Adsorption of cadmium (II) on organic xerogel microspheres: effect of adding sepiolite or vermiculite and operating conditions on the adsorption capacity.

The organic xerogel (OX) was synthesized through sol-gel polymerization of formaldehyde and resorcinol in inverse emulsion using Na2CO3 as a catalyst. Meanwhile, OX containing sepiolite (OX-Sep) and vermiculite (OX-Ver) were prepared similarly to OX but adding clays during synthesis. All materials were mesoporous and presented spherical morphology, and the surface of these materials exhibited an acidic character because the concentration of acidic sites was higher than those of basic sites. Cd(II) adsorption from aqueous solutions onto OX, OX-Sep, and OX-Ver was examined, and the OX-Sep showed the highest adsorption capacity towards Cd(II) of 189.7 mg/g, being 1.5, 2, and 36 times higher than that of OX-Ver, OX, and Sep. The OX-Sep capacity for adsorbing Cd(II) was significantly lessened by decreasing the pH from 7 to 4 and raising the ionic strength from 0.01 N to 0.1 N. This trend was ascribed to electrostatic attraction between the Cd+2 in water and the negatively charged surface of OX-Sep. Besides, desorption studies at pH 4 showed that the average desorption percentage of Cd(II) adsorbed on OX-Sep was 80%. The characterization results and the effect of the operating conditions on the adsorption capacity proved that electrostatic attraction and cation exchange play a crucial role in the adsorption mechanism.

Adsorption capacity of different types of carbon nanotubes towards metronidazole and dimetridazole antibiotics from aqueous solutions: effect of morphology and surface chemistry.

The effect of surface chemistry and morphology of carbon nanotubes (CNTs) on their adsorption capacity towards dimetridazole (DTZ) and metronidazole (MNZ) antibiotics from water solutions was investigated in this work. The CNTs studied were single-walled carbon nanotubes (SWCNTs), CNTs doped with nitrogen (N-CNTs), multiwalled CNTs (MWCNTs), and MWCNTs functionalized with carboxylic groups (MWCNT-COOH). The experimental adsorption equilibrium data were best interpreted with the Redlich-Peterson (R-P) isotherm model. At T of 25 °C and pH of 7, the capacities of adsorption decreased as follows: SWCNT > MWCNT > N-CNT ≈ MWCNT-COOH, and the maximum capacities of SWCNT towards MNZ and DTZ were 101 mg/g and 84 mg/g, correspondingly. The SWCNT had the highest adsorption capacity because SWCNT presented the largest surface area, and was the only nanomaterial with a basic surface. The adsorption of both antibiotics on the CNTs was predominantly ascribed to the π-π stacking. The basic groups promoted the π-π stacking interactions and favored the adsorption capacity towards MNZ and DTZ. The capacity of SWCNT for adsorbing MNZ was lessening substantially by reducing the pH from 11 to 2, and the electrostatic interactions caused this trend. The Sheindorf-Rebuhn-Sheintuch adsorption model interpreted the data for the competitive adsorption of DTZ and MNZ on SWCNT adequately.