Environmental Engineering Reference
In-Depth Information
TABLE 6.1.1
Aqueous Solubility of Metal Phosphate Compounds
Metal
Solid
Reaction
K
sp
a, b
Pb
Hydroxypyromorphite
Pb
10
(PO
4
)
6
(OH)
2
-76.8
Chloropyromorphite
Pb
10
(PO
4
)
6
(Cl)
2
-85.4
Fluoropyromorphite
Pb
10
(PO
4
)
6
(F)
2
-71.6
Ca
Hydroxyapatite
Ca
10
(PO
4
)
6
(OH)
2
-55.9
Fluorapatite
Ca
10
(PO
4
)
6
(F)
2
-110.2
U
Autunite
Ca(UO
2
)
2
(PO
4
)
2
·10H
2
O
-50
Chernikovite
(H
3
O)
2
(UO
2
)(PO
4
)
2
·6H
2
O
-23
Sr
Strontium phosphate
Sr
5
(PO
4
)
3
(OH)
-51.3
Cd
Cadmium phosphate
Cd
3
(PO
4
)
2
-32.6
a
For a solid, X
a
Y
b
Z
c
, the solubility reaction is given by: X
a
Y
b
Z
c
↔ aX
+ bY + cZ. The solubility product constant K
sp
is defined as: K
sp
=
[X]
a
[Y]
b
[Z]
c
.
b
Data from References 1-7.
phosphate into the surface can treat the surface of a contaminated site. In a similar
manner, the subsurface can also be treated by injection of phosphate into the con-
taminated soil. Groundwater contaminants can also be treated by either phosphate
injection or by use of a permeable barrier.
In situ
stabilization has several advantages
over conventional technologies, including 1) the ability to treat small and very large
areas; 2) minimal exposure of workers to the contaminants; 3) low implementation
and operational cost at depths of 5 to 50 m or more; 4) minimal required site
excavation; and 5) no transportation or disposal cost for the contaminated soil. Two
novel
in situ
stabilization techniques using soluble forms of phosphate have been
proposed.
15,16
The secondary minerals in the oxidized zones of lead ore deposits, as assem-
blages around ore bodies, in soil, sediments, and phosphate beds prove the stability
of naturally occurring metal phosphates.
4,17
Studies indicate metal phosphate com-
pounds can be extremely stable and remain unchanged for thousands to hundreds
of thousands of years in different environmental settings.
2,3,18-22
The most commonly investigated materials have been phosphoric acid, natural
phosphate rock, bone char, and apatite. Treatment studies are also reported using
organophosphorous, monocalcium phosphate, calcium oxyphosphate, and triple
superphosphate. Phosphate fertilizers are readily available, inexpensive materials
produced in very large quantities throughout the world. Phosphate rock is mined as
a raw material for fertilizer production and as such is readily available and inexpen-
sive. Bone charcoal is a mixture of activated carbon and apatite prepared by the
calcination of animal bone in the absence of oxygen and is an inexpensive, widely
available material used for the refinement of raw sugar.
Hydroxyapatite, Ca
10
(PO
4
)
6
(OH)
2
, is a calcium phosphate mineral that exhibits
a high stability under reducing and oxidizing conditions and very low water solubility
(K
sp
< 10
-48
) under alkaline conditions. Many substitutions can be made in the apatite
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