Adsorption behavior of Cr(VI) by N-doped biochar derived from bamboo

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.

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.

The specific surface area (S_BET), mesopore volume (V_meso) and micropore volume (V_micro) 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).

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.

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.

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).

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 (S_BET) 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.

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 (X_m) of Cr(VI) onto BZ-9.5AG-30 min was larger than the X_m 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).

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.

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).

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 (Q_t) 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.

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 2p_3/2 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.

Figure 5(d) shows the Cr 2p_3/2 spectrum, and the Cr 2p_3/2 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).

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).

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.

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.