Chelating agents EDTA and oxalic acid for enhanced Fenton treatment of landfill leachate at neutral pH

Sanitary landfill leachate is a complex effluent characterized by high concentrations of recalcitrant organic matter, particularly humic substances, and ammoniacal nitrogen. Among Advanced Oxidation Processes (AOPs), the Fenton process (FP) is effective for the degradation of refractory compounds; however, its application is typically restricted to acidic conditions due to iron precipitation. This study evaluated the effects of ethylenediaminetetraacetic acid (EDTA) and oxalic acid (OA) as chelating agents in the Fenton treatment of biologically pretreated landfill leachate, aiming to enable operation at near-neutral pH and maintain iron solubility. The investigation was conducted in three stages: (I) assessment of reaction pH (5.0 and 7.0); (II) optimization of chelating agent dosage at neutral pH; and (III) a fractional factorial design evaluating the effects of H₂O₂, Fe²⁺, and chelant concentrations. The results showed that EDTA did not enhance Fenton performance for the evaluated leachate, achieving a maximum overall COD removal of 21.9% at an Fe:EDTA ratio of 1:0.2. In contrast, OA significantly improved treatment efficiency at pH 7.0. Under optimized conditions identified in Stage 3 (25 mM Fe²⁺, 290 mM H₂O₂, and 16 mM OA), an overall COD removal of 72.4% was achieved, with 35.4% attributed to the chemical oxidation step. These findings demonstrate that OA effectively expands the operational range of the Fenton process to near-neutral pH while allowing the use of lower iron concentrations, thereby reducing reagent consumption and sludge generation.