Heterogeneous Fenton-Like Catalysis of Electrogenerated H2O2 for Dissolved RDX Removal

Abstract

New insensitive high explosives pose great challenges to conventional explosives manufacturing wastewater treatment processes and require advanced methods to effectively and efficiently mineralize these recalcitrant pollutants. Oxidation processes that utilize the fundamental techniques of Fenton chemistry optimized to overcome conventional limitations are vital to provide efficient degradation of these pollutants while maintaining cost-effectiveness and scalability. In this manner, utilizing heterogeneous catalysts and in-situ generated H₂O₂ to degrade IHEs is proposed. For heterogeneous catalyst optimization, varying the surface chemistry of activated carbon for use as a catalyst removes precipitation complications associated with iron species in Fenton chemistry while including removal by adsorption. Activated carbon impregnated with 5% MnO₂ in the presence of H₂O₂ realized a high concentration of hydroxyl radical formation - 140 μM with 10 mM H₂O₂ - while maintaining low cost and relative ease of synthesis. This AC-Mn5 catalyst performed effectively over a wide pH range and in the presence of varying H₂O₂ concentrations with a sufficient effective lifetime. In-situ generation of H₂O₂ removes the logistical and economic constraints associated with external H₂O₂, with hydrophobic carbon electrodes utilizing generated gaseous O₂ for 2-electron oxygen reduction reactions. In a novel flow-through reactor, gaseous O₂ is generated on a titanium/mixed metal oxide anode with subsequent H₂O₂ electrogeneration on a hydrophobic microporous-layered carbon cloth cathode. This reactor is able to electrogenerate 2 mM H₂O₂ at an optimized current intensity of 150 mA and over a wide range of flow rates, influent pH values, and through multiple iterations. Coupling these two optimization methods realizes the production of highly oxidative hydroxyl radicals by Fenton-like catalysis of electrogenerated H₂O₂ on the surface of an MnO₂-impregnated activated carbon catalyst. This method incorporates electrochemically induced oxidation of munitions in addition to removal by adsorption while maintaining cost-effectiveness and scalability. It is anticipated this platform holds great promise to eliminate analogous contaminants.

Keywords: Explosives; RDX; electroperoxidation; heterogenous catalysts; oxidation processes.

FIGURE 1 |

Setup for combined H₂O₂ generation in a plug flow reactor and subsequent model pollutant removal in packed AC-Mn5 column.

FIGURE 2 |

H₂O₂ electrogeneration optimization for (A) Current intensity (B) Influent pH values (C) Influent flow rate, and (D) H₂O₂ mass flow rate.

FIGURE 3 |

(A) Reusability of single CCMPL cathode for H₂O₂ electrogeneration and (B) corresponding steady state current efficiencies (CE) for each non-continuous consecutive run of H₂O₂ electrogeneration.

FIGURE 4 |

(A) H₂ O₂ concentration on hydroxyl radical formation and (B) Hydroxyl radical formation at varying pH values.

FIGURE 5 |

Batch 25 mg/L RDX removal due to (A) 10 mg (8 mg AC-Mn5/mg RDX) (B) 25 mg (20 mg AC-Mn5/mg RDX), and (C) 50 mg (40 mg AC-Mn5/mg RDX) of AC-Mn5 catalyst.

FIGURE 6 |

Batch 25 mg/L RDX removal with 10 (8 mg AC-Mn5/mg RDX) and 25 mg (20 mg AC-Mn5/mg RDX) AC-Mn5 catalyst for 5 h adsorption and subsequent 5-min catalysis with 2 mM H₂O₂ (A) RDX Ct/Co (B) Normalized RDX removal per minute.

FIGURE 7 |

Combined H₂O₂ generation and RDX removal by adsorption + H₂O₂/AC-Mn5 catalysis under flow (A) Sustained H₂O₂ electrogeneration with optimized CCMPL cathode (B) RDX Ct/Co utilizing 1 gram AC-Mn5 with/without in-situ H₂O₂ electrogeneration, and (C) Mass RDX breakthrough of columns with/without in-situ H₂O₂ electrogeneration.