In situ loading metal oxide particles on bio-chars: Reusable materials for efficient removal of methylene blue from wastewater

Journal of Cleaner Production 220 (2019) 460e474

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In situ loading metal oxide particles on bio-chars: Reusable materials for efficient removal of methylene blue from wastewater Shimin Zhai a, Min Li a, b, *, Dong Wang a, Liping Zhang a, Yi Yang a, Shaohai Fu a, b, ** a

Jiangsu Engineering Research Center For Digital Textile Inkjet Printing, Key Laboratory of Eco-Textile, Jiangnan University, Ministry of Education, Wuxi, Jiangsu, 214122, China b Suzhou Sunmun Technology Co.,Ltd, Kunshan, Suzhou, Jiangsu, 215337, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 6 October 2018 Received in revised form 20 January 2019 Accepted 14 February 2019 Available online 16 February 2019

Dyeing wastewater produced from textile industry has attracted considerable attention due to its pollution on environment. In this study, bio-char (BC), TiO2/bio-char (BC-T), TiO2/MgO/ZnO/bio-char composites (BC-TMZ) were prepared and utilized to remove methylene blue (MB). The surface morphology and composition of BC, BC-T and BC-TMZ were described by XRD, EDS, SEM, BET and TEM. Furthermore, the synergistic removal effects of as-prepared materials in MB solutions were explored under different bio-char dosage, initial MB concentration, reaction time and different light irradiation conditions. Finally, the residual MB solutions after treatment are characterized by LC-MS. The results indicated that the metal oxide particles (Fe2O3, ZnO, MgO and TiO2) have been loaded into pores of the BC-TMZ. Moreover, the BC-TMZ which act as both adsorbent and catalytic, possesses excellent reusability (reused for 4 times) to remove MB from solution. The adsorption behaviors of BC-TMZ were described by Pseudo-second order model and Langmuir model well. The maximum adsorption-catalytic capability of BC-TMZ (0.8 g/L) for MB (50 mg/L) was 62.5 mg/g with UV light irradiation (253.7 nm, 40 W) in 4 h. The MB molecules can be decomposed completely by BC-TMZ. The BC-TMZ is potential to be used as a reusable material for the removal of pollutants. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Metal oxide crystals Bio-char composites Methylene blue Removal capability Wastewater treatment

1. Introduction Due to low dye-uptake, a large amount of colored wastewater is produced during the dyeing process (Mouni et al., 2018). Because of high toxicity, dyeing wastewater is poorly biodegradable and cannot be directly treated by traditional biological methods (such as aerobic and anaerobic microbial treatment) (Punzi et al., 2015). Hence, highly effective and economic pretreatment before biological treatment for dyeing wastewater is especially important. The conventional pretreatment methods can be categorized into physical methods (ion exchange, coagulation/flocculation, membrane separation, etc.) and chemical methods (electrochemical

* Corresponding author. Jiangsu Engineering Research Center For Digital Textile Inkjet Printing, Key Laboratory of Eco-Textile, Jiangnan University, Ministry of Education, 1800 Lihu Avenue, Wuxi, Jiangsu, 214122, China. ** Corresponding author. Jiangsu Engineering Research Center For Digital Textile Inkjet Printing, Key Laboratory of Eco-Textile, Jiangnan University, Ministry of Education, 1800 Lihu Avenue, Wuxi, Jiangsu, 214122, China. E-mail addresses: [email protected] (M. Li), [email protected] (S. Fu). https://doi.org/10.1016/j.jclepro.2019.02.152 0959-6526/© 2019 Elsevier Ltd. All rights reserved.

techniques, adsorption, photo-catalysis, etc.). Among the dyes, methylene blue (MB) is a kind of cationic dye used in textile dyeing, ink manufacture and indicator. Because of its stable chemical property and acute toxicity, the MB wastewater has low biodegradability. Actually, adsorption has been proven to be an effective pretreatment method for MB wastewater. Bio-char is one of the novel adsorption materials with high surface area, rich functional groups and pore structures (Valix et al., 2006). Presently, some functional bio-chars have been prepared to enhance its efficiency, such as magnetic bio-chars, bio-apatite loaded bio-chars and thermal-alkaline pretreated bio-char (Xiao et al., 2018; Shen et al., 2017; Liu et al., 2018a), etc. However, most bio-char materials cannot be reused without desorption or secondary carbonization, because the pores and adsorption sites were filled or occupied after adsorption. Presently, large numbers of used bio-chars are not recycled since the high costs of regeneration (Sühnholz et al., 2018). The used bio-chars are piled up or buried in the ground as solid wastes, which may have a massive impact on carbon cycling. If the modified bio-char can decompose the adsorbed organic matters, it may overcome the problem of solid wastes and reduce the cost of bio-char.

S. Zhai et al. / Journal of Cleaner Production 220 (2019) 460e474

More recently, metal oxides have attracted increased attention on the decomposition of pollutants (P and.Y, 2003). The way to decompose organic pollutants by metal oxides have been investigated previously. Some metal oxides can decompose pollutants without light irradiation, such as MgO, CaO and Al2O3, and some have photocatalytic ability, such as TiO2. So far, many researches of catalytic behavior on metal oxides have been conducted for their high efficiency, low cost and no secondary contamination. Liu et al. have prepared a flexible and breathable fabric that can decompose mustard gas by in-situ growing MgO nanoparticles on the surface of poly (m-phenylene Isophthalamide) (PMIA) (P and.Y, 2003). Wu et al. have synthesized a porous copper ferrite foam to efficiently remove arsenic from solution (Wu et al., 2018). Nevertheless, some problems are still hindering the practical application of nanometer photo-catalytic materials, such as high rate of electron-hole recombination, aggregation of nanoparticles and low utilization ratio of sunlight (Zhang and Park, 2017). Metal doping is an effective way to resolve aforementioned problems, which can endow nanoparticles with high efficient, stability and electrical conductivity (P and.Y, 2003; Houskova et al., 2007; Singh et al., 2011). For example, Solmaz et al. have fabricated visiblelight-driven TiO2/Ag2WO4/AgBr photocatalysts for Rhodamine B decomposition with efficient use of sunlight (Feizpoor and HabibiYangjeh, 2017). Mahvi A.H. et al. have synthesized FeNi3@SiO2@TiO2 nano-composites to degrade tetracycline successfully (Nasseh et al., 2018). Bamboo is a kind of fast-growing plant, which is prolific, cheap, and easy to obtain. Moreover, for the bamboo material, the pyrolysis process has less air pollution and stronger consistency of composition than multiple composition biomass (such as sewage sludge, scrap tires and petroleum wastes). Hence, the bamboo is used as the raw material to prepare bio-char. In this paper, porous adsorption material (BC), adsorption/photocatalytic material (BC-T) and adsorption/photocatalytic/catalytic material (BC-TMZ) were prepared from bamboo material and metal oxides. The synergistic effects and reusability of as-prepared materials on MB wastewater decomposition were explored. The study consists of adsorption process and catalytic process. The impacts of adsorption time, biochar dosage and initial MB concentration on the removal of MB will be examined at first. Then, the adsorption kinetics data and isotherm data will be analyzed using relevant equation models. The reusable abilities of two bio-char composites will be assessed under UV light irradiation. Finally, the likely pathway of MB degradation is proposed.

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Table 1 The elemental and ingredient composition of bamboo powder. Bamboo composition

Percentage (%)

Elements

C N O H S

45.52 0.35 48.79 5.13 0.22

Ingredient

Hemicellulose Lignin Cellulose

21.18 17.72 43.78

2. Experimental 2.1. Materials and chemicals Bamboo used in the experiment was grown in the city of Jinhua (located in Zhejiang Province, China). The bamboo was crushed into cylinder-shaped powder (about 50 mm long, 7 mm width) as Fig. S1 in supplement materials by a pulverizer (SE-750, Shengxiang Electric Appliance Co., China). In Table 1, it can be seen that the bamboo material consists of hemicellulose (21.18%), lignin (17.72%) and cellulose (43.78%). The main elements of bamboo powders are C (45.52%), N (0.35%), O (48.79%), H (5.13%) and S (0.22%). Methylene blue (C16H18ClN3S,
99.5%, lmax is 664 nm), as an adsorbate, was purchased from National Drug Group Chemical Reagent Co., Ltd. (Shanghai, China). The KOH (
95%), using as an activator for the bio-char, was purchased from (carat Martha) Shanghai Vibration Spectrum Biotechnology Co. Ltd. The iron powder (150 mm,
96%), tetrabutyl titanate (C16H36O4Ti,
98%), acetic acid (CH3COOH,
99.5%), magnesium chloride (MgCl2,
98%), zinc chloride (ZnCl2,
98%) and ammonium hydroxide (NH3H2O, 25%e28%) were all purchased from National Drug Group Chemical Reagent Co., Ltd. (Shanghai, China) and used without further purification. 2.2. Preparation of bio-char and bio-char composites The preparation of bio-char and composite materials are illustrated by flow chart as Fig. 1 (Liu et al., 2018b; Bogatu et al., 2017). To make sure the unsaturated carbons of bamboo were transformed into a relatively stable C (Chen et al., 2016), the alkalinepretreated bamboo powder was pyrolyzed at 700  C ±5  C for 1 h.

Fig. 1. Preparation of BC, BC-T and BC-TMZ.

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In Fig. 1, the pyrolysis and calcination processes were conducted in an Electrical Laboratory Furnace (STM-25-14, Luoyang Shante Co.,  China) with heating rate of 5 C/min under airtight environment. The mixing process is conducted by a pulverizer (SE-750, Shengxiang Electric Appliance Co., China) for 5 min. The solids were separated from suspension liquids by a centrifuge (TG-16-WS, Shanghai Lu Xiangyi Centrifuge Instrument Co., China) at 4000 rpm for 10 min. Three materials were prepared and the yield of bio-char pyrolyzed from alkaline-pretreated bamboo powder was 42.25%, that of BC-T made from BC was 59.89%, and that of BC-TMZ prepared from BC-T was 98%. All of the products were washed by deionized water for three times and dried at 60  C for 12 h before use.

2.3.1. Effect of the bio-char dosage on MB adsorption The BC, BC-T and BC-TMZ (0.02e0.16 g) were mixed with MB solution (100 ml, 50 mg/L) at rotate speed of 120 rpm for 24 h at room temperature (5  C), respectively. Then, the mixture was centrifuged at 4000 rpm for 10 min and the residual MB concentration in the filtrate was measured, respectively. Aforementioned processes were repeated for three times to get the average values of residual MB concentration. 2.3.2. Effect of the adsorption time on MB adsorption The BC, BC-T and BC-TMZ (0.08 g) were mixed with MB solution (100 mL, 50 mg/L) at rotate speed of 120 rpm for various adsorption time (10e1440 min) at room temperature (5  C), respectively. The methods of separation and concentration measurement were similar to those mentioned previously. Aforementioned processes were repeated for three times to get the average values of residual MB concentration. 2.3.3. Effect of initial MB concentration on adsorption The as-prepared materials (0.08 g) were mixed with MB solution (100 mL, 25e400 mg/L) at rotate speed of 120 rpm for 24 h at room temperature (5  C), respectively. The methods of separation and concentration measurement were similar to those mentioned previously. Aforementioned processes were repeated for three times to get the average values of residual MB concentration. 2.4. Adsorption kinetics Based on above results in section 2.3.2, the adsorption process were fitted by the Pseudo-first-order, Pseudo-second-order, Intraparticle diffusion and Elovich models as Eqs. (1)e(4) (Son et al., 2018):

Pseudo  first  order model lnðQ e  Q t Þ ¼ lnQ e  K1 t Pseudo  second  order model:

t 1 t ¼ þ Q t K2 Q 2e Q e

Intra  particle diffusion model:Q t ¼ K3 t0:5 þ C 1

b

lnðabÞ þ

According to above experimental results in section 2.3.3, the adsorption process were described by Langmuir, Freundlich, Temkin and Dubinin-Radushkevich isotherm models as follows (Son et al., 2018):

Langmuir isotherm equation:

Ce 1 1 ¼ þ Ce Q e Q m KL Q m

1

b

lnt

(1) (2)

(3)

(5)

1 Freundlich isotherm lnQ e ¼ lnKf þ lnCe n Temkin isotherm equation:Q e ¼

2.3. Adsorption experiment

Elovich model:Q t ¼

2.5. Adsorption isotherms

(6)

RT RT lnKt þ lnCe bt bt

(7)

D  R isotherm lnQ e ¼ lnQ m  bε2

(8)


 1 ε ¼ RTln 1 þ Ce

(9)

1 Ea ¼ pffiffiffiffiffiffi 2b

(10)

In Eq. (5)-(11), KL (L/mg), Kf (L/mg), bt, b (mol2/kJ2) and 3 are the constants in corresponding equations, respectively. Kt (L/mg) is equilibrium binding constant. RL is a separation factor of Langmuir isotherm model and C0 (mg/L) is the initial MB concentration. R is universal gas constant (8.314 J/mol/K). Qe (mg/g), Ce (mg/L) are the equilibrium adsorption capacity and equilibrium concentration, respectively. n reflect the adsorption ability, Qm (mg/g) is the maximum adsorption capacity, T (K) is absolute temperature. Ea (kJ/ mol) is the free energy per molecule of adsorbate. 2.6. Methylene blue photocatalysis decomposition with UV light 2.6.1. Effect of bio-char dosage on MB photocatalysis The BC, BC-T and BC-TMZ (0.02 g-0.16 g) were mixed with MB solution (100 mL, 50 mg/L) under UV light irradiation (253.7 nm) by a UV lamp (QB801, 40 W) for 4 h at 120 rpm, respectively. The separation and concentration measurement methods were similar to those mentioned previously. The separated solids were dried at 60  C for 12 h before reuse. To ensure the UV irradiation is evenly distributed, the reactor was designed as Fig. S2. 2.6.2. Reusable test with UV light irradiation and analysis of MB degradation products The as-prepared materials (0.08 g) were mixed with MB solution (100 ml, 50 mg/L) at 120 rpm for 4 h with UV light irradiation at room temperature (5  C), respectively. The solids were separated and the residue MB concentrations were measured as those mentioned previously. The separated solids were dried at 60  C for 12 h before reuse. Then, the separated solids were reused for 4 times on the MB treatment. Finally, the constituents of initial MB solution (50 mg/L) and residue MB solutions pretreated by the BCTMZ with UV light irradiation were analyzed by LC/MS method.

(4)

In Eqs. (1)e(4), Qt (mg/g) is the adsorption capability at time t, K1 (/min), K2 (/min) and K3 (mg/(g$min1/2)) are the constants in corresponding equations, respectively. C (mg/g) is the film thickness of Intra-particle diffusion adsorption, a (mg/(g $ min)) and b (g/mg) are the adsorption and desorption constants in Elovich model, respectively. t (min) is the adsorption time.

2.7. Calculations The removal capacity (Q, mg/g) of MB was calculated as equation (11):

Q¼

ðC0  Ce Þ  V m

(11)

S. Zhai et al. / Journal of Cleaner Production 220 (2019) 460e474

In the equation, C0 (mg/L) and Ce (mg/L) are the initial and residual MB concentration, respectively. V (L) is MB solution volume and m (g) is the bio-char dosage. The error analysis was carried out by estimating the normalized deviation (ND) and normalized standard deviations (NSD) as follow:

ND ¼

  100 XqeðexpÞ  qeðpredÞ      n qeðexpÞ

NSD ¼ 100

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi uP . 2 u qeðexpÞ  qeðpredÞ qeðexpÞ t n

(12)

(13)

In equations (1) and (2), the qe(exp) and qe(pred) are the experimental and predicted urea sorption capacity (mg$g1), respectively, and n is the number of observations made. 2.8. Analytical methods Elemental analyses of the bamboo powder were performed using an elemental analyzer (vario EL III, Hanau, Germany). The composition of bamboo powder was tested by the Van Soest method. The pore diameters and surface areas of the bio-char composites were characterized by a surface area analyzer (Tristar Ⅱ 3020, Mike Murray Feldman Instrument Co., USA) using the nitrogen gas adsorption isotherms (Analysis Bath Temperature 195.85  C, Equilibration Interval 10s, Warm Free Space 20.9245 cm3 and Cold Free Space 50.09 cm3). The TGA were analyzed using a (Q500 V20.13 Build 39, USA) TG analyzer at heating rate of 10 K/min in N2 atmosphere. X-Ray Diffractions (XRD) of the bio-char materials were conducted by an x-ray diffractometer (D8, Brook AXS Co., Ltd., Germany) under normal conditions (maximum output power 3 kW, Cu target, light tube power 2.2 kW, 2q scan range 15o-80 and at the rate of 0.1 /min). Scanning electron microscopies (SEM) of the bio-char composite was conducted using a SEM (SU8010, Hitachi, Japan) with an acceleration voltage of 10 kV. Energy dispersive spectroscopy and element mapping were measured by an Energy Dispersive Spectrometer (IE250X-Max50, Oxford, UK). Transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and selected area electron diffraction (SAED) images were recorded on a TF20 Jeol2100F field emission transmission electron microscope. XPS was performed on a Thermo ESCALAB 250XI photoelectron spectrometer with a monochromated Al- Ka source at a residual gas pressure of less than 108 Pa. All the binding energies were referenced to the C 1 s peak at 284.6 eV of the surface adventitious carbon. The residual MB concentrations were measured using an ultraviolet spectrophotometer (UV-2600, Shimadzu, Japan) at 664 nm. The performance liquid chromatography and mass spectrometry analysis of residue MB solutions were conducted using an HPLC-MS/MS Instrument (MALDI SYNAPT MS, Waters, USA). All of the other parameters were measured following standard methods (APHA, 2005). 3. Results and discussion 3.1. MB adsorption of the BC-T and BC-TMZ The adsorption capabilities of as-prepared materials at different bio-char dosage, initial MB concentration and reaction time were shown as Fig. 2. It can be seen that BC-T and BC-TMZ have higher removal capability than that of BC at the same adsorption condition. The improvement of removal capability may attribute to the

463

adsorption ability of metal oxide crystals. Moreover, some adsorbed MB molecules were decomposed by MgO crystals and some adsorption sites on the bio-char were freed up for the re-adsorption (Mita et al., 1994). In Fig. S3 (supplement materials), the MB removal percent increases with increasing bio-char dosage. When the dosage of BC-TMZ is above 0.8 g/L, the removal percent of MB is near 100%. It means the suitable bio-char dosage is 0.8 g/L at this MB concentration (50 mg/L). And the MB removal quantity of BC, BC-T and BC-TMZ at 0.8 g/L were 62.5, 52.9 and 15.8 mg/g, respectively (Fig. 2-A). For the adsorption activities with different adsorption time in Fig. 2 B, the MB removal capabilities of the BCTMZ are lower than those of the BC-T from 10 min to 60 min, stating that the MB is harder to be adsorbed by BC-TMZ firstly. These may due to the fact that the metal crystals loaded into BCTMZ reduced the surface area and increased the enthalpy of MB adsorption (Zhu et al., 2013; Yang et al., 2016). However, when the absorption time is beyond 60 min, comparing with BC-T, the BCTMZ has higher removal capabilities because the absorbed MB are decomposed by ZnO and MgO crystals. In Fig. 2 C, from 25 mg/L to 400 mg/L, the removal capability increased with increasing of initial dye concentration. Because the adsorbent has higher driving force for MB molecule transfer at higher MB concentration (Niu et al., 2012). In addition, the BC-TMZ has a higher removal capability than that of BC-T and BC, which also reveals that the metal crystals (ZnO and MgO) play an important role for MB wastewater treatment. 3.2. Adsorption kinetics The kinetics of MB adsorption onto the as-prepared materials are investigated as Fig. 3 and Table 2. Four adsorption kinetic models were used in this section: pseudo-first order model, pseudo-second order model, particle diffusion model and Elovich model. For the BC, only the Particle diffusion model exhibited high correlation between predicted and experimental adsorption capability (R2 ¼ 0.9547). However, for the BC-T and BC-TMZ, the pseudosecond order model has the highest degree of fitting (R2 > 0.99). The pseudo-second order model assumes chemisorption of MB onto BC-T and BC-TMZ, which may attributed to the metal oxide crystals exist in the bio-char materials. Moreover, from the normalized deviation (ND) and normalized standard deviations (NSD) of BC-T and BC-TMZ, the particle diffusion kinetic model can fit the experimental data best. These mean that the molecule diffusion inside adsorbent also was one of the key rate limiting steps for the MB adsorption onto BC-T and BC-TMZ (Pirhashemi and Habibi-Yangjeh, 2017). It can be concluded that both of intraparticle diffusion and chemisorption control the sorption onto BC-T and BC-TMZ. 3.3. Adsorption isotherms Four adsorption isotherm models (Langmuir, Freundlich, Temkin and Dubinin-Radushkevich isotherm models) were used to describe the equilibrium between adsorbent and adsorbate. The results were shown as Fig. 4 and Table 33. From the correlation coefficients (R2), Langmuir isotherm mode provides the best fit for the MB adsorption onto the BC (R2 ¼ 0.98), BC-T (R2 ¼ 0.98) and BCTMZ (R2 ¼ 0.99). The Langmuir isotherm equation assumes the single molecular or monolayer adsorption (Podlogar et al., 2016). But, not only one model provide excellent correlation between predicted and experimental removal capacity (R2 > 0.95). According to the normalized deviation (ND) and normalized standard deviations (NSD) values, the Dubinin-Radushkevich model can describe the MB adsorption onto BC, BC-T and BC-TMZ best. The Ea values of D-R isotherm model illustrate that the type of MB

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Fig. 2. Effect of bio-char dosage (A) (MB solution volume is 100 ml, initial concentration is 50 mg/L, reaction time is 24 h), adsorption time (B) (bio-char dosage is 0.8 g/L, initial concentration is 50 mg/L and MB solution volume is 100 ml) and initial MB concentration (C) (bio-char dosage is 0.8 g/L, MB solution volume is 100 ml, reaction time is 24 h) on the MB adsorption without light irradiation.

adsorption onto bio-char composites are physical adsorption (Ea < 8 kJ/mol) (Veldurthi et al., 2012). Moreover, to investigate the effect of changing temperature, the experiments of MB adsorption onto BC-TMZ were conducted at 5  C, 20  C and 30  C (Tables S3eS5 and Fig. S4 in the supplement materials). The estimated values of DG0 were 23.63, 22.85 and 25.75 kJ/mol at 278.15, 293.15 and 303.15 K, respectively. The negative values indicated that the adsorption onto BC-TMZ is spontaneous process. Besides, the positive value for DH0 (62.96 kJ/mol) indicates that the adsorption of MB onto BC-TMZ is an endothermic process. The positive value of entropy (292.66 J/mol/K) reflects the affinity of the adsorbent for MB (Bulut and Aydın, 2006) (see Table 3). 3.4. Photo-catalysis decomposition experiment To explore the suitable bio-char dosage with UV light irradiation, the catalysis experiments of three samples were conducted at different bio-char dosages (0.2e1.6 g/L). In Fig. 5A, the removal capabilities of the BC-TMZ are higher than these of the BC-T and BC at low bio-char dosages (below 0.8 g/L). However, when the biochar dosage is above 0.8 g/L, the removal capabilities of BC-T and BC-TMZ were same, indicating that the MB in solution is removed

completely. The phenomena indicates that the BC-TMZ and BC-T have stronger abilities to remove MB with UV irradiation than BC, which may due to the decomposition for MB. The results were consistent with the HPLC analysis for intermediate products of BC, BC-T and BC-TMZ in Fig. S5. Besides, the bio-char dosage at 0.8 g/L is suitable for the MB reuse experiments with UV light irradiation. Based on the results in Fig. 5 A, the reusability of as-prepared materials at bio-char dosage of 0.8 g/L with UV irradiation were shown as Fig. 5 B. In Fig. 5 B, the removal capabilities of the BC-TMZ are higher than these of the BC and BC-T. The results were consistent with the effects of each metal oxides on the reusability in Fig. S6. In addition, the reusability of BC-TMZ (R-1st, R-2nd and R3rd) in the experiments are close to that of the initial use (62.5 mg/ g), however, the removal capabilities of BC and BC-T decreased rapidly, which means that the BC-TMZ has the potential to reusable remove MB from wastewater. The pictures of residual MB solution after being treated by BC-TMZ were shown as Fig. S7. It can be seen that the MB solution was discolored completely by BC-TMZ for its initial use, 1st reuse and 2nd reuse (Table S6). But, the residual solution was wathet blue when the BC-TMZ was reused for the 3rd time. However, it also can be concluded that the BC-TMZ has excellent reusability. The enhancement and reusable remove

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Fig. 3. Kinetic adsorption model-fitting curves of the as-prepared materials.

Table 2 Adsorption kinetics parameters of four models. Adsorption Model

Fitting Parameters 1

BC

BC-T

BC-TMZ

Pseudo-first order model

K1 (min ) Qe (mg/g) R2 ND NSD

0.0050 5.5011 0.8945 77.5479 78.6715

0.0121 22.1803 0.9195 71.2138 72.3958

0.0092 38.9006 0.9919 59.4488 61.8044

Pseudo-second order model

K2 (min1) Qe (mg/g) R2 ND NSD

5.5969 9.8232 0.9262 28.2119 33.1705

0.0016 50.6842 0.9969 7.3664 11.8941

0.0006 63.7762 0.9950 9.1235 14.7170

Particle diffusion model

K3 (mg/(g$min1/2)) C (mg/g) R2 ND NSD

0.2768 5.3887 0.9547 2.0248 2.7389

1.2705 29.4207 0.9886 1.3144 1.3825

2.5045 20.3679 0.9836 2.8236 3.2857

Elovich model

a(mg/(g $ min)) b(g/mg)

23.2484 0.9084 0.8796 3.9432 4.7805

220.0915 0.1923 0.9757 1.8723 2.2334

11.2831 0.0981 0.9567 5.1658 6.8131

R2 ND NSD

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Fig. 4. Isotherm adsorption model-fitting curves of the as-prepared materials.

Table 3 Fitting parameters of four isotherm models. Isotherm Model

Fitting Parameters

BC

BC-T

BC-TMZ

Langmuir

KL (L/mg) Qm (mg/g) R2 ND NSD

0.0159 27.1297 0.9844 6.1436 6.8865

0.0536 87.2601 0.9896 14.5478 26.8818

0.1208 114.4165 0.9957 18.4696 38.0172

Freundlich

Kf (L/mg) 1/n R2 ND NSD

4.2493 0.2907 0.9592 3.0591 3.9631

45.2155 0.0959 0.7742 5.9543 6.1493

77.4242 0.0547 0.8855 4.2495 6.0264

Temkin

Kt (L/mg) bt (mg/g) R2 ND NSD

0.2616 474.9435 0.9576 4.5807 5.5131

486.7417 345.0338 0.7322 7.2136 7.5745

3.78*107 499.9222 0.8212 5.8619 7.6499

Dubinin-Radushkevich

Qm (mg/g) Ea (kJ/mol) b (mol2/kJ2) R2 ND NSD

40.9684 1.2197 0.3361 0.9589 3.0602 3.9737

95.5433 2.1004 0.1134 0.7842 5.8301 6.0124

128.7299 2.3137 0.0934 0.9507 3.0333 3.9177

mechanism of BC-TMZ could be explain for the following reason: During the MB removal process, the BC-TMZ can adsorb MB molecules easily due to its rich porous structures and high surface area. The adsorbed MB were degraded by MgO/ZnO crystals and TiO2 nanoparticles under UV light irradiation. Hence, the adsorption sites on the surface of BC-TMZ can reusable adsorb MB molecules and the ‘adsorption-degradation-adsorption’ process could enhance the MB removal capability. 3.5. Mechanisms and pathways of MB degradation For the BC-TMZ, the degradation products of MB were analyzed by HPLC-MS spectra (Fig. 5 C-F). In Fig. 5 C, the composition changing of residue MB solution was correlated with shifts and changes in intensity of three HPLC peaks, including peak 1 (the value of m/z is 123), peak 2 (the value of m/z is 284) and peak 3 (the value of m/z is 227). According to the mass spectra in Fig. 5DeF, peak 2 at 7.3 min was assigned to methylene blue (C16H18ClN3S), peak 1 at 1.4 min was assigned to 2, 3- phenolic hydroxyl aniline (C6H7O2N) and peak 3 at 10.4 min was assigned to 3, 7- amino phenothiazine (C12H10N3S). From Fig. 5 C, it can be seen that peak 2 appeared in HPLC of initial MB solution (MB) but disappeared in R-2nd and R-3rd, indicating that the MB was completely decomposed. However, the peak 2 reappear with the increasing of repeat time (reused for 3e4 times), indicating that

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467

Fig. 5. Effect of bio-char dosage with UV light irradiation (A); Removal capabilities of prepared materials with UV irradiation (B) (bio-char dosage is 0.8 g/L, MB solution volume is 100 ml, initial concentration is 50 mg/L, reaction time is 4 h); High-performance liquid chromatography (HPLC) of MB initial solution and the residue MB solutions after reused for 1e4 times (C), and mass spectrum at the three peaks in HPLC (D, E and F).

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Fig. 6. Possible decomposition processes in the BC-TMZ.

Fig. 7. Nitrogen adsorption/desorption isotherm (Analysis bath temperature 195.85  C, Equilibration Interval 10 s) of BC (A), BC-T (B), BC-TMZ (C) and their pore diameter distributions (D).

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Table 4 Properties of composite bio-chars. Samples

SBET (m2/g)

Total pore volume (cm3/g)

Average pore diameter (nm)

Reference

Bamboo charcoal Sludge bio-char BC BC-T BC-TMZ

70.00 89.91 296.10 159.44 86.27

0.04 0.19 0.28 0.16 0.06

2.15 9.66 3.29 3.26 2.64

Yang et al. (2014) Xiao et al. (2018) In this study In this study In this study

Fig. 8. XRD patterns of as-prepared materials in range of 15o-80 (Cu target, light tube power 2.2 kW, at the rate of 0.1 /min) (A); FTIR spectra of the BC, BC-T and BC-TMZ (B).

the MB was not degraded completely and decomposition capability of BC-TMZ declined with the increasing of repeat time. Moreover, the peak 1 and peak 3 did not occur in HPLC of initial MB solution (MB), R-1st and R-2nd but appeared in R-3rd and R4th, meaning that 2, 3- phenolic hydroxyl aniline and 3, 7- amino phenothiazine were not components of initial MB solution but intermediate products during the MB decomposition. The presence of intermediate products in R-3rd and R-4th also indicate that the decomposition capability of BC-TMZ declined and some intermediate products of MB were not decomposed completely. In conclusion, the MB can be decomposed completely by the BC-TMZ with UV light irradiation, but the degradation capability declined with the increasing of reuse time. Based on the above research results, the possible processes of MB decomposition by BC-TMZ are shown as Fig. 6. Three decomposition mechanisms (Fe-C microcell effect, photocatalysis and metal oxide catalysis) may be included to support the MB decompose process. Firstly, for the TiO2 photocatalysis process with UV irradiation, plenty of charges that separated from electron-hole pairs were generated on the TiO2 particles. Then, the electrons are captured by O2 and react with MB (Zhang and Park, 2017; Seow and Lim, 2016). Secondly, for a Fe-C micro-battery, the REDOX reaction happened when the electrons are transferred from negativev electrodes (Fe) to positive electrodes (C), which could decompose MB in the solution (Mahmoud et al., 2016). Thirdly, the bonds of MB molecules can be broken by the magnesium oxide crystals and the incorporated zinc oxide crystals can decrease the activation energies of catalysis (Fonseca et al., 2017; Devi and Saroha, 2017). As a result, the MB molecules may be decomposed into small molecules

like H2O and CO2 (Sun et al., 2014).

3.6. Characterization of the bio-char composites Nitrogen adsorption/desorption isotherm was used to investigate the textural properties of as prepared materials (BC, BC-T and BC-TMZ) in Fig. 7(AeC) and the related data are recorded in Table 4. All of the three adsorption/desorption curves were type IV isotherm as per IUPAC classification (Fan et al., 2017). In Fig. 7(AeC), the sharp increase for the quantity adsorbed at the low pressure area indicate that rich micropores occur on the materials. Moreover, H4 type hysteresis loop is observed on the nitrogen adsorption/desorption isotherms, which means that the three materials are hierarchically porous materials. The BET curve of the sample BCT and BC-TMZ were not fully closed. The possible reason is that the loaded metal oxides changed the porous channel and the structures were too complex to desorb N2. According to the pore diameter distributions (Fig. 7 (D)), the pore structures have highest pore volume when pore diameters are in the range of 2e50 nm, which reveals that the bio-char composites also have mesoporous structures (Nourmoradi et al., 2012). From Table 4, BET surface areas, total pore volumes, average pore diameter and micro-pore volumes of BC are higher than those of BC-T and BC-TMZ. The results indicate that the pores on the bio-char composites were partially filled by the metal oxide particles. Additionally, the average pore diameters of BC-TMZ (2.64 nm), BC-T (3.26 nm) and BC (3.29 nm) demonstrate that the prepared materials have mesoporous structures (2e50 nm), which were also consistent with the analysis results in Fig. 7 (D). In conclusion, the prepared materials have both

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Fig. 9. SEM images of BC (A), BC-T (B) and BC-TMZ (C); Energy Dispersive Spectroscopy of BC-T (D) and BC-TMZ (E); EDX elemental mappings of BC-T (F) and BC-TMZ (G) from selected areas for C, Fe, Ti, Mg and Zn.

micropores and mesopores, and the loaded metal oxides decreased the BET surface area of biochars. The X-ray diffraction patterns of as-prepared materials in the wide angle range of 15o-80 were shown as Fig. 8 A. In the XRD patterns of BC, BC-T and BC-TMZ, the (210) peak at 30.1 corresponds to the graphite which was formed during the pyrolysis process. Comparing with BC, more peaks at 25.2 (101), 62.1 (213) in the BC-T and BC-TMZ represented its anatase components, whereas the peaks at 35.6 (110), 43.5 (202) and 57.1 (122) exhibited the hematite that derived from iron powder added before pyrolysis (Sangeeta et al., 2017). For the BC-TMZ, the (003) peak at 21.2 and the (100) peak at 31.2 indicated the coexistence of MgO

(21.2 ) and ZnO (31.2 ) crystals in the composite. Besides, the conversion of MgCl2 into MgO crystals were also proved by TGA in Fig. S 8, which were consistent with the XRD analysis. In conclusion, the TiO2 and Fe2O3 crystals were existed in BC-T, and the TiO2, Fe2O3, MgO, ZnO crystals have been loaded on the BC-TMZ. The incorporated metal oxide crystals have changed the crystalline phase structures of bio-char substantially. Besides, the TG analysis in Fig. S8 also show that the MgCl2 has been converted into MgO crystal, which is consistent with the XRD analysis. FTIR was conducted on as-prepared materials to observe functional groups and the FTIR spectrums are summarized in Fig. 8 B. Comparing with the BC, new broad peak appeared in

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471

Fig. 10. FESEM, HRTEM and SAED images of loaded TiO2 (A, B and C), MgO (D, E and F) and ZnO (G, H and I) crystals.

500e700 cm1 for the BC-T and BC-TMZ, which was attributed to stretching vibration of metal oxide (Ti-O-Ti (400e700 cm1), MgO-Mg (400-631 cm1) and Zn-O-Zn (333-600 cm1)) covalent
re et al., 2017). bonds (Sultan et al., 2018; Kim et al., 2018; Perrie These indicated that the metal oxide crystals have been loaded on the BC-T and BC-TMZ successfully. Furthermore, for the three samples, the peaks at 1620 cm1 and 3420 cm1 were attributed to the stretching vibration of adsorbed H2O and hydroxyl (-OH), the intense band at the range of 800e1250 cm1 was caused by the bending and stretching modes of C-C, and the frequently peaks near 2920 cm1 indicated that -CH2 groups presented in the biochars. Most components of the three samples were similar beside metal oxides, which demonstrated that the loaded metal oxide particles have little impact on the other functional groups in the materials. The surface morphology of as-prepared materials were characterized by SEM observation. As shown in Fig. 9 (AeC), all of the three samples exhibit rich pore structures, which were consistent with the high BET surface area in Table 4. Comparing with the BC (Fig. 9 (A)), the surface are rough with many nanoparticles in the BC-T and BC-TMZ ( Fig. 9(B and C)), which may indicate the existence of TiO2 nanoparticles on the bio-char surface. Moreover, many acicular and granular crystals are found in the pores of the BC-TMZ (Fig. 9 (C)), which may suggest that the zinc oxide (acicular crystals) and magnesium oxide crystals (granular crystals) have been loaded into the BC-TMZ. Because the distribution of TiO2 nanoparticles on the BC-TMZ are little changed, the ZnO and MgO

crystals loading process may have a small effect on the existence of TiO2 nanoparticles (Lee et al., 2018; Yang et al., 2014). However, due to the existence of the metal oxides in the pores, the BET surface area and pore volumes of BC-TMZ may be decreased, which was also consistent with the results in Table 4. Furthermore, to further verify the composition of nanoparticles and crystals loaded on the BC-T and BC-TMZ shown as SEM observation, several elements on the bio-chars from selected area were analyzed by energy dispersive spectroscopy (EDS) and elemental mapping ( Fig. 9 FeG. For the BC-T (Fig. 9 F) and BC-TMZ (Fig. 9 G), the elements of C, Fe, O, Ti and K are observed, indicating the TiO2, KCO3, Fe2O3 and Fe3O4 were existed. However, comparing with the BC-T, more elements of Mg and Zn are founded on the BCTMZ (Fig. 9 G), which means that ZnO and MgO crystals are loaded. The results verified the existence of metal oxide on the BC-T and BCTMZ, which were accorded with previous characterization. The nanoparticles, granular and acicular crystals observed in the SEM images were further characterized by FE-TEM as shown in Fig. 10(A, D and G). It is observed that the diameter of nanoparticles are about 7.310 nm, the granular crystals are 312.84 nm long and 182.68 width, and the acicular crystals are 1.931 mm long and 0.383 mm width. Furthermore, the HRTEM image (Fig. 10-B) of nanoparticles in Fig. 10-A reveals lattice fringes with distances of 0.349 and 0.248 nm, corresponding to the (101) plane of TiO2 and the (110) plane of Fe2O3, respectively. Besides, their relevant SAED pattern in Fig. 10-C also confirms the existence of TiO2 and Fe2O3 components on the bio-char. The lattice fringe patterns in Fig. 10-E

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Fig. 11. High-resolution XPS of C 1 s (A), O 1 s (B), Ti 2p (C), Mg 2p, Zn 2p (D) and as-prepared materials (E) in the binding energy regions.

with spacing's of 0.290 and 0.232 nm confirm the presences of (220) and (222) planes of MgO respectively, which are consistent with the SAED pattern in Fig. 10-F. The result mean that the major component of granular crystals in Fig. 10-D is MgO. And the interplanar separation obtained from the HRTEM image (Fig. 10-H) of acicular crystals are 0.247 and 0.190 nm, which are corresponded to (101) and (102) lattice planes of ZnO crystal. Moreover, the SAED pattern in Fig. 10-I also coincides with the interplaner distance in

corresponding HRTEM image and XRD pattern in the paper. The interactions between elements or components in the BC, BC-T and BC-TMZ were studied by high-resolution XPS in the C 1s, O 1s, Ti 2p, Mg 2p and Zn 2p binding energy regions. The C 1s XP spectra of as-prepared materials were shown as Fig. 11-A. All spectra exhibited two major components with binding energies of 284.6 and 286.7 eV, attributed to C-C and C-O moieties respectively. The C-C and C-O groups were typical products of pyrolysis for

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bamboo powder. For O 1s XPS of the three samples (Fig. 11-B), two peaks with the binding energies of 530.6 and 532.2 eV can be attributed to the lattice-oxygen and hydroxyl-oxygen. Thereinto, the lattice-oxygen is attributed to the metal oxides (such as Fe2O3, TiO2, MgO and ZnO) and the hydroxyl-oxygen is attributed to the adsorbed water or hydroxyl on the bio-char. Besides, for the Ti 2p XPS of BC-T and BC-TMZ (Fig. 11-C), the peaks are split into two components, 2p3/2 and 2p1/2, at 458.5 and 464.3 eV respectively, which are in good agreement with anatase TiO2. The Mg 2p and Zn 2p XPS of BC-TMZ are shown as Fig. 11-D. Four peaks with binding energies of 50.3 eV, 56.6 eV, 1022.5 eV and 1045.5 eV occur, which can be assigned to Mg 2p, Fe 3p, Zn 2p3/2 and Zn 2p1/2 states respectively. The element states are identical to the binding energy for Mg 2p (Mg2þ) in MgO, Fe 3p (Fe3þ) in Fe2O3 and Zn 2p (Zn2þ) in ZnO, respectively (Chi et al., 2019; Montero et al., 2016; Brunckova et al., 2019). The above analysis were consistent with the results in Fig. 11-E. 4. Conclusion As the Fe2O3, TiO2, ZnO and MgO crystals are loaded into pores of the bio-char, the BC-T and BC-TMZ have lower BET surface areas and total pore volumes than the BC. However, comparing with BC, the BC-T and BC-TMZ have higher removal capability and the highest removal capacities of adsorption are 85.1 mg/g (BC-T) and 114.5 mg/g (BC-TMZ), respectively. The adsorption kinetics and isotherm curves of BC-T and BC-TMZ indicate that the MB adsorption are determined by chemisorption and monolayer adsorption. Finally, the adsorbed MB can be decomposed into small molecules by BC-TMZ. The BC-TMZ can be used for 4 times to remove MB from solution completely with UV light irradiation. Considering the economy and reusability, the BC-TMZ is potential to use in a large scale in the future. Acknowledgements This research was supported by National Key R&D Program of China [2017YFB0309700]; the Fundamental Research Funds for the Central Universities [JUSRP51514]; and the Scientific Research Projects of Jiangsu Province [BY2016022-31]; China Postdoctoral Science Foundation Funded Project [2018M630520]; Postgraduate Research & Practice Innovation Program of Jiangsu Province [ KYCX18_1826 ]. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jclepro.2019.02.152. References APHA, 2005. Standard Methods for the Examination of Water and Wastewater, 21th ed. American Public Health Association, Washington DC, USA. Bogatu, C., Perniu, D., Sau, C., Iorga, O., Cosnita, M., Duta, A., 2017. Ultrasound assisted sol-gel TiO2 powders and thin films for photocatalytic removal of toxic pollutants. Ceram. Int. 43, 7963e7969. Brunckova, H., Kanuchova, M., Kolev, H., Mudra, E., Medvecky, L., 2019. XPS characterization of SmNbO4 and SmTaO4 precursors prepared by sol-gel method. Appl. Surf. Sci. 473, 1e5. Bulut, Y., Aydın, H., 2006. A kinetics and thermodynamics study of methylene blue adsorption on wheat shells. Desalination 194, 259e267. Chen, D., Yu, X., Song, C., Pang, X., Huang, J., Li, Y., 2016. Effect of pyrolysis temperature on the chemical oxidation stability of bamboo bio-char. Bioresour. Technol. 218, 1303e1306. Chi, M., Sun, X., Sujan, A., Davis, Z., Tatarchuk, B.J., 2019. A quantitative XPS examination of UV induced surface modification of TiO2 sorbents for the increased saturation capacity of sulfur heterocycles. Fuel 238, 454e461. Devi, P., Saroha, A.K., 2017. Utilization of sludge based adsorbents for the removal of various pollutants: a review. Sci. Total Environ. 578, 16e33.

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