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Kinetics, Catalysis, and Reaction Engineering

Activation of Peroxymonosulfate by Fe3O4-CsxWO3/ NiAl Layered Double Hydroxides Composites for the Degradation of 2,4-Dichlorophenoxyacetic Acid Guoqing Zhao, Xiao-Qing Chen, Jiao Zou, Caifeng Li, Lukai Liu, Taiheng Zhang, Jin-Gang Yu, and Feipeng Jiao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b04453 • Publication Date (Web): 09 Nov 2018 Downloaded from http://pubs.acs.org on November 11, 2018

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Industrial & Engineering Chemistry Research

Activation of Peroxymonosulfate by Fe3O4-CsxWO3/NiAl Layered Double Hydroxides Composites for the Degradation of 2,4-Dichlorophenoxyacetic Acid

Guoqing Zhao, Xiaoqing Chen, Jiao Zou, Caifeng Li, Lukai Liu, Taiheng Zhang, Jingang Yu, and Feipeng Jiao*

Hunan Provincial Key Laboratory of Efficient and Clean Utilization of Manganese Resources, School of Chemistry and Chemical Engineering, Central South University, Changsha 410083, People’s Republic of China

*Corresponding author address: Prof. & Dr. Feipeng Jiao School of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China Tel.: +86 731 88830833; Fax: +86 731 88830833. E-mail: [email protected] (F. Jiao)

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Novel Fe3O4-CsxWO3/NiAl layered double hydroxides composites (FCW/LDH) for the activation of peroxymonosulfate (PMS) and the succedent degradation of 2,4-dichlorophenoxyacetic acid (2,4-D) was synthesized through a simple method. The composites prepared were characterized by XRD, SEM, EDS, FT-IR, UV-vis DRS, and N2 physisorption, respectively. It was found that FCW/LDH could effectively catalyze PMS to generate sulfate radicals (SO4-·) to degrade 2,4-D. The added 2,4-D (20 mg/L) was almost completely removed (with a removal of 90.53%) in 180 min by using 1.00 g/L FCW/LDH and 0.50 g/L PMS. Several related factors (PMS concentration, initial pH, disturbing anions) were tested in order to understand their effect on the degradation performance. Furthermore, the photocatalysts exhibited good reusability and stability after four recycles. Finally, a possible degradation mechanism was provided according to above experimental results. This desired FCW/LDH composites showed promising application in purifying the water. Keywords: Peroxymonosulfate, Degradation, Fe3O4, CsxWO3, Layered double hydroxides.


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1. INTRODUCTION Nowadays, environmental pollution, especially antibiotics, chemical intermediate, organic dyes, pesticide, leather wastewater, is a significant problem,

and

has

led

overpowering

threat

to

human

health.

2,4-Dichlorophenoxyacetic acid (2,4-D), as one typical pesticide, was widely recognized for its well performance to control broad-leaved weeds present in fields.1-3 2,4-D is widely known as common universal herbicide and it has been used in all kinds of weeds. Moreover, the features of low cost, as well as remarkable performance of 2,4-D application have increased its worldwide application. Up to now, more than 600 commercial product types of 2,4-D are manufactured and used as herbicides in agricultural field.4 However, various 2,4-D herbicides are misemployed and improperly treated, which lead to plenty of herbicides releasing into aquatic ecosystem. The presence of 2,4-D in the water bodies affects aquatic organisms and other living-forms through drinking the contaminated water.5 In addition, previous research has been written about 2,4-D can be teratogenic and carcinogenic for mankind and it is also can cause endocrine disorders.6-8 Several methods including adsorption, flocculation, filtration, coagulation and chlorination are not able to get rid of 2,4-D from water nicely, they fairly transfer it from one phase to another (adsorption, filtration, etc.).9,10 The disadvantages of conventional approaches to 3 ACS Paragon Plus Environment


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remove 2,4-D have resulted in efforts to develop alternative methods such as advanced oxidation processes (AOPs). Simultaneously, lots of AOPs studies, like photocatalytic

oxidation,

sonochemical degradation,

Fenton/photo-Fenton degradation, and ozonation, have been explored for 2,4-D elimination from the body water.10-15 Nowadays, carbocatalytic activation of persulfate degradation pollutants can be considered as one of typical AOPs based on the mass generation of sulfate radicals (SO4-·) (SR-AOPs) active substances has been developed for the removal of 2,4-D.16 SO4-· possesses superior redox-potential, half-time than ·OH radicals.17

SO4-· shows better

specific to the excessive electron half than ·OH for SO4-· usually participated some chemical reaction.18 Peroxymonosulfate (PMS) is easily to generate SO4-· by UV-light irradiation, AC-based catalyst, and transition metals due to its unsymmetrical structure.19,20 Transition metals (Fe, Cu, and Co, etc.) obtained concerned as the activating agents for PMS to remove the target contaminants.21 S2O82- + heat/hv/microwave → 2SO4-·

(1)

S2O82- + e- → 2SO4-· + SO42-

(2)

Mn+ + S2O82- → M(n+1)+ + SO4-· + SO42-

(3)

Mn+ + HSO5- → M(n+1)+ + SO4-· + OH-

(4)

SO4-· + H2O →·OH + SO42- + H+

(5)

Where, M represents transition such as Fe, Ni, Cu and Co, etc. 4 ACS Paragon Plus Environment

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LDH, Fe3O4 and CsxWO3 all exhibited superior photocatalytic performance for PMS based SO4-· generation according to Eqs (1)-(5).22-24 Moreover, the magnetic nature of Fe3O4 makes the catalysts recycled from reaction systems easily. Besides, it is worth noting that the quantity of active sites and electrical conductivity of pure LDH, Fe3O4 and CsxWO3 are limited, which conceivable decreases its electron transmit rate and photocatalytic performance, respectively. Co-doping Fe3O4 and CsxWO3 into the lattices of NiAl LDH composites not only restrict the size of particles to nanoscale to expose large amount of active sites but also promote electron transfer in the photocatalysts. Above all, LDH-based catalysts (FCW/LDH) are never prepared as yet. Hence, in order to add the number of catalysis active sites of LDH composites, FCW/LDH composites are considered to be synthesized. In this paper, the FCW/LDH composites as the high catalytic activity and environmental-friendly photocatalysts were synthesized and characterized for the first time. 2,4-D was degraded by FCW/LDH composites as an activator of PMS systematically. The photodegradation mechanism was inspected according to the experimental results. The impacts of certain key parameters on the degradation process including PMS concentration, initial pH, anions, catalysts dosages and stability are considered.



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2. EXPERIMENTAL SECTION 2.1. Reagents and Chemicals 2,4-D (CAS No. 94-75-7, C8H6Cl2O3) was purchased from Sigma-Aldrich

Chemical

Co.,

Ltd.

Peroxymonosulfate

(PMS,

KHSO5·0.5KHSO4·0.5K2SO4, purity ≥47% KHSO5) were provided by Saen Technology Co., Ltd. (Shanghai, China). Ni(NO3)2·6H2O and Al(NO3)3·9H2O were received from Fengchuan Chem. Reagent Co., Ltd. (Tianjin, China). WCl6 and CsOH·H2O were purchased from Alfa Aesar. Iron (Ⅲ) chloride hexahydrate (FeCl3·6H2O), polyethylene glycol 6000 (PEG 6000), MeOH, TBA, KI, BQ and NaOH were supplied by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Ultrapure water (18.2 MΩ), collected from a Milli-Q water purification system (Millipore, Bedford, MA, USA), was used to prepare all aqueous solutions. Sodium acetate, anhydrous ethanol, and ethylene glycol were purchased from KeMiOu Chemicals Co., Ltd. (Tianjin, China). The other reagents were of analytical grade. 2.2. Preparation of Photocatalysts The target photocatalysts were prepared by a hydrothermal method in the previous literatures.25-27 The detail processes are described in the Supplementary materials. The preparation method of the samples was showed in Scheme 1. 2.3. Characterization Methods 6 ACS Paragon Plus Environment

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The crystal phase and structure composition of the samples were obtained from X-ray diffraction (XRD, Bruker D8, Germany). Morphology of target catalysts was studied by methods of scanning electron microscopy (SEM, TESCAN MIRA3 LMU). An energy dispersive X-ray spectrometer (EDS) attached to the scanning electron microscope (SEM) operated at 20 KV was used to analyze the target catalysts. The pore characteristics and specific surface area of products were recorded by the N2 adsorption-desorption specific surface area analyzer (Micromeritics ASAP 2020 surface area). The FT-IR spectra of catalysts were recorded on an AVATAR 360 spectrometer (Nicolet Instrument Corporation, America) in the region 4000-400 cm-1. The UV-vis DRS spectra of all the catalysts were tested on the UV diffused reflectance spectroscopy (UV-vis DRS, Shimadzu 2401 models) to evaluate electronic structures and optical properties. 2.4. Photocatalytic Performance Test The phtocatalytic degradation performance of all the catalysts was tested

in

photocatalytic

reactor

(Models:

YM-GHX-V).

The

photodegradation performance was tested by degrading 50 mL of 20 mg/L 2,4-D simulated solution. The initial pH was confirmed by 0.1 M NaOH and HCl solution. Apart from inspecting the impact of initial pH for experiments, the pH was always ca. 7.0. Meanwhile, 50 mg of catalysts and a certain amount of PMS were added into the thermostatic 7 ACS Paragon Plus Environment


reactor that with 50 mL of 2,4-D and then started the stirrer for 30 min for the adsorption-desorption equilibrium established. The photocatalytic degradation process was continued under 500 W xenon lamps. 3 mL of the mixture was collected every 30 min. the mixture solutions were filtered through a 0.22 μm membrane to remove the catalysts. The concentration of 2,4-D was measured by an UV-9600 spectroscopy.

3. RESULTS AND DISCUSSION 3.1. Characterization of Photocatalysts The XRD patterns of Fe3O4, LDHs and FCW/LDH composites were exhibited in Figure. S1. The diffraction peaks of the Fe3O4 are at 2θ =30.2º 35.5º, 43.1º, 57º, and 62.7º, which were well corresponding to lattice plane (2 2 0), (3 1 1), (4 0 0), (5 1 1) and (4 4 0).28 In Figure. S1b, we can find the peaks at 11.3º, 23.2º, and 34.9º fitted well to the LDH with basal diffraction planes of (0 0 3), (0 0 6) and (0 0 9). Moreover, it can be easier observed that the diffraction peaks could be well conformed to hexagonal cesium tungsten bronze (JCPDS No. 831334).29 In Figure. S1c -S1f, the XRD analysis suggested the co-existence of Fe3O4, CsxWO3, and LDH phase in the pre-synthesized FCW/LDH composites and no other impure peaks were observed. The surface morphologies of all the catalysts were characterized by SEM (Figure. S2). As shown in Figure. S2a, the morphology of the 8 ACS Paragon Plus Environment

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microspheres could be observed without any severe damage.30 From Figure. S2b, the CsxWO3 particles were dispersed on the LDH surface, and the other parts of CsxWO3 particles might be assembled into the inside frame of the CW/LDH composites, which were well fitted our previous work.26,

31

It could be easily discovered that structure of

FCW/LDH was more uniform than that of CW/LDH (Figure. S2c). Besides, the spherical shape of FCW/LDH composites was clearly observed in the Figure and the surface of FCW/LDH was rough and somewhat agglomerated. Meanwhile, the EDS of 0.5% FCW/LDH composites were shown in Figure. S2d, the elements of Ni, Al, Fe, W, Cs and O were observable. As for Table S1, the atomic ratios of Ni, Al, Fe, W, Cs and O were in good agreement to their stoichiometric ratios. The FT-IR spectra of Fe3O4, LDHs, CW/LDH and FCW/LDH composites were displayed in Figure. S3. The strong peaks at 3448 cm-1 were attributed to the O-H stretching vibrations of the hydroxyl groups, which was derived from the layered and interlayer water molecules. The absorption peaks at 1632 cm-1 can be assigned to a water deformation.32 Additional a sharp band at 1380 cm-1 was ascribed to the CO32- and -OH group, proving that some CO32- ions consisted in LDHs molecules.33 The strong bands from 400 to 800 cm-1 corresponded to the bending vibration of M-O and M-OH.34 Moreover, the intense adsorption peaks at around 585 cm-1 was regarded as the Fe-O stretching and bending vibration of 9 ACS Paragon Plus Environment


Fe3O4.35 Similarly, the vibration of Fe-O was also observed in the FCW/LDH composites (Figure. S3d). Nitrogen uptake isotherms and pore size of the catalysts were presented in Figure. 1. The obvious hysteresis loop was appeared in FCW/LDH graphs in the relative pressure from 0.4 to 1.0, which was a standard Ⅳ-type isotherm. Clearly, the mesoporous structure of FCW/LDH was obtained. The pore size distribution was chiefly concentrated 3.4 nm (Figure. 1d, inset). BET specific surface area of Fe3O4, LDHs and CW/LDH were 58.21, 28.45, 200.2 m2/g, respectively. However, the BET specific surface area of FCW/LDH was 146.5 m2/g, which may be due to the Fe3O4 occupied the pores of the composites. On the other hand, the decrease of surface area might be thanks to the increase size of composites and the agglomeration of catalysts.36 The optical absorption properties of Fe3O4, LDHs, CW/LDH, and 0.5% FCW/LDH composites were investigated by using UV-vis absorption spectrum and the consequences are exhibited in Figure. 2. The original LDHs and CW/LDHs had an optical response in the ultraviolet region. Conversely, Fe3O4 and FCW/LDH composites had wide absorption ability in visible light region (Figure. 2a). Accordingly, the band gap energy (Eg) of the LDHs, CW/LDH, Fe3O4 and FCW/LDH composites counted by the Kubelka-Munk equation are 4.8, 3.2, 3.1and 2.7 eV, respectively, as shown in Figure. 2b. It can be observed that the 10 ACS Paragon Plus Environment

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Eg of LDHs composites were narrow down after introducing CsxWO3 particles on the surface (from 4.8 eV to 3.2 eV), and the Eg of FCW/LDH composites decreased to 2.7 eV after Fe3O4 particles were coated on the surface. It indicated that the introduction of Fe3O4 and CsxWO3 had remarkable effects on decreasing the Eg and hence, improving the visible-light absorption. 3.2. Preliminary Study of Difference Process The 2,4-D degradation efficiency by PMS activated with catalysts was investigated, without controlling the solution pH (initial solution pH=7.0), the results were displayed in Figure. 3. It can be observed that the self-degradation of 2,4-D was almost negligible without the addition of catalyst throughout the blank experiment. Besides, the adsorption performance of 2,4-D on the FCW/LDH was tested in the dark. It reflected that the period of 30 min was long enough to obtain the adsorption-desorption

equilibrium

before

photodegradation.

The

conversion of 2,4-D was approximately 90.53% after 150 min visible light irradiation when the PMS and 0.5% FCW/LDH composites were added simultaneously, which was higher than those of Fe3O4, LDHs and CW/LDH with about 77.07%, 55.57%, and 61.99%, respectively. One part was a concerted reaction between PMS and products to generate radicals in the whole reaction system. On the other hand, Fe3O4, LDHs, CsxWO3 coupled substance might promote the effective separation of 11 ACS Paragon Plus Environment


electrons and holes. The visible light driving activation PMS degradation of 2,4-D accords with pseuso-first-order kinetic model.37,38 The highest rate constant was 0.01311 min-1 for 0.5% FCW/LDH composites which is almost 1.7 times higher than those of composites 1% FCW/LDH (0.00775 min-1), 3% FCW/LDH (0.00759 min-1), 5% FCW/LDH (0.00761 min-1), 2.8 times, 1.5 times and 2.4 times than that of LDHs, Fe3O4 and CW/LDH, respectively (Figure. 3b & c). The above phenomena verified that the coupling interface between Fe3O4, LDHs and CsxWO3 is a crucial factor. 3.3. Effect of Initial PMS Dosage The degradation of 20 mg/L target solution with different PMS dosage addition under visible light irradiation is exhibited in Figure. 4. As for Figure. 4a, the photocatalytic degradation rates of 2,4-D increased from 67.00% to 90.05% with the mPMS addition increasing from 0.25 to 2.00 g/L in 180 min. In Figure. 4b & c, the k value of 2,4-D removal increased from 0.01136 to 0.01452 min-1 with the addition of mPMS increasing from 0.25 to 2.00 g/L, and then the k was nearly unchanged with the addition of PMS further increasing. Hence, we obtained a conclusion that the optimal interval of k value at 0.25 to 2.00 g/L, which was due to the content of PMS was a significant parameter to produce active substances during the degradation reaction, and a great deal of 12 ACS Paragon Plus Environment

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PMS can produce abundant active substances. However, when the addition of PMS was exceeded 2.00 g/L, the k value was almost unchanged even if the concentration of PMS increasing. Because, at CPMS>2.00 g/L, the incremental removal of 2,4-D was not significant. This is because of the scavenging of SO4-· and ·OH radicals were by HSO5-, and the formation of less reactive SO4-· radicals. 3.4. Effect of Initial pH As we all know, pH is a significant role in SR-AOPs, which can influence the components of the organics and the generation of fundamental radical types to affect the removal rates.39,40 The impacts of initial pH on 2,4-D removal were investigated from 3.0 to 11.0, and the results were displayed in Figure. 5. In Figure. 5a, it is apparent to obtain that the removal rates of 2,4-D were 93.31%, 92.30%, 90.53%, 89.09%, 88.29% by 0.5% FCW/LDH composites at different initial pH 3.0, 5.0, 7.0, 9.0, and 11.0, respectively. Besides, as for Figure. 5b and 5c, the k value of 2,4-D degradation was 0.01513, 0.01426, 0.01311, 0.01235, and 0.01217 min-1 with the original pH changing from 3.0 to 11.0, respectively. It means that the composites possessed the higher 2,4-D degradation efficiency and k value at initial pH=3.0 than others. Obviously, the acidic condition exhibited high removal rates for 2,4-D degradation. These conclusions were acquired that the FCW/LDH composites surface with positive charge can absorb 2,4-D, because 2,4-D 13 ACS Paragon Plus Environment


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was anionic in acidic form.5 Activated PMS removed 2,4-D thoroughly on the catalysts surfaces. Furthermore, the removal efficiency of 2,4-D was reduced under alkaline conditions (pH 9.0 and 11.0). The conclusions also indicated that 2,4-D was unhooked validly beyond pH=7, which was assigned to the ·OH become the main active substance and mounts of the SO4-· was reacted in the process.41,42 SO4-· + OH- → ·OH + SO42-

(6)

3.5. Effect of Anions Three different anions (CO32-, HCO3- and Cl-) were used to investigate the influence of degradation performance. The results of photocatalytic

degradation

activity

and

k

values

for

2,4-D

photodegradation at different addition of anions were exhibited in Figure. 6 and Figure. S4-S6. The influences of different addition of CO32- on the elimination of 2,4-D at original pH 7.0 were exhibited in Figure. 6a and Figure. S4. In Figure. 6a, the elimination efficiency of 2,4-D was 90.53%, 81.19%, 75.54%, 75.04% and 69.80% at dosages of CO32- at 0, 2, 4, 8 and 16 mg/L. respectively. Also, in Figure. S4, a clear decline of 2,4-D removal was investigated when CO32- was introduced in the degradation solution. The k value was minished from 0.01311 to 0.00608 min-1 when the content of CO32- aggrandized from 0 to 16 mg/L. The other sides, as located in Figure. 6b, the elimination rates of 2,4-D were 90.53%, 14 ACS Paragon Plus Environment

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42.11%, 37.06%, 33.64% and 26.81% at dosages of HCO3- at 0, 2, 4, 8 and 16 mg/L. respectively. Moreover, the k value was added from 0.00184 to 0.01311 min-1 when the content of HCO3- decreased from 16 to 0 mg/L (Figure. S5). Obviously, the addition of CO32- and HCO3- were severely prohibited the degradation efficiency of 2,4-D. The reasons were probably due to CO32- and HCO3- greatly scrambled for SO4-· and ·OH (Eqs. (7)-(12)43,44) to produce a weak oxidant (CO3·-).45 The results were all conformed to the previous study that bicarbonate obstructed BPA photocatalytic degradation in UV/persulfate system.46 CO32- + hVB+ → CO3·-

(7)

CO32- + SO4-· → CO3·- + SO42-

(8)

CO32- + ·OH → CO3·- + OH-

(9)

HCO3- + hVB+ → CO3·- + H2O

(10)

HCO3- + SO4-· → CO3·- + H + SO42-

(11)

HCO3- + ·OH → CO3·- + H2O

(12)

In Figure. 6c, the elimination rates of 2,4-D were 90.53%, 93.61%, 95.66%, 97.77% and 99.14% with adding content of Cl- from 0 to 16 mg/L in 180 min. Figure. S6 proved that the removal of 2,4-D was arresting enhanced with different content of Cl-, the k value of 2,4-D degradation improved immensely from 0.01311 to 0.02485 min-1 with the content aggrandizing from 0 to 16 mg/L. Hence, the presence of Clenable improve the degradation rates of 2,4-D in PMS photodegradation 15 ACS Paragon Plus Environment


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reaction, which was accorded with the results of previous reports.47,48 The main effects of anions can be explained that the three kinds radicals (Cl·, ClOH·-, and Cl2·) were generated by SO4-· and ·OH (Eqs. (13)-(18)49), in which Cl· might conducive to the photodegradation. Moreover, the generated chlorine free radicals could further produce more SO4-·.50 Hence, the excellent elimination rates of 2,4-D was enhanced by the two-factor influence of Cl· and SO4-· under overweight SO4-· condition. Cl- + hVB+ → Cl·

(13)

Cl- + ·OH → ClOH·-

(14)

Cl- + SO4-·→ Cl· + SO42-

(15)

Cl- + Cl· → Cl2·

(16)

ClOH·- + H+ → Cl· + H2O

(17)

ClOH·- + Cl- → Cl2· + OH-

(18)

3.6. Stability and Reusability of Catalysts Though the prepared catalysts presented good activation property of PMS for the removal of 2,4-D, it was an important task to investigate the stability and reusability of the catalysts. The consistency of catalysts was tested for four continuous circulations under the criteria of pH=7.0, 20 mg/L(2,4-D), 0.50 g/L PMS, and 1 g/L 0.5% FCW/LDH. After each experiment, the recycled catalysts were rinsed with ethanol and aqua distillate repeatedly to scrub any impurities. As demonstrated in Figure. 7, the removal efficiency of 2,4-D in 180 min was able to maintain beyond 16 ACS Paragon Plus Environment

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75% after four consecutive cycles. The main reasons for this decline might ascribe the loss of the active photocatalytic sites, belonging to the core substances of motivating PMS. In addition, the surface of recycled catalysts might were covered by 2,4-D and its intermediates, resulting in a decrease in degradation activity.51 Generally speaking, 0.5% FCW/LDH composites still had good stability and reusability, indicating the as-prepared samples had potential practical application in environmental treatment. 3.7. Degradation Mechanism of 2,4-D under FCW/LDH+PMS+UV System It is known that the reactive species (h+, SO4·-, ·OH, and O2·-) in PMS activation degradation of organic contaminants. In order to better understand the specific details of these active substance during 2,4-D removal process, four active substance scavengers (TBA, KI, MeOH and BQ) were adopted according to previous reported.52 Based on previous literatures, MeOH was confirmed to extinct with ·OH and SO4·-, TBA was only deemed to eliminate ·OH radicals.53 Figure. 8, 2,4-D removal efficiency was decreased from 90.53% (without any scavenger) to 56.27%, 59.19%, 67.43% and 83.46% (in the existence of 10 mM MeOH, TBA, KI and BQ), respectively. Hence, both of SO4·- and ·OH were produced during the reaction and the primary radicals in the 2,4-D degradation process. 17 ACS Paragon Plus Environment


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Based on the experimental results and other literatures, the possibly mechanism on enhanced PMS activation with FCW/LDH composites was proposed in Figure. 9. Typically, under the excitation of visible light, the e-/h+ pairs in FCW/LDH composites were generated in Fe3O4, CsxWO, and NiAl LDH semiconductors (Eqs. (19)). These electrons and holes can react with H2O2, H2O, OH- and O2 to further generate ·OH and O2·reactive species (Eqs. (20)-(22)).54 In addition, the generated O2·- may transferred into H2O2 and HO2· radicals (Eqs. (23)-(25)). Then, the H2O2 molecules reacted with e-, generating the ·OH radicals (Eqs. (26)). Moreover, the O2·- can directly decomposed PMS and H2O to produce SO4·- and ·OH radicals, respectively (Eqs. (27)-(28)). FCW/LDH + hv → hVB+ + eCB-

(19)

FCW/LDH (hVB+) + H2O → ·OH + H+

(20)

FCW/LDH (hVB+) + OH- → ·OH

(21)

FCW/LDH (eCB-) + O2 → O2·-

(22)

2O2·- + 2H+ → H2O2 + O2

(23)

O2·- + H+ → HO2·

(24)

HO2· + 2H+ → H2O2 + O2

(25)

FCW/LDH (eCB-) + H2O2 → ·OH + H2O

(26)

HSO5- + O2·- → SO4·- + HO2·

(27)

O2·- + H2O → ·OH + OH-

(28)

On the other hand, the generated electrons can also captured by the 18 ACS Paragon Plus Environment

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PMS activation to generate SO4·- and ·OH radicals, shown in Eqs. (29)-(30). Similarly, the SO4·-, SO5·- and ·OH radicals can also be produced by motivation of PMS by catalyst under visible light driven (Eqs. (31)-(32)). Moreover, the SO5·- also can weiter react with itself to produce PS and SO5·- (Eqs. (33)-(34)). Hence, in turn, a great deal of SO4·- and ·OH radicals would formed.55 FCW/LDH (eCB-) + HSO5- → ·OH + OH-

(29)

FCW/LDH (eCB-) + HSO5- → ·OH + SO42-

(30)

HSO5- + hv → SO4·- + ·OH

(31)

FCW/LDH (hVB+) + HSO5- → SO5·- + H+

(32)

2SO5·- → S2O82- + O2

(33)

2(SO5·-) → SO5·- + SO5·- + O2

(34)

The initial pollutant (2,4-D) absorbed in the Fe3O4 particles could generate electron to Fe (Ⅲ), which connected with the generated Fe (Ⅱ) and SO4·- radicals. Thus, a Fenton-like reaction was happened between Fe (Ⅱ) and PMS at the surface of FCW/LDH composites, forming SO4·- and Fe (Ⅲ), respectively. In addition, the occurred reaction can also generate a certain amount of ·OH, which was a beneficial species in the degradation process.56 As the introduction of CsxWO3 and NiAl LDH, it acted as holes catcher and electron transmitter and helped to produce Fe (Ⅱ).57 Also, it could able to decompose the PS as well as generating many radicals at the surface of catalysts. Meanwhile, the generated 19 ACS Paragon Plus Environment


Page 20 of 45

radicals might into the aqueous solution. All in all, all the active radicals played significant character in removal of 2,4-D. The detail process was exhibited as Eqs. (35)-(43): 58-60 2,4-D + Fe (Ⅲ) → 2,4-D·+ + Fe (Ⅱ)

(35)

Fe (Ⅲ) + HSO5- → Fe (Ⅱ) + SO5·- + H+

(36)

Fe (Ⅱ) + HSO5- → Fe (Ⅲ) + SO4·- + OH-

(37)

SO4·- + H2O → ·OH + SO42- + H+

(38)

SO4·- + OH- → ·OH + SO42-

(39)

2,4-D + SO4·- → 2,4-D·+ + SO42-

(40)

2,4-D·+ + H2O → (OH)2,4-D· + H+

(41)

2,4-D + ·OH → (OH)2,4-D·

(42)

(OH)2,4-D·→ Intermediate products → CO2 + H2O + Cl-

(43)

4. CONCLUSION In summary, FCW/LDH composites were first synthesized as heterogeneous catalysts of PMS. It was discovered that FCW/LDH composites had a higher catalytic activity towards the degradation of 2,4-D in the presence of PMS in comparison with LDH, Fe3O4 and CW/LDH composites. 90.53% of 20 mg/L 2,4-D could be degraded in 180 min by using 1.00 g/L 0.5% FCW/LDH to catalyze PMS (0.50 g/L). Furthermore, the photodegradation rates of 2,4 D via PMS activation is able to maintain beyond 75% after four circulation runs. Moreover, a 20 ACS Paragon Plus Environment

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conceivable

mechanism

for

2,4-D

degradation

by

FCW/LDH

composites-activated PMS was proposed as well. More important, the catalysts with many advantages can apply to organic pollutants degradation and water environment remediation.

ACKNOWLEDGMENTS The authors would like to thank the National Natural Science Foundation of China (No. 21476269, 21776319), the Hunan Provincial Science and Technology Plan Project, China (No. 2016TP1007), the Fundamental Research Funds for the Central Universities of Central South University (No. 2017zzts777) and the work also supported by the Open-End Fund for the Valuable and Precision Instruments of Central South University (No. CSUZC201832).

ASSOCIATED CONTENT Supporting Information. The detail synthesis process of photocatalysts; The detailed EDX data see the table; XRD patterns of the samples; SEM images of the samples and the EDX images of the products; FT-IR spectra of the prepared composites; Pseudo-first-order degradation kinetic for 2,4-D with different initial interfering ions, used to estimate Langmuir-Hinshelwood coefficients and The rate constant k of 2,4-D with different initial interfering ions. 21 ACS Paragon Plus Environment


Page 22 of 45

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FIGUREURE CAPTIONS Scheme 1 Illustration of the synthesis process of the FCW/LDH composites. Figure 1 The N2 adsorption-desorption isotherm curve of (a) Fe3O4, (b) LDHs, (c) CW/LDH, and (d) 0.5 % FCW/LDH composites Figure 2 (a) UV-vis diffuse reflectance spectra of Fe3O4, LDHs, CW/LDH, and 0.5 % FCW/LDH composites; (b) the plot of (Ahv)2 vs hv of the samples. Figure 3 (a) Comparative studies of the photocatalytic activity of the various catalysts under visible light, (b) Pseudo-first-order degradation kinetic

for

2,4-D

with

different

catalysts,

used

to

estimate

Langmuir-Hinshelwood coefficients, (c) The rate constant k of 2,4-D with different catalysts. Figure 4 (a) Effect of initial PMS dosage on the degradation of 2,4-D , (b) Pseudo-first-order degradation kinetic for 2,4-D with different initial PMS dosage, used to estimate Langmuir-Hinshelwood coefficients, (c) The rate constant k of 2,4-D with different initial PMS dosage. photocatalysts dosage. Figure 5 (a) Effect of initial pH on the degradation of 2,4-D, (b) Pseudo-first-order degradation kinetic for 2,4-D with different initial pH, used to estimate Langmuir-Hinshelwood coefficients, (c) The rate 33 ACS Paragon Plus Environment


constant k of 2,4-D with different initial pH. Figure 6 (a) Effect of initial CO32- dosage on the degradation of 2,4-D, (b) Effect of initial HCO3- dosage on the degradation of 2,4-D, (c) Effect of initial Cl- dosage on the degradation of 2,4-D. Figure 7 Repeated recycling of 0.5% FCW/LDH composites with PMS for the removal of 2,4-D. Figure 8 Within 180 min reaction (Reaction conditions: initial 2,4-D concentration 20 mg/L, PMS 0.50 g/L, catalyst loading 1 g/L, pH=7.0 and quencher dosage 10 mM). Figure 9 Schematic diagram of the photocatalytic process occurring on the surface of the Fe3O4-CsxWO3/NiAl-LDH composites.


Page 34 of 45

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Scheme 1 Illustration of the synthesis process of the FCW/LDH composites.



120

b a

0.005

100

0.006

0.004

0.002

0.000

30

0

10

20

30

40

50

60

70

Pore diameter (nm)

20

adsorption desorption

10 0 0.0

0.2

0.4

0.6

0.8

0.004

(cm3g-1nm-1)

40

adsorbed (cm3g-1)

dV/dD (cm3g-1nm-1)

0.008

50

80

dV/dD

0.010

Volume

Volume

adsorbed (cm3 g-1)

60

60

0.003

0.002

0.001

0.000 0

50

100

adsorption desorption 0.2

0.005 0.000 -0.005 60

80

Pore diameter (nm)

100


50

0.6

0.8

1.0

60

(cm 3g-1nm -1)

40

70

d

0.030

dV/dD

0.010

adsorbed (cm3g-1)

0.015

80

Volume

dV/dD (cm 3g-1nm -1)

0.020

40

0.4

0.035

90

0.025

20

300

Relative pressure (P/P0)

0.030

0

250

0 0.0

0.035

150

200

20

1.0

c 200

150

Pore diameter (nm)

40


Volume adsorbed (cm3g-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 45

0.025 0.020 0.015 0.010 0.005 0.000 0

20

40

60

80

Pore diameter (nm)

50


30 20

0 0.0

0.2

0.4

0.6

0.8

1.0

0.0


0.2

0.4

0.6

0.8

1.0


Figure. 1 The N2 adsorption-desorption isotherm curve of (a) Fe3O4, (b) LDHs, (c) CW/LDH, and (d) 0.5 % FCW/LDH composites.


Page 37 of 45

a

2.0

0.5 %FCW/LDH

1.8

Absorbance (a.u.)

1.6

CW/LDH

1.4 1.2 1.0

LDHs

0.8 0.6 0.4 200

Fe3O4 300

400

500

600

700

800

Wavelength (nm)

b

0.5 %FCW/LDH CW/LDH

(Ahv)2 (ev2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fe3O4

LDHs

2

3

4

5

6

hv (eV) Figure. 2 (a) UV-vis diffuse reflectance spectra of Fe3O4, LDHs, CW/LDH, and 0.5 % FCW/LDH composites; (b) the plot of (Ahv)2 vs hv of the samples.



a

100

C/C0 (%)

80

60

40

Blank LDH Fe3O4 CW/LDH 0.5% FCW/LDH 1% FCW/LDH 3% FCW/LDH 5% FCW/LDH

20

0 0

20

40

60

80

100

120

140

160

180

120

140

160

180

Time (min)

2.5

b

LDH Fe3O4

- ln (C/C0)

2.0

CW/LDH 0.5% FCW/LDH 1% FCW/LDH 3% FCW/LDH 5% FCW/LDH

1.5

1.0

0.5

0.0 0

20

40

60

80

100

Time (min)

c

0.014

0.01311

0.012 0.010

0.00862

k (min-1)

0.00775 0.00759 0.00761

0.008 0.006

0.00539

0.00463 0.004

H W/ FC 5%

LD W/ FC 3%

LD

H

H LD W/ FC 1%

%F 0 .5

H /L D CW

Fe 3O 4

H

0.000

CW

/L D

H

0.002

LD

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 45

Figure. 3 (a) Comparative studies of the photocatalytic activity of the various catalysts under visible light, (b) Pseudo-first-order degradation kinetic for 2,4-D with different catalysts, used to estimate Langmuir-Hinshelwood coefficients, (c) The rate constant k of 2,4-D with different catalysts.


Page 39 of 45

a

100

C/C0 (%)

80

60

40

Blank 0.25 g/L 0.50 g/L 1.00 g/L 2.00 g/L

20

0 0

20

40

60

80

100

120

140

160

180

120

140

160

180

Time (min)

b

2.5

0.25 0.50 1.00 2.00

- ln (C/C0)

2.0

g/L g/L g/L g/L

1.5

1.0

0.5

0.0 0

20

40

60

80

100

Time (min)

c

0.016

0.01452 0.014 0.012

k (min-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.01311

0.01338

0.50 g/L

1.00 g/L

0.01136

0.010 0.008 0.006 0.004 0.002 0.000

0.25 g/L

2.00 g/L

Figure. 4 (a) Effect of initial PMS dosage on the degradation of 2,4-D , (b) Pseudo-first-order degradation kinetic for 2,4-D with different initial PMS dosage, used to estimate Langmuir-Hinshelwood coefficients, (c) The rate constant k of 2,4-D with different initial PMS dosage.



a

100

C/C0 (%)

80

60

Blank pH=3 pH=5 pH=7 pH=9 pH=11

40

20

0 0

20

40

60

80

100

120

140

160

180

120

140

160

180

Time (min)

b 2.5

pH=3 pH=5 pH=7 pH=9 pH=11

- ln(C/C0)

2.0

1.5

1.0

0.5

0.0 0

20

40

60

80

100

Time (min)

c

0.016

0.01513 0.01426

0.014

0.01311

0.012

0.01235

0.01217

0.010

k (min-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 45

0.008 0.006 0.004 0.002 0.000

pH=3

pH=5

pH=7

pH=9

pH=11

Figure. 5 (a) Effect of initial pH on the degradation of 2,4-D, (b) Pseudo-first-order degradation kinetic for 2,4-D with different initial pH, used to estimate Langmuir-Hinshelwood coefficients, (c) The rate constant k of 2,4-D with different initial pH.


Page 41 of 45

a

100

80

C/C0 (%)

60

Blank 0 mg/L 2 mg/L 4 mg/L 8 mg/L 16 mg/L

40

20

0 0

20

40

60

80

100

120

140

160

180

120

140

160

180

120

140

160

180

Time (min)

b

100

C/C0 (%)

80

60


40

20

0 0

20

40

60

80

100

Time (min)

c

100

80

C/C0 (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60


40

20

0 0

20

40

60

80

100

Time (min)

Figure. 6 (a) Effect of initial CO32- dosage on the degradation of 2,4-D, (b) Effect of initial HCO3dosage on the degradation of 2,4-D, (c) Effect of initial Cl- dosage on the degradation of 2,4-D.



100

80

60

C/C0 (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 45

40

20

0

1

2

3

4

Cycles

Figure. 7 Repeated recycling of 0.5% FCW/LDH composites with PMS for the removal of 2,4-D.


Page 43 of 45

100

80

2,4-D degradation (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60

40

20

0

No MeOH scavenger

TBA

KI

BQ

Figure. 8 Within 180 min reaction (Reaction conditions: initial 2,4-D concentration 20 mg/L, PMS 0.50 g/L, catalyst loading 1 g/L, pH=7.0 and quencher dosage 10 mM).



Figure. 9 Schematic diagram of the photocatalytic process occurring on the surface of the Fe3O4-CsxWO3/NiAl-LDH composites


Page 44 of 45

Page 45 of 45 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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