Sorption computer modeling

Computer modeling techniques are a substantial aid in zeolite structure solutions or refinements, and a means of extracting structural insight from difiraction or other anal ftical experiments. Sorption results, particle shapes in some cases, diffraction or scattering data, as well as optical, NMB and EXAFS spectra can all be simulated based on an atomic structure and, conversely, analytical data of these various types can be used to guide the development or detailing of an appropriate structural model. 

What is the chemical composition and range of compositions of soil waters and what concentrations of sorbate species of interest are or might be present Does computer modeling of the water chemi.stry indicate that a potential sorbate species is at or above saturation with respect to a possible mineral Sorption of that species cannot be meaningfully examined, except at concentrations below mineral saturation. 

Piwoni et al. (1986) found that nitrobenzene did not volatilize in their microcosms simulating land-application of wastewater, but was totally degraded. Enfield et al. (1986) employed a calculated Henry s law constant of 1.30 x 10 kPa m mol, and arrived at a biodegradation rate coefficient greater than 8 day . They predicted that 0.2% of the added nitrobenzene could be accounted for in volatiles. The EXAMS computer model (Burns et al. 1981) predicts volatilization half-lives of 12 days (river) to 68 days (eutrophic lake) and up to 2% sediment sorption for nitrobenzene. 

Reactions of metal ions in aqueous media have been show n to be strongly influenced by surface sorption reactions. The adequate description of metal ion behavior in systems where particulates have been included is an important step in the application of laboratory data to natural systems of wide environmental interest. In this study, data on the adsorption of aqueous nickel, Ni, onto oxides of silica and iron is presented. Of special interest are the effects which various ligands have on the adsorption reactions. Data are analyzed through the use of the chemical equilibrium computer model REDEQL2 making use of the solvent-ion model of adsorption. 

The first section deals specifically with the spectroscopic/ microscopic tools that can be used in concert with macroscopic techniques. The second section emphasizes computer models that are used to elucidate surface mediated reaction mechanisms. The remainder of the volume is organized around reaction type. Sections are included on sorption/desorption of inorganic species sorption/desorption of organic species precipitation/dissolution processes heterogeneous electron transfer reactions photochemically driven reactions and microbially mediated reactions. What follows are a few highlights taken from the work presented in this volume. 

The objects of the investigation by Jansen et al. (Chapter 4) were perfiuorinated copolymers of Hyflon AD. The authors reported novel data on free volume, presented the results of computer modelling and the gas permeation parameters. It should be stressed that such comprehensive smdy of a polymer becomes more and more popular today if one wants to understand transport and sorption parameters of a membrane material. This chapter will give much food for future comparisons with other perfiuorinated polymers as well as conventional glassy polymers. 

This paper studied the sorption/desorption of n-pentane in polystyrene and polypropylene using the sheet samples. The solubility of n-pentane in PS and PP was measured. And the concentration-dependent diffusivity of n-pentane in PS and PP was computationally modeled. Results exhibit that the simulated profiles match well with the experimental data while the penetrant concentration is above 30%. 

Based on the aforementioned procedures of sorption and desorption experiments, the approach to implement the desorption experiments is more accurate. Consequently, the desorption profile was selected for computational modeling. While Equation (5) is utilized, the desorption curves are plotted in Figure 6 where 2L equals 0.03cm under a variety of diffusivities from 10 to 10 cmVs. Apparently, the experimental curve can not be well predicted through Equation (5) using the concentration-independent diffusivity. 

The Freundlich isotherm (or Freundlich model) is an empirical description of species sorption similar to the K, approach, but differing in how the ratio of sorbed to dissolved mass is computed. In the model, dissolved mass, the denominator in the ratio, is raised to an exponent less than one. The ratio, represented by the Freundlich coefficient Kf, is taken to be constant, as is the exponent, denoted f, where 0< f <1. As before, the masses of dissolved and sorbed species are entered, respectively, in units such as moles per gram of dry sediment and moles per cm3 fluid. Since the denominator is raised to an arbitrary exponent rif, the units for Kf are not commonly reported, and care must be taken to note the units in which the ratio was determined. 

Modern N2 sorption porosimeters are very sophisticated and generally reliable. Typically they come supplied with customized user-friendly software which enables the experimental data to be readily computed using the above models and mathematical expressions. Usually the raw isotherm data is displayed graphically along with various forms of the derived pore size distribution curve and tabulated data for surface area, pore volume and average pore diameter. 

First, the computer calculated uncertainties shown for the calculated values of kj, k and kg are an indication that the model has considerable validity for describing the kinetics of the system, at least over one half-life in the disappearance of chlorpyrifos. Second, the values of k and kj are all similar and their magnitude indicates that in this case the assumption of rapid sorption/desorption kinetics compared to hydrolysis is valid. 

Perrone et al. (2001) modelled Ni(II) adsorp-tion to synthetic carbonate fluoroapatite (CaI0 ((P04)5(C03))(0H,F). The solid phase had a pHIEP of 6.3 and a ZPC of 6.4 with an SSA of 8.8m2/g, an estimated sorption site density of 3.1 sites/nm2. They conducted 8-day isotherms in closed vessels at Ni concentrations of 5 x 10-10 to 1 x 10 8 M, constant I (0.05, 0.1 or 0.5 M), constant solid phase concentrations of 10 g/dm3 at pH values of 4 to 12. As Ni sorption occurred, no significant release of Ca was seen. Sorption was reversible. Rather than precisely characterize surface functional groups, they elected to describe their sorbent surfaces using acid-base reactions for the average behaviour of all sites involved in protonation and deprotonation. Potentiametric titration data were used to estimate the constants with the FTTEQL computer code ... 

Demontis et al. (94) reported an early MD study of the sorption and mobility of benzene in zeolite NaY. The zeolite was modeled with a Si/Al ratio of 3.0, as in previous calculations for Xe and methane. The zeolite and benzene molecules were treated as rigid. The authors supported the assumption of a rigid zeolite lattice by quoting structural studies (95), in which the cell parameter of NaY zeolite was found to contract little upon uptake of benzene. It is, however, more than possible that the lattice undergoes substantial deformation without an overall change in volume quantum chemical calculations (96) have shown that the Si-O-Si bending potential is very soft. When these calculations were performed, the assumption of a rigid lattice was more a matter of computational necessity than it is today. 

Some results of application of theoretical models to trace metal speciation were presented in the section on metal-inorganic interactions (tables 2 and 3) a collection of papers dealing with chemical modelling in aqueous systems, including speciation, sorption, solubility and kinetics was edited by 3enne (1979). Recently Nordstrom and Ball (198 0 summarized 58 aqueous chemical equilibrium computer programs of which 19 were dealing with trace metals. 

In summary while conceptually appealing, the application of complex multi-solute models for Sr sorption to zeolite is in the early stages of development. While preliminary results are encouraging, additional work is required to develop more efficient computational methods and develop an improved database for parameter estimation. The remainder of this section focuses on the simpler retardation factor approach. 

Mattigod, S. V. and Sposito, G. Chemical modeling of trace metal equilibria in contaminated soil solutions using the computer program GEOCHEM, j[n Jenne, E.A., ed., "Chemical Modeling in Aqueous Systems. Speciation, Sorption, Solubiliiy, and Kinetics." Amer. Chem. Soc., 1978 (This volume). 

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