Adsorption of Zinc Ions from Water Using Zeolite/Iron

Reprinted from

Adsorption Science & Technology2007 Volume 25 Number 10

Multi-Science Publishing Co. Ltd.5 Wates Way, Brentwood, Essex CM15 9TB, United Kingdom

Adsorption of Zinc Ions from Water Using

Zeolite/Iron Oxide Composites

Denise A. Fungaro and José E. A. Graciano

729

†First presented at the 6th Brazilian Meeting on Adsorption held in Maringá, State of Paraná, Southern Brazil on August13–16, 2006.*Author to whom all correspondence should be addressed. E-mail: [email protected].

Adsorption of Zinc Ions from Water Using Zeolite/Iron Oxide Composites†

Denise A. Fungaro* and José E. A. Graciano Center of Environmental Chemistry, Institute for Energy and

Nuclear Research, Av. Prof. Lineu Prestes, 2242, CEP 05508-000, São Paulo, SP, Brazil.

ABSTRACT: The adsorption characteristics of zeolites synthesized from flyash were combined in a composite with the magnetic properties of iron oxidesto produce adsorbents which were magnetic materials. Such zeolite/ironoxide magnetic composites were prepared with weight ratios of 3:1, 2:1 and 1:1.The experimental data for the equilibrium adsorption isotherms of Zn2+ ionsonto the composites were modelled using the Freundlich and Langmuirequations. The presence of iron oxide had no significant effect on the adsorptioncapacities of the magnetic composites.

The experimental data were also employed to determine the kineticcharacteristics of the adsorption process. The adsorption of Zn2+ ions was foundto follow pseudo-second-order type kinetics. Although intra-particle diffusionoccurred in the adsorption processes, it could not be accepted as the primaryrate-determining step. The evaluated thermodynamic parameters indicated thatthe adsorption of Zn2+ ions onto zeolite/iron composites was spontaneous andendothermic.

INTRODUCTION

The application of modified magnetic materials to solve environmental problems has receivedconsiderable attention in recent years. Magnetic composites can be used to adsorb contaminantsfrom aqueous or gaseous effluents and, after adsorption, can be separated from the medium by asimple magnetic process. Some examples of this technology are the use of poly(oxy-2,6-dimethyl-1,4-phenylene) for the adsorption of organic dyes (Safarik et al. 1995), polymer-coated magneticparticles for oil spill remediation (Orbell et al. 1997), polyacrylic acid-bound iron oxide magneticnano-particles for the adsorption of Methylene Blue (Mak and Chen 2004) and the use ofmontmorillonite–Cu(II)/Fe(III) oxide magnetic materials for the removal of humic acid (Xianjiaet al. 2006).

Magnetic composites based on activated carbon/iron oxide, clay/iron oxide and NaYcommercial zeolite/iron oxide have been prepared via a simple precipitation method and used forthe removal of contaminants from aqueous effluents (Oliveira et al. 2002, 2003, 2004). Thesematerials possessed a high surface area and a high adsorption capacity.

In the present work, the magnetic properties of iron oxides have been combined with theadsorption features of zeolites synthesized from fly ash to produce a novel magnetic compositeadsorbent. It is possible to convert fly ash into zeolitic products by hydrothermal treatment in analkaline medium (Henmi 1987; Querol et al.1997; Poole et al. 2000; Rayalu et al. 2000; Kolay et al.

2001; Murayama et al. 2002). The products showed a significantly increased surface area and cation-exchange capacity when compared to the raw ash.

Zeolitic materials have been used as low-cost adsorbents for the removal of metal from aqueoussolution (Singer and Berkgaut 1995; Lin and Hsi 1995; Amrhein et al. 1996; Querol et al. 2001, 2002;Fungaro and Silva 2002; Fungaro et al. 2004, 2005). The zeolite/iron oxide composites were used asadsorbents for the removal of Zn(II) ions from water. The Langmuir and Freundlich isotherm modelswere examined for their applicability to model the isotherm data. The experimental kinetic data wereevaluated by applying the pseudo-first- and pseudo-second-order and intra-particle diffusion models.

EXPERIMENTAL

Materials

All the chemicals used in the present study were of Analytical Reagent grade. A stock solution ofZn(II) ions was prepared by dissolving a weighed quantity of the respective nitrate salt inde-ionized water (Millipore Milli-Q). Samples of fly ash from a baghouse filter were obtainedfrom a coal-fired power plant located at Figueira County in Paraná State, Brazil. The chemicalcomposition of the fly ash was determined by X-ray fluorescence analysis (XRFS RIX 3000 –Rigaku) and is listed in Table 1.

The fly ash from the baghouse filter had a very low SiO2/Al2O3 ratio compared with most coalfly ashes (ca. 2 wt/wt%). This feature, coupled with the relatively low impurities content (Fe, Caand S oxides), indicates a high potential for the use of this sample as a starting material for thesynthesis of low-Si zeolites.

Synthesis of composites

Coal fly ashes were used as starting materials for zeolite synthesis by means of hydrothermaltreatment. In a typical synthesis, 20 g ash was heated in an oven for 24 h at 100°C, cooled in adesiccator and then treated with 160 m� of 3.5 mol/� NaOH solution. The zeolitic material wasthen washed repeatedly with de-ionized water and dried at 100°C for 24 h.

The composites were prepared by the co-precipitation method, by adding a 5 mol/� NaOHsolution into a mixed solution consisting of 0.25 mol/� ferrous chloride and 0.5 mol/� ferricchloride (molar ratio 1:2) at room temperature until a pH value of 11 was attained. The resulting

730 D.A. Fungaro and J.E.A. Graciano/Adsorption Science & Technology Vol. 25 No. 10 2007

TABLE 1. Chemical Composition of Coal Fly Ash

Components Composition Components Composition (wt%) (wt%)

SiO2 20.4 K2O 2.94Al2O3 15.9 TiO2 0.471Fe2O3 6.99 SO3 0.582Na2O 1.18 MgO 0.483CaO 0.86 ZnO 0.131

SiO2/Al2O3 1.3

slurry was filtered and washed repeatedly with distilled water until the wash solution had a pHvalue of 7. The particles thus obtained were re-dispersed in aqueous solution and zeolite wasadded. The amount of zeolite was adjusted in order to obtain zeolite/iron oxide weight ratios of1:1, 2:1 and 3:1, respectively. The composites were filtered and the materials obtained were driedin an oven at 100°C.

Batch adsorption experiments

Batch adsorption experiments were carried out by shaking 1 g of composite with 100 m� of solutionscontaining Zn(II) ions. After shaking, the suspensions were centrifuged and the supernatantconcentrations were established titrimetrically by EDTA methods. Adsorption isotherms of Zn(II)ions at initial concentrations in the range 260–520 mg/� were determined after shaking thesuspensions for 24 h.

RESULTS AND DISCUSSION

Adsorption isotherms

The analysis and design of adsorption processes requires data obtained under equilibriumconditions to obtain a better understanding of the adsorption process. Such data allow fundamentalphysiochemical calculations for evaluating the applicability of the adsorption process as an unitoperation to be undertaken. In the present study, adsorption isotherms were determined for theadsorption of Zn2+ ions onto pure zeolite and zeolite/Fe oxide composites. The results are shownin Figure 1. It will be seen that the adsorption capacities increased with increasing equilibriumZn2+ ion concentrations and eventually attained a plateau value.

The adsorption capacity for the Zn2+ ion decreased in the order zeolite > 3:1 composite > 2:1composite > 1:1 composite. It is interesting to note that the presence of iron oxide had nosignificant effect on the adsorption capacity of the zeolite in the composites. The lower adsorption

Adsorption of Zinc Ions from Water Using Zeolite/Iron Oxide Composites 731

40

20

00 100 200 300

q e (

mg/

g)

Ce (mg/�)

Figure 1. Equilibrium adsorption isotherms for Zn2+ ions onto pure zeolite (�) and onto the 3:1 (�), 2:1 (�) and 1:1 (�)zeolite/Fe oxide composites.

capacity of the composites is probably related to the decrease in surface area caused by thepresence of iron oxide. Similar results have been obtained for the adsorption of Zn2+ ions ontoclay/iron oxide composites (Oliveira et al. 2003).

The shapes of the isotherms are largely determined by the adsorption mechanism and cantherefore be used to diagnose the nature of the adsorption process (Giles et al. 1960). Thus,solution adsorption isotherms may be sub-divided into four main classes relating to their shapes;these are termed S, L, H and C with the subgroups 1, 2, 3, 4 or max. Figure 1 clearly shows thatthe adsorption isotherms obtained in the present studies were of L2 type. In processes where suchisotherms are obtained, the adsorption of solute onto the adsorbent proceeds until a monolayer isestablished; the formation of further layers is not possible in this case (Giles et al. 1960).

The equilibrium data obtained in the present study were analyzed using the Freundlich andLangmuir isotherms. The linear forms of the expressions for these isotherms may be expressed bythe following equations:

Freundlich: (1)

where KF [(mg/g)(�/mg)1/n] and n are the Freundlich constants related to the adsorption capacityand the intensity of adsorption of the adsorbents:

Langmuir: Ce/qe = 1/bQ0 + Ce/Q0 (2)

where qe is the concentration of the solid-phase adsorbate at equilibrium (mg/g), Ce is theconcentration of the aqueous-phase adsorbate at equilibrium (mg/�), Q0 (mg/g) is the maximumamount of adsorbate per unit weight of adsorbent necessary to form a complete monolayer on thesurface, and b (�/mg) is the Langmuir isotherm constant related to the affinity of the adsorption sites.

Figures 2 and 3 depict linear Langmuir and Freundlich plots for the adsorption of Zn2+ ions ontopure zeolite and the zeolite/Fe oxide composites, respectively. The isotherm constants andcorrelation coefficients calculated for the Freundlich and Langmuir equations are listed in Table 2.

log log logq K Ce F n e= + 1


Ce (mg/�)

Ce/

q e (g

/�)

00

10

5

100 200 300

Figure 2. Langmuir plots for the adsorption of Zn2+ ions onto pure zeolite (�) and onto the 3:1 (�), 2:1 (�) and 1:1 (�)zeolite/Fe oxide composites.

The data shown in the table indicate that the correlation coefficients (R2) demonstate that theLangmuir model fitted the experimental data better than the Freundlich model.

The Langmuir isotherm can also be represented in terms of a dimensionless constant separationfactor or an equilibrium parameter:

RL = 1/(1 + bC0) (3)


log Ce (mg/�)

log

q e (

mg/

g)

0.51.3

1.4

1.5

1.6

1.0 1.5 2.0 2.5

Figure 3. Freundlich plots for the adsorption of Zn2+ ions onto pure zeolite (�) and onto the 3:1 (�), 2:1 (�) and 1:1 (�)zeolite/Fe oxide composites.

TABLE 2. Langmuir and Freundlich Parameters for the Adsorption of Zn2+

Ions onto Pure Zeolite and Zeolite/Iron Oxide Composites

Adsorbent Langmuir isotherm

Qo (mg/g) b (�/mg) RL R2

1:1 zeolite:Fe 27.8 0.062 0.668 0.99952:1 zeolite:Fe 28.6 0.108 0.536 0.99793:1 zeolite:Fe 30.0 0.175 0.417 0.9979Zeolite 36.8 0.113 0.016 0.9991

Adsorbent Freundlich isotherm

KFa n R2

1:1 zeolite:Fe 53.4 8.75 0.86962:1 zeolite:Fe 17.8 12.4 0.73743:1 zeolite:Fe 21.1 16.5 0.8240Zeolite 19.3 8.42 0.9828

aExpressed in (mg/g)(�/mg)1/n units

where b is the Langmuir constant and C0 is the initial concentration of Zn2+ ions. Values of RLbetween 0 and 1 indicate favourable adsorption (Hall et al. 1966). It will be seen from Table 2 thatthe calculated RL values were between 0 and 1 for all the adsorbents studied, indicating favourableconditions for the adsorption process. The Freundlich parameters KF and n are also presented inTable 2. All the n values were higher than 1, indicating that the Zn2+ ions were adsorbedfavourably onto the adsorbents.

The 2:1 zeolite/iron oxide composite was chosen for the studies of the kinetic andthermodynamic characteristics of the adsorption process.

Effect of contact time and concentration

Figure 4 shows the effect of the initial Zn2+ ion concentration at different contact time on theadsorption capacity of the 2:1 zeolite/iron oxide composite. It will be seen that the extent ofadsorption increased rapidly over the initial stages before 5 h and then approached equilibriumafter 10 h. During the initial stages of adsorption, a large number of vacant surface sites areavailable on the adsorbent. However, after this stage, the remaining vacant surface sites becomedifficult to fill due to repulsive forces between the solute molecules on the solid surface and thebulk phase. In addition, metal ions are adsorbed into those pores that are virtually saturated duringthe initial adsorption stage. Thereafter, the metal ions have to traverse further and deeper into thepores, thereby encountering considerable resistance. This results in the slow down of theadsorption process during its later stages.

It will be seen from the figure that the amount adsorbed (mg/g) increased as the agitation timeand concentration increased, and then remained virtually constant after equilibrium had beenattained. An increase in the initial Zn2+ ion concentration led to an increase in the adsorptioncapacity of the composite towards Zn2+ ions. Thus, the initial concentration is an important drivingforce in overcoming all mass-transfer resistances towards the transfer of ions from aqueoussolution to the solid phase. This means that higher initial ion concentrations will lead to anenhancement of the sorption process. The time to reach equilibrium was independent of the initial


00

10

20

30

40

5 10

Time (h)

q (m

g/g)

15 20 25

Figure 4. Kinetic plots for the adsorption of Zn2+ ions onto the 2:1 zeolite/iron oxide composite. The data points relate tothe following initial Zn2+ ion concentrations: (�) 520 mg/�; (�) 390 mg/�; (�) 330 mg/�; and (�) 260 mg/�.

Zn2+ ion concentration although the extent of adsorption decreased from 86% to 54% when theZn2+ ion concentration increased from 260 mg/� to 520 mg/�. Hence, an increase in the initial Zn2+

ion concentration results in the rapid attainment of composite saturation and a higher residual Zn2+

ion concentration in the equilibrium solution. Furthermore, it should be noted that the removalcurves are single, smooth and continuous, thereby suggesting the formation of a monolayer ofadsorbate on the surface of the adsorbent.

Adsorption kinetics

A study of the adsorption kinetics is desirable as it provides information about the mechanism ofthe process, which is important for its efficiency. Predicting the rate at which contamination isremoved from aqueous solutions is important in order to design an adsorption treatment plant.Several models can be applied to determine the adsorption kinetics, viz. the pseudo-first-ordermodel, the pseudo-second-order model and the intra-particle diffusion model.

The pseudo-first-order model can be expressed by the relationship (Ho and McKay 1998a):

log(qe – q) = log qe – k1t/2.303 (4)

where q and qe represent the amount of Zn2+ ions adsorbed (mg/g) at a given time, t (h), and atequilibrium, respectively, while k1 represents the sorption rate constant (h–1).

The kinetic data were further analyzed using the pseudo-second-order relationship proposed byHo et al. (1996), which is represented by:

(5)

where k2 is the pseudo-second-order rate constant [g/(mg h)] and qe and q represents the amountsof Zn2+ ions adsorbed (mg/g) at equilibrium and a given time t (h). The constant k2 is used tocalculate the initial adsorption rate, h [mg/(g h)], via the equation:

h = k2q2e (6)

Because the two above equations cannot provide definite mechanisms, the data obtained werealso examined by the intra-particle diffusion model. According to Weber and Morris (1963), theintra-particle diffusion coefficient, Ki, can be defined as:

Ki = q/t0.5 (7)

Hence, the value of Ki [mg/(g h0.5)] can be obtained from the slope of the plot of q (mg/g) versust0.5 (h0.5). Previous studies have shown that such plots may present multilinearity, characterizingtwo or more steps involved in the adsorption process (Allen et al. 1989).

Figures 5–7 present the plots for the adsorption of Zn2+ ions onto the 2:1 zeolite/iron oxidecomposite applying the pseudo-first-order kinetic model, the pseudo-second-order kinetic modeland the intra-particle diffusion model, respectively. The calculated values of the kinetic constants,the diffusion coefficient and the corresponding linear regression correlation constants are listed inTable 3. As seen from the table, the values of the correlation coefficients obtained from theapplication of pseudo-second-order model were higher than those for the pseudo-first-order andintra-particle diffusion models. The best fit to the kinetic data arising from the studies of the

t

q k q qt

e e

= +1 1

22


adsorption of Zn2+ ions onto the composite was provided by the pseudo-second-order rateexpression. This suggests that the Zn2+ ion/composite system may involve an activated orchemisorption process (Ho and McKay 1998b).

The linearity of the fitting lines obtained from the application of the diffusion model (Figure 7)points to the presence of intra-particle diffusion in the system. However, the fact that the lines donot pass through the origin of the plots indicates that, although intra-particle diffusion may beinvolved in the adsorption process, it was not the rate-controlling step (Weber and Morris 1963).


00.0

0.5

1.0

1.5

5 10Time (h)

log(

q e –

q)

(mg/

g)

Figure 5. Pseudo-first-order kinetic plots for the adsorption of Zn2+ ions onto the 2:1 zeolite/iron oxide composite. The datapoints relate to the following initial Zn2+ ion concentrations: (�) 520 mg/�; (�) 390 mg/�; (�) 330 mg/�; and (�) 260 mg/�.

0 2 4 6 8 10

Time (h)

t/q

[(h

g)/m

g]

0.0

0.1

0.2

0.3

0.4

Figure 6. Pseudo-second-order kinetic plots for the adsorption of Zn2+ ions onto the 2:1 zeolite/iron oxide composite.The data points relate to the following initial Zn2+ ion concentrations: (�) 520 mg/�; (�) 390 mg/�; (�) 330 mg/�; and(�) 260 mg/�.


TABLE 3. Kinetic Constants for the Adsorption of Zn2+ Ions onto the 2:1 Zeolite/IronOxide Composite

Zn2+ ion Pseudo-first-order kineticsconc. (mg/�)

k1 (h–1) R2

1

260 3.14 × 10–1 0.9837330 3.34 × 10–1 0.9791390 2.61 × 10–1 0.9804520 2.04 × 10–1 0.9756

Pseudo-second-order kinetics

k2 [g/(mg h)] h [mg/(g h)] qe (mg/g) R22

260 2.62 × 10–2 0.176 25.9 0.9954330 2.14 × 10–2 0.164 27.7 0.9962390 2.18 × 10–2 0.193 29.8 0.9979520 0.164 × 10–2 1.76 32.6 0.9961

Intra-particle diffusion

Ki [mg/(g h0.5)] R2i

260 7.86 0.9742330 8.23 0.9809390 9.04 0.9787520 9.53 0.9848

Time0.5 (h0.5)

q (m

g/g)

00

10

20

30

1 2 3 4

Figure 7. Intra-particle diffusion plots for the adsorption of Zn2+ ions onto the 2:1 zeolite/iron oxide composite. Thedata points relate to the following initial Zn2+ ion concentrations: (�) 520 mg/�; (�) 390 mg/�; (�) 330 mg/�; and(�) 260 mg/�.

Thermodynamic parameters

In any adsorption process, both energy and entropy considerations must be taken into account inorder to determine whether that process will occur spontaneously. Values of the thermodynamicparameters are important indicators regarding the practical application of a process. Accordingly,the adsorption of Zn2+ ions from aqueous solution onto the 2:1 zeolite/iron oxide composite wasstudied at different temperatures. The observed variation in the extent of adsorption with respectto temperature was explained on the basis of changes in the standard free energy (∆G0), enthalpy(∆H0) and entropy (∆S0). These may be calculated via the following equations:

–∆G0 = 2.303RT log Kc (8)

(9)

(10)

where R is the gas constant and T is the temperature (K). The value of Kc was calculated using theequation:

(11)

where Cads is the equilibrium concentration of Zn2+ ions on the adsorbent (mg/�) and Ce is theequilibrium concentration of Zn2+ ions in the solution (mg/�). The symbols Kc, Kc1

and Kc2

correspond to the equilibrium constants at temperatures T, T1 and T2, respectively. The calculatedvalues of the thermodynamic parameters are presented in Table 4.

The negative, small values of the free energy change (∆G0) listed in this table indicate thespontaneous nature of the adsorption process. Similarly, the positive values of the standardenthalpy change (∆H0) at different temperatures indicate the endothermic nature of the adsorptionprocess. Finally, the positive values of ∆S0 suggest an increased randomness at the solid/solutioninterface during the adsorption of Zn2+ ions onto the composite.

KC

Ccads

e

=

∆∆ ∆

SH G

T0

0 0

=−

∆H RT T

T T

K

Kc

c

0 1 2

2 1

2 303 2

1

=−

. log


TABLE 4. Thermodynamic Parameters for the Adsorption of Zn2+

Ions onto the 2:1 Zeolite/Iron Oxide Composite

Temperature ∆G0 ∆H0 ∆S0

(°C) (kJ/mol) (kJ/mol) [J/(K mol)]

25 –3.90 6.42 34.630 –4.07 47.8 17140 –5.78

CONCLUSIONS

The results obtained in these studies show that the adsorption features of zeolite synthesized fromfly ash can be combined with magnetic iron oxides in a composite to produce magneticadsorbents. The preparation of such adsorbents involves a simple precipitation procedure,requiring readily available chemicals such as iron salts, sodium hydroxide and fly ash. Isothermsfor the adsorption of Zn2+ ions onto the adsorbents were best modelled by the Langmuir equation.The composites showed adsorption capacities towards Zn2+ ions from aqueous solution whichwere in the range 27.8–30.0 mg/g, with increasing presence of iron oxides producing a decreasein the adsorption capacity of the zeolite. The coal fly ash-based magnetic adsorbent can be usedas an alternative to more costly adsorbents for the treatment of wastewater.

ACKNOWLEDGEMENTS

The support of Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Brazil,is gratefully acknowledged. The authors are grateful to the Carbonífera do Cambuí Ltda. forsupplying the coal ash samples.

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Adsorption of Zinc Ions from Water Using Zeolite/Iron