Perchlorate uptake by synthetic layered double hydroxides and organo-clay minerals

Abstract

The perchlorate uptake of hydrotalcite- and hydrocalumite-type layered double hydroxides (LDHs), and surfactant modified clay minerals were compared. Perchlorate uptake by both hydrotalcite- and hydrocalumite-type LDHs was in the range of 0.011 to 0.197meq/g from a 2mM perchlorate solution. The nitrate form of Mg:Al LDH had the highest uptake of 0.197±0.033meq/g, and the carbonate form of LDH the lowest uptake of 0.011±0.003meq/g. Octadecyltrimethylammonium (ODTMA), Dodecyltrimethylammonium (DoDTMA), and hexadecyltrimethylammonium (HDTMA) clay minerals removed perchlorate in the range of 0.025 to 0.348meq/g from a 2mM perchlorate solution. Synthetic HDTMA (5.0 CEC) Na-1-mica exhibited the highest adsorption of perchlorate with 0.348±0.011meq/g, while DoDTMA montmorillonite had the lowest adsorption with 0.025±0.009meq/g. Tested under the same conditions, a surfactant modified carbon showed an uptake of 0.303±0.005meq/g and this carbon is currently being used for filtration of drinking water.Hydrophobic organo-clay minerals with larger interlayer distance resulted in higher perchlorate uptake by exchange/adsorption on the residual charge of the cationic surfactants. The uptake by LDH was by anion exchange on the surfaces and, for the nitrate form of hydrotalcite, also from the interlayer spaces. The mechanisms of uptake were confirmed by X-ray diffraction (XRD) and scanning electron microscopy. This study indicated that cationic surfactant containing organo-clay minerals may be useful materials for the removal of perchlorate from water and could serve as alternatives to surfactant modified-activated carbons.