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heterogeneous Fe-based Fenton catalysts toward rapid degradation of organic pollutants at near neutral pH
Chemosphere 245 (2020) 125663
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In-situ generation of multi-homogeneous/heterogeneous Fe-based Fenton catalysts toward rapid degradation of organic pollutants at near neutral pH Xuqing Li, Bing Xiao, Meng Wu, Lin Wang, Rufen Chen, Yu Wei, Hui Liu* School of Chemistry and Material Science, Key Laboratory of Inorganic Nanomaterials of Hebei Province, National Demonstration Center for Experimental Chemistry Education, Hebei Normal University, Shijiazhuang, 050024, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A modified Fe2þ Fenton reaction system was constructed. Pollutants with high concentration and different structures were degraded at near neutral pH. In-situ-formed multi-Fenton centers accelerated the decomposition of H2O2 and generation of OH. After degradation, the added ferrous ions were transformed to hematite, goethite and magnetite.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 August 2019 Received in revised form 1 December 2019 Accepted 12 December 2019 Available online 18 December 2019
In this study, an in-situ generated multi-homogeneous/heterogeneous Fe-based catalytic system was developed, which exhibited a high efficiency for the production of OH and rapid degradation of various organic pollutants in a near neutral pH range (5e8). The mechanism for the rapid decomposition of H2O2 and the generation of OH were investigated in detail. The results indicated that, besides the introduced Fe2þ, the in-situ generated various iron species including Fe(OH)þ, Fe(OH)2, Fe3þ, ferrihydrite (Fh), gFeOOH and a-FeOOH as well as FeII/Fh, FeII/g-FeOOH and FeII/a-FeOOH could simultaneously act as homogeneous and heterogeneous Fenton reaction catalysts. The dropwise addition manner of Fe2þ greatly improved the catalytic efficiency of Fe2þ ions in near neutral pH environment, while the in-situ generated nanosized Fh, g-FeOOH and a-FeOOH could supply numerous active catalytic sites. After degradation, the ferrous ions could be transformed to various crystalline iron oxides by the catalytic phase transformation. This study presents a method towards the rational design of novel Fenton catalysts for wastewater treatment. © 2019 Elsevier Ltd. All rights reserved.
Handling Editor: Jun Huang Keywords: In-situ generation Multi-homogeneous/heterogeneous catalyst Degradation Organic pollutant Mechanism
1. Introduction Advanced oxidation processes (AOPs) are considered as one of the most effective technologies to treat organic pollutants from
* Corresponding author. E-mail address: [email protected] (H. Liu). https://doi.org/10.1016/j.chemosphere.2019.125663 0045-6535/© 2019 Elsevier Ltd. All rights reserved.
wastewater. As an important AOP, the homogeneous Fenton system can produce highly oxidizing hydroxyl radical (OH) for nonselective degradation of organic pollutants (Dong et al., 2018; Li et al., 2018; Pham et al., 2012; Qin et al., 2015). However, the classic Fenton reaction has efficient reactivity only at an acid pH such as pH 2.5e3.5, and the deposition of large amounts of Fe3þ sludge limits its applications (Chen et al., 2011; Yang et al., 2013). To overcome these limitations, two strategies have been put forward.
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further purification.
One is to develop heterogeneous Fenton catalysts, and the other is to reuse iron-containing sludge after appropriate treatment. For the first strategy, various heterogeneous Fenton catalysts such as zero-valent iron (Xu and Wang, 2011), Fe2O3 (Zhang et al., 2010), FeOOH (Wang et al., 2015a, 2015b), g-Fe2O3 (Cao et al., 2015) have been reported. The advantage of these heterogeneous catalysts lies in their easy separation from the reaction system. However, iron oxides also exhibit a high catalytic efficiency in an acid pH environment and with pH increasing, the inert catalytic activity was displayed (Bokare and Choi, 2014; Ma et al., 2015). Although other transitional metals or nonmetallic materials have been developed for the degradation of organic pollutants (Lyu et al., 2018; Nie et al., 2009, 2010), their catalytic performance usually needs the aids of specific equipment and energy such as ultrasonic and/or UV light irradiation etc. (Nie et al., 2015). Relatively, homogeneous Fenton system is still a proper candidate catalyst to degrade organic pollutants from the angle of the operation cost, equipment and working efficiency, especially for the treatment of pollutants with high concentration. For the second strategy, one example is the reuse of ferric sludge as an iron source for the Fenton-based process in wastewater treatment reported (Bolobaev et al., 2014). This method could minimize the production of hazardous ferric-containing wastes and hence reduce the overall cost of the treatment process. Another example is that the ferric sludge was dewatered, dried and baked at 350e400 C and the residual solids were dissolved in sulfuric acid to form the reusable catalyst for Fenton and Fenton-like reactions (Cao et al., 2009). Moreover, Feng et al. investigated a cycling Fenton process for the degradation of organic pollutants, in which Fe species could be repeatedly used as a homogeneous catalyst after acid dissolution of its Fe-containing sludge (Feng et al., 2017). However, there are still some problems for the reuse of ironcontaining sludge, such as longer precipitation time, high baked temperature and consumption of large amounts of acid. Our group has explored the catalytic formation of various iron oxides in liquid phase and their application in the environmental field (Cao et al., 2015; Liu et al., 2005, 2007, 2010). We found that trace of Fe(II) adsorbed on ferrihydrite (Fh) could obviously accelerate its transformation (called catalytic phase transformation) (Liu et al., 2005, 2007). The products might be lepidocrocite, goethite, hematite and magnetite, which depended on pH, temperature and the amount of Fe(II) (Liu et al., 2007). Some studies have demonstrated that when Fe(II) was adsorbed on the surface of iron oxide or iron mineral, it displayed enhanced strong reductive abilities towards environmental contaminants compared with aqueous Fe2þ alone (Boland et al., 2014; Jones et al., 2016; Silvester et al., 2005; Stewart et al., 2018). Based on these research results, we modified the classical homogeneous Fenton system. In the modified system, Fe2þ ions were still used as an iron source, and various organic pollutants with different structures (e.g. Glyphosate, 2.4.6Trichlorophenol, Bright blue, Rhodamine B, etc.) could be rapidly degraded at near neutral pH values. After degradation, those iron ions could be transformed to crystalline iron oxides by the catalytic phase transformation.
The concentrations of pollutants Bright blue (BB), Medium yellow 10 (MY10), Acid Green 25 (AG25), Phenol red (PR), Rhodamine B (RhB), Malachite green (MG) and Cationic Pink FG (CPFG) and pNitrophenol in the solution were determined using a UVevis spectrophotometer (UV-8000 Shanghai Metash Instruments Co. Ltd.) at 628, 355, 296, 432, 553, 617, 522 and 317 nm, respectively. In a typical EPR experiment, 0.06 g Fh was added to 100 mL of H2O2 (8 mmol/L) solution and its pH was adjusted to 7. Then, 1 mL of DMPO (Sigma Aldrich, 100 ppm) was immediately injected into the reaction system. The supernatant filtered through a Naflon membrane of 0.22 mm was transferred to a 100 mL capillary tube, which was then fixed in the resonant cavity of the spectrometer.
2. Experimental
3. Results and discussion
2.1. Materials
3.1. Design of the modified Fenton reaction
All chemicals were analytical grade. FeSO4∙7H2O and NaOH were purchased from Tianjin Yongda Chemical Corp. Bright blue (BB), Medium yellow 10 (MY10), Acid Green 25 (AG25), Phenol red (PR), Rhodamine B (RhB), Malachite green (MG), Cationic Pink FG (CPFG), Glyphosate (Gly) and 2.4.6-Trichlorophenol (Trp) were purchased from J&K. All chemicals were used as received without
Scheme 1 shows the modified Fenton reaction process. In this scheme, Fe2þ and OH solutions were added simultaneously into a solution including pollutant and H2O2. The addition rates of the two solutions were controlled via peristaltic pump by maintaining near neutral pH. There are two paths to form Fh in this process. First, the added Fe2þ ions can react directly with H2O2 to generate OH and
2.2. Characterization of samples X-ray diffraction (XRD) measurement was carried out at room temperature using a D8ADVANCE diffractometer with CuKa radiation (l ¼ 0.15418 nm). Scanning Electron Microscopy (SEM) images were taken with a S-4800 scanning electron microscope (Japan Hitachi LTD). The DMPO trapped EPR spectrum was carried out at a Magnettech MS-5000 electron paramagnetic resonance spectroscopy (Germany). 2.3. Degradation of organic pollutants Degradation of pollutant was carried out in a beaker placed in a water bath with magnetic stirring under room light. In a typical procedure, 0.1e20 mmol of H2O2 was added into 100 mL of the pollutant solution (10e5000 mg/L). Then, Fe2þ (0.04e10 mmol) and (0.2e1.0) mol/L NaOH solutions were added simultaneously into the reaction system until Fe2þ solution was exhausted. The addition rates of the two solutions were controlled via peristaltic pump by maintaining pH 5e8. The reaction lasts from several to 20 min, depending on the addition velocity of Fe2þ and concentration of pollutant. Hereafter, part of the supernatant was used to determine the concentration of pollutants. The remaining reaction system was heated until boiling and the reflux was maintained for 1e2 h. The resulted product was thoroughly washed with deionized water and dried at about 70e80 C. The concentration of pollutants (e.g. glyphosate (Gly) and 2.4.6Trichlorophenol (Trp)) was determined by using a LC1260 HPLC (USA Agilent Technologies) equipped with a C18 column and UV detector. Water (98%) and methanol (2%) were mixed as the mobile phase for glyphosate, while methanol (60%) and 0.1% acetic acid (40%) were used as the mobile phase for 2.4.6-Trichlorophenol. The wavelength was 240 and 294 nm for glyphosate and 2.4.6trichlorophenol, respectively, using a UVevis spectrophotometer. Before determination, glyphosate was quantified after pre-column derivatization with NaNO2 (Eq. (1)) according to a method of Wang et al. (2015a, 2015b). KBr
C3 H8 NO5 P þ NaNO2 þ HCl ! C3 H7 N2 O6 P þ NaCl þ H2 O
(1)
X. Li et al. / Chemosphere 245 (2020) 125663
Scheme 1. Schematic of the modified Fenton reaction.
Fe3þ ions (Eq. (2)) (Dong et al., 2018). Although Fe3þ ions can also react with H2O2 to regenerate Fe2þ ions through Eq. (3), Eq. (2) dominates due to its much larger reaction rate than Eq. (3). Thus, Fe3þ ions were fast precipitated to form Fh due to its very small ksp (1.1 1038) and the addition of NaOH. Second, those added Fe2þ ions may react directly with NaOH to generate Fe(OH)2, which is also quickly oxidized into Fh in the absence of inert gas protection (Zhang et al., 2019). Fe2þ þ H2O2 / Fe3þ þ OH þ OH k1 ¼ 40e80 L mol1 s1
(2)
Fe3þ þ H2O2 / Fe2þ þ HO2 þ Hþ k2 ¼ 9.1 107 L mol1 s1 (3) When subsequent Fe2þ ions were added into the system, part of them could be adsorbed on the surface of Fh (Liu et al., 2005, 2007) to form FeII/Fh. On the one hand, these FeII ions have a strong reducing ability (Boland et al., 2014; Jones et al., 2016; Silvester et al., 2005; Stewart et al., 2018), thus, FeII/Fh more easily reacts with H2O2 to produce OH and then degrade pollutants. On the other hand, the particle size of Fh is about 2e5 nm (Liu et al., 2014), resulting in numerous heterogeneous catalytic sites to improve the decomposition of H2O2. After degradation, the FeII/Fh could be transformed to crystal products of iron oxides by the catalytic phase transformation (Liu et al., 2005, 2007). 3.2. Degradation of various pollutants The catalytic performance of the modified Fenton system was evaluated by degrading several model pollutants with different structures. Table 1 shows the degradation rate and experimental conditions of different pollutants. All experimental results show that the current modified Fenton system exhibits high degradation efficiency for the selected pollutants. The dosage of Fe2þ and H2O2
3
depends on the concentration of pollutant. Even for a 5000 ppm of glyphosate, a high degradation rate still reaches (Table S1). To evaluate that the high removal rate of glyphosate results from its degradation instead of adsorption, three experiments (e.g. TOC, SI()-MS and EPR) were conducted. The result of TOC indicated that the mineralization rate is 65e60% for a 300e5000 mg/L solution of glyphosate (Table S1). To gain the degradation product of glyphosate, Fe2þ ions were added in batch mode. For example, 10 mmol of Fe2þ ions was used to completely degrade 5000 ppm of glyphosate solution. First, 5 mmol of Fe2þ ions were added into the reaction system and then the degradation products were detected by SI()-MS. Second, another 5 mmol of Fe2þ ions were added into the reaction system and then the degradation products were again detected by SI()-MS. The results are shown in Fig. 1a. Before degradation the species with m/z of 199.2 was attributed to the derivatization product of glyphosate (nitroglyphosate). When 5.0 mmol of Fe2þ ions were added into 5000 ppm of glyphosate solution, five species with m/z of 61, 75.1, 77.1, 119.1 and 199.2 were found in the supernatant (Fig. 1a). When 10.0 mmol of Fe2þ ions were added, only three species with m/z 61, 75.1 and 77.1 were found, suggesting that the species with m/z 119.1 was an intermediate. Among them, the species with m/z of 119.1 was attributed to N-nitrososarcosine, the derivatization product of sarcosine. The species with m/z of 61, 75.1 and 77.1 were acetic acid, glyoxylic acid and hydroxyacetic acid, respectively. Based on the experimental results, the degradation path of glyphosate was described in Fig. 1b. Fig. 1c presents EPR spectrum of the current system. Incubation of H2O2 and DMPO with the current system resulted in a strong fourline EPR signal with a 1:2:2:1 peak-to-peak intensity pattern. The four lines has approximately equal spacing of ca. 1.50 mT and the g value is 2.0060, which is consistent with the hyperfine splitting reported for the DMPO-OH spin adduct (Voinov et al., 2011 Yan et al., 2012). Considering the potential contribution of adsorption of pollutants on their removals, two representative pollutants (cationic dye CPFG and anionic dye BB) were selected to evaluate their adsorption capacity. The result indicated that only several ppm of BB and CPFG was adsorbed (Fig. S1). All the above results suggested that the high removal rate of pollutant was resulted from its degradation instead of adsorption. In addition, the modified reaction procedure was used to degrade a mixed solution including two or three pollutants. Fig. S2 presents UVevis absorption spectra and HPLC spectra of mixed solution of BB and MY 10 before and after degradation. It can be seen that almost the two pollutants have been completely degraded. For a mixed solution of three kinds of pollutants (AG, MG and CPFG), a high degradation rate can be realized (Table S2 and Fig. S3). The mineralization rate is larger than 60%. 3.3. Transformation of Fe2þ The
transformation products
of
the
added
Fe2þ
were
Table 1 Degradation of Various Organic pollutants. Pollutants
Concentration of pollutants (ppm)
FeSO4 (mmol)
H2O2 (mmol)
pH
Degradation rate (%)
Glyphosate 2.4.6-Trichlorophenol Bright blue Medium yellow 10 Alizarin green Phenol red Rhodamine B Malachite green Cationic Pink FG
100 80 80 167 167 167 167 167 167
0.20 1.0 1.0 0.6 0.6 0.8 0.6 0.3 0.3
0.50 1.2 1.2 0.8 0.9 1.0 0.9 0.5 0.4
6e7 5e6 6e7 7e8 7e8 6e7 6e7 5e6 5e6
94.59 90.63 88.68 91.85 92.72 90.40 88.80 91.02 94.76
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Fig. 2. (a) XRD patterns of the products obtained by adding different amount of Fe2þ to degrade 200 mg/L of Orange G, H2O2 (1.2 mmol), pH 7, 25 C, #: Fh, *: lepidocrocite, D: goethite; (b) XRD patterns of the products obtained by boiling and refluxing for 1e2 h after degradation. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
dosage of FeII ions and pH. When nFe(II) ¼ 2e18 mmol, the transformation product is hematite at pH 6e8. When nFe(II) ¼ 2025 mmol, the products are goethite at pH 6e7 and the mixture of goethite and hematite at pH 7e8 (not shown). When nFe(II) ¼ 50 mmol, the transformation product is magnetite at pH 6e8. Fig. S4 presents SEM images of the products. The as-prepared hematite particles are composed of a large number of uniform spheres with diameters of approximately 60e80 nm (Fig. S4a). The as-prepared goethite is present as rod-like particles (Fig. S4b), and the obtained magnetite shows uniform spheres with diameters of approximately 30e50 nm (Fig. S4c). Fig. 1. (a) ESI()-MS of glyphosate sample withdrawn at 0, 5 and 10 mmol of Fe2þ addition in the presence of H2O2. (b) The degradation path of glyphosate. (c) EPR spectrum over the current system.
determined by XRD patterns after degradation of pollutants (Fig. 2a). For 100 mL Orange G solution (200 mg/L), Fh was obtained when the degradation reaction was conducted at room temperature and the dosage of Fe2þ ions was 15 mmol. With increasing the dosage of Fe2þ ions, poor-crystallization lepidocrocite and goethite were obtained (Fig. 2a), suggesting that the initial transformation product of Fe2þ ions should be Fh. Once Fh was formed, subsequently added Fe(II) could be adsorbed on surface of Fh and accelerate its transformation to lepidocrocite and goethite (Liu et al., 2005, 2007). To obtain well-crystallization iron oxides, the reaction system after degradation was heated and kept boiling and refluxing for 1e2 h. XRD patterns of the obtained products were shown in Fig. 2b. The added Fe2þ ions can be transformed to hematite, goethite and magnetite, respectively, which depends on the
3.4. Analysis on iron species in the current system Ferrous ions in aqueous solution exist in different forms such as Fe2þ, Fe(OH)þ, Fe(OH)2 and Fe(OH) 3 , which depends on pH (Cornell and Schwertmann, 2003). When pH 5, ferrous ions predominantly present as Fe2þ. With increasing pH, the proportion of Fe2þ diminishes while that of Fe(OH)þ increases in the range of pH 5e8. In this pH range, Fe(OH)2 is probably formed. When pH 10, Fe(OH) 3 species appeared. Because the pH of the current system is controlled within 5e8 (Table 1), the main ferrous species should be Fe(OH)þ and Fe(OH)2. However, the formation of Fe(OH)2 depends on both pH and the concentration of ferrous ions. Table S3 presents the saturation concentration of Fe2þ calculated from Ksp (8 1016) of Fe(OH)2 at 25 C and the real concentration of Fe2þ added into the current system (Table 1). The latter was much less than the former, suggesting that the precipitation of Fe(OH)2 is unstable even if it could be formed in the range of pH 5e8. Actually, the added Fe2þ ions could rapidly react with H2O2 to form Fe3þ ions (Eq. (2)) and then to form Fh due to its very little Ksp value (1.1 1038).
X. Li et al. / Chemosphere 245 (2020) 125663
Because Fe2þ ions were continuously added into the system, the subsequently added Fe2þ ions could be partly adsorbed on the surface of Fh to form FeII/Fh. On the one hand, these FeII ions have a strong reducing ability (Boland et al., 2014; Jones et al., 2016; Silvester et al., 2005; Stewart et al., 2018) and could react with H2O2 to produce OH (Eq. (4)). On the other hand, the FeII in FeII/Fh could catalyze the transformation of Fh to form lepidocrocite and goethite (Eq. (5)) (Liu et al., 2007). The two reactions are parallel and compete each other. FeII/Fh þ H2O2 / FeIII/Fh þ OH þ OH
5
in the absence of pollutant. When the amount of H2O2 is 0.1 mmol, the main product is g-FeOOH (Fig. 3b). With increasing dosage of H2O2, the formation of g-FeOOH is suppressed and Fh gradually dominates (Fig. 3b). These results indicate that Eq. (4) takes precedence over Eq. (5). In summary, when Fe2þ solution is added into the system according to the designed procedure, the coexisted iron species include FeII (e.g. Fe2þ and FeOHþ), Fe(OH)2, Fe3þ (e.g. Fe3þ, Fe(OH)2- and Fe(OH)-2), Fh, g-FeOOH and a-FeOOH as well as FeII/Fh, FeII/g-FeOOH and FeII/a-FeOOH under the condition of pH 5e8. Except Fe2þ ions, all of other iron species could be in-situ generated.
(4) 3.5. The mechanism for the rapid production of hydroxyl radicals
FeII =Fh / g FeOOH þ a FeOOH þ FeII
(5)
To confirm the above analysis, several experiments were conducted either in the absence or presence of H2O2. Fig. 3a presents XRD patterns of the products obtained with different amount of Fe2þ and in the absence of H2O2. When nFe(II) ¼ 0.1 mmol, Fh and gFeOOH coexist in the product. When nFe(II) ¼ 1.0 mmol, the main product is g-FeOOH. When nFe(II) ¼ 5.0 mmol, the main product is g-FeOOH along with small amount of a-FeOOH. These results indicate that Fh is the initially formed product. The formation of gFeOOH and a-FeOOH in Fig. 3a should be attributed to the catalysis of FeII to the transformation of Fh (Liu et al., 2007). Subsequently, the changes of the adsorption rate of Fe2þ ions on the Fh with pH values were determined under nitrogen protection (Fig. S5). About 25% of Fe2þ ions are adsorbed at pH 5 and about 50% at pH 6 and almost all of Fe2þ ions could be adsorbed at pH 7e8. The results in Figs. 3a and S5 indicate that once Fh is formed, the subsequently added Fe2þ could be adsorbed on the surface of Fh to catalyze the transformation of Fh. Fig. 3b presents XRD patterns of the transformation products of Fe2þ ions by adding different amount of H2O2
Fig. 3. (a) XRD patterns of the products obtained by adding different amount of Fe2þ in the absence of H2O2 and pollutant. pH 7, 25 C, #: Fh, *:g-FeOOH, D: a-FeOOH; (b) XRD patterns of the products obtained by adding different amount of H2O2 in the absence of pollutant. pH 7, 25 C, Fe2þ: 1.0 mmol.
All of iron species mentioned in Section 3.4 could react with H2O2 to produce OH and degrade pollutants. However, the catalytic performance of iron species varies with species. First, Fe2þ could directly react with H2O2 to produce OH (Eq. (2)). In a near neutral pH environment, the resulting Fe3þ ions by Eq. (2) are rapidly precipitated to form Fh. Thus, the contribution of aqueous Fe3þ alone to the production of OH should be minor. Second, Fe2þ may directly react with NaOH to form Fe(OH)2, and Fe(OH)2 could also be used as heterogeneous Fenton catalyst (Yan et al., 2012). However, the proportion of Fe(OH)2 in the current system at pH 5e8 is very low (Cornell and Schwertmann, 2003). In addition, Fe(OH)2 itself is easily oxidized to form Fh under the condition without the protection of inert gases. Thus, the contribution of Fe(OH)2 is also minor. Third, Fh, g-FeOOH and a-FeOOH have been reported as heterogeneous Fenton catalysts, but all of them displayed relatively inert catalytic activity at neutral pH value (He et al., 2018). Actually, their catalytic action for the production of OH in the current system mainly comes from the contribution of FeII/Fh, FeII/g-FeOOH and FeII/a-FeOOH. To evaluate the difference in the production of OH between Fh and FeII/Fh, g-FeOOH and FeII/ g-FeOOH as well as a-FeOOH and FeII/a-FeOOH, the EPR spectra of the above systems were determined (Fig. 4). The magnitude of the DMPO-OH adduct EPR signals in FeII/Fh, FeII/g-FeOOH and FeII/aFeOOH systems increased by 5.36, 10.88 and 5.72-fold in comparison with that of the systems without FeII, respectively, indicating that Fh, g-FeOOH and a-FeOOH combined with FeII exhibit stronger catalytic ability for the decomposition of H2O2 than themselves. Subsequently, according to the experimental condition in Table 1, the solid product formed in BB degradation system was collected and characterized, and the catalytic performance of the solid product was evaluated in the presence and absence of Fe2þ (Fig. S6). XRD pattern in Fig. S6a indicates that the collected solid product is a mixture of Fh, g-FeOOH and a-FeOOH. The degradation rate of BB was 4% at 30 min when using the solid product as catalyst, while the degradation rate reaches 93% in the coexistence of the solid product and Fe2þ (Fig. S6b). Among them, Fe2þ itself contributes 64% of degradation. It is well known that the homogeneous Fenton reaction has efficient reactivity only at an acid pH (2.5e3.5). The results in Fig. S6b meant that the dropwise addition manner of Fe2þ solution greatly improves the catalytic efficiency of Fe2þ ions in near neutral pH environment. As for the three iron (hydr)oxides, the contribution of FeII/Fh on the degradation should be the largest since Fh is initially formed in the degradation process and the reaction between FeII/Fh and H2O2 (Eq. (4)) takes precedence over the catalytic transformation of Fh (Eq. (5)). In a word, the current system is a complex one, in which the addition of Fe2þ, the formation of various iron species and the degradation of pollutant proceeded simultaneously. The species and quantity of the solid products changed with the reaction process. Hence, the above results are only a rough evaluation to the contributions of different iron species. In summary, when Fe2þ ions were introduced into the system
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Fig. 5. Scheme of the rapid production of hydroxyl radicals.
were simultaneously dripped into the system with pollutant and H2O2 by controlling the dripping rate to keep a constant neutral pH. The designed reaction system exhibited a high degradation efficiency for various organic pollutants. The degradation time for the pollutants depended on the dripping rate of Fe2þ solution. Usually, it only needed about 10e20 min to complete the degradation of pollutant with 100e5000 ppm in concentration. The degradation rate could reach 99% by adjusting the amount of Fe2þ and H2O2. The mineralization rates of various organic pollutants were more than 60%. The rapid generation of OH was attributed to in-situ generated Fe-based homogeneous and heterogeneous Fenton reaction centers, including FeII (e.g. Fe2þ and FeOHþ), Fe(OH)2, Fe3þ (e.g. Fe3þ, Fe(OH)2- and Fe(OH)-2), Fh, g-FeOOH and a-FeOOH as well as FeII/Fh, FeII/g-FeOOH and FeII/a-FeOOH. After degradation, the iron ions were effectively transformed into hematite, goethite and magnetite. Author contributions section Xuqing Li: Main Data, Bing Xiao: Data, Writing, Original draft preparation. Meng Wu: Data, Lin Wang: Data, Rufen Chen: Reviewing, Yu Wei: Supervision, Hui Liu: Idea, Conceptualization, Methodology, Reviewing and Editing Acknowledgments Xuqing Li and Bing Xiao contributed equally. The authors are grateful to the National Natural Science Foundation of China (No. 21277040 and 21477032), Natural Science Foundation of Hebei Province (No. B2017205166) and Technology Innovation Project of Hebei Normal University (No. L2018K06). Appendix A. Supplementary data Fig. 4. EPR spectra of different systems at pH ¼ 7. (a) Fh and FeII/Fh, g ¼ 2.0060; (b) gFeOOH and FeII/g-FeOOH, g ¼ 2.0063; (c) a-FeOOH and FeII/a-FeOOH, g ¼ 2.0064.
including pollutant and H2O2 according to the designed produce, Fe2þ itself and the in-situ generated Fe(OH)2, Fh, g-FeOOH, aFeOOH, FeII/Fh, FeII/g-FeOOH and FeII/a-FeOOH with very small particle size and high specific surface area constructed numerous catalytic centers of homogeneous and heterogeneous Fenton reaction, leading to a rapid degradation of pollutants. After degradation, the added Fe2þ ions were transformed to hematite, goethite and magnetite particles by simply refluxing process. The efficient production mechanism of OH in the current system can be exhibited by Fig. 5. This study presents a novel approach toward the rational design of catalysts for wastewater treatment.
4. Conclusion In this study, a rapid degradation path of pollutants in a neutral pH range (5e8) was developed, in which Fe2þ and OH solution
Supplementary data to this article can be found online at https://doi.org/10.1016/j.chemosphere.2019.125663. References Bokare, A.D., Choi, W., 2014. Review of iron-free Fenton-like systems for activating H2O2 in advanced oxidation processes. J. Hazard Mater. 275, 121e135. Boland, D.D., Collins, R.N., Glover, C.J., Payne, T.E., Waite, T.D., 2014. Reduction of U(VI) by Fe(II) during the Fe(II)-accelerated transformation of ferrihydrite. Environ. Sci. Technol. 48, 9086e9093. Bolobaev, J., Kattel, E., Viisima, M., Goi, A., Trapido, M., Tenno, T., Dulova, N., 2014. Reuse of ferric sludge as an iron source for the Fenton-based process in wastewater treatment. Chem. Eng. J. 255, 8e13. Cao, S.S., Kang, F.F., Li, P., Chen, R.F., Liu, H., Wei, Y., 2015. Photoassisted heteroFenton degradation mechanism of Acid Blue 74 by a g-Fe2O3 catalyst. RSC Adv. 5, 66231e66238. Cao, G.M., Sheng, M., Niu, W.F., Fei, Y.L., Li, D., 2009. Regeneration and reuse of iron catalyst for Fenton-like reactions. J. Hazard Mater. 172, 1446e1449. Chen, L., Ma, J., Li, X., Zhang, J., Fang, J., Guan, Y., Xie, P., 2011. Strong enhancement on Fenton oxidation by addition of hydroxylamine to accelerate the ferric and ferrous iron cycles. Environ. Sci. Technol. 45, 3925e3930. Cornell, R.M., Schwertmann, U., 2003. The Iron Oxides, second ed. WILEY-VCH, New York. Dong, C.C., Ji, J.H., Shen, B., Xing, M.Y., Zhang, J.L., 2018. Enhancement of H2O2
X. Li et al. / Chemosphere 245 (2020) 125663 decomposition by the Co-catalytic effect of WS2 on the Fenton reaction for the synchronous reduction of Cr(VI) and remediation of phenol. Environ. Sci. Technol. 52, 11297e11308. Feng, J.X., Li, S.Y., Sheng, Y., Xiong, Y., Lan, S.Y., Tian, S.H., Kong, L.J., Fan, C., 2017. Remarkable improvement of cycling Fenton process for catalytic degradation of phenol: tuning of triggering effect. Appl. Catal. Gen. 542, 21e27. He, D.L., Wu, X.J., Chen, Y.F., Situ, Y., Zhong, L., Huang, H., 2018. In-situ growth of lepidocrocite on Bi2O3 rod: a perfect cycle coupling photocatalysis and heterogeneous fenton-like process by potential-level matching with advanced oxidation. Chemosphere 210, 334e340. Jones, A.M., Kinsela, A.S., Collins, R.N., Waite, T.D., 2016. The reduction of 4chloronitrobenzene by Fe(II)-Fe(III) oxide systems - correlations with reduction potential and inhibition by silicate. J. Hazard Mater. 320, 143e149. Li, X.N., Huang, X., Xi, S.B., Miao, S., Ding, J., Cai, W.Z., Liu, S., Yang, X.L., Yang, H.B., Gao, J.J., Wang, J.H., Huang, Y.Q., Zhang, T., Liu, B., 2018. Single cobalt atoms anchored on porous N-doped graphene with dual reaction sites for efficient Fenton-like catalysis. J. Am. Chem. Soc. 140, 12469e12475. Liu, H., Li, X.L., Wang, Y., Yang, X., Zhen, Z., Chen, R.F., Hou, D.L., Wei, Y., 2014. New insight into the effect of the formation environment of ferrihydrite on its structure and properties. RSC Adv. 4, 11451e11458. Liu, H., Li, P., Zhu, M.Y., Wei, Y., Sun, Y.H., 2007. Fe(II)-induced transformation from ferrihydrite to lepidocrocite and goethite. J. Solid State Chem. 180, 2121e2128. Liu, H., Wang, Y., Ma, Y., Wei, Y., 2010. The microstructure of ferrihydrite and its catalytic reactivity. Chemosphere 79, 802e806. Liu, H., Wei, Y., Sun, Y.H., 2005. The formation of hematite from Ferrihydrite using Fe(II) as a catalyst. J. Mol. Catal. A Chem. 226, 135e140. Lyu, L., Yu, G., Zhang, L., Hu, C., Sun, Y., 2018. 4-phenoxyphenol-functionalized reduced graphene oxide nanosheets: a metal-free Fenton-like catalyst for pollutant destruction. Environ. Sci. Technol. 5, 747e756. Ma, Z.C., Ren, L.M., Xing, S.T., Wu, Y.S., Gao, Y.Z., 2015. Sodium dodecyl sulfate modified FeCo2O4 with enhanced Fenton-like activity at neutral pH. J. Phys. Chem. C 119, 23068e23074. Nie, Y., Hu, C., Qu, J., Zhao, X., 2009. Photoassisted degradation of endocrine disruptors over CuOxeFeOOH with H2O2 at neutral pH. Appl. Catal. B Environ. 87, 30e36. Nie, Y., Hu, C., Zhou, L., Qu, J., Wei, Q., Wang, D., 2010. Degra-dation characteristics of humic acid over iron oxides/Fe0 core-shell nanoparticles with UVA/H2O2. J. Hazard Mater. 173, 474e479. Nie, Y., Zhang, L., Li, Y.Y., Hu, C., 2015. Enhanced Fenton-like degradation of
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refractory organic compounds by surface complex formation of LaFeO3 and H2O2. J. Hazard Mater. 294, 195e200. Pham, A.L.T., Doyle, F.M., Sedlak, D.L., 2012. Kinetics and efficiency of H2O2 activation by iron-containing minerals and aquifer materials. Water Res. 46, 6454e6462. Qin, Y., Song, F., Ai, Z., Zhang, P., Zhang, L., 2015. Protocatechuic acid promoted alachlor degradation in Fe(III)/H2O2 Fenton system. Environ. Sci. Technol. 49, 7948e7956. hin, A., Grene che, J.M., Liger, E., 2005. Silvester, E., Charlet, L., Tournassat, C., Ge € ssbauer spectrometry of FeII adsorbed Redox potential measurements and mo onto FeIII (oxyhydr)oxides. Geochem. Cosmochim. Acta 69, 4801e4815. Stewart, S.M., Hofstetter, T.B., Joshi, P., Gorski, C.A., 2018. Linking thermodynamics to pollutant reduction kinetics by Fe2þ bound to iron oxides. Environ. Sci. Technol. 52, 5600e5609. n, J.O.S., Morrison, E., Smirnova, T.I., Smirnov, A.I., 2011. SurfaceVoinov, M.A., Paga mediated production of hydroxyl radicals as a mechanism of iron oxide nanoparticle biotoxicity. J. Am. Chem. Soc. 133, 35e41. Wang, Y., Gao, Y.W., Chen, L., Zhang, H., 2015a. Goethite as an efficient heterogeneous Fenton catalyst for thedegradation of methyl orange. Catal. Today 252, 107e112. Wang, F.X., Hu, M., Liu, C.M., Wan, Y., Yuan, X.Q., Rao, Y.L., Yu, Y.L., Fu, G.M., 2015b. Comparison of analysis methods for glyphosate in herbicide products by nitrosation derivatization. Agrochemicals 54, 334e336. Xu, L., Wang, J., 2011. A heterogeneous Fenton-like system with nanoparticulate zero-valent iron for removal of 4-chloro-3-methyl phenol. J. Hazard Mater. 186, 256e264. Yan, J.C., Tang, H.Q., Lin, Z.F., Anjum, M.N., Zhu, L.H., 2012. Efficient degradation of organic pollutants with ferrous hydroxide colloids as heterogeneous Fentonlike activator of hydrogen peroxide. Chemosphere 87, 111e117. Yang, X.J., Xu, X.M., Xu, J., Han, Y.F., 2013. Iron oxychloride (FeOCl): an efficient Fenton-like catalyst for producing hydroxyl radicals in degradation of organic contaminants. J. Am. Chem. Soc. 135, 16058e16061. Zhang, G.K., Gao, Y.Y., Zhang, Y.L., Guo, Y.D., 2010. Fe2O3-pillared rectorite as an efficient and stable Fenton-like heterogeneous catalyst for photodegradation of organic contaminants. Environ. Sci. Technol. 44, 6384e6389. Zhang, T.T., Zhao, Y.L., Kang, S.C., Li, Y.J., Zhang, Q.W., 2019. Formation of active Fe(OH)3 in situ for enhancing arsenic removal from water by the oxidation of Fe(II) in air with the presence of CaCO3. J. Clean. Prod. 227, 1e9.
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In-situ generation of multi-homogeneous/heterogeneous Fe-based Fenton catalysts toward rapid degradation of organic pollutants at near neutral pH Xuqing Li, Bing Xiao, Meng Wu, Lin Wang, Rufen Chen, Yu Wei, Hui Liu* School of Chemistry and Material Science, Key Laboratory of Inorganic Nanomaterials of Hebei Province, National Demonstration Center for Experimental Chemistry Education, Hebei Normal University, Shijiazhuang, 050024, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A modified Fe2þ Fenton reaction system was constructed. Pollutants with high concentration and different structures were degraded at near neutral pH. In-situ-formed multi-Fenton centers accelerated the decomposition of H2O2 and generation of OH. After degradation, the added ferrous ions were transformed to hematite, goethite and magnetite.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 August 2019 Received in revised form 1 December 2019 Accepted 12 December 2019 Available online 18 December 2019
In this study, an in-situ generated multi-homogeneous/heterogeneous Fe-based catalytic system was developed, which exhibited a high efficiency for the production of OH and rapid degradation of various organic pollutants in a near neutral pH range (5e8). The mechanism for the rapid decomposition of H2O2 and the generation of OH were investigated in detail. The results indicated that, besides the introduced Fe2þ, the in-situ generated various iron species including Fe(OH)þ, Fe(OH)2, Fe3þ, ferrihydrite (Fh), gFeOOH and a-FeOOH as well as FeII/Fh, FeII/g-FeOOH and FeII/a-FeOOH could simultaneously act as homogeneous and heterogeneous Fenton reaction catalysts. The dropwise addition manner of Fe2þ greatly improved the catalytic efficiency of Fe2þ ions in near neutral pH environment, while the in-situ generated nanosized Fh, g-FeOOH and a-FeOOH could supply numerous active catalytic sites. After degradation, the ferrous ions could be transformed to various crystalline iron oxides by the catalytic phase transformation. This study presents a method towards the rational design of novel Fenton catalysts for wastewater treatment. © 2019 Elsevier Ltd. All rights reserved.
Handling Editor: Jun Huang Keywords: In-situ generation Multi-homogeneous/heterogeneous catalyst Degradation Organic pollutant Mechanism
1. Introduction Advanced oxidation processes (AOPs) are considered as one of the most effective technologies to treat organic pollutants from
* Corresponding author. E-mail address: [email protected] (H. Liu). https://doi.org/10.1016/j.chemosphere.2019.125663 0045-6535/© 2019 Elsevier Ltd. All rights reserved.
wastewater. As an important AOP, the homogeneous Fenton system can produce highly oxidizing hydroxyl radical (OH) for nonselective degradation of organic pollutants (Dong et al., 2018; Li et al., 2018; Pham et al., 2012; Qin et al., 2015). However, the classic Fenton reaction has efficient reactivity only at an acid pH such as pH 2.5e3.5, and the deposition of large amounts of Fe3þ sludge limits its applications (Chen et al., 2011; Yang et al., 2013). To overcome these limitations, two strategies have been put forward.
2
X. Li et al. / Chemosphere 245 (2020) 125663
further purification.
One is to develop heterogeneous Fenton catalysts, and the other is to reuse iron-containing sludge after appropriate treatment. For the first strategy, various heterogeneous Fenton catalysts such as zero-valent iron (Xu and Wang, 2011), Fe2O3 (Zhang et al., 2010), FeOOH (Wang et al., 2015a, 2015b), g-Fe2O3 (Cao et al., 2015) have been reported. The advantage of these heterogeneous catalysts lies in their easy separation from the reaction system. However, iron oxides also exhibit a high catalytic efficiency in an acid pH environment and with pH increasing, the inert catalytic activity was displayed (Bokare and Choi, 2014; Ma et al., 2015). Although other transitional metals or nonmetallic materials have been developed for the degradation of organic pollutants (Lyu et al., 2018; Nie et al., 2009, 2010), their catalytic performance usually needs the aids of specific equipment and energy such as ultrasonic and/or UV light irradiation etc. (Nie et al., 2015). Relatively, homogeneous Fenton system is still a proper candidate catalyst to degrade organic pollutants from the angle of the operation cost, equipment and working efficiency, especially for the treatment of pollutants with high concentration. For the second strategy, one example is the reuse of ferric sludge as an iron source for the Fenton-based process in wastewater treatment reported (Bolobaev et al., 2014). This method could minimize the production of hazardous ferric-containing wastes and hence reduce the overall cost of the treatment process. Another example is that the ferric sludge was dewatered, dried and baked at 350e400 C and the residual solids were dissolved in sulfuric acid to form the reusable catalyst for Fenton and Fenton-like reactions (Cao et al., 2009). Moreover, Feng et al. investigated a cycling Fenton process for the degradation of organic pollutants, in which Fe species could be repeatedly used as a homogeneous catalyst after acid dissolution of its Fe-containing sludge (Feng et al., 2017). However, there are still some problems for the reuse of ironcontaining sludge, such as longer precipitation time, high baked temperature and consumption of large amounts of acid. Our group has explored the catalytic formation of various iron oxides in liquid phase and their application in the environmental field (Cao et al., 2015; Liu et al., 2005, 2007, 2010). We found that trace of Fe(II) adsorbed on ferrihydrite (Fh) could obviously accelerate its transformation (called catalytic phase transformation) (Liu et al., 2005, 2007). The products might be lepidocrocite, goethite, hematite and magnetite, which depended on pH, temperature and the amount of Fe(II) (Liu et al., 2007). Some studies have demonstrated that when Fe(II) was adsorbed on the surface of iron oxide or iron mineral, it displayed enhanced strong reductive abilities towards environmental contaminants compared with aqueous Fe2þ alone (Boland et al., 2014; Jones et al., 2016; Silvester et al., 2005; Stewart et al., 2018). Based on these research results, we modified the classical homogeneous Fenton system. In the modified system, Fe2þ ions were still used as an iron source, and various organic pollutants with different structures (e.g. Glyphosate, 2.4.6Trichlorophenol, Bright blue, Rhodamine B, etc.) could be rapidly degraded at near neutral pH values. After degradation, those iron ions could be transformed to crystalline iron oxides by the catalytic phase transformation.
The concentrations of pollutants Bright blue (BB), Medium yellow 10 (MY10), Acid Green 25 (AG25), Phenol red (PR), Rhodamine B (RhB), Malachite green (MG) and Cationic Pink FG (CPFG) and pNitrophenol in the solution were determined using a UVevis spectrophotometer (UV-8000 Shanghai Metash Instruments Co. Ltd.) at 628, 355, 296, 432, 553, 617, 522 and 317 nm, respectively. In a typical EPR experiment, 0.06 g Fh was added to 100 mL of H2O2 (8 mmol/L) solution and its pH was adjusted to 7. Then, 1 mL of DMPO (Sigma Aldrich, 100 ppm) was immediately injected into the reaction system. The supernatant filtered through a Naflon membrane of 0.22 mm was transferred to a 100 mL capillary tube, which was then fixed in the resonant cavity of the spectrometer.
2. Experimental
3. Results and discussion
2.1. Materials
3.1. Design of the modified Fenton reaction
All chemicals were analytical grade. FeSO4∙7H2O and NaOH were purchased from Tianjin Yongda Chemical Corp. Bright blue (BB), Medium yellow 10 (MY10), Acid Green 25 (AG25), Phenol red (PR), Rhodamine B (RhB), Malachite green (MG), Cationic Pink FG (CPFG), Glyphosate (Gly) and 2.4.6-Trichlorophenol (Trp) were purchased from J&K. All chemicals were used as received without
Scheme 1 shows the modified Fenton reaction process. In this scheme, Fe2þ and OH solutions were added simultaneously into a solution including pollutant and H2O2. The addition rates of the two solutions were controlled via peristaltic pump by maintaining near neutral pH. There are two paths to form Fh in this process. First, the added Fe2þ ions can react directly with H2O2 to generate OH and
2.2. Characterization of samples X-ray diffraction (XRD) measurement was carried out at room temperature using a D8ADVANCE diffractometer with CuKa radiation (l ¼ 0.15418 nm). Scanning Electron Microscopy (SEM) images were taken with a S-4800 scanning electron microscope (Japan Hitachi LTD). The DMPO trapped EPR spectrum was carried out at a Magnettech MS-5000 electron paramagnetic resonance spectroscopy (Germany). 2.3. Degradation of organic pollutants Degradation of pollutant was carried out in a beaker placed in a water bath with magnetic stirring under room light. In a typical procedure, 0.1e20 mmol of H2O2 was added into 100 mL of the pollutant solution (10e5000 mg/L). Then, Fe2þ (0.04e10 mmol) and (0.2e1.0) mol/L NaOH solutions were added simultaneously into the reaction system until Fe2þ solution was exhausted. The addition rates of the two solutions were controlled via peristaltic pump by maintaining pH 5e8. The reaction lasts from several to 20 min, depending on the addition velocity of Fe2þ and concentration of pollutant. Hereafter, part of the supernatant was used to determine the concentration of pollutants. The remaining reaction system was heated until boiling and the reflux was maintained for 1e2 h. The resulted product was thoroughly washed with deionized water and dried at about 70e80 C. The concentration of pollutants (e.g. glyphosate (Gly) and 2.4.6Trichlorophenol (Trp)) was determined by using a LC1260 HPLC (USA Agilent Technologies) equipped with a C18 column and UV detector. Water (98%) and methanol (2%) were mixed as the mobile phase for glyphosate, while methanol (60%) and 0.1% acetic acid (40%) were used as the mobile phase for 2.4.6-Trichlorophenol. The wavelength was 240 and 294 nm for glyphosate and 2.4.6trichlorophenol, respectively, using a UVevis spectrophotometer. Before determination, glyphosate was quantified after pre-column derivatization with NaNO2 (Eq. (1)) according to a method of Wang et al. (2015a, 2015b). KBr
C3 H8 NO5 P þ NaNO2 þ HCl ! C3 H7 N2 O6 P þ NaCl þ H2 O
(1)
X. Li et al. / Chemosphere 245 (2020) 125663
Scheme 1. Schematic of the modified Fenton reaction.
Fe3þ ions (Eq. (2)) (Dong et al., 2018). Although Fe3þ ions can also react with H2O2 to regenerate Fe2þ ions through Eq. (3), Eq. (2) dominates due to its much larger reaction rate than Eq. (3). Thus, Fe3þ ions were fast precipitated to form Fh due to its very small ksp (1.1 1038) and the addition of NaOH. Second, those added Fe2þ ions may react directly with NaOH to generate Fe(OH)2, which is also quickly oxidized into Fh in the absence of inert gas protection (Zhang et al., 2019). Fe2þ þ H2O2 / Fe3þ þ OH þ OH k1 ¼ 40e80 L mol1 s1
(2)
Fe3þ þ H2O2 / Fe2þ þ HO2 þ Hþ k2 ¼ 9.1 107 L mol1 s1 (3) When subsequent Fe2þ ions were added into the system, part of them could be adsorbed on the surface of Fh (Liu et al., 2005, 2007) to form FeII/Fh. On the one hand, these FeII ions have a strong reducing ability (Boland et al., 2014; Jones et al., 2016; Silvester et al., 2005; Stewart et al., 2018), thus, FeII/Fh more easily reacts with H2O2 to produce OH and then degrade pollutants. On the other hand, the particle size of Fh is about 2e5 nm (Liu et al., 2014), resulting in numerous heterogeneous catalytic sites to improve the decomposition of H2O2. After degradation, the FeII/Fh could be transformed to crystal products of iron oxides by the catalytic phase transformation (Liu et al., 2005, 2007). 3.2. Degradation of various pollutants The catalytic performance of the modified Fenton system was evaluated by degrading several model pollutants with different structures. Table 1 shows the degradation rate and experimental conditions of different pollutants. All experimental results show that the current modified Fenton system exhibits high degradation efficiency for the selected pollutants. The dosage of Fe2þ and H2O2
3
depends on the concentration of pollutant. Even for a 5000 ppm of glyphosate, a high degradation rate still reaches (Table S1). To evaluate that the high removal rate of glyphosate results from its degradation instead of adsorption, three experiments (e.g. TOC, SI()-MS and EPR) were conducted. The result of TOC indicated that the mineralization rate is 65e60% for a 300e5000 mg/L solution of glyphosate (Table S1). To gain the degradation product of glyphosate, Fe2þ ions were added in batch mode. For example, 10 mmol of Fe2þ ions was used to completely degrade 5000 ppm of glyphosate solution. First, 5 mmol of Fe2þ ions were added into the reaction system and then the degradation products were detected by SI()-MS. Second, another 5 mmol of Fe2þ ions were added into the reaction system and then the degradation products were again detected by SI()-MS. The results are shown in Fig. 1a. Before degradation the species with m/z of 199.2 was attributed to the derivatization product of glyphosate (nitroglyphosate). When 5.0 mmol of Fe2þ ions were added into 5000 ppm of glyphosate solution, five species with m/z of 61, 75.1, 77.1, 119.1 and 199.2 were found in the supernatant (Fig. 1a). When 10.0 mmol of Fe2þ ions were added, only three species with m/z 61, 75.1 and 77.1 were found, suggesting that the species with m/z 119.1 was an intermediate. Among them, the species with m/z of 119.1 was attributed to N-nitrososarcosine, the derivatization product of sarcosine. The species with m/z of 61, 75.1 and 77.1 were acetic acid, glyoxylic acid and hydroxyacetic acid, respectively. Based on the experimental results, the degradation path of glyphosate was described in Fig. 1b. Fig. 1c presents EPR spectrum of the current system. Incubation of H2O2 and DMPO with the current system resulted in a strong fourline EPR signal with a 1:2:2:1 peak-to-peak intensity pattern. The four lines has approximately equal spacing of ca. 1.50 mT and the g value is 2.0060, which is consistent with the hyperfine splitting reported for the DMPO-OH spin adduct (Voinov et al., 2011 Yan et al., 2012). Considering the potential contribution of adsorption of pollutants on their removals, two representative pollutants (cationic dye CPFG and anionic dye BB) were selected to evaluate their adsorption capacity. The result indicated that only several ppm of BB and CPFG was adsorbed (Fig. S1). All the above results suggested that the high removal rate of pollutant was resulted from its degradation instead of adsorption. In addition, the modified reaction procedure was used to degrade a mixed solution including two or three pollutants. Fig. S2 presents UVevis absorption spectra and HPLC spectra of mixed solution of BB and MY 10 before and after degradation. It can be seen that almost the two pollutants have been completely degraded. For a mixed solution of three kinds of pollutants (AG, MG and CPFG), a high degradation rate can be realized (Table S2 and Fig. S3). The mineralization rate is larger than 60%. 3.3. Transformation of Fe2þ The
transformation products
of
the
added
Fe2þ
were
Table 1 Degradation of Various Organic pollutants. Pollutants
Concentration of pollutants (ppm)
FeSO4 (mmol)
H2O2 (mmol)
pH
Degradation rate (%)
Glyphosate 2.4.6-Trichlorophenol Bright blue Medium yellow 10 Alizarin green Phenol red Rhodamine B Malachite green Cationic Pink FG
100 80 80 167 167 167 167 167 167
0.20 1.0 1.0 0.6 0.6 0.8 0.6 0.3 0.3
0.50 1.2 1.2 0.8 0.9 1.0 0.9 0.5 0.4
6e7 5e6 6e7 7e8 7e8 6e7 6e7 5e6 5e6
94.59 90.63 88.68 91.85 92.72 90.40 88.80 91.02 94.76
4
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Fig. 2. (a) XRD patterns of the products obtained by adding different amount of Fe2þ to degrade 200 mg/L of Orange G, H2O2 (1.2 mmol), pH 7, 25 C, #: Fh, *: lepidocrocite, D: goethite; (b) XRD patterns of the products obtained by boiling and refluxing for 1e2 h after degradation. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
dosage of FeII ions and pH. When nFe(II) ¼ 2e18 mmol, the transformation product is hematite at pH 6e8. When nFe(II) ¼ 2025 mmol, the products are goethite at pH 6e7 and the mixture of goethite and hematite at pH 7e8 (not shown). When nFe(II) ¼ 50 mmol, the transformation product is magnetite at pH 6e8. Fig. S4 presents SEM images of the products. The as-prepared hematite particles are composed of a large number of uniform spheres with diameters of approximately 60e80 nm (Fig. S4a). The as-prepared goethite is present as rod-like particles (Fig. S4b), and the obtained magnetite shows uniform spheres with diameters of approximately 30e50 nm (Fig. S4c). Fig. 1. (a) ESI()-MS of glyphosate sample withdrawn at 0, 5 and 10 mmol of Fe2þ addition in the presence of H2O2. (b) The degradation path of glyphosate. (c) EPR spectrum over the current system.
determined by XRD patterns after degradation of pollutants (Fig. 2a). For 100 mL Orange G solution (200 mg/L), Fh was obtained when the degradation reaction was conducted at room temperature and the dosage of Fe2þ ions was 15 mmol. With increasing the dosage of Fe2þ ions, poor-crystallization lepidocrocite and goethite were obtained (Fig. 2a), suggesting that the initial transformation product of Fe2þ ions should be Fh. Once Fh was formed, subsequently added Fe(II) could be adsorbed on surface of Fh and accelerate its transformation to lepidocrocite and goethite (Liu et al., 2005, 2007). To obtain well-crystallization iron oxides, the reaction system after degradation was heated and kept boiling and refluxing for 1e2 h. XRD patterns of the obtained products were shown in Fig. 2b. The added Fe2þ ions can be transformed to hematite, goethite and magnetite, respectively, which depends on the
3.4. Analysis on iron species in the current system Ferrous ions in aqueous solution exist in different forms such as Fe2þ, Fe(OH)þ, Fe(OH)2 and Fe(OH) 3 , which depends on pH (Cornell and Schwertmann, 2003). When pH 5, ferrous ions predominantly present as Fe2þ. With increasing pH, the proportion of Fe2þ diminishes while that of Fe(OH)þ increases in the range of pH 5e8. In this pH range, Fe(OH)2 is probably formed. When pH 10, Fe(OH) 3 species appeared. Because the pH of the current system is controlled within 5e8 (Table 1), the main ferrous species should be Fe(OH)þ and Fe(OH)2. However, the formation of Fe(OH)2 depends on both pH and the concentration of ferrous ions. Table S3 presents the saturation concentration of Fe2þ calculated from Ksp (8 1016) of Fe(OH)2 at 25 C and the real concentration of Fe2þ added into the current system (Table 1). The latter was much less than the former, suggesting that the precipitation of Fe(OH)2 is unstable even if it could be formed in the range of pH 5e8. Actually, the added Fe2þ ions could rapidly react with H2O2 to form Fe3þ ions (Eq. (2)) and then to form Fh due to its very little Ksp value (1.1 1038).
X. Li et al. / Chemosphere 245 (2020) 125663
Because Fe2þ ions were continuously added into the system, the subsequently added Fe2þ ions could be partly adsorbed on the surface of Fh to form FeII/Fh. On the one hand, these FeII ions have a strong reducing ability (Boland et al., 2014; Jones et al., 2016; Silvester et al., 2005; Stewart et al., 2018) and could react with H2O2 to produce OH (Eq. (4)). On the other hand, the FeII in FeII/Fh could catalyze the transformation of Fh to form lepidocrocite and goethite (Eq. (5)) (Liu et al., 2007). The two reactions are parallel and compete each other. FeII/Fh þ H2O2 / FeIII/Fh þ OH þ OH
5
in the absence of pollutant. When the amount of H2O2 is 0.1 mmol, the main product is g-FeOOH (Fig. 3b). With increasing dosage of H2O2, the formation of g-FeOOH is suppressed and Fh gradually dominates (Fig. 3b). These results indicate that Eq. (4) takes precedence over Eq. (5). In summary, when Fe2þ solution is added into the system according to the designed procedure, the coexisted iron species include FeII (e.g. Fe2þ and FeOHþ), Fe(OH)2, Fe3þ (e.g. Fe3þ, Fe(OH)2- and Fe(OH)-2), Fh, g-FeOOH and a-FeOOH as well as FeII/Fh, FeII/g-FeOOH and FeII/a-FeOOH under the condition of pH 5e8. Except Fe2þ ions, all of other iron species could be in-situ generated.
(4) 3.5. The mechanism for the rapid production of hydroxyl radicals
FeII =Fh / g FeOOH þ a FeOOH þ FeII
(5)
To confirm the above analysis, several experiments were conducted either in the absence or presence of H2O2. Fig. 3a presents XRD patterns of the products obtained with different amount of Fe2þ and in the absence of H2O2. When nFe(II) ¼ 0.1 mmol, Fh and gFeOOH coexist in the product. When nFe(II) ¼ 1.0 mmol, the main product is g-FeOOH. When nFe(II) ¼ 5.0 mmol, the main product is g-FeOOH along with small amount of a-FeOOH. These results indicate that Fh is the initially formed product. The formation of gFeOOH and a-FeOOH in Fig. 3a should be attributed to the catalysis of FeII to the transformation of Fh (Liu et al., 2007). Subsequently, the changes of the adsorption rate of Fe2þ ions on the Fh with pH values were determined under nitrogen protection (Fig. S5). About 25% of Fe2þ ions are adsorbed at pH 5 and about 50% at pH 6 and almost all of Fe2þ ions could be adsorbed at pH 7e8. The results in Figs. 3a and S5 indicate that once Fh is formed, the subsequently added Fe2þ could be adsorbed on the surface of Fh to catalyze the transformation of Fh. Fig. 3b presents XRD patterns of the transformation products of Fe2þ ions by adding different amount of H2O2
Fig. 3. (a) XRD patterns of the products obtained by adding different amount of Fe2þ in the absence of H2O2 and pollutant. pH 7, 25 C, #: Fh, *:g-FeOOH, D: a-FeOOH; (b) XRD patterns of the products obtained by adding different amount of H2O2 in the absence of pollutant. pH 7, 25 C, Fe2þ: 1.0 mmol.
All of iron species mentioned in Section 3.4 could react with H2O2 to produce OH and degrade pollutants. However, the catalytic performance of iron species varies with species. First, Fe2þ could directly react with H2O2 to produce OH (Eq. (2)). In a near neutral pH environment, the resulting Fe3þ ions by Eq. (2) are rapidly precipitated to form Fh. Thus, the contribution of aqueous Fe3þ alone to the production of OH should be minor. Second, Fe2þ may directly react with NaOH to form Fe(OH)2, and Fe(OH)2 could also be used as heterogeneous Fenton catalyst (Yan et al., 2012). However, the proportion of Fe(OH)2 in the current system at pH 5e8 is very low (Cornell and Schwertmann, 2003). In addition, Fe(OH)2 itself is easily oxidized to form Fh under the condition without the protection of inert gases. Thus, the contribution of Fe(OH)2 is also minor. Third, Fh, g-FeOOH and a-FeOOH have been reported as heterogeneous Fenton catalysts, but all of them displayed relatively inert catalytic activity at neutral pH value (He et al., 2018). Actually, their catalytic action for the production of OH in the current system mainly comes from the contribution of FeII/Fh, FeII/g-FeOOH and FeII/a-FeOOH. To evaluate the difference in the production of OH between Fh and FeII/Fh, g-FeOOH and FeII/ g-FeOOH as well as a-FeOOH and FeII/a-FeOOH, the EPR spectra of the above systems were determined (Fig. 4). The magnitude of the DMPO-OH adduct EPR signals in FeII/Fh, FeII/g-FeOOH and FeII/aFeOOH systems increased by 5.36, 10.88 and 5.72-fold in comparison with that of the systems without FeII, respectively, indicating that Fh, g-FeOOH and a-FeOOH combined with FeII exhibit stronger catalytic ability for the decomposition of H2O2 than themselves. Subsequently, according to the experimental condition in Table 1, the solid product formed in BB degradation system was collected and characterized, and the catalytic performance of the solid product was evaluated in the presence and absence of Fe2þ (Fig. S6). XRD pattern in Fig. S6a indicates that the collected solid product is a mixture of Fh, g-FeOOH and a-FeOOH. The degradation rate of BB was 4% at 30 min when using the solid product as catalyst, while the degradation rate reaches 93% in the coexistence of the solid product and Fe2þ (Fig. S6b). Among them, Fe2þ itself contributes 64% of degradation. It is well known that the homogeneous Fenton reaction has efficient reactivity only at an acid pH (2.5e3.5). The results in Fig. S6b meant that the dropwise addition manner of Fe2þ solution greatly improves the catalytic efficiency of Fe2þ ions in near neutral pH environment. As for the three iron (hydr)oxides, the contribution of FeII/Fh on the degradation should be the largest since Fh is initially formed in the degradation process and the reaction between FeII/Fh and H2O2 (Eq. (4)) takes precedence over the catalytic transformation of Fh (Eq. (5)). In a word, the current system is a complex one, in which the addition of Fe2þ, the formation of various iron species and the degradation of pollutant proceeded simultaneously. The species and quantity of the solid products changed with the reaction process. Hence, the above results are only a rough evaluation to the contributions of different iron species. In summary, when Fe2þ ions were introduced into the system
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X. Li et al. / Chemosphere 245 (2020) 125663
Fig. 5. Scheme of the rapid production of hydroxyl radicals.
were simultaneously dripped into the system with pollutant and H2O2 by controlling the dripping rate to keep a constant neutral pH. The designed reaction system exhibited a high degradation efficiency for various organic pollutants. The degradation time for the pollutants depended on the dripping rate of Fe2þ solution. Usually, it only needed about 10e20 min to complete the degradation of pollutant with 100e5000 ppm in concentration. The degradation rate could reach 99% by adjusting the amount of Fe2þ and H2O2. The mineralization rates of various organic pollutants were more than 60%. The rapid generation of OH was attributed to in-situ generated Fe-based homogeneous and heterogeneous Fenton reaction centers, including FeII (e.g. Fe2þ and FeOHþ), Fe(OH)2, Fe3þ (e.g. Fe3þ, Fe(OH)2- and Fe(OH)-2), Fh, g-FeOOH and a-FeOOH as well as FeII/Fh, FeII/g-FeOOH and FeII/a-FeOOH. After degradation, the iron ions were effectively transformed into hematite, goethite and magnetite. Author contributions section Xuqing Li: Main Data, Bing Xiao: Data, Writing, Original draft preparation. Meng Wu: Data, Lin Wang: Data, Rufen Chen: Reviewing, Yu Wei: Supervision, Hui Liu: Idea, Conceptualization, Methodology, Reviewing and Editing Acknowledgments Xuqing Li and Bing Xiao contributed equally. The authors are grateful to the National Natural Science Foundation of China (No. 21277040 and 21477032), Natural Science Foundation of Hebei Province (No. B2017205166) and Technology Innovation Project of Hebei Normal University (No. L2018K06). Appendix A. Supplementary data Fig. 4. EPR spectra of different systems at pH ¼ 7. (a) Fh and FeII/Fh, g ¼ 2.0060; (b) gFeOOH and FeII/g-FeOOH, g ¼ 2.0063; (c) a-FeOOH and FeII/a-FeOOH, g ¼ 2.0064.
including pollutant and H2O2 according to the designed produce, Fe2þ itself and the in-situ generated Fe(OH)2, Fh, g-FeOOH, aFeOOH, FeII/Fh, FeII/g-FeOOH and FeII/a-FeOOH with very small particle size and high specific surface area constructed numerous catalytic centers of homogeneous and heterogeneous Fenton reaction, leading to a rapid degradation of pollutants. After degradation, the added Fe2þ ions were transformed to hematite, goethite and magnetite particles by simply refluxing process. The efficient production mechanism of OH in the current system can be exhibited by Fig. 5. This study presents a novel approach toward the rational design of catalysts for wastewater treatment.
4. Conclusion In this study, a rapid degradation path of pollutants in a neutral pH range (5e8) was developed, in which Fe2þ and OH solution
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