Optimization of PAHs oxidation from contaminated soil using modified nanoscale zero-valent iron combined with potassium permanganate

Abstract

A novel synergistic oxidation technology based on modified nanoscale zero-valent iron (nZVI) and potassium permanganate (KMnO₄) was developed for polycyclic aromatic hydrocarbons (PAHs) remediation in actual contaminated soil. In this study, three surfactants were used as dispersants to modify nZVI, including poly acrylic (PAA), sorbitan monolaurate (SPAN-20) and sugar esters (SE). The following parameters were studied to optimize the coupling oxidation process: dispersants/nZVI ratio, dosage of oxidant based on soil oxidation demand (SOD), amount of modified nanomaterials added in the coupling system. By using zeta potential, XRD, SEM, BET characterization methods, the results show that nZVI successfully coated with 5% PAA, 20% SE and 10% SPAN-20 have the best stability and mobility to effectively reduce the agglomeration effect. The conditions for treating PAH contaminated soil with the three best modified nanocomposites combined with KMnO₄ were studied. The optimal conditions were defined as [SE-nZVI] = 10% and [KMnO₄] = 40% SOD_max for 24 h at 25 °C. The synergistic oxidation process under these optimal conditions and the two unoptimized processes of KMnO₄ and nZVI-KMnO₄ degraded 85%, 58.9% and 62% of PAHs, respectively. This showed that the treatment effect of the optimized oxidation process was improved by 1.3–1.5 times. Further, by using gas chromatography-mass spectrometry (GC-MS), adsorption and electrophilic substitution reaction were speculated as the oxidation mechanism of PAHs treated by the coupling system of SE-nZVI-KMnO₄. PAHs could finally be decomposed into 9-methylene-9H-fluorene, fluoranthene and 1,5-diphenyl-1,4-pentadiyn-3-one and reached a safer status in the soil.