Effect of heavy metals on the surface free energy and zeta potential of volcanic glass: implications on the adhesion and growth of microorganisms

We studied the surface properties of eruptive material from Deception Island (South Shetland Islands, Antarctica). The surface free energy and zeta potential were analyzed in obsidian and pumice stones immersed in different electrolytes (NaCl, BaCl₂, AlCl₃, VCl₂, PbCl₂, CdCl₂, FeCl₃, and CrCl₃). The results show that obsidian has a monopolar character. The value of the obsidian electron-donor component increased in the presence of heavy metals, reaching 61.0 mJ/m² in the case of CrCl₃. This means that the heavy metals transform the surface of obsidian into a hydrophilic material in almost all cases, except in the case of FeCl₃, which makes the obsidian surface hydrophobic. The pH variation of the liquid phase has a significant influence on the zeta potential, not only changing its value, but also the sign of the surface electric charge. A significant result is that a variation of two in pH (between 5 and 7) changes the sign of the charge from positive to negative or vice versa, depending on the electrolyte used. In general, the presence of electrolytes changes the surface properties of solid materials and consequently the adhesion and growth of microorganisms on coastal rocks.