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Sorption of Hexavalent and Trivalent Chromium onto New Activated Clinoptilolite Based Materials: Kinetic, Thermodynamic and Characterizaion Aspects, pp. 603-624 $100.00
Authors:  (J. C. Torres, A. I. Pérez-Cordovés - International Institute of Ecology, São Paulo, Brazil and others)
Abstract:
Altered quantities of chromium in the environment is generally caused by the
widespread use of this metal in various industries such as metallurgical, refractory,
chemical, photography, fungicide and tanning industries.
Trivalent chromium (Cr(III)) is considered to be an essential trace element for living
organisms, whereas the hexavalent form of the metal (Cr(VI)) exerts toxic effects on
biological systems. After recent studies suggesting ingestion of Cr(VI) may cause
chromate induced cancers, there has been a renewed interest in the ability to separate,
identify, and quantify Cr(III) and Cr(VI) in a broad range of sample matrices.
A natural mineral of clinoptilolite was activated by chemical treatment followed by
calcination. The resulting material (NaCC1) was employed in the sorption of Cr(III) and
a surfactant (HDTMA)-modified form of NaCC1 (SMAC) was used in the sorption of
Cr(VI) from aqueous solutions at different total normalities (TN) and 30°C. The NaCC1
was characterized for total element composition and water. pH, dynamic and equilibrium
experiments were conducted to study the ion-exchange sorption and desorption of each
absorbent for Cr(VI) and Cr(III) species at different TN´s. The adsorption of Cr(VI) (TN
= 50 and 300 mg l-1 Cr) showed negligible variations with solution pH in the interval 2-
12, whereas Cr(III) at TN = 100 mg l-1 showed an adsorption maximum at pH 5.3.
Kinetic studies showed a maximum loading of 54 mmol Kg-1 for Cr(VI) and 35.2 mmol
Kg-1 for Cr(III). An stoichiometric (2:1) replacement of Br- by Cr(VI) was evidenced by
the linear correlation (r = 0.997) between the equivalent concentrations of the two anions.
Both Cr(VI) and Cr(III) adsorption isotherms were well described by the Langmuir
model. Cr(VI): Sm = 43 mmol Kg-1, KL = 30 l mol-1; Cr(III): Sm = 70 mmol Kg-1. Diffuse-
Reflectance IR spectra of the SMAC and Cr(VI)-SMAC samples showed absorption
bands in the interval 3000-2850 cm-1 ascribed to C-H vibrations of saturated
hydrocarbons, which demonstrate surfactant molecules sorbed onto the activated
clinoptilolite surface. The UV-Vis spectra of the Cr(VI)-SMAC samples showed
absorption bands at 263 and 347 nm, corresponding to charge transfer transitions by the
CrO4
2- anions from oxygen p-orbitals to higher energy empty Cr d-orbitals (O → Cr
electron hop), which were weakened due to chromate oxygen atoms bonded to positively
charged HDTMA surfactant head groups. 


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Sorption of Hexavalent and Trivalent Chromium onto New Activated Clinoptilolite Based Materials: Kinetic, Thermodynamic and Characterizaion Aspects, pp. 603-624