Adsorptive Removal of Malachite Green Dye Using Durian Seed-Based Activated Carbon
Article in Water Air and Soil Pollution · August 2014
DOI: 10.1007/s11270-014-2057-z
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Adsorptive Removal of Malachite Green Dye Using Durian Seed-Based Activated Carbon
Mohd Azmier Ahmad&Norhidayah Ahmad&
Olugbenga Solomon Bello
Received: 1 March 2014 / Accepted: 2 July 2014 / Published online: 19 July 2014
#Springer International Publishing Switzerland 2014
Abstract Chemically prepared activated carbon de- rived from durian seed (DSAC) was used as adsorbent to adsorb Malachite green (MG) dye. The prepared DSAC was characterized using Brunauer–Emmet–Tell- er (BET), Fourier transform infrared (FTIR), scanning electron microscope (SEM), and proximate analysis, respectively. Batch adsorption studies were carried out for the removal of MG dye from aqueous solutions by varying operational parameters like contact time, initial MG dye concentration, solution temperature, and initial solution pH. Maximum dye removal of 97 % was ob- tained at pH 8. Experimental data were analyzed by eight model equations—Langmuir, Freundlich, Temkin, Dubinin–Radushkevich, Radke–Prausnitz, Sips, Vieth–
Sladek, and Brouers–Sotolongo isotherms—and it was found that the Freundlich isotherm model fitted the adsorption data the most. Adsorption rate constants were determined using pseudo-first-order and pseudo- second-order rate equations, Elovich, intraparticle diffu- sion, and Avrami kinetic model. The results clearly showed that the adsorption of MG dye onto DSAC followed the pseudo-second-order model, and the mech- anism of adsorption was controlled both by film diffu- sion and intraparticle diffusion. Thermodynamic
parameters such asΔG,ΔH, andΔSwere also calculated for the adsorption process. The process was found to be spontaneous and endothermic in nature. This work pro- vided an attractive adsorbent for the removal of MG dye from wastewaters.
Keywords Durian seeds . Malachite green dye . Adsorption . Spontaneous
1 Introduction
Dyeing is a fundamental operation during textile pro- cessing. There are various types of dyes that are com- monly used such as basic dye, reactive dye, and acid dye. Basic dyes such as Malachite green (MG) and methylene blue (MB) have chromophores with positive ion and amino group in their molecule which are water soluble. It has favorable characteristics of high brilliance and intensity of colors; it is highly visible even in very low concentrations. It is used for dyeing silk, cotton, nylon, coir, and wool (Ahmad and Alrozi 2011). Ac- cording to Nethaji et al. (2010), about 10,000 different dyes weighing approximately 0.7 million tons/year were produced annually worldwide with 10–15 % of these dyes released into the effluent during the dyeing pro- cess. These large amounts of spent dye baths resulted from inefficiency of the dyeing process. The discharge of these compounds into water causes several ecological and environmental problems owing to their toxic, non- biodegradable, and potentially carcinogenic nature (Baskaralingam et al. 2006). Such colored effluents DOI 10.1007/s11270-014-2057-z
M. A. Ahmad
:
N. Ahmad:
O. S. BelloSchool of Chemical Engineering, Universiti Sains Malaysia, Engineering Campus, 14300 Nibong Tebal, Penang, Malaysia O. S. Bello (*)
Department of Pure and Applied Chemistry, Ladoke Akintola University of Technology,
P.M.B 4000, Ogbomoso, Oyo State, Nigeria e-mail: [email protected]
consume dissolved oxygen from the stream and prevent reoxygenation in receiving water. The complex molec- ular structure of dyes makes them resistant to biological or even chemical degradation. This also renders the treatment methods insufficient and costly (Koprivanac and Kusic2008). Several adsorbents have been used to remove Malachite green dye from solutions, and some of them include the following: potato peel (Guechi and Hamdaoui2011) of potato (Solanum tuberosum), plant wastes (Gupta et al.2011), fruit waste (Parimaladevi and Venkateswaran,2011), cashew nut bark Alfa Universal (Parthasarathy et al. 2011), Annona squamosa seed (Santhi et al. 2011), and Limonia acidissima (wood apple) shell (Sartape et al.2013). In a quest for a better and efficient adsorbent for MG dye, modified durian seed was selected.
Durian (Durio zibethinusMurray) is the most popu- lar seasonal fruit in Southeast Asia, particularly Malay- sia, Indonesia, Thailand, and the Philippines, and be- longs to the family Bombacaceae (Booncherm &
Siriphanich, 1991). A significant percentage of the planted durian fruit crop is wasted each year. Only one third of durian fruit is edible, whereas the seeds (20–
25 %) and the shells are usually discarded. The wasted durian seed represents a significant potential for the development of value-added products. To the best of our knowledge, no report has been documented on the removal of Malachite green dye using durian seed;
hence, this study investigates the preparation of durian seed-based activated carbon using physicochemical ac- tivation, characterization of the prepared adsorbent, and study of various adsorption parameters as follows: initial dye concentration, contact time, temperature, solution pH, adsorption isotherms, and kinetic and thermody- namic studies of the adsorption of MG dye onto durian peels.
2 Materials and Methods
2.1 Activated Carbon Preparation
Durian fruits (DS) were purchased from a local market in Parit Buntar area, Perak, Malaysia. The fruit was dehusked (the rind was cut open), by cutting along the suture on the back of the lobules. Durian seeds were removed, cleaned, and rinsed thoroughly with distilled water. It was then dried to constant weight and stored in an air-tight container for further use. Ten grams of
durian seed was placed in a vertical tubular reactor.
Nitrogen gas was purged into the reactor to create inert condition. The flow rate of nitrogen gas and the heating rate were held at 150 cm3/min and 10 °C/min, respec- tively. The temperature was ramped from room temper- ature to 700 °C and held for 1 h. Then the reactor was cooled down to room temperature. The char produced was stored in an air-tight container for further treatment.
The char produced was then impregnated. The impreg- nation ratio (IR) was calculated using
IR¼wKOH
wchar ð1Þ
wherewKOH wKOHis the dry weight (g) of potassium hydroxide pellet andwcharis the dry weight (g) of char.
The char and KOH powder (depending on the IR) were mixed together with deionized water in a 250-mL bea- ker. The mixture was stirred thoroughly until it dried in an oven overnight at 105 °C for dehydrating purpose.
The KOH-impregnated char was placed inside the ver- tical tubular reactor for activation process. The system was purged under nitrogen flow of 150 cm3/min. The temperature was ramped from ambient temperature to the activation temperature at a heating rate of 10 °C/min.
Once the desired activation temperature was reached, the gas flow was switched to carbon dioxide at a flow rate of 150 cm3/min to complete the activation process.
Then, the reactor was cooled to room temperature under nitrogen flow. The sample was washed with 0.1 M HCl.
It was further washed with deionized water several times until the pH of the washing solution reached 6.5–7. The pH was measured using a pH meter (Model Delta 320, Mettler Toledo, China). Filter paper and filter funnel were used in the washing process. The washed sample was kept in an oven at 105 °C for 12 h. The dried sample which was the activated carbon derived from durian seed (DSAC) was stored in air-tight containers for fur- ther characterization and adsorption studies. The DSAC yield was calculated using the equation:
Yield %ð Þ ¼ wc
w0100 ð2Þ
wherewcandw0are the dry weight of DSAC (g) and the dry weight of precursor (g), respectively.
2.2 Adsorbate Used
MG was used as adsorbate to determine the adsorption performance of the prepared activated carbon. The prop- erties of MG dye used are listed in Table1.
2.3 Batch Equilibrium Studies
The effects of initial dye concentration, contact time, solution temperature, and solution pH on the adsorption uptake of MG dye on DSAC were studied. Sample solutions were withdrawn at intervals to determine the residual concentration by using a UV–visible spectro- photometer at the maximum wavelength of 618 nm. The amount of dye adsorbed at equilibrium,qe(mg/g) was calculated as:
qe¼ðCo−CeÞV
W ð3Þ
whereCoandCe(mg/L) are the liquid-phase concentra- tions of initial adsorbate and equilibrium, respectively.V is the volume of the solution (dm3) andWis the mass (g) of DSAC used.
2.4 Effect of Initial Adsorbate Concentration and Contact Time
One hundred milliliters of MG dye solution with known initial concentration was placed in a series of 250 mL Erlenmeyer flasks. The amount of adsorbent that was added into each flask was fixed at 0.1 g. The flasks were placed in an isothermal water bath shaker (Model Protech, Malaysia) at a constant temperature of 30 °C, with rotation speed of 120 rpm, until an equilibrium
point was reached. Samples are withdrawn at intervals to determine the residual concentration of the dye at 6 1 8 n m w a v e l e n g t h u s i n g a U V–v i s i b l e spectrophotometer.
2.5 Effect of Solution Temperature
The effect of solution temperature on the adsorption process was carried out by varying the adsorption tem- perature at 30, 45, and 60 °C by adjusting the tempera- ture controller of the water bath shaker, while other operating parameters such as adsorbent dosage (0.1 g) and rotation speed (120 rpm) were kept constant.
2.6 Effect of Solution pH
Solution pH was studied by varying the initial pH of solution from 2 to 12. The pH was adjusted by 0.1 M NaOH or 0.1 M HCl and measured by using a pH meter.
The adsorbent dosage, rotation speed, solution temper- ature, and initial dye concentration were fixed at 0.1 g, 120 rpm, 30 °C, and 100 mg/L respectively.
2.7 Adsorption Isotherm Studies
This was carried out by fitting the equilibrium data to the Langmuir (Langmuir, 1918), Freundlich (Freundlich, Table 1 Properties of
malachite green (Santhi et al.2010a)
Properties
Chemical name 4-[(4-Dimethylaminophenyl)-phenyl-methyl]-N,N-dimethyl-aniline
Common name Malachite green hydrochloride
Generic name Basic green 4
CAS number 123333-61-9
Color index number 42000
Ionization Basic
Maximum wavelength 618 nm
Empirical formula C23H26N2O·HCl
Molecular weight 382.93 g/mol
Chemical structure
1906), Temkin (Temkin and Pyzhev, 1940), Dubinin–
Radushkevich (Dubinin and Radushkevich,1947), Sips (Sips, 1948), Vieth–Sladek (Vieth and Sladek, 1965), Brouers–Sotolongo (Brouers et al.2005), and Radke–
Prausnitz (Radke and Prausnitz, 1972) isotherms. The applicability and suitability of the isotherm equation to the equilibrium data were compared by judging the values of the correlation coefficients,R2andΔqe. Linear regression was carried out by using Microsoft Excel spreadsheet with Solver add-in to determine the iso- therm parameters.
2.8 Batch Kinetic Studies
This procedure is similar to that of batch equilibrium studies. The difference is that the adsorbent–adsorbate solution was taken at preset time intervals and the con- centration of the solution was measured. The amount of adsorption at time t, qt (mg/g), was calculated using Eq.4:
qt¼ðCo−CtÞV
W ð4Þ
whereCoandCt(mg/L) are the liquid-phase concentra- tions of adsorbate at initial and at any timet, respective- ly.Vis the volume of the solution andWis the mass of adsorbent used. The adsorption kinetics of the dye on adsorbent was investigated using the pseudo-first-order model (Lagergren 1898), pseudo-second-order model (Ho and McKay, 1999), Avrami model (Avrami, 1940), and Elovich model (Aharoni and Ungarish 1976), respectively.
2.8.1 Validity of the Kinetic Model
The applicability and fitting of the isotherm equation to the kinetic data was compared by judging from theR2 values and the normalized standard deviationΔqt(%) calculated from Eq.5. The normalized standard devia- tion,Δqt(%), was used to verify the kinetic models used to describe the adsorption process. It is defined as:
Δqtð Þ ¼% 100
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ΣnT qexp−qcal
.
qexp
h i2
r
n−1 ð5Þ
wherenis the number of data points, andqexpandqcal
(mg/g) are the experimental and calculated adsorption
capacity values. A lower value ofΔqtindicates a good fit between the experimental and calculated data.
2.9 Adsorption Thermodynamics
The experimental data obtained from batch adsorption studies performed earlier were analyzed by using the thermodynamic equations as expressed by Eqs.6:
ΔG¼−RTlnKL ð6Þ
LnK¼ΔS R −ΔH
RT ð6aÞ
ΔGwas calculated using Eq.6. The values ofΔH and ΔS can be obtained, respectively, from the slope and intercept of Van’t Hoff plot of lnKLversus 1 / T.
Values ofKLmay be calculated from the relation lnqe/ Ce at different solution temperatures of 30, 45, and 60 °C, respectively. Arrhenius equation has been ap- plied to evaluate the activation energy of adsorption representing the minimum energy that reactants must have for the reaction to proceed, as shown by the fol- lowing relationship:
lnk2¼lnA−Ea
RT ð6bÞ
wherek2is the rate constant obtained from the pseudo- second-order kinetic model (g/mg h),Eais the Arrhenius activation energy of adsorption (kJ/mol),Ais the Arrhe- nius factor, R is the universal gas constant (8.314 J/
mol K), andTis the absolute temperature. When lnk2
is plotted against 1 /T, a straight line with slope of−Ea/ Ris obtained.
2.10 Adsorption Mechanism
The adsorption mechanisms of MG dye on the adsor- bent were investigated using intraparticle diffusion and Boyd models represented by Eqs.7and8, respectively.
The applicability and fitting of the model throws more light into the mechanism of MG dye adsorption onto the DSAC prepared.
2.10.1 Intraparticle Diffusion Model
The intraparticle diffusion model (Weber and Morris, 1962) is expressed as shown in Eq.7
qt¼kpit12þCi ð7Þ
where Ci is the intercept and kpi (mg/g h1/2) is the intraparticle diffusion rate constant, which can be eval- uated from the slope of the linear plot ofqtversust1/2. Theqtis the amount of solute adsorb per unit weight of adsorbent per time (mg/g), andt1/2is the half adsorption time (g/h mg). The intercept of the plot reflects the boundary layer effect. The larger the intercept, the great- er the contribution of the surface sorption in the rate- controlling step. If the regression of qt versus t1/2 is linear and passes through the origin, then intraparticle diffusion is the sole rate-limiting step. If the linear plots at each concentration did not pass through the origin, it indicates that the intraparticle diffusion was not only a rate-controlling step (Wu et al.2005).
2.10.2 Boyd Model
In order to identify the slowest step in the adsorption process, Boyd kinetic equation (Boyd et al.1947) was applied, and it is expressed as:
F¼1− 6
π2eBt ð8Þ
F¼qt
qe ð8aÞ
whereqtis the amount of the adsorbate adsorbed at time t(mg/g),Fis the fraction of adsorbate adsorbed at timet, andBtis the mathematical function ofF. Equation8can be rearranged by taking the natural logarithm to obtain the following equation:
Bt ¼ −0:4977−ln 1ð − FÞ ð8bÞ
3 Results and Discussions
3.1 Characterization of the Prepared Adsorbent 3.1.1 Surface Area and Pore Characteristics
The Brunauer–Emmett–Teller (BET) surface area, mesopore surface area, total pore volume, and average pore diameter of DSAC are shown in Table 2. The surface area of DSAC was 980.62 m2/g with a high pore volume of 0.528 cm3/g. The average pore diameter is 3.40 nm. According to IUPAC classification (IUPAC et al. 1972), DSAC belongs to the mesopore region which has significant mesopores suitable for MG dye
adsorption. A similar result was obtained in the removal of MG dye usingCocos nuciferashell-based activated carbon (Bello and Ahmad,2012).
The surface area of DSAC prepared was comparable with other AC obtained from literature. The physio- chemical activation process had contributed to the high surface area and total pore volume of the prepared DSAC. The DSAC expansion porosity is associated with gasification reaction (Basta et al.2009). The diffu- sion of carbon-KOH and CO2molecules into the pores promotes reactions which developed more pores in the DSAC. The KOH melts and its oxide component reacts with carbon dioxide, which assists in the enhancement of the surface area of the samples. This result was in agreement with the findings of Ahmad and Alrozi (2011), who reported the BET surface area, total pore volume, and average pore diameter of the rambutan peel-based AC as 908.35 m2/g, 0.52 cm3/g, and 2.63 nm, respectively, thus confirming that the combi- nation of both chemical and physical activating agents of KOH and CO2promotes the formation of mesopores and enhances the surface area of DSAC.
3.1.2 Proximate Analysis
The precursors were found to be rich in moisture and volatile matter. However, the moisture and volatile con- tent were found to decrease significantly from precursor to activated carbon. During carbonization and activation processes, organic substances become unstable as a result of the heat causing the molecules to break their bonds and linkages. During the activation step, the volatile matter is released as gas, and liquid products evaporate off leaving the material with a high carbon content (Ahmad and Alrozi,2010). Proximate analysis clearly indicates that the physiochemical activation has successfully increased the fixed carbon content and decreased the volatile matter (Table3).
Table 2 Surface area and pore characteristics of the samples Sample BET
surface area (m2/g)
Mesopore surface area (m2/g)
Total pore volume (cm3/g)
Average pore diameter (nm)
DS 9.44 – – –
DS char 197.80 106.36 0.198 3.27
DSAC 980.62 516.28 0.528 3.40
DSdurian seed,–not available
3.1.3 Surface Chemistry
Table4 shows the assignment of absorption bands for the Fourier transform infrared (FTIR) spectrum of DSAC (Fig. 1). The spectrum of the samples shows the presence of several functional groups. These spectra revealed either a reduction, disappearance, or broaden- ing of the peaks after the process of impregnation.
The long bandwidth around 3,601–3,861 cm−1in- dicated that the main functional groups found on DS were O–H stretching vibration of hydroxyl functional groups including hydrogen bonding. Other major peaks detected at bandwidths of 2,399–2,102, 1,732–1,901, 1,635, and 1,361 cm−1were respective- ly attributed to C≡C stretching of the alkyne group;
vibration of carboxylic C = O stretching of lactones, ketones, and carboxylic anhydrides; C= C of aromat- ic ring; and C–H stretching in alkanes or alkyl group.
The disappearance of ether and phenol in the DSAC samples shows that these functional groups are ther- mally unstable. DS exhibits peaks at band position 1,635 cm−1, which was assigned to the aromatic group but disappeared in DS char and DSAC. The aromatic group disappeared due to oxidative degra- d a t i o n o f a r o m a t i c r i n g s d u r i n g c h e m i c a l
impregnation and heating stage. The medium peaks around 1,014 and 867–590 cm−1found on the spectra were due to the presence of C–O groups stretching in ester, ether, or phenol group and C–H out-of-plane bending in benzene derivatives, respectively. A sim- ilar result was obtained in the preparation of activated carbon from cherry stones by chemical activation with ZnCl2(Olivares et al.2006).
3.1.4 Surface Morphology
The surface morphologies of DS and DSAC are shown in Fig.2. The surface structures of both precursors were rough and uneven. Different imaging magnifications between DS and DSAC were due to the modification using an activating agent. A significant pore structure exists with a series of rough cavities distributed over the surface of DSAC. This was due to the breakdown of lignocellulose at high temperature followed by evapo- ration of volatile compounds leaving samples with well- developed pores. During the activation process, the C- KOH reaction rate was increased, thus resulting in car- bon“burn off,”thereby developing good pores on the sample. The C-KOH reaction also increased the porosity of DSAC as well as created new pores due to loss of volatile components, carbon in the form of CO, and CO2 (Auta and Hameed, 2011). The physiochemical treat- ment was able to produce porous adsorbent thus increas- ing the surface area. Kilic et al. (2011) found out that KOH assists in widening the porosity of tobacco residue AC. The tobacco residue was found to have very little porosity, whereas the surface structure of TRAC was full with cavities. Physiochemical activation produces a porous adsorbent resulting in increased uptake in the adsorption of MG dye.
Table 3 Proximate analysis of the precursor, char, and activated samples
Sample Proximate analysis (%)
Moisture Volatile matter Fixed carbon Ash
DS 8.94 69.70 17.26 4.10
DS char 5.14 29.65 61.39 3.82
DSAC 3.10 22.41 70.89 3.60
Table 4 FTIR spectrum band assignment for DS, DS char, and DSAC
Assignment Band position (cm−1)
DS DS char DSAC
O–H stretching of hydroxyl group 3,861–3,738 3,857–3,614 3,861–3,610
C≡C stretching of alkyne group 2,287–2,102 2,399–2,295 2,337
C=O stretching of lactones, ketones, and carboxylic anhydrides 1,901 1,739 1,732
C=C of aromatic ring 1,635 – –
C–H stretching in alkanes or alkyl group 1,361 – –
C–O groups stretching in ester, ether, or phenol group 1,014 – –
–C≡C–H–C–H bend in functional group 867–767 597 590
(a)
(b)
(c)
Fig. 1 FTIR spectra:aDS,bDS char, andcDSAC
3.2 Batch Equilibrium Studies
3.2.1 Effect of Contact Time and Initial Concentration of Adsorbate
Figure 3 shows the MG dye adsorption uptake with respect to time at various initial dye concentrations (25–500 mg/L) by DSAC at three different temperatures of 30, 45, and 60 °C, respectively. The percentage of MG dye removal with respect to time is shown in Fig.4.
At 30 °C, the amount of MG dye adsorbed as well as the percentage of dye removal increased with the increase in initial dye concentration. As the initial dye concentra- tion increased from 25 to 500 mg/L, the adsorption uptake at equilibrium for MG dye was found to increase from 22.0 to 332.08 mg/g. This was due to the increase in mass transfer driving force with increase in initial dye concentration. The initial dye concentration provides a driving force to overcome mass transfer resistance of dyes between the aqueous and solid phases. The adsorp- tion curves were smooth and continuous profiles leading to equilibrium (Djeribi and Hamdaoui,2008). When an equilibrium condition was reached, the dye molecule in the solution is in a state of equilibrium with the molecule adsorbed by the DSAC. The higher MG dye concentra- tion provides a strong driving force of the gradient;
hence, an enhanced adsorption capacity was observed (Mahmoud et al.2012). Similar trends were observed at higher temperatures as shown in Fig.3b, c.
The MG dye adsorption uptake curves have three parts. In the first 6 h, about 50 % of MG dye adsorption occurred at temperatures of 30, 45, and 60 °C, respec- tively. In the second part (6 h<t< 22 h), the slopes became gentle resulting in an additional 20 % adsorp- tion. In the third part (t>22 h), the dye adsorption uptake (qt) was steady and the adsorption reached equilibrium.
There was an initial sharp increase of MG dye adsorp- tion uptake (qt) due to the large number of available active sites. The gradual slower adsorption at the end is probably due to the saturation of active sites and a decrease in dye concentration (Li et al.2009; Khattri and Singh,2009). The remaining surface sites are diffi- cult to be occupied. This was due to the repulsion between the solute molecules and bulk phases of DSAC.
Similar results were obtained in the adsorption of MG dye on neem sawdust (Khattri and Singh, 2009) and banana stalk-activated carbon (Bello et al.2011a,b).
The contact time needed for dye solutions with initial concentrations of 25–200 mg/L to reach equilibrium was around 6 h. However, for dye solutions of higher initial concentrations (400–500 mg/L), equilibrium times of 20–24 h were required. This is due to the fact that the initial dye molecules have to first encounter the boundary layer effect and then have to diffuse into the boundary layer film onto the adsorbent surface. Finally, it has to diffuse into the activated carbon and adsorbed onto the binding surface (Santhi et al.2010a,b). There- fore, a dye solution with higher initial concentrations would take relatively longer contact time to achieve equilibrium due to the higher amount of adsorbate mol- ecules. The larger surface area and the total pore volume of DSAC relatively highly enhanced the adsorption process. A similar result was obtained in the adsorption of MB dye using periwinkle shells (Bello et al.2008).
3.2.2 Effect of Solution Temperature
Figure5shows the MG dye adsorption capacity against the solution temperature for DSAC at 30 °C. As tem- perature increased from 30 to 60 °C, the amount of MG dye adsorbed increased from 332.08 to 370.94 mg/g.
The adsorption capacity (qe),increased with increase in (b)
(a)
Fig. 2 SEM micrographs ofaDS (×2,000) andbDSAC (×2,000)
temperature indicating that the adsorption was an endo- thermic process. As the solution temperature increased, dye mobility increased to undergo interaction with the active site at the DSAC’s surface (Mahmoodi et al.
2011). In addition, the rate of dye molecule diffusion across the external boundary layer and in the internal pores of the AC particle increased (Dogan et al.2009).
The diffusion rate of adsorbate molecules across the external boundary layer and in the internal pores of the adsorbent increased as the viscosity of the solution decreased at high temperature. In addition, different temperatures tend to change the equilibrium capacity of the adsorbent for a particular adsorbate (Alkan et al.
2008). Adsorption capacity by DSAC was high because of the larger surface area and total pore volume of DSAC. A similar trend was reported for adsorption of basic dye on AC prepared from rambutan peel by
Ahmad and Alrozi (2011) and rice straw by Basta et al. (2009).
3.2.3 Effect of Initial pH
The pH point of zero charge of DSAC was found to occur at pH 3.2, implying that DSAC surface has a positive charge in solution up to pH 3.2 and a negative charge above this pH (Nowicki et al.2010) (figure not shown). The contents of oxygen-containing function- al groups with various acidic groups which were determined using the Boehm titration (Boehm, 2002) are as follows: carboxyl (0.1432 meq g−1), lac- tonic (0.4315 meq g−1), phenolic (0.2214 meq g−1), total acidic group (0.7961 meq g−1), and total amount of the basic groups (0.3546 meq g−1). The acidic group content is much higher than the basic group
a
b
c
Fig. 3 Plot of MG dye adsorption uptake against adsorption time at various initial concentrations ata30,b45, andc 60 °C on DSAC
content. This is consistent with a value of pHpzc3.2, showing the dominance of acidic groups. The effect of initial pH on MG dye removal onto DSAC is shown in Fig. 6. The maximum MG dye removal was recorded at pH 8 to be 97 %. At this pH, more OH− ions accumulate on the adsorbent surface resulting in the interaction between the negatively charge adsorbent surface and cationic MG dye mole- cule, thus an enhanced MG dye removal. However, at low pH, low MG dye uptake was due to the presence
of excess H+ions competing with the cationic groups on the MG dye for adsorption sites. As the solution pH increases, the surface charge density decreased and the electrostatic repulsion between the positively charged dye and the surface of the DSAC was lowered, thus resulting in high MG dye removal. A similar result was obtained in the removal of Mala- chite green from an aqueous solution by activated carbon prepared from the epicarp of Ricinus communis(Santhi et al.2010a,b).
a
b
c
Fig. 4 MG dye percentage removal against adsorption time at various initial concentrations at a30,b45, andc60 °C on DSAC
Fig. 5 MG dye adsorption capacity against the solution temperature for DSAC at different temperatures
Fig. 6 Effect of initial pH on MG removal on DSAC
Fig. 7 Plots ofaLangmuir,bFreundlich,cTemkin,dDubinin–Radushkevich,eRadke–Prausnitz,fSips,gVieth–Sladek, andhBrouers– Sotolongo isotherms for MG dye adsorption on DSAC
3.3 Adsorption Isotherm Studies
The adsorption isotherm signifies the amount of mole- cules distributed between the liquid phase and the solid phase when the adsorption process reaches equilibrium state. Langmuir, Freundlich, Temkin, Dubinin–
Radushkevich, Sips, Vieth–Sladek, Brouers–Sotolongo, and Radke–Prausnitz isotherm models were chosen to
fit the experimental equilibrium data. Figure7shows the plots of the linearized equation isotherm parameters for MG dye removal at temperature 30 °C. Similar plots were obtained for the adsorption systems at 45 and 60 °C, respectively (figure not shown). Table5 shows the values of maximum monolayer coverage (Qm), cor- relation coefficient (R2), normalized standard deviation (Δqe), and other parameters as calculated from isotherms Table 5 Isotherm parameters for the adsorption of MG dye onto DSAC at 30 °C
DSAC-MG
Langmuir Freundlich Temkin Dubinin– Radushkevich
Sips Vieth–Sladek Brouers– Sotolongo
Radke– Prausnitz Qm 476.19 1/nF 0.679 B 77.716 bDR 0.0035 Qm 480.20 Kvs 0.4856 Qm 532.30 KRP 0.0083 KL 0.0118 KF 0.3727 A 0.2542 E 5.3430 Ks 0.0116 Qm 445.60 kBS 9.9×10−3 Qm 516.52 R2 0.9467 R2 0.9995 R2 0.9061 qs 11.95 ms 1.1106 βvs 9.20×10−3 α 0.907 mRP 0.974
RL 0.1449 R2 0.8194 R2 0.892 R2 0.985 R2 0.982 R2 0.970
Δqe 3.215 Δqe 0.544 Δqe 5.481 Δqe 2.154 Δqe 8.174 Δqe 6.221 Δqe 5.841 Δqe 4.987
Table 6 Comparison of adsorption capacities of MG dye using various adsorbents
Adsorbent Capacity qo(mg/g) Conditions References
pH Temp (K)
Carbon prepared fromBorassusbark 20.70 6 303 Arivoli et al. (2009)
Caulerpa racemosavar.cylindracea(marine alga) 25.67 6 318 Bekci et al. (2009)
Saccharomyces cerevisiae 17.00 5 308 Godbole and Sawant (2006)
Potato peel 32.39 4 298 Guechi and Hamdaoui (2011)
Leaves ofSolanum tuberosum 33.3 7 303 Gupta et al. (2011)
Unsaturated polyester Ce (IV) phosphate 1.01 8 300 Khan et al. (2010)
Neem sawdust 4.35 7.2 303 Khattri and Singh (2009)
Rubber wood sawdust 36.45 – 305 Kumar and Sivanesan (2007)
Commercial activated carbon 8.27 7 303 Mall et al. (2005)
Waste fruit residues 37.03 5–8 300 Parimaladevi and Venkateswaran (2011)
Dried cashew nut bark carbon 20.09 6.60 – Parthasarathy et al. (2011)
Tamarind fruit shell 1.95 5 303 Saha et al. (2010)
Epicarp ofRicinus communis-activated carbon 27.78 – – Santhi et al. (2010a) Annona squamosaseed-activated carbon 25.91 6 300 Santhi et al. (2010a)
Annona squamosaseed 25.91 6 300 Santhi et al. (2011)
Cellulose powder 2.42 7.2 298 Sekhar et al. (2009)
Kapok hull-activated carbon 30.16 6.7 300 Syed (2011)
Chlorella-based biomass 18.4 7 298 Tsai and Chen (2010)
Carbon prepared fromArundo donaxroot 8.69 5–7 303 Zhang et al. (2008)
Wood apple shell 34.56 7.5 299 Sartape et al. (2013)
DSAC 476.19 8.0 298 This work
at temperature 30 °C. Freundlich isotherm fitted well with the data (R2>0.99) for MG dye adsorption onto DSAC. The maximum adsorption capacity (Qm) values obtained from isotherm were in range of 445.6–
532.3 mg/g for DSAC-MG (Table 5). Freundlich iso- therm shows the lowest Δqe values in addition to the highestR2value.
From the experimental data, the Freundlich iso- therm model indicated the heterogeneous nature of DSAC surface, in which the energy term in the Lang- muir equation varies as a function of the surface coverage (Wang et al. 2010). A higher value of KF
indicates an adsorbent having a higher capacity for adsorption (Tunç et al. 2009). Table 5 shows the calculated values ofKFsuggesting a high adsorptive capacity for MG dye uptake. The 1/n value for Freundlich was 0.679 for DSAC, indicating that the physical process was favorable for MG dye adsorp- tion on DSAC. The 1/nvalue was below 1, indicating that the adsorbate was favorable on the adsorbent (Tunç et al.2009).
The fundamental feature of the Langmuir isotherm is expressed in terms separation factor (RL).RLindicates isotherm to be either unfavorable (RL>1), linear (RL= 1), favorable (0<RL<1), or irreversible (RL=0). TheRL
for DSAC onto MG dye ranged between 0 and 1, which indicates that the adsorption was favorable at the con- centration range studied. TheRLvalues decreased as the initial concentration dye concentration increased which indicates that the sorption process was favorable at high initial dye concentration. Table6compares the various adsorption capacities of different adsorbents with the one obtained in the present study. DSAC gave a higher value ofqoabove other adsorbents used in the removal of MG dye.
3.4 Batch Kinetic Studies
The adsorption equilibrium studies are important in determining the effectiveness of adsorption. Four dif- ferent kinetic models were used to predict the adsorp- tion process: pseudo-first-order, pseudo-second-
(a) (b)
(c) (d)
Fig. 8 Linearized plots ofapseudo-first-order,bpseudo-second-order,cAvrami, anddElovich kinetic model for MG adsorption on DSAC-MG at 30 °C
order, Elovich, and Avrami models. As shown by plots in Fig. 8, the pseudo-second-order kinetic models fit well with data when compared to other kinetic models. The experimental data in Table 7 showed that the MG dye adsorption parameters gave good agreement with the pseudo-second-order kinetic model withR2value close to unity and low values of Δqt, and this reaction is significant in the rate- controlling step. The rate coefficient, k2, of pseudo- second-order was found to decrease with increase in initial dye concentration. It indicates that as the initial concentration increases, the electrostatic interaction decreased on the site, thus lowering the dyes’affinity toward the adsorbent (Chu, 2002). In addition, this behavior was due to the lower competition for the sorption sites at lower concentration. At higher
concentrations, the competition for the surface active sites was high, and consequently, lower sorption rates were obtained. However, Elovich desorption con- stant, βEl, increases as the initial dye concentration increases, and this is related to the extent of surface coverage and activation energy for the chemisorption process. On the other hand, the Elovich constant,αEl, decreased with increase in initial dye concentration.
This indicates that the adsorption process involves more than one mechanism. The findings were in agreement with several studies on the adsorption of MG dye ontoArundo donaxroot (Zhang et al.2008), maize cob (Sonawane and Shrivastava,2009), flower biomass (Nethaji et al.2010), rambutan peel (Ahmad and Alrozi, 2011), and ginger waste (Ahmad and Kumar,2010) respectively.
Table 7 Kinetic model constant values for DSAC-MG adsorption at 30 °C
Model Kinetic parameters Initial MG dye concentration (mg/L)
25 50 100 200 400 500
Pseudo-first-order qeexp (mg/g) 21.26 41.63 78.43 144.50 270.17 313.46
k1(min−1) 0.507 0.353 0.308 0.312 0.283 0.303
qecal (mg/g) 17.15 34.66 66.50 127.58 239.80 284.05
R2 0.956 0.958 0.94 0.961 0.961 0.968
Δqt(%) 20.195 22.508 13.921 15.822 27.281 10.921
Pseudo-second-order qecal (mg/g) 18.05 29.76 52.35 97.08 172.41 212.76
k2(min−1) 0.162 0.096 0.052 0.022 0.012 0.008
h(mg/g min) 52.91 84.74 142.85 204.08 357.14 357.14
R2 0.982 0.984 0.989 0.981 0.994 0.988
Δqt(%) 4.876 2.739 9.435 7.895 8.843 7.354
Elovich αEl(mg/g min) 13.50 3.89 1.51 0.61 0.280 0.213
βEl(g mg−1) 3.224 6.560 12.74 24.33 46.27 54.85
R2 0.966 0.963 0.972 0.967 0.969 0.980
Δqt(%) 24.482 16.142 16.449 21.796 18.442 20.982
Avrami nAV 0.585 0.523 0.540 0.568 0.588 0.646
kAV(min−1) 75.52 124.43 155.6 180.62 198.22 192.41
R2 0.976 0.956 0.970 0.982 0.982 0.987
Δqt(%) 14.283 27.871 33.683 30.561 27.580 29.792
Table 8 Thermodynamic param- eters for MG dye adsorption onto DSAC at different temperatures
Activated carbon ΔH(kJ/mol) ΔS(J/mol K) Ea(kJ/mol) −ΔG(kJ/mol)
303 K 318 K 333 K
DSAC-MG 10.64 12.56 5.05 11.43 11.42 11.51
3.5 Thermodynamic Studies
Thermodynamic adsorption parameters such as change in enthalpy (ΔH), entropy (ΔS), Gibbs free energy (ΔG), and activation energy (Ea) for the ad- sorption of MG dye onto DSAC are listed in Table8.
From the table, the ΔH value obtained is positive indicating that the process is endothermic in nature.
This was due to the decrease in the viscosity of the solution at higher temperature (Wang et al.2005).ΔH for physisorption is generally less than 80 kJ/mol, whereas for chemisorption, it falls within the range 80–420 kJ/mol (Tan, 2008). Values obtained forΔH are less than 80 kJ/mol, indicating that the adsorption process follows the physisorption mechanism. The positive values of ΔS indicate that the degrees of freedom increased at the solid–liquid interface during dye adsorption onto adsorbent. The negative values ofΔGindicate the spontaneous nature of the adsorp- tion processes at the range of the temperature studied.
The magnitude ofΔGvalues obtained in this study is
between −20 and 0 kJ/mol which confirmed the physisorption nature of the adsorption process (Tan, 2008).
Another characteristic of the physical adsorption mechanism is low activation energy (5–40 kJ/mol), while higher activation energy (40–800 kJ/mol) sug- gests a chemisorption mechanism (Hameed et al.
2007).Eavalue in Table8 was between 5 and 40 kJ/
mol, indicating that the adsorption of MG dye onto Table 9 Intraparticle diffusion model constant for adsorption of MG dye onto DSAC at 30 °C
Activated carbon MG initial concentration (mg/L) 25 mg/L 50 mg/L 100 mg/L 200 mg/L 400 mg/L 500 mg/L
DSAC Kp1(mg/g h1/2) 25.2896 46.1531 70.7080 110.5409 171.7696 200.5876
Kp2(mg/g h1/2) 1.577 4.1183 6.5271 11.2141 26.1921 28.9647
Kp3(mg/g h1/2) 0.0001 0.0004 0.0004 0.0004 0.0004 0.0004
C1 1.0564 1.8288 2.4792 4.4507 6.7326 3.9944
C2 10.2004 14.6260 25.3041 41.8429 73.9450 88.6514
C3 21.2570 41.6356 78.4290 144.4972 270.1659 313.464
(R1)2 0.8565 0.8676 0.9093 0.8773 0.8992 0.9710
(R2)2 0.8383 0.9530 0.9697 0.9323 0.9627 0.9233
(R3)2 0.9643 1.0000 0.9231 0.9999 0.9643 0.9998
Fig. 9 Plots of intraparticle diffusion model for MG adsorption on DSAC at 30 °C
Fig. 10 Boyd plots for adsorption of MG dye onto DSAC
DSAC follows a physisorption process.
3.6 Adsorption Mechanism
3.6.1 The Intraparticle Diffusion Model
The kinetic mechanisms were further analyzed by using the intraparticle diffusion model. The first, sharper re- gion within the first 15 min was termed rapid adsorption due to the strong electrostatic attraction between the dye and the external surface of the adsorbent. The second region, the gradual adsorption stage, indicates the intraparticle diffusion of dye molecules through the pores of the adsorbent which is the rate-controlling step (Ahmad et al. 2009). The third region was the final equilibrium stage when intraparticle diffusion starts to slow down due to the very low adsorbate concentration in the solution (Wang et al.2010). It can be seen that the linear lines of the second and third stages did not inter- cept through the origin. It shows that intraparticle diffu- sion was not only a rate-limiting mechanism in the adsorption process. Table9shows the intraparticle mod- el constant for adsorption of MG dye onto DSAC. The kpvalues and interceptCvalues for all initial concen- tration were found to increase from the first stage of adsorption toward the third stage. Increase in dye con- centration results in an increase in the driving force, thereby increasing the dye diffusion rate. The constant, C, was found to increase with increase in initial dye concentration, indicating the boundary layer effect (Wang et al.2010) (Fig.9).
3.6.2 Boyd Model
The Boyd model which is a linearity test ofBtversus time plots was employed to distinguish between film diffusion and particle diffusion-controlled adsorptions.
If the plot of Bt versus time is a straight line passing through the origin, then the adsorption rate is governed by particle diffusion mechanism; otherwise, it is governed by film diffusion mechanism. This plot is useful in determining whether external diffusion or intraparticle diffusion controls the rate of adsorption.
Figure10illustrates the Boyd plot for MG dye adsorp- tion onto DSAC at 30 °C. Similar plots are obtained at 45 and 60 °C (figure not shown). The linear lines obtained at different dye concentrations did not pass through the origin, suggesting the difference in the rate of mass transfer in the first and second stages by external
mass transfer (film diffusion) where the particle diffu- sion was the limiting step. The first stage of adsorption is attributed to the diffusion in the macropore and the second stage to micropore diffusion (Ahmad et al.
2009). A similar observation was obtained in the ad- sorption of MG dye onto treated ginger waste (Ahmad and Kumar2010).
4 Conclusion
The adsorption process of DSAC in the removal of MG dye was very effective at pH 8 resulting in 97 % removal efficiency. Adsorption isotherm data fitted the Freundlich model the most. The dimensionless separa- tion factor of Langmuir isotherm,RL, lies between 0 and 1. The Freundlich constant (1/n) is less than 1, indicating that the adsorption process is favorable. The kinetics of adsorption followed the pseudo-second-order rate equa- tion. Both intraparticle diffusion and boundary layer diffusion govern the mechanism of the adsorption pro- cess. The negative value ofΔGrevealed the spontaneity of the process, while the positive value ofΔHindicated that the process is endothermic. The positive value ofΔS showed the increased randomness of the adsorbate mol- ecules on the solid surfaces than in the solution. The prepared adsorbent can be effectively used in the remov- al of MG dye from industrial effluents.
Acknowledgments The financial support in the form of grants from USM; the 3 months USM-TWAS Visiting Researcher Fel- lowship, FR number: 3240268492 awarded to the corresponding author; and the accumulated leave granted to Dr. O.S Bello by his home institution to utilize the fellowship is thankfully recognized.
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