Correlations Between Structures And Sorption Capacities Of Organobentonites And Their Molecular Interaction Mechanisms

Organobentonites have many potential applications in environmental protectionbecause of their excellent sorption capacities toward hydrophobic organic pollutants.Some problems need to be further resolved before organobentonite can be widelyused in wastewater treatment,such as improving their sorption capacities especiallyfor water soluble organic contaminants and reducing their costs.To resolve theseproblems,it is necessary to understand the correlation of structural characteristics andsorption capacities of organobentonites.In this dissertation,adsorption characteristicsand mechanisms of siloxane surfaces of bentonites were investigated.And aqueoussorption of organic contaminants to a typical organobentonite was characterized usingLinear Solvation Energy Relationship (LSER) approach.A unique organobentonitewith great sorption capacity toward water soluble aromatic contaminants wasprepared.The main original conclusions of this work are as follows:(1) Adsorption mechanisms of organic comtaminants on the siloxane surfaces ofTMA⁺-bentonites are proposed.Reduction of the layer charge and saturation ofbentonite interlayers with TMA⁺ modify the interlayer microenvironments,whichdramatically promote adsorption of NOCs.Specific mechanisms (i.e.,stericrestriction and phenyl-effect) control the adsorption of NOCs onto internal siloxanesurfaces of TMA⁺-bentonites from water.Adsorption mechanism on the interlayer ofTMA⁺-bentonites varies with solute-loading,from polarity-selective toaromaticity-preferable.Significant contribution of phenyl-effect between adsorbedsolutes to aromatics adsorption on TMA-bentonites is found.Solvent polarity effecton the aggregation of TMA⁺-bentonites and aniline adsorption demonstrated that theexternal siloxane surfaces (generally being omitted) are actually exploited to favoradsorption in nonpolar solvent.(2) Sorption mechanism of organic contaminants onto a typical organobentonite(a bentonite modified by cetyltrimethylammonium (CTMA⁺)) from water wascharacterized using a linear solvation energy relationship (LSER) model.The fitted LSER equation:logK_d^cal=-0.464+1.694R₂+2.277V_x-0.435Ï€₂^H+0.591∑α₂^H-1.532∑β₂^H wasobtained by a multiple regression of the partition coefficients of 16 probe solutesagainst the solvation parameter of the solutes.The coefficients of the LSER equationshow that the sorption of organic solutes onto CTMA-bentonite is dominated byÏ€-/n-electron pair donor-accepter interaction,cavity/dispersion interactions andhydrogen-bond donor effect.The influence of the properties of organic compounds ontheir sorption onto CTMA-bentonite is recognized to be dominated by theÏ€-/n-electron pair donor-accepter interaction.This conclusion was also evidenced by¹H-NMR.The greater chemical shift change of CTMA⁺ with 1-nitroaniline thannitrobenzene,1-naphtylamine than N,N-dimethylaniline and aniline thancyclohexanone in ¹H-NMR spectroscopy demonstrated the significant role ofÏ€-/n-electron interaction in organic contaminants sorption onto CTMA-bentonite.(3) A unique organobentonite (65BTMA) was synthesized by intercalatingbenzyltrimethylammonium cation (BTMA⁺) into the interlayer of a reduced-chargebentonite with cation exchange capacity (CEC) of 65 cmol/kg.Phenol,aniline andtoluene were used as model compounds of water soluble aromatic contaminants.Theremoval efficiencies by 65BTMA were achieved at 83.3%,89.2% and 97.3%respectively,at the initial concentration of 20 mg/L.To 65BTMA,sorptionmagnitudes of aromatic contaminants were much greater than that of aliphaticcompounds with similar size;and dramatically higher than those to otherorganobentonites at low pollutant concentrations.These observations revealed thatthe strong phenyls interactions contributed significantly to sorb the aqueous solublearomatic contaminants to 65BTMA (＞90%),and which favored to design uniquelypowerful sorbents.