Comparison of USEPA digestion methods to heavy metals
in soil samples
Ygor Jacques Agra Bezerra da Silva &
Clístenes Williams Araújo do Nascimento &
Caroline Miranda Biondi
Received: 18 February 2013 /Accepted: 15 July 2013
#Springer Science+Business Media Dordrecht 2013
Abstract The use of appropriate analytical methods is
of paramount importance for risk assessment and mon-
itoring of potentially toxic metals in soils. In this sense,
the objective of this study was to compare the effective-
the Brazilian legislation for the management of contam-
inated areas (CONAMA 2009), aiming at the determi-
nation of environmentally available metal concentra-
tions (USEPA 3050B,USEPA 3051A),aswellasa total
digestion method (USEPA 3052). Samples from 10
classes of soils were analyzed for Cu, Zn, Cd, Pb, Ni,
and Hg. The results showed that the USEPA method
3051A is moreefficient thanthe USEPA method 3050B
in the extraction of levels considered environmentally
higher recovery of these elements, the method requires
shorter digestion time, lower consumption of acids, and
reduced risk of contamination. The USEPA method
3051A showed greater efficiency in Hg extraction in
soils with higher clay content. Therefore, it is suitable
for situations where a wide range of soils with different
mineralogical characteristics are analyzed or in order to
The increasing occurrence of soil contamination by
heavy metals has heightened concerns about environ-
ment quality, as these elements cause changes in the
health risks to humans. In this context, the use of accu-
rate methods for monitoring heavy metals in soils is of
great importance for risk assessment (Guven and
Görkem 2011). Such studies must carefully consider
the attention required in the laboratory analytical deter-
mination, which involves digestion methods, choice of
adequate instrumentation as well as precision and accu-
racy in metal determination in the samples.
In general, acid digestion procedures are used to
convert solid samples into liquid extracts to quantify
the total or pseudototal concentrations of metallic ele-
ments in soils. This principle consists of releasing the
metals present in the solid matrix to the acidic solution
during the extraction process, this procedure being nec-
Environ Monit Assess
Y. J. A. B. da Silva
Departamento de Agronomia, UFRPE,
Endereço: Rua Dom Manuel de Medeiros, S/N,
Dois Irmãos, Brazil
C. W. A. do Nascimento (*):C. M. Biondi
UFRPE, Endereço: Rua Dom Manuel de Medeiros,
S/N, CEP: 52171-900 Dois Irmãos, Recife, PE, Brazil
C. W. A. Nascimento
C. M. Biondi
such as inductively coupled plasma optical emission
spectrometry or atomic absorption spectroscopy.
Several acid digestion methods for the determination of
heavy metals in soils have been described in the litera-
ture. They range from milder attacks, such as aqua regia
in an open system, to the use of hydrofluoric acid in a
closed system, which is considered a total digestion for
the large variation in metal content obtained by different
methods, the digestion of samples is the principal factor
contributing to the uncertainty of analytical results
(Kántor 2001; Axelsson and Rodushkin 2001; Belarra
et al. 2002; Al-Harahsheh et al. 2009). Currently, this
variation in methods hinders the comparison of obtained
data; thus, it is essential that the regulatory agencies
standardize the method used to determine metal concen-
trations in soils. In Brazil, the National Environment
Council (CONAMA 2009) stipulates that for regulatory
purposes, the USEPA methods 3050 and 3051 or their
updates must be used in the digestion of soil samples for
heavy metal determination.
The USEPA methods 3050 and 3050B are consid-
ered conventional procedures because they are
conducted in an open system, in which the elements in
the solid phase are extracted by a heat source in the
presence of nitric and hydrochloric acids. This method
has the disadvantage of atmospheric contamination risk
and loss of more volatile elements (Nieuwenhuize et al.
1991), such as Hg. The USEPA method 3051A is a
modification of the method 3051, requiring the addition
of hydrochloric acid (HCl) with nitric acid (HNO3), to
improve the recovery of Ag, Al, Fe, and Sb (USEPA
1997). Performed in a closed microwave system, this
method provides higher temperature and pressure
(Berghoff-Tetra 2004), resulting in a faster, safer, and
more efficient digestion, in addition to being less sus-
ceptible to the loss of volatile elements. However, it
should be noted that the methods 3050B and 3051A
are not total digestion techniques, because they do not
recover 100 % of the element in the soil sample
(Sawhney and Stilwell 1994). The USEPA method
3052, also carried out in a microwave, is recommended
for a total digestion, by promoting the sample’s total
decomposition due to the hydrofluoric acid (HF) pres-
ence (USEPA 1995b).
Appreciable differences in the recovery of metals
are observed between these methods (Scancar et al.
2000; Chen and Ma 2001; Campos et al. 2003; Tighe
et al. 2004; Chander et al. 2008), many times with poor
correlations between them, indicating a possible depen-
dence of the metal recovery with the soil mineralogical
composition and the nature of the metal. Therefore, it is
important to evaluate these digestion methods using
soils with different characteristics. In this sense, the
objective of this study was to compare the effectiveness
ofthree digestion methods (USEPA 3050B,3051A,and
3052) for determination of Cu, Zn, Cd, Pb, Ni, and Hg
in10 soilsamples withdifferent chemical, physical, and
mineralogical characteristics, as well as to provide sub-
sidies to the Brazilian legislation on the subject
(CONAMA Resolution no. 420 of 28 Dec. 2009), with
respect to analytical methodologies.
Materials and methods
Samples were collected from the surface horizons of 10
soil classes in several municipalities in the state of
Pernambuco: Mollisols (Nazaré da Mata), Oxisols (Rio
Formoso), Ultisols (Camutanga), Ultisols (Aliança),
Histosols (Ipojuca), Gleysols (Ipojuca), Oxisols
(Caruaru), Ultisols (Garanhuns), Entisols Fluvents
(Ibimirim), and Vertisols (Bodocó). The physical and
chemical characteristics of these soils are shown in
Table 1. The air-dried samples were sifted on a 2-mm
mesh nylon sieve. The aliquot was macerated in an agate
mortar and sifted with a stainless steel 0.3-mm mesh
sieve (ABNT no. 50), in order to avoid contamination.
Three different methods of sample digestion were
assessed, which are described below. All digestions
were performed in duplicate.
USEPA 3050B (USEPA 1996) Pulverized soil samples
(0.5 g) were transferred to Teflon beakers, where
10 mL of 50 % HNO3was added. The solutions were
heated on a hot plate at 95 °C±5 with a ribbed watch
glass, allowing them to evaporate (without boiling) to
about 5 mL, for 2 h. Subsequently, 2 mL of ultrapure
water and 3 mL of hydrogen peroxide (30 % H2O2)
were added to the beakers. The solutions were again
heated until the effervescence reduced; aliquots of
1 mL of 30 % H2O2were added until the effervescence
was minimal or the sample’s appearance suffered no
further changes. After, the heating procedure was re-
peated, thus evaporating (without boiling) the solu-
tions to about 5 mL, for 2 h. Finally, 10 mL of concen-
trated HCl was added to the solutions, followed by hot
plate heating (95 °C±5) for 15 min.
Environ Monit Assess
USEPA 3051A (USEPA 1998) Pulverized soil samples
(0.5 g) were transferred to Teflon tubes, where 9 mL of
HNO3and 3 mL of HCl were added. They were kept in
a closed system, a microwave oven (MarsXpress) for
8 min 40 s on the temperature ramp, the necessary time
to reach 175 °C; then this temperature was maintained
for an additional 4 min 30 s.
USEPA 3052 (USEPA 1996) Pulverized soil samples
(0.5 g) were placed in Teflon tubes, where 9 mL of
HNO3and 3 mL of concentrated HF, of high analytical
purity, were added. After, samples were submitted to
microwave irradiation for 5.5 min to reach 180 °C,
attaining a maximum pressure of 16 atm, and 4.5 min
digestion with constant temperature and pressure.
After digestion, all extracts were transferred to
50-mL certified flasks (NBR ISO/IEC), filling with
ultrapure water (Millipore Direct-Q System) and filter-
ing in a slow filter paper (Macherey Nagel®). High-
purity acids were used in the analyses (Merck PA).
Table 1 Physical and chemical characteristics of the soil samples
P. Profile, 1 Mollisols, 2 Oxisols, 3 Ultisols, 4 Ultisols, 5 Histosols, 6 Gleysols, 7 Oxisols, 8 Ultisols, 9 Entisols Fluvents, 10 Vertisols
bKCl 1 mol L−1(De Filippo and Ribeiro 1997)
cMehlich-1(De Filippo and Ribeiro 1997)
dUSEPA 3051A (EPA 1998)
Fig. 1 Mean concentration
of heavy metals extracted
by the USEPA methods
3050B, 3051A, and 3052 in
10 soil samples. Means with
the same letter are equally
significant by the Tukey test
(5 % probability)
Environ Monit Assess
Glassware was cleaned and decontaminated in a 5 %
Calibration curves for metal determination were pre-
pared from standard 1,000 mg L−1(Titrisol®, Merck).
calibration curve was higher than 0.999. After initial
calibration, it was checked again after 10 samples were
equipment was recalibrated.
The concentrations of Zn, Ni, Cu, Pb, and Cd in the
extracts of the three methods were determined by an
atomic absorption spectrophotometer (PerkinElmer
AAnalyst™ 800) using the flame technique. Hg was
determined in the same equipment coupled to a hydride
with an electrodeless discharge lamp.
The results were submitted to descriptive statistics
(mean and standard deviation), Pearson’s linear correla-
tion between the metals in each method, and analysis of
Table 2 Hg (in microgram per
kilogram), Zn, Ni, Pb, Cu, and
Cd (in milligram per kilogram)
concentrations in samples of 10
soils digested by the methods
3050B, 3051A, and 3052
Results are expressed as mean ±
standard deviation. Detection
limits (in microgram per kilo-
gram): Hg (0.009), Pb (15), Zn
(1.5), Cu (1.5), Ni (6), Cd (0.8)
1 Mollisols, 2 Oxisols, 3 Ultisols,
4 Ultisols, 5 Histosols, 6 Gleysols,
7 Oxisols, 8 Ultisols, 9 Entisols
Fluvents, 10 Vertisols
Profile 3050B3051A3052 3050B3051A3052
Environ Monit Assess
probability, using the Statistical Analysis System (SAS)
Learning Edition version 2.0, was performed as a means
Results and discussion
Most elements analyzed showed significantly higher
concentrations when extracted by the total decomposi-
tion method (USEPA 3052), except for Cu and Pb,
which showed concentrations statistically equal with
method 3051A (Fig. 1). As the method 3052 destroys
the silicates and liberates the metals linked to this more
the amounts of Cu and Pb bound to silicate minerals are
very low or negligible. Metals which participate in the
crystal structure of silicate minerals can only be
completely recovered by total digestion involving HF
(Hewitt and Reynolds 1990; Sawhney and Stilwell
1994). For other elements, the silicate fractions appear
to have greater participation in the total metal content in
soil, with the metal distributionindifferentsoilfractions
being dependent on soil characteristics, such as the
content and type of clay, as well as the very nature of
the metal. These results corroborate Chen et al. (1998),
40 soils from Florida, but also did not detect differences
in relation to Cu.
All determinedelements, excepting Hg,showed mean
concentrations significantly higher when extracted by
method 3051A than by method 3050B (Fig. 1). This
lower metal recovery for the method 3050B is due to a
less aggressive acid attack, held in an open system at
atmospheric pressure, as well as to the greater possibility
of analyte loss, when compared to the microwave closed
Zn, Cu, Ni, Pb, and Cd in soil, also found higher recov-
to the open system digestion. They attributed these dif-
ferences to losses by volatilization and oxidation of some
elements during hot plate digestion. In contrast, Sastre
et al. (2002) obtained similar recoveries for Zn, Cu, Pb,
and Cd in a microwave digestion with the aqua regia
method (open system), in various certified materials and
environmental matrices (soils, sediments, and plants).
Contrary to Nieuwenhuize et al. (1991), there was
no statistical difference for the mean of the 10 soils
between the methods 3050B and 3051A, resulting
from Hg loss during the open system digestion. Chen
et al. (1998) also found no significant differences in the
Hg content determined in soil samples digested by the
two methods. However, comparing the Hg levels in
samples within the same class of soil, a greater recov-
ery by method 3051A was observed in more clayey
soils (Table 2). For example, the Red Ultisol Eutrophic
Nitosol with twice the silt + clay concentration of the
other two Ultisols, which had sand content in excess of
650 g kg−1(Table 1), showed a Hg recovery 40 %
superior when extracted by the method 3051A.
Similarly, for Gleysol with 710 g kg−1of clay, the Hg
recovery by the method 3151A was 22 % superior to
that obtained with the method 3050B (Table 2).
Although there is a high positive correlation be-
tween these methods for Hg (Table 3), given the
greater extraction efficiency for most analyzed metals
and the contamination risk reduction, the use of the
3051A method is recommended (Chen et al. 1998).
Additionally, Guven and Görkem (2011) observed
that in the USEPA method 3050B, soil samples were
extracted in about 180 to 200 min, including evapo-
ration and cooling time, with acid consumption rang-
ing between 35 and 50 mL. However, all microwave
heating programs had total digestion time of only
Positive correlations were observed between methods
3051A and 3050B for all elements analyzed, except for
Cd (Table 3). The low correlation coefficient for this
element can be explained by the low Cd concentration
in the studied soils (Table 2), which leads to high vari-
ability of results. The highest concentration of Cd
Table 3 Correlationcoefficients fortheconcentrationsofHg,Zn,
Ni, Pb, Cu, and Cd extracted from 10 soil samples by the USEPA
methods 3050B, 3051A, and 3052
Element3050B × 3051A3050B × 3052 3051A × 3052
aSignificant at <0.1 %
bSignificant at 5 %
Environ Monit Assess
(>50 %) in the studied soils was found in silicate frac-
tions, accessed only by the method 3052. This corrobo-
rates the low recovery by the other two tested methods
andexplains thelackofcorrelation between the methods
3051A, 3050B, and 3052, for this element.
Given the higher concentration of recovered metals and
od 3052 showed the best performance among the evalu-
ated methods. However, by overestimating the environ-
mentally available concentrations, this method is not
recommended for regulatory purposes by the Brazilian
legislation, which adopts the USEPA methods 3050 and
3051, for the metals analyzed in this work (except Hg).
The results showed that the USEPA method 3051A is
more efficient than the USEPA method 3050B in the
extraction of concentrations considered environmentally
available of Zn, Cu, Cd, Pb, and Ni, because in addition
to providing higher recoveries of these elements, it re-
quires less digestion time, lower consumption of acids,
and reduced risk of contamination and element losses.
The USEPA method 3051A demonstrated an even great-
er efficiency in Hg extraction in soils with higher clay
content. Therefore, it is recommended for situations
where a wide range of soils with different mineralogical
characteristics are analyzed or in order to decrease losses
due to element volatilization in open systems.
Conflict of interests
conflict of interest.
The authors declare that they have no
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