Tuning the hydrophobicity of mesoporous silica materials for the adsorption of organic pollutant in aqueous solution

Abstract

The ability of various as-prepared and organically modified MCM-41 and HMS mesoporous silica materials to behave as efficient adsorbents for organic pollutants in aqueous solution was investigated by using different surface functionalization procedures, so as to adjust their hydrophilic/hydrophobic balance. The hydrophilic and organophilic properties of the parent silica materials and their corresponding surface functionalized counterparts were studied by using water and toluene adsorption isotherms. Their quantification was determined by the hydrophobic static index value (HI static), as well as by the silanol and organic group densities after the functionalization step. A clear correlation could be found between the HI static values and either the superficial silanol density, or the amount of organic moieties grafted or incorporated to the silica materials. For the highly organically functionalized samples, the residual superficial silanol groups (<50%) are sufficiently isolated from each other so as to prevent the water capillary condensation within the pores, thereby leading to an increased hydrophobic character of the resulting mesoporous silica. Those hydrophobic samples, for which the water liquid meniscus formation within the mesopores was minimized or avoided, exhibited a storage capacity for an organic pollutant (N,N-diethyl-m-toluamide, DEET) in aqueous solution more than 20 times higher than that of the corresponding unmodified sample, independently of the silica nature (MCM-41 or HMS). For all calcined and silylated samples, the DEET maximum adsorption capacities determined by the Langmuir model could be correlated with the silica surface coverage by trimethylsilyl groups and thus with the remaining silanol amount.