Bovine carbonic anhydrase (BCA) was covalently immobilized onto OAPS (octa(aminophenyl)silsesquioxane)-functionalized Fe3O4/SiO2 nanoparticles by using glutaraldehyde as a spacer. The Fe3O4 nanoparticles were coated with SiO2, onto which was grafted OAPS, and the product was characterized using SEM, TEM, XRD, IR, X-ray photoelectron spectroscopy (XPS), and magnetometer analysis. The enzymatic activities of the free and Fe3O4/SiO2/OAPS-conjugated BCA (FeCA) were investigated by hydrolyzing p-nitrophenylacetate (p-NPA), and hydration and sequestration of CO2 to CaCO3. The CO2 conversion efficiency and reusability of the FeCA were studied before and after washing the recovered FeCA by applying a magnetic field and quantifying the unreacted Ca2+ ions by using ion chromatography. After 30 cycles, the FeCA displayed strong activity, and the CO2 capture efficiency was 26-fold higher than that of the free enzyme. Storage stability studies suggested that FeCA retained nearly 82 % of its activity after 30 days. Nucleation of the precipitated CaCO3 was monitored by using polarized light microscopy, which revealed the formation of two phases, calcite and valerite, at pH 10 upon addition of serine. The magnetic nanobiocatalyst was shown to be an excellent reusable catalyst for the sequestration of CO2.