Sorption desorption isotherms

Sorbed pesticides are not available for transport, but if water having lower pesticide concentration moves through the soil layer, pesticide is desorbed from the soil surface until a new equiUbrium is reached. Thus, the kinetics of sorption and desorption relative to the water conductivity rates determine the actual rate of pesticide transport. At high rates of water flow, chances are greater that sorption and desorption reactions may not reach equihbrium (64). NonequiUbrium models may describe sorption and desorption better under these circumstances. The prediction of herbicide concentration in the soil solution is further compHcated by hysteresis in the sorption—desorption isotherms. Both sorption and dispersion contribute to the substantial retention of herbicide found behind the initial front in typical breakthrough curves and to the depth distribution of residues. 

The most fundamental manner of demonstrating the relationship between sorbed water vapor and a solid is the water sorption-desorption isotherm. The water sorption-desorption isotherm describes the relationship between the equilibrium amount of water vapor sorbed to a solid (usually expressed as amount per unit mass or per unit surface area of solid) and the thermodynamic quantity, water activity (aw), at constant temperature and pressure. At equilibrium the chemical potential of water sorbed to the solid must equal the chemical potential of water in the vapor phase. Water activity in the vapor phase is related to chemical potential by... 

Generation of water sorption/desorption isotherms in a controlled relative humidity environment can be carried out either gravimetrically or volumetrically. Gravimetric methods require... 

There are a good number of sorption/desorption isotherm models which were developed in order to reflect the actual sorption/desorption processes occurring in the natural environment. Some models have a sound theoretical basis however, they may have only limited experimental utility because the assumptions involved in the development of the relationship apply only to a limited number of sorption/desorption processes. Other models are more empirical in their derivation, but tend to be more generally applicable. 

SORPnON-DESORPTION ISOTHERMS. Sorption-desorption isotherms for the untreated, O-methylated, and O-octylated extracts are presented in Figures 6-8, where the relative vapor pressure of benzene (p/Po) is plotted against the... 

Figure 6. Sorption-desorption Isotherm for Benzene and Extract of Illinois No. 6 Coal...

The sorption-desorption isotherm for the O-octylated extract is noticably different. The shape of the sorption curve is concave upward and the hysteresis effect is much less pronounced. Nearly all of the benzene (97% wt) could be desorbed under vacuum at 30" C. Clearly, this material is responding much differently than the untreated and O-methylated extracts. 

Chemical and microbiological transformations of the pesticide while the experiment is being conducted could also cause nonsingularity in pesticide-soil sorption-desorption isotherms. Hamaker and Thompson (1972) theorized that a portion of a sorbed pesticide is more strongly held than... 

Water sorption-desorption isotherms can be carried out by using thermobalances. Now specific instruments allow to measure water sorption-desorption isotherms at different constant temperatures (e.g., dynamic vapor sorption instrument (DVS), Surface Measurement Systems Ltd., Monarch Beach, US). 

Some isotherms of water vapours sorption-desorption are presented in Fig, 1. In the first cycle of sorption, the isotherms have a pronounced stepwise character, and the values of water vapour sorption are unusually low for wood specimens. This is connected with the strict drying conditions of the specimens 378K at a rapid rise in temperature) after the exposure procedure, which resulted in the fixation of the nonequilibrium structure. After saturation with water vapours and the subsequent vacuum treatment at 295K during the procedure of the sorption experiment, the second cycle of isotherms was carried out, and this state was regarded as an equilibrium one. Thus, the analysis of the specimens structure was performed on the basis of the sorption-desorption isotherms of the second cycle. 

Figure 1 Sorption-desorption isotherm of carboxymethylcellulose sodium.

Figure 1 Sorption-desorption isotherm for colloidal silicon dioxide. O Sorption Desorption...

Figure 2 Sorption-desorption isotherm of magnesium aluminum silicate (Pharmasorb).

Figure 2 Sorption-desorption isotherm for mannitol. Sorption equilibrium moisture Desorption equilibrium moisture...

Figure 3 Sorption-desorption isotherm of wheat starch.

Figure 1 Moisture sorption-desorption isotherm of powdered sucrose.

Nitrogen sorption/desorption isotherms of membrane samples (carefully dried at room temperature) were obtained by Micrometries Instrument Corp., Norcross, Georgia. About 1.5g of each membrane samples was used for measurements of BET surface areas, 21 point sorption and 22 point desorption isotherms. The Instrument used was DigiSorb 2500 with fully automated control. The most significant experimental error introduced was from saturation pressure variation of about + 1.5 mm Hg (that is, about 0.2% error in relative pressure). 

Nitrogen sorption/desorption isotherms of membranes A, B and C exhibit narrow hysteresis loops in regions close to saturation points (Figures 3a,b,c). The experimental points on both branches of the three isotherms were fitted by an analytical function. In each case, correlation coefficients were greater than 0,9995. This allowed not only averaging of experimental data, but also simplified numerical procedures of isotherm analysis. Analysis was performed only in the regions for which experimental points were available. 

Aristov et al. (17) developed a method for calculating sorption/ desorption isotherms for beds of regularly-packed uniform spheres. 

Figure 2. Room temperature sorption-desorption isotherms of acid Nafion. Key , room temperature dried absorption , 220° C dried primary absorption , 220° C dried desorption Q, 220° C dried secondary absorption.

Determine the solvation/desolvation behaviour depending on the vapour pressure of the solvent (hydrates moisture sorption/desorption isotherm). 

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