Competitive solution/surface complexation

Heavy-lanthanide enrichments in seawater are reasonably explained by eqs. (1) and (2) wherein the extent of lanthanide solution complexation increases between La and Lu to a much greater degree than is the case for surface complexation. It is probable that competitive solution/surface complexation exerts the dominant role in heavy-lanthanide enrichments in seawater. However, it should be noted, as well, that lanthanide phosphate co-precipitation also appears to promote heavy-lanthanide enrichments in solution. Although the existence of lanthanide phosphate precipitation in seawater has not been directly demonstrated, such controls are consistent with observed total lanthanide concentrations in some environments (Byrne and Kim 1993, Johannesson et al. 1995) and are consistent, as well, with the general features of lanthanide fractionation in the oceans. 

The biovailability of an element in the soil is the result of a competition between surface complexation at the plant root system, at various soil solid phases, and that remaining in solution. Organic and humic materials in soils act as ligands and the resulting complexes can be important in the movement of metal ions (Lindsay, 1974). At present there is little information on the relationship between concentration of the complexed species and their uptake rate by plants though some general evidence sug-... 

Using SFS, Davies and co-workers [77-79] reported enhanced adsorption and competitive adsorption at the hydrophobic surface, reminiscent of that seen at the air-solution interface. For the SDS/PEO mixture [79], competitive adsorption was observed at low concentrations, whereas at higher SDS concentrations, PEO was depleted from the surface. Similar observations were made from IR-ATR measurements by Poirier et al. [80] on CieTAB/PSS mixtures at the silica-solution interface. However, the technique could not distinguish between depletion or surface complex formation. Similar trends were also reported by Fielden et al. [76] for SDS/AM-MAPTC mixtures on mica. For the PEI/SDS mixture at the hydrophobic interface [76], the SFS measurements indicated a higher degree of order and hence adsorption due to complexation at the interface. This was also shown to be strongly pH dependent [81],... 

Inner-sphere complexes are relatively stable in comparison to outer-sphere complexes under equivalent solution conditions (i.e. pH, ionic strength), and in a competitive situation will tend to displace less stable adsorbates. This is a fundamental property of coordination reactions, and explains the observed trends in metal uptake preference observed in lichen studies (Puckett et al., 1973). Metal sorption results previously attributed to ion exchange reactions are more precisely described as resulting from competitive surface complexation reactions involving multiple cation types. Strictly speaking, each metal adsorption reaction can be described using a discrete mass law relation, such as... 

More recently, Brennsteiner et al. [ 175] noted that the electrochemical removal efficiency for nickel is dependent on the pH of the contaminant solution. Maximum efficiency was achieved at pH = 7.0, but only when the carbon electrode was preplated with a layer of copper the role of surface chemistry was not investigated. Seco et al. [172] did characterize the surface chemistry of a commercial activated carbon (pHp r = 6.1) and studied its uptake of heavy metals (Ni, Cu, Cd, Zn), as well as of some binary systems. They interpreted the monotonic uptake increase with pH to be consistent with the surface complexation model a decrease in competition between proton and metal species for the surface sites and a decrease in positive surface charge, which results in a lower cou-lombic repulsion of the sorbing metal. In the binary uptake studies, they concluded that Ni (as well as Cd and Zn) is not as strongly attracted to the. sorbent as Cu. 

Many of the solutes commonly present in groundwater can influence adsorption of As. Anions compete directly with As for surface complexation sites, the stronger the anion adsorption the greater the competition. Adsorption of cations can increase adsorption of As by increasing the surface charge of a solid. 

As detailed above, the adsorption behavior of most actinides varies widely with solution pH, Eh, complexation, competitive adsorption and ionic strength, and the surface properties of sorbent phases. For this reason, many researchers have modeled actinide adsorption using surface complex-ation (SC) models that can quantitatively account for such variables. These models include the constant capacitance (CC), diffuse-layer (DL), and triple-layer (TL) models (Chap. 10). Much of the ra-... 

Adsorption equilibria for polymers out of concentrated solutions as function of concentration frequently exhibit very pronounced maxima (Fig. 12). These unusual curves can be accounted for if one assumes that the adsorbed species are in aggregation equilibrium in the solution, depending upon the amount of surface area per unit volume of solution. Hence one expects that the adsorption equilibrium out of concentrated polymer solution may not only be approached with "infinite slowness but is also a function of the system characteristics, and the definition of reproducible conditions contains many more variables than one is used to from the more common work with dilute solution. This complexity is particularly awkward when one deals with the important case of competitive adsorption of polymers out of concentrated multicomponent solutions, a common phenomenon in many industrial processes, such as paint adhesion, corrosion prevention, lubrication, especially wear prevention, etc. 

Competitive systems include suspensions with more than one solute (electrolyte ions, proton, and hydroxide not counted). These may be systems (1) in which two or more metal ions compete for surface sites (e.g., studied by Benjamin [117] or Yang and Davis [79]), (2) in which two or more ligands compete for surface sites (e.g., studied by Goldberg [81] or by Mesuere and Fish [118] and Hiemstra and van Riemsdijk [119]), and (3) systems in which metals and ligands are present, which at least under certain experimental conditions adsorb simultaneously without forming common surface complexes [14] (i.e., they compete). 

It is now generally agreed that the preceding condition 1 and 2 are fulfilled by the formation of surface compounds between the metallic element in various oxidation states and species, essentially anions and solvent molecules, present in the solution. The complex pH and anion dependence of the dissolution rate is assumed to reveal the competitive role of various types of mechanisms [24]. According to Sato [25] ... 

Many models try to provide a quantitative analysis of adsorption phenomena on oxide surfaces. Most of these models are based on the formation of surface complexes [1,13,29]. They consider the competition between the variation in free enthalpy due to chemical and electrostatic interactions and the variation in solvation energy of the adsorbed catiotis [30]. Some models also consider equilibrium constants derived from the mass-action law (the triple-layer model) [31,32]. Combining surface complexation and precipitation of cations in solution is a way of treating the problem continuously over a very wide range of concentrations [33]. 

Lanthanide distribution models (Elderfield 1988, Byrne and Kim 1993, Erel and Morgan 1991, Erel and Stolper 1993) provide a basis for the comparative shale-normalized lanthanide concentrations (lanthanide fractionations) which are observed in the oceans. Lanthanide fractionation models are formulated in terms of competitive complexation equilibria involving, on one hand, solution complexation, and on the other, surface complexation on marine particles. Following the developments of Elderfield (1988) and Byrne and Kim (1993), shale-normalized lanthanide concentrations (Mj)sn in seawater can be expressed as... 

The extensive fractionation of trivalent lanthanides between Sargasso Sea water and suspended particles can be qualitatively explained by a solution/surface competition model (fig. 15). The progressive decrease from La to Lu in the extent to which dissolved lanthanides are removed by suspended particles results from an increase in the solution complexation constants as predicted by chemical models (e.g., sect. 4 of this chapter Erel and Morgan 1991, Erel and Stolper 1993, Byrne and Li 1995). 

The adsorbed layer at G—L or S—L surfaces ia practical surfactant systems may have a complex composition. The adsorbed molecules or ions may be close-packed forming almost a condensed film with solvent molecules virtually excluded from the surface, or widely spaced and behave somewhat like a two-dimensional gas. The adsorbed film may be multilayer rather than monolayer. Counterions are sometimes present with the surfactant ia the adsorbed layer. Mixed moaolayers are known that iavolve molecular complexes, eg, oae-to-oae complexes of fatty alcohol sulfates with fatty alcohols (10), as well as complexes betweea fatty acids and fatty acid soaps (11). Competitive or preferential adsorption between multiple solutes at G—L and L—L iaterfaces is an important effect ia foaming, foam stabiLizatioa, and defoaming (see Defoamers). 

---