Waste Management & Research (1997) 15, 383–394 PERFORMANCE OF NATURAL ZEOLITES FOR THE TREATMENT OF MIXED METAL-CONTAMINATED EFFLUENTS Sabeha Kesraoui Ouki 1 and Mark Kavannagh 2 1 Department of Civil Engineering, University of Surrey, Surrey GU2 5XH, U.K. 2 Centre for Environmental Control & Waste Management, Imperial College of Science, Technology & Medicine, London SW1 2BU, U.K. (Received 4 December 1995, accepted in revised form 5 September 1996) Two natural zeolites, clinoptilolite and chabazite, have been evaluated with respect to their selectivity and removal performance for the treatment of effluents con- taminated with mixed heavy metals, namely Pb, Cd, Cu, Zn, Cr, Ni and Co. Parameters such as metal concentration, pH and presence of competing ions were examined and removal performance was determined in terms of the zeolites, ion- exchange capacity measured at room temperature. The study showed that at metal concentrations ranging from 1 mg l -1 to 10 mg l -1 the zeolites exhibited an optimum removal efficiency at metals concentration of 10 mg l -1 . Clinoptilolite and chabazite exhibited different selectivity profiles for all metals studied except for Pb for which both zeolites performed exceptionally well. The selectivity sequences for clinoptilolite and chabazite are summarized as follows: chabazite (Pb>Cd>Zn>Co>Cu>Ni>Cr); and clinoptilolite (Pb>Cu>Cd>Zn>Cr>Co>Ni). The study also showed that the chabazite exchange capacity is superior to that of clinoptilolite which is mainly due to the higher AI substitution of Si which provides chabazite with a negative framework favourable to higher exchange capability. Solution pH was found to have an effect on metal removal as pH can influence both the character of the exchanging ions and the zeolite itself. The metal removal mechanism was demonstrated to be controlled by ion exchange and precipitation was negligible. The results also showed that Ca was a major competing cation for ion exchange for both clinoptilolite and chabazite when concentrations exceeded 1000 mg l -1 . Overall, chabazite and clinoptilolite removal efficiency was not affected by the presence of more than one heavy metal in solution which demonstrates their potential application in the treatment of effluents contaminated with mixed heavy metals. 1997 ISWA Key Words—Natural zeolites, heavy metals, ion exchange, removal efficiency, effluent treatment, competing cations. Introduction Zeolites have been intensively studied in the last half century although attention has concentrated mainly on synthetic zeolites and it is only in recent years that natural zeolites have started gaining interest (Ouki et al. 1994). Natural zeolites are capable of removing quantities of cations from aqueous solutions by utilizing the phenomenon of ion-exchange (Ouki et al. 1995). Clinoptilolite for example has received extensive attention due to its attractive selectivities for certain heavy metal ions such as Pb 2+ , Zn 2+ , Cd 2+ , Ni 2+ , Fe 2+ and Mn 2+ (Semmens & Seyfarth 1978; Aielo et al. 1980; Blanchard et al. 1984; Zamzow & Eichbaum 1990). However, application of natural zeolites on an industrial scale has been limited to the removal of ammonia from municipal wastewaters (Bray & Fullman 1971; Gaspard et al. 1983; Ciambeli et al. 1985), and to a smaller extent, for the removal of Cs and Sr from radioactive wastes 0734–242X/97/040383+12 $25.00/0 wm960094 1997 ISWA
Transcript
Waste Management & Research (1997) 15, 383–394
PERFORMANCE OF NATURAL ZEOLITES FOR THETREATMENT OF MIXED METAL-CONTAMINATED
EFFLUENTS
Sabeha Kesraoui Ouki1 and Mark Kavannagh2
1Department of Civil Engineering, University of Surrey, Surrey GU2 5XH, U.K. 2Centre forEnvironmental Control & Waste Management, Imperial College of Science, Technology &
Medicine, London SW1 2BU, U.K.
(Received 4 December 1995, accepted in revised form 5 September 1996)
Two natural zeolites, clinoptilolite and chabazite, have been evaluated with respectto their selectivity and removal performance for the treatment of effluents con-taminated with mixed heavy metals, namely Pb, Cd, Cu, Zn, Cr, Ni and Co.Parameters such as metal concentration, pH and presence of competing ions wereexamined and removal performance was determined in terms of the zeolites, ion-exchange capacity measured at room temperature. The study showed that at metalconcentrations ranging from 1 mg l−1 to 10 mg l−1 the zeolites exhibited an optimumremoval efficiency at metals concentration of 10 mg l−1. Clinoptilolite and chabaziteexhibited different selectivity profiles for all metals studied except for Pb for whichboth zeolites performed exceptionally well. The selectivity sequences for clinoptiloliteand chabazite are summarized as follows: chabazite (Pb>Cd>Zn>Co>Cu>Ni>Cr);and clinoptilolite (Pb>Cu>Cd>Zn>Cr>Co>Ni). The study also showed that thechabazite exchange capacity is superior to that of clinoptilolite which is mainly dueto the higher AI substitution of Si which provides chabazite with a negative frameworkfavourable to higher exchange capability. Solution pH was found to have an effecton metal removal as pH can influence both the character of the exchanging ionsand the zeolite itself. The metal removal mechanism was demonstrated to becontrolled by ion exchange and precipitation was negligible. The results also showedthat Ca was a major competing cation for ion exchange for both clinoptiloliteand chabazite when concentrations exceeded 1000 mg l−1. Overall, chabazite andclinoptilolite removal efficiency was not affected by the presence of more than oneheavy metal in solution which demonstrates their potential application in thetreatment of effluents contaminated with mixed heavy metals. 1997 ISWA
Key Words—Natural zeolites, heavy metals, ion exchange, removal efficiency, effluenttreatment, competing cations.
Introduction
Zeolites have been intensively studied in the last half century although attention hasconcentrated mainly on synthetic zeolites and it is only in recent years that naturalzeolites have started gaining interest (Ouki et al. 1994). Natural zeolites are capable ofremoving quantities of cations from aqueous solutions by utilizing the phenomenon ofion-exchange (Ouki et al. 1995). Clinoptilolite for example has received extensiveattention due to its attractive selectivities for certain heavy metal ions such as Pb2+,Zn2+, Cd2+, Ni2+, Fe2+ and Mn2+ (Semmens & Seyfarth 1978; Aielo et al. 1980;Blanchard et al. 1984; Zamzow & Eichbaum 1990). However, application of naturalzeolites on an industrial scale has been limited to the removal of ammonia frommunicipal wastewaters (Bray & Fullman 1971; Gaspard et al. 1983; Ciambeli et al.1985), and to a smaller extent, for the removal of Cs and Sr from radioactive wastes
(Koon & Kaufman 1974; Grant & Skriba 1987). Many industries such as metal finishing,mining and mineral processing, coal mining and oil refining, have problems associatedwith heavy metals contamination of process and runoff waters. New approaches andtechnologies must therefore be developed to assist in both the removals and recoveryof valuable metals from process and wastewaters.
Ion exchange is feasible when an exchanger has a high selectivity for the metal to beremoved and the concentration of competing ions is low. In certain cases special resinsmay be manufactured which have a very high selectivity for a particular heavy metal.The metal may then be recovered by incinerating the metal-saturated resin and naturallythe cost of such a process limits application to only the more expensive metals. Inmany cases however, the heavy metals are not valuable enough to warrant the use ofspecial selective exchangers and they are not easily separated from other metal ions bytheir chemical character. In such cases a selective cation exchange such as naturalzeolites may provide an economical means of removing mixed heavy metals fromeffluents.
Consequently, the main objective of this study is to investigate the natural zeolites’potential for the treatment of effluent contaminated with mixed heavy metals, namelyPb, Cd, Zn, Ni, Co, Cr and Cu. Important parameters such as metals concentration,pH and the presence of competing alkali cations will be examined in order to understandthe removal mechanisms involved, and to optimize the overall removal efficiency of thesystem for best performance.
2. Experimental investigation
2.1 Materials, analytical and experimental methods
All chemicals, salts, acids and pH buffers used in the study were “AnalaR” grade andwere supplied by BDH Laboratory Chemicals (U.K.). All solutions, standards anddilutions were made using distilled water. pH values were measured with a Jenway pHmeter, model 3010 digital. All metals were analysed using Inductively Coupled PlasmaAtomic Emission Spectrometry (ICP-AES).
2.2 Conditioning
Conditioning has been proved to improve both the exchange capacity and the removalefficiency when operating at metal concentrations exceeding 250 mg l−1 (Kesraoui-Oukiet al. 1993). The aim of conditioning is to produce a homo-ionic compound for eachnatural zeolite. The process of conditioning involved grinding and sieving the zeolitesamples to produce particles with diameter <500 lm followed by a thorough distilledwater wash to remove any fine particulate and addition of a 2 N NaCl solution to thezeolite samples. The suspension is rotated in an end-over-end rotary shaker equippedwith a variable speed for a period of 24 hours after which the zeolites are separatedfrom the supernatant by filtration using Whatman #1 paper. It is then followed by athorough distilled water wash to remove any excess NaCl present on the zeolites surface.Finally, the zeolites are oven dried at 105°C and stored until required for use.
2.3 Effect of metal concentration on zeolite exchange performance
The rate of uptake of metals by zeolites from aqueous solution was investigated todetermine the optimum time required for the metal–zeolite system to reach equilibrium.
Performance of natural zeolites 385
A 100-ml aliquot of a mixed solution of the seven metals originating from their nitratesalts, were equilibrated with 0.5 g chabazite and clinoptilolite. The concentration rangestudied for each metal in the solution was 1, 5, 10, 15 and 30 mg l−1. The equilibriumstudies were performed for the time intervals ranging from 1 to 240 min and 24 hourslater. At the designated time the two phases were separated by filtration using Whatman#1 paper and the initial and final metals concentration remaining in solution wereanalysed by ICP-AES (Kesraoui-Ouki et al. 1993).
2.4 Effect of pH on zeolite exchange performance
A 100-ml aliquot of a mixed solution of the above mentioned metals originating fromtheir nitrate salts, was prepared containing 10 mg l−1 of each metal (10 mg l−1 was foundto be the optimum concentration for metal removal). The solutions were adjusted withreagent grade nitric acid or sodium hydroxide to pH values ranging from 3 to 6 andwere equilibrated with 0.5 g of chabazite and clinoptilolite. The experiments wereperformed for the time intervals of 1 min to 240 min and 24 hours. At the designatedtime the two phases were separated by filtration using Whatman #1 paper and theinitial and final concentration of the metals were analysed for by ICP-AES.
2.5 Effect of Ca on metal removal
To study the effect of Ca2+ addition on the removal performance of the zeolites, thesame experimental procedure as previously described was followed using Ca2+ cationsoriginating from its hydroxide form, at concentrations ranging from 40 mg l−1 to12 000 mg l−1. For more details please refer to Kesraoui-Ouki et al. (1993).
3. Results and discussion
3.1 Effect of metal concentration on removal performance
The results of the analysis of the effects of metal concentration on the kinetics ofremoval are shown in Fig. 1[(a)–(e)] for chabazite and Fig. 2[(a)–(e)] for clinoptilolite.The data clearly show that chabazite is more efficient than clinoptilolite for metalsremoval as it is illustrated by the corresponding fast kinetics and low residual metalconcentrations achieved. This is mainly attributed to the window size that controlsaccess to the pore system which is larger in chabazite than in clinoptilolite. In addition,chabazite has a higher AI substitution of Si, which in turn will provide a negativeframework favourable to higher ion-exchange capability. The time required to reachequilibrium was assumed to be 24 hours; however, the results demonstrated that morethan 90% of the removal was achieved in the first 15 min when using clinoptilolite andin the first 5 min when using chabazite. This result again revealed the chabazite highpotential in achieving best removal performance in a very short period of time. Thedata also show that for both zeolites, 10 mg l−1 is the optimum concentration for bestmetal removal efficiency. Table 1 illustrates the maximum capacity achieved for eachmetal using clinoptilolite and chabazite.
Figure 3 exhibits clinoptilolite removal efficiency versus the initial metal concentration.The results clearly show that high selectivities were obtained for lead, copper, cadmium,zinc and chromium and slightly lower values for nickel and cobalt. The clinoptiloliteselectivity sequence is as follows: Pb>Cu>Cd>Zn>Cr>Co>Ni. Chabazite, on the other
hand, is much more efficient than clinoptilolite and can remove in excess of 95% of theinitial Pb, Cd, Zn, Co, Cu and Cr present in the mixed metals solution (Fig. 4).The corresponding selectivity sequence for chabazite was found to be as follows:Pb>Cd>Zn>Co>Ni>Cr. The selectivity series could be the result of various factorswhich influence ion-exchange behaviour in zeolites. One factor is the frameworkstructure of the zeolite itself. The dimensions of the channel formed by the tetrahedralunits which make up the zeolite must be large enough to allow passage of a hydratedmetal ion. There is some question as to the space available in the main channels ofclinoptilolite because the structure has not been determined but is thought to be relatedto heulandite (Breck 1974). If clinoptilolite has channels with dimensions similar toheulandite then few of the hydrated metals would be able to move within the structure.Some of the waters of hydration would have to be removed in order to accommodatethe metal ion (Semmens 1981).
Eisenman (1962) has shown that the specificity of a surface containing fixed charges
Performance of natural zeolites 387
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TABLE 1Maximum adsorption capacity of chabazite and clinoptilolite
for alkali metal ions can be accounted for in terms of ion hydration and electrostaticbond energies. Sherry (1969) extended this model for divalent ions. According to themodel, if a cation exchanger such as a zeolite is placed in a solution containing severaldifferent metal salts, the preference of the exchanger for one metal over another dependson whether the difference in their hydration free energy or coulombic free energy ofinteraction with fixed anionic exchange sites predominates.
Since the silica/aluminium ratio in clinoptilolite is relatively high (4.25–5.25) (Breck,1974), and the volumetric capacity is correspondingly low, the ionic field within thezeolite structure is relatively weak, and the electrostatic interactions are not expectedto be as important as hydration free energy. The metals with the highest free energy ofhydration should therefore prefer to remain in the solution phase where their hydrationrequirements may be better satisfied. According to hydration energies listed in Table 2the selectivity series for the metals considered should be: Pb>Cd>Co>Zn>Cu. Thehydration energy model does explain the much higher selectivity of clinoptilolite forlead. It also predicts that clinoptilolite will show similar selectivity for cadmium, copper,zinc and cobalt. However, the model does not explain the low selectivity the clinoptiloliteexhibited for nickel. Perhaps the electrostatic interaction of the cation with the ionicsite on the zeolite is of same significance. Also, the Eisenman model does not take intoaccount the changes in water content and entropy of ions in the zeolite phase whichcould also be factors in determining selectivity.
A common factor that prevents a material being adsorbed by a zeolite is the size ofthe ion. If the ion size is greater than that of the pore, the species will be excluded. Bycomparison of the hydrated ion sizes of the metals examined (Table 2) the followingselectivity sequence is generated: Pb>Ni>Cu>Co>Cd>Zn>Cr. This sequence is verysimilar to the one produced by the experimental tests using chabazite and the lowselectivity chabazite exhibited for nickel can be explained by the reasons mentionedabove.
The effect of concentration on metals removal is clearly seen in the case of clinoptilolite(Fig. 3). For example, only 15% of cobalt is removed when the initial concentration is30 ppm, while 59% of the metal is removed when the initial concentration is 10 ppm,and 88% of zinc is removed when the initial concentration is 10 ppm, while only 36%of the metal is removed from the solution when the initial concentration is 30 ppm.Clinoptilolite performed poorly in the case of Ni (minimum removal efficiency 6%,maximum 30%) and better in the case of chromium (minimum 29%, maximum 80%)and Cd (minimum 72%, maximum 92%). However Pb removal efficiency was unaffected
Performance of natural zeolites 389
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by the initial metals concentration and 100% removal was achieved in most cases. Inthe case of chabazite, concentration did not seem to have a significant effect since thiszeolite can achieve much higher removal efficiencies than clinoptilolite even at 1 or30 ppm (Figs 3 & 4). The percentage of removal of Zn, for example, ranges from 91%to 99%, as concentration changes from 1 to 30 ppm. Chabazite can achieve very highremoval efficiencies for Cu (minimum 93%, maximum 99%), Cd (minimum 96%,maximum 99%), Pb (minimum >99%, maximum 100%) and Co (minimum 91%,maximum 98%). On the other hand, its removal performance was slightly lower in thecase of Ni (minimum 6%, maximum 99%) and chromium (minimum 56%, maximum99%).
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3.2 Effect of pH on metal removal
Figures 5 and 6 show the change of concentration versus time at pH values 3, 4, 5 and6 for both clinoptilolite and chabazite, respectively. pH has a significant impact onmetals removal by zeolites since it can influence both the characters of the exchanging
Performance of natural zeolites 391
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Fig. 8. Effect of alkali Ca on metals removal efficiencyof clinoptilolite. See Fig. 1 for explanation of symbols.
ions and the character of the zeolite itself. Chemically, the solution pH influences metalspeciation. Heavy metals may form complexes with inorganic ligands such as OH− forexample. The extent of complex formation varies with pH, the ionic composition andthe particular metal for concern. The exact speciation of a metal has a significantimpact on the removal efficiency of the zeolites and the selectivity of the metal for thezeolite will also be influenced by the character of the metal complex that predominatesat a particular solution pH. Furthermore, the kinetics of removal may be adverselyaffected if a large complex is formed and this may in turn reduce the effective exchangecapacity of the zeolite for a given contact time (Semmens 1981).
The impact of the pH on the zeolite itself is poorly understood at the present time.The zeolite surface (approximately 15–20 m2 g−1 for clinoptilolite) will be influenced bythe ambient pH which is not equal to the external solution value. Surface functionalgroups may dissociate at higher pH values leaving anionic surface sites that may makea significant contribution to the metal removal. Many investigators (Schindler 1976)have observed a strong pH dependence on metal removal by inorganic adsorbents suchas silica. Metal removal typically increases with increasing pH and the steepest rise inmetal uptake is noted to coincide with the isoelectric point of the adsorbent (Huang &Ostovic 1978).
Zeolites such as clinoptilolite and chabazite are not only influenced by pH but inturn are capable of affecting solution pH especially in batch systems. Calcium andsodium are the major cations released in solution and bicarbonates/carbonates are themajor anions. This points to the major problem in researching natural zeolites; theyare not pure products but rather contain a variety of impurities such as calciumcarbonate, unaltered glass, clays, etc. which are occluded during the formation of thezeolite. Unlike synthetic ion exchange resins which tend to have an internal pH slightlylower than that measured in solution because of the influence of the Donnan potential,natural zeolites tend to have a higher internal pH. The influence of the zeolite on pHmust be recognized when metal removal behaviour is interpreted and in certain casesit makes data analysis difficult. Typically, heavy metal exchange isotherms are collectedat low pH in batch systems but because of the zeolites tendency to elevate the solutionpH, the solution must be manually restored to the original desired value many times.The system cannot be buffered at the desired pH since both the inorganic and organicbuffers may complex or precipitate the heavy metal of interest. In addition, the ionic
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strength of the solution will vary with the amount of zeolite added to the solution.Another concern is the possibility of internal precipitation. The higher internal pH ofthe zeolite combined with high internal metal concentrations may cause hydroxide orcarbonate precipitation within the channels of the zeolite and at the zeolite surface.
The experimental results indicated that the actual removal was adversely affected bydecreasing the solution pH. Metal removal efficiency using clinoptilolite were as follows:Cr (minimum 61%, maximum 79%), Co (minimum 45%, maximum 55%) and Ni(minimum 6%, maximum 36%). However, the pH affected to a lesser extent the removalof Cu (minimum 81%, maximum 95%), Zn (minimum 65%, maximum 85%), Cd(minimum 75%, maximum 89%) and Pb (100% in most cases). Chabazite could achievehigh removal efficiency for most metals even at low pH values (Figure 6). A slighteffect was noticed with Cu (minimum 90.5%, maximum 99%) and to a lesser extentwith Cr (minimum 95%, maximum 99%), Zn (minimum 94.5%, maximum >99%), Cd(minimum 99%, maximum 100%), Co (minimum 99%, maximum 100%), Ni (minimum97%, maximum 99%) and Pb (100% in all cases).
It should be noted that precipitation occurred at pH 11.7 according to a titrationtest conducted for a solution containing 10 ppm of each metal. Therefore, one canconclude that the predominant mechanism for metal removal for both zeolites is ionexchange with precipitation being negligible. These results could be explained by twotheories: (i) the zeolites were highly selective for H3O+ ions and when their concentrationwas increased, the metal exchange was reduced accordingly; and (ii) the zeolites wereweakly acidic in character and at low pH values the capacity of the zeolites for exchangewas consequently reduced.
3.3 Effect of Ca cations on metal removal
The effect of Ca on metals removal by chabazite and clinoptilolite is shown in Figs 7and 8. The initial concentration of each metal in the mixed metals solution was 10 mg l−1.The results showed that at calcium concentrations less than 400 mg l−1, the effect onthe removal efficiency was negligible for most metals of concern for both zeolitesstudied. However, a drastic decrease in the removal efficiency, from in excess of 98%to almost nil, was observed when calcium concentrations exceeded 1000 mg l−1. Thesefindings suggest that at high concentrations (>1000 mg l−1, calcium can be a majorcompeting cation for ion exchange and consequently could significantly affect thezeolites removal performance for heavy metals.
4. Conclusions
The study revealed that metals removal efficiencies exceeding 99% could be achievedby both clinoptilolite and chabazite and the selectivity series for the various metalcations was found to be independent of their initial concentration. The results alsoindicated that chabazite has higher exchange capacity than clinoptilolite which is mainlydue to its higher Al substitution of Si which provides a negative framework favourableto superior ion exchange capability. Metals removal efficiencies were examined forseveral concentrations ranging from 1 ppm to 30 ppm for each metal for which bothzeolites showed that optimum removal was achieved at an initial metals concentrationof 10 ppm.
Clinoptilolite is highly selective for lead, copper and cadmium and its selectivitysequence is as follows: Pb>Cu>Cd>Zn>Cr>Co>Ni. Chabazite is highly selective for
Performance of natural zeolites 393
lead, cadmium, zinc, cobalt, copper and less selective for nickel and chromium. Theselectivity sequence for chabazite is a follows: Pb>Cd>Zn>Co>Ni>Cr. pH was foundto have a significant impact on metals removal by zeolite since it can influence boththe character of the exchanging ions and the character of the zeolite itself. Equilibriumstudies conducted at various pH values showed that optimum removal is achieved whenoperating at pH value between 4 and 5. Titration tests showed that precipitation occurredat pH 11.7 and therefore metal precipitation during the ion-exchange mechanism isvirtually negligible as the experimental pH increased from an initial value of 3 to anequilibrium value less than 8. Consequently it was concluded that metal removalmechanism in this case is predominantly controlled by ion exchange. Calcium wasfound to be a major competing cation for ion exchange at concentrations exceeding1000 mg l−1 with both clinoptilolite and chabazite.
5. Recommendations
For a natural zeolite to be useful for the removal of metals from an effluent, thefollowing properties are all desirable: (i) it should have a substantial exchange capacityfor these ions; (ii) it is desirable for the exchanger to be readily capable of regeneration,as not only can the metals be recovered in concentrated form but the exchanger mayalso be re-used. Thus a zeolite having a very high selectivity for a metal is not necessarilya good choice because of regeneration difficulties; and (iii) it is also essential that theexchange reaction be reversible. Therefore for any successful applications to be de-veloped, further research on regeneration of the zeolites is imperative and required.
Acknowledgement
This project was undertaken at the Centre for Environmental Control and WasteManagement, Imperial College, London. Acknowledgements are also extended to MrCostas Papakonstantinou for his help with the laboratory experiments and the metalanalysis by ICP-AES.
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