This work proposes a new separation concept denoted as nanolevel high gradient magnetic separation (HGMS) or magnetic adsorption. A magnetic heteroflocculation model describes the magnetic forces between two spherical particles with different sizes and magnetic properties, and reveals the feasibilities and limitations of nanolevel HGMS. The adsorbent particles, composed of antiferromagnetic magnetite, are modeled as large, immobile spheres on the order of 100-500 nm in radius. The adsorbate, paramagnetic colloidal Fe(OH)(2) particles, are treated as freely diffusing small spheres on the order of 20-80 nm in radius. The model assumes that the magnetite particles are dispersed throughout a porous, nonmagnetic, solid matrix and that they are free of convective forces. The model also assumes that magnetic forces alone act on the Fe(OH), particles, opposed only by Brownian motion. When the magnetic force is attractive and overwhelms the randomizing Brownian force, adsorption occurs. The results from this model show the importance of the external field strength, the sizes of the adsorbent and adsorbate particles, and their magnetic properties in developing a practical nanolevel HGMS process. (C) 1997 Elsevier Science B.V.