Design, Development, and Operation of an Electrochemical Urea Removal Reactor for the Application of Portable Dialysis

Abstract

Electrochemical investigations of Pt and Ni-based catalysts were performed in pursuit of designing a reactor capable of oxidizing 15 grams of urea per day for the use of portable dialysis. The electrocatalytic experiments were primarily performed in an alkaline aqueous three-electrode cell utilizing cyclic voltammetry and chronoamperometry; this served to elucidate the oxidation reaction and deactivation mechanisms and measure the kinetic parameters of urea oxidation on various catalysts including Pt, Ni, Ni(OH)2, and 1 mol% iron-doped Ni(OH)2. The hierarchy of urea oxidation activity in alkaline media is summarized as Ni(OH)2 = Fe-Ni(OH)2 > Ni >> Pt. Creatinine, the second most concentrated solute in urine, was also electrochemically investigated on Ni foam-based electrodes. The creatinine activity is summarized as Fe-Ni(OH)2 > Ni(OH)2 > Ni. Creatinine oxidizes on Fe-Ni(OH)2 and exhibits a concentration and time dependent catalyst deactivation mechanism. Application of these electrokinetic studies led to design and development of a bench-scale urea removal cell (URC) utilizing a Ni(OH)2 foam anode, a Pt/C supported on carbon cloth cathode, and an alkaline exchange membrane electrolyte. The URC achieves urea removal rates of about 35 mg/cm2/day from an anolyte feed of 10 mM urea in 0.1 M KHCO3 operating at a pH of 7.4, a temperature of 37 °C, a flowrate of 20 ml/min, and a cell voltage of 1.9 V. The addition of chloride into the anolyte feed induced pitting corrosion of the anode due to a combination of the neutral pH and applied voltage. Early corrosion experiments indicated that even 1% of physiological Cl– concentrations in the URC create a corrosive environment, primarily due to the Ni foam substrate rather than Ni(OH)2 catalyst. The reported electrocatalytic urea oxidation research and its application to an electrochemical reactor are directly applicable to the general field of wastewater remediation via electrolysis, which has less strict physiological conditions and portability requirements.