Impact of sulfate ion addition on electrochemical oxidation of anaerobically treated landfill leachate using boron-doped diamond anode

Anaerobic biological process is commonly applied for treating leachate generated from solid waste landfill. However, it is often found less effective to degrade leachate contaminants with low biodegradability, high salinity, and rich nutrient constituents. High concentration of organic and nitrogen still remained in the effluent of landfill leachate treated by anaerobic processes. Therefore, further treatment is required to efficiently meet the effluent standard prior to release into water bodies. In this study, electrochemical oxidation process using boron-doped diamond anode was investigated as a post-treatment for the treatment of anaerobically treated leachate effluent. Boron-doped diamond anode is able to generate of active species, such as OH^·, Cl^·, Cl₂^·−, and SO₄^·− as well as other oxidative agents to promote more indirect oxidation processes of non-biodegradable organic contaminants. The effect of sulfate ion addition on the overall performance of the electrochemical oxidation was investigated in a lab-scale of three-compartment electrochemical reactor, consisting of anode, cathode, and central compartments. A 2-L of the pre-treated leachate was recirculated in a batch mode by applying constant current density of 25, 37.5, and 50 mA cm⁻². The optimum removal of nitrogen and organic contaminants were obtained about 59% and 93%, respectively, at current density of 37.5 mA cm⁻², molar ratio 1:1 of [SO₄²⁻]:[Cl⁻] sulfate ion addition based on specific energy estimation and removal efficiency. The implied removal of 2.28 g COD and 1.77 g total inorganic nitrogen with the total energy required was 5.7 Wh g⁻¹ COD and 7.5 Wh g⁻¹ N, respectively. The results also revealed that addition of sulfate ion enhances organic removal through indirect oxidation and leads to the formation of nitrate without affecting overall total nitrogen removal.