Materiais híbridos nanoestruturados sintetizados a partir da funcionalização de esmectitas para imobilização de espécies inorgânicas e orgânicas

Abstract

The present work was aimed at synthesizing nanostructured hybrid materials derived from the functionalization of smectite clays with ligands containing thiol (-SH) and amino (-NH2) groups for application in adsorption processes. Two montmorillonite samples (a commercialAmerican clay-Cloisite-Na and an in nature Brazilian clay from Campina Grande, PB) and one synthetic, commercial clay (Laponite) were investigated. The modification routes were based on the grafting reaction between hydroxyl groups present on clay surface and the hydrolyzable alkoxy group of the (3-mercaptopropyl)trimethoxysilane or (3-aminopropyl)triethoxysilane in dry toluene or alcohol solution under reflux. The present work showed that the proposed modification routes were effective for the grafting of the thiol and amino groups in the clay structure. The functionalization degree varied according to the clay sample, acidpre-treatment, modifier type and dispersing media. A detailed characterization of the materials indicated that the functionalization does not modify the original structure of the clays but altered their original hydrophilic nature to hydrophobic. The smectite clays showedan immobilization capacity comparable to those reported for silica gel. The immobilization capacities of the mercaptopropyl groups were calculated as 1.76 mmol/g, 1.45 mmol/g and 1.2 mmol/g for the Brazilian clay, Cloisite and Laponite, respectively. With aminopropyl, theimmobilization capacities were determined as 2.0 mmol/g and 1.5 mmol/g for the Cloisite and Laponite samples, respectively. The specific surface areas and pore volumes of all clay samples decreased significantly upon grafting. The accessibility to the reactive SH groupswas successfully determined by using the Volhard method. It ranged from 60% to 75%,depending on the organic content, for the H-montmorillonite samples and to 100% for the SH-laponite sample. Similar values of accessibility were observed with the amino-modified clays. The thiol-functionalized montmorillonite samples showed average binding capacities 160% and 1100% higher, respectively, with respect to Cd(II) and Ag(I) to those obtained with the ungrafted material. These results support a mechanism of adsorption involving primarilyion complexation by the thiol groups (specific) instead of cation exchange (unspecific). Adsorption experiments have demonstrated the selectivity of a thiol-modified synthetic clay by As(III) species. The highest loading capacity (22.3 mg/g) and speciation has occurred at pH 4-5, where the trivalent species are present in its neutral form (H3AsO3). The proposedmodification processes are suggested for the preparation of novel adsorbent materials with controlled selectivity and specificity and therefore with good potential for separation and preconcentrationpurposes.