High-Efficient removal of mercury in water by the flexible NH2-MIL-53(Al): Exploiting the gate opening effect

•Flexible MOF NH2-MIL-53(Al) can largely enhance the adsorption capacity towards Hg(II)based on a gate-opening effect.•Large-pore NH2-MIL-53(Al) could achieve much faster Hg(II) uptake than narrow-pore NH2-MIL-53(Al).•Amino groups and oxygen centers provides affinity sites in the Hg(Ⅱ) removal.•The size of the nanopore has a direct influence on diffusivity of Hg(Ⅱ) transport via molecular dynamics simulation.