Mechanistic insights into the pH-driven radical transformation of the Fe(II)/nCP in groundwater remediation.

Calcium peroxide nanoparticles (nCP) as a versatile and safe solid H₂O₂ source, have attracted significant research interst for their application potential in groundwater remediation. Compared to the traditional Fenton system, the nCP-based Fenton-like system has a wider pH-working window for contaminants degradation. This results from the dominant radical transformation under different pH. Unlike the traditional Fenton system which is only effective in acid conditions with hydroxyl radical (•OH) as the main active species, the release of H₂O₂ and O₂ from nCP provides multiple contaminants degradation pathways. In acidic environments, •OH and Fe(IV) predominate as the active species, facilitated by substantial H₂O₂ production which activates the Fenton reaction. In neutral or alkaline conditions, the production of H₂O₂ was dramatically decreased. While the O₂ released from nCP can be catalyzed by Fe(II) to form superoxide radical (•O₂^-), which subsequently generate singlet oxygen (¹O₂). The formation pathway of •O₂^- was tracked by O¹⁸ isotope labeling experiment. The impact of the water matrix on radical generation in the Fe(II)/nCP Fenton-like system was also studied. This research deepens the understanding of the radical formation mechanisms in nCP-based Fenton-like system, offering insights to support their application in remediating contaminated groundwater.