The feasibility is examined of using high-gradient magnetic separation (HGMS) to recover about 8-nm magnetite nanoparticles that are tailored specifically to extract target solutes (polymer-coated nanoparticles for the extraction of soluble organic contaminants from water and phospholipid-coated particles for the selective extraction of proteins). A general model for nanoparticle capture based on calculating the limit of static nanoparticle buildup around the wires in an HGMS column is presented. Model predictions are compared successfully with experimental results from a bench-scale HGMS column for both the polymer- and phospholipid-coated particles. A minimum diameter for successful particle capture is derived for both individually dispersed nanoparticles and aggregates of nanoparticles. The individually dispersed polymer-coated nanoparticles are less easily captured in the HGMS column than are the phospholipid-coated particles, which exist as submicron aggregates. (C) 2004 American Institute of Chemical Engineers.