Magnetite nanoparticles with a size distribution of 15–21 nm were synthesized and decorated onto surface of graphene oxide by ultrasound assisted precipitation. Size and size distribution of the obtained M–GO hybrid were appreciably finer than the hybrids prepared by stirring method. M–GO is a superparamagnetic material with saturation magnetization of 31 emu g⁻¹. The Langevin equation was successfully applied for estimation of size of Fe₃O₄ nanoparticles in M–GO hybrid, with maximum error of 17.5%. The study put forward a formation mechanism for M–GO, based on instrumental analyses. Adsorption isotherms of Sr²⁺ and Co²⁺ ions, which were fitted by Langmuir monolayer model, displayed two-fold higher capacity for Co²⁺ ions, presumably due to its similarity to Fe²⁺ (of Fe₃O₄ component). Uptake of both Co²⁺ and Sr²⁺ ions were endothermic, and spontaneous, however the former proceeded through inner-shell complex formation, while the latter took place via ion exchange mechanism. Rate of adsorption of Co²⁺ was faster, but for both ions, chemical reaction was the rate determining step. Sorption of Sr²⁺ and Co²⁺ ions greatly increased at pHs above 5, where (1) surface zeta potential changed its sign, and (2) deprotonating reactions at the surface became complete.