Effect of reactor configuration on the kinetics and nitrogen byproduct selectivity of urea electrolysis using a boron doped diamond electrode. (1st January 2020)

• Record Type: Journal Article
• Authors: Schranck, Andrew; Doudrick, Kyle
• Is Part Of: Water research. Volume 168(2020)
• Publication Date: 2020-01-01
• Subjects: Urea -- Urine -- Electrolysis -- Boron doped diamond -- Undivided reactor -- Nitrogen byproducts; Water -- Pollution -- Research -- Periodicals; 363.7394
• DOI: 10.1016/j.watres.2019.115130
• Languages: English
• ISSNs: 0043-1354

Abstract

Abstract: Electrochemical systems have emerged as an advantageous approach for decentralized management of source-separated urine with the possibility of recovering or removing nutrients and generating energy. In this study, the kinetics and byproduct selectivity of the electrolytic removal of urea were investigated using a boron doped diamond working electrode under varied operational conditions with a primary focus on comparing undivided and divided reactors. The urea removal rate in the undivided and divided reactors was similar, but the divided reactor had an increased required cell voltage needed to maintain the equivalent current density. The current efficiency was similar for 0.1, 0.25, and 0.5 A (33.3, 83.3, 167 mA/cm 2 ), suggesting no interference from competing reactions at higher potentials. In a divided reactor, increasing the anolyte pH reduced the urea removal rate presumably from hydroxyl radical scavenging by hydroxide. Further, for all divided reactor experiments, the final pH was less than 1, suggesting that the transport of protons across the ion exchange membrane to the cathode was slower than the oxidation reactions producing protons. The nitrogen byproduct selectivity was markedly different in the undivided and divided reactors. In both reactors, nitrate (NO3 − ) formed as the main byproduct at the anode, but in the undivided reactor it was reduced at the stainless steel cathode to ammonia. In the presence of 1 M chloride, the urea removal kinetics improved from the generation of reactive chlorine species, and the byproduct selectivity was shifted away from NO3 − to presumably chloramines and N2 . Overall, these results indicate that the electrochemical reactor configuration should be carefully considered depending on the desired outcome of treating source-separated urine (e.g., nitrogen recovery, H2 generation). Graphical abstract: Image 1 Highlights: Urea removal rates were similar in undivided and divided reactors. Total ammonia nitrogen was the main urea removal byproduct in the undivided reactor. Nitrate was the main urea removal byproduct in the divided reactor. The presence of Cl − increased urea oxidation kinetics and shifted the byproduct to presumably chloramines and dinitrogen.