Adsorption behavior of tetracycline from aqueous solution on ferroferric oxide nanoparticles assisted powdered activated carbon

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Adsorption behavior of tetracycline from aqueous solution on ferroferric oxide nanoparticles assisted powdered activated carbon ⁎⁎

Jiahui Zhoua, Fang Maa, , , Haijuan Guob, a b

⁎

State Key Laboratory of Urban Water Resource and Environment, School of Environment, Harbin Institute of Technology, Harbin 150090, China College of Energy and Environmental Engineering, Hebei University of Engineering, Handan 056038, China

H I GH L IG H T S

G R A P H I C A L A B S T R A C T

oxide nanoparticles as• Ferroferric sisted powdered activated carbon was

• • •

applied to adsorb tetracycline for the first time. The existence of ferroferric oxide nanoparticles improved the adsorption capacity of powdered activated carbon. Chemical adsorption played the dominant role in the adsorption process. Ferroferric oxide nanoparticles assisted powdered activated carbon was suggested as a regenerable and efficient adsorbent.

A R T I C LE I N FO

A B S T R A C T

Keywords: Ferroferric oxide nanoparticles (FONP) Adsorption Powdered activated carbon (PAC) Tetracycline (TC)

The typical chemical co-precipitation method was adopted for the synthesis of ferroferric oxide nanoparticles assisted powdered activated carbon (FONP-PAC). The crystal structure, surface profile, and element composition were systematically investigated to reveal FONP-PAC’s structural characteristics and tetracycline (TC) adsorption mechanism. We found the formation of FONP distributed on the surface/pores/channels of PAC. The adsorption behavior was dramatically fast and could be described by Freundlich isotherm and Elovich kinetic models. The thermodynamics investigation indicated that the adsorption process was spontaneous endothermic reaction, which was mainly carried out via chemical sorption mechanism. Our research of the superior adsorption capacity of magnetically separable FONP-PAC for TC from aqueous solution proposed a feasible adsorbent instead of PAC for antibiotic wastewater, and the good regeneration ability of FONP-PAC provided the feasibility of its application in engineering.

1. Introduction Antibiotics are considered to be the persistent micro-pollutants in water environment, most of them cannot be absorbed and metabolized completely by the bodies of humans and animals, which cause large amount of them to be released into environment through urine and

⁎

feces [1–3]. Among the various antibiotics, tetracycline (TC) is the second most common antibiotic group in both production and usage throughout the world which shows antimicrobial activities against various kinds of pathogenic bacteria [4,5]. While, most of TC enter into environment due to its poor metabolization or adsorption by humans and animals, which will not only cause negative impact on the water

Corresponding authors. E-mail addresses: [email protected] (F. Ma), [email protected] (H. Guo).

https://doi.org/10.1016/j.cej.2019.123290 Received 12 August 2019; Received in revised form 24 October 2019; Accepted 25 October 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Jiahui Zhou, Fang Ma and Haijuan Guo, Chemical Engineering Journal, https://doi.org/10.1016/j.cej.2019.123290

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2.2. Characterization of PAC and FONP-PACs

environment, but also suppress the progress of many water treatment [5,6]. Moreover, the abuse of TC may lead to the increase of resistance to microorganism, and then become a big public health crisis [7–9]. Hence, the removal of TC from water is urgently needed. In recent years, various techniques have been investigated to remove TC from wastewater, including biological treatment, coagulation, sedimentation, ozonation, photocatalysis, electrochemical degradation, and adsorption [10–16]. Among them, adsorption has been considered as one of the most prospective approaches due to its attractive advantages such as low-cost effectiveness, environmental friendliness, and simple operation [8,17,18]. Xiong et al. synthesized magnetic carbonαFe/Fe3C by a carbonization method and achieved high TC removal efficiency with the pore filling effect and electrostatic as the dominate roles [8]. Ye et al. have reported that the synthesized g-MoS2/PGBC exhibited considerable TC removal efficiency with electrostatic interaction, π-π stacking, hydrogen bonding, and pore-filling, which were involved in the attraction and collection of TC molecules onto the material surface [14]. It was also reported that the interactions of multiple pollutants in adsorption behavior consisted of the competition and surface modification, the competitions usually increased the mobility of pollutants, and the surface modified by the co-occurring pollutant showed different adsorption properties for other pollutants [19]. As one of the most widely used adsorbents, powdered activated carbon (PAC) process was also considered to be a reliable solution for removing TC due to its strong adsorption performance, mild reaction conditions, and little by-products [20]. While, its intrinsic difficulties of separation and regeneration limited its application. Several researchers combined PAC with magnetic iron nanoparticles such as Fe0, Fe2O3, or Fe3O4 to achieve easy magnetic separation performance of PAC from wastewater under the action of outside magnetic field [21–25]. Magnetic carbon composites have been widely used in the removal of various contaminants from water environment, such as heavy metals, dyes, natural organic matter (NOM), and antibiotics [26–29]. However, the influence of ferroferric oxide nanoparticles (FONP) on the TC adsorption capacity of FONP-PAC is still unclear. In addition, there has been few reports about systematic studies of TC adsorption behavior on FONP-PAC composites. In this study, the FONP-PAC composites with different FONP/PAC mass ratios of 1/1, 2/1 and 3/1 were prepared using a chemical coprecipitation method. The characteristics change during preparation of FONP-PAC composites were investigated by XRD, FTIR, XPS, SEM-EDS, Raman, VSM, and BET. The effect of pH was investigated to determine the optimum adsorption condition, and the effects of coexisting ions and coexisting natural organic matter were also investigated. The TC adsorption isotherms of PAC and 1FONP-PAC were investigated, with the adsorption capacities of TC studied in the meantime. Moreover, the adsorption kinetics and thermodynamics have been fitted and analyzed. Finally, the reusability of 1FONP-PAC was discussed. The significance of this research was to assess the feasibility of FONP-PAC as a substitute adsorbent of PAC for antibiotics removal and reveal the TC adsorption mechanism.

The X-ray diffraction (XRD) patterns of PAC and FONP-PACs were recorded by a Panalytical X’Pert-Pro MPD analyzer. The Fourier transform-infrared spectroscopy (FTIR) was employed by a Perkin Elmer Spectrum One analyzer to identify the surface properties of PAC and FONP-PACs, with the wavenumber ranging from 400 to 4000 cm−1. The X-ray photoelectron spectroscopy (XPS) was carried out by a Thermo Fisher ESCALab 250Xi X-ray photoelectron spectroscope to analyze the elemental quantity and state of PAC and FONPPACs. The surface morphology and elemental distribution of PAC and FONP-PACs were evaluated by the scanning electron microscopy (SEM) using a Hitachi S-4800 microscope coupled with an energy-dispersive spectrometer (EDS). The Raman spectra were obtained by a Thermo Fisher DXR2xi confocal Raman microscope (633 nm). The saturation magnetization was evaluated by a vibrating sample magnetometer (VSM). The specific surface area was also measured by a Micromeritics ASAP 2020 apparatus, and the samples were calculated by Brunauer–Emmett–Teller (BET) equation. 2.3. Adsorption isotherms, kinetics and thermodynamics The pH value played an important role in the adsorption efficiency of TC by FONP-PAC. Firstly, the optimum pH value was ensured by single-factor experiment. All the following experiments were conducted under the condition of the optimum pH. For isotherm batch adsorption: (30–200) mg/L of TC and 0.5 g/L 1FONP-PAC were added in a thermostatic mechanical stirring bath at 20 °C (293 K), 400 rpm until adsorption reached equilibrium. After adsorption, the adsorbent was magnetic separated by outside magnetic field, the remaining solution was filtered using 0.45 μm millipore membrane filters, and the residual concentration of TC was analysed quantitatively by high performance liquid chromatography (HPLC). In the same manner, the experiments were repeated at different temperatures: 20 °C (293 K), 30 °C (303 K), and 40 °C (313 K). For the kinetics batch adsorption: the experiments were repeated at different initial concentrations of TC: 50 mg/L, 100 mg/L, 150 mg/L, and 200 mg/L. The residual concentrations of TC for different hydraulic retention time (10 min-300 min) were analyzed. 3. Results and discussion 3.1. Characterization of the adsorbents As XRD patterns presented in Fig. 1a, in contrast to the amorphous structure of PAC, there were some typical diffraction peaks for FONPPACs. The diffraction peaks of FONP-PACs at 2θ = 30.1°, 35.5°, 43.2°, 53.3°, 57.0° and 62.7° matched well with the following planes: (220), (311), (400), (422), (511), and (440), indicating the existed structure of inverse spinel-type Fe3O4 in FONP-PACs (JCPDS Card No: 19-0629) [30,31]. FTIR spectra were measured to investigate the functional groups on the surface of PAC and FONP-PACs. As showed in Fig. 1b, the bands at 3399, 1701, 1578, and 1367 cm−1 were assigned to the O–H, C]O, C]C, and C–C stretching vibrations, respectively [3,32–36]. Compared with PAC, many bands in FONP-PACs appeared, disappeared, or shifted. For example, the band of the C–O (984 cm−1) vibration mode appeared, the band of the C]O (1701 cm−1) vibration mode disappeared, and the band of the C]C (1578 cm−1) vibration mode shifted, indicating that FONP interacted with the surface functional groups of PAC during the synthesis process. The bands at 443 and 587 cm−1 were assigned to the adsorption peaks of Fe-O [35–39], indicating that FONP existed on the surface of PAC. The XPS full spectra (Fig. 1c) showed that PAC was comprised of C 1s and O 1s, FONP was comprised of Fe 2p and O 1s, while FONP-PACs consisted of C 1s, O 1s and Fe 2p. Furthermore, after the calibration of binding energy was done with the C 1s peak at 284.6 eV, the various

2. Materials and methods 2.1. Preparation of FONP/PAC composites The 200 mesh wooden PAC with a methylene blue absorbing value of 285.8 mg/g was chosen and vacuum dried at 105 °C for 24 h as the carbon matrix for FONP-PACs preparation. FONP-PACs were prepared by a chemical co-precipitation method with different FONP/PAC mass ratios of 1/1, 2/1 and 3/1. Detailed information of the preparation process is shown in the Supporting Information. For convenience, the compositions (FONP/PAC mass ratios of 1/1, 2/1 and 3/1) were named as 1FONP-PAC, 2FONP-PAC, and 3FONP-PAC, respectively.

2

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Fig. 1. XRD patterns (a); FTIR spectra (b); and XPS full spectra (c) of PAC, FONP, and FONP-PAC with different FONP/PAC mass ratios of 1/1, 2/1, and 3/1.

Fig. 2. XPS spectra of C 1s (a); and Fe 2p (b) for PAC, FONP, and FONP-PAC with different FONP/PAC mass ratios of 1/1, 2/1, and 3/1.

[41–44]. And the highest binding energy peak at 724.7 eV was attributed to Fe3+ (Fe 2p1/2) of Fe3O4 [43,44]. Furthermore, the small shakeup satellite peak of Fe3+ (Fe 2p3/2) was identified at 718.8 eV [44,45]. The results indicated the existence of Fe3O4, which was consistent with the results of XRD. After fitting the XPS Fe 2p spectra with these iron species signature, the 2p3/2/2p1/2 ratios of FONP, 1FONP-PAC, 2FONPPAC, and 3FONP-PAC were found to be 2.21–2.32, close to its predicted value of 2, and the Fe2+/Fe3+ ratios were 0.41, 0.47, 0.45, and 0.40, respectively. The Fe2+/Fe3+ ratio of 1FONP-PAC was the closest to the value of 0.5 expected from the stoichiometry of Fe3O4. The SEM images shown in Fig. 3 revealed that there were some nanospheres formed on the surface, pores, and channels of FONP-PACs with an average particle size of 20–50 nm. Furthermore, the amount of nanospheres observed increased with the increasing proportion of FONP in FONP-PACs. The element (C, O, and Fe) composition and distribution of FONP-PACs were investigated by EDS elemental mapping (Fig. 4). The results indicated that most of Fe and O appeared simultaneously on the surface of FONP-PACs. There was also some area contained C and O without Fe, indicating the existence of oxygencontaining functional groups on the surface of PAC, which was similar to the results of Fig. 1b and Fig. 2a. Combining with XRD, FTIR, and XPS results, it could be deduced that the synthesized FONP distributed uniformly on the surface, pores, and channels of PAC when FONP-PAC was prepared by the chemical co-precipitation method. The Raman spectra of PAC, FONP, and FONP-PACs were shown in Fig. S1. Two diffraction peaks around 1337 and 1592 cm−1 were observed, corresponding to the D and G bands of the amorphous PAC, respectively [43]. The characteristic diffraction peak of Fe3O4 around 690 cm−1 was also detected, which was consistent with the results of XRD and XPS. Fig. S2 presented the magnetic hysteresis curves of FONP-PACs with different FONP/PAC mass ratios of 1/1, 2/1, and 3/1. The saturation magnetization values (Ms) reached 22.2, 29.2, and 32.7 emu.g−1,

Table 1 XPS results for the relative atomic percentages of C 1s and Fe 2p species on PAC, FONP, and FONP-PAC with different FONP/PAC mass ratios of 1/1, 2/1, and 3/ 1: C 1 s (a); Fe 2p (b). (a) C 1s Sample PAC 1FONP-PAC 2FONP-PAC 3FONP-PAC

C–C/C]C

C–O

O–C]O

62.8% 47.1% 56.7% 73.4%

21.3% 35.6% 32.3% 15.5%

15.9% 17.3% 11.0% 11.1%

(b) Fe 2p Sample

FONP 1FONP-PAC 2FONP-PAC 3FONP-PAC

2+

Fe (Fe 2p3/2)

Fe3+ (Fe 2p3/2)

Fe3+ sat

Fe3+ (Fe 2p1/2)

26.0% 28.4% 27.8% 25.8%

35.3% 33.3% 34.7% 36.9%

11.0% 10.8% 10.2% 10.3%

27.7% 27.5% 27.3% 27.0%

states of C 1s and Fe 2p were determined via deconvolution using the software CasaXPS 2.3.16. Fig. 2a and Table 1a showed that the C 1s spectra all consisted of C–C/C]C (284.6 eV), C–O (285.7 eV), and O–C]O (288.6 eV) [28,40]. The contents of them in PAC were 62.8%, 21.3%, and 15.9%, respectively. With FONP proportion increased, the contents of C–O and O–C]O were 35.6% and 17.3% (1FONP-PAC), 32.3% and 11.0% (2FONP-PAC), and 15.5% and 11.1% (3FONP-PAC), respectively; meanwhile, the content of C–C/C]C was 47.1%, 56.7%, and 73.4%, respectively. The Fe 2p spectra of FONP and FONP-PACs were shown in Fig. 2b, and the relative content of various species of Fe 2p was summarized in Table 1b. The peaks located at the binding energies of 710.6 eV and 712.3 eV were assigned to Fe2+ and Fe3+ (Fe 2p3/2), respectively 3

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Fig. 3. SEM images of PAC (a); 1FONP-PAC (b); 2FONP-PAC (c); and 3FONP-PAC (d).

the results of XRD and SEM analysis. The type IV isotherms of FONPPACs with a type H3 hysteresis were obtained in Fig. 5a, and the pore size distributions of FONP-PACs were mainly concentrated in 3.87 nm (Fig. 5b), which demonstrated the existence of mesoporous structure [8,46].

respectively, indicating the magnetic properties were influenced by the content of FONP in FONP-PACs. In addition, the superparamagnetic characteristic and negligible coercivity of FONP-PACs indicated they could be separated rapidly from the aqueous solution by outside magnetic field, which was of great significance for the application in engineering. It can be seen from Fig. 5 and Table S1 that the formation of FONP inside the porous structure of PAC caused a reduction in BET surface area and pore volume of FONP-PACs. FONP has a relatively smaller surface area compared with that of PAC. Thus, the specific surface area of FONP-PACs decreased significantly with the increase of FONP mass fraction. The phenomenon showed that the surface properties of FONPPACs were apparently different from those of PAC, which conformed to

3.2. Effect of pH During the adsorption of TC, aqueous pH affected the adsorption process, including the form of TC and the surface charge of 1FONP-PAC [47–49]. In order to study the effect of pH on TC adsorption, various pH values (2, 3, 4, 5, and 7) were applied. The results of the pH effect upon the TC adsorption were shown in Fig. 6. The adsorption efficiency of

Fig. 4. SEM-EDS elemental mapping images of 1FONP-PAC (a); 2FONP-PAC (b); and 3FONP-PAC (c). 4

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Fig. 5. N2 adsorption-desorption isotherms (a); and BJH desorption dV/dlog(D) pore volume (b) of FONP-PAC with different FONP/PAC mass ratios of 1/1, 2/1, and 3/1.

160

1FONP-PAC was investigated. As shown in Fig. S5, the effect of the ionic strength (Na+, K+, Mg2+, 2+ Ca , Cl−, NO3−, SO42−, and HPO42−) of eight background ions on TC adsorption was investigated due to their wide distribution in environmental water. The experiments were conducted with chloride salts for cations and sodium salts for anions. As can be seen from Fig. S5a, Na+ and K+ had no significant competition effects on TC adsorption onto 1FONP-PAC. The adsorption capacity of TC decreased by 30.6% and 27.2% after adding 250 mM Mg2+ and Ca2+, respectively. Generally, cations have a priority sequence for binding to organic matter as alkalimetal cations (e.g., Na+ and K+) < H+ < alkaline-earth cations (e.g., Mg2+ and Ca2+) [53]. Hence, alkaline-earth cations (e.g., Mg2+ and Ca2+) had bigger hydrated radius and occupied more adsorption sites than those of alkali-metal cations (e.g., Na+ and K+), and led to greater electronic screening effect which more strongly inhibited TC adsorption on 1FONP-PAC [52]. For coexisting electrolyte anions, Cl− and NO3− had no obvious effects on TC adsorption onto 1FONP-PAC (Fig. S5b). In contrast, the coexisting SO42− and HPO42− anions could slightly enhance the adsorption capacity of TC on 1FONP-PAC. With the addition of 250 mM SO42− and HPO42−, the adsorption capacity of TC increased by 13.5% and 10.9%, respectively. The experimental results herein provided further evidence that the TC adsorption process on 1FONPPAC was dominated by surface complexation/chemical adsorption rather than ion exchange/physical sorption.

140 120

80

150 140

60

pH 2 pH 3 pH 4 pH 5 pH 7

40 20

130

qe(mg/g)

qt (mg/g)

100

120 110

1FONP-PAC

100 90

2

3

4

5

6

7

pH

0 0

60

120

180

240

300

360

420

t (min) Fig. 6. Effect of pH on the adsorption process.

1FONP-PAC increased first and then decreased after pH > 3, thus the best adsorption capacity of 1FONP-PAC for TC (140.2 mg/g at 300 min) was observed in the pH value of 3. For 1FONP-PAC, the increase of pH led to the deprotonation of –OH and –COOH on adsorption sites [49,50]. For TC, the molecule morphology changed at different pH ranges. As shown in Fig. S3, when pH was lower than 3.3, TC was a positively charged molecule. At pH 3.3–7.7, TC was a zwitterion and nearly neutral. When pH was higher than 7.7, TC became negative [51,52]. At low pH, the electrostatic interaction should be weak, because the protonation of –OH and –COOH on 1FONP-PAC. At high pH, both TC and 1FONP-PAC were negatively charged. Thus, the electrostatic repulsion occurred in both. Therefore, only in the middle pH range, the adsorption reached high capacity. pH 3 was selected as the best value in the following adsorption experiment. Furthermore, at the pH value of 3, the iron leaching was 1.85 mg/L (Fig. S4), accounting for only 1.02% of total iron in 1FONP-PAC, which guaranteed the stability of the 1FONP-PAC components.

3.4. Effect of natural organic matter Because of the wide distribution of natural organic matter (NOM) in environmental water, the coexisting NOM might also affect the TC adsorption on 1FONP-PAC. In this work, the influence of NOM, represented by humic acid (HA) at various concentrations (i.e., 2.0, 4.0, 6.0, 8.0, and 10.0 mg/L, respectively), was evaluated on the adsorption performance. This concentration range covered the environmental concentration of HA. As can be seen in Fig. S6, the TC adsorption capacity decreased from 208.3 mg/g to 188.4 mg/g after adding 2 mg/L HA, which might be due to the competition for adsorption sites between HA and TC. It was considered that TC had strong complexation with HA under acidic and neutral condition [52,54], therefore, HA might act as a bridge between 1FONP-PAC and TC under acidic condition, which led to the increasing of TC adsorption capacity to 209.3 mg/g with the increasing HA concentration to 10 mg/L.

3.3. Effect of coexisting ions On account of the heterogeneity and complexity of environmental water, various ions exist in the natural water and wastewater systems. The electrolyte cations therein may compete for the binding sites of solid particles; moreover, electrolyte anions always exhibit strong complexation ability to the surface of various adsorbents [53]. Based on these reasons, the effect of coexisting ions on TC adsorption onto

3.5. Adsorption isotherms The equilibrium data were analyzed with the help of mathematical models used to explain the TC distribution between the adsorbent and liquid. As shown in Fig. 7 and Table S2, the isothermal data were 5

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Fig. 7. The Langmuir isotherm model (a); and the Freundlich isotherm model (b) for TC adsorption. The pseudo-first order plots (c); the pseudo-second order plots (d); the Elovich model (e); and the intra-particle diffusion model (f) for TC adsorption on 1FONP-PAC.

analyzed by Langmuir model (R2 = 0.69 and R2 = 0.62) and Freundlich model (R2 = 0.98 and R2 = 0.99), indicating that both the equilibrium data of 1FONP-PAC and PAC were well described by the empirical Freundlich isotherm model. Freundlich isotherm model is based on the assumption that the adsorbent surface consists of adsorptive sites with different energies [55], indicating the multilayer adsorption behavior of TC on 1FONP-PAC and PAC. Compared to PAC, the FONP/ PAC mass ratio of 1FONP-PAC was 1/1, the adsorption data indicated that the existence of FONP could improve the adsorption capacities by increasing the adsorption sites [52]. For the Freundlich adsorption isotherms, the adsorption constant KF of 1FONP-PAC and PAC were 111.92 and 199.25 mg/g(L/mg)1/n, respectively. The separation factor 1 related to the adsorption intensity or surface uniformity were 0.1456

by plotting q vs ln t and obtaining t0, by trial and error, such that q vs ln (t + t0) becomes a straight line. The parameter βE is obtained from the slope of that line, and then the initial sorption rate, αE, can be calculated from t0 and βE [57,58]. The parameters of the Elovich model for the adsorption process were shown in Table S4. In order to investigate the rate-limiting step of the adsorption process by means of the intra-particle diffusion model, t1/2 for qt was schematized, with the results presented in Fig. 7f and Table S4. In general, the adsorption could be divided into two stages: surface adsorption and slow intra-particle diffusion. As fitted by intra-particle diffusion model, the straight line did not pass through the coordinate origin, indicating that intra-particle diffusion was not the only step to control the adsorption process, which was also controlled by other adsorption stages [48,59].

n

and 0.0962 (0 < 1 < 1), respectively, which suggested strong intern action between TC and adsorbents [52,56]. In addition, the adsorption performance of TC on 1FONP-PAC has been compared with other reported adsorbents, and the results were shown in Table S3. It was obvious that the adsorption performance of 1FONP-PAC was the best, and the preparation process of 1FONP-PAC was relatively simpler at a lower cost, which was crucial to its practical application.

3.7. Adsorption thermodynamics The influence of temperature on the adsorption of TC by 1FONPPAC was investigated. As shown in Fig. S7, higher temperature resulted in higher adsorption capacity, indicating the adsorption process of TC on 1FONP-PAC was endothermic in nature. By fitting the data, thermodynamic parameters were obtained (Table S5). The △G value was calculated as −0.85, −3.03, and −5.21 kJ·mol−1 at 293, 303, and 313 K, respectively, indicating the adsorption of TC on FONP-PAC was spontaneous [60]. The positive △H value was 63.15 kJ·mol−1, consistent with the endothermic nature. As discussed in the literature, when the |△H| value > 50 kJ·mol−1, the adsorption process was mainly carried out via chemical sorption mechanism [60,61], which was consistent with the XPS results of 1FONP-PAC before and after adsorption experiment (Fig. S8 and Table S6). The △S value was 0.22 kJ·(mol·K)−1, indicating the randomness of TC adsorbed state and the increasing degree of freedom during the adsorption process. The adsorption of TC on FONP-PAC would reduce the disorder of TC molecules but significantly increased the disorder of water molecules surrounding the surface of 1FONP-PAC, which leaded to the △S > 0 [62].

3.6. Adsorption kinetics To model the adsorption data, pseudo-first order, pseudo-second order, Elovich model, and intraparticle diffusion equations were adopted (Fig. 7 and Table S4). The pseudo-first order model fitted failed because the correlation coefficients were determined in the range of 0.929–0.958. Moreover, as can be seen from Fig. 7c and Table S4, the calculated qe,cal values varied greatly from the experimental qe,exp values. Compared with the pseudo-first order model, the pseudo-second order model had higher R2 values of 0.978–0.992, and the qe,cal values were closer to qe,exp values. The Elovich model was also used to describe the adsorption behavior of TC on 1FONP-PAC, the results showed that the calculated values were closest to the true values. Thus, the adsorption process was quite appropriate for the Elovich model (R2 > 0.996). In this work, in order to more accurately find kinetic parameters of the Elovich equation (αE, βE, and t0), a further treatment is necessary. The testing of the Elovich equation is usually carried out 6

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3.8. Regeneration of 1FONP-PAC

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To investigate the regeneration ability of 1FONP-PAC, several cycles of adsorption-desorption experiments were performed. As shown in Fig. S9, the TC removal efficiency dropped from 98.78% to 52.71% after five cycles, indicating that 1FONP-PAC could be regenerated. After each cycle, the adsorbent could be recycled rapidly from the aqueous solution by outside magnetic field, which provided the feasibility for its engineering application in the future. 4. Conclusions In this study, FONP assisted PAC (FONP-PAC) were successfully synthesized with different FONP/PAC mass ratios using a chemical coprecipitation method. The surface properties of FONP-PACs were apparently different from those of PAC. FONP were not uniformly covering the surface of PAC, they formed clusters and patches on it and were also inside the carbon matrix. The adsorption process of TC on 1FONP-PAC was comprehensively investigated. The results indicated the high adsorption capacity of 1FONP-PAC for TC from aqueous solutions. The aqueous pH was the key factor affecting the adsorption process of TC on 1FONP-PAC. It was found that the adsorption process was well described by the Freundlich isotherm model and the Elovich kinetic model. Additionally, the thermodynamic studies indicated that the adsorption behavior of TC on 1FONP-PAC was spontaneous endothermic, and the chemical adsorption played the dominant role in it. The studies of the effects of coexisting ions and HA demonstrated that 1FONP-PAC could maintain satisfactory TC adsorption capacity in various complex environmental water. Most significantly, the good regeneration ability and magnetic separation property of 1FONP-PAC made it a feasible adsorbent instead of PAC for potentially wide application in engineering. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The research was financially funded by the State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (QA201319), the National Natural Science Foundation of China (No. 51478140), and the National Key Research and Development Program of China (No. 2016YFC0401102). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.123290. References [1] X.D. Zhu, Y.C. Liu, C. Zhou, G. Luo, S.C. Zhang, J.M. Chen, A novel porous carbon derived from hydrothermal carbon for efficient adsorption of tetracycline, Carbon 77 (2014) 627–636, https://doi.org/10.1016/j.carbon.2014.05.067. [2] Z.Y. Zhang, H.J. Liu, L.Y. Wu, H.C. Lan, J.H. Qu, Preparation of amino-Fe(III) functionalized mesoporous silica for synergistic adsorption of tetracycline and copper, Chemosphere 138 (2015) 625–632, https://doi.org/10.1016/j. chemosphere.2015.07.014. [3] V.T. Nguyen, T.B. Nguyen, C.W. Chen, C.M. Hung, T.D.H. Vo, J.H. Chang, C.D. Dong, Influence of pyrolysis temperature on polycyclic aromatic hydrocarbons production and tetracycline adsorption behavior of biochar derived from spent coffee ground, Bioresour. Technol. 284 (2019) 197–203, https://doi.org/10.1016/j. biortech.2019.03.096. [4] J.Y. Cao, L.D. Lai, B. Lai, G. Yao, X. Chen, L.P. Song, Degradation of tetracycline by peroxymonosulfate activated with zero-valent iron: performance, intermediates, toxicity and mechanism, Chem. Eng. J. 364 (2019) 45–56, https://doi.org/10.

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