Environmental Engineering Reference
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20. Eq. 7.10 and Eq. 7.20 are used to describe the liquid-liquid extraction and vapor-liquid
equilibrium, respectively. Illustrate the similarity between these two formulas.
21. Which of the following chemical(s) need azotropic distillation for its separation from an
aqueous sample: (a) hydrophobic pesticide, (b) acetone, (c) acetonitrile, (d) methyl
isobutyl ketone, and (d) pyrene.
22. A clean soil sample was used as a matrix to develop an extraction method for pyrene (a
hydrophobic compound), using methylene chloride (MECl) extraction. The soil has no
previous contamination of this compound, so the background concentration of pyrene
can be assumed zero. This analyst spiked 1.025 g soil sample with 5 mL of 10 mg/L
pyrene (dissolved in acetone) stock solution, let the acetone evaporate under hood, and
then extract into 10 mL MECl with a sonic extractor. The soil extract was then
determined to contain 2.0 mg/L pyrene by HPLC method. Calculate the % recovery
of the extraction. Is this an acceptable extraction method?
23. 100 mL of aqueous solution containing 2,4,6-trinitrotoluene (TNT) at an initial con-
centration of 0.05 ppm is shaken with 10 mL hexane. After phase separation, it was
determined that the organic phase has 0.4 ppm. What is the partition coefficient, and
what is the percent extracted? If the equilibrium concentration (rather than initial
concentration) is 0.05 ppm, this would simplify the calculation. What would then be the
partition coefficient and percent extracted?
24. Liquid-liquid extraction (LLE) has long been a standard method for concentrating
hydrophobic organic compounds such as pesticide and polycyclic aromatic hydrocar-
bons (PAHs) from water. A sample of water contaminated with a pesticide is extracted
with methylene chloride for determination of this pesticide.
(a) In experiments with a standard solution of a known concentration, it is found that
when a 100 mL water sample is extracted with 30 mL of methylene chloride, 75% of
this pesticide is removed. What is the partition coefficient (D) of this pesticide?
What fraction of this pesticide would be removed if three extractions of 10 mL of
methylene chloride each were done instead?
(b) The extraction efficiency of LLE depends on the hydrophobicity (hence D) of the
chemical to be extracted as well as the volume and number of extraction.
Demonstrate this by filling in the blanks of the table below regarding the total
extraction efficiency (assume water sample volume¼100 mL).
Extraction efficiency if D (unitless) ¼
10
25
50
100
1,000
One extraction with 30 mL
Three extractions, 10 mL each
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