New principals on the adsorption of alkyl compound by Mg–Al oxide: Adsorption kinetics and equilibrium studies

The uptake of ethylsulfate (ES⁻), hexylsulfate (HS⁻), and dodecylsulfate (DS⁻) anions by Mg–Al oxide in aqueous solution was examined through adsorption equilibrium and kinetic modelling. The removal of ES⁻, HS⁻, and DS⁻ by Mg–Al oxide proceeded as diffusion-controlled reactions exhibiting Langmuir-type adsorption isotherms. The maximum amounts of adsorption are ordered as DS⁻ > HS⁻ > ES⁻. Mg–Al oxide preferentially reacts with the organic anion with lower charge density, because the effect of hydrophobic interactions in these cases is stronger than that of electrostatic interactions. In the first stage of reaction, Mg–Al oxide, which is prepared by the thermal decomposition of Mg–Al layered double hydroxides (LDHs) intercalated with CO₃²⁻ (CO₃·Mg–Al LDH), rehydrates and combines with alkylsulfate to reconstruct the LDH structure. Then the alkylsulfate anions intercalated in the interlayer of the LDH attracted other anions in the solution via hydrophobic interactions, thereby promoting their removal by Mg–Al oxide. DS⁻ was removed with the highest efficiency followed by HS⁻, because the hydrophobic attraction between DS⁻ anions is stronger than that between HS⁻, which is stronger than that between ES⁻. At low initial concentrations (C₀), the intercalated alkylsulfates are likely oriented parallel to the brucite-like host layers of Mg–Al LDH. With increasing C₀, pairs of alkyl chains in DS⁻ and HS⁻ may overlap due to the hydrophobic interaction, while the alkyl chains in DS⁻ are likely arranged perpendicular to the host layers to form bilayers.