Significance of thermodynamic and physical characteristics on permeation of ions during membrane separation: Hydrated radius, hydration free energy and viscous effects

Publisher Summary This chapter focuses on the nature of interaction among trace metals in soil solution, dissolved organics in soil solution, and solid surfaces. The interaction between metal cations and dissolved polyfunctional organic compounds of low molecular weight is important because of its role in mineral-weathering and soil-forming processes and its potential role in heavy metal contamination of soil and groundwater. The chapter presents the organics and metals in the soil solution. Dissolved organics that interact with soil constituents and trace metal ions are of two major kinds: a range of low-molecular-weight organic acids—including polyphenols, simple aliphatic acids, amino acids, sugar acids, and hydroxamate siderophores; and a series of soluble humic/fulvic acids. Numerous environmental issues arise in relation to the interaction of metal ions with soluble organics. Some of these include the phytoavailability of metals, plant nutrient availability, toxicological effects of coordinated metal ions on aquatic and marine organisms, and transport of contaminants, particularly in relation to implications for surface and groundwater quality and soil genesis. All of these issues are highly dependent on the nature and concentration of the contaminant in the soil solution phase. Extant research indicates that low-molecular-weight ligands in soil solution may either enhance or retard reactions with solid surfaces—depending on the functional groups on the organic molecule, soil surface properties, and soil solution conditions. It is imperative that increased research efforts be devoted to evaluating the effects of these organics on metal reactions in the soil.

Role of Metal-Organic complexation in metal sorption by Soils

Low molecular weight organic acids and their Al-complexes in soil solution—composition, distribution and seasonal variation in three podzolized soils

The crystalline swelling properties and interlayer structure of a cesium montmorillonite clay were investigated using molecular computer simulations. Two classes of dry-clay structures, proposed previously to explain X-ray diffraction and NMR experiments, were identified using Monte Carlo annealing calculations. Hydrated clays with water contents ranging from 0.044 to 0.440 gH2O/gclay were investigated using molecular dynamics simulations. Layer spacings calculated as a function of water content were found to be similar to experimental swelling curves, showing a distinct plateau at the monolayer-hydrate spacing. Hydration energies were calculated as a function of water content and expressed in three complementary forms. The immersion energy form was found to be most useful, revealing an apparently global minimum in the swelling-coordinate energy that corresponds to the monolayer hydrate. This is in agreement with experimental measurements and may help clarify the energetic origins of discrete, crystalline...

Molecular Computer Simulations of the Swelling Properties and Interlayer Structure of Cesium Montmorillonite

https://hal.archives-ouvertes.fr/hal-00974813/document

Possible role of organic matter in radiocaesium adsorption in soils.

A critical demand in environmental modeling and a desirable but elusive goal of research on the ion exchange properties of the charged solid surface has been to determine the selectivity coefficient from fundamental properties of the ions and surface. We developed a Hard and Soft Acid and Base (HSAB) Model to describe exchangeable cation selectivity on solid surfaces. Our previous work has shown that the model quantitatively describes alkali cation exchange on clay minerals in terms of the absolute etectronegativity and softness of the exchangeable cations and two fitting parameters: a and/3. This study was conducted to determine the relationship between a and/3 and surface charge characteristics of 2:1 clays. The layer charge and cation selectivity of seven smectites and one vermiculite were used. The regression of log K ~ against four combinations of charge properties was performed and the appropriate relationship between a,/3, and surface charge was selected based on both statistical criteria (R 2) and their consistency with the assumptions of the HSAB model. The selected model was then cross-validated using separate cation exchange data from the literature. It was found that a and/3 are linearly related to the amount of charge arising from mineral tetrahedral and octahedral sites, respectively. These results make it possible to predict the alkali cation selectivity of 2:1 clay minerals from their chemical composition data and the alkali cation properties.

Alkali cation selectivity and surface charge of 2:1 clay minerals

Experimental support was obtained for a quantitative ion-exchange model based on the principle of hard and soft acids and bases (HSAB) applied to monovalent cation exchange on clay minerals, oxides, and exchange resins. According to the model, the thermodynamic exchange constant (Kex) can be obtained from the differences in absolute electronegativity (x) and absolute softness (S) between the exchanging cations in binary exchange as follows: where the subscripts M and R denote exchange and reference cations, respectively, and and are coefficients dependent only on the exchanger surface properties, temperature, pressure, and ionic strength. This study was conducted to test the applicability of this model to monovalent cation exchange on clay minerals and ionicexchange resins. Exchange isotherms for Li, K, Rb, Cs, Ag, and Tl vs. Na on vermiculite, smectites, hydrous Zr oxides, silica gel, and ion-exchange resins were obtained experimentally or from the literature and and were obtained by fitting the data to the above equation. An excellent fit (R 0.954, n = 5) of the alkali metal cation-exchange data to the model resulted on all of the tested exchangers, whereas Ag and Tl exchange was successfully fit only on hard surfaces such as a vermiculite and on highly hydrated surfaces such as certain ion exchange resins. According to the HSAB model, coefficients and determine the relative contribution to Kex from ionic or covalent bonding in surface complexation. The ratio depended on the layer charge of clay minerals and on the acid strength of the exchange resins and hydrous oxides. View complete article To view this complete article, insert Disc 5 then click button8

Hard and Soft Acid-Base Model Verified for Monovalent Cation Selectivity

Monovalent Cation Selectivity Quantitatively Modeled According to Hard/Soft Acid/Base Theory

Labile and Nonlabile Aqueous Silica in Acid Solutions: Relation to the Colloidal Fraction

Microelectrophoresis has been used for more than 60 years to study the behavior of aluminosilicate clays, yet there is still widespread disagreement concerning the ability of this simple measurement to describe the nature of the aqueous/solid interface. The disagreement stems from questions regarding the validity of the equations used to transform the measured property, electrophoretic mobility (µ), into the zeta potential (ζ). In electrophoresis, ζ is the average electrostatic potential at the shear plane between a hydrated particle moving in response to an electric field and the stationary water through which the particle moves. To make the mathematical analysis of the forces involved in electrophoresis tractable, the shear plane must be defined as an imaginary plane separating the hydration sheath of the moving particle from the bulk water in which it is moving (Hunter, 1981).

Shihe Xu

Papers

Alkali cation selectivity and surface charge of 2:1 clay minerals

Hard and Soft Acid-Base Model Verified for Monovalent Cation Selectivity

Monovalent Cation Selectivity Quantitatively Modeled According to Hard/Soft Acid/Base Theory

Labile and Nonlabile Aqueous Silica in Acid Solutions: Relation to the Colloidal Fraction

Microelectrophoresis Applied to the Surface Chemistry of Clay Minerals