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

Highlights: • Distribution of persistent free radicals in HA was described. • Transformation of goethite by various fractions of HA was investigated. • High molecular weight fractions of HA had higher reducing capacity than other fractions. • Persistent free radicals in HA induce the reduction of goethite. Humic substances, the most abundant component of soil organic matter, play vital roles in the biogeochemical cycles 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 acid (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 (16 spins/g, far less than those of bulk HA (3.04 × 10¹⁶ spins/g) and 14000-retentate (1.30 × 10¹⁶ spins/g). The total concentrations of reactive oxygen species (ROS) induced by PFRs in HA fractions ranged from 8.04 × 10¹⁶ (in 14000-retentate) to 32.35 × 10¹⁶ 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.