Fe3O4 nanocrystals with narrow particle size distribution (10–15 nm) were produced in high yield by using a simple solution-phase method at room temperature. In this approach, the iron oxide nanocrystals can be continually prepared through controlling the pH value of the solution. The as-synthesized Fe3O4 nanoparticles are single-crystalline and exhibit high saturation magnetization. New adsorbents of Fe3O4 nanocrystals immobilized on bacterial cellulose (FNBC) were prepared by a simple blending method in the hope of removing heavy metal ions from aqueous solutions. The FNBC showed significantly higher affinity for removing Cd2+ compared to BC. In addition, the effect of characteristic parameters such as pH value, adsorption time by simulating the adsorptive kinetics, and adsorptive isotherms were also investigated.