High-efficiency removal of Cr(VI) by modified biochar derived from glue residue

Journal Pre-proof High-efficiency removal of Cr(VI) by modified biochar derived from glue residue Yueyue Shi, Rui Shan, Lili Lu, Haoran Yuan, Hong Jiang, Yuyuan Zhang, Yong Chen PII:

S0959-6526(19)34805-X

DOI:

https://doi.org/10.1016/j.jclepro.2019.119935

Reference:

JCLP 119935

To appear in:

Journal of Cleaner Production

Received Date: 5 May 2019 Revised Date:

2 December 2019

Accepted Date: 29 December 2019

Please cite this article as: Shi Y, Shan R, Lu L, Yuan H, Jiang H, Zhang Y, Chen Y, High-efficiency removal of Cr(VI) by modified biochar derived from glue residue, Journal of Cleaner Production (2020), doi: https://doi.org/10.1016/j.jclepro.2019.119935. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Author Contributions Section Yueyue Shi: hi performed writing – original draft and conceptualization. Rui Shan: Shan performed writing – review and editing. Lili Lu and Jiang Hong: Hong Software, Validation. Haoran Yuan: Supervision. Yuyuan Zhang and Yong Chen: conceptualization

Graphical Abstracts Soaked in chemicals (e. g. KOH, ZnCl , HCl) 2

Glue residue

Mass ratio of modifier and glue residue (i. e. 1:1, 2:1, 3:1)

Pyrolysis temperature (300/500/700/900 )

Modified biochar (e. g. Zn2GT700)

Reduction

Cr O 2

3

Desorption

COOH

Regeneration

Surface complexation Electrostatic interaction

Cr(VI)

Cr(III)

High-efficiency removal of Cr(VI) by modified biochar derived from glue residue Yueyue Shi a, c,d,e,1, Rui Shan a,b,c,d,1, Lili Lu a, Haoran Yuan a,b,c,d*, Hong Jiang f, Yuyuan Zhang g, Yong Chen a,b,c,d a

Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences,

Guangzhou 510640, China b

Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou),

Guangzhou, 511458, China c

CAS Key Laboratory of Renewable Energy, Guangzhou 510640, China

d

Guangdong Provincial Key Laboratory of New and Renewable Energy Research and

Development, Guangzhou 510640, China e

Nano Science and Technology Institute, University of Science and Technology of China,

Suzhou 215123, China f

Department of Chemistry, University of Science and Technology of China, Hefei

230026, China g

College of Materials Science and Energy Engineering, Foshan University, Foshan,

Guangdong 528000, China

*

Corresponding authors. Email address: [email protected] (H.R. Yuan). 1 Both authors contributed equally to this work. 1

Abstract: A series of cost-effective, high-efficiency and sustainable adsorbent for Cr(VI) were prepared from waste glue residue. Considering that chemical modification has great influence on the properties of biochar, several modifiers (HCl, KOH, and ZnCl2) were used to improve the adsorption capacity of biochar. Besides that, the effect of prepare conditions (i.e., pyrolysis temperature and impregnation ratio of modifier and glue residue) were also investigated. Emission scanning electron microscopy, transmission electron microscopy, X-ray diffractometer, X-ray photoelectron spectroscopy and Autosorb1-MP Quantachrome were used to characterize the physicochemical properties of waste glue residue biochar. Reaction time, solution pH, adsorbent dosage, and initial adsorbate concentration on contaminant removal were also examined. The maximum sorption capacity of ZnCl2 modified glue residue biochar reached 325.54 mg/g, which is higher than previous reported adsorbents. The recycling experiment demonstrated that the removal efficiency of the optimal biochar was 90% after six cycles. Hence, waste glue residue exhibits a great potential in in Cr(VI) contaminated water’s management. Keywords: Glue residue; Modified biochar; Cr(VI) removal; Adsorption mechanism

2

1 Introduction With the increase of industrialization and urbanization, water shortage and pollution are of great public concern. Heavy metal such as chromium is of specific concern due to its high toxicity, tendency to bioaccumulate, and persistency in water (Sharafi et al., 2019; Singh & Kumar, 2017;Wu et al., 2019). As an important industrial material, Cr has an extensive use in paper, electroplating, dye manufacture, and paint industries (Huang et al., 2016; Li et al., 2019). Chromium usually exists in compounds in trivalent and hexavalent, and hexavalent chromium is much more toxic than trivalent chromium. Cr(VI) contamination can lead to serious health problems, may even cause cancer (Zhu et al., 2018; Hilbrandt et al., 2019).Chemical precipitation, physical separation, and bioremediation are common methods for the elimination of Cr(VI) from water (Liu et al., 2019). Of these methods, sorption could be an efficient and economical method (Renu et al., 2017). Activated carbon (AC), silica gel, alumina, and biochar are common adsorbents (Ahmed et al., 2016). Among these adsorbents, biochar has attracted extensive attention due to its low cost, good adsorption effect and no harms of environment (Ahmad et al., 2014; Mohan et al., 2014). Biochar are generally derived from biomass, industrial waste, agricultural waste, or municipal sludge (Zhang et al., 2017a). Glue residues as an industrial waste, which are abundant in automobile and electronic manufacturing industries, contain toxic components, improper treatment may lead to pollution of the environment and hurt to human health (Liu et al., 2018). Industrial 3

polymer waste treatment by physical regeneration and chemical decomposition processes are costly, resource wasting and harmful to the environment. While pyrolysis could be a promising way to reuse waste glue residues, which can not only accelerate waste material utilization but also be useful in solving some environmental and ecological problems (Singh et al., 2012). The pyrolysis product of glue residue has a high carbon content, rich functional groups and high stability, which could be used in sewage treatment. But there are not ample amount of literature available regarding such studies. Several studies have been reported about the application of biochar in heavy metal removal from wastewater (Aghababaei et al., 2017; Ai et al., 2019). Dong et al. (2011) found that Cr(VI) adsorption capacity by sugar beet tailing biochar reached 123 mg/g in acidic condition. Zhang et al. (2018) proved magnetic biochar had a high removal efficiency under a low Cr(VI) concentration (10 mg/L). Shi et al. (2018) indicated that polyethylene imine modification of corn cobs hydrochar had an improvement of Cr(VI) removal, and the maximum adsorption capacities is 33.663 mg/g. However, the low adsorption capacity, complex preparation processes and poor recycling performance have limited large-scale use of these biochar, and the adsorption mechanism of Cr(VI) was not clear. Hence, it is important to develop a reusable adsorbent that is of high efficiency and low cost. Previous researches have suggested that modification can change the physicochemical performance of biochar, and may improve its adsorption capacity. The 4

various physical and chemical methods to modify biochar include steam activation, surface oxidation, acid activation, mineral dipping, and base treatment (Anfruns et al., 2014; Gokce & Aktas, 2014; Han et al., 2015; Trubetskaya et al., 2019; Wang & Kaskel, 2012). Modification can change functional groups, specific surface area, and pore structure of biochar (Singh & Kumar, 2017). Xia et al. (2016) proved that ZnCl2-activated biochar exhibited a good adsorption of As(III), Liu et al. (2018) found that the operation condition had a great influence on the characteristics of biochar. However, the mechanism of modification and the relationship between biochar preparation condition and Cr(VI) adsorption capacity has not been studied yet. In this study, glue residue biochar was prepared and used in Cr(VI) adsorption, and KOH, HCl and ZnCl2 were used as modifiers in the biochar production. The objectives summarized as: (1) characterize the physicochemical properties of biochar prepared in different condition, and evaluated their capacity to adsorb Cr(VI); (2) explore the mechanisms of modification and Cr(VI) adsorption; (3) analyze Cr(VI) adsorption process by the studies of adsorption kinetics, adsorption thermodynamics and adsorption isotherm; (4) examines the potential for efficient regeneration of the adsorbent and adsorbate recovery. This study can provide a new method for glue residue biochar preparation and Cr(VI) removal. 2 Materials and methods 2.1 Materials Glue residue was acquired from Foxconn Technology Group, Guangzhou, 5

Guangdong province (China). The glue residue was placed in an oven and dried at 105 °C for 24 h. The analytical reagents used including: HCl, ZnCl2, KOH, NaOH, HNO3, H2SO4 and K2Cr2O7, these were obtained from Shanghai Macklin Biochemical Co. Ltd.. 2.2 Preparation of biochar and modified biochar Biochar and modified biochar were prepared by oxygen-limited pyrolysis. For the control sample, a sample of glue residue was pyrolyzed in Nitrogen gas in a tube furnace. It took 70 min for the tube furnace to warm from 30 °C to 700 °C and keep the temperature constant for 120 min at 700 °C. The prepared biochar was labelled GB700. Modifier type, pyrolysis temperature, and impregnation ratio all influence modified biochar properties (Liu et al., 2015). The biochar modified by HCl, ZnCl2, and KOH were labelled HxGTy, ZnxGTy, and KxGTy, respectively, where x represents the mass ratio of the modifier to the glue residue, and y represents the pyrolysis temperature. To prepare the ZnCl2 modified adsorbents, 4-12 g ZnCl2 and 4 g glue residue were dissolved in 10 mL deionized water, and stirred for 24 h to mix them evenly, then dried in an oven for 12 h at 90 °C. The dry mixture was put into a tube furnace under a N2 flow for pyrolysis, and the reactor was heated from 30 °C to the terminal temperature (between 300 °C and 900 °C) at a rate of 10 °C/min and held for 2 h. When cooled to room temperature, the materials were washed in 0.1 M HNO3 or NaOH and deionized water several times to neutralize. For biochar labelling (e.g. Zn2GT700), the number after Zn refers to the mass ratio of ZnCl2 to glue residue (e.g. 6

2), and the number after GT represents the pyrolysis temperature (e.g. 700 °C). 2.3 Adsorbent characterization Surface morphology and element composition of biochar were explored by field emission scanning electron microscopy (Hitachi S-4800 FESEM, Japan) and energy-dispersive X-ray spectroscopy (EDS), respectively. The internal morphology of the sample was observed by transmission electron microscopy. The surface image and crystal structure of biochar were determine by X 'pert Pro MPD X-ray diffractometer from PANalytical company in the Netherlands. Fourier infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS, ESCA Lab 250Xi, Thermo Fisher, USA) were used to study the types and contents of functional groups and to analyze adsorption mechanisms. The mass loss was detected by a SDT Q600 thermogravimetric analyzer (TGA). The specific surface area, pore size and total volume were calculated by adsorption of N2 using an Autosorb1-MP Quantachrome. A laser particle analyzer was used to measure zeta potentials of the prepared biochar at different pH values. The zero point charge (ZPC) of the biochar was evaluated based on the pH shift method of Pashai Gatabi et al. (2016). The total Cr content was detected by ICP, and the content of Cr(VI) was detected though a spectrophotometry. Cr(VI) can react with diphenylcarbazide formed a purple complex, which can be determined by a UV spectrophotometer at the wavelength of 540 nm. The content of Cr(III) was calculated by the difference between the total Cr content and Cr(VI).

7

2.4 Adsorption and desorption experiments K2Cr2O7 was dissolved into deionized water to obtain a 1000 mg/L Cr (VI) stock solution. And the stock solution was diluted to the desired concentration (between 50 to 1000 mg/L). Into a 15 mL vial, 10 mL Cr(VI) solution and 20 mg of the prepared biochar were added. Then this was oscillated at 80 rpm for 1440 min on a tube mixer. The suspension was filtered through a 0.22 µm syringe filter before the final concentration was tested. The pH of the Cr(VI) stock solution was varied from 1 to 10, jumping up with value of 1 at each time. The reaction time was varied from 10 min to 1440 min, and the sorbent dosage ranged from 10 mg to 80 mg. For the adsorption kinetics test, Cr(VI) solution was 500 mg/L initially; the sorbent concentration was 2 g/L in their optimal pH and the reaction times were set between 5 min and 720 min. Adsorption isotherms were implemented at 25 °C by adding 20 mg biochar to 10 mL Cr(VI) solution, and the initial concentration was within the range 50-1000 mg/L. The desorption experiments of Cr(VI) were carried out in accordance with Dong et al. (2011). To estimate Cr(VI) removal capabilities of prepared biochar, qe (mg/g; the quantity of contaminant adsorbed per unit weight of biochar) was computed by Eq. (1):

qe = (c0 − ce )V / m The percentage of heavy metals removed (E) was calculated by Eq. (2): 8

(1)

E = (c0 − ce )100% / c0

(2)

where c0 and ce are Cr(VI) concentration initially and at equilibrium (mg/L), separately; V is the aqueous solution volume (L), and m is the biochar mass (g). 3 Results and discussion 3.1 Adsorbent characterization Table 1 shows the physical characteristics of different biochar. Three chemical reagents were used for modified biochar preparation. The BET surface area (SBET) of the HCl, ZnCl2 and KOH modified biochar were 85.93, 694.03 and 860.45 m2/g at 700°C, respectively. KOH-modified biochar and AC mainly contained microporous, while macropores and mesopores were the major pore structure in ZnCl2-modified biochar. When pyrolysis temperature increased from 300 °C to 700 °C, the SBET of the ZnCl2-modified biochar increased and the ratio of micropores decreased. This may be due to the reaction of ZnCl2 with organic compounds in the glue residue, which promoted the formation of a mesoporous structure with the increase in temperature. According to the N2-adsorption curve, biochar prepared at 900 °C had more micropores, so its specific surface area was larger than biochar prepared at 700 °C. Under high temperature, more micropores might be formed, and mesoporous might collapse into macropores, resulting in a decrease in mesoporous and total pore volume. Therefore, biochar (mass ratio of ZnCl2 to glue residue of 2:1) pyrolysis at 700 °C exhibited the largest SBET and more highly mesoporous structure compared to the

9

other preparations.

Table 1 The specific surface area and pore size of different biochar BET surface area

Micropore surface

Total pore volume

Average pore

(m2/g)

area (m2/g)

(cm3/g)

diameter (nm)

GB700

4.79

0

0.00745

3.1108

H2GT700

85.93

41.752

0. 0653

1.5202

K2GT700

860.45

738.36

0.6124

1.4231

Zn2GT300

507.28

317.92

0.3628

1.4303

Zn2GT500

512.86

254.21

0.4756

1.3596

Zn2GT700

694.03

160.559

0.8952

2.5797

Zn2GT900

730.86

364.19

0.4756

1.2483

Zn1GT700

521.08

195.08

0.5710

2.1915

Zn3GT700

488.26

123.232

0.496

2.2539

AC

886.34

798.749

0.4791

1.0810

Sorbent

The FTIR spectrum of samples are presented in Figure 1. The peaks at 3419 cm-1, 1632 cm-1 and 1356 cm-1 could be respectively attributed to the vibrations of –OH, C=O and COO– (Lin et al., 2017). After modification by KOH, HCl and ZnCl2 reagents, the strength of these peaks for H2GT700, K2GT700 and Zn2GT700 were significantly enhanced compared to the unmodified biochar (GB700), and Zn2GT700 was of the strongest peaks. The strengths of the –OH, C=O and COO– peaks for ZnCl2-modified biochar was in the order of: Zn2GT300 > Zn2GT700 > Zn2GT500 > Zn3GT900. After Cr(VI) adsorption, the peaks of Zn2GT700 surface groups declined sharply, which might be because the functional groups of the biochar surface became complexed with Cr. In the case of AC, the peaks of its surface functional groups were 10

of low intensity, which may be one of the reasons for its poor Cr(VI) adsorption capacity. Scanning electron microscope images were used to examine the microstructure of GB700, Zn2GT700, and Zn2GT700+Cr. Elements at the surface of these biochar were also studied by EDS (Figure 2). The GB700 biochar exhibited a blocky surface structure (Figure 2a). After ZnCl2 modification, porous structures were observed on Zn2GT700 surface (Figure 2c), consistent with N2-adsorption. After Cr(VI) adsorption (Figure 2e) bright small particles were observed on the surface of Zn2GT700. EDS images showed that C, O, Al, and Si were the main elements on the surface of GB700 (Figure 2b) and Zn occurred on the surface of Zn2GT700 (Figure 2d). Comparison of the surface elements on Zn2GT700 before and after Cr adsorption, Zn content on the surface of Zn2GT700 decreased significantly, and Cr appeared after adsorption reaction, which proved that Cr was adsorbed onto the biochar surface (Figure 2f). Crystalline structures of the modified biochar were evaluated by examination of XRD patterns. The crystal structure of ZnCl2 modified biochar varied with the pyrolysis temperature (Figure 3a). The Zn2GT300 peaks at 2θ = 11.2°, 22.2°, 24.8°, 32.9°, 33.6° and 37.9° correspond with Zn5(OH)8Cl2·2H2O (Xia et al., 2016). The peaks at 31.6°, 34.2°, 36.1°, 47.3°, 56.4°, 62.7°, 67.8° and 68.9° in Zn2GT500, correspond with ZnO (Long et al., 2017). The ZnO diffraction peaks were also observed in Zn2GT700 and Zn2GT900, while Zn2GT700 showing the strongest 11

intensity. The peaks at 21.9°, 25.3°, 38.7° and 48.8° in Zn2GT700 and Zn2GT900 were attributed to Zn2SiO4 based on Jade software. The characteristic peaks of GB700, K2GT700, H2GT700 and AC (Figure 3b) at 24.8° and 43.9° were attributed to amorphous carbon (Zhang et al., 2017b). After Cr(VI) sorption, the strength of ZnO peaks on the surface of Zn2GT700 decreased dramatically, which could be due to precipitation of Cr(III) ions onto the surface of Zn2GT700. 3.2 Effects of preparation conditions on adsorption capacity of biochar 3.2.1 Effect of modifier type Figure 4a shows the removal of Cr(VI) by different biochar (modified by an acid (HCl), a salt (ZnCl2) and a base (KOH)). The modified biochar all had better removal efficiency of Cr(VI) than the raw biochar, and the ZnCl2 modified biochar had the largest adsorption amount of Cr(VI) at both low and high concentrations. ZnCl2 has strong dehydration capacity, which can reduce the carbonization temperature of organic components, restrain the formation of tar, retain more carbon and form more functional groups on biochar surface (Al-Lagtah et al., 2016). Moreover, ZnCl2 can make organics swell up at low temperatures, and intrude into the organics interior, which lead to the breaking of chain and the depolymerization of organic compounds, thus forming a molten mixture. ZnCl2 has a relatively low melting and boiling point (263 and 732 °C, respectively), which forms molten salts during pyrolysis process and distributed in the carbon structure, increased the metal cation on the surface of biochar, and facilitated the formation of mesoporous (Liu et 12

al.,2015). The pyrolysis process of glue residue and the mixture of ZnCl2/glue residue (mass ratio of 2:1) in a N2 atmosphere were analyzed with the help of TG and DTG (Figure 5). The first 10% of weight loss between 214 °C and 259 °C in pure glue residue (Figure 5a) was due to evaporation of water, small molecules, and gases from the sample. The second weight loss around 330–450 °C was attributed to the decomposition of macromolecular chains. For ZnCl2/glue residue, the initial mass loss below 100 °C was due to the vaporization of moisture (Figure 5b). The weight loss between 180 °C and 300 °C could be attributed to ZnCl2 reacting with glue reside, and formed Zn5(OH)8Cl2·2H2O. In the range of 400-580 °C, the mixture lost 50% of its weight, and a sharp peak in DTG was observed at about 520 °C, which corresponded to the decomposition of Zn5(OH)8Cl2·2H2O into ZnO. When the pyrolysis temperature exceeded 800 °C, ZnO may react with C to form Zn vapor and CO/CO2, which resulted in the weight loss of 10%. Furthermore, ZnO could react with SiO2 to form Zn2SiO4 (Sun et al., 2013). The analysis of TG and DTG were consistent with XRD results. The potential pyrolytic process of ZnCl2 in the mixture may be summarized as:

5ZnCl2 +10H2O → Zn5 (OH )8 Cl2 2H2O + 8HCl

(3)

Zn5 (OH )8 Cl2 2H2O → 5ZnO + 2HCl + 5H2O

(4)

2ZnO + SiO2 → Zn2 SiO4

(5)

13

3ZnO + 2C → 3Zn ↑ +CO ↑ +CO2 ↑

(6)

KOH treatment can increase the hydrophilicity of biochar and contribute to the formation of micropores. When glue residue decomposed at a low temperature, some volatiles such as CO2 could release and react with KOH to form K2CO3. As the temperature rose, KOH could reacted with C to form K vapor and CO. At a higher temperature, K2CO3 could react with C and decompose into K2O; and then K2O might be reduced to K by CO (Wang et al., 2012). The escape of gas facilitates the formation of micropores.

2KOH + CO2 → K2CO3 + H2O

(7)

2 KOH + 2C → 2 K ↑ +2CO ↑ + H 2 ↑

(8)

K2CO3 + C → K2O + 2CO ↑

(9)

K2O + C → CO ↑ +2K ↑

(10)

HCl treatment can increase the acidic groups on the surface of biochar, and enlarge its specific surface area by removing the mineral deposition on the surface of biochar. However, the specific surface area of HCl modified biochar (H2GT700) is smaller than KOH and ZnCl2 modified biochar, and after HCl treatment, the metal ions on the surface of biochar decreased. The adsorption capacity of modified biochar (K2GT700, H2GT700 and Zn2GT700) were higher than that of the un-modified glue residue biochar (GB700), 14

indicating the affirmative effect of modification. The differences in results could be related to the surface functional groups and pore structures in the biochar. Cr(VI) exists as Cr2O72-, CrO4- and CrO42- in aqueous solution, whose radii are large, which made them hard to be adsorbed by micropores. This could explain why K2GT700 and AC, both with abundant micropores, but had low adsorption capacities. The ZnCl2 modified biochar (Zn2GT700) had the richest surface functional groups, was mesoporous and had the best Cr(VI) removal performance (Table 1 and Figure 1). And the adsorption capacity of biochar modified by HCl was much lower than those modified by KOH and ZnCl2. Therefore, ZnCl2 was selected as the reasonable modifier for the preparation of modified biochar in the following test. 3.2.2 Effects of pyrolysis temperature Figure 4b shows the removal of Cr(VI) by ZnCl2-modified biochar prepared at different temperatures. As the preparation temperature increased, the removal efficiency and adsorption amount of Cr(VI) increased and reached to the largest value at 700 °C. However, at the higher temperature of 900 °C, both of them decreased. This can be explained by the increases in temperature promoting the development of mesoporous structure and surface functional groups, but when the temperature is too high, more micropores formed, mesoporous and the functional groups decreased (Ahmad et al., 2018). Considering mesoporous and functional groups were significant in Cr(VI) adsorption, of the tested temperatures, 700 °C is the most reasonable temperature for ZnCl2-modified biochar preparation. 15

3.2.3 Effect of impregnation ratio The mass ratio of modifier (ZnCl2) and glue residues could also be an important factor affecting the adsorption capacity of biochar. The effects on Cr(VI) removal by biochar prepared at different ZnCl2: glue residue impregnation ratios (1:1, 2:1 and 3:1) are compared in Figure 4c. Cr(VI) adsorption capability increased when the impregnation ratio was augmented from 1:1 to 2:1, and Zn2GT700 showed the best performance. Then the adsorption amount decreased at the impregnation ratio of 3:1. Co-pyrolysis of ZnCl2 and organic components increased the active sites on the biochar surface, making it more conducive to contaminant adsorption. However, when the ZnCl2 ratio became too high, the viscosity of the molten salt solution increased, which will inhibit the mixing of ZnCl2 and glue residue, and decreasing the activation sites. With excessive ZnCl2 impregnation, some crystalline salts may remain on the biochar, blocking the pore structure and affecting adsorption efficiency (Duan et al., 2019). Therefore, 2:1 was selected as the most suitable dipping ratio. Based on the results presented in section 3.2, sample Zn2GT700 had the largest Cr(VI) removal capacity and was selected for the further experimentation. 3.3 Cr(VI) removal by Zn2GT700 in different reaction conditions 3.3.1. Influence of environment pH on Cr(VI) removal Figure 6a shows the removal results of Cr(VI) by Zn2GT700 in the pH values from 1 to 10. The maximum adsorption (206.7 mg/g) of Cr(VI) occurred at a solution pH of 2. At pH values above 2, Cr(VI) removal dropped rapidly with increasing pH 16

values. This is consistent with the observations of Li et al., (2013) and which may be related to the surface characteristics of the biochar and presence of different Cr(VI) species at diverse pH values. At low solution pH (pH < 2), the main species of Cr(VI) was Cr2O72-, at pH values in the range of 2-6, both Cr2O72- and HCrO4- existed; while pH values exceeds 6, CrO42- becomes dominant (Shi et al., 2018). Considering that the pHzpc of Zn2GT700 was 7.07 (Figure 7), in acidic conditions, the high H+ concentration made for ample protonation of biochar surface functional groups, and the positive charges were conducive to HCrO4- adsorption. In an alkaline environment, the negative charge of the biochar surface increased due to the high proportion of OH-, which led to electrostatic repulsion between CrO42- and active sites on the surface of biochar (Liu et al., 2019). These results indicate that in an acidic solution, electrostatic interaction is more important in Cr(VI) adsorption, whereas in an alkaline solution the ion exchange is more important. 3.3.2 Influence of Zn2GT700 dose on Cr(VI) adsorption The adsorbent dose is considered a significant factor that can affect the adsorption process(Zhang et al., 2017b). Figure 6b shows the results of Cr(VI) adsorption by Zn2GT700 at different doses. With an increase in adsorbent dosage, the quantity of contaminants adsorbed per unit adsorbent decreased and the removal efficiency increased. When adsorbent dose was increased from 10 mg to 20 mg, the adsorption capacity reduced slightly (from 325.54 mg/g to 208.86 mg/g) and removal efficiency increased markedly (from 68.28% to 85.04%). At 40 mg and 80 mg of adsorbent, the 17

adsorption capacity reduced considerably and the percent removal increased slightly. Therefore, 20 mg was selected as the adsorbent dosage in the adsorption experiments. 3.4. Adsorption isotherm, thermodynamics and kinetic study of Cr(VI) adsorption

Table 2 Adsorption isotherm and kinetic models mentioned in this study

Adsorption isotherm models

Adsorption kinetic models

Names

Equations

Freundlich

qe = k F ce1/ nF

Langmuir

qe = qmkLce (1+ kLce )

Pseudo-first order

qt = qe (1 − e− k1t )

Pseudo-second order

qt = k2 qe 2t / (1 + k2 qet )

Elovich

qt = A + B ln t

Intraparticle diffusion

qt = ki t1/2 + C

Table 3 Fitting parameters of the adsorption isotherm studies Adsorption isotherm parameters Freundlich

kF = 67.886 mg(1-n)L/g

nF = 0.2286

R2= 0.9619

Langmuir

qm= 307.80 mg/g

kL= 0.08587

R2 = 0. 9385

Table 4 Fitting parameters of the adsorption kinetics studies 18

Adsorption kinetic parameters pseudo-first order

,

209.61

pseudo-second order

Elovich

intraparticle diffusion

qe1 = 191.18 mg/g

qe2 =202.79 mg/g

A= 31.45 mg/g

C=107.33 mg/g

k1 =0.03407

k2= 0.0002545

B=25.99

ki=3.27

R2 = 0.8491

R2 = 0.9632

R2 = 0.9560

R2 = 0.7167

(mg/g)

Two adsorption isotherm (Langmuir and Freundlich) and four adsorption kinetic (pseudo-first order, pseudo-second order, Elovich and intraparticle diffusion) models were used in the study of adsorption processes. The equations of these models are shown in Table 2. The Langmuir and Freundlich isotherm models were selected to analyze the Cr(VI) adsorption performance by Zn2GT700, and to help understand the adsorbate-adsorbent interactions. Figure 8a shows that the Freundlich model was more suitable to describe the adsorption of Cr(VI) by Zn2GT700, with a correlation coefficient of 0.9619 (Table 3). The maximum Langmuir adsorption amount (qm) was 307.80 mg/g, which is higher than for Melia azedarach wood biochar (130.5 mg/g: (Zhang et al., 2018)), ramie biochar (82.23 mg/g: (Zhou et al., 2016)) and nanoscale iron sulfide biochar (150 mg/g: (Lyu et al., 2017)). The Freundlich isotherm model suggests multilayer adsorption on heterogeneous surfaces, whose adsorption curve was nonlinear (Kumari et al., 2019). The nF value of the Freundlich isotherm model was less than 1, indicating that the interaction between adsorbate and adsorbent was chemical adsorption, and Cr(VI) was easily absorbed on the surface of Zn2GT700. 19

Adsorption kinetics studies were used to explore the mechanisms between Zn2GT700 and Cr(VI), too. Figure 8b and Table 4 show that the observed Cr(VI) adsorption data is fit by the pseudo-second order (PSO) kinetic model (R2 = 0.9632) better than by the pseudo-first order (PFO) kinetic model (R2 = 0.8491). Besides that, the adsorption values of calculated by pseudo-second order kinetic model (qe2 = 202.79 mg/g) was closer to the experimental value (qe,exp = 209.61 mg/g) than the pseudo-first order kinetic model (qe1 = 191.18 mg/g). This suggests that Cr(VI) adsorption capacity was related to adsorption points on the surface of the biochar and that chemical adsorption dominated the reaction process. The fitting curve of Elovich model is shown in Figure 8c. Elovich model can also well describe the adsorption process, whose R2 is 0.956 (Table 4), which means that the mechanisms of Cr(VI) adsorption are complicated (Xia et al., 2016). The fitting results of the intraparticle diffusion model are shown in Figure 8d. The line fitted by qt and t1/2 does not pass the origin, indicating that intraparticle diffusion is not the speed limiting step, and the adsorption process is under the common control of other adsorption mechanisms (Zhou et al., 2017). The results of adsorption kinetic model is consistent with the adsorption isotherm. Table 5 Adsorption thermodynamics parameters

Temperature (K)

lnkd

∆G (kJ/mol)

∆H (kJ)

∆S (J∙mol-1K-1)

293

0.952

-2.319

4.547

23.39

20

303

0.993

-2.493

313

1.073

-2.784

Adsorption thermodynamics is important for describing the adsorption properties of sorbents, and ∆G, ∆H and ∆S are the main parameters which can be calculated by the following formula:

∆G = −RT ln kd

(11)

∆S ∆H − R RT

(12)

ln k d =

kd =

qe ce

(13)

where R is a constant, whose value is 8.314 J/(mol∙ K); T is absolute temperature; qe and ce are equilibration adsorption amount and equilibration adsorbate concentration. Table 5 showed the adsorption thermodynamics parameters. It can be found that the value of ∆G is negative at different temperatures, indicating that the adsorption process is thermodynamically spontaneous. And with the increase of temperature, the value of ∆G decreased, which suggested that higher temperature was beneficial to the adsorption reaction.

∆H represents the change of enthalpy of a reaction; ∆H > 0 means that the adsorption of Cr(VI) is endothermic; ∆S is the change in entropy of the reaction, whose value is greater than 0 implying that the disorder in solid/liquid interface 21

increased (Chen et al., 2019). 3.5 Adsorption mechanisms of Cr(VI) XPS was used to investigate the components and chemical states of the biochar before and after adsorption. The XPS full spectrum analysis of biochar (Figure 9a) showed that that the major peaks for GB700 were C, O, Si. For Zn2GT700, there was also a distinct Zn peak. After adsorption, an apparent decline in C1s and Zn2p was observed, and the Cr2p peak emerged in Zn2GT700. This suggested that Zn2GT700 had remarkable adsorption effect on Cr(VI). The C and Zn on the surface of Zn2GT700 might be related to contaminant adsorption. The C1s signal (Figure 9b) could be split into three peaks: C–C/C–H(284.8eV), C–O (286.2eV), and O– C=O(288.4eV). Associated with the FTIR results, C–C/C–H, –OH and –COOH, as strong electron donors, are reductive and can oxidize Cr(VI) into Cr(III) (Li et al., 2017). For Zn2p, the binding energies of 1045.2 eV and 1021.9 eV (Figure 9c) were attributed to Zn2p1/2 and Zn2p3/2 of ZnO, respectively (Xia et al., 2016). The analysis of XRD and XPS data showed that ZnO mainly existed on the surface of Zn2GT700. After Cr(VI) adsorption, both Cr(VI) and Cr(III) could be detected on Zn2GT700 surface. The Cr2p signal could be divided into Cr2p1/2(III), Cr2p3/2(VI), and Cr2p3/2(III) (Figure 9d), whose binding energies were 587.2 eV, 579.4 eV, and 577.3 eV, respectively (Campos et al., 2019). The significant reduction in ZnO content after adsorption might be put down to the ion exchange between Zn2+ and Cr(III). Cr adsorbed on the surface of Zn2GT700 was split into 18% Cr(VI) and 82% Cr(III). 22

The transformation from Cr(VI) to Cr(III) can be summarized as:

Cr2O72− + 14H + + 6e− → 2Cr 3+ + 7 H 2O

(14)

HCrO4− + 7 H + + 3e− → Cr 3+ + 4 H 2O

(15)

CrO42− + 8H + → Cr 3+ + 4 H 2O

(16)

Transmission electron microscope spectra and element maps of Zn2GT700 adsorbed with Cr(VI) are provided in Figure 10. Graphitized carbon was observed, and Zn-based particles were uniformly distributed on the surface of Zn2GT700 (Figure 10a and Figure 10b). After adsorption of Cr(VI), C, O, Si, Al, Zn and Cr were found on the surface of Zn2GT700. Oxygen, Zn and Si were distributed in similar densities, indicating that they mainly existed on the surface as ZnO and Zn2SiO4 particles, in-keeping with XRD analyses. The density distribution of Cr was positively correlated with other elements on the surface, possibly due Cr(III) deposition by ion exchange and co-precipitation (Cr2O3 formation). Zn2GT700 excelled at the removal of Cr(VI). Figure 11 showed the adsorption mechanisms of Cr(VI), which can be summarized as follows: Firstly, Cr(VI) was adsorbed onto protonated functional groups on the surface of biochar by electrostatic interaction, which was mostly reduced to Cr(III); Secondly, a significant amount of Cr(III) was adsorbed onto the Zn2GT700 surface by ion-exchange with Al3+, H+, and Zn2+, and complexation with surface functional groups, or adsorption via the pore structure. The remaining Cr(III) was released into solution. 23

3.6 Desorption and regeneration studies The goal of regeneration is not only to recover the adsorption capacity of depleted adsorbent, but also to recycle valuable adsorbate. Since solution pH had significant influence on Cr(VI) adsorption by Zn2GT700, it is necessary to control pH during desorption. Both 0.1 mol/L NaOH and H2SO4 solutions were used as elution to better understand Cr desorption process (Figure 12a and Figure 12b).

After elution with NaOH solution, Cr(VI) and Cr(III) could be detected, and their concentration increased with reaction time. Cr(VI) concentration was higher than Cr(III) in the alkaline solution. After 24 h, Cr(VI) and Cr(III) concentrations in the solution reached 4.1 and 12.9 mg/L, respectively. This may be due to oxidation of Cr(III) adsorbed onto Zn2GT700 surface to Cr(VI) under alkaline conditions and leached into solution (Park et al., 2005). This proved the existence of the two main states of Cr on the biochar, further support the XPS analysis results. After being eluted by NaOH, the surface of biochar is deprotonated, and Cr(VI) and Cr(III) adsorbed by electrostatic effect escaped from the surface of the biochar into the solution, allowing large amounts of ZnO to be re-released. Cr2O3 on the surface of biochar, is oxidized under alkaline conditions and desorbed into solution, and then the surface functional groups of biochar is reduced negatively. Considering the concentration of chromium in industrial waste water, we choose the initial solution with concentration of 200 mg/L Cr(VI) for adsorption-desorption cycle experiment (the maximum adsorption amount of Zn2GT700 is 325.54 mg/g). After the first adsorption, many active 24

adsorption sites on the surface of biochar were not occupied by pollutants, which provided a supplement for the loss of Zn and functional groups in the subsequent cycle. The ZnO and functional groups on the surface of Zn2GT700 will play an important role on the next adsorption. As shown in Figure 12b, during H2SO4 desorption, the content of Cr(III) in solution was much higher than that of Cr(VI). After 24 h, Cr(III) and Cr(VI) concentrations were 100.6 and 0.2 mg/L, respectively. Both Cr(III) and Cr(VI) were adsorbed on the surface of Zn2GT700 in a strong acidic environment, and Cr(VI) could be easily reduced to Cr(III). Considering that the toxicity of Cr(III) is much lower than Cr(VI) and Cr-containing wastewater is generally acidic, Zn2GT700 could be a simple, safe and efficient adsorbent for Cr(VI) elimination from polluted water. The regeneration experiments were carried out according to the methods of Hu et al. (2005), 0.1M NaOH solution was used as an eluent. Figure 12c shows the effects on Cr(VI) adsorption by Zn2GT700 during six runs. After six cycles of adsorption/desorption, the removal efficiency of Cr(VI) by Zn2GT700 only decreased from 98% to 90%, which could be explained by a great mass of reversible sites on the surface of the adsorbent. Moreover, Cr(VI) could also be concentrated for recycling via desorption. The results of regeneration experiment indicated that the Zn2GT700 biochar was reusable and highly efficient in removing Cr.

4. Conclusions Modified glue residue biochar showed high efficiency in Cr(VI) removal from 25

aqueous solution. The influence of modifier type, operating conditions on biochar characteristics and Cr(VI) removal was investigated. The results showed that biochar (Zn2GT700) derived from glue residue modified by a 2:1 ratio of ZnCl2 and pyrolyzed at 700°C has the highest specific surface area and the richest surface functional groups, which proved to be an efficient and recyclable adsorbent in Cr(VI) adsorption. When pH value is 2, the maximum adsorption capacity of Zn2GT700 is 325.54 mg/g for Cr(VI), and the highest removal efficiency reached 98%, which were higher than AC and other modified biochar. The adsorption data fitted the PSO kinetic and Freundlich isotherm models well, and the adsorption thermodynamics study showed that Cr(VI) adsorption on Zn2GT700 is spontaneous. For the adsorption mechanism analysis, ZnO promoted the removal of Cr(VI) by electrostatic interaction and ion exchange, and surface functional groups such as -OH and COOH are conducive to reducing Cr(VI) to Cr(III). Besides that, co-precipitation and physical adsorption played important roles in Cr(VI) removal. Zn2GT700 also showed a good regeneration, Cr(VI) removal efficiency remained 90% after six cycles. The modified glue biochar can be expected to be potential application in Cr(VI) polluted wastewater treatment.

Acknowledgements This research was financially supported by State's Key Project of Research and Development Plan, China (y804091001), National Natural Science Funds of China (51776211), Key Special Project for Introduced Talents Team of Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou), (GML2019ZD0101). 26

27

References Aghababaei, A., Ncibi, M.C., Sillanpaa, M., 2017. Optimized removal of oxytetracycline

and

cadmium

from

contaminated

waters

using

chemically-activated and pyrolyzed biochars from forest and wood-processing residues. Bioresour. Technol. 239, 28-36. Ahmad, M., Rajapaksha, A.U., Lim, J.E., Zhang, M., Bolan, N., Mohan, D., Vithanage, M., Lee, S.S., Ok, Y.S., 2014. Biochar as a sorbent for contaminant management in soil and water: a review. Chemosphere 99, 19-33. Ahmad, M., Zietzschmann, F., Liu, R.J., Liu, G.J., Yousaf, B., Abbas, Q., 2018. Operating conditions-induced changes in product yield and characteristics during thermal-conversion of peanut shell to biochar in relation to economic analysis. J. Clean. Prod. 193, 479-490. Ahmed, M.B., Zhou, J.L., Ngo, H.H., et al. 2016. Progress in the preparation and application of modified biochar for improved contaminant removal from water and wastewater. Bioresour. Technol. 214: 836-851. Ai, T., Jiang, X., Liu, Q., Lv, L., Wu, H., 2019. Daptomycin adsorption on magnetic ultra-fine wood-based biochars from water: Kinetics, isotherms, and mechanism studies. Bioresour. Technol. 273, 8-15. Al-Lagtah, N.M.A., Al-Muhtaseb, A.H., Ahmad, M.N.M., Salameh, Y., 2016. Chemical and physical characteristics of optimal synthesised activated carbons 28

from grass-derived sulfonated lignin versus commercial activated carbons. Micropor. Mesopor. Mat. 225, 504-514. Anfruns, A., García-Suárez, E.J., Montes-Morán, M.A., Gonzalez-Olmos, R., Martin, M.J., 2014. New insights into the influence of activated carbon surface oxygen groups on H2O2 decomposition and oxidation of pre-adsorbed volatile organic compounds. Carbon 77, 89-98. Campos, A.F.C., de Oliveira, H.A.L., da Silva, F.N., da Silva, F.G., Coppola, P., Aquino, R., Mezzi, A., Depeyrot, J., 2019. Core-Shell Bimagnetic Nanoadsorbents for Hexavalent Chromium Removal from Aqueous Solutions. J. Hazard. Mater. 362, 82-91. Chen B, Chen S, Zhao H, et al. A versatile β-cyclodextrin and polyethyleneimine bi-functionalized magnetic nanoadsorbent for simultaneous capture of methyl orange and Pb (II) from complex wastewater[J]. Chemosphere, 2019, 216: 605-616. Dong, X., Ma, L.Q., Li, Y., 2011. Characteristics and mechanisms of hexavalent chromium removal by biochar from sugar beet tailing. J. Hazard. Mater. 190(1-3), 909-915. Duan, X.L., Yuan, C.G., Jing, T.T., Yuan, X.D., 2019. Removal of elemental mercury using large surface area micro-porous corn cob activated carbon by zinc chloride activation. Fuel. 239, 830-840. Gokce, Y., Aktas, Z., 2014. Nitric acid modification of activated carbon produced from 29

waste tea and adsorption of methylene blue and phenol. Appl. Surf. Sci. 313, 352-359. Han, Z., Sani, B., Mrozik, W., Obst, M., Beckingham, B., Karapanagioti, H.K., Werner, D., 2015. Magnetite impregnation effects on the sorbent properties of activated carbons and biochars. Water. Res. 70, 394-403. Hilbrandt, I., Ruhl, A.S., Zietzschmann, F., Molkenthin, M., Jekel, M., 2019. Competition in chromate adsorption onto micro-sized granular ferric hydroxide. Chemosphere. 218, 749-757. Hu, J., Chen, G., Lo, I.M., 2005. Removal and recovery of Cr(VI) from wastewater by maghemite nanoparticles. Water. Res. 39(18), 4528-4536. Huang, X., Hou, X., Song, F., Zhao, J., Zhang, L., 2016. Facet-Dependent Cr(VI) Adsorption of Hematite Nanocrystals. Environ. Sci. Technol. 50(4), 1964-1972. Kumari, U., Behera, S.K., Meikap, B.C., 2019. A novel acid modified alumina adsorbent with enhanced defluoridation property: Kinetics, isotherm study and applicability on industrial wastewater. J. Hazard. Mater. 365, 868-882. Li, H., Dong, X., da Silva, E.B., de Oliveira, L.M., Chen, Y., Ma, L.Q., 2017. Mechanisms of metal sorption by biochars: Biochar characteristics and modifications. Chemosphere 178, 466-478. Li, K.X., Huang, Z.A., Zhu, S.Y., Luo, S.L., Yan, L.S., Dai, Y.H., Guo, Y.H., Yang, Y.X., 2019. Removal of Cr(VI) from water by a biochar-coupled g-C3N4 nanosheets composite and performance of a recycled photocatalyst in single and combined 30

pollution systems. Appl. Catal. B 243, 386-396. Li, Y., Zhu, S., Liu, Q., Chen, Z., Gu, J., Zhu, C., Lu, T., Zhang, D., Ma, J., 2013. N-doped porous carbon with magnetic particles formed in situ for enhanced Cr(VI) removal. Water. Res. 47(12), 4188-4197. Lin, L., Qiu, W., Wang, D., Huang, Q., Song, Z., Chau, H.W., 2017. Arsenic removal in aqueous solution by a novel Fe-Mn modified biochar composite: Characterization and mechanism. Ecotoxicol. Environ. Saf. 144, 514-521. Liu, R., Liu, G., Yousaf, B., & Abbas, Q., 2018. Operating conditions-induced changes in product yield and characteristics during thermal-conversion of peanut shell to biochar in relation to economic analysis. J. Clean. Prod. 193, 479-490. Liu, W., Jin, L.D., Xu, J., Liu, J., Li, Y.Y., Zhou, P.P., Wang, C.C., Dahlgren, R.A., Wang, X.D., 2019. Insight into pH dependent Cr(VI) removal with magnetic Fe3S4. Chem. Eng. J. 359, 564-571. Liu, W.J., Jiang, H., Yu, H.Q., 2015. Development of Biochar-Based Functional Materials: Toward a Sustainable Platform Carbon Material. Chem. Rev. 115(22), 12251-12285. Liu, Y., Li, K., Guo, J., Xu, Z.M., 2018. Impact of the operating conditions on the derived products and the reaction mechanism in vacuum pyrolysis treatment of the organic material in waste integrated circuits. J. Clean. Prod. 197, 1488-1497. Long, L., Xue, Y.W., Zeng, Y.F., Yang, K., Lin, C.J., 2017. Synthesis, characterization and mechanism analysis of modified crayfish shell biochar possessed ZnO 31

nanoparticles to remove trichloroacetic acid. J. Clean. Prod. 166, 1244-1252. Lyu, H., Tang, J., Huang, Y., Gai, L., Zeng, E.Y., Liber, K., Gong, Y., 2017. Removal of hexavalent chromium from aqueous solutions by a novel biochar supported nanoscale iron sulfide composite. Chem. Eng. J. 322, 516-524. Mohan, D., Sarswat, A., Ok, Y.S., Pittman, C.U., Jr., 2014. Organic and inorganic contaminants removal from water with biochar, a renewable, low cost and sustainable adsorbent--a critical review. Bioresour. Technol. 160, 191-202. Park, D., Yun, Y.S., Jo, J.H., Park, J.M., 2005. Mechanism of hexavalent chromium removal by dead fungal biomass of Aspergillus niger. Water. Res. 39(4), 533-540. Pashai Gatabi, M., Milani Moghaddam, H., Ghorbani, M., 2016. Point of zero charge of maghemite decorated multiwalled carbon nanotubes fabricated by chemical precipitation method. J. Mol. Liq. 216, 117-125. Renu, Agarwal, M., Singh, K., 2017. Heavy metal removal from wastewater using various adsorbents: a review. J. Water. Reuse. Desal. 7(4), 387-419. Sharafi, K., Nodehi, R.N., Yunesian, M., Hossein Mahvi, A., Pirsaheb, M., Nazmara, S., 2019. Human health risk assessment for some toxic metals in widely consumed rice brands (domestic and imported) in Tehran, Iran: Uncertainty and sensitivity analysis. Food. Chem. 277, 145-155. Shi, S., Yang, J., Liang, S., Li, M., Gan, Q., Xiao, K., Hu, J., 2018. Enhanced Cr(VI) removal from acidic solutions using biochar modified by Fe3O4@SiO2-NH2 particles. Sci. Total. Environ. 628-629, 499-508. 32

Shi, Y., Zhang, T., Ren, H., Kruse, A., & Cui, R., 2018. Polyethylene imine modified hydrochar adsorption for chromium (VI) and nickel (II) removal from aqueous solution. Bioresour. Technol. 247, 370-379. Singh, S., Wu, C.F., Williams, P.T., 2012. Pyrolysis of waste materials using TGA-MS and TGA-FTIR as complementary characterisation techniques. J. Anal. Appl. Pyrol. 94, 99-107. Singh, U.K., Kumar, B., 2017. Pathways of heavy metals contamination and associated human health risk in Ajay River basin, India. Chemosphere 174, 183-199. Sun, L., Tian, C., Li, M., Meng, X., Wang, L., Wang, R., Yin, J., Fu, H., 2013. From coconut shell to porous graphene-like nanosheets for high-power supercapacitors. J. Mater. Chem. A. 1, 6462−6470. Trubetskaya, A., Kling, J., Ershag, O., Attard, T.M., Schroeder, E., 2019. Removal of phenol and chlorine from wastewater using steam activated biomass soot and tire carbon black. J. Hazard. Mater. 365, 846-856. Wang, J.C., Kaskel, S., 2012. KOH activation of carbon-based materials for energy storage. J. Mater. Chem. 22(45). Wu, J., Cheng, X., Yang, G., 2019. Preparation of nanochitin-contained magnetic chitosan microfibers via continuous injection gelation method for removal of Ni(II) ion from aqueous solution. Int.J biol. macromol. 125, 404-413. Xia, D., Tan, F., Zhang, C.P., Jiang, X.L., Chen, Z., Li, H., Zheng, Y.M., Li, Q.B., Wang, Y.P., 2016. ZnCl2 -activated biochar from biogas residue facilitates aqueous As(III) 33

removal. Appl. Surf. Sci. 377, 361-369. Zhang, H.Z., Chen, C.R., Gray, E.M., Boyd, S.E., 2017a. Effect of feedstock and pyrolysis temperature on properties of biochar governing end use efficacy. Biomass. Bioenergy 105, 136-146. Zhang, J., Chen, S., Zhang, H., Wang, X., 2017b. Removal behaviors and mechanisms of hexavalent chromium from aqueous solution by cephalosporin residue and derived chars. Bioresour. Technol. 238, 484-491. Zhang, X., Lv, L., Qin, Y., Xu, M., Jia, X., Chen, Z., 2018. Removal of aqueous Cr(VI) by a magnetic biochar derived from Melia azedarach wood. Bioresour. Technol. 256, 1-10. Zhou, L., Liu, Y., Liu, S., Yin, Y., Zeng, G., Tan, X., Hu, X., Hu, X., Jiang, L., Ding, Y., Liu, S., Huang, X., 2016. Investigation of the adsorption-reduction mechanisms of hexavalent chromium by ramie biochars of different pyrolytic temperatures. Bioresour. Technol. 218, 351-359. Zhou, Y., Liu, X., Xiang, Y., Wang, P., Zhang, J., Zhang, F., Wei, J., Luo, L., Lei, M., Tang, L., 2017. Modification of biochar derived from sawdust and its application in removal of tetracycline and copper from aqueous solution: Adsorption mechanism and modelling. Bioresour. Technol. 245, 266–273. Zhu, Y., Li, H., Zhang, G., Meng, F., Li, L., Wu, S., 2018. Removal of hexavalent chromium from aqueous solution by different surface-modified biochars: Acid washing, nanoscale zero-valent iron and ferric iron loading. Bioresour. Technol. 34

261, 142-150.

35

Zn2GT700+Cr AC

Transmittance (%)

Zn2GT900 Zn2GT500 Zn2GT300 Zn2GT700 K2GT700 H2GT700

GB700 1356 1632

500

1000

1500

3419

2000

2500

3000

Wavenumber (cm-1)

Figure 1

3500

4000

Figure 2

(a) ★ ★ ★

Zn2GT900

☆☆

★

★

★ ★★

Intensity (a.u)

☆ ★ ★ ★

Zn2GT700

★

★

★ ☆ ☆

★ ★

☆

★

★ ★ ★ ★

★

★

★● ★

★

Zn2GT500 ★ ★

●

10

● ●

20

★●●★★ ●

30

40

50

60

Zn2GT300 70

80

2θ (°) (b)

Intensity (a.u)

★ZnO ★ ☆Zn2SiO4 ●Zn5(OH)8Cl2· 2H2O ★ ★

★ ★ ☆ ☆ ☆☆

★

Zn2GT700 ★ ★

☆

★

★★★ ★

Zn2GT700+Cr AC K2GT700 H2GT700 GB700

10

20

30

40

2θ (°)

Figure 3

50

60

70

80

100

(a)

qe(c0=100 mg/L) 90 qe(c0=500 mg/L) 80 E(c0=100 mg/L)

80 70

qe (mg/g)

60

E(c0=500 mg/L)

50

70 60 50

40

40

30

30

20

20

10

10

0

E (%)

90

H2GT700 Zn2GT700 K2GT700

GB700

AC

0

Adsorbents 90

100

(b)

80

90

70

80

qe (mg/g)

60

50

50 40

40

30

30

20

20

10

10

0

E (%)

70

60

Zn2GT300

Zn2GT500

Zn2GT700

Zn2GT900

0

Adsorbents 90

100

(c)

80

90

70

80

qe (mg/g)

60

50

50 40

40

30

30

20

20

10

10

0

Zn1GT700

Zn2GT700

Adsorbents

Figure 4

Zn3GT700

0

E (%)

70

60

(a)

100 80

Weight (%)

-0.4 60 -0.8 40 20

-1.2

0

-1.6 100

200

300

400

500

600

700

800

Derive weight (%/℃ )

0.0

900 1000

Temperature (℃ ) 0.1

(b)

100 80

-0.1

60

-0.2 -0.3

40 -0.4 20 0

Weight (%) Derive weight (%/℃ ) 100

200

300

400

500

-0.5

600

700

Temperature (℃ )

Figure 5

800

-0.6 900 1000

Derive weight (%/℃ )

Weight (%)

0.0

100

(a)

210

90

180

80

qe (mg/g)

60 120

50

90

40

E (%)

70

150

30

60

20 30

10

0

0 2

4

6

8

10

pH 100

(b)

300

95

250

90

200

qe (mg/g) E (%)

150

85 80

100

75

50

70

0

10

20

30

40

50

60

Adsorbents dose (mg)

Figure 6

70

80

65

E (%)

qe (mg/g)

350

4 3 2

∆pH

1

pHPZC=7.07

0 -1 -2 -3 2

3

4

5

6

7

8

pHZPC initial pH value

Figure 7

9

10

11

(a)

350

200

300

180

qe (mg/g)

qe (mg/g)

250 200

Adsorption isotherm Freundlich Langmuir

150 100

160 140 120

pseudo-first-order kinetic pseudo-second-order kinetic

100

50 0

(b)

220

80

0

200

400

600

800

1000

1200

0

1400

200

400

600

ce (mg/L) (c)

220

800

1000 1200 1400 1600

t (min) (c)

240 220

200

200 180

qt (mg/g)

qt (mg/g)

180 160 140

qe Elovich

120

160 140 120

100

100

80

80 2

3

4

5

6

7

8

ln t (ln min)

qe Intraparticle diffusion model

0

5

10

15

20

25 1/2

t1/2 (min )

Figure 8

30

35

40

(b)

(a)

C1s

Zn2p3

O1s Cr2p

C－C/C－H

GT350-700+Cr

Intensity (a.u)

Intensity (a.u)

C1s Zn2p3 O1s GT350-700

C1s

O1s

Si2p 0

200

C－O O-C=O

GB700 400

600

800

1000

280

1200

282

284

Binding energy (eV)

286

290

292

294

Binding energy (eV)

(c)

(d)

Cr2p1/2

Cr2p3/2

Zn2p 3/2

Cr( )

Zn2p1/2

Cr( )

Cr2p3/2 Cr( )

Intensity (a.u)

Intensity (a.u.)

288

Zn2GT700

Zn2GT700+Cr

1015

1020

1025

1030

1035

1040

1045

1050

570

Binding energy (eV)

575

580

585

Binding energy (eV)

Figure 9

590

595

Figure 10

Cr2O3

Reduction

Precipitation

COOH

Surface Ion exchange Electrostatic interaction Cr(VI)

Cr( ) Figure 11

complexation

Cr Concentration (mg/L)

20

(a)

16

Total Cr Cr(℃ ) Cr(℃ )

12

8

4

0

0

200

400

600

800

1000

1200

1400

1000

1200

1400

t (min) 120

(b)

Cr Concentration (mg/L)

100 80 60 40 20 0

0

200

400

600

800

t (min) (c)

100

E (%)

80

60

40

20

0

1

2

3

4

Number cycles

Figure 12

5

6

Figure Captions Figure 1 FTIR spectrum of modified biochar. Figure 2 (a) SEM image of GB700; (b) EDS of GB700; (c) SEM image of Zn2GT700; (d) EDS of Zn2GT700; (e) SEM image of Zn2GT700+Cr; (f) EDS of Zn2GT700+Cr. Figure 3 The XRD patterns of (a), (b) modified biochar. Figure 4 Cr(VI) removal performance by different adsorbents: (a) biochar modified by different modifiers; (b) ZnCl2-modified biochar produced at different temperatures; (c) modified biochar prepared at different mass ratio (ZnCl2: glue residues). The sorbent content was 2 g/L and reaction time was 24 h. Each experiment repeated three times. Figure 5 TG curves of (a) glue and (b) ZnCl2/glue reside. Figure 6 Influence of reaction conditions on Cr(VI) removal by Zn2GT700 (a) influence of pH and (b) influence of adsorbent dose. The initial concentration of Cr(VI) was 500 mg/L, the reaction time was 24 h, and the volume of solution was 10 mL. When considering the influence of pH, the addition amount of adsorbent is 20mg. While considering the effect of adsorbent addition amount, the value of pH was 2. Each experiment repeated three times. Figure 7 pHPZC determination curve. Figure 8 (a) Adsorption isotherm study of Cr(VI) adsorption by Zn2GT700; (b) pseudo first and second adsorption kinetic study of Cr(VI) adsorption by Zn2GT700; (c) Elovich model of Cr(VI) adsorption by Zn2GT700; (d) intraparticle diffusion model of Cr(VI) adsorption by Zn2GT700. Figure 9 XPS spectra (a), spectra analysis of Zn 2p (b) and (c), (d) Cr 2p in biochar before and after Cr(VI) adsorption. Figure 10 The TEM spectrums of Zn2GT700 after adsorption of Cr(VI) (a), (b) and (c); and mapping diagrams of different elements. Figure 11 Cr(VI) adsorption mechanisms by Zn2GT700.

Figure 12 Desorption of Zn2GT700+Cr by (a) 0.1M NaOH, (b) 0.1M H2SO4, and (c) regeneration studies. The initial concentration of Cr(VI) in the regeneration experiments was 200 mg/L, the sorbent content was 2 g/L reaction time was 24 h and the pH values of each adsorption was 2.

(a)

350

200

300

180

qe (mg/g)

250

qe (mg/g)

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Figure 8 (a) Adsorption isotherm study of Cr(VI) adsorption by Zn2GT700; (b) pseudo first and second adsorption kinetic study of Cr(VI) adsorption by Zn2GT700; (c) Elovich model of Cr(VI) adsorption by Zn2GT700; (d) intraparticle diffusion model of Cr(VI) adsorption by Zn2GT700.

Highlights 1. Provided a new and feasible method for the waste glue residue treatment. 2. Cr(VI) removal effect by different modified glue residue biochar were compared. 3. ZnCl2 modified biochar showed the maximum adsorption of Cr(VI) (325mg/g). 4. The adsorbent maintained a high adsorption efficiency (90%) after six cycles.

Declaration of interests ☒ 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. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: