Environmental Engineering Reference
In-Depth Information
Water-insoluble Phosphate Compounds
Many apatite [Ap, Ca
10
(PO
4
)
6
(OH,F,Cl)
2
] based materials, such as natural
mineral phosphate and synthetic apatite, bind Cd and other PTE ions from
aqueous solutions and soils (Mandjiny et al., 1995; Chen et al., 2007a; Chen et
al., 2007b; Peld et al, 2004; Knox et al, 2003). Calcium-hydroxyapatite (HAp,
Ca
10
(PO
4
)
6
(OH)
2
) has also attracted considerable interests because of its high
adsorption capacity, biological compatibility, low solubility in basic and
neutral media, excellent buffer property, high stability and low cost. Both
synthetic and natural HAp reduced bioavailability of soil Cd, limiting its
uptake by crops (Chlopecka, Adriano 1997; Keller et al, 2005). In general,
HAp has demonstrated the best removal efficiency due to its moderate
solubility—between highly insoluble and highly soluble phosphate bearing
materials such as phosphate rock and phosphate fertilizers, respectively
(Hodson et al, 2000).
Several mechanisms were proposed to explain the sorption process of Cd
on apatites, including superficial sorption, ion exchange, and precipitation.
The sorption process was studied in detail by Jeanjean et al (1995) and
Fedoroff et al. (1999). A detailed description of the structure of calcium-
hydroxyapatite was provided by Beevers and McIntyre (1945) and later by
Kay and Young (1964) and Hughes et al. (1989). Calcium-hydroxyapatite
crystallizes in the hexagonal system, where Ca
2+
occupies two different
crystallographic sites, Ca(1) and Ca(2). Calcium(1) is found on ternary axes at
x=1/3, y =2/3 whereas Ca(2) is found at sites with symmetry m at z=1/4,
z=3/4; HO
-
ions are found in channels along the hexagonal screw-axes, at
z=0,198. Sorbed Cd ions substitute for Ca, which is released into the soil
solution (Takeuchi, Arai 1990; Mandjiny et al, 1998). The ion exchange
reaction mechanism can be expressed as (Equation 1):
Ca
10
(PO
4
)
6
(OH)
2
+ x Cd
2+
→ (Cd
x
,Ca
10−x
)(PO
4
)
6
(OH)
2
+ x Ca
2+
(Equation 1)
For low sorbed quantities, Cd is located in Ca(2) sites, while for larger
quantities, it is located in both Ca(1) and Ca(2) sites (Fedoroff et al., 1999).
Scanning electron microscope observations showed that there is no
modification of the crystallite morphology after Cd fixation. This process is
only partly reversible and takes place in a large pH range. In addition, Cd may
diffuse into the bulk of apatite crystals. Nuclear microprobe measurements
showed that Cd penetrated into the whole thickness of the crystals (Toulhoat et
al., 1996). Fedoroff et al. (1999) concluded that Ca(2) sites, which are adjacent
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