SREL Reprint #2279

 

 

 

Fourier Transform Raman Spectroscopy of Sorbed HDTMA and the Mechanism of Chromate Sorption to Surfactant-Modified Clinoptilolite

ENID J. SULLIVAN, DOUGLAS B. HUNTER, AND ROBERT S. BOWMAN
Department of Earth and Environmental Science and Geophysical Research Center,
New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801,
and Division of Biogeochemistry, University of Georgia,
Savannah River Ecology Laboratory, Aiken, South Carolina 29801

 

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 (Fr) 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 that 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 v1 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.

SREL Reprint #2279

Sullivan, E.J., D.B. Hunter, and R.S. Bowman. 1998. Fourier transform raman spectroscopy of sorbed HDTMA and the mechanism of chromate sorption to surfactant-modified clinoptilolite. Environmental Science and Technology 32:1948-1955.

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