RADIONUCLIDES AND HEAVY METAL CONTENTS IN PHOSPHOGYPSUM SAMPLES...
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SEÇÃO IX - POLUIÇÃO DO SOLO E
QUALIDADE AMBIENTAL
RADIONUCLIDES AND HEAVY METAL CONTENTS IN
PHOSPHOGYPSUM SAMPLES IN COMPARISON TO
CERRADO SOILS(1)
Vanusa Maria Feliciano Jacomino(2), Kerley Alberto Pereira de
Oliveira(3), Maria Helena Tirollo Taddei(4), Maria Célia Siqueira (5), Maria
Eleonora Deschamps Pires Carneiro(6), Marcos Roberto Lopes
Nascimento(7), David Faria da Silva(8) & Jaime Wilson Vargas de Mello(9)
ABSTRACT
Phosphogysum (PG) or agricultural gypsum, a solid waste from the phosphate
fertilizer industry, is used as soil amendment, especially on soils in the Cerrado
region, in Brazil. This material may however contain natural radionuclides and
metals which can be transferred to soils, plants and water sources. This paper
presents and discusses the results of physical and chemical analyses that
characterized samples of PG and compares them to the results found in two typical
soils of the Cerrado, a clayey and sandy one. These analyses included: solid waste
classification, evaluation of organic matter content and of P, K, Ca, Mg, and Al
concentrations and of the mineralogical composition. Natural radionuclides and
metal concentrations in PG and soil samples were also measured. Phosphogypsum
was classified as Class II A - Not Dangerous, Not Inert, Not Corrosive and Not
Reactive. The organic matter content in the soil samples was low and potential
acidity high. In the mean, the specific 226Ra activity in the phosphogypsum samples
(252 Bq kg-1) was below the maximum level recommended by USEPA, which is
370 Bq kg-1 for agricultural use. In addition, this study verified that natural
(1)
Recebido para publicação em março de 2009 e aprovado em junho de 2009.
Pesquisadora Titular da Comissão Nacional de Energia Nuclear, Centro de Desenvolvimento da Tecnologia Nuclear – CNEN/
CDTN. Av. Presidente Antônio Carlos 6.627, Campus da UFMG, Pampulha, CEP 31270-901 Belo Horizonte (MG). E-mail:
vmfj@cdtn.br
(3)
Bolsista da Comissão Nacional de Energia Nuclear, CNEN/CDTN. E-mail: kapo@cdtn.br
(4)
Pesquisadora Titular da Comissão Nacional de Energia Nuclear, Laboratório de Poços de Caldas –CNEN/LAPOC. Rod. Poços
de Caldas km 13, Andradas, CEP 37701-970 Poços de Caldas (MG). E-mail: mhtaddei@cnen.gov.br
(5)
Bolsista do Departamento de Química da Universidade Federal de São Carlos – UFSCar. Rodovia Washington Luis km 235,
Monjolinho, CEP 13565-905 São Carlos (SP). E-mail: mc.ufscar@gmail.com
(6)
Gerente da Gestão de Resíduos Sólidos, Fundação Estadual do Meio Ambiente – FEAM. Rua Espírito Santo 495, CEP 30160030 Belo Horizonte (MG).E-mail:eleonora.deschamps@meioambiente.mg.gov.br
(7)
IPesquisador Titular da Comissão Nacional de Energia Nuclear, CNEN/LAPOC. E-mail: pmarcos@cnen.gov.br
(8)
Bolsista do Departamento de Solos, Universidade Federal de Viçosa – UFV. Av. Peter Henry Rolfs s/n, CEP 36570-000 Viçosa
(MG). E-mail: davidf.agro@hotmail.com
(9)
Professor Adjunto do Departamento de Solos, UFV. E-mail: jwvmello@ufv.br
(2)
R. Bras. Ci. Solo, 33:1481-1488, 2009
1482
Vanusa Maria Feliciano Jacomino et al.
radionuclides and metals concentrations in PG were lower than in the clayey Oxisol
of Sete Lagoas, Minas Gerais, Brazil. These results indicated that the application
of phosphogypsum as soil amendment in agriculture would not cause a significant
impact on the environment.
Index terms: phosphogysum, soil fertility, radionuclides, metals, contamination.
RESUMO:
CONTEÚDO DE RADIONUCLÍDEOS E METAIS PESADOS EM
AMOSTRAS DE FOSFOGESSO, COMPARATIVAMENTE AO DE
SOLOS DE CERRADO
O fosfogesso ou “gesso agrícola” – um resíduo sólido da indústria de produção de fertilizantes
fosfatados – é usado como condicionador de solos, em especial do solo da região do Cerrado,
Brasil. Entretanto, esse material contém radionuclídeos naturais e metais que podem ser
transferidos para o solo, as plantas e o lençol freático. Neste trabalho são apresentados e
discutidos os resultados de análises químicas e físicas para caracterização do fosfogesso e
compara seus resultados com aqueles encontrados em solos típicos do Cerrado, sendo um
argiloso e outro arenoso. As análises incluíram: a classificação do resíduo sólido, a avaliação
do conteúdo de matéria orgânica e da concentração de P, K, Ca, Mg e Al das amostras de solo,
bem como a composição mineralógica e das concentrações de radionuclídeos naturais, metais
e metaloides presentes nas amostras selecionadas. O fosfogesso foi classificado como Classe II
A - Não Perigoso, Não Inerte, Não Corrosivo e Não Reativo. Os solos analisados apresentaram
baixo conteúdo de matéria orgânica e baixa fertilidade natural. A atividade específica média
do 226Ra (252 Bq kg-1) das amostras de fosfogesso ficou abaixo do limite recomendado pela
Agência Ambiental Americana (USEPA) para uso na agricultura, cujo valor é de 370 Bq kg-1.
Além disso, o estudo mostrou que as concentrações de radionuclídeos naturais e de metais
apresentaram-se inferiores aos valores obtidos para o Latossolo Vermelho-Amarelo de textura
argilosa coletado em Sete Lagoas – MG. Esses resultados indicam que a aplicação do fosfogesso
na agricultura como condicionador do solo não resulta em impacto significativo ao meio
ambiente.
Termos de indexação: fosfogesso, fertilidade do solo, radionuclídeos, metais, contaminação.
INTRODUCTION
The main raw material in Brazilian phosphoric
fertilizer industries is the apatite, of which
approximately 80 % are of igneous origin (Canut, 2006).
The most commonly used process in the production of
phosphoric fertilizers is the attack of the phosphate
rock with concentrated sulfuric acid and water. In
this case, the main products from chemical reactions
are phosphoric acid (H3PO4), and simple superphosphate (SSP). Dehydrated calcium sulphate
(phosphogysum) and hydrofluoric acid (HF) are byproducts of phosphate rock processing.
The phosphogysum (PG) generation rate is
approximately 4.8 t t-1 of phosphoric acid produced.
The annual world production is estimated to be 150
million tons, approximately 12 million tons of which
are produced in Brazil (Mazzilli et al., 2000). At
present, this material is stored in piles near the
factories. However, this practice represents a
potential risk of contamination, mainly to organisms
and hydrological systems in the surroundings of the
storage sites.
R. Bras. Ci. Solo, 33:1481-1488, 2009
The global demand has spurred interest in
processes that generate great amounts of both H3PO4
and chemical fertilizers. However, there are several
fundamental environmental questions regarding PG
disposal that remain unaddressed. Phosphogysum,
although being mainly composed of dehydrated
calcium sulfate, also contains fluoride, heavy metals,
and radionuclides (of the 238U and 232Th natural
radioactive series). These impurities can percolate
through the pile and contaminate groundwater,
eventually causing damage to human health (Mazzilli
et al., 2000). Another problem is 222Rn exhalation,
which can affect people in the surroundings of the
storage sites (Santos et al., 2006), especially workers.
The possibility of using PG in agriculture (FIRP,
1996; El-Mrabet et al., 2003; Papastefanou et al., 2006)
has been the focus of research in several countries
and in Brazil, especially with regard to the agriculture
in the Cerrado (Ribeiro et al., 2000; Ernani et al.,
2001). The soil properties in this region are
compatible with the use of this PG. These are acidic
soils (pH between 4.3 and 6.2) with high levels of
exchangeable Al and low levels of plant-available P;
RADIONUCLIDES AND HEAVY METAL CONTENTS IN PHOSPHOGYPSUM SAMPLES...
they are also poor in Ca and Mg, elements involved in
root growth (Silva &Azevedo, 2002).
According to Kochian et al., 2004, acidity is the
greatest limitation of soils in extensive tropical and
temperate areas.
Toxic concentrations of soluble Al in soils that
negatively influence crops can be avoided with the
addition of acidity correctives, which buffer the soil
pH over 5.0. Rocks that contain alkaline constituents
such as oxides, hydroxides, carbonates and silicates
of Ca, and, or, Mg are commonly employed to
neutralize soil acidity (Bellingieri & Bertin, 2003).
Calcareous materials are included in this category
and are known as limestone that contains calcium
and, or, magnesium carbonates. However, it is more
difficult and expensive for farmers to obtain limestone,
mainly due to its use in more profitable industrial
sectors, such as cement production, metallurgic sites,
building sites, etc.
Various industrial residues have been studied as
possible substitutes for limestone. Among them, the
industrial waste phosphogysum (PG), or “agricultural
gypsum” (AG), has been considered mainly for a
combined application with limestone, to reduce the
effect of subsuperficial acidity on root growth. In
Brazil, PG has been applied as soil conditioner for
several years (Embrapa, 2000; Ernani et al., 2001).
Nevertheless, due to the presence of toxic elements
(e.g., heavy metals, metalloids and radionuclides) in
the substance, and taking into account the
environmental aspects related to the reuse of residues
in agriculture (CETESB, 1999; Andrade & Abreu,
2006), a study has been carried out to evaluate whether
PG application can affect human health and the
environment.
This paper aims to present and discuss the results
of a set of experiments involving the physical,
chemical, and radiological characterization of two soil
types (clayey and sandy) of the Cerrado. These results
are compared with those related to the characterization
of phosphogysum samples collected from a fertilizer
plant in the industrial area of Uberaba, MG, Brazil,
which have been used by the local growers as soil
conditioner.
MATERIALS AND METHODS
Sample collection and pre-treatment
PG samples were collected in a fertilizer plant that
produces H3PO4 by means of a wet method. The
H3PO4 rock used at the industrial site is of igneous
origin and comes from the Alkaline-Carbonate site in
Tapira, MG, Brazil (Canut, 2006). Thirty samples
were collected from the surface of the piles at different
locations, according to the Environmental Protection
Agency guidelines (USEPA, 1988). PG samples were
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then dried in the laboratory at 60 °C for 48 h and
sieved through 30 and 60 mesh (590 and 250 μm).
Afterwards, small fractions of the samples were
mixed, and divided to form a composed sample.
The soil samples were collected from two different
locations in the municipal district of Sete Lagoas
(clayey Yellow Rhodic Ferralsol (Hapludox) - LVSL)
and of João Pinheiro (sandy Rhodic Ferralsol (Hapludox)
- LVTM), that represent typical soils of the Cerrado
region. The samples were air-dried and sieved through
2 mm mesh. In the chemical analysis the following
properties were measured: pH in water (1:2.5); P and
K extractable by Mehlich-1; exchangeable Ca, Mg, and
Al by extraction with 1 mol L -1 KCl; Sum of
Exchangeable Bases (SB); Cation Exchange Capacity
at pH 7.0 (CEC); Base Saturation Index (V);
Aluminum Saturation Index (m), and Organic Matter
Content (OM).
Leaching and dissolution/solubilization tests
A composed PG sample was submitted to leaching
and solubilization tests for waste classification.
Brazilian regulations consider two waste categories:
(1) Class I - dangerous solid waste, with hazardous
properties such as inflammability, corrosiveness,
reactivity, toxicity, and, or, pathogeneticity; (2)
Class II – Harmless solid waste - IIA (non-inert) and
IIB (inert), wastes with none of the above hazardous
properties.
Leaching and dissolution/solubilization were tested
according to ABNT NBR 10005/2004 and 10006/2004
Norms. All substances listed in the F and G Annexes
of ABNT NBR 10004/2004 were analyzed and
classified according to the toxicity by leaching and
dissolution of inorganic and organic substances
(pesticides and other chemicals).
Mineralogical analysis
For the mineralogical analysis, soil and PG
samples were first disassembled by hand on a table
using a cylindrical roll and then sieved through
16 meshes. A fraction of each sample was removed
for mineralogical analysis by X-ray diffraction (powder
method). These analyses were carried out using a
Rigaku X-ray Diffractometer with a Cu tube. The
crystalline phases were identified by comparing
sample diffractograms with those of the International
Center for Diffraction Database (JCPDS, 2000). These
analyses took into account intensities of main
reflections and comparison of diffractograms to
determine the relative concentrations of different
minerals in the samples.
Determination of radionuclide activity
concentrations
The 226Ra, 228Ra, and 210Pb activity concentrations
in PG and soil samples were determined by Gamma
R. Bras. Ci. Solo, 33:1481-1488, 2009
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Vanusa Maria Feliciano Jacomino et al.
Spectrometry using an HPGe detector (relative
efficiency 45 %). The samples were sealed in plastic
containers for 30 days, the period required to ensure
the radioactive balance between 226 Ra and its
daughters, 214Pb and 214Bi. Energy photopeaks at
609, 1,020 and 351 keV were used to determine 226Ra,
corresponding to 214Bi and 214Pb, respectively. A
911 keV energy photopeak for 228Ac (T1/2 = 6.12 h) was
used to determine 228Ra; a 46.5 keV energy photopeak
was used for 210Pb determination.
The 210Po activity concentration was measured
using an Alpha Spectrometry system, by
determination of the powder deposited in a silver plate,
using a reducing HCl and ascorbic acid medium. The
matrix was purified with a specific SR-Spec
chromatographic resin from Eichrom.
The 232Th activity concentration in both PG and
soil samples were determined by the Neutron
Activation Analysis technique (AAN), k0 method
(Menezes et al., 2003), which is a sensitive multielement analytical technique used for both qualitative
and quantitative analyses of major, minor, trace and
rare elements. In this case, 200 mg of each sample
were weighed in polyethylene tubes, sealed and
irradiated in a TRIGA MARK I IPR-R1 reactor (at
the Centro de Desenvolvimento da Tecnologia Nuclear
- CDTN/CNEN), at 100 kW and a thermal neutron
flow equivalent to 6.35 x 1011 neutrons cm-2 s-1, for 8 h.
After irradiation and an appropriate decline time, the
medium and long radionuclide half-life were
determined. Gamma Spectrometry was applied using
an HPGe detector with 15 % efficiency. KAYZERO/
SOLCO software was used to calculate the
concentration of the elements.
The 238U activity concentration was determined
by retarded neutron fission activation, and samples
were irradiated with a flow of thermal neutrons. This
method uses fast irradiation followed by a reading of
retarded neutrons. Samples were irradiated for 50 s,
with decline time of 30 s, and counting took 60 s. An
automated pneumatic system was used for irradiation
and counting and a 10BF3 detector to count retarded
neutrons. Uranium concentration was calculated
based on linear regression, adjusted according to
established patterns.
absorption spectrophotometry coupled with a graphite
furnace (GF-AAS) to determine Cd and Pb.
Calibration solutions were prepared with 1,000 mg L-1
Merck certified solutions. A lake sediment reference
material IAEA-SL-1, supplied by the International
Atomic Energy Agency, was analyzed to check the
accuracy of the method. Values measured were within
the confidence interval (95 %) of the recommended
values.
Total Hg was determined by cold steam generation
coupled with Atomic Absorption Spectrophotometry
(CV-AAS). Samples were processed according to the
adapted procedure of USEPA (1995), which involved
the addition of 1.0 mL of water purified in a Milli-Q®
system and 2.5 mL of a 3:7 mixture of concentrated
nitric and sulfuric acids (0.0000005 % maximum Hg
level) per approximately 200 mg of sample.
These samples were later heated to 75 °C for 4 h.
After that, 4.0 mL of a 5 % (m/v) aqueous potassium
permanganate solution was added along with 2.0 mL
of 5 % (m/v) aqueous potassium persulfate solution.
The excess of permanganate was eliminated by adding
1 mL of 20 % (m/v) hydroxylamine and the final
volume was adjusted to 50.0 mL. An aliquot was
centrifuged for 10 min (2,000 rpm) and the floating
material was analyzed by CV-AAS.
Calibration curves were prepared from a certified
1,000 μg mL-1 Hg Merck standard solution (1.0 to
5.0 ηg mL-1), measuring patterns under the same
conditions as the samples. Method accuracy was
verified by analyzing a CRM 320 reference material,
a river sediment provided by the BCR Community
Bureau of Reference.
As and Se concentrations were analyzed by the
Neutron Activation Analysis (AAN), ko method. In
this case, two 200 mg aliquots of each sample were
weighed in polyethylene tubes and sealed; one tube
was used to determine As and the other Se
concentrations. Afterwards, the samples were sent
to the CDTN/CNEN, for irradiation in the TRIGA
MARK I IPR-R1 reactor, at 100 kW, with a thermal
neutron flow of 6.35 x 1011 neutrons cm-2 s-1, for 8 h,
as described above. The reference material IAEA/Soil7 was analyzed together with the PG and soil samples
to test the accuracy of the method.
Determination of metal and metalloid
concentrations
The metal and metalloids (Ni, Cr, Cd, Pb, Hg, As,
and Se) concentrations in soil and PG samples were
analyzed as described below.
For the determination of Ni, Cr, Cd, and Pb
concentrations, 1.0 g (dry weight) of each sample was
digested with repeated additions of nitric acid (HNO3)
and hydrogen peroxide (H2O2), according to the
USEPA Method 3050B (USEPA, 1996). Inductively
coupled plasma - atomic emission spectrometry (ICPAES) was used to determine Ni and Cr, and atomic
R. Bras. Ci. Solo, 33:1481-1488, 2009
RESULTS AND DISCUSSION
Results from solid waste classification tests showed
that PG was considered neither corrosive nor reagent,
according to ABNT norms. It was classified as noninert waste, based on NBR 10004/2004 (Class II A,
non-dangerous/non-inert) due to the presence of As,
F, Al, Fe, Mn, and sulfates above threshold limits
in the dissolution/solubilization test (Oliveira,
2008).
RADIONUCLIDES AND HEAVY METAL CONTENTS IN PHOSPHOGYPSUM SAMPLES...
Chemical characterization revealed that both soils
were acidic and fertility levels low (Table 1). This is
related to the low nutrient levels of (Ca, Mg, K, and
P), as well as low CEC values. The low base saturation
indexes (V) indicated low proportions of exchangeable
Ca2+, Mg2+, and K+ occupying negative charges sites
of the colloids. Conversely, exchangeable Al3+ was
dominant, as indicated by a relatively high Al3+
saturation index (m > 30 %). The fertility of the clayey
soil was slightly higher, with higher Ca and organic
matter (OM) levels. In general, both soils had high
Al3+ levels and potential acidity. These are typical
characteristic of acidic Cerrado soils, where
exchangeable Al3+ is considered toxic to most cultivated
plants.
Mineralogical analysis revealed that anydrite and
bassanite were the dominant mineral phase in PG
(Table 2). PG has a yellowish white coloration and
the granulometry distribution analyses showed that
it consists mainly of particles with a diameter between
53 and 74 μm (Oliveira, 2008).
The mineralogy of the sandy soil was dominated
by quartz (above 30 %). Granulometry distribution
analyses revealed that this soil contained particles
with a diameter varying from 105 to 250 μm. The
main constituent of the clayey soil was kaolinite (more
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than 30 %), basically composed of fine particles of
approximately 250 μm diameter.
These results corroborated the soil chemical
characterization analyses, which indicated low CEC
(Table 1). The mineralogical composition was typical
of Ferralsols (Hapludox) of the Cerrado region.
Wasserman et al. (2002) demonstrated that the
presence of minerals such as kaolinite and gibbsite
influences the transfer of pollutants in the soil-plant
system. This fact can be related to the low capacity
of these minerals to retain cations. On the other hand,
Fe and Al oxides, such as goethite and gibbsite, may
have high adsorption capacities due to the formation
of “inner sphere” surface type complexes with some
metals and metalloids in the soil, such as As. These
surface complexes are quite stable and can hinder the
transfer of soil pollutants to plants as well as alter
their bioavailability.
The 238U concentration in PG was well below the
concentration found in the clayey soil. It is important
to mention that: (1) PG samples usually have a low
238 U concentration compared to other natural
radionuclides. In phosphoric rock, members of the
natural series of 238U and 232Th are in radioactive
equilibrium. During acid attack to phosphoric rocks,
H3PO4 is enriched with 238U, while 232Th, radium
Table 1. Chemical characterization of the soil samples
Table 2. Results of the mineralogical composition analyses
R. Bras. Ci. Solo, 33:1481-1488, 2009
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Vanusa Maria Feliciano Jacomino et al.
isotopes, and 210Pb tend to concentrate in PG (Mazzilli,
2000); (2) Brazilian phosphoric rock is of igneous
origin, with lower 238U concentrations than 232Th
concentrations; and (3) the clayey soil used in this
study was developed from the weathering of over
2,700 million-year-old granite rocks, typically
containing 5–10 mg kg-1 238U.
In general, the PG generated by H3PO4 industries
in Brazil has natural radionuclide activity
concentrations well below the values observed in other
countries (Silva et al., 2001; Papastefanou et al., 2006).
The highest 232Th activity concentration in relation
to 238U activity concentration can be explained by the
igneous origin of the rock used in the H 3PO 4
production process.
Metal (Cd, Pb, Cr, Ni, and Hg) and metalloid (As
and Se) concentrations in PG and soil samples are
listed in table 4. The values were compared with the
contaminant limits for soil conditioners established
by the Brazilian Agriculture Department (Brazil,
2006), as well as the maximum heavy metal limits in
sewage sludge for use in agriculture (CONAMA, 2006).
All values were below the recommended limits.
Table 3. Natural radionuclide activity concentrations
In all cases, concentrations of toxic elements found
in the clayey soil were higher than in the sandy soil.
This is probably due to the higher CEC and organic
matter contents in the clay soil. The concentrations
of toxic elements in PG were generally lower than in
the soils studied, especially in the clayey soil.
Concentrations of toxic elements in PG samples were
below the limits established by MAPA Normative
Instruction SDA number 27 (Brazil, 2006).
As previously mentioned, the analytical results
were verified by analyzing certified reference materials
as quality control. Experimental results were similar
to the reference values, indicating the accuracy of the
methods.
CONCLUSIONS
1. This study showed that PG is classified as nonHazardous, non-Inert waste (Class IIA), with As, F,
Al, Fe, Mn, and sulfate concentrations in the
solubilization test above the maximum limit
established by ABNT Norm 10004/2004. Both soil
types (sandy and clayey Ferralsols) are acidic with
low fertility and the Al concentration level was
considered toxic to plants.
2. Mineralogical and granulometric analyses
indicated that anydrite and bassanite are the
dominant phases in PG in particles varying from 53
to 74 μm diameter. The sandy soil consists of quartz
(> 30 %). The main constituent of the clayey soil is
kaolinite (> 30 %).
3. The 226Ra (251 Bq kg-1) and 228Ra (226 Bq kg-1)
activity concentrations in PG samples were well below
the EPA limits recommended for agricultural purposes
(USEPA, 1988). The 210 Pb and 210 Po activity
concentration values were similar to the values found
for 226Ra and 228Ra. The 238U concentration in PG
samples was below the concentration found in the
Table 4. Metal and metalloid concentration analyses
R. Bras. Ci. Solo, 33:1481-1488, 2009
RADIONUCLIDES AND HEAVY METAL CONTENTS IN PHOSPHOGYPSUM SAMPLES...
clayey soil. 232Th activity concentration was higher
than 238U, due to igneous origin of the rock which is
used in the production of H3PO4. The activity
concentration for natural radionuclides was higher
in the clayey than in the sandy soil. The same was
observed regarding the concentration of other toxic
elements. These results were related to
physicochemical characteristics of the soil samples.
In addition, all heavy metal and metalloid
concentrations were lower than the limits established
by the Brazilian Agriculture Department (Brazil, 2006)
and by the National Environmental Council
(CONAMA, 2006). These results, together with those
related to the concentration of natural radionuclides,
indicate that, in principle, the use of PG for agriculture
in the Cerrado does not represent a significant impact
on the environment.
ACKNOWLEDGEMENTS
This study was funded by the company Fosfertil
and Conselho Nacional de Desenvolvimento Científico
e Tecnológico (CNPq), grants nº 410021/2006-7. The
authors would like to thank Dr Helena Eugênia L.
Palmieri and Dr Maria Ângela Braaros Menezes of
the Centro de Desenvolvimento da Tecnologia Nuclear
for determining the Hg concentration and for
performing the Neutron Activation Analysis, as well
as the Fundação de Amparo à Pesquisa do Estado de
Minas Gerais (FAPEMIG) for the financial support
for publishing this paper.
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