A method of purifying a biological composition includes: disposing loose cationic ligand-functionalized staple fibers and a biological composition within a mixing volume of a vessel; agitating the biological composition and the loose cationic ligand-functionalized staple fibers while they are in intimate contact with each other within the mixing volume to provide modified fibers and a purified biological composition; and separating at least a portion of the purified biological composition from the modified fibers and any loose cationic ligand-functionalized staple fibers with which it is in contact. The loose cationic ligand-functionalized staple fibers have a modified surface layer comprising a grafted acrylic polymer comprising 10 to 100 percent by weight of a cationically-ionizable monomer unit. An article for purifying a biological composition includes: a vessel having a mixing volume disposed therein; and the loose cationic ligand-functionalized staple fibers disposed within the mixing volume.