Sedimentation of Fe₃O₄ nanosized magnetic particles in water solution enhanced in a gradient magnetic field

The sedimentation dynamic of magnetite (Fe₃O₄) nanoparticles in water was investigated, both in the presence of a vertical gradient magnetic field and in the gravitational field only. The nanopowders (four samples with average particle diameter ranging from 16 to 84 nm) were prepared by a gas-condensation synthesis method. The sedimentation was monitored by measuring the light transmission coefficient k of the suspension as a function of time. The sedimentation process is of rather complex character for both the large and the small particles. Specifically, the light transmission reflects the different stages of the particle aggregation. Magnetite nanoparticles tend to aggregate into micron-sized aggregates which sediment rather rapidly in high concentrated suspensions (for example 5 g/L), even in the absence of a magnetic field. Gradient magnetic fields (for example H = 6 kOe, dH/dz = 1.6 kOe/cm) help to increase the sedimentation rate tremendously and reduce the total sedimentation time from several days up to several minutes—here for an average particle size of 16 nm. An effective removal of heavy metal pollutants (Cr, Cu, etc.) from water can be achieved using the optimal combination of the magnetite particle size, particle concentration in water suspension, and magnetic field strength and gradient.