Experimental and DFT studies on the equilibrium properties, kinetics, and mechanism of nitric oxide removal using metal-EDTA and ferrous thiochelates

Abstract

Aqueous solutions of various metal chelate complexes (i.e., Fe(II)-EDTA, Co(II)-EDTA, Cu(II)-EDTA, Fe(II)-(DMPS)2, and Fe(II)-(DMSA)2) were tested for their ability to absorb NO, the main component of NOx. The equilibrium and kinetic rate constants of absorption were compared for these chelates at a fixed pH of 7 and at 298 K. In terms of the effect of the core metal on the equilibrium and rate constants, values obtained for Fe(II)-EDTA were higher than those of Co(II)-EDTA and Cu(II)-EDTA. In terms of the influence of the chelate ligand on the equilibrium and rate constants, ferrous thiochelates such as Fe(II)-(DMPS)2 and Fe(II)-(DMSA)2 were found to exhibit dual sites for NO binding, with two equilibrium constants being determined; this resulted in these complexes exhibiting higher NO absorption capacities than Fe(II)-EDTA. The kinetic rate constants of the ferrous chelates were comparable within the same order of magnitude. Density functional theory (DFT) calculations and Fourier-transform infrared (FT-IR) analyses were employed to theoretically determine the mechanisms of NO absorption in the chelate solution and to confirm the trends of NO absorption capacity corresponding to different metal chelate complexes, wherein a reasonable agreement with the experimental results was obtained.