Efficiently removing dissolved organic pollutants in landfill leachate concentrate using dual-anode Fe2+/HClO system: The significance of insolubilization based on oxidative coupling of organics

Abstract

The strategy to mineralize dissolved organics for the decontamination of landfill leachate concentrate (LLC) is expensive. Because mineralization requires high stoichiometric ratio of oxidants (e.g., •OH) or electron transfer (i.e., energy) to the pollutant molecules. Whereas, transferring the dissolved organic pollutants into separable insoluble matters is an energy-saving strategy. Herein, we developed a dual-anode Fe2+/HClO system for removing dissolved organic pollutants in LLC through insolubilization based on oxidative coupling. In this system, the majority of dissolved organic pollutants were transferred into insoluble substances, which could be removed easily. As a result, 89.8 % of the dissolved organic carbon (DOC) in LLC was fast removed in 3 h with a low specific energy consumption (SEC, 4.0 kWh kg−1 COD and 17.6 kWh kg−1 DOC), and the formation of toxic by-products can be well controlled to total organic chlorine (TOCl) < 0.1 mg/L. The formation of insoluble substances was attributed to the oxidative coupling of organic small molecules into the macromolecular ones. This was evidenced by the generation of insoluble compounds with the larger molecular mass, and the formation of coupling or polymerization products of the model pollutant (guaiacol). The predominant mechanism was proposed that Fe(IV) was generated in the dual-anode Fe2+/HClO system, and Fe(IV) oxidized pollutants to produce organic radicals, then the organic radicals coupled and polymerized to form larger molecules and insoluble compounds. This work offers a novel electrochemical method for efficient removal of organic pollutants in LLC with minimal toxic by-products generated, and revealed the significance of oxidative coupling process in the electrochemical Fe2+/HClO system.