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