In-situ Electrochemical Synthesis of H2O2 for p-nitrophenol Degradation Utilizing a Flow-through Three-dimensional Activated Carbon Cathode with Regeneration Capabilities

Abstract

The growing ubiquity of recalcitrant organic contaminants in the aqueous environment poses risks to effective and efficient water treatment and reuse. A novel three-dimensional (3D) electrochemical flow-through reactor employing activated carbon (AC) encased in a stainless-steel (SS) mesh as a cathode is proposed for the removal and degradation of a model recalcitrant contaminant p-nitrophenol (PNP), a toxic compound that is not easily biodegradable or naturally photolyzed, can accumulate and lead to adverse environmental health outcomes, and is one of the more frequently detected pollutants in the environment. As a stable 3D electrode, granular AC supported by a SS mesh frame as a cathode is hypothesized to 1) electrogenerate H₂O₂ via a 2-electron oxygen reduction reaction on the AC surface, 2) initiate decomposition of this electrogenerated H₂O₂ to form hydroxyl radicals on catalytic sites of the AC surface 3) remove PNP molecules from the waste stream via adsorption, and 4) co-locate the PNP contaminant on the carbon surface to allow for oxidation by formed hydroxyl radicals. Additionally, this design is utilized to electrochemically regenerate the AC within the cathode that is significantly saturated with PNP to allow for environmentally friendly and economic reuse of this material. Under flow conditions with optimized parameters, the 3D AC electrode is nearly 20% more effective than traditional adsorption in removing PNP. 30 grams of AC within the 3D electrode can remove 100% of the PNP compound and 92% of TOC under flow. The carbon within the 3D cathode can be electrochemically regenerated in the proposed flow system and design thereby increasing the adsorptive capacity by 60%. Moreover, in combination with continuous electrochemical treatment, the total PNP removal is enhanced by 115% over adsorption. It is anticipated this platform holds great promises to eliminate analogous contaminants as well as mixtures.

Keywords: Electrochemical oxidation; Fenton; activated carbon; electrochemical regeneration; heterogeneous catalysts.

Figure 1.

Flow removal of 30 mg/L PNP utilizing 5 mM Na₂SO₄ electrolyte and the large plug flow reactor detailed by A) comparing adsorption and ACSS mesh cathode mechanisms for PNP removal, (B) mg PNP removed per gram of AC from the C_t/C_o data in part A, (C) varying applied current density with 20g of AC mass within ACSS mesh for PNP removal, (D) mg PNP removed per ampere of electrical power for each timestep associated with part C, (E) varying mass of AC within the ACSS mesh cathode with a current density of 10 mA/g AC, and (F) mg PNP removed per gram of AC of applied current for each timestep associated with part F

Figure 2.

Flow removal of 30 mg/L PNP utilizing 5 mM Na₂SO₄ electrolyte and the small plug flow reactor detailed by A) comparing adsorption and ACSS mesh cathode mechanisms for PNP removal varying AC mass with a flowrate of 2 mL/min, (B) 10 run iterations of 10g ACSS mesh cathode longevity versus adsorption for 2-hr PNP removal at 10 mA/g current density and 2 mL/min (C) varying flowrate with 10g ACSS mesh cathode at 10 mA/g current density for PNP removal, (D) mg PNP removed per gram of AC for each flowrate determination in part C, (E) varying influent pH value with 10g ACSS mesh cathode with a current density of 10 mA/g AC and 2 mL/min, and (F) 2-hr TOC removal data for experiments represented in parts A and C

Figure 3.

Plug-flow regeneration of AC contained with the ACSS mesh utilizing 3 gram AC and varying applied currents/regeneration times to include regeneration of previously utilized ACSS mesh cathodes (applied current densities of 10 mA/g) that have become saturated. A flowrate of 2 mL/min was utilized for all trials