This work highlights the chemical activation of hydrothermally carbonized biomass with phosphoric acid under static air as activating atmosphere. Argan nut shells were hydrothermally carbonized, and then chemically activated with phosphoric acid under oxidizing atmosphere. The activated carbons displayed excellent specific surface areas (from 1200 m2g−1 to 1880 m2g−1) and the material yield were in acceptable level (between 27 and 50%). The varied activation conditions provided a distinctive possibility for creating and controlling the mesoporosity of the activated carbons. At 500 °C, a mesoporosity around of 93% could be reached by an H3PO4 impregnation ratio of 4. The activating temperature 500 °C and the impregnation ratio of 3 were observed as optimum for achieving the best textural properties. The optimized material HC-500-3 exhibited a specific area of 1880 m2g−1, pore volume and average pore diameter of 1.36 cm3g−1 and 2.9 nm, as well as good amount of oxygen functional groups (2.25 mmolg−1) including carboxyl, phenol, and lactone groups. The adsorption performance of optimized material was evaluated using Bisphenol A as a target molecule. The adsorption process could be described by Weber Morris and Langmuir isotherm models. At a temperature of 25 °C, Langmuir monolayer adsorption capacity was observed to be 420 mgg−1 which is comparable or higher than those reported previously. The estimated thermodynamic parameters indicate spontaneous and exothermic adsorption process (∆H° = −19.39 kJmol−1; −5.58 kJmol−1 ≤ ∆G° ≤ −4.65 kJmol−1). The mesoporous activated carbon prepared by chemical activation of biomass-derived hydrochar under atmospheric air proved to be very potential adsorbent for Bisphenol A removal.