Sorption of nonionic organic solutes from water to tetraalkylammonium bentonites: Mechanistic considerations and application of the Polanyi-Manes potential theory

This work describes the role of quaternary alkylammonium amendment length on sorption mechanisms of modified bentonites for four nonionic organic compounds; benzene, carbon tetrachloride, TCE, and 1,2-DCB. Tetramethyl to tetrabutyl alkyl amendments were studied and an important mechanistic shift occurred at the propyl chain length for all four solutes studied. Three- and four-carbon-chain functional groups on the ammonium cation resulted in a linear, rather than a curvilinear isotherm. The uptake on tetrapropyl and tetrabutylammonium clays was noncompetitive in binary systems and showed negligible sensitivity to temperature variations, indicating the linear isotherms describe a partitioning uptake mechanism for these organoclays. The adsorptive organoclays (tetramethyl and tetraethylammonium clays) were fit with the Dubinin-Radushkevich equation to investigate the application of the Polanyi-Manes potential theory to organoclay adsorption. It was found that TCE and carbon tetrachloride, with similar physical and chemical characteristics, behaved according to the Polanyi-Manes theory. Benzene showed an anomalously high adsorption volume limit, possibly due to dense packing in the adsorption space or chemisorption to the short chain alkyl groups.