Abstract

In this study, N-doped biochar BZ-9.5AG-30 min was prepared from bamboo by using ZnCl2 as activator and heat treated at 950 °C under NH3 gas flow for the removal of Cr(VI). The adsorbent was characterized by BET, and the amount of introduced nitrogen content and nitrogen species on BZ-9.5AG-30 min was examined by CHN elemental analyzer and X-ray photoelectron spectroscopy, respectively. Herein, the obtained BZ-9.5AG-30 min had a high specific surface area (1,610 m2/g) and high N content (4.52%). The pH of the solution had a great influence on the adsorption process, indicating that the acid condition is conducive to the adsorption process of Cr(VI). Adsorption equilibrium data of Cr(VI) were analyzed by the Langmuir and the Freundlich models. The adsorption equilibrium data were well described by the Langmuir model, and BZ-9.5AG-30 min has excellent adsorption capacity for Cr(VI) (4.31 mmol/g). BZ-9.5AG-30 min showed superior recyclability, and after five times regenerations, the adsorption capacity of BZ-9.5AG-30 min still had 63% of the initial adsorption capacity.

INTRODUCTION

Heavy metal wastewater pollution triggered by organic dyes has been recognized as a global issue of concern (Farajzadeh & Monji 2004). Cr(VI) is a common toxic heavy metal, which widely exists in manufacturing and dyeing wastewater, electroplating wastewater and other industrial wastewater (Lyu et al. 2017; Xiao et al. 2018). Cr(VI) has different compounds in aqueous solution, such as Cr2O72−, CrO42−, and HCrO4 (Peng et al. 2017), and Cr(VI) has approximately 100 times higher toxicity than Cr(III) (Shen & Wang 1995; Dong et al. 2017). Cr(VI) causes harmful effects on the human body due to its strong oxidizing characteristic and can be easily absorbed and accumulated (Naghipour et al. 2018), especially by the kidneys, stomach, and liver (Miretzky & Cirelli 2010). WHO has set the Cr(VI) lower limit of drinking water to be 0.05 mg/L or less (Bansal et al. 2009). Therefore, it is necessary to carry out effluent treatment before discharging Cr(VI) wastewater into the environment.

At present, conventional treatment methods for Cr(VI)-containing wastewater include reduction (Pinos et al. 2016), ion exchange (Rapti et al. 2016), biological treatment method (Zeng et al. 2019), and adsorption (Ramavandi et al. 2014). Among these, adsorption is a better method because of its high efficiency, low cost and simplicity of design (Yan et al. 2017; Jaafari & Yaghmaeian 2019). Carbon adsorbents generally include activated carbon, carbon molecular sieves and carbon-containing nanomaterials (Mohan et al. 2005). Activated carbon is the most widely used carbon adsorption material (Lalvani et al. 1998). Activated carbon is mostly made from bamboo, wood and other raw materials, which are activated after carbonization at high temperature (Salas-Enríquez et al. 2019). Generally, activated carbon can be divided into granular activated carbon and powdered activated carbon according to particle size. Activated carbon has a high specific surface area of up to 1,500 m2/g, and there are many kinds of oxygen-containing functional groups on the surface.

Biochar is an activated carbon like adsorbent (Tan et al. 2016). However, due to the lack of porous structure, the specific surface area of most biochar is relatively low. To overcome this shortcoming, the primary activator used is ZnCl2, H3PO4, KOH, and NaOH. Among them, ZnCl2 activation method is relatively sophisticated method (Su et al. 2015). Recent studies have reported that nitrogen (N) doping can enhance the adsorption capacity of activated carbon for anions such as nitrate (Goto et al. 2017). Since the presence of quaternary nitrogen on the surface of nitrogen-doped activated carbon, it acts as a positively charged adsorption site on carbon, surface which is beneficial to the adsorption of various anions (Yoo et al. 2018).

Bamboo could be regarded as cheaply available biochar material (Su & Dong 2019). A large number of low-value bamboo wastes will be produced in the process of bamboo exploitation, processing, and utilization. The effective utilization rate of bamboo wastes is only about 35–40%. Bamboo waste, if discarded, decayed, buried or burned, not only will cause environmental pollution, but also a waste of bamboo resources. Bamboo contains a lot of cellulose and lignin, and its resource utilization is extensive. Biochar materials can be prepared by treating bamboo forest solid wastes, which have good adsorption effect on heavy metals and organic pollutants in water.

Based on these considerations, in this work, the commercial bamboo chips were further activated by ZnCl2 and heat treated at 950 °C under NH3 gas flow, in order to increase the adsorption capacity for Cr(VI) in aqueous solution.

MATERIALS AND METHODS

Materials

The feedstock for the production of biochar was willow residues pruned from moso bamboo. Moso bamboo obtained in Aichi prefecture, Japan, was cut into chips and used as a precursor for preparation of adsorbent.

The bamboo chips were dried in an oven at 110 °C for 1 h. In this study, using the ZnCl2 as an activator, the prepared bamboo chips were impregnated with ZnCl2 solution at a ratio of 3 g-ZnCl2/g-bamboo, and then the mixture was dried in an oven at 110 °C overnight. The resulting mixture was pyrolyzed for 1 h at 400 °C under N2 atmosphere in a tubular furnace. The prepared biochar was placed in 1 M HCl solution and stirred for 1 h, rinsed in a Soxhlet extractor for 24 h, and dried at 110 °C overnight. The obtained samples were referred to as BZ.

N-doped biochar were prepared from BZ. The prepared BZ was heated from room temperature to 600 °C under a helium atmosphere in a tubular furnace, then the helium gas was directly replaced by NH3 gas, then heated up to 950 °C and kept there for 0, 15, 30 and 45 minutes. Finally, the NH3 gas was changed to helium and the samples were cooled to room temperature. The obtained samples were referred to as BZ-9.5AG-xmin, where x is the heating retention time.

Biochar characterization

The specific surface area (SBET), mesopore volume (Vmeso) and micropore volume (Vmicro) of the prepared biochar were calculated based on N2 adsorption/desorption isotherms at −196 °C using a Bellsorp-mini II (MicrotracBEL, Co., Ltd., Japan) surface area analyzer. The elemental composition of C, H and N of the adsorbents was determined with a Perkin Elmer 2400 II (Perkin Elmer Japan, Co., Ltd., Japan), and O content was obtained by difference. X-ray photoelectron spectroscopy (XPS) analysis was performed using a JEOL JPS-9030 spectrometer. The concentrations of Cr(VI) solution after batch experiments were determined with an atomic adsorption spectrometer novAA300 (Analytik Jena AG, Germany).

Batch adsorption

The removal of Cr(VI) by the biochar was examined by measuring the concentrations of Cr(VI) in a batch system. All batch experiments were carried out in Erlenmeyer flasks. Batch experiments were conducted in 30 mL Erlenmeyer flasks, with 0.02 g of the sample put into a flask containing 20 mL of Cr(VI) solution with various initial concentrations. The flasks were agitated in a water bath shaker at 25 °C at the speed of 100 rpm. The adsorbed amount of Cr(VI) per unit mass of adsorbent was calculated by Equation (1): 
formula
(1)
where m (g) is the mass of the biochar, V (L) is the volume of the Cr(VI) solution, C0 (mmol/L) represents the initial concentration of Cr(VI), Ce (mmol/L) is the adsorption equilibrium concentration of Cr(VI).

The effect of solution pH on Cr(VI) adsorption was examined by mixing 20 mg of adsorbent with 20 mL of 9 mmol/L Cr(VI) solution. The mixed solution had different pH values, ranging from 2 to 9. The initial pH of Cr(VI) solution was adjusted by 0.1 M HCl and 0.1 M NaOH.

The theoretical equilibrium adsorption capacity of activated carbon for heavy metal ions is an important index to measure its adsorption capacity. Under isothermal conditions, the adsorption phenomena on the adsorbent surface in solution, and the relationship between the amount of adsorption on the adsorbent surface and the equilibrium concentration of the solution, were characterized. The commonly used fitting models are the Langmuir isotherm model and the Freundlich isotherm model. The Langmuir isotherm model is a theoretical formula, which is based on three assumptions: (1) the adsorbent surface is uniform, and the adsorption energy is the same everywhere; (2) the adsorption is a monolayer, and when the adsorbent surface is saturated with the adsorbate, its adsorption capacity reaches the maximum; (3) there is no interaction between the adsorbed molecules (Wu et al. 2010). The equation is as follows: 
formula
(2)
where, Xm (mmol/g) represents the theoretical maximum adsorbed amount of Cr(VI), and KL (L/mmol) is the Langmuir constant related to adsorption energy.
The Freundlich isotherm model is an empirical formula, which shows that the adsorption occurs on heterogeneous surfaces and is multiphase adsorption (Luo et al. 2018). The expression is as follows: 
formula
(3)
where KF is the Freundlich constant and 1/n is the heterogeneity factor. The greater the value of n, the better the adsorption performance.
The adsorption kinetics of Cr(VI) on biochar samples were studied by mixing 0.2 g of adsorbent with 200 ml of 9 mmol/L Cr(VI) solution, and the concentration was analyzed at disparate time intervals (1, 2, 3, 4, 5, 10, 15, 20, 25, 30, 60, 120, 180, 240, 300, 480, 1,500, and 1,920 min). The data were analyzed using both the pseudo-first-order and the pseudo-second-order models. The linearized forms of the models are shown as follows: 
formula
(4)
 
formula
(5)
where Qe is the amount of Cr(VI) adsorbed at equilibrium (mmol/g), Qt is the adsorbed amount of Cr(VI) (mmol/g) at time t (min). k1 and k2 are the rate constant of the pseudo-first-order model (L/min) and the pseudo-second-order model (g/mmol•min).

In order to test the regeneration capacity of the biochar, 300 mg of adsorbent was added to an Erlenmeyer flask containing 300 mL of Cr(VI) solution with an initial pH of 2 and an initial concentration of 9 mmol/L. The flasks were shaken at 100 rpm at 25 °C for 24 h to reach apparent equilibrium. Then the mixture was filtered with 0.45 μm filter paper, in the desorption agent, the filtered biochar was washed with distilled water and placed in 300 mL of 1 mol/L NaOH solution at 90 °C for 2 h. The regeneration experiment was repeated in successive adsorption-desorption cycles that were conducted five times.

RESULTS AND DISCUSSION

Characterization of biochar

The elemental composition of oxidized biochar with different NH3 retention times is shown in Table 1. It can be seen from Table 1 that the N content of BZ was only 0.4%; by contrast, the N content of biochar after NH3 treatment increased. As the NH3 retention time increased from 0 min to 30 min, the N content increased from 3.06% to 4.52%. And as the NH3 retention time increased to 45 min, the N content of biochar decreased to 2.83%. This shows that the N content of biochar reached its maximum when the NH3 retention time was 30 min. Therefore, BZ-9.5AG-30 min is expected to have higher removal efficiency of Cr(VI).

Table 1

Elemental composition of BZ-9.5AG-30 min

AdsorbentCHNOa
[wt%][wt%][wt%][wt%]
BZ 90.37 1.41 0.20 7.92 
BZ-9.5AG-0 min 91.29 0.49 3.06 5.16 
BZ-9.5AG-15 min 90.70 0.44 3.50 5.36 
BZ-9.5AG-30 min 88.66 0.39 4.52 6.43 
BZ-9.5AG-45 min 90.58 0.41 2.83 6.18 
AdsorbentCHNOa
[wt%][wt%][wt%][wt%]
BZ 90.37 1.41 0.20 7.92 
BZ-9.5AG-0 min 91.29 0.49 3.06 5.16 
BZ-9.5AG-15 min 90.70 0.44 3.50 5.36 
BZ-9.5AG-30 min 88.66 0.39 4.52 6.43 
BZ-9.5AG-45 min 90.58 0.41 2.83 6.18 

aCalculated by difference.

The nitrogen adsorption-desorption isotherm curve of the BZ-9.5AG-xmin is shown in Figure 1. From Figure 1, a type I isotherm was observed, which indicates the existence of a mainly microporous structure. The specific surface area (BET) and micro-/mesoporous structure parameters of the adsorbent were calculated and are shown in Table 2. It can be seen that the specific surface area (SBET) of biochar decreased after the beginning of NH3 gas introduction, but the specific surface area of biochar increased with the increased NH3 retention time. This may be due to the formation of N-containing functional groups on the surface of the biochar at the beginning of NH3 introduction, thus reducing the surface area. With the increased NH3 retention time at high temperature, the microporous structure was developed continuously and the specific surface area of the biochar increased.

Table 2

Textural and surface properties of each prepared biochar

AdsorbentSBETVtotalVmicroVmeso
[m2/g][cm3/g][cm3/g][cm3/g]
BZ 1,399 0.75 0.74 0.01 
BZ-9.5AG-0 min 1,122 0.55 0.55 0.01 
BZ-9.5AG-15 min 1,277 0.63 0.63 0.01 
BZ-9.5AG-30 min 1,610 0.85 0.85 0.01 
BZ-9.5AG-45 min 1,869 1.00 0.99 0.01 
AdsorbentSBETVtotalVmicroVmeso
[m2/g][cm3/g][cm3/g][cm3/g]
BZ 1,399 0.75 0.74 0.01 
BZ-9.5AG-0 min 1,122 0.55 0.55 0.01 
BZ-9.5AG-15 min 1,277 0.63 0.63 0.01 
BZ-9.5AG-30 min 1,610 0.85 0.85 0.01 
BZ-9.5AG-45 min 1,869 1.00 0.99 0.01 
Figure 1

Nitrogen adsorption isotherm of BZ-9.5AG-30 min.

Figure 1

Nitrogen adsorption isotherm of BZ-9.5AG-30 min.

Adsorption isotherms and adsorption capacity

Adsorption isotherms of Cr(VI) for the BZ and BZ-9.5AG-30 min are shown in Figure 2. Both Langmuir and Freundlich models were used to fit the adsorption data. The adsorption process of Cr(VI) at 25 °C and the different isotherm constants determined are presented in Table 3. The R2 values of the Langmuir model were larger than those of the Freundlich model, indicating that the Langmuir models can be a good description of Cr(VI) adsorption behavior. The Langmuir isotherm is usually used to describe monolayer adsorption on a homogenous surface, indicating that Cr(VI) adsorption onto BZ and BZ-9.5AG-30 min tended to be monolayer adsorption. BZ and BZ-9.5AG-30 min all exhibited high adsorption capacity for Cr(VI). Comparatively, the maximum adsorbed amount (Xm) of Cr(VI) onto BZ-9.5AG-30 min was larger than the Xm of BZ, which indicated that N-doping biochar has better adsorption capacity for Cr(VI). The increase in the adsorbed amount can be explained by the increase of specific surface area and nitrogen content. Compared with BZ, BZ-9.5AG-30 min had a larger specific surface area and total pore volume, so it can provide more adsorption sites. With the increase of nitrogen content, more quaternary nitrogen (N-Q) can be formed on the surface of the biochar. Positive charges on quaternary nitrogen can provide more adsorption sites for anions, thus increasing the adsorption of Cr(VI) (Yoo et al. 2018).

Table 3

Langmuir and Freundlich adsorption isotherm parameters of Cr(VI) at 298 K in aqueous solutions

Langmuir model
Freudlich model
Xm (mmol/g)KL (L/mmol)R21/nKFR2
BZ 3.81 0.38 0.95 0.64 0.73 0.80 
BZ-9.5AG-30 min 4.31 0.32 0.98 0.66 0.69 0.92 
Langmuir model
Freudlich model
Xm (mmol/g)KL (L/mmol)R21/nKFR2
BZ 3.81 0.38 0.95 0.64 0.73 0.80 
BZ-9.5AG-30 min 4.31 0.32 0.98 0.66 0.69 0.92 
Figure 2

Adsorption isotherms of BZ (a) and BZ-9.5AG-30 min (b) for Cr(VI).

Figure 2

Adsorption isotherms of BZ (a) and BZ-9.5AG-30 min (b) for Cr(VI).

Comparison with values in the literature (Table 4) shows that BZ-9.5AG-30 min is among the highest carbon adsorbents for Cr(VI) (Karthikeyan et al. 2005; Rangabhashiyam & Selvaraju 2015; Zhu et al. 2016; Chu et al. 2018; Yu et al. 2018; Zhang et al. 2018), so it has great potential in practical water treatment applications.

Table 4

Comparison with other similar adsorbents for Cr(VI) removal

AdsorbentQe (mmol/g)Reference
Swietenia mahagoni shell 1.13 Rangabhashiyam & Selvaraju (2015)  
Hevea brasilinesis sawdust AC 0.85 Karthikeyan et al. (2005)  
Biochar derived from waste water hyacinth 0.46 Yu et al. (2018)  
Bismuth modified biochar 0.23 Zhu et al. (2016)  
Biochar derived from Melia azedarach wood 0.49 Zhang et al. (2018)  
Ion exchange resin HP555 3.69 Chu et al. (2018)  
N-doped biochar BZ-9.5AG-30 min 4.31 This work 
AdsorbentQe (mmol/g)Reference
Swietenia mahagoni shell 1.13 Rangabhashiyam & Selvaraju (2015)  
Hevea brasilinesis sawdust AC 0.85 Karthikeyan et al. (2005)  
Biochar derived from waste water hyacinth 0.46 Yu et al. (2018)  
Bismuth modified biochar 0.23 Zhu et al. (2016)  
Biochar derived from Melia azedarach wood 0.49 Zhang et al. (2018)  
Ion exchange resin HP555 3.69 Chu et al. (2018)  
N-doped biochar BZ-9.5AG-30 min 4.31 This work 

Effect of pH

The initial pH of the solution usually has a great influence on the adsorption capacity of biochar. The effect of initial pH on the adsorption of Cr(VI) by BZ-9.5AG-30 min is shown in Figure 3. For adsorption of Cr(VI), the adsorbed amount of Cr(VI) on BZ-9.5AG-30 min decreased as pH increased from 2 to 9. The effect of initial pH on adsorption can be explained by the surface charge of biochar and the ionic forms of Cr(VI). Cr(VI) exists primarily as an oxyanion, and it exists in different ionic forms in aqueous solution. When solution pH is greater than 6, it presents as CrO42−; when pH is ranging between 1.0 and 6.0, it exists as Cr2O72− and HCrO4. Therefore, when the solution pH has changed the form of the Cr(VI) ions it will influence the Cr(VI) uptake capacity of the biochar. Moreover, amino groups exist on the surface of N-doped biochar. Under acidic conditions, amino groups present as NH3+, with positive sites, which are also beneficial to the adsorption of Cr(VI) (Zhao et al. 2011). With the increase of pH, the amount of OH ions in the solution increases, and the competition between OH ions and Cr(VI) ions results in a decrease of the adsorption capacity of biochar to Cr(VI) (Liu et al. 2017).

Figure 3

Effect of initial pH value on Cr(VI) removal.

Figure 3

Effect of initial pH value on Cr(VI) removal.

Effect of contact time and adsorption kinetics

The effects of contact time on the adsorption capacity of BZ-9.5AG-30 min for Cr(VI) were also investigated. As shown in Figure 4, is the determination of the relationship between the amount of adsorption of Cr(VI) and time in the range of 0–2,000 min. The adsorption rate of Cr(VI) was initially higher, reaching 86% of the maximum adsorption capacity at 480 minutes, and the adsorption capacity (Qt) would not change after 1,000 minutes. So the optimum adsorption time of BZ-9.5AG-30 min for Cr(VI) was about 1,000 min. The experimental data in Figure 4 were fitted with the pseudo-first-order model and pseudo-second-order model, and the results of the kinetic analysis are shown in Table 5. As shown in Table 5, the R2 values of the pseudo-first-order model and the pseudo-second-order model are both higher than 0.95; however, the R2 value of the pseudo-second-order model was larger than that of the pseudo-first-order model and larger than 0.99, which indicates that the pseudo-second-order model can better describe the adsorption of BZ on Cr(VI). These facts suggest that the adsorption behavior is mainly governed by diffusion control mechanism in Cr(VI) solution.

Table 5

Kinetic parameters for Cr(VI) adsorption onto BZ-9.5AG-30 min

Pseudo-first order
Pseudo-second order
Qe (mmol/g)k1 (min−1)R2Qe (mmol/g)k2 (g/mmol min)R2
2.89 0.003 0.97 2.92 0.008 0.998 
Pseudo-first order
Pseudo-second order
Qe (mmol/g)k1 (min−1)R2Qe (mmol/g)k2 (g/mmol min)R2
2.89 0.003 0.97 2.92 0.008 0.998 
Figure 4

Cr(VI) adsorption kinetic by BZ-9.5AG-30 min.

Figure 4

Cr(VI) adsorption kinetic by BZ-9.5AG-30 min.

XPS analysis

The XPS spectra measurement was performed to investigate the surface chemical state and elemental changes of BZ-9.5AG-30 min before and after adsorption. As shown in Figure 5(a), before the adsorption of Cr (VI), the XPS spectra of biochar showed three peaks, C 1s (284 eV), N 1s (400 eV) and O 1s (531 eV), respectively. After the experiment of Cr(VI) adsorption, a very obvious peak appeared at 578 eV, which is assigned to the Cr 2p3/2 energy level, suggesting that Cr(VI) was successfully adsorbed on the biochar. The N1s spectra of BZ-9.5AG-30 min before and after the removal of Cr(VI) are shown in Figure 5(b) and 5(c). The N 1s spectrum was resolved into four individual peaks at 398.7 eV, 400.4 eV, 401.3 eV and 403.3 eV, which are assigned to pyridinic nitrogen (N-6), pyrrolic nitrogen (N-5), quaternary nitrogen (N-Q) and pyridine-N-oxide (N-X), respectively. Among them, because N-Q has a positive charge, the adsorption capacity of anions should be increased. Notably, as shown in Table 6, the percentage of N-Q in BZ-9.5AG-30 min decreases from 22.3% to 5.5% after adsorption, and this would be because the reduction process of Cr(VI) consumes the positive charge on the adsorbent surface, resulting in the decrease of N-Q.

Table 6

Relative surface atomic ratios of different N species in BZ-9.5AG-30 min before and after Cr(VI) adsorption

N-6 (%)N-5 (%)N-Q (%)N-X (%)
Before adsorption 37.1 11.4 22.3 29.2 
After adsorption 25.9 50.4 5.5 18.2 
N-6 (%)N-5 (%)N-Q (%)N-X (%)
Before adsorption 37.1 11.4 22.3 29.2 
After adsorption 25.9 50.4 5.5 18.2 
Figure 5

(a) XPS survey spectra of BZ-9.5AG-30 min before and after Cr(VI) adsorption; (b) N 1 s spectrum of BZ-9.5AG-30 min before Cr(VI) adsorption; (c) N 1 s spectrum of BZ-9.5AG-30 min after Cr(VI) adsorption; (d) Cr 2p spectrum of BZ-9.5AG-30 min after Cr(VI) adsorption.

Figure 5

(a) XPS survey spectra of BZ-9.5AG-30 min before and after Cr(VI) adsorption; (b) N 1 s spectrum of BZ-9.5AG-30 min before Cr(VI) adsorption; (c) N 1 s spectrum of BZ-9.5AG-30 min after Cr(VI) adsorption; (d) Cr 2p spectrum of BZ-9.5AG-30 min after Cr(VI) adsorption.

Figure 5(d) shows the Cr 2p3/2 spectrum, and the Cr 2p3/2 spectrum is divided into two peaks at 577.6 eV and 579.5 eV, which were assigned to Cr(III) and Cr(VI). The adsorbed chromium predominantly existed in Cr(III), and only a small amount was present as Cr(VI). As shown in Figure 5(d), 69.7% of the adsorbed chromium existed in trivalent form, and the remaining 30.3% was present as Cr(VI).

Regeneration experiment

In order to evaluate the recoverability of N-doped biochar BZ, five cycles of adsorption-desorption experiments were carried out on N-doped biochar BZ-9.5AG-30 min. Figure 6 shows the adsorption capacity of BZ-9.5AG-30 min for Cr(VI) in the function of the adsorption-desorption cycles. After five time regenerations, the adsorption capacity of BZ-9.5AG-30 min still had 63% of the initial adsorption capacity, so it had better regeneration ability than our previous study (Chu et al. 2018).

Figure 6

The regeneration adsorptive performance of BZ-9.5AG-30 min after five cycles.

Figure 6

The regeneration adsorptive performance of BZ-9.5AG-30 min after five cycles.

CONCLUSIONS

In conclusion, we have successfully prepared N-doped biochar adsorbent BZ-9.5AG-30 min for heavy metal Cr(VI) removal. The Cr(VI) removal experiments showed that BZ-9.5AG-30 min had a high adsorption capacity (4.31 mmol/g) of Cr(VI), which was much higher than previous reports. The effect of pH has shown that the highest adsorption capacity of Cr(VI) by BZ-9.5AG-30 min was at pH 2. The adsorption reaction conformed to the Langmuir model, and the kinetic analyses indicated that the adsorption of Cr(VI) by BZ-9.5AG-30 min fitted well with the pseudo-second-order model. After five regeneration experiments, BZ-9.5AG-30 min showed superior recyclability, and therefore, BZ-9.5AG-30 min is expected to be a promising adsorbent for the efficient removal of Cr(VI) from wastewater.

ACKNOWLEDGEMENTS

Gratitude is greatly extended to Prof. Dr. Fumio Imazeki, the head of Safety and Health Organization, Chiba University, for his financial support on our study. The first author also acknowledges the kind support of the Japanese Government (MEXT) for the scholarship.

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