Chelates, Can't Live With 'em, Can't Do Without 'em
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:
Sequestering agent Complexing agent Ligand (the scientifically
correct term) Coordination agent
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:
Chloride (weak, but if present in large enough quantities it can work)
Ammonia (NH3) Monoethanolamine EDTA Citrate Cyanide Quadrol (Used
in some Electroless Copper baths)
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 Replacement agents Reducing agents
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 sulfate) 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! |
