Clay Mineral Type and Organic Compound Sorption by Hexadecyltrimethlyammonium‐Exchanged Clays

Abstract In order to relate mineralogy to sorbent efficiency, organo‐clays were prepared from reference vermiculite, illite, smectite, and kaolinite clay minerals using the organic cation hexadecyltrimethylammonium (HDTMA). Adsorption isotherms using 14 C‐HDTMA indicated stoichiometric adsorption of HDTMA up to the cation‐exchange capacity (CEC). Organo‐clays were prepared by adding HDTMA equivalent to the CEC and were evaluated as sorbents for nonionic organic compounds (NOCs) dissolved in water. X‐ray diffraction analysis of the HDTMA clays revealed basal spacings of 28 Å for the vermiculite and 23, 20, and 18 Å for the high‐charge, intermediate‐charge, and low‐charge smectites, respectively. The HDTMA vermiculite, illite, and smectites were all highly effective sorbents for NOCs, whereas Mg smectite was ineffective. The sorption isotherms of benzene, toluene, ethylbenzene, propylbenzene, butylbenzene, t ‐butylbenzene, naphthalene, and biphenyl on the HDTMA clays indicated that sorption occurred by partition interactions with the HDTMA‐derived organic phase. In general, both the greater HDTMA content and the larger basal spacings of high‐charge HDTMA clays increased NOC sorption. Mineral‐charge effects on the sorption of unsubstituted aromatic compounds (benzene, naphthalene, and biphenyl) were less evident than for alkyl‐benzenes. Greater sorption of the alkylbenzenes by high‐charge HDTMA clays can be attributed to the capability of the large basal spacings to accommodate larger solute molecules. The formation of organic cation exchanged soil clays derived from vermiculite, illite, or smectite may greatly improve the ability of soils to immobilize organic contaminants.