Speciation adsorbed species

Mouvet, Ch., and A. C. M. Bourg (1983), "Speciation (Including Adsorbed Species) of Copper, Lead, Nickel and Zinc in the Meuse River", Water Res. 17/6, 641 -49. 

MINTEQA2 http //www.epa.gov/ceampubl/mmedia/minteq/index.htm MINTEQA2 is an equilibrium speciation model that can be used to calculate the equilibrium composition of dilute aqueous solutions in the laboratory or in natural aqueous systems. The model is useful for calculating the equilibrium mass distribution among dissolved species, adsorbed species, and multiple solid phases under a variety of conditions including a gas phase with constant partial pressures. 

The adsorption data is often fitted to an adsorption isotherm equation. Two of the most widely used are the Langmuir and the Freundlich equations. These are useful for summarizing adsorption data and for comparison purposes. They may enable limited predictions of adsorption behaviour under conditions other than those of the actual experiment to be made, but they provide no information about the mechanism of adsorption nor the speciation of the surface complexes. More information is available from the various surface complexation models that have been developed in recent years. These models represent adsorption in terms of interaction of the adsorbate with the surface OH groups of the adsorbent oxide (see Chap. 10) and can describe the location of the adsorbed species in the electrical double layer. 

Simple ligands can adsorb on iron oxides to form a variety of surface species including mononuclear monodentate, mononuclear bidentate and binuclear mono or bi-dentate complexes (Fig. 11.2) these complexes may also be protonated. How adsorbed ligands (and cations) are coordinated to the oxide surface can be deduced from adsorption data, particularly from the area/adsorbed species and from coadsorption of protons. Spectroscopic techniques such as FTIR and EXAFS can provide further (often direct) information about the nature of the surfaces species and their mode of coordination. In another approach, the surface species which permit satisfactory modelling of the adsorption data are often assumed to predominate. As, however, the species chosen can depend upon the model being used, this method cannot provide an unequivocal indication of surface speciation confirmation by an experimental (preferably spectroscopic) technique is necessary. 

Mouvet C, Bourg ACM. 1983. Speciation (including adsorbed species) of copper, lead, nickel, and zinc in the Meuse River. Water Research 17 641-649. 

Once a choice of molecular model for adsorbed species activity coefficients has been made and its parameters are measured, an equilibrium speciation calculation can be performed by exact analogy with that for aqueous species (see Table 9.3). Computationally, adsorbent species and adsorbed species (SR M OH H in... 

Thus, the adsorbent species selected as components are SO (s) and SOI 12(s), the first being classified formally as a ligand (because it reacts with metals), while the second is classified formally as a metal (because it reacts with ligands), insofar as a speciation calculation is concerned. Both of the adsorbed species on the right in equation (9.9) are interpreted molecularly as inner-sphere surface... 

The formal similarity between adsorption and complexation reactions can be exploited to incorporate adsorbed species into the equilibrium speciation calculations described in Sections 2.4 and 3.1. To do this, a choice of adsorbent species components (SR r in Eq. 4.3) must be made and equilibrium constants for reactions with aqueous ions must be available. A model for computing adsorbed species activity coefficients must also be selected.8 Once these choices are made and the thermodynamic data are compiled, a speciation calculation proceeds by adding adsorbent species and adsorbed species (SR Mp(OH)yHxLq in Eq. 4.3) to the mole-balance equations for metals and ligands, and then following the steps described in Section 2.4 for aqueous species. For compatibility of the units of concentration, njw) in Eq. 4.2 is converted to an aqueous-phase concentration through division by the volume of aqueous solution. 

This dilemma emphasizes the two essential prerequisites for a successful molecular probe of surface speciation (1) it must be able to distinguish species that are stationary at an interface for longer than 10 ps from those that are not, and (2) it must be able to disaggregate the behavior of oxygen atoms bonding to an adsorbed species into contributions from complexing surface groups and those from coordinated water molecules. The ESR techniques succeed with the time scale criterion (diffuse-ion species are mobile, whereas... 

While Davis and his colleagues illustrated the significance of soil metal speciation in risk assessment, Morrison et al. (1989) pointed out that the toxicity of metals is related to the forms in which they exist in the aqueous phase. This is because the interaction of metals with intracellular compartments is highly dependent on chemical speciation. Some species may be able to bind chemically with extracellular proteins and other biological molecules, some may adsorb onto cell walls, and others may diffuse through cell membranes. Consequently, toxicity is more related to the concentrations of metals in a particular species, than to the total concentrations. Geochemical modeling... 

The Pb dust analysis demonstrated the applicability of XAFS, p-XRF and p-XRD to determine the speciation of trace metals in heterogeneous samples. The X-ray techniques successfully identihed the speciation and potential sources of Pb in house dust samples, and provided an explanation for the observed high bioaccessibility. Linear combination fitting of the EXAFS i (k) k data of a Pb-elevated house dust sample show that Pb is complexed in two environments, as both a mineral (hydrocerussite) and an adsorbed species (adsorbed onto an iron oxide). The main findings from the Pb dust analysis can be summarized as follows ... 

The same argument has been advanced more quantitatively for the respecia-tion of gold chlorides adsorbed onto alumina [27]. The adsorbed Au complexes contain less chloride than those in solution at the same bulk pH, however, when the data for the adsorbed species was replotted at the estimated surface pH, the speciation agreed reasonably well with the bulk-solution species. 

Table 3 summarizes the results of Solar et al. [18], which clearly illustrate the dramatic improvement in molybdenum adsorption once the electrostatic repulsion is eliminated. Thus, for example, the surface coverage of the as-received carbon black increased by more than an order of magnitude at pH < pHpzc despite the uncertainties regarding the surface area occupied by the adsorbed species and the complex speciation diagram of [72], these high values could well be inter-... 

---