PolyQuat and Shocking with Chlorine

[chem geek]

I received the following response from Buckman Laboratories:

Mr. Falk:
 
Sorry about the delay in getting back to you.  I've been checking into some
literature references and trying to contact some of our water treatment
specialists to find some answers to your questions.
 
1)    The Polyquat 60 product that you mentioned is typically used along with
chlorine at normal use concentrations (low ppm levels of both substances) in
swimming pools.  Chlorine provides a faster kill of bacteria, while the
Polyquat 60 provides better control of algae as well as better water clarity.
The different advantages of these two pool treatments complement each other.
 
2)    While chlorine and Polyquat 60 have been shown to have very little
interaction at low concentrations, there can be some reaction or degradation
at higher concentrations.  For example, Polyquat 60 and bleach should not
be mixed neat, as an undesirable reaction would be expected to occur.
 
3)    In an alkaline solution containing an oxidizing agent, the polymer that
comprises the active ingredient in Polyquat 60 can be cleaved at the sites of
the quaternary ammonium groups, forming shorter polymer chains.  These
shorter chains are still cationic; and, according to some internal lab data
here at Buckman Laboratories, these shorter chains retain their biocidal
activity.  As the chains become shorter, the Polyquat test kit will not longer
respond to and detect them; however, the lack of a response by the test kit
does not necessarily indicate that the biocidal activity has been lost.
 
4)    In our recommended approach for winterization, the pool should be
"shocked" with about 5-10 ppm chlorine to destroy any lingering oxidant
demand, to remove any residual ammonia or chloramine, etc.  After a
couple of days, adjust the pH back to 7.2-7.6 and add Polyquat 60 at the
maximum rate allowed by the label.  This approach allows you to have the
maximum benefit from both chemicals.
 
If you have any other questions, please do not hesitate to contact us.
 
Tom McNeel
Principal Scientist
Buckman Laboratories International, Inc.

So there does appear to be consumption of chlorine by the PolyQuat (shock chlorine has high FC levels and increases the pH of the pool) and this is probably cleavage at the Nitrogen site, but the remaining chains are still effective as an algaecide. However, the recommendation is to first shock with chlorine (mostly to get rid of most organics and/or ammonia in the pool) and then wait for the FC to return to normal levels and make sure the pH is in the normal range. Then, add the PolyQuat in the recommended amount for closing the pool.

In addition to the E-mail, I talked with Dr. McNeel and asked about the method of action of PolyQuat on algae and he said that the cationic nature of the molecule (that means it has positive net charge) binds to and interferes with the negatively charged cell exteriors of most algae and bacteria and prevents the normal transport of food into the cell and waste out of the cell. This process is somewhat slow so it's fine for inhibiting algae, but not quite fast enough to quickly kill bacteria so chlorine is still required for disinfection. This method of action is for the hydrophillic (i.e. "water loving") PolyQuat molecules. There are other quarternary ammonium compounds that are hydrophobic (i.e. "water hating") that operate with a different method of action, but we didn't get into details on that.

He also said that while PolyQuat inhibits and kills algae, it does not harm people, either through skin exposure nor consumption (i.e. drinking pool water that has some PolyQuat in it). [EDIT] Dr. McNeel sent a follow-up E-mail saying that after adding PolyQuat, the circulation pump should be kept running for at least 24 hours to thoroughly distribute it through the pool. My guess is that the heavy molecular weight of the PolyQuat (it's a long-chain polymer) makes it slower to diffuse so lots of circulation is needed. [END-EDIT]

Thanks,
Richard