Large Internal Resistance Research Articles

Gold recovery with ion exchange used resins

In this paper one strong acidic, one strong basic and one weak basic ion-exchange resins, considered as exhausted in an industrial demineralizing plant, are screened for gold recovery from cyanide solutions. Based on the observed ability for the recovery and on the ease of regeneration, the weak base anion exchanger Purolite A-100 is selected. This spent resin is stable until 60°C and, after regeneration, conserves its physical properties as compared with a new one. Equilibrium data for the resin are determined, proving the very high capacity of the resin for gold (∼500 mg Au/g dry resin) and modeled by the Freundlich and mass action isotherm models. A kinetic experiment is conducted in a batch adsorber and modeled with an equivalent Fick's diffusivity using the linear driving force approximation, showing that the film resistance to mass transfer controls the operation. Finally, a fixed bed adsorber is saturated with gold aurocyanide and regenerated with a potassium hydroxide solution. The model used for the simulation of both steps incorporates axial dispersion and the same equivalence for the ionic diffusivity. During the elution process, precipitation of dihydrated potassium aurocyanide occurs inside the resin, increasing the intraparticle resistance to mass transfer. The model is able to reasonably represent the experimental elution results when a large internal resistance to mass transfer is used. A 25-fold concentration of the initial gold solution is obtained in this saturation/elution process, albeit the precipitation, showing the feasibility of the method for the recovery of gold, increasing the useful life of the resins and decreasing pollution. The anionic Purolite A-100 resin showed also a significant capacity for removing silver, although less than for gold, probably because silver cyanide complexes occupy, on the average, more than one ion exchange site in the resin.

Anatomy of CO2 diffusion in leaf photosynthesis

In some C3 species, Cc can be as low as half the ambient CO2 in the air. This clearly indicates that resistance to CO2 diffusion within the leaf is substantial. Affinity of Rubisco to CO2 is low. The presence of oxygen further lowers the carboxylation rate. Moreover, the photorespiration path consumes energy and releases CO2 . Clearly, it is advantageous for the leaf to minimize resistance to CO2 diffusion to increase photosynthesis. We will discuss problems concerning the resistance to CO2 diffusion from the mesophyll cell surface to the chloroplast stroma ('internal' resistance) from several viewpoints. 1) There is a correlation between cell wall thickness and the internal resistance. The clear correlation was also observed during the leaf development. In broad-leaved evergreen trees and some alpine plants, large internal resistance may be mainly attributed to their thick cell walls. 2) When Hg2+, an inhibitor of the water channel, was fed via the petiole of Phaseolus vulgaris, the internal resistance of the leaf increased by several, which suggests that CO2 would diffuse through the water channels. 3) Sun leaves should have larger chloroplast surface area facing the intercellular spaces per unit leaf area (Sc) than shade leaves to conduct efficient photosynthesis at high Cc level. This may be an answer to the question why sun leaves are thicker. We also show how sun and shade leaves differentiate in response to light environment. The mature leaves rather than the differentiating leaves per se sense the light environment.

Development and operation of a 150 W air-feed direct methanol fuel cell stack

A five-cell 150 W air-feed direct methanol fuel cell (DMFC) stack was demonstrated. The DMFC cells employed Nafion 117® as a solid polymer electrolyte membrane and high surface area carbon supported Pt-Ru and Pt catalysts for methanol electrooxidation and oxygen reduction, respectively. Stainless steel-based stack housing and bipolar plates were utilized. Electrodes with a 225 cm2 geometrical area were manufactured by a doctor-blade technique. An average power density of about 140 mW cm−2 was obtained at 110 °C in the presence of 1 M methanol and 3 atm air feed. A small area graphite single cell (5 cm2) based on the same membrane electrode assembly (MEA) gave a power density of 180 mW cm−2 under similar operating conditions. This difference is ascribed to the larger internal resistance of the stack and to non-homogeneous reactant distribution. A small loss of performance was observed at high current densities after one month of discontinuous stack operation.

Color removal with natural adsorbents: modeling, simulation and experimental

The adsorption in some natural materials containing chitin namely, Squid (Loligo vulgaris) and Sepia (Sepia officinalis) pens, and Anodonta (Anodonta cygnea) shells for color removal from textile wastewaters was studied. A reactive and a direct green dyestuff, the Cibacron green T3G-E (CI reactive green 12) and the Solophenyl green BLE 155% (CI direct green 26) from CIBA, respectively, were selected for this study. Continuous experiments in a packed column at 20°C with the natural materials showed a large internal resistance to mass transfer. In order to improve the adsorbents performances, the materials were submitted to chemical treatment (demineralization and/or deproteinization). Isotherms at 20°C were determined for all systems and compared with the ones using the materials after chemical treatment. These results were fitted by both Langmuir and Freundlich models. The determined parameters showed that equilibrium adsorption capacities increased at least five times. These results and the mathematical simulation of the column runs showed that there are improvements both in equilibrium and kinetic data. The adsorbents physical and chemical properties before and after chemical treatment were briefly characterized in order to investigate the changes responsible for those improvements. Biodegradation of the direct dyestuff was observed during the column operation using both the Anodonta shell and Sepia pen. For these two systems the chemical treatment of the materials did not improve the color removal. Biodegradation was included in the developed model and the influence of the model parameters on the system behavior was analyzed.

Allocation of nitrogen and carbon in leaves of Metrosideros polymorpha regulates carboxylation capacity and δ13C along an altitudinal gradient

1. Metrosideros polymorpha (O’hia), the dominant tree species in Hawaiian forest ecosystems, grows from sea level to treeline (2500 m). Consistent changes in its morphology and anatomy occur along this altitudinal/temperature gradient. Patterns of variation in photosynthetic gas exchange, leaf nitrogen content, nitrogen‐use efficiency, δ13C, and morphological and anatomical characteristics were determined across the elevational gradient. In addition, on‐line carbon isotope discrimination studies of high and low elevation M. polymorpha were performed. 2. Observed trends with increasing altitude were: (1) progressively higher carboxylation efficiency, leaf N content on an area basis, leaf mass per unit area (LMA), less negative foliar δ13C and (2) progressively smaller leaf size. Net CO2 assimilation (A) expressed on an area basis, leaf dry mass and N content per leaf remained relatively constant along the gradient. 3. Foliar δ13C became less negative with increasing elevation (– 30‰ at low elevation to – 24‰ at high elevation) and was strongly correlated with foliar N and LMA. Foliar δ13C was also correlated with variations in the ratio of intercellular to ambient partial pressure of CO2 (pi/pa), as determined by field gas‐exchange studies. 4. Results from on‐line fractionation experiments suggested that the relatively large internal resistance to CO2 diffusion did not differ between high and low elevation populations, despite differences in LMA. Less negative values of δ13C at high elevations and corresponding lower values of pi/pa were associated with increased carboxylation efficiency and N content on a unit leaf area basis. 5. Two major homeostatic responses in M. polymorpha plants along elevational/temperature gradients were observed: (1) maintenance of similar photosynthetic rates per unit leaf surface area despite suboptimal conditions for CO2 assimilation at high elevation and (2) similar N content per leaf despite lower soil N availability at high elevations. These homeostatic mechanisms allow M. polymorpha to maintain a relatively high level of growth‐related activities at high elevation, despite limiting environmental conditions.

Numerical simulation of a coupled-quantum-well resonant tunneling transistor

The quantum distributed model of the resonant tunneling transistor is improved based on a generalized S-matrix method. As a result, a model of the base current and the collector current is formulated in a consistent way and incoherent tunnelings are also considered. This model is used to study the coupled-quantum-well resonant tunneling transistor, and it is found that the resonance in the collector current disappears because of the spatial dependence of the resonance in the collector current when there is large internal resistance or a large scattering rate. It is also found that inclusion of the hole current is important in calculating the device characteristics of the resonant tunneling transistor.

Photovoltaic and electrical properties of aluminum/Langmuir-Blodgett films/silver sandwich cells incorporating either chlorophyll a, chlorophyll b, or zinc porphyrin derivative

The photovoltaic properties of sandwich cells of Langmuir-Blodgett films of chlorophyll a, chlorophyll b, and zinc porphyrin derivative have been measured. The rectifying devices were prepared by interposing the pigment multilayers between two semitransparent metal electrodes, i.e., aluminum and silver with work functions ([phi]) such that [phi][sub Al] < [phi][sub pigment] < [phi][sub Ag]. All sandwich cells exhibit small short-circuit photocurrents and large open-circuit photovoltages (as high as 1.1 V). The low-power conversion efficiency of all three pigments (0.001--0.030%) is related to the large internal resistance of the photovoltaic cells (30 to more than 1,000 M[Omega]). This suggests that the poor photovoltaic properties of the sandwich cells are extrinsic to the molecular structure of pigments and should rather be related to the presence of the insulating layers of aluminum and chrome oxide and of the monolayer of cadmium arachidate. In addition, a theoretical treatment of the action spectra is presented. The multiple internal reflections of light between the semitransparent metal electrodes strongly modify the photovoltaic properties of the sandwich cells. The characterization of the metal/semiconductor junction was carried out with the low-frequency oscillographic method. Large variations in the relations between 1/C[sup 2] and V were observed as a function ofmore » the light intensity, the frequency, and the amplitude of the applied bias. These variations have been related to the presence of the insulating layer which modifies the voltage applied to the junction. 49 refs., 11 figs., 1 tab.« less

MONOLAYER STUDIES OF 5‐(4‐CARB‐ OXYPHENYL)‐10,15,20‐TRITOLYL‐PORPHYRIN‐II. PHOTOVOLTAIC STUDY OF MULTILAYER SANDWICH CELLS*

AbstractThe photovoltaic behavior of multilayer sandwich cells fabricated from the title compound has been examined. These multilayers are amorphous and contain a wide range of aggregated species. The action spectrum of the photocurrent agrees qualitatively with the absorption spectrum. This indicates that the photocurrent arises from all species in the film.The shape of the dark current‐voltage characteristics varied with the porphyrin thickness. For thin films, (1 and 3 monolayers) the current can be described by a tunnelling mechanism. For thick cells (29 monolayers) the current‐voltage characteristic can be fitted using either of two models: a modified Shockley equation (suggesting that the dye acts as a p‐type semiconductor) or a combination of Schottky and Frenkel–Poole emission (suggesting that the porphyrin acts as an insulator). The former interpretation is preferred because it is consistent with the observed rectification characteristics; nevertheless, the latter is in agreement with the magnitude of the conductivity of the films.The power‐conversion efficiencies of these cells were quite low (∼4 × 10‐4% for incident white light of intensity 2.72 W m‐2) owing to the very large internal resistance of the cells. The high resistance can be explained by observing that the molecules are in a very unfavorable orientation; maximum conductivity would be expected for the porphyrin ring systems perpendicular to the direction of current flow to give maximum overlap of the systems.

Photosynthesis and Transpiration of Pinus Radiata D. Don Under Plantation Conditions in Southern Australia. I. Response to Irrigation With Waste Water.

A plantation of P. radiata growing on sandy soil responded by greatly increased growth to applications of waste water over 3 years. Foliar concentrations of phosphorus and potassium increased following imgation but, although the waste water contained sodium up to 200 mg l-�, the concentration of sodium in the foliage remained low. Maximum rates of photosynthesis of branches within the canopy were 1 .8 �mol m-� s-� (imgated trees) and 0.7 �mol m-� s-� (unimgated trees). Needle conductance (g,) in imgated trees was 20-50 mmol m-� s-� and, in unimgated trees, less than 10 mmol m-2 s-�. Greater rates of photosynthesis in imgated trees might be explained by greater values of g*l; variations in the rate of photosynthesis during the course of a day did not, however, always follow variations in g*l particularly in irrigated trees. Needle water potential (�*p) of imgated trees reached a minimum of -2.0 MPa during the day, indicating a large internal resistance to the transport of water through the tree. Minimum �*p of unirrigated trees was -2.6 MPa; substantial rainfall during the measurement period did not reduce the difference in �*p between imgated and unimgated trees.

Influence of diode capacitance on performance of balanced microwave mixers

The effect of the diode capacitance parasitics on the performance of microwave double balanced lattice mixers, driven by a local oscillator having a large internal resistance and a current sinusoidal waveform, is examined. It is found that the current present in the diodes is modified by the diode capacitance and this in turn influences the performance of the mixers. To reduce the effect of the diode capacitance on the performance of the mixer it is necessary to reduce the internal resistance of the local oscillator. There is therefore a compromise between the need of a low noise figure, and hence a requirement of having a large internal resistance of the local oscillator, and the undesired effect on the diode current, and hence on the performance of the mixer.

Electron tube Amplifiers

Up-to-date electrometer tube amplifiers are a perfect equipment for measuring currents and voltages from sources with a large internal resistance, and they can be used for solving a wide range of problems related to electrometric measurements. The current sensitivity of mass-produced tube electrometers amounts to 10−15−10−14 A. Several problems, such as obtaining a logarithmic scale and high-speed systems for measuring very small currents are best solved by means of tube electrometers. It is true that in recent years tube electrometers have acquired an important rival consisting of electrometers with field-effect transducers; however, the latter are as yet in the development stage and one can expect that tube electrometers will for a long time occupy the leading position among other systems used for similar purposes.

Utilization of operational amplifiers for raising the input impedance of measuring circuits

The analysis and testing of a matching circuit consisting of two operational dc amplifiers have shown that it is possible by means of this circuit to match dc voltage circuits with a large internal resistance (up to 10 meg) to circuits with a small internal resistance (up to 10 kohm).