Sorption sites, specific

Although in many. soils with tow P availability, significant desorption of sparingly soluble Pi forms requires at least millimolar concentration levels of specific carboxylates (e.g., citrate, oxalate) in the soil solution, much lower concentrations (0.1 mM) were necessary to reduce. soil adsorption of Pi, which was applied simultaneously with carboxylates (100). Thus, competition of carboxylates with Pi for P sorption sites in the soil matrix may be a mechanism that can. 

U.S. EPA defines MNA as the reliance on natural processes, within the context of a carefully controlled and monitored site cleanup approach, to achieve site-specific remediation objectives within a time frame that is reasonable compared to that offered by other more active methods. The natural processes include biodegradation, dispersion, dilution, sorption, volatilization, stabilization, and transformation. These processes reduce site risk by transforming contaminants to less toxic forms, reducing contaminant concentrations, and reducing contaminant mobility and bioavailability. Other terms for natural attenuation in the literature include intrinsic remediation, intrinsic bio-remediation, passive bioremediation natural recovery, and natural assimilation. 30... 

In all of the workshops, but especially in the FAT and Exposure Assessment workshops, the need for better understanding and model representation of soil systems, including both unsaturated and saturated zones, was evident. This included the entire range of processes shown in Table II, i.e., transport, chemical and biological transformations, and intermedia transfers by sorption/desorption and volatilization. In fact, the Exposure Assessment workshop (Level II) listed biological degradation processes as a major research priority for both soil and water systems, since current understanding in both systems must be improved for site-specific assessments. 

Surface area and moisture uptake have been related to the disintegration properties of excipients such as crosspovidone, starch, and alginic acid [17]. The surface areas of the three materials were measured, and a linear correlation was found between the maximum moisture sorption and specific surface area for the three disintegrants. The greater the surface area of the material, the more numerous were the sites for capillary attraction of water to its surface. It was postulated that the capillary action appears to be responsible for the disintegration properties of the materials. 

Equation (9.15) was written for macro-pore diffusion. Recognize that the diffusion of sorbates in the zeoHte crystals has a similar or even identical form. The substitution of an appropriate diffusion model can be made at either the macropore, the micro-pore or at both scales. The analytical solutions that can be derived can become so complex that they yield Httle understanding of the underlying phenomena. In a seminal work that sought to bridge the gap between tractabUity and clarity, the work of Haynes and Sarma [10] stands out They took the approach of formulating the equations of continuity for the column, the macro-pores of the sorbent and the specific sorption sites in the sorbent. Each formulation was a pde with its appropriate initial and boundary conditions. They used the method of moments to derive the contributions of the three distinct mass transfer mechanisms to the overall mass transfer coefficient. The method of moments employs the solutions to all relevant pde s by use of a Laplace transform. While the solutions in Laplace domain are actually easy to obtain, those same solutions cannot be readily inverted to obtain a complete description of the system. The moments of the solutions in the Laplace domain can however be derived with relative ease. 

Conversely, with appropriate site-specific information, reasonable estimates of the magnitude of sorption coefficients can be made (see below). 

The importance of quadrupole interaction in zeolitic sorption has been pointed out by Barrer and Stuart (20). Such effects are clearly illustrated by the data for sorption of nitrogen and carbon dioxide in both H-chabazite and 5A zeolite. For these molecules, which have large quadrupole moments, the experimental values of K0 are much smaller than the theoretical values predicted from the idealized model suggesting either localized sorption at specific sites within the cavity or restricted rotational freedom. 

The sorption of a penetrant by a class (a) membrane can be treated most simply, if the processes of sorption in the polymer matrix and into each kind of specific sorption site within it can be treated as mutually independent and hence additive. Under these conditions, each sorbed species may similarly be expected to contribute additively to the total diffusion flux. Hence, if there are N — 1 kinds of specific sorption sites, the overall sorption and permeability or diffusion coefficients can be written as... 

Here, S and P, or DXj characterize the ith sorbed penetrant species (i = 1 refers to the polymer matrix and i = 2, 3,. .., N identify each kind of specific sorption site). 

Earlier work on the application of the concept of dual mode sorption and diffusion to glassy polymer-gas systems has been reviewed in detail 6) and important aspects of more recent work have been dealt with in more recent reviews 7 10). Eq. (5) was first applied by Michaels et al U). Sorption in the polymer matrix and in the specific sorption sites was represented by linear (Henry s law) and Langmuir isotherms respectively so that Sj in Eq. (5) is given by... 

However, more recently competitive sorption has been reported and is thought to be the result of site-specific sorption occurring in soil organic matter (Xing et al., 1996 Xing and Pignatello, 1998). 

Estimate the site-specific sorption coefficient (Kd or Kf) from the estimated K(k and the organic carbon content of the soil or sediment, using Equation (3). 

The extrapolation methods related to differences in media and matrices are mainly governed by sorption processes. Therefore, uncertainty analysis should try to address toxicant properties related to sorption (KD, A"ow, Ka, etc.) and matrix-specific characterization of sorption sites (qualitative and quantitative). Some of the computerized models for exposure and effects assessment explicitly include options for treating key variables as probability distributions (e.g., AQUATOX). 

It is also evident that the collision frequency (or pre-exponential factor) differs between different metal oxides for comparable surface areas. Hence, the active site density depends on the type of metal oxide and most likely also on pre-treatment and manufacturing of the oxide particles. In practice, determining the active site density is very difficult or even impossible since it is based on BET measurements of the specific surface area. The BET surface area can give a measure of the sorption site density which deviates significantly from the actual specific surface area. Again, this demonstrates the difficulties encountered when trying to compare the reactivity of different solid materials at a given temperature on the basis of the specific surface area. 

From the previous sections it is evident that radionuclides of high specific activity often represent very small amounts (microamounts, non-weighable amounts < 1 pg) of matter, especially if the half-lives are short. Handling of such microamounts requires special precautions, because in the absence of measurable amounts of carriers the radionuclides are microcomponents and their chemical behaviom may be quite different from that observed for macrocomponents. This aspect is of special importance if the system contains liquid/solid, gas/solid or liquid/liquid interfaces. The percentage of radionuclides sorbed on the walls of a container depends on the chemical form (species) of the radionuclide, its concentration and specific activity, and on the properties of the container material. At high specific activity of a radionuclide in solution, the surface of a glass beaker generally offers an excess of surface sorption sites. 

The large number of measurements in various systems lead to the result that the interaction of radionuclides with solids is rather complex and depends on many parameters the species of the radionuclides in the solution, their properties and their dispersion, the components of the solid, the surface area of the particles, the nature of the sorption sites, the presence of organic substances and of microorganisms, and the interference or competition of other species. Therefore, an investigation of the behaviour of the radionuclides in the specific system of interest is unavoidable, if reliable information about their migration behaviour is required. 

Sorption overview. Both empirical and mechanistic approaches have emerged since the 1970s to describe interactions between radionuclides and geomedia. These are based on conditional constants, which are valid for specific experimental conditions, or more robust intrinsic constants, which are valid over a wider range of conditions. The empirical approach involves measurements of conditional radionuclide distribution or sorption coefficients or Rsite-specific water-rock systems using synthetic or natural ground waters and crushed rock samples. Mechanistic-based approaches produce intrinsic, thermodynamic surface-complexation constants for simple electrolyte solutions with pure mineral phases. 

The preceding analysis suggests that water, indeed, penetrates throughout the amorphous regions of these materials and undergoes a specific interaction with available sorption sites, most likely the available hydroxyl groups on the anhydroglucose units. Differential heat of sorption results for various starches ... 

Other supportive evidence for a specific water-solid interaction is available from thermal studies showing the amount of non-freezeable water,nuclear magnetic resonance,and diffusion studies. The evidence is less clear, however, concerning whether there is distinct binding of water to sorption sites with discrete energy levels or whether there is a continuum of states where water interacts to a lesser extent with increasing amount sorbed. In any event, it is clear that sorbed water behaves with a considerable degree of mobility, and hence, questions the use of the term bound water. ... 

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