This study investigates the valorisation of Malus domestica debris from the Ifrane region of Morocco for the production of H3PO4 activated biochar and its application in the elimination of methyl orange (MO) from aqueous solutions via batch adsorption. A comprehensive characterization of the activated biochar was conducted using specific surface area analysis, Boehm titration, point of zero charge (pHpzc) determination, thermogravimetric analysis (TGA) and X-ray diffraction (XRD) measurements. The findings indicate that phosphoric acid activation enhances both the specific surface area and functional group density of the biochar. Kinetic and equilibrium models were used to assess the adsorption process, and the Freundlich, Langmuir, and Temkin isotherms were fitted to the equilibrium data. The results demonstrate that adsorption capacity rises with time until equilibrium is attained following ninety minutes of contact. In addition, phosphoric acid activation enhanced the quantity of MO adsorbed per gram of biochar. Under optimized conditions, equilibrium adsorption kinetics followed a pseudo-second-order model, while the Langmuir isotherm provided the best fit for describing MO adsorption, with a maximum adsorption capacity of 143.65 mg·g⁻¹.Thermodynamic analysis revealed that the adsorption process is endothermic and primarily governed by physical interactions. These findings highlight the potential of H3PO4 activated biochar derived from Malus domestica debris as a highly effective and sustainable adsorbent for the removal of methyl orange from aqueous solutions.