Sorption—desorption developments

WASP/TOXIWASP/WASTOX. The Water Quality Analysis Simulation Program (WASP, 3)is a generalized finite-difference code designed to accept user-specified kinetic models as subroutines. It can be applied to one, two, and three-dimensional descriptions of water bodies, and process models can be structured to include linear and non-linear kinetics. Two versions of WASP designed specifically for synthetic organic chemicals exist at this time. TOXIWASP (54) was developed at the Athens Environmental Research Laboratory of U.S. E.P.A. WASTOX (55) was developed at HydroQual, with participation from the group responsible for WASP. Both codes include process models for hydrolysis, biolysis, oxidations, volatilization, and photolysis. Both treat sorption/desorption as local equilibria. These codes allow the user to specify either constant or time-variable transport and reaction processes. 

Sorption and desorption are usually modeled as one fully reversible process, although hystersis is sometimes observed. Four types of equations are commonly used to describe sorption/desorption processes Langmuir, Freundlich, overall and ion or cation exchange. The Langmuir isotherm model was developed for single layer adsorption and is based on the assumption that maximum adsorption corresponds to a saturated monolayer of solute molecules on the adsorbent surface, that the energy of adsorption is constant, and that there is no transmigration of adsorbate on the surface phase. 

The Elovich model was originally developed to describe the kinetics of heterogeneous chemisorption of gases on solid surfaces [117]. It describes a number of reaction mechanisms including bulk and surface diffusion, as well as activation and deactivation of catalytic surfaces. In solid phase chemistry, the Elovich model has been used to describe the kinetics of sorption/desorption of various chemicals on solid phases [23]. It can be expressed as [118] ... 

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. 

To study rapid reactions, traditional batch and flow techniques are inadequate. However, the development of stopped flow, electric field pulse, and particularly pressure-jump relaxation techniques have made the study of rapid reactions possible (Chapter 4). German and Japanese workers have very successfully studied exchange and sorption-desorption reactions on oxides and zeolites using these techniques. In addition to being able to study rapid reaction rates, one can obtain chemical kinetics parameters. The use of these methods by soil and environmental scientists would provide much needed mechanistic information about sorption processes. 

Most soil-pesticide sorption-desorption studies have used batch techniques, which create several problems. In many batch studies the slow portion of the soil-pesticide interactions may not be seen if observation times are too short (McCall and Agin, 1985). Additionally, desorption is usually begun by centrifuging the equilibrated soil-pesticide system, removing a known volume of pesticide solution, replacing with the same volume of pesticide-free solution, and resuspending the soil-pesticide solution. This procedure is then repeated to develop desorption isotherms initiated from a particular point on the sorption isotherm. Then there is... 

Selim et al. (1976b) developed a simplified two-site model to simulate sorption-desorption of reactive solutes applied to soil undergoing steady water flow. The sorption sites were assumed to support either instantaneous (equilibrium sites) or slow (kinetic sites) first-order reactions. As pore-water velocity increased, the residence time of the solute decreased and less time was allowed for kinetic sorption sites to interact (Selim et al., 1976b). The sorption-desorption process was dominated by the equilib-... 

The automated sorption apparatus developped can be useful for easy and rapid adsorption and desorption measurements of solutes in supercritical fluids. 

The conventional inverse gas chromatography (IGC) is based on equations that assume equilibrium is established during the course o the chromatograph. Consequently, those stationary phases that exhibit marked hysteresis in sorption/desorption give IGC sorption data at considerable variance with long-term gravimetric methods. A modified frontal procedure was developed that avoids the assumption of equilibrium to enable studies of interaction kinetics of gas phase components with a stationary phase, such as a biopolymer, having entropic as well as enthalpic relations affected by concentration shifts and time dependent parameters. 

Three new methods to characterize the pore structure and pore size distribution in the top layer of asymmetric membranes have been developed or refined in our laboratory during the past few years a) the gas ad-sorption/desorption method, b) thermoporometry and o) selective permeation (fractional rejection). 

It is therefore highly desirable to develop more quantitative methods for characterization of pore structures. The results of recent investigations, including ultrafiltration (water flux and rejection of a polydisperse solute), high-resolution SEM and nitrogen sorption/desorption analysis, are described below. 

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

A starting point for any discussion of the kinetics and mechanisms of sorption-desorption processes is an understanding of the terminology that is central to the topic. There are well-developed terms associated with sorption-desorption processes, and a brief introduction is warranted. 

Differences in the temporal development of release rates for the individual elements are connected with their sorption/desorption behaviour, which is primarily due to pH-effects but may also be influenced by com-plexation, e.g. by elevated concentrations of chloride ions. With respect to the pH-effects, however, there are significant differences in the response of the various solid substrates to the addition of H+-ions, and it may be argued that the pH-values on the solid surfaces -which can be estimated from "pH-titration tests" - are decisive for the behavior of the particular element rather than the pH-values determined in solution. 

The moisture-solid interaction is an inevitable aspect of pharmaceutical development. Elucidation of moisture-induced physical alterations in amorphous pharmaceuticals is crucial, especially for ASD. Gravimetric measurement on the rate and extent of moisture gain (sorption) by or loss (desorption) from amorphous samples as a function of RH or as a function of time at a constant RH (isohumic condition) can provide a wealth of information of ASD. The key structural properties of ASD measureable by moisture sorption/desorption are drug-polymer interactions, moisture-induced glass transition, crystallization, hydrate formation/dehydration, etc. while that associated with particulate or bulk properties are hygroscopicity, diffusivity, pore size, surface area, etc. (Burnett et al. 2009). 

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