|Hydrogen peroxide (H2O2) is an effective, environmentally friendly oxidant used directly and in advanced oxidation processes for water and wastewater treatment. Although most H2O2 is currently manufactured offsite, concentrated, and trucked to sites where it must be safely stored and handled, it may also be generated on site electrochemically, commonly over a gas diffusion electrode (GDE). This technology has the potential to make processes more economical, feasible, available, and flexible depending on the application and the site.
H2O2 electrogeneration has been heavily studied but there are particular knowledge gaps around performance of commercially available, unmodified, metal-free GDEs, contradictory evidence about optimum pH and cathode potential, a lack of studies using continuous mode reactors, weak quantification mass flows inside reactors, and a lack of studies looking at in situ treatment that take advantage of anodic oxidation and that do not require advanced oxidation. The present work addresses these gaps in two phases of experimentation (Chapters 3 and 4) and provides a brief comment on economic viability (Chapter 5)
Firstly, effects of hydraulic retention time, cathode potential, reactor geometry, and pH in a continuous mode dual chamber reactor, including kinetic quantification and mass balance modelling are studied in Chapter 3. Performance shows a tradeoff where concentrations up to ~6500 mg L-1 may be generated but at a CE approaching 0% while CE near 100% can be maintained when H2O2 is produced at ~22 mg L-1. Additionally, a microbial electrochemical cell (MEC) is demonstrated to have a comparable current density to abiotic tests.
Secondly, in Chapter 4, sulfur(IV) is demonstrated to be treated by both electrogenerated H2O2 and anodic oxidation in a single chamber electrolysis cell. CE is improved 3-8 times compared to H2O2 production alone, and near complete removal (at a low CE of 61.1%) or near 200% CE (at a low removal rate of only 27%) are achieved under various operating conditions, with intermediate values obtained by changing operating conditions.
In summary, this work is establishes the maximum performance H2O2 electrogeneration under realistic conditions and shows how in situ treatment improves system efficiency by reducing H2O2 loss and taking advantage of anodic oxidation. The advantages of in situ production (and treatment) are complemented by a predicted comparable operating cost to traditional H2O2 technologies, suggesting that this technology is ready for scaling up and commercialization and has to potential to help secure safe water resources for the future.