21 November 2008 Biogeochemical cycling of arsenic in sedimentary basin of southwestern Taiwan Wang,...

21 November 2008

Biogeochemical cycling ofBiogeochemical cycling ofarsenic in sedimentary basin ofarsenic in sedimentary basin of

southwestern Taiwansouthwestern Taiwan Wang, Sheng Wei

Department of Bioenvironmental Systems Engineering

National Taiwan University

Department of Bioenvironmental Systems Department of Bioenvironmental Systems Engineering NTUEngineering NTU

Introduction

Arsenic is carcinogenic, and can cause other human health effects

As(III), inorganic species in anaerobic environment, is generally considered more mobility and toxic than As(V), the primary form in aerobic environment

Most frequently found in groundwater As released from natural sources to groundwater is

the dominant cause


Introduction

As contamination in groundwater of the world As contamination in groundwater of the world


Introduction

As in Bangladesh As in Bangladesh


C h o u sh u i r iv e r

Ta i

wan

Str

a it

Cen

tra l

Mou

n ta i

n

3

2 175

2 98 1 31 1 73 22 39 3 7

3 02 62

3 63 51 91 56

3 9

2 8

2 4

2 21 4

4 4 03 4

4 13 83 32 71 8

1 61 011

3 12 52 01 2

1

23 4

56

789

1 0

111 21 31 41 5

1 6

1 7 1 8

1 92 0 2 1

2 22 3

2 4 2 52 6 2 7

2 82 9 3 0

3 13 23 3

0 1 0 2 0 k m

S ou th ern C h ou sh u i r iver a llu v ia l fan

C h ian an p la in

P e ik an g riv e r

P ach an g r iv e r

T sen g w en riv e r

E rn jen r iv e r

T a iw a n

Introduction

High As in deep wells of depth 60-300m has been proved to

be associated with blackfoot

disease of the Chianan plain

High As in shallow wells of

depth 30-40m have been found in the southern Choushui river alluvial fan (Yun-Lin county)


Aquaculture water

High As content groundwater

Effect human health

Sediment

Food

Fish/ShellfishBioaccumulation

Ingestion

Aquaculture Ecosystem

Drinking

As

As As

Introduction

Exposure pathway of As in southwest TaiwanExposure pathway of As in southwest Taiwan


-350

-300

-250

-200

-150

-100

-50

0

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

As (mg/L)D

epth

(m

)

-300

-250

-200

-150

-100

-50

0

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

As (mg/L)

Dep

th (

m)

High As

High As

High As

High As

Chianan plain Chianan plain

Choushui river alluvial fan Choushui river alluvial fan

The vertical distribution of As in groundwater in Taiwan The vertical distribution of As in groundwater in Taiwan


Study areasC h o u sh u i r iv e r

Ta i

wan

Str

a it

Cen

tra l

Mou

n ta i

n

3

2 175

2 98 1 31 1 73 22 39 3 7

3 02 62

3 63 51 91 56

3 9

2 8

2 4

2 21 4

4 4 03 4

4 13 83 32 71 8

1 61 011

3 12 52 01 2

1

23 4

56

789

1 0

111 21 31 41 5

1 6

1 7 1 8

1 92 0 2 1

2 22 3

2 4 2 52 6 2 7

2 82 9 3 0

3 13 23 3

0 1 0 2 0 k m

S o u th ern C h o u sh u i r iv er a llu v ia l fa n

C h ia n a n p la in

P e ik an g riv e r

P ach an g riv e r

T sen g w en riv e r

E rn jen r iv e r

A A '

B

B '

The southern Choushui

river alluvial fan 41 hydrological stations 107 monitoring wells

The Chianan plain 33 hydrological stations 100 monitoring wells


Chianan plain

Chianan plain

Choushui river alluvial fan

Choushui river alluvial fan

Hydrogeological profile

Hydrogeological profile

Study areas


Topic

11Where is the As in ground water from ?Where is the As in ground water from ?

33Have the As problemsalready been solved ? Have the As problemsalready been solved ?

22How dose the As releaseinto groundwater ?How dose the As releaseinto groundwater ?

Characterization of groundwater quality

Composition of sedimentary deposits



Taiwan Sugar Company (2003; 2004) As concentrations of groundwater

0.12±0.14 mg/L in the southern Choushui river alluvial fan

0.30±0.35 mg/L in the Chianan plain

Monitoring wells of that As > 0.01 mg/L 70% in the southern Choushui river alluvial fan> 95% in the Chianan plain


Factor analysis Liu et al. (2003): The Choushui river alluvial fan 33 hydrological stations, 100 monitoring wells of the Chianan

plain 3 layers: 0-70m, 70-170m, and >170m EC, Eh, pH, TDS, Alk, TOC, NH4

+, SO42-, Cl-, Ca2+, Mg2+, Na+,

K+, As, Fe



Factor analysis

Factor EigenvaluePercentageof variance

Cumulative percentageof variance

1 7.55 50.34 50.34

2 2.79 18.62 68.96

3 1.28 8.56 77.52

4 0.79 5.24 82.75

5 0.68 4.51 87.27

6 0.48 3.18 90.45

7 0.42 2.77 93.22

8 0.31 2.08 95.30

9 0.27 1.83 97.12

10 0.20 1.33 98.45

11 0.12 0.80 99.25

12 0.08 0.52 99.76

13 0.03 0.17 99.93

14 0.01 0.04 99.98

15 0.00 0.02 100.00


Variable Factor 1 Factor 2 Factor 3

EC 0.85 0.02 0.14

Eh -0.42 -0.67 0.15

pH -0.09 0.37 -0.79

TDS 0.98 0.02 0.09

Alk -0.14 0.82 -0.07

SO42- 0.89 -0.12 -0.05

Cl- 0.97 0.04 0.13

Ca2+ 0.79 -0.18 0.23

Mg2+ 0.93 -0.12 0.01

Na+ 0.97 0.05 0.10

K+ 0.95 -0.06 0.02

As -0.19 0.68 0.07

Fe 0.18 0.40 0.72

TOC 0.00 0.91 0.09

NH4+ 0.61 0.18 0.38

Loadings for three-factors Loadings for three-factors

As enrichment factorAs enrichment factorSalinization factorSalinization factor

Factor analysis


Redox zoning

Redox zoning Chen and Liu (2003): The Choushui river alluvial fan Definition of redox zones were same with Chen and Liu (2003) zone 1: Eh >0, and DO or NO3

->0.5 mg/L

zone 2: Eh <0, and with detectable dissolved sulfide zone 3: Eh <0, and without detectable dissolved sulfide


Redox zoning

1 5 0 0 0 0 1 6 0 0 0 0 1 7 0 0 0 0 1 8 0 0 0 0 1 9 0 0 0 0 2 0 0 0 0 02 5 3 0 0 0 0

2 5 4 0 0 0 0

2 5 5 0 0 0 0

2 5 6 0 0 0 0

2 5 7 0 0 0 0

2 5 8 0 0 0 0

2 5 9 0 0 0 0

2 6 0 0 0 0 0

2 6 1 0 0 0 0

1

234 5

7891 0 111 2

1 3 1 41 5

1 6 1 7

1 8 1 9 2 02 1

2 22 3

2 42 52 6 2 7

2 8 2 93 0

3 1

3 2

1 5 0 0 0 0 1 6 0 0 0 0 1 7 0 0 0 0 1 8 0 0 0 0 1 9 0 0 0 0 2 0 0 0 0 02 5 3 0 0 0 0

2 5 4 0 0 0 0

2 5 5 0 0 0 0

2 5 6 0 0 0 0

2 5 7 0 0 0 0

2 5 8 0 0 0 0

2 5 9 0 0 0 0

2 6 0 0 0 0 0

2 6 1 0 0 0 0

3 3

3 1

3 0 2 92 8

2 7 2 5

2 3 2 22 1

2 01 9

1 61 5

1 411

9 8 7

64 3

1

1 5 0 0 0 0 1 6 0 0 0 0 1 7 0 0 0 0 1 8 0 0 0 0 1 9 0 0 0 0 2 0 0 0 0 02 5 3 0 0 0 0

2 5 4 0 0 0 0

2 5 5 0 0 0 0

2 5 6 0 0 0 0

2 5 7 0 0 0 0

2 5 8 0 0 0 0

2 5 9 0 0 0 0

2 6 0 0 0 0 0

2 6 1 0 0 0 0

1

4 5

891 0 111 2

1 31 5

1 6 1 7

1 8 1 9 2 02 1

2 22 3

2 5

2 8 2 93 0

3 23 3

1 7 0 0 0 0 1 8 0 0 0 0 1 9 0 0 0 0 2 0 0 0 0 0 2 1 0 0 0 0 2 2 0 0 0 0

2 6 0 0 0 0 0

2 6 1 0 0 0 0

2 6 2 0 0 0 0

2 6 3 0 0 0 0

2 6 4 0 0 0 0

2 6 5 0 0 0 0

2 6 6 0 0 0 0

2 6 7 0 0 0 0

3

2 175

2 98 1 31 1 73 22 39 3 7

3 02 62

3 63 51 91 56

3 9

2 8

2 4

2 21 4

4 4 03 4

4 13 83 32 7

1 8

1 61 011

3 12 52 01 2

1 7 0 0 0 0 1 8 0 0 0 0 1 9 0 0 0 0 2 0 0 0 0 0 2 1 0 0 0 0

2 6 0 0 0 0 0

2 6 1 0 0 0 0

2 6 2 0 0 0 0

2 6 3 0 0 0 0

2 6 4 0 0 0 0

2 6 5 0 0 0 0

2 6 6 0 0 0 0

2 6 7 0 0 0 0

3

2 175

2 98 1 31 1 73 22 39 3 7

3 02 62

3 63 51 91 56

3 9

2 8

2 4

2 21 4

4 4 03 4

4 13 83 32 7

1 8

1 61 011

3 12 52 01 2

1 7 0 0 0 0 1 8 0 0 0 0 1 9 0 0 0 0 2 0 0 0 0 0 2 1 0 0 0 0

2 6 0 0 0 0 0

2 6 1 0 0 0 0

2 6 2 0 0 0 0

2 6 3 0 0 0 0

2 6 4 0 0 0 0

2 6 5 0 0 0 0

2 6 6 0 0 0 0

2 6 7 0 0 0 0

3

2 175

2 98 1 31 1 73 22 39 3 7

3 02 62

3 63 51 91 56

3 9

2 8

2 4

2 21 4

4 4 03 4

4 13 83 32 7

1 8

1 61 011

3 12 52 01 2

0-70 m 70-170 m >170 m

Zone 1

Zone 2

Zone 3

Distribution of redox zones at different depths Distribution of redox zones at different depths

Zone 1 Eh >0 DO or NO3

->0.5 mg/LZone 2 Eh <0 with sulfideZone 3 Eh <0 without sulfide

Aquifer 1 Aquifer 3Aquifer 2



13 drilling stations 655 samples

3 m intervals, <100m 10 m intervals, >100m

Analysis AAS+HG

11

32

1

5

1012

7

9

13

6 8

4

0 1 0 2 0 3 0

C h o u sh u i R iv e r

P e ik ang R iv e r

Tai

wan

Str

a it

Cen

tra l

Mou

n ta i

n

Y u n -L in co u n ty

T a iw a n

k m



As distribution of core samples

As distribution of core samples

Log-normaldistribution

Log-normaldistribution

Geometric mean: 2.27±1.43 mg/kg

Max: 590 mg/kg


0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0-3 0 0

-2 5 0

-2 0 0

-1 5 0

-1 0 0

-5 0

0

Dep

th(m

)

-0 .2

0 .7 5

0 .8 6

6 7 8 9 1 0 11 1 2 1 3

lo g (A s(m g /k g ))

D is tan ce (x 1 0 0 m )

0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0

D is tan ce (x 5 0 m )

-3 0 0

-2 5 0

-2 0 0

-1 5 0

-1 0 0

-5 0

0

Dep

th(m

)

-0 .2

0 .7 5

0 .8 6

lo g (A s(m g /k g ))

1 2 3 4 5 6


Log CAs(mg/kg) distributions in the west-east and north-south cross section Log CAs(mg/kg) distributions in the west-east and north-south cross section

11

32

1

5

1012

7

9

13

6 8

4

0 1 0 2 0 3 0

C h o u sh u i R iv e r

P e ikang R iv e r

Ta i

wan

Str

a it

Cen

tral

Mou

n tai

n


T a iw a n

k m


Station Depth (m) As (mg/kg)

1 54 590.00

3 162 117.00

3 180 107.50

8 130 102.60

1 63 76.80

1 6 75.35

10 39 75.08

10 272 74.64

10 152 67.48

11 30 61.88

10 21 56.55

8 12 54.90

1 60 52.35

ShallowShallow

DeepDeep

BothBoth

Tabulate CAs > 50 mg/kg of different drilling stations and depths Tabulate CAs > 50 mg/kg of different drilling stations and depths


non-marine sequence 1 (<3,000 years B.P.)

marine sequence 1 (3,000-9,000 years B.P.)

non-marine sequence 2 (9,000-35,000 years B.P.)

marine sequence 2 (35,000-50,000 years B.P.)

non-marine sequence 3 (>50,000 years B.P.)

UnknownClaySandGravel

11

32

1

5

1012

7

9

13

6 8

4

0 .0 0 0 1 0 0 0 0 .0 0 0 2 0 0 0 0 .0 0 0 3 0 0 0 0 .0 0 0

C h o u -S h u i R iv e r


T a iw a n

100 20 30km

Plain view of the marine and non-marine sequences distribution (Huang, 1996)

Plain view of the marine and non-marine sequences distribution (Huang, 1996)


SandSiltClay

log(

As

Con

cent

rati

on)

3.0

2.5

2.0

1.5

1.0

.5

0.0

-.5

□﹕75% and 25%□﹕50%┬﹕non-outlier Max┴﹕non-outlier Min○﹕outlier 1.5~3 × box＊﹕ extremes >3 × box


Box-plot of log CAs of clay, silt and sand types of core samples Box-plot of log CAs of clay, silt and sand types of core samples


Station Depth (m) Age (year) As (mg/kg)1 108.4 >52000 5.32

3 32.7 2931±81 29.34

3 48.9 8819±99 51.35

3 117.9 43900±1500 13.16

5 157.8 >45000 5.43

6 39.4 3936±64 11.31

7 23.0 5364±89 4.83

7 42.7 8440±90 8.27

7 111.4 39900±1100 110

7 129.6 >50000 1.71

8 91.6 >50000 34.04

9 25.1 7370±100 10.89

9 25.1 7620±80 10.89

9 48.8 9230±60 2.61

9 68.6 36400±500 6.63

9 105.1 >50000 17.22

13 38.2 8330±60 11.51

13 38.8 7920±70 11.51

13 55.0 7090±60 3.37

13 60.0 7850±60 3.01

13 99.0 43300±130 4.61

Geological dating v.s. CAs of core samples Geological dating v.s. CAs of core samples


R2 = 0.51

0.0

0.5

1.0

1.5

2.0

2.5

-2.0 -1.5 -1.0 -0.5 0.0

log (As) in groundwater (mg/L)

log

(As)

of c

ore

sam

ples

in a

quita

rds

(mg/

kg)

As of core samples in aquitards v.s. As of ground water

As of core samples in aquitards v.s. As of ground water

As of core samples in aquifers v.s. As of ground water

As of core samples in aquifers v.s. As of ground water

R2 = 0.21

0.0

0.5

1.0

1.5

2.0

2.5

-2.0 -1.5 -1.0 -0.5 0.0

log (As) in groundwater (mg/L)

log

(As)

of co

re s

ampl

es in

aqui

fers

(mg/

kg)



Smedley and Kinniburgh (2002) Extreme As concentrations in natural water are rare, but are most

frequently observed in groundwater

Welch et al. (2000); Nordstrom (2002) Release from natural sources is the dominant cause of elevated As in

groundwater

Edmonds and Francesconi (1997); Francesconi et al. (1998) High As in the marine formation may be attributed to the

bioaccumulation and biotransformation of As in the sea organisms


Topic

22


33Have the As problemsalready been solved ? Have the As problemsalready been solved ?

How dose the As releaseinto groundwater ?How dose the As releaseinto groundwater ?


As release to groundwater

Mandal et al. (1996); Loeppert (1997); Wilkie and Hering (1998) The oxidation of As-bearing pyrite minerals

Nickson et al. (2000); McArthur et al. (2001) Reductive dissolution of As-rich Fe oxy-hydroxides

Acharyya et al. (1999) Competitive exchange of adsorbed As on aquifer minerals with

phosphate ions that migrate into aquifers from the application of

fertilizers to surface soil

Harvey et al. (2002); Smedley and Kinniburgh, (2002) Reductive dissolution of Fe oxy-hydroxides under reducing conditions

is the most probable reason of As accumulation in groundwater


Geochemical modeling

Geochemical calculation: PHREEQC Chemical speciation Saturation index (SI)

SI<0: the potential for dissolution SI>0: the potential for precipitation

3 monitoring wells of 3 layers of the Chianan plain and the southern Choushui river alluvial fan pH, Eh, temperature, DO, Alk, SO4

2-, Cl- , Ca2+, Mg2+, Na+, K+, As, Fe, Mn, NH4

-, NO3- and HS-



1 7 0 0 0 0 1 8 0 0 0 0 1 9 0 0 0 0 2 0 0 0 0 0 2 1 0 0 0 0

2 6 0 0 0 0 0

2 6 1 0 0 0 0

2 6 2 0 0 0 0

2 6 3 0 0 0 0

2 6 4 0 0 0 0

2 6 5 0 0 0 0

2 6 6 0 0 0 0

2 6 7 0 0 0 0

3 8

2 66

1 7 0 0 0 0 1 8 0 0 0 0 1 9 0 0 0 0 2 0 0 0 0 0 2 1 0 0 0 0 2 2 0 0 0 0

2 6 0 0 0 0 0

2 6 1 0 0 0 0

2 6 2 0 0 0 0

2 6 3 0 0 0 0

2 6 4 0 0 0 0

2 6 5 0 0 0 0

2 6 6 0 0 0 0

2 6 7 0 0 0 0

4 12 7

6

1 7 0 0 0 0 1 8 0 0 0 0 1 9 0 0 0 0 2 0 0 0 0 0 2 1 0 0 0 0

2 6 0 0 0 0 0

2 6 1 0 0 0 0

2 6 2 0 0 0 0

2 6 3 0 0 0 0

2 6 4 0 0 0 0

2 6 5 0 0 0 0

2 6 6 0 0 0 0

2 6 7 0 0 0 0

4 1

2 3

6

150000 160000 170000 180000 190000 2000002530000

2540000

2550000

2560000

2570000

2580000

2590000

2600000

2610000

1 2

2 1

2 8

150000 160000 170000 180000 190000 2000002530000

2540000

2550000

2560000

2570000

2580000

2590000

2600000

2610000

1 9 2 0

2 7

150000 160000 170000 180000 190000 2000002530000

2540000

2550000

2560000

2570000

2580000

2590000

2600000

2610000

1 6

2 83 0



Chemical species of groundwater in 3 layersChemical species of groundwater in 3 layers



Saturation index of groundwater in 3 layersSaturation index of groundwater in 3 layers


Step Extractant Target phaseMg 1M MgCl2 Ionically bound As

PO4 1M NaH2PO4 Strongly adsorbed As

HCl 1N HCl Coprecipitated with AVS, carbonates, Mn oxide

Ox 0.2M ammonium oxalate/oxalic acid Coprecipitated with amorphous Feoxyhydroxides

Ti 0.05M Ti( )-citrate-EDTA-Ⅲbicarbonate

Coprecipitated with crystalline Feoxyhydroxides

HF 10M HF As oxides and As coprecipitated withsilicates

HNO3 16N HNO3

Coprecipitated with pyrite andamorphous As2S3

hot HNO3 16N HNO3+30% H2O2Orpiment and remaining recalcitrant

As minerals

Sequential extraction

Surface analysesSurface analyses X-ray diffractometer (XRD) X-ray fluorescence (XRF) High resolution x-ray photoelectron spectrometer (HR-XPS)High resolution x-ray photoelectron spectrometer (HR-XPS) Scanning electron microscope (SEM-EDS)Scanning electron microscope (SEM-EDS)


Oremland and Stolz (2003)

Bacteria-mediated mobilization of As into groundwater Bacteria-mediated mobilization of As into groundwater



Islam et al. (2004) Reduction of ferric iron took place

before the As release to groundwater The iron-reducing bacteria played a

major role in the subsequent reduction

and release of As The delivery of surface - drived organic

carbon into subsurface communities

may have a dramatic role in As mobility



Herbel and Fendorf (2006)

The effects of bacterial processes on arsenic mobilityin iron (hydr)oxide systems

The effects of bacterial processes on arsenic mobilityin iron (hydr)oxide systems



Groundwater of YL7 was collected for IRB enrichment Geometric mean of As: 360.57 ppb

C h o u sh u i r iv e r

Y L 7

0 1 0 2 0 k m

S ou th ern C h ou sh u i r iv er a llu v ia l fa n

P e ik an g r iv e r

T a iw a n(R O C )

screen

-25

-5

-10

-15

-20

0

gravalsilty sandclay

As release to groundwater- Batch experiments of IRB


Inject 1 ml groundwater into 10 ml autoclaved IRB

medium H2 or acetate or citrate + Fe3+ → HCO3

- + Fe2+ + H+

Orange →colorless (about 10 days) Transfer 4 times



Lu (2007) Amorphous Fe (hydr)oxides (HFO) are the primary Fe

mineral and correlate well with the As of core samples 0.5M NaOH was dropwised to 100ml of 0.05M Fe(NO3)3

until pH=7.5-8 HFO reducing experiments (anaerobic condition)

A: HFO+citrate+IRB B: HFO+citrate C: HFO+acetate+IRB D: HFO+acetate



As(V) reducing experiments E: As(V)+citrate+IRB F: As(V)+citrate G: As(V)+acetate+IRB H: As(V)+acetate

[As(V)-HFO] reducing experiments I: [As-HFO]+citrate+IRB J: [As-HFO]+citrate K: [As-HFO]+acetate+IRB L: [As-HFO]+acetate

As(III) and As(V) were analyzed by AAS+HG Fe(II) was measured colorimetrically by ferrozine method



HFO reducing experiments

0

50

100

150

200

250

0 5 10 15

Time (days)

Fe(

Ⅱ)

(mg/

l)

HFO+IRB+citrateHFO+citrateHFO+IRB+acetateHFO+acetate



(a)0

5

10

15

20

25

30

0 5 10 15 20

Time (days)

As

(mg/

l)

As (Ⅲ )

As (Ⅴ )

Total As

(c)0

5

10

15

20

25

30

0 5 10 15 20

Time (days)

As

(mg/

l)

As (Ⅲ )

As (Ⅴ )

Total As

As(V) reducing experiments

(b)0

5

10

15

20

25

30

0 5 10 15 20

Time (days)

As

(mg/

l) As (Ⅲ )

As (Ⅴ )

Total As

(d)0

5

10

15

20

25

30

0 5 10 15 20

Time (days)

As

(mg/

l)As (Ⅲ )

As (Ⅴ )

Total As

As(V) + citrate + IRB

As(V) + citrate + IRB

As(V) + acetate + IRB

As(V) + acetate + IRB

As(V) + citrate As(V) + citrate As(V) + acetate As(V) + acetate



Results and disscusions

Direct enzymatic microbial reduction of As(V) (arsC)

Detoxification pathway (arsC pathway)

Respiratory pathway (arrA pathway)

As(V)-reducing bacteria are simultaneously cultured of the enrichment experiments

Plausible microbial-mediated processes of As(V) reduction



[As(V)-HFO] reducing experiments

(a)0

2

4

6

8

10

12

0 5 10 15 20

Time (days)

As

(mg/

l)

0

50

100

150

200

250

Fe(Ⅱ

) (m

g/l)

As (Ⅲ )As (Ⅴ )Total AsFe (Ⅱ )

(b)

0

2

4

6

8

10

12

0 5 10 15 20

Time (days)

As

(mg/

l)

0

50

100

150

200

250Fe

(Ⅱ) (

mg/

l)


(c)

0

1

2

3

4

5

0 5 10 15 20

Time (days)

As

(mg/

l)

0

20

40

60

80

100

Fe(

Ⅱ)

(mg/

l)


(d)

0

1

2

3

4

5

0 5 10 15 20

Time (days)

As

(mg/

l)

0

20

40

60

80

100

Fe(Ⅱ

) (m

g/l)


[As(V)-HFO] + citrate + IRB

[As(V)-HFO] + citrate + IRB

[As(V)-HFO] + acetate[As(V)-HFO] + acetate[As(V)-HFO] + citrate[As(V)-HFO] + citrate

[As(V)-HFO] + acetate + IRB

[As(V)-HFO] + acetate + IRB


Microbial reduction ofAs(V) by IRB

Reduction of As-rich Feoxy-hydroxides

Biogeochemical processes for As release

As(V) reduction took place after Fe(III) reduction and As(V)desorption, because IRB reduce aqueous As(V) following the

consumption of Fe(III) mineral

Direct enzymatic Indirect processes



This reduction reactions are promoted by IRB Mobilization:

Citrate: desorption of As from the surface of HFO is the

main process of As release Acetate: As release caused by reductive dissolution of HFO

via IRB driven

Transformation: Some IRB have the ability to reduce aqueous As(V) after the

reduction of Fe(III) minerals



Topic

33


Have the As problemsalready been solved ? Have the As problemsalready been solved ?

22How dose the As releaseinto groundwater ?How dose the As releaseinto groundwater ?


Future works

amorphous iron hydroxide (HFO)

Fe(III) Fe(III)

As(V) As(V)

Fe(II) (decrease As(V) sorption sites)

As(V)

As(III)

macromoleculeorganic carbon

degradatedorganic carbon

decrease of sorption sites

e- donor

Mobilization: desorption

aquifer

aquitardanaerobic environmentlow Eh

e- donor

Transport

:competitive desorption :reductive dissolution by IRB

competition of sorption sites

Transformation: reduced by IRB

Conceptual model of As mobilization and transformation in groundwaterConceptual model of As mobilization and transformation in groundwater


21 November 2008

Thanks for your attention

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21 November 2008 Biogeochemical cycling of arsenic in sedimentary basin of southwestern Taiwan Wang,...

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