PROBLEMES OF DISSOLUTION AND CRYSTALLIZATION OF STRUVITE
D. Matýsek, H.Raclavská
VŠB – Technical University Ostrava, Institute of Geological Engineering, 17.listopadu, 708 33 Ostrava – Poruba,
Czech Republic
Email: Dalibor.matysek@vsb.cz and helena.raclavska@vsb.cz
ABSTRACT
Struvite represents very interesting phosphate which has potential possibility for recovering of
orthophosphates from wastewaters. Its spontaneous crystallization can however cause considerable
problems. Results of modelling of dissolution and crystallization of struvite in water by means of reagents
based on equilibrium state are presented. It is demonstrated that struvite is mineral with high solubility.
For its crystallization it is necessary oversaturation of water especially by ions NH4+.
Keywords: Sludge, phosphorus, modelling, PhreeqC
INTRODUCTION
Struvite is very interesting phosphate which represents the potential possibility of recovery of
dissolved orthophosphates from wastewaters. On the other side its crystallization in waste tubins is often
causing troubles. This short contribution (or several remarks) is dealing with equilibrium modelling of
struvite behaviour in solution. It originated as a reaction on some articles presented in previous issues of
the Scope Newsletter. The aim of this contribution is also to demonstrate some possibilities of utilization
of equilibrium (thermodynamics) modelling of some hydrochemical processes.
The modelling of hydrochemical processes is based on the existence of equilibrium state and it is strongly
limited by knowledge of equilibrium constants and enthalpy of all reactions, which take place in aqueous
environment. In spite of the fact that possibility of equilibrium state existence is rather controversial and
in the value equilibrium constant (particularly for Ksp of some phases) there exists dispersion up to
several orders, the modelling provides in most cases utilizable results. The advantage of the modelling is
flexibility and possibility to obtain the precise estimations of activities of all complex ion pairs, complex
ions and ion associates in the solution. Several programs exist for this modelling and they are
continuously improved (e. g. PhreeqC, Minteq, Chess, Mineql+ and others). The more detailed
description of calculation algorithms and conditions of utilization of equilibrium modelling cannot be
presented here for the lack of space.
MODELLING OF HYDROCHEMICAL PROCESSES
The presented results were calculated by means of program PhreeqC 2.0.0 with interface PhreeqcI
R_0_30 [2] using databases of equilibrium constants Minteq. For struvite there was used value pKsp 13.26
[1], for hydroxyapatite value pKsp 44.199 (Minteq). The solution contained in all cases indifferent
electrolyte (0.1 mol l-1 LiCl) in order to adjust pH by means of saturation index HCl (numerical titration).
In the introduction to the description of obtained results it is necessary to mention the strong
ability of phosphates to form complexes in water. They create complex ions or ion associates with almost
all cations, which can be present in the solution. Activities or concentrations of these associates cannot be
neglected in any case. The direct calculation of solution composition in equilibrium with phosphate
minerals from the equilibrium constant provide too low, not realistic concentrations of dissolved
phosphorus due to the formation of associates.
Equilibrium of struvite in water in dependence on pH.
The results of modelling are presented in the Figure 1. Struvite belongs in this case among relatively high
soluble phosphates. The optimum – minimal concentration of phosphorus in the solution is located at the
value pH = 9.47 and the solution in equilibrium with struvite contains the total concentrations 1.408
mmol/l Ptot, Mgtot and Ntot (which corresponds to 133.72 mg l-1 PO4-3, 34.25 mg l-1 Mg2+ and 25.4 mg l-1
NH4+). Concentrations, activities of ion associates and their share from the total content are listed in Table
1.
Equilibrium of struvite at the nonstoichiometric addition of Mg2+ and NH4+.
The influence of addition of ions Mg2+ or NH4+ to the solution was studied in equilibrium with struvite
(see previous paragraph – pH = 9.47). This addition causes non-equilibrium in the system, which is
compensated by precipitation of struvite. The results are presented in Figure 2 and 3. The diagrams
represent projection of general 3D surface on the two axes directions. It was found that additions have
substantial influence on phosphorus concentration in equilibrium solution. An impact of addition of NH4+
is pronouncedly higher (up to approx. 13x), probably because it is not participating on the origin of
associates. The similar influence will have additions of other alkalies, particularly potassium in the
equilibrium solution, due to the ability of isomorphous substitution. Certain changes of acidobasic
reaction value of solution optimum for struvite were also observed in case of additions. The addition of
Mg2+ is decreasing the pH value of equilibrium state (it is influenced by forming of hydroxy-complexes of
Mg) while addition of NH4+ is moderately increasing it (influence on distribution NH4+ - NH3). Diagrams
in Figure 2 and 3 also demonstrate that reaching of environmentally acceptable concentration of
phosphorus in waste water (approx. 0.1 mg l-1) by precipitation of struvite is probably not possible and
after recovery of dissolved phosphorus from wastewater it will be necessary to perform further
purification.
Equilibrium of struvite at the presence of addition of Ca2+ ions
Ions of calcium form common component of all surface waters and waste waters. In respect to the high
stability of calcium phosphates, the concentrations of Ca2+ ions have extreme influence on the
crystallization of struvite. Some problem in modelling of behaviour of minerals from the apatite group in
solutions represents high dispersion in published values of dissolution constant which is caused beside
other reasons by formation of amorphous or poorly crystallized precursors. Already small addition of
Ca2+ ions to the solution in equilibrium with struvite is leading to the over passing of saturation indexes of
apatite minerals (hydroxyapatite or chlorapatite) at the simultaneous dissolution of struvite. Struvite
represents the stable phase in relationship to hdroxyapatite only under conditions that log (Mg/Ca) >
0.3723 x pH, e. g. at pH = 4, Mg/Ca = 30.85 and at pH = 10, Mg/Ca = 5284. This is demonstrated also in
Figure 3 where to the solution in equilibrium with struvite it was added equal amounts (32 mmol l-1) of
NH4+, Mg2+ and 0 (line a) or 32 mmol l-1 of Ca2+ ions (line b). The equilibrium concentration of
phosphorus is in relation to struvite and in the case of line c with relation to both struvite and
hydroxyapatite.
Conclusion
From the modelling presented above there result some facts which can be utilized for recovery of struvite
from waste waters and also for solution of problems which struvite can cause by its precipitation in
pipelines. Struvite represents relatively well soluble phosphate. It can be precipitated in larger scale
probably only at the non-stoichiometric concentrations of its components when decisive influence is
exerted by concentration of ammonia ions. Maximum recovery of phosphorus can be expected under
conditions when molar concentrations Mg = P and concentration of NH4+ or K+ are as high as possible.
Calcium ions are suppressing very strongly formation of struvite with preferential origin of
hydroxyapatite. The decreasing of non-desirable precipitation of struvite is possible by decreasing of
water oversaturation by struvite by means of adjustment of acidobasic reaction value. In addition, it is
possible to influence – increase redox state and to limit amount of ammonium ions in solution e. g. by
aeration. In respect to the prevalently amorphous character of precipitated apatite minerals it is necessary
to verify influence of Ca2+ ions on the formation of precipitates in pipes.
REFERENCES
1. Ohlinger K., Struvite precipitation kinetics. SCOPE, 36 Wastewater chemistry. (2000)
2. Parkhurst D. L., Appelo C.A.J., User’s guide to Phreeqc (version 2) - A computer program for
speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations. WaterResources Investigations Report 99-4259. U.S. Department of the Interior, U. S. Geological Survey.
Denver Colorado, 326 pp. (1999).
Table 1. Concentrations and activities of ion associates in the solution in equilibrium with struvite
Element
Mg
total
1.408
mmol l-1
N total 1.408
P
total
1.408
mmol l-1
Associate
Mg+2
MgPO4MgHPO4
MgOH+
MgH2PO4+
NH3
NH4+
HPO4-2
MgPO4MgHPO4
PO4-3
H2PO4MgH2PO4+
H3PO4
Molality
[mmol/l]
7.50E-01
5.90E-01
6.53E-02
1.95E-03
2.01E-05
7.75E-01
6.33E-01
7.47E-01
5.90E-01
6.53E-02
3.43E-03
1.92E-03
2.01E-05
6.97E-11
Activity [mmol l-1]
Share of molality [%]
3.22E-01
4.64E-01
6.69E-02
1.56E-03
1.58E-05
7.94E-01
4.69E-01
2.78E-01
4.64E-01
6.69E-02
3.72E-04
1.51E-03
1.58E-05
7.14E-11
53.27
41.90
4.64
0.14
< 0.01
55.04
44.96
53.05
41.90
4.64
0.24
0.14
< 0.01
< 0.01
Following pages : Figure 1. Behaviour of struvite in water in dependence on value of acidobasic reaction
Figure 2. Equilibrium of struvite in solution with addition of ion Mg2+ and NH4+
Figure 3. Equilibrium of struvite in solution with addition of ion Mg2+ and NH4+
Figure 4. Influence of Ca2+ ions on equilibrium concentration of total phosphorus and equilibrium of
struvite in solution
Fig 1 :
Fig 2
Fig 3
Fig 4