Cd Sorption from Aqueous Solutions by Natural Clinoptilolite and Its Modified Forms Clinoptilolite-Na and Clinoptilolite-NH 4

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1 Journal of Mineral Metal and Material Engineering, 6,, -9 Cd Sorption from Aqueous Solutions by Natural Clinoptilolite and Its Modified Forms Clinoptilolite-Na and Clinoptilolite-NH 4 D. Ruiz-Serrano and R. Ramírez-Bon * Centro de Investigación y de Estudios Avanzados del IPN, Unidad Querétaro, Apdo. Postal -798, Querétaro, Qro., 76 México Abstract: A natural zeolite, clinoptilolite, from a deposit in Sonora, México was used for the removal of Cd ions from aqueous solution with a Cd concentration in the range -, ppm. To improve the sorption properties, the natural clinoptilolite was conditioned by means of cation exchange in Na and NH 4 solutions. Natural and the two treated forms of clinoptilolite were used in Cd sorption experiments, and the results were compared to assess the effects of the conditioning processes. The reduction of Cd in the aqueous solution, measured by atomic absorption spectroscopy, and the corresponding increase of Cd loaded in the clinoptilolite powders, as measured by EDS, as a function of cationexchange time, revealed the Cd sorption properties of the three types of clinoptilolite powders. The results show that natural clinoptilolite has the capacity to remove Cd from the aqueous solutions, and that such capacity improves when it is modified to the clinoptilolite-na and clinoptilolite-nh 4 forms. Both the percentage of Cd removed from the aqueous solution and the amount of Cd loaded in clinoptilolite are higher for the modified Na and NH 4 forms in the complete range of Cd aqueous solution concentrations analyzed. Among the modified clinoptilolite forms, the NH 4 showed the better Cd sorption properties with higher percentages of Cd removed from the water solutions and higher Cd loaded into the powders after sorption experiments. Based on this work, it is concluded that natural clinoptilolite is a promising mineral for the removal of heavy metals, such as Cd, from contaminated water. Keywords: Clinoptilolite, Heavy metals, Cation exchange, Sorption, Cadmium.. INTRODUCTION Natural zeolites are readily available and low-cost minerals, the properties of which can be exploited for a wide range of applications [-8]. They are composed of three of the most abundant elements on earth, Si, Al and O, arranged in SiO 4 and AlO 4 basic tetrahedra, forming a porous, crystalline structure of cages and channels with molecular dimensions. The unbalanced charge produced by the AlO 4 tetrahedral molecules is stabilized by monovalent and divalent extra-frame cations, the amount of which is directly related to the silicon-aluminum (Si/Al) ratio of the natural zeolite. Because of their natural origin, the types of cations in natural zeolites varies, Na, K, Mg and Ca being the most common. These extra-frame cations are weakly linked to the framework structure and therefore can be easily exchanged for other types of cations by means of simple processes. Their porous structure, along with the cation-exchange properties, give natural zeolites the ability to be used for inclusion chemistry, allowing their easy modification for a wide diversity of important applications. Natural zeolites have volcanic origin; they are formed by the interaction of volcanic rocks and ash with alkaline groundwater. There are around 4 types of * Address correspondence to this autor at the Centro de Investigación y de Estudios Avanzados del IPN. Unidad Querétaro, Apdo. Postal - 798, Querétaro, Qro., 76 México; Tel: (5) ; rrbon@cinvestav.mx E-ISSN: 44-5/6 natural zeolite frameworks, the most common being clinoptilolite, heulandite, chabazite, and phillipsite. The chemical composition and purity of natural zeolite tuffs are subject to some degree of variation, depending on their origin region. Independent of origin, it is very common to find other minerals, such as quartz, or metals in the natural zeolite tuffs, as well as mixtures with other types of natural zeolite. This is one of the main disadvantages of natural zeolites, although it has not discouraged their use because of their great abundance and availability. Using proper pretreatments it is possible to improve the purity and cation-exchange properties of natural zeolites. Nowadays, natural zeolites are used as additives in cattle food, additives in concrete, sorbents in agriculture and mining, for removing heavy metals from water and soil, for removing of dyes, for removing of radioactive isotopes from radioactive wastes, as bactericide products, and more [-9]. Clinoptilolite has been one of the most studied natural zeolites because its abundancy in many regions of the world. It has a crystalline structure corresponding to the monoclinic crystalline system and a generic chemical formula (NaKMgCaFe) -6 [(SiO ) (AlO ) 6 ] 4H O. Three types of channels form the porous structure of clinoptilolite. One of the channels is formed by eight-membered adjacent rings with free access of.6 x.47 nm. Two other parallel channels are formed by eight members with window access of. x.46 nm and members with window access of. x.76 nm. There 6 Synchro Publisher

2 4 Journal of Mineral Metal and Material Engineering, 6, Vol. Ruiz-Serrano and Ramírez-Bon are three minerals associated with clinoptilolite, depending on the major cation in its chemical composition, namely: clinoptilolite-na, clinoptilolite-k and clinoptilolite-ca. Clinoptilolite has a high Si/Al ratio, with values larger than 4. The principal properties of clinoptilolite are high chemical and thermal stability, and resistance against ionizing radiation and corrosion. It also has great capability for heavy-metal-cation exchange, including radioactive isotopes. Based on these properties, clinoptilolite can be used in many applications in agriculture, medicine and industry. Among the most important ones are improvement and ph-regulation of soil, elaboration of fertilizers, elaboration of detergents and deodorants, molecular sieve, sorption of heavy metals and dyes, food additive, and sorption of toxins [-4,6-8]. Due to its large Si/Al ratio and to its polycationic characteristic, the cation-exchange properties of clinoptilolite are rather deficient. That is why, for most applications, clinoptilolite in its natural form has to be modified by means of appropriate pretreatments [- ]. The purpose of the treatments is basically to convert clinoptilolite from polycationic to monocationic by means of cation-exchange processes. Thus, it is possible to obtain the so-called clinoptilolite-na, clinoptilolite-ca, and clinoptilolite-h forms by suitable cationexchange processes. The effectiveness of these treatments depends on several parameters, such as temperature and time of the ion-exchange process, as well as the concentration of the cationic solution. However, because of the difficulty in exchanging some natural cations in clinoptilolite, the monocationic character is difficult to attain, and therefore these forms of clinoptilolite are referred to as Na, Ca or H-enriched clinoptilolite. In a previous paper [] we reported the structural, chemical composition, and chemical state modifications of clinoptilolite samples produced by two types of cation-exchange processes. The analyses were done on a natural clinoptilolite from Sonora, México, and the modifications were produced by cation exchange in Na + and NH 4 + solutions to obtain clinoptilolite-na and clinoptilolite-nh 4 forms, respectively. In this work, we have applied the natural clinoptilolite and the Na and NH 4 forms to the removal of heavy Cd ions from aqueous solutions. The efficiency of the removal of Cd by the three forms of clinoptilolite was determined from the amount of Cd measured in the Cd aqueous solution after exchange processes, as a function of time. Chemical composition measurements on the clinoptilolite powders after the exchange processes also provided information about the sorption of Cd ions by the clinoptilolite samples.. EXPERIMENTAL DETAILS The clinoptilolite powder used in this work was obtained from a tuff which origin was a mineral deposit located in Sonora, Mexico. To obtain clinoptilolite in powders form, the tuff material was ground and sieved to average particle diameters of.75 mm ( US- Mesh) and. mm (5 US-Mesh). The ground natural clinoptilolite powder was labeled CLI. The ground natural powder was modified by two different processes to obtain clinoptilolite-na and clinoptilolite-nh 4 forms, labeled CLI-Na and CLI-NH 4, respectively. To obtain CLI-Na, the CLI powder was subjected to a treatment of cation exchange in a.5 M NaCl solution and stirred at reflux temperature for min. The Na-exchanged clinoptilolite powder was separated by filtration and successively washed with deionized water. Finally, the obtained CLI-Na powder was dried at room temperature. To obtain CLI-NH 4, CLI powder was mixed in a.5 M NH 4 Cl solution and stirred at reflux temperature for min. After this process the CLI-NH 4 powder was filtered, washed and dried in the same way as for the Na-exchange process. For the Cd sorption experiments, CdCl aqueous solutions were prepared at.,. and. M concentrations, respectively. To ml of each of these solutions, g of the three clinoptilolite powder was added and magnetically stirred at room temperature. This was done for the CLI, CLI-Na and CLI-NH 4, producing nine solutions. The sorption experiments were performed for, 5, 5, and 6 min, after which both the aqueous solution and clinoptilolite powders were collected. The clinoptilolite powders were washed and dried in the same manner as used in the exchange processes. The residual atomic percentage of Cd in the aqueous solutions was measured by atomic absorption spectroscopy ( Perkin Elmer atomic absorption spectrometer). The amount of Cd removed by the clinoptilolite samples from the Cd aqueous solutions was measured by energy-dispersive X-ray spectroscopy (EDS) (Phillips ESEM XL microscope). The crystalline structure of the clinoptilolite powders was analyzed by X-ray diffraction (XRD) measurements (Rigaku DMAX diffractometer).. RESULTS AND DISCUSSION To confirm that the crystalline structure of clinoptilolite was not modified after the Na and NH 4 processes,

3 Cd Sorption from Aqueous Solutions Journal of Mineral Metal and Material Engineering, 6, Vol. 5 the X-ray diffraction patterns of the three samples were measured and are shown in Figure. As anticipated, the three patterns are similar, displaying the same diffraction peaks, confirming that the crystalline structure of CLI-Na and CLI-NH 4 is not affected by the cation-exchange processes. From these patterns, it can be concluded that the main crystalline phase in the natural zeolite is clinoptilolite-ca (JCPDS #9-8), mixed with some heulandite-ca (JCPDS #4-57), a mixture that is common in natural clinoptilolite tuffs [- 5]. Intensity (Arb. Units) Theta (Degrees) Clinoptilolite-Ca Clinoptilolite-Na Heulandite-Ca CLI CLI-Na CLI-NH 4 Figure : XRD patterns of the natural and modified clinoptilolite powders. three starting concentrations of Cd in the aqueous solutions. The Cd concentration decreases quickly for the first 5 min of the sorption experiments and stabilizes after this time. The decrease is around one order of magnitude in the Cd solutions with. and. M concentration. In the case of the solution with. M concentration, the decrease is less than one order of magnitude, however the absolute amount of Cd removed from the solution by clinoptilolite is much higher in this case. The percentage of Cd removed from the aqueous solutions by the CLI samples as a function of time was calculated as x (C -C)/C, where C is the initial concentration of Cd in the solution and C is the Cd concentration after time t. The results are shown in Figure, where it is observed that highest percentages of Cd removed from solution are 85.6 and 94.4%, attained at 5 and minutes for the solutions with. and. M concentrations, respectively. The highest percentage of Cd removed from the. M Cd solution is 44.7% and it is attained quickly, at min, after which the values vary between and 6%. It is likely that at this Cd concentration an equilibrium in the cation-exchange processes is reached, such that the number of Cd ions exchanged from the solution to the clinoptilolite pores is equal to the number of Cd ions in the opposite direction. For this, the natural cations of clinoptilolite undergo similar cation exchange equilibrium conditions. CLI. M Cd. M Cd. M Cd CLI Cd concentration (ppm). M Cd (995 ppm). M Cd (4 ppm). M Cd (.4 ppm) Cd Removal (%) Figure : Cd concentration (in ppm) of the aqueous solutions as a function of exchange time, for three starting Cd concentrations, using natural clinoptilolite (CLI). Figure shows the Cd concentration, measured in ppm, in the residual water as a function of time after the Cd sorption experiments with CLI powders, for the 5 5 Figure : Percentage of Cd removed from the aqueous solutions as a function of exchange time, for three starting Cd concentrations, using natural clinoptilolite (CLI). The weight percentage (wt %) of Cd in the CLI powders, measured by EDS, as a function of exchange 6

4 6 Journal of Mineral Metal and Material Engineering, 6, Vol. Ruiz-Serrano and Ramírez-Bon Table : Amount of Cd (wt %) in the CLI Powder, Determined by EDS, as a Function of Exchange Time, for Three Starting Cd Concentrations of the Aqueous Solution Cd Concentration in Solution (M)... Exchange wt % of Cd in CLI wt % of Cd in CLI wt % of Cd in CLI.5 ±. ND.9 ± ±..9 ±.4.4 ± ±.. ±.. ±.7.68 ±.9. ±..5 ± ±.. ±.8. ±.6 time, is given in Table. These are the amounts of Cd captured by clinoptilolite from the aqueous solution by cation exchange with the natural cations of clinoptilolite, mainly Na and Ca cations. As with the amount of Cd measured in the residual water, the amount of Cd in the CLI powders varies quickly (in this case increases) for the first 5 minutes and subsequently stabilizes, which can be related with cation-exchange equilibrium. A larger amount of Cd loaded into the CLI powders in the Cd solutions with higher Cd concentration, and there was a slight increase with exchange time in the three Cd solutions. The highest values of wt % of Cd in the CLI samples were. ±.7,. ±. and.74 ±. for the.,. and. M Cd concentrations, respectively. Figure 4 shows the Cd concentration as a function of time measured in the residual water of the Cd sorption experiments with CLI-Na powders. The behavior is Cd Concentration (ppm). M Cd (995 ppm). M Cd (4 ppm). M Cd (.4 ppm) CLI-Na + Figure 4: Cd concentration in ppm of the aqueous solutions as a function of exchange time, for three starting Cd concentrations, using clinoptilolite-na (CLI-Na). similar to the CLI powders (Figure ), with the Cd concentration decreasing quickly for the first 5 min and then stabilizing. The corresponding percentages of Cd removed from the aqueous solutions as a function of time are shown in Figure 5. The Na-modified clinoptilolite produces a slightly different behavior than the natual clinoptilolite (CLI). In this case, after a fast increase for the first minute, the increase continues, albeit slowly, rather than stabilizing. The percentage of Cd removed from the solutions increases with Cd concentration; the highest values were 57.8, and 96.% for the.,. and. M concentration solutions, respectively. Cd Removal (%) M Cd. M Cd 5. M Cd 5 CLI-Na Figure 5: Percentage of Cd removed from the aqueous solutions as a function of exchange time, for three starting Cd concentrations, using clinoptilolite-na (CLI-Na). The weight percentages (wt %) of Cd in the CLI-Na powders as a function of exchange time are shown in Table. The higher values of wt % of Cd in the CLI-Na samples, as compared with the CLI samples, indicate that the Cd sorption properties of clinoptilolite were 6

5 Cd Sorption from Aqueous Solutions Journal of Mineral Metal and Material Engineering, 6, Vol. 7 Table : Amount of Cd (wt %) in the CLI-Na Powder, Determined by EDS, as a Function of Exchange Time for Three Starting Cd Concentrations of the Aqueous Solution Cd Concentration in Solution (M)... Exchange wt % of Cd in CLI-Na wt % of Cd in CLI-Na wt % of Cd in CLI-Na.5 ±.. ±.5.5 ± ±..5 ±.. ± ±.7.97 ±..46 ±.4.68 ±.9.7 ±.5.69 ±. 6.8 ±.8.6 ±..9 ±. improved by its modification in the NaCl solutions to acquire the CLI-Na form. The highest amounts of Cd in the CLI-Na samples were.8,.7 and.9 wt % for the.,. and. M concentration solutions, respectively. These values are 45, 5 and 8% higher than those for the corresponding CLI samples. Figure 6 shows the evolution of the Cd concentration in the solutions where CLI-NH 4 powders were used to remove Cd ions. The trend of the Cd concentration is similar to the CLI and CLI-Na cases, although at the lowest Cd concentration (. M) the values increases with time after initially decreasing quickly. That is, Cd ions are released after being exchanged by the CLI-NH 4 powder. From these results, the corresponding percentages of Cd removed from the aqueous solutions as a function of time were determined and plotted in Figure 7. Here the. and. M Cd solutions display a fast initial increase followed by a CLI-NH + 4 slight increase, while the. M Cd solution stabilizes after an initial increase. The percentage of Cd removed from the solutions also increases with Cd concentration, and the highest values were 4., 8. and 97. % for the.,. and. M concentration solutions, respectively. Except for the value of the lowest solution concentration, these percentages of Cd removed from the solutions using CLI-NH 4 samples are higher than those obtained with CLI and CLI-Na samples. Therefore, the CLI-NH 4 form has improved sorption properties to remove Cd ions from aqueous solutions. Cd Removal (%) M. M. M CLI-NH + 4 Cd Concentration (ppm). M Cd (995 ppm). M Cd (4 ppm). M Cd (.4 ppm) 5 5 Figure 7: Percentage of Cd removed from the aqueous solutions as a function of exchange time, for three starting Cd concentrations, using clinoptilolite-nh 4 (CLI-NH 4) Figure 6: Cd concentration in ppm of the aqueous solutions as a function of exchange time, for three starting Cd concentrations, using clinoptilolite-nh 4 (CLI-NH 4). This conclusion is also reached via the wt % of Cd in the CLI-NH 4 powders as a function of exchange time (Table ). For this clinoptilolite form, the values of wt % of Cd loaded in the powders after the sorption experiments are higher than those of the other two types of clinoptilolite samples. The highest amounts of Cd in the CLI-NH 4 samples were.,.7 and.8 wt % for

6 8 Journal of Mineral Metal and Material Engineering, 6, Vol. Ruiz-Serrano and Ramírez-Bon Table : Amount of Cd (wt %) in the CLI-NH 4 Powder, Determined by EDS, as a Function of Exchange Time for Three Starting Cd Concentrations of the Aqueous Solution Cd Concentration in Solution (M)... Exchange wt % of Cd in CLI-NH 4 wt % of Cd in CLI-NH 4 wt % of Cd in CLI-NH 4.55 ±.5.6 ±..7 ± ±.9. ±.7.6 ±. 5.8 ±.5.7 ±..78 ±.8.59 ±..4 ±.. ± ±..8 ±.4. ±. the.,. and. M concentration solutions, respectively. These values are 67, 4 and % higher than those corresponding to the CLI samples, and 6, 8 and 4% higher than those corresponding to the CLI- Na samples. Therefore, these results show that the CLI-NH 4 form has better properties for the sorption of Cd ions from aqueous solutions in a large range of Cd concentrations (-, ppm). 4. CONCLUSIONS In this work we have applied a natural zeolite clinoptilolite from Sonora, México, in the natural and Na and NH 4 cation-exchange-modified forms, to the Cd sorption from aqueous solutions. The Na and NH 4 forms of clinoptilolite were obtained by simple cationexchange processes in NaCl and NH 4 Cl solutions, respectively. The Cd sorption from aqueous solutions was produced by cation-exchange processes in the clinoptilolite powder samples. The Cd sorption efficiency of the three forms of clinoptilolite samples was assessed from measurements of the Cd concentration (ppm) in the residual water and by the Cd loading (wt %) in the clinoptilolite powders after the cation exchange processes. It was found that both modified forms, CLI-Na and CLI-NH 4, improve the Cd sorption efficiency of natural zeolite (CLI), CLI-NH 4 being the better at removing Cd from aqueous solutions. The highest percentages of Cd removed from the solutions by CLI-NH 4 samples were 4., 8. and 97.% for the.,. and. M concentration solutions, respectively. The largest amounts of Cd in the CLI-NH 4 samples were.,.7 and.8 wt % for the.,. and. M concentration solutions, respectively. These values are 67, 4 and % higher than those corresponding to the CLI samples, and 6, 8 and 4% higher than the CLI-Na samples. The results reported in this work show that natural clinoptilolite is a promis- ing mineral to be applied to solving ambient contamination, specifically for the removing of heavy metals, such as Cd, from contaminated water. Furthermore, it was shown that the conditioning pretreatments of natural clinoptilolite improved its sorption properties for these applications. ACKNOWLEDGEMENTS The helpful technical assistance of Carlos Avila Herrera and Martin Adelaido Hernandez is acknowledged. We specially thank Dr. Mike Boldrick for his helpful English review and technical comments on the paper. REFERENCES [] Reháková M, Čuvanová S, Dzivák M, Rimár J, Gavalová Z. Curr. Opin. Agricultural and agrochemical uses of natural zeolite of the clinoptilolite type. Solid State Mater Sci. 4; 8: [] Mozgawa W, Bajda T. Application of vibrational spectra in the studies of cation sorption on zeolites. J Mol Struct. 6; 79-79: [] Erdem E, Karapinar N, Donat R. The removal of heavy metal cations by natural zeolites. J Colloid Interface Sci. 4; 8: [4] Faghihian H, Ghannadi Maragehb M, Kazemian H. The use of clinoptilolite and its sodium form for removal of radioactive cesium, and strontium from nuclear wastewater and Pb+ Ni+ Cd+ Ba+ from municipal wastewater. Appl Radiat Isot. 999; 5: [5] Top A, Ulku S. Silver, zinc, and copper exchange in a Naclinoptilolite and resulting effect on antibacterial activity. Appl Clay Sci. 4; 7: [6] Castaldi P, Santona L, Enzob S, Melis P. Sorption processes and XRD analysis of a natural zeolite exchanged with Pb+, Cd+ and Zn+ cations. J Hazard Mater. 8; 56:

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