Speciation soil particle surfaces

Hunter DB, Bertsch PM (1998) In situ examination of uranium contaminated soil particles by micro-X-ray absorption and micro-fluorescence spectroscopies. J Radioanal Nucl Chem 234 237-242 Hunter DB, Bertsch PM, Kemner KM, Clark SB (1997) Distribution and chemical speciation of metals and metalloids in biota collected from contaminated environments by spatially resolved XRF, XANES, and EXAFS. JPhys IV 7 (Colloque C2, X-Ray Absorption Fine Structure, Vol. 2) 767-771 Hunter KA (1980) Microelectrophoretic properties of natural surface-active organic matter in coastal seawater. Limnology and Oceanography 25 807-822... 

The solid-water interface, mostly established by the particles in natural waters and soils, plays a commanding role in regulating the concentrations of most dissolved reactive trace elements in soil and natural water systems and in the coupling of various hydrogeochemical cycles (Fig. 1.1). Usually the concentrations of most trace elements (M or mol kg-1) are much larger in solid or surface phases than in the water phase. Thus, the capacity of particles to bind trace elements (ion exchange, adsorption) must be considered in addition to the effect of solute complex formers in influencing the speciation of the trace metals. 

Species distribution studies have shown that trace element (e.g. metals) concentrations in soils and sediments vary with physical location (e.g. depth below bed surface) and with particle size. In these speciation studies the total element content of each fraction was determined using a suitable trace element procedure, for example, solid sample analysis by X-ray emission spectroscopy or neutron activation analysis, or alternatively by dissolution of sample and analysis by ICPOES, AAS or ASV. The type of sample fraction analysed can vary, and a few... 

In addition to the soluble chemical species and possible solid phase species described in the previous sections no discussion on speciation can be complete without the consideration of surface species. These include the inorganic and organic ions adsorbed on the surface of particles. Natural systems such as soils, sediments and waters abound with colloids such as the hydrous oxides of iron, aluminium, manganese and silicon which have the potential to form surface complexes with the various cationic and anionic dissolved species (Evans, 1989). 

A prototypical example of a molecular probe used extensively to study the mineral adsorbent-solution interface is the ESR spin-probe, Cu2+ (Sposito, 1993), whose spectroscopic properties are sensitive to changes in coordination environment. Since water does not interfere significantly with Cu11 ESR spectra, they may be recorded in situ for colloidal suspensions. Detailed, molecular-level information about coordination and orientation of both inner- and outer-sphere Cu2+ surface complexes has resulted from ESR studies of both phyllosilicates and metal oxyhydroxides. In addition, ESR techniques have been combined with closely related spectroscopic methods, like electron-spin-echo envelope modulation (ESEEM) and electron-nuclear double resonance (ENDOR), to provide complementary information about transition metal ion behaviour at mineral surfaces (Sposito, 1993). The level of sophistication and sensitivity of these kinds of surface speciation studies is increasing continually, such that the heterogeneous colloidal particles in soils can be investigated ever more accurately. 

Surface waters, soils, and sediments are often divided into "parcels" that are modeled as open qrstems (16). Inputs and outputs of water, water-borne solutes, and water-borne particles change over time. As a consequence, chemical conditions within the parcel change. If metal ion exchange and ligand exchange reactions take place quickly, equilibrium descriptions of chemical speciation are appropriate. If these reactions take place slowly, however, then an appraisal of speciation requires knowledge of reaction kinetics (17). 

---