In this research, cadmium oxide nanowires loaded on activated carbon (CdO-NW-AC) has been synthesizedby a simple procedure and characterized by different techniques such as XRD, SEM and UV–vis spectrometry.This new adsorbent has been efficiently utilized for the removal of the Direct Yellow 12 (DY-12) fromwastewater. To obtain maximum DY-12 removal efficiency, the influences of variables such as pH, DY-12concentration, amount of CdO-NW-AC, contact time, and temperature have been examined and optimizedin a batch method. Following the variable optimization, the experimental equilibrium data (at different con-centration of DY-12) was fitted to conventional isotherm models such as Langmuir, Freundlich and Tempkin.The applicability of each method is based on the R2 and error analysis for each model. It was found that theexperimental equilibrium data well fitted to the Langmuir isotherm model. The dependency of removal pro-cess to time and the experimental data follow second order kinetic model with involvement of intraparticlediffusion model. The negative value of Gibbs's free energy and positive value of adsorption enthalpy show thespontaneous and endothermic nature of adsorption process.
Wastewater containing large amount of dye generally producedfrom different sources led to generation and creation of some problemssuch as toxicity, allergy, skin irritation and mutagenic and/or carcino-genic hazards [1,2]. DY-12 (Fig. 1) belongs to azo dye categories widelyapplied in dying silk, wool, leather, biological stain, dermatologicalagent, veterinarymedicine and green inkmanufacture [3]. The entranceof this dye to the aquatic environment causes permanent injury tohumans' and animals' eyes [4–7]. Therefore, treatment of wastewaterscontaining this dye is a challenging requirement.
Large scale production and extensive applications of syntheticdyes cause considerable environmental pollution. Dyes are difficultto decompose biologically & chemically [8,9]. Most of the previousdye treatment procedures either generate more toxic compounds(degradation procedure) or produce huge amount of waste in addi-tion to only transfer dyes from aqueous solutions to other regions ofthe environment [10,11]. The complete removal requires new treat-ment methods. Most of the previous dye removal procedures alsoneed expensive instrument or are time consuming [12].
Adsorption as one of the most popular and prominent wastewatertreatment method is based on the transfer of dye molecule to a low
cost adsorbent and high adsorption capacity [8–12]. In our opinion,the application of metallic center nanomaterial via binding dye mole-cules admits their quantitative and complete accumulation on thesurface of adsorbent. Application of low cost adsorbent in nanoscaleextensively improves the efficiency of adsorption.
This procedure is superior to other conventional wastewatertreatments such as chemical oxidation and biodegradation methodin terms of initial cost, simplicity, and ease of adsorbent operation[13]. In this regard, application of low cost and high adsorption capac-ity with high amount of vacant surface atom led to improvement inthe performance and efficiency of the adsorption process. The nano-technology has created great excitement and expectation in the lastdecade at the nanoscale fundamental property changes [14]. Due tohigh dependency of properties and function to their size and shape,nano-dimensional semiconductor materials were greatly applied inoptoelectronics, electronics, sensing, energy storing and harvestingapplications. Binary semiconductor oxides such as ZnO, CdO and SnO2
have innumerable applications and are now widely used as transparentconductive oxides. Being of high optoelectronic efficiency relative tothe indirect band gap of group IV crystals, it is considered as a reliablematerial for visible andnear ultraviolet applications. Application of nano-scale material significantly enhances their role and activity. Oxide basednanomaterial has been used as catalysts and starting materials for prep-aration of structural ceramics [15]. CdO (n-type semiconductor) withdirect and indirect band gaps of 2.4 eV and 1.98 eV, respectively [16]was applied for preparation of catalysts [17], sensors [18], nonlinear
2259M. Ghaedi et al. / Materials Science and Engineering C 33 (2013) 2258–2265
materials [19], solar cells [20] and other optoelectronic devices. Thephysical and chemical properties of CdO nanoparticles depend to thestoichiometry and particle shape and size. All of these properties signifi-cantly are influenced by conditions and preparation methods [21–26].Generally, metal oxide nanoparticles are produced via soft chemicalmethods such as co-precipitation, sol–gel and hydrothermal synthesis[27–32].
The present research focus describes a safe and efficient procedurefor synthesis and characterization of cadmium oxide nanowires loadedon activated carbon (CdO-NW-AC). This new adsorbent following fullidentification and evaluation was successfully applied for the removalof DY-12. The effect of variables such as pH, amount of adsorbent andcontact time was investigated by one at a time optimization method.The kinetic, isothermand thermodynamic datawere analyzed accordingto different models to study the adsorption mechanism and thermody-namic feature of the process.
2. Experimental
2.1. Instruments and reagents
DY-12 (CASnumber=2870-32-8;molecularweight: 680.66 gmol−1,Purity: 99%) was purchased from Nantong Chemical Zone, Jiangsu,China. The pH measurements were done using pH/Ion meter model-686 and absorption studies were carried out using Jusco UV–Visiblespectrophotometer model V-570. All chemicals including activatedcarbon (charcoal), Cd(CH3COO)2.2H2O, NH3, NaOH, HCl and KCl of ana-lytical reagent grade are purchased from Merck, Darmstadt, Germany.
The morphology of the CdO nanowires was investigated usingscanning electron microscopy with model HITACHI S-4160 FE-SEMwith cold field emission cathode (acceleration voltage 15 kV) by anAuto Fine Coater (JFC-1300, JEOL). XRD patterns were taken with anautomated Philips X'Pert X-ray diffractometer with Cu Kα radiation(40 kV and30 mA). Reflective UV–vis absorption spectrawere recordedon a Hitachi U-3310 spectrophotometer.
2.2. Preparation of CdO nanowires
In a typical synthesis, at first an aqueous solution of Cd(CH3COO)2.2H2O (50 mL, 0.15 mol L−1) was prepared, and aqueous ammoniasolution (25%) was added drop wise to the above solution under con-stant stirring. A white precipitate was initially observed, which subse-quently dissolved back into solution upon further addition of the NH3
solution. The pH of the solution was adjusted at 12 using the ammo-nia solution. Finally, the mixture was maintained at a pH of 12 and at25 °C for 5 days, resulting in the gradual growth of the Cd(OH)2nanowires in the solution. The precipitate inclusive of Cd (OH)2nanowires was isolated by centrifuging from the aqueous alkalinesolution and washed with double distilled water and absolute alcohol
three times, respectively. Finally, the solid product was dried underair atmosphere at room temperature for 12 h. The CdO nanowireswere formed by calcinations of the precipitate at 400 °C (electricfurnace) under air atmosphere for 1 h till formation of a yellowproduct.The heating rate through the calcination process was 20 °C/min,while the cooling rate was 10 °C/min. Finally this compound washomogenously mixed with AC at the mass ratio of 1:10 for 1 h till ahomogenous adsorbent was obtained. Powder XRD patterns of thenanowire samples were taken with Cu Kbalpha> radiation (40 kV and30 mA) for 2btheta> values over 20–100°. XRD measurements werecarried out at proper scan mode (step size: 0.040°, time per step:0.40 s, and scan rate: 0.100°s−1). To record FE-SEM with cold fieldemission cathode (acceleration voltage 15 kV), it is necessary to coatthe CdO nanowires by gold (by an Auto Fine Coater, JFC-1300, JEOL).Reflective UV–vis absorption spectra were recorded in the range of300–800 nm with a resolution of 1 nm.
2.3. Measurements of dye uptake
All experiments were carried out using an optimum value of theadsorbent (0.1 and 0.14 g L−1) at 25 °C in 50 mL beakers in an IKAmagnetic stirrer operating at 300 rpm to elucidate the optimum con-ditions (pH, contact time and initial dye concentration). The test solu-tions were prepared daily by its dilution, while its concentration wasdetermined at 398 nm using calibration curve obtained at the sameconditions. UV–Visible spectra were recorded in the range of 300–500 nm with a resolution of 1 nm. 100 mg DY-12 was dissolved in100 mL double distilled water to prepare a stock of 1000 mg L−1
solution.The actual amount and removal percentage of DY-12 was recorded
after a desired contact time, following centrifuging and analyzingthe supernatant solution spectrophotometrically for evaluation ofDY-12 concentration. The concentration of DY-12 was determined at398 nm. The influence of contact time on the removal percentage ofDY-12 by CdO-NW-AC as new adsorbent from 50 mL of 25 and40 mg L−1 of DY-12 at pH 1 using 0.1 and 0.14 g L−1 of this new adsor-bent was investigated.
The DY-12 adsorption capacities of CdO-NW-AC were determinedat certain time intervals (1, 2, 3, 4, 6, 7, 8 and 9 min) at temperaturesin the range of 10–60 °C at optimum values of all variables. The effectof pH on adsorption was studied by adjusting DY-12 solutions(25 mg L−1) to different pH values (1–5) via addition of HCl and/orNaOH. The solution was agitated with 0.1 g L−1 of CdO-NW-AC for9 min. The influence of ionic strength at various KCl concentrations(0, 0.25 and 0.5 M) at 25 mg L−1 of DY-12, pH 1, 0.1 g L−1 ofCdO-NW-AC and contact time of 9 min was investigated. The influ-ence of temperature in the range of 288–335 K at 25 and 40 mg L−1
of DY-12 concentration, pH of 1.0 of 0.1 and 0.14 g L−1 CdO-NW-ACand 10 min contact timewas examined. DY-12 adsorption experimentswere also accomplished to obtain isotherms at room temperature overthe concentration range of 15–45 mg L−1 and the equilibriumadsorbed value qe (mg g−1) was calculated based on the followingmass balance relationship:
qe ¼ C0−Ceð ÞV=W ð1Þ
where C0 and Ce are the initial and equilibrium remainedDY-12 concen-trations in solution, respectively (mg L−1), V the volume of the solution(L) and W is the mass of the adsorbent (g).
The cadmium content was determined by Shimadzu 680 AA witha cadmium hollow cathode lamp that was run under manufacturerecommended conditions of wavelength, 228.3 nm; lamp current,5.0 mA; slit width, 0.2 nm; acetylene flow, 1.5 L min−1; and air flowas oxidant, 3.5 L min−1.
2260 M. Ghaedi et al. / Materials Science and Engineering C 33 (2013) 2258–2265
2.4. Determination of point of zero charge pH (pHpzc)
The final pH drift method was used for determination of point ofzero charge (pHpzc) of CdO-NW-AC. For that purpose, the initial pHvalue of 50 mL of the aqueous solution was adjusted in the range of1.0–7.0 by HCl or NaOH solutions. After addition of a certain amountof CdO-NW-AC to these solutions, the mixture was stirred for 10 minand the difference between the final pH and initial pH of the solutionwas plotted against the initial pH of the solution. It is well known thatthe surface of CdO-NW-AC is neutral when the solution pH is equal topHpzc, and the surface is negatively charged at pH values higher thanpHpzc and positively charged at pH values lower than pHpzc [29].
3. Results and discussion
Fig. 2A shows a reflective UV–vis absorption spectrum of the CdOnanowires on a quartz substrate. By investigation of the UV–vis
A
B
Fig. 2. A: Absorption spectrum and the plot of (αhυ)2 versus hυ (inset) of the CdOnanowires, B: X-ray diffraction (XRD) pattern of the CdO nanowires.
spectrum, we observed that an absorption edge was located atabout 500 nm. From the absorption spectrum, the band gap energy(Eg) of the CdO nanowires was obtained using the following rela-tion [33]:
αhυð Þ2 ¼ A Eg−hυ� �
ð2Þ
where A is a constant, Eg is the separation between the valence andconduction bands (the band gap energy), h is the Planck constant, αis the absorption coefficient, and υ is the frequency of the absorptionlight. The band gap energy (Eg) of the CdO nanowires was calculatedby plotting the square of the product of absorption coefficients inphoton energy (αhυ)2 against photon energy (hυ) shown in theinset of Fig. 2A. As can be seen, (αhυ)2 varies linearly with hυabove the band gap energy. Accordingly, the band gap energy isobtained by extrapolating the straight portion of the curve to zeroabsorption coefficients. The obtained band gap energy of the CdOnanowires was about 2.50 eV, which is in good agreement with thereported value for the direct band gap of bulk CdO at 2.44 eV [34].CdO nanoparticles with diameters of 4–18 nm have been shownto exhibit pronounced quantum confinement effects, yet our CdOnanowires are too thick and long to observe such effects [35].
The standard XRD pattern for the powdered calcinated precipitateof CdO (Fig. 2B) shows eight peaks in the diffractogram at around33.21°, 38.49°, 55.59°, 66.05°, 69.65°, 82.30°, 91.61°, and 94.59°assigned to the planes (111), (200), (220), (311), (222), (400),(331), and (420) respectively belonging to the cubic lattice structureof the compound [16]. The standard XRD pattern for CdO (JointCommittee for Powder Diffraction Standards, JCPDS card No. 01-1049)is given at the bottom of Fig. 2B. The absence of other peaks (Cdpeaks) shows that the product is completely composed of CdO andsharp peaks indicating the crystalline nature of this new material.
Fig. 3A and B shows two low and high magnificence FE-SEM imagesof the CdO nanowires. As shown in these images, the preparednanowires are fairly uniform and straight, with diameters in the rangeof 50–120 nm. The CdO nanowires are ultra-long, having lengths ofmore than 2 μm,which virtually correspond to an aspect ratio of greaterthan 20.
The toxicity of cadmium based adsorbent as new adsorbent and itsefficiency and durability for removal of DY-12 from wastewater sig-nificantly improved via impregnation and loading of CdO nanowireson activated carbon. It is mentionable that the loaded nanowiresonly diffuse back to solution only in highly acidic medium. The pro-posed CdO nanowires completely loaded homogenously with activat-ed carbon at the 1:10 mass ratio and vigorously mixed with pasteland mortar. Such loading strongly accelerated in the presence ofdye molecule that may be attributed to the complexation of dyemolecule with cadmium metal ions. Such result has been reportedby other researchers [36–40].
It was seen that both CdO-NW-AC and AC (the same mass) aresole adsorbents having removal efficiencies of 78 and 86%, while theproposed adsorbent (1:10 mass ratio) has removal percentage of98%. This synergic effect encourages us to use this new adsorbentfor DY-12 removal from wastewater. The applied adsorbent hashigh stability under applied conditions. The amount of cadmiumbladed to solution after 25 time using was lower than 0.008 andafter 50 time using is lower than 0.015 mg L−1 which is lower thanthe World Health Organization threshold value in phytotherapeuticformulations (b0.3 mg kg) [34]. This point shows the suitability ofthis new adsorbent for DY-12 removal without marginal toxicity forecosystems. On the other hand CdO-NW-AC is a recyclable adsorbentsuch that after washing the used adsorbent with 10 mL of NaOH pre-pared in acetonitrile, it can be used at least for 10 times in adsorptionstudies.
Fig. 3. Low (A) and high (B) magnificence FE-SEM images of the CdO nanowires.
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3.1. Effect of contact time
Contact time and removal rate are important factors for selectionand design of economical adsorbent for wastewater treatment [35].
It was seen, that increasing contact time till 9 min led to improve-ment in the adsorption of DY-12 and further addition has no signifi-cant influence on DY-12 removal percentage. It is observed thatmore than 90% of DY-12 removal was occurred in the first 2 minand the equilibrium was achieved at 9 and 10 min for 25 and40 mg L−1 of DY-12 initial concentration (Fig. 4). High rate of adsorption
Fig. 4. Effect of contact time on DY-12 removal at 0.1 and 0.14 g L−1 of CdO-NW-AC atpH 1, at room temperature and DY-12 concentrations of 25 and 40 mg L−1.
at the initial stage of removal process is of concern to the availablehigh vacant reactive sites. The active sites for adsorption are the cad-mium center and different functional groups of AC. With rising contacttime probably due to the electrostatic repulsion between the adsorbednegatively charged sorbate species onto the surface of adsorbent andbulk in addition to pore diffusion the rate of adsorption decreased[23,24,34].
3.2. Effect of pH
The dye solution pH plays an important role on the adsorptionprocess and adsorption capacity via affecting the extent of ionizationof the acidic and basic compound functional groups [35,41]. In general,initial pH value may enhance or depress the dye uptake due to thechange in the charge of the adsorbent surface and dye molecule. Thepossible mechanism of adsorption process is through binding of cadmi-um atomwith nitrogen atoms of the DY-12molecule (soft–soft interac-tion) and/or dye through interaction with AC functional groups or π–πinteraction or hydrogen bonding. In this regard, the influence of pH onthe DY-12 removal at optimum values of other variables in the rangeof 1–5 was studied and results are shown in Fig. 5. It was seen thathigh removal percentage of DY-12 occurs at pH=1.0. Probably at lowpH either via strong electrostatic attraction between the AC functionalgroups (carboxylic, hydroxyl) with DY-12 anionic dye or throughbinding with cadmium center of adsorbent the DY-12 removal signifi-cantly increased [42]. The results for the determination of pHpzc ofCdO-NW-AC were illustrated in Fig. 6. It is obvious, that the proposedadsorbent has positive charge and change in pH 1.0 [42]. On the otherhand, the adsorbent surface exhibits relatively positive charge valuesat pH higher than 1.0 and this parameter slightly decreased aroundpH (~1–2) and the surface shows gradually the more negative valuesas the pH increased. Therefore, at higher pH the DY-12 dye moleculehas strong repulsion with adsorbent surface. A lower adsorption athigher pH valuesmay bedue to the abundance of OH− ions and becauseof ionic repulsion between the negatively charged surface and the an-ionic dye molecules. There are also no more exchangeable anions onthe outer surface of the adsorbent at higher pHvalues and consequentlythe adsorption decreases. At lower pH, more protons will be available,thereby increasing electrostatic attractions between negatively chargeddye anions and positively charged adsorption sites and causing anincrease in dye adsorption [26,28,29,34,43].
3.3. Effect of ionic strength
The ionic strength significantly reduces the fluctuation of pH andimproves the precision of measurement of pH [29]. On the other
80
85
90
95
100
0 1 2 3 4 5 6pH
DY
12
rem
oval
(%
)
Fig. 5. Effect of pH on the removal of DY-12 by CdO-NW-AC at room temperature,contact time of 9.5 min, adsorbent dosage of 0.005 g in 50 mL and dye concentrationof 25 mg L−1.
Fig. 6. pHzpc of CdO-NW-AC (pHi=initial pH before removal and pHf=forward pH ofremoval).
A
B
Fig. 7. A: Effect of CdO-NW-AC dosage on DY-12 removal at the dye concentrationof 25 mg L−1, at pH 1, and room temperature, B: Effect of CdO-NW-AC dosage onDY-12 removal at the dye concentration of 40 mg L−1, at pH 1 at room temperature.
75
80
85
90
95
100
15 20 25 30 35 40 45 50
Initial dye concentration (mg/L)
Rem
oval
(%
)
Fig. 8. Effect of initial dye concentration on removal of DY-12 at 0.005 g of CdO-NW-ACin 50 mL, pH 1 and room temperature.
2262 M. Ghaedi et al. / Materials Science and Engineering C 33 (2013) 2258–2265
hand, electrolyte acts as salting out agent and improves the removalpercentage of dye [34]. Ionic strength influences on both electrostaticand non-electrostatic interactions between the adsorbate and theadsorbent surface [29,34]. The removal percentages of DY-12 at 0,0.25 and 0.5 mol L−1 were 96.2, 98.8 and 98.9, respectively. It wasobserved that the removal percentage of DY-12 onto CdO-NW-ACwas not significantly affected with increasing KCl concentrationfrom 0 to 0.5 M. This phenomenon shows that Cl− ions do not com-pete with DY-12 functional groups for adsorption on CdO-NW-AC.Therefore, this process can be applied for DY-12 removal from waste-water samples with normally high salt.
3.4. Effect of adsorbent dosage
The influence of amount of CdO-NW-AC at two different DY-12concentrations of (25 and 40 mg L−1) on its removal efficiency wasstudied by changing the quantity of adsorbent in the ranges of 20 to160 mg L−1 and 80 to 220 mg L−1 for 25 and 40 mg L−1 respectively.As it can be seen (Fig. 7A, B), by increasing the amount of CdO-NW-ACtill 0.1 and 0.14 g L−1 for 25 and 40 mg L−1 of DY-12 enhancementin the DY-12 removal percentage was achieved and further additionof adsorbent amount does not change the removal percentage signifi-cantly. The increase in actual amount of adsorbedDY-12 and its removalpercentage can be attributed to the high amount of vacant site with re-spect to DY-12 molecule and availability of more adsorption sites[44,45].
3.5. Effect of initial dye concentration on adsorption of DY-12
The diffusion and convection are the main tools for mass transferof dye molecule to the adsorbent. Among them the concentration gra-dient was significantly influenced by the surface area of the adsorbentand concentration of the dye molecule controls the rate of dye trans-fers to the adsorbent surface [45,47].
Effect of initial DY-12 concentration on its adsorption and removalpercentage in the range of 15 to 45 mg L−1 at room temperature wasexamined and results are shown in Fig. 8. It is evident from thepresented result that the amount of adsorbed DY-12 at low initialconcentration was smaller than the corresponding amount at higherinitial value, while the removal percentages decrease significantlywith increasing initial DY-12 concentration. By raising DY-12 concen-tration although the concentration gradients increase, but due tosaturation of CdO-NW-AC the removal percentages significantly de-crease. At a low concentration of DY-12 due to high surface area ofCdO-NW-AC the fast adsorption rate and short extraction time wereachieved. These results clearly indicate that the adsorption of DY-12from its aqueous solution depends to its initial concentration[23,29,46–48].
3.6. Effect of temperature
Various textile dye effluents are produced at relatively high andvarious temperatures. Therefore, the influence of temperature for
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0 2 4 6 8 10 12
Ce (mg/L)
Ce/q
e (g
/L)
Fig. 9. Langmuir isotherm for adsorption of DY-12 onto 0.005 g of CdO-NW-AC in50 mL of different initial dye concentrations, room temperature, pH 1 (qe: experimentaldata of the equilibrium capacity (mg g−1) and Ce: dye concentration (mg L−1) atequilibrium).
2263M. Ghaedi et al. / Materials Science and Engineering C 33 (2013) 2258–2265
the real application of the CdO-NW-AC for DY-12 removal was inves-tigated. The endothermic or exothermic nature of the adsorption pro-cess was examined by conducting similar experiments at varioustemperatures in the range of 283–333 K. Improvement in DY-12 re-moval percentage with raising temperature shows the endothermicnature of the adsorption process that may be attributed to increasingdiffusion coefficient of dye molecule and its dehydration that in-creases dye reactivity [23,35].
3.7. Adsorption equilibrium study
The isotherm analysis of the equilibrium data was examined byfitting the experimental data to Langmuir, Freundlich and Tempkinisotherms to find the suitable model [34,49,50]. The Langmuir plot(Ce/qe vs. Ce) for DY-12 adsorption at room temperatures gives astraight line and the value of Qm and KL constants and the correlationcoefficients for this model are presented in Table 1 and Fig. 9. The iso-therms of DY-12 on CdO-NW-AC were found to be linear over thewhole concentration range studies with extremely high correlationcoefficients (R2>0.998) confirmed by error analysis.
Table 1 shows the Freundlich adsorption isotherm constant andits respective correlation coefficients. Log qe is plotted against Log ceand the data are treated by linear regression analysis [35], while1/nF and KF constants are determined from the slope and intercept.KF (mg g−1 (L/mg)1/n) approximately shows adsorption capacityand 1/n indicates the adsorption intensity. The magnitude of theexponent (1/n) represents the adsorption favorability. Values of n>1represent favorable adsorption [51]. The values of KF, n and the linearregression correlation (R) for the Freundlich model are given inTable 1. The value of 1/nF is known as the heterogeneity factor andranges between 0 and 1. For more heterogeneous surface, the value of1/nF is close to 0 [51], and the values higher than unity of 1/nF showphysical nature of the adsorption process. Also, Table 1 shows theTempkin adsorption isotherm constant and its respective correlationcoefficients [52,53]. The low R2 values of thesemodels show their inap-plicability for interpretation of experimental data.
Tempkin and Pyzhev [54] based on decreasing adsorption heat viarespective interaction between them in linear form is represented byan equation presented in Table 1.
Values of B1 and KT were calculated from the plot of qe against LnCe (Table 1). As shown in Table 1 the Tempkin isotherm R2>0.9 isnear the value of Langmuir and shows the strong interaction betweenadsorbate molecules and adsorbent surface.
3.8. Kinetic study
The nature of the kinetic and rate limiting step of the adsorptionprocess was examined by fitting the experimental data to variouskinetic models such as pseudo-first and second-order, Elovich andinterparticle diffusion kinetic models. In pseudo-first-order kinetic
Table 1Isotherm parameters and correlation coefficients calculated by various models.
Isotherm equation Condition
Langmuir-1: Ce/qe=(1/KL Qm)+Ce/Qm This assumes that in Langmuir adsorption takhomogeneous sites within the adsorbent andinteraction among adsorbed species and thatsaturated after one layer of adsorbate molecuadsorbent surface.
Freundlich: ln qe=ln KF+(1/n) ln Ce The Freundlich isotherm model takes the muheterogeneous adsorption into account.
Tempkin: qe=BT ln KT+BT ln Ce The Tempkin isotherm model assumes effectinteractions among adsorbate particles and sudecrease in the heat of adsorption of all the mlayer, due to these interactions.
model as widely applicable kinetic model [55], plotting the value ofLog (qe−qt) versus t gives a linear relationship that k1 and qe canbe determined from its slope and intercept. If the intercept dose isnot equal (qe) then the reaction is not likely to be first-order reaction,even if this plot has high correlation coefficient [56]. The kinetic datawere further analyzed using Ho's pseudo-second-order kineticsmodel [57]. From the slope and intercept of plot of t/q versus t theconstants like k2 and also qeq (mg g−1) were calculated and theirvalue in addition to linear regression correlation coefficient (R2)values is summarized in Table 2. The increase in their values byincreasing initial dye concentration shows high tendency of adsorbentfor adsorption of DY-12 molecules. This agreement of experimentaland calculated equilibrium adsorption capacity (qe) value shows suit-ability of the second order kinetic model (Fig. 10) for interpretation ofexperimental data. The high correlation coefficients of this model atall concentrations (greater than 0.99) show its applicability for inter-pretation of the entire adsorption data. The intraparticle diffusion(another kinetic model) should be used to study the rate-limiting stepof the adsorption process [58,59]. The rate constant Kid was evaluatedfrom the slope of the second regression line and bounding layerobtained from intercept show the thickness of the boundary layer(Table 2). The intraparticle diffusion is the sole rate-limiting step, ifthe plot of qt versus t 0.5 plots pass through the origin. In the presentstudy such observation has not seen. This result show that surface ad-sorption and intraparticle diffusion were concurrently control theDY-12 adsorption onto CdO-NW-AC.
The Elovich equation is another rate equation based on the ad-sorption capacity [49–61]. Plot of qt versus ln (t) should yield a linearrelationship that the slope of (1/β) and intercept of (1/β) ln (αβ)used for evaluating this model are constant. The Elovich constants
Parameters Value
es place at specificthere is no significantthe adsorbent isles formed on the
Qm Theoretical maximum adsorptioncapacity (mg g−1).
ltilayer and 1/n Heterogeneity factor (L/g) 0.1071KF Isotherm constant indicate the capacity
parameter (mg g−1)276.503
R2 The correlation coefficients 0.9137of some indirectggests a linearolecules in the
BT Related to the heat of adsorption 26.014KT Equilibrium binding constant (L mg−1) 5.8×104
R2 Correlation coefficients 0.923
Table 2Adsorption kinetic parameters at different initial DY-12 onto 0.1 g L−1 of CdO-NW-AC at pH 1, room temperature and DY-12 concentrations of 25 and 40 mg L−1.
Model Initial DY-12 concentration (mg L−1)
25 40
First-order kineticlog(qe−qt)=log(qe)−(k1/2.303) t
k1 Rate constant of pseudo-first order adsorption (L min−1). 6.4484×10−3 8.0605×10−3
qe (calc) Adsorption capacity at equilibrium (mg g−1) 87.3575 272.27R2 Correlation coefficients 0.9337 0.931
Second-order kinetic(t/qt)=1/k2qe2+1/qe(t)
k2 Rate constant of pseudo-second order adsorption (L min−1). 1.42416 5.806×10−5
qe (exp) Experimental data of the equilibrium capacity (mg g−1) 245.1589 392.8865
0
0.5
1
1.5
2
2.5
3
3.5
0 100 200 300 400 500 600 700 800 900
Time (s)
t/qt
(g/
mg)
40ppm25ppm
Fig. 10. Plot of pseudo-second-order kinetics for adsorption of DY-12 onto 0.005 g ofCdO-NW-AC at pH 1, at room temperature and at dye concentrations of 25 and40 mg L−1.
2264 M. Ghaedi et al. / Materials Science and Engineering C 33 (2013) 2258–2265
obtained from the slope and the intercept of the straight line arereported in Table 2.
3.9. Thermodynamic study
Increasing temperature by decreasing the solution viscosity in-creased the rate of diffusion of the adsorbate molecules across theexternal boundary layer and the internal pores of the adsorbent particle.In addition, changing temperature will change the equilibrium capacityof the adsorbent for a particular adsorbate [62]. Thermodynamicparameters for the present system were calculated and the respectivevalue is presented in Table 3 based on the van't Hoff equation:
LnK∘ ¼ ΔS∘=R−ΔH∘=RT: ð2Þ
Table 3Thermodynamic parameters for adsorption of DY-12 onto 0.1 g L−1 CdO-NW-AC at pH 1 a
Adsorbent C0 (mg L−1) Parameter Temperature (K)
283.15
CdO-NW-AC 25 Ko 10.104840 2.9530
CdO-NW-AC 25 ΔG° (kJ/mol) −5.44540 −2.5491
C0 (mg L−1) ΔS° (J/mol K)
25 365.64740 152.478
The slope and intercept of the van't Hoff plot are equal to ΔH°/Rand ΔS°/R, respectively where R is the universal gas constant(8.314 J/(mol K) and T is the absolute temperature (K).
The change in standard free energy (ΔG°) of adsorption wascalculated from the following equation:
ΔG∘ ¼ −RT lnK∘ ð3Þ
where ΔG° is the free energy change (kJ mol−1) and Ko is the ther-modynamic equilibrium constant.
The negative value of ΔG° and the positive value of enthalpy showspontaneous endothermic nature of the adsorption process. As it canbe seen (Table 3) increasing the initial DY-12 concentration led tonegative value of Gibbs free energy.
The values of activation energy (Ea) and sticking probability (S*)were estimated from the experimental data and the calculated valuebased on modified Arrhenius type equation according to surface cov-erage (θ) is as follows [63,64]:
S� ¼ 1−θð Þe− Ea=RTð Þ: ð4Þ
The sticking probability (S*) value lies in the range of 0bS*b1which depends on the system temperature. The parameter S* indi-cates the measure of the potential of an adsorbate to remain on theadsorbent indefinite. The surface coverage, θ, can be calculated fromthe following equation:
θ ¼ 1−Ce=Co½ �: ð5Þ
The activation energy and sticking probability were estimatedfrom a plot of ln (1−θ) vs. 1/T.
2265M. Ghaedi et al. / Materials Science and Engineering C 33 (2013) 2258–2265
4. Conclusion
CdO-NW-AC has been synthesized and characterized with SEM,XRD and UV–vis techniques. This new adsorbent has been utilizedfor the removal of DY-12 in a batch sorption process and the influenceof parameters such as initial DY-12 concentration, contact time, initialpH and the amount of CdO-NW-AC on DY-12 removal has been inves-tigated. The experimental data were analyzed by the Langmuir,Freundlich and Tempkin isotherm models and the results showedthat the equilibriumdatawere best fitted and described by the Langmuirisotherm model. The adsorption kinetics of DY-12 onto CdO-NW-ACfollowed the pseudo second order model. The CdO-NW-AC is applicablefor quantity removal of DY-12 for a contact time of 10 min with adsorp-tion capacity of 357.142 mg g−1. On the other hand the experimentalresults showed that CdO-NW-AC can be used at least for 10 times with-out any considerable change in its adsorption properties. By consideringall of these results, it can be concluded that CdO-NW-AC can be utilizedas a new and effective adsorbent in removal of DY-12, a highly toxicdye, from waters and wastewaters.
Acknowledgments
The authors express their appreciation to the Graduate School andResearch Council of the University of Yasouj for financial support ofthis work. We also have deep sincere and regard to Dr. Celal Duranand Dr. Duygu Ozdes for their help in English editing.
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