Biochar is a multifunctional carbon-pore material that can be used as a soil amendment and an adsorbent biomaterial for water treatment. Nitrogen is one of the nutrients that causes water eutrophication in aquatic environments. In this study, corncob biochar (CBB) characterizations were modified using phosphoric acid (P-CCB) and hydrogen peroxide (H-CCB) to improve ammonium removal efficiency from synthetic wastewater. The biochar physiochemical properties were investigated, including ash content, pH, total elements, functional groups, surface area, and morphological structure. A batch kinetic trial extended to 24 h was carried out to evaluate the adsorption efficiency of ammonium (40 mg L^-1) by the three biochar types; moreover, four kinetic models were tested. The results revealed that the acid oxidation process reduced ash content, pH, and total carbon content values. At the same time, oxygen percentage and molar ratios of O/C and (O+N)/C were increased relative to the original corncob biochar. The efficiency of the activated biochars to remove NH₄⁺-N was higher than that of CCB, and there was not much difference between P-CCB and H-CCB. The adsorption capacity (Q_e) of NH₄⁺-N by CCB reached 4.47 mg g^-1 after 24 h, whereas this amount increased due to the activation and reached 6.88 and 6.61 mg g^-1 for P-CCB and H-CCB, respectively. The Pseudo-Second order model displayed well-fitting results to elucidate the adsorption results depending on the R² value (0.99–1.00). These results suggest that corncob-activated biochar by phosphoric acid and hydrogen peroxide could effectively eliminate ammonium-nitrogen from wastewater.