Speciation Studies of Metal Ions in Environmental and Biological Media Using Supported Liquid Membrane Extraction

In ecological risk assessment for chemical pollutants, it is important to quantify the concentrations of freely dissolved substances in aqueous samples for approximate characterization of the bioavailable fraction. Such measurements are important because the concentration and toxicity of a chemical in an organism’s body are linked to the bioavailable fraction and its chemical form within the surrounding environment. Metal ions, unlike organic pollutants, can be distributed in the surrounding environment in forms such as free-hydrated ions, dissolved inorganic and organic complexes, and metals associated with colloidal particles. Furthermore, some of these forms can exist in different oxidation states depending on the metal ion and environmental conditions. These include chromium(III) and chromium(VI), vanadium(IV) and vanadium(V), and arsenic(III) and arsenic(V). All of these forms play an important role in assessing the health hazards and toxicity of metal ions in ecosystems, and any analytical method must be based upon them. A good example is the contrast between chromium(III), a nutrient element used to control glucose and lipid metabolism in the cell membrane and chromium(VI), which is carcinogenic.1 However, many trace-metal analytical methods, such as inductively coupled plasma spectrometry or atomic-absorption spectrometry have no speciation capabilities. These methods demand sample preparation techniques capable of isolating the chemical and bioavailable form of the metal ion from the matrix before its determination. Speciation studies are therefore challenging, especially at trace levels and in complicated samples, such as biological fluids. Liquid membrane–based techniques have proved to be promising for speciation studies.2–4 Prominent among several types of liquidmembrane devices is the supported liquid membrane (SLM) technique.2,3