Sorption of Hexavalent and Trivalent Chromium Onto New Activated Clinoptilolite-Based Materials: Kinetic, Thermodynamic and Characterization Aspects

Aqueous chromium is generally found as Cr(III) and/or Cr(VI) species, which are severely different in charge, physicochemical properties, as well as chemical and biochemical reactivity.

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 normality (TN) and 30°C. The NaCC1 was characterized for total element composition and water. The pH, dynamic and equilibrium experiments were conducted to study the ion-exchange sorption and desorption of each absorbent for both Cr(VI) and Cr(III) species at different TNxs. 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 of 3000-2850 cm-1 ascribed to C-H vibrations of saturated hydrocarbons, which evidenced surfactant molecules sorbed onto the activated clinoptilolite surface. The UV-Vis spectra of the Cr(VI)-SMAC samples exhibited absorption bands at 263 and 347 nm, corresponding to electronic d-transitions of chromate oxygen atoms bonded to positively charged HDTMA surfactant head groups.