Sorption values, mineral surfaces

Using the limited empirical observations of the magnitudes of Kimin values, one may examine the question of when nonpolar chemical sorption to mineral surfaces from... 

Sorption to mineral surfaces (as opposed to NOM) is generally viewed as more of a displacement than a dissolution phenomenon. Because mineral surfaces tend to be more polar than NOM, sorption to the former is more substantial for polar and ionic compounds than for those that are more hydrophobic (Curtis et al., 1986 Chiou, 1998). Furthermore, since most NOM and mineral surfaces exhibit either a neutral or negative charge, sorption to soils and sediments is considerably stronger for pesticide compounds that are positively charged in solution—such as paraquat or diquat—than for neutral species, and weaker still for anions. As a consequence, measured values in soils exhibit little dependence upon pH for pesticide compounds that are not Brpnsted acids or bases (Macalady and Wolfe, 1985 Haderlein and Schwarzenbach, 1993). 

Deviations from organic carbon correlations at these Iow/qc values have been attributed to sorption at mineral surfaces (6, 9). Furthermore, the reliability of K(l( predictions at higher/oc values may be affected by differences in sorption behavior attributed to differences in the organic matter associated with different types of natural. solid phases and, potentially, with different fractions of any particular natural sorbent (3, 10, 11). 

It has been proposed that the extent to which mixed-cation hydroxide compounds actually do form in aquatic and terrestrial environments is limited more by slow rates of soil mineral dissolution, a necessary preliminary step, than by lack of thermodynamic favorability (57). Because the dissolution rates of clays and oxide minerals are fairly slow, the possibility of mixed-cation hydroxide formation as a plausible "sorption mode" in 24 hour-based sorption experiments (and also most long-term studies) containing divalent metal ions such as Mg, Ni, Co, Zn, and Mn and Al(III)-, Fe(III)-, and Cr(III)-(hydr)oxide or silicate minerals has been ignored in the literature 16,17). This study and others recently published (77), however, suggests that metal sorption onto mineral surfaces can significantly destabilize surface metal ions (A1 and Si) relative to the bulk solution, and therefore lead to an enhanced dissolution of the clay and oxide minerals. Thus, predictions on the rate and the extent of mixed-cation hydroxide formation in aquatic and terrestrial environments based on the dissolution rate of the mineral surface alone are not valid and underestimate the true values. 

Chemical reactions of adsorbed species are of importance in vast areas of science, the involvement of adsorbed metal ions in catalysis being one example of great economic value. In addition reactions involving adsorbed species can sometimes produce products that may be either difficult or impossible to prepare away from the mineral surface. Therefore, an understanding of the chemical processes that occur in such systems is of potential economic benefit to industrial operations. Such knowledge is also of much wider significance, however, because the movement of ions in most environmental situations is controlled by sorption processes, and aluminosilicate minerals play a major role in many situations. 

The dependence of Kss on sorbed SDS levels appears to be qualitatively consistent with proposed surfactant structures at mineral surfaces (Fuerstenau and Wakamatsu, 1975 Holsen et al., 1991 Chandar et al., 1987). For example, the configuration of adsorbed SDS in Region I is expected to be different from that in Regions II and III, where micelle-like structures are thought to exist at these relatively higher sorption densities. Apparently, these differences in sorbed surfactant structure that result from regional sorption mechanisms and sorption densities lead to regional differences in Kss values. 

Although sorption of TCB by the silica surfaces can increase values of KDi over those predicted from /oc and K()C alone, the effect is only weakly apparent until very large quantities of mineral surface and very low/oc values are obtained. Some caution must be exercised in extrapolating these comparisons between the reactions of organic contaminants with organic and mineral surfaces to solutes less hydrophobic than TCB. As noted earlier, specific interactions with mineral surfaces may be more important determinants of sorption for less hydrophobic contaminants. 

Most of the recent adsorption literature has emphasized the importance of the acid-base properties of oxide surfaces when explaining or estimating their sorption behavior. However, Sveijensky (1993) has shown that log values for the adsorption of a specific cation by multiple mineral sorbents are a simple linear function of l/e, where e is the dielectric constant of each mineral. He has used this approach to estimate CC and TL model K"" values for the adsorption of up to 18 cations on 7 oxide and silicate mineral surfaces. 

A reduction in system pH enhances the solubility of PR, making the precipitation of pyromorphite minerals possible. However, the sorption of Pb decreases sharply as the system pH decreased, producing a sigmoidal function, usually referred to as an adsorption edge, which reflects the affinity of a metal species for a mineral surface (Sposito, 1984). The ability of Pb to form inner-sphere surface complexes is related to the ability of a species in solution to form hydroxides. In fact, it has been shown that surface affinity of metal cations for Fe-oxide and Fe-hydroxide surfaces agrees with their hydrolysis values (Hayes and Katz, 1996). An analogy between solution complexation and surface complexation is represented in the following reactions (Hayes and Katz, 1996) ... 

The paper summarizes eiforts started to deliver a profound chemical base for risk assessment, namely to properly take into account the physico-chemical phenomena governing the contamination source term development in time and space. One major aspect there is the substitution of conventional distribution coefficients (IQ values) for the empirical description of sorption processes by surface complexation models, in combination with other thermodynamic concepts. Thus, the framework of a Smart Kd is developed for complex scenarios with a detailed explanation of the underl3dng assumptions and theories. It helps to identify essential processes and the associated most critical parameters, easing further refinement studies. The presented case studies cover a broad spectrum of contamination cases and successfully demonstrate the applicability of the methodology. The necessity to create a mineral-specific sorption database to support the Smart IQ approach is derived and a first prototype for such a digital database introduced, combining numeric data with a knowledge base about the relevant theories, experimental methods, and structural information. 

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