Activation effect of halides on chromium electrodeposition from chromic acid baths

The current efficiency of chromium electrodeposition and cathodic polarization curves were determined in halide-chromic acid-sulphuric acid systems. The composition of the cathodic films formed was determined by XPS and AES. The results show that sulphate is an effective catalyst for the deposition of bright chromium and that the current efficiency of the chromium deposition increases remarkably when F^– and Cl^– are added to the bath, and also that F^– and Cl^– participate in the formation of the films. The depth profile curves of the film show that halide is distributed in the inner layer of the film, and SO₄ ^2– in the surface layer. It is deduced that F^– and Cl^– form a bridged complex, [C^III-X^–-Cr^III], in which electron transition is easily carried out.     

Pulse electrodeposition of titanium on carbon steel in the LiF–NaF–KF eutectic melt

Electrodeposition of titanium was carried out in the K₃TiF₆–LiF–NaF–KF melt using both direct (DC) and unipolar pulse current (PC) techniques. Dense and smooth titanium coatings were obtained by PC plating at 750 °C whereas DC plating led to rough and dendritic deposits. The best results were obtained using a 100C cm^–2 pulse charge and a cathodic current density of 50 and 75mA cm^–2. The cathodic current efficiency was in the range 60–65%. The titanium deposits obtained under such conditions behaved similarly to CP-titanium in NaCl and HNO₃ solutions at room temperature.     

Nitrous oxide emissions from paddy soil in three rice-based cropping systems in China

Rice-flooding fallow, rice-wheat, and double rice-wheat systems were adopted in pot experiment in an annual rotation to investigate the effects of cropping system on N₂O emission from rice-based cropping systems. The annual N₂O emission from the rice-wheat and the double rice-wheat cropping systems were 4.3 kg N ha^–1 and 3.9 kg N ha^–1, respectively, higher than that from rice-flooding fallow cropping system, 1.4 kg N ha^–1. The average N₂O flux was 115 and 118 g N m^–2 h^–1 for rice season in rice-wheat system and early rice season in double rice-wheat system, respectively, 68.6 and 35.3 g N m^–2 h^–1 for the late rice season in double rice-wheat system and rice season in rice-flooding fallow, respectively, and only 3.1–5.3 g N m^–2 h^–1 for winter wheat or flooding fallow season. Temporal variations of N₂O emission during rice growing seasons differed and high N₂O emission occurred when soil conditions changed from upland crop to flooded rice.     

Effects of rice cultivars on methane fluxes in a paddy soil

CH₄ emission and its relevant processes involved (i.e. CH₄ production, rhizospheric CH₄ oxidation and plant-mediated CH₄ transport) were studied simultaneously to comprehensively understand how rice cultivars (Yanxuan, 72031, and 9516) at growth stages (early and late tillering, panicle initiation, ripening, and harvest stage) affect CH₄ emission in a paddy soil. Over the entire rice-growing season, Yanxuan had the highest CH₄ emission flux with 5.98 g CH₄ m^–2 h^–1 followed by 72031 (4.48 g CH₄ m^–2 h^–1) and 9516 (3.41 g CH₄ m^–2 h^–1). The highest CH₄ production rate of paddy soils planted to Yanxuan was observed with 18.0 g CH₄ kg_{{ (d.w.soil)}} h^–1 followed by the soil planted to 9516 (17.5 g CH₄ kg_{{ (d.w.soil)}} h^–1). For each cultivar, both rhizospheric CH₄ oxidation ability and plant-mediated CH₄ transport efficiency varied widely with a range of 9.81–76.8% and 15.5–80.5% over the duration of crop growth, respectively. Multiple regression analyses showed that CH₄ emission flux was positively related with CH₄ production rate and rice plant-mediated CH₄ transport efficiency, but negatively with rhizospheric CH₄ oxidation (R ²=0.425 for Yanxuan, P<0.01; R ²=0.426 for 72031, P<0.01; R ²=0.564 for 9516, P<0.01). The contribution of rice plants to CH₄ production seems to be more important than to rhizospheric CH₄ oxidation and plant-mediated transport in impact of rice plants on CH₄ emission.     

Measurement of nitrous oxide emissions from two rice-based cropping systems in China

A field experiment was conducted to investigate the effects of winter management and N fertilization on N₂O emission from a double rice-based cropping system. A rice field was either cropped with milk vetch (plot V) or left fallow (plot F) during the winter between rice crops. The milk vetch was incorporated in situ when the plot was prepared for rice transplanting. Then the plots V and F were divided into two sub-plots, which were then fertilized with 276 kg urea-N ha^–1 (referred to as plot VN and plot FN) or not fertilized (referred to as plot VU and plot FU). N₂O emission was measured periodically during the winter season and double rice growing seasons. The average N₂O flux was 11.0 and 18.1 g N m^–2 h^–1 for plot V and plot F, respectively, during winter season. During the early rice growing period, N₂O emission from plot VN averaged 167 g N m^–2 h^–1, which was eight- to fifteen-fold higher than that from the other three treatments (17.8, 21.0 and 10.8 g N m^–2 h^–1 for plots VU, FN, and FU, respectively). During the late rice growing period, the mean N₂O flux was 14.5, 11.1, 12.1 and 9.9 g N m^–2 h^–1 for plots VN, VU, FN and FU, respectively. The annual N₂O emission rates from green manure-double rice and fallow-double rice cropping systems were 3.6 kg N ha^–1 and 1.3 kg N ha^–1, respectively, with synthetic N fertilizer, and were 0.99 kg N ha^–1 and 1.12 kg N ha^–1, respectively, without synthetic N fertilizer. Generally, both green manure N and synthetic fertilizer N contribute to N₂O emission during double rice season.     

Mass transfer and electrocrystallization analyses of nanocrystalline nickel production by pulse plating

A comparison between the experimental process parameters employed for the pulse plating of nanocrystalline nickel and the solution-side mass transfer and electrokinetic characteristics has been carried out. It was found that the experimental process parameters (on-time, off time and cathodic pulse current density) for cathodic rectangular pulses are consistent and within the physical constraints (limiting pulse current density, transition time, capacitance effects and integrity of the waveform) predicted from theory with the adopted postulates. This theoretical analysis also provides a means of predicting the behaviour of the process subject to a change in the system, kinetic and process parameters. The product constraints (current distribution, nucleation rate and grain size), defined as the experimental conditions under which nanocrystalline grains are produced, were inferred from electrocrystallization theory. High negative overpotential, high adion population and low adion surface mobility are prerequisites for massive nucleation rates and reduced grain growth; conditions ideal for nanograin production. Pulse plating can satisfy the former two requirements but published calculations show that surface mobility is not rate-limiting under high negative overpotentials for nickel. Inhibitors are required to reduce surface mobility and this is consistent with experimental findings. Sensitivity analysis on the conditions which reduce the total overpotential (thereby providing more energy for the formation of new nucleation sites) are also carried out. The following lists the effect on the overpotential in decreasing order: cathodic duty cycle, charge transfer coefficient, Nernst diffusion thickness, diffusion coefficient, kinetic parameter () and exchange current density.Nomenclature A constant employed in Fig. 8, (nFi₀)/(RT _e C _a)(s^–1) - B constant in Equation 38 (V²) - C cation concentration (molcm^–3) - C _a capacitance of double layer (µFcm^–2) - C _s cation surface concentration (molcm^–3) - C _s ^* dimensionless cation surface concentration, C _s/C (–) - C cation bulk concentration (molcm^–3) - D diffusion coefficient of cation (cm²s^–1) - E total applied potential (V) - E ⁰ standard cell potential (V) - F Faraday constant (Cmol^–1) - function defined in Appendix C(–) - Fr frequency of waveform (Hz) - f _i,p function defined in Appendix C for pth period (–) - f _i, function defined in Appendix C for p period (–) - G _j function defined in Appendix B (–) - gi function defined in Appendix B (–) - i current density (Acm^¨) - i _ac unsteady fluctuating a.c. current density (Acm^–2) - i _c capacitance current density (Acm^–2) - i _dc steady time-averaged d.c. current density (Acm^–2) - i _F Faradaic current density (Acm^–2) - i _lim limiting d.c. current density (Acm^–2) - i ₀ exchange current density (Acm^–2) - i _PL limiting pulse current density, i ₁{Cs = 0 at t = (p – 1) T + t ₁(Acm^–2) - i ₁ cathodic pulse current density (Acm^–2) - i ₂ relaxed or low current pulse current density (Acm^–2) - iin anodic pulse current density (Acm^–2) - i ^* dimensionless current density, i/|i _lim| (–) - i ₀ ^* dimensionless exchange current density, i _dc/|i _lim| (–) - i _dc ^* dimensionless steady time-averaged d.c. current density, i _dc/|i _lim| (–) - i _PL ^* dimensionless limiting cathodic pulse current density, i _PL/|i _lim| (–) - i _PL,p ^* dimensionless limiting pulse current density at pth period, i ₁(C _s = 0)/|i _lim| (–) - i _PL, ^* dimensionless limiting pulse current density for p , i ₁(C _s = 0)/|i _lim| (–) - i ₁ ^* dimensionless cathodic pulse current density, i ₁/|i _lim| (–)     

Effect of changing groundwater levels caused by land-use changes on greenhouse gas fluxes from tropical peat lands

Monthly measurements of carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) fluxes in peat soils were carried out and compared with groundwater level over a year at four sites (drained forest, upland cassava,upland and lowland paddy fields) located in Jambi province, Indonesia. Fluxes from swamp forest soils were also measured once per year as the native state of this investigated area. Land-use change from drained forest to lowland paddy field significantly decreased the CO₂ (from 266 to 30 mg C m^–2 h^–1) and N₂O fluxes (from 25.4 to 3.8 g N m^–2 h^–1), but increased the CH₄ flux (from 0.1 to 4.2 mg C m^–2 h^–1) in the soils. Change from drained forest to cassava field significantly increased N₂O flux (from 25.4 to 62.2 g N m^–2 h^–1), but had no significant influence on CO₂ (from 266 to 200 mg C m^–2 h^–1) and CH₄ fluxes (from 0.1 to 0.3 mg C m^–2 h^–1) in the soils. Averaged CO₂ fluxes in the swamp forests (94 mg C m^–2 h^–1) were estimated to be one-third of that in the drained forest. Groundwater levels of drained forest and upland crop fields had been lowered by drainage ditches while swamp forest and lowland paddy field were flooded, although groundwater levels were also affected by precipitation. Groundwater levels were negatively related to CO₂ flux but positively related to CH₄ flux at all investigation sites. The peak of the N₂O flux was observed at –20 cm of groundwater level. Lowering the groundwater level by 10 cm from the soil surface resulted in a 50 increase in CO₂ emission (from 109.1 to 162.4 mg C m^–2 h^–1) and a 25% decrease in CH₄ emission (from 0.440 to 0.325 mg C m^–2 h^–1) in this study. These results suggest that lowering of groundwater level by the drainage ditches in the peat lands contributes to global warming and devastation of fields. Swamp forest was probably the best land-use management in peat lands to suppress the carbon loss and greenhouse gas emission. Lowland paddy field was a better agricultural system in the peat lands in terms of C sequestration and greenhouse gas emission. Carbon loss from lowland paddy field was one-eighth of that of the other upland crop systems, although the Global Warming Potential was almost the same level as that of the other upland crop systems because of CH₄ emission through rice plants.     

Electrodeposition of chromium from Cr (III) electrolytes in the presence of formic acid

Chromium coatings deposited from sulphate, chloride and perchlorate electrolytes based on the [Cr(H₂O)₄(HCOO)]²⁺ complex ion were investigated. The current efficiency reached 30% in the case of chloride electrolyte for various current densities in the range 4–10 A dm^-2 depending on pH. Such a large current efficiency is due to the catalytic effect of the chloride ions on the electroreduction of the chromium complex to metallic chromium. Deposition of chromium from sulphate electrolyte took place with a current efficiency of 16% which rose for higher pH and lower current densities. Semi-bright and bright coatings with thickness of approximately 10m of good adhesion to a copper electrode were deposited from chloride and sulphate electrolytes.     

Electrodeposition and characterization of amorphous Fe-Cr-P-C alloys

Electrodeposition of iron-chromium alloys was investigated in a divided and undivided cell. The influence of the current density on the composition of the alloy was determined. Energy dispersive spectroscopy and Auger electron spectroscopy were used to evaluate alloy composition. These alloys were determined to be amorphous by X-ray diffraction and transmission electron microscopy. The corrosion behaviour of the alloys was studied in 0.5m HCl, and they were found to be more corrosion resistant than conventional chromium electrodeposits, with corrosion currents ranging between 40–60 A cm^–2, compared to 1850 A cm^–2 for conventional chromium. The alloy electrodeposition mechanism was studied by cyclic voltammetry.     

Experimental investigation of the primary and secondary current distribution in a rotating cylinder Hull cell

A rotating cylinder cell having a nonuniform current distribution similar to the traditional Hull cell is presented. The rotating cylinder Hull (RCH) cell consists of an inner cylinder electrode coaxial with a stationary outer insulating tube. Due to its well-defined, uniform mass-transfer distribution, whose magnitude can be easily varied, this cell can be used to study processes involving current distribution and mass-transfer effects simultaneously. Primary and secondary current distributions along the rotating electrode have been calculated and experimentally verified by depositing copper.List of symbols c distance between the cathode and the insulating tube (cm) - F Faraday's constant (96 484.6 C mol^–1) - h cathode length (cm) - i local current density (A cm^–2) - i _L limiting current density (A cm^–2) - i _ave average current density along the cathode (A cm^–2) - i ₀ exchange current density (A cm^–2) - I total current (A) - M atomic weight of copper (63.54 g mol^–1) - n valence - r _p polarization resistance () - t deposition time (s) - V _c cathode potential (V) - Wa _T Wagner number for a Tafel kinetic approximation - x/h dimensionless distance along the cathode surface - z atomic number Greek symbols _a anodic Tafel constant (V) - _c cathodic Tafel constant (V) - solution potential (V) - overpotential at the cathode surface (V) - density of copper (8.86 g cm^–3) - electrolyte conductivity ( cm^–1) - deposit thickness (cm) - _ave average deposit thickness (cm) - surface normal (cm)     

Electrochemical reduction of solutions of ReF6 in fused LiF–NaF–KF eutectic mixture

Electrolyte was prepared by introducing gaseous ReF₆ into the molten LiF–NaF–KF eutectic at 600 °C. The electrochemical properties of the solutions were studied by voltammetric techniques. The reduction of ReF₈ ^2– to Re occurred via a single irreversible step with diffusion controlled mass transfer. The diffusion coefficient of ReF₈ ^2– was 8 × 10^–10 m² s^–1 and the cathodic transfer coefficient was 0.13. Well-crystallized pure rhenium layers, up to 50 m thick, were obtained on W, Ag, graphite and vitreous carbon substrates and were examined by SEM and X-ray diffraction techniques.     

An experimental study of mass transfer in pulse reversal plating

An experimental study has been made of the limiting pulse current density for a periodic pulse reversal plating of copper on a rotating disc electrode from an acidic copper sulfate bath containing 0.05m CuSO₄ and 0.5M H₂SO₄. The measurements were made over a range of the electrode rotational speeds of 400–2500 r.p.m., pulse periods of 1–100 ms, cathodic duty cycles of 0.25–0.9, and dimension-less anodic pulse reversal current densities of 0 to 50. The experimental limiting pulse current data were compared to the theoretical prediction of Chin's mass transfer model. A satisfactory agreement was obtained over the range of a dimensionless pulse period ofDT/ ²=0.001–1; the root mean square deviation between the theory and 128 experimental data points was ±8.5%.Notation C _b bulk concentration of the diffusing ion (mol cm^–3) - C _s surface concentration of the diffusing ion (mol cm^–3) - D diffusivity of the diffusing ion (cm² s^–1) - F Faraday's constant (96 500C equiv^–1) - i current density (A cm^–2) - i ₁ cathodic pulse current density (A cm^–2) - i ₃ anodic pulse reversal current density (A cm^–2) - i ₃ ^* dimensionless anodic pulse reversal density defined asi ₃/i _lim - i _lim cathodic d.c. limiting current density (A cm^–2) - i _{lim, a} anodic d.c. limiting current density (A cm^–2) - i _PL cathodic limiting pulse current density (A cm^–2) - i _PL ^* dimensionless limiting pulse current density defined asi _PL/i _lim - m dummy index in Equation 1 - n number of electrons transferred in the electrode reaction (equiv/mol) - l time (s) - t ₁ cathodic pulse time (s) - i ₃ anodic pulse reversal time (s) - T pulse period equal tot ₁+t ₃ (s) - T ^* pulse period defined asDT/ ² (dimensionless) Greek letters thickness of the steady-state Nernst diffusion layer (cm) - electrode potential (V) - _de time-averaged electrode potential (V) - _m eigenvalues given by Equation 2 (dimensionless) - ₁ cathodic duty cycle (dimensionless) - ₃ anodic duty cycle in pulse reversal plating (dimensionless) - kinematic viscosity (cm² s^–1) - electrode rotational speed (rad s^–1)     

Long service life IrO2/Ta2O5 electrodes for electroflotation

Ti/IrO₂-Ta₂O₅ electrodes prepared by thermal decomposition of the respective chlorides were successfully employed as oxygen evolving electrodes for electroflotation of waste water contaminated with dispersed peptides and oils. Service lives and rates of dissolution of the Ti/IrO₂-Ta₂O₅ electrodes were measured by means of accelerated life tests, e.g. electrolysis in 0.5M H₂SO₄ at 25°C and j = 2 A cm^–2. The steady-state rate of dissolution of the IrO₂ active layer was reached after 600–700 h (0.095 g Ir h^–1 cm^–2) which is 200–300 times lower than the initial dissolution rate. The steady-state rate of dissolution of iridium was found to be proportional to the applied current density (in the range 0.5–3 A cm^–2 ). The oxygen overpotential increased slightly during electrolysis (59–82 mV for j = 0.1 A cm^–2 ) and the increase was higher for the lower content of iridium in an active surface layer. The service life of Ti/IrO₂ (65 mol%)-Ta₂O₅ (35 mol%) in industrial conditions of electrochemical devices was estimated to be greater than five years.List of symbols a constant in Tafel equation (mV) - b slope in Tafel equation (mV dec^–1) - E potential (V) - f mole fraction of iridium in the active layer - j current density (A cm^–2) - l number of layers - m _Ir content of iridium in the active layer (mg cm^–2) - r dissolution rate of the IrO₂ active layer (g Ir h^–1 cm^–2) - T _c calcination temperature (°C) - _{O 2} oxygen overpotential (mV) - _{O 2} difference in oxygen overpotential (mV) - _A service life in accelerated service life tests (h) - _S service life in accelerated service life tests related to 0.1 mg Ir cm^–2 (h) - _p polarization time in accelerated service life tests (h)     

The mechanism of copper powder formation in potentiostatic deposition

A mechanism for copper powder formation in potentiostatic deposition is proposed, and the critical overpotential of copper powder formation is determined. A good agreement between theoretical and experimental results has been obtained.List of symbols C ₀ bulk concentration (mol cm^–3) - D diffusion coefficient (cm² s^–1) - F Faraday's constant (C mol^–1) - h height of protrusion (cm) - h _c height at which dendrites crack (cm) - h _i height (cm) - h ₀ initial height of protrusion (cm) - h _j,t elevation at pointj and timet (cm) - h _j,0 initial elevation at pointj (cm) - I limiting diffusion current (A) - I ₀ initial limiting diffusion current (A) - i limiting current density (A cm^–2) - i _d current density on the tip of dendrite of height h (A cm^–2) - i _t total current (A cm^–2) - j number - k proportionality factor [cm (mol cm^–3)^m] - k constant - M number of dendrites - m number - N number of elevated points - n number of electrons - p concentration exponent - Q _c quantity of electricity (C) - R gas constant (J mol^–1 K^–1) - S electrode surface area (cm²) - T temperature (K) - t time (s) - t _a longest time in which approximation h is valid (s) - t _i induction time (s) - V molar volume (cm³ mol^–1) - surface tension (J cm^–2) - thickness of diffusion layer (cm) - overpotential (V) - _c,p critical overpotential of powder formation (V) - fraction of flat surface - apparent induction time (s)     

Electrical measurements in bipolar trickle reactors

Current potential curves for the total current flowing through the reactor and for the current passing through a single ring of the column packing have been measured using solutions containing the ferroferricyanide couple. The theoretical formulation of current-potential plots has been extended to incorporate a fast reversible reaction in the presence of diffusion polarization. A method for deriving the film thickness and mass transfer limiting current from these plots has been provided.List of symbols a integration constant in Equation 6 - b integration constant in Equation 8 - E applied potential (V) - E _r potential of the ring electrode with respect to the feeder electrode at the entry position of the reactor (V) - E ₁,E ₂ reversible potentials of an anodic and cathodic reaction, respectively - F Faraday constant - h film thickness (cm) - I current passing through segmented rings (mA) - I _F Faradaic current per unit length of wetted perimeter (A cm^–1) - I _NF non-Faradaic current per unit length of wetted perimeter (A cm^–1) - I _T total current per unit length of wetted perimeter (A cm^–1) - L half-length of Raschig ring (cm) - i _D limiting mass-transfer controlled current (A cm^–1) - i _D limiting mass transfer controlled current at the end of the rings (A cm^–2) - i_{D, 1mM} limiting mass transfer controlled current for 1 mM of redox couple - i_o1, i_o2 exchange current for two reactions (one anodic and the other cathodic) - n number of electrons transferred in an electrochemical reaction - n ₁,n ₂ number of electrons transferred in two reactions (one anodic and the other cathodic) - n _c number of mmol of ferro-ferricyanide - n _r number of graphite Raschig rings in a single layer of a packed column - r reaction rate (mol cm^–2 s^–1) - R gas constant (8.314JK^–1mol^–1) - r _o,r _i radii of the outer and inner perimeter of the ring (cm) - (Re)_f film Reynolds number (dimensionless) - T temperature (K) - v volumetric liquid flow rate (cm³ min^–1) - x axial co-ordinate along Raschig ring (cm) - ₁, ₂ transfer coefficients for two reactions (one anodic and the other cathodic) (dimensionless) - fraction of the end areas of the rings which overlap (dimensionless) - electrode overpotential (V) - _T total overpotential for half of a bipolar ring (V) - v kinematic viscosity (cm² s^–1) - solution resistivity ( cm) - _s potential in the solution phase (V)     

Electrochemical mass transfer in solutions containing drag-reducing polymers

Electrochemical mass transfer was studied in the presence of polyethylene oxide as a drag-reducing agent using the cathodic reduction of K₃Fe(CN)₆ at a rotating cylinder electrode over a range of Reynolds numbers from 4100—41 000. Solutions containing polymer showed a lowered mass transfer coefficient than that without polymer. Mass transfer data in solutions containing polymers was found to fit the correlation: (St) = 0.475 (Re)^–0.3(Sc)^–0.644.A comparison was made between the reduction in friction and the rate of mass transfer; it was found that at relatively low (Re) values, the reduction in the rate of mass transfer is higher than the reduction in friction, whilst at relatively high (Re) values, the reverse is true.List of symbols I _L limiting current density, A cm^–2 - Z number of electrons involved in the reaction - F Faraday (96 500C) - K mass transfer coefficient, cm s^–1 - V linear velocity of the cylinder, cm s^–1 - angular velocity, rad s^–1 - D diffusion coefficient, cm² s^–1 - v kinematic viscosity, cm² s^–1 - d diameter of the cylinder, cm - , ₀ viscosity of solutions with and without polymers respectively, P - density, g cm^–3 - C concentration of Fe(CN) ₆ ^3– ions, mol cc^–1 - (St) Stanton number =K/V - (Sc) Schmidt number =v/D - (Re) Reynold number =Vd/     

Fe3+--gluconate and Ca2+-Fe3+--gluconate complexes as mediators for indirect cathodic reduction of vat dyes – Cyclic voltammetry and batch electrolysis experiments

The indirect cathodic reduction of dispersed vat dyes CI Vat Yellow 1 and CI Vat Blue 5 was investigated by cyclic voltammetry and with batch electrolysis experiments. 0.01molL^–1 solutions of the complexes Fe³⁺--gluconate and Ca²⁺-Fe³⁺--gluconate were studied. The addition of dispersed dyestuff to the mediator solution lead to a catalytic current. While the cathodic peak currents of both complexes is comparable, Fe³⁺-DGL shows higher enhancement factors, which are defined as quotient of catalytic peak current and cathodic peak current of the mediator system (I_p)_c/(I_p)_d. In the presence of 0.5gL^–1 of dispersed vat dye enhancement factors of 1.8 were determined at scan rates of 0.005–0.010Vs^–1. Galvanostatic batch reduction experiments with use of a laboratory multi-cathode cell confirmed the favourable properties of the Fe³⁺-DGL in comparison to the binuclear Ca²⁺-Fe³⁺-DGL. With use of the Fe³⁺-DGL mediator complete dyestuff reduction could be achieved. The batch reduction process was followed experimentally by photometry and redox potential measurement in the catholyte.     

HBr electrolysis in the Ispara Mark 13A flue gas desulphurization process: electrolysis in a DEM cell

The potential application of a DEM cell for the electrolysis of hydrogen bromide in the Ispra Mark 13A process for flue gas desulphurization has been tested in a number of laboratory experiments and in long-duration tests in a bench-scale plant of the process. Satisfactory electrode materials have been found, i.e. Hastelloy C 276 for the cathode and a RuO₂ coating on titanium for the anode. Both electrode materials showed a good stability during a 1500 hours experiment. Cell voltage/current density relationships have been determined during bench-scale plant operation. A typical value is 1.5V at a current density of 2.5 kA m^–2. It has been shown that in an undivided cell a cathodic back reaction occurs which causes a decrease of the current efficiency. Under normal operation conditions current efficiencies of about 90% are obtained.A simplified flow model for the DEM cell was developed which is useful in understanding the phenomena which occur during scale-up of the cell. An industrial size installation for the production of 170 kg h^–1 of bromine at a current density of 2 kA m^–2 was constructed and has been in operation since August 1989.Nomenclature a _x thermodynamic activity of the constituentx (mol cm^–3) - C bromine concentration (mol l^–1) - e _z local current efficiency - e _ov overall cell efficiency - E _a ⁰ anodic standard potential (V) - E _c ⁰ cathodic standard potential (V) - E _a ^c equilibrium anode potential (V) - E _e ^c equilibrium cathode potential (V) - F Faraday number (C mol^–1) - g _a anodic overpotential (V) - g _c cathodic overpotential (V) - G electrolyte flow rate (l h^–1) - i current density (A m^–2) - K _c cathodic back reaction rate factor (l mol^–1) - L cell width (m) - n number of electrons involved (n=2) - R gas constant (J K^–1 mol^–1) - R _cell cell resistance (ohm m²) - R _c circuit resistance (ohm m²) - w _b local cathodic back reaction rate (mol m^–2 h^–1) - w _th local theoretical reaction rate (mol m^–2 h^–1) - W _th overall theoretical reaction rate (mol h^–1) - T temperature (K) - Z cell length (m)     

Rates and controls on denitrification in an agricultural soil fertilized with liquid lagoonal swine waste

Under laboratory conditions, we studied rates and controls on denitrification and denitrification potential (denitrifying enzyme activity, DEA) in agricultural soils in the southeastern United States that had been repeatedly fertilized with liquid lagoonal swine effluent. This is a waste management practice commonly employed by large-scale swine production facilities that have proliferated regionally in the past 10 years. The microbial community was rapidly responsive to the added waste, as denitrification N flux (N₂ + N₂O) from intact soil cores increased from about 200 to as high as 2850 g N m^–2 h^–1, usually within 1 day of application. Elevated rates of denitrification were short-lived (3 days), as the combination of coarse soil texture (rapid drainage) and low mineralization potential (low organic content) of the waste rapidly restored aerobic conditions. Although <2% of the fertilizer-N was lost to denitrification by the time rates had returned to pre-fertilization values after 8–12 days, soil NO₃^––N levels increased from 5 g N g_dw soil^–1 to as high as 43 g N g_dw soil^–1, providing not only substrate for additional denitrification following rainfall, but also a mobile N source for both offsite transport by surface and groundwater and assimilation by plants. Both N₂O and N₂ production from denitrification were unresponsive to changes in soil moisture until field capacity was approached or exceeded. Temperature coefficients (Q₁₀) for DEA varied from 1.6 to 2.8 between 7 and 30 °C, depending on the temperature interval, while high DEA between 20 and 40 °C pointed to a denitrifying community well-adapted to regional summer soil temperatures. Glucose-C or NO₃^––N amendments proved equally stimulatory to DEA in homogenized soils relative to water-only controls. However, addition of the combined substrates gave the best response, indicating that these chemical factors were equally important controls on potential denitrification in these soils once anaerobic conditions had become established.     

Nitrous oxide emissions from fertilized upland fields in Thailand

Nitrous oxide (N₂O) emission from fertilized maize fields was measured using a closed chamber at four experimental sites in Thailand. The average measured N₂O flux from unfertilized plots through crop season was 4.16 ± 1.52, 5.05 ± 1.65, 5.25 ± 1.68 and 6.74 ± 2.95 g N₂O-N m^-2 h^-1, at Nakhon Sawan, Phra Phutthabat, Khon Kaen and Chiang Mai, respectively. Increased N₂O emissions by the application of nitrogen fertilizer were 0.22–0.44, 0.19–0.38%, 0.12–0.24 and 0.08–0.15% of the applied N, respectively. Compared to other data, N₂O emission rate to applied nitrogen was not significantly different between the data of Thailand and the Temperate Zone.