Application of response surface methodology (RSM) for optimization of Cu 2+ , Cd 2+ , Ni 2+ , Pb 2+ , Fe 2+ , and Zn 2+ removal from aqueous solution using microwaved olive stone activated

Research ArticleReceived: 10 January 2013 Revised: 27 February 2013 Accepted article published: 14 March 2013 Published online in Wiley Online Library:

(wileyonlinelibrary.com) DOI 10.1002/jctb.4073

Application of response surface methodology(RSM) for optimization of Cu2+, Cd2+, Ni2+,Pb2+, Fe2+, and Zn2+ removal from aqueoussolution using microwaved olive stoneactivated carbonTamer M. Alslaibi,a Ismail Abustan,a∗ Mohd Azmier Ahmadb andAhmad Abu Foulc

Abstract

BACKGROUND: The efficiencies of removal of Cu2+, Cd2+, Ni2+, Pb2+, Fe2+, and Zn2+ from aqueous solution with olive stoneactivated carbon (OSAC) were investigated in this work. A central composite design (CCD) method was used to optimize thepreparation of OSAC using microwave assisted potassium hydroxide.

RESULTS: The optimum conditions obtained were 565 W radiation power, 7 min radiation time, and 1.87 impregnation ratio.This resulted in 98.55% removal of Cu2+, 95.32% of Cd2+, 98.19% of Ni2+ 98.83% of Pb2+, 99.32% of Fe2+, 98.36% of Zn2+,and 85.15% of OSAC yield. The surface characteristics of the AC prepared under optimized conditions were examined by porestructure analysis, scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FT-IR). The BET surfacearea, total pore volume and average pore diameter of the prepared AC were 1280.71 m2 g−1, 0.604 cm3 g−1 and 4.63 nm,respectively. The equilibrium data of the adsorption was well fitted to the Langmuir equation and the highest value ofadsorption capacity (Q) on the OSAC was found for Fe2+ (62.50 mg g−1), followed by Pb2+ (23.47 mg g−1), Cu2+ (22.73 mg g−1),Zn2+ (15.08 mg g−1), Ni2+ (12.00 mg g−1), and Zn2+ (11.72 mg g−1).

CONCLUSIONS: OSAC prepared by microwaves can be used for the removal of metals from contaminated wastewater.c© 2013 Society of Chemical Industry

Keywords: activated carbon; heavy metals adsorption; microwave; olive stone; response surface methodology (RSM)

INTRODUCTIONWastewater pollution is one of the big issues in environmentalconservation. Hospital wastewater carries several types of con-taminants, including heavy metals, biological oxygen demand,chemical oxygen demand, color, and suspended solids. Heavymetals, such as Cu2+, Cd2+, Ni2+, Pb2+, Fe2+ and Zn2+ are toxic tohuman beings and other living organisms if their concentrationsexceed acceptance limits. These heavy metals appear in wastew-ater discharged from hospitals1 and industries such as smelting,metal plating, Cd–Ni battery production, phosphate fertilizermanufacture, pigment mining, stabilizer production, and alloymanufacturing.2

In the field of water pollution the removal of toxic metals,which cause serious degradation of the environment, fromwastewater is a matter of great interest.3 Regarding environmentalcompartments, heavy metals constitute an ecological andhuman health issue, because heavy metals do not undergobiological degradation, compared with certain organic pollutants.4

Cancer, anemia, and liver and kidney damage are among themajor human health problems that are caused by long-term

exposure to heavy metals. Thus, these toxic heavy metalsshould be removed from wastewater to protect people and theenvironment.5

Activated carbons (AC) have been used as adsorbent materialsbecause of their large surface area, microporous structure,high degree of surface reactivity and high adsorption capacity.Nevertheless, application fields of commercially available ACare still limited due to its high cost, resulting from the use ofnon-renewable and relatively high cost starting material such

∗ Correspondence to: I. Abustan, School of Civil Engineering, Engineering Campus,Universiti Sains Malaysia, 14300 Nibong Tebal, Pulau Pinang, Malaysia.E-mail: [email protected]

a School of Civil Engineering, Engineering Campus, Universiti Sains Malaysia,14300 Nibong Tebal, Pulau Pinang, Malaysia

b SchoolofChemicalEngineering,EngineeringCampus,UniversitiSainsMalaysia,14300 Nibong Tebal, Pulau Pinang, Malaysia

c Environmental Engineering, Islamic University of Gaza, Palestine

J Chem Technol Biotechnol (2013) www.soci.org c© 2013 Society of Chemical Industry

www.soci.org T M Alslaibi et al.

as coal. Conscientious effort has been made by researchersto produce cheaper, more effective and environment-friendlyACs that are equivalent to those commercially available. Thishas taken various approaches, which includes using variousprecursors such as agricultural byproducts, use of variouschemicals for activation and devising various methods ofpreparations.

Olive stone waste residue as a raw material for the productionof AC can be considered as one of the best among theagricultural wastes because it is cheap and abundant especiallyin Mediterranean countries. According to the international olivecouncil, the world annual production of olive oil in 2010 was morethan 3 million tons, translating to approximately 15 million tonsof olive cake as the byproduct.6 The Mediterranean region is oneof the top olive- and olive oil-producing regions in the world, withabout 95% of the world’s olive trees.7,8

For preparation of AC, a conventional heating method isusually adopted, in which the energy is produced by an electricalfurnace. This method requires high energy consumption and along processing time. This has prompted the preparation of ACusing microwave technology. In microwave heating, a tremendousthermal gradient from the interior of the char particle to itscool surface allows the microwave-induced reaction to proceedquickly and effectively at a low bulk temperature; therefore, energysavings and a shortened processing time are the key advantages ofmicrowave heating.9 Recently, microwave heating technology hasbeen applied to the fabrication of AC for dyes removal using low-cost waste and agricultural byproducts such as bamboo waste,10,11

waste tea,12 rice husk,13 oil palm fibre,14 pistachio nut shells,15

cotton stalk,16 date stones17 and mangosteen peel.18 However, aliterature review claims that no study has been done on removalof heavy metals using AC prepared by microwave technology.19,20

Furthermore, the use of olive stone for the preparation of activatedcarbon with this process has not been reported. The same is truewith studies concerning optimization of AC preparation conditionsfor heavy metals removal using response surface methodology(RSM). The optimization of RSM is particularly useful when allthe independent variables and their levels and responses are notclearly known.21 A standard RSM design called a central compositedesign (CCD) is suitable for creating a quadratic surface and ithelps to analyze the interaction between parameters, as well asto optimize the effective parameters with a minimum number ofexperiments.22

The main objective of this study is to optimize the preparationconditions of olive stone activated carbon (OSAC) by microwavesfor optimal removal of a group of metals, namely Cu2+, Cd2+, Ni2+,Pb2+, Fe2+ and Zn2+ from aqueous solution. CCD was selected tostudy simultaneously the effects of three AC preparation variables(radiation power, radiation time and chemical impregnationratio) on these responses. Empirical models correlating the Cu2+,Cd2+, Ni2+, Pb2+, Fe2+, Zn2+ removal and OSAC yield to thethree variables were then developed. The characteristics andthe adsorption ability of the OSAC prepared under optimizedconditions were investigated, as well.

MATERIAL AND METHODSAqueous solutionA stock solution of metals was prepared by dissolvingappropriate amounts of NiCl2.6H2O(s), CdCl2.H2O(s), Pb(NO3)2(s),CuCl2.2H2O(s), FeCl2.4H2O(s) and Zn(NO3)2.6H2O in deionized

water. The stock solution was diluted with deionized water toobtain the desired concentration, 20 mg L−1.

Preparation of activated carbonOlive stone waste was obtained from Gaza, Palestine. The OS wastewas rinsed three times with hot water, three times with cold water,and dried in an oven at 105 ◦C for 24 h to remove moisture content.Then, it was ground and sieved for a particle size of 2.0 mm to4.75 mm. The carbonization step was carried out by loading 500 gof dried precursor in a stainless steel vertical tubular reactor placedin a tube furnace at 600 ◦C for 1 h under purified nitrogen (99.99%)flow. A chemical activation method using potassium hydroxide(KOH) was used to activate the char because KOH appears to bethe best activation agent in terms of both adsorption capacity andcarbon yield.23 Char (30 g) was impregnated by specified amountof KOH. The amount of KOH used was adjusted to give a specificimpregnation ratio (weight of activating agent:weight of char) of0.5:1, 1.25:1 and 2:1. The impregnation ratio is given by

Impregnation ratio (IR) = 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. Theactivation step of the impregnated char was carried out usinga modified commercial microwave with a frequency of 2.45 GHzat power levels ranging from 264 W to 616 W and times from4 min to 8 min under nitrogen flow 300 cm3 min−1 as shown inFig. 1. The ranges of preparation conditions were selected based onprevious work.24 The sample was then cooled to room temperatureunder nitrogen flow and washed with hot deionized water and0.1 mol L−1 HCl until the pH of the washed solution was within therange 6.5 to 7.

Design of experimental using RSMThe Design Expert Software (version 6.0.7) was used for thestatistical design of experiments and data analysis. In this study,RSM was used to assess the relationship between responses(Cu2+, Cd2+, Ni2+, Pb2+, Fe2+, Zn2+ removal, and OSAC yield, %)and independent variables, as well as to optimize the relevantconditions of variables in order to predict the best value ofresponses. Among other designs under RSM, CCD was used tostudy the individual and synergetic effect of the three factors onthe responses.12 CCD is suitable for fitting a quadratic surfaceand it helps to optimize the effective parameters with a minimumnumber of experiments, in addition to analyzing the interactionbetween parameters.10 The variables considered were radiationpower (X1), radiation time (X2) and chemical impregnation ratio(X3). These three variables together with their respective rangeswere chosen based on the literature and our preliminary studies.The ranges and the levels of the variables investigated are givenin Table 1.

Performance of the process was evaluated by analyzing Cu2+,Cd2+, Ni2+, Pb2+, Fe2+, Zn2+ removal efficiencies, and theOSAC yield. Each independent variable was varied over threelevels between −1 and +1 at the determined ranges based onsome preliminary experiments. The total number of experimentsobtained for the three factors was 20 (=2k + 2k + 6), where k is thenumber of factors (k = 3). Fourteen experiments were enhancedwith six replications to assess the pure error. Considering that

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Removal of metals from aqueous solution with olive stone activated carbon www.soci.org

Figure 1. Schematic diagram of the preparation of activated carbon from olive stones with KOH activation by microwave irradiation.

Table 1. Independent variables and their coded levels for CCD

Coded variable levels

Variables (factors) Code Units −1 0 1

Radiation power X1 (W) 264 440 616

Radiation time X2 (min) 4 6 8

Impregnation ratio X3 - 0.5 1.25 2

there are only three levels for each factor, the appropriate modelis the quadratic model:

Y = bo +∑k

i=1bixi +

∑k

i=1biix

2i +

∑k−1

i=1

∑k

j=i+1bijxixj + ei

(2)where Y is the predicted response, b0 is the constant coefficient,bi is the linear coefficient, bij is the interaction coefficient, bii isthe quadratic coefficient, Xi and Xj are the coded values of the ACpreparation variables, and ei is the error.

Analysis of variance (ANOVA) was used for graphical analyses ofthe data to obtain the interaction between the process variablesand the responses. The quality of fit of the polynomial modelwas expressed by the coefficient of determination (R2). The modelF-value (Fisher variation ratio), probability value (Prob > F), andadequate precision (AP) were the main indicators demonstratingthe significance and adequacy of the used model.25

Activated carbon yieldThe yield of the OSAC was calculated based on the followingequation:

Yield (%) = dry weight (g) of final activated carbon

dry weight (g) of char× 100

(3)

Batch equilibrium studiesThe batch adsorption experiments were undertaken in a seriesof Erlenmeyer flasks. 100 mL of the model aqueous solution withinitial Cu2+, Cd2+, Ni2+, Pb2+, Fe2+ and Zn2+ concentration of20 mg L−1 was placed in each flask. Each of the prepared OSACsamples (0.25 g) was added to individual flasks, which were thenkept in an isothermal shaker at 200 rpm, pH 5 and 30 ◦C untilequilibrium was reached after 3 h where the rate of metals removal

remained constant. These operational parameters were identifiedbased on literature and preliminary experiments. After agitation,the solid was removed by filtration through a 0.45 µm pore sizeWhatman filter paper. The final metal concentrations in the filtratesand in the initial solution were determined using an inductivelycoupled plasma optical emission spectroscopy system (Varian,715-ES). The sorbed metal concentrations were obtained from thedifference between the initial and final metal concentrations insolution. The percentage removal at equilibrium was calculated asfollows:

Removal (%) = c0 − ce

c0× 100 (4)

where, Co and Ce are the liquid-phase concentrations at initialstate and at equilibrium (mg L−1), respectively.

The amount of metals adsorbed per unit mass of adsorbent atequilibrium conditions, qe (mg g−1), was calculated by

qe = (Co − Ce) V

W(5)

where qe (mg g−1) is the amount of solute adsorbed per unitweight of adsorbent; Co and Ce (mg L−1) are the liquid-phaseconcentrations of adsorbate at initial and equilibrium conditions,respectively; V (L) is the volume of the solution; and W (g) is themass of adsorbent used.

The effects of pH and OSAC dosage on metals removal weretested, respectively, by varying the pH from 2 to 6 and dosagefrom 0.025 to 2 g, with initial metals concentration of 20 mg L−1

and adsorption temperature 30 ◦C. The initial pH of the metalssolution was adjusted by addition of 0.10 mol L−1 HCl or NaOH.

Adsorption isothermLangmuir and Freundlich models were used to investigate theequilibrium behavior of metals adsorption on the preparedOSAC. The Langmuir adsorption isotherm assumes monolayeradsorption, with no lateral interaction and steric hindrancebetween the adsorbed molecules, even on the adjacent sites.The form of the Langmuir isotherm equation is

qe = QbCe

1 + bCe(6)

where Ce (mg L−1) is the equilibrium liquid-phase concentration ofmetals, qe (mg g−1) is the equilibrium uptake capacity, Q (mg g−1)

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is the Langmuir constant related to adsorption capacity, andb (L mg−1) is the Langmuir constant related to the energy ofsorption, which quantitatively reflects the affinity between thesorbent and the sorbate.

Another characteristic parameter of the Langmuir isotherm isthe dimensionless factor RL, related to the shape of the isotherm. Itsvalue indicates either unfavorable (RL > 1), linear (RL =1), favorable(0 < RL < 1), or irreversible (RL = 0) adsorption and is evaluated as26

RL = 1

(1 + bCo)(7)

where b is the Langmuir constant and Co is the initial pollutantconcentration (mg L−1).

The Freundlich isotherm is an empirical model describingmultilayer adsorption, with non-uniform distribution of adsorptionheat and affinities over the heterogeneous surface. The Freundlichmodel is based on sorption on a heterogeneous surface of differentaffinities. The form of Freundlich model is given as

qe = Kf C1/ne (6)

where qe (mg g−1) is the amount of metals adsorbed at equilibrium,Ce (mg L−1) is the adsorbate concentration, Kf (m g−1)(L mg−1)1/n

is the Freundlich constant related to adsorption capacity, and1/n is the Freundlich constant related to sorption intensity of thesorbent.

BET, SEM and FTIR of the prepared activated carbonThe surface area, pore volume and average pore diameter of thesamples were determined using a Micromeritics ASAP 2020 vol-umetric adsorption analyzer. The BET surface area was measuredfrom the adsorption isotherm using the Brunauer–Emmett–Tellerequation. The total pore volume was estimated to be the liquid

volume of nitrogen at a relative pressure of 0.98. The surfacemorphology of the samples was examined using a scanningelectron microscope (Quanta 450 FEG, Netherland). Chemicalcharacteristics of surface functional group of the activated carbonwas detected by diluting in K-Br pellets and recorded witha FTIR spectroscope (IR Prestige 21 Shimadzu, Japan) in the400–4000 cm−1 wave number range.

RESULTS AND DISCUSSIONA total of 20 runs of the CCD experimental design were conducted.The results are shown in Table 2. The observed percentage removalefficiencies varied between 72.73% and 98.64% for Cu2+, 24.11and 94.15% for Cd2+, 23.99 and 98.23% for Ni2+, 75.55 and 98.55%for Pb2+, 79.95 and 99.61% for Fe2+, 42.07 and 98.39% for Zn2+,and between 84.64 and 91.38% for OSAC yield.

Analysis of varianceANOVA was carried out to justify the adequacy of the models.The results of the second-order response surface model fittingin the form of ANOVA are given in Tables 3 for Cu2+, Cd2+,Ni2+, Pb2+, Fe2+, Zn2+ removal, and OSAC yield. The quality ofthe model developed was evaluated based on the coefficient ofdetermination, R2 and also the standard deviation values. Datagiven in Table 3 demonstrate that all the models were significantat the 5% confidence level, given that P values were less than0.05. The closer the R2 value to unity and the smaller the standarddeviation, the more accurate the response predicted by the model.The values of coefficient of determinations (R2 = 0.9744, 0.9878,0.9856, 0.9780, 0.9840, 0.9903, and 0.9386) obtained in the presentstudy for Cu2+, Cd2+, Ni2+, Pb2+, Fe2+, Zn2+ removal, and OSACyield were higher than 0.80. For good fit of a model, the coefficientof determination should be a minimum of 0.80. A high R2 value

Table 2. Experimental factors and experimental responses

Variables Responses

Run no. Type

X1:

Power (W)

X2:

time (min) X3: IR

Cu removal

Y1 (%)

Cd removal

Y2 (%)

Ni removal

Y3 (%)

Pb removal

Y4 (%)

Fe removal

Y5 (%)

Zn removal

Y6 (%)

Yield

Y7 (%)

1 Center 440 6 1.25 93.36 83.43 88.00 95.37 98.41 91.57 88.14

2 Center 440 6 1.25 92.34 84.11 88.15 94.34 97.53 90.59 87.56

3 Center 440 6 1.25 92.21 84.75 87.91 94.18 97.22 91.19 88.78

4 Center 440 6 1.25 93.55 84.36 89.39 95.62 98.58 91.39 88.88

5 Center 440 6 1.25 92.97 84.63 88.62 94.94 97.84 90.99 87.86

6 Center 440 6 1.25 93.62 83.92 87.78 95.59 98.38 90.89 88.94

7 Axial 440 6 2.00 97.12 87.99 91.99 98.22 99.21 93.99 88.26

8 Axial 440 4 1.25 89.15 52.73 57.50 91.19 94.17 73.94 89.16

9 Axial 440 6 0.50 87.77 50.60 61.32 89.81 92.74 64.94 90.34

10 Axial 616 6 1.25 93.65 87.84 92.90 95.71 98.83 92.99 86.70

11 Axial 264 6 1.25 88.72 68.35 79.04 90.73 94.69 82.28 89.28

12 Axial 440 8 1.25 97.50 86.64 90.41 98.42 99.55 92.59 86.48

13 Fact 616 4 0.50 81.45 30.53 38.55 83.35 86.42 56.30 89.22

14 Fact 264 4 0.50 72.73 24.11 23.99 75.55 79.95 42.07 91.38

15 Fact 616 4 2.00 91.21 65.75 75.55 93.15 96.22 88.74 87.36

16 Fact 616 8 2.00 98.64 94.15 98.23 98.55 99.61 98.39 84.64

17 Fact 616 8 0.50 88.66 54.03 77.18 90.67 93.59 79.15 85.62

18 Fact 264 4 2.00 83.87 52.72 51.55 85.80 88.90 78.94 88.74

19 Fact 264 8 0.50 82.68 39.47 48.25 84.78 87.68 66.70 89.38

20 Fact 264 8 2.00 90.23 67.62 80.90 90.17 93.37 83.06 88.94



Table 3. ANOVA for analysis of variance and adequacy of the quadratic model for OSAC yield, Cu2+ , Cd2+ , Ni2+ , Pb2+ , Fe2+ and Zn2+ removal

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

Cu removal (%) Model 705.48 6 117.58 82.57 <0.0001 SD = 1.19,

X1 125.18 1 125.18 87.90 <0.0001 CV = 1.32,

X2 154.43 1 154.43 108.45 <0.0001 R2 = 0.9744,

X3 228.27 1 228.27 160.30 <0.0001 Adj R2 = 0.9626,

X21 34.72 1 34.72 24.38 0.0003 AP =34.69.

X22 5.48 1 5.48 3.85 0.0715

X23 14.41 1 14.41 10.12 0.0072

Residual 18.51 13 1.42 – –

Pure error 1.87 5 0.37 – –

Cd removal (%) Model 8406.70 8 8406.70 110.89 <0.0001 SD = 3.08,

X1 640.44 1 640.44 67.58 <0.0001 CV = 4.50,

X2 1347.08 1 1347.08 142.15 <0.0001 R2 = 0.9878,

X3 2872.93 1 2872.93 303.16 <0.0001 Adj R2 = 0.9788,

X21 51.66 1 51.66 5.45 0.0395 AP =35.41.

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 – –

Ni removal (%) Model 8078.44 5 1615.69 191.48 <0.0001 SD = 2.90,

X1 973.64 1 973.64 115.39 <0.0001 CV = 3.88,

X2 2185.34 1 2185.34 258.99 <0.0001 R2 = 0.9856,

X3 2218.26 1 2218.26 262.89 <0.0001 Adj R2 = 0.9804,

X22 656.81 1 656.81 77.84 <0.0001 AP = 49.71.

X23 432.43 1 432.43 51.25 <0.0001

Residual 118.13 14 8.44 – –

Pure error 1.82 5 0.36 – –

Pb removal (%) Model 627.39 7 89.63 76.38 <0.0001 SD = 1.08,

X1 118.39 1 118.39 100.90 <0.0001 CV = 1.18,

X2 112.60 1 112.60 95.96 <0.0001 R2 = 0.9780,

X3 174.05 1 174.05 148.33 <0.0001 Adj R2 = 0.9652,

X21 30.72 1 30.72 26.18 0.0003 AP = 32.02.

X22 8.48 1 8.48 7.23 0.0197

X23 17.78 1 17.78 15.16 0.0021

X2X3 5.73 1 5.73 4.88 0.0473

Residual 14.08 12 1.17 – –

Pure error 1.98 5 0.40 – –

Fe removal (%) Model 521.47 7 74.50 105.68 <0.0001 SD = 0.84,

X1 90.53 1 90.53 128.43 <0.0001 CV = 0.89,

X2 79.22 1 79.22 112.39 <0.0001 R2 = 0.9840,

X3 136.30 1 136.30 193.37 <0.0001 Adj R2 = 0.9747,

X21 14.37 1 14.37 20.39 0.0007 AP = 35.84.

X22 13.13 1 13.13 18.62 0.0010

X23 25.86 1 25.86 36.69 <0.0001

X2X3 6.18 1 6.18 8.76 0.0119

Residual 8.46 12 0.70 – –

Pure error 1.5 5 0.3 – –

Zn removal (%) Model 4013.73 6 668.95 221.11 <0.0001 SD = 1.74,

X1 391.00 1 391.00 129.24 <0.0001 CV = 2.12,

X2 638.43 1 638.43 211.03 <0.0001 R2 = 0.9903,

X3 1794.98 1 1794.98 593.31 <0.0001 Adj R2 = 0.9858,

X22 120.40 1 120.40 39.80 <0.0001 AP =53.72.

X23 315.82 1 315.82 104.39 <0.0001

X2X3 142.11 1 142.11 46.97 <0.0001

Residual 39.33 13 3.03 – –

Pure error 0.63 5 0.13 – –



Table 3. Continued

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

OSAC yield Model 44.15 7 6.31 26.19 <0.0001 SD = 0.49,

X1 20.11 1 20.11 83.50 <0.0001 CV = 0.56,

X2 11.66 1 11.66 48.44 <0.0001 R2 = 0.9386,

X3 6.40 1 6.40 26.58 0.0002 Adj R2 = 0.902,

X22 2.02 1 2.02 8.37 0.0135 AP = 21.25.

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 – –

close to 1 demonstrates good agreement between the calculatedand observed results within the range of experiment, and showsthat a desirable and reasonable agreement with adjusted R2 isnecessary.27,28 From the statistical results obtained, it was shownthat the above models were adequate to predict the Cu2+, Cd2+,Ni2+, Pb2+, Fe2+, Zn2+ removal and OSAC yield within the range ofvariables studied. The adequate precision (AP) ratio of the modelsvaries between 32.02 and 53.72 for metals removal and is 21.25for OSAC yield, which is an adequate signal for the model. APvalues higher than 4 are desirable, and confirm that the predictedmodels can be used to navigate the space defined by the CCD.29

The coefficient of variance (CV) as the ratio of the standard errorof estimate to the mean value of the observed response (as a per-centage) identifies reproducibility of the model. A model typicallycan be considered reproducible if its CV is not more than 10%.30

According to Table 3, the CV values obtained for all responsesstudied are relatively small with none of them exceeding 4.50%.

In the present study, seven quadratic models are significantmodel terms (Table 3). Insignificant model terms, which havelimited influence, were excluded from the study to improvethe models. Based on the results, the response surface modelsconstructed to predict Cu2+, Cd2+, Ni2+, Pb2+, Fe2+ and Zn2+removal, and OSAC yield were considered reasonable. The finalregression models, in terms of their coded factors, are expressedby the following second-order polynomial equations

Cu2+ removal (%) = +93.70 + 3.54 X1 + 3.93 X2 + 4.78 X3 − 3.55 X12 − 1.41 X2

2 − 2.29 X32

Cd2+ removal (%) = +83.49 + 8.0 X1 +11.61 X2 + 16.95 X3 − 4.33 X12 − 12.74 X2

2 − 13.13 X32 + 2.70 X1 X2 + 2.32 X1 X3

Ni2+ removal (%) = +87.84 +9.87 X1 + 14.78 X2 + 14.89 X3 − 14.33 X22 − 11.62 X3

2

Pb2+ removal (%) = +95.63 + 3.44 X1 + 3.36 X2 + 4.17 X3 − 3.34 X12 − 1.76 X2

2 − 2.54 X32 − 0.85 X2 X3

Fe2+ removal (%) = +98.41 +3.01 X1 + 2.81 X2 + 3.69 X3 − 2.29 X12 − 2.18 X2

2 − 3.07 X32 − 0.88 X2 X3

Zn2+ removal (%) = +90.07 + 6.25 X1 + 7.99 X2 + 13.40 X3 − 6.13 X22 − 9.93 X3

2 − 4.21 X2 X3

Yield (%) = +88.34 − 1.42 X1 − 1.08 X2 − 0.80 X3 − 0.79 X22 + 0.69 X3

2 − 0.57 X1 X2 + 0.38 X2 X3

Diagnostic plots such as the predicted versus actual values helpto judge the level of model satisfaction. The predicted versusactual values plots of metals removal and OSAC yield show anadequate agreement between real data and those gained fromthe models. Hence, all predictive models can be used to navigatethe design space defined by the CCD.

Removal efficiency and OSAC yieldTo assess the interactive relationships between independentvariables and the responses of certain models, three-dimensional(3D) surface response and contour plots utilizing DesignExpert 6.0.7 software were constructed. ANOVA analysis for

Cu2+, Cd2+, Ni2+, Pb2+, Fe2+ and Zn2+ removal shows thatthe impregnation ratio (X3) presented the largest ranges ofF-value (148.33–593.31), followed by radiation time (X2) rangesof (95.96–258.99), while radiation power (X1) presented the leastranges of (67.58–129.24). In contrast, ANOVA analysis for OSACyield shows that the radiation power (X1) presented the largestF-value of 83.50, followed by radiation time (X2) of 48.44, andimpregnation ratio (X3) of 26.58. This means that the impregnationratio and radiation time had a higher significant effect on Cu2+,Cd2+, Ni2+, Pb2+, Fe2+ and Zn2+ removal for the prepared AC,compared with radiation power. While the radiation power had ahigher significant effect in OSAC yield followed by radiation timeand impregnation ratio. As shown in Fig. 2(a)–(g), in each plot,one variable was kept constant while the other two were variedwithin the experimental ranges. Figure 2(a)–(g) shows the three-dimensional response surface of the combined effect of radiationpower and impregnation ratio, while the radiation time was keptat optimum level (t = 7 min). The maximum observed removal ofCu2+, Cd2+, Ni2+, Pb2+, Fe2+, Zn2+ and OSAC yield were 98.55%,95.32%, 98.19%, 98.83%, 99.32%, 98.36%, and 85.15%, respectively,at radiation power 565 W, impregnation ratio 1.87, pH 5, contacttime 3 h, shaking speed 200 rpm and OSAC dosage 0.25 g. Mean-while, the minimum predicted Cu2+, Cd2+, Ni2+, Pb2+, Fe2+, Zn2+removal efficiencies (81.50%, 41.51%, 55.01%, 84.12%, 87.84%,and 65.82, respectively) were obtained at radiation power 264 W

and impregnation ratio 0.5. Figure 2(g) shows that the carbon yielddecreased from 90.32% to 85.41% with increasing radiation powerfrom 264 W to 616 W and increasing impregnation ratio from 0.5to 2. The contour plots demonstrate that the improvement ofremoval efficiencies for Cu2+, Cd2+, Ni2+, Pb2+, Fe2+ and Zn2+ isattributed to the increase in radiation power and impregnationratio (Fig. 2(a)–(f)) while the improvement in OSAC yield isattributed to the decrease in radiation power and impregnationratio (Fig. 2(g)). One possible reason is that the reaction betweenKOH and the char was fiercer at higher microwave power, whichfacilitated the development of the pore structure and more activesites formed. In addition, the removal of some components from



(a) (b)

(c) (d)

Cu

rem

oval

%

Radiation power(watt)

Impregnationratio

81.50

85.88

90.26

94.64

99.02

264352

440528

616

0.50

0.88

1.25

1.63

2.00

Ni r

emov

al %


Impregnationratio

55.01

66.26

77.50

88.75

100.00

264352

440528

616

0.50

0.88

1.25

1.63

2.00


Impregnationratio

Pb r

emov

al %

84.12

87.90

91.68

95.46

99.24

264352

440

528616

0.50

0.88

1.25

1.63

2.00


Impregnationratio

Cd

rem

oval

%

41.51

55.39

69.27

83.15

97.03

264352

440528

616

0.50

0.88

1.25

1.63

2.00

(e)(f)

(g)

Zn

rem

oval

%


Impregnationratio

65.82

74.36

82.91

91.45

100.00

264352

440528

616

0.50

0.88

1.25

1.63

2.00

Impregnationratio


Yie

ld %

85.41

86.64

87.87

89.09

90.32

264

352440

528616

0.50

0.88

1.25

1.63

2.00


Impregnationratio

Fe r

emov

al %

87.84

90.88

93.92

96.96

100.00

264352

440528

616

0.50

0.88

1.25

1.63

2.00

Figure 2. Three-dimensional response surface plot: (a) Cu2+ removal, (b) Cd2+ removal, (c) Ni2+ removal, (d) Pb 2+ removal, (e) Fe2+ removal, (f) Zn2+removal, and (g) OSAC yield (effect of radiation power and chemical impregnation ratio, t = 7 min).

the activation process such as tar and volatile matter was easierat higher radiation power, which also promoted the activationprocess. Therefore, the adsorption capacity of OSAC would beincreased with increase of radiation power and impregnation ratio.When microwave power reached a certain level, overall energy

could burn a small quantity of carbon, and the structure of thepores was destroyed. Therefore, the microwave radiation powerlevel of 565 W was chosen to prepare the AC from olive stone.Similar results have been obtained by other researchers.11,16,31

Similarly, as the KOH dosage increased, more potential sites could



be penetrated and occupied by the activating agents. However,excessive KOH could not promote the activation further since aninsulating layer might be formed due to the presence of K2Oand K2CO3 or K residues on the pore surface of char.11,32 Generalchemical reaction between KOH and carbon material are as follows:

4KOH + C → K2 CO3 + K2 O + 2H2 (9)

The three-dimensional response surface of the combined effectof radiation time and impregnation ratio was investigated whenthe radiation power was kept at optimum level (power = 565 W).The contour plots demonstrate that the improvement of removalefficiencies for Cu2+, Cd2+, Ni2+, Pb2+, Fe2+ and Zn2+ can beattributed to the increase in radiation time and impregnationratio while the improvement in OSAC yield can be attributedto the decrease in radiation time and impregnation ratio. Theactivation degree was much more dependent on the microwaveradiation time. One possible reason is that with prolongationof the microwave radiation time, many more pores and activesites developed in the surface of OSAC. Therefore, the removalefficiency of OSAC would increase and the yield would decreasewith prolongation of microwave radiation time. However, whenthe microwave radiation time reached a certain value, the pores ofcarbon would be burnt by microwave heating, which would lowerthe removal efficiencies and the yield of AC. Similar results havebeen obtained by other researchers.11,16,31,33

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 with the increase in solution pH frompH 3 to pH 6 and the highest metals removals were achieved atpH 5. According to Bozic et al.34 at low pH < 3 the minimal removalmay be an effect of the higher concentration and high mobilityof H+, which competes with metal ions on the active sites on thesorbent surface, resulting in its preferential adsorption rather thanthe metal ions. Therefore, H+ ions react with anionic functionalgroups on the surface of OSAC and result in the reduction of thenumber of binding sites available for the adsorption of Cu2+, Cd2+,Ni2+, Pb2+, Fe2+ and Zn2+. This increase may have been an effectof the presence of negative charge on the surface of the adsorbentthat may have been responsible for the metal binding becausesolution pH can affect the charge of OSAC surfaces.35 In addition, athigher pH values, the lower number of H+ and greater number ofligands with negatives charges result in greater metal adsorption.The same trend was observed by several researchers who stud-ied metal sorption by different biomaterials, namely, copper bysawdust,36 zinc, lead, and cadmium by jute fibers,37 lead and cad-mium by fruit shell,38 and cadmium by orange wastes.39 Similarly,the percentages of metals removal increased as a result of increas-ing dosage of the OSAC from 0.025 to 0.25 g, after which it becameconstant. This is because the number of available adsorption siteswas increased through increase in the adsorbent dose.40

OptimizationThe optimization process was carried out to determine theoptimum value of OSAC preparation conditions via microwavefor optimal removal of Cu2+, Cd2+, Ni2+, Pb2+, Fe2+ and Zn2+from aqueous solution and OSAC yield using the Design Expert6.0.7 software. According to the software optimization step, thedesired goal for each operational condition (radiation power,radiation time, impregnation ratio) was chosen ‘within the range’.The responses (Cu2+, Cd2+, Ni2+, Pb2+, Fe2+ and Zn2+ removal,

Table 4. Verification of experimental and predicted values of theactivated carbon preparedunder the optimum conditions (565 W,7 min, 1.87 IR) predicted by RSM

Response Experimental Predicted Error (%) Desirability

OSAC yield (%) 85.15 86.05 1.06

0.99

Cu2+ removal (%) 98.55 98.76 0.22

Cd2+ removal (%) 95.32 96.25 0.97

Ni2+ removal (%) 98.19 99.99 1.83

Pb2+ removal (%) 98.83 99.09 0.26

Fe2+ removal (%) 99.32 99.99 0.67

Zn2+ removal (%) 98.36 99.16 0.81

and OSAC yield) were defined as maximum to achieve the highestperformance. The value of desirability obtained (0.99) shows thatthe estimated function may represent the experimental modeland desired conditions. The predicted and experimental results ofCu2+, Cd2+, Ni2+, Pb2+, Fe2+ and Zn2+ removal, and OSAC yieldobtained at optimum conditions are listed in Table 4. The optimumAC was obtained using the following preparation conditions: 565 Wradiation power, 7 min activation time, and 1.87 impregnationratio, which experimentally resulted in 98.55% of Cu2+, 95.32%of Cd2+, 98.19% of Ni2+ 98.83% of Pb2+, 99.32% of Fe2+, 98.36%of Zn2+ removal, and 85.15% of OSAC yield. It was observed thatthe experimental values obtained were in good agreement withthe values predicted by the models with relatively small errors(between 0.22% and 1.83% for metals removal and 1.06% forOSAC yield). This means that the model can be used to predict theremoval efficiencies of metals under the experimental conditionsused. In this study, a shorter preparation time is required and higherBET surface area is obtained compared with that in literature.11,17,33

Characterization of OSAC prepared under optimumconditionsFigure 3(a)–(d) shows SEM images of the precursor, char, thederived microwaved OSAC and the exhausted OSAC preparedunder optimum conditions (565 W radiation power, 7 min radiationtime and 1.87 KOH: char impregnation ratio), respectively. It canbe seen that a well developed and uniform surface is formed,with an orderly pore structure in the microwave irradiated samplecompared with the original precursor and char which was uneven,rough and undulating with very few pores available on the surface.This is due to the activation process using microwave and KOH,which were effective in creating well developed pores on thesurface of the OSAC, leading to large surface area AC with goodporous structure (mesopores). Similar observations were reportedby other researchers in their work preparing ACs from date stone,17

pistachio nut shells,15 cotton stalk,16,33,41 bamboo11 and oil palmempty fruit bunches.42 After heavy metals adsorption, pores werefilled with contaminants.

The BET surface area, mesopore surface area, total porevolume and 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,respectively. The maximum value of AC yield was found to be85.15%. The average pore diameter of the AC was 4.63 nm,indicating that the AC prepared was in the ‘mesopores’ region.The International Union of Pure and Applied Chemistry hasrecommended a classification for porous materials where poresof less than 2 nm in diameter are termed ‘micropores’, those withdiameters between 2 and 50 nm are termed ‘mesopores’, and



(a) (b)

(c) (d)

Figure 3. Scanning electron micrograph: (a) OS raw, (a) OS char, (c) microwaved OSAC, and (d) exhausted OSAC (magnifications: 4000×).

those greater than 50 nm in diameter are termed ‘macropores’.43

The AC resulting from OS waste contained relatively large surfacearea and total pore volume compared with commercially availableACs such as F100 and BPL from Calgon Corporation with BETsurface area of 957 and 972 m2 g−1 in addition to total porevolume of 0.526 and 0.525 cm3 g−1, respectively.

The FTIR spectrum of OSAC obtained (Fig. 4) revealed peaksbetween 3861 and 3734, 2318 and 2102, 1905 and 1886, 1762and 1242 and 999 and 624 cm−1, corresponding to the presenceof –OH (hydroxyl), C ≡ C (alkynes), –COOH (carboxylic acids),in-plane –OH, and C–O–C (esters, ether or phenol) functional

groups. Based on experimental results and the speciation of metalions, metal removal by OSAC may have occurred through com-plexation between the negatively charged functional groups, suchas carboxylic groups (−COOH) and hydroxyl groups (−OH),39,44

and metal cations, such as Me+2 and Me(OH)+. At pH higher than 3to 5, carboxylic groups are deprotonated and negatively charged.Accordingly, the attraction of positively charged metal ions wouldbe improved.45 In other words, the adsorptive characteristic wasinfluenced by the surface functionalities, which may serve as thechemical binding sites for the adsorption process. Besides, thepresence of hydroxyl, carbonyl, and alkyl groups could dissociate

Figure 4. FTIR of OSAC.



Table 5. Langmuir and Freundlich isotherm parameters for the adsorption of Fe2+ , Pb2+ , Cu2+ , Zn2+ , Ni2+ , and Cd2+ onto OSAC.

Langmuir Isotherm model Freundlich Isotherm model

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

Fe2+ 62.50 4.706 0.989 0.011 47.62 0.477 0.936

Pb2+ 23.47 3.981 0.982 0.012 14.84 0.278 0.951

Cu2+ 22.73 3.548 0.982 0.014 14.07 0.378 0.956

Zn2+ 15.08 3.098 0.940 0.016 9.60 0.223 0.989

Ni2+ 12.00 3.051 0.991 0.016 8.33 0.144 0.894

Cd2+ 11.72 1.956 0.996 0.025 7.70 0.156 0.879

as negatively charge sites, and contribute to electrostatic attractionbetween the ACs and the positively charge metal ions.46,47

Adsorption studyThe Langmuir and Freundlich isotherms were fitted to theexperimental data. The constants together with the linearregression correlation (R2) values are listed in Table 5. The highestvalue of adsorption capacity (Q) on the OSAC was found forFe2+ (62.50 mg g−1), followed by Pb2+ (23.47 mg g−1), Cu2+

(22.73 mg g−1), Zn2+ (15.08 mg g−1), Ni2+ (12.00 mg g−1), andZn2+ (11.72 mg g−1). Therefore, the adsorption capacity series,Fe > Pb > Cu > Zn > Ni > Cd, found in this study is in agreementwith the first hydrolysis constant (Me(OH)+) series, which isFe > Pb > Cu > Zn > Ni > Cd.48 Furthermore, the order of thisseries is similar to the relative affinity order of OSAC found onthe basis of b values, which is Fe > Pb > Cu > Zn > Ni > Cd. TheRL values for the adsorption of Fe2+, Pb2+, Cu2+, Zn2+, Ni2+, andCd2+ on the OSAC were between 0 and 1 indicating that theadsorption is a favorable process.

The values of K , 1/n, and R2 for the Freundlich model are alsopresented. The adsorption intensities of OSAC indicate a normalFreundlich isotherm, where the 1/n values of Fe2+, Pb2+, Cu2+,Zn2+, Ni2+, and Cd2+ are less than one. The adsorption of Fe2+,Pb2+, Cu2+, Zn2+, Ni2+, and Cd2+ was reasonably explained bythe Langmuir and Freundlich isotherms. However, the Langmuirmodel yielded the best fit given that the R2 values were relativelyhigh (close to unity). Larger values of k mean greater capacities ofadsorption.49 Therefore, sorption capacity of OSAC for the studiedmetal ions may be in the order Fe > Pb > Cu > Zn > Ni > Cd.The results obtained agreed with the work by Li,50 in which thefollowing order for the sorption of metals onto sawdust andmodified peanut husk was reported: Pb > Cu > Cr. Another studyby Broun51 examined peanut hull pellets and found a similar orderof affinity: Pb > Zn > Cu > Cd. Similarly, Tuzen52 reported that theaffinity order of the metal ions towards carbon nanotubes wasCu > Pb > Zn > Co > Ni > Cd > Mn.

CONCLUSIONIn the present study, a CCD method was used to optimizethe preparation of AC from olive stone with KOH as activator.The impregnation ratio and radiation time were more highlysignificant factors on metals removal for the prepared AC thanradiation power. Radiation power had the greatest impact onOSAC yield, followed by radiation time and impregnation ratio.The optimum conditions were radiation power 565 W, radiationtime 7 min, and impregnation ratio 1.87. N2 adsorption showedthat the BET surface area of the prepared AC was 1280.71 m2 g−1.

Scanning electron microscopy (SEM) and Fourier transforminfrared spectroscopy (FT-IR) investigation found the presenceof an open-pore structure and different functionalities on thecarbon surfaces compared with those of olive stone. Langmuirisotherms better fitted the experimental equilibrium data of metalsadsorption on the prepared OSAC. The maximum adsorptioncapacity (Q) on the OSAC was found for Fe2+ (62.50 mg g−1),followed by Pb2+ (23.47 mg g−1), Cu2+ (22.73 mg g−1), Zn2+(15.08 mg g−1), Ni2+ (12.00 mg g−1), and Zn2+ (11.72 mg g−1).OSAC prepared by microwaves can be used for the removal ofmetals from contaminated wastewater. The optimization resultsobtained by RSM can be used to prepare AC to be usedfor heavy metals removal in large-scale columns in treatmentplants.

ACKNOWLEDGEMENTThe authors wish to thank the Universiti Sains Malaysia (USM)for its financial support under the USM and TWAS Fellowshipscheme and RU-PRGS grant scheme (No. 8045048) and theMinistry of Higher Education, Malaysia for providing LRGS grant No.(203/PKT/670006) and (03-01-05-SF0502) to conduct this study.

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Documents

Application of response surface methodology (RSM) for optimization of Cu 2+ , Cd 2+ , Ni 2+ , Pb 2+ , Fe 2+ , and Zn 2+ removal from aqueous solution using microwaved olive stone activated