Spring 2011 Closure
***Q. I read in previous student’s question that the 4-phase model includes NAPLs in addition to soil, air, and water. What are some examples of NAPLs? Or does this term usually only pertain to petroleum? What about natural gas? What do you call contaminated coal or do you just consider it soil and factor in the large organic content?
A. Petroleum is the most common NAPL, but any chemical that separates from water and floats is an L-NAPL, light NAPL. There are also D [dense] NAPLs that sink, such as the chlorinated solvents.
** Q. Fugacity. What is the definition of fugacity and what is the difference between fugacity and solubility? Does fugacity just refer to chemical partitioning based on media?
A. Fugacity was a chapter in the back of our thermodynamics book that none of the professors ever got to. Chemical engineers do spend some time with it, but no one else. I refer to Dr. Mackay and the “Patron saint of fugacity.” He proselytizes for constantly. Now the ouch, since I should be able to explain it. Fugacity refers to the “[chemical] activity” or the “pressure” that tends to cause a substance to partition out of one media. It is related to solubility and everything else, but is different. Go to http://en.wikipedia.org/wiki/Fugacity and glance at the equations, then let’s go skiing.
** Q. In sub module 3A page 5, how will the system behave if the lake is not well stirred and how the concentration of lake will very?
A. A “well stirred model” means that the concentration leaving is the same as the concentration in the system. It is an assumption required in order to use the mathematics. If it is not well stirred, it becomes a much more complicated problem. The concentration in that case would vary with location, both vertically and horizontally.
* Q. I had a question / observation about how chemical concentrations decrease with time. I was wondering how the solubility of the chemical in the water would affect concentration rate. Also, it would seem that a chemical with a density higher than water would just sink to the bottom and stay there. I realize that the C = C0e-Q/V t equation works for an ideal situation. How would you even begin to figure out the concentration rates of things like PCB’s in the Hudson River which have been there for decades already?
A. Simple, you wind up with a “k” that is very very small – a half-life of thousands of years. If it sunk like a ball bearing, the concentration in the water would be zero (for the heavier PCB that is true), but the concentration in the sediment would go up.