Peroxi-electrocoagulation for treatment of trace organic compounds and natural organic matter at neutral pH

Abstract

Iron-based oxidation technologies can be advantageous for mitigating trace organic compounds (TOrCs) during water and wastewater treatment due to their production of hydroxyl radicals. However, iron-based oxidation often occurs at acidic pH to promote Fenton's reaction, which limits the processes' feasibility for treatment applications. This study focused on utilizing iron-electrocoagulation (EC) paired with ex situ H₂O₂ addition (peroxi-electrocoagulation [EC:H₂O₂]) to promote oxidative reactions at neutral pH conditions. The hydroxyl radical probe para-chlorobenzoic acid (pCBA) was used to gauge oxidant activity and serve as a representative TOrC. The impact of water pH, current density, iron dose, H₂O₂ dose (i.e., [H₂O₂]_initial/[Fe²⁺]_generated ratio), and the presence of natural organic matter (NOM) were evaluated. Multivariable regressions showed that high levels of H₂O₂ relative to iron (i.e., [H₂O₂]_initial/[Fe²⁺]_generated ratio >0.7) inhibited the rate of pCBA oxidation, likely due to additional radical quenching from extra H₂O₂. Oxidation of pCBA was confirmed at neutral pH conditions, indicating that EC:H₂O₂ may potentially serve as a multi-mechanistic treatment technology capable of oxidation. Experiments were also conducted in real-world water samples to gauge EC:H₂O₂ applications for treating groundwater, river water, and primary treated wastewater. Overall, H₂O₂ addition enhanced the oxidative degradation of TOrCs while still removing NOM. The one exception was the primary effluent sample, which had the highest degree of oxidant scavenging of all matrices tested. The electrical energy per order (E_EO) metric demonstrated that EC:H₂O₂ is competitive with other TOrC oxidation technologies, with the added benefit of NOM mitigation in the same unit process.