First cadmium coordination compound as an efficient flocculant for Congo red

•New Cd(II) coordination compound of 2-phenylimidazo[4,5-f][1,10]phenanthroline was synthesized via hydrolysis of acetonitrile to acetate.•This compound proves to be the first coordination compound for rapid, specific and complete flocculation of Congo red.•The flocculation mechanism is rationally proposed based on various methods.