Disposal of plastic Waste has become an ever-increasing concern throughout the world. There has been considerable interest or demand for recycling plastic waste into higher-value products. In this study, an attempt has been made to convert waste polystyrene (PS) into porous and functionalized granular adsorbent for a potential application in humic acid removal. Porous PS granules were prepared via a dissolution/precipitation and etching/oxidation method, and their surfaces were modified with cross-linked polyethylenimine (PEI). The surface morphologies of PS granules, oxidized PS granules, and PEI-immobilized PS granules were examined with a scanning electron microscope (SEM), and the surface properties of these granules were investigated through Fourier transform infrared (FTIR) spectroscopy and differential scanning calorimetry (DSC). The prepared PS granules had a more-or-less spherical shape with sizes in the range of 1.3-2.4 mm, depending on the polymer concentrations used. The etching/oxidation process resulted in highly porous granules, and their surfaces were activated with polar chemical groups, which facilitated the immobilization of PEI on the external and internal surfaces of the granules in a simple solution reaction process. The PEI-functionalized PS granules were demonstrated to be an effective adsorbent for humic acid removal.