REGENERATION OF ZEOLITE LOADED WITH LEAD AND ZINC AFTER WASTEWATER TREATMENT E. KATSOU, S. MALAMIS, M. TZANOUDAKI, K.J. HARALAMBOUS, M. LOIZIDOU School of Chemical Engineering, National Technical University of Athens, 9 Iroon Polytechniou St., Zografou Campus, PC 157 73, Athens, Greece SUMMARY: The aim of this work is to examine the potential regeneration of natural zeolite which has been loaded with lead and zinc contained in primary wastewater, effluent and aqueous solutions. Several desorbents were examined with respect to their efficiency for the removal of Pb(II) and Zn(II) from zeolite and the highest desorption was obtained for 3M and 1M KCl respectively. The desorption process depended on the type and concentration of desorbent, on the metal desorbed and also on the liquid medium where the adsorption process took place. Alternative regeneration cycles resulted in the significant reduction of desorption efficiency by 5% after 8 cycles for lead and 4 cycles for zinc. 1. INTRODUCTION Lead and zinc are among the most common pollutants met in industrial effluents that are associated with pollution and toxicity problems. Therefore, these metals must be efficiently removed from wastewater. Various techniques have been employed for heavy metal removal, including precipitation, coagulation flocculation, electrochemical treatment, chelation, biosorption, adsorption, ion exchange, membrane filtration, solvent extraction and reverse osmosis (Lesmana et al., 29; Kurniawan et al., 26; Rubio et al., 22). These techniques have their inherent advantages and limitations in application. Adsorption is an effective process for heavy metal removal particularly in the cases where low-cost adsorbents are employed. Natural zeolites are effectively used in ion exchange processes as sorbents for Pb(II) and Zn(II) removal, owing to their high reserves, advantageous ion exchange capacities and low-cost. Blöcher et al. (23) combined hybrid flotation, membrane separation and adsorption, using zeolite as a bonding agent for the removal of Cu, Ni and Zn from wastewater. The sorption capacity of synthetic zeolite for Zn(II) uptake was 25 mg/g. The adsorption capacities of natural clinoptilolite and Fe-over-exchanged clinoptilolite for Zn(II) uptake from drinking water were 71.3 mg/g and 94.8 mg/g respectively (Dimirkou & Doula, 28). Castaldi et al. (28) deduced that the maximum adsorption capacity of natural zeolite for Pb(II), Zn(II) and Cd(II) in batch tests was 247 mg/g, 96 mg/g and 13 mg/g respectively.
Günay et al. (27) found that the adsorption capacity of raw and pre-treated clinoptilolite for Pb(II) removal from aqueous solutions was 8.9 mg/g and 122.4 mg/g, respectively. Sprynskyy et al. (26) determined the sorption capacity of clinoptilolite for Pb removal to be 27.7 mg/g in multi-component aqueous solutions. The findings of the above mentioned studies revealed that clinoptilolite is an effective adsorbent for Zn(II) and Pb(II) uptake. A drawback of adsorption is that the mineral is not recycled. Clinoptilolite regeneration would enable metal and sorbent recovery from the resulting concentrated regenerant. Such studies are therefore important for future practical use of the mineral (Gedik & Imamoglu, 28). A small number of studies have been performed on the regeneration of clinoptilolite after Zn(II) and Pb(II) removal. Gedik & Imamoglu (28) tested four different chemicals (1M of NaCl, KCl, CaCl 2 of HCl) for clinoptilolilite regeneration and cadmium recovery with NaCl exhibiting the highest adsorbent regeneration efficiency (72 97%). Regeneration of clinoptilolite and Pb(II) recovery was obtained using 3 g/l NaCl solution at ph 11.5 in fixed bed column, achieving 95% regeneration efficiency (Turan et al., 25). Eight regeneration cycles of natural zeolite from Pb(II) were performed in fixed bed column employing 15 g/l NaNO 3 as desorbing solution, achieving 3-4 times smaller volume of regenerant used than the metal solution volume per cycle (Medvidović et al., 26). Cui et al. (26) studied the adsorption/desorption of zinc for clinoptilolite in a laboratory slurry bubble column using NaCl as desorbent accomplishing a desorption efficiency of 56%. Li et al. (27) investigated the adsorption/desorption of Zn(II) for clinoptilolite and found that EDTA was the most effective desorbing medium followed by NaCl. The aim of this work is to investigate the regeneration of zinc and lead contaminated clinoptilolite with the use of suitable desorbing agents. This way the mineral can be employed several times for the removal of heavy metals from wastewater. Also heavy metals can be diverted from wastewater to a low-volume desorbing solution that can be easily handled. 2. MATERIALS AND METHODS 2.1 Adsorbent and chemicals employed Natural (clinoptilolite) zeolite was the adsorbent employed in this work. The mineral was obtained by S&B Industrial Minerals S.A. Zeolite was sieved to the desired size (1. mm - 1.4 mm), rinsed with distilled water, dried at 8 C for at least 24 h and stored in a desiccator until its use. Zeolite was used in its natural form without any chemical modification. The chemicals and reagents employed for the experiments were of analytical grade and were supplied by Merck. Metal solutions were prepared by dissolving the appropriate amount of salts {Pb(NO 3 ) 2 and Ν 2 Ο 6 Ζn6Η 2 Ο} in deionized water, while liquid media under examination were enriched with 32 mg/l of each of the two metals. The chosen concentration is a typical one that is met in industrial wastewater. Desorption experiments were performed using HNO 3 at concentrations 5% and 1% and NaCl, KCl and NH 4 Cl at concentrations.1m,.5m, 1M, 3M and 5M. In order to adjust the ph of Pb(II) and Zn(II) solutions HNO 3 was used. 2.2 Batch adsorption and desorption studies Adsorption experiments were performed in aqueous solutions, primary wastewater and secondary effluent. Primary wastewater was collected from the overflow of a primary sedimentation tank of a municipal wastewater treatment plant. The secondary effluent was collected from permeate of a Membrane Bioreactor (MBR) treating municipal wastewater. The adsorption experiments were conducted in batch reactors where the liquid media were enriched with 32 mg/l Pb(II) or Zn(II) and 1 g/l zeolite was added. The solution ph was adjusted at 3.5 to avoid chemical precipitation so that adsorption can be studied. The batch systems remained at
room temperature (25 C) without agitation until equilibrium was reached. Then samples were filtered through Whatman membranes with pore size.45 μm. The filtrate was measured in AAS of VARIAN AA24OFS for its Pb(II) and Zn(II) content. The metal-loaded zeolite was collected from the membrane, dried at 8 C to remove moisture. The removal rate of the divalent cation was calculated as the percentage difference between the concentration of the corresponding cation initially (C initial = 32 mg/l) and after (C final ) the adsorption process. The amount of metal adsorbed onto the clinoptilolite, q a (mg/g) is given by: ( Cinitial C final ) qa (1) m where m is the amount of adsorbent employed (g/l). Desorption experiments were conducted by placing zeolite polluted with Zn(II) or Pb(II) ions into the desorbing solution in batch reactors. The solution ph was adjusted at 3.5 and the solution remained without agitation at 25 o C until equilibrium was reached. Samples were then filtered through Whatman membranes and the filtrate was analyzed for its Zn(II) or Pb(II) content. The metal content in solution represents the amount of metal desorbed. Desorption is given as a percentage of the amount of metal desorbed from the amount of metal initially adsorbed: Cadsorbed Cdesorbed (%) Desorption = 1 (2) Cadsorbed where C adsorbed (mg/l) is the metal adsorbed by clinoptilolite and C desorbed (mg/l) is the metal detected in the desorbing agent. The amount of each metal desorbed from clinoptilolite q d (mg/g) is calculated by: C q desorbed d m (3) 2.3 Zeolite regeneration The effect of regeneration was tested via operation of successive Pb(II) and Zn(II) adsorption/desorption cycles. The solutions exhibiting the highest recovery in each metal (i.e. 1M KCl for Zn & 3M KCl for Pb) were used to examine the mineral regeneration potential and metal recovery. In this process, repeated cycles of loading mineral with metal (32 mg/l Pb and Zn) and desorption experiments using the best desorption solution were conducted. The repeated cycles were conducted for all three liquids under examination (i.e. aqueous solution, effluent and primary wastewater). At the completion of each cycle the regenerated clinoptilolite samples were used for Zn(II) and Pb(II) removal following the procedure of Section 2.2. 3. RESULTS AND DISCUSSION In this Section the experimental results obtained are discussed and assessed. This includes i) characterization of zeolite used as adsorbent, ii) determination of the adsorption capacity of clinoptilolite for Zn(II) and Pb(II) uptake, iii) testing the performance of desorbing agents used for mineral regeneration and metal recovery, iv) assessment of the regeneration of metal-loaded clinoptilolite to sustain metal removal capacity in successive adsorption/desorption cycles. The results of this study can be used for the design of a sustainable, cost-effective wastewater treatment system, enabling the recovery of both the sorbent and the metal.
Table 1a - Chemical composition of natural zeolite obtained by XRF Table 1b - Structural composition of natural zeolite obtained by XRD and used JCPDS data file Substances (%) Mineral name Structural formula JCPDS file number CEC [meq/g] SiO 2 71.3 Clinoptilolite, Ca-type Ca 3.16 Si 36 O 72 21.8H 2 O 7-1859 2.36 Al 2 O 3 12.1 KNa 2 Ca 2 (Si 29 Al 7 )O 72 24H 2 O 39-1383 3.5 Fe 2 O 3.9 Clinoptilolite, Na-type (Na,K,Ca) 5 Al 6 Si 3 O 72 18H 2 O 47-187 6.61 CaO 3.4 Feldspar (Rb.811 Al.62 )(Al.997 Si 3.3 O 8 ) 7-856 Na 2 O.3 Quartz SiO 2 82-1234 MgO.7 Illite K(AlFe) 2 AlSi 3 O 1 (OH) 2 H 2 O 15-63 K 2 O 3.7 Chabazite Ca 2 Al 4 Si 8 O 24 12H 2 O 34-137 TiO 2.1 2.4 Mineral characterization The major constituents of zeolite were silica (SiO 2 ) and alumina (Al 2 O 3 ) (Table 1a). The amounts of other minor oxides and elements constitute the remaining composition. Zeolite was characterized by poor sodium and magnesium content and higher calcium and potassium. Intensities and positions of Bragg peaks of crystalline phases were identified by comparing with those listed in the Joint Committee on Powder Diffraction Standards (JCPDS) data files. Natural zeolite mainly consisted of clinoptilolite with minor quantities of feldspar, quartz, illite and chabazite. Due to overlapping of diffraction maxima, the presence of some minor contents could not be included. On the basis of structural formulas presented in minerals, the theoretical cationexchange capacities are given in Table 1b. The Si/Al ratio of zeolite was 6.67. 2.5 Desorption of lead and zinc ions Desorption studies were performed to select the optimum chemical compound to be used in successive regeneration cycles. Sorption was conducted in three liquid media: (a) deionized water (b) primary wastewater and (c) MBR effluent. The characteristics of primary wastewater and MBR effluent are summarized in Table 2. The average amounts of zinc and lead ions adsorbed onto zeolite are given in Figure 1. It is observed that mineral adsorption capacity was higher for Pb(II) compared to Zn(II) in all liquid media, which also agrees with previous findings (Cincotti et al., 26). The preference of clinoptololite for Pb(II) is due to its lower hydration energy (Semmens & Seyfarth, 1978). Adsorption of Pb(II) and Zn(II) onto clinoptilolite occurred by ion-exchange and it was found to be reversible. Mineral adsorption efficiency followed the order: aqueous solution > effluent > primary wastewater. This is reasonable since wastewater contained NH 4 + ions that were easily removed by zeolite, thus competing with lead or zinc ions for the available adsorption sites. Thus, adsorption of heavy metals onto zeolite was lower in primary wastewater. The final effluent contained Ca ++, Mg ++ and Na + cations that competed to some extent with lead and zinc during the adsorption process, compared to aqueous solutions where there were no cations. Furthermore, the solution ph was low (3.5) and did not favour the formation of insoluble complexes between metal ions and wastewater compounds. Desorption of lead and zinc from natural zeolite was examined in batch reactors for five (5) different desorbing agents. In Figure 2 the desorption percentage of lead is shown when the solutions of HNO 3, KCl, NaCl and NH 4 Cl were employed. The deviation of mineral adsorption capacity for the same liquid media was low and thus the percentage desorption achieved for each desorbing agent could be used as a measure of the performance of each desorbing solution. The use of HNO 3 could achieve effective desorption of lead adsorbed onto zeolite in aqueous
solutions (>99%) showing that H + are very competitive and can replace the metal from zeolite. Nevertheless, HNO 3 was not effective for desorption of lead from zeolite eluted from primary wastewater. KCl was very effective for lead desorption, irrespective of the liquid medium employed. In particular, the desorption efficiency was 6-8% for the concentrations of.1m and 1M, while it reached 1% for 3M for all three liquid media. The desorption efficiency was high (>8%) when 3M & 5M NaCl and NH 4 Cl were employed only in aqueous solutions, while it was limited for the other two liquid media. In the case of NaCl the increase of concentration from 3M to 5M resulted in a deterioration of lead desorption efficiency. The most effective desorbing agent was 3M KCl resulting in complete desorption of Pb(II) for all three liquid media. Significant chemical concentrations are required to achieve effective lead desorption. In most cases the increase in desorbent concentration resulted in an increase in metal desorption efficiency. At higher NaCl, NH 4 Cl and KCl concentrations there were more Na +, NH 4 + and K + ions in the solution, favouring ion exchange for Pb(II) and Zn(II) ions from clinoptilolite. In Figure 3 the desorption percentage of zinc achieved by employing the solutions of HNO 3, KCl, NaCl and NH 4 Cl is given. KCl 1M was very effective (>98.5%) in zinc desorption from zeolite for all three liquids, while NH 4 Cl performed well (>8%) for all the examined concentrations and liquid media. The use of 1M and 3M NaCl was effective (>8%) for zinc desorption from clinoptilolite for all three media. Comparing the desorption efficiency between zinc and lead for the same desorbing agent, it is deduced that lower concentrations of desorbing agent were required for the effective removal of Zn(II). Mineral adsorption capacity was lower for zinc uptake, but its release from the solid to the liquid phase was easier showing the preference of zinc for the solution phase. Zinc ions are characterized by smaller ionic radius than lead ions, but similar hydrated ionic radius (Kiellend, 1937). Water molecules are more easily detached from zinc, favouring the desorption process of zinc ions. Potassium chloride was found to be the best desorbent, indicating that zeolite is very selective for K +. Table 2 - Composition of primary wastewater and MBR effluent Parameter Primary wastewater MBR Effluent ph 7.2 (7. 7.6) 7.1 (6.9 7.4) TSS (mg/l) 252 (15 325) n.d. VSS (mg/l) 195 (15 35) - COD (mg/l) 549 (465 619) 21 (16 27) NH 4 -N (mg/l) 53 (48 59) <.5 NO 3 -N (mg/l) <.5 45 (39 48) TN (mg/l) 69 (64-75) 52 (48 57) Na + (mg/l) 196 (11 31) 135 (93 25) Ca ++ (mg/l) 213 (145 295) 147 (85 251) Mg ++ (mg/l) 37 (17 56) 26 (11 46) K + (mg/l) 5 (2 16) 2 ( 8) q a (mg/g) 3 2 1 Pb(II) Zn(II) Water Effluent Wastewater Figure 1. Adsorption of Pb(II) and Zn(II) onto 1 g/l zeolite for initial metal concentration of 32 mg/l, ph = 3.5 / T=25 o C in aqueous solutions, MBR effluent and primary wastewater
The results showed that even when the same desorbing agent was employed at the same concentration, zeolite eluted from wastewater in some cases performed worst than that from aqueous solution in terms of its desorption efficiency. This shows that the adsorption process modified the active sites on mineral surface with respect to metal removal. In particular, the presence of significant NH 4 -N concentration (5 6 mg/l) in primary wastewater resulted in the uptake of ammonia ions by zeolite that were subsequently released in the desorbing agent, thus limiting the desorption efficiency. Occasional measurements of NH 4 + in the desorbing agent confirmed this. Ammonia and Pb(II) are more selective than zinc for removal with clinoptilolite as adsorbent (Langella et al., 2). Also, during the sorption process zeolite may adsorb apart from lead or zinc ions other cations and organic substances present in primary wastewater and effluent (e.g. Ca 2+, Mg 2+ ), which are then released in the desorbing agent. This can explain the lower performance of effluent compared to aqueous solutions that was observed in certain cases. Pb(II) desorption (%) 1 8 6 4 2 (a) HNO3 5% 1% Pb(II) desorption (%) 1 8 6 4 2 (b) KCl.1M.5Μ 1M 3M Pb(II) desorption (%).1M 1M 3M 5M 1 (c) NaCl 8 6 4 2.1 M 1M 3M 5M 1 (d) NH 4Cl 8 6 4 2 Figure 2. Pb(II) desorption achieved by (a) HNO 3, (b) KCl, (c) NaCl and (d) NH 4 Cl Zn(II) desorption (%) Zn(II) desorption (%) 5% 1% 1 (a) HNO3 8 6 4 2.1M.5M 1M 3M 1 (c) NaCl 8 6 4 2 Figure 3. Zn(II) desorption (%) achieved by (a) HNO 3, (b) KCl, (c) NaCl & (d) NH 4 Cl Pb(II) desorption (%) Zn(II) desorption (%) Zn(II) desorption (%) 1 8 6 4 2 1 8 6 4 2.1M.5M 1M (b) KCl.1M.5M 1M 3M (d) NH 4Cl
The results showed that desorption depended on the type and concentration of the desorbing agent, on the metal and also on the liquid medium where the adsorption process took place. In most cases the desorbing solutions more effectively desorbed zinc than lead, although the mineral adsorption capacity of zeolite was higher for Pb(II). The most effective chemical compound for zinc desorption was 1M KCl, while for lead it was 3M KCl. For these solutions, several regeneration cycles were performed in order to determine the number of sorption/desorption cycles that can be effectively conducted before disposing the mineral. 3.1 Regeneration cycles Several (>1) adsorption/desorption cycles were conducted for lead and zinc to determine the adsorption/desorption capacity of the mineral, once it has been regenerated several times. The desorbent employed was 1M KCl for zinc and 3M KCl for lead, since these solutions exhibited the highest desorption performance. In Figures 4a and 4b lead adsorption and desorption is given respectively for twelve regeneration cycles, while the respective amounts of zinc adsorbed/ desorbed are given in Figures 5a and 5b. During the first two adsorption/desorption cycles the adsorption capacity of mineral increased since the use of KCl as desorbing agent enhanced the metal uptake capability of zeolite as the solution modified the mineral structure by replacing mobile ions originally present in the mineral. This finding agrees with previous research work (Li et al., 27). The regeneration efficiency reduced as the number of regeneration cycles increased. This behaviour was observed for both zinc and lead. However, in the case of zinc the decrease in adsorption/desorption with increasing cycles was steeper. Regeneration efficiency for lead reduced within the first three cycles to approximately 8-85%, while it required 8 regeneration cycles to reduce to 5%. In the case of zinc the abrupt reduction of regeneration efficiency resulted in recovery rates lower than 5% in 4 adsorption/desorption cycles. Zeolite eluted from aqueous solution performed better compared to zeolite eluted from wastewater and effluent, indicating that the liquid media in which adsorption took place influenced not only the adsorption processes but also the desorption process. The worse performance of zinc compared to lead can be attributed to the lower amount of zinc that was adsorbed by zeolite and was thus available for the desorption process (Figure 1). It seems that it was more difficult to achieve higher regeneration efficiencies for lower metal concentrations. It is important to notice that although the regeneration efficiency decreased significantly with ascending cycles, zeolite still had significant adsorption capacity particularly in the case of lead. The adsorption capacity of zeolite at the 9 th cycle was greater than 1 mg/g for all three liquid media, since more than 1 mg/l of lead ions could be removed from the solution. In some cases (e.g. 6 th cycle for lead, 5 th cycle for zinc) the desorption of Pb(II) and Zn(II) exceeded the adsorption for a given cycle. This was probably attributed to changes in mineral surface or mineral structure, caused by successive adsorption/desorption cycles, which resulted in enhanced desorption that released metal ions adsorbed in previous cycles. 4. CONCLUSION The desorption process depends on the type and concentration of the desorbing agent, on the metal that must be desorbed, and also on the liquid medium where the adsorption process takes place. The results showed that for most desorbents zinc was more effectively desorbed than lead, while the mineral adsorption capacity of zeolite was higher for Pb(II) than for Zn(II), since lead is more selective than zinc. KCl was the most effective solution for the desorption process. Alternative regeneration cycles resulted in significant reduction (>5%) of desorption efficiency after 9 cycles for lead and 4 cycles for zinc.
rption (%) H2SO4 5% 1 1 (b) K 8 8 6 6 qa (mg/g) 3 2 1 (a) Pb Wastewater Effluent Water q d (mg/g) 3 2 1 (b) Pb Wastewater Effluent Water 1 2 3 4 5 6 7 8 9 1 11 12 Regeneration cycles Figure 4. Pb(II) (a) adsorbed and (b) desorbed during successive regeneration cycles qa (mg/g) 16 12 8 4 (a) Zn Wastew ater Effluent Water 1 2 3 4 5 6 7 8 9 1 11 12 Regeneration cycles Figure 5. Zn(II) (a) adsorbed and (b) desorbed during successive regeneration cycles qd (mg/g) 16 12 8 4 1 2 3 4 5 6 7 8 9 1 11 12 Regeneration cycles (b) Zn Wastewater Effluent Water 1 2 3 4 5 6 7 8 9 1 11 12 Regeneration cycles REFERENCES Blöcher C., Dorda J., Mavrov V., Chmiel H., Lazaridis N.K. & Matis K.A. (23) Hybrid flotation membrane filtration process for the removal of heavy metal ions from wastewater. Wat. Res. Vol. 37, pp. 418 426. Castaldi P., Santona L., Enzo S. & Melis P. (28) Sorption processes and XRD analysis of a natural zeolite exchanged with Pb 2+, Cd 2+ and Zn 2+ cations. J. Hazard. Mater. Vol. 156, pp. 428-434. Cincotti A., Mameli A., Locci A.M., Orru R. & Cao G. (26) Heavy metals uptake by Sardinian natural zeolites: Experiment and modelling. Ind. Eng. Chem. Res. Vol. 45, pp. 174 184. Cui H., Li L.Y. & Grace J.R. (26) Exploration of remediation of acid rock drainage with clinoptilolite as sorbent in a slurry bubble column for both heavy metal capture and regeneration. Wat. Res. Vol. 4, pp. 3359-3366. Dimirkou A. & Doula M.K. (28) Use of clinoptilolite and an Fe-overexchanged clinoptilolite in Zn 2+ and Mn 2+ removal from drinking water. Desalination. Vol. 224, pp. 28-292. Gedik K. & Imamoglu I. (28) Removal of cadmium from aqueous solutions using clinoptilolite: Influence of pretreatment and regeneration. J. Hazard. Mater. Vol. 155, pp. 385-392. Günay A., Arslankaya E. & Tosun I. (27) Lead removal from aqueous solution by natural and pre-treated clinoptilolite: Adsorption equilibrium and kinetics J. of Hazard. Mater. Vol. 146, pp. 362-371. Kielland, J. (1937) Individual activity coefficients of ions in aqueous solutions, J. Am. Chem. Soc. Vol. 59, pp. 1675-1678 Kurniawan T.A., Chan G.Y.S., Lo W.-H. & Babel S. (26) Comparisons of low-cost adsorbents for treating wastewaters laden with heavy metals. Sci. Total Environ. Vol. 366, pp. 49 426. Langella A., Pansini M., Cappelletti P., De Gennaro B., De' Gennaro M. & Colella C. (2) NH 4 +, Cu 2+, Zn 2+, Cd 2+ and Pb 2+ exchange for Na + in a sedimentary clinoptilolite, North Sardinia, Italy. Microporous Mesoporous Mater. 37 (3), 337-343 Lesmana S.O., Febriana N., Soetaredjo F.E., Sunarso J. & Ismadji S. (29) Studies on potential applications of biomass for the separation of heavy metals from water and wastewater. Biochem. Eng. J. Vol. 44, pp. 19 41. Li L.Y., Chen M., Grace, J.R., Tazaki K., Shiraki K., Asada R. & Watanabe H. (27) Remediation of acid rock drainage by regenerable natural clinoptilolite. Water Air Soil Pollut. Vol. 18, pp. 11-27. Medvidović N.V., Peric J. & Trgo M. (26) Column performance in lead removal from aqueous solutions by fixed bed of natural zeolite clinoptilolite. Sep. Purif. Technol. Vol. 49, pp. 237 244. Rubio J., Souza M.L. & Smith R.W. (22) Overview of flotation as a wastewater treatment technique. Miner. Eng. Vol. 15, pp. 139 155. Semmens M.J. & Seyfarth M. (1978) The selectivity of clinoptilolite for certain heavy metals. In: Natural Zeolites: Occurrence, Properties, Use. Sand, Mumpton (Eds), Pergamon Press, Elmsford, N. York pp. 517-526. Sprynskyy M, Buszewski B., Terzyk A.P. & Namieśnik J. (26) Study of the selection mechanism of heavy metal (Pb 2+, Cu 2+, Ni 2+ and Cd 2+ ) adsorption on clinoptilolite. J. Colloid Interface Sci. Vol. 34, pp. 21-28. Turan M., Mart U., Yuksel B. & Celik M.S. (25) Lead removal in fixed-bed columns by zeolite and sepiolite. Chemosphere. Vol. 6, pp. 1487 1492.