Understanding of the mechanism of crystal violet adsorption on modified halloysite: Hydrophobicity, performance, and interaction

Halloysite was processed at 600 °C and then by acid leaching with HCl solutions of different concentrations, i.e. 0.5, 3 and 5 N (H600-xN; x = 0.5, 3 or 5). The resulting materials underwent chemical, textural, and laser diffraction analyses and were used in crystal violet (CV) adsorption. Bath experiments were conducted to evaluate the parameters influencing adsorption. A hydrophobicity study by adsorption of water/toluene and a spectroscopic investigation by FTIR and Raman were conducted, to understand the interaction mechanism. The affinity for CV is as follows: H600-0.5N (115 m²g^-1) > H600-3N (434 m²g^-1) > H600-5N (503 m²g^-1) > H600-0N (61 m²g^-1). The maximum adsorption of H600-0.5N would be explained by optimal hydrophilic and hydrophobic properties. Dealumination leads to the creation of more silanols responsible for hydrophilicity. Dehydroxylation at 600 °C combined with dealumination would induce a partial transformation of silanols into siloxanes which are responsible for organophilicity. The CV-H600-0.5N interaction implies two mechanisms: hydrophobic interactions and hydrogen bond. This study focused on hydrophobic interaction as a non-negligible component governing the interaction of organic contaminants with 1:1 clay minerals, while it was not sufficiently considered in the scientific literature.