Measuring mercury ion concentration with a carbon nano tube paste electrode using the cyclic voltammetry method

A simply prepared carbon nano tube paste electrode (CNTPE) was utilized for monitoring mercury ion concentration using the cyclic voltammetry (CV) method and the square wave anodic stripping voltammetric (SWASV) method. The CNTPE was compared with various conventional electrodes. The CNTPE method was applied to determine the concentration of trace levels of Hg(II) in several water samples, which yielded a relative error of 0.6% with a concentration of 0.20 mg L^–1 Hg(II). It was deposited at –0.5 V (vs Ag/AgCl), which was subsequently reduced to +0.20 V to strip it on the CNTPE. The optimal experimental conditions for the analysis were found to be as follows: pH value of 4 for the medium; deposition potential of –0.5 V; deposition time of 210 s; SW frequency of 40 Hz; SW amplitude of 100 mV, and step potential of 25 mV. Given these optimum conditions, a linear range was observed within the concentrations of 1.0–25.0 g L^–1 and 40.0–200.0 g L^–1. The detection limit was found to be 0.42 g L^–1.     

Nitrogen application and underground water contamination in some agricultural soils of South Western Nigeria

The effect of nitrogen fertilizer application on nitrate leaching and contamination of underground and surface waters in a continuously cropped lowland area of South Western Nigeria has indicated a high potential for nitrate leaching.It was estimated that with 100 kg N ha^–1 applied, as much as 29.5 kg N ha^–1 could be lost through leaching below the root zone of a maize crop, Over a 3 year period the applied nitrogen contributed to nitrate pollution of underground water significantly in excess of the maximum level accepted for potable water. This was particularly high in valley bottoms where the nitrate nitrogen content ranged from 12.8 to 24.6 mg L^–1. Contribution to adjacent stream was, however, not significant.     

Ammonium nitrate wastewater treatment by coupled membrane electrolysis and electrodialysis

A process coupling membrane electrolysis and electrodialysis is implemented to treat ammonium nitrate wastewater. Membrane electrolysis produces ammonia and nitric acid while electrodialysis reconcentrates the depleted salt solution. Ammonia is removed continuously by in situ stripping; thus allowing gas production with a constant current efficiency (about 70%). Nitric acid up to 8 mol L^–1 is obtained. The current efficiency of acid production depends on nitric acid concentration. When this concentration varies from 1 to 8 mol L^–1 the average current efficiency is about 58%. Electrodialysis produces a rejected stream containing less than 3 × 10^–3 mol L^–1 of ammonium nitrate.     

Nitrogen fertilization and nitrate pollution in groundwater in Japan: Present status and measures for sustainable agriculture

During the last two decades, nitrate nitrogen (NO₃-N) concentrations in groundwater in Japan have increased steadily due to the development of intensive agriculture. In some areas, they have reached or even exceeded the unacceptable level for drinking water, 10 mg l^–1. In 2000, the Environment Agency showed that 5.6% (173 of 3,374) tested wells and 4.7% (64 of 1,362) wells used for drinking water exceeded the standard level in 1999. The highest value of NO₃-N in the wells was 100 mg l^–1. Many researches have shown that NO₃-N pollution of groundwater was widely observed in Japan, except the paddy field regions. Farming practices in Kagamigahara city of Gifu prefecture have been typical ones for reducing NO₃-N pollution in groundwater. In the east district of the city, NO₃-N concentration was low in 1966, but reached 27.5 mg l^–1 in June, 1974. The farmers in this district began to reduce the nitrogen fertilizers in carrot cultivation, going from 256 kg N ha^–1 in 1970 to 153 kg N ha^–1 in 1991. The use of controlled release fertilizer increased fertilizer-nitrogen efficiency compared with common compound fertilizer and NO₃-N concentration in the groundwater began to decrease steadily. It was discussed that in order to decrease the NO₃-N pollution of groundwater, it is necessary to refocus not only agricultural technology but also agricultural policy, toward sustainable agriculture and rural development.     

Electrosynthesis of propylene oxide in a bipolar trickle-bed reactor

The synthesis of propylene oxide by electrolysis of dilute sodium bromide solution with propylene gas was investigated in an electrochemical flow-by bipolar reactor consisting of six parallel fixed beds of graphite particles separated by polypropylene felt diaphragms. The reactor was operated in a single pass mode with a two-phase co-current flow of propylene and sodium bromide solution through the beds of graphite particles. The maximum pressure in the system was 2.2 atm absolute.The effects of superficial current density (0.4–2.7 kAm^–2), sodium bromide concentration (0.2 and 0.5 M), electrolyte load (4.4–13.2 kg m^–2s^–1), propylene gas load (0.08–1.66 kg m^–2s^–1), reactor outlet temperature (30 and 60°C), bed thickness (5–15 mm) and different carbon types (Union Carbide and Ultra Carbon) on the space-time yield and selectivity for propylene oxide were measured. Depending on conditions, the space-time yield for propylene oxide was between 5.5 and 97 kg m^–3h^–1, and the selectivity was between 55 and 87%. The current efficiency and the specific energy consumption varied from 14 to 58% and from 6 to 60 kWh kg^–1 of propylene oxide. The space-time yield for propylene oxide increased with increasing current density, increasing gas flow and decreasing bed thickness.The current efficiencies for hydrogen, oxygen, dibromopropane, hypobromite, bromite and bromate were also determined.Paper first presented at the AIChE Symposium on Electrochemical Reaction Engineering, Seattle, Washington, 15 August 1985.     

Corrosion of Mild Steel in low Conductive Media simulating Natural Waters

The corrosion of a mild steel was examined in two aerated neutral aqueous solutions, defined as reference solution (0.2 g L^–1NaCl) and as -solution (1.3 g L^–1NaCl + 0.63 g L^–1NaHCO₃ + 0.27 g L^–1Na₂SO₄). Their composition was chosen on the basis of the physical and chemical properties of certain natural waters. The solutions simulated the least (reference solution) and the most (-solution) aggressive waters of the Sebou river in Morocco, as determined after a four-year examination (1991–94), at 13 pump stations located along the river. Various experimental methods were used to determine the corrosion mechanism. Cathodic range voltammetry using a rotating disc allowed the kinetics of oxygen reduction process to be determined. Since the conductivity of the solutions were low, the potential was corrected for ohmic drop estimated through the high frequency limit in the Nyquist diagrams (electrochemical impedance spectroscopy) as well as the current interrupter method. After correction, the polarization curves revealed a diffusion plateau attributed to dissolved oxygen reduction. At the plateau, a two-step mechanism was derived involving oxygen diffusion through the hydrodynamic layer and through a porous inner layer formed by the corrosion products. This inner layer could not be observed by SEM, but both EIS and EHD (electrohydrodynamic impedance) confirmed the presence of a thin porous dielectric layer. At the open circuit potential, the corrosion rate was determined by the diffusion rate of dissolved oxygen in the -solution, and by charge transfer in the reference solution. This shows that the corrosion mechanism strongly depends on the electrolyte and its conductivity.     

Assessing phosphorus bioavailability in agricultural soils and runoff

Bioavailable phosphorus (BAP) transported in agricultural runoff can accelerate surface water eutrophication. Although several algal assays and chemical extractions have been proposed to estimate BAP, procedural and theoretical limitations have restricted widespread BAP measurement. Thus, a routine method was developed to estimate BAP, which uses iron oxide-impregnated paper strips (Fe-oxide strips) as a P-sink for BAP in runoff. In the proposed method BAP is determined by shaking 50 mL of unfiltered runoff with one Fe-oxide strip for 16 h. Phosphorus is removed from the strip by 0.1M H₂SO₄ and measured. The BAP content of runoff from 20 agricultural watersheds in the Southern Plains was related to the growth of P-starved algae incubated for 29 d with runoff as the sole source of P. Acting as a P sink, Fe-oxide strips may have a stronger theoretical basis than chemical extraction in estimating BAP. The method may also have potential use as an environmental soil P test to indicate soils liable to enrich runoff with sufficient P to accelerate eutrophication. Bioavailable P loss in runoff was lower from no till (438 g ha^–1 yr^–1) than from conventional till (1288 g ha^–1 yr^–1). Kinetic and enrichment ratio approaches accurately predicted (r² of 0.93) BAP transport in runoff during 1988 to 1990. Use of the Fe-oxide strip method will facilitate estimation of BAP transport in runoff and thereby, improve assessment of the resultant impact on the biological productivity of receiving surface waters.Abbreviations A Degree of soil aggregation (unitless) - B Soil bulk density (Mg m^–3) - BAP Bioavailable P content of runoff (mg L^–1 and kg ha^–1) - BIOP Bioavailable P content of soil (mg kg^–1) - BPP Bioavailable particulate P content of runoff (mg L^–1 and kg ha^–1) - D Effective depth of interaction between runoff and surface soil (mm) - DP Dissolved P content of runoff (mg L^–1 and kg ha^–1) - ER Enrichment ratio (unitless) - P Phosphorus - P_a Bray I available P content of 0-50 mm depth of soil (mg kg^–1) - PP Particulate P content of runoff (mg L^–1 and kg ha^–1) - STP Soil test P, plant available (mg kg^–1) - TP Total P content of runoff (mg L^–1 and kg ha^–1) - t Duration of runoff event (min) - W Runoff water/soil (suspended sediment) ratio (L g^–1) - K,, ß, i Constants of the equation describing the kinetics of soil P desorption     

Kinetic study of electrochemical reactions at catalyst-recast ionomer interfaces from thin active layer modelling

The objective of the following study was to test proton exchange membrane fuel cell catalysts. A mixture of supported catalyst and recast ionomer (Nafion®) was deposited on a rotating disc electrode (RDE). The resulting thin active layer was immersed in a dilute sulphuric acid solution. The RDE technique allows correction of mass transfer limitation in solution. To calculate the kinetic parameters from the current-potential relation, a mathematical model was written taking into account gas diffusion, ohmic drops and interfacial kinetics within the thin layer. Analytic and/or numerical expressions for the effectiveness factor and for the current-potential relation were obtained. The oxygen reduction reaction at various Pt/C-recast Nafion® interfaces demonstrates the validity of this test procedure.Nomenclature b Tafel slope (V dec^–1) - c Local gas concentration (mol cm^–3) - c _s gas concentration at the electrolyte side (mol cm^–3) - D gas diffusion coefficient within the layer (cm² s^–1) - F Faraday constant (96 500 C mol^–1) - i total current density based on the geometric area (A cm^–2) - i ₀ ^* exchange current density per real catalyst area (A cm^–2) - I dimensionless total current density - j local ionic current density based on the geometric area (A cm^–2) - K ionic conductivity within the layer (S cm^–1) - K _e electronic conductivity within the layer (S cm^–1) - L layer thickness (cm) - m mass fraction of catalyst in the catalytic powder - n total number of electrons involved in reaction - R gas constant (8.31 J K^–1 mol^–1) - S specific catalyst area (m² g^–1) - T temperature (K) - u, v and w dimensionless parameters in Equations 8 and A4 - y dimensionless abscissa Greek symbols - cathodic transfer coefficient - effectiveness factor - local dimensionless overpotential - real catalyst area/geometric area ratio - local overpotential (V) - Nafion® volume fraction - tortuosity factor     

Electrochemical remediation of metal-bearing wastewaters Part I: Copper removal from simulated mine drainage waters

A bench-scale electrochemical cell for plating heavy metals, such as copper from dilute wastewaters, was designed and tested. Optimization tests were performed on simulated mine-drainage water (pH 2.6, 0.1 M Na₂SO₄, 0.02 ^–1 cm^–1, 150 mg L^–1 Cu²⁺) using a vertically oriented, flow-through cell containing a carbon felt cathode. Results obtained for optimized conditions of applied potential and volume flow rate demonstrated greater than 99.9% recovery of copper metal from feed solutions at an ohmic corrected potential of –0.70 V vs Ag/AgCl and flow rates approaching the design maximum of about 0.30 mL s^–1. The effluent concentration of copper under conditions of optimum potential and flow rate could be routinely reduced to a target level of 50 g L^–1.     

Nutrient cycling in a cropping system with potato, spring wheat, sugar beet, oats and nitrogen catch crops. II. Effect of catch crops on nitrate leaching in autumn and winter

The Nitrate Directive of the European Union (EU) forces agriculture to reduce nitrate emission. The current study addressed nitrate emission and nitrate-N concentrations in leachate from cropping systems with and without the cultivation of catch crops (winter rye: Secale cereale L. and forage rape: Brassica napus ssp. oleifera (Metzg.) Sinksk). For this purpose, ceramic suction cups were used, installed at 80 cm below the soil surface. Soil water samples were extracted at intervals of ca 14 days over the course of three leaching seasons (September – February) in 1992–1995 on sandy soil in a crop rotation comprising potato (Solanum tuberosum L.), spring wheat (Triticum aestivum L.), sugar beet (Beta vulgaris L.) and oats (Avena sativa L.). Nitrate-N concentration was determined in the soil water samples. In a selection of samples several cations and anions were determined in order to analyze which cations primarily leach in combination with nitrate. The water flux at 80 cm depth was calculated with the SWAP model. Nitrate-N loss per interval was obtained by multiplying the measured nitrate-N concentration and the calculated flux. Accumulation over the season yielded the total nitrate-N leaching and the seasonal flux-weighted nitrate-N concentration in leachate. Among the cases studied, the total leaching of nitrate-N ranged between 30 and 140 kg ha^–1. Over the leaching season, the flux-weighted nitrate-N concentration ranged between 5 and 25 mg L^–1. Without catch crop cultivation, that concentration exceeded the EU nitrate-N standard (11.3 mg L^–1) in all cases. Averaged for the current rotation, cultivation of catch crops would result in average nitrate-N concentrations in leachate near or below the EU nitrate standard. Nitrate-N concentrations correlated with calcium concentration and to a lesser extent with magnesium and potassium, indicating that these three ion species primarily leach in combination with nitrate. It is concluded that systematic inclusion of catch crops helps to decrease the nitrate-N concentration in leachate to values near or below the EU standard in arable rotations on sandy soils.     

Axial dispersion in flow porous electrodes

The coefficient of axial dispersionD _L in a porous electrode, composed of rolled 80-mesh platinum screen, was determined using the process of the flow electrolysis of 2.0×10^–3 M K₃Fe(CN)₆ in 1 MKCl in water. The results were analysed in the light of an earlier model for flow electrodes.List of symbols a Electrode cross-sectional area (cm²) - b Empirical constant - c ₀ Initial concentration of substrate (mol ml^–1) - D _L Axial dispersion coefficient (cm² s^–1) - D ^* Effective dispersion coefficient (cm² s^–1) - F Faraday constant (C mol^–1) - I ₁ Limiting current (A) - L Electrode height (cm) - R Limiting degree of conversion of substance - v Volume flow rate (ml s^–1) - Empirical constant - Electrode porosity     

Applications of magnetoelectrolysis

A broad overview of research on the effects of imposed magnetic fields on electrolytic processes is given. As well as modelling of mass transfer in magnetoelectrolytic cells, the effect of magnetic fields on reaction kinetics is discussed. Interactions of an imposed magnetic field with cathodic crystallization and anodic dissolution behaviour of metals are also treated. These topics are described from a practical point of view.Nomenclature 1, 2 regression parameters (-) - B magnetic field flux density vector (T) - c concentration (mol m^–3) - c bulk concentration (mol m^–3) - D diffusion coefficient (m² s^–1) - d _e diameter of rotating disc electrode (m) - E electric field strength vector (V m^–1) - E _i induced electric field strength vector (V m^–1) - E _g electrostatic field strength vector (V m^–1) - F force vector (N) - F Faraday constant (C mol^–1) - H magnetic field strength vector (A m^–1) - i current density (A m^–2) - i _L limiting current density (A m^–2) - i _L ⁰ limiting current density without applied magnetic field (A m^–2) - I current (A) - I _L limiting current (A) - j current density vector (A m^–2) - K reaction equilibrium constant - k reaction velocity constant - k _b Boltzmann constant (J K^–1) - _{m 1}, m ₂ regression parameters (-) - n charge transfer number (-) - q charge on a particle (C) - R gas constant (J mol^–1 K^–1) - T temperature (K) - t time (s) - V electrostatic potential (V) - v particle velocity vector (m s^–1) Greek symbols transfer coefficient (–) - velocity gradient (s^–1) - _MS potential difference between metal phase and point just inside electrolyte phase (OHP) - diffusion layer thickness (m) - ₀ hydrodynamic boundary layer thickness without applied magnetic field (m) - density (kg m^–3) - electrolyte conductivity (^–1 m^–1) - magnetic permeability (V s A^–1 m^–1) - kinematic viscosity (m² s^–1) - vorticity     

A feasibility study of mercury deposition in a lead fluidized bed electrode

Following previous work on the recovery of copper from very dilute solutions using a copper fluidized bed electrode, the behaviour of a lead fluidized bed electrode (FBE) is described, for the recovery of mercury from chloride solutions, as typified by chlor-alkali plant effluent.Injection of known quantities of Hg(II) into the FBE catholyte and integration of the current vs time response followed by chemical analysis, allowed mean current efficiencies for mercury deposition to be determined as a function of:feeder electrode potential, Hg(II) concentration, flow rate, bed depth, particle size range, and reservoir volume. By judicious choice of these experimental variables, particularly by limiting bed depths to 20 mm, (potentiostatic) current efficiencies for Hg(II) deposition of 99% could be achieved.Nomenclature a cross sectional area of FBE cell (1.26×10^–3 m²) - A area per unit volume of FBE electrode (m^–1) - c(x) concentration at distancex from feeder electrode (mol m^–3) - c ₀ inlet concentration (mol m^–3) - c _XL outlet concentration (mol m^–3) - D diffusion coefficient (m²s^–1) - I current density (A m^–2) - L static bed length (mm) - t time (s) - T catholyte temperature (K) - u electrolyte superficial linear velocity (mm s^–1) - V electrolyte volume (m³) - XL expanded bed length (mm) - diffusion layer thickness (m) - characteristic length (u/DA) (m) - (lead) density (11.4×10⁶ g m^–3)     

Photocatalytic degradation of oxalic acid on a semiconductive layer of n-TiO2 particles in a batch plate reactor. Part III: Rate determining steps and nonsteady diffusion model for oxygen transport

The effect of oxygen concentration on the photocatalytic degradation rate of oxalic acid on a fixed layer of TiO₂ particles in a batch mode plate photoreactor was investigated at various light intensities. The regions where the photocatalytical decomposition rate is controlled by the flux of oxygen, photons, or both, were identified. For low oxygen concentration (0–0.15 mol m^–3) and photon flux intensity in the range from 10 to 24 × 10^–5 einstein m^–2 s^–1 the experimentally determined photocatalytical decomposition rate was in agreement with that theoretically calculated assuming the process to be controlled by the limiting flux of oxygen to the TiO₂ surface. At higher concentrations of oxygen (0.15–0.94 mol m^–3) the rate of photocatalysis was controlled simultaneously by both the flux of oxygen and photons. The influence of the oxygen concentration decreased with decreasing photon flux. For low photon flux intensities (3.5 × 10^–5 einstein m^–2 s^–1), the reaction rate was controlled by the photon flux. The concentration profile of oxygen in the diffusion layer along the reactor plate was calculated and showed a significant decrease in oxygen concentration on the TiO₂ surface.     

Effect of gas evolution on mass transfer in an electrochemical reactor

Experiments were conducted to study the effect of gas bubbles generated at platinum microelectrodes, on mass transfer at a series of copper strip segmented electrodes strategically located on both sides of microelectrodes in a vertical parallel-plate reactor. Mass transfer was measured in the absence and presence of gas bubbles, without and with superimposed liquid flow. Mass transfer results were compared, wherever possible, with available correlations for similar conditions, and found to be in good agreement. Mass transfer was observed to depend on whether one or all copper strip electrodes were switched on, due to dissipation of the concentration boundary layer in the interelectrode gaps. Experimental data show that mass transfer was significantly enhanced in the vicinity of gas generating microelectrodes, when there was forced flow of electrolyte. The increase in mass transfer coefficient was as much as fivefold. Since similar enhancement did not occur with quiescent liquid, the enhanced mass transfer was probably caused by a complex interplay of gas bubbles and forced flow.List of symbols A electrode area (cm²) - a constant in the correlation (k = aRe ^m , cm s^–1) - C _{R, bulk} concentration of the reactant in the bulk (mol^–1 dm^–3) - D diffusion coefficient (cm² s^–1) - d _h hydraulic diameter of the reactor (cm) - F Faraday constant - Gr Grashof number =gL ³/² (dimensionless) - g gravitational acceleration (cm s^–2) - i _g gas current density (A cm^–2) - i _L mass transfer limiting current density (A cm^–2) - k mass transfer coefficient (cm s^–1) - L characteristic length (cm) - m exponent in correlations - n number of electrons involved in overall electrode reaction, dimensionless - Re Reynolds number =Ud _h^–1 (dimensionless) - Sc Schmidt number = D ^–1 (dimensionless) - Sh Sherwood number =kLD ^–1 (dimensionless) - U mean bulk velocity (cm s^–1) - x distance (cm) - _N equivalent Nernst diffusion layer thickness (cm) - kinematic viscosity (cm² s^–1) - density difference = (_L – ), (g cm^–3) - _L density of the liquid (g cm^–3) - average density of the two-phase mixture (g cm^–3) - void fraction (volumetric gas flow/gas and liquid flow)     

Mass transfer at carbon fibre electrodes

Mass transfer at carbon fibre electrodes has been studied using the mass transfer controlled reduction of potassium hexacyanoferrate(III) to potassium hexacyanoferrate(II). Different geometrical configurations have been assessed in a flow-by mode, namely bundles of loose fibres with liquid flow parallel to the fibres, carbon cloth with flow parallel to the cloth and carbon felt with liquid flow through the felt. For comparison, mass transfer rates at a single fibre have been measured; the experimental data fit the correlationSh=7Re ^0.4. The same correlation can be used as a first approximation for felts. Mass transfer for fibre bundles and cloth under comparable conditions is much lower owing to channelling.Nomenclature c reactant concentration (mol m^–3) - c ₀ reactant concentration atx=0 (mol m^–3) - c _L reactant concentration atx=L (mol m^–3) - d fibre diameter (m) - D diffusion coefficient (m² s^–1) - F Faraday number (96 487 C) - h depth of the electrode (m) - i current density (A m^–2) - I current (A) - k mass transfer coefficient (m s^–1) - L length of the electrode (m) - n number of electrons - S specific surface area (m² m^–3) - u (superficial) velocity (m s^–1) - V _R reactor volume (m³) - w width of electrode (m) - x distance in flow direction (m) - current efficiency - electrode efficiency - characteristic length (m) - v kinematic viscosity (m² s^–1) - _s ⁿ normalized space velocity (m³ m^–3s^–1) - Re Reynolds number (ud/v) - Sh Sherwood number (kd/D) - Sc Schmidt number (v/D)     

The effect of graphitization catalyst on the structure and porosity of SiC derived carbons

A number of carbide-derived carbon (CDC) samples were synthesized through the reaction between α-SiC and gaseous chlorine at temperatures 900, 1000 and 1100 °C and by varying the amount of catalyst. The chlorides of Co(II), Ni(II) and Fe(III) were used as catalytic additives in a range of concentration of 0.1–5 wt%. The structural differences of the obtained carbons were studied by low-temperature nitrogen adsorption, X-ray diffraction and Raman spectroscopy. Results showed that porosity, specific surface area and graphitization degree of the CDC materials is a function of chlorination temperature and catalyst concentration, which agrees with previous results. It was shown that the catalytic graphitization only weakly influences the L_a value of the crystallites, which according to the Raman scattering is 4–5 nm in both the highly disordered SiC derived carbons and in fully graphitic carbons made from SiC containing 15 wt% of surface-contacted Co–Ni–Fe catalyst. The surface area of the CDC materials can be controlled in the range of 300–1350 m² g⁻¹, depending on the amount of catalysts used.     

Modelling of time-dependent performance criteria in a three-dimensional cell system during batch recirculation copper recovery

A three-dimensional electrode cell with cross-flow of current and electrolyte is modelled for galvanostatic and pseudopotentiostatic operation. The model is based on the electrodeposition of copper from acidified copper sulphate solution onto copper particles, with an initial concentration ensuring a diffusion-controlled process and operating in a batch recycle mode. Plug flow through the cell and perfect mixing of the electrolyte in the reservoir are assumed. Based on the model, the behaviour of reacting ion concentration, current efficiency, cell voltage, specific energy consumption and process time on selected independent variables is analysed for both galvanostatic and pseudopotentiostatic modes of operation. From the results presented it is possible to identify the optimal values of parameters for copper electrowinning.List of symbols a specific surface area (m^–1) - A cross-sectional area (mu²) - a _a Tafel constant for anode overpotential (V) - a _II Tofel constant for hydrogen evolution overpotential (V) - b _a Tafel coefficient for anode overpotential (V decade^–1) - b _H Tafel coefficient for hydrogen evolution overpotential (V decade^–1) - C _e concentration at the electrode surface (m) - C _L cell outlet concentration (m) - C ₀ cell inlet concentration (m) - C ₀ ⁰ initial cell inlet concentration att = 0 (m) - d _p particle diameter (m) - e, e _p current efficiency and pump efficiency, respectively - E specific energy consumption (Wh mol^–1) - E solution phase potential drop through the cathode (V) - F Faraday number (C mol^–1) - h interelectrode distance (m) - i, i _L current density and limiting current density, respectively (A m^–2) - I, I _L current and limiting current, respectively (A) - I _H partial current for hydrogen evolution (A) - k _L mass transfer coefficient (m s^–1) - L bed height (m) - l bed depth (m) - M molecular weight (g mol^–1) - N power per unit of electrode area (W m^–2) - n exponent in Equation 19 - P pressure drop in the cell (N m^–2) - Q electrolyte flow rate (m³ h^–1) - R Universal gas constant (J mol^–1 K^–1) - r _e electrochemical reaction rate (mol m^–2 h^–1) - t _c critical time for operating current to reach instantaneous limiting current (s) - t _p process time to reach specified degree of conversion (s) - T temperature (K) - u electrolyte velocity (m s^–1) - U total cell voltage (V) - U ₀ reversible decomposition potential (V) - U _ohm ohmic voltage drop between anode and threedimensional cathode (V) - V volume of electrolyte (m³) - z number of transferred electrons Greek letters ratio of the operating and limiting currents - _{A, a} anodic activation overpotential (V) - _{c, e} cathodic concentration overpotential (V) - bed voidage - _H void fraction of hydrogen bubbles in cathode - constant (Equation 2) - ₀ electrolyte conductivity (ohm^–1 m^–1) - v electrolyte kinematic viscosity (m² s^–1) - _d diaphragm voltage drop (V) - _H voltage drop due to hydrogen bubble containing electrolyte in cathode (V) - electrolyte density (kg m^–3) - _p particle density (kg M^–3) - reservoir residence time (s)     

Electrochemical mass transfer to regular packings in a bubble column

The behaviour of regular packings constructed from corrugated metal sheets was investigated since they constitute an attractive packing material for the electrochemical absorption of gases. Mass transfer coefficients for the regular packing contained in a circular electrolytical cell were determined by the electrochemical method with simulation of the absorption process by bubbling nitrogen through the column. Correlations for the mass transfer rate as a function of fluid dynamic parameters and fluid properties are presented.List of symbols A electrode surface area (m²) - c ₀ bulk concentration (mol m^–3) - D diffusivity (m² s^–1) - E gas hold-up = volume fraction of gas - F Faraday constant (As mo^–1) - Fr Froude number = Vs ²/gL - g gravitational acceleration (m s^–2) - Ga Galileo number = L ³ g/v² - I limiting current (A) - mass transfer coefficient (ms^–1) - L characteristic length (m) - Re Reynolds number = V _s L/ - Sc Schmidt number = /D - Sh Sherwood number = L/D - St Stanton number = /V _S - V _S superficial gas velocity (ms^–1) - z valence change in electrochemical reaction - kinematic viscosity (m² s^–1)     

Nitrogen uptake efficiency in maize production using irrigation water high in nitrate

In order to achieve efficient use of nitrogen (N) and minimize pollution potentials, producers of irrigated maize (Zea mays L.) must make the best use of N from all sources. This study was conducted to evaluate crop utilization of nitrate in irrigation water and the effect N fertilizer has on N use efficiencies of this nitrate under irrigated maize production. The study site is representative of a large portion of the Central Platte Valley of Nebraska where ground water nitrate-N (NO₃-N) concentrations over 10 mg L^–1 are common. Microplots were established to accommodate four fertilizer N rates (0, 50, 100, and 150 kg ha^–1) receiving irrigation water containing three levels of NO₃-N (0, 10, 20 mg L^–1). Stable isotope¹⁵N was applied as a tracer in the irrigation water for treatments containing 10 and 20 mg L^–1 NO₃-N. Plots that did not receive nitrate in the irrigation water where tagged with¹⁵N fertilizer as a sidedress treatment. Sidedressed N fertilizer significantly reduced irrigation-N uptake efficiencies. When residual N uptake is added to first year plant usage, total irrigation NO₃-N uptake efficiencies are similar to total sidedress N fertilizer uptake efficiencies for our cropping system over the two year period. Efficiency of irrigation-N use depends on crop needs and availability of N from other sources during the irrigation season.