esm222_04_5_equil

esm222_04_5_equil - ESM 222 Pollutant-Water Equilibrium...

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1 © Arturo A. Keller ESM 222 Equilibrium Distribution of Pollutants in the Environment 2 © Arturo A. Keller Pollutant-Water Equilibrium ± Solubility in Water ± Non-ionic compounds ± organic compounds except acids, bases and some alcohols and aldehydes ± most gases ± Ionic compounds ± acids and bases ± many inorganic compounds 3 © Arturo A. Keller Pollutant-Water Equilibrium ± Non-ionic compounds ± Aqueous solubility is usually available in handbooks ± Older handbooks may indicate “ND” = non-detectable, when in fact they are slightly soluble and toxic even at that low level (e.g. alkylates, PCBs, pesticides) 4 © Arturo A. Keller Pollutant-Water Equilibrium from Schwarzenbach et al., 1993 5 © Arturo A. Keller Pollutant-Water Equilibrium ± Effect of salts (Na + , K + ,Ca 2+ , Mg 2+ , Cl - , SO 4 2- , HCO 3 - ) ± a concern in saline environments (e.g. seawater, salt lakes, wetlands, brines) ± salts always decrease the aqueous solubility of nonpolar organics ± the “salting out” effect depends on the compound; in general the more soluble it is the more it is “salted out” (i.e. its solubility decreases more) 6 © Arturo A. Keller Pollutant-Water Equilibrium ± Calculation of mass of pollutant in solution is: M x,w = V w C sat x,w M x,w = mass of x in water (g) V w = volume of water (m 3 ) C sat x,w = solubility of x in water (g/m 3 )
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7 © Arturo A. Keller Pollutant-Water Equilibrium ± Effect of other organic compounds ± If the other organic compound is present in large amounts (> 10% by volume), then it is a cosolvent => large effect, increasing dissolution ± In very small amounts (< 0.1% by volume), it is a cosolute => no effect 8 © Arturo A. Keller Pollutant-Water Equilibrium from Schwarzenbach et al., 1993 9 © Arturo A. Keller Pollutant-Water Equilibrium ± Dissolution of mixtures ± gasoline, diesel, jet fuel, PCBs, gasoline with MTBE or EtOH ± the more soluble compounds (e.g. benzene) will dissolve first and may be cosolutes/cosolvents for others ± the remaining organic mixture has the heavier (MW) molecules 10 © Arturo A. Keller Pollutant-Air Equilibrium 11 © Arturo A. Keller Pollutant-Air Equilibrium ± If we keep an organic compound in a bottle, it is usually in the liquid or solid phases ± If we open the bottle, most organics will slowly evaporate completely into the available air volume ± The tendency to transfer to the gas phase is measured by the vapor pressure, P sat , which is typically measured in atmospheres (atm). 12 © Arturo A. Keller Pollutant-Air Equilibrium ± At the normal boiling or sublimation (evaporation from the solid to the gas phase) point of a compound P sat = 1 atm ± Below the boiling or sublimation point, we can find the molar (volume) fraction of the compound in the gas phase, y, from P sat : y = P sat / P
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13 © Arturo A. Keller Pollutant-Air Equilibrium ± Consider only a fixed volume of air in contact with the organic phase ± for short term (not allowing transport) ± confined conditions (closed bottle, soil vapors) ± amount of organic in the air phase: Mass of pollutant in air at equilibrium (kg/m 3 ): M x,a = V a C sat x,a
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esm222_04_5_equil - ESM 222 Pollutant-Water Equilibrium...

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