Adsorption of Three Flavonoids from Aqueous Solutions onto

Article pubs.acs.org/jced

Adsorption of Three Flavonoids from Aqueous Solutions onto Mesoporous Carbon Yin Li,*,†,‡ Shenghua Tang,† Yiyi Bao,† Shengdao Shan,‡ Ruiqin Yang,† Jianwei Mao,† Ju Zhu,† and Qing Ge† †

Zhejiang Provincial Key Lab for Chemical and Biological Processing Technology of Farm Products, School of Biological and Chemical Engineering and ‡Key Laboratory of Recycling and Eco-treatment of Waste Biomass of Zhejiang Province, Zhejiang University of Science and Technology, Hangzhou, 310023, China S Supporting Information *

ABSTRACT: A mesoporous carbon adsorbent was synthesized for the adsorption of myricetrin, puerarin, and naringin from aqueous solutions. The mesoporous carbon can effectively adsorb the flavonoids, and the adsorption capacities at the feed temperature of 298 K and equilibrium concentration of 0.05 mg·mL−1 were from 63 to 206 mg·g−1. Adsorption thermodynamic parameters were calculated. Adsorption kinetics and adsorption dynamic breakthrough experiments further confirmed the efficient adsorption of these molecules on the carbon adsorbent. A complex mechanism involving physical adsorption and chemical adsorption was suggested for the adsorption of myricetrin, puerarin, and naringin on the mesoporous carbon. flavonoids from natural resources,20−22 and development of adsorbents for efficient separation and purification of natural products such as flavonoids is a meaningful task. Carbonaceous materials could physically adsorb natural products and other organic compounds from water via Van de Waals forces, among these materials, activated carbon is widely applied for adsorptive removal or purification of organic compounds including flavonoids from aqueous solutions.23−26 However, micropores, which are the dominant pore structures of activated carbons, would not benefit the diffusion of compounds with higher molecular weight, and result in a slow mass transport rate and low adsorption capacity. Mesoporous (pore diameter between 2 and 50 nm) carbon materials have regulated pore structures, large specific surface areas and pore volumes, relatively larger pore sizes, and high mechanical stability,27,28 are believed to be a kind of promising materials as highly efficient adsorbents, and have been applied on adsorptive removal of aromatic compounds, dyes, and heavy metals from wastewater.29−32 Owing to their physicochemical properties, especially well-developed mesopores, mesoporous carbons offer great potential for selective adsorption of large species that were limited by the micropores of activated carbons.33 Furthermore, according to the adsorption properties of alkaloids on a series of mesoporous carbons from our previous work,34−36 mesoporous carbons are suggested as

1. INTRODUCTION Flavonoids are secondary metabolites from natural resources containing a diphenylpropane (C6C3C6) skeleton, based on their chemical structures, they can be classified into flavonols, flavanones, anthocyanidins, flavones, isoflavones, and chalcones.1−3 Flavonoids widely exist in plants1 and have antiinflammatory,4,5 antioxidant,6,7 antivirus,8 antiproliferative,9 pro-apoptotic,10 and antitumor activities.11 They are consumed in a healthy diet in many countries,1 and some of them are the main bioactive components of medicinal plants.12 Myricetrin, puerarin, and naringin are three typical flavonoids belonging to flavonols, isoflavones, and flavones, respectively, and are present as the form of glycosides. Myricetrin and puerarin have one glycosyl. Myricetrin widely exists in citrus fruits,13 stem bark, and leaves of plants14,15 and has exhibited strong antioxidative activity.14 Puerarin is a major active constituent of the Chinese herbal medicine Ge-gen (Radix Puerariae, RP), which is used for the treatment of fever, pain, diabetes, and so on.16,17 Naringin has two glycosyls, is present in grapefruit and other citrus fruits, and possesses anti-inflammatory, hypocholesterolemic, and neuroprotective effects.13,18 Flavonoids are generally obtained from natural resources through solvent extraction and further purification. The conventional separation approach of flavonoids is a multistep process normally including solid−liquid extraction, liquid− liquid extraction, and column chromatography separation. However, this process is time and solvent consuming, and is difficult to achieve high recovery of the products.19 Adsorption is one of the most widely used methods for isolation of © 2017 American Chemical Society

Received: March 16, 2017 Accepted: July 6, 2017 Published: July 20, 2017 3178

DOI: 10.1021/acs.jced.7b00268 J. Chem. Eng. Data 2017, 62, 3178−3186

Journal of Chemical & Engineering Data

Article

and pore diameter of the mesoporous carbon were determined by N2 adsorption−desorption isotherms at 77 K on an ASAP 2020 system (Micromeritics, U.S.A.). The carbon sample was degassed at 573 K for 6 h prior to the measurement. Fourier transform IR (FTIR) spectroscopy of the sample was collected in FTIR spectrometer Bruker Vertex 70. Transmission electron microscopy (TEM) images were collected on a Joel JEM-2100F microscope. Small-angle X-ray diffraction (XRD) was carried out on a Rigaku Ultima IV diffractometer with CuKα radiation (40 kV, 20 mA). Elemental analysis was conducted on a Vario MICRO cube Elementar, and oxygen content was calculated by difference. 2.4. Adsorption Isotherms and Desorption. About 0.0100 g of the carbon sample was introduced into a conical flask containing 100 mL of flavonoid aqueous solution of known concentration. The flasks were covered and shaken at 220 rpm in a TENSUC TS-2102C shaker for 9 h, and the feed temperature was set at 298, 308, or 318 K. The absorbances of the adsorbed solutions were detected by a UV−vis spectrometer at 206.5, 282, and 249 nm for myricetrin, naringin, and puerarin, respectively, the equilibrium concentrations (Ce, mg·mL−1) of the flavonoid solutions were calculated from their absorbances using the standard curve method. The adsorption capacities (Qe, mg·g−1) were calculated as follows

promising adsorbents for adsorptive purification of natural products; therefore, they may be potential adsorbents for separation and purification of flavonoid compounds from water. However, no report is currently available about adsorption of flavonoids on mesoporous carbons. The main aim of this work was to evaluate mesoporous carbon as an adsorbent for separating flavonoids from aqueous solutions. A mesoporous carbon was synthesized, characterized, and tested for adsorption isotherms, kinetics, and column breakthrough curves of myricetrin, puerarin, and naringin from aqueous solutions for the first time. The results were compared with those obtained on macroporous resins. The results obtained could provide references for applying mesoporous carbons to separate and purify flavonoid compounds and other natural compounds from extracts of natural resources.

2. EXPERIMENTAL SECTION 2.1. Materials. Triblock copolymer Poloxamer 407 was obtained from BASF Corp., and tetraethyl orthosilicate (TEOS) and formalin (36.5−38 wt % formaldehyde) were provided by Sigma-Aldrich. Ethanol was from Hangzhou Gaojing Fine Chemical Co., Ltd., HF was from Shanghai Ling Feng Chemical Reagent Co., Ltd., HCl was from Zhejiang Zhongxing Chemical Reagent Co., Ltd., phenol was from Sinopharm Chemical Reagent Co., Ltd., NaOH was from Hangzhou Xiaoshan Chemical Reagent Corp., and all chemicals were analytical reagent grade. Myricetrin was a product of Nanjing Zelang Medical Technology Co., Ltd. (Nanjing, China), and puerarin and naringin were supplied by Xi’an Huilin Biotech Co., Ltd. (Xi’an, China); they all have a higher than 98% purity. Their main properties including IUPAC systematic names, formula, structure, chemical name, molecular weight, and molecular size calculated from Gaussian software20 are listed in Table S1 (chemical sample description table) in Supporting Information. All reagents were used as received. 2.2. Synthesis of the Mesoporous Carbon. Twenty-four grams of phenol and 15.72 g of formalin were mixed with 1.03 g of a NaOH solution (20 wt %) to prepare a soluble phenolic resin through a reported approach.34,37 The obtained product was dispersed in alcohol to form a 20% mass fraction solution. Tetraethyl orthosilicate (TEOS) (44.58 mL) was mixed with 15.0 g of a HCl solution (0.2 mol·L−1) and 75.95 mL of alcohol, then the mixture was then prehydrolyzed for 5 h at ambient temperature, followed by the addition of 75.0 g of the phenolic resin resol alcoholic solution, 16.0 g of Poloxamer 407, and 151.00 mL of alcohol under stirring until a solution was formed. The solution was transferred into a Petri dish without cover to evaporate the alcohol at ambient temperature for 24 h, the product was then heated in an oven at 373 K for 24 h. After that, the obtained film was calcined in a nitrogen atmosphere at 623 K for 5 h, then 1173 K for 4 h, and the heating rate was set to be 5 °C·min−1 throughout the whole calcination process. The product was leached with a HF solution (10 wt %) for 24 h to dissolve silica constituent, then filtrated, washed, and dried. The final carbon product was crushed into small particles, sieves were applied to separate carbon particles of different size classes, and the carbon particles with sizes between 0.18 and 0.45 mm were collected for subsequent use. 2.3. Characterization. The specific surface area calculated using Brunauer−Emmett−Teller (BET) method, pore volume,

Q e = (C0 − Ce) ·V /m

(1)

where C0 (mg·mL−1) is the initial concentration of the flavonoids, V (mL) is the volume of the solution and m (g) is the mass of the mesoporous carbon. The flavonoid loaded mesoporous carbon was collected by filtration and desorbed with a 50 mL of 70% (volume fraction) ethanol aqueous solution under shaking for 9 h at 298 K. Three consecutive adsorption−desorption cycles were conducted. The concentration of the desorbed flavonoid (Cd, mg·mL−1) in the ethanol aqueous solution was analyzed by the UV−vis spectrometer and the desorption rate was calculated as D=

Cd·Vd (C0 − Ce) ·V

(2)

where Vd (mL) is the volume of the eluent (mL); D is the desorption rate (%). 2.5. Adsorption Kinetics. About 0.0800 g of the adsorbent sample was added into a 200 mL of flavonoid aqueous solution with a known initial concentration (0.1 mg·mL−1 for myricetrin and naringin, and 0.3 mg·mL−1 for puerarin) in a glass flask. The mixture was then shaken at a set temperature until a preliminary adsorption equilibrium was established. At each preset interval, 1.00 mL of the flavonoid solution was withdrawn, the concentration (Ct, mg·mL−1) at the contact time was detected, and the adsorption capacity (Qt, mg·g−1) was calculated by

Q t = (C 0 − C t ) · V / m

(3)

2.6. Dynamic Breakthrough Curves. Three milliliters (Bed volume, BV) of the adsorbent was packed in a glass column (internal diameter: 10 mm) and rinsed by 9.0 mL of deionized water. A flavonoid water solution with a set concentration (0.14 mg·mL−1 for myricetrin, 0.4 mg·mL−1 for puerarin, and 0.15 mg·mL−1 for naringin) was passed through the column bed at a flow rate of 0.5 mL/min (10 BV·h−1). The concentration (C, mg·mL−1) of the flavonoid model 3179



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compounds in the effluent was continuously determined until it reached to the initial value. Dynamic adsorption capacities of myricetrin, puerarin, and naringin were calculated from their breakthrough curves using trapezoidal numerical integration method.

3. RESULTS AND DISCUSSION 3.1. Characterization. The nitrogen adsorption−desorption isotherms and the pore diameter distribution calculated Figure 3. TEM images of the mesoporous carbon.

Figure 1. Nitrogen adsorption−desorption isotherms of the mesoporous carbon at 77K (a) and pore diameter distribution of the mesoporous carbon (b).

Figure 4. Experimental adsorption isotherms and model fitting curves of (a) myricetrin, (b) puerarin, and (c) naringin on the mesoporous carbon at 298, 308, and 318 K: time = 540 min.

Figure 2. FT-IR spectroscopy of the mesoporous carbon. 3180



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Table 1. Adsorption Isotherm Fitting Parameters for Myricetrin, Puerarin, and Naringin from Langmuir and Freundlich Modelsa Langmuir model temperature (K)

Qm (mg·g−1)

KL (mL·mg−1)

R2

KF [(mg·g−1)·(mL·mg−1)1/n]

1/n

R2

myricetrin

298 308 318 298 308 318 298 308 318

272.4 311.7 218.4 127.7 141.7 125.3 209.3 178.9 234.6

56.98 32.53 130.8 21.42 16.07 21.59 66.72 165.4 67.37

0.9802 0.9917 0.9778 0.9673 0.9951 0.9815 0.9906 0.9996 0.9840

808.8 988.8 497.7 279.7 317.4 282.0 421.3 274.0 484.1

0.4562 0.5433 0.3144 0.4979 0.5565 0.5099 0.3304 0.1872 0.3371

0.9902 0.9937 0.9901 0.9917 0.9980 0.9881 0.9996 0.9929 0.9978

puerarin

naringin

a

Freundlich model

flavonoids

Uncertainty: ΔT = 1 K.

Figure S1 in Supporting Information, no diffraction peak could be readily observed for the mesoporous carbon despite the mesopore structure from nitrogen adsorption−desorption isotherms, which might due to the disordered mesostructure of the mesoporous carbon. 3.2. Adsorption Isotherms and Desorption. Figure 4 depicts the adsorption isotherms of myricetrin, puerarin and naringin on the mesoporous carbon at 298, 308, and 318 K, it can be found that the mesporous carbon can effectively adsorb the three flavonoids from their aqueous solutions. Figure 4a shows that the equilibrium adsorption capacities of myricetrin on the carbon are between 194 and 206 mg·g−1 at 0.05 mg·mL−1 (equilibrium concentration), which are higher than that on some commercial macroporous resins such as HPD300, HPD100B, AB-8, and HPD722 at similar conditions obtained in our previous work.20 The adsorption capacities of puerarin (Figure 4b) at 0.05 mg·mL−1 are between 60 and 63 mg·g−1, which are a slight lower than that on HPD300 but higher than that on HPD100B, AB-8, and HPD722.20 The BET surface area of the mesoporous carbon synthesized in this study is 857.8 m2·g−1, which is equal to the surface area of HPD300 (800−870 m2·g−1) and larger than that of HPD100B, AB-8, and HPD722 and this might be the reason for the effective adsorption of myricetrin and puerarin on the carbon sample. However, the adsorption capacities of naringin (Figure 4b) at 0.05 mg·mL−1 are ranging from 156 to 178 mg·g−1, which are not as high as that on HPD300, HPD100B, AB-8, and HPD722.20 The two glycosyls of naringin should make it exhibit higher solubility in water than myricetrin and puerarin and lead to a relatively lower adsorption capacity on the mainly hydrophobic mesoporous carbon. Moreover, naringin has the largest molecular size (Table S1) among all the three flavonoids, which could make it difficult to be adsorbed through pore adsorption, and this could be another reason for the lower adsorption capacity. These results suggested that the mesoporous carbon might be more suitable for the adsorptive separation of flavonoids with few glycosyls and small molecular size.

Figure 5. Adsorption−desorption cycles of naringin on the mesoporous carbon at 298 K: adsorption and desorption time = 540 min, initial concentration of naringin = 0.1 mg·mL−1, 70% alcohol aqueous solution for desorption.

from the desorption branch of the mesoporous carbon were plotted in Figure 1a,b, respectively. The BET specific surface area of the mesoporous carbon sample was measured to be 857.8 m2·g−1, the pore volume was determined to be 0.91 cm3·g−1, the pore diameter was 4.20 nm, and mesopores with diameters between 2 and 5 nm are the major pore structures in the prepared carbon sample. The FTIR spectra was shown in Figure 2, the mesporous carbon is poor in functional groups, only few oxygen containing groups such as OH (around 3435 cm−1), CO (between 1000 and 1200 cm−1), and CO (between 1600 and 1700 cm−1) could be observed, and the weak peaks at 2853 and 2922 cm−1 and the peaks between 1370 and 1470 cm−1 are ascribed to CH bonds. As seen from the elemental compositions listed in Table S1 in Supporting Information, the mesoporous carbon has an oxygen content of 37.42%, which further confirms the oxygenic groups on the surface of the mesoporou carbon. TEM images (Figure 3) reveal a disordered and homogeneous porous structure in the interior of the mesoporous carbon. As shown in the small-angle XRD pattern displayed in

Table 2. Calculated Thermodynamic Parameters of Myricetrin, Puerarin, and Naringin According to Freundlich Model ΔG0 (KJ·mol−1) flavonoids

298 K

308 K

318 K

ΔH0 (KJ·mol−1)

myricetrin puerarin naringin

−25.89 −22.55 −26.43

−25.73 −22.59 −28.75

−28.93 −23.85 −28.44

19.95 −2.881 3.119

3181

ΔS0 (J·mol−1·K

−1

)

151.9 65.31 100.6 DOI: 10.1021/acs.jced.7b00268 J. Chem. Eng. Data 2017, 62, 3178−3186


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Freundlich isotherm: Q e = KFCe1/ n

where Qm (mg·g−1) is the theoretical maximum monolayer adsorption capacity, KL (mL·mg−1) is the Langmuir constant related to the adsorption energy reflecting the affinity between the adsorbate and adsorbent,38,39 KF ((mg·g −1)·(mL·mg−1)1/n) and n are the Freundlich constants. KF is an indicator of the relative adsorption capacity, n is related to the magnitude of the adsorption driving force and heterogeneity of the binding sites,38 and 1/n indicates the favorability of the adsorption. The parameter values obtained with the correlation coefficients (R2) for the Langmuir and Freundlich equations were summarized in Table 1. The Freundlich isotherm equation yields a slightly better correlation coefficient than the Langmuir model for all the three flavonoids, which indicates that the adsorption of the flavonoids on the mesoporous carbon is not a simple monolayer adsorption process. On the other hand, the 1/n values from the Freundlich model for myricetrin, puerarin, and naringin at 298, 308, and 318 K are smaller than 1, which represents favorable adsorption for all the three flavonoids on the mesoporous carbon.38 Moreover, the 1/n values of myricetrin, puerarin, and naringin at 298 K were calculated to be 0.4562, 0.4979, and 0.3304, respectively, which are less than 0.5, implying that the adsorption of the three flavonoids on the carbon adsorbent at 298 K could take place easily.40 The results of three consecutive adsorption−desorption cycles of naringin on the mesoporous carbon were presented in Figure 5. It could be seen that the desorption rate after one cycle is higher than 99%, which is similar to that from macroporous resins,20 and both the percentage adsorption capacity and desorption rate of naringin after two cycles are higher than 90%, implying a good reusability of the mesoporous carbon. However, the adsorption capacity and desorption rate decreased faster after three cycles, which might be due to the weak chemical interaction between naringin and the mesoporous carbon. Other desorption solvents would be attempted for more efficient desorption of flavonoid compounds from the mesoporous carbon. 3.3. Adsorption Thermodynamics. Adsorption thermodynamic parameters such as adsorption enthalpy (ΔH0), adsorption entropy (ΔS0) and adsorption Gibbs free energy (ΔG0) were calculated using following equations reported in literatures41

Figure 6. Experimental adsorption kinetics and model fitting curves of (a) myricetrin, (b) puerarin, and (c) naringin on the mesoporous carbon at 298, 308, and 318 K: initial concentration of myricetrin and naringin = 0.1 mg·mL−1, initial concentration of puerarin = 0.3 mg·mL−1.

As can be seen in Figure 4, adsorption capacities of the three flavonoids do not present a linear correlation with the feed temperatures, indicating that the adsorption of myricetrin, puerarin, and naringin on the mesoporous carbon is not a total physical adsorption process. Myricetrin, puerarin, and naringin are three flavonoid glycosides containing multiple hydroxyl groups, which might form hydrogen bonds with the residual O-containing functional groups on the mesoporous carbon and offer weak chemical adsorption for the three adsorbates. Langmuir and Frendlich adsorption equations were applied to analyze the experimental isotherm data of the three flavonoids. The two models are given as Langmuir isotherm: Q e =

ΔG° = −RT ln Keq

(6)

ΔG = ΔH ° − T ΔS °

(7)

Keq

⎛ = K f M w (T )⎜ ⎜ ⎝

⎞(1 − 1/ n) ⎟ MW (T) ⎟ 1000 ⎠ 1

(8)

where, T (K) is the temperature, R (8.314 J·mol−1·K−1) is the ideal gas constant, Keq is the adsorption equilibrium constant, Kf ((mg·g−1)·(L·mg−1)1/n) and n are the Freundlich constants, Kf = KF·1000(−1/n), Mw(T) (mg·L −1) (g) is the mass of water per liter at a temperature T. The calculated values of the thermodynamic parameters were summarized in Table 2. The negative ΔG values of all the three flavonoids at 298, 308, and 318 K imply a spontaneous adsorption process for all the three adsorbates on the

Q mKLCe 1 + KLCe

(5)

(4) 3182



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Table 3. Adsorption Kinetic Fitting Parameters of Myricetrin, Puerarin, and Naringin from Pseudo-First-Order and Pseudo-Second-Order Rate Equationsa pseudo-first-order flavonoids

temperature (K)

myricetrin

298 308 318 298 308 318 298 308 318

puerarin

naringin

a

k1 (min−1) 1.131 1.161 1.457 1.041 1.288 1.109 7.810 8.440 9.700

× × × × × × × × ×

pseudo-second-order

qe (mg·g−1)

−2

10 10−2 10−2 10−2 10−2 10−2 10−3 10−3 10−3

R2

164.8 143.1 161.4 121.4 134.4 116.3 123.9 157.6 147.0

0.9500 0.9553 0.9420 0.9889 0.9879 0.9880 0.9928 0.9959 0.9977

qe (mg·g−1) 201.7 174.5 191.0 156.6 166.8 147.6 160.6 202.3 184.9

k2 (g·mg−1·min−1) 6.000 7.000 9.000 6.000 8.000 7.000 4.000 4.000 5.000

× × × × × × × × ×

−5

10 10−5 10−5 10−5 10−5 10−5 10−5 10−5 10−5

v0 (mg·g−1·min−1)

R2

2.441 2.132 3.283 1.471 2.226 1.525 1.032 1.637 1.709

0.9725 0.9785 0.9742 0.9797 0.9778 0.9881 0.9980 0.9984 0.9980

Uncertainty: ΔT = 1 K.

mesoporous carbon, and the ΔS values for the three flavonoids are positive, which comfirms this deduction. The ΔH0 value of puerarin on the mesoporous carbon is negative that suggests an exothermic process for the adsorption of puerarin. However, the positive ΔH0 values for myricetrin and naringin imply an endothermic adsorption process of these two flavonoids, which might due to the nonlinear correlation between the adsorption capacities and the feed temperatures for both myricetrin and naringin, and these results further confirm the chemical interaction such as weak hydrogen bonding interaction between the flavonoids and the surface of the mesoporous carbon. 3.4. Adsorption Kinetics. Figure 6 presents the adsorption kinetic curves of myricetrin, puerarin and naringin on the mesoporous carbon at 298, 308, and 318 K. The adsorption of all the three flavonoids on the mesoporous carbon is fast, and the time needed to establish adsorption equilibrium for myricetrin, puerarin and naringin at 298 K is 300, 300, and 360 min (up to 95% uptake), respectively, which is equivalent to the time needed for myricetrin, puerarin and naringin at 298 K on commercial macroporous resins HPD300, HPD100B, AB-8, and HPD722.20 The good kinetic properties of the mesoporous carbon indicate its application as an efficient adsorbent for adsorptive separation of flavonoid compounds. Two kinetic models, pseudo-first-order model 42 and pseudo-second-order model43 were used to elucidate the experimental data, and these two models can be expressed as follows Pseudo‐first‐order: Q t = qe(1 − e−k1t ) Pseudo‐second‐order: −1

(9)

t 1 1 = + t Qt qe k 2qe2 −1

describe the adsorption of divalent metal ions and dyes on peat43,44 and the main assumption of this model was that the adsorption rate limiting step may be due to chemical adsorption through valence forces.38,42 The higher correlation coefficients from pseudo-second-order model for myricetrin further confirm the weak chemical adsorption of myricetrin on the mesoroporous carbon. In contrast, pseudo-first-order equation is slightly better than pseudo-second-order model for fitting the kinetic data of puerarin. Puerarin has the least amount of hydroxyl groups among the three flavonoids, which might result in a weaker chemical interaction between puerarin and the mesoporous carbon. The correlation coefficients from both the pseudo-first-order and pseudo-second-order models for naringin exceeded 0.99, implying that both equations can be used to describe the adsorption kinetics of naringin. Moreover, k1 and k2 for myricetrin and puerarin at 298 K have similar values while the values of k1 and k2 for naringin at 298 K are smaller, which agrees with the order of the required time for adsorption equilibrium of the three flavonoids and indicats a faster adsorption rate of myricetrin and puerarin than that of naringin on the mesoporous carbon. These results might be because that naringin has a larger molecular size than myricetrin and puerarin, which could result in a lower diffusion rate of naringin in the pores and further lead to a slower adsorption. 3.5. Dynamic Adsorption. Figure 7 presented the dynamic adsorption breakthrough curves of myricetrin, puerarin, and naringin on mesoporous carbon columns at room temperature. The breakthrough point (C·C0−1 = 0.05) of myricetrin (Figure 7a) was measured to be 124 BV, and the dynamic adsorption capacity of myricetrin was calculated to be 173.3 mg·g−1. The breakthrough point of puerarin (Figure 7b) was 115 BV, and the dynamic adsorption capacity of puerarin was calculated to be 336.7 mg·g−1, which is much larger than the theoretical monolayer adsorption capacity obtained from the fitting results of Langmuir model at 298 K (127.7 mg·g−1), similar results have been reported in previous literature,45 and these results indicate a nonmonolayer adsorption process of puerarin on the mesoporous carbon. For naringin (Figure 7c), the breakthrough point was 78 BV, its dynamic adsorption capacity was 251.5 mg·g−1, and the exhaustion point (95% final concentration) was up to 500 BV that further confirmed the slow diffusion rate of naringin on the mesoporous carbon. The breakthrough curves of the three flavonoids were described through Adams-Bohart, Thomas, Yoon and Nelson, and Clark models,46−48 which are expressed as follows

(10)

−1

where k1 (1·min ) and k2 (g·mg ·min ) are the rate constants of pseudo-first-order rate equation and pseudosecond-order rate equation, respectively, qe (mg·g−1) is the theoretical adsorption capacity at equilibrium. The parameters obtained from the two kinetic models along with the correlation coefficients R2 were listed in Table 3. As shown in Table 3, the correlation coefficients of pseudofirst-order kinetic model for myricetrin are lower than those from pseudo-second-order model, which implies the applicability of pseudo-second-order equation on the adsorption kinetics of myricetrin on the mesoporous carbon. The pseudosecond order equation was first proposed by Ho and McKay to 3183



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Clark model ⎛ ⎞1/ n − 1 C 1 ⎟ =⎜ − rt ⎝ 1 + Ae ⎠ C0

(14)

where C (mg·mL −1 ) in all models is the flavonoid concentration in the effluent. In the Adams-Bohart model, k [mL/(min·mg)] is the Adams-Bohart kinetic constant, Z (cm) is the column length, N0 (mg·mL−1) is the saturation concentration, and U (cm·min−1) is the linear flow rate. In the Thomas model, kT (mL·min−1·mg−1) is the Thomas rate constant, q0 (mg·g−1) is the adsorption capacity at complete exhaustion point, mC (g) is the adsorbent mass, and Q (mL·min−1) is the flow rate. In the Yoon and Nelson model, kYN (min−1) is the Yoon-Nelson rate constant and t0.5 (min) is the time at C/C0 = 0.5. A and r in the Clark model are the Clark constants related to the breakthrough time. The fitness of the four models was displayed in Figure 7 and the model parameters were presented in Table 4. All the four models can fit the breakthrough curves of myricetrin and puerarin very well (R2 > 0.999), and the four fitting curves completely overlapped. Moreover, both the N0 values from the Adams-Bohart model (35.95 and 69.41 mg·mL−1 (177.5 and 342.7 mg·g−1) for myricetrin and puerarin, respectively) and the q0 values from the Thomas model (177.5 and 342.7 mg·g−1 for myricetrin and puerarin, respectively) are closed to the experimental dynamic adsorption capacities of myricetrin and puerarin, and the t0.5 values from the Yoon and Nelson model (1541 and 1041 min for myricetrin and puerarin, respectively) are also close to the experimental values of 1542 and 1043 min. These results imply that all these models are suitable for the dynamic behavior prediction of myricetrin and puerarin in a mesoporous carbon column. On the other side, the R2 values of the Adams-Bohart, Thomas, Yoon and Nelson models for the breakthrough curve of naringin (R2 > 0.992) are slightly higher than that from the Clark model (R2 > 0.985), indicating a better validity of these three models for describing the dynamic breakthrough curves of naringin.

4. CONCLUSIONS The capability of the mesoporous carbon for adsorbing myricetrin, puerarin, and naringin was investigated. The BET specific surface area, pore volume, and average pore diameter of the synthesized mesoporous carbon adsorbent were measured to be 857.8 m2·g−1, 0.91 cm3·g−1, and 4.20 nm, respectively. The adsorption equilibrium capacities of myricetrin, puerarin, and naringin on the mesoporous carbon at 0.05 mg·mL−1 and 298 K are 206, 63, and 156 mg·g−1, respectively, the adsorption isotherms of the three flavonoids were found to be in a better fitness with the Freundlich isotherm model, and the mesoporous carbon should be more suitable for the adsorptive separation of small molecular size flavonoids. In addition, the adsorption of the flavonoids on the mesoporous carbon is fast, 300, 300, and 360 min are needed to establish adsorption equilibrium at 298 K for myricetrin, puerarin, and naringin, respectively. The column adsorption experiments confirmed the effective adsorption of the three flavonoids on the mesoporous carbon. The results obtained could be applied to the adsorptive separation and purification of flavonoids using mesoporous carbons as adsorbents.

Figure 7. Experimental adsorption breakthrough curves of (a) myricetrin, (b) puerarin, and (c) naringin in water from mesoporous carbon columns at room temperature: initial concentration of myricetrin = 0.14 mg·mL−1, initial concentration of puerarin = 0.4 mg·mL−1, initial concentration of naringin = 0.15 mg·mL−1.

Adams-Bohart model C 1 = kN Z C0 1 + exp(ln(exp U0 − 1) − kC0t )

( )

(11)

Thomas model C 1 = C0 1 + exp[k T

(

q0mc Q − C0t

)

(12)

Yoon and Nelson model exp(k YNt − k YNt0.5) C = C0 1 + exp(k YNt − k YNt0.5)

(13) 3184



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Table 4. Fitting Parameters for the Breakthrough Curves of Myricetrin, Puerarin, and Naringin from Adams-Bohart, Thomas, Yoon and Nelson, and Clark Models Adams-Bohart model

■

flavonoids

k (mL·min−1·mg−1)


0.04434 0.02098 0.01297

Thomas model

N0 (mg·mL−1) 35.95 69.41 53.51 Yoon and Nelson model

R2

kt (mL·(min·mg)−1)

0.999 0.999 0.992

0.04434 0.02098 0.01297

0.999 0.999 0.992

t0.5 (min)

R2

A

r

R2

0.006210 0.008390 0.001940

1541 1041 2133

0.999 0.999 0.992

6465 6101 11.57

0.005860 0.008380 0.001590

0.999 0.999 0.985


(3) Rice-Evans, C. A.; Miller, N. J.; Paganga, G. Structure-antioxidant activity relationships of flavonoids and phenolic acids. Free Radical Biol. Med. 1996, 20, 933−956. (4) Harborne, J. B.; Williams, C. A. Advances in flavonoid research since 1992. Phytochemistry 2000, 55, 481−504. (5) Pan, M. H.; Lai, C. S.; Ho, C. T. Anti-inflammatory activity of natural dietary flavonoids. Food Funct. 2010, 1, 15−31. (6) Nijveldt, R. J.; van Nood, E.; van Hoorn, D. E.; Boelens, P. G.; van Norren, K.; van Leeuwen, P. A. Flavonoids: a review of probable mechanisms of action and potential applications. Am. J. Clin. Nutr. 2001, 74, 418−425. (7) Prochazkova, D.; Bousova, I.; Wilhelmova, N. Antioxidant and prooxidant properties of flavonoids. Fitoterapia 2011, 82, 513−523. (8) Savi, L. A.; Caon, T.; de Oliveira, A. P.; Sobottka, A. M.; Werner, W.; Reginatto, F. H.; Schenkel, E. P.; Barardi, C. R.; Simoes, C. M. Evaluation of antirotavirus activity of flavonoids. Fitoterapia 2010, 81, 1142−1146. (9) Gao, T.; Jin, X.; Tang, W.; Wang, X.; Zhao, Y. New geranylated flavanones from the fruits of Paulownia catalpifolia Gong Tong with their anti-proliferative activity on lung cancer cells A549. Bioorg. Med. Chem. Lett. 2015, 25, 3686−3689. (10) Passreiter, C. M.; Suckow-Schnitker, A.-K.; Kulawik, A.; AddaeKyereme, J.; Wright, C. W.; Wätjen, W. Prenylated flavanone derivatives isolated from Erythrina addisoniae are potent inducers of apoptotic cell death. Phytochemistry 2015, 117, 237−244. (11) Buer, C. S.; Imin, N.; Djordjevic, M. A. Flavonoids: new roles for old molecules. J. Integr. Plant Biol. 2010, 52, 98−111. (12) Xu, F. M.; Matsuda, H.; Hata, H.; Sugawara, K.; Nakamura, S.; Yoshikawa, M. Structures of New Flavonoids and Benzofuran-Type Stilbene and Degranulation Inhibitors of Rat Basophilic Leukemia Cells from the Brazilian Herbal Medicine Cissus sicyoides. Chem. Pharm. Bull. 2009, 57, 1089−1095. (13) Zhao, J. Y.; Li, L.; Jiao, F. P.; Ren, F. L. Human plasma protein binding of water soluble flavonoids extracted from citrus peels. J. Cent. South Univ. 2014, 21, 2645−2651. (14) Chung, S. K.; Kim, Y. C.; Takaya, Y.; Terashima, K.; Niwa, M. Novel Flavonol Glycoside, 7-O-Methyl Mearnsitrin, from Sageretia theezans and Its Antioxidant Effect. J. Agric. Food Chem. 2004, 52, 4664−4668. (15) Aderogba, M. A.; Ndhlala, A. R.; Rengasamy, K. R.; Van Staden, J. Antimicrobial and selected in vitro enzyme inhibitory effects of leaf extracts, flavonols and indole alkaloids isolated from Croton menyharthii. Molecules 2013, 18, 12633−12644. (16) Teng, Y.; Cui, H.; Yang, M.; Song, H.; Zhang, Q.; Su, Y.; Zheng, J. Protective effect of puerarin on diabetic retinopathy in rats. Mol. Biol. Rep. 2009, 36, 1129−1133. (17) Xiong, F.; Sun, X.; Gan, L.; Yang, X.; Xu, H. Puerarin protects rat pancreatic islets from damage by hydrogen peroxide. Eur. J. Pharmacol. 2006, 529, 1−7. (18) Rong, W.; Wang, J.; Liu, X.; Jiang, L.; Wei, F.; Hu, X.; Han, X.; Liu, Z. Naringin treatment improves functional recovery by increasing BDNF and VEGF expression, inhibiting neuronal apoptosis after spinal cord injury. Neurochem. Res. 2012, 37, 1615−1623.

ASSOCIATED CONTENT

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.7b00268. Small-angle XRD pattern of the mesoporous carbon; properties of myricetrin, puerarin, and naringin; main properties of the mesoporous carbon; experimental nitrogen adsorption−desorption isotherm data of the mesoporous carbon; pore diameter distribution data of the mesoporous carbon; experimental adsorption isotherm data of myricetrin, puerarin, and naringin; experimental data of adsorption−desorption cycles for naringin; experimental adsorption kinetic data of myricetrin, puerarin, and naringin; experimental dynamic adsorption breakthrough data of myricetrin, puerarin, and naringin (PDF)

AUTHOR INFORMATION

Corresponding Author

*Tel: 86-571-85070380. E-mail address: cherryli1986@126. com. ORCID

Yin Li: 0000-0001-6920-9388 Funding

S.S. received funding from International Science and Technology Cooperation Program of China (2014DFE90040), R.Y. received funding from Zhejiang Province Research Project of Public Welfare Technology Application (2015C33006), Y.L. received funding from Foundation of Zhejiang Provincial Key Lab. for Chem. and Bio. Processing Technology of Farm Products, Zhejiang University of Science and Technology (2016KF0021), J.Z. received funding from Science and Technology Project of Zhejiang Province (2014C37063), and Q.G. received funding from Analysis and Test Funding Project of Zhejiang Province (2015C37059). Notes

The authors declare no competing financial interest.

■

177.5 342.7 263.2 Clark model

R2

KYN (min−1)

S Supporting Information *

■

q0 (mg·g−1)

REFERENCES

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Adsorption of Three Flavonoids from Aqueous Solutions onto

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