Coating superparamagnetic iron oxide NPs with SiO2 has been established in order to confer stability in acidic media. Acid stability tests were carried out between pH 1 and pH 7 to determine the effectiveness of the SiO2 passivating layer to protect the magnetic Fe3O4 core. Transmission Electron Microscopy (TEM) and zeta potential measurements have shown that uncoated Fe3O4 NPs exhibit rapid agglomeration and dissolution when exposed to acidic media, moving from a zeta potential of - 26 mV to a zeta potential of + 3 mV. In contrast, the SiO2 coating of the Fe3O4 NPs shows a very high degree of stability for over 14 months and the zeta potential of these NPs remained at ∼- 39 mV throughout the acid exposure and they showed no loss in magnetisaton. Due to the use of these NPs as a potential tool for heavy metal extraction, the stability of the surface functionalisation (in this case a phosphate complex) was also assessed. With a constant zeta potential of ∼ - 29 mV for POx-SiO2@Fe3O4 NP complex, the phosphate functionality was shown to be highly stable in the acidic conditions simulating the environment of certain nuclear wastes. ATR-FTIR was conducted after acid exposure confirming that the phosphate complex on the surface of the NPs remained present. Finally, preliminary sorption experiments were carried out with Pb(II), where the NP complexes shown complete removal of the heavy metals at pH 3 and pH 5.