Hierarchically assembled mesoporous ZnO nanorods for the removal of lead and cadmium by using differential pulse anodic stripping voltammetric method

Powder Technology 239 (2013) 208–216

Contents lists available at SciVerse ScienceDirect

Powder Technology journal homepage: www.elsevier.com/locate/powtec

Hierarchically assembled mesoporous ZnO nanorods for the removal of lead and cadmium by using differential pulse anodic stripping voltammetric method K. Yogesh Kumar a, H.B. Muralidhara a,⁎, Y. Arthoba Nayaka b, J. Balasubramanyam a, H. Hanumanthappa a a b

Centre for Nanosciences, K.S. Institute of Technology, Visvesvaraya Technological University, Bangalore 560 062, India Department of PG Studies and Research in Chemistry, School of Chemical Sciences, Kuvempu University, Shankaraghatta 577 451, India

a r t i c l e

i n f o

Article history: Received 27 August 2012 Received in revised form 25 January 2013 Accepted 2 February 2013 Available online 8 February 2013 Keywords: Zinc nanorods Adsorption Lead Cadmium DPASV Regeneration

a b s t r a c t Mesoporous hierarchical ZnO nanorods (ZnOs) with specific surface areas of 15.75 m 2 g−1 and pore volumes of 0.038 cm 3 g−1 were successfully synthesized by hydrothermal method. The ZnOs were characterized by XRD, SEM, TEM, SAED, BET surface area analyze and EDX. The ZnOs were investigated as adsorbents for the removal of Pb(II) and Cd(II) from aqueous solutions. Batch experiments were conducted under different adsorbate concentrations, contact times, adsorbent dosages, pHs and temperature conditions. Equilibrium data were best fitted with the Langmuir and Freundlich isotherm models and maximum adsorption capacities of Pb(II) and Cd(II) were determined to be 160.7 and 147.25 mg g−1 respectively. Adsorption kinetics of both metal ions followed the pseudo-second-order model. The ZnOs remained effective for heavy metal ion adsorption after regeneration by an acid wash. Pb(II) and Cd(II) loading capacities of the recycled ZnOs have an average two-thirds that of the original capacities. The metal ion adsorption capacity and reusability of ZnOs make them promising adsorbents for wastewater cleanup. © 2013 Elsevier B.V. All rights reserved.

1. Introduction With the rise in industrial productions, large quantities of heavy metals have been released into the natural environment. Heavy metals are mostly toxic, even at very low concentrations [1], and can not only have harmful effects on organisms living in water, but also accumulate throughout the food chain and may affect human beings as well [1,2]. Some metals associated with these activities are cadmium, chromium, lead and mercury [3]. The occurrence of heavy metals, especially lead and cadmium, in industrial effluents beyond permissible limits brings serious environmental pollution, threatening human health and ecosystem [4,5]. Therefore, these pollutions must be removed before the effluents are discharged into the environment. Many techniques, such as ion exchange, precipitation, adsorption, membrane processes, reverse osmosis, sedimentation, and electrodialysis, have been employed for separation of heavy metals from wastewater. With the increase in environmental pollution, there is a growing demand to develop novel adsorbents of higher efficiency for removal of heavy metal ions from aqueous media over those commercially available [6,7].

⁎ Corresponding author. Tel.: +91 9739315239; fax: +91 80 28435723. E-mail addresses: [email protected] (K.Y. Kumar), [email protected] (H.B. Muralidhara), [email protected] (Y.A. Nayaka), [email protected] (J. Balasubramanyam), [email protected] (H. Hanumanthappa). 0032-5910/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.powtec.2013.02.009

Many efforts have been devoted to the synthesis, characterization and application of the uniform mesoporous materials over the last decade [8,9]. Hierarchical porous metal oxides have attracted attention in many practical fields including material adsorption, selection, sensing, removal, storage, and release. This is due to their regular pore structures, enhanced surface areas, and controlled organization of primary building units with various dimensions into ordered superstructures [10–14]. ZnO is a fascinating semiconducting material which has been much investigated in recent years due to its extensive important applications as catalysis, electrode materials, gas sensors and electrochromic films [15–17]. Recently, various chemical and physicochemical methods have been employed to produce ZnO nanomaterials including nanoparticles, nanorods, nanowires, and nanosheets [18]. Among the various hierarchical structures, ZnOs remain of significance to chemists and material researchers due to their ability to form nanomaterials easily and role in device applications [19]. Particularly, ZnOs have many functional hydroxyl groups. All the protons of these hydroxyl groups may be readily exchanged with heavy metal ions in aqueous solutions. As an environmentally friendly material, ZnOs are promising adsorbents for contaminant removal and environmental remediation [20,21]. Therefore, developing a facile and template-free method to prepare hierarchical ZnO structures is of scientific and practical importance. Hydrothermal method has several advantages over other methods of preparing nanoparticles (such as sol–gel, solid state and chemical precipitation), because of the use of simple equipment, catalyst-free

K.Y. Kumar et al. / Powder Technology 239 (2013) 208–216

growth, low cost, easy control of particle size, environmental friendliness and being less hazardous [22]. Many techniques, such as spectral [23,24] and electrochemical methods, have been used for the detection of trace Pb(II) and Cd(II) in wastewater. But traditional spectral methods are somewhat cumbersome, time consuming and especially not suitable for in situ measurement because of their complicated and ponderous instruments. In contrast, the electroanalytical methods have more attractive features such as low cost, favorable portability and easy operation procedures. Differential pulse anodic stripping voltammetry (DPASV) is considered to be one of the most sensitive electroanalytical methods in trace analysis of heavy metals [25,26]. However, so far, the studies using electrochemical techniques to remove heavy metals in aqueous solutions are limited. Potential applications of adsorption onto ZnOs have not been explored in detail. The purpose of this study is to investigate the feasibility of ZnOs as high efficient adsorbent for the removal of Pb(II) and Cd(II) from aqueous solutions. The effect of various operational conditions such as contact time, pH and adsorbent dosage was systematically studied. Adsorption kinetics and isotherms were also analyzed to reveal the adsorption mechanisms. 2. Methods and materials 2.1. Reagents All the reagents used in the experiments were of analytical grade and purchased from Fisher Scientific India Pvt. Ltd. (Mumbai, India). Stock solution (1000 mg L −1) of Pb(II) or Cd(II) was prepared by dissolving stoichiometric amount of corresponding nitrate, Pb(NO3)2 or Cd(NO)3·4H2O, in deionized water and further diluted to the desired concentrations for the experiments. 2.2. Instruments Metal ion concentrations were analyzed by DPASV using an electrochemical work station (CHI 660D). A three electrode system consisted of a glassy carbon (Ø= 3 mm) as working electrode, saturated calomel as reference electrode and platinum wire as auxillary electrode. X-ray diffraction (XRD) patterns were obtained on a Bruker D2 Phaser XRD system. Surface morphology (SEM) was studied using scanning electron microscope (JEOL JSM 840A), coupled with energy dispersive X-ray analyzer (EDX). Transmission electron microscope (TEM) and selected area diffraction patterns (SAED) were recorded by using Philips CM-200 instrument. BET surface area, total pore volume and average pore size of the ZnOs were measured using ASAP 2010 Micrometrics instrument by Brunauer–Emmett–Teller (BET) method. Finally, the metal ion concentration of real water sample was analyzed by Atomic Absorption Spectroscopy (AAS) using Shimadzu AA-6300.

209

of parameters that influence the process of adsorption. In this method, a series of 250 mL of Erlenmeyer flasks was filled with 100 mL of metal ion solution of varying concentrations (100–400 mg L − 1). Then an equal amount of ZnOs (250 mg L − 1) was added into each flask and subjected to agitation using incubator shaker at 200 rpm, until equilibrium was reached (90 min). The resultant solutions were centrifuged and the supernatant liquids were determined for Pb(II) and Cd(II) ions. For all the experimental studies, the reaction temperature was kept constant at 303 K, and pH of the solution maintained unaltered unless and otherwise mentioned. The amount of adsorbate adsorbed at equilibrium condition, qe (mg g − 1) was calculated in the following equation: qe ¼

ðC0 −Ce Þ V W

ð1Þ

where C0 and Ce are the initial and equilibrium concentrations (mg L − 1) respectively. V is the volume of solution (L) and W is the mass of adsorbent used (g). The effect of pH on the adsorption capacity of metal ion was evaluated by agitating 200 mg L −1 metal ion solution with 250 mg L −1 of ZnOs for predetermined equilibrium time at pH ranging from 4.0 to 8.0. The pH of metal ion solution was adjusted by using 0.5 M HCl or 0.5 M NaOH. Similarly, the effect of adsorbent dosage (250, 500, 750, 1000 and 1250 mg L −1) was also studied in batch experiments. The amount of metal ion adsorbed at equilibrium qt (mg g−1) and the metal ion removal efficiency R(%) were computed by Eqs. (2) and (3) respectively. qt ¼

ðC0 −Ct Þ V W

Rð% Þ ¼

ð2Þ

ðC0 −Ce Þ  100 C0

ð3Þ

where Ct (mg L −1) is the equilibrium concentration of metal ion. 3. Results and discussion 3.1. Characterization of ZnOs XRD patterns of the above mentioned ZnOs are summarized in Fig. 1. All the diffraction peaks in the patterns can be exactly indexed as hexagonal wurtzite ZnOs with lattice constants a = 3.253 nm and c = 5.209 nm, which are in accordance with the values in the standard card (JCPDS 80-0075). No characteristic peaks were observed for the other impurities. In addition, all peaks of the samples are much higher

24000

(101)

22000

2.3. Preparation of hierarchical zinc oxide nanorods

20000 18000

Intensity (a.u)

ZnOs were synthesized via the hydrothermal method in the presence of Triton X-100. Experimental details are described as follows: 20 mL of 0.2 mol L − 1 NaOH solution was slowly added into a 20 mL of 0.1 mol L − 1 [Zn(NO)3·6H2O] solution containing 0.004 mol L − 1 of Triton X-100 with constant stirring. After vigorous stirring for 3 h, the mixture was autoclaved at 200 °C for 5 h. After cooling the reaction system to room temperature gradually, the white precipitate was filtered from the solution and washed thoroughly with deionized water and absolute alcohol several times and then dried in an oven at 50 °C for 12 h.

16000

(100)

14000 12000

(002)

10000 (110)

8000 (103) (112)

6000

(102)

4000

(201) (200)

2000

(202) (104)

0

2.4. Adsorption experiments The adsorption experiments of Pb(II) and Cd(II) on ZnOs were conducted in batch method, which permits the complete evaluation

20

30

40

50

60

2θ Fig. 1. Typical XRD pattern of ZnOs.

70

80

210

K.Y. Kumar et al. / Powder Technology 239 (2013) 208–216

and narrower shaped, which indicate that they have higher crystallinity and lower surface defects. The average crystallite size (Dp in nm) of ZnO particles can be estimated according to the diffraction reflection by using the Debye–Scherrer equation

depending on the parameters, such as the concentration of Zn2+ and the pH value [30]. 3.3. Adsorption studies

!

Kλ β1=2 cos θ

DP ¼

ð4Þ

where K is a constant equal to 0.89, λ is the X-ray wavelength equal to 1.54 Å, β1/2 is the full width at half maximum and θ is the half diffraction angle. The average particle size and lattice parameters are summarized in Table 1. Fig. 2 represents the morphology of the ZnOs synthesized by the hydrothermal method. Typical FESEM images of the ZnOs at two different magnifications are shown in Fig. 2(a) and (b). It is clear from the images that a very high percentage of the nanorods which are highly dispersed in the space without any aggregation are aligned [27]. In addition, their dimensions are very much identical and several nanorods are slanted or tipped over. TEM was used to further investigate the microstructure of the synthesized ZnOs. It revealed the formation of nanorods with hexagonal crystal structure as shown in Fig. 3. The diffraction pattern of a selected area (SAED) is shown in the inset, confirming their high crystallinity, and that are growing along the 001 plane, which is a common fast growth direction of ZnOs. Porous structures of the resulting samples were studied by nitrogen sorption. Fig. 4(a) and (b) presents the nitrogen adsorption– desorption isotherms and BJH pore size distribution curve of the ZnOs respectively. The prepared nanoparticles exhibit Type IV adsorption isotherm with an H3 hysteresis loop according to BDDT classification, which is a typical characteristic of mesoporous materials. Herein, formation of such mesoporous structure is attributed to the aggregation of the primary nanoparticles [28]. The surface area and average pore diameters derived from isotherm are 15.75 m 2 g −1 and 9.73 nm respectively. 3.2. Growth mechanism of ZnOs Based on the experimental results, a possible mechanism for the formation of ZnOs has been proposed. Morphologies of the material are mainly determined by, the crystal nucleation and crystal growth direction. Large quantities of the growth units ([Zn(OH)4] 2−) were formed under the condition of heavy alkaline solution, which contribute to the growth of ZnO nuclei and the crystal nuclei exhibiting fast growth orientation (001), [29]. Zn

2þ

−

þ 2OH ↔ZnðOHÞ2

ð5Þ

ZnðOHÞ2 þ 2OH↔½ZnðOHÞ4  ½ZnðOHÞ4 

2−

2−

ð6Þ

2−

↔ZnO2 þ 2H2 O

ð7Þ

−

2−

ZnO2 þ H2 O↔ZnO þ 2OH

ð8Þ

Significant reactions involved in the growth are illustrated in the above equations. As in Eq. (6), the product is not necessarily Zn(OH)42−, but could also be in the form of Zn(OH)+, Zn(OH)2 or Zn(OH)3−,

3.3.1. Effect of initial concentration The dependence of the adsorption capacity of ZnOs on the equilibrium concentrations of Pb(II) and Cd(II) is presented in Fig. 5. The results reveal that adsorption capacities increased steadily with metal ion concentration. This can be attributed to the increase in the concentration gradient which acts as a driving force for the adsorption process. At higher concentration of metal ions, the active sites of ZnOs were surrounded by much more metal ions, and the number of metal ions competing for available binding sites increases leading to an increased uptake of metal ions from the solution. Therefore, the values of qe increased with the increase of initial metal ion concentrations (C0). The results show that the adsorption capacity obtained for Pb(II) 1295.44 mg g −1 is higher than that of Cd(II) 992 mg g −1. The selectivity of ZnOs is, therefore, in the order of Pb>Cd which could be attributed to the higher electronegativity of lead (2.33) than cadmium (1.69), which has a stronger affinity for the metal oxides [31,32]. 3.3.2. Effect of adsorbent dose The dependence of Pb(II) and Cd(II) adsorption was studied by varying the amount of adsorbents from 250 to 1250 mg L −1, while keeping metal ion concentration constant (200 mg L −1). From Fig. 6, it can be observed that removal efficiency of the adsorbent generally improved by increasing its dosage. This is expected due to the fact that the higher the dose of adsorbents in the solution, the greater is the availability of exchangeable sites for the ions. Results showed no further increase in adsorption after a certain amount of adsorbent was added. The maximum percentage removal of Pb(II) was about 96%, while for Cd(II) it was 91% at the dosage of 1250 mg L −1. These results also suggest that after a certain dose of adsorbent, the maximum adsorption sets in and hence the amount of ions bound to the adsorbent and the amount of free ions in the solution remain constant even with further addition of the dose of adsorbent. 3.3.3. Effect of pH The pH of the solution is recognized as an important parameter that significantly affects adsorption of metal ions. For this reason, the effect of pH on adsorption of Pb(II) and Cd(II) was studied. As shown in Fig. 7, the pH of the solution was varied from 4.0 to 8.0, which was appropriate to prevent precipitation of metals in the form of hydroxides under higher pH conditions. Generally, adsorption of tested metallic species increases with increase in the pH. The low adsorption in the acidic region can be partially attributed to the competition between hydrogen and metal ions for the same sites. The increase in pH makes the ZnO surface more negative, thus enhancing electrostatic interactions between metal cations and the adsorbent, resulting in higher retention of metal species [33]. The effect of solution pH on adsorption could be explained on the basis of surface charge of the adsorbent and the degree of speciation of adsorbates. At lower pH the adsorbent surface, hydrous oxide (MOH) surface will be completely covered by H + ions (MOH +2), at higher pH, hydroxide ions react with the hydrous oxide to produce deprotonated oxide (MO −) as shown in the following reactions [34]. þ

Table 1 Parameters derived from XRD of ZnOs.

þ2

ð9Þ

−

ð10Þ

MOH þ H →MOH

Adsorbent

2θ

hkl

Size

Structure

ZnOs

34.40 36.27 47.46

(002) (101) (102)

21.9 24.5 22.9

Hexagonal

−

MOH þ OH →MO þ H2 O þ2

MOH

−

−

þ 2OH →MO

ð11Þ

K.Y. Kumar et al. / Powder Technology 239 (2013) 208–216

(a)

211

(b)

Fig. 2. Low (a) and high (b) magnification SEM micrographs of ZnOs.

3.4. Adsorption isotherms The equilibrium adsorption isotherms are one of the most useful data to understand the mechanism of the adsorption. Several isotherm equations are available and two important isotherms are chosen in this study, namely the Langmuir and Freundlich isotherms. All the isotherm model parameters for the adsorption are listed in Table 2.

constant (L g−1), and aL is the Langmuir constant (L mg−1). Therefore, a plot of Ce/qe versus Ce gives a straight line of slope aL/KL and intercepts 1/KL. The theoretical maximum adsorption capacity (qmax) corresponding to Langmuir constants is numerically equal to KL/aL. Essential characteristics of the Langmuir isotherm parameters can be used to predict the affinity between the sorbate and sorbent using

3.4.1. Langmuir isotherm The theoretical Langmuir adsorption isotherm is based on three assumptions, namely: adsorption cannot proceed beyond monolayer coverage, all surface sites are equivalent and can accommodate at most one adsorbed atom, and the ability of a molecule to adsorb at a given site is independent of the occupation of neighboring sites. At equilibrium, there is no net charge of surface coverage. The non-linear equation of Langmuir isotherm model is expressed as follows [35] qe ¼

KL Ce 1 þ aL Ce

Adsorption Desorp tion

7

Volume Adsorbed (cm³/g)



8

 ð12Þ

where qe is the solid phase equilibrium concentration (mg g−1), Ce is the liquid phase equilibrium concentration (mg g−1), KL is the Langmuir

(a)

6 5 4 3 2 1 0 0.2

0.4

0.6

0.8

1.0

Relative Pressure (P/Po) 0.0010

(b) Pore volume (cm3/g-nm)

0.0008

0.0006

0.0004

0.0002

0.0000 5

10

15

20

Pore diametre (nm) Fig. 3. Typical TEM micrograph of ZnOs; the inset figure shows the SAED patterns.

Fig. 4. Nitrogen adsorption-desorption isotherm (a) and BJH pore size distribution curve (b) of ZnOs.

212

K.Y. Kumar et al. / Powder Technology 239 (2013) 208–216

94 100

92

Removal efficiency in %

Removal efficiency in %

90 80

60 2+

100 ppm Pb 2+ 200 ppm Pb 2+ 300 ppm Pb 2+ 400 ppm Pb 2+ 100 ppm Cd 2+ 200 ppm Cd 2+ 300 ppm Cd 2+ 400 ppm Cd

40

20 2+

Pb 2+ - - - - Cd

0

88 86 84 82 80 78 76 2+

2+

200 ppm Pb 2+ 200 ppm Cd

Pb 2+ - - - - Cd

74 72 70

0

20

40

60

80

4

100

5

6

7

8

Variation of pH

Time in minutes Fig. 5. Effect of contact time on adsorption of Pb(II) and Cd(II) by ZnOs at different initial concentrations.

Fig. 7. Variation in removal efficiency of Pb(II) and Cd(II) on ZnOs as a function of pH.

separation factor or dimensionless equilibrium parameter, “RL”, expressed as in the following equation:

energy) and intensity of the sorbent, respectively [36]. The equation may be linearized by taking the logarithm on both sides:

RL ¼

1 1 þ aL Ce

ð13Þ

the values of RL indicate the shapes of isotherms to be either unfavorable (RL > 1), linear (RL =1), favorable (0b RL b 1) or irreversible (RL =0). RL was found to be in the range of 0–1 which indicates the favorable adsorption. 3.4.2. Freundlich isotherm The Freundlich isotherm is an empirical equation used to describe adsorption at multilayer and on a heterogeneous surface 1=n

qe ¼ KF Ce

ð14Þ

where qe (mg g−1) is the equilibrium value for removal of adsorbate per unit weight of adsorbent, Ce (mg L−1) is the equilibrium concentration of metal ion in solution, and KF and n are Freundlich isotherm constants which are related to the adsorption capacity (or the bonding 96 94

Removal efficiency in %

92 90 88 86 84

log qe ¼

1 log Ce þ log KF n

ð15Þ

where Ce is the equilibrium concentration (mg L−1), qe is the amount of metal ion adsorbed at equilibrium (mg g−1), KF is the Freundlich constant related to the adsorption capacity of the adsorbent, and 1/n is the Freundlich constant related to adsorption intensity. If the value of 1/n is smaller than 1, it indicates favorable adsorption of metal ion on to the surface of the adsorbent [37]. The values of KF and 1/n were determined from the slope and intercept of the lnqe vs. lnCe plots. The compounds having greater KF values have high affinity toward the adsorbent as compared to others having low KF value; the relatively high correlation coefficients indicate that the experimental data agree well with the Freundlich isotherm model. 3.5. Adsorption kinetics Several steps can be used to examine the controlling mechanism of adsorption process, such as chemical reactions, diffusion control and mass transfer. Kinetic models are used to test experimental data from the adsorption of Pb(II) and Cd(II) onto ZnOs. The kinetic parameters which are helpful for the prediction of adsorption rate give important information for designing and modeling the adsorption processes [38]. To evaluate kinetics of the adsorption process, the pseudo first-order, pseudo second-order and intraparticle diffusion models were tested to interpret the experimental data. 3.5.1. Pseudo-first-order kinetic model The adsorption kinetic data were described by the Lagergren pseudo-first-order model, which is the earliest known equation

82 80 78 2+

2+

Pb 2+ - - - - Cd

76 74

200 ppm Pb 2+ 200 ppm Cd

Table 2 Adsorption isotherm model parameters for the adsorption of Pb(II) and Cd(II) on ZnOs. Metal ion

72 200

400

600

800

1000

1200

Q0 (mg g

1400

Adsorbent dose Fig. 6. Effect of adsorbent dosage on adsorption of Pb(II) and Cd(II) by ZnOs.

Langmuir

Pb(II) Cd(II)

160.77 147.25

Freundlich

−1

)

KL (L mg 0.01 0.14

−1

)

R

2

0.93 0.98

KF (mg g−1)

nF

R2

186.33 51.76

2.30 1.68

0.98 0.99

Metal ion concentration = 200 mg L−1, adsorbent dose = 250 mg L−1.

K.Y. Kumar et al. / Powder Technology 239 (2013) 208–216

describing the adsorption rate based on the adsorption capacity. The equation for linear form is log ðqe −qt Þ ¼ log qe −

k1 t 2:303

ð16Þ

where qe and qt are the amounts of adsorbate adsorbed (mg g −1) at equilibrium and at contact time t (min) respectively, and k1 is the pseudo-first-order rate constant (min −1). The first-order-rate constant k1 can be obtained from the slope of the plot ln(qe − qt) vs t. In first-order kinetics the rate of occupation of adsorption sites is proportional to the number of unoccupied sites [39]. The results obtained from applying the first order kinetic model indicated that the correlation coefficient (R 2) values of fitting the first-order rate model are not high for the different Pb(II) and Cd(II) concentrations; furthermore, the estimated values of qe calculated from the equation differed from the experimental values, which show that the model is not appropriate to describe the adsorption process (Table 3). 3.5.2. Pseudo-second-order kinetic model The pseudo-second-order kinetic model can be represented in the following form: t 1 1 ¼ þ t qt k2 q2e qe

ð17Þ

where k2 is the rate constant of pseudo-second-order adsorption (g(mg min) −1). The pseudo-second-order rate constants were determined experimentally by plotting t/qt against t. Some experiments fitted using pseudo-second-order model are given in Table 3. In most systems, the correlation coefficients were higher than 0.99. Moreover, the calculated qe values agreed very well with the experimental data. As such, in comparison to pseudo-first-order kinetic this model is considered more appropriate to represent the kinetic data in adsorption systems. This tendency comes as an indication that the rate limiting step in adsorption of heavy metals is the chemisorptions involving valence forces through the sharing or exchange of electrons between sorbent and sorbate [40]. 3.5.3. Intraparticle diffusion model The intraparticle diffusion model proposed by Weber and Morris is applied to study the adsorption process, which is written as: 1=2

Q t ¼ kid t

þC

ð18Þ

where Kid is the intraparticle diffusion rate constant (mg g−1 min−1/2) and C is the intercept. The sorption of any metal ions from aqueous phase onto solid phase is a multi-step process involving transport of metal ions from aqueous phase to the surface of the solid particles (bulk diffusion) and then, diffusion of the metal ions via the boundary layer to the surface of the solid particles (film diffusion) followed by transport of the metal ions from the solid particle surface to its interior pores (pore diffusion or intraparticle diffusion), which is likely to be a

213

slow process, therefore, it may be the rate-determining step. In addition, sorption of metal ion at an active site on the solid phase surface could also occur through chemical reactions such as ion-exchange, complexation and chelation. The metal ion sorption is controlled usually by either the intraparticle (pore diffusion) or the liquid-phase mass transport rates (film diffusion). If the experiment is a batch system with rapid stirring, there is a possibility that intraparticle diffusion is the rate determining step. If the intraparticle diffusion is involved in the sorption process, then the plot of qt vs. t1/2 would result in a linear relationship, and the intraparticle diffusion would be the controlling step if this line passed through the origin. The shape of Fig. 8 confirms that straight lines do not pass through the origin for all studied adsorption processes. The deviation of straight lines from the origin, as shown in the figure, may be because of the difference between the rate of mass transfer in the initial and final steps of the sorption processes. The initial adsorption rate is controlled by the film diffusion, which is followed by the pore diffusion [41]. 3.6. Thermodynamic studies Thermodynamic studies are used to interpret any reaction in a better way. In the present studies also, thermodynamic studies were performed and the parameters namely free energy change (ΔG 0), enthalpy (ΔH 0), and entropy (ΔS 0), were determined at 303, 313 and 323 K. Thermodynamic parameters were calculated by using the following equations [42]: kc ¼

Ca Cb

ð19Þ

0

ΔG ¼ −RT 1n Kc

ð20Þ

s0 ΔH0 − R RT

ð21Þ

1n kc ¼

where kC is the distribution coefficient for the adsorption, Cb is the equilibrium concentration in solution (ppm), and Ca is the solid phase concentration at equilibrium. ΔH 0 is the enthalpy change, ΔS 0 is the entropy change, ΔG 0 is the Gibb's free energy change, R is the gas constant, and T is the absolute temperature. The values of Kc increased as the temperature is increased, indicating the endothermic nature of the process of removal (Fig. 9). The values of these parameters are given in Table 4. The positive value of ΔS 0 suggests that the process of adsorption is spontaneous and thermodynamically favorable. The positive entropy change also illustrates the increased randomness at the solid/solution interface. The increase in the entropy after the adsorption is presumably due to the release of aqua molecules after the metal ion is entrapped onto the surface of ZnOs. It is noted that values of ΔG 0 decrease by increasing temperature. This reveals that a greater adsorption can be obtained at higher temperatures.

Table 3 Kinetic parameters for the adsorption of Pb(II) and Cd(II) on ZnOs. Kinetic models

Parameters

First order kinetic model

qe, cal (mg g−1) k1 (min−1) R2 qe, exp (mg g−1) qe, cal (mg g−1) k2 (min−1) R2 qe, exp (mg g−1)

Second order kinetic model

Pb(II) concentration (mg L−1)

Cd(II) concentration (mg L−1)

100

200

300

400

100

200

300

400

221.3 0.03 0.98 380.6 418.4 0.08 0.99 380.6

676 0.05 0.84 701.1 793.6 0.07 0.99 701.1

812.8 0.05 0.87 1004 1123 0.09 0.99 1004

1412 0.05 0.91 1295 1515 0.06 0.99 1295

436.5 0.06 0.93 313.2 357.1 0.01 0.99 313.2

918.3 0.06 0.92 584 699.3 0.07 0.99 584

1548 0.06 0.84 800.4 934.5 0.07 0.99 800.4

1862 0.05 0.73 992 1200 0.06 0.99 992

Metal ion concentration = 200 mg L−1, adsorbent dose = 250 mg L−1.

214

K.Y. Kumar et al. / Powder Technology 239 (2013) 208–216

1500

2+

100 ppm Pb 2+ 200 ppm Pb 2+ 300 ppm Pb 2+ 400 ppm Pb 2+ 100 ppm Cd 2+ 200 ppm Cd 2+ 300 ppm Cd 2+ 400 ppm Cd

1400 1300 1200 1100

Qt

1000 900

Table 4 Thermodynamic parameters for the adsorption of Pb(II) and Cd(II) on ZnOs.

2+

Pb 2+ - - - - Cd

800 700 600

Temperature (°C)

ΔG0 (kJ mol−1)

ΔS0 (JK−1 mol−1)

ΔH0 (kJ mol−1)

Pb(II) 30 °C 40 °C 50 °C

−8.41 −9.42 −10.07

82.51

−17.04

Cd(II) 30 °C 40 °C 50 °C

−5.99 −6.50 −7.14

56.97

−11.63

Metal ion concentration = 200 mg L−1, adsorbent dose = 250 mg L−1.

500 400 300 200 3

4

5

6

7

8

9

10

t1/2 Fig. 8. Intraparticle diffusion model fitting of the adsorption kinetics Pb(II) and Cd(II) on ZnOs.

The mechanism of desorption might be attributed to the replacement of H + ions on the metal loaded adsorbents. With the decrease of pH, there was an increase in H + ion concentration. The abundant H + ions in the solution would compete with the metal ions for the exchange sites. As H + ions occupied the sites in the adsorbents, the adsorbed metal ions were released into the aqueous solution. The aforementioned results confirmed the possibility of fast recovery of the ZnOs by reducing pH, which should be of significance to practical applications [43].

3.7. EDX analysis

3.9. Comparison with other adsorbents

Fig. 10 shows the EDX spectra of ZnO adsorbent before and after loading with Pb(II) and Cd(II) ions respectively. Spectra clearly shows the peak for the presence of zinc and oxygen as major constituents before adsorption. Comparing the spectra of the ZnOs loaded with Pb(II) and Cd(II) with that of unloaded one, the lead peak and cadmium peak could be observed. It was suggested that heavy metals had been adsorbed on the surface of ZnOs successfully. Moreover, before and after adsorption no characteristic peaks were observed for any impurities.

The comparison between the maximum uptake capacities for Pb(II) and Cd(II) with ZnOs and other adsorbents reported in the literature is given in Table 5. The results show that the maximum adsorption capacity obtained in this study is comparable to the result from the reported low-cost adsorbents, since oxides are also highly stable under higher temperatures and even to acidic treatment.

3.8. Regeneration studies To evaluate the regeneration performance of the ZnOs, Pb(II) and Cd(II), loaded ZnOs were treated with 0.1 mol L −1 HCl solution (pH ≤ 2) and shaken in digitally controlled water bath shaker for a period of 1 h. The removal percentage after first and second usages were up to 46.21 and 34.16% for Pb(II) and Cd(II) respectively. 3.8 3.7 3.6 3.5 3.4 3.3

lnKC

3.2 3.1 3.0

3.10. Real sample analysis The proposed DPASV method was applied to the removal of Cd(II) from the ground water sample collected from Peenya Industrial area of Bangalore, India. The water sample had Cd(II) concentration (0.004 mg L −1) above the maximum acceptable concentration for drinking water (0.003 mg L −1). In this study, ZnO (250 mg L −1) was added into 100 mL of real water sample containing Cd(II) and the adsorption experiments were conducted at optimum conditions. The concentrations of the Cd(II) were determined by calibration plot of the stripping current against the concentrations of the metal standard. The peak potential of Cd(II) for the standards and samples was not obtained at a definite potential characteristics of the metal but at varying close potentials to it [52]. To examine the reliability and accuracy of the method, the removal percent of analyte ion was also evaluated using AAS method. The result obtained at DPASV method (93%) showed good agreement with the AAS measurements (95%), indicating the potential application of hierarchical ZnOs for the removal of Cd(II) or other heavy metal ions in real water samples by using DPASV method [52]. 4. Conclusion

2.9 2.8 2+

Pb 2+ - - - - Cd

2.7 2.6 2.5 2.4 0.00300

0.00305

0.00310

0.00315

0.00320

1/T(K-1) Fig. 9. Effect of temperature on adsorption of Pb(II) and Cd(II) on ZnOs.

In summary, ZnOs have been successfully synthesized via a low-cost approach in a mixed solution of zinc nitrate, sodium hydroxide, and Triton X-100 as capping agent. The results of various characterizations showed the formation of hierarchical nanorods. Adsorption of Pb(II) and Cd(II) was dependent on metal ion concentration, adsorbent dose, contact time, pH and temperature. The Langmuir and Freundlich adsorption isotherm models were used for the mathematical explanation of the adsorption equilibrium of Pb(II) and Cd(II) ions. The maximum adsorption capacities of Pb(II) and Cd(II) were determined to be 160.7 and 147.25 mg g−1 respectively. Kinetic studies demonstrated

K.Y. Kumar et al. / Powder Technology 239 (2013) 208–216

215

(a)

(c)

(b)

Fig. 10. EDX patterns of ZnOs (a) after adsorption of Pb(II) (b) and after adsorption of Cd(II) (c) ions on ZnOs.

that the mechanism for adsorption of Pb(II) and Cd(II) followed the pseudo-second-order rate model, which provided the best fit for the experimental data. Intraparticle diffusion model suggested that the initial adsorption rate was controlled by the film diffusion, which was followed by pore diffusion. The thermodynamic studies revealed that

Table 5 Comparison of monolayer maximum capacities of some adsorbents for Pb(II) and Cd(II) from aqueous solutions. Adsorbents

Adsorbate

Adsorption capacity (mg g−1)

Macro fungus biomass Activated carbon Manganese oxide-coated zeolite Manganese oxide-coated carbon nanotubes ZnOs Activated carbon Mango peel waste Chitosan Kaolinite modified with tripolyphosphate ZnOs

Pb(II) Pb(II) Pb(II) Pb(II)

38.4 43.85 60.09 78.74

Pb(II) Cd(II) Cd(II) Cd(II) Cd(II)

160.70 19.50 68.92 105 113.28

Present study [48] [49] [50] [51]

Cd(II)

147.25

Present study

References

[44] [45] [46] [47]

the adsorption of Pb(II) and Cd(II) on ZnOs is endothermic and spontaneous in nature. Based on the above results a conclusion can be made that ZnOs were excellent adsorbents for wastewater cleanup. Acknowledgment The authors are grateful to Visvesvaraya Technological University for providing financial support under research grant scheme (project no VTU/Aca/2010-11/a-9/11353). The authors are also grateful to K.S. Institute of Technology, Bangalore for their support and IIT Kanpur, IIT Bombay and PPRI Bangalore for providing instrumental facilities. References [1] E. Munaf, T. Takeuchi, Monitoring of university effluents, in: T. Korenaga, et al., (Eds.), Hazardous Waste Control in Research and Education, Lewis Publisher, USA, 1994. [2] Y.H. Chen, F.A. Li, Kinetic study on removal of copper(II) using goethite and hematite nano-photocatalysts, Journal of Colloid and Interface Science 347 (2010) 277–281. [3] R. Zein, R. Suhaili, F. Earnestly, E. Munaf Indrawati, Removal of Pb(II), Cd(II) and Co(II) from aqueous solution using Garcinia mangostana L. fruit shell, Journal of Hazardous Materials 181 (2010) 52–56. [4] M.K. Mondal, Removal of Pb(II) ions from aqueous solution using activated tea waste: adsorption on a fixed-bed column, Journal of Environmental Management 90 (2009) 3266–3271.

216

K.Y. Kumar et al. / Powder Technology 239 (2013) 208–216

[5] M. Hamidpour, M. Kalbasi, M. Afyuni, H. Shariatmadari, P.E. Holm, H.C.B. Hansen, Sorption hysteresis of Cd(II) and Pb(II) on natural zeolite and bentonite, Journal of Hazardous Materials 181 (2010) 686–691. [6] W.S. Peterlene, A.A. Winkler-Hechenleitner, E.A.G. Pineda, Adsorption of Cd(II) and Pb(II) onto functionalized formic lignin from sugar cane bagasse, Bioresource Technology 68 (1999) 95–100. [7] A. Sari, M. Tuzen, Kinetic and equilibrium studies of biosorption of Pb(II) and Cd(II) from aqueous solution by macrofungus (Amanita rubescens) biomass, Journal of Hazardous Materials 164 (2009) 1004–1011. [8] G.J. Soler-Illia, A.A. De, C. Sanchez, B. Lebeau, J. Patarin, Chemical strategies to design textured materials: from microporous and mesoporous oxides to nanonetworks and hierarchical structures, Chemical Reviews 102 (2002) 4093–4138. [9] A. Stein, Advances in microporous and mesoporous solids highlights of recent progress, Advanced Materials 15 (2003) 763–775. [10] L.S. Zhong, J.S. Hu, H.P. Liang, A.M. Cao, W.G. Song, L.J. Wan, Self-assembled 3D flowerlike iron oxide nanostructures and their application in water treatment, Advanced Materials 18 (2006) 2426–2431. [11] L.S. Zhong, J.S. Hu, A.M. Cao, Q. Liu, W.G. Song, L.J. Wan, 3D Flower like ceria micro/nanocomposite structure and its application for water treatment and CO removal, Chemistry of Materials 19 (2007) 1648–1655. [12] C.Y. Cao, Z.M. Cui, C.Q. Chen, W.G. Song, W. Cai, Ceria hollow nanospheres produced by a template-free microwave-assisted hydrothermal method for heavy metal ion removal and catalysis, Journal of Physical Chemistry C 114 (2010) 9865–9870. [13] W. Cai, J. Yu, M. Jaroniec, Template-free synthesis of hierarchical spindle-like γ-Al2O3 materials and their adsorption affinity towards organic and inorganic pollutants in water, Journal of Materials Chemistry 20 (2010) 4587–4594. [14] Y. Wang, G. Wang, H. Wang, W. Cai, C. Liang, L. Zhang, Template-induced synthesis of hierarchical SiO2@γ-AlOOH spheres and their application in Cr(VI) removal, Nanotechnology 20 (2009) 155604. [15] X.D. Wang, C.J. Summers, Z.L. Wang, Large-scale hexagonal-patterned growth of aligned ZnO nanorods for nano-optoelectronics and nanosensor arrays, Nano Letters 4 (2004) 423–426. [16] Z.L. Wang, J.H. Song, Piezoelectric nanogenerators based on Zinc oxide nanowire arrays, Science 312 (2006) 242–246. [17] R. Yakimova, G. Steinhoff, R.M. Petoral Jr., C. Vahlberg, V. Khranovskyy, G.R. Yazdi, K. Uvdal, A. Lloyd Spetz, Novel material concepts of transducers for chemical and biosensors, Biosensors and Bioelectronics 22 (2007) 2780–2785. [18] T. Kawano, H. Imai, A simple preparation technique for shape-controlled zinc oxide nanoparticles: formation of narrow size-distributed nanorods using seeds in aqueous solutions, Colloids and Surfaces A: Physicochemical and Engineering Aspects 319 (2008) 130–136. [19] C.H. Hung, W.T. Whang, A novel low-temperature growth and characterization of single crystal ZnO nanorods, Materials Chemistry and Physics 82 (2003) 705–710. [20] T. Sheela, Y. Arthoba Nayaka, R. Vishwanatha, S. Basavanna, T.G. Venkatesha, Powder Technology 217 (2012) 163–170. [21] X. Wang, W. Cai, Y. Lin, G. Wang, C. Liang, Mass production of micro/nanostructured porous ZnO plates and their strong structurally enhanced and selective adsorption performance for environmental remediation, Journal of Materials Chemistry 20 (2010) 8582–8590. [22] D. Polsongkram, P. Chamninok, S. Pukird, L. Chow, O. Lupan, G. Chai, H. Khallaf, S. Park, A. Schulte, Effect of synthesis conditions on the growth of ZnO nanorods via hydrothermal method, Physica B: Condensed Matter 403 (2008) 3713–3717. [23] G.A. Zachariadis, E. Sahanidou, Multi-element method for determination of trace elements in sunscreens by ICP-AES, Journal of Pharmaceutical and Biomedical Analysis 50 (2009) 342–348.

[24] Y. Chen, H. Chang, Y. Shiang, Y. Hung, C. Chiang, C. Huang, Colorimetric assay for lead ions based on the leaching of gold nanoparticles, Analytical Chemistry 81 (2009) 9433–9439. [25] J. Wang, Stripping Analysis, VCH, Deerfield Beach, Florida, 1985. [26] T. Sheela, Y. Arthoba Nayaka, Kinetics and thermodynamics of cadmium and lead ions adsorption on NiO nanoparticles, Chemical Engineering Journal 191 (2012) 123–131. [27] X. Wu, H. Chen, L. Gong, F. Qu, Y. Zheng, Low temperature growth and properties of ZnO nanorod arrays, advances in natural sciences, Nanoscience and Nanotechnology 2 (2011) 035006, (4 pp.). [28] T. Lopez, J. Manjarrez, D. Rembao, E. Vinogradova, A. Moreno, R.D. Gonzalez, An implantable sol–gel derived titania–silica carrier system for the controlled release of anticonvulsants, Materials Letters 60 (2006) 2903–2908. [29] Y. Wang, X. Li, N. Wang, X. Quan, Y. Chen, Separation and Purification Technology 62 (2008) 727–732. [30] S. Xu, Z.L. Wang, Nano research (2011), http://dx.doi.org/10.1007/s12274-011-0160-7. [31] V.O. Njoku, E.E. Oguzie, C. Obi, O.S. Bello, A.A. Ayuk, Australian Journal of Basic and Applied Sciences 5 (2011) 344–346. [32] J. Lim, H.M. Kang, L.H. Kim, S.O. Ko, Environmental Engineering Resource 13 (2008) 79–84. [33] K.P. Ska, M. Bystrzejewski, Colloids and Surfaces A: Physicochemical and Engineering Aspects 362 (2010) 102–109. [34] S. Inan, Y. Altas, Separation Science and Technology 45 (2010) 269–276. [35] P.S. Kumar, S. Ramalingam, S.D. Kirupha, A. Murugesan, T. Vidhyadevi, S. Sivanesan, Chemical Engineering Journal 167 (2011) 122–131. [36] Y.G. Koa, Y.J. Chunb, C.H. Kima, U.S. Choia, Journal of Hazardous Materials 194 (2011) 92–94. [37] J. Lin, Y. Zhan, Z. Zhu, Colloids and Surfaces A: Physicochemical and Engineering Aspects 384 (2011) 9–16. [38] H.K. Boparai, M. Joseph, D.M. Carroll, Journal of Hazardous Materials 186 (458) (2011) 465. [39] M.F. Elkady, M.M. Mahmoud, H.M. Abd-El-Rahman, Journal of Non-Crystalline Solids 357 (2011) 1118–1129. [40] J. Febrianto, A.N. Kosasih, J. Sunarso, Y.H. Ju, N. Indraswati, S. Ismadji, Journal of Hazardous Materials 162 (2009) 616–645. [41] Q.S. Liu, T. Zheng, P. Wang, J.P. Jiang, N. Li, Chemical Engineering Journal 157 (2010) 348–356. [42] P.P. Selvam, N. Morad, K.A. Tan, Journal of Hazardous Materials 186 (2011) 160–168. [43] L. Xiong, C. Chen, Q. Chen, J. Ni, Journal of Hazardous Materials 189 (2011) 741–748. [44] A. Sari, M. Tuzen, Journal of Hazardous Materials 164 (2009) 1004–1011. [45] J. Acharya, J.N. Sahu, C.R. Mohanty, B.C. Meikap, Chemical Engineering Journal 149 (2009) 249–262. [46] W.H. Zou, R.P. Han, Z.Z. Chen, J. Shi, H.M. Liu, Journal of Chemical Engineering Data 51 (2006) 534–541. [47] S.G. Wang, W.X. Gong, X.W. Liu, Y.W. Yao, B.Y. Gao, Q.Y. Yue, Separation and Purification Technology 58 (2007) 17–23. [48] M.M. Rao, A. Ramesh, G.P.C. Rao, K. Seshaiah, Journal of Hazardous Materials 129 (2006) 123–129. [49] M. Iqbal, A. Saeeda, S.I. Zafar, Journal of Hazardous Materials 164 (2009) 161–171. [50] J.R. Evans, W.G. Davids, J.D. MacRae, A. Amirbahman, Water Resources 36 (2002) 3219–3226. [51] E.I. Unuabonah, B.I. Olu-Owolabi, K.O. Adebowale, A.E. Ofomaja, Colloids and Surfaces A: Physicochemical and Engineering Aspects 292 (2007) 202–211. [52] C.O. Ehi-Eromosele, W.O. Okiei, Resources and Environment 2 (2012) 82–86.