Advancing Fenton and photo-Fenton water treatment through the catalyst design

The review is devoted to modern Fenton, photo-Fenton, as well as Fenton-like and photo-Fenton-like reactions with participation of iron species in liquid phase and as heterogeneous catalysts. Mechanisms of these reactions were considered that include hydroxyl radical and oxoferryl species as the reactive intermediates. The barriers in the way of application of these reactions to wastewater treatment were discussed. The following fundamental problems need further research efforts: inclusion of more mechanism steps and quantum calculations of all rate constants lacking in the literature, checking the outer sphere electron transfer contribution, determination of the causes for the key changes in the homogeneous Fenton reaction mechanism with a change in the reagents concentration. The key advances for Fenton reactions implementation for the water treatment are related to tremendous hydrodynamical effects on the catalytic activity, design of ligands for high rate and completeness of mineralization in short time, and design of highly active heterogeneous catalysts. While both homogeneous and heterogeneous Fenton and photo-Fenton systems are open for further improvements, heterogeneous photo-Fenton systems are most promising for practical applications because of the inherent higher catalyst stability. Modern methods of quantum chemistry are expected to play a continuously increasing role in development of such catalysts.

Computational analysis of non-heme iron-oxo formation by direct NO release in nitrite reduction

A direct NO-releasing reaction of nitrite catalyzed by [N(afa^Cy)₃Fe(OTf)]⁺ (afa (azafulvene-amine); OTf (trifluoromethanesulfonate); Cy (cyclohexyl)) was investigated using density functional theory (DFT) with D3 dispersion correction. The complex featured a secondary coordination sphere that facilitated the formation of the iron-oxo product [N(afa^Cy)₃FeO]⁺ with three (Fe)OH-N hydrogen bonds. As a high-spin iron(ii), the O-binding initial intermediate Fe(O)-nitrito was thermodynamically favorable in the S = 2 state. The cleavage of the (Fe)O-NO bond was performed by a β-electron shift to produce Fe(iii)-O by electron rearrangement in the S = 5/2 state. The different electron configurations are responsible for the structural properties, the valence of iron in the complexes, and the pathways of the reactions. Moreover, the two different H-bonds, (Fe)OH-N and (Fe)O-HN (by O-protonation), in the product complexes played a role in determining the reaction channels by impacting the N-H bond rotation. Thus, an exothermic sequence of conversions Fe(ii) → Fe(iii)-O → Fe(iii)-OH → Fe(iii)-O was established for the targeted product formation. This process provided a clue to build two key intermediates, iron-oxo and iron-hydroxo, in a variety of biological and synthetic systems. The results of this study are in agreement with experimental observations and describe the roles of H-bonding in nitrite reduction catalyzed by the non-heme iron complex.