Advanced Reduction Processes for Degradation of Refractory Organics in Landfill Leachate

Hydroxyl radicals generated from advanced oxidation processes (AOPs) are broadly applied to mitigation of refractory organics in landfill leachate. However, following treatment by AOPs, the wastewater still contains highly oxidized organics that are recalcitrant to further chemical oxidation, thereby providing a challenge to established wastewater-treatment technologies. This study aimed to validate whether advanced reduction processes (ARPs), an emerging chemical degradation process driven by highly reductive hydrated electrons (${\mathrm{e}}_{\mathrm{aq}}^{-}$), can effectively decompose persistent and complex organics in a Fenton pretreated landfill leachate. Results showed that ARPs poorly cleave C-C bonds in the Fenton pretreated leachate organic matter (LOM) due to low reactivity of the mixed organics toward ${\mathrm{e}}_{\mathrm{aq}}^{-}$. However, ${\mathrm{e}}_{\mathrm{aq}}^{-}$ and ultraviolet (UV) irradiation during the ARP treatment could attack certain chromophores to reduce ${\mathrm{UV}}_{254}$ absorbance by 53%. The transformation particularly occurred to the low-molecular-weight ($<10\mathrm{kDa}$) and hydrophobic LOM fractions. Meanwhile, the carboxylic content declined after the ARP treatment, whereas the phenolic concentration remained constant, suggesting that ${\mathrm{e}}_{\mathrm{aq}}^{-}$ principally reacted with electron withdrawing groups rather than electron donating moieties on the Fenton pretreated LOM. This study highlights that ARPs can serve as a promising post-treatment after AOPs for treatment of refractory organic wastes in landfill leachate.