Environmental Engineering Reference
In-Depth Information
contrary, when the liquid water is heated to high temperature (B) under constant pressure,
its density decreases and the liquid becomes less and less dense. Above a given tempera-
ture that depends on the pressure, the gas is the stable phase. Above a point located at
221 atm and 375°C, the liquid and the water vapor are the same phase. There is no more
difference between the gas and the liquid. This point is called the critical point. The den-
sity of the luid at the critical point is around 0.32 g/cm 3 .
3.2.2 Supercritical Water and Organic Compounds
Close to the critical point and above, radicals are formed by the dissociation of the H 2 O
molecule. The dissociation reactions depend a lot on the species present in the water. The
addition of even a very low amount of an oxidizing or reducing product changes the reac-
tions and the radicals formed. Highly oxidizing radicals are produced and then react with
organic species. This is the principle of water remediation in critical water. We will see
later than the use of nanocatalysts can contribute to the oxidation of organic compounds
in subcritical water and that a remediation treatment can be performed in these “softer”
conditions.
In these conditions, even the more stable organic molecules are oxidized to produce
simple molecules such as H 2 O, CO 2 , N 2 , and heat. In particular, all types of hormones and
drugs are degraded to simple molecules in supercritical water. When the pollutant is an
organic-inorganic material, the organic part is broken down to H 2 O, CO 2 , and N 2 , and the
inorganic part generally precipitate as an oxide. The presence of sulfur or phosphorous
leads to the production of SO 2− and PO 3− ions.
3.2.3 Sequestration of Toxic Elements in Nanoparticles of Complex Oxides
An important parameter is drastically changing with the temperature of water: the dielec-
tric constant. The relative dielectric constant ε r of water decreases from room temperature
to the critical point. Table 3.1 gives ε r values for water.
As ε r decreases, the solubility of nonpolar or slightly polar molecules increases. At the
critical point, water is no longer a polar solvent and all the ionic species dissolved in water
will precipitate. As the medium is highly oxidizing, oxides will be formed and complex
oxides can be formed depending on the ions present in the water.
A speciic interest is in the sequestration of toxic elements such as lead, cadmium, mer-
cury, or arsenic. These elements easily form complex oxides with iron, manganese, tita-
nium, aluminum, etc. The process to remove these elements from water is to precipitate a
solution containing the toxic element and an element making the structure of the oxide.
Complex oxide nanoparticles (or a solid solution of the toxic element in the oxide structure)
are formed in the critical water conditions and can be removed by iltration.
TABLE 3.1
Values of Relative Dielectric Constant ε r of Water
Temperature (°C)
Pressure
25°C, ε r
300°C, ε r
375°C, ε r
400°C, ε r
1 atm
79
300 atm
80
22
10
6
 
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