THE BEHAVIOR OF THE VOLCANIC TUFF BASED ON CLINOPTILOLITE IN THE ION EXCHANGE PROCESS WITH Cr3+AND

Fe3+ IONS FROM WASTEWATERS

Claudia Cobzaru1, Ioan Iordache2, Adriana Marinoiu3, 1Universitatea Tehnică “Gh. Asachi”, Facultatea de Inginerie Chimică şi Protecţia Mediului, B-dul. D. Mangeron, nr. 71, 70050, Iaşi

2 ICSI, Rm Valcea, str Uzinei nr.4 ,240050, Romania3S.C. OLTCHIM, Rm Vâlcea- Centrul de Cercetare, St Uzinei, nr 1, Rm Vâlcea 240050.

INTRODUCTION

REFERENCES

The commercial interest in the natural zeolites is presently of an unprecedent level due to their advantages against the synthetic catalytical materials. Since the natural zeolites are already synthesized the obtaining prices by industrial processes are connected with their preparation only (washing, crushing, screening, applied treatments, etc). The synthesis stage which is often very expensive can thus be eliminated. The properties of natural zeolites and their potential applications were described in previous papers where the following domains are mentioned: agriculture [1-3], purification of waste waters [4-9], pharmacy [10-12], separation and adsorption of gases [13,14], catalysis [15-19], etc. The applications of natural zeolites are based on their versatile properties, such as: ion-exchange capacity, adsorption capacity, high content of microelements, microporosity, acido-basic or redox properties of the surface, etc.

The clinoptilolite is the best known among the natural zeolites being present especially in volcanic rocks in 60-90% content. The volcanic tuffs also contain feldspaths, clays, glass and quartz. The natural clinoptilolite was found to show high capacity of retaining by ion-exchange the undesired ions frequently present in the industrial waste waters [20-25]. The studies performed made evident the ion-exchange capacities and exchange kinetics to be strongly affected by several factors connected to the zeolite chemical and structural properties (composition, porosity, pretreatment, etc) as well as to the exchange process parameters (ion concentrations, temperature, time, solution/zeolite ratio, presence of other ions, pH, etc).

These considerations would sustain the desirability of our study aimed to investigate the behaviour of the clinoptilolite volcanic tuff, either native or in Na + form, in the ion exchange process with Cr3+ and Fe3+ ion known as a serious polluting agent in industrial waste waters.

CONCLUSIONS

The behaviour of the volcanic tuff in the exchange process with Cr3+ and Fe3+ ions was followed. The variation of the exchange capacity in time for these metals is presented in Figure 1.

RESULTS AND DISCUSSIONS

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formaldehyde over alkali and alkaline earth metal ion modified basic zeolites, Catalysis Communications, 9, 532-538.20. Rodriguez-Iznaga I., Gómez A., Rodriguez-Fuentes G., Benitez-Aguilar A., Serrano-Ballan J., (2002), Natural clinoptilolite as an

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EXPERIMENTAL

1. MaterialThe volcanic tuff of a 70% clinoptilolite content taken in the present study

was mined in the Mirsid (Romania) deposits. Its average chemical composition is the following: 68,12%-70,06% SiO2; 11,77%-12,62% Al2O3; 0,67%-1,06% Fe2O3; 3,36%-4,82% CaO; 0,55%-0,82% MgO; 2,20%-3,52% K2O; 0,40%-0,60% Na2O; 0,18%-0,24 TiO2 [26]. A volcanic tuff sample of 0.25-0.50 mm was prepared.

2. Ionic exchangeThe volcanic tuff was firstly washed repeatedly with distilled water, then

dried at 60oC for 3 hours, crushed and screened. The resulting product was put into contact with an aqueous 1N sodium chloride solution in the tuff: solution ratio of 1:10 (by vol.) at 80oC for 24 hours under periodical stirring. Every 6 hours the solution was replaced by a newly prepared one. When the exchange process was over the volcanic tuff was repeatedly washed with distilled water and dried at 60oC. The modified tuff was thus obtained as its sodium form. The ionic exchange capacity of the tuff was estimated by the analysis of the sodium amount remaining after the exchange process and the Na+ ions was analysed by flamphotometric method. The content of exchanged ions (as sodium equivalent) was of 2.05 mEq Na+/ g tuff.

The iron form of the tuff was obtained from the sodium form by ionic exchange with 0.1N FeCl3 in the ratio of 1:10 (by vol.) at 80oC for three hours under periodical stirring. Also, the chromium form of the tuff was obtained from the sodium form by ionic exchange with 0.1N CrCl3 • 6H2O in the ratio of 1:10 (by vol.) at 80oC for three hours under periodical stirring. The Cr3+ and Fe3+ cation was analysed by analytical method [27].

In the present paper the ionic exchange kinetic of the volcanic tuff used for retaining of Cr3+ and Fe3+ ions in aqueous solutions was studied. In this exchange process, the tuff in the sodium form was more efficient for retaining of Cr3+ ion than Fe3+ ion. The exchange process was also influenced by the tuff grain size the exchange rate being higher with small-sized grains, which would be indicative of the role of diffusion processes into the small pores of clinoptilolite.

Table 1. Ionic radii of metallic cations [28].

This behaviour could be admitted to be mainly caused by the diffusion of ions through the solid pores. It could also be assumed that the ions of longer radius have no access to certain regions in the clinoptilolite internal structure. Here we have to remember that the clinoptilolite contains a two-dimensional system of intersecting channels of 8-10 tetrahedrons and the effective diameter of the access windows into channels and cavities is of 4.4x3 Å and 7.9x3.5 Å respectively [26].

0

0.5

1

1.5

2

0 30 60 90 120 150 180 210

temp, min

mE

q m

etal

/g tu

ff

Cr

Fe

Figure 1. Exchange process kinetics for Cr3+ and Fe3+ ions on volcanic tuff

The reaction which lies at the basis of exchange process is: n Na+(zeol) + Men+ (sol) n Na+(sol) + Men+(zeol)where: Men+ = Fe3+, Cr3+.It can be seen from Figure 1 that the metalic ions exchanges to makes

very fast and the mounts reserved were visibly. The ionic exchange capacity of the Na+ form of tuff for the Cr3+ and Fe3+ ions are seen to be different and decreases in the following order Cr3+Fe3+. These results correlated with the data in Table 1 make evident a correlation between the exchange capacity and the ionic radius. Thus, the smaller ionic radius the higher exchanging rates and ionic exchange capacity are.

Cations Na+ Cr3+ Fe3+

Ionic radius [Å] 0.98 1.18 1.17