Determination of Henry's Law Constants of Odorous Contaminants and Their Application to Human Perception


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Virginia Tech


Although utilities attempt to avoid offensive smelling compounds in consumer's drinking water, their efforts are often hampered by a lack of data or knowledge of the physical, chemical, and sensory properties of odorants. Many factors affect the ability of a consumer to detect odors, including: concentration, presence of chlorine/other odorants, temperature, and the individual's sensitivity.

This research developed a simplified static-headspace technique to determine Henry's Law constants at multiple temperatures and then use these data to calculate the enthalpy of solution so that new Henry's Law constants can be calculated at any temperature using the van't Hoff Equation. The method was applied to three taste-and-odor compounds of moderate water solubility (about 100 mg/L). 2-Methylisoborneol (2-MIB) is a methylated monoterpene alcohol that is produced by actinomycetes and blue-green algae and has a musty odor that is detectable at 4-10 ng/L water. Geosmin, also produced by actinomycetes and blue-green algae, has a detectable earthy odor at 5-10 ng/L. trans-2, cis-6-Nonadienal is enzymatically synthesized from poly-unsaturated fatty acids by diatoms like Synura and has cucumber and fishy odors detectable at 10-40 ng/L levels.

The new static headspace method uses standard glassware used in odor-analyses. 500 mL wide-mouth Erlenmeyer flasks were modified with septum sampling ports to measure vapor phase concentrations by SPME/GC-MS. Unitless Henry's Law constants were determined at multiple temperatures using the vapor and aqueous phase concentrations. From the Henry's Law constants, the enthalpies of reactions were calculated. For these compounds, the values for Henry's Law constants ranged from 0.002 to 0.02 for four temperatures between 20 to 45 °C with geosmin and 2-MIB having similar and higher values than for nonadienal. The constants increased with increasing temperature. The enthalpies of vaporization from the aqueous phase were determined to be in the range of 50-80 kJoule/mole.

The experiments were repeated with fulvic acid added to the aqueous media at different concentrations. The Henry's Law constants were decreased with the presence of fulvic acid; however no correlation between the concentration of fulvic acid and the decrease was observed. The decrease in constants for 2-MIB and geosmin were very small compared to nonadienal.

Finally the measured Henry's Law constants were used to predict gas phase concentrations of odorants for known aqueous concentrations of geosmin, 2-MIB, and nonadienal. The results were correlated to the human sensory data obtained from flavor profile analysis. The data demonstrated that as the gas phase concentration increased, the perceived odor intensity also increased, but only up to a certain point. The vapor phase concentration increased linearly as the aqueous phase concentration increased, but the FPA intensity increased at a lower rate and leveled-off. The increase in the FPA rating at 25ï °C was greater than at 45 °C although the vapor phase concentration was greater at 45 °C. For samples containing 400 and 600 ng/L geosmin, 400 and 600 ng/L, 2-MIB, 100 and 200 ng/L nonadienal, the increase in gas phase concentration did not increase the FPA ratings of the panelists.

It was concluded that, utilities will be challenged to assess and treat high concentrations of geosmin, 2-MIB, and nonadienal. Sensory analysis will not be predictive of aqueous or vapor concentration at high levels and may be misleading if used to determine a treatment strategy. Chemical analyses, especially solid phase microextraction technique is very effective in measuring these compounds even at low ng/L levels. The temperature-related Henry's Law constants can be used to assess remediation systems, human exposure and sensory perception by predicting gas phase concentration in a variety of situations, such as showering and washing dishes.



Drinking Water, Geosmin, 2-MIB, Henry's Law Constant, Taste-and-Odor, Nonadienal