Ligand effects on nitrate reduction by zero-valent iron: Role of surface complexation

Surface passivation is a key limiting factor in the application of zero-valent iron (ZVI) for water remediation. Addition of ligands is a useful approach to overcome this issue. In this work, a small amount of acetylacetone (AA) (0.5 mM) was found highly efficient to enhance the reduction of nitrate by ZVI at near neutral conditions (pH 6.0) with the formation of considerable black coating on ZVI. At an initial nitrate concentration of 20 mg N/L, the pseudo first-order reduction rate constant of nitrate in the ZVI-AA-NO₃^- system was 0.0991 h^-1, which was 52 times higher than that in the ZVI-NO₃^- system. Under otherwise identical conditions, the other five ligands, including EDTA, formate, acetate, oxalate, and phosphate, had negligible effects. Based on the pK_a values of these ligands and the final species of iron, the ligand effects on nitrate reduction by ZVI were summarized from three aspects: (1) the ability to offer potentially dissociable protons from the ligands; (2) the complexation ability to eliminate iron (hydr)oxide precipitates from the surface of ZVI; and (3) the ability to lower down the redox potentials of iron species. The good performance of AA in these three aspects makes it advantage over the other ligands. A cycle test up to six runs demonstrates that AA could continuously take effect in the ZVI system. The results here point out the potential of AA as an effective ligand in ZVI system for enhanced contaminant transformation.