Ferrate(VI) based chemical oxidation for the remediation of aged PCB contaminated soil: Comparison with conventional oxidants and study of limiting factors

Ferrate (Fe(VI)) has emerged as an efficient oxidant to treat organic pollutants in aqueous solution. However, its application has never been assessed to remediate the contaminated soils. Here, we report the first study to use Fe(VI) for chemical oxidation of PCBs in historically contaminated soils obtained from an industrial wasteland. The first part of this study explores the efficiency of ferrate(VI) to degrade PCBs under various experimental conditions (liquid/solid ratio, oxidant dose, temperature and reaction time). Integrated application of Fe(VI) with conventional oxidants (hydrogen peroxide H2O2, persulfate S2O82− and peroxymonosulfate HSO5−) was also tested. Conventional oxidants resulted in only 2-12% PCB degradation while Fe(VI) removed 30% of PCBs. Integrated use of Fe(VI) and conventional oxidants improved the degradation efficiency which was the highest (40% removal) by Fe(VI)/HSO5−. This could be attributed to two-step degradation proceeding firstly by electron transfer with Fe(VI) followed by radical attack of conventional oxidants. To evaluate if PCB degradation is limited by soil factors, role of carbonate content, overall soil matrix, and PCB availability were assessed as a second part of this study. Negative role of soil matrix was highly prominent for conventional oxidants than Fe(VI). An increase in PCB availability improved the treatment efficiency (45% removal by Fe(VI) alone) highlighting the role of soil factors. All these results indicate the higher suitability and efficiency of Fe(VI) as compared to the conventional oxidants for soil remediation. Since use of Fe(VI) is innovative for both soil remediation and PCB oxidation, degradation mechanisms are also proposed.