title: Sorption of organic cations to phyllosilicate clay minerals: CEC-normalization, salt dependency, and the role of electrostatic and hydrophobic effects.
abstract: Sorption to the phyllosilicate clay minerals Illite, kaolinite, and bentonite has been studied for a wide variety of organic cations using a flow-through method with fully aqueous medium as the eluent. Linear isotherms were observed at concentrations below 10% of the cation-exchange capacity (CEC) for Illite and kaolinite and below 1 mmol/kg (<1% CEC) for bentonite. Sorption to clays was strongly influenced by the electrolyte composition of the eluent but with a consistent trend for a diverse set of compounds on all clays, thus allowing for empirical correction factors. When sorption affinities for a given compound to a given clay are normalized to the CEC of the clay, the differences in sorption affinities between clays are reduced to less than 0.5 log units for most compounds. Although CEC-normalized sorption of quaternary ammonium compounds to clay was up to 10-fold higher than CEC-normalized sorption to soil organic matter, CEC-normalized sorption for most compounds was comparable between clays and soil organic matter. The clay fraction is thus a potentially relevant sorption phase for organic cations in many soils. The sorption data for organic cations to clay showed several regular trends with molecular structure but also showed quite a few systematic effects that we cannot explain. A model on the basis of the molecular size and charge density at the ionized nitrogen is used here as a tool to obtain benchmark values that elucidate the effect of specific polar moieties on the sorption affinity.
authors.affiliation: Department of Analytical Environmental Chemistry, Helmholtz Centre for Environmental Research-UFZ , Permoserstrasse 15, 04318 Leipzig, Germany.
authors.forename: Steven T J
authors.initials: ST
authors.lastname: Droge
authors.forename: Kai-Uwe
authors.initials: KU
authors.lastname: Goss
mesh_terms: D000327:Adsorption; D000538:Aluminum Silicates / Q000737:chemistry*; D001546:Bentonite / Q000737:chemistry; D002412:Cations / Q000737:chemistry; D000077215:Clay; D004573:Electrolytes / Q000737:chemistry; D057927:Hydrophobic and Hydrophilic Interactions; D007616:Kaolin / Q000737:chemistry; D008903:Minerals / Q000737:chemistry*; D008970:Molecular Weight; D009930:Organic Chemicals / Q000737:chemistry*; D000644:Quaternary Ammonium Compounds / Q000737:chemistry; D012492:Salts / Q000737:chemistry*; D012989:Soil Pollutants / Q000032:analysis; D055672:Static Electricity
chemical_list: D000538:Aluminum Silicates; D002412:Cations; D004573:Electrolytes; D008903:Minerals; D009930:Organic Chemicals; D000644:Quaternary Ammonium Compounds; D012492:Salts; D012989:Soil Pollutants; C099089:illite; D001546:Bentonite; D007616:Kaolin; D000077215:Clay