Nanoporous biochar with high specific surface area based on rice straw digestion residue for efficient adsorption of mercury ion from water

•High-efficiency adsorbent was prepared using rice straw biogas residue.•Loose lignocellulosic structure contributed to high specific surface area.•KRSB adsorbed mercury ions in multiple ion systems with high selectivity.•Complexation of mercury ion with KRSB as the dominant adsorption mechanism.•Adsorption process obeyed pseudo-second order kinetic and Langmuir isotherm.