Magnetic sedimentation of nonmagnetic TiO2 nanoparticles in water by heteroaggregation with Fe-based nanoparticles

A new approach to remove TiO2 nanoparticles from water is studied. Composite Fe3O4-SiO2 and Fe-C-SO3H nanoparticles with magnetic cores (5 nm) were designed to be used as flocculants for magnetic sedimentation of nonmagnetic TiO2 nanoparticles in water. The sedimentation dynamics of heterogeneous water suspensions (starting solid phase concentration of 100 mg/l) of (TiO2)(1-x)(Fe3O4-SiO2), (x = 0.15-0.55) and (TiO2)(1-x)(Fe-C-SO3H)(x) (x = 0.01-0.17) was studied in a gradient magnetic field (B = 0.3 T, dB/dz < 0.16 T/m) for pH = 3-6. The NMR relaxometry and the UV - spectrophotometric methods were used to monitor the residual concentration of Fe3O4-SiO2 , Fe-C-SO3H and TiO2 nanoparticles in water after magnetic sedimentation. The hydrodynamic sizes of the heteroaggregates (d(h)) depend on the pH value and on the mass content of the magnetic component, x. For the mixed (TiO2)(1-x)(Fe3O4-SiO2)(x) water suspensions the maximum of d(h) = 5 mu m and the fastest sedimentation are reached for x = 0.35-0.5 at pH = 3, 4, and for x = 0.2 at pH = 5. At this regime, the highest efficiency of TiO2 particles removal from water, that is, 99.7% for 30 min, was observed. For the mixed (TiO2)(1-x)(Fe-C-SO3H)(x) water suspensions the largest heteroaggregates and the fastest sedimentation were observed for the smaller mass content of the magnetic flocculant, x = 0.03-0.05 at pH = 3, 4 and for x = 0.03 at pH = 5. However, by adding Fe-C-SO3 H nanoparticles flocculant the achieved minimum residual concentration of TiO2 nanoparticles in water is two orders of magnitude higher. These results are discussed by assuming a specific distribution of Fe - based nanoparticles by heteroaggregation with TiO2 nanoparticles.