Adsorption Of Cadmium On Acid-modified Transcarpathian Clinoptilolite
Adsorption of Cadmium on Acid-Modified Transcarpathian Clinoptilolite
V. O. Vasylechko, G. V. Gryshchouk, Yu. B. Kuz\’ma, V. P. Zakordonskij,
L. O. Vasylechko, L. O. Lebedynets, M. B. Kalytovs\’ka
Ivan Franko National Univrsity of L\’viv, L\’viv, Ukraine
The hydrogen forms of zeolites considerably differ from the non-modified zeolites by the adsorption properties. During the treatment of the zeolites by the acid on the first stage cations move of from the exchange positions. The next stage is characterized by dialuminizing the frame without its visible change. On the closing stage the frame is destroyed with forming of the amorphous phase. On the acid modification of the zeolites it may be also increased their porosity. One of the most distributed methods of obtaining the H-forms of the zeolites is their treatment by acid solutions. But the use of this method is bounded by the instability of the most of the natural zeolites grates acid acting. By this simple method it is possible to obtain the H-forms only of the most high-silicic and comparatively acid-steady zeolites (such as clynoptilolite, mordenite). The H-forms may be also obtained through the NH4-forms by the thermal destruction of the NH4-ions with the isolation of the ammonia.
The Transcarpathian clinoptilolite (deposit near the village of Sokyrnytsa) stands out from other natural zeolites by their elevated acid stability, thermal stability and mechanical strength. It has been known that aluminium, calcium and to a lesser degree potassium most intensively get free during the treatment of the Sokyrnytsa clinoptilolite by acid. The conditions of the H-forms of the Transcarpathian clinoptilolite (v. Sokyrnytsa). have been investigated. The optimum conditions of concentrating the trace amounts of Cd(II) on the Transcarpathion acid-modified clinoptilolite from water solutions at the dynamic and at the static regime have been established. Sulphuric, hydrochloric and nitric acids were used as the acid-modifies. It has been shown that the adsorption capacity of the H-form of the clinoptilolite is dependant on the nature and the concentration of the acid and also on the time of the contact between the acid and zeolite during the modification. H-form of the clinoptilolite obtained by the 1M HCl is the most effective adsorbent of Cd(II).
The highest adsorption capacity of the acid-modified clinoptilolite is reached after the 5-hour contact of the zeolite with the acid solution. Adsorption capacity value of the H-forms of the clinoptilolite obtained through its NH4-forms, obtained in the result of the modification by 1M HCl. Obviously the adsorption distinctive properties between the studied H-forms of the clinoptilolite are caused by the different degrees of accessibility of the cations positions for the exchange ions of Cd(II).
Adsorption properties of the natural zeolites are dependent oh their prior thermal treatment. Maximum of the adsorption capacity of the unmodified clinoptilolite to Cd(II) agrees satisfactorily with the temperature interval of 120-1700C, that by the data of the termogravimetric analysis corresponds to removal of physically connected water [1]. At the same time, prior roasting the samples of the H-form of the clinoptilolite tends only to decreasing their adsorption capacity. These distinctive features may be caused by the difference in the processes rehydration between the non-modified and the H-form of the clinoptilolite. Among other factors DTG-maximum asymmetry is more clear for the H-forms. Three shoulders corresponding to three processes divided on the temperature scale with quas-maxima at 110-120, 147 and 200-2200C. It may be inferred that there are at least three types of surface physically adsopbed water.
Under static conditions the H-forms of the clinoptilolite adsorbs Cd(II) from water solutions with pH 5,2 at their best. At this value of pH 99% of cadmium is adsorbed. The solution of NaCl of mixture of HCl and HNO3 solutions may be used as the desorbent of cadmium.
Under dynamic conditions (flow rate of solution through adsorbent=3ml/min; pH of Cd2+ solution =5,6; diameter of adsorbent grains=0,20-0,315mm) the adsorption capacity of the H-forms of clinoptilolite is 10mg/g, that is almost an order more then the capacity of natural non-modified clinoptilolite [2].
Based on the obtained results the methods of concentrating trace quantities of cadmium from water have been proposed. The high amount of the adsorption capacity of the H-forms of the clinoptilolite gives grounds to propose this adsorbent as the filter for cleaning water from cadmium.
References
1. Vasylechko V.O., Gryshchouk G.V., Lebedynets L.O. et al. Adsorption of Copper on Transcarpathian Clinoptilolite., Adsorp. Sci. Technol., 1999, V.17, N 2, P.125-134.
2. Vasylechko V.O., Gryshchouk G.V., Kuz’ma Yu.B. et al. Adsorption of Cadmium on Transcarpathian Clinoptilolite., Materials IV Polish-Ukrainian Symposium “Theoretical and Experimental Studies on Interfacial Phenomena and their Technological Applications” (Lublin, Polska, September 1-3, 1999). - Lublin.: UMCS, 1999. -O-9.
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