Water desorption mechanism

Scheidegger AM, Sparks DL (1996) A critical assessment of sorption-desorption mechanisms at the soil mineral/water interface. Soil Sci 161 813... 

Radium may be transported in the atmosphere in association with particulate matter. It exists primarily as a divalent ion in water, and its concentration is usually controlled by adsorption-desorption mechanisms at solid-liquid interfaces and by the solubility of radium-containing minerals. Radium does not degrade in water other than by radioactive decay at rates that are specific to each isotope. Radium may be readily adsorbed by earth materials consequently, it is usually not a mobile constituent in the environment. It may be bioconcentrated and bioaccumulated by plants and animals, and it is transferred in food chains from lower trophic levels to humans. 

In accordance with these features, the film deposition mechanism presented in Figure 9.7 was suggested. To explain the decrease in the film growth rate and the variation of the composition of chemisorbed MC1X surface groups with temperature, it was assumed that the concentration of surface OH groups is controlled by the dehydroxylation (water desorption) reaction (Figure 9.8). 

The synthesis of phenolic-formaldehyde and melamine-formaldehyde resins in the presence of fumed silica allows obtaining porous organic materials with a differentiated porous structure and surface properties. The pore characteristics of the studied resins in dry state were determined from nitrogen adsorption isotherms. The differences in surface character of the synthesized polymers were estimated satisfactorily by XPS spectra showing the presence of various functional groups. The adsorption/desorption mechanism of water and benzene on the investigated porous polymers was different due to differentiated hydrophobicity of the bulk material. 

The operation pressure can influence the second part of the desorption mechanism. If the partial vapor pressure inside the product is equal to the value of the desorption isotherm at that temperature, no desorption can take place. Therefore, the pressure in the chamber has to be small compared to this equilibrium pressure. The necessary pressure drop from the place of desorption to the pressure in the chamber is difficult to evaluate theoretically because water molecules will be transported not only by diffusion in the capillaries but by migration on the solid structure. To transport the possible volume of water vapor would require a high chamber pressure. On the other hand, a large gradient in concentration for the diffusion of molecules would require a very low pressure in the chamber. In most practical cases the pressure limit given by the condenser temperature (T ) will be used, e.g., = -55°C limits the chamber pressure to 0.02 mbar. This may not be... 

Partitioning of a xenobiotic from the aquatic phase into biota or into the sediment phase is not a terminal process indeed, from an environmental point of view these represent merely the introduction of the xenobiotic into a complex network of interactions. At least three important factors should be evaluated (1) direct desorption mechanisms from the sediment phase into the aquatic phase, (2) resuspension of particulate matter from the sediment phase into the water column, and (3) the role of interstitial water in the sediment phase. 

The hydroperoxide formation on Pt(100) implies that at negative potentials, the remaining oxo-species (Figure 4.5) would be negatively charged and after the H+-ion coordination, water desorption would occur. The mechanism for this reaction is similar to that proposed on Pt(lll), and it justifies the simultaneous desorption of water from Pt(100) and the detection of the only oxygen reduction product from Pt(l 11). 

O Day PA, Carroll SA, Waychunas GA (1998) Rock-water interactions controlling zinc, cadmium, and lead concentrations in surface waters and sediments, U.S. Tri-State Mining District. I. Molecular identification using X-ray absorption spectroscopy. Env Sci Tech 32 943-955 O Reilly S, Strawn DG, Sparks DL (2001) Residence time effects on arsenate adsorption/desorption mechanisms on goethite. Soil Sci Soc Am J 65 67-77... 

Speciation and solubility of chromium in wetlands and aquatic systems is governed by the competition among chromium oxidation states, adsorption/desorption mechanism, and soil/sediment redox-pH conditions. Chromium (VI) is reduced to chromium (HI) at approximately +350 mV in soils and sediment. Reduced Cr(III) can be rapidly oxidized to the tetravalent chromate and dichromate forms by manganese compounds. Cr(III) is much less soluble in natural system than the hexavalent form and has a much lower toxicity. Chromium is less likely to be a problem in wetlands than in nonwetlands because the reducing conditions cause its reduction or conversion to the more insoluble Cr(III) form. This is depicted in Figure 12.15, which shows changes in water-soluble chromium as affected by the soil redox potential. 

The fractal approach was also used to investigate adsorption and desorption mechanisms of water vapor on active carbons that were derived from coconut shell, coal, coke and pitch fiber featuring a wide range of BET specific surface areas [78]. A values were measured for the water clusters adsorbed on primary carbon centers. Values ranging from 1.64 to 1.67 implied a diffusion-limited aggregation model on a pore wall plane, whereas higher A values (up to 1.86), measured at a relative pressure X = 0.95, implied the formation of water clusters that were partly merged vertically to the walls. 

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