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Journal of Environmental Radioactivity 101 (2010) 1061e1069

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Journal of Environmental Radioactivity

journal homepage: www.elsevier .com/locate/ jenvrad

Study of 63Ni adsorption on NKF-6 zeolite

Hui Zhang, Xianjin Yu, Lei Chen*, Yongjie Jing, Zhiwei GeSchool of Chemical Engineering, Shandong University of Technology, Zibo 255049, PR China

a r t i c l e i n f o

Article history:Received 21 April 2010Received in revised form24 August 2010Accepted 26 August 2010Available online 24 September 2010

Keywords:63NiNKF-6 zeoliteAdsorptionpHTemperature

* Corresponding author.E-mail address: songjia19831204@163.com (L. Che

0265-931X/$ e see front matter � 2010 Elsevier Ltd.doi:10.1016/j.jenvrad.2010.08.009

a b s t r a c t

The adsorption of 63Ni from aqueous solutions using NKF-6 zeolite was investigated by a batch techniqueunder ambient conditions. The adsorption was investigated as a function of contact time, pH, ionicstrength, foreign ions, humic substances (FA/HA) and temperature. The kinetic adsorption was welldescribed by the pseudo-second-order rate equation. The adsorption of 63Ni on NKF-6 zeolite wasstrongly dependent on pH and ionic strength, and the adsorption of 63Ni increased with increasing NKF-6zeolite content. At low pH values, the presence of FA enhanced the adsorption of 63Ni on NKF-6 zeolite,but the presence of HA had no drastic effect. At high pH values, the presence of FA or HA decreased theadsorption of 63Ni on NKF-6 zeolite. The adsorption isotherms were well represented by the Langmuirmodel. The thermodynamic parameters (i.e., DH0, DS0 and DG0) for the adsorption of 63Ni were deter-mined from the temperature dependent isotherms at 293.15, 313.15 and 333.15 �K, respectively, and theresults indicate that the adsorption reaction was favored at high temperature. The results suggest thatthe adsorption process of 63Ni on NKF-6 zeolite is spontaneous and endothermic.

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1. Introduction

Nickel is a non-biodegradable toxic metal ion present in waste-water. The main source of nickel pollution in water derives fromindustrial production processes such as galvanization, smelting,mining, batteries manufacturing and metal finishing. The presenceand accumulation of nickel in industrial effluents has a toxic (Yanget al., 2009a) or carcinogenic (Yang et al., 2009a) effect on livingspecies. The presence of nickel in drinking water above thepermissible limit of 0.02 mg L�1 (drinking-water quality standardsGB5749-2005) may cause adverse health impacts such as anemia,diarrhea, encephalopathy, hepatitis and the dysfunction of centralnervous system. The isotope 63Ni (T1/2 ¼ 96a) is an importantnuclear fission product in nuclear power plants, and the research isessential to evaluate the behavior of 63Ni (Tan et al., 2008a). Becauseenvironmental effects are possible, it is of great importance toeliminate nickel ions from wastewaters. As an economical andefficient method, adsorption techniques have been widely appliedto remove metal ions from wastewaters. Many sorbents have beeninvestigated for the removal ofNi2þ, such as activated carbon (Hasar,2003), bentonite (Yang et al., 2009b), carbon nanotubes (Tan et al.,2008b), olive stone waste (Fiol et al., 2006), rectorite (Chang et al.,2007; Tan et al., 2008c) and mordenite (Wang et al., 2007).

n).

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Zeolites are microporous crystalline solids with well-definedstructures. Generally they contain silicon, aluminium and oxygen intheir framework and cations, water and/or other molecules withintheir pores (Shao et al., 2009a). Many kinds of zeolites occurnaturally as minerals, and are extensively mined in many parts oftheworld. Others are synthetic (Martucci et al., 2003) and aremadecommercially for specific uses, or produced by research scientiststrying to understand more about their chemistry. NKF-6 zeolite(b-zeolite) is the only high-silica zeolite possessing a three-dimensional system of large rings (rings of 12 oxygen atoms as theminimum constricting apertures) (Treacy and Newsam, 1988), andit draws much attention because of its unique characteristics, inparticular, its acidity and potential for acid catalysis (Kiricsi et al.,1994). With regard to the numerous possibilities of various tech-nical applications, it is nothing unusual for the consideration oftheir application for environmental pollution cleaning. Zeoliteshave aroused researchers’ widespread attention as a new type ofadsorbent and exhibit outstanding capability for the removal ofmetal ions (Shao et al., 2009b) and textile dyes (Alpat et al., 2008)fromwastewater. The results indicated that zeolite is a very suitablematerial in wastewater management. However, the mechanism ofmetal ion adsorption on zeolite is still ambiguous.

The results of Ni2þ adsorption on different adsorbents showedthat theadsorptionwasdependent onpH, and thepresence of humicsubstances affected the adsorption. Many mechanisms have beensynchronously postulated for 63Ni adsorption, including surfacecomplexation, ion exchange, surface precipitation/coprecipitation

Fig. 1. FT-IR spectrum of NKF-6 zeolite.

H. Zhang et al. / Journal of Environmental Radioactivity 101 (2010) 1061e10691062

and diffusion into particle micropores (Fan et al., 2009a; Stumm,1992). However, to the best of our knowledge, the study of 63Ni onNKF-6 zeolite, especially the thermodynamic data of 63Ni adsorptionon NKF-6 zeolite and the effect of humic substances on 63Ni uptaketo humicezeolite hybrids, is still scarce.

Thebasic objectivesof thepresent researchare: (1) to characterizeNKF-6 zeolite using FTIR and XRD; (2) to investigate the adsorptionkinetics and to analyze the experimental datawith a pseudo-second-order equation; (3) to study the adsorption of 63Ni on NKF-6 zeoliteby varying the experimental conditions contact time, pH, ionicstrength, foreign ions and temperature; (4) to compare the influenceof humic acid and fulvic acid on 63Ni adsorption; and (5) to presumethe adsorption mechanism of 63Ni on NKF-6 zeolite.

2. Experimental

2.1. Material

All chemicals used in the experiments were purchased as analytical purity, andused without further purification. The NKF-6 zeolite was derived from NankaiUniversity Catalyst Co., Ltd (China). The zeolite sample was purified to remove thesoluble impurityby shaking in1mol L�1HNO3 for 24hat roomtemperature and rinsedwithMilli-Q water up to pH 6.0. The sample was then dried in oven at 110 �C for 24 h.

The Ni stock solution (60 mg L�1) was prepared by dissolving 70.24 mg Ni(NO3)2$6H2O (purity >99.9%) into 250 mL Milli-Q water. Radionuclide 63Ni waspurchased from the Institute of Atomic Energy of China, and was used in theexperiments as radiotracer. The carrier free 63Ni was dissolved in HNO3 to achievea radioactive nickel nitrate solution, which was the same as the nickel nitrate stocksolution used in the experiments. The radiotracer 63Ni was added to the stable Nistock solution to achieve the concentration ratio of 63Ni: stable Ni w1:10, and thenthe stock solution was diluted with Milli-Q water to obtain different Ni solutionswith concentrations ranging from 2 to 20 mg L�1. The concentration of radionuclide63Ni was analyzed by liquid scintillation counting using a Packard 3100 TR/AB LiquidScintillation Analyzer (PerkinElmer). The scintillation cocktail was ULTIMA GOLD AB(Packard). Because the sorption properties of 63Ni and stable Ni on NKF-6 are thesame, the analysis of 63Ni concentration before and after sorption experiments canbe used to evaluate the sorption behavior of stable Ni on NKF-6 zeolite.

Soil humic and fulvic acids were extracted from the soil of Gansu province(China), and had been characterized in detail (Tan et al., 2008d, 2009a,b). The maincomponents of HA are: C 60.44%, H 3.53%, N 4.22%, O 31.31% and S 0.50%; and thoseof FA are: C 50.15%, H 4.42%, N 5.38%, O 39.56% and S 0.49%.

2.2. Characterization

The NKF-6 zeolite sample was characterized using Fourier Transform Infraredspectra (FTIR) (Perkin Elmer spectrum 100, America) in pressed KBr pellets. Thespectral resolution was set to 1 cm�1, and 150 scans were collected for eachspectrum.

The X-ray powder diffraction (XRD) pattern of NKF-6 zeolite crystal was recor-ded on a MAC Science Co. M18XHF diffractometer. The XRD analysis was performedwith Cu Ka radiation (l ¼ 0.15406 nm) with a Rigaku diffractometer. The 2q-scan-ning rate was 2 min�1. The XRD pattern was identified by comparison to the JCPDstandards.

2.3. Experimental procedures

All the experiments were carried out by using a batch technique in polyethylenecentrifuge tubes under ambient conditions. The stock solutions of NKF-6 zeolite andNaClO4 were pre-equilibrated for 24 h and then the 63Ni stock solutionwas added toachieve the desired concentrations of different components. The pH was adjusted todesired values by adding negligible volumes of 0.1 or 0.01 mol L�1 HClO4 or NaOH.After the suspensions were shaken for 24 h, the solid and liquid phases wereseparated by centrifugation at 9000 rpm for 30min. It was necessary to note that theadsorption of 63Ni on the tube wall was negligible according to the test of 63Niadsorption in the absence of NKF-6 zeolite.

The amount of 63Ni adsorbed on NKF-6 zeolite was calculated from the differ-ence between the initial concentration and the equilibrium one. The adsorptionpercent (%) and distribution coefficient (Kd) are calculated from equations:

Adsorption% ¼ C0 � CeC0

� 100% (1)

Kd ¼ C0 � CeCe

� Vm

(2)

where C0 is the initial concentration, Ce is the final concentration in the supernatantafter centrifugation, m is the mass of NKF-6 zeolite, and V is the volume of the

suspension. All experimental data were the averages of duplicate determinations.The relative errors of the data were about 5%.

3. Results and discussion

3.1. Characterization of NKF-6 zeolite

The FT-IR spectrum of NKF-6 zeolite is shown in Fig. 1. Theadsorption band at 587 cm�1 indicates the formation of D5R (doublefive ring) by tetrahedral SiO4 and AlO4 units. The bands at 517, 587,and 815 cm�1 indicate the complete crystalline structure of NKF-6zeolite. The band at 1182 cm�1 suggests the formation of crystallineNKF-6 zeolite. The peak at 2463 cm�1 is assigned to the vibration ofadsorbed CO2 species on NKF-6 zeolite surface. It is also well knownthat thepeaks at 3429and1632 cm�1 in Fig.1 correspond to thebendvibration of zeolitic water in the NKF-6 zeolite (Shao et al., 2009a,c).

Fig. 2 shows the XRD pattern of NKF-6 zeolite. The XRD patternof NKF-6 zeolite exhibits the most intense diffraction peaks at2q ¼ 20e32� and thus confirms the BEA structure of NKF-6 zeoliteas well as its good crystalline nature (Eswaramoorthi et al., 2003). Itwas found to be in good agreement with standard references(Treacy and Higgins, 2001).

3.2. Adsorption kinetics

The effect of contact time on 63Ni adsorption to NKF-6 zeolite isshown in Fig. 3. The adsorption of 63Ni increased rapidly during thefirst 4 h, and then maintained a high level of adsorption withincreasing contact time. The quick adsorption of 63Ni on NKF-6zeolite suggests chemical adsorption rather than physical adsorp-tion (Chen et al., 2009; Zhao et al., 2009). The results indicate thata period of several hours was enough to achieve the equilibration of63Ni adsorption on NKF-6 zeolite. In the following experiments,48 h was selected to achieve the adsorption equilibrium.

The pseudo-second-order rate equation was used to simulatethe kinetic adsorption process (Ho andMcKay,1999; Li et al., 2009):

tqt

¼ 12K 0q2e

þ 1qet (3)

where qt (mg g�1) is the amount of 63Ni adsorbed on NKF-6 zeoliteat time t, qe (mg g�1) is the equilibrium adsorption capacity, and K0

(gmg�1 h�1) is the pseudo-second-order rate constant. The straightline plot of t/qt vs. t (insert in Fig. 3) indicates that the kinetic

Fig. 4. Variation in adsorption percentage (%) and distribution coefficient (Kd) of 63Nias a function of NKF-6 zeolite content, pH ¼ 6.25 � 0.10, T ¼ 293.5 K, I ¼ 0.01 mol L�1

NaClO4, C[63Ni] (initial) ¼ 1.59 � 10�4 mol L�1.Fig. 2. XRD pattern of NKF-6 zeolite.

H. Zhang et al. / Journal of Environmental Radioactivity 101 (2010) 1061e1069 1063

adsorption of 63Ni on NKF-6 zeolite can be well described by thepseudo-second-order rate equation. The correlation coefficient ofthe pseudo-second-order rate equation for the linear plot is quiteclose to 1. The values of K’ and qe calculated from the intercept andslope of Eq. (3) are 1.28 g mg�1 h�1 and 15.13 mg g�1, respectively.The value of K’ also indicates that the adsorption process achievesequilibrium very quickly.

3.3. Effect of solid content

The adsorption of 63Ni on NKF-6 zeolite as a function of solidcontent is shown in Fig. 4. The adsorption percentage of 63Niincreased with increasing NKF-6 zeolite content. With increasingsolid content, the amount of functional groups at NKF-6 zeolitesurfaces increases, and more groups are available for binding 63Ni.The result is consistent with 63Ni adsorption on GMZ bentonite(Yang et al., 2009b). The distribution coefficient (Kd) as a functionof NKF-6 zeolite content is also plotted in Fig. 4. It is interesting tonotice that the Kd values increase weakly with increasing NKF-6zeolite contents under the experimental uncertainties. However,

Fig. 3. Effect of contract time on the adsorption of 63Ni to NKF-6 zeolite,pH ¼ 6.25 � 0.10, T ¼ 293.5 K, I ¼ 0.01 mol L�1 NaClO4, m/V ¼ 0.6 g L�1, C[63Ni](initial) ¼ 1.59 � 10�4 mol L�1.

Xu et al. (2006) reported that the Kd values of Co(II) adsorption onbentonite decreased slightly with increasing solid content, andChen and Wang (2006) found that the Kd values of Ni(II) adsorp-tion on rectorite decreased with increasing solid content. Consid-ering the physicochemical properties of the distributioncoefficient, the Kd values should be independent of solid content atvery low solid content. It is well known that NKF-6 zeolite presentsin solution as fibrillar structures (Fan et al., 2009a) and the siteswhich participate in the adsorption of metal ions locate not onlyon the surfaces but also in the channel of the colloids. Withincreasing NKF-6 zeolite content, local high concentration of 63Nimay be formed on solid surfaces and as a result precipitation orthe formation of macromolecular colloids may occur, and therebycause the distribution coefficient to increase slightly withincreasing solid content. Nevertheless, the interpretation is stillquestionably; further investigation is necessary.

3.4. Effect of pH

Fig. 5 shows the influence of pH on the removal of 63Ni to NKF-6zeolite. The pH value played an important role on the adsorption.The adsorption increased gradually with increasing pH at pH <5,then increased abruptly at pH 5.0e8.5, and at last maintaineda high level at pH >8.5.

The effect of pH can be explained in terms of pHzpc (zero pointcharge) of the adsorbent. The pHzpc value of NKF-6 zeolite is aboutpH6.5� 0.1 (Chutia et al., 2009). At pH<pHzpc, the surface charge ofthe adsorbent is positive. The ion exchange between 63Ni ions andHþ/Naþon zeolite surfaces can lead to the adsorptionof 63Ni onNKF-6 zeolite. At pH>pHzpc, it is easy for the positively charged 63Ni to beadsorbed on the negatively charged NKF-6 zeolite surface. Theelectrostatic attraction between 63Ni species and NKF-6 zeoliteparticles would also lead to an increase adsorption of 63Ni ions.Another reason is that the aluminol and silanol groups of the zeoliteare less protonated with increasing pH values. Hence, these groupsaremore available to retain 63Ni ions. Surface complexationbetween63Ni ions and NKF-6 zeolite is facilitated and the adsorption isthereby enhanced. In general, the effect of pH on the adsorption of63Ni on zeolite is the result of the combination of these factors (asmentioned above). The adsorption of 63Ni on NKF-6 zeolite may beindicative of many surface complexes because the sorption edges of63Ni on NKF-6 zeolite are spread over three pH units (Kowal-Fouchard et al., 2004; Chang et al., 2007; Tan et al., 2008d). The

H. Zhang et al. / Journal of Environmental Radioactivity 101 (2010) 1061e10691064

adsorption mechanism of 63Ni on NKF-6 zeolite will be discussed inmore detail in the following section.

3.5. Effect of ionic strength

In order to evaluate the influence of background electrolyte ionson 63Ni adsorption, the pH-dependent adsorption of 63Ni on zeolitewas investigated in 0.001, 0.01 and 0.1 mol L�1 NaClO4 solutions. Ascan be also seen from Fig. 5, the adsorption of 63Ni was affected byionic strength at pH <8.5, and was independent of ionic strength atpH >8.5. From the ionic strength dependence, one can deduce thation exchange is the main mechanism for 63Ni adsorption on NKF-6zeolite at pH <8.5 (Echeverría et al., 2003; Fan et al., 2009b), whichis also supported by the very slow increase of 63Ni adsorption at thispH range. Below pH 8.5, adsorption may be via ion exchange withhydrogen and sodium ions that saturate the exchange sites(Mohapatra and Gupta, 2005). The fact that the amount of 63Niadsorbed on NKF-6 zeolite was the highest in 0.001 mol L�1 NaClO4and was the lowest in 0.1 mol L�1 NaClO4 also supports thishypothesis. At pH >8.5, surface complexation probably contributesto the uptake of 63Ni to NKF-6 zeolite surface. The exchangeinvolving hydrated Ni2þ can be depicted by two elementary reac-tions (Fan et al., 2009a):

(_) to exchange with hydronium ions:

2hS� OHþ Ni2þ/ðhS� OÞ2Niþ 2Hþ (4)

(__) to exchange with Naþ ions at zeolite surfaces:

2hS� ONaþ Ni2þ/ðhS� OÞ2Niþ 2Naþ (5)

(___) hydrolysis of Ni2þ in solution:

Ni2þ þ nH2O/NiðOHÞmðH2OÞ2�mn�m þmHþ (6)

(_V) Being n > m, and to exchange with hydrolyzed species:

hS� ONaþ NiðOHÞmðH2OÞ2�mn�m/hS� ONiOHþ Naþ (7)

It is necessary to note that the hydroxylated surface groups varyat different pH values because of the protonation/deprotonationprocesses as follows:

hSOHþHþ4hSOHþ2 (8)

Fig. 5. Effect of pH and ionic strength on 63Ni adsorption to NKF-6 zeolite,T ¼ 293.15 K, m/V ¼ 0.6 g L�1, C[63Ni] (initial) ¼ 1.59 � 10�4 mol L�1.

hSOH4hSO� þHþ (9)

With increasing pH values, the concentrations of surface species(hSOH uncharged surface groups; hSOH2

þ positively chargedsurface groups; hSO� negatively charged surface groups) becomedifferent. The concentration of hSOH2

þ decreases with increasingpH, whereas hSO� increases with increasing pH.

To illustrate the variation and relationship of pH, Ce (mol L�1, theconcentration of 63Ni remained in solution), and qe (mol g�1, theconcentration of 63Ni adsorbed on solid phase), the experimentaldata in Fig. 5 are plotted as three-dimensional plots of qe, Ce, and pH(Fig. 6). On the pH-qe plane, one can see that adsorption of 63Ni onNKF-6 zeolite was strongly dependent on pH values, which is quitesimilar to the results shown in Fig. 5; On the pH-Ce plane, theconcentration of 63Ni remaining in solution decreased withincreasing pH at pH <8.5, and maintained a low level at pH >8.5.The projection on the pH-Ce plane is just the inverted image of theprojection on the pH-qe plane. On the Ce-qe plane, the projection isa straight line containing adsorption data under the three ionicstrength conditions. It is known that the initial concentration of63Ni for each experimental point was the same. The followingequation can describe the relationship of Ceeqe:

VC0 ¼ mqe þ VCe (10)

Eq. (10) can be rearranged as:

qe ¼ C0Vm

� CeVm

(11)

where V is the volume andm is the mass of NKF-6 zeolite. From Eq.(11), it is clear that the experiment data of Ceeqe lies in a straightline with a slope (�V/m) and intercept (C0,V/m). The slope andintercept calculated from the Ceeqe line are �1.67 and 1.67 � 10�2,which are in accordance with the values of m/V ¼ 0.6 g L�1 andC0 ¼ 1.59 � 10�4 mol L�1. The 3D plots show the relationship of pH,Ce, and qe very clearly, i.e., all the data of Ceeqe lie in a straight linewith slope (�V/m) and intercept (C0V/m) for the same initialconcentration of 63Ni and the same NKF-6 zeolite content (Shenget al., 2009a).

3.6. Effect of foreign ions

In order to investigate the influence of background electrolyteions on 63Ni adsorption, the adsorption of 63Ni on NKF-6 zeolite was

Fig. 6. 3D plots of 63Ni adsorption to NKF-6 zeolite as a function of pH and ionicstrengths, T ¼ 293.15 K, m/V ¼ 0.6 g L�1, C[63Ni] (initial) ¼ 1.59 � 10�4 mol L�1.

H. Zhang et al. / Journal of Environmental Radioactivity 101 (2010) 1061e1069 1065

studied as a function of pH values in 0.01 mol L�1 NaClO4, NaCl,NaNO3, KClO4 and LiClO4, respectively. Fig. 7A shows that theadsorption of 63Ni on NKF-6 zeolitewas influenced by the cations. AtpH <8.5, the adsorption percentages of 63Ni on NKF-6 zeolite underthe same pHvalueswere in the following sequence: LiþzNaþ> Kþ,indicating that the cations can alter the surface properties of NKF-6zeolite and thus can influence theadsorptionof 63Ni onNKF-6 zeolitesurfaces (Tan et al., 2007a). The adsorption of 63Ni on NKF-6 zeolitecan be considered as a competition of 63Ni with Liþ (or Naþ, Kþ) atzeolite surfaces. The hydration radius of the three cations areKþ ¼ 2.32 Å, Naþ ¼ 2.76 Å and Liþ ¼ 3.4 Å (Esmadi and Simm,1995).The hydration radius of Kþ is smaller than those of the other twocations and therefore the influence of Kþ on 63Ni adsorption is largerthan those of Naþ and Liþ. In general, the influence of monovalentalkali ions on the adsorption of bivalent 63Ni should be weak.However, in this work the influence of Liþ, Naþ and Kþ on 63Niadsorptionwas drastic. Tan et al. (2007a,b) investigated the effect ofLiþ, Naþ andKþon the adsorption of Th(IV) onTiO2, respectively, andalso found similar results. Before the addition of 63Ni ions, the stocksolution of NKF-6 zeolite was pre-equilibrated with the alkali ions.The adsorption of 63Ni on NKF-6 zeolite can be considered as theexchange of 63Ni with alkali ions and other reactions. Thereby, it isreasonable that the alkali ions can affect 63Ni adsorption.

Fig. 7B shows that foreign anions affected 63Ni adsorptiondrastically. The adsorption of 63Ni on NKF-6 zeolite was the lowest

Fig. 7. Effect of cations (A) and anions (B) on the adsorption of 63Ni on NKF-6 zeolite asa function of pH, T ¼ 293.15 K, m/V ¼ 0.6 g L�1, C[63Ni] (initial) ¼ 1.59 � 10�4 mol L�1.

in 0.01 mol L�1 NaCl solution and was the highest in 0.01 mol L�1

NaClO4 solution at pH <7.5. This may be attributed to: (1) Cl� andNO3

� that can form soluble complexes with 63Ni (e.g. NiClþ andNiNO3

þ), whereas ClO4� can not, and 63Ni has higher affinity with Cl�

than NO3� and ClO4

�; (2) compared with NO3� and ClO4

�, Cl� is easierto form idiocratic adsorption on the solid surface, which changesthe surface state of NKF-6 zeolite and decreases the availability ofbinding sites; and (3) the inorganic acid radical radius order isCl� < NO3

� < ClO4�, and the smaller radius inorganic acid radicals

takes up more ionic exchange sites and leads to the decrease of 63Niadsorption on NKF-6 zeolite (Sheng et al., 2008)

3.7. Effect of humic substances

Effect of FA on 63Ni adsorption as a function of pH is shown inFig. 8. The presence of FA enhanced the adsorption of 63Ni on NKF-6zeolite at pH<7.5, but suppressed 63Ni adsorption at pH>7.5. In thepresence of FA, 63NieFA complexes may be formed in solution or onzeolite surfaces, which is dependent on the interaction of FA withzeolite at different pH values. At low pH, the negatively charged FAis easily adsorbed on the positively charged surfaces of zeolite. The63Ni can easily form complexes with FA adsorbed on the surface ofNKF-6 zeolite, which enhances the adsorption of 63Ni on FAezeolitehybrids. Additionally, the surface adsorbed FA may induce a morenegative surface charge on NKF-6 zeolite, allowing more 63Ni to beadsorbed due to favorable electrostatic interaction. At high pH, asthe solubility of FA increases and zeolite becomes negativelycharged. The negatively charged FA is weakly adsorbed on NKF-6zeolite due to electrostatic repulse and a part of stable FA-Nicomplexes are formed in solution, which leads to the decrease of63Ni adsorption on NKF-6 zeolite.

Based on the above results and the surface properties of zeolite,the complex mechanism between FA, NKF-6 zeolite and 63Ni can beexpressed as follows (Yu et al., 2008):

hS� OHþ2 þ �OOC� FA4hS� OOC� FAþ H2O (12)

hS�OHþ2

�OH þ HOO� FA4hS�O��O�C� FAþ 2H2O (13)

The presence of FA in solution affects the adsorption of 63Ni onNKF-6 zeolite, and the species of 63Ni on NKF-6 zeolite dependmainly on both FA and NKF-6 zeolite. Ternary complexes, such as

Fig. 8. Influence of pH on 63Ni adsorption on NKF-6 zeolite in presence/absence of FA/HA, T ¼ 293.15 K, m/V ¼ 0.6 g L�1, I ¼ 0.01 mol L�1 NaClO4, C[63Ni](initial) ¼ 1.59 � 10�4 mol L�1.

H. Zhang et al. / Journal of Environmental Radioactivity 101 (2010) 1061e10691066

^SOeOeFAeNi, are probably formed on the surface of NKF-6zeolite in the presence of FA (Xu et al., 2007). This result is similar tothat of Th(IV) adsorption on SiO2 in the presence of HA/FA at pH>6(Chen and Wang, 2007). The adsorption of 63Ni on NKF-6 zeolite inthe presence of HA as a function of pH is also shown in Fig. 8. Nodrastic effect of HA on 63Ni uptake to NKF-6 zeolite at pH <7.5 wasobserved. At pH >7.5, the presence of HA decreased the adsorptionof 63Ni on HAezeolite hybrids. At low pH values, HA generallypresents as spherical shapes in solution or adsorbed on solidparticles (Chen et al., 2007). The spherical structure makes most ofthe functional groups not available to form complexes with metalions (Eswaramoorthi et al., 2003), thereby the presence of HA doesnot have a large effect 63Ni uptake to NKF-6 zeolite. At high pHvalues, the negatively charged HA is difficult to be adsorbed on thenegatively charged surface of NKF-6 zeolite. The fraction of HAremained in solution increases and thus increases the formation ofstable HAeNi complexes in solution, which decreases the adsorp-tion of 63Ni on NKF-6 zeolite.

One can also see from Fig. 8 that the influence of FAwas strongerthan HA at the same pH values under the same FA/HA concentra-tions. It is necessary to note that the samples of FA and HA wereextracted from the same soil sample, and probably had similarfunction groups such as carboxylic and phenolic groups. Theinfluence of FA and HA on 63Ni adsorption to zeolite should be verysimilar. It may be concluded that the presence of FA/HA decreased63Ni adsorption to FA/HAezeolite hybrids at pH >7.5, and theinfluence of FA/HA on 63Ni adsorption at pH<7.5 wasweaker ratherthan that of FA/HA at pH>7.5. The complicated structures of FA, HAand zeolite make it difficult to get an unambiguous and exactconclusion about the FA/HA influence on 63Ni adsorption to zeolite.Nevertheless, humic substances are ubiquitous in environment andtheir influence on the physicochemical behavior of metal ionscannot be neglected.

3.8. Effect of temperature and thermodynamic data

It is well known that temperature is one of the most importantparameters that dominate the physicochemical behaviors of metalions in the environment. The adsorption isotherms of 63Ni on NKF-6 zeolite at 293.15, 313.15 and 333.15 K are shown in Fig. 9. Theadsorption of 63Ni increased with the rise of temperature. Theadsorption isotherm was the highest at T ¼ 333.15 K and is thelowest at T ¼ 293.15 K. The results indicate that the adsorptionreaction was endothermic. Three different models, i.e., Langmuir,

Fig. 9. Adsorption isotherms of 63Ni on NKF-6 zeolite at three different temperatures,pH ¼ 6.2 � 0.10, m/V ¼ 0.6 g L�1, I ¼ 0.01 mol L�1 NaClO4.

Fig. 10. Langmuir (A), Freundlich (B) and D-R (C) isotherm fitting isotherms of 63Niadsorption on NKF-6 zeolite at three different temperatures, pH ¼ 6.25 � 0.10,m/V ¼ 0.6 g L�1, I ¼ 0.01 mol L�1 NaClO4.

Freundlich and D-R isotherm equations, were employed to simulatethe adsorption isotherms and to establish the relationship betweenthe amount of 63Ni adsorbed on solid phase and the concentrationof 63Ni remained in solution.

The Langmuir isothermmodel is used to describe themonolayeradsorption process. Its form can be expressed by the followingequation (Hu et al., 2009; Sari et al., 2007):

Table 1The parameters for Langmuir, Freundlich and D-R isotherms.

T(�K) Langmuir Freundlich D-R

qmax (mol g�1) b(L mol�1) R kF(mol1�n Ln g�1) n R b(mol2 kJ�2) qmax(mol g�1) R

293.15 1.45 � 10�4 1.66 � 104 0.997 3.88 � 10�3 0.41 0.972 2.02 � 10�3 5.66 � 10�4 0.979313.15 1.74 � 10�4 2.55 � 104 0.998 3.32 � 10�3 0.36 0.964 2.84 � 10�3 6.46 � 10�4 0.972333.15 1.84 � 10�4 4.2 � 104 0.999 2.21 � 10�3 0.29 0.949 3.79 � 10�3 6.11 � 10�4 0.959

H. Zhang et al. / Journal of Environmental Radioactivity 101 (2010) 1061e1069 1067

qe ¼ bqmaxCe1þ bCe

(14)

Eq. (14) can be expressed in linear form:

1qe

¼ 1qmax

þ 1bqmax

$1Ce

(15)

where Ce is the equilibrium concentration of metal ions remainingin the solution (mol L�1); qe is the amount of metal ions adsorbedon per weight unit of solid after equilibrium (mol g�1); qmax, themaximum adsorption capacity, is the amount of metal ions atcomplete monolayer coverage (mol g�1) and b (L mol�1) is a con-stant that relates to the heat of adsorption.

The Freundlich isotherm model represents properly theadsorption data at low and intermediate concentrations onheterogeneous surfaces (Hu et al., 2009; Sheng et al., 2009b):

qe ¼ kFCne (16)

Eq. (16) can be expressed in linear form:

logqe ¼ logkF þ nlogCe (17)

where kF (mol1�n g�1 Ln) represents the adsorption capacity whenmetal ion equilibrium concentration equals to 1 and n representsthe degree of dependence of adsorption with equilibriumconcentration.

The D-R isotherm model is more general than the Langmuirisotherm since it does not assume a homogeneous surface orconstant adsorption potential (Wang et al., 2009a,b):

qe ¼ qmaxexp��b32

�(18)

or in the linear form:

lnqe ¼ lnqmax � b32 (19)

Fig. 11. Linear plots of lnKd versus Ce, pH ¼ 6.25 � 0.10, m/V ¼ 0.6 g L�1,I ¼ 0.01 mol L�1 NaClO4.

where qe and qmax are defined above, b is the activity coefficientrelated to the mean adsorption energy (mol2 kJ�2), and 3 is thePolanyi potential, which is equal to:

3 ¼ RTln�1þ 1

Ce

�(20)

where R is ideal gas constant (8.3145 J mol�1 K�1), and T is theabsolute temperature in Kelvin (K).

E (kJ mol�1) is defined as the free energy change that is requiredto transfer 1 mol of ions from solution to the solid surfaces. Therelation can be described as follow:

E ¼ 1ffiffiffiffiffiffi2b

p (21)

The experimental data of 63Ni adsorption (Fig. 9) are analyzedwith the Langmuir, Freundlich and D-R models, and the results aregiven in Fig.10. The related parameters are listed in Table 1. It can beconcluded from the correlation coefficients that Langmuir modelrepresented the experimental data better than Freundlich and D-Rmodels. The fact that the Langmuir isotherm fits the experimentaldata very well suggests almost complete monolayer coverage of theadsorbent particles. Moreover, zeolite has a limited adsorptioncapacity, thus the adsorption could be better described by Langmuirmodel than by Freundlich and D-R models, since an exponentiallyincreasing adsorption was assumed in the Freundlich model (Chenand Wang, 2007). The values of qmax obtained from the Langmuirmodel for 63Ni adsorption on zeolitewere the highest at T¼ 333.15 Kand the lowest at T¼ 293.15 K,which indicates that the adsorption isenhancedwith increasing temperature. In the Freundlichmodel, thevalue of n is lower than 1, which indicates that a nonlinear adsorp-tion takes place on zeolite surfaces. The magnitude of E is animportant factor for estimating the sorption mechanism. The Evalues obtained from Eq. (21) are 15.7 (at T ¼ 333.15 K), 13.3 (atT ¼ 313.15 K) and 11.5 kJ mol�1 (at T ¼ 293.15 K), which are in theadsorption energy range of chemical ion exchange reaction (Donatet al., 2005; Ozcan et al., 2006). This suggests that 63Ni adsorptiononto zeolite should be attributed to chemical adsorption rather thanphysical adsorption. The adsorption capacities (qmax) derived fromthe D-R model were higher than those derived from the Langmuirmodel. This may be attributed to the different assumptions consid-ered in the formulation of the isotherms.

The thermodynamic parameters (DH0, DS0 and DG0) for 63Niadsorption on NKF-6 zeolite can be determined from the temper-ature dependent adsorption. Free energy change (DG0) is calculatedfrom the relationship as follow (Fan et al., 2008):

DG0 ¼ �RTlnK0 (22)

Table 2Constants of linear fit lnKd vs. Ce (lnKd ¼ A þ BCe) for 63Ni adsorption on NKF-6zeolite.

T (�K) A B R

293.15 3.20 � 0.02 �2034 � 117 0.990313.15 3.42 � 0.03 �2548 � 192 0.983333.15 3.59 � 0.04 �3146 � 311 0.972

Table 3Values of thermodynamic parameters for the adsorption of 63Ni on NKF-6 zeolite.

T (�K) DG0 (kJ mol�1) DH0(kJ mol�1) DS0(J mol�1 K�1)

293.15 �7.81 7.88 53.5313.15 �8.91 7.85 53.5333.15 �9.95 7.88 53.5

H. Zhang et al. / Journal of Environmental Radioactivity 101 (2010) 1061e10691068

where R is the ideal gas constant (8.314 J mol�1 K�1), K0 is theadsorption equilibrium constant. The values of lnK0 are obtained byplotting lnKd versus Ce (Fig. 11) and extrapolating Ce to zero (Yuet al., 2008). Constants of linear fit of lnKd versus Ce for adsorp-tion of 63Ni to NKF-6 zeolite are listed in Table 2. Although Kd isnot a thermodynamical parameter, the adsorption equilibriumconstant (K0) is generally related to the distribution coefficient (Kd)when the hypothesis that the adsorption reaction is surface reac-tion and the thermodynamical values are obtained withoutdeducing the surface released protons neutralization is made.Thereby, it is reasonable to use the distribution coefficient to derivethe adsorption equilibrium constant. Its intercept with the verticalaxis gives the value of lnK0. Standard entropy change (DS0) iscalculated using the equation:

DS0 ¼ � vDG0

vT

!p (23)

The average standard enthalpy change (DH0) is then calculatedfrom the relationship:

DH0 ¼ DG0 þ TDS0 (24)

The values obtained from Eqs. (22e24) are tabulated in Table 3.A positive value of the standard enthalpy change indicates that theadsorption was endothermic. One possible explanation to thisphenomenon is that 63Ni is solved well in water, and the hydrationsheath of 63Ni has to be destroyed before its adsorption on NKF-6zeolite. This dehydration process needs energy, and it is favored athigh temperature. This energy exceeds the exothermicity ofcations to attach to the solid surface. This assumption indicatesthat the endothermicity of the desolvation process was higherthan the enthalpy of adsorption by a considerable extent. TheGibbs free energy change (DG0) was negative as expected fora spontaneous process under the conditions applied. The value ofDG0 becomes more negative with the increase of temperature,indicating more efficient adsorption at high temperature. At hightemperature, cations are readily desolvated and hence itsadsorption becomes more favorable. The positive value of entropychange (DS0) suggests the affinity of zeolite toward 63Ni ions inaqueous solutions and may suggest some structure changes on theadsorbents (Chen and Wang, 2006, 2007). The results of 63Niadsorption on zeolite indicate that the adsorption of 63Ni on NKF-6 zeolite was endothermic.

4. Conclusions

From the results of 63Ni adsorption on NKF-6 zeolite, one candraw the following conclusions:

(1) The adsorption of 63Ni on NKF-6 zeolite achieved equilibriumrapidly. The kinetic adsorption of 63Ni on NKF-6 zeolite wasdescribed well by the pseudo-second-order model.

(2) The adsorption of 63Ni onNKF-6 zeolitewas strongly dependenton pH values. The adsorption increased with increasing pHvalues at pH<8.5, and thenmaintained a high level at pH>8.5.

(3) The adsorption of 63Ni on NKF-6 zeolite was dependent onforeign ions at low pH values, and independent of foreign ionsat high pH values.

(4) The adsorption of 63Ni on NKF-6 zeolite was dependent onionic strength at low pH and was independent of ionic strengthat high pH values. The adsorption of 63Ni was dominated by ionexchange or outer-sphere surface complexation at low pHvalues, and by inner-sphere surface complexation at high pHvalues.

(5) At low pH values, the presence of FA enhanced the adsorptionof 63Ni on NKF-6 zeolite, while the presence of HA had nodrastic effect. At high pH values, the presence of FA or HAdecreased the adsorption of 63Ni on NKF-6 zeolite.

(6) The thermodynamic parameters indicate that the adsorption of63Ni on NKF-6 zeolite was a spontaneous and endothermicprocess.

Acknowledgment

Financial support from National Natural Science Foundation ofChina (J0630962) is acknowledged.

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