Hexavalent chromium adsorption by tetrahexylphosphonium modified beidellite clay

Abstract

The fixation of the hexavalent chromium (CrVI), a toxic component in the wastewater, from the aqueous solutions into the interlayer space of beidellite (Bd) clay was studied, combining theoretical and experimental methods. The structure of intercalated CrVI anionic species to beidellite modified by tetrahexylphosphonium cation (Cr-THP-Bd) was examined using the density functional theory (DFT) calculations on representative structural models. The computed basal spacing (d001) of the Cr-THP-Bd model (15.5 Å) showed a good agreement with the experimental value (15.3 Å) obtained by the XRD method. Calculated intercalation energy of −214.2 kJ/mol demonstrated strong stabilization of chromate anion in the THP-Bd structure. C–H⋅⋅⋅O and O–H⋅⋅⋅O hydrogen bond interactions present in the Cr-THP-Bd model significantly affected the structural stability of the whole studied system. The calculated vibrational spectra from the ab initio molecular dynamics (AIMD) helped identify the Cr–O stretching and O–Cr–O bending vibrational modes of (CrO4)2− anion in the complex FTIR spectra. The amount of (CrO4)2− anion determined by the experiment clearly confirmed that Bd modified by the THP surfactant significantly increased the adsorption of the chromate anion in comparison to pure Na-Bd clay.