Nonlinear Sorption Mechanisms Of Polar Organic Compounds Phenols/Anilines By CTAB-bentonites

Organobentonites are widely used in wastewater treatment because of their low cost and superior sorptive properties.Thus,it is significant to explore the sorption behavior and the sorption mechanism of organic compounds by organobentonites.In literature,sorption isotherms of apolar organic compounds?e.g.,benzene and toluene?by organobentonites were linear,while sorption isotherms of polar organic compounds?e.g.,phenols and anilines?by organobentonites were nonlinear.The linear isotherms of organic compounds by the sorbent should be attributed to linear partition mechanism,i.e.,a process similar to the dissolving of organic compounds into an organic solvent,where the intercalated surfactants in organobentonites is a partition medium like organic solvent.Adsorption mechanism on the exposed hydrophobic siloxane surface of organobentonites was employed to explain the nonlinear isotherms of polar organic compounds by organobentonites.However,in this study,by investigating the sorption,desorption and competitive sorption behaviors of phenols,anilines,PAHs and nitrobenzenes by CTAB-bentonites prepared with various CTAB exchange amounts,we observed that the nonlinear sorption of phenols/anilines by CTAB-bentonites should be attributed to nonlinear partition and space limitation,rather than adsorption,which give new insights into the nonlinear sorption mechanisms of polar organic compounds by organobentonites.The concrete results of this study were as follows:?1?Nonlinear sorption of phenols/anilines by CTAB-bentonites should be attributed to nonlinear partition derived from hydrogen-bonding interaction and space limitation,rather than adsorption.Concrete results are:?i?surface area of CTAB-bentonites was small?SA-N₂?8.4 m²/g,SA-CO₂?59.8 m²/g?,while nonlinear sorption capacity of phenols/anilines by CTAB-bentonites was up to 300 mg/g;?ii?linear isotherms of PAHs/nitrobenzenes and nonlinear isotherms of phenols/anilines by CTAB-bentonites were observed;?iii?sorption coefficient log K_d was positively correlated with octanol-water distribution coefficient log K_OW and organic carbon content f_oc of CTAB-bentonites(log K_d=S×log K_OW+I,I=X×f_oc+Y),implying that CTAB-bentonites was a partition medium;?iv?sorption capacity log Q⁰ was positively correlated with chemicals octanol solubility log S_o?log S_o<2.80?and organic carbon content f_oc of CTAB-bentonites(log Q⁰=J×f_oc+K),in consistent with the characteristic of partition;?v?space limitation made an effect on the nonlinear partitioning of polar organic compounds with log S_o>2.80 into CTAB-bentonites;?vi?sorption affinity E was positively correlated with hydrogen-bonding donor ability?_m of organic compounds?E=B×?_m+C?,which indicated that hydrogen-bonding interaction in addition to Van der Waals force was responsible for the isotherm nonlinearity of phenols/anilines by CTAB-bentonites.?2?Desorption hysteresis was not observed for polar?i.e.,phenols&anilines?and apolar?i.e.,PAHs&organic compounds?organic compounds by 100CTAB,while competitive sorption of phenols,anilines and PAHs was observed.Hydrogen-bonding interaction and space limitation are responsible for the competitive sorption between organic compounds by 100CTAB.Competition of 2,4-dichlorophenol decreased the sorption capacity of 4-chlorophenol by 100CTAB because both hydrogen bond competition and space competition exist in the nonlinear partitioning of 2,4-dichlorophenol and 4-chlorophenol into 100CTAB.Competitive sorption was not observed between 4-chlorophenol and 1,3-dinitrobenzene since space competition of1,3-dinitrobenzene to 4-chlorophenol was negligible.Competition of phenanthrene for interlayer available space decreased the sorption capacity of 4-chlorophenol by100CTAB at relatively high concentration.Competitive sorption was not observed between 1,3-dinitrobenzene and 1,3,5-trinitrobenzene.