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Ind. Eng. Chem. Res. 2009, 48, 1458–1463
Fe(III)-Immobilized Collagen Fiber: A Renewable Heterogeneous Catalyst for the Photoassisted Decomposition of Orange II Xiaohu Liu, Rui Tang, Qiang He, Xuepin Liao,* and Bi Shi* National Engineering Laboratory for Clean Technology of Leather Manufacture, Sichuan UniVersity, Chengdu 610065, P. R. China
A novel catalyst for the Fenton reaction was prepared by immobilizing Fe(III) onto collagen fiber, and its catalytic activity for the photoassisted decomposition of Orange II was investigated. The results indicated that this catalyst, Fe(III)-immobilized collagen fiber (FICF), follows an adsorption-decomposition mechanism so that it can effectively accelerate the decoloration and mineralization rates of Orange II in aqueous solution. Catalyzed by FICF (0.5 g/L, 91 mg/g Fe loading), the TOC of Orange II solution (1000 mL, 0.2 mM, pH 6.42) was 62.9% removed within 90 min under UVC irradiation (254 nm, 10 W) in the presence of H2O2 (5.0 mM); meanwhile, the dye solution was completely decolorized. FICF can be recycled 10 times with little activity loss, and its catalytic activity can be easily recovered by reimmobilization of Fe(III). Therefore, FICF could act as an efficient and cost-effective catalyst for the photoassisted decomposition of Orange II. 1. Introduction Dyes are widely employed in industries such as the printing and dyeing of textiles, the manufacturing of leather, and so on, and they are usually applied in excess to ensure full penetration/ reaction so as to guarantee the quality of the product. The excess dyes used inevitably remain in the effluents whose high pollution loads1,2 and deep colors are very difficult to remove by conventional chemical and biological methods.3 Therefore, an effective treatment is necessary for the dye-containing effluents before they are discharged or mixed with other wastewaters, in order to prevent the possibly serious pollution of nearby bodies of water.4 Many measures including coagulation,5,6 adsorption,7,8 and oxidation processes9,10 have been studied for the treatment and decoloration of dye-containing wastewaters. Among them, photoassisted oxidation has been found to be a most effective approach, especially in terms of cost, for degrading dyes and eliminating the deep colors of wastewaters.4 Some metal ions such as Fe(III) are employed as catalysts for the photoassisted decomposition of dyes, and they are usually immobilized onto some supporting matrixes such as resin11 and zeolite12 to avoid secondary metal pollution. Such supported catalysts greatly advance the application of photoassisted oxidation technology in dye waste treatments, although the stability of the immobilized metal ions should be further improved. As an abundant natural biomass, collagen fiber mainly comes from the skins of domestic animals and is traditionally used as a raw material in the leather industry. This natural biomass contains plenty of functional groups such as sOH, sNH2, and sCOOH, and it is capable of chemically reacting with many kinds of metal ions, including Cr(III), Fe(III), and Zr(IV),13 according to the principles of leather processing. It has been demonstrated that collagen fiber primarily combines Fe(III) through the formation of hydroxyl complexes between Fe(III) and the sCOOH groups on the collagen side chains.14 Therefore, Fe(III) can be stably immobilized onto collagen fiber so as to serve as a heterogeneous catalyst for the photoassisted oxidation/decomposition of dyes. Because collagen fiber is able * To whom correspondence should be addressed. Tel.: +86-2885405508. Fax: +86-28-85400356. E-mail:
[email protected] (B.S.),
[email protected] (X.P.L.).
to adsorb dyes based on a physical or chemical mechanism,15 it is reasonable to deduce that the adsorption effect could promote the catalytic decomposition of dyes by immobilized Fe(III). In this work, a novel heterogeneous catalyst, Fe(III)immobilized collagen fiber (FICF), was prepared and characterized by UV-vis diffuse reflectance spectroscopy (DRS), X-ray diffraction (XRD), and differential scanning calorimetry (DSC). Orange II, a typical organic dye, was used as a model compound for the photoassisted decomposition study. The adsorption of Orange II onto FICF and the corresponding catalytic decomposition reaction were investigated. The reaction conditions including pH, FICF dosage, and H2O2 concentration were optimized. Furthermore, the stability of FICF and the catalytic activity of recycled FICF were also investigated for the purposes of practical industrial application. 2. Materials and Methods 2.1. Materials and Reagents. Orange II (C.I. 15510, Acid Orange 7, dye content > 95%), hydrogen peroxide, and other reagents (AR) were purchased from Kelong Chemical Reagents Co., Ltd. (Chengdu, China). Collagen fiber was prepared according to the procedures reported in our previous work.16 Catalyst FICF was prepared according to the following process: Collagen fiber (15.0 g) was first soaked in distilled water (400 mL, preadjusted to pH 1.7-2.0) at room temperature for 24 h. Then, the immobilization reaction of Fe(III) onto collagen fiber was initiated by adding 5.0 g of Fe2(SO4)3 into the solution and stirring the mixture constantly at 40 °C for 4 h. Subsequently, an appropriate amount of NaHCO3 solution (15% w/w) was gradually added into the reaction system over the course of 2 h in order to increase the pH to 3.0-3.5, and the reaction continuously proceeded at 40 °C for another 4 h. Then, the catalyst, denoted as FICF5, was collected by filtering, washed with distilled water, and dried at 50 °C for 12 h. Similarly, FICF10, FICF15, and FICF20 were prepared using 10.0, 15.0, and 20.0 g, respectively, of Fe2(SO4)3 to react with 15.0 g of collagen fiber. Catalyst FICF5 was employed for the adsorption and catalytic decomposition experiments in this research, unless otherwise indicated.
10.1021/ie801330m CCC: $40.75 2009 American Chemical Society Published on Web 01/08/2009
Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009 1459
Figure 1. Schematic of photoreactor: (1) sample inlet, (2) inner quartz tube (40 × 4 cm i.d.), (3) low-pressure mercury vapor lamp, (4) outer glass tube (45 × 8 cm i.d.), (5) air pump, (6) valve, (7) bubbler, (8) sample outlet with filter
2.2. Adsorption of Orange II to FICF. The adsorption of Orange II to FICF was conducted by adding 1 g of FICF into 1000 mL of Orange II solution (0.2 mM, pH 6.42) and shaking (130 rpm) the mixture constantly at room temperature for 90 min. The concentration of Orange II remaining in solution was monitored using a UV-vis spectrophotometer (Shimadzu UV2501PC, λ ) 485 nm). The dye-adsorbed FICF was collected by filtration and then analyzed by UV-vis DRS (TU-1901). 2.3. Photoreactor and Catalytic Decomposition of Orange II by FICF. The photoreactor was self-designed, as shown in Figure 1. Catalyst FICF, hydrogen peroxide, and 1000 mL of Orange II solution were mixed in the outer glass tube of the reactor. The stirred mixture was irradiated with air by a low-pressure mercury vapor lamp (10 W, λ ) 254 nm) installed inside the inner quartz tube. The photoassisted decomposition of Orange II was conducted at room temperature, and 5 mL of reaction solution was sampled at indicated intervals for UV-vis spectroscopy, total organic carbon (TOC), and inductively coupled plasma Auger electron spectroscopy (ICP-AES), as well as the determination of H2O2 concentration. 2.4. Analysis Method. The structure of FICF was characterized by UV-vis DRS (TU-1901) and X-ray diffraction (X’Pert Pro, PANalytical, Almelo, The Netherlands). The thermal stability (denaturation temperature) of FICF was measured using differential scanning calorimetry (2000PC, NETZSCH, Frankfurt, Germany). To determine the amount of Fe(III) loaded on the FICF, the catalyst was completely dissolved in aqua regia, and then the content of Fe(III) was analyzed using a PerkinElmer Optima 2100DV spectrometer. ICP-AES was used to detect the leakage of Fe ions from FICF during the process of catalytic photoassisted decomposition of Orange II. The concentration of Orange II remaining in solution was monitored by UV-vis spectroscopy. A TOC analyzer (LiquiTOC, Elementar, Hanau, Germany) was used to measure the total organic carbon (TOC) in solution, in order to characterize the mineralization degree of Orange II. The residual concentration of H2O2 in the reaction solution was monitored according to the iodometric method.17 3. Results and Discussion 3.1. Immobilization of Fe(III) onto Collagen Fiber. The Fe loading on FICF varied with the amount of Fe2(SO4)3 used in immobilization process, and it was found to be 91, 130, 154, and 170 mg/g for FICF5, FICF10, FICF15, and FICF20, respectively. As shown in Figure 2, the results of UV-vis DRS
Figure 2. UV-vis diffuse reflectance spectra of FICF and collagen fiber.
Figure 3. XRD patterns of collagen fiber and FICF.
indicate an obvious difference in absorbance between collagen fiber and FICF. Collagen fiber has a very low absorbance within 400 and 700 nm, whereas all of the FICF samples with varying Fe loadings showed a marked absorbance in this wavelength range. This observation confirms a well-known fact that Fe(III) can chemically combine with collagen fiber.14 As shown in Figure 3, the intensity of the diffraction peak of FICF was greatly reduced due to the cross-linking between Fe(III) and collagen fiber.18 Moreover, no new diffraction peak was found, which suggests that the Fe(III) loaded on FICF is in a complex state and that no crystal structure was generated. The denaturation temperature of FICF, determined by DSC, was increased to 82-87 °C compared with that of collagen fiber (60-65 °C), indicating an increased thermal stability of FICF due to the complex reaction between Fe(III) and collagen fiber. All of these results confirmed that Fe(III) can be immobilized onto collagen fiber and that the FICF obtained is chemically and physically stable to be used as catalyst for the photoassisted decomposition of Orange II. 3.2. Adsorption of Orange II to FICF. As a typical azo dye, Orange II contains some polar groups such as sSO3- and sOH, and therefore, it should be adsorbed onto FICF through hydrogen bond or ionic bond.19 As presented in Table 1, Orange
1460 Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009 Table 1. Adsorption of Orange II onto FICF* time (min)
residual Orange II (mM)
adsorption ratio (%)
pH
0 10 30 50 70 90
0.2000 0.1300 0.1150 0.1068 0.1074 0.1072
0 35.0 42.5 46.6 46.3 46.4
6.42 3.89 3.89 3.87 3.85 3.83
* Adsorption conditions: 0.2 mM Orange II, initial pH 6.42, 1 g/L FICF, room temperature. Values are the means of three replicates.
Figure 5. Effect of FICF dosage on extent of decoloration from Orange II solution (0.2 mM Orange II, 5 mM H2O2, pH 6.42, 254-nm UVC, 10 W; values are the means of three replicates).
Figure 4. UV-vis DRS results for (a) FICF, (b) Orange II-adsorbed FICF, and (c) FICF collected after 90 min of catalytic decomposition (reaction conditions: 0.2 mM Orange II, pH 6.42, 1 g/L FICF, 5 mM H2O2, 254-nm UVC, 10 W, room temperature).
II showed rapid adsorption to FICF, and 35% of it was removed from solution within 10 min. The adsorption equilibrium was attained within 50 min, at which point more than 46% of the Orange II was adsorbed onto the FICF. The adsorption of Orange II onto FICF was accompanied by a change in the pH of the dye solution, which quickly decreased from 6.42 to 3.89 in the first 10 min and then kept steady at 3.89 around. After adsorption, FICF exhibited a new absorbance peak near 480 nm in its UV-vis DRS spectrogram, as shown in Figure 4b. This new peak is in accordance with the characteristic absorbance of Orange II, which is a further demonstration of the adsorption of the dye by FICF. It was found that the catalyst FICF was fully colored by Orange II in experiments when it was soaked in the dye solution, owing to the adsorption effect. This adsorption effect should occur along with the catalytic photoassisted decomposition of Orange II by FICF. Consequently, the catalyst would be decolorized as a result of the decomposition of Orange II adsorbed on FICF. In fact, these phenomena were observed by UV-vis DRS analyses. The peak of Orange II-adsorbed FICF at 480 nm (see Figure 4b) disappeared after the decomposition reaction, as shown in Figure 4c, which indicates that the active surface of FICF was recovered. The photoassisted decomposition of Orange II catalyzed by FICF is discussed in detail in the next section. 3.3. Photoassisted Decomposition of Orange II Catalyzed by FICF. 3.3.1. Decoloration and Photoassisted Decomposition of Orange II Solution in the Presence of FICF. As shown in Figure 5, FICF led to the removal of Orange II in the presence of H2O2 and UVC, and consequently, the dye solution was gradually decolorized. This decoloration can be
Figure 6. Effect of FICF dosage on extent of TOC removal from Orange II solution (0.2 mM Orange II, 5 mM H2O2, pH 6.42, 254-nm UVC, 10 W; values are the means of three replicates).
attributed to two effects: adsorption and the catalytic decomposition of Orange II by FICF. TOC analyses demonstrate that Orange II was mineralized during the decomposition process, as shown in Figure 6. The Orange II solution was decolorized/ decomposed with decreased absorbance at 485 nm once the chromophoric groups in the dye molecules were broken. However, the decomposition of Orange II into intermediates does not mean that it has been mineralized. As a result, the TOC removal rate of a dye solution is generally lower than the decoloration rate (Figures 5 and 6) In general, a higher decolorizing/mineralizing efficiency was observed when more FICF was employed during the first 50 min of the decomposition reaction, but the different dosages of FICF nearly resulted in the same total decoloration of Orange II solution with a TOC removal extent of 60-64% within 90 min (Figures 5 and 6). A higher loading of the FICF catalyst can promote the adsorption and catalytic decomposition of Orange II, allowing for faster decoloration of the dye solution. However, a higher dosage of FICF also prevents UVC from penetrating the solution and weakens the mineralization of Orange II.20 So, the increase of FICF from 0.25 to 1.00 g in 1 L of Orange II solution did not lead to an obvious change of the TOC removal extent after 90 min of reaction. However, in
Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009 1461 Table 2. Extent of TOC Removal from Orange II Solution during Photoassisted Decompositiona TOC removal (%) time (min)
FICF5
FICF10
FICF15
FICF20
10 50 90
25.9 44.9 62.9
27.6 55.1 66.1
28.8 62.7 69.2
25.7 68.1 73.8
a Conditions: 0.2 mM Orange II, 5 mM H2O2, pH 6.42, 254-nm UVC, 10 W, 0.5 g/L FICF. Values are the means of three replicates.
another series of experiments, an increasing extent of Orange II mineralization was observed when the Fe load on the catalyst was increased. As presented in Table 2, the TOC removal extents of Orange II solution (0.2 mM) achieved within 90 min using FICF5, FICF10, FICF15, and FICF20 (0.5 g/L) were 62.9%, 66.1%, 69.2%, and 73.8%, respectively. All of these results indicate that FICF can effectively catalyze the photoassisted decomposition of Orange II so as to decolorize and mineralize the dye solution. 3.3.2. Effect of pH on Leakage of Fe from FICF during the Photoassisted Decomposition of Orange II. As discussed above, the photoassisted decomposition of Orange II can be improved by increasing the Fe loading on FICF. In contrast, the leakage of Fe from FICF can reduce its catalytic activity, especially for repeated uses of the catalyst. It is wellknown that Fe(III) can be immobilized on collagen fiber through the formation of hydroxyl complexes between Fe(III) and the sCOOH groups on the collagen fiber14 and that the hydroxyl complexes remain stable within the pH range of 3.0-7.0. A certain amount of Fe(III) can leak from FICF during the photoassisted decomposition of Orange II as a result of the conversion of oxidation and reduction states between Fe(III) and Fe(II) in the Fenton reaction system, as shown in eqs 1 and 2.21,22 The leaked Fe(III) can be reimmobilized onto collagen fiber when the environmental pH is in the range of 3.0-7.0. However, such a reimmobilization would not take place under more acidic condition, because the sCOOH groups on collagen fiber are not ionized and therefore hydroxyl complexes between Fe(III) and the sCOOH groups are seldom formed.23 So, the photoassisted decomposition of Orange II, catalyzed by FICF, should be performed in a suitable pH range. The natural pH of Orange II solution (0.2 mM) is 6.42, and it quickly decreased to 3.74 in the first 10 min of the photoassisted decomposition reaction because of the adsorption of the dye onto FICF, which is similar to the observation discussed above. Then, the pH gradually decreased to 3.69 in 90 min of reaction in the presence of H2O2 (5 mM) and FICF (1.0 g/L). Meanwhile, about 64.0% of TOC was removed, and the Fe leakage from the FICF remained at a very low level in solution (
