Environmental Engineering Reference
In-Depth Information
Surface treatment
Phosphate injection
or mixing
in situ barrier
Contaminated
Soil
Migration of waste
Water
Body
Groundwater Flow
FIGURE 6.1.1
Phosphate treatment of contaminated soil using three different methods:
surface treatment, subsurface treatment, and as an
in situ
permeable reactive or sorptive barrier.
structure, including Sr
2+
, Na
+
, and Mg
2+
for Ca
2+
; CO
3
2-
and HPO
4
2-
for PO
4
3-
; and
F
-
, Cl
-
and CO
3
2-
for OH
-
. High-purity hydroxyapatite can be produced synthetically
by precipitation from oversaturated solutions of calcium and phosphate,
23
from high-
temperature solid-state reactions,
24
or from calcination of animal bones.
25,26
Figure
6.1.2 presents transmission electron microscopy (TEM) images of synthetic
hydroxyapatite and hydroxyapatite prepared by calcination of cattle bone at 500 to
1100°C. As calcination temperature increases from 500 to 1100°C, the average
crystal size of the material increases from approximately 0.20 μm to 0.50 μm and
the surface area decreases from 101 m
2
/g to 2.2 m
2
/g, indicating a large internal
pore space in the material treated at 500˚C. For the bone treated at 1100°C, small
crystals of MgO can be seen on the surface (Figure 6.1.2f). Mg is a component of
bone and is expelled from the apatite crystal at high temperatures and forms MgO.
A compilation of the available literature studies for heavy metal and radionuclide
sorption, stabilization, and immobilization using phosphates is given in Table 6.1.2.
The list includes laboratory studies, field trials, and reports on remediation of
contaminated sites.
In conclusion, it has been well demonstrated through laboratory experiments,
field trials, and remediation activities that phosphate treatment for stabilization and
immobilization of heavy metal and radionuclide wastes is effective and economical.
Phosphate treatment significantly reduces the solubility and bioavailability of con-
taminants in soil, groundwater, and wastes. Studies and applications of phosphate
treatment have mainly centered on immobilization of contaminants in soil and
removal of contaminants in groundwater.
In situ
treatment of contaminated soil is
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