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Monothioarsenate occurrence in Bangladesh groundwater and its removal by ferrous and zero-valent iron technologies Britta Planer-Friedrich, Jörg Schaller, Fabian Wismeth, Judith Mehlhorn, and Stephan J. Hug Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 19 Apr 2018 Downloaded from http://pubs.acs.org on April 19, 2018
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Environmental Science & Technology
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Journal: Environmental Science and Technology
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Monothioarsenate occurrence in Bangladesh groundwater and its removal by ferrous
4
and zero-valent iron technologies
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Britta Planer-Friedricha*, Jörg Schallera, Fabian Wismetha, Judith Mehlhorna, and Stephan J.
7
Hugb
8 9 10
a
11
(BayCEER), Bayreuth University, Universitaetsstrasse 30, 95440 Bayreuth, Germany
Environmental Geochemistry, Bayreuth Center for Ecology and Environmental Research
12 13
b
14
Ueberlandstrasse 133, 8600 Duebendorf, Switzerland
Eawag,
Swiss
Federal
Institute
for
Environmental
Science
and
Technology,
15 16 17 18
* Corresponding author: phone +49 921 553999, fax +49 921 552334, b.planer-
19
[email protected] 20
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Abstract
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In most natural groundwaters, sulfide concentrations are low and little attention has been
28
paid to potential occurrence of thioarsenates (AsVSn-IIO4-n3- with n=1-4). Thioarsenate
29
occurrence in groundwater could be critical with regard to the efficiency of iron (Fe)-based
30
treatment technologies because previous studies reported less sorption of thioarsenates to
31
preformed Fe-minerals compared to arsenite and arsenate. We analyzed 273 groundwater
32
samples
33
monothioarsenate (MTA), likely formed via solid-phase zero-valent sulfur, in almost 50% of
34
all samples. Concentrations ranged up to >30 µg L-1 (21% of total As). MTA removal by
35
locally used technologies in which zero-valent or ferrous Fe is oxidized by aeration and As
36
sorbs or co-precipitates with the forming Fe(III)hydroxides was indeed lower than for
37
arsenate. The presence of phosphate required up to three times as much Fe(II) for
38
comparable MTA removal. However, in contrast to previous sorption studies on preformed
39
Fe minerals, MTA removal, even in the presence of phosphate, was still higher than that of
40
arsenite. The more efficient MTA removal is likely caused by a combination of co-
41
precipitation and adsorption rendering the tested Fe-based treatment technologies suitable
42
for As removal also in the presence of MTA.
taken
from
different
wells
in
Bangladesh
over
a
year
and
detected
43 44
Introduction
45
Arsenic (As) is a ubiquitous element and even at trace concentrations carcinogenic to
46
humans. Many aquifers worldwide contain naturally high As concentrations and drinking
47
water is one of the main exposure routes for As to humans. The As problem is especially
48
severe in Southeast Asian countries
49
concentrations up to several mg L-1 As have been reported for groundwater 5. The local
50
drinking water limit is 50 µg L-1 As, which is exceeded in one third of all tubewells
51
leads to a 1% additional cancer risk
52
limit to the 10 µg L-1 recommended by the World Health Organization 8. The two As species
1-4
. In Bangladesh, one of the worst affected countries,
6, 7
5
and still
. Worldwide, many countries have lowered their As
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typically determined in groundwater are the reduced species arsenite (non-charged below pH
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9.2 9, H3AsIIIO30,) and the oxidized species arsenate (typically negatively charged, pK
55
(H2AsVO4-/HAsVO42-) = 7.2 9).
56 57
Many of the technologies used for As removal from drinking water rely on iron (Fe)-based
58
treatment strategies. Thereby, As is either adsorbed to pre-formed Fe(III) (hydr)oxide phases
59
or the Fe(III) mineral is formed “concurrently”, i.e. in the presence of As, from added zero-
60
valent iron (ZVI) or ferrous iron (Fe(II)). Arsenic, if present as arsenite, is first oxidized to
61
arsenate and then removed by a combination of sorption and co-precipitation. The major
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challenge in developing countries like Bangladesh without central water supply and central
63
water treatment facilities is to provide a low-cost and simple technology to affected
64
households that does not require the use of potentially hazardous chemicals. Two examples
65
for concurrent removal technologies are the SORAS method 10 and SONO filters which are
66
more widely used in Bangladesh
67
oxidation of arsenite to arsenate promoted by UV-light and co-precipitation with Fe(III)
68
formed form naturally occurring Fe(II) in polyethylenterephthalate (PET) bottles
69
filters are buckets filled with layers of sand and ZVI, pre-treated with acid to enhance
70
formation of Fe(III) phases and thereby As removal. Total As removal efficiency in all
71
methods might be negatively affected by competition with other groundwater constituents.
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Silicate hereby mainly affects arsenite, while phosphate has a strong effect on arsenate and
73
arsenite 13.
11, 12
. SORAS (solar oxidation and removal of As) uses
10
. SONO
74 75
A group of As species, typically not considered in the context of natural groundwater
76
occurrence, are thioarsenates. Thioarsenates (AsVS-IInO4-n3-, with n=1-4) are structural
77
analogues to arsenate. At near-neutral pH, thioarsenates do not form directly by addition of
78
sulfide to arsenate, but in two steps via As(III) species
79
exchange for hydroxyl groups first forms thioarsenites (AsIIIS-IInO3-n3-, with n=1-3). Addition of
80
zero-valent sulfur then leads to formation of thioarsenates; As(III) is oxidized to As(V) and
14, 15
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. Starting from arsenite, sulfide
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S(0) is reduced to S(-II) 14, 15. The only thioarsenate species that does not require free sulfide
82
for formation is monothioarsenate (MTA) which forms from arsenite and zero-valent sulfur. It
83
also is the most stable thioarsenate species with very slow transformation rates upon
84
acidification 16, aeration
85
environments, such as in hot springs
86
be the dominant As species. For groundwaters, the only evidence for thioarsenate
87
occurrence so far comes from contaminated sites
88
for groundwater As remediation
89
sulfide concentrations such as in Southeast Asia (1-2 µM 28).
17
, heating 17, or in the absence of zero-valent sulfur 18, 19
22-27
or stagnant terminal lakes
15
20, 21
14
. In sulfide-rich
, thioarsenates can
or sites where sulfate has been applied
but not from natural groundwaters with typically low
90 91
If thioarsenates occur in groundwater used as drinking water their removal must be
92
addressed besides that of arsenite and arsenate because of similar toxicological risk
93
potential concern could be that thioarsenates are removed less efficiently by Fe-based As
94
removal technologies. Thioarsenates were reported to decrease As removal on concurrently
95
formed mackinawite (Fe(II)S)
96
arsenite and arsenate
97
32, 33
98
arsenate, but these experiments were conducted at higher As concentrations than typically
99
encountered in groundwater (0.1 mM
32
31
29, 30
.A
and showed less sorption to pre-formed mackinawite than
. With regard to pre-formed Fe(III) phases, monothioarsenate (MTA)
and trithioarsenate
32
showed slower and less sorption to goethite than arsenite and
32
and 0.5 mM
32, 33
). To our knowledge, there are to
100
date no studies on thioarsenate removal with concurrently formed Fe(III) phases and
101
competition with silicate and phosphate is unknown.
102 103
The present study focuses on MTA, the only thioarsenate species that can form in the
104
absence of free sulfide and the most stable thioarsenate species. It addresses two questions:
105
1) Does MTA form in natural groundwaters of a field site in Bangladesh and if so, does its
106
occurrence correlate with a certain well depth, season, or other hydrogeochemical
107
parameters? 2) How is MTA in comparison to arsenite and arsenate removed with Fe(II) and
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ZVI in the absence and presence of competitors, such as silicate and phosphate?
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Materials and Methods
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Field sampling groundwater in Bangladesh. Groundwater samples were collected from
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seven monitoring wells drilled to different depths (9, 15, 21, 26, 27, 35, 85 m) at a field site in
113
the Titas district in Bangladesh (N23°35,809’, E90°47,857’). Sampling was done almost
114
weekly from the middle of the dry season in February 2007, through the rainy season which
115
peaks in August, to the beginning of the following dry season in December 2007 (31
116
sampling times). There is a gap in our monitoring from July, 25th and September, 2nd where
117
for logistic reasons we could not obtain samples. Additionally, 48 irrigation and drinking water
118
wells in an area of approximately 10 km2 around the field site were sampled between
119
February and May 2007; eight wells were sampled a second time in July 2007. For a detailed
120
description on well installation at the field site, sedimentology, climate, as well as methods
121
and results of general hydrogeochemistry with
122
publication 34.
2
H/18O isotope data see our previous
123 124
Samples for As speciation by ion chromatography coupled to inductively-coupled plasma
125
mass spectrometry (IC-ICP-MS) were filtered (0.2 µm cellulose-acetate), acidified to 2% HCl
126
and stored refrigerated until analysis at the latest 3-4 weeks later following our routine
127
method
128
thioarsenate species. Higher thiolated species, if they were present (see discussion below),
129
would have transformed to arsenite upon acidification
130
that also preserve higher thiolated species such as flash-freezing
131
filtered samples in nitrogen-purged bottles
132
remoteness of the field site, political instability, and daily power cuts. Total Fe, Fe(II), and
133
sulfide were determined photometrically on-site by the HACH ferrozine, phenanthroline, and
134
methylene-blue method, respectively 36.
19
. The use of acid for sample stabilization limited us to detecting MTA as the only
35
16
. Alternative stabilization methods 19
or ethanol addition to
could unfortunately not be realized due to the
135 136
Lab experiments.
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Experiments included 1) fixed-time batch experiments with varying concentrations of Fe(II)
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with and without phosphate and silicate, or with and without UV light and addition of citrate,
139
2) time-resolved batch experiments at a fixed Fe(II) concentration with and without
140
phosphate and silicate, 3) time-resolved batch and 4) column experiments using ZVI with
141
and without phosphate addition.
142 13
was used. CaCO3 and MgO (both Sigma-
143
For all experiments, synthetic groundwater
144
Aldrich p.a.) were dissolved in nano-pure water (Barnstead nano-pure TOC-UV) by stirring
145
and purging with CO2 (resulting in a pH of 5-6 and final concentrations of 2.5 mM Ca and
146
1.65 mM Mg). Silicate was added under rapid stirring as a stock solution of Na2SiO3 (Aldrich
147
>98%) to final concentrations of 0.71 mM (20 mg L-1). Selected experiments were conducted
148
without silicate addition. Stirring under CO2 was continued for at least 5 h. The water was
149
then aerated and brought to pH 7.0 by purging with air. Phosphate (if applicable) was added
150
from a stock solution of NaH2PO4 or KH2PO4 (both Fluka puriss p.a.) with a final
151
concentration of 67.7 µM P (3 mg L-1), which did not change the pH. The selected
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concentration of 3 mg L-1 reflects typical concentrations in As-affected groundwater in
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Bangladesh (1.9±1.8 mg L-1)
154
sampling (data in Supporting Information, Table S1 & S2; data from the 35 m deep well were
155
excluded since this well is influenced by the overlying peat layer 34). Arsenate or arsenite was
156
added from Na2HAsO4 (Fluka puriss. p.a.) or AsNaO2 (Fluka, purum p.a.) stock solutions,
157
respectively, and MTA was added from a stock solution prepared according to Suess et al. 37.
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Standard experimental concentrations were 6.7 µM (500 µg L-1 As). Selected experiments
159
were conducted with mixtures of all three As species in equal percentages at total
160
concentrations of 6.7 and 20 µM (500 and 1500 µg L-1), respectively.
1
as well as concentrations observed during our own field
161 162
Fe(II) addition and SORAS batch experiments. For the fixed-time Fe(II) batch
163
experiments, Fe(II) was added from a stock solution of FeSO4*7H2O (Fluka analytics) to the
164
As-containing synthetic groundwater to yield initial Fe(II) concentrations of 1, 2, 3, 5, 7, 10,
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and 15 mg L-1. Solutions (250 mL) were allowed to react for 4 hours in closed PET bottles
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(330 mL) with ambient air headspace and shaken intermittently. For the time-resolved Fe(II)
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batch experiments, a fixed Fe(II) concentration of 15 mg L-1 was chosen and samples were
168
taken after 10, 20, 30, 45, 60, 90, 140, 200, and 300 min. To induce photooxidation as used
169
in the SORAS method, 200 mL samples with varying Fe(II) concentrations were filled into
170
330 mL PET bottles with ambient air headspace and exposed for 5 hours to UV-light in an
171
air-ventilated box with 8 Philips TL20W/05 (actinic blue) lamps. The lamps emit UV-light with
172
a similar spectral composition as UV in sunlight and the intensity in the range of 320-400 nm
173
was roughly equivalent to sunlight at 24° latitude in summer between 9 am and 4 pm (70-95
174
W m-2)
175
to an initial concentration of 50 µM. Citrate was tested because in previous experiments
176
photolysis of Fe(III) citrate complexes had been reported to lead to formation of oxidants and
177
promote Fe(III) flocculation and precipitation 10.
10
. In further experiments, citrate (C6H6Na2O7*1.5H2O, Fluka puriss. p.a.) was added
178 179
MTA oxidation by Fe(III). To test whether Fe(III) formed from Fe(II) acts as oxidizing agent
180
and determines MTA species transformations in solution, we conducted two additional
181
experiments using a lower MTA concentration (100 µg L-1 instead of 500 µg L-1) with 5 mg L-1
182
Fe(III) (molar ratio Fe/As = 67) and 50 mg L-1 Fe(III) (molar ratio Fe/As = 670). Sampling was
183
done after 0.1, 1, 2, 3, and 4 h reaction time. To avoid precipitation, these experiments were
184
conducted at pH 2.
185 186
Zero-valent iron (SONO) experiments. To simulate the patented composite Fe matrix used
187
in SONO-filters
188
and unspecified composition, Fe turnings were produced in Eawag’s mechanics shop from a
189
clean cylinder of CK-45 steel (98.5 % Fe, 0.42 % C, 0.49 % Mn, 0.26 % Si, 0.15 % Cu, all
190
other elements < 0.1 %), without cutting fluid, resulting in clean turnings of 1 mm width,
191
0.1 mm thickness and 50-100 mm length. The turnings were broken up to short pieces
192
logKarsenite 4.5. Mixtures with equal percentages of arsenate, MTA, and arsenite at final
312
concentrations of 500 and 1500 µg L-1 reflect the overall behavior expected from the
313
contributing individual species; at 15 mg L-1 Fe(II), 90 % and 88.5 % of initial total As was
314
removed (see Supporting Information, Figure S2).
13, 43
, arsenate removal by Fe(II) was much more efficient
Fe(II) was required to decrease the initial concentration of arsenate by 90% to less than 50
315 316
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Arsenate MTA
80
Arsenite
60 40 20 0 0.0
2.5
5.0
7.5
10.0
12.5
15.0
Fe(II) mg L-1
317 318
Figure 2 Removal of arsenate, MTA, and arsenite with increasing concentrations of Fe(II) (all
319
experiments were done in the absence of phosphate and the presence of silicate); symbols
320
represent experimental data points, lines represent the output from the model calculation
321
(see Supporting Information, Table S3).
322 323
Influence of phosphate and silicate on monothioarsenate removal by Fe(II)
324
The presence of phosphate significantly increased both the required amount of Fe(II) and
325
kinetics of MTA removal with Fe(II) (Figure 3 & Supporting Information, Figure S3). To
326
remove 90% of initial MTA, about three times as much Fe(II) was required in the presence of
327
phosphate (15 mg L-1 Fe(II)) than in its absence (5 mg L-1 Fe(II)) and it took about five times
328
as long (Figure 3). The logKMTA 5.1 obtained in the presence of phosphate (fit and data
329
shown in Supporting Information, Figure S3 and Table S3, respectively) was lower than in its
330
absence.
331 11, 13
332
Similar effects of phosphate have been described for arsenate and arsenite before
333
the presence of phosphate, the amount of Fe(II) required to remove 90% of initial arsenate
334
and arsenite increased by a factor of five from 2-2.5 to 10-12 mg L-1 Fe(II) and by a factor of
335
1.5 from 30-40 to 50-55 mg L-1 Fe(II), respectively
336
constants for arsenate (logKarsenate 5.6) and arsenite (logKarsenite 3.7) obtained in these
337
previous studies under similar silicate and phosphate conditions
338
determined in the present study (logKMTA 5.1). The observed higher amount of Fe(II) in
339
MTA+P compared to MTA-P experiments is approximately what is needed to form the
11, 13
. In
. The modeled sorption equilibrium
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frame that of MTA
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(at 3 mg P L-1 or 96.9 µM =
340
previously described P-Fe precipitate with a P/Fe ratio of 0.7
341
138 µM or 7.7 mg L-1 Fe). The different shape of the removal curves (Fig. 3a & S3) with a
342
steep initial decline in the absence of phosphate but an initial lag phase with very little
343
removal in the presence of phosphate also is a clear indication that, first, phosphate is
344
removed and only after its depletion, MTA follows.
345 Si+ P+
Si- P+
Si+ P-
Si- P-
(a)
80 60 40 20
100
% of initial monothioarsenate
% of initial monothioarsenate
100
0
80 60 40 20 0
0.0
346
(b)
2.5
5.0
7.5
10.0
12.5
15.0
0
50
Fe(II) (mg L-1)
100
150
200
250
300
Time (min)
347
Figure 3 a) Effect of phosphate and silicate on required amount of Fe(II) (after a reaction time
348
of 4 hours) and b) kinetics (at 15 mg L-1 Fe(II) addition) of MTA removal by Fe(II); Si+ reflects
349
addition of 20 mg Si L-1 and P+ addition of 3 mg L-1 phosphate to solution; no silicate or
350
phosphate was added to the Si- and P- treatments
351 352
Silicate, in contrast to phosphate, had almost no effect on MTA removal efficiency (Figure 3),
353
which is in line with previous observations for arsenate
354
charged arsenite, but not as effectively with anions such as arsenate or, like in our case,
355
MTA. In case of As removal with Fe(II), previous studies have even shown only a minor
356
effect of silicate on the removal of arsenite because there is little direct arsenite sorption but
357
mainly oxidation of arsenite to arsenate before removal by Fe(III) phases
358
consistent effect of silicate was, however, observed in the treatments without phosphate.
359
Both at high Fe(II) concentrations (> 7 mg L-1) and for longer reaction times (> 90 min), MTA
360
concentrations in solution re-increased in the absence of silicate, but not in its presence. A
361
similar silicate effect was reported for arsenate removal before 45. The authors showed that in
362
the absence of silicate the observed As re-increase in solution is explained by a
363
transformation from amorphous Fe phosphate and arsenate phases (with high sorption site
13
. Silicate competes with the non-
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. A small, but
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364
densities) to ferrihydrite and poorly crystallized lepidocrocite (with low sorption site densities)
365
45, 46
366
reaction times, no re-increase in As could be observed. However, overall in contrast to the
367
large effect of phosphate on MTA removal, silicate only played a minor role.
. Silicate slows down this transformation rate which explains why for comparable
368 369
Monothioarsenate species transformation during removal by Fe(II)
370
Chromatographic analysis of the MTA standard used for the As removal experiments showed
371
that its purity was 95 % (besides 3.5 % arsenite and 1.5 % arsenate). Speciation analysis
372
showed only little transformation without Fe(II) addition during 4 hours reaction times with
373
ambient air headspace (purity 93% at 0 mg L-1 Fe(II) in Figure 4a). This observation is in
374
accordance with previous reports on the stability of MTA upon exposure to atmospheric
375
oxygen
376
largest part this is removal by sorption to Fe(III) phases as discussed above, there is also
377
some species transformation leading to increases in absolute concentrations of arsenite and
378
(despite better removal compared to MTA) also of arsenate. Relative shares at 5 mg L-1
379
added Fe(II) were 74% MTA, 6% arsenite, and 20% arsenate (Figure 4a).
17
. With increasing Fe(II) addition, MTA concentrations decrease. While for the
100
(a)
100
10
0.0
2.5
5.0
7.5
10.0
12.5
15.0
As concentration in µg L-1
As concentration in µg L-1
1,000
(b)
10
1
0
50
100
Fe(II) (mg L-1)
380
MTA
Arsenite
150
200
250
Time (min) Arsenate
MTA
Arsenite
Arsenate
381 382
Figure 4 a) Monothioarsenate species transformation after a reaction time of 4 hours at
383
Fe(II)/As molar ratios of 0, 5, 13, and 40 at pH 7 and b) after a reaction time of 0.1, 1, 2, 3, 4
384
hours for an Fe(III)/As molar ratio of 67 at pH 2 (the low pH in this experiment was chosen to
385
avoid Fe(III) precipitation/sorption which would have affected As species distribution in
386
solution due to the different removal efficiencies for each species); note the logarithmic y-axis
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In a separate experiment we tested whether Fe(III) formed from Fe(II) acts as oxidizing agent
389
and determines MTA species transformations in solution. Figure 4b shows that the first step
390
of Fe(III)-driven MTA transformation is disproportionation to arsenite (and zero-valent sulfur,
391
not measured here, but confirmed in previous reports
392
disproportionation is likely further oxidation of zero-valent sulfur by Fe(III). The sulfur
393
oxidation product was not determined in this experiment, but in a previous study we
394
confirmed that oxidation of zero-valent sulfur by Fe(III) yields thiosulfate
395
oxidation of zero-valent sulfur leads to a shift in equilibrium between MTA and arsenite/zero-
396
valent sulfur to the side of arsenite. Arsenite thereby only accumulates temporarily, seen e.g.
397
with a maximum share of 44 % after 1 hour at a Fe/As ratio of 67 (Figure 4b). The extent of
398
MTA dissociation vs. onward oxidation of arsenite to arsenate depends on the Fe/As ratio
399
and time. Higher Fe/As ratios (Supporting Information, Figure S4) and longer reaction times
400
(Figure 4a) promote the second step of further Fe(III)-driven oxidation of arsenite to
401
arsenate, as has been observed before
402
oxidation apparently was so fast that arsenite only appeared in minor shares as intermediate
403
product and sorption of MTA and arsenate determined the overall As removal.
12, 13, 48
14
). The reason for the initiation of
47
. The onward
. For all our sorption experiments, secondary
404 405
The influence of UV light and citrate addition on monothioarsenate removal by Fe(II)
406
(SORAS)
407
Including UV light and citrate addition in the Fe(II) removal experiments to mimic exactly the
408
SORAS method, did not change total MTA removal efficiency (Supporting Information, Figure
409
S5a) nor MTA species transformation (Supporting Information, Figure S5b). This observation
410
is in line with the known stability of MTA under ambient conditions even upon exposure to
411
atmospheric oxygen and hours of heating
412
oxidizing agents like H2O2
413
double-bond sulfur, leading to MTA transformation to arsenite. For the overall removal
414
efficiency by the SORAS method, this means UV light and citrate addition can be applied as
17
17
, or acidification to pH 2
9, 16
.
Only strong
or the above described Fe(III) are capable of oxidizing the
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to oxidize arsenite to arsenate without yielding any additional undesired
415
described before
416
arsenite from MTA transformation.
417 418
Monothioarsenate removal by ZVI – batch experiments
419
Removal rates in batch experiments with ZVI followed the same order as observed for the
420
Fe(II) batch experiments with the highest removal rate for arsenate, closely followed by MTA,
421
and the lowest removal rate for arsenite (Figure 5a). Phosphate, again, decreased the
422
removal rates, stronger for arsenate and MTA than for arsenite. Still, despite significant
423
differences in removal kinetics, independent of As species and absence or presence of
424
phosphate, the high amount of reactive Fe applied (6.7 g L-1 ZVI compared to max. 15 mg L-1
425
Fe(II)) was sufficient to remove more than 99% of the initially applied As in a maximum
426
reaction time of 4 hours. 1,000
(a) As concentration in µg L-1
% of initial concentration
120 100 80 60 40 20
(b)
100
10
1
0 0
50
100
150
200
250
0
50
100
Time (min)
427
150
200
250
Time (min)
MTA P+
Arsenite P+
Arsenate P+
MTA P-
Arsenite P-
Arsenate P-
MTA
DTA
Arsenite
Arsenate
428
Figure 5 a) Removal of arsenate, MTA, and arsenite by ZVI (6.7 g L-1) in the absence (P-)
429
and presence (P+) of phosphate and b) species distribution in the MTA solution in the
430
absence of phosphate; note the logarithmic y-axis. All experiments with 20 mg L-1 Si.
431 432
During reaction with ZVI, a slight increase in total arsenite concentrations was observed both
433
in the absence (Figure 5b) and with a slight temporal delay, also in the presence of
434
phosphate (Supporting Information, Figure S6b). Interestingly, minor amounts (2-10 µg L-1
435
corresponding to 3-5% of total As) of dithioarsenate (AsVS-II2O23-, DTA) were observed in the
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first 30 min. DTA can only form if some of the zero-valent sulfur liberated during MTA ACS Paragon Plus Environment
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transformation to arsenite was further reduced to sulfide, reacted with arsenite to form
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monothioarsenite (AsIIIS-IIO23-) which in turn reacted with zero-valent sulfur to form the DTA.
439
Dissolved arsenate in comparison to arsenite contributed little (at maximum 6%) and was not
440
detectable from 60 min on. In comparison to the Fe(II) experiments, a much higher share of
441
arsenite remained in solution. It contributed almost 50% to total dissolved As after 30 min,
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80% after 60 min and was the only detectable dissolved species from 90 min on. The reason
443
could be either the significantly better sorption of MTA and arsenate versus arsenite or less
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on-ward oxidation of arsenite to arsenate owed to the fact that more oxygen is required to
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transform ZVI first to Fe(II) before it is further oxidized to Fe(III) phases. Overall, however,
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direct sorption of MTA (and arsenate) was so fast that the formation of arsenite did not
447
significantly influence the overall As removal efficiency (when it became dominant > 30 min,
448
total As concentrations had already dropped to about 10% of the original concentration).
449 450
Monothioarsenate removal by ZVI – column experiments (SONO filters)
451
The ZVI column experiments mimicking the SONO filter confirmed the order observed in all
452
previous batch experiments. In total, the column was loaded 20 times in 10 days. As reported
453
before
454
oxidation to Fe(II) and various mixed-valence Fe(II/III) and Fe(III)-phases. After some initial
455
variations, As concentrations at the outflow stabilized and were lowest for arsenate (6.2 ± 2.3
456
µg L-1), followed by MTA (28 ± 2.7 µg L-1), and highest for arsenite (76 ± 5.2 µg L-1) (all
457
values are averages over the last six column loadings in Figure 6). The MTA breakthrough in
458
the presence of phosphate was initially very high (maximum values of 146 ± 4.5 µg L-1), but
459
dropped after 15 column loadings to average values of 53 ± 11.4 µg L-1. The same delayed
460
removal of MTA in the presence of phosphate has been observed in the ZVI batch
461
experiments (Figure 5).
12
, the periodic change between flowing and stagnant conditions promotes ZVI
462 463
Speciation analyses of the outflow solutions of each experiment after 2, 7, 14, and 20 column
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loadings (Supporting Information, Table S4) confirmed the high removal efficiency for MTA ACS Paragon Plus Environment
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and arsenate compared to low removal efficiency for arsenite. From the 2nd to the 21st
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column loading with MTA (original purity 95%), absolute outflow concentrations of MTA
467
decreased from 55 µg L-1 to 8 µg L-1, while arsenite remained almost constant (11 and 14 µg
468
L-1). The preferential removal of MTA led to a relative increase in the share of arsenite from
469
14% to 54% in total As remaining in solution, while MTA dropped from 72% to 30%. Some
470
non-adsorbed arsenate (3 µg L-1 = 4% of total As) and minor reduction to arsenite (2 µg L-1 =
471
19% of total As) was also observed when applying a pure arsenite or a pure arsenate
472
solution, respectively (Supporting Information, Table S4).
473 180
Arsenate PMTA PArsenite PMTA P+
160
As in µg L-1
140 120 100 80 60 40 20 0 0
5
10
15
20
Column loadings
474 475 476
Figure 6 Concentrations of arsenate, MTA, and arsenite in the absence of phosphate (P-)
477
and MTA in the presence of phosphate (P+) over time at the outflow of the ZVI column; one
478
column loading equals approximately 90 mL synthetic groundwater solution with the
479
respective As spike (first column loading was discarded due to dilution effects). Inflow As-
480
concentrations were 500 µg L-1. All experiments with 20 mg L-1 Si.
481 482
Implications of MTA occurrence in groundwater
483
The present study has shown that MTA can occur in significant quantities in natural
484
groundwaters even when there is only low or no detectable free sulfide. The reason is that it
485
does not form from arsenate and sulfide but from arsenite and zero-valent sulfur which is
486
most likely bound to Fe phases or organic matter surfaces (solid phase vs. aqueous phase
487
driven formation). Clearly, sulfide is not an appropriate proxy to exclude existence of all
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thioarsenates and more studies are needed to understand occurrence and fate of MTA in
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natural groundwaters worldwide. Knowing about MTA occurrence in groundwater is ACS Paragon Plus Environment
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important for choosing the appropriate As removal technology. For groundwaters containing
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MTA, removal technologies should be chosen where the Fe sorbent is formed in the
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presence of As because co-precipitation seems to be much more efficient than sorption for
493
MTA removal.
494 495
Acknowledgement
496
This work was funded by the German Research Foundation Grant PL 302/2-1 and PL 302/3-
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1 (Emmy Noether Program).
498 499 500
Associated Content – Supporting Information
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Hydrogeochemistry of wells in Bangladesh; modelled sorption equilibrium constants, species
502
distribution at the outflow of a SONO filter column; PCA analysis for hydrogeochemistry;
503
removal efficiency for mixtures of arsenate, MTA, and arsenite with increasing concentrations
504
of Fe(II), removal of arsenate, MTA, and arsenite in the absence of phosphate, of MTA in the
505
absence and presence of phosphate and of phosphate (in the presence of MTA); MTA
506
species transformation over time for an Fe/As ratio of 670; MTA removal efficiency and
507
speciation with UV light and citrate addition; MTA species transformation by ZVI in the
508
absence and presence of phosphate. The Supporting Information is available free of charge
509
on the ACS Publications website at ###.
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