Microwave-enhanced Fe⁰/PDS system for treating refractory organic matter in biotreated landfill leachate effluent

The effluent produced from treating landfill leachate via biological methods still contains a large amount of refractory organic matter and needs to be further treated. In this study, we used a microwave-enhanced zero-valent iron/peroxydisulfate (MW/Fe⁰/PDS) system for the further treatment of biotreated landfill leachate effluent (BTLE). We investigated the efficiency and mechanism of removing refractory organic matter from BTLE. The results show that under the conditions of MW = 240 W, [PDS]₀ = 40 mM, [Fe⁰]₀ = 10 mM, and t = 12 min, the MW/Fe⁰/PDS system best removed organic matter in the BTLE. The removal rates of UV absorbance at 254 nm, color number, and total organic carbon reached 76.94%, 93.50%, and 63.08%, respectively. The aromaticity, degree of humification, molecular weight, and degree of polymerization of the organic matter decreased significantly. The surface morphology and elemental valence analysis of Fe⁰ before and after the reaction revealed that iron-based oxides (i.e., Fe₂O₃, Fe₃O₄, FeOOH, and Fe (OH)₃) formed on the surface of Fe⁰. The refractory organic matter was removed by homogeneous and heterogeneous Fenton-like reactions and the adsorption-precipitation of iron-based colloids. The thermal and nonthermal effects of MW promoted direct PDS decomposition, accelerated the formation of Fe²⁺ on the surface of Fe⁰, promoted the redox cycle of Fe³⁺/Fe²⁺, and generated more sulfate radicals (SO₄ ^•-). Together, these effects improved the removal efficiency  of refractory organic matter. The results of this study can serve as a theoretical basis for the use of the MW/Fe⁰/PDS system to treat refractory organic matter in BTLE. PRACTITIONER POINTS: A microwave-enhanced Fe⁰/PDS system is effective for removing organic matter from landfill leachate. Colored and fluorescent organic matter in the landfill leachate was efficiently degraded. Heterogeneous and homogeneous effects Fenton reactions are involved in the system.