molecules Article
Green Synthesis of Zinc Oxide Nanoparticles for Enhanced Adsorption of Lead Ions from Aqueous Solutions: Equilibrium, Kinetic and Thermodynamic Studies Susan Azizi 1 , Mahnaz Mahdavi Shahri 2, * and Rosfarizan Mohamad 1,3, * 1 2 3
*
Department of Bioprocess Technology, Faculty of Biotechnology and Biomolecular Sciences, Universiti Putra Malaysia, UPM Serdang, Selangor 43400, Malaysia;
[email protected] Department of Chemistry, Shiraz Branch, Islamic Azad University, Shiraz 74731-71987, Iran Laboratory of Biopolymer and Derivatives, Institute of Tropical Forestry and Forest Products, Universiti Putra Malaysia, UPM Serdang, Selangor 43400, Malaysia Correspondence:
[email protected] (M.M.S.);
[email protected] (R.M.); Tel.: +98-91-5124-7470 (M.M.S.); +60-1-3263-6029 (R.M.)
Academic Editor: Wei Zhang Received: 20 April 2017; Accepted: 15 May 2017; Published: 8 June 2017
Abstract: In the present study, ZnO nanoparticles (NPs) were synthesized in zerumbone solution by a green approach and appraised for their ability to absorb Pb(II) ions from aqueous solution. The formation of as-synthesized NPs was established by X-ray diffraction (XRD), Transmission Electron Microscopy (TEM), and UV–visible studies. The XRD and TEM analyses revealed high purity and wurtzite hexagonal structure of ZnO NPs with a mean size of 10.01 ± 2.6 nm. Batch experiments were performed to investigate the impact of process parameters viz. Pb(II) concentration, pH of solution, adsorbent mass, solution temperature, and contact time variations on the removal efficiency of Pb(II). The adsorption isotherm data provided that the adsorption process was mainly monolayer on ZnO NPs. The adsorption process follows pseudo-second-order reaction kinetic. The maximum removal efficiencies were 93% at pH 5. Thermodynamic parameters such as enthalpy change (∆H0 ), free energy change (∆G0 ), and entropy change (∆S0 ) were calculated; the adsorption process was spontaneous and endothermic. The good efficiency of the as-synthesized NPs makes them attractive for applications in water treatment, for removal of heavy metals from aqueous system. Keywords: ZnO nanoparticles; heavy metals; adsorption; green chemistry
1. Introduction The removal of heavy metals from water and wastewater is a matter of concern worldwide. These heavy metals are of serious health and environmental concern and there is a need to discover new and effective methods for their removal from industrial effluents [1]. Heavy metals reach tissues through the food chain and accumulate in the human body. If the metals are ingested beyond the permitted concentration, they can cause serious health disorders [2]. Lead ion is one of the heavy metals considered toxic to humans and aquatic life when present in high quantities in water. The presence of lead ions in drinking water above the acceptable limit (5 ng/mL) may cause harmful health effects such as anaemia, encephalopathy, hepatitis, nephritic syndrome. Various approaches have been developed for the removal of heavy-metal ions from water/wastewater which include chemical precipitation/coagulation, membrane technology, electrolytic reduction, ion exchange and adsorption. Adsorption is one of the most common techniques used to removal heavy-metal ions due to its simplicity and high efficiency, as well as the ease of use of a wide range of adsorbents. Molecules 2017, 22, 831; doi:10.3390/molecules22060831
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Numerous studies have explored various nanoparticles for adsorption of heavy metals, owing to the simplicity of modifying their surface functionality and their high surface area to volume ratio for enhanced adsorption capacity and efficiency [3]. Nanosized metal oxides, including nanosized manganese oxides, ferric oxides, aluminum oxides, magnesium oxides and cerium oxides are considered as the capable ones for heavy metals adsorption from aqueous systems [4]. To date, these nanomaterials are extensively explored as highly efficient absorbents for heavy metal ions removal from water/wastewater. They display some advantages such as high ability, unsaturated surfaces, simple operation, rapid kinetics, and desirable sorption toward heavy metals in water and wastewater [5]. As a low-toxicity material, ZnO has various applications in different areas including catalyst industry [6], gas sensors [7] solar cells [8] and medicine [9]. As an adsorbent, ZnO proved to be the most effective than other absorbents such as phosphate, iron oxide, and activated carbon for sulfur compounds removal and H2 S, due to more favorable sulfidation thermodynamics [10]. Recently in some literatures have been reported that ZnO NPs could efficiently absorb heavy metals from aqueous systems [11]. Use of plants extracts in synthesis of nanoparticles is relatively novel leading to accurately green chemistry as it is eco-friendly, low cost and smoothly scaled up for large scale production [12,13]. In addition, the biomolecules of plant extract will chemically bind to the surface of the nano-structures, stabilize the nanoparticles and prevent their aggregation as well leads to arising consequent surface effects during their application [14]. In this paper, we are the first to report the green synthesis of ZnO-NPs using zerumbone extract. Zerumbone is a monocyclic sesquiterpenoid which can be found plentifully in rhizomes mostly from Zingiber zerumbet Smith and Zingiber aromaticum [15,16]. Additionally, the potential use of green synthesized ZnO NP as a nano-adsorbent (ZnO-Nano-A) in the removal of Pb(II) by determining the maximum adsorption capacity was investigated. Langmuir and Freundlich adsorption isotherm models were applied to fit the equilibrium isotherm. The adsorption kinetics models and the thermodynamics of adsorption for Pb(II) ions were also evaluated. 2. Results and Discussion 2.1. Characterization of ZnO NPs Formation of ZnO NPs was visually evident from the color change of reaction mixture from colorless to white after 30 min of reaction. The presence of carbonyl groups and extensive number of π electrons in the zerumbone molecular structure can enable the complexation of zinc cations (Zn2+ ) to molecules of zerumbone followed by hydrolysis, and finally formation of ZnO NPs through thermal decomposition of Zn(OH)2 complex as a unique source precursor. This structure helps zerumbone to stabilize zinc particles and eventually ZnO NPs while preventing their extreme aggregation or crystal growth In addition, the stabilizing of ZnO NPs by zerumbone, which acts as donor of electrons, leads to formation of active adsorption sites to absorb heavy metal ions from aqueous system (Figure 1). The XRD pattern of the bioformed ZnO-NPs is shown in Figure 2a. The XRD pattern shows the all of the characteristics peaks of ZnO-NPs with Miller indices (100), (002), (101), (102), (110), (103), (200), (112), and (201) which can be indexed to reflections of the ZnO wurtzite structure (JCPDS 36-1451). The presence of zerumbone on the surface of nanoparticles was confirmed with a halo of the typical peak at 2θ of 22.93◦ . Line broadening and sharpness of the diffraction peaks are evidences respectively, which the as-synthesized particles are in the nanometer range and crystals. The crystal size of the ZnO NPs was calculated using Debye-Scherrer equation which was around 10 nm. The morphology and structure of the ZnO NPs were investigated by transmission electron microscopy (TEM). The particle size and size distribution of ZnO NPs were calculated by measuring the diameters of around 100 nanoparticles chosen randomly through the TEM images. Figure 2b,c show typical TEM images of bioformed ZnO NPs in two different magnifications. The micrograph 2b shows the polydispersed nanostructures with a regular hexagonal surface shape which are in agreement with the XRD result.
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112 112 201 201
200 200
103 103
102 102
110 110
101 101
Intensity Intensity(a.u) (a.u)
aa
100 100 002 002
Figure 1. 1. Schematic Schematic illustration illustration of of the the synthesis synthesis and and functionalization functionalization of of the the zerumbone-stabilized zerumbone-stabilized Figure functionalization the zerumbone-stabilized 2+ ions to zerumbone. ZnO NPs, and possible chelating of Pb 2+ 2+ ZnO NPs, and possible chelating of Pb ions ionsto tozerumbone. zerumbone.
30 30
40 40
50 50
2θ (degree (degree)) 2θ
60 60
70 70
80 80
Figure 2. 2. (a) (a) The XRD pattern; (b,c) TEM images and (inset) particle size distribution histogram of Figure (a)The TheXRD XRDpattern; pattern;(b,c) (b,c)TEM TEMimages imagesand and(inset) (inset) particle size distribution histogram particle size distribution histogram of ZnO-NPs. of ZnO-NPs. ZnO-NPs.
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At higher micrograph (Figure (Figure 2c), 2c), the with aa At higher magnification magnification micrograph the well-defined well-defined surface surface modification modification with thickness of ~4 nm, revealing the presence of coating, was obvious. The results are consistent with the thickness of ~4 nm, revealing the presence of coating, was obvious. The results are consistent with concept that that the ZnO nanoparticles are firmly coatedcoated with zerumbone and this process inhibitsinhibits further the concept the ZnO nanoparticles are firmly with zerumbone and this process aggregation or agglomeration between the final nanoparticles. Particle size distribution histogram further aggregation or agglomeration between the final nanoparticles. Particle size distribution revealed anrevealed average particle diameter of ZnO of 10.01 ± 2.6ofnm (inset Figure 2c), which is well histogram an average particle diameter of ZnO 10.01 ± 2.6 nm (inset Figure 2c), matched which is with the measured crystal diameter obtained from XRD result. well matched with the measured crystal diameter obtained from XRD result.
a
Figure 3. (A) (A) UV–Vis UV–Vis spectrum spectrum ((a) ((a) zerumbone zerumbone (b) (b) bio-synthesized bio-synthesized ZnO Figure 3. ZnO NPs, NPs, and and (inset) (inset) band band gap gap estimation) FTIR spectrum zerumbone before and estimation) (B) (B) zeta zeta potential potential of of ZnO ZnO NPs NPs and and (C) (C) the the FTIR spectrum of of pure pure zerumbone before and after of ZnO ZnO NPs. NPs. after biosynthesis biosynthesis of
The UV–vis absorption spectrum (Figure 3A) shows a typical absorption peak of ZnO at a wavelength of 353 nm which can be ascribed to the band-gap absorption of ZnO due to the electron transitions from the valence band to the conduction band (O2p-Zn3d) [17]. Furthermore, this peak
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The UV–vis absorption spectrum (Figure 3A) shows a typical absorption peak of ZnO at a wavelength of 353 nm which can be ascribed to the band-gap absorption of ZnO due to the electron transitions from the valence band to the conduction band (O2p-Zn3d) [17]. Furthermore, this peak shows that the particles are nanoscale, with a narrow particle size distribution. The absorption peak of zerumbone at 250 nm, which is related to the π→π * transitions in the sesquiterpene system can hardly be seen in the UV spectrum of ZnO NPs, may be due to change in the sesquiterpene structure and absence of this π→π * transitions. The band gap of ZnO NPs was obtained through the first derivative of the absorbance in regard to photon energy and the maximum in the derivative band was seen at the lower energy edges. The derivative of the absorbance of the ZnO NPs is shown in inset figure and it illustrates a band gap of 3.34 eV for the ZnO NPs. Zeta potential (ZP) value (Figure 3B) displays information about the surface charge and stability of bioformed ZnO NPs. The average ZP value of −57.70 mV specified that the surfaces of ZnO NPs are coated with molecules which are mainly included of negatively charged groups and similarly responsible for stability of the nanoparticles [18]. This result showed nanoparticles have considerable active adsorption sites to absorb heavy metal ions from aqueous systems. ICP-AES analysis of Zn2+ content after formation of ZnO by extract was found to be 185 ppm at pH 7. The percent yield (%Y) was calculated as 92%. The FTIR spectrum of pure zerumbone before and after biosynthesis of ZnO NPs is shown in Figure 3C. The characteristic FTIR peaks of zerumbone alone over the range 500–4000 cm−1 appear at 1654 cm−1 (α,β-unsaturated ketone and ethylenic bands) [19]. The peak at 1457 cm−1 is due to the C=C groups and the peaks below 1000 cm−1 resulted from C–H bending. The peak close 2935 cm−1 indicates a CH2 stretching vibration and the broad peak at 3465 cm−1 is due to an OH stretching vibration probably originating from the residual alcohol. After synthesis of ZnO NPs important differences in the intensity, shape and position of peaks indicate the contribution of the functional groups of zerumbone to the formation and coating of nanoparticles. Most of the peaks below 1750 cm−1 disappeared, shrank and shifted, indicating the C=O and C=C groups participated in the production and coating of nanoparticles. In fact the π electrons of C=O and C=C groups can transfer to the free orbital of Zn2+ ions. Such interactions would decrease the mobility of ions and after formation of ZnO NPs inhibit the growth of large particles. The appearance of a strong band at 1071 cm−1 attributed to C–O vibration is an evidence for the binding of C–O groups to the surface of the nanoparticles, which this surface structure can contribute for the absorption of metal ions. These observations are in consistent with the UV-Vis absorption result. The formation of ZnO NPs is clearly confirmed by an intense band at 410 cm−1 . 2.2. Evaluation of Adsorption Mechanism 2.2.1. Effect of Contact Time The kinetic study for adsorption of Pb(II) ions in aqueous solution at different concentration 5, 15 and 25 mg L−1 at 30 ◦ C with fixed adsorbent mass is shown in Figure 4. The results show that the adsorption rate initially increased rapidly with increasing contact time, and then adsorption equilibrium was reached after 60 min for Pb(II) ions. In addition, the adsorption capacity achieved a constant value after equilibrium had been reached. This probably resulted from saturation of nano-adsorbent surfaces with metal ions followed by adsorption and desorption processes that occur after saturation. The results indicate that the adsorption capacity of Pb(II) increased from 1.26 to 3.82 mg g−1 with an increase in initial concentration from 5 to 25 mg L−1 . This is attributed to the fact that the driving force, which depends on the concentration gradient, increases with the increasing initial Pb(II) concentration [20].
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66 ppm 2525ppm
qt (mgg-1-1) qt (mgg )
55
ppm 1515ppm
ppm 5 5ppm
44 33 22 11 00
00
3030
6060
9090
120 120
150 150
180 180
210 210
240 240
Time(min (min Time )) Figure 4.Effect Effect contact time onthe theadsorption adsorption Pb(II)onto ontoZnO-Nano-A ZnO-Nano-A different Figure ofofcontact time ofofPb(II) atatdifferent Figure 4.4.Effect of contact time on theon adsorption of Pb(II) onto ZnO-Nano-A at different concentration: −1; initial pH, 5.0; solution volume, 25 mL; temperature, −1 − 1 ◦ concentration: (adsorbent mass, 0.1 g L °C). concentration: (adsorbent mass, 0.1 g L ; initial pH, 5.0; solution volume, 25 mL; temperature, 3030°C). (adsorbent mass, 0.1 g L ; initial pH, 5.0; solution volume, 25 mL; temperature, 30 C).
2.2.2.Effect Effectof Massof Adsorbent 2.2.2. Effect ofofMass Mass ofofAdsorbent Adsorbent 2.2.2. Adsorbent dosage animportant important parameter because determines thecapacity capacity anadsorbent adsorbent Adsorbent dosage isis an important parameter because determines the capacity ofofan an adsorbent Adsorbent dosage is an parameter because ititit determines the of for a given initial concentration of the adsorbate at the operating conditions. To determine the foraagiven given initial concentration of adsorbate the adsorbate the operating conditions. To determine the for initial concentration of the at the at operating conditions. To determine the optimum −1 −1) ) optimumconcentration concentration ZnO-Nano-A, different amountsofof adsorbent gLL were added −(0.02–0.2 1 ) were g optimum ofofZnO-Nano-A, different adsorbent were added concentration of ZnO-Nano-A, different amounts of amounts adsorbent (0.02–0.2 g L(0.02–0.2 added to 25.0 mL −1 Pb(II) ions. Figure 5 shows that adsorption −1 to 25.0 mL of aqueous solution containing 10.0 mg L to aqueous 25.0 mL solution of aqueous solution 10.0 containing mgions. L Pb(II) ions. Figurethat 5 shows that adsorption of containing mg L−110.0 Pb(II) Figure 5 shows adsorption increased −1−1 increasedwith gradually withadsorbent increasing adsorbent mass,totoaat amaximum maximum at0.1 0.1ggLL forPb(II). Pb(II). Afterthis this increased gradually with increasing adsorbent mass, for After gradually increasing mass, to a maximum 0.1 g L−1 at for Pb(II). After this maximum maximum equilibrium value, the adsorption capacity did not increase with increasing adsorbent maximum equilibrium value, the adsorption capacity did not increase with increasing adsorbent equilibrium value, the adsorption capacity did not increase with increasing adsorbent mass. It is mass.ItItisthat isapparent apparent thatthe theadsorption adsorption capacity metal ionsincreases increasesrapidly rapidly within increase the mass. that ofofmetal ions with increase ininthe apparent the adsorption capacity ofcapacity metal ions increases rapidly with increase the dose of dose theadsorbents adsorbents due the greateravailability availability theexchangeable exchangeable sitesor orsurface surface areathat and dose ofofthe due totothe greater ofofthe sites area and the adsorbents due to the greater availability of the exchangeable sites or surface area and after after that increasing these sites had no effect after equilibrium was reached [21]. after that increasing sites hadafter no effect after equilibrium was reached [21]. increasing these sitesthese had no effect equilibrium was reached [21]. 1.4 1.4
qe (mgg-1-1) qe (mgg )
1.2 1.2 11 0.8 0.8 0.6 0.6 0.4 0.4 0.2 0.2
00
0.05 0.05
0.1 0.1
0.15 0.15
0.2 0.2
-1-1) (gL
Adsorbentmass mass(gL ) Adsorbent
Figure Effectofofthe themass massofofadsorbent adsorbentononthe thePb(II) Pb(II)adsorption adsorptiononto ontoZnO-Nano-A: ZnO-Nano-A:(solution (solutionvolume, volume, Figure 5.5.Effect Figure 5. Pb(II) adsorption −1 ◦ − 1 −1 ;contact contacttime, time,111h). h). mL; initial pH, 5.0; temperature, °C; initial concentration, mg 25 mL; initial pH, 5.0; temperature, 30 C; initial concentration, 10 mg LLL;;contact time, h). 2525 mL; initial pH, 5.0; temperature, 3030°C; initial concentration, 1010 mg
2.2.3.Effect Effectof pH 2.2.3. Effect ofofpH pH 2.2.3. The pH ofthe the aqueous solution hasbeen beenidentified identified asthe the mostimportant important parameter that The pH aqueous solution most parameter that The pH ofofthe aqueous solution has has been identified as theas most important parameter that controls controls the adsorption process. The adsorption behavior of Pb(II) onto ZnO-Nano-A has been controls the adsorption process. The adsorption of ZnO-Nano-A Pb(II) onto ZnO-Nano-A has been the adsorption process. The adsorption behavior ofbehavior Pb(II) onto has been investigated investigated different pHsranging ranging from 2.0toto66.0. 6.0. Figure shows thatuptake themetal metal uptakeincreased increased investigated atatdifferent from 2.0 Figure 6 6shows that the uptake at different pHs rangingpHs from 2.0 to 6.0. Figure shows that the metal increased with the with the increase of solution pH. The maximum equilibrium uptake for Pb(II) ions was at pH 5.0, with the increase of solution pH. The maximum equilibrium uptake for Pb(II) ions was at pH increase of solution pH. The maximum equilibrium uptake for Pb(II) ions was at pH 5.0, while5.0, at while at pH 2.0 the adsorption capacity was much lower, because large quantities of protons compete while pHadsorption 2.0 the adsorption waslower, much because lower, because large quantities of protons compete pH 2.0atthe capacitycapacity was much large quantities of protons compete with with the nano-adsorbent and decrease the absorption of metal cations [22]. Depending on the solution with the nano-adsorbent and decrease the absorption of metal cations [22]. Depending on the solution the nano-adsorbent and decrease the absorption of metal cations [22]. Depending on the solution pH, pH,the thesurface surfaceofofnano-adsorbent nano-adsorbentcan canundergo undergoprotonation. protonation.As Asthe thepH pHofofsolution solutionincreases, increases,the the pH, number of protons on the surface of nano-adsorbent decreases and thus more negative groups for number of protons on the surface of nano-adsorbent decreases and thus more negative groups for
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the surface of nano-adsorbent can undergo protonation. As the pH of solution increases, the number of complexation of metal cations are provided. From these results, the divalent Pb were bound to active protons on the surface of nano-adsorbent decreases and thus more negative groups for complexation surfaces on the lone pair electrons of oxygen from ZnO as well, C–O− functional groups attached on of metal cations are provided. From these results, the divalent Pb were bound to active surfaces on the its surface after synthesis with zerumbone, which mainly acts as active site for the complex formation lone pair electrons of oxygen from ZnO as well, C–O− functional groups attached on its surface after with Pb ions [23]. synthesis with zerumbone, which mainly acts as active site for the complex formation with Pb ions [23].
120
Adsorption (%)
100 80 60 40 20 0 0
2
4
6
8
pH Figure Figure 6. 6. Effect Effect of of pH pH on on the the Pb(II) Pb(II) adsorption adsorption onto onto ZnO-Nano-A: ZnO-Nano-A: (initial (initial concentrations concentrationsof of Pb(II), Pb(II), −1 −1 − 1 − 1 ◦ 10 volume, 2525 mL; temperature, 3030 °C;C; contact time, 1 h). 10 mg mg L L ; adsorbent ; adsorbentmass, mass,0.1 0.1ggLL ; solution ; solution volume, mL; temperature, contact time, 1 h).
2.3. 2.3. Metal Metal Adsorption Adsorption Characteristics Characteristics 2.3.1. 2.3.1. Adsorption Adsorption Kinetics Kinetics Modeling Modeling Study Study Kinetics Kinetics and and equilibrium equilibrium of of adsorption adsorption are are the the two two major major parameters parameters to to evaluate evaluate adsorption adsorption dynamics. dynamics. The Thekinetic kineticconstants constantsofofPb(II) Pb(II) ions ions adsorption, adsorption, which which could could be be used used to to optimize optimize the the residence residence time time of of an an industrial industrial wastewater wastewater treated treated with with ZnO-Nano-A, ZnO-Nano-A, were were computed computed using using the the experimental data. The adsorption equilibrium was reached with a minimum solid-solution contact experimental data. The adsorption equilibrium was reached with a minimum solid-solution contact time equilibrium time, thethe Pb(II) adsorption ontoonto nano-adsorbent at pH time of ofapproximately approximately60 60min. min.AtAt equilibrium time, Pb(II) adsorption nano-adsorbent at 5.0 found to be TheThe twotwo kinds of of kinetic models used pHwas 5.0 was found to 93%. be 93%. kinds kinetic models usedininthis thisstudy studyare arepseudo-first-order pseudo-first-order and and pseudo-second-order pseudo-second-order equations. equations. Lagergren-first-order Lagergren-first-orderequation equationisis the the most most popular popular kinetics kinetics equation equation [22]. [22]. The The Equation Equation is is expressed expressed as: as: (1) ln q − q = lnq − k t ln qe − qt = lnqe − k1 t (1) −1 where qe and qt (mg g ) are the amounts of adsorption at equilibrium and time t (min), respectively −1) q where qe and (mgrate g−1constant ) are the amounts of adsorption at equilibrium and time t (min), respectively and k1 (min ist the of pseudo-first order adsorption. − 1 and k ) is therate rateconstant constantand of pseudo-first order adsorption. The adsorption k1 can be determined experimentally by plotting of ln(qe − qt) 1 (min The adsorption rate constant and k can be determined experimentally by plotting of data ln(qeusing − qt ) against t. A pseudo second-order kinetic1 model [24] was used to fit the adsorption kinetic against t. A pseudo second-order kinetic model [24] was used to fit the adsorption kinetic data using the following Equation: the following Equation: 1 t t t = 1 + t (2) = + (2) kk qq2 qq qqt 2 e e −1). where k2 is the rate constant of the pseudo second-order model (g mg−1−min where k2 is the rate constant of the pseudo second-order model (g mg 1 min−1 ). Kinetic parameters of these models for different concentrations of lead can be determined Kinetic parameters of these models for different concentrations of lead can be determined experimentally from the slope and intercepts of the linear plots of ln(qe − qt) against t and t/qt versus t, experimentally from the slope and intercepts of the linear plots of ln(qe − qt ) against t and t/qt respectively and are shown in Table 1. The rate law for a pseudo second-order kinetic model best versus t, respectively and are shown in Table 1. The rate law for a pseudo second-order kinetic model described the experimental data with the higher correlation coefficients (R2). Also the calculated values best described the experimental data with the higher correlation coefficients (R2 ). Also the calculated of qe estimated from the pseudo second-order kinetic model is much closer to the experimental values values of qe estimated from the pseudo second-order kinetic model is much closer to the experimental of qe than that of pseudo-first-order model (Table 1). Consequently, the adsorption of Pb(II) by ZnONano-A in this study was better fitted to the pseudo-second-order model.
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values of qe than that of pseudo-first-order model (Table 1). Consequently, the adsorption of Pb(II) by ZnO-Nano-A in this study was better fitted to the pseudo-second-order model. Molecules 2017, 22, 831
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Table 1. Kinetic parameters of Pb(II) adsorbed onto ZnO-Nano-A.
Table 1. Kinetic parameters of Pb(II) adsorbed onto ZnO-Nano-A. Pseudo-First Order
Conc (mg L−1 )
Pseudo-Second Order
Pseudo-First qe exp (mg g−1 ) k1Order (min−1 )
−1 Conc (mg 10 L ) 10 20 30 20 30
−1 qe exp 2.1 (mg g ) 3.72.1 4.83.7 4.8
qe (cal)
−2 ) (min 4.8 k ×1 10 2 −2 4.810×−10 2.3 × 2 −2 1.4 × 2.310×−10
1 min−1 ) Order R2 qe (cal) k2 (g mg−Pseudo-Second
R2
q1.3 e (cal) 0.868R2 0.868 4.21.3 0.817 5.84.2 0.959 0.817 5.8 0.959
−1
1.4 × 10−2
−1) 2 k22.7 2 (g mg−1 × 10−min 2 −2 10.722.7 × 10×−10 2 −2 4.810.7 × 10×−10 4.8 × 10−2
qe2.2 (cal) 2.2 3.8 4.5 3.8 4.5
R 0.999 0.999 0.989 0.997 0.989 2
0.997
2.3.2. Adsorption Isotherm Modeling 2.3.2. Adsorption Isotherm Modeling An analysis of the relationship between adsorption capacity of nano-adsorbent and metal ion An analysis ofperformed the relationship between adsorption capacity of nano-adsorbent and metal ion concentration was using the Langmuir adsorption equations [23] as: concentration was performed using the Langmuir adsorption equations [23] as: 1 C Ce C = 1 +C e (3) qe = bQm + Qm (3) q Q bQ and the the Freundlich Freundlich adsorption adsorption equation equation [23] [23] as: as: and
11 lnqe = = lnK lnKF + + lnC lnq lnCe nn
(4)(4)
The Langmuirmodel modelpredicts predicts formation an adsorbed solute monolayer and the The Langmuir the the formation of an of adsorbed solute monolayer and the Freundlich Freundlich model considers the existence of a multilayered structure. The Langmuir and Freundlich model considers the existence of a multilayered structure. The Langmuir and Freundlich isotherm isotherm parameters were calculated from the and of intercept of linear of Ce/qCe versus Ce parameters were calculated from the slope andslope intercept linear plots of Ceplots /qe versus e and lnqe and lnqlnC e versus lnCe (Figure 7), and are given in Table 2. versus (Figure 7), and are given in Table 2. e
20
a
18
R² = 0.9666
Ce/qe (gL-1)
16
R² = 0.9783
14 12
R² = 0.9866
10 8 6 4 2
30oC
50oC
70oC
0 0
50
100
Ce
150
(mgL-1)
Figure 7. Cont.
200
250
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6
b 5
ln qe
4
R² = 0.919 R² = 0.913 R² = 0.861
3 2 1 30oC
50oC
70oC
0 0
1
2
3
4
5
6
7
ln Ce Figure 7. Linearized Langmuir (a)(a) and Freundlich (b) isotherms Figure 7. Linearized Langmuir and Freundlich (b) isothermsfor forPb(II) Pb(II)adsorption adsorptiononto ontoZnO-NanoZnO-Nano-A −11 .. A at various temperatures: contact time, 1 h; initial pH, 5.0; adsorbent mass: mass: 0.1 g L− Table 2. 2. Isotherm constants of of Pb(II) adsorbed onto ZnO-Nano-A. Table Isotherm constants Pb(II) adsorbed onto ZnO-Nano-A. Langmuir Isotherm Freundlich Isotherm Langmuir Isotherm Freundlich Isotherm 2 Temp (°C) Qm (mg g−1) B (L mg−1) R2 KF (mg g−1) n R ◦ − 1 − 1 2 Temp Q ) 5.93 ×B10 (L−2mg 0.988 ) R KF (mg g−1 )2.34 n R2 m (mg g 30 ( C) 15.65 1.98 0.929 2 50 30 17.84 7.37 ×5.93 10−2× 10−0.999 2.54 2.34 0.961 15.65 0.988 2.94 1.98 0.929 −2 − 2 0.999 3.16 2.81 2.54 0.914 70 50 19.65 8.39 × 10 17.84 0.999 2.94 0.961 7.37 × 10 70 19.65 0.999 3.16 2.81 0.914 8.39 × 10−2
From the significant correlation coefficients, the Langmuir equation was used to describe the adsorption whichcorrelation fit the adsorption datathe better than the Freundlich for Pb(II) Fromisotherms, the significant coefficients, Langmuir equation was equation used to describe the adsorption. Similar to the adsorption kinetics, the maximum equilibrium adsorption capacity (Q m) adsorption isotherms, which fit the adsorption data better than the Freundlich equation for Pb(II) was obtained Similar 19.65 mg g−1 adsorption for Pb(II). kinetics, Since thethe value of n isequilibrium greater than 1, it indicates favorable adsorption. to the maximum adsorption capacity (Qm ) was adsorption of metal ions on the surface of adsorbent [25]. This result is comparable with obtained 19.65 mg g−1 for Pb(II). Since the value of n is greater than 1, it indicates favorable previous adsorption studies which lead of ions from aqueous solutions NiFe2Owith 4 nanoparticles with 99% of metal ions removed on the surface adsorbent [25]. This result isusing comparable previous studies which efficiency during 1 h [26], CuO nanoparticles with maximum adsorption capacity 37.027 mg g−1 during after removed lead ions from aqueous solutions using NiFe2 O4 nanoparticles with 99% efficiency 3 1hh[27] nanoparticles with maximum capacity g−1 and at [26],and CuOdiatomite nanoparticles with maximum adsorptionadsorption capacity 37.027 mg gof−1103.1 after 3mg h [27] equilibrium time of 90 min [28]. − 1 diatomite nanoparticles with maximum adsorption capacity of 103.1 mg g at equilibrium time of 90 min [28]. 2.3.3. Thermodynamic Study 2.3.3. Study on the adsorption characteristics of Pb(II) was investigated by TheThermodynamic effect of temperature determining the adsorption isotherms 50, and 70 °C to obtain theof thermodynamic parameters,by The effect of temperature on at the30,adsorption characteristics Pb(II) was investigated which were evaluated using the Van’t Hoff equation: determining the adsorption isotherms at 30, 50, and 70 ◦ C to obtain the thermodynamic parameters, which were evaluated using the Van’t Hoff equation: ∆S ∆H (5) ln K = + R 0 RT 0 ∆S ∆H Kdthe = absolute + temperature (K) and Kd is an equilibrium(5) ), T where R is the gas constant (8.314 Jmol K−1ln R RT constant obtained by multiplying Langmuir constants Qm and b (L mol−1). The value of the change 0) and R is (ΔH the gas constant (8.314 Jmol(ΔS K−01)),during T the absolute temperature Kd is an equilibrium inwhere enthalpy entropy change the binding process (K) wasand determined from the − 1 constant multiplying Langmuir constants Qm and b (L mol The value of theat change gradient ofobtained the plotsby between lnKd versus T−1. The plot shown in Figure 8 for).Pb(II) was linear the 0 ) and entropy change (∆S0 ) during the binding process was determined from the in enthalpy (∆H range of investigated temperatures according to Van’t Hoff equation. gradient of the plots between lnKd versus T−1 . The plot shown in Figure 8 for Pb(II) was linear at the range of investigated temperatures according to Van’t Hoff equation.
Molecules Molecules2017, 2017,22, 22,831 831
1010ofof1413
7.4
7.2
ln Kd
R² = 0.906 7
6.8
6.6 0.0028
0.003
0.0032
0.0034
T-1 Figure 8. 8. Plot ofof lnK T−1 for the adsorption of Pb(II) onto ZnO-Nano-A. d vs. Figure Plot lnK d vs. T−1 for the adsorption of Pb(II) onto ZnO-Nano-A. 0 )0 of the adsorption process is related to the equilibrium The TheGibbs Gibbsfree freeenergy energychange change(∆G (ΔG ) of the adsorption process is related to the equilibrium constant constantby bythe theclassical classicalVan’t Van’tHoff Hoffequation: equation:
(6) ∆G 0 = −RT ln K ∆G = −RT ln Kd (6) The calculated thermodynamic parameters such as ΔH0, ΔS0 and ΔG0 for the adsorption system 0 , ∆S0 and ∆G0 for the adsorption system The calculated parameters such has as ∆H are given in Tablethermodynamic 3. The adsorption of lead ions been found to increase with an increase in are given in Table 3. The adsorption of lead ions has been found to increase withwas an positive increase and in temperature from 30 to 70 °C. It can be seen from Table 3 that ΔH0 values obtained ◦ C. It can be seen from Table 3 that ∆H0 values obtained was positive and temperature from 30 to 70 also observed that the distribution coefficient values, Kd increased with increase in temperature also observed distributionnature coefficient values, Kd increased with increase which which showsthat thethe endothermic of the adsorption. The positive valueinoftemperature ΔS0 suggested the 0 shows the endothermic nature of the adsorption. The positive value of ∆S suggested the increasing increasing randomness at the solid/liquid interface during the adsorption of Pb(II) ions on ZnOrandomness the solid/liquid interface during adsorption was of Pb(II) ions onand ZnO-Nano-A. 0 indicated Nano-A. Theatnegative values of ΔG that the the adsorption spontaneous the decrease 0 The negative that the adsorption was that spontaneous and the in the in the valuevalues of ΔGof0 ∆G withindicated increasing temperature shows the reaction wasdecrease favorable at value higher oftemperature ∆G0 with increasing temperature shows that the reaction was favorable at higher temperature [29]. [29]. Table Table3.3.Thermodynamic Thermodynamicparameters parametersofofPb(II) Pb(II)adsorbed adsorbedonto ontoZnO-Nano-A. ZnO-Nano-A. 1 ) −1) ∆G 0 0(kJ −1−1 Temp (kJ − mol ΔG (kJmol mol Temp (K) (K) ∆H0ΔH (kJ0 mol ))
30 30 50 50 70 70
11.13 11.13
–16.19 –16.19 −−18.40 18.40 –19.91 –19.91
−−1 1K ΔS00(J(Jmol mol K−−11)) ∆S
0.7 0.7
3.3.Materials Materialsand andMethods Methods 3.1. 3.1.Materials Materials Pb(NO Pb(NO3 )32)2 was was purchased purchased from from Fluka Fluka (Morris (Morris Plains, Plains, NJ, NJ,USA). USA).All Allother otherchemicals chemicalswere wereofof analytical reagent grade and were used without further purification. Zingiber zerumbet rhizome analytical reagent grade and were used without further purification. Zingiber zerumbet rhizomewas was obtained obtainedfrom froma alocal localmarket marketininKuala KualaLumpur, Lumpur,Malaysia. Malaysia. 3.2. Extraction of Zerumbone 3.2. Extraction of Zerumbone Zerumbone crystals were prepared according to process described in previous study [30] and Zerumbone crystals were prepared according to process described in previous study [30] and yielded 1.45 g/kg rhizome. Briefly, Fresh Zingiber zerumbet rhizome (1 kg) was washed several times yielded 1.45 g/kg rhizome. Briefly, Fresh Zingiber zerumbet rhizome (1 kg) was washed several times with water, cut to small pieces and boiled with deionized water in a distillation hydration apparatus with water, cut to small pieces and boiled with deionized water in a distillation hydration apparatus to obtain the essential oil. The oil was crystallized using absolute 100% n-hexane (Sigma-Aldrich, to obtain the essential oil. The oil was crystallized using absolute 100% n-hexane (Sigma-Aldrich, Kuala Lumpur, Malaysia) and the solution was then left to evaporate in a fume hood (Novaire, Kuala Lumpur, Malaysia) and the solution was then left to evaporate in a fume hood (Novaire, Newton, MA, USA). Recrystallization was carried out three times to achieve pure zerumbone crystals. Newton, MA, USA). Recrystallization was carried out three times to achieve pure zerumbone crystals. The purity of zerumbone was determined using a high performance liquid chromatography
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The purity of zerumbone was determined using a high performance liquid chromatography (HPLC) system (Waters, Milford, MA, USA) was 99.97%. Pure zerumbone crystals were collected in clean glass bottles and kept at 4 ◦ C. 3.3. Synthesis of ZnO NPs Zerumbone crystals (about 1 g) was dissolved in ethanol (98%, 100 mL) under gentle stirring at room temperature. After complete dissolution, zinc acetatedehydrate (Zn(Ac)2 ·2H2 O, 2.19 g) was added to react with the zerumbone solution for 2 h under continuous magnetic stirring at 70 ◦ C. The white color solid product was collected by centrifugation at 8000 rpm for 15 min and washed carefully with ethanol to remove surplus zerumbone and then dried at 100 ◦ C for 2 h. 3.4. Characterization of ZnO NPs The ZnO NPs were characterized by PXRD (Philips, X’pert, Almelo, The Netherlands), at 40 kV and 30 mA from 2θ = 10◦ to 80◦ with nickel-filtered Cu (λ = 1.542 Å) at room temperature. FT-IR spectra of the samples were recorded over the range of 400–4000 cm−1 by a Model spectrum 100 series (Perkin Elmer, Waltham, MA, USA) FTIR spectrophotometer. UV-vis spectra of ZnO NPs powders were measured using a spectrophotometer (Lambda 25-Perkin Elmer) in wavelength between 200 and 800 nm. The morphology and size of ZnO-NP samples were examined by using a Transmission Electron Microscope (TEM, Hitachi H-700, Tokyo, Japan) in 120 kV. The particle electrostatic charge was evaluated using the laser doppler electrophoresis technique, whereby 100 µL of the solution was diluted in 1.5 mL of water. Then it was poured into a Zeta sizer-nano instrument cuvette (Malvern, UK); the results are stated as zeta potential (ZP). Concentration of Zn2+ ions before and after addition of extract was measured using Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) model Perkin Elmer 1000. Following Equation was used to calculate percentage yield (%Y) using initial concentration (IC) and final concentration (FC) of Zn2+ ions: %Y =
Ic − Fc × 100% Ic
(7)
3.5. Adsorption Experiments The adsorption of Pb(II) ions on ZnO NPs as nano-adsorbent was studied by a batch technique. The stock solution of 1000 mg L−1 Pb(II) was prepared by dissolving a weighed quantity of Pb(NO3 )2 in distilled water. The metal solutions were prepared in distilled water by gradually diluting the stock solution to desired concentrations. The adsorption behaviors of Pb(II) ions by ZnO-Nano-A were investigated in the pH range 2.0 to 6.0 at 30 ◦ C. The effect of several parameters such as pH, concentrations, contact time and adsorbent mass on the adsorption were studied. The initial pH of the adsorbate solution was adjusted using 1 M HCl or 1 M NaOH aqueous solution without any further adjustment during the adsorption process. Adsorption isotherm of Pb(II) on ZnO-Nano-A was carried out from batch experiments by contacting 0.1 g of the ZnO-Nano-A with 25 mL of varying concentrations of Pb(II) from 5 to 250 mg L−1 for 1 h on a mechanical stirrer at different temperature (30, 50 and 70 ◦ C). The pH of the solution was adjusted to an optimum pH. After reaching adsorption equilibrium, the adsorbent was removed through centrifuge, and the concentration of metal ions remaining in absorbent was measured using ICP-AES. For Pb(II) adsorption kinetics studies, 0.1 g of ZnO-Nano-A was contacted with 25 mL of Pb(II) solution of varying concentrations in a flask and stirred continuously at different times. At the end of the pre-determined time intervals, the adsorbent was separated by centrifuge. The residual concentration of Pb(II) in the absorbent was determined using ICP-AES. The results of these studies were used to obtain the optimum conditions for maximum
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metal removal from aqueous solution. The amount of metal ions adsorbed onto the unit amount of the asorbents, qe (mg g−1 ), was calculated using the following equation: qe =
(C0 − Ce )V W
(8)
where C0 and Ce are initial and final metal ions concentrations (mg L−1 ), respectively, V is the volume of lead solution (L), and W is the dry mass of the adsorbent (g). The percent adsorption of metal ions was calculated as follows: C0 − Ce Adsorption% = × 100 (9) C0 4. Conclusions Nanoparticles provide an efficient technique for the removal of toxic heavy metals from wastewater. In this study, ZnO NPs were successfully prepared with a safe, simple and economic process using of zerumbone extract and evaluated their efficiency as novel nano-adsorbent for removal of lead ions in aqueous solution. The biosynthesized nanoparticles exhibited an excellent adsorption for the Pb(II) ions that followed Langmuir adsorption model and pseudo-second-order equation. The maximum adsorption capacity of Pb(II) was found to be 19.65 mg g−1 under pH of 5, and temperature of 70 ◦ C in aqueous solution. The enhanced adsorption at higher temperature indicates endothermic adsorption process. Acknowledgments: The authors are grateful to the Department of Bioprocess Technology, Faculty of Biotechnology and Biomolecular Sciences for the laboratory facilities. Author Contributions: S.A. and M.M.S. carried out the synthesis, characterization of samples and prepared the draft of manuscript. R.M. supervised research and revised the draft for important intellectual content, and gave final approval of the version to be published. All authors read and approved the final manuscript. Conflicts of Interest: The authors declare no conflict of interest.
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Sample Availability: Samples of the compounds are not available from the authors. © 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
