DESORPTION OF COPPER FROM ILLITE CLAY USING A CHELATING RESIN ENCAPSULATED BETWEEN POROUS MEMBRANES
Toxicity studies have established the need for information concerning species distribution of heavy metals in the environment. In aquatic environments, the nature of trace metals, whether free, adsorbed, or complexed controls impact on biota. Often, major portions of the trace metals present in an aquatic system are immobilized on suspended particulates. Changes in environmental conditions such as pH, soluble ligand concentration and type, and concentration of suspended particulate material in the water column, can affect speciation of a metal. Filter feeding organisms are especially susceptable to these environmental changes. High bioconcentrations of trace metal species which are available to organisms can retard growth or cause mortality as well as posing a hazard to human ingestion. In an attempt to investigate the bioavailability of particulate-bound metals under varying environmental conditions, we have developed a syringe drive system employing a membrane sandwiched chelating resin. The device consists of a cassette containing 100 mg of Chelex-100 sandwiched between two 0.40 um Polycarbonate membranes. While the device is similar to the one described by Kerfoot and Vaccaro (1975), it has been modified from a passive system dependent upon diffusional processes to an active mode in which a syringe draws solution through the cassette and then expels it to the bulk solution. Free metal ions, metal complexes in solution, and some particulates which are smaller than 0.40 um pass the membrane and are retained in the cassette. The construction allows a great deal of versatility in choice of membranes, their porosity, and the chelating resin used. The exchange rate with the primary solution may also be varied within a large range by appropriate selection of the step-function speed reducer connected to the drive motor. This versatility enables the experimenter to match the performance of the device to his needs. Early experiments with pure copper nitrate solutions established the reproducibility and effectiveness of the method as a trace metal scavenging technique. Mathematical simulation results were matched closely by experimental data obtained with atomic absorption spectrometry. To study the release characteristics of trace metals, illite clay was "contaminated" with known amounts of trace metals and suspended in aqueous solutions. Continuous agitation was provided. As the cassette removes metal from solution, the clay is driven by equilibrium considerations to desorb more metal ions to the system. In numerous trials, it has been possible to achieve mass balance for metal collected in the cassette compared to metal lost by the clay. The desorption curves generated at various pH levels show features which can be interpreted to yield speciation characteristics of the solutions. Ion selective electrode work in conjunction with atomic absorption spectrometry has aided in elucidating the ratio of free to total trace metal content of the solutions. Results from this research may be utilized in predicting the impact of the changes in environmental variables such as pH and the disposal of metal-rich wastes.
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