22 research outputs found
Heats of K/Ca And K/Pb Exchange in Two Tropical Soils as Measured By Flow Calorimetry
Flow calorimetry can provide useful information about surface chemical reactions in soils that cannot be obtained readily by other methods. When flow calorimetry is conducted over a range of surface coverages, different sorption heats can be calculated to yield information about how binding energies vary with coverage, i.e., surface heterogeneity. The purpose of this study was to determine heats of exchange for K/Ca and K/Pb systems using flow calorimetry and to evaluate the degree of surface heterogeneity with respect to cation exchange. Surface horizon samples from a Typic Acrorthox and Typic Tropohumult from Puerto Rico were used. Lead was adsorbed specifically in both soils, but no adsorption heat was detected for this reaction in either soil. However, heats associated with reversible cation exchange between K and Pb were observed. Heats for K/Ca exchange were greater than those generated for K/Pb exchange in both soils. Heats of exchange were greater in the Ultisol than in the Oxisol. The differential heats of exchange were independent of exchange composition for both K/Pb and K/Ca exchange in the Oxisol, indicating that all cation exchange sites were similar energetically. In the Ultisol, the differential heats of exchange increased as exchangeable K decreased, indicating that the exchange sites were not similar energetically. These differences were attributed to the presence of smectite in the Ultisol, which was able, in part, to collapse when saturated with K
Point Of Zero Charge Determination in Soils and Minerals via Traditional Methods and Detection of Electroacoustic Mobility
Points of zero charge were determined on two highly weathered surface soils from Puerto Rico, an Oxisol and Ultisol, as well as mineral-standard kaolinite and synthetic goethite using three methods: (1) potentiometric titration measuring the adsorption of H+ and OH− on amphoteric surfaces in solutions of varying ionic strength (I) (point of zero salt effect), (2) direct assessment of surface charges via non-specific ion adsorption as a function of pH and I (point of zero net charge), and (3) electroacoustic mobility of reversible particles as it varies with pH and I (isoelectric point). The first two methods yielded points of zero charge for kaolinite (2.7–3.2) and synthetic goethite (7.4–8.2) comparable to those reported previously, indicating the reliability of these analyses. The soil values ranged from 3.9 to 4.4 for the Oxisol and 2.3 to 3.7 for the Ultisol. Electroacoustic mobility, as measured by the AcoustoSizer™, is a parameter that has yet to be thoroughly tested for mineral or soil systems as a viable alternative to PZC assessment. The points of zero charge from electroacoustic mobility of kaolinite (3.8–4.1) and synthetic goethite (8.1–8.2) were similar to values obtained by electrophoretic mobility. Furthermore, the values found for the Oxisol (3.4–3.5) and Ultisol (2.6–2.7) were in the range expected for these soils
Mechanisms of Lead, Copper, and Zinc Retention by Phosphate Rock
The solid–liquid interface reaction between phosphate rock (PR) and metals (Pb, Cu, and Zn) was studied. Phosphate rock has the highest affinity for Pb, followed by Cu and Zn, with sorption capacities of 138, 114, and 83.2 mmol/kg PR, respectively. In the Pb–Cu–Zn ternary system, competitive metal sorption occurred with sorption capacity reduction of 15.2%, 48.3%, and 75.6% for Pb, Cu, and Zn, respectively compared to the mono-metal systems. A fractional factorial design showed the interfering effect in the order of Pb \u3e Cu \u3e Zn. Desorption of Cu and Zn was sensitive to pH change, increasing with pH decline, whereas Pb desorption was decreased with a strongly acidic TCLP extracting solution (pH=2.93). The greatest stability of Pb retention by PR can be attributed to the formation of insoluble fluoropyromorphite [Pb10(PO4)6F2], which was primarily responsible for Pb immobilization (up to 78.3%), with less contribution from the surface adsorption or complexation (21.7%), compared to 74.5% for Cu and 95.7% for Zn. Solution pH reduction during metal retention and flow calorimetry analysis both supported the hypothesis of retention of Pb, Cu, and Zn by surface adsorption or complexation. Flow calorimetry indicated that Pb and Cu adsorption onto PR was exothermic, while Zn sorption was endothermic. Our research demonstrated that PR can effectively remove Pb from solutions, even in the presence of other heavy metals (e.g. Cu, Zn).
‘‘Capsule’’: Phosphate-induced formation of fluoropyromorphite is primarily responsible for Pb immobilization by phosphate rock, whereas Cu and Zn retention is mainly attributable to the surface adsorption or complexation
Mechanisms of lead, copper, and zinc retention by phosphate rock
Mitteilungen des Förderkreises Archive und Bibliotheken zur Geschichte der Arbeiterbewegung, Nr. 37 / März 2010. Ausführliche Rezension hier in der Tageszeitung junge welt vom Montag 29. März 2010
Selectivities of Potassium-Calcium and Potassium-Lead Exchange in Two Tropical Soils
Measurement of cation selectivity in soils provides important information about the affinity and binding strength of a particular cation on soil surfaces. Gaines-Thomas (KGT) selectivity coefficients were determined for a variety of K/Ca and K/Pb ratios on an Oxisol and Ultisol soil from Puerto Rico. The calculated KGT values indicated a preference for K+ over Ca2+ or Pb2+. The selectivity for Pb2+ was significantly greater than that for Ca2+ due to Pb2+\u27s larger hydrated charge density relative to that of Ca2+. The patterns of selectivity were independent of metal type. The selectivity of the Oxisol for Ca2+ or Pb2+ exhibited no trend and changed little with changes in divalent metal surface coverage. The Ultisol displayed a decrease in selectivity for Ca2+ and Pb2+ with increasing surface coverage of these ions. This was attributed to the presence of smectite in the Ultisol, which was able to partially collapse when K+ saturated. Some of the Pb sorption in the soils was due to chemisorption. The Oxisol chemisorbed 3000 mg Pb kg-1 while that value for the Ultisol was ≈1900 mg kg-1. The differences were due to the greater quantities of Fe/Al oxides and organic matter in the Oxisol relative to the Ultisol. Scanning electron microscopy-energy dispersive X-ray (SEM-EDX) spectroscopy detected discrete Pb-C phase in both soils. The C was from organic matter. Under experimental conditions, any Pb-carbonate phase would not have been stable. It was possible Pb was associated with organic sulfhydral groups. The selectivity exhibited by soil systems for various nutrient and heavy metals is important in elucidating how available these metals will be for plant/animal uptake as well as their mobility and stability in the soil environment
The Science Performance of JWST as Characterized in Commissioning
This paper characterizes the actual science performance of the James Webb
Space Telescope (JWST), as determined from the six month commissioning period.
We summarize the performance of the spacecraft, telescope, science instruments,
and ground system, with an emphasis on differences from pre-launch
expectations. Commissioning has made clear that JWST is fully capable of
achieving the discoveries for which it was built. Moreover, almost across the
board, the science performance of JWST is better than expected; in most cases,
JWST will go deeper faster than expected. The telescope and instrument suite
have demonstrated the sensitivity, stability, image quality, and spectral range
that are necessary to transform our understanding of the cosmos through
observations spanning from near-earth asteroids to the most distant galaxies.Comment: 5th version as accepted to PASP; 31 pages, 18 figures;
https://iopscience.iop.org/article/10.1088/1538-3873/acb29
The Science Performance of JWST as Characterized in Commissioning
This paper characterizes the actual science performance of the James Webb Space Telescope (JWST), as determined from the six month commissioning period. We summarize the performance of the spacecraft, telescope, science instruments, and ground system, with an emphasis on differences from pre-launch expectations. Commissioning has made clear that JWST is fully capable of achieving the discoveries for which it was built. Moreover, almost across the board, the science performance of JWST is better than expected; in most cases, JWST will go deeper faster than expected. The telescope and instrument suite have demonstrated the sensitivity, stability, image quality, and spectral range that are necessary to transform our understanding of the cosmos through observations spanning from near-earth asteroids to the most distant galaxies
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The time-concentration interaction of Al toxicity in wheat root meristems
The time-concentration interaction of Al toxicity in the root
meristems of Brevor wheat was studied using the L.D. 50 as a criteria
for Al damage. The L.D. 50 represented a precise, well defined
degree of biological damage which was easily reproduced under conditions
where factors like temperature, pH, nutrient concentration,
Al concentration, and time of exposure were rigidly controlled.
A diffusion-absorption model was developed to explain the
time-concentration interaction of Al toxicity in wheat root meristems.
The model attributed the L.D. 50 to the accumulation within meristematic
cells of a fixed quantity of Al. The model predicted that
above a certain time, the t(L.D. 50)- reciprocal concentration relationship
should be linear with the data extrapolating to positive intercepts
on the time axis. Wheat roots of the variety, Brevor, were found to
yield data that when plotted as t(L.D. 50) versus reciprocal
concentration were linear for Al concentrations as high as 50 ppm and
did have positive intercepts on the time axis. A linear relationship
was obtained regardless of the salt concentration or the pH.
It was observed that lateral roots were much more sensitive to
Al than primary roots at all concentrations used in this study. The
t(L.D. 50)- reciprocal concentration relationship for lateral roots not
only had a smaller slope than that for primary roots, but also extrapolated
to smaller intercepts on the time axis. This behavior was shown
to be in agreement with the predictions of the model where meristematic
size was the only variable.
The model suggested that the effect of salt concentration on Al
toxicity in Brevor wheat is on the uptake of Al by meristematic cells.
The model could not account for the salt effect by changes in the diffusion
coefficient or the complexing of Al by cell walls or other
material outside the cell. The model further suggested that the Al
uptake rate is "hyperbolic" with concentration. This would be in
agreement with what is known about the uptake of many other solutes,
both organic and inorganic as well.
The pH was found to have two effects on the toxicity of Al. One
was an effect during pretreatment prior to Al exposure. Increasing
the pH at which the seeds were germinated from 4 to 5 resulted in a
small but consistent increase in the t(L.D. 50) obtained during a subsequent
Al exposure. The mechanism by which pretreatment pH affects the subsequent toxicity of Al was speculated to be a detrimental
effect of H+ on the membranes during their development. The other
effect of pH was observed during the Al treatment. Increasing the pH
of an Al solution from 4.0 to 4.5 increased the toxicity of Al. The
effect of pH on the t(L.D.50)-reciprocal concentration relationship
was to lower the slope but have no effect on the extrapolated intercept.
The increase in toxicity was shown to be too small to be accounted for by increases in the concentration of AlOH⁺² or Al(OH)⁺¹
₂ calculated
from known thermodynamic equilibrium constants. In addition,
increasing the pH from 4. 5 to 4.7 did not further increase the toxicity
of Al. This lack of response of Al toxicity between pH 4.5 and 4.7
was apparently not the result of precipitation of Al under these experimental
conditions. The evidence suggested that there may be other
processes in addition to hydrolysis which affect the toxicity of Al as
the pH is raised. Experiments using the metabolic inhibitor, DNP,
during an Al treatment suggested that Al uptake by meristematic cells
is probably the result of passive diffusion across the plasmalemma.
Under these conditions, the permeability of cell membranes would be
a controlling factor in the rate of Al uptake. A mechanism was proposed
in which the increased toxicity of Al with pH was attributed to
both the hydrolysis of Al and the known effects of pH on membrane
permeability