DSpace at VNU: Mobilization of arsenic and iron from Red River floodplain sediments, Vietnam

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Geochimica et Cosmochimica Acta 74 (2010) 3367–3381
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Mobilization of arsenic and iron from Red River
floodplain sediments, Vietnam
Dieke Postma a,*, Søren Jessen a, Nguyen Thi Minh Hue b, Mai Thanh Duc b,
Christian Bender Koch c, Pham Hung Viet b, Pham Quy Nhan d, Flemming Larsen a
a

Dept. of Geochemistry, National Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen, Denmark
b
Research Centre for Environmental Technology and Sustainable Development (CETASD), Hanoi University of Science,
VNU, 334-Nguyen Trai, Thanhxuan Dist., Hanoi, Viet Nam
c
Dept. of Basic Sciences and Environment, University of Copenhagen, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Denmark
d
Hanoi University of Mining and Geology, Dong Ngac Dist., Hanoi, Viet Nam
Received 25 September 2009; accepted in revised form 19 March 2010; available online 30 March 2010

Abstract
Sediments from the Red River and from an adjacent floodplain aquifer were investigated with respect to the speciation of
Fe and As in the solid phase, to trace the diagenetic changes in the river sediment upon burial into young aquifers, and the
related mechanisms of arsenic release to the groundwater. Goethite with subordinate amounts of hematite were, using
Mo¨ssbauer spectroscopy, identified as the iron oxide minerals present in both types of sediment. The release kinetics of
Fe, As, Mn and PO4 from the sediment were investigated in leaching experiments with HCl and 10 mM ascorbic acid, both
at pH 3. From the river sediments, most of the Fe and As was mobilized by reductive dissolution with ascorbic acid while HCl
released very little Fe and As. This suggests As to be associated with an Fe-oxide phase. For oxidized aquifer sediment most
Fe was mobilized by ascorbic acid but here not much As was released. However, the reduced aquifer sediments contained a
large pool of Fe(II) and As that is readily leached by HCl, probably derived from an unidentified authigenic Fe(II)-containing

mineral which incorporates As as well. Extraction with ascorbic acid indicates that the river sediments contain both As(V) and
As(III), while the reduced aquifer sediment almost exclusively releases As(III). The difference in the amount of Fe(II) leached
from river and oxidized aquifer sediments by ascorbic acid and HCl, was attributed to reductive dissolution of Fe(III). The
reactivity of this pool of Fe(III) was quantified by a rate law and compared to that of synthetic iron oxides. In the river mud,
Fe(III) had a reactivity close to that of ferrihydrite, while the river sand and oxidized aquifer sediment exhibited a reactivity
ranging from lepidocrocite or poorly crystalline goethite to hematite. Mineralogy by itself appears to be a poor predictor of
the iron oxide reactivity in natural samples using the reactivity of synthetic Fe-oxides as a reference. Sediments were incubated, both unamended and with acetate added, and monitored for up to 2 months. The river mud showed the fastest release
of both Fe and As, while the effect of acetate addition was minor. This suggests that the presence of reactive organic carbon is
not rate limiting. In the case of the river and aquifer sediments, the release of Fe and As was always stimulated by acetate
addition and here reactive organic carbon was clearly the rate limiting factor. The reduced aquifer sediment apparently
can sustain slower but prolonged microbially-driven release of As. The highly reactive pools of Fe(III) and As in the river
mud could be due to reoxidation of As and Fe contained in the reducing groundwater from the floodplain aquifers that
are discharging into the river. Deposition of the suspended mud on the floodplain during high river stages is proposed to
be a major flux of As onto the floodplain and into the underlying aquifers.
Ó 2010 Elsevier Ltd. All rights reserved.

*

Corresponding author. Tel.: +45 38142784.
E-mail address: (D. Postma).

0016-7037/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.gca.2010.03.024


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1. INTRODUCTION

The widespread contamination of groundwater with
arsenic in the floodplain sediments from SE Asia has been
extensively documented and the associated health risks
are a matter of deep concern (Yu et al., 2003; Polya
et al., 2005; Ahmed et al., 2006; Berg et al., 2007). In order
to alleviate this threat it is of great importance to elucidate
the processes controlling the mobilization of arsenic. Overall, our conceptual understanding of arsenic mobilization
into the groundwater is as follows. Sediments (sand and
mud) are transported by the rivers from the Himalayan
mountain range to the flood plains. The sediments are
deposited on the floodplain and develop aquifers and aquitards, which become anoxic and start to release arsenic to
the groundwater. Most researchers ascribe the mobilization
of arsenic to the reduction of As-containing Fe-oxides or
Fe-oxyhydroxides, henceforth collectively called Fe-oxides,
by organic carbon oxidation (Nickson et al., 1998, 2000;
McArthur et al., 2001; Dowling et al., 2002; Harvey
et al., 2002; Swartz et al., 2004; Postma et al., 2007). However, the more detailed mechanisms that are involved remain unclear. One possible scenario is that arsenic
initially is present as As(V) within the Fe-oxide crystal lattice and during reductive dissolution of the Fe-oxide the
arsenate ions are released. The liberated arsenate ion may
be reduced to arsenite either in aqueous solution or at the
surface of the Fe-oxide. In addition, both arsenate and arsenite can adsorb or desorb to the surface of the remaining
Fe-oxide (Dixit and Hering, 2003; Stachowicz et al.,
2008). An alternative scenario is that both arsenate and
arsenite initially are present at the surface of the Fe-oxide
and as the surface of the Fe-oxide diminishes due to reduction, the surface species must go into solution. Pedersen
et al. (2006) co-precipitated trace amounts of arsenate with
Fe-oxides and found for ferrihydrite that most As(V) coordinated to the surface, whereas for goethite a large part was
incorporated in the crystal lattice. Pedersen et al. (2006) and
Tufano and Fendorf (2008) showed that, during the reductive dissolution of an As-containing-Fe-oxide, the releases
of As and Fe are non-stoichiometric. These scenarios indicate a large number of possible pathways for arsenic mobilization, which even for simple synthetic systems are poorly

understood. Also, the role of microbes in the reduction of
arsenate and the coupling between the processes of Fe(III)
and arsenate reduction are extensively discussed (Akai
et al., 2004; Islam et al., 2004; van Geen et al., 2004;
Polizzotto et al., 2006; Heimann et al., 2007; Lear et al.,
2007; Sutton et al., 2009). Finally, other mineral species
in the sediments like carbonates (Roma´n-Ross et al.,
2006; Alexandratos et al., 2007; Sø et al., 2008) or silicates
(Goldberg and Glaubig, 1988; Chakraborty et al., 2007)
may also adsorb arsenic, thereby further complicating the
process of arsenic release into the groundwater.
Hydrogeological pathways may be part of the mobilization scenario as well. Polizzotto et al. (2008) suggested that
most arsenic in the 6000-year-old Mekong delta aquifer is
mobilized in surface soil layers and is subsequently transported down through the sandy aquifer. In this case the
retardation of aqueous arsenic species becomes important.

On the other hand, Postma et al. (2007) observed in a very
young Red River aquifer that the mobilization of arsenic
did occur within the aquifer.
There is a clear need for a more mechanistic insight into
the water–sediment interactions that are involved in the
mobilization of arsenic into groundwater. As a first step a
better understanding of the solid phase speciation of As
and Fe in the sediments should be sought. Our current
knowledge is based on bulk sediment analysis (Kocar
et al., 2008), sequential extraction data (Dowling et al.,
2002; Akai et al., 2004; Swartz et al., 2004), and spectroscopic techniques like EXAFS and XANES (Polizzotto
et al., 2006; Itai et al., 2010). Currently, there is not much
information concerning the mineralogy of the Fe-oxides
in the Holocene aquifer sediments to which As is supposed

to be associated (Akai et al., 2004; Polizzotto et al., 2006;
Rowland et al., 2008). How the solid phase data are to be
interpreted in terms of mobilization of As is also not clear.
The two approaches so far employed are desorption of As
from the sediment in the laboratory (Polizzotto et al.,
2006) or the field (Harvey et al., 2002), and sediment incubation studies (Islam et al., 2004; van Geen et al., 2004;
Gault et al., 2005; Anawar et al., 2006; Radloff et al.,
2007). Although such experiments have demonstrated the
release of arsenic from the sediment, the mechanisms
involved have not been well defined.
The objective of this study is to improve our understanding of the mobilization of arsenic from the Holocene
aquifer sediments into groundwater. Our field site is located along the Red River, Vietnam, where previous work
has shown extensive mobilization of arsenic in the floodplain aquifer (Postma et al., 2007). We compare river and
aquifer sediments in order to detect diagenetic changes in
the sediment composition that may reveal the processes involved. The methods employed, range from mineralogical
investigations using Mo¨ssbauer spectroscopy, extractions
over time in ascorbic acid and HCl and finally sediment
incubation. The leaching experiments with ascorbic acid
and HCl give new insight concerning the kinetics of iron
and arsenic mobilization, provide data about the redox
speciation of arsenic in the solid phase and allow the determination of the reactivity of the Fe(III) pool in the sediment. Finally, incubation experiments explore the release
of Fe and As on a larger time scale under close to in situ
conditions.
2. METHODS
2.1. Sediment sampling
The sediments were collected at our field site along the
Red River, 30 km upstream of Hanoi (Postma et al.,
2007). The site is located between the river and the dyke
and is subject to seasonal flooding. Aquifer sediments were
sampled and stored as described by Postma et al. (2007).

Results are presented for one sample from the oxidized
zone (6.6–7.5 m depth) and one from the reduced zone
(9.5–10.0 m depth) of the sand aquifer. Aquifer sediments
from 15 m depth have been dated to be around 460 years
old (Larsen et al., 2008) and consequently the aquifer sedi-


Mobilization of arsenic

ment used for experiments here must be even younger. The
river sand and mud were sampled nearby at the same low
energy bend of the river as surficial sediments in shallow
water. River and aquifer sediments were stored in O2impermeable Al-laminate bags. In this state the samples
were transported to Denmark where they were kept refrigerated at 10 °C.
2.2. Mineralogy of fine size fractions
All handling of the samples was done inside an anoxic
glove box. The fine size fractions (<20 lm) of the sandy
samples were separated by sedimentation in deoxygenated
water and freeze dried. Mud samples were freeze dried without pre-treatments. Mo¨ssbauer spectra at temperatures between 80 and 20 K were obtained using closed cycle
cryostats. The samples were transferred into Perspex sample holders and frozen in liquid nitrogen before being removed from the glove box. Handling and measuring the
samples at 80 K and below, did not cause any changes to
the sample, as checked by remeasuring samples. The spectrometer was calibrated using the center and splitting of
the room temperature spectrum of a thin foil of natural
iron. The spectra were fitted using a combination of
Lorentzian shaped doublets and sextets constrained to be
pair-wise identical, and having area ratios of 1:1 and
3:2:1, respectively. Assuming that all components have
identical f-factors at 20 K, the relative spectral areas can
be directly converted into abundances of Fe in the identified
phases. For XRD analyses a part of the dry sample was exposed to air (causing at least partial oxidation of ferrous

iron) and scanned using a Siemens D5000 diffractometer
equipped with Co Ka radiation and a diffracted beam
monochromator.
2.3. Kinetic experiments
Leaching experiments with HCl and ascorbic acid were
carried in a 1-L reaction vessel equipped with an automatic
titrator to maintain a constant pH of 3 in the suspension.
For each sediment sample, two parallel experiments were
carried out; one with a 1 mM HCl solution (pH 3) and
one with a 10 mM ascorbic acid solution adjusted to pH
3 with HCl, similar to the procedure used by Larsen et al.
(2006).
2.4. Incubation experiments
Known amounts of wet sediment were loaded into 1 L
glass bottles, filling the bottles about halfway with sediment. The bottles were then topped up with water from
the Red River. For each type of sediment parallel incubations were done, one unamended and one with 4 g/L
CH3COONa added (Islam et al., 2004). After flushing with
nitrogen to expel oxygen left in the bottles, all bottles were
sealed with septum caps. The bottles with incubated samples were shaken every day. Samples were collected by
penetrating the septum using a nitrogen flushed syringe
and needle.

3369

2.5. Analytical procedures
Water samples were collected at various times in 50 mL
polypropylene syringes and filtered through 0.2 lm Sartorius Minisart cellulose acetate filters. Aqueous As(V) and
As(III) were separated by filtering the water sample
through first a 0.2-lm membrane filter and then a disposable anion exchange cartridge at a flow rate of approximately 6 mL/min using the syringe. The anion exchange
cartridge was mounted directly onto the filter and the

assembly was carefully flushed by N2 before use to avoid
oxidation of Fe2+. The cartridges contained 0.8 g aluminosilicate adsorbent that selectively adsorbs As(V) but not
As(III) (Meng and Wang, 1998). Arsenite was determined
as the As concentration in the water filtered through a cartridge, and As(V) was calculated as the difference between
the total As and As(III) concentrations.
Ferrous iron and phosphate concentrations were measured spectro-photometrically immediately after sampling,
using a Hach DR/2010 instrument. Ferrous iron was measured by the Ferrozine method (Stookey, 1970), and phosphate using the molybdenum blue method. The detection
limits were 1.8 and 1.1 lM, respectively. Arsenic was analyzed with a Shimadzu Atomic Absorption Spectrophotometer AAS-6800 using a HVG hydride generator and a
graphite furnace or alternatively a flow injection system
(FIHG-AAS), using a Perkin-Elmer 5000 with a deuterium
background corrector. Ca, Fe and Mn were determined by
ICP-OES. Detection limits were as follows; As 0.013 lM,
Mn 0.91 lM and Ca 0.50 lM.
3. RESULTS
This study reports on sediment composition and its relation to arsenic mobilization in the Red River floodplain.
Results are presented from; (a) recent sediments from the
Red River, one sandy sample and one mud sample, and
(b) aquifer sediments from the boring reported on by
Postma et al. (2007) selecting one sample from the oxidized
zone and one from the reduced zone.
3.1. Iron mineralogy
Powder XRD did not reveal any peaks indicating the
presence of iron oxides, siderite, pyrite or vivianite. However, preferentially orientated XRD did indicate the presence of chlorite, smectite, illite and kaolinite as the major
phyllosilicate minerals. The Mo¨ssbauer spectra of the samples, measured at 20 K, can be fitted well by the use of two
doublets and two sextets (Fig. 1). The doublets (isomer
shifts of 0.45 and 1.28 mm/s and quadrupole splittings of
0.81 and 2.84 mm/s, respectively) indicate ferric and ferrous
ions within phyllosilicates (Murad and Cashion, 2004). The
Mo¨ssbauer parameters of the doublets do not permit the
identification of specific types of phyllosilicates and also

the discrimination towards a number of other structures
with octahedrally coordinated iron is difficult. The parameters of the sextets are typical for sedimentary hematite
(magnetic hyperfine field of 53.0 T, quadrupole shift
À0.09 mm/s and isomer shift of 0.49 mm/s) and goethite


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D. Postma et al. / Geochimica et Cosmochimica Acta 74 (2010) 3367–3381

Fig. 1. Mo¨ssbauer spectra, obtained from the <20 lm fraction at 20 K, of river and aquifer sediments. Peaks indicative for the presence of
goethite (G) and hematite (H) as well as of Fe in clay are marked.

(magnetic hyperfine field of 49.6 T, quadrupole shift
À0.12 mm/s and isomer shift of 0.48 mm/s) (Murad and
Cashion, 2004). The central parts of the spectra were also
measured at smaller amplitudes and at a number of temperatures between 80 and 20 K in a search for characteristics
indicating additional minerals, in particular ferrihydrite,
but no signals were resolved. If other oxide phases are present, they exist in very small abundances (a maximum of 2%
of the Fe is a conservative estimate). Mo¨ssbauer spectros-

copy also did not detect any siderite or other non-silicate
phases that could have formed authigenically.
The hyperfine parameters of oxide phases enable a
description of the crystallinity of the mineral, using the line
width of the sextet. Because of the limited overlap between
lines 1 and 6 of the 2 sextets we have chosen to use the width
of line 1 (and 6) to compare the samples. For goethite, the
dominant oxide phase, the width of line 1 is; river mud:
0.84 mm/s, river sand: 0.92 mm/s, oxidized aquifer:

Mobilization of arsenic

0.81 mm/s and reduced aquifer: 0.70 mm/s. For comparison,
the experimental line width of a well crystalline natural goethite is approximately 0.30 mm/s and the increased line
widths are due to lattice substitutions and defects. Accordingly all samples contain poorly crystalline goethite and the
goethite in the river sand clearly has the poorest crystallinity.
The distribution of iron over the various iron-containing
minerals in the sediments was calculated from the fit of the
Mo¨ssbauer spectra at 20 K and is expressed as percentage
of total Fe in Fig. 2. Overall, the distribution of iron in
the river sediments is quite similar to that in the aquifer
sediments. Goethite is the predominant iron oxide and represents 30–53% of the iron. It is accompanied by a smaller
amount of hematite (7–12%) in all samples. The river sediments contain somewhat more goethite and less hematite.
Fe(II) in silicates constitutes 14–31% and Fe(III) in silicates
23–28% of the iron. The silicate bound iron is probably
contained in the phyllosilicates illite and smectite identified
by XRD. The ratio of Fe(II) to total Fe in the phyllosilicates varies between approximately 38% for the river mud
and sand to approximately 50% for the aquifer samples.
The relatively higher Fe(II) content in the aquifer sediments
may have two contributions: (1) solid state reduction of
Fe(III) in the minerals and (2) tentatively the formation
of an authigenic smectite phase in the sediment.
3.2. Kinetics of iron and arsenic release by ascorbic acid/HCl
The sediments were leached with excess ascorbic acid at
pH 3 to provide kinetic information concerning the release
of elements during the reductive dissolution of Fe(III)-minerals (Postma, 1993; Larsen et al., 2006). A parallel extraction with HCl, adjusted to pH 3, reveals if some of the
released Fe2+ originates from proton induced dissolution
of Fe(II)-containing minerals like siderite or vivianite. This

% Fe
river sand

60

river mud

aq ox

aq red

50

40

30

20

10

0

hematite

goethite

Fe(II) clay

Fe(III) clay

Fig. 2. The relative distribution of iron over various minerals as
calculated from Mo¨ssbauer spectroscopy results.

3371

treatment would also dissolve FeS, but the smell of escaping hydrogen sulfide was never noted. Neither pyrite nor
iron oxyhydroxides dissolve significantly in HCl at pH 3
(Postma, 1993).
The river mud shows a substantial release of Fe(II) by
ascorbic acid from the start of the experiment (Fig. 3). This
Fe(II)-release of 90–100 lmol/g is about four times higher
than found in the other samples and suggests the presence
of a large and highly reactive pool of Fe-oxide. HCl also releases some Fe from the river mud, amounting to 10–15%
of the Fe(II) liberated by ascorbic acid. HCl-leaching of
the river sand and the oxic aquifer sediment liberates little
Fe (Figs. 4 and 5) but in the presence of ascorbic acid there
is a strong release of iron, with a maximum Fe concentration of 20–25 lmol/g, indicating the reductive dissolution
of Fe-oxides in the sediments. The reduced aquifer sediment
(Fig. 6) behaves entirely differently from all other samples.
Here the amount of Fe(II) extracted by HCl and ascorbic
acid is more or less the same. The somewhat higher Fe release by HCl, as compared to ascorbic acid, may be due to
sediment heterogeneity which makes it difficult to obtain
identical subsamples. The Fe release was immediate, indicating the presence of an easily dissolving Fe(II)-phase in
the reduced aquifer sediment. It also follows that the
amount of easily reducible Fe(III) in the reduced aquifer
sediment by comparison is small at the time scale of a
few days, the duration of the experiments.
Manganese is leached from the sediments in much smaller amounts than Fe. Most Mn is leached from the river
mud with concentrations of up to 12 lmol/g. Again the difference between ascorbic acid and HCl can be attributed to

reductive dissolution of Mn-oxides (van der Zee and
van Raaphorst, 2004). Both river sand and oxidized aquifer
sediment release about 1 lmol/g Mn by HCl (Figs. 4 and 5)
while ascorbic acid releases an additional 1 lmol/g Mn.
This suggests the existence of a small amount of reducible
Mn, present either as a separate phase or as a constituent
of the dissolving Fe-oxides. In the river mud (Fig. 3) most
of the Mn is already released by HCl but ascorbic acid creates a small initial excess of Mn. Since the Mn concentration decreases at longer times, either readsorption or
some precipitation of Mn must occur. For the reduced
aquifer sediment the amounts of Mn extracted by HCl
and ascorbic acid are about the same (Fig. 6).
In laboratory studies of the reductive dissolution of synthetic As(V)-containing Fe-oxides it was found that ascorbate did not reduce arsenate to arsenite (Jung and Zheng,
2006; Pedersen et al., 2006). Assuming that the same is true
for arsenic associated with the iron oxides in the sediments,
the leaching experiments can provide information concerning the arsenic redox speciation in the sediments. The river
sediments (Figs. 3 and 4) show a release pattern of arsenic
which resembles that of iron. For both mud and sand, HCl
liberates only a small portion of the As that is released by
ascorbic acid and exclusively as As(III). The mud sample
leached by ascorbic acid, shows a strong initial increase in
total As with a higher As/Fe ratio than later in the experiment. This initial peak is present as As(V) and constitutes
about 30% of the total arsenic release. Later on in the
experiment As(V) appears to become readsorbed by the


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D. Postma et al. / Geochimica et Cosmochimica Acta 74 (2010) 3367–3381

Fe(II) µmol/g

Mn µmol/g

100

15

80
10
60
40
5
20
0

0
0

1
HCl

Asc

2

3

0

4

Asc

1
HCl

2

3

1
HCl

2

3

4

PO4 nmol/g

As nmol/g
70

300

60

250

50

200

40
150
30
100

20

50

10

0

0
0

1

2

As(tot) Asc

As(tot) HCl

As(V) Asc

As(V) HCl

3

0

4

Time (days)

Asc

4

Time (days)

Fig. 3. Results of parallel extractions of river mud with 10 mM ascorbic acid for reductive dissolution and for proton induced dissolution with
HCl. Both experiments were done at pH 3. For arsenic, the difference between As(tot) and As(V) is present as As(III).

Fe(II) µmol/g

Mn µmol/g

25

3

20
2
15

10
1
5
0

0
0

1

Asc

2

3

4

0

5

1

Asc

HCl

As nmol/g

2

3

4

5

2

3

4

5
Time (days)

HCl

PO4 nmol/g

6

120

5

100

4

80

3

60

2

40

1

20
0

0
0

1

2

3

As(tot) Asc

As(tot) HCl

As(V) Asc

As(V) HCl

4

5
Time (days)

0

1

Asc

HCl

Fig. 4. Results of parallel extractions of river sand with 10 mM ascorbic acid for reductive dissolution and for proton induced dissolution
with HCl. Both experiments were done at pH 3. For arsenic, the difference between As(tot) and As(V) is present as As(III).


Mobilization of arsenic
Fe(II) µmol/g

3373

Mn µmol/g
3

60
50

2

40
30
20

1

10
0

0
0
Asc

1
HCl

2

0

3

Asc

As nmol/g

PO4 nmol/g

12

25

10

20

1
HCl

2

1

2

3

8
15
6
10
4
5

2
0

0
0

1

2

As(tot) Asc

As(tot) HCl

As(V) Asc

As(V) HCl

3

0

Time (days)

Asc

3

Time (days)

HCl

Fig. 5. Results of parallel extractions of oxidized aquifer sediment with 10 mM ascorbic acid for reductive dissolution and for proton induced

dissolution with HCl. Both experiments were done at pH 3. For arsenic, the difference between As(tot) and As(V) is present as As(III).

Mn µmol/g

Fe(II) µmol/g

3

60
50

2

40
30
20

1

10
0

0
0

1
Asc

0

2

Asc

HCl

As nmol/g

1

2

1

2

HCl

PO4 nmol/g

12

120

10

100

8

80

6

60

4

40

2

20
0

0
0

1
As(tot) Asc

As(tot) HCl

As(V) Asc

As(V) HCl

2

Time (days)

0
Asc

HCl

Time (days)

Fig. 6. Results of parallel extractions of reduced aquifer sediment with 10 mM ascorbic acid for reductive dissolution and for proton induced
dissolution with HCl. Both experiments were done at pH 3. For arsenic, the difference between As(tot) and As(V) is present as As(III).


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D. Postma et al. / Geochimica et Cosmochimica Acta 74 (2010) 3367–3381

sediment and As(III) becomes the predominant redox species. Generally, the ratio of released As/Fe in the mud is
much higher than found in the other samples. From the river sand (Fig. 4), ascorbic acid releases both Fe(II) and total
As in a similar pattern. In this case roughly half of the arsenic is found as As(V).
The behavior of the aquifer sediments (Figs. 5 and 6) is
different in the oxidized and the reduced zones. In the oxidized zone there is very little release of arsenic, even when
ascorbic acid is reducing a considerable amount of iron
oxide. In the reduced aquifer sediment (Fig. 6) we noted
earlier that ascorbic acid and HCl release more or less the
same amount of Fe(II) indicating the dissolution of a
Fe(II)-phase. Apparently, this Fe(II)-phase also contains
As(III) since a similar release pattern is observed, while
no As(V) is leached from the sediment.
The release pattern of phosphate (Figs. 3–6) resembles
that of iron. The presence of ascorbic acid seems to be required to mobilize phosphate. Therefore, phosphate must

be associated with the iron oxides and first during reductive
dissolution of the iron oxides, phosphate is released. As for
arsenate, readsorption of phosphate to remaining Fe-oxides
could be expected, but no decrease in phosphate over time
was observed. In the case of the reduced aquifer sediment,
the phosphate already dissolves with HCl (Fig. 6), similar
to the behavior of iron, which is consistent with the dissolution of a vivianite type of phase. The amount of released
phosphate is, however, much smaller than that of Fe and
therefore if vivianite dissolution takes place then it can only
constitute a tiny fraction of the dissolving Fe(II)-phase.

m t µmol/g

3.2.1. Reactivity of iron oxides in sediments
The reactivity of the pool of Fe-oxide in the sediment
was determined using the approach of Larsen et al. (2006)
where the rate of reduction of iron oxides is described by
the rate law:
 c
J
m
¼ k0
:
ð1Þ
m0
m0

15

À1

0

Here J is the reduction rate (mol s ), k the apparent rate
constant (sÀ1), m0 the initial sum of reactive iron oxides, m
the remaining mass at a given time (both in mol). For synthetic Fe-oxides, the exponent c varies with the crystal
structure and the distribution of the crystal population
(Larsen and Postma, 2001). In natural sediments c characterizes a reactive continuum of dissolving Fe(III)-phases
(Postma, 1993). The rate law (Eq. (1)) is based on the fraction of undissolved iron oxide (m/m0) but the experimental
data yields the dissolved amount, mt that equals m0 À m.
The difference in the amount of Fe extracted by respectively
HCl and ascorbic acid, both at pH 3, was attributed to the
reductive dissolution of Fe-oxides and is therefore equal to
mt. The data for mt versus time obtained in this way
(Fig. 7), was fitted to the integrated form of Eq. (1) after
substitution of mt = m0 À m:
h
i
1
ð2Þ
for c – 1 : mt ¼ m0 1 À ð1 À k 0 ð1 À cÞtÞ1Àc :
Eq. (2) is the same as Eq. (9) of Larsen et al. (2006) but corrected for a typographical error (the last square parenthesis

r iver m ud

80

60

40

m 0 76 µmol/g
k' 4.1×10-4s-1
γ 0.99

20

0
0

200,000

100,000

300,000

400,000

r iver sand

25
20
15
10

m0 24 µmol/g
-5
k' 2.1×10 s-1
γ 1.45

5
0
0

100,000

200,000

300,000

400,000

500,000

aquifer ox

25
20

10

m0 31 µmol/g
-5
k' 2.4×10 s-1
γ 3.35

5
0
0

50,000 100,000

150,000

200,000 250,000 300,000
Time (sec)

Fig. 7. The reactivity of Fe(III) in the river and oxidized aquifer
sediments. Data points are obtained as the difference between
ascorbate and HCl extractable Fe (Figs. 3–5). The lines are data fits
of Eq. (2). The inserts give the rate parameters fitted for Eq. (2).
From top to bottom r2 is 0.945, 0.994 and 0.995, respectively.

should be placed at the end of the equation; not before the
exponential term). Eq. (2) contains three fit parameters; m0,
k0 and c.
The data for mt versus time and the data fit of Eq. (2) are
displayed in Fig. 7. Naturally, this procedure could not be
followed for the reduced aquifer sediment where the difference between ascorbic acid and HCl extractable Fe was
zero. Fitting the data measured for the river sand and the
oxic aquifer sediment yield more or less the same values
for m0 and k0 but the c in the oxic aquifer sediment is twice
as high as in river sand indicating a significant heterogeneity in the pool of iron oxides. The rate constant k0 for the
river mud is much higher than for the other samples. This


Mobilization of arsenic

3375

resulted in an enhanced release of As and Fe, quite similar
to the pattern observed in the oxidized zone, and again with
a significant part of the As being present as As(V).

fine grained sediment apparently contains a much higher
reactive surface area of Fe-oxides than the sandy sediments.
3.3. Incubation experiments

4. DISCUSSION
Sediment incubation experiments were carried out in parallel for unamended sediments and sediments with added acetate. The objective of acetate addition was to check whether
the availability of reactive organic carbon limited the rate of
the redox processes in the system.
The unamended river mud sample (Fig. 8) showed an almost linear increase in both Fe2+ and total As over the entire incubation period of 30 days, after an initial lag period
of a few days. A low concentration of As(V) was identified
after 13 days, but generally nearly all arsenic is present in
solution as As(III). Addition of acetate did not have a great
influence on the As and Fe release rates although the Fe release seemed to be enhanced somewhat. In the case of the
incubated river sand, (Fig. 8) low Fe and As contents appear in solution after 5 days but there is no further increase
in concentration over time. Addition of acetate resulted in
the stimulation of both the release of Fe and As. Again
most arsenic is present in solution as As(III).
The unamended oxidized aquifer sediment (Fig. 9) did
not show a significant release of As or Fe over the 50 day
incubation period. However, addition of acetate resulted,
after an initial lag period of a few days, in a rapid increase
in both Fe2+ and As. About 70% of the arsenic in solution
is present as As(III) while the remainder is in the As(V) oxidation state. In case of the unamended reduced aquifer
sediment there is a small but steady increase in As and Fe
over the incubation period. Again the addition of acetate

Fe(II) µmol/g

4.1. Iron oxide mineralogy, reactivity and reductive
dissolution
One of the problems in assessing the role of reductive
dissolution of iron oxides as a mechanism for the mobilization of arsenic in the Holocene aquifers of SE Asia has been
to identify and characterize the Fe-oxides present in the
sediments. Different approaches have been attempted ranging from mineralogical methods like X-ray diffraction
(XRD) and transmission electron microscopy (TEM) (Akai
et al., 2004), and indirect methods like chemical extractions
(Dowling et al., 2002; Akai et al., 2004; Horneman et al.,
2004; Swartz et al., 2004; Postma et al., 2007), and reflectance and magnetic properties (Horneman et al., 2004).
There are only very few studies that have positively identified iron oxides minerals in the reduced Holocene aquifer
sediment. In Bangladesh, Akai et al. (2004) identified
ferrihydrite, goethite and hematite by TEM in Holocene
aquifer sediments, while Rowland et al. (2008) identified
goethite by XRD. In contrast, Polizzotto et al. (2006) used
both extended X-ray absorption fine structure (EXAFS)
and X-ray absorption near-edge structure (XANES) spectroscopy, but failed to identify any Fe-oxide minerals in
Holocene reduced aquifer sediment from Bangladesh. As
a consequence they concluded that reductive dissolution
of Fe-oxides probably played a minor role. It is not clear

As nmol/g

r iver m ud

1
0.8

Fe(II) µmol/g

As nmol/g

river mud + acetate

1

20

20

0.8

15

0.6

15

0.6
10

10
0.4

0.4
5

0.2

0
0

5

10

15

20

25

Fe(II) µmol/g

0
30

0
0

As nmol/g

r iver sand

5

0.2

5

10

15

20

0
30

25

Fe(II) µmol/g

As nmol/g

river sand + acetate

4

0.2

0.15

3

0.15

3

0.1

2

0.1

2

0.05

1

0.05

1

0
30

0

0.2

0
0

5

10

15

Fe(II)

As(tot)

As(V)

20

25

Time (days)

0

5

10

15

20

4

25

0
30

Time (days)

Fig. 8. Incubation of river sediments. Results displayed in right hand panels have been amended with addition of acetate.


3376

D. Postma et al. / Geochimica et Cosmochimica Acta 74 (2010) 3367–3381
Fe(II) µmol/g

As nmol/g

aquifer ox

0.2

4

0.15

0.1

0
10

20

30

40

Fe(II) µmol/g

0.1

2

1 0.05

1

4

0.1

0
Fe(II)

20
As(tot)

30
As(V)

40

20

30

40

Fe(II) µmol/g

50

As nmol/g

aquifer red + acetate

0.2

4

3

0.1

2

1 0.05

1

0
10

10

3 0.15

2

0.05

0
0

As nmol/g

0.15

0

0

50

aquifer red

0.2

4

3

0
0

As nmol/g

aquifer ox + acetate

3 0.15

2

0.05

Fe(II) µmol/g
0.2

50

0

0
0

10

20

30

Time (days)

40

50

Time (days)

Fig. 9. Incubation of aquifer sediments. Results displayed in right hand panels have been amended with addition of acetate.

whether these conflicting results for Bangladesh Holocene
aquifer sediments are due to differences in detection limit
for the various methods, or caused by local variations in
sediment composition.
In the case of the Red River floodplain sediments, the
high sensitivity of Mo¨ssbauer spectroscopy allowed us to
confirm the presence of goethite with subordinate amounts
of hematite in the sediments of both the Red River and the
Dan Phuong aquifer (Fig 1), while XRD could not identify
any Fe-oxides. Overall, there was no major difference in the
iron speciation between the different sediments. However,
line broadening indicates that the goethite present in river
sand and mud is more disordered. Likewise, van der Zee
et al. (2003) observed considerable line broadening in Mo¨ssbauer spectra for goethite present in recent sediments. The
<2 lm fraction of the river mud was investigated by Mo¨ssbauer spectroscopy to obtain an even higher resolution.
Compared to the <20 lm fraction, the <2 lm fraction
had a higher Fe-oxides content, but also in this case only
goethite and hematite were identified. The presence of the
more stable iron oxides like goethite and hematite, and particularly the absence of ferrihydrite, in the Dan Phuong
aquifer sediment is in good agreement with the modeling
work of Postma et al. (2007), who found that the observed
sequence of redox processes with methanogenesis proceeding concomitantly with iron reduction in the Dan Phuong
aquifer is only possible thermodynamically in the presence
of the more stable iron oxides. Van der Zee et al. (2003)

investigated a range of recent lake and marine sediments
using Mo¨ssbauer spectroscopy and also noted the absence
of ferrihydrite and the presence of what they call nanogoethite.

Fig. 10 compares the reactivity of the Fe(III) pool in the
sediments with the reactivity determined for synthetic
-log(J/m0 )
3

4

5

6

7

0.0

0.5

1.0
-log(m/m0 )

Fig. 10. The reactivity of Fe(III) in river and aquifer sediments as
compared to that of synthetic iron oxides, compiled from Larsen
and Postma (2001) and Pedersen et al. (2005, 2006).


Mobilization of arsenic

Fe-oxide minerals (Larsen and Postma, 2001; Pedersen
et al., 2005, 2006). In this diagram the rate law (Eq. (1))
yields a straight line where the intersection with Y-axis gives
the initial rate, k0 , and the slope is determined by c. The Xaxis spans from 0% to 90% of the initial mass of Fe-oxides
being dissolved. The river mud shows an Fe(III) reactivity
resembling that of ferrihydrite, but with a somewhat lower
initial rate. The river sand and the oxidized aquifer sediment have initial rates between those for lepidocrocite
and a poorly crystalline goethite but with an increasing
fraction being dissolved, the rates decrease to the range
for goethite to hematite. The sediment Fe(III) reactivity
determinations are only to a limited extent in agreement
with the behavior predicted from the Fe-oxide mineralogy.
For the river sand and the oxidized aquifer sediment, the
Fe(III) reactivity ranges between a poorly crystalline goethite and hematite and is in reasonable agreement with
the results from Mo¨ssbauer spectroscopy. However, the river
mud (Fig. 10) displayed an Fe(III) reactivity close to ferrihydrite while Mo¨ssbauer only found goethite and hematite. It
could be argued that the high reactivity is due to the presence
of undetectable small amounts of ferrihydrite in the mud.
However, when the Mo¨ssbauer spectrum of the <2 lm river
mud fraction was remeasured after reaction with ascorbic
acid a significant reduction, from 0.86 to 0.70 mm/s, in the
width of line 1 (and 6) of goethite was found. This indicates
that the most poorly crystalline part of the goethite has been
removed by reaction with ascorbic acid. Apparently the high
reactivity of the goethite is related to the extent of crystallographic disorder. It appears that bulk mineralogy by itself is a
poor predictor of the iron oxide reactivity in natural samples
using the reactivity of synthetic Fe-oxides as a reference.
Rather it is the mineral surface chemistry and the extent
and distribution of crystallographic disorder that control

the water–mineral interactions. Therefore, direct measurements of mineral reactivity are required in order to predict
the controlling processes in water–sediment systems, and
characterization methods sensitive to disorder must be used
to identify the mineralogy.
Generally, the Fe(III) reactivity in the aquifer is in the
low end of the range previously reported for aquifer sediments (Larsen et al., 2006) and estuarine sediments (Hyacinthe et al., 2006). The high heterogeneity in Fe(III)
reactivity in the oxidized aquifer sediment, reflected by c
or the slope of the line in Fig. 10, suggests repeated cycles
of reductive dissolution during the wet season with a high
water table and reoxidation of aqueous Fe(II) during the
dry season. While it is likely that the least crystalline Feoxides are dissolved preferentially during the wet season
with reducing conditions, they are apparently formed again
during the dry season. Thompson et al. (2006) found that
soil Fe-oxides became more crystalline during repeated redox oscillations, but they did not attempt to determine
the reactivity of the Fe-oxides.
The incubation experiments with added acetate (Figs. 8
and 9), that lasted up to 50 days, shows a much smaller
release of iron than found in the reductive dissolution
experiments with ascorbic acid, with a duration of up to
5 days (Figs. 3–6), suggesting the abiotic reduction by
ascorbic acid to be a stronger reductive process than the

3377

microbially mediated process in the presence of acetate, as
found previously by Hyacinthe et al. (2006). In evaluating
the effect of acetate addition, it became clear that in all
incubations, except for the river mud, the redox processes
are rate limited by the availability of reactive organic carbon since the presence of acetate as additional substrate
for the micro-organisms did stimulate the processes substantially. Only with the river mud did the presence of acetate have little effect. In this case, reactive organic carbon is

apparently present in the sediment and the availability of
reactive iron oxide becomes rate limiting. In agreement with
the ascorbate experiments, the incubated river mud releases
much more Fe and As compared to the sandy samples. The
acetate-amended incubation of the reduced aquifer sediment (Fig. 9) did over a period of 45 days show a significant
increase in iron which suggests that the reduced aquifer sediment still contains a potential for further reductive dissolution of iron oxides. In the HCl and ascorbic acid
experiments this effect would be overshadowed by the more
than 300 times larger release of Fe by HCl.
The different assays used to describe the pool of Fe(III)
in the sediments address different questions. Mo¨ssbauer
spectroscopy may determine the Fe(III) mineralogy in the
sediment as well as the extent of crystallographic disorder
but provides no direct information concerning mineral
reactivity. Conversely reactivity determinations calibrated
on synthetic Fe-oxide minerals, are of little use for obtaining information concerning the mineralogy of the sedimentary Fe(III) pool. The HCl/ascorbate extractions do
provide kinetic information on the intrinsic reactivity of
the sedimentary Fe(III) pool. In particular they allow a
more nuanced picture of Fe(III) reactivity in sediments than
what is possible by sequential batch extractions. However,
they cannot directly predict the extent of reaction under
in situ conditions. Finally, the incubations provide information on the microbial processes that occur in the aquifer.
An important conclusion from the incubation experiments
is that in the sandy sediments, Fe(III) reduction is carbon
limited, while this is not the case for the river mud. The
reactive transport model for the aquifer by Postma et al.
(2007) predicted a rate of Fe(III) reduction of 0.03 mg Fe
per gram sediment per pore volume of 40 years duration,
which corresponds to 0.013 lmol/(g sed. year). This is on
the same order of magnitude as the amount of Fe(II) mobilized from the river sand during incubation (Fig. 8) but
much smaller than that reduced by ascorbate (Fig. 4).

Therefore, the interpretation of the incubation and leaching
results is also a matter of the time scales considered where
the former deals with the current situation on the time scale
of years, while the latter relate mostly to a time scale ranging from hundreds to thousands of years.
4.2. Mobilization of arsenic from sediments
The identification of goethite and hematite as the
Fe-oxides present in the sediments also places some thermodynamic constraints on the mobilization of arsenic as compared to iron. Fig. 11 shows a redox diagram for aqueous
arsenic speciation with superimposed lines for Fe(II)/Feoxide equilibria drawn for an aqueous Fe2+ concentration


3378

D. Postma et al. / Geochimica et Cosmochimica Acta 74 (2010) 3367–3381

pe
20

10

0

-10
2

4

6

8

10

12

pH
Fig. 11. The stability lines for the Fe2+–goethite, Fe2+–hematite
and Fe2+-ferrihydrite equilibria, drawn for an Fe2+ activity of
10À4 M, overlain by the stability diagram for aqueous arsenic from
Appelo and Postma (2005). Boundaries between arsenic species are
for an equal concentration of both ions.

of 0.1 mM, which is close to the concentration commonly
found in the reduced Holocene aquifers contaminated with
arsenic. The aquifers have a near neutral pH and, as shown
in Fig. 11, under these conditions As(V) can only be a stable
aqueous phase in the presence of ferrihydrite. Our sediments contain goethite and hematite and therefore As(III)
must be the stable aqueous phase in the groundwater. In
good agreement, most of the arsenic in the groundwater at
Dan Phuong is As(III) but small amounts of As(V) were also
present (Postma et al., 2007). It also follows from Fig. 11 that
during the sequential reduction of As and Fe species, the
reduction of As(V) to As(III) thermodynamically should
proceed before the reduction of either goethite or hematite
to Fe(II), as also deduced by Kocar and Fendorf (2009),
while in the presence of ferrihydrite, the two processes may
take place more or less simultaneously. With goethite and
hematite there is a kinetic constraint when As(V) is contained
within the Fe-oxides because iron reduction is required to release As(V). Once As(V) is liberated from the Fe-oxide crystal lattice, thermodynamics predict that it should be reduced
and as a result As(III) and Fe2+ will be released simultaneously during the reductive dissolution of goethite or hematite containing As(V). However, if the Fe and As reduction
processes are uncoupled, i.e., As(V) is adsorbed on the surface of the iron oxide, then in the presence of goethite or

hematite, the reduction of As(V) should proceed before the
reduction of Fe-oxide and As(III) should be released to the
solution before Fe2+. A complication is the possible adsorption of part of the produced As(III) or Fe(II) as well as
desorption of As(III) and As(V) as the Fe-oxide surface area
diminishes during reduction (Pedersen et al., 2006; Tufano
et al., 2008). These idealized scenarios are compared with
experimental evidence next.

In terms of the sequential release of As and Fe to the solution, incubation studies have produced diverging results. The
incubated river sediments and oxidized aquifer sediment in
our study show a concomitant release of As and Fe (Figs. 8
and 9), corresponding to the situation where a goethite or a
hematite containing As is being reduced in a coupled process.
A simultaneous appearance of As and Fe in solution during
aquifer sediment incubation was also reported by van Geen
et al. (2004), Gault et al. (2005) and Anawar et al. (2006).
In contrast, Radloff et al. (2007) reported results from incubation of Holocene aquifer sediments where the release of
As started before the release of Fe, corresponding to the situation where As(V) reduction is uncoupled and starts before
the reduction of a Fe-oxide phase with a stability in the range
goethite to hematite. Conversely, Islam et al. (2004) reported
results from aquifer sediment incubation experiments showing the release of Fe(II) before that of As(III). This release sequence during reduction is thermodynamically only possible
in the presence of highly unstable iron oxides like ferrihydrite
or when all As(III) produced initially becomes adsorbed. The
fact that different sediments during reduction yield dissimilar
As/Fe release patterns must indicate a variability in both the
Fe-oxide mineralogy and the arsenic speciation in the
sediments.
Leaching of the sediment with ascorbic acid is expected to
preserve the As(V)/As(III) speciation in the iron oxides (Jung
and Zheng, 2006; Pedersen et al., 2006). The river sand

(Fig. 4) did release about equal amounts of As(V) and
As(III). Since As(III) is not thermodynamically stable within
the crystal structure of hematite or goethite it must have been
present on the surface of the iron oxides. The leached As(V)
could originate from both the Fe-oxide crystal lattice and
from desorption from the Fe-oxide surface. Pedersen et al.
(2006) found that structurally released As(V) did readsorb
strongly on the goethite surface. This is probably observed
in the river mud (Fig. 3), where As(V) is mobilized during
the first hour of the reduction experiment, followed by a decrease in As(V) possibly due to readsorption. In contrast to
the results of the ascorbate leaching experiments, in the incubations of river sediment, no As(V) appeared in solution as it
apparently is quickly reduced to As(III) (Fig. 8). Heimann
et al. (2007) investigated the presence of arsenate reducing
microbes in the reduced sediment at the end of the incubation
and found the presence of a viable population of microbial
As(V) respirers. This indicates that microbial As(V) reduction is an important process in the aquifer
The reduced aquifer sediment released almost equal
amounts of As in HCl and ascorbic acid (Fig. 6) indicating
the dissolution of an As(III) containing phase which probably contains Fe(II) as well. Polizzotto et al. (2006)
suspended reduced aquifer sediment in deionized water
and found a significant desorption of As which could be
interpreted as a dissolving As containing phase in our
experiments. However, while Polizzotto et al. (2006) concluded that the reductive dissolution of As-containing Feoxides in the reduced part of Holocene aquifers is of minor
importance, our incubation results with acetate amended
reduced aquifer sediment (Fig. 9) suggest that there is an
additional potential for arsenic mobilization by Fe(III)
reduction in the aquifer.


Mobilization of arsenic

4.3. Cycling of As and Fe between the river and floodplain
aquifer
An underlying idea in this study was to consider the
sediment deposited by the Red River as the source of arsenic
for release during the oxidation of organic matter in the
aquifer and therefore to compare the composition of the
sand sampled in the river bed with the sand in the aquifer.
Overall the mineralogy of the Fe-oxides did not vary greatly
between the sandy river sediments and the aquifer sediments
(Fig. 1). However, line broadening in the Mo¨ssbauer spectra
for goethite was somewhat less in the aquifer sediments than
in the river sediments suggesting the removal of the most
reactive Fe-oxide by reduction after deposition. The small
difference in mineralogy may be due to the young age, less
than 460 years (Larsen et al., 2008), of the floodplain sediments along the Red River. An important difference is the
accumulation of an authigenic Fe(II)-containing phase in
the reduced aquifer sediments. Unfortunately, the mineralogy of this phase was not identified, by neither Mo¨ssbauer
spectroscopy nor by XRD. Reactive transport modeling
(Postma et al., 2007) suggest the precipitation of siderite
to be a reasonable option, in agreement with the findings
of Sengupta et al. (2004), but the amount of siderite precipitated as predicted by the model is small and difficult to detect in solid phase analysis.
The leaching experiments of the Red River mud with
ascorbic acid revealed the presence of a large and strongly
reactive pool of iron oxides with a high As content (Figs.
3 and 7). One may speculate why the reactivity of the iron
oxide in the river mud is so much higher than in the sand
deposits. The occurrence of such a pool of highly reactive,
poorly crystalline, iron oxide seems not really consistent
with long distance sediment transport in the Red River

and points rather towards a more local source. Weathering
of primary minerals like silicates or sulfide minerals in the
river environment could be an option. Another likely explanation could be that it is the result of reoxidation of the
Fe(II) contained in the groundwater that is discharged into
the river (Datta et al., 2009). As described by Larsen et al.
(2008) for the Red River at Dan Phuong, and by Polizzotto
et al. (2008) for the Mekong, there is a large flux of groundwater, high in Fe(II) and As, discharging from the floodplain aquifers into the river. The discharging anoxic
groundwater mixes with the oxic river water and poorly
crystalline iron oxides will precipitate that may incorporate
arsenic. The resulting small particles of iron oxyhydroxides
will remain suspended in the river water. In times with a
high river stage the muddy sediment will deposit on the
floodplain, on the surface of fields and in particular accumulate in ponds and ox bows. For a field site at the shores
of the Mekong river in Cambodia, Polizzotto et al. (2008)
calculated that the flux of As discharging with the groundwater into the river is more or less balanced by the flux of
As from the river to the floodplain by sedimentation.
The mud sediment contains both a highly reactive pool
of iron oxide as well as very reactive organic matter as
was demonstrated by unamended sediment incubations
(Fig. 8). These microcosms became anoxic in a matter of
days and both arsenic and iron were readily released. This

3379

combination of a highly reactive iron oxide phase, containing arsenic, being present together with reactive organic
carbon makes it potentially a source for arsenic contamination. After deposition in topographic depressions, like
ponds and along channels, the mud will quickly turn anoxic
and start to release Fe and As. Subsequent infiltration
through these mud layers may yield high arsenic groundwater in good agreement with the model proposed by
Polizzotto et al. (2008) for plumes of high arsenic groundwater, emitted from ponds and spreading through the aquifer. In the case of our aquifer, however, most infiltration,

but not all, proceeds through fractured overbank clay
deposits (Postma et al., 2007; Larsen et al., 2008) which
apparently become rapidly leached in As along the flowpath since the oxidized aquifer sediment (Fig. 5) did not
contain arsenic that was easily mobilized.
5. CONCLUSIONS
Goethite and hematite were identified as Fe-oxides
occurring in sediments from the Red River and adjacent
aquifer by Mo¨ssbauer spectroscopy, and spectral line
broadening indicates that goethite possesses a relatively
high degree of disorder. Kinetic analysis shows the river
mud to contain highly reactive Fe-oxides, equivalent to synthetic ferrihydrite, while in the river sand and oxidized aquifer sediment the Fe-oxides had a reactivity comparable to a
range from disordered goethite to hematite.
The release of As from the sediments is directly coupled
to the reduction of iron oxides and, except for the river
mud, the rate of arsenic release is limited by availability
of reactive organic carbon. In the reduced aquifer sediment
an unidentified authigenic Fe(II)-phase accumulates which
apparently also contains As(III).
The Red River mud contains both very reactive Feoxides with a high As content, and reactive organic carbon.
Potentially this constitutes a likely source for arsenic contamination. After deposition of the mud, the sediment will
quickly turn anoxic and start to release both Fe and As.
Deposition on the floodplain may occur in ponds and channels, and at sites where infiltration into the aquifer is high, it
will introduce arsenic into the groundwater.
ACKNOWLEDGMENTS
This work has been supported by DANIDA as part of a
research capacity building project “Water Resources Research in
Vietnam”. Kristian Anker Rasmussen carried out the draft work.
The reviewers, in particular Lex van Geen, are thanked for their
constructive comments.

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Associate editor: Michael L. Machesky