by Rudy Sedlak
The term "chelating agent", as used in the printed circuit board industry, has a much broader definition than the definition in text books. The PCB industry uses the term chelate to mean any agent that will form a compound with a heavy metal ion that is not precipitated in a conventional pH adjust/precipitation waste water treatment facility.
Other terms used to describe these "agents" that form these compounds include:
The scientific definition of a chelate is a Ligand that forms a compound with the metal that is a ring structure, like EDTA. This ring structure makes the compound one of the more stable coordination compounds (or ligands), and thus one of the more difficult to remove from solution.
Any free metal ion (M++) in solution has water molecules coordinated around it in this kind of manner:
M(H2O)x++
A ligand replaces these water molecules with another compound that forms a more stable structure. This prevents the metal ion from reacting with the OH- ion present in the precipitator at high pH. Since the metal ion is "tied up" tightly by the ligand, it cannot form the insoluble metal hydroxides [M(OH)2], which precipitate the metals out of solution.
There are many different ligands, for example:
Of these ligands listed above, only EDTA, Quadrol and Citrate are "chelates" in the strict scientific sense of the word, but all of them will carry heavy metals through most waste treatment systems, and thus are called chelates by the PCB industry.
The PCB industry treats waste streams with various chemicals to try to overcome the problem of ligands carrying metal through waste treatment. These chemicals can be broken down into various categories:
Precipitants, like Thiocarbamates, and sulfide form an insoluble compound that is even more stable than the chelate-metal compound, thus effectively "stealing" the metal away from the chelate, and dropping it out of solution. Precipitants can be highly effective, but it is important to realize that just because the metal has been removed, does not mean that the ligand is in any way inactivated. Thus, if the treated waste solution is later mixed with metal bearing solution, it will pickup and hold the newly introduced metal just as tightly as it did the old, and carry it through waste treatment just as easily.
Replacement agents (like ferrous sulphate) depend on the fact that ligands have a preference list for metals. For instance, given the choice of copper or ferrous Iron to react with, EDTA will react with Iron preferentially. This means that if enough ferrous Iron is present, it will tie up all the EDTA (and certain other Ligands), freeing the Copper to react with the OH- in the precipitator, and fall out of the solution as the Copper Hydroxide [Cu(OH)2)]. The key to making this work is to make sure that the replacement agent being used is preferred by all the ligands present over all the heavy metals that need to be removed.
Reducing agents (like Sodium Borohydride) work by converting the heavy metals from the water soluble ion form back to the metal, which then falls out of solution. This approach is also very effective, but again does not disable the ligand, thus leaving it free to pickup metals at some point down stream, and carry them through waste treatment.
Many products used in the PCB and metal finishing industries absolutely require ligands (chelates) to work and, in fact, the proper ligand (chelate) is frequently considered to be the critical factor in determining the performance of a product, thus is often considered, by the manufacturer, to be a closely guarded trade secret. Further, since few of the ligands are hazardous by themselves, it is not required that they be mentioned on the MSDS. This, of course, makes management of this problem by the user more difficult. The only way to reliably determine the presence of ligands is to take fresh, ready-to-use solutions, adjust them to the pH of the clarifier/precipitator and dose them with high levels of copper, or other heavy metals in use, and see what amount of the heavy metal stays in solution. And lastly, although it is hard to believe, "chelates" were not invented by sanitary district people to insure that they get to levy high fines on PCB shops for exceeding their discharge limits!
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