Soil surface speciation

Johnston and Sposito (26) arrived at similar conclusions in a review of approaches to soil surface speciation and then went on to suggest an obvious alternative to the endless archiving of adsorption data ... 

Arai Y, Sparks DL (2002) Residence time effects on arsenate surface speciation at the aluminum oxide-water interface. Soil Sci 167 303-314... 

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. 

A deeper perception of the mechanistic implications of equation (9.2) can be had if the rational activity coefficients are described on the molecular level using the methods of statistical mechanics. This approach is the analogue of the statistical mechanical theory of activity coefficients for species in aqueous solution (Sposito, 1983). Fundamental to it is the prescription of surface speciation and the dependence of the rational activity coefficient on surface characteristics. Three representative molecular models of adsorption following this paradigm are summarised in Table 9.8. Each has been applied with success to describe the surface reactions of soil colloids (Goldberg, 1992). 

This is not to say that no progress has been made in surface speciation kinetics related to soils (Sparks and Suarez, 1991). Adaptation of elementary reaction mechanisms established for aqueous complexes, for example, has proved to be an effective means of classifying kinetic surface species and establishing rate laws for... 

Over the last three decades, two general approaches have been proposed in the literature for describing the interactions of sulfate in soils. The first approach is that of a chemical nature where thermodynamic interrelationships with speciation of cations and anions present in soil solution and the interaction with the soil surface are the major mechanisms. These models may be referred to as chemical models. Examples of such models include that of Cosby et al. (1986), Reuss and Johnson (1986), De Vries et al. (1994), among others. A common feature of these models is that both ion exchange and aluminum hydrolysis reactions are similar. Their capability of quantifying these processes varies according to whether the interactions are... 

The purpose of this chapter is to present several of the most common models used to describe metal and metalloid ion adsorption by soil components. Empirical models used in soil chemistry are described and their limitations discussed. Common chemical models used to describe metal adsorption on soil minerals are described and their advantages over empirical approaches discussed. Methods for obtaining model parameters are provided. Methods for establishing adsorption mechanisms and surface speciation are addressed. Limitations and approximations in the application of chemical models to natural systems are presented. 

The sampling of water and soil for speciation analysis is not significantly different than for total metal content determinations. The distribution of metals ranges from truly dissolved to particulate-bound species. This includes inorganic complexes or complexes with organic molecules, surface-bound metals (organic and inorganic... 

Al-Hamdan AZ, Reddy KR. (2005). Surface speciation modeling of heavy metals in kaolin Imphcations for electrokinetic soil remediation processes. Adsorption 11 529-546. 

Hie parameters ois, oos and at, reflect the disposition of ions in the soil solution after they have become incorporated into the interfacial region. Therefore, these surface charge densities represent the net charging effects of the surface speciation of the ions. By analogy with the use of speciation models (ion-association models) to estimate the distribution of ionic charge in aqueous phases like soil. solutions, surface speciation models (surface... 

The CA and GC modeling approaches can differ in the selection of surface species to fit experimental data. Surface speciation in the CA modeling approach is normally fixed by a previous fit of an SCM to a reference mineral phase, such as ferrihydiite (Waite et al., 1994 Dzombak Morel, 1990). Ideally, spectroscopic data (XAS, FTIR, and others) are used to constrain the selection of surface species in CA models for reference minerals, so that molecular scale details of the bonding are included within the model as well as EDL terms however, as is illustrated in the CA model calculations presented here and elsewhere (Waite et al., 2000 Davis et al., 1998), application of CA models to soils and sediments is not straightforward. Without very detailed characterization of the physical and chemical characteristics of the surface, several assumptions need to be made to apply a CA model. 

Berner RA, Holdren GR Jr (1979) Mechanism of feldspar weathering. II. Observations of feldspars from soils. Geochim Cosmochim Acta 43 1173-1186 Bevan J, Savage D (1989) The effect of organic acids on the dissolution of K-feldspar under conditions relevant to burial diagenesis. Mineral Mag 53 415-425 Blum AE, Lasaga AC (1988) The role of surface speciation in the low-temperature dissolution of minerals. Nature 4 431-433... 

Soil geochemistry is widely applied in mineral exploration, and with advancing knowledge of speciation and residence phases of trace elements in soils, a variety of partial and selective extractions for chemical analysis have been developed over the past decades. Each of these methods has been designed to target and dissolve only those elements that are adsorbed onto labile phases in soil, from carrier fluids and gases that transported them from a deposit to the surface (e.g. Hall etal. 1996). 

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 ... 

The concentrations of the four A1 species occurring in each layer of the soil profile are expressed as a percentage of the A1 content of the specific soil layer. While Alpstot increased with depth of the profile, the contribution of the operationally defined A1 species towards speciation however decreased with depth (Figure 3a-d). The maximum differentiation of Alpstotas A1, A1, A1, and A1 was 8.51% which was observed in the surface layer while the mean contribution was 5.43 1.60%. On the contrary, in 37-57 cm depth which had the most abundant Alpstot, the mean share of the A1 species (2.72 0.75%) was the lowest contribution to A1 in all the layers of the soil profile. This implied that pseudo total A1 in these depths are predominantly bound as silicates and hence are not available for speciation under the experimental conditions. 

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. 

Manceau, A. Lanson, B. Schlegel, M.L. Harge, J.C. Musso, M. Eybert-Berard, L. Haze-marm, J.-L. Chateigner, D. Lamble, G.M. (2000) Quantitative Zn speciation in smelter-contaminated soils by EXAFS spectroscopy. Am. J. Sd. 300 289-343 Manceau, A. Nagy, K.L. Spadini, L. Ragnars-dottir, K.V. (2000 a) Influence of anionic layer structure of Fe-oxyhydroxides on the structure of Cd surface complexes. J. Colloid Interface Sd. 228 306-316... 

Alvarez E, Perez A, Calvo R. 1993. Aluminum speciation in surface waters and soil solutions in areas of sulphide mineralization in Galicia (N.W. Spain). Sci Total Environ 133 17-37. 

Boudot J-P, Merlet D, Rouiller J, et al. 1994. Validation of an operational procedure for aluminum speciation in soil solutions and surface waters. Sci Total Environ 158 237-252. 

Lu JY, Chakrabarti CL, Back MH, et al. 1996. Speciation of some metals in river surface water, rain and snow, and the interactions of these metals with selected soil matrices. J Anal Atom Spectrom 11 1189-1201. 

Rodushkin IV, Moiseenko TI, Kudryavtseva LP. 1995b. Changes in trace element speciation in Kola Morth surface waters during snow melt. Water Air Soil Pollut 85 731-736. 

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