ArsenicSpeciationAnalysisforSiteCharacterization_update4_ph.pptx

Office of Research and Development
Arsenic Speciation Analysis for Site
Characterization
Peng Ho1,*, Robert Ford2, Rick Wilkin1
Office of Research and Development, Center for Environmental Solutions and Emergency Response
1 Groundwater Characterization & Remediation Division, Ada, OK 74820
* National Research Council
2 Land Remediation & Technology Division, Cincinnati, OH 45268

• Arsenic geochemistry in groundwater
• Species
• Toxicity
• Geochemical behavior: mobilization vs immobilization
• Questions from technical support requests
• Why/When/How do we measure arsenic speciation?
• Issues (e.g., modeling arsenic speciation and arsenate mobility)
• Case studies
• East Helena [Region 8]
• Vineland [Region 2]
• Delatte [Region 6]
• Arsenic speciation analysis
• EPA 1632 method vs ORD method
• Sample collection, preservation and holding times
• Summary
Outline

Common Aqueous Arsenic Species

Eh-pH diagram for aqueous As species in the system As–H2O at 25 °C and 1 bar total pressure.
Smedley and Kinniburgh, 2002 (Figure 1, As concentration & ionic strength not specified)
groundwater Arsenate-As(V)
H2AsO4
-
HAsO4
2-
Arsenite-As(III)
H3AsO3
0
oxic
anoxic
Inorganic As Species (only oxyanions)

• Low or negligible in groundwater
• Increased methylated As in
enriched organic-carbon systems
• MMAs(V) and DMAs(V) used for
herbicides and pesticides
• MMAs(III) and DMAs(III) are
unstable under oxidizing
conditions
Methylated Arsenic Species
• Former herbicide production plant in
Marinette, Wisconsin
• MMAs(III) 3.9-274 mg/L
• MMAs(V) 40.6 – 490 mg/L

Mineral Dissolution
Enargite/Luzonite Cu3AsS4
Smithite AgAsS2
Orpiment As2S3
Proustite Ag3AsS3
Thioarsenates
Thioarsenites
H3AsIIIOxS3-x, x = 0-2
H3AsVOxS4-x, x = 0-3
Mineral images from mindat.org
Thioarsenic Species in Sulfidic Waters
S substitution for O

Arsenite decreases with increasing H2S =>
Production of thioarsenic species
The solubility of amorphous As2S3
Eary, 1992
Delatte Superfund site (Region 6);
PRB treatment
?
Thioarsenic Species in Sulfidic Waters

Arsenic Toxicity
Thioarsenic species
Arsenate-As(V)
• Arsenite-As(III) MMAs(V) DMAs(V)
> > =
• MMAs(III) DMAs(III)
> Arsenite-As(III) =
• Marine bacterium: Thioarsenite is not bioavailable and reduces arsenite toxicity (Rader et al.,
2004)
• Plant root: arsenite > monothioarsenate > arsenate; at concentrations ≥ 25 μM As (Planer-
Friedrich et al., 2017)
• Human cell: methylated thioarsenates are highly toxic (Naranmandura et al., 2007)
Toxicity order

Arsenic Behavior: Immobilization
Controlling Factors: Solid surface, pH, Eh, concentrations of As and competing ions, and As speciation
Reduction
Oxidation
abiotic/biotic
Arsenate-As(V)
H2AsO4
-
HAsO4
2-
Arsenite-As(III)
H3AsO3
0
aqueous
solid
Arsenate
(Ca, Fe, Mn)
precipitates
Orpiment
Realgar
Adsorption
Al, Fe, Mn oxides
As(V)
Al, Fe, Mn oxides,
sulfides
As(III)
Coprecipitation Coprecipitation Precipitation
Precipitation
Minerals
&
Natural Organic Matter
As(V) As(III)

Arsenic Behavior: Mobilization
Controlling Factors: pH, Eh, competing ions
Processes: alkali desorption, reductive dissolution, sulfide oxidation, competitive adsorption
Reduction
Oxidation
abiotic/biotic
Arsenate-As(V)
H2AsO4
-
HAsO4
2-
Arsenite-As(III)
H3AsO3
0
Orpiment
Realgar
aqueous
solid
Arsenate
(Ca, Fe, Mn)
precipitates
Minerals
&
Natural Organic Matter
As(V) As(III)
Al, Fe, Mn oxides
As(V)
Al, Fe, Mn oxides,
sulfides
As(III)
Dissolution Desorption Dissolution Dissolution
Dissolution
Thio-As(III) & Thio-As(V)

• Competitive adsorption onto mineral surfaces
• Forms a complex (As-NOM and As-metal-
NOM)
• NOM used as a substrate for in-situ
bioremediation changes redox potentials
which may mobilize As
NOM Role in As Mobilization and Immobilization
Mobilization Immobilization
• As is complexed with solid phase NOM
and accumulates in sediments
Natural Organic Matter (NOM) can be
• Dissolved phase, colloidal phase, solid phase and bound to mineral surfaces in soils
and sediments
• Source for microbe growth
• Electron shuttle

• Why do we need to
measure As speciation?
• When should speciation
measurements be
requested?
• How is it measured &
what are the sample
collection issues?
At sites where arsenic is a COC and remediation is anticipated.
Accurate lab-based speciation measurements inform the CSM
and RI/FS process; critical information for evaluating &
designing remediation strategies in the FS
As soon as possible in the RI process; after arsenic is
established as a COC. Perhaps triggered by 5-year review or an
optimization study
Contract labs can perform this analysis (ORD can assist as well).
General Questions from Technical Support Requests
Field filtered; HCl-preserved;
amber-plastic, cold (for oxyanions
in most groundwater matrices;
note acid preservative not used for
sulfide-containing samples)

• Field measurements of pH and
ORP, along with lab-measurements
of dissolved arsenic can be used to
calculate arsenate & arsenite
• Use Phreeq-C, MinteqA2, or
Geochemist’s Workbench® for
equilibrium calculations
• Many studies have shown this
approach to be inaccurate
• An uncertainty in the ORP
measurement of 20 mV
(conservative) translates to an
uncertainty in the log ratio of 1.4
or a factor of 13x
from Wilkin et al. (2008) EPA/R-08/093 from Beak and Wilkin (2009)
Issue: “We do not need to measure it because we can model it”
It = arsenic speciation (or any redox couple)

• Use field measurements of pH and Fe(II) to calculate Eh and
estimate the As(III)/As(V) ratio
• Does not rely on ORP measurements; this approach assumes
“equilibrium” between Fe(II) in solution and Fe(III) in a solid
phase (i.e., hematite, goethite, hydrous ferric oxide)
• An uncertainty in the Eh estimation of >200 mV results from using
the spectrum of hematite to ferrihydrite as the assumed solid
phase; thus, need to constrain mineralogy
Issue: “We do not need to measure it because we can model it”
Using groundwater Fe(II) to constrain Eh, and redox couples

Some truth here; however, arsenate, like arsenite, is very soluble over most conditions. Because
arsenate is a charged anion it will tend to sorb to charged surfaces, like Fe(III) oxyhydroxides.
Key questions to ask w/respect to site characterization: how much sorption capacity is available and
how much capacity could be produced through time? (molar Fe(II)/As ratios > 8 are desirable)
Arsenate is soluble
from Cheng et al. (2016) Water Research, v. 96, p. 22-31
Arsenate sorption has limits
Issue: “Arsenic present as arsenate will be less mobile
in the environment”

Case Study 1 ― East Helena/smelter site (Region 8)
• Complex smelter site [former ASARCO facility]
• Long history (>100 y); multiple sources, mainly arsenic
(groundwater), selenium (groundwater), lead (soil), cadmium
(soil/groundwater)
• Arsenic (and selenium) speciation analysis was used to develop
conceptual site model; understand plume structure; and
understand transport & fate
• Accurate speciation information is
needed for in-situ remedy selection
• Wilkin (ORD) & Betsy Burns (R8)

• L-shaped As-Se concentration
relationship
• Caused by site redox conditions
=> governs As/Se speciation
and mobility

Case Study 2 ― Vineland [Region 2]
• ORD STLR Project w/Region 2, Army Corps of
Engineers, and ORD
• Large-scale pilot testing of air sparging in low-pH,
Fe(II) groundwater – Vineland Superfund Site (NJ)
• Solid-phase Arsenic speciation was determined on
core samples after digestion in 0.1 M HCl and in
water
• Analysis showed that most of the extracted arsenic
was present as As(III) with some As(V)
• Wilkin (ORD), Nica Klaber (R2), Diana Cutt (ORD/R2),
Tricia North & Lily Sehayek (Army Corps)
EPA Report 600/R-19/102

• SEM images and EDX maps of particles in
samples show that As is closely associated
with Fe-rich coatings present on the aquifer
particles
• The speciation analysis indicated that during
air sparging:
 Groundwater Fe(II) was oxidized and
precipitated as Fe(III)-O solids
 Groundwater Arsenite sorbed to the Fe(III)
solids and mainly remained as As(III); i.e.,
transformation to As(V) is kinetically limited
Case Study 2 ― Vineland [Region 2]

Case Study 3 ― Delatte [Region 6]
• Site impacted by historical battery recycling activities
• Groundwater impacted by metals (Pb, Cd, Ni) and
acidic pH (pH 2.8 - 3.7)
• 700 ft (long) x 14 ft (deep) x 6 ft (width) full-scale cow
manure-limestone PRB installed in Spring 2003
• PRB designed to produce alkalinity and neutralize
acidity along with promoting sulfate reduction to
stimulate precipitation of metal sulfides
• Ludwig/Wilkin/Ross (ORD); Brian Mueller (R6)

Case Study 3 ― Delatte [Region 6]
• PRB successful in neutralizing acidic
groundwater and precipitating metals
(Pb, Cd, Ni)
• Stimulation of sulfate reduction
produced dissolved H2S
concentrations at sufficient levels to
produce more mobile thioarsenic
species in groundwater
• Important to consider potential
generation of secondary arsenic
plumes for in-situ remedy designs
that stimulate conditions conducive
to arsenic mobilization

• Measure Inorganic As (arsenite + arsenate), MMAs(V)
and DMAs(V)
• Arsenate concentration is calculated
• Arsenate = Inorganic As – Arsenite
• QA/QC
• Initial precision and recovery
• Quality Control sample (secondary source)
• Calibration verification
• Blanks (calibration blanks, method blanks, field
blanks, equipment blanks)
• Matrix Spike and Matrix Spike Duplicate
• Ongoing precision and recovery
“How do we measure it?” EPA Method 1632 - Hydride Generation Quartz
Furnace Atomic Adsorption Spectrometry

“How do we measure it?” ORD Method
Arsenic Speciation Analysis using IC-TQ-ICP-MS
A known issue: 40Ar35Cl interference on
75As using SQ-ICP-MS
Mass shift reactions that move the
analyte of interest to a different m/z
using TQ-ICP-MS
Arsenic Species Separation
Arsenic Determination
Arsenic Chromatography Display
Ion Chromatography
TQ-ICP-MS
150Nd+, 91Zr+
75As++  91[AsO]+
75As+
Q1
Q2
Q3
O2
150Nd++, 150Sm++
75ArCl+
91[AsO]+
Detector

C18 column
Isocratic Method
Eluent: 5 mM tetrabutylammonium hydroxide,
3 mM malonic acid in 5% methanol (pH: 5.70)
Arsenite Peak at ~ 93 sec.
Arsenate Peak at ~ 242 sec.
PRP-X100 column
Isocratic Method
Eluent: 10 mM ammonium phosphate monobasic
and ammonium nitrate (pH: 4.75)
Arsenite Peak at ~ 139 sec.
Arsenate Peak at ~ 386 sec.
“How do we measure it?” ORD Method
Arsenic Speciation Analysis using IC-TQ-ICP-MS

• Samples are acidified in both methods; this is a concern for samples containing free
dissolved hydrogen sulfide/bisulfide
Comparison between EPA Method 1632 and
ORD method
Method EPA1632 ORD Method
Analytes Measurement 1 (>13 mins)
Inorganic As [As(III)+As(V)], MMAs(V), DMAs(V)
Only 1 measurement (7-10 mins)
As(III), As(V), MMAs(V), DMAs(V)
Measurement 2 (>7 mins)
As(III)
Calibration Range 0.01 – 50 µg/L 0.05 – 2000 µg/L
Minimum level of
Quantitation
<1 µg/L <1 µg/L
Note As(V) is not directly measured
As(V) = Inorganic As – As(III)
pH range (4.7-5.7) of eluents is compatible
with Fe-containing samples

• Filtration (< 0.45 μm); Dark condition or opaque bottle
• These methods may preserve MMAs(V) and DMAs(V), but NOT thioarsenic species (sensitive to pH change)
EPA
Method 1632
ORD Method McCleskey
et al., 2004
Gault et al.,
2005
Bednar et
al., 2002
Samanta and
Clifford, 2005
Stetson et
al., 2021
Reagent HCl (pH <2) HCl (pH <2) HCl HCl EDTA EDTA-HAc EDTA
Temperature (°C) 0-4 0-4 4 4 Room Temp. Room Temp. 4
Holding time 28 days 14 days 15 weeks 3 months 30 – 85 days < 15 days
Sample
characteristic
Not sulfidic;
Fe-containing
Not sulfidic;
Fe-containing
Not sulfidic;
Fe-
containing
High-Fe,
low-Eh, Not
sulfidic
Fe-
containing
Fe-containing Low Fe
(1mg/L)
Comment (1) EDTA forms complexes with other cations
(2) Too much EDTA addition changes the
matrix and dilutes the As concentration
Preservation for arsenate and arsenite in water samples –
chemical preservative methods

• Filtered and unacidified samples with no headspace are
collected in amber glass vials
• Keep in a cool condition (4°C) until analysis
• Thioarsenic species is analyzed using ion
chromatography with a sample/eluent pH matching
method
 Analytical column: AS22
 Isocratic method: 60 mM NH4NO3 (eluent pH is
adjusted to match sample pH with NH4OH addition)
 All elute (arsenite, arsenate and thioarsenites) in < 10
minutes
Preservation and analysis for thioarsenic
species in water samples
Arsenic chromatography (m/z = 75) of arsenic speciation in natural
sulfidic matrices using the AS22 column and sample pH/eluent pH
matching.
Na-Ca-HCO3-type water (pH 7.02) with 31 μM/L ΣH2S
Na-Cl-type water (pH 7.15) with 46 μM/L ΣH2S.

0
250
500
750
1000
0 250 500 750 1000
Aresnite
+
Arsenate
(ppb)
total dissolved As (ppb)
C18 PRP-X100
filtered & acidified
Total dissolved As concentration using TQ-
ICP-MS
Arsenite and arsenate concentration using IC-
TQ-ICP-MS
𝑅𝑒𝑐𝑜𝑣𝑒𝑟𝑦 (%) =
𝑎𝑟𝑠𝑒𝑛𝑖𝑡𝑒+𝑎𝑟𝑠𝑒𝑛𝑎𝑡𝑒
𝑡𝑜𝑡𝑎𝑙 𝑑𝑖𝑠𝑠𝑜𝑙𝑣𝑒𝑑 𝑎𝑟𝑠𝑒𝑛𝑖𝑐
× 100
Important: Are arsenite and arsenate
effectively preserved?
Samples from
Ft. Devens, Nov.
2021
C18: 75-110 %
(median: 87%)
PRP-X100: 70-100%
(median: 78%)
 If the recovery is outside of 70-130%, then the
sample results need to be re-evaluated.
Preservation for arsenate and arsenite in non-sulfidic
water samples

Biological sulfate reduction technologies
Conceptual iron transformation and sulfur redox cycle,
and arsenic partitioning among different phase. (Sun et
al., 2016)
The importance of thioarsenic compounds in
sulfide-rich groundwater

• As speciation changes with redox and pH gradients, which subsequently affect As mobilization
and immobilization in groundwater. Many remedial technologies manipulate redox and pH of
groundwater that might cause an unintended As plume.
• As speciation analysis for site characterization can provide useful information.
• CSM and RI/FS process
• evaluating & designing remediation strategies in the FS
• As speciation measurements should be conducted:
• as soon as possible in the RI process
• after arsenic is established as a COC
• 5-year review or an optimization study
• Sample collection method:
• Field filtered; HCl-preserved; amber-plastic, cold (for oxyanions in most groundwater
matrices; not suitable for sulfide-containing samples).
• Send samples to Contract Labs and/or ORD can perform As speciation analysis for groundwater
studies.
Summary

Dr. Peng Ho
National Research Council Research Associate
Groundwater Characterization & Remediation Division, Ada, OK
580-436-8607
ho.peng@epa.gov
Dr. Richard T. Wilkin
US EPA Office of Research and Development
Center for Environmental Solutions and Emergency Response
Groundwater Characterization & Remediation Division, Ada, OK
580-436-8874
wilkin.rick@epa.gov
32
Contact Information
Dr. Robert Ford
US EPA Office of Research and Development
Land Remediation & Technology Division
Cincinnati, OH
513-569-7501
ford.robert@epa.gov
Disclaimer: The views expressed in this presentation are those of the
authors and do not necessarily represent the views or policies of the U.S.
Environmental Protection Agency. Any mention of trade names or
commercial products does not constitute endorsement or
recommendation for use.

Issue: “Will in-situ redox manipulation result in
mobilization of naturally occurring arsenic?”
• Stimulation of iron- and/or sulfate-
reducing conditions to induce
organic contaminant degradation
• Iron-reducing conditions:
dissolution of iron oxides in aquifer
solids and mobilization of adsorbed
arsenic
• Sulfate-reducing conditions: change
arsenic speciation to more mobile
forms (e.g., thioarsenic species)
• Extent of problem depends on
capacity of downgradient aquifer to
re-attenuate mobilized arsenic
Reductive dissolution of iron oxide mobilizes adsorbed
arsenite:
 FeOOH-H3AsO3 + 3H+ + e- = Fe2+ + H3AsO3 + 2H2O
Conversion of arsenite to thioarsenic during sulfate
reduction coupled to organic matter oxidation:
 H3AsO3 + 3SO4
2- + 3CH4 = H3AsS3 + 3CO2 + 6OH- + 3H2O

• Generally, it is important to collect both unfiltered
and filtered samples when metal COCs are
identified
• The question to answer is whether metals are
associated with particulates that might be
captured during well purging
• If metals are associated with particulates, i.e.,
MeTotal > MeFiltered
• Could be metal-containing particles are
transported in groundwater
• Could be metals are concentrated in particles
formed or transported into the well bore,
often associated with Fe, Mn, or Al
• Could be aquifer particles that come into the
screened interval and are captured during the
well purge
• Specific to Arsenic speciation analysis,
in most cases, it makes the most sense
to filter samples in the field and
preserve
• It is possible to extract unfiltered
samples, filter, and carry out speciation
analyses
• Samples that contain particulates must
be avoided during chromatography
Sample Collection Issue: “Total (Unfiltered) vs Filtered”

Potential Problems
• Ion-exchange capacity is
exhausted by other anions
(i.e., SO4
2-, Cl-)
• Organic arsenic species (e.g.,
MMAs(V), DMAs(V)) elute
with arsenite
AsIII
AsV
H
3
As
III
O
3
0
Filtered
Acidified or EDTA
(EDTA-HAc)
Anion-exchange
column
H
2
As
V
O
4
-
HAs
V
O
4
2- eluent
Anion-exchange
column
H
2
As
V
O
4
-
HAs
V
O
4
2-
Preservation for arsenate and arsenite in water samples –
Field-speciation methods

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