Particle-induced nanobubble generation for material-selective nanoparticle flotation

Abstract

The presented work investigates particle-induced nanobubble generation in order to extend the applicability of material-selective pressure flotation towards nanoscale materials. During a gas treatment procedure, filtered compressed air in the range of 0 bar–2 bar overpressure was introduced into colloidal suspensions in ultra-purified water until saturation was obtained. Nanobubble formation in the suspension occurred after slow depressurization with a gas vent. Suspensions of platinum nanoparticles and polystyrene beads were used to examine the effects of particle hydrophobicity and roughness. Using nanoparticle-tracking-analysis (NTA), the evaluation of particle size and scattered light intensity showed the occurrence of nanobubbles, both, as a separate phase (bulk nanobubbles) and attached to the particles’ surface. The sharply separated nanobubble peak was observed in number density NTA plots of gas treated platinum nanoparticles. The size of the hydrophilic platinum particles remained unchanged. In contrast, for hydrophobic polystyrene particles, the hydrodynamic diameter increased during a gas treatment procedure. The experimental results are understood, on the one hand, as the nucleation of bulk nanobubbles in the cavities of hydrophilic platinum particles and, on the other, as surface-growth of nanobubbles attached to the hydrophobic polymer particles.