Solute adsorption, macroscopic

In surface-complexation models, the relationship between the proton and metal/surface-site complexes is explicitly defined in the formulation of the proposed (but hypothetical) microscopic subreactions. In contrast, in macroscopic models, the relationship between solute adsorption and the overall proton activity is chemically less direct there is no information given about the source of the proton other than a generic relationship between adsorption and changes in proton activity. The macroscopic solute adsorption/pH relationships correspond to the net proton release or consumption from all chemical interactions involved in proton tranfer. Since it is not possible to account for all of these contributions directly for many heterogeneous systems of interest, the objective of the macroscopic models is to establish and calibrate overall partitioning coefficients with respect to observed system variables. 

Macroscopic Descriptions of Solute Adsorption and the Net Proton Coefficient. The macroscopic proton coefficient plays two important roles in our macroscopic descriptions of surface processes. First,... 

Because of its importance, it is not surprising that the study of the neat liquid surface, as well as of solute adsorption, spectroscopy, and reactivity, goes back many years. However, up until the last decade of the twentieth century most of the experimental studies involved the measurement of macroscopic properties such as surface tension and surface potential, and generally speaking, the spectroscopic techniques employed lacked the specificity and... 

In their description of metal ion adsorption, Benjamin and Leckie used an apparent adsorption reaction which included a generic relationship between the removal of a metal ion from solution and the release of protons. The macroscopic proton coefficient was given a constant value, suggesting that x was uniform for all site types and all intensities of metal ion/oxide surface site interaction. Because the numerical value of x is a fundamental part of the determination of K, discussions of surface site heterogeneity, which are formulated in terms similar to Equation 4, cannot be decoupled from observations of the response of x to pH and adsorption density. As will be discussed later, It is not the general concept of surface-site heterogeneity which is affected by what is known of x> instead, it is the specific details of the relationship between K, pH and T which is altered. 

The chemical complexity of most natural systems often requires that adsorption reactions be described using semi-empirical, macroscopic models. A common approach is to describe the net transfer of an adsorbate from the solution phase to the solid/water interface with a single stoichiometric expression. Such stoichiometries include a generic relationship between the adsorption of a solute and the release or consumption of protons. 

Measurements of the chemical composition of an aqueous solution phase are interpreted commonly to provide experimental evidence for either adsorption or surface precipitation mechanisms in sorption processes. The conceptual aspects of these measurements vis-a-vis their usefulness in distinguishing adsorption from precipitation phenomena are reviewed critically. It is concluded that the inherently macroscopic, indirect nature of the data produced by such measurements limit their applicability to determine sorption mechanisms in a fundamental way. Surface spectroscopy (optical or magnetic resonance), although not a fully developed experimental technique for aqueous colloidal systems, appears to offer the best hope for a truly molecular-level probe of the interfacial region that can discriminate among the structures that arise there from diverse chemical conditions. 

Solubility and kinetics methods for distinguishing adsorption from surface precipitation suffer from the fundamental weakness of being macroscopic approaches that do not involve a direct examination of the solid phase. Information about the composition of an aqueous solution phase is not sufficient to permit a clear inference of a sorption mechanism because the aqueous solution phase does not determine uniquely the nature of its contiguous solid phases, even at equilibrium (49). Perhaps more important is the fact that adsorption and surface precipitation are essentially molecular concepts on which strictly macroscopic approaches can provide no unambiguous data (12, 21). Molecular concepts can be studied only by molecular methods. 

Negative adsorption occurs when a charged solid surface faces an ion in an aqueous suspension and the ion is repelled from the surface by Coulomb forces. The Coulomb repulsion produces a region in the aqueous solution that is depleted of the anion and an equivalent region far from the surface that is relatively enriched. Sposito (1984) characterized this macroscopic phenomenon through the definition of the relative surface excess of an anion in a suspension, by... 

This example illustrates the qualitative nature of information that can be gleaned from macroscopic uptake studies. Consideration of adsorption isotherms alone cannot provide mechanistic information about sorption reactions because such isotherms can be fit equally well with a variety of surface complexation models assuming different reaction stoichiometries. More quantitative, molecular-scale information about such reactions is needed if we are to develop a fundamental understanding of molecular processes at environmental interfaces. Over the past 20 years in situ XAFS spectroscopy studies have provided quantitative information on the products of sorption reactions at metal oxide-aqueous solution interfaces (e.g., [39,40,129-138]. One... 

Transfer of solute with and relative to the moving water and competitive adsorption of solutes are central to amelioration of saline and alkali soils, agricultural chemical location in soils and management of wastes in soils. This paper illustrates how space-like coordinates based on the distribution of the solid and the water help analyse these problems. We focus on the macroscopic or Darcy scale of discourse [6], which permits unambiguous measurement of the key elements of the flow equations, and we restrict ourselves to 1-dimensional flow, because that seems to limit analysable experiments. 

Strictly speaking, there are only two ways in which gases can be in a sample. If gas atoms are mixed with host atoms on a microscopic scale, the gas is dissolved if gas atoms are on the surfaces that bound the sample, the gas is adsorbed. There are quite a few complications and variations on these basic themes, however, as will be discussed. From a macroscopic viewpoint, and from the viewpoint of laboratory practice, it is sometimes difficult to distinguish between solution and adsorption,... 

Equation 4.3 is formally similar to a complexation reaction between SR(s) and the aqueous solution species on the left side. Indeed, the solid-phase product on the right side can be interpreted on the molecular level as either an outer-sphere or an inner-sphere surface complex. The latter type of adsorbed species was invoked in connection with the generic adsorption-desorption reactions in Eqs. 3.46 and 3.61, which were applied to interpret mineral dissolution processes. In general, adsorbed species can be either diffuse-layer ions or surface complexes,7 and both species are likely to be included in macroscopic composition measurements based on Eq. 4.2. Equation 4.3, being an overall reaction, does not imply any particular adsorbed species product, aside from its stoichiometry and the electroneutrality condition in Eq. 4.4. 

Elements 108 - 116 are homologues of Os through Po and are expected to be partially very noble metals. Thus it is obvious that their electrochemical deposition could be an attractive method for their separation from aqueous solutions. It is known that the potential associated with the electrochemical deposition of radionuclides in metallic form from solutions of extremely small concentration is strongly influenced by the electrode material. This is reproduced in a macroscopic model [70], in which the interaction between the microcomponent A and the electrode material B is described by the partial molar adsorption enthalpy and adsorption entropy. By combination with the thermodynamic description of the electrode process, a potential is calculated that characterizes the process at 50% deposition ... 

Protein function at solid-liquid interfaces holds a structural and a dynamic perspective [31]. The structural perspective addresses macroscopic adsorption, molecular interactions between the protein and the surface, collective interactions between the individual adsorbed protein molecules, and changes in the conformational and hydration states of the protein molecules induced by these physical interactions. Interactions caused by protein adsorption are mostly non-covalent but strong enough to cause drastic functional transformations. All these features are, moreover, affected by the double layer and the electrode potential at electrochemical interfaces. Factors that determine protein adsorption patterns have been discussed in detail recently, both in the broad context of solute proteins at solid surfaces [31], and in specific contexts of interfacial metalloprotein electrochemistry [34]. Some important elements that can also be modelled in suitable detail would be ... 

Chromatographic separations of the isotopic isomers of molecules are of interest first because they afford a convenient technique for analysis of mixtures, secondly because scale up in the future might afford economically feasible separations of macroscopic amounts of material, and finally because the values of the separation factors and their temperature coefficients are of intrinsic theoretical interest. This last follows because such data are straightforwardly related to the understanding of isotope effects on solution and adsorption processes and of the intermolecular forces which give rise to these effects. We have approached the general problem... 

Similar simulations for finite gold nanocrystals with 561 atoms led to a similar hysteresis between adsorption and desorption. Moreover, the adsorption starts at a lower thiol concentration for the nanocrystals than for the macroscopic crystal surface. Adding the solvent led to significant changes. There results a competitive adsorption between the thiols and the solvent and the interactions between the tails are modified. Thus, as the authors conclude, phenomena that are observed in vacuum may be different from those observed in a solution. 

Solid-phase As can be found in many different forms in aquifer sediments examples include (1) stoichiometric arsenic minerals (2) solid solution of arsenic in minerals or x-ray amorphous phases from trace (< 1000 ppm) to atom percent levels (3) coprecipitation of As with minerals during their formation and (4) adsorption of As on particle surfaces. This section gives a brief background on the macroscopic and spectroscopic techniques that are commonly used for ascertaining As species in these phases. 

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