Bioreactors for low-pH Fe(II) oxidation also remove remarkable amounts of total Fe

Bill Burgos, Yizhi Sheng, Brad Kaley – Environmental Engineering

Appalachian Research Initiative for Environmental Science (ARIES)

Rates of Fe(II) oxidation have been measured at many field sites in the Appalachian Bituminous Coal Basin

“Natural” terraced iron formations (TIFs) “Engineered” (TIFs)

Rates of Fe(II) oxidation have also been measured at 3 field sites in the Iberian Pyrite Belt (IPB)

Cerro Colorado, Rio Tinto mining district, Spain

Rio Tintillo, Huelva Province, Spain

AMD from the IPB has much higher conductance and dissolved metals, and lower pH values compared to Appalachian sites

Cravotta (2008), Applied Geochemistry, 23:166-202 (n=99)Sánchez-España et al. (2005), Applied Geochemistry, 20:1320-1356 (n=40)

We measured a suite of geochemical parameters as a function of distance downstream from each AMD source

We measured water velocity at each sample location to transform [conc]-vs-distance to [conc]-vs-time

stopwatch tracer tape measure

10

Assuming the stream reach behaves as a plug flow reactor, and Fe(II) oxidation is a first-order reaction

[Fe(II)]top

[Fe(II)]bottom

kFe(II)

Accum = In – Out ± Rxn

d[Fe(II)]/dt *V = 0 – 0 – kFe(II)*[Fe(II)]*V

kFe(II) = LN(Fe(II)]bottom/Fe(II)]top) / travel time

travel time =distance/velocity

2.0 2.5 3.0 3.5 4.0 4.5-3

-2

-1

0US sites

IPB sites

-1*pH

pH

log

kFe(

II) (1

/min

)

Fe(II) oxidation was fastest at lower pH values

2.0 2.5 3.0 3.5 4.0 4.5-3

-2

-1

0US sites

IPB sites

-1*pH

pH

log

kFe(

II) (1

/min

)

Fe(II) oxidation was fastest at lower pH values

2.0 2.5 3.0 3.5 4.0 4.5-3

-2

-1

0 US sitesIPB sites

pH

log

kFe(

II) (1

/min

)

We enriched Fe(II)-oxidizers from two sites for lab tests

Scalp Levelfastest site

Brubaker Run“average” site

Microbes were extracted from site sediments and enriched in “fed-batch” mode where Fe(II) was re-spiked into reactor

52 54 56 58 60 62 64 66 680

5

10

15

200

5

10

15

0

210

420

630

52 54 56 58 60 62 64 66 68

Tota

l Fe(

T) R

emov

alRa

te (m

g Fe

(T)/

L/h)

Time(d)

Oxi

datio

n ra

te(m

g Fe

(II)/

L/h)

Fe(II

) (m

g/L)

A chemostatic (constant pH, T) flow-through bioreactor was then used to measure Fe(II) oxidation rates under varied conditions

Reactor pH and the influent [Fe(II)] were varied systematically through a series of set points to measure Fe(II) oxidation rates

16

Brubaker Run Set points varied

Reactor pH[Fe(II)]in = 300 mg/L

2.9, 2.6, 2.3, 2.6, 2.9, 3.2, 3.5, 3.8, 4.1, 3.8, 3.5

Influent [Fe(II)]pH = 2.9

300, 80, 300, 600, 1200, 2400, 1200, 600

Scalp Level Set points varied

Reactor pH[Fe(II)]in = 300 mg/L

2.7, 2.4, 2.1, 2.4, 2.7, 3.0, 3.3, 3.6, 3.9, 4.2

Influent [Fe(II)]pH = 2.7

300, 66, 300, 600, 1200, 2400, 1200

17

0 50 100 150 200 250 300

0.0

0.2

0.4

0.6

0.8

1.03.5

Pore volume

Total Fe (T) Dissolved Fe(II)

C out/C

in

3.84.13.83.53.22.92.62.3pH= 2.9 2.6

Steady-state conditions for effluent [Fe(II)] were achieved for each reactor pH set point (all experiments run at ΘH = 6 h)

Brubaker Run

18

0 50 100 150 200 250 300 350 400 450 500 550

0.0

0.2

0.4

0.6

0.8

1.0

4.23.93.63.33.02.72.42.1

C out/C

in

Pore volume

Total Fe (T) Dissolved Fe(II)

pH=2.7 2.4

Steady-state conditions for effluent [Fe(II)] were also achieved with the Scalp Level chemostat

A general rate law can be written for biological Fe(II) oxidation

19

Simplified as follows:• Cbacteria remained ~constant (107 cell/mL)• DO averaged ~5 mg/L >> limiting concentration• pH purposefully varied in experiments

=

20

Assuming the bioreactor is completely-mixed, at steady state, and Fe(II) oxidation is a first-order reaction

Q[Fe(II)]in

Q[Fe(II)]out

V

kFe(II)

Accum = In – Out ± Rxn

0 = Q*[Fe(II)]in – Q*[Fe(II)]out – kFe(II)*[Fe(II)]out*V

kFe(II) = ([Fe(II)]in – [Fe(II)]out)*Q

[Fe(II)]out*V

21

2.0 2.5 3.0 3.5 4.0 4.5-3

-2

-1

0 Brubaker Run chemostat

Scalp Level chemostat

-1*pH

pH

log

kFe(

II) (1

/min

)

Fe(II) oxidation was fastest at lower pH values

22

2.0 2.5 3.0 3.5 4.0 4.5-3

-2

-1

0 US sites

Brubaker Run chemostat

Scalp Level chemostat

pH

log

kFe(

II) (1

/min

)

Fe(II) oxidation was slower in the lab versus the field

23

2.0 2.5 3.0 3.5 4.0 4.5-3

-2

-1

0 US sites

Brubaker Run chemostat

Scalp Level chemostat

pH

log

kFe(

II) (1

/min

)

Lab rates were similar from sites with different field rates

Scalp Level

Brubaker Run

Reactor pH and the influent [Fe(II)] were varied systematically through a series of set points to measure Fe(II) oxidation rates

24

Brubaker Run Set points varied

Reactor pH[Fe(II)]in = 300 mg/L

2.9, 2.6, 2.3, 2.6, 2.9, 3.2, 3.5, 3.8, 4.1, 3.8, 3.5

Influent [Fe(II)]pH = 2.9

300, 80, 300, 600, 1200, 2400, 1200, 600

Scalp Level Set points varied

Reactor pH[Fe(II)]in = 300 mg/L

2.7, 2.4, 2.1, 2.4, 2.7, 3.0, 3.3, 3.6, 3.9, 4.2

Influent [Fe(II)]pH = 2.7

300, 66, 300, 600, 1200, 2400, 1200

0 50 100 150 2000.0

0.2

0.4

0.6

0.8

1.0600

Total Fe (T) Dissolved Fe(II)

C out/C

in

Pore volume

Fe(II)in= 300mg/L

80 300 600 1200 2400 1200

Experiments were also conducted with variable influent [Fe(II)] (all at pH 2.90 for Brubaker Run)

26

0 500 1000 1500 2000 25000

1E-7

2E-7

3E-7

4E-7

5E-7

6E-7

7E-7

8E-7

Influent Fe(II) (mg/L)

chemostat_SL chemostat_BRO

xida

tion

Rate

s (m

ol/L

/s)

Fe(II) oxidation rates were highest at higher influent Fe(II) concentrations

Scalp Level chemostat pH 2.7Brubaker Run chemostat pH 2.9

From all these chemostat experiments, we attempted to parameterize a multi-Monod rate law for Fe(II) oxidation

27

0.000 0.002 0.004 0.006 0.008

1.0E-7

1.2E-7

1.4E-7

1.6E-7

1.8E-7

2.0E-7

2.2E-7

2.4E-7

2.6E-7

Oxi

datio

n Ra

te (m

ol/L

/s)

H+ (mol/L)

chemostat_SL chemostat_BR SL data fit BR data fit All data fit

Measured rates display “saturation effect” of [H+]

Scalp Level chemostatBrubaker Run chemostat

KH+

0 500 1000 1500 2000 25000.0

2.0E-7

4.0E-7

6.0E-7

8.0E-7

1.0E-6 chemostat_SL chemostat_BR SL data fit BR data fit All data fit

Oxi

datio

n Ra

te (m

ol/L

/s)

Influent Fe(II) (mg/L)

Measured rates also display saturation effect of [Fe2+]

Scalp Level chemostatBrubaker Run chemostat

KFe(II)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.70

1E-7

2E-7

3E-7

4E-7

5E-7

6E-7

7E-7

8E-7

9E-7

Oxi

datio

n Ra

te (m

ol/L

/s)

chemostat_SL_pH series chemostat_BR_pH series chemostat_SL_Fe(II) series chemostat_BR_Fe(II) series Linear Fit

y=8.93289*10-7

x+4.01526*10-8

r=0.91623

+in

+in

+Fe(II) inH

[Fe(II) ][H ]*

(K H ) (K Fe(II) )

We can now predict rate of low-pH Fe(II) oxidation based solely on pH and influent [Fe2+]

k*bio

aerationAMD Fe(II) oxidation limeneutralization sedimentation discharge

low-pH Fe(II) oxidizing bioreactor

AMD schwertmanniteremoval

limeneutralization sedimentation discharge

A

B

Low-pH Fe(II) oxidation can be incorporated into an active treatment plant

Low-pH Fe(II) oxidation can be engineered like a conventional wastewater treatment unit

Janneck, E., I. Arnold, T. Koch, J. Meyer, D. Burghardt and S. Ehinger (2010) “Microbial synthesis of schwertmannite from lignite mine water and its utilization for removal of arsenic from mine waters and for production of iron pigments.” Mine Water and Innovative Thinking IMWA 2010, p. 131-134.

Previous failures caused by accumulation of heavy mineral precipitates can be overcome and exploited by design modifications

Photograph of vertically-oriented growth media totally encrusted with iron minerals that are harvested and sold as pigments or used for arsenic removal

Reactor schematic from group in Germany

Bioreactor systems proposed for low-pH Fe(II) oxidation and low-pH Fe removal, followed by sulfate-reduction for metal recovery from acid pit lakes

pH 2.1 2.3 NaOH pH 3.5 pH 3.5 3.2

Hedrich and Johnson (2012) Bioresource Technology 106:44–49

34

Fe(T) removal was calculated as a zero-order rate, and normalized based on plan area of the bioreactor

Q[Fe(T)]in

Q[Fe(T)]out

V

GDM = g Fe(T) removed per day per m2 land area

GDM = ([Fe(T)]in – [Fe(T)]out)*Q

Atop

Atop

2.0 2.5 3.0 3.5 4.0 4.50

20

40

60

80

100

120

140

160

180

GD

M (g

Fe(

T)/d

/m2 )

chemostat_SL chemostat_BR

pH

Fe(T) removal was exceptionally high ~pH 3

GDM = 4 – 8sheet-flow“chunk” reactors

GDM = 20settling pond

Scalp Level chemostat 300 mg/L Brubaker Run chemostat [Fe(II)]in

Fe(T) removal increased with influent Fe(II)

GDM = 20settling pond

0 500 1000 1500 2000 25000

100

200

300

400

500G

DM

(g F

e(T)

/d/m

2 )

Influent Fe(II) (mg/L)

chemostat_SL chemostat_BR

Scalp Level chemostat pH 2.7Brubaker Run chemostat pH 2.9

Terraced iron formations (TIFs) formed via low-pH Fe(II) oxidation can be used in passive treatment systems

natural TIF Alkalinityinfluent effluent

natural TIF Alkalinityinfluent effluent

Scalp Level – very low emergent pH Upper Red Eyes – pH 4.1

2

3

4

5

pH[F

e2+]

or [F

e3+]

distance

[Tot

al F

e]

Fe2+

Fe3+

[Fe2

+]or

[Fe3

+]

distance

[Tot

al F

e]

Fe2+

Fe3+

2

3

4

5

pH

Conclusions • Low-pH Fe(II) oxidation is fastest at lower pH and

higher [Fe(II)]• Rates of Fe(II) oxidation can be predicted based on pH

and [Fe(II)]in

• At pH 3 and 300 mg/L Fe(II), suspended-growth bioreactors can remove Fe(T) at 100 – 400 g Fe/d*m2

• Attached-growth bioreactors will likely remove even more Fe(T)

• “Engineered” terraces are also effective in passive treatment systems