FOURIER-TRANSFORM RAMAN-SPECTROSCOPY OF SORBED HDTMA AND THE MECHANISM OF CHROMATE SORPTION TO SURFACTANT-MODIFIED CLINOPTILOLITE

We examined sorption of the cationic surfactant hexadecyltrimethylammo nium bromide (HDTMA) to clinoptilolite zeolite and the subsequent sorp tion of the chromate anion to surfactant-modified zeolite (SMZ). We us ed Fourier transform (FT) Raman spectroscopy and batch sorption method s to elucidate the structure of sorbed HDTMA and to determine the mech anisms of chromate sorption. At high HDTMA loading levels (above the z eolite's external cation exchange capacity, ECEC), the Raman spectra i ndicated th at sorbed HDTMA was similar in conformation to solution mi celles and, thus, may contain anion exchange sites. Sorbed HDTMA showe d less structuring of tail groups and a decrease in head group hydrati on. At lower loadings, the sorbed HDTMA tail groups tended to have mor e disorder, similar to solution monomers. When HDTMA loading rates wer e greater than 100% of the ECEC, chromate sorbed onto SMZ with near-eq uivalent Br- counterion exchange. A peak in the Raman spectrum at 902 cm(-1) indicated the presence of sorbed Cr2O72-, although no bulk solu tion oligomerized chromate species should have been present at a solut ion pH of 7. A 30 cm(-1) shift in the nu(1) peak for sorbed versus sol ution chromate may indicate that surface-enhanced Lewis acid-base inte ractions were responsible for some chromate sorption in addition to th e predominant anion-exchange mechanism.