1. Crystallization as a tool for Phosphorous Recovery
from Wastewater: Kinetics and Uptake of
Contaminants
Ioannis Bountasa,b, Petros G.Koutsoukosa,b
a Department of Chemical Engineering, University of Patras, GR-26504 Patras, Greece
b Foundation of Research and Technology Hellas-Institute of Chemical Engineering and High-
Temperature Chemical Processes, Stadiou str., Platani Achaias, GR-26504 Patras, Greece
FORTH/ICE-HT
1
2. Phosphorus:
•Indispensable element for living organisms
•Toxic when bound to organic compounds (organophosphorus)
•Pollutant of water – contributes to eutrophication
•Useful industrial material for the production of a significant
number and various types of products
2
3. • The problem of water pollution due to
phosphorus enrichment becomes increasingly
acute and there is urgent demand for the
optimization of the removal processes.
• Recent legislation of EC Urban Waste Water
Treatment Directive concerning the quality
characteristics of wastewater became very
strict.
• Wastewater (municipal) rich in ammonia,
phosphorus and divalent cations (Ca and Mg)
3
4. Struvite: Three faces in one: The good, the bad and
the…ugly
One compound, three qualifications…Struvite = magnesium
ammonium phosphate hexahydrate, MgNH4PO46H2O 4
7. Struvite - MgNH4PO46H2O (MAP)
• logKsp = -13,68
• White prismatic crystals
• Mean size 25μm
• Found in biological systems
(undesirable, how can we avoid it?) as
scaling in industrial components
(undesirable: How can we eliminate its
formation??)
• Fertilizer (We want it! How to
optimize the conditions and the quality
of the obtained crystals??) 7
8. Strategy to obtain answers to the
questions
• Measurement of the kinetics of crystal growth over a wide
pH range, emphasizing acid pH, aqueous solutions
• Measurement of the stability of acidic (pH 6.50)
supersaturated solutions
• Batch reactors
• Investigate the effect of the presence of impurities on the
kinetics of struvite crystal growth
• Investigate interactions between struvite and pollutants (heavy
metals, As)
8
9. Experimental Procedure
• Synthesis and characterization of MAP seeds
• Thermodynamic calculations of supersaturation for series of
solution compositions at pH 6,5 and 8,5
• Preparation and stability investigation of the
supersaturated solutions with and without heavy metals (at
very low concentrations)
• Assessment of the impurity-struvite interactions with
experimental measurements of the adsorption and of the
MAP crystal growth rates in the presence of the impurities
• Equilibration- separation of the equilibrated solids from the
liquid phase when equilibrium is reached – chemical analysis,
solid characterization
• Calculation of isotherms and fitting of experimental data in
adsorption models 9
10. Experimental Procedure for the
measurement of crystal growth rates
Mg2++ NH4
+ + H2PO4
- MgNH4PO4
.6H2O +2H+
protons released into the solution
Drop of pH
• Seeded growth methodology-constant pH- constant supersaturation
In solutions supersaturated with respect to struvite, a drop of pH
exceeding 0.01 pH units triggers the addition of titrant solution(s)
from the burette(s) of the computerized automatic titrator
10
11. 0 50 100 150 200 250 300
1.60E-008
2.00E-008
2.40E-008
2.80E-008
3.20E-008
Rate
of
Crystal
Growth/mol
min
-1
m
-2
Mass of seed crystals/ x10
-3
g
Seeded crystal growth of struvite on struvite seed crystals; pH
6.5 , θ=25oC. Dependence of the rates of crystal growth on the
amount of seed crystals introduced. Solution volume 0.5 L 11
14. n
MAP
σ
MAP
p,
k
MAP
p,
R
• kp,MAP the apparent rate constant
• n the apparent order of the rate of
crystal growth of MAP
Kinetics
n indicative of the mechanism of crystal growth
Surface diffusion controlled, predominant
14
15. Results
• induction time: The
time lapsed between the
introduction of struvite
seed crystals in the
supersaturated solution
and the first addition of
titrant solutions.
• Initial rate of
precipitation:
Calculated from the slope
of the curve of titrants
addition as a function of
time
0 20 40 60 80 100 120 140 160
0
2
4
6
8
10
12
Volume
(ml)
time (min)
15
16. 0.5 0.6 0.7 0.8 0.9 1.0
0
1
2
3
Rate
of
Precipitation
/x10
-5
mol
min
-1
Relative Supersaturation,
pH 8.50
pH 6.5
Kinetics of spontaneous precipitation of struvite in SWW pH 6.5 ,
θ=25oC. Data at pH 8.50 Kofina A.N.; Demadis K.D.; Koutsoukos P.G.,
The Effect of Citrate and Phosphocitrate on Struvite Spontaneous
Precipitation, Crystal Growth &Design, 2007,7,2705-2712 16
17. Blank Ni Mn Cr As
0.0
5.0x10
-10
1.0x10
-9
1.5x10
-9
2.0x10
-9
2.5x10
-9
3.0x10
-9
Rate
of
Struvite
Crystal
Growth
/
mol
s
-1
m
-2-1
m
-2
Element present in the supersaturated solutions
Rate of seeded growth of struvite on struvite seed crystals in the absence (blank)
and in the presence of heavy metals (concentrations given as totals) Ω=1,2 , pH
6,5 ,θ=25oC)
17
18. SEM picture of struvite seed
crystals prepared and aged in
the lab
XRD pattern of struvite
seed crystals
18
19. Experimental setup for the measurement of the uptake isotherms
of the pollutants
• Saturated solution at pH 6,5
and 8,5 – background
electrolyte NaNO3
• 30 mg MAP at 50ml solution
• θ=25oC
19
20. As Ni Cd Cr Mn
0.0
5.0x10
-5
2.0x10
-4
2.5x10
-4
Concetration/
mol
dm
-3
Additives in solution
Initial
Equilibrium
As Ni Fe Cd Cr Mn
0.0
5.0x10
-5
1.0x10
-4
1.5x10
-4
2.0x10
-4
2.5x10
-4
3.0x10
-4
Concetration/
mol
dm
-3
Additives in solution
Initial
Equilibrium
pH 8,5 pH 6,5
Initial and equilibrium concentrations for the adsorption of
heavy metals on struvite crystals pH 8.5 and 6.5 ; Ι=0.05Μ ,
θ=25oC.
Results
20
21. Adsorption isotherm of Ni on struvite crystals; pH 8,5 and 6,5 -
Ι=0,05Μ - θ=25oC
0.0 1.0x10
-5
2.0x10
-5
3.0x10
-5
4.0x10
-5
5.0x10
-5
6.0x10
-5
-2.0x10
-7
0.0
2.0x10
-7
4.0x10
-7
6.0x10
-7
8.0x10
-7
1.0x10
-6
1.2x10
-6
1.4x10
-6
1.6x10
-6
1.8x10
-6
(mole
m
-2
)
[Ni]eq
( mole L
-1
) 1,0x10
-5
2,0x10
-5
3,0x10
-5
4,0x10
-5
2,0x10
-6
4,0x10
-6
6,0x10
-6
8,0x10
-6
1,0x10
-5
(
mole
m
-2
)
[Ni]eq
/ M
pH 8,5
pH 6,5
Γ=
V∙(𝐶0−𝐶𝑒𝑞)
𝑊∙𝑆𝑆𝐴
V volume
𝑪𝟎 initial concetration
𝑪𝒆𝒒 equilibrium concentration
W seed’s mass
SAA seed’s specific surface area
Adsorption isotherms
21
22. • The Freundlich isotherm is applicable to both
monolayer (chemisorption) and multilayer
adsorption (physisorption)
• Is based on the assumption that the adsorbate
adsorbs onto the heterogeneous surface of an
adsorbent
logΓ = logKf +
𝟏
𝒏
∙ log𝑪𝒆𝒒
• Γ surface concetration
• 𝐂𝒆𝒒 equilibrium
concentration
• Kf and n are Freundlich
isotherm constants related
to adsorption capacity and
adsorption intensity
Freundlich
Adsorption isotherm models
22
26. • The kinetics of crystal growth of MAP at conditions of
constant supersaturation and in acid or alkaline pH showed
that the rates of crystal growth were not affected by the
presence of Fe3+, Cd2+, Cr6+, and Mn2+ ions at
concentrations up to the limit of precipitation of other
mineral phases or oxides
• As(V) ions also failed to show any effect on the rates of
crystal growth at concentrations as high as 250M,
although at alkaline pH it seems that there is As uptake
by struvite crystals
• Ni2+ ions were an exception in that they were adsorbed
and they showed a slight acceleration of the crystal
growth process suggesting a different type of interaction
of this metal with MAP surface.
Conclusions
26
27. Acknowledgements
• Secretariat General for Research and Technology of
Ministry of Education for ARISTEIA II program (project
4420 SPM)
• The Municipal Water and Wastewater Treatment Company,
Patras Municipality
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