Aniline is an important industrial material and widely used in the industry of pharmacy, dyestuff and textile, etc. Because of its toxicity, it is necessary to reduce its spread in the environment. The adsorption is one of the easiest and most effective methods to remove deleterious substances from wastewater. And the effective, economical adsorbents will be used to remove deleterious substances from wastewater.

The bamboo-charcoal was the pyrogenation product of bamboo with wide surface area and strong adsorption ability. As a novel adsorbent, the bamboo charcoal is widely applied in waste water treating(Zhang et al., 2001; Jiang et al., 2002; Hu, 2002). The bamboo charcoal is an economical adsorbent for removing organic substances from wastewater because of its abundance and availability. The adsorption properties of organic substances on bamboo-charcoal have been reported in recent years(Xu et al., 2002; Zhang et al., 2006; Kei et al., 2004; Asada et al., 2002; Yang et al., 2005; Zhang, 2001; Jiang, 2001; 2002; Kang, 1998).

In this article, the adsorption properties of bamboo-charcoal for aniline were investigated. The effect of contact time, pH, adsorption temperature, and initial concentration of aniline were examined in this study.

1 Materials and methods 1.1 Instruments

723 spectrophotometer, Sartorius PB-20 pH meter, SHZ-B temperature constant shaking machine, WD900SL23-2 gelanshi household microwave oven.

1.2 Materials

A standard solution of 2.0 mg·mL^－1 aniline was prepared with aniline (AR) and 0.05 mol·L^－1 H₂SO₄. Buffer solution with pH 3.2~6.4 wa s prepared by HAc-NaAc, other reagents were AR grade.

The machine-kiln bamboo charcoal was provided by Zhejiang Forest Bamboo Charcoal Limited Company, the experiments adopted bamboo-charcoal that average diameters were 0.90~0.60 mm and 0.45~0.355 mm. They were soaked in dilute NaOH overnight and then washed thoroughly with deionized water until the pH became neutral. After the pretreatment, bamboo charcoal was then dried at 250 ℃ in an electric oven until weight invariableness.

1.3 Analytical method

A sample solution of aniline was accurately added into a 25 mL colorimetric tube, subsequently adding 0.8 mL of 50 g·L^－1 KHSO₄ and 0.6 mL of 0.1 mol·L^－1 NaNO₂, which was shaken for 5 min. And then 2 mL of 2 mol·L^－1 carbamide solution was added, which was shaken again until air bubble disappeared. And then 1 mL of 0.2% 8-Hydroxy-quinoline and 0.5 mL of 6 mol·L^－1 NaOH were added. After the addition of deionized water to the mark to colorimetric tube, the absorbency was determined in a 1 cm colorimetric vessel at wavelength of 488 nm and compared with blank test.

1.4 Sorption equilibrium experiment

A desired amount of treated bamboo charcoal was weighted and added into a conical flask, then a desired volume of buffer solution with pH 5.9 and required amount of standard solution of aniline were added. The flask was shaken in a shaker at constant temperature. The upper layer of clear solution was taken out for analysis until adsorption equilibrium was reached (Hu et al., 1999). The adsorption amounts [Q/(mg·g^－1)] and adsorption rate(E/%) was calculated as following:

c₀:initial concentration of aniline in solution(mg·mL^－1); c_e:equilibrium concentration of aniline in solution(mg·mL^－1); V:total volume of solution(mL); m:mass of bamboo charcoal(g).

2 Results and discussion 2.1 Influence of pH on adsorption

The test was carried out according to the above mentioned method. The influence of pH on the sorption behavior of bamboo charcoal for aniline was shown in Fig. 1.

The results shows that the bamboo-charcoal has high ability of adsorption for aniline at a wide pH range(pH 3.2~6.4), the adsorption amounts increases with the increase of pH. The adsorption amounts was highest at pH 5.9 in HAc-NaAc medium. All following experiments were carried out at pH 5.9.

2.2 Determination of equilibrium time and influence of size of bamboo charcoal on adsorption

According to the experimental condition as shown in Fig. 2 and the one mentioned above, 50.0 mg of bamboo charcoal was weighted accurately, then the upper layer solution was taken out at intervals and the remains concentrations were determined. After all of the volumes were collected, a series of data was obtained as shown in Fig. 2.

The result shows that equilibrium time is 360 min. The adsorption amounts increases with the decrease of size of bamboo charcoal.

2.3 Influence of mass of bamboo charcoal on adsorption efficiency

The different mass of bamboo charcoal(0.45~0.355 mm, i.d.) was weighted accurately, according to the experimental condition shown in Fig. 3 and the above mentioned. After equilibrium, the upper layer clear solution was taken out for the determination of remains concentrations and a series of adsorption efficiency was calculated and shown in Fig. 3.

The result shows that adsorption efficiency increases for 5.7%, 4.1% and 3.2%, respectively with the increase of mass of bamboo charcoal from 50.0 to 80.0 mg per 10.0 mg. Which shows that the increase of adsorption efficiency decreases with the increase of mass of bamboo charcoal.

2.4 Influence of initial concentration of aniline on adsorption

Different amounts of the standard solution of aniline were added into nine conical flasks respectively that containing 50.0 mg of bamboo charcoal. The experiment was carried out by the above-mentioned method at 298 K. When the sorption equilibrium was reached, equilibrium concentration was determined and corresponding sorption amounts and adsorption efficiency was calculated and shown in Fig. 4.

The result shows that adsorption increases along with the increases of initial concentration of aniline. When c₀≤100 mg·L^－1, the change of adsorption was even bigger. However, when c₀ > 100 mg·L^－1, the change of adsorption amounts would become smaller. But adsorption efficiency decreases from slow to quick with the increase of concentration of aniline.

2.5 Influence of temperature on sorption and the Freundlich isotherm model

2.0, 2.5, 3.0, 4.0 mL of the standard solution of aniline and 30 mL of buffer solution with pH 5.9 were added to four conical flasks respectively that containing 50.0 mg of bamboo charcoal(0.45~0.355 mm, i.d.). which was diluted to 50 mL with deionized water. The experiment was carried out by using the above-mentioned method at the temperature 298, 308, 318 K respectively. When the sorption equilibrium was reached, equilibrium concentration was determined and corresponding sorption amounts was calculated and shown in Tab. 1.

According to Freundlich isotherm model Q=ac^1/b, i.e., lgQ=1/b(lgc)+lga, the slope of straight line, which is gotten from the figure lgQ versus lgc_e, yields the 1/b at the temperature of 298, 308, 318 K. The Freundlich isotherm model of the different temperature and the correlation coefficient of straight line were shown in Tab. 1.

The result shows that adsorption amounts decrease with the increase of adsorption temperature. It means that the sorption process is an exothermic process. The adsorption behavior of aniline on bamboo-carbon obeys the Freundlich isotherm model at the temperature of 318, 308, 298 K. Thus the slope of the straight line was 1/b, 1/b(318 K)=0.498, 1/b(308 K)=0.430, 1/b(298 K)=0.325, and b(318 K)=2.0, b(308 K)=2.3, b(298 K)=3.1. The fact b is between 2 and 10 indicates that aniline is easy to be adsorbed(Cooney, 1999; Zhang et al., 2003).

2.6 Influence of different adsorption ways on sorption ratio

Three different adsorptive ways as followed were carried out in the solution that containing 200 mg bamboo charcoal (0.60~0.45 mm, i.d.) and 100 mL aniline solution with concentration of 60 mg·L^－1. Fig. 5 shows different adsorption ways on sorption ratio.

The result shows that the adsorption effect will be improved with increasing contact opportunity between adsorbent and adsorbate. The main influence factor was diffusion rate of aniline in isothermal fixed adsorption way. Since lack of contact opportunity between bamboo charcoal and aniline, this way shows lower adsorption effect. However, it shows better adsorption effect in the second and third ways because of abundance contact opportunity between bamboo charcoal and aniline. Sorption ratios were all above 98%.In column adsorption, sorption ratio was increased with the flow rate of aniline decrease.

2.7 Regeneration and reuse of bamboo charcoal

After aniline was adsorbed on bamboo charcoal, the bamboo charcoal was eluted by 10 mL of 0.4~2.0 mol·L^－1 H₂SO₄, and it was heating by microwave for 5 min, then the bamboo charcoal was washed several times by deionized water and was dried at 250 ℃ at last. The bamboo charcoal after regeneration was used again in adsorption of aniline, the sorption amounts and regeneration efficiency was calculated and shown in Tab. 2.

The result shows that bamboo charcoal can be regenerated quantitatively. The optimized eluant concentration of H₂SO₄ was 0.8 mol·L^－1, and the regeneration efficiency can up to 97%. The regenerated bamboo charcoal is effective and can be reused.

3 Conclusion

The adsorption property of aniline on bamboo-charcoal was studied in aqueous solution. The result shows that the bamboo-charcoal has high adsorption ability for aniline at a wide pH range(pH 4.0~6.4), the optimized adsorption acidity of bamboo-charcoal for aniline is pH 5.90 in HAc-NaAc medium.

Adsorption equilibrium time is 360 min. The adsorption amounts increases with the decrease of size of bamboo charcoal. The adsorption efficiency decreases with the increase of mass of bamboo charcoal in a certain concentration and a certain volume of aniline solution. The adsorption amounts increases with the increases of initial concentration of aniline.

As the adsorption temperature increases, the adsorption decreased. And it has been proved that the adsorption process is an exothermic process. The adsorption behavior of bamboo-carbon for aniline obeys the Freundlich isotherm model.

The used bamboo-charcoal can be treated with microwave by heating in 0.8 mol·L^-1 H₂SO₄ and the adsorption ability can be regenerated to 97%.

The equilibrium adsorption amount is 44.5 mg·g^－1 when initial concentration of aniline is 0.060 mg·mL^－1. The aniline can be removed effectively from aqueous solution with bamboo-charcoal.