Efficient peroxymonosulfate activation and bisphenol A degradation derived from mineral-carbon materials: Key role of double mineral-templates

Journal Pre-proof Efficient peroxymonosulfate activation and bisphenol A degradation derived from mineral-carbon materials: key role of double mineral-templates Shanshan Yang (Conceptualization) (Investigation) (Data curation) (Writing - original draft), Xiaodi Duan (Conceptualization) (Investigation) (Writing - review and editing), Junqin Liu (Investigation) (Data curation), Pingxiao Wu (Methodology) (Funding acquisition) (Writing - review and editing), Chunquan Li (Formal analysis) (Writing - review and editing), Xiongbo Dong (Investigation) (Visualization), Nengwu Zhu (Funding acquisition) (Supervision), Dionysios D. Dionysiou (Methodology) (Funding acquisition) (Writing - review and editing)

PII:

S0926-3373(20)30116-8

DOI:

https://doi.org/10.1016/j.apcatb.2020.118701

Reference:

APCATB 118701

To appear in:

Applied Catalysis B: Environmental

Received Date:

16 October 2019

Revised Date:

24 December 2019

Accepted Date:

27 January 2020

Please cite this article as: Yang S, Duan X, Liu J, Wu P, Li C, Dong X, Zhu N, Dionysiou DD, Efficient peroxymonosulfate activation and bisphenol A degradation derived from mineral-carbon materials: key role of double mineral-templates, Applied Catalysis B: Environmental (2020), doi: https://doi.org/10.1016/j.apcatb.2020.118701

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Efficient peroxymonosulfate activation and bisphenol A degradation derived from mineral-carbon materials: key role of double mineral-templates Shanshan Yanga,b, Xiaodi Duanb, Junqin Liua, Pingxiao Wua,c*, Chunquan Lid, Xiongbo Dongb,d, Nengwu Zhua,c, Dionysios D. Dionysioub,* College of Environment and Energy, South China University of Technology, Guangzhou 510006,

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a

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P.R. China

Environmental Engineering and Science Program, Department of Chemical and Environmental

The Key Lab of Pollution Control and Ecosystem Restoration in Industry Clusters, Ministry of

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Education, Guangzhou 510006, P.R. China

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Engineering, University of Cincinnati, Cincinnati, OH 45221-0012, USA

School of Chemical and Environmental Engineering, China University of Mining and

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Technology (Beijing), Beijing 100083, P.R. China

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Graphical abstract

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Highlight 

BPA can be removed rapidly by NPC-FeOOH/Mt activated PMS.

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The synergistic effect of double mineral-templates plays a key role on NPC-FeOOH/Mt structure.

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Activation mechanism includes free radical and non-radical pathways.

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Abstract

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This study aimed to explore the catalytic performance of mineral-carbon materials towards

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peroxymonosulfate (PMS) activation and investigate the synergistic effect of different types of minerals on the structure of the formed carbon materials. Tetracycline (TC) was used as carbon

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source, and montmorillonite (Mt), goethite (FeOOH), and goethite pillared montmorillonite

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(FeOOH/Mt) were used as different types of minerals. With the aid of double mineral-templates, the obtained mineral-carbon materials (FeOOH/Mt-TC-C) showed superior activation ability

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towards PMS for bisphenol A degradation with pseudo-first-order kinetic constant (k) of 0.1151 min-1. The active constituent is the formed carbon material (NPC-FeOOH/Mt). The synergistic effect of double mineral-templates played a key role on the structure of NPC-FeOOH/Mt, which

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exhibited an ultra-thin carbon nanosheet structure with hierarchical pores (average pore volume of 1.99 cm2∙g-1), various defective sites (C=O and graphitic N) and good electric conductivity. Activation mechanism of NPC-FeOOH/Mt towards PMS includes free radical and non-radical pathways.

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Keywords: Peroxymonosulfate; Double mineral-templates; Nitrogen-doped; Hierarchical pores; Water treatment

1 Introduction

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Water pollution caused by endocrine disruption chemicals has attracted increasing attention.

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Bisphenol-A (BPA), a plasticizer, is widely used in plastic industry and has been detected in various environmental media including water and sediment [1]. BPA is also a typical kind of

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endocrine disruption chemical, which has negative effect on environment and human health. Hence,

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it is essential to removal BPA from the environment. Advanced oxidation processes (AOPs) have

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exhibited promising prospect in water treatment. Peroxymonosulfate (PMS) as a promising oxidizing agent has been widely studied in AOPs due to its excellent stability and good oxidation

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ability [2-4].

Clay minerals (montmorillonite, sepiolite, kaolinite, etc.) and iron oxyhydroxide minerals (goethite, hematite, etc.) are the two main types of minerals in soil and have been extensively

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investigated in environment remediation [5-9]. Most clay minerals possess a layered or microporous structure and have unique characteristics such as chemical and mechanical stability, expansible interlayer space, high cation exchangeable ability, and negatively charged layers [10]. Organic contaminants can be adsorbed on their surface or intercalated into their layer through surface complexation, cation exchange, and electrostatic attraction [11, 12]. Besides, iron

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oxyhydroxide minerals are also of great importance for regulating the migration of contaminants in natural environment owing to their high reactivity and surface area [13]. The adsorption of organic contaminants on iron oxyhydroxide minerals is mainly ascribed to chemical complexation between their versatile oxygen groups [14]. Therefore, clay and iron oxyhydroxide minerals can

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both act as appropriate adsorbents for accumulation of organic contaminants. In addition, organic compounds on the surface of minerals can serve as carbon source to

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prepare carbon materials when using minerals as inorganic templates during pyrolysis process [15,

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16]. Most of previous studies used polymers intercalated into clay minerals as carbon source to

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synthesize mineral-carbon materials, which would increase the production cost and is not beneficial for scale-up production [17, 18]. It should be noted that organic contaminants

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accumulated on minerals can also serve as carbon source to form carbon material, and the environmental risk of organic contaminants/mineral composite would also be reduced significantly

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with the complete conversion of organic contaminants [19]. The physicochemical properties of the obtained mineral-carbon composites, especially for the formed carbon materials, are highly dependent on the structure of minerals. For example, the template of iron nanoclusters can also

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play the role of porogen to produce carbon materials with high surface area and abundant large micropores due to the phase conversion of iron oxide [16]. Hongping He’s group reported that the layer structure of montmorillonite is beneficial to carbonize organic compounds to form twodimensional layered graphene-like carbon materials [20, 21]. However, the synergistic effect of different types of minerals on the structure of the formed carbon materials has been rarely 4

evaluated. The application of the obtained mineral-carbon composites in environment remediation has also been investigated and mainly focused on adsorption[22, 23]. The catalytic performance of mineral-carbon composites in PMS-based AOPs should also be investigated to expand the

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application of mineral-carbon composites to environment remediation. In this study, montmorillonite (Mt) and goethite (FeOOH) were chosen as representatives of clay and iron

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oxyhydroxide minerals, respectively. Adsorption experiments were conducted to load organic

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contaminants on mineral surfaces, which were then pyrolyzed under N2 atmosphere to fabricate

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mineral-carbon composites. Our main objectives were: (1) to explore the catalytic performance of mineral-carbon composites in PMS-based AOPs; (2) to distinguish the active constituent of

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mineral-carbon composites towards PMS activation; and (3) to explore the synergistic effect of double mineral-templates on the structure of the formed carbon materials. The results of this work

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will not only provide a significant insight into the effect of double minerals-template on the physicochemical property of the formed carbon materials, but also expand the applications of mineral-carbon composites and the formed carbon materials in environment remediation.

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2 Experiment methods

2.1 Reagents and materials Details of all reagents and materials were described in Supporting Information (SI).

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2.2 Pyrolysis of organic contaminants/mineral composite The preparation of organic contaminants/mineral composites was conducted by adsorption experiments. In this study, tetracycline (TC) was used as carbon source, and Mt, FeOOH and goethite pillared montmorillonite (FeOOH/Mt) were used as different types of mineral templates.

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Typically, 0.2 g mineral was added into a 50 mL centrifuge tube with 25 mL of TC solution at 4 g·L-1. Second, centrifuge tubes were shaken horizontally at 180 rpm and 30 ± 0.5 °C for 24 h.

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Lastly, the obtained organic contaminants/mineral composites were collected by centrifugation

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and dried at 80 °C. The amount of TC adsorbed on different minerals was 379.37, 154.08 and

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215.56 mg∙g-1 for Mt, FeOOH and FeOOH/Mt, respectively, as shown in Fig. S1. The pyrolysis of organic contaminants/mineral composites was performed by tube furnace with a predetermined

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temperature program under N2 atmosphere. The initial temperature of the tube furnace was set at 30 ℃, then increased to 700 ℃ at a heating rate of 5 °C∙min-1 and held at 700 °C for 180 min. The

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pyrolysis products of organic contaminants/mineral composites were labelled as Mt-TC-C, FeOOH-TC-C and FeOOH/Mt-TC-C, respectively. Besides, the pyrolysis of mineral including Mt, FeOOH and FeOOH/Mt was also conducted under the same condition, which were labelled as Mt-

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C, FeOOH-C, and FeOOH/Mt-C, respectively. Etching experiments were used to liberate the formed nitrogen-doped porous carbon material from mineral-carbon composites, which were named as NPC-Mt, NPC-FeOOH and NPC-FeOOH/Mt. In addition, the pyrolysis of TC was also conducted to prepare carbon material under the same condition, which was labelled as NC. Detailed information of etching experiment was described in SI. 6

2.3 Catalytic performance of the pyrolysis products of mineral-organics BPA was chosen to estimate the activation ability of mineral-carbon composites towards PMS. Detailed information of degradation experiments and radical scavenger experiments were described in SI. NPC-FeOOH/Mt was recycled by filtration, washed with ultrapure water for five

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times, and then dried for the next use and characterization. Ultraperformance liquid chromatography-quadrupole-time-of flight premier mass spectrometer (UPLC-Q-TOF/MS,

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Waters Corp., Milford, MA, USA) and gas chromatography-mass spectrometry (GC/MS-QP2010

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Ultra, Shimadzu, Japan) were used to identify the degradation products of BPA. Besides, detailed

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description of degradation experiments, measurement of BPA and PMS, electron paramagnetic resonance spectrometry (EPR) analysis, and GC/MS analysis methods were the same as our

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previous study [24]. The concentration of furfuryl alcohol was measured by high performance liquid chromatograph (HPLC, SHIMADZU 2030C 3D) with an Athena C18 column (150×4.6 mm,

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5μm). The mobile phase was methanol/0.1% phosphoric acid mixture (60: 40, V/V) with a flow rate at 1.0 mL∙min−1 under column temperature at 30 ℃. Detailed information of UPLC-QTOF/MS is provided in SI.

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2.4 Characterization of the formed graphene-like carbon materials Characterization of the formed carbon materials was conducted to investigate the effect of the

type of mineral on their structure. The (i) chemical composition, (ii) morphological characteristics, (iii) specific surface area and pore-size distribution, (iv) crystalline structure, and (v) surface chemical states were measured by (i) C/H/N elemental analysis, thermo-gravimetric analysis (TG) 7

and inductively coupled plasma-optical emission spectroscopy (ICP-OES), (ii) high resolution transmission electron microscopy (TEM), (iii) N2 adsorption/desorption isotherm analysis, (iv) Xray power diffraction (XRD) analysis, and (v) X-ray photoelectron spectroscopy (XPS), respectively. Electrochemical measurements, including electrochemical impedance spectroscopy

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(EIS) and chronoamperometric measurement, were also conducted to explore the electrochemical properties of the formed carbon materials.

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3 Results and discussion

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3.1 Catalytic performance of the mineral-carbon materials

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Our previous study [25] reported that FeOOH and TC in FeOOH/Mt-TC could be converted

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into mixed-valence iron oxide and graphene-like carbon materials, respectively. FeOOH/Mt-TCC exhibited good catalytic performance for BPA degradation in Fenton system owing to the

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synergistic effect between Fe3O4 and graphene-like carbon materials. As depicted in Fig. 1a and d, FeOOH/Mt-TC-C also exhibited effective activation performance towards PMS for BPA degradation with pseudo-first-order kinetic constant (k) of 0.1151 min-1. To further explore the

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active constituent of FeOOH/Mt-TC-C for efficient BPA degradation, control experiments were conducted. From Fig. 1a, Mt-C, FeOOH-C, Mt/FeOOH-C, and NC exhibited poor activation performance toward PMS, indicating that the pyrolysis product of minerals or organic compounds (TC) alone could not act as the active constituent. NPC-FeOOH/Mt, which etched from FeOOH/Mt-TC-C, presented excellent catalytic activity towards PMS for rapid BPA degradation.

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Moreover, k of NPC-FeOOH/Mt (0.4589 min-1) was four times higher than that of FeOOH/MtTC-C (0.1151 min-1). Therefore, the active constituent of FeOOH/Mt-TC-C is the formed carbon material (NPC-FeOOH/Mt), which was generated from TC with the aid of double mineraltemplates during the pyrolysis process.

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To explore the influence of mineral-template on the activation ability of mineral-carbon composites towards PMS, the catalytic activities of Mt-TC-C and FeOOH-TC-C were firstly

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determined (Fig. 1b and d). The k of studied materials in PMS-based system ranked in the order

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of FeOOH/Mt-TC-C (0.1151 min-1) > FeOOH-TC-C (0.0507 min-1) > Mt-TC-C (0.007 min-1).

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Subsequently, the catalytic performance of the formed carbon materials in Mt-TC-C and FeOOHTC-C was measured. From Fig. 1c, both the adsorption of the formed carbon materials and the

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oxidation of PMS contributed to the removal of BPA. The removal efficiency of BPA by NPCFeOOH/Mt was the best and followed by NPC-FeOOH and NPC-Mt. These results indicated that

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the catalytic activity of mineral-carbon composites was highly dependent on the catalytic activity of the formed carbon materials. Moreover, the type of minerals presented great effect on the structure of the formed carbon materials, resulting in different catalytic performances towards

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PMS activation.

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Fig. 1 Removal of BPA by FeOOH/Mt-TC-C, Mt-C, FeOOH-C, FeOOH/Mt-C, NC and NPC- FeOOH/Mt in PMS-based system (a); Removal of BPA by FeOOH/Mt-TC-C, Mt-TC-C and FeOOH-TC -C in PMS-based

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system (b); Removal of BPA by NC, NPC-Mt, NPC-FeOOH and NPC-FeOOH/Mt in PMS-based system (c); Pseudo first order kinetic curves of BPA degradation by the pyrolysis product of mineral-organics composites and the formed carbon materials in PMS-based system (d). Conditions: dosage of FeOOH/Mt-TC-C, Mt-C, FeOOH-C, FeOOH/Mt-C: 0.1 g∙L-1; dosage of NC, NPC-Mt, NPC-FeOOH and NPC-FeOOH/Mt: 0.05 g∙L-1;

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PMS: 1mM; pH: 6.0; Temperature: 30 ℃.

3.2 Effects of mineral types on the structure of formed carbon materials The content of carbon materials in mineral-carbon composites and their thermal stabilities

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were investigated by TG. As shown in Fig. 2, after calcination in air atmosphere, the weight loss was 33.42%, 2.33%, and 32.53% for Mt-TC-C, FeOOH-TC-C, and FeOOH/Mt-TC-C, respectively. This result showed that the conversion efficiency of TC towards carbon materials with the template of FeOOH was lower than that with the template of Mt. Besides, compared with Mt-TC-C, significant weight loss occurred at around 400 ℃, which indicated the thermal stability 10

of mineral-carbon composites improved with the template of FeOOH. Chemical composition of the formed carbon materials was measured by C/H/N elemental analysis and ICP-OES. As shown in Table S1, all samples were mainly composed by C, H, and N. Meanwhile, trace of Si and Fe were detected in NPC-Mt, NPC-FeOOH and NPC-FeOOH/Mt, owing to their incomplete removal

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during etching.

Fig. 2 TG profiles of Mt-TC-C, FeOOH-TC-C, and FeOOH/Mt-TC-C

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Based on the results of TEM, the morphologies of the formed carbon materials varied greatly with different mineral-templates. As exhibited in Fig. 3a, TEM image of NC showed a flake-like carbon nanosheet structure with a high degree of aggregation. The image of NPC-Mt (Fig. 3b)

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exhibited graphene-like crinkled structure with wrinkles at the edge of carbon materials. The wrinkles originated from the structural defects after the doping of nitrogen into carbon framework [26]. Besides, the degree of aggregation decreased significantly because the template of Mt was beneficial to exfoliate the carbon layers. NPC-FeOOH presents many pores in carbon framework, as indicated by the blue circles (Fig. 3c). FeOOH is converted into mixed-valence iron oxide after 11

pyrolysis, which might further act as activating porogen to induce the porous structure [27]. In addition, a tiny proportion of iron particles (as labelled with yellow rectangles) can be still observed. These particles are well anchored within the interior of graphitic carbon layers. This is consistent with the result of ICP-OES analysis (Table S1). As depicted in Fig. S2b-c, the thickness

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of NPC-FeOOH/Mt was measured to be about 1.6 nm. With the double mineral-templates of FeOOH and Mt, NPC-FeOOH/Mt (Fig. 3d) exhibits an ultra-thin carbon nanosheet structure with

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abundant wrinkles and pores. As depicted in HRTEM image of the edge of carbon aggregates (Fig.

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3e), some anchored iron particles of much smaller size are observed compared with NPC-FeOOH.

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This can be attributed to the presence of the layer template of Mt which can enhance the dispersion of iron particles within the interior of graphitic carbon layers. The crystallographic characteristics

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of NPC-FeOOH/Mt were further investigated by selective-area electron diffraction analysis (SAED, Fig. 3f), which showed broad and dispersed ring patterns corresponding to (002), (101)

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and (110) facets of graphite. Meanwhile, the HRTEM of NPC-FeOOH/Mt (Fig. S2a) further revealed the (002) facet of graphite with a lattice fringe of 0.34 nm, further confirming the formation of multiple graphitic carbon layers [28]. The high angle annular dark field scanning

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TEM (HAADF-STEM) image demonstrated the tiny proportion of iron particles on NPCFeOOH/Mt. Besides, as shown in Fig. 3g, the elements of C, N, O, and Fe were evenly dispersed on NPC-FeOOH/Mt. The signal of Fe was very weak, which could not be observed clearly, demonstrating that the etching process effectively realized the dissolution and leaching of metal elements. 12

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Fig. 3 TEM images of NC (a), NPC-Mt (b), NPC-FeOOH (c), and NPC-FeOOH/Mt (d); HRTEM (e),

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SAED (f) and HAADF-STEM (g) images of NPC-FeOOH/Mt.

XRD analysis was conducted to investigate the crystallographic and structural characteristics

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of the formed carbon materials (Fig. S3). The XRD spectrum of NC did not present any characteristic peaks, suggesting that NC comprised of amorphous carbon. NPC-FeOOH/Mt exhibited a broad peak positioned approximately at 26.58˚, corresponding to the (002) crystalline

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facet of graphite (JCPDS, No. 41-1487), which demonstrated the formation of graphene-like carbon structure. Among all the samples, the peak at 26.58˚ for NPC-Mt was the most strong and sharp, confirming that the layer template of Mt was beneficial to carbonize organics to form carbon materials of high extent of graphitization. For NPC-FeOOH, the peak at 26.58˚ was the broadest and shifted to lower angle, which indicated that the porous structure generated from the phase 13

conversion of FeOOH could produce structural defects of graphene-like carbon and lead to weak crystallinity. In addition, the peak at around 44.96˚ indexed to (101) facet of graphite was too weak to be identified in all samples, which might be ascribed to the decreased crystallinity caused by the doping of nitrogen into carbon framework and the presence of amorphous carbon in the formed

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graphene-like carbon materials [29]. The surface areas of NC and NPC-FeOOH/Mt were determined by N2 adsorption/desorption

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isotherms, as depicted in Fig. 4a. NC could hardly adsorb N2 in the relative pressure (P/P0) range

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of 0.0-1.0, suggesting its low surface area. The use of double mineral-templates of Mt and FeOOH

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resulted in significantly higher surface area of the synthesized composite NPC-FeOOH/Mt. Besides, the isotherm curves of NPC-FeOOH/Mt exhibited an upward trend at P/P0 < 0.4, slow

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adsorption in the range of 0.4 < P/P0 < 0.8, and rapid trend at P/P0 > 0.8 with a clear H3 hysteresis loop at P/P0 > 0.6. Based on IUPAC classification, NPC-FeOOH/Mt presented a hybrid type Ⅰ/Ⅳ

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profile, indicating a hierarchical pore size distribution. The pore size distribution of NPCFeOOH/Mt (Fig. 4b) further confirmed that the hierarchical porous graphene-like carbon materials contained both large micropores (distribution pore size at 1.5-4 nm) and mesopores (distribution

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pore size at 4-40 nm). Furthermore, the hierarchical porous structure would produce more defect sites at the edges of graphene-like carbon framework [30]. To explore the role of different mineral-templates in the increased surface area and hierarchical pore size distribution of NPC-FeOOH/Mt, the surface areas and pore size distributions of NPC-Mt and NPC-FeOOH were also measured. NPC-Mt presented a type Ⅳ N2 adsorption14

desorption isotherms with a clear H3 hysteresis loop at P/P0 of 0.5-0.9, indicating the presence of mesopores. Based on the results of TEM, the slit mesopores were created by the stacking of layered graphene-like carbon materials, which was consistent with the layer template of Mt [21]. The isotherm curves of NPC-FeOOH exhibited a similar upward trend with that of NPC-FeOOH/Mt.

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However, the pore size distribution of NPC-FeOOH indicated formation of both micropores (peak pore at 2-6 nm) and mesopores (peak pore at 6-30 nm), which were generated from the phase

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conversion of FeOOH [27] and the leaching of iron oxides by the etching treatment. Hence, we

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speculated that the pores of NPC-FeOOH/Mt were generated from the stack of layered carbon

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materials, phase conversion of FeOOH, and the etching process. Compared with NPC-Mt and NPC-FeOOH/Mt, the formation of hierarchical porous structure of NPC-FeOOH/Mt was ascribed

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to the synergistic effect of double mineral-templates. Due to the exfoliation of layer template of Mt, the aggregation degree of FeOOH decreased and the size of formed iron oxide particles became

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smaller. Therefore, smaller iron oxide particles would generate more pores and smaller pore size when acted as an inner template and activating porogen during the pyrolysis process. The detailed pieces of information about the textural properties of NC, NPC-Mt, NPC-FeOOH, NPC-

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FeOOH/Mt are listed in Table S1. The significantly enhanced surface area and hierarchical porous structure of NPC-FeOOH/Mt could facilitate the accumulation of contaminants or PMS on the NPC-FeOOH/Mt surface and improve the removal efficiency of contaminants by activated PMS [31, 32].

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Fig. 4 Nitrogen adsorption-desorption isotherms (a) and pore size distribution curves (b) of NC, NPC-Mt,

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NPC-FeOOH and NPC-FeOOH/Mt

As depicted in Fig. S4, the Raman spectra of the obtained carbon materials showed similar

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patterns with two characteristic peaks at around 1349 cm-1 and 1585 cm-1, which are ascribed to

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the D and G bands, respectively. The intensity ratio of D and G bands (ID/IG) was also calculated

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to evaluate the structural defects and graphitization degree of all samples [33]. Compared with NC, the ID/IG of NPC-Mt decreased, indicating the template of Mt was beneficial to improve the

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graphitization degree of carbon materials. The ID/IG of NPC-FeOOH/Mt was a little higher than that of NPC-Mt, which could be explained by that NPC-FeOOH/Mt with hierarchical porous structure presented more defective sites. Meanwhile, the ID/IG values of NPC-FeOOH/Mt and NC

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were the same, suggesting a consistent carbon molecular skeleton. XPS spectra were obtained to determine the surface element composition and chemical states

of the obtained carbon materials. Three characteristic peaks located at 285, 400, and 532 eV were observed in the wide scan spectra (Fig. S6), corresponding to C1s, N1s, and O1s, respectively. The presence of N further confirmed that N heteroatoms were successfully doped into graphitic carbon 16

framework and NPC-FeOOH/Mt exhibited the highest N content among all the samples. Compared with NC, the increased oxygen content of NPC-FeOOH indicated that the template of FeOOH facilitated the generation of structural defects, such as oxygen groups. The presence of different oxygen groups also promoted the adsorption of BPA on the obtained carbon materials

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[34]. The inset table in Fig. S6 reports data on the presence of trace amounts of Fe in NPC-Mt, NPC-FeOOH, and NPC-FeOOH/Mt. This shows some iron particles could not be removed during

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etching, which is consistent with the TEM results. High-resolution C1s and N1s XPS spectra were

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also obtained to get more insights on functional groups. For NC and NPC-Mt (Fig. 5a-b), the C

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signals could be divided into three types of components with binding energies at 284.6, 285.6, and 289 eV, which correspond to C-C bond, C-O-C/C-N functional group, and π-π shake-up,

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respectively. For NPC-FeOOH and NPC-FeOOH/Mt (Fig. 5c-d), a new component at around 287.0 eV corresponding to electron-donating groups (C=O) was observed, which could serve as

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Lewis basic sites due to its high redox potential [35]. In addition, the FT-IR spectra of NPCFeOOH and NPC-FeOOH/Mt (Fig. S5) showed that a weak and broad peak located at 1720-1660 cm-1, which was corresponded to the stretching vibration of C=O [36]. Hence, the presence of

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transition metal (Fe) in template of FeOOH was beneficial to generate C=O at the edges of graphene-like carbon materials. As depicted in Fig. 5e-h, two kinds of N bonding configuration including pyridinic N (398.3 eV) and graphitic N (400.5 eV) were observed in all four samples. Graphitic N has smaller radius and stronger electronegativity compared with pyridinic N, which could modulate the electron density of neighbor carbon configuration and circulate the electron 17

flow [37]. With the simultaneous aid of Mt and FeOOH template, NPC-FeOOH/Mt exhibited the highest content of graphitic N, highest level of structural defects, and appropriate amount of electron-donating C=O group. The half-peak width of the characteristic XPS peak in all samples

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are listed in Table S2.

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The electrical conductivity of the formed graphene-like carbon materials was further elucidated by EIS. The Nyquist plots of all samples as well as the equivalent circuit model are shown in Fig. 6. The equivalent circuit consisted of an ohmic resistance (Ro), a charge transfer resistance (Rct), and a Warburg impedance (W) [38]. Based on the results of EIS analysis, the

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electrical conductivity of the four materials ranked in the order of NC ≈ NPC-Mt > NPCFeOOH/Mt ≈ NPC-FeOOH. This result indicated that the increased structural defects had a

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negative effect on electrical conductivity. The good electrical conductivity of NPC-FeOOH/Mt

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enabled this material to act as an electron mediator to accelerate electron transfer between

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contaminants and PMS, inducing non-radical oxidation route. Fig. 7 shows an illustration of the formed nitrogen doped hierarchical porous carbon materials with the aid of double mineral-

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template during the pyrolysis process.

Fig. 6 Nyquist plots of NC, NPC-Mt, NPC-FeOOH and NPC-FeOOH/Mt and the corresponding equivalent circuit

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double mineral-template during pyrolysis process

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Fig. 7 Illustration of the formation of hierarchical porous N-doping graphene-like materials with the aid of

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3.3 Identification of free radical and non-radical pathway in NPC-FeOOH/Mt

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- PMS system

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Carbonaceous materials have been widely investigated when employed as catalysts to activate PMS. The physicochemical properties of carbonaceous materials, such as defective sites,

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heteroatom doping, and electrical conductivity, play a vital role on their activation performance and induce different activation mechanisms, including free radical and non-radical pathways [35,

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36, 39, 40].

Firstly, EPR analyses using 5,5-dimethyl-1-pyrrolidine N-oxide (DMPO) or 2,2,6,6tetramethyl-4-piperidone (TEMP) as spin trap agents were conducted to elucidate the formation of

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predominant reactive species. As shown in Fig. 8a, no signal could be detected for PMS alone. The signals of DMPO-OH adduct (the intensities of 1:2:2:1) were observed in NPC-FeOOH/MtPMS system and the peak intensity increased with the increasing reaction time. However, the signals of DMPO-SO4 were not found within 30 s of reaction time. After 1 min, the signals of DMPO-SO4 could be also detected. Hence, •OH and SO4•− could be generated only by activated 21

PMS. From Fig. 8b, similar phenomena were observed. With the addition of PMS or NPCFeOOH/Mt alone, no signal was detected. In NPC-FeOOH/Mt-PMS system, a characteristic triplet of TEMP-1O2 signal was also observed (the intensities of 1:1:1), indicating the generation of 1O2 [41]. The intensity of TEMP-1O2 signal strengthen as time went by, which suggested the amount

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of 1O2 also increased with the increasing time. Various radical scavenger experiments were further conducted to elucidate the type of radicals

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generated in NPC-FeOOH/Mt-PMS system (Fig. 8c). Dimethylsulfoxide (DMSO) was used to

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quench the surface-bound active species. The significant inhibitory phenomenon of DMSO (5 mM)

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indicated that the accumulation of BPA or PMS on the surface of NPC-FeOOH/Mt was important for BPA degradation by activated PMS [31]. Furthermore, with the addition of various quenchers

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(1M TBA, 1M phenol and 1 mM p-BQ), all degradation efficiencies of BPA decreased, indicating the contribution of free radicals to BPA degradation. As depicted in Fig. S7a, k decreased from

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0.1984 to 0.1755, 0.1214 min-1 with the addition of TBA and phenol, respectively. Moreover, the inhibitory effect of phenol was more obvious than TBA, which showed that both •OH and SO4•− were produced in NPC-FeOOH/Mt-PMS system. Interestingly, k increased from 0.1984 to 0.2265

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min-1 with the addition of 1 mM p-BQ. This is because PMS could be activated by p-BQ to form 1

O2 due to its ketone structure containing two carbonyl groups [42]. To demonstrate whether 1O2 was generated during PMS activation, oxidation degradation of

furfuryl alcohol (FFA), as a 1O2 indicator [39], was conducted in NPC-FeOOH/Mt-PMS system. As depicted in Fig. 8d, almost 12% FFA were oxidized by PMS alone after 10 min reaction time, 22

suggesting the oxidation ability of PMS towards FFA is limited. With the addition of NPCFeOOH/Mt in PMS system, the degradation of FFA become more rapidly and the removal efficiency was about 90%, which indicated the generation of 1O2. In addition, it has been verified that the lifetime of 1O2 can be markedly extended in D2O [43]. The degradation of BPA was

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conducted in 4:1 (V:V) D2O and H2O, respectively. From Fig. S7b, BPA was almost degraded by 2 min. Compared with the BPA degradation in water, BPA degradation efficiency was enhanced

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slightly. Therefore, it was speculated that 1O2 was generated in NPC-FeOOH/Mt-PMS system. 1O2

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is a non-radical reactive species due to its properties, such as milder oxidative potential, substrate-

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specificity and resistance to the generally used free radical quenchers [44]. Hence, the non-radical pathway should include the generation of 1O2.

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Besides, the excellent electrical conductivity of carbonaceous materials can endow such materials as electron mediators to induce non-radical pathway reactions [36]. Chronoamperometric

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measurements were conducted to verify the electron transfer process through non-radical pathway and results are presented in Fig. 8e. When a bare glassy carbon disk was used as the working electrode, there was no obvious change of current with the addition of BPA and PMS. This

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suggested that no redox reaction occurred between BPA and PMS. When NPC-FeOOH/Mt was used as the working electrode, a minor positive current formed with the addition of PMS or BPA alone, indicating the electron transfer between BPA (or PMS) and NPC-FeOOH/Mt. However, when PMS, NPC-FeOOH/Mt, and BPA coexisted, an obvious enhanced positive current flow was observed, which suggested the accelerated oxidation reaction and faster electron transfer on NPC23

FeOOH/Mt surface resulting from the establishment of ternary system. This result implied NPCFeOOH/Mt worked as an electron mediator in a non-radical pathway, which can mediate the electron transfer from BPA to PMS, leading to the oxidation of BPA [32, 45]. The consumption of PMS by NPC-FeOOH/Mt in the presence or absence of BPA was further

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measured to investigate the activation mechanism. As depicted in Fig. 8f, almost 50% PMS was consumed in the presence of only NPC-FeOOH/Mt. This indicated that PMS was activated by

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NPC-FeOOH/Mt to generate free radicals. With the addition of BPA, the consumption of PMS

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increased to 60%, indicating BPA could serve as an electron donor to improve PMS activation [32],

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which was well matched with results from the chronoamperometric measurements. Based on the above results, when PMS is mixed with NPC-FeOOH/Mt together, two pathways including free

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radical and non-radical will take place simultaneously.

24

Fig. 8 EPR spectra obtained by using DMPO (a) and TEMP (b) as spin-trapping agents in PMS/NPCFeOOH/Mt system with different reaction time; Removal of BPA with the addition of different kinds of scavengers in PMS/NPC-FeOOH/Mt system (c); Removal of FFA in PMS-based system (d); Chronoamperometric measurements with the addition of PMS or/and BPA in PMS-based system (e); Consumption of PMS in different PMS-based systems (f). Conditions of PMS-based system: dosage of NPCFeOOH/Mt: 0.05 g∙L-1; BPA: 20 mg∙L-1; FFA: 0.05 mM; PMS: 1mM; pH: 6.0; Temperature: 30 ℃.

3.4 Investigation of active sites in NPC-FeOOH/Mt and activation mechanism

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To explore the active sites in NPC-FeOOH/Mt towards PMS, five-cycle experiments were firstly conducted to investigate its performance for BPA removal in water. As depicted in Fig. 9a,

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the BPA removal efficiency decreased gradually with the increase in number of NPC-FeOOH/Mt

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reuse. k decreased from the first use to the fifth use of NPC-FeOOH/Mt, which suggested changes

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of both numbers and types of active sites (Fig. 9b). To explore the change of active sites before and after reaction, XPS spectra of the five times used NPC-FeOOH/Mt were measured. As shown

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in Fig. S8a-c, electron-donating C=O functional groups disappeared, and the content of nitrogen and graphitic N decreased from 4.67% and 3.71% to 0.82% and 0.42%, respectively. This indicated

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C=O and graphitic N were the main active sites for PMS activation. As a previous study reported, electron-donating C=O at the edges of graphene-like carbon materials could work as Lewis basic sites and donate the electrons to PMS to cleave the peroxide O-O bond, resulting in the production

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of •OH and SO4•−[35]. In addition, nitrogen doping in carbonaceous materials, especially for the successful doping of graphitic N, would improve the electron transfer between NPC-FeOOH/Mt and PMS to form the metastable intermediates of NPC-FeOOH/Mt-PMS, which would decompose and generate 1O2 [26, 46]. Therefore, we speculated that C=O and graphitic N were the main active sites of NPC-FeOOH/Mt, which induced the generation of •OH, SO4•− and 1O2 for BPA degradation. 25

Reactivation of the reused NPC-FeOOH/Mt was conducted by dichloromethane (DCM) desorption and recalcination in N2 atmosphere. The detailed information was described in SI. From Fig. 9c-d, the adsorption of BPA on NPC-FeOOH/Mt increased slightly after DCM desorption. However, the degradation rate of BPA did not increase, which indicated that the changes of active

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sites were the main reasons for the decreased catalytic performance. After recalcination in N2 atmosphere, both adsorption capacity and degradation rate of BPA increased, confirming that

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recalcination of reused NPC-FeOOH/Mt in N2 atmosphere was helpful to remove the adsorbed

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intermediates and regenerate the active sites. Change of NPC-FeOOH/Mt surface properties

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induced by different reactivation methods were measured by XPS. After DCM desorption, there was no obvious change of active sites (C=O, doping nitrogen and graphitic N) for NPC-FeOOH/Mt

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compared with five times used NPC-FeOOH/Mt (Fig. S8d-e). However, for NPC-FeOOH/Mt after recalcination in N2 atmosphere (Fig. S8f-g), the content of C=O, doping nitrogen, and graphitic N

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increased slightly, leading to a moderate enhancement of removal efficiency of BPA in PMS-based system. Hence, these results further demonstrated that active sites of NPC-FeOOH/Mt, such as

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C=O and doping nitrogen especially for graphitic N, played a vital role in BPA degradation.

26

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Fig. 9 Removal of BPA (a) and pseudo first order kinetic curves (b) by different used time of NPC-FeOOH/Mt

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in PMS-based system; Removal of BPA (c) and pseudo first order kinetic curves (d) by first used NPCFeOOH/Mt, fifth used NPC-FeOOH/Mt, reused after DCM desorption and reused after N2 recalcined in PMSbased system. Conditions of PMS-based system: dosage of NPC-FeOOH/Mt: 0.05 g∙L-1; BPA: 20 mg∙L-1; PMS: 1mM; pH: 6.0; Temperature: 30 ℃.

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The electrical conductivity of carbonaceous materials is very important for non-radical pathway. Therefore, the electrical conductivities of NPC-FeOOH/Mt after five times reused and regeneration by recalcination were measured through EIS analysis. Fig. S9 showed that the

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electrical conductivity of the five times reused NPC-FeOOH/Mt decreased significantly compared with that of the fresh NPC-FeOOH/Mt. After regeneration, the electrical conductivity improved. Hence, the gradual decrease of the degradation rate of BPA during reuse of NPC-FeOOH/Mt could be also attributed to the decrease of electrical conductivity of the material, and the electrical conductivity of NPC-FeOOH/Mt is an important property for PMS activation in non-radical 27

pathway. Based on these results, the activation mechanism of PMS by NPC-FeOOH/Mt is proposed in Fig. 10. First, BPA can be accumulated on the surface of NPC-FeOOH/Mt due to its high surface area and hierarchical porous structure. Then with the addition of PMS, both free radical and non-

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radical pathways are induced. In one pathway, active sites of NPC-FeOOH/Mt, such as C=O and graphitic N, donate electrons to PMS to cleave the peroxide O-O bond, resulting in the generation

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of •OH and SO4•− (free radical pathway). Then these free radicals could abstract electrons from

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BPA and lead to the oxidation of BPA. In another pathway, graphitic N may also become powerful

re

tools to extract electrons from PMS to form the metastable intermediates of NPC-FeOOH/MtPMS, which would further decompose and generate 1O2. Besides, NPC-FeOOH/Mt with excellent

lP

conductivity can act as electron mediator and enable PMS to abstract electrons directly from BPA,

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resulting in BPA oxidation (non-radical pathway).

Fig. 10 Proposed activation mechanism of PMS by NPC-FeOOH/Mt

28

3.5 Effects of several parameters on catalytic performance of NPC-FeOOH/Mt To evaluate the catalytic performance of NPC-FeOOH/Mt for BPA degradation in PMS-based process, the effects of several parameters (i.e., dosages of catalyst, concentration of PMS, initial pH) as well as the role of water quality matrix were explored. First, the effect of the dosage of

of

NPC-FeOOH/Mt on BPA degradation was evaluated. Results presented in Fig. S11a-b showed that increase of NPC-FeOOH/Mt dosage led to an enhancement of BPA removal efficiency. With the

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dosage increased from 0.02 to 0.1 g∙L-1, the k values increased from 0.0466 to 1.3771 min-1. This

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proved that higher catalyst dosage substantially accelerated the degradation kinetics by providing

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more active sites. The influence of PMS initial concentration was also investigated, as shown Fig. S11c-d. There were negligible differences in BPA degradation with the increase in PMS initial

lP

concentration, which could be ascribed to the limited active sites on NPC-FeOOH/Mt during PMS activation [47]. Moreover, the catalytic activities of PMS-based systems were reported to be

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dependent on initial solution pH [4]. As displayed in Fig. S11e-f, NPC-FeOOH/Mt still presented good removal ability for BPA at different initial pH in PMS-based system. The results indicated that the accumulation of BPA on NPC-FeOOH/Mt surface and the generation of reactive species

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were not affected by pH. Therefore, NPC-FeOOH/Mt can work within a wide pH range for oxidation of contaminants, presenting certain advantages in environmental remediation. Moreover, to explore the degradation of BPA by NPC-FeOOH/Mt-activated PMS in real groundwater, water samples from Pearl River (Guangdong in China) were used as testing matrix. The water quality parameters of Pearl River are listed in Table S3. As shown in Fig. 11a, the background water 29

constituents in Pearl River have minor effect on BPA degradation by activated PMS, indicating the promising application of this catalyst in real matrix (field) water treatment. To further evaluate the catalytic performance of NPC-FeOOH/Mt towards other kinds of contaminants, 2,4-dichlorophenol (2,4-DCP) and Orange G (OG) were also chosen as target

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contaminants. From Fig. 11b, NPC-FeOOH/Mt still exhibited excellent removal ability towards 2,4-DCP and OG in PMS-based system, including adsorption and degradation process. The k of

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2,4-DCP was the highest among the target contaminants and was twice higher than that of BPA.

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Besides, the catalytic activities of some commonly used catalysts, such as metal-based catalysts

re

(Co3O4 and Fe3O4) and certain carbonaceous materials (GO and CNT) [48], were also measured for BPA oxidation by activated PMS. Fig. 11c shows that the adsorption capacity and degradation

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rate of BPA by NPC-FeOOH/Mt were much higher than those of these catalysts under the same

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conditions. Therefore, NPC-FeOOH/Mt present excellent advantages in PMS activation.

Fig. 11 Effect of the background water constituents in Pearl River on the degradation of BPA (a); Removal of

other kinds of contaminants in PMS/NPC-FeOOH/Mt system (b); Removal of BPA by other kinds of catalysts in PMS-based system (c). Conditions of PMS-based system: dosage of catalyst: 0.05 g∙L-1; BPA: 20 mg∙L-1; DCP: 20 mg∙L-1; OG: 50 mg∙L-1; PMS: 1mM; pH: 6.0; Temperature: 30 ℃.

Transformation products of BPA degradation were tentatively identified by analyzing the m/z data from GC-MS and UPLC-Q-TOF/MS results. Five aromatic products were detected including 30

phenol (Cpd 1, m/z 93.03), p-benzoquinone (Cpd 2, m/z 108.02), hydroquinone (Cpd 3, m/z 110.05), 4-isopropenylphenol (Cpd 4, m/z 133.06), and the isomers of phenol 2,4 bis 1,1, dimethyl ethyl (Cpd 5, m/z 191.10), as shown in Fig. S11-12. . According to our previous study, •OH first attacks the C-C bond of isopropyl group between two phenyl groups in BPA, leading to the

of

formation of phenol radicals and isopropyl phenol radicals [25]. The attack of SO4•− on BPA occurs via direct electron transfer reaction with the formation of hydroxycyclohexadienyl radical, which

ro

then is converted into phenol radical and isopropyl phenol radical by β-scission reaction [49]. The

-p

H abstraction from the isopropyl phenol radical lead to the formation of 4-isopropenylphenol (m/z

re

133.06). Phenol radical also generates phenol (m/z 93.03), hydroquinone (m/z 110.05), pbenzoquinone (m/z 108.02), and the isomers of phenol 2,4 bis 1,1, dimethyl ethyl (m/z 191.10)

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with the attack of free radicals. In addition, 1O2 can decompose BPA rapidly through hydroxylation and β-scission reaction, which can produce phenol (m/z 93.03), and 4-isopropenylphenol (m/z

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133.06) [50]. Hence, the possible degradation pathways of BPA by free radicals are proposed in Fig. S13 according to the experimental results of this work and relevant information for previous studies [51, 52].

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4 Conclusions

In summary, with the aid of double mineral-templates during pyrolysis process, the obtained

mineral-carbon materials (FeOOH/Mt-TC-C) showed excellent activation ability towards PMS for BPA degradation with k of 0.1151 min-1. The active constituent of FeOOH/Mt-TC-C for PMS

31

activation could be ascribed to the formed graphene-like carbon material (NPC-FeOOH/Mt) generated from the conversion of TC. NPC-FeOOH/Mt exhibited an ultra-thin carbon nanosheet structure with the doping of nitrogen, hierarchical pores, many defective sites, and excellent electrical conductivity. The template of layer Mt was beneficial to overcome the aggregation of

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graphene-like carbon materials, enhance the graphitization degree, and improve the electrical conductivity. The phase conversion of FeOOH could act as an inner template and activating

ro

porogen to generate hierarchical porous structure with high surface area. Besides, the presence of

-p

transition metal (Fe) in template of FeOOH facilitated the generation of structural defects, such as

re

electron-rich C=O group. Activation mechanism of NPC-FeOOH/Mt towards PMS includes free radical and non-radical pathways. The good enrichment ability of BPA on the surface of NPC-

lP

FeOOH/Mt and the excellent electrical conductivity made NPC-FeOOH/Mt as an electron mediator to induce non-radical pathway. Active sites of NPC-FeOOH/Mt, such as C=O and

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graphitic N, donate electrons to PMS to generate •OH, SO4•−, and 1O2. In addition, NPCFeOOH/Mt still exhibited effective removal ability towards other kinds of contaminants, such as 2,4-dichlorophenol and Orange G. Hence, our work not only provides a significant insight into the

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synergistic effect of different types of mineral-template composites on the physicochemical property of the formed carbon materials, but also expands the potential applications of mineralcarbon composites and the formed carbon materials in environment remediation.

32

Authors contributiuon Shanshan Yang: Conceptualization, Investigation, Data curation, writing-original draft; Xiaodi Duan: Conceptualization, Investigation, Writing-review & editing; Junqin Liu: Investigation, Data curation, Pingxiao Wu: Methodology, Funding acquisition, Writing-review & editing; Chunquan Li: Formal analysis, Writing-review & editing; Xiongbo Dong: Investigation, Visualization; Nengwu Zhu: Funding acquisition, Supervision; Dionysios D. Dionysiou:

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Methodology, Funding acquisition, Writing - review & editing.

Declaration of interests

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The authors declare that they have no known competing financial interests or personal

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relationships that could have appeared to influence the work reported in this paper.

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Acknowledgements

The authors are grateful for financial support from the National Natural Science Foundation

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of China (Grant No. 41972037, 41673092 and 41472038), the Science and Technology Plan of Guangdong Province, China (Grant No. 2016B020242004), Guangdong special support program for millions of leading engineering talents (Grant No. 201626011). The first author is also grateful

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for financial support from the China Scholarship Council (201806150128). D. D. Dionysiou also acknowledges support from the University of Cincinnati through a UNESCO co-Chair Professor position on “Water Access and Sustainability” and the Herman Schneider Professorship in the College of Engineering and Applied Sciences.

33

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