Herein, triblock copolymer surfactant (F127) and mesoporous silica (MCM-41) as soft and hard templates were employed to synthesize of mesoporous CuO/g-C(3)N(4 )heterostructures with large surface areas for Hg (II) photoreduction in existence of formic acid as a holes sacrificial. TEM image for mesoporous CuO/g-C3N4 indicated that CuO NPs are homogeneously distributed with spherical shape in particle size similar to 5 nm onto the surface of g-C3N4 . Mesoporous 2%CuO/g-C(3)N(4 )heterostructure was achieved a high Hg (II) photoreduction rate of 628.74 mu molg (1)h (1) and high photoreduction efficiency of similar to 100% within 50 min compared with the pure either mesoporous CuO NPs (130.11 mu molg (1)h (1 ), 21%) and g-C3N4 (88.54 mu molg (1)h (1) ,14%). The highest Hg(II) photoreduction rate achieved was 628.74 timolg (1)h (1) , which is 4.83 and 7.1 magnitudes stronger than mesoporous CuO NPs and g-C3N4. The excellent photocatalytic performance of mesoporous CuO/g-C3N4 heterostructures for Hg(II) photoreduction is referred to highly dispersed mesoporous CuO NPs with small particle size onto g-C3N4, narrow bandgap, large surface area, a rapid transfer of Hg(II) ions and HCOOH to easily reach the active sites due to the facile penetration through the mesostructure, thus promoting the utilization of porous structure of CuO/g-C3N4 heterostructures for efficient diffusion of Hg(II) ions. The intense interaction between mesoporous CuO NPs and porous g-C3N4 confirms the durability of the CuO/g-C3N4 heterostructures during recyclability for five times. (C) 2020 Elsevier Inc. All rights reserved.