Journal of Environmental Chemical Engineering 1 (2013) 589–599

Cadmium removal from aqueous solution using microwaved olive stone activatedcarbon

Tamer M. Alslaibi a, Ismail Abustan a,*, Mohd Azmier Ahmad b, Ahmad Abu Foul c

a School of Civil Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Pulau Pinang, Malaysiab School of Chemical Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Pulau Pinang, Malaysiac Environmental Engineering, Islamic University of Gaza, Palestine

A R T I C L E I N F O

Article history:

Received 9 May 2013

Received in revised form 25 June 2013

Accepted 27 June 2013

Keywords:

Activated carbon

Cadmium adsorption

Microwave

Olive stone

Response surface methodology (RSM)

A B S T R A C T

Contamination of natural aquatic ecosystems by wastewater containing heavy metals is a major

environmental and human health issue. The removal of heavy metals using adsorption techniques with

microwave-irradiated low-cost adsorbents has a few numbers of studies. In this study, the removal

efficiency for cadmium (Cd2+) from aqueous solution using olive stone activated carbon (OSAC) prepared

by microwave was investigated. Central composite design (CCD) with response surface methodology

(RSM) was applied to evaluate the interaction and relationship between operating variables (i.e.,

radiation power, radiation time, and impregnation ratio), and to develop the optimum operating

condition. Equilibrium isotherms in this study were analyzed using the Langmuir and Freundlich. Kinetic

data were obtained and analyzed using pseudo-first-order and pseudo-second-order equations. Based

on statistical analysis, Cd2+ removal model proved to be significant with very low probability values

(<0.0001). The surface characteristics of the AC prepared under optimized condition were examined by

scanning electron microscopy and Fourier transform infrared spectroscopy. The optimum conditions

obtained were 565 W radiation power, 7 min radiation time, and 1.87 impregnation ratio. This resulted

in 95.32% removal of Cd2+ and 85.15% of OSAC yield. The process via microwave requires significantly

lesser holding time as compared to conventional heating method to produce activated carbon of

comparable quality. The prediction results fitted well with experimental findings. The adsorption

isotherm data fitted the Langmuir isotherm well, and the monolayer adsorption capacity was found to be

11.72 mg/g. Microwaved olive stone can be used for the efficient removal of Cd2+ from contaminated

wastewater.

� 2013 Elsevier Ltd All rights reserved.

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Journal of Environmental Chemical Engineering

jou r n al h o mep ag e: w ww .e lsev ier . co m / loc ate / jec e

Introduction

Heavy metals, such as copper, cadmium, nickel, lead, and zincare toxic to human beings and other living organisms if theirconcentrations exceed the acceptance limit. These heavy metalsappear in wastewater discharged from hospitals [1] and differentindustries, including smelting, metal plating, Cd–Ni battery andalloy manufacturing [2]. In the field of water pollution the removalof toxic metals from wastewater is a matter of great interestbecause it causes serious degradation of the environment [3].Heavy metals present ecological and human health issue becausethey do not undergo biological degradation unlike certain organicpollutants [4]. Renal toxicity, hepatotoxicity and carcinogenicity,

* Corresponding author. Tel.: +60 4 5996259; fax: +60 4 5941009.

E-mail addresses: tamer_2004@hotmail.com (T.M. Alslaibi),

ceismail@eng.usm.my (I. Abustan), chazmier@eng.usm.my (M.A. Ahmad),

afoul@iugaza.edu.ps (A.A. Foul).

2213-3437/$ – see front matter � 2013 Elsevier Ltd All rights reserved.

http://dx.doi.org/10.1016/j.jece.2013.06.028

lung disease, lung cancer, and damage to human respiratorysystems are the major human health issues that can be caused bylong term exposure to Cd2+, so this toxic heavy metal should beremoved from the wastewater to protect the people and theenvironment [5,6].

Activated carbons (AC) are used as adsorbent materials becauseof their large surface areas, microporous structures, high degree ofsurface reactivates and high adsorption capacities [7]. Neverthe-less, commercially available activated carbons are still limited dueto high cost resulting from the use of non-renewable and relativelyhigh cost starting materials such as coal. But conscientious effortsare made by researchers to produce cheaper, more effective andenvironmental friendly activated carbons [8–11]. These have takenvarious approaches which include: using various precursors suchas agricultural byproducts materials; use of various chemicals foractivation; and devising various methods of preparations.

Olive stone waste residue as a raw material for the productionof activated carbon can be considered as one of the best candidateamong the agricultural wastes because it is cheap and quite

Fig. 1. Schematic diagram of the preparation of activated carbon from OS with KOH

activation by microwave irradiation.

T.M. Alslaibi et al. / Journal of Environmental Chemical Engineering 1 (2013) 589–599590

abundant, especially in Mediterranean countries. According to theinternational olive council, the annual production of olive oil in theworld at the year 2012 was more than 3 million tons, translating toapproximately 15 million tons of olive cakes as byproducts [12].

The conventional heating method usually adopted for thepreparations of ACs involves production of energy by an electricalfurnace and requires high energy consumptions and longprocessing times, thereby prompting researchers to study thepreparation of AC using microwave technology [13,14]. Duringmicrowave heating, a tremendous thermal gradient from theinterior of the char particle to its cool surface allows themicrowave-induced reaction to proceed quickly and effectivelyat a low bulk temperature; energy savings and a shortenedprocessing time are the key advantages of microwave heating [15].Recently, microwave heating technology has been applied to thefabrication of AC for dyes removal using low-cost wastes andagriculture byproducts such as almond husk, waste tea [16], sugarbeet bagasse [17], oil palm fiber [18], pistachio nut shells [19],cotton stalk [20], date stones [21] and mangosteen peel [22].However, few studies on heavy metal removal using AC preparedby microwave technology have been done. Furthermore, the use ofolive stone (OS) for AC preparation via microwave technology hasnot been reported and studies concerning the optimization of ACpreparation conditions for Cd2+ removal using response surfacemethodology (RSM) are limited. The optimization of RSM isparticularly useful when all the independent variables and theirlevels and responses are not clearly known [23]. A standard RSMdesign called a central composite design (CCD) is suitable forcreating a quadratic surface and it helps to analyze the interactionbetween the parameters, as well as to optimize the effectiveparameters with a minimum number of experiments [24].

The main objectives of the present study include the following:

1. to investigate the efficiency of OSAC prepared via microwave forremoving Cd2+ from aqueous solution;

2. to build equations of Cd2+ removal efficiency from aqueoussolution and OSAC yield with respect to OSAC preparationconditions (i.e., radiation power, radiation time, and chemicalimpregnation ratio);

3. to determine the optimum operational conditions of the studiedapplication; and

4. to determine suitable isotherm and kinetic models, and evaluateparameters models describing the isotherms and kinetics ofmicrowaved OSAC.

Materials and methods

Aqueous solution

A stock solution 1000 mg/L of Cd2+ was prepared by dissolvingappropriate amount of CdCl2�H2O(s) in deionized water. The stocksolution was diluted with deionized water to obtain desiredconcentration of 20 mg/L.

Preparation of activated carbon

OS waste was obtained from Gaza, Palestine. The OS waste wasrinsed thrice with hot water and thrice with cold water and thendried in an oven at 105 8C for 24 h to remove moisture. The sampleswere ground and sieved to a particle size between 2.0 mm and4.75 mm. Carbonization was carried out by loading 500 g of driedprecursor into a stainless steel vertical tube reactor placed in a tubefurnace at 600 8C for 1 h under purified nitrogen (99.99%) flow.Potassium hydroxide (KOH) was used to activate the char via thechemical activation method. The amount of KOH used wasadjusted to yield a certain impregnation ratio (weight of activating

agent: weight of char) of 0.5:1, 1.25:1 and 2:1, as calculated usingthe following equation:

Impregnation ratio ðIRÞ ¼ dry weight of KOH pellets

dry weight of char(1)

Deionized water was then added to dissolve all the KOH pellets.Impregnation was performed for 24 h at room temperature, thusincorporating all the chemicals into the core of the particles.Activation of impregnated char was carried out using a modifiedcommercial microwave with a frequency of 2.45 GHz at differentpower level ranging from 264 W to 616 W and various radiationtime ranging from 4 min to 8 min under a nitrogen flow of300 cm3/min, as shown in Fig. 1. The sample was then cooled toroom temperature (28 8C) under nitrogen flow and washed withhot deionized water and 0.1 M HCl until the pH of the washedsolution ranged from 6.5 to 7.

Design of experimental using RSM

The Design Expert Software (version 6.0.7) was used for thestatistical design of experiments and data analysis. The consideredvariables were radiation power (X1), radiation time (X2) andchemical impregnation ratio (X3). These three variables togetherwith their respective ranges were chosen based on the literatureand our preliminary studies. The ranges and the levels of thevariables investigated are given in Table 1.

Performance of the process was evaluated by analyzing Cd2+

removal efficiency and OSAC yield. Each independent variable wasvaried over three levels between �1 and +1 at the determinedranges based on some preliminary experiments. The total numberof experiments obtained for the three factors was 20 (=2k + 2k + 6),where k is the number of factors (k = 3). Fourteen experimentswere enhanced with 6 replications to assess the pure error. Sinceeach factor only had three levels, the appropriate model is thequadratic model, expressed as the following equation

Y ¼ b0 þXk

i¼1

bixi þXk

i¼1

biix2i þ

Xk�1

i¼1

Xk

j¼iþ1

bi jxix j þ ei (2)

where Y is the predicted response, b0 is the constant coefficient, bi

is the linear coefficient, bij is the interaction coefficient, bii is thequadratic coefficient, and Xi and Xj are the coded values of the ACpreparation variables, and ei is the error.

The quality of the fit of polynomial model was expressed by thecorrelation coefficient (R2). The model F-value (Fisher variationratio), probability value (Prob > F), and adequate precision (AP) arethe main indicators demonstrating the significance and adequacyof the used model [25].

Table 1Independent variables and their coded levels for CCD.

Variables (factors) Code Units Coded variable levels

�1 0 1

Radiation power X1 (watt) 264 440 616

Radiation time X2 (min) 4 6 8

Impregnation ratio X3 – 0.5 1.25 2

T.M. Alslaibi et al. / Journal of Environmental Chemical Engineering 1 (2013) 589–599 591

Activated carbon yield

The yield of the OSAC was calculated based on the followingequation:

Yieldð%Þ ¼ dry weightðgÞ of final activated carbon

dry weightðgÞ of char� 100 (3)

Batch equilibrium studies

Batch adsorption was performed in 20 flasks of 250 mLErlenmeyer flasks. In each flask, we placed 100 mL of the aqueoussolution with an initial Cd2+ concentration of 20 mg/L. Each of theprepared AC samples (0.25 g) was added to individual flasks, whichwere then kept in an isothermal shaker at 200 rpm and 30 8C untilequilibrium was reached at 3 h. After agitation, the solid wasremoved by filtration through a 0.45 mm pore size Whatmanmembrane filter paper. The final metal concentrations in thefiltrates and in the initial solution were determined using aninductively coupled plasma optical emission spectroscopy system(Varian, 715-ES). The sorbed metal concentrations were obtainedfrom the difference between the initial and final metal concentra-tions in solution. The percentage removal at equilibrium wascalculated as following equation:

Removalð%Þ ¼ co � ce

co� 100 (4)

where Co and Ce are the liquid-phase concentrations at initial stateand at equilibrium (mg/L), respectively.

The amount of metals adsorbed per unit mass of adsorbent atequilibrium conditions, qe (mg/g), was calculated by equation:

qe ¼ðc p � ceÞv

w(5)

Table 2Experimental factors and responses.

Run no. Type Factors

X1: power (W) X2: time (Min

1 Center 440 6

2 Center 440 6

3 Center 440 6

4 Center 440 6

5 Center 440 6

6 Center 440 6

7 Axial 440 6

8 Axial 440 4

9 Axial 440 6

10 Axial 616 6

11 Axial 264 6

12 Axial 440 8

13 Fact 616 4

14 Fact 264 4

15 Fact 616 4

16 Fact 616 8

17 Fact 616 8

18 Fact 264 4

19 Fact 264 8

20 Fact 264 8

where qe (mg/g) is the amount of solute adsorbed per unit weightof adsorbent; Co and Ce (mg/L) are the liquid-phase concentrationsof adsorbate at initial and equilibrium conditions, respectively; V

(L) is the volume of the solution; and W (g) is the mass of adsorbentused.

The effects of pH and OSAC dosage on metals removal weretested respectively by varying the pH from 2 to 6 and dosage from0.025 to 2 g, with initial metals concentration of 20 mg/L andadsorption temperature of 30 8C. The initial pH of the metalssolution was adjusted by addition of 0.10 M HCl or NaOH.

BET, SEM and FTIR of the prepared activated carbon

The surface area, pore volume and average pore diameter of thesamples were determined by using Micromeritics ASAP 2020volumetric adsorption analyzer. The BET surface area wasmeasured from the adsorption isotherm using Brunauer–Emmett–Teller equation. The total pore volume was estimatedto be the liquid volume of nitrogen at a relative pressure of 0.98.The surface morphology of the samples was examined using ascanning electron microscope (Quanta 450 FEG, Netherland).Chemical characteristics of surface functional group of theactivated carbon was detected by diluting in KBr pellets wererecorded with FTIR spectroscopy (IR Prestige 21 Shimadzu, Japan)in the 400–4000 cm�1 wave number range.

Results and discussion

A total of 20 runs of the CCD experimental design wereconducted. The results are shown in Table 2. The observed percentremoval efficiencies varied between 24.11 and 94.15% for Cd2+

removal and 84.64 and 91.38% for OSAC yield.

Responses

) X3: IR Cd removal Y1 (%) Yield Y2 (%)

1.25 83.43 88.14

1.25 84.11 87.56

1.25 84.75 88.78

1.25 84.36 88.88

1.25 84.63 87.86

1.25 83.92 88.94

2.00 87.99 88.26

1.25 52.73 89.16

0.50 50.60 90.34

1.25 87.84 86.70

1.25 68.35 89.28

1.25 86.64 86.48

0.50 30.53 89.22

0.50 24.11 91.38

2.00 65.75 87.36

2.00 94.15 84.64

0.50 54.03 85.62

2.00 52.72 88.74

0.50 39.47 89.38

2.00 67.62 88.94

Table 3ANOVA for analysis of variance and adequacy of the quadratic model for Cd2+ removal and OSAC yield.

Response Source of data Sum of squares Degree of freedom Mean square F-value Prob. > F Comment

Cd2+ removal (%) Model 8406.70 8 8406.70 110.89 <0.0001 SD = 3.08, CV = 4.50, R2 = 0.9878,

Adj R2 = 0.9788, AP = 35.41X1 640.44 1 640.44 67.58 <0.0001

X2 1347.08 1 1347.08 142.15 <0.0001

X3 2872.93 1 2872.93 303.16 <0.0001

X21 51.66 1 51.66 5.45 0.0395

X22 446.48 1 446.48 47.11 <0.0001

X23 474.17 1 474.17 50.04 <0.0001

X1X2 58.49 1 58.49 6.17 0.0303

X1X3 43.18 1 43.18 4.56 0.0561

Residual 104.24 11 9.48 – –

Pure error 1.19 5 0.24 – –

OSAC yield (%) Model 44.15 7 6.31 26.19 <0.0001 SD = 0.49, CV = 0.56, R2 = 0.9386,

Adj R2 = 0.902, AP = 21.25X1 20.11 1 20.11 83.50 <0.0001

X2 11.66 1 11.66 48.44 <0.0001

X3 6.40 1 6.40 26.58 0.0002

X22 2.02 1 2.02 8.37 0.0135

X23 1.51 1 1.51 6.26 0.0278

X1X2 2.55 1 2.55 10.61 0.0069

X2X3 1.19 1 1.19 4.92 0.0465

Residual 2.89 12 0.24 – –

Pure error 1.72 5 0.34 – –

Studentized Residuals

No

rma

l %

Pro

ba

bili

ty

-2.28 -1.20 -0.11 0.97 2.06

1

510

2030

50

7080

9095

99

Studentized Residuals

No

rma

l %

Pro

ba

bili

ty

-1.68 -0.91 -0.13 0.64 1.41

1

510

2030

50

7080

9095

99

Actual

Pre

dic

ted

16.08

35.60

55.11

74.63

94.15

16.08 35.60 55.11 74.63 94.15

Actual

Pre

dic

ted

84.64

86.33

88.01

89.69

91.38

84.64 86.33 88.01 89.69 91.38

(c) (d)

(a) (b)

Fig. 2. Design Expert plot; normal probability plot of the studentized residual for (a) Cd2+ removal and (b) OSAC yield; predicted versus actual values plot for (c) Cd2+ removal

and (d) OSAC yield.

T.M. Alslaibi et al. / Journal of Environmental Chemical Engineering 1 (2013) 589–599592

(a) (b)

Devia�on from reference point

-1.850 -1.312 -0.774 -0.236 0.302

84.64

86.33

88.01

89.69

91.38

A

A

B

B

C

C

Devia�on from reference point

Cd re

mov

al %

OSA

C yi

eld

%

-1.850 -1.312 -0.774 -0.236 0.302

24.111

41.7489

59.3867

77.0246

94.6624

A

A

B

B

C

C

X1

X1

X1

X3

X3

X2

X2

X1X2

X2X3

X3

Fig. 3. Perturbation plot for (a) Cd2+ removal, and (b) OSAC yield.

T.M. Alslaibi et al. / Journal of Environmental Chemical Engineering 1 (2013) 589–599 593

Statistical analysis

Analysis of variance (ANOVA) was carried out to justify theadequacy of the models. The results of the second-order responsesurface model fitting in the form of ANOVA are given in Table 3 forCd2+ removal and OSAC yield. The quality of the model developedwas evaluated based on correlation coefficient, R2 and standarddeviation. Data given in Table 3 demonstrate that the two modelswere significant at the 5% confidence level, i.e., P values < 0.05. Thecloser the R2 to unity and the smaller the standard deviation, themore accurate the response could be predicted by the model.

The correlation coefficients for Cd2+ removal (R2 = 0.988) andfor OSAC yield (R2 = 0.939) obtained in the present study werehigher than 0.80, indicating that only 1.2% and 6.1% of the totaldissimilarity are not explained by the empirical model for Cd2+

removal and OSAC yield, respectively. According to Bashir et al.[26], for a model to feature good fit, the correlation coefficient mustbe a minimum of 0.80. An R2 value close to 1 demonstratesfavorable agreement between the calculated and observed resultswithin the experimental range.

ANOVA results for the quadratic response surface model forCd2+ removal yielded a model F-value of 110.89, indicating that themodel is significant. Values of probability > F less than 0.05indicated that the model terms are significant. In this study, X1, X2,X3, X2

1 ; X22 ; X2

3 , X1 X2, and X1 X3 were significant model terms.Insignificant model term, which has limited influence, such as X2

X3, was excluded from the study to improve the model. Based onthe results, the response surface model constructed in this studyfor predicting Cd2+ removal efficiency was considered reasonable.ANOVA results for the quadratic response surface model for OSACyield (Table 3) showed a model F-value of 26.19, indicating that themodel was also significant. In this study, X1, X2, X3, X2

2 ; X23 , X1 X2, and

X2 X3 were significant model terms. Insignificant model terms,which have limited influence, such as X2

1 and X1 X3, were excludedfrom the study to improve the model.

The AP ratio of the models was 35.41 for Cd2+ removal and 21.25for OSAC yield, which is an adequate signal for the model. AP valueshigher than 4 are desirable and confirm that the predicted modelscan be used to navigate the space defined by the CCD [26]. Thecoefficient of variance (CV), which is calculated as the ratio of thestandard error of the estimate to the mean value of the observedresponse (as a percentage), identifies the reproducibility of amodel. A model typically can be considered reproducible if its CV isnot more than 10% [27]. According to Table 3, the CV valuesobtained for the Cd2+ removal and OSAC yield were relatively small

with none exceeding 4.5%. Based on the statistical results obtained,the aforementioned models were adequate to predict Cd2+ removaland OSAC yield within the range of variables studied. The finalregression models, in terms of their coded factors, are expressed bythe following second-order polynomial equations below.

Cd2þ removalð%Þ ¼ þ83:49 þ 8:0X1 þ 11:61X2 þ 16:95X3

� 4:33X21 � 12:74X2

2 � 13:13X23 þ 2:70X1X2

þ 2:32X1X3

Yieldð%Þ ¼ þ88:34 � 1:42X1 � 1:08X2 � 0:80X3 � 0:79X22

þ 0:69X23 � 0:57X1X2 þ 0:38X2X3

Confirming whether or not the selected model providesadequate approximation of the real system is generally important.The model adequacy can be judged by applying the diagnostic plotsprovided by Design Expert 6.0.7 software, such as normalprobability plots of the studentized residuals and predicted versusactual value plots. The normal probability plots of the studentizedresiduals for Cd2+ removal and OSAC yield are shown in Fig. 2a andb. A normal probability plot indicates that the residuals follow anormal distribution and the points follow a straight line. Sinceslight scattering is expected even with normal data, as shown inFig. 2a and b, the data can be assumed to be normally distributed.

Diagnostic plots, such as predicted versus actual values, helpdetermine whether or not a model is satisfactory. The plots ofpredicted versus actual values for Cd2+ removal and OSAC yield areshown in Fig. 2c and d, respectively. These plots show adequateagreement between real data and data gained from the models.Thus, all predictive models can be used to navigate the designspace defined by CCD.

The perturbation plot shows how the response changes as eachfactor moves from the chosen reference point, with all other factorsheld constant at the reference value. Perturbation effect curveswere produced with the vertical axis representing Cd2+ removaland OSAC yield and the horizontal axis representing preparationconditions X1, X2 and X3. By overlying all of the perturbation curves,we obtained a perturbation plot. Fig. 3 shows the perturbation plotof the factors used in this study. Although all factors showedsignificant quadratic effects, the curve with the most prominentchange for Cd2+ removal was the perturbation curve of impregna-tion ratio (X3) compared to those of the other factors fixed at their

(a) (b)

Impregna�on ra�o

Radia�on power(wa�)

Yiel

d %

85.41

86.64

87.87

89.09

90.32

352

440

528

616

0.50 264

0.88

1.25

1.63

2.00

(c) (d)

Impregna�on ra�o

Radia�on �me(min)

84.92

86.16

87.39

88.63

89.86

5

6

7

8

0.50 4

0.88

1.25

1.63

2.00

Radia�on power(wa�)

Impregna�on ra�o

Cd re

mov

al %

Yiel

d %

Cd re

mov

al %

41.51

55.39

69.27

83.15

97.03

352

440

528

616

0.50 264

0.88

1.25

1.63

2.00

Radia�on �me(min)

Impregna�on ra�o

28.63

45.87

63.11

80.35

97.59

5

6

7

8

0.50 4

0.88

1.25

1.63

2.00

Fig. 4. Three-dimensional response surface plot: (a) Cd2+ removal, and (b) OSAC yield (effect of radiation power and chemical impregnation ratio, t = 7 min); (c) Cd2+ removal,

and (d) OSAC yield (effect of radiation time and chemical impregnation ratio, radiation power = 565 W).

T.M. Alslaibi et al. / Journal of Environmental Chemical Engineering 1 (2013) 589–599594

maximum levels. Thus, we believe that X3 was the most significantfactor that contributed to the removal of Cd2+ and had the mostpronounced quadratic effect. The radiation power (X1) showed theleast prominent change compared to the other two factors, but itstill showed a significant quadratic effect. Meanwhile, the curvewith the most prominent change for OSAC yield was theperturbation curve of radiation power (X1) compared to those ofthe other factors fixed at their maximum levels. Thus, we believethat X1 was the most significant factor that contributed to the OSACyield and had the most pronounced quadratic effect. Theimpregnation ratio (X3) showed the least prominent changecompared to the other two factors, but it still showed a significantquadratic effect.

Cadmium removal efficiency and OSAC yield

To assess the interactive relationships between independentvariables and the responses of certain models, three-dimensional(3D) surface response and contour plots utilizing Design Expert6.0.7 software were constructed. As shown in Fig. 4, in each plot,one variable was kept constant while the two others were variedwithin the experimental ranges.

Fig. 4a and b shows the 3D response surface of the combinedeffects of radiation power and impregnation ratio when theradiation time was kept at optimum level (t = 7 min). Themaximum observed removal rate of Cd2+ was 97.03% at a radiationpower of 616 W and impregnation ratio of 2.0. Meanwhile, theminimum predicted Cd2+ removal efficiency 41.51% was obtainedat a radiation power of 264 W and impregnation ratio of 0.5. As forOSAC yield, the maximum observed OSAC yield was 90.32% at aradiation power of 264 W and impregnation ratio of 0.5.Meanwhile, the minimum OSAC yield 85.41% was obtained at aradiation power of 616 W and impregnation ratio of 2.0. Thecontour plots demonstrate that the improvement in removalefficiency of Cd2+ may be attributed to increases in radiation powerand impregnation ratio (Fig. 4a) and that the improvement in OSACyield may be attributed to decreases in radiation power andimpregnation ratio (Fig. 4b). A possible explanation for this effect isthat the reaction between KOH and the char is greater at higherradiation power, thereby facilitating the development of the porestructure and resulting in the formation of a larger number ofactive sites. In addition, the removal of several components fromthe activation process, such as tar and volatile matter, is easier athigher radiation power, which also promotes the activation

Fig. 5. Scanning electron micrograph: (a) OS raw, (b) OSAC 2–4.75 mm (magnifications: 2000�).

T.M. Alslaibi et al. / Journal of Environmental Chemical Engineering 1 (2013) 589–599 595

process. Similar results have been obtained by other researchers[20,28,29].

Fig. 4c and d shows the 3D response surface of the combinedeffect of radiation time and impregnation ratio, while the radiationpower was kept at optimum level (power = 565 W). The contourplots demonstrate that the improvement in removal efficiency ofCd2+ may be attributed to increases in radiation time andimpregnation ratio (Fig. 4c) and that the improvement in OSACyield may be attributed to decreases in radiation time andimpregnation ratio (Fig. 4d). The activation degree significantlydepended on radiation time. A possible explanation for this resultis that with increased radiation time, significantly more pores andactive sites develop on the OSAC surface. Therefore, the removalefficiency of OSAC increases (Fig. 4c) and the yield decreases(Fig. 4d) with increasing radiation time due to more release ofvolatile matters. Similar results were obtained by other research-ers [20,28–30].

Solution pH also affects adsorption by regulating the adsorbentsurface charge as well as degree of ionization of the adsorbatemolecules. The percentages of metals removals using OSAC werefound to increase significantly from 32.57% to 94.27% with theincrease in solution pH from pH 3 to pH 6 and the highest metalsremovals 95.32% were achieved at pH 5. According to Bozic et al.[31] at low pH < 3 the minimal removal may be an effect of thehigher concentration and high mobility of the H+, which competeswith metal ions on the active sites on the sorbent surface, resultingin its preferential adsorption rather than the metal ions. Therefore,H+ ions react with anionic functional groups on the surface of OSACand results in the reduction of the number of binding sitesavailable for the adsorption of Cd2+. This increase may have beenan effect of the presence of negative charge on the surface of theadsorbent that may have been responsible for the metal bindingbecause solution pH can affect the charge of OSAC surfaces [32]. Inaddition, at higher pH values, the lower number of H+ and greaternumber of ligands with negatives charges result in greater metaladsorption.

Table 4Verification of experimental and predicted values of prepared activated carbon under

Response Experamintal Pr

Cd2+ removal (%) 95.32 96

OSAC yield (%) 85.15 86

Optimization

Optimization was carried out to determine the optimum OSACpreparation conditions for the optimal removal of Cd2+ fromaqueous solution and OSAC yield using Design Expert 6.0.7software. According to the software optimization step, the desiredgoal for each operational condition (radiation power, radiationtime, and impregnation ratio) was selected to be ‘‘within’’ therange. The responses (Cd2+ removal and OSAC yield) were definedas maximum to achieve the highest performance. The obtainedvalue of desirability (0.99) showed that the estimated functionrepresents the experimental model and desired conditions. Thepredicted and experimental results of Cd2+ removal and OSAC yieldobtained at optimum conditions are listed in Table 4. As shown inTable 4, 96.25% Cd2+ removal and 86.05% OSAC yield werepredicted according to the model under optimized preparationconditions (radiation power of 565 W, radiation time of 7 min, andimpregnation ratio of 1.87). From the laboratory experiment,95.32% Cd2+ removal and 85.15% MIOS yield were obtained; theseresults agree well with the predicted response values withrelatively small errors of 0.97% for Cd2+ removal and 1.06% forOSAc yield. In this study, a shorter preparation time was appliedcompared with that used in the literature [21,28,30].

Characterization of activated carbon prepared under optimum

conditions

Fig. 5a and b shows the SEM images of the precursor and thederived OSAC prepared under optimum conditions (565 Wradiation power, 7 min radiation time and 1.87 KOH: charimpregnation ratio), respectively. It can be found that themicrowave irradiated sample has well developed and uniformsurface with an orderly pore structure, compared to the originalprecursor which was uneven, rough and undulating with very littlepores available on the surface. This might be due to the activationprocess using KOH, which were effective in creating well

the optimum conditions (565 W, 7 min, 1.87 IR) predicted by RSM.

edicted Error (%) Desirability

.25 0.97 0.99

.05 1.06

Fig. 6. FTIR spectrums; (a) OS and (b) OSAC.

T.M. Alslaibi et al. / Journal of Environmental Chemical Engineering 1 (2013) 589–599596

developed pores on the surfaces of the OSAC, hence leading to largesurface area activated carbon with good porous structure(mesopores). Similar observations were reported by otherresearchers in their works of preparing activated carbons fromdate stone [21], pistachio nut shells [19], cotton stalk [20,30,33]bamboo [28] oil palm empty fruit bunch [34].

The BET surface area, mesopore surface area, total pore volumeand average pore diameter of the prepared AC were1280.71 m2 g�1, 883.49 m2 g�1, 0.604 cm3 g�1 and 4.63 nm, re-spectively. The maximum value of AC yield was found to be 85.15%.Besides, the average pore diameter of the AC was found to be4.63 nm, indicated that the AC prepared was in the mesoporesregion according to the International Union of Pure and AppliedChemistry (IUPAC), pores are classified as micropores (<2 nmdiameter), mesopores (2–50 nm diameter) and macropores(>50 nm diameter) [35].

The obtained FTIR spectrum of OSAC as shown in Fig. 6 revealedthe peaks between 3861–3734, 2318–2102, 1905–1886, 1762–1242and 999–624 cm�1, corresponds to the presence of –OH (hydroxyl),CBC (alkynes), –COOH (carboxylic acids), in-plane –OH, and C–O–C

(esters, ether or phenol) functional groups. Based on experimentalresults and the speciation of metal ions, metal removal by OSAC mayhave occurred through the complexation between the negativelycharged functional groups, such as carboxylic groups (–COOH) andhydroxyl groups (–OH) [36,37], and metal cations, such as Me+2 andMe(OH)+. At pH higher than 3–5, carboxylic groups are deprotonatedand negatively charged. Accordingly, the attraction of positivelycharged metal ions would be improved [38]. In other words, theadsorptive characteristic was influenced by the surface functionali-ties, which may serve as the chemical binding sites for theadsorption process. Besides, the presence of hydroxyl, carbonyl,and alkyl groups could dissociate as negatively charge sites, andcontributing to electrostatic attraction between the activatedcarbons and the positively charge metal ions [39,40].

Adsorption isotherms

The Langmuir model is based on the assumption thatadsorption energy is constant and independent of surfacecoverage. Maximum adsorption occurs once the surface is covered

Table 5Langmuir and Freundlich isotherm parameters for Cd2+adsorption onto OSAC.

Parameter Langmuir Isotherm model Freundlich Isotherm model

Q (mg/g) b (L/mg) R2 RL K (mg/g) (L/mg) 1/n 1/n R2

Cd2+ 11.72 1.956 0.996 0.025 7.70 0.156 0.879

Table 6Comparison of the biosorption capacity of different biosorbents.

Sorbent Activation method BET surface area (m2/g) Adsorption capacity (mg/g) Ref.

Bagasse fly ash Conventional heating 450 1.24 [45]

Petiolar felt-sheath of palm Conventional heating NA 10.8 [46]

Peanut hull pellets Conventional heating NA 6 [47]

Mangosteen shell Conventional heating 704 3.15 [48]

Olive stone Conventional heating 790.25 1.85 [2]

Clinoptilolite (zeolites) – NA 3.7 [49]

Olive stone Microwave heating 1280.71 11.72 This work

NA: not available.

Table 7Pseudo-first order and pseudo-second order kinetic model parameters for the adsorption of Cd2+ onto OSAC at a concentration of 20 mg/L.

Parameter qe,exp (mg/g) Pseudo-first order model Pseudo-second order model

K1 (1/h) qe,cal (mg/g) R2 K2 (g/mg h) qe,cal (mg/g) R2

Cd2+ 7.618 1.058 6.406 0.949 0.244 8.696 0.997

T.M. Alslaibi et al. / Journal of Environmental Chemical Engineering 1 (2013) 589–599 597

by a monolayer of adsorbate [41]. The linear form of the Langmuirisotherm equation is given as equation:

1

ðqeÞ¼ 1

QbCeþ 1

Q(6)

where Ce (mg/L) is the equilibrium liquid-phase concentration ofmetals, qe (mg/g) is the equilibrium uptake capacity, Q (mg/g) is theLangmuir constant related to adsorption capacity, and b (L/mg) isthe Langmuir constant related to the energy of sorption, whichquantitatively reflects the affinity between the sorbent and thesorbate. The equilibrium data were fitted to the Langmuir isothermby plotting 1/(qe) versus 1/Ce using a straight line represented byEq. (6). Q was evaluated from the intercept and b was determinedfrom the slope. The constants together with the linear regressioncorrelation (R2) values are listed in Table 5. The characteristics ofthe Langmuir isotherm can be expressed using the equilibriumparameter RL [42] equation:

RL ¼1

ð1 þ bCoÞ(7)

where b is the Langmuir constant and Co is the initial pollutantconcentration (mg/L). The value of RL indicates whether theisotherm is unfavorable (RL > 1), linear (RL = 1), favorable(0 < RL < 1), or irreversible (RL = 0). The RL values for the adsorptionof Cd2+ on the OSAC were 0.06 indicating that the adsorption is afavorable process.

The Freundlich model is based on sorption on a heterogeneoussurface of varied affinities. The linear form of Freundlich model isgiven as equation:

logqe ¼ logK þ 1

nlogCe (8)

where qe (mg/g) is the amount of metals adsorbed at equilibrium,Ce (mg/L) is the adsorbate concentration, Kf (m/g)(L/mg)1/n is theFreundlich constant related to adsorption capacity, and 1/n is theFreundlich constant related to sorption intensity of the sorbent.Larger values of Kf indicate greater adsorption capacities [43].

The slope of 1/n, ranging between 0 and 1, is a measure of theadsorption intensity or surface heterogeneity. This slope becomes

more heterogeneous as its value approaches 0; 1/n < 1 indicates anormal Freundlich isotherm, whereas 1/n > 1 indicates coopera-tive adsorption [44]. The plot of log qe versus log C gives a straightline with a slope of 1/n. The value of K was calculated from theintercept value. The values of K, 1/n, and R2 for the Freundlichmodel are given in Table 5. The results indicate that the adsorptionintensity was derived from the Freundlich coefficient, where the 1/n value of Cd2+ (0.156) is less than one, which indicates a normalFreundlich isotherm.

Table 5 shows that the Langmuir isotherm fits the data betterthan the Freundlich isotherm. This result is also confirmed by thehigh R2 of the Langmuir model (0.986) compared to Freundlich(0.879), indicating that the adsorption of Cd2+ on OSAC takes placeas monolayer adsorption on a surface that is homogeneous inadsorption affinity. As a result, the adsorption isotherm data werebest described by Langmuir isotherm and the adsorption capacitywas determined to be 11.72 mg/g.

Table 6 lists the comparison of Cd2+ adsorption for variousactivated carbons. The results obtained in the present work werecomparable with the works reported in the literature. The variationin the Cd2+adsorption might be due to the different precursors aswell as the activation methods and/or conditions used to preparethe activated carbons.

Adsorption kinetics

Adsorption kinetics is of great significance in evaluating theperformance of a certain adsorbent and in gaining insight into theunderlying mechanisms [50]. Hameed [51] reported that kineticmodeling is generally used to investigate the mechanism ofadsorption and the potential rate-controlling processes, such asmass transfer and chemical reaction. In the present study, themodeling of the kinetics of the adsorption of Cd2+ on OSAC wasinvestigated using two common models, namely, pseudo-first-order and pseudo-second-order models. The pseudo-first-ordermodel is illustrated as following equation:

logðqe � qtÞ ¼ logðqeÞ � K1t

2:303(9)

al Chemical Engineering 1 (2013) 589–599

A pseudo-second-order model is described as followingequation:

t

qt

¼ 1

k2q2e

þ t

qe

(10)

where qe and qt (mg/g) are the amounts of adsorbate adsorbed atequilibrium and at any time, t (h), respectively, k1 (1/h) and k2 (g/mg h) are the equilibrium rate constants of pseudo-first-order andpseudo-second-order models, respectively and t (h) is the contacttime. The linear plot of log (qe � qt) versus t provides a slope of k1

and intercept of log qe. The values of k1 and R2 obtained from theplot for adsorption of Cd2+ on the adsorbent are reported in Table 7.The R2 value (0.949) obtained for the pseudo-first-order model wasnot high and the experimental qe did not agree with the calculatedvalue obtained from the linear plot. This finding shows that theadsorption of Cd2+ on the adsorbent does not follow a pseudo first-order kinetic model.

Based on Table 7, R2 (0.997) value obtained from the pseudo-second-order model were close to unity, indicating that theadsorption of Cd2+ on OSAC fits this model well and that theadsorption process is controlled by chemisorption [37].

Conclusion

In the present study, optimization of Cd2+ removal fromaqueous solution using OSAC prepared by microwave wasinvestigated. Integration of microwave heating promotes porositydevelopment over a short heating period (7 min). The interactionbetween OSAC preparation variables, such as radiation power,radiation time, and impregnation ratio and responses weredetermined during optimization using RSM with CCD. Statisticalanalysis of the interaction between model responses (Cd2+ removaland OSAC yield) and preparation parameters was significant at Pvalue < 0.05. R2 values of 0.988 for Cd2+ removal and 0.939 forOSAC yield showed that the actual data fitted the predicted datawell. The adsorption data fitted the Langmuir isotherm well withR2 = 0.986. Adsorption kinetics followed the pseudo-second-order,demonstrating that chemisorption is the rate-controlling stepduring Cd2+ adsorption. The optimum results attained from themodel indicate that 7.0 min of contact time is required to achieve96.25% of Cd2+ removal and 86.05% of OSAC yield when theradiation power and impregnation ratio are 565 W and 1.87,respectively, rendering the process via microwave requiressignificantly lesser holding time as compared to conventionalheating method to produce activated carbon of comparablequality. According to this study, OSAC prepared by microwavecan be used for the efficient removal of Cd2+ from contaminatedwastewater. The optimization results obtained by RSM can be usedto prepare activated carbon to be used for heavy metals removal inlarge-scale columns in treatment plants.

Acknowledgements

The authors wish to acknowledge the Universiti Sains Malaysia(USM) for its financial support under the USM and TWASFellowship scheme and RU-PRGS grant scheme (No. 8045048)and acknowledge Ministry of Higher Education, Malaysia forproviding LRGS grant No. (203/PKT/670006) and (03-01-05-SF0502) to conduct this study.

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