Chapter 15 - Adsorption of Surfactants, Dyes and Cationic Herbicides on Sepiolite and Palygorskite: Modifications, Applications and Modelling

Abstract

Abstract Adsorption of organic molecules, such as surfactants, dyes and cationic herbicides on sepiolite and palygorskite, is reviewed with an emphasis on modelling and applications. The experimental procedures employed in the reviewed studies included adsorption, XRD, SEM, HRTEM, FTIR, zeta potential and thermal analysis measurements. The surfactants whose adsorption and interactions with the clay minerals were studied included anionic, neutral and cationic ones. These two clays are structurally similar, having a similar content of zeolitic and bound water and IR absorption pattern; the differences include a larger b axis and a larger surface area per weight, a larger fraction of MgO and smaller fractions of Al2O3 and Fe2O3 in the case of sepiolite. Modelling of adsorption of organic cations by the clay minerals considered the electrostatic equations of Gouy–Chapman and specific binding in a closed system. In the case of monovalent organic cations, the model considers neutral complexes formed from binding of a cation to a negatively charged surface site, and positively charged ones which are formed by a binding of another cation to the neutral complex. Both clay minerals have an abundance of neutral silanol (Si OH) sites on the external surfaces. The number of these N sites exceeds by several-fold the number of negatively charged sites, that is, the CEC (cation-exchange capacity). The model accounts for the ability of charged organic monovalent cations to adsorb to neutral binding sites on the silicate layer. The adsorbed amounts of two cationic dyes, methylene blue and crystal violet were more than fourfold of the CEC. In contrast, the largest adsorbed amounts of the divalent organic cations paraquat, diquat and methyl green on sepiolite were between 100% and 140% of its CEC. This outcome suggested that these divalent organic cations do not interact with the neutral sites of sepiolite. This assumption was confirmed by IR measurements, which did not show changes of the peaks arising from the vibrations of external Si OH groups of the clay, when the divalent organic cations were added, unlike changes which were clearly observed when monovalent organic cations were added to the mineral. The model succeeded to simulate and yield predictions for adsorption of organic cations by sepiolite and palygorskite.