The present study aims to synthesize an activated carbon adsorbent (S-AC, P-AC and OH-AC) from polystyrene divinyl benzene waste under different carbonization temperature (450^oC and 900 ^oC and activation conditions (H₂SO4, H₃PO₄ or NaOH). It being characterized and applied as an adsorbent for manganese removal from groundwater. FTIR results indicate that the changes in the peak intensities clear that the binding process was occurring on the surface of the adsorbent. Different factors such as solution pH, adsorbent dosage, contact time, temperature and Mn(II) initial concentration were investigated. The results showed that manganese adsorption capacity decreases with the increase of the carbonization temperature (900^oC) and of activating agent H₂SO4 > H₃PO4 >NaOH. Kinetically, it was shown that the activation temperature of 450^oC is the best temperature for activating the adsorbent carbons. The pseudo-first-order model is appropriate for predicting the adsorption process of Mn(II) onto the P-AC 450^oC, P-AC 900^oC and OH-AC 900^oC, while the pseudo-second-order model is appropriate for predicting the adsorption process of Mn(II) onto the both S-AC 450^oC, S-AC 900^oC, OH-AC 450^oCand OH-AC 900^oC. Thermodynamic calculations affirmed that Mn(II) adsorption onto S-AC and P-AC was an endothermic process while onto OH-AC was exothermic process. Finally, the results suggested that the prepared S-AC has high adsorption capacities for Mn(II) compared with other adsorbents, such as P-AC and OH-AC. Therefore, S-AC 450^oCcan be used in the groundwater treatment unit.