Remediation of PNP-contaminated groundwater using a modified CaO₂/Fe(II) Fenton system: Reactive principles, degradation performance and potential pathways

Calcium peroxide (CP, CaO₂), acting as a source of H₂O₂, was combined with Fe(II) to provide a novel modified Fenton system. The removal of p-nitrophenol (PNP) in groundwater as a model pollutant was investigated using this system while addressing effect of reagents on water quality, mechanism of reactive oxygen species formation, possible degradation pathways, impact of geochemistry and process parameters on the removal efficiency. The dissolved oxygen, oxidation-reduction potential, pH, electrical conductivity and H₂O₂ concentration in groundwater were all increased significantly due to the addition of CaO₂ and Fe(II), especially in the early stage and under acidic conditions. Within 30 min, 96% of an initial 40 mg/L PNP contamination was removed using the optimum conditions, consisting of 250 mg/L CaO₂, 50 mg/L Fe(II), an initial pH of 2.6, oscillation frequency of 170 rpm and temperature of 298 K. The batch experiments revealed that the initial pH and the Fe(II) and CaO₂ concentrations all decisively affected removal efficiency, while the groundwater dynamics, temperature and illumination conditions had little effect. The presence of hydroxyl radicals (∙OH) and singlet oxygen (¹O₂) was confirmed by electron paramagnetic resonance spectroscopy. In addition, ∙OH quenching experiments indicated that the former rather than the latter played a major role in PNP removal. The generation of ¹O₂ in this system was also predicted by classical molecular orbital theory. These findings would provide useful technical support for the remediation of groundwater contamination.