High adsorptivity and recycling performance activated carbon fibers for Cu(II) adsorption

Journal Pre-proof High adsorptivity and recycling performance activated carbon fibers for Cu(II) adsorption

Junwei Yu, Chong Chi, Bo Zhu, Kun Qiao, Xun Cai, Yuan Cheng, Shuhan Yan PII:

S0048-9697(19)34403-1

DOI:

https://doi.org/10.1016/j.scitotenv.2019.134412

Reference:

STOTEN 134412

To appear in:

Science of the Total Environment

Received date:

23 June 2019

Revised date:

10 September 2019

Accepted date:

10 September 2019

Please cite this article as: J. Yu, C. Chi, B. Zhu, et al., High adsorptivity and recycling performance activated carbon fibers for Cu(II) adsorption, Science of the Total Environment (2018), https://doi.org/10.1016/j.scitotenv.2019.134412

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© 2018 Published by Elsevier.

Journal Pre-proof

High adsorptivity and recycling performance activated carbon fibers for Cu(II) adsorption Junwei Yu,1, 2 Chong Chi,1, 2 Bo Zhu,1, 2 Kun Qiao,3, * Xun Cai,4 Yuan Cheng,5 Shuhan Yan,1, 2 1 Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials (Ministry of Education), Shandong University, Jinan 250061, China 2 Carbon Fiber Engineering Research Center, School of Materials Science and Engineering, Shandong University, Jinan 250061, China

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3 School of Mechanical, Electrical & Information Engineering, Shandong University, Weihai

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264209, China

4 School of Computer Science and Technology, Shandong University, Jinan 250101, China

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5 Shandong University Library, Jinan 250061, China

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Correspondence to: K. Qiao, School of Mechanical, Electrical & Information Engineering, Shandong University, No. 180 Cultural West Road, Huancui District, Weihai.

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E-mail Addresses: [email protected] (K. Qiao)

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Abstract: In order to develop adsorbent materials with high Cu(II) adsorptivity and renewable recycling for Cu(II) ， nitric acid oxidation process is optimized and

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ameliorated by microwave and sonication to obtain an efficient modification and regeneration processes. Microwave-assisted nitric acid oxidation process has the most significant enhancement effect to the Cu(II) adsorptivity of activated carbon fiber felts (ACFFs), which can reach 23.13 mg/g and 4.55 times of pristine felts. It is due to this process can greatly increase the ultramicropore volume and polar oxygen-containing groups. In addition, sonication-assisted-pickling regeneration process achieves efficient regenerations and enhancements of Cu(II) adsorptivity for ACFFs. The Cu(II) adsorptivity and regeneration rate of ACFFs are still up at 25.51 mg/g and 379.59% after five times recycling by the process of sonication-assisted pickling regeneration process. 1 / 23

Journal Pre-proof Keywords: activated carbon fiber felts; nitric acid oxidation process; microwaveassisted; Cu(II) adsorptivity; recyclability 1 Introduction Cu(II) is a common harmful pollutant, excessive intake of Cu(II) may cause liver damage and acute poisoning in humans (Li and Bai, 2005; Hu et al., 2014). Governments have developed uniform Cu(II) emission standards, such as 2 mg/L in the US and 3 mg/L in Japan, and this standard will continue to raise with the need of

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human health. The field of Cu(II) purification faces a series of problems such as

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difficult to purify, high cost and non-recyclable (Wang et al., 2012; Li et al., 2012).

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The most promising purification process is the adsorption method, while activated

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carbon fiber felts (ACFFs) is highly efficient for specific adsorption (Macíasgarcía et al., 2017; Bhatnagar et al., 2013; Huang and Su, 2010).

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Sorption mainly includes physisorption derived from the adsorption potential of the

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pore structure and chemisorption derived from the polar action of the surface chemical group (Yu et al., 2018; Julien et al., 1998). Activated carbon fiber felts

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(ACFFs), as a porous adsorbent based on microporous structure, have a large number of micropores and abundant oxygen-containing groups on the surface, which determine its excellent physical adsorption properties and chemical adsorption properties (Kumar et al., 2015; Miyamoto et al., 2005). Its ultramicropores are basically the interlayer pore of graphite-like microcrystals and ultramicropores are mainly formed by the disorderly arrangement of graphite-like microcrystals. Therefore, porous adsorbent materials with high adsorption properties for specific adsorbents can be obtained by oxidation modification, which can enhance the content of pore structures and surface polar functional groups simultaneously (Kadirvelu et al., 2000). 2 / 23

Journal Pre-proof Chen et al. (2003) modified granular activated carbon with 1.0 M critic acid. The Cu(II) adsorptivity was enhanced by 140% and there was no research on recycling performance. Lu et al. (2017) modified activated carbon fibers by chemical grafting with amidoxime, which increased U(VI) adsorptivity by 172%. Mugisidi et al. (2007) modified activated carbon by treatment with 15% sodium acetate, which increased Cu(II) adsorptivity by 120%. For the application of specific ion adsorption, the existing modification methods of activated carbon fiber are relatively simple, and the

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research on regeneration process is less. Therefore, activated carbon fibers having

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high adsorption properties and recycling properties for specific metal ions are in

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urgent need to be exploited.

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The objective of this study is to investigate an effective modification process and regeneration process of ACFFs, which aims to enhance the Cu(II) adsorptivity and

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achieve recycling of ACFFs for the purification of Cu(II). In this article, ACFFs

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modified by nitric acid oxidation, ultrasonic-assisted nitric acid oxidation process and microwave-assisted nitric acid oxidation process were studied by using 50 mg/L Cu(II)

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aqueous solution as adsorbent, and the technological parameters such as nitric acid concentration, nitric acid dosage, oxidation time and oxidation temperature were optimized. Moreover, sonication-assisted-pickling regeneration process was studied for the regeneration and reutilization of ACFFs for Cu(II) adsorption. 2. Materials and methods 2.1 Chemicals and materials ACFFs with specific surface area of 1055.65 m2/g were provided by Carbon Fiber Engineering Technology Research Center, Shandong University. ACFFs was pretreated in a vacuum oven under the specific conditions of 0.1 MPa vacuum at the temperature of 403 K for 3 h before nitric acid oxidation process. All the chemical 3 / 23

Journal Pre-proof reagents

such

as

nitric

acid,

copper

sulfate

pentahydrate,

sodium

diethyldithiocarbamate, ammonia were provided by China Pharmaceutical Group Chemical Reagents Co., Ltd.. High pure nitrogen was provided by Jinan Deyang Gas Co., Ltd.. Deionized water was self-made in the laboratory. 2.2 Nitric acid oxidation process Under the condition of water bath heating, nitric acid oxidation modification of ACFFs (2 g for ACFFs dosage) were carried out in nitric acid solution with different

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concentrations (22%, 45%, 68%) and different dosages (10 mL, 20 mL, 30 mL). The

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effects of oxidation treatment time (1 h, 2 h, 3 h) and oxidation treatment temperature

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(303 K, 318 K, 333 K) on the Cu(II) adsorptivity of ACFFs were studied, and the

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optimum technological parameters of nitric acid oxidation process for improving the Cu(II) adsorptivity of ACFFs were explored. Moreover, the microwave-assisted nitric

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acid oxidation process and sonication-assisted nitric acid oxidation process were

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proposed and studied on the basis of the optimal nitric acid oxidation process. The samples of ACFFs treated by nitric acid oxidation process were named N-

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ACFF in this manuscript in this article. In detail, the ACFFs modified by the optimal nitric acid oxidation process under water bath heating conditions, microwave-assisted and sonication-assisted nitric acid oxidation process are recorded as NO-ACFF, NMACFF and NS-ACFF separately, and pristine felts was named as ACFF0. 2.2 Sonication-assisted-pickling regeneration process Sonication-assisted-pickling regeneration process is combined the traditional pickling regeneration process with the ultrasonic vibration regeneration method to obtain a more suitable regeneration process for metal-absorbed ACFFs. Three samples of ACFF0, NO-ACFF and NM-ACFF after 50 mg/L Cu(II) adsorption were used as research objects for regeneration process. 0.2 g of Cu(II)-absorbed ACFFs were 4 / 23

Journal Pre-proof placed in 500 mL of deionized water, 5 wt%, 10 wt% and 15 wt% nitric acid solution separately. After sealing, the sonication-assisted-pickling regeneration process was carried out in an ultrasonic system at a temperature of 298 K for 3 h with 100 W of ultrasonic power. Cu(II) re-adsorption performance test of regenerated ACFFs was carried out after sufficient washing and drying. 2.3 Evaluation of Cu(II) adsorptivity 2.3.1 Evaluation principle

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Ultraviolet-visible spectrophotometry can determine the absorbance of Cu(Ⅱ) in

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the wavelength range of 190-800 nm, according to Lambert-Beer law (Formula 1), the

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concentration of Cu(II) is proportional to the intensity of absorption wavelength. In

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equation 1, A is absorbance, transmittance T=I(intensity of emission) /I0(intensity of incident light), K is molar absorbance coefficient; c is concentration of absorbent (unit

(1)

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mol/L), L is thickness of absorption layer (in cm).

A chelating system of DDTC-Na-Cu(II) was established by using sodium

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diethyldithiocarbamate (DDTC-Na) as a chelating agent in ammonia environment (pH=9+1). According to the absorption spectra, it was found that the chelating system had the maximum absorption intensity at 452 nm. Furthermore, the absorbance of Cu(II) standard working fluids at different concentrations was determined in the wavelength range of 420-480 nm in this paper. The standard curve of absorbanceconcentration was drawn by taking the absorbance value at 452 nm as the ordinate and the concentration value of Cu(II) standard working fluids as the abscissa. 2.3.2 Evaluation of Cu(II) adsorptivity ACFFs were pretreated at a high temperature of 403 K for 3 h in a vacuum environment of 0.01 MPa before each evaluation tests of Cu(II) adsorptivity. 0.2 g 5 / 23

Journal Pre-proof ACFFs pretreated samples were placed in the Cu(II) standard working fluid of 50 mg/L (c0) with a volume of 200 mL. The samples were adsorbed in a constant temperature shaking table for 2 h at 333 K and the shaking speed used 120 rpm. Upon completion of adsorption, the absorbance value at 452 nm of the remaining Cu(II) solution was tested and the concentration of the remaining Cu(II) solution ct was calculated by equation 2. (2)

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Cu(II) adsorption value per ACFFs unit mass Q, Cu(II) removal efficiency R and

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regeneration rate η were calculated by equation 3-5 separately. In equation 3-5, m is

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the dosage of ACFFs in units of g, Qt and Q0 are Cu(II) adsorption values of ACFFs

2.4 Characterization

(3) (4) (5)

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before and after regeneration.

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Specific surface area and aperture distribution of ACFFs were measured by ASAP2020 automatic surface area and mesoporous/microporous analyzer. The setting of test parameters was referred to the national standard of China GB/T 19587-2004 and the pretreatment temperature was set at 405 K. The surface chemistry of ACFFs was analyzed by an X-ray Photoelectron Spectroscopy (XPS) spectrometer (ESCALAB-250, Thermo Fisher, USA), and it was performed on a PHI5700 ESCA system equipped with aluminum anode (Kα=1486.6 eV). All the binding energies were calibrated using contaminated carbon. During data collection, the pressure in the analysis chamber was maintained below 1.0×10-7 Pa. In addition, the fourier transform infrared spectra analysis was based on the TENSOR37 infrared 6 / 23

Journal Pre-proof spectroscopy from Bruker Company. 3 Results and discussion 3.1 Characterization of ACFFs The diameter of free Cu(II) in aqueous phase is 0.146 nm, and the physical adsorption of Cu(II) by ACFFs mainly comes from the adsorption potential energy of microporous structure. In this part, ultramicropores (0-0.7 nm) and supermicropores (0.7-2 nm) of ACFFs were analyzed by N2 adsorption test. As shown in Fig. 1c and

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Tab. 1, the order of specific surface area, micropore volume and total pore volume is

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NS-ACFF>NM-ACFF>ACFF0>NO-ACFF, the order of ultramicropore volume is NM-

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ACFF>NO-ACFF>ACFF0>NM-ACFF.

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ACFF>NS-ACFF>ACFF0>NO-ACFF and the order of supermicropore volume is NS-

Ultramicropore volume of NO-ACFF decreases by 0.0699 cm3/g (20.54%)

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compared to ACFF0, and the pore size distributions (PSDs) of ultramicropore around

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0.51 nm becomes weaker after oxidation (Fig. 1a). In addition, the supermicropore volume increases by 0.035 cm3/g (49.65%) and PSDs of supermicropores at 0.76,

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0.81, 0.89 and 1.14 nm improve obviously (Fig. 1b). The data show that the specific surface area and total pore volume reduction of NO-ACFF treated by nitric acid oxidation process are mainly caused by the destruction of ultramicroporous pore walls. The collapse of the ultramicropore wall makes the pore size of the ultramicropores to become supermicropores or disappear. Sonication-assisted nitric acid oxidation process has the most obvious effect on the specific surface area (12.55%), micropore volume (22.05%) and total pore volume (22.7%) of ACFFs, which is related to the formation of high speed micro-jets and the high-energy environment of local high temperature and high pressure. Further, pore volume of ultramicropore and supermicropore of NS-ACFF increase 0.0373 cm3/g 7 / 23

Journal Pre-proof (10.96%) and 0.0533 cm3/g (75.6%) simultaneously by sonication-assisted nitric acid oxidation process. As Fig. 1a and 1b shows, PSDs of NS-ACFF’s ultramicropores and supermicropores have no obvious change in distribution, except for the pore volume in the original concentrated pore size enhance markedly. The high-energy nitric acid solution under the sonication field makes more carbon atoms, which are difficult to be etched into holes, participate in the oxidation reaction. The effect of sonicationassisted nitric acid oxidation process on the formation of pore structure is stronger

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than the destruction of ultramicropore wall, and the effect is manifested by a

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substantial increase ultimately.

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Microwave-assisted nitric acid oxidation process has relatively faintish effect on

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the specific surface area, micropore volume and total pore volume in contrast to N SACFF, howbeit, NM-ACFF has the greatest enhancement effect on ultramicropore

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volume. Its ultramicropore volume is 16.19% larger than ACFF0 and 4.71% larger

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than NS-ACFF. PSDs of NS-ACFF’s ultramicropores show bimodal distribution near 0.49 and 0.56 nm, and the pore volume of ultramicropores at 0.49 nm is significantly

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increased. The adsorption isotherms of ACFF0, NO-ACFF, NM-ACFF and NS-ACFF are typical I-type isotherms (Fig. 1d). Microporous adsorption and capillary condensation occur at lower relative pressure (P/P0<0.2) and nitrogen adsorption can be basically completed, the adsorption amount of N2 reaches equilibrium when the pressure continues to increase. The equilibrium height of the adsorption capacity depends on the micropore volume of the ACFFs, and the order of equilibrium height is NS-ACFF>NM-ACFF>ACFF0>NO-ACFF, which is consistent with the results of Table 2. Fig. 1 Pore structure characteristics of ACFFs before and after oxidation. (a) PSDs of ultramicropores (b) PSDs of supermicropores (c) volumes of micropores, ultramicropore and supermicropores (d) adsorption isotherm. 8 / 23

Journal Pre-proof Tab. 1 Surface structure parameters of ACFFs before and after oxidation.

In order to further study the effect of surface chemical structure on Cu(Ⅱ) adsorptivity of N-ACFFs, the surface chemical structure analysis is carried out by IR and XPS tests. The surface element content obtained by XPS is shown in Tab. 2. The content of N and O elements increased significantly after oxidation treatment, especially the content of O element. The content increments of O element on the surface of NO-ACFF, NM-ACFF and NS-ACFF are 4.68%, 5.5% and 2.7%,

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respectively, and the N element increments are 1.3%, 0.59%, and 0.68%, respectively.

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It can be seen that the content of oxygen-containing functional groups and nitrogen-

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containing functional groups on the surface of ACFFs are significantly increased after

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nitric acid oxidation modification, wherein the increment of oxygen-containing functional groups content of NM-ACFF is greater than NS-ACFF.

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Fig. 2a shows the IR spectra of ACFF0, NO-ACFF, NS-ACFF and NM-ACFF. Peak

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position changes of IR spectra mainly include two vanished characteristic peaks of 1446 cm-1 (δO-H) and 3433 cm-1 (σO-H or σN-H), three newly generated characteristic

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peaks of 1710 cm-1 (σC=O), 1568 cm-1 (antisymmetric σNO2) and 1337 cm-1 (symmetric σNO2) (Zhang et al., 2005; Barrosobogeat et al., 2016). From the appearance of C=O, R-NO2 characteristic peaks and the disappearance of O-H stretching vibration and bending vibration characteristic peaks, it can be concluded that the nitric acid oxidation process can not only oxidize the surface chemical structure, but also accompany the nitrification reaction of R-OH bond or C-H bond. This conclusion can be confirmed from the O1s peak separation results (Nguyen et al., 2005; Zhang et al., 2009; Yang et al., 2016). As Fig. 2c- Fig. 2f shows, 4.29% R-NO2 appears after treated by nitric acid oxidation process, and the content of R-NO2 further increases after microwave-assisted nitric acid oxidation process and sonication-assisted nitric 9 / 23

Journal Pre-proof acid oxidation process. The results indicated that the technologies of microwaveassisted and sonication-assisted have the effect of promoting nitrification reaction in the nitric acid oxidation process. As Fig. 2b shows, peak positions changed were mainly 3433, 1710, 1568 and 1337 cm-1 after adsorption of Cu(II). The possible reasons of the decrease of C=O characteristic peak (1710 cm-1) and the appearance of O-H or N-H characteristic peak (3433 cm-1) is Cu(II) and H+ absorbed by C=O bond, then forms enol structure

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ultimately. The decrease of antisymmetric σNO2 characteristic peak (1568 cm-1) and

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the disappearance of symmetric σNO2 characteristic peak (1337 cm-1) suggest that the

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nitro group is reduced to an amino group under the action of H+ and Cu(II).

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Fig. 2 Infrared spectrum of (a) ACFFs before and after different oxidation processes (b) ACFFs before and after adsorption of Cu(II); O1s peak separation results of (c) ACFF0, (d) NO-ACFF, (e)

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NS-ACFF and (f) NM-ACFF.

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Fig. 3 A schematic summary for the oxidation and regeneration process. Tab. 2 Surface element content of ACFFs before and after nitric acid oxidation.

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3.2 Parameter optimization of nitric acid oxidation process The extent and intensity of nitric acid oxidation process affect the pore structure and surface chemical structure of ACFFs. Surface structure of ACFFs is further oxidized and etched to form new pore structure in the nitric acid oxidation process of ACFFs, accompanied by the unfolding of original closed pore structure opened and the growth of surface polar group. As Fig. 4a shows, Q and R of N-ACFFs are approximately linear positive correlation with c(HNO3). The results show that Q of NACFFs reaches the maximum (Qmax=11.03 mg/g) when the nitric acid concentration is 68%. As shown in Fig. 4b, Q of N-ACFFs reaches the maximum (Qmax=15.78 mg/g) when the nitric acid volume is 20 mL. As shown in Figure 4c, Q and Ar of N-ACFFs increase first and then decrease with the increase of oxidation time. The optimal 10 / 23

Journal Pre-proof volume of nitric acid oxidation treatment is 2 hours. As shown in Fig. 4d, Q of NACFFs decreases from 15.78 mg/g to 6.73 mg/g with the increase of ACFFs dosage, while R increases from 31.55% to 67.29%. The study of Cu(II) adsorptivity of ACFFs is mainly based on the Cu(II) adsorption value Q of per unit mass of ACFFs in this article, therefore, m(ACFFs)=0.2 g are used in the evaluation of Cu(II) adsorptivity. Due to the small thickness of most pore walls of ACFFs, as the strength of nitric acid oxidation (nitric acid concentration, nitric acid volume) increases and fully

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proceeds (oxidation reaction time), the pore walls are easily collapsed by nitric acid

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oxidation (Yu et al., 2018; Huang et al., 2009; Collins et al., 2013). The destruction of

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the pore wall structure leads to a larger pore diameter, a smaller specific surface area

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and pore volume, resulting in the decrease of Cu(II) adsorptivity of N-ACFFs. Therefore, the Cu(II) adsorptivity (Q and R) of N-ACFFs does not necessarily

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increase with the increase of the nitric acid oxidation reaction strength and deepening.

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Based on the parametric optimization results of nitric acid oxidation process, the optimum parameters for nitric acid oxidation process of ACFFs are as follows: nitric

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acid concentration 68%, nitric acid volume 20 mL, oxidation reaction temperature 333 K and oxidation reaction time 2 h. Q(NO-ACFF) is 15.779 mg/g, which 3.11 times that of ACFF0.

Fig. 4 Cu(II) adsorptivity of N-ACFFs under different technological parameters of (a) nitric acid solution concentration, (b) nitric acid solution volume, (c) oxidation time and (d) ACFFs dosage.

3.3 Cu(II) adsorptivity of NM-ACFF and NS-ACFF Microwaves (electromagnetic waves with a frequency of 0.3-300 GHz and a wavelength of 0.1-1 m) have wave-particle duality and three basic characteristics of penetration, absorption and reflection (Balaji et al., 2007; Eilola et al., 2012; Goessler et al., 2003). Microwave irradiation has been applied for the rapid preparation of 11 / 23

Journal Pre-proof various materials for the detection and adsorption of heavy metal ions (Deng et al., 2019; Deng et al., 2016; Deng et al., 2016). Sonication (frequency range 2×104-1×109 Hz, wave velocity about 1500 m/s) propagating in liquid has the function of accelerating liquids’ mechanical motion and cavitation effect. Cavitation effect can produce local high temperature, local high pressure, strong shock wave and high speed micro-jet, which provides a high-energy reaction environment for the progress of physicochemical reaction (Zhang et al., 2018; Deng et al., 2009; Rinaldi et al.,

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2011; Dalodière et al., 2016).

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As shown in Fig. 5, Q of NO-ACFF, NM-ACFF and NS-ACFF are 4.55, 3.64 and

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3.11 times of Q(ACFF0) respectively. NM-ACFF has the largest Cu(II) adsorptivity

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(Q(NM-ACFF)=23.13 mg/g), ΔQ of NM-ACFF is 46.57% compared to Q(NO-ACFF). The results show that sonication-assisted and microwave-assisted nitric acid oxidation

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process can further improve the Cu(II) adsorptivity of ACFFs than nitric acid

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oxidation process under water bath heating conditions, and microwave-assisted nitric acid oxidation process has the best promotion function on the Cu(II) adsorptivity of

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ACFFs. Consistent with the variation of ultramicropore volume, Q(NM-ACFF) is 355.31% higher than Q(ACFF0) and 25.03% higher than Q(NS-ACFF). The pore size for effective adsorption of Cu(II) is in the range of 0.25-0.44 nm (1.7-3 dCu(II)) in the physical adsorption of ACFFs, thus, the ultramicropore volume is the main influence factor of physical adsorption for Cu(II) (Ko et al., 2004; Machida et al., 2005; Lima et al., 2011). Due to the strong microwave absorption capability of ACFFs, the "body heating" method of microwave heating can not only rapidly heat up the inside and outside of ACFFs, but also form a plurality of hot spots with high energy on the surface of the ACFFs. Thereby more surface atoms undergo oxidation reaction, thus microwave12 / 23

Journal Pre-proof assisted nitric acid oxidation process can further improve the Cu(II) adsorptivity of ACFFs (Dallinger et al., 2007; Oliver 2008; Hoz et al., 2005; He et al., 2017). On the other hand, nitric acid solution produces intense motion and cavitation bubbles as the ultrasound propagates. Strong shock waves and high speed micro-jets (greater than 110 m/s) are produced when the bubbles burst, accompanied by the formation of local high temperature above 5200 K and local high pressure about 5×107 Pa, which provides a special high energy environment for nitric acid oxidation of ACFFs. In

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addition, active hydroxyl radicals (∙OH) with strong oxidation ability are produced in

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nitric acid solution when ultrasound propagates (Shibata et al., 2004; Sesis et al., 2013;

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Torres et al., 2007; Kim et al., 2008). Therefore, sonication-assisted nitric acid

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oxidation process has a significant promotion effect on the Cu(II) adsorptivity of ACFFs.

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Fig. 5 Q of ACFF0、NO-ACFF、NM-ACFF and NS-ACFF.

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Fig. 6a shows the effect of nitric acid concentration on Qt and Rt of ACFF0 regenerated by sonication-assisted-pickling regeneration process. The results show

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that 15 wt% nitric acid as pickling solution has the best effect on the regeneration of ACFF0 (η=136.7%). Thus nitric acid solution of 15 wt% is selected as pickling solution to research the effect of regeneration time on Cu(II) adsorptivity and recycling performance of ACFFs. As Fig. 6b shows, Qt of ACFF0 has a positive correlation with regeneration time, and the regeneration rate η increases from 136.7% to 379.59% with the increase of regeneration time. Besides, NO-ACFF and NM-ACFF maintain a high Cu(II) adsorption value on the whole with regeneration time. Qt of NO-ACFF and NM-ACFF reach the maximum after the second regeneration synchronously, Qt maximum of NO-ACFF and NM-ACFF are 26.05 mg/g (η=164.04%) and 27.07 mg/g (η=117.03%) separately. 13 / 23

Journal Pre-proof Hence one can see that sonication-assisted-pickling regeneration process achieves efficient regenerations and enhancements of Cu(II) adsorptivity for ACFF0, NO-ACFF and NM-ACFF. ACFF0 and N-ACFFs have a commendable recycling performance and can obtain a higher Cu(II) adsorptivity by this regeneration process. The environment of local high-pressure and high-temperature shock wave generated by the burst of "cavitation bubble" contribute to the removal of the adsorbate on the surface of ACFFs, and also assist the pickling solution further modify ACFFs to enhance Cu(II)

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adsorptivity.

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Fig. 6 Influence factors of sonication-assisted-pickling regeneration process of (a)

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pickling solution concentration and (b) regeneration time

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Acknowledgments

This work was supported by National Natural Science Foundation of China (Grant

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No. 51473088), National Key Research and Development Plan of China (Project No.

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2016YFC0301402), Key Research and Development Plan of Shandong Province (Project No. 2018GGX102029), Key Research and Development Plan of Shandong

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Province (Project No. 2017CXGC0409) and Shandong Post-doctoral Innovation Fund (Project No. 2017030759). References

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mesoporous materials, characterization and photocatalytic properties. Appl. Cata. A-Gen. 284, 193-198.

Barrosobogeat, A., Alexandrefranco, M., Fernándezgonzález, C., Macíasgarcía, A., Gómezserrano, V., 2016. Preparation of activated carbon-SnO2, TiO2 and WO3 catalysts. study by FT-IR spectroscopy. Ind. Eng. Chem. Res. 55, 5200-5206. Nguyen, T.X., Bhatia, S.K., 2005. Characterization of activated carbon fibers using argon adsorption. Carbon. 43, 775-785. Zhang, Y., Xiao, P., Zhou, X., Liu, D., Garcia, B., Cao, G., 2009. Carbon monoxide annealed TiO2 nanotube array electrodes for efficient biosensor applications. J. Mater. Chem. 19, 948-953. 16 / 23

Journal Pre-proof Yang, S., Lei, L., Xiao, T., Di, Z., Zhang, Y., 2016. Role of surface chemistry in modified ACF (activated carbon fiber)-catalyzed peroxymonosulfate oxidation. Appl. Surf. Sci. 383, 142-150. Yu, J., Qiao, K., Zhu, B., Cai, X., Liu, D., Li, M., Yuan, X., 2018. Structural research of activated carbon fibers during a novel phosphoric acid reactivation process assisted by sonication. Funct. Mater. Lett. 11, 1850066. Huang, G., Shi, J.X., Langrish, T.A.G., 2009. Removal of Cr(Ⅵ) from aqueous

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electrochemical oxidation process: electro-oxidation of cerium(Ⅲ) to cerium(Ⅳ) in nitric acid medium and a study on phenol degradation by cerium(Ⅳ) oxidant.

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Journal Pre-proof Lima, L.S.D., Araujo, M.D.M., Quináia, S.P., Migliorine, D.W., Garcia, J.R., 2011. Adsorption modeling of Cr, Cd and Cu on activated carbon of different origins by using fractional factorial design. Chem. Eng. J. 166, 881-889. Dallinger, D., Kappe, C.O., 2007. Microwave-assisted synthesis in water as solvent Chem. Rev. 107, 2563-2591. Oliver, K.C., 2008. Microwave dielectric heating in synthetic organic chemistry. Chem. Soc. Rev. 37, 1127-1139.

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Journal Pre-proof

Fig. 1 Pore structure characteristics of ACFFs before and after oxidation. (a) PSDs of ultramicropores (b) PSDs of supermicropores (c) volumes of micropores,

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ultramicropore and supermicropores (d) adsorption isotherm.

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Fig. 2 Infrared spectrum of (a) ACFFs before and after different oxidation processes (b) ACFFs before and after adsorption of Cu(II); O1s peak separation results of (c) ACFF0, (d) NO-ACFF, (e)

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NS-ACFF and (f) NM-ACFF.

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Fig. 3 A schematic summary for the oxidation and regeneration process. Fig. 4 Cu(II) adsorptivity of N-ACFFs under different technological parameters of (a) nitric

dosage.

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acid solution concentration, (b) nitric acid solution volume, (c) oxidation time and (d) ACFFs

Fig. 5 Q of ACFF0、NO-ACFF、NM-ACFF and NS-ACFF.

Fig. 6 Influence factors of sonication-assisted-pickling regeneration process of (a)

pickling solution concentration and (b) regeneration time.

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Journal Pre-proof Tab. 1 Surface structure parameters of ACFFs before and after oxidation. ACFF0

NO-ACFF

NS-ACFF

NM-ACFF

BET surface area /m2∙g-1

1083.4

971.63

1219.4

1171.8

total pore volume /cm3∙g-1

0.4230

0.3902

0.5190

0.4549

micropore volume /cm3∙g-1

0.4108

0.3759

0.5014

0.4436

ultramicropore volume /cm3∙g-1

0.3403

0.2704

0.3776

0.3954

supermicropore volume /cm3∙g-1

0.0705

0.1055

0.1238

0.0482

mesopore volume /cm3∙g-1

0.0121

0.0142

0.0175

0.0112

average aperture /nm

1.5617

1.6063

1.7025

1.5527

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adsorbent

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Journal Pre-proof Tab. 2 Surface element content of ACFFs before and after nitric acid oxidation. C/%

N/%

O/%

ACFF0

87.82

1.20

10.98

NO-ACFF

81.83

2.50

15.66

NS-ACFF

84.44

1.88

13.68

NM-ACFF

81.74

1.79

16.48

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element

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Journal Pre-proof

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Graphical Abstract

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