The Environmentalist

, Volume 31, Issue 4, pp 349–357

Optimization of activated carbon production from empty fruit bunch fibers in one-step steam pyrolysis for cadmium removal from aqueous solution

Authors

    • Bioenvironmental Engineering Research Unit (BERU), Department of Biotechnology Engineering, Faculty of EngineeringInternational Islamic University Malaysia
  • Suleyman A. Muyibi
    • Bioenvironmental Engineering Research Unit (BERU), Department of Biotechnology Engineering, Faculty of EngineeringInternational Islamic University Malaysia
  • Jeminat Omotayo Amode
    • Bioenvironmental Engineering Research Unit (BERU), Department of Biotechnology Engineering, Faculty of EngineeringInternational Islamic University Malaysia
Article

DOI: 10.1007/s10669-011-9342-9

Cite this article as:
Alkhatib, M.F., Muyibi, S.A. & Amode, J.O. Environmentalist (2011) 31: 349. doi:10.1007/s10669-011-9342-9

Abstract

The fast growth of the palm oil industry in Malaysia is associated with various waste products, namely the empty fruit bunches (EFB), which have a negative impact on the environment. Therefore, these wastes were utilized as a cheap raw material for the production of activated carbon (AC) with less energy consumption. One-step steam pyrolysis was used to produce AC from oil palm empty fruit bunch fibers (EFBF) by varying the operating parameters of temperature, steam flow rate, and activation time using two-level full factorial experimental design (FFD). Ten samples of AC were prepared and the optimized production conditions were chosen based on the ability to adsorb and remove cadmium. Physical activation comprised of carbonization for 30 min using nitrogen gas (N2), followed by activation with steam at different flow rates (2.0, 3.0, and 4.0 ml/min), temperatures (600, 750, and 900°C) and times (15, 30, and 45 min). The AC sample produced at an activation temperature of 900°C with a steam flow rate of 2.0 ml/min and activation time of 15 min was selected as the best adsorbent with a total yield of 21.7%. It had adsorbed more than 97% of total cadmium from aqueous solution within 2 min of the contact time. Characterization of EFBF-based AC by SEM and BET surface area analysis had shown a good-quality adsorbent with highly active sites and well-developed pores with BET surface area of 635.16 m2/g. Experimental results indicated that the prepared AC from EFBF provide a promising solution in water and wastewater treatment.

Keywords

Activated carbonAdsorptionCadmium removalOil palm empty fruit bunchesSteam activationStatistical optimization

1 Introduction

Activated carbon (AC) is still the main noted adsorbent for the removal of pollutants from polluted gaseous and liquid streams. The high adsorption capacity is mainly due to the high surface area and the existence of functional groups on its active sites. The challenge in utilizing AC is, however, to cater to the demands with reasonable costs for end-users. AC production costs can be reduced by either choosing a cheap raw material and/or by applying a proper production method (Lafi 2001), nevertheless, it is still a challenge to prepare such AC having very specific characteristics; with a given pore size distribution and using low-cost raw materials processed at less energy costs (Sudaryanto et al. 2006). Therefore, it is of extreme relevance to find suitable, low-cost raw materials that are economically attractive and at the same time possessing similar characteristics to the conventional ones. The use of waste materials for the preparation of AC is also very attractive from the perspective of their contribution to decrease the costs of waste disposal, thus promoting environmental protection, given that these waste materials are available in large quantities.

Malaysia is the largest palm oil producer in the world and generates one of the most abundant organic residues, which is the empty fruit bunches (EFB), amounting to 12.4 million ton per year (Suhaim and Ong 2001). Some of these solid wastes are usually used as fuel for the boiler to produce process steam and/or electricity in palm oil mills, while most are still left unused and disposed in landfills, which in the end created a huge impact on the environment and caused major disposal problems. However, application of oil palm EFB AC as an adsorbent offers highly effective technological means in dealing with the pollution of the aqua-environment with only minimal investment required. Conversion of EFB to a value-added product such as AC would directly solve part of the environmental problems by utilizing the abundance and turning these by-products into a resource for another industry. Alam et al. (2009) used carbon dioxide (CO2) activation to produce powdered activated carbon (PAC) from oil palm EFB. The AC produced at optimum conditions, at a temperature of 900°C and activation time of 15 min had the highest surface area of only 375 m2/g.

AC is produced, amongst other methods, by two-step physical activation, where the precursor is carbonized in a separate step producing the low surface area char. The produced char is then activated by gasification with an oxidizing agent at relatively high temperatures (800–1,000°C) in an oxidizing environment to develop its porosity (Byrne and Marsh 1995). However, some researchers (Gergova et al. 1993; Fan et al. 2004; Tennant and Mazyck 2003; David 2008) used a one-step pyrolysis/steam activation process to produce AC from the agricultural by-product, where the precursor is carbonized and immediately activated in the same reactor system. This method is highly preferred as compared to the two-stage treatment from an economical point of view because it involves less energy consumption.

AC produced from different precursors has been widely studied as an adsorbent material for its ability to remove heavy metals (Goel et al. 2005; Gueu et al. 2007; Issabayeva et al. 2007; Mondal et al. 2008). In fact, here in Malaysia, industries dealing with electroplating, electronics, batteries, and metal treatment or fabrication are the major sources of heavy metals contamination. Many of these industries are located on the western coast of peninsular Malaysia, which includes the areas of Klang Valley, Malacca, Johor Bahru, and Penang (Onundi et al. 2010). For these reasons, the interest in utilizing abundant agro-based wastes as a source of AC to remove heavy metals could be a feasible alternative in an industrially growing country like Malaysia.

In this study, oil palm empty fruit bunch fibers (EFBF) were used as a precursor for AC production by first carbonization in an inert environment followed sequentially by steam as the activating agent in the one-step pyrolysis, with varying temperature from 600 to 900°C. The product was characterized for ash content, bulk density, and subjected to SEM, FTIR, EDX, and BET surface area analysis. It is aimed that the produced AC will be able to substantially remove cadmium from the aqueous solution. The selection of cadmium was on the basis that it was found in concentrations higher than the allowed levels (0.01 mg/l) in some rivers like Jejawi River, located in the state of Penang, with an average concentration of 0.14 mg/l (Alkarkhi et al. 2008).

2 Materials and methods

2.1 Preparation of raw material

The oil palm empty fruit bunch fibers (EFBF) sample was collected from a palm oil producer (Seri Ulu Langat Palm Oil Mill Sdn. Bhd.) in Dengkil, Selangor, Malaysia. Samples were collected in plastic bags and stored in a laboratory cold-room at 4°C. Prior to pyrolysis, the EFBF was washed several times with tap water to remove dirt from its surface followed by drying at 105°C for 24 h in an oven to remove excess water content until it reached constant weight.

2.2 Production of AC

EFBF was used for the production of PAC by pyrolysis and steam activation in one step. Design Expert® 6.0.8 software (Stat Ease Inc., Minneapolis, MN, USA) was used to prepare a two-level full factorial design (FFD) with two central points to optimize the production of PACs. The range and level of the variables are shown in Table 1.
Table 1

Experimental factors and their levels for fractional factorial design (FFD)*

Factors

Name

Actual (low)

Actual (high)

A

Temperature (°C)

600

900

B

Activation time (min)

15

45

C

Steam flow rate (ml/min)

2.0

4.0

Central point Average of high and low values for each parameter (duplicate)

A total of 150 g of raw fibers (EFBF) was carbonized and activated in a quartz tube (OD: 95 mm, ID: 90 mm, L: 1,100 mm) using a horizontal high temperature CSC—Split tube (Lenton Thermal Design, UK). The system setup is shown in Fig. 1.
https://static-content.springer.com/image/art%3A10.1007%2Fs10669-011-9342-9/MediaObjects/10669_2011_9342_Fig1_HTML.gif
Fig. 1

Schematic diagram of the AC production system

Once the tube furnace was switched on and set at required temperatures (600, 750 and 900°C), nitrogen gas (N2) was supplied to the system at 2.5 l/min (Phan et al. 2006). This was continued for 30 min at a heat rate of 23°C/min. Upon the completion of carbonization, the N2 gas flow was stopped and steam was introduced for activation. The activation process was carried out at varying flow rates of steam 2.0, 3.0, and 4.0 ml/min and activation times of 15, 30, and 45 min according to the matrix design of experiment in Table 1. At the end of each interval, the steam generator was stopped and switched back to the N2 gas and the system was allowed to cool to below 50°C before removing the AC from the quartz tube to allow it to cool in a desiccator. The AC produced was ground and sieved to size fractions less than 250 μm (Alam et al. 2007). The weights of all samples were taken before and after production. The AC yield was calculated by the following expression:
$$ {\text{Yield}}\,(\% ) = \frac{\text{mass of activated carbon produced }}{\text{Initial mass of EFB fibers }} \times 100 $$
(1)

2.3 Cadmium adsorption test by PAC

The adsorption potential of cadmium by the produced PAC was tested by carrying out the experimental runs in a factorial design as shown in Table 2.
Table 2

The runs for PAC production as designed by Design Expert software

Runs

(AC no.)

A

Activation temperature (°C)

B

Activation time (min)

C

Steam flow rate (ml/min)

1

750

30

3

2

900

45

4

3

900

15

2

4

600

45

2

5

600

15

4

6

600

45

4

7

600

15

2

8

900

15

4

9

900

45

2

10

750

30

3

Adsorption studies were conducted at varying times from 2 to 10 min, at 2-min intervals. In this adsorption test, 0.05 g of PAC was added to 50 ml aqueous solution of cadmium at pH 5 and concentration of 0.3 mg/l contained in 100 ml conical flask. The agitation speed was set at 150 rpm in a rotary shaker (Goel et al. 2005). At the end of each interval of time, the sample was filtered using Whatman filter paper No. 40. The residual cadmium in the aqueous solution was analyzed through atomic adsorption spectrometry using an ALPH-4—Flame Atomic Adsorption Spectrophotometer at wave lengths 228.8 nm against reagent blank using an acetylene-air flame according to standard methods. The adsorption capacity of cadmium, q (mg/l) and percentage removal were calculated from the difference between the initial and final adsorbate (cadmium) concentration using the relationship as follows:
$$ {\text{Adsorption capacity at time}},t,q_{t} = [\left( {C_{\text{i}} - C_{\text{e}} } \right) \times V]/M $$
(2)
$$ {\text{Cadmium removal }}\left( \% \right) \, = \, [(C_{ 0} - C_{\text{f}} )/C_{0} ] \times 100 $$
(3)
where qt is the adsorption capacity at each time interval (mg/g), C0, Ci, and Ce, Cf are the initial and final concentrations of cadmium, respectively (mg/l), M is the adsorbent dosage (g) and V is the solution volume (l). All the ten PAC samples produced were tested for cadmium adsorption.

2.4 Characterization of PAC

Selected samples from the produced EFBF-AC were characterized for morphology by scanning electron microscope (SEM) (JSM-500, JEOL). The BET surface area and pore size of selected samples of PAC was measured by Autosorb-1 (Quantachrome). FTIR spectroscopy (PerkinElmer precisely spectrum of model 100) in the wave number range of 4,000–400 cm−1 and potassium bromide (KBr) pellet technique were used to detect the surface functional groups present in the AC. For FTIR analysis, about 0.001–0.01 g of finely ground sample was well mixed with about 0.05 mg of KBr powder. The mixture was then pressed continuously at a pressure of 10 tonnes for 1 min to form a transparent pellet using a Perkin Elmer hydraulic press. The pellet was analyzed immediately after being prepared.

3 Results and discussion

3.1 Effect of production conditions on PAC yield

The different yields obtained for PACs produced are shown in Table 3. It is observed that the yield of PAC decreased as the temperature and time increased. This could be explained by the increase in the release of volatile organic compounds (VOCs) with the increase in temperature and time, noting that the percentage in decrease in yield with time is higher at higher temperatures, e.g., when the time increased from 15 to 45 min, the yield decreased by 12% at 600°C, while at 900°C the yield decreased 32%.
Table 3

Yield of PACs produced at different conditions

PAC sample no.

Steam flow rate

(ml/min)

Activation temp

°C

Activation time

(min)

Yield

(%)

1

3

750

30

24

2

4

900

45

17

3

2

900

15

22

4

2

600

45

36

5

4

600

15

33

6

4

600

45

30

7

2

600

15

41

8

4

900

15

23

9

2

900

45

15

10

3

750

30

23

Similarly, the steam flow rate had an analogous effect on yield as that of time and temperature, i.e., the yield decreased with an increase in steam flow rate. However, this effect was less at higher temperatures. Steam, besides acting as an activating agent to increase the active sites on PAC, also acts to remove tars entrapped in between the pores of PAC during the carbonization process resulting in reduction of PAC yield. It seems that at higher temperatures, more VOCs, which include tars, are released during the carbonization process, resulting in a reduction in the effect of steam flow rate on the PAC yield.

3.2 Cadmium adsorption test by PAC

The cadmium percentage removal with time by different samples of PAC is presented in Fig. 2, which shows that the highest percentage removal of cadmium was achieved during the first 2 min for all samples except sample AC 10, in which after 2 min the performance gradually increased slightly and became almost constant at 4 min. This observation is in support of the findings reported by several authors (Alam et al. 2009; Periasamy and Namasivayam 1994; Muyibi et al. 2008; Ameen et al. 2008). It is also observed that the high removal percentage of cadmium (97–100%) was by PAC produced at 900°C (AC2, AC3, AC8, and AC9), where AC2 at 900°C, 45 min and 4 ml/min steam flow rate had the highest percentage removal of 100%. However, the lowest percentage removal of cadmium was recorded by AC7 produced at 600°C, 15 min, and a 2.0 ml/min steam flow rate.
https://static-content.springer.com/image/art%3A10.1007%2Fs10669-011-9342-9/MediaObjects/10669_2011_9342_Fig2_HTML.gif
Fig. 2

Cadmium removal (%) with time of different AC samples (1–10 as in Table 3)

3.3 Statistical analysis of physical conditions for production of PACs

A two-level full factorial design (FFD) with two central points was selected for the optimization of PACs production to obtain the relation between preparation parameters and the adsorption capacity and yield. Table 4 shows the adsorption capacities for all PACs produced. It is obvious that high adsorption was found within 2 min of contact time in most cases. Hence, the optimum physical conditions were determined by considering the adsorption capacities within the 2 min. The range for maximum adsorption capacity of total cadmium was observed to be from 0.085 to 0.284 mg/g, while the carbon yield obtained ranged from 14.8 to 41.48%.
Table 4

Experimental design matrix for PAC production with yield, adsorption capacities, and percentage of removal

Run no.

Steam flow rate (ml/min)

Activation temp (°C)

Activation time (min)

Yield (%)

Adsorption capacity (mg/g)

Removal (%)

1

3.00

750.00

30.00

23.79

0.250

89.40

2

4.00

900.00

45.00

17.21

0.284

100.00

3

2.00

900.00

15.00

21.77

0.273

97.17

4

2.00

600.00

45.00

35.57

0.269

95.83

5

4.00

600.00

15.00

33.17

0.152

56.68

6

4.00

600.00

45.00

30.45

0.158

58.77

7

2.00

600.00

15.00

41.48

0.085

34.53

8

4.00

900.00

15.00

22.64

0.279

99.04

9

2.00

900.00

45.00

14.80

0.277

98.39

10

3.00

750.00

30.00

23.20

0.252

90.15

Table 5 refers to the corresponding analysis of variance (ANOVA) for cadmium adsorption capacity, with A; steam flow rate (ml/min), B; activation temperature (°C) and C; activation time (min). The very high model F value of 2,465.40 implies the model is significant. Values of “Prob > F” less than 0.05 indicate model terms are significant. This expresses that activation temperatures (B) are highly significant since it’s “Prob > F” value is 0.0063 and has major effect on the adsorption properties of AC samples produced. In this case; B, C, AB, AC, BC, and ABC are significant model terms. Since a signal-to-noise ratio (S/N) greater than 4 is desirable, S/N value of 133.580 obtained in this model implies very adequate precision. This model can therefore be used to navigate the design space.
Table 5

Analysis of variance (ANOVA) for selected factorial model for adsorption capacity

Source

Sum of squares

DF

Mean square

F value

Prob > F

Status

Model

0.043

7

6.11E−03

2465.4

0.0155

Significant

A

1.22E−04

1

1.22E−04

49.23

0.0901

 

B

0.025

1

0.025

10193.01

0.0063

Significant

C

4.98E−03

1

4.98E−03

2012.6

0.0142

Significant

AB

4.23E−04

1

4.23E−04

170.69

0.0486

Significant

AC

3.84E−03

1

3.84E−03

1552.3

0.0162

Significant

BC

4.08E−03

1

4.08E−03

1647.01

0.0157

Significant

ABC

4.04E−03

1

4.04E−03

1632.98

0.0158

Significant

Curvature

1.33E−03

1

1.33E−03

535.69

0.0275

Significant

Residual

2.48E−06

1

2.48E−06

   

Cor total

0.044

9

 

Adeq precision

133.58

 

R-squared

0.9999

  

Std. dev.

0.0016

 

Adj R-squared

0.9995

     

Pred R-squared

0.9652

     

A: is steam flow rate (ml/min), B: activation temperature (°C) and C: activation time (min)

The model equation indicates that the very high R2 statistics of 0.9999 showed that 99.99% of the variations in adsorption capacity can be explained by the independent variables; activation temperature, activation time, and steam flow rate. The model also showed that R2 is in reasonable agreement with adjusted R2 values of 0.9995 for adsorption capacity. The standard deviation of the models was 0.0016 as presented in Table 5. The closer the R2 value to unity and the smaller the standard deviation, the better the model. This implies that the theoretical values will be closer to the experimental values for the response. According to these criteria, the best model can therefore be identified.

The regression model for the adsorption capacity (mg/g) in terms of the factors to be optimized is developed in the following equation:
$$ {\text{Adsorption}}\,{\text{capacity}}\,\left( {{\text{mg}}/{\text{g}}} \right) = 0. 2 2- 0. 3 9 {\text{A}} + 0.0 5 6 {\text{B}} + 0.0 2 5 {\text{C}} + 0.00 7 3 {\text{AB}} - 0.0 2 2 {\text{AC}} - 0.0 2 3 {\text{BC}} + 0.0 2 2 {\text{ABC}} $$
(4)
where A is steam gas in ml/min, B and C are in activation temperature degree Celsius (°C) and time in minute, respectively. The coefficients with one factor represent the effect of the particular factor, while the coefficients with two factors represent the interaction between the two factors. The positive sign in front of the terms indicates a synergistic effect whereas a negative sign indicates an antagonistic effect. Hence, as activation temperature and time increases, adsorption capacity is expected to increase, which is contrary to the effect of steam.
The graphical representations of the model equations facilitate an examination of the effects of PAC preparation parameters on the adsorption capacity. A three-dimensional (3D) surface plot between the production parameters in Fig. 3a–c illustrates the responses of different experimental variables that can be used to identify the major interactions between the variables. The 3D response surface in Fig. 3a–c showed that the adsorption capacity of AC samples obviously increased with the increase in activation temperatures of 600–900°C. The results also showed that adsorption capacity of PACs generally increased with an increase in activation times 15–45 min and decreased with a steam flow rate of 2–4 ml/min. The result also clearly showed that the activation temperatures have a significant effect on the adsorption properties of the PACs produced while the activation time with steam flow rate had little influence. This result agreed with the results of Alam et al. (2009).
https://static-content.springer.com/image/art%3A10.1007%2Fs10669-011-9342-9/MediaObjects/10669_2011_9342_Fig3_HTML.gif
Fig. 3

3D plots showing effect of PAC production conditions on its adsorption capacity. a Effects of activation time and steam flow rate. b Effects of activation temperature and steam flow rate. c Effects of activation time and activation temperature

The statistical analysis using the Design-Expert software gave numerical solutions of optimum conditions for the factors as follows; steam flow rate 4.00 ml/min, activation temperature of 900°C, and activation time of 45 min. However, if compared to the conditions at a steam flow rate 2 ml/min, activation temperature of 900°C, and activation time of 15 min (Table 4), it could be noticed that the difference in adsorption capacity is not high (0.284 and 0.273 mg/g, respectively), yet the yield is much lower for the former (17.21 and 21.77%, respectively). Also, by comparing energy consumption, it will be noticed that the latter requires much less energy with respect to the steam flow rate and activation time (4 ml/min and 45 min vs. 2 ml/min and 15 min, respectively). On the other hand, the removal percentage for no. 2 in Table 6 is sufficient to meet the effluent discharge requirements by the Department of Environment Malaysia for cadmium discharge of 0.01 mg/l. Therefore, the optimum conditions chosen in this work are a steam flow rate of 2 ml/min, activation temperature of 900°C, and activation time of 15 min.
Table 6

Comparison of cadmium removal and BET surface area of selected PAC samples

No.

Steam flow rate (ml/min)

Activation temp (°C)

Activation time (min)

Removal %

BET surface area (m2/g)

AC2

4.00

900.00

45.00

100.00

635.16

AC3

2.00

900.00

15.00

97.17

603.76

AC5

4.00

600.00

15.00

56.68

384.14

AC7

2.00

600.00

15.00

34.53

175.38

3.4 BET surface area

The physical characterization of the best-quality PAC produced was analyzed with Quantachrome Autosorb 1 surface area analyzer by nitrogen adsorption at 77 K. Prior to the analysis, the sample was degassed at 200°C for 3 h. The surface area was calculated by the Brunauer, Emmett and Teller (BET) equation using the nitrogen adsorption data. The highest BET surface area of the PAC produced was found to be 635.16 m2/g. The results obtained agreed with previous studies which produced steam-activated carbon from palm oil shell with BET surface area of 670.1 m2/g at 900°C and 1 h (Panyawatanakit 1997), and it was much higher for the same raw material (EFBF) when produced with CO2 activation at optimum conditions of temperature at 900°C and activation time of 15 min, where the highest surface area was 375 m2/g (Alam et al. 2009). The results also showed an increase in cadmium removal with an increase in BET surface area. The highest removal was achieved by AC sample with the largest surface area (635.16 m2/g). It is eminent that high BET surface area is due to the presence of high volumes of micropores (diameter < 2 nm) where adsorption mainly occurs (Alcañiz-Monge et al. 2002), as the energy of adsorption is substantially higher than that for adsorption on mesopores, which causes a particularly large increase of adsorption capacity for small equilibrium pressures of adsorbate (Jankowsak et al. 1991).

3.5 Scanning electron microscope (SEM)

Scanning electron microscopy (SEM) technique was employed to observe the surface physical morphology of the derived AC sample. Figure 4 shows the 2,000× magnification of cross section for (a) raw EFBF, (b) selected PAC sample produced at 900°C activation temperature, 15-min activation time, and 2.0 ml/min steam flow rate. It can be observed that raw EFBF had very little pores available on the surface. However, pores of different sizes and different shapes could be observed on the PAC sample, as a result of the carbonization as well as diffusion of steam as activating agent through the carbonaceous matrix enhancing the removal of impurities and the activation of the carbon to create porosity.
https://static-content.springer.com/image/art%3A10.1007%2Fs10669-011-9342-9/MediaObjects/10669_2011_9342_Fig4_HTML.jpg
Fig. 4

a Cross section image of raw EFBF. b Best selected AC at 900°C for 15-min activation and 2 ml/min steam flow

3.6 FTIR analysis

A larger number of oxygen-containing functional groups on the surface of AC will increase the ion-exchange capability of a carbon material (Li et al. 2003). This increases the adsorption capacity of cadmium and therefore FTIR analysis was carried out to study the major functional groups that contributed to cadmium adsorption. Figure 5 shows a comparison between the FTIR curves for samples AC2 and AC7, the samples with the highest and lowest cadmium removal, respectively.
https://static-content.springer.com/image/art%3A10.1007%2Fs10669-011-9342-9/MediaObjects/10669_2011_9342_Fig5_HTML.gif
Fig. 5

FTIR spectrum of selected PAC (% transmittance vs. wave number in cm−1)

It can be observed that peaks appear in the range of 1,850–2,000 cm−1 and 1,750–1,845 cm−1, corresponding to the C=O (Kim et al. 2006; Tan and Xiao 2009). Other peaks appear at 2,970, 1,738, 1,434, 1,365, and 1,216 cm−1, where the first peak corresponds to the –OH, while the other peaks correspond to the –COOH (Wu et al. 2007).

FTIR showed that the relative intensity of hydroxyl and carboxyl groups that appeared at 3,616–3,840 and 3,200–3,550 cm−1 is higher for the AC2 as compared to AC7, which indicates that the samples with higher activation temperature and steam flow rate have a larger number of these functional groups and thus a higher removal capacity.

It can be concluded that the hydroxyl and carboxyl groups are the main contributors to the removal of heavy metals (Yongbin et al. 2010; Tan and Xiao 2009; Li et al. 2003). In fact, when the AC is placed in water, the acidic surface groups, hydroxyl and carboxyl groups, undergo ionization, producing H+ ions and leaving the carbon surface with negatively charged sites, as shown in Eqs. (5) and (6).
$$ > {\text{C}} - {\text{COOH}} \to > {\text{C}} - {\text{COO}} - + {\text{ H}}^{ + } $$
(5)
$$ > {\text{C}} - {\text{COH}} \to > {\text{C}} - {\text{CO}} - + {\text{ H}}^{ + } $$
(6)
These negatively charged sites produce a competition between the H+ ions and the Cd+2 ions for the carbon surface, and the more the availability and concentration of these sites on the activated carbon surface, the higher is the adsorption of the Cd ions on the surface (Bansal and Goyal 2005).

Moreover, increasing the activation time and flow rate results in increase of the active functional groups and thus higher heavy metals removal. The results agreed with the surface chemistry of other agricultural by-products (Alam et al.2009; Bouchelta et al. 2008; Sivabalan et al. 2008; Al-Degs et al. 2000; Sellitti et al. 1990)

Relevant characterization of the best selected novel EFBF-based PAC prepared at 900°C activated temperature, 15-min activation time, and 2.0 ml/min steam flow rate are summarized in Table 7
Table 7

Physical and chemical composition of the best selected EFBF-based PAC

Parameter

Unit

EFBF-based PAC

BET surface area

m2/g

603.76

Total pore volume

cc/g

0.341

Porosity volume

cm3 g−1

0.1622

Medium pore diameter

Å

40

Mean particle size

μm

40

Bulk density

g/l

1

Ash content

%

3.34

Carbon yield

%

21.8

Elemental composition

% wt

87.9 carbon, 0.21 potassium, 0.41 silica, oxygen 4.56 and nitrogen 0.19

Surface chemistry

–OH groups; carbonyl compound; presence of –NH2; vibration of the bond C–Cl, C=C and saturated carbon i.e., C–H

4 Conclusions

An AC sample produced at an activation temperature of 900°C with a steam flow rate of 2.0 ml/min and an activation period of 15 min was selected as the best-quality adsorbent among the ten different samples. It adsorbed more than 97% of the total cadmium from the aqueous solution of 0.3 mg/l concentration within 2 min of contact time. Characterizations of EFBF-based PAC had shown good-quality adsorbent with highly active sites, well-developed pores with the highest BET surface area obtained from the resulting AC, 635.2 m2/g, with about 17% yield. The activation temperature of the PAC produced was found to have the most significant effect on the yield compared to the activation time and steam flow rate.

Acknowledgments

The authors are very grateful to the Ministry of Higher Education Malaysia for the provision of research grant under the Fundamental Research Grant Scheme under FRGS 0207-55 for this project. The authors acknowledge the valuable input of Mrs. Iman Husain on the FTIR results. The authors are also grateful to Ms. Noor Yuslida binti Hazahari for proofreading the article.

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© Springer Science+Business Media, LLC 2011