Electrooxidation of urea at the ruthenium titanium oxide electrode

Abstract

Urea adsorbs on the ruthenium titanium oxide electrode, depressing the observed current. For artificial kidney dialysate concentrations of urea and NaCl (0.50 kg/m3 and 100 mol/m3, respectively), the major electrolysis products are N, , CO,, 0,, and H, , and the reaction mechanism is solution oxidation of urea by anodically generated active chlorine. A nitrogen-yielding direct electrode reaction is observed at high urea concentration (30 kg/m3) and low NaCl concentration (10-100 m o~ / m3). J. C. Wright, A. S. Michaels SCOPEUrea is the major nitrogenous waste metabolite produced by the body. Bioengineering applications of urea oxidation by electrochemical means include water reclamation from urine for extended space flight (Lockheed, 1977), implantable bioelectrochemical sensors (Marincic et al., 1979), and dialysate regeneration for the artificial kidney (Yao et al., 1974;Bizot and Sausse, 1975; Fels, 1978). Previous investigations of urea electrooxidation have almost exclusively involved the use of platinum electrodes. Most investigators have claimed that the chloride in physiological solutions is first oxidized to active chlorine, and that urea is oxidized by the active chlorine in a bulk solution reaction (Bizot and Sausse, 1975;Lockheed, 1977; Fels, 1978;Quellhorst et al., 1978). However, urea adsorption on platinum electrodes has been confirmed by radiotracer experiments (Gromyko et al., 1979;Horanyi et al., 1979), and direct electrode reaction of urea has been reported (Gromyko et al., 1973(Gromyko et al., , 1974Keller et al., 1980). High surface area ruthenium titanium oxide electrodes (Beer, 1966(Beer, , 1967) provide a potential alternative to platinum electrodes. In the present study the electrochemistry of urea at the ruthenium titanium oxide electrode primarily was investigated for aqueous solution concentrations of urea and chloride found in artificial kidney dialysate; more concentrated solutions also were studied. CONCLUSIONS AND SIGNIFICANCEUrea (0.50-30.0 kg /m3) depressed the observed anodic current, thus indicating urea adsorption on the ruthenium titanium oxide electrode. In the absence of a cell separator, electrolysis of solutions representative of artificial kidney dialysate (pH 7.5, 100 mol/m3 NaCl + 0.5 kg/m3 urea) above the chloride discharge potential yielded N, and CO, (from urea oxidation) and 0, (from water oxidation) at the anode; H, is produced at the cathode. The N, evolved matched the urea con- 1450September 1986 sumed within experimental error (+ 15%). Coulombic efficiencies for urea oxidation of 18 to 54% were observed. For these solutions, the mechanism for urea electrooxidation at the ruthenium titanium oxide electrode involves solution oxidation of the urea by anodically generated active chlorine in a series of reaction steps. The rate of the last overall step, which converts chlorinated nitrogen compounds to molecular nitrogen, increases with pH. At high urea concentrations (30 kg/ rn3) and low NaCl concentrations (10-100 mol/m3), a direct electrode reaction is observed...

Effects of carbonate on the electrolytic removal of ammonia and urea from urine with thermally prepared IrO2 electrodes

“…For instance, on RuO 2 anodes, a more efficient indirect oxidation was observed at higher pH values and high NaCl concentrations [33].…”

“…7, or indirect oxidation, via electro-generated active chlorine, as indicated by Eq. 8 [32,33]. Simka and Piotrowski [22] further differentiate indirect urea electrolysis through chlorine adsorbed at the anodic surface and through active chlorine present in the bulk of the solution.…”

“…The mechanism of urea oxidation and its products depend strongly on the process parameters, and especially on the initial concentration of chloride, the pH value of the solution, the applied current density, and the applied anodic potential [31,33]. For instance, on RuO 2 anodes, a more efficient indirect oxidation was observed at higher pH values and high NaCl concentrations [33].…”

Urea removal from aqueous solutions—a review

“…The rate and effectiveness of urea decomposition increased with the acidity of the solution and with the process temperature [53,56]. When the process was realized on a surface of RuO 2 -TiO 2 -coated titanium electrode, urea decomposed into CO 2 and N 2 with the formation of chlorourea and subsequently dichlorourea [57]. Hernlem studied an electrolytic system in which a urea-contaminated solution was treated with the addition of a chloride species [47].…”

“…Wright et al [57] investigated the decomposition of urea from artificial kidney dialysate in solutions with different amounts of urea and sodium chloride using RuO 2 -SnO 2 -TiO 2 electrodes. The urea was decomposed by anodically generated active chlorine and the major products of the electrolysis were N 2 , CO 2 , H 2 , and O 2 .…”