Heterogeneous activation of peroxymonosulfate by Fe-Co layered doubled hydroxide for efficient catalytic degradation of Rhoadmine B

Chemical Engineering Journal 321 (2017) 222–232

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Heterogeneous activation of peroxymonosulfate by Fe-Co layered doubled hydroxide for efficient catalytic degradation of Rhoadmine B Cheng Gong a,b, Fei Chen a,b, Qi Yang a,b,⇑, Kun Luo c,⇑, Fubing Yao a,b, Shana Wang a,b, Xiaolin Wang a,b, Jiawei Wu a,b, Xiaoming Li a,b, Dongbo Wang a,b, Guangming Zeng a,b a b c

College of Environmental Science and Engineering, Hunan University, Changsha 410082, PR China Key Laboratory of Environmental Biology and Pollution Control (Hunan University), Ministry of Education, Changsha 410082, PR China Department of Biological and Environmental Engineering, Changsha College, Changsha 410003, PR 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


FeCo-LDH was used as heterogeneous

catalyst to active PMS.
The FeCo-LDH/PMS system showed

high performance for dye degradation.
Operating parameters of FeCo-LDH/ PMS system for RhB degradation were optimized.
Reaction mechanism and the stability of FeCo-LDH/PMS were studied in detail.

a r t i c l e

i n f o

Article history: Received 23 January 2017 Received in revised form 23 March 2017 Accepted 25 March 2017 Available online 28 March 2017 Keywords: Sulfate radical Fe-Co layered doubled hydroxide Heterogeneous activation Peroxymonosulfate Rhodmine B

a b s t r a c t Nowadays, increasing attention has been paid to the sulfate radical (SO 4 ) due to its high oxidation efficiency for refractory organic pollutants. In this study, a novel heterogeneous catalyst, Fe-Co layered doubled hydroxide (FeCo-LDH) was prepared by co-precipitation method for the activation of peroxymonosulfate (PMS). The characterization results showed that FeCo-LDH with Fe:Co ratio of 1:2 had high purity and crystallographic structure. With Rhodmine B (RhB) as the model organic pollutant, FeCo-LDH (1:2)/PMS system exhibited much superior degradation performance, which was comparative to homogeneous Co(II)/PMS system. The effect of various parameters, such as temperature, initial pH, initial RhB concentration, PMS dosage and catalyst loading on the RhB degradation was discussed in detail. The RhB oxidation in FeCo-LDH/PMS system can be described well by pseudo-first-order kinetic and the activation energy was calculated as 59.71 kJ/mol. The quenching experiments using ethanol and tert butyl alcohol (TBA) as radical scavengers indicated that both SO 4 and OH radicals were generated in radicals acted as the predominant reactive species. The good stability FeCo-LDH/PMS system and SO 4 and catalytic activity of as-prepared solid catalyst after used repeatedly as well as in different water sources revealed its potential in practical application. Ó 2017 Elsevier B.V. All rights reserved.

1. Introduction ⇑ Corresponding authors at: College of Environmental Science and Engineering, Hunan University, Changsha 410082, PR China (Q. Yang). E-mail addresses: [email protected] (C. Gong), [email protected] (Q. Yang), [email protected] (K. Luo). http://dx.doi.org/10.1016/j.cej.2017.03.117 1385-8947/Ó 2017 Elsevier B.V. All rights reserved.

Synthetic dyes are not only extensively used in the production of textile, leather, paper, photoelectrochemicals cells and hair colorings, but also widely applied in food technology and agriculture research [1,2]. Due to their toxicity and non-biodegradability [3], it

C. Gong et al. / Chemical Engineering Journal 321 (2017) 222–232

would pose potential threats to human beings when these dyes enter into the domestic water systems, especially drinking water. So, as the representative refractory organic pollutants, the dyes in wastewater must be removed or degraded to an acceptable level before being discharged into the natural water. Many methods including adsorption, biological degradation and photocatalytic treatment have been adopted to treat the dyes in water [4,5]. However, each of them are more or less constrained by some disadvantages such as slow process, secondary waste and high cost, etc. Therefore, it’s necessary to find more effective and economical method to remove the dyes from the contaminated environment. Recently, much attention has been paid to producing hydroxyl and sulfate radical for refractory organic pollutant degradation due to their strong oxidizing property. Hydroxyl radical (HO), generated from the Fenton or Fenton-like process, is highly efficient to decompose nearly all organic compounds [6]. However, the practical application of Fenton process is significantly suppressed because of the massive sludge production and strict acid environment. Heterogeneous Fenton process can solve the abovementioned drawbacks to a certain degree, but the treatment efficiency is still unsatisfactory [7,8]. Compared to HO with standard redox potential of 1.8–2.7 eV, sulfate radical (SO 4 ) has higher redox potential (2.5–3.1 eV) and can achieve better oxidation efficiency to the refractory organics. Gong et al. reported that activated carbon fibers (ACFs) supported iron (Fe@ACFs) could well heterogeneously activate the peroxymonsulfate (PMS) to produce SO 4 and showed almost sevenfold higher rate than H2O2 system in the degradation of organic contaminant. Simultaneously, there were negligible irons (less than 0.5 ppm) in the solution [9]. So, in this work, PMS was selected as the oxidant to achieve the refractory organic pollutants degradation through the sulfate radicaladvanced oxidation process (SR-AOP). According to previous reports [10], PMS could be activated to produce SO 4 by ultraviolet (UV), heat, ultrasound, base granular activated carbon or transition metals (Fe, Co, et. al.). Among these activation methods, the activation of PMS by transition metals is the most common. Cobalt (Co) shows highly effective to activate PMS [11]. Su et al. found that the higher cobalt content in the catalyst could lead more SO 4 production from the PMS activation, which was in favor of Rhoadmine B (RhB) removal through SR-AOP [12,13]. However, the dissolve cobalt in the solution maybe causes some health problems [14]. Yang et al. reported that CoFe2O4 exhibited good heterogeneous activation ability to PMS in the decomposition of 2,4dichlorophenol [15]. Further, strong Fe-Co interactions suppressed Co leaching. So exploring the heterogeneous catalysts is an ideal method to prevent the leaching of heavy metal during SR-AOP activated by transition metals. Layered double hydroxides (LDHs), is usually referred to as hydrotalcite-like compounds. It is a kind of anionic layer materials made up of a main body layer board with the metal hydroxide and an interlayer region contain compensating anions and solvation molecules. The general formula of these materials are written as x [M(II)1xM(III)x(OH)2]x+[An x/n] mH2O, where M(II) and M(III) are divalent and trivalent cations, An is the interlayer anion and x can generally have values between 0.2 and 0.4 [16]. Since their unique structure and relatively simple synthesis process, as-synthesized LDHs and their modified forms have been widely investigated as heterogeneous catalysts for various applications. For example, Chen et al. [17] synthesized ZnCr-LDH and found it had enhanced adsorption and photocatalytic activity in methylene blue (MB) removal. Sivashunmugam [18] and Wang [19] reported that the calcined LDHs were the reusable base catalysts for the abundant production of Fatty Acid Methyl Esters (FAME). To the best of our knowledge, there are almost no reports on the LDHs as the heterogeneous catalyst for the activation of PS or PMS.

223

Herein, in this study, a facile synthesis method was provided to prepare Fe-Co layered doubled hydroxide (FeCo-LDH) as the heterogeneous catalyst to activate the PMS. The crystalline structure, morphology, and textural properties of as-prepared catalysts were investigated through corresponding characterization technologies. In order to evaluate the catalytic performance of FeCoLDH, the common azo industrial dye, Rhodamine B (RhB) was selected as the target refractory organic pollutant. The influences of several main parameters, including reaction temperature, initial pH, initial pollutant concentration, PMS dosage and solid catalyst loading on RhB degradation were discussed in detail. Simultaneously, RhB degradation kinetics under different conditions in FeCo-LDH/PMS system was also studied. In addition, the stability and reusability of the catalyst was tested by cycle experiments and in different wastewater source. Lastly, the involved predominant active species were identified, the generation way of radical  intermediates (SO 4 , HO ) and their attack pattern to the target pollutant also discussed. This work might provide a newly, valuable and feasible approach to the dye wastewater treatment. 2. Experimental 2.1. Chemicals Oxone (2KHSO5KHSO4K2SO4, 95.0%), a double potassium salt, was manufactured by DuPont, in which the active ingredient is the peroxymonosulfate (KHSO5, 47.0%). Rhodamine B dye (C28H31ClN2O3), cobalt nitrate hexahydrate (Co(NO3)26H2O, 98.5%), iron(III) nitrate nonahydrate (Fe(NO3)39H2O, 98.5%), sodium (NaOH, 99.0%), sodium carbonate (NaCO3, 99.0%), ethanol (CH3CH2OH, 99,7%), tert-butylalcohol (TBA) ((CH3)3OH, 99.0%), D-(+)Glucose (C6H12O6, 100.0%), ammonium sulfate ((NH4)2SO4, 98.0%), calcium chloride (CaCl2, 98.0%), potassium chloride (KCl, 98.5%), magnesium sulfate (MgSO4, 98.0%) and calcium monohydrogenphosphate (CaHPO4, 99.0%) were all analytical grade. Pure methanol (CH3OH, 99.8%) was used as a quenching agent to stop the reaction. All of these chemical were purchased from Ainopharm Chemical Reagent Co., Ltd. Double distilled water was used throughout in the experiment. 2.2. Catalyst preparation Fe/Co-LDH was synthesized via co-precipitation method. Typically, desired of Fe(NO3)39H2O (1 mmol) and Co(NO3)26H2O (2 mmol) were thoroughly dissolved in deionized water under constant stirring, which was named as solution A. Simultaneously, alkaline solution B (containing NaOH (0.35 mol/L) and NaCO3 (0.15 mol/L)) was also prepared. The pH values of solution A was strictly adjusted to be 10 using solution B. The mixture was stirred for 30 min at room temperature and the co-precipitation happened, then the suspension was aged at 65 °C for 24 h under water bath conditions. Finally, the products Fe/Co-LDHs were obtained by filtering, washing with absolute ethanol and deionized water for several times and finally dried at 70 °C in an oven for 24 h. 2.3. Catalyst characterization The crystallographic properties of samples were investigated by power X-ray diffraction (XRD) on a Rigaka D/max 2500v/pc (Cu, Ka, k = 0.154 nm, 40KV, 40 mA) at a scan rate of 0.1° 2h s1. Fourier transform infrared spectroscopy (FTIR) was measured on an IR Prestige-21 spectrometer (Shimadzu, Japan) at room temperature using the standard KBr disk in the 4000–500 cm1 range. Morphological analysis was performed by a field emission scanning electron microscope (FESEM, Hitachi S-4800) with 5.0 kV scanning

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voltages. The transmission electron microscopy (TEM) was performed on a transmission electron microscope (TEM, FEI Tecnai G20) at an accelerating voltage of 200 kV. The UV–vis absorption spectra were recorded by an UV–vis spectrometer (UV-4100 Shimada) with an integrating sphere for diffuse-reflectance spectroscopy (DRS), using BaSO4 as the reference. The X-ray photoelectron spectroscopy (XPS) was carried out on a Thermo ESCALAB 250 XI spectrometer with Al Ka source. 2.4. Catalytic activity test Degradation experiment was conducted at room temperature (25 ± 2 °C) in a 200 mL glass reactor with constant stirring. The reaction temperature was controlled by recirculation water. In a typical run, RhB solution (100 mL) with the initial concentration ranging from 20–100 mg/L was transferred into the reactor and then a certain amount of catalyst (0.2–1.5 g/L) were added. Before the reaction, 30 min ultrasonic treatment was carried out to achieve the adsorption-desorption equilibrium of RhB on the catalyst, and then appropriate amounts of PMS (0.15–1.5 g/L) were added to start the catalytic process. The solution pH was adjusted by NaOH and HNO3 because the presence of PMS could lead to the obvious changes of pH. At predetermined intervals of degradation time, 4 mL sample were withdraw from suspension and quickly injected pure methanol (1 mL) as the quenching reagent to prevent further reaction. After this, the samples were centrifuged at 4000 rpm for 5 min to separate the catalyst. The residual RhB concentration was detected by measuring the absorbance at the maximum absorption wavelengths 554 nm with the UV–vis spectrophotometer. All the measurements were conducted in triplicate to ensure the veracity of experimental results. For recycling experiment, the used catalyst in the suspension was gathered from after each run by vacuum filtration, and washed several times by the double distilled water until neutral pH. Then the washed catalyst dried overnight in oven at 65 °C. For the quenching experiments, a known amount of quench reagent TBA (tert-butyl alcohol) and ethanol was added into the RhB solution before the addition of PMS. 3. Results and discussion 3.1. Characterization of catalyst Fig. 1a shows the XRD patterns of Fe-Co LDH with different Fe3+/Co2+ molar ratio1:2, 1:1, 3:1, 6:1, respectively. Based on the previous reports [20], a typical XRD pattern of LDHs always has a strong absorption peak in low angle and other two weaker absorp-

tion peaks with equal angle interval between the three peaks, and also possesses two relatively weaker peaks around 60°. It was observed that Fe-Co (1:2) performed better crystallographic structure than Fe-Co (1:1), the characteristic peaks of LDH (JCPDS 50-0235) appeared at 2h of 11.75°, 23.57°, 34.11°, 38.76°, 46.28°, and 60.70°. In contrast, the Fe-Co (1:1) sample presented slightly weaker intensity and mildly peak. However, with the Fe3+/Co2+ molar ratio increasing to 3:1 and 6:1, the characteristic peaks disappeared and even no peaks turned up in the Fe-Co (6:1) sample, indicating that continual increase of Fe3+/Co2+ molar ratio would be not beneficial to form unique LDH crystal structure. In order to confer high-crystallized of the best molar ration in these samples, Fig. 1b illustrated the FT-IR spectra of the prepared catalysts. Fe-Co (1:2) and Fe-Co (1:1) presented almost the same curve. The broad band at about 3427–3503 cm1 can be assigned to the stretching vibration of hydroxyl groups in the brucite-like layers. The strong peak at 1630 cm1 is attributed to the hydroxyl deformation mode of the interlayer water molecules. The obtained results demonstrated that OH groups were found in the samples and water molecules presented in the interlayers. And the characteristic absorption peak at 1356 cm1, corresponded to the vibrational absorption of interlayer CO23 [21]. Furthermore, the bands observed in the range from 1000 to 400 cm1 are interpreted as the vibration modes attributed to metal-oxygen (M-O) or metalhydroxyl (M-OH) group vibrations, 758 cm1 is Co-OH stretching mode, 678 cm1 is Co-O stretching mode and 518 cm1 probably belongs to Fe-O stretching mode in the brucite-like layers [20]. The above information was in good accordance with the XRD analysis. The sample Fe-Co (1:2) morphological features are recorded in Fig. 2 by SEM and TEM technologies. As previous studies reported [21], LDHs generally possess octahedral symmetric whose basic construction unit is octahedron. Metal ions are located in the center while hydroxyl ions in vertex angle of the octahedron. Adjacent octahedrons connect together depending on sharing the edges to form continuous two dimensional octahedral coordination layers which determine the lamellate morphology of layered double hydroxides. From the Fig. 2a and b, it can be seen that the sample possess lamellar structure. As displayed in Fig. 2c and d, TEM images showed that unlike the rigid lamellar structure of LDH, the lamellas of the obtained materials were also presented faint and fragmentary morphology. It may be reasonable to speculate that the non-uniformly of alkali caused by the reaction time or no-enough constant stirring attached to the surface of the well dispersed regular hexagonal shape, and corroded it. Overall, combined with the results of XRD and FT-IR, it could infer that the LDHs were well fabricated and could be used for the following applications.

Fig. 1. (a) XRD pattern and (b) FT-IR spectra of FeCo-LDHs with different Fe/Co molar ratios.

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Fig. 2. (a) (b) SEM images of FeCo-LDH (1:2), (c) (d) TEM images of FeCo-LDH (1:2).

X-ray photoelectron spectroscopy (XPS) was performed to reveal the chemical state of the elements in the compound. Fig. 3a displays the XPS survey spectrum obtained from Fe-Co LDH (1:2), whose peaks are mainly attributed to C 1s, O 1s, Fe 2p and Co 2p regions. In the Co 2p spectrum, there were two main peaks Co 2p3/2 (780.8 eV) and Co 2p1/2 (796.7 eV), respectively. Meanwhile, a satellite bands at 785.7 eV and 802.9 eV was recorded. These values matched well with reported data for Co(OH)2, implying that the cobalt in the compounds was presented a high-spin Co2+ state [15,22]. It was hard to identified the state of Fe for the peak asymmetries, complex multiple splitting, shake-up and overlapping bonding energies [23]. Fig. 3c shows the XPS spectrum for Fe 2p where two peaks located at 710.6 eV and 724.4 eV accompanied by satellite bands. Generally, the appearance of satellite bands near the Fe 2p main peaks is regarded as an indicator of Fe3+ valence state. Those results suggest that the obtained material already exist metal elements Fe3+ and Co2+, and also coincides with the data stated before. 3.2. Catalyst activity of the prepared LDH RhB degradation experiments were carried out in different systems, including homogeneous Fe(III), Co(II) and Fe(III)+Co(II)/PMS systems and heterogeneous FeCo-LDH (1:2)/PMS system. In homogeneous PMS systems, the dosage of Fe3+ or Co2+ equaled to the metal amount of the obtained catalyst. As shown in Fig. 4a, although PMS is a strong oxidizing agent with a standard reduction potential slightly higher than that of H2O2 [7,24], the RhB degradation efficiency in bare PMS could be negligible for the absence of

activator. In parallel, alone addition of solid catalyst, the RhB removal was less than 2% after 2 h, suggesting that dye adsorption on the surface of solid catalyst was negligible. The RhB was almost completely removed within 10 min when the heterogeneous catalyst FeCo-LDH (1:2) and PMS was simultaneously present. The result indicated that FeCo-LDH (1:2) might act as an efficient activator of PMS to activate more sulfate radical production and enhance the RhB degradation. In previous studies, the same phenomena were also presented in PMS activation by metal ions, such as Mg, Al, Co, Mn, Pt, Fe [11,25]. Compared with PMS or solid catalyst alone, the degradation of RhB was much higher in the homogeneous or heterogeneous PMS systems and the efficiency was comparative except for the homogeneous Fe(III)/PMS system. Nevertheless, a serious problem existed in homogeneous Co(II) and Fe (III)+Co(II)/PMS systems that the Co2+ would dissolve out after reaction and bring threat to environment and human beings. Furthermore, about 71.0% RhB could be degraded in the homogeneous Fe(III)/PMS systems, which might attribute that Fe3+ could not directly activate the PMS to produce the sulfate radical. And they need initially to be reduced to Fe2+ by the following reaction: þ Fe3þ þ HSO5 ! Fe2þ þ SO 5 þH

ð1Þ

Fig. 4c demonstrated the degradation efficiency of RhB in FeCoLDH/PMS system with different Fe3+/Co2+ molar ratios. Fe-Co (1:2), Fe-Co (1:3) and Fe-Co (1:6) samples showed the better removal rate for RhB, reaching to 99.4, 99.5 and 99.6% within 10 min, respectively. And Fig. 4d stated the enlargement of overlap parts in the Fig. 4c, it can be observed that almost the same removal efficiency was achieved in different Fe3+/Co2+ molar ratios. Thus,

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Fig. 3. XPS spectrum of FeCo-LDH (1:2) (a) survey spectrum; (b) Co 2p; (c) Fe 2p.

taking degradation efficiency, cost and potential in actual application into consideration, FeCo-LDH with Fe3+/Co2+ molar ratio 1:2 was selected an optimal catalyst for the heterogeneous activation of PMS. This also accorded with the previous reported results that the best synthesis condition of FeCo-LDH was the Fe3+/Co2+ molar ratio between 1:2 and 1:3 [22,24,26]. Meanwhile, it is also observed from Fig. 4b that for Fe-Co (1:2) catalyst, the intensity of characteristic absorbance peaks of the dye in the visible spectral region decreased rapidly with the reaction proceeding and fully disappeared after 255 s, indicating that the catalyst effectively activated PMS, the chromophoric group in catalyst was broken down, and the dye in water was degraded completely.

LDH. It was reasonable that multiple hydroxyl surface complexes was formed by the larger number of OH at alkaline condition and impeded the direct contact between the catalysts and PMS [27], resulting in the lower degraded efficiency. Besides, the PMS self-dissociation through the non-radical pathways in higher pH solution would additionally reduce the degradation efficiency of target contaminant. The 30.6% of RhB degradation at solution initial pH 1 should ascribe to the stabilization effect of H+ on the HSO 5 [28]. In order to estimate the kinetic parameters of the reactions, a pseudo-first-order mode (Eq. (2)) was employed to evaluate the catalytic reaction kinetics [29].

3.3. Effect of reaction parameters on RhB degradation

ln

3.3.1. Effect of reaction temperature and solution initial pH The effect of solution initial pH on RhB degradation was investigated at pH 1–11 and the results displayed in Fig. 5a. It can be seen from Fig. 5a that the removal efficiency of RhB was beyond 80% at the initial pH 4 to 9, and the best catalytic performance was achieved at neutral condition. Furthermore, RhB degradation apparently happened at first 4 min, revealing that the PMS can be quickly activated by the as-prepared solid catalyst and produce more SO 4 radicals. Interestingly, the degradation efficient at blank (3.42) was comparative to that at initial pH 4, which means that PMS activation induced by FeCo-LDH can go on at the ordinary condition and its potential to practical applications is more than H2O2 [7]. However, further increasing the solution initial pH to 11 made RhB degradation efficiency decrease to 2.4%, implying that higher pH was adverse to the activation of PMS by FeCo-

where C is the concentration of RhB at time t, C0 is the initial RhB concentration, and k is the pseudo-first-order reaction rate constant. The temperature can remarkably enhance the activation of PMS. As shown in Fig. 5b, the RhB degradation increased with increasing reaction temperature. When experiments took place at room temperature (about 25 °C), RhB degradation efficiency could reach 100% within 10 min. The time for 100% RhB removal reduced to be 6 min at 35 °C. With the temperature increasing to 45 °C, the corresponding time was further shortened to 4 min. RhB degradation in FeCo-LDH/PMS process was well formulated by the pseudofirst-order kinetics, the reaction rate constants were found to be 0.01111 min1 at 25 °C, 0.02313 min1 at 35 °C, and 0.05005 min1 at 45 °C, respectively (Table 1), suggesting that higher reaction temperature significantly enhanced the activation of PMS, and



C C0

 ¼ kt

ð2Þ

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227

Fig. 4. (a) RhB removal in different systems; (b) UV–vis spectra changes of RhB in FeCo-LDH(1:2)/PMS system; (c) RhB removal with different Fe/Co molar ratios; (d) enlargement of overlap section in (c) [Experimental conditions: initial RhB concentration = 20 mg/L, PMS dosage = 0.15 g/L, catalyst loading = 0.2 g/L, initial pH = 3.42, room temperature(25 °C)].

then improved the RhB removal efficiency. Because rate constants increased at elevated temperatures, the Arrhenius equation Ea

(k ¼ A  eRT , where k is rate constant of a chemical reaction, T temperature in kelvin, A the prefactor, Ea the activation energy, and R the universal gas constant) was employed to determine the correlation between the rate constant and reaction temperature. Based on it, the activation energy (Ea) of RhB degradation was calculated to be 59.71 kJ/mol, which is higher than the activation energy of the diffusion-controlled reactions (usually ranging from 10–13 kJ/mol). These results illustrated that the apparent reaction rate is dominated by the rate of intrinsic chemical reactions on the oxide surface rather than the rate of mass transfer [29].

3.3.2. Effect of PMS dosage and catalyst loading Fig. 5c illustrated that the decolorization of RhB at varying concentration of oxidant. The RhB degradation almost happened within the short duration time of 240 s no matter how the dosage of PMS (0.15–1.0 g/L) changed. The sharply degraded curves reveal the positive effect of oxone on RhB degradation. But compared with the results at PMS dosage of 0.15 g/L, the RhB degradation efficiency within 100 s improved 14.4, 23.6, 41.4% at the PMS dosage 0.3, 0.6, 1.0 g/L, respectively. It suggested that the RhB degradation independed on the PMS dosage due to the present of adequate oxidants. However, when PMS dosage further increased to 1.5 g/L, there was no obvious improvement of RhB degradation, which should be the reason that excessive PMS was activated by itself. The extra HSO-5 in solution can react with SO 4 to generate

 SO 5 , which has lower oxidation than SO4 , resulting in the worse RhB degradation. The influence of catalyst loading on the RhB degradation was studied by varying the catalysts loading from 0.2 to 1.5 g/L. Fig. 5d illustrated that higher catalyst loading had an obvious positive effect on RhB degradation. The pseudo-first-order rate constants stated in Table 2 also confirmed the conclusion. More catalysts were added, higher RhB removal efficiency and reaction rate were achieved. From the inset in Fig. 5d, more straightforward degradation tendency within 240 s could be observed. The RhB degradation rate improved dramatically with the catalyst loading increasing from 0.2 to 0.8 g/L. However, when the loading increased to 1.5 g/L, the overlapped curves revealed that the degradation performance in this system couldn’t significantly improve because more catalyst loading made the supply of PMS in system insufficient.

3.3.3. Effect of initial RhB concentration The RhB degradation in FeCo-LDH(1:2)/PMS system was evaluated at initial RhB concentration ranging from 20 to 100 mg/L. As seen from Fig. 6a, higher initial RhB concentration resulted in worse RhB degradation efficiency. 20 mg/L RhB was nearly completely degraded within 10 min, while within same reaction time, RhB degradation efficiency was only 95.5, 78.5, 36.8, 28.2% at initial RhB concentration of 30, 50, 80, 100 mg/L, respectively. As previously mentioned, RhB degradation in FeCo-LDH (1:2)/PMS system was depended on the amount of sulfate radicals. Increase in dye concentration will increase the number of dye molecules,

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Fig. 5. Effects of (a) initial pH; (b) reaction temperature; (c) PMS dosages; (d) catalysis loading on the degradation of RhB in FeCo-LDH(1:2)/PMS system.

Table 1 Pseudo-first-order rate constants (K), correlation coefficients (R2), and degradation efficiencies (DE) of RhB at 90 s and 10 min under different temperature and PMS concentration in FeCo-LDH/PMS system. RhB (ln Ct = Kt + ln C0, pseudo-first-order reaction)

Parameters

K (min1)

R2

Standard errora

DE

Temperature (°C)

25 35 45

0.01111 0.02313 0.05005

0.9747 0.9914 0.9666

0.000799 0.000965 0.004630

53.40 88.67 99.41

99.81 99.84 99.90

PMS concentrationc (g/L)

0.15 0.30 0.60 1.00 1.50

0.00878 0.02274 0.02629 0.03083 0.04678

0.9612 0.9540 0.9589 0.9344 0.9704

0.000728 0.002220 0.002430 0.003630 0.004070

62.82 68.51 77.73 95.55 97.32

99.52 99.62 99.68 99.87 99.94

b

a b c

90s

(%)

DE

10min

(%)

(I) The standard error referred to that of the slope in the equations. (II) Experiment conditions: 100 mL 20 mg/L RhB solution, 15 mg PMS, 20 mg catalyst, and no initial pH adjustment. (III) Experiment conditions: 100 mL 20 mg/L RhB solution, 20 mg catalyst, room temperature (25 °C), and no initial pH adjustment.

which will form the competition to SO 4 in the solution. High concentration of RhB in solution will require more time to achieve the complete removal, thus lowering RhB degradation efficiency [24]. 3.4. Reaction quenching studies on radical mechanism of RhB degradation According to the previous literatures [30,31], there were three main types of reaction radicals relevant to the degradation of pollutants by the catalyst-mediated activation of PMS, namely sulfate  (SO 4 ), hydroxyl (OH) and peroxymonosulfate (SO5 ). To get a deeper insight into the degradation mechanism and the predominant radical in FeCo-LDH/PMS system, further experiments were con-

ducted in the same process as RhB degradation described above except for the addition of two radical scavengers, TBA (tert-butyl alcohol) and ethanol. It is widely accepted that ethanol readily reacts with both sulfate and hydroxyl radicals at high rates whereas TBA are mainly reactive toward hydroxyl and about 1000-fold slower with sulfate radical. The contribution of peroxymonosulfate to RhB degradation can be neglected due to its lower redox potential [31]. Thus, ethanol and TBA were used to distinguish with the sulfate and hydroxyl radicals. Fig. 6b demonstrates the RhB degradation efficiency at FeCo-LDH/PMS system in the presence of both quenching reagents at different volumes. It can be see that when no quenching agent was added, 50 mg/L RhB was degraded approximately 85.3% within 10 min. However, the

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Table 2 Pseudo-first-order rate constants (K), correlation coefficients (R2), and degradation efficiencies (DE) of the RhB at 90 s and 10 min under different catalyst loading and initial pH in FeCo-LDH/PMS system. RhB (ln Ct = Kt + ln C0, pseudo-first-order reaction)

Parameters

K (min1)

R2

Standard errora

DE

Catalyst dosage (g/L)

0.20 0.40 0.80

0.01119 0.01305 0.03677

0.9760 0.9874 0.9941

0.000783 0.000740 0.001420

54.16 65.46 95.87

98.93 98.67 99.52

Initial pHc

Blank 1 4 7 9 11

0.80725 0.03373 0.75981 0.76717 0.30925 0.00197

0.9578 0.9718 0.9495 0.9960 0.9975 0.9404

0.00613 0.00729 0.01805 0.00101 0.00811 0.00248

61.54 10.76 79.62 84.46 45.94 1.960

99.18 30.70 99.47 99.30 84.44 1.910

b

a b c

90s

(%)

DE

10min

(%)

(I) The standard error referred to that of the slope in the equations. (II) Experiment conditions: 100 mL 20 mg/L RhB solution, 15 mg PMS, room temperature (25 °C), and no initial pH adjustment. (III) Experiment conditions: 100 mL 20 mg/L RhB solution, 15 mg PMS, 20 mg catalyst, room temperature (25 °C).

Fig. 6. Effects of (a) initial RhB concentration on the degradation of RhB; (b) Effect of different radicals scavengers on RhB degradation in FeCo-LDH(1:2)/PMS system [experiment conditions: initial RhB concentration = 50 mg/L, PMS dosage = 0.15 g/L, catalyst loading = 0.2 g/L, initial pH = 3.56, room temperature (25 °C)].

dye degradation was inhibited with the addition of ethanol and the negative effect was more apparent with the increasing ethanol concentration. RhB degradation efficiency respectively dropped 6.5, 28.0, 35.3 and 50.3% within the same reaction time when 3.8, 7.6, 15.2 and 30 mL ethanol was added, indicating that sulfate and hydroxyl radicals were involved in the oxidation degradation of RhB. On contrary, the addition of TBA even at the highest dosage of 30 mL showed slighter negative effect on RhB degradation. These results confirmed that sulfate and hydroxyl radicals were involved in the FeCo-LDH/PMS system and the sulfate radicals were the primary species, which are responsible for the RhB degradation. In order to further reveal the reaction mechanism and under stand how SO 4 or OH generated and reacted with the target pollutant, the sequencing reactions for PMS activation by Fe/Co-LDH are listed as follows:  Co2þ þ HSO5 ! Co3þ þ SO 4 þ OH

ð3Þ

þ Co3þ þ HSO5 ! Co2þ þ SO 5 þH

ð4Þ

 Fe2þ þ HSO5 ! Fe3þ þ SO 4 þ OH

ð5Þ

2  þ SO 4 þ H 2 O ! SO4 þ H þ OH

ð6Þ

2 SO 4 þ OH  þRhB ! By  products þ CO2 þ H2 O þ SO4

ð7Þ

Fe3þ þ e ! Fe2þ ; E0 ¼ 0:77 V

ð8Þ

Co2þ  e ! Co3þ ; E0 ¼ 1:83 V

ð9Þ

Fe3þ þ Co2þ ! Fe2þ þ Co3þ ; E0 ¼ 1:04 V

ð10Þ

As shown in Eq. (5), PMS is activated to generate the reactive radicals SO 4 mainly by the Fe(II), which captured electron from the Fe(III). On the other hand, Co(II) lost electron to activate PMS  to form the SO 4 (Eq. (3)), and the SO5 was generated by the reactions between high state of metal irons and the PMS (Eqs. (1), (4)), which also have some contributions to the degradation of RhB. Then, the OH was formed by the generated SO 4 reacting with the water molecule in the system (Eq. (6)). Finally, the strong oxi dizing SO 4 and OH simultaneously attacked target pollutant, made the rapidly degradation of RhB (Eq. (7)). The above reactions are illustrated in Fig. 7. Based on the unique structure of LDHs, the octahedral site can easily accommodate Co(II), Co(III), Fe(II) and Fe (III), allowing the Fe and Co species to be reversibly oxidized and reduced while keeping the same structure. According to the standard reduction potentials for metals (Eqs. (8)–(10)), it can be considered that the oxidation and reduction between Co(II) and Fe(III) is thermodynamically favorable [12]. The efficient regeneration of the surface Co(II) by this process could be responsible to the remarkable activation of PMS and enhanced degradation of RhB in FeCo-LDH/PMS system. And the scheme of heterogeneous of PMS to attacked RhB was stated in Fig. 8.

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Fig. 7. Mechanism of the sulfate radical chain reaction.

3.5. Reusability of the catalysts and practical application Considering the practical implementation of the heterogeneous catalytic system, it is essential to evaluate the reusability of used catalysts. RhB degradation in four consecutive cycling runs is displayed in Fig. 9a. After the previous degradation process finished, the used catalyst was filtrated from the solution, washed several times with distilled water and dried overnight before next run. Since the amounts of catalysts used in the reaction was little, there were several parallel runs at the same conditions to ensure enough amount of recycled catalyst for the next run. Compared with the fresh catalyst, the RhB degradation efficiency within 10 min was still as high as 98.6% after fourth runs. No remarkable reduction in the catalytic activity implied that the solid catalyst had good stability and long-life. However, the gradually smooth curves and the slight decrease in the removal efficiency indicated that similar to previous studies [32–35], the small amounts of cobalt and iron leached from the catalyst surface and the active catalytic sites were slightly poisoned for the adsorbed organic species.

In order to test the feasibility of solid materials in real wastewater, the same process as RhB degradation described above was operated in sewage water, river water and tap water. Water sample were obtained from Xiangjiang, drinking water supply system in the campus of Hunan University and the first sewage treatment plant in Changsha, respectively. And the composition of water sample was stated in Table 3. According to the Fig. 9b, no matter in what kind of water sample, the higher removal of target contaminant all was achieved. In detail, the RhB removal efficiency after 10 min was more than 99.8, 98.7, 96.2, 95.1, 95.8% in de-ionized water, tap water, Xiangjing river water, municipal sewage and simulated sewage, respectively. As expected, RhB removal was optimal in de-ionized water and worst in municipal sewage. The possible reason could be explained as that the impurity such as the natural organic matters (NOMs), dissolved organic matters (DOMs) and other trace irons existed in real water sample, which could be adsorbed on the surface of catalysts and impeded the direct communication between the PMS and Fe-Co/LDH. So the production of SO 4 was inhibited in real

Fig. 8. The scheme of PMS activation by heterogeneous catalyst FeCo-LDH.

C. Gong et al. / Chemical Engineering Journal 321 (2017) 222–232

231

Fig. 9. (a) Degradation of RhB by multiple-used FeCo-LDH (1:2) catalyst; (b) RhB degradation in FeCo-LDH(1:2)/PMS system with water resources (a De-ionized water; b Taper water; c River water; d Municipal sewage; e Simulated sewage) [Experiment conditions: initial RhB concentration = 20 mg/L, PMS dosage = 0.15 g/L, catalyst loading = 0.2 g/L, no solution pH adjustment, room temperature (25 °C)].

Table 3 The composition of simulated sewage water.

Substrate Target contaminant Other nutrients and trace elements

References

Matters

Content (mg/L)

Glucose RhB (NH4)2SO4, Na2CO3 CaCl2, KCl, MgSO4,CaHPO4

500 20 100 25

water, thus causing a relatively lower removal rate for RhB. However, above 90.0% removal rate in different water source gave us adequate confidence that as-prepared catalytic material was favorable for the purification of RhB-contaminated wastewater in practical. 4. Conclusions In this study, we successfully synthesized the FeCo-LDH with high purity and good crystal through a facile chemical process and employed it as the supernal efficient heterogeneous catalyst of PMS activation to degrade target contaminant RhB. Within shorter reaction time 10 min, 20 mg/L RhB was removed more than 99% in FeCo-LDH(1:2)/PMS system. Further investigations indicated that RhB degradation in FeCo-LDH(1:2)/PMS system was affected by pH, temperature, initial RhB concentration or PMS dosage and the degradation process could be well described by the pseudo- first order kinetics with the activation energy of 59.71 kJ/mol. Radial quenching studies demonstrated that SO 4 and OH radical simultaneously participated in the oxidation reaction process in the FeCo-LDH/PMS system, and SO 4 played the dominating role in the ultrafast degradation of RhB. The catalyst showed higher catalytic activity to activate PMS even after recycling for four times, indicating its excellent reusability and stability. Furthermore, more than 90% of RhB degradation in FeCoLDH/PMS system all was maintained in different water resources such as tap water, simulated sewage, municipal sewage or real river water. Acknowledgments This research was financially supported by the project of National Natural Science Foundation of China (NSFC) (Nos. 51378188, 51478170, 51508178), Doctoral Fund of Ministry of Education of China (20130161120021) and Planned Science and Technology Project of Hunan Province, China (No. 2015SK20672).

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