Distribution of persistent free radicals in different molecular weight fractions from peat humic acids and their impact in reducing goethite

Humic substances, the most abundant component of soil organic matter, play vital roles in the biogeochemical cydes and pollutant redox reactions. However, the knowledge regarding the distribution of persistent free radicals (PFRs) and redox capacity in different molecular weight fractions (MWF) of humic add (HA) and their impact on the transformation of iron oxide minerals remains unknown. In this study, we separated bulk HA into various MWF using dialysis methods and systematically investigated their PFRs properties, redox capacity, and the impact in reducing goethite. The results showed that the PFRs in the low MWF ( <3500, <7000, and <14,000 Da) can be assigned to oxygen-centered organic radicals while those in the bulk and high MWF (14000-retentate) were assigned to carbon-centered organic radicals. The PFRs concentrations of low MWF were 0.20-0.45 x 10(16) spins/g, far less than those of bulk HA (3.04 x 10(16) spins/g) and 14000-retentate (1.30 x 10(16) spins/g). The total concentrations of reactive oxygen species (ROS) induced by PFRs in HA fractions ranged from 8.04 x 10(16) (in 14000-retentate) to 32.35 x 10(16) spins/g (in bulk HA). Compared with the low MWF, the 14000-retentate fractions had the higher reducing capacity, which was positively related to the content of PFRs and phenolic hydroxyl in HA. The results obtained provide valuable insight into the geochemistry processes of Fe-containing minerals during their interaction with HA in the natural environment. Overall, the results obtained provide valuable insight into the geochemical behaviors of HA-associated PFRs under natural conditions. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

The study revealed the presence of different types of persistent free radicals in humic acid fractions with different molecular weight fractions, with lower molecular weight fractions having lower PFR concentrations compared to bulk HA and higher molecular weight fractions. In terms of redox capacity, higher molecular weight fractions exhibited greater reducing capacity, positively correlated with the content of PFRs and phenolic hydroxyl groups.