Chromium (VI) uptake by polyhexamethylene-guanidine-modified natural zeolitic materials

The Cr(VI) uptake from aqueous solutions by polyhexamethylene-guanidine-modified natural zeolitic materials from Metaxades, Greece (PHMG-MTX) and Tedzami, Georgia (PHMG-TZ) was investigated using batch techniques. Spectrophotometry using 1,5-diphenylcarbazide for colour development was used for Cr determination. The Cr(VI)-solution concentration varied between 5 and 50 mg/L and the initial pH was adjusted to 3.0, 4.0 and 5.0. Additional experiments were conducted in the presence of competing anions (background electrolyte I = 0.1 M established by 0.1 M NaNO3 and 0.033 M Na2SO4) on solutions with unadjusted pH. For comparison, the Cr(VI) adsorption by Tedzami tuffs modified by octadecylpolyhexamethylene-guanidine-chloride (ODPHMG-TZ) was also investigated. All investigated materials adsorbed considerable amounts of Cr(VI)-oxyanions. It was shown that the Cr(VI)-uptake strongly depended on pH and the presence of competing anions. The Cr(VI)-oxyanion uptake by PHMG-TZ was slightly higher than that observed for ODPHMG-TZ, whereas the sorption capacity of PHMG-MTX was of the same order of magnitude as Metaxades tuffs modified by hexadecyl-trimethylammonium-bromide. The experimental data obtained for pHinit = 3.0 were selected for modeling the adsorption behavior of the materials using six isotherm equations (Freundlich (F), Langmuir (L), Langmuir–Freundlich (L–F), Dubinin–Radushkevich (D–R), Redlich–Peterson (R–P), Toth (T)). The obtained modeling results indicated that, although the three-parameter models, taking into account the surface heterogeneity, provided the closest approach to the measurement data, the parameters estimates could be highly biased. On the other hand, the two-parameter Langmuir isotherm was sufficient to describe the process equilibrium in terms of smallest number of adjustable parameters and approximation of the sorption capacity.