Abstract
Interactions between cationic dyes and negatively charged mineral surfaces have long attracted great attention from clay mineralogists, environmental scientists, and chemical engineers. In this study, the interactions between a cationic dye toluidine blue (TB) and palygorskite and sepiolite were investigated under different experimental conditions. The results showed that in addition to cation exchange, the specific surface area (SSA) of the minerals, particularly the formation of dimer molecules on the surface of both minerals, also accounted for the much higher TB uptake in comparison to their cation exchange capacities (CEC). The TB molecules were sorbed to the external surfaces, as no d-spacing expansion was observed in X-ray diffraction analyses. FTIR analyses showed strong interactions between the C=N or N-(CH3)2 group and the mineral surfaces, suggesting net electrostatic interactions if either of these functional groups bears a positive charge. Results from molecular dynamic simulations suggested dense monolayer TB formation on palygorskite because of its limited SSA and large CEC values. In comparison, a loosely dimeric formation was revealed on sepiolite for its large SSA and limited CEC values. Therefore, palygorskite is a better carrier for the sorption of cationic dyes, as evidenced by Maya blue paintings.
Highlights
The interactions between a cationic dye toluidine blue and fibrous clay minerals palygorskite and sepiolite in aqueous solution were studied under different experimental conditions
The isotherm study showed that the toluidine blue (TB) sorption capacity was much higher than the cation exchange capacity (CEC) values of the minerals, for SEP
It is the dimeric formation of TB on both minerals that contributes to the significantly higher
Summary
Clay minerals, due to their large specific surface area (SSA), high cation exchange capacity (CEC), and bulk quantities, are of great potential in practical applications in modern society. As such, they have been studied extensively as sorbents for sorptive removal of different types of contaminants from water. Those of a cationic nature could be effectively removed by clay minerals via cation exchange processes. Water-soluble dyes could be cationic, zwitterionic, or anionic in nature. More than 10,000 types of dyes are used in different industries [1]. As different dyes have different physico-chemical properties, active approaches to remove dyes from solution include physical [2], chemical [1], and biological processes [3], and could be achieved via either sorption or degradation
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