Journal of Minerals & Materials Characterization & Engineering, Vol. 7, No.1, pp 59-70, 2007 Printed in the USA. All rights reserved
Characterization of Chromium in Contaminated Soil Studied by SEM, EDS,
XRD and Mössbauer Spectroscopy
L. R. Reyes-Gutiérrez
, E. T. Romero-Guzmán
A. Cabral-Prieto
and R. Rodríguez-Castillo
Universidad Nacional Autónoma de México, Instituto de Geofísica, Posgrado en Ciencias de la
Tierra, UACPyP-CCH. México.;
Universidad Autónoma del Estado de Hidalgo. Centro de Investigaciones en Ciencias de la
Tierra. Carretera Pachuca-Tulancingo Km. 4.5, Pachuca Hidalgo. C.P. 42184. Tel: 01 771 71
72000 ext. 6622, Fax: 01 771 71 721 33.
Instituto Nacional de Investigaciones Nucleares. Gcia. Ciencias Básicas, Depto. de Química
Carretera México-Toluca km36.5, Salazar Estado de México
AP 18-1027. C.P. 52045. Tel: +52 (55) 53 29 72 00, ext. 2266, Fax: +52 (55) 53 29 73 01.
Chromium is important from the environmental point of view since its behavior and toxicity
properties depend on its oxidation states. The Cr(VI) concentration in wells of Buenavista,
Guanajuato, Mexico, is higher than the permissible level of it for drinking water, 0.05mg/L. The
objective of this research was to determine the elution of chromium with deionized water from
contaminated soil samples and to determine the oxidation state of Fe, which is an element that
can limit the mobility of chromium. These results will be considered in a pump and treat
remediation scheme for this site. Chromium contaminated soil samples were obtained from an
industrial area of Leon, Guanajuato, México. O, Na, Mg, K, Al, Si, Ca, Cr and Fe were found in
the chemical analysis by EDS of the contaminated samples. In the soluble species only O, Na, S,
Ca and Cr were found. The oxidation state of iron was determined by Mössbauer spectroscopy
(MS) in the soil contaminated with chromium, in the soil washed with deionizer water and also in
the soluble samples. CaCrO
was found in the soluble fraction, as a single crystalline phase by
XRD. MS indicated that at least two iron species were present, one insoluble and the other
sparingly soluble.
Keywords: Cr(VI), soil contamination, Mössbauer, EDS, SEM, DRX
60 L. R. Reyes-Gutiérrez,, E. T. Romero-Guzmán, A. Cabral-Prieto
and R. Rodríguez-Castillo Vol.7, No.1
Chromium (Cr) is present in nature mainly in the form of highly insoluble Cr(III) minerals. The
other stable oxidation state of chromium is VI, which forms very soluble chromate compounds.
However, the ocurrence of appreciable amounts of Cr(VI) in soil and water suggests that artificial
addition of chromate ions due to industrial activities has ocurred. For instance, the leather
industry generates solid residues with a high content of Cr(III) oxides. These Cr(III) oxides
dissolve at pH values below 6. The Cr(VI) compounds dissolve in acid and basic media and they
are readily reduced to Cr(III) species under acid conditions. Thus, chromium as contaminat may
be found in water, either as dissolved chromate (VI) species and/or as poorly soluble Cr(III)
oxides deposited on the sediments. Cr(VI) is potentially more dangerous to living organisms than
Cr(III), because of its high solubility. The highest permissible level of total soluble chromium
concentration in drinking water has been set to 0.05 mg/L [1-4].
The chromite, FeCr
, a natural source of Cr [5], can be oxidized to Cr(VI) in presence of Mn
(III/IV) minerals, which are frequently found in soils [6-8]. On the other hand, organic
compounds, ferrous species and sulfide compounds may be responsible for the reduction of
Cr(VI) to Cr(III), i. e., these redox reactions may occur in contaminated waters, sediments and
soils [9,10].
In this paper a soil contaminated with Cr from an industry that produces chromium compounds
located in Buenavista, Guanajuato State, México was analyzed. In this site, there are two large
deposites of Cr(III) and Cr(VI). One deposit is on protected surface without connecting with the
other, a container in an excavated place. The Cr(VI) container was not properly designed and
built into a clayey unit. Rainwater may enter to the container, forming a leakage that reaches the
local aquifer. This aquifer is not mostly used as drinking water source [2,11]. The wastes of this
last container were place in other one by the end of 1994.
The lack of strictict environmenatl regulations has allowed that the permisible levels of total
soluble chromium concentrations in drinking water had been higher than 0.05 mg/L. There are
several examples where high concentrations of Cr(VI) has been the cause of cancer in childs and
building workers [1,2,4]. Armienta et al., (1996) detected Cr in local vegetation but reduced
Cr(III) was not observed in soils [12]. This research is the second part of the previous study [3]
looking for the remediation of this contaminated site. These studies are important to prevent risks
to the human health since chromium can be accumulated on skin, lungs, muscles fat, and it
accumulates in liver, dorsal spine, hair, nails and placenta [1].
Therefore, the aim of this work was to identify the chemical specie of chromium as well as to
determine the degree of association that exists between the chromium and iron, this knowledge
will be used in a treatment scheme for this site.
2.1. Site Description: The container of Cr(VI) wastes under study is located near the Turbio
river, a few kilometers away from Buenavista, Leon City, Guanajuato state, Mexico. The Cr(VI)
wastes from the industrial processes are located inside the industry area facilities. The
stratigraphic nature of the site is composed of fine to coarse grain sand with variable quantities of
silt, clays and graves [13]. The hydraulic conductivity (Kx, Ky), the distribution coefficient (Kd)
and the α
ratio of this site have been reported by Reyes (1998) [3]. The container consists of
an excavation without a membrane; 70 m long, 30 m width, and 6 m deep and it is located only 3
m over watertable (Figure 1). The initial Cr concentration of the leakage, 90 mg/L was
Vol.7, No.1 Characterization of Chromium in Contaminated Sois 61
determined by means of computational modeling of solute transport. The highest Cr(VI)
concentration was measured in 1992 in the piezometer II, 30 m away from the container.
Although by a sensibility analysis of the numerical modeling results, a value of 160 mg/L was
reported [3]. The Cr(VI) concentration in sediments is up to 1300 mg/kg. The isoconcentration
levels of Cr(VI) in groundwater over last ten years are showed in Figure 1.
0200 400 600 80010001200140016001800
Distance (m)
ground surface
water level
t = 10 years
La Hulera
= 50.0 m
= 2.5 m
Kx= 100 m/d
Ky= 50 m/d
Kd= 0.007 mL/g
Clayey Lens
Clayey Package
Co= 90 mg/L
α /α = 20
Figure 1. Diagram of the deposit of Cr(VI) and its plume of contamination formed through a
period of 10 years (Reyes, 1998). (Concentration contours in mg/L)
2.2. Sampling and Leaching: Three samples of 2 kg of soil were collected. The samples were
kept at –4°C in order to maintain the same chemical conditions of the container before the
analysis. A representative composed sample was prepared with the collected samples from the
superficial zone (with the highest Cr content), intermediate zone and from the bottom of the
container. The samples were separated using a mesh no. 40, for homogeneity purposes. A glass
column of 1.5 cm in diameter and 20 cm length was packed with 5 gr of sample for leaching
experiments. The experiment was reproduced 10 times. Deionized water was passed through the
column to elute Cr (VI). 10 mL aliquots were collected directly from the column for Cr analysis.
The aqueous eluted fraction was evaporated to dryness and a yellow powder residue was
obtained. The fractions: contaminated soil, treated soil and the leached fraction were analyzed.
2.3. Analysis: Original, treated and soluble samples were analyzed by Scanning Electron
Microscopy (SEM), X-Ray Energy Dispersive Spectrometry (EDS), X-Ray Diffraction (XRD)
and Mössbauer Spectroscopy (MS). The SEM analysis was done using a PHILLIPS XL-30
microscope with a 3.5 nm of resolution. Chemical composition was determined by EDS using an
EDAX spectrometer, the detection limit was 0.01 %. The mineralogical analysis was made using
a SIEMENS D-5000 X-Ray Diffractometer, the detection limit was 3-5 % wt. Finally, the
oxidation state of iron was obtained by a conventional Mössbauer spectrometer using a
source. The isomeric shift is relative to Fe
, the detection limit was 0.1 %.
3.1. Soil Contaminated With Chromium
62 L. R. Reyes-Gutiérrez,, E. T. Romero-Guzmán, A. Cabral-Prieto
and R. Rodríguez-Castillo Vol.7, No.1
The untreated samples were analyzed by SEM to determine its morphological characteristics.
Some micrographs illustrate the shape of the soil particles (Figure 2). The soil is a heterogeneous
material containing a large particle size distribution. Analyses of these untreated particles, with
EDS, indicate that chromium is present (Figure 2a). The EDS analysis is presented in Table 1. 10
individual analyses from different zones of the sample were performed by EDS. The element
content in the soil is Si, Al, O, Fe, Ca, K, Mg, Na and Cr. In all EDS elemental analyses appear
the usual composition of the aluminosilicates minerals consisting of Si, Al, O, Na and Ca as
shown in Figure 4. It is interesting to note, that there are particles which do not contain neither K
nor Ba in the untreated sample, Figures 2b and 2c. On the other hand, Figure 2b shows a particle
with a high Ba and Cr content. It is important to indicate, that it was only a punctual analysis of a
brilliant particle (marked with an arrow). The presence of Ba
has been reported in other similar
studies [14,15]. Ba
is related to clay in soils, probably forming a chromate mineral phase,
, which can be a Cr(VI) source [16,17]. The presence of Ba
could associate to the
erosion of intrusive rocks from the north of the study area [18]. The Fe content is also variable,
and based on the EDS results; the untreated soil has the highest content of Fe, Figure 2c. This
sample was studied by Mössbauer spectroscopy; its content is the highest in to identify the iron
oxidation state. The Mössbauer spectrum is shown in Figure 3. The Mössbauer parameters are
characteristic of Fe
, with the Isomeric Shift, IS=0.38 mm/s and the Quadropole Splitting QS
=0.53 mm/s.
Table 1. Average composition of untreated and treated soils
Element Wt %
Untreated soil Treated soil
O 45.85 ± 1.35
53.13 ± 0.82
Na 1.09 ± 0.54
0.14 ± 0.14
Mg 0.89 ± 0.07
0.44 ± 0.13
Al 9.94 ± 0.48
8.65 ± 0.48
Si 27.39 ± 1.30
30.43 ± 1.34
K 2.76 ± 0.77
2.09 ± 0.57
Ca 3.73 ± 1.26
2.33 ± 0.17
Cr 4.10 ± 1.07
Fe 4.23 ± 0.64
2.79 ± 0.12
Vol.7, No.1 Characterization of Chromium in Contaminated Sois 63
Figure 2. SEM images from untreated soil observed at a) 100 X, b) 1284 X and c) 1588 X. The
sections show agglomerates of particles of soil; and their corresponding EDS analysis.
64 L. R. Reyes-Gutiérrez,, E. T. Romero-Guzmán, A. Cabral-Prieto
and R. Rodríguez-Castillo Vol.7, No.1
Figure 3. Mössbauer spectrum of untreated soil.
Figure 4. SEM image of the treated soil and its EDS analysis.
-8 -6-4 -202468
Velocity (mm/s)
Transmission (%)
+ 3
IS = 0.38 mm/s/Fe
QS = 0.53 mm/s
I = 8 %
Vol.7, No.1 Characterization of Chromium in Contaminated Sois 65
3.2. Treated Soil
Table 1 and Figure 4 show that the treated soil with deionized water was chromium free. The
treated sample formed aggregates of particles with a smaller particle size distribution than the
untreated sample. The EDS analysis indicated that the washed soil sample is mainly composed by
Si, Al, O, Fe, Ca, K, Mg and Na. Cr(VI) or Cr(III) were not found in the washed soil samples. It
is important to point out that, a fraction of the iron content (2.7 weight %) remained in this
treated soil; that is, as insoluble material. The rest of the iron content (1.4 weight %) was in
soluble form, Table 1. The chemical form of this soluble iron is not clear. Since the sample was
taken 30 cm below the ground level, it means that Cr(VI) is eluted through the unsaturated zone
and eventually it should contaminate the groundwater.
Figure 5 shows the micrographs of the treated soil where two types of particles are observed. One
of them is formed by superposed layers corresponding to silicate structures, Figure 5a. The EDS
analysis was similar to the untreated soil, Figure 4. Figure 5b shows the other particle, a smooth
material, characteristic of silica as its EDS analysis suggests. Mg, Ca, and Fe were detected in
these particles with a low weight percentage. Although, the EDS analysis is a local method,
chromium was not present in the treated soil, indicating that the separation method used to
remove it from soil was efficient. The Mössbauer spectrum is shown in Figure 6 and the
Mössbauer parameters are also characteristic of Fe
, IS=0.37 mm/s and the QS =0.51 mm/s. A
small contribution appears in the spectrum which could be due to the presence of iron oxides.
Figure 5. SEM images of treated soil observed at 1000 X. Sections show a. superposed layers
corresponding to silicate structures and b. a smooth material.
66 L. R. Reyes-Gutiérrez,, E. T. Romero-Guzmán, A. Cabral-Prieto
and R. Rodríguez-Castillo Vol.7, No.1
-8-6-4-202 46 8
Velocity (mm/s)
Transmission (%)
Figure 6. Mössbauer spectrum of treated soil.
3.3. Soluble Fraction
The micrographs of the soluble residues, the yellow powder, show hexagonal crystals of different
size (Figure 7). Their composition includes O, Na, S, Ca and Cr. The X-ray diffraction analysis
showed chromatite, CaCrO
(Joint Committee on Powder Diffraction Standards, JCPDS card 8-
0458) [19] like a main crystalline phase, also reported by Bajda, 2005 [20], Figure 8. The
formation in aqueous solution of the chromatite, CaCrO
according Puigdomenech (2004) [21] is
log = 2.266.
Fe was not detected in this fraction by SEM and XRD techniques, but it was detected by
Mössbauer spectroscopy. The Mössbauer parameters are also characteristic of Fe
, with IS=0.38
mm/s and the QS =0.54 mm/s, Figure 9. In this case, the FeOHCrO
, a salt with Fe
, was
determined. The presence of this last compound was reported by Rock et al. (2001) [22]. In
agreement with the pH (6.5-8.5) of the eluates during the leaching of chromium from
contaminated soil, it was verified that the iron was not as oxide but as hydroxide chemical specie.
has a log K=-36.5, according to Puigdomenech, (2004) [21], which indicates that
in solution this chemical specie should not be formed, nevertheless it precipitated when the
eluted solution was evaporated.
Vol.7, No.1 Characterization of Chromium in Contaminated Sois 67
Figure 7. SEM image of chromium removed from soil and its EDS analysis.
68 L. R. Reyes-Gutiérrez,, E. T. Romero-Guzmán, A. Cabral-Prieto
and R. Rodríguez-Castillo Vol.7, No.1
10 2030 40 50 60 70
Intensity (a.u.)
Figure 8. X-Ray Diffraction pattern of chromatite, CaCrO
-8 -6 -4 -202468
Transmission (%)
Velocity (mm/s)
IS = 0.38 mm/s/Fe
QS = 0.54 mm/s
I = 6 %
Figure 9. Mössbauer spectrum of the soluble yellow powder fraction.
Vol.7, No.1 Characterization of Chromium in Contaminated Sois 69
According to the results, the soil contaminated with chromium can be described as aggregates,
layers and inorganic particles distributed in the bulk. The soil components are based on O, Na,
Mg, K, Al, Si, Ca, Cr and Fe. All soluble species were leached from the soil. The soluble yellow
powder was composed by hexagonal crystals indicating that the oxidation state of Cr was VI,
which represents the highest risk to the environment for its toxicity.
The results revealed the efficient of leaching chromium from soil using deionized water. Fe
detected by MS in the soluble yellow powder sample but iron was no detected by SEM and XRD.
These results indicate that there are two species in the eluted fraction. The compound determined
was the chromatite CaCrO
which was formed by dissolution of calcium carbonates from soil in
the presence of heavily soils polluted with Cr(VI) and FeOHCrO
in a lower concentration.
Therefore, in this case, the degree of association that exists between the chromium and iron is
weak, because the Cr(VI) is leaching easily from the soil particles.
The authors would like to thank Chemical Central industry for their technical helpful.
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