An improved process for removal and recovery of heavy petroleum from solids using a ferrate-based hybrid oxidant

Persulfate oxidants are widely used in soil remediation and wastewater treatment but perform poorly in degrading polycyclic aromatic hydrocarbons (PAHs), especially heavy fractions in solids. Herein, we propose the utilization of a green peroxymonosulfate-ferrate-FeS (PFI) oxidant as a promising process aid for remediating soils contaminated with heavy petroleum components, including asphaltenes and resins. The PFI oxidant could degrade heavy petroleum fractions because of dual activation of the peroxymonosulfate and ferrate by FeS at ambient conditions. Nevertheless, when dealing with soil with high oil content (>10%), the degradation efficiency remains limited (<30%) regardless of the quantity of oxidants employed. Surface elemental analysis shows that a coating of secondary products (Fe(OH)₃, Fe₂O₃) on the surface and in pores of the soil-pollutant matrix explains the failure of oxidation and inefficient use of oxidant. To address this issue, a strategy of pre-solvent extraction-oxidation hybrid process with sequent acidic washing is proposed, where dichloromethane serves as the solvent, and PFI acts as the oxidant. In this system over 90% of the oil could be recovered with an oxidation efficiency of 80% by alleviating the problem of iron oxide coating the matrix surface. The oxidant consumption is also reduced to 70 wt% of the sludge. The PFI oxidant is found to exhibit excellent universality in treating oily sludge with low petroleum content (<2%), reducing the petroleum content in the residue to less than 0.3 wt% (meeting the national standards). The degradation of low oil content sludge by the PFI oxidant followed pseudo first-order kinetics. These findings not only elucidate the failure of PFI oxidation for high oil content oily sludge and identify its potential engineering application range, but also offer a practical strategy for processing petroleum-contaminated soil with varying oil contents through wet oxidation.