Nonlinear sorption of phenols and anilines by organobentonites: Nonlinear partition and space limitation for partitioning

Abstract

Organobentonites, i.e., bentonites coated with surfactants such as cetyltrimethylammonium (CTAB), are superior and low-cost sorbents for removal of organic contaminants from wastewater. Nonlinear sorption of polar organic compounds such as phenols and anilines by organobentonites were widely observed and interpreted by adsorption mechanism. However, in this study, it was observed that the nonlinear sorption of phenols and anilines by CTAB coated bentonites (CTAB-bentonites) should be attributed to nonlinear partition mechanism with the additional space limitation in CTAB-bentonites for nonlinear partitioning, rather than adsorption mechanism. This nonlinear partition mechanism is supported by that (i) organobentonites is a partition medium, identified by the linear isotherms of polycyclic aromatic hydrocarbons (PAHs) and nitrobenzenes; (ii) sorption coefficients (logKd), the ratio of adsorbed amount (qe) to equilibrium concentration (Ce), and Dubinin-Ashtakhov (DA) model fitted sorption capacity (logQ0) of organic compounds, by a given CTAB-bentonite, are positively correlated with their octanol-water distribution coefficients (logKOW) and solubility in octanol (logSo) respectively; (iii) logKd and logQ0 of a given organic compound by CTAB-bentonites are positively correlated with organic carbon contents (foc) of CTAB-bentonites, but not specific surface area. Specific interaction (i.e., hydrogen-bonding interaction), in addition to van der Waals force, is responsible for the nonlinear partitioning of phenols and anilines into CTAB-bentonites, because of the positively linear relationship between DA model fitted sorption affinity (E) and hydrogen-bonding donor parameter (αm) of organic compounds. These results could help the recognizing of the nonlinear sorption behaviors of organic compounds by organobentonites and promote their environmental applications in wastewater treatment.