Field-Based Arsenic Speciation in Water Samples and the Relationship
to Human Health
J.
1
O'Reilly* ,
Dr. M.
2
Watts
and Prof. N. I.
1
Ward
1.
ICP-MS Facility, Chemistry, School of Biomedical and Molecular Sciences, University of Surrey,
Guildford, Surrey, GU2 7XH
2. British Geological Survey, Keyworth, Nottingham, NG12 5GG
INTRODUCTION:
A field-based method for the separation and preservation of four arsenic species (As3+, DMA, MMA and As5+) in waters has been
developed, utilising disposable Bond Elut SPE cartridges. On-site separation is achievable through strong cation exchange (SCX)
and strong anion exchange (SAX) phases connected in series. The arsenic species are then eluted and analysed in the laboratory for
total arsenic by ICP-MS. Arsenic species identification was confirmed using HPLC-ICP-MS. The field-based method was
employed on natural/acid mine drainage waters collected from two areas: the Devon Great Consol near Tavistock in Devon and
rural regions of Argentina. Populations in certain regions of Argentina (La Pampa and San Juan) display human health conditions
linked to arsenic exposure, including skin legions, pigmentation changes, hyperkeratosis and various forms of cancer.
FIELD-BASED METHODOLOGY:
Cartridge conditioning is carried out to promote adsorption of the
arsenic species onto the silica-based resin:
• SCX: 15ml 50% v/v MeOH; 15ml 1M H3PO4; 10ml DI water
• SAX: 15ml 50% v/v MeOH; 10ml DI water
1Elution Processes
Elution
Processes
Elute with
Elute with
acetic acid
HNO3
and HNO3
Sample
SCX Cartridge
(DMA)
Extraction
SAX Cartridge
(MMA and As5+)
SCX
As3+
SAX
MMA
DMA
As5+
Figure 1: Field-Based Set-up
SPIKED RECOVERY TESTS:
Solutions spiked with 10 μg/l arsenic were passed through the
field-based set-up (Figure 1). Recoveries for the 4 arsenic species
ranged from 85-98%.
CALIBRATION:
Arsenic species measured over a range of 0 – 100 μg/l using a
VG Elemental PQ ExCell ICP-MS gave an r2 value > 0.998.
ARGENTINA STUDY SITE:
Some remote populations of
Argentina rely solely on artesian
drinking water supplies.
Total Arsenic (AsT) Levels
Analysis of waters from La Pampa
(artesian-tap)
and
San
Juan
Figure 5: La Pampa
(artesian/river) show high total
arsenic levels, artesian (713-729
μg/l La Pampa; 59-352 μg/l San
Juan) and river (67-132 μg/l Rio
Blanco, San Juan). Background
levels (Rio Negro) were 26.0 - 40.0
µg/l AsT. The highest AsT level was
found in Edvardo Castex (La Figure 6: Rio Blanco, San Juan
Pampa) tap water with 700 µg/l
(Figure 7). The WHO level for
drinking water is 10 µg/l AsT.
Arsenic Speciation
SPE results showed for La Pampa
5+
3+
tap water As > As > MMA >
3+
DMA, San Juan river water As >
MMA > As5+ > DMA and for San
Figure 7: Arsenic levels in
3+
5+
Juan tap water As > As > MMA
Argentina
> DMA (Figure 8).
Arsenic Fractions (μg/l)
Total AsFraction levels are
Total (As )
Total
Site
As
DMA MMA As
F/UA (µg/l)
(As
) generally higher than AsT,
La Pampa (tap)
136.6 394.2
4.7
49.5 706.5
1154.9 with a significant variation
San Juan (river)
115.5 137.8
3.3
6.3
3.5
150.9
San Juan (tap)
332.8 219.4
1.4
57.4
63.9
342.1 in levels for La Pampa.
Artesian/tap
800
700
T
3+
5+
Frac.
3+
5+
Figure 4a: 10 µg/l arsenic std.
HPLC-ICP-MS
Figure 4 shows the As speciation for site 3.
Filtered water shows As5+ as the predominant
species, whereas field SPE analysis (Figure
3) shows As3+ and As5+. It is suggested that
3+
As oxidation takes place with water sample
storage and transport to the laboratory.
Initial preservation studies using 1%
ascorbic and citric acids, shows acidification
also influences As speciation (Figure 4).
a) Filtered
As5+
b) 1% Ascorbic
As5+
c) 1 % Citric
MMA
Figure 4: Arsenic species
for site 3
River
(Rio Blanco)
300
200
100
T
3+
Rio Blanco - East
Rio Blanco - West
Rio Blanco - 30cm Depth
Rio Blanco - 20cm Depth
Rio Blanco - 10cm Depth
Rio Blanco - Surface
Ed. Castex Tank
Ed. Castex Garden
Ed. Castex House
Encon Water 6
Encon Water 5
Encon Water 4
Encon Water 3
Encon Water 2
Encon Water 1
0
5+
Frac.
Figure 8: SPE cartridge results
Tap water analysed from La Pampa is used by a family in
which 8 members have recently died of arsenic poisoning.
CONCLUSION:
• Comparative arsenic studies were carried out between waters
analysed from Devon and Argentina using SPE cartridges.
• Variable SPE recoveries were reported between spiking tests
and real samples.
• Use of ascorbic and citric acids as preservatives alters the
type of species identified by HPLC-ICP-MS.
• Tap water AsT exceeds WHO guidelines. Studies need to be
carried out to assess the effect on human and animal health.
As5+
DMA
400
Caucete 2
The Devon Great Consol (Figure 2) is a
former arsenic mine, ceasing production
around 1925. Eight sites were selected
within the vicinity of the arsenic mine.
Total Arsenic (AsT) Levels
AsT
levels
(filtered
and
Figure 2: Study site 7
unacidified, F/UA) ranged from
Arsenic Fractions (µg/l)
5.5 – 1866.3 µg/l. Site 3 was Site Total (As ) As DMA MMA As Total
F/UA (µg/l)
(As
)
adjacent to the mine tailings area.
1
5.5
6.4
0.2
0.9
10.5
18.0
Background levels were 5.0 - 20.0 2
13.1
30.6
7.9
8.1
44.0
90.5
3
1866.3 1902.3
8.3
257.4 1665.1
3833.1
µg/l AsT.
4
2.3
16.2
6.8
7.4
26.4
56.8
Arsenic Speciation
5
18.2
96.5
16.0
2.7
14.6
129.7
71.6
36.9
5.6
9.0
40.4
91.9
SPE results generally showed As3+ > 67
9.6 2930.6
7.9
2.1
10.6
2951.2
60.4 160.1
5.1
5.0
170.2
As5+ > DMA > MMA (Figure 3). 8
Site 3 had the highest As species Figure 3: SPE cartridge results
DMA MMA
AsB
levels, similar to AsT. Total AsFraction
As
As
levels are generally higher than AsT.
500
Caucete 1
DEVON STUDY SITE:
As Conc. (µg/l)
600
REFERENCES:
• P. L. Smedley et al., Applied Geochemistry, 2002, 17, 517
• Francesconi et al., Analyst, 2004, 129, 373
• Martinez-Bravo et al., J. Chrom. A, 2001, 926, 265