Electro-catalytic oxidation of artificial human urine by using BDD and IrO2 electrodes

Urea catalytic oxidation for energy and environmental applications

Urea is one of the most essential reactive nitrogen species in the nitrogen cycle and plays an indispensable role in the water-energy-food nexus. However, untreated urea or urine wastewater causes severe environmental pollution and threatens human health. Electrocatalytic and photo(electro)catalytic urea oxidation technologies under mild conditions have become promising methods for energy recovery and environmental remediation. An in-depth understanding of the reaction mechanisms of the urea oxidation reaction (UOR) is important to design efficient electrocatalysts/photo(electro)catalysts for these technologies. This review provides a critical appraisal of the recent advances in the UOR by means of both electrocatalysis and photo(electro)catalysis, aiming to comprehensively assess this emerging field from fundamentals and materials, to practical applications. The emphasis of this review is on the design and development strategies for electrocatalysts/photo(electro)catalysts based on reaction pathways. Meanwhile, the UOR in natural urine is discussed, focusing on the influence of impurity ions. A particular emphasis is placed on the application of the UOR in energy and environmental fields, such as hydrogen production by urea electrolysis, urea fuel cells, and urea/urine wastewater remediation. Finally, future directions, prospects, and remaining challenges are discussed for this emerging research field. This critical review significantly increases the understanding of current progress in urea conversion and the development of a sustainable nitrogen economy.

Regulating the Innocuity of Urea Electro-Oxidation via Cation-mediated Adsorption

Urea electrolysis is an emerging technology that bridges efficient wastewater treatment and hydrogen production with lower electricity costs. However, conventional Ni-based catalysts could easily overoxidize urea into the secondary contaminant NOx - , and enhancing the innocuity of urea electrolysis remains a grand challenge to be achieved. Herein, we tailored the electrode-electrolyte interface of an unconventional cation effect on the anodic oxidation of urea to regulate its activity and selectivity. Smaller cations of Li+ were discovered to increase the Faradaic efficiency (FE) of the innocuous N2 product from the standard value of ~15 % to 45 %, while decreasing the FEs of the over-oxidized NOx - product from ~80 % to 46 %, pointing to a more sustainable process. The kinetic and computational analysis revealed the dominant residence of cations on the outer Helmholtz layer, which forms the interactions with the surface adsorbates. The Li+ hydration shells and rigid hydrogen bonding network interact strongly with the adsorbed urea to decrease its adsorption energy and subjection to C-N cleavage, thereby directing it toward the N2 pathway. This work emphasizes the tuning of the interactions within the electrode-electrolyte interface for enhancing the efficiency and sustainability of electrocatalytic processes.

Electrochemical oxidation of pharmaceuticals in synthetic fresh human urine: Using selective radical quenchers to reveal the dominant degradation pathways and the scavenging effects of individual urine constituents

Electrochemical oxidation of fresh human urine is a promising method to prevent pharmaceuticals from being discharged into the environment. Here, we evaluate the importance of electro-generated oxidants and direct anodic oxidation for degradation of four pharmaceutical (cyclophosphamide (CP), carbamazepine (CBZ), sulfamethoxazole (SMX) and ibuprofen (IBP)) accounting for the scavenging effect of urine constituents using boron-doped diamond (BDD) and IrO₂ electrodes. Allyl alcohol and tert-butanol were used as selective quenchers for adsorbed and dissolved radicals, respectively. In electrolyte containing only chloride and pharmaceuticals, we found that CBZ and SMX are primarily oxidized by electro-generated Cl₂ in the fluid boundary layer , and CP and IBP are primarily oxidized by physisorbed ^•OH or chemisorbed chlorine (IrO₃-Cl). Regarding the effects of other fresh urine constituents, urea, creatinine, and uric acid quench the dissolved reactive chlorine species (Cl^•/Cl₂^•‒, HOCl, Cl₂, etc.). However, SO₄^2‒ shows no effect on pharmaceutical degradation while H₂PO₄^‒ and citrate ions quench IrO₃-Cl resulting in a mixed kinetic and mass-transfer limiting oxidation of pharmaceuticals on IrO₂. Citrate ions only quench the dissolved oxidants (surface adsorbed radicals are the dominant oxidants) leading to the pharmaceutical degradation limited by the mass transfer of pharmaceutical to BDD surface. This work provides an understanding of the significance of various pathways for pharmaceutical degradation, scavenging effect of urine constituents, and strategies for rapid pharmaceutical degradation in human urine.

A novel electrocatalytic system with high reactive chlorine species utilization capacity to degrade tetracycline in marine aquaculture wastewater

The problems of high salinity and coexistence of antibiotics in mariculture wastewater pose a great challenge to the traditional wastewater treatment technology. Herein, an electrocatalytic system based on cathodes to sustain reactive chlorine species (RCS) in a high chlorine environment was proposed. The results show that the content of RCS is affected by cathodes. The electrocatalytic system with FeNi/NF as cathode has the largest RCS retention capacity when compared with other cathode systems (carbon felt, nickel foam, copper foam, stainless steel, and nickel-iron foam). This is related to FeNi/NF's higher hydrogen production activity, which inhibits the reduction reaction of RCS. Furthermore, the degradation of tetracycline by the proposed FeNi/NF system maintained long-term effective performance across 20 cycles. Thus, the application of high chlorine resistance electrocatalysis system provides a possibility for practical electrocatalysis treatment of mariculture wastewater.

High-Efficiency Water Recovery from Urine by Vacuum Membrane Distillation for Space Applications: Water Quality Improvement and Operation Stability

Water recovery by membrane distillation (MD) is an attractive alternative to existing urine treatment systems because it could improve the water recovery rate and reliability in space missions. However, there are few studies of urine MD, particularly on the removal of the remaining contaminants from distillate water and the assessment of its long-term performance. In this study, the influences of various operation parameters on distillate water quality and operation stability were investigated in batch mode. The low pH of feedstock reduced the conductivity and total ammonium nitrogen (TAN) in distillate water because the low pH promoted the ionization of ammonia to ammonium ions. However, the low pH also facilitated the formation of free chlorine hydride, which resulted in the minor deterioration of the conductivity in the distillate due to the increasing volatility of chlorine hydride in the feedstock. Thirty batches of vacuum membrane distillation (VMD) experiments demonstrated that the permeate flux and the distillate water quality slightly decreased due to the small range of membrane wetting but still maintained an over 94.2% and 95.8% removal efficiency of the total organic carbon (TOC) and TAN, and the conductivity was <125 μs cm−1 in the distillate water after 30 test batches. VMD is a feasible option for urine treatment in space missions.

Deciphering and Suppressing Over-Oxidized Nitrogen in Nickel-Catalyzed Urea Electrolysis

Urea electrolysis is a prospective technology for simultaneous H₂ production and nitrogen suppression in the process of water being used for energy production. Its sustainability is currently founded on innocuous N₂ products; however, we discovered that prevalent nickel-based catalysts could generally over-oxidize urea into NO₂ ^- products with ≈80 % Faradaic efficiencies, posing potential secondary hazards to the environment. Trace amounts of over-oxidized NO₃ ^- and N₂ O were also detected. Using ¹⁵ N isotopes and urea analogues, we derived a nitrogen-fate network involving a NO₂ ^- -formation pathway via OH^- -assisted C-N cleavage and two N₂ -formation pathways via intra- and intermolecular coupling. DFT calculations confirmed that C-N cleavage is energetically more favorable. Inspired by the mechanism, a polyaniline-coating strategy was developed to locally enrich urea for increasing N₂ production by a factor of two. These findings provide complementary insights into the nitrogen fate in water-energy nexus systems.

Removal of naphthalene and phenanthrene in synthetic solutions by electro-oxidation coupled with membrane bioreactor

Naphthalene (NAPH) and phenanthrene (PHEN) are two of the most abundant polycyclic aromatic hydrocarbons (PAHs) found in nature, and they are considered in the list of US EPA priority pollutants. The contribution of this research lies in the comprehensive analysis of a strategy for the coupling of electro-oxidation (EO) and biodegradation in a submerged membrane bioreactor (SMBR) with the objective to remove PAHs, using NAPH and PHEN as model compounds. The electrochemical degradation of NAPH and PHEN in aqueous synthetic solution has been carried out using two different anodes: Ti/IrO₂ and Ti/SnO₂. The effects of EO operating parameters (current density, reaction time, and pH) on the NAPH and PHEN removals were investigated applying 2³ factorial design with both electrodes. Additionally, the EO effluents were analyzed for COD, NH₄-N, and biodegradability (respirometry tests). The highest removals of both compounds were reached with Ti/IrO₂ anode, at acidic conditions (pH of 2), current density of 50 mA cm^-2, and electrolysis time of 60 min. However, the Ti/SnO₂ anode allowed greater reduction of the biomass inhibition, which means that the enhancement of the EO effluent biodegradability was reached; therefore, this electrode was selected for the coupled EO-SMBR system, applying the operating conditions that improved the biodegradability of the effluent. The EO process allowed NAPH and PHEN removal efficiencies of 96 ± 5% and 94 ± 3%, respectively. The membrane bioreactor was operated with organic load of 0.6 ± 0.1 gCOD gVSS^-1 d^-1, hydraulic retention time of 6 h, and solid retention time of 30 d, obtaining average COD, NH₄-N, NAPH, and PHEN removals of 98±0.5%, 91±6.4%, 99.1±0.96%, and 99.7±0.4% respectively. The sorption of phenanthrene onto the biomass had a low contribution, 0.9±0.2%, concluding that biodegradation was the main removal mechanism in the bioreactor. The coupled system EO-SMBR allowed high NAPH and PHEN removal efficiencies of 99.99±0.01 and 99.99±0.02%, respectively.

Selective oxidation of pharmaceuticals and suppression of perchlorate formation during electrolysis of fresh human urine

Urine comprises only a small (~1%) volumetric fraction of municipal wastewater, but represents a dominant source of pharmaceuticals, many of which may pass through conventional wastewater treatment and pose risks to aquatic ecosystems. Point-source treatment of source-separated urine presents a unique opportunity to degrade pharmaceuticals before dilution with wastewater, and electrochemical advanced oxidation processes are one increasingly investigated option. However, they often lead to the formation of oxidation byproducts including chlorate, perchlorate at very high concentrations. Here, we show that the high urea content of fresh human urine suppresses the formation of oxychlorides by inhibiting formation of HOCl/OCl^‒ during electrolysis, while still enabling pharmaceutical degradation due to the slow rate of urea oxidation by ^•OH. This results in improved performance compared to equivalent treatment of hydrolyzed aged urine. This electrochemical oxidation scheme is shown to degrade the model contaminants cyclophosphamide and sulfamethoxazole with surface-area-to-volume-normalized pseudo-first-order rate constants greater than 0.08 cm/min in authentic fresh human urine. It results in ~100 × decrease in pharmaceutical concentrations in 2 h while generating ~1000 × lower oxychloride byproduct concentrations in synthetic fresh urine than synthetic hydrolyzed aged urine matrixes. Importantly, this proof-of-principle shows that simple and safe electrochemical methods can be used for point-source-remediation of pharmaceuticals in fresh human urine (before storage and hydrolysis), without formation of significant oxychloride byproducts.

Application of Ti/IrO2 electrode in the electrochemical oxidation of the TNT red water

Via the thermal sintering, a nanocrystalline IrO₂ coating was formed on the Ti substrate to successfully prepare a Ti/IrO₂ electrode. Based on the electrochemical analysis, the prepared Ti/IrO₂ electrode was found to have powerful oxidation effect on the organics in the TNT red water, where the nitro compound was oxidized through an irreversible electrochemical process at 0.6 V vs. SCE. According to the analysis of the nitro compound content, the UV-vis spectra, and the FTIR spectra of 2,4,6-trinitrotoluene (TNT) red water with electrolytic periods, the degradation mechanism of the dinitrotoluene sulfonate (DNTS) was developed. And the intermediates were characterized by UPLC-HRMS. The DNTS mainly occurred one electron transfer reaction on the Ti/IrO₂ electrode. At the early stage of the electrolysis, the polymerization of DNTS was mainly dominated. The generated polymer did not form a polymer film on the electrode surface, but instead it promoted a further reduction. After electrolyzing for 30 h, all NO₂ function group in the TNT red water was degraded completely.

Urea removal strategies for dialysate regeneration in a wearable artificial kidney

The availability of a wearable artificial kidney (WAK) that provides dialysis outside the hospital would be an important advancement for dialysis patients. The concept of a WAK is based on regeneration of a small volume of dialysate in a closed-loop. Removal of urea, the primary waste product of nitrogen metabolism, is the major challenge for the realization of a WAK since it is a molecule with low reactivity that is difficult to adsorb while it is the waste solute with the highest daily molar production. Currently, no efficient urea removal technology is available that allows for miniaturization of the WAK to a size and weight that is acceptable for patients to carry. Several urea removal strategies have been explored, including enzymatic hydrolysis by urease, electro-oxidation and sorbent systems. However, thus far, these methods have toxic side effects, limited removal capacity or slow removal kinetics. This review discusses different urea removal strategies for application in a wearable dialysis device, from both a chemical and a medical perspective.

Formation of ammonium ions by electrochemical oxidation of urea with a boron-doped diamond electrode

Ammonium ions were formed electrochemically from urea with a boron-doped diamond electrode and increased by using photocatalyst together.

Effect of reactor configuration on the kinetics and nitrogen byproduct selectivity of urea electrolysis using a boron doped diamond electrode

Electrochemical systems have emerged as an advantageous approach for decentralized management of source-separated urine with the possibility of recovering or removing nutrients and generating energy. In this study, the kinetics and byproduct selectivity of the electrolytic removal of urea were investigated using a boron doped diamond working electrode under varied operational conditions with a primary focus on comparing undivided and divided reactors. The urea removal rate in the undivided and divided reactors was similar, but the divided reactor had an increased required cell voltage needed to maintain the equivalent current density. The current efficiency was similar for 0.1, 0.25, and 0.5 A (33.3, 83.3, 167 mA/cm²), suggesting no interference from competing reactions at higher potentials. In a divided reactor, increasing the anolyte pH reduced the urea removal rate presumably from hydroxyl radical scavenging by hydroxide. Further, for all divided reactor experiments, the final pH was less than 1, suggesting that the transport of protons across the ion exchange membrane to the cathode was slower than the oxidation reactions producing protons. The nitrogen byproduct selectivity was markedly different in the undivided and divided reactors. In both reactors, nitrate (NO₃^-) formed as the main byproduct at the anode, but in the undivided reactor it was reduced at the stainless steel cathode to ammonia. In the presence of 1 M chloride, the urea removal kinetics improved from the generation of reactive chlorine species, and the byproduct selectivity was shifted away from NO₃^- to presumably chloramines and N₂. Overall, these results indicate that the electrochemical reactor configuration should be carefully considered depending on the desired outcome of treating source-separated urine (e.g., nitrogen recovery, H₂ generation).

Electrochemical removal of amoxicillin using a Cu doped PbO2 electrode: Electrode characterization, operational parameters optimization and degradation mechanism

This work investigated the electrochemical degradation of amoxicillin (AMX) in aqueous solution with Cu-PbO2 electrode. The main influence factors on the degradation of AMX, such as Na2SO4 concentration, initial AMX concentration, current density and initial pH value, were analyzed in detail. Under the optimal conditions, the removal rates of AMX and chemical oxygen demand (COD) reached 99.4% and 46.3% after 150 min treatment. The results indicated that the electrochemical degradation of AMX fitted pseudo-first-order reaction kinetics. Compared with undoped PbO2 electrode, Cu-PbO2 electrode had a smaller crystal size, more proportion of hydroxyl oxygen species, greater AMX and chemical oxygen demand (COD) removal efficiency, higher average current efficiency (ACE) and lower electrical efficiency per log order (EE/O). Electrochemical oxidation using Cu-PbO2 electrodes was an effective way to eliminate amoxicillin in aqueous solution. Moreover, a possible degradation pathway including ring open and mineralization was proposed by intermediate products determined by GC-MS method. This paper could provide basic data and technique reference for the amoxicillin wastewater pollution control.

Degradation of chloramphenicol using Ti-Sb/attapulgite ceramsite particle electrodes

Ti-Sb/attapulgite ceramsite particle electrodes were prepared for the efficient degradation of chloramphenicol (CAP) in wastewater. To observe the surface morphology and structural characteristics of the Ti-Sb/attapulgite ceramsite particle electrodes, Fourier transform infrared spectroscopy, X-ray fluorescence, scanning electron microscopy, and X-ray diffraction were used for characterization. Parameters affecting the degradation efficiency and the energy consumption of the Ti-Sb/attapulgite ceramsite particle electrodes, such as current density, electrode distance, initial pH, conductivity, air flow, and packing ratio, were examined. Results showed that the optimal conditions were 20 mA/cm² current intensity, 3 cm electrode distance, 5,000 μS/cm conductivity, 2.0 L/min air flow, 50.0% packing ratio, and initial pH 1. The removal efficiency of chloramphenicol was 73.7% under the optimal conditions, and the energy consumption was 191.3 (kW h)/kg CAP. PRACTITIONER POINTS: Attapulgite ceramsite with good physical performance was prepared. The removal efficiencies for CAP in water by Ti-Sb/attapulgite ceramsite particle electrodes were studied. CAP was effectively removed by 3-D electrode system. CAP removal was significantly influenced by operational parameters. Three-dimensional electrode system shows good degradation ability.

Electrochemical carbamazepine degradation: Effect of the generated active chlorine, transformation pathways and toxicity

Carbamazepine (CBZ) is a biorecalcitrant pharmaceutical compound frequently detected in wastewater and water bodies which has numerous negative effects on living organisms. In this investigation the effect of electrocatalytically generated active chlorine on CBZ degradation was studied using Nb/BDD or Ti/IrO₂ anodes. Subsequently, a response surface methodology based on a factorial plan and central composite design was carried out to determine the contribution of individual factors and to obtain the optimal experimental parameters for CBZ abatement. Electric current and treatment time were found to be the pivotal parameters influencing the degradation efficiency with respective contributions of 45.19% and 35.44%. The anode material had lower influence on the response, however, using an Nb/BDD anode, the oxidation was more effective due to the increased production of OH radicals as well as HClO, Cl and ClO^- species. Considering CBZ degradation and energetic consumption, the percentage of degraded CBZ was 88.70 ± 0.35% consuming 1.07 kWh m^-3 (at 1.0 A, NaCl concentration of 14 mM after 12.45 min, using Nb/BDD anode). First order kinetic constant (k) value of 0.189 min^-1 was obtained at optimal conditions when demineralized water was used for the NaCl supporting electrolyte, while k was lower when tap water or treated wastewaters were used for this purpose. Oxidation of CBZ yielded six aromatic intermediates, identified by gas chromatography - mass spectrometry technique and degradation pathways were proposed. The performed acute toxicity tests indicated an increase during the treatment, which was demonstrated to be mainly attributed to the remnant active chlorine.

Nitrate and carbon matter removals from real effluents using Si/BDD electrode

This work investigated the electrochemical treatment of four real effluents which were municipal wastewater (MWW), human urine (HU), river water (Wadi), and slaughterhouse wastewater (SHWW). The treatment was performed on a Boron-Doped Diamond (BDD) as anode/cathode material with an applied current density of 35.7 mA cm^-2 and without any reagent addition. Effluent characterization before treatment indicated that nitrogen pollution existed essentially as ammonium/ammonia ions, low level of nitrate, and in some cases as nitrite form. Organic pollution was also determined by COD values which were 920, 7300, 320, and 2280 mg O₂ L^-1 for MWW, HU, Wadi, and SHWW effluents, respectively. The effectiveness of the electrochemical oxidation/reduction was assessed by nitrogenous compounds and COD removals. Obtained removals underlined the simultaneous oxidation and reduction at the BDD interfaces of the main species present in the real effluents as well as the electro-generated ones. Results confirmed the high performance of BDD electrode for removal of coexistent pollutants from the studied matrix. The oxidation of organic matter and ammonium/ammonia as well as the kinetic of COD degradation was rapid in acidic medium (HU case) than that in neutral and alkaline medium (MWW, Wadi, and SHWW). Moreover, the phytotoxicity test showed less toxic behavior only in the cases of MWW and SHWW with a Germination Index equal to 58.8 and 72.2 %, respectively. The EC and ACE were also evaluated for all studied wastewaters, and the lowest EC value (0.03 kWh (g COD)^-1) was obtained for the more charged effluent (HU).

Formation of Chlorination Byproducts and Their Emission Pathways in Chlorine Mediated Electro-Oxidation of Urine on Active and Nonactive Type Anodes

Chlorination byproducts (CBPs) are harmful to human health and the environment. Their formation in chlorine mediated electro-oxidation is a concern for electrochemical urine treatment. We investigated the formation of chlorate, perchlorate, and organic chlorination byproducts (OCBPs) during galvanostatic (10, 15, 20 mA · cm(-2)) electro-oxidation of urine on boron-doped diamond (BDD) and thermally decomposed iridium oxide film (TDIROF) anodes. In the beginning of the batch experiments, the production of perchlorate was prevented by competing active chlorine and chlorate formation as well as by direct oxidation of organic substances. Perchlorate was only formed at higher specific charges (>17 Ah · L(-1) on BDD and >29 Ah · L(-1) on TDIROF) resulting in chlorate and perchlorate being the dominant CBPs (>90% of initial chloride). BDD produced mainly short chained OCBPs (dichloromethane, trichloromethane, and tetrachloromethane), whereas longer chained OCBPs (1,2-dichloropropane and 1,2-dichloroethane) were more frequently found on TDIROF. The OCBPs were primarily eliminated by electrochemical stripping: On BDD, this pathway accounted for 40% (dichloromethane) to 100% (tetrachloromethane) and on TDIROF for 90% (1,2-dichloroethane) to 100% (trichloromethane) of what was produced. A post-treatment of the liquid as well as the gas phase should be foreseen if CBP formation cannot be prevented by eliminating chloride or organic substances in a pretreatment.