Mechanistic Insight into Humic Acid-Enhanced Hydroxyl Radical Production from Fe(II)-Bearing Clay Mineral Oxygenation

Hydroxyl radical (center dot OH) production by electron transfer from Fe(II)-bearing clay minerals to oxygen has been increasingly reported. However, the influence of ubiquitous coexisting humic acid (HA) on this process is poorly understood. Here, we investigated the effect of different HA on center dot OH production during the oxygenation of reduced nontronite NAu-2 (rNAu-2), montmorillonite, and sediment. Results showed that HA could enhance center dot OH production, and the enhancement was related to the content of reactive Fe(II) in rNAu-2 and the electron-accepting capacity of HA. Coexisting HA leads to a new electron-transfer pathway from Fe(II) in rNAu-2 to HA (instead of the HA-Fe complex) and then to O-2, changing the first step of O-2 reduction from one- to two-electron transfer process with H2O2 as the main intermediate. Reduced HA decomposes H2O2 to center dot OH at a higher yield (13.8%) than rNAu-2 (8.8%). Modeling results reveal that the HA-mediated electron-transfer pathway contributes to 12.6-70.2% of H2O2 generation and 13.2-62.1% of center dot OH formation from H2O2 decomposition, with larger contributions at higher HA concentrations (5-100 mg C/L). Our findings implicate that HA-mediated electron transfer can expand the area of center dot OH production from the mineral surface to the aqueous phase and increase the yield of center dot OH production.