Environmental Engineering Reference
In-Depth Information
With the same assumptions the approaches (
7.13
) and (
7.14
) also lead to
ordinary differential equations:
2
@
c
@x
¼
ag
c
cþ
b
þ
ad
1
v
d
2
c
cþ
b
(7.24)
v
d
2
and
@c
@x
¼
1
c
cþ
b
2
1
(7.25)
respectively.
Sidebar 7.2: Gadolinium-DTPA Steady Transport and Degradation
Diethylene-triamine-pentaacetic acid (DTPA) is used in the pulp and paper
industry, where its application increased dramatically with the introduction of
H
2
O
2
as a substitute for the bleaching agent chlorine. Chelating agents,
particularly DTPA, are added to bind heavy metals and thus prevent the
decomposition of H
2
O
2
during the bleaching process (van Dam et al.
1999
).
After processing, DTPA remains a component of the effluent reaching sew-
age treatment plants and downstream surface water bodies such as rivers and
lakes, as well as connected aquifers. Finally the substance enters the water
supply systems.
Gadolinium (Gd) is a rare earth element (REE), which rarely occurs in
natural environments. Therefore Gd has become an indicator for human
impact in metropolitan areas; Gd is widely used in medical applications.
Since 1988 most contrast agents for magnetic resonance imaging in medicine
contain gadolinium. Gd is complexed with DTPA to form Gd-DTPA,
an aqueous soluble stable complex. Gd-DTPA does not accumulate in the
human body but is eliminated without significant chemical change via the
kidneys within a day.
Gd-DTPA reaches sewage treatment plants after having passed the sewage
systems. The sewage treatment processes are not sufficient to degrade the Gd-
DTPA complex. In densely populated areas increased concentrations of Gd-
DTPA can be found in surface water bodies that are partially recharged by
effluents of sewage plants. Even aquifers contain Gd-DTPA introduced by
infiltration of surface water. Such a path is of particular concern, where
drinking water is pumped from well galleries in the vicinity of surface
water bodies, where bank filtration is used for public water supply.
(continued)
