United States
Environmental Protection
Agency
Engineering Issue
Biotransformation of Dimethylarsinic Acid
Table Contents
1.0 PURPOSE 1
2.0 INTRODUCTION 1
3.0 BACKGROUND 2
3.1 Chemical Properties 2
3.2 Toxicity 3
4.0 BIOTRANSFORMATION OF DMA(V) 4
4.1 Microbial Arsenic Tolerance 6
4.2 Biotransformation of DMA(V) and Related
Arsenic-Containing Compounds 6
4.2.1 Studies in Aerobic
Biotransformation 7
4.2.2 Studies in Anaerobic
Biotransformation 7
5.0 FATE AND TRANSPORT IN
THE ENVIRONMENT 10
6.0 SUMMARY 10
7.0 ACKNOWLEDGEMENTS 10
9.0 REFERENCES 11
ACRONYMS AND ABBREVIATIONS 13
Disclaimer: The U.S. Environmental Protection Agency, through its Office
of Research and Development, funded and managed the literature review
on the state of the science described herein. It has been subjected to the
Agency's peer and administrative review. Any opinions expressed in this
report are those of the author(s) and do not necessarily reflect the views of the
Agency, therefore, no official endorsement should be inferred. Any mention
of trade names or commercial products does not constitute endorsement or
recommendation for use.
1.0 PURPOSE
The U.S. Environmental Protection Agency (EPA) Engineering Issue
Papers (EIPs) are a series of documents that summarize the available
information on specific contaminants, selected treatment and site
remediation technologies, and related issues. This EIP is intended to
provide remedial project managers (RPMs), on-scene coordinators
(OSCs), contractors, and other state or private remediation managers
with an overview regarding the biotransformation of dimethylarsinic
acid (DMA[V]). The (V) suffix in DMA(V) denotes the +5 oxidation
state of arsenic.
This EIP summarizes the state of the science regarding the
biotransformation of DMA(V) and was developed from peer-reviewed
literature, scientific documents, EPA reports, internet sources, input
from experts in the field, and other pertinent sources. This EIP
includes a review of the current understanding of biologically-mediated
transformation of DMA(V) and its metabolites. Given the challenges
remaining in transitioning from laboratory studies to field applications,
this EIP provides summary guidance for implementing currently
recommended remediation strategies for DMA(V) at contaminated
sites.
The table of contents shows the type of information covered in this EIP.
Important information has been summarized, while references and web
site links are provided for readers interested in additional information.
The web site links, verified as accurate at the time of publication, are
subject to change.
2.0 INTRODUCTION
Historically, DMA(V) and its salts have been used as herbicides
and defoliants and became one of the most popular herbicides used
worldwide in terms of volume [1-3]. It is estimated that during the
1970s and 1980s, 10 to 12 million acres were treated annually with 2.1
million kilograms (kg) of monomethylarsonic acid (MMA[V]) and
DMA(V) in the U.S. [4]. At present, large amounts of organic arsenical
herbicides are used for agricultural and aesthetic reasons (e.g., golf
course maintenance) [5, 6]. DMA is also one of the primary sources
of arsenic in orchards, and is likely a source of arsenic in apples and
juice that the public became keenly aware of in 2011. In 2011, it was
estimated that 100,000 pounds of DMA(V) were commercially used
in the U.S. in over 150 products [5]. However, EPA banned the use of
organic arsenicals (including DMA[V]) after December 31, 2013.
-------
A mixture of DMA(V) and sodium cacodylate was applied
to crops during the Vietnam conflict. This herbicidal
mixture was named 'Agent Blue . Between 1962 and 1971,
an estimated 1.2 million gallons of varying concentrations
of Agent Blue were released under the herbicide program
known as "Operation Trail Dust" [3]. Information
available regarding this program indicates that Agent Blue
was applied to crops using low-flying aircraft equipped
with sprayers. The intention of this herbicide application
program was to reduce crop growth through desiccation.
In addition to anthropogenic sources of DMA(V), it
is likely that biotransformation processes within the
environmental arsenic cycle are significant sources of
organic arsenicals [5]. For example, phytoplankton have
been observed to methylate arsenate to MMA(V) and
DMA(V) within marine environments [7]. Within the
arsenic biogeochemical cycle, it is important to note that
pentavalent organoarsenicals are less toxic than inorganic
arsenates, whereas trivalent inorganic arsenic species
are generally both more toxic and more mobile in the
environment than pentavalent inorganic arsenic species.
Thus, understanding the potential biotransformation
pathways of organoarsenicals within the environment as
well as the fate, transport, and risk associated with the
various arsenic species will help to guide remediation
strategies for DMA(V). This EIP discusses the following
in sections below:
1. Chemical properties and toxicity;
2. Biological transformation processes, and;
3. Fate of these compounds in the environment.
3.0 BACKGROUND
3.1 Chemical Properties
DMA(V) is comprised of a single arsenic atom connected
by three single covalent bonds (two methyl groups and
one hydroxyl group) and a double bond to oxygen (see
Figure 1). This results in a +5 oxidation state for arsenic.
DMA(V) is an amphoteric compound (exhibiting
properties of both an acid and base) with an acid
dissociation constant (pKa) value of 6.4. DMA(V) exists
in a white crystalline solid form with a melting point of
195°C. Its water solubility is 2,000 g/L at 25°C [8]. The
soil organic carbon-water partitioning coefficient (Koc)
for DMA(V) is reported to be 43.89 L/kg [9].
H3C
H3C As =
/
HO
Figure 1. Chemical structure of DMA(V)
There are a relatively small number of related organic and
inorganic arsenic compounds which have been identified
as the main chemical species involved in the transformation
of DMA(V) (see Table 1). The compounds in Table 1
contain arsenic in either the +3 or +5 valence state. These
compounds include inorganic forms (As[III] and As[V])
as well as mono- and di-methyl species (MMA[III],
MMA[V], DMA[III] and DMA[V]), and one relatively
non-toxic and volatile trimethyl species (TMAO). These
compounds have been chosen for further discussion as
they represent the main products of transformations of
DMA(V) in the natural environment. They have been
detected and quantified in laboratory studies relating to
the amount and rate of biotransformation of DMA(V).
Engineering Issue: Biotransformation of Dimethylarsinic Acid
-------
Table 1. Chemical Properties of Select Inorganic and Organic Arsenic Compounds
Compound
Arsenite (As[lll])
Arsenate (As[V])
Monomethylarsonous acid (MMA[III])
Monomethylarsonic acid (MMA[V])
Dimethylarsinous acid (DMA[III])
Dimethylarsinic acid (DMA[V])
Molecular Formula
As033
As043
As(CH3)(OH)2
AstCH^OHJjO
As(CH3)2(OH)
As(CHJ2(OH)0
hc./ru \ n
AS(OH3)3U
Arsenic
Oxidation State
3+
5+
3+
5+
3+
5+
5+
Chemical Structure
(fully protonated forms)
\
OAc
\
/
\
1 \C\ A r
HO
\
HO
\
Hf~ A r
/
Ho
\
/
HO
\
H3C
3.2 Toxicity
Overall, DMA(V) and other pentavalent organoarsenicals
are less toxic than inorganic arsenic. All trivalent forms of
arsenic species (both inorganic and organic) are generally
both more mobile and more toxic to humans than the
pentavalent forms of arsenic compounds (organic and
inorganic). This is apparent in both the minimum risk level
(MRL) values derived by the Agency for Toxic Substances
and Disease Registry [8] and from EPA regional screening
levels (RSLs) for soil and tap water. The MRL is an estimate
of the daily human exposure to a hazardous substance that
is likely to be without appreciable risk of adverse, non-
cancer health effects over a specified duration of exposure.
Table 2 summarizes available MRLs and RSLs for inorganic
arsenic and organic arsenic (V) species discussed in this
paper.
Biotransformation of Dimethylarsinic Acid: Engineering Issue
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Table 2. Summary of MRL and RSL Values for Inorganic Arsenic and Organoarsenicals
MRL, acute duration (14 days or less) oral
exposure [8]
MRL, intermediate duration (15 to 364
days) oral exposure [8]
MRL, chronic duration (365 days or more)
oral exposure [8]
Residential Soil RSL [9]
Tapwater RSL [9]
Inorganic Arsenic
0.005 mg As/kg/day
NE
0.0003 mg As/kg/day
0.39 mg/kg
0.045 |jg/L
MMA(V)
NE
0.1 mg MMA/kg/day
0.1 mg MMA/kg/day
610 mg/kg
160|jg/L
DMA(V)
NE
NE
0.2 mg DMA/kg/day
1,200 mg/kg
3,100 |jg/L
NE - Not established
4.0 BIOTRANSFORMATION OF DMA(V)
The term "biotransformation" refers to biologically-
mediated reactions which change the composition and/
or distribution of contaminants in the environment.
"Bioremediation" is a process where microorganisms
biotransform hazardous contaminants to other comp ounds
that are intended to be less hazardous than the parent
material. Therefore, the term "biotransformation" should
not be interpreted as a synonym for "bioremediation".
Understanding biotransformation processes for DMA(V)
will help to inform the optimum operating conditions for
potential application in remediation efforts.
As presented in Table 3 and Figure 2, the four
biotransformation reactions associated with arsenic are:
1. Methylation. The process in which a compound
gains a methyl group. For example, methylation
of the organic compound MMA(V) to the organic
compound DMA(V).
2. Demethylation. The process in which a compound
loses a methyl group. For example, demethylation
of the organic compound MMA(V) to the inorganic
compound As(V).
3. Oxidation. The process in which the valence state of
the compound is increased. For example, oxidation
of the organic compound MMA(III) to the organic
compound MMA(V) increases the valence state of
arsenic from +3 to +5.
4. Reduction. The process in which the valence state of
the compound is decreased. For example, reduction
of the inorganic compound As(V) to the inorganic
compound As(III) decreases the valence state from
+5 to +3.
Organic and inorganic arsenic compounds can undergo
any of these four reactions based on environmental
conditions. In general, methylation and demethylation of
arsenic are facilitated by microorganisms via enzymatic
processes [7, 10]. As illustrated in Figure 2, DMA(V)
can be demethylated to MMA(V), which can be further
demethylated to inorganic As(V) biotically Conversely,
inorganic As(V) has been shown to be biotically
methylated to MMA(V) and DMA(V). While arsines
can be formed from As(V) compounds, the contribution
of this mechanism to the fate of DMA(V) is minimal and
therefore will not be described in detail in this paper [10].
Oxidation and reduction reactions may occur abiotically
when environmental conditions change, such as
photochemical oxidation of reduced arsenic compounds.
However, oxidation and reduction processes can be a result
of biotic processes. As shown in Figure 2, microorganisms
have been observed to reduce As(V) to As(III) as well as
oxidize As(III) to As(V) [7]. As shown in Table 2, As(V)
species (organic and inorganic) are less toxic than As(III)
species (organic and inorganic). The general toxicity
trend by species increases in the TMAO(V) < DMA(V)
< MMA (V)< [As(V), As(III)] < MMA(III) [10]. Given
the wide variety of potential reactions in the arsenic
biogeochemical cycle, the following sections focus on the
role that microorganisms play with respect to DMA(V)
biotransformation processes.
Engineering Issue: Biotransformation of Dimethylarsinic Acid
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Table 3. Summary of the Potential Biotransformation Reactions for DMA(V) and Related Compounds
with Respect to Relative Toxicity
Reactant
DMA(V)
DMA(V)
DMA(V)
MMA(V)
MMA(V)
MMA(V)
MMA(V)
MMA(V)
Inorganic As(V)
Inorganic As(V)
Inorganic As (V)
Inorganic As (V)
Inorganic As (III)
Inorganic As (III)
MMA(III)
MMA(III)
MMA(III)
TMAO(V)
Transformation Reaction
Demethylation
Demethylation
Reduction
Demethylation
Methylation
Methylation
Reduction
Reduction
Methylation
Methylation
Reduction
Reduction
Methylation
Oxidation
Demethylation
Oxidation
Oxidization
Reduction
Product
MMA(V)
Inorganic As (V)
Arsines
Inorganic As (V)
TMAO(V)
DMA(V)
Arsines
MMA(III)
MMA(V)
DMA(V)
Arsines
Inorganic As (III)
MMA(III)
Inorganic As (V)
Inorganic As (III)
MMA(V)
TMAO(V)
MMA(III)
Relative Toxicity
More Toxic
More Toxic
More Toxic
More Toxic
Less Toxic
Less Toxic
More Toxic
More Toxic
Less Toxic
Less Toxic
More Toxic
More Toxic
More Toxic
Less Toxic
Less Toxic
Less Toxic
Less Toxic
More Toxic
Biotransformation of Dimethylarsinic Acid: Engineering Issue
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Demethylated
As(V)
-»-MMA(V)-«— —*- |>\1A(V)
Methylated
I M V I, •. .
0ne-WtyAr$#iig (As) Ttanslocmation
Two-Wajf As TransJocmaflion
.vim i.
Higher Toxicity
ne. (111)
Figure 2. Overview of Biological Transformation Processes of Organic and Inorganic Arsenic Compounds
4.1 Microbial Arsenic Tolerance
Biotransformation processes can only occur if active
biological species capable of transforming DMA(V) and
related compounds thrive within the environment. For
example, microbial arsenic-tolerant species have been
isolated from gold and antimony mines [11, 12], highly-
contaminated soils and sediments [13-16], and golf course
greens [6, 17]. In general, it is unusual for organisms
to be arsenic-tolerant as arsenic is generally used as an
antimicrobial agent. These microorganisms are either
tolerant of arsenic uptake or have developed the ability to
detoxify arsenic. One detoxification mechanism uses an
arsenic-resistance operon (denoted 'ars') that facilitates
biological transformation [18, 19]. Tolerance to arsenic-
containing compounds is highly species-, environment-,
and compound-specific [20, 21]. For example, reported
arsenic tolerance for Stenotrophomonas maltophilia
ranged over two orders of magnitude [11]. Additionally,
phytoplankton are thought to accumulate inorganic
arsenic species and generate organoarsenic compounds
as a detoxification mechanism [22, 23]. Regardless of the
detoxification mechanism, microbial action contributes
to the arsenic biogeochemical cycle and can include
methylated, demethylated, oxidized or reduced forms of
arsenic.
4.2 Biotransformation of DMA(V) and Related
Arsenic-Containing Compounds
Tables 4 and 5 present key aspects of research conducted
over the past 40 years under controlled conditions to study
methylation, demethylation, oxidation and reduction. The
tables include, to a lesser extent, oxidation and reduction
of organoarsenic compounds related to DMA(V). The
studies reviewed here have been separated into those
conducted under aerobic (or oxic) conditions (Table 4) and
those conducted under anaerobic (or anoxic) conditions
(Table 5). The following discussion will provide insight
Engineering Issue: Biotransformation of Dimethylarsinic Acid
-------
into the research work conducted in understanding the
complexity of biotransformation of DMA(V) and related
organoarsenicals.
4.2.7 Studies in Aerobic Biotransformation
Research conducted in aerobic environments has
studied the ability of indigenous microorganisms from
contaminated soils, sediments and aqueous environments
to transform organoarsenicals. Overall, DMA(V)
biotransforms to MMA(V), inorganic As(V), and arsines
over time in the environment with inorganic As(V) as
the predominant end product. The reverse pathways of
methlyation of inorganic As(V) to MMA(V) and DMA(V)
are also possible; however, anaerobic methlyation of As(V)
appears more prevalent in the environment [10].
In an early study by Von Endt et al. [24], demethylation rates
were observed to be much higher in pure cultures than in
soils as would be expected. In later studies, three similar
soil types were used to evaluate DMA(V) degradation
and three metabolites were identified: As(V), MMA(V),
and arsines [25, 26]. Although the organisms responsible
for DMA(V) transformation were not specifically
identified, a large consortium of microorganisms was
assumed responsible for the biotransformation. Specific
organisms responsible for the biotransformation of these
compounds were not identified until recently when
advanced microbiological techniques were employed [10].
Additionally, impacts of amendments such as manure,
hay or sewage sludge were investigated on DMA(V)
biotransformation [26]. Although no difference in
transformation rate was observed between amendments,
DMA(V) was degraded more rapidly in unamended soils.
This difference is assumed to be due to reduced DMA(V)
bioavailability through adsorption to amended organic
matter. More recent studies used low concentrations
of DMA(V) as the reactant organic arsenic compound.
Huang et al. [27] studied the demethylation and reduction
of 755 to 1,035 parts per billion concentrations of DMA(V)
in soils collected from Germany. In contrast to Gao
and Burau shown in Table 4 [28], Huang et al. observed
the sequential formation of MMA(V) before complete
demethylation to inorganic forms of arsenic.
With respect to other organoarsenicals, MMA(V) was the
subject of two studies in both soil and pure culture [29,30].
In soil, parts per million concentrations of MMA(V) were
demethylated to As(V). Separate pure cultures of bacteria
(Mycobacterium neoaurum and Scopulariopsis koningii)
as well as three fungi (Fomitopsis pinicola, Puccinia gladioli
and Scopulariopsis brevicaulis) methylated MMA(V)
finally to TMAO. These organisms were also found to
oxidize and methylate MMA(III) to TMAO. Therefore,
the dynamic nature of the arsenic biogeochemical cycle
should be recognized when investigating a contaminated
site such that analyzing for specific arsenic compounds
and oxidation states is recommended.
4.2.2 Studies in Anaerobic Biotransformation
As with aerobic studies, research conducted in anaerobic
environments evaluated the ability of indigenous
microorganisms to transform organoarsenicals. Overall,
the entire range of potential biotransformation pathways
(see Figure 2) can be observed under anaerobic conditions.
Demethylation of DMA(V) as well as methylation of As(V)
can occur under the appropriate conditions. Anoxic
contaminated sites with arsenic contamination should be
monitored for all As(III) and As(V) species to understand
the distribution of arsenic compounds.
Demethylation of DMA(V) can produce in MMA(V) and
inorganic As(V). Woolson and Kearney [25] repeated
their earlier aerobic experiments on three soils under
anoxic conditions. As with the aerobic conditions,
DMA(V) was converted to inorganic As(V) as well as
arsines, however DMA(V) transformations were not as
efficient under anaerobic conditions. Furthermore, As(V)
reduction to As(III) has been observed by iron-reducing,
sulfate-reducing, and fermenting populations as well
as methanogens [10]. This indicates that reduction to
the more toxic trivalent forms is a possibility. Another
potential pathway for As(III) formation includes the
reduction of MMA(V) to MMA(III) as an intermediate
compound before producing inorganic As(III).
Studies have also been completed to determine the
anaerobic transformation of MMA(V) [2, 5, 15, 29] and
inorganic forms of arsenic [31,32]. In the case of MMA(V),
transformation products include either inorganic arsenic
or volatile arsines. Yoshinaga et al. [5] identified two
specific bacteria that were necessary and sufficient for
transformation of MMA(V) to As(III). Reduction of
MMA(V) to MMA(III) by Burkholderia species was
first required before demethylation of MMA(III) to
As(III) by Streptomyces species. This indicates that full
transformation of some organoarsenic compounds may
not be accomplished by the same organism, but rather
as sequential steps of methylation/demethylation and
oxidation/reduction.
Biotransformation of Dimethylarsinic Acid: Engineering Issue
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00 Table 4. Summary of Selected Biotransformation Studies of DMA(V) and Related Compounds in Aerobic Environments
Initial Arsenic
Species
DMA(V)
MMA(V)
MMA(III)
Biotransformation
Demethylation
Demethylation/
Arsine
Demethylation/
Reduction
Demethylation
Methylation
Methylation
Transformation
Pathway
(see Figure 2)
DMA(V)^As(V)
DMA(V)^As(V),Ars
DMA(V) -> MMA(V) ->
iAs
MMA(V)^As(V)
MMA(V) -> TMAO
MMA(III) -> TMAO
Microorganism
Fungus, Actinomycete,
Bacterium
Consortium
Consortium
Consortium
Consortium
Consortium
Consortium
Consortium
Mycobacterium neoaurum,
Scopulariopsis koningii, F.
pinicola, P. gladioli,
S. brevicaulis
Mycobacterium neoaurum,
Scopulariopsis koningii, F.
pinicola, P. gladioli,
S. brevicaulis
Initial
Concentration
10 ppm
10 ppm, 100 ppm
10ppb
184 ppm
1 ppm, 10 ppm,
100 ppm
1545 ppm
755 ppb, 1035 ppb
3.7 ppm, 9.2 ppm
920 ppb
930 ppb
Temperature
30 °C
28-30 °C
20 °C
5°C,25°C
25 °C
25 °C
5°C
15°C,30°C
21 °C
21 °C
Percent
transformed/
3-20%/11 d
1.7-10%/60d
37%/24h
5-90%/70 d
76%/168d
25%/91d
80%/100d
72%/120d
3-1 7% / 28 d
8-38% / 28 d
Media
Pure Culture
Soil
Mixed
Culture
Soil
Soil
Mixed
Culture
Soil
Soil
Pure Culture
Pure Culture
Reference
[24]
[24]
[33]
[28]
[25]
[34]
[27]
[29]
[30]
[30]
DMAr(lll) = dimethylarsine
Ars = volatile arsine compounds (arsine, methylarsine, dimethylarsine, trimethylarsine)
iAs = inorganic arsenic
100% removal indicates final concentration was nondetect for the method employed
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Table 5. Summary of Selected Biotransformation Studies of DMA(V) and Related Compounds in Anaerobic Environments
Initial Arsenic
Species
DMA(V)
MMA(V)
As(V)
As(V)
Biotransformation
Demethylation/
Reduction
Demethylation
Demethylation/
Methylation
Demethylation/
Reduction
Demethylation/
Reduction
Demethylation/ Meth-
ylation
Reduction
Reduction/ Methyla-
tion/Oxidation
Methylation
Transformation
Pathway
(see Figure 2)
DMA(V) -> iAs
DMA(V) -> MMA(V) ->
iAs
DMA(V) -> MMA(V) ->
iAs
DMA(V) -> MMA(V)
DMA(V) -> iAs and Ars
DMA(V) -+ MMA(V),
As(V) and Ars
DMA(V)^Ars
MMA(V) -+ MMA(III) -+
As(lll)
MMA(V) -+ MMA(III) -+
As(lll)
MMA(V) -+ iAs
MMA(V)^As(V)and
Ars
MMA(V) -+ MMA(III)
As(V)^As(lll)^
MMA(V) -+ DMA(V) -+
Ars
As(V)^Ars
Microorganism
Phytoplankton
Consortium
Consortium
Consortium
Consortium
Consortium
Consortium
Streptomyces sp.,
Burkholderia sp.
Pseudomonas putida
strain KT2240, Burkhold-
eria sp. MR1
9 separate isolates of
As-resistant bacteria
Consortium
Consortium
Methanobacterium strain
M.o.H
Desulfovibrio vulgaris
strain 8303
Initial
Concentration
138 ppb
1545 ppm
755 ppb, 1035 ppb
138 ppm
21 .4 ppm
10 ppm
1 ppm, 10 ppm, 100
ppm
140 ppb
140 ppb
140 ppb
3.7 ppm, 9.2 ppm
400 ppm, 2000 ppm
75 ppb
75 ppb
Temperature
4-30 °C
25 °C
5°C
30 °C
Ambient
Ambient
25 °C
Ambient
Ambient
20 °C
15°C,30°C
30 °C
Ambient
Ambient
Percent trans-
formed/time
100%/21d
10%/91d
66%/100d
75-95%/217d
85-89%/59 d
60%/60d
61%/168d
100%/7d
100%/7d
5-100%/14d
100%/120d
24-49%/240d
Not quantified
Not quantified
Media
Aqueous
Mixed
Culture
Soil
Sludge
Soil
Soil
Soil
Soil Ex-
tracts
Soil Ex-
tracts
Pure
Culture
Soil
Sludge
Pure
Culture
Pure
Culture
Reference
[14]
[34]
[27]
[2]
[35]
[26]
[25]
[5]
[5]
[15]
[29]
[2]
[31]
[31]
Ars = volatile arsine compounds (arsine, methylarsine, dimethylarsine, trimethylarsine)
iAs = inorganic arsenic
100% removal indicates final concentration was nondetect for the method employed
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5.0 FATE AND TRANSPORT IN THE ENVIRONMENT
The fate of DMA(V) in the environment is difficult to assess
in detail because of the number of interrelated abiotic and
biological processes that occur simultaneously. Phase
transfer, advective, and diffusive transport, methylation,
demethylation, oxidation and reduction are examples of
these.
While highly soluble in water, both DM A( V) and MM A( V)
have high sorption capacities (MMA > DMA) in soil and
sediment systems. Organoarsenic as well as inorganic
arsenic sorption is dependent on clay and mineral content
(i.e., ferrihydrite and alumina) of the soil, as well as pH of
the system. For example, DMA(V) [36] and MMA(V) [6]
adsorption increased as iron oxide and alumina content
increased. Overall, inorganic As(V) species are found
in oxic conditions and can strongly adsorb to soil and
sediment in acidic and neutral environments, whereas
inorganic As(III) species are weakly retained by soil
and sediment, and are mobile in both oxic and anoxic
environments. Thus, the potential for desorption and
remobilization of arsenic into the aqueous environment
by reductive transformations of As(V) to As(III) species
should be included in evaluating impacts to site conditions
when considering remedial technologies.
6.0 SUMMARY
Biotransformation of DMA(V) is a significant part of the
arsenic biogeochemical cycle and can produce a variety
of end products. Site specific conditions, in particular
the predominant redox condition, will direct the most
prevalent forms of arsenic species present.
Under aerobic conditions, DMA(V) is predominately
demethlyated to inorganic As(V). While inorganic As(V)
is more toxic than the organoarsenicals (DMA[V] and
MMA[V]), this valence of inorganic As strongly sorbs
to iron, aluminum, and manganese oxyhydroxides,
and clay minerals under acidic and neutral conditions.
Thus, pentavalent species are generally immobile in the
aerobic environment. Therefore, maintaining an aerobic
environment may aid in reducing transport of arsenic in
the environment.
Under anaerobic conditions, DMA(V) is also demethylated
to inorganic arsenic. Although the transformation
pathways incorporate a variety of intermediate
compounds, both inorganic As(V) and As(III) can be
produced. Trivalent arsenic [As(III)] species (both
organic and inorganic) are generally both more toxic and
more mobile than pentavalent arsenic species (organic
and inorganic). Therefore, all potential arsenic species
produced from biotransformation processes should be
considered in the remedy selection process when altering
a site's oxidation/reduction environment and pH.
These results indicate that transitioning from aerobic
to anaerobic conditions may increase the toxicity and
mobility of arsenic in the environment. As such, redox
conditions play an important role during remedial
alternative evaluation as the redox conditions impact the
mobility and toxicity of the arsenic species. Therefore,
understanding the potential biotransformation pathways
of organoarsenicals within the environment as well as the
fate, transport, and risk associated with the various arsenic
species are important when assessing remedial alternatives
for sites contaminated with DMA(V).
Predominant remediation strategies for As currently
include removal and immobilization [37]. Removal
consists of excavating the zone identified as contaminated.
Once removed, contaminated soil may be treated through
ex-situ means or sent to a hazardous waste landfill. Ex-
situ arsenic treatment includes (from low to high cost)
iron seeding for co-precipitation with aqueous As, soil
washing with acid extraction, membrane filtration, media
adsorption, ion exchange, and pyrometallurgical recovery.
In-situ remediation approaches rely on immobilization of
arsenic to reduce the bioavailability of the contaminant
and decrease the associated risk. Immobilization can
include (from low to high cost) phytoremediation
(if concentrations are low), biological treatment/
biotransformation, installation of a permeable reactive
barrier, soil flushing, amendments for precipitation or co-
precipitation, solidification and stabilization, electrokinetic
treatment, and vitrification. In-situ amendments for
precipitation or co-precipitation include zero valent iron
(ZVI), titanium dioxide (TiO2), and ZVI coupled with
a sulfate compound. These immobilization methods are
undergoing evaluation to ensure that they can serve as
long term, stable sinks for arsenic despite fluctuations in
geochemistry.
7.0 ACKNOWLEDGEMENTS
This EIP was prepared by the EPA, Office of Research
and Development, National Risk Management Research
Laboratory. Dr. John McKernan served as the EPA
Technical Project Manager.
10
Engineering Issue: Biotransformation of Dimethylarsinic Acid
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The following technical experts contributed their time
and expertise by providing review comments:
John Lenhart, Ph.D., The Ohio State University
Heather Rectanus, Ph.D., P.E., Battelle
Ryan Fimmen, Ph.D., Geosyntec Consultants
Kirk Scheckel, Ph.D., EPA
Brian Yates, Battelle
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12
Engineering Issue: Biotransformation of Dimethylarsinic Acid
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ACRONYMS AND ABBREVIATIONS
As(III) Inorganic/elemental arsenic (arsenite;
oxidation state +3)
As (V) Inorganic/elemental arsenic (arsenate;
oxidation state +5)
DMA(V) dimethylarsinic acid (oxidation state +5)
EPA U.S. Environmental Protection Agency
kg Kilogram
MMA(III) monomethylarsonous acid
(oxidation state +3)
MMA(V) monomethylarsonic acid (oxidation state +5)
MRL minimum risk level
OSC on-scene coordinator
ppb parts per billion
ppm parts per million
RPM Remedial Project Manager
RSL regional screening levels
TMAO trimethylarsine oxide
U.S. United States
Biotransformation of Dimethylarsinic Acid: Engineering Issue
13
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United States
Environmental Protection
Agency
Office of Research and Development
National Risk Management
Research Laboratory
Cincinnati, OH 45268
Official Business
Penalty for Private Use
$300
EPA/600/R-14/219
December 2014
www.epa.gov
PRESORTED STANDARD
POSTAGE & FEES PAID
EPA
PERMIT NO. G-35
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