Comparative adsorption of polycylic aromatic compounds on organo-vermiculites modified by imidazolium- and pyridinium-based gemini surfactants

Abstract

Comparative adsorption of 2-naphthol (NP) and naproxen (NPX) was carried on two organo- vermiculites (DDBI-Vt and DDBP-Vt) functionalized by two gemini surfactants with different headgroups (imidazolium and pyridinium) after determining their structures and properties by a variety of characterizations. The maximum adsorption amounts of NP were 206.82 and 200.33 mg/g on DDBI-Vt and DDBP-Vt, respectively, which were 74.48 and 52.99 mg/g for NPX on DDBI-Vt and DDBP-Vt, respectively under the conditions of pH = 6 at 25 °C. The key influence factors (e.g. modifier dosage, contact time, pH, concentration of adsorbate, and temperature) were studied, combining with the kinetics, isotherms and thermodynamics to explore the adsorption mechanism. Adsorption results manifested that DDBI-Vt was much superior to DDBP-Vt for removal of NP and NPX. The adsorption data were well correlated by pseudo-second-order kinetic model, Freundlich and Redlich Peterson isotherms. Thermodynamic parameters certified that all adsorption processes were exothermic and randomness-decreased physical adsorption involving in electrostatic attraction, hydrophobic and π-π interactions. The structure-activity relationships can be given: (i) for the difference of modifiers, comparing with the pyridinium ring of DDBP-Vt, imidazolium ring of DDBI-Vt were more π-electron-deficient which showed a stronger π-π interaction with naphthalene ring; (ii) in view of the π-electron density discrepancy between NP and NPX, the strength of π-π interaction betwixt NP and N-heteroaromatic ring was stronger than that of NPX; (iii) steric hindrance of adsorbate molecules can affect the strength of π-π interaction and adsorption rate. This work aims at comparing the adsorption behavior from the structures of the adsorbate and the adsorbent, which displays the advantage of imidazolium-based organo-Vt for removing electron-rich pollutants.