Polystyrene waste, such as Styrofoam® waste, and carbon nanofibers may be used to produce highly hydrophobic compositions or composites that can separate oil and water. Methods for purifying an aqueous solution may include: passing the aqueous solution, including a hydrophobic organic substance, over or through a surface including a polystyrene-CNF composition, thereby producing an aqueous product including less of the hydrophobic organic substance; and optionally, passing the aqueous product over or through the surface at least one more time.