Magnetic sedimentation and aggregation of Fe3O4@SiO2 nanoparticles in water medium

•Vertical gradient magnetic field enhances sedimentation of Fe3O4@SiO2 nanoparticles in water.•The sedimentation related to nanoparticle aggregation can be tuned by salt additions.•The results are valuable for developing of water purification technologies.

The sedimentation dynamics of Fe3O4@SiO2 nanoparticles (20/30 nm) in water with additions of Na2SO4, CaCl2, NaH2PO4 salts with concentration of (5–100) mM were studied in a vertical gradient magnetic field (B1 ⩽ 0.3 T, dB/dz ⩽ 0.13 T/cm). The presence of magnetic field leads to nanoparticle sedimentation time decreasing from 103 h to 101 h and to 10−2 h when the salts are added. The results are interpreted on the basis of aggregate formation caused by electrostatic, steric and magnetic inter-particle interactions in water medium.While the stability of Fe3O4@SiO2 nanoparticles in pure water is dependent on steric inter-particle interactions involving polymeric Si–O groups on the surface of the composite particles, the presence of Na+ and Ca2+ cations in water leads to a decrease of electrostatic repulsion because the charge on the nanoparticle surfaces decreases. Magnetic moments, induced in the magnetic field, contribute to the enlargement of the aggregates and, in interacting with the magnetic field gradient, facilitates a quicker sedimentation of the suspension.The data obtained are useful for the development of methods aimed at the magnetic separation of magnetic nanosorbents in water purification technology.