Metal removal with chelating agent

Pamela Slavings of the Dow Chemical Company in Midland, Michigan, performs the metal analysis required following the removal of metals with chelating agents. 

As an alternative to acid washing, soils can also be flushed with chelating agents. Examples of effective chelating agents include ethylenediaminetetraacetic acid (EDTA) and nitrilotriacetic acid (NTA), both of which readily bind and solubilize metals. Using this approach, Peters Shem (1992) have recently reported on the removal of lead from a contaminated soil. In this study, 0.1M EDTA removed 60% of the lead in a soil containing 10 000 mg lead/kg. 

It is possible in many metalloenzymes to substitute the native metal ion with another first transition ion either by removing the original ion with chelating agents or by exchange dialysis. In addition to certain interesting physical properties that can be studied by this method, an opportunity is provided to discover what kind of specificity for catalysis resides in the electronic properties of the metal ion. Studies of this type have been conducted with several metalloenzymes. 

A complete removal of oxygen in aqueous solution is not feasible and thus in practice, the metal ions are frequently complexed with chelating agents (EDTA, disodium edate, citric acid, etc.). 

A metal atom is essential to the catalytic activity of carboxypeptidase A (53). The enzyme, as isolated, contains one gram atom of zinc per molecular weight of 34,600. Removal of the metal atom, either by dialysis at low pH or by treatment with chelating agents, gives a totally inactive apoenzyme. Activity can be restored by readdition of zinc or a number of other divalent metal ions (Table VII). The dual activity of carboxypeptidase towards peptides and esters is quite sensitive to the particular activating metal ion. Thus, the cobalt enzyme has twice the activity of the native zinc enzyme toward peptides but the same activity toward esters. Characteristic peptidase and esterase activities are also observed for the and Mn enzymes as well while the Cd ", Rh ", and Pb " en-... 

Lead can accumulate in the bones and other body tissues unless removed soon after ingestion. In some cases, treatment with chelating agents such as EDTA has been used to remove lead, mercury, or other heavy metals from the body. Discuss the advantages and disadvantages of such treatment. Include both thermodynamic and kinetic arguments in your answer. 

Indications of a structural role for metals in a multichain enzyme were actually noted nearly a decade ago e.g., the removal of zinc from yeast alcohol dehydrogenase with chelating agents resulted in dissociation of the protein into subunits (6). During the intervening years, additional investigations have suggested that this structural role for metals may be rather common and, indeed, have important implications to enzymatic function. 

Unsaturated fatty acids undergo a chemical change known as auto-oxidation. The process requires oxygen (air) and is accelerated by the presence of trace metals. Vegetable oils resists this process because they contain antioxidants, such as tocopherol. Fats and oils often are treated with chelating agents such as citric acid to remove the metal catalysts. 

This was significant in the preparation of l,2-dimethyl-3-hydroxy-4-pyridone, employed clinically as an iron chelating agent. The aminoreductone is obtained by reaction of methylamine with maltol. Traces of metal within the system readily form highly colored complexes with reactant or product and these are difficult to remove. With the CMR, the preparation was achieved in 65 % yield without the need for decolorizing charcoal and the product was crystallized by collecting the effluent in acetone (Scheme 2.10) [22]. 

The most obvious treatment of poisoning from excessive metal exposure is to remove the metal from the body, thus the development of chelating agents. While treatment may be necessary, it is far more desirable to prevent exposure. In fact the best treatment for low-level exposure is often to identify the source of exposure and eliminate contact with the metal. An excellent example of this principle is lead, where the most important action is to reduce or eliminate exposure. 

Amines such as diethylamine, morpholine, pyridine, and /V, /V, /V, /V -tetramethylethylene-diamine are used to solubilize the metal salt and increase the pH of the reaction system so as to lower the oxidation potential of the phenol reactant. The polymerization does not proceed if one uses an amine that forms an insoluble metal complex. Some copper-amine catalysts are inactivated by hydrolysis via the water formed as a by-product of polymerization. The presence of a desiccant such as anhydrous magnesium sulfate or 4-A molecular sieve in the reaction mixture prevents this inactivation. Polymerization is terminated by sweeping the reaction system with nitrogen and the catalyst is inactivated and removed by using an aqueous chelating agent. 

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