Electrolytic oxidation as a sustainable method to transform urine into nutrients

© 2020 by the authors. In this work, the transformation of urine into nutrients using electrolytic oxidation in a single-compartment electrochemical cell in galvanostatic mode was investigated. The electrolytic oxidation was performed using thin film anode materials: boron-doped diamond (BDD) and dimensionally stable anodes (DSA). The transformation of urine into nutrients was confirmed by the release of nitrate (NO3-) and ammonium (NH4 +) ions during electrolytic treatment of synthetic urine aqueous solutions. The removal of chemical oxygen demand (COD) and total organic carbon (TOC) during electrolytic treatment confirmed the conversion of organic pollutants into biocompatible substances. Higher amounts of NO3-and NH4 + were released by electrolytic oxidation using BDD compared to DSA anodes. The removal of COD and TOC was faster using BDD anodes at different current densities. Active chlorine and chloramines were formed during electrolytic treatment, which is advantageous to deactivate any pathogenic microorganisms. Larger quantities of active chlorine and chloramines were measured with DSA anodes. The control of chlorine by-products to concentrations lower than the regulations require can be possible by lowering the current density to values smaller than 20 mA/cm2. Electrolytic oxidation using BDD or DSA thin film anodes seems to be a sustainable method capable of transforming urine into nutrients, removing organic pollution, and deactivating pathogens

The contribution of mediated oxidation mechanisms in the electrolytic degradation of cyanuric acid using diamond anodes

In this work, the contribution of mediated oxidation mechanisms in the electrolytic degradation of cyanuric acid using boron-doped diamond (BDD) anodes was investigated in different electrolytes. A complete mineralization of cyanuric acid was obtained in NaCl; however lower degrees of mineralization of 70% and 40% were obtained in Na2SO4 and NaClO4, respectively. This can be explained by the nature of the oxidants electrogenerated in each electrolyte. It is clear that the contribution of active chlorine (Cl2, HClO, ClO−) electrogenerated from oxidation of chlorides on BDD is much more important in the electrolytic degradation of cyanuric acid than the persulfate and hydroxyl radicals produced by electro-oxidation of sulfate and water on BDD anodes. This could be explained by the high affinity of active chlorine towards nitrogen compounds. No organic intermediates were detected during the electrolytic degradation of cyanuric acid in any the electrolytes, which can be explained by their immediate depletion by hydroxyl radicals produced on the BDD surface. Nitrates and ammonium were the final products of electrolytic degradation of cyanuric acid on BDD anodes in all electrolytes. In addition, small amounts of chloramines were formed in the chloride medium. Low current density (≤ 10 mA/cm2) and neutral medium (pH in the range 6–9) should be used for high efficiency electrolytic degradation and negligible formation of hazardous chlorate and perchlorate

Mechanism and kinetics of electrochemical degradation of uric acid using conductive-diamond anodes

Uric acid (UA) is one of the principal effluents of urine wastewaters, widely used in agriculture as fertilizer, which is potentially dangerous and biorefractory. Hence, the degradation of UA (2,6,8-trihydroxy purine) in aqueous solution of pH 3.0 has been studied by conductive-diamond electrochemical oxidation. Hydroxyl radicals formed from water oxidation at the surface of boron-doped diamond anodes were the main oxidizing agents. Effects of current density and supporting electrolyte on the degradation rate and process efficiency are assessed. Results show that the increase of current density from 20 to 60 mA cm–2 leads to a decrease in the efficiency of the electrochemical process. In addition, the best degradation occurred in the presence of NaCl as conductive electrolyte. Interestingly, an almost total mineralization of 50 ppm UA was obtained when anodic oxidation was performed at low current densities (20 mA cm–2) and in the presence of NaCl. This result confirmed that the electrolysis using diamond anodes is a very interesting technology for the treatment of UA. The identification of UA transformation products was performed by high-performance liquid chromatography (HPLC). HPLC analysis of treated solutions revealed that oxalic acid and urea were the two intermediates found. Oxalic acid was the most persistent product. Based on detected intermediates and bibliographic research, a mechanism of UA mineralization by anodic oxidation has been proposed. Ionic chromatography analysis confirmed the release of NO− 3 and NO+ 4 ions during UA mineralization

Electrochemical Oxidation/Disinfection of Urine Wastewaters with Different Anode Materials

In the present work, electrochemical technology was used simultaneously for the deactivation of microorganisms and the destruction of micro-pollutants contained in synthetic urine wastewaters. Microorganisms (E. coli) were added to synthetic urine wastewaters to mimic secondary treated sewage wastewaters. Different anode materials were employed including boron-doped diamond (BDD), dimensionally stable anode (DSA: IrO2 and RuO2) and platinum (Pt). The results showed that for the different anode materials, a complete deactivation of E. coli microorganisms at low applied electric charge (1.34 Ah dm−3) was obtained. The complete deactivation of microorganisms in wastewater seems to be directly related to active chlorine and oxygen species electrochemically produced at the surface of the anode material. Complete depletion of COD and TOC can be attained during electrolyses with BDD anode after the consumption of specific electric charges of 4.0 and 8.0 Ah dm−3, respectively. Higher specific electric charges (>25 Ah dm−3) were consumed to removal completely COD and about 75% of TOC during electrolyses with DSA anodes (IrO2 and RuO2). However, the electrolysis using Pt anode can partially remove and even after the consumption of high specific electric charges (>40 Ah dm−3) COD and TOC did not exceed 50 and 25%, respectively. Active chlorine species including hypochlorite ions and chloramines formed during electrolysis contribute not only to deactivate microorganisms but also to degrade organics compounds. High conversion yields of organic nitrogen into nitrates and ammonium were achieved during electrolysis BDD and DSA anodes. The results have confirmed that BDD anode is more efficient than with IrO2, RuO2 and Pt electrodes in terms of COD and TOC removals. However, higher amounts of perchlorates were measured at the end of the electrolysis using BDD anode

The electrolytic treatment of synthetic urine using DSA electrodes

In this work, the electrochemical treatment of synthetic urine by anodic oxidation using dimensionally stable anode (DSA) was studied. The influence of current density and temperature on the kinetics and efficiency of anodic oxidation was examined. The intermediates and the evolution of products were also monitored by HPLC and IC. Anodic oxidation achieves the almost total removal of COD, which is mainly transformed into carbon dioxide and oxalic acid. This latter compound explains the occurrence of residual TOC at large current densities. The electrochemical technology also attains a very significant removal of nitrogen, which is transformed into gaseous nitrogen through the formation of chloramines. The increase of current density leads to a decrease in the efficiency of the electrochemical process. Furthermore, the increase of the temperature has no significant influence on the oxidation rate but it contributes to a higher mineralization. The most efficient conditions were found to be low current densities (20Ahdm−3) and low temperatures (25°C), which in addition prevents the formation of chlorate and perchlorate ions in the reaction system.the Junta de Comunidades de Castilla La Mancha government and EU through projects PEII-2014-039-P, FEDER 2007-2013 PP201010 (Planta Piloto de Estación de Estación de Regeneración de Aguas Depuradas), INNOCAMP

Oxidative Degradation of Tannic Acid in Aqueous Solution by UV/S2O82− and UV/H2O2/Fe2+ Processes: A Comparative Study

Tannic acid (TA) is a major pollutant present in the wastewater generated from vegetable tanneries process and food processing. This work studied TA degradation by two advanced oxidation processes (APOs): UV irradiation at the wavelength of 254 nm in the presence of hydrogen peroxide (H2O2) and ferrous iron (photo-Fenton) and in the presence of potassium persulfate. The influence of certain experimental parameters such as K2S2O8, H2O2, Fe2+, and TA concentrations, initial pH and temperature was evaluated in order to obtain the highest efficiency in terms of aromatics (decay in UV absorbance at 276 nm) and TOC removals. Chemical oxidation of TA (0.1 mM) by UV/persulfate achieved 96.32% of aromatics removal and 54.41% of TOC removal under optimized conditions of pH = 9 and 53.10 mM of K2S2O8 after 60 min. The treatment of TA by photo-Fenton process successfully led to almost complete aromatics removal (99.32%) and high TOC removal (94.27%) from aqueous solutions containing 0.1 mM of TA at natural pH = 3 using 29.4 mM of H2O2 and 0.18 mM of Fe2+ at 25 °C after 120 min. More efficient degradation of TA by photo-Fenton process than UV/persulfate was obtained, which confirms that hydroxyl radicals are more powerful oxidants than sulfate radicals. The complete removal of organic pollution from natural waters can be accomplished by direct chemical oxidation via hydroxyl radicals generated from photocatalytic decomposition of H2O2

Treatment of Synthetic Urine by Electrochemical Oxidation using Conductive-Diamond Anodes

In this work, the electrochemical oxidation of synthetic urine by anodic oxidation using boron-doped diamond (BDD) as anode and stainless steel as cathode was investigated. Results show that complete depletion of COD and TOC can be attained regardless of the current density applied in the range 20 - 100 mA cm-2. Oxalic and oxamic acids, and, in lower concentrations, creatol and guanidine were identified as the main intermediates. Chloride ions play a very important role as mediators and contribute not only to obtain a high efficiency in the removal of the organics but also to obtain an efficient removal of nitrogen by the transformation of the various raw nitrogen species into gaseous nitrogen through chloramine formation. Main drawback of the technology is the formation of chlorates and perchlorates as final chlorine products. The increase of current density from 20 mA cm-2 to 60 mA cm-2 led to an increase in the rate of COD and TOC removals although the process becomes less efficient in terms of energy consumption (removals of COD and TOC after applying 18 Ah dm-3 were 93.94 and 94.94%, respectively at 20 mA cm-2 and 89.17 and 86.72%, respectively, at 60 mA cm-2)