Most of the data detailing metals concentration in the environment is for total metals.  For many applications it is important to know the probable mocular form or chemical of the metal of interest as it may exist under a specified set of environmental conditions.  The particular chemical form of the elemental constituent may affect such chemical and physical parameters or its volatility, aqueous solubility, bioauailabiuty, toxicity, etc, and thus affect its impact on human and ecological health.  These same parameters may determine the way that the constituents will respond to remediation technologies.  An example is mercuric sulfide, a relatively nonvolatile, highly stable form of mercury that has a low bioavailability, which represents a much lower health risk than other forms of toxic mercury such as methyl mercury.  Similarly, hexavalent chromium is much greater hazard then trivalent chromium.

Liquid Samples

In the solution phase, speciation may involve the determination of the constituent of interest, the concentrations of various ligands that may form soluble complexes or insoluble mineral phases with the constituent of interest, anionic or cationic nature of the complex formed, and the measurement of a set of “master parameters” (e.g., pH value, redox condition, dissolved oxygen level, solution temperature, etc.) that control the overall chemical composition of the various probable constituents at equilibrium.  Often, these parameters may be entered into a predictive thermodynamic model to estimate the relative predominance of the possible chemical forms of the constituent of interest.  Similar predictions may be made for the solid phases that may exist at equilibrium.

Solid Samples

Particle speciation is an analysis method used to identify the host mineral or amorphous compound that contains a target element.  The target element is typically at a minor or trace concentration in the bulk sample.  Materials and Chemistry Laboratory, Inc. (MCLinc) uses x-ray diffraction (XRD) to determine the presence crystalline phases in samples.  MCLinc also uses scanning electron microscopy (SEM), transmission electron microscopy (TEM), in conjunction with energy dispersive spectroscopy (EDS), and selected area electron diffraction (SAED) to determine the size, elemental association, matrix association, and composition of particulates.  MCLinc also uses x-ray photoelectron spectroscopy (SPS or ESCA) to determine the valence state of the target element, which can lead to phase identification.

Often the first task is to employ one of a number of sample preparation techniques, which are used to increase the concentration of the host compound and thereby make identification more efficient.  If a sample contains a trace amount of, as an example uranium, a sample preparation scheme that concentrates the host compound such as density separation can enrich the sample by upwards of 100 times.  This will allow XRD, SEM, TEM, and XPS techniques to identify compounds at levels that were initially below the level of detection.

These speciation methods have been applied to a variety of projects including mercury speciation in stream deposits, permeable barrier characterization, and plutonium speciation in soil.