Surface Processes Control the Fate of Reactive Oxidants Generated by Electrochemical Activation of Hydrogen Peroxide on Stainless-Steel Electrodes

Abstract

Low-cost stainless-steel electrodes can activate hydrogen peroxide (H 2 O 2 ) by converting it into a hydroxyl radical ( • OH) and other reactive oxidants. At an applied potential of +0.020 V, the stainless-steel electrode produced • OH with a yield that was over an order of magnitude higher than that reported for other systems that employ iron oxides as catalysts under circumneutral pH conditions. Decreasing the applied potential at pH 8 and 9 enhanced the rate of H 2 O 2 loss by shifting the process to a reaction mechanism that resulted in the formation of an Fe(IV) species. Significant metal leaching was only observed under acidic pH conditions (i.e., at pH <6), with the release of dissolved Fe and Cr occurring as the thickness of the passivation layer decreased. Despite the relatively high yield of • OH production under circumneutral pH conditions, most of the oxidants were scavenged by the electrode surface when contaminant concentrations comparable to those expected in drinking water sources were tested. The stainless-steel electrode efficiently removed trace organic contaminants from an authentic surface water sample without contaminating the water with Fe and Cr. With further development, stainless-steel electrodes could provide a cost-effective alternative to other H 2 O 2 activation processes, such as those by ultraviolet light.

Electrochemical Hydrogen Peroxide Generation and Activation Using a Dual-Cathode Flow-Through Treatment System: Enhanced Selectivity for Contaminant Removal by Electrostatic Repulsion

“…The performance of the treatment train consisting of an air-diffusion electrode operated in series with a Pt/Ti electrode followed by a stainless-steel electrode was evaluated under various applied potentials using carbamazepine and atrazine as representative uncharged trace organic contaminants. When the stainless-steel electrode was operated at +0.02 V vs SHE, the potential found to be most efficient in conversion of H 2 O 2 into · OH, , the rate of removal of the contaminants increased as the potential of the air-diffusion cathode decreased from +0.02 to −0.10 V vs SHE (Figure a).…”

“…Although the divided cell configuration minimized H 2 O 2 loss, sending water from the air-diffusion electrode chamber directly to the second (i.e., stainless-steel) cathode was not conducive to contaminant oxidation because the relatively high pH of water leaving the first cathode (Figure S11), which decreased the efficiency of conversion of H 2 O 2 into · OH . Because anodically formed HOCl resulted in a relatively small loss of H 2 O 2 and acid production in the anode lowered the pH to values that were conducive to contaminant transformation, , a semidivided configuration was employed with the air-diffusion cathode placed between the Pt/Ti anode and the stainless-steel cathode (Figure d). A Pt mesh was placed downstream of the stainless-steel electrode to serve as the counter electrode for the stainless-steel cathode.…”

“…Additionally, use of a dual-cathode system avoids inefficiencies inherent in single electrode systems in which the same potential is applied for reductive processes that have different electrochemical potentials (Text S1). As a result of this inefficiency, systems employing single composite electrodes often produce excess H 2 O 2 or apply current to solutions after H 2 O 2 has been depleted. , In addition to increasing energy consumption, the mismatch between H 2 O 2 production and use can result in the presence of an unwanted oxidant in treated water (i.e., H 2 O 2 ) as well as the production of other unwanted products (e.g., H 2 ). , Finally, electrochemical generation of H 2 O 2 increases localized solution pH values, which can decrease the efficiency of H 2 O 2 activation …”

“…Recently, a low-cost, commercially available material, stainless-steel scrubbing pads, was investigated as an electrode for converting H 2 O 2 to · OH for contaminant oxidation. , Notably, the stainless-steel electrode demonstrated higher yields for converting H 2 O 2 into · OH (i.e., ∼70%) than those typical of other anodes or heterogeneous catalysts. For example, the observed efficiency for · OH production was over an order of magnitude higher than that reported for heterogeneous Fenton systems . Although the structure of the electrode lends itself to use as a flow-through electrode, the system was only studied in a batch reactor configuration.…”

“…Finally, the efficiency of advanced oxidation processes is limited by the low selectivity of · OH, especially in the presence of natural organic matter (NOM)a ubiquitous component of surface and groundwater that competes with contaminants for · OH. ,, In an electrochemical treatment system, the electric potential gradient can be used to separate charged species from uncharged species. The negatively charged cathode may be able to repel negatively charged NOM away from its surface and prevent its reaction with · OH produced on the electrode surface.…”