Fourier transform Raman spectroscopy of sorbed HDTMA and the mechanism of chromate sorption to surfactant-modified clinoptilolite

We examined sorption of the cationic surfactant hexadecyltrimethylammonium bromide (HDTMA) to clinoptilolite zeolite and the subsequent sorption of the chromate anion to surfactant-modified zeolite (SMZ). We used Fourier transform (FT) Raman spectroscopy and batch sorption methods to elucidate the structure of sorbed HDTMA and to determine the mechanisms of chromate sorption. At high HDTMA loading levels (above the zeolite's external cation exchange capacity, ECEC), the Raman spectra indicated th at sorbed HDTMA was similar in conformation to solution micelles and, thus, may contain anion exchange sites. Sorbed HDTMA showed less structuring of tail groups and a decrease in head group hydration. At lower loadings, the sorbed HDTMA tail groups tended to have more disorder, similar to solution monomers. When HDTMA loading rates were greater than 100% of the ECEC, chromate sorbed onto SMZ with near-equivalent Br- counterion exchange. A peak in the Raman spectrum at 902 cm(-1) indicated the presence of sorbed Cr2O72-, although no bulk solution oligomerized chromate species should have been present at a solution pH of 7. A 30 cm(-1) shift in the nu(1) peak for sorbed versus solution chromate may indicate that surface-enhanced Lewis acid-base interactions were responsible for some chromate sorption in addition to the predominant anion-exchange mechanism.