Waste polystyrene containing additives was converted into a polymeric flocculant by a chemical modification. Specifically, waste polystyrene and waste high-impact polystyrene (HI-PS) containing rubber components or a coloring agent were sulfonated to produce a water-soluble polymer whose molecular weight ranged from 400 000 to 700 000. The polymer provides superior purification of the supernatant after flocculating a kaolin suspension than a conventional polymeric flocculant such as a partially hydrolyzed polyacrylamide (hereafter abbreviate PAA). Moreover, using the polymer and a PAA together provides a higher sedimentation rate and a purer supernatant in the treatment of actual wastewater than using each one separately. A new reclamation technology to convert waste plastic into a functional polymer is reported.