Retention of inorganic oxyanions by organo-kaolinite

A natural kaolinite (KGa-1b) was treated with the surfactant hexadecyltrime thylammonium bromide (HDTMA-Br) to a level twice that of the cation exchang e capacity (CEC). Sorption of nitrate. arsenate, and chromate by the result ant organo-kaolinite was then quantified. Sorption of each oxyanion was wel l-described by the Langmuir isotherm. Sorption of nitrate was the greatest. with a Langmuir sorption maximum of 24 mmol/kg, although chromate showed t he highest sorption affinity of 20 L/kg. Sorption of nitrate. arsenate, and chromate on organo-kaolinite was at least two orders of magnitude greater than their sorption on unmodified kaolinite. Desorption of the bromide coun terion indicated that each of the oxyanions was retained by ion exchange on an HDTMA bilayer formed on the organokaolinite. Chromate sorption on the o rgano-kaolinite was unaffected by solution pH in the range 5-9, but decreas ed at pH 11 due to competition of OH- for anion exchange sites. Similarly, chromate exchange by organo-kaolinite was reduced in the presence of high b ackground levels of chloride. Chromate was effectively retained when flowin g through a packed bed of organo-kaotinite: after an input of more than 40 pore volumes. the effluent concentration of chromate was less than 10% of t he input concentration. and 90% of the original HDTMA remained on the organ o-kaolinite. The results demonstrate that properly prepared organoclays can remove oxyanions, as well as nonpolar organics, from contaminated waters. (C) 2001 Elsevier Science Ltd. All rights reserved.