In previous blogs I discussed being surprised on at least two
occasions. One dealt with trying to
determine the ventilation rate in a newly constructed experimental room of bare
wallboard using C02 as a tracer gas.
The second covered the development of an indoor air model of a wood
preservative. These are available in
previous blogs in this series. This
week I want to talk about what was perhaps the biggest surprise I have received to
date. It deals with the measurement of
a very high level of airborne contaminant from a source that was not expected
to present anything near the level actually measured.
The product of interest was lipophilic and slightly water
soluble molecule (300 ppm w/w in water) which at 500 µg/m3 in air
was considered to be irritating to the upper respiratory tract. The product was being tested for its ability
to treat thousands of gallons of water in a plant. It was mixed into this large quantity of treated water systems at 5 ppm.
Part of the testing was to have IH monitoring of the
operators and various places within the plant where airborne exposure to the
product might occur.
Before the monitoring, I modeled the potential exposure and even
considering a reasonably large activity coefficient, I did not anticipate that we would have any
airborne product above about 50 µg/m3.
As you might imagine I was very surprised when one of the
results (and area sample) came back at approximately 600 µg/m3. Indeed, the head-space concentration of the
concentrated product was only about 900 µg/m3. So we had a measured airborne concentration
that was greater than 50% of headspace for concentrated product over an aqueous
solution of the compound at only 5 ppm or
0.005%!
We double checked everything and found no mistakes. I had to understand this result. I was not present during the sampling but I
trusted the IH that did the work. I felt
like I had no choice but to get on a 2 hour plane ride to the plant and
reproduce the work done by the IH. The
single high concentration sample was taken in a man-way located above a sump
which contained a majority of the treated water.
As soon as I saw the water in the sump and breathed the air
above it, I started to understand what was happening. There was a 3 inch layer of foam on top of
the water. If you put your head near the
surface you could hear the bubbles popping much in the same way as you can here
soap bubble pop in a bubble bath.
The data and the above observation of foam provided a hypothesis
as to what might be happening. The foam
was obviously a lipophilic or at least a surface active compound that was
present in the water along with the compound of interest. The compound, which has a relatively high
octanol-water partition coefficient (see previous blog on Kow) was partitioning
into this layer. The foam bubbles were
breaking or “popping” into the air above the foam.
As they did so they were releasing aerosol particles that were rich in
the compound. These resulted in a very
high concentration of airborne compound both as an aerosol and as a vapor as
the aerosol particles evaporated.
To test this hypothesis we sampled the foam. It had concentrated the compound by over 3
orders of magnitude above the concentration in the water so that there were thousands of ppm of the compound dissolved within
the foam.
After doing some research on this phenomenon, I found it has
a name: foam fractionation. I
presented it here just to show how Mother Nature can through us some curves on
occasion and when she does we get a chance to learn.
Questions for the LinkedIn Group:
Have you ever come across foam fractionation in your work?
What have been your biggest surprises in the realm of
chemical exposure assessment?