Arsenic(III) methylation in betaine-nontronite clay-water suspensions under environmental conditions.

Abstract

This paper reports arsenic methylation in betaine-nontronite clay-water suspensions under environmental conditions. Two nontronites (<0.05 mm), NAu-1 (green color, Al-enriched) and NAu-2 (brown color, Al-poor, contains tetrahedral Fe) from Uley Mine - South Australia were selected for this study. Betaine (pK(a)=1.83) was selected as methyl donor. The reaction between 5 g L(-1) clay, 20 ppm As(III), and 0.4M betaine at 7< or =pH(0)< or =9 under anoxic conditions was studied. The presence of nontronite clays were found to favor As(III) conversion to monomethylarsenic (MMA). Arsenic conversion was found to be as high as 50.2 ng MMA/ng As(III)(0). Conversion of As was found to be more quantitative in the presence of NAu-2 ((Na(0.72)) [Si(7.55) Al(0.16)Fe(0.29)][Al(0.34) Fe(3.54) Mg(0.05)] O(20)(OH)(4)) than NAu-1 ((Na(1.05)) [Si(6.98) Al(0.95)Fe(0.07)][Al(0.36) Fe(3.61) Mg(0.04)] O(20)(OH)(4)). The inherent negative charge at the nontronite tetrahedral layer stabilizes positively charged organic intermediate-reaction species, thereby leading to decreases in the overall methylation activation energy. The outcome of this work shows that nontronite clays catalyze As methylation to MMA via non-enzymatic pathway(s) under environmental conditions.

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Journal of Hazardous Materials 178 (2010) 450–454
Contents lists available at ScienceDirect
Journal of Hazardous Materials
journal homepage: www.elsevier.com/locate/jhazmat
Arsenic(III) methylation in betaine–nontronite clay–water suspensions under
environmental conditions
Javiera Cervini-Silvaa,b,c,d,∗, Jessica Hernández-Pinedae, María Teresa Rivas-Valdése,
Hilda Cornejo-Garridob, José Guzmánf, Pilar Fernández-Lomelíng, Luz Maria Del Razoh
aUniversidad Autónoma Metropolitana, Unidad Cuajimalpa, Mexico
bPosgrado en Ciencias de la Tierra, Universidad Nacional Autónoma de México, Mexico
cEarth Sciences Division, Lawrence Berkeley National Laboratory, United States
dNASA Astrobiology Institute, United States
eFacultad de Química, Universidad Nacional Autónoma de México, Mexico
fCentro de Investigación en Ciencia Aplicada y Tecnología Avanzada, Unidad Legaria, Mexico
gInstituto de Geografia, Universidad Nacional Autónoma de México, Mexico
hDepartamrnto de Toxicologia, CINVESTAV, Mexico
a r t i c l e i n f o
Article history:
Received 5 November 2009
Received in revised form 4 January 2010
Accepted 20 January 2010
Available online 28 January 2010
Keywords:
Methyl transfer
Abiotic
Remediation
Toxicity
Soils
Sediments
a b s t r a c t
This paper reports arsenic methylation in betaine–nontronite clay–water suspensions under environ-
mental conditions. Two nontronites (<0.05mm), NAu-1 (green color, Al-enriched) and NAu-2 (brown
color, Al-poor, contains tetrahedral Fe) from Uley Mine - South Australia were selected for this study.
Betaine (pKa=1.83) was selected as methyl donor. The reaction between 5gL−1clay, 20ppm As(III),
and 0.4M betaine at 7≤pH0≤9 under anoxic conditions was studied. The presence of nontronite clays
were found to favor As(III) conversion to monomethylarsenic (MMA). Arsenic conversion was found
to be as high as 50.2ngMMA/ng As(III)0. Conversion of As was found to be more quantitative in the
presence of NAu-2 ((Na0.72) [Si7.55Al0.16Fe0.29][Al0.34Fe3.54Mg0.05] O20(OH)4) than NAu-1 ((Na1.05) [Si6.98
Al0.95Fe0.07][Al0.36Fe3.61Mg0.04] O20(OH)4). The inherent negative charge at the nontronite tetrahedral
layer stabilizes positively charged organic intermediate-reaction species, thereby leading to decreases in
the overall methylation activation energy. The outcome of this work shows that nontronite clays catalyze
As methylation to MMA via non-enzymatic pathway(s) under environmental conditions.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
The environmental toxicity of arsenic depends heavily on spe-
ciation. As(III)-bearing compounds are two to three times more
toxic than As(V)-bearing compounds [1–3], while reduced, inor-
ganic arsenic found in sulfide minerals is relatively low in toxicity.
Inorganic forms of arsenic, either As(III) or As(V) compounds,
are significantly more toxic than many As-organic complexes.
For instance, arsenobetaine (Me3As+CH2CO2; (trimethylarso-
nio)acetate; LD50=10gkg−1, mouse, oral; [4]), which has been
found in high levels in fishery products, presents an acute toxi-
∗Corresponding author at: Departamento de Procesos y Tecnología, División de
Ciencias Naturales e Ingeniería, Universidad Autónoma Metropolitana, Unidad Cua-
jimalpa (UAM-C), Artificios No. 40, 6◦Piso, Col. Miguel Hidalgo, Delegación Álvaro
Obregón, C.P. 01120 México, D.F., Mexico. Tel.: +52 55 26 36 38 00x3827;
fax: +52 55 26 36 38 00x3832.
E-mail addresses: jcervini@correo.cua.uam.mx, jcervinisilva@yahoo.com
(J. Cervini-Silva).
city approximately one three-hundredth that of arsenic trioxide
[arsenite; As2O3: LD50=0.03gkg−1, mouse, oral]. Arsenobetaine is
chemically stable, has a low affinity for animal and human tis-
sues, and is rapidly excreted from the human body [5–7]. On the
other hand, other arsenochemicals such as As(III)-methyl species
[e.g., methylarsenoic acid (MMAs=H2AsO2CH3) or dimethylarsi-
nous acid [DMAs=HAsO2(CH3)2] present comparable toxicity or
higher than inorganic species, iAs(III) or iAs(V) [8–13].
In nature, the methylation of arsenic has been attributed to the
metabolic activity of fungi, bacteria, molds, mammals, or aquatic
organisms [14–16]. The biological methylation of arsenic has been
proposed to occur by alternating reduction of As(V) to As(III),
and oxidative methylation [16–18], where methyltransferase and
reductase act as electron-transfer mediators [19]. To the authors’
knowledge, little work has been conducted to elucidate arsenic
methylation pathway(s) at the pore water interface under abi-
otic environmental conditions. The affinity of soluble inorganic
arsenic for environmental iron minerals [20,21], and humic acids
or low-molecular weight organic matter [16,22] is well known.
A recent report compares the adsorption behavior of inorganic
0304-3894/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jhazmat.2010.01.102

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451
Table 1
Chemical composition, structural formulae, and particle-size distribution of NAu-1 and NAu-2.
Structural formulaea
NAu-1
NAu-2
(Na1.05) [Si6.98Al0.95Fe0.07][Al0.36Fe3.61Mg0.04]O20(OH)4
(Na0.72) [Si7.55Al0.16Fe0.29][Al0.34Fe3.54Mg0.05]O20(OH)4
Chemical composition (%)a,b
SiO2
Fe2O3
Al2O3
CaOMgO TiO2
Na2O
NAu-1
NAu-2
51.4
56.2
35.9
37.8
8.1
3.1
3.6
2.3
0.2
0.25
0.2
0.02
0.03
0.14
Particle-size distributionc
>2?m 0.5–2?m0.2–0.5?m<0.2?m
NAu-1
NAu-2
8
5
4
5
6
9
82
82
Mineralogical compositiond
NAu-1NAu-2e
ABCDEABCDE
Nontronite
Kaolin
Quartz
Biotite
Goethite
90
4
2
<1
3
49
4
13
2
31
86
7
1
1
5
93
4
<1
(···)
3
>99
(···)f
(···)
(···)
(···)
Nontronite
Plagioclase
Quartz
Biotite
Talc
95
5
<1
(···)
(···)
10
68
10
4
3
83
10
2
(···)
5
99
(···)
<1
(···)
(···)
>99
(···)
(···)
(···)
(···)
aKeeling et al. [26].
bMnO (%)≤0.02, K2O (%)≤0.01, P2O5(%)≤0.01, SO3(%)≤0.004, ZrO2(%)≤0.01, Sr (%)≤0.003.
cn=6, C.V.=3%.
dColumns A, B, C, D, and E refer to size fractions bulk, 2?m, 0.5–2?m, 0.2–0.5?m, <0.2?m, respectively.
eIlmenite (5%) was found to be present in fraction B only.
f(···)=below detection limits.
[iAs, i.e., As (III) or As(V)], monomethyl [MMAs], and [DMAs] on
goethite and 2-line ferrihydrite surfaces at 3≤pH≤11 [23]. Unlike
for the case of inorganic or organic As(V) species, MMAs(III) and
DMAs(III) showed negligible adsorption on goethite or ferrihydrite
at 4≤pH≤7 (0.44gFeL−1, 13.3mmolAs(III)0L−1). By contrast, the
adsorption behavior of As approached quantitative adsorption
across at 3.5≤pH≤11, and surface excess values (n) at pH=4
and 7 corresponded to 0.14 and 0.2molAsmolFe−1, respectively
(0.2molL−1[As]eq). Arguably, methylation conveys alterations on
As(III)bindingmechanism(s).ThepooradsorptionofAs(III)organic
species by either mineral surface was postulated as evidence for
non-specific adsorption.
Clays (d<2?m) are ubiquitous naturally occurring small-sized
particles and are commonly found in wide range of environmen-
tal compartments, from sediments in the bottom of the ocean
to atmospheric aerosols in the upper stratosphere. Clays consti-
tute the most important earth’s reservoir of iron, a key element
for triggering fundamental metabolic pathways to sustain life.
Little quantitative information exists, however, on whether iron-
rich smectite clay minerals can induce arsenic methylation. Early
work has reported that organic arsenicals such as arsanilic acid
(p-aminophenylarsenic acid) can be retained in up to 80% in the
soil fraction by 1:1 or 2:1 phyllosilicates, kaolinite, montmoril-
lonite or vermicullite [14,24]. The extent of adsorption of disodium
methanearsonate has been found to correlate with the soil clay
content (kaolinite>vermiculite>montmorillonite; [25]). Report-
edly, shales and clays can accumulate up to 500ppm arsenic [24].
Phyllosilicates have been shown to enhance arsenite oxidation
to arsenate [20]. Thus, studying the biogeochemistry and envi-
ronmental fate of arsenic and, in particular, its speciation and
transformation pathways at the clay–water interface deserve fur-
ther scrutiny.
We selected two iron-rich nontronite clays to study the plausi-
blemethylationofinorganicarsenicinaqueoussolutionatambient
temperature. The chemical composition of these clays differs pri-
marily because the total aluminum and tetrahedral Fe content.
As evidenced by infrared spectroscopy, NAu-2 contains significant
quantities of tetrahedral Fe [26]. Betaine (N,N,N-trimethylglycine;
pKa=1.83) was selected as a methyl donor.
2. Materials and methods
2.1. Source of clays
Nontronites(NAu-1,greencolor,Al-enrichedandNAu-2,brown
color,Al-poor,containstetrahedralFe)fromUleyMine-SouthAus-
tralia were purchased from the Source Clays Repository of the Clay
MineralsSociety(PurdueUniversity,WestLafayette,Indiana).NAu-
1 contains minor kaolin and quartz. NAu-2 contains fewer total
impurities in the form of carbonate and iron oxyhydroxides ([26],
Table 1). The clays were used as received.
2.2. Reaction of inorganic arsenic, nontronite clay, and methyl
donors
Suspensions containing 20ppm As(III) (AsNaO2, Fluka Chemika,
Switzerland), 5gL−1nontronite (NAu-1 or NAu-2), and 20ppm
betaine (Sigma–Aldrich, Milwaukee, WI) were prepared. Suspen-
sions were placed in 50-mL Nalgene bottles and stirred at 150rpm
for 48h, then centrifuged at 4000rpm for 50min or until separa-
tion of the clay fraction was achieved. The supernatant solution
wasdecanted,filteredusing0.022?m-Milliporesyringefilters,and
analyzed for arsenic speciation (below).
All solutions and suspensions were prepared using N2-purged
nanopure water. Adjustments of pH values were achieved by
adding aliquots of 0.01M HCl or NaOH standard solutions. Experi-
ments were conducted by duplicates and samples were discarded
after sacrificed.

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J. Cervini-Silva et al. / Journal of Hazardous Materials 178 (2010) 450–454
Table 2
Nomenclature of arsenic species determined in As suspensions using HG-CT-AAS:
inorganic arsenic, iAs (iAsIII+iAsV); monomethyl arsenic, MMAs (MAsIII+MAsV);
and dimethyl arsenic, DMAs (DMAsIII+DMAsV).
Name AbbreviationChemical formulae
Arsenite, arsenious acid
Arsenate, arsenic acid
Monomethylarsonic acid
Monomethylarsonous acid
Dimethylarsinic acid
Dimethylarsinous acid
iAsIII
iAsV
MMAsV
MMAsIII
DMAsV
DMAsIII
As(OH)3
AsO(OH)3
CH3AsO(OH)2
CH3As(OH)2
(CH3)2AsO(OH)
(CH3)2AsOH
2.3. Arsenic speciation
Arsenic species in supernatant solutions were analyzed by
HG-CT-AAS using a PerkinElmer Model 3100 AA spectrometer
(PerkinElmer, Norwalk, CT, USA) equipped with a conventional
quartz tube atomizer as described elsewhere [27]. Briefly, hydrides
(i.e.,arsineandthemethyl-substitutedarsines)weregeneratedina
reactionwithsodiumborohydride(NaBH4;EMScience,Gibbstown,
NJ, USA) in the presence of concentrated HCl (Sigma–Aldrich,
St. Louis, MO, USA). Under these conditions, hydrides are gener-
ated from both trivalent and pentavalent arsenic species [27,28].
Arsenic species were analyzed by a recently developed automated
HG-CT-AAStechniqueusingaPerkinElmerModel5100PCAAspec-
trometer equipped with the multiatomizer and a FIAS200 flow
injection accessory [29,30]. Unlike the conventional HG-AAS, the
new method provides low detection limits (DLs) needed for anal-
ysis of arsenic species in small samples. Before analysis, each
supernatant solution was treated with 2% l-cysteine hydrochlo-
ride (EMD Chemicals Inc., Darmstadt, Germany) for 70min at room
temperature. Treatment with cysteine reduces all pentavalent
arsenic species to trivalency. Hydrides were generated from 0.5-
mL aliquots of cysteine-treated samples by reaction with NaBH4in
a Tris–HCl (Sigma–Aldrich) buffer (pH 6) as previously described
[29,30]. HG-CT-AAS was developed for the oxidation-state-specific
speciation analysis of arsenic, but under current operating con-
ditions both procedures described above determined total iAs
[?
ibration and method validation are reported elsewhere [29].
Concentrations of iAs, MMA, and DMA were expressed as
nanograms of arsenic per millilitre of supernatant solution.
Stata 8.0 (Stata Corp., College Station, TX, USA) and Instat
(GraphPad Software Inc., San Diego, CA, USA) statistical software
packages were used for data analyses. Differences in the per-
centages of arsenic species and the species ratios were evaluated
by unpaired t-test. The nonparametric Spearman correlation was
used to analyze associations between the concentrations of arsenic
species, the percentage of arsenic species, or ratios of arsenic
species in supernatant solutions. Differences or correlation with
p<0.05 were considered to be statistically significant.
iAs=iAs(III)+iAs(V)], MMA [?
MMA=MMA(III)+MMA(V)], and
DMA [?
DMA=DMA(III)+DMA(V)] (Table 2). Details on the cal-
2.4. Total soluble Fe analysis
Determinations of total soluble Fe that were conducted by
atomic absorption spectrometry using a Varian, SpectrAA 110
equipped with an acetylene-air oxidant flame (AAS-F) at 248.3nm.
2.5. Surface analyses
Surface and chemical analyses of clay samples were conducted
by energy dispersive spectroscopy (EDS). A Cambridge-Leica Stere-
oscan 440 Scanning Electron Microscope equipped with an Oxford,
model Pentafet was used. Backscattering-electron micrographs
were obtained for all samples.
3. Results and discussion
3.1. Arsenic methylation in nontronite suspensions
NontronitesampleswerereactedwithAs(III)andbetaineatpH7
or 9 and extracted for analyses. As evidenced by HG-CT-AAS results
(Table 3 top, 5th and 6th columns), nontronites facilitated trans-
formation of As(III) to MMA. The formation of MMA in clay-free
experiments was found below detection limits. Recoveries of total
As [inorganic (iAs)+organic] varied from 5.05 to 55.1%. The extent
of As methylation ranged from 4.7 to 50.2ng MMA per ng As(III)0.
The conversion of As to MMA was found to be more quantitative in
the presence of NAu-2 over NAu-1.
The effect of the extent of clay dissolution on As speciation was
evaluated. Elemental analysis of NAu-1 and NAu-2 supernatant
solutions adjusted at pH0=7 showed the accumulation of Fe after
24htoapproximate2and12ppm,respectively.Thepresentresults
are in agreement with the idea that the dissolution of NAu-2 con-
veys a faster depletion rate of the total surface sites. At the same
time, data showing a higher extent of As conversion in NAu-2 sus-
pensions (Table 3 top, 5th and 6th columns) can be attributed to
a higher number of adsorption active sites available for organic
arsenic, and that the formation of MMA is surface-controlled. A
similar scenario is proposed to explain the relative recovery of
total arsenic over As(III)0. Results listed in Table 3 (top, 3rd to
6th columns) show that recoveries for iAs are lower than organic
arsenic, within error [29,30], regardless of the initial reaction con-
ditions.
The precipitation of arsenic at the clay surface was also
evaluated. EDS analyses of nontronite samples showed: O–K
25.4±2.7% (w/w) (44.1±3.1% atomic, n=7); Mg–K 0.94±0.2%
(w/w) (1.01±0.3% atomic, n=7); Al–K 3.3±1.8% (w/w) (3.3±1.8%
atomic, n=7); Si–K 34±1.8% (w/w) (33.2±2.1% atomic, n=7);
Ca–K 1.7±0.4 (1.01±0.4, n=6); 32.3±2 (15.8±1.7, n=6); As–L
0.18±0.09(0.08±0.04,n=5);Cu–K0.4,n=1;K–K7(5,n=1);W–M
3(0.5,n=1);andC.V.8.67%.Arseniccontentsinclaysampleseither
before or after reaction ([As(III)]0=20ppm, 0.5M betaine at pH07
or9)werefoundtobecomparable.Wedonotdiscardthepossibility
of small-sized arsenic precipitates below detection limits (<0.01%),
however. How the presence of positively charged iron oxides
such as goethite (p.z.c.=7.8 [31]; Table 3) may affect the reaction
mechanism(s) of arsenic methylation in nontronite–methyl donor
suspensions deserves further scrutiny.
3.2. Mechanism of arsenic methylation in betaine–nontronite
clays aqueous suspensions
The formation of MMA can be explained because arsenic
methylation–oxidation reaction according to [32]:
As(OH)3+ CH+
At 7≤pH≤9, methane arsenic acid (pKa1=3.61; pKa2=8.24 at
18◦C; [25]) is predominantly in the mono- and di-protonated
forms. Betaine (pKa=1.83) methyl group transfer is favored in
the presence of nontronite (point of zero charge, p.z.c.mont=2.5;
[31]). The negative charge at the clay surface stabilizes positively
charged organic intermediate-reaction species, thereby leading
to decreases in the activation energy for methyl transfer. Dehy-
drochlorination of 1,1,1-trichlorethane to 1,1-dichlorethene by
chemically and microbially reduced smectite minerals has been
explained because the stabilization of carbocation intermediates
at the clay–water interface [33]. Gaussian-2 COSMO calculations
[31] show that the second methylation of As(III)0[as As(OH)3] is
the most endothermic. Those findings help explaining in part the
lack of evidence for the formation of DMA (pKa=6.2) (Table 3, top).
3→ (CH3)AsO(OH)2+ H+
(1)

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453
Table 3
Arsenic methylation in betaine–nontronite clay suspensions.a.
NontroniteArsenic compartmentalizationb
pH0
iAs (?g)MMA (ng)
ngiAsgclay−1
ngiAsngAs(III)0−1
ngMMAgclay−1
ngiAsngAs(III)0−1
NAu-1
NAu-2
720.2
9.5
5
3
(···)c
19
(···)
5
NAu-1
NAu-2
90.1
19.5
0.03
5
95
201
24
50
Soluble-reaction products identificationd
Solution composition
m/z values
491.3374.2 257.1 4693 352.2235.1 118.1
Betaine
Betaine+As(III)
Betaine+As(III)+NAu-2
18.42
3.05
5.92
10.53
2.44
3.95
85.82
20.73
24.34
11.84
6.71
11.18
43.42
38.41
35.52
100
100
100
5.92
4.88
5.92
aReaction conditions: Arsenic initial concentration, [As(III)]0, 20ppm; clay concentration=5gL−1; betaine initial concentration=0.4M.
bIn all samples the concentrations of DMAs was found to be below detection limits<0.2ngL−1.
c(···)=below detection limits<0.1ngL−1supernatant solution after extraction.
dReaction conditions: Arsenic initial concentration, [As(III)]0, 20ppm; clay concentration=5gL−1; betaine initial concentration=0.4M. In all cases the initial pH, pH0, was
adjusted to 9.
ResultslistedinTable3(top,5thand6thcolumns)showahigher
extent of As(III) conversion in the presence of NAu-2 over NAu-1.
These results agree well with reports on the higher reactivity of
NAu-2 over NAu-1 as evidenced by their relative ability to induce
lipid peroxidation, a major indicator of oxidative stress [34], for
example.ExistingstructuraldifferencesbetweenNAu-1andNAu-2
are associated with the distribution of structural iron over the total
structuralironcontentalone(35.21and37.85%Fe2O3,respectively;
[3]).ReportedX-raypre-edgespectraforNAu-1andNAu-2samples
reveal differences in intensity, position, and shape. Enhancements
of the Fe K pre-edge intensity in NAu-2 and the change in pre-edge
profile[35]hasbeenattributedtoincreasedstructuraldisorderdue
to lowering in symmetry around 6-coordinated structural Fe(III)
[36], or to appreciable amounts of Mg and/or Al within octahedral
sites [37]. Lastly, experimental Fe(III)-O waveforms for NAu-2 was
found to be lower in amplitude and shifted in comparison to NAu-
1 [35]. Waveforms for NAu-2 are right-shifted at low Å−1. Such
phase shifts are consistent with shorter average Fe(III)–O bond dis-
tances and lower average coordination for NAu-2.Finally, a direct
comparison of mass spectrometry analyses for suspensions bear-
ing betaine, As and betaine, and As, clay, and betaine (Table 3,
bottom)providesaclearindicationofmultiplefragmentationpath-
ways. These results are consistent with evidence provided above
showing that ubiquitous clay surfaces (e.g., smectites) can act as
catalysts for the transformation of inorganic arsenic in the pres-
ence of a naturally occurring methyl donor (e.g., betaine). The
present work adds to current knowledge on the conversion of
inorganic arsenic to trimethylarsine oxide (97–99%) and tetram-
ethylarsonium (CH4As+, 1%) in coenzyme B12-bearing suspensions
[7]; transformation of trimethylarsine oxide into arsenobetaine
after arsenic carboxymethylation in a 2.0×10−7M trimethylar-
sine oxide/5.0×10−3M glutathione/5.0×10−3M iodoacetic acid
solution at pH 5 and 37◦C for 2h [7]; demethylation of arseno-
betaine (0.5 and 5mgAsL−1; m/z 75) to dimethylarsine (m/z 139)
and dimethylarsinoylacetate (m/z 181) with clinoptilolite zeolite
((Na,K,Ca)2–3Al3(Al,Si)2Si13O36·12(H2O)) at 25◦C after 14d [38];
degradation of arsenobetaine to dimethylarsine, dimethylarsi-
noylacetate, and a demethylation product (m/z 165, 133, 121,
105, 89, 79) by a mixture of clinoptilolite and modernite zeo-
lites after 56d. In this regard, clinoptilolite zeolites are known to
present a strong exchange affinity for NH4+. Mordenite zeolites
((Ca,Na2,K2)Al2Si10O24·7H2O),ontheotherhand,arecommercially
used to catalyze acid-catalyzed isomerisation of alkanes and aro-
matics.
4. Conclusions
Nontronite clay minerals facilitated arsenic methylation in
water suspensions at ambient temperature. The outcome of this
work provides evidence to show non-enzymatic methylation of
arsenic. The conversion of As(III) to methyl arsenic (MMA) was
explained because methyl transfer at the clay–water interface. The
negative charge of the clay surface helps to stabilize positively
charged organic intermediate species. Additional work is need to
assess how the role of dissolving vs. structural Fe on the complex-
ation of arsenic by betaine at the clay surface and/or supernatant
solution. Because small-sized particles present higher-dissolution
rates than bulk material [39], differences in reactivity as function
of particle size are predicted.
The present work provides evidence to show that non-
enzymatic methylation of arsenic can proceed in up to 50% within
24h. Iron-redox cycling in smectite clay minerals occurs as a result
of burial, submersion, wetting, drying, and other events in natu-
ral soils and sediments. Electron transfer at the water interface of
iron-rich clay minerals conveys electron delocalization through-
out siloxane groups and the concomitant formation of channels
and holes. A consequence is the polarization of adsorbed water.
As the surface of smectite clay minerals hydrates, water molecules
reorganize to participate in hydrogen bonding interactions with
otherwatermolecules,justasinbulkwater,andtoformhydrogen-
bridging interactions with organic solutes [40,38]. Because clays
canactaselectronacceptorsordonors[41]andcatalyzeavarietyof
chemicalreactions,itfollowsthatclay-surfacechemistrywillaffect
thefateofsolutessuchasarsenic(inorganicororganic)susceptible
to redox transformations present in colloidal environs [42].
Acknowledgements
The authors express gratitude to Dr. Rebecca Sutton (Environ-
mental Working Group) and Daria Kibanova (School of Chemistry,
UNAM) for helpful comments, and M en C Luz C Sanchez Penia
(Depto Toxicologia, CINVESTAV) for technical assistance. JHP and
HCG thank the support of a DGAPA-UNAM undergraduate scholar-
ship. This project was supported in part by UNAM (PUNTA-PAPIIT,

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J. Cervini-Silva et al. / Journal of Hazardous Materials 178 (2010) 450–454
Grant No: IN116007-2), CONACYT (SEP-CONACYT Ciencia Básica
2006, Grant No: 61670), CONACYT-CNRS grant program, and by
ECACORE 2020 (SEMARNAT-CONACYT).
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