Effects of the counterion on dielectric spectroscopy of a montmorillonite suspension over the frequency range 105–1010 Hz

Controversial Issues Related to Dye Adsorption on Clay Minerals: A Critical Review

This critical review points out the most serious and problematic issues to be found in the literature on the adsorption of dyes on clay minerals. The introduction draws attention to the fundamental problems, namely the insufficient characterization of adsorbents, the influence of impurities on the adsorption of dyes, and the choice of inappropriate models for the description of the very complex systems that clay minerals and their systems represent. This paper discusses the main processes accompanying adsorption in colloidal systems of clay minerals. The relationship between the stability of the colloidal systems and the adsorption of dye molecules is analyzed. The usual methodological procedures for determining and evaluating the adsorption of dyes are critically reviewed. A brief overview and examples of modified clay minerals and complex systems for the adsorption of organic dyes are summarized. This review is a guide for avoiding some faults in characterizing the adsorption of organic dyes on clay minerals, to improve the procedure for determining adsorption, to evaluate results correctly, and to find an appropriate theoretical interpretation. The main message of this article is a critical analysis of the current state of the research in this field, but at the same time, it is a guide on how to avoid the most common problems and mistakes.

Complex dielectric permittivity of metal and polymer modified montmorillonite

Owing to its high surface area and high surface charge density, clay, either contaminated with heavy metal ions or modified with organic additives as barrier materials, is difficult to assess and monitor. Complex dielectric permittivity (κ*) showed promising potential in tackling the above issues. In this study, the complex dielectric permittivity (κ*) of clays modified with a surfactant, four polymers, and four metal ions was measured at frequencies from 0.2-20 GHz. With the addition of polymer and metal ions, increasing frequency caused a slight decrement in real permittivity but a significant decrement in effective permittivity. A modified linearity polynomial equation, which considered the particle conductivity, was developed to fit the relationship between effective conductivity (σeff) and porosity ranging from 0.7 to 1.0. A three-dimensional Cole-Cole plot (κ'-κeff″-w) shows Cole-Cole circle expansion at higher water content. The resonance strength of modified clays was observed to increase with water content, which suggests that the number of water molecules in the diffuse layer of polymer or metal ions saturated clay increased. However, sorbed polymer and metal ions have an insignificant influence on the resonance time τ_s and stretching exponent 1-α. κ* can provide nondestructive characterization of metal or polymer modified clays.