Geochemical Speciation and Sorption

A contaminant can occur in many different forms in an environmental compartment, each of them having different mobility, transfer coefficients to and between living matter, and even toxicity. This variety of existing forms for a given element is termed chemical speciation, defined as the distribution of one or more elements between all its possible species (distinct chemical entities) in a given system. It should be noted that the term speciation is also used in another context to describe all the experimental methods applied to investigate the above discussed species distribution. 

The various processes listed above can be described quantitatively by their respective functional terms, each requiring a unique set of parameters. Fortunately, many parameters are used simultaneously for many or all of the above reactions, like temperature or concentrations. In general, the parameters can be divided into system-specific parameters (subdivided into the stationary state and the dynamic evolution) and into reaction-specific parameters. Many of these parameters also depend on the chemical or physical models applied to the system. The resulting very complex pattern usually has to be simplified when it comes to integrated modelling exercises of contaminant reaction paths and migration, based on coupled transport codes. 

Whereas the simplest (and older) sorption models do not distinguish between the basic processes contributing to the overall sorption, newer model approaches try to address all relevant processes separately, namely physisorption, chemisorption, coprecipitation, inclusion, diffusion, surface-precipitation, or the formation of solid solutions. Sorption models in a strict sense are usually grouped into two classes, the phenomenological models, and the surface complexation models. 

Phenomenological adsorption models comprise different flavours of the afore mentioned distribution coefficient (K j) model. The K j framework is built on the concept of distribution (or retardation) coefficients. This is defined as the experimentally determined ratio of the sorbed (fixed, immobilized) and unsorbed (free, truly dissolved) fraction of a component (chemical element) under equilibrium conditions. 

Sorption isotherms at constant pH (most often used types are Langmuir, Freimdlich, and Frumkin) are a step toward a more realistic description of surface phenomena, expressing the Kj as a function of sorbent concentration. Their to the best semi-empirical foimdations allow them to be rather simple in terms of defining equations and amoimt of model parameters. Of course this often does not reflect reality with the necessary accuracy, so their usefulness is restricted to only a few cases. 

---

Fig. 15.5. Calculated metal sorption curves for Pb, Cu and Cd onto the bacterium Bacillus subtilis, shown as a function of pH versus the concentration of sorbed metal. Curves are calculated based on experimental metal sorption data of Fein et al. (1997), and were computed using the geochemical speciation programme JCHESS. The solution depicted contains 1 g 1 bacteria dry wt (155 m g surface area, 8.0 Cm electrical double layer capacitance), 1 mM dissolved CaC03 and 1 iM dissolved lead, copper and cadmium. Adsorption was calculated using a CCM treatment.

Geochemical speciation combined with ion exchange Sorption edges and sorption isotherms MINTEQ Study of sorption of divalent metals in calcite [8]... 

Langmuir, D., Techniques of estimating thermodynamic properties for some aqueous complexes of geochemical interest, in Chemical Modeling in Aqueous Systems Speciation, Sorption, Solubility and Kinetics, Jenne, E.A., Ed., ACS Symposium, American Chemical Society, Washington, DC, 1979, pp. 353-387. 

Mattigod, S. V. and Sposito, G. Chemical modeling of trace metal equilibria in contaminated soil solutions using the computer program GEOCHEM, j[n Jenne, E.A., ed., "Chemical Modeling in Aqueous Systems. Speciation, Sorption, Solubiliiy, and Kinetics." Amer. Chem. Soc., 1978 (This volume). 

The redistribution and availability of mercury in the aquatic environment is directly affected by the role suspended and bottom sediments play in the adsorption-desorption process. Sorption phenomena are affected by a variety of bio-geochemical factors including, but not limited to, mercury speciation, associated ionic concentrations, oxygen concentrations, H2S availability, the pH and Eh of the bulk and interstitial waters, bacterial activity, chloride concentrations and sediment type and size. 

Such Fe(in) and Al speciation has important implications for the geochemical evolution of AMD. At typical conditions of pH (1.5 ) and sulfate concentrations (0.01-0.1 M SO "), the mineralogy of the precipitates will be dominated by sulfates and hydroxysulfates, instead of oxides or hydroxides, as described in the next section. Further, this metal speciation can imply important differences in their sorption behavior. For example, at sulfate activities of 10 " to 10 , aluminum is present as free aqueous cation, being essentially conservative. However, at higher activities of the SO " anion (from 10 to >10 ), Al forms bisulfate anionic species (A1(S04)2) which can be sorbed onto positively charged mineral surfaces at low pH [14]. 

Experimental conditions must be controlled carefully to ensure that the data acquired are relevant to the geochemical system of interest. It is particularly important to control the concentration of carbonate in experimental solutions because the aqueous speciation of some key radioelements, such as uranium(VI) is extremely sensitive to this parameter and this can have a strong impact on sorption behaviour (Hsi Langmuir 1985). 

---