Surface chemical speciation

These three types of surface species—inner-sphere complex, outer-sphere complex, and diffuse-layer—represent three modes of adsorption of small aqueous ions that contribute to the formation of the electrochemical double layer on clay mineral surfaces. No inference of special planes containing adsorbed ions is required by these surface chemical speciation concepts, nor is detailed molecular structure implied, other than the general notions of surface complexes and vicinal dissociated ions. It is sometimes convenient, although not necessary, to group the two types of surface complex into a Stern layer to distinguish them from diffuse-layer ions [18]. This geometric partitioning of surface species, however, should not be taken to mean that diffuse-layer ions necessarily approach a particle surface less closely than do Stern-layer ions. 

SALI compares fiivorably with other major surface analytical techniques in terms of sensitivity and spatial resolution. Its major advantj e is the combination of analytical versatility, ease of quantification, and sensitivity. Table 1 compares the analytical characteristics of SALI to four major surfiice spectroscopic techniques.These techniques can also be categorized by the chemical information they provide. Both SALI and SIMS (static mode only) can provide molecular fingerprint information via mass spectra that give mass peaks corresponding to structural units of the molecule, while XPS provides only short-range chemical information. XPS and static SIMS are often used to complement each other since XPS chemical speciation information is semiquantitative however, SALI molecular information can potentially be quantified direedy without correlation with another surface spectroscopic technique. AES and Rutherford Backscattering (RBS) provide primarily elemental information, and therefore yield litde structural informadon. The common detection limit refers to the sensitivity for nearly all elements that these techniques enjoy. 

An earlier study on the speciation of heavy metals and geochemical mapping of the total metal content of the surface soils of Delhi7 showed the total Al content to vary between 1.87 to 5.34% with a mean of 3.57%. In view of the above, the objective of this research was to ascertain primarily the status of Aluminum in Delhi soils by studying its chemical speciation in the soil profile in order to improve the understanding of its distribution in the solid-phase pool of the soil. More precisely, the objectives were ... 

Rates of reductive dissolution of transition metal oxide/hydroxide minerals are controlled by rates of surface chemical reactions under most conditions of environmental and geochemical interest. This paper examines the mechanisms of reductive dissolution through a discussion of relevant elementary reaction processes. Reductive dissolution occurs via (i) surface precursor complex formation between reductant molecules and oxide surface sites, (ii) electron transfer within this surface complex, and (iii) breakdown of the successor complex and release of dissolved metal ions. Surface speciation is an important determinant of rates of individual surface chemical reactions and overall rates of reductive dissolution. 

Advantages brought about by the direct analysis of solid samples as compared with the analysis of dissolved samples include a shorter total analysis time (prior dissolution steps are not required), low cost (chemical reagents are not used), less risk of contamination and less destruction of the sample. In addition, some techniques can extract information about chemical speciation e.g. XPS provides information about oxidation states and chemical bonds) and spatial composition, i.e. information with lateral resolution allowing mapping of the surface and analysis with depth resolution, of particular interest for thin-film analysis. 

Table 9.10 Results of a chemical speciation calculation involving inner-sphere surface ...

Using XRF and EPMA in conjunction with SEM and EDX the antiwear films were found to consist of P, S, O, and Zn (Brown et al., 1992 Rounds, 1993). The application of XPS and AES surface techniques promoted deeper understanding of the antiwear mechanism elemental composition of the chemical species, valence of the elements, and depth distribution. Chemical speciation, e.g., phosphate and S (sulfide or sulfate) can be obtained from binding energies. 

Tipping, E., Lofts, S., and Lawlor, A.J. (1998) Modeling the chemical speciation of trace metals in the surface waters of the Humber system. Sci. Total Environ. 210/211, 63-77. 

Table 4.2 Results of a Chemical Speciation Calculation at pH 7.5 Involving Specific Adsorption (Inner-Sphere Surface Complexation)3...

Ion adsorption and desorption at the mineral-water interface are important processes in soils, sediments, surface waters, and groundwater. By capturing or releasing ions, mineral surfaces play key roles in soil fertility, soil aggregation, chemical speciation, weathering, and the transport and fate of nutrients and pollutants in the environment. Proton adsorption is a very specific form of ion adsorption. This area is so important... 

The chemical speciation of copper in river water and model solutions was investigated by a titration technique in which cupric ion activities were measured at constant pH as the total copper concentration ([Cujoj]) was varied by incremental additions of CUSO4. pCu(-log cupric ion activity) was measured with a cupric ion-selective electrode (Orion 94-29) and pH with a glass electrode (Beckman 39301) both coupled to a single junction Ag/AgCl reference electrode (Orion 90-01) in a temperature controlled (25 + 0.5°C) water bath. Total copper concentrations in the titrated solutions were determined directly by atomic absorption spectrophotometry (Perkin Elmer 603) using a graphite furnace (Perkin Elmer 2200). Measurement of total copper concentrations is necessary because of adsorptive loss of copper from solution onto container and/or electrode surfaces. 

Finally, we must somehow resolve the vexing problem of the chemical speciation of trace elements in seawater what is the chemical nature of the various metal chelators whose existence has been demonstrated by electrochemistry Is the chemistry of several metals in surface seawater really controlled by metallophores released by prokaryotes Or are dissolved metals chiefly present as parts of metalloproteins in the process of remineralization How do metal chelators affect the residence times of metals (in particular, scavenged elements such as iron and cobalt) and in turn how do those chelators influence the global carbon cycle via changes in marine primary productivity ... 

A)jS, whether sampled from probability distribution functions or calculated by regression equations or surface-complexation models, can be used in many contaminant transport models. Alternate forms of the retardation factor equation that use a (Equation (3)) and are appropriate for porous media, fractured porous media, or discrete fractures have been used to calculate contaminant velocity and discharge (e.g., Erickson, 1983 Neretnieks and Rasmuson, 1984). An alternative approach couples chemical speciation calculations... 

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