Photoformation of persistent free radicals on a montmorillonite-humic acid complex simulated as particulate organic matter in an aqueous solution

Abstract

This study investigates the formation of persistent free radicals (PFRs) on particulate organic matter (POM) under irradiation in water. A montmorillonite–humic acid complex (Mnt–HA complex) was prepared to simulate POM, and the generated PFRs were detected by the electron paramagnetic resonance (EPR) technique. EPR signals with the trend of an initial increase and then a decrease were observed under irradiation for 8 days, and the g factors were in the range of 2.0034–2.0039, which indicated the generation of carbon-centered radicals with electrophilic moieties. Different concentrations and types of halophenols and transition-metal ions were respectively adsorbed on the Mnt–HA complex to probe their influence on the formation of PFRs. The amount of PFRs generated in the Mnt–HA complexes was in the order: 2-bromophenol (2-BP) > 2,4-dibromophenol (2,4-DBP) > 2,4-dichlorophenol (2,4-DCP), which implied that halogen substitution and the number of substituents in the halophenols could affect the generation of PFRs. The effects of transition-metal ions that resulted in the reduction of PFRs when adsorbed on the Mnt–HA complex were as follows: Fe³⁺ > Zn²⁺ > Cu²⁺ > Mn²⁺, and this is in agreement with their redox capacity. Analyzing the induced generation of reactive oxygen species (ROS) and electrons on POM, it is found that halophenols and transition metal ions also affected this process under irradiation. These findings indicate that the photoformation of PFRs on POM could be a source of PFRs in aqueous environments and requires further attention.