The effect of layer charge origin and density, type of interlayer cations, and occupancy of the octahedral sheet of smectites on the adsorption of pyocyanin

Abstract

Inactivation of pyocyanin, a virulence compound produced by Pseudomonas aeruginosa, has been proposed as a novel alternative approach to combat the multi-drug resistant Pseudomonas aeruginosa infections. It has been shown that montmorillonite was able to bind pyocyanin. Yet, the adsorption mechanism and the influence of the structural properties of smectites — type of interlayer cation, charge density, source of charge, and occupancy of the type of octahedral sheet — on the adsorption of pyocyanin is not clear. Batch adsorption experiments, variable-temperature X-ray diffraction, UV spectroscopic measurements, and electron dispersive spectroscopy analysis were conducted to investigate the role of several smectite structural properties on pyocyanin adsorption — affinity (K), capacity (Qmax), the adsorption mechanism, and the transformation of pyocyanin on smectite surface. The results indicated that interlayer adsorption of pyocyanin in smectite occurred by two types of adsorption mechanisms. 1) protonation of zwitterionic pyocyanin (PYO) and ion-exchange of PYOH+ for interlayer cations. This mechanism was dominant in alkali and alkaline-earth metal smectites. 2) direct coordination of zwitterionic PYO with interlayer cations. This mechanism was dominant for transition/heavy metal smectite. Among the structural factors investigated, 1) the type of interlayer cation showed the greatest effects on pyocyanin adsorption. Except for Cu2+, Qmax was similar for all the cation-saturated smectites (∼0.30–0.56 mol/kg) while the affinity K was higher in monovalent cations compared to the divalent cations. The Cu-smectite recorded the highest Qmax of ∼0.70 mol/kg. 2) smectites with different layer charge densities showed similar Qmax but the K was higher in low charge density smectite suggesting the importance of hydrophobic domain for pyocyanin adsorption. 3) It appeared that the layer charge source or the types of octahedral occupation did not show distinct effects on pyocyanin adsorption, but the presence of accessory minerals may mask the effects.