Role of Humic Acid in Enhancing Dissolved Air Flotation for the Removal of TiO2 Nanoparticles

The particle separation efficiency by flotation sharply decreases or even completely fails when the diameter of dispersed particles falls into the nanoscale. In the present laboratory work, humic acid was used to enhance the removal of TiO2 nanoparticles from suspension in a chemical coagulant-free dissolved air flotation process. Without humic acid, merely 63.8% of TiO2 nanoparticles were removed. For the humic acid-assisted dissolved air flotation, the pH of humic acid solution significantly influenced the removal efficiency: more than 90% of nanoparticles could be separated when the pH of the humic acid stock solution was acidic; however, the basic solutions resulted in rather poor performance. In the acidic solution, the fiberlike humic acid might form colloids through the attraction between hydrophobic moieties. They possibly acted as a fishnet and trapped nanoparticles, leading to the great measured bubble–particle attachment efficiency. In all the effluents, a low residual dissolved organic carbon was observed, revealing a good participation of humic acid in flotation. Moreover, a higher air-to-solid ratio could improve the nanoparticle elimination by offering a larger surface area of air bubbles. The fractal dimension of flotation flocs demonstrated that the aggregates with compact structure took greater advantage in the flotation separation of nanoparticles.