Mercury (Hg) immobilization using stabilized iron sulfide (FeS) nanoparticles was investigated through a series of batch and column experiments. The nanoparticles were prepared using a low-cost, food-grade cellulose (sodium carboxymethyl cellulose, CMC) as the stabilizer. The hydrodynamic diameter of fresh FeS-CMC nanoparticles was measured to be 38.5+/-5.4nm. Batch tests showed that the nanoparticles can effectively immobilize Hg in a clay loam sediment. The Hg distribution coefficient for the nanoparticles was determined to be 8930+/-1480L/g, which is >4 orders of magnitude greater than for the sediment. When the Hg-laden sediment was treated at an FeS-to-Hg molar ratio of 26.5, the Hg concentration leached into water was reduced by 97% and the TCLP (toxicity characteristic leaching procedure) leachability of Hg was reduced by 99%. Column tests showed that water-leachable mercury from the sediment containing 3120mg/L Hg was reduced by 67% and the TCLP leachability by >77% when the sediment was treated with 67 pore volumes (PVs) of a 0.5g/L FeS nanoparticle suspension. Column tests proved that the stabilized nanoparticles were highly mobile in the sediment and full breakthrough of the nanoparticles occurred at approximately 18 PVs.