Three-compartment Cell Research Articles

Membrane characterization for electrochemical LiOH production from Li2SO4 with simultaneous H2SO4 valorization

This work characterized different cation- and anion-exchange membranes to improve the efficiency for the electrochemical conversion of Li2SO4 into LiOH and simultaneously recover H2SO4 as a byproduct, an essential process for sustainable alternatives for lithium−ion battery recycling. The membrane’s ability to block H+ and OH− migration over the membrane to the feed stream of the electrolyzer was investigated. Simultaneously, the membrane resistance was measured to assess its impact on the cell voltage and overall energy consumption. The best CEM, Sx-2301-Wn, enabled to concentrate LiOH up to 1.7M with a current efficiency (CE) of 77.3%, while Fumasep FAB-130-PK, the best AEM, was able to concentrate H2SO4 up to 0.6M with a CE of 74.6%. The recirculation of LiOH into the middle compartment to maintain a constant pH was also investigated and showed to improve both Li+ (4.2%–8%) and SO42- (5.1%) migration, but pH higher than 3 led to an increased membrane resistance. The results of this work contributed to the selection of a suited membrane and ideal operational conditions for producing LiOH and H2SO4 through a three-compartment membrane electrolysis cell.

Selection of suitable organic electrolyte for CO2 electro-reduction to CO in three-compartment electrolysis cell with NaOH and Cl2 yielded as byproducts

Developing high performance electrolyte is essential in improving the performance of CO2 electro-reduction. In this work, we have constructed a three-compartment electrolysis cell for CO2 reduction to CO in organic electrolyte, with NaOH and Cl2 produced as byproducts. The generated CO and Cl2 can be served as feedstock for the production of phosgene, which is a widely used acylating agent for producing valuable downstream products. In order to improve the performance of the electrolysis cell, various commonly used organic solvents and supporting electrolytes were investigated. Consequently, 0.72 M Bu4NCF3SO3 in propylene carbonate (PC) was selected due to its high CO2 solubility. This electrolyte enabled the achievement of a current density of 27 mA/cm2 at a potential of −2.0 V (vs. SHE), with a stable Faradaic efficiency of 93 % for CO production. Compared with an aqueous catholytes, the altered double electric layer in the organic electrolyte prevents cathode poisoning, which is crucial for practical applications.

Insight on Zn-Al LDH as electrocatalyst for CO2 reduction reaction: An in-situ ATR-IR study

Electrochemical reduction of CO2 (CO2RR) is expected to play a key role among the various strategies being explored to limit global warming. In this scenario, Layered Double Hydroxides (LDHs) are emerging as a promising class of electrocatalysts to replace the most used noble metals. In this work three Zn-Al LDH with different Zn2+/Al3+ ratio were synthesized and characterized by means of XRD, STEM-EDX and HR-TEM. Their suitability for CO2RR to CO was assessed by means of a custom-made three-compartment cell, showing an increase in CO selectivity by decreasing the Zn2+/Al3+ ratio. The CO2 interaction with the samples was firstly characterized by means of volumetric adsorption measurements, exhibiting an increase in capture capacity by decreasing the Zn2+/Al3+ ratio. The evolution of the samples in interaction with a CO2-saturated liquid flow was then deeply investigated by means of in-situ ATR-IR spectroscopy. The samples displayed a different evolution in the vibrational region of the carbonate-like species (1800–1200 cm−1). To better discriminate the different carbonate cyclohexane was also employed. A definitive assignment of the main IR bands of the carbonate was carried out by studying the spectral behavior of the different bands observed in the ATR-IR experiments and by comparing these results with the existing literature. Interestingly, Zn-Al 1:2 LDH, the most efficient electrocatalyst for CO2RR, is also the sole sample exhibiting a higher monodentate to total bidentate carbonates ratio, suggesting that the existence of a higher content of low coordination oxygen anions with stronger basic character can influence the final catalytic activity.

Electrochemically Mediated Ammonia Removal and Recovery from Wastewater Using Functional Membranes

Wastewater treatment plants (WWTPs) typically use anaerobic digestion (AD) to treat sludge, but ammonia produced during AD process returns to the headworks of WWTPs. The amount of ammonia in the liquid stream produced after dewatering the stabilized sludge, commonly referred to as dewatering sidestream, could contribute as high as 20% of the overall ammonia loading to WWTPs. Several sidestream treatment technologies are being developed not only to prevent additional ammonia inputs to WWTPs, but also to valorize ammonia for fertilizer production. Electrochemically mediated processes can achieve this goal in two simultaneous steps: (1) ammonium separation from wastewater through a cation-exchange membrane and (2) ammonium-to-ammonia conversion for its recovery by membrane stripping. Although electrochemical reactions in electrodialysis or flow-electrode capacitive deionization have been shown to drive the separation and increase the solution pH, an acid solution must be externally fed in the subsequent stripping process. Here, we demonstrated the use of bipolar membrane electrodialysis (BMED) to separate ammonium while simultaneously producing acid and base streams. Using a three-compartment BMED cell (effective repeating unit area = 29.2 cm2) with a synthetic sidestream containing 50 mM total ammonia nitrogen (TAN), we achieved TAN removal up to 80% with energy consumption less than 10 kWh kg−1 N at a productivity of 34.2 L m−2-repeating unit membrane area h−1 (LMH). The separated ammonia was recovered (>90%) in a subsequent membrane contactor that was fed with acid and base streams produced from the BMED cell. We specifically focused on increasing TAN concentration in the final product stream by changing relative flow rate of acid/base streams, which allowed for concentrating TAN nearly 8 times greater than the feed. These results show the use of BMED coupled with membrane stripping enables ammonia recovery without the need for chemical inputs, representing a promising sidestream treatment technology.

Core-shell nanostructured Cu-based bi-metallic electrocatalysts for co-production of ethylene and acetate.

Direct electrocatalytic CCU routes to produce a myriad of valuable chemicals (e.g., methanol, acetic acid, ethylene, propanol, among others) will allow the chemical industry to shift away from the conventional fossil-based production. Electrofuels need to go beyond the current electroreduction of CO2 to CO, and we will here demonstrate the continuous flow electroreduction of syngas (i.e., CO and H2), which are the products from CO2-to-CO, with enhanced product selectivity (∼90% towards ethylene). To overcome current drawbacks, including bicarbonate formation that resulted in low CO2 utilisation and low C2+ product selectivity, the development of nanostructured core-shell bi-metallic electrocatalysts for direct electrochemical reduction of syngas to C2+ is proposed. Electrosynthesis of ethylene is performed in a state-of-the-art continuous flow three-compartment cell to produce ethylene (cathodic gas phase product) and acetate (cathodic liquid phase product), simultaneously.

Electrodialysis of LiH2PO4 for High-Purity LiOH and Green H2 Production

The current soda-lime evaporation process to recover lithium carbonate from continental brines by treatment with Solvay has a low efficiency due to the high solubility of Li2CO3, unlike precipitation of Li3PO4 after removal of alkaline earth cations, which has a very high efficiency. The main goal of this work is to provide a proof of concept for replacing the present method of LiOH.H2O production, i.e. replacing the reaction of Li 2 CO 3 with Ca(OH)2, by a single step electrodialysis of LiH2PO4 from insoluble Li3PO4 with no waste generation, recycling of phosphoric acid, better efficiency than the lithium carbonate pathway and green hydrogen generation. Experimental results have been complemented with numerical simulation of the electro-dialysis system solving the time-dependent concentration and potential profiles for two configurations: two compartments using a cation-selective membrane and a three-compartment cell with cation and anion-selective membranes operating at constant current, along with the voltage evolution and current efficiency for LiOH production.

Electrodialysis of Lithium Sulphate Solution: Model Development and Validation

A comprehensive mathematical model is proposed to study the transport phenomena in an Electrodialysis (ED) process employed to recover lithium hydroxide and sulfuric acid from the lithium sulphate solution derived from a recycling process of spent lithium-ion battery material. The model is developed based on the conservation equations of mass and ions, and considers electrolyte solutions consisting of mono- and multivalence ions. The concentration polarization at ion exchanged membranes (IEMs) and their adjacent diffusion boundary layers as a function of the applied current, inlet concentrations and flow rate are computed. Experimental data from a three-compartment ED cell are used for validation. A parametric study is performed to evaluate the impact of parameters on transmembrane fluxes of ion and water. It is revealed that increasing current leads to the enhancement of the transmembrane water and concentration polarization across IEMs. Feeding solutions consisting of smaller ions result in lower water transfer through IEMs. Raising the lithium concentration at the dilute channel increases the LiOH concentration due to reduced transmembrane water transfer. Using the uncertainty propagation method, it is found that current and counter-ion radius are the most influential parameters affecting the outlet concentration of concentrate channel and transmembrane water transfer.

Flocculating-filtration-processed mesoporous structure in laminar ion-selective membrane for osmosis energy conversion and desalination

Ion-selective membrane based technologies, such as electrodialysis and reverse electrodialysis technology, show great potential in addressing challenge of the water-energy nexus. However, the ion-selective membrane usually suffers from the trade-off between ionic selectivity and ion transport capacity. Here, we prove that mesoporous structure in lamellar ion-selective membrane can increase the transmembrane ionic transport with no loss of ionic selectivity. The mesoporous structure is constructed via adjusting the pH of graphene oxide/ionic polymer solution and self-assembly of flocculating solution. Negatively or positively charged graphene oxide based membrane with a thickness of 4 μm was used in a three-compartment electrochemical cell for osmosis energy conversion and desalination. An output power density of 5.87 W m−2 is achieved by mixing artificial seawater (0.5 M NaCl) and river water (0.01 M NaCl). The conductivity of seawater can quickly decrease from 12.3 mS cm−1 to 36 μS cm−1 in typical electrodialysis process, showing the excellent desalination ability. This work proves a new strategy of enhancing transmembrane ionic transport in ion-selective membrane and provides new opportunities for 2D nanomaterials.

Electrochemical production of formic acid from carbon dioxide: A life cycle assessment study

Even though lab-scale experimental studies have shown the technological potential of the electrochemical conversion of carbon dioxide (CO2) to formic acid (FA), there are few studies that have evaluated the environmental feasibility of such a conversion. This study develops an integrated process consisting of a three-compartment cell reaction system and a pressure-swing distillation separation system to produce commercial-grade FA from CO2. A total of 18 environment impact categories, including climate change and fossil depletion, are estimated for the process configurations on a system level. The electrochemical process has a 12.4–99.7% reduced environmental impact except for terrestrial ecotoxicity, compared to the conventional FA production process using carbon monoxide obtained from fossil fuels. In 17 categories, except that of ionizing radiation, electricity was identified as a major factor (17–97%) affecting the environment. A reduction in the electricity requirements or supply of renewable electricity, such as hydropower, will enable more environment friendly FA production from CO2.

A NOVEL CLOSED LOOP PROCESS FOR RECYCLING SPENT Li-ION BATTERY CATHODE MATERIALS

ABSTRACT Hydrometallurgical recycling of spent lithium cathode material commonly uses sodium hydroxide () and sodium carbonate () as reagents for impurity removal, metal extraction, and lithium recovery, which results a final processing solution containing high concentration of that needs to be treated before discharge or reuse. The present study reports the development of a novel closed-loop hydrometallurgical recycling of spent lithium cathode material process by adapting lithium hydroxide () or lithium carbonate () to replace in impurity removal and metal extraction, as well as using a three-compartment electrodialysis cell to regenerate and sulfuric acid () in recovering lithium. The regenerated and can be reused in the hydrometallurgical recycling process, making this a closed-loop process. Three different scales are studied to understand the reaction kinetics: 1-l batch, 40-l batch, and 73-l continuous batch. With as a reagent, impurities, such as Al and Fe, can be removed at pH 4.0–5.5. Metal extraction of Ni, Co, and Mn as carbonate or hydroxide can be achieved at pH 8–10. Up to 0.9 M of can be generated with a three-compartment electrodialysis cell with a 78% current efficiency. The proposed closed-loop hydrometallurgical process reduces costs through reagent regeneration, offering economic attractiveness, and provides an environmentally friendlier approach to recycling spent lithium-ion battery cathode material.

Tuning Sn3O4 for CO2 reduction to formate with ultra-high current density

Abstract In light of environmental concerns and energy demand, significant progress has been made toward CO2 reduction with high activity and selectivity. Particular interest has been focused on liquid products because of the easier separation process and higher value comparing to traditional gaseous products. Formic acid (or formate) can be one of the most economically viable reduction products. Nevertheless, few catalysts can meet the requirement for commercial-viable production, especially the current density. Herein, a hierarchical-Sn3O4 nanosheet (H–Sn3O4 NS) electrocatalyst is synthesized and applied into a three-compartment CO2 flow cell electrolyzer with ultra-high current density. The as-synthesized H–Sn3O4 NS electrocatalyst enables electrochemical CO2 reduction to formate with the selectivity of 91.1% at −1.02 V versus reversible hydrogen electrode (RHE) as well as a partial current density of 421 mA cm−2, whose performance is among the best that has been reported for CO2 electrocatalysts. The superior performance is due to the three-dimensional (3D) hierarchical structure, which supplies a large electrochemical surface area (ECSA) to facilitate mass and charge transfer. And the stability study with in-situ Raman spectroscopy and XRD characterization exhibited that the Sn3O4 materials could still maintain its original oxidation state under the maximum formate Faradaic efficiency of CO2 reduction reaction (CO2RR). The further theoretical calculation shows that the catalyst surface is beneficial for the formation of OCHO* (the intermediate of formate) than COOH* (the intermediate of CO) or H* (the intermediate of H2), which is in favor of the high activity and selectivity for formate in the experiments.

Recovery of Li and Co from LiCoO2 via Hydrometallurgical–Electrodialytic Treatment

Lithium-ion batteries play an important role in our modern society as the main option to power portable electronic devices and electric vehicles. The growing demand for these batteries encourages the development of more efficient recycling processes, aiming to decrease the environmental impact of the spent batteries and recover their valuable components. In this paper, a combined hydrometallurgical-electrodialytic method is proposed for processing battery waste. In the combined technique, the amount of leaching solution is reduced as acid is generated via electrolysis. At the same time, the use of ion-exchange membranes and the possibility of electroplating allows for a selective separation of the target metals. Experiments were performed using LiCoO2, which is one of the most used cathodes in lithium-ion batteries. First, 0.1 M HCl solution was used in batch extractions to study the kinetics of LiCoO2 dissolution, reaching an extraction of 30% and 69% of cobalt and lithium, respectively. Secondly, hydrometallurgical extraction experiments were carried out in three-compartment electrodialytic cells, enhanced with cation-exchange membranes. Experiments yielded to a selective recovery in the catholyte of 62% of lithium and 33% of cobalt, 80% of the latter electrodeposited at the cathode.

Recovery of phosphorus and metallic nickel along with HCl production from electroless nickel plating effluents: The key role of three-compartment photoelectrocatalytic cell system

A three-compartment photoelectrocatalytic (PEC) cell system combined with ion exchange and chemical precipitation was proposed to recover phosphorus and nickel from electroless nickel plating effluents containing hypophosphite (H2PO2−) and nickel ions (Ni2+). Ion exchange was used to concentrate and separate Ni2+ and H2PO2−. As a key unit, the established PEC system consisted of TiO2/Ni-Sb-SnO2 photoanode and Ti cathode. With 25.8 mM NaH2PO2 and 500 mM NiCl2, 100 % H2PO2− was oxidized to PO43− in the anode cell, 78 % Ni2+ was recovered as metallic Ni in the cathode cell, and 900 mM HCl was obtained in the middle cell within 24 h at 3.0 V. Based on quenching experiments and ESR technique, OH radicals were mainly responsible for H2PO2− oxidation. In situ Raman spectroscopy indicated that Ni2+ initially reacted with OH− to form α-Ni(OH)2, which was gradually reduced to metallic Ni. Fortunately, a slight pH decrease in the cathode cell in the three-compartment cell system was beneficial for Ni2+ reduction to Ni°. The obtained PO43− was recovered by chemical precipitation. Finally, recovery of phosphorus and metallic nickel along with HCl production from an actual electroless nickel plating effluents in terms of efficiency, cost-benefit, and stability assessment were demonstrated.

Alkalinity of Electro-Activated Aqueous Solutions

Electro-activated alkaline (basic) solution known as catholyte has been reported to have a number of unique properties. The changes in the pH and alkalinity of the catholyte were monitored in the current study. A full-factorial design was applied in order to examine the effects of current, time of treatment, and salt concentration on the electro-activated solutions treated in 4 types of configurations. Two configurations were constructed as three-compartment cells with chambers separated by anion exchange membrane (AEM) and cation exchange membrane (CEM) with different dispositions in the cell. The other two configurations were two-compartment cells with chambers separated by either an AEM or a CEM. For pH increase, time was the most significant factor, whereas for the alkalinity the cell configuration had a significant impact. Thus, when CEM was used to separate central and cathodic compartments in a three-cell reactor, or anodic and cathodic compartments in a two-cell reactor only current and time were found to be highly significant. On the other hand, when the AEM was used the salt concentration appeared to be an important factor too. Maximum values for pH (pH 12) and alkalinity (25.46 ± 0.31 mmol/L) were obtained after 60 min of treatment and 0.2 A current, and regardless of NaCl concentrations when CEM was used, whereas maximum values for the AEM membrane were achieved only with maximum NaCl concentration of 1 M. Despite highly alkaline medium (pH ~ 12) the strength of the solution was comparable to dilute NaOH solution (~0.03 M).

From Laboratory to Industrial Scale: A Prospective LCA for Electrochemical Reduction of CO2 to Formic Acid.

CO2-based production technologies unveil the possibility of sustainable production in the chemical industry. However, so-called carbon capture and utilization (CCU) options do not inevitably lead to improved environmental performance, which is especially uncertain for emerging technologies compared to present production practices. Thus, far, emerging CCU technologies have been environmentally assessed with conventional life cycle assessment (LCA). Therefore, this study aims to develop a methodology for applying prospective LCA to emerging production technologies from the laboratory to industrial scale. The developed four-step approach for implementing prospective LCA is applied to the case of electrochemical formic acid (FA) production via supercritical CO2 (scCO2) under consideration of different reactor designs to guide process engineers from an environmental standpoint. While using prospective LCA, the underlying modeling approach relies on consequential LCA (cLCA). Fourteen out of the 15 analyzed impact categories (IC) reveal lower environmental impacts for the scale-ups, which are based on the best-case assumptions and on a flow-through regime compared to the conventional FA production. Nevertheless, the impacts of the scale-ups that are based on a batch reactor (BR) and a three compartment cell (TCC) are higher than for the best case and the flow-through reactor scale-up.

Rational Design of Electrocatalytic Interfaces: Cd UPD Mediated Nitrate Reduction on Pd:Au Bimetallic Surfaces

INTRODUCTION Efforts in our laboratories have been focused on the rational design and assembly of bifunctional electrocatalytic interfaces incorporating two electrocatalysts acting in series as a means of improving both the overall activity and specificity of redox processes involving multiple transfer of electrons. This strategy was recently implemented to promote the reduction of NO3 - on Au NP dispersed on the surface of glassy carbon, GC, by adsorbed hemin (Hm), an iron porphyrin, in aqueous electrolytes containing Cd2+.1 This work represents an extension of this previous study in which Hm has been replaced by Pd nanoparticles, NP, co-dispersed with Au NP on GC, a tactic that virtually eliminates the formation of Au|Pd alloys. Impetus for this selection was provided by the much lower overpotential for NO2 - reduction on Pd compared to that on adsorbed Hm, a factor that would lead to overall activities surpassing those found earlier for the Cd(UPD)|Hm system. EXPERIMENTAL All electrochemical measurements were performed in an all-glass, three-compartment cell, filled with deaerated (Ar, Airgas, PP300) 0.1 M HClO4 solutions prepared from ultrapure perchloric acid (OmniTrace Ultra) and ultra-high purity water, UPW, generated by a Barnstead system (18.2 MΩ cm). A carbon rod (Alfa Aesar, diameter 6.15 mm) and a Ag/AgCl (Basi, MF2052) were used as counter and reference electrodes, respectively. Experiments were carried out with a GC disk electrode, GCE, (Pine Instruments, disk area, 0.196 cm2) using a commercial rotator (Pine Instruments, Model AFMSRX) and a Metrohm Autolab (PGSTAT302N). Cadmium perchlorate, Cd(ClO4)2 (Sigma-Aldrich, 99.999%), and sodium nitrate, NaNO3, (Sigma Aldrich, ACS reagent) were used as received. Nanoparticles, NP of Au and Pd were synthesized and cleaned using a methodology previously described in which an aqueous solution of the corresponding metallic precursor (HAuCl4 or H2PdCl4) was reduced with ice-cold and freshly prepared NaBH4 solution in the presence of trisodium citrate.2 Stock suspensions of each type of NP were prepared by mixing 1 volume of the original, as received, NP, with 3 volumes of UPW. To prepare either Au or Pd dispersions on the GCE, denoted hereafter as Au|GCE or Pd|GCE, 10 µL of the diluted NP aqueous suspensions were delivered to the GC surface using a micropipette, and then allowed to dry with a gentle stream of Ar. To prepare the co-dispersed Pd and Au NP on a GC disk, Pd:Au|GCE, 500 µL of each dilute NP suspension were mixed in a vial and following sonication, 10 µL of the mixture then delivered on the GCE using a micropipette, and allowed to dry under Ar. RESULTS AND DISCUSSION Shown in 1 are cyclic voltammograms recorded with Au|GCE (magenta), Pd|GCE (black) and Pd:Au|GCE (green) in pristine 0.1 N HClO4, displayed features believed to be characteristic of the metals involved. Integration of the characteristic reduction peaks in green in Fig 1 were found to be approximately half of those in either the Au|GCE or Pd|GCE (see magenta and black curves), and thus consistent with the relative amounts of each of the NP in the Pd:Au|GCE. Shown in Panel A, Fig. 2 are cyclic voltammograms obtained with these NP modified GCE surfaces in 0.1 M HClO4 containing 1 mM Cd(ClO4)2 and 2 mM NaNO3. Cursory inspection of these data reveals much higher activity for the Pd:Au|GCE, even though the amount of each NP was decreased to half, compared to the single metal surfaces. The same overall trend was found for measurements performed under forced convection, as shown in Panel B in this figure which reinforces the view that the two catalysts are indeed acting in series, i.e., Cd UPD on Au is responsible for the conversion of NO3 - to NO2 -, whereas Pd reduces NO2 - further to products of lower oxidation states. REFERENCES Y. Chen, H. Zhu, M. Rasmussen and D. Scherson, J. Phys. Chem. Lett., 1, 1907 (2010).C. M. Sanchez-Sanchez, F. J. Vidal-Iglesias, J. Solla-Gullon, V. Montiel, A. Aldaz, J. M. Feliu and E. Herrero, Electrochim. Acta, 55, 8252 (2010). ACKNOWLEDGEMENTS: This work was supported by a grant from NSF, CHE-1412060. Figure 1

Enhanced Electro-Oxidation of Urea Using Hollow Structured Nickel Manganese Oxide Catalysts

The utilization of urea has attracted more attention due to its various applications such as fuel cell, hydrogen production, wastewater remediation, and electrochemical sensors. [1] Especially, urea is a crucial biomarker to detect various human metabolic disorders such as renal function and liver disease. Therefore, the measurement of the urea level plays an important role in monitoring metabolic activity of human. Electrochemical sensing method is considered one of the most promising technique for urea detection due to its simplicity and reliable sensing. [2] The enzyme-based catalyst is widely used for urea oxidation. However, the enzyme has the stability issue due to the denaturing of the enzyme. Hence, non-enzymatic biosensors by metal-based catalysts have been studied. Compared with noble metal catalysts, nickel-based catalysts are inexpensive and abundant. Furthermore, bimetallic nickel-based oxides (nickel cobalt oxide [3], nickel molybdenum oxide [4], and nickel manganese oxide [5]) have been developed for efficient urea oxidation reaction in alkaline medium. Despite many efforts to achieve higher catalytic effect with bimetallic oxides, the improvement of catalytic property of urea oxidation is still challenging due to the low exposure of active sites and low electrical conductivity The hollow structured catalyst can improve the performance of urea detection by providing structural stability of catalyst and facile transport channels for electrolyte with exploiting its inner and outer surface as active sites. With introducing the template-free hydrothermal method with SDBS, Ni-Mn-based oxide is synthesized. Due to the synergetic catalytic effect between NiO and MnO and morphological effect of hollow structure, higher oxidation current and lower onset potential could be achieved. In this study, nickel manganese oxide catalyst with the variation of Ni:Mn atomic ratios were synthesized by one-pot template-free hydrothermal method. Structure and morphologies of synthesized nickel and manganese oxide hollow structure were characterized by XRD and SEM. And the atomic ratio of nickel manganese oxide was confirmed by EDS. CV measurement was conducted in a three-compartment cell with a potentiostat 1M KOH with 0.33M urea. CA measurement was also performed to measure the selectivity and limit of detection. Detailed mechanism and discussion of the formation of structure and sensing properties of nickel manganese oxide catalyst will be presented.

Removal of nitrates from copper-containing aqueous acidic leach solutions by electrodialysis

ABSTRACT The north of Chile is characterised by a large content of caliche minerals which represent high amounts of sodium and potassium nitrates. The presence of these nitrates, in copper-containing aqueous acid leach solutions, are responsible for serious problems in the hydrometallurgical extraction of copper-from-ore. The main problems are: extractant losses due to nitration and/or hydrolysis and negative effects upon bioleaching microorganisms such as lower growth cell and higher microbial lag times. On the basis of these problems, the objective of this work was to evaluate electrodialysis (ED) as an alternative method for removal for nitrates from copper-containing aqueous acid leach solutions. For this purpose, ED experiments in a three-compartment cell using both real and synthetic solutions at different current densities, flow rate and temperature were studied. It was found that high temperature, high flow rate and low current densities favour the ED performance, under the range investigated. Thus, optimum operating conditions were found at 40 °C, 120 Lh-1and 150 A m−2, where current efficiencies (CE) of 96.4% and 89.4% were obtained for synthetic and real catholyte, respectively. Likewise, the energy consumption (EC) was 25% lesser (0.75 kWh kg−1) with real catholyte versus 1.01 k Wh kg−1 consumed using synthetic catholyte. These results demonstrated that electrodialysis could be applied for removal of nitrates from copper-containing aqueous acid leach solutions as an alternative technology.

Efficient and Selective Electrochemically Driven Enzyme-Catalyzed Reduction of Carbon Dioxide to Formate using Formate Dehydrogenase and an Artificial Cofactor.

Increasing levels of carbon dioxide in the atmosphere and the growing need for energy necessitate a shift toward reliance on renewable energy sources and the utilization of carbon dioxide. Thus, producing carbonaceous fuel by the electrochemical reduction of carbon dioxide has been very appealing. We have focused on addressing the principal challenges of poor selectivity and poor energy efficiency in the electrochemical reduction of carbon dioxide. We have demonstrated here a viable pathway for the efficient and continuous electrochemical reduction of CO2 to formate using the metal-independent enzyme type of formate dehydrogenase (FDH) derived from C andida boidinii yeast. This type of FDH is attractive because it is commercially produced. In natural metabolic processes, this type of metal-independent FDH oxidizes formate to carbon dioxide using NAD+ as a cofactor. We show that FDH can catalyze the reverse process to generate formate when the natural cofactor NADH is replaced with an artificial cofactor, the methyl viologen radical cation. The methyl viologen radical cation is generated in situ, electrochemically. Our approach relies on the special properties of methyl viologen as a "unidirectional" redox cofactor for the conversion of CO2 to formate. Methyl viologen (in the oxidized form) does not catalyze formate oxidation, while the methyl viologen radical cation is an effective cofactor for the reduction of carbon dioxide. Thus, although the thermodynamic driving force is favorable for the oxidized form of methyl viologen to oxidize formate to carbon dioxide, the kinetic factors are not favorable. Only the reverse reaction of carbon dioxide reduction to formate is kinetically viable with the cofactor, methyl viologen radical cation. Binding free energy calculated from atomistic molecular dynamics (MD) simulations consolidate our understanding of these special binding properties of the methyl viologen radical cation and its ability to facilitate the two-electron reduction of carbon dioxide to formate in metal-independent FDH. By carrying out the reactions in a novel three-compartment cell, we have demonstrated the continuous production of formate at high energy efficiency and yield. This cell configuration uses judiciously selected ion-exchange membranes to separate the reaction compartments to preserve the yields of the methyl viologen radical cation and formate. By the electroregeneration of the methyl viologen radical cation at -0.44 V versus the normal hydrogen electrode, we could produce formate at 20 mV negative to the reversible electrode potential for carbon dioxide reduction to formate. Our results are in sharp contrast to the large overpotentials of -800 to -1000 mV required on metal catalysts, vindicating the selectivity and kinetic facility provided by FDH. Formate yields as high as 97% ± 1% could be realized by avoiding the adventitious reoxidation of the methyl viologen radical cation by molecular oxygen. We anticipate that the insights from the electrochemical studies and the MD simulations to be useful in redesigning the metal-independent FDH and alternate artificial cofactors to achieve even higher rates of conversion.

Comparison of two- and three-compartment cells for electrodialytic removal of heavy metals from contaminated material suspensions.

Electrodialytic remediation can be applied to remove heavy metals from contaminated particulate materials in suspension. The applied electric current is the cleaning agent and the heavy metals are removed by electromigration. In this study, a two-compartment cell was compared to a three-compartment cell, for several contaminated materials such as soils, sediments, mine tailings and ashes and totally 20 experiments were conducted. The pH decrease was faster in the two-compartment cell, but the metal removal was higher in the three-compartment cell since anionic metal species are removed from the material suspension in this cell set-up. For materials with relatively high chloride content, fly ash and harbour sediments, up to 38% of the metals were found in the filtrate in the two-compartment cell. Up to 9% of the current was used to transport heavy metal ions in the experiments and the current was mainly carried by H+ and Ca2+. Even with the lower pH in the two-compartment cell experiments, there was little difference in the percentage of the current carried by the metal ions between the two set-ups. Multivariate analysis showed that the choice of cell set-up depends on the metals targeted by remediation and the material characteristics.