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Published online 4 January 2008
Published in J Environ Qual 37:90-97 (2008)
DOI: 10.2134/jeq2007.0210
© 2008 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
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TECHNICAL REPORTS

Organic Compounds in the Environment

Reactivity of the Plant Growth Regulator Paclobutrazol (Cultar) with Two Tropical Soils of the Northeast Semiarid Region of Brazil

Mônica Lúcia Milfonta, Jean Manuel Fonseca Martinsb,*, Antonio Celso Dantas Antoninoa, Ester Ribeiro Gouveiac, André Maciel Nettoa, Véronique Guinéb, Hélène Masb and Maria Betânia Galvão dos Santos Freired

a Universidade Federal de Pernambuco, Departamento de Energia Nuclear, Av. Prof. Luiz Freire 1000, Cid. Universitária, CEP 54740-540, Recife, PE, Brazil
b Laboratoire d'étude de Transferts en Hydrologie et Environnement (LTHE), Univ. Grenoble I, CNRS, BP 53, 38041, Grenoble Cedex 09, França
c Universidade Federal de Pernambuco, Departamento de Antibióticos, Av. Prof. Arthur de Sá, Cid. Universitária, Recife, PE, Brazil
d Universidade Federal Rural de Pernambuco, Departamento de Agronomia, Rua Dom Manoel de Medeiros, s/n, Dois Irmãos, CEP 52171-900, Recife, PE, Brazil

* Corresponding author: (jean.martins{at}hmg.inpg.fr).

Received for publication April 30, 2007.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Materials and Methods
 Results and Discussion
 Conclusion
 REFERENCES
 
The reactivity of paclobutrazol (PBZ, a plant growth retardant) with a Yellow Ultisol and a Vertisol from the semiarid northeast region of Brazil was evaluated through batch sorption experiments and modeling. Although not instantaneous, the sorption kinetic of PBZ (pure and formulated) was fast (a few hours) in both soils. The sorption kinetics were well described by a second-order (Formula=k2(Se2–St)2) but not by a first-order model. The sorption isotherms were found to be linear and the calculated KD values were 8.8 ± 0.11 and 7.4 ± 0.2 L kg–1 for pure PBZ in the Ultisol and the Vertisol, respectively. The corresponding KOC values were 1275 ± 34 (logKOC = 3.11) and 1156 ± 49 (logKOC = 3.06) L kg–1, respectively. Considering the very different texture of the two soils and the similar KOC values determined, these results showed that in both soils, the sorption of PBZ is dominantly controlled by organic matter, although some interactions of PBZ with iron oxides (goethite) were observed in the Ultisol. Based on these sorption parameters a low leachability potential of PBZ in soils is anticipated, as they correspond to a groundwater ubiquity score (GUS) ranging from 2.0 to 2.7, i.e., moderately to not mobile, in contradiction with the actual groundwater situation in Brazil. This work stresses the need to evaluate and predict the risk associated with aquifer contamination by this widely used plant growth regulator.

Abbreviations: PBZ, Paclobutrazol • HPLC, high-performance liquid chromatography • TOC, total organic carbon content • BSSS, Brazilian Society of Soil Science


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Materials and Methods
 Results and Discussion
 Conclusion
 REFERENCES
 
THE mango is the tropical fruit that is most produced in the world, being approximately 50% of all tropical fruits produced. It is an important agricultural product for the economy of the developing countries in the tropics, for both domestic trade and export (Jedele et al., 2003). In 2005 FAO (FAO, 2005) estimated mango world production over 26 million tons, and Brazil is the third largest mango exporter, after Mexico and India, with 138,189 tons, i.e., 16% of global mango exports (FAO, 2005). A large percentage of Brazilian mango is produced in the semiarid zone of the northeast region of Brazil, in the irrigated fruit agriculture hub of the São Francisco River Valley. In this region, large amounts of pesticides are used (Laabs et al., 2002) and especially plant growth regulators that are used to control mango production. Plant growth regulators are organic compounds non-toxic to plants and are used to regulate plant growth without changing developmental patterns (Rademacher, 2000). The largest group of plant growth regulators consists of chemicals antagonistic to the hormone gibberellins (Fletcher et al., 2000). Among this group, paclobutrazol (PBZ) is the growth regulator mostly used in the irrigated mango orchards of the São Francisco River Valley. Paclobutrazol is generally applied annually as root and foliar treatments, although soil application has shown better efficiency for the enhancement of a plant's reproductive growth (Singh and Ram, 2000). Both soil and foliar applications require the use of highly concentrated solutions (up to 7.5 g L–1) of plant growth regulators (Orzolek and Kaplan, 1988), resulting in high levels of growth regulator residues in both crops, superficial waters, and soils (Davis et al., 1988, EMBRAPA, 2001). Residues of growth regulators in roots and shoots of edible crops are known to be hazardous to human and other animal health (Castro et al., 2004). This is why application of growth regulators to plants is now restricted (Davis et al., 1988). The increase in soil areas cultivated with mangoes all over the world, even in Brazil, has led to an increasing use of these types of chemicals in the last two decades and especially that of PBZ (Voon et al., 1992). Recent studies on the contamination of superficial waters and aquifers of the São Francisco River Valley demonstrated the potential, ample presence of PBZ, evidencing this chemical as a major contaminant of aquifers in this region (EMBRAPA, 2001; Ferracini et al., 2001). Indeed, after application, PBZ is partially taken up by plants (Attiya et al., 1983; Orzolek and Kaplan, 1988; Singh and Ram, 2000; Sharma and Awasthi, 2005) and quite quickly transported to the underlying aquifer by rainfall washing (Jones et al., 1989; Salazar-Garcia and Vazquez-Valvidia, 1997) and infiltration. The first point is quite well documented, as plant uptake of PBZ is important and known to be dependent on rain washing efficiency when applied as a foliar spray (Jones et al., 1989; Salazar-Garcia and Vazquez-Valvidia, 1997). Surprisingly and despite the huge amounts of PBZ applied to crops and the groundwater contamination risk associated with PBZ-like molecules (Voon et al., 1992; EMBRAPA, 2001; Ferracini et al., 2001; Paraíba et al., 2003; Armas et al., 2005), its fate and toxicity is still unclear and poorly documented. Indeed, most studies on PBZ deal with degradation processes (Jackson et al., 1996; Silva et al., 2003a, Silva et al., 2003b), uptake by plants, or formation of bounded residues in both soils and fruits (Adriansen and Ogaard, 1997; Jacyna and Dodds, 1999; Singh and Ram, 2000; Sancho et al., 2003; Armas et al., 2005). Little is known about PBZ reactivity with soil minerals and organic matter and how it affects transport processes related to rainfall infiltration (Hornsby et al., 1996; Dy, 2003; Shalini and Sharma, 2006). Concerning PBZ persistence and degradation, some field studies have demonstrated that paclobutrazol can be considered as highly persistent with field half-life values ranging from 180 to 973 d (Hornsby et al., 1996; Jacyna and Dodds, 1999; Sharma and Awasthi, 2005). However, other studies have shown that PBZ can be rapidly degraded by pure cultures of specific soil microorganisms such as Pseudomonas sp. or Acinetobacter and sometimes to almost undetectable levels with resulting half-life values of 13 to 95 d (Jackson et al., 1996; Silva et al., 2003a; Silva et al., 2003b). Some studies on PBZ sorption in soil have shown that its sorption can be controlled by clay and total organic carbon (TOC) content but varies strongly with soil type (Hornsby et al., 1996; Dy, 2003; Shalini and Sharma, 2006), providing a large range of linear or nonlinear soil/water distribution coefficients (KD and Kf) ranging from 140 to 940 (Hornsby et al., 1996; Dy, 2003). These wide ranges of sorption and degradation parameters make accurate predictions of PBZ transport very difficult. To accurately evaluate the risk associated with the increasing use of PBZ worldwide (Voon et al., 1992), there is a strong need to study the leaching potential of such chemicals in unsaturated soils (Hinz et al., 1998) and especially of PBZ (Jackson et al., 1996; Lanchote et al., 2000; EMBRAPA, 2001; Sharma and Awasthi, 2005). In particular, the knowledge of PBZ reactivity with soil constituents and its transport in the soil solution or through soil runoff would be of great interest for the assessment of the risk associated with PBZ application in mango orchards.

The main objective of the present work was to evaluate and model the reactivity (sorption isotherms and kinetics) of pure and formulated PBZ with the constituents of two contrasted soils. These soils originated from irrigated mango orchards that present a high risk of aquifer contamination and are representative of the northeast region of Brazil. The effects of soil texture and PBZ formulation (presence of mobile colloidal particles) on sorption kinetics and isotherms were evaluated. The leaching potential of PBZ in relation with these sorption parameters was evaluated, on the field scale, on the basis of the calculated GUS risk indices (Gustafson, 1991).


   Materials and Methods
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results and Discussion
 Conclusion
 REFERENCES
 
Chemicals and Solutions
Paclobutrazol ([2RS, 3RS]-1-(4-chlorphenyl)-4–4dimethyl-2-(1H-1,2,4-triazole-1-yl)pentan-3-ol, CASRN 76738–62–0) is a growth regulator of various fruit plants, such as mango, citrus, avocado, etc. Paclobutrazol (C15H20N3ClO) is generally applied to trees as foliar or root spray at concentrations up to 6 g a.i. L–1. Paclobutrazol is the active ingredient (26.5%, w/w) of Cultar 250SC formulation (250 g L–1 active ingredient and 750 g L–1 inert ingredients) produced and provided by Syngenta, Proteção de Cultivos LTDA, São Paulo, Brazil. Paclobutrazol is widely used in mango growth control in Brazil. Pure white crystals (99.5%) of PBZ (Fig. 1 ) were obtained from Sigma Aldrich (France) and were used to prepare standard solutions and perform sorption experiments. Paclobutrazol is photo (>10 D, pH 7) and thermally (>6 mo, 50°C) stable. The main physical chemical properties of PBZ are as follows (Worthing and Hence, 1994; Magnitskiy, 2004): molecular mass, 293.8 g mol–1; solubility in water at 20°C, 0.026 to 0.035 g L–1; melting point, 165 to 166°C, average KOC 540 ± 400 L kg–1; vapor pressure at 20°C, 7.5 10–9 Pa; average half-life (T1/2) in soil, 180 to 973 d.


Figure 1
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  		Fig. 1. Chemical structure of paclobutrazol.

 
Paclobutrazol Analysis
Using a 1100 HPLC system (Agilent), an analytical method (Vaz et al., 2007) was adapted from Silva et al. (2003a) to detect and quantify PBZ in soil solution. The HPLC was equipped with a C18 ODS column 250 x 4.6 mm x 5 µm (Beckman Ultrasphere). The HPLC method was based on an isocratic elution pattern (methanol/water, 70/30) 0.75mL min–1 and a UV-VIS detector (PBZ detection at 225 nm). Methanol was added to the samples in a sample/methanol ratio of 70/30. When necessary, soil suspensions were centrifuged for 10 min at 9000 g to remove soil particles and filtered at 0.2 µm before HPLC analysis (the absence of adsorption on the cellulose filters was checked with PBZ standards). The detection limit of the PBZ analysis procedure was estimated as 0.01 mg L–1.

Cultar Formulation Analysis
As they were not available from the manufacturer, some properties of the Cultar formulation were characterized by laser granulometry, XRD analysis, and optical microscopy and are reported in the Results section. The particles of Cultar formulation were characterized by means of a Malvern Mastersizer 2000 Laser Granulometer, equipped with a variable-speed fluid module that measured the particle size of water-borne Cultar particles by the diffraction of laser light into 52 photodiode detectors. The instrument routinely measures particles in the size range 0.02 to 2000 µm. The technique is nondestructive and rapid, with a sample processing time of 3 to 4 min. The basic operating procedure requires samples to be dispersed in water to form slurry into the fluid module. A light microscope (Axioscope, Zeiss France) equipped with an interferential phase contrast (IPC) device, and a digital camera (DP50, Zeiss) was used for image recording to analyze the shape and size of water dispersed Cultar particles.

Similarly, the mineralogy analysis of the particles of Cultar was performed on air-dried samples on a glass slide using a Bruker D501 diffractometer equipped with a Kevex Si (Li) solid-state detector. Intensities were recorded at 0.02°2 steps, from 5 to 80°2, using a counting time of 4 s per step.

Soils
The soils samples were collected in irrigated mango orchards (Mangifera indica L. cv. Tommy Atkins) at the Bebedouro and Mandacaru Experimental Stations of the Brazilian Organization for Agriculture and Animal Research (EMBRAPA Semi-Arido) in the Municipalities of Petrolina (9°09' S, 40°22' W), Pernambuco state, and Juazeiro (9°24' S, 40°24' W), Bahia state, located in the São Francisco river valley, northeast Brazil. These soil types represent over 60% of Brazilian agricultural soils. The region's climate is hot and semiarid, with an annual average air temperature, Ta, of 26.2°C (± 0.9) and an annual rainfall, r, of 535.8 mm (± 180.3) (Azevedo et al., 2003) concentrated in the rainy season period between January and April. The two representative soils, a Yellow Ultisol (Bebedouro) and a Vertisol (Mandacaru), present no historical use of PBZ. In April 2000, disturbed soil samples were collected at the soil surface (0–0.2m) in triplicate and pooled before analysis (Table 1 ). After collection the soils were spread out on trays, air dried, sieved at 2 mm, and stored at room temperature. Mineralogy analysis of the soils was performed on selected air-dried samples. The XRD patterns were recorded using the same method used to analyze Cultar particles and given above (Bruker D501 diffractometer equipped with a Kevex Si (Li) solid-state detector).


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  		Table 1. Main physical-chemical properties of the two Brazilian soils (CEC, cation exchange capacity).

 
Sorption Kinetics
The sorption kinetic studies were conducted at 25°C in batch reactors of 100 mL with a soil/solution ratio of 5/50, using the procedure described by Martins and Mermoud (1998). Sorption kinetics were determined at 25 mg L–1 for both pure and formulated paclobutrazol. The procedure involved the collection of 1 mL (in triplicate) of soil suspension from the flasks at 0, 0.08, 0.17, 0.33, 0.75, 0.83, 1.2, 4, 8, and 24 h, followed by centrifugation at 9000 g for 15 min and filtration at 0.2 µm (cellulose filter) of the supernatants. 0.3 mL of MeOH were added to 0.7 mL of the sample and analyzed immediately by HPLC.

Sorption Isotherms
Adsorption isotherms were determined according to the experimental procedure described by Martins and Mermoud (1999). Three grams of dry soil were mixed with 30 mL of PBZ solution (soil/solution ratio of 10) at different concentrations in 50 mL polypropylene centrifuge tubes. The samples were prepared in triplicate at initial concentrations of 0, 0.1, 0.25, 1.0, 2.5, 10, and 25 mg L–1. The soil suspensions were shaken at 200 rpm on a rotary tumbler during 24 h (sufficient to achieve sorption equilibrium as shown by sorption kinetics) at 25°C and then centrifuged for 15 min at 9000 g. The supernatant was filtered through a 0.2 µm cellulose filter and then followed the same preparation as for the sorption kinetic samples. The quantity of pesticide sorbed to the soil was assumed to be the difference between that initially present in solution (measured in triplicate with controls) and that remaining in solution after equilibrium (Eq. [2]). Indeed, PBZ degradation is unlikely to occur within 24 h, since literature reported T1/2 values for PBZ ranging between 180 and 973 d. Sorption reversibility was not evaluated, so that irreversible adsorption processes cannot be totally avoided.

Sorption Modeling
The sorption process can be defined as the partitioning of a chemical between solid and liquid phases. At environmental concentrations (low concentrations), the adsorption isotherm of organic pollutants in soil can frequently be considered as linear. In this case, sorption isotherms can be represented by (Martins and Mermoud, 1998):

Formula 1[1]
where S is the sorbed chemical concentration (M m–1), Ce is the liquid chemical concentration at equilibrium (M L–3), and KD is a distribution coefficient (L3 m–1).

S is determined using Eq. [2] (Martins and Mermoud, 1998):

Formula 2[2]
where Co is the initial chemical concentration (M M–1), and FD is a factor of dilution.

As the major sorbing phase in natural soils for organics is the organic matter, a more global parameter can be used to describe the soil sorption capacity, KOC, calculated as follows (Martins and Mermoud, 1998):

Formula 3[3]
where OC% is the total organic carbon content of the soil.

The kinetics of sorption processes can be represented mathematically using the following first order model (Lagergren, 1898, Yaneva and Koumanova, 2006):

Formula 4[4]
where Se1 and St are the sorption capacity (M M–1) at equilibrium and at time t, respectively, and k1 is the rate constant of first order sorption (T–1). After integration and application of the boundary conditions t = 0 to t = t and St = 0 to St = St, Eq. [4] becomes:

Formula 5[5]
To obtain the sorption rate constants, ln(Se1 – St) was plotted against t for PBZ, k1 being the slope of the obtained linear plot.

If the sorption rate follows a second-order mechanism, the sorption kinetic is expressed as (Lagergren, 1898, Yaneva and Koumanova, 2006):

Formula 6[6]
where Se2 and St are the sorption capacity (M M–1) at equilibrium and at time t, respectively, and k2 is the rate constant of second-order sorption (M M–1T–1). After integration and application of the boundary conditions t = 0 to t = t and St = 0 to St = St, Eq. [6] becomes:

Formula 7[7]
It can be rearranged in a linear form:

Formula 8[8]
where ks can be regarded as the initial sorption rate St/t->0 (Lagergren, 1898; Yaneva and Koumanova, 2006)


   Results and Discussion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
Conclusion
 REFERENCES
 
Soil Analysis
The results of the analysis of the two tropical soils are presented in Table 1. The two soils had almost neutral pH and quite low, but similar TOC and nitrogen contents. The Yellow Ultisol and the Vertisol strongly differ in clay content and in cation exchange capacity (CEC), resulting in a better aggregation of the Vertisol, and suggesting a potentially higher reactivity of this soil with chemicals. The major crystallized minerals of these two soils were identified by XRD analysis (Table 1). The Yellow Ultisol appears to be composed of quartz, kaolinite, calcite, goethite, and potassium feldspath. The Vertisol contains mainly quartz, kaolinite, and potassium feldspath, but no goethite (no iron oxides). The Yellow Ultisol and the Vertisol were respectively classified as sand and clay soils, according to the classification of both the Brazilian and American soil science societies. The particle size distribution curves of the two soils (data not shown), measured by laser granulometry, confirmed that contrarily to the Yellow Ultisol, the Vertisol is well structured and contains a high amount of colloidal particles of less than 0.2 µm in diameter.

Cultar Formulation Analysis and Solubilization
The particle size distribution curve of a Cultar suspension obtained by laser granulometry is presented in Fig. 2 . The curve shows that most particles of the Cultar formulation present a size ranging between 0.2 and 10 µm. At least two main particle classes containing PBZ can be detected, with sizes ranging from 0.2 to 1 µm and from 1 to 10 µm. The size of Cultar particles is an important parameter since PBZ is associated with these particles that are formulated by the manufacturer to regulate PBZ release to the plants in the field. Thus, they control PBZ solubilization kinetics, and possibly its facilitated transport.


Figure 2
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  		Fig. 2. Size distribution of Cultar formulation particles measured by laser granulometry (Mastersizer 2000 laser Granulometer, Malvern, UK).

 
The observation of Cultar particles under light microscopy (x1000) revealed that most of the observed particles presented a size under 10 µm, the most numerous being the 0.2- to 1.0-µm diameter particles. This result thus confirms the laser granulometry results. In the field, the mobility of formulated PBZ is therefore probably linked to particle mobility that depends strongly on their shape and size, especially those, numerous ones, that present colloidal size and properties, as already shown for various other pollutants (e.g., Citeau et al., 2003; Guiné et al., 2003).

Unfortunately, the XRD analysis of Cultar formulation did not permit the identification of these particles, as it corresponded to none of the available references.

To account for the solubilization kinetic of PBZ from Cultar in the sorption kinetics, the time needed to solubilize the formulated PBZ (Cultar) in distilled water was measured by HPLC. The results (data not shown) showed that concentration reaches the solubility of PBZ (36 mg L–1) almost instantaneously, i.e., within less than 1 h, that is the shortest time needed to process the samples until HPLC analysis. This result suggests that the PBZ solubilization process from Cultar should not affect the sorption kinetics of formulated PBZ.

Sorption Kinetics
To estimate the time needed to achieve sorption equilibrium, the kinetics of PBZ sorption in the two soils were performed at 25 mg L–1 and are presented in Fig. 3 . The results showed that at equilibrium, the Yellow Ultisol sorbs slightly more PBZ (formulated or not) than the Vertisol (Se2 ranged between 90.9 and 92.6 mg kg–1 for the Yellow Ultisol and between 84 and 86.2 mg kg–1 for the Vertisol). Furthermore, it appeared that the sorption equilibrium of both pure and formulated PBZ is achieved within 8 h. Although fast, the sorption process can thus not be considered as instantaneous, suggesting that the sorption is slightly under diffusion-kinetic control. To account for this kinetic effect in the sorption process, the curves were fitted with both first-order (Eq. [4] and [5]) and second-order (Eq. [6] and [7]) models. The corresponding parameters are presented in Table 2 . The first-order model was unable to adequately fit the data points as shown by the much underestimated values of Se1 (Table 2). Contrarily, the second-order model (solid and dashed lines in Fig. 4 ) fitted the data very well and presented R2 values close to 1 for both soils and for pure and formulated PBZ. The applicability of this model is indeed validated by the linearity observed in Fig. 4, when plotting t/St vs. t (Eq. [8]).


Figure 3
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  		Fig. 3. Sorption kinetics of pure (a) and formulated (b) paclobutrazol in aqueous suspensions of Yellow Ultisol and Vertisol. Lines correspond to best fits of the data obtained with Eq. 6. Error bars correspond to ± 1 standard deviation, calculated from triplicates.

 

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  		Table 2. Sorption parameters of pure and formulated Paclobutrazol.

 

Figure 4
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  		Fig. 4. Linearized sorption kinetics of pure (a) and formulated (b) paclobutrazol in aqueous suspensions of Yellow Ultisol and Vertisol. The data were linearized using Eq. [7] describing a second-order model of sorption kinetics.

 
The shape of the PBZ sorption kinetics is very similar in both soils suggesting the involvement of similar sorption mechanisms. Furthermore, these results suggest that the soil structure is not involved in the sorption kinetics. Indeed, contrarily to the Yellow Ultisol that is a dispersed sandy soil, the Vertisol (a clayey soil) is very well structured and aggregated as confirmed by laser granulometry (data not shown). As the adsorption capacity is very similar in both soils despite their different aggregation, it suggests that the sorption kinetics of PBZ is mostly controlled by the chemical sorption process itself, rather by a diffusion process.

Consequently, an equilibration time of 24 h was chosen for performing sorption isotherms as it permits sorption equilibrium and is short enough to neglect PBZ degradation.

Sorption Isotherms
The sorption isotherms of PBZ in the two tropical soils were found to be linear for both formulations (Fig. 5 ). Modeled sorption isotherms were derived from a least squares regression of the data using Eq. [1] and are presented in Fig. 5 (solid and dashed lines) with the data obtained from the sorption kinetic results (closed symbols). Calculated KD and KOC values (Eq. [3]) are presented in Table 2. The calculated KD values were 8.8 ± 0.11 and 7.4 ± 0.2 L kg–1 for pure PBZ in the Yellow Ultisol and the Vertisol, respectively. The corresponding KOC values were 1275 ± 34 (logKOC = 3.11) and 1156 ± 49 (logKOC = 3.06) L kg–1, respectively. For the formulated PBZ (dissolved from Cultar), the calculated KD values were 9.2 ± 0.1 and 7.8 ± 0.21 L kg–1 in the Yellow Ultisol and the Vertisol, respectively. The corresponding KOC values were 1333 ± 33 (logKOC = 3.12) and 1218 ± 52 (logKOC = 3.08) L kg–1, respectively. These values are in the upper range of the few available literature data (Hornsby et al., 1996; Adriansen and Ogaard, 1997; Dy, 2003). These findings thus confirm the important reactivity of this chemical with natural soil constituents. It should be noted that since adsorption reversibility was not evaluated, irreversible sorption processes cannot be totally eliminated.


Figure 5
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  		Fig. 5. Sorption isotherms of pure (a) and formulated (b) paclobutrazol in aqueous suspensions of Yellow Ultisol and Vertisol (soil/solution ratio = 10). Error bars correspond to ± 1 standard deviation, calculated from triplicates.

 
Paired comparisons were made between the two soils and between the pure and formulated PBZ. In paired comparisons of the two soils with similar pH and TOC, although the KD values are close for the two soils, the Vertisol was less sorptive despite a much larger percentage of clay (48.1% compared to 7% for the Yellow Ultisol). This is somewhat surprising and suggests that PBZ does not undergo any interaction with the clay minerals of this soil contrarily to what has been previously reported (Adriansen and Ogaard, 1997). Probably the nature and reactivity of the clay minerals present in their soils were different from those of the Vertisol used in the present study. Indeed, the XRD analysis of these soils (Table 1) showed that the Vertisol contains mainly kaolinite that is known to be poorly reactive due to its low specific surface.

Furthermore, considering the very different texture of the two soils, and since the measured KOC values are close, these results show that, whatever the formulation, PBZ sorption is dominantly controlled by the organic matter in both soils, probably through hydrophobic bonding. Indeed, although slightly polar, this molecule is mainly composed of apolar groups probably responsible for its preferential affinity for soil organic matter as already reported for many other chemicals (e.g., Martins and Mermoud, 1998; Rice et al., 2004; Nkedi-Kizza et al., 2006). The formation of PBZ-bound residues in soils induced by interactions with soil organic matter have also been reported (Jacyna and Dodds, 1999; Singh and Ram 2000; Sancho et al., 2003), thus supporting the hypothesis of a major implication of OM in PBZ retention in these soils. This result is also supported by the sorption kinetic results since they showed that PBZ sorption is mostly controlled by the adsorption process rather by a kinetic process (quite fast sorption process, <24 h).

Besides PBZ sorption onto OM, some interactions can also be attributed to iron oxides (goethite) in the Yellow Ultisol. This secondary sorption mechanism is responsible for the slightly higher reactivity of PBZ with this soil compared to the Vertisol that was estimated as 10% of the total reactivity, on the basis of the KD values. The polar groups (mostly the azole and hydroxyl groups) of the molecule are probably responsible for this type of interaction with the positively charged iron oxides (Hofstetter et al., 2003). This phenomenon was also observed in a parallel study involving the same soils, with a bromide tracer, which was shown to adsorb to the Ultisol only (Milfont et al., 2006).

In paired comparisons of pure PBZ and Cultar, KD and KOC values were found to be quite close but significantly different and higher for the formulated PBZ. This suggests that the formulation material only slightly enhances PBZ sorption, probably through a solubilization control that modifies PBZ reactivity with the soil constituents. This process would need further investigations with specific attention to the identification of the formulation constituents.

Evaluation of Water Contamination Risk from Paclobutrazol
Because of the quite strong sorption of PBZ measured in the two tropical soils and its low degradation, this plant growth retardant is likely to persist in surface soils for years, as already suggested (Jackson et al., 1996; Dy, 2003; Sharma and Awasthi, 2005). The risk of PBZ leaching to surface runoff is therefore important and may lead to PBZ transport to nearby water bodies or downward to aquifers. To evaluate this risk, we calculated the groundwater ubiquity score (GUS indices) of PBZ using the following equation (Gustafson, 1991):

Formula 9[9]
To calculate PBZ GUS indices, we used the sorption parameters determined in the present study (average KOC = 3.1) and an average half-life value of 200 d taken from Hornsby et al. (1996). The calculated GUS values ranged from 2.0 to 2.7 for both pure and formulated PBZ. This result indicates that PBZ is intermediately to not mobile, following Gustafson (1991).

This study thus confirms that PBZ is likely to persist in vulnerable tropical soils in an adsorbed form, protected by organic matter. As such it may be subject to transport associated with dissolved or particulate organic matter, like other highly reactive contaminants (Monrozier et al., 1993, Spack et al., 1998). Furthermore, as a persistent molecule, it may also affect mango growth in short-term rotation crops (Jackson et al., 1996; Singh and Bhattacherjee, 2005).

As a consequence, the study of PBZ transport in such soils appears urgent to evaluate the real risk of groundwater contamination associated with this molecule and its potential toxicity to humans through bio-magnification.


   Conclusion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
 Conclusion
REFERENCES
 
The present study concerned the characterization and modeling of the reactivity of PBZ, a plant growth retardant, with a Yellow Ultisol and a Vertisol, representative of the semiarid northeast region of Brazil. Sorption results showed that, although not instantaneous, the sorption kinetics of PBZ (pure and formulated) was fast (a few hours) in both soils, and well described by a second-order model but not by a first-order model. The sorption isotherms were found to be linear with quite high KD and Koc values for both pure and formulated PBZ in the Ultisol and in the Vertisol. Considering the very different texture of the two soils and the similar KOC values determined, this study showed that in both soils, the sorption of PBZ is dominantly controlled by organic matter, although some interactions of PBZ with iron oxides (goethite) were observed in the Ultisol. Based on these sorption parameters a low leachability potential of pure or formulated PBZ in soils is anticipated, as they correspond to a GUS indices ranging from 2.0 to 2.7, i.e., moderately to not mobile, in contradiction with the actual groundwater situation in Brazil. This work stresses the need to evaluate comprehensively and predict the risk associated with aquifer contamination by this widely used plant growth regulator.


   ACKNOWLEDGMENTS
 
This work was supported by the CNPq CAPES-COFECUB grant No. 550160/02-7, Edital do CT-HIDRO No. 01/2001, 303790/02-4, 140968/2002-4 e 505452/03-0. The authors gratefully acknowledge Nicolas Geoffroy (LGIT, CNRS, France) for XRD analysis of soils and Cultar suspensions, as well as Antonio Marques da Silva and Ricardo Paixão da Silva for their useful technical help. The authors would like to thank Thomas H. Greenhalgh (DEN, UFPE, Brazil) for language revision.


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 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
 Conclusion
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