Effect of chloride ions on the degradation of phenanthrene by persulfate activated by zero-valent iron nanoparticles loaded with carbon

Advanced oxidation processes (AOPs) utilizing iron-based catalysts for the activation of persulfate (PS) have been widely applied for the remediation of soils and groundwater. The presence of chloride ions (Cl⁻) in groundwater can affect the reactivity of the sulfate radical system, yet the explanation for this behavior remains inconsistent and its mechanism was not well understood. Therefore, this study investigated the effects of Cl⁻ and pH on the degradation of phenanthrene (PHE) in a biochar-supported nanoscale zero-valent iron activated PS (BC@nZVI/PS) system, based on the free radicals generated in the system. Additionally, the influence of the reaction pathway on the properties of degradation products or intermediates was examined. The results showed that Cl⁻ had a dual effect on the degradation of PHE, and the release of Fe²⁺ was accelerated at Cl⁻ concentrations greater than 1 mM, and the degradation efficiency was usually higher at low pH conditions, but there was a potential risk of secondary pollution. The presence of Cl⁻ altered the types and concentration distribution of free radicals in the system. Gas chromatography-mass spectrometry (GC-MS) identified chlorinated by-products, and density functional theory (DFT) calculations were used to hypothesize the degradation pathway of PHE. These findings are significant for evaluating the practical applications of iron-based catalyst-activated PS technologies.