Supercritical fluid extraction of iron and copper from aqueous samples

An understanding of the toxicity, bioavailability, bioaccumulation and transport of elements in the environment is vital to understanding ecological systems. This study investigates the movement of inorganic metal salts under modified supercritical fluid extraction conditions, in an effort to provide experimental guidelines for extraction of metallo-organics from natural samples. Experiments involved the extraction of iron and copper from aqueous samples deposited on filter paper. The effects of pressure (100-300 atm), temperature (44.6$\sp\circ$-74.2$\sp\circ$C), and modifier content on the extraction efficiency were determined. The introduction of modifiers by on-line injection or premixed with carbon dioxide, was evaluated by determining analyte recoveries. Wet and dry samples were tested for metal extractability. Results demonstrated that the physical state of the sample, and extraction parameter such as the type of modifier and its content, and pressure and temperature extraction parameters can determine the selectivity and efficiency of metal recoveries. CO$\sb2$ without addition of modifier gave low overall iron(III) recoveries. However, with careful selection of modifier (cosolvent) and extraction conditions, quantitative recoveries could be obtained. For instance, at 100 atm, 44.6$\sp\circ$C and with 10% (v/v) methanol in CO$\sb2$, iron was recovered almost quantitatively (84%). For copper, polar modifiers had little effect on extraction efficiency. Optimum extraction conditions are independent of percent modifier content. In general, extraction efficiencies are better for wet samples than for dry samples, for both iron and copper. Extraction experiments showed that copper can be recovered from wet samples with 5 mL supercritical carbon dioxide. The acidity of the samples and the oxidation state of the metal analyte have little effect on analyte extraction selectivity or efficiency. Higher modifier content is required for metal analyte extraction from dry samples than wet. Counter-ions have a small effect. For copper there is a selectivity factor. However, for iron, counter-ions that were tested have no effect on extraction efficiency.