Induced generation of hydroxyl radicals from green rust under oxic conditions by iron-phosphate complexes

Abstract

The transformation of green rust (GR) to form ferric oxyhydroxide at the oxic/anoxic interface, is an important process of iron cycles in subsurface environments. However, little is known in its electron transfer process and environmental impacts. Herein, we demonstrate that the GR oxygenation can generate hydroxyl radical (•OH; 1.6 μM) with a low rate of 0.012 μM/min. Interestingly, a strong correlation between the adsorbed phosphate on GR and the •OH generation is found; the presence of 20 mM phosphate significantly enhance the generation rate of •OH up to 0.13 μM/min, and a high cumulative concentration up to 20.9 μM. Our results demonstrate that the formation of iron-phosphate complexes is likely a key species determining •OH generation. Electrochemical analysis and DFT calculations indicate that phosphate can decrease the electron utilization ratio of GR but notably increase the electron selectivity for •OH production under oxic conditions. Subsequently, the generation of •OH radicals plays a role in oxidatively detoxifying diverse organic pollutants (e.g. chlorophenol and pharmaceuticals). Our findings provide new insights into the important role of oxyanions such as phosphate in determining electron transfer during iron cycles and consequently the fate of pollutants in subsurface environments.