Silica-based aerogels as adsorbents for phenol-derivative compounds

• Highly porous silica-based aerogels with tailored degrees of hydrophobicity.
• Adsorbents with promising removal capacity for several phenol-derivatives.
• Pseudo-second order kinetics and Langmuir form isotherm for adsorption of SDBS.
• Important role of the hydrophobic interactions in the adsorption process.
• Increased adsorption of 4-chlorophenol exhibited by aerogels with β-cyclodextrin.

The simultaneous presence of hydrophobic and hydrophilic chemical moieties in phenolic compounds imply some challenges in the choice of the more selective absorbent for their removal from industrial wastewaters. In this work, the adsorption capacities of chemically tailored aerogels/xerogels towards several phenolic compounds – phenol, p-cresol and 4-clorophenol – were evaluated to study the various kinds of interactions in the adsorption process as a function of the functional groups in the gel structure. The surfactant sodium dodecylbenzenesulfonate (SDBS) was also tested as an adsorbate due to its amphiphilic character. Highly porous silica-based aerogels/xerogels with different degrees of hydrophobicity/hydrophilicity were synthesized, by using a combination of methyltrimethoxysilane (MTMS) and tetramethyl orthosilicate (TMOS) precursors in different proportions, in order to provide a variable number of methyl and hydroxyl groups in the aerogels structure. β-cyclodextrin functionalization of the aerogels was also tested due to the recognized suitability of this macrocycle cavity for phenol retention. In this way, a systematic and innovative study was carried out based on a wide range of contact angles of the adsorbent matrices and different hydrophobic/hydrophilic nature of adsorbates, in order to have an increased understanding of the dominant adsorption interactions of phenolic compounds with these silica-based adsorbents. The sol–gel synthesis followed a one-step NH4OH-catalyzed procedure and the drying of the gels was accomplished by supercritical fluid drying/extraction with CO2, to obtain aerogels, and evaporative drying, to produce xerogels. When using SDBS as adsorbate in the xerogels with intermediate wetting behaviour, good adsorption capacity is achieved and the adsorption kinetics is well described by a pseudo-second order model. The aerogels with intermediate and high contact angles show higher adsorption capacities, with favourable isotherms, for the phenolic compounds with higher hydrophobicity, which shows the important role of hydrophobic interactions in this process. The best results were obtained for the aerogel functionalized with β-cyclodextrin when adsorbing the 4-chlorophenol, proving the positive effect of the presence of the β-cyclodextrin hydrophobic cavity in the adsorption of this highly hydrophobic pollutant.

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