Post on 17-Feb-2018
Removal of Cr(VI) by Fe(II) reductive precipitation from groundwaters containing natural organic matter
M. Langer, A. Gröhlich, M. Ernst, M. Mitrakas, A. Zouboulis, I. Katsoyiannis
16th International Conference on Chemistry and the Environment 18.06.-22.06.2017, Oslo, Norway
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Chromate in drinking water
Worldwide people suffer from groundwater contaminated with chromium
Sources: Broad application in industry and erosion of natural ophiolitic or ultramafic rocks
Predominant redox species are Cr(VI) and Cr(III)
Recent studies reclassified Cr(VI) as toxic, carcinogenic and mutagenic
Current discussion about reducing the acceptable limits in in drinking water
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50
75
Current Future ?
Ch
rom
ium
[µ
g/L
]
50 µg/L
<< 10 µg/L
1 I
ntr
od
uct
ion
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Chromium removal
Reduction through Fe(II):
Resulting Cr(III) has low solubility: Precipitates as Cr(III) hydroxide, adsorbs onto or co-precipitates with iron hydroxides
Removal of produced solids e.g. by filtration
Known to be influenced by:
Fe(II) reductive precipitation widely used, but so far, role of natural organic matter rarely investigated
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1 I
ntr
od
uct
ion
Cr(VI) + 3 Fe(II) Cr(III) + 3 Fe(III)
pH value
Fe(II)/ Cr(VI) ratio
Temperature
Water composition
[Cr(VI)]:[Fe(II)] = 1:3
(Mitrakas et al. 2011)
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Natural organic matter in water
Commonly found in natural waters and soils
Wide variety of substances (e.g. lignin, polysaccharides, lipids); Most important humic and fulvic acids
Influence on chromium removal:
Formation of soluble complexes of Fe(III), Cr(III) and humic substances (Buerge, Hug 1998)
Catalyzing/Accelerating effect on Fe(II) reductive precipitation at pH<6 (Hori et al. 2015)
Dependent on the type of organic substances
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Redox cycling of ferrous and ferric iron in the presence of soil humic acids (SHS) (Wittbrodt, Palmer 1996)
1 I
ntr
od
uct
ion
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Materials and Methods
Model groundwater spiked with Cr(VI) and humic acid (Carl Roth)
Procedure and conditions:
Analytics:
Cr(VI) photometric with DPC complexation (100mm cuvette)
Cr(total) ICP-MS or GFAAS
5
2 M
ater
ials
an
d M
eth
od
s
Cr(VI): 100 µg/L
Humic acid: 0-5 mg/L DOC
pH: 6.5 - 8.0
FeSO3 dose:
[Cr(VI)]:[Fe(II)]
0.25 - 2.0 mg/L Fe(II)
1:2.3 - 1:18.6
at TUHH and AUTh
Model water
Cr(VI), HA Fe(II)
250 rpm 2 min
50 rpm 60 min
Filtration 0.45 µm
1.8 L
Model water
Cr(VI), HA Fe(II)
250 rpm2 min
50 rpm60 min
Filtration0.45 µm
1.8 L
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Influence of pH and Fe(II) dose
With 1 mg/L Fe(II) total Cr concentrations of 5 µg/L can be achieved over wide pH range when humic acid is absent
At pH 8 Fe(II) oxidation competition through oxygen
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3 R
esu
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and
Dis
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ion
0
20
40
60
80
100
0 10 20 30 40 50 60
Cr
(VI)
(µ
g/L)
Time (min)
0.25
0.5
1.0
1.5
2.0
Fe(II)-dosage in mg/L
Cr(VI) Fe(II) dose [mg/L]
0.25 1:2.3
0.5 1:4.7
1.0 1:9.3
1.5 1:14.0
2.0 1:18.6
0
15
30
45
60
75
6,5 7 7,5 8
Cr(
VI)
[µ
g/L
]
pH [-]
0
15
30
45
60
75
6,5 7 7,5 8
Cr(
tota
l) [
µg
/L]
pH [-]
Cr(total) [Cr(VI)]:[Fe(II)]
(Gröhlich et al. 2017)
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Influence of humic acid on Cr(VI) reduction
Reduction of Cr(VI) to Cr(III) only influenced by HA at very low Fe(II) dosage (substoichiometric) and at pH 8
No explicit catalyzing effects on Cr(VI) reduction through humic acid observed
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esu
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and
Dis
cuss
ion
Fe(II) dose [mg/L]:
0.25
1.0
2.0
0
20
40
60
80
100
0 1 2 3 4 5
Cr(
VI)
[µ
g/L
]
HA [mg DOC/L]
detection limit*
0
20
40
60
80
0 1 2 3 4 5
Cr(
VI)
[µ
g/L
]
HA [mg DOC/L]
0.25
1.0
2.0
detection limit pH 7
0
20
40
60
80
100
0 1 2 3 4 5
Cr(
VI)
[µ
g/L
]
HA [mg DOC/L]
detection limit*
pH [-]:
8.0
7.0
6.5
0
20
40
60
80
100
0 1 2 3 4 5
Cr(
VI)
[µ
g/L]
HA [mg DOC/L]
pH 8
pH 7
pH 6.5
detection limit
2 mg/L Fe(II)
Fe(II) variation pH variation
*Detection limit increased due to interferences with humic acid
(Gröhlich et al. 2017)
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Influence of humic acid on Cr(III) removal
Already at DOC of 1 mg/L removal of Cr(III) heavily impaired
When Cr(VI) fully converted into Cr(III), 30 t 80 µg/L Cr(III) left in solution
Even with 2 mg/L Fe(II) total Cr values <30 µg/L not achievable
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esu
lts
and
Dis
cuss
ion
Cr(III) Fe(II) dose [mg/L]:
0.25
1.0
2.0
0
20
40
60
80
0 1 2 3 4 5
Cr(
VI)
[µ
g/L
]
HA [mg DOC/L]
0.25
1.0
2.0
detection limit
0
20
40
60
80
100
0 1 2 3 4 5
Cr(
tota
l) [
µg
/L]
HA [mg DOC/L]
Cr(total)
pH 7
0
20
40
60
80
100
0 1 2 3 4 5
Cr(
III)
[µg
/L]
HA (mg DOC/L)
(Gröhlich et al. 2017)
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Conclusion
Cr(VI) is realbly converted into Cr(III) under a wide range of process conditions
Removal of Cr(III) was observed to be slower and much more prone to be influenced by pH and humic acid
Without HA Cr(total) values of 5µg/L easily achieved,
whereas with HA (1-5 mg/L DOC) 30-80 µg/L Cr(III) remain in solution
As a likely result of complexation, high amounts of residual Fe (> 0.1 mg/L) were observed
Optimization of Cr(III) removal necessary for a versatile application of Fe(II) reductive precipitation
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4 C
on
clu
sio
n a
nd
Fu
ture
Pro
spec
ts
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Future Prospects
Lab-scale pilot experiments including recirculation and submerged microfiltration:
Larger contact surface area for Cr(III) adsorption and removal of undesirable and interfering DOC expected
First experiences: At high sludge concentrations (1 g/L Fe(III)) lower residual concentrations of Cr(VI), Cr(III) and Fe possible
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4 C
on
clu
sio
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nd
Fu
ture
Pro
spec
ts
modelwater
Fe(II)
1 L/h0.25-1 mg/L
1 L/h
0.1 µm
1 L
6 min
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Margarethe Langer Institute for Water Ressources and Water Supply Technische Universität Hamburg
Am Schwarzenberg-Campus 3 21073 Hamburg, Germany
margarethe.langer@tuhh.de
Thanks for you attention
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References
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Buerge, I. J.; Hug, S. J. (1998): Influence of organic ligands on chromium(VI) reduction by iron(II). In Environmental Science & Technology 32 (14), pp. 2092–2099. DOI: 10.1021/es970932b.
Gröhlich, A.; Langer, M.; Mitrakas, M.; Zouboulis, A.; Katsoyiannis, I.; Ernst, M. (2017): Effect of Organic Matter on Cr(VI) Removal from Groundwaters by Fe(II) Reductive Precipitation for Groundwater Treatment. In Water 9 (6). DOI: 10.3390/w9060389
Hori, M.; Shozugawa, K.; Matsuo, M. (2015): Reduction process of Cr(VI) by Fe(II) and humic acid analyzed using high time resolution XAFS analysis. In Journal of Hazardous Materials 285, pp. 140–147. DOI: 10.1016/j.jhazmat.2014.11.047.
Mitrakas, M. G.; Pantazatou, A. S.; Tzimou-Tsitouridou, R.; Sikalidis, C. A. (2011): Influence of pH and temperature on Cr(VI) removal from a natural water using Fe(II): A pilot and full scale case study. In Desalination and Water Treatment 33 (1-3), pp. 77–85. DOI: 10.5004/dwt.2011.2620.
Wittbrodt, P. R.; Palmer, C. D. (1996a): Effect of temperature, ionic strength, background electrolytes, and Fe(III) on the reduction of hexavalent chromium by soil humic substances. In Environmental Science & Technology 30 (8), pp. 2470–2477. DOI: 10.1021/es950731c
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Reaction time
13
Bac
k-u
p
0
20
40
60
80
100
0 10 20 30 40 50 60
Cr
(VI)
[µ
g/L
]
Time (min)
0
20
40
60
80
100
0 10 20 30 40 50 60
Cr(
tota
l) [
µg
/L]
Time [min] 0
20
40
60
80
100
0 10 20 30 40 50 60
Cr
(VI)
(µ
g/L)
Time (min)
0.25
0.5
1.0
1.5
2.0
Fe(II)-dosage in mg/L
0
20
40
60
80
100
0 10 20 30 40 50 60
Cr(
tota
l) [
µg
/L]
Time [min]
0.25 0.5 1.0 1.5 2.0
Cr(VI) Cr(total)
DOC
Fe(II) dose [mg/L]:
0.25
0.5
1.0
1.5
2.0
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Influence of humic acid and pH
14
3 R
esu
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and
Dis
cuss
ion
Cr(VI) pH [-]:
8.0
7.0
6.5
0
20
40
60
80
100
0 1 2 3 4 5
Cr(
VI)
[µ
g/L
]
HA [mg DOC/L]
detection limit
0
20
40
60
80
100
0 1 2 3 4 5
Cr(
VI)
[µ
g/L]
HA [mg DOC/L]
pH 8
pH 7
pH 6.5
detection limit
0
20
40
60
80
100
0 1 2 3 4 5
Cr(
tota
l) [
µg/
L]
HA [mg DOC/L]
Cr(total)
2 mg/L Fe(II)