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Novel reduction of Cr(VI) from wastewater using a naturally derived microcapsule loaded with rutin–Cr(III) complex
Journal of Hazardous Materials 285 (2015) 336–345
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Novel reduction of Cr(VI) from wastewater using a naturally derived microcapsule loaded with rutin–Cr(III) complex Yun Qi a,∗ , Meng Jiang a , Yuan-Lu Cui b,∗∗ , Lin Zhao a , Shejiang Liu a a Tianjin Technological Engineering Center on Biomass-Derived Gas and Oil, Faculty of Environmental Science and Engineering, Tianjin University, No. 92, Weijin Rd., Nankai District, Tianjin 300072, China b Tianjin State Key Laboratory of Modern Chinese Medicine, Tianjin University of Traditional Chinese Medicine, Tianjin 300193, China
h i g h l i g h t s • • • •
Low-cost hexavalent chromium reduction microcapsule was prepared with natural material. Rutin–Cr(III) complex was synthesized and characterized. Rutin–Cr(III) complex exhibited strong reducibility of Cr(VI). Rutin–Cr(III) alginate–chitosan microcapsule can reduce Cr(VI) steadily and prolongedly.
a r t i c l e
i n f o
Article history: Received 17 August 2014 Received in revised form 12 November 2014 Accepted 6 December 2014 Available online 10 December 2014 Keywords: Rutin Flavonoid–metal complex Rutin–Cr(III) Alginate–chitosan microcapsule Cr(VI) reduction
a b s t r a c t The harmfulness of carcinogenic hexavalent chromium (Cr(VI)) is dramatically decreased when Cr(VI) is reduced to trivalent chromium (Cr(III)). Rutin, a natural flavonoid, exhibits excellent antioxidant activity by coordinating metal ions. In this study, a complex containing rutin and Cr(III) (rutin–Cr(III)) was synthesized and characterized. The rutin–Cr(III) complex was much easier to reduce than rutin. The reduction of the rutin–Cr(III) complex was highly pH-dependent, with 90% of the Cr(VI) being reduced to Cr(III) in 2 h under optimal conditions. A biodegradable, sustained-release system encapsulating the rutin–Cr(III) complex in a alginate–chitosan microcapsule (rutin–Cr(III) ACMS) was also evaluated, and the reduction of Cr(VI) was assessed. This study also demonstrated that low-pH solutions increased the reduction rate of Cr(VI). The environmentally friendly microcapsules can reduce Cr(VI) for prolonged periods of time and can easily biodegrade after releasing the rutin–Cr(III) complex. Given the excellent performance of rutin–Cr(III) ACMS, the microcapsule system represents an effective system for the remediation of Cr(VI) pollution. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Chromium and chromium compounds are used extensively in leather tanning, electroplating, textile dyeing, stainless steel production and pigmentation industries. Thus, large quantities of chromium are discharged into the environment [1]. In China, approximately 1.34 × 104 t of chromium was discharged into water bodies from 1990 to 2009, mainly from the fabricated metal and leather tanning industries [2]. Under natural conditions, chromium generally exists in one of two valence states, trivalent chromium (Cr(III)) or hexavalent chromium (Cr(VI)). The hexavalent
∗ Corresponding author. Tel.: +86 22 27891291/13752219121. ∗∗ Corresponding author. Tel.: +86 22 59596170. E-mail addresses: [email protected] (Y. Qi), [email protected] (Y.-L. Cui). http://dx.doi.org/10.1016/j.jhazmat.2014.12.008 0304-3894/© 2014 Elsevier B.V. All rights reserved.
oxidation state of chromium (Cr(VI)) is known to be highly toxic and carcinogenic, thereby posing a threat to both humans and animals [3]. Cr(VI) is listed among the top 20 contaminants on Superfund Priority List of Hazardous Substances. In contrast, Cr(III) is estimated to be 100 times less toxic than Cr(VI) and has a lower solubility. Cr(III) is also known to be an essential dietary element [4]. Thus, the reduction of Cr(VI) to Cr(III) is a feasible method for minimizing chromium pollution. Current approaches to reducing Cr(VI) concentrations involve the reduction of Cr(VI) to Cr(III) using various chemical, physical or biological methods, such as reduction, ion exchange, membrane separation, precipitation and adsorption electrodialysis [5]. However, most of these procedures are expensive or have significant disadvantages, such as incomplete metal removal, high reagent consumption, secondary waste product generation and high energy requirements. Therefore, the
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few studies have evaluated the use of rutin–metal complex in the reduction of oxidized ions in water treatment systems [13]. Alginate is a natural binary copolymer made from linear chains of ␣-l-guluronic acid (G) and -d-mannuronic acid (M) in varying ratios. In the presence of divalent cations (such as Ca2+ ), an ionotropic effect will occur between sodium alginate — the salt form of alginates — and Na+ , leading to a regular threedimensional net structure bound by divalent cations [14]. Chitosan is a cationic polysaccharide composed of poly(-1-4)-2-amino-2deoxy-d-glucopyranose. Due to the formation of complex between protonated amine and anionic polysaccharide in sodium alginate under acidic conditions, chitosan has been shown to be an efficient adjuvant for extending the residence time in drug release systems and to reinforce the biological activity of drugs [15]. Alginate–chitosan microcapsules are traditionally used as a sustained release drug delivery system in the pharmaceutical industry [16] because they are easy to prepare, highly stable, and allow for high drug loading [17]; however, these microcapsules are seldom used in wastewater disposal with flavonoids. In this study, rutin–Cr(III) complex encapsulated in a alginate–chitosan microcapsule system (rutin–Cr(III) ACMS) were prepared using an internal gelation technique. Through this method, the rutin–Cr(III) complex was maintained in aqueous solution and Cr(VI) was reduced. 2. Materials and methods 2.1. Chemicals Rutin (purity > 95%), sodium alginate and chitosan (90% deacetylation degree) were supplied by Sangon Biotech, Co., Ltd. (Shanghai, China). Micronized CaCO3 (40 nm) was provided by Beijing Wangyong Technology Ltd. (Beijing, China). Liquid paraffin was supplied by Beifang Tianyi Chemical Reagent Company (Tianjin, China). Span 80 and Tween 80 were purchased from Sigma–Aldrich Co. (St. Louis, USA). All remaining chemicals used in this study were of analytical grade and were used without further purification. All solutions were prepared with deionized water. A dialysis membrane with a MWCO value of 7000 was provided by Beijing Solarbio Science and Technology Co., Ltd. (Beijing, China). Fig. 1. Chemical structures of (a) flavonoids, (b) rutin, and (c) rutin–Cr(III) complex.
2.2. Synthesis of rutin–Cr(III) and rutin–Cr(III) ACMS
development of a low-cost method that uses readily available materials to remove and recover Cr(VI) is warranted. The literature suggests that Cr(VI) can be removed using natural biomaterials through both direct and indirect reduction mechanisms [6]. Flavonoids are abundant natural compounds found in most plants and are concentrated in plants’ seeds, fruit skin, fruit peel, bark and flowers. A large number of flavonoids are polyphenols with a basic structure composed of two aromatic rings (benzo-␥-pyrone and benzene) linked together by three carbon atoms with a simplified configuration of C6 C3 C6 (Fig. 1(a)). In recent years, flavonoids have generated interest among researchers in many fields for their antioxidant and free-radical scavenging activities. The 3-OH groups at 5,3 ,4 and the carbonyl at position 4 in rutin (Fig. 1(b)), which is a typical flavonoid, have two possible coordination sites (i.e., catechol and 4-oxo-5-OH, as indicated by arrows) through which they can easily form complexes with metal ions [7,8]. Kostyuk et al. [9] reported that increased efficiency was achieved using rutin complexes containing Zn(II), Cu(II), Fe(II), or Fe(III) in free radical scavenging assays due to the generation of extra superoxide dismutase centers. Many studies have also reported that rutin–metal complex act as antioxidants [10–12], but
Rutin–Cr(III) was synthesized by following the luteolin–Cr(III) procedure described by Gao et al. [18]. Minor changes were made to the reaction between 1 mmol rutin and 1.5 mmol CrCl3 ·6H2 O. Based on the method used in our previous study [19], rutin–Cr(III) ACMS was prepared using a water-in-oil emulsion gelation technique by combining 0.2 g micronized CaCO3 with 20 mL 1.5% sodium alginate aqueous solution while stirring using a magnetic stir bar and heating at 45 ◦ C. Micronized CaCO3 was then uniformly dispersed, and 0.1 g rutin–Cr(III) was added (mixture A). At the same time, 100 mL of paraffin oil containing 1% Span 80 was prepared in a 500 mL three-neck round-bottom flask with a stirring speed of 300 rpm at 37 ◦ C (mixture B). Subsequently, mixture A was added to mixture B using a 10 mL injector (matched needle) with a rate of 30 drop/min at a drop height of 10 cm. The solution was then left undisturbed for 15 min to enable emulsification to occur before 0.5 mL glacial acetic in 5 mL paraffin oil was dropped to release Ca2+ [14]. The mixture was set aside for sedimentation separation. After precipitating completely, the upper oily liquid was removed, and the lower alginate microspheres were washed with 1% (v/v) glaciated Tween 80 solution three times to remove excess oil. The washing procedure was then repeated three times using chilled water.
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A 1% (w/v) chitosan solution was prepared using 1% (v/v) glacial acetic while stirring at 45 ◦ C for 4–6 h. The solution was then stored in a refrigerator overnight. The pH was adjusted to 5.5 with 1 M NaOH. The alginate microspheres obtained previously were dispersed into the chitosan solution at a ratio of 1:6 (v/v) and gently oscillated for 50 min to allow for coating with chitosan. The chitosan solution was later centrifuged, and the alginate chitosan microspheres were washed with chilled water three times. To obtain alginate chitosan liquid-core microcapsules, the microspheres were immersed in a 55 mM sodium citrate solution prepared with 5% NaCl (w/v). The ratio of microspheres to sodium citrate solution was 1:8 (v/v). The mixture was gently oscillated for 30 min. After centrifugation, the microcapsules were washed with chilled water and lyophilized. 2.3. Analytical methods 2.3.1. Characterization of rutin–Cr(III) ACMS morphology The width of the wet microcapsules’ particle distribution was estimated by a particle size analyzer (Mastersizer 2000, Malvern, UK), with the span value calculated using Eq. (1); a small span value corresponds to a highly uniform distribution. Span =
(D(0.9) − D(0.1)) D(0.5)
(1)
D(0.1, 0.5, 0.9) – volume percentage of microcapsules with a diameter D(n) of n%(%). A field emission scanning electron microscope (SEM; Nova NanoSEM 430, FEI, USA) and inverted biological microscope (Eclipse Ti-U, Nikon, Japan) were used to characterize the morphology of the microcapsules. The lyophilized microcapsules prepared in Section 2.2 were fixed with double side adhesive carbon tape and coated with gold under vacuum before SEM observation. The SEM images were observed at accelerating voltage of 3.0 kV. Lyophilized microcapsules were embedded in epon and ultrathin sections were obtained by Ultramicrotome (Lecia EM UC6, Leica, Germany). Transmission electron microscopy (TEM; H7650B, Hitachi, Japan) was applied to investigate the inner structure of the microcapsules at 80 kV. 2.3.2. UV–vis spectra UV–vis spectra were recorded in the spectral range of 200–500 nm on a UV–vis spectrometer (DU800, Beckman, USA) with a 10 mm quartz cell. The contents of rutin and the rutin–Cr(III) complex were measured in methanol and water (pH 3) with blank subtraction. 2.3.3. FT-IR analysis FT-IR spectra of samples were recorded on a FT-IR spectrometer (Nicolet 6700, Thermo Scientific, USA) with a DTGS detector. The spectra were collected in the range of 4000–400 cm−1 with a resolution of 2 cm−1 at 40 scans per spectrum. 2.3.4. Thermogravimetric analysis Thermogravimetric data (TG and DTA) for samples were obtained using a Thermogravimetric/Differemtial Thermal Analyzer (Pyris/Diamond TG/DTA, PerkinElmer, USA). All tests were conducted with samples mass of 2–5 mg at 10 ◦ C/min over a range of 20–700 ◦ C. The measurements were carried out in N2 atmosphere with a flow rate of 100 ml/min. 2.3.5. Quantitative analysis of Cr in rutin–Cr(III) complex and rutin–Cr(III) ACMS The C, H and Cr contents of the rutin–Cr(III) complex were determined using a elemental analyzer (Vario EL, Elementar, Germany).
A polarized Zeeman atomic absorption spectrophotometer (Model 180-80, Hitachi, Japan) was used to determine the Cr content in the rutin–Cr(III) complex and rutin–Cr(III) ACMS, and the loading of rutin–Cr(III) in ACMS was calculated by Eq. (2) according to the percentage score of Cr(III) in the rutin–Cr(III) complex. Rutin-Cr(III)Loading =
W1 × 100% W2
(2)
W1 – weight of rutin–Cr(III) complex in the microcapsules (mg), W2 – weight of microcapsules (mg). 2.3.6. Determination of antioxidant activity of complex by DPPH method and FRAP assay The DPPH free radical scavenging assay and the ferric reducing ability of plasma (FRAP) assay were conducted in 96 well plates to investigate the antioxidant properties of rutin and rutin–Cr(III) complex. An microplate reader (Infinite 200, Tecan, Switzerland) was used to collect the data [20]. In the DPPH free radical scavenging experiment, 0.6 L 3 × 10−4 mol/L of rutin, rutin–Cr(III) complex, and Trolox methanol solution (as a control sample) were added to 234 L 6 × 10−4 mol/L of DPPH, followed by 10 s of oscillation for complete mixing. The data were collected at 515 nm, and the temperature was held at 27 ± 0.2 ◦ C during the experiments. The percentage of DPPH remaining was calculated by Eq. (3): RemainingDPPH% =
[DPPH]t × 100% [DPPH]0
(3)
t – reaction time(min), [DPPH]0 – initial concentration of DPPH(10−4 mol/L), [DPPH]t – concentration of DPPH at t (10−4 mol/L). In the FRAP section, 5 L 0.15 × 10−4 mol/L, 0.3 × 10−4 mol/L, 0.9375 × 10−4 mol/L, 1.875 × 10−4 mol/L, 3.75 × 10−4 mol/L, 7.5 × 10−4 mol/L or 15 × 10−4 mol/L of rutin was added to 180 L FRAP working solution. The mixture was incubated at 37 ◦ C for 4 min after 10 s of mixing, and the absorbance value was measured at 593 nm. The total relative antioxidant capacity of rutin was determined by comparing with the absorbance of a 1.0 × 10−3 mol/L FeSO4 standard solution and calculated using Eq. (4). The antioxidant capacity of the rutin–Cr(III) complex was also investigated using the same methods. Relative total antioxidant capacity =
C0 CA
(4)
C0 – concentration of FeSO4 standard solution, 1 × 10−3 mol/L, A – absorbance value of 1 × 10−3 mol/L FeSO4 standard solution, CA – corresponding concentration of rutin or rutin–Cr(III) complex at the same absorbance value of A. 2.4. Reduction of Cr(VI) by rutin–Cr(III) complex and rutin–Cr(III) ACMS All experiments were performed in 500 mL volumes at 1.93 × 10−5 mol/L Cr(VI) with an agitation rate of 200 rpm. Spectrophotometric methods were applied to determine the concentration of Cr(VI) in aqueous solution [21]. The percentage of Cr(VI) reduction was calculated using Eq. (5). Percentage of Cr(VI)reduction(%) =
(C0 − Ct ) × 100% C0
(5)
C0 – initial concentration of Cr(VI) (mol/L), Ct – concentration of Cr(VI) at t (mol/L).
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Fig. 2. Morphology of rutin–Cr(III) microcapsules (ACMS). (a) Optical photomicrograph of rutin–Cr(III) ACMS, (b) SEM micrographs of rutin–Cr(III) ACMS, (c) SEM micrographs of microcapsule surfaces and morphology, (d) TEM images of the cross-section morphology of the microcapsule, (e) TEM images of the boundary regions of the microcapsule membrane. The chitosan layer was indicated by arrows, and (f) TEM images of the structural details of inner alginate. The rutin–Cr(III) complex was indicated by circles and by arrows.
2.5. XPS analysis
3. Results and discussion
The valence state of the Cr after reduction of Cr(VI) by rutin–Cr(III) complex was determined by the X-ray photoelectron spectrometer (PHI 1600, PerkinElmer, USA). The solid form of the reaction products were obtained by vacuum drying the mixture of 1.93 × 10−5 mol/L Cr(VI) and 1.23 × 10−5 mol/L rutin–Cr(III) at pH 1. CrCl3 ·6H2 O and K2 Cr2 O7 (Sangon Biotech, China) were used as Cr(III) and Cr(VI) reference compounds, respectively. Experiment was carried out with an AlK˛ (h = 1667.0 eV) X-ray source at 250.0 W and pass energy of 29.35 eV for high-resolution analyses. The vacuum in the analysis chamber was kept below 3 × 10−6 Pa during measurement. The binding energy of the spectra was calibrated with aliphatic carbons C1s(284.6 eV).
3.1. Morphology characterization of rutin–Cr(III) ACMS The morphology and structure of rutin–Cr(III) ACMS was studied by optical inverted microscope, SEM and TEM (Fig. 2). The spherical rutin–Cr(III) ACMS under wet conditions was observed by inverted microscope, which was showed in Fig. 2(a). The D(0.5) of rutin–Cr(III) ACMS calculated using Eq. (1) was 235 m and the computed span value was 1.10, which suggested that the microcapsules obtained in this experiment were uniform in size. The SEM results presented in Fig. 2(b) indicated that the lyophilized microcapsules exhibited good spherical shape in geometry and it was consistent with its image in wet conditions. The microcapsule
Fig. 3. FT-IR spectra of (a) alginate, chitosan, and blank microcapsules, (b) rutin and rutin–Cr(III) complex.
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Fig. 4. DTA-TG-DTG of (a) sodium alginate, (b) chitosan, (c) blank microcapsules, (d) rutin, and (e) rutin–Cr(III) complex.
presented homogeneous properties generally though some bulges observed on the surface. Detailed examination of the membrane topology in Fig. 2(c) illustrated compact and denser chitosan layer in the outmost of microcapsules [22]. The internal morphology analysis of ultrathin sections was conducted by the TEM (Fig. 2(d)–(f)). The cross-section observation in Fig. 2(d) and (f) exhibited that rutin–Cr(III) complex was distributed evenly in the microcapsules. The remaining clumps of complex may be the aggregation of rutin–Cr(III) complex caused by freeze-drying. Fig. 2(e) showed clear microcapsule membrane on the boundary region. Thus, this roughly 200 nm thick film demonstrated the presence of surface chitosan layer [23,24]. One approach to provide information on molecular interaction of microcapsules was to monitor changes in the FT-IR spectra of their component parts. The FT-IR spectra of alginate, chitosan and blank microcapsules were shown in Fig. 3(a). In the spectra of alginate, the wide band at 3442.50 cm−1 was assigned to the hydroxyl groups. The distinct peaks at 1611.57 cm−1 and 1414.38 cm−1 were related to the asymmetric and symmetric carboxyl groups (COO ) vibrations, respectively. The absorption band at 1030.83 cm−1 corresponds to the saccharide (COH) stretching. In the chitosan spectra, the broad band observed at 3434.56 cm−1 can be attributed to the overlapped peaks of amine groups and hydroxyl groups. The peaks present at 1656.40 cm−1 , 1602.81 cm−1 and 1422.81 cm−1 were recognized as the N H stretching of amide and ether bonds [25]. After the interaction between alginate and chitosan, the peak at 1611.57 cm−1 , 1414.38 cm−1 in the spectra of alginate and 1656.40 cm−1 , 1602.81 cm−1 , 1422.81 cm−1 in that of chitosan disappeared and a new peaks arose at 1624.15 cm−1 ,
1415.96 cm−1 , which suggested the formation of a polyelectrolyte complex between alginate and chitosan [26,27]. The presence of rutin–Cr(III) in the microcapsules was not observed in the FT-IR spectra (data not shown). Thermogravimetry is another valid method to acquire interpretation of the isolated polyelectrolytes and their complexes form. The thermogravimetric behaviors of sodium alginate were pre-
Fig. 5. UV–vis spectra of rutin and rutin–Cr(III) complex. Inset: stability of pH 1 aqueous solution of rutin–Cr(III) monitored for 20 days.
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Fig. 6. DPPH scavenging activities of rutin and rutin–Cr(III) complex. (a) Antioxidant activities of rutin, rutin–Cr(III) complex and Trolox against DPPH free radicals. (b) Proposed oxidation mechanisms of rutin and rutin–Cr(III) complex by DPPH free radicals.
sented in Fig. 4(a), which illustrated the dehydration process and the degradation of the saccharide rings. It corresponded to the endothermic peak at 68.84 ◦ C and exothermic peaks at 251.69 ◦ C, respectively [28]. From the TG and DTA curves of chitosan in Fig. 4(b), a significant mass loss and a exothermic peak were observed at 305.94 ◦ C, which was due to the decomposition of chitosan. This decomposition may be caused by the conjoint effects of depolymerization of the polymeric units and combustion of the saccharide rings [29,30]. As it was illustrated in Fig. 4(c), the transformation from broad exothermic peaks at 257.71 ◦ C and 327.49 ◦ C, along with a new peak arising at 432.13 ◦ C in the blank microcapsules showed evidence of complex between alginate and chitosan [31]. 3.2. Characterization of rutin–Cr(III) complex The UV–vis spectra shown in Fig. 5 demonstrate that rutin exhibits two major absorption bands in the ultraviolet/visible region at 357 nm (band I) and 255 nm (band II), and is consistent with the result in Panhwar’s study [32]. These bands corresponded to the cinnamoyl system of ring B and the benzoyl system of ring A, which accounted for the –* transition of a benzene ring conjugated system. Compared with the bands in the rutin spectrum, both of the bands were shifted to higher wavelengths in the Cr(III)–rutin complex spectrum (424 nm and 265 nm). The two most probable chelating sites in rutin are the 4-C O/5OH and the 3 C OH/4 C OH. The former forms a six-member ring with Cr(III), and the latter, with a more acidic proton in the 3 C OH group, forms a five-member ring with Cr(III) [7]. The coordination bond formed between Cr(III) and O consists of an unshared pair of electrons unilaterally provided by O and the unoccupied orbital of Cr(III). The abovementioned chelated state causes electrons from a high-energy atomic orbital to transfer to a lower-energy molecular orbital according to the lowest energy principle in hybrid orbital theory, which leads to a bathochromic shift in both bands of rutin. A comparison of the FT-IR spectra of rutin and the rutin–Cr(III) complex was performed. The significant changes presented in Fig. 3(b) suggested a coordination between Cr(III) with rutin. The (C O) vibration frequency was significantly shifted by 28.92 cm−1 from 1654.62 cm−1 for rutin to 1625.70 cm−1 for the rutin–Cr(III) complex. This remarkable spectral shift indicates the involvement of 4-C O in the chelation of the Cr(III) ion. A new band observed at 493.69 cm−1 was ascribed to Cr O stretching vibrations. It indicated a coordination reaction. In addition, it should be noted that the (C C) stretching vibration frequency of the benzene ring skeleton in rutin was shifted to low wave
numbers, from 1600.63 cm−1 to 1569.77 cm−1 and 1504.20 cm−1 to 1475.28 cm−1 after forming a complex, which was due to the enhanced conjugation effect caused by chelation. This finding provided further evidence of the chelation between the 4-C O/5-OH and 3 C OH/4 C OH with Cr(III). The (C O) and OH frequencies associated with in-plane deformation coupling showed the same tendency. However, no obvious shift in the (O C O) vibration frequency was observed, which suggested that ring oxygen did not participate in the coordination. The broad bands at 3403.74 cm−1 in the complex and 3423.03 cm−1 in rutin were representative of the stretching vibration of OH/H2 O groups. The C, H and Cr contents were determined using an elemental analyzer and an atomic absorption spectrometer. The calculated ratio of ligands to Cr(III) was 2:3, thus, the proposed molecular formula of the rutin–Cr(III) complex was Cr3 C54 H56 O32 Cl2 ·22H2 O. The analytical value/calculated values were (a) C: 35.84%/35.24%; (b) H: 5.92%/5.49%; and (c) Cr: 8.35%/8.48%. Thus, using Eq. (2), the loading of the rutin–Cr(III) complex in rutin–Cr(III) ACMS was approximately 48%. According to Malesev and Kuntic [33], the major site for metal chelation (benzoyl moiety or catechol moiety) was greatly affected by experimental conditions, such as the type of medium and pH used, as well as the ratio and properties of the metal ion itself. The results of this experiment illustrated that Cr(III) was simultaneously chelated to both 5-OH/4-C O and 3 C OH/4 C OH [7]. The proposed molecular structure of the rutin–Cr(III) complex is shown in Fig. 1(c). TG and DTA of rutin and rutin–Cr(III) shown in Fig. 4(d) and (e) were investigated to validated the elemental analysis result and the composition of the complex. Rutin underwent a process of melting and decomposition around 274.41 ◦ C after a slow loss of moisture. The wide exothermic band from 350 ◦ C to 700 ◦ C indicated that rutin was subject to gradual destruction with a rise in temperature. At the same time, a broad and intense end effect at 166.13 ◦ C in the DTA curve of the complex were observed and assigned to a lossweight rate of 20.64%, which is consistent with the 22 mol crystal water (21.53%) in the calculated molecular formula. The exothermic peak at 410.28 ◦ C represents the combustion of the parent rutin structure. The composition of residue was C and Cr2 O3 . DPPH assay is used as a valid and easy way to estimate scavenging activity of antioxidants partially since it does not have to be generated compared with other radical scavenging assays [34]. In this study, the DPPH scavenging activities of equimolar rutin, rutin–Cr(III) complex and Trolox (Fig. 6(a)) were compared, and their activities were defined as the time required to decrease the initial DPPH· concentration by 50%. The results were calculated using Eq. (3), which indicated that this 50% reduction was first
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Fig. 7. Optimization of reducing conditions for Cr(VI). (a) Percentage reduction as a function of time for various pH values; the initial concentration of the rutin–Cr(III) complex was 8.25 × 10−6 mol/L. (b) Percentage reduction as a function time at various concentrations of the rutin–Cr(III) complex; the initial pH was 1.
achieved by rutin–Cr(III) in 43 min, when rutin showed a 39.54% reduction and Trolox showed a 30.45% reduction. Some work also had been done by Souza to compare the antioxidant activity of flavonoids with its different metal complex, and consistent result was observed [35,36]. And herein the DPPH scavenging activity of rutin can be attributed to the 3 C OH/4 C OH on the B ring. The H atom in the 4 -OH is donated to DPPH to form a more stable DPPH H molecule, and rutin is converted to a semiquinone radical. The increased antioxidant activity of rutin–Cr(III) compared to that of rutin indicates that Cr(III) ions improved the structure of rutin for the formation of reaction centers, which facilitated the departure of the H atom [36,37]. The proposed oxidation process is shown in Fig. 6(b). FRAP is another method used to determine antioxidant capacity. During the reaction, Fe3+ -TPTZ complexes are reduced to blue Fe2+ -TPTZ complexes that strongly absorb at 593 nm. Therefore, the relationship between the absorbance value at 593 nm and the concentration of antioxidant complexes can be established to determine antioxidant capacity. Using Eq. (4), the relative total antioxidant capacity of rutin and rutin–Cr(III) were calculated to be 2.62 and 0.99. However, according to Panhwar, rutin and rutin–Cr(III) possessed an almost equal total antioxidant capacity measured by the phosphomolybdenum method [32]. This difference may be caused by different test method [38].
576.5–579.5 eV and 586.0–589.0 eV, which were corresponded to Cr 2p3/2 and Cr 2p1/2 orbitals, respectively. The Cr 2p3/2 orbitals were designated at 577.2 eV (CrCl3 ) and 576.2–576.5 eV (Cr2 O3 ) for Cr(III) compounds, while Cr(VI) forms were characterized by higher binding energies such as 578.1 eV (CrO3 ) or 579.2 eV (K2 Cr2 O7 ) [41,42]. The XPS spectra of reaction products showed that the chemical state of Cr was trivalent. Thus, it confirms the reduction of Cr(VI) by rutin–Cr(III) complex. In the reaction between rutin–Cr(III) and Cr(VI), the 5-OH/4 O region is not reported to be involved in the reaction due to the high stability attributed to the formation of four bonds with Cr(III). However, for the 3 -OH/4 -OH region of the B ring, the Cr(III) ion may become dissociated and provide an electron, which reduces Cr(VI) to Cr(III). As a reducing agent, rutin–Cr(III) is oxidized to quinine [13]. Studies have also reported that 2-(3,4-dihydroxybenzoyl)2,4,6-trihydroxy-3(2H)-benzofuranone is the oxidation product of flavonoids by metallic ions [43,44]. Fig. 9 shows the proposed reduction scheme of rutin–Cr(III) complex. The 3 -OH/4 -OH chelation section is oxidized first, followed by the extraction of two electrons and a single proton, which contributes to the formation of (a) [45]. The para-quinone (b) and other quinonoid (c), (d) and (e) have analogous molecular structures under acidic conditions
3.3. Reduction of Cr(VI) with rutin–Cr(III) complex 3.3.1. Effect of initial pH on Cr(VI) reduction efficiency Before investigating the effect of initial pH on reduction of Cr(VI), the stability of rutin–Cr(III) complex at pH 1 was examined by determining its absorbance at 265 nm and 424 nm. And result in Fig. 5 (inset) showed that it can keep stability at examined pH. As shown in Fig. 7(a), the percentage of Cr(VI) reduction was calculated using Eq. (5). Increased efficiency was observed at low pH. Approximately 91.45% of the Cr(VI) was reduced at pH 1 in 180 min. The corresponding rates at pH 2–4 were 59.17%, 30.98% and 11.09%, respectively. These results showed that an acidic medium facilitates the reduction of Cr(VI) by rutin–Cr(III) complex. The existing forms of Cr(VI) and Cr(III) are primarily dependent on pH, which affects the reduction of Cr(VI) and the formation of Cr(III) [39]. Under acidic conditions, the potential (E0 ) for the reduction of Cr(VI) to Cr(III) is greater than +1.30 V but drops to 0.36 V under near-neutral conditions [40]. As shown in Fig. 8, the chemical states of Cr were further analyzed by XPS spectra. Two bands arose at binding energies of
Fig. 8. Cr2p XPS of K2 Cr2 O7 , CrCl3 , rutin–Cr(III) complex and reaction products.
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Fig. 9. Proposed oxidation processes of rutin–Cr(III) complex. “-Ru” represents the rhamnoglucoside of ring C in Fig. 1.
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4. Conclusions A rutin–Cr(III) complex was synthesized with a proposed Cr(III)-to-rutin ratio of 3:2. This study demonstrated that low solution pH increased the reduction rate of Cr(VI). Using an emulsification/internal gelation technique, a novel and effective sustained-release rutin–Cr(III) ACMS system was developed for Cr(VI) reduction. The environmentally friendly microcapsules are biodegradable following the release of rutin–Cr(III). However, further studies on the mechanism of Cr(VI) reduction are warranted. For practical applications, simpler rutin–Cr(III) ACMS preparation methods are also needed. Acknowledgments
Fig. 10. Reduction of Cr(VI) by rutin–Cr(III) ACMS and blank ACMS at different initial pH values. The amount of dried microcapsules used in each group was 0.133 g.
The authors gratefully acknowledge financial support from National Natural Science Foundation of China (51108310) and Major Science and Technology Program for Water Pollution Control and Treatment of China (2014ZX07203-009). References
[46]. Subsequently, water facilitates the formation of hemiketals (f), which form a more stable benzofuranone isomer (g) and (h) [47].
3.3.2. Effect of initial rutin–Cr(III) complex concentration on Cr(VI) reduction efficiency The sharp decrease in the concentration of Cr(VI) shown in Fig. 7(b) indicated that 1.23 × 10−5 mol/L and 1.65 × 10−5 mol/L rutin–Cr(III) were sufficient concentrations for the complete reduction of Cr(VI). This result suggested that the molar concentration of rutin–Cr(III) required for the reduction of 1 M Cr(VI) is 0.93. This concentration is lower than the concentrations required of traditional reducing agents such as H2 S and Fe(II) — 1.5 and 3, respectively, — to achieve the same reduction results [48,49].
3.4. Reduction of Cr(VI) by rutin–Cr(III) ACMS As shown in Fig. 10, the initial pH affected the reduction of Cr(VI) by rutin–Cr(III) ACMS. As the pH increased from 1 to 3, the time for complete reduction increased from 22 h to 168 h. The time required to reduce 90% of the Cr(V) at pH 1 was 16 h using rutin–Cr(III) ACMS, which was eight times slower than that required using rutin–Cr(III) alone. At pH 5.6, the decrease in Cr(VI) was notably less pronounced. According to Gonzalez et al. [50], the extent of drug release in alginate–chitosan microcapsules increased with pH, which was caused by the disintegration of the microsphere system and the deprotonation of alginic acid. The results obtained in this experiment can be explained by that the effect of pH dominated the process of Cr(VI) removal by rutin–Cr(III) ACMS. Moreover, low solution pH significantly inhibited the adsorption ability of alginate–chitosan [51,52]. The results of control experiments illustrated that this contribution was minor. This environmentally friendly rutin–Cr(III) ACMS is a micronsized rutin–Cr(III) source that can be easily used to release rutin–Cr(III) continuously in contaminated sites. Thus, the concentration of rutin–Cr(III) can be maintained at a certain level for longer periods of time. Furthermore, this system has practical importance for flowing wastewater applications because it prevents the rutin–Cr(III) components from being swept away with wastewater. A continuous flow reactor for Cr(VI) removal will be useful for further studies.
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Novel reduction of Cr(VI) from wastewater using a naturally derived microcapsule loaded with rutin–Cr(III) complex Yun Qi a,∗ , Meng Jiang a , Yuan-Lu Cui b,∗∗ , Lin Zhao a , Shejiang Liu a a Tianjin Technological Engineering Center on Biomass-Derived Gas and Oil, Faculty of Environmental Science and Engineering, Tianjin University, No. 92, Weijin Rd., Nankai District, Tianjin 300072, China b Tianjin State Key Laboratory of Modern Chinese Medicine, Tianjin University of Traditional Chinese Medicine, Tianjin 300193, China
h i g h l i g h t s • • • •
Low-cost hexavalent chromium reduction microcapsule was prepared with natural material. Rutin–Cr(III) complex was synthesized and characterized. Rutin–Cr(III) complex exhibited strong reducibility of Cr(VI). Rutin–Cr(III) alginate–chitosan microcapsule can reduce Cr(VI) steadily and prolongedly.
a r t i c l e
i n f o
Article history: Received 17 August 2014 Received in revised form 12 November 2014 Accepted 6 December 2014 Available online 10 December 2014 Keywords: Rutin Flavonoid–metal complex Rutin–Cr(III) Alginate–chitosan microcapsule Cr(VI) reduction
a b s t r a c t The harmfulness of carcinogenic hexavalent chromium (Cr(VI)) is dramatically decreased when Cr(VI) is reduced to trivalent chromium (Cr(III)). Rutin, a natural flavonoid, exhibits excellent antioxidant activity by coordinating metal ions. In this study, a complex containing rutin and Cr(III) (rutin–Cr(III)) was synthesized and characterized. The rutin–Cr(III) complex was much easier to reduce than rutin. The reduction of the rutin–Cr(III) complex was highly pH-dependent, with 90% of the Cr(VI) being reduced to Cr(III) in 2 h under optimal conditions. A biodegradable, sustained-release system encapsulating the rutin–Cr(III) complex in a alginate–chitosan microcapsule (rutin–Cr(III) ACMS) was also evaluated, and the reduction of Cr(VI) was assessed. This study also demonstrated that low-pH solutions increased the reduction rate of Cr(VI). The environmentally friendly microcapsules can reduce Cr(VI) for prolonged periods of time and can easily biodegrade after releasing the rutin–Cr(III) complex. Given the excellent performance of rutin–Cr(III) ACMS, the microcapsule system represents an effective system for the remediation of Cr(VI) pollution. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Chromium and chromium compounds are used extensively in leather tanning, electroplating, textile dyeing, stainless steel production and pigmentation industries. Thus, large quantities of chromium are discharged into the environment [1]. In China, approximately 1.34 × 104 t of chromium was discharged into water bodies from 1990 to 2009, mainly from the fabricated metal and leather tanning industries [2]. Under natural conditions, chromium generally exists in one of two valence states, trivalent chromium (Cr(III)) or hexavalent chromium (Cr(VI)). The hexavalent
∗ Corresponding author. Tel.: +86 22 27891291/13752219121. ∗∗ Corresponding author. Tel.: +86 22 59596170. E-mail addresses: [email protected] (Y. Qi), [email protected] (Y.-L. Cui). http://dx.doi.org/10.1016/j.jhazmat.2014.12.008 0304-3894/© 2014 Elsevier B.V. All rights reserved.
oxidation state of chromium (Cr(VI)) is known to be highly toxic and carcinogenic, thereby posing a threat to both humans and animals [3]. Cr(VI) is listed among the top 20 contaminants on Superfund Priority List of Hazardous Substances. In contrast, Cr(III) is estimated to be 100 times less toxic than Cr(VI) and has a lower solubility. Cr(III) is also known to be an essential dietary element [4]. Thus, the reduction of Cr(VI) to Cr(III) is a feasible method for minimizing chromium pollution. Current approaches to reducing Cr(VI) concentrations involve the reduction of Cr(VI) to Cr(III) using various chemical, physical or biological methods, such as reduction, ion exchange, membrane separation, precipitation and adsorption electrodialysis [5]. However, most of these procedures are expensive or have significant disadvantages, such as incomplete metal removal, high reagent consumption, secondary waste product generation and high energy requirements. Therefore, the
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few studies have evaluated the use of rutin–metal complex in the reduction of oxidized ions in water treatment systems [13]. Alginate is a natural binary copolymer made from linear chains of ␣-l-guluronic acid (G) and -d-mannuronic acid (M) in varying ratios. In the presence of divalent cations (such as Ca2+ ), an ionotropic effect will occur between sodium alginate — the salt form of alginates — and Na+ , leading to a regular threedimensional net structure bound by divalent cations [14]. Chitosan is a cationic polysaccharide composed of poly(-1-4)-2-amino-2deoxy-d-glucopyranose. Due to the formation of complex between protonated amine and anionic polysaccharide in sodium alginate under acidic conditions, chitosan has been shown to be an efficient adjuvant for extending the residence time in drug release systems and to reinforce the biological activity of drugs [15]. Alginate–chitosan microcapsules are traditionally used as a sustained release drug delivery system in the pharmaceutical industry [16] because they are easy to prepare, highly stable, and allow for high drug loading [17]; however, these microcapsules are seldom used in wastewater disposal with flavonoids. In this study, rutin–Cr(III) complex encapsulated in a alginate–chitosan microcapsule system (rutin–Cr(III) ACMS) were prepared using an internal gelation technique. Through this method, the rutin–Cr(III) complex was maintained in aqueous solution and Cr(VI) was reduced. 2. Materials and methods 2.1. Chemicals Rutin (purity > 95%), sodium alginate and chitosan (90% deacetylation degree) were supplied by Sangon Biotech, Co., Ltd. (Shanghai, China). Micronized CaCO3 (40 nm) was provided by Beijing Wangyong Technology Ltd. (Beijing, China). Liquid paraffin was supplied by Beifang Tianyi Chemical Reagent Company (Tianjin, China). Span 80 and Tween 80 were purchased from Sigma–Aldrich Co. (St. Louis, USA). All remaining chemicals used in this study were of analytical grade and were used without further purification. All solutions were prepared with deionized water. A dialysis membrane with a MWCO value of 7000 was provided by Beijing Solarbio Science and Technology Co., Ltd. (Beijing, China). Fig. 1. Chemical structures of (a) flavonoids, (b) rutin, and (c) rutin–Cr(III) complex.
2.2. Synthesis of rutin–Cr(III) and rutin–Cr(III) ACMS
development of a low-cost method that uses readily available materials to remove and recover Cr(VI) is warranted. The literature suggests that Cr(VI) can be removed using natural biomaterials through both direct and indirect reduction mechanisms [6]. Flavonoids are abundant natural compounds found in most plants and are concentrated in plants’ seeds, fruit skin, fruit peel, bark and flowers. A large number of flavonoids are polyphenols with a basic structure composed of two aromatic rings (benzo-␥-pyrone and benzene) linked together by three carbon atoms with a simplified configuration of C6 C3 C6 (Fig. 1(a)). In recent years, flavonoids have generated interest among researchers in many fields for their antioxidant and free-radical scavenging activities. The 3-OH groups at 5,3 ,4 and the carbonyl at position 4 in rutin (Fig. 1(b)), which is a typical flavonoid, have two possible coordination sites (i.e., catechol and 4-oxo-5-OH, as indicated by arrows) through which they can easily form complexes with metal ions [7,8]. Kostyuk et al. [9] reported that increased efficiency was achieved using rutin complexes containing Zn(II), Cu(II), Fe(II), or Fe(III) in free radical scavenging assays due to the generation of extra superoxide dismutase centers. Many studies have also reported that rutin–metal complex act as antioxidants [10–12], but
Rutin–Cr(III) was synthesized by following the luteolin–Cr(III) procedure described by Gao et al. [18]. Minor changes were made to the reaction between 1 mmol rutin and 1.5 mmol CrCl3 ·6H2 O. Based on the method used in our previous study [19], rutin–Cr(III) ACMS was prepared using a water-in-oil emulsion gelation technique by combining 0.2 g micronized CaCO3 with 20 mL 1.5% sodium alginate aqueous solution while stirring using a magnetic stir bar and heating at 45 ◦ C. Micronized CaCO3 was then uniformly dispersed, and 0.1 g rutin–Cr(III) was added (mixture A). At the same time, 100 mL of paraffin oil containing 1% Span 80 was prepared in a 500 mL three-neck round-bottom flask with a stirring speed of 300 rpm at 37 ◦ C (mixture B). Subsequently, mixture A was added to mixture B using a 10 mL injector (matched needle) with a rate of 30 drop/min at a drop height of 10 cm. The solution was then left undisturbed for 15 min to enable emulsification to occur before 0.5 mL glacial acetic in 5 mL paraffin oil was dropped to release Ca2+ [14]. The mixture was set aside for sedimentation separation. After precipitating completely, the upper oily liquid was removed, and the lower alginate microspheres were washed with 1% (v/v) glaciated Tween 80 solution three times to remove excess oil. The washing procedure was then repeated three times using chilled water.
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A 1% (w/v) chitosan solution was prepared using 1% (v/v) glacial acetic while stirring at 45 ◦ C for 4–6 h. The solution was then stored in a refrigerator overnight. The pH was adjusted to 5.5 with 1 M NaOH. The alginate microspheres obtained previously were dispersed into the chitosan solution at a ratio of 1:6 (v/v) and gently oscillated for 50 min to allow for coating with chitosan. The chitosan solution was later centrifuged, and the alginate chitosan microspheres were washed with chilled water three times. To obtain alginate chitosan liquid-core microcapsules, the microspheres were immersed in a 55 mM sodium citrate solution prepared with 5% NaCl (w/v). The ratio of microspheres to sodium citrate solution was 1:8 (v/v). The mixture was gently oscillated for 30 min. After centrifugation, the microcapsules were washed with chilled water and lyophilized. 2.3. Analytical methods 2.3.1. Characterization of rutin–Cr(III) ACMS morphology The width of the wet microcapsules’ particle distribution was estimated by a particle size analyzer (Mastersizer 2000, Malvern, UK), with the span value calculated using Eq. (1); a small span value corresponds to a highly uniform distribution. Span =
(D(0.9) − D(0.1)) D(0.5)
(1)
D(0.1, 0.5, 0.9) – volume percentage of microcapsules with a diameter D(n) of n%(%). A field emission scanning electron microscope (SEM; Nova NanoSEM 430, FEI, USA) and inverted biological microscope (Eclipse Ti-U, Nikon, Japan) were used to characterize the morphology of the microcapsules. The lyophilized microcapsules prepared in Section 2.2 were fixed with double side adhesive carbon tape and coated with gold under vacuum before SEM observation. The SEM images were observed at accelerating voltage of 3.0 kV. Lyophilized microcapsules were embedded in epon and ultrathin sections were obtained by Ultramicrotome (Lecia EM UC6, Leica, Germany). Transmission electron microscopy (TEM; H7650B, Hitachi, Japan) was applied to investigate the inner structure of the microcapsules at 80 kV. 2.3.2. UV–vis spectra UV–vis spectra were recorded in the spectral range of 200–500 nm on a UV–vis spectrometer (DU800, Beckman, USA) with a 10 mm quartz cell. The contents of rutin and the rutin–Cr(III) complex were measured in methanol and water (pH 3) with blank subtraction. 2.3.3. FT-IR analysis FT-IR spectra of samples were recorded on a FT-IR spectrometer (Nicolet 6700, Thermo Scientific, USA) with a DTGS detector. The spectra were collected in the range of 4000–400 cm−1 with a resolution of 2 cm−1 at 40 scans per spectrum. 2.3.4. Thermogravimetric analysis Thermogravimetric data (TG and DTA) for samples were obtained using a Thermogravimetric/Differemtial Thermal Analyzer (Pyris/Diamond TG/DTA, PerkinElmer, USA). All tests were conducted with samples mass of 2–5 mg at 10 ◦ C/min over a range of 20–700 ◦ C. The measurements were carried out in N2 atmosphere with a flow rate of 100 ml/min. 2.3.5. Quantitative analysis of Cr in rutin–Cr(III) complex and rutin–Cr(III) ACMS The C, H and Cr contents of the rutin–Cr(III) complex were determined using a elemental analyzer (Vario EL, Elementar, Germany).
A polarized Zeeman atomic absorption spectrophotometer (Model 180-80, Hitachi, Japan) was used to determine the Cr content in the rutin–Cr(III) complex and rutin–Cr(III) ACMS, and the loading of rutin–Cr(III) in ACMS was calculated by Eq. (2) according to the percentage score of Cr(III) in the rutin–Cr(III) complex. Rutin-Cr(III)Loading =
W1 × 100% W2
(2)
W1 – weight of rutin–Cr(III) complex in the microcapsules (mg), W2 – weight of microcapsules (mg). 2.3.6. Determination of antioxidant activity of complex by DPPH method and FRAP assay The DPPH free radical scavenging assay and the ferric reducing ability of plasma (FRAP) assay were conducted in 96 well plates to investigate the antioxidant properties of rutin and rutin–Cr(III) complex. An microplate reader (Infinite 200, Tecan, Switzerland) was used to collect the data [20]. In the DPPH free radical scavenging experiment, 0.6 L 3 × 10−4 mol/L of rutin, rutin–Cr(III) complex, and Trolox methanol solution (as a control sample) were added to 234 L 6 × 10−4 mol/L of DPPH, followed by 10 s of oscillation for complete mixing. The data were collected at 515 nm, and the temperature was held at 27 ± 0.2 ◦ C during the experiments. The percentage of DPPH remaining was calculated by Eq. (3): RemainingDPPH% =
[DPPH]t × 100% [DPPH]0
(3)
t – reaction time(min), [DPPH]0 – initial concentration of DPPH(10−4 mol/L), [DPPH]t – concentration of DPPH at t (10−4 mol/L). In the FRAP section, 5 L 0.15 × 10−4 mol/L, 0.3 × 10−4 mol/L, 0.9375 × 10−4 mol/L, 1.875 × 10−4 mol/L, 3.75 × 10−4 mol/L, 7.5 × 10−4 mol/L or 15 × 10−4 mol/L of rutin was added to 180 L FRAP working solution. The mixture was incubated at 37 ◦ C for 4 min after 10 s of mixing, and the absorbance value was measured at 593 nm. The total relative antioxidant capacity of rutin was determined by comparing with the absorbance of a 1.0 × 10−3 mol/L FeSO4 standard solution and calculated using Eq. (4). The antioxidant capacity of the rutin–Cr(III) complex was also investigated using the same methods. Relative total antioxidant capacity =
C0 CA
(4)
C0 – concentration of FeSO4 standard solution, 1 × 10−3 mol/L, A – absorbance value of 1 × 10−3 mol/L FeSO4 standard solution, CA – corresponding concentration of rutin or rutin–Cr(III) complex at the same absorbance value of A. 2.4. Reduction of Cr(VI) by rutin–Cr(III) complex and rutin–Cr(III) ACMS All experiments were performed in 500 mL volumes at 1.93 × 10−5 mol/L Cr(VI) with an agitation rate of 200 rpm. Spectrophotometric methods were applied to determine the concentration of Cr(VI) in aqueous solution [21]. The percentage of Cr(VI) reduction was calculated using Eq. (5). Percentage of Cr(VI)reduction(%) =
(C0 − Ct ) × 100% C0
(5)
C0 – initial concentration of Cr(VI) (mol/L), Ct – concentration of Cr(VI) at t (mol/L).
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Fig. 2. Morphology of rutin–Cr(III) microcapsules (ACMS). (a) Optical photomicrograph of rutin–Cr(III) ACMS, (b) SEM micrographs of rutin–Cr(III) ACMS, (c) SEM micrographs of microcapsule surfaces and morphology, (d) TEM images of the cross-section morphology of the microcapsule, (e) TEM images of the boundary regions of the microcapsule membrane. The chitosan layer was indicated by arrows, and (f) TEM images of the structural details of inner alginate. The rutin–Cr(III) complex was indicated by circles and by arrows.
2.5. XPS analysis
3. Results and discussion
The valence state of the Cr after reduction of Cr(VI) by rutin–Cr(III) complex was determined by the X-ray photoelectron spectrometer (PHI 1600, PerkinElmer, USA). The solid form of the reaction products were obtained by vacuum drying the mixture of 1.93 × 10−5 mol/L Cr(VI) and 1.23 × 10−5 mol/L rutin–Cr(III) at pH 1. CrCl3 ·6H2 O and K2 Cr2 O7 (Sangon Biotech, China) were used as Cr(III) and Cr(VI) reference compounds, respectively. Experiment was carried out with an AlK˛ (h = 1667.0 eV) X-ray source at 250.0 W and pass energy of 29.35 eV for high-resolution analyses. The vacuum in the analysis chamber was kept below 3 × 10−6 Pa during measurement. The binding energy of the spectra was calibrated with aliphatic carbons C1s(284.6 eV).
3.1. Morphology characterization of rutin–Cr(III) ACMS The morphology and structure of rutin–Cr(III) ACMS was studied by optical inverted microscope, SEM and TEM (Fig. 2). The spherical rutin–Cr(III) ACMS under wet conditions was observed by inverted microscope, which was showed in Fig. 2(a). The D(0.5) of rutin–Cr(III) ACMS calculated using Eq. (1) was 235 m and the computed span value was 1.10, which suggested that the microcapsules obtained in this experiment were uniform in size. The SEM results presented in Fig. 2(b) indicated that the lyophilized microcapsules exhibited good spherical shape in geometry and it was consistent with its image in wet conditions. The microcapsule
Fig. 3. FT-IR spectra of (a) alginate, chitosan, and blank microcapsules, (b) rutin and rutin–Cr(III) complex.
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Fig. 4. DTA-TG-DTG of (a) sodium alginate, (b) chitosan, (c) blank microcapsules, (d) rutin, and (e) rutin–Cr(III) complex.
presented homogeneous properties generally though some bulges observed on the surface. Detailed examination of the membrane topology in Fig. 2(c) illustrated compact and denser chitosan layer in the outmost of microcapsules [22]. The internal morphology analysis of ultrathin sections was conducted by the TEM (Fig. 2(d)–(f)). The cross-section observation in Fig. 2(d) and (f) exhibited that rutin–Cr(III) complex was distributed evenly in the microcapsules. The remaining clumps of complex may be the aggregation of rutin–Cr(III) complex caused by freeze-drying. Fig. 2(e) showed clear microcapsule membrane on the boundary region. Thus, this roughly 200 nm thick film demonstrated the presence of surface chitosan layer [23,24]. One approach to provide information on molecular interaction of microcapsules was to monitor changes in the FT-IR spectra of their component parts. The FT-IR spectra of alginate, chitosan and blank microcapsules were shown in Fig. 3(a). In the spectra of alginate, the wide band at 3442.50 cm−1 was assigned to the hydroxyl groups. The distinct peaks at 1611.57 cm−1 and 1414.38 cm−1 were related to the asymmetric and symmetric carboxyl groups (COO ) vibrations, respectively. The absorption band at 1030.83 cm−1 corresponds to the saccharide (COH) stretching. In the chitosan spectra, the broad band observed at 3434.56 cm−1 can be attributed to the overlapped peaks of amine groups and hydroxyl groups. The peaks present at 1656.40 cm−1 , 1602.81 cm−1 and 1422.81 cm−1 were recognized as the N H stretching of amide and ether bonds [25]. After the interaction between alginate and chitosan, the peak at 1611.57 cm−1 , 1414.38 cm−1 in the spectra of alginate and 1656.40 cm−1 , 1602.81 cm−1 , 1422.81 cm−1 in that of chitosan disappeared and a new peaks arose at 1624.15 cm−1 ,
1415.96 cm−1 , which suggested the formation of a polyelectrolyte complex between alginate and chitosan [26,27]. The presence of rutin–Cr(III) in the microcapsules was not observed in the FT-IR spectra (data not shown). Thermogravimetry is another valid method to acquire interpretation of the isolated polyelectrolytes and their complexes form. The thermogravimetric behaviors of sodium alginate were pre-
Fig. 5. UV–vis spectra of rutin and rutin–Cr(III) complex. Inset: stability of pH 1 aqueous solution of rutin–Cr(III) monitored for 20 days.
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Fig. 6. DPPH scavenging activities of rutin and rutin–Cr(III) complex. (a) Antioxidant activities of rutin, rutin–Cr(III) complex and Trolox against DPPH free radicals. (b) Proposed oxidation mechanisms of rutin and rutin–Cr(III) complex by DPPH free radicals.
sented in Fig. 4(a), which illustrated the dehydration process and the degradation of the saccharide rings. It corresponded to the endothermic peak at 68.84 ◦ C and exothermic peaks at 251.69 ◦ C, respectively [28]. From the TG and DTA curves of chitosan in Fig. 4(b), a significant mass loss and a exothermic peak were observed at 305.94 ◦ C, which was due to the decomposition of chitosan. This decomposition may be caused by the conjoint effects of depolymerization of the polymeric units and combustion of the saccharide rings [29,30]. As it was illustrated in Fig. 4(c), the transformation from broad exothermic peaks at 257.71 ◦ C and 327.49 ◦ C, along with a new peak arising at 432.13 ◦ C in the blank microcapsules showed evidence of complex between alginate and chitosan [31]. 3.2. Characterization of rutin–Cr(III) complex The UV–vis spectra shown in Fig. 5 demonstrate that rutin exhibits two major absorption bands in the ultraviolet/visible region at 357 nm (band I) and 255 nm (band II), and is consistent with the result in Panhwar’s study [32]. These bands corresponded to the cinnamoyl system of ring B and the benzoyl system of ring A, which accounted for the –* transition of a benzene ring conjugated system. Compared with the bands in the rutin spectrum, both of the bands were shifted to higher wavelengths in the Cr(III)–rutin complex spectrum (424 nm and 265 nm). The two most probable chelating sites in rutin are the 4-C O/5OH and the 3 C OH/4 C OH. The former forms a six-member ring with Cr(III), and the latter, with a more acidic proton in the 3 C OH group, forms a five-member ring with Cr(III) [7]. The coordination bond formed between Cr(III) and O consists of an unshared pair of electrons unilaterally provided by O and the unoccupied orbital of Cr(III). The abovementioned chelated state causes electrons from a high-energy atomic orbital to transfer to a lower-energy molecular orbital according to the lowest energy principle in hybrid orbital theory, which leads to a bathochromic shift in both bands of rutin. A comparison of the FT-IR spectra of rutin and the rutin–Cr(III) complex was performed. The significant changes presented in Fig. 3(b) suggested a coordination between Cr(III) with rutin. The (C O) vibration frequency was significantly shifted by 28.92 cm−1 from 1654.62 cm−1 for rutin to 1625.70 cm−1 for the rutin–Cr(III) complex. This remarkable spectral shift indicates the involvement of 4-C O in the chelation of the Cr(III) ion. A new band observed at 493.69 cm−1 was ascribed to Cr O stretching vibrations. It indicated a coordination reaction. In addition, it should be noted that the (C C) stretching vibration frequency of the benzene ring skeleton in rutin was shifted to low wave
numbers, from 1600.63 cm−1 to 1569.77 cm−1 and 1504.20 cm−1 to 1475.28 cm−1 after forming a complex, which was due to the enhanced conjugation effect caused by chelation. This finding provided further evidence of the chelation between the 4-C O/5-OH and 3 C OH/4 C OH with Cr(III). The (C O) and OH frequencies associated with in-plane deformation coupling showed the same tendency. However, no obvious shift in the (O C O) vibration frequency was observed, which suggested that ring oxygen did not participate in the coordination. The broad bands at 3403.74 cm−1 in the complex and 3423.03 cm−1 in rutin were representative of the stretching vibration of OH/H2 O groups. The C, H and Cr contents were determined using an elemental analyzer and an atomic absorption spectrometer. The calculated ratio of ligands to Cr(III) was 2:3, thus, the proposed molecular formula of the rutin–Cr(III) complex was Cr3 C54 H56 O32 Cl2 ·22H2 O. The analytical value/calculated values were (a) C: 35.84%/35.24%; (b) H: 5.92%/5.49%; and (c) Cr: 8.35%/8.48%. Thus, using Eq. (2), the loading of the rutin–Cr(III) complex in rutin–Cr(III) ACMS was approximately 48%. According to Malesev and Kuntic [33], the major site for metal chelation (benzoyl moiety or catechol moiety) was greatly affected by experimental conditions, such as the type of medium and pH used, as well as the ratio and properties of the metal ion itself. The results of this experiment illustrated that Cr(III) was simultaneously chelated to both 5-OH/4-C O and 3 C OH/4 C OH [7]. The proposed molecular structure of the rutin–Cr(III) complex is shown in Fig. 1(c). TG and DTA of rutin and rutin–Cr(III) shown in Fig. 4(d) and (e) were investigated to validated the elemental analysis result and the composition of the complex. Rutin underwent a process of melting and decomposition around 274.41 ◦ C after a slow loss of moisture. The wide exothermic band from 350 ◦ C to 700 ◦ C indicated that rutin was subject to gradual destruction with a rise in temperature. At the same time, a broad and intense end effect at 166.13 ◦ C in the DTA curve of the complex were observed and assigned to a lossweight rate of 20.64%, which is consistent with the 22 mol crystal water (21.53%) in the calculated molecular formula. The exothermic peak at 410.28 ◦ C represents the combustion of the parent rutin structure. The composition of residue was C and Cr2 O3 . DPPH assay is used as a valid and easy way to estimate scavenging activity of antioxidants partially since it does not have to be generated compared with other radical scavenging assays [34]. In this study, the DPPH scavenging activities of equimolar rutin, rutin–Cr(III) complex and Trolox (Fig. 6(a)) were compared, and their activities were defined as the time required to decrease the initial DPPH· concentration by 50%. The results were calculated using Eq. (3), which indicated that this 50% reduction was first
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Fig. 7. Optimization of reducing conditions for Cr(VI). (a) Percentage reduction as a function of time for various pH values; the initial concentration of the rutin–Cr(III) complex was 8.25 × 10−6 mol/L. (b) Percentage reduction as a function time at various concentrations of the rutin–Cr(III) complex; the initial pH was 1.
achieved by rutin–Cr(III) in 43 min, when rutin showed a 39.54% reduction and Trolox showed a 30.45% reduction. Some work also had been done by Souza to compare the antioxidant activity of flavonoids with its different metal complex, and consistent result was observed [35,36]. And herein the DPPH scavenging activity of rutin can be attributed to the 3 C OH/4 C OH on the B ring. The H atom in the 4 -OH is donated to DPPH to form a more stable DPPH H molecule, and rutin is converted to a semiquinone radical. The increased antioxidant activity of rutin–Cr(III) compared to that of rutin indicates that Cr(III) ions improved the structure of rutin for the formation of reaction centers, which facilitated the departure of the H atom [36,37]. The proposed oxidation process is shown in Fig. 6(b). FRAP is another method used to determine antioxidant capacity. During the reaction, Fe3+ -TPTZ complexes are reduced to blue Fe2+ -TPTZ complexes that strongly absorb at 593 nm. Therefore, the relationship between the absorbance value at 593 nm and the concentration of antioxidant complexes can be established to determine antioxidant capacity. Using Eq. (4), the relative total antioxidant capacity of rutin and rutin–Cr(III) were calculated to be 2.62 and 0.99. However, according to Panhwar, rutin and rutin–Cr(III) possessed an almost equal total antioxidant capacity measured by the phosphomolybdenum method [32]. This difference may be caused by different test method [38].
576.5–579.5 eV and 586.0–589.0 eV, which were corresponded to Cr 2p3/2 and Cr 2p1/2 orbitals, respectively. The Cr 2p3/2 orbitals were designated at 577.2 eV (CrCl3 ) and 576.2–576.5 eV (Cr2 O3 ) for Cr(III) compounds, while Cr(VI) forms were characterized by higher binding energies such as 578.1 eV (CrO3 ) or 579.2 eV (K2 Cr2 O7 ) [41,42]. The XPS spectra of reaction products showed that the chemical state of Cr was trivalent. Thus, it confirms the reduction of Cr(VI) by rutin–Cr(III) complex. In the reaction between rutin–Cr(III) and Cr(VI), the 5-OH/4 O region is not reported to be involved in the reaction due to the high stability attributed to the formation of four bonds with Cr(III). However, for the 3 -OH/4 -OH region of the B ring, the Cr(III) ion may become dissociated and provide an electron, which reduces Cr(VI) to Cr(III). As a reducing agent, rutin–Cr(III) is oxidized to quinine [13]. Studies have also reported that 2-(3,4-dihydroxybenzoyl)2,4,6-trihydroxy-3(2H)-benzofuranone is the oxidation product of flavonoids by metallic ions [43,44]. Fig. 9 shows the proposed reduction scheme of rutin–Cr(III) complex. The 3 -OH/4 -OH chelation section is oxidized first, followed by the extraction of two electrons and a single proton, which contributes to the formation of (a) [45]. The para-quinone (b) and other quinonoid (c), (d) and (e) have analogous molecular structures under acidic conditions
3.3. Reduction of Cr(VI) with rutin–Cr(III) complex 3.3.1. Effect of initial pH on Cr(VI) reduction efficiency Before investigating the effect of initial pH on reduction of Cr(VI), the stability of rutin–Cr(III) complex at pH 1 was examined by determining its absorbance at 265 nm and 424 nm. And result in Fig. 5 (inset) showed that it can keep stability at examined pH. As shown in Fig. 7(a), the percentage of Cr(VI) reduction was calculated using Eq. (5). Increased efficiency was observed at low pH. Approximately 91.45% of the Cr(VI) was reduced at pH 1 in 180 min. The corresponding rates at pH 2–4 were 59.17%, 30.98% and 11.09%, respectively. These results showed that an acidic medium facilitates the reduction of Cr(VI) by rutin–Cr(III) complex. The existing forms of Cr(VI) and Cr(III) are primarily dependent on pH, which affects the reduction of Cr(VI) and the formation of Cr(III) [39]. Under acidic conditions, the potential (E0 ) for the reduction of Cr(VI) to Cr(III) is greater than +1.30 V but drops to 0.36 V under near-neutral conditions [40]. As shown in Fig. 8, the chemical states of Cr were further analyzed by XPS spectra. Two bands arose at binding energies of
Fig. 8. Cr2p XPS of K2 Cr2 O7 , CrCl3 , rutin–Cr(III) complex and reaction products.
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Fig. 9. Proposed oxidation processes of rutin–Cr(III) complex. “-Ru” represents the rhamnoglucoside of ring C in Fig. 1.
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4. Conclusions A rutin–Cr(III) complex was synthesized with a proposed Cr(III)-to-rutin ratio of 3:2. This study demonstrated that low solution pH increased the reduction rate of Cr(VI). Using an emulsification/internal gelation technique, a novel and effective sustained-release rutin–Cr(III) ACMS system was developed for Cr(VI) reduction. The environmentally friendly microcapsules are biodegradable following the release of rutin–Cr(III). However, further studies on the mechanism of Cr(VI) reduction are warranted. For practical applications, simpler rutin–Cr(III) ACMS preparation methods are also needed. Acknowledgments
Fig. 10. Reduction of Cr(VI) by rutin–Cr(III) ACMS and blank ACMS at different initial pH values. The amount of dried microcapsules used in each group was 0.133 g.
The authors gratefully acknowledge financial support from National Natural Science Foundation of China (51108310) and Major Science and Technology Program for Water Pollution Control and Treatment of China (2014ZX07203-009). References
[46]. Subsequently, water facilitates the formation of hemiketals (f), which form a more stable benzofuranone isomer (g) and (h) [47].
3.3.2. Effect of initial rutin–Cr(III) complex concentration on Cr(VI) reduction efficiency The sharp decrease in the concentration of Cr(VI) shown in Fig. 7(b) indicated that 1.23 × 10−5 mol/L and 1.65 × 10−5 mol/L rutin–Cr(III) were sufficient concentrations for the complete reduction of Cr(VI). This result suggested that the molar concentration of rutin–Cr(III) required for the reduction of 1 M Cr(VI) is 0.93. This concentration is lower than the concentrations required of traditional reducing agents such as H2 S and Fe(II) — 1.5 and 3, respectively, — to achieve the same reduction results [48,49].
3.4. Reduction of Cr(VI) by rutin–Cr(III) ACMS As shown in Fig. 10, the initial pH affected the reduction of Cr(VI) by rutin–Cr(III) ACMS. As the pH increased from 1 to 3, the time for complete reduction increased from 22 h to 168 h. The time required to reduce 90% of the Cr(V) at pH 1 was 16 h using rutin–Cr(III) ACMS, which was eight times slower than that required using rutin–Cr(III) alone. At pH 5.6, the decrease in Cr(VI) was notably less pronounced. According to Gonzalez et al. [50], the extent of drug release in alginate–chitosan microcapsules increased with pH, which was caused by the disintegration of the microsphere system and the deprotonation of alginic acid. The results obtained in this experiment can be explained by that the effect of pH dominated the process of Cr(VI) removal by rutin–Cr(III) ACMS. Moreover, low solution pH significantly inhibited the adsorption ability of alginate–chitosan [51,52]. The results of control experiments illustrated that this contribution was minor. This environmentally friendly rutin–Cr(III) ACMS is a micronsized rutin–Cr(III) source that can be easily used to release rutin–Cr(III) continuously in contaminated sites. Thus, the concentration of rutin–Cr(III) can be maintained at a certain level for longer periods of time. Furthermore, this system has practical importance for flowing wastewater applications because it prevents the rutin–Cr(III) components from being swept away with wastewater. A continuous flow reactor for Cr(VI) removal will be useful for further studies.
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