675.7 224.2 158.7
0.005 35 0.007 64 0.0153 0.0215
0.9899 0.9850 0.9983 0.9962
0.7890−0.1108 07236−0.0803 0.5666−0.0418 0.4819−0.0301
18.7501 34.9459 34.4197 29.8355
2.1209 2.3596 3.6484 4.0156
0.8410 0.7120 0.7528 0.6264
retained after regeneration, which proved that SBA-HBP is an efficient and sustainable adsorbent.
the Langmuir constant that represents the energy of the adsorption process. Qmax and kL were calculated from the slope and intercept of the Langmuir isotherm (Figure 8e), and their values were listed in Table 3. Another essential parameter, RL, called the separation factor, is determined by the relation RL =
1 1 + kLC i
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CONCLUSION In this study, a functionalized mesoporous silica SBA-15 based on the surface modification with organosilane and terminated amino hyperbranched polymer was developed to obtain a versatile and high efficiency adsorbent. Compared with SBA-15 and SBA-CAR, the resultant SBA-HBP adsorbent shows quite high adsorption capacity for both anionic and cationic dyes, as well as heavy metal ions due to its three-dimensional hyperbranch-substrate nanonetwork with abundant adsorption sites, providing the complex chemical and physical interactions such as electrostatic attraction, hydrogen bonds, complexation, etc. For SBA-HBP, the adsorption rate is fast, and adsorption equilibrium can be reached within 2 h for MB and CR, 4 h for Fe3+, and 3 h for Cu2+; meanwhile, the saturated adsorption capacities are 452.1 mg/g for MB, 593.4 mg/g for CR, 224.2 mg/g for Fe3+, and 158.7 mg/g for Cu2+, respectively. The adsorption kinetics and isotherm studies have shown that the adsorption process obeys the pseudo-second-order kinetics and Langmuir isotherm. With consideration of the versatile, high efficiency, and regeneration-sustainable adsorption, SBA-HBP is believed to be a promising candidate for application in the removal of various pollutants from industrial effluents.
(6)
where RL indicates the type of the isotherm to be either unfavorable (RL > 1), linear (RL = 1), favorable (0 < RL < 1), or irreversible (RL = 0).51 The Freundlich isotherm model is an empirical relationship depicting the adsorption for solutes from liquid to solid surface, which presumes that multilayer adsorption occurs on a heterogeneous surface. The linear form of the Freundlich is ln Q e = ln KF +
1 ln Ce n
(7)
where KF is the Freundlich constant and n is the heterogeneity factor, which are determined by the intercept and slope of the linear plot, respectively (Figure 8f). The corresponding parameters are summarized in Table 3. Compared with the Freundlich isotherm, the Langmuir isotherm is considered the more favorable model for depicting the adsorption processes, owing to the high correlation coefficient (R2 > 0.99), reasonable Langmuir constant (kL > 0), and suitable separation factor (0 < RL < 1). In Figure 8, for both kinetics and isotherm experiments, the pH of adsorbate solutions is the initial value without adjustment, which is equal to 6.8 for MB, 9.5 for CR, 5.5 for Fe3+, and 5.8 for Cu2+. Desorption and Regeneration Study. The regeneration of adsorbent is considered a crucial factor in sustainable applications. After desorption, the regenerated SBA-HBP was reused for adsorption in succeeding cycles. Notably, as shown in Figure 9, even if the adsorption capacities of regenerated SBA-HBP were found to be decreased slightly during the five cycles, more than 90% of the initial adsorption capacity was
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: +86-512-67487152. Fax: +86-512-67487152. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National High Technology Research and Development Program of China (No. 2012AA030313) and Innovation Program for Postgraduate Cultivation of Jiangsu Province (No. ZY32002114).
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REFERENCES
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Figure 9. Adsorption capacity of regenerated SBA-HBP in different cycles. G
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