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Geochemical fractionation of
210
Pb in oxic estuarine sediments
of Coatzacoalcos River, Gulf of Mexico
J. F. Ontiveros-Cuadras •A. C. Ruiz-Ferna
´ndez •
J. A. Sanchez-Cabeza •L. L. Wee-Kwong •
L. H. Pe
´rez-Bernal
Received: 3 May 2011 / Published online: 18 February 2012
Akade
´miai Kiado
´, Budapest, Hungary 2012
Abstract
210
Pb activities were analyzed in surface sedi-
ments from the Coatzacoalcos River (Gulf of Mexico) to
evaluate its distribution according to sediment grain size
and in different geochemical compartments by using
sequential extraction techniques. The geochemical frac-
tionation experiments provided compatible results: by
using the Tessier’s method [1] more than 90% of the
210
Pb
activity in the samples was found the residual fraction
(primary and secondary minerals) and the remaining
(\10%) in the iron and manganese oxides fraction of the
sediments; whereas using the Huerta-Diaz and Morse
method [2] the
210
Pb content was found in comparative
amounts in the reactive, the silicate, and the pyrite fractions
(accounting together for [80%), and the rest was found in
the residual fraction. The grain size fractionation analyses
showed that the
210
Pb activities were mostly retained in the
clay fraction, accounting up to 60–70% of the
210
Pb total
activity in the sediment sample and therefore, it is con-
cluded that the separation of the clay fraction can be useful
to improve the analysis of low
210
Pb content sediments for
dating purposes.
Keywords
210
Pb Sequential extraction Grain size
Estuarine sediments Gulf of Mexico
Introduction
Radionuclides are powerful tools which can provide
information about physical, chemical, biological, and sed-
imentological processes in aquatic systems. They are
especially useful because they include the time dimension
of processes, being like clocks associated to the system [3].
The most used tool to study processes spanning the past
100–150 years is the natural fallout radionuclide
210
Pb, a
member of the
238
U radioactive series with a half-live of
22.26 years. Unsupported
210
Pb is produced in the atmo-
sphere from the decay of
222
Rn emanated from continental
rocks and soils [4]. Atmospheric
210
Pb fluxes are known to
be predominantly regulated by climate features of a region
such as rainfall, and the oceanic or continental origin of the
dominant air mass. However, in low latitude regions
(30N–30S)
210
Pb concentration in air are low in com-
parison with northern mid latitudes (60N–30N) because
the low continent/ocean ratio (23%) and the atmospheric
zonal circulation [5].
Over the last decades the
210
Pb dating methodology has
been a basic tool to reconstruct recent chronologies of
lacustrine and marine coastal sediments, and to model
some aquatic processes. Humic acids are the major organic
fraction carrying unsupported
210
Pb (from atmospheric
fallout or produced in the water column) especially in lake
J. F. Ontiveros-Cuadras
Posgrado en Ciencias del Mar y Limnologı
´a, Universidad
Nacional Auto
´noma de Me
´xico (UNAM), Calz. Joel Montes
Camarena s/n, 82040 Mazatla
´n, Mexico
A. C. Ruiz-Ferna
´ndez (&)J. A. Sanchez-Cabeza
L. H. Pe
´rez-Bernal
Instituto de Ciencias del Mar y Limnologı
´a, Universidad
Nacional Auto
´noma de Me
´xico (UNAM), Ciudad Universitaria,
04510 Coyoaca
´n, Me
´xico, D.F, Mexico
e-mail: caro@ola.icmyl.unam.mx
J. A. Sanchez-Cabeza
Institut de Cie
`ncia i Tecnologia Ambientals, and Departament de
Fı
´sica, Universitat Auto
`noma de Barcelona (UAB), 08193
Bellaterra, Spain
L. L. Wee-Kwong
Environment Laboratories, International Atomic Energy
Agency, 4 Quai Antoine 1er, 98000 Monaco, Monaco
123
J Radioanal Nucl Chem (2012) 292:947–956
DOI 10.1007/s10967-012-1668-3
and marine sediments where acidic conditions prevail,
whereas the supported
210
Pb (in equilibrium with its parent
radionuclide
226
Ra) is strongly bound to clay minerals [6,
7]. According to some studies [8–10] unsupported
210
Pb
activity increases with decreasing grain size, and the
degree of association with a particular grain size depend on
Fe and Mn-oxyhydroxides and organic material contents.
One of the best methods to study the partition of ra-
dionuclides in environmental matrices is the sequential
extraction or fractionation technique. Sequential extraction
procedures consist on subjecting a sediment sample to a
series of increasingly selective chemical reagents under
specified conditions [1,2,11–13]. The basic assumption of
selective extraction techniques is that the reagents used are
able to selectively destroy one phase (specificity) without
solubilization of the others [14]. These studies have shown
that the designation of any extracted to a given substrate
does not necessarily reflect the whole scavenging action of
discrete sediment phases, but rather should be considered
as operationally defined by the method of extraction. The
objective of this paper is to contribute to a better under-
standing of the geochemical distribution of
210
Pb in surface
estuarine oxic sediments, which can be useful to improve
the analysis of
210
Pb for dating applications in areas where
the
210
Pb signal is characteristically low.
Materials and methods
Study area
The Coatzacoalcos River Estuary is located in the state of
Veracruz, southern Gulf of Mexico (Fig. 1) and has an
estimated length of 40 km. The Coatzacoalcos River
originates at more than 2,000 m elevation in Oaxaca State
and several smaller streams (Corte, Chichihua, Almoloya,
Malatango Sarabia, Jaltepec, Uxpanapa, and Calzada)
discharge waters into its main branch, draining an area of
21,120 km
2
, with an annual volume of discharge of 32
752 m
3
[15]. In the upper estuary, width and depth reach
213 and 18 m, respectively; in the river mouth, width and
depth reach 530 and 11 m, respectively. Several studies
show that the Coatzacoalcos River and wetlands sur-
rounding the Minatitla
´n-Coatzacoalcos industrial area are
contaminated by trace metals and petroleum hydrocarbons
due to the discharges of untreated wastes and oil spills [16,
17]. Discharges of untreated urban and industrial wastes
include, among other pollutants, 6.2, 1.5, and 25.3 ton per
day of suspended solids, heavy metals, and sulfates,
respectively [15]. The Coatzacoalcos River basin is com-
posed of different types of rock outcrops such as sand-
stones, siltstones, shale, and limestone [18]. The weather in
Fig. 1 Location of
Coatzacoalcos River sampling
stations: 1Calzadas River, 2
joint of Calzadas-Coatzacoalcos
River, 3San Francisco stream, 4
joint of San Antonio-
Coatzacoalcos River, 5joint of
Uxpanapa-Coatzacoalcos river,
6Coatzacoalcos River
948 J. F. Ontiveros-Cuadras et al.
123
the area is warm, with an average temperature of 24 C; the
area is characterized by heavy rains and hurricanes from
June to September and a dry season during spring (March
to May) when saline waters can be detected 45 km
upstream [19]. Northeastern trade winds prevail in the area;
however, strong cold winds from the north (NE, called
‘‘nortes’’), peaking at over 100 km h
-1
, strike the region
from September through March; and hot–dry air masses
(SE, called ‘‘suradas’’) sweep in from the south toward the
coast, during the dry season [20]. Previous studies [21,22]
show that the coastal zone of the Gulf of Mexico has low
atmospheric
210
Pb fluxes. In the Coatzacoalcos River
catchment area the prevailing winds have an oceanic origin
and, therefore are especially depleted in
210
Pb [5].
Sampling
Six samples of surface sediments were collected along the
Coatzacoalcos River bed in February 2008 (Fig. 1)by
using a Van Veen dredge. The sediment samples were
removed from the upper layer (1 cm) at the middle part of
the bulk sample, in such a way that sediments had no
contact with the metallic parts of the dredge. Sediments
were transferred to plastic bags and kept at 4 C until
analysis.
Experimental and laboratory analysis
Grain size analysis was performed on fresh sample aliquots
by using the standard sieve and pipette method [23]. Other
sample aliquots were freeze-dried, ground to a powder with
a porcelain mortar and pestle, and stored in acid-washed
(2 M, HNO
3
) plastic bags. Total carbon (TC) was mea-
sured by using a TruSpec CN 630-100-100 and following
the method described by Van Iperen and Helder [24]. TC
was determined by analyzing 10 mg aliquots of dry and
ground sediment. For organic carbon (C
org
) analy-
sis,*500 mg of sediment (w
i
) was treated with 10 mL of
1 N HCl to remove carbonate content present in the sam-
ple, the residue was dried on a hotplate overnight (70 C)
and weighted (w
f
), and aliquots of 10 mg of this residue
(acidified sample) were analyzed with a TruSpec analyzer
(C
org initial
). The C
org
value was estimated as follows:
Corg ¼Corg initial wf=wi
ðÞ
where w
f
/w
i
is a correction factor for weight increase after
acidification, due to conversion of carbonate to chloride
and to hydration. The difference between TC and C
org
values was assumed to represent the contents of inorganic
carbon (C
inorg
). Values obtained from replicate analyses
(n=8) of the standard reference material EUROVECTOR
E1135-A showed good agreement with the certified carbon
content (accuracy =96%; precision =4.6%). Trace metal
element analyses were performed by X-ray fluorescence
(XRF) by using a Spectro X lab 2000 (Spectro Analytical
Instruments) at IAEA-MEL, Monaco. Pressed sediment
powder (4 g aliquots) was placed into a low-density
polyethylene cell, in which the bottom had been previously
wrapped up with ProleneT membrane film. The spectral
analysis and quantitative calculations were performed
using the systems software’s calibrated internal evaluation
method, by performing a combination of three measure-
ments with different X-ray targets, and enabled the deter-
mination of elemental concentrations of atomic numbers
ranging from 13 (Al) to 92 (U). Replicate analysis (n=6)
of the reference materials IAEA-158, IAEA-356, IAEA-
405, and IAEA-433 indicated good agreement between
certificated and analytical values for most metals, with
accuracies above 90% and uncertainties below 8% for most
of the analyzed metals (Table 1).
210
Pb determination
210
Pb was determined by alpha counting of
210
Po deposited
onto silver discs [25,26] assuming secular equilibrium
between both radionuclides. Sediment aliquots of 0.3 g
were spiked with
209
Po as yield tracer and then digested
using a 5:4:1 mixture of HNO
3
?HCl ?HF in Teflon
PFA containers hermetically closed and heated on a hot-
plate (150 C) overnight. The acid mixture was evaporated
and converted to a chloride salt by repeated evaporation
with 12 M HCl; the residue was resuspended in HCl 0.5 N
and separated by centrifugation. The solution was trans-
ferred to a glass beaker and 0.2 g of ascorbic acid was
added. A silver disc was placed into the solution to allow
Po isotopes to deposit spontaneously in orbital agitation at
room temperature overnight. The
210
Po levels were mea-
sured by a-particle spectrometry using Ortec-Ametek sili-
con surface barrier detectors (Model 920E) coupled to a PC
running under the Maestro
TM
data acquisition software.
Replicate analyses (n=27) of the standard reference
material IAEA 300 confirmed good agreement with the
certified
210
Pb activity (accuracy =84.3%, precision =
5.1%).
210
Pb in grain size fractions
Sediment bulk samples were gently disaggregated and
passed through a 63 lm sieve to separate the sand fraction
([63 lm) from the mixture of silt (2–63 lm) and clay
(\2lm). The mixture of silt and clay were collected and
homogenized with MilliQ water and were placed inside of
an ultrasonic bath during 10 min to induce the flotation and
separation of fine particles [27]. The suspension was dilu-
ted to 1 L and decanted in order to separate clays from
silts. The two fractions were dried at 40 C, ground to a
Geochemical fractionation of
210
Pb in oxic estuarine sediments 949
123
Table 1 Analysis of metals and radionuclides in certified reference materials (CRM)
Element IAEA-158
a
IAEA-356
b
IAEA-405
c
IAEA-433
d
Certified value Measured value Certified value Measured value Certified value Measured value Certified value Measured value
Al (g kg
-1
) 50.2 ±2.8 34.5 ±0.6 39.0 ±3.1 28.8 ±5.1 NA 71.7 ±4.0 78.2 ±4.2 66.8 ±2.0
Fe (g kg
-1
) 26.3 ±1.4 28.2 ±0.1 24.1 ±0.6 26.7 ±0.1 37.4 ±0.3 40.4 ±0.1 40.8 ±1.9 43.5 ±0.5
Mn (mg kg
-1
) 356 ±24 354 ±8 312 ±9 314 ±13 495 ±5 501 ±25 316 ±16 314 ±7
Br (mg kg
-1
) 224 ±15 213 ±2 76.1 ±8.9 76.2 ±1.5 NA 85.4 ±1.1 67 ±16 62.8 ±0.9
Pb (mg kg
-1
) 39.6 ±4.7 44.5 ±1.9 374 ±16 288 ±3 74.8 ±1.1 78.8 ±1.4 26.0 ±2.7 28.2 ±1.1
Th (mg kg
-1
) 8.8 ±0.5 7.9 ±0.5 6.6 ±0.1 6.7 ±1.2 NA 12.2 ±1.2 9.7 ±0.5 8.7 ±0.6
U (mg kg
-1
) 2.4 ±0.2 \5.9 3.2 ±0.2 \5.9 NA \5.9 2.4 ±0.2 \5.9
IAEA-300
e
IAEA-313
f
IAEA-384
g
IAEA-385
h
Radionuclide Certified value Measured value Certified 95% CI Measured median
i
Certified 95% CI Measured median
i
Certified 95% CI Measured median
i
210
Pb (Bq kg
-1
) 201 ±12.7 181 ±6.1
226
Ra (Bq kg
-1
) 307–379 372 (n=5) 2.0–2.9 2.2 (n=15) 21.6–22.4 23.5 (n=15)
NA not available
The IAEA reference materials are:
a
Trace metals and methylmercury in marine sediment (coastal UK)
b
Major, trace elements, and methylmercury compounds in polluted marine sediments (Venice Lagoon, Italy)
c
Trace elements and methylmercury in estuarine sediments (Tagus Estuary, Portugal)
d
Trace elements and methylmercury in marine sediments (coastal Algeria)
e
Radionuclides in the Baltic Sea sediments (Bothnian Sea)
f 226
Ra, Th and U in stream sediments (Sumatra, Indonesia)
g
Radionuclides in Fangataufa Lagoon sediment (French Polynesia)
h
Radionuclide in Irish Sea sediment
i
Data obtained from reference [28]
950 J. F. Ontiveros-Cuadras et al.
123
powder with a porcelain mortar and pestle and the activity
of
210
Pb was measured in the individual size fractions.
226
Ra determination
226
Ra was determined with the ultralow background liquid
scintillation system Quantulus 1220
TM
(Wallac, Turku,
Finland) using alpha/beta discrimination [28]. Sediment
aliquots (0.2 g) were totally digested with a mixture 5:2:3
of HNO
3
?HCl ?HF in closed vessels at high pressure
using microwave heating. The extract was evaporated to
incipient dryness and small amounts of 0.5 M HCl were
added to dissolve the residue and eliminate HNO
3
. The
tracer solutions were prepared by gravimetrically spiking
226
Ra 2 M HNO
3
(NIST, SRM4967, U.S.A.) into known
amounts of deionized water contained in 20 mL low-dif-
fusion PE counting vials. The total volume was then
adjusted to 10 mL followed by the addition of 10 mL
OptiScint HiSafe cocktail to form two immiscible liquids.
The mixture was stored for 3 weeks in a dark temperature-
controlled area to allow in-growth and equilibrium of the
radioactive progenies. The background solutions were
prepared with 10 mL of deionized water, acidified to match
the standard solutions, to which 10 mL of the scintillation
cocktail was added. In all cases, quenching was changed by
adding different amounts of CCl
4
, ranging from 0 to
200 lL. All reagents used in the experiments were of
analytical grade (Fisher Scientific). Counting was per-
formed with Wallac OptiScint HiSafe III, diisopropyl
naphthalene based aqueous immiscible cocktail, and low-
diffusion PE counting vials (Packard BioScience) [29]. The
method accuracy was checked against recovery of certified
materials (IAEA-313, IAEA-384, and IAEA-385); results
are shown in Table 1.
210
Pb sequential extraction procedures
Aliquots of 5.0 g freeze-dried sediment were treated as
follows:
Method I [2]
(i) Reactive fraction. It comprises amorphous and crys-
talline iron and manganese oxyhydroxides, carbonates
and hydrous aluminosilicates. It was obtained after
digestion of the sediment sample with 40 mL of 1 M
HCl for 16 h at room temperature.
(ii) Silicate fraction. It comprises clay minerals, and was
extracted after two consecutive leachings of the
sediments with 60 mL of 10 M HF for 1 and 16 h
at room temperature, respectively. The precipitated
fluorides were redissolved with 10 g of H
3
BO
4
.
(iii) Pyrite fraction. This fraction comprises pyrite and
associated trace metals. It was obtained after diges-
tion of the silicate fraction residue with 20 mL of
concentrated HNO
3
for 2 h at room temperature. In
the present study, the oxic conditions of the surface
sediments in Coatzacoalcos River would prevent
pyrite precipitation. Therefore, we considered that
this extraction corresponded to the organic matter
fraction since pyrite formation is primarily con-
trolled by the deposition of organic matter; and
reagents, normally used to dissolve pyrite, also
oxidize organic matter [30,31].
(iv) Remnant fraction. This is the solid residue after the three
previous fractions have been removed. Although this
was not considered in the original reference, we
considered important to analyze it since no complete
digestion is attained without using HF [32]. The residue
was digested with a mixture 5:4:1 of HNO
3
?HCl ?
HF and heated on a hotplate (150 C) overnight.
Method II [1]
(i) Exchangeable fraction. This chemical treatment step
changes the water ionic composition to promote the
remobilization of those metals adsorbed to the
exposed sediment surfaces. The sediment was
extracted at room temperature for 1 h with 80 mL of
magnesium chloride solution (1 M MgCl
2
, pH 7.0)
with continuous agitation.
(ii) Carbonates fraction. It comprises the components
incorporated to sediment carbonates by co-precipita-
tion reactions. The residue from fraction (i) was
leached at room temperature for 2 h under continuous
agitation with 160 mL of 1 M NaAc (CH
3
COONa)
adjusted to pH 5.0 with HAc (HCH
2
COOH).
(iii) Iron and manganese oxides fraction. These oxides
are important sinks and transport modes of heavy
metals in the environment because hydrous metal
oxides are pH sensitive and thermodynamically
unstable under anoxic conditions [33]. The residue
from the previous fraction was extracted with
100 mL of 0.04 M NH
2
OH-HCl in 25% (v/v) HAc
at 96 ±3C for 6 h with continuous agitation.
(iv) Organic matter fraction. This fraction comprises
colloidal and particulate organic material that has
the capacity to form organo-trace element complexes.
To the solid residue from fraction (iii) we added
15 mL of 0.02 M HNO
3
and 25 mL of 30% H
2
O
2
,
and adjusted to pH 2 with HNO
3
. The mixture was
heated to 85 ±2C for 2 h with occasional agitation.
A second aliquot of 15 mL of 30% H
2
O
2
(pH 2 with
HNO
3
) was added and the sample was heated again to
Geochemical fractionation of
210
Pb in oxic estuarine sediments 951
123
85 ±2C for 3 h with intermittent agitation. After
cooling, 25 mL of 3.2 M NH
4
OAc (CH
3
COONH
4
)in
20% (v/v) HNO
3
was added, the sample was diluted to
100 mL and agitated continuously for 30 min.
(v) Residual fraction. Once the previous four fractions
have been removed, the residual solid should contain
mainly primary (quartz, feldspars, micas) and sec-
ondary (detrital silicate minerals, resistant sulfides)
minerals and refractory organic material [34] which
may hold trace metals within their crystal structure.
These metals are not expected to be released in
solution over a reasonable time span under the
conditions normally encountered in nature. The last
solid residue was digested with a mixture 5:4:1 of
concentrated HNO
3
?HCl ?HF and heated on a
hotplate (150 C) overnight.
The residues from all the aqueous phases were centrifuged and
each supernatant were transferred to Teflon PFA containers,
where the solutions were evaporated to incipient dryness on a
hot plate at \80 C. The
210
Pb analyses of the dry residues
were performed following the methodology described in Sect.
2.3.2.
209
Po was added as internal tracer to each supernatant of
the chemical extractions before acid digestion.
Results and discussion
Sediment characterization
In Table 2we present the geochemical properties and
activity concentrations of natural radionuclides in the
sediments samples from Coatzacoalcos River Estuary. The
samples were predominantly silt-sandy, with clay (\2lm)
contents \30%. The C
org
contents were low, ranging from
0.7 to 1.3%, whereas C
inorg
contents ranged from 0.09 to
0.45%. Aluminum and Fe showed rather small variation
between stations suggesting a relatively constant mineral-
ogical sediment composition and origin. Bromine contents
showed increased concentrations toward Coatzacoalcos
River mouth, showing an increasing influence of the
intrusion of sea water from the Gulf of Mexico; contrarily,
Th concentrations decreased in the same direction, indi-
cating a lower influence of terrigenous sources by runoff as
the water approached the river mouth [35]. Total activity
concentration of the radionuclides
210
Pb and
226
Ra ranged
from 20.4 to 31.5 Bq kg
-1
and from 15.9 to 25 Bq kg
-1
,
respectively. Sediment samples from stations 1, 3, 5, and 6
showed
210
Pb concentrations very close to
226
Ra activities,
indicating that the
210
Pb present corresponded mainly to
the
210
Pb supported fraction, which is mostly controlled by
the mineralogy of the catchment bedrock and not the
atmospheric deposition of
210
Pb.
210
Pb fractionation according to sediment grain size
The distribution of
210
Pb activities by grain size fractions is
presented in Fig. 2. The
210
Pb concentrations found in the
clay fractions (58.9 ±0.5 to 91.6 ±0.4 Bq kg
-1
,
64–73%) were considerably higher than the activities in the
silt fraction (14.6 ±0.5 to 30.1 ±0.5 Bq kg
-1
, 16–26%)
and the sand fraction (9.1 ±0.5 to 16.8 ±0.5 Bq kg
-1
,
8–14%) thus reflecting the nature of radionuclide
Table 2 Chemical characterization and activity concentration of natural radionuclides in Coatzacoalcos River Estuary surface sediments
(uncertainty corresponds to 1r)
Magnitude Station
123456
Distance to river mouth (km) 7.8 7.2 15.2 16.7 21.8 34.4
Al (g kg
-1
) 62.9 ±5.6 55.0 ±5.8 50.5 ±4.8 74.7 ±6.9 47.9 ±8.4 77.5 ±7.9
Fe (g kg
-1
) 35.2 ±0.1 45.4 ±0.1 32.2 ±0.1 41.6 ±0.1 36.6 ±0.1 44.2 ±0.1
Mn (mg kg
-1
) 471 ±21 887 ±23 376 ±19 434 ±14 478 ±21 706 ±20
Br (mg kg
-1
) 4.9 ±0.6 13.3 ±0.7 7.6 ±0.6 5.4 ±0.4 2.6 ±0.5 4.9 ±0.5
Pb (mg kg
-1
) 7.7 ±1.6 12.8 ±1.6 10.7 ±1.6 15.1 ±1.3 10.8 ±1.6 15.2 ±1.5
Th (mg kg
-1
) 3.3 ±0.9 4.1 ±0.8 4.8 ±0.9 5.7 ±0.8 8.2 ±1.0 7.8 ±0.9
U (mg kg
-1
)\5.9 \5.9 \5.9 \5.9 \5.9 \5.9
C
org
(%) 0.72 ±0.003 1.14 ±0.004 1.3 ±0.005 0.9 ±0.003 0.93 ±0.004 1.1 ±0.004
C
inorg
(%) 0.09 ±0.001 0.19 ±0.003 0.45 ±0.007 0.3 ±0.004 0.3 ±0.004 0.15 ±0.002
Clay (%) 13.0 ±0.2 30.5 ±0.5 5.5 ±0.1 32.5 ±0.5 18.4 ±0.3 28.7 ±0.4
Silt (%) 11.7 ±1.5 43.9 ±5.7 36.6 ±4.7 39.2 ±5.1 36.2 ±4.7 36.9 ±4.8
Sand (%) 75.3 ±1.2 25.1 ±0.4 57.8 ±0.9 27.9 ±0.5 44.4 ±0.7 33.9 ±0.6
210
Pb (Bq kg
-1
) 20.4 ±0.5 29.8 ±0.5 21.4 ±0.5 31.5 ±0.4 23.3 ±0.5 28.7 ±0.5
226
Ra (Bq kg
-1
) 17.1 ±0.4 16.4 ±0.6 16.8 ±0.5 15.9 ±0.7 25.0 ±0.5 23.3 ±0.7
952 J. F. Ontiveros-Cuadras et al.
123
adsorption by sediment particles, i.e., when the adsorption
of ions by soil particles is associated with cation exchange
or similar processes, the specific surface area (or particle
size) of the particles exerts a primary control on the con-
centration adsorbed [36]. Also, heavy and transitional
metals (including Pb) are mainly related to the fine-grained
clayey sediments, while the coarse-grained sediments (first
of all, sand) are, on the contrary, depleted in these metals
because quartz, prevailing in sand and quartz silts, dilutes
the metal concentrations in sediments [37].
The
210
Pb activity of the sediment samples as a whole
was compared to the calculated
210
Pb activity (
210
Pb
calc
)
distributed among the grain size fractions of each sample
[
210
Pb
calc
=(
210
Pb
clay
•clay content) ?(
210
Pb
silt
•silt
content) ?(
210
Pb
sand
•sand content)] and were found
significantly equivalent (t-student test, P\0.05) at stations
1, 4, and 5 (Table 3); however, small differences were
observed for the rest of the stations, which were related to
heterogeneities of the sediment samples.
Fractionation of
210
Pb
Fractionation of
210
Pb: method I
The
210
Pb activities (Bq kg
-1
) in the fractions defined for
method I, are compiled in Table 4. In stations 1, 2, 3, and 6,
the highest
210
Pb activities were found in the reactive and
silicate fractions in comparable ranges (between 28 and
41%, and from 31 to 45%, respectively) whereas the
210
Pb
activity ranges for pyrite (organic matter) and remnant
fractions were low, 11–25% and 12–15% correspondingly.
On the other hand, in stations 4 and 5, the
210
Pb activities
were associated to a greater extent to the silicate fraction
(31 and 47%, respectively), whilst the rest was distributed
among the pyrite (26 and 22%), the reactive (27 and 13%),
and the remnant (16 and 17%) fractions. The reactive
fraction comprises amorphous and crystalline iron and
manganese oxyhydroxides, carbonates, and hydrous alu-
minosilicates. Iron and manganese oxyhydroxides are
known to concentrate heavy metals on their surface [38]
and it is also known, that
210
Pb behavior is tightly coupled
to the reduction/oxidation cycle of manganese oxides
rather than iron oxides [39,40]. Regarding the silicate
fraction, it comprises clay minerals, which may form a
stable metal–ion binding, and behave as a template for
adsorption and chemical interactions [41]. Also, the total
decomposition method (5:4:1; HNO
3
?HCl ?HF)
applied in the remnant fraction, dissolves the silicates that
remain in the mineral matrix [2].
Fractionation of
210
Pb: method II
When using method II (Table 5) the
210
Pb radionuclide
activities in the fractions operationally defined as
exchangeable, carbonates and organic matter were below
the minimum detectable activity (MDA) of the technique
(1.6 910
-3
to 2.3 910
-2
Bq kg
-1
). Similar results were
observed for the fractionation of
210
Pb in soils with dif-
ferent loads of organic matter, where the activities of the
radionuclide in the more labile fractions were below the
MDA [42]. The activity of
210
Pb in the fraction of iron and
manganese oxides was lower than 10%. The relative low
association of
210
Pb with the oxides fraction could be a
result of an incomplete extraction with hydroxylamine
hydrochloride (NH
2
OH–HCl 0.04 M), since it generally
attacks only the more labile fraction of Fe and Mn oxides
[12] but excludes other more resistant forms such as the Fe
Fig. 2
210
Pb activity distribution in clay, silt and sand fractions in
sediments from the Coatzacoalcos River
Table 3
210
Pb activity in the different grain size fractions
Station Size fraction (%)
210
Pb activity (Bq kg
-1
) Total
210
Pb activity (Bq kg
-1
)
Clay Silt Sand Clay Silt Sand Estimated Measured
1 13.0 ±2.6 11.7 ±2.3 75.3 ±15.1 77.0 ±1.4 30.1 ±0.1 9.1 ±0.7 20.3 ±2.1 20.4 ±0.2
2 30.5 ±6.1 43.9 ±8.8 25.1 ±5.0 79.9 ±0.7 18.9 ±0.1 13.8 ±0.2 36.2 ±1.7 29.7 ±0.8
3 5.5 ±1.1 36.6 ±7.3 57.8 ±11.6 6.9 ±4.2 14.7 ±0.1 10.6 ±0.2 15.2 ±1.5 21.3 ±0.5
4 32.5 ±6.5 39.2 ±7.4 27.9 ±5.6 91.6 ±0.7 23.1 ±0.3 16.8 ±0.7 43.5 ±1.9 31.5 ±2.9
5 18.4 ±3.7 36.2 ±7.2 45.0 ±9.0 58.9 ±2.4 19.9 ±0.6 12.8 ±1.4 23.7 ±1.7 23.3 ±0.5
6 28.7 ±5.7 36.9 ±7.4 33.9 ±6.8 74.7 ±2.6 24.4 ±0.7 14.1 ±0.1 35.2 ±2.1 28.6 ±1.0
Geochemical fractionation of
210
Pb in oxic estuarine sediments 953
123
attached to silicates [43]. The residual fraction showed the
highest concentration of
210
Pb in all the stations, with
values ranging from 92 to 98% of the total activity. The
closeness between the activity of
210
Pb and
226
Ra in all
sediment samples suggests that
210
Pb activities were
mainly related to the mineral matrix (supported fraction),
which comes from
226
Ra decay. The strong association of
supported
210
Pb with the residual fraction is in agreement
with other studies [44,45].
Correspondence between both methods
According to the Pearson correlation test, there is a sig-
nificant correspondence between the
210
Pb contents in the
sediment fractions defined for both methods: (a) the iron
and manganese oxide fraction (method II) presented a
direct correlation (r=0.91; P\0.05) to the reactive
fraction (method I) which integrates amorphous and crys-
talline forms of Fe and Mn oxides; and (b) the residual
fraction (method II), which includes detrital sili-
cates ?sulfides ?refractory organic matter, was directly
correlated (r=0.88; P\0.05) to the silicate fraction
(method I), primarily composed by clay minerals. These
correlations suggest that there is consistency between the
geochemical components in both sequential extraction
schemes.
Conclusions
Surface sediment samples from Coatzacoalcos River
Estuary are predominantly silt–sandy, with clay (\2lm)
contents less than 30%. The
210
Pb activities in the different
grain fractions were closely related to the particle specific
areas, as the radionuclide is preferentially adsorbed by finer
grain particles (clays), with activities ranging from
58.9 ±0.5 to 91.6 ±0.4 Bq kg
-1
. The results of the
sequential extraction by method I indicated that
210
Pb was
mostly associated to both silicates and clays, and the
reactive fraction (Mn and Fe oxy-hydroxide ?carbon-
ates ?Al-silicates) accounted for up to 60–80% of the
210
Pb present in the samples. A non-negligible 10–25%
fraction appeared to be retained by the organic matter
fraction. The results of the sequential extraction of method
II indicated that
210
Pb in the sediments samples was mainly
associated with the residual fraction (detrital silicate min-
erals ?resistant sulfides ?refractory organic material)
with relative abundance between 92 and 98% and, to a
lesser degree, to the iron and manganese oxides fraction
(2.1–7.7%). The significant correlation between the
210
Pb
content in the sediment fractions defined for both methods:
(a) the iron and manganese oxide fraction and the reactive
fraction and (b) the residual fraction and silicate fraction,
confirmed that there was consistency between the
Table 4 Fractionation of
210
Pb (Bq kg
-1
) by method I (Huerta-Dı
´az
and Morse [2]), in surface sediments from Coatzacoalcos River
Estuary, Mexico
Station Fraction
* 210
Pb (Bq kg
-1
) Ratio** (%)
1 1 2.94 ±0.27 18.41 ±3.68
2 7.18 ±0.71 45.01 ±9.00
3 3.92 ±0.33 24.56 ±4.91
4 1.92 ±0.06 12.02 ±2.40
2 1 5.27 ±0.72 26.71 ±8.01
2 6.19 ±1.56 31.37 ±9.41
3 5.12 ±0.23 25.96 ±7.78
4 3.15 ±0.21 15.95 ±4.78
3 1 2.47 ±0.20 13.03 ±1.30
2 8.92 ±0.37 47.17 ±4.71
3 4.24 ±0.21 22.40 ±2.24
4 3.29 ±0.27 17.40 ±1.74
4 1 7.37 ±1.01 28.34 ±5.66
2 10.69 ±0.16 41.13 ±8.22
3 4.39 ±0.24 16.89 ±3.37
4 3.55 ±0.34 13.64 ±2.72
5 1 6.60 ±0.27 38.09 ±3.80
2 6.63 ±0.01 38.26 ±3.82
3 1.82 ±0.08 10.51 ±1.05
4 2.28 ±0.15 13.13 ±1.31
6 1 9.88 ±0.55 41.13 ±4.11
2 7.56 ±0.02 31.47 ±3.14
3 3.58 ±0.33 14.92 ±1.49
4 3.00 ±0.14 12.48 ±1.24
*
Fraction 1: reactive; Fraction 2: silicates; Fraction 3: pyrite; Frac-
tion 4: remnant
**
The ratio is the percentage of
210
Pb activity in each fraction with
respect of the
210
Pb activity of the whole sample (
210
Pb in the frac-
tion/total
210
Pb) 9100
Table 5 Fractionation of
210
Pb (Bq kg
-1
) by method II (Tessier et al.
[1]), in surface sediments from Coatzacoalcos River Estuary, Mexico
Station Fe and Mn Oxides Residual
210
Pb
(Bq kg
-1
)
Ratio* (%)
210
Pb
(Bq kg
-1
)
Ratio* (%)
1 0.19 ±0.02 2.08 ±0.20 9.09 ±0.07 97.92 ±9.79
2 0.38 ±0.03 4.20 ±0.42 8.55 ±0.94 95.80 ±9.58
3 0.45 ±0.02 3.74 ±0.37 11.6 ±0.95 96.26 ±9.62
4 0.76 ±0.12 4.71 ±0.94 15.4 ±0.85 95.29 ±19.05
5 0.61 ±0.04 7.71 ±0.77 7.27 ±0.35 92.29 ±9.22
6 1.07 ±0.10 7.62 ±0.76 13.0 ±0.93 92.38 ±9.23
*
The ratio is the percentage of
210
Pb activity at each fraction with
respect of the
210
Pb activity of the whole sample (
210
Pb in the frac-
tion/total
210
Pb) 9100
954 J. F. Ontiveros-Cuadras et al.
123
geochemical components in both sequential extraction
schemes.
It is clear that the separation of the coarser fraction in
the sediment samples resulted in higher activities of
210
Pb
in the sample, and therefore, the analysis of the fine frac-
tion of the sediments could be useful to improve the
analysis of
210
Pb for dating purposes, especially in areas
where the atmospheric flux of
210
Pb is characteristically
low. This may improve the precision of
210
Pb chronologies
in these areas.
Acknowledgments This work was financially supported by the
program UNAM-DGAPA PAPIIT 105009 and the International
Atomic Energy Agency (IAEA) project RLA/7/012. The scholarship
of J.F. Ontiveros-Cuadras was provided by the Consejo Nacional de
Ciencia y Tecnologı
´a (CONACyT-Me
´xico). Thanks are due to F.
Pa
´ez-Osuna for his help in field work, to A. Abreu-Grobois for
reviewing the statistic section, and to M.C. Ramı
´rez-Ja
´uregui, G.
Ramı
´rez-Rese
´ndiz, and H. Bojo
´rquez-Leyva for their technical
assistance.
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