Enhanced sorption of naphthalene and nitroaromatic compounds to bentonite by potassium and cetyltrimethylammonium cations

Sorption of naphthalene, p-nitrotoluene, nitrobenzene and m-dinitrobenzene from water to original bentonite, original bentonite in KCl solutions, and an organobentonite (i.e., 100CTMAB) was compared. The affinities of sorbates with original bentonite were extremely weak and similar (sorption coefficient (Kd) = 0.41-0.94 mL/g) regardless of their properties and structures. Sorption was dramatically enhanced by 100CTMAB due to strong retention of CTMA+-derived organic phase and the exposed-siloxane surfaces. The resultant Kd increased by 5360, 780, 40 and 200 times, respectively, for naphthalene, p-nitrotoluene, nitrobenzene and m-dinitrobenzene, reversed with their aqueous solubility. The presence of K+ promoted sorption of nitroaromatic compounds (50-400 times). The enhanced-mechanism for nitrobenzene at low concentrations and p-nitrotoluene was derived mainly from weak van der Waals interaction between aromatic rings and siloxane surfaces, while for nitrobenzene at high concentrations and m-dinitrobenzene the mechanisms were attributed to primary complexation of -NO2 group with K+ and then strong interactions by electron donor-acceptor. Sorption of m-dinitrobenzene enhanced linearly with K+ concentrations of 0.005-0.20 mol/L, and overshadowed the role of 100CTMAB when K+ > 0.5 mol/L. These will eventually facilitate the application of modified-bentonite to abate pollutants in environments.