The embodiments provide a modified filter membrane for separating a crude solution of a biological product and a viral contaminant. The filter membrane has a cellulosed based porous surface, and at least one divalent metal ion bound to the cellulose based porous surface of the filter membrane to form a modified filter membrane cellulose based porous surface, wherein the modified cellulose based porous surface separates the crude solution by retaining a viral contaminant greater than 15 nm in diameter while allowing a biological product smaller than 15 nm in diameter to pass through. The embodiments also provide a method of filtering a crude solution of a biological product and a viral contaminant using a modified filter membrane by adding a divalent metal ion to a filter membrane porous surface to form a modified filter membrane porous surface with a pore size in the range of 1 to 15 nm in size, and filtering the crude solution of the biological product and the viral contaminant through the porous surface of the modified filter membrane, wherein the modified filter membrane retains the viral contaminant on the porous surface while allowing the biological product to pass through.
