Light-driven urea oxidation for a wearable artificial kidney

For the development of a wearable artificial kidney (WAK) that uses a small dialysate volume that is continuously regenerated, it is essential that urea, one of the main uremic retention solutes, is removed. Despite advances in sorbent technology or electro-oxidation no safe, efficient and selective method for urea removal has been re-ported that allows miniaturization of the artificial kidney to wearable proportions. Here we have developed a flow cell for light-driven, photo-electrocatalytic (PEC) urea removal for use in a WAK. We use a photo-active material (hematite) coated with a catalyst (NiOOH) as working electrode for selective urea oxidation and a silver-chloride (AgCl) cathode. The use of the AgCl counter electrodes eliminates the need for an external bias voltage, and allows operation under light illumination only. Using LED illumination (460 nm) we show that urea is selectively oxidized over chloride. N-2 formation is confirmed by gas-phase analysis of the headspace of the sample vial, using mass spectrometry. Other nitrogen containing products include nitrite but importantly ammonia and nitrate are not detected. Using the PEC concept a urea removal rate of 2.5 mu mol/cm(2)h (or 0.15 mg/ cm(2)h) has been achieved. Extrapolating our results to an upscaled system, a surface area of 0.5 m(2) would enable efficient removal of the daily produced amount of urea (similar to 300 mmol) urea within 24 h, when driven by LED illumination only.