Single and simultaneous adsorption of basic dyes by novel organo-vermiculite: A combined experimental and theoretical study

A multi-hydroxyl-containing gemini surfactant (1,3-bis(dodecyldimethylammonio)-2,2-dihydroxymethylpropane dibromide, BDHMP) was designed to modify Na-vermiculite (Na-Vt) for single/simultaneous adsorption of methyl blue/crystal violet (MB/CV). The superiority of dihydroxyl-containing spacer was unveiled in terms of both the modifier availability and cost-effectiveness of BDHMP-Vt. The adsorption capacities of BDHMP-Vt towards MB/CV reach as high as 531/956 mg/g in single system, which are interpreted by partition process and noncovalent interactions (OH-pi, CH-pi or pi-pi stacking interactions) based on density functional theory (DFT), adsorption kinetics, isotherms and thermodynamics. The dominant role of strong OH-pi interaction is proved by the higher values of Log K-OM than Log K-OW . Multi-molecular adsorption of MB/CV is happened on the adsorptive activate site due to secondary physical bonding mechanisms such as pi-pi stacking, which, further functions effectively in binary-dye system. The total mmol of adsorbed dyes on per unit mass of BDHMP-Vt are 2.84 mmol (or 2.95 mmol) in binary-dye system, higher than single system (1.52/2.31 mmol for MB/CV, respectively). Competitive (or synergistic) effect of MB to CV (or CV to MB) is revealed by influence strength factor (phi). Molecular simulation is conducted to calculate the adsorption isosteric heat and describe the process, which is suitable to predict the adsorption trends and corroborate with experimental results on a macro level. Results in this study throw lights on the exploration of organo-Vt adsorbents to removal basic dyes, in aspects of not only the optimization of modifier architecture, but also theoretical analysis methods with mechanic insights.