Removal of Lead and Nickel Ions Using Zeolite ... - Wiley Online Library

J. Chem. T ech. Biotechnol. 1997, 69, 27È34

Removal of Lead and Nickel Ions Using Zeolite Tuff Ahmad Al-Haj Ali* & Ribhi El-Bishtawi Department of Chemical Engineering, University of Jordan, Amman, Jordan (Received 1 April 1996 ; revised version received 24 September 1996 ; accepted 14 November 1996)

Abstract : The capacity of Jordanian zeolite tu† for the removal of lead and nickel ions from aqueous solutions has been investigated under di†erent conditions, namely zeolite particle size, initial solution pH, initial metal ion concentration, slurry concentration and solution temperature. Equilibrium data obtained have been found to Ðt both the Langmuir and Freundlich adsorption isotherms. It has been found that this zeolite (phillipsite) tu† is an efficient ion exchanger for removing both lead and nickel ions. Its removal capability is considerably higher for lead ions than for nickel ions under all conditions tested ; however, the actual exchange capacities are far below the theoretical values. The Ðner the zeolite particles used, the higher the metal exchange capacity. An initial solution pH of 4É0 is favourable for obtaining high metal removal. Key words : heavy metals, lead, nickel, zeolite, ion exchange, sorption, equilibrium isotherms

NOTATION

icity.1 Major anthropogenic sources of heavy metals in the environment include metal extraction, metal fabrication and surface Ðnishing, paints and pigments, as well as the manufacture of batteries. The most common methods for the removal of heavy metals are ion exchange and chemical precipitation. The main advantages of ion exchange over chemical precipitation are recovery of metal value, selectivity, less sludge volume produced and the meeting of strict discharge speciÐcations. In ion exchange systems, polymeric resins as well as zeolites are usually employed. The availability of natural zeolites in many countries provides low-cost treatment by ion exchange systems. Zeolites are aluminosilicate minerals containing exchangeable alkaline and alkaline earth metal cations (normally Na, K, Ca and Mg) as well as water in their structural framework. The physical structure is porous, enclosing interconnected cavities in which the metal ions and water molecules are contained.2 They have been used in water softening applications for a long time. Recently, research has focused on the utilization of zeolite in the decontamination of metal-laden waste waters.3h7 Jordan is a country with limited water resources and this necessitates that much e†ort is put into water conservation and environmental protection. The expansion

AEC

Actual exchange capacity (meq metal ion g~1 zeolite) Ce Equilibrium metal ion concentration (meq dm~3) Ci Initial metal ion concentration (meq dm~3) K , n Freundlich model parameters F K , a Langmuir model parameters L qe Equilibrium removal (meq metal ion g~1 zeolite) S Slurry concentration (g zeolite dm~3) TEC Theoretical exchange capacity (meq metal ion g~1 zeolite)

INTRODUCTION Industrial wastewaters often contain considerable amounts of heavy metals that would endanger public health and the environment if discharged without adequate treatment. Heavy metals are elements such as Pb, Hg, Cr, Ni, Cd, Cu and Zn which have high atomic densities and which are usually associated with tox* To whom correspondence should be addressed.

27 J. Chem. T ech. Biotechnol. 0268-2575/97/$17.50 ( 1997 SCI. Printed in Great Britain

A. A.-H. Ali, R. El-Bishtawi

28 of industrial activities, including metal-based industries, requires the availability of low-cost technology and materials for wastewater treatment. However, the country has huge reserves of zeolite tu†s, especially in the north-east where phillipsite is the dominant mineral.8 The main objective of this study is to evaluate the capacity of Jordanian zeolite tu† for removing lead and nickel ions from aqueous solutions (prepared by dissolving metal nitrates in distilled water) under di†erent operating conditions.

EXPERIMENTAL

RESULTS AND DISCUSSION IdentiÐcation of zeolite X-ray di†raction analysis indicated that this zeolite sample is rich in phillipsite with some non-zeolite materials such as calcite and clays. The XRD chart is shown in Fig. 1. A recent study by Zamzow and Murphy5 who tested 24 zeolite minerals for the removal of Pb, Cd, Cu and Zn has indicated that phillipsite is the most efficient. The chemical compositions of sieved and treated zeolite fractions obtained from XRF analysis are given in Table 1. These data are used to calculate the theoretical exchange capacity.

Zeolite treatment and analysis The raw zeolite sample was ground using a morter and pestle then sieved into several size fractions in the range of 45È710 km using standard ASTM sieve series. A sample of each fraction was immersed in NaCl solution (25 g dm~3) overnight, Ðltered, gently washed by distilled water and then oven-dried until constant weight at room temperature. The treated (brinated) zeolite was used as a sorbent for lead and nickel ions. NaCl treatment was conducted based on the Ðndings of previous studies that sodium ions are the most e†ective exchangeable ions for heavy metal removal.3,5 X-ray di†raction (XRD) analysis was carried out on raw zeolite (unsieved) to identify the mineral type. However, X-ray Ñuorescence (XRF) analysis was carried out on two samples (di†erent size fractions) of treated zeolite to determine their chemical compositions.

Equilibrium removal of metal ions The amount of metal ion removed by zeolite at equilibrium (qe, meq g~1) was calculated from the following equation which is based on AAS analysis data : qe \ (Ci-Ce)/S

(1)

The e†ect of various variables on qe is discussed below.

Shake-Ñask experiments and solution analysis A volume of 150 cm3 of metal nitrate solution with a concentration in the range 50È400 mg metal ion dm~3 (0É483È3É861 and 1É704È13É629 meq dm~3 for Pb and Ni ions, respectively) was placed in a 250 cm3 Erlenmeyer Ñask. The pH of the solution was adjusted to the desired level using 0É01 N HNO solution. An 3 accurately-weighed zeolite sample (within the range 0É15È1É20 g) was then added to the solution to give the desired slurry concentration (in the range 1É0È8É0 g zeolite dm~3). A series of such Ñasks was then agitated at a constant speed of 300 strokes per minute in a shaking water bath, the temperature of which was kept constant at a prespeciÐed value. After a contact time of 5 days (the equilibrium time determined by preliminary experiments), the zeolite was separated by Ðltration through a cellulose acetate membrane Ðlter, the pH of the Ðltrate (Ðnal solution pH) measured and the Ðltrate analysed for the remaining (equilibrium) metal ion concentration by atomic absorption spectrophotometry (AAS). For some experiments, the concentrations of Na, K, Ca and Mg were also determined by AAS.

Fig. 1. X-ray di†raction chart for zeolite tu†.

TABLE 1 Chemical Composition (Wt%) of Treated Zeolite Samples (obtained by XRF analysis) Compound

90È180 km

355È710 km

SiO 2 Fe O 2 3 Al O 2 3 MgO CaO TiO K O 2 Na O 2 MnO P O 2 5

35É63 11É71 9É71 11É27 6É65 1É73 1É29 1É42 0É12 0É53

37É35 6É48 12É15 6É81 9É03 1É01 2É18 2É71 0É06 0É25

Removal of lead and nickel ions using zeolite tu†

29

E†ect of particle size Experiments were carried out for the determination of qe for lead and nickel ions using a wide range of particle sizes of treated zeolite. The results are presented in Table 2. These data indicate that a decrease in particle size from the 355È710 km fraction to the 180È355 km fraction results in increases of 8% and 14% in qe for lead and nickel ions, respectively. A further decrease in size to the 90È180 km fraction produces a 20% and 90% increase in qe for lead and nickel ions respectively. Ultimately, a decrease to the size range 45È90 km leads to an increase of 25% and 180% in qe for lead and nickel ions, respectively. Metal uptake by zeolite takes place at sites on the exterior surface of the particle as well as sites within the particle. However, only a fraction of the internal sites is accessible to metal ions due to pore di†usion resistance. Thus, decreasing the particle size increases the external surface area, which means increasing the number of available sites for metal uptake. For further investigation, the 90È180 km size fraction was selected because of its high metal removal capacities as well as easy handling. E†ect of initial pH Shake-Ñask experiments were carried out using metal ion solutions at di†erent initial pH levels (maintaining all other conditions constant) to determine the equilibrium metal ion removal and the results are shown in Fig. 2 from which it is obvious that qe is low at low pH levels for either metal ion. This can be explained by the competition by protons for sites on the zeolite particle. The value of qe is increased by increasing the initial pH level for both lead and nickel ions with a more profound e†ect of initial pH in the case of lead ions. It is apparent that using solutions with an initial pH of 4É0È 4É5 gives the highest qe values. These results are in agreement with several previous investigations on metal

Fig. 2. E†ect of initial pH on equilibrium metal ion removal.

sorption by a variety of materials which revealed that the sorption capacity is low at pH levels below 4É0.9h11 Moreover, low pH levels are undesirable in zeolite applications because this would a†ect the chemical structure of zeolite.3,8 E†ect of initial metal ion concentration Several experiments were conducted using the 90È 180 km particle size zeolite with di†erent initial concentrations, Ci (meq dm~3), for each of the metal ions considered. The results obtained are plotted in Figs 3A and 3B. The plotted data indicate that raising Ci values from 0É483 to 3É861 meq Pb ion dm~3 increases qe by 5É5 times whereas raising Ci values from 1É704 to 13É629 meq Ni ion dm~3 increases qe by 2É2 times only. In addition, qe varies linearly with Ci up to about 2É4 meq dm~3 for lead ions, beyond which qe starts to level o†. For nickel ions, there is a non-linear dependency of qe on initial concentration. The values of 2É413 meq Pb ion dm~3 and 8É518 meq Ni ion dm~3 (250 ppm) were selected as initial metal concentrations in all other experiments.

TABLE 2 Equilibrium Metal Ion Removals by Di†erent Size Fractions of Zeolitea Size (km)

355È710 180È355 90È180 45È90

qe (meq g~1)

Final pH

Pb

Ni

Pb

Ni

0É927 1É004 1É115 1É158

0É358 0É409 0É681 1É005

5É95 6É05 6É20 6É25

6É90 7É05 7É10 7É15

a Initial pH \ 4É0, S \ 2 g dm~3, T \ 20¡C and Ci \ 250 mg dm~3 (2É413 meq Pb dm~3 or 8É518 meq Ni dm~3).

Fig. 3A. E†ect of initial concentration on equilibrium lead ion removal.


30

2É0 g dm~3 results in a decrease of 47% in equilibrium lead ion removal (qe). Further increases in S to 4É0 and to 8É0 g dm~3 decrease qe by 71 and 86%, respectively. This is due to the decline in the remaining lead ion concentration in solution with increasing sorbent dose. The equilibrium removal of Ni ions decreases by 17 and 47% when the slurry concentration is increased from 1É0 to 2É0 and from 1É0 to 4É0 respectively. Further increase in S does not a†ect qe. E†ect of solution temperature

Fig. 3B. E†ect of initial concentration on equilibrium nickel ion removal.

E†ect of slurry concentration Experiments were carried out using di†erent slurry concentrations, S (g zeolite dm~3 of solution), keeping other conditions constant. The experimental data are plotted in Figs 4A and 4B. It is apparent from these data that raising the slurry concentration from 1É0 to

Equilibrium removals for lead and nickel ions were determined under isothermal conditions in the temperature range 20È35¡C. The data obtained for both metal ions are shown in Fig. 5 which indicates that qe for lead ions is independent of temperature in the tested range ; however, qe for nickel ions is only slightly increased with increasing the solution temperature. These results are in agreement with those obtained by Maliou et al.7 for the removal of lead and cadmium ions using clinoptilolite zeolite. This e†ect of temperature can be attributed to the enhancement of ion di†usivity. Final solution pH

Fig. 4A. E†ect of slurry concentration on equilibrium lead ion removal.

Fig. 4B. E†ect of slurry concentration on equilibrium nickel ion removal.

The addition of zeolite is expected to raise the solution pH due to the release of alkaline metal ions from the mineral. The pH should not be allowed to rise to levels at which chemical precipitation would occur. Lead and nickel ions would start precipitation as hydroxides from dilute solutions at pH 6É0 and 6É7, respectively. Other metal ions follow the same behaviour at other particular pH values.12 The experimental results presented in Fig. 6 show the variation of Ðnal pH (measured at equilibrium) with initial solution pH. It can be concluded from this Ðgure that starting with an initial pH of 4É5 (the highest value

Fig. 5. E†ect of temperature on equilibrium metal ion removal.


31

Fig. 6. Dependency of Ðnal pH on initial pH.

Fig. 7A. E†ect of initial lead ion concentration on Ðnal pH of solution.

tested), the Ðnal pH values approach 6É0 and 6É7 for lead and nickel ions, respectively. This indicates that using an initial solution pH below 4É5 is necessary to avoid chemical precipitation of metal ions as hydroxides during the sorption process. For the considerations mentioned above, an initial pH of 4É0 was selected for all other experiments. The e†ect of initial concentration on Ðnal solution pH can be understood by considering the hydrolysis reactions of lead and nickel ions, the solution species involved and their equilibrium concentrations. The hydrolytic reactions of divalent metal cations can be represented by the general equation :13 M2` ] nH O ¡ M(OH)2~n ] nH` 2 n

Figure 8 shows the e†ect of slurry concentration on the Ðnal solution pH. This Ðgure indicates that increasing the slurry concentration from 1É0 to 6É0 g zeolite dm~3 increases the Ðnal solution pH linearly from 5É7 to 7É8 in the case of lead ions. Further increase in slurry concentration does not increase the Ðnal pH. Figure 8 also indicates that slurry concentration should be low to avoid chemical precipitation of lead as hydroxide at

(2)

However, based on the equilibrium constants, only the Ðrst hydrolysis reaction (n \ 1) is considered signiÐcant for lead and nickel ions :13,14 Pb2` ] H O ¡ PbOH` ] H` 2

(3)

Ni2` ] H O ¡ NiOH` ] H` 2

(4)

These equations indicate that starting with a high metal ion concentration results in a high equilibrium concentration of the same ion and hence a high hydrogen ion concentration (low pH) and vice versa. Figures 7A and 7B present values of the Ðnal pH obtained using solutions of di†erent initial lead and nickel ion concentrations, respectively. Starting at a Ðxed initial pH of 4É0, the Ðnal pH decreases with increasing Ci of lead ions. This is due to the fact that at a Ðxed slurry concentration, the equilibrium concentration (Ce) of lead ions is high for high Ci values. Therefore, the Ðnal pH is low, as explained by the hydrolysis reactions above. It is noteworthy that low initial lead ion concentrations (below about 2É4 meq dm~3) result in Ðnal pH values that give rise to lead ion removal by chemical precipitation in addition to ion exchange sorption. In the case of nickel ions, however, the Ðnal pH varies only slightly with initial metal ion concentration.

Fig. 7B. E†ect of initial nickel ion concentration on Ðnal pH of solution.

Fig. 8. E†ect of slurry concentration on Ðnal pH during metal ion removal.


32 pH values higher than 6É0. At S \ 2É0 g dm~3, the solution pH is only slightly above pH 6É0 at the end of metalÈsorbent contact and therefore, the precipitation is expected to be negligible. In the case of nickel ions, however, the e†ect of slurry concentration on Ðnal solution pH is rather slight and the values are around 7É0. In either case, the increase in Ðnal pH is due to the increase in the concentrations of alkaline ions which are released from zeolite during the ion exchange process. In addition, alkaline species (mainly calcium) are expected to be released by the calcite impurities in the zeolite tu†. Moreover, Table 2 presents the values of Ðnal solution pH obtained using di†erent particle size fractions. The data indicate negligible variations in Ðnal pH due to particle size for both metal ions. Adsorption isotherms The equilibrium removal of metal ions considered can be mathematically expressed in terms of adsorption isotherms. Adsorption isotherm data are commonly Ðtted to the Langmuir model (eqn (5)) or the Freundlich model (eqn (6)) : qe \ K Ce/(1 ] aCe) L

(5)

qe \ K Cen F

(6)

Fig. 9B. Linearized Langmuir isotherm for nickel ion removal by zeolite.

ions. The values of K , a, K and n that best Ðtted the L F data as well as the corresponding correlation coefficients are presented in Table 3. Values of qe predicted by both models as well as the experimental values are

These models are rearranged to the linear form as follows : Ce/qe \ (1/K ) ] (a/K )Ce L L

(7)

log qe \ log K ] n log Ce F

(8)

Plotting the experimental data using eqn (7) (Figs 9A and 9B) and eqn (8) (Figs 10A and 10B) indicates that both models give good Ðt for the data for both metal

Fig. 10A. Linearized Freundlich isotherm for lead ion removal by zeolite.

Fig. 9A. Linearized Langmuir isotherm for lead ion removal by zeolite.

Fig. 10B. Linearized Freundlich isotherm for nickel ion removal by zeolite.


33

TABLE 3 Langmuir and Freundlich Models Parameters for Best Fit and the Corresponding Correlation Coefficients

L angmuir model K L a r2 Freundlich model K F n r2

Pb ion

Ni ion

34É187 25É233 1É000

0É401 0É460 0É991

1É629 0É322 0É892

0É334 0É336 0É987

shown in Figs 11A and 11B for lead and nickel ions, respectively. In summary, the equilibrium removal of lead ions by the investigated zeolite can be represented by the following empirical formulae : qe \ 34É187Ce/(1 ] 25É233Ce)

(9)

qe \ 1É629Ce0Õ322

(10)

For nickel ions, the empirical formulae obtained are : qe \ 0É401Ce/(1 ] 0É460Ce)

(11)

qe \ 0É334Ce0Õ336

(12)

Exchange capacity of zeolite

Fig. 11A. Adsorption isotherms for lead ion removal.

The theoretical exchange capacity (TEC, meq g~1) is considered as the sum of the amounts of Na, K and Ca ions present in 1 g of zeolite. TEC values for the 90È 180 km and the 355È710 km fractions are calculated from the chemical composition of zeolite samples obtained from the XRF analysis (Table 1). These values are found to be 3É1 and 4É5 meq g~1, respectively. The actual exchange capacity (AEC, meq g~1) is taken as the number of milliequivalents of metal ion (Pb or Ni) removed by 1 g of zeolite at equilibrium conditions. Values of the actual exchange capacity obtained from typical equilibrium experiments giving the highest equilibrium metal ion removal (see Figs 3A and 3B) are compared with the TCE values in Table 4. It is obvious that the percentage ratios of (AEC/TEC) are quite low for both particle size fractions for both metal ions. However, the ratio for lead ions is approximately 2È3 times that for nickel ions.

CONCLUSIONS

Fig. 11B. Adsorption isotherms for nickel ion removal.

Jordanian zeolite tu† (shown to be phillipsite) was found to be an efficient ion exchanger for removing lead

TABLE 4 Actual (AEC) and Theoretical (TEC) Exchange Capacities of Treated Zeolitea Size (km)

Ion

T EC (meq g~1)

AEC (meq g~1)

(AEC/T EC) ] 100

90È180

Pb Ni Pb Ni

3É1 3É1 4É5 4É5

1É31 0É76 1É09 0É40

42É3 24É5 24É2 8É9

355È710

a Initial pH \ 4É0, S \ 2É0 g dm~3, T \ 20¡C and Ci \ 400 mg dm~3 (3É861 meq Pb dm~3 or 13É629 meq Ni dm~3).


34 and nickel ions from aqueous solutions. The mineral has a higher removal efficiency for lead than for nickel ions regardless of the operating conditions. However, the actual exchange capacity is far below the theoretical value. The Ðner the zeolite particles used, the higher the metal ion removal. High metal uptake can be achieved using an initial solution pH of 4É0È4É5 with careful selection of other conditions, especially metal ion and slurry concentrations to avoid masking of sorption by chemical precipitation. Equilibrium removal by zeolite followed typical adsorption isotherms. Detailed studies are needed for further evaluation of this potentially cost-e†ective ion exchanger.

7.

ACKNOWLEDGEMENTS

8.

3. 4. 5. 6.

The authors are grateful to the Deanship of Academic Research at the University of Jordan for the Ðnancial support of this study. Thanks are due to the Directorate of Laboratories in the Natural Resources Authority of Jordan for supplying the zeolite samples and conducting the XRF analysis.

11.

REFERENCES

12.

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