Clay properties sorption

In most mathematical analyses used to establish bounds for radionuclide migration rates through the abyssal red clays, the sorption properties of the sediment are generally represented mathematically by the sorption equilibrium distribution coefficients for each of the species involved. These coefficients are usually denoted by Kp. and are defined by... 

The engineered barriers of a repository (waste form, container, clay buffer and backfill material and repository stmcmre) which will degrade with time due to various physico-chemical processes, strongly affect the geochemistry of fluids and rocks of the near field environment (redox properties, sorption properties, solubility limits, equilibrium chemistry, etc.) and determine the speciation and the subsequent migration behaviour of the radionuclides into the far field. Extensive... 

The barrier properties of the PCL-based composites were investigated. The transport properties, sorption and diffusion, were measured by a microgravimetric method . The studied model permeants were methylene chloride and water vapour for which the zero concentration diffusion coefficient Dq was determined. The presence of clay (hydrophilic platelets) in the composite gives rise to specific sites on which water molecules can be entrapped and immobilized, thus the water sorption increases on increasing the clay content, particularly for microcomposites containing Cloisite Na It was found out that the microcomposites as well as the intercalated nanocomposites have diffusion parameters for water vapour very near to those of pure PCL. 

Many factors affect the mechanisms and kinetics of sorption and transport processes. For instance, differences in the chemical stmcture and properties, ie, ionizahility, solubiUty in water, vapor pressure, and polarity, between pesticides affect their behavior in the environment through effects on sorption and transport processes. Differences in soil properties, ie, pH and percentage of organic carbon and clay contents, and soil conditions, ie, moisture content and landscape position climatic conditions, ie, temperature, precipitation, and radiation and cultural practices, ie, crop and tillage, can all modify the behavior of the pesticide in soils. Persistence of a pesticide in soil is a consequence of a complex interaction of processes. Because the persistence of a pesticide can govern its availabiUty and efficacy for pest control, as weU as its potential for adverse environmental impacts, knowledge of the basic processes is necessary if the benefits of the pesticide ate to be maximized. 

The sorption behavior of 11 PAH compounds (a training set, Table 11) on various solid phases (e.g., three soils and two sediments) with different properties to relevant sorption (e.g., organic carbon content, clay content, pH, cation exchange capacity CEC Table 12), was determined by batch equilibrium studies [1]. Batch equilibrium tests were designed to determine rates of equilibrium sorption under conditions of high mixing and high surface areas of the solid particles (see Chap. 3). 

The amount of adsorbed chemical is controlled by both properties of the chemical and of the clay material. The clay saturating cation is a major factor affecting the adsorption of the organophosphorus pesticide. The adsorption isotherm of parathion from an aqueous solution onto montmorillonite saturated with various cations (Fig. 8.32), shows that the sorption sequence (Al > Na > Ca ) is not in agreement with any of the ionic series based on ionic properties. This shows that, in parathion-montmoriUonite interactions in aqueous suspension, such factors as clay dispersion, steric effects, and hydration shells are dominant in the sorption process. In general, organophosphorus adsorption on clays is described by the Freundhch equation, and the values for parathion sorption are 3 for Ca +-kaoUnite, 125 for Ca -montmorillonite, and 145 for Ca -attapulgite. 

Barron,V Rendon, J.L. Torrent, J. Serna, C.J, (1984) Relation of infrared, crystallochemical, and morphological properties of Al-substi-tuted hematites. Clays Clay Min. 32 475-479 Barrow, J.J. Cox,V.C. (1992) The effects of pH and chloride concentration on mercury sorption. I. Goethite. J. Soil Sci. 43 437-450 Barrow, N. Madrid, L. Posner, A.M. (1981) A partial model for the rate of adsorption and desorption of phosphate by goethite. J. Soil Sci. 32 399-407... 

Celis, R. Cornejo, J. Hermosin, M.C. (1998) Textural properties of synthetic clay-ferrihy-drite associations. Clay Miner. 33 395-407 Celis, R. Hermosin, M.C. Cox, L. Cornejo, J. (1999) Sorption of 2,4-Dichlorophenoxyacetic acid by model particles simulating naturally occurring soil colloids. Environ. Sci. Techn. 31 1200-1206... 

Another field with a large potential for improvements concerns aluminosilicate minerals, which are of great importance in determining the chemistry of water in many types of rock. In backfill clays, aluminosilicates are responsible for the retention (sorption, incorporation) of trace elements and may affect both oxidation potential (incorporation of Fe(II)/Fe(III)) and pH (hydrolysis of silicate and/or exchange of H+). Related classes of compounds (i.e., calcium silicates and calcium aluminates) form the chemical backbone of cementitious materials. The thermodynamic properties of these substances are still largely unexplored. 

It was felt that the presence of residual salts in the clay would complicate the analysis of experimental data. Therefore, in order to remove such salts prior to using the clay, the samples of sediment were dialyzed (using deionized water) until a twentyfold concentration of the dialyzing solution did not yield a precipitate upon addition of silver nitrate. (Also, no precipitate was observed upon concentration of the solution.) The solids were then dialyzed once more, vacuum dried, and stored in sealed containers in a desiccator until needed. (The preceding procedure may have resulted in some alteration of the sorption properties of the red clay, particularly with regard to the hydrous oxides. It is intended to assess the extent of such alteration, if any, during the course of future work.)... 

General. The abyssal red clays were, in part, originally selected for study because previous investigations of their sorption properties had been encouraging (6,7), and because generally the types of minerals occurring in the sediment exhibit very favorable ion-exchange and adsorption properties. In... 

Zhuang, J. and Yu, G.-R. (2002) Effects of surface coatings on electrochemical properties and contaminant sorption of clay minerals. Chemosphere, 49(6), 619-28. 

Studies on sorption of triazines by individual soil constituents and by model sorbents have been very helpful in evaluating sorption mechanisms and in assessing the potential contribution of various constituents to triazine sorption by soils. However, intimate associations between organic substances, silicate clays, and oxyhydroxide materials modify the sorptive properties of the individual constituents. Associations between soil constituents influence soil properties - such as pH, specific surface area, and functional group availability - which in turn influence triazine sorption behavior. For instance, atrazine and simazine sorption behavior is different for synthetic mixtures of model soil... 

The type of clay present in a soil influences triazine sorption (Brown and White, 1969). Furthermore, variations in surface properties among different samples of the same clay type greatly influence sorption. For instance, sorption of atrazine on 13 clay samples, of which smectite was the dominant mineral, ranged from 0% to 100% of added atrazine (Figure 21.7), and was inversely correlated to the surface charge density of the smectites (Laird et al., 1992). Such data illustrate the complexity of sorption processes and the reason why simple predictive models relying on % OC, % clay, or surface area normalizations may fail to predict accurately the sorption of triazine by a particular soil. 

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