Unveiling the interfacial and coordinatively unsaturated effect in iron diselenide-based hierarchical heterojunction for enhanced Hg0 removal

•2D hierarchical FeSe2/g-C3N4 nano-heterojunction was synthesized via rationally engineered integration of structural and electronic properties.•FeSe2/g-C3N4 exhibited excellent sulfur resistance under a high SO2 concentration.•Phase interface of heterojunction can achieve effective and fast electron transportation.•The oxygen-containing functional groups on g-C3N4 nanosheets were activated.•The coordinatively unsaturated Se1− ligands was the predominant active ingredient for Hg0 immobilization.