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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rom

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[µ

g/L

]

50 µg/L

<< 10 µg/L

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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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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)

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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

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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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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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Fe(II) dose [mg/L]:

0.25

1.0

2.0

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20

40

60

80

100

0 1 2 3 4 5

Cr(

VI)

[µ

g/L

]

HA [mg DOC/L]

detection limit*

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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

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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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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

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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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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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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

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Cr

(VI)

[µ

g/L

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Time (min)

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0 10 20 30 40 50 60

Cr(

tota

l) [

µg

/L]

Time [min] 0

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Cr

(VI)

(µ

g/L)

Time (min)

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1.5

2.0

Fe(II)-dosage in mg/L

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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

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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)