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• Eggshell-modified biochar shows prominent phosphorous adsorption performance. • Chemical adsorption process is the controlled step for eggshell-modified biochar. • Ca-P precipitation, hydrogen bonding, electrostatic attraction are adsorption step. • Adsorbed phosphates are mainly distributed inside pore size range of 15–50 nm. Biochar modified with calcium source is acted as an effective adsorbent for phosphorous recovery. In this research, eggshell is used as a low-cost and environmentally friendly calcium source to replace chemical reagents such as CaCO 3 , Ca(OH) 2 and CaCl 2 used in the modified biochar production. Biochar derived from rape straw and modified with eggshell shows prominent phosphorous adsorption performance (e.g., equilibrium adsorption amount, 109.7 mg/g). The kinetic and isotherm analysis demonstrate that chemical adsorption process is performed as the main controlled step for the modified biochar adsorption, and the phosphate adsorption process is composed of both monolayer adsorption and multi-layer adsorption. Moreover, it is found from the physicochemical structures comparison before and after phosphate adsorption that Ca-P precipitation, hydrogen bonding and electrostatic attraction are identified as main adsorption mechanisms. In addition, the adsorbed phosphates are mainly distributed inside the space with pore sizes of 15–50 nm. [ABSTRACT FROM AUTHOR]