Environmental Engineering Reference
In-Depth Information
An example is arsenic removal from water. The maximum content of arsenic in drinking
water is set to 10 to 50 μg of arsenic per liter, depending on the country. Arsenic may be
present under two oxidation states: As(III) and As(V), this last one having higher afinity
with iron oxide or manganese oxide than As(III). In critical water, As(III) will be irst oxi-
dized to As(V). Then, mixing of the water low containing As(V) with a supercritical water
solution containing iron or manganese ions will lead to Fe-As or Mn-As nanooxides. For
instance, Mn
3
(AsO
4
)
2
oxide is formed with manganese.
3.2.4 Process
The process for water remediation in the supercritical state is based on the heating of water
containing organic pollutants near the critical point. The process steps are indicated in
Figure 3.2.
The process is generally driven in a continuous mode, and all the steps are conducted in
a tubular reactor. This allows a very small volume of hot pressurized water and reduces
the risks linked to the handling of a large volume of supercritical water. The reactor can be
very compact, and eficient heat exchange can be performed to decrease the energy intake.
Technically, the process can be achieved in two ways:
• First, by heating the low of polluted water up to the critical point of water (Figure
3.3a). The process steps are as follows:
1.
Compression from the atmosphere up to the critical pressure of 221 atm
2.
Heating from room temperature to the critical temperature of 375°C
3.
Cooling down to room temperature
4.
Pressure release
Oxidizing radicals are produced in the hotter part of the heating device. With
the increase of temperature along the heating device, the concentration of radicals
increase and more oxidizing radicals are formed. Then, the oxidation of the organ-
ics starts with the less stable organic molecules and goes on with the more stable
Pressure
(atm)
Liquid
(water)
Supercritical state
2
221
Critical point
221 atm, 375°C
3
Solid
(ice)
1
4
0.006
Room
conditions
Gas
(water vapor)
Temperature
°C
375
0.01
FIGURE 3.2
Process steps in the water phase diagram. 1: compression; 2: heating; 3: cooling; 4: pressure release.
