Electro-oxidation of cyanide on active and non-active anodes: Designing the electrocatalytic response of cobalt spinels

[EN] The feasibility of the electrochemical technologies for wastewater treatment greatly relies on the design of efficient but inexpensive electrocatalysts. It is generally accepted that the so-called ¿non-active¿ anodes (like the boron-doped diamond (BDD) or SnO2-based anodes), producing highly oxidizing hydroxyl radicals, are the most promising candidates for pollutants abatement. In this work, the electrocatalytic performance of various cobalt oxides, pure and doped with Cu or Au, for CN¿ oxidation has been studied and compared with that of conventional graphite, BDD, SnO2-Sb and SnO2-Sb-Pt. The metal oxide electrodes were prepared by thermal decomposition of the salt precursors onto Ti. For the M-doped Co3O4 electrodes, the nominal M/Co ratios were Cu/ Co=0.07¿1.00; and Au/Co=0.05¿0.20. The electrodes were characterized by different techniques (XRD, SEM, EDX, XPS) and their electrocatalytic response was studied by cyclic voltammetry and galvanostatic electrolysis in a H-type cell in aqueous 0.1M NaOH. The obtained results show that the nature of the dopant plays a key role on the electrocatalytic behavior of cobalt spinels. Thus, while Cu catalyzes the CN¿ electro-oxidation, Au declines it. This is explained by the fact that, unlike Au (which segregates as Au-rich particles), Cu is effectively incorporated into the spinel structure by forming a solid solution (CuxCo3-xO4). In this solid solution, atomic scale Cu(spinel)-CN¿ specific interactions occur to catalyze the reaction, whereas in segregated Au particles the oxidation is hindered probably by a too-strong adsorption of cyanide and/or its inaccessibility to oxide active sites. Electrolysis runs have revealed that ¿active¿ over-saturated Cu-doped spinels (Cu/Co=1.00) exhibit higher current efficiencies than conventional graphite and ¿non-active¿ BDD and SnO2-based anodes. Hence, we hereby demonstrate that an inexpensive ¿active¿ electrocatalyst can show even higher efficiency than the most powerful BDD anode. These results highlight the significance of anode design in the application of the electrochemical technique for wastewater treatment.Financial support from the Spanish Ministerio de Economia y Competitividad and FEDER funds (MAT2016-76595-R, IJCI-2014-20012) is gratefully acknowledgedBerenguer, R.; Quijada, C.; La Rosa-Toro, A.; Morallón, E. (2019). Electro-oxidation of cyanide on active and non-active anodes: Designing the electrocatalytic response of cobalt spinels. Separation and Purification Technology. 208:42-50. https://doi.org/10.1016/j.seppur.2018.05.024S425020

Electrooxidation of cyanide on cobalt oxide anodes

Oxidation of cyanide ions at a Ti/Co3O4 electrode in aqueous base solution has been investigated. The cyclic voltammetric curve for the oxidation of cyanide at Ti/Co3O4 shows a well formed wave prior to oxygen evolution at a potential where the spinel surface itself undergoes oxidation. Using a flow cell it is confirmed that the conversion of cyanide (CN-) to cyanate (CNO-) can be achieved galvanostatically with a reasonable current efficiency. As an example, at a current density of 100 A m(-2), CN- concentration can be lowered from 10 to 0.2 mM with an electric energy consumption of about 18 kWh kg(-1) of CN- oxidized and a global current efficiency of 28.5%. The oxide coating appears to be quite stable during repeated electrolyses

Potential use of carbon felt in gold hydrometallurgy

The use of carbon felt as a three-dimensional electrode appears to be very promising for the recovery of heavy metals, and toxic compounds removal from dilute solutions, considering its favourable physico-chemical properties : high specific surface area, good fluid permeability and compressibility, chemical inertness and good electrical conductivity. This work presents the contribution of the carbon felt electrode in two different steps of the gold cyanidation recovery process : firstly as a cathode for the electrowinning of gold solution obtained after the elution of loaded carbon; secondly as an anode for the electro-oxidation of cyanide ions present at low concentration (200-300mg/l) in waste streams. In the first case, more than 10 kg of gold per m(2) of felt (2000 kg of gold per m(3) of felt) can be loaded at 400 A/m(2) from dilute gold solutions (30 mg/l Au) with classic Faradic yields (6-12 %) and high overall extraction efficiency (> 90 %). The felt homogeneously loaded with adhesive gold deposit can be smelted in an electric furnace without addition of fluxes since the carbon felt decomposes in gaseous products without formation of ashes. In the second case, the cyanide ion concentration can be lowered to as low as 10 mg/l by electro-oxidation to cyanate form (CNO-) at 400 A/m2 in the anodic compartment of a divided cell. The presence of copper ion significantly improves the current efficiency of this electro-oxidation and sharply reduces the oxidation of the carbon felt. Copper ions accelerate the oxidation rate of free cyanide ions through the formation of easily oxidizable complexes owing to a mechanism in which the Cu-I/Cu-II redox couple acts as an electron transfer mediator. (C) 1999 Elsevier Science Ltd. All rights reserved

Oliguria in Critically Ill Patients: Impact on AKI Classification and Outcomes Prediction

Introduction. Current definition and staging of acute kidney injury (AKI) considers both serum creatinine (sCr) and urinary output (UO) alterations. However, the relevance of oliguria-based criteria is disputed. Objectives. To determine the contribution of oliguria, as defined by KDIGO criteria, to AKI diagnosis, severity assessment and mortality prediction. Methods. We conducted a cohort study including all adult patients admitted within our multi-disciplinary intensive care unit between January 1st 2010 and June 15th 2020. Daily sCr and hourly UO measurements along with socio-demographic characteristics and severity scores were extracted from our electronic medical charts. Long-term mortality was assessed by cross-referencing our database with the Swiss national death registry. We determined the onset and severity of AKI according to KDIGO classification using UO and sCr criteria separately and assessed their agreement. Using a multivariable model accounting for baseline characteristics, severity scores and sCr stages, we evaluated the relative influence of UO criteria on 90-day mortality. Sensitivity analyses were conducted to assess the impact of missing sCr, body weight and UO values. Results. Among the 15′620 patients included in the study [10′330 (66.1%) males, median age 65.0 years (IQR, 53.0-75.0), median SAPS score 40.0 (IQR, 30.0-53.0), median follow-up 67.0 months (IQR, 34.0-100.0)], 12′143 (77.7%) fulfilled AKI criteria. SCr and UO criteria had poor agreement on AKI diagnosis and staging (Cohen's weighted kappa = 0.36, 95% CI 0.34-0.37, p < 0.001). Compared to the isolated use of sCr criteria, consideration of UO criteria enabled to identify AKI in 5′630 (36.0%) patients. Those patients had a higher 90-day mortality than no-AKI patients (respectively 12.9% and 8.3%, p < 0.001). On multivariable analysis accounting for sCr stage, comorbidities and illness severity, UO stage 2 and 3 were associated with a higher 90-day mortality [OR 2.4 (1.6-3.8), p < 0.001, and 6.2 (3.7-10.5), p < 0.001, respectively]. These results remained significant in all sensitivity analyses. Conclusion. Oliguria lasting more than 12 h (KDIGO stage 2 or 3) has major diagnostic and prognostic implications, irrespective of sCr elevations