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Slow pyrolysis magnetization of hydrochar for effective and highly stable removal of tetracycline from aqueous solution Si-Qin Chen, Ya-Li Chen, and Hong Jiang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b04683 • Publication Date (Web): 03 Mar 2017 Downloaded from http://pubs.acs.org on March 5, 2017

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Industrial & Engineering Chemistry Research

Slow pyrolysis magnetization of hydrochar for effective and highly stable removal of tetracycline from aqueous solution

Si-Qin Chen, Ya-Li Chen, Hong Jiang* CAS Key Laboratory of Urban Pollutant Conversion, Department of Chemistry, University of Science and Technology of China, Hefei 230026, China

*Corresponding author: Dr. Hong Jiang, Fax: +86-551-63607482; E-mail: [email protected]

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ABSTRACT Although biochar has been intensively studied as an inexpensive adsorbent for diverse organic pollutants in aqueous solution, synchronously achieving the high adsorption capacity, separability, and stability is still a challenge. Herein, we partially addressed this issue via an integrated activation and pyrolytic magnetization of sawdust hydrochar, during which the surface area of magnetic activated sawdust hydrochar (M-SDHA) increases from 1.7 to 1710 m2 g−1, and the weight loss decreases from 70% to 5% at 700 oC. Correspondingly, the maxmium adsorption capacity of M-SDHA toward tetracycline (TC) reaches 423.7 mg g−1 and remains constant at pH of 5 to 9. Multiple characterizations show that the fine pore structure and surface functional groups of M-SDHA were maintained during pyrolysis magnetization process, which is responsible for the high adsorption capacity. In the pyrolysis magnetization process, the FeCl3 was reduced to Fe3O4 which endowed M-SDHA with the magnetism and may simultaneously improve the thermostability of M-SDHA. In addition, acidic-basic stability of M-SDHA may be responsible for the stable adsorption toward TC at different pH based on FTIR results. These results along with the column adsorption experiment show that M-SDHA is an effective and practical adsorbent for TC removal.

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1. INDRODUCTION In recent years, the presence and fate of antibiotics in the environment have received growing attention. The tetracycline (TC) group is the second most widely-used antibiotics applied in human and animal therapies 1. Only a small portion of TC is metabolized and absorbed in the body, while the majority is unmetabolized and excreted to environment 2-4. For example, Hamscher’s report showed that the TC concentration in surface soil with liquid manure as a fertilizer was 86.2 µg kg−1 on average 5, and Karthikeyan and Bleam’s report found levels as high as 4 µg L−1 of TC detected in municipal wastewater 6. The presence of low-concentration antibiotics and their transformation products in the environment have raised significant concern for potential toxic effects as well as the potential for the development and transfer of antibiotic resistant genes among microorganisms 7, 8. Compared with current treatment technologies for TC, such as electron pulse radiolysis 9, oxidation degradation

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, and biodegradation by activated sludge

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, photo catalytic

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, adsorption is effective,

economic, requires simple operation, and is a practical approach for TC removal

13

.

Activated carbon, carbon nanotubes, and graphene have been reported to be good adsorbents for TC removal 14-16. However, many of these adsorbents are prohibitively expensive for the treatment of a large amount of TC-contaminated water. Therefore, low-cost adsorbents are urgently needed. Hydrochar prepared from the hydrothermal carbonization of biomass wastes has been intensively investigated for energy and environmental applications including as supercapacitor, catalyst support, electrode materials for battery, and as adsorbent 17-19. Though possessing abundant functional groups on the surface, such as hydroxy, carboxy, and amino groups 20, the hydrochar materials generally have low surface area and almost no open porosity 21-23, leading to the low adsorption capacity for pollutants. 3



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For this reason, KOH activation method is often used to obtain microporous carbon with higher specific surface area (SSA) and better porosity allowing enhanced adsorption 24, 25. On the other hand, due to the very fine particles and low density of activated hydrochar, after mixing hydrochar with a solution, it is difficult to separate the hydrochar for recycling. Zhu et al. hence prepared magnetic porous carbon with maghemite particles by simultaneous activation and magnetization 26. Compared with traditional activated hydrochar, it is easier to separate and recycle magnetic activated hydrochar by using external magnetic fields

27

. However, the SSA of the obtained

magnetic porous carbon was drastically decreased (349 m2 g−1) because of the interaction of maghemite and KOH, significantly lower than the SSA of the material without magnetization (1000-3000 m2 g−1)

28, 29

, resulting in an adsorption capacity

for TC that was less than 30 mg g−1. In addition, the spent adsorbents containing organic pollutants are commonly regenerated at high temperature 30-32, requiring the high thermal stability of hydrochar. Furthermore, the pH of practical TC wastewater is varied, and the chemical stability of hydrochar is important. However, the hydrochar which simultaneously possesses high adsorption capacity, feasible magnetic separability, excellent thermal and chemical stability is rarely reported. In our previous work, magnetization was obtained through the slow pyrolysis of FeCl3-preloaded biomass. During the slow pyrolysis, a reducing atmosphere is produced and the Fe (Ⅲ) can be transferred to Fe3O4, γ-Fe2O3, and Fe0 33. The modified pyrolytic biochar efficiently removed TC from aqueous solution 34, 35. In this work, we proposed an approach of activation and magnetization for the preparation of magnetic sawdust hydrochar (M-SDHA) using fir sawdust, a widely available forestal 4


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and agricultural byproducts. After the activation of the sawdust hydrochar by KOH, the obtained activated sawdust hydrochar (SDHA) was preloaded with FeCl3 and slowly pyrolyzed under inert atmosphere to prepare M-SDHA. The remaining organic moieties in SDHA were volatilized during pyrolysis magnetization to increase thermal stability of M-SDHA. We employed multiple approaches to characterize the physicochemical properties of M-SDHA. Batch adsorption experiments of M-SDHA for TC removal were carried out and the effect of environmentally relevant pH was investigated to illustrate the high adsorption capacity and the chemical stability. In addition, a column experiment was also performed to demonstrate the continuous adsorption performance of M-SDHA.

2. METHODS AND MATERIALS 2.1. Materials All chemicals were obtained from Sinopharm Chemical Reagent Co., Shanghai, China. The fir sawdust, an abundant lignocellulosic biomass, was gathered from a timber treatment plant in Hefei, China. The sawdust was initially washed, and removed moisture in a dryer at 105 oC for 24 h. The dried sawdust was crushed using a high-speed rotary grinder, and sawdust particles with sizes smaller than 100 mesh were collected. 2.2. SDHA Preparation Sawdust hydrochar (SDH) was obtained by the hydrothermal carbonization of sawdust. An aqueous solution/dispersion (320 g L−1) of the sawdust was filled into a stainless steel autoclave with Telfon lining and heated to 250 oC in an oven, holding at 250 oC for 2 h. The SDH was filtered and fully washed by distilled water and then at 120 oC for 4 h and crushed into powder. Secondly, 2 g SDH and 8 g KOH was 5



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completely and put into a corundum crucible. Then it was placed in a horizontal and heated to 800 °C with a heating rate of 3 °C min−1, holding at 800 °C for 1h. The nitrogen carrier gas flowed continuously until the furnace cooled down to room temperature. The resulting activated products was washed by 10 wt% HCl to remove inorganic salts and then washed with distilled water to reach a neutral pH, and then dried in an oven at 120 oC for 3 h to obtain activated sawdust hydrochar (SDHA). 2.3. M-SDHA Preparation 100 mL of FeCl3 solution with a concentration of 50 mmol L−1 was put into a flask with 1.0 g of SDHA and the mixture was shaken for 300 min to achieve complete impregnation. The moisture of the mixture was removed by evaporating and subsequent drying at 105

o

C overnight and the Fe-loaded SDHA with an

approximately 5.0 mmol g−1 Fe content was obtained. The M-SDHA was obtained by pyrolyzing Fe-loaded SDHA at 700 oC for 1 h under N2 flow at a heating rate of 3 K min−1 in a horizontal furnace. 2.4. Characterization Nitrogen adsorption–desorption isotherms at 77 K with a Micromeritics Gemini apparatus (ASAP 2020M+C, Micromeritics, Co., USA) were used to determine the specific surface areas of SDH, SDHA, and M-SDHA. The hydrogen, carbon, nitrogen, and oxygen contents in SDH, SDHA, and M-SDHA were determined by an elemental analyzer (VARIO EL III, Elementar Inc., Germany). The surface functional groups of SDH, SDHA, and M-SDHA (before and after TC adsorption) were characterized by Fourier transform infrared spectrometer (FTIR, EQUIVOX55 IR spectroscopy, Bruker, Germany) with a detection range from 4000 to 400 cm−1 and a resolution of 2 cm−1. X–ray photoelectron spectroscopy (XPS, ESCALAB250, Thermo-VGScientific Inc., UK) using monochromatized Al Kα 6


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radiation (1486.9 eV) was used to analyze the compositions and chemical state changes of C 1s, O 1s in the SDH, SDHA, and M-SDHA. A DTG-60H/DSC-60 thermogravimetric analyzer (TGA, Shimadzu Co., Japan) was employed to evaluate the thermal stability of the SDH, SDHA, and M-SDHA. 2.5. Batch Adsorption Experiment of TC 100 mg L−1 TC solutions (background ionic strength was provided by 0.01 M CaCl2) were prepared freshly for each batch test. The adsorption kinetic experiment was performed using a batch protocol. Briefly, 20 mg of adsorbent samples were respectively mixed with 50 mL of 100 mg L–1 TC solution in 100 mL vials and shaken with a rotating speed of 180 rpm at 25 oC for 2 h. The control experiment was conducted using TC/CaCl2 solution without adsorbent. All experiments were performed in duplicate. TC concentrations of the samples were determined by UV–vis spectroscopy after the samples were filtered through a 0.22 µm membrane filter .The absorbance of TC was measured at 355 nm. 2.6. Column Adsorption Experiment of TC The fix-bed adsorption experiment was performed to further study the practical implications of M-SDHA for TC removal. For column experiment, a column with a length of 20 cm and a diameter of 9.0 mm was used, which was filled with M-SDHA sample (0.804 g) and the effective bed volume (BV) was 8 cm3. The influent concentration of TC in the solution was controlled to 100 mg L−1 and the down-flow through the column was maintained by vacuum filtration at a constant flow rate of 1mL min−1. The effluent was collected and analyzed for TC concentration using a UV–vis spectrometer at 355 nm.

3. RESULTS AND DISCUSSION 7



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3.1. Characterization of Composites

Fig.1. Nitrogen adsorption–desorption isotherms of SDH, SDHA, and M-SDHA. The nitrogen adsorption–desorption isotherms of the prepared SDH, SDHA, and M-SDHA are presented in Fig. 1. At low relative pressure, the N2 isotherms of M-SDHA and SDHA display the shape of type I, implying the existence of structure. At high relative pressure, both the isotherms of M-SDHA and SDHA are possessed with a small hysteresis loop, which is nearly a type Ⅳ isotherm and indicates the presence of a small quantity of mesopores. For the pattern of SDH, the isotherm is a typical type II pattern, which suggests SDH almost has no pores.

Table

shows the textural characteristic of SDH, SDHA, and M-SDHA. The calculated Brunauer–Emmett–Teller (BET) surface area of SDH, SDHA, and M-SDHA are 1.7, 2193.0, and 1710.3 m2 g−1, respectively. Comparison of the parameters of SDHA and SDH showed that the activation of SDH caused a significant increase of the surface area and micropore volume. This can be attributed to the removal of volatile organic substances by KOH at high temperature during activation. Although the surface area M-SDHA was obviously decreased after magnetization, the pore volume and average pore size remained unchanged. This indicated that the presence of Fe on the surface or in the pores of SDHA did not dramatically destroy the pore structure after 8


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magnetization. Table 1 Element analysis and textural characteristic of SDH, SDHA, and M-SDHA. Textural characteristic

Element analysis（wt. %） Sample SDH SDHA M-SDHA

C

O

H

N

57.74 58.23 55.14

41.29 24.97 17.12

5.85 1.43 0.94

0.41 0.41 0.33

BET surface area (m2g−1) 1.7 2193.0 1710.3

a

b

c

d

e

f

Pore volume (cm3 g−1) 0.003 1.301 0.969

Average pore size (nm) 5.76 2.37 2.26

Fig.2. XPS spectra of SDH, SDHA and M-SDHA XPS survey was used to show changes of the surface element content and groups of SDH before and after activation and magnetization. Deconvolution of the 2p and C 1s peaks of XPS were analyzed (Fig.2). Figures 2a, b, and c show the wide scan of SDH, SDHA, and M-SDHA, respectively. The wide scan XPS spectra of SDH and SDHA showed two obvious peaks: C 1s (285.1 eV) and O 1s (533.12 eV) peaks. Compared with SDH, the relative intensity of O 1s of SDHA is obviously lower than SDH, indicating the decrease of O content in SDHA due to the removal of volatile compounds during KOH activation. The C 1s (284.8 eV) and O 1s (530.8 eV) peaks also contained on the surface of M-SDHA, and the content of oxygen on the surface 9



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M-SDHA significantly decreased relative to SDH. The Fe 2p XPS spectra of SDH, SDHA, and M-SDHA are shown in the inset curves in Figs. 3a, b, and c, respectively. No Fe 2p signals are detected in SDH, but strong Fe 2p peaks appear on the wide scan of M-SDHA. The weak Fe 2p peaks in SDHA may result from performing the KOH activation in a steel tube. For M-SDHA, the XPS spectra of Fe 2p signal can be deconvoluted into two peaks, 711.6 eV (Fe 2p1/2) and 725.5 eV (Fe 2p3/2), which can assigned to Fe3O4. The XPS spectra of C 1s of SDH, SDHA, and M-SDHA are presented in Figs. 2c, d, and e, respectively. High-resolution spectra of C 1s of SDH be deconvoluted into four peaks: 283.3, 284.9, 285.6, and 286.8 eV, which indicates presence of C=C, C-C, C-O, and C-O-C. The C 1s spectra of SDHA can be deconvoluted into four peaks: 284.2, 284.7, 285.4, and 288.5 eV, which can be to C=C sp2，C-C sp3，C-O, and COOR. The C 1s of M-SDHA spectra can be deconvoluted into four slightly shifted peaks, 284.4, 284.9, 285.8, and 289.2 eV, can be also assigned to C=C, C-C, C-O, and COOR (Fig.2f). The change between the 1s spectra of SDH, SDHA, and M-SDHA can be attributed to the strong oxidation environment during activation 36, 37.

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Fig.3. (a) XRD spectra of SDH, SDHA, and M-SDHA. (b) Magnetization curve of M-SDHA, and a photo of the magnetic separation of M-SDHA. As shown in Fig. 3a, the XRD patterns of SDH, SDHA, and M-SDHA have different peaks, indicating the presence of mineral crystals. Before activation and magnetization, the wider peak centered at ~23º indicates that the microstructure of the SDH is amorphous 16. After activation and magnetization, the major crystalline phase in M-SDHA was Fe3O4. A single peak around 45°can be attributed to Fe0. The formation of Fe0 is mainly due to the reductive atmosphere during slow pyrolysis. The magnetic property of M-SDHA was determined by hysteresis curve (Fig. 3b). The saturation magnetization of M-SDHA was 8.2 emu g−1 and the material exhibited excellent magnetic responsivity when attracted briefly by a permanent magnet (the inset figure in Fig. 4b). 3.2. Adsorption Performance Toward TC

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Fig.4. Adsorptive kinetics and isotherms. (a) Adsorption kinetics of M-SDHA, SDHA and SDH, the inset figure is the fitting results of kinetics data of M-SDHA by the pseudo-second-order kinetics model. (b) Adsorption isotherms for the adsorption of TC by M-SDHA. (c) TC concentration of the effluent of the column test with 0.8 g M-SDHA as the column filler and a TC solution flow rate of 1.0 mL/min. Figure 4a shows the TC adsorption kinetics of SDH, SDHA and M-SDHA. The TC removal efficiencies at different time intervals (0-120 min) were obtained. It is 12


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obviously found that SDH shows almost no TC adsorption capacity, while after activation and magnetization, M-SDHA exhibited an excellent TC removal capacity. Equilibrium was achieved within approximately 30 min, the corresponding TC adsorption amount was almost 250 mg g−1 and the removal efficiency was 96%. 4a also shows the adsorption kinetics of SDHA and it can be seen that SDHA reached the adsorption equilibrium in 10 min and shared the same adsorption amount of 250 g-1 as M-SDHA. It should be noted that M-SDHA reached the adsorption equilibrium more slowly than SDHA, which was probably due to that iron oxides on the surface reduced the rate of adsorption. Compared the adsorption performance of M-SDHA with SDHA, we can conclude that the adsorption of TC was mainly attributed to the activated hydrochar. The pseudo-second-order model expressed by Eq. (1) was used to fit the kinetics data of M-SDHA: (1) where qt and qe represent the amount of TC adsorbed (mg g−1) on M-SDHA at time t (min) and at equilibrium, respectively; K2 (g mg−1 min−1) is the corresponding adsorption rate constant. The equilibrium adsorption capacity (qe) and the adsorption rate constant (K2) were calculated by the intercept and slope of the t/qt versus t plot, which are shown in the inset figure of Fig. 4a. The correlation coefficient (R2) is 0.999. The equilibrium adsorption capacity calculated by this model (qe,cal=256.4 mg g−1) is close to the result obtained from experiments (qe,exp=250.5 mg g−1), confirming the validity of the pseudo-second-order model which is based on the hypothesis that chemical sorption is the rate-limiting step of the adsorption process. A series of adsorption experiments with different initial TC concentrations from 50 to 600 mg L−1 were conducted to obtain the adsorption isotherms. Langmuir and 13



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Freundlich isotherm models, Eq. (2) and (3), were employed to match the data : (2)

(3) where KL (L mg−1) represents the Langmuir constant associated with the free adsorption energy and qmax (mg g−1) is the maximum adsorption capacity. KF [mg g−1(L mg−1) 1/ n] and n are the Freundlich constants relevant to the adsorption capacity and adsorption intensity, respectively. Parameters of Langmuir and Freundlich models were calculated by Eq. (2) and (3) and the fitting result are shown in Fig. 4b. The Langmuir isotherm model was applied to estimate the maximum adsorption capacity corresponding to complete monolayer coverage on the M-SDHA surface. The correlation coefficient (R2) is 0.972, and the maximum adsorption capacity calculated from Langmuir model is 423.7 mg g−1, which is much higher than most reported adsorbents (Table S1). The Freundlich equation gives a good fit, too, with a lower correlation coefficient (R2) of 0.856. The n value is >1, thereby indicating nonlinearity in the isotherms similar to other studies

38

. Both of the two models can fit the

equilibrium data, but the Langmuir model has a higher correlation coefficient, indicating monolayer coverage of adsorbate on planar surfaces of adsorbents

38

. As

shown in Fig S1, SDHA has a maximum adsorption capacity of 454.5 mg g-1. The adsorption capacity of SDHA is a little higher than that of M-SDHA, which is probably because the surface area of SDHA is a bit larger than that of M-SDHA. Similar adsorption capacity of SDHA and M-SDHA also indicates that the activated hydrochar was the main contributor for TC adsorption and the magnetization step did not significantly affect the adsorption ability of the activated hydrochar. 14


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Although batch experiment is necessary to obtain the adsorption capacity of sorbents for given adsorbate condition present in fluid phases, it is also important to evaluate the adsorption performance in a continuously operated fixed bed for the potential practical application of M-SDHA. As seen in Fig. 4c, TC breaks through the M-SDHA column when the effective filtration volume is about 1000 bed volumes (BV) from the breakthrough curves, and the effective treatment volume for the M-SDHA is about 280 BV. The concentration of TC in the effluent is below 0.1 mg g−1, demonstrating the excellent adsorption peformance of M-SDHA toward TC.

3.3. Thermal Stability of M-SDHA

Fig.5. TG (a) and DTG (b) curves of SDH, SDHA, and M-SDHA. (c) Raman spectra of SDH, SDHA, and M-SDHA. The thermodynamic stability of adsorbents is closely related to their recyclability. 15



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SDH, SDHA, and M-SDHA were characterized by TG-DTG and Raman, and the results are shown in Fig. 5. In the range of test temperature (room temperature to 700 o

C), the weight loss for SDH, SDHA, and M-SDHA were about 70%, 10%, and 5%,

respectively. The significant reduction in the weight loss of SDHA and M-SDHA compared to that of SDH can be attributed to the complete removal of volatile compounds during activation and magnetization (Fig. 5a). There are two weight loss stages in the TG and DTG curves of SDH. The moisture evaporated in the first stage in Fig. 2b, 37-133 oC) and the lignin decomposed in the second stage (B in Fig. 5b, 133-668 oC). SDHA and M-SDHA underwent only a single weight loss, in the range 37-133 oC, corresponding to the evaporation of water, indicating that SDHA and M-SDHA showed little decomposition at high temperature. The TG and DTG results indicate that pyrolysis magnetization can improve the thermal stability of SDHA, which enhances thermal regeneration performance and broadens the potential application of this material. The thermal stability can be enhanced by better graphitization 39, 40. Raman spectra can clearly show the graphitization of M-SDHA. The Raman spectra of SDH, SDHA, and M-SDHA are shown in Fig. 5c. Characteristic bands at 1600 cm−1 (G-band) and 1360 cm−1 (D-band) are associated with the vibration of sp2-hybridized carbon atoms and breathing mode of aromatic rings, respectively

41, 42

. The ratio of the integrated

intensities of the D and G bands (ID/ IG) reveals the graphitization degree of the char. shown in Fig.5c, the ID/ IG value of SDH, SDHA, and M-SDHA are 0.49, 1.00 and respectively. Compared with SDH, SDHA has a lower graphitization degree, which may be caused by the activation that causes the hydrochar to burn out after long exposure time at high temperature. The value of ID/ IG is 0.73 for M-SDHA, indicating a higher graphitization degree relative to SDHA. This suggests that the dislocations 16


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defects in M-SDHA were eliminated after the pyrolysis magnetization. 3.4. Chemical Stability of M-SDHA The adsorption of TC onto M-SDHA at different solution pH values was investigated, and the results are shown in Fig. 6a. TC is an amphoteric molecule with three values of pKa (3.4, 7.7, and 9.7), and has differently charged group at different pH range. Thus it can exist as: H4TC+ at pH

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