## Seletividade da Clinoptilolita natural por metais tóxicos em sistemas aquosos.

##### Abstract

Natural zeolites are the most important inorganic cationic exchangers exhibiting high
ion exchange capacity, selectivity and environment compatibility.
Heavy metals are well known for toxicity and their disposal is a significant industrial
waste problem.
The goal of this work was directed to evaluate the selectivity of a purified homo-ionic
clinoptilolite mineral for aqueous Pb2+, Zn2+, Cu2+ and Na+ ions at 0,005 eq/L and 303 K,
interpreted through the application of empirical thermodynamic models to the zeolite phase
(Margules, Van Laar, Wilson) coupled with a well established ion-interaction approach for
the electrolyte solution (Pitzer).
The present study considered the following stages: (1) adsorbent material: preparation
and characterization; (2) aqueous solutions: nitrates of sodium, lead, zinc and copper; (3)
equilibration of weighed amounts of homo-ionic clinoptilolite with a series of solutions
containing the two competing cations; (4) analysis for aqueous cations by AAE; (5)
construction of the equilibrium points; isotherm analysis; (6) test for thermodynamic
reversibility; (7) empirical models for the zeolite phase (admitted as a solid solution) jointed
to the ion-interaction model chosen for the aqueous solution; (8) equilibrium constant and
Gibbs free energy for the ion-exchange reactions; phenomenological interpretation of the
thermodynamic parameters obtained by means of the application of empirical models to the
zeolite phase. The above procedure was, in the same way, followed for the ternary systems.
The results obtained in this work shown that the empirical models adopted for the solid
phase coupled to Pitzer s model for the activity coefficients in the electrolyte solution
describe successfully the binary ion-exchange equilibria. The calculated equilibrium constant
and the corresponding Gibbs free energy for each binary-exchange reaction resulted in a
selectivity sequence, at the normality and temperature of this study, easily deduced as:
2 2 2 Pb Na Cu Zn + + + + > > . Besides, the parameters estimated applying the Margules , Van
Laar s and Wilson s equations for cations in the solid binary mixture resulted in useful values
quantifying adequately the cation zeolite framework interactions, thus, an alternative way to
interpret the adsorbent selectivity from the charge and cationic radius effect.
The ternary parameters obtained applying multi-component empirical models do not
explain properly the non-ideality of ions in a solid mixture containing more than two
components. This is in accordance with the results encountered in a number of publications on
crystal structure of heulandite-group zeolites: these aluminosilicates are found to contain
crystallographically distinct set of sites throughout the exchanger framework and that
normally each set of sites is partially populated by the exchanging ions. As a consequence,
activity coefficients for a multi-component exchange reaction cannot be predicted from
appropriate binary data for a heterogeneous exchanger, since the phenomenological binary
coefficients are complicated functions of each site set, population and composition, and both
these properties will change on introducing other species of ion in the exchanger. In this
sense, and from what were obtained here, is believed that multi-component solid phase nonideality
must, at least, be interpreted through the application of statistical thermodynamic
models considering the energetic heterogeneity of a number of site set and the charge density
of the specific zeolite framework.