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Adsorption of fluoride on
iron hydroxides/oxides
Ina Beate Jenssen
HYDROMET project meeting
October 5th, 2016
Porsgrunn
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Outline
• Adsorption of fluoride
• Initial experiments
– Preparation of ferric hydroxides
• Fluoride adsorption
– Effect of pH, adsorbent, surface area, and temperature
• BET surface area
• Summary of results
• Hydromet in education at NTNU
• Further work
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Adsorption of fluoride
• Results from SINTEF – even distribution of fluoride on theprecipitate– Adsorption?
• Literature research– Iron hydroxides as adsorbent for fluoride in water treatment*
• Adsorption (IUPAC definition):– Increase in the concentration of a substance at the interface of a
condensed and a liquid or gaseous layer owing to the operation of surface forces.
– Physisorption
– Chemisorption
*Mohapatra, M., Anand, S., Mishra, B. K., Giles, D. E., & Singh, P. (2009). Review of fluoride removal from drinking water. Journal of Environmental Management, 91(1), 67-77.
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Adsorption of fluoride
• Proposed mechanism
– Ligand exchange*
• Important factors
– pH
– Surface area
– Competing anions
– Fe(II)/Fe(III) ratio
– Temperature
*Ding, X. (2012). Identity of Fluoride and Phosphate-Binding Sites at FeOOH Surfaces.
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Initial experiments – effect of pH
• From the literature:
Kumar, E., Bhatnagar, A., Ji, M., Jung, W., Lee, S.-H., Kim, S.-J., . . . Jeon, B.-H. (2009). Defluoridationfrom aqueous solutions by granular ferric hydroxide(GFH). Water research, 43(2), 490-498.
Tang, Y., Wang, J., & Gao, N. (2010). Characteristics and model studies for fluoride and arsenic adsorption on goethite. Journal of Environmental Sciences, 22(11), 1689-1694.
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Initial experiments – preparation of
ferric hydroxides• Goethite
– 1 M Fe(NO3)3
– 5 M NaOH
– pH 12, 80 °C, 3 days
SEM picture of goethite crystals
XRD diagram of goethite
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Initial experiments – preparation of
ferric hydroxides
• Ferrihydrite (2-line)
– 1 M Fe(NO3)3
– 1 M NaOH added until pH 7
SEM picture of ferrihydrite
XRD diagram of ferrihydrite
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"Felleslab" experiment
• 3rd year students in chemical engineering and
biotechnology master program
“Separation of fluoride – adsorption of fluoride
on iron hydroxides”
• Study effect of pH, adsorbent and temperature
– Ferrihydrite, goethite, Al(OH)3
– 10 groups
– pH range 2 - 10, temperatures 25, 50 and 80 C
– Each group: 3 pH values, 1 temperature
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Effect of pH
0
0,5
1
1,5
2
2,5
3
3,5
4
4,5
0 2 4 6 8 10
mg
F ad
sorb
ed /
g a
dso
rben
t
pH
T=80 °C Goethite
Stronglydecreasingadsorption withincreasing pH
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Effect of adsorbent –
mg F adsorbed /g adsorbent
0
0,5
1
1,5
2
2,5
3
3,5
4
4,5
0 2 4 6 8 10
mg
F ad
sorb
ed /
g a
dso
rben
t
pH
T=80 °C Goethite
T=80 °C Ferrihydrite
Not taking surfacearea into account, goethite and ferrihydrite equallygood at low pH, ferrihydrite better at higher pH
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Effect of adsorbent –
mg F adsorbed / m2
0
0,02
0,04
0,06
0,08
0,1
0,12
0,14
0 2 4 6 8 10
mg
F ad
sorb
ed /
m2
pH
T=80 °C Goethite
T=80 °C Ferrihydrite
Taking surface area into account, goethiteis the best adsorbent
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Effect of temperature
0
0,002
0,004
0,006
0,008
0,01
0,012
0,014
0,016
0,018
0 2 4 6 8
mg
F ad
sorb
ed /
m2
pH
T=25 °C Ferrihydrite
T=50 °C Ferrihydrite
T=80 °C Ferrihydrite
No significantdifferences in adsorption at different temperatures
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Effect of pH, adsorbent and
temperature
0
0,5
1
1,5
2
2,5
3
3,5
4
4,5
5
0 2 4 6 8 10
mg
F ad
sorb
ed /
g a
dso
rben
t
pH
T=25 °C Goethite
T=50 °C Goethite
T=80 °C Goethite
T=25 °C Ferrihydrite
T=50 °C Ferrihydrite
T=80 °C Ferrihydrite
• Surface area not taken into account
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Effect of pH, adsorbent and
temperature
0
0,02
0,04
0,06
0,08
0,1
0,12
0,14
0,16
0 2 4 6 8 10
mg
F ad
sorb
ed /
m2
pH
T=25 °C Goethite
T=50 °C Goethite
T=80 °C Goethite
T=25 °C Ferrihydrite
T=50 °C Ferrihydrite
T=80 °C Ferrihydrite
• Surface area taken into account
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• Goethite is more affected by pH than ferrihydrite
• Adsorption capacity (mg/m2) higher for goethite
than ferrihydrite
• Type of surface area matters!
• Best alternative: goethite with high surface area
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BET surface area
• Theoretical surface areas*
– Goethite: 20 m2/g
– Ferrihydrite: 200 m2/g
• Standard pretreatment: degassing at 200 C
overnight
– Avoid humidity in the BET equipment
* Schwertmann, U. and R. M. Cornell (2007). Iron Oxides in the Laboratory, Wiley-VCH VerlagGmbH.
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BET surface area
• Theoretical surface areas*
– Goethite: 20 m2/g
– Ferrihydrite: 200 m2/g
* Schwertmann, U. and R. M. Cornell (2007). Iron Oxides in the Laboratory, Wiley-VCH VerlagGmbH.
BET surface area (m2/g)
De-gas temperature (°C) Goethite Ferrihydrite
200 80 0.74
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BET surface area
• Theoretical surface areas*
– Goethite: 20 m2/g
– Ferrihydrite: 200 m2/g
* Schwertmann, U. and R. M. Cornell (2007). Iron Oxides in the Laboratory, Wiley-VCH VerlagGmbH.
BET surface area (m2/g)
De-gas temperature (°C) Goethite Ferrihydrite
200 80 0.74
180 81 7.8
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BET surface area
• Theoretical surface areas*
– Goethite: 20 m2/g
– Ferrihydrite: 200 m2/g
* Schwertmann, U. and R. M. Cornell (2007). Iron Oxides in the Laboratory, Wiley-VCH VerlagGmbH.** Weidler, Peter G. "BET sample pretreatment of synthetic ferrihydrite and its influence on the determination of surface area and porosity." Journal of Porous Materials 4.3 (1997): 165-169.
BET surface area (m2/g)
De-gas temperature (°C) Goethite Ferrihydrite
200 80 0.74
180 81 7.8
120** 35 272
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XRD diagrams of goethite before and after BET analysis:
BeforeBET
200 C
180 C
• Pretreatment at 200 C hematite
• Pretreatment at 180 C goethite + hematite
Before BET
200 C
180 C
XRD diagrams of ferrihydrite before and after BET analysis:
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TGA
70
75
80
85
90
95
100
105
0 100 200 300 400 500 600 700
Mas
s/%
Temperature (°C)
Ferrihydrite - TGA
BeforeTGA
After TGA
XRD diagrams of ferrihydrite before and after TGA
After TGA: Hematite!
340 CTransformation at approx. 340 C
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Summary
• Important factors affecting the adsorption are:
– pH: adsorption capacity decreases with increasing pH
– Surface area and type of surface
• Goethite has the best adsorption properties, when taking
the surface area into account
• Temperature does not seem to have an effect
– Need to study this closer
• Surface area determination by BET is not straight
forward
• Heating and dehydration of goethite and
ferrihydrate hematite
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Hydromet in education at NTNU
• 2 project students – specialization projects fall
2016, master thesis spring 2017
• 3rd year master students Chemical Engineering
and Biotechnology – experiment in TKP4105
Separation Technology
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Specialization projects
Boliden“Fluoride adsorption on iron hydroxide/oxide precipitates”
• How is the supersaturation affected by Fe3+/Fe2+-ratio, pH and temperature?
• Particle size and crystalline properties
• Transformation of precipitated iron hydroxides to different phases
• Experiments performed in batch reactor
• Determination of Fe2+-concentration by UV-VIS
• XRD – type of iron hydroxide
• SEM – shape and size of particles
• BET – surface area
• Adsorption of fluoride at different pH
• Is surface area the determining factor, or is the type of surface area also important?
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Visual MINTEQ
• Chemical equilibrium software
• Modelling and planning experiments
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Specialization projects
Glencore“Mechanisms and particle properties for the iron hydroxide/oxide precipitation at Glencore Nikkelverk”
• Ferrous iron from raw material is oxidised with Cl2 (g), pH is adjusted with NiCO3, and the iron precipitates as ferric hydroxide/oxide:
2 FeCl2 + Cl2 (g) + 3H2O + 3 NiCO3 (s) = 2 Fe(OH)3 (s) + 3 NiCl2 +3 CO2 (g)
• How is the supersaturation affected by Fe3+/Fe2+-ratio, pH and temperature?
• Particle size and crystalline properties
• Transformation of precipitated iron hydroxides to different phases
• Experiments performed in batch reactor• Will not use Cl2 (g) due to HSE issues
• Determination of Fe2+-concentration by UV-VIS
• XRD – type of iron hydroxide
• SEM – shape and size of particles
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Further work
• Effect of contact time
• Closer study of "felleslab"-experiments
• Precipitation under controlled conditions
– Which factors determines size and surface area?
• Ratio of Fe(II)/Fe(III)
– Effect on precipitation and adsorption
• Adsorption mechanism
• Experiments with Boliden’s conditions
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Thank you for your attention!