solubility of organics

ENVIRONMENTAL GEOCHEMISTRY AT TEXAS A&M UNIVERSITY

http://environmentalgeochemistry.pbworks.com//

Solubility of Organics

Bruce HerbertGeology & Geophysics



Organic Contaminants In Aqueous Solution

■ Solubility of organic contaminants is a major factor determining their environmental behavior and fate.

■ We use thermodynamics to predict the speciation of a geologic system

■ Aqueous solubility: abundance of chemical per unit volume in aqueous phase when the solution is in equilibrium with the pure compound in its actual aggregation state (solid, liquid, or gas) at STP.



Solubility and Organic Structure



Thermodynamic description of solubility

■ Chemical potential in phase ■ At equilibrium, the chemical potential of the organic compound

is equal in the two phase

is the molar free energy of solution



Thermodynamic description of solubility

■ The mole fraction solubility, at equilibrium, would be given by

where is the activity coefficient of the organic liquid compound in water at saturation.



Solubility of Organics

■ Saturation can be expressed in terms of molar concentrations as

■ The solubility of an organic compound in water is inversely related to its activity coefficient, which is one of the most important environmental parameters to know.

■ Note that the activity coefficient, , is for a saturated solution which is not necessarily identical to the activity coefficient for a very dilute solution, because of possible organic-organic interactions in saturated conditions.



Molecular Description of Organic Solubility



Temperature Effects

■ Temperature effects on the aqueous solubilities of organic solids and gases are more important due to temperature effects on phase conversion.

■ where a constant and Vw have been assumed and units have been switched to molar units.

■ Solubility of solids increases with increasing T since the enthalpy cost of melting solids decreases at higher T.

■ Solubility of gases decreases with increasing T since the enthalpy cost of condensing gases increases at higher T.



Inorganic Salt Effect

■ The solubility of neutral, nonpolar organics always decreases in solutions of increasing ionic strength.

■This effect is termed salting out.

■The effect is due to the increased difficulty in creating a cavity in a solution of higher ionic strength, where more of the water molecules form complexes with the dissolved ions.

■ The salting-out effect is greater for salts that form stronger and larger hydration spheres.



Dissolved Organic Solutes and Cosolvents

Organic solutes and cosolvents include species such as dissolved humics, surfactants, and other organic solvents such as methanol

1. Other organic molecules are present in large abundances (> 10% by volume).

■These molecules then act as solvent molecules themselves and partially surround the organic molecule.

■This generally has large effect on increasing organic solubility.

2. The cosolvents are less abundant.

■The organic molecule can share hydration spheres with the cosolvent, increasing its solubility.

■Example: n-octanol at 4.5x10-3 M increases hexachlorobenzene (2x) and DDT (3x) solubility.

3. The cosolvents have very low abundance (10-3 volume fraction).

■Generally no effect on solubility.




■ Example: the effect of gasoline additives on BTEX solubility in groundwater

■ The EPA, through the Clean Air Act, is requiring the addition of oxygenates to be added to gasoline to increase the combustion of gasoline.

■ The oxygenates include methyl-tert butyl ether and ethanol. They are added to gasoline at rates about 10% by volume.




■ Solutions containing greater than 0.1% cosolvent increased BTEX solubilities with the greatest increases in solubilities for the more nonpolar organic solutes.

■ 0.1% cosolvent increase BTEX solubility roughly 1%, 10% cosolvent increased BTEX solubility by 100%.

■ MTBE had a greater effect on BTEX solubilities than ethanol or methanol.

■ High cosolvent concentrations could result in the subsurface from spills or a leaking UST to groundwater by infiltrating water.




■ Some molecules, called surfactants have special properties because they aggregate in solution to form nonpolar and polar regions.

■ Surfactants are organic molecules that have polar and nonpolar regions.

■ Surfactants aggregate in aqueous solutions to form micelles at the critical micelle concentration (CMC).

■ Partial aggregation can occur for some surfactants at lower concentrations when they form hemi-micelles.



Example: Soil Flushing with Surfactants

■ Soil flushing enhances contaminant recovery in conventional pump and treat systems by increasing the quantity of contaminant with the pumped water. This occurs because:

■ The interfacial tensions between NAPLs and water are reduced by the surfactants

■ The contaminant solubilities are increased by the surfactants



Soil Flushing with Surfactants

■ Soil flushing can be used for contaminant removal of low solubility contaminants, pools and discontinuous ganglia or globules of NAPLs, and sorbed contaminants.

■ The amount of contaminant mobilized depends on:

■ The chemical structure of the cosolvent or surfactant

■ The cosolvent or surfactant concentrations

■ Geochemical conditions

■ The chemical structure of the contaminant

■ The temperature Bacteria in sewage treatment plant releasing natural surfactants.

http://www.ethlife.ethz.ch/archive_articles/080715-nano_in_klaeranlage/index_EN



Soil Flushing with Surfactants

■ Limitations

■ Geologic conditions can limit soil flushing, especially those of low hydraulic conductivity.

■ Higher degree of porous media heterogeneity will also limit technique.

■ Permeability reduction, coating of aquifer surfaces, and water quality impacts through the use of cosolvents or surfactants can also be important.

solubility of organics

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