Effect of background electrolytes on the adsorption of nitroaromatic compounds onto bentonite

To further elucidate interaction of nitroaromatic compounds with mineral surface, the sorption of m-dinitrobenzene (m-DNB) and nitrobenzene to original bentonite in aqueous solution containing different electrolytes (i.e., KCl, NH₄Cl, CaCl₂ and Tetramethylammonium bromide (TMAB)) was studied. The sorption of m-DNB was greatly enhanced with the presence of KCl and NH₄Cl, while little influence was observed with CaCl₂ and TMAB, following the order of KCl > NH₄Cl TMAB, CaCl₂, or DI water. For nitrobenzene, sorption enhancement only occurred at high nitrobenzene concentrations in the presence of KCl, and the solute equilibrium concentration at inflexion point was lowered with increasing KCl concentration. These sorption enhancements were significantly promoted with the increase of electrolyte concentration. The salting-out effect is insufficient to account for the sorption enhancement by original bentonite with increasing KCl or NH₄Cl concentration. X-ray diffraction patterns of bentonite suspensions indicated that the sorption enhancement of m-DNB was attributed to the intercalation of K⁺ or NH₄⁺ into bentonite interlayer and then dehydration with m-DNB to form inner-sphere complexes, which caused previously expanded bentonite interlayers to collapse in aqueous suspension, thus further enhanced the interaction of phenyl with siloxane surface. In comparison, the sorption enhancement of NB is attributed to the formation of outer-sphere complexes with K⁺ at high solute-loadings (> 200–400 mg/kg). The sorption of m-DNB to initially modified TMA⁺-bentonite and K⁺-bentonite was almost the same as respective sorption to original bentonite in solution containing TMA⁺ and K⁺.