Ligands may increase the yields of reactive oxygen species(ROS)in zero-valent iron(ZⅥ)/O2 systems.To clarify the relationship between the properties of ligands and their effects on the oxidative removal of contaminants,five common ligands(formate,acetate,oxalate,ethylenediaminetetraacetic acid(EDTA),and phosphate)as well as acetylacetone(AA)were investigated with arsenite(As(Ⅲ))as the target contaminant at three initial pH values(3.0,5.0,and 7.0).The addition of these ligands to the ZⅥ/O2 system resulted in quite different effects on As(Ⅲ)removal.EDTA enhanced the oxidation of As(Ⅲ)to arsenate(As(Ⅴ))but inhibited the removal of As(Ⅴ).Oxalate was the only ligand in this work that accelerated both the removal of As(Ⅲ)and As(Ⅴ).By analyzing the ligand effects from the four aspects:dissolution of surface iron(hydr)oxides,corrosion of ZⅥ,reaction with ROS,and interference with precipitation,the following properties of ligands were believed to be important: ability to provide dissociable protons,complexation ability with iron,and reactivity with ROS.The complexation ability is a double-edged sword.It could enhance the generation of ROS by reducing the reduction potential of the Fe(Ⅲ)/Fe(Ⅱ)redox couple,but also could inhibit the removal of arsenic by coprecipitation.The elucidated relationship between the key property parameters of ligands and their effects on the ZⅥ/O2 system is helpful for the rational design of effective ZⅥ/ligand/02 systems.