Fate of PAHs under subcritical and supercritical conditions in landfill leachate: Removal or formation?

[Display omitted] •The removal of COD and PAHs with the SCWO process was not directly correlated.•Both subcritical and supercritical conditions could lead to the formation of PAHs at low oxidant dose.•The SCWO process has a high potential for degradation and removal of PAHs in controlled conditions. In this study, fate of eight polycyclic aromatic hydrocarbons (PAHs) described as priority pollutants by the European Union during the subcritical and supercritical water oxidation (SCWO) of landfill leachate was investigated. The experiments were conducted under the temperature of 250–500 °C, pressure of 10–35 MPa, residence time of 2–18 min and dimensionless oxidant dose (DOD) of 0.2–2.3. The results revealed that the SCWO process had a high potential for degradation of PAHs except for four and more rings from leachate having high organic and inorganic pollutant loads. The removal efficiencies of chemical oxygen demand (COD), naphthalene (NAP), anthracene (ANT) and fluoranthene (FL) increased up to 98%, 97%, 95%, and 87%, respectively. However, the high removal efficiencies of COD did not directly correlate with the removal of PAHs. 2- to 4-ring PAHs increased in subcritical and supercritical conditions at 0.4 DOD. Additionally, PAH compounds such as pyrene and chrysene not found in raw leachate were detected in some treated samples at low DOD and high temperature. It was concluded that aromatic substances detected in the leachate could transform to naphthalene, anthracene and fluoranthene with merger/separation reactions. On the other hand, aldehyde, ketone and phenol-based compounds were detected as the transformation products when the oxidation mechanism was dominant. Therefore, optimum experimental conditions that aimed to both minimize PAHs formation and maximize COD removal were determined as 7 min, 2 DOD, 375 °C and 22.5 MPa.