Elimination of TiO₂ nanoparticles with the assist of humic acid: influence of agglomeration in the dissolved air flotation process

With recent advances in nanotechnology, environmental and health consequences of nanomaterial disposal merit close attention. In the search for environmentally-friendly reagent, this study investigates the use of humic acid (HA) as an assist of dissolved air flotation (DAF) in the TiO₂ nanoparticle (TNP) elimination. To determine mechanisms of TNPs interacting with HA, surface modification experiments were firstly carried out; thereafter, laboratory scaled DAF tests were applied to remove TNPs with HA assisting. Results of surface modification experiments showed that the zeta potential of TNP suspension system had a reversal trend due to counter ions of TNP and anions offered by the HA stock solution. The surface modified suspension was not easy to restabilize because of the close combination of TNPs and HA through sphere linkages or hydrogen-bonded surface complexes. Agglomeration took place more readily along with increasing HA concentration in the optimum dosage range (7.8-9.15 mg/L DOC). The flotation performance revealed that HA could improve the DAF efficiency in the optimum dosage range of HA. The interaction between TNPs and HA (Na(+)-humate), including surface charge neutralization (electrostatic interactions), sphere linkages or hydrogen-bonded surface complexes, hydrophobic interactions, and van der Waals interactions, played dominant roles.