Insight on the generation of reactive oxygen species in the CaO₂/Fe(II) Fenton system and the hydroxyl radical advancing strategy

Calcium peroxide (CaO₂) is a stable hydrogen peroxide (H₂O₂) carrier, and the CaO₂/Fe(II) system has been applied for treatment of various pollutants. It is commonly reported in the literature that hydroxyl radical (HO^[rad]) and superoxide radical anions (O₂ ^[rad]−) are the two main reactive oxygen species (ROSs) generated in the CaO₂/Fe(II) system. However, many of the reported results were deduced from degradation performance rather than specific testing of radical generation. Thus, the specific generation of ROSs and the influence of system conditions on ROSs yield are still unclear. To our knowledge, this is the first study specifically focusing on the generation of HO^[rad] and O₂ ^[rad]− in the CaO₂/Fe(II) system. Experimental conditions were optimized to investigate the production of HO^[rad] and O₂ ^[rad]−. The results showed the influences of CaO₂, Fe(II), and solution pH on HO^[rad] and O₂ ^[rad]− generation, and the HO^[rad] generation efficiency was reported for the first time. In addition, the ROSs generation pathways in the CaO₂/Fe(II) system were elucidated. A strategy for enhancing HO^[rad] yield is developed, based on the continuously dosing Fe(II). This proposed strategy has implications for the effective application of in situ chemical oxidation employing CaO₂/Fe(II) for groundwater remediation.