Eucalyptus sawdust derived biochar generated by combining the hydrothermal carbonization and low concentration KOH modification for hexavalent chromium removal

Journal of Environmental Management 206 (2018) 989e998

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Research article

Eucalyptus sawdust derived biochar generated by combining the hydrothermal carbonization and low concentration KOH modification for hexavalent chromium removal Xiaojuan Zhang, Lei Zhang*, Aimin Li** Key Laboratory of Industrial Ecology and Environmental Engineering, School of Environmental Science and Technology, Dalian University of Technology, Linggong Road 2, Dalian 116024, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 30 June 2017 Received in revised form 22 November 2017 Accepted 29 November 2017 Available online 7 December 2017

In this study, Eucalyptus sawdust was hydrothermally carbonized, and the resulting biochar was modified by a low concentration potassium hydroxide. The morphology and surface property was characterized by SEM-EDS, BET, FTIR and XPS techniques. A series of batch adsorption experiments were conducted to screen out the optimum conditions, and to investigate the isotherm, kinetics and thermodynamic behaviors. The results indicated that a high adsorption capacity of hexavalent chromium (qe 45.88 mg/g) was achieved by the combining of hydrothermal carbonization at 220
C and 0.05 N potassium hydroxide modification, and a high biochar yield (47.61%) was obtained. The isotherm, kinetics and thermodynamic studies suggested that the spontaneously and endothermically chemical adsorption was the main mechanism, which was partially supported by BET, FTIR and XPS results. This finding suggested that the combination of hydrothermal carbonization and a subsequent low alkali modification was an effective method to prepare a high-performance adsorbent for hexavalent chromium removal. © 2017 Elsevier Ltd. All rights reserved.

Keywords: Eucalyptus sawdust Hydrothermal carbonization Hexavalent chromium Low concentration potassium hydroxide Adsorption

1. Introduction Cr(VI) is a highly toxic metal, causing carcinogenic, mutagenic and teratogenic effects on biological system (Kalidhasan et al., 2016). Cr(VI) is released to the environment through various industrial processes including leather tanning, electroplating, manufacturing of chromate and dye and petroleum refining (Zhang et al., 2015a). Low Cr(VI) levels for water environment (below 0.1 mg/L) and for drinking water (below 0.05 mg/L) were set by the US Environmental Protection Agency. Therefore, it is necessary to remove the excessive Cr(VI) from supply water or wastewater for drinking or discharging into water body. Conventionally, the methods that have been utilized to remove the heavy metals include oxidation, precipitation, coagulation, ion exchange, membrane technology, adsorption, etc. (Ihsanullah et al., 2016). Among them, adsorption method is considered as a versatile and widely used technology for metal ion removal. It has the advantages of easy operation and high removal efficiency. For

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (L. Zhang), [email protected] (A. Li). https://doi.org/10.1016/j.jenvman.2017.11.079 0301-4797/© 2017 Elsevier Ltd. All rights reserved.

adsorption process, a high-performance adsorbent is the key factor, determining pollutant removal efficiency and operating cost. Several adsorbents have been employed for the adsorption of heavy metals, which can be classified into organic type (including activated carbon, polymeric resins, chitosan, etc.) and inorganic type (including clay based adsorbents, etc.). Therefore, it is important to develop efficient adsorbents for Cr(VI) removal. Activated carbon has been employed as an excellent adsorbent for the removals of heavy metals and organic pollutants. Typically, activated carbon is a carbonaceous material obtained from thermal degradation of coal or biomass, followed by activation step. Its adsorption capacity depends on the raw material and processing conditions for activated carbon production. In this field, pyrolysis is the most popular thermal conversion method for organic materials, because the resulting pyrolytic char has a well-developed pore structure. The subsequent activation process can further improve the pore structure and surface area for activated carbon production. Although activated carbon is effective for heavy metal removal like Cr(VI), some drawbacks relating to raw material and manufacturing process have to be taken account. For example, coal derived activated carbon is limited by the exhaustion risk of fossil fuel. The low yield and high cost of biomass derived activated carbon via

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pyrolysis and activation treatment are the barrier for widely application. Biochar is the solid residue of thermal conversion of biomass. Due to the special surface characteristics, recently, biochar is directly used as a low-cost and environmentally friendly sorbent instead of activated carbon for the removal of heavy metals (Inyang et al., 2015). In general, biochar was produced from pyrolysis of biomass, called pyrochar, which contains rich aromatic functional groups. However, the biochar yield from pyrolysis was very low (slow pyrolysis yield 30%; fast pyrolysis yield 12e26%) (Cha et al., 2016). More recently, the attention of biochar production from high-temperature pyrolysis gradually shifted to hydrothermal carbonization, which is also an effective thermo-chemical conversion process for carbonaceous material production. Assisted by hydrothermal carbonization, various carbon materials with a large number of reactive oxygen functional groups were synthesized from waste biomass (Hu et al., 2010). Biochar produced from hydrothermal carbonization at a mild temperature possessed a high yield and abundant surface functional groups (Liu et al., 2013). Different from pyrochar mainly containing aromatic functional groups, the biochar from hydrothermal treatment was composed of more alkyl moieties (Kambo and Dutta, 2015). The researchers compared the copper adsorption capacity of hydrothermal biochar with that of pyrolytic biochar, and found that the adsorption capacity of hydrothermal biochar was much higher than pyrolytic biochar (Liu et al., 2010). Therefore, the hydrothermal biochar might be a better candidate for Cr(VI) removal. Although biochar is effective for pollutants removal, the indigenous inorganics, impurities, and organic debris in the raw biochar decreased the efficiency of adsorption. A post treatment is an effective way to improve the adsorption capacity. The activation step for activated carbon production is one kind of severe treatment. Physical activation and chemical activation are two typical methods to improve the specific surface area and pore size distribution of biochar. For example, switch grass was carbonized at 500
C and 700
C and the resulting biochar was subsequently steam activated at 800
C for 45 min. The activated biochar (also called activated carbon) exhibited better adsorption performance than biochar for copper and phenol removal (Han et al., 2013). As mentioned above, biochar yield is very low, and the activation step will cause additional weight loss. In addition, high chemical cost and/or energy cost (steam supply) during the activation process will decrease the economic feasibility. This fact suggested that harsh activation treatment for biochar might not be the best approach to enhance its adsorption capacity. Surface modification is a more attractive choice for biochar treatment to increase its adsorption capacity by a low-strength treatment than activation. The modified biochar by a simple postprocessing showed a better adsorption performance for pollutants removal. For example, surface oxidation is a typical surface modification method using oxidizing agents such as air, HNO3, H2O2, Fe(NO3)3 (Huang et al., 2009). Using H2O2 treatment on raw biochar increased the lead sorption ability from 1.04 mg/g to 22.82 mg/g (Xue et al., 2012). Alkali, acid or metal salt soaking is another modification method. A high-alkali concentration (2 N) to treat biochar for copper and cadmium removal, and high adsorption capacities for cadmium and copper removal (34 mg/g and 31 mg/g) were achieved (Regmi et al., 2012). However, it also has some disadvantages. The modification of biochar by strong oxidants or high concentrations of alkali/acid will inevitably lead to a great weight loss. The high dosage of chemicals is another main economic limitation. Therefore, it is important to develop a highyield, low-cost, and high-performance biomass-based adsorbent. The aim of this study is to develop a biochar based absorbent from hydrothermal carbonization of wood sawdust with a high

yield and high performance. Firstly, a raw biochar was produced from hydrothermal carbonization of Eucalyptus sawdust at different hydrothermal temperatures. Then, an alkali or acid modification method was employed to clean up the raw biochar. A series of batch adsorption experiments were conducted to evaluate the effects of hydrothermal carbonization temperature and alkali concentration on Cr(VI) removal. In addition, the isotherms, kinetics and thermodynamics of Cr(VI) adsorption were performed, and by combining the different characterization methods, e.g. BrunauerEmmett-Teller (BET), Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectrometer (XPS) (K-Alpha, Thermofisher Scienticfic Company, USA) analysis the mechanism of Cr(VI) removal by the modified biochar was tentatively elucidated. 2. Materials and methods 2.1. Biochar preparation Eucalyptus sawdust was provided by the Guangxi Yulin Wood Manufacturing Factory, China. It was ground to pass through a 0.83 mm screen. Hydrothermal carbonization experiments were carried out in a 250-mL stirred batch reactor (Nickel alloy 625, GCFtype, Dalian Controlled Plant, China). For each run, 80 g of dry Eucalyptus sawdust and distilled water (biomass-to-water ratio 1:7) were weighed and transferred into the reactor. The reactor was purged with N2 for 5 min and magnetically stirred at 100 rpm. Then it was heated to 160
C, 190
C, 220
C, or 250
C for 1 h. After the reaction, the reactor was cooled down with cooling water. The gas in the headspace of the reactor was released and the solid-liquid mixture was filtered with a 0.45 mm membrane filter. The solid residue, named as a biochar, was washed with deionized water until the pH was neutral. The biochar was dried in an oven at 105
C for 24 h, and stored in an air-tight plastic tube. 2.2. Modification of raw material and biochar The biochar (or raw material) and different concentrations of KOH (or HCl) solution with a ratio 1:50 (w/v) were mixed, and stirred for 1 h at 30
C. The solution was transferred into a Buchner funnel, and filtered by a 0.45-mm membrane. Then the filtered biochar was washed with deionized water until the pH of filtrate remained neutral. Finally, the washed biochar was dried in an oven at 105
C for 24 h, and named as a modified biochar. The yield of the modified biochar was calculated according to Equation (1):

Yield ¼ ðmass of solid residue=mass of dried feedstockÞ  ðmass of modified solid residue=mass of biocharÞ  100%

(1)

2.3. Cr(VI) adsorption experiment The biochar (or raw material) was used as an adsorbent for Cr(VI) removal from solution. A series of adsorption experiments were conducted to evaluate the adsorption capacity of different adsorbents. The typical adsorption parameters of pH, contact time, and adsorption temperature were examined. For comparison, two commercial activated carbons, commercial coconut activated carbon and coaly activated carbon, were tested as controls. The batch experiments were carried out in a 100-mL centrifuge tube with a sealing film. The Cr(VI) solutions with certain concentrations was obtained by diluting a stock solution of Cr(VI) (1000 mg/L). A mixture of 0.1 g biochar and 50 mL Cr(VI) solution was added into

X. Zhang et al. / Journal of Environmental Management 206 (2018) 989e998

the tube. The pH was adjusted by adding HCl or KOH solution, and then placed in a thermostatically shaking water bath (SHA-CA, Kexi Instrument Co. Ltd, China) at a speed of 150 rpm and a designated temperature. To examine the effects of pH on Cr(VI) adsorption, the pH values were varied from 2 to 9, which was considered as a practical range. Isotherm experiments were carried out at different initial Cr(VI) concentrations (50, 100, 150, 200, 300, 400, 500, 600, and 800 mg/ L) for an adsorption period of 8 h. Kinetics experiments were performed with an initial Cr(VI) solution concentration of 100 mg/L for 8 h, during which the samples were taken at different time intervals (0.08, 0.17, 0.25, 0.5, 0.75, 1, 1.5, 2, 3, 4, 5, and 6 h). For thermodynamic studies, in addition, another set of experiments, which was same as kinetics studies, was performed at different temperatures (25, 45, and 65 ± 3
C). The removal efficiency and the amount of adsorption, qe (mg/g) of Cr(VI) were determined by Equations (2) and (3), respectively:

Removal efficiency ¼ ðC0  Ce Þ=C0  100%

(2)

qe ¼ ðC0  Ce Þ  V=M

(3)

where, C0 and Ce are the initial and equilibrium concentration of Cr (VI) in solution, respectively. V (mL) is the volume of solution, and M (mg) is the mass of the adsorbent. 2.4. Analytical method After the designated reaction time, the biochar was separated from the solution by vacuum filtration, and then the solution passed through a 0.45 mm syringe membrane filter. The concentration of Cr(VI) was determined by a UVevisible spectrophotometer at 540 nm according to 1, 5-diphenylcarbazide method. Surface morphology of raw and biochar samples were observed using a scanning electron microscope (SEM, HITACHI S-4800, Japan). Fourier transform infrared spectroscopy (FTIR) (Nicolet 6700, Thermo Fisher Scientific Inc, USA) was used to identify the changes of functional groups on the raw and modified biochar, and the spectra were determined in the range of 400e4000 cm1 using the KBr pellet technique. The Brunauer-Emmett-Teller (BET) surface area of biochar was measured by N2 adsorption isotherm at 77 K with a Quadrasorb SI analyzer (Quantachrome instruments, USA). X-ray photoelectron spectrometer (XPS) (K-Alpha, Thermofisher Scienticfic Company, USA) was performed to determine the surface chemical composition of biochar. 3. Results and discussion 3.1. Comparison of different modification methods on Cr(VI) adsorption capacity In order to screen out an appropriate modification method, different treatment methods including acid and alkali modification methods were employed to biochar and Eucalyptus sawdust, and their effectiveness was evaluated by Cr(VI) adsorption experiments. Two commercial activated carbons, coaly activated carbon and coconut activated carbon were run as controls for evaluating Cr(VI) adsorption capacity. The adsorption performance of different adsorbents is summarized in Table 1, in which some representative reference data are also included. As for raw biomass, its adsorption capacity of Cr(VI) was 15.27 mg/ g, which greatly decreased to 6.94 mg/g and 4.35 mg/g after a 0.05 N KOH and 2 N KOH modification, respectively. This might be because the fiber structure was destroyed by alkaline treatment, by which some components of biomass such as lignin were dissolved. In

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contrast, the raw biochar obtained from hydrothermal carbonization exhibited a better adsorption performance than raw material (Eucalyptus sawdust), and the adsorption capacity was increased by 45.97% from 15.27 mg/g for raw biomass to 22.29 mg/g for raw biochar. These results suggested that hydrothermal carbonization alone even without a modification was also an effective method for enhancing the Cr(VI) adsorption capacity of biomass. In order to further increase the adsorption ability, the biochar from hydrothermal carbonization was subjected to alkali or acid modification. Interestingly, as listed in Table 1, the adsorption capacity of the alkali modified biochar was greatly increased by more than two folds. More importantly, with a very low KOH modification (0.05 N), Cr(VI) adsorption capacity was dramatically increased from 22.29 mg/g for raw biochar to 45.88 mg/g for 0.05 N KOH modified biochar. This might be due to the removal of organic debris in the raw biochar by alkaline solution, as reflected by the increased surface area (see Table 6 in BET analysis section). These results suggested that the alkali treatment was an efficient modification method for the enhanced Cr(VI) adsorption of biochar. As acid treatment was reported to be an effective method to remove indigenous inorganics or impurities, thus introducing more heterogeneous pores (Rajapaksha et al., 2016). HCl solutions with the same concentrations as KOH solution were employed to modify the raw biochar. Different from the alkali modification, as shown in Table 1, the acid modification on raw biochar did not show any positive effects on Cr(VI) adsorption. When the raw biochar was treated by a 0.05 N HCl solution, Cr(VI) adsorption capacity also maintained at a constant value. With further increasing the acid solution strength from 0.05 N to 2 N, the Cr(VI) adsorption capacity even decreased from 22.77 mg/g to 19.36 mg/g. The biochar from hydrothermal carbonization had low inorganics content that might weaken the modification effect of HCl. In addition, according to the experimental results (Section 3.4), acid modification destroyed the mesoporous structure on the surface of biochar and reduced the surface area, thus decreasing the adsorption capacity of Cr(VI) for the resulting biochar. Cr(VI) adsorption capacity of the modified biochar was also compared with two commercial activated carbons, as shown in Table 1, the alkali modified biochar in this study had a much higher adsorption capacity (45.88 mg/g) than coconut activated carbon (21.13 mg/g) and coaly activated carbon (33.42 mg/g). Compared with the listed absorbents from previous literature reports, the alkali modified biochar in this study showed a better Cr(VI) adsorption capacity, even though some of them were produced by costly chemical activation. These results suggested a unique, superior effect of alkali modification on the Cr(VI) adsorption performance. As mentioned above, the yield of adsorbent is an important parameter to determine the economic feasibility, so the char yields of pyrolysis, hydrothermal carbonization and their following treatments from different sources are compared, and listed in Table 2. The char yield from pyrolysis of biomass was only half of that from hydrothermal carbonization (Table 2). In order to improve adsorption properties, such as specific surface area and pore-size distribution, currently, the biochar was activated by subjecting it to a high temperature in the presence of steam or chemicals. After activation process, the biochar yield (or activation carbon yield) was further reduced. As shown in Table 2, activated biochar yields (or activated carbon yield) were 16.22% using K2CO3 and 5.75% using KOH as activators, respectively. These values were far below the biochar yield (47.61%) of the KOH modified biochar from hydrothermal carbonization in this study. As the traditional activated carbon preparation process (pyrolysis followed by activation) had the disadvantages of a high energy consumption, high demand of chemical activator and low product yield, therefore, the approach of combining hydrothermal carbonization and a subsequent mild alkali modification had a great potential to make an

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Table 1 Adsorption property comparison of various adsorbents for Cr(VI) removal. Types of adsorbent and treatment conditions

qe (mg/g)

Removal efficiency (%)

Blank 0.05 N KOH 2 N KOH

15.27 ± 0.87 6.94 ± 1.16 4.35 ± 0.58

30.35 ± 1.74 13.88 ± 2.32 8.69 ± 1.16

Blank 0.05 N KOH 2 N KOH 0.05 N HCl 2 N HCl

22.29 45.88 46.08 22.77 19.36 21.13 33.42 7.76 16.26 39.10

Data sources

Eucalyptus sawdust

This study

Biochar

CCACa CACb Eucalyptus grandis sawdust Peanut shell Alumina materials a b

microwave-assisted activation with ZnCl2 chemical activation with KOH template-free hydrothermal method

± ± ± ± ± ± ±

1.01 0.08 0.29 0.58 0.10 0.38 1.58

44.59 91.77 92.16 45.54 38.72 42.27 66.83 e e e

± ± ± ± ± ± ±

2.03 0.16 0.58 1.16 0.19 0.77 1.15

This study

This study Chen et al., 2015 Al-Othman et al., 2012 Zhang et al., 2015b

Commercial coconut activated carbon. Coaly activated carbon.

Table 2 Biochar or activated carbon yields from different thermo-chemical and modified processes. Solid yield (%)

Ref.

12 (fast)-35 (slow) 25-35 (slow) 20-40 (slow); 10e20 (fast)

Libra et al., 2011 Funke and Ziegler, 2010 Qian et al., 2015

16.22 5.75

Tay et al., 2009

50e80 45e70 30e60 47.61

Libra et al., 2011 Funke and Ziegler, 2010 Qian et al., 2015 This study

Pyrolysis

After activation process K2CO3 KOH Hydrothermal carbonization

Low-concentration KOH modification

adsorbent for Cr(VI) adsorption.

3.2. Optimization of hydrothermal carbonization and alkali modification conditions 3.2.1. Effect of hydrothermal carbonization temperature The hydrothermal temperature is the most important factor

affecting the yield and properties of the carbonized product. During the hydrothermal carbonization process, the hydrolysis, dehydration and decarboxylation of lignocellulosic biomass occurred, which will influence the oxygen-containing functional groups (OFG) of the biochar surface. These characteristics will have a great impact on the subsequent adsorption performance. With the increasing temperature of hydrothermal treatment, the density of OFG and OH groups of biochar from lignocellulosic biomass was reduced (Kang et al., 2012). Therefore, the functional groups were strongly dependent on the reaction temperature. So, the hydrothermal carbonization temperature should be carefully chosen. According to literature reports, Falco et al. (2011) and our previous study, the temperatures in the range of 160e250
C were examined for hydrothermal carbonization. The effects of hydrothermal carbonization temperature on the biochar yield and Cr(VI) adsorption capacity are present in Fig. 1. The solid residue yield of hydrothermal carbonization without modification treatment decreased with increasing the reaction temperatures (data not shown). After a low-concentration alkali treatment, additional 13e23% of biochar was dissolved. The total yield of two steps was 65.75% at 160
C, and decreased to 34.06% at 250
C. The changes of hydrothermal temperature also significantly affected Cr(VI) adsorption capacity. Adsorption capacity increased from 26.39 mg/g to 45.88 mg/g with increasing the hydrothermal carbonization temperatures from 160
C to 220
C. With the further increasing of hydrothermal carbonization temperature, the Cr(VI) adsorption capacity and removal efficiency decreased. Considering the fact that hydrothermal process involves the reaction pathways

Table 3 Kinetics constant parameters for Cr(VI) adsorption on the modified biochar. T (
C)

25 45 65

Pseudo-first-order

Table 5 Thermodynamic parameters for Cr (VI) adsorption on the modified biochar.

Pseudo-second-order

k1 (min1)

qe (mg/g)

R2

k2 (g/mg min)

qe (mg/g)

R2

0.0346 0.0832 0.1300

42.988 47.368 49.420

0.90167 0.93516 0.98213

0.00083 0.00261 0.00559

49.184 50.495 50.780

0.96027 0.98708 0.98975

DG0 (kJ mol1) 25
C

45
C

65
C

5.10

13.10

23.93

DH0 (kJ mol1)

DS0 (J K1 mol1)

134.72

467.81

Table 4 Fitting parameters for different isotherm models. T (
C)

25 45 65

Langmuir model

Freundlich model

qm (mg/g)

k1 (L/mg)

R2

kf (mg11/nL1/ng1)

1/n

R2

120.35 152.76 212.20

0.05259 0.04747 0.02345

0.78710 0.87116 0.92265

44.471 47.578 49.701

0.15786 0.19138 0.22489

0.98284 0.98443 0.98005

X. Zhang et al. / Journal of Environmental Management 206 (2018) 989e998

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Table 6 Surface area and pore distribution properties of the samples.

Raw material Biochar Modified biochar (0.05 N KOH) Modified biochar (0.05 N HCl) CCACa CACb a b

Surface area (m2/g N2)

Pore volume (cm3/g)

Pore diameter (nm)

12.607 12.198 18.698 1.827 535.348 508.564

0.025 0.032 0.040 0.031 0.159 0.170

3.857 2.519 3.486 4.955 3.120 3.114

Commercial coconut activated carbon. Coaly activated carbon.

of dehydration, decarboxylation, condensation and polymerization (Libra et al., 2011), at a higher temperature, the degree of hydrothermal reaction became more severe, resulting in a loss of oxygen functional group of biochar (Funke and Ziegler, 2010), which might be the reason causing the decreased Cr(VI) adsorption capacity. Thus, a moderate hydrothermal temperature is favorable for Cr(VI) adsorption. Balancing the total yield of biochar and Cr(VI) adsorption capacity, in this study, 220
C was chosen as the optimum hydrothermal carbonization temperature for following experiments.

the key to obtain a high-performance and high-yield biochar based adsorbent. Considering the Cr(VI) adsorption capacity, chemical cost and biochar yield, 0.05 N KOH modification was chosen as the optimum condition. 3.3. Batch adsorption experiment

3.2.2. Effect of different KOH concentration As mentioned above, the alkali modification of biochar has two main purposes, one is to clean up the blocked pores of biochar to increase the porosity, and the other is to modify the surface chemical characteristics of biochar such as surface functional groups and active sites. In order to optimize the alkali modification, different KOH concentrations were employed to treat the raw biochar from hydrothermal carbonization, and the Cr(VI) adsorption capacity and biochar yield are present in Fig. 2. Interestingly, a very low KOH modification (0.01 N KOH) greatly increased the Cr(VI) adsorption capacity from 22.29 mg/g for raw biomass to 36.39 mg/g. With further increasing KOH concentrations, Cr(VI) adsorption capacity steadily increased to 41.98 mg/g for 0.02 N KOH modification, and was stabilized at 45.88 mg/g for 0.05 N KOH modification. When KOH concentration was further increased to 2 N KOH, although a slightly higher Cr(VI) adsorption capacity of 46.08 mg/g was achieved, the biochar yields dramatically dropped from 78.5% for 0.05 N KOH modification to 61.8%. This might be due to the dissolution of carbonaceous material from hydrothermal carbonization of biomass in a high strength alkali solution. This result suggested that a low and moderate alkali modification was

3.3.1. Effect of solution pH and contact time As shown in Fig. 3a, the solution pH values greatly affected the Cr(VI) adsorption performance. Maximum Cr(VI) removal efficiency of 91.77% was achieved at the lowest pH 2. When the solution pH increased from 2 to 4, the removal efficiency sharply decreased from 91.77% to 5.69%, and beyond pH 5 the adsorption was negli2 gible. Cr(VI) has high mobility, and exists as HCrO 4 and Cr2O7 species in acidic medium and CrO2 species at alkaline pH, 4 respectively (Kalidhasan et al., 2016). Some researchers noted that Cr(VI) was adsorbed on the activated carbon surface mostly in the form of HCrO 4 at pH 2 (Zhang et al., 2015a). Under acidic conditions, the negative charges on biochar surface were neutralized and protonated with a large number of Hþ ions, thus effectively attracting negatively charged chromium species through a high electrostatic attraction force (Chen et al., 2015). At high pHs, the deprotonation of functional groups of biochar led to a repelling negative charge, thus decreasing removal efficiency (Saha and Orvig, 2010). Considering the Cr(VI) removal efficiency and chemical addition for pH adjustment, pH 2 was considered as the optimum value for Cr(VI) adsorption. The effect of contact time on Cr(VI) adsorption was estimated by using a 50-mL solution containing 100 mg/L Cr(VI). The initial solution pH was 2 and absorbent dosage was 2 g/L. The results are presented in Fig. 3b. The Cr(VI) adsorption rapidly increased in the range from 0.08 to 3 h, and more than 85% of Cr(VI) was removed

Fig. 1. Effect of hydrothermal temperature on the biochar yield, removal efficiency and adsorption capacity of Cr(VI).

Fig. 2. Effect of KOH concentrations on biochar yield and Cr(VI) adsorption capacity (initial Cr(VI) concentration: 100 mg/L, dosage of modified biochar: 2 g/L, contact time 5 h, pH 2).

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X. Zhang et al. / Journal of Environmental Management 206 (2018) 989e998

(0.75 h at 45
C, 0.5 h at 65
C). In order to quantitatively evaluate the adsorption kinetics and elucidate the adsorption mechanisms, the experimental data at three temperatures were fitted by pseudofirst-order and pseudo-second-order equation, respectively, which are expressed as follows:

  qt ¼ qe 1  ek1 t t 1 t ¼ þ qt k2 q2e qe

(4)

(5)

where, qe and qt (mg/g) are the amounts of Cr(VI) adsorbed per unit mass of the adsorbent at equilibrium and at time t, respectively; k1 (min1) and k2 (g mg1 min1) are the pseudo-first-order and pseudo-second-order rate constant, respectively. The fitting kinetic parameters from the two models are present in Table 3. The pseudo-first-order kinetics model describes that the adsorption rate is based on the solid capacity at the solid/liquid interface. The pseudo-second-order kinetics model assumes that the rate limiting step is chemical sorption through sharing or exchanging of electrons between adsorbate and polar functional groups on adsorbent (such as aldehydes, ketones, acids and

Fig. 3. Cr(VI) removal efficiency and adsorption capacity of the modified biochar under different pH values (a), and different contact times (b) (initial Cr(VI) concentration: 100 mg/L, dosage of modified biochar: 2 g/L, solution volume 50 mL).

from aqueous solution in three hours. With further increasing contact time from 5 h to 8 h, the adsorption process slowed down. After 8 h the adsorption curve became flattened, and more than 97% of Cr(VI) was removed. The similar adsorption progress of activated carbon prepared from peanut shell for Cr(VI) adsorption was reported (Al-Othman et al., 2012). For their results, in contact time from 0.17 to 7 h, the adsorption of Cr(VI) increased, became slow until 20 h, and the adsorption process reached an equilibrium after 24 h. For this study, in contrast, the modified biochar had a shorter period for the rapid adsorption. In this process, the number of Cr(VI) ions transported from the bulk solution onto the surface of adsorbent, and were adsorbed on the binding sites of the modified biochar. As the active sites were gradually occupied, the adsorption rate began to slow down. According to our experimental results, the contact time of 8 h was considered to be optimum for a real application, in which Cr(VI) adsorption capacity and removal efficiency of the modified biochar were 48.58 mg/g and 97.18%, respectively. 3.3.2. Kinetics studies As indicated in Fig. 3b, a rapid adsorption process occurred, and the Cr(VI) removal efficiency was 79.48% in the first 2 h at 25
C. For comparison, the adsorption progresses of another two higher temperatures of 45
C and 65
C were examined, and the results are presented in Fig. 4. As shown in Fig. 4, as the temperatures increased, a shorter time was needed to obtain 80% removal rate

Fig. 4. Pseudo-first-order kinetics model (a) and pseudo-second-order kinetic model (b) for Cr(VI) adsorption at different temperatures.

X. Zhang et al. / Journal of Environmental Management 206 (2018) 989e998

phenolics) (Ho and McKay, 2000). The correlation coefficient R2 of the pseudo-second-order kinetics model was 0.96, 0.99 and 0.99 (for 25
C, 45
C and 65
C) which were all higher than those of the pseudo-first-order kinetics model (0.90 for 25
C, 0.94 for 45
C, 0.98 for 65
C). This indicated that the adsorption was well described by the pseudo-second-order kinetics model other than pseudo-first-order kinetics model. The calculated values of qe were 49.18 mg/g for 25
C, 50.50 mg/g for 45
C and 50.78 mg/g for 65
C which were close to the experimental values (48.58 mg/g for 25
C, 49.97 mg/g for 45
C, 49.97 mg/g for 65
C). This result suggested the mechanism of Cr(VI) sorption on the modified biochar was mainly attributed to chemical sorption through sharing or exchanging of electrons between modified biochar and Cr(VI). In addition, adsorption rate constant k2 greatly increased from 0.00083 g mg1 min1 at 25
C to 0.00261 g mg1 min1 at 45
C 0.00559 g mg1 min1 at 65
C with increasing the temperatures. The increased adsorption rate constant might be attributed to two factors of the increased transporting of the Cr(VI) in the aqueous phase and the decreased viscosity of the solution at increased temperatures. The adsorption rate was increased as temperature increased suggested that the Cr(VI) adsorption process of the modified biochar was an endothermic process, which was further confirmed by the following thermodynamic analysis. For the further confirmation, activation energy (Ea) was determined based on the pseudo-second-order rate constants. According to Arrhenius Equation (6), Ea of the adsorption process was calculated.

ln k2 ¼ ln A 

Ea RT

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maximum adsorption capacity (mg/g), kl (L/mg) and kf (mg11/nL1/ g ) are the Langmuir constant and Freundlich constant, respectively, and n is the constant with the relevant intensity. The constants and corresponding correlation coefficients of the two models are listed in Table 4. The correlation coefficient R2 of Freundlich isotherm at different temperatures (0.98 at 25
C, 0.98 at 45
C, 0.98 at 65
C) were all higher than that of Langmuir (0.79 at 25
C, 0.87 at 45
C, 0.92 at 65
C), indicating that the experimental data were better to fit Freundlich isotherm than Langmuir isotherm model. Freundlich isotherm suggested that the uptake of Cr(VI) ions occurred more on the heterogeneous adsorbent surface of the modified biochar, thus resulting in multilayer adsorption on the surface of binding sites between the surface functional groups of the modified biochar and Cr(VI) ions. Freundlich constant, n, reflects the adsorption intensity, which lies between 1 and 10 for a favorable adsorption process (Kumar et al., 2011). In this study, the Freundlich constant n at three temperatures were 6.33 (at 25
C), 5.23 (at 45
C) and 4.45 (at 65
C), respectively, suggesting that an effective interaction process occurred between the adsorbent and Cr(VI) ions. n 1

3.3.4. Thermodynamic studies Thermodynamic analysis was employed to study the mechanism involved in the process of Cr(VI) adsorption on the modified

(6)

where, k2 (g mg1 min1) is the rate constant of pseudo-secondorder kinetics model; A is the pre-exponential factor; T is the absolute temperature; and R is the universal gas constant (8.314 J mol1 K1). Ea was calculated from the slope and intercept of ln k2 versus 1/T (Fig. S1). The activation energy for Cr(VI) adsorption on the modified biochar was 40.05 kJ/mol, suggesting that Cr(VI) was adsorbed onto the surface of the modified biochar chemically other than physically (Boparai et al., 2011). 3.3.3. Adsorption isotherm The effect of different initial Cr(VI) concentrations on Cr(VI) adsorption capacity at different temperatures is shown in Fig. 5. The adsorption capacity of Cr(VI) increased with the increasing of initial Cr(VI) concentration and adsorption temperature. For example, as the initial Cr(VI) concentrations increased from 50 mg/L to 800 mg/ L, the adsorption capacity of Cr(VI) increased from 24.94 mg/g to 120.55 mg/g at 25
C. In order to explore the adsorption mechanism, the adsorption isotherms (Langmuir and Freundlich models) were used to fit the experimental data. Langmuir model assumes that the surface contains identical sorption sites, and the uptake of adsorbate occurs on mono-molecular layer by chemical adsorption without interaction between the adsorbed molecules. Freundlich model is suitable for describing in equivalent adsorption sites, and predicts the adsorption capacity increases with the increasing of adsorbate concentrations (Figueira et al., 2011). The two models are expressed as follows:

Langmuir : qe ¼

qm kl Ce 1 þ kl Ce 1=n

Freundlich : qe ¼ kf Ce

(7)

(8)

where, Ce is the equilibrium concentration (mg/L), qe is the amount of Cr(VI) adsorbed by biochar at equilibrium (mg/g), qm is the

Fig. 5. Langmuir adsorption isotherms (a) Freundlich adsorption isotherms (b) for Cr(VI) adsorption at different temperatures.

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X. Zhang et al. / Journal of Environmental Management 206 (2018) 989e998

the adsorption process was spontaneous, and the increased absolute value of DG0 (5.10 kJ mol1 at 25
C, 13.10 kJ mol1 at 45
C and 23.93 kJ mol1 at 65
C) implied that the driving forces for Cr(VI) adsorption increased with the increasing of temperatures. The thermodynamic parameters mean the affinity between the modified biochar and Cr(VI) ions, and the higher temperature caused the more frequent contact between the modified biochar and Cr(VI) ions. In addition, the high temperature may increase the number of adsorption sites because the internal bonds of adsorbent were broken (Jain et al., 2010). 3.4. Morphology and structural characterization of biochar

Fig. 6. FTIR spectra of raw material (Eucalyptus sawdust) (a); biochar from hydrothermal carbonization (at 220
C) (b); modified biochar with low concentration KOH (0.05 N) (c); modified biochar with HCl (0.05 N) (d); commercial coconut activated carbon (CCAC) (e); and coaly activated carbon (CAC) (f).

biochar. Thermodynamic parameters, DG0 (standard free energy change), DH0 (standard enthalpy change) and DS0 (standard entropy change) are calculated in the temperature range from 25
C to 65
C by the following equations:

DG0 ¼ DH0  T DS0 DG0 ¼ RT ln K 0

(9) (10)

aqe K ¼ Ce 0

(11)

DH 0

ln K 0 ¼ 

RT

þ

3.4.1. SEM images The surface morphology of raw material, biochar and modified biochar are displayed in the SEM images (Fig. S3). The raw material (Fig. S3a) had continuous and flat surfaces. After hydrothermal carbonization, the fibrous structure of raw material was destroyed. The resulting biochar had much rougher surface, and its particles were highly heterogeneous (Fig. S3b). Some porous structure was found on the surface of the biochar modified by alkali solution (Fig. S3c). However, the surface of the acid modified biochar was very smooth (Fig. S3d). This can be partially confirmed by the following BET results. In order to observe the surface morphology of biochars before and after adsorption, additional the scanning electron microscope energy dispersive spectrometer (SEM-EDS) (FEI Quanta 450, Japan) analysis were carried out. Although additional SEM observation indicated that no noticeable changes were observed on the biochars before and after Cr(VI) adsorption, the EDS results confirmed that chromium ions were anchored on the surface of biochars after adsorption reaction at different temperatures.

D S0 R

(12)

where, T is the absolute temperature; and R is the universal gas constant (8.314 J mol1 K1), a is the adsorbent dose (2 g/L), K0 is the sorption equilibrium constant. The values of DH0 and DS0 were calculated from the slope and intercept of ln K0 versus 1/T in Fig. S2, and are present in Table 5. The positive value of DH0 (134.72 kJ mol1) demonstrated that the sorption process was endothermic, which coincided with the fact that high temperatures facilitated the adsorption process of Cr(VI) on the modified biochar. Furthermore, the positive value of DS0 (467.81 J K1 mol1) reflected that the disorder of solid-solution interface of the adsorption process increased. The negative values of DG0 indicated that

3.4.2. BET analysis As for Brunauer-Emmett-Teller (BET) surface area, no difference between raw material and unmodified biochar was found (Table 6). After potassium hydroxide modification, the BET surface area increased from 12.198 m2/g to 18.698 m2/g. However, the BET surface area of biochar modified by acid (0.05 N HCl) was reduced from 12.198 m2/g to 1.827 m2/g. The porous structural parameters are extracted from Barret-Joyner-Halenda (BJH) pore size distribution curve, and listed in Table 6. The pores of three samples mainly consisted of mesopores and a few micropores. The diameter of the modified biochar had a broader pore size distribution than that of the raw biochar, and the pore volume of the alkali modified biochar was greater than others. This might be due to the alkali modification cleaning up the blocked pores of biochar from hydrothermal carbonization, thus improving the porosity of biochar. Interestingly, it was worth noting that the BET surface area of commercial coconut activated carbon (535.348 m2/g) and coaly activated carbon (508.564 m2/g) was much larger than that of the

Fig. 7. Schematic diagram illustrated the process of KOH modified on the biochar surface and Cr(VI) adsorption.

X. Zhang et al. / Journal of Environmental Management 206 (2018) 989e998

alkali modified biochar, but the adsorption capacity of Cr(VI) was lower than that of the modified biochar (see Table 1). This suggested that physical properties such as BET surface were not the dominant factor determining the Cr(VI) adsorption capacity of the modified biochar. Therefore, the main mechanism of Cr(VI) adsorption was ascribed to chemical adsorption mainly controlling by the surface functional groups of the modified biochar.

997

by which a high biochar yield (47.61%) and a high adsorption capacity of hexavalent chromium (qe 45.88 mg/g) were achieved. Isothermal, kinetics and thermodynamics studies suggested that the adsorption mechanism of the alkali modified biochar was spontaneously and endothermically chemisorption, which was partially supported by SEM, BET and FTIR analysis. Acknowledgment

3.4.3. FTIR and XPS analysis The relationship of BET surface area and Cr(VI) adsorption capacity suggested that the adsorption ability of Cr(VI) might be due to the surface functional groups of biochar. In this section, the surface functional groups of the samples were determined by FTIR analysis, and the spectra are illustrated in Fig. 6. According to the previous literature (Kang et al., 2012; Reza et al., 2015), the adsorption peaks at 3417 cm1 was assigned to the eOH stretching vibrations, the signals at 2856e2920 cm1 were attributed to aliphatic CeH stretching, the band at 1633 cm1 corresponded to carbonyl from the carboxyl group (Dong et al., 2011), and the weak bands at 1513, 1454, 1369 and 1317 cm1 were related to arylalkyl ethers (eOCH3), aromatic C]C ring stretching, C]O stretch of carboxylate, and phenolic hydroxyl bending vibration, respectively. The band at 1272 cm1 could be attributed to acetyl groups. A sharp band at 1058 cm1 was connected with CeO stretching vibration. The functional groups of the modified biochar were more abundant than those of the conventional activated carbons, and the FTIR results also indicated that the intensity of the carbonyl peak increased after potassium hydroxide modification. The carbonyl groups could be used as an electron donor to provide coordination electrons to form a stable complex with chromium ions (Miretzky and Cirelli, 2010). For the alkali modified biochar, the rich oxygen containing functional groups such as eCOOH, eOH, eOCH3 were protonated in acidic environment, which would strongly electrostatically interact 2 with HCrO 4 and Cr2O7 species (Song et al., 2015; Sun et al., 2016). The chemical properties of the modified biochar contributed to the increased Cr(VI) removal efficiency. To further explore the mechanism, the surface chemical composition of biochar was determined using XPS. The atomic C and O were observed from XPS results for biochars before and after adsorption, and Cr appeared after adsorption. The C 1 s XPS spectra could be attributed into CeC/CeH, CeO, C]O and C]OeO. There were no significant differences on species before and after adsorption, but their relative contents changed. As for O 1 s XPS, its binding energy was slightly shifted to the lower-energy region after adsorption, indicating the local binding environment were changed due to Cr(VI) adsorption. Based on the adsorption performance and characterization results, a schematic diagram was drawn as shown in Fig. 7, which pictured the alkali modification on surface properties and subsequent adsorption mechanisms. The alkali modification might promote the dissociation of weakly-bonded parts and ionization of phenol and humus in the biochar (Lin et al., 2012), and increase the BET surface area by removing the organic debris. The protonation at acidic conditions and rich functional groups could enhance the Cr(VI) adsorption through electrostatic attraction and complexation. 4. Conclusion The combination of hydrothermal carbonization of woody biomass, Eucalyptus sawdust, and subsequent modification by a low concentration alkali is an effective method for making highperformance adsorbent for Cr(VI) removal. The optimum hydrothermal carbonization temperature was 220
C, and the alkali concentration for modification was 0.05 N of potassium hydroxide,

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