Learning from Surprises III – Foam Fractionation

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?