Dithionite extractable iron responsible for the production of hydroxyl radicals in soils under fluctuating redox conditions

Abstract

Iron is a redox-active metal that is abundantly present in soils. Many recent studies have reported the production of reactive oxygen species (ROS) during the oxygenation of structural ferrous iron (Fe(II)). In soils, structural Fe(II) is mainly formed through microbial reduction of iron(III) under anoxic conditions, however, little is known about which iron species are preferred to be reduced by dissimilatory iron-reducing bacteria and further induce the production of ROS during oxygenation. The present study employed a variety of soils to investigate the microbial reduction of iron and the following production of ROS. Sustained production of hydroxyl radical (•OH) was observed during redox cycles, positively correlated to the contents of bio-reduced Fe(II). Furthermore, the yields of •OH, hydrogen peroxide, and Fe(II) were positively correlated to the content of crystalline Fe extracted using sodium dithionite rather than the total Fe in the soils. In addition, clay size fractions with higher dithionite extractable Fe contents were found to have greater contributions to the production of •OH compared to the coarse fractions. This study demonstrates the presence of •OH production in soils under fluctuating redox conditions and suggests that dithionite extractable Fe could serve as a feasible indicator to evaluate the potential of •OH production.