9 Struvite Crystallization: An Effective Technology …
149
Table 9.3 Properties of struvite
Property
Description
Chemical name
Magnesium ammonium phosphate hexahydrate
Chemical formula MgNH 4 PO 4 · 6H 2 O
Colour
White, yellow, brownish or light grey, based on crystallization media (Lee
et al. 2009)
Molecular weight 245.43 g/mol
Specific gravity
1.711 (Borgerding 1972)
Solubility
Low in neutral/alkaline medium (0.018 g/100 ml at 25 °C). High in acids
(0.178 g/100 ml at 25 °C in 0.01 N HCl)
9.3.2 Struvite Formation
The general equation of struvite formation is as given in Eq. (9.1):
(9.1)
MAP precipitation reaction lowers the pH of solution due to crystallization of
soluble orthophosphate (PO 4
3− ) (Le Corre et al. 2009), which means that the dominant form of P in struvite formation reaction would be HPO
2−
4 or H 2 PO
−
4 instead of
PO
3−
4 , which is explained through Eqs. (9.2) and (9.3) (Wilsenach et al. 2007):
(9.2)
(9.3)
The formation and crystallization of struvite follow two main stages, namely,
nucleation and crystal growth. Nucleation is the process of crystals birth in a solution
where ions combine to form embryos. In fact, this process is basically controlled by
reaction kinetics. Crystal growth is the subsequent process that corresponds to the
development of crystals until reaching equilibrium. Fundamentally, the final size of
crystals, hence its settle ability, is governed by crystal growth rate (Le Corre et al.
2009).
149
Table 9.3 Properties of struvite
Property
Description
Chemical name
Magnesium ammonium phosphate hexahydrate
Chemical formula MgNH 4 PO 4 · 6H 2 O
Colour
White, yellow, brownish or light grey, based on crystallization media (Lee
et al. 2009)
Molecular weight 245.43 g/mol
Specific gravity
1.711 (Borgerding 1972)
Solubility
Low in neutral/alkaline medium (0.018 g/100 ml at 25 °C). High in acids
(0.178 g/100 ml at 25 °C in 0.01 N HCl)
9.3.2 Struvite Formation
The general equation of struvite formation is as given in Eq. (9.1):
(9.1)
MAP precipitation reaction lowers the pH of solution due to crystallization of
soluble orthophosphate (PO 4
3− ) (Le Corre et al. 2009), which means that the dominant form of P in struvite formation reaction would be HPO
2−
4 or H 2 PO
−
4 instead of
PO
3−
4 , which is explained through Eqs. (9.2) and (9.3) (Wilsenach et al. 2007):
(9.2)
(9.3)
The formation and crystallization of struvite follow two main stages, namely,
nucleation and crystal growth. Nucleation is the process of crystals birth in a solution
where ions combine to form embryos. In fact, this process is basically controlled by
reaction kinetics. Crystal growth is the subsequent process that corresponds to the
development of crystals until reaching equilibrium. Fundamentally, the final size of
crystals, hence its settle ability, is governed by crystal growth rate (Le Corre et al.
2009).