Effect of ligands on the production of oxidants from oxygenation of reduced Fe-bearing clay mineral nontronite

Oxygenation of reduced iron-bearing clay minerals produces highly reactive oxidants that are capable of transforming a range of organic compounds, which is of great environmental importance, but the effect of Fe-chelating ligands on this process has not been studied. In this work, oxidant production from oxygenation of reduced nontronite was investigated in the presence of four common ligands. Addition of phosphate, tripolyphosphate (TPP), nitrilotriacetic acid (NTA) and ethylene diaminetetraacetic acid (EDTA) all significantly increased the oxidant yields, but the specific mechanisms varied, depending on the ligand types. NTA and EDTA addition promoted dissolution of structural Fe, forming aqueous NTA-Fe2+ and EDTA-Fe2+ complexes. Upon exposure to air, these complexed species rapidly oxidized through homogeneous Fenton reaction, forming chelated soluble Fe3+ species, which were rapidly reduced back to Fe2+ by electrons transferred from structural Fe(II) in reduced nontronite. With increasing NTA and EDTA loadings, homogeneous Fenton reaction gradually dominated over heterogeneous oxidation of structural Fe(II). Because of rapid air oxidation of NTA-Fe2+ and EDTA-Fe2+ complexes, these species were maintained at low levels in aqueous solution, thus minimizing the quenching effect of oxidants by excess Fe2+ and the probability of non-oxidant generation pathway, both of which contributed to the observed increase of the oxidant yields. In contrast, phosphate increased the oxidant yield through sorption to the surface of reduced NAu-2 and a subsequent shift of weak oxidant [possibly Fe(IV)] to strongly reactive hydroxyl radicals (HO center dot). TPP played a dual role by both changing the surface catalytic properties of nontronite through sorption and enhancing homogeneous Fenton reaction by chelating with aqueous Fe2+ and Fe3+. These results shed lights on how commonly present natural and synthetic ligands affect the oxidation process of reduced iron-bearing clay minerals and oxidant production, hence providing a theoretical basis for understanding oxidant-promoted transformation mechanisms of organic matter in either natural or engineered systems. (C) 2019 Elsevier Ltd. All rights reserved.