Chemistry and EcologyVol. 28, No. 2, April 2012, 153–167

Adsorptive features of banana (Musa paradisiaca) stalk-basedactivated carbon for malachite green dye removal

Olugbenga Solomon Belloa,b*, Mohd Azmier Ahmada and Norhidayah Ahmada

aSchool of Chemical Engineering, Engineering Campus, Universiti Sains Malaysia, Penang, Malaysia;bDepartment of Pure and Applied Chemistry, Ladoke Akintola University of Technology,

Ogbomoso, Nigeria

(Received 24 May 2011; final version received 27 September 2011)

Chemically prepared activated carbon derived from banana stalk (BSAC) was used as an adsorbent toremove malachite green (MG) dye from aqueous solution. BSAC was characterised using thermogravi-metric analyser, Brunauer Emmett Teller, Fourier transform infrared spectrometry, scanning electronmicroscopy, pHpzc, elemental analysis and Boehm titration. The effectiveness of BSAC in adsorbingMG dye was studied as a function of pH, contact time, temperature, initial dye concentration and repeateddesorption–adsorption processes. pHpzc of BSAC was 4.5 and maximum dye adsorption occurred at pH 8.0.The rate of dye adsorption by BSAC was very fast initially, attaining equilibrium within 120 min follow-ing a pseudo-second-order kinetic model. Experimental data were analysed by Langmuir, Freundlich andDubinin–Raduschevich isotherms. Equilibrium data fitted best into the Langmuir model, with a maximumadsorption capacity of 141.76 mg·g−1. �G0 values were negative, indicating that the process of MG dyeadsorption onto BSAC was spontaneous. The positive values of �H0 and �S0 suggests that the processof dye adsorption was endothermic. The regeneration efficiency of spent BSAC was studied using 0.5 MHCl, and was found to be in the range of 90.22–95.16% after four cycles. This adsorbent was found to beboth effective and viable for the removal of MG dye from aqueous solution.

Keywords: banana stalk; dye; adsorption; endothermic; regeneration

1. Introduction

Colour is one of the characteristics of an effluent; it affects the nature of water, inhibits sunlightpenetration and reduces photosynthetic activity [1,2]. Dyes are carcinogenic and mutagenic; theyare generally stable to biological degradation [1,3]. Hence, their removal from industrial effluentsbefore discharge into the environment is extremely important. Malachite green (MG), triphenylmethane dye, has been widely used for the dyeing of leather, wool and silk, as well as in distilleries[4]. In addition, MG is also used as a fungicide and antiseptic in the aquaculture industry to controlfish parasites and disease [5]. However, MG is very dangerous and highly cytotoxic to mammaliancells; it also acts as a liver tumour-enhancing agent. Existing techniques for the removal ofdyes from dye-containing wastewater, for example chemical coagulation/flocculation, ozonation,oxidation processes, chemical precipitation, ion exchange, reverse osmosis and ultrafiltration,

*Corresponding author. Email: osbello@yahoo.com

ISSN 0275-7540 print/ISSN 1029-0370 online© 2012 Taylor & Francishttp://dx.doi.org/10.1080/02757540.2011.628318http://www.tandfonline.com

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have serious restrictions such as high cost, the formation of hazardous by-products or intensiveenergy requirements [6]. Adsorption processes using activated carbons are widely used to removepollutants from wastewaters. However, commercially available activated carbon is expensive.Therefore, the development of efficient, low-cost and environmentally friendly technologies toreduce dye content in wastewater becomes necessary. In recent years, there has been a growinginterest in finding low-cost alternatives to carbon-based adsorbents [7,8]. A number of non-conventional adsorbents has been studied for their capacity to remove MG dye from aqueoussolutions, such as de-oiled soya [9], agro-industry waste, mangosteen peel [10], rambutan peel[11], rubber seed coat [12], iron humate [13], modified rice straw [14] and banana stalk [15,16].

Banana (Musa paradisiaca) is native to Southeast Asia, most of the bananas produced areconsumed locally and ∼10% are exported [17]. Massive amounts of stalk (as waste products) aredisposed, resulting in a serious environmental problem through the emission of foul odours whenit is left to decompose. We attempted to use banana stalk, an abundantly available lignocellulosicagricultural waste, as a cheap and renewable precursor for activated carbon preparation for MGdye removal. This would help in placing value on this agricultural waste and provide a potentiallycheap alternative to existing commercial activated carbons. To the best of our knowledge, noreport has been documented on the use of banana stalk-activated carbon (BSAC) as a low-costalternative adsorbent for MG dye removal from aqueous solution. The effects of initial MG dyeconcentration, solution temperature, pH and repeated desorption–adsorption studies on MG dyeadsorption onto BSAC were investigated. Kinetics, isotherms and thermodynamic parametersgoverning the adsorption process were also studied and reported.

2. Materials and methods

2.1. Adsorbate

MG dye was supplied by Sigma-Aldrich (M) Sdn Bhd, Malaysia and used as received. MG hasa chemical formula of C52H54N4O12, a molecular mass of 927.00 g·mol−1 and λmax = 618 nm.Stock solutions were prepared by dissolving accurately weighed samples of dye in deionisedwater.

2.2. Adsorbent preparation

Banana stalk was washed and dried at 120 ◦C overnight to remove the moisture content. It wascut into small pieces and then carbonised at 700 ◦C under nitrogen flow (99.999%) for 1 h (firstpyrolysis). A certain amount of produced char was then soaked with potassium hydroxide (KOH)at an impregnation ratio of 1:1 (KOH pellets : char). The mixture was dehydrated in an ovenovernight at 105 ◦C, then pyrolysed in a stainless steel vertical tubular reactor placed in a tubefurnace under nitrogen flow of 150 cm3·min−1 (second pyrolysis) to a final temperature of 800 ◦C.Once the final temperature was reached, the gas flow was switched to CO2 and activation wascontinued for 2 h. The sample was then cooled to room temperature under nitrogen flow andwashed with 0.1 M HCl. It was later washed with deionised water until the pH of the washingsolution reached 6.5–7.

2.3. Batch equilibrium studies

Batch equilibrium tests were carried out on adsorption of MG dye on BSAC. The effects of initialdye concentration, contact time, temperature and solution pH on dye uptake were investigated.

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Sample solutions were withdrawn at equilibrium to determine residual concentrations. Solutionswere filtered prior to analysis in order to minimise the interference of the carbon fines. Forequilibrium studies, the experiment was carried out for 4 h to ensure that equilibrium was reached.The linear Beer–Lambert relationship between absorbance and concentration with the calibrationcurve was established by plotting the graph of absorbance versus concentration of the dye solution.The concentration of MG dye solution before and after adsorption was determined using a doubleUV-Vis spectrophotometer (UV-1800 Shimadzu, Japan). The amount of MG dye adsorbed atequilibrium, qe (mg·g−1) was calculated according to Equation (1).

qe = (C0 − Ce)V

W(1)

where Co and Ce (mg·L−1) are the liquid-phase concentrations of MG dye initially and at equi-librium, respectively. V (L) is the volume of the solution and W (g) is the mass of dry adsorbentused. For the batch kinetic studies, the same procedure was followed, but the aqueous sampleswere taken at preset time intervals. The concentrations of MG dye were similarly measured. TheMG dye uptake at any time, qt (mg·g−1), was calculated.

2.4. Effect of initial MG dye concentration, contact time and solution temperature

One hundred millilitres of MG dye solution with initial concentrations of 50–400 mg·L−1 wereprepared in a series of 250-mL Erlenmeyer flasks. 0.20 g of the BSAC was added to each flask,covered with glass stopper and the flasks were then placed in an isothermal water bath shaker(Model Protech, Malaysia) at 303 K with rotation speed of 120 rpm, until equilibrium was reached.The effect of solution temperature on the MG dye adsorption process was examined by varyingthe adsorption temperature at 303, 318 and 333 K, respectively. This was done by adjusting thetemperature control unit of the water bath shaker.

2.5. Effect of solution pH

The effect of solution pH on the MG dye adsorption process was studied by varying the initialpH of the solution from 3 to 10. The pH was adjusted by adding 0.1 M hydrochloric acid and/or0.1 M sodium hydroxide, and was measured using pH meter (Model Delta 320, Mettler Toledo,China). The MG dye initial concentration was fixed at 100 mg·L−1 with an adsorbent dosage of0.2 g. The solution temperature was maintained at 303 K. Percentage dye removal was calculatedusing Equation (2).

Removal(%) = (C0 − Ce)

C0× 100 (2)

2.6. Characterisation of BSAC

Fourier transform infrared (FTIR) spectroscopic analysis was performed using FTIR spectro-scope (FTIR-2000, Perkin–Elmer). The spectra were measured from 4000 to 400 cm−1. Texturalcharacterisation of BSAC was determined by using N2 adsorption at 77 K using Autosorb I sup-plied by Quantachrome Corp., USA. The Brunauer Emmett Teller (BET) (N2, 77 K) is the mostusual procedure used in characterising activated carbon. The surface morphology of the sampleswas examined using a scanning electron microscope (Model VPFESEM Supra 35VP). Proximateanalysis was carried out using a thermogravimetric analyser (TGA) (Perkin–Elmer TGA7, USA)and elemental analysis was performed using elemental analyser (Perkin–Elmer Series 11, 2400,USA). Fifty millilitres of a 0.01 M NaCl solution was placed in each of a set of 10 Erlenmeyer

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flasks (250 mL). The pH was adjusted to between 2 and 12 by adding 0.1 M HCl or 0.1 M NaOHsolution. Then 0.15 g of each BSAC sample was added and agitated at 150 rpm for 48 h underatmospheric conditions. The final pH was measured and the results were plotted with pH (initialpH – final pH) against final pH. The pHpzc is the point where the curve pHfinal versus pHinitial

crosses the line pHinitial = pHfinal [18]. The total surface basicity and acidity of the samples weredetermined by titration with NaOH and HCl using Boehm titration method [19]. BSAC (0.15 g)was mixed with 50 mL of 0.05 N NaOH or HCl solutions for 48 h with continuous stirring. A 10-mL aliquot of each filtrate were then titrated against 0.05 N HCl or NaOH, using phenolphthaleinas indicator. Capacities for H+ and OH− were then measured.

3. Results and discussion

3.1. The effect of pH

The pH of the system and the zero point charge of the adsorbent (pHpzc) exert profound influenceon the adsorptive uptake of MG dye molecule presumably due to its influence on the surfaceproperties of the BSAC and ionisation/dissociation of the MG dye molecule. The zero pointcharge was found to occur at pH 4.5 meaning that BSAC surface has a positive charge in solutionup to pH 4.5 and a negative charge above this pH (Figure 1). Oxygen functional groups areimportant characteristics of activated carbon because they determine the surface properties ofcarbons and hence their quality as ion exchangers, adsorbents, catalysts, and catalyst supports[20]. The contents of oxygen-containing functional groups with various acidic groups (carboxyl,lactonic and phenolic), as well as the total amount of the basic groups are presented in Table 1.The acidic groups content is much higher than the basic groups content. This is consistent with avalue of pHpzc 4.5, showing the dominance of acidic groups. The effect of pH on the adsorption

Figure 1. Zeta potential vs. pH curve of BSAC.

Table 1. Characteristics of BSAC.

BSAC Surface chemistry

Carboxylic (meq·g−1) 0.1832Lactonic (meq·g−1) 0.2418Phenolic (meq·g−1) 0.2749Acidic (meq·g−1) 0.6999Basic (meq·g−1) 0.1582pHpzc 4.5

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Figure 2. Effect of pH on the adsorption of MG dye by BSAC. Conditions: adsorbent dosage, 0.2 g; 120 rpm, agitationtime, 120 min; temperature, 303 K.

of MG dye on BSAC was studied using MG dye initial concentration of 100 mg·L−1 and 0.2 gBSAC in the pH range 2–9 (Figure 2). Under acidic conditions (pH 3.0–4.0) the amount of MGdye uptake was very small, whereas adsorption was enhanced at pH >4.0. At lower pH values,the surface charge of BSAC is positive, thus, MG dye adsorption on BSAC is not favourablebecause of electrostatic repulsion between the BSAC surface and the positively charged MG dyecation. When pH was increased, electrostatic repulsion between the MG dye and BSAC surfacesites and the competing effect of hydronium ions decreased, the BSAC surface became negativelycharged facilitating binding of the MG dye cation via electrostatic attraction, hence, MG dyeuptake increased. A similar result was obtained for the adsorption of MG dye onto activatedcarbon derived from Borassus aethiopum flower biomass [21].

3.2. Effect of temperature and initial dye concentration

Experiments were performed at three different temperatures; 303, 318 and 333 K (Table 2). Max-imum equilibrium sorption capacity was obtained at 333 K. The amount of MG dye adsorbed atequilibrium increased from 35.41 to 335.67 mg·g−1 (Table 3). This is mainly due to the increasedsurface activity, suggesting that adsorption of MG dye onto BSAC was an endothermic process. Asimilar trend was observed with the use of dead plane tree leaves for the sorption of MG dye [22].Figure 3 shows the effect of different initial dye concentrations (50–400 mg·L−1) on the amountof dye adsorbed on BSAC. It is evident from Figure 3 that the amount of MG dye adsorbedincreases with an increase in the initial concentration of dye. Equilibrium was attained in 90 minat an MG dye concentration of 50–200 mg·L−1. However, equilibrium was attained in 120 min at300–400 mg·L−1 dye concentration. The initial dye concentration provides the necessary drivingforce to overcome resistance to the mass transfer of MG dye between the aqueous phase and thesolid phase. The increase in the initial dye concentration also enhances the interaction betweenMG and BSAC. Therefore, an increase in the initial concentration of MG dye enhances its adsorp-tion uptake. This is due to an increase in the driving force of the concentration gradient, as a resultof increase in initial dye concentration. A similar trend has been reported previously [23].

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Table 2. Isotherm constants and correlation coefficient for the adsorp-tion of MG onto BSAC at different temperatures.

T (K)

Models 303 318 333

Langmuir isothermqmax (mg·g−1) 125.14 134.33 141.76KL (L·mg−1) 0.0044 0.0051 0.0062RL 0.12 0.15 0.16R2 0.99 0.99 0.99

Freundlich isothermkF 28.19 31.33 33.52n 2.29 2.32 2.38R2 0.97 0.98 0.97

D-R isothermqo (mg·g−1) 58.73 66.18 75.02β (mol2·kJ−2) 0.056 0.048 0.039R2 0.98 0.98 0.98Ea (kJ·mol−1) 2.99 3.23 3.58

Table 3. Comparison of pseudo-first-order, pseudo-second-order and intraparticle diffusionkinetic model rate constants calculated from experimental data at different concentrations.

Co (mg·L−1)

Models 50 100 200 300 400

Pseudo-first orderqe,(exp) (mg·L−1) 35.41 72.31 167.71 225.67 335.67k1 (min−1) 0.19 0.27 0.38 0.47 0.52qe,(cal) (mg·L−1) 26.54 58.19 141.63 198.87 312.39R2 0.93 0.94 0.97 0.97 0.97

Pseudo-second orderk2 (g·mg−1·min−1) 0.67 0.61 0.56 0.43 0.36qe,(cal) (mg·L−1) 35.76 73.45 168.96 226.84 337.68R2 0.99 0.99 0.99 0.99 0.99

Intraparticle diffusionkdiff (mg·g−1·min−1/2) 1.78 2.62 3.01 3.89 4.52C (mg·g−1) 6.25 7.58 8.92 10.77 12.19R2 0.84 0.89 0.82 0.85 0.91

Figure 3. Effect of initial concentration on the adsorption of MG dye by BSAC. Conditions: adsorbent dosage, 0.2 g;120 rpm, agitation time, 240 min; temperature, 303 K.

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3.3. Characterisation of BSAC

Results of proximate and elemental analyses of samples are shown in Table 4. From the proximateanalysis, it was found that moisture, ash and volatile matter contents are low, however, the fixedcarbon content is satisfactory for dye adsorption when compared with raw and charred bananastalks. Elemental analysis showed high elemental carbon with insignificant values for hydrogenand sulfur. The specific surface area of the BSAC was calculated using the BET equation [24]. Itwas found that the BET surface area, pore volume and average pore diameter of the BSAC were684.11 m2·g−1, 0.482 cm3·g−1 and 2.84 nm, respectively (Table 5). The surface of BSAC containswell-developed pores for MG dye to be adsorbed. BSAC also possess a high surface area, makingit more efficient for MG dye removal. The high BET surface area and total pore volume of theBSAC could be attributed to the activation process used. Pore development in the char duringcarbonisation also helped enhance the BET surface area and pore volume of the activated carbonby promoting the diffusion of KOH and CO2 molecules into the pores, thus creating more poresin the activated carbon.

The FTIR spectroscopic characteristics are shown in Table 6. The FTIR spectroscopic analysisindicated a broad band at 3422 cm−1, representing bonded –OH groups. The band observed at∼2920–2849 cm−1 was assigned to the aliphatic C−H group. The peak around 1628 cm−1 corre-sponds to the C=O stretch.The peaks observed at 1544 and 1517 cm−1 correspond to the secondaryamine group. Symmetric bending of CH3 is observed to shift to 1450, 1432 and 1347 cm−1. Thus,it seems that this type of functional group is likely participating in MG dye binding. The peaksobserved at 1228, 1143 and 637 cm−1 were assigned to –SO3 stretching, C−O stretching of ethergroups and –CN stretching, respectively. As seen in Table 6, spectral analysis before and afterMG dye adsorption indicated that mostly the bonded –OH groups, C=O stretching, secondaryamine group and symmetric bending of CH3 were involved in MG dye adsorption. There wereclear band shifts and an intensity decrease for the band at 3422, 1628, 1544 and 1450 cm−1. Theseband shifts suggest that there is attachment of MG dyes on BSAC [25]. The SEM image (Figure S1,Supplementary material, available online only) at 1000× magnification revealed the nature of thesurface of the raw banana stalk and BSAC, respectively. From Figure S1a (Supplementary mate-rial, available online only), the pores on the raw banana stalk were not well developed, however,in Figure S1b (Supplementary material, available online only), BSAC has well-developed surfacewith heterogeneous pores and cavities. This indicates that there is a good possibility for MG dyeto be trapped and adsorbed into BSAC surface.

Table 4. Proximate and elemental analysis of BSAC.

Proximate analysis (%) Elemental analysis (%)

Sample Moisture Volatile Fixed carbon Ash C H S (N + O)a

Raw BS 14.36 73.91 5.83 5.90 9.32 4.38 0.10 86.20BS char 6.54 31.25 59.35 2.86 66.31 3.65 0.06 29.98BSAC 2.91 18.64 73.92 4.53 75.28 1.44 0.05 23.23

Note: aEstimated by difference.

Table 5. Surface area and pore characteristics of BSAC.

BET surface area Total pore volume Average pore diameterSample (m2·g−1) (cm3·g−1) (nm)

BS char 145.97 0.225 2.68BSAC 684.11 0.482 2.84

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Table 6. FTIR spectra characteristics.

Wave number (cm−1)

IR peak Before adsorption After adsorption Differences Assignment

1 3422 3417 −5 Bonded –OH groups2 2920 2923 +2 Aliphatic C−H group3 2849 2850 +1 Aliphatic C−H group4 1628 1625 −3 CO stretching5 1544 1534 −20 Secondary amine group6 1517 1515 −2 Secondary amine group7 1450 1445 −5 Symmetric bending of CH38 1432 1433 +1 Symmetric bending of CH39 1347 1367 +20 Symmetric bending of CH3

10 1228 1225 −3 –SO3 stretching11 1143 1143 0 C−O stretching of ether groups12 637 637 0 –CN stretching

3.4. Adsorption kinetics

3.4.1. The pseudo-first-order kinetics model

The pseudo-first-order equation is generally expressed as follows [26]

ln(qe − qt) = qe − k1t (3)

where qe and qt are the adsorption capacities at equilibrium and time t, respectively (mg·g−1).k1 is the rate constant for pseudo-first-order adsorption (min−1). Plots of ln(qe − qt) againsttat various dye concentrations and temperatures resulted in linear graphs with negative slopes(Figure 4). k1 and qe are calculated from the slopes and intercepts respectively. Although thecorrelation coefficients (R2) were high, the qe calc and the qe exp. values do not agree (Table 3).Therefore, the adsorption of MG dye onto BSAC does not follow the pseudo-first-order kinetics.

3.4.2. The pseudo-second-order kinetics model

The pseudo-second-order kinetics equation is expressed as [27].

dqt

dt= k2(qe − qt)

2 (4)

Figure 4. Pseudo-first-order plot for the adsorption of MG dye on BSAC at different concentrations.

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where k is the rate constant of pseudo-second-order adsorption (g·mg−1·min−1). For the boundaryconditions t = 0 to t = t and qt = 0 to qt = qt , the integrated form of the equation becomes

1

qe − qt= 1

qe+ k2t (5)

Rearranging this equation, we have a linear form:

t

qt= 1

k2q2e

+ 1

qet (6)

plots of t/qt versus t gave linear graphs from which qe and k2 were estimated from the slopes andintercepts of the plot (Figure 5). The R2 values were as high as 0.99 and there were good agreementbetween qe cal and qe exp (Table 3). The good agreement shows that the pseudo-second-orderkinetics equation fits the adsorption data well.

3.4.3. Intraparticle diffusion model

The intraparticle diffusion equation is given as [28]

qt = kdiff t1/2 + C (7)

where qt is the amount of MG dye adsorbed (mg·g−1) at time t, and kdiff (mg·g−1·min−1/2) is therate constant for intraparticle diffusion. The value of Cgives an idea about the thickness of theboundary layer, the larger the intercept the greater the boundary layer effect. According to thismodel, the plot of uptake, qt , versus the square root of time, t1/2 should be linear if intraparticlediffusion is involved in the adsorption process, and if these lines pass through the origin, thenintraparticle diffusion is the rate controlling step. The rate constant kdiff , C and R2 are shown inTable 3. The correlation coefficients (R2) for the intraparticle diffusion model are between 0.82and 0.91, indicating that adsorption of MG dye onto BSAC may be controlled by an intraparticlediffusion model. Figure 6 shows the plot of uptake for MG dye versus square root time at differentinitial dye concentrations. The plots do not pass through the origin, this is indicative of some degreeof boundary layer diffusion.

Figure 5. Pseudo-second-order plot for the adsorption of MG dye on BSAC at different concentrations.

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Figure 6. Intraparticle diffusion plots for the adsorption of MG dye on BSAC at different initial dye concentrations.Conditions: temperature, 303 K; adsorbent dosage, 0.2 g; 120 rpm.

3.5. Adsorption isotherms

3.5.1. Langmuir isotherm model

The linearised form of the Langmuir adsorption model is expressed as [29]

Ce

qe= Ce

qo+ 1

qoKL(8)

where Ce is the MG dye concentration in the solution at equilibrium (m·L−1), qe is the MG dyeconcentration on the adsorbent at equilibrium (mg·g−1), qo is the monolayer adsorption capacityof adsorbent (mg·g−1) and KL is the Langmuir adsorption constant (L·mg−1). A plot of Ce/qe

against Ce gave a straight line with a slope 1/qo and an intercept of 1/qoKL (Figure not shown).Values of qo and KL are calculated and reported in Table 2. The value of qo obtained was comparedwith those from other adsorbents. BSAC proved to be a good adsorbent for the adsorption of MGdye from aqueous solutions (Table 7). To confirm the favourability of the adsorption process, thedimensionless equilibrium parameter (RL) expressed by Equation (9) was used

RL = 1

(1 + KLCo)(9)

where Co is the highest initial dye concentration in solution, is used to confirm the favourabilityof the adsorption process, that is (0 < RL < 1) favourable, RL = 1 linear, RL = 0 irreversible orRL > 1 unfavourable [29]. The values of RL reported in Table 2 obtained at various temperatureswere <1, indicating that the adsorption of MG dye onto BSAC is favourable.

3.5.2. Freundlich isotherm model

The linearised form of Freundlich model is expressed as [30].

log qe = 1

nlog Ce + log kf (10)

where qe is the amount of MG dye adsorbed at equilibrium (mg·g−1), Ce is the equilibriumconcentration of the adsorbate (mg·L−1); kf and n are constants incorporating the factors affectingthe adsorption capacity and the degree of non-linearity between the solute concentration in the

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Table 7. Comparison of adsorption capacities of MG dye untovarious adsorbents.

Adsorbent qo (mg·g−1) Ref

Lemon peel 51.7 [34]Jute fibre carbon 137 [35]Oil palm fibre 150 [36]Bentonite 7.72 [37]Chitosan beads 93.5 [38]Sugarcane dust 4.88 [39]Activated charcoal 0.180 [40]Iron humate 19.2 [41]Activated slag 74.2 [42]Cyclodextrin-based adsorbent 91.9 [43]BSAC 141.76 This study

solution and the amount adsorbed at equilibrium respectively. Plots of log qe versus log Ce gavelinear graphs (Figure not shown) with high R2. Comparing the R2 values of the two isotherms,the adsorption data fits the Langmuir model better (Table 2). The values of kf and nobtained fromthe slopes and intercepts of the graph are reported in Table 2. Values of n > 1 indicate that theadsorption is favourable.

3.5.3. Dubinin–Radushkevich isotherm model (D–R model)

The D–R model [31] was used to estimate the characteristic porosity of BSAC and the apparentenergy of adsorption. It is expressed as

ln qe = ln qo − βε2 (11)

where β is the free energy of sorption per mole of the sorbate as it migrates to the surface of BSACfrom an infinite distance in the solution (mol2·kJ−2), qo is the maximum adsorption capacity andε is the Polanyi potential (J·mol−1) that is expressed as

ε = RT ln

(1 + 1

Ce

)(12)

where R and T are the gas constant (kJ·mol−1·K−1) and the absolute temperature (K), respectively.A plot of ln qe versus ε2 gave a linear plot (not shown) where β and qo are obtained from the slopesand intercepts respectively (Table 2). Similarly, the β value obtained was then used to estimatethe mean free energy of adsorption, E (Table 2).

E =√

1

2β(13)

The values of E were found to be in the range 2.99–3.58 kJ·mol−1 over the range of temperaturesused in this study. Because E < 8 kJ·mol−1, it suggests that the adsorption mechanism is physicalin nature [32].

3.6. Thermodynamic studies

Thermodynamic parameters; �G0, �H0 and �S0 were also determined to investigate the fea-sibility, spontaneity and the nature of the interaction. This was achieved by using the equations

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Figure 7. Van’t Hoff plot of the adsorption of MG dye onto BSAC.

below

ln KL = �S0

R− �H0

RT(14)

�G0 = −RT ln KL (15)

where KL is the Langmuir constant, T is the temperature in K and R is the gas constant. Aplot of ln KL against 1/T gave linear plot (Figure 7) from which �H0 and �S0 values wereobtained from the slope and intercept respectively (Table 8). The negative values of �G0 indicatethat the adsorption of MG dye onto BSAC is spontaneous and thermodynamically favoured.Moreover, when the temperature increases from 303 to 333 K, �G0 changes from −20.94 to−23.97 kJ·mol−1, suggesting that adsorption is more spontaneous at higher temperature. Thepositive value of �H0 (8.924 kJ·mol−1) indicates that the process is endothermic in nature, whichis supported by the increase in the adsorption capacity of BSAC for MG dye with increasingtemperature as observed from Table 3. Moreover, the positive value of �S00.128 kJ·mol−1·K−1

suggests that there was increased randomness at the solid-liquid interface during the adsorptionof MG dye onto BSAC.

3.7. Desorption of MG dye from BSAC

In other to decrease adsorption process cost and the dependence of the process on continuoussupply of adsorbent, desorption studies were embarked upon. This process was also aimed atmaking the adsorbent retain its original form, and ensuring that it does not lose its adsorptioncapacity. The desorption process requires proper selection of eluent; which strongly depends onthe type of adsorbent and the mechanism of adsorption [33]. The selected eluent must be effective,

Table 8. Thermodynamic parameters for the adsorption of MG unto BSAC at different temperatures.

Temp (◦C) K (L·mol−1) �G0 (kJ·mol−1) �H0 (kJ·mol−1) �S0 (kJ·mol−1·K−1)

303 4078.8 −20.943 8.924 0.128318 4727.7 −22.370333 5747.4 −23.966

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Table 9. Percentage MG dye adsorbed on regenerated BSAC.

Percentage MG dye desorbed

Repeated adsorption–desorption studies

with 0.5 M HCl

Dye Adsorbent without HCl I Cycle II Cycle III Cycle IV Cycle

MG dye BSAC 54.71 90.22 92.74 94.89 95.16

harmless to the adsorbent, non-polluting and cheap. For this purpose, dilute solutions of mineralacids such as HCl, H2SO4, CH3COOH and HNO3 are always employed. In this study, HCl solutionwas selected as an eluent to desorb MG dye ions from BSAC. In acidic conditions, MG dye isdisplaced by protons from their binding sites. It was subjected to adsorption–desorption cyclesthrough regeneration step in between. The regeneration efficiency increased from 90.22 to 95.16%when 0.5 M HCl was used in desorbing MG dye from BSAC. There was a decrease in efficiencywhen the adsorbent was used without HCl treatment (Table 9).

4. Conclusion

The study shows that optimum dye removal was at pH 8.0 with MG dye removal efficiency of97.3%. The equilibrium time for adsorption of MG dye from aqueous solution was achieved within90 and 120 min contact time. Sorption energy obtained from D–R isotherm for the adsorptionprocess were found to range from 2.99–3.58 kJ·mol−1 indicating that the mechanism of adsorptionof MG dye onto BSAC is physical in nature. The study revealed that the efficiency of adsorptionwas retained only when BSAC after adsorption was given a treatment with 0.5 M HCl solutionat least after four cycles. The study showed the suitability of BSAC for MG dye contaminatedwastewaters.

Acknowledgements

The authors gratefully acknowledge the financial support received in the form of research grants from Universiti SainsMalaysia that resulted in this article. The 1-year Post Doctoral Fellowship jointly awarded by USM-TWAS to Dr OlugbengaSolomon Bello (FR Number: 3240223483 inYear 2009) of the Department of Pure andApplied Chemistry LadokeAkintolaUniversity of Technology, P.M.B 4000, Ogbomoso, Oyo State. Nigeria and the 12-month study leave granted him by hishome institution to honor this fellowship are both acknowledged.

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