Efficient degradation of soil polycyclic aromatic hydrocarbons by the nZVI/HO/OA system: The key role of oxalic acid.

Soil contamination with polycyclic aromatic hydrocarbons (PAHs) is severe, and their high biotoxicity and recalcitrance made effective remediation challenging. The heterogeneous Fenton process utilizing nanoscale zero-valent iron (nZVI) has been recognized as a promising technology for organic pollutant removal in various remediation media (e.g., soil, groundwater, sediments). However, nZVI cannot release sufficient Fe(II), limiting its ability to activate hydrogen peroxide (HO). Therefore, this study proposes the addition of oxalic acid (OA), which is economical, environmentally friendly, and readily available, to enhance nZVI-activated HO for remediating PAHs-contaminated soil. Results showed that the addition of OA to the nZVI/HO system increased the removal efficiency of benzo [a]pyrene (BaP) from 39.28 % to 95.33 % within 60 min, with over 80 % efficiency for various PAHs in both actual and spiked soils. Free radical scavenging experiments indicated that hydroxyl radicals (•OH) were the primary contributors to BaP removal. Further investigation of the mechanism revealed that OA promoted the dissolution of large amounts of Fe(II) from nZVI through the formation of a FeCO shell layer with high proton (H) conductivity on the surface of nZVI as well as the lowering of pH to provide H to nZVI. Based on the results of mass spectrometry analysis and ECOSAR and T.E.S.T. software, BaP degradation intermediates and their toxicity were identified. The toxicity of all degradation intermediates was lower than that of BaP. These findings provided a simple and cost-effective advanced oxidation method for the removal of PAHs from soil.