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Activation of Na2S2O8 for dye degradation by Fe complexes fixed on polycarboxylic acids modified waste cotton
Carbohydrate Polymers 181 (2018) 103–110
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Carbohydrate Polymers journal homepage: www.elsevier.com/locate/carbpol
Research Paper
Activation of Na2S2O8 for dye degradation by Fe complexes fixed on polycarboxylic acids modified waste cotton
MARK
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Guangzeng Liua, Yongchun Donga,b, , Peng Wanga, Liran Biana a b
Division of Textile Chemistry & Ecology, School of Textiles, Tianjin Polytechnic University, Tianjin 300387, China Key Laboratory of Advanced Textile Composite of Ministry of Education, Tianjin Polytechnic University, Tianjin 300387, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Polycarboxylic acid Industrial waste cotton Complex Na2S2O8 activation Dye degradation
Three aliphatic polycarboxylic acids (PCAs) including tartaric acid (TA), citric acid (CA) and 1,2,3,4-butanetetracarboxylic acid (BTCA) were used for the surface modification of waste cotton fiber by an industrialized pad-dry-cure process to introduce carboxyl groups, which then coordinated with Fe3+ ions to obtain three PCA modified cotton fiber Fe complexes. TA modified cotton fiber could easily react with Fe3+ ions to form its complex with high Fe content. Furthermore, TA modified cotton fiber Fe complex showed a better enhanced effect of activated Na2S2O8 to produce free oxygen radicals for the degradation of an azo dye, Reactive Red 195 than the other two complexes. Different critical Fe contents were found for three complexes to obtain the best enhanced effect.
1. Introduction Polymer metal complexes (PMCs) are defined as complexes consisting of a polymer ligand and metal ions, in which the metal ions are attached to the polymer ligand by a coordinate bond. The metal ions introduced into a polymer chain often causes structural diversity, thus improving polymer chemical and physical behavior, and causing the uneven and complicated surface morphology (El-Sawy & Ali, 2010). Over the past two decades, PMCs have received considerable attention due to their unique electric, mechanical, thermal and catalysis properties. Some PMCs have been developed as the efficient and environment friendly heterogeneous catalysts in order to avoid some drawbacks of homogeneous catalysts such as corrosion, difficulty in separation and deficiency of regeneration (Islam, Roy, Mondal, & Salam, 2012). Furthermore, the properties of PMCs were found to depend on the nature of the metal ion and its content in the compounds. On the other hand, the selection of appropriate ligands is crucial to determine the structures and properties of the resulting complexes (Song et al., 2017; Wang, Zhang et al., 2015; Wang, Wang et al., 2015). Polycarboxylic acids (PCAs) as the carboxylate ligands have been widely used in the synthesis of PMCs due to their diversity of coordination modes and high structural stability (Du, Liu, Li, & Fang, 2013; Song et al., 2017; Wang et al., 2013; Wang, Zhang et al., 2015; Wang, Wang et al., 2015). Moreover, many aromatic PCAs were employed to react with phenanthroline, triazine or pyridyl compounds for the synthesis of complex photocatalysts (Song et al., 2017; Wang, Zhang
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et al., 2015; Wang, Wang et al., 2015; Zhang, Wang, Wang, & Gao, 2015). However, these complexes are known to be non-biodegradable or toxic to aquatic plants and animals because of their high composition ratio of aromatic rings present in their coordination structure. Besides, the difficulty in recovery of the catalyst from the reaction mixture and oxidative self-destruction of the catalyst in the oxidizing media are often considered as two main drawbacks. It is well known that H2O2 is the most commonly used oxidizing agent in the advanced oxidation processes (AOPs) for degrading organic pollutants in water because it is a cheap and environmentally friendly oxidant. However, it is fact that the handling of concentrated H2O2 solutions is not desirable for safety problems, and H2O2 rarely has a half-life more than 48 h (Chen et al., 2011; Habibi, Zolfigol, Safaiee, Shamsian, & Ghorbani-Choghamarani 2009; Wu et al., 2014). Therefore, an attempt should be made to find alternative oxidizing agents with higher stability and similar activity instead of H2O2 for AOPs. Persulfate has been applied for AOPs technologies due to their advantages of cost effectiveness, high activity and the environmentally friendly nature (Bjoersvik, Merinero, & Liguori, 2005; Gao et al., 2016; Matzek & Carter, 2016; Wu et al., 2014; Yang, Yang, Shao, Niu, & Wang, 2011). Additionally, it is relatively stable solids and is thus easy to handle on a large-scale and permit accurate dosage (Bjoersvik et al., 2005; Habibi et al., 2009). Specifically, sodium persulfate (SPS) has been found to be activated by heat, light irradiation or transition metal ions to produce free sulfate radicals (%SO4−) with a high oxidation potential (E0 = 2.6 V), which is comparable to that of %OH (E0 = 2.7 V)
Corresponding author at: Division of Textile Chemistry & Ecology, School of Textiles, Tianjin Polytechnic University, 399 Bingshui West Road, Xiqing District, Tianjin 300387, China. E-mail addresses: [email protected], [email protected] (Y. Dong).
http://dx.doi.org/10.1016/j.carbpol.2017.10.060 Received 18 August 2017; Received in revised form 7 October 2017; Accepted 16 October 2017 Available online 18 October 2017 0144-8617/ © 2017 Published by Elsevier Ltd.
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Fig. 1. Esterification of cotton fabric with three PCAs containing 2, 3 and 4 carboxyl groups.
(Huie, Clifton, & Neta, 1991; Wu et al., 2014). Furthermore, %SO4− is more efficient than %OH to oxidize some refractory organic compounds due to its selective oxidation ability (Neta, Huie, & Ross, 1998; Wu et al., 2014). On the other hand, cotton fiber is the commonly used cellulose fiber in the modern textile industry. Cotton fiber with production of more than 27 million tons per year, accounts for 30% of total fiber production worldwide (Wanassi, Azzouz, & Hassen, 2016). Huge production capacity of cotton textile industry has lead to higher output of waste cotton fibers because different cotton wastes are generated from the initial stage of harvesting cotton crop to the final stage of finished textile product. In recent years, industrial waste cotton has been regarded as the one of the major sources of the cellulosic wastes for the recycling. Some attempts have been made to reuse industrial waste cotton for the production of porous carbonaceous materials, biofuels or cellulose nanocrystals and fibers, etc (Ismail & Talib, 2016; Nikolić et al., 2017; Thambiraj & Shankaran, 2017; Zheng, Zhao, & Ye, 2014). However, these recycling processes were often conducted at high temperature or require the complicate high-cost facilities, which are more energy consuming. The main purpose of this work here is to fabricate a sustainable, low-cost and highly effective PCAs immobilized cellulose materialbased catalyst using the typical industrial waste cotton, comber noil from yarn spinning process, which are produced when cotton is combed in comber machine to remove short fibers. Hence, three aliphatic PCAs, namely tartaric acid, citric acid and 1,2,3,4-butanetetracarboxylic acid were used for the surface modification of waste cotton fiber through an industrialized pad-dry-cure process because these aliphatic PCAs are the safe and non-toxic chemicals (Shen, Xu, Kong, & Yang, 2015; Soleimani-Gorgani & Karami, 2016). The modified cotton fibers were then coordinated with Fe3+ ions to prepare three carboxylic cotton fiber Fe complexes as the heterogeneous photocatalysts. Thus, waste cotton fiber is considered as an ideal supporting material or carrier for the PCA Fe complex catalyst in the practical application due to its low cost, environmental friendliness, suitable mechanical stability and easy industrialized modification process. The effect of different PCAs on the surface structures and enhanced activation of three complexes for Na2S2O8 used in the oxidative degradation of a typical azo dye, Reactive Red 195 in water was investigated. Besides, the oxidation mechanisms were studied and proposed.
application, waste cotton fibers were firstly spun into 80 tex yarns, which were then weaved into a cotton mesh fabric. The open mesh structure showed an advantage of better hydrodynamic properties of significance to industrial scale application. The obtained cotton fabric was further treated with a solution containing 10.0gL−1 NaOH and 5.0 gL−1 anionic detergent at the boil for 60 min to remove the impurities from its surface, then thoroughly washed with cold water and dried at ambient temperature before use. Tartaric acid (TA), citric acid (CA), 1,2,3,4-butanetetracarboxylic acid (BTCA), NaH2PO2, Fe2(SO4)3, H2O2 (30%wt), Na2S2O8, isopropyl alcohol (IPA), tert-butyl alcohol (TBA) and 1, 4-benzoquinone (BQ) were of analytical grade and used without further purification. 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) was purchased from Micklin Biochemical Technology Co., Ltd (Shanghai, China). A typical commercial azo dye, Reactive Red 195 (RR 195, CAS: 93050-79-4) was used after purification by the re-precipitation method (Greaves, Churchley, Hutchings, Phillips, & Taylo, 2001). All solutions were prepared in deionized water. 2.2. Fabrication of PCA modified cotton fiber Fe complexes In a typical procedure, cotton fabric was modified in a consecutive two stage process to produce a complex: Stage (1): Modification of cotton fabric with PCAs Cotton fabric was first immersed in an aqueous solution containing various concentration of PCA and NaH2PO2 (5.0%w/w) at room temperature for 10 min, then pressed through a two-roll laboratory mangle (Mathis AG, Switzerland) to give a wet pick-up of 75–80% based on the weight of fabric. Afterward, the padded fabric was dried at 80 °C for 5 min, and cured at 140 °C for 1.5 min to obtain PCA modified cotton fiber (denoted as PCA-cotton), which was washed thoroughly, rinsed, and dried. The modification of cotton fabric with different PCAs was described in Fig. 1. Carboxyl group content in PCA-Cotton samples (QCOOH, mmolg−1) was determined using a titration method described in our previous work (Li, Dong, & Li, 2015). Briefly, about 0.50 g of the dried PCACotton pieces were dispersed in 50 mL of 0.10molL−1 NaOH aqueous solution under a nitrogen atmosphere and stirred for 2 h at room temperature. The amount of the unneutralized NaOH in solution was then determined by titration with standardized 0.10 mmolL−1HCl aqueous solution using an automatic titrator (Shanghai Jingmi Instrumental Co. China). QCOOH values of the samples were calculated as: QCOOH = (V1C1-V2C2)/m, where C1 and C2 are the concentrations of NaOH and HCl aqueous solution (molL−1), respectively. V1 is the volume of NaOH solution (50 mL). V2 is the volume of HCl solution consumed in titration (mL) and m is the mass of dry PCA-Cotton used (g). Stage (2): Coordination of PCA-Cotton with Fe3+ ions 5.0 g of PCA-Cotton was impregnated in 150 mL of the given concentration of Fe3+ ion aqueous solution. The mixtures were kept at
2. Experimental 2.1. Materials and chemicals Waste cotton fibers (comber noil) were collected from a local spinning and weaving industry mills in Tianjin area, China. To enable easier access of reagents to the fiber surface during the preparation and 104
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Fig. 2. Fabrication of Fe-PCA-Cotton at different experimental conditions.
reaction and their contribution to dye degradation, three radical scavengers IPA (%OH and %SO4− scavenger), TBA (%OH scavenger, but not %SO4−) (Buxton, Greenstock, Helman, & Ross, 1998; Monteagudo, Durán, Martin, & Carnicer, 2011) and BQ (O2−% scavenger) (Buxton et al., 1988; Wu et al., 2014) were adopted in the dye degradation experiments according to their reactive activities with each of the free oxygen species potentially present in Fe-PCA-Cotton/SPS or H2O2 system. All the experiments were carried out in triplicate.
50 °C for 3 h under continuous agitation. The obtained PCA-Cotton Fe complex (denoted as Fe-PCA-Cotton) was then taken out, washed with deionized water repeatedly and dried under vacuum at 60 °C for 3 h. The residual concentration of Fe3+ ions in the coordinating solution was determined using a Varian Vista-MPX inductively-coupled plasma optical emission spectroscopy (ICP-OES), for calculating the Fe content (QFe) of the complex. 2.3. Characterization of PCA-Cotton Fe complexes
2.5. Analytical methods The surface morphology of Fe-PCA-Cotton was observed by S-4800 scanning electron microscope (Hitachi High-Tech Co., Japan) operating at 15 kV. The composition of Fe-PCA-Cotton was verified by a Nicolet Magna-560 Fourier transform spectrometer (Nicolet Instrument Co., USA) with 4 cm−1 resolution. The X-ray diffraction measurement and the binding energy analysis of Fe-PCA-Cotton were also conducted on a Rigaku Xd/Max-2500 X-ray diffractometer (Rigaku Co., Japan) and a PHI 5600 X-ray photoelectron spectrometer (Pekin Elmer Inc., USA), respectively. The contact angle of a water droplet on the surface of sample was also measured using a liquid-solid contact angle analyzer (DSA100; Kruss, Germany) equipped with a high speed camera. Approximately 0.60 μL of deionization water was transferred to each fiber.
The concentration of RR 195 in test solution during the degradation experiment was determined using a UV-2401 spectrophotometer (Shimadzu Co., Japan) at 522 nm (maximum absorption wavelength of RR195). Dye decoloration percentage at a given time was calculated as: D% = (1 − Ct/C0) × 100%
(1)
where C0 was the initial concentration of the dye (0.05 mmolL−1), and Ct was the concentrations of the dye in test solution at a given time (mmolL−1). The content of total organic carbon (TOC) of test solution was assayed by a TOC-VCPH analyzer (Shimadzu Co., Japan). TOC removal percentage of the test solution was calculated as follows: TOC Removal (%) = (1 − TOCt/TOC0) × 100%
(2)
−1
where TOC0 is the initial TOC value (mgL ), and TOCt is the residual TOC value of test solution during the reaction (mgL−1), respectively. ESR (Electron Spin Resonance) spectra of the radical spin trapped by DMPO were examined to identify the main free radicals present in the reaction using a JES-FA200 ESR spectrometer (JEOL, Japan) with an irradiation source of Quanta-Ray Nd:YAG pulsed laser system (λ = 532 nm).
2.4. Catalytic evaluation Fe-PCA-Cotton was assessed as a catalyst for SPS or H2O2 activation by the degradation of RR195 conducted in a specially designed photoreaction system presented in our previous work (Han, Dong, & Dong, 2011). A cut-off filter was used to ensure that a complete visible light irradiation (λ > 420 nm). Light intensity inside photoreaction system was measured to be 9.65 mWcm−2 using FZ-A radiometer (BNU Light and Electronic Instrumental Co., China), and the temperature in reaction vessel was kept at 25 ± 1 °C. Unless noted otherwise, all the degradation experiments were performed in 100 mL of test solution containing 0.05 mmolL−1 RR195 and 3.0 mmolL−1 SPS or H2O2 at an initial pH of 6.0, with 0.50 g of Fe-PCA-Cotton. To investigate the free oxygen radicals generated during the
3. Results and discussion 3.1. Fabrication of PCA-Cotton Fe complexes Cotton fabric were modified with three PCAs including TA, CA and BTCA, respectively to introduce the carboxyl groups, which 105
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Fig. 3. SEM images of original cotton fiber (a) and three PCACotton Fe complexes including Fe-TA-Cotton (b), Fe-CACotton (c) and Fe-BTCA-Cotton (d).
coordinated with Fe3+ ions for obtaining three PCA-Cotton Fe complexes, and the results were shown in Fig. 2. Fig. 2(a) shows the effect of PCA concentration on QCOOH values of the modified cotton fabrics. It was clear that QCOOH values of three PCACottons increased with increasing PCAs concentration, and the increasing tendency became level at the PCA concentration of 1.00 molL−1, proposing the maximum QCOOH values (1.14 mmolg−1 for TACotton, 1.51 mmolg−1 for CA-Cotton, 2.16 mmolg−1 for BTCA-Cotton,) of three PCA-Cottons were achieved. This is because higher concentration of PCAs could enhance the esterification between carboxyl groups of PCAs and hydroxyl groups in cellulose chains on cotton fiber (Bolin, Peixin, Kelu, & Gang, 2015), thus leading to a gradual enhancement in their QCOOH values. Furthermore, when the same concentration of PCA was used, their QCOOH values ranked in this order: BTCA-Cotton > CA-Cotton > TA-Cotton, suggesting that increasing the length of the backbone and number of carboxyl groups in the PCAs structure could enhance the QCOOH value of the resulting PCA-Cotton. It was reported that aliphatic PCAs with more than two carboxyl groups have better crosslink ability than dicarboxylic acids, and BTCA with 4 carbons in its backbone and 4 carboxyl groups showed the better crosslinking effects than CA with 3 carbons in its backbone and 3 carboxyl groups, leading to the strongest intermolecular interaction (Shen et al., 2015; Song, Xu, Xie, & Yang, 2016), thus produced crosslinked cotton fiber with high QCOOH value. BTCA or CA first forms a cyclic anhydride by dehydration of the adjacent two carboxyl groups at high temperature, which then reacts with one or two hydroxyl groups of cotton fiber to form PCA-Cotton containing one or two carboxyl groups for causing the crosslink (Li et al., 2015). While dicarboxylic acid is unable to crosslink celluloses because its short backbone restricted its range of crosslinking (Ji, Zhao, Yan, & Sun, 2016; Song et al., 2016; Yang, Qian, & Lickfield, 2001). However, TA with 2 carbons in its backbone and 2 carboxyl groups is able directly to react with cellulose molecule to impart only one carboxyl group (Ji et al., 2016), thus leading to a low QCOOH value. Fig. 2(b) displays a different change in QFe values of the obtained complexes prepared by three PCA-Cottons with varied QCOOH values. QFe value of Fe-TA-Cotton was higher than those of the other complexes. Moreover, higher QCOOH value of TA-Cotton gradually increased the QFe value of the resulting complex. This is because high QCOOH value could provide more carboxyl groups, which may prefer to react with
Fe3+ ions by intermolecular coordination, thus enhancing the immobilization of Fe3+ ions on fiber surface. By contrast, CA-Cotton and BTCA-Cotton have the different coordinating performance with Fe3+ ions from TA-Cotton. QFe values of the two complexes increased with increasing their QCOOH values, and a highest QFe value was achieved at QCOOH value of 1.0 mmolg−1 for CA-Cotton or 0.75 mmolg−1 for BTCACotton. This was due mainly to the space steric hindrance resulted by their complicated crosslink structure between cellulose chains, which could limit the coordination of Fe3+ ions with their carboxyl groups. Furthermore, higher crosslink degree of BTCA-Cotton than CA-Cotton as mentioned above was responsible for lowest QFe value of the resulting complex. As shown in Fig. 2(c), when three PCA-cotton with similar QCOOH values (approximately 0.85 mmolg−1) being used, increasing initial concentration of Fe3+ ions caused a gradual increment in QFe value of the resulting complexes, and increasing tendency becomes level at 150 mmolL−1of Fe3+ ions for TA-Cotton and CA-Cotton, and 125 mmolL−1of Fe3+ ions for BTCA-Cotton, which was attributed to higher crosslink degree of BTCA-Cotton, thus retarding the penetration of Fe3+ ions into the modified cotton fiber from water during the reaction. Besides, at a given initial concentration of Fe3+ ions, QFe values of three complexes were ranked as follow: Fe-TA-Cotton > FeCA-Cotton > Fe-BTCA-Cotton. This is mainly owing to the big difference in the crosslink degree between these PCA-Cotton samples. 3.2. Surface morphology and composition analysis Fig. 3(a) shows that a round surface of original cotton fiber is comparatively smooth. A mud-like layer was found from Fig. 3(b–d) on cotton fiber to cause the rough and uneven surface of three Fe-PCACottons, demonstrating the modification of cotton fiber with PCAs and subsequent coordination with Fe3+ ions. Fig. 4(A–B) shows several characteristic absorption peaks of original cotton fiber at 3340, 2900, 1431, 1316, 1158, 1061, 1033 and 905 cm−1, owing to the stretching of OH, CH, CO and CeOeC, respectively (Li et al., 2015; Sun, Lin, Deng, & Li, 2008). A peak around 1720 cm−1representing carbonyl stretching vibration of the carboxyl groups and ester carbonyl bands was found in the spectra of all the PCA-Cotton, confirming that the carboxyl groups have been introduced into surface structure of cotton fiber by ester linkage with PCAs. More importantly, this peak became much less intensive, and a new peak centered at 1630 cm−1 appeared 106
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Fig. 4. FTIR spectra of three PCA-Cottons (A) and their Fe complexes (B), XPS (C) and XRD (D) spectra of cotton fiber and three complexes.
As shown in Fig. 5(A), D% values in the presence of only H2O2 or SPS in 90 min were less than 20% (curves a-b), proposing that little degradation of RR195 took place. While Fe-PCA-Cotton was used, D% values significantly increased with the prolonging of reaction time (curves c-h), especially in the presence of SPS. Moreover, D% values with SPS/Fe-PCA-Cotton systems (curves d, g and h) were higher than those with H2O2/Fe-PCA-Cotton systems (curves c, e and f), correspondingly. This suggested that Fe-PCA-Cotton exhibited a stronger activity for SPS than H2O2 used in dye degradation. According to the D % values, three complexes were ranked in this order: Fe-TACotton > Fe-CA-Cotton > Fe-BTCA-Cotton in the case of H2O2 or SPS, indicating that Fe-TA-Cotton has a better activation effect for H2O2 or SPS than the other two complexes at the same conditions. To investigate the impact of the QFe value of the complexes on their photocatalytic capacity, the degradation of RR 195 with SPS was performed in the presence of three Fe-PCA-Cotton containing different QFe values, respectively. The pseudo-first-order rate constants, k of dye degradation were calculated with all regression coefficients greater than 0.95 and shown in Fig. 5(B). Increasing the QFe value is accompanied with an almost linear enhancement in k values. And highest k values were obtained at different critical QFe values (QFe-c: 0.53 mmolg−1 for Fe-TACotton, 0.49 mmolg−1 for Fe-CA-Cotton and 0.37 mmolg−1 for Fe-BTCA-Cotton). These indicated that three complexes could promote the SPS decomposition to generate various free radicals, which are
in the spectra of three complexes, especially Fe-TA-Cotton and Fe-CACotton, revealing that Fe3+ ions have participated in the coordination with hydroxyl groups of cellulose chains. According to their XPS spectra shown in Fig. 4(C), cotton fiber surface was composed mainly of carbon and oxygen elements, and a small amount of Fe element was observed for three complexes. The binding energy of the Fe2p in the complexes are between 709.95 eV and 710.93 eV, lower than that (711.3 eV) of the Fe2p in FeCl3 (Dong, Han, Liu, & Du, 2010). On the other hand, the binding energy of the O1 s in the complexes shifted to higher binding energy level, proposing that oxygen atom as the electron donor reacted with Fe3+ ions to form complexes. Besides, it was found from Fig. 4(D) that the peaks at 22.95° of three complexes were lower than that of untreated cotton fiber, suggesting that the modification and coordination may decrease their crystallinity, thus possibly reducing their mechanical property. 3.3. Activation of SPS by different PCA-Cotton Fe complexes Three Fe-PCA-Cotton containing similar QFe values (about 0.35 mmolg−1) were prepared by optimizing the modification and coordination processes and then tested as the heterogeneous photocatalysts for the activation of H2O2 or SPS used in the degradation of RR 195 under visible irradiation, respectively. D% values of RR 195 during the reactions were measured and shown in Fig. 5(A).
Fig. 5. (A): D% values at different systems (a) H2O2, (b) SPS, (c) H2O2/Fe-BTCA-Cotton, (d) SPS/FeBTCA-Cotton, (e) H2O2/Fe-CA-Cotton, (f) H2O2/FeTA-Cotton, (g) SPS/Fe-CA-Cotton, (h) SPS/Fe-TACotton; (B): Effect of QFe on k values in the presence of different Fe-PCA-Cotton.
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intensity or low fluxes during the reaction. To further understand the generation of various free radicals and their contribution to dye degradation in SPS/Fe-TA-Cotton system, the scavenger tests were employed to elucidate the role of free radicals based on the use of an excess of scavenger compounds, and ensuring that all of the radicals were captured by scavenger compound. The scavenger tests were performed through repeating RR 195 degradation in SPS/Fe-TA-Cotton system with the addition of radical scavengers IPA, TBA or BQ, respectively. Meanwhile, control tests without a scavenger were also carried out, and the results are presented in Fig. 7(b). Fig. 7(b) shows that in the absence of scavengers, RR 195 was almost completely degraded within 120 min in the control test. The addition of IPA (2.50molL−1) led to a significant loss in the dye degradation compared to control test, and D% value in 120 min in the presence of IPA was 17.43%. Moreover, presence of TBA (2.00molL−1) decreased D% value in 120 min from 96.67% to 22.45%. Comparing the difference in D% value in 120 min between the addition of TBA and IPA, it was observed that %OH and %SO4− contributed 56.79% and 5.02% to the dye degradation, respectively in this system. In contrast, when excess BQ (20.0 mmolL−1) being used, a small loss (4.25%) in dye degradation in 120 min was found, demonstrating that the dye degradation is slightly inhibited by BQ, and O2−% was involved in the dye degradation. It was concluded that although %SO4− and O2−% oxidation also led to the dye degradation, they showed much smaller contribution to dye degradation than %OH at the same conditions. Besides, it should be pointed out that the small amount of dye degradation caused by non-reactive species was found in SPS/Fe-TA-Cotton system since it was not eliminated by the addition of scavengers, which may be owing to the generation of high-valent iron (Groves 2006) or reductant (Wu et al., 2014) during the reaction.
responsible for the dye degradation. However, excess QFe values reduced k values, particularly for Fe-BTCA-Cotton. It was worth noticing that the k values for three complexes exhibited the same order as the QFe values mentioned above. This further demonstrated that increasing the number of carboxyl groups from 2 to 4 and the number of carbons in the backbone from 2 to 4 of the PCAs used, the catalytic activity of the resulting Fe-PCA-Cotton decreased remarkably, and leads to a subsequent slow dye degradation efficiency. This is ascribed to a big difference in the coordination structure such as the extent of conjugation, the coordination environment, and the steric hindrance around the active metal centers (Wang, Zhang et al., 2015; Wang, Wang et al., 2015; Wu et al., 2011) as well as surface performance between these complexes. Higher crosslink degree of BTCA-Cotton and CA-Cotton may result in a more complicated structure of their complexes than TACotton. Besides, the uncoordinated carboxyl groups of the complexes constructed from PCAs may capture hydroxyl (%OH) radicals, limiting the catalysis (Song et al., 2017). Fe-BTCA-Cotton and Fe-CA-Cotton have more uncoordinated carboxyl groups than Fe-TA-Cotton because BTCA and CA were PCAs with more than two carboxyl groups, which regarded as the scavengers for %OH radicals. Another reason is that the complexes exhibited a reverse rank in water contact angle (θ) (Fig. 6), and high QFe values increased their θ levels, proposing that the surface of PCA-Cotton became hydrophobic after Fe3+ ion coordination. Moreover, Fe-BTCA-Cotton had a highest hydrophobic surface possibly owing to the lack of hydroxyl groups in its structure (Fig. 1) and its higher crosslink with cotton fibers, thus reducing the adsorption of SPS or dye molecules onto the complex, slowing down the dye degradation rate. 3.4. Possible activation mechanism of SPS by PCA-Cotton Fe complex Fe-TA-Cotton was selected to investigate the SPS activation mechanism due to its highest phtocatalytic performance. ESR spectroscopy coupled with DMPO as a spin trapping agent was applied to detect the radical species generated and presented in Fig. 7(a). The signals with intensity of 1:2:2:1 corresponding to the characteristic peaks of DMPO%OH adducts were found in the dark, and visible irradiation significantly increased the peak intensity, which confirms that the hydroxyl radicals (%OH) have been formed in the presence of Fe-TACotton, and when irradiated with visible light, SPS was further activated to produce %OH. However, signals of DMPO-%SO4− and O2−% is not identified in the ESR spectra, which may be due to their weak
3.5. UV–vis absorbance spectrum and TOC analysis for degradation of RR 195 The photocatalytic degradation process of RR 195 in SPS/Fe-TACotton system under visible irradiation was investigated by UV–vis spectrum and TOC analysis, and shown in Fig. 8. Fig. 8(a) shows that both the absorbance intensities at 200–400 nm and 400–800 nm of the dye decreases gradually with prolonging irradiation time, proposing that SPS/Fe-TA-Cotton system caused not only the breaking of azo linkages, but also the decomposition of aromatic parts including benzene and naphthalene rings in dye molecular Fig. 6. Water contact angles for three Fe-PCA-Cotton containing different QFe values.
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Fig. 7. (a) ESR spectra of free radicals generated in SPS/FeTA-Cotton system, (b) Effect of scavengers on RR 195 degradation performance with SPS.
Fig. 8. Changes in UV–vis spectra (a) and TOC removal (b) of RR 195 solution with SPS/Fe-TA-Cotton system.
References
structure. Moreover, the peak at 522 nm degreased much more rapidly than the peak at 294 nm, suggesting that the decoloration of RR 195 was almost completed within 30 min, and the aromatic content decomposed more slowly than azo linkages in its molecular structure. Similar results have been reported in the oxidation process of H2O2 catalyzed by the other complexes (Dong et al., 2010; Li, Dong, & Ding, 2010). Meanwhile, TOC values were measured during the reaction, and the corresponding TOC removal percentage was calculated (Fig. 8(b)). It was apparent that TOC removal percentage significantly increased and exceeded 80% within 8 h, indicating that RR 195 molecules were effectively mineralized by SPS/Fe-TA-Cotton system under visible irradiation. Accordingly, it is believed that Fe-TA-Cotton could enhance the activation of SPS to generate various free radicals, thus leading to a remarkable breaking of azo linkages and aromatic parts of RR 195 and then converts them into H2O and CO2. Moreover, the most degradation products within 6 h were found to the simple aliphatic organic acids and almost not toxic according to the preliminary GC–MS analysis and the toxicity evaluation by the inhibition of vegetable seeds germination.
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4. Conclusions Three PCAs were reacted with waste cotton fabric to incorporate carboxyl groups, which could coordinate with Fe3+ ions to prepare FePCA-Cotton for the activation of SPS and H2O2 used in dye degradation. High concentration of PCAs, especially BTCA significantly increased their QCOOH values. TA-Cotton reacted with Fe3+ ions more easily to form its Fe complex with high QFe value. Three Fe-PCA-Cottons could enhance the activation of SPS to accelerate the dye degradation. Fe-TACotton showed a better enhanced effect than the other two complexes. Different critical QFe values were found for three complexes to obtain the best enhanced effect.
Acknowledgement This research was supported by Innovation & Pioneering Talents Plan of Jiangsu Province (2015-340).
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Contents lists available at ScienceDirect
Carbohydrate Polymers journal homepage: www.elsevier.com/locate/carbpol
Research Paper
Activation of Na2S2O8 for dye degradation by Fe complexes fixed on polycarboxylic acids modified waste cotton
MARK
⁎
Guangzeng Liua, Yongchun Donga,b, , Peng Wanga, Liran Biana a b
Division of Textile Chemistry & Ecology, School of Textiles, Tianjin Polytechnic University, Tianjin 300387, China Key Laboratory of Advanced Textile Composite of Ministry of Education, Tianjin Polytechnic University, Tianjin 300387, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Polycarboxylic acid Industrial waste cotton Complex Na2S2O8 activation Dye degradation
Three aliphatic polycarboxylic acids (PCAs) including tartaric acid (TA), citric acid (CA) and 1,2,3,4-butanetetracarboxylic acid (BTCA) were used for the surface modification of waste cotton fiber by an industrialized pad-dry-cure process to introduce carboxyl groups, which then coordinated with Fe3+ ions to obtain three PCA modified cotton fiber Fe complexes. TA modified cotton fiber could easily react with Fe3+ ions to form its complex with high Fe content. Furthermore, TA modified cotton fiber Fe complex showed a better enhanced effect of activated Na2S2O8 to produce free oxygen radicals for the degradation of an azo dye, Reactive Red 195 than the other two complexes. Different critical Fe contents were found for three complexes to obtain the best enhanced effect.
1. Introduction Polymer metal complexes (PMCs) are defined as complexes consisting of a polymer ligand and metal ions, in which the metal ions are attached to the polymer ligand by a coordinate bond. The metal ions introduced into a polymer chain often causes structural diversity, thus improving polymer chemical and physical behavior, and causing the uneven and complicated surface morphology (El-Sawy & Ali, 2010). Over the past two decades, PMCs have received considerable attention due to their unique electric, mechanical, thermal and catalysis properties. Some PMCs have been developed as the efficient and environment friendly heterogeneous catalysts in order to avoid some drawbacks of homogeneous catalysts such as corrosion, difficulty in separation and deficiency of regeneration (Islam, Roy, Mondal, & Salam, 2012). Furthermore, the properties of PMCs were found to depend on the nature of the metal ion and its content in the compounds. On the other hand, the selection of appropriate ligands is crucial to determine the structures and properties of the resulting complexes (Song et al., 2017; Wang, Zhang et al., 2015; Wang, Wang et al., 2015). Polycarboxylic acids (PCAs) as the carboxylate ligands have been widely used in the synthesis of PMCs due to their diversity of coordination modes and high structural stability (Du, Liu, Li, & Fang, 2013; Song et al., 2017; Wang et al., 2013; Wang, Zhang et al., 2015; Wang, Wang et al., 2015). Moreover, many aromatic PCAs were employed to react with phenanthroline, triazine or pyridyl compounds for the synthesis of complex photocatalysts (Song et al., 2017; Wang, Zhang
⁎
et al., 2015; Wang, Wang et al., 2015; Zhang, Wang, Wang, & Gao, 2015). However, these complexes are known to be non-biodegradable or toxic to aquatic plants and animals because of their high composition ratio of aromatic rings present in their coordination structure. Besides, the difficulty in recovery of the catalyst from the reaction mixture and oxidative self-destruction of the catalyst in the oxidizing media are often considered as two main drawbacks. It is well known that H2O2 is the most commonly used oxidizing agent in the advanced oxidation processes (AOPs) for degrading organic pollutants in water because it is a cheap and environmentally friendly oxidant. However, it is fact that the handling of concentrated H2O2 solutions is not desirable for safety problems, and H2O2 rarely has a half-life more than 48 h (Chen et al., 2011; Habibi, Zolfigol, Safaiee, Shamsian, & Ghorbani-Choghamarani 2009; Wu et al., 2014). Therefore, an attempt should be made to find alternative oxidizing agents with higher stability and similar activity instead of H2O2 for AOPs. Persulfate has been applied for AOPs technologies due to their advantages of cost effectiveness, high activity and the environmentally friendly nature (Bjoersvik, Merinero, & Liguori, 2005; Gao et al., 2016; Matzek & Carter, 2016; Wu et al., 2014; Yang, Yang, Shao, Niu, & Wang, 2011). Additionally, it is relatively stable solids and is thus easy to handle on a large-scale and permit accurate dosage (Bjoersvik et al., 2005; Habibi et al., 2009). Specifically, sodium persulfate (SPS) has been found to be activated by heat, light irradiation or transition metal ions to produce free sulfate radicals (%SO4−) with a high oxidation potential (E0 = 2.6 V), which is comparable to that of %OH (E0 = 2.7 V)
Corresponding author at: Division of Textile Chemistry & Ecology, School of Textiles, Tianjin Polytechnic University, 399 Bingshui West Road, Xiqing District, Tianjin 300387, China. E-mail addresses: [email protected], [email protected] (Y. Dong).
http://dx.doi.org/10.1016/j.carbpol.2017.10.060 Received 18 August 2017; Received in revised form 7 October 2017; Accepted 16 October 2017 Available online 18 October 2017 0144-8617/ © 2017 Published by Elsevier Ltd.
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Fig. 1. Esterification of cotton fabric with three PCAs containing 2, 3 and 4 carboxyl groups.
(Huie, Clifton, & Neta, 1991; Wu et al., 2014). Furthermore, %SO4− is more efficient than %OH to oxidize some refractory organic compounds due to its selective oxidation ability (Neta, Huie, & Ross, 1998; Wu et al., 2014). On the other hand, cotton fiber is the commonly used cellulose fiber in the modern textile industry. Cotton fiber with production of more than 27 million tons per year, accounts for 30% of total fiber production worldwide (Wanassi, Azzouz, & Hassen, 2016). Huge production capacity of cotton textile industry has lead to higher output of waste cotton fibers because different cotton wastes are generated from the initial stage of harvesting cotton crop to the final stage of finished textile product. In recent years, industrial waste cotton has been regarded as the one of the major sources of the cellulosic wastes for the recycling. Some attempts have been made to reuse industrial waste cotton for the production of porous carbonaceous materials, biofuels or cellulose nanocrystals and fibers, etc (Ismail & Talib, 2016; Nikolić et al., 2017; Thambiraj & Shankaran, 2017; Zheng, Zhao, & Ye, 2014). However, these recycling processes were often conducted at high temperature or require the complicate high-cost facilities, which are more energy consuming. The main purpose of this work here is to fabricate a sustainable, low-cost and highly effective PCAs immobilized cellulose materialbased catalyst using the typical industrial waste cotton, comber noil from yarn spinning process, which are produced when cotton is combed in comber machine to remove short fibers. Hence, three aliphatic PCAs, namely tartaric acid, citric acid and 1,2,3,4-butanetetracarboxylic acid were used for the surface modification of waste cotton fiber through an industrialized pad-dry-cure process because these aliphatic PCAs are the safe and non-toxic chemicals (Shen, Xu, Kong, & Yang, 2015; Soleimani-Gorgani & Karami, 2016). The modified cotton fibers were then coordinated with Fe3+ ions to prepare three carboxylic cotton fiber Fe complexes as the heterogeneous photocatalysts. Thus, waste cotton fiber is considered as an ideal supporting material or carrier for the PCA Fe complex catalyst in the practical application due to its low cost, environmental friendliness, suitable mechanical stability and easy industrialized modification process. The effect of different PCAs on the surface structures and enhanced activation of three complexes for Na2S2O8 used in the oxidative degradation of a typical azo dye, Reactive Red 195 in water was investigated. Besides, the oxidation mechanisms were studied and proposed.
application, waste cotton fibers were firstly spun into 80 tex yarns, which were then weaved into a cotton mesh fabric. The open mesh structure showed an advantage of better hydrodynamic properties of significance to industrial scale application. The obtained cotton fabric was further treated with a solution containing 10.0gL−1 NaOH and 5.0 gL−1 anionic detergent at the boil for 60 min to remove the impurities from its surface, then thoroughly washed with cold water and dried at ambient temperature before use. Tartaric acid (TA), citric acid (CA), 1,2,3,4-butanetetracarboxylic acid (BTCA), NaH2PO2, Fe2(SO4)3, H2O2 (30%wt), Na2S2O8, isopropyl alcohol (IPA), tert-butyl alcohol (TBA) and 1, 4-benzoquinone (BQ) were of analytical grade and used without further purification. 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) was purchased from Micklin Biochemical Technology Co., Ltd (Shanghai, China). A typical commercial azo dye, Reactive Red 195 (RR 195, CAS: 93050-79-4) was used after purification by the re-precipitation method (Greaves, Churchley, Hutchings, Phillips, & Taylo, 2001). All solutions were prepared in deionized water. 2.2. Fabrication of PCA modified cotton fiber Fe complexes In a typical procedure, cotton fabric was modified in a consecutive two stage process to produce a complex: Stage (1): Modification of cotton fabric with PCAs Cotton fabric was first immersed in an aqueous solution containing various concentration of PCA and NaH2PO2 (5.0%w/w) at room temperature for 10 min, then pressed through a two-roll laboratory mangle (Mathis AG, Switzerland) to give a wet pick-up of 75–80% based on the weight of fabric. Afterward, the padded fabric was dried at 80 °C for 5 min, and cured at 140 °C for 1.5 min to obtain PCA modified cotton fiber (denoted as PCA-cotton), which was washed thoroughly, rinsed, and dried. The modification of cotton fabric with different PCAs was described in Fig. 1. Carboxyl group content in PCA-Cotton samples (QCOOH, mmolg−1) was determined using a titration method described in our previous work (Li, Dong, & Li, 2015). Briefly, about 0.50 g of the dried PCACotton pieces were dispersed in 50 mL of 0.10molL−1 NaOH aqueous solution under a nitrogen atmosphere and stirred for 2 h at room temperature. The amount of the unneutralized NaOH in solution was then determined by titration with standardized 0.10 mmolL−1HCl aqueous solution using an automatic titrator (Shanghai Jingmi Instrumental Co. China). QCOOH values of the samples were calculated as: QCOOH = (V1C1-V2C2)/m, where C1 and C2 are the concentrations of NaOH and HCl aqueous solution (molL−1), respectively. V1 is the volume of NaOH solution (50 mL). V2 is the volume of HCl solution consumed in titration (mL) and m is the mass of dry PCA-Cotton used (g). Stage (2): Coordination of PCA-Cotton with Fe3+ ions 5.0 g of PCA-Cotton was impregnated in 150 mL of the given concentration of Fe3+ ion aqueous solution. The mixtures were kept at
2. Experimental 2.1. Materials and chemicals Waste cotton fibers (comber noil) were collected from a local spinning and weaving industry mills in Tianjin area, China. To enable easier access of reagents to the fiber surface during the preparation and 104
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Fig. 2. Fabrication of Fe-PCA-Cotton at different experimental conditions.
reaction and their contribution to dye degradation, three radical scavengers IPA (%OH and %SO4− scavenger), TBA (%OH scavenger, but not %SO4−) (Buxton, Greenstock, Helman, & Ross, 1998; Monteagudo, Durán, Martin, & Carnicer, 2011) and BQ (O2−% scavenger) (Buxton et al., 1988; Wu et al., 2014) were adopted in the dye degradation experiments according to their reactive activities with each of the free oxygen species potentially present in Fe-PCA-Cotton/SPS or H2O2 system. All the experiments were carried out in triplicate.
50 °C for 3 h under continuous agitation. The obtained PCA-Cotton Fe complex (denoted as Fe-PCA-Cotton) was then taken out, washed with deionized water repeatedly and dried under vacuum at 60 °C for 3 h. The residual concentration of Fe3+ ions in the coordinating solution was determined using a Varian Vista-MPX inductively-coupled plasma optical emission spectroscopy (ICP-OES), for calculating the Fe content (QFe) of the complex. 2.3. Characterization of PCA-Cotton Fe complexes
2.5. Analytical methods The surface morphology of Fe-PCA-Cotton was observed by S-4800 scanning electron microscope (Hitachi High-Tech Co., Japan) operating at 15 kV. The composition of Fe-PCA-Cotton was verified by a Nicolet Magna-560 Fourier transform spectrometer (Nicolet Instrument Co., USA) with 4 cm−1 resolution. The X-ray diffraction measurement and the binding energy analysis of Fe-PCA-Cotton were also conducted on a Rigaku Xd/Max-2500 X-ray diffractometer (Rigaku Co., Japan) and a PHI 5600 X-ray photoelectron spectrometer (Pekin Elmer Inc., USA), respectively. The contact angle of a water droplet on the surface of sample was also measured using a liquid-solid contact angle analyzer (DSA100; Kruss, Germany) equipped with a high speed camera. Approximately 0.60 μL of deionization water was transferred to each fiber.
The concentration of RR 195 in test solution during the degradation experiment was determined using a UV-2401 spectrophotometer (Shimadzu Co., Japan) at 522 nm (maximum absorption wavelength of RR195). Dye decoloration percentage at a given time was calculated as: D% = (1 − Ct/C0) × 100%
(1)
where C0 was the initial concentration of the dye (0.05 mmolL−1), and Ct was the concentrations of the dye in test solution at a given time (mmolL−1). The content of total organic carbon (TOC) of test solution was assayed by a TOC-VCPH analyzer (Shimadzu Co., Japan). TOC removal percentage of the test solution was calculated as follows: TOC Removal (%) = (1 − TOCt/TOC0) × 100%
(2)
−1
where TOC0 is the initial TOC value (mgL ), and TOCt is the residual TOC value of test solution during the reaction (mgL−1), respectively. ESR (Electron Spin Resonance) spectra of the radical spin trapped by DMPO were examined to identify the main free radicals present in the reaction using a JES-FA200 ESR spectrometer (JEOL, Japan) with an irradiation source of Quanta-Ray Nd:YAG pulsed laser system (λ = 532 nm).
2.4. Catalytic evaluation Fe-PCA-Cotton was assessed as a catalyst for SPS or H2O2 activation by the degradation of RR195 conducted in a specially designed photoreaction system presented in our previous work (Han, Dong, & Dong, 2011). A cut-off filter was used to ensure that a complete visible light irradiation (λ > 420 nm). Light intensity inside photoreaction system was measured to be 9.65 mWcm−2 using FZ-A radiometer (BNU Light and Electronic Instrumental Co., China), and the temperature in reaction vessel was kept at 25 ± 1 °C. Unless noted otherwise, all the degradation experiments were performed in 100 mL of test solution containing 0.05 mmolL−1 RR195 and 3.0 mmolL−1 SPS or H2O2 at an initial pH of 6.0, with 0.50 g of Fe-PCA-Cotton. To investigate the free oxygen radicals generated during the
3. Results and discussion 3.1. Fabrication of PCA-Cotton Fe complexes Cotton fabric were modified with three PCAs including TA, CA and BTCA, respectively to introduce the carboxyl groups, which 105
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Fig. 3. SEM images of original cotton fiber (a) and three PCACotton Fe complexes including Fe-TA-Cotton (b), Fe-CACotton (c) and Fe-BTCA-Cotton (d).
coordinated with Fe3+ ions for obtaining three PCA-Cotton Fe complexes, and the results were shown in Fig. 2. Fig. 2(a) shows the effect of PCA concentration on QCOOH values of the modified cotton fabrics. It was clear that QCOOH values of three PCACottons increased with increasing PCAs concentration, and the increasing tendency became level at the PCA concentration of 1.00 molL−1, proposing the maximum QCOOH values (1.14 mmolg−1 for TACotton, 1.51 mmolg−1 for CA-Cotton, 2.16 mmolg−1 for BTCA-Cotton,) of three PCA-Cottons were achieved. This is because higher concentration of PCAs could enhance the esterification between carboxyl groups of PCAs and hydroxyl groups in cellulose chains on cotton fiber (Bolin, Peixin, Kelu, & Gang, 2015), thus leading to a gradual enhancement in their QCOOH values. Furthermore, when the same concentration of PCA was used, their QCOOH values ranked in this order: BTCA-Cotton > CA-Cotton > TA-Cotton, suggesting that increasing the length of the backbone and number of carboxyl groups in the PCAs structure could enhance the QCOOH value of the resulting PCA-Cotton. It was reported that aliphatic PCAs with more than two carboxyl groups have better crosslink ability than dicarboxylic acids, and BTCA with 4 carbons in its backbone and 4 carboxyl groups showed the better crosslinking effects than CA with 3 carbons in its backbone and 3 carboxyl groups, leading to the strongest intermolecular interaction (Shen et al., 2015; Song, Xu, Xie, & Yang, 2016), thus produced crosslinked cotton fiber with high QCOOH value. BTCA or CA first forms a cyclic anhydride by dehydration of the adjacent two carboxyl groups at high temperature, which then reacts with one or two hydroxyl groups of cotton fiber to form PCA-Cotton containing one or two carboxyl groups for causing the crosslink (Li et al., 2015). While dicarboxylic acid is unable to crosslink celluloses because its short backbone restricted its range of crosslinking (Ji, Zhao, Yan, & Sun, 2016; Song et al., 2016; Yang, Qian, & Lickfield, 2001). However, TA with 2 carbons in its backbone and 2 carboxyl groups is able directly to react with cellulose molecule to impart only one carboxyl group (Ji et al., 2016), thus leading to a low QCOOH value. Fig. 2(b) displays a different change in QFe values of the obtained complexes prepared by three PCA-Cottons with varied QCOOH values. QFe value of Fe-TA-Cotton was higher than those of the other complexes. Moreover, higher QCOOH value of TA-Cotton gradually increased the QFe value of the resulting complex. This is because high QCOOH value could provide more carboxyl groups, which may prefer to react with
Fe3+ ions by intermolecular coordination, thus enhancing the immobilization of Fe3+ ions on fiber surface. By contrast, CA-Cotton and BTCA-Cotton have the different coordinating performance with Fe3+ ions from TA-Cotton. QFe values of the two complexes increased with increasing their QCOOH values, and a highest QFe value was achieved at QCOOH value of 1.0 mmolg−1 for CA-Cotton or 0.75 mmolg−1 for BTCACotton. This was due mainly to the space steric hindrance resulted by their complicated crosslink structure between cellulose chains, which could limit the coordination of Fe3+ ions with their carboxyl groups. Furthermore, higher crosslink degree of BTCA-Cotton than CA-Cotton as mentioned above was responsible for lowest QFe value of the resulting complex. As shown in Fig. 2(c), when three PCA-cotton with similar QCOOH values (approximately 0.85 mmolg−1) being used, increasing initial concentration of Fe3+ ions caused a gradual increment in QFe value of the resulting complexes, and increasing tendency becomes level at 150 mmolL−1of Fe3+ ions for TA-Cotton and CA-Cotton, and 125 mmolL−1of Fe3+ ions for BTCA-Cotton, which was attributed to higher crosslink degree of BTCA-Cotton, thus retarding the penetration of Fe3+ ions into the modified cotton fiber from water during the reaction. Besides, at a given initial concentration of Fe3+ ions, QFe values of three complexes were ranked as follow: Fe-TA-Cotton > FeCA-Cotton > Fe-BTCA-Cotton. This is mainly owing to the big difference in the crosslink degree between these PCA-Cotton samples. 3.2. Surface morphology and composition analysis Fig. 3(a) shows that a round surface of original cotton fiber is comparatively smooth. A mud-like layer was found from Fig. 3(b–d) on cotton fiber to cause the rough and uneven surface of three Fe-PCACottons, demonstrating the modification of cotton fiber with PCAs and subsequent coordination with Fe3+ ions. Fig. 4(A–B) shows several characteristic absorption peaks of original cotton fiber at 3340, 2900, 1431, 1316, 1158, 1061, 1033 and 905 cm−1, owing to the stretching of OH, CH, CO and CeOeC, respectively (Li et al., 2015; Sun, Lin, Deng, & Li, 2008). A peak around 1720 cm−1representing carbonyl stretching vibration of the carboxyl groups and ester carbonyl bands was found in the spectra of all the PCA-Cotton, confirming that the carboxyl groups have been introduced into surface structure of cotton fiber by ester linkage with PCAs. More importantly, this peak became much less intensive, and a new peak centered at 1630 cm−1 appeared 106
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Fig. 4. FTIR spectra of three PCA-Cottons (A) and their Fe complexes (B), XPS (C) and XRD (D) spectra of cotton fiber and three complexes.
As shown in Fig. 5(A), D% values in the presence of only H2O2 or SPS in 90 min were less than 20% (curves a-b), proposing that little degradation of RR195 took place. While Fe-PCA-Cotton was used, D% values significantly increased with the prolonging of reaction time (curves c-h), especially in the presence of SPS. Moreover, D% values with SPS/Fe-PCA-Cotton systems (curves d, g and h) were higher than those with H2O2/Fe-PCA-Cotton systems (curves c, e and f), correspondingly. This suggested that Fe-PCA-Cotton exhibited a stronger activity for SPS than H2O2 used in dye degradation. According to the D % values, three complexes were ranked in this order: Fe-TACotton > Fe-CA-Cotton > Fe-BTCA-Cotton in the case of H2O2 or SPS, indicating that Fe-TA-Cotton has a better activation effect for H2O2 or SPS than the other two complexes at the same conditions. To investigate the impact of the QFe value of the complexes on their photocatalytic capacity, the degradation of RR 195 with SPS was performed in the presence of three Fe-PCA-Cotton containing different QFe values, respectively. The pseudo-first-order rate constants, k of dye degradation were calculated with all regression coefficients greater than 0.95 and shown in Fig. 5(B). Increasing the QFe value is accompanied with an almost linear enhancement in k values. And highest k values were obtained at different critical QFe values (QFe-c: 0.53 mmolg−1 for Fe-TACotton, 0.49 mmolg−1 for Fe-CA-Cotton and 0.37 mmolg−1 for Fe-BTCA-Cotton). These indicated that three complexes could promote the SPS decomposition to generate various free radicals, which are
in the spectra of three complexes, especially Fe-TA-Cotton and Fe-CACotton, revealing that Fe3+ ions have participated in the coordination with hydroxyl groups of cellulose chains. According to their XPS spectra shown in Fig. 4(C), cotton fiber surface was composed mainly of carbon and oxygen elements, and a small amount of Fe element was observed for three complexes. The binding energy of the Fe2p in the complexes are between 709.95 eV and 710.93 eV, lower than that (711.3 eV) of the Fe2p in FeCl3 (Dong, Han, Liu, & Du, 2010). On the other hand, the binding energy of the O1 s in the complexes shifted to higher binding energy level, proposing that oxygen atom as the electron donor reacted with Fe3+ ions to form complexes. Besides, it was found from Fig. 4(D) that the peaks at 22.95° of three complexes were lower than that of untreated cotton fiber, suggesting that the modification and coordination may decrease their crystallinity, thus possibly reducing their mechanical property. 3.3. Activation of SPS by different PCA-Cotton Fe complexes Three Fe-PCA-Cotton containing similar QFe values (about 0.35 mmolg−1) were prepared by optimizing the modification and coordination processes and then tested as the heterogeneous photocatalysts for the activation of H2O2 or SPS used in the degradation of RR 195 under visible irradiation, respectively. D% values of RR 195 during the reactions were measured and shown in Fig. 5(A).
Fig. 5. (A): D% values at different systems (a) H2O2, (b) SPS, (c) H2O2/Fe-BTCA-Cotton, (d) SPS/FeBTCA-Cotton, (e) H2O2/Fe-CA-Cotton, (f) H2O2/FeTA-Cotton, (g) SPS/Fe-CA-Cotton, (h) SPS/Fe-TACotton; (B): Effect of QFe on k values in the presence of different Fe-PCA-Cotton.
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intensity or low fluxes during the reaction. To further understand the generation of various free radicals and their contribution to dye degradation in SPS/Fe-TA-Cotton system, the scavenger tests were employed to elucidate the role of free radicals based on the use of an excess of scavenger compounds, and ensuring that all of the radicals were captured by scavenger compound. The scavenger tests were performed through repeating RR 195 degradation in SPS/Fe-TA-Cotton system with the addition of radical scavengers IPA, TBA or BQ, respectively. Meanwhile, control tests without a scavenger were also carried out, and the results are presented in Fig. 7(b). Fig. 7(b) shows that in the absence of scavengers, RR 195 was almost completely degraded within 120 min in the control test. The addition of IPA (2.50molL−1) led to a significant loss in the dye degradation compared to control test, and D% value in 120 min in the presence of IPA was 17.43%. Moreover, presence of TBA (2.00molL−1) decreased D% value in 120 min from 96.67% to 22.45%. Comparing the difference in D% value in 120 min between the addition of TBA and IPA, it was observed that %OH and %SO4− contributed 56.79% and 5.02% to the dye degradation, respectively in this system. In contrast, when excess BQ (20.0 mmolL−1) being used, a small loss (4.25%) in dye degradation in 120 min was found, demonstrating that the dye degradation is slightly inhibited by BQ, and O2−% was involved in the dye degradation. It was concluded that although %SO4− and O2−% oxidation also led to the dye degradation, they showed much smaller contribution to dye degradation than %OH at the same conditions. Besides, it should be pointed out that the small amount of dye degradation caused by non-reactive species was found in SPS/Fe-TA-Cotton system since it was not eliminated by the addition of scavengers, which may be owing to the generation of high-valent iron (Groves 2006) or reductant (Wu et al., 2014) during the reaction.
responsible for the dye degradation. However, excess QFe values reduced k values, particularly for Fe-BTCA-Cotton. It was worth noticing that the k values for three complexes exhibited the same order as the QFe values mentioned above. This further demonstrated that increasing the number of carboxyl groups from 2 to 4 and the number of carbons in the backbone from 2 to 4 of the PCAs used, the catalytic activity of the resulting Fe-PCA-Cotton decreased remarkably, and leads to a subsequent slow dye degradation efficiency. This is ascribed to a big difference in the coordination structure such as the extent of conjugation, the coordination environment, and the steric hindrance around the active metal centers (Wang, Zhang et al., 2015; Wang, Wang et al., 2015; Wu et al., 2011) as well as surface performance between these complexes. Higher crosslink degree of BTCA-Cotton and CA-Cotton may result in a more complicated structure of their complexes than TACotton. Besides, the uncoordinated carboxyl groups of the complexes constructed from PCAs may capture hydroxyl (%OH) radicals, limiting the catalysis (Song et al., 2017). Fe-BTCA-Cotton and Fe-CA-Cotton have more uncoordinated carboxyl groups than Fe-TA-Cotton because BTCA and CA were PCAs with more than two carboxyl groups, which regarded as the scavengers for %OH radicals. Another reason is that the complexes exhibited a reverse rank in water contact angle (θ) (Fig. 6), and high QFe values increased their θ levels, proposing that the surface of PCA-Cotton became hydrophobic after Fe3+ ion coordination. Moreover, Fe-BTCA-Cotton had a highest hydrophobic surface possibly owing to the lack of hydroxyl groups in its structure (Fig. 1) and its higher crosslink with cotton fibers, thus reducing the adsorption of SPS or dye molecules onto the complex, slowing down the dye degradation rate. 3.4. Possible activation mechanism of SPS by PCA-Cotton Fe complex Fe-TA-Cotton was selected to investigate the SPS activation mechanism due to its highest phtocatalytic performance. ESR spectroscopy coupled with DMPO as a spin trapping agent was applied to detect the radical species generated and presented in Fig. 7(a). The signals with intensity of 1:2:2:1 corresponding to the characteristic peaks of DMPO%OH adducts were found in the dark, and visible irradiation significantly increased the peak intensity, which confirms that the hydroxyl radicals (%OH) have been formed in the presence of Fe-TACotton, and when irradiated with visible light, SPS was further activated to produce %OH. However, signals of DMPO-%SO4− and O2−% is not identified in the ESR spectra, which may be due to their weak
3.5. UV–vis absorbance spectrum and TOC analysis for degradation of RR 195 The photocatalytic degradation process of RR 195 in SPS/Fe-TACotton system under visible irradiation was investigated by UV–vis spectrum and TOC analysis, and shown in Fig. 8. Fig. 8(a) shows that both the absorbance intensities at 200–400 nm and 400–800 nm of the dye decreases gradually with prolonging irradiation time, proposing that SPS/Fe-TA-Cotton system caused not only the breaking of azo linkages, but also the decomposition of aromatic parts including benzene and naphthalene rings in dye molecular Fig. 6. Water contact angles for three Fe-PCA-Cotton containing different QFe values.
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Fig. 7. (a) ESR spectra of free radicals generated in SPS/FeTA-Cotton system, (b) Effect of scavengers on RR 195 degradation performance with SPS.
Fig. 8. Changes in UV–vis spectra (a) and TOC removal (b) of RR 195 solution with SPS/Fe-TA-Cotton system.
References
structure. Moreover, the peak at 522 nm degreased much more rapidly than the peak at 294 nm, suggesting that the decoloration of RR 195 was almost completed within 30 min, and the aromatic content decomposed more slowly than azo linkages in its molecular structure. Similar results have been reported in the oxidation process of H2O2 catalyzed by the other complexes (Dong et al., 2010; Li, Dong, & Ding, 2010). Meanwhile, TOC values were measured during the reaction, and the corresponding TOC removal percentage was calculated (Fig. 8(b)). It was apparent that TOC removal percentage significantly increased and exceeded 80% within 8 h, indicating that RR 195 molecules were effectively mineralized by SPS/Fe-TA-Cotton system under visible irradiation. Accordingly, it is believed that Fe-TA-Cotton could enhance the activation of SPS to generate various free radicals, thus leading to a remarkable breaking of azo linkages and aromatic parts of RR 195 and then converts them into H2O and CO2. Moreover, the most degradation products within 6 h were found to the simple aliphatic organic acids and almost not toxic according to the preliminary GC–MS analysis and the toxicity evaluation by the inhibition of vegetable seeds germination.
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4. Conclusions Three PCAs were reacted with waste cotton fabric to incorporate carboxyl groups, which could coordinate with Fe3+ ions to prepare FePCA-Cotton for the activation of SPS and H2O2 used in dye degradation. High concentration of PCAs, especially BTCA significantly increased their QCOOH values. TA-Cotton reacted with Fe3+ ions more easily to form its Fe complex with high QFe value. Three Fe-PCA-Cottons could enhance the activation of SPS to accelerate the dye degradation. Fe-TACotton showed a better enhanced effect than the other two complexes. Different critical QFe values were found for three complexes to obtain the best enhanced effect.
Acknowledgement This research was supported by Innovation & Pioneering Talents Plan of Jiangsu Province (2015-340).
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