Treatment of Complexed Metals

Complexed metals form a group of wastewater pollutants that contain complexing agents. Complexing agents prevent the metals from being precipitated. In fact, almost all groups of wastewaters from metal finishing operations contain inorganic and organic complex formers that may interact or interfere with many of the treatment methods. In some cases, metals can be effectively removed independently of how strong the metal binding is, as in the case of sulfide precipitation. Therefore, complexed metal definition is made relative to a reference. This reference is mainly hydroxide precipitation. If a wastewater containing complexing agents cannot be treated to remove metals by hydroxide precipitation within the limits of usual operation, it is considered to be complexed metal wastewater. While some weak complexing agents such as citrate and tartrate may not interfere with hydroxide precipitation [30], most of the ligands presented in Table 6 modify the precipitation performance to yield unacceptable effluent metal concentrations. Many of these complex formers are chelates, which bind the metals in more than one position to form stable structures. Because treatment of complexed metals tends to be expensive, they need to be segregated. The treatment methods applicable to complexed metals wastewaters can be classified as:

• pretreatment followed by hydroxide precipitation;

• modified hydroxide precipitation; and

• other methods of treatment.

Pretreatment of complexed metal wastewaters aims to retain hydroxide precipitation as the main or central treatment. Thus, the purpose of pretreatment is to destroy the complexing agents or to convert them into a form or into compounds such that they cannot interfere with the precipitation. One of the methods being used is the addition of complex breakers such as sodium dimethyldithiocarbamate (DTC). This method may be expensive and some complex breakers may have toxic effects on the environment. Another pretreatment method is chemical reduction, which is based on a complexing equilibrium. Many of the complex formers are weak acids, which convert to their acid forms at low pH, thus freeing the metal ion. If the metal is reduced at this pH, using chemical reduction agents, to an oxidation state in which it cannot re-combine with complexing agents, then it can be separated by conversion into a compound that is separable by precipitation or by using other means.

Oxidation is another method aimed at the destruction of complex formers, generally not to mineralization, that is, carbon dioxide and water, but to structures for which they have no capacity of complexing or at least to a much weaker extent than the initial organic matter. Strong oxidants such as chlorine or ozone are used for oxidation. Existing metals may serve as catalysts. Oxidation may be conducted at pH values, usually acid, beyond the range of hydroxide precipitation. The method may be useful in removing organic priority pollutants. Selective ion exchange, as in the case of ammonia removal by clinoptilolite, may also prove useful [31].

Modified hydroxide precipitation is based on the addition of chemicals to which complexing agents preferentially bind, allowing freed metal ions to be precipitated. The theory and application basis of the hydroxide precipitation of complexed metals is given in the literature [32]. The added chemicals are usually metals having affinity to the complexing agents at least to a comparable extent with those of the metals to be precipitated. The added metal generally does not have a greater capacity than the existing metals. Therefore, it cannot favorably compete with the existing metals at the usual pH values for their hydroxide precipitation, but increasing the pH shifts the equilibrium in favor of the added metal and causes the existing metals to be freed and precipitated as hydroxides. This application is therefore also known as high pH precipitation [3,32]. The mechanism is explained by the ligand-sharing effect of the added metal. Figures 7 and 8 illustrate the mechanism by which ligand sharing is carried out by calcium. These figures were drawn to present ligands of varying strength [33]. In Figure 7, succinic acid is seen to increase the cadmium concentration at pH 9.0, particularly, for high concentrations of succinic acid. The ligand-sharing effect of calcium at this pH does not seem to be effective. However, increasing the pH to 11.0 causes the succinic acid to be bound almost totally by calcium, and cadmium solubility returns to its noncomplexed state. Succinic acid is a relatively weak ligand and the pH elevation needed to completely overcome its effect is up to the usual optimum pH of cadmium. For this case, an extra pH increase is not needed, as long as calcium is used for ligand sharing. However, for NTA, a strong complex former, the pH increase needed for cadmium to turn back to normal solubility is seen to be one unit more than the optimum pH. The effect of calcium and increased pH on hydroxide precipitation of nickel is seen for the common strong ligands of NTA and EDTA in Figure 8 [33]. Calcium is one of the strongest ligand-sharing metals. The ability of other common metals for ligand sharing action has been investigated [34]. Calcium, ferrous and ferric ions, Mn2+ and Mg2+ were theoretically

Figure 7 Cadmium-succinic acid and NTA. (From Ref. 33.)

Figure 7 Cadmium-succinic acid and NTA. (From Ref. 33.)

Figure 8 Nickel-EDTA and NTA. (From Ref. 33.)

and experimentally evaluated in terms of their effectiveness in precipitating complexed metals. Calcium is found to be the most effective, while Fe2+ and Mn2+ have limited capacity. Fe2+ was also reported to be useful in the destabilization of the Cu-EDTA complex [35]. However, as noted in the literature, Fe2+ and Mn2+ are readily oxidizable at alkali pH, thus losing their ability to bind complex formers [34].

The use of other methods that are less affected or not affected by the existence of complexed metals is the third alternative. Sulfide precipitation is the best precipitation method applicable to complexed metals [3]. Membrane processes may be used for the separation of complexed metals [3], and developing methods of adsorption and ion exchange may also prove useful for this purpose.

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