Trends in Analytical Chemistry, Vol. 23, No. 10–11, 2004
Trends
Extraction and analysis of arsenic
in soils and sediments
K.A. Hudson-Edwards , S.L. Houghton, A. Osborn
The ability to extract arsenic (As) from soils and sediments, and analyze it
with accuracy and precision, is of paramount importance, given the high risk
that As, even in relatively low concentrations, poses to pore waters and
biota. A large number of methods exist for extracting and analyzing total As,
and As associated with a variety of operationally defined phase associations,
in soils and sediments. We give an overview of methods used at present, and
consider potential problems. We strongly recommend adoption of universal
standard techniques and certified reference materials, especially for
sequential extraction schemes.
ª 2004 Elsevier Ltd. All rights reserved.
Keywords: Analysis; Arsenic; Extraction; Sediment; Sequential extraction; Soil
1. Introduction
K.A. Hudson-Edwards*
School of Earth Sciences,
Birkbeck College, University of
London, Malet Street, London
WC1E 7HX, UK
Wolfson Laboratory for
Environmental Geochemistry,
Research School of Earth
Sciences at UCL-Birkbeck,
University of London, Gower
Street, London WC1E 6BT, UK
S.L. Houghton, A. Osborn
Department of Earth Sciences,
UCL, Gower Street, London
WC1E 6BT, UK
Wolfson Laboratory for
Environmental Geochemistry,
Research School of Earth
Sciences at UCL-Birkbeck,
University of London, Gower
Street, London WC1E 6BT, UK
*Corresponding author.
Tel.: +44 207 679 7715;
Fax: +44 207 383 0008;
E-mail: k.hudson-edwards@
geology.bbk.ac.uk
Although arsenic (As) is only the 20th
most abundant element in the continental
crust, it occurs in detectable, but generally
low, amounts in virtually all soils and
sediments. Higher concentrations of As
are recorded in soils and sediments that
have been affected by anthropogenic activities, or where the soils and sediments
overlie or are derived from As-rich
mineralized rocks. Anthropogenic sources
of arsenic include coal and metal mining,
sewage, phosphate fertilizers, pesticides,
wood preservatives and paints. Both high
(up to 500–10,000 ppm) and low
concentrations of As in soils and
sediments are potentially of concern,
because they may contribute to high
concentrations of As in pore or surface
waters through desorption or dissolution,
plants through growth and uptake, or
animals (including humans) through
ingestion (e.g., [1,2]). One of the most
important examples of this that has been
highlighted in recent years is the worldwide mass poisoning of tens of millions of
people through drinking As-contaminated
ground water (e.g., Bangladesh, Vietnam
[3,4]). In these cases, the source of the
0165-9936/$ - see front matter ª 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.trac.2004.07.010
aqueous As is sediment that contains low
concentrations of As (generally less than
25 ppm).
The pathways of As from soil and sediment to water, plants and animals depend
on the solid-phase partitioning of the As.
Oxidation states of As in oxidized or
weakly reduced soils and sediments are
arsenite (III) and arsenate (V), while in
strongly reduced soils, As(III) and As(0)
may occur [5]. The major complexes
formed by arsenite (As(OH)3 , As(OH)4
and AsO2 (OH)2 ) and arsenate (AsO43 )
are sorbed onto common soil minerals
including clays [6] and hydrous Al, Fe and
Mn oxides [7,8].
In order to be able to properly understand the global distribution of As in soils
and sediments and its pathways to water,
plants and animals, it is vitally important
to be able to collect accurate and precise
total and solid-phase partitioning data for
As at high and low concentrations. To this
end, we present in this article an overview
of current methods used for the extraction
and analysis of total As, and single and
sequential extraction schemes for the
solid-phase partitioning of As, in soils and
sediments.
2. Methods for extracting and
analyzing total As in soils and
sediments
A wide variety of methods exist for
extracting and analyzing of total As in
soils and sediments. Colorimetry has been
recommended by the European Commission (EEC, Directive 90/515/EEC 1990) for
the analysis of As in soils. X-ray fluorescence spectrometry (XRF) is used by some
workers [9] and government surveys [10].
Discs composed of homogenized, powdered
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Trends in Analytical Chemistry, Vol. 23, No. 10–11, 2004
sediment or soil sample, and a sample binder, such as
Molwiol, are subjected to high pressure (about 5 tonnes)
and analyzed using an XRF instrument. The precision of
XRF for samples with concentrations less than 500 ppm
is very good, at ±2 relative error [9].
The most common methods used to extract the total
As from soils and sediments for later analysis involve wet
ashing of the sample using one or a combination of the
acids H2 SO4 , HNO3 , H2 O2 , HCl, H3 BO3 and HF. Some
methods add a dry-ashing step prior to the wet ashing.
The acids are used to achieve complete destruction of all
As-bearing phases in the soil and sediment. Examples of
these methods, which are used by governments and
researchers, are summarized in Table 1. Recoveries for
these methods, assessed through analysis of standard
reference materials (SRMs) or certified reference materials (CRMs), are reported to be in the range 74–110%
(Table 1) (e.g., [11–13]). These variations are ascribed to
sample heterogeneity [14] and to loss of volatile As
during extraction [13]. The latter can be limited by the
use of H2 SO4 , which prohibits the formation of volatile
species of As, and by the use of microwave-digestion
instruments ([13], see below).
The ashing methods can be carried out using a
hotplate or microwave-digestion ovens [15,16]. Microwave methods are often recommended because there is
little to no loss of volatile As during the extraction,
analytical blanks are lower, and less time and less acid
are needed to carry out the extraction. The extraction
vessels used are typically made of fluoropolymers (e.g.,
Teflon, TFM or PFA), which can withstand high
temperatures and high pressures (e.g., up to 260C for
Teflon [17]; or, 500 psi [18]) so that more efficient
extraction of As is achieved.
It is normally recommended that, following ashing
and extraction, solutions be stored in polyethylene or
polypropylene containers prior to analysis, since the As
in glass can be leached and thus contaminate the
sample. Extractant solutions are analyzed by a number
of analytical instruments, including graphite furnace
atomic absorption spectrometry (GF-AAS), inductively
coupled plasma atomic emission spectrometry (ICP-AES)
and inductively coupled plasma mass spectrometry
(ICP-MS). All of these methods may be combined with
hydride generation (HG).
Flame atomic absorption spectrometry (FAAS) is not
commonly used for the analysis of As extracts, because
of interferences and poorer detection limits (<1 mg/l As)
than GF-AAS. HG-FAAS is used [16], as are GF-AAS and
HG-GF-AAS [19]. Matrix modifiers, such as nitrates of
Ni, Pd or Mg, are added to each sample on injection to
avoid losing As in the ashing stage (As sublimes at
613C), and to ensure that consistent signal intensities
are achieved. With AAS techniques, background
corrections can be made via the deuterium lamp as the
wavelength (193.7 nm) for As is low and susceptible to
interference from light scattering and matrix effects.
HG is widely used because it can improve detection
limits by up to 100 times. Many elements are difficult to
analyze by FAAS as their primary atomic lines are below
200 nm, where the lines from the flame gases are also
strong. HG is a powerful technique that utilizes chemical
properties typical of the metalloid group of elements
(e.g., As, Bi, Sb, Se, Te, Ge and Sn) to form volatile
hydrides (arsine in the case of As). These hydrides are
carried using a stream of inert gas to the atomizer and on
into the flame. This method not only lowers the amount
of matrix interference taken into the atomizer, but it also
provides a longer residence time for atoms in the flame
during their measurement, greatly improving sensitivity.
ICP-AES is not used to a great extent for As analysis,
because emission spectra suffer from interferences that
give erroneous results. However, HG-ICP-AES is used to
reduce or to eliminate these interferences and to improve
detection limits [20].
ICP-MS is used extensively for the analysis of
As-bearing extracts [12,13], but samples with high Cl
concentrations lead to molecular interferences from
Table 1. Examples of extraction and analysis methods for total As in soils and sediments
Reference
Extraction method
Analysis method
Recovery
Repeatability/
reproducibility
United States Environmental
Protection Agency (US-EPA)
206–5, 1974
US-EPA 7060A, 1994
US-EPA 3050B, 1996
United States Geological Survey
(USGS); [53]
United States Department of
Agriculture, 2001 (CLG-ARS.03)
[14]
[14]
[24]
H2 SO4 –HNO3
AAS
Not available
Not available
H2 O2 –HNO3
HNO3 –HCl
HNO3 –H2 O2 ,
H2 SO4 –HF–HCl
HNO3 –HCl
GF-AAS
GF-AAS or ICP-MS
HG–FAAS
Not available
100–102%
Not available
Not available
Not available
Not available
AAS
HNO3 –H2 SO4 –HClO4
HNO3 –HCl
H2 O2 –HNO3
AAS
AAS
GF-AAS
80–110%
(acceptable)
96 ± 3%
74 ± 1%
Not available
Repeatability (CV) 6 10
Reproducibility 6 20
Not available
Not available
RSD 11–15%
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Trends in Analytical Chemistry, Vol. 23, No. 10–11, 2004
40
Ar35 Clþ in lower-resolution quadrupole instruments. It
is therefore recommended that digests carried out with
HCl should not be analyzed on such instruments.
Techniques such as addition of nitrogen to the carrier
gas [21] can be employed to reduce the interferences.
3. Single and sequential extractions for As in soils
and sediments
Although it is undoubtedly important to know the total
concentrations of As in soils and sediments, these
concentrations do not give any information about the
solid-phase partitioning and potential mobility of As
within the soils. This is particularly important for As,
which in many areas is in too low abundance or is
associated with such fine-grained solid phases that
characterization by standard mineralogical techniques is
extremely difficult or time-consuming. Despite the
well-known pitfalls of such sequential extractions
(re-adsorption, poor reproducibility, lack of selectivity)
[22,23], many workers have devoted time and effort to
devising both single and sequential extraction chemical
procedures for As in soils and sediments to estimate the
operationally defined phase associations, solubility and
availability of As, with a view to understanding the
factors controlling As mobility. At present, there is no
universally agreed standard method for single or
sequential extractions of As in soils and sediments
[24–26].
Many sequential extraction schemes used for As are
based on conventional schemes used to extract metals
and other elements that form cations [22,27–29].
However, other schemes recognize the anionic behavior
of As in soils and sediments, and are based on extraction
procedures for P [30–35]. Schemes have also been
developed for As specifically [15,19], and are based on
the knowledge that As is stable over a smaller range of
Eh and pH than P, As has a greater propensity to form
bonds with S and C than P, and organic As is less
common than organic P in soils [19,30,36].
A summary of a selection of sequential extraction
schemes used presently for As in soils and sediments is
presented in Table 2. Attempts have been made in Table
2 to assign the reagents used by each author or group of
authors to a standard descriptor (in the heading of the
table); if the authors describe the extractant differently,
the name that they use for the extraction step is indicated
in parentheses. The most obvious feature of the schemes
is the different numbers of steps involved, and different
extractions used. Shaking times for each step are also
vastly different, but are not shown in the table; readers
are referred to the specific articles for more information.
Virtually all of the schemes extract an easily sorbed
phase, an Al-, Fe- and/or Mn-oxyhydroxide phase and a
residual phase. Other operationally defined phases
Trends
extracted include water- or easily soluble, acid volatile
sulphide, organic matter, acid-soluble, Ca-associated, As
oxide and silicate, and As or Fe sulphide (Table 2). In
general, most soil or sediment As is extracted by the
Fe oxyhydroxide (reducible) phase, reflecting the
well-known association of As for Fe oxyhydroxides.
The chemicals used for sequential extractions are
introduced in increasing strengths and varying pHs, are
chosen to minimize re-adsorption or precipitation
between steps, and vary depending on the preference
of the authors (Table 2) (e.g., ionically bound or
exchangeable As is extracted using MgCl2 [22,24],
(NH4 )2 SO4 [15], NaNO3 [37] and anion exchange
membrane strips [19,35]). All of these extractants rely
on the principle of ion exchange, whereby loosely bound
As (indiscriminate with respect to mineral type) is
exchanged with one of the components of the chemical
extractants or membrane strips.
Keon et al. [24], Wenzel et al. [15] and Cai et al. [37]
have also extracted a specifically adsorbed As fraction,
using NaH2 PO4 , NH4 H2 PO4 and KH2 PO4 , respectively
(Table 2). The basis of this fraction is the competitive
exchange between phosphate (PO43 ) and arsenate
(AsO43 ) in soils, where, because of the smaller size and
higher charge density of phosphate, arsenate is preferentially desorbed over phosphate [38].
Acid volatile sulphides, carbonates, Mn oxides and
amorphous Fe oxyhydroxides are extracted using a
variety of chemicals (Table 2). By contrast, Al-associated
As is almost always extracted by NH4 F [19,35], because
of the stability of Al–Fe complexes [39]. Wenzel et al.
[15] tested and subsequently questioned the ability of
NH4 F to extract Al-bound As. They found no microscopic evidence for As–Al association, and suggested
that the correlation between As and Al extracted by
NH4 F was circumstantial for the soils that they were
studying. Furthermore, they felt that significant
re-adsorption might occur between an NH4 F extraction
and subsequent, often NaOH, extraction, and decided not
to use it in their extraction scheme. However, they did
suggest that NH4 F should probably be used in As-extraction schemes for soils rich in Al-clays.
To extract As associated with amorphous and/or
crystalline Fe oxyhydroxides, NaOH [19,35,40], oxalate/
oxalic acid [24], Ti(III)-citrate–EDTA-bicarbonate [24],
NH4 þ -oxalate–ascorbic acid [15] and Na citrate–
NaHCO3 –HNO3 [19,35] are used as reagents (Table 2).
Some of these reagents are oxidizing, promoting ligand
dissolution (e.g., oxalate/oxalic acid of Keon et al. [24]),
while others rely on reductive dissolution to release the
As (e.g., Ti(III)-citrate–EDTA-bicarbonate of Keon et al.
[24]; Na citrate–NaHCO3 –HNO3 of Cappuyns et al. [35]
and Van Herreweghe et al. [19]). Most of these reagents
are regarded efficient in terms of removing Fe oxyhydroxide-associated As, but it should be noted that weaker
chemicals, such as NaH2 PO4 and NH4 H2 PO4 , used for
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Table 2. Examples of sequential extraction schemes for arsenic
Reference
(Water)
Soluble
Chang and
Jackson [42]
(1) 1 M
NH4Cl
Tessier et al.
[22]
(1) 1 M
MgCl2
Ionicallybound/
exchangeable
Strongly
adsorbed
Acid volatile
sulphides,
carbonates,
Mn oxides
and
amorphous
Fe oxyhydroxides
Al-associated Organic
Amorphous
matter/
and/or
oxidisable crystalline
Fe oxyhydroxides/
reducible
Low (or
acid)
soluble
(2) 0.5 M
NH4F
(4) 2 M
H2SO4 (Caassociated)
(5) CBD
(reductantsoluble Feassociated)
(3) 0.1 M
NaOH
(4) 8.8 M
H2O2 /
HNO3 +
0.8 M
NH4OAc
(2) 1 M
NaOAc
(carbonate)
(2)
(NH4)2CO3
(sulphides &
As bound to
Al
compounds)
(3) Trilon B
(enhanced
dissolution of
carbonates)
(2) (NH4)2CO3
(sulphides &
As bound to
Al
compounds)
(4) NaOH
(organic
matter and
Fe
hydroxides)
Orpiment
and other
recalcitrant
minerals
(5) H2SO4
(1) 0.25 M
HN2OH·HC
l/0.2 M
HCl/0.025
M H3PO4
(metal
oxides)
(6) residual
(2) aqua
regia + 8.8
M H2O2
(metal
sulphides)
Trends in Analytical Chemistry, Vol. 23, No. 10–11, 2004
(1)
NH4Cl
Pyrite and
amorphous
As2S
(5) HF/
HClO4
(3) 0.04 M
NH2OH·HCl
Amacher and
KotubyAmacher [44]
Il’yin and
Konarbayeva
[39]
Arsenic
oxides
and
silicates /
residual
Gleyzes et al.
[41]
Cai et al. [37]
(1)
Ultrapure
water
(2) 0.1 M
hydroxylamine
hydrochloride
(3) 0.2 M
ammonium
oxalate
(1) 0.1 M
KH2PO4/
K2HPO4
(easily
extractable)
(2) 0.2 M
oxalate/oxalic
acid
(4) 0.3 M
H3PO4
(1) 0.1 M (2) 0.1 M
KH2PO4
NaNO3
Keon et al.
[24]
(1) 1 M
MgCl2
(2) 1 M
NaH2PO4
(3) 1 M HCl
Wenzel et al.
[15]
(1) 0.05M
(NH4)2SO4
(nonspecifically
sorbed)
(2) 0.05 M
NH4H2PO4
(specifically
sorbed)
(3) 0.2 M
+
NH4 -oxalate
buffer in the
dark
(amorphous
and poorlycrystalline
hydrous
oxides of Fe
and Al)
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Cappuyns
et al. [35]
and Van
Herreweghe
et al. [19]
(based on
Manful [34])
Scheme I
(1) 1 M
NH4Cl
(easily
soluble)
Cappuyns
et al. [35]
and Van
Herreweghe
et al. [19]
(based on
Manful [34])
Scheme II
(1) H2O
(2) 2 anionexchange
membrane
(AEM)
strips
(6) 10 M
HF
(4) 0.2 M
oxalate/oxalic
acid
(5) 0.05 M
Ti(III)-citrateEDTAbicarbonate
(4) 0.2 M
+
NH4 oxalate buffer
+ ascorbic
acid (wellcrystallized
hydrous
oxides of Fe
and Al)
(2) 0.5 M
NH4F (pH 8.2)
(NH4Fextractable)
(3) 0.1 M
NaOH
(NaOH or Febound)
(3) 0.5 M
NH4F (NH4Fextractable)
(4) 0.5 M Na
citrate & 1 M
NaHCO3 while
adding 0.5 g
Na2S2O4·2H2O
(4) 0.1 M
NaOH
(NaOH or Febound)
(5) 0.5 M Na
citrate + 1 M
NaHCO3
0.02 MHNO3
(6) 8.8 M
H2O2 +
HNO3
(0.02 M)
(7) 16 M
HNO3
(8) 16 M
HNO3 + 30%
H2O2
(5)
HNO3/H2O2
(microwave
digestion)
(residual)
(5) 0.25 M
H2SO4
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Montperrus
et al. [26]*
(6) HCl /
HNO3 / HF
(residual)
(7) HCl /
HNO3 / HF
(residual)
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749
Where the descriptor of the operationally defined extraction for a given reference is different from that in the heading of the table, the difference name is given in
parentheses after the chemical extractant used.
* Carried out in a single extraction rather than in sequential extractions.
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strongly or specifically adsorbed As, are also thought to
remove As sorbed to Fe phases (e.g., ferrihydrite and
goethite) [15]. Furthermore, even with the care taken
with extraction schemes and quality-assurance
measures, the extractions are not always selective. Van
Herreweghe et al. [19] believed that they extracted
Pb-arsenate during their NaOH extraction, which was
designed to extract only Fe oxyhydroxide-associated As.
H2 SO4 is used by some workers to extract Ca-associated, or acid-soluble, As [19,34,35,40]. Ca-arsenates are
stable only in highly oxidizing and alkaline environments, and are more soluble than Al- and Fe-arsenates.
Because of this, Van Herreweghe et al. [19] suggested
that any Ca-arsenates would dissolve in weaker
extractants (e.g., NH4 F) used earlier in sequential
extraction schemes, so the H2 SO4 would not selectively
extract Ca-arsenates but, rather, As bound to Fe
oxyhydroxides. Van Herreweghe et al. [19] subsequently
abandoned extraction of Ca-arsenates in favor of an
operationally defined As oxide and silicate phase
(Scheme II of Van Herreweghe et al. [19]; Table 2).
Oxidizing, often concentrated, acids and reagents (e.g.,
H2 O2 , HF, HNO3 and aqua regia) are used to extract As
bound to relatively insoluble phases and minerals,
including sulphides, silicates, oxides and ‘residual’
phases (Table 2). These are similar reagents to those
used to extract total As from soils and sediments.
Many of the sequential extraction methods and
extractants used have been evaluated by Gleyzes et al.
[41] and Van Herreweghe et al. [19], with conflicting
results. Gleyzes et al. [41] carried out careful comparisons of different extraction schemes on contaminated
soils, and concluded that a cation-based scheme (slightly
modified from [22]) was more convenient than the
anionic, P-based scheme of Chang and Jackson [42] in
evaluating the mobilization potential of As for their
samples. Van Herreweghe et al. [19] compared the
Community Bureau of Reference (BCR) cation-based
scheme to an anion-based scheme specific to the
properties of As, and recommended the latter (Table 2).
Van Herreweghe et al. [19] also evaluated other
sequential extraction schemes for As. Although they
criticized the method of Keon et al. [24] as being
‘circumstantial’ and using very hazardous reagents (HF
and Ti(III) chloride), in fact, Keon et al. [24] did carry
out rigorous quality assurance procedures. Moreover,
the Keon et al. [24] method was used by Harvey et al.
[43] to evaluate the solid-phase partitioning of As in
sediments responsible for ground water As contamination. Harvey et al. [43] found that the sum of extractions
was 120 ± 39% of the total As determined by XRF,
which was regarded as acceptable.
The two-step method of Amacher and KotubyAmacher [44] that was developed to selectively extract
As associated with metal oxides and metal sulphides
(Table 2) was also criticized by Van Herreweghe et al.
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[19] because, during the reductive dissolution of amorphous Fe oxide by warm, acidified hydroxylamine
hydrochloride in the first step, As re-adsorbed to
goethite. Van Herreweghe et al. [19] used the method of
Jackson and Miller [45] and recommended that 0.1 M
PO4 be added to step one to prevent this re-adsorption.
A number of techniques are used to test the efficiency
of sequential extraction schemes for As in soils and
sediments, including:
• intra-method reproducibility tests between
different types of sample [24],
• adding known amounts of common As-bearing
minerals and phases to sediment sub-samples
[24],
• testing the extraction of As at different molar
extractant strengths and times [15], and most
commonly,
• comparing the sums of data for each extraction
step with data for ‘total’ elements [13,15,
19,24,43].
Coefficients of variation and relative standard deviations for replicates are reported by some researchers to
vary from less than 5% to 10%, and accuracies from
88% to more than 90% [15,19,24]. Van Herreweghe
et al. [19] found that the coefficient of variation was
poorer for heavily contaminated natural samples
(11–20%), and attributed this to sample heterogeneity.
Many researchers have strongly stressed the need for
complementary mineralogical (X-ray diffraction, scanning electron microscopy with energy dispersive X-ray
analysis, electron microprobe analysis), and spectroscopic methods (X-ray absorption near edge structure,
[XANES] and X-ray absorption fine structure [EXAFS]) to
validate sequential extraction data for As [13,19,24,26].
Two-step sequential extractions are used for
bioavailability tests of As in humans, which in several
cases are based on the fact that one of the major pathways of soils and sediments to humans (especially
children) is ingestion through hand-to-mouth activity
[46]. Rodriguez et al. [47] evaluated the bioavailability
of As from mine- and smelter-contaminated soils by
means of a sequential extraction scheme involving
simulated gastric (0.15 M NaCl and 1% porcine pepsin)
and intestinal solutions (NaHCO3 and porcine bile
extract). Ruby et al. [48] devised a physiologically based
extraction to simulate the conditions in the stomach (pH
2.5) and small intestine (pH 7). Simulated gastric
solutions are prepared using HCl, pepsin, citrate, malate,
lactic acid and acetic acid, and NaHCO3 is used to
simulate intestinal conditions. Tests such as these are
very useful bases for human risk-assessment studies.
Single extractions are employed to determine the
amounts of As that may be released from soils and
sediments under particular environmental conditions.
Some of these are based on portions of the sequential
extraction schemes described above, while others have
Trends in Analytical Chemistry, Vol. 23, No. 10–11, 2004
been developed for a specific purpose. Many workers
have sought to assess the degree of affinity of As for Fe
oxides in soils and sediments, because reduction of such
As-bearing, amorphous Fe oxides may lead to release of
As to water systems. Acid ammonium oxalate is used for
this purpose [12], but it has been recognized that it may
be too harsh an extractant and not selective enough,
since it dissolves clay minerals and more crystalline Fe
minerals (e.g., magnetite) [49]. Hydroxylamine hydrochloride is also used to extract Fe oxide-associated As
[26,50]. The weakly acid-extractable concentrations of
As in soils are determined using acetic acid [25,51].
Bhattacharya et al. [52] used sodium pyrophosphate
(Na2 P4 O7 ) to extract organically associated As from
alluvial sediments, and achieved accuracy and precision
of ±5%.
Obviously, the choice of single or sequential extraction
scheme for As will depend on the types of soils and
sediments being analyzed. This was elegantly shown by
Gleyzes et al. [41] and by Montperrus et al. [26], who
found that orthophosphoric acid was the most efficient
extractant for As in river sediment and sludge, and
ammonium oxalate the most efficient for As in soil. A
substantial amount of information about the soil or
sediment should therefore be procured prior to undertaking the sequential extraction work. This should
comprise analysis of the organic matter and nutrient (N,
P) content, mineralogy, cation-exchange capacity, and
the major and trace element geochemistry of the whole
sediment or soil (e.g., soils with high concentrations of
Fe oxides should employ a scheme that includes an extraction for As bound to Fe oxyhydroxides, and soils with
high concentrations of Al and Al-clays, an extraction for
As bound to Al-phases).
4. Conclusions
A wide variety of methods exists for extracting and
analyzing total As and operationally defined solid fractions of As in soils and sediments. Extraction for total As
is carried out by colorimetry, XRF, or wet ashing using
acids on a hotplate or a microwave-digestion instrument.
Analysis of acid extracts is carried out by FAAS, GF-AAS,
ICP-AES or ICP-MS, with or without HG, which generally
improves detection limits by up to 100-fold. Studies
focusing on the determination of the solid-phase partitioning of As in soils and sediments employ sequential
chemical extraction schemes, of which a wide variety
exist. Schemes are based on those for extraction of cations
or the anions P and As. Single extractions are also used to
estimate bioavailable As or As associated with a particular, operationally defined phase, such as Fe oxides.
We strongly recommend that complementary techniques (mineralogical and spectroscopic analysis, pore
water analysis, and platinum-electrode potential [24]) be
Trends
carried out to validate the operationally defined
sequential extraction results for As. Moreover, efforts
should be made to adopt universal standard methods for
total and sequential extraction schemes of As in soils and
sediments, because of its globally hazardous effects and
scientific efforts to understand its biogeochemical
cycling. There may be a need to adopt more than one
sequential extraction scheme, because of the widely
varying characteristics of As-bearing soils and
sediments. Once the sequential extraction schemes are
adopted, CRMs should be manufactured for each
operationally defined phase of the scheme, and these
should be made widely available.
Acknowledgements
We acknowledge support from the UK Natural Environment Research Council (NERC), The Royal Society,
British Council, University of London Central Research
Fund and Birkbeck College Faculty of Science that we
have received over the years for our As-related research.
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