The most reactive oxidant, the hydroxyl radical (•OH), is crucial to the biogeochemical cycle of elements and the attenuation of environmental pollutants. In recent years, it was discovered that even in the absence of foreign hydrogen peroxide, the redox reaction of iron in deep sediments can naturally form •OH.
However, little is known about how the process is affected by soil organic matter (SOM) that is connected with minerals.
In a study published in Environmental Science & Technology, a research team led by Prof. Zhu Yongguan and Li Gang from the Institute of Urban Environment of the Chinese Academy of Sciences reported the influence of humic substances, the major components of SOM, on the microbially mediated iron reduction and reoxidation processes, and established the pathway of ·OH production in different SOM-containing system.
The researchers used fulvic acid (FA), humic acid (HA), and humin (HM), components of humic substances operationally separated from soil, to evaluate the influence of SOM characteristics on iron redox processes.
They discovered that whereas HA with high electron donating capacity reduced the yield of •OH, high electron exchange capacity of FA and HA boosted the microbial iron reduction process.
The researchers discovered various paths for •OH formation in SOM-containing systems using the scavengers of potential intermediary involved in its synthesis. They discovered that in the FA-containing system, the one-electron transfer process dominated the formation of •OH, but in the HA and HM-containing systems, both one and two-electron transfer processes were present.
The researchers also discovered that the characteristics of dissolved fractions of SOM were altered by microbially driven iron redox processes, and that the aromaticity of dissolved fractions of HA was reduced because of its high reactivity with •OH.
According to the high-resolution transmission electron microscope and X-ray diffractometer, SOM prevented the creation of higher stable and crystalline iron oxy(hydr)oxides by inhibiting the formation of ferrous secondary minerals.
The understanding of iron redox mechanisms involving SOM and •OH generation is improved by this work. When analyzing the impact of potentially created •OH on pollutants degradation in redox fluctuating environments, the mechanisms disclosed need to be taken into account.
