Metal adsorption and desorption characteristics of surfactant modified clay complexes

Several modified clays have been designed and created for selective removal and recovery of heavy metals such as Cd, Cu, Cr, etc. These surfactant-clay complexes were prepared using hectorite or montmorillonite as the base clay. A simple two-step approach has been developed to synthesize these modified-clay complexes through ion exchange and hydrophobic anchoring of several surfactants such as long-chain alkyldiamines, long-chain dialkylamines, and long-chain carboxylic adds onto the clay matrices. The adsorption capacities and affinity constants of the modified clays can be found to approach those of commercial chelating resin (Chelex 100, Bio-Rad). Using cadmium as a model metal and montmorillonite-cetylbenzyldimethylammonium-palmitic acid (M-CBDA-PA) as a model modified-clay complex, the maximum adsorption capacity of the modified clay is found to be 42 +/- 0.8 mg/g of clay and the affinity constant is 3.0 +/- 0.1 mg/L. The metal adsorption has been shown to be mainly through chemical complexation rather than ion exchange. The immobilization of the metal ions is pH dependent, and thus, pH can act as a molecular switch to regenerate the modified-clay complexes.