SORPTION OF CHROMATE AND OTHER INORGANIC ANIONS BY ORGANO-ZEOLITE

We performed batch sorption experiments that showed a significantly enhanced removal of inorganic oxyanions from aqueous solution by clinoptilolite-dominated zeolite modified by the quaternary amine hexadecyltrimethyl-ammonium (HDTMA). Since HDTMA is too large to enter into the internal portion of the zeolite, sorption of the amine only occurred on the zeolite's external exchange sites. HDTMA was exchanged with extrastructural cations of the zeolite up to the external cation-exchange capacity. The HDTMA-modified surface was stable when exposed to extremes in pH and ionic strength and to organic solvents. While the natural zeolite had no affinity for the oxyanions, the HDTMA-modified zeolite showed significant removal of chromate, selenate, and sulfate from 0.005 M CaCl2 aqueous solution. Sorption data for each anion were well-described by the Langmuir isotherm equation. We found that anion sorption was highest when the zeolite was modified such that HDTMA satisfied 100% of its external cation-exchange capacity. The mechanism of anion retention appears to be the formation of an HDTMA-anion precipitate on the zeolite surface.