Removal of organic dye by biomass-based iron carbide composite with an improved stability and efficiency

Accepted Manuscript Title: Removal of organic dye by biomass-based iron carbide composite with an improved stability and efficiency Authors: Nan Zhao, Feiran Chang, Boyuan Hao, Lian Yu, Jean Louis Morel, Jing Zhang PII: DOI: Reference:

S0304-3894(19)30213-4 https://doi.org/10.1016/j.jhazmat.2019.02.077 HAZMAT 20346

To appear in:

Journal of Hazardous Materials

Received date: Revised date: Accepted date:

3 November 2018 18 February 2019 21 February 2019

Please cite this article as: Zhao N, Chang F, Hao B, Yu L, Morel JL, Zhang J, Removal of organic dye by biomass-based iron carbide composite with an improved stability and efficiency, Journal of Hazardous Materials (2019), https://doi.org/10.1016/j.jhazmat.2019.02.077 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Removal of organic dye by biomass-based iron carbide composite with an improved stability and efficiency Nan Zhao,a, b Feiran Chang,c Boyuan Hao,d LianYu,e Jean Louis Morel,f Jing Zhang *,a, g a

Key Laboratory of Environmental Nano-technology and Health Effect, Research Center for Eco-Environmental

b

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Sciences, Chinese Academy of Sciences, Beijing 100085, China

School of Environmental Science and Engineering, Guangdong Provincial Key Lab of Environmental Pollution

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Control and Remediation Technology, Sun Yat-sen University, Guangzhou 510275, China

College of Life Science, Peking University, Beijing 100871, China

d

School of Chemical and Environmental Engineering, China University of Mining & Technology, Beijing

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c

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100083, China

National Center for Nanoscience and Technology, Beijing 100190, China

f

Université de Lorraine, INRA, Laboratoire Sols et Environnement, 2, avenue de la forêt de Haye - BP 20163,

g

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54505 Vandœuvre-lès-Nancy, France

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A

e

National Engineering Laboratory for VOCs Pollution Control Materials & Technology, University of Chinese

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Academy of Sciences, Beijing 101408, China

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Corresponding Author *Tel: +86 10 62919003

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E-mail: [email protected]

Graphical abstract

1

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Highlights

A novel method was developed to improve the stability and reactivity of Fe0.

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The oxidation of Fe0 was suppressed by Fe3C in the treatment of methyl orange.

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Fe3C revealed an excellent removal efficiency for methyl orange in a wide pH

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

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range.

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ABSTRACT

The efficiency of zero-valent iron (Fe0) for the degradation of contaminants in water or soil can be

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highly reduced by side reactions with oxygen or water. This work was conducted to test whether

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this drawback can be effectively suppressed by the carbonation of Fe0 with pyrolyzed biomass, which forms a Fe3C composite. The composite Fe3C was characterized and its reactivity and stability were assessed in batch tests with methyl orange (MO) as a model pollutant. The results

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indicated that the removal rate of MO on Fe3C composite was higher than that of Fe0 (7.587 mg/(g·min) vs. 4.306 mg/(g·min)) at pH 4, where the degradation mechanism was confirmed by high-performance liquid chromatography-mass spectrometry. More importantly, the produced iron oxide in the Fe3C composite was highly suppressed. Regeneration studies showed that after three 2

times of cycling, the removal efficiency of MO on Fe3C composite was kept to 99.42%, but Fe0 almost lost its reactivity. In situ chemical reduction of a colorimetric redox probe (indigo-5, 5’-disulfonate, I2S) quantitatively demonstrated that Fe3C composite has the reduction kinetics of I2S obviously slower than Fe0, indicating that Fe3C composite improved the stability of

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KEYWORDS: Fe3C composite; Fe0; degradation; stability; methyl orange

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incorporated Fe0 to resist the side oxidation.

1. INTRODUCTION

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The application of zero-valent iron (Fe0) in water and soil remediation has attracted huge

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attention due to its high efficiency [1]. However, due to a series of drawbacks of Fe0, such as poor

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stability, ion leaching and easy aggregation, in environmental applications at field scale remains

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limited [2-3]. Fe0 can be readily oxidized by air or water, producing an oxide shell, which hinders

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the electron flow from the metallic iron, and dramatically decreases the reactive lifetime of Fe0 [4-5]. Several attempts to improve the stability and reactivity of Fe0 were initiated to reduce the

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oxidization processes in water. For example, doping nano Fe0 with Pd and Pt was tested for the

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treatment of chlorinated organic pollutants. The palladized nano Fe0 exhibited a higher reactivity than Pt/Fe particles [6]. Polymeric stabilizers, such as carboxymethyl cellulose (CMC), polyacrylic acid and polyacrylamide, were also used on nano Fe0 [7]. Polystyrenesulfonate

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modified nano Fe0 exhibited a poor stability as compared to CMC modified Fe0, due to the existence of hydrophobic chains of polystyrenesulfonate molecules [7-8]. Bio-surfactant of rhamnolipids was also used to stabilize nano Fe0, and preserve the high nano Fe0 mobility [9]. Among the various methods developed to enhance the reactivity and stability of Fe0, the 3

modification by sulfur compounds has received increased interest as sulfidation of Fe0 could decrease the corrosion rate. However, the application of dithionite would also convert the nano Fe0 into FeS, which could reduce the capacity of Fe0 to a certain extent [10]. An alternative to improve the stability and reactivity of Fe0 is the use of carbon-rich residue

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biochar. Biochar can be derived from the pyrolysis of a large range of organic materials, including biomass. It has been recognized as a cost-effective and environmental friendly adsorbent for the

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removal of contaminants [11-13]. However, powdered biochar is difficult to recover from water or

soil. Biochar can be applied as a support to stabilize the Fe0, and the combination of biochar with

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iron can provide extra active sites for the degradation/sorption of pollutants. In order to increase

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the biochar removal efficiency, many compounds have been tested, such as iron oxide-biochar

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composite, nanoscale Fe0 supported porous carbon, and Ni-Fe0 magnetic biochar [11, 14-15].

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However, the stability and reusability of these materials have not been investigated.

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This work was conducted to determine the properties and efficiency of a Fe3C composite made with biochar for the removal of organic contaminants from water in comparison with Fe0. The

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Fe3C composite was produced from a mixture of 300oC corn straw biochar and Fe(NO3)3

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pyrolyzed at 800 oC, and methyl orange was used as a model of organic contaminants. Additionally, the adsorption and degradation mechanisms of methyl orange on Fe3C composite were further studied. Finally, the potential of Fe3C composite as a promising adsorbent for

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wastewater treatment and contaminated soil remediation was discussed on the basis of the removal efficiency of MO.

2. Materials and methods 4

2.1. Preparation of Fe0, Fe3C composite, and 800 oC biochar Fe0 was prepared by dissolving 1 g FeSO4·7H2O in 50 mL of distilled water in a 3-neck flask, and by adding drop by drop a saturated 50 mL of 0.5 mol/L NaBH4 solution under N2, and agitation with a magnetic stirrer to keep a uniform concentration. Stirring was prolonged for 2 h to

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ensure a complete reaction. Then, the solid residue was separated by filtration and washed with ethanol. The obtained Fe0 was dried in an oven at 105 oC and stored in an air-tight bottle.

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Fe3C composite was obtained by mixing 0.4 g of 300 oC corn straw biochar with 50 mL of 0.04 mol/L Fe(NO3)3·9H2O solution at pH 3. After an 8 h of stirring at room temperature, the mixture

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was dried in a convection oven for 48 h. Solid mixture was then heated in a electric resistance

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furnace at 800 oC for 2 h under N2 atmosphere. The obtained Fe3C composite was stored in an

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air-tight bottle.

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The 800 °C biochar was produced by putting a 300 oC corn straw biochar in a electric resistance

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furnace and pyrolyzing at 800 oC for 2 h under N2 atmosphere. 2.2. Removal experiments

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In a first series of removal dynamics experiment of MO (Sinopharm Chemical Reagent Co.,

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Ltd.), 0.0200 g of sample was mixed with 15 mL of 25 mg/L MO solution at pH 8. All experiments were performed using 50 mL glass tubes with a shaking speed of 150 rpm in the dark at 25 ± 1 °C. After shaking for certain time, the mixtures were centrifuged at 3000 rpm for 5 min

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to collect the supernatant which was filtered through a 0.22 µm membrane. The final MO concentration was tested using UV-vis spectroscopy at 465 nm. All the removal experiments were conducted in triplicate. The total organic carbon (TOC) concentration was tested with a total organic carbon analyzer (Shimadzu TOC-VCPH). In the second series, the MO solution was 5

adjusted to pH 4 by adding 0.1 M HCl dropwise to study the effects of different pH on the removal dynamics of MO. The MO concentration was tested using UV-vis spectroscopy at 477 nm. The other procedures were the same as in the first series. Isotherm measurements were performed with 15 mL of MO solutions (28 to 350 mg/L). A

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known amount of samples (0.0200 g) was added into the 50 mL glass tubes and then agitated for

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240 min. The other procedures were the same as in the first series.

3. Results and discussion

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3.1. Characterization of Fe3C composite

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The SEM image showed evenly distributed iron spheres, which were half-embedded or

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completely enclosed into the porous char matrix (Fig. 1a). The TEM image further indicated that

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Fe3C composite was composed of nanoparticle and film-like carbon material. The dark and

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quasi-spherical iron spheres were enveloped by concentric shells. The core diameters of iron spheres were ranging from 12~43 nm and the thicknesses of shells were from 3 -12 nm (Fig. 1b).

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The results for TEM and SEM suggested that iron nanoparticles formed in the biochar matrix

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could be interpreted as iron cores encapsulated in a graphite shell structure, which were coincident with the previous studies [16-17]. This composite structure could avoid the direct contact between iron nanoparticles and dissolving oxygen in solution and thus stabilize the reactive iron

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nanoparticles. On the other hand, the activity (e.g. reductive ability) of iron core would be hardly suppressed by the carbon shell, due to the excellent electron conductivity of carbon structure. The N2 adsorption and desorption isotherm constituted typical IV type curves with hysteresis loop of H4 for Fe3C composite and Fe0, indicating the existence of mesoporous structures (Fig. 6

S2). As for biochar, it was obvious that this gave a type I isotherm. The analysis results of BET surface areas, total pore volumes, and average pore sizes of Fe3C composite, Fe0 and 800 oC biochar were summarized in Table S1. The BET surface area of Fe3C composite was 218.73 m2/g, significantly higher than that of Fe0 (46.68 m2/g). But the surface area of Fe3C composite was

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smaller than that of 800 oC biochar (603.63 m2/g), probably because the formation of iron spheres blocked the pores of the biochar. The average pore size of Fe3C composite, Fe0 and 800 oC biochar

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samples were 4.13 nm, 7.05 nm and 2.17 nm, respectively.

The X-ray diffraction (XRD) patterns of Fe3C composite, patterns exhibited three parts:

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graphite, Fe0 (2θ = 44.67o, 64.98o and 82.41o) and Fe3C (2θ = 44.73o, 44.57o, 42.89o, 43.75o,

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45.87o, 37.67o, 48.56o, 49.16o and 51.88o). The peak at 2θ = 25.94o confirmed the existence of

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graphite, which indicated that the carbonaceous materials in biochar were converted to graphite

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(Fig. 1c) [16]. For Fe0, which was prepared with the reduction of NaBH4 method, showed a typical

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sharp peak at 2θ = 44.67o (Fig. 1d). There was no obvious diffraction peak for 800 oC biochar suggesting its amorphous nature (Fig. S1) [18]. Therefore, the addition of iron could cause the

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carbonization/graphitization of the biochar [19]. The XRD results revealed that after the thermal

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treatment of iron-impregnated biochar, the partial dissolution of carbon into the crystal lattice of Fe0 led to the formation of Fe3C [16, 20]. The alloy-structural Fe3C was formed as an interface between the carbonaceous shell and the iron core and might act as a catalyst to further increase the

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reactivity of Fe0/C composite for degradation of pollutants. The XPS survey spectra revealed that no peak of Fe 2p existed on the surface of Fe3C composite, confirming that Fe3C was encapsulated by graphite, and the shell structure was formed (Fig. 2a, b) [17]. During the formation of the carbonaceous shell, Fe3C was formed in the outer 7

layer of Fe0 [16]. Two major peaks at 710.58 eV and 724.08 eV, corresponding to Fe2p1/2 and Fe2p3/2 respectively, were observed along with a weak satellite peak at 720.18 eV (Fig. 2c). This indicated that the surface of Fe0 was covered by oxidized iron [21]. 3.2. Removal experiments of MO

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The removal dynamics of MO on Fe3C composite, Fe0 and 800 oC biochar at pH 8 is given in Fig. 3a. Both Fe0 and Fe3C composite showed a high removal efficiency for MO, as compared to

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biochar. The removal efficiency of MO on the Fe3C composite (99.52%) was detectably higher than that of Fe0 (95.33%) at equilibrium state. This result indicated that the removal of MO was

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enhanced by the presence of graphite or Fe3C. 800 oC biochar showed a very low removal

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efficiency for MO, and its highest removal efficiency was only 24.97%, which could ascribe to the

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adsorption effect. Fe3C composite could remove 72.21% of MO within 5 min while Fe0 removed

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only 58.69% (Fig. 3b). The best results for degradation of MO by Fe0 and Fe3C composite were

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obtained at pH 4. This was probably because at acidic conditions, Fe0 and Fe3C composite would produce more H2 to reduce and degrade MO [22]. For a given experimental time, the removal rate

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of MO on Fe3C composite was faster than on Fe0.

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The kinetics data obtained from those equilibrium experiments were fitted by a pseudo first-order kinetic model and pseudo second-order kinetic model. The removal process was well described with the pseudo second-order kinetic model for Fe3C composite and Fe0 (Table 1). At

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pH 4, the removal rate constant of MO by Fe3C composite was 0.022 g/(mg• min) , which was 1.7 times that of Fe0. The removal rate decreased with the increase of pH. As the removal efficiency of MO by biochar was much lower than those by Fe3C composite and Fe0, the removal quickly reached equilibrium. Thus, biochar had a removal rate constant of 0.023 g/(mg• min), as high as 8

that of Fe3C composite. The removal of MO was reduced at pH 4 for biochar as compared to pH 8, because of pH-dependent ionization of MO [24]. This was consistent with the adsorption of MO on paper and pulp sludge biochar [25]. The removal rate of Fe3C obtained in this study was relatively higher, as compared to the previous report, such as carbon coated monolith and

compared with those of the available adsorbents, as described in Table S2.

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m-CS/γ-Fe2O3/MWCNTs (TableS2). The removal rate constants obtained in this work were

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The isotherms of MO for 800 oC biochar, Fe0 and Fe3C composite were presented in Fig. S3. The isotherms of MO adsorbed on the biochar was fitted with Freundlich and Langmuir models,

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and the fitted parameters were shown in Table S3. The goodness-of-fit for the isotherm models

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was estimated by the coefficient of determination (R2). The value of R2 (0.97) showed that the

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Freundlich isotherm was suitable for modeling the adsorption process of MO on the biochar. It

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was multilayer adsorption and the adsorption heat was non-uniform distributed on the surface of

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biochar. For Fe0 and Fe3C composite, both adsorption and degradation of MO occurred, so the removal of MO could not be fit and explained by the classical adsorption isotherm equations.

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Especially, the removal of MO on Fe0 was mainly dominated by degradation.

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3.3. Removal mechanisms of MO The UV-vis absorption spectra of MO at pH 8 revealed that the concentration of MO decreased

greatly with Fe3C composite and Fe0 after 20 min (Fig. 4). It also showed that these two materials

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had a high efficiency to remove MO, and removal of MO was faster on Fe0 than on Fe3C composite. The high removal rates were mainly due to chemical interactions [26]. A strong absorbance band was observed at 465 nm, which represented conjugated structure formed by an azo band and the peak at 271 nm was the π-π* transition of aromatic rings (Fig. 4a, b) [27]. After 9

20 min of reaction, this band became weaker and the peak at 271 nm disappeared. Meanwhile, a new peak appeared at 248 nm which was ascribed to amines [28]. This phenomenon suggested the cleavage of the azo bond and the formation of a new product of sulfanilic acid [29]. Thus, the chromophore group and conjugated system were destroyed. The absorbance of amines increased

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with the reaction time for the removal of MO on Fe0, showing the direct decomposition of MO in the solution. Sulfanilic acid would be produced and released into the solution during the

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degradation process [30]. Interestingly, the absorbance of amines decreased with the increasing of reaction time, confirming that the adsorption and degradation happened simultaneously for the

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removal of MO on Fe3C composite. For 800 oC biochar, the UV-vis absorption spectra did not

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change with increasing contact time showing that only adsorption occurred which was controlled

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by physical interaction.

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The absorbance band for MO showed red shifts (465→477 nm, 271→275 nm) at pH 4 as

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compared to pH 8, due to the increase of delocalization for the MO molecules [30]. The decreased magnitude of the absorbance at the peak of 477 nm for Fe3C composite was greater than that of

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Fe0, which was opposite to the case at pH 8 (Fig. 5). This implied that the initial pH of the solution

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had a significant effect on the degradation of MO on Fe3C composite. The change trends of the UV-vis spectra for the band at 275 nm were the same as those at pH 8. The XRD patterns of Fe3C composite, Fe0 and 800 oC biochar also showed changes after the

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reaction at pH 8 (Fig. 6a, b, c). The peak intensity at 2θ = 44.67o decreased for Fe3C composite, indicating that iron was involved in the reaction. Fe3C kept its state and there was no iron oxide (2θ = 35.39o) formed during the adsorption and degradation processes, suggesting that Fe3C composite offered a suitable stability. Fe0 became iron oxide after 160 min, suggesting that redox 10

reaction took place in the degradation process of MO. For biochar, no change happened during the reaction with MO for XRD pattern. This further confirmed that MO was adsorbed by biochar and that the process was controlled by physical interaction. Even in the acidic condition, there was no obvious change for the XRD patterns of Fe3C composite (Fig. 7a). Yet, the iron oxide appeared at

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80 min for Fe0 sample. To further understand the removal mechanisms of MO on Fe3C composite, we investigated the

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XPS spectra of Fe3C composite and Fe0 after the reaction with MO (Fig. 8). Three new peaks at 398.88 (N 1s), 711.68 (Fe2p1/2) and 725.28 (Fe2p3/2) eV appeared as compared to the XPS

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spectrum in Fig. 3a [31,32]. The increased content of N showed that amines were adsorbed on the

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surface of Fe3C composite by forming N-C bond [30]. The XPS full spectrum obtained for Fe0

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showed no peak at 398.88 eV, but the appearance of N 1s narrow scan indicated few amounts of

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amines were adsorbed by Fe0 (Fig. 8b). At the same time, the oxidation of Fe0 to iron oxide in the

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Fe3C composite during the degradation process was coupled with the reduction of MO (Fig. 8c). To further understand the changes after the reaction with MO and Fe3C or Fe0, it was necessary to

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fit the Fe2p1/2 spectra. Peaks A, B and C were assigned to Fe(II) and Fe(III) in oxide iron, and

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peak D was due to Fe0 [33]. The contents of Fe0 were 17.43% in the surface of Fe3C and 7.59% in Fe0 (Fig. 8c, d). This further demonstrated that Fe3C was stable and corrosion was unlikely. From the above analysis, it could be concluded that the inner sphere of Fe0 in Fe3C composite was

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involved in the degradation of MO, and it was oxidized to iron oxide. Meanwhile, MO was reduced to sulfanilic acid, which can be adsorbed by graphite or Fe3C through the formation of a N-C bond. HPLC-MS analyses were conducted by using Fe3C composite and Fe0 to further study the 11

degradation process and recognize the possible degradation products. Fig. 9 showed the ion chromatograms relative to HPLC-MS. The methyl orange peak was observed (MO, m/z 304, 6.30/6.00 min Retention Time (RT)), indicating that some of the starting MO molecules remained without undergoing degradation at 20 min. Two degradation products were detected from the Fe0

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sample. Product A is a demethylated intermediate formed through the cleavage of a methyl group from the MO molecule (m/z 290, 5.20 min RT). Product B is the intermediate compound formed

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by azo bond-breaking (m/z 172, 1.58 min RT). For Fe3C composite, only one degradation product B (m/z 172, 1.56 min RT) was detected.

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As shown in Fig. 10, after 280 min of reaction in the Fe3C system, the removal of TOC

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amounted to 90.2%, while the removal efficiencies of TOC by Fe0 and biochar were only 2.16%

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and 8.54%, respectively. The TOC results revealed that Fe3C composite had a prevailing

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degradation/adsorption effect on the removal of MO, as compared to Fe0 and biochar. In the Fe0

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and Fe3C composite systems, the reduction cleavage of N=N azo bond was the main mechanism to destruct and decolorize of azo dyes, while the mineralization of MO was weak by the reduction

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effect. Therefore, the TOC obtained here mainly stood for the adsorption capacity of the applied

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materials to remove the organic after degradation. According to the results by TOC measurement, we could see that the advantages of Fe3C composite were to provide both degradation and adsorption capacities to remove MO. But for Fe0, its low TOC removal efficiency could be

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attributed to the weak adsorption ability to the partially decomposed products of MO. The TOC results revealed that, the degradation of MO into the intermediate compounds with smaller molecular weight might help to the adsorption by the biochar part of Fe3C composite. In contrast, the adsorption of raw MO by the biochar was less favorable. 12

As described in Fig. 11, the reduction of MO by Fe0 underwent three steps (Fig. 11a). In the first step, MO was reduced into the intermediate A by demethylation. Before the broken of azo bond of MO, a transitional compound (Ar-NH-NH-Ar’) resulting from the incomplete reduction of MO was formed. The unstable transitional compound was finally reduced and cleavaged into degradation

products

(sulfanilic

acid

and

N-methyl-p-phenylenediamine/N,

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two

N-dimethyl-p-phenylenediamine). As a contrast, the reduction of MO by Fe3C composite occurred

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in two steps without demethylation (Fig. 11b). This probably originated from the strong

adsorption of MO on the surface of Fe3C composite, which activated the cleavage of azo bond.

composite,

which

was

confirmed

by

TOC

(Fig.

10).

The

product

of

N

Fe3C

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After the degradation, the products with smaller molecular weight were also strongly adsorbed by

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N-methyl-p-phenylenediamine or N, N-dimethyl-p-phenylenediamine was hardly detected by

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3.4. Regeneration of Fe3C composite

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HPLC in solution, probably because the product was adsorbed by Fe0 and Fe3C composite.

Regeneration studies were performed to assess the potential reusability of the Fe 3C

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composite. The primary objective of regeneration was to restore the removal capacity of the

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exhausted material. A secondary objective was to recover valuable components present in the adsorbed phase [31]. The MO removal efficiency on Fe3C composite remained almost constant for four cycles (Fig. 12a). It was above 99% even after the third usage. However, for

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Fe0, the removal efficiency decreased to 77% after the second usage. For the third usage, Fe 0 had lost its degradation capacity, and showed only adsorption capacity (Fig. 12c). The XRD patterns showed that Fe0 became iron oxide after the first usage, but Fe3C composite kept the original state (Fig. 13). After the second, third and fourth interactions of MO with Fe3C 13

composite, the peak intensities of Fe0 at 2θ=44.67o, 64.98o and 82.41o increased. Because the high-temperature desorption technology promoted the activation of Fe3C composite and the iron ion or iron oxide on the surface of Fe3C composite were reduced to Fe0. 3.5. Stability of Fe3C composite

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The effects of pH on the removal of MO and the crystal structures were tested to compare the stability of Fe3C composite and Fe0. At a given pH, the removal efficiency of MO on Fe3C

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composite was higher than that of Fe0. No matter acidic or alkaline condition, the removal

efficiencies of MO on Fe3C composite were above 98%. The Fe3C composite stayed in its original

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state at pH 3, 4, 8 and 14 (In Fig. 14b). Only the peak intensity of Fe0 in Fe3C composite

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decreased at pH 2, because the strong acidity could lead to the reaction of Fe0 with H+. This

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qualitatively indicated that Fe3C composite was stable at different pH values. However, the crystal

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structure of Fe0 disappeared at pH 2 and 3 suggesting that it was almost fully dissolved (Fig. 14c).

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The leaching of Fe happened easily at low pH condition [34]. Fe0 would become iron oxide at pH 4 and 8. At pH 14, the passivation layer of iron hydroxide covering the Fe0 surface offered a

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protection against corrosion [35-36]. Fe3C composite had a core-shell structure of Fe0 coated by

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Fe3C, which was embedded in the matrix of biochar. So, it could stabilize at both acid and alkaline conditions. Moreover, it could provide both high adsorption and degradation capacities to remove MO, due to its high surface area and pore volume (Table S1) for the adsorption, as well as reactive

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Fe0/Fe3C for the degradation. As a comparison, biochar could only adsorb MO and reach saturated, while Fe0 could be easily oxidized with the dissolved oxygen and lose its reactivity, as proved by the regeneration experiment. The quantitative analysis of the stability of Fe3C composite and Fe0 was carried out according to 14

the in situ chemical reduction method. Indigo-5, 5’-disulfonate (I2S) was used as chemical redox probe. The oxidized form of I2S (I2Sox) which was bright blue could be reduced to reduced form (I2Sred) with pale yellow by Fe3C composite and Fe0 [37]. The reaction between Fe0 and I2Sox was as follows: 2Fe(III) + 3[I2Sred] + 6OH-

(1)

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2Fe0 + 3[I2Sox] + 6H2O

The reduction efficiency was measured over time by monitoring aqueous absorbance of I2Sox.

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The reduction efficiency quickly increased and approached a plateau at about 4 min, indicating that Fe0 was easily consumed and the redox reaction was fast (Fig. 14d). On the contrary, for I2S

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reduction by Fe3C composite, the apparent kinetics of I2S reduction was slower and the plateau

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was reached in 10 min, presumably due to the slower reaction of I2S with Fe3C composite than

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Fe0. The formation of a carbonaceous shell could also inhibit the redox reaction. We chose to fit

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the data with the pseudo first-order kinetic model, which gave reduction rate constants of 1.11

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min-1 for Fe0 and 0.46 min-1 for Fe3C. The reduction rate of indigo-5, 5’-disulfonate on Fe3C

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composite was smaller than that of Fe0, indicating the good stability of Fe3C composite.

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4. Conclusions

The challenge for the application of Fe0 lies in how to increase its stability while keep the

reactivity during the degradation of pollutants. This work presented a novel strategy for the above

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issue by employing biomass-based Fe3C composite. The iron spheres uniformly dispersed on biochar and they were half-embedded or completely enclosed in the porous char matrix. Our Fe3C composite has a good stability, high removal efficiency for MO and good recycling performance. Importantly, it revealed an excellent removal efficiency for MO under different pH condition by 15

the dual effects of adsorption and degradation. The dual modes of adsorption and degradation of Fe3C composite provide an effective strategy for the treatment of organic pollutants. The usage of Fe3C composite has the potential to solve the problems of stability that limit the application of Fe0. The findings of this study provide a new material for the transformation and removal of

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contaminants. Further work on the efficiency of Fe3C composites for degradation of complex pollutants, e.g. halogenated organic compounds, or for the in-situ remediation of contaminant sites

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will be explored in future studies.

authors

acknowledge

the

financial

support

from

National Key Research and

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The

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Acknowledgments

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Development Program of China (2016YFA0203101, 2017YFA0207204), the National Natural

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Science Foundation of China (Grant No. 21876190, 21836002, and 41807113), the Key Research

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and Development Program of Ningxia (2017BY064), and the “One Hundred Talents Program” in Chinese Academy of Sciences, Natural Science Foundation of Guangdong Province

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(2018A030313940).

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zero-valent iron particles. Chem. Eng. J. 320 (2017) 290-299.

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[5] D. Ribas, M. Cernik, J.A. Benito, J. Filip, V. Marti, Activation process of air stable nanoscale

[6] W.X. Zhang, C.B. Wang, H.L. Lien, Treatment of chlorinated organic contaminants with

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nanoscale bimetallic particles. Catal. Today 40 (1998) 387-395.

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[7] C.M. Cirtiu, T. Raychoudhury, S. Ghoshal, A. Moores, Systematic comparison of the size,

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surface characteristics and colloidal stability of zero valent iron nanoparticles pre- and

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post-grafted with common polymers. Colloid Surface A 390 (2011) 95-104.

ED

[8] A. Ditsch, P.E. Laibinis, D.I.C. Wang, T.A. Hatton, Controlled clustering and enhanced stability of polymer-coated magnetic nanoparticles. Langmuir 21 (2005) 6006-6018.

PT

[9] S. Bhattachatjee, M. Basnet, N. Tufenkji, S. Ghoshal, Effects of rhamnolipid and

CC E

carboxymethylcellulose coatings on reactivity of palladium-doped nanoscale zerovalent iron particles. Environ. Sci. Technol. 50 (2016) 1812-1820. [10] D.M. Fan, G.O. Johnson, P.G. Tratnyek, R.L. Johnson, Sulfidation of nano zerovalent iron

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(nZVI) for improved selectivity during in-situ chemical reduction (ISCR). Environ. Sci. Technol. 50 (2016) 9558-9565. [11] B.L. Chen, Z.M. Chen, S.F. Lv, A novel magnetic biochar efficiently sorbs organic pollutants and phosphate. Bioresource Technol. 102 (2011) 716-723. 17

[12] F. Rees, F. Watteau, S. Mathieu, M.P. Turpault, Y. Le Brech, R.L. Qiu, J.L. Morel, Metal immobilization on wood-derived Biochars: distribution and reactivity of carbonate phases. J. Environ. Qual. 46 (2017) 845-854. [13] N. Zhao, C.F. Zhao, Y.Z. Lv, W.F. Zhang, Y.G. Du, Z.P. Hao, J. Zhang, Adsorption and

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coadsorption mechanisms of Cr(VI) and organic contaminants on H3PO4 treated biochar. Chemosphere 186 (2017) 422-429.

SC R

[14] Z.G. Liu, F. Zhang, S.K. Hoekman, T.T. Liu, C. Gai, N.N. Peng, Homogeneously dispersed

zerovalent iron nanoparticles supported on hydrochar-derived porous carbon: simple, in situ

U

synthesis and use for dechlorination of PCBs. ACS Sustain. Chem. Eng. 4 (2016) 3261-3267.

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[15] P. Devi, A.K. Saroha, Simultaneous adsorption and dechlorination of pentachlorophenol from

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effluent by Ni-ZVI magnetic biochar composites synthesized from paper mill sludge. Chem. Eng.

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J. 271 (2015) 195-203.

ED

[16] Q.G. Yan, C.X. Wan, J. Liu, J.S. Gao, F. Yu, J.L. Zhang, Z.Y. Cai, Iron nanoparticles in situ encapsulated in biochar-based carbon as an effective catalyst for the conversion of

PT

biomass-derived syngas to liquid hydrocarbons. Green Chem. 15 (2013) 1631-1640.

CC E

[17] G.Y. Ren, X.Y. Lu, Y.N. Li, Y. Zhu, L.M. Dai, L. Jiang, Porous core-shell Fe3C embedded N-doped carbon nanofibers as an effective electrocatalysts for oxygen reduction reaction. ACS Appl. Mater. Inter. 8 (2016) 4118-4125.

A

[18] B. Chen, Z.L. Zhu, S.X. Liu, J. Hong, J. Ma, Y.L. Qiu, J.H. Chen, Facile hydrothermal synthesis of nanostructured hollow iron-cerium alkoxides and their superior arsenic adsorption performance. ACS Appl. Mater. Inter. 6 (2014) 14016-14025. [19] A. Ōya, S. Ōtani, Influences of particle size of metal on catalytic graphitization of 18

non-graphitizing carbons. Carbon 19 (1981) 391-400. [20] Y.F. Shen, Carbothermal synthesis of metal-functionalized nanostructures for energy and environmental applications. J. Mater. Chem. A 3 (2015) 13114-13188. [21] W. Yan, A.A. Herzing, C.J. Kiely, W.X. Zhang, Nanoscale zero-valent iron (nZVI): aspects of

IP T

the core-shell structure and reactions with inorganic species in water. J. Contam. Hydrol. 118 (2010) 96-104.

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[22] H.N. Liu, G.T. Li, J.H. Qu, H.J. Liu, Degradation of azo dye Acid Orange 7 in water by Fe0/

granular activated carbon system in the presence of ultrasound. J. Hazard. Mater. 144 (2007)

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[23] K.V.G. Ravikumar, S. Santhosh, V.S. Shruthi, V.N. Yarlagadda, P. Mrudula, C. Natarajan, M.

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Environ. Chem. Eng. 6 (2018) 1683-1689.

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Amitara, Biogenic nano zero valent iron (bio-nZVI) anaerobic granules for textile dye removal. J.

ED

[24] J. Ma, F. Yu, L. Zhou, L. Jin, M. Yang, J. Luan, Y. Tang, H. Fan, Z. Yuan, J. Chen, Enhanced adsorptive removal of methyl orange and methylene blue from aqueous solution by

PT

alkali-activated multiwalled carbon nanotubes. Appl. Mater. Inter. 4 (2012) 5749-5760.

CC E

[25] N. Chaukura, E.C. Murimba, W. Gwenzi, Synthesis, characterisation and methyl orange adsorption capacity of ferric oxide-biochar nano-composites derived from pulp and paper sludge. Appl. Water Sci. 7 (2017) 2175-2186.

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[26] W. Yao, S.J. Yu, J. Wang, Y.D. Zou, S.S. Lu, Y.J. Ai, N.S. Alharbi, A. Alsaedi, T. Hayat, X.K. Wang, Enhanced removal of methyl orange on calcined glycerol-modified nanocrystallined Mg/Al layered double hydroxides. Chem. Eng. J. 307 (2017) 476-486. [27] C. Galindo, P. Jacques, A. Kalt, Photodegradation of the aminoazobenzene acid orange 52 by 19

three advanced oxidation processes: UV/H2O2 UV/TiO2 and VIS/TiO2 - comparative mechanistic and kinetic investigations. J. Photochem. Photobiol. A-Chem. 130 (2000) 35-47. [28] Y.Y. Sha, I. Mathew, Q.Z. Cui, M. Clay, F. Gao, X.J. Zhang, Z.Y. Gu, Rapid degradation of azo dye methyl orange using hollow cobalt nanoparticles. Chemosphere 144 (2016) 1530-1535.

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[29] J. Fan, Y.H. Guo, J.J. Wang, M.H. Fan, Rapid decolorization of azo dye methyl orange in aqueous solution by nanoscale zerovalent iron particles. J. Hazard. Mater. 166 (2009) 904-910.

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[30] J. Del Nero, R.E. de Araujo, A.S.L. Gomes, C.P. de Melo, Theoretical and experimental

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[31] J. Hu, G.H. Chen, I.M.C. Lo, Removal and recovery of Cr(VI) from wastewater by

A

maghemite nanoparticles. Water Res. 39 (2005) 4528-4536.

M

[32] F.S. Guo, P.J. Yang, Z.M. Pan, X.N. Cao, Z.L. Xie, X.C. Wang, Carbon-doped BN nanosheets

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for the oxidative dehydrogenation of ethylbenzene. Angew Chem. Int. Edit. 56 (2017) 8231-8235. [33] H. Zhao, L. Qian, Y. Chen, Q. Wang, G. Zhao, Selective catalytic two-electron O2 reduction

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for onsite efficient oxidation reaction in heterogeneous electro-fenton process. Chem. Eng. J. 332

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(2018) 486-498.

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Water 45 (2017) 1600826. [35] J. Farrell, M. Kason, N. Melitas, T. Li, Investigation of the long-term performance of zero-valent iron for reductive dechlorination of trichloroethylene. Environ. Sci. Technol. 34 (2000) 514-521. 20

[36] W.L. Yan, A.A. Herzing, C.J. Kiely, W.X. Zhang, Nanoscale zero-valent iron (nZVI): aspects of the core-shell structure and reactions with inorganic species in water. J. Contam. Hydrol. 118 (2010) 96-104. [37] D.M. Fan, S.W. Chen, R.L. Johnson, P.G. Tratnyek, Field deployable chemical redox probe

IP T

for quantitative characterization of carboxymethylcellulose modified nano zerovalent iron.

A

CC E

PT

ED

M

A

N

U

SC R

Environ. Sci. Technol. 49 (2015) 10589-10597.

21

Figures and legends Fig. 1. SEM (a) and TEM images (b) of Fe3C composite, XRD patterns of Fe3C composite (c) and Fe0 (d). Fig. 2. XPS spectra of Fe3C composite (a, b) and Fe0 (c). Fig. 3. The removal dynamics of MO at pH 8 (a) and 4 (b).

IP T

Fig. 4. The UV-vis absorption spectra of MO at pH 8 by Fe3C composite (a), Fe0 (b), and biochar (c).

SC R

Fig. 5. The UV-vis absorption spectra of MO at pH 4 by Fe3C composite (a), Fe0 (b), and biochar (c).

Fig. 6. XRD patterns of Fe3C composite (a), Fe0 (b), and biochar (c) reacting with MO at different

U

time at pH 8.

N

Fig. 7. XRD patterns of Fe3C composite (a) and Fe0 (b) reacting with MO at different time at pH

A

4.

Fig. 8. XPS analysis of used Fe3C composite (a) and Fe0 (b), shirley background-subtracted Fe

M

2p1/2 spectrum of used Fe3C composite (c) and Fe0 (d).

ED

Fig. 9. Liquid chromatograph observed after 20 min for Fe3C composite (a), Fe0 (b). Fig. 10. TOC removal efficiency of MO by Fe3C composite, Fe0, and biochar. Fig. 11. Proposed degradation pathway of MO by Fe0 (a) and Fe3C composite (b). Dotted square

PT

means the product was not detected in the solution, which might be adsorbed on the surface of Fe0 and Fe3C composite.

CC E

Fig. 12. Regeneration studies of Fe3C composite and Fe0 (a), UV-vis spectra of MO by Fe3C composite (b) and Fe0 (c) before and after four cycles. Fig. 13. XRD patterns of Fe3C composite (a) and Fe0 (c) before and after four cycles. (b) presents

A

the zoom-in XRD patterns of Fe3C composite in the range of 2θ=40-85o. Fig. 14. The removal efficiency of MO at different pH (a), XRD patterns of Fe3C composite (b) and Fe0 (c) at different pH, and (d) the reduction efficiency of indigo-5, 5’-disulfonate on Fe3C composite and Fe0.

22

IP T (d)

Fig. 1

A

CC E

PT

ED

M

10 20 30 40 50 60 70 80 90 2 Theta (degree)

23

SC R U

PDF 01-071-8483

N

A

PDF 01-085-1317

Intensity (a.u.)

Intensity (a.u.)

(c)

10 20 30 40 50 60 70 80 90 2 Theta (degree)

(a)

C 1s

200 400 600 800 1000 1200 Binding energy (eV)

SC R

0

M

A

N

U

Intensity (a.u.)

(b)

ED

700 705 710 715 720 725 730 735 740 Binding energy (eV)

(c)

Fe 2p1/2 Fe 2p3/2

PT

Intensity (a.u.)

CC E A

IP T

Intensity (a.u.)

O 1s

700 705 710 715 720 725 730 735 740 Binding energy (eV) Fig. 2

24

0

Fe

100 80 60 40

40

80 120 160 200 240 280 Time (min)

Fe3C

0

Fe

Biochar

(b)

100

U

80

N

60

A

40

100 150 Time (min) Fig. 3

A

CC E

PT

ED

50

M

20 0 0

IP T

20 0 0

Removal efficiency (%)

(a)

Biochar

SC R

Removal efficiency (%)

Fe3C

25

200

250

2.1

20 min 160 min 240 min

40 min 180 min 280 min

(a)

1.5 1.2

t

0.9 0.6 0.3

20 min 160 min 240 min

1.2

t

A

0.9 0.6

ED

240 280 320 360 400 440 480 520 Wavelength (nm)

PT

0.0

2.1

25 mg/L 80 min 200 min

20 min 160 min 240 min

Intensity (a.u.)

40 min 180 min 280 min

(c)

t

CC E

1.8

(b)

N

1.5

0.3

A

40 min 180 min 280 min

M

Intensity (a.u.)

1.8

25 mg/L 80 min 200 min

SC R

2.1

240 280 320 360 400 440 480 520 Wavelength (nm)

U

0.0

1.5 1.2 0.9 0.6 0.3 0.0

240 280 320 360 400 440 480 520 Wavelength (nm) Fig. 4 26

IP T

Intensity (a.u.)

1.8

25 mg/L 80 min 200 min

25 mg/L 20 min 160 min

2.1

10 min 80 min 240 min

(a)

1.5 1.2

t

0.9 0.6 0.3

25 mg/L 20 min 160 min

2.1 1.5

t

A

0.9 0.6

PT

2.1

240 280 320 360 400 440 480 520 Wavelength (nm)

ED

0.0

25 mg/L 20 min 160 min

5 min 40 min 200 min

10 min 80 min 240 min

(c) t

1.5

CC E

Intensity (a.u.)

1.8

(b)

N

1.2

0.3

A

10 min 80 min 240 min

M

Intensity (a.u.)

1.8

5 min 40 min 200 min

SC R

240 280 320 360 400 440 480 520 Wavelength (nm)

U

0.0

1.2 0.9 0.6 0.3 0.0

240 280 320 360 400 440 480 520 Wavelength (nm) Fig. 5 27

IP T

Intensity (a.u.)

1.8

5 min 40 min 200 min

(a)

280 min

Intensity (a.u.)

240 min 200 min 160 min

Fe3C

SC R

10 20 30 40 50 60 70 80 90 2 Theta (degree) 280 min

(b)

Intensity (a.u.)

240 min

U

200 min

20 min 0

Fe

M

A

N

160 min

ED

10 20 30 40 50 60 70 80 90 2 Theta (degree)

(c)

PT

280 min

Intensity (a.u.)

CC E A

IP T

20 min

200 min 20 min

Biochar

10 20 30 40 50 60 70 80 90 2 Theta (degree)

Fig. 6

28

(a)

240 min

Intensity (a.u.)

200 min 160 min 80 min

Fe3C

SC R

10 20 30 40 50 60 70 80 90 2 Theta (degree) (b)

240 min

Intensity (a.u.)

U

200 min

M

A

N

160 min 80 min 5 min 0

Fe

ED

10 20 30 40 50 60 70 80 90 2 Theta (degree)

Fig. 7

A

CC E

PT

IP T

5 min

29

O 1s

Intensity (a.u.)

N 1s

O 1s Fe 2p

C 1s

N 1s

IP T

Fe 2p

Intensity (a.u.)

Intensity (a.u.)

(b)

(a)

C 1s

390 393 396 399 402 405 408 Binding energy (eV)

0

200 400 600 800 1000 1200 Binding energy (eV)

200 400 600 800 1000 1200 Binding energy (eV)

SC R

0

(c)

C

A

D

708 710 712 714 Binding energy (eV)

716

Fig.8

A

CC E

PT

ED

706

M

A

N

C D

U

B

A

Intensity (a.u.)

Intensity (a.u.)

B

(d)

30

706

708 710 712 714 Binding energy (eV)

716

(a)

B: m/z=172

3.0x107

m/z=304

2.5x107 2.0x107 1.5x107

IP T

1.0x107 5.0x106 0.0 0

1

SC R

Relative Abundance

3.5x107

2 3 4 5 6 7 8 Retention Time (min)

M

8.0x107 7

4.0x107 2.0x107

PT

0.0 0

1

m/z=304

A: m/z=290

2 3 4 5 6 7 8 Retention Time (min) Fig. 9

A

CC E

U A

1.0x108 6.0x10

(b)

N

1.2x10

B: m/z=172

8

ED

Relative Abundance

1.4x108

9 10

31

9 10

80 60 Fe0 Biochar

40

0 0

50

SC R

20

IP T

Fe3C

100 150 200 250 300 Time (min)

A

CC E

PT

ED

M

A

N

Fig. 10

U

TOC removal efficiency (%)

100

32

A ED

PT

CC E Fig. 11

33

IP T

SC R

U

N

A

M

100 80 60 40

1

1.8

2 3 Cycle number

4

20 mg/L 1 cycle 2 cycle 3 cycle 4 cycle

1.5

(b)

U

1.2

IP T

20 0

0.9

N

0.6 0.3

A

Intensity (a.u.)

0

Fe Fe3C

(a)

SC R

Removal efficiency (%)

120

M

0.0

ED

240 280 320 360 400 440 480 520 Wavelength (nm) 1.8

A

CC E

PT

Intensity (a.u.)

1.5 1.2

20 mg/L 1 cycle 2 cycle 3 cycle 4 cycle

(c)

0.9 0.6 0.3 0.0

240 280 320 360 400 440 480 520 Wavelength (nm) Fig. 12

34

(a)

4 cycle

Intensity (a.u.)

3 cycle 2 cycle 1 cycle

SC R

10 20 30 40 50 60 70 80 90 2 Theta (degree) (b)

Intensity (a.u.)

4 cycle

M

60 70 2 Theta (degree)

ED

(c)

50

A

N

U

3 cycle

40

2 cycle

1 cycle

80 4 cycle

Intensity (a.u.)

3 cycle

2 cycle

PT CC E A

IP T

Fe3C

1 cycle 0

Fe

10 20 30 40 50 60 70 80 90 2 Theta (degree) Fig. 13

35

100

(b)

0

Fe3C

Fe

Intensity (a.u.)

pH 2

60 40 20 0

pH 14

10 20 30 40 50 60 70 80 90 2 Theta (degree)

pH 14 (c) pH 2

Intensity (a.u.)

pH 8

pH 3

Fe3C

100 80 60

0

Fe

(d)

N

pH 4

SC R

pH 8

pH 4

U

pH 4

pH 3

IP T

80

Reduction efficiency (%)

Removal efficiency (%)

(a)

40

A

pH 8

M

pH 14

Fig. 14

A

CC E

PT

ED

10 20 30 40 50 60 70 80 90 2 Theta (degree)

36

20 0 0 2 4 6 8 10121416182022 Time (min)

Table 1. Kinetics parameters for the removal of MO on Fe3C composite, Fe0 and 800 oC biochar

K2 (g/(mg·min))

qe (mg/g)

v0 (mg/(g·min))

R2

17.29±0.140

0.963

0.005±0.001

18.66±0.072

1.741±0.466

0.992

0.084±0.008

17.03±0.032

0.975

0.009±0.001

17.87±0.133

3.134±0.474

0.991

0.013±0.001

4.797±0.078

0.963

0.007±0.001

4.992±0.112

0.174±0.006

0.953

0.233±0.010

17.78±0.004

0.956

0.022±0.001

18.57±0.015

7.587±0.454

0.990

0.154±0.013

17.24±0.124

0.954

0.013±0.001

18.20±0.148

4.306±0.008

0.993

0.212±0.001

1.98±0.050

0.171

0.023±0.002

0.149±0.016

0.979

SC R

0.057±0.009

2.525±0.030

K 1 and K2 were the kinetic removal rate constants, qt expressed the removal quantity at different

A

a

R2

N

Fe3C (pH 8) Fe0 (pH 8) Biochar (pH 8) Fe3C (pH 4) Fe0 (pH 4) Biochar (pH 4)

qe (mg/g)

IP T

K 1 (min-1)a

Pseudo second-order kinetic t/ qt=1/K2 q e 2 +t/q e =1/v0+t/q e

U

Sample

Pseudo first-order kinetic qt= q e [1-exp(-K 1 t)]

A

CC E

PT

ED

Values are mean ± standard deviation.

M

contact time, qe represented the removal amount at equilibrium, v0 was the removal rate [23].

37