Abstracts
Sugarcane is an important crop for sugar and biofuel production in Brazil. Growers depend greatly on herbicides to produce it. This experiment used herbicide physical-chemical and sugarcane plant physiological properties to simulate herbicide uptake and estimate the bioconcentration factor (BCF). The (BCF) was calculated for the steady state chemical equilibrium between the plant herbicide concentration and soil solution. Plant-water partition coefficient (sugarcane bagasse-water partition coefficient), herbicide dilution rate, metabolism and dissipation in the soil-plant system, as well as total plant biomass factors were used. In addition, we added Tebuthiuron at rate of 5.0kg a.i. ha-1 to physically test the model. In conclusion, the model showed the following ranking of herbicide uptake: sulfentrazone > picloram >tebuthiuron > hexazinone > metribuzin > simazine > ametryn > diuron > clomazone > acetochlor. Furthermore, the highest BCF herbicides showed higher Groundwater Ubiquity Score (GUS) index indicating high leaching potential. We did not find tebuthiuron in plants after three months of herbicide application
herbicide; bioconcentration factor; plant uptake; model; bagasse adsorption
A cana de açúcar é uma cultura importante para produção de açúcar e biocombustíveis no Brasil e exige elevada utilização de herbicidas. Utilizamos modelo matemático para ajudar na compreensão da absorção de herbicida dessa cultura. Propriedades físico-químicas dos herbicidas e propriedades fisiológicas das plantas de cana foram usados para estimar a absorção e também o fator de bioconcentração, bioconcentration factor (BCF), calculado para o equilíbrio químico entre a concentração do herbicida na planta e na solução do solo. O coeficiente de partição planta/água, a taxa de diluição de herbicida, o metabolismo e a dissipação no sistema solo-planta e biomassa total das plantas foram adicionados ao modelo. O herbicida tebuthiuron aplicado ao solo na dose de 5,0kg ha-1 i.a. foi utilizado para testar o modelo. A absorção dos herbicidas mostrada pelo modelo indicou em ordem o seguinte: sulfentrazone> picloram> tebuthiuron> hexazinone> metribuzin> simazina> ametryn> diuron> clomazone> acetochlor. Esses herbicidas com alto índice (BCF) também apresentaram alto índice de potencial de lixiviação para água subterrânea "Groundwater Ubiquity Score" (GUS). Tebuthiuron não foi encontrado nas plantas após três meses de aplicação
herbicida; fator de bioconcentração; absorção; estimativa; adsorção bagaço
INTRODUCTION:
In Brazil, sugarcane area encompasses more than 5 million hectares. Farmers produce more
than 570 million tons of sugarcane yearly. The industry uses this crop to produce sugar,
alcohol and other derivates (ALCOPAR, 2012ALCOPAR. Estatísticas: histórico de produção da cana-de-açúcar no Brasil
e no Paraná. 2012. Available from: <http://www.alcopar.org.br>. Accessed: Mar.
20, 2012.
http://www.alcopar.org.br...
). This
production level reflects the sugarcane favorable climate in many parts of Brazil.
However, those same environmental conditions also favor the growth of several weed
species, and growers use herbicides to manage them.
Sugarcane requires large amounts of water during its vegetative cycle and has a high
transpiration rate. Normally water is also the vehicle of herbicide uptake by the
plants. The herbicides used in sugarcane cultivation have high water solubility, long
soil half-life, and high accumulation potential in plants (TRAPP, 1995TRAPP, S.; MATTHIES, M. Generic one-compartment model for uptake of
organic-chemicals by foliar vegetation. Environmental Science & Technology, v.29,
n.9, p.2333-2338, 1995. Available from:
<http://pubs-acs-org.ez103.periodicos.capes.gov.br/doi/abs/10.1021/es00009a027>.
Accessed: Oct. 3, 2014, DOI: 10.1021/es00009a027.
http://pubs-acs-org.ez103.periodicos.cap...
; COUSINS & MACKAY,
2001COUSINS, I.T.; MACKAY, D. Strategies for including vegetation
compartments in multimedia models. Chemosphere, v.44, n.4, p.643-654, 2001. Available
from:
<http://www-sciencedirect-com.ez103.periodicos.capes.gov.br/science/article/pii/S0045653500005142>.
Accessed: Oct. 3, 2014. DOI: 10.1016/S0045-6535(00)00514-2.
http://www-sciencedirect-com.ez103.perio...
).
Mathematical models predict plant herbicide concentrations (TRAPP, 1995TRAPP, S.; MATTHIES, M. Generic one-compartment model for uptake of
organic-chemicals by foliar vegetation. Environmental Science & Technology, v.29,
n.9, p.2333-2338, 1995. Available from:
<http://pubs-acs-org.ez103.periodicos.capes.gov.br/doi/abs/10.1021/es00009a027>.
Accessed: Oct. 3, 2014, DOI: 10.1021/es00009a027.
http://pubs-acs-org.ez103.periodicos.cap...
). Several models simulate any substance uptake by plants
(TRAPP & MATTHIES, 1995TRAPP, S.; MATTHIES, M. Generic one-compartment model for uptake of
organic-chemicals by foliar vegetation. Environmental Science & Technology, v.29,
n.9, p.2333-2338, 1995. Available from:
<http://pubs-acs-org.ez103.periodicos.capes.gov.br/doi/abs/10.1021/es00009a027>.
Accessed: Oct. 3, 2014, DOI: 10.1021/es00009a027.
http://pubs-acs-org.ez103.periodicos.cap...
; FUJISAWA et al., 2002FUJISAWA, T. et al. Improved uptake models of nonionized pesticides to
foliage and seed of crops. Journal of Agricultural and Food Chemistry, v.50, n.3,
p.532-537, 2002. Available from:
<http://pubs-acs-org.ez103.periodicos.capes.gov.br/doi/abs/10.1021/jf010985j>.
Accessed: Oct. 3, 2014. DOI: 10.1021/jf010985j.
http://pubs-acs-org.ez103.periodicos.cap...
; TRAPP et al., 2003TRAPP, S. et al. Fruit tree model for uptake of organic compounds from
soil. Sar and Qsar in Environmental Research, v.14, n.1, p.17-26, 2003. Available
from: <http://www.ncbi.nlm.nih.gov/pubmed/17514576>. Accessed: Oct. 3, 2014.
DOI: 10.1080/10629360701303693.
http://www.ncbi.nlm.nih.gov/pubmed/17514...
; TRAPP,
2007_____. Fruit Tree model for uptake of organic compounds from soil and
air. Sar and Qsar in Environmental Research, v.18, n.3-4, p.367-387, 2007. Available
from: <http://www.ncbi.nlm.nih.gov/pubmed/17514576>. Accessed: Oct. 3, 2014.
DOI: 10.1080/10629360701303693.
http://www.ncbi.nlm.nih.gov/pubmed/17514...
; PARAIBA & KATAGUIRI, 2008PARAIBA, L.C.; KATAGUIRI, K. Model approach for estimating potato
pesticide bioconcentration factor., Chemosphere v.73, n.8, p.1247-1252, 2008.
Available from:
<http://www-sciencedirect-com.ez103.periodicos.capes.gov.br/science/article/pii/S0045653508009156>.
Accessed: Oct. 3, 2014. DOI: 10.1016/j.chemosphere.2008.07.026.
http://www-sciencedirect-com.ez103.perio...
).
Some researchers developed models to simulate specific substance uptake by leaves (TRAPP, 1995TRAPP, S.; MATTHIES, M. Generic one-compartment model for uptake of
organic-chemicals by foliar vegetation. Environmental Science & Technology, v.29,
n.9, p.2333-2338, 1995. Available from:
<http://pubs-acs-org.ez103.periodicos.capes.gov.br/doi/abs/10.1021/es00009a027>.
Accessed: Oct. 3, 2014, DOI: 10.1021/es00009a027.
http://pubs-acs-org.ez103.periodicos.cap...
), roots and tubers, (TRAPP et al., 2003TRAPP, S. et al. Fruit tree model for uptake of organic compounds from
soil. Sar and Qsar in Environmental Research, v.14, n.1, p.17-26, 2003. Available
from: <http://www.ncbi.nlm.nih.gov/pubmed/17514576>. Accessed: Oct. 3, 2014.
DOI: 10.1080/10629360701303693.
http://www.ncbi.nlm.nih.gov/pubmed/17514...
; PARAIBA & KATAGUIRI, 2008PARAIBA, L.C.; KATAGUIRI, K. Model approach for estimating potato
pesticide bioconcentration factor., Chemosphere v.73, n.8, p.1247-1252, 2008.
Available from:
<http://www-sciencedirect-com.ez103.periodicos.capes.gov.br/science/article/pii/S0045653508009156>.
Accessed: Oct. 3, 2014. DOI: 10.1016/j.chemosphere.2008.07.026.
http://www-sciencedirect-com.ez103.perio...
) or by roots and leaves (FUJISAWA et al., 2002FUJISAWA, T. et al. Improved uptake models of nonionized pesticides to
foliage and seed of crops. Journal of Agricultural and Food Chemistry, v.50, n.3,
p.532-537, 2002. Available from:
<http://pubs-acs-org.ez103.periodicos.capes.gov.br/doi/abs/10.1021/jf010985j>.
Accessed: Oct. 3, 2014. DOI: 10.1021/jf010985j.
http://pubs-acs-org.ez103.periodicos.cap...
; TRAPP,
2007_____. Fruit Tree model for uptake of organic compounds from soil and
air. Sar and Qsar in Environmental Research, v.18, n.3-4, p.367-387, 2007. Available
from: <http://www.ncbi.nlm.nih.gov/pubmed/17514576>. Accessed: Oct. 3, 2014.
DOI: 10.1080/10629360701303693.
http://www.ncbi.nlm.nih.gov/pubmed/17514...
). However, none of these models estimate the herbicide bioconcentration
factor and uptake in sugarcane.
The bioconcentration factor of a substance (BCF) in an organism is a
coefficient that describes the increase in concentration of herbicides in the organism
in relation to concentration in the medium, estimated by the limit in time in the
chemical steady state equilibrium. In case of plants for food, the BCF
permits scientists to evaluate the human's daily ingestion of pesticide establishing
safe limits for concentration in the medium (PARAIBA
& KATAGUIRI, 2008PARAIBA, L.C.; KATAGUIRI, K. Model approach for estimating potato
pesticide bioconcentration factor., Chemosphere v.73, n.8, p.1247-1252, 2008.
Available from:
<http://www-sciencedirect-com.ez103.periodicos.capes.gov.br/science/article/pii/S0045653508009156>.
Accessed: Oct. 3, 2014. DOI: 10.1016/j.chemosphere.2008.07.026.
http://www-sciencedirect-com.ez103.perio...
). This research used this model to estimate sugarcane
herbicide absorption from soil solution. This work evaluated the BCF of
herbicide in sugarcane through physical and chemical properties of herbicides and
physiological characteristics of the crop. We chose tebuthiuron as an indicator because
it is commonly used in sugarcane cultivation in Brazil and has high mobility (CERDEIRA et al., 2007CERDEIRA, A.L. et al. Leaching and half-life of the herbicide
tebuthiuron on a recharge area of Guarany aquifer in sugarcane fields in Brazil.
Journal of Environmental Science and Health Part B-Pesticides Food Contaminants and
Agricultural Wastes, v.42, n.6, p.635-639, 2007. Available from:
<http://www.ncbi.nlm.nih.gov/pubmed/17701698>. Accessed: Oct. 3, 2014. DOI:
10.1080/03601230701465593.
http://www.ncbi.nlm.nih.gov/pubmed/17701...
).
MATERIALS AND METHODS:
The BCF model
This paper's estimation of sugarcane herbicide bioconcentration relies on several
assumptions: degradation in the soil and its metabolism and dilution in the plant are
described by first order kinetic equation; the plant uses the water transpiration stream
to uptake the herbicide from the soil solution; soil solution herbicide present in
concentrations that are available for plant uptake; and the plant distributes the
herbicide throughout itself by transpiration. These assumptions reflect the results of
several studies (TRAPP, 2007; REIN et al., 2011REIN, A. et al. New concepts for dynamic plant uptake models. Sar and
Qsar in Environmental Research, v.22, n.1-2, p.191-215, 2011. Available from:
<http://www.ncbi.nlm.nih.gov/pubmed/21391147>. Accessed: Oct. 3, 2014. DOI:
10.1080/1062936X.2010.548829.
http://www.ncbi.nlm.nih.gov/pubmed/21391...
;
TRAPP & LEGIND, 2011TRAPP, S.; LEGIND, C.N. Uptake of organic contaminants from soil into
vegetables and fruits. In:; __________ (Ed.). Dealing with contaminated sites: from
theory towards practical application, 2011. p.369-408. ISBN
978-90-481-9756-9.; TRAPP & EGGEN, 2013TRAPP, S.; EGGEN, T. Simulation of the plant uptake of organophosphates
and other emerging pollutants for greenhouse experiments and field conditions.
Environmental Science and Pollution Research, v.20, n.6, p.4018-4029, 2013. Available
from: <http://www.ncbi.nlm.nih.gov/pubmed/23212267>. Accessed: Oct. 3, 2014.
DOI: 10.1007/s11356-012-1337-7.
http://www.ncbi.nlm.nih.gov/pubmed/23212...
).
We obtained the BCF for the steady state chemical equilibrium, which we estimated using the time limit of the quotient between the plant herbicide and the soil solution herbicide concentration. We calculated the total balance of the herbicide's mass through the following equations:
(TRAPP et al., 2003TRAPP, S. et al. Fruit tree model for uptake of organic compounds from
soil. Sar and Qsar in Environmental Research, v.14, n.1, p.17-26, 2003. Available
from: <http://www.ncbi.nlm.nih.gov/pubmed/17514576>. Accessed: Oct. 3, 2014.
DOI: 10.1080/10629360701303693.
http://www.ncbi.nlm.nih.gov/pubmed/17514...
; PARAIBA, 2007PARAIBA, L.C. Pesticide bioconcentration modelling for fruit trees.,
Chemosphere v.66, n.8, p.1468-1475, 2007. Available from:
<http://www-sciencedirect-com.ez103.periodicos.capes.gov.br/science/article/pii/S0045653506011878>.
Accessed: Oct. 3, 2014. DOI: 10.1016/j.chemosphere.2006.09.017.
http://www-sciencedirect-com.ez103.perio...
; PARAIBA et al.,
2010PARAIBA, L.C. et al. Bioconcentration factor estimates of polycyclic
aromatic hydrocarbons in grains of corn plants cultivated in soils treated with
sewage sludge. Science of the Total Environment, v.408, n.16, p.3270-3276, 2010.
Available from:
<http://www-sciencedirect-com.ez103.periodicos.capes.gov.br/science/article/pii/S0048969710004043>.
Accessed: Oct. 3, 2014. DOI: 10.1016/j.scitotenv.2010.04.026.
http://www-sciencedirect-com.ez103.perio...
), where M
P (kg ha-1) is the total plant fresh biomass, Q (L
day-1 ha-1) is the plant transpiration rate,
TSCF
soil is the herbicide concentration factor in the transpiration stream in relation
to the soil solution, C
W (mg L-1) is the soil solution herbicide concentration,
k
E (day-1) is the herbicide transformation rate in the plant,
k
G (day-1) is the plant growth rate, C
P (mg kg-1) is the plant herbicide concentration, and
K
PW (L kg-1) is the herbicide plant-water sorption coefficient or
herbicide bagasse-water sorption coefficient measured as described in the sorption
experiments.
The TSCF
soil was estimated from the herbicide octanol-water partition coefficient using the
equation, given by (BURKEN & SCHNOOR, 1998BURKEN, J.G.; SCHNOOR, J.L. Predictive relationships for uptake of
organic contaminants by hybrid poplar trees. Environmental Science & Technology,
v.32, n.21, p.3379-3385, 1998. Available from:
<http://pubs-acs-org.ez103.periodicos.capes.gov.br/doi/abs/10.1021/es9706817>.
Accessed: Oct. 3, 2014. DOI: 10.1021/es9706817.
http://pubs-acs-org.ez103.periodicos.cap...
)
where logK OW is the logarithm of herbicide octanol-water partition coefficient and TSCF is the concentration factor of pesticide in the transpiration stream without interference of soil elements. The present paper calculates the TSCF soil is developed by
(NICHOLLS, 1984NICHOLLS, P.H. Physicochemical evaluation: the environment, and expert system for pesticide preregistration assessment. In: BRIGHTON CROP PROTECTION CONFERENCE: PESTS AND DISEASES, 1984, Brighton, UK. Proceedings... Brighton: British Crop Protection Council, 1984. p.1337-1342.), where K OC (L kg-1) is the herbicide sorption coefficient in the soil organic carbon, f OC (g g-1) is the soil organic carbon volumetric fraction,
(kg L-1) is the total soil density, and (g g-1) is the soil water volumetric content.The experiment measured the herbicide plant-water partition coefficient K PW (L kg-1) as follows (Table 1). Initially, it was determined the sorption coefficient of the herbicides in the bagasse (dry pulp that remains after juice extraction) for the development of the model that describes the mass balance of the herbicide in the soil-plant in sugarcane. We selected the following herbicides to evaluate the BCF: acetochlor, ametryn, clomazone, diuron, hexazinone, metribuzin, picloram, simazine, sulfentrazone and tebuthiuron. For this experiment we first washed and processed the sugarcane cane in a manual mill to separate the juice from the bagasse. We then dried the resulting bagasse in an oven with air circulation at 60°C for 72h. After complete drying, we grounded the residue using a knife mill. Then we washed the bagasse with distilled water and filtered it through qualitative filter paper (Whatman n. 1), using a Büchner funnel. After washing, we again dried the residue under the same conditions described above and sieved in a 1mm sieve.
The soil solution herbicide concentration followed a first order kinetic equation given by C W (t) = C W(0) exp (-k S t), where C W(0) (mg L-1), C W = C W(t) (mg L-1) and k S (day-1) are the initial soil solution herbicide concentration, the current soil solution herbicide concentration, and the herbicide dissipation rate in the soil, respectively. The equation
describes the plant herbicide concentration, where
(L kg day-1 ha-1) and
(day-1). The A constant is the herbicide uptake rate by plants and the B constant is herbicide dilution and metabolic rate in the plants.
We calculated the bioconcentration factor for the steady state chemical equilibrium by the time limit of the quotient between the plant herbicide and soil solution herbicide concentrations, as follows
where BCF (L kg-1) is the plant herbicide bioconcentration
factor and k
EGS
= k
E
+ k
G- k
S (day-1) is the herbicide dilution rate, metabolism and dissipation
in the soil-plant system (PARAIBA, 2007PARAIBA, L.C. Pesticide bioconcentration modelling for fruit trees.,
Chemosphere v.66, n.8, p.1468-1475, 2007. Available from:
<http://www-sciencedirect-com.ez103.periodicos.capes.gov.br/science/article/pii/S0045653506011878>.
Accessed: Oct. 3, 2014. DOI: 10.1016/j.chemosphere.2006.09.017.
http://www-sciencedirect-com.ez103.perio...
; PARAIBA et al., 2010PARAIBA, L.C. et al. Bioconcentration factor estimates of polycyclic
aromatic hydrocarbons in grains of corn plants cultivated in soils treated with
sewage sludge. Science of the Total Environment, v.408, n.16, p.3270-3276, 2010.
Available from:
<http://www-sciencedirect-com.ez103.periodicos.capes.gov.br/science/article/pii/S0048969710004043>.
Accessed: Oct. 3, 2014. DOI: 10.1016/j.scitotenv.2010.04.026.
http://www-sciencedirect-com.ez103.perio...
).
Previous research provided the herbicide half-life time values and the soil sorption coefficients (HORNSBY et al., 1995HORNSBY, A.G. et al. Pesticide properties in the environment. New York: Springer-Verlag, 1995. 277p.). This experiment uses the half-life time to estimate the degradation rate, k S values, through the expression
, where t 1/2 (day) is the soil herbicide half-life (HORNSBY et al., 1995HORNSBY, A.G. et al. Pesticide properties in the environment. New York: Springer-Verlag, 1995. 277p.). Although polar herbicides K oc varies with soil pH with geographical regions, normally crop soils pHs are adjusted to near 6 worldwide, thus validating the possible use of literature for this estimation (OLIVEIRA et al., 2001OLIVEIRA, R.S. et al. Sorption and leaching potential of herbicides on Brazilian soils. Weed Research, v.41, n.2, p.97-110, 2001. Available from: <http://onlinelibrary-wiley-com.ez103.periodicos.capes.gov.br/doi/10.1046/j.1365-3180.2001.00219.x/abstract;jsessionid=FDF9396E2DDBF647125925011587ABD2.f02t01> Accessed: Oct. 3, 2014. DOI: 10.1046/j.1365-3180.2001.00219.x.http://onlinelibrary-wiley-com.ez103.per... ).
Sorption kinetics and chromatographic analysis of the herbicides
The equation
determines the sorption coefficients, where C P (mg µg-1) is the concentration of the herbicides the bagasse sorbed, C W (mg mL-1) is the concentration in water solution, and K PW is the partition coefficient bagasse-water of the herbicides measured. The equation provides the coefficients, where X (mg) is the amount of herbicide sorbed, and M (µg) is the weight of bagasse and C fW (mg mL-1) is the final concentration of herbicide in solution. The formula measures the amount of herbicide sorbed, where C iW (mg mL-1) is the initial concentration of the herbicide in solution and V (mL) is the volume of solution (Table 1) (TRAPP et al., 2001TRAPP, S. et al. Sorption of lipophilic organic compounds to wood and implications for their environmental fate., Environmental Science & Technology v.35, n.8, p.1561-1566, 2001. Available from: <http://pubs-acs-org.ez103.periodicos.capes.gov.br/doi/abs/10.1021/es000204f>. Accessed: Oct. 3, 2014., DOI: 10.1021/es000204f.http://pubs-acs-org.ez103.periodicos.cap... ).
We performed sorption kinetics and isotherms curves for each individual herbicide. We kept the rate of bagasse:solution at 1:15 (1g bagasse - 15mL of solution of 1µg mL-1 of herbicide in water), and maintained the samples at a temperature of 25°C, shaking at 185rpm. To quantify and determine the point on time of equilibrium of herbicide absorbed, took the aliquots and filtered in 0.45μm sieve for herbicide at regular intervals. The initial time of the experiment (t = 0) was when we applied the herbicide to the media. We performed the analyses on HPLC using a UV-Vis detector (Shimadzu, model SPD 10AVvp), Supelco-Lichosorb RP-18.5µm (250mm-4.6mm) column, flow rate of 1.0mL min-1, room temperature and injection volume of 20µL. Each herbicide had a specific chromatographic conditions (mobile phase and UV wavelength). All herbicides reached equilibrium within a period of 24 hours. For construction of the isotherms, seven standard concentrations, ranging from 0.1 to 8.0µg mL-1, in duplicate were used.
Tebuthiuron sugarcane uptake experiment
To conduct the experiment we used containers of 40×103cm3 capacity with organic sugarcane so as to avoid interference with other chemicals watered to maintain at field capacity, IAC2480 variety. The experiment was conducted at pH 5.9 (CaCl2), soil organic carbon of 0.012g g-1, total soil density of 1.3kg L-1; and soil moisture of 0.28g g.-1 The herbicide tebuthiuron was applied at of 5.0kg a.i. ha-1, a rate that although higher than the recommended application rate did not cause phytotoxicity to the plant. We used higher rates to simulate a worst case scenario. We harvested the sugarcane every three months after application in order to quantify the herbicide until 20 months old.
RESULTS AND DISCUSSION:
We applied the model to the sugarcane with the following soil and plant parameters: soil
organic carbon=0.012g g-1; total soil density = 1.3kg L-1; soil
humidity =0.28g g-1; total plant fresh biomass = 80,000kg ha-1;
average plant transpiration rate = 32,000L day-1 ha-1; and average
relative plant growth rate = 0.05 day-1 (CABRAL et al., 2003CABRAL, O.M.R. et al. Fluxos turbulentos de calor sensível, vapor de
água e CO2 sobre plantação de cana-de-açúcar (Saccharum sp.) em Sertãozinho-SP.
Revista Brasileira de Meteorologia, v.18, n.1, p.61-70, 2003.). The plant herbicide metabolism rate k
E was calculated by k
E
= k
S
/16 (JURASKE et al., 2008JURASKE, R. et al. Estimating half-lives of pesticides in/on vegetation
for use in multimedia fate and exposure models. Chemosphere, v.70, n.10, p.1748-1755,
2008. Available from:
<http://www-sciencedirect-com.ez103.periodicos.capes.gov.br/science/article/pii/S0045653507010843>.
Accessed: Oct. 3, 2014. DOI: 10.1016/j.
http://www-sciencedirect-com.ez103.perio...
).
The data (Table 1) were used to produce two curves correlation of herbicide properties such as water solubility, partition coefficient octanol-water, and sorption coefficient in bagasse. The equations log K PW =0.45+0.46xlogK OW, with P<0.001, n=8, correlation coefficient R - s =79.75%, standard deviation=0.15 (Equation 1) and logK PW = 0.29+0.046xlogSol+0.54x logK OW, with P<0.001, n = 8, correlation coefficient R-s=82.93%, standard deviation =0.155 (Equation 2), were developed.
Equations 1 and 2 show the relationship of the sorption coefficient plant/water or bagasse/water with the coefficient octanol-water. Due to the high solubility of the herbicides picloram and acetochlor, the analytical method did not work as for the other herbicides. For this reason, we used the equations to estimate sorption coefficient bagasse/water for the herbicides: acetochlor (Sol=0.233g L-1 and logK OW =4.14) and picloram (Sol=0.430g L-1 and logK OW =19). Sorption coefficient bagasse-water (K PW) of acetochlor was 230 (Equation 1) and 313L kg-1 (Equation 2). Picloram was 21 (Equation 1) and 19L kg-1 (Equation 2). The Pesticide Manual provided the values of Sol (soubility) and logK OW (octanol-water partition coefficient) of the herbicides (TOMLIN, 2000TOMLIN, C.D.S. The pesticide manual: a world compendium. 12. Farnham, Surray, United Kingdom: British Crop Protection Council and Royal Society of Chemistry, 2000. 1250p.). The values obtained from Equation 1 for picloram and acetochlor were used to feed the model.
Sugarcane herbicide BCF varied between 0.0081L kg-1 (acetochlor) and 0.8570L kg-1 (sulfentrazone) (Table 2). Based on the BCF values measured (Table 2), we concluded that the herbicides that would most probably be found in plant are sulfentrazone > picloram > tebuthiuron > hexazinone > metribuzin > simazine> ametryn > diuron > clomazone > acetochlor. Herbicides with small K PW and K OC and high TSCF soil are the ones with the highest BCF, such as sulfentrazone, picloram, tebuthiuron, hexazinone, and metribuzin (Table 2). The herbicides ametryn, diuron, clomazone, and acetochlor have the lowest BCF and have in common high K OC, K OW , K PW, and low TSCF soil (Table 2).
The BCF (Table 2) values permit an estimation of the herbicide daily intake (DI), per body weight, from sugarcane products consumption establishing acceptable limits. For example, a soil solution containing 1.0mg L-1 (C W) of tebuthiuron leads to a sugarcane concentration (C P) of 0.4159mg kg-1 (C P =BCFxC W ) and a daily herbicide intake of 0.003 mg kg-1 (mg of tebuthiuron per kg body weight) considering a 70kg person with a daily sugarcane juice consumption (DI) of 0.5kg, measured by DI=0.5X C P /70.
Besides being absorbed, herbicides could leach to groundwater. We can estimate the
herbicide leaching potential through the GUS index (GUSTAFSON, 1989GUSTAFSON, D.I. Groundwater ubiquity score - a simple method for
assessing pesticide leachability. Environmental Toxicology and Chemistry, v.8, n.4,
p.339-357, 1989. Available from:
<http://onlinelibrary.wiley.com/doi/10.1002/etc.5620080411/abstract>. Accessed:
Oct. 3, 2014. DOI: 10.1897/1552-8618(1989)8[339:GUSASM]2.0.CO;2.
http://onlinelibrary.wiley.com/doi/10.10...
), given by GUS=(4-log K
OC) x log t
1/2. Depending on the GUS index numerical value, we may classify the
herbicide as a high leaching potential (GUS≥2.8), non-leaching
(GUS≤1.8)or with undetermined leaching potential (transient)
(1.8GUS<2,8). Therefore, herbicides with low soil sorption
coefficients and high water solubility, are classified as leaching pesticides because of
their GUS index values (Table
2). High soil herbicide sorption makes them adsorbed in soil matrix, and thus,
unavailable for lixiviation or plant uptake (TRAPP,
1995TRAPP, S.; MATTHIES, M. Generic one-compartment model for uptake of
organic-chemicals by foliar vegetation. Environmental Science & Technology, v.29,
n.9, p.2333-2338, 1995. Available from:
<http://pubs-acs-org.ez103.periodicos.capes.gov.br/doi/abs/10.1021/es00009a027>.
Accessed: Oct. 3, 2014, DOI: 10.1021/es00009a027.
http://pubs-acs-org.ez103.periodicos.cap...
). Six of the ten studied herbicides (60%) are potentially leaching. One
(10%) is a non-leaching herbicide and others transient (Table 2). Overall herbicides with the highest BCF or
potential to be absorbed are also the ones with highest potential to leach to
groundwater (Table 2).
Tebuthiuron bioconcentration
We applied Tebuthiuron to the soil at 5.0kg a.i. ha-1, a rate higher than the
recommended, without causing phytotoxicity to the plant. We harvested the plants every
three months after application for herbicide quantification. We were able to measure and
detect the herbicide only during the first harvesting date, three months after
application, at a level of 0.5 mg kg-1. Although, in our study, the herbicide
had a significant BCF of 0.42, it also had a high GUS
index rating, 5.36, indicating that most likely it would leach or break down (SILVA et al., 2010SILVA, M.R.A. et al. Photo-Fenton degradation of the herbicide
tebuthiuron under solar irradiation: Iron complexation and initial intermediates.
Water Research, v.44, n.12, p.3745-3753, 2010. Available from:
<http://www-sciencedirect-com.ez103.periodicos.capes.gov.br/science/article/pii/S0043135410002782>.
Accessed: Oct. 3, 2014. DOI: 10.1016/j.watres.2010.04.025.
http://www-sciencedirect-com.ez103.perio...
) before the plant uptakes the
herbicide further.
Other authors found tebuthiuron residues in adult sugarcane plants cultivated in soils
treated with 2.2kg. a.i. ha-1 (recommended rate) and 11.2kg ha-1
with herbicide residue concentrations of 0.074 and 0.026µg g-1, respectively
(CAUX et al., 1997CAUX, P.Y. et al. Canadian water quality guidelines for tebuthiuron.
Environmental Toxicology and Water Quality, v.12, n.1, p.61-95, 1997. DOI:
10.1002/(SICI)1098-2256(1997)12:1<61::AID-TOX9>3.0.CO;2-6.
https://doi.org/10.1002/(SICI)1098-2256(...
). In this same experiment,
the authors' found in their sugarcane bagasse, juice and syrup (11.2kg ha-1)
contained concentrations of 0.063, 0.076 and 0.193µg g-1, respectively. They
found no residues in the sugar obtained from these plants. However, researchers detected
residues of three tebuthiuron metabolites in grasses and bushes grown in soils of a
semiarid region of Arizona, 11 years after herbicide application (JOHNSEN & MORTON, 1991JOHNSEN, T.N.; MORTON, H.L. Long-term Tebuthiuron content of grasses and
shrubs on semiarid rangelands. Journal of Range Management, v.44, n.3, p.249-253,
1991. Available from:
<https://journals.uair.arizona.edu/index.php/jrm/article/view/8598> Accessed:
Oct. 3, 2014. DOI: 10.2307/4002952.
https://journals.uair.arizona.edu/index....
). We found no other literature regarding
experiments on the application of tebuthiuron to sugarcane plants similar to those
reported by CAUX et al. (1997CAUX, P.Y. et al. Canadian water quality guidelines for tebuthiuron.
Environmental Toxicology and Water Quality, v.12, n.1, p.61-95, 1997. DOI:
10.1002/(SICI)1098-2256(1997)12:1<61::AID-TOX9>3.0.CO;2-6.
https://doi.org/10.1002/(SICI)1098-2256(...
) and JOHNSEN & MORTON (1991JOHNSEN, T.N.; MORTON, H.L. Long-term Tebuthiuron content of grasses and
shrubs on semiarid rangelands. Journal of Range Management, v.44, n.3, p.249-253,
1991. Available from:
<https://journals.uair.arizona.edu/index.php/jrm/article/view/8598> Accessed:
Oct. 3, 2014. DOI: 10.2307/4002952.
https://journals.uair.arizona.edu/index....
).
Another practical information generated by this study is related to herbicide efficacy for weed control, since herbicides with high K PW such as measured in this study (Table 2) can be less efficient for weed control. These herbicides are pre plant soil applied over a straw left by mechanical harvesting of sugarcane, are adsorbed by the straw residues left and therefore failing to reach the weed seeds that should be controlled (CORREIA et al., 2007CORREIA, N.M. et al. Crop residues aging on soil and their effects on pre-emergence herbicides. Bragantia, v.66, n.1, p.101-110, 2007.).
CONCLUSION:
This study presents a model equation to estimate the BCF. This equation depends directly on the herbicide concentration factor in the transpiration stream, the transpiration stream volume and the plant-water partition coefficient; and indirectly on transpiration stream volume, herbicide dilution rate, metabolism and dissipation in the soil-plant system, the plant-water partition coefficient and the plant biomass. This model allows identification of herbicides that might potentially bioconcentrate in sugarcane and sugar products. The herbicides with highest BCF that would most probably be found in plant are sulfentrazone > picloram > tebuthiuron > hexazinone > metribuzin > simazine > ametryn > diuron > clomazone > acetochlor. Overall herbicides with the highest BCF or potential to be absorbed are also the ones with highest potential to leach to groundwater.
ACKNOWLEDGEMENTS:
This work was supported by The State of São Paulo Research Foundation (FAPESP), Brazil.
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Publication Dates
-
Publication in this collection
Apr 2015
History
-
Received
29 June 2012 -
Accepted
02 Dec 2013