Experimental Design To Optimize the Preparation of Activated

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Experimental Design To Optimize the Preparation of Activated Carbons from Herb Residues by Vacuum and Traditional ZnCl2 Chemical Activation Juan Yang and Keqiang Qiu* School of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China ABSTRACT: Doehlert matrix was used to optimize the experimental conditions for the preparation of activated carbons from herb residues by vacuum chemical activation and traditional chemical activation using ZnCl2 as activation agent. The effects of activation temperature and impregnation ratio, the most influential factors of ZnCl2 chemical activation, were studied. The obtained activated carbons were characterized by total yield and methylene blue and iodine adsorption value. Each response has been described by a second-order model, and the predicted model presented a good agreement with experimental data. The results showed that activated carbons prepared by vacuum chemical activation have higher yield and better adsorption capacity. The removal percentages of methylene blue for the two optimal activated carbons and a commercial activated carbon were determined. The activated carbon prepared by vacuum chemical activation exhibited the highest methylene blue removal efficiency.

1. INTRODUCTION Activated carbon is a well-known material used for adsorption of pollutants within gaseous and liquid phases.1,2 For water and wastewater treatment, activated carbons with relatively higher surface area and better adsorption capacity are needed. The qualities and characteristics of activated carbon depend on the physical and chemical properties of the starting materials and the activation methods used. Hence considerable effort has been devoted to finding suitable precursors and modifying the existing methods to obtain activated carbon with better properties.3-11 In general, activated carbon can be prepared from a variety of carbonaceous materials, such as coal, peat, wood, coconut shell, rice husk, and any other materials rich in the element carbon. In recent years, there has been a growing interest in the production of activated carbons from agricultural wastes because of their abundant resources and cheap prices.12-17 As the cradle of Chinese medicine, a large number of herb residues are generated annually in China. According to statistics, the production of herb residues has exceeded 13 000 000 tons every year.18 The herb residues have high moisture content and many nutrient components, which are easy to decay and thus lead to a serious environmental problem. Therefore, it is an important topic to make full use of the herb residues in an environmentally friendly manner. In this work, we focus on the preparation of activated carbon from herb residues. As mentioned before, many activation methods have been employed to obtain activated carbons from agricultural byproducts; chemical activation method is one of the most widely used.3-6,8 However, most of the studies at present are carried out under atmospheric conditions (traditional chemical activation). Our previous paper has indicated that activated carbon obtained by vacuum chemical activation (prepared under vacuum condition) has better properties (e.g., higher Brunauer-EmmettTeller (BET) surface area and better adsorption capacity) than that prepared by traditional chemical activation.5,10,11 As far as is known to the authors, no study has been done on comparison of the activated carbons prepared from herb residues r 2011 American Chemical Society

by vacuum chemical activation and traditional chemical activation. The main objective of the present study was to obtain activated carbons from herb residues by the two activation methods using ZnCl2 as activation agent. Doehlert matrix19 was used to optimize the experimental conditions for the preparation of activated carbons because it has a number of advantages such as the following: (i) the possibility to present a uniform distribution of experimental points in the space studied, (ii) the ability to explore the whole of the domain, (iii) the usefulness of interpolating the response, and (iv) the possibility of adding new factors without altering the quality of the matrix.20 The preparation of activated carbon is influenced by many factors, among which activation temperature (U1) and impregnation ratio (U2) are the most important two factors.21 Thus, a series of experiments were carried out with values of U1 and U2 included in the suitable range. Total yield and methylene blue and iodine adsorption values were used to characterize the obtained activated carbon. The optimal experimental conditions for preparing activated carbons suitable for use as adsorbents were determined. The adsorption results of optimal activated carbons prepared by both vacuum and traditional chemical activation were compared with those of commercial activated carbon.

2. EXPERIMENTAL SECTION 2.1. Material. The herb residues used in this study were obtained from a local pharmaceutical factory. The main ingredients include the following: echinacea, red clover, salvia, angelica, scutellaria, schisandra, lucidum, and so forth. Though many kinds of herbs are involved, for one pharmaceuticals production, the proportions of the required ingredients are fixed; thus, the Received: July 18, 2010 Accepted: February 7, 2011 Revised: December 17, 2010 Published: February 24, 2011 4057

dx.doi.org/10.1021/ie101531p | Ind. Eng. Chem. Res. 2011, 50, 4057–4064

Industrial & Engineering Chemistry Research

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Table 1. Proximate and Ultimate Analyses of Raw Material proximate analysis (wt %)

ultimate analysis (wt %)

moisture

4.74

carbon

ash

5.62

hydrogen

6.10

72.62 17.02

nitrogen sulfur

3.53 1.08

volatile fixed carbon

36.06

compositions of the herb residues can be controlled. The fresh herb residues had a high content of water. Prior to use, the precursor was dried naturally and crushed to a particle size of 0.15-0.85 mm. The proximate and ultimate analyses of raw material are shown in Table 1. 2.2. Preparation. Zinc chloride pellets were dissolved in water and then impregnated into the herb residues with certain impregnation ratio (mass ratio of ZnCl2 to herb residues). The mixture was maintained at room temperature for 48 h and then evaporated to dryness at 105 °C. The impregnated herb residues were placed in a stainless steel reactor (self-manufactured) which was inserted into an electrical furnace. The reactor was connected to a cold trap, which was used to collect liquid product (biomass-oil). The end of the cold trap was connected to a mechanical vacuum pump. In the experiment, the system was first vacuumed to a system pressure of 20 kPa. Then the sample was heated from room temperature to a preselected final temperature with a heating rate of 5 °C/ min. The material was activated at the final temperature for a holding time of 60 min, before it was subsequently cooled to room temperature again. The heating and cooling processes were conducted under the same vacuum condition. The resulting sample was thoroughly washed with 0.1 M hydrochloric acid and distilled water to remove the residual ZnCl2 until the pH value of the washed solution was between 6 and 7 and was then dried for further analysis. To compare the differences between the activated carbons obtained by vacuum chemical activation and traditional chemical activation, a series of experiments were also carried out under argon atmosphere. The reactor was first purged with high-purity argon gas at a high flow rate to avoid sample oxidation, and then the argon flow was adjusted to the desired rate (150 cm3(STP)/ min). The other processes and conditions were the same to those of the vacuum case except that the activation and cooling processes were carried out under the argon atmosphere. 2.3. Characterization. The yield of activated carbon (Y1), which is regarded as an indicator of the process efficiency for activated carbon preparation, can be estimated according to the following equation: Y1 =% ¼

WC 100 WS

ð1Þ

where WC is the weight of the obtained activated carbon and WS is the weight of herb residues used. The methylene blue adsorption value and iodine absorption value of activated carbon were determined according to China national standards GB/T 12496.10-1999 and GB/T 12496.81999, respectively. The specific surface area and pore structure characteristic of activated carbons were determined by nitrogen adsorption at -196 °C (Micromeritics ASAP2020). The BET surface area was calculated from the isotherms using the BET equation. The total pore volume (VT) was assessed by

converting the amount of nitrogen gas adsorbed at a relative pressure (∼0.99) to the volume of liquid adsorbate. The t-plot method was used to calculate the micropore volume (VMi). The mesopore volume (VMe) was determined by subtracting the micropore volume from the total pore volume while the micropore and mesopore percentages were based on the total pore volume. To evaluate the adsorption capacity of the obtained activated carbons and compare with commercial activated carbon (purchase form Aladdin), a batch of experiments was carried out by adding different amounts of activated carbon (0.02-0.13 g) into each 100 mL methylene blue solution (200 mg/L). The test was conducted at a temperature of 25 °C. A certain amount of activated carbon was first added into the solution and shaken at 270 rpm in a shaker for 24 h. The sample was separated by filtration, and the concentration of methylene blue at equilibrium (Ce) was determined by using a UV-vis spectrometer at a wavelength of 665 nm. The removal percentage of methylene blue was calculated according to the following equation: R=% ¼

C0 - C e 100 C0

ð2Þ

where C0 and Ce are the initial and equilibrium concentrations of methylene blue (mg/L), respectively. 2.4. Methodology of Experimental Design. Response surface methodology (RSM) is a collection of mathematical and statistical techniques that are useful for modeling and analysis of problems in which a response of interest is influenced by several variables. One of the RSM design called Doehlert matrix was applied in this work to study the variables for preparing the activated carbons. The Doehlert design describes a spherical experimental domain, and it stresses uniformity in space filling.19 For ZnCl2 chemical activation, the most influential experimental factors on the characteristics of activated carbons are activation temperature (U1) and impregnation ratio (U2). Therefore, a bivariate Doehlert matrix, which consists of several central points and six points forming a regular hexagon, was used to represent the responses studied in all experimental regions of these two factors.22 The factors are given in the form of coded variables (Xi) with no units in order to compare the factors of different natures. The relationship between coded and real values is given by ( ) Ui - Ui0 R ð3Þ Xi ¼ ΔUi where Xi is the coded value for the level of factor i, Ui is its real value in an experiment, U0i is the real value at the center of the experimental domain, ΔUi is the step of variation of the real value, and R is the coded value limit for each factor; in the present work R = 1. The number of experiments required (N) is given by N = k2 þ k þ C, where k is the number of variables and C is the number of center points. The center points are used to determine the experimental error and the reproducibility of the data. In this study k = 2 and C = 3; therefore, the matrix was comprised of nine experiments. Doehlert’s experimental matrix and the corresponding experimental conditions are given in Table 2, together with the experimental results. The experimental data were analyzed by response surface methodology. Each response (Y) can be described by a secondorder model adequate for predicting the responses in all 4058

dx.doi.org/10.1021/ie101531p |Ind. Eng. Chem. Res. 2011, 50, 4057–4064


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Table 2. Doehlert’s Experimental Matrix and the Corresponding Experimental Conditions and Responsesa chemical activation vacuum U1 (°C)

U2

1

550

2

500

3

350

expt no.

X1

X2

1.000

1.0

1.433

0.5

1.000

-1.0

traditional

YV1 (%)

YV2 (mg/g)

YV3 (mg/g)

YT1 (%)

YT2 (mg/g)

YT3 (mg/g)

0

31.0

263

828

30.7

192

731

0.866

30.4

307

988

30.5

232

845

0

36.0

99

900

32.9

24

865

4

400

0.567

-0.5

-0.866

35.0

165

968

34.2

95

872

5

500

0.567

0.5

-0.866

33.1

179

882

31.9

191

866

6

400

1.433

-0.5

0.866

32.3

202

932

31.5

129

889

7

450

1.000

0

0

32.8

296

998

31.8

205

930

8 9

450 450

1.000 1.000

0 0

0 0

32.9 33.0

305 299

1014 991

31.8 32.1

201 212

918 933

a U1 = activation temperature. U2 = impregnation ratio. YV1 (YT1) = total yield in the activated carbon preparation. YV2 (YT2) = methylene blue adsorption capacity of obtained activated carbon. YV3 (YT3) = iodine adsorption capacity of obtained activated carbon.

experimental regions Y ¼ a0 þ a1 X1 þ a2 X2 þ a11 X1 2 þ a22 X2 2 þ a12 X1 X2

ð4Þ

where Y is the predicted response; Xi is the coded value related to Ui; a0 is the intercept term, a constant that corresponds to the response when Xi is zero for each factor; ai is the coefficient of the linear term that determines the influence of Ui; aii is the coefficient of the quadratic term which can be regarded as a curve “shape” parameter; and a12 the coefficient of interaction between the two factors. For each response (Yi), the coefficients of the postulated model were calculated on the basis of the experimental responses by least-squares regression using the Design Expert software (Stat-Ease Inc., Minneapolis, MN, USA). 2.5. Optimization Method. The desirability function approach is one of the most widely used methods for the optimization of multiple response processes. It involves transformation of each predicted response, Yi, to a dimensionless partial desirability function, di, which includes the researcher’s priorities and desires when building the optimization procedure. When a response is to be maximized, the partial desirability of Yi at condition x can be defined as 8 > 0 if Yi ðxÞ e Yi, min > > !r > > < Y ðxÞ - Y i i, min if Yi, min e Yi ðxÞ e Yi, max di ðxÞ ¼ > Y Y i, max i, min > > > > :1 if Yi ðxÞ g Yi, max ð5Þ where Yi,min and Yi,max are, respectively, the lower and upper acceptability bounds for response i and r is the weight. In this work we chose weights equal to 1 for all of the responses. For each response Yi(x), a desirability function di(x) assigns numbers between 0 and 1, with di(x) = 0 representing a completely undesirable value of Yi and di(x) = 1 representing a completely desirable or ideal response value. The individual desirabilities are then combined using the geometric mean, which gives the overall desirability D(x): DðxÞ ¼ ½d1 ðxÞ d2 ðxÞ ::: dn ðxÞ1=n

ð6Þ

with n denoting the number of responses. If any response Yi is completely undesirable, then the overall desirability is zero. A value of D(x) different from zero implies that all responses are in a desirable range simultaneously. The overall desirability is a function of condition x. Once the maximized value of D(x) is found, the optimal conditions are obtained.

3. RESULTS AND DISCUSSION 3.1. Response Analysis and Interpretation. The responses studied were total yield (Y1), adsorption of methylene blue (Y2), and adsorption of iodine (Y3). The experimental results for these parameters of the obtained activated carbons are given in Table 2. In this study, the subscripts “V” and “T” represent the activated carbon prepared by vacuum chemical activation and traditional chemical activation, respectively. Replicated experiments 7-9 at the center point were performed in order to determine the experimental error. 3.1.1. Total Yield (Y1). The models of total yield obtained by least-squares regression corresponding to the experimental results can be described as follows:

YV1 ¼ 32:9 - 2:3X1 - 1:6X2 þ 0:6X1 2 - 0:5X2 2

ð7Þ

YT1 ¼ 31:9 - 1:3X1 - 1:2X2 - 0:1X1 2 þ 0:2X2 2 þ 0:8X1 X2 ð8Þ The correlation coefficients (r2) of the two models are 0.990 and 0.980, respectively, which indicate a good coherence between theoretical and experimental responses. According to the established model, the contour plots and response surfaces can be drawn as shown in Figure 1. In general, for traditional chemical activation, the total yield of activated carbon decreases with increasing activation temperature and impregnation ratio.23 As expected, the effects of activation temperature and impregnation ratio are negative (a1 < 0, a2 < 0), for both of the activation methods. Indeed, the increase in activation temperature quickens the release of volatiles and causes a decrease in the carbon yield. Though ZnCl2 impregnation can inhibit the formation of tars and enhance the yield of carbon, for a higher impregnation ratio, the movement of the volatiles through the pore passages will not be hindered and 4059



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Figure 1. Contour plots, response surfaces, and predicted vs experimental plots for total yield (Y1): (a-c) for vacuum chemical activation and (d-f) for traditional chemical activation.

volatiles will be subsequently released from the carbon surface during activation, resulting in a lower carbon yield.8 Unlike the independent factors, the interaction effects between the two variables are less significant. For YV1, a12 = 0, indicating that there is little interaction effect. On the whole, the maximum yields are obtained at low activation temperature and impregnation ratio. From the contour plots we can see that the total yield for vacuum chemical activation varies between 29.8 and 36.2%, which are close to 30.5 and 35.0% for traditional chemical activation. Nevertheless, under the same temperature and impregnation ratio, the yields for the activated carbons prepared under vacuum are mostly higher than those obtained at atmosphere. In the vacuum case, the real activation temperature is lower than the temperature set for the furnace, leading to a reduced release of volatile, which contributes to the increase of activated carbon yield.8 Parts c and f of Figure 1 show the predicted values versus the experimental values of total yield for the activated carbons prepared by the two methods. As can be seen, the predicted values obtained were quite close to the experimental values, indicating that the models developed were successful in capturing the correlation between the activated carbon preparation variables and the total yield. 3.1.2. Adsorption of Methylene Blue (Y2). The methylene blue molecule has a minimum molecular cross-section of about 0.8 nm, and it has been estimated that the minimum pore diameter it can enter is 1.3 nm.24 According to IUPAC classification, the borderline of micropore and mesopore is 2 nm. Therefore, it can only enter the largest micropores, and most of it is

likely to be adsorbed in mesopores. Thus, the methylene blue test can be used to predict the organic compound adsorption (e.g., microcystin and color bodies) and to provide a simple method for screening for a specific carbon in water treatment.25 The methylene blue adsorption for the two preparation methods can be described by the following equations: YV2 ¼ 300 þ 74:5X1 þ 47:6X2 - 119X1 2 - 76X2 2 þ 52:5X1 X2 ð9Þ YT2 ¼ 206 þ 89:2X1 þ 21:6X2 - 98X1 2 - 26:3X2 2 þ 4X1 X2 ð10Þ The correlation coefficients (r2) of YV2 and YT2 are 0.991 and 0.995, respectively. The closer the r2 value is to 1, the better the model fits the experimental data, and the less the difference between the predicted and observed values. Therefore, the predicted value for Y2 would be more accurate and closer to its actual value, compared to Y1 and Y3 (in the section 3.1.3 discussion). As can be seen from the models, both of the two factors have positive linear term coefficients (a1 > 0, a2 > 0) and negative quadratic term coefficients (a11 < 0, a22 < 0), but the effects of activation temperature are more important than those of the impregnation ratio (a1 > a2, |a11| > |a22|). In addition, the two factors show a more significant synergistic effect for vacuum chemical activation than that for the traditional chemical activation (aV12 . aT12). 4060



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Figure 2. Contour plots, response surfaces, and predicted vs experimental plots for methylene blue adsorption capacity (Y2): (a-c) for vacuum chemical activation and (d-f) for traditional chemical activation.

The response surfaces and contour plots of methylene blue adsorption value (Y2) are shown in Figure 2. For the two methods, in the low-temperature range, the methylene blue adsorption capacity increases with increasing activation temperature. Increased temperature can enhance the dehydration of ZnCl2 and increase the release of volatile matter, which may facilitate the development of pore structure. However, a higher temperature will cause the heat shrinkage of carbon structure and subsequently result in the decrease of surface area and pore volume,26 which was indicated by the decreased methylene blue adsorption value. For impregnation ratio, it has a similar effect on the methylene blue adsorption value. The adsorption value increases first with increasing impregnation ratio and then decreases gradually when it got to a maximum. Furthermore, the effect of impregnation ratio is more noticeable under high temperature, especially for atmospheric condition. As Figure 2d shows, in the low-temperature range, nearly straight lines parallel to the impregnation ratio axis reveal that the impregnation ratio has little influence on the methylene blue adsorption value. The response surfaces in Figure 2b,e show clear peaks, suggesting that the optimal conditions for maximum methylene blue adsorption value are well inside the design boundary. The maximum predicted adsorption capacity indicated by the response surface is confined in the smallest ellipse in the contour diagram. The optimal point for YV2 is equal to 326 mg/g corresponding to an activation temperature of 491 °C and an impregnation ratio of 1.230 (Figure 2a). And the point giving the maximum adsorption value found for YT2 was 232 mg/g at an activation temperature of

496 °C and an impregnation ratio of 1.225 (Figure 2d). The activated carbon prepared under vacuum showed a higher methylene blue adsorption capacity than that obtained at atmosphere. Since methylene blue molecule is likely to be adsorbed in mesopores, this result indicates that activated carbon prepared by vacuum chemical activation has a higher volume of mesopores. The predicted versus actual plots of methylene blue adsorption value for the activated carbons prepared under vacuum and atmosphere are shown in Figure 2c,f, respectively. The observed plots are distributed near the straight line, revealing that the predicted values were quite close to the experimental values. 3.1.3. Adsorption of Iodine (Y3). Compared with methylene blue, the iodine molecule (about 0.27 nm) is much smaller, permitting its penetration into narrower micropores (pore diameter less than 1 nm).25 Therefore, the iodine adsorption capacity can give an indication on microporosity and reveal the adsorption of pollutants of small molecular size. The responses are described by the following equations: YV3 ¼ 1001 - 29X1 þ 20:2X2 - 137X1 2 - 32:3X2 2 þ 82X1 X2 ð11Þ YT3 ¼ 927 - 53X1 - 1:2X2 - 129X1 2 - 35:7X2 2 - 21:9X1 X2 ð12Þ The correlation coefficients (r2) of the responses are 0.988 and 0.947, respectively. There is some difference between vacuum chemical activation and traditional chemical activation. For YV3, 4061



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Figure 3. Contour plots, response surfaces, and predicted vs experimental plots for iodine adsorption capacity (Y3): (a-c) for vacuum chemical activation and (d-f) for traditional chemical activation.

the linear term coefficient of activation temperature is negative (aV1 < 0) and the linear term coefficient of impregnation ratio is positive (aV2 > 0); they show comparative linear influence as the absolute values of aV1 and aV2 are close. For YT3, the linear term coefficients of both variables are negative (aT1 < 0, aT2 < 0), and the linear influence of activation temperature is more important than the impregnation ratio. Furthermore, the interaction effect of YV3 is much more noticeable than YT3. Figure 3 shows the contour plots and response surfaces of iodine adsorption value (Y3). The variation of iodine adsorption capacity in relation to activation temperature and impregnation ratio is similar to that of methylene blue adsorption capacity. The reasons for this phenomenon are almost the same as those for methylene blue adsorption. As can be seen from Figure 3, the maximum iodine adsorption value of 1004 mg/g is achieved at an activation temperature of 448 °C and an impregnation ratio of 1.145 for the activated carbon prepared under vacuum. And the maximum value for activated carbon obtained at atmosphere is 933 mg/g also in the selected experimental region. The corresponding activation temperature and impregnation ratio are 429 °C and 1.025, respectively. For the two methods, both the maximum methylene blue and iodine adsorption values appear in the experimental region, indicating the selected design region is very suitable for optimization. The models obtained for the iodine adsorption value are satisfied as the predicted versus actual value plot approximates along a straight line as shown in Figure 3c,f. 3.2. Optimization. In the production of commercial activated carbons, relatively high product yield and good adsorption

capacity are expected, because high product yield can help to reduce the cost of activated carbon and good adsorption properties will improve the competitiveness of activated carbon in the market place. In this study, the starting material used is cheap and easy to get, and the total yields obtained are almost above 30%, which is relatively a high yield for activated carbon preparation. Thus, the total yield (Y1) was not taken into account in the optimization. Nevertheless, it is still difficult to optimize the other two responses (Y2 and Y3) under the same conditions because the interest regions of them are different. To compromise between the two responses, the desirability function was applied using Design Expert software. To optimize, the lower and upper acceptability bounds of the responses for optimal activated carbon must be fixed first. According to the experimental results, we established the range of variation for these two responses of 50-350 mg/g for methylene blue adsorption and 600-1200 mg/g for iodine adsorption. The two experimental conditions with the highest desirability were obtained according to the calculation method used. As shown in Figure 4, the optimal point indicated by the model corresponds to an activation temperature of 474 °C and an impregnation ratio of 1.225 for vacuum chemical activation. Those for traditional chemical activation are activation temperature of 467 °C and impregnation ratio of 1.105. To test the validity of this method, we have prepared activated carbon samples under the above experimental conditions. The characteristics of the optimal activated carbons are shown in Table 3 together with those calculated from the model. It shows a good agreement between the experimental values and those 4062



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Figure 4. Variation of the desirability function: (a) for vacuum chemical activation and (b) for traditional chemical activation.

Table 3. Calculated and Experimental Response Values Corresponding to the Optimal Activated Carbons conditionsa

responses

Y1 (%)

Y2 (mg/g)

Y3 (mg/g)

Table 4. BET Surface Areas and Pore Volumes of Commercial Activated Carbon and Optimal Activated Carbons Prepared by Different Methods

474 °C/1.225

calculated

31.6

323

997

experimental

32.2

316

994

Traditional Chemical Activation 467 °C/1.105 a

SBET (m2/g)

VT (cm3/g)

VMi (cm3/g)

VMe (cm3/g)

VMi/ VT

VMe/ VT

vacuum

1551

0.814

0.424

0.390

52.1

47.9

commercial

1350

0.720

0.419

0.301

58.2

41.8

traditional

1125

0.628

0.355

0.273

56.5

43.5

activated carbon

Vacuum Chemical Activation

calculated

33.3

222

911

experimental

34.0

206

896

Conditions: activation temperature/impregnation ratio.

calculated from the model. Comparing the responses of these optimal activated carbons, it is clear that activated carbon obtained under vacuum has higher methylene blue and iodine adsorption values than that prepared at atmosphere, indicating activated carbon prepared by vacuum chemical activation has higher volume of both micropores and mesopores. It is conjectured that the volatiles released can be quickly removed from the pores and the sample surface under vacuum condition, thereby reducing the possibility of volatile deposition on the pores and sample surface which may otherwise block up the pores and reduce the pore surface area and volume. The results obtained were in agreement with our previous study, in which the activated carbon obtained under vacuum has better properties than that prepared under atmospheric condition.5 In this case, the yield of optimal activated carbon for vacuum chemical activation is a little lower than that for traditional chemical activation. This can be attributed to the different preparation conditions: the activation temperature and impregnation ratio used under vacuum are both higher than those at atmosphere. Though vacuum condition can increase the carbon yield, it competes with the negative effect of temperature and impregnation ratio. The antagonistic effect leads to a reduced activated carbon yield. 3.3. Comparative Study with Commercial Activated Carbon. To compare between the activated carbons obtained by the two methods and the commercial activated carbon, nitrogen adsorption was carried out to characterize their pore structures. Table 4 shows the BET surface areas and pore volumes of activated carbons. We can see that vacuum condition is beneficial to improve the BET surface area and pore volumes. The BET

Figure 5. Effects of activated carbon mass on removal efficiency of methylene blue (200 mg/L, 100 mL).

surface area and total pore volume of activated carbon prepared by vacuum chemical activation are 37.9 and 29.6%, respectively, higher than those of activated carbon obtained by traditional chemical activation. The commercial activated carbon has medium performances among the three activated carbons. However, all of samples have comparable pore volume distributions. Furthermore, to compare the practical adsorption efficacies, the behavior of these activated carbons in adsorption of methylene blue was determined. Figure 5 shows the removal percentage of methylene blue solution plotted to activated carbons of different mass. It is apparent that, with an increase in the activated carbon mass, the removal percentage of methylene blue increased. The activated carbon prepared by vacuum chemical activation shows the highest methylene blue removal efficiency. When 0.085 g of activated carbon (prepared by vacuum chemical 4063


Industrial & Engineering Chemistry Research activation) was added into 100 mL of methylene blue solution, the removal percentage can reach to over 99%. The dosages are 0.12 and 0.10 g for activated carbon prepared by traditional chemical activation and commercial activated carbon, respectively, to reach the same removal percentage. Though the adsorption efficacy for activated carbon prepared by traditional chemical activation cannot compare with that for commercial activated carbon, we can use the vacuum chemical activation and thus get activated carbon with better adsorption property. The results fully indicate that vacuum chemical activation is an improvement of traditional chemical activation.

4. CONCLUSIONS Activated carbons were prepared from herb residues by vacuum chemical activation and traditional chemical activation using ZnCl2 as the activation agent. Process optimization was carried out by means of Doehlert experiment design. The experimental values obtained were found to agree satisfactorily with the values predicted by the models. The optimal conditions for vacuum chemical activation were an activation temperature of 474 °C and an impregnation ratio of 1.225, which were 467 °C and 1.105 for traditional chemical activation. The obtained activated carbons show high methylene blue and iodine absorption values, suggesting herb residue is a suitable precursor for activated carbon production. At the same activation condition, activated carbons obtained by vacuum chemical activation exhibited better adsorption properties than that prepared by traditional chemical activation, indicating vacuum chemical activation is very effective in improving the quality of activated carbon. ’ AUTHOR INFORMATION Corresponding Author

*Tel.: þ86 731 88836994. Fax: þ86 731 8836994. E-mail: qiuwhs@ sohu.com.

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