Surface precipitation, distinguishing from adsorption

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. 

When a precipitate separates from a solution, it is not always perfectly pure it may contain varying amounts of impurities dependent upon the nature of the precipitate and the conditions of precipitation. The contamination of the precipitate by substances which are normally soluble in the mother liquor is termed co-precipitation. We must distinguish between two important types of co-precipitation. The first is concerned with adsorption at the surface of the particles exposed to the solution, and the second relates to the occlusion of foreign substances during the process of crystal growth from the primary particles. 

Sposito, G. Distinguish adsorption from surface precipitation. In Geochemical Processes at Mineral Surfaces. J.A. Davis, K.F. Hayes, eds. Washington D.C. Americal Chemical Society Symposium Series No. 323,1986. 

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. 

A central problem in the chemistry of natural water systems is the establishment of experimental methods with which to distinguish adsorption from surface precipitation (1-3). Corey ( 2) has written a comprehensive review of this problem which should be read as an introduction to the present essay, particularly for his set of six conclusions that set out general conditions likely to result in adsorption or precipitation. The discussion to follow is not a comprehensive review, but instead focuses on three popular approaches to the adsorption/surface precipitation dichotomy. The emphasis here is on the conceptual relationship of each approach to the defining statements made above To what extent is an approach capable of distinguishing adsorption from surface precipitation ... 

SPOSITO Distinguishing Adsorption from Surface Precipitation... 

Solubility and kinetics methods for distinguishing adsorption from surface precipitation have the common features of being essentially macroscopic in nature and of not utilizing a direct examination of sorbed material. The essential difference between an adsorbate and a surface precipitate lies with molecular structure, however, and it is inevitable that methodologies not equipped to explore that structure directly will produce ambiguous results requiring ad hoc assumptions in order to interpret them. The principal technique for... 

Sposito, G. 1986. Distinguishing adsorption from surface precipitation. In Davis, J. A. Hayes, K. F. (eds) Geochemical Processes at Mineral Surfaces. American Chemical Society, Washington, DC, 217-228. 

The processes of adsorption, precipitation and coprecipitation are difficult to distinguish on that basis from the analysis of the diminution of the ions from the solution, changes of pH and kinetics. Only the spectroscopic investigations of the molecular interactions between adsorbent and adsorbate may help to distinguish a type of the process [146,147]. As an adsorption of the ions, is assumed process of the two-dimensional structure formation, whereas for three-dimensional structures precipitation or surface precipitation takes place. From this reason an AFM method may be useful at investigations of the morphology changes of the adsorbate surface [147]. 

Sposito, G. (1984). The Surface Chemistry of Soils, Oxford University Press, New York. Sposito, G. (1986). Distinguishing adsorption from surface precipitation. In Geochemical... 

Sposito, G. 1986. Distinguishing adsorption from surface precipitation, p. 217-228. In J.A. 

Sparks CJ Jr (1980) X-ray fluorescence microprobe for chemical analysis. In Winick H, Doniach S (eds) Synchrotron Radiation Research, Plenum Press, New York, p 459-512 Spngberg D, Hermansson K, Lindqvist-Reis P, Jalilehvand F, Sandstroem M, Persson I (2000) Model extended X-ray absorption fine structure (EXAFS) spectra from molecular dynamics data for Ca2+ and Al3+ aqueous solutions. J Phys Chem 104 10467-10472 Sposito G. (1986) Distinguishing adsorption from surface precipitation. In Geochemical Processes at Mineral Surfaces, Davis JA, Hayes KF (eds) ACS Symposium Series 323, The American Chemical Society, Washington, DC, p 217-228... 

Chemically, asphaltenes are polycyclic molecules that are disc shaped, and have a tendency to form stacked aggregates. The tendency of asphaltenes to self-aggregate distinguishes them from other oil constituents. Asphaltene aggregation is the cause of complex non-linear effects in such phenomena as adsorption at solid surfaces, precipitation, fluid s rheology, emulsion stability, etc P . ti). Asphaltenes are regarded to be polar species, formed by condensed poly aromatic structures, containing alkyl chains, hetero atoms (such as O, S and N) and some metals. 

The natural question to pose at this juncture is, what experimental evidence is sufficient to determine whether a substance lost from the soil solution has, in fact, been adsorbed An answer to this difficult and complex question can perhaps best be introduced by a discussion of experimental criteria that cannot be used to distinguish adsorption from precipitation in soils. This kind of discussion should be of help in any critical reading of the abundant literature on sorption studies, as well as in the design of experiments on surface phenomena in soils. 

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