In an improved process for immobilizing a chelator moiety on a silica substrate, the substrate is reacted in a liquid reaction medium with an alkyltrialkoxysilane compound having a functional group that provides an attachment site for covalently binding the chelator moiety to the substrate wherein the improvement comprises providing a particulate silica substrate having a surface area less than about 50 square meters per gram. An improved extracorporeal device for removing metal ions from blood and other fluids includes a cartridge (10) having an inlet (32) and an outlet (34) and containing a plurality of tubular anisotropic membrane fibers (22) that extends from the inlet (32) to the outlet (34) and supported by a macroporous structure (24) that contains a chelator moiety immobilized on a particulate silica substrate having a surface area of less than about 50 square meters per gram. The anisotropic membrane forms a diffusion barrier permeable to metal cations, but impermeable to high molecular weight components.